Mechanisms of intersystem crossing in aromatic hydrocarbons

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Abstract
A rate expression for intersystem crossings S‐T in aromatic hydrocarbons is formulated by means of a Green’s function technique. The rate constant involves not only diagonal terms of spin–orbit coupling and nonadiabatically induced spin–orbit coupling but also cross terms between them that are usually neglected. It is shown that promoting modes for intersystem crossings can be classified into two groups according to the perturbations causing intersystem crossings. One group is governed by vibronic spin–orbit coupling matrix elements and the other by vibronic coupling matrix elements which involve the nuclear momentum operator. An approximate calculation of vibronic spin–orbit coupling matrix elements is carried out for the S1(1B2u) ‐T intersystem crossing in benzene. The results indicate that C–H out‐of‐plane vibration ν10 acts as a dominant promoting mode for both S1‐T1 direct and S1‐T2‐T1 indirect mechanisms.