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-1,2-dihydro-4H,5H-pyrano-[3:4-c][I]-be nzopyrans) in solutions at room temperature, in frozen ethanol matrix at 77 K, in solid phase and in PVC films are reported. The low fluorescence quan tum yield of the compounds investigated in solution is explained as a resul t of internal conversion from the fluorescent S-2(pi pi*) State to the lowe r lying S-1(n pi*) state. The phosphorescence with a life time in the order of seconds is connected with intersystem crossing S-1(n pi*) - T-1(pi pi*) . The absorption Franck Condon transitions in solid phase and in PVC matrix are only weekly batochromically shifted according to solution.