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Keywords:

  • azobenzene;
  • isomerization;
  • conical intersection;
  • Raman excitation profile

Abstract

Azobenzene and its derivatives have widely been recognized as systems with potential applications such as optical switching elements, and the mechanism of trans-cis photoisomerization, which is the basis for all these applications, has long been sought. The long-standing controversy on the relaxation mechanism of π–π* (S2) excited trans-azobenzene (TAB) is addressed in this paper. The role of S1[BOND]S2 intersection and the possible involvement of a third state in the S2 photochemistry are investigated. The excited state structure and dynamics of TAB are extracted by modeling the Raman excitation profiles (REPs) of the S1 and S2 states and the absorption spectrum of TAB by Heller's time-dependent formalism. The REPs of S2 are modeled in the single resonant state approximation by explicitly including the eight most Franck–Condon (FC) active modes. The obtained NN and CN bond lengths of S2 and the intial dynamics in these coordinates following photoexcitation to S2 are in agreement with recent theoretical calculations. The structural parameters of S1 are obtained by exploiting the nature of interference in the REPs of S1. From these parameters a three-mode–two-state vibronic coupling model, based on harmonic diabatic potentials and linear coupling, is formulated to study the S2[BOND]S1 internal conversion dynamics. The results indicate that the S1[BOND]S2 intersection is not directly accessible within electronic dephasing time of S2 to cause significant population decay from S2 to S1. The current conception of π–π* relaxation mechanisms in TAB is reviewed and revised on the basis of our observations and also from recently reported results of time-resolved and theoretical studies. The role of a third state (S3), which appears to be an integral part of in the relaxation mechanism of S2, is pointed out and the feasibility of two competing relaxation pathways for S2 state is identified. One is the direct rotationless S2[BOND]S1(C2h) internal conversion and the other is S2[BOND]S3 internal conversion along rotational pathway followed by a branching of the S3 population to S0 and S1. Copyright © 2008 John Wiley & Sons, Ltd.