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

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 en ergy above 200 cm(-1) in the first excited state S-1, intramolecular vibrat ional redistribution leads to emission from interconverting conformers. For the hot isolated molecules, the emission spectrum is dominated by active m odes of 350, 810, and 1150 cm(-1) and the distributions of oscillator stren gth for absorption and emission cross at 25 420 +/- 40 cm(-1). This value s hould be used in solvation studies for the electronic S-1 energy of the bar e molecule. The oscillator distributions change with temperature indicating a dependence of Franck-Condon factors for high-frequency modes on a low-fr equency coordinate. Calculations of electronic structure with density-funct ional 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 julolidi ne group in the S-1 state on the 200 fs time scale.