Molecular scale organized polyconjugated polymer–heteropolyacid composites

Hybrid organic–inorganic molecular-organized materials based on a conductive polymer – poly[2-metoxy-5(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV) – doped with Keggin-type heteropolyacids (polyoxometalates) were synthesized. It is shown that the rate of chemical interaction between MDMO-PPV and heteropolyacids correlates with the oxidative activity and acidity of the latter and leads to the formation of tertiary structures due to a strong electrostatic interaction of each 4–6 monomer units of the polymer with the heteropolyacid anion cluster. This interaction manifests itself in the appearance of an intensive EPR signal characteristic of the unpaired electron localized in the conjugated polymer matrix as well as in the changes in optical, photoelectrochemical, and photoluminescence properties of the composite films.

Electron paramagnetic resonance (EPR) spectra were recorded on a Varian E-3 X-band spectrometer at 77 K and at room temperature.The standards for the calculation of g-factor values were a 2,2-diphenyl-1picrylhydrazyl sample (g = 2.0036) and Mn 2+ ions in MgO matrix.The content of paramagnetic centres in the samples was measured by double integration of the spectra using subsequent comparison with a CuCl 2 • 2H 2 O single crystal standard having a known number of spins.
Photoluminescence (PL) spectra were recorded with a Spex Fluorolog-3 spectrofluorometer and optical absorbance spectra with a Specord M-40 spectrophotometer.Electrochemical measurements were conducted with the use of a PI-50-1 potentiostat, a PR-8 programmer, and a standard three-electrode cell equipped with a saturated Ag/AgCl reference electrode and a Pt counter electrode.
Electro-and photoelectrochemical measurements were performed with a PI-50-1 potentiostat and a PR-8 programmer; a quartz three-electrode cell was equipped with a saturated Ag/AgCl reference electrode and a Pt counter electrode.As a light source a 1 kW xenon arc lamp with a double-grating monochromator was used.

RESULTS AND DISCUSSION
The interaction between the solution of the polymer in chloroform and the methanolic solution of heteropolyacids results in the formation of black lustrous deposits insoluble in water and organic solvents.The fastest reaction was observed in the case of phosphomolybdinum derivatives; it proceeded practically instantly.In general, the rate of interaction decreased in the order H The most complete interaction between MDMO-PPV and heteropolyacid took place in the case of a molar ratio of the compounds equal to one mole of heteropolyacid per 4-5 moles of the monomer units of the polymeric chain.It should be pointed out that this stoichiometry is in a good agreement with the results of the chemical analysis of the obtained deposit.Its annealing in the air at 900 °C for 30 min gave a solid (21% by weight), which consisted of pure MoO 3 units corresponding to one heteropolyacid molecule per four monomer units of the polymer chain.On the other hand, the chemical elemental analysis of the organic moiety of the MDMO-PPV-heteropolyacid deposits indicated that MDMO-PPV did not undergo essential chemical transformations in its interaction with polyoxometalates, because the percentages of carbon (78.8%), hydrogen (9.7%), and oxygen (11.5%) in the organic moieties of the composite remained practically the same as in the untreated MDMO-PPV (79.1%, 9.8%, and 11.1%, correspondingly).However, other properties of the polymer -colour, solubility, etc. -dramatically changed.
The changes in the polymer matrix are additionally illustrated by the data obtained by EPR spectroscopy.It is well known that neither MDMO-PPV nor phosphomolibdic heteropolyacids show an EPR signal at room or low (77 K) temperatures.Contrary to this, MDMO-PPV-heteropolyacid composites exhibit EPR signals (Fig. 1) corresponding to two types of paramagnetic centres.An intensive narrow single line with a g-factor of 2.0025 and a line width ∆H of 1.1 G is characteristic of an unpaired electron localized in a conjugated polymer matrix.The second broad (∆H = 20 G) signal at a higher magnetic field can be attributed to Mo 5+ ions; its g-value of 1.955 is in a good agreement with literature data (g = 1.951) [12].
Variations in the molar ratio of starting components in the range of 0.02 to 1.0 practically did not influence the line width.This fact allows us to conclude that there is no dipole-dipole interaction between stabilized electrons and Mo 5+ centres in the heteropolyacid anion, which means that the distance between the neighbouring paramagnetic centres is larger than 2.5−3 nm.In the case of MDMO-PPV doped with H 3 PVMo 11 O 40 (Fig. 2, curve 2), vanadium-substituted phosphomolibdinum anions reveal a strong EPR signal characteristic of V 4+ ions, which originally exist in the heteropolyacid molecule.The introduction of these vanadium-containing clusters led to a substantial increase in the width of  the line attributed to paramagnetic centres of MDMO-PPV, which reflects the existence of a rather weak magnetic dipole-dipole coupling between the unpaired electrons of the polymer cation radicals and V 4+ centres.In accordance with this dipole broadening, a calculation of the average distance between the neighbouring radicals and V 4+ centres in the composite showed that the distance between spins was not less than 1.8-2.0nm.
Figure 2 shows the dependence of the concentration of stabilized unpaired electrons in MDMO-PPV on the molar ratio of heteropolyacids and MDMO-PPV (calculated for the monomer unit of the polymer) used in the synthesis of the composites.As can be seen from the plot, the concentration of paramagnetic centres grows with increasing content of polyoxometalate in the reaction mixtures only up to a MDMO-PPV-to-polyoxometalate ratio of approximately (4-5) : 1.
As indicated above, this ratio of the components is in close agreement with the data of the chemical analysis of MDMO-PPV-polyoxometalate composites prepared at a 1 : 1 ratio of the starting components.In these conditions, if the content of polyoxometalate in the reaction mixture is higher than one molecule per four to six monomer units of the polymer, excess polyoxometalate molecules will not react with the polymer and thus will not cause a further increase of the concentration of paramagnetic centres.Hence, the EPR data, along with the results of chemical analysis, indicate a distinct relation between the content of heteropolyacid and polymeric components in the composites.
To summarize, the interaction between MDMO-PPV and heteropolyacids is characterized by a distinct stoichiometry; it does not influence the chemical formulation of the polymer but it leads to a sharp decrease in solubility and to the appearance of unpaired electrons.Based on these facts we can suggest that the main result of this interaction is the formation of a charge-transfer complex between heteropolyacid anion clusters and conformationally distorted polymer chains.
For the investigation of optical, photoluminescence, and photoelectrochemical properties of a charge-transfer complex thin polymer films deposited onto optically transparent quartz substrates were employed.The rate of the interaction between MDMO-PPV films and heteropolyacids as well as the depth of film transformation crucially depended on the solvent used.After the treatment of MDMO-PPV films with methanolic solutions of heteropolyacids no visible changes of the polymer were observed.Evidently it is connected with the insolubility of MDMO-PPV in methanol; hence heteropolyacid clusters could penetrate the MDMO-PPV film only to a depth close to the monomolecular surface layer.
Contrary to this, if the solution of the heteropolyacid in the chloroform-methanol mixture was used, the treated films lost their red colour and became dark grey.In this case heteropolyacid clusters could penetrate into the bulk of the polymer, and a bulk heterostructure was formed.
This process manifested itself in pronounced differences in the optical absorbance spectra of MDMO-PPV films before and after their treatment with the solution of heteropolyacid in the chloroform-methanol mixture.As Fig. 3a shows, the main peak of MDMO-PPV absorption sharply decreases, and a new wide longwavelength peak appears at approximately 700-800 nm.In the case of heteropolyacids dissolved in the chloroform-methanol mixture, 1 : 2, these spectral transformations were usually completed in several minutes for H 3 PMo 12 O 40 and in 14-16 h in the case of H 3 PW 12 O 40 .
Figure 3b illustrates an enhancement of the longwavelength absorbance and, simultaneously, a reduction of the main peak of the MDMO-PPV absorbance as a function of increasing chloroform content in the heteropolyacid solution used for treating the polymer.Evidently, in this case the depth of the penetration of heteropolyacid acids into the bulk of the polymer was a determinating factor for completing the transformation of the polymer to the polymer-heteropolyacids composite.
The formation of the bulk heterostructure led to significant changes in the photoluminescence spectra (Fig. 4a).In these spectra there was no peak at 593 nm with a shoulder at 640 nm characteristic of MDMO-PPV, instead a new short-wavelength peak at 525 nm appeared.The treatment of PPV film with H 3 PMo 12 O 40 dissolved in the methanol-chloroform mixture led to an intensity of luminescence decreased by two orders of magnitude with increasing chloroform content (Fig. 4b).
Similar spectral behaviour was also observed in the case of MDMO-PPV films treated with tungsten phosphate and other polyoxometalates in the form of heteropolyacids.These changes are due to the transformations in the conjugated polymer chain, which can also influence the charge-transfer processes.We investigated this effect with electrochemical techniques using PPV films deposited onto optically transparent quartz substrates covered with an ITO (indium-tin oxide) layer.Comparison of cyclic voltammograms of MDMO-PPV film electrodes registered in 0.1 M (C 4 H 9 ) 4 NBF 4 acetonitrile electrolyte reveals that both anodic and cathodic currents markedly grow after the treatment of the polymer with the heteropolyacid solutions in a methanolchloroform mixture [3].On the other hand, after passing a cathodic charge through the composite MDMO-PPVpolyoxometalate film, the bluish-grey colour of the composite film turned red, characteristic of pure MDMO-PPV.Then, after passing an anodic charge at potentials higher than + 1 V, the red colour of the polymer film transformed again to the bluish-grey characteristic of the MDMO-PPV-heteropolyacid composite.The change in colour of the composite film from bluish-grey to red is attributed to an electron injection from the ITO conduction band into the composite, which led to cathodic reduction of Mo(VI) to Mo(V) and, as a result, to disrupting the charge transfer complex.The anodic polarization, in turn, caused the oxidation of Mo(V) to Mo(VI), accompanied by a proton injection from the electrolyte to the composite film, which led back to the formation of a charge transfer complex involving heteropolyanions and polymer molecule.Such reversibility may serve as another substantial argument in favour of the fact that the interaction of PPV with polyoxometalates does not lead to destructive chemical transformations of the polymer in spite of strong alterations in its chromophore (polyconjugated) system.
An enhancement of the electrocatalytic activity of the polymer due to bulk heterojunction and chargetransfer complex formation allows improvement of the functional characteristics of photoelectrochemical cells based on the use of MDMO-PPV as a semiconductive photosensitive element.As is seen from Fig. 5a, the treatment of a MDMO-PPV film with a heteropolyacid solution enhances the values of photocurrent to be measured in the photoelectrochemical cell.We connect this increase in photoresponse with a strongly enhanced electrocatalytic activity of the modified surface and, consequently, with a decreased overvoltage at the polymer-electrolyte interface.Additionally, in the photovoltage spectrum (Fig. 5b) one can observe the appearance of a long-wavelength photoresponse attributed to a photogeneration process involving the charge-transfer complex.Moreover, the long-term stability of hybrid electrodes increases markedly (2-5 times) due to the formation of a thin surface layer consisting of the products of the polymer-heteropolyacid interaction.As a result, it becomes possible to perform long-term electro-and photoelectrochemical experiments even in the presence of strong donor and acceptor additives in the electrolyte.Evidently such protective treatment could be useful for various applications -electrochemical sensors, electrocatalytic and other systems in which the polymer is in contact with reactive media.

CONCLUSIONS
Based on the experimental data obtained with the use of optical absorbance and luminescence spectroscopy as well as on the results of electrochemical and EPR measurements and the data of chemical analysis, we can conclude that the interaction of MDMO-PPV with heteropolyacids leads to the formation of strong chargetransfer complexes.This interaction does not cause irreversible destructive transformations in the polymer chain.The introduction of polyoxometalate clusters into a PPV matrix is not equivalent to classical acid doping well known for polyaniline-heteropolyacid systems but it causes reversible alterations in the polymer electronic structure, which lead to substantial changes in the optical, photoluminescence, and photoelectrochemical spectra, as well as to an increase of free charge carrier concentrations and to an enhancement of the electrocatalytic activity of the polymer surface.A novel hybrid material obtained in the framework of this work may be prominent as an active component of photoelectrochemical cells and other electro-and photoelectrochemical devices.

Fig. 3 .Fig. 4 .
Fig. 3. Optical absorbance spectra of MDMO-PPV films before and after their treatment with H 3 PMo 12 O 40 solutions with various methanol-to-chloroform molar ratios (a); dependence of the absorbance of MDMO-PPV films treated with H 3 PMo 12 O 40 solutions at long-and short-wavelength peaks (750 and 500 nm, correspondingly) on the chloroform content in methanol-chloroform mixtures (b).