Photoinduced Electron Transfer in Self-Assembled Monolayers of

Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, ... and Life Science, Graduate School of Engineering, Osaka Universi...
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Langmuir 2005, 21, 6385-6391

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Photoinduced Electron Transfer in Self-Assembled Monolayers of Porphyrin-Fullerene Dyads on ITO Vladimir Chukharev,* Tommi Vuorinen, Alexander Efimov, and Nikolai V. Tkachenko Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, 33101 Tampere, Finland

Makoto Kimura and Shunichi Fukuzumi Department of Material and Life Science, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan

Hiroshi Imahori Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, PRESTO, Japan Science and Technology Agency (JST), Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Helge Lemmetyinen Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, 33101 Tampere, Finland Received January 11, 2005. In Final Form: April 22, 2005 Two porphyrin-fullerene dyads were synthesized to form self-assembled monolayers (SAMs) on indiumtin oxide (ITO) electrode, with either ITO-porphyrin-fullerene or ITO-fullerene-porphyrin orientations. The dyads contain two linkers for connecting the porphyrin and fullerene moieties and enforcing them essentially to similar geometries of the donor-acceptor pair, and two linkers to ensure the attachment of the dyads to the ITO surface with two desired opposite orientations. The transient photovoltage responses (Maxwell displacement charge) were measured for the dyad films covered by insulating LB films, thus ensuring that the dyads interact only with the ITO electrode. The direction of the electron transfer was from the photoexcited dyad to ITO independent of the dyad orientation. The response amplitude for the ITO-fullerene-porphyrin structure, where the primary intramolecular electron-transfer direction coincides with the direction of the final electron transfer from the dyad to ITO, was 25 times stronger than that for the opposite ITO-porphyrin-fullerene orientation of the dyad. Static photocurrent measurements in a liquid electrochemical cell, however, show only a minor orientation effect, indicating that the photocurrent generation is controlled by the processes at the SAM-liquid interface.

Introduction Molecular self-assembling on surfaces has recently received attention as an important tool in nanotechnology and molecular engineering.1-6 The method allows one to * Corresponding author. E-mail: [email protected]. (1) (a) Delmarre, D.; Meallet, R.; Bied-Charreton, C.; Pansu, R. B. J. Photochem. Photobiol., A 1999, 124, 23. (b) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441. (c) Kang, J. F.; Perry, J. D.; Tian, P.; Kilbey, S. M., II Langmuir 2002, 18, 10196. (d) Liu, X.; Neoh, K. G.; Kang, E. T. Langmuir 2002, 18, 9041. (e) Facchetti, A.; van der Boom, M. E.; Abbotto, A.; Beverina, L.; Marks, T. J.; Pagani, G. A. Langmuir 2001, 17, 5939. (f) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.; Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams, R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257. (g) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. 2003, 107, 12105. (h) Hasobe, T.; Kamat, P. V.; Troiani, V.; Solladie, N.; Ahn, T. K.; Kim, S. K.; Kim, D.; Kongkanand, A.; Kuwabata, S.; Fukuzumi, S. J. Phys. Chem. B 2005, 109, 19. (2) (a) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955. (b) Weber, R.; Winter, B.; Hertel, I. V.; Stiller, B.; Schrader, S.; Brehmer, L.; Koch, N. J. Phys. Chem. B 2003, 107, 7768. (c) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319. (d) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (3) Imahori, H.; Kashiwagi, Y.; Endo, Y.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Araki, Y.; Ito, O.; Fukuzumi, S. Langmuir 2004, 20, 73.

achieve desired functionality at a molecular scale by organizing specific molecular groups with respect to the surface. One of the most successful applications of this kind of molecular arrangements is organic solar cell, where the functional groups are light sensors, the electron donors and acceptors, which are assembled on an electrode surface to generate current under light illumination.3,7 Metal electrodes are a natural choice for assembling molecular layers with photoelectric functions (such as donoracceptor systems). In particular, thiol-modified gold is successfully used to assemble great variety of organic molecules.2 Thin gold layers are rather transparent to incident light, allowing illumination of the active layer through them. They, however, strongly interact with the (4) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (5) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Chem. Commun. 2000, 1921. (6) Hasobe, T.; Imahori, H.; Ohkubo, K.; Yamada, H.; Sato, T.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. J. Porphyrins Phthalocyanines 2003, 7, 296. (7) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15.

10.1021/la0500833 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/21/2005

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chromophores and quench efficiently the excited state.1-3 To overcome this problem, semiconductor electrodes with a large energy gap, for example, indium-tin oxide (ITO), are successfully used for the solar cell applications.3,7 The photocurrent generation at the electrode surface modified by functional molecules is typically a multistep reaction. It includes the light absorption by the molecules on the surface, the primary charge separation inside the organic layers, the charge transfer to the anode electrode, and the charge transfer from the cathode electrode to the molecular layer. The primary charge separation inside the molecular layer has a few advantages over the direct interaction (electron transfer) of an excited molecule with the electrode. First, the interacting donor and acceptor moieties can be placed close to each other to promote the fast and efficient charge separation (CS) at the molecular level, thus reducing negative effects of other quenching mechanisms, typical for densely packed molecular assemblies (aggregates).4,8 Second, by applying multistep charge-transfer strategy, one can achieve a longer charge separation distance and reduce significantly the back charge-transfer probability. Thus, the total efficiency of the photocurrent generation increases, although this advantage comes at the expense of energy loss at each CS step. Finally, a technical advantage is that the primary charge separation in the molecules can be studied and fine-tuned in solutions before building the whole solid structure. Ideally, the molecular layer assembled on the electrode surface should provide a high absorption of the incident light and an efficient primary charge separation in the layer. The direction of the primary CS should be in accord with the direction of the charges generation/recombination at the electrode interfaces to support the total current generation property of the device. Candidates to form molecular layers with properties presented above are porphyrin-fullerene compounds. Recently, we synthesized9 and studied10 dyads with porphyrin and fullerene covalently linked together by two chains, and observed fast and efficient interactions between the donor (porphyrin) and the acceptor (fullerene) moieties. After some chemical modifications of the dyads, it became possible to attach the dyads to ITO either the porphyrin or the fullerene moiety on the ITO side, and to study how these orientations affect the properties of the device. The selection of the dyads for these experiments was based on the observed donor-acceptor interactions in solutions.10 Herein, we report the synthesis of a pair of porphyrinfullerene dyads, both with two linkers, and the preparation of the self-assembled monolayers (SAM) of these molecules on ITO and glass. The molecules, that is, dyads, can be attached to the indium-tin oxide (ITO) surface either by the porphyrin end or by the fullerene end of the molecule, resulting in opposite directions of the primary charge transfer. The photoelectrical measurements include the photocurrent generation in an electrochemical cell of SAM on ITO as the working electrode, and the time-resolved Maxwell displacement charge generation in the photovoltage mode. To the best of our knowledge, the two (8) (a) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, M. J. Phys. Chem. B 2000, 104, 1253. (b) Willert, A.; Bachilo, S.; Rempel, U.; Shulga, A.; Zenkevich, E.; von Borczyskowski, C. J. Photochem. Photobiol., A 1999, 126, 99. (9) Efimov, A.; Vainiotalo, P.; Tkachenko, N. V.; Lemmetyinen, H. J. Porphyrins Phthalocyanines 2003, 7, 610. (10) Chukharev, V.; Tkachenko, N. V.; Efimov, A.; Guldi, D. M.; Hirsch, A.; Scheloske, M.; Lemmetyinen, H. J. Phys. Chem. B 2004, 108, 16377.

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methods have not yet been used together. The effect of the orientation of the donor-acceptor pair with respect to the solid-state electrode on the photoelectric signal is studied and discussed. Results and Discussion The chemical structures and configurations of the selfassembled monolayers are presented in Figure 1. The following abbreviations are used: IP for the structure ITO/porphyrin, IPF for the structure ITO/porphyrin/ fullerene, and IFP for the structure ITO/fullerene/ porphyrin. Similarly for the SAMs on glass, the abbreviations are gP, gPF, and gFP, respectively. The procedure of the SAMs preparation is described in the Materials and Methods section. Absorptions of SAMs. In the absorption spectra of the monolayer, on both the glass and the ITO surfaces, one has to take into account that the layers are formed on each side of the substrates. In addition, the ITO layer itself absorbs light with the same intensity as SAMs. The effects of these artifacts were taken into account in all spectra and other presented results. Absorption spectra of the SAMs on the glass substrates show clear Q and Soret bands (Figure 2). The Soret band of porphyrin in the gP sample (Figure 2a) is broader, and its maximum is shifted by 5 nm toward longer wavelengths as compared to that in toluene solution. Similar but smaller changes are seen for the SAMs of the dyads on glass as compared to those in solutions (Figure 2b and c). It should be noted that the Soret bands of the dyads themselves are broader and somewhat less intensive as compared to reference porphyrins.10 The absorption of the ITO layer is higher than absorptions of the SAMs, causing relatively big inaccuracy in the resulting spectra. Still, the absorptions at the Soret band maxima provide a possibility to estimate the mean molecular areas (mma) of the molecules in the SAMs on ITO, as well as in the SAM on glass. The estimations were done by using the absorptivities of the compounds in toluene solution ( ) 3.4 × 105 M-1 cm-1 for porphyrin, and ca. 2 × 105 M-1 cm-1 for the dyads at the Soret band) and are summarized in Table 1. It is important to notice that the positions of the Soret bands in SAM on ITO are very close to those in solution, and closer than those for SAMs on glass. Previously, significant red shifts were observed in self-assembled monolayers of porphyrin and porphyrin-fullerene dyad, and attributed to the interchromophore interactions in the tightly packed monolayers.4,8 In those studies, the molecules were attached to the substrate by a single linker, whereas in the present study dyads are equipped with two or four linkers, as in the case of porphyrin alone. Therefore, we can expect that porphyrin macrocycles are aligned mostly parallel to the surface, in contrast to the single linker compounds, where the perpendicular orientations are most probable. This difference in the orientations of the porphyrin macrocycles may result in much weaker interchromophore interactions and smaller perturbations of the absorptions in the present case. The larger mean molecular areas for the both porphyrin and dyads support the proposed in-plane alignment of the porphyrin macrocycles. Emission Spectra. The emission spectra (Figure 3) of the studied molecules in the self-assembled monolayers resemble the spectra of the corresponding compounds in low to moderate polar solvents. The spectrum of gP has two maxima, one at 650, and the other at 710 nm. SAMs of the dyads have a recognizable exciplex emission band

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Figure 1. Chemical structures of the molecules constituting self-assembled monolayers IPF, IFP, and IP.

at 720-820 nm,10,11 which dominates the emission of gFP. For the sample with the reverse dyad structure, gPF, this band is weaker but still appreciable. The two bands of the porphyrin emission for dyad SAMs are shifted by about 10 nm to longer wavelengths and are lower in intensity as compared to gP. At least a 100-fold intensity decrease at 650 nm is observed for all dyads samples as compared to gP. It is also important to notice that the exciplex emissions of the SAMs on ITO are weaker than those on glass, indicating that the ITO surface interacts with SAM stronger than the glass. (11) (a) Tkachenko, N. V.; Rantala, L.; Tauber, A. Y.; Helaja, J.; Hynninen, P. H.; Lemmetyinen, H. J. Am. Chem. Soc. 1999, 121, 9378. (b) Vehmanen, V.; Tkachenko, N. V.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. Chem. Phys. Lett. 2001, 345, 213. (c) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067. (d) Vehmanen, V.; Tkachenko, N. V.; Efimov, A.; Damlin, P.; Ivaska, A.; Lemmetyinen, H. J. Phys. Chem. A 2002, 106, 8029. (e) Tkachenko, N. V.; Lemmetyinen, H.; Sonoda, J.; Ohkubo, K.; Sato, T.; Imahori, H.; Fukuzumi, S. J. Phys. Chem. A 2003, 107, 8834. (f) Imahori, H.; Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.; Lemmetyinen, H.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105, 1750. (g) Armaroli, N.; Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.; Diederich, F. Chem.-Eur. J. 2000, 6, 1629. (h) Vehmanen, V.; Tkachenko, N. V.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. Spectrochim. Acta, Part A 2001, 57, 2227.

Cyclic Voltammetry of the SAMs. Cyclic voltammograms (CV) of SAMs on ITO were recorded in a 0.2 M dichloromethane solution of n-Bu4NPF6, Figure 4. The porphyrin oxidation peaks were observed at 1.1 V for IPF and IFP, and at 1.2 V for IP (vs Ag/AgCl in saturated KCl), close to the expected potentials3,4 and measurements in solutions. The CV measurements can be used for an alternative estimation of the mean molecular area.4 The integral of the current peak (see Figure 4, Q1 and Q2 areas) gives the charge accumulated by the molecular layer on the electrode surface and in the capacitance of the electrodes. Assuming that one electron corresponds to one molecule, and neglecting the capacitance, one can calculate the number of molecules on the ITO electrode N ) Q/e, where e is electron charge. For the studied samples, the electrode areas were 0.48 cm2, and roughness coefficient of ITO was assumed to be 1.3.4 The estimations of the mean molecular areas (mma) are collected in Table 1. The mma values estimated from the absorption and CV measurements are in good agreement. All three compounds have a higher mma value on ITO as compared to that on glass. On average, the coverage of the surface is

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Figure 2. Normalized absorption spectra of the SAMs on solid substrates and in toluene. (a) Structures of IP, gP, and porphyrin in toluene. (b) Structures of IPF, gPF, and the dyad in toluene. (c) Structures of IFP and gFP.

40% lower than the value in the case of ITO. However, the average mma values are small enough to assume that most of the ITO surface is covered by the SAMs for all of the samples. Based on the absorption and emission spectra of the SAMs on ITO and glass, as well as on the CV data of the SAMs on ITO, one can conclude that the monolayers contain the desired molecules. The properties of the dyads were not disturbed essentially by attaching them onto the surface of the corresponding substrate. The mean molecular area estimations, as expected, give the values that are close to the area of porphyrin moiety oriented parallel to the surface (ca. 2.3 nm2)12,13 rather than the perpendicular orientation (ca. 0.8 nm2).4,12,13 Transient Photovoltage Responses. For the timeresolved Maxwell displacement charge (TRMDC) experiments, the SAM layer of each dyad on ITO was covered by 20 layers of octadecylamine (ODA), made by the Langmuir-Blodgett method.14 The ODA layers insulated the SAMs from the second electrode, which was a drop of liquid InGa alloy brought to a contact with the sample surface.14 (12) Chou, H.; Chen, C.-T.; Stork, K. F.; Bohn, P. W.; Suslick, K. S. J. Phys. Chem. 1994, 98, 383.

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The photovoltage response signals of the samples are shown in Figure 5. The signs of the responses indicate that for all of the samples the photoexcitation results in an electron transfer from the SAM to the ITO electrode. It should be noted that when the dyads were insulated from both electrodes by 10 ODA monolayers, the signs of the photoresponses followed the orientations of the dyads,15 similar to the previously reported LB films of phytochlorin-fullerene dyads.14 In the transient photovoltage method, the signal amplitude is proportional to the density of charges (electrons) transferred from the organic monolayer to the ITO electrode and to the distance of the charge transfer. Therefore, the amplitude of the photoresponse can be used as a measure of relative efficiencies of the electron transfer (ET) in different samples. The IPF structure yields a 4.4 times higher and the IFP structure about 110 times higher amplitudes as compared to the reference IP structure (see Figure 5a). The ratio of the amplitudes for IFP and IPF is about 25. Accounting for the direction of the photoinduced ET, one can assume that in the final CS state the electron is shifted to the ITO electrode, whereas the positive charge is localized on porphyrin moiety of the dyad. In the frame of this simplified model, the photoresponse of the SAMs should be proportional to the distance from porphyrin moiety to the ITO electrode surface. However, the distance alone cannot explain the difference in signal amplitudes between IPF and IFP SAMs. The shortest possible distance in IPF structure is the van der Waals length, or >3 Å. Insertion of the fullerene between the ITO and porphyrin in IFP structure may increase the distance by 10 Å, thus providing the distance increase by not more than 4 times, which is much lower than the measured value, 25 times. Thus, the excited dyad in the IFP structure transfers an electron to ITO with higher probability, than excited dyad in the IPF structure. This seems natural because the primary intramolecular ET in IFP is from porphyrin to fullerene, the direction toward ITO. In the IPF structure, the primary ET is opposite. The decays of the photovoltage signals are relatively slow as illustrated in the double logarithmic plot in Figure 5b. A fast decay after 100 ms delay is due to the instrumental time constant. Qualitatively, the fastest relaxation was observed for the porphyrin SAM. The dyad films are relaxing with rather similar rates during the first 10 µs after the excitation, but at longer delays the IPF structure shows a slower relaxation rate. Photocurrent Responses. The static photocurrent measurements were performed using a three-electrode electrochemical cell in conditions similar to those previously used for porphyrin-fullerene SAMs on ITO.4-6 Anodic photocurrent was measured at 0.5 V bias in a 0.1 M Na2SO4 aqueous electrolyte containing 50 mM triethanolamine, saturated by argon. Cathodic photocurrent was measured at 0 V bias in a 0.1 mM Na2SO4 aqueous solution containing 5 mM 1,1′-dihexyl-4,4′-dipyridinium dibromate, saturated by oxygen. In general, the results of the measurements were similar to those previously reported for porphyrin and/or the porphyrin-fullerene dyads on (13) Tran-Thi, T. H.; Lipskier, J. F.; Houde, D.; Pepin, C.; Langlois, R.; Palacin, S. J. Chem. Soc., Faraday Trans. 1992, 88, 2529. (14) Tkachenko, N. T.; Vuorimaa, E.; Kesti, T.; Alekseev, A. S.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. J. Phys. Chem. B 2000, 104, 6371. (15) Vuorinen, T.; Kaunisto, K.; Tkachenko, N. V.; Efimov, A.; Alekseev, A. S.; Hosomizu, K.; Imahori, H.; Lemmetyinen, H. Langmuir 2005, 21, 5383.

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Table 1. Mean Molecular Areas, Estimated from Absorption and Cyclic Voltammetry, and Quantum Yields of Photocurrent in Electrochemical Cell structure

absorption maximum at Soret band

IP IPF IFP gP gPF gFP

0.020 0.010 0.017 0.031 0.018 0.025

a

mean molecular area, nm2 from absorption from voltammetry 3.7 ( 0.5 4.2 ( 1 2.7 ( 0.6 2.4 2.4 1.8

3.0 4.8 3.0

quantum yield of photocurrent, % anodic (0.5 V)a cathodic (0 V)a 1.0 2.1 2.8

0.25 0.36 0.45

Bias voltage is given vs Ag/AgCl (sat. KCl) reference electrode.

Figure 3. Normalized emission spectra of SAMs, gFP (dotted line), gPF (dashed line), IFP (thin solid line), and gP (thick line).

Figure 4. Cyclic voltammogram of IP vs Ag/AgCl (sat. KCl), 100 mV/s. Q1 and Q2 present the charges used for mean molecular area calculations.

ITO.4-6 Figure 6 presents some examples of the electrochemical current response on photoexcitation at different biases for the IPF sample. The photocurrent action spectra are given in Figure 7. In general, they follow the absorption spectra, except the wavelength area from 550 to 700 nm, where the signal is

higher relative to the corresponding Q-band absorption. Considering the ratio of the Soret band to the Q-band maxima to be the same for SAMs on ITO and SAMs on glass, one can conclude that the efficiency of the anodic photocurrent generation with the excitation at longer wavelengths is at least 5 times higher than that for the excitation at the Soret band. It is still unclear why the photocurrent generation is so efficient for the Q-bands excitation. The quantum yield of the photocurrent generation was calculated as a ratio of number of electrons in the photocurrent to number of absorbed photons.6 The quantum yields for the cathodic currents are 0.36% and 0.45%, and for the anodic currents, 2.1% and 2.8%, for the IPF and IFP structures, respectively (excitation at Soret band). For the IP structure, the quantum yields are 0.25% and 1.0%, respectively. In other words, fullerene increases the photocurrent generation efficiency 2-3 times, and the orientation of the porphyrin-fullerene dyad on the ITO surface changes the efficiency by 25-35%. Comparison of Photocurrent and Photovoltage Responses. The difference in the photocurrent generation between the samples is apparently much smaller than the difference in the photovoltage responses of the similar samples. The essential difference between the transient photovoltage and the static photocurrent measurements is that in the former case the only process is ET at the ITO-SAM interface (including intramolecular ET in SAM), whereas in the static photocurrent measurements the current flows through the whole electrochemical cell. The latter includes ET at the SAM-electrolyte interface and the carrier transport to the counter electrode in addition to primary ET in SAM and ET at the ITO-SAM interface. Therefore, one can conclude that in the case of the static photocurrent the limiting step is the interaction of SAM with the electrolyte rather than the primary intramolecular electron transfer or electron transfer from SAM to ITO.

Figure 5. Time-resolved Maxwell displacement charge decays in IFP (solid line), IPF (dashed line), and IP (dash-doted line). (a) Fast part in linear scale, note magnification factors for IPF and IP. (b) Full time range in double logarithmic scale. Fast decay after 100 ms is due to the instrumental time constant.

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Figure 6. Anodic current of IPF in electrochemical cell with stepwise light excitation at different biases vs Ag/AgCl (sat. KCl) reference electrode in 0.1 M Na2SO4 aqueous solution with 50 mM triethanolamine.

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different in these two cases. In the case of IP SAM, the primary electron transfer should involve an interaction of the excited porphyrin (in a singlet or triplet state) with either ITO or electrolyte, and thus needs a long lifetime of the excited state of porphyrin. In the case of dyads, the locally excited state of porphyrin is quenched efficiently (the lifetime is as short as 200 fs) by formation of an intramolecular exciplex.10 The lifetime of the exciplex in nonpolar solvents (e.g., toluene) is around 3 ns. In polar solutions, the exciplex transforms very fast (time constant 6-10 ps) to a complete charge-separated state, which has a lifetime around 0.5 ns in benzonitrile.10 The photocurrent generation measurements were done in water solutions, and water is polar in nature, so therefore one can expect the lifetimes of the exciplex and the charge-separated states to be also short. It can be even shorter in water than in benzonitrile because CS state relaxation for porphyrin-fullerene dyads is in the Marcus inverted region.1f,11e,16 Therefore, the interaction of SAM with the electrolyte becomes the ratelimiting step in the total current flow. To overcome this step, one has to increase the lifetime of the chargeseparated state. Materials and Methods The synthesis and characterization of porphyrin-fullerene dyads and reference porphyrin, activated for making selfassembled monolayers on ITO, as well as the procedure of SAMs preparation, are provided in the Supporting Information. Spectroscopy and Electrochemistry. Absorption spectra were measured on Shimadzu UV3100 and UV2501PC spectrophotometers. Emission spectra were measured with a Fluorolog 3 (SPEX Inc.) fluorimeter with a cooled photomultiplier (Hamamatsu R2658). Spectra were corrected by using manufacturer-supplied correction functions. The formula used for the estimations of mean molecular area from the absorption is mma ) kr/NAA, where kr is the surface roughness assumed to be 1.3,4 NA is Avogadro’s constant, and A is the absorbance of the monolayer. It follows from the Beer-Lambert law and from the fact that mma is inversed molecular surface density, which in turn is concentration times layer thickness. The photocurrent and electrochemical measurements were made in two similar systems, consisting of a voltammetric analyzer CV-50W (Bioanalytical Systems, Inc.) and a threeelectrode cell with the active ITO electrode (area 0.48 cm2), platinum wire counter electrode, and Ag/AgCl (sat. KCl) reference electrode. The photocurrent was induced by stepwise excitation from a xenon lamp coupled with a monochromator (Ritsu MC-10N). Time-resolved photovoltage measurements were carried out using the time-resolved Maxwell displacement charge technique described elsewhere.17 The samples were excited by 10 ns laser pulses at 430 nm. Deposition of 20 LB monolayers of octadecylamine (ODA) was done at a surface pressure of 20 mN/m from phosphate buffer subphase at a temperature of 20 °C with an LB 5000 system (KSV Instruments, Finland).

Conclusions Figure 7. Photocurrent action spectra for structures IFP, IPF, and IP. HV2+ (2) presents cathodic current in oxygen saturated 0.1 M Na2SO4 water solution with 5 mM hexyl viologenBr2. TEA (O) presents anodic current in Ar saturated 0.1 M Na2SO4 aqueous solution with 20 mM triethanolamine. Solid line presents absorption spectrum of corresponding SAM on ITO (right scale). (a) Structure IFP. (b) Structure IPF. (c) Structure IP (reference).

The photocurrent generation efficiency for the IP structure is almost the same as that for the IPF sample. Apparently, the mechanism of the photoconductivity is

The synthesized porphyrin-fullerene dyads form qualitatively good self-assembled monolayers on glass and ITO with reasonably high surface coverage. Dyad SAMs on ITO transfer electrons to ITO under excitation by the laser pulse. The efficiency of such photovoltage generation for the IFP structure is 25 times higher than that for the IPF structure, and 110 times than that for the IP (16) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607. (17) Tkachenko, N. V.; Tauber, A. Y.; Hynninen, P. H.; Sharonov, A. Y.; Lemmetyinen, H. J. Phys. Chem. A 1999, 103, 3657.

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structure. Photocurrent generation for IFP in electrochemical cell is 30% higher than that for IPF, and approximately 2 times higher than that for the IP structure. Acknowledgment. This work was supported by the Academy of Finland and the National Technology Agency of Finland, and partially by a Grant-in-Aid (No. 16205020)

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from the Ministry of Education, Culture, Sports, Science, and Technology (Japan). Supporting Information Available: The synthesis and characterization of molecules for making self-assembled monolayers, and the procedure of SAMs preparation. This material is available free of charge via the Internet at http://pubs.acs.org. LA0500833