$S_{2} \rightarrow S_{1}$ FLUORESCENCE IN AZULENE

Abstract

The $S_{2} \rightarrow S_{1}$ fluorescence spectrum of azulene has been measured at $4.2^\circ K$ in a naphthalene mixed crystal and in pentane, hexane, and polycrystalline methylcyclohexane Shpol’skii matrices. The vibrational analysis of the naphthalene mixed crystal spectrum is in excellent agreement with the $S_{1} \leftarrow S_{0}$ absorption measurements of Small and Kusserow;$^{1}$ at least 21 of the 25 in-plane $S_{1}$ vibrations (excluding C-H stretches) are active in the fluorescence spectrum. However, there are at least 10 additional vibronic origins in emission and we ascribe these to out-of-plane fundamentals induced by coupling to $\sigma\Pi^{*}$ and/or $Pi\sigma*$ excited states. The total $S_{2} \leftrightarrow S_{1}$ oscillator strength is $\sim 10^{-5}$, the majority of which is vibronically induced.