Intramolecular vibrational relaxation in aryl alkyl ketones as revealed by excess energy dependence of T1→S intersystem crossing rate

Abstract

T 1(nπ*)→S0 intersystem crossing in dilute vapors of aryl alkyl ketones has been studied as a function of excess vibrational energy (in T1) to determine the relative efficiency of intramolecular vibrational energy redistribution. The excess energy dependence of the nonradiative decay rate, as measured by α=d ln k/dE, is strongly molecular structure dependent. Molecules with a large alkyl group exhibit very large α for S0→S1(nπ*) excitation and relatively small α for S0→S2(ππ*) excitation. Molecules with a small alkyl group (acetophenone) exhibit an intermediate value of α, which is independent of the mode of excitation. These results indicate that the rate and the extent of communication between carbonyl and ring vibrations are significantly smaller for molecules with a bulky alkyl group. The possible origin of this ’’size-dependent’’ vibrational relaxation is discussed.