The Azulene S₁ State Decays via a Conical Intersection:  A CASSCF Study with MMVB Dynamics

The anomalous fluorescence of azulene emission from S 2 rather than S 1 was first recognized by Beer and Longuet-Higgins 40 years ago. Femtosecond laser studies and spectroscopic line width measurements have now established that radiationless decay from S 1 to the ground state takes place in less than 1 ps. In this paper we show how such ultrafast S 1 decay can be explained by relaxation through an unavoided S 1 /S 0 crossing (i.e., a conical intersection). The S 1 relaxation dynamics of azulene from its Franck−Condon structure are modeled semiclassically using a hybrid quantum-mechanical/force-field potential (MMVB) which reproduces the structure of the ab initio CASSCF potential energy surface. This simulation suggests that the S 1 → S 0 decay takes place in femtoseconds before a single oscillation through the crossing is completed.