PHOTOPHYSICAL TRIPLET-STATE PROCESSES OF 4H-1-BENZOPYRANE-4-THIONE INA PERFLUOROALKANE .3. EXCITATION WAVELENGTH DEPENDENCE OF LUMINESCENCE QUANTUM YIELDS

The phosphorescence excitation spectrum of 4H-1-benzopyrane-4-thione ( BPT) in perfluoro-1,3-dimethylcyclohexane coincides with the absorptio n spectrum in the range of the transitions S-0 --> T-1 and S-0 --> S-1 . Hence the quantum yield phi(P) of the phosphorescence from BPT is in dependent of the primary excited state, T-1 or S-1, in contrast to a r eported lower value of phi(P) for excitation to S-1. The ratio phi(PF2 )/phi(P) of the quantum yields phi(PF2) of the prompt S-2 --> S-0 fluo rescence and phi(P) of the phosphorescence decreases with decreasing e xcitation wavelength.