Mixed crystal spectroscopy of 1,3-diazaazulene

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Abstract
The 4500 Å 1B1?1A1 absorption and fluorescence systems of 1,3‐diazaazulene are analyzed in three host crystals, naphthalene, quinazoline, and p‐dichlorobenzene. Extensive vibronic coupling activity by a1 and b1 fundamentals is discussed in terms of the assignments for the higher energy excited states. The mirror symmetry breakdown between the absorption and fluorescence spectra is marked and while the fluorescence spectrum is medium independent, a portion of the totally symmetric vibrational structure in the absorption spectrum is strongly dependent on the medium. The overall mirror symmetry breakdown is attributed to both intrastate and interstate Duschinsky excited state normal coordinate rotations, interferences between allowed and forbidden transition moments, and the vibronically induced moving level effect.