The Journal of
Physical Chemistry
0 Copyright, 1985, by the American Chemical Society
VOLUME 89, NUMBER 23 NOVEMBER 7,1985
LETTERS Photoresponse of Pt Electrodes Coated by Electropolymerized Polypyridyl Complexes of Ruthenium( II)-Containing Pyrrole Groups in the Presence of an External Quencher. Film Thickness Effect S. Cosnier; A. Deronzier,*t and J.-C. Moutet*+ Laboratoires de Chimie, Laboratoire d’Electrochimie Organique et Analytique (UA CNRS 321). DZpartement de Recherche Fondamentale. 85 X , F.38041 Grenoble CZdex, France (Received: May 2, 1985; In Final Form: August 16, 1985)
Visible photolysis of Pt electrodes coated by oxidatively electropolymerizedpolypyridyl complexes of ruthenium(I1)-containing pyrrole groups give appreciable cathodic photocurrents when irradiated in the presence of 4-methylbenzenediazonium tetrafluoroborate (4-CH3C6H4N2+BF4-) in acetonitrile. Film thickness studies reveal that the beneficial effect on the photocurrent intensity brought by increasing film thickness is rapidly cancelled by the resulting electron diffusion limitation.
Introduction It has been recently shown that metallic electrodes coated by polymeric films containing a photosensitizer such as polypyridyl complexes of ruthenium(I1) can act as photoelectrodes under visible irradiation. A photoresponse, as a photocurrent, is obtained as a consequence of the electron transfer between the excited state of the photosensitizer in the film and redox quenchers either in In the latter case, when solutionI4 or incorporated in the the photosensitizer and the quencher are arranged as a bilayer the photoelectrode acts as a molecular phot~diode.~,’We have recently reported that polymeric films containing an electroactive center could be obtained by direct anodic electropolymerization of pyrrole groups covalently bound to this electroactive enter.^,^ The following complexes [ R ~ ( b p y ) ~ ( L2+,~ )[Ru(bpy) ~] and [Ru(L2)J2+ as their tetrafluoroborate salts, where bpy = 2,2’-bipyridine, gave by anodic polymerization very stable, adherant, and permeable films? The resulting modified electrodes
’+,
Present address: Laboratoire d’Electrochimie Organique et de Photochimie Redox (UA CNRS 321), Universitd Scientifique et Mddicale de Grenoble, B. P. No. 68, 38402 Saint-Martin d”5res CMex, France.
0022-3654/85/2089-4895$01.50/0
Ll
L2
present the regular electroactivity of polypyridyl complexes of ruthenium(I1) while polypyrrolic chains do not exhibit their usual (1) M. Kaneko, M. Ochiai, and A. Yamada, Makromol. Chem., Rapid Commun., 3, 299 (1982). (2) M. Kaneko, A. Yamada, N. Oyama, and S. Yamaguchi, Makromol. Chem., Rapid Commun., 3, 769 (1982). (3) T. D. Westmoreland, J. M. Calvert, R. W. Murray, and T. J. Meyer, J . Chem. Soc., Chem. Commun., 65 (1983). (4) L. D. Margerum, T. J. Meyer, and R. W. Murray, J . Electroanab Chem., 149, 279 (1983). ( 5 ) N. Oyama, S.Yamaguchi, M. Kaneko, and A. Yamada, J . Electroanal. Chem., 139, 215 (1982). (6) M. Kaneko, S. Moriya, A. Yamada, H. Yamamoto, and N. Oyama, Elecfrochim. Acta, 29, 115 (1984).
0 1985 American Chemical Society
4896
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985
Letters
electroactivity and cond~ctivity.~ In this present paper we report the photoresponse of these new modified electrodes under visible illumination in the presence of an irreversible oxidative quencher in aprotic medium. It has been shown that by use of a sacrificial acceptor instead of a reversible one gave markedly larger photocurrent^.^ This system allowed us to demonstrate the influence of the film thickness on the photoresponse of the electrode especially in terms of photocurrent intensities.
Experimental Section Electrochemical equipment and purification of acetonitrile and tetrabutylammonium perchlorate have been previously described.I0 Synthesis and purification of the ruthenium(I1) monomer complexes were reported e l ~ e w h e r e . ~4-Methylbenzenediazonium tetrafluoroborate was prepared and purified by a standard proFilm preparations and photoelectrochemical and cedure. 1 I spectroelectrochemical measurements were made by using a conventional sandwich-type cell.12 All potentials were referred to the Ag/Ag+ 10 m M in CH3CN reference electrode. Film Preparations. All experiments were run in a drybox under an argon atmosphere. Thin films of polymers were prepared by controlled-potential oxidation a t 1.0 V of 0.5 m M Ru(I1) complexes in CH,CN containing 0.1 M n-Bu4NC104on a platinum disk (area 1 cm2) polished with 1-Mm diamond paste. The auxiliary electrode was another platinum disk mounted parallel without a separating-compartment system. The length of the cell was 1 cm. After passing the required quantity of electricity, the resulting modified electrode was removed, washed several times with CH,CN, and transferred in a clean cell containing pure supporting electrolyte CH3CN solution. Apparent surface concentrations of electroactive species (hence the film thickness) r (mol.cm-2) were determined from the charge under the anodic Ru"/'I' cyclic voltammogram peak.I3 Photoelectrochemical Measurements. In these experiments an optical flat disk was used instead of the platinum disk auxiliary electrode, while a platinum wire served as the auxiliary electrode. The cell was prepared in a drybox under an argon atmosphere and was unstirred. The polymeric film electrode was irradiated with a 250-W Hg lamp through UV and IR cutoff filters with a surface light intensity of 175 mW*cm-2. The electrode potential was adjusted to 0.3 V by a potentiostat. The photoresponse was recorded with a Xt recorder. Spectroelectrochemical Measurements. Thin films of ruthenium(I1) polymer complexes were deposited by the electrochemical technique described above using an optically transparent doped indium oxide electrode (OTE) instead of the platinum disk. After preparation the resulting modified electrode was transferred in a clean cell mounted with an opticall disk. The absorption spectra Ru"') and were recorded while applying potentials of 1.2 (Ru" -1.9 V (Rut' Ru'), respectively.
-
-
Results and Discussion First of all we have verified that these films are stable in their one-electron oxidized and reduced forms. This point is especially crucial for the oxidized form since it is involved in the photoelectrochemical process. For instance, Figure 1 shows the evolution of the absorption spectrum of a film of p ~ l y [ R u ( L ~ ) , ]when ~+ different potentials are applied. Absorptions exhibited by the oxidized (Figure la, curve 4) and the reduced (Figure lb, curve ~
~~
(7) K. Murao and K. Suzuki, Polym. Prep., Am. Chem. SOC.,Diu. Polym. Chem., 25, 260 (1984). (8) G. Bidan, A. Deronzier, and J. C. Moutet, J . Chem. SOC.,Chem. Commun., 1185 (1984). (9) G . Bidan, A. Deronzier, and J. C. Moutet, Nouu. J. Chim., 8, 502 (1984); S. Cosnier, A. Deronzier, and J. C. Moutet, J . Electroanal. Chem., in press. (IO) J. C. Moutet, J . Electroanal. Chem., 161, 181 (1984). (1 1) A. Roe In 'Organic Reactions", R. Adams, Ed., Wiley, New York, 1949. (12) G. A. Gruver and T. Kuwana, J . Electroanal. Chem., 36,85 (1972). (13) H. D. Abrufia, T. J. Meyer, and R. W. Murray, Inorg. Chem., 18, 3233 (1979).
a
I
400
500
c 700
600
h/nm Figure 1. Evolution of the absorption spectra of OTE/poly[Ru(L,)!]*+ electrode (r = 2.4 X mol-cm-2)when different potentials are applied. (a) E = +1.3 V, curve 1: before oxidation; curves 2 and 3: film partially oxidized; curve 4: film fully oxidized. (b) E = -1.9 V, curve 1: before reduction; curve 2: film partially reduced; curve 3: film fully reduced.
hv on
hV off
60
120
I80
240
300
time/s Figure 2. Photocurrent response of a Pt electrode coated with a poly[Ru(Lz),l2' film (I? = 5.8 X mol.cm-*) with 4-CH,C6H4N2+(c = 2 X IO-, M) in C H 3 C N solution.
3) forms of the film are quite similar to those for the Ru(bpy)?+ and Ru(bpy)3+ species in solution.14 The initial absorption is fully restored when the film is reduced or oxidized back, showing that there is no loss of polymeric material during oxidative or reductive processes. Figure 2 shows a typical response of a p~ly[Ru(L,),~+]/Pt electrode (I'= 5.8 X mol-cm-*) in the presence of 2 X M 4-methylbenzenediazonium tetrafluoroborate (4CH3C6H4N2+BF4-)in CH3CN containing lo-' M n-Bu4NC10, under visible photolysis. The steady-state value I, of the photocurrent is quickly reached but is only moderately stable with time (loss of -60% after 1 h of irradiation). However, at the same time the loss of electroactivity of the film is only 20% as measured (14) See, for example, M. Neumann-Spallart, K. Kalyanasundaram, C. Gratzel, and M. Grltzel, Helu. Chim. Acta, 63, 1 1 1 1 (1980), for the Ru( b ~ y ) , ~spectrum + and H. Cano-Yelo and A. Deronzier, N o w . J . Chim.,7, 147 (1983), for the Ru(bpy),+ spectrum.
Letters
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985 4897
SCHEME 1 I
Pt electrode
I j
coated polymer
1
1
solution
by cyclic voltammetry. Furthermore no new peak resulting from photosubstitution appears on the ~oltammogram.'~Addition of an extra amount of 4-methylbenzenediazonium tetrafluoroborate did not cause any enhancement of the photocurrent. The photocurrent decrease vs. time could be due to changes in the permeability and photophysical properties of the film resulting from its chemical modifications caused by radicals formed following the quenching by electron transfer (see below). Photolysis in the absence of 4-CH3C6H4N2+or of a bare, uncoated platinum electrode in the presence of 4-CH3C6H4N2+did not cause any photocurrent. Moreover, if after an irradiation at the polymeric film electrode the same solution is irradiated at a bare platinum electrode no photocurrent could be measured. This last observation proves that the photocurrent is not due to the presence of some Ru( 11) complexes in solution resulting from a partial dissolution of the polymer. The photocurrent should be regarded as the result of the irreversible oxidative quenching of the excited state of the ruthenium polymer by a 4-CH3C6H4N2+which diffuses in the film. It is well-known that benzenediazonium salts are excellent oxidative quenchers for Ru(bpy),,+* in solution.I6 The mechanism for the cathodic photocurrent response obtained can be represented by Scheme I. As previously observed by Meyer et a1: the steady-state photocurrent increases with increased concentration of added quencher and reaches an upper limit at [4-CH3C6H4N2+]= 6 X lo-' M. In their experiment they used a platinum electrode coated with a chemically prepared ruthenium(I1) polymer in the presence of CQ(C,O~),~in aqueous medium. Figure 3 shows the evolution of the steady-state photocurrent I , vs. the film thickness using an identical amount of 4CH3C6H4N2+.As for a conventional photoelectrochemical cell using a photosensitizer in solution," I , increases first with the photosensitizer concentration (here the film thickness for very thin films). These results are consistent with a photoreduction of the diazonium salt which occurs within the bulk polymeric film as well as at the film/solution interface. After a limiting value for Z, is reached, in contrast with what is observed for a solution photoelectrochemical cell, Z, decreases slowly as the film thickness (15) See, for example, 0.Haas, M. Kriens, and J. G. Vos, J. Am. Chem. Soc., 103, 1318 (1980).
(16) H. Cano-Yelo and a Deronzier, J . Chem. SOC., Perkin Trans. 2, 1093 (1984); J. Chem. Soc., Faraday Trans. 1,80,3011 (1984); Tetrahedron Lett., 25, 5517 (1984). (17) A. Deronzier and F. Esposito, N o w . J. Chim., 7, 15 (1983).
2
4
8
8
10
Figure 3. Plots of the steady-state photocurrents I, vs. poly[Ru(bpy),P O ~ Y [ R ~ ( ~ P Y ) ( L (*) ~ ) ~and I * ' P O ~ Y [ R W ~ ) ,( IX~I +thick(L1)2I2+ M in all cases. ness. [4-CH,C6H4N2+] = 2 X (@)t
increases. The most obvious explanation for this phenomena is that increasing the film thickness results in more difficulty for the electrons to propagate by self-exchange from the Pt/polymer interface to the photogenerated Ru3+ species in the film. The beneficial effect brought by increasing film thickness is quickly limited by the resulting electron diffusion limitation. This situation can be compared well with the effect of film thickness with redox polymer electrodes in electrochemical catalysis.'* It should be noted that I, increases in the following order: poly[Ru(bpy)2(L1)2]2+< poly[R~(bpy)(L~)~]*+ < p o l y [ R ~ ( L ~ ) , ] ~This + . order can be interpreted in terms of excited-state lifetimes of the ruthenium(I1) species in the films. The lower steady-state photo),] is in current Z, is obtained with p ~ l y [ R u ( b p y ) ~ ( L ~ which agreement with the lower lifetime of complexes containing two pyridine and two bipyridine as ligands compared with complexes containing three bipyridine ligands.lg As excited-state selfquenching in films seems to be a deciding factor for the excited state lifetime: higher cross-linking in the poly [Ru(L,),] 2+ films could decrease the in comparison with poly[R~(bpy)(L~)~]~+~*~~*~~ mobility of the fixed Ru(I1) sites hence the self-quenching. However, differences in film composition and hence electron acceptor mobility and steady-state concentration within the film could also be the origin of Z, variations. Finally, our observations afford a new type of example demonstrating the importance of film thickness on the electron transfer efficiency implying polymeric redox materials. Acknowledgment. We thank Professor G. Cauquis for his interest in this work and Professor J. Kelly of the University of Dublin for fruitful discussions. (18) See, for example, C. P. Andrieux, J. M. Dumas-Bouchiat, and J.-M. Saveant, J . Electrocanal. Chem., 114,159 (1980); C . Degrand and L. L. Miller, J . Am. Chem. SOC.,102, 5728 (1980); and ref 10. (19) J. M. Calvert, J. V. Caspar, R. A. Binstead, T. D. Westmoreland, and T. J. Meyer, J . Am. Chem. SOC.,104, 6620 (1982). (20) T. Ikeda, R. Schmehl, P. Denisevich, K. Willman, and R. W. Murray, J . Am. Chem. Soc., 104, 2683 (1982). (21) J. M. Calvert, R. H. Schmehl, B. P. Sullivan, J. S. Facci, T. J. Meyer, and R. W. Murray, Inorg. Chem., 22, 2151 (1983).
