WO2010006698A1

WO2010006698A1 - Conjugated low band-gap copolymers and relative preparation process

Info

Publication number: WO2010006698A1
Application number: PCT/EP2009/004710
Authority: WO
Inventors: Riccardo Po'; Giuliana Schimperna; Maria Caldararo; Maria. Anna Cardaci
Original assignee: Eni S.P.A.
Priority date: 2008-06-30
Filing date: 2009-06-29
Publication date: 2010-01-21
Also published as: ITMI20081211A1

Abstract

Alternating conjugated copolymers are described, comprising: naphthalene diimide units A having general formula (I) wherein R and R', the same or different, are alkyl groups, preferably branched, containing from 1 to 36 carbon atoms, preferably from 4 to 24, more preferably from 6 to 18 carbon atoms, or aryl groups, preferably phenyls, substituted with alkyl radicals having from 1 to 24 carbon atoms; at least one conjugated electron-donor structural unit B, wherein unit A is connected to unit B, in the alternating copolymer, in any of the positions 2, 3, 6 or 7.

Description

CONJUGATED LOW BAND-GAP COPOLYMERS AND RELATIVE PREPARATION PROCESS

The present invention relates to conjugated low- band-gap copolymers and the relative preparation process. The present invention falls within the field of photo-active materials which can be used in the construction of photovoltaic devices.

Photovoltaic devices are devices capable of converting the energy of a light radiation into electric energy. At present, most photovoltaic devices which can be used for practical applications exploit the physico- chemical properties of photo-active materials of the inorganic type, in particular high-purity crystalline silicon. As a result of the high production costs of silicon, scientific research has been orienting its efforts towards the development of alternative organic materials having a polymeric structure (so-called "polymer photovoltaic cells") . Unlike high-purity crystalline silicon, in fact, organic polymers are characterized by a relative synthesis facility, a low production cost, a reduced weight of the relative photovoltaic device, in addition to allowing the recycling of said polymer at the end of the life-cycle of the device in which it is used. The functioning of polymer photovoltaic cells is based on the combined use of an electron acceptor compound and an electron donor compound. In the state of the art, widely-used electron donor and acceptor compounds in photovoltaic devices are π-conjugated polymers belonging to the groups of poly (para-phenylene vinylene)s and polythiophenes . The former can be used as either acceptor or donor compounds, on the basis of the electronic properties determined by the substituent groups of the polymer chain. The latter are normally used as donor compounds. Derivatives of fullerene are the most widely-used acceptor compounds .

The basic conversion process of light into electric current in a polymer photovoltaic cell takes place through the following steps: 1. absorption of a photon on the part of the donor compound with the formation of an exciton, i.e., an "electron-hole" pair;

2. diffusion of the exciton in a region of the donor compound in which its dissociation can take place; 3. dissociation of the exciton in the two separated charge carriers (electron (-) and hole ( +)) ; 4. transport of the charges thus formed to the cathode (electron, through the acceptor compound) and anode (hole, through the donor compound) , with the generation of an electric current in the circuit of the device. The photo-absorption process with the formation of the exciton and subsequent yielding of the electron to the acceptor compound leads to the excitation of an electron from the HOMO (Highest Occupied Molecular Orbital) to the LUMO (Lowest Unoccupied Molecular Orbital) of the donor and subsequently the transfer from this to the LUMO of the acceptor.

As the efficiency of a polymer photovoltaic cell depends on the number of free electrons which are generated by dissociation of the excitons, one of the structural characteristics of the donor compounds which mostly influences said efficiency is the difference in energy existing between the HOMO and LUMO orbitals of the donor (so-called band-gap) . The wave-length of the photons which the donor compound is capable of absorbing and effectively converting into electric energy (so- called "photon harvesting" or "light-harvesting" process) depends on this difference. In order to obtain acceptable electric currents, the band-gap between HOMO and LUMO must not be too high, but at the same time, it must not be too low, as an excessively low gap would jeopardize the voltage obtained at the electrodes of the device.

The flow of photons of solar radiation which reaches the surface of the Earth is maximum for energy values of around 1.8 eV (corresponding to radiations having a wave- length of about 700 nm) . Due to the high band-gap values, however, (generally higher than 2 eV) which characterize polymeric materials currently known and used as donor compounds in photovoltaic devices, the light harvesting process of this spectral field is not very efficient and only a fraction of the overall solar energy (generally that of 350 to 650 nm) is converted into electric energy. Among the polymers most widely-used as donor compounds, for example, the polymer MDMO-PPV (poly [2-methoxy-5- (3 , 7- dimethyloctyloxy) -1 , 4-phenylene] -alt- (vinylene) ) has a band-gap equal to 2.2 eV, whereas the polymer P3HT (poly (3-hexylthiophene) has a band-gap of 1.9 eV. These compounds, used in combination with acceptor compounds based on fullerenes, are capable of obtaining maximum conversion efficiencies of solar radiation equal to about 3.5%.

In order to improve the yield of the light harvesting process and consequently the efficiency of photovoltaic devices, it is consequently fundamental to find new donor compounds capable of capturing and converting solar radiations having a lower energy, i.e. donor compounds characterized by lower band-gap values than those of organic polymers typically used as donors.

Patent application US 2008/0021220 Al describes naphthalene compounds of the mono- or di-imide type which can be used as semiconductor materials for the production of LEDs, transistors and photovoltaic devices. The compounds described can be optionally substituted on the naphthalene rings with functional groups and/or electron- acceptor species. Optionally, these compounds can also be N- substituted. The compounds of said patent, however, have band-gap values of about 3 eV, consequently their efficiency in photovoltaic devices is relatively low.

An objective of the present invention is to overcome the drawbacks specified by the state of the art.

An object of the present invention therefore relates to alternating conjugated copolymer comprising: - naphthalene diimide units A having general formula (I)

wherein R and R' , the same or different, are alkyl groups, preferably branched, containing from 1 to 36 carbon atoms, preferably from 4 to 24, more preferably from 6 to 18 carbon atoms, or aryl groups, preferably phenyls, substituted with alkyl radicals having from 1 to 24, preferably from 4 to 18 carbon atoms; at least one conjugated electron-donor structural unit B, wherein unit A is connected to unit B, in the alternating copolymer, in any of the positions 2, 3, 6 or 7.

The average number of A units in the copolymer of the present invention preferably ranges from 2 to 1,000, more preferably from 5 to 1,000.

The electron-donor structural units B are preferably of the thiophene, fluorene, phenothiazine, pyrrole, carbazole type, possibly also condensed and optionally substituted with alkyl groups.

The electron-donor structural units B can be selected, for example, from those of the following list:

wherein Y is one or more groups selected from: alkyl, alkoxy, alkylamino, alkylthio having from 1 to 20 carbon atoms;

wherein X is an O or S atom;

or a 1,2-vinylene group; wherein the substituent indicated with R is selected from alkyl groups, preferably branched, containing from 1 to 36 carbon atoms, preferably from 4 to 24, more preferably from 4 to 16, carbon atoms, or aryl groups, preferably phenyls, substituted with alkyl radicals having from 1 to 24 carbon atoms, preferably from 4 to 18 carbon atoms.

Examples of alkyl groups are: hexyl , heptyl, octyl, dodecyl, hexadecyl, 2-ethylhexyl, 2-ethyloctyl, 2 -ethyl - dodecyl, 2-ethylhexadecyl, 2-butylhexyl, 2-butyldodecyl, 1-hexylhexyl, 1-octyloctyl, 1-dodecyldodecyl, 1-hexa- decylhexadecyl , l-octadecyloctadecyl, 1, 1, 3 , 3-tetra- methylbutyl .

Aryl groups can be, for example, 2,2-diiso- propylphenyl . In order to guarantee the solubility of the copolymers according to the present invention, the ratio between the sum of all the carbon atoms of the alkyl chains variably present in the repetitive base units of the copolymer and the number of aromatic rings present in the same unit ranges preferably from 3.5 to 12. The alternating conjugated copolymers of the present invention are preferably of the linear type, in which the divalent units A and B are linearly alternating along the polymeric chain. Each A unit is bound to two B units and vice versa, except when the A unit or B unit form terminal units of the polymeric chain. In this latter case, the terminal A unit or B unit are bound to a single B or A unit respectively, and the remaining valence is saturated by a terminal substituent whose structure depends on the preparation method of the polymer and can be easily identified by an expert in the field. In most cases, said substituent is H or Br.

When an A unit is not a terminal unit of the polymer chain, it is bound to two B units, the same or different, in any two of the positions 2, 3, 6 and 7 of formula I, preferably in positions 2,6 or 2,7, the other two positions normally, but not necessarily, being saturated by an H atom, or a hydrocarbon group having from 1 to 10 carbon atoms . The alternating conjugated copolymers of the present invention preferably have a structure of the repetitive base unit of the (A-B)_n, (B'-A-B')n or (B' -B" -A-B" ) _n type, wherein A and B have the meaning previously illustrated, B' and B" are B-type conjugated subunits, combined with each other to form more complex B units, and n is an integer varying from 2 to 1,000, preferably from 5 to 1,000. B" is preferably a 1,2-vinylene or 2 , 5- thienylene unit.

The copolymers poly [2, 7- (9, 9-bis- (2-ethylhexyl) -9H- fluorene) -2' , 5' -thienylene-3" , 7"- (N, N' -heptyl-1" , 2" , 5" , - 6" -tetracarboxydiimmide)naphthylene-2' ' ' ,5' ' ' -thienylene] (copolymer 1), and poly (3 , 7-dithienyl- (N, N' -heptyl- 1 , 2 , 5, 6-tetracarboxydiimide) naphthylene) (copolymer 2) are particularly preferred for the light harvesting properties shown. These copolymers are characterized by band-gap values lower than 2.0 eV and can therefore be advantageously used as electron-donor compounds in photovoltaic devices, in particular to exploit solar radiation with a higher wave-length. When the copolymer according to the present invention is the following:

a possible method for obtaining it is that corresponding to the process indicated in the following scheme A:

Scheme A wherein Pd TTP is Pd (0) tetrakis (triphenylphosphine) and NBS is dibromo isocyanuric acid.

This reaction scheme is also used for substituents on the nitrogen which are different from C₇Hi₅ starting from the appropriate precursor.

When the copolymer according to the present invention is the following (structure of the type (B'-A-B')π) :

A possible method for obtaining it is that corresponding to the process indicated in the following scheme B:

Scheme B wherein R' is an alkyl group having from 1 to 6 carbon atoms, normally methyl or butyl, whose tin derivatives are commercial products.

When the copolymer according to the present invention is the following (structure of the type (B' -

B " Α _ Ϊ3 " ^ \ •

it can be obtained by means of the process indicated in the following scheme C:

fl

Scheme C wherein "Pd TTP" is Pd (0) tetrakis (triphenylphosphine) and NBS is dibromo isocyanuric acid. This reaction scheme is also used for substituents on the nitrogen which are different from C₇Hi₅ starting from the appropriate precursor.

When the copolymer according to the present invention is the following,

it can be obtained by means of the process indicated in the following scheme D:

Scheme D wherein Pd(OAc)₂ is palladium acetate, TOP is tri-o- tolyl-phosphine, TBA is tributylamine, DMF is dimethylformamide and NMP is N-methylpyrrolidone .

Alternating copolymers of the present invention having structures different from those previously indicated can be obtained by means of processes of the radical or redox type corresponding to those described or in any case which can be easily deduced by experts in the field on the basis of the known methods of organic chemistry.

A second object of the present invention relates to a compound having general formula (II)

R1

wherein:

Ri and R₂, the same or different, are linear or branched alkyl groups containing from 1 to 36 carbon atoms, optionally containing heteroatoms of O , S , N ;

R₃ and R₄, the same or different, are hydrogen, halogen or linear or branched alkyl and/or alkoxy groups containing from 1 to 20 carbon atoms; R₅ and R₆, the same or different, are hydrogen or linear or branched alkyl groups containing from 1 to 10 carbon atoms.

The compound having general formula (II) is in fact the reaction intermediate according to the schemes B and C indicated above which allows the preparation, for example, of copolymer 1 and copolymer 2 according to the present invention.

The compound having general formula (II) can be substituted in both position N (substituents R₁ e R₂) and in one or more of the positions available on the naphthalene aromatic rings (R₅ e R₆) . These substituents are selected in relation to the structure to be obtained in the final copolymer.

In a preferred embodiment, Ri e R₂ are a linear or branched alkyl group having from 4 to 36 carbon atoms, for example, 2-ethylhexyl, 2-ethyloctyl, 2-ethyldodecyl,

2-ethylhexadecyl, 2-butylhexyl, 2-butyldodecyl, 1- hexylhexyl, 1-octyloctyl , 1-dodecyldodecyl, 1- hexadecylhexadecyl , 1-octadecyloctadecyl due to the greater solubility they confer to the polymer. In a preferred embodiment, in the compound having general formula (II) Ri e R₂, the same or different, are a C₇Hi₅ o C₈Hi₇ alkyl group, whereas R₃, R₄, R₅ and R₆ represent a hydrogen atom.

In a further preferred embodiment, in the compound having general formula (II), Ri and R₂, the same or different, are a C₇Hi₅ or C₈Hi₇ alkyl group, R₃, R₅ and R₆ are an H atom and R₄ is a Br atom.

Said compound having general formula (II) is obtained by means of the following process:

wherein:

Ri_/ R_2/ R_3/ R_4/ ^R ₅ ^and R₆, not shown for greater simplicity in the corresponding positions of the formulae of the reagents, have the meanings previously indicated, suitably selected in relation to the product to be synthesized.

In the SnR' ₃ substituent group, the radical R' is an alkyl group having from 1 to 6 carbon atoms, normally methyl or butyl, whose tin derivatives are commercial products .

The SnR' ₃ group is preferably a tri-butylstannyl radical .

The copolymers according to the present invention have favourable physico-chemical properties which enable its use as photo-active materials, in particular as electron-donor compounds within photovoltaic devices.

Thanks to the low band-gap value that characterizes them, these copolymers are also capable of effectively collecting and converting solar radiation with a higher wave-length into electric energy, unlike the known donor compounds used in the state of the art .

A last object of the present invention therefore relates to a photovoltaic device comprising any of the copolymers of the present invention.

As will be better illustrated in the following examples, the copolymers can be easily synthesized starting from compounds having general formula (II) according to the process schemes illustrated above. The compound having general formula (II) can also be synthesized starting from the corresponding anhydride of naphthalene carboxy-diimide .

The following embodiment examples are provided for purely illustrative purposes of the present invention and should not be considered as limiting the protection scope . Method for determining the HOMO-LUMO band-gap

The copolymers according to the present invention were characterized by means of UV-Vis-NIR spectroscopy to determine the energy value of the HOMO-LUMO band-gap according to the following procedure.

The polymer is dissolved in toluene at a concentration of about 10^"4 M, transferred to a Suprasil 1.0 cm quartz cuvette and analyzed in transmission by means of a Perkin Elmer λ 19 double-beam UV-Vis-NIR spectrophotometer and double monochromator , within the range of 190-900 nm with a pass -through band of 2.0 nm, scanning rate of 120 nm/min and step of 1 nm, using an identical cuvette filled with pure solvent, as reference. The band-gap is estimated from the diffuse reflectance spectra by measuring the absorption edge corresponding to the transition of the valence band (VB) and the conduction band (CB) . For determining the edge, resort was made to the intersection with the axis of the abscissa of the tangent line at the absorption band in the flexpoint.

The flexpoint (λ_F, y_F) is determined on the basis of the coordinates of the minimum of the spectrum in first derivative, indicated with λ'_min ed y'_min. The equation of the tangent line at the UV-Vis spectrum in the flexpoint (λ_F, y_F) is:

y = y ' min λ + y_F - y ' _min λ ' _min

Finally, from the intersection condition with the axis of the abscissa y = 0, the following is obtained:

λ_EDGE = (y'_min λ'_mxn - y_F)/y'mxn

Therefore, by measuring the coordinates of the minimum of the spectrum in first derivative and the corresponding absorbance value y_F from the UV-Vis spectrum, λ_EDG_E is obtained directly by substitution. The corresponding energy is:

EβDGE = hVEDGE ⁼ h C / AEDGE wherein h = 6.626 10^~34 J s c = 2.998 10⁸ m s^"1 i.e. E_EDGE = 1-988 10^"16 j/λ_EDGE(nm) . Finally, remembering that 1 J = 6.24 10¹⁸ eV, the following is obtained:

EED_GE = 1240 eV / λ_EDGE(nm) EXAMPLE 1

Alternating copolymer bis (2-ethylhexyl) fluorene - N, N- diheptyl naphthylimide . The following products were charged, in order and under an inert atmosphere, into a three-necked 250 ml flask equipped with a magnetic stirrer and reflux condenser: - 384 mg (0.80 mraoles) of 9, 9-bis (2 ' -ethyhexyl) -2 , 7-di- boronic acid;

629 mg (0.80 mmoles) of 2 , 6-dibromo-N, N' -diheptyl- naphthalene-1, 4,5, 8-tetracarboxydiimmide;

- 187 mg (4.4 mmoles) of aliquat 336 dissolved in 50 ml of toluene;

- 9 ml of potassium carbonate 1 M (the solution was prepared using de-aerated water) ;

- 49 mg (0.041 mmoles) of palladium (0) tetrakis (tri- phenylphosphine) . The reaction mixture is heated to reflux temperature and under intense stirring for 24 hours.

At the end of the period indicated, the mixture was concentrated to about half of its volume and poured into

300 ml of methanol. The solid obtained was filtered and dried in an oven under vacuum at 55⁰C for 16 hours. 71 mg of dark magenta-coloured solid were obtained.

The band-gap measured for the copolymer thus obtained is 2.3 eV.

Schematically, the reaction which took place is the following: acid

EXAMPLE 2

Alternating copolymer bis (2-ethylhexyl) fluorene - thiophene - N, N-diheptyl naphthylimide - thiophene

(copolymer 1) .

The following products were charged, in order and under an inert atmosphere, into a two-necked 100 ml flask equipped with a magnetic stirrer and reflux condenser: - 96 mg (0.200 mmoles) of 9, 9-bis (2 ' -ethylhexyl) -2 , 7-di- boronic acid;

- 157 mg (0.200 mmoles) of 2 , 6-bis (5" -bromo-2" -thienyl) - N, N' -diheptylnaphthalene-1, 4,5, 8- tetracarboxy diimide both dissolved in 25 ml of toluene and 1 ml of ethanol . The mixture was heated for 15 minutes to 90⁰C. After cooling for a couple of minutes in a nitrogen flow, the following products were added:

- 0.5 ml of K₂CO₃ 2 M (the solution was prepared using de-aerated water) ; - 24 mg of palladium tetrakis (triphenylphosphine) (0.02 mmoles) dissolved in about 1 ml of de-aerated toluene.

The mixture was brought to a temperature of 100⁰C and the reaction continued for about 24 hours. At the end, the mixture was concentrated until dry, dissolved in 5 ml of methylene chloride and added dropwise to 90 ml of methanol. The solid precipitated was filtered, washed with warm methanol, water and finally again with methanol. The product was finally dried under vacuum at 55⁰C for 8 hours. 83 mg of deep blue solid were obtained.

The band-gap measured for the copolymer thus obtained is 1.9 eV.

Schematically the reaction which took place is the following :

EXAMPLE 3

Alternating copolymer vinylene - N-octyl-3,7- phenothiazine - vinylene - N,N-diheptyl naphthylimide .

The following products were charged into a 25 ml flask equipped with a reflux condenser:

- 81 mg (0.223 mmoles) of N-octyl-3 , 7-divinyl phenothi- azine,-

- 124 mg (0.223 mmoles) of 2, 6-dibromo-N, N' -diheptyl- naphthalene- 1, 4,5, 8-tetracarboxydiimide,-

- 20 mg (0.067 mmoles) of tri-o- tolylphosphine,-

- 2.5 mg (0.0114 mmoles) of palladium acetate;

- 1 ml of de-aerated tributylamine ;

- 4 ml of de-aerated N,N-dimethylformamide; - 2 ml of de-aerated N-methylpyrrolidone .

The mixture was degassed and heated in a nitrogen atmosphere to 100⁰C for 32 hours and to 120⁰C for 16 hours. After cooling, the reaction mixture was poured into 200 ml of methanol and centrifuged to recover the solid fraction. The solid was dissolved in 6 ml of tetrahydrofuran and re-precipitated in 80 ml of methanol. The mixture was centrifuged and the solid was washed with methanol and dried in an oven under vacuum at 5O⁰C for 16 hours. 84 mg of dark red-coloured product were obtained. The band-gap measured for the copolymer thus obtained is 2.0 eV .

Schematically the reaction which took place is the following :

EXAMPLE 4

Alternating copolymer thiophene -N,N-diheptyl naphthylimide - thiophene (copolymer 2) .

210 mg (1.29 mmoles ) of FeCl₃ and 20 ml of methylene chloride were charged, in an inert atmosphere, into a 100 ml two-necked flask equipped with a reflux condenser and dropping funnel.

A solution of 203 mg (0.323 mmoles) of 2,6-bis(2"- thienyl) -N, N' -diheptylnaphthalene-1, 4,5, 8-tetracarboxy- diimide dissolved in 40 ml of methylene chloride was added under stirring to the mixture.

The reaction was carried out at room temperature for 24 hours. At the end of this period, the mixture was concentrated until dry and the solid was washed with methanol until the solvent recovered was colourless. The solid thus obtained was dissolved in toluene, filtered and repeatedly extracted with toluene. The organic phases coming from the different extractions were joined, concentrated to about 100 ml and subsequently extracted with water, 5% ammonia, a solution of EDTA 0.5 M and finally with water. The toluene solution was dried on calcium sulfate, concentrated to about 10 ml and added dropwise to 300 ml of methanol. The solid precipitated was filtered and dried in an oven under vacuum at 50⁰C. 12 mg of purple-coloured solid were obtained.

The band-gap measured for the copolymer thus obtained is 1.8 eV.

Schematically the reaction which took place is the following :

EXAMPLE 5 (COMPARATIVE)

The band-gap of the regioregular compound poly (3- hexylthiophene) (Aldrich Co.) was determined according to the determination method described above. The band-gap value proved to be equal to 2.3 eV . EXAMPLE 6

The compound 2, 6-bis (2' -thienyl) -N, N' -diheptyl- 1, 4 , 5 , 8-tetracarboxynaphthalenediimide (intermediate II) n-C7H 15

was synthesized according to the following reaction scheme :

wherein NBS is dibromo isocyanuric acid and -SnR' ₃ is a tributylstannyl radical.

The dibromo isocyanuric acid was synthesized according to the following reaction: OH O

N^" TST Br. Br

^*N N^'

^{H0 N 0H} O- ^{^}N- -O

I H

The following products were reacted: 6.5 g (50.4 mmoles) of cyanuric acid, 4.3 g (104.8 mmoles) of lithium hydroxide, 31 g (193.7 mmoles) of bromine and 0.5 1 of water.

Bromine was slowly added dropwise to the acqueous solution of cyanuric acid and lithium hydroxide. After 12 hours at 4⁰C, the dibromoisocyanuric acid was isolated, as a white solid, by filtration. After washing the precipitate with water and drying it, 9.0 g of dibromoisocyanuric acid were obtained (yield = 62%) .

The intermediate compound 2 , 6-dibromo-l , 4.5.8- tetracarboxynaphthalenedianhydride (intermediate 12) was obtained by reaction of 1, 4, 5, 8-tetracarboxydiimide with NBS according to the following reaction scheme and operative conditions:

The following reagents were used: 3.0 g (11.2 mmoles) of 1, 4 , 5, 8-naphthalenetetracarboxydianhydride, 6.3 g ((21.9 mmoles) of dibromoisocyanuric acid and 36 ml of concentrated sulfuric acid (97%) .

The compound 1, 4 , 5, 8-naphthalenetetracarboxydian- hydride suspended in 18 ml of concentrated sulfuric acid was slowly added dropwise to a suspension of dibromoisocyanuric acid in 18 ml of concentrated sulfuric acid. The temperature of the mixture was brought to 130⁰C. After 10 hours, the mixture was poured into ice. The precipitate was filtered and washed with water. 4.7 g of 2, 6-dibromo-l, 4,5, 8- tetracarboxynaphthalenedianhydride (intermediate 12) (quantitative yield) were obtained, after drying under vacuum.

The compound 2 , 6-dibromo-N, N' -diheptyl-1, 4 , 5 , 8- tetracarboxynaphthalenediimide (intermediate 13) was obtained starting from the intermediate 12 according to the following reaction:

The following reagents were used: 4.8 g (11.2 mmoles) of 2, 6-dibromo-l, 4 , 5, 8-tetracarboxynaphthalene- dianhydride, 3.2 g (27.8 mmoles) of heptylamine and 50 ml of glacial acetic acid.

The heptylamine was added, in an inert atmosphere, to the solution of 2, 6-dibromo-l, 4 , 5, 8-tetracarboxy- naphthelenedianhydride in acetic acid. The temperature was brought to 100⁰C. After 8 hours, the precipitate was filtered and the solid wash various times with ethyl ether and finally with toluene. After removing the toluene by means of distillation at reduced pressure, 2.0 g of 2, 6-dibromo-N,N' -dieptil-1, 4 , 5, 8-tetracarboxynaph- thalenediimide (yield = 29%) (intermediate 13) were obtained.

It is also possible to obtain intermediate 13 starting from 1, 4 , 5 , 8- tetracarboxydiimide, by first effecting the reaction with heptylamine and subsequently the reaction with dibromoisocyanuric acid.

The compound 2,6- bis (2 ' -thienyl) -N, N' -diheptyl- 1,4, 5, 8-tetracarboxynaphthalenediimide (Intermediate II) was finally obtained starting from the intermediate 13 according to the following reaction scheme and operative conditions :

The following reagents were used: 2.O g (3.2 mmoles) of 2, 6-dibromo-N,N' -diheptyl-1, 4,5, 8- tetracarboxynaph- thalenediimide, 4.51 g (12.1 mmoles) tributyl stannylthiophene, 180.0 mg (0.16 mmoles) palladium tetrakis (triphenylphosphine) and 120 ml of anhydrous toluene .

The tributyl stannylthiophene was added in an inert atmosphere, to the toluene solution containing 2,6- dibromo-N,N' -diheptyl-1, 4,5, 8-tetracarboxynaphthalene-di- imide. After removing the air present by means of vacuum/nitrogen cycles, palladium tetrakis (triphenylphosphine) was added. The temperature was then brought to 100⁰C. After 7 hours, water was added to the mixture, which was subsequently extracted with ethyl ether. After washing it to neutrality, the organic extract was dried on sodium sulfate. After purification by column chromatography ( SiO2; heptane/dichloromethane = 20/80) , 0.5 g of 2 , 6-bis (2 ' -thienyl) -N, N' -diheptyl- 1, 4 , 5, 8-tetracarboxynaphthalene-diimide (yield = 24 %) (intermediate II) and 0.5 g of 2- (2 ' -thienyl) -N, N' - diheptyl-1, 4 , 5, 8-tetracarboxynaphthal-enediimide (yield = 30%) were obtained.

The intermediate Il was characterized by means of IH-NMR (200MHz; CDCl₃), obtaining the following spectrum: 8.8 ppm (2H, s) ; 7.6 ppm (2H, d) ; 7.2 (4H,m) ; 4.1 ppm (4H,t) ; 1.7 ppm (4H, m) ; 1.3 ppm (16H, m) ; 0.9 ppm (6H, t) .

Claims

CLAIMS 1. An alternating conjugated copolymer comprising:

- naphthalene diimide units A having general formula

(I) :

(D wherein R and R' , the same or different, are alkyl groups, preferably branched, containing from 1 to 36 carbon atoms, preferably from 4 to 24, more preferably from 6 to 18 carbon atoms, or aryl groups, preferably phenyls, substituted with alkyl radicals having from 1 to 24 carbon atoms,- at least one conjugated electron-donor structural unit B, wherein unit A is connected to unit B, in the alternating copolymer, in any of the positions 2, 3, 6 or 7.

2. The copolymer according to claim 1, wherein the average number of A units in the copolymer ranges from 2 to 1,000, preferably from 5 to 1,000.

3. The copolymer according to claim 1, wherein the electron-donor structural units B are units of the thiophene, fluorine, phenothiazine, pyrrole, carbazole type, possibly also condensed and optionally substituted with alkyl groups.

4. The copolymer according to claim 1, wherein the conjugated electron-donor structural units B are selected within the following group:

wherein Y is one or more groups selected from: alkyl, alkoxy, alkylamino, alkylthio;

wherein X is an O or S atom;

or a 1,2-vinylene group; wherein the substituent indicated with R in the above structures is selected from alkyl groups, preferably branched, containing from 1 to 36 carbon atoms, preferably from 4 to 24, more preferably from 4 to 16, carbon atoms, or aryl groups, preferably phenyls, substituted with alkyl radicals having from 1 to 24 carbon atoms, preferably from 4 to 18 carbon atoms.

5. The copolymer according to claim 4, wherein R is selected from: heptyl, octyl, dodecyl, hexadecyl,

2-ethylhexyl, 2-ethyloctyl, 2-ethyldodecyl, 2 -ethyl - hexadecyl, 2-butylhexyl, 2-butyldodecyl, 1-hexylhexyl , 1-octyloctyl , 1-dodecyldodecyl, l-hexadecylhexadecyl, 1-octadecyloctadecyl, 1, 1, 3, 3-tetramethylbutyl, 2,2-di- isopropylphenyl .

6. The copolymer according to any of the previous claims, wherein each non- terminal A unit in the polymeric chain is bound to two B units, the same or different, in any two of the positions 2, 3, 6 and 7 of formula I, preferably in positions 2.6 or 2.7.

7. The copolymer according to any of the previous claims, wherein the structure of the repetitive base unit is represented by one of the formulae (A-B)_n, (B'-A-B')_n or (B' -B" -A-B" )_π, wherein A and B have the meaning previously illustrated, B' and B" are B-type conjugated subunits, and n is an integer varying from 2 to 1,000, preferably from 5 to 1,000.

8. The copolymer according to any of the previous claims, wherein the ratio between the sum of all the carbon atoms of the alkyl chains present in the repetitive base units and the number of aromatic rings present in the same unit varies from 3.5 to 12.

9. The copolymer according to claim 1 having the following structure