2783is concluded that we are observing the combined contributions of increasing k, and decreasing knr,The wavelengths of the absorption maximum of the J-band in samples of 40 and 70 OC agitation are 645 and 652 nm, respectively, corresponding to transition energies of 1.92 and 1.90 eV. This small difference of 0.02 eV, which might potentially be translated into a size-dependent AE, is too small, however, to explain the observed change of k,, as can be seen from the energy gap dependence shown in Figure 2.The mechanism of exciton-tapping ~u p e r s e n s i t i z a t i o n~'~~~ as-sumes that k, decreases with increasing aggregate size. Terminal molecules of a given aggregate act as traps for the highly mobile excitons and enhance the probability of electron injection into the conduction band of silver halide (AgX) to give (AgX)-and (dye),+. It is therefore considered that the observed decrease in k, with aggregate size supports the mechanism of the excitontrapping supersensitization. Registry No.The dependence of fluorescence lifetime and relative quantum yield on temperature and aggregate size has been investigated for the J-aggregate of pseudoisocyanine (PIC) on an AgBr surface, varying the average physical size of the aggregate in a statistical sense by diluting it with a close structural analogue. The dominant feature controlling the excited-state dynamics is found to be energy transfer to a defect state which is nonradiative at room temperature. The rate of this transfer process increases with aggregate size. At large aggregate sizes, a weak superradiant enhancement of the J-aggregate radiative rate is also observed, with a temperature dependence which suggests strong coupling of the J-aggregate exciton to a low-frequency phonon. Since both the energy transfer to the defect state and the radiative decay compete with the desired process of electron transfer from the aggregate excited state to the AgBr conduction band, the sensitizing efficiency of the J-aggregate is expected to decrease with increasing aggregate size. Measurement of this size-dependent sensitizing efficiency shows a smaller loss than expected, indicating that the electron-transfer rate from the aggregate excited state to the AgBr conduction band increases with increasing aggregate size.

Heterogeneous electron transfer involves the coupling of a dense manifold of highly delocalized electronic levels of the solid state to a discrete molecular state as well as an abrupt change in phase in the reaction coordinate. These features make this problem unique relative to homogeneous solution phase or gas phase reaction mechanisms which involve coupling between discrete states within a uniform medium. Recent advances in time domain optical methods are discussed in the context of studying interfacial charge transfer processes at single crystal semiconductor surfaces as a means to probe the primary processes governing heterogeneous electron transfer. Two distinct boundary conditions are discussed: charge injection into a semiconductor from an adsorbate and charge emission from a semiconductor to an acceptor. The reaction dynamics are investigated using a combination of nonlinear spectroscopies with an emphasis on mapping the electron transport and transfer and investigating the role of nuclear vs electronic relaxation mechanisms in the bamer crossing dynamics. A fundamental understanding at this level seeks to determine the criteria for fully optimizing charge separation at surfaces.

The primary process in conventional photography involves electron transfer from an excited dye molecule into the conduction band of a silver halide microcrystal. Repeated events of this type ultimately lead to formation of a small, stable cluster of silver atoms in the silver halide that acts as the latent image, along with the one-electron oxidized forms of the dye molecules. Here we describe a new concept for increasing the efficiency of photographic systems, two-electron sensitization, which makes use of the chemical potential stored in the oxidized dyes. In conventional photography, subsequent reactions of the oxidized dyes are not controlled and may in fact include counterproductive return electron transfer reactions (recombination). In the two-electron sensitization scheme, an appropriately designed electron donor molecule, X−Y, that is added to the photographic dispersion transfers an electron to the oxidized dye to give a radical cation, X−Y•+. The X−Y•+ then undergoes a fragmentation reaction to give a radical, X•, and a stable cation, Y+. The radical X• is chosen to be sufficiently reducing so that it can inject an electron into the silver halide conduction band. In this way, the oxidized dye, which is a strong oxidant, is replaced by the radical, X•, which is a strong reductant. The two-electron transfer scheme has the potential of doubling the photographic speed because two electrons are injected per absorbed photon. Here we describe the mechanistic details of the two-electron sensitization scheme and the structural and energetic criteria for the X−Y molecules. Several electron-rich carboxylate molecules that meet these criteria have been identified. Solution-phase experiments to determine the fragmentation (decarboxylation) kinetics and the reducing power of the resultant radicals are described. Photographic data demonstrate that increases in sensitivity by factors approaching 2 can be obtained, confirming the viability of the two-electron sensitization concept.

Experimental and computational studies of photophysical processes in silver halide imaging materials are presented. Recent investigations that have refined our understanding of carrier recombination paths, exciton behavior, quantum confinement effects, and the structure and function of small surface silver clusters are detailed. The theory and mechanism of electron and hole injection from photoexcited surface-adsorbed dye molecules are outlined. The carrier trapping properties and subsequent photophysics of transition-metal dopant complexes are presented. Particular emphasis is given to the role of relaxation processes in electron and hole trapping events.