PHOTOPHYSICS OF THE S-2 STATE OF THIOCOUMARIN - A VIBRATIONALLY UNEQUILIBRATED LUMINOPHORE

A weak fluorescence from the S-2(pi,pi) state of thiocoumarin in perf luorohydrocarbons and n-hexane solutions, of a quantum yield similar t o 6.5 x 10(-5), and similarly weak phosphorescence from the T-1(pi,pi ) state, at room temperature, were measured. A distinct dependence of the shape and position of the fluorescence band on the energy of vibra tional excitation within the S-2(pi,pi) state was found and interpret ed as a result of emission from the unrelaxed vibrational levels. The lifetime of the S-2 State was estimated as similar to 1 ps, which is a bout 20 times lower than the value expected on the basis of the ''ener gy gap law'' for Delta E(S-2-S-1). To give a satisfactory interpretati on of the properties of thiocoumarin in the relaxed vibrational config uration of the S-2 state, a very efficient electronic relaxation shoul d have been considered for which a few possible mechanisms were discus sed. A short lifetime of excited vibrational levels was not a conseque nce of electronic relaxation processes but was rather directly related to the rate of intramolecular vibrational redistribution.