Knowledge Bank

The $S2\to S1$ 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 $S1\leftarrow S0$ absorption measurements of Small and Kusserow;$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ at least 21 of the 25 in-plane $S1$ 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 $\sigma \Pi \ast$ and/or $Pi\sigma \ast$ excited states. The total $S2\leftrightarrow S1$ oscillator strength is $\sim 10-5$, the majority of which is vibronically induced.