ITMI20111518A1 - CONJUGATED PHOTOACTIVE COMPOSITE INCLUDING INDOLICITY UNITS - Google Patents

Description

PHOTOACTIVE CONJUGATED COMPOUND INCLUDING INDOLIC UNITS

DESCRIPTION

The present invention relates to a photo-active conjugate compound. More particularly, the present invention relates to a photoactive conjugate compound comprising indole units.

The present invention also relates to the use of said photoactive conjugate compound comprising indole units in the construction of photovoltaic devices such as, for example, photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on rigid support and on flexible support.

Furthermore, said photoactive conjugate compound comprising indole units can also be used in the construction of photovoltaic cells known as Gràtzel cells or DSSC ("Dye-Sensitized Solar Cells").

Photovoltaic devices are devices capable of converting the energy of a light radiation into electrical energy. Currently, most photovoltaic devices that can be used for practical applications exploit the chemical-physical properties of photoactive inorganic materials, in particular high purity crystalline silicon. Due to the high production costs of silicon, however, scientific research has for some time been orienting its efforts towards the development of alternative organic materials having a conjugate, oligomeric or polymeric structure, in order to obtain organic photovoltaic devices such as, for example , organic photovoltaic cells. In fact, unlike high purity crystalline silicon, said organic materials are characterized by a relative ease of synthesis, a low production cost, a reduced weight of the related organic photovoltaic devices, as well as allowing the recycling of said materials of the type organic at the end of the life cycle of the organic photovoltaic device in which they are used.

The above advantages make the use of said organic materials energetically and economically attractive despite any lower efficiencies (η) of the organic photovoltaic devices thus obtained compared to inorganic photovoltaic devices.

The operation of organic photovoltaic devices such as, for example, organic photovoltaic cells, is based on the combined use of an electron acceptor compound and an electron donor compound. In the state of the art, the electron acceptor compounds most used in organic photovoltaic devices are fullerene derivatives, in particular PC61BM (6.6 phenyl-C61-methyl butyric ester) or PC71BM (6.6 phenyl-C71-methyl ester butyric), which led to the greatest efficiencies when mixed with electron donor compounds selected from π-conjugated polymers such as, for example, polythiophenes (η> 5%), polycarbazoles (η> 6%), poly (thienothiophene) derivatives benzodithiophene (PTB) (η> 8%).

The elementary process of converting light into electric current in an organic photovoltaic cell takes place through the following stages:

1. absorption of a photon by the electron donor compound with the formation of an exciton, ie a pair of “electron-electron hole (or hole)” charge carriers;

2. diffusion of the exciton in a region of the electron donor compound up to the interface with the electron acceptor compound;

3. dissociation of the exciton in the two charge transporters: electron (-) in the accepting phase (i.e. in the electron acceptor compound) and electronic hole (or hole) (+)] in the donor phase (i.e. in the electron donor compound);

4. transport of the charges thus formed to the cathode (electron through the electron acceptor compound) and to the anode [electronic hole (or hole) through the electron donor compound], with the generation of an electric current in the organic photovoltaic cell circuit.

The photoabsorption process with the formation of the exciton and subsequent transfer of the electron to the electron acceptor compound involves the excitation of an electron from the HOMO ("Highest Occupied Molecular Orbitai") to the LUMO ("Lowest Unoccupied Molecular Orbitai") of the compound electron donor and, subsequently, the transition from this to the LUMO of the electron acceptor compound.

Since Γ efficiency of an organic photovoltaic cell depends on the number of free electrons that are generated by dissociation of the excitons, which in turn are directly correlated to the number of absorbed photons, one of the structural characteristics of electron donor compounds that most affects this efficiency is the difference of energy existing between the HOMO and LUMO orbitals of the electron donor compound, or the so-called “band-gap”. In particular, the maximum value of the wavelength at which the electron donor compound is able to collect and efficiently convert photons into electrical energy, or the so-called "light harvesting" or "photon" process, depends on this difference. harvesting ". 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 small. In fact, a too small "band-gap" would correspond to a too high HOMO which would penalize the voltage obtainable at the device's electrodes (this voltage is in fact correctable to the difference between HOMO of the electron donor compound and LUMO of the electron acceptor compound).

In the simplest way of operating, organic photovoltaic cells are manufactured by introducing between two electrodes, usually consisting of indiotain oxide (ITO) (anode) and aluminum (Al) (cathode), a thin layer (about 100 nanometers) of a mixture of the electron acceptor compound and of the electron donor compound (architecture known as “bulk heterojunction”). Generally, in order to create a layer of this type, a solution of the two compounds is prepared and, subsequently, a photoactive film is created on the anode [indium-tin oxide (ITO)] starting from said solution, resorting to suitable quab deposition techniques, for example, "spincoating", "spray-coating", "ink-jet printing", and the like. Finally, on the dried film, the counter electrode is deposited [i.e. the aluminum cathode (Al)]. Optionally, between the electrodes and the photoactive film, other additional layers capable of performing specific functions of an electrical, optical or mechanical nature can be introduced.

Generally, in order to favor the reaching of the anode [indium-tin oxide (ITO)] by the electron holes (or holes) and at the same time block the transport of electrons, thus improving the collection of charges by the electrode and inhibiting the recombination phenomena, before creating the photoactive film starting from the mixture of the acceptor compound and the donor compound as described above, a film starting from an aqueous suspension of PEDOT: PSS [poly (3,4 -ethylenedioxythiophene) polystyrene sulfonate], resorting to suitable deposition techniques such as, for example, "spin-coating", "spray-coating", "ink-jet printing", and the like. Finally, the counter electrode [cathode (Al)] is deposited on the dried film.

The most commonly used electron donor compound in the production of organic photovoltaic cells is the regioregular poly (3-hexylthiophene) (P3HT). This polymer has excellent electronic and optical characteristics (good values of the HOMO and LUMO orbitals, good absorption coefficient), a good solubility in the solvents that are used to manufacture the photovoltaic cells and a fair mobility of the electronic gaps.

Other examples of polymers that can be advantageously used as electron donor compounds are: the polymer PCDTBT {poly [N-9 "-heptadecanyl-2,7-carbazole-alt-5,5- (4 ', 7'-di- 2-thienyl-2 ', r, 3'-benzothia-diazole]}, the PCPDTBT {poly [2,6- (4,4-bis- (2-ethylhexyl) -4 // - cyclopenta [2, 1 -b; 3, 4-b '] -dithiophene) -alt-4,7- (2, 1,3-benzothiadiazole)]}.

However, said electron donor compounds can have some drawbacks.

In fact, the photon flux of solar radiation reaching the surface of the earth is maximum for energy values around 1.8 eV (corresponding to radiation having a wavelength of about 700 nm). Due to the high "band-gap" values, generally higher than 2 eV, which characterize the aforementioned electron-donating compounds, the "light harvesting" or "photon harvesting" process in this spectral range is not very efficient and only one fraction of the total solar energy, generally between 350 nm and 650 nm, is converted into electrical energy. By way of example, among the polymers most used as electron donor compounds, the aforementioned regioregular poly (3-hexylthiophene) (P3HT) has a band-gap equal to 1.9 eV. These compounds, used in combination with fullerenes-based electron accepting compounds such as, for example, the fullerrenic derivatives listed above, are capable of achieving maximum solar radiation conversion efficiencies of about 5%.

To improve the yield of the "light harvesting" or "photon harvesting" process and, consequently, the efficiency of organic photovoltaic devices, it is therefore essential to identify new electron donor compounds capable of capturing and converting the wavelengths of the solar radiation having lower energy, ie electron donor compounds characterized by lower "band-gap" values than those of organic polymers typically used as electron donor compounds.

The Applicant therefore posed the problem of finding electron donor compounds having a low "band gap" value, i.e. a "band gap" value less than or equal to 2 eV.

The Applicant has now found photoactive conjugated compounds comprising indole units, said compounds having a low "band gap" value, i.e. a "band gap" value lower than or equal to 2 eV, which can be advantageously used in the construction of photovoltaic devices such as, for example, photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on rigid support and on flexible support .

The Applicant has also found that said photoactive conjugated compounds comprising indole units can also be advantageously used in the construction of photovoltaic cells known as Gratzel cells or DSSC ("Dye-Sensitized Solar Cells").

Therefore, the object of the present invention is a photoactive conjugate compound comprising indole units having general formula (I):

in which:

A represents a 1,2-vinyl group; or it can be selected from divalent cyclic or polycyclic groups containing one or more aromatic or heteroaromatic rings, said aromatic or heteroaromatic rings being optionally substituted;

- FGi and FG2, the same or different from each other, represent a hydrogen atom;

or they are selected from: C1-C32 alkyl groups, preferably C4-C24, linear or branched; acid groups -COOH; ester groups having the general formula -COOR wherein R represents a C1-C32 alkyl group, preferably C4-C24, linear or branched; groups having the general formula -Ri-0-R2 in which R1 represents a C1-C32 alkylene group, preferably C4-C24, linear or branched, and R2 represents a C1-C32 alkyl group, preferably C4-C24, linear or branched; groups having the general formula -OR3 in which R3 represents a C1-C32 alkyl group, preferably C4-C24, linear or branched; polyoxyalkylene groups having general formula -0 - [- CH2-CH2-0] m-R or -0 - [- CH2-CH (CH3) -0] m-R or - [- CH2-CH2-0] m-R or - [- CH2- CH (CH3) -0] m-R in which R has the same meanings described above and m is an integer between 1 and 10, preferably between 1 and 3; arylene or heteroarylene groups having the general formula -Y-NHR4o -Y-N (Rs) 2 in which Y represents an optionally substituted arylene group or an optionally substituted heteroarylene group, and R4 and R5, equal or different from each other, represent a C-C32 alkyl group, preferably C4-C24, linear or branched; arylene or heteroarylecarboxylic groups having the general formula -Y-COOH or -Y-R6-COOH in which Y has the same meanings described above and R6 represents a C-C32 alkylene group, preferably C4-C24, linear or branched; groups having the formula -OPO (OH) 2o -PO (OH) 2; groups having the general formula -OPO (OR8) 2o -PO (OR8) 2 in which R8 represents a C1-C32, preferably C4-C24, linear or branched alkyl group; arylene or heteroarylene groups having the general formula -Y-R9 in which Y has the same meanings described above and R9 represents a hydrogen atom or a C1-C32 alkyl group, preferably C4-C24, linear or branched.

It should be noted that the dashed lines in the aforementioned general formula (I) indicate the attachment points of groups in the case of oligomers and / or polymerization in the case of polymers.

In accordance with a preferred embodiment of the present invention, said photoactive conjugate compound comprising indole units can be selected from monomers having general formula (Π):

in which:

- A, FGi and FG2, have the same meanings described above;

Bi and B2, the same or different from each other, represent a hydrogen atom; a 1,2-vinyl group; or they can be selected from optionally substituted ardic groups; optionally substituted heteroaryl groups.

In accordance with a further preferred embodiment of the present invention, said photoactive conjugate compound containing indole units can be selected from polymers having general formula (IH):

A, FGi and FG2, have the same meanings described above;

B represents a 1,2-vinyl group; or it can be selected from optionally substituted arylene groups, or from optionally substituted heteroaryene groups;

n is an integer between 10 and 300, preferably between 25 and 250.

For the purpose of the present description and of the following claims, the definitions of the numerical ranges always include the extremes unless otherwise specified.

The term "divalent cyclic or polycyclic groups containing one or more aromatic or heteroaromatic rings" means groups containing an optionally substituted aromatic ring or an optionally substituted heteroaromatic ring, or groups containing several optionally substituted aromatic rings or several optionally substituted heteroaromatic rings, said aromatic or heteroaromatic rings being condensed or joined together by simple bonds or by binding groups. Specific examples of cyclic or polycyclic groups containing one or more aromatic or heteroaromatic rings are the following:

in which X and Y, the same or different from each other, represent a sulfur atom, an oxygen atom, a selenium atom, or an NR 'group in which R' represents a linear or branched C-C32 alkyl group and the substituents R ", The same or different from each other, represent a hydrogen atom, or a linear or branched C-C32 alkyl group.

The term "C-C32 alkyl groups" refers to alkyl groups having from 1 to 32 carbon atoms, linear or branched. Specific examples of C-C32 alkyl groups are: methyl, ethyl, propyl, isopropyl, bufyl, t-butyl, hexyl, heptyl, octyl, decyl, tetradecyl, dodecyl, hexadecyl, octadecyl, eicosyl, 1-ethylpropyl, 1-butylpentyl, 1 -hexylheptyl, 1-octylnonyl, 1-dodecyltridecyl, 1-hexadecylheptadecyl, 1-octadecylnonadecyl 2-ethylhexyl, 2-ethyloctyl, 2-ethyldecyl, 2-ethyldodecyl, 2-butylhexyl, 2-butyldodecyl, 2-hexyloctyl, 2-hexyl octyl 2-octyldecyl, 2-decyldodecyl, 4-butylhexyl, 4-butyldecyl, 4-butyldecyl, 4-butyldodecyl, 4-hexyldecyl.

The term "polyoxyalkylene groups" refers to groups having oxyalkylene units in the molecule. Specific examples of polyoxyalkylene groups are the following:

where m is an integer between 1 and 10, preferably between 1 and 3.

The term "aryl groups" refers to aromatic carbocyclic groups. Said aromatic carbocyclic groups can optionally be substituted with one or more groups, the same or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C32 alkyl groups; C-C32 alkoxy groups; cyan groups; animino groups; nitro groups. Specific examples of aryl groups are the following:

in which the substituents R ”, the same or different from each other, have the same meanings described above.

The term "heteroaryl groups" means aromatic, penta- or hexa-atomic heterocyclic groups, including benzocondensed or heterobicyclic, containing from 1 to 4 eternatoms chosen from nitrogen, oxygen, sulfur, silicon, selenium, phosphorus. Said heteroaryl groups can optionally be replaced with one or more groups, the same or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C32 alkyl groups; C-C32 alkoxy groups; cyan groups; animino groups; nitro groups. Specific examples of heteroaryl groups are the following:

in which X represents a sulfur atom, an oxygen atom, a selenium atom, or an NR 'group in which R' represents a linear or branched C-C32 alkyl group and the substituents R ", equal or different from each other, represent a hydrogen atom, or a linear or branched C 1 -C32 alkyl group.

The term “arylene groups” refers to divalent aromatic carbocyclic groups. Said aromatic carbocyclic groups can optionally be substituted with one or more groups, the same or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C10 alkyl groups; C1-C10 alkoxy groups; cyan groups; amino groups; nitro groups. Specific examples of arylene groups are:

The term "heteroarylene groups" means aromatic, penta- or hexa-atomic heterocyclic groups, also benzocondensed or heterobicyclic, containing from 1 to 4 eternatoms chosen from nitrogen, oxygen, sulfur, silicon, selenium, phosphorus, said groups being divalent. Said heteroarylene groups can optionally be replaced with one or more groups, the same or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C32 alkyl groups; C-C32 alkoxy groups; cyan groups; amino groups; nitro groups. Specific examples of etroaryene groups are:

in which X represents a sulfur atom, an oxygen atom, a selenium atom, or an NR 'group in which R' represents a linear or branched C-C32 alkyl group and the substituents R ", equal or different from each other, represent a hydrogen atom, or a linear or branched C 1 -C32 alkyl group.

In accordance with a preferred embodiment of the present invention, said polymers having general formula (ΠΓ) can have a weight average molecular weight (Mw) between 2000 Dalton and 150000 Dalton, preferably between 10000 Dalton and 70000 Dalton. Said weight average molecular weight (Mw) can be determined as reported below.

Said photoactive conjugate compound having general formula (I), in particular said monomer having general formula (Π) and said polymer having general formula (IH), are characterized by "band-gap" values lower than 2.0 eV and, therefore , can be advantageously used as electron donor compounds, in photovoltaic devices, in particular to exploit low-energy solar radiation, therefore at a higher wavelength.

Compounds having general formula (I) can be synthesized through processes known in the art.

The monomers having general formula (Π) can be synthesized, for example, by a process which comprises the reaction of an indole having general formula (IIa) with an indole having general formula (II'a) and with a halide having general formula ( Ilb), according to the following Scheme 1: Scheme 1

wherein FGi, FG2, BI, B2 and A have the same meanings described above, X represents a halogen atom selected from bromine, chlorine, fluorine, iodine, preferably bromine. Said reaction is generally carried out in the presence of a solvent such as, for example, dimethylformamide (DMF), at a temperature between 80 ° C and 150 ° C, in the presence of a copper-based catalyst. More details relating to this procedure can be found, for example, in the article by Shen and others: "Synthesis of tetrarylsilanes and its usage as blue emitters in electroluminescence", Synthetic Metals "(2008), Vol. 158, pg.

1054-1058.

Polymers having general formula (IH) can be synthesized, for example, starting from a compound containing indole groups having general formula (IHb) according to the following Scheme 2:

Scheme 2

in which in which FGi, FG2, B, A and n have the same meanings described above, Ra has the meanings described below, ADBIC is dibromoisocyanuric acid.

Polymerization reactions of type (1) and type (2) are also known as Stille reaction (in the case where B or R <a> represent -Sn (R <a> 3) where R <a> is a linear or branched C1-C6 alkyl group, preferably a tributylstannyl or trimethylstannyl group); or as a Suzuki reaction [in the case where B or R <a> represent -B (OH) 2, or a -B (OR <b>) 2 where R <b> is a linear or branched C-C alkyl group] and are generally catalyzed by palladium complexes.

Generally, the aforesaid Stille and Suzuki reactions are catalyzed by PdCl2 (PPh3) 2, as it is or prepared in situ starting from PdCl2 and triphenylphosphine; or from palladium-tetrakistriphenylphosphine [Pd (PPh3) 3], or from Pd (OAc) 2 and other phosphines such as, for example, tri-ortho-tolylphosphine or tris-para-tolyl phosphine.

The Stille and Suzuki reactions can be carried out in solvents selected, for example, from: ethers (for example, 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran); hydrocarbons (toluene, xylene); dipolar aprotic solvents (N, N-dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide). Generally, the reaction temperatures are between 80 ° C and 160 ° C.

Generally, in the case of the Suzuki reaction, it is necessary to add a saturated aqueous solution of sodium bicarbonate or potassium bicarbonate, or a saturated aqueous solution of sodium carbonate or potassium carbonate.

Generally, at the end of the above reactions, the solvent can be removed by distillation and the crude product obtained can be dissolved in the minimum quantity of a suitable solvent such as, for example, chloroform, and precipitated in a suitable solvent such as, for example, methanol.

Further details relating to the synthesis process of said monomer having general formula (II) and of said polymer having general formula (IH), are reported below in the following examples.

Said photoactive conjugate compound having general formula (I), in particular said monomer having general formula (Π), and / or said polymer having general formula (III), can be used in the construction of photovoltaic devices such as, for example, photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on rigid support and on flexible support, as well as in the construction of photovoltaic cells known as Gràtzel cells or DSSC (“Dye-Sensitized Solar Celi”).

A further object of the present invention is therefore the use of said photoactive conjugate compound having general formula (I), in particular of said monomer having general formula (Π), and / or of said polymer having general formula (ΙΠ), in the construction of photovoltaic devices such as, for example, photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on rigid support and on flexible support, as well as in the construction of photovoltaic cells known as Gràtzel cells or DSSC ("Dye-Sensitized Solar Celi ").

As mentioned above, thanks to the low "band-gap" value of said conjugated compound having general formula (I), in particular of said monomer having general formula (Π) and of said polymer having general formula (IH), said compound, as well as said monomer and said polymer, they are able to collect and efficiently convert even low-energy solar radiation, therefore at a higher wavelength, into electrical energy.

A further object of the present invention is therefore a photovoltaic device which can be chosen, for example, among photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on a rigid support and on a flexible support, comprising at least one photoactive conjugate compound having general formula (I), in particular at least one monomer having general formula (Π), and / or at least one polymer having general formula (ΠΓ), described above.

Furthermore, a further object of the present invention is a photovoltaic cell known as a Gratzel cell or DSSC ("Dye-Sensitized Solar Cell"), comprising at least one photoactive conjugate compound having general formula (I), in particular at least one monomer having general formula (Π), and / or at least one polymer having general formula (ΠΓ), described above.

In order to better understand the present invention and to put it into practice, some illustrative and non-limiting examples thereof are reported below.

Characterization of the monomers and polymers obtained

Determination of the molecular weight

The molecular weight of the monomers and polymers obtained by operating in accordance with the following examples, was determined by "Gel Permeation Chromatography" (GPC) on a WATERS 150C instrument, using HT5432 columns, with trichlorobenzene eluent, at 80 ° C.

The weight average molecular weight (Mw) and the polydispersity index ("Polydispersion Index" - PDI), corresponding to the Mw / Mn ratio (Mn = number average molecular weight) are reported.

Determination of the optical "band-gap"

The monomers and polymers obtained by operating in accordance with the following examples, were characterized by UV-Vis-NIR spectroscopy to determine the energy entity of the optical "band-gap" in solution or on thin film according to the following procedure.

If the "optical band-gap" was measured in solution, the monomer or polymer was dissolved in toluene, chloroform, chlorobenzene, dichlorobenzene, trichlorobenzene, or other suitable solvent. The solution thus obtained was placed in a quartz cuvette and analyzed in transmission by means of a double beam UV-Vis-NIR spectrophotometer and double monochromator Perici n Elmer λ 950, in the range 200 nm - 850 nm, with a bandwidth of 2 , 0 nm, scanning speed of 220 nm / min and step of 1 nm, using as a reference an identical quartz cuvette containing only the solvent used as a reference.

In the case in which the "optical band-gap" was measured on a thin film, the monomer or polymer was dissolved in toluene, chloroform, chlorobenzene, dichlorobenzene, trichlorobenzene, or other suitable solvent, obtaining a solution having an equal concentration at about 10 mg / ml, which was deposited, by spin-coating, on a Suprasil quartz slide. The thin film thus obtained was analyzed in transmission by means of a double beam UV-Vis-NIR spectrophotometer and double monochromator Perici n Elmer λ 950, in the range 200 nm - 850 nm, with a passband of 2.0 nm, speed scan of 220 nm / min and step of 1 nm, using as reference an identical Suprasil quartz slide, as it is, as a reference.

The optical "band-gap" was estimated from the transmission spectra by measuring the absorption edge corresponding to the transition from the valence band (VB) to the conduction band (CB). For the determination of the Edge, the intersection with the abscissa axis of the tangent line to the absorption band at the inflection point was used.

The inflection point (λρ, yi) was determined on the basis of the coordinates of the minimum of the spectrum in first derivative, denoted by λ'π ^ and y'mm-The equation of the tangent line to the UV-Vis spectrum at the inflection point (λρ, yi) is the following:

y <=> y min λ yF <—> y min λ min

Finally, from the condition of intersection with the abscissa axis ψ = 0, it was obtained:

λππθΗ <=> (y min λ min <"> YFVY min

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

EEDGE - EVEDGE - h c / λ] EDGE

in which:

h = 6.626 10-34 J s;

c = 2.998 108 m s <1>;

or:

EEDGE = 1-988 10-16 JA, EDGE (nm).

Finally, remembering that 1 J = 6.24 1018 eV, we have:

EEDGE = 1240 eV / AEDGE (nm).

Determination of HOMO and LUMO

The determination of the HOMO and LUMO values of the monomers and polymers obtained by operating in accordance with the following examples, was carried out by means of the cyclic voltammetry (CV) technique. With this technique it is possible to measure the values of the formation potentials of the root cation and the root anion of the sample under examination. These values, inserted in a specific equation, allow to obtain the HOMO and LUMO values of the monomer or polymer in question. The difference between HOMO and LUMO gives the value of the electrochemical "band-gap".

The values of the electrochemical "band-gap" are generally higher than the values of the optical "band-gap" since during the execution of the cyclic voltammetry (CV), the neutral compound is loaded and undergoes a conformational reorganization, with an increase of the " energy gap ", while the optical measurement does not lead to the formation of charged species.

The cyclic voltammetry (CV) measurements were performed with an Autolab PGSTAT12 potentiostat (with GPES Ecochemie software) in a three-electrode cell. In the measurements carried out, an Ag / AgCl electrode was used as a reference electrode, a platinum wire as a counter electrode and a glassy graphite electrode as a working electrode. The sample to be analyzed was dissolved in a suitable solvent and, subsequently, was deposited, with a calibrated capillary, on the working electrode, so as to form a film. The electrodes were immersed in a 0.1M electrolytic solution of 95% tetraflurobate tetrabutylammonium in acetonitrile. The sample was subsequently subjected to a cyclic potential in the shape of a triangular wave. At the same time, according to the applied potential difference, the current was monitored, which signals the occurrence of oxidation or reduction reactions of the species present.

The oxidation process corresponds to the removal of an electron from HOMO, while the reduction cycle corresponds to the introduction of an electron into the LUMO. The potential for the formation of the root cation and the root anion were derived from the value of the peak onset (Eonset), which is determined by molecules and / or chain segments with HOMO-LUMO levels closest to the edges of the bands. The electrochemical potentials to those relating to the electronic levels can be correlated if both refer to the vacuum. For this purpose, the potential of ferrocene in vacuum, known in literature and equal to -4.8 eV, was taken as a reference. The inter-solvent redox ferrocene / ferrocinium pair (Fc / Fc <+>) was chosen because it has an oxidation-reduction potential independent of the working solvent.

The general formula for calculating the energies of the HOMO-LUMO levels is therefore given by the following equation:

E (eV) = -4.8 [E1⁄2Ag / Agci (Fc / Fc <+>) - Eonset Ag / Agci (polymer)] where:

E = HOMO or LUMO depending on the Eonset value inserted;

EI / 2Ag / Agci = half-wave potential of the peak corresponding to the redox ferrocene / ferrocinium couple measured under the same analysis conditions as the sample and with the same theme of electrodes used for the sample;

EonsetAg / Agci = onset potential measured for the polymer in the anodic zone when we want to calculate ΓΗΟΜΟ and in the cathode zone when we want to calculate the LUMO.

EXAMPLE 1

Synthesis of the alternating copolymer benzothiadiazole-indole-bithio-dihexyl-phenylenindole (polymer 1) having formula (1)

In a 25 ml double-necked flask, equipped with a magnetic stirrer and reflux refrigerant, the following components were introduced, under a nitrogen atmosphere:

28.4 mg (0.04 mmol) of l, l- (3,3'diesyl-2,2'-bithiophene-5,5'diyl) -bis- (3-bromo- 1 H-indole);

15.5 mg (0.04 mmol) of 4,7-bis-pinacolboronic-2, 1,3-benzothiadiazole ester;

- 0.5 ml of anhydrous n-propanol;

0.5 ml of a 4 M aqueous solution of potassium bicarbonate (K2C03); 0.5 ml of deaerated toluene.

The reaction mixture obtained was heated to 70 ° C, for 15 minutes, and subsequently 7.1 mg (0.006 mmol) of palladiotetraki strifenilfosfina [Pd (PPh)] were added. Subsequently, the reaction mixture obtained was left at 70 ° C for 40 hours, obtaining a solution.

The solution obtained was cooled to room temperature (25 ° C), poured into 50 ml of methanol, obtaining a solid which was subsequently filtered. 15 mg of solid product (polymer 1) of black color were obtained. The optical "band-gap", measured in solution after dissolving the product obtained in dichlorobenzene, is equal to 1.85 eV.

EXAMPLE 2

Synthesis of diindolyl-dithienyl-benzothiadiazole (monomer 1) having formula (la)

In a 50 ml double-necked flask, equipped with a magnetic stirrer and reflux refrigerant, the following components were introduced under a nitrogen atmosphere:

101.4 mg (0.22 mmol) of 4,7-bis- (2-bromo-5-thienyl) -2,1,3-benzothiadiazole; 105.1 mg (mmol) of indole;

98.7 mg of potassium hydroxide (KOH);

- 50 mg of copper powder;

20 mg of 1,4,7,10,13,16-hexoxacyclooctadecane (18-crown-6);

3 ml of dimethylformamide (DMF).

The reaction mixture was heated to 140 ° C, for 6 hours, and subsequently extracted with 200 ml of water and 200 ml of dichloromethane, obtaining an organic phase and an aqueous phase.

The organic phase obtained was separated, anhydrified with sodium sulphate and subsequently filtered in order to eliminate the salt, obtaining a concentrated solution. The concentrated solution obtained was precipitated in methanol to obtain a violet solid which was purified by elution chromatography (eluent: from heptane / ethyl acetate = 9/1 vol / vol to dichloromethane) on a silica gel chromatographic column. 27 mg of violet-colored powder product (monomer 1) were obtained. The optical "band-gap", measured on a thin film after dissolving the product obtained in chloroform, is equal to 1.95 eV.

Claims (10)

CLAIMS 1. Photoactive conjugate compound comprising indole units, called indole units having the following general formula (I): in which: A represents a 1,2-vinyl group; or it can be selected from divalent cyclic or polycyclic groups containing one or more aromatic or heteroaromatic rings, said aromatic or heteroaromatic rings being optionally substituted; - FGi and FG2, the same or different from each other, represent a hydrogen atom; or they are selected from: Linear or branched C1-C32 alkyl groups; acid groups -COOH; ester groups having general formula -COOR wherein R represents a linear or branched C1-C32 alkyl group; groups having the general formula -Ri-O-R2 in which R1 represents a linear or branched C1-C32 alkylene group, and R2 represents a linear or branched C1-C32 alkyl group; groups having the general formula -OR3 in which R3 represents a linear or branched C1-C32 alkyl group; polyoxyalkylene groups having general formula -0 - [- CH2-CH2-0] m-R or -0 - [- CH2-CH (CH3) -0] m-R or - [- CH2-CH2-0] m-R or - [- CH2 -CH (CH3) -0] m-R in which R has the same meanings described above and m is an integer between 1 and 10; arylene or heteroarylene groups having the general formula -Y-NHR4o -Y-N (Rs) 2 in which Y represents an optionally substituted arylene group or an optionally substituted heteroarylene group, and R4 and R5, equal or different from each other, represent a linear C-C32 alkyl group or branched; arylene carboxyl or heteroarylecarboxylic groups having the general formula -Y-COOH or -Y-R6-COOH in which Y has the same meanings described above and R6 represents a linear or branched C-C32 alkylene group; groups having the formula -OPO (OH) 2o -PO (OH) 2; groups having the general formula -OPO (OR8) 2o -PO (OR8) 2 in which R8 represents a linear or branched C 1 -C32 alkyl group; arylene or heteroarylene groups having the general formula -Y-R9 in which Y has the same meanings described above and R9 represents a hydrogen atom or a linear or branched C1-C32 alkyl group. 2. Photoactive conjugate compound comprising indole units according to claim 1, wherein said compound is selected from monomers having general formula (Π): in which: A, FGi and FG2, have the same meanings described above; Bi and B2, the same or different from each other, represent a hydrogen atom; a 1,2-vinyl group; or they can be selected from optionally substituted ardic groups; optionally substituted heteroaryl groups. 3. Photoactive conjugate compound comprising indole units according to claim 1, wherein said compound is selected from polymers having general formula (IH): in which: A, FGi and FG2, have the same meanings described above; B represents a 1,2-vinyl group; or it can be selected from optionally substituted arylene groups, or from optionally substituted heteroaryene groups; n is an integer between 10 and 300. 4. Photoactive conjugate compound comprising indole units according to claim 3, wherein n is a number between 25 and 250. 5. Photoactive conjugate compound comprising indole units according to claim 3 or 4, wherein said compound has a weight average molecular weight (Mw) comprised between 2000 Dalton and 150000 Dalton. 6. Photoactive conjugate compound comprising indole units according to claim 5, wherein said compound has a weight average molecular weight (Mw) comprised between 10000 Dalton and 70000 Dalton. 7. Use of the photoactive conjugate compound comprising indole units as per any one of the preceding claims, in the construction of photovoltaic devices such as photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on rigid support and on flexible support. 8. Use of the photoactive conjugate compound containing indole units referred to in any one of the preceding claims, in the construction of photovoltaic cells known as Gràtzel cells or DSSC (“Dye-Sensitized Solar Cells). 9. Photovoltaic device selected from photovoltaic cells, photovoltaic modules, solar cells, solar modules, both on a rigid support and on a flexible support, comprising at least one photoactive conjugate compound referred to in any one of claims 1 to 6. 10. Photovoltaic cell known as Gràtzel cell or DSSC (“Dye-Sensitized Solar Cells), comprising at least one photoactive conjugate compound referred to in any one of claims 1 to 6.