Picosecond fluorescence depletion spectroscopy. II. Intramolecular vibrational relaxation in the excited electronic state of fluorene

Abstract

This paper presents the results of the time-resolved study of intramolecular vibrational relaxation in the molecule fluorene. The results represent the first extensive study of a molecule using the technique which we have developed known as the time-resolved fluorescence depletion technique. Fluorescence depletion decays and dispersed fluorescence spectra of 19 vibronic features of fluorene are presented. The decays show a progression of dynamic behavior including stationary behavior at low densities of states, quantum beating at intermediate densities, and fast decay of the initially prepared state at high state densities. The data allow us to assign IVR lifetimes to several vibronic levels of fluorene from 27 ps at 1425 cm−1 of excess vibrational energy to ≤10 ps at ∼2000 cm−1. The degree of spectral congestion in the associated dispersed fluorescence spectra is shown to be related to the dynamic behavior of the vibronic features. In addition, the effects of intermolecular rotational coherences on the time-resolved fluorescence depletion decays of fluorene are explored. With these results we are able to confirm the assignments of the excited state rotational constants which we have made using rotational band contour simulations.