Photophysical properties of some coumarin derivatives: 5,7-dimethoxycoumarin, 4′,5′-dihydropsoralen, 8-methoxypsoralen (8-MOP) and 8-MOPTHD C4-cyclomonoadduct (THD  thymidine)

The photophysical properties of a 4t,5′-C 4 -cyclomonoadduct (F-2) of 8-methoxypsoralen (8-MOP) with thymidine were compared with those of 5,7-dimethoxycoumarin (DMC), 4t,5′-dihydropsoralen (DHP) and 8-MOP. The magnitudes of the fluorescence quantum yields are in the order DMC > DHP > F-2 > 8-MOP, which is exactly the opposite of the order for the temperature dependence of the fluorescence in ethanol. For DMC, DHP, 8-MOP and F-2 the ratios of the phosphorescence quantum yields to the fluorescence quantum yields are 0.056, less than 0.01, 0.68 and 5.6 and the phosphorescence lifetimes are 0.88 s, 0.88 s, 0.76 s and 1.2 s respectively, in ethanol at 77 K. The phosphorescence-to-fluorescence ratio increases sharply while the phosphorescence lifetime decreases by the external heavy-atom effect. At 77 K, the fluorescence maximum is very red shifted in isopentane compared with that in ethanol, and phosphorescence is not observed in isopentane. The solvent dependence of the fluorescence of the compounds is probably due to a change in the rate of internal conversion rather than to intersystem crossing.