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Concentration Effects of Porphyrin Monolayers on the Structure and Photoelectrochemical Properties of Mixed Self-Assembled Monolayers of Porphyrin and Alkanethiol on Gold Electrodes Hiroshi Imahori,* Taku Hasobe, Hiroko Yamada, Yoshinobu Nishimura,† Iwao Yamazaki,*,† and Shunichi Fukuzumi* Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, Japan Science and Technology Corporation, Suita, Osaka 565-0871, Japan, and Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received January 3, 2001. In Final Form: April 16, 2001 A systematic series of mixed self-assembled monolayers (SAMs) of porphyrin and alkanethiol have been prepared to examine the concentration effects of porphyrin monolayers on the structure and photoelectrochemical properties of the mixed SAMs on a gold electrode. The measurements of ultraviolet-visible absorption spectroscopy in transmission mode and cyclic voltammetry of mixed SAMs of porphyrin and alkanethiol have revealed that the porphyrin interaction in the mixed SAMs decreases with increasing the relative ratio of the alkanethiol to the porphyrin. Photoelectrochemical measurements were performed in an argon-saturated Na2SO4 aqueous solution containing methyl viologen as an electron carrier using the modified gold working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode. The quantum yields of the photocurrent generation remain constant (0.26 ( 0.04%) with an increase in the relative ratio of the alkanethiol to the porphyrin in the mixed SAMs, which is consistent with the constant porphyrin fluorescence lifetime (∼40 ps), irrespective of the porphyrin ratio in the mixed SAMs on the gold electrode.
Introduction Electron transfer (ET) in organized molecular assemblies constitutes a topic of great interest owing to its vital role in biological processes.1 In particular, photoinduced ET reactions in organized media have been studied extensively toward the construction of artificial photosynthesis. Well-defined arrangements of chlorophylls and quinones in reaction center complexes of photosynthesis are responsible for the vectorial multistep ET relay from the special pair (chlorophyll dimer) to the quinones embedded in the protein matrix.2 This generates a longlived, charge-separated state across the membrane with nearly 100% quantum yield, which eventually leads to the production of chemical energy or the equivalent.2 The kinetic and spectroscopic studies on ET processes in the corresponding artificial molecular assemblies have provided useful information for understanding the relationship between structure and function as well as for application as artificial photosynthetic materials.3-12 A number of artificial photosynthetic molecular assemblies have been developed using conventional methodologies such as Langmuir-Blodgett films and lipid bilayer * To whom correspondence should be addressed.E-mail: imahori@ ap.chem.eng.osaka-u.ac.jp;
[email protected]; fukuzumi@ ap.chem.eng.osaka-u.ac.jp. † Hokkaido University. (1) (a) A New Developments in Construction and Functions of Organic Thin Films; Kajiyama, T., Aizawa, M., Eds.; Elsevier: Amsterdam, 1996. (b) A Molecular Electronics; Jortner, J., Ratner, M., Eds.; Blackwell: Oxford, 1997. (c) Electron Transfer in Chemistry; Balzani, V. Ed.; Wiley-VCH: Weinheim, 2001. (2) (a) Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.; Kluwer Academic Publishers: Dordrecht, 1995. (b) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, 1993. (c) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517.
membranes to improve the conversion efficiency of light energy into photocurrent or chemical energy as macro(3) (a) Seta, P.; Bienvenue, E.; Moore, A. L.; Mathis, P.; Bensasson, R. V.; Liddell, P. A.; Pessiki, P. J.; Joy, A.; Moore, T. A.; Gust, D. Nature 1985, 316, 653. (b) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1997, 385, 239. (c) SteinbergYfrach, G.; Rigaud, J.-L.; Durantini, E. N.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1998, 392, 479. (d) Seta, P.; Bienvenue, E.; Maillard, P.; Momenteau, M. Photochem. Photobiol. 1989, 49, 537. (4) (a) Fujihira, M.; Nishiyama, K.; Yamada, H. Thin Solid Films 1985, 132, 77. (b) Yamazaki, I.; Tamai, N.; Yamazaki, T.; Murakami, A.; Mimuro, M.; Fujita, Y. J. Phys. Chem. 1988, 92, 5035. (c) Fujihira, M. Mol. Cryst. Liq. Cryst. 1990, 183, 59. (d) Aoki, A.; Abe, Y.; Miyashita, T. Langmuir 1999, 15, 1463. (e) Morita, T.; Kimura, S.; Imanishi, Y. J. Am. Chem. Soc. 1999, 121, 581. (5) (a) Hsu, Y.; Penner, T. L.; Whitten, D. G. J. Phys. Chem. 1992, 96, 2790. (b) Kim, Y.-S.; Liang, K.; Law, K.-Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 984. (c) Choudhury, B.; Weedon, A. C.; Bolton, J. R. Langmuir 1998, 14, 6192. (d) Choudhury, B.; Weedon, A. C.; Bolton, J. R. Langmuir 1998, 14, 6199. (e) Fungo, F.; Otero, L. A.; Sereno, L.; Silber, J. J.; Durantini, E. N. J. Mater. Chem. 2000, 10, 645. (f) Martı´n, M. T.; Mo¨bius, D. Supramol. Sci. 1997, 4, 381. (6) (a) Fox, M. A. Top. Curr. Chem. 1991, 159, 68. (b) Fox, M. A. Acc. Chem. Res. 1992, 25, 569. (c) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141. (d) Fox, M. A. Acc. Chem. Res. 1999, 32, 201. (7) (a) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (b) Bonhote, P.; Moser, J.-E.; Humphry-Baker, R.; Vlachopoulos, N.; Zakeeruddin, S. M.; Walder, L.; Gra¨tzel, M. J. Am. Chem. Soc. 1999, 121, 1324. (c) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (8) (a) Mallouk, T. E.; Harrison, D. J. In Interfacial Design and Chemical Sensing; Mallouk, T. E., Harrison, D. J., Eds.; ACS Symposium Series 561; American Chemical Society: Washington, DC, 1994. (b) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (c) Kaschak, D.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435. (d) Kaschak, D. M.; Johnson, S. A.; Waraksa, C. C.; Pogue, J.; Mallouk, T. E. Coord. Chem. Rev. 1999, 185-186, 403. (9) (a) Molecular Level Artificial Photosynthetic Materials; Meyer, G. J., Ed.; Wiley: New York, 1997. (b) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. J. Phys. Chem. B 1997, 101, 2591. (c) Kleverlaan, C. J.; Indelli, M. T.; Bignozzi, C. A.; Pavanin, L.; Scandola, F.; Hasselman, G. M.; Meyer, G. J. J. Am. Chem. Soc. 2000, 122, 2840.
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scopic quantity.3-12 Self-assembled monolayers (SAMs) seem to be alternative attractive candidates for artificial photosynthetic materials, since they are relatively stable and can be readily formed in a highly ordered manner on conducting or semiconducting substrates.13 SAMs of porphyrins on flat gold substrates or equivalents have thereby attracted considerable attention as artificial photosynthetic materials.14-20 We have reported a variety of photoactive SAMs comprising porphyrin16 or fullerene17 or both18 that reveal photoinduced multistep ET and energy transfer (EN) on gold or ITO19 surfaces. The quantum yields of the photocurrent generation, based on the adsorbed photons, in the ferrocene-porphyrin-C60 triad cells were reported to be 20-50%,18c,d,g which are the highest values ever reported for photocurrent generation at monolayer-modified metal electrodes and artificial photosynthetic mem(10) (a) Tkachenko, N. V.; Tauber, A. Y.; Hynninen, P. H.; Sharonov, A. Y.; Lemmetyinen, H. J. Phys. Chem. A 1999, 103, 3657. (b) Pevenage, D.; Van der Auwear, M.; De Schryver, F. C. Langmuir 1999, 15, 4641. (c) Tkachenko, N. V.; Vuorimaa, E.; Kesti, T.; Alekseev, A. S.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. J. Phys. Chem. B 2000, 104, 6371. (11) (a) Kamat, P. V. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, Germany, 1998; p 207. (b) Kamat, P. V.; Barazzouk, S.; Hotchandani, S.; Thomas, K. G. Chem. Eur. J. 2000, 6, 3914. (12) (a) Hurst, J. K.; Thompson, D. H. P.; Connolly, J. S. J. Am. Chem. Soc. 1987, 109, 507. (b) Mauzerall, D.; Hwang, K. C. Nature 1993, 361, 138. (c) Khairutdinov, R. F.; Hurst, J. K. Nature 1999, 402, 509. (13) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (14) (a) Byrd, H.; Suponeva, E. P.; Bocarsly, A. B.; Thompson, M. E. Nature 1996, 380, 610. (b) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258. (c) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J.; Katz, E.; Willner, I.; Du¨rr, H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480. (15) (a) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (b) Kondo, T.; Yanagida, M.; Nomura, S.-i.; Ito, T.; Uosaki, K. J. Electroanal. Chem. 1997, 438, 121. (c) Koide, Y.; Terasaki, N.; Akiyama, T.; Yamada, S. Thin Solid Film 1999, 350, 223. (d) Morita, T.; Kimura, S.; Kobayashi, S.; Imanishi, Y. J. Am. Chem. Soc. 2000, 122, 2850. (e) Kondo, T.; Yanagida, M.; Zhang, X.-Q.; Uosaki, K. Chem. Lett. 2000, 964. (f) Hatano, T.; Ikeda, A.; Akiyama, T.; Yamada, S.; Sano, M.; Kanekiyo, Y.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2000, 909. (16) (a) Akiyama, T.; Imahori, H.; Sakata, Y. Chem. Lett. 1994, 1447. (b) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335. (c) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J. Phys. Chem. B 2000, 104, 1253. (d) Imahori, H.; Arimura, M.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 335. (17) (a) Imahori, H.; Azuma, T.; Ozawa, S.; Yamada, H.; Ushida, K.; Ajavakom, A.; Norieda, H.; Sakata, Y. Chem. Commun. 1999, 557. (b) Imahori, H.; Azuma, T.; Ajavakom, A.; Norieda, H.; Yamada, H.; Sakata, Y. J. Phys. Chem. B 1999, 103, 7233. (c) Hirayama, D.; Yamashiro, T.; Takimiya, K.; Aso, Y.; Otsubo, T.; Norieda, H.; Imahori, H.; Sakata, Y. Chem. Lett. 2000, 570. (18) (a) Akiyama, T.; Imahori, H.; Ajavakom, A.; Sakata, Y. Chem. Lett. 1996, 907. (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. Soc. Jpn. 1999, 72, 485. (c) Imahori, H.; Yamada, H.; Ozawa, S.; Ushida, K.; Sakata, Y. Chem. Commun. 1999, 1165. (d) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B, 2000, 104, 2099. (e) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537. (f) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445. (g) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (19) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Chem. Commun. 2000, 1921. (20) (a) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek, B. M.; Wall, C. G.; Ou, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem. 1994, 33, 3952. (b) Chrisstoffels, L. A. J.; Adronov, A.; Fre´chet, J. M. Angew. Chem., Int. Ed. 2000, 39, 2163. (c) Imahori, H.; Nishimura, Y.; Norieda, H.; Karita, H.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. Chem. Commun. 2000, 661. (d) Morita, T.; Kimura, S.; Kobayashi, S.; Imanishi, Y. Chem. Lett. 2000, 676.
Figure 1. Bis(porphyrin) disulfide 1, undecanethiol 2, porphyrin reference 3, and mixed SAMs of 1 and 2 on Au(111) (1,2/Au).
branes.3-20 In this system, however, the incident photon to photocurrent efficiency (IPCE) has been limited by the poor light-harvesting efficiency of the single-component SAMs. To improve the overall efficiency for the energy conversion, it is highly desired to develop multicomponent SAMs each of which has a different function such as lightharvesting or charge separation. The structure and photoelectrochemical properties of one component may or may not be affected by another component of mixed SAMs. However, such concentration effects of mixed SAMs have not been fully understood.18g,20 We focus on the concentration effects of the porphyrin component on the structure and photoelectrochemistry of mixed SAMs of bis(porphyrin) disulfide 1 and alkanethiol 2 on gold electrodes (Figure 1) in detail based on the UVvis absorption spectroscopy, cyclic voltammetry, fluorescence lifetime, and photoelectrochemical measurements. We have previously reported the chain length effects on the structures and photoelectrochemical properties of the porphyrin SAMs in depth.16c In particular, porphyrin disulfide 1 has been extensively studied to provide wellordered monolayers on the gold electrodes,16c as in the case of conventional alkanethiols with a long methylene chain on a gold surface.13 Thus, 1 combined with 2, which has a similar length of methylene spacer, on the gold electrodes provides an ideal system for evaluating the concentration effects of 1 on the structures and the photoelectrochemical properties of the mixed SAMs. Experimental Section Materials and Preparation. The synthesis and characterization of 1 and 3 were described previously.16c Undecanethiol
Concentration Effects of Porphyrin Monolayers 2 was obtained from Tokyo Kasei Organic Chemicals. Glucose, glucose oxidase, and catalase were purchased from Wako Pure Chemical Industries, Ltd. The gold electrodes (Au(111)) were prepared by a vacuum deposition technique with gold (1000 Å) onto fresh mica (roughness factor (R))1.1) or with gold (200 Å) onto transparent glass slide (R ) 1.5) for the measurements of absorption spectra in transmission mode.16c The gold electrodes on mica were annealed with hydrogen flame for 30 s immediately prior to immersion into the solutions. Monolayers of mixtures of 1,2/Au were formed by the coadsorption onto Au(111)/mica substrates. The coadsorption onto the gold surface was carried out from CH2Cl2 solutions containing 1 and 2 with the total concentration of 10 µM for 20 h to complete the mixed SAM formation. After soaking, the gold substrate was washed well with CH2Cl2 and dried with a stream of argon. Estimate of Surface Coverage. All electrochemical studies were performed on a Bioanalytical Systems, Inc. CV-50W voltammetric analyzer using a standard three-electrode cell with a modified gold working electrode (0.48 cm2), a platinum wire counter electrode, and an Ag/AgCl (saturated KCl) reference electrode.16c The adsorbed amount of 1 in 1,2/Au was determined from the charge of the first anodic peak of the porphyrin, whereas 2 in 1,2/Au was also estimated using the calculated value of 1 at the gold surface, assuming that the molecular area of 2 in the mixed SAMs is the same as that of the densely packed pure alkanethiols (21 Å2).21 Photoelectrochemical Measurements. Photoelectrochemical measurements were performed in a one-compartment Pyrex UV cell (5 mL) under argon atmosphere.16c The cell was illuminated with monochromatic excitation light through interference filters (MIF-S, Vacuum Optics Corporation of Japan) by a 180 W UV lamp (Sumida LS-140UV) on the SAM of 0.48 cm2. Unless otherwise stated, an argon-saturated 0.1 M Na2SO4 and 5 × 10-3 M methyl viologen aqueous electrolyte solution was used. The photocurrent was measured with a three-electrode arrangement: a modified gold working electrode, a platinum wire counter electrode (the distance between the electrodes is 0.3 mm.), and an Ag/AgCl (saturated KCl) reference. The light intensity was monitored by an optical power meter (Anritsu ML9002A) and corrected. Oxygen was removed by addition of excess glucose, glucose oxidase, and catalase into 5 mL of argonsaturated 0.1 M Na2SO4 aqueous electrolyte solution to examine the oxygen effect as an electron carrier.12b,22 Quantum yields were calculated on the basis of the number of photons absorbed by the chromophore on the gold electrodes at 428.8 nm using the input power (6.0 mW cm-2), the photocurrent density, and the absorbance determined from the absorption spectrum on the gold electrode. The absorbance at 428.8 nm was converted into the net absorbance, including contribution of the reflection on the gold surface, using the reflectivity of the incident light (38.7% at 428.8 nm), and the roughness factor of the gold electrodes. Fluorescence Lifetime Measurements. Fluorescence decays were measured by using a femtosecond pulse laser excitation and a single-photon counting system for fluorescence decay measurement.23 The laser system was a mode-locked Ti:Sa laser (Coherent, Mira 900) pumped by an argon ion laser (Coherent, Innova 300). The repetition rate of a laser pulse was 2.9 MHz with a pulse picker (Coherent, Model 9200). The second harmonic generated by an ultrafast harmonic system (Inrad, Model 5-050) was used as an excitation source. The excitation wavelength was set at 435 nm, and temporal profiles of fluorescence decay and rise were recorded by using a microchannel plate photomultiplier (Hamamatsu R3809U). Full-width at half-maximum (fwhm) of the instrument response function was 36 ps, where the time interval of the multichannel analyzer (CANBERRA, Model 3501) was 2.6 ps in the channel number. The fluorescence decays were measured at 655 nm for the porphyrin moiety. Criteria for the best fit were the values of χ2 and the Durbin-Watson parameters, obtained by nonlinear regression. (21) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (22) Benesch, R. E.; Benesch, R. Science 1953, 118, 447. (23) (a) Boens, N.; Tamai, N.; Yamazaki, I.; Yamazaki, T. Photochem. Photobiol. 1990, 52, 911. (b) Nishimura, Y.; Yasuda, A.; Speiser, S.; Yamazaki, I. Chem. Phys. Lett. 2000, 323, 117.
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Figure 2. Absorption spectra of mixed SAMs of 1 and 2 on the gold surface from the CH2Cl2 solution with molar ratios of (a) 100:0 (top) and (b) 98:2 (middle) together with absorption spectrum of 3 in CH2Cl2 (bottom). The spectrum of 3 in CH2Cl2 is normalized at the Soret band (top) for comparison. Action spectrum of the Au/1,2 (98:2)/MV2+/Pt cell (middle) is shown as dotted line with circles.
Results and Discussion Spectroscopic Studies on Porphyrin SAMs. Monolayers of mixtures of 1 and 2 were formed by the coadsorption of 1 and 2 onto the gold surfaces (hereafter denoted as 1,2/Au, where / represents an interface). The coadsorption onto the gold surfaces was carried out from CH2Cl2 solutions containing 1 and 2 with the total concentration of 10 µM [molar ratio of 1:2 ) (a) 100:0, (b) 98:2, (c) 90:10, (d) 75:25, (e) 50:50] for 20 h to complete the mixed SAM formation. After soaking, the gold substrate was washed well with CH2Cl2 and dried with a stream of argon. Figure 2 shows the absorption spectra of 1/Au (1:2 ) (a) 100:0, top)16c and 1,2/Au (1:2 ) (b) 98:2, middle) in transmission mode and of porphyrin reference 316c (Figure 1) in CH2Cl2 (bottom). The Soret band of 1/Au on the gold surface [(a) λmax ) 428 nm] is broadened and red-shifted by 7 nm relative to the corresponding spectrum of 3 in CH2Cl2 (λmax ) 421 nm).24 Similar red-shift and broadening were observed for 1,2/Au (entries b-e).25 The results are summarized in Table 1. It is interesting to note that the Soret band becomes blue-shifted [λmax ) (a) 428 nm, (b) 425 nm, (c) 423 nm, (d) 422 nm, (e) 422 nm] together with decreasing the intensity [absorbance at the Soret peak ) (a) 0.034, (b) 0.017, (c) 0.011, (d) 0.0068, (e) 0.0045] when the relative ratio of the alkanethiol to the porphyrin in the soaking solutions increases from (a) 0% to (e) 50%. This indicates that the π-π interaction between the porphyrin decreases with an increase in the relative ratio of the alkanethiol to the porphyrin in the mixed SAMs. Electrochemistry. The cyclic voltammetric measurements of 1,2/Au in CH2Cl2 containing 0.1 M n-Bu4NPF6 (24) The red-shift (7 nm) and broadening of the SAM of 1 on Au(111) [1:2 ) (a) 100: 0] were reported to be due to the partially stacked sideby-side porphyrin aggregation in the SAM.16c (25) Owing to the poor signal-to-noise ratio of the Q-bands on the gold electrodes, it was impossible to obtain reliable spectra at the wavelength longer than 500 nm.
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Table 1. Concentration Dependence of Spectral Change and Surface Coverage
molar ratio 1:2a solution
1:2b mixed SAM
(a) 100:0
100:0
(b) 98:2
43:57
(c) 90:10
28:72
(d) 75:25
17:83
(e) 50:50
14:86
electrochemistry surface coverage Γ/× 10-10 mol cm-2 (molecular area/Å2 molecule-1) 1c 2c 1.5 (110) 1.2 (140) 1.0 (160) 0.78 (210) 0.68 (240)
3 in CH2Cl2
1.6 (100) 2.6 (64) 3.8 (44) 4.3 (39)
absorption spectroscopy surface coverage Γ/× 10-10 mol cm-2 absorbanced (λmax/nm e) 0.034 (428) 0.017 (425) 0.011 (423) 0.0068 (422) 0.0045 (422) (421)
1f 1.5 0.75 0.49 0.30 0.20
a
Molar ratio of 1:2 in CH2Cl2 solutions with the total concentration of 10 µM for formation of the mixed SAMs. b Molar ratio of 1:2 on the gold electrodes estimated from the electrochemical measurements. c Estimated from the electrochemical measurements, assuming that one molecule of the porphyrin and alkanethiol moieties in the mixed SAMs occupies the same area as those of pure SAMs of 1 (110 Å2 molecule-1) and 2 (21 Å2 molecule-1), respectively. d Absorbance at the Soret band. e λmax at the Soret band. f Estimated from the UV-vis spectroscopic measurements, assuming that the molar absorption coefficient of the porphyrin at the Soret band in the mixed SAMs is the same as that in the pure SAM of 1. The molar absorption coefficient was calculated from the absorbance at the Soret band and the surface coverage obtained using the electrochemical measurements.
Figure 3. Cyclic voltammograms of mixed SAMs of 1 and 2 on the gold surface from the CH2Cl2 solution with molar ratios of (a) 100:0 (top) and (b) 98:2 (middle and bottom). The cyclic voltammograms of 1,2/Au in CH2Cl2 containing 0.1 M n-Bu4NPF6 electrolyte are obtained with a sweep rate of 50 mV-1 (electrode area, 0.48 cm2).
were performed with a sweep rate of 50 mV s-1 (electrode area, 0.48 cm2) to estimate the surface coverage. Two successive redox couples, corresponding to the first and the second oxidation of the porphyrin moiety [E1/2 ) +1.10, +1.33 V vs Ag/AgCl (saturated KCl)], are observed for 1/Au (1:2 ) (a) 100:0; Figure 3, top), as described previously.16c The cyclic voltammogram of 1,2/Au (1:2 ) (b) 98:2) in CH2Cl2 is characterized by two successive anodic waves showing a well-defined current maximum, but no corresponding cathodic waves are observed on the reversed scan due to the instability of the radical cation produced by the anodic oxidation (Figure 3, middle).26 This agrees well with the fact that a first scan limited to the potential sweep of the first oxidation step is reversible
(Figure 3, bottom). Similar irreversible redox behavior was also observed for 1,2/Au [1:2 ) (c) 90:10, (e) 50:50]. The intensity of the anodic current decreases with an increase in the relative ratio of the alkanethiol to the porphyrin in the mixed SAMs. The redox potentials of porphyrin 3 in the CH2Cl2 solution have been reported to be E1/2 ) +0.91, +1.25 V vs Ag/AgCl (saturated KCl), which are shifted to the negative direction by 80-190 mV compared to those of 1/Au [1:2 ) (a) 100:0].16c This difference can be explained as a consequence of the decreased dielectric constant in the nonpolar monolayer as compared to that of bulk solution.27 The successive negative shift of the anodic peak potentials [Ep ) (a) 1.13, 1.35 V, (b) 1.07, 1.30 V, (c) 1.06, 1.30 V, (e) 1.03, 1.30 V] with increasing the relative ratio of the alkanethiol to the porphyrin is consistent with an increase in the dielectric constant around the porphyrin moiety, due to the exposure of the porphyrin into the electrolyte solution (Table 2). Integration of the area under the anodic surface waves due to the first oxidation of the porphyrin provides an estimate of the surface coverage of the porphyrin, Γ. The results are summarized in Table 1. The Γ values of the porphyrin ((0.68-1.5) × 10-10 mol cm-2) become smaller as the relative ratio of the alkanethiol to the porphyrin increases in the soaking solutions. The Γ values of the porphyrin ((0.20-1.5) × 10-10 mol cm-2) were also estimated from the UV-visible spectroscopic data, provided that the absorption coefficient of the porphyrin at the Soret band in the mixed SAMs is the same as that in the pure porphyrin SAM (2.3 × 108 mol-1 cm2).28 Assuming that (i) molecular areas of the porphyrin and the alkanethiol in the mixed SAMs are the same as those of densely packed pure SAMs of 1 (110 Å2)16c and 2 (21 Å2)21 (26) Similar irreversible electrochemical behavior has been reported for porphyrin SAMs on the gold electrodes where the methylene spacer between the porphyrin and the gold electrode is short.16c (27) Synthetic porphyrin dimers in close proximity generally exhibit the negative shift of the oxidation potential due to the interaction between the porphyrins.16c However, the peak potentials due to the oxidations of the porphyrin in the SAMs are shifted to the positive direction rather than the negative direction. This indicates the weak interaction between the porphyrins in the mixed SAMs.16c (28) The surface coverage estimations of 1 obtained from electrochemical and absorption measurements contradict each other. This may be ascribed to the possible inaccuracy of the molar absorption coefficient of the porphyrin in the mixed SAMs as well as the inhibited ion transport and the reduced electrochemical accessbility of the densely packed porphyrin in the SAM of 1.16c,29
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Table 2. Electrochemical and Photoelectrochemical Data molar ratioa 1:2 (a) 100:0 (b) 98:2 (c) 90:10 (e) 50:50 3 in CH2Cl2
potential/mVb Epc (P0/P1+) Epc (P1+/P2+) 1130 (1100)d 1070 (1060)d 1060 1030 (1020)d (910)d
photocurrent density/nA cm-2 e
absorbance at 428.8 nmf
quantum yield/%g
fluorescence lifetime/psh 40
1350 (1330)d 1300
460
0.040 ( 0.006
0.25 ( 0.04
260
0.020 ( 0.003
0.27 ( 0.04
1300 1300
120 50
0.010 ( 0.002 0.0043 ( 0.0007
0.27 ( 0.04 0.26 ( 0.04
(1250)d
36 39 9700 (benzene) 9800 (THF)
a Molar ratio of 1:2 in CH Cl solutions with the total concentration of 10 µM for formation of the mixed SAMs. b Measured in CH Cl 2 2 2 2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 50 mV s-1 using Ag/AgCl (saturated KCl) reference. c Peak potential of the oxidative d e wave due to the porphyrin moiety. Redox potential (E1/2) of the oxidations of the porphyrin. Obtained in the three-electrode systems under excitation with λ ) 428.8 ( 3.9 nm light of 6.0 mW cm-2 and -100 mV vs Ag/AgCl (saturated KCl). f Absorbance of the monolayers estimated at 428.8 nm including the contribution of reflection and the experimental error. g φ ) (i/e)/[I(1-10-A)], I ) (Wλ)/(hc), where i is the photocurrent density, e is the elementary charge, I is number of photons per unit area and unit time, λ is the wavelength of light irradiation, A is absorbance of the adsorbed dyes at λ nm, W is light power irradiated at λ nm, c is the light velocity, and h is the Planck constant. h Excited at 435 nm and monitored at 655 nm.
and (ii) both 1 and 2 are well-packed on the gold electrodes, the relative ratios of 1:2 in the mixed SAMs prepared from CH2Cl2 solutions containing different ratios of 1:2 are estimated to be (a) 100:0, (b) 43:57, (c) 28:72, (d) 17:83, and (e) 14:86 (Table 1), respectively, based on the electrochemical studies.29 The estimated ratios of 1:2 in the mixed SAMs are significantly smaller than those of the solutions: (b) 98:2, (c) 90:10, (d) 75:25, and (e) 50:50. Although the spacer moieties of 1 and 2 are similar, 1 has an additional bulky porphyrin moiety. Taking into concert the optimized molecular areas of pure 1 (110 Å2)16c and 2 (21 Å2)21 on the gold electrodes, the thermodynamic preference for the alkanethiol adsorption over the porphyrin on the gold surface can be explained by stabilization energy due to about four extra S-Au interactions per an exchange of the porphyrin alkanethiolate with the undecanethiolate. Photoelectrochemical Measurements. Photoelectrochemical measurements were performed for 1,2/Au in an argon-saturated 0.1 M Na2SO4 aqueous solution containing 5 mM methyl viologen (MV2+) as an electron carrier using the modified gold electrode as a working electrode, a platinum counter electrode, and an Ag/AgCl (saturated KCl) reference electrode (hereafter, denoted as Au/1,2/MV2+/Pt cell). The inset in Figure 4 displays currents produced by on-and-off illumination of the Au/ 1,2 (98:2)/MV2+/Pt cell under the excitation of light at a wavelength of 428.8 ((3.9) nm with a power density of 6.0 mW cm-2. The SAM showed a cathodic photoelectronic response when switching the light on and off, which is similar to that of the Au/1,2 (100:0)/MV2+/Pt cell.16c There is a good linear relationship between the intensities of the photocurrent and of the light intensity at each wavelength (from 0.03 to 6.0 mW cm-2). In the threeelectrode systems, an increase in the net cathodic photocurrent with an increase of the negative bias (from 600 to -100 mV) to the gold electrode (Figure 4) demonstrates that the photocurrent flows from the gold electrode to the counter electrode through the SAM and the electrolyte. The dark current is much lower than the net photocurrent within the range of the applied potential to the gold electrode. However, when the more negative or the positive potential was applied to the system, the photoelectrochemical response was not reproducible, implying the collapse of the monolayer structure. (29) The looser packing at higher concentrations of 2 may affect the accuracy of the surface coverage estimations (vide infra).
Figure 4. Photocurrent vs applied potential curves for the Au/1,2 (98:2)/MV2+/Pt cell (solid line with open circles) with monochromic light of 428.8 nm with 6.0 mW cm-2: working electrode, Ag/AgCl (saturated KCl); electrolyte, 0.1 M Na2SO4 containing 5 mM methyl viologen under argon atmosphere. The dark current is shown as a dotted line with triangles. The inset depicts the photoelectrochemical response at applied potential -100 mV vs Ag/AgCl (saturated KCl).
The concentration dependence of the net photocurrent upon methyl viologen was examined for the Au/1,2 (98: 2)/MV2+/Pt cell. Under argon-saturated conditions the net photocurrent at -100 mV vs Ag/AgCl increased linearly with an increase of methyl viologen (up to 2 mM) and became saturated at around 5 mM of methyl viologen, as shown in Figure 5. Such a saturated dependence may be ascribed to formation of a complex between the porphyrin and methyl viologen.30 This shows that methyl viologen acts as electron carrier in the present systems.31 The net photocurrent was enhanced under the oxygen-saturated conditions, demonstrating that oxygen is also an electron (30) For a complex formation between a porphyrin and methyl viologen, see: Andersson, M.; Davidsson, J.; Hammarstro¨m, L.; KorppiTommola, J.; Peltola, T. J. Phys. Chem. B 1999, 103, 3258. (31) It is well-known that methyl viologen and O2 act as an electron acceptor in the similar photoelectrochemical cells.16c O2 bubbling of the electrolyte solution in the Au/1,2 (100:0)/Pt cell increased the photocurrent by ca. 20-30%, and successive argon bubbling of the solution decreased the current nearly to the initial state both in the absence and the presence of MV2+.16c These results indicate that O2 is an efficient electron carrier in the Au/1,2 (100:0)/Pt cell. Addition of MV2+ (up to 5 mM) in the Au/1,2 (100:0)/Pt cell increased the photocurrent by about 15%, showing that MV2+ is also an electron carrier.16c However, further addition of MV2+ did not change the intensity of the photocurrent significantly.16c
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Langmuir, Vol. 17, No. 16, 2001
Figure 5. Concentration dependence of net photocurrent upon methyl viologen for the Au/1,2 (98:2)/MV2+/Pt cell with monochromic light of 428.8 nm with 6.0 mW cm-2 at an applied potential of -100 mV vs Ag/AgCl (saturated KCl): working electrode, Ag/AgCl (saturated KCl); electrolyte, 0.1 M Na2SO4 under argon-saturated (solid line with circles) and oxygensaturated (dotted line with triangles) conditions. The contribution from the dark current has been subtracted.
carrier.31,32 Under the oxygen-saturated conditions, the net photocurrent increased linearly with an increase of methyl viologen (up to 0.5 mM) and then remained constant (>1 mM). Under the excitation of light at a wavelength of 428.8 ((3.9) nm with a power density of 6.0 mW cm-2 at -100 mV bias potential, we obtained a net current density of 260 nA cm-2. Given an absorbance of 0.020 ( 0.003, including the reflection at 428.8 nm for the 1,2 (98:2)/Au, we can estimate the quantum yield of the Au/1, 2 (98: 2)/MV2+/Pt cell to be 0.27 ( 0.04% (Table 2). The action spectrum agrees with the absorption spectrum of 1,2 (98: 2)/Au in the range of 380-500 nm, showing that the porphyrin is the photoactive species (Figure 2, middle). The small difference in shape and the peak position between the two spectra may be due to the difference between the actual spectrum obtained in the transmission mode in air and the real spectrum on the gold surface under the photoelectrochemical conditions, where the porphyrins are in contact with the electrolyte solution.5b,33 Similar photoelectrochemical behavior was observed for the Au/1,2 (90:10 or 50:50)/MV2+/Pt cells. However, dark cathodic current becomes more prominent as the relative ratios of the alkanethiol to the porphyrin increase. Under argon-saturated conditions the dark current in the Au/ 1,2 (98:2)/MV2+/Pt cell increases linearly with an increase of methyl viologen (up to 0.2 mM) and then leveled off (>2 mM). Similar behavior was observed for bare gold electrode, but the intensity is larger by a factor of 4-6 than the former. This indicates direct electron flow from gold electrode to electron carriers in the electrolyte due to the loose packing of the monolayers. The intensities of the net photocurrents at -100 mV and the quantum yields are summarized in Table 2. It should be noted here that the quantum yields reveal no appreciable concentration effects of 1 diluted by the alkanethiol 2 in the mixed SAMs. Fluorescence Lifetime Measurements. To probe the excited singlet state of the porphyrin in the mixed SAMs, (32) When oxygen was scrubbed out from the argon-saturated 0.1 M Na2SO4 aqueous solution with glucose, glucose oxidase, and catalase, which are known to remove oxygen from solutions,12b,22 the effect (i.e., removal of oxygen) for the photocurrent could not be confirmed because of the unstable and large dark current. (33) A similar shift in action spectra was observed for Au/1,2 (90:10 or 50:50)/MV2+/Pt cells.
Imahori et al.
Figure 6. Photocurrent generation diagram indicating the path of electron flow.
time-resolved, single-photon counting fluorescence measurements were performed for 1,2/Au [1:2 ) (a) 100: 0, (c) 90:10, (e) 50:50] as well as 3 in solutions with the excitation wavelength at 435 nm, where the light is absorbed by the porphyrin moiety. In each case the decay of the fluorescence intensity at λobs ) 655 nm (due to the singlet excited states of the porphyrin) can be monitored. The decay curve could be fitted as single exponential, and the results are summarized in Table 2. The fluorescence lifetimes of 1,2 (100:0)/Au at 655 nm (40 ps)16c is much shorter than that of 3 (9.8 ns in THF and 9.7 ns in benzene). This indicates that the excited singlet state of the porphyrin moiety in the SAMs is efficiently quenched by an EN to the gold surface, as described previously.16c Interestingly, the fluorescence lifetimes of the porphyrin moiety in 1,2/Au at 655 nm remain almost constant, irrespective of the relative ratio of the alkanethiol to the porphyrin [(a) 40 ps; (c) 36 ps; (e) 39 ps]. These results show that the interaction between the porphyrin and the gold surface rather than the interaction between the porphyrins is a dominant factor for determining the lifetimes of the porphyrin excited singlet state on the gold surface. Photocurrent generation in the present systems can be explained as follows (Figure 6). Taking into account the potentials of the excited singlet (1P*/P•+ ) -0.80 V vs Ag/ AgCl) and triplet (3P*/P•+ ) -0.30 V vs Ag/AgCl) states of the porphyrin,16c photoirradiation of the modified gold electrode results in intermolecular ET from only the singlet excited state of the porphyrin to the methyl viologen (E0red ) -0.62 V vs Ag/AgCl) or O2 (E0red ) -0.48 V vs Ag/AgCl).16c The reduced electron carrier (MV•+ or O2•-) diffuses to release an electron to the platinum counter electrode, whereas the resultant porphyrin radical cation (P•+) (+1.10 V vs Ag/AgCl) captures an electron from the gold electrode, generating the cathodic current flow. However, there exists a competitive deactivation pathway in the porphyrin excited singlet state by the gold electrode via an EN judging from the extremely short fluorescence lifetimes of the porphyrins (ca. 40 ps) on the gold surface as compared to those (ca. 1-10 ns) on a quartz or semiconductor surface.34 Thus, the quantum yields of the photocurrent generation in the present system may be determined largely by the extent of the porphyrin quenching by the gold surface rather than the interaction between the porphyrins, which (34) Since the bulky tert-butyl groups are introduced at the metapositions of the meso-phenyl groups on the porphyrin ring, self-quenching due to the porphyrin aggregation is significantly reduced, thereby resulting in no apparent effect on the porphyrin fluorescence lifetimes under the present conditions.16c,19 See: Anikin, M.; Tkachenko, N. V.; Lemmetyinen, H. Langmuir, 1997, 13, 3002.
Concentration Effects of Porphyrin Monolayers
decreases with increasing the ratio of the alkanethiol to the porphyrin as indicated by the spectroscopic and electrochemical measurements (vide supra). This is supported by the constant porphyrin fluorescence lifetimes on the gold electrodes irrespective of the ratio of the alkanethiol to the porphyrin.34 In fact, the quenching rate of the porphyrin singlet excited state by ET to methyl viologen is estimated as ∼5 × 107 s-1 at the concentration of methyl viologen (∼5 mM) assuming the photoinduced ET rate constant is diffusion-limited (∼1010 M-1 s-1). Since the photoinduced ET process competes with the quenching by the gold surface (∼2.5 × 1010 s-1), the quantum yield for the photoinduced ET to methyl viologen is estimated to be ∼0.2%, which is consistent with the quantum yield for the photocurrent generation (∼0.2%). In conclusion, the present study has clearly demonstrated that porphyrin interaction in the mixed SAMs is
Langmuir, Vol. 17, No. 16, 2001 4931
not a crucial factor controlling photocurrent generation efficiencies. This conclusion is important for the further development of integrated mixed SAMs exhibiting efficient photoinduced EN and ET, since we can only focus on optimization of each component of mixed SAMs (i.e., antenna molecule and charge separation molecule) without paying attention to each interaction in the mixed monolayers. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research and a Millennium Project (No. 12310) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. H. I. thanks the Sumitomo Foundation for financial support. LA010006H
