Vibronic spectral behavior of molecules. XIII. Theoretical contribution to the vibronic coupling and the dushinsky effect on the S1 ← S0 absorption and the S1 → S0, S2 → S1, and S2 → S0 fluorescences of azulene

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Abstract
Based on the completely optimized S0, S1, and S2 molecular geometries of azulene, the vibronic structure of the S1 ← S0 absorption as well as of the S1 → S0, S2 → S1, and S2 → S0 fluorescences is investigated theoretically within the adiabatic approximation. By means of theory‐experiment comparisons, the influence of non‐Condon terms and of the Dushinsky effect on the vibronic structure of azulene spectral behavior is discussed. Typical for the S1 ← S0 absorption and the S1 → S0 fluorescence are vibronic transition moment contributions of Condon type, whereas the interpretation of azulene S2 → S1 and S2 → S0 fluorescences is successful only within the scope of the Herzberg–Teller approach by taking into account vibronic coupling terms and, additionally, the Dushinsky effect in the latter case. An analysis of the relevant vibrational modes is given.