Radiationless singlet deactivation in isolated large molecules. I Naphthalene, naphthol, and naphthylamine

Abstract

Fluorescence lifetimes of naphthalene, β-naphthol, and β-naphthylamine have been measured under collision-free conditions in the vapor. The lifetimes were then combined with the measured quantum yield (for naphthalene), or with the estimated radiative decay rate (for β-naphthylamine), to deduce the nonradiative decay rate of the molecule as a function of excitation energy. The results indicate that (1) the nonradiative decay probability increases, first slowly then very rapidly, as the excess vibrational energy in the lowest excited singlet state (S1) is varied over a range of nearly 20 000 cm−1, and (2) a plot of nonradiative decay rate versus excitation energy exhibits a break at photon energy corresponding to the second excited singlet state of a molecule. Observation (1) is interpreted in terms of two different pathways of radiationless transition that compete in the singlet deactivation of the molecules. It is argued that, in the region of low excess energies, the intersystem crossing to the triplet manifold dominates over the internal conversion to the ground state owing to a more favorable Frank-Condon factor. As the excess vibrational energy is increased the internal conversion, which is expected to display greater energy dependence than the intersystem crossing, becomes increasingly more important and it finally becomes the dominant pathway of nonradiative decay at high excess energies. This model of competing radiationless transitions is shown to be consistent with the observed deuterium isotope effect on the nonradiative decay rate of naphthalene vapor. Observation (2), which confirms our earlier results [J. Chem. Phys. 58, 1247 (1973)], suggests that S2→S1 internal conversion leads to changes in vibrational distribution, which is substantially maintained during the time scale of radiationless decay of S1. Thus, the intramolecular vibrational energy redistribution, leading to the randomization of vibrational levels, does not appear to be a fast process even for large molecules with high density of states.