Photoinduced Bond Dissociation of 4-Methylcoumarin Derivatives in Solution Studied by Laser Flash Photolysis and DFT Calculations

Abstract

Photochemical properties of 4-coumarinylmethyl derivatives (CM-X, X = Br, Cl, OH, OPh, SH, and SPh) in solution were studied by laser flash photolysis and DFT calculations. It was found that CM-Br and CM-SPh undergo photoinduced bond dissociation in the lowest excited singlet state, yielding the 4-coumarinylmethyl radical (CMR) and the corresponding radical with quantum yields of approximately 0.25. Laser flash photolysis of CM-Cl, OH, OPh, and SH provided no or very little transient absorption of the corresponding triplet state. Upon triplet sensitization of CM-X using benzophenone (BP) as a triplet sensitizer, efficient formation of triplet CM-OH and -OPh was seen, whereas CM-SH and -SPh underwent the C-S bond cleavage in the lowest triplet (T(1)) state, resulting in production of CMR and the corresponding radicals with efficiencies (alpha(rad)) of >or=0.66. CM-Br and -Cl efficiently quenched triplet BP without formation of appreciable intermediates. On the basis of the results of laser flash photolysis and DFT calculations, photochemical features of triplet CM-X were discussed in detail.