Photophysics of indole in polar solvent: Analysis of fluorescence lifetime distributions and time-dependence spectral shifts

Abstract

The fluorescence lifetime distribution of indole in isobutanol is described by the Maximum Entropy Method (MEM) by a main slow emission decay component and a second fast one which is due to spectral kinetics processes. These latter components are associated with positive amplitudes at short emission wavelengths and with negative amplitudes with the same mean decay time value at the long‐wavelength edge of the fluorescence spectrum. Both positive and negative rapid components 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 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 the centers of the time‐dependent emission spectrum, can be described by MEM as a sum of two or three discrete components depending on the temperature and not by continuous distributions. The longuest one displays a similar value as that of the longuest longitudinal relaxation time of isobutanol 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 are consistent with a mechanism of general dielectric solvent relaxation rather than formation of binary excited state complexes in the time range studied.