Langmuir 1998, 14, 5335-5338
5335
Chain Length Effect on Photocurrent from Polymethylene-Linked Porphyrins in Self-Assembled Monolayers Hiroshi Imahori,*,† Hiroyuki Norieda,† Shinichiro Ozawa,† Kiminori Ushida,‡ Hiroko Yamada,† Takayuki Azuma,† Koichi Tamaki,† and Yoshiteru Sakata*,† The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan, and The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received March 30, 1998. In Final Form: July 17, 1998 Disulfides, with a series of alkyl spacers containing porphyrins at both ends, were prepared to evaluate the effect of the spacer length on the interfacial structure and photoelectrochemical properties of selfassembled monolayers (SAMs) on a gold electrode. The structure of the SAMs was investigated using UV-visible absorption spectroscopy, cyclic voltammetry, and photoelectrochemical studies. These measurements showed that as the length of the spacers increases, the SAMs tend to form a highly ordered structure on the gold electrode. Photoelectrochemical studies, using modified Au and Pt electrodes, were carried out in the presence of methyl viologen as an electron carrier. The photocurrents decrease dramatically with a decrease in the spacer length, indicating that there are two competitive deactivation pathways for the excited porphyrin, i.e., the quenching by the electrode and electron carrier.
In recent years self-assembled monolayers (SAMs) have attracted much attention because of their potential applications in molecular technologies.1 It is well established that thiols and disulfides covalently link to gold surfaces using a S-Au linkage, leading to the formation of highly ordered SAMs. This technique is useful for constructing organized molecular assemblies containing photo- and/or electroactive molecules on gold electrodes. There are a number of reports of SAMs of photoactive molecules, such as porphyrins,2 azobenzenes,3 and so on,4 on gold electrodes. However, a systematic study of the effect of the interfacial structure on the electro- and photochemical properties of these chemically modified electrodes is still lacking.4a,b,5 Here we report on the structure and photoelectrochemistry of SAMs of porphyrins on gold electrodes (Figure 1). The effect of the chain length of the SAMs was systematically investigated using † The Institute of Scientific and Industrial Research, Osaka University. ‡ The Institute of Chemical Research (RIKEN).
(1) In A Introduction to Ultrathin Organic Films; Ulman, A., Ed.; Academic Press: San Diego, CA, 1991. (2) (a) Luttrull, D. K.; Graham, J.; DeRose, J. A.; Gust, D.; Moore, T. A.; Lindsay, S. M. Langmuir 1992, 8, 765. (b) Zak, J.; Yuan, H.; Ho, M.; Woo, K.; Porter, M. D. Langmuir 1993, 9, 2772. (c) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277. (d) Akiyama, T.; Imahori, H.; Sakata, Y. Chem. Lett. 1994, 1447. (e) Chambrien, I.; Cook, M. J.; Russell, D. A. Synthesis 1995, 1283. (f) Shimazu, K.; Takechi, M.; Fujii, H.; Suzuki, M.; Saiki, H.; Yoshimura, T.; Uosaki, K. Thin Solid Films 1996, 273, 250. (g) Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir 1996, 12, 5742. (h) Akiyama, T.; Imahori, H.; Ajavakom, A.; Sakata, Y. Chem. Lett. 1996, 907. (i) Kondo, T.; Ito, T.; Nomura, S.; Uosaki, K. Thin Solid Films 1996, 284-285, 652. (j) Simpson, T. R. E.; Cook, M. J.; Petty, M. C.; Thorpe, S. C.; Russell, D. A. Analyst 1996, 121, 1501. (k) Simpson, T. R. E.; Revell, D. J.; Cook, M. J.; Russell, D. A. Langmuir 1997, 13, 460. (l) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C.-h.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W. Langmuir 1997, 13, 2143. (m) Katz, E.; Willner, I. Langmuir 1997, 13, 3364. (n) Yuan, H.; Woo, L. K. J. Porphyrins Phthalocyanines 1997, 1, 189. (o) Imahori, H.; Azuma, T.; Ushida, K.; Takahashi, M.; Akiyama, T.; Hasegawa, M.; Okada, T.; Sakata, Y. SPIE 1997, 3142, 104. (p) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (q) Crossley, M. J.; Prashar, J. K. Tetrahedron Lett. 1997, 38, 6751.
the UV-visible absorption spectroscopy, cyclic voltammetry, and photoelectrochemistry. Disulfides 1 with different spacer lengths were synthesized as previously described.2d Their structures were verified using analytical methods including 1H NMR and matrix-assisted laser desorption ionization time of flight mass spectroscopy.6 Porphyrins 1 spontaneously self-assembled to form monolayer films on a Au(111)/Cr/Si(100) surface.7 In a typical experiment, a gold electrode was soaked in a (3) (a) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (b) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1996, 12, 5838. (c) Yu, H.-Z.; Wang, Y. Q.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. Langmuir 1996, 12, 2843. (d) Morigaki, K.; Liu, Z.-F.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1995, 99, 14771. (e) Wang, Y.-Q.; Yu, H.-Z.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Liu, Z.-F. Langmuir 1996, 12, 5466. (f) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211. (g) Ye, Q.; Fang, J.; Sun, L. J. Phys. Chem. B 1997, 101, 8221. (h) Yu, H. Z.; Shao, H. B.; Luo, Y.; Zhang, H. L.; Liu, Z. F. Langmuir 1997, 13, 5774. (4) (a) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (b) Ravenscroft, M. S.; Finklea, H. O. J. Phys. Chem. 1994, 98, 3843. (c) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955. (d) Karpovich, D. S.; Blanchard, G. J. Langmuir 1996, 12, 5522. (e) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937. (f) Li, W.; Lynch, V.; Thompson, H.; Fox, M. A. J. Am. Chem. Soc. 1997, 119, 7211. (g) Fox, M. A.; Wooten, M. D. Langmuir 1997, 13, 7099. (h) Yamada, S.; Koide, Y.; Matsuo, T. J. Electroanal. Chem. 1997, 426, 23. (i) Fox, M. A.; Whitesell, J. K.; Mckerrow, A. J. Langmuir 1998, 14, 816. (5) (a) Li, T. T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 6107. (b) Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. J. Phys. Chem. 1993, 97, 6564. (c) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286. (d) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563. (6) Spectral data for 1e: 1H NMR (270 MHz, CDCl3) δ 8.89 (s, 8H), 8.88 (d, J ) 5 Hz, 4H), 8.84 (d, J ) 5 Hz, 4H), 8.17 (d, J ) 8 Hz, 4H), 8.07 (s, 12H), 7.86 (d, J ) 8 Hz, 4H), 7.77 (s, 6H), 7.44 (br.s, 2H), 2.70 (t, J ) 8 Hz, 4H), 2.43 (t, J ) 8 Hz, 4H), 1.4-1.9 (m, 140H), -2.7 (br.s, 4H). TOF-MS 2333 (M + H+). (7) The gold electrode was prepared by vacuum deposition technique with 2000 Å thickness of gold on Si(100) wafer precoated with 50 Å of chromium. Atomic force microscopy (AFM) and X-ray diffraction studies showed that the gold electrode has (111) surface mainly with roughness factor of 1.1.
S0743-7463(98)00351-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/21/1998
5336 Langmuir, Vol. 14, No. 19, 1998
Letters
Figure 3. Cyclic voltammograms of 1a/Au (dotted line) and 1d/Au (solid line) in dichloromethane: 100 mV s-1, electrode area 0.57 cm2, initial potential 0 V.
Figure 1. Self-assembled monolayers of porphyrins 1a-e on gold electrodes
Figure 2. UV-visible absorption spectra of 1a-e/Au. The absorption spectrum of 1e in dichloromethane (dotted line) and action spectrum in Au/1e/MV/Pt cell (dashed line with circles) are shown for a comparison.
dichloromethane solution of 1 (0.1 mM) for 20 h to reach equilibrium.8 After soaking, the electrode was washed thoroughly with dichloromethane and ethanol. Figure 2 shows the absorption spectra of 1/Au as a function of spacer length. The absorbance at the Soret band increases gradually with an increase in the number of methylenes (n ) 1,3,5), while 1c-e/Au (n ) 5,7,10) does not exhibit a substantial change in the intensities. Assuming that the molar absorptivities of 1a-e/Au are similar, this suggests that the amounts of the adsorbed molecules become saturated at around n ) 5. The Soret bands are broadened and red-shifted by about 15 nm relative to the (8) UV-visible absorption measurements were made using a Hitachi U-3500 spectrometer at reflection mode. The time profile of amount of the adsorbed molecules on the Au substrate was monitored using the UV-visible absorption spectroscopy.
corresponding spectra in a dichloromethane solution. A similar red-shift was reported for SAMs containing porphyrins as terminal chromophores.2b,f,p The red-shift of the Soret band is probably due to the excitonic interactions among the porphyrins and aggregation in the monolayer microenvironment.9,10 A small red-shift (up to 3 nm) of the Soret band peak position was observed as the chains became longer. This shift may be ascribed to an increase in the π-π interaction among the porphyrin moieties, despite the existence of the bulky tert-butyl groups at the meso-phenyl groups. Cyclic voltammetry of 1/Au was carried out in dichloromethane solution containing 0.1 M n-Bu4NPF6. Two successive waves, corresponding to the first and the second oxidation of the porphyrin moiety, were clearly seen. The redox waves were irreversible in 1a/Au, while they were reversible in 1b-e/Au.11 A typical example is shown in Figure 3. The differences between the anodic and cathodic peak potentials (∆Ep ) 100-110 mV) are larger than the ideal value (60 mV), indicating slow kinetics of the couple. Integration of the area under the anodic surface waves provides an estimate of the surface coverage, Γ.12 The values of Γ obtained here using the cyclic voltammetry are quite consistent with those obtained using the UVvisible absorption spectroscopy and those of the similar system reported previously.2b-d,f,p The somewhat smaller Γ values obtained using the absorption spectroscopy may be attributed to the reduced molar absorptivity of the porphyrins on the gold surface compared with that in the solution. Assuming that the porphyrins are densely packed with a perpendicular orientation to the gold surface, then the area of one molecule is calculated to be 200 Å2 (20 Å × 10 Å). The expected value of the molecular area is in good agreement with those of 1d/Au and 1e/Au, indicating that densely packed and ordered monolayers are formed in 1d/Au and 1e/Au, whereas a less well(9) (a) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (b) Kroon, J. M.; Sudho¨lter, E. J. R.; Schenning, A. P. H. J.; Nolte, R. J. M. Langmuir 1995, 11, 214. (c) Zhang, Z.; Verma, A. L.; Yoneyama, M.; Nakashima, K.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 4422. (10) Osuka, A.; Maruyama, K. J. Am. Chem. Soc. 1988, 110, 4454. (11) Ill-defined and broad redox waves due to the porphyrin in the similar SAMs were observed in 0.1-1 M HClO4.2b, c (12) It is known that densely packed monolayer films exhibit inhibited ion transport and reduced electrochemical accessibility.3f Thus, coverage is a low limit estimate, and occupied area is an upper limit estimate. Considering that the porphyrins are planar large molecules with bulky substituents at the meso positions, even in the densely packed monolayer films the electrochemical accessibility may be similar to that in the ideal system where surface-confined redox-active molecules are completely electrochemically accessible in the film.
Letters
Langmuir, Vol. 14, No. 19, 1998 5337
Table 1. Surface Coverage and Molecular Occupied Area entry
n
Γ/10-11 mol cm-2 a
occupied areab/Å2 molecule-1
Γ/10-11 mol cm-2 c
1a/Au 1b/Au 1c/Au 1d/Au 1e/Au
1 3 5 7 10
4.4 6.4 6.6 7.9 7.7
380 260 250 210 220
2.1 3.9 4.9 5.4 5.0
a Estimated on the basis of the integration of the cyclic voltammetric curve due to the first oxidation of the porphyrin moiety. b Calculated from the values of Γ obtained using cyclic voltammetry. c Estimated using UV-visible absorption spectroscopy, assuming that the molar absorptivities of 1a-e/Au at the Soret band are the same as those of 1a-e ( ) 8.8 × 105 M-1 cm-1) in dichloromethane.
Figure 5. Schematic representation of photoelectrochemical cell indicating the path of electron flow.
Figure 4. Photocurrents irradiated with a monochromic light of 438.5 nm with 4.4 mW cm-2: working electrode, 1a-e/Au; counter electrode, Pt; electrolyte, 0.1 M Na2SO4 containing 5 mM methyl viologen under argon atmosphere.
ordered structure is formed in 1a-c/Au (Table 1). The fact that the value of 1d/Au and 1e/Au is somewhat larger than that for the compact packing suggests a tilted structure, where the face-to-face stacked porphyrins are inclined to the gold surface with an angle less than 90°.13 Photoelectrochemical measurements were carried out for 1a-e/Au in 0.1 M Na2SO4 solution containing 5 mM methyl viologen (MV) as an electron carrier using the modified gold electrode as a working electrode, a platinum counter electrode, and a Ag/AgCl reference electrode (hereafter, Au/1a-e/MV/Pt, where / represents an interface). In the three-electrode systems, an increase in the cathodic photocurrent with an increase of the negative bias to the gold electrode demonstrates that the photocurrent flows from the cathode to the anode through the electrolyte. Figure 4 shows photocurrents produced by on-off illumination of the Au/1a-e/MV/Pt cell. The intensities of the short-circuit photocurrents for the Au/ (13) Surface coverage of 1e/Au estimated using UV-visible absorption spectroscopy as well as cyclic voltammetry deviates slightly from the continuous trend with an increase of the chain length. This may be related to the orientation of the terminal porphyrin where the number of methylene is even or odd. A similar effect has been observed in selfassembled monolayers with a terminal methyl group. Reflectance Fourier transform infrared spectroscopic experiments as well as atomic force microscopy and scanning tunneling microscopy are in progress to establish the molecular orientation. (a) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (b) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284.
1/MV/Pt cell are an order of magnitude larger than those of the Au/1/Pt cell, indicating the involvement of MV for the former case. Under the excitation of light at a wavelength of 438.5 ((4.9) nm and a power density of 4.4 mW cm-2, we obtained a short-circuit current (Jsc) of 160 nA cm-2 and an open-circuit voltage (Voc) of 230 mV from the Au/1e/MV/Pt cell. Given an absorbance of 0.051 at 438.5 nm for the 1e/Au, we estimate the quantum yield of the Au/1e/MV/Pt cell to be 0.001. The agreement between the action spectrum and the absorption of 1/Au in the range 400-500 nm shows that porphyrin is the photoactive species (Figure 2). The SAM showed a photoelectronic response when switching the light on and off. The photocurrent decreased exponentially, reaching a constant value of approximately half the initial intensity, and maintained during irradiation of 1 h. The intensity of the photocurrent from the Au/1/MV/Pt cell decreased as the alkane chain length decreased, and eventually a negative photocurrent was observed in the Au/1a/MV/Pt cell. A similar negative photocurrent was also observed in bare Au/MV/Pt and bare Au/Pt cells, implying that direct excitation of the bare gold electrode is primarily responsible for the small negative photocurrents. The photocurrent generation trend can be explained using the following mechanism as shown in Figure 5. An electron transfer occurs from the excited singlet (S1 ) 1.90 eV) or triplet state (T1 ) 1.44 eV) of the porphyrin to the MV.14 The reduced MV diffuses to the Pt electrode and transfers the electron, resulting in the generation of the photocurrent. However, there may be a competitive deactivation pathway in the excited porphyrin by the gold electrode. As the number of methylenes decreases, electronic coupling between the porphyrin and the electrode increases, leading to the efficient quenching of the excited porphyrin by the electrode, probably via energy transfer or electron transfer through the spacer.4g,i,15 Therefore, it is concluded that the dramatic decrease of the photocurrent with a decrease of the alkyl chain length is responsible for the more efficient quenching of the excited porphyrin by the electrode rather than the slight (14) The free energy change from the singlet and triplet excited states to the charge-separated states were calculated to be 0.21 and -0.25 eV, respectively. No corrections have been made for any Coulombic effects because it is difficult to assess the effect at the interface. (15) (a) Waldeck, D. H.; Alivisatos, A. P.; Harris, C. B. Surf. Sci. 1985, 158, 103. (b) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Acc. Chem. Res. 1990, 23, 327.
5338 Langmuir, Vol. 14, No. 19, 1998
decrease in the amount of the adsorbed molecules on the gold surface. In conclusion, a chain length effect was observed for SAMs of porphyrins using UV-visible absorption spectroscopy, cyclic voltammetry, and photoelectrochemistry. The relationship between the photocurrent and the interfacial structure of the SAMs has been elucidated for the first time. We are currently investigating the relationship between the SAM’s structure, including the
Letters
packing and the orientation of the porphyrins, and function in detail. Acknowledgment. This work was supported by Grant-in-Aid for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces from Ministry of Education, Science, Sports and Culture, Japan. LA980351F
