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Structural Characterization and Photoelectrochemical Properties of the Self-Assembled Monolayers of Tris(2,2′-bipyridine)ruthenium(II)-Viologen Linked Compounds Formed on the Gold Surface Nao Terasaki,† Tsuyoshi Akiyama,‡ and Sunao Yamada*,‡ Department of Materials Physics and Chemistry and Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan Received May 12, 2002. In Final Form: July 26, 2002 Steady-state spectroscopic studies of tris(2,2′-bipyridine)ruthenium(II)-viologen (Ru2+-V2+) linked thiol derivatives with different spacer-chain lengths (denoted as RuCnVCmS, where (n, m) ) (11, 2), (7, 6), (7, 2), and (3, 6)) and different number ratios of Ru2+:V2+ ) 1:2, Ru(C7VC6S), have been carried out, together with the corresponding reference compounds without Ru2+ or V2+. Absorption and cyclic voltammetric (CV) measurements indicated no appreciable electronic interactions between the two chromophores (Ru2+ and V2+) in the ground state. While luminescence spectral measurements revealed the occurrence of efficient photoinduced electron transfer from the photoexcited Ru2+(*Ru2+) to V2+. The electron transfer efficiency was also better for the 1:2 complex as compared with the corresponding 1:1 complex. Self-assembled monolayers (SAMs) of these compounds were fabricated on a gold electrode. XPS and CV measurements revealed immobilization of the compounds on the gold surface via gold-sulfur bonding. In the presence of triethanolamine, all modified electrodes with the Ru2+-V2+ linked compounds showed clear anodic photocurrents. The presence of the V2+ moiety was crucial for obtaining larger photocurrents A shorter spacer-chain length between the V2+ moiety and the terminal thiol group (m ) 2) afforded a larger photocurrent, while a moderate spacer-chain length between the Ru2+ and the V2+ moieties was also a key for obtaining a larger photocurrent. A possible mechanism for photocurrent generation is discussed.
Introduction Photoinduced charge-separation between an electron donor and an acceptor molecule is one of the most important photoreactions for the conversion of light to chemical potential energy, as has been verified in the reaction center of photosynthesis. The conversion of light to electricity has been realized by spatially and unidirectionally arranging the electron donor-acceptor pair on a conductive support by using, for example, the Langmuir-Blodgett (LB) technique.1 However, the LB method seems to be somewhat disadvantageous in terms of film stability, uniformity, manipulation, and cost for instrumentation. Modification of a conductive (especially gold) surface with an organosulfur compound (so-called Au-S selfassembling) is rather new and also has been successful for easily fabricating a monolayer assembly on the gold surface.2 Recently, electron donor-acceptor linked sulfur compounds such as porphyrin-quinone,3 porphyrinferrocene,4 porphyrin-fullerene5 diad systems, and even a fullerene-porphyrin-ferrocene triad system6 have been † Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University. ‡ Department of Applied Chemistry, Graduate School of Engineering, Kyushu University.
(1) Fujihira, M. Photoelectric Conversion with Langmuir-Blodgett Films. In Photochemical Processes in Organized Molecular Systems; Honda, K., Ed.; Elsevier Science Publishers: Amsterdam, 1991; p 463. (2) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (b) Ulman, A. Chem. Evol. 1996, 96, 1533. Finklea, H. O. Electrochemistry of Organized Monolayers of Thiols and Related Molecules on Electordes. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19. (3) Kondo, T.; Ito, T.; Nomura, S.; Uosaki, K. Thin Solid Films 1996, 284/285, 652-655.
prepared, and the photoelectrochemical responses from their self-assembled monolayers (SAMs) on the gold electrodes have been studied. The pair of tris(2,2′-bipyridine)ruthenium and viologen (Ru2+ and V2+) is a well-known couple for photoinduced charge-separation.7 We previously reported preliminary investigations on photoelectrochemical properties of the SAMs of Ru2+ and V2+nonlinked,8 and linked 1:19 and 1:210 systems on the gold surface. In this paper, we have extended the steady-state spectral characterization and photoelectrochemical properties of the SAMs of a series of Ru2+ and V2+ linked compounds. In particular, the effects of spacer-chain lengths between the Ru2+ and V2+ moieties and between the V2+ moiety and the electrode surface have been investigated. (4) (a) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367-8368. (b) Yanagida, M.; Kanai, T.; Zhang, X.-Q.; Kondo, T.; Uosaki, K. Bull. Chem. Soc. Jpn. 1998, 71, 2555-2559. (c) Kondo, T.; Kanai, T.; Iso-o, K.; Uosaki, K. Z. Phys. Chem. 1999, 212, 23-30. (d) Kondo, T.; Yanagida, M.; Zhang, X.-Q.; Uosaki, K. Chem. Lett. 2000, 964-965. (5) (a) Akiyama, T.; Imahori, H.; Ajawakom, A.; Sakata, Y. Chem. Lett. 1996, 907-908. (b) Imahori, H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Bull. Chem. Jpn. 1999, 72, 485-502. (6) (a) Imahori, H.; Yamada, H.; Ozawa, S.; Ushida, K.; Sakata, Y. Chem. Commun. 1999, 1165-1166. (b) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B. 2000, 104, 2099-2108. (c) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100-110. (7) Matsuo, T. J. Photochemistry 1985, 29, 41. (8) Yamada, S.; Kohrogi, H.; Matsuo, T. Chem. Lett. 1995, 639-640. (9) (a) Yamada, S.; Koide, Y.; Matsuo, T. J. Electroanal. Chem. 1997, 426, 23-26. (b) Koide, Y.; Terasaki, N.; Akiyama, T.; Yamada, S. Thin Solid Films. 1999, 350, 223-227. (c) Kuwahara, Y.; Akiyama, T.; Yamada, S. Langmuir 2001, 17, 5714. (10) Terasaki, N.; Akiyama, T.; Yamada, S. Chem. Lett. 2000, 668.
10.1021/la025936v CCC: $22.00 © 2002 American Chemical Society Published on Web 10/04/2002
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Experimental Section Materials and Preparation. Synthesis and characterization of RuCnVCmS ((n, m) ) (7, 6), (3, 6)), Ru(C7VC6S), and RuC17S have been described previously.8,9,11 Other linked compounds RuCnVCmS ((n, m) ) (11, 2), (7, 2)) could be prepared in similar manners as before and will not be described here. The products were isolated as dimer (disulfide) forms. 1H NMR and elemental analysis data are as follows. RuC11VC2S. 1H NMR (400 MHz, CD3CN): δ ) 1.42 (br, 6H), 1.69 (br, 2H), 2.02 (br, 2H), 2.53 (s, 3H), 2.80 (t, 2H, J ) 7.79 Hz), 3.40 (t, 2H, J ) 6.59 Hz), 4.70 (t, 2H, J ) 7.32 Hz), 4.99 (t, 2H, J ) 6.59 Hz), 7.23 (s, 2H), 7.38 (dd, 4H, J ) 6.60, 13.6 Hz), 7.54 (d, 2H, J ) 5.89 Hz), 7.70 (dd, 4H, J ) 3.20, 11.2 Hz), 8.05 (m, 4H), 8.46 (m, 6H), 8.51 (d, 4H, J ) 8.42 Hz), 8.96 (d, 2H, J ) 6.96 Hz), 9.00 (d, 2H, J ) 6.96 Hz). Anal. Calcd for C50H51O16N8SCl4Ru‚H2O: C, 45.74; H, 4.07; N, 8.53. Found: C, 45.44; H, 4.14; N, 8.44. RuC7VC2S. 1H NMR (400 MHz, CD3CN): δ ) 1.36 (br, 14H), 1.68 (br, 2H), 1.99 (br, 2H), 2.53 (s, 3H), 2.78 (t, 2H, J ) 7.69 Hz), 3.40 (t, 2H, J ) 6.59 Hz), 4.61 (t, 2H, J ) 7.69 Hz), 4.98 (t, 2H, J ) 6.59 Hz), 7.22 (s, 2H), 7.38 (dd, 4H, J ) 5.85, 13.2 Hz), 7.55 (d, 2H, J ) 5.50 Hz), 7.72 (dd, 4H, J ) 3.20, 10.8 Hz), 8.05 (m, 4H), 8.46 (m, 6H), 8.51 (d, 4H, J ) 8.22 Hz), 8.94 (d, 2H, J ) 6.96 Hz), 8.99 (d, 2H, J ) 6.96 Hz). Anal. Calcd for C54H59O16N8SCl4Ru: C, 48.00; H, 4.40; N, 8.29. Found: C, 47.88; H, 4.61; N, 8.04. Spectroscopic Measurements. Absorption and fluorescence spectra were recorded with a Shimadzu UV-2500 spectrophotometer and a Hitachi F-3010 fluorescence spectrophotometer, respectively. X-ray photoelectron spectroscopy (XPS) was carried out by a Shimadzu ESCA-850 system. Electrochemical Measurements. Cyclic voltammograms (CVs) and differential pulse voltammograms (DPVS) were measured in a three-electrode cell with a potentiostat (BAS CV50WH). Platinum disk (diameter 1.5 mm) (working), platinum wire (diameter 1.5 mm) (counter), and silver wire (diameter 1 mm) (reference) electrodes were used for the measurements in an acetonitrile solution containing the compound (1 × 10-4 M) and tetra(n-butylammonium) perchlorate (TBAP) (0.1 M). For the modified electrode, the measurements were carried out by replacing it with the platinum working electrode. Measurements were carried out under nitrogen atmosphere. Preparation of Modified Electrodes. The gold electrode was prepared by vacuum deposition of gold onto a mica plate (1.5 × 0.9 × 0.1 cm) at 300 °C.12 The roughness factor of the gold surface was evaluated from the charge for the reduction of gold oxide to be ∼2.7. It was immersed into an acetonitrile solution containing each compound (1 × 10-3 M, as monomer unit) for 1 day. The electrode was then removed from the solution, washed with acetonitrile and then methanol, and dried in air. Photocurrent Measurements. Photocurrent measurements for the modified electrodes were taken in aqueous solutions containing triethanolamine (TEOA) (5 × 10-2 M) NaClO4 (0.1 M) under nitrogen atmosphere unless otherwise noted, the reference and counter electrodes were same as those in the CV measurements of the modified electrodes. The light from a Xe lamp (300 W ILC) was passed through a monochromator and irradiated the modified electrode (∼0.2 cm2 irradiation region). The photocurrent intensity was measured at 470 nm (∆λ ) (16 nm). The photo current action spectra were measured by changing the excitation wavelength (∆λ ) (16 nm). All measurements were carried out at room temperature.
Results and Discussion Absorption spectra of RuC11VC2S, RuC3VC6S, RuC17S, C12VC2S, and Ru(C7VC6S) in acetonitrile (1 × 10-5 M) are shown in Figure 1. Broad bands in the ∼400-∼500 nm region, characteristic of the metal-to-ligand chargetransfer (MLCT) transition, essentially overlap each other in RuCnVCmS, Ru(C7VC6S), and RuC17S. In Figure 1c, absorption spectra of Ru(C7VC6S), RuC7VC6S, and RuC17S (11) Kuwahara, Y.; Akiyama, T.; Yamada, S. Thin Solid Films 2001, 393 273. (12) Naohara, H.; Ye, S,; Uosaki, K. Colloids Surf. A 1999, 154, 201.
are compared. The V2+ moiety has an absorption band at ∼260 nm, as is verified from the spectrum of C12VC2S (Figure 1b). The band around 260 nm reasonably increases as the number of the V2+ moiety increases. The strong band at ∼280 nm is due to the 2,2′-bipyridine ligands of the Ru2+ moiety. These observations indicate that the spectra of the linked complexes are simply the sum of those of Ru2+ and V2+ moieties. The results also suggest no appreciable electronic interactions between Ru2+ and V2+ moieties at the ground state, independent of the spacer-chain length between the two chromophores in the range of n ) 3 to 11. Typical cyclic voltammograms, measured in the acetonitrile/TBAP (0.1 M) solution in the presence of Fc (1 × 10-4 M) as a marker molecule, are shown in Figure 2. Four couples of one-electron redox processes for metal oxidation (Ru3+/2+, denoted as I), viologen reduction (II for the fist reduction, V2+/+; III for the second one, V+/0), and the reduction of the 2,2′-bipyridine ligand (IV), in the range of +1.5 to -1.6 V vs Ag wire, were clearly observed for RuCnVCmS ((n, m) ) (11, 2), (3, 6)), while two redox waves due to metal oxidation (I) and ligand reduction (IV) were observed for RuC17S (c). The redox potentials of Ru3+/ Ru2+ (1.37 V) were almost identical in RuCnVCmS and RuC17S, and the first (-0.20 ( 0.01 V) and the second (-0.63 ( 0.01 V) reduction potentials of the V2+ moiety were also apparently identical in RuCnVCmS and C12VC2S.
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Terasaki et al. Table 1. Redox Potentialsa of the Compounds in Acetonitrile compound
Ru3+/2+
V2+/+
V+/0
ligand
RuC11VC2S RuC7VC6S RuC7VC2S RuC3VC6S Ru(C7VC6S) RuC17S C12VC2S
+1.37 +1.37 +1.37 +1.37 +1.37 +1.37
-0.19 -0.21 -0.19 -0.19 -0.19
-0.63 -0.64 -0.62 -0.62 -0.63
-1.14 -1.14 -1.14 -1.16 -1.14 -1.15
-0.19
-0.62
a
Values ((0.01 V) were taken as the average of cathodic and anodic peak potentials vs Ag wire in acetonitrile containing 0.1 M TBAP, with a sweep rate of 100 mV/s
Figure 1. Absorption spectra of the compounds in acetonitrile (1 × 10-5 M as monomer unit): (a), RuC11VC2S and RuC3VC6S; (b) RuC17S and C12VC2S; (c) Ru(C7VC6S), RuC7VC6S, and RuC17S.
Figure 2. Cyclic voltammograms in acetonitrile/TBAP (0.1 M): (a) RuC11VC2S + Fc; (b) RuC3VC6S + Fc; (c) RuC17S + Fc; (d), C12VC2S + Fc. Concentrations of compounds are 1 × 10-4 M. Scan rate: 0.1 v/s.
The results also suggest no appreciable electronic interactions between Ru2+ and V2+ moieties of RuCnVCmS at the ground states. CVs of Ru(C7VC6S) also afforded the four redox potentials as shown in Table 1, but the peak intensities of the V2+ reduction waves were not well correlated with the number of the V2+ moieties. Thus,
Figure 3. Differential pulse voltammograms of Ru(C7VC6S)/ Au (a) and RuC7VC6S/Au (b) in acetonitrile/TBAP (0.1 M). Scan rate: 0.02 v/s.
DPVs were compared between Ru(C7VC6S) and RuC7VC6S, which were immobilized on the Au electrode (as modified electrodes). The results are shown in Figure 3. The peaks at +1.5 and -0.25 V correspond to Ru3+/2+ and V2+/+, respectively. The ratios of these peak intensities are well correlated the number ratio of Ru2+ and V2+ moieties (1:2), and the results are quite consistent with the absorption spectral data (Figure 1c). Luminescence spectra of Ru(C7VC6S), RuC7VC6S, and RuC17S excited at 470 nm are shown in Figure 4. It is clear that the luminescence intensity is largest in RuC17S and becomes lower with decreasing the spacer-chain length (n ) 11, 7, 3) between Ru2+ and V2+ moieties. The luminescence intensity of RuC3VC6S was about 1/11th of that of RuC11VC2S. Both Ru2+ and V2+ moieties are dicationic, and thus they will be spatially separated from each other by electrostatic repulsion, by taking some extended conformations of the spacer-chain. Thus, electron transfer from the photoexcited Ru2+ (*Ru2+) to the V2+ moiety must be favorable for shorter spacer-chain lengths in RuCnVCmS, as has been suggested by Yonemoto et al.13 The luminescence intensity of Ru(C7VC6S) is remarkably smaller than that of RuC7VC6S. Since the spacer-chain lengths between Ru2+ and V2+ moieties are identical, the (13) (a) Yonemoto, E. H.; Riley, R. L.; Kim, Y.; I. Atherton, S. J.; Schmehl, R. H.; Mallouk, T. E.J. Am. Chem. Soc. 1992, 114, 8081. (b) Yonemoto, E. H.; Saupe, G. B.; Schmehl, R. H.; Hubig, S. M.; Riley, R. L.; Iverson, B. L.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 4786.
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Figure 4. Luminescence spectra of RuCnVCmS, Ru(C7VC6S), and RuC17S in acetonitrile (1 × 10-5 M): (a) RuC17S; (b) RuC11VC2S; (c) RuC7VC6S; (d) RuC7VC2S; (e) RuC3VC6S; (f) Ru(C7VC6S). λex ) 470 nm.
Figure 6. Cyclic and differential pulse (inset) voltammograms of modified electrodes: (a) C12VC6S/Au; (b) C12VC2S/Au; (c) RuC7VC6S/Au. Scan rate: 100-500 mV/s. [NaClO4] ) 0.1 M. Table 2. XPS Binding Energies of the S 2p Regions for the SAMs Formed on the Gold Electrodes
Figure 5. X-ray photoelectron spectra of the S 2p region: (a) RuC7VC6S/Au; (b) C16S/Au; (c) Ru(C7VC6S)/Au; (d) SC16S/Au.
result clearly indicates that the electron transfer quenching of *Ru2+ by V2+ occurs much more effectively in Ru(C7VC6S). Figure 5 shows the S 2p regions of XPS spectra of the modified electrodes with RuC7VC6, C16S, Ru(C7VC6S), and SC6S, as typical examples. The RuC7VC6S/Au sample shows a broad band whose peak is in the 162.0-163.0 eV region. This is very close to the S 2p3/2 (162.0 eV) and the S 2p1/2 (163.0 eV) peaks of C16S/Au (b). These peak positions are also quite similar to those of thiol derivatives immobilized on the gold surface, for example, 161.8 eV (S 2p3/2) and 163.2 eV (S 2p1/2) for benzenethiol on Au,5b suggesting the existence of the binding state of the S-Au linkage. A somewhat poor signal-to-noise ratio for the RuC7VC6S/Au as compared with the C16S/Au electrode is due to lower surface coverage (vide infra). The Ru(C7VC6S)/Au sample showed a quite similar spectrum with the RuC7VC6S/Au sample, though the signal-to-noise ratio was much poorer due to lower surface coverage.10 As to the SC6S/Au electrode, the S 2p peaks are observed in the 161.5-165.0 eV region (peaks: 162.0, 163.2, and 164.8 eV), a slightly higher energy region than the others. This
compound
S 2p3/2
S 2p1/2
RuC11VC2S RuC7VC6S RuC7VC2S RuC3VC6S RuC17S Ru(C7VC6S) C16S/Au HSC6S/Au
161.2 161.7 161.2 161.5 161.4 161.7 162.0 161-163
162.2 163.1 162.2 162.6 162.7 162.7 163.0 163-165
is due to the sum of the peaks based on the free S (no S-Au linkage) and the bound S (S-Au linkage) species; the former peak appears in somewhat higher energy region,14 for example, 164.0 (S 2p3/2) and 165.3 eV (S 2p1/2) for ethanethiol.5b The results also suggest the formation of a S-Au linkage for the RuC7VC6S/Au and the Ru(C7VC6S)/Au. Other compounds showed similar bands and the peak positions are summarized in Table 2. Some CVs of the modified electrodes, RuC7VC6S/Au, C12VC6S/Au, and C12VC2S/Au, measured in the aqueous solution of 0.1 M NaClO4, are shown in Figure 6, where the DPVs for the C12VC2S/Au are shown in the inset of Figure 6b. In each electrode, the redox waves due to V2+/+ were clearly observed in the -0.4 to -0.5 V region. The peak currents were proportional to the scan rate, indicat(14) Photoinduced Electron Transfer; Fox, M. A., Chanon, X, Eds.; Elsevier: Amsterdam, Vol. D, 1988.
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Table 3. Surface Coverage and Photocurrent Intensity of SAM Formed on the Gold Electrode
compound
surface coveragea/×10-11 mol/cm2
relative photocurrent intensityb
RuC11VC2S RuC7VC6S RuC7VC2S RuC3VC6S RuC17S C12VC6S C12VC2S
4.0 8.9 3.1 5.3 11.8 12.1 7.8
3.4 4.5 6.0 1.6 1 negligible negligible
a Evaluated from the integration of the CV curves of V2+/V+ and of the DPV curves of Ru3+/2+. b Measured at 0.5 V. Intensity of RuC17S/Au was normalized to unity (corresponding to nA/cm2). c Ru(C VC S) 3.6 × 10-11 mol/cm2. 7 6
ing immobilization of the compound on the gold surface. Integration of the areas of the reduction waves of CV peaks as well as those of the DPV peaks allowed an evaluation of the surface coverages of the compounds. The results are summarized Table 3. All linked compounds gave comparable values in the (3-9) × 10-11 mol/cm2 region. The surface coverage of RuC17S (1.2 × 10-10 mol cm-2, determined from DPV measurements) was considerably larger than RuCnVCmS, probably because the electrostatic repulsion among the assembly was reduced at least to some extent due to the lack of the V2+ moiety. The surface coverage of RuC17S was further larger than that of Ru2+V2+ linked systems, because unwanted electrostatic repulsion was smaller in RuC17S as compared with Ru2+V2+ linked systems. A considerably lower coverage for Ru(C7VC2S) may be at least in part due to increased intramolecular and intermolecular electrostatic repulsions among the positively charged Ru2+ and V2+ moieties. The surface coverage of C12VC6S is about 1.5 times higher than that of C12VC2S. This is most probably attributed to the difference of the electrostatic effects among the viologen moieties in the monolayer assembly. The effect of electrostatic repulsion must be larger for the assembly of C12VC2S having the shorter spacer-chain length between the viologen moiety and the sulfur atom. The viologen moiety of C12VC2S ought to be assembled in looser packing on the electrode surface, because the conformational change for relieving the electrostatic repulsion must be unlikely after anchoring on the electrode surface via the thiol group. Similar tendencies for the surface coverages are also seen in the Ru2+-V2+ linked systems. Photocurrent action spectra for RuC7VC2S/Au and RuC17S/Au electrodes at 0.2 V vs Ag/AgCl (saturated KCl) in the presence of TEOA (5 × 10-2 M) as the sacrificial reagent are shown in Figure 7. The photocurrent was negligibly small in the absence of TEOA under the deaerated condition. Clearly, the action spectrum of the RuC7VC2S/Au electrode correlated well with the corresponding absorption spectrum in solution. Thus, the Rucomplex moiety certainly acts as the photoactive site. In the case of the RuC17S/Au electrode, the anodic photocurrent was substantially smaller than that from the RuC11VC2S/Au electrode (vide infra), though the broad peak of the action spectrum was still observed around the absorption peak (Figure 7b). Figure 8a shows applied potential dependencies of the photocurrents per one Ru2+ unit in the modified electrodes: RuCnVCmS/Au and RuC17S/Au. Intensities of photocurrents at 0.5 V were in the order RuC7VC2S/Au > RuC11VC2S/Au > RuC7VC6S/Au > RuC3VC6S/Au > RuC17S/Au electrode. If one assumes that all immobilized compounds are taking extended conformations as to the electrode surface, the photocurrent is larger for a smaller
Figure 7. Photocurrent action spectra of (a) RuC7VC2S/Au and (b) RuC17S/Au electrode and their corresponding electronic absorption spectra (solid lines) in acetonitrile solutions. Conditions for photocurrent measurements: ∆λ ) (16 nm; E ) 0.2 V vs Ag/AgCl (saturated KCl); [TEOA] ) 5 × 10-2 M; [NaClO4] ) 0.1 M.
Figure 8. Applied potential dependencies of photocurrents from RuCnVCmS/Au, Ru(C7VC6S)/Au, and RuC17S/Au electrodes: a set of n and m for RuCnVCmS are shown in as (n, m) in the figure. Conditions: λex ) 470 ( 16 nm; [TEOA] ) 5 × 10-2 M; [NaClO4] ) 0.1 M.
distance between the viologen moiety and the electrode and for moderate distance between Ru2+ and V2+ moieties. Photocurrents are also compared with the different number of the V2+ moieties, as shown in Figure 8b. The Ru(C7VC6S)/Au electrode gave roughly two times photocurrents than the RuC7VC6S/Au electrode. As for Ru(C7VC2S), there are two pathways for the photoinduced electron transfer from *Ru2+ to V2+. In fact, the efficient electron transfer quenching is observed in Ru(C7VC2S) (Figure 4). Judging from the luminescence spectra, the
Self-Assembled Monolayers
Figure 9. Photocurrent generation diagram including the electron flow.
photoinduced electron transfer occurs more effectively in the presence of the two V2+ moieties; this is not contradictory to the larger photocurrent in the Ru(C7VC6S)/Au electrode. Unfortunately, it is not clear at this stage how effectively the V+ and the V2+ moieties cooperate with each other to retard the reverse electron transfer. Anyway, a larger ratio of V2+ to Ru2+ can be favorable for obtaining a larger photocurrent under the identical coverage of the SAM. The mechanism for the observed anodic photocurrent for the RuCnVCmS/Au electrode is shown in Figure 9. First, a redox pair Ru3+-V+ will be generated by photoinduced electron transfer from *Ru2+ to V2+. The oxidized Rucomplex, Ru3+, will be easily reduced to the initial Ru2+ by TEOA, since the Ru2+ moiety is exposed to the bulk solution. Accordingly, the reverse electron transfer should be suppressed and the concomitant electron transfer from V+ to the electrode will be increased, responsive to the anodic photocurrent. The electron transfer from V+ to the electrode must be superior for a shorter distance between V+ and the electrode, because the RuC7VC2S/Au electrode afforded the larger photocurrent than the RuC7VC6S/Au electrode. On the other hand, the orders of photocurrents from the linked systems were as follows: RuC7VC2S/Au > RuC11VC2S/Au, and RuC7VC6S/Au > RuC3VC6S/Au. The direct electron transfer from *Ru2+ to the electrode is
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substantially smaller as compared with that from *Ru2+ to V2+, because the photocurrent from the RuC17S/Au electrode is much smaller than those from the RuCnVCmS/ Au electrodes. In general, forward and reverse electron transfer rates decrease with increasing the spacer-chain length between the donor and the acceptor.14 As has been determined previously, forward and reverse electron-transfer reaction rates (kf and kb) between Ru2+ and V 2+ in the Ru2+(CH2)n-V2+ linked system (quite similar to RuCnVCmS) are comparable (ln kf ) 21.4, ln kb ) 22.6) in the case of n ) 3, while the forward rate is remarkably smaller for n ) 7 (ln kf ) 17.66, kb not determined).13 Since the reverse electron-transfer rate from V+ to Ru3+ must be smaller for a longer spacer-chain length, Ru3+ will be effectively reduced to Ru2+ by TEOA for the larger n values in RuCnVCmS. On the other hand, a larger distance between Ru2+ and V 2+ chromophores is unfavorable for the forward electron transfer from *Ru2+ to V 2+. Thus, there must be an optimum distance between the two chromophores for obtaining the largest photocurrent. In conclusion, the present study demonstrated that there seemed to be some optimum conditions on the spatial distance and the number ratios between Ru2+ and V2+ chromophores to achieve efficient photocurrent generation from the SAMs on the gold electrode. The present results are very instructive for the further improvement of the photocurrent efficiency in the SAMs. At the same time, time-resolved spectroscopic studies must be essential for a skillful design of the donor-acceptor linked thiol derivative. Acknowledgment. The present study was partially supported by Grant-in-Aids for Scientific Research (No. 11167266 and 13022254) from The Ministry of Education, Culture, Sports, Science, and Technology. The authors also thank the center of Advanced Instrumental Analysis, Kyushu University, for NMR measurements and Mr. H. Horiuchi for his skill in preparing a photoelectrochemical cell. LA025936V
