Luminescence Properties of Some Coumarins with 3:4-Fused Ring System

The photophysical characteristics of a group of coumarins with a 3:4-fused ring system (2-substituted-4,5-dioxo-l,2-dihydro-4H,5H-pyrano-[3:4-c] [l]-benzopyrans) in solutions at room temperature, in frozen ethanol matrix at 77 K, in solid phase and in PVC films are reported. The low fluorescence quantum yield of the compounds investigated in solution is explained as a result of internal conversion from the fluorescent S2(TCJt*) state to the lower lying S^nJi*) state. The phosphorescence with a life time in the order of seconds is connected with intersystem crossing S[ (n;i*) T^JtJt*). The absorption Franck Condon transitions in solid phase and in PVC matrix are only weekly batochromically shifted according to solution.