Photoionization and photosensitized electron-transfer reactions of psoralens and coumarins

The radical cations of several psoralens (furocoumarins) and coumarins have been generated via direct excitation (lambda(ex) = 355 nm) in aqueous solution and/or via photosensitized electron transfer in acetonitrile and characterized using transient absorption spectroscopy. Significant photoionization yields were observed for 8-methoxy-psoralen (8-MOP) and three methoxy-substituted coumarins in aqueous buffer and for 4,5',8-trimethylpsoralen in aqueous acetonitrile. Of those, 6,7-dimethoxycoumarin (67-DMC) was found to have the highest quantum (0.2 +/- 0.03) yield for photoionization in aqueous buffer following direct excitation at both 355 and 308 nm. Laser energy dependence plots for the photoionization process are linear and pass through (0,0) for lambda(ex) = 355, 308, and 266 nm, providing strong evidence for a monophotonic process. The photoionization results are compared with both the one-electron oxidation potentials of the compounds (obtained via cyclic voltammetry) and the fluorescence quantum yields and lifetimes (obtained via single photon counting). The results demonstrate that photoionization is of general importance in psoralen and coumarin photochemistry and varies substantially in efficiency, depending on the structure, photophysical parameters, and environment of the substrate. The observation of photoionization upon low-intensity UVA irradiation is clearly significant with respect to the clinical use of these compounds in psoralen UVA therapy.