SIMULATIONS OF SOLVENT EFFECTS ON FLUORESCENCE SPECTRA AND DYNAMICS OF INDOLES

摘要

The effect of solvent in the first two π*←π excited states of indole and 3-methylindole, (1{sup left}L{sub}a and 1{sup left}L{sub}b), has been studied using a hybrid theoretical method that couples molecular dynamics and a semiempirical molecular orbital procedure. This method yields information about the mechanism and the time scales involved in bulk solvent reorganization after excitation of the indole chromophore to the 1{sup left}L{sub}a, (or 1{sup left}L{sub}b), state, as well as the solvent induced inhomogeneous broadening. The fluorescence red shifts predicted in several solvents, (water, methanol, butanol and dimethyl ether), agree reasonably well with experimental values. These time resolved calculations also predict inversion of the excited states with respect to vacuum and indicate that the solvent relaxation has two components: the first one is inertial in character, with a Gaussian shape and a half-width at half-maximum of ～15 fs for water, and of 100-300 fs for the other solvents. The second component shows an exponential decay behavior and seems related to the longitudinal relaxation time of the solvent. The correlation times for this component are ～170 fs for water and a few picoseconds for the other solvents. Calculations performed in butanol at 0 K indicate that we can expect a fast, (1-5 ps), red shift in the fluorescence, as large as 1000-2000 cm{sup}(-1), even when the solvent is rigid, because the inertial response is still possible in this environment. Absorption shifts are not calculated well using this method because it does not take into account the electronic polarizability of the solvent, which would be responsible for half of the absorption shift.