WO2021237309A1

WO2021237309A1 - Multi-stimuli responsive metal-organic frameworks

Info

Publication number: WO2021237309A1
Application number: PCT/AU2021/050527
Authority: WO
Inventors: Lyndon HALL; Dylan A. Sherman; Ryuichi Murase; Eleanor KEARNS; Qinyi GU; Deanna M. D'alessandro
Original assignee: The University Of Sydney
Priority date: 2020-05-28
Filing date: 2021-05-28
Publication date: 2021-12-02

Abstract

This invention relates to multi-stimuli responsive metal-organic frameworks (MOFs) comprising a repeat unit of the formula M2(L1)2(L2)2. Also disclosed are methods of preparation of multi-stimuli responsive MOFs of the formula M2(L1)2(L2)2 as well as their uses.

Description

Multi-stimuli responsive metal-organic frameworks

This application claims priority to Australian provisional patent application no. 2020901751 (filed on 28 May 2020), the entire contents of which is hereby incorporated by reference. Field of the invention

The present invention relates generally to the fields of chemistry and materials science. More particularly, the invention concerns metal-organic frameworks capable of undergoing reversible photo-induced cycloaddition reactions and uses thereof.

Background of the invention Metal-organic frameworks are a class of functional materials derived from the reaction of metal nodes with organic linkers to form supramolecular assemblies that exhibit properties including nanoscale porosity, large internal surface areas and well- defined cavities and voids throughout their structures. Many thousands of metal-organic frameworks have been investigated for use in gas storage and separation processes. There is a need for metal-organic frameworks with improved tunability.

Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the invention

Metal-organic frameworks (MOFs)

In a first aspect the present invention provides a metal-organic framework (MOF) comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion;

L1 is a ligand comprising at least two carboxylates,

1

Substitue Sheets

(Rule 26) RO/AU wherein the at least two carboxylates are selected from the group consisting of aryl carboxylate(s), heteroaryl carboxylate(s), and combinations thereof; wherein two of the at least two carboxylates are linked via a conjugated system that is substantially linear or linear; and wherein oxygen atoms of two of the at least two carboxylates are coordinated to one or both of M;

L2 is a ligand that i

wherein pyr is an optionally substituted pyridyl;

Z is selected from the group consisting of S, Se and Te;

X is optional and when present is a linker such that pyr is conjugated with is a bond;

such that

wherein pairs of L2 ligands are at least substantially co-facial to one another; or

2

Substitue Sheets

(Rule 26) RO/AU pairs of L2 ligands are bonded to each other as follows:

wherein each M is coordinated to two L2 ligands via the basic nitrogen of the pyridyl.

L1

Preferably, L1 comprises two carboxylates.

The at least two carboxylates of L1 may be aryl or heteroaryl carboxylates. The at least two aryl or heteroaryl carboxylates of L1 may be aryl carboxylates. The at least two aryl or heteroaryl carboxylates of L1 may be heteroaryl carboxylates. Preferably, the at least two carboxylates of L1 have pKa’s from about 4 to about

6, preferably from about 4 to about 5.5. More preferably, the at least two carboxylates of L1 have pKa’s of about 5.

When the carboxylate(s) are aryl, the carboxylate(s) may be substituents on the functionality selected from the group consisting of phenylene, naphthylene, or combinations thereof. When the carboxylates are aryl, the carboxylates are preferably substituents on the functionality phenylene.

When the carboxylate(s) are heteroaryl, the carboxylates may be substituents on the functionality selected from the group consisting of thiophenylene, selenophenylene, tellurophenylene, furanylene, pyrrolylene, or combinations thereof. When the carboxylate(s) are heteroaryl, the carboxylates may be substituents on the functionality selected from the group consisting of thiophenylene, selenophenylene, or combinations

3

Substitue Sheets (Rule 26)

RO/AU thereof. When the carboxylate(s) are heteroaryl, the carboxylates are preferably substituents on the functionality thiophenylene.

The aryl or heteroaryl carboxylates may be fused and remain substantially linear or linear. The aryl or heteroaryl carboxylates may be bridged and remain substantially linear or linear. When the aryl or heteroaryl carboxylates are bridged, they may be bridged by an optionally substituted carbon chain that may be interrupted by one or more heteroatoms, or by an optionally substituted heteroatom. When the aryl or heteroaryl carboxylates are bridged, they are preferably bridged by a sulfone.

The substantially linear or linear conjugated system of L1 may be a bond or may comprise an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may comprise an alkylene group, a phenylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may consist of one or more alkylene groups, one or more alkynylene groups, one or more phenylene groups, one or more napthylene groups, one or more ether groups, one or more amine groups, one or more imine groups, one or more azo groups, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may consist of one or more alkylene groups, one or more phenylene groups, one or more ether groups, one or more amine groups, one or more imine groups, one or more azo groups, or combinations thereof.

L1 may be selected from the group consisting of:

4

Substitue Sheets

(Rule 26) RO/AU

wherein, Y is optional and when present is a substantially linear or linear linker such that the two carboxylates of L1 are conjugated with one another; and the aromatic or heteroaromatic rings may be optionally substituted.

L1 may be selected from the group consisting of:

wherein the aromatic or heteroaromatic rings may be optionally substituted.

5

Substitue Sheets

(Rule 26) RO/AU L1 may be selected from the group consisting of:

wherein, Y is optional and when present is a substantially linear or linear linker such that the two carboxylates of L1 are conjugated with one another; and the aromatic rings may be optionally substituted.

Preferably, L1 is

In some embodiments where Y is present, Y comprises an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof. In some embodiments where Y is present, Y is one or more alkylene groups, one or more alkynylene groups, one or more phenylene groups, one or more napthylene groups, an ether group, an amine group, an imine group, or an azo group. In some embodiments where Y is present, Y is an alkylene group, an alkynylene group, a

6

Substitue Sheets (Rule 26)

RO/AU phenylene group, a napthylene group, an ether group, an amine group, an imine group, or an azo group. In some embodiments where Y is present, Y is an alkylene group, a phenylene group, an ether group, or an azo group. In some embodiments where Y is present, Y is an alkylene group. In some embodiments where Y is present, Y is a phenylene group. In some embodiments where Y is present, Y is an ether group. In some embodiments where Y is present, Y is an azo group.

In some embodiments where Y is present, Y is selected from the group

, . some embodiments where Y is present, Y is selected from the group consisting of,

, and combinations thereof. In some embodiments where Y is present, Y is selected from the group consisting of,

, and

wherein pairs of L2 ligands are at least substantially co-facial to one another.

L2 may be such that pairs of L2 ligands are bonded to each other as follows:

7

Substitue Sheets

(Rule 26) RO/AU

When

pairs of L2 ligands are preferably co-facial to one another.

respective 2,3 and 6,7 double bonds of each

moiety may be separated from one another by a distance of about 3.5 to about 5.0 A. pyr may be unsubstituted, pyr may be an optionally substituted 4-pyridyl group, pyr is preferably an unsubstituted 4-pyridyl group. Z may be selected from the group consisting of S and Se. Z may be Se. Z is preferably S.

8

Substitue Sheets (Rule 26) RO/AU When X is present, X may comprise an alkylene group, an alkynylene group, a phenylene group, a naphthylene group, or combinations thereof. Preferably, X comprises an alkylene group, a phenylene group, or combinations thereof.

When X is present, X may be one or more alkenyl groups, or one or more phenylene groups. Preferably, X is an alkenyl group or a phenylene group.

9

Substitue Sheets (Rule 26) RO/AU In preferred embodiments of the first aspect, the present invention provides a metal-organic framework (MOF) comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion;

L1 is a ligand that is

wherein Y is selected from the group consisting of a bond,

combinations thereof;

L2 is a ligand that is

10

Substitue Sheets (Rule 26) RO/AU wherein X is selected from the group consisting of a bond,

combinations thereof; and each M is coordinated to 2 L2 ligands, and wherein pairs of L2 ligands are co- facial to one another. In preferred embodiments of the first aspect the present invention provides a metal-organic framework (MOF) comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion;

L1 is a ligand that is

wherein Y is selected from the group consisting of a bond,

and combinations thereof;

L2 is a ligand that is

11

Substitue Sheets

(Rule 26) RO/AU

wherein X is selected from the group consisting of a bond,

combinations thereof; and each M is coordinated to 2 L2 ligands; wherein the dotted line in L2 signifies that pairs of L2 ligands are bonded

other as follows:

12

Substitue Sheets

(Rule 26) RO/AU

Further embodiments of the first aspect

The following statements apply to the first aspect:

Y may be selected from the group consisting of a bond,

and

. Y may be selected from the group consisting of a bond,

Y may be a bond. Y may

may be

. Y may

may

may be

Y may

. When Y is a bond, L1 is biphenyl-4, 4’-

13

Substitue Sheets (Rule 26) RO/AU dicarboxylate. When Y is

L1 is 4,4’- stilbenedicarboxylate. When Y is

, L1 is 4,4’- azobisbenzoate. When Y

L1 is 4,4’-oxybisbenzoate.

X may be selected from the group consisting of a bond,

. X may

Each member of the pairs of L2 ligands may be coordinated to separate metal ions. In the pairs of L2 ligands co-facial to one another the respective 2,3 and 6,7 double bonds of each tetrathiafulvalene moiety may be separated from one another by a distance of about 3.5 to about 4.2 A.

2 L1 ligands of the repeat unit may be bridging ligands such that each M of the repeat unit is coordinated to 3 L1 ligands. As used herein, bridging ligand refers to a ligand that is coordinated to more than one metal ion through a single functional group. For instance, a ligand that is coordinated to two different metal ions through the two different oxygen atoms of the same carboxylate group is a bridging ligand.

14

Substitue Sheets

(Rule 26) RO/AU The 2 L2 ligands of the repeat unit that are coordinated to each M of the repeat unit may be coordinated to each M through the nitrogen atom on one of the pyridyl rings of each of the 2 L2 ligands.

The L1 ligands of the repeat unit may be coordinated to each M of the repeat unit through one of carboxylate groups of each L1.

The repeat unit may comprise a cyclic moiety of the formula

wherein LT is the remainder of the ligand L1 that does not include the carboxylate group depicted in the cyclic moiety.

The cyclic moiety of the repeat unit may further comprise bonds to a further L1 ligand, such that the cyclic moiety may be of the formula

The metal ions may possess octahedral geometry. The apical positions of the metal ions may be occupied by the L2 ligands.

The equatorial positions of the metal ions may be occupied by the L1 ligands.

15

Substitue Sheets (Rule 26) RO/AU Repetition of the M2(L1)₂ moiety may create undulating sheets in the MOF. The term ‘undulating sheets’ refers to sheets that undulate to deviate from a 2-dimensional plane in an amplitude and period that is substantially repeating. An undulating sheet of the invention is depicted in Figure 1c. The pairs of L2 ligands of the repeat unit may link the undulating sheets so as to form a structure that repeats in 3 dimensions.

The repeat unit of the MOF may comprise one or more guest molecules.

The one or more guest molecules of the repeat unit of the MOF may be solvent molecules, gas molecules, or combinations thereof. The guest solvent molecule(s) of the repeat unit of the MOF may be dimethylformamide (DMF), dimethylacetamide (DMA), diethylformamide (DEF) or combinations thereof. Preferably the guest solvent molecule is DMF.

The guest gas molecule(s) of the repeat unit of the MOF may be carbon dioxide, methane, hydrogen, nitrogen, oxygen or combinations thereof. The MOF may comprise from one to six DMF molecules per repeat unit. The

MOF may comprise one or two DMF molecules per repeat unit. The MOF may comprise one DMF molecule per repeat unit. The MOF may comprise two DMF molecules per repeat unit.

The MOF may comprise about 1 , about 1.2, about 1.4, about 1.5, about 1.6, about 1.8, or about 2 solvent molecules per repeat unit. Preferably the solvent molecule is DMF.

The framework of the MOF may occupy approximately one third of the unit cell volume.

The repeat unit of the MOF may possess the monoclinic space group P2₁/n. The repeat unit of the MOF may possess the monoclinic space group P2₁/c. The repeat unit of the MOF may possess the orthorhombic space group Pcc2.

The MOF may be an activated MOF, for example an activated MOF for gas adsorption. As used herein, an activated MOF is a MOF that substantially does not

16

Substitue Sheets (Rule 26)

RO/AU comprise guest solvent molecules. The pores of an activated MOF are open. Methods of activating MOFs are known in the art. These include solvent exchange to a solvent removable under reduced pressure and/or elevated temperature, supercritical CO₂ exchange, and freeze-drying. M may be one or more of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc,

Ru, Rh, Pd, Ag or Cd. In one embodiment, M is Cd, Zn or Co. In another embodiment M is Cd. Cd is preferred when L1 is:

In another embodiment M is Zn. Zn or Cd is preferred when L1 is

ferred. The metal ions may be in the +2 oxidation state.

Methods of preparation

In a second aspect, the present invention provides a method for preparing a MOF as defined in the first aspect,

17

Substitue Sheets

(Rule 26) RO/AU wherein

wherein pairs of L2 ligands are at least substantially co-facial to one another, the method comprising heating a mixture comprising: a metal salt; a compound of formula (III), or an acid thereof; a compound of formula (II); wherein formula (III) is a compound comprising at least two carboxylates, wherein the at least two carboxylates are selected from the group consisting of aryl carboxylate(s), heteroaryl carboxylate(s), and combinations thereof; and wherein two of the at least two carboxylates are linked via a conjugated system that is substantially linear or linear; wherein formula (II) is a compound that is

wherein pyr is an optionally substituted pyridyl;

Z is selected from the group consisting of S, Se and Te;

18

Substitue Sheets (Rule 26) RO/AU X is optional and when present is a linker such that pyr is conjugated with the

group of L2.

Formula (III)

Preferably, formula (III) comprises two carboxylates.

The at least two carboxylates of formula (III) may be aryl or heteroaryl carboxylates. The at least two aryl or heteroaryl carboxylates of formula (III) may be aryl carboxylates. The at least two aryl or heteroaryl carboxylates of formula (III) may be heteroaryl carboxylates.

Preferably, the at least two carboxylates of formula (III) have pKa’s from about 4 to about 6, preferably from about 4 to about 5.5. More preferably, the at least two carboxylates of formula (III) have pKa’s of about 5.

When the carboxylate(s) are heteroaryl, the carboxylates may be substituents on the functionality selected from the group consisting of thiophenylene, selenophenylene, tellurophenylene, furanylene, pyrrolylene, or combinations thereof. When the carboxylate(s) are heteroaryl, the carboxylates may be substituents on the functionality selected from the group consisting of thiophenylene, selenophenylene, or combinations thereof. When the carboxylate(s) are heteroaryl, the carboxylates are preferably substituents on the functionality thiophenylene.

The aryl or heteroaryl carboxylates may be fused and remain substantially linear or linear. The aryl or heteroaryl carboxylates may be bridged and remain substantially linear or linear. When the aryl or heteroaryl carboxylates are bridged, they may be bridged by an optionally substituted carbon chain that may be interrupted by one or

19

Substitue Sheets (Rule 26) RO/AU more heteroatoms, or by an optionally substituted heteroatom. When the aryl or heteroaryl carboxylates are bridged, they are preferably bridged by a sulfone.

The substantially linear or linear conjugated system of formula (III) may be a bond or may comprise an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may comprise an alkylene group, a phenylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof.

The substantially linear or linear conjugated system of formula (III) may be a bond or may consist of one or more alkylene groups, one or more alkynylene groups, one or more phenylene groups, one or more napthylene groups, one or more ether groups, one or more amine groups, one or more imine groups, one or more azo groups, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may consist of one or more alkylene groups, one or more phenylene groups, one or more ether groups, one or more amine groups, one or more imine groups, one or more azo groups, or combinations thereof.

Formula (III) may be selected from the group consisting of:

,

20

Substitue Sheets (Rule 26) RO/AU wherein, Y is optional and when present is a substantially linear or linear linker such that the two carboxylates of formula (III) are conjugated with one another; and the aromatic or heteroaromatic rings may be optionally substituted.

Formula (III) may be selected from the group consisting of:

, wherein the aromatic or heteroaromatic rings may be optionally substituted.

Preferably, formula (III) is

, or an acid thereof; wherein the aromatic rings may be optionally substituted.

Preferred optional substituents include C1-C4 alkyl and C2-C4 alkylene.

In some embodiments where Y is present, Y comprises an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof.

21

Substitue Sheets

(Rule 26) RO/AU In some embodiments where Y is present, Y is one or more alkylene groups, one or more alkynylene groups, one or more phenylene groups, one or more napthylene groups, an ether group, an amine group, an imine group, or an azo group. In some embodiments where Y is present, Y is an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, or an azo group. In some embodiments where Y is present, Y is an alkylene group, a phenylene group, an ether group, or an azo group. In some embodiments where Y is present, Y is an alkylene group. In some embodiments where Y is present, Y is a phenylene group. In some embodiments where Y is present, Y is an ether group. In some embodiments where Y is present, Y is an azo group.

In some embodiments where Y is present, Y is selected from the group

in some embodiments where Y is present, Y is selected from the group consisting of

, and combinations thereof. In some embodiments where Y is present, Y is selected from the group consisting of

, and

Formula (II)

Z may be selected from the group consisting of S and Se. Z may be Se. Z is preferably S.

When X is present, X may comprise an alkylene group, an alkynylene group, a phenylene group, a naphthylene group, or combinations thereof. Preferably, X comprises an alkylene group, a phenylene group, or combinations thereof.

22

Substitue Sheets

(Rule 26) RO/AU When X is present, X may be one or more alkenyl groups, or one or more phenylene groups. Preferably, X is an alkenyl group or a phenylene group.

The ^pyr groups may be trans with respect to each other on the

pyr

Embodiments concerning other parameters of the second aspect

The metal salt may be a nitrate, a sulfate or a halide. In one embodiment the metal salt is a nitrate salt. The metal salt may be a hydrate.

23

Substitue Sheets (Rule 26) RO/AU The mixture may further comprise one or more solvents. The one or more solvents may comprise one or more polar aprotic solvents, one or more linear or branched alcohols, water and combinations thereof. The one or more solvents may comprise one or more polar aprotic solvents. The polar aprotic solvents may be DMF, DMA and DEF and combinations thereof. The one or more solvents may comprise one or more linear or branched alcohols. The one or more linear or branched alcohols may be methanol, ethanol, n-propanol, 2-propanol and combination thereof. The one or more solvents may comprise water.

The mixture may further comprise:

• DMF, DMA and DEF and combinations thereof; and

• methanol, ethanol, n-propanol, FI2O, 2-propanol and combination thereof.

The mixture may further comprise:

• DMF, DMA and DEF and combinations thereof; and

• methanol, ethanol, n-propanol, and combination thereof.

The combination of DMF and ethanol in the mixture is preferred.

The metal salt, compound of formula (III) and compound of formula (II) may be present in a molar ratio of about 1 :1 :1.

The mixture may comprise a ratio of

• DMF, DMA and DEF and combinations thereof; to

• methanol, ethanol, n-propanol and combinations thereof of from about 10:1 to about 2:1 , or from about 8:1 to about 3:1 , or from about 6:1 to about 5:1.

Fleating may be performed in the absence of light.

Heating may be performed in a solvothermal reactor, optionally followed by heating in a convection oven.

24

Substitue Sheets

(Rule 26) RO/AU A portion of the heating may be performed in a solvothermal reactor. A portion of the heating may be performed in a convection oven.

Heating in the solvothermal reactor may be performed at a temperature from about 100 °C to about 200 °C, or at a temperature from about 120 °C to about 140 °C, or at about 130 °C.

Heating in the convection oven may be performed at a temperature from about 50 °C to about 150 °C, or at a temperature from about 70 °C to about 100 °C, or at about 80 °C.

Heating in the solvothermal reactor may be performed for a period of time from about 1 minute to about 1 hour, or from about 2 minutes to about 30 minutes, or about 10 minutes.

Heating in the convection oven may be performed for a period of time from about

4 hours to about 7 days, from about 4 hours to about 24 hours, from about 6 hours to about 18 hours, from about 1 day to about 7 days, or from about 2 days to about 6 days, or about 4 days.

In some embodiments heating may be performed for from about 3 days to about

5 days at a temperature from about 60 °C to about 100 °C. In one embodiment, heating is performed for about 4 days at about 80 °C.

In preferred embodiments of the second aspect, the compound of formula (III) is a compound of formula (Ilia) and the compound of formula (II) is a compound of formula (lla). In such embodiments the present invention provides a method for preparing a MOF as defined in the first aspect, the method comprising heating a mixture comprising: a metal salt; a compound of the following formula (Ilia):

acid thereof;

Substitue Sheets (Rule 26) RO/AU and a compound of the following formula (lla):

wherein Y is selected from the group consisting of a bond,

and combinations thereof; and

X is selected from the group consisting of a bond,

combinations thereof.

In embodiments where Y is present, Y may be selected from the group consisting of

, and combinations thereof. In embodiments where Y is present, Y may be selected from the group consisting of

may be selected from the group consisting of a bond,

26

Substitue Sheets (Rule 26) RO/AU

Y may be a bond. Y may

may be

may

may be

Y may Y may be . Y may be Y may be When Y is a bond, L1 is biphenyl-4,4’-

dicarboxylate. When Y

is L1 is 4,4’- stilbenedicarboxylate. When Y is L1 is 4,4’- azobisbenzoate. When Y

L1 is 4,4’-oxybisbenzoate.

X may be selected from the group consisting of a bond,

27

Substitue Sheets

(Rule 26) RO/AU X may be a bond. X may

may be

. X may

may

In a third aspect the present invention provides a method for preparing a MOF as defined in the first aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

the method comprising irradiation with light of a MOF as defined in the first aspect, wherein

Conversion from the MOF of the first aspect wherein L2 is

the MOF of the first

28

Substitue Sheets (Rule 26) RO/AU aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

SC-SC transformation. The light may be white light, visible light, ambient light, NIR, UV light or light with a spectral range of 300-600 nm. In one embodiment the light is UV light. In one embodiment the light has a spectral range of 300-600 nm.

Irradiation may be performed for at least about 2 minutes. Irradiation may be performed for at least about 15 minutes. Irradiation may be performed for about 1 hour. Irradiation may be performed for about 2 hours.

Irradiation may be performed for between about 15 minutes and about 2 hours, or for between about 15 minutes and 1 hour. Irradiation may be performed for at least about 2 minutes and about 2 hours, or for between about 2 minutes and about 1 hour. Irradiation may be performed for between about 1 hour and about 2 hours. Irradiation under ambient light may be performed for between about 1 hour and about 2 hours. A person skilled in the art would appreciate that irradiation times could vary depending upon the intensity and the wavelength of the light of irradiation.

In a fourth aspect the present invention provides a method for preparing a MOF as defined in the first aspect wherein L2 is

29

Substitue Sheets (Rule 26) RO/AU

pyr the method comprising heating a MOF as defined in the first aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

Heating may be carried out at a temperature between about 100 °C and about 200 °C, or at about 150 °C.

Heating may be carried out for a period of time between about 5 minutes and about 72 hours, or between about 10 minutes and about 48 hours, or about 48 hours.

The MOF may be heated as a slurry comprising DMF, DMA, DEF or combinations thereof. A slurry comprising DMF is preferred.

The heating may be performed in the absence of light.

In a fifth aspect the present invention provides a method for reversibly switching between a MOF defined in the first aspect wherein L2 is

30

Substitue Sheets (Rule 26) RO/AU

defined in the first aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

the method comprising:

(i) irradiation of the MOF defined in the first aspect wherein L2 is

, with light to provide the MOF of the first aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

31

Substitue Sheets (Rule 26) RO/AU

(ii) heating the MOF defined in the first aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

the MOF of the first aspect wherein L2 is

Conversion from the MOF of the first aspect wherein L2 is

32

Substitue Sheets (Rule 26) RO/AU

, to the MOF of the first aspect wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

transformation. The light may be white light, visible light, NIR, UV light or light with a spectral range of 300-600 nm. In one embodiment the light is UV light. In one embodiment the light has a spectral range of 300-600 nm.

Irradiation with light may be performed for at least about 2 minutes. Irradiation with light may be performed for at least about 15 minutes. Irradiation with light may be performed for about 1 hour. Irradiation with light may be performed for about 2 hours.

Irradiation with light may be performed for between about 15 minutes and about 2 hours, or for between about 15 minutes and 1 hour. Irradiation with light may be performed for at least about 2 minutes and about 2 hours, or for between about 2 minutes and about 1 hour. Irradiation with light may be performed for between about 1 hour and about 2 hours.

Irradiation under ambient light may be performed for between about 1 hour and about 2 hours. A person skilled in the art would appreciate that irradiation times could vary depending upon the intensity and the wavelength of the light of irradiation. Kinetics

33

Substitue Sheets (Rule 26) RO/AU information obtained for the series of MOFs of the invention incorporating the different L1 ligands shows that the rate of the photocyclisation is sensitive to the precise structural relationship of the two tetrathiafulvalene based chromophores.

The MOF may be heated as a slurry comprising DMF, DMA, DEF or combinations thereof. A slurry comprising DMF is preferred. The heating may be performed in the absence of light.

Use of MOFs

In a sixth aspect the present invention provides use of a MOF defined in any one of the previous aspects.

The use may be for electrochromic sensors, chemical monitoring systems, electrocatalysis, optoelectronics and electronic components.

The use may be for nanocarriers containing photo-valves for remote control drug delivery; ion channels for separation of alkali metal ions relevant to battery technologies and/or for desalination; ion channels for separation of lanthanoid ions, low-energy light and electrically-driven gas separation; sunlight driven actuators; or sorption or separation of gas molecules.

Use for ion channels for separation of alkali metal ions relevant to battery technologies is preferred. Use for ion channels for separation of alkali metal ions relevant to desalination is preferred. The alkali metal ions may be selected from the group consisting of lithium ions, sodium ions, potassium ions, and combinations thereof. Use for ion channels for separation of lanthanoid ions is preferred. The lanthanoid ions may be selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and combinations thereof.

34

Substitue Sheets (Rule 26)

RO/AU In a seventh aspect the present invention provides use of a MOF defined in the first or second aspect for sorption or separation of gas molecules.

The gas molecules may be one or more of: carbon dioxide, methane, nitrogen, oxygen and hydrogen.

The MOF of the invention may be stable in one or more oxidation states. The properties of the MOF of any one of the aspects of the invention may be further tuned by modifying the MOF's oxidation state.

In some embodiments the oxidation state of L2 is modified. In some embodiments L2 is subjected to conditions that oxidise L2 relative to the oxidation state of L2 prior to being subjected to those conditions. In some embodiments L2 is subjected to conditions that reduce L2 relative to the oxidation state of L2 prior to being subjected to those conditions.

In some embodiments L2 is in oxidation state L2°. In some embodiments L2 is in oxidation state L2-⁺ radical. In some embodiments L2 is in oxidation state L2²⁺. L2²⁺ is oxidised relative to L2-⁺ which in turn is oxidised relative to L2°. L2° is reduced relative to L2-⁺ which in turn is reduced relative to L2²⁺.

In some embodiments, L2 is in a combination of oxidation states selected from L2°, L2-⁺ and L2²⁺. In some embodiments, L2 is in a combination of oxidation states L2° and L2-⁺. In some embodiments, L2 is in a combination of oxidation states L2-⁺ and L2²⁺.

Definitions

refers to an alkylene that is of either E- or Z-geometry. Starting materials of a single isomer can isomerise undergo the conditions used to form the MOFs of the invention. This is tolerated by the MOFs of the invention. The structures

represent equivalent stereochemical requirements in azo and imino structures, respectively.

35

Substitue Sheets (Rule 26) RO/AU When Y is a bond, the two phenylene groups of L1 are directly bound to one another.

When X is a bond, the pyridyl groups of L2 are directly bound to the

group. “Substantially linear” refers to a feature that may deviate up to 65° from linearity.

The inventors have found that bent ligands with that deviate 55° from linearity can be used in MOFs of the invention and expect that a deviation of up to 65° will be tolerated. The skilled person will appreciate that chemical structures may adopt different conformations. Thus, “substantially linear” with respect to a chemical structural feature refers to the substantial linearity of a conformation that is a local minimum in energy. The substantially linear conformation need not be an absolute minimum in energy. “Substantially linear” with respect to a chemical structural feature does not refer to moieties other than those specified, for instance, it does not refer to substituents other than those specified or side chains. A “substantially linear or linear” feature may be substantially linear. A “substantially linear or linear” feature may be linear.

“At least substantially co-facial” refers to arrangements that are either substantially co-facial or co-facial. An arrangement that is “at least substantially cofacial” may be substantially co-facial. An arrangement that is “at least substantially cofacial” may be co-facial. Tetrachalcafulvalene refers to a structure

, wherein Z is a chalcogenide. Tetrathiafulvalene [TTF] is the S-containing member of the group.

Preferred optional substituents include C1 -C4 alkyl, C2-C4 alkylene and halo (F, Cl, Br, I). When the optional substituent is halo, Cl and Br are preferred. As used herein, except where the context requires otherwise, the term "comprise" and variations of the

36

Substitue Sheets (Rule 26) RO/AU term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

In the context of this specification the terms "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

The term "about" is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result.

Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 : Crystal structure of Compound 1 showing a) the single building unit, b) a single net, and c) undulating sheets of {Cd(bpdc)}n.

Figure 2: The cofacially arranged Py₂TTF moieties of a) Compound 1 , b) Compound 2 and c) Compound 3. The view down the c-axis of d) Compound 1 , e) Compound 2 and f) Compound 3. Flydrogen atoms and solvent molecules in each of the frameworks have been omitted for clarity.

Figure 3: TGA of Compound 1 under nitrogen.

Figure 4: TGA of Compound 3 under nitrogen.

Figure 5: a) Light-irradiated PXRD of Compound 1 over a period of 17 hours. The starting pattern is shown in the front and the last pattern is shown in the back, b) Light-irradiated Raman (785 nm) of Compound 1. The first and the last spectra are shown in the front (0 seconds) and the back (630 seconds), respectively, c) Isothermal Raman (785 nm) of Compound 3 at 180 °C. The first and the last spectra are shown in the front (0 seconds) and the back (660 seconds), respectively.

37

Substitue Sheets (Rule 26)

RO/AU Figure 6: PXRD of Compound 1 at 298 K (top) as a slurry in DMF-EtOH and the calculated powder pattern of Compound 1 (bottom).

Figure 7: PXRD of Compound 3 at 298 K (top) and the calculated powder pattern of Compound 3 (bottom). Figure 8: Calculated PXRD patterns of Compound 1 (bottom) and Compound 3

(top) between 20 = 5 - 20°. Selected (hkl) indices have been labelled.

Figure 9: PXRD pattern of calculated Compound 3 (bottom), as-synthesised Compound 3 (second from bottom), calculated Compound 1 (second from top) and Compound 3 heated at 150 °C for 48 hours as a slurry in DMF (top). Figure 10: a) Diffuse reflectance spectrum of Compound 1 (black- peak at approximately 20000 cm^-1) and Compound 3 (lighter). Cyclic voltammogram of b) Compound 1 and c) Compound 3 in 0.1 M [(n-C4H₉)₄N]PF₆/CH₃CN recorded at 200 mV/s. Arrows indicate the direction of the forward scan, d) Vis/NIR SEC of Compound 1 in 0.1 M [(n-C4hl9)4N]PF6/CFl3CN between applied potentials of +0.3 to +0.6 V, inset - 1 at 0.0 V (left), +0.6 to +0.8 V (middle) and +0.8 to +0.9 V, inset - Compound 1 at +0.8 V (right), e) Vis/NIR SEC of Compound 3 between applied potentials of 0.0 to +0.7 V. Inset - starting sample of Compound 3 at 0.0 V (left) and the sample at +0.7 V (right). Arrows indicate the direction of the spectral progression.

Figure 11 : Cyclic voltammogram of Compound 1 in 0.1 M [(n- C₄H₉)₄N]PF₆/CH₃CN at various scan rates.

Figure 12: Square wave voltammogram of Compound 1 in 0.1 M [(n- C4Fl9)4N]PF6/CFl3CN. Arrows indicate the direction of the forward scan.

Figure 13: Light-irradiated Raman (785 nm) of Compound 1 between a) 600 to 1150 cm^-1 and b) 1150 to 1630 cm^-1. The first and the last spectrum is shown in the front (0 seconds) and back (630 seconds), respectively.

Figure 14: Isothermal Raman (785 nm) of Compound 3 at 180 °C between a) 550 to 1050 cm^-1 and b) 1050 to 1750 cm^-1. The first and the last spectrum is shown in the front (0 seconds) and back (660 seconds), respectively.

38

Substitue Sheets (Rule 26) RO/AU Figure 15: Depictions of the structure of Compound 4. Figure 15a: Crystal structure of Compound 4 showing {Cd(bdc)}n sheets. Figure 15b: Crystal structure of one net of Compound 4. Figure 15c: Crystal structure of the cofacial Py₂TTF units. Figure 15d: Crystal structure of interpenetrated nets of Compound 4. The independent nets have been highlighted in different shades. Solvent and hydrogen molecules have been excluded for clarity.

Figure 16: PXRD of Compound 4 (top) and calculated pattern (bottom).

Figure 17: Calculated Raman spectra of the Py₂TTF (bottom) and (Py₄C₁₂S₈H₄) (top) fragments.

Figure 18: PXRD patterns demonstrating repeated conversion between Compound 1 and Compound 3. Calculated PXRD patterns of Compound 1 (Fig 18b, bottom) and Compound 3 (Fig 18a, bottom); PXRD patterns of a sample of Compound 1 converted to Compound 3 (Fig 18a, second from bottom), then retro-converted to Compound 1 (Fig 18b, top), then re-converted to Compound 3 (Fig 18a, second from top), then re-retro-converted to Compound 1 (Fig 18b, middle), then converted a third time to Compound 3 (Fig 18a, top).

Figure 19: Voltammograms of the retro conversion of Compound 3 to compound 1. Arrows indicate the direction of the forward scan. Figure 19a: Square wave voltammogram of a sample of Compound 3 retro-converted to Compound 1 in 0.1 M [(n- C₄H₉)₄N]PF₆/CH₃CN. Figure 19b: Cyclic voltammogram of a sample of Compound 3 retro-converted to Compound 1 in 0.1 M [(n-C4H₉)₄N]PF₆/CH₃CN at 100 mV/s at scan rates of 100-1600 mV/s.

Figure 20: Square wave voltammograms of Compound 1 (Figure 20a), Compound 2 (Figure 20b) and Compound 3 (Figure 20c) in 0.1 M [(n- C₄H₉)₄N]PF₆/CH₃CN. Arrows indicate the direction of the forward scan.

Figure 21 : Cyclic voltammogram of Compound 4 in 0.1 M [(n- C₄H₉)₄N]PF₆/CH₃CN at 100 mV/s (Figure 21a) and at scan rates of 100-1600 mV/s (Figure 21b). Arrows indicate the direction of the forward scan. The data shows the presence of two distinct one-electron processes at 0.12 and 0.30 V (vs. Fc/Fc+), attributed to the oxidation of Py₂TTF to its radical cation, and dication, respectively.

39

Substitue Sheets (Rule 26)

RO/AU Figure 22: Vis/NIR SEC of Compound 4 in 0.1 M [(n-C4H₉)₄N]PF₆/CH₃CN between 0 to 0.2 V (Figure 22a) and 0.2 to 0.7 V (Figure 22b). Arrows indicate the direction of the spectral change. The first process which occurs between 0.0 to 0.2 V {vs. Ag/Ag+) shows an intensification of the band at 22560 cm^-1 (Figure 21). Increasing the potential to 0.7 V {vs. Ag/Ag+) results in the decrease in the bands at 18570 and 22560 cm^-1 due to oxidation of Py₂ TTF^'+ to Py₂TTF2+ (Figure 21). Notably, no changes were observed in the NIR region in our study. The absence of IVCT in this framework likely originates from the less favourable orientation and the longer distance between Py₂TTF units. Figure 23: Left to right: crystals of Zn-1A exposed to a light source with a colour temperature of 5600 K and 1100 Im. Images taken initially and then every 20 s until 120 s has elapsed.

Figure 24: PXRD patterns of Zn-1 A (bottom) and Zn-1 B (top).

Figure 25: (a) TGA of Zn-1 A (solid line) and Zn-1 B (dotted line); (b) solid state diffuse reflectance spectra of Zn-1 A (solid line) and Zn-1 B (dotted line).

Figure 26: (a) Solid state voltammogram of Zn-1 A recorded at 25 mV/s in a 0.1 M TBAPF6/MeCN electrolyte; (b) (a) Solid state voltammogram of Zn-1 B recorded at 100 mV/s in a 0.1 M TBAPF₆/MeCN electrolyte.

Figure 27: (a) Solid state squarewave voltammogram of Zn-1 A (a) and Zn-1 B (b) in a 0.1 M TBAPF6/MeCN electrolyte. Oxidation = top, reduction = bottom.

Figure 28: Raman spectra of Zn-1 A (solid line) and Zn-1 B (dotted line). Arrows highlight key peaks for distinguishing Py₂TTF and Py₄C₁₂S₈H₄.

Figure 29: (a) Zn-1 A secondary building unit; (b) two interpenetrated nets of Zn- 1A; (c) Zn-1 A viewed down the a axis; (d) Zn-1 A viewed down the b axis; top (e) and side (f) view of a Py₂TTF dimer in Zn-1 A. Green = Zn, black = C, red = O, blue = N, yellow = S; hydrogen atoms and solvent molecules omitted for clarity.

Figure 30: Zn-2A secondary building unit; (b) two interpenetrated nets of Zn-2A; (c) Zn-2A viewed down the c axis; (d) Zn-2A viewed down the b axis; top (e) and side (f)

40

Substitue Sheets (Rule 26) RO/AU view of a Py₂TTF dimer in Zn-2A. Green = Zn, black = C, red = O, blue = N, yellow = S; hydrogen atoms and solvent molecules omitted for clarity.

Figure 31 : A crystal of Zn-2A exposed to a light source with a colour temperature of 5600 K and 1100 Im. Images taken initially and then every 3 min until 27 min had elapsed. Image progression is left to right and top row then bottom row.

Figure 32: PXRD patterns of Zn-2A (black- bottom) and Zn-2B (lighter- top).

Figure 33: (a) TGA of Zn-2A (solid line) and Zn-2B (dotted line), (b) Solid state diffuse reflectance spectra of Zn-2A (solid line) and Zn-2B (dotted line).

Figure 34: Solid state cyclic voltammograms of Zn-2A (a) and Zn-2B (b) in a 0.1 M TBAPF6/MeCN electrolyte, recorded at 100 mV/s.

Figure 35: Solid state squarewave voltammograms of Zn-2A (a) and Zn-2B (b) recorded at room temperature in a 0.1 M TBAPF6/MeCN electrolyte. Oxidation = top, reduction = bottom.

Figure 36: Raman spectra of Zn-2A (solid line) and Zn-2B (dotted line). Arrows highlight key peaks for distinguishing Py₂TTF and Py₄C₁₂S₈H₄.

Figure 37: Plot of gas uptakes of Compound 3 at 278 K.

Detailed description of the embodiments

Of the many coordination frameworks reported to date, very few have exploited light as a stimulus for cycloaddition reactions to modulate the physical and chemical properties of the framework. The design of such multifunctional frameworks requires the integration of photoactive components, in addition to the necessary structural and photophysical prerequisites required for a chemical reaction. Whilst the design of such a combination of features within a coordination framework is challenging, the integration of photocycloadducts provides framework materials with an additional dimension of complexity and utility. A prospect yet to be considered in framework chemistry is the concept of employing single-crystal-to-single-crystal (SC-SC) photocyclisation in metal- organic frameworks to introduce functionality such as reversible switching and property modulation. If the properties of the framework could be further modulated through other stimuli, for instance modulation of oxidation state, then this would be further desirable.

41

Substitue Sheets (Rule 26)

RO/AU Against this background, the present inventors have developed a framework structure that undergoes a reversible single-crystal-to-single-crystal (SC-SC) transformation via a double [2+2] photocyclisation. As used herein, a single-crystal-to- single-crystal transformation is a solid state phase transition in which the integrity and long range structural order of the crystalline states are maintained throughout the entire transformation process.

Metal-organic frameworks (MOFs)

M is a metal ion;

L1 is a ligand comprising at least two carboxylates, wherein the at least two carboxylates are selected from the group consisting of aryl carboxylate(s), heteroaryl carboxylate(s), and combinations thereof; wherein two of the at least two carboxylates are linked via a conjugated system that is substantially linear or linear; and wherein oxygen atoms of two of the at least two carboxylates are coordinated to one or both of M;

L2 is a ligand that i

wherein pyr is an optionally substituted pyridyl;

Z is selected from the group consisting of S, Se and Te;

42

Substitue Sheets (Rule 26) RO/AU X is optional and when present is a linker such that pyr is conjugated with

wherein pairs of L2 ligands are at least substantially co-facial to one another; or pairs of L2 ligands are bonded to each other as follows:

L1

Preferably, L1 comprises two carboxylates.

43

Substitue Sheets (Rule 26)

RO/AU The at least two carboxylates of L1 may be aryl or heteroaryl carboxylates. The at least two aryl or heteroaryl carboxylates of L1 may be aryl carboxylates. The at least two aryl or heteroaryl carboxylates of L1 may be heteroaryl carboxylates.

Preferably, the at least two carboxylates of L1 have pKa’s from about 4 to about 6, preferably from about 4 to about 5.5. More preferably, the at least two carboxylates of

L1 have pKa’s of about 5.

The substantially linear or linear conjugated system of L1 may be a bond or may comprise an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may comprise an alkylene group, a phenylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof.

44

Substitue Sheets

(Rule 26) RO/AU The substantially linear or linear conjugated system of L1 may be a bond or may consist of one or more alkylene groups, one or more alkynylene groups, one or more phenylene groups, one or more napthylene groups, one or more ether groups, one or more amine groups, one or more imine groups, one or more azo groups, or combinations thereof. The substantially linear or linear conjugated system of L1 may be a bond or may consist of one or more alkylene groups, one or more phenylene groups, one or more ether groups, one or more amine groups, one or more imine groups, one or more azo groups, or combinations thereof.

L1 may be selected from the group consisting of:

L1 may be selected from the group consisting of:

45

Substitue Sheets (Rule 26) RO/AU

wherein the aromatic or heteroaromatic rings may be optionally substituted.

Preferably, L1 is

wherein the aromatic rings may be optionally substituted.

Preferred optional substituents include C1-C4 alkyl and C2-C4 alkylene.

In some embodiments where Y is present, Y is one or more alkylene groups, one or more alkynylene groups, one or more phenylene groups, one or more napthylene groups, an ether group, an amine group, an imine group, or an azo group. In some embodiments where Y is present, Y is an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, or an azo group. In some embodiments where Y is present, Y is an alkylene group, a phenylene group, an ether group, or an azo group. In some embodiments where Y is present, Y is an alkylene group. In some embodiments where Y is present, Y is a

46

Substitue Sheets (Rule 26)

RO/AU phenylene group. In some embodiments where Y is present, Y is an ether group. In some embodiments where Y is present, Y is an azo group.

In some embodiments where Y is present, Y is selected from the group

. in some embodiments where Y is present, Y is selected from the group consisting of

, and

L2 may be such that pairs of L2 ligands are bonded to each other as follows:

47

Substitue Sheets

(Rule 26) RO/AU

When

pairs of L2 ligands are preferably co-facial to one another.

respective 2,3 and 6,7 double bonds of each

48

The

groups may be trans with respect to each other on the

49

Substitue Sheets (Rule 26) RO/AU In preferred embodiments of the first aspect the present invention provides a metal-organic framework comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion;

L1 is a ligand that is

wherein Y is selected from the group consisting of a bond,

and combinations thereof;

L2 is a ligand that is

50

combinations thereof; and each M is coordinated to 2 L2 ligands, and wherein pairs of L2 ligands are cofacial to one another. In preferred embodiments of the first aspect the present invention provides a metal-organic framework (MOF) comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion;

L1 is a ligand that is

wherein Y is selected from the group consisting of a bond,

and

combinations thereof;

L2 is a ligand that is

51

Substitue Sheets (Rule 26) RO/AU

wherein X is selected from the group consisting of a bond,

other as follows:

52

Substitue Sheets (Rule 26) RO/AU

In some embodiments, X is a bond.

In embodiments of the first aspect, MOFs may possess one or more of the following characteristics: · A cyclic moiety of the formula a cyclic moiety of the formula

the repeat unit, wherein L1’ is the remainder of the ligand L1 that does not include the carboxylate group depicted in the cyclic moiety.

• Octahedral geometry of the metal ions. · Each member of pairs of L2 ligands, wherein the pairs of L2 ligands are bonded to each other as follows:

53

Substitue Sheets (Rule 26) RO/AU

coordinatedeparate metal ions.

• In the pairs of L2 ligands substantially co-facial to one another the respective 2,3 and 6,7 double bonds of each tetrathiafulvalene moiety may be separated from one another by a distance of about 3.5 to about 5.0 A , preferably about 3.5 to about 4.2 A.

• L2 ligands occupying the apical positions of the metal ions.

• L1 ligands occupying the equatorial positions of the metal ions.

• L1 ligands are bridging ligands such that each M is coordinated to 3 L1 ligands. · Undulating sheets in the MOF due to repetition of the M₂(L1 )₂ moiety.

• Pairs of L2 ligands link the sheets so as to form a structure that repeats in 3 dimensions.

• One or more guest molecules, for example solvent molecules such as DMF.

• Approximately one third of the unit cell volume occupied by the framework. · The monoclinic space groups P2₁/n or P2₁/c, or the orthorhombic space group

Pcc2; preferably the monoclinic space group P2₁/n.

54

Substitue Sheets (Rule 26)

RO/AU In one embodiment the MOF of the first aspect has the following repeat unit:

wherein the wavy line

indicates the border of the repeat unit, such that the portion of the ligands outside of the wavy line are part of different repeat units. In one embodiment the MOF of the first aspect has the following repeat unit:

55

Substitue Sheets

(Rule 26) RO/AU

wherein the wavy line

indicates the border of the repeat unit, such that the portion of the ligands outside of the wavy line are part of different repeat units.

Surprisingly, it has been found by the inventors that the MOF of the first aspect, wherein

undergoes SC-

SC double [2+2] photocyclisation of pairs of tetrachalcafulvalene (where tetrathiafulvalene [TTF] is the S-containing member of the group) moieties to provide the cyclised MOF of the first aspect, wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

56

Substitue Sheets (Rule 26) RO/AU

The photocyclisation reaction is facilitated by the substantially co-facial arrangement of the tetrachalcafulvalene (such as TTF) ligands in which the double bonds are oriented in parallel and within a distance sufficient to maximise p_z orbital overlap. Such an arrangement optimises the conditions required for the SC-SC cyclisation.

To the best of the inventors' knowledge, this is the first example of a double [2+2] photocyclisation of a tetrachalcafulvalene (such as TTF) core and the first example of a 3D porous MOF exhibiting double [2+2] photocyclisation. Even more surprisingly, the cyclisation is reversible simply by heating. To the best of the inventors' knowledge, the reversibility also represents the first such example in a tetrachalcafulvalene (such as TTF) core and in a 3D porous MOF.

The reversible structural change of the tetrachalcafulvalene (such as TTF) core may act as a switch thereby opening up many potential applications for the MOFs.

Preparation of MOFs of the first aspect, wherein L2 is

MOFs in accordance with the first aspect, wherein L2 is

57

Substitue Sheets (Rule 26) RO/AU

, may be prepared by mixing a metal salt, a suitable compound of formula (III) depending on the desired L1 ligand, and suitable compound of formula (II) depending on the desired L2 ligand in a suitable solvent and heating. The metal of the metal salt may be a metal as defined herein.

In the some embodiments the compound of formula (III) is an acid. In some embodiments the compound of formula (III) is biphenyl-4, 4’-dicarboxylic acid. In some embodiments the compound of formula (III) is 4,4’-stilbenedicarboxylic acid. In some embodiments the compound of formula (III) is 4,4’-oxybisbenzoic acid. In some embodiments the compound of formula (III) is 4,4’-azobisbenzoic acid. In some embodiments the compound of formula (III) is 2,5-thiocarboxylic acid. In some embodiments the compound of formula (III) is 2,2’-sulfone-4,4’-biphenyldicarboxylic acid.

In some embodiments the solvent may be a mixture of DMF and ethanol. In other embodiments the DMF may be replaced with dimethylacetamide (DMA) or diethylformamide (DEF). The reaction may, for example, be carried out in a solvothermal reactor and/or in a convection oven. In one embodiment, the reaction is carried out in a solvothermal reactor and a convection oven. Typically, the reaction is carried out in the absence of light. In some embodiments MOFs of the first aspect, wherein L2 is pyr

, wherein pairs of L2 ligands are at least substantially co-facial to one another, are prepared as follows:

58

Substitue Sheets (Rule 26) RO/AU • Forming a mixture of a metal salt, a compound of formula (III) and a compound of formula (II) in a 4:1 to 6:1 mixture of DMF:ethanol;

• heating the mixture in a solvothermal reactor in the absence of light for between about 5 minutes and about 20 minutes at a temperature between about 120 °C and about 140 °C; and

• heating the mixture in a convection oven in the absence of light for between about 3 days and 5 days at a temperature between about 70 °C and about 90 °C.

In other embodiments MOFs of the first aspect, wherein L2 is

• Forming a mixture of a metal salt, a compound of formula (III) and a compound of formula (II) in a 4.5:1 to 5.5:1 mixture of DMF:ethanol;

• heating the mixture in a solvothermal reactor in the absence of light for between about 5 minutes and about 15 minutes at a temperature between about 120 °C and about 140 °C; and

• heating the mixture in a convection oven for between about 3.5 days and about 4.5 days at a temperature between about 75 °C and about 85 °C.

In further embodiments MOFs of the first aspect, wherein L2 is

59

Substitue Sheets (Rule 26)

RO/AU • Forming a mixture of a metal salt, a compound of formula (III) and a compound of formula (II) in a about a 5.3:1 mixture of DMF:ethanol;

• heating the mixture in a solvothermal reactor in the absence of light for about 10 minutes at a temperature of about 130 °C; and · heating the mixture in a convection oven in the absence of light for about 4 days at a temperature of about 80 °C.

In the above embodiments the molar ratio of the metal salt:compound of formula (lll):compound of formula (II) may be about 1 :1 :1.

Preparation of MOFs of the first aspect by cyclisation such that L2 ligands are bonded to each other as follows:

The double [2+2] photocyclisation may conveniently be performed by subjecting the MOF of the first aspect wherein L2 is

, wherein pairs of L2 ligands are at least substantially co-facial to one another, to light, such as for example white light, visible light, NIR light, UV light or light with a spectral range of 300-600 nm. It has been found that exposing a bulk sample of a MOF of the first aspect wherein L2 is

60

Substitue Sheets (Rule 26) RO/AU

, wherein pairs of L2 ligands are at least substantially co-facial to one another, (Compound 1 ) to a 20 W UV lamp for 1 hour was sufficient to drive cyclisation to completion. Cyclisation is typically accompanied by a colour change from bright red/orange (uncyclised form) to light yellow (cyclised form).

The Vis-NIR spectrum of Compound 1 exhibits a broad adsorption band at about 25,000 cnr¹ (400 nm) which is assigned to a HOMO-LUMO transition. This suggests that UV light is likely required to induce the cyclisation.

It has also been found that a combination of cyclised and non-cyclised MOFs may be obtained by solvothermal synthesis in ambient lighting conditions.

Reversal of the cyclisation to provide MOFs of the first aspect such that L2 is

are at least substantially co-facial to one another,

Reversion from the cyclised MOF back to the uncyclised MOF may be conveniently achieved by heating the MOF of the first aspect, wherein L2 ligands are bonded to each other as follows:

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Substitue Sheets (Rule 26) RO/AU

, for example at a temperature between about 100 °C and about 200 °C, or at about 150 °C for a period of about 48 hours. The cyclised MOF may be heated in a solvent slurry, such as for example a slurry in DMF. Typically, heating is performed in the absence of light. The reversible structural change of the tetrachalcafulvalene (such as TTF) core may act as a switch to control, modulate and optimise characteristics including thermal stability, redox properties, optical properties and porosity. Accordingly, the invention also embraces a method for reversibly switching between a MOF defined in the first aspect, wherein

MOF defined in the first aspect, wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

62

Substitue Sheets (Rule 26) RO/AU

the method comprising the following steps:

(i) irradiation of the MOF defined in the first aspect, wherein L2 is

, with light to provide the MOF of the first aspect, wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

(ii) heating the MOF defined in the first aspect, wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

63

Substitue Sheets (Rule 26) RO/AU

to provide the

MOF of the first aspect, wherein L2 is

Steps (i) and (ii) may be carried out as described above for irradiation and heating of the MOFs of the first aspect.

Applications and uses of the MOFs

Potential applications stemming from the switching capability range from electrochromic sensors to chemical monitoring systems, electrocatalysis, optoelectronics and electronic components. By applying an external UV or heat stimulus the desired properties can be switched on or off for a particular application, or the framework itself could be used as an indicator for the external stimulus as a quality control type material. Once switched to a desired state, the material may then be further tuned again by modifying the material's redox state. Further more specific examples are as follows: Nanocarriers containing photo-valves for remote control drug delivery.

Nanoporous solids including silicas have been used previously for stimuli-responsive drug release, however such carriers often require use of UV-light. The MOFs described

64

Substitue Sheets (Rule 26) RO/AU herein are responsive to visible light thereby offering a safer option for drug release. Examples include therapeutics for cancer and bone diseases.

Ion channel for separation of alkali metal ions relevant to battery technologies and for desalination. The MOFs described herein may be capable of selectively separating alkali earth ions such as lithium, sodium and potassium from complex groundwater basins and separating salt from seawater. Irradiation with visible light offers a convenient means of either capturing or releasing the sequestered ions.

Lanthanoid separation of complex mixtures of ions. The MOFs described herein may be capable of selectively separating lanthanoid ions of the elements lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Flo), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Irradiation with visible light provides a means of either capturing or releasing the sequestered ions.

Low-energy light and electrically-driven gas separation. Current methods for separating gases rely on temperature or pressure swing methods that often require significant energy input. Exploiting the light- and redox-functionality of the material developed herein offers a potentially cheaper, more environmentally benign method for gas separations.

Sunlight driven actuators. Actuators are typically controlled by low energy signals such as current, electric voltage or pressure. The MOFs described herein are a rare example of a nanoporous system which alters its structural properties in response to sunlight. This presents an opportunity to develop nano-components of machines that are driven by light.

As a result of their highly porous nature, MOFs have also found use in gas storage and separation. MOFs in accordance with the second aspect have been found to have thermal stability above 300 °C, thereby allowing for activation to access open voids in the structure. For example, Compound 3 herein demonstrated porosity and selectivity for carbon dioxide capture, with a BET surface area of 922 m² g ¹ (calculated based on the N₂ isotherm at 77 K). Compound 3 exhibited N₂ and CO₂ uptakes at 278 K of 0.35 and 3.87 mmol/g, respectively; and a CO₂/N₂ selectivity of 25.2 (see Figure 37).

65

Substitue Sheets (Rule 26) RO/AU Similarly, Compound 3 exhibited CH₄ and H₂ uptakes at 278 K of 1.24 and 0.0946 mmol/g, respectively.

Examples

Synthesis Cd(NO₃)₂-4H₂O and biphenyl-4, 4’-dicarboxylic acid (bpdc) were purchased from

Sigma-Aldrich and used without further purification. All solvents were of reagent grade or higher and used without further purification unless otherwise stated. Acetonitrile was distilled from CaH₂ and degassed prior to use. [(n-C₄H₉)₄N]PF₆ was recrystallized three times from EtOH prior to use. ¹H NMR spectra were recorded on a Bruker AVANCE 300 spectrometer operating at 300 MFIz. ¹FI NMR chemical shifts were referenced against an internal residual solvent resonance. All spectra were recorded at room temperature and are reported in ppm (±0.05 Hz). All deuterated solvents were obtained from Cambridge Stable Isotopes. C, H and N microanalyses were carried out at the Advanced Analytical Centre at James Cook University, Australia or the Chemical Analysis Facility in the Department of Chemistry and Biomolecular Science at Macquarie University, Australia.

2,6-bis(4'-pyridyl)-tetrathiafulvalene (Py₂TTF) was synthesised according to the following scheme:

66

Substitue Sheets (Rule 26) RO/AU Scheme 1 : Synthesis of Py₂TTF.

Synthesis of 4-Bromoacetylpyridine

4-Acetyl pyridine (15 imL, 0.136 mol) and acetic acid (150 mL, 2.625 mol) were cooled to 0 °C. HBr (5 mL, 48%) was then added and the mixture allowed to stir for 15 minutes. Br₂/CH₃COOH (5 mL, 5 mL) was then added dropwise and the resulting solution stirred at 0 °C for 20 min, followed by stirring at room temperature for 1 hour. The formed precipitate (orange) was collected by vacuum filtration, washed with diethyl ether, and heated at 80 °C to remove excess Br₂ for 1 hour to yield 4- bromoacetylpyridine as a white microcrystalline powder (46.7 g, 86.5%). ¹H NMR (D₂O, 300 MHz): δ 3.73 (s, 2H), 8.21 (m, 2H), 8.80 (m, 2H).

Synthesis of K-O-isopropyl xanthate

KOH (26 g, 0.46 mol) was dissolved in isopropanol (300 mL, 3.945 mol) to which CS₂ was added (50 mL, 0.828 mol) at 0 °C. The resulting mixture was stirred for 1 hour and brought to room temperature. The mixture was filtered and the resulting product recrystallised from EtOH to yield K-O-isopropyl xanthate (55 g, 58.1%).

Synthesis of 4-(Pyridine-4-yi)- 1,3-dithiol-2-one

To a suspension of 4-bromoacetylpyridine (22.74 g, 0.113 mol) in DCM (180 mL), triethylamine (13.6 mL, 0.094 mol) was then added dropwise followed by the addition of K-O-isopropyl xanthate (16.7 g, 0.096 mol). The solution was stirred for 4 h and turned dark red. The resulting mixture was washed with H₂O (2 x 100 mL) and the organic layer isolated. After further washing (H₂O, 6 x 100 mL) the isolated organic layer was heated to remove solvent producing a dark red oil. The oil was dried under N₂, affording a black solid to which H₂SO₄ (98%, 11.5 mL) was added dropwise. After 10 minutes, ice was added until the hot reaction flask was cool. The mixture was neutralised with NaHCO₃ (100 mL, 0.1 M) and the precipitate formed was extracted with DCM (3 x 50 mL). The DCM solution was washed with H₂O (6 x 100 mL), solvent was evaporated under vacuum and the remaining product recrystallised from EtOH twice, yielding 4-(pyridine-4-yl)-1 ,3-dithiol-2-one as a bright red solid (3.63 g, 2.2%).

67

Substitue Sheets (Rule 26) RO/AU Alternative Synthesis of 4-(Pyridine-4-yi)- 1,3-dithiol-2-one

4-(Bromoacetyl)pyridine hydrobromide (6.26 g, 23.6 mmol) and potassium isopropylxanthate (4.11 g, 23.6 mmol) were dissolved in 250 mL H₂O (250 mL) and heated at 50 °C for 90 min. The resulting mixture was extracted with DCM (3 c 50 mL) and washed with H₂O (3 c 50 mL). The organic layer was dried with MgSO₄ and solvent removed under vacuum to afford a red solid to which H₂SO₄ (98%, 12 mL) was added dropwise. After 15 min the solution was neutralised with saturated aqueous NaHCO₃ and extracted with DCM (3 ^c 50 mL). The organic phase was washed with H₂O (3 ^c 50 mL), dried (MgSO₄) and solvent removed under vacuum. The red solid was recrystallised from isopropanol to afford 4-(4-pyridyl)-1 ,3-dithiol-2-one as brown crystals (2.36 g, 51.2%). ¹H NMR (CDCl₃, 300 MHz): δ 7.15 (s, 1 H), 7.28 (dd, 2H, J = 4.5, 1.7 Hz), 8.64 (dd, 2H, J = 4.5, 1.6 Hz). ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 116.1 , 120.3, 132.3, 139.5, 150.8, 190.9.

Synthesis of 2, 6-bis(4'-pyridyl)-tetrathiafulvalene

4-(Pyridine-4-yl)-1 ,3-dithiol-2-one (3.63 g, 0.019 mol) was suspended in P(OEt)3 (25 mL, 0.15 mol) and toluene (25 mL) and refluxed under N₂ for 6 h in the absence of light. The resulting product was filtered and washed with MeOH yielding 2,6-bis(4'- pyridyl)-tetrathiafulvalene as a red solid (1.96 g, 18.3 %, m.p. >260 °C). ESI-MS (ESI+, MeOH): m/z 357 [M], ¹H NMR (300 MHz, CDCl₃): δ 8.65 (d, 2H), 8.61 (d, 2H), 7.31 (d, 2H), 7.13 (d, 2H). 6.88 (s, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 148.5, 144.1 ; 136.9, 129.1 , 120.5. IR (ATR) (cm^-1): 2975 (m), 2777 (w), 2456 (w), 2355 (w), 1916 (w), 1584 (s), 1555 (s), 1535 (s), 1403 (m), 1250 (m), 1217 (m), 1166 (w), 1038 (m), 991 (m), 935 (m), 811 (s), 781 (s), 766 (s), 721 (m), 690 (s), 661 (m), 616 (s). Elemental Analysis calculated for C₁₆H₁₀N₂S₄ (%): C 53.60, H 2.81 , N 7.81 , S 35.77; Found (%): C 52.74, H 2.98, N 7.54, S 35.12.

Alternative synthesis of 2, 6-bis(4'-pyridyl)-tetrathiafulvalene

4-(4-Pyridyl)-1 ,3-dithiol-2-one (2.36 g, 12.1 mmol) was added to a degassed solution of P(OEt)₃ (16 mL, 93.3 mmol) in dry toluene (16 mL) and heated at 110 °C under N₂ for 6 h in the absence of light. The resulting solid was filtered and washed with MeOH (3 x 15 mL) then diethyl ether (2 x 15 mL), affording 2,6-bis(4'-pyridyl)- tetrathiafulvalene as a red solid (1 .26 g, 58.1%).

68

Substitue Sheets (Rule 26)

RO/AU General synthesis of a MOF wherein L2 is

A suitable salt of M such as a nitrate (1 equiv), a compound of formula (II) (1 equiv) and a compound of formula (III) (1 equiv) may be placed in a vessel and dissolved in a suitable solvent (preferably a polar aprotic solvent such as DMF). Preferably the reaction is performed in the absence of light. The reaction is heated (for instance, to 80 °C), affording a MOF of the invention wherein L2 is

. The inventors have found that this process is robust and expect the same conditions to be applicable for different MOFs of the inventions wherein L2 is

Synthesis of a MOF having a repeat unit wherein M is Cd(ll), L1 is biphenyl-4, 4'- dicarboxylic acid (bpdc) and L2 is Py2TTF (denoted herein as [Cd₂(Py₂TTF)₂(bpdc)₂] or Compound 1 ) Cd(NO₃)₂-4H₂O (11.6 mg, 0.0375 mmol), Py₂TTF (13.5 mg, 0.0375 mmol) and biphenyl-4, 4’-dicaryboxylic acid (9.1 mg, 0.0375 mmol) were placed in a 21 mL solvothermal vial which was covered with aluminium foil and dissolved in 4 mL of DMF

69

Substitue Sheets (Rule 26)

RO/AU and 0.75 imL of EtOH. The mixture was heated to 130 °C for 10 minutes and further heated at 80 °C in a convection oven with no exposure to light, yielding red plate-like crystals after four days. Yield: 20 mg (30.4 % based on Cd(ll)). IR (ATR) (cm^-1): 3058 (w), 2928 (w), 2852 (w), 1941 (w), 2468 (w), 1671 (s), 1605 (s), 1576 (s), 1524 (s), 1498 (w), 1393 (s), 1252 (w), 1224 (w), 1176 (m), 1094 (s), 1018 (w), 939 (w), 851 (m), 818

(w), 770 (m), 729 (w), 682 (m), 661 (m), 628 (m). Elemental Analysis calculated for C6oH₄2Cd₂N40₈S8 (%): C 50.46 H 2.96 N 3.92 O 8.96 S 17.96 Cd 15.74.

Synthesis of a MOF based on Compound 1 in which the Py2TTF moieties are partially cyclised (denoted herein as [ [Cd₂(Py₄C₁₂S₈F₁₄)_x(Py₂TTF)_x(bpdc)₂] or Compound 2). Samples of Compound 2 for single crystal X-ray diffraction studies were generated from wetted crystals of Compound 1 in DMF via light irradiation for 6 minutes using white light (25 W) from a microscope lamp. The partially cyclised product was then transferred immediately from the microscope slide to the diffractometer in paratone oil under dark conditions for analysis. Synthesis of a MOF based on Compound 1 in which the Py2TTF moieties are cyclised (denoted herein as [Cd2(Py4Ci2S8H4)₂(bpdc)₂] or Compound 3).

Single crystals of Compound 3 were formed by irradiating single crystals of Compound 1 with white light (25 W) using the beam from a microscope. Bulk Compound 3 was formed by placing a sample of Compound 1 in direct sunlight for 2 days or under a UV lamp (20 W) for 1 h. IR (ATR) (cm^-1 ): 3054 (w), 2918 (w), 2847 (w), 1663 (s), 1605 (m), 1576 (s), 1522 (m), 1384 (s), 1254 (w), 1174 (w), 1090 (m), 1020 (w), 884 (w), 849 (m), 770 (s), 735 (w), 706 (w), 680 (m), 659 (m), 624 (m). Elemental Analysis calculated for C_6oH₄₂Cd₂N₄O₈S₈ (%): C 50.46 H 2.96 N 3.92 O 8.96 S 17.96 Cd 15.74.

70

Substitue Sheets (Rule 26) RO/AU Alternative synthesis of a MOF having a repeat unit wherein M is Cd(ll), L1 is biphenyl- 4,4'-dicarboxylic acid (bpdc) and L2 is Py2TTF (denoted herein as [Cd₂(Py₂TTF)₂(bpdc)₂] or Compound 1)

All solvothermal MOF syntheses were performed in 21 mL glass scintillation vials and heated in a bloc heater at the heat and times specified. Cd(NO₃)₂-4H₂O (7.7 mg, 0.025 mmol) was dissolved in EtOH (1.0 mL). H₂BPDC (6.1 mg, 0.025 mmol) and Py₂TTF (9.0 mg, 0.025 mmol) were dissolved in 3.0 mL and 2.0 mL of N,N-dimethylformamide (DMF), respectively. The three solutions were combined, sealed and heated at 90 °C for 48 h in the absence of light. The resulting red, plate-like crystals of Compound 1 were purified by successive solvent washes with DMF, which was replaced every 24 h until it remained colourless (12 mg, 32%).

Crystals of Compound 1 were irradiated with white light LEDs for approximately 1 h to afford the cyclised framework, Compound 3, as yellow crystals.

Synthesis of a MOF having a repeat unit wherein M is Cd(ll), the moiety equivalent to L1 is 1 ,4-benzenedicarboxylate (bdc) and L2 is Py₂TTF (denoted herein as [Cd₂(Py₂ TTF)₂(bdc)₂] or Compound 4)

Synthesis of [Cd₂(Py₂TTF)₂(bdc)₂]-DMF (Compound 4). Cd(NO₃)₂-4H₂O (11.6 mg, 0.0375 mmol), 2,6-bis(4-pyridyl)-tetrathiafulvalene (13.5 mg, 0.0375 mmol) and 1 ,4-benzenedicarboxylate (6.4 mg, 0.0375 mmol) were dissolved in DMF (4 mL) and EtOFI (0.75 mL). The mixture was sealed and after sonication for 5 min was heated in a solvothermal oven at 80 °C for 72 h to yield red plate-like crystals suitable for X-ray diffraction (11.2 mg, 11.3% based on Cd(ll)). IR (ATR) (cm-1): 3045 (w), 2930 (w), 2864 (w), 2485 (w), 1195 (w), 1663 (m), 1605 (m), 1549 (s), 1376 (s), 1219 (m), 1090 (s), 1015 (s), 941 (m), 824 (s), 748 (s), 690 (s), 659 (s), 626 (s). Analysis calc, for Cd₂C57H₄8N701 2S8 (%): C 45.51 , H 3.22, N 6.52, S 17.05; Found (%): C 45.49, H

3.15, N 6.45, S 17.08.

Synthesis of (E)-4,4'-bis(4-(pyridin-4-yl)phenyl)-2,2'-bi( 1 ,3-dithiolylidene)

(Py2Ph2TTF) Py₂Ph2TTF was synthesised in the same manner as Py₂TTF with the exception that a Suzuki reaction was performed prior to bromination.

71

Substitue Sheets

(Rule 26) RO/AU 1-[4-(4-Pyridinyl)phenyl]ethenone: To degassed 1 ,4-dioxane (44 mL) and water (11 imL) was added 4-Pyridineboronic acid (4.44 g, 0.036 mol), 4-bromoacetophenone (7.22 g, 0.036 mol), K₂CO₃ (13.3 g, 0.096 mol), and Pd(PPh₃)₄ (50 mg, cat.). The resulting mixture was refluxed under an inert atmosphere for 16 hours. Upon completion the reaction mixture was diluted with water and extracted with diethyl ether (3 x 20 mL). The combine organic layers were washed with water (3 x 50 ml), dried over MgSO₄, filtered, and their solvent volume was reduced in vacuo. The title compound was obtained as an off-white powder (7 g, 0.035 mol, 97 % yeild).

2-bromo-1-(4-(pyridin-4-yl)phenyl)ethan-1-one: To a solution of 1-[4-(4- Pyridinyl)phenyl]ethenone (7g 0.035 mol) in acetic acid (150 mL) at 0°C was added HBr (3.5 mL, 48% in acetic acid). The reaction mixture was stirred for 15 minutes prior to the addition of Br2 ( 0.0672 mol, 10.92 g, 3.5 mL). The reaction temperature was slowly increased from 0°C to 80°C and heated until the distinct red colour of Br2 was no longer visible. The reaction mixture was allowed to cool to room temperature and the resulting solid collected by vacuum filtration and washed with EtOH (3 x 50 mL) and diethyl ether (3 x 50 mL). The title compound was obtained as a fluffy pale beige solid (0.016 mol, 4.514 g, 47%).

4-(4-Pyridinyl)-1 ,3-dithiol-2-one: Et3N (2.21 mL) was added to a suspension of 2-bromo- 1-(4-(pyridin-4-yl)phenyl)ethan-1-one (4.514 g, 0.016 mol) in CH₂CI₂ (100 mL). The resulting reaction mixture was stirred for 15 minutes prior to the addition of K- isopropylxanthate (2.61 g, 0.015 mol). Following the K-sopropylxanthate addition the reaction mixture was stirred overnight. Upon completion the reaction mixture was washed with water (1 x 50 mL) which was back extracted with CH₂CI₂ (3 x 20 mL). The combined organic layers were washed with water until the washings came back colourless. The organic phase was dried over MgSO₄ and reduced to a dark red oil by rotary evaporation. The oil was dried under N₂ until a shiny black solid was obtained, cone. H₂SO₄ () was added and the resulting oil neutralized in a rapidly stirring solution of NaHCO₃. The aqueous solution was extracted with CH₂CI₂ (5 x 20 mL) and the combined organic layers were washed with water until the washings came back colourless. The organic phase was dried over MgSO₄ and the solvent volume was reduced in vacuo. 4-(4-(pyridin-4-yl)phenyl)-1 ,3-dithiol-2-onewas obtained as an orange brown solid ( 0.96 g, 0.0035 mol, 22%).

72

Substitue Sheets

(Rule 26) RO/AU (E)-4,4'-bis(4-(pyridin-4-yl)phenyl)-2,2'-bi(1 ,3-dithiolylidene) (Py₂Ph₂TTF): To a degassed solution of toluene (10 imL) and triethylphosphite (10 imL), (E)-4,4'-bis(4- (pyridin-4-yl)phenyl)-2,2'-bi(1 ,3-dithiolylidene) (0.96 g, 0.0035 mol) was added. The resulting mixture was refluxed with stirring under an inert atmosphere for 6 hours. The resulting red powder was collected by vacuum filtration and washed with cold MeOH (3 x 5 mL). Py₂Ph2TTF was obtained as a bright-red solid (300 mg, 0.0006 mol, 17 %).

[Cd₂(SBPDC)₂(Py₂TTF)₂]xDMF (Cd-2A) (SBPDC = 2-sulfone-4,4'-biphenyl dicarboxylate; x = number of DMF solvent molecules). Cd(NO₃)₂-4H₂O (6.2 mg, 0.020 mmol) was dissolved in EtOH (1.5 mL). 2,2’-Fl2SBPDC (6.1 mg, 0.020 mmol) and Py₂TTF (7.2 mg, 0.020 mmol) were dissolved in 1.5 mL and 2.0 mL of DMF, respectively. The three solutions were combined, sealed and heated at 70 °C for 48 h in the absence of light. The resulting red, block-like crystals were purified by successive solvent washes with DMF, which was replaced every 24 h until it remained colourless (9.0 mg, 29%). IR (ATR): v_max (cnr¹) 415 (s), 441 (m), 482 (w), 525 (m), 548 (m), 586 (m), 593 (m), 629 (m), 662 (m), 691 (m), 778 (m), 820 (m), 887 (m), 939 (w), 1017 (m), 1043 (m), 1067 (m), 1126 (m), 1173 (m), 1221 (w), 1255 (w), 1290 (m), 1387 (s), 1414 (m), 1470 (w), 1553 (s), 1588 (m), 2901 (w), 2972 (w).

[Zn₂(SBPDC)₂(Py₂TTF)₂]· 6DMF (Zn-1A). Zn(NO₃)₂-6H₂O (6.0 mg, 0.020 mmol) was dissolved in EtOFI (0.65 mL). FI2SBPDC (6.1 mg, 0.020 mmol) and Py₂TTF (7.2 mg, 0.020 mmol) were dissolved in 1.5 mL and 1.6 mL of DMF, respectively. The three solutions were combined, sealed and heated at 100 °C for 48 h. The resulting red, plate-like crystals were purified by successive solvent washes with DMF, which was replaced every 24 h until it remained colourless (7.7 mg, 20%). Elemental analysis: Found (%): C 53.70, H 4.34, N 3,21 ; Calc, for C₈₀H₈₂N₄O₁₇S₁₀Zn₂ (%): C 52.70, H 4.53, N 3.07. IR (ATR): v_max (cnr¹) 421 (m), 442 (m), 539 (w), 559 (m), 592 (m), 631 (m), 662 (s), 694 (m), 729 (w), 777 (s), 797 (w), 824 (w), 883 (m), 940 (m), 1019 (w), 1044 (m), 1069 (m), 1133 (m), 1172 (s), 1223 (w), 1252 (w), 1296 (m), 1380 (s), 1422 (w), 1465 (w), 1533 (w), 1556 (m), 1602 (s), 1645 (m), 2901 (m), 2969 (m).

[Zn₂(SBPDC)₂(Py4Ci₂S8H4)]-6DMF (Zn-1B). Powdered Zn-1A was irradiated with white light LEDs (4 W) for approximately 16 h to afford the cyclised framework, Zn-1 B, as a yellow powder. IR (ATR): Vmax (cm^-1) 419 (m), 492 (m), 561 (w), 593 (m), 626 (w), 662 (m), 733 (w), 774 (s), 833 (w), 854 (w), 884 (m), 1023 (w), 1069 (m), 1127 (m), 1172

73

Substitue Sheets (Rule 26)

RO/AU (m), 1255 (w), 1297 (m), 1374 (s), 1419 (m), 1469 (w), 1555 (w), 1601 (s), 1652 (m), 3265 (br).

[Cd2(TDC)₂(Py2TTF)₂]xDMF (Cd-3A) (TDC = 2,5-thiophenedicarboxylate; x = number of DMF solvent molecules). Cd(NO₃)₂-4H₂O (5.6 mg, 0.018 mmol) was dissolved in EtOH (0.7 mL). H₂TDC (3.1 mg, 0.018 mmol) and Py₂TTF (6.5 mg, 0.018 mmol) were dissolved in 1.0 mL and 1.5 mL of DMF, respectively. The three solutions were combined, sealed and heated at 70 °C for 48 h. The thin, red, plate-like crystals which formed were purified by successive solvent washes with DMF, which was replaced every 24 h until it remained colourless (6.2 mg, 26%). IR (ATR): v_max (cm^-1) 442 (s), 482 (m), 532 (w), 556 (m), 628 (m), 663 (m), 689 (m), 727 (w), 770 (s), 819 (m), 875 (w),

940 (w), 1017 (m), 1070 (m), 1124 (w), 1223 (m), 1337 (m), 1373 (s), 1423 (m), 1525 (m), 1553 (s), 1602 (m), 2902 (w), 2967 (w).

[Zn₂(TDC)₂(Py₂TTF)₂]-2DMF (Zn-2A). Zn(NO₃)₂-6H₂O (5.4 mg, 0.018 mmol) was dissolved in EtOFI (0.7 mL). FI2TDC (3.1 mg, 0.018 mmol) and Py₂TTF (6.5 mg, 0.018 mmol) were dissolved in 1.5 mL and 2.0 mL of DMF, respectively. The three solutions were combined, sealed and heated at 100 °C for 48 h in the absence of light. The resulting bright red, block-like crystals were purified by successive solvent washes with DMF, which was replaced every 24 h until it remained colourless (6.6 mg, 27%). Elemental analysis: Found (%): C 44.61 , H 2.84, N 6.27; Calc, for C_44.8H_35.2N_5.6O₉.₆S₁₀Zn₂ (%): C 44.91 , H 2.72, N 6.01 . IR (ATR): v_max (cnr¹) 443 (s), 500 (m), 558 (m), 629 (s), 664 (m), 691 (m), 730 (w), 771 (s), 802 (m), 813 (m), 832 (w), 873 (w), 942 (w), 1021 (m), 1037 (m), 1067 (m), 1221 (m), 1332 (s), 1358 (s), 1401 (m), 1423 (w), 1495 (w), 1525 (s), 1554 (s), 1596 (s), 1663 (m), 2902 (m), 2968 (m).

[Zn₂(TDC)₂(Py₄C₁₂S₈H₄)]-2DMF (Zn-2B). Powdered Zn-2A was irradiated with white light LEDs (4 W) for approximately 16 h to afford the cyclised framework, Zn-2B, as a yellow/orange powder. IR (ATR): Vmax (cm^-1) 459 (m), 493 (m), 530 (w), 598 (m), 626 (m), 661 (w), 697 (w), 730 (w), 770 (s), 798 (w), 815 (m), 831 (w), 848 (w), 883 (w), 1023 (w), 1068 (w), 1091 (w), 1217 (w), 1336 (s), 1363 (s), 1419 (w), 1490 (w), 1525 (m), 1576 (s), 1594 (s), 1663 (s), 3065 (w).

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Substitue Sheets

(Rule 26) RO/AU Characterisation

Single Crystal X-ray Diffraction. Measurements of single crystal X-ray diffraction data for Compound 1 was undertaken on an Oxford Supernova diffractometer with Cu- Ka radiation at 100 K. Diffraction data for Compound 3 was collected on a Bruker Apexll FR591 diffractometer with Mo-Ka radiation at 100 K. Crystals were extracted from the mother liquor into paratone oil and mounted onto the goniometer for data collection. In the case of Compound 1 , the crystals were cold-mounted under dry ice to prevent degradation of the crystals. The structures were solved using SHELXT¹ and refined using a full-matrix least squares procedure based upon F².² Structure solution and refinement was performed within the WinGX system of programs.³ Crystal information and details relating to the structural refinements are presented in Table 1 below.

Table 1 : Crystal data and structure refinement form Compounds 1 , 2, 3 and 4

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Substitue Sheets

(Rule 26) RO/AU

Powder X-ray Diffraction. Powdered samples were loaded in a 0.5 mm diameter capillary and sealed. For Compound 1 , the sample was loaded into the capillary as a slurry in DMF. Single point measurements were performed over the 5-50° 20 range with a 0.02° step size and 2° min^-1 scan rate on a PANalytical X'Pert Pro diffractometer fitted with a solid-state PIXcel detector 40 kV, 30 mA, 1° divergence and anti-scatter slits, and 0.3 mm receiver and detector slits using Cu-Ka (l = 1.5406 A) radiation.

Light Irradiated Powder X-ray Diffraction. Light-irradiated PXRD analysis was undertaken on a PANalytical X'Pert Pro diffractometer fitted with a solid-state PIXcel detector 40 kV, 30 mA, 1° divergence and anti-scatter slits, and 0.3 mm receiver and detector slits using Cu-Ka (A = 1.5406 A) radiation. A powdered sample of Compound 1

76

Substitue Sheets (Rule 26)

RO/AU in a slurry of DMF was loaded into a 0.3 mm diameter capillary and 4 x 3 blue LED (12 x 0.28 W) strips were mounted around the capillary. Single point measurements were taken at 30 minute intervals with continuous light irradiation onto the sample.

Electrochemistry and spectroscopy. Cyclic voltammograms were collected on a BASi Epsilon electrochemical analyser. All measurements were recorded in 0.1 M [(n- C₄H₉)4N]PF₆/CH₃CN electrolyte using a glassy carbon electrode, platinum counter electrode and an Ag reference electrode. All potentials are reported in volts versus Fc/Fc⁺ couple. UV-Vis-NIR spectra were collected on an Agilent CARY5000 Spectrometer with a Harrick Omni-Diff probe. Spectra were collected between 5000- 25000 cm^-1 and are reported as the Kubelka-Munk transform, where F(R) = (1 -R)²/2R.

Solid-State Electrochemistry. Solid state electrochemical measurements were performed using a Basi Epsilon electrochemical analyser. Argon was bubbled through solutions of 0.1 M [(n-C₄H₉)₄N]PF₆/CH₃CN. The CVs were recorded using a glassy carbon working electrode (1.5 mm diameter), a platinum wire auxiliary electrode and an Ag reference electrode. The sample was mounted on the glassy carbon working electrode by dipping the electrode into a paste made of the powder sample in the supporting electrolyte.

Solid-State UV-Vis-NIR Spectroscopy. UV-Vis-NIR spectra were obtained on powdered samples at room temperature using an Agilent CARY5000 Spectrophotometer equipped with a Harrick Omni-Diff Probe accessory over the wavenumber range 5000-25000 cm^-1. Spectra are reported as the Kubelka-Munk transform, where F(R) = (1 -R)²/2R.

Solid-State Spectroelectrochemistry.⁴ In the solid state, the diffuse reflectance spectra of the electrogenerated species were collected in situ in a 0.1 M [(n- C₄H₉)₄N]PF₆/CH₃CN electrolyte over the range 5000-25000 cm^-1 using a Harrick Omni Diff Probe attachment and a custom built solid state spectroelectrochemical cell. The cell consisted of a Pt wire counter electrode and an Ag/AgCI reference electrode in 3 M NaCI aqueous solution. The solid sample was immobilised onto a 0.1 mm thick indium tin oxide (ITO) coated glass slide (which acted as the working electrode) using a thin strip of Teflon tape. The applied potential was controlled using an eDAQ e-corder 410 potentiostat. Continuous scans of the sample were taken on the CARY5000

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RO/AU spectrometer and the potential increased gradually until a change in the spectrum was observed. Spectra are reported as the Kubelka-Munk transform, where F(R) = (1 -R) 2/2R

Raman Spectroscopy. Single point Raman spectra were measured using an inVia Renishaw Confocal Raman microscope. The laser (785 nm) was focused onto the sample using the Raman microscope (x50 magnification). The Raman spectra were recorded over the 100-3200 cm^-1 range with 10 seconds exposure time and 10 % laser power over 1 accumulation.

Light-irradiated Raman Spectroscopy. Light irradiated Raman spectra were measured using an inVia Qontor Confocal Raman microscope. The laser (785 nm) was focused onto freshly prepared crystals of Compound 1 which were wetted with DMF using the sample stage microscope (x50 magnification). The Raman spectra were recorded over the 500-1700 cm^-1 range with 1 second exposure time and 10 % laser power over 1 accumulation. A white light from a microscope lamp (25 W) was directed at the sample. Spectra were collected between 30 second intervals of light irradiation. The light from the sample stage cavity and room were turned off to ensure no other source of light was irradiating the sample during the experiment.

Isothermal Raman Spectroscopy. Isothermal Raman spectra were measured using an inVia Qontor Confocal Raman microscope. The laser (785 nm) was focused onto the crystals of Compound 3 using the sample stage microscope (x50 magnification). The Raman spectra were recorded over the 500-1700 cm^-1 range with 1 second exposure time and 10 % laser power over 1 accumulation. The samples were placed onto a heating stage and the temperature was set by an external controller. Upon heating the sample, each spectra were collected in 30 second intervals.

Quantification of the structural transformation. The percentage conversion of framework from Compound 1 to Compound 3 was calculated by monitoring the peak centred at 1527 cm^-1 in in the light irradiated Raman spectra. The conversion to Compound 3 was deemed complete when full recession or no further change in the 1527 cm^-1 peak was observed. Deconvolution of the peaks yielded the integrated area which was then used to calculate the percentage conversion as a fraction r relative to the starting spectrum of Compound 1. Spectral deconvolutions were undertaken in

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RO/AU OriginPro software using the Lorentzian model with fixed baselines. The modelled peak positions for each spectrum were adjusted to give the best possible fit with the maximum deviation being 4 cm^-1.

The same procedure was used to quantify the conversion of Compound 3 to Compound 1 from the isothermal Raman experiment. In this case, the percentage conversion was calculated as a fraction relative to the spectrum of Compound 3. A summary of the parameters is provided in Tables 5 and 6.

Results

The solvothermal reaction of Cd(NO₃)₂-4H₂O, Py₂TTF and H₂bpdc in a DMF- EtOFI mixture at 80 °C for 4 days yielded bright orange plate-like crystals of Compound 1. Elucidation of the crystal structure via single crystal X-ray diffraction revealed Compound 1 to possess the monoclinic space group, P2-i/n, with unit cell parameters a = 10.1851 (4) A, b= 28.1899(11) A, c= 14.9875(5) A, b = 97.675° and V= 4264.6(3) A.

The structure of Compound 1 possesses an octahedral Cd(ll) ion coordinated by two bpdc and two Py₂TTF ligands, wherein the two bpdc ligands are made up of 1 full bpdc carboxylate and 2 halves of a bpdc carboxylate (the other two halves bridging to another Cd(ll) ion). Two of these Cd(ll) centers form an 8-membered {(Cd-O-C-O)₂) ring consisting of two Cd(ll) and four bpdc units (see Figure 1a). The repetition of these single building units creates grid-like 2-D sheets of {Cd(bpdc)}n that propagate in the a-b direction. While two opposing bpdc ligands coordinated to the Cd dimer single building unit remain in-plane of each other, the others bend in opposing directions creating an undulating 2-D sheet (see Figure 1c). Additionally, a rotation of ca. 38° about the central C-C bond between phenyl groups in the bpdc ligand is found, likely to accommodate for the puckering in the {Cd(bpdc)}n sheets. The apical positions of Cd(ll) are occupied by Py₂TTF ligands which act to pillar the 2-D sheets to form a 3-D framework as shown in Figure 1b. Closer inspection of the 8-membered ring reveals the Cd...Cd separation to be only 3.869 A. This allows for closer alignment of the Py₂TTF ligands, optimising the cofacial arrangement of Py₂TTF dimers (see Figure 2a). The dimers adopt a face-to-face tt-stacking orientation with a slight translational shift that staggers the ligands, aligning corresponding S atoms of each TTF core, with a proximal S...S distance of 3.77 A.

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Substitue Sheets (Rule 26) RO/AU The large rectangular cavities that run in all three directions afford a framework that occupies approximately only a third of the unit cell volume. Thus, a second interpenetrating framework, independent of the first, is found, translated approximately mid-way along the unit cell length (see Figure 2d). Despite the presence of this second network, the framework of Compound 1 retains open channels along the c-direction (26.6% void volume, based on the Van der Waals surface). DMF molecules are found in these channels for the as-synthesised material and the formula unit was calculated to be [Cd₂(Py₂TTF)₂(bpdc)₂]-5DMF. This is supported by the thermogravimetric analysis of Compound 1 which exhibits a 16.6% weight loss between 50-130 °C corresponding to approximately one DMF molecule per formula unit (see Figure 3). Rapid weight loss is observed at 275 °C upon further heating, indicative of framework decomposition.

When exposed to natural light over a period of four days the crystals comprising Compound 1 turned from bright orange to light yellow in colour generating the structure of [Cd₂(Py₄C₁₂S₈F₁₄)(bpdc)₂] (Compound 3; Figure 2f). Initial attempts to reproduce this effect under simulated sun light using a desk lamp or UV LEDs for a prolonged time (>1 hour) led to the loss of single crystal character. Flowever, crystals of suitable quality were obtained by exposing crystals of Compound 1 to a 40 W microscope lamp for 15 minutes. Elucidation of this structure reveals a crystal system reminiscent of Compound 1 which possesses the same monoclinic space group, P2₁/n, with unit cell parameters of a = 9.250(19) (4) A, b = 28.297(6) A, c = 15.864(3) A and b = 95.75(3)°. This translates to an overall decrease in the unit cell volume by 133.2 A.

Remarkably, elucidation of this structure revealed a significant SC-SC structural transformation at the cofacial Py₂TTF ligands of independent nets, whereby the original olefinic bonds that cap each pair of sulfur atoms around the C=C bond in the TTF core have cyclised to form cyclobutane rings, chemically bonding the previously distinct Py₂TTF dimer units together (see Figure 2f). The once planar Py₂TTF ligands now also exhibit puckering at the S atoms and the pyridyl groups adopt a V-like conformation to retain coordination to the Cd(ll) centers. Distortion of the 8-membered single building unit is also apparent whereby the Cd... Cd separation is lengthened significantly to 4.126 A. With these structural changes, the pore apertures of Compound 3 are also slightly modified, with the channels that run along the c-direction experiencing a contraction (Figures 2d and 2f). Whilst solvent molecules could not be satisfactorily

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RO/AU modelled, the SQUEEZE25⁵ program in PLATON₂6⁶ together with evidence from the elemental analysis and TGA suggest the presence of two DMF molecules per formula unit (Figure 4). Crystals of 3 showed superior thermal stability as evidenced by TGA, and were stable in air indefinitely. By exposing crystals of Compound 1 to 6 minutes of light from a 100 W halogen lamp source a partially cyclised form of [Cd₂(Py₄C_i2S₈F₁₄)_o.84{(Py₂TTF)₂}_o.32(bpdc)₂]-DMF (Compound 2) was generated and characterised using synchrotron radiation. The framework of Compound 2 still retains the symmetry of Compound 1 and the unit cell parameters were found to lie between that of Compounds 1 and 3. These changes are summarised in Table 2. In examining the structure of Compound 2, it is noted that the Py₂TTF ligand has undergone an incomplete [2+2] cycloaddition reaction with half of the Py₂TTF exhibiting disorder. As depicted in Figure 2b, the disorder alludes to a 70 % conversion (calculated from the refined site occupancies) to the dimerised (Py₄C₁₂S₈H₄) form with the remaining 30 % existing as the original Py₂TTF form. Notably, the disorder is only found on opposing diagonals of the (Py₄C₁₂S₈H₄) moiety suggesting that the cycloaddition may occur in a concerted manner. This would appear to be preferential to the stepwise mechanism, as cyclisation of a single side of the unsaturated alkene would cause the uncyclised side to adopt an outward bend, likely prohibiting further reaction. Table 2: Unit cell parameters of Compounds 1 to 3 obtained upon light- irradiation

The in situ conversion and monitoring of the bulk powder of Compound 1 was confirmed by light-irradiated powder X-ray diffraction and Raman studies. A powdered

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Substitue Sheets (Rule 26) RO/AU sample of Compound 1 in a slurry of DMF-EtOH was exposed to light from twelve blue LEDs (0.28 W and 51 lumens each; 300-600 nm spectral range) and the evolution of the powder patterns was monitored over a 17.5 hour period at 300 K. A comparison of the calculated powder patterns for Compounds 1 and 3 revealed the most significant Bragg reflections are observed in the 20 = 5-20° range (Figures 5a, 6, 7 and 8). The experimental powder patterns collected at room temperature show slight deviation in the position of the peaks in contrast to the calculated plot due to unit cell expansion. As shown in Figure 5a, the most prominent difference is observed for the 010 peak which shifts in 20 from 6.8° to ca. 6.5° upon irradiation, becoming a shoulder to the 020 reflection. A second significant shift is that of the 10-1 peak from 20 = 9.6° to 10.2°, becoming more intense. The 111 peak is also seen to diminish and arise as a less intense peak around 20 = 12°. The 121 peak in Compound 1 diminishes as a new peak at 20 = 12.7° increases due to the 121 peak in Compound 3. A change is also seen from the 11-2 and 131 combined peaks of the starting pattern of Compound 1 , which appeared to shift to higher angles, forming the predicted 11 -2 and 131 peaks of Compound 3 as two overlapping peaks. Notably, a lowering in intensity of the 151 peak from Compound 1 is also seen during irradiation by UV, and a new higher angle peak forms at the position where the 151 reflection lies for Compound 3. These changes correlate well with the expected powder pattern of Compound 3. Additionally, the complete regress of the 011 peak is indicative of quantitative generation of Compound 3.

For light-irradiated Raman measurements, the same 25 W microscope lamp was employed. A spectrum was collected using 785 nm laser excitation with 30 second intervals of light irradiation between spectral collections over the course of 10.5 minutes. Table 3: Experimental and vibrational mode assignments of Compound 1.

Vibrational modes (v) of Py₂TTF and (Py₄C₁₂S₈H₄) are shown in Scheme 2.

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^*b = bending, d = deformation, st = stretch, w = wagging, s = scissoring

Table 4: Experimental and vibrational mode assignments of Compound 3. Vibrational modes (v) of Py₂TTF and (Py₄C₁₂S₈H₄) are shown in Scheme 2.

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Substitue Sheets (Rule 26) RO/AU 1559 Pyridyl ring (w)

^*b = bending, d = deformation, st = stretch, w = wagging, s = scissoring

Scheme 2: Vibrational modes (v) of Py₂TTF (top) and (Py₄C₁₂S₈H₄) (bottom). As shown in Figure 5b, a number of distinct changes were observed in the

Raman spectra upon light irradiation over the course of 10 minutes. Firstly, the regress

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RO/AU of four peaks at 831 , 847, 940 and 1527 cm^-1 was observed. These peaks are assigned to the C-S stretching (v3), C-C stretching (v7), C-S stretching (v6) and pyridyl ring wagging, respectively. The growth of new peaks at 735, 783, 1078 and 1559 cm^-1 was also observed. These peaks correspond to the stretching of the newly formed C-C bond (v7), C-S stretching (v3), pyridyl ring bending and pyridyl ring wagging, respectively. Overall, the spectral changes observed are consistent with the transformation of Py₂TTF in Compound 1 to the cyclised (Py₄C₁₂S₈H₄) form. Notably, the growth of the peak at 783 cm^-1 is most significant indication of the photocyclisation reaction. Other aforementioned changes are closely related to new C-C bond or the pyridyl rings which undergo a geometry change from Compound 1 to Compound 3.

At room temperature, both in ambient light and when kept in the dark, samples of Compound 3 remained stable and did not exhibit any retro-conversion to the original state. Heating crystals of Compound 3 as a slurry in DMF in dark conditions at 150 °C for 48 h saw a complete retro-conversion of all crystals to the cofacial Compound 1 structure. The retro-conversion was visually observable with a clear change in the crystal colour from the yellow Compound 3 to the dark red Compound 1. Analysis of the retro-conversion was confirmed by PXRD of the heated sample which revealed a diffraction pattern corresponding to Compound 1 (Figures 9 and 18). The reverse process was successfully studied in situ using Raman spectroscopy (785 nm). Heating (180 °C) and concurrent monitoring of the crystals of Compound 3 yielded the spectral changes shown in Figure 5c over a period of ca. 4.5 minutes. These changes correspond well to the light-irradiated in situ Raman discussed above and the final spectrum resembling that of the Raman spectrum of Compound 1.

A sample of Compound 1 was also cycled through two complete structural switches (Compound 1 - Compound 3 - Compound 1 - Compound 3 - Compound 1), with PXRD data showing no loss of crystallinity after each conversion (Figure 18). After the second retro-conversion the sample retained an ability to cyclise without loss of crystallinity for a third time.

To better understand the physical property changes that accompany the SC-SC transformation, the solid state spectroscopic, electrochemical and spectroelectrochemical properties of Compounds 1 and 3 were probed.

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Substitue Sheets (Rule 26) RO/AU The optical properties of framework materials can provide insight into their fundamental electronic properties. Bulk samples of Compound 1 and Compound 3 retain the stark colour changes observed in single crystals, with Compound 1 existing as a dark red microcrystalline powder while Compound 3 is a light yellow powder. As shown in Figure 10a, three transitions are observed in the as-synthesised framework of Compound 1. The bands at 20120 and 23310 cm^-1 are assigned to a p-p* and an intramolecular charge transfer transition (ICT) of the Py₂TTF ligand, respectively. The lower energy band at 17400 cm^-1 is attributed to the small presence of radical Py₂TTF^'+ in the framework. In studying the diffuse reflectance spectrum of Compound 3, a stark contrast in the spectra of the two frameworks is found with Compound 3 exhibiting an overall blue-shift of the spectrum. Additionally, the two transitions that were present at 17400 and 20120 cm^-1 are completely diminished. This is consistent with the structural changes associated with the Py₂TTF becoming a cyclised (Py₄C₁₂S₈H₄) dimer which likely foregoes the ability to undergo a two successive one-electron oxidation and possess significantly different charge-transfer properties to the archetypal TTF. In the far-visible/UV region a band at >25000 cm^-1 is observed for Compound 3. Although the origin of this band is still unknown, it likely arises from the Py₄C₁₂S₈H₄ ligand and not from the other components present in Compound 3.

Investigating the solid state cyclic voltammogram of Compound 1 in 0.1 M [(n- C₄H₉)₄N]PF₆/CH₃CN revealed two quasi-reversible one electron processes reminiscent of the electrochemistry of Py₂TTF (Figure 10b and 11 ). Scanning anodically, at 400 mV/s the first process at 0.34 V (vs. Fc/Fc⁺) is attributed to the generation of the Py₂TTF^'+ radical. This is followed by the second oxidation to the Py₂TTF²⁺ at 0.69 V (vs. Fc/Fc⁺). In contrast to the traditional electrochemistry of Compund 1 , the cyclic voltammogram of Compound 3 (Figure 10c) reveals only one irreversible oxidation at 0.83 V (vs. Fc/Fc+). The cyclic voltammogram of a sample of Compound 3 retro- converted to Compound 1 (Figure 19) shows that the electrochemical properties of the material return to those of the as-synthesised sample of Compound 1 , demonstrating an ability to switch the material’s redox activity by the reversible double [2 + 2] photocyclisation.

In contrast to the more traditional electrochemistry of Compound 1 , the cyclic voltammogram of Compound 3 in 0.1 M [(n-C4H₉)₄N]PF₆/CH₃CN reveals a more

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RO/AU convoluted and complex redox process. The 400 mV/s voltammogram shows the presence of multiple processes when scanning anodically (Figure 10c). Owing to the large capacitance, a further square wave voltammetry (SWV) study was undertaken which revealed four oxidative and three reductive processes in the reverse scan (Figures 12 and 16). Based on the cyclic voltammogram of Py₂TTF and the position of these faradaic processes, the first and third oxidative processes at 0.26 and 0.52 V (vs. Fc/Fc⁺), respectively, can be attributed to the redox processes for Py₂TTF. This likely arises from a small proportion of the uncyclised cofacial form still present in the framework of Compound 3. The assignment of the quasi-reversible and irreversible oxidation at 0.25 and 0.83 V (vs. Fc/Fc⁺) cannot definitively be identified. Flowever, the removal of conjugation from the TTF core in Compound 3 likely reduces or completely inhibits the ability for electron delocalisation, hence the ability to stabilise oxidation states accessible to Py₂TTF. Although the irreversible oxidation at 0.83 V cannot definitively be identified, the process is consistent with the reported electrochemical behaviour of a one-sided [2 + 2] photocyclised TTF based molecule. This intermediate state, however, does demonstrate that the degree of cyclisation of the material can be used to control the material’s electrochemical properties.

A solid state Vis-NIR spectroelectrochemistry (SEC) study of Compound 1 was conducted in 0.1 M [(n-C4H₉)₄N]PF₆/CH₃CN to observe the effect of electrochemical modulation on its optical properties. Akin to the de novo diffuse reflectance spectrum of Compound 1 (Figure 10a), the starting spectrum shows three transitions at 18400, 20970 and 23560 cm^-1 attributed to the aforementioned transitions, as shown in Figure 10d. Upon application of oxidative potential bias, three main processes were observed. The first, which occurs when a potential of 0.3-0.6 V (vs. Ag/Ag⁺) is applied, involves the formation of Py₂TTF^'+ radical as indicated by the increase of the band at 18400 cm^-1 in addition to the two other bands in the Vis/UV region. Upon increasing this potential to +0.7 V (vs. Ag/Ag⁺), a more complex second process is observed. The band at 18400 cm^-1, which is attributed to the p-p* transition, experiences a further increase in intensity as well as a red-shift to 17900 cm^-1. The second process is also accompanied by the formation of two new bands at 6880 ca. 14900 cm^-1. The new peak at 14900 cm^-1 is attributed to the radical Py₂TTF^'+ form whilst the lower energy band in the NIR region is assigned to a through-space Intervalence Charge Transfer (IVCT) transition between the cofacial Py₂TTF moieties in Compound 1. Ligand-based IVCT in

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RO/AU TTF oligomers and discrete complexes are more commonly encountered, although the number of framework materials to possess such features are still scarce. For the framework of Compound 1 , the origin of the IVCT band is likely due to a localised interaction between the partially oxidised cofacial TTF⁰/ TTF ⁺ cores. A final process occurs between 0.8-0.9 V (vs. Ag/Ag⁺), leading to a decrease in the band at 14900 and 18400 cm^-1, suggestive of Py₂TTF²⁺ formation. Additionally, the previously red crystals of Compound 1 had obtained a darker brown in colour (Figure 10d, inset). Partial regeneration of the starting spectrum was observed when a cathodic potential to -0.5 V (vs. Ag/Ag⁺) was applied, indicating the rate of reduction may be significantly slower than that of the oxidation process.

In the related structure of Compound 4 which precludes the cofacial arrangement of Py₂TTF ligands found in Compound 1 and adopts a significantly larger intermolecular distance between the ligands, both the electrochemical and Vis-NIR data lend support to our interpretation that Compound 4 does not cyclise (Figures 21 and 22). The Py₂TTF ligands in Compound 4 behave as isolated, non-interacting units with no evidence for the IVCT interaction found in Compound 1.

The Vis-NIR SEC of Compound 3 in 0.1 M [(n-C4H9)4N]PF₆/CH₃CN showed one process which occured upon applying an anodic potential between +0.3-0.7 V (vs. Ag/Ag⁺) (Figure 10e). During this process a new broad band arises between ca. 13000-16000 cm^-1, whilst the ICT transition present in the neutral state increases in intensity at 24000 cm^-1. The process was characterised by a colour change from orange to brown (Figure 10e, inset). A slight intensification of the band at 22100 cm^-1 is also observable and is tentatively assigned to an MLCT transition.

To quantitatively monitor the completeness of the cyclisation reaction upon conversion of Compound 1 to Compound 3, a photo-irradiated Raman experiment was performed. The 1527 cm^-1 peak in the Raman spectra proved to be the most suitable given its full regression upon completion. By deconvoluting this peak, it was found that almost full conversion (ca. 98.6%) of Compound 1 to Compound 3 was achieved over 540 s of light irradiation (Table 5).

Table 5: summarising the quantification of the conversion of Compound 1 to Compound 3 from the photo-irradiated Raman spectra.

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RO/AU

Using the same approach to the photo-irradiated Raman experiment, deconvolution of the 1527 cm^-1 peak enabled evaluation the retro-conversion, and full conversion was observed over 270 s of light irradiation (Table 6). Note that this method is semi-quantitative with an estimated uncertainty limit based on the accuracy with which band areas could be calculated of ±5%. In both the PXRD and Raman methods, regeneration of Compound 1 required the samples to be wetted with DMF as the absence of the solvent led to loss of crystallinity.

Table 6. Table summarising the quantification of the retro-conversion of Compound 3 to Compound 1 from the isothermal Raman spectra.

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Substitue Sheets (Rule 26) RO/AU Comparison between Compound 1 and Compound 4

To provide further insight into the structural and energetic requirements for the transformation between Compound 1 and Compound 3, a model framework of [Cd₂(Py₂TTF)(bdc)₂] (Compound 4) was synthesised using the 1 ,4- benzenedicarboxylate co-ligand and similar reaction conditions to that for Compound 1. 1 ,4-Benzenedicarboxylate is more rigid than biphenyl-4, 4'-dicaryboxylic acid, the coligand for Compound 1. The most significant difference between the structures of Compound 1 and Compound 4 is the orientation of the Py₂TTF dimers, which were found to exist in a herringbone stacking arrangement in Compound 4 (Figure 15). The staggered arrangement of these ligands leads to a significantly distant intermolecular separation between the TTF units (C=C, 4.46-4.74 A). Light irradiation of the bulk material using both sunlight and a 25W light source showed no evidence of a structural transformation. In relation to the necessary conditions for [2+2] photocyclisation to occur, the framework of Compound 4 significantly exceeds the geometric proximity required for photocyclisation to occur.

Characterisation of [Cd₂(Py₂TTF)₂(bdc)₂]-DMF (Compound 4). Single crystal X- ray diffraction revealed Compound 4 to possess the monoclinic space group P2₁/c with unit cell dimensions of a = 19.3285(8) A, b = 19.4632(6) A, c = 17.0969(6) A and b = 98.710(4)°. PXRD revealed the homogeneity of the bulk product. The structure of Compound 4 is depicted in Figure 15. Examination of the crystal structure revealed Compound 4 to possesses the same {(Cd-O-C-O)₂} structural building unit with Py₂TTF ligands coordinating on the apical position of the Cd(ll) ions. Though significant similarity in connectivity exists, Compound 4 was found to possess less undulation in its {Cd(bdc)₄}n sheets when compared with Compound 1. This is owed to the additional rotational freedom provided by the bdc co-ligand to the framework, which helps to alleviate framework strain. Additionally, the shorter bdc ligand gives rise to an almost halving of the void space in the framework of Compound 4 compared to Compound 1. A second interpenetrating framework is also found in Compound 4 which exists approximately midway between the first net.

Characterisation of [Zn2(SBPDC)₂(Py2TTF)₂]

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Substitue Sheets (Rule 26) RO/AU The solvothermal reaction of Zh(Nq3)₂·6H₂q, Py₂TTF and 2-sulfone-4,4'- biphenyldicarboxylic acid (H₂SBPDC) in a DFM/EtOH solution at 100 °C for 48 h yielded red plate-like crystals of [Zn2(SBPDC)₂(Py₂TTF)₂] (hereafter referred to as Zn-1A) suitable for SCXRD. Zn-1A crystallises in the monoclinic space group, P21/c, with unit cell parameters a = 18.8827(5) A, b = 16.4226(5) A, c= 27.3013(9) A, b = 106.975(3)°, and V = 8097.4(4) A3 (further details provided in Table 7). Zn-1A possesses the same basic structure as Compound 1 , featuring [Zn-O-C-O]2 nodes linked by undulating sheets of dicarboxylate ligands, joined by pillars of cofacial Py₂TTF dimers (Figure 29). The Zn N bond lengths (2.12-2.17 A) are shorter than the Cd - N bonds in Compound 1 (2.29-2.32 A), consistent with the better overlap of the N 2 p orbital with the Zn 3d orbital compared to the Cd Ad orbital. Due to the length of the ligands, Zn-1A is doubly interpenetrated, with the second net located approximately halfway along the unit cell length. Despite this Zn-1A still possess 3060 A3 void volume per unit cell, corresponding to 37.7% porosity (calculated with PLATON). This pore space is filled with six DMF molecules per asymmetric unit. The bulk purity of Zn-1A was assessed via PXRD, with a Le Bail refinement confirming that the single crystal structure is representative of the bulk material (Rwp = 4.80).

Table 7: Table summarising data collection and refinement statistics for Zn-1A and Zn-2A:

Zn-IA Zn-2A

Empirical formula C39H37N5O9S5Z11 C25H19N3O5S5Z11 Formula weight 1889.8 5336.8 Collection temperature (K) 100 100 Wavelength (A) 1.54178 1.54178 Crystal system Monoclinic orthorhombic Space group P2₁/c Pcc2 a (A) 18.8827(5) 17.8539(2) b (A) 16.4226(5) 18.9972(3) c (A) 27.3013(9) 16.6443(2)

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Substitue Sheets (Rule 26) RO/AU a (A) 90 90 b (A) 106.975(3) 90 g (A) 90 90

Volume (A³) 8097.4(4) 5645.32(13)

Z 4 1 m (mm^-1) 3.781 5.022

F(000) 3900 2720

Crystal size (mm) 0.204x0.069x0.019 0.099x0.075x0.059

Theta range (°) 3.385-72.558 4.313-72.396

Reflections measured 32029 28067

Independent reflections 15639 9899

Complete to theta (%) 99.6 99.9

Absorption correction Gaussian Gaussian

Max. and min. transmission 1.000, 0.824 1.000, 0.805

Refinement method F² F²

Data/restraints/parameters 15639/25/1068 9899/7/681

Goodness-of-fit on F² 1.108 1.314

Final R(F) indices [I>2s(I)] 0.0890 0.0500 wR2 (F²) indices (all data) 0.3090 0.1530

Largest difference between

1.806/- 1.638 1.161/-0.793 peak and hole (e/A³)

Despite the broad similarities in their overall structure, there are several key differences between Zn-1 A and Compound 1 . Most obvious is the increased slanting of the Py₂TTF dimers in Zn-1 A due to a larger b angle ( ca . 107.0 vs 97.7°). Moreover, the ligands in each dimer are inequivalent, with one Py₂TTF mostly flat and the other curving inwards at the centre. Both display twisting between the TTF cores and pyridyl rings, with torsion

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RO/AU angles ranging from 1.44 to 12.09°. The Py₂TTF dimers are also not perfectly coplanar, with the TTF core of one ligand rotated about its long axis by 4.6 ° with respect to the other. Additionally, the Zn -Zn separation is larger than the Cd- Cd distance in Compound 1 (4.11 vs 3.87 A).

Response of Zn-1 A ([Zn₂(SBPDC)₂(Py₂TTF)₂]) to light irradiation

When crystals of Zn-1 A were exposed to white light, their colour changed from red to colourless (Figure 23). PXRD confirmed a structural change had occurred and that crystallinity was partially maintained (Figure 24). The new material is hereafter referred to as Zn-1 B. Unfortunately, the light-induced structural switch caused cracking in crystals of Zn-1B, preventing determination of the crystal structure by SCXRD. Interestingly, the transition appears to propagate through the crystal as a unified front, possibly indicating a cooperative mechanism where the change in one unit cell aids the switch in adjacent unit cells.

The Py₂TTF dimers in Zn-1 A contain two inequivalent pairs of double bonds with separations (measured between the centroids of each alkene) of 3.74 and 4.14 A, respectively, both of which satisfy Schmidt’s distance criteria for [2+2] photocycloadditions. To further understand the likely extent of orbital overlap between the pairs of double bonds, more comprehensive geometrical parameters were calculated and compared with those for Compound 1 (Table 8). Both qi and 02 are close to their ideal values of 0 and 90°, respectively, for all pairs of reacting alkenes in Zn-1 A and Compound 1. Flowever, due to the offset of the TTF cores normal to the double bond axis in these MOFs, their 03 values deviate noticeably from the optimal 90° orientation. This effect is exacerbated for the double bonds in Zn-1 A with 4.14 A separation due to the outwards bending of that C₃S₂ ring. Nevertheless, [2+2] photocycloadditions have occurred in systems with even less favourable double bond orientations, provided the crystal structure allows sufficient movement to take place (see for instance, Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87 (2), 433-481 )The structure of Compound 3 reveals that the [Cd-O-C-O]2 ring can deform to accommodate the geometry of the cyclised dimers; presumably the [Zn-O-C-O]2 ring could do the same. Therefore, if Compound 1 is used as a benchmark for a system which undergoes a double [2+2] photocycloaddition between cofacial TTF units, it is feasible that the same is occurring in Zn-1 A.

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RO/AU Table 8: Geometrical parameters for the TTF ring alkenes in Compound 1 (Cd- 1 A), Zn-1 A and Zn-2A:

Evidence fora Double [2+2] Photocycloaddition from Zn-1A to Zn-1B TGA of Zn-1A and Zn-1 B revealed initial weight losses of 14.5 and 17%, respectively, due to evacuation of pore solvent (Figure 25a). Solvent loss in Zn-1 B continued at higher temperatures than for Zn-1A; one possible explanation is that Zn-1 B has an altered pore structure which hinders the escape of solvent molecules from the framework. Both MOFs exhibit similar decomposition behaviour, with the onset occurring in two stages: 336 and 368°C in Zn-1 A and 320 and 368°C in Zn-1 B.

The solid state UV-vis-NIR spectrum of Zn-1 A reveals a broad peak at 20800 crrr¹, which is assigned to an intramolecular transition in the TTF core of Py₂TTF (Figure 25b). The band is completely diminished in Zn-1 B, which instead exhibits a small shoulder band at 24800 cnr¹, demonstrating an alteration of the electronic character of the TTF core. This observation is consistent with the behaviour of Compound 1 and Compound 3 and is clear evidence that the TTF cores are directly involved in the structural transition, consistent with the occurrence of double [2+2] photocycloadditions between Py₂TTF dimers. Furthermore, both Zn-1 A and Zn-1 B display bands at approximately 30300, 33100 and 39900 cnr¹, all of which can be assigned to aromatic TT-TT* transitions in SBPDC, emphasising that the structural transition is confined to the Py₂TTF ligands.

Since cyclisation between Py₂TTF dimers should result in a change in the oxidation processes associated with the TTF cores, electrochemical experiments were conducted on both Zn-1 A and Zn-1 B. The cyclic voltammogram of Zn-1 A exhibits two quasi-

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RO/AU reversible redox processes, assigned to the formation of Py₂TTF^#+ and Py₂TTF²⁺, respectively (Figure 26a). Squarewave voltammetry confirmed they occurred at E1/2 = +0.16 V and E1/2 = +0.46 V vs Fc/Fc⁺, respectively (Figure 27a), consistent with a wide range of previously reported TTF-containing systems. In contrast, the electrochemistry of Zn-1 B exhibits only one minor, irreversible oxidative process occurring at +0.90 V vs Fc/Fc⁺ (Figure 26b and Figure 27b). This is very similar to the electrochemistry of Compound 3 and is further evidence that [2+2] photocyclisation has occurred in Zn-1 B. If cyclisation had occurred between only one pair of double bonds in Zn-1 A, one redox process would still be expected of the remaining C₃S₂ ring. As this was not observed in the electrochemical experiments, it appears that both C₃S₂ rings partake in the structural switch.

Raman spectroscopy has been used to follow the progress of cyclisation between Compound 1 and Compound 3 by monitoring several vibrational modes which are unique to either Py₂TTF or the cyclised dimer, Py₄C₁₂S₈H₄. Therefore, assuming that the presence of Zn instead of Cd has a negligible impact on the frequency of these vibrational modes, a similar approach may be applied to Zn-1 A and Zn-1 B. The transition from Zn-1 A to Zn-1 B is accompanied by the regression of peaks at 841 , 941 and 1524 cm^-1 in the spectrum of Zn-1 A (Figure 27), which correspond to two C-S stretching modes (vz and nb) and the TTF ring alkene stretch (vs), respectively. The peak at 479 cm^-1 is also diminished but its assignment was less clear; it is possibly due to C-H bending on the TTF core (V4). Additionally, vibrational modes attributed to Py₄Ci2S8Fl4 appear in the spectrum of Zn-1 B at 501 , 539, 732, 1077 and 1556 cm^-1, corresponding to a C-S bend (V2), pyridyl ring wagging, stretching of the newly formed C-C bond (vg), pyridyl ring bending, and a second pyridyl wagging mode, respectively. These observations are comparable with observations of Compound 1 and Compound 3, suggesting that Zn-1 A undergoes light-induced double [2+2] photocycloadditions in the same fashion.

Taken together, the preceding evidence is consistent with Zn-1 A undergoing a double [2+2] photocycloaddition between Py₂TTF dimers when exposed to light.

Transition from Zn-1 B to Zn-1 A

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Substitue Sheets (Rule 26) RO/AU Retro-conversion of the cyclobutane rings in Compound 3 occurs upon heating at 150 °C, yielding the original framework, Compound 1. To determine whether this switching property was replicated in Zn-1 B, a sample was heated at 150 °C in DMF for 1 h. The material turned red (reminiscent of Zn-1 A), however PXRD indicated that Zn-1 A was not reformed. A large amorphous component was also observed, suggesting that some decomposition had occurred, and when Zn-1 B was left at 150 °C overnight, it completely dissolved. However, when Zn-1 B was left at 80 °C overnight the material turned red (reminiscent of Zn-1 A). The inventors expect that Zn-1 A will re-form from Zn- 1 B upon heating at lower temperatures such as 80 °C.

Characterisation of [Zn2(TDC)₂(Py2TTF)₂]

A solvothermal reaction between Zh(Nq3)₂·6H₂q, Py₂TTF and 2,5- thiophenedicarboxylic acid (H₂TDC) in DFM/EtOH at 100 °C for 48 h yielded red crystals of [Zn2(TDC)₂(Py₂TTF)₂] (hereafter referred to as Zn-2A) suitable for SCXRD. Zn-2A crystallises in the orthorhombic space group Pcc2 with unit cell parameters a = 17.8539(2) A, b = 18.9972(3) A, c= 16.6443(2)A, and V = 5645.32 A3 (further details provided in Table 7). Like Zn-1 A, Zn-2A contains 2D sheets of dicarboxylates liking [Zn- 0-C-O]2 units, bridged by pillars of cofacial Py₂TTF dimers (Figure 30). The Zn N bonds (2.11-2.15 A) are slightly shorter than those in Zn-1 A. The MOF is also doubly interpenetrated, but due to the shorter length of TDC compared to SBPDC, the void space is only 1392 A3 per unit cell, corresponding to 24.7% porosity (calculated with PLATON). These pores contain two DMF solvent molecules per asymmetric unit. The bulk purity of Zn-2A was confirmed by a Le Bail refinement of the PXRD pattern (Rwp = 3.18).

In contrast to both Compound 1 and Zn-1 A, the Py₂TTF dimers in Zn-2A are crossed over, causing the TTF ring alkenes to be completely offset, with no direct overlap (Figure 30e). The crossover point is not at the centre of each ligand, leading to two inequivalent pairs of double bonds with average separations of 4.39 and 4.89 A, respectively (measured between the centroids of each alkene). Owing to the orthorhombic topology, the Py₂TTF pillars are not slanted. However, both Py₂TTF ligands in the dimer are twisted, with torsion angles between the TTF cores and pyridyl rings ranging from 2.24-21.29°. This causes a deviation from ideal coplanarity, with the TTF core of one ligand rotated about its long axis by 5.1 ° with respect to the other.

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RO/AU However, the two TTF cores are still substantially co-facial with respect to each other. The Zn -Zn spacing of 3.91 A lies between the M -M distances observed in Compound 1 and Zn-1 A.

Response of Zn-2A to light irradiation

Given the crossed nature of its Py₂TTF dimers (Figure 30e), it was expected that [2+2] photocycloadditions would be impossible in Zn-2A, making it non-responsive to light exposure. Remarkably, however, when crystals of Zn-2A were irradiated with white light, their colour changed from red to yellow/orange (Figure 31). The resulting material is hereafter referred to as Zn-2B. PXRD confirmed a structural transition had occurred, albeit with a slight loss of crystallinity (Figure 32). As with Zn-1 B, the structural transition in Zn-2B was accompanied by crystal cracking. Consequently, single crystals suitable for SCXRD could not be achieved, despite efforts to minimise cracking such as using a less intense light source to slow conversion, and encasing crystals in a polyvinylidene fluoride membrane prior to irradiation. Hence, a crystal structure of Zn-2B was not obtained. The structural switch appears to travel through the crystals of Zn-2A in a concerted manner (similar to observations of Zn-1 A), again suggesting a cooperative mechanism.

Since the double bonds in Zn-2A required to form the cyclised dimer have no direct overlap, the merit of applying the q-i, 02, and 03 parameters to predict whether [2+2] photocycloadditions will occur is dubious. In particular, accurate derivation of 02 and 03 both require the four carbons of the reacting double bonds to form a parallelogram. This is not the case in Zn-2A, hence the reported values should be considered as approximate (Table 8). Nevertheless, in order to enable some comparison with Compound 1 and Zn-1 A, a brief discussion is useful. In Zn-2A, the combined deviation of d, 01, and 02 from their optimal range is far greater than in any reported TTF derivative which undergoes [2+2] photocycloaddition. Evidently, for a double [2+2] photocycloaddition between Py₂TTF dimers to occur in Zn-2A, an unprecedented level of molecular movement would be required to attain sufficient orbital overlap - far exceeding previous reports for reactions of this nature.

The topochemical postulate states that “reactions in crystals proceed with a minimum of atomic and molecular movement” and is often invoked to emphasise the limited capacity

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RO/AU for movement afforded by the crystal lattice. While this is valid for densely packed crystals of simple organic or inorganic molecules, certain MOF topologies do afford a reasonable degree of molecular motion, as evidenced by the large number of flexible or breathable frameworks reported to date. Additionally, the concept of ‘dynamic preformation’ - where the excited state can undergo movements not accessible to the ground state - could also help explain the observations in relation to Zn-2B.

One scenario is that a Py₂TTF ligand can rotate around its long axis in the excited state, resulting in a better alignment of the TTF ring olefins and enabling cyclisation. Alternatively, electronic excitation may induce trans/cis isomerisation in Py₂TTF, leading to the rotation occurring in two stages or even generating a singly cyclised dimer.

Investigating the structural transition from Zn-2A to Zn-2B

The TGA curves for Zn-2A and Zn-2B are very similar, with both MOFs decomposing at 303°C (Figure 33a). Their identical thermal stability suggests that the structural change induced in Zn-2B does not affect the primary decomposition mechanism.

The solid state UV-vis-NIR spectrum of Zn-2A is characterised by peaks at 19700 and 35400 cm^-1, with a shoulder at 30100 cm^-1 (Figure 33b). The bands at 19700 and 30100 cm^-1 are attributed to intramolecular excitations in the TTF core of Py₂TTF, while the band at 35400 cm^-1 arises from a combination of aromatic TT-TT* transitions in TDC and Py₂TTF pyridyl rings. In the spectrum of Zn-2B, the bands at 19700 and 30100 cm ¹ are considerably diminished (Figure 33b), suggesting that the TTF cores are involved in the structural transformation. Flowever, in contrast to Zn-1 B, the shoulder at ca. 25000 cm^-1 extends further into the visible region, possibly signalling that a small amount of Py₂TTF has remained unreacted.

Despite containing redox-active TTF cores, solid state cyclic voltammetry of Zn-2A only revealed one quasi-reversible oxidative process, with E1/2 = +0.07 V vs Fc/Fc+ (Figure 34a). This process was poorly defined in squarewave voltammetry, suggesting a small current flow (Figure 35a). Due to the smaller pore size of Zn-2A compared to other TTF- containing MOFs such as Compound 1 and Zn-1A, a possible explanation is that the pores can only accommodate one PF6^' counterion. By inhibiting the diffusion of a second PF6^' anion, oxidation to the TTF²⁺ dication is prevented. Attempts to detect the second expected oxidation process using a more concentrated electrolyte (0.25 M), or

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RO/AU an electrolyte with a smaller counteranion (tetrabutylammonium bromide), were unsuccessful. In contrast, the solid state cyclic voltammogram of Zn-2B appears devoid of redox activity (Figure 34b and Figure 35b). This implies that the TTF cores are modified in response to light irradiation. Moreover, this behaviour is similar to that exhibited by Compound 1 and Zn-1 B.

Comparing the Raman spectra of Zn-2A and Zn-2B, it is apparent that peaks at 841 , 943, and 1523 cm^-1 in the spectrum of Zn-2A are greatly diminished in the spectrum of Zn-2B (Figure 36). The assignment of these peaks to Py₂TTF vibrational modes is the same as for Zn-1 A. Moreover, peaks appear at 502, 541 , 731 , 1074 and 1559 cm^-1 in the spectrum of Zn-2B, consistent with the established assignment of vibrational modes in Py₄Ci2S8Fl4 described above. These observations are in excellent agreement with both the Compound 1 /Compound 3 and Zn-1 A/Zn-1 B systems, suggesting that a double [2+2] photocycloaddition has occurred in Zn-2B. Flowever, the presence of peaks at 451 , 989 and 1325 cm ¹ could not be assigned to either Py₂TTF, Py₄C₁₂S₈H₄, or TDC. Therefore, cyclisation may not be the exclusive process taking place.

Transition from Zn-2B to Zn-2A

When Zn-2B was heated at 150 °C in DMF for 1 h, its colour changed from orange to red (the colour of Zn-2A). Flowever, despite these observations, the PXRD of the material remained essentially unchanged. Flowever, the inventors expect that Zn-2A will re-form from Zn-2B upon heating at lower temperatures such as 80 °C.

References

1. Sheldrick, G. Acta Crystallographica Section A 2015, 71 (1), 3-8 DOI: doi:10.1107/S2053273314026370.

2. Sheldrick, G. M. Acta Crystallographica Section C 2015, 71 (1), 3-8 DOI: 10.1107/S2053229614024218.

3. Farrugia, L. Journal of Applied Crystallography 2012, 45 (4), 849-854 DOI: doi:10.1107/S0021889812029111.

4. Usov, P. M.; Fabian, C.; D'Alessandro, D. M. Chemical Communications 2012, 48 (33), 3945-3947 DOI: 10.1039/c2cc30568b.

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RO/AU 5. Spek, A. L. Acta Crystallogr C Struct Chem 2015, 71 (Pt 1), 9-18 DOI: 10.1107/S2053229614024929.

6. Spek, A. L. Acta Crystallogr D Biol Crystallogr 2009, 65 (Pt 2), 148-55 DOI: 10.1107/S090744490804362X. 7. Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87 (2), 433-481.

Although the invention has been described with reference to specific embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Itemised list of embodiments

1. A metal-organic framework comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion;

L1 is a ligand that is

wherein Y is selected from the group consisting of a bond,

and combinations thereof;

L2 is a ligand that is

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wherein X is selected from the group consisting of a bond,

combinations thereof; and each M is coordinated to 2 L2 ligands, and wherein pairs of L2 ligands are co- facial to one another.

2. The metal-organic framework of item 1 , wherein the pairs of L2 ligands co-facial to one another are coordinated to separate metal ions.

3. The metal-organic framework of item 1 or item 2, wherein in the pairs of L2 ligands co-facial to one another respective 2,3 and 6,7 double bonds of each tetrathiafulvalene moiety are separated from one another by a distance of about 3.5 to 4.2 A.

4. A metal-organic framework comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion; L1 is a ligand that is

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Substitue Sheets (Rule 26) RO/AU

wherein Y is selected from the group consisting of a bond,

and combinations thereof;

L2 is a ligand that is

wherein X is selected from the group consisting of a bond,

other as follows:

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5. The metal-organic framework of item 4, wherein the pairs of L2 ligands are coordinated to separate metal ions.

6. The metal-organic framework of any one of items 1 to 5, wherein X is a bond. 7. The metal-organic framework of any one of items 1 to 6, wherein 2 L1 ligands are bridging ligands such that each M is coordinated to 3 L1 ligands.

8. The metal-organic framework of any one of items 1 to 7, wherein the repeat unit comprises a cyclic moiety of the formula

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Substitue Sheets (Rule 26) RO/AU 9. The metal-organic framework of any one of items 1 to 8, wherein the metal ions possess octahedral geometry.

10. The metal-organic framework of item 9, wherein apical positions of the metal ions are occupied by the L2 ligands. 11. The metal-organic framework of item 9 or item 10, wherein equatorial positions of the metal ions are occupied by the L1 ligands.

12. The metal-organic framework of any one of items 1 to 11 , wherein repetition of the M2(L1)₂ moiety creates undulating sheets in the metal-organic framework.

13. The metal-organic framework of item 12, wherein the pairs of L2 ligands link the sheets so as to form a structure that repeats in 3 dimensions.

14. The metal-organic framework of any one of items 1 to 13, further comprising one or more guest molecules.

15. The metal-organic framework of any one of items 1 to 14, wherein the framework occupies approximately one third of a unit cell volume. 16. The metal-organic framework of any one of items 1 to 15 which possesses the monoclinic space group P2₁/n.

17. The metal-organic framework of any one of items 1 to 16, wherein M is selected from the group consisting of: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc,

Ru, Rh, Pd, Ag and Cd. 18. The metal-organic framework of item 17, wherein M is Cd, Zn or Co.

19. The metal-organic framework of item 18, wherein M is Cd.

20. The metal-organic framework of any one of items 1 to 19, wherein the metal- organic framework is activated.

21. A method for preparing a metal-organic framework as defined in item 1 , the method comprising heating a mixture comprising: a metal salt;

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Substitue Sheets (Rule 26) RO/AU a compound of the following formula (III):

acid thereof; and a compound of the following formula (II):

wherein Y is selected from the group consisting of a bond,

X is selected from the group consisting of a bond,

and

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Substitue Sheets (Rule 26) RO/AU 22. The method of item 21 , wherein the metal salt is a nitrate salt.

23. The method of item 21 or item 22, wherein the mixture further comprises DMF and ethanol.

24. The method of any one of items 21 to 23, wherein the metal salt, compound of formula (III) and compound of formula (II) are present in a molar ratio of about 1 :1 :1.

25. The method of item 23 or item 24, wherein the mixture comprises a ratio of DMF to ethanol that is between about 10:1 and about 2:1 , or between about 8:1 and about 3:1 , or between about 6:1 and about 5:1.

26. The method of any one of items 21 to 25, wherein heating is performed in the absence of light.

27. The method of any one of items 21 to 26, wherein heating is performed in a solvothermal reactor, followed by a convection oven.

28. The method of item 27, wherein heating in the solvothermal reactor is performed at a temperature between about 100 °C and about 200 °C, or at a temperature between about 120 °C and about 140 °C, or at about 130 °C.

29. The method of item 27 or item 28, wherein heating in the convection oven is performed at a temperature between about 50 °C and about 150 °C, or at a temperature between about 70 °C and about 100 °C, or at about 80 °C.

30. The method of any one of items 27 to 29, wherein heating in the solvothermal reactor is performed for a period of time between about 1 minute and about 1 hour, or between about 2 minutes and about 30 minutes, or about 10 minutes.

31. The method of any one of items 27 to 30, wherein heating in the convection oven is performed for a period of time between about 1 day and about 7 days, or between about 2 days and about 6 days, or about 4 days. 32. The method of any one of items 21 to 26, wherein heating is performed for between about 3 days and about 5 days at a temperature between about 60 °C and about 100 °C.

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Substitue Sheets (Rule 26) RO/AU 33. A method for preparing a metal-organic framework as defined in item 4, the method comprising irradiation of a metal-organic framework as defined in item 1 with light.

34. The method of item 33, wherein conversion from the metal-organic framework of item 1 to the metal-organic framework of item 4 is via SC-SC transformation.

35. The method of item 33 or item 34, wherein the light is UV light.

36. The method of any one of items 33 to 35, wherein irradiation is performed for at least about 15 minutes.

37. The method of any one of items 33 to 36, wherein irradiation is performed for between about 15 minutes and about 2 hours, or for about 1 hour.

38. A method for preparing a metal-organic framework as defined in item 1 , the method comprising heating a metal-organic framework as defined in item 4.

39. The method of item 38, wherein heating is carried out at a temperature between about 100 °C and about 200 °C, or at about 150 °C. 40. The method of item 38 or item 39, wherein heating is carried out for a period of time between about 5 minutes and about 72 hours, or between about 10 minutes and about 48 hours, or about 48 hours.

41. The method of any one of items 38 to 40, wherein the metal-organic framework is heated as a slurry comprising DMF. 42. The method of any one of items 38 to 41 , wherein heating is performed in the absence of light.

43. A method for reversibly switching between a metal-organic framework of item 1 and a metal-organic framework of item 4, the method comprising:

(i) irradiation of the metal-organic framework of item 1 with light to provide the metal-organic framework of item 4; and,

(ii) heating the metal-organic framework of item 4 to provide the metal-organic framework of item 1.

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Substitue Sheets (Rule 26) RO/AU 44. The method of item 43, wherein conversion from the metal-organic framework of item 1 to the metal-organic framework of item 4 is via SC-SC transformation.

45. The method of item 43 or item 44, wherein the light is UV light.

46. The method of any one of items 43 to 45, wherein irradiation is performed for at least about 15 minutes.

47. The method of any one of items 43 to 46, wherein irradiation is performed for between about 15 minutes and about 2 hours, or for about 1 hour.

48. The method of any one of items 43 to 47, wherein heating is carried out at a temperature between about 100 °C and about 200 °C, or at about 150 °C. 49. The method of any one of items 43 to 48, wherein heating is carried out for a period of time between about 5 minutes and about 72 hours, or between about 10 minutes and about 48 hours, or about 48 hours.

50. The method of any one of items 43 to 49, wherein the metal-organic framework is heated as a slurry comprising DMF. 51. The method of any one of items 43 to 50, wherein heating is performed in the absence of light.

52. Use of a metal-organic framework defined in item 4 for sorption or separation of gas molecules.

53. The use of item 52, wherein the gas molecules are one or more of: carbon dioxide, methane, nitrogen, oxygen and hydrogen.

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Claims

1. A metal-organic framework (MOF) comprising a repeat unit of the formula: M₂(L1)₂(L2)₂, wherein:

M is a metal ion; L1 is a ligand comprising at least two carboxylates, wherein the at least two carboxylates are selected from the group consisting of aryl carboxylate(s), heteroaryl carboxylate(s), and combinations thereof; wherein two of the at least two carboxylates are linked via a conjugated system that is substantially linear or linear; and wherein oxygen atoms of two of the at least two carboxylates are coordinated to one or both of M;

L2 is a ligand that i

wherein pyr is an optionally substituted pyridyl;

Z is selected from the group consisting of S, Se and Te;

X is optional and when present is a linker such that pyr is conjugated with the

group of L2;

is a bond;

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Substitue Sheets (Rule 26) RO/AU such that L2 is

wherein pairs of L2 ligands are at least substantially co-facial to one another;

pairs of L2 ligands are bonded to each other as follows:

2. The MOF of claim 1 , wherein Z is S. 4. The MOF of claim 1 or 2, wherein pyr is an optionally substituted 4-pyridyl.

5. The MOF of any one of the preceding claims, wherein X is optional and when present comprises an alkylene group, an alkynylene group, a phenylene group, a naphthylene group, or combinations thereof.

6. The MOF of any one of the preceding claims, wherein each member of a pair of L2 ligands at least substantially co-facial to one another is coordinated to separate metal ions; or

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Substitue Sheets (Rule 26) RO/AU wherein each member of the pair of L2 ligands bonded to each other as follows:

is coordinated to separate metal ions.

7. The MOF of any one of the preceding claims, wherein the substantially linear or linear conjugated system of L1 is a bond or comprises an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof.

8. The MOF of any one of the preceding claims, wherein the at least two carboxylates of L1 are aryl carboxylates. 9. The MOF of any one of the preceding claims, wherein L1 is selected from the group consisting of:

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(Rule 26) RO/AU wherein, Y is optional and when present is a substantially linear or linear linker such that the two carboxylates of L1 are conjugated with one another; and the aromatic rings may be optionally substituted.

10. The MOF of any one of the preceding claims, wherein L1 is:

11. The MOF of claim 10, wherein Y is optional or comprises an alkylene group, an alkynylene group, a phenylene group, a napthylene group, an ether group, an amine group, an imine group, an azo group, or combinations thereof.

12. The MOF of any one of claims 1-7, wherein the at least two carboxylates of L1 are heteroaryl carboxylates.

13. The MOF of any one of claims 1 -7 and 12, wherein L1 is

wherein the heteroaromatic ring may be optionally substituted.

14. The MOF of any one of the preceding claims, wherein L1 ligands are bridging ligands such that each M is coordinated to three L1 ligands.

15. The MOF of any one of preceding claims, wherein L2 is:

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(Rule 26) RO/AU

16. The MOF of any one of claims 1-14, wherein pairs of L2 ligands are bonded to each other as follows:

17. A method for preparing a MOF of any one of claims 1-14 and 16, wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

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Substitue Sheets (Rule 26) RO/AU the method comprising irradiation with light of a MOF of any one of claims

1-15, wherein

18. A method for preparing a MOF of any one of claims 1-15, wherein L2 is

the method comprising heating a MOF of any one of claims 1-14 and 16 wherein L2 is such that pairs of L2 ligands are bonded to each other as follows:

19. A method for preparing a MOF of any one of claims 1 -15, wherein

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(Rule 26) RO/AU wherein pairs of L2 ligands are at least substantially co-facial to one another, the method comprising heating a mixture comprising: a metal salt; a compound of formula (III), or an acid thereof; a compound of formula (II); wherein formula (III) is a compound comprising at least two carboxylates, wherein the at least two carboxylates are selected from the group consisting of aryl carboxylate(s), heteroaryl carboxylate(s), and combinations thereof; and wherein two of the at least two carboxylates are linked via a conjugated system that is substantially linear or linear; wherein formula (II) is a compound that is

wherein pyr is an optionally substituted pyridyl;

Z is selected from the group consisting of S, Se and Te;

X is optional and when present is a linker such that pyr is conjugated with the

group of L2.

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20. Use of a MOF of any one of claims 1 -16 for nanocarriers containing photo-valves for remote control drug delivery; ion channels for separation of alkali metal ions relevant to battery technologies and/or for desalination; ion channels for separation of lanthanoid ions, low-energy light and electrically-driven gas separation; sunlight driven actuators; or sorption or separation of gas molecules.

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