Subpicosecond vibrational relaxation of the S1 states of azulene and guaiazulene in solution

Abstract

The S1 population decay times of azulene and 1,4-dimethyl-7-isopropylazulene (guaiazulene) in solution have been determined as a function of their initial vibrational energy content, [Formula: see text] using a pump–probe experiment with subpicosecond time resolution. The S1 lifetime of azulene does not depend on [Formula: see text] for energies up to 1760 cm−1, whereas the lifetime of the shorter-lived S1 state of guaiazulene is independent of [Formula: see text] only for energies up to ~1000 cm−1. At higher energies, the lifetime decreases with increasing [Formula: see text] and exhibits the same behavior in two structurally different solvents. It is suggested that internal conversion to the electronic ground state from a vibrationally unrelaxed S1 state is responsible for the effects observed in guaiazulene, and that intramolecular vibrational redistribution occurs with a time constant of several hundred femtoseconds. Keywords: pump–probe spectroscopy, azulene, femtosecond processes.