J. Phys. Chem. 1988, 92, 255-256 complex so much increases the density of rovibronic levels that all resolution of AlO-based vibrational level structure is lost in excited-state complex formation.I0
255
various degrees from weak binding van der Waals interactions to strong chemical bonding. The spectra in Figure 3 suggest that C 0 2 complex formation or solvation presents only a small perturbation to the electronic spectrum of the metal oxide. Not only do these results indicate that we are dealing with relatively weak binding of the C 0 2 to the product metal oxide producing a highly perturbed metal oxide excited electronic state, but also, in conjunction with recent results obtained when the transition metals are entrained in C O and subsequently oxidized with O3or N02,6these results indicate that the spectra of MCO, M(CO),, and M-C02 complexes might be observed through a variety of spectroscopies in the vicinity of known atomic transitions for the metal atoms of interest.
Conclusion The results obtained thus far indicate that using an extrapolation of the techniques we have outlined here, it will be possible to (1) evaluate nontransition-metal M(C0)2 (x = 1,2) binding energies, (2) study long-lived metal oxide-carbon dioxide solvation complexes formed in metal-C02 complex oxidation, and (3) characterize the spectroscopy of M-CO and M-C02 complexes. We believe that these metal atom based solvation complexes present an opportunity to study the middle ground region extending in
Photoinduced Electron Transfer from Colloidal Cadmium Sulfide to Methylviologen: A Picosecond Transient Absorption Study Yoshio Nosaka,* Hajime Miyama, Department of Chemistry, Technological University of Nagaoka, Nagaoka, 940-21 Japan
Mamoru Terauchi, and Takayoshi Kobayashi Department of Physics, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku Tokyo, 113 Japan (Received: September 29, 1987)
Picosecond absorption spectra were measured for methylviologen (MVz+)on illuminated colloidal CdS. The reduction of MV2+was found to occur within 100 ps after the excitation. Thz rate of reduction is larger than the reported value by more than 1 order of magnitude. The reducible MVZ+is adsorbed on the surface but not in solution as has been reported.
Study of semiconductor particles is attractive because of possible applications for the direct conversion of solar energy into chemical energy.' Therefore, it seems important to investigate the processes such as interfacial electron transfer and electron-hole recombination occurring at illuminated semiconductor colloidal particles. For that purpose, time-resolved laser spectroscopy for transparent colloidal semiconductor solution is very useful.2 Serpone and co-workers3first reported picosecond absorption spectra of reduced methylviologen ( M Y + ) on illuminated colloidal CdS and claimed that the photoinduced electron transfer from CdS to methylviologen (MV2+) occurred at the rate of lo9 s-l. Rossetti and Bms4 reported that the rise time of MV'+ radical is between 5 ns and 20 ps from a picosecond resonance Raman scattering study. Recently, Ramsded has reported that MVZ+is not adsorbed on colloidal CdS stabilized with sodium hexametaphosphate (HMP) from the analysis of nanosecond laser flash photolysis. The authors have reported evidence of the adsorption of MV2+ on colloidal CdS stabilized with various anionic reagenk6 Furthermore, we found that poly(acry1ic acid) (PAA) was superior as a stabilizing agent of CdS colloid and that the colloidal CdS stabilized with HMP, which was used by Serpone3 and Ramsden,5 was frequently aggregated in our experiments. If MV2+ is adsorbed on the surface of colloidal CdS, the electron transfer to MVZ+ is expected to be faster than 1 ns in contrast to their ( I ) Graetzel, M. Acc. Chem. Res. 1981, 14, 376-384. (2) (a) Pelizzetti, E.; Serpone, N., Eds.Homogeneous and Heterogeneous Photocatalysis; Reidel: Dordrecht, 1986; NATO AS1 Ser. C, Vol. 174. (b) Kalyanasundaram, K.; Graetzel, M.; Pelizzetti, E. Coord. Chem. Rev. 1986, 69, 57-125. (3) Serpone, N.; Sharma, D. K.; Jamieson, M. A,; Graetzel, M.; Ramsden, J. Chem. Phys. Lett. 1985, 115, 473-476. (4) Rossetti, R.; Brus, L. E. J . Phys. Chem. 1986, 90, 558-560. (5) Ramsden, J. J. Proc. R. SOC.London, A 1987, 410, 89-103. (6) Nosaka, Y.; Fox, M. A. Langmuir, in press.
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However, in our previous study, the direct observation of MV2+reduction failed because of a poor time resolution of the ~ measuring system (1 n ~ ) . Then, we measured picosecond absorption spectra of PAA-stabilized CdS colloid containing MVZ+ and report here briefly some evidence obtained about the mechanism of photoinduced interfacial electron transfer. Cadmium nitrate (Cd(N03),), sodium sulfide (Na2S.9H20), methylviologen (Nakarai Chemicals Co.), and poly(acry1ic acid) (25%, Wako Chemicals Co.) were used as received. Colloidal CdS was prepared by a similar method as reported previ~usly.~ Into the mixture of 10 mL of 0.3% PAA solution buffered with NaOH and 15 mL of 4 mM NazS solution was poured 15 mL of 4 mM Cd(N03)2solution in the dark, and then MV2+was added to the colloidal CdS solution. The resultant solution showed no light scattering, and the band edge of the absorption spectrum was at about 500 nm. The sample solution was placed in a 3-mm quartz cell and bubbled with N2 gas for more than 15 min before the measurement. In order to stir the sample solution, the bubbling was continued during the measurement. The apparatus used for picosecond transient absorption measurement is already described elsewhere.8 The excitation light for CdS was frequency-tripled (355 nm) light from a mode-locked Nd:YAG laser. The excitation beam of 0.25 mJ/pulse was focused on the size of 1.5" diameter at the front surface of the sample cell. The spectra were obtained by averaging over 32 or 64 shots. Figure 1 shows the picosecond transient absorption spectra obtained before and after the 30-ps excitation. Since the reduced form of MV2+ has an absorption band at about 600 nm,9 these (7) Nosaka, Y.; Fox, M. A. J . Phys. Chem. 1986, 90, 6521-6522.
(8) Iwai, I.; Ikeuchi, M.; Inoue, Y.; Kobayashi, T. In Protochlorophyllide Reduction and Greening; Sironval, C., Brouers, M., Eds.; Martinus Nijhoff/Dr. W. Junk Pubiishers: The Hague, 1983; p 99. (9) Kosower, E. M.; Cotter, J. L. J . Am. Chem. Sot. 1964, 86, 5524.
0 1988 American Chemical Society
J . Phys. Chem. 1988, 92, 256-260
256 0.1 5
I
-
I LL: V
Z
4
m
a: m m
0
I
I I 0
5 50
600
6 50
700
WAVELENGTH/ nm
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
