SUBPICOSECOND VIBRATIONAL-RELAXATION OF THE S-1 STATES OF AZULENE ANDGUAIAZULENE IN SOLUTION

The S-1 population decay times of azulene and 1,4-dimethyl-7-isopropyl azulene (guaiazulene) in solution have been determined as a function o f their initial vibrational energy content, E(vib)(i), using a pump-pr obe experiment with subpicosecond time resolution. The S-1 lifetime of azulene does not depend on E(vib)(i) for energies up to 1760 cm(-1), whereas the lifetime of the shorter-lived S-1 state of guaiazulene is independent of E(vib)(i) only for energies up to similar to 1000 cm(-1 ). At higher energies, the lifetime decreases with increasing E(vib)(i ), and exhibits the same behavior in two structurally different solven ts. It is suggested that internal conversion to the electronic ground state from a vibrationally unrelaxed S-1 state is responsible for the effects observed in guaiazulene, and that intramolecular vibrational r edistribution occurs with a time constant of several hundred femtoseco nds.