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.

Excited state enol-keto isomerization in salicylic acid ͑SA͒ monomer and dimer has been studied in a supersonic free jet expansion. Two carboxylic group rotamers of SA with significantly different photophysical properties are found in the expansion. Rotamer I, the major form of SA in the expansion, has an intramolecular hydrogen bond and can undergo excited state tautomerization reaction. Its S 1 origin is at 335.34 nm. Single vibronic level emission spectra are dominated by progressions in OH stretching ͑3230 cm Ϫ1 ͒, and in-plane bending of the carboxylic group ͑240 cm Ϫ1 ͒. The spectra appear to consist of two components, normal ͑UV͒ and tautomer ͑BLUE͒ emissions, even at the origin. The intensity of the BLUE relative to the UV emission depends on the vibronic state, rather than the excess vibrational energy between the origin and 1100 cm Ϫ1. The fluorescence decay time profiles for both the emission components of rotamer I are identical within ϳ1 ns experimental time resolution. A nonradiative decay process with an activation energy of ϳ1100 cm Ϫ1 is deduced from an abrupt decrease in fluorescence lifetimes above this energy. The rotamer II cannot undergo excited state tautomerization. Its electronic origin is at 311.52 nm and emits only UV fluorescence. Upon increasing the concentration of the SA sample, a new spectrum is observed. Due to a nonlinear concentration dependence of the intensity and the propensity of SA to form dimers in solution, it is assigned to the SA dimer. This spectrum shows possible evidence of double proton transfer in the S 1 state.

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Experimental and theoretical (PM3) studies of 7-hydroxyquinoline in glycerol and in ethylene glycol show the occurrence of a proton-transfer reaction in the ground as well as in the first singlet electronically excited states. Both studies indicate that the H-bond bridge formed in the 1:1 complex provides a stabilization of the keto form in the S0 state (λabs= 420 nm). In S1, a photoinduced proton-transfer reaction solely occurs in the bridged or well-prepared H-bonded enol form, producing a fraction of the keto tautomer that emits a largely Stokes shifted band (λemis = 530 nm). The time-resolved fluorescence measurements show that the dynamics of this proton-hopping reaction is viscosity-dependent (0.5 ns in ethylene glycol and 0.8 ns in glycerol). Theoretical calculations indicate the coexistence of cis and trans rotamers of the dye in the gas phase, in agreement with the observation in a jet-cooled molecular beam. The optimized geometries of the 1:1 complexes of both cis-enol and keto tautomers with both solvents indicate that proton-transfer dynamics involves a global nuclear motion of the H-bond bridge. In both associated tautomers, the 2-OH group of the glycerol molecule does not participate in the H-bond bridge involved in the tautomerization. Analysis of the HOMO and LUMO shows that the driving force of the proton-hopping reaction originates in a partial intramolecular charge transfer from the proton-donating site to the accepting group within the dye molecule.