J . Phys. Chem. 1988, 92, 256-260
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Figure 1. Picosecond transient absorption spectra for 1.5 mM CdS containing 2 mM MV2+ and 0.075% polyacrylate. The numbers for the spectra indicate the delay time before (-100 ps) and after (100,200, 1000 ps) the excitation laser pulse of 30 ps.
transition spectra are attributable to the MV'+ radical. The fact that the absorption band was rather broad may be responsible for the aggregation of MV" because the MV" dimer has an absorption band at 535 nm.9 As reported in the previous paper, 10-ns laser excitation on a similar solution causes the absorption bands attributable to MV'+ and its dimer. In order to confirm that the absorption is attributable to the reduced methylviologen, the quantum yield for electron transfer is estimated as follows. The change in the absorbance with laser excitation was about 0.1 at 606 nm. By using the molar extinction coefficient of 13 700 M-' cm-' at 606 nm,Io the concentration of MV" was calculated to be 24 pM. The energy density of excitation pulse was measured to be 14 mJ/cm2. Since the absorbance of the sample solution was 2.5 cm-' at the excitation (10) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617-2620.
wavelength, the concentration of photon absorbed in 3-mm path length is calculated to be 114 MM. Thus, the quantum yield for electron transfer, a, is estimated to be about 0.2. This value of @ agrees with those obtained for the MV'+ in the previous As shown in Figure 1, the transient spectrum recorded at 100 ps after the pulse is almost the same absorbance as those at 200 ps and at 1 ns. This implies that the MV2+ is reduced by the photoinduced conduction-band electrons within 100 ps after the excitation. The square-mean diffusion distance x of MV2+ in 100 ps is calculated to be 0.6 nm, by using the equation x = (4Dt)]i2,where the diffusion constant D is 8.9 X 10-Io m2/s.5 This diffusion distance is almost the same size as the MV2+molecule. Therefore, we may conclude that the photoinduced electrons transfer to surface-adsorbed MV2+ but not to that in bulk solution via diffusion. Presumably, this scheme is also adoptable to HMP-stabilized CdS colloid which is employed by Serpone et aL3 and R a m ~ d e n because ,~ the apparent association constant, 1O4 M-I, is the same order of magnitude as that for the PAA-stabilized CdS colloid.6 In the previous study," an analysis of the dependences of @ on the pulse width and on the excitation laser intensity revealed that the photoinduced conduction-band electrons disappear within 3 ps after the excitation. By adopting the data for the colloidal CdS stabilized with styrene-maleic anhydride copolymer," the rise time of the MV2+reduction is between 20 and 100 ps. Thus, the dynamic mechanism of the photoinduced reduction of MV2+on the surface of colloidal CdS is summarized as follows: Conduction-band electrons induced by the irradiation are trapped within 3 ps at some site on the surface," which is likely a sulfur vacancy or a surface Cd2+ion. Then, the trapped electron transfers to the surface-adsorbed MV2+in the time range between 20 and 100 ps after the excitation. (11) Nosaka, Y.; Fox, M. A. J . Phys. Chem., in press.
Picosecond Pump-Probe Spectroscopy of Dyes on Surfaces: Electronic Energy Relaxation in Aggregates of Pseudoisocyanine on Colloidal Sliica Edward L. Quitevis,* Miin-Liang Horng,t and Sun-Yung Chen Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 (Received: October 12, 1987; In Final Form: November 20, 1987)
Picosecond optical pumpprobe spectroscopy was used to measure electronic energy relaxation in aggregates of pseudoisocyanine on 40-A-diameter colloidal silica particles. The adsorption of pseudoisocyanine on colloidal silica wds accompanied by the appearance of a sharp band at 569 nm, called the J-band, in the absorption spectrum. The J-band arises from an optical transition to a single-exciton state of aggregates absorbed on colloids. Nonexponential signals obtained from transient optical bleaching of the J-band are discussed in terms of a polariton model.
Introduction
The spectroscopy of molecules adsorbed on surfaces has provided a wealth of information concerning molecular interactions at solid-liquid interfaces.' In particular, studies of electronic energy relaxation of organic molecules adsorbed on surfaces are essential in understanding the efficiency of dye sensitizers on semiconductor electrodes in liquid-junction solar cells.24 This paper describes the first picosecond optical pump-probe measurements of electronic energy relaxation in aggregates of l , 1'-
* Author to whom correspondences should be addressed. 'Robert A. Welch Foundation Predoctoral Fellow. 0022-3654/88/2092-0256$01.50/0
diethyl-2,2'-cyanine, or pseudoisocyanine. (PIC), adsorbed on colloidal silica. Cyanine dyes are of interest because of their (1) Thomas, J. K. J . Phys. Chem. 1987, 91, 267 and references cited therein. (2) (a) Liang, Y.; Ponte Goncalves, A. M.; Negus, D. K. J . Phys. Chem. 1983,87, 1. (b) Liang, Y.; Ponte Goncalves, A. M. J. Phys. Chem. 1985.89, 3290. ( 3 ) (a) Anfinrud, P. A.; Causgrove, T. P.; Struve, W. S . J . Phys. Chem. 1986, 90, 5887. (b) Anfinrud, P.; Crackel, R. L.; Struve, W. S. J . Phys. Chem. 1984,88,5873. (c) Crackel, R. L.; Struve, W. S. Chem. Phys. Lett. 1985, 120, 473. (4) Alivisatos, A. P.; Arndt, M. F.; Efrima, S.; Waldeck, D. H.; Harris, C. B. J . Chem. Phys. 1987, 86, 6540.
0 1988 American Chemical Society
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Letters aggregation properties in solution and on surface^.^ Cyanine dye aggregates have been used as spectral sensitizers of semiconductor electrodes and silver halide emulsion^.^^' Transparent colloidal dispersions are convenient systems in which to apply a variety of spectroscopic techniques in order to study molecular interactions at surfaces.' Colloids are well-suited for spectroscopic studies because the small particle dimensions (40-500 A) help to minimize light scattering, the large surface-to-volume ratio enhances the optical detectability of surface interactions, and they can be readily prepared or are commercially available. Colloidal silica was chosen because surface interactions can be studied in the absence of either surface-enhanced optical effects* or electron transfer between adsorbed molecules and the c ~ l l o i d . ~The physical properties of colloidal silica have been well-documented,10 and there have been a number of photochemical studies on colloidal silica during the past several The dynamics of charge- and energy-transfer processes on colloidal silica have been recently investigated because of applications to light-to-energy conversion schemes.12 Silica particles have been compared to anionic micelles. At pH >6, the silanol groups on the surface are ionized and an electrical double layer is formed. Cationic organic molecules, such as PIC, will bind very strongly to the negatively charged surface of colloidal silica, principally through electrostatic attraction. Because the adsorbed molecules cannot penetrate the solid silica surface, these molecules face a water environment. Rather than dispersing uniformly on the silica surface as in anionic micelles, the adsorbed cationic dye molecules tend to cluster.'la Although this property is undesirable when studying isolated adsorbed molecules, it is advantageous when studying aggregates of organic molecules on surfaces. In alcohol and dilute aqueous solution (
