The charge injection dynamics of dye sensitization from a surface-bound dye (coumarin 343 (C343)) to the conduction band (CB) of the TiO2 is reported here for the first time. Ultrafast fluorescence dynamics demonstrate that the charge injection from the C343 dye to the CB of the TiO2 occurs on a time scale of ca. 200 fs. The charge injection efficiency is attributed to strong electronic coupling between the dye and TiO2 energy levels. The results yield a rate of injection of 5 × 1012 s-1.

Absorption and fluorescence properties of rhodamine B in submono-, mono-and multilayer systems adsorbed on fused quartz plates have been studied at 295 and 77 K. Existence of fluorescent and nonfluorescent dimers of rhodamine B at 295 K and a structural change of the aggregate geometry at low temperatures have been postulated. Rates of nonradiative decay and photoinduced electron transfer of dimers adsbrbed on organic crystals and their possible role in hole injection are discussed.

Intermolecular electron transfer (ET), which occurs faster than solvation dynamics, has been investigated using the fluorescence up-conversion technique. The ultrafast ET processes have been observed from the electron-donating solvents to the excited coumarin dyes. In this work we have mainly focused our attention on the roles of the substitution of electron-donating solvent molecules in the ET dynamics. We have used aniline, N-monoalkylanilines, and N,N-dialkylanilines as electron-donating solvents and five 7-amino coumarins as acceptor molecules. For these systems the free energy gaps have been estimated from the cyclic voltammetry measurement and the steady-state absorption and fluorescence measurements. The experimental results indicate that the ET rate depends largely on the substituent groups of the solvent molecules. In N,N-dialkylanilines the ET rate gets smaller as the size of the alkyl substituent group becomes larger. For N-monoalkylanilines, however, the ET dynamics are not changed by the different alkyl substituent groups. In many donor−acceptor combinations we recognized that the ET rate is much faster than the solvation time. We simulated the results by the two-dimensional ET model with solvent and nuclear coordinates and found that the N-alkyl substituent effect on the ET rate appears to be mainly due to the changes in the electronic matrix element. We have also investigated the deuterium isotope effect on the ET dynamics using N-deuterated-N-monoalkylanilines as donor solvents. For the fastest ET the isotope effect is hardly observable, whereas the effect is quite large (∼20%) for slower ET. The deuterium isotope effect seems to mainly come from the change of stabilization energies in intermolecular hydrogen-bonding interaction by deuteration. The extent of deuterium isotope effect on ET is similar for all N-monoalkylanilines used. This result indicates that the size of the alkyl groups does not affect much the hydrogen-bonding interaction.

Recent theoretical and experimental progress concerning electron transfer (ET) in solution is reviewed by focusing on the mechanism of ET, which occurs much faster than solvation dynamics. Theories of ET in solution are briefly reviewed with particular emphasis placed on the relation to solvent dynamics. Experimental methods to investigate solvent polarization relaxation are described. Ultrafast intramolecular ET, which is found in back ET from the photo-induced charge-transfer state to the ground state, is described concerning highly polar betaines and mixed-valence compounds. Ultrafast intermolecular ET has been observed for the systems of various dyes in electron-donating solvents. A non-exponential process with a significant temperature dependence was observed in aniline. A faster ET with a single exponential decay as fast as 1013 s−1 was observed with no temperature dependence in a system of oxazine 1 in N,N-dimethylaniline. The ET rate constants of excited coumarins in electron-donating solvents drastically depend on the substituent groups of the coumarin. Relatively small Stokes shifts in steady-state fluorescence spectra of ultrafast reacting molecules in solution are evidence of a “chemical timing” effect; namely, the reaction occurs in a non-equilibrium configuration of the solvent. These experimental observations are explained in terms of the extended Sumi–Marcus theory, in which the effect of solvation dynamics and low- and high-frequency vibrational modes are taken into account.

We have investigated intermolecular electron transfer (ET) from electron-donating solvents (aniline and N&-dimethylaniline) to coumarins in the excited state by means of the femtosecond fluorescence up-conversion technique. The coumarins we studied have a variety of structures with different substituents in the 4-and 7-positions. The ET occurs on a time scale ranging from a few nanoseconds to a couple of hundred femtoseconds depending on the structure of the coumarins and solvent. As for the 7-position, as the length of the alkyl chain on the amino group is longer, the ET is slower, and when the amino group is fixed by a double-hexagonal ring, it is slowest. When the electron-accepting ability of the substituent in the 4-position is increased, the reaction occurs faster. The origin of this substituent effect is mainly attributed to the variation of the energy gap between the reactant and product states. This is confirmed by theoretical calculations in terms of the extended Sumi-Marcus two-dimensional model. Good agreement between the experiment and calculation indicates that some of the reactions take place from the relaxed vibrational state of reactant to the excited vibrational states of high-frequency modes of product states. The simulated population decays for nonequilibrium configuration of solvents agreed well with experimental data. In the steady-state fluorescence spectra was also observed an effect of very fast fluorescence quenching due to ET; i.e., the amount of fluorescence Stokes shift depends on the rate of ET because the excited state is quenched in competition with thermal equilibration of the solvent configuration. We regard this spectral shift as the result of the "chemical timing" effect in solution.