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

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* (S<SUB>2</SUB>) excited trans-azobenzene (TAB) is addressed in this paper. The role of S<SUB>1</SUB>S<SUB>2</SUB> intersection and the possible involvement of a third state in the S<SUB>2</SUB> photochemistry are investigated. The excited state structure and dynamics of TAB are extracted by modeling the Raman excitation profiles (REPs) of the S<SUB>1</SUB> and S<SUB>2</SUB> states and the absorption spectrum of TAB by Heller's time-dependent formalism. The REPs of S<SUB>2</SUB> 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 S<SUB>2</SUB> and the intial dynamics in these coordinates following photoexcitation to S<SUB>2</SUB> are in agreement with recent theoretical calculations. The structural parameters of S<SUB>1</SUB> are obtained by exploiting the nature of interference in the REPs of S<SUB>1</SUB>. From these parameters a three-mode-two-state vibronic coupling model, based on harmonic diabatic potentials and linear coupling, is formulated to study the S<SUB>2</SUB>— S<SUB>1</SUB> internal conversion dynamics. The results indicate that the S<SUB>1</SUB>— S<SUB>2</SUB> intersection is not directly accessible within electronic dephasing time of S<SUB>2</SUB> to cause significant population decay from S<SUB>2</SUB> to S<SUB>1</SUB>. 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 (S<SUB>3</SUB>), which appears to be an integral part of in the relaxation mechanism of S<SUB>2</SUB>, is pointed out and the feasibility of two competing relaxation pathways for S<SUB>2</SUB> state is identified. One is the direct rotationless S<SUB>2</SUB>— S1(C<SUB>2h</SUB>) internal conversion and the other is S<SUB>2</SUB>—S<SUB>3</SUB> internal conversion along rotational pathway followed by a branching of the S<SUB>3</SUB> population to S<SUB>0</SUB> and S<SUB>1