ELECTRON-TRANSFER THRESHOLD FOR SPECTRAL SENSITIZATION OF SILVER-HALIDES BY MONOMERIC CYANINE DYES

The energetic requirements for electron transfer to the conduction ban d of silver bromide from a series of cyanine dyes have been studied us ing both picosecond time-resolved and steady state fluorescence techni ques. The selected dyes exhibit monomeric absorption spectra in the ad sorbed state and have excited state donor energies that are calculated to span a range about the conduction band edge of AgBr. Rate constant s and yields for these electron transfers were extracted from the pico second fluorescence lifetime measurements. Yields were also obtained t hrough independent measurements of the steady state fluorescence of th ese dyes and were correlated with estimates of the free energy of the reaction to depict a threshold for electron transfer with respect to t he conduction band of AgBr. The threshold compares well with model cur ves from electron transfer theory and with literature reports of photo graphic relative quantum yield measurements on similar monomeric syste ms at silver halide surfaces. This correlation supports the electron t ransfer model for spectral sensitization of the silver halides. Contra sts are drawn with the sharp energetic thresholds expected for aggrega ted dyes.