PICOSECOND KINETICS OF LIGHT-INDUCED ELECTRON-TRANSFER FROM J-AGGREGATED CYANINE DYES TO AGBR MICROCRYSTALS - EFFECT OF AGGREGATE SIZE

The light-induced electron transfer from 5,5'-dichloro-9-ethylthiacarbocyanine (dye 1) and related dyes to octahedral silver bromide microcrystals was studied in a photographic emulsion (i.e., a suspension of the microcrystals in an aqueous gelatin matrix). The energy gap dependence of the quantum yield of electron transfer [phi(r) = k(s)/(k(s) + k(L)), where k(s) and k(L) are the rate constants of the electron transfer and competing deactivation channels, respectively] obeyed the Marcus theory with a very small total rearrangement energy of 0.05 eV. The aggregate size of dye 1 on the microcrystals was increased by increasing the agitation temperature of the emulsion. The effect of the aggregate size on the kinetics of electron transfer was studied by means of a time-correlated single-photon-counting system. The film samples were scanned at several centimeters per second to avoid photoinduced damage. For a J-aggregate consisting of approximately 6 molecules, phi(r), k(s), and k(L) were 0.44, 1.69 x 10(10) s-1, and 2.17 x 10(10) s-1, respectively, and for a larger J-aggregate of approximately 14 molecules, we obtained 0.20, 1.03 x 10(10) s-1, and 4.23 x 10(10) s-1, respectively. It is thought that the proportionality of the radiative rate constant of the J-aggregate to its size, as given by exciton theory, made a substantial contribution to the increase in k(L) and to the decrease in phi(r) with aggregate size. The decrease in k(s) with aggregate size is consistent with the mechanism of exciton-trapping supersensitization.