J. Chem. SOC., Faraday Trans. 1, 1984,80, 240-2415 Light-induced Electron-transfer Reactions at Electrodes Coated with Macromolecular Thionine and Ruthenium Systems BY R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN* Department of Inorganic Chemistry, University of Madras, Guindy Campus, Madras 600 025, Tamil Nadu, India Received 22nd August, 1983 Thiazine dyes have been condensed with the macromolecules poly(N-methylolacrylamide), pol y(N-meth ylolacr ylamide-co-acrylic acid) and pol y(N-meth ylolacrylamide-co-vin ylp yridine). Cyclic voltammograms of polymeric thionine-coated electrodes indicate the formation of a complex by the dye with ferric and ferricyanide ions at the electrode. The peak potential of quinone is not affected by the polymer-dye-coated electrode.When the electrode is exposed to light cathodic behaviour at the electrode is observed, indicating a change in the polarity of the electrode in comparison with the reaction at a platinum electrode with thionine and iron(1r) present in the homogeneous solution. It is proposed that the generation of a charge-separated thionine-iron complex is stabilised by the macromolecular network. As a comparison polymeric ruthenium(I1) bipyridyl complexes have been prepared and their cyclic-voltammetric and photoelectrochemical behaviour has been investigated. The nature of the polymeric network and of the dye bound to it appears to be an important factor for efficient charge transfer at the electrode. Charge transfer across a photoconductor/liquid interface has important applications in energy-conversion systems' and in imaging processes.2 There have been several attempts to prepare electrodes coated with dyes and to use them to catalyse electron transport using Although photoconductivity is observed in several cases these systems have serious limitations5 owing to the poor light absorption exhibited by the thin films used in these studies and their instability on prolonged irradiation.Films of thicknesses of more than a few hundred Angstroms exhibit resistance to the photoinjection of charge at the electrode. Thus only a.fraction of the incident light is used for charge transport across the junction. Recently,s using macromolecular films containing dyes it has been found possible to observe a fairly appreciable current density using films of 10 pm thickness.More recently7-10 considerable effort has been made to understand electron transfer across polymer-coated electrodes. These studies reveal that charge transfer across the polymer interface is controlled by the nature of the polymer, the extent of its solvent-induced swelling and the distribution of the charge carriers in the polymer film. It is recognized that chemically modified electrode systems should facilitate the separation and stabilization of the charge carriers produced photochemically. In this report we present our results for the photoelectro- chemical properties of different types of chromophores attached to the polymeric films. 24052406 ELECTRON TRANSFER AT COATED ELECTRODES EXPERIMENTAL PREPARATION OF POLYMER SAMPLES Acrylamide (B.D.H.) obtained commercially was allowed to react with paraformaldehyde to obtain N-methylolacrylamide.N-methylolacrylamide was polymerized in aqueous solution using potassium peroxodisulphate as the initiator to obtain poly(N-methylolacrylamide), P(h1AAM). The copolymer poly(N-methylolacrylamide-co-acrylic acid), P(MAAM-co-AA), was prepared by copolymerizing N-methylolacrylamide with acrylic acid. P(MAAM-co-AA) was precipitated by adding the solution to a mixture of ethylacetate and dioxane (4: 1 v/v). The homopolymer P(MAAM) was soluble in this solvent mixture. Any poly(acry1ic acid) present in the copolymer as an impurity was removed by stirring the copolymer in dioxane in which poly(acry1ic acid) dissolved. The separated copolymer was dried and stored in a vacuum desiccator.Poly(N-methylolacrylamide-co-4-vinylpyridine), P(MAAM-co-VP), was obtained by copolymerizing N-methylolacrylamide with 4-vinylpyridine, freshly vacuum distilled from a commercial sample. The polymer in solution was precipitated using an acetone + water (4: 1 v/v) mixture. The homopolymer P(MAAM) was soluble in this solvent mixture and poly(4-vinylpyridine) was removed by stirring the precipitate in dry ethanol. The purified copolymer, P(MAAM-co-VP), was dried in a vacuum at room temperature. Since the colour of the polymer slowly changed to yellowish brown on exposure to the atmosphere it was stored in a vacuum desiccator. PREPARATION OF POLYMER-DYE COMPOUNDS Poly(acrylamidomethylt~onine-co-methylolacry1amide) (I) was prepared by the following procedure.ll Thionine obtained commercially (Riedel) was purified by repeated recrystallization in propan-2-01.Purified thionine was added to an aqueous solution of the polymer P(MAAM) in the desired molar ratio and the mixture was kept at 90 "C for 5 h. Hydroquinone was added to this mixture to prevent cross-linking of the polymer. The resulting polymer-bound thionine was precipitated in large amounts of methanol and purified by repeated precipitation. Poly(acrylamidomethylthionine-co-methylolacrylamide-co-acrylic acid) (11) and poly(acry1- amidomethylthionine-co-methylolacrylamide-co-4-vinylpyridine) (111) were prepared by follow- ing a procedure similar to that used for (I). The polymerdye complex (11) was precipitated using an ethylacetate + acetone (4: 1 v/v) mixture.The copolymers were purified by repeated precipitation. The uncondensed dye from the polymerdye compounds was removed by dialysing the solution of the copolymer in a dialysis sack for 8-10 days in water. Dialysis was continued until the solution outside the sack showed no absorption for thionine (A,,, = 600 nm). The thionine dye attached to the macromolecular chain was not removed easily even under rigorous conditions. Aqueous solutions of the polymer-dye complex were allowed to stand for several months in the laboratory and were subsequently dialysed against water; they did not show any trace of dye passing through the membrane, thus indicating the stability of the polymer-bound dye. The concentration of thionine bound to the macromolecule was estimated by a titrimetric method.I2 A solution containing a known amount of ferrous ammonium sulphate in ortho- phosphoric acid was titrated against a known volume of the polymer-bound thionine solution.The colour of the polymer-bound thionine disappeared owing to the reduction of thionine to leucothionine by ferrous ion in the presence of phosphoric acid. The end point was the appear- ance of a rose-red colour, the colour of polymer-bound leucothionine in orthophosphoric acid. From the titration data the amount of thionine present in the solution was calculated. A known volume of the original polymerdye solution was evaporated and the weight of the residue was taken as the amount of polymer-dye complex present in the original solution. The number of thionine units bound to a polymer chain consisting of a given number of monomer units (the ratio m / d ) was calculated, knowing the amounts of polymer and thionine present per unit volume of solution.The thickness of the film was measured by a micro-screw gauge.R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2407 PREPARATION OF POLYMER-BOUND RUTHENIUM(II) COMPLEXES Poly(4-vinylpyridine), PVP, was obtained by the polymerization of 4-vinylpyridine using potassium peroxodisulphate as initiator. Vacuum-distilled 4-vinylpyridine (5 cm3) was dissolved in 50 cm3 of an acetone+water mixture (1 : 1 v/v) in a round-bottomed flask. Ca. 50 mg of potassium peroxodisulphate was added and the mixture was kept at 50+ 1 "C for 6 h. The resulting PVP was precipitated by pouring the viscous solution into a large amount of water.PVP, purified by being dissolved in a minimum amount of ethanol and reprecipitated in water, was dried in a vacuum oven at 25 "C and stored in a vacuum desiccator. Poly(4-vinylpyridine- co-acrylamide) was prepared by the copolymerization of 4-vinylpyridine and acrylamide. Appropriate molar ratios of 4-vinylpyridine and acrylamide were dissolved in an acetone + water mixture (1 : 1 v/v) and the solution was deaerated for 20 min by the passage of purified nitrogen. To thw solution was added 50mg of potassium peroxodisulphate and the deaeration was continued for 10 min. The solution was then allowed to remain at 45 "C for 6 h. The resulting viscous solution was poured into the appropriate non-solvents and the polymer P(VP-co-AM) was precipitated.In the case of P(V,P-co-AM) the ratio of 4-vinylpyridine to acrylamide (VP: AM) was 1 : 1.94 and the non-solvent used for precipitating the polymer was dioxane; for P(VP-co-AM) with VP:AM = 1 : 15.4 and 1 :40 the non-solvent used was acetone. The copolymer ratio (VP: AM) was found by determining the amount of vinylpyridine present in a given amount of P(VP-co-AM), taking into account that vinylpyridine absorbs at 254 nm (E = 3597 dm3 mol-1 cm-l) and that acrylamide exhibits no absorption at this wavelength. The c-dichlorobis(bipyridine)ruthenium(Ir) complex was attached to PVP13 and P(VP-co-AM) by treating stoichiometric quantities of the ruthenium complex with PVP or P(VP-co-AM) in appropriate solvents. The polymer PVP and c-dichlorobis(bipyridine)ruthenium(rI) were taken in the required stoichiometric ratio and dissolved in methanol.The methanolic solution was then refluxed for 90 h. The resultant clear solution was dialysed with cold water present outside the membrane for ca. 1 week to remove the uncondensed ruthenium complex under dark conditions. After dialysis the methanolic solution was evaporated to dryness in a thin-film evaporator at 40 "C. The film was then stored in a vacuum desiccator at room temperature. When P(VP-co-AM) was used instead of PVP, P(VP-co-AM) and Ru(bpy),Cl, were taken in methanol and water was added until P(VP-co-AM) was completely dissolved; condensation was then carried out as in the case of PVP. The products [R~(bpy)~(VP-co-AM),1~+ and [RU(~~~>,(PVP),]~+ were characterized by their visible absorption spectra, which had absorption maxima at 460 nm.The percentage ruthenium content was calculated spectrophotometrically assuming that the molar absorptivity of [Ru(bpy),(VP-co-AM),I2+ at 460 nm is equal to that of [RU(bPY)2(PY)2I2+. ANALYTICAL PROCEDURES Cyclic voltammograms were run using the PAR modules: a model 173 potentiostat, a model 175 universal programmer and a model 176 current follower. The working electrodes employed were a platinum plate (1 cm2), polymer-bound ruthenium or thionine-coated platinum (1 cm2). A platinum plate (1 cm2) was used as the counter-electrode and a saturated calomel electrode was used as the reference electrode. The latter was connected to the potentiostat through a PAR model 178 electrometer.The photovoltaic effects of the polymer-coated electrodes were studied using a cell consisting of a 1 cm2 platinum plate coated with either polymer-bound thionine or polymer-bound ruthenium and another 1 cm2 platinum electrode. The distance between the two electrodes was maintained at 1 0.1 mm. The polymer-bound thionine or polymer-bound ruthenium was coated onto the platinum by taking a known concentration of the polymer complex on the surface of the electrolytically cleaned and rinsed platinum and drying this at 90 "C in a vacuum. The evaporation of the solvent left an insoluble stable film on the electrode. In the case of polymer-bound thionine the electrodes were immersed in a cell containing 10 cm3 of 0.5 x lo-, mol dmP3 sulphuric acid and lo-, mol dm-3 ferrous solution.The solution was deaerated for 30 min by the passage of oxygen-free nitrogen. The photovoltaic effect of the polymer-bound ruthenium film was studied by keeping the electrodes in 0.1 mol dm-3 perchloric acid and 5 x mol dm-3 ferric perchlorate. The solution was deaerated by the2408 ELECTRON TRANSFER AT COATED ELECTRODES 0.6 \ 9 4 0.3 0 tlmin tlmin t/min Fig. 1. (a) Photocurrent for iron-thionine system, (b) photocurrent for iron-polymer-thionine system and (c) photocurrent for electrode coated with polymer-thionine complex for ' light on ' and ' light off' conditions. Table 1. Behaviour of electrodes coated with the polymer-dye complex substratea AE,,/mV I,,/pA Pmax/pW A 37.4b 2.84 0.027 3 1 .4c 3.70 0.029 58.gd 2.88 0.042 50.0e 3.9 0.049 SO.@ 3.7 0.046 B 32.0b 1.80 0.014 29.6c 2.76 0.020 C 4.0b - - - - 3.0c a (A) Poly(N-acrylamidomethylthionine-co-methylolacrylamide), (B) poly(N-acrylamido- methylthionine-co-methylolacrylamide-co-acrylic acid) and (C) poly(N-acrylamidomethylthio- nine-co-methylolacrylamide-co-vinylpyridine) ; stirring conditions; stationary conditions; d p H 1; e p H 2 ; f p H 3 .passage of oxygen-free nitrogen for 30 min. The irradiation source was a 300 W tungsten lamp in the case of the polymer-bound thionine film and a 1000 W tungsten lamp in the case of the polymer-bound ruthenium film. RESULTS POLY MER-BOUND THIONINE SYSTEMS The polymeric thionine film is not soluble in aqueous solution. Irradiation of the electrode coated with polymer-bound thionine immersed in a cell containing an aqueous solution of ferrous ion using a 300 W tungsten lamp shows a positive photopotential and the current flows from the uncoated platinum electrode to the coated electrode as shown in fig.1 (c). The open-circuit photopotential, AE,,, short-circuit current, Isc, and maximum power output for the coated electrode, Pmax, are given in table 1 . The photovoltaic behaviour of the electrode coated withR. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2409 Table 2. Behaviour of electrodes coated with polymer-dye complexes on irradiation with various reductan tsa FeSO, 32b FeSO, 1 oc K,Fe(CN), 2b H2Q 20b a Reductant concentration 1 x mol dm-3, pH 2.0; in aqueous solution ; in a 50 % water + acetonitrile mixture.Table 3. Cyclic-voltammetric behaviour at coated and uncoated platinum electrodesa reaction uncoated coated FelI1 + e- -+ FeL1 0.37 0.26 Q + 2e-+ 2H+ + H,Q 0.24 0.24 Fe(CN)g- + e- --+ Fe(Cn)4,- 0.14 0.00 a Medium 0.25 mol dm-3 H,SO,, sweep rate 50 mV s-l ; Epc, cathodic peak potential. polymer-bound thionine was studied using the reducing agents FeSO,, K,Fe(CN), and hydroquinone, and the results are summarized in table 2. The cyclic-voltammetric behaviour of the coated electrodes in the presence of these reductants is given in table 3. POLYMER-BOUND RUTHENIUM(II) COMPLEXES [Ru(bpy),(VP-co-AM),I2+ was coated onto a platinum electrode and cyclic voltam- mograms were obtained in acetonitrile + water mixtures (v/v). The wave shapes were found to be of a diffusional character, as shown for a typical case in fig. 2.For all the samples the cathodic peak is observed at 1.02 V or close to that potential. The peak current strongly depends upon the concentration of sulphuric, acid present in the medium. The cathodic peak current, peak potentials and separation of peak potentials at various acid concentrations are listed in table 4. The characteristics of these cyclic voltammograms are given in table 5 for different acetonitrile +water mixtures. The amount of RuII present in the film is determined from the known amount of RuII solution in contact with the platinum plate knowing the molar absorptivity of [Ru(bpy),(py),12+ at 460 nm. Details of cyclic voltammograms of the polymer with different amounts of ruthenium present are given in table 6 .The photopotential of polymer-bound ruthenium@) coated onto a platinum electrode dipped into ferric ion solution using a 1000 W tungsten-halogen lamp was found to be 5 mV in water or acetonitrile +water solution.2410 too 80 60 % u s" 40 20 0 ELECTRON TRANSFER AT COATED ELECTRODES I* 3 0.7 EfV vs SCE 0 3 6 9 12 I5 o-t/(mV s-+ Fig. 2. (a) Cyclic voltammogram for [(R~(bpy),(VP-co-AM,)]~+-coated electrode. 0.08 mol dm-3 H,SO,, VP:AM = 1 : 15.4, scan rates (in mV s-l) as follows: (1) 5, (2) 10, (3) 20, (4) 50, '(5) 100 and (6) 200. (b) Plot of cathodic peak current against (scan rate)&.R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 241 1 Table 4. Cyclic-voltammetric data for polymer-bound ruthenium complexes coated onto electrodes at various acid concentrationsa sample VP:AM H2S04 EpcIV EpaIV AEp/mV ZpcIPA l:o 0.1 0.15 0.2 1 : 1.94 0.1 0.15 1 : 15.4 0.1 0.15 1:40 0.1 0.2 1.02 1.02 1.015 1.015 1.025 1.015 1.025 1.03 1.045 1.066 1.07 1.07 1.08 1 .O n 1.08 1.085 1.09 1.09 46 50 55 65 50 65 60 60 45 470 550 570 205 297 35 50 10 15 a Sweep rate 20 mV s-l, solvent composition CH3CN:H20 = 9: 1 v/v. Table 5. Cyclic-voltammetric data for polymer-bound ruthenium complexes coated onto electrodes in various solvent mediaa solvent CH,CN : H20 (v/v) ZpcIPA EpcIV EpaIV AEp/mV 7: 3 8:2 9: 1 125 1.05 1.107 57 125 1.04 1.095 55 60 1.02 1.08 68 _ _ _ _ _ _ _ _ _ ~ ~ ~~ ~ a Sample composition VP:AM = 1 : 15.4, sweep rate 50 mV s-l, [H2S04] = 0.05 mol dm-3. Table 6. Cyclic-voltammetric data for polymer-bound ruthenium complexes coated onto electrodes amount of Ru'I sample % present VP:AM condens- in film (v/v> ZpclPA EpcIV EPa/V AEp/mV ation /mol dm-3 l:o 470 1.02 1.066 47 9.06 2.37 x lod6 1 : 15.4 35 1.015 1.08 65 1.04 2.43 x lo-' 1:40 10 1.03 1.09 60 0.58 7.98 x 1: 1.94 205 1.015 1.08 65 3.89 7.74 x 10-7 DISCUSSION Thionine dye, which absorbs strongly in the visible region with a maximum at 600 nm, and tris(2,2-bipyridine)ruthenium(11), with a maximum at 450 nm, do not undergo any net decomposition on steady photolysis (4 < 10-5).14915 In the case of both systems efficient transient charge separation processes occur on irradiation with visible light.This property makes these compounds very attractive for study as model241 2 ELECTRON TRANSFER AT COATED ELECTRODES photochemical systems for solar-energy conversion.Both dyes undergo electron transfer from the excited states. Thionine undergoes a photoinduced reduction which is followed by a disproportionation reaction to yield leucothionine and thionine via a two-electron reduction. The redox potentials are TH+/TH;+ = 0.2 V and TH,+/THi = 0.575 V us NHE. *Ru(bpy):+ ion undergoes a one-electron oxidation, with the redox potentials for the couple *Ru(bpy):+/Ru(bpy)i+ = -0.82 V and Ru(bpy):+/Ru(bpy)i+ = 1.27 V us NHE. Although no economically viable solar cells have yet been made using these compounds, attemptP1* have been made to use their derivatives in energy-conversion devices. Recentlylga other macromolecular thionine films have been prepared, and under heterogeneous conditions the dye-coated electrode behaves as a cathode.Different polymer-bound RuII-bipyridyl complexes have been prepared and their photoelectrochemical properties have also recently been reported.lgb CYCLIC-VOLTAMMETRY STUDIES OF ELECTRODES COATED WITH POLYMER-DYE COMPLEXES The cyclic-voltammetric behaviour of electrodes coated with polymeric thionine in the presence of ferrous ion reveals that the cathodic peak potential of the FeI1/Fe1I1 couple shifts cathodically. Similar behaviour has been observed for inert elec- trodes coated with monomeric thionine derivatives.20 The cathodic peak of the Fe(CN):-/Fe(CN)i- couple also shifts negatively at the polymeric thionine electrode. These results are explained by the formation of a complex between the ferric ion and dye in the former case and by the formation of an ion pair between the ferrocyanide and thionine in the latter system.The quinone system does not form an adduct with the polymeric thionine and hence its redox potential is not affected at the polymer electrode. Note that the peak currents decrease in all cases because part of the electrode surface is covered by the inert polymer network. In acidic solution pyridine groups in the PVP chain which are not coordinated to the ruthenium become protonated. Owing to quarternization of the pyridine centres the structure of the polymer chain itself is changed. With protonated pyridine units present in the macromolecule a more linear conformation is attained by the polymer. In the PVP chain, in which some of the pyridines are coordinated to the ruthenium, the film is likely to be composed of a random tangle of polymer chains.Cyclic voltammograms of ruthenium(I1)-bound polymers containing varying amounts of acrylamide in a given solvent at constant pH show a decrease in cathodic peak current (table 6). For the film with VP:AM = 1: 1.94 the cathodic peak current is decreased by a factor of ca. 2 in comparison with the ruthenium-bound homopolymer of PVP even though the total ruthenium content in the film is decreased by a factor of ca. 3. For the polymer sample with VP:AM = 1 : 15.4 the total amount of ruthenium in the film and the extent of metallation in the polymer sample are decreased by a factor of three. However, the cathodic peak current is decreased by a factor of 7 when compared with the sample in which VP: AM = 1 : 1.94.Thus it appears that below a certain ruthenium content in the polymer chain the current flow is drastically affected. Presumably as the percentage of ruthenium in the polymer chain is decreased the average distance between the ruthenium centres increases and the rate of charge transfer to the electrode through the ruthenium is decreased. This explanation is in line with the proposed mechanism for charge transfer involving electron exchange between adjacent pairs of redox centres.21R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2413 PHOTOVOLTAIC EFFECT OF A POLYMER FILM CONTAINING MACROMOLECULES BOUND TO LIGHT ABSORBERS Light-induced electron transport across thick films of polymers containing light absorbers is of considerable interest and not many systems are known where this effect has been shown for film thicknesses exceeding few hundred In addition to the photoinduced current, polymeric thionine films coated onto inert electrodes also show a heterogeneous photoredox reaction not exhibited in a homogeneous solution containing macromolecular thionine.On one-electron reduction thionine produces semithionine which disproportionates in homogeneous solution. In homogeneous solution the electrode reaction is believed to involve the two-electron-reduced leu~othionine.~~ In contrast in the polymeric film it appears that disproportionation does not occur. Thus the electrode reaction occurring under heterogeneous conditions brings about a change in the polarity of the electrodes and hence a change in the direction of the current flow as shown in fig.1. The electrode reactions occurring at the electrode coated with polymeric thionine and when the dye is present in the bulk solution (at the uncoated platinum electrode) are shown in schemes 1 and 2. Scheme 1. Electrode coated with polymer-thionine complexes h Y P-TH+ -+ *P-TH+ H+ *P-TH+ + FeII + [P-TH2-FeI4+ [P-TH2-FeI4+ + e- + P-TH;+ + FeI1 (cathode) FeII -+ FelI1 + e- (anode) P-TH;+ + FeIII -+ P-TH+ + FeII + H+. Scheme 2. Polymer-thionine complex in homogeneous solution H+ *P-TH+ + FeII -+ P-TH;+ + FelI1 2P-THi+ + P-TH; + P-TH+ P-TH; -+ P-TH+ + 2H+ + 2e- (anode) FeIrl +e- -+ FeII (cathode). At the heterogeneous electrode the dye present in the macromolecular network is excited on light absorption and is reduced by the ferrous ion present in the solvent.The fact that the electrode functions as a cathode suggests that at the uncoated platinum electrode ferrous ion is oxidized to ferric ion. The species which is reduced at the polymer-dye electrode is proposed to be a complex between thionine and iron. When the dye is present in the macromolecular film attached to the electrode a new type of electrode reaction is seen which indicates a possibility of preparing chemically modified electrodes of specific character. The thionine-iron complex formed by the absorption of light is stabilized by the polymer network and at the electrode this complex is further reduced. The following reaction occurs : [P-TH2-FeI4+ + e- -+ FeII + P-TH;+ .2414 ELECTRON TRANSFER AT COATED ELECTRODES inert electro :trolyte swelled polymer film Fig, 3.Electrode coated with polymer4ye complex: 0, semithionine; 0, FeI1 and 0, FeI". The polymer-semithionine complex is oxidized by the ferric ion produced at the anode, thus completing the cycle. [R~(bpy),(PvP)~]~+ complexes show photosubstitutional and excited-state electron-transfer proce~ses.~~~ 25 Illumination of the electrode coated with [Ru(bpy),(PVP),I2+ immersed in aqueous solution containing FeIII ions does not show any appreciable photopotential. The meagre photopotential and direction of the current observed indicate that the electrode reaction is similar to that for the Ru(bpy);+-Fe*II photogalvanic cell. The absence of an appreciable photopotential when the macromolecular ruthenium complex is coated onto the electrode may be due to the following reasons.Some of the absorbed light is used in the photolabilization of the PVP ligand in the swollen film. Also, it is likely that the photoproduced ruthenium(II1) and the ferrous ion recombine at a faster rate without taking part in the electrode reaction. It thus seems that macromolecular structure alters the nature of the reactivity of the photoproducts in the case of thionine polymer, whereas in the case of the ruthenium polymer the macromolecular environment has not altered the nature of the electrode process.The dye is incorporated into the polymer-coated electrode as shown in fig. 3. The ruthenium and thionine polymers are both water soluble and the cross-linked films coated onto the electrode are swollen, allowing the free penetration of ions through the polymer network.In the case of thionine the dye centres interact to some extent with each other as shown by the absorption spectra,s whereas in the case of ruthenium no such interactions are It is apparent that the macromolecular network plays an important role in the efficient charge injection at the thionine polymer electrode. It is seen to be an essential condition since on irradiation monomeric thionine-coated electrodes do not show any change in the polarity of the thionine-coated electrode or a high current density.26 Work is underway to determine the efficiency of the thionine-coated electrode for the conversion of light and to prepare polymer-coated electrodes with different light absorbers.R.TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. 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