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 obtain relatively narrow peaks, which originate first from the main emission decay component and second from additional short components which are due to spectral kinetics processes. These latter components are characterized by positive amplitudes at short emission wavelengths and by negative amplitudes (with the same mean time component value) at long-wavelength edge of the fluorescence spectrum. For both positive and negative components, the effect of red-edge excitation is strongly pronounced: 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 temperature decreases. The variation is linear in Arrhenius coordinates and results in activation energies of 7.5 kcal/mol. The shifts of emission spectra as a function of time (TRES) have allowed to obtain relaxation times in the same range as observed in lifetime distributions. The C(t) function defined by using the barycenters of the emission spectra can be described by MEM as a sum of two or three discrete species depending on the temperature. The longest one displays a value similar to that of the longest longitudinal relaxation time of isobutyl alcohol as determined by dielectric measurements in the same temperature range. The excited state stabilization energy is around 3.7 kcal/mol. This set of results is consistent with a mechanism of general dielectric solvent relaxation rather than formation of binary excited state complexes in the time range studied.