Photophysical and photochemical properties of some skin-sensitizing agents

Abstract

The photophysical properties of $4``, 5``-C_4$-cyclomonoadduct(F-2) of 8-methoxypsoralen(8-MOP) with thymine were compared with those of 5,7-dimethoxycoumarin(DMC), 4``,5``-dihydropsoralen(DHP), and 8-MOP. The magnitude of fluorescence quantum yields is in the order of $DMC > DHP >> F-2 > 8-MOP$ which is exactly opposite to that of the temperature dependence of fluorescence in ethanol. The ratios of the phosphorescence to fluorescence quantum yields are $0.056, < 0.01$, 0.68, and 5.6 and the phosphorescence lifetimes are 0.88, 0.88, 0.76, and 1.2 s for DMC, DHP, 8-MOP, and F-2, respectively, in ethanol at 77K. The phosphorescence to fluorescence ratio increases sharply while phosphorescence lifetime decreases by external heavy atom effect. At 77 K, the fluorescence maximum is very much red-shifted in isopentane compared to that in ethanol, and phosphorescence is not observed in isopentane. The solvent dependence of fluorescence of the compounds is probably due to the change of internal conversion rate rather than intersystem crossing. The Stern-Volmer constants for fluorescence quenching by tetramethylethylene decrease in order of $DMC >> DHP > F-2 > 8-MOP$. The same order was observed for the quantum yields of [2 + 2] cycloaddition reaction with tetramethylethylene on direct irradiation. In [2 + 2] photocycloaddition of F-2 with tetramethylethylene in ethanol, the ratio of quantum yields deduced from singlet and triplet of F-2, $\Phi_3^\circ\,/\,\Phi_1^\circ$, is about 5. The excited triplet state is the reactive state for the [2 + 2] photocycloaddition of F-2 with tetramethylethylene in solution but the excited singlet state of F-2 becomes very important in biological environments. The fluorescence quantum yields of khellin and khellin-dimethyl fumarate $C_4$-cycloadduct(KDF) are very sensitive to temperature and to the nature of solvents, especially the proton-donating ability in solute-to-solvent hydrogen bonding. The fluorescence quantum yields in ethanol ...