Coumarin 153 in the gas phase: optical spectra and quantum chemical calculations

Abstract

We report the absorption spectrum of Coumarin 153 in the gas phase at 383 K and its fluorescence spectra in a supersonic jet. At excess vibrational energy above 200 cm^-1 in the first excited state S₁, intramolecular vibrational redistribution leads to emission from interconverting conformers. For the hot isolated molecules, the emission spectrum is dominated by active modes of 350, 810, and 1150 cm^-1 and the distributions of oscillator strength for absorption and emission cross at 25420±40 cm^-1. This value should be used in solvation studies for the electronic S₁ energy of the bare molecule. The oscillator distributions change with temperature indicating a dependence of Franck-Condon factors for high-frequency modes on a low-frequency coordinate. Calculations of electronic structure with density-functional theory show that the second excited state is well separated from the first. It is proposed that spectral changes observed during nonequilibrium polar solvation are caused by intramolecular reorganisation of the julolidine group in the S₁ state on the 200 fs time scale.