Ultrafast dynamics and photochemistry of π-π* excited trans- azobenzene-a comprehensive analysis of resonance Raman intensities

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 ππ* (S₂) excited trans-azobenzene (TAB) is addressed in this paper. The role of S₁S₂ 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— S₁ internal conversion dynamics. The results indicate that the S₁— 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 ππ* 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— S1(C_2h) internal conversion and the other is S₂—S₃ internal conversion along rotational pathway followed by a branching of the S₃ population to S₀ and S₁.