Influence of large amplitude vibrational motion on the rate of intersystem crossing: A study of single vibronic level fluorescence from aniline-h7, aniline N, N-d2, aniline-d5, and aniline-d7

Abstract

Results of single vibronic level fluorescence measurements in aniline‐h₇, aniline‐N, N‐d₂, aniline‐d₅, and aniline‐d₇ are reported. For each excited level the radiative and nonradiative lifetimes and fluorescence yields have been obtained. The dependence of the nonradiative lifetime of the excited level on the excess energy in aniline‐h₇ is seen to behave similarly to that in benzene for states independent of inversion mode excitation. Levels involving inversion amplitude must be treated separately; we interpret their lifetimes by postulating that a nonplanar triplet state is involved in the intersystem crossing. Deuteration in the ring and amino positions affects the nonradiative lifetimes to different degrees and leads to the rejection of crossing to the ³B₂ state as the rate determining step. A detailed study of the dependence of the nonradiative process on excitation of the inversion mode was carried out using the methods of Heller, Freed, and Gelbart. The numerical calculations were based on Franck‐Condon overlap factors appropriate for transitions from a near planar initial state to a significantly bent final state. The experimental trends observed with excitation of inversion mode were qualitatively reproduced for all the molecules studied except aniline N, N‐d₂.