J. Phys. Chem. 1992,96, 9591-9594 15% and it decreases for lower temperatures and more polar solvents, the effect of cis absorption at 416 nm (molar absorption for cis is 3.5 times less than that for trans) on the calculated dipole moment would not exceed 2%. Lastly, it is interesting to compare the dc conductivity method with the transient microwave conductivity technique (TRMC).* The principal difference between these methods in studying excited-state dipoles is that in the latter the signal is proportional to the concentration of excited dipoles while in the dc conductivity approach the signal varies with the time derivative of the concentration (see eq 5 ) . The TRMC technique has more limited time resolution but greater sensitivity for long-lived dipoles.* Because of that, the two methods should be considered to be complementary. TRMC has advantages on longer time scales and dc conductivity in studying fast processes. Moreover, since the dc conductivity photoresponse is expected to be similar for both permanent and induced dipoles, the technique should be useful in studying excited states with large electronic polarizabilities.
Conclusions Measurements of the photocurrent caused by the rotational relaxation of excited-state dipoles in liquids can be made with enough time resolution to see the molecules rotate into their new equilibrium distribution. The dipole moment of the first excited singlet state of DMANS is found to be in a good agreement with that from other methods. Since the dc photocurrent method does not require a fluorescent species and allows the use of polar solvents, it should be applicable to the study of a wide variety of systems where information about the dipole moments of the species created by excitation is important. Acknowledgment. The authors are very grateful to W. Casey for helpful discussions about cell designs and to D. Collins for loaning us the Tektronix TDS 540 oscilloscope. Tom Scott provided the design for the Raman shifter, and its usefulness in
9591
dipole experiments was demonstrated by Karyn Grzeskowiak. Grant DE-FG02-86ER13592 from the Office of Basic Energy Sciences, Division of Chemical Sciences, US-DOE, supported this work.
References and Notes Brown, S.C.; Braun, C. L. J . Phys. Chem. 1991, 95, 511. de Haas, M. P.; Warman, J. M. Chem. Phys. 1982, 73, 35. Debye, P. Polar Molecules; Dover: New York, 1929. Willig, F. Ber Bunsenges. Phys. Chem. 1988,92, 1312. Bitterling, K.; Willig, F. J . Electroanal. Chem. 1986, 204, 211. (5) Sarbacher, R. I.; Edson, W. A. Hyper and Ultrahigh Frequency Engineering, J. Wiley & Sons, Inc.: New York, 1944. (6) Maier, M.; Kaiser, W.; Giordmaine, J. A. Phys. Rev. Lett. 1966, 17, 26. Culver, W. H.; Vanderslice, J. T. A.; Townsend, V. W. T. Appl. Phys. Lett. 1968, 12, 189. (7) Kawski, A,; Kaminski, J.; Kukulievski, Z . Narurforsch. 1979,3411,702. (8) Liptay, W. Excited Stares; Academic Press: New York, 1974; Vol. (1) (2) (3) (4)
1.
(9) Kawski, A.; Grycrzynski, I.; Jung, Ch.; Heckner, K.-H. Z . Narurforsch. 1977, 32a, 420. (10) Czekalla, J.; Wick, G. Ber. Bunsenges. Phys. Chem. 1961.65, 727. (1 1) Lippert, E. Z . Elektrochem. 1957,61,962. Moll, F.; Lippert, E. Z . Elektrochem. 1954, 58, 853. (12) Onsager, L. J . Am. Chem. SOC.1936, 58, 1486. (13) Kirkwood, J. J . Chem. Phys. 1939, 7, 911. (14) Frohlich, H. The Theory of Dielectrics; Clarendon Press: Oxford, U.K.,1949. (15) Osipov, 0. A. Zh. Fir. Khim. 1957, 31, 1542. Minkin, V. 1.; Osipov,
0.A.; Zhdanov, Yu. A. Dipole Moments in Organic Chemistry; Plenum Press: New York, 1970; p 30. (16) Bottcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1973. (17) LeFevre, R. J. Dipole Moments; Methuen: London, 1948. (18) Marcus, Y. Ion Solvation; J. Wiley & Sons: Chichester, U.K., 1985; p 130. (19) Perrin, F. J. Phys. Radium 1934,5,497. Tao, T. Biopolymers 1969, 8, 609. Dutt, G. B.; Doraiswamy, S. J. Chem. Phys. 1992, 96, 2475. (20) Rossini, F. D., et ai. Selected Properties of Hydrocarbons; US Government Printing Office: Washington, DC, 1957; p c-K-3200. (21) Smirnov, S.N.; Braun, C. L. Manuscript in preparation. (22) Schulte-Frohlinde, D.; Blume, H.; Gusten, H. J . Phys. Chem. 1962, 66, 2486. Gorner, H.; Schulte-Frohlinde, D. J . Mol. Srrucr. 1982, 84, 227.
CdS-Particle-Mediated Transmembrane Photoelectron Transfer in Surfactant Vesicles Ottd Honfth' and Janos H. Fender* Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 (Received: July 28, 1992; In Final Form: October 13, 1992) Cadmium sulfide (CdS) particles have been in situ generated in surfactant vesicles prepared from dihexadecyl phosphate (DHP). Absorption and emission spectra indicated the diameters of CdS particles to be 2.5-5.0 nm. Fluorescence decays of CdS particles in DHP vesicles have been found to be multiexponential and unaffected by the incorporation of cetylviologen cations, C16MV2+.Conversely, fluorescence intensities have been observed to decrease upon the addition of CI6MV2+, indicating the predominance of static quenching in photoelectron transfer in DHP vesicles. Photoelectron transfer has been observed by monitoring the development of CI6MV'+upon the band gap irradiation of CdS particles, confined in the inner bilayers of DHP vesicles, in the presence of benzyl alcohol as a sacrificial electron donor. Transmembrane photoelectron transfer has been demonstrated in dissymmetrical DHP vesicles which contained size-quantized CdS particles in their inner bilayers, c I 6 M v 2 in + their inner and outer bilayers, and Ag+ on their outer surfaces. Band gap irradiation in the presence of benzyl alcohol has resulted in the reduction of Ag+ on the DHP vesicle outer surface.
Introduction The importance of biological and chemical energy conversion and storage has prompted the vigorous investigation of transmembrane electron transfer in model systems.*" Unilamellar surfactant vesicles have been found to be particularly useful since they provide distinct compartments in their aqueous interiors, inner and outer surfaces, and within their bilayers for the incorporation of sensitizers, electron donors, and acceptor^.^ Irradiation of dihexadecyl phosphate (DHP) surfactant vesicles containing ruthenium tris(2,2'-bipyridme) and methylviologen (MV2+)dications as sensitizers and electron acceptors on their inner and outer surfaces, respectively, for example, was proposed to involve 0022-365419212096-9591$03.00/0
transmembrane photoelectron t r a n ~ f e r . ~The observed transmembrane diffusion of the reduced electron acceptor (MV") rendered the interpretation of this experiment less than straightf~rward.~~~ More recently, transmembrane photoelectron transfer has been reported from CdS particles, attached in high concentration to the outer surface of DHP vesicles, to MV*+ entrapped therein.8 However, photolysis has been accompanied by MV'+ dimerization and occurred only in low quantum yields (ca. 0.05).8 Evidence is presented, in this Letter, for photoelectron transfer from size-quantized CdS particles to silver ions (As+) located across DHP vesicles which had surface-active N'-methyl-N0 1992 American Chemical Society
9592 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
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WAVELENGTH, nm Figure I . Absorption (-), uncorrected emission A,, = 370 nm), and excitation (- - -;A,, = 5 0 0 nm) of CdS particles incorporated in the inner and outer bilayers of DHP vesicles. Emission and excitation spectra have been normalized to the absorption scale. Vesicles were prepared by presonication (70 W, 82 "C) of 2.0 X M DHP (5 min), followed by further sonication (70 W, 82 "C) upon the addition of stoichiometric amounts of 0.1 M NaOH (5 min) and 4.0 X M CdCI, (10 min). Solutions were bubbled for 45 min prior to and during sonication with a gentle stream of argon. Traces of titanium, released by the tip of the sonicator, were removed by centrifugation (30 min at 3000 rpm). CdS particles were generated by the subsequent addition of 4.0 X IO4 M H2S. (e-;
hexadecyl-4,4'-bipyridinium (cetylmethylviologen) cations (CI6MV2+)anchored in both sides of their bilayers and which contained benzyl alcohol as a sacrificial electron donor. Experimental Section Dihexadecyl phosphate (DHP, Sigma), CdC12.5Hz0 (Baker), NaOH (Fisher), methylviologen dichloride (MVC12, Aldrich), AgN03 (Aldrich), and benzyl alcohol (Baker) were of analytical grade and were used without further purification. Gaseous H2S (99.5% Matheson) was used as received. N-Methyl-N'-hexadecyl-4,4'-bipyridinium methyl sulfate (CI6MV2+CH3SO3-) was synthesized as described in the l i t e r a t ~ r e .Triply ~ distilled water was purified by a Millipore Milli-Q system containing a 0.4-pm Millistack filter at the outlet. Preparation of DHP vesicles, in the absence and in the presence of CdS (selectively placed at the interior surfaces or located both at the interior and exterior vesicle surfaces), and/or CI6MV2+CH3SO< followed the established methodol~gies.~.~ Vesicles, when needed, were concentrated in an Amicon ultrafiltration cell equipped with a Diaflo membrane of 100000 nominal molecular weight cutoff. Absorption spectra were taken on a Hewlett-Packard 8450A diode array spectrophotometer. Cadmium ion concentrations were determined by atomic absorption spectroscopy on an Instrument Laboratory S11 AA/AE photometer. Hydrodynamic diameters of the vesicles were determined by dynamic light scattering using a Brookhaven BI 2030 AT system. Steady-state fluorescence measurements were camed out on a Spex 1681 Fluorolog equipped with a Tracor Northern TN-6500 rapid scan detector system (using a 390-nm cutoff filter in the emission pathway). Fluorescence lifetimes were determined on a laser flash photolysis system using an excimer-pumped dye laser as the excitation source (Aex = 343 nm). Electrochemical experiments were performed by using an EG&G PAR 273 potentiostat/galvanostat.1° Vesicle samples were irradiated by a 200-W Xe-Hg lamp (Oriel) via a 390-nm cutoff filter and a 20-cm water filter (to absorb the IR output of the lamp). Typically, 3.0 mL of degassed stirred solutions was irradiated in a quartz cuvette at r c " temperature. Quantum yields at 410 nm were determined as described in the literature.' Results and Discussion Typical absorption, excitation, and emission spectra of CdS particles, incorporated into DHP vesicles, are shown in Figure
2 0 Lc " c (
-1.50 -2.00 -2.50 0
100
200
300
400
500
TIME, ns Figure 2. Fluorescence decays of CdS particles incorporated into the inner and outer bilayers of DHP vesicles containing 0 M (a), 5.0 X M (b), 1.0 X lo4 M (c), and 2.0 X lo4 M (d) Cj6MV2+(Aex = 343 nm, A,, = 500 nm). Vesicle solutions were prepared as described in the legend to Figure 1 with the exception that the appropriate amounts of C,,MV2+ were introduced prior to the addition of CdCI,.
1. This particular sample was prepared by cosonicating 2.0 X M DHP and 4.0 X M Cd2+at pH = 7.8 and subsequently introducing 4.0 X lo4 M H2S; thus, it contained CdS particles at both the interior and the exterior vesicle surfaces. The absorption edge a t 475 nm corresponds to CdS particles having approximately 5.0-nm diameters.l' The presence of shoulders (at 350 and 420 nm) in the absorption spectrum is indicative of CdS particles with diameters in the 2.5-4.0-nm range. These CdS particles are clearly in the size-quantized regime.l2J3 Fluorescence was observable, as reported p r e v i o ~ s l y , only ~~J~ for CdS particles which were located in the interior of DHP vesicles. It can originate in the recombination of the trapped charge carriers or in the excitonic state.16J7 The former manifests in the appearance of a broad and Stokes-shifted band.17 In contrast, the spectrum due to excitonic fluorescence appears as a sharp band near the absorption onset and is considered to arise from the detrapping of trapped e1e~trons.l~The observed broad-structured emission spectrum reflects contributions of both excitonic and trapped fluorescence of the CdS particles incorporated in DHP vesicles (Figure 1). The correspondence of the maximum in the excitation spectrum with the shoulder in the absorption spectrum (Figure 1) is indicative of the predominant presence of a given population of size-quantized CdS particles." Fluorescence decays of CdS particles in DHP vesicles were found to be multiexponential, as expected.16J7 Incorporation of long chain CI6MV2+into DHP vesicles decreased the fluorescence intensities but did not affect the decay times (Figure 2). Apparently, electron transfer to cetylmethylviologen occurs predominantly by static quenching in DHP vesicles. This is not surprising since anchoring C16Mv2+into the matrix of the DHP bilayer considerably hindered the reactant mobility and, hence, diminished the possibility of dynamic quenching. It should be pointed out that all quenching experiments were carried out 30 min subsequent to the in situ formation of CdS (Le., 30 min subsequent to the introduction of HzS). Fluoescence experiments carried out on the same system a week after the preparation of the CdS indicated a much less (at least 10 times) quenching efficiency of C16MV2+.This phenomenon suggests that aging changes the morphology and/or location of the sizequantized CdS particles such that they become less accessible to the quencher. Hydrodynamic diameters of vesicles, prepared from 2.0 X M DHP (95 f 5 nm), remained unaltered for vesicles prepared from 2.0 X M DHP and 2.0 X lo4 M CI6MV2+.Addition of 4.0 X lo4 M Cd2+increased, however, the mean hydrodynamic diameters of DHP vesicles to 114 f 6 nm. Additional introduction of 1% (v/v) benzyl alcohol increased the mean hydrodynamic diameters of DHP vesicles somewhat more (to 130 f 7 nm). Further addition of benzyl alcohol (up to 4%, v/v) did not affect the sizes of DHP vesicles; they remained at 130 f 7 nm.
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9593
Letters 2*oo
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WAVELENGTH, nm Figure 3. Absorption spectra of degassed solutions of CI6MV2+,incorporated into the inner and outer bilayers of DHP vesicles which contained CdS particles exclusively in their inner bilayers, subsequent to 0 s (c), IO s (b), and 60 s (a) irradiation (200-W Xe-Hg lamp, 390-nm cutoff and water filters) in the presence of 1.2% (v/v) benzyl alcohol. Vesicles were prepared by the presonication (70 W, 82 "C) of 2.0 X lo-' M DHP ( 5 min), followed by further sonication (70 W, 82 "C) upon the addition of stoichiometric amounts of 0.1 M NaOH ( 5 min) and 2.0 X lo4 M C,6MV2t and 2 X lo4 M CdCI2 (10 min). Solutions were bubbled for 45 min prior to and during sonication with a gentle stream of argon. Traces of titanium, released by the tip of the sonicator, were removed by centrifugation (30 A n at 3000 rpm). Cd2+ions were removed from the outer vesicle surfaces by passing the dispersions through a Bio-Rad AG5OW-X2 (100-200 mesh, H-form, 16-cm length, 1.0-cm diameter) cation-exchange resin column. The Cd2+concentration in vesicles, subsequent to the ion-exchange procedure, was determined to be 1.0 X M. CdS Darticles were generated by the subsequent addition of 4.0 X 10-4 M H~S.
8z 4 1 4
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400
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700
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WAVELENGTH, nm Figure 5. Absorption spectra of degassed solutions of CI6MV2+,incorporated into the inner and outer bilayers of DHP vesicles which contained CdS particles exclusively in their inner bilayers and 4.0 X IO4 M Agt on their outer surfaces, prior (-) and subsequent (---) to 5 min of irradiation (390-nm cutoff and water filters) in the presence of 1.6% (v/v) benzyl alcohol. The differential spectrum is also shown. (-e-)
electron transfer in DHP vesicles. Indeed, photoreduction of Fe(CN)6*, entrapped in the internal pools of DHP vesicles, could be mediated by Zn(TPPS)4-, distributed in the bulk aqueous solution, by transmembrane charge relay via CI6MV2+incorporated into the inner and outer vesicle bilayers.21 Similarly, photoelectron transfer from CdS particles, located at the exterior surfaces of DHP vesicles, to short chain viologens, entrapped in the vesicle interiors, was unequivocally demonstrated by in situ simultaneous optical and electrochemical monitoring.6 Although we reproduced these experiments, no photocurrent could be measured on using cetylmethylviologen. Apparently,while reduced methyl and short chain viologens diffuse to the anode, the longer 0.30 chain C & f V ' + cannot. This observation is in accord with the 0 failure to detect cyclic voltammetric responses for DHP-vesicled incorporated cetylmethylviologen.22 Transmembrane photoelectron transfer was demonstrated in dissymmetricalDHP vesicles which contained sizequantized CdS Oa20 particles in their inner bilayers, CI6MVz+in their inner and outer bilayers, and Ag+ on their outer surfaces. Irradiation (A > 390 2 0.10 8 nm) in the presence of 1.6% (v/v) benzyl alcohol led to the development of a new broad absorbance, with a maximum of 420 nm, a minimum of 325 nm (interband), and a shoulder around 500 nm (Figure 5 ) , which corresponded to colloidal silver par5 0.00 t i ~ l e s . ~A~ much - * ~ slower (>5 times) Ago formation was observed Q 0.0 0.5 1 .o 1.5 2 .O in identically prepared DHP vesicles, even in the absence of CI6MV2+,upon irradiation with identical light energy. Diffusion BENZYL ALCOHOL CONC. (vel.%) of silver ions from the exterior to the interior vesicle surface is Figure 4. Quantum yields for the formation of C I 6 M V t in degassed unlikely to play a role in this proccss since independent experiments solutions of Cl6MV2+,incorporated into the outer and inner bilayers of demonstrated the quantitative removal of Ag+ externally added DHP vesicles which contained CdS particles exclusively in their inner to DHP vesicles.2s Further blank tests showed that all other bilayers, as a function of added benzyl alcohol Concentrations, Vesicle components in the system were necessary for silver ion photoresolutions were prepared as described in the legend to Figure 3. duction. Using the 420-nm absorbance of the colloidal silver Photoelectron transfer was also observed by monitoring the particle formed, and taking = 3100 M-I cm-1,26permitted the development of reduced cetylmethylviologen (Cl6MV.+) upon estimation of the quantum yield for the overall process to be 0.15.2' band gap irradiation of CdS particles, confined in the inner bilayers Transmembrane photoelectron transfer involved the following of DHP vesicles, in the presence of 1.2% (v/v) benzyl alcohol as steps (see Figure 6): (i) band gap irradiation into size-quantized a sacrificial electron donor (Figure 3). The absorption spectra CdS particles, located within the inner surface of 130 f 7 nm in Figure 3 indicate the presence of both monomeric (characterized diameter DHP vesicles, to produce conduction-band electrons and by absorption maxima at 398 and 602-605 nm) and dimeric valenceband holes; (ii) nonproduotive electron-hole recombination; (characterized by broad bands centered at 370 and 530 nm) (iii) transfer of a portion of conduction-band electrons to neighCI6MV". Increasing the irradiation times from 10 to 60 s deboring CI6MV2+molecules to produce C,6MV*+;(iv) quenching creased the concentrations of monomeric CI6MV'+ from 73% to of a portion of valence-band holes by benzyl alcohol; (v) trans53%.18 This situation is characteristic for long-chain vi010gens.~~-~~ membrane electron exchange between cetylmethylviologen m o m From the initial rate of viologen cation radical accumulation, the and dications; and (vi) reduction of Ag+ at the outer DHP vesicle quantum yield was estimated at various added benzyl alcohol (hole surface by CI6MV'+. Although electron transfer across cetylscavenger) concentrations (Figure 4). The effect of the conmethylviologen-containing egg-lecithin vesicles has recently been centration of the added hole scavenger can be attributed to its rationalized in terms of a mechanism which involved the ratetransmembrane diffusion in a rate-determining step. determining disproportionation2C16MV'+= CI6MVO+ C16MV2+ CdS-mediated cetylmethylviologen photoreduction offers, at and the subsequent diffusion of CI6MVoacross the bilayer,29in least in principle, the possibility of demonstrating transmembrane our system it cannot play an appreciable role since the equilibrium
3
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9594 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 INSIDE
OUTSIDE
Letters through a DHP vesicle without destroying it, and its oxidation product, benzaldehyde, can be blown off of vesicle solutions.
Acknowledgment. Support of this work by a grant from the
US. Department of Energy is gratefully acknowledged. O.H. is indebted to the Szkhenyi Istvdn Scholarship Foundation for making his work in Syracuse possible. We thank Michael Brandt and Sdndor Ntmeth for their competent technical assistance.
References and Notes
M A P + /
1-
Figure 6. Schematic of the proposed transmembrane photoelectron transfer. See the penultimate paragraph of the Results and Discussion section for details. The wiggles extending from MV2+ and MV'+ represent the alkyl chains in C t 6 M v 2 +and CI6MV'+.
constant for the disproportionationis extremely low (10-~-lO-4).~~ Since the quantum yield for the accumulation of CI6MV'+in the presence of 1.6% benzyl alcohol (but in the absence of added Ag+) is ca. 0.25 (Figure 4), the efficiency of the transmembrane silver ion reduction (i.e., steps v and vi) is about 60%. This value is in accord with an earlier observation of transmembrane electron transfer in the opposite direction (Le., from the outside to the inside)21and permits the assessment of 0.4 for the upper limit of the quantum yield for transmembrane photoelectron transfer from the published quantum yields (O.6'9*J'J1) for CdS-mediated, direct (Le,, CdS and viologens are being in contact on the same side of the vesicles) photoreduction. The present system offers many advantages for the systematic investigation of transmembrane electron transfer. First, and most importantly, it provides a well understood, reproducible, and beneficial matrix for compartmentalization of the components of the electron-transfer apparatus. Second, it allows the in situ generation of size-quantized semiconductor particles in the inner bilayer from their metal ion precursors, confined within the aqueous pools of the vesicles. Third, it utilizes cetylmethylviologen as an anchored electron-transfer mediator. Unlike its shorter chain analogues, CI6MV2+cannot be ion exchanged from the vesicles. Fourth, it operates with AB+, which is easily reduced by CI6MV'+ and electrostatically attached to the outer DHP surface, and, importantly, its photochemistry and colloid chemistry are well understood. Finally, benzyl alcohol is employed in the present system as an efficient hole scavenger since it is chemically and photochemically (&rradiation > 390 nm) inactive, it is able to diffuse
(1) Permanent address: Department of General and Inorganic Chemistry, University of Veszprh, H-8200. Veszpr6m Pf. 158, Hungary. (2) Baral, S.;Fendler, J. H. In Photoinduced Electron Transfec Chanon, M., Fox, M. A., Eds.; Elsevier Science Publishers: Amsterdam, The Netherlands, 1988; Part B, p 541. (3) Hurst, J. K. In Kinetics and Catalysis in MicroheterogeneousSystems; Gritzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991; p 183. (4) Fendler, J. H. J . Phys. Chem. 1985,89,2730. (5) Tunuli, M. S.;Fendler. J. H. J . Am. Chem. Soc. 1981, 103, 2507. (6) Lee,L. Y. C.; Hurst, J. K.; Politi, M.; Kurihara, K.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 370. (7) Tricot, Y.-M.; Manassen, J. J. Phys. Chem. 1988,92, 5239. (8) Tricot, Y.-M.; Porat, Z.; Manassen, J. J. Phys. Chem. 1991,95,3242. (9) Pileni, M.-P.; Braun, A. M.; Gritzel, M. fhotochem. Photobiol. 1980, 31, 423. (IO) Chang, A.-C.; Fendler, J. H. J . Phys. Chem. 1989, 93, 2538. (1 1) Fojtik, A.; Weller, H.; Koch, U,; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 969. (12) Henglein, A. Chem. Rev. 1989, 89, 1961. (13) Steigerwald, M. L.; Bms, L. E. Acc. Chem. Res. 1990, 23, 183. (14) Tricot, Y.-M.; Fendler, J. H. J. Am. Chem. Soc. 1984, 106, 2475. (15) Tricot, Y.-M.; Fendler, J. H. J. Am. Chem. Soc. 1984, 106, 7359. (16) ONeil, M.; Marohn, J.; McLendon, G.J. Phys. Chem. 1990, 94, 4356. (17) EychmuUer, A.; Hasselbarth, A,; Katsikas, L.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1991, 95. 79. (18) % monomer = [100(A60Snm/AS52nm - 0.S9)/0.98].19 (19) Lei, Y.;Hurst, J. K. J . Phys. Chem. 1991, 95, 7918. (20) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J . Phys. Chem. 1988, 92,6978. (21) Hurst, J. K.; Lee, L. Y.-C.; Gritzel, M. J. Am. Chem. Soc. 1983,105, 7048. (22) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J . Eleciroanal. Chem. 1988, 246, 337. (23) Henglein, A.; Linnert, T.; Mulvaney, P. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1449. (24) Henglein, A. Isr. J . Chem., in press. (25) Meldrum, F.; Horvsth, 0.;Fendler, J. H. Unpublished results. (26) Yonezawa, Y.; Sato, T.; Ohno, M.; Ha&, H. J. Chem. Soc.,Faraday Trans. 1 1987,83, 1559. (27) The uncertainty of these values is rather high since the spectrum of the colloidal silver is sensitive to the medium in which it is generatedS2* (28) Vogler, A.; Quett, C.; Kunkely, H. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1486. (29) Hammarstrom, L.; Almgrem, M.; Norrby, T. J . Phys. Chem. 1992, 96, 5017. (30) Henglein, A. J . Phys. Chem. 1982, 86, 2291. (31) Youn, H.-C.; Tricot, Y.-M.; Fendler, J. H. J . Phys. Chem. 1987, 91, 581.
