J. Phys. Chem. 1995,99, 16704-16708
16704
Estimation of Exciton Sizes in Squaraine Monolayers by Intralayer Photoinduced Electron Transfer Kangning Liang? Kock-Yee Law,*.$and David G. Whitten*9+ NSF Center for Photoinduced Charge Transfer, Department of Chemistry, Uniuersiry of Rochester, Rochester, New York 14627, and Wilson Center for Research and Technology, Xerox Corporation, 800 Phillips Road, 114-390, Webster, New York 14580 Received: May 9, 1995; In Final Form: August 16, 1995@
The effect of intralayer electron acceptor and donor on the photocurrent generation of monolayer modified SnOz electrodes of DSSQ (4-(distearylamino)phenyl-4'-(dimethyl~no)phenylsquar~ne) has been investigated in solution photoelectrochemical experiments. While intralayer electron acceptors, such as C14MV2' (1methyl- l'-tetradecyl-4,4'-bipyridinium dichloride), are shown to promote cathodic photocurrent generations under both ambient and deaerated conditions, CI8PDA (N,N-dioctadecyl-p-phenylenediamine), an intralayer donor, is found to quench the cathodic photocurrent under ambient condition and make the photocurrent anodic when the electrolyte solution is deaerated with N2. From the effects of the concentration of the donor/ acceptor on the photocurrent generation process, and the assumption that the squaraine chromophores are in a "card pack" (translation or glide layer packing) arrangement in the monolayers, the size of the exciton in the aggregated monolayer is estimated to consist of -25 DSSQ molecules. Evidence is provided that aggregates with higher quantum efficiency of photocurrent generation can be obtained when the monolayer is prepared at subphase temperatures higher than 55 "C. The increase in quantum efficiency is discussed in terms of the increase in the exciton diffusion length.
Introduction Organic pigments, which absorb strongly in the visible region, are often found to be useful as photoconductors in electrophotographic devices.' These pigments include phthalocyanines, squaraines, perylenes, azo pigments, etc. They are usually used as microcrystalline solids embedded in a polymer matrix. Excitation of the photoconductor leads to the formation of excitons, which formally dissociate into electron-hole pairs and are captured for electrostatic imaging. Basic studies aimed at understanding the photogeneration mechanism of organic photoconductors have been documented. The emerging trend based on results from phthalocyanines,2-6 perylenes,' and more recently azo pigments8-'* suggests that the primary step in the photogeneration process involves an electron-transfer reaction between the exciton and either an electron acceptor or donor. Factors such as the molecular arrangement of the photoconductive molecules in the m i c r ~ c r y s t a l , ' ~and - ' ~ the size, shape, and crystallinity of the microcrystal^,'^.'^ are known to influence the photogenerationefficiency. Very little is known about these effects on the exciton, the primary species in the photogeneration process. Using aggregates formed in the monolayers of 4-(distearylamino)phenyl-4'-(dimethylamino)phenylsquaraine (DSSQ), we observed cathodic photocurrent generations in DSSQ modified SnO2 electrodes in solution photoelectrochemicalexperiments.I8 The cathodic photocurrent is attributed to an electron transfer from the excited squaraine aggregate to 0 2 , followed by a subsequent transfer of an electron from the conduction band of the ,51102 electrode to the holes in the squaraine aggregate. While electron acceptors (e.g., methyl viologen) are shown to enhance the cathodic photocurrent, electron donors (e.g., hydroquinone) quench it under ambient conditions. In the absence of 0 2 , the
* To whom correspondence should be addressed.
' Universitv
of Rochester.
= Xerox ~ d r p . @
Abstract published in Advance ACS Absrraca, October 15, 1995.
ClsPDA
added donor in the electrolyte solution can reverse the flow of the electrons and make the photocurrent anodic. In this work, we extend our investigation to electron-transfer reactions within the monolayer assembly by incorporating either an electron donor or acceptor in the DSSQ monolayer. The structures of these acceptors and donors (C14MV2+,C1p,MV2+, 2C18MV2+, C I ~ P D Aand , C18NFc) are shown in Scheme 1. Their effects on the photocurrent generation process were found to be similar to those obtained in homogeneous solution. The concentration dependence of these acceptors and donors on the photogeneration process has been used to estimate the size of the squaraine exciton in monolayers. An increase in photocurrent generation efficiency is obtained when the monolayer is prepared at subphase temperatures higher than 55 "C. Evidence is provided that the increase in quantum efficiency is due to the size increase in the exciton, presumably due to the formation of larger aggregated structures at higher temperatures.
0 1995 American Chemical Society 0022-3654/95/2099-16704$09.00/0
Exciton Sizes in Squaraine Monolayers
J. Phys. Chem., Vol. 99, No. 45, I995 16705
Experimental Section Materials. DSSQ (4-(distearylamino)phenyl-4'-(dimethylamin0)phenylsquaraine) was synthesized by condensing 1-(p(dimethylamino)phenyl)-2-hydroxycyclobutene-3,4-dione with N,N-dioctadecylaniline in refluxing 2-propanol using tributylorthoformate as a dehydrating reagent.I9 C14MV2+(l-methyll'-tetradecyl-4,4'-bipyridinium dichloride) and 2 c 1&V2+ ( 1,l'dioctadecyl-4,4'-bipyridinium dichloride) were purchased from Fluka and Aldrich, respectively. They were used as received. C18MV2+(l-methyl-l'-octadecyl-4,4'-bipyridinium dichloride) was synthesized by reacting 1-methyl-4,4'-bipyridine with n-octadecyl bromide followed by an ion-exchange process.20 C 18PDA (N,N-dioctadecyl-p-phenylenediamine) was synthesized by dialkylating 4'-aminoacetanilide (Aldrich) with n-octadecyl bromide followed by hydrolyzing the resulting product with HCL2I (Ferrocenylmethy1)dimethyloctadecyla"onium hexafluorophosphate, cI8NFc, was a sample generously provided by Dr. J. S. Facci (Xerox) and the synthetic procedure has been described elsewhere.22 The spreading solvent was chloroform (HPLC, pentene stabilized) and was purchased from Fisher. Distilled water was in-house deionized water purified by passing through a Millipore water purification system. The electrolyte for the electrochemical experiment was NaNO3 and was obtained from Fisher. General Techniques. Monolayers and mixed monolayers of DSSQ were obtained by spreading chloroform solutions of DSSQ M) containing varying amounts of C14MV2+,c18MV2+, 2C18MV2+, C I ~ P D A or , cl8NFc onto an aqueous subphase, which contained cadmium chloride (3 x M) and sodium bicarbonate (5 x M) in a KSV 5000 film balance. The temperature of the subphase was -35 "C unless specified. The transfer ratios on hydrophilic SnOz electrodes were 1.0 f 0.1 and all the DSSQ-Sn02 electrodes studied consisted of a single layer of DSSQ deposited onto precleaned SnO2 electrodes. The aggregation of the squaraine chromophore in the monolayer was studied by absorption spectroscopy using a HP8452A diode array spectrophotometer. Photoelectrochemicalexperiments on various DSSQ modified SnO;? electrodes were carried out on an in-house assembled apparatus consisting of an electrochemical analyzer, Model BAS lOOB from Bioanalytical Systems, and a xenon arc lamp system (Model A1010) from Photon Technology International Inc. The irradiated area on each electrode is -1 cm2 and the light intensity at 530 nm is -2 x 10l6 photons/(cm2.s). The details of the apparatus and the measuring procedures have been reported earlier.I8 Results Characterization of Squaraine Aggregates in Mixed Monolayers. Monolayers and mixed monolayers of DSSQ were obtained by spreading chloroform solutions of DSSQ (M) containing varying amounts of C 14MV2+,C 1 8MV2+, 2C18MV2+,ClsPDA, or ClSNFC onto an aqueous subphase in a film balance at a subphase temperature of 35 "C. Modified SnO2 electrodes were prepared by transferring the monolayers onto precleaned hydrophilic SnO2 substrates. Details of the procedures have been given earlier.I8 Figure 1 shows a typical absorption spectrum of a mixed monolayer of DSSQ and C14MV2+ on a SnO2 electrode. The I,,, lies at 530 nm and is blue-shifted from the solution absorption in chloroform (A,, = 633 nm). The absorption spectrum of the mixed monolayer is identical to that of a pure DSSQ m ~ n o l a y e r , ' ~indicating .'~ that incorporation of a small amount of C14MV2+ in the monolayer of DSSQ has very little effect on the aggregation. We have examined all the DSSQ-C14MV2+ modified SnOz
0'08 0.06
-
0.04
-
002
-
01
;
' 5:
n 4
0.00
300
400
500
600
700
800
Wavelength (nm)
Figure 1. Absorption spectrum of a mixed monolayer of DSSQ and C,4MV2+ (1O:l) on a Sn02 electrode.
-w-
*
3
N2 deaerated
I00
" 0.
0 05
01
0 15
02
0 25
Molar Ratio (C14MV2+/DSSQ)
Figure 2. Effect of C14MV2+concentration on the photocurrent generation of DSSQ-C14MV2+ modified SnO? electrodes (bias voltage = 0 against AgCVAg reference electrode).
electrodes used in this work. Our data suggest that the variations of both Amax and absorbance are small and are within the experimental uncertainty. Very similar spectral data are also obtained for DSSQ mixed monolayers containing C18MV2+,2Cl8MV2+,ClsPDA, and c18NFC . Photocurrent Generation from Mixed Monolayer Modified Sn02 Electrodes. Electron Acceptor. Cathodic photocurrents are obtained when modified SnO;?electrodes containing mixed monolayers of DSSQ and C14MV2+are irradiated. The action spectrum coincides with the absorption spectrum of the modified electrode with a maximum photoresponse at 530 nm. The effect of the concentration of C14MV2+on the photocurrent is plotted in Figure 2. Under both ambient and N2 deaerated conditions, the photocurrent generation efficiency increases as [C14MV2+] increases and levels off when the [C14MV2+)/ [DSSQ] ratio reaches 0.02. It is important to note that we observe higher cathodic photocurrents from these mixed monolayers as compared to that obtained from a pure DSSQ monolayer under both ambient and deaerated conditions. The result indicates that C14MV2+ is an electron acceptor for the cathodic photogeneration process, in addition to 0 2 . The effect is much more obvious when the electrolyte solution is deaerated with N2. Contrasting to the pure DSSQ monolayer results,I8 we observed significant cathodic photocurrents from the mixed monolayer in the absence of 0 2 when the [CI~MV~']/[DSSQ] ratio is 20.02. The cathodic photocurrent is presumably promoted by an electron-transfer process, from the excited squaraine aggregate to C14MV2+ within the monolayer. The saturation effect in Figure 2 suggests that for every 50 DSSQ
16706 J. Phys. Chem., Vol. 99, No. 45, 1995
400-
9
2- :3 -
0
0 0
Liang et al. N2 deaerated ambient
-
0
5
q
-
200-
a!\ 2M)-
D
2 "
1
0
Ob1
do2
0'03
0'04
-4037
005
Molar Ratio (2C18MVZ+/DSSQ)
Figure 3. Effect of 2C18MV2+ concentration on the photocument generation of DSSQ-2C18MV2+ modified Sn02 electrodes under ambient conditions (bias voltage = 0 against Ag/AgCl reference
_"""
electrode). molecules, only 1 C I ~ M V molecule + is needed to promote the optimal cathodic photocurrent. C14MV2+ is known to be slightly soluble in water.23 Since the temperature of the subphase used in the above experiments is -35 "C, one of the concerns is the dissolution of C14MV2+ into the subphase during film fabrication. To examine the seriousness of this solubility problem, we also studied DSSQ mixed monolayers containing C18MV2+,which is known to be less water soluble than C14MV2+. A very similar concentration dependence on the cathodic photocurrent process is observed. Specifically, C I8MV2+also promotes the cathodic photocurrent under both ambient and deaerated conditions. Similar to C14MV2+, the sensitized cathodic photocurrent becomes optimal when the [C18MV2+]/[DSSQ]ratio is 20.02. Figure 3 shows the effect of 2C18MV2+concentration in the mixed monolayer on the cathodic photocurrent generation process. Similar to C14MV2+ and Ci8MV2+, 2C18MV2+promotes cathodic photocurrent too, suggesting that there is no orientation effect on the electron-transfer process. The most significant observation is that the sensitized cathodic photocurrent levels off at [2C18MV2+]/[DSSQ] L 0.01. The result suggests that 2c18MV2+is twice more effective in sensitizing the photocurrent. Electron Donors. Two surfactant electron donors, ClgNFc and C 18PD.4, were studied. While incorporation of C 1 8NFc into the monolayers of DSSQ has practically no effect on the cathodic photogeneration process, interesting results were obtained for the CI~PDA-DSSQ monolayers. CIRPDAis shown to quench the cathodic photocurrent generation under ambient conditions and makes the photocurrent anodic when the electrolyte solution is deaerated with nitrogen. The effect of the concentration of ClgPDA on the photocurrent is plotted in Figure 4. It is interesting to note that whether it is a quenching of the cathodic photocurrent or a sensitization of the anodic photocurrent, the donor effect from ClsPDA becomes optimal when the [Cl8PDA]/[DSSQ] ratio exceeds 0.02. Effect of Subphase Temperature. Experiments have been extended to increase the temperature of the subphase during the preparation of the DSSQ-Sn02 modified electrodes. The results (Figure 5) show that there is a factor of -6 increase in cathodic photocurrent generation when the modified electrode is prepared at subphase temperatures 2 5 5 "C.
Discussion Effects of Intralayer Donors and Acceptors on the Photocurrent Generation Process. We have previously demonstrated that, under the ambient condition, the cathodic photo-
2500
-
2000
-
I500 1000
-
500 -
30
40
50
60
Temperature (OC)
Figure 5. Effect of the temperature of the subphase on the cathodic
photocurrent generation of the DSSQ-Sn02 electrode (ambient conditions). current generation from the DSSQ-Sn02 electrode involves an electron transfer from the excited squaraine aggregates to oxygen to form the superoxide anion radical followed by a subsequent electron transfer from the conduction band of the electrode to the holes in the squaraine aggregates. Addition of methyl viologen in the electrolyte solution was shown to enhance the cathodic photogeneration process under both ambient and N2 deaerated conditions. The photocurrent enhancement was attributed to the electron transfer reaction from the excited squaraine aggregates to methyl viologen. The photocurrent results in Figures 2 and 3 suggest that similar electron-transfer reactions also occur between the excited squaraine aggregates and the viologen groups in c & f V 2 + C , 1 8MV2+,and 2 c I 8MV2+ within the monolayer assembly. The most significant finding is that, according to the concentration effect (Figure 2 and 3), 2C18MV2+ is twice more effective in promoting the cathodic photocurrent. This issue will be discussed in the next section. In the cases of electron donors, while ClsNFc is found to be ineffective in influencing the photocurrent, C I8PDA quenches the cathodic photocurrent under ambient conditions and makes the photocurrent anodic in the absence of oxygen. The quenching of the cathodic photocurrent suggests that the photogenerated holes produced by the electron-transfer reaction between the excited squaraine aggregate and 0 2 are scavenged by the electron donor. The electron transfer between C18PDA and the holes short-circuits the cathodic photocurrent. In the absence of oxygen, electron transfer from ClsPDA to the excited squaraine aggregate produces a radical anion, which transfers an electron to the conduction band of the SnO2 electrode to make the photocurrent anodic.I8 The lack of an effect from Cl8 N F C may be attributable to the unfavorable energetics in the electron-transfer process. Its oxidation potential is 0.5 1 V vs Ag/AgCl and is higher than that of C I ~ P D A(0.17 V vs Ag/ AgCl). Interestingly, the results in Figure 4 show that the
J. Phys. Chem., Vol. 99, No. 45, 1995 16707
Exciton Sizes in Squaraine Monolayers squaraine aggregates
f
sitesfor additional donors or acceptors
Figure 6. A schematic for the “extended aggregates” of DSSQ in mixed monolayer containing C14MV2+,C1gMV2+,or C1gPDA on ,31102 electrodes (top view).
intralayer donor effect on the photogeneration process also becomes optimal when the [ClsPDA]/[DSSQ] ratio reaches 0.02. The implication of this ratio to the size of the exciton will be discussed. Estimation of the Size of Squaraine Excitons in Monolayers. One of the most intriguing results in Figure 2 and 4 is the saturation of the sensitizatiodquenching effect on the photocurrent generation process as the concentration of the electron acceptor/donor in the monolayer increases. For electron acceptors such as C14MV2+and C1sMV2+,and electron donors such as ClsPDA, an optimal effect is attained at a ratio of 1 to 50 (0.02). It is important to note that these amphiphilic acceptors and donors all consist of an electroactive group that is expected to reside vertically along the long molecular axis in the monolayer. Geometrically, they are similar to DSSQ. Our studies of other amphiphilic squaraines suggest that the “unit aggregate” responsible for the sharp aggregated absorption at 530 nm is a cyclic tetramer.24,25These studies as well as molecular simulations on squaraine monolayer clusters suggest that the “extended” aggregate of the pure dye monolayer may be a “mosaic” of these unit aggregates, which has a glide plane or herringbone arrangement. Clearly the current results imply that the exciton must be delocalized over areas larger than the “unit aggregate” by either a true delocalization or a very rapid “hopping” mechanism. We propose that each squaraine exciton consists of -25 DSSQ molecules and the amphiphilic acceptor or donor resides at the boundary of the exciton. A schematic showing the molecular arrangement is depicted in Figure 6. Essentially, at the lowest optimal concentration (1:50), each acceptor or donor is hypothesized to sensitize or quench the photocurrent generation process of two excitons. As the concentration of the amphiphilic acceptor or donor increases, they will reside at the boundary of the exciton, leading to the leveling off effect observed in Figures 2 and 4. The proposed model is supported by the concentration dependence results in Figure 3. 2C18MV2+is found to be twice more effective in sensitizing the cathodic photocurrent. In other words, each 2ClsMV2+molecule can sensitize as many as -100 DSSQ molecules. The finding is actually consistent with the model shown in Figure 6. For example, as a result of the change in orientation for 2C18MV2+ (e.g., vertically along the short axis), each 2C1sMV2+ may be surrounded by four “extended aggregates” and each “extended aggregate” consists of -25 DSSQ molecules. The exciton size estimated in this work, which is -25, is consistent with other results reported in the literature. For example, Mobius and Kuhn26 showed by the energy-transfer technique that the excited energy of an oxacyanine dye extends over -10 molecules at room temperature. Analogously, Sat0 and co-workers2’ found that the excitons of several J-aggregates of cyanine dyes consist of -20 monomer units in liposomes. Effect of Exciton Size on the Photocurrent Generation Efficiency. The results in Figure 5 show that there is a factor
0
0.005
0.01
I 0.015
0.02
0.025
Molar Ratio (C1sMV2+/DSSQ)
Figure 7. Effect of C18MV2+ concentration on the photocurrent generation of DSSQ-ClgMV2+ modified SnO2 electrodes prepared at 55 “C subphase temperature (ambient conditions, bias voltage = 0 against AgCl/Ag reference electrode).
of -6 increase in quantum efficiency for the cathodic photocurrent generation process when the monolayer DSSQ-Sn02 electrode is prepared at subphase temperatures 255 “C. To comprehend the finding, we studied the effect of [C18MV2+] on the photocurrent using a series of mixed monolayers (containing DSSQ and ClsMV2+) prepared at 55 “C. The concentration dependence plot is given in Figure 7. Under the most optimal conditions, we observed a cathodic photocurrent of -6200 nA/cm2 from the monolayer modified electrode. This corresponds to a quantum yield of -2.4%, suggesting that we have improved the quantum efficiency of the photoelectrochemical cell by -8 times by using a higher subphase temperature and by incorporating a small amount of electron acceptor in the DSSQ monolayer. The concentration dependence plot for the mixed monolayers prepared at 55 “C (Figure 7) is similar to analogous mixed monolayers prepared at 35 “C (e.g., Figure 2). The biggest difference is the [CI~MV*+]/[DSSQ] ratio at which the photocurrent becomes optimal. The ratio in Figure 7 is 0.01, suggesting that the exciton prepared at 55 “C consists of -100 DSSQ molecules and is a factor of 2 larger than those prepared at 35 “C. We attribute the increase in the exciton size to the greater fluidity of the monolayer on water at temperatures higher than 55 “C, which enables better (annealed) molecular arrangements. This observation is not limited to aggregates assembled by the Langmuir-Blodgett film technique. For example, Tani et al.2srecently reported that the size of the J-aggregates of 5,5’dichloro-9-ethylthiacarbocyanine on AgBr increases from -6 to 14 molecules when the agitation temperature of the dye/AgBr emulsion increases from 40 to 70 “C. It is believed that the increased temperature enhances growth of the J-aggregates in the emulsion. The overall result on the increase in quantum yield for excitons of larger sizes can be rationalized in terms of the increase in the exciton diffusion length. For instance, assuming that the interplanar distance between the squaraine chromophores is -4 this would correspond to the increase of exciton diffusion length from -100 to -200 A. As the exciton diffusion length increases, the probability of transferring an electron to oxygen, which leads to cathodic photocurrent generation, increases. This interpretation is consistent with the finding of Muenter and c o - w o r k e r ~ ,who ~ ~ reported that the electrontransfer rate from an exciton of a cyanine dye to the AgBr conduction band increases with increasing exciton diffusion length, Incidentally, Yanagi et al.30also showed in photoelectrochemical experiments that the photocurrent quantum yield for chloroaluminum phthalocyanine increases by -25 times for
16708 J. Phys. Chem., Vol. 99, No. 45, 1995
larger crystallites, attributable to the longer exciton diffusion length and the high charge carrier mobility.
Concluding Remarks This work reports the use of a very simple technique, solution photoelectrochemistry,to estimate the size of squaraine excitons in monolayers. At room temperature, each squaraine exciton is estimated to consist of -25 molecules. The size of the exciton increases to -50 monomer units when the monolayer is prepared at subphase temperatures higher than 55 "C. The increase in size is shown to be responsible for the increase in photocurrent generation efficiency, owing to the increase in the exciton diffusion length.
Acknowledgment. The authors thank the National Science Foundation for the financial support in the form of a grant from the Center for Photoinduced Charge Transfer (CHE-9120001). References and Notes (1) Law, K. Y. Chem. Rev. 1993, 93, 449. (2) Loutfy, R. 0.; Menzel, E. R. J . Am. Chem. Soc. 1980, 102, 4967. (3) Ahuja, R. C.; Hauffe, K. Ber. Bunsen-Ges. Phys. Chem. 1980,84, 68. (4) Mizuguchi, J. Jpn. J. Appl. Phys. 1982, 21, 822. ( 5 ) Mizuguchi, J. Jpn. J . Appl. Phys. 1981, 20, 1855, 2065, and 2073. (6) Fujimaki, Y. lS&T Proc. 7th lnt. Congr. Adv. Non-lmpact Printing Technol. 1991, 269. (7) Popovic, Z. D.; Hor, A. M.; Loutfy, R. 0. Chem. Phys. 1988, 127, 451. (8) Umeda, U.,Niimi, T.; Hashimoto, M. Jpn. J . Appl. Phys. 1990, 29, 2746. (9) Niimi, T.; Umeda, U. J . Appl. Phys. 1993, 74, 465.
Liang et al. (10) Umeda, M.; Hashimoto, M. J. Appl. Phys. 1992, 72, 117. (11) Umeda, J.; Niimi, T. J. Imaging Sci. Technol. 1994, 38, 281. (12) Law, K. Y.; Tamawskyj, I. W.; Popovic, Z. D. J. lmaging Scr. Technol. 1994, 38, 118. (13) Law, K. Y. J . Phys. Chem. 1988, 92, 4226. (14) Dulmage, W. L.; Light, W. A.; Manno, S. J.; Salzberg, C. D.; Smith, D. L.; Staudenmayer, W. J. J. Appl. Phys. 1978, 49, 5543. (15) Borsenberger, P. M.; Regan. M. T.; Staudenmayer, W. J. US Patent 4,578,334 (1986). (16) Mizuguchi, J.; Rochat, A. C. J. Imaging Sci. 1988, 32, 135. (17) Enokida, T.; Hirohashi, R.; Mizukami, S. J. lmaging Sci. 1991, 35, 235. ( 18) Kim, Y, S.; Liang, K.; Law, K. Y.; Whltten, D. G. J . Phys. Chem. 1994, 98, 984. (19) Law, K. Y.; Chen, C. C. J. Phys. Chem. 1989, 93, 2533. (20) Pileni, M. P.; Braun, A. M.: Gratzel, M. Photochem. Photobiol. 1980, 33, 423. (21) Naito, K.; Miura, A,: Azuma, M. J. Am. Chem. Soc. 1991, 113, 6386. (22) Facci, J. F.; Falcigno, P. A.: Gold, J. M. Langmuir 1988, 2, 732. (23) The critical micelle concentrations for C14MV2+and C,8MV2+are -7.5 x M and -8.2 x M, respectively; see: Krieg, M.; Pileni, M. P.: Braun, A. M.: Gratzel, M. J. Colloid Interface Sci. 1981, 83, 209. (24) Chen. H.; Law, K. Y.; Perlstein. J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257. (25) Chen. H.: Farahat. M. S.: Law. K. Y.: Perlstein. J.: Whitten. D. G. J . Am.'Chem. Soc., submitted for publication. (26) Mobius, D.; Kuhn, H. lsr. J. Chem. 1979, 18, 375. (27) Sato, T.; Kurahashi, M.; Yonezawa, Y. J . Phys. Chem. 1993, 97, 3395. (28) Tani, T.; Suzumoto, T.: Kemnitz, K.; Yoshihara, K. J . Phys. Chem. 1992, 96, 2778. (29) Muenter, A. A.; Brumbaugh, D. V.; Apolito, J.; Hom, L. A,: Spano, F. C.; Mukamel, S . J. Phys. Chem. 1992, 96, 2783. (30) Yanagi, H.; Douko, S.; Ueda, Y.; Ashida, M.; Wohrle, D. J . Phys. Chem. 1992, 96, 1366.
JF95 1298W
