SOLVENT RELAXATION AROUND THE EXCITED-STATE OF INDOLE - ANALYSIS OF FLUORESCENCE LIFETIME DISTRIBUTIONS AND TIME-DEPENDENCE SPECTRAL SHIFTS

Analysis of the fluorescence lifetime distributions of indole in polar protic solvents by the maximum entropy method (MEM) has allowed to ob tain relatively narrow peaks, which originate first from the main emis sion decay component and second from additional short components which are due to spectral kinetics processes. These latter components are c haracterized by positive amplitudes at short emission wavelengths and by negative amplitudes (with the same mean time component value) at lo ng-wavelength edge of the fluorescence spectrum. For both positive and negative components, the effect of red-edge excitation is strongly pr onounced: they sharply decline or even disappear if the excitation is performed at the long-wavelength edge of the absorption spectrum (300 nm). As expected, these components are very sensitive to temperature. The observed relaxational component shifts to longer values as the tem perature decreases. The variation is linear in Arrhenius coordinates a nd results in activation energies of 7.5 kcal/mol. The shifts of emiss ion spectra as a function of time (TRES) have allowed to obtain relaxa tion times in the same range as observed in lifetime distributions. Th e C(t) function defined by using the barycenters of the emission spect ra can be described by MEM as a sum of two or three discrete species d epending on the temperature. The longest one displays a value similar to that of the longest longitudinal relaxation time of isobutyl alcoho l as determined by dielectric measurements in the same temperature ran ge. The excited state stabilization energy is around 3.7 kcal/mol. Thi s set of results is consistent with a mechanism of general dielectric solvent relaxation rather than formation of binary excited state compl exes in the time range studied.