Triplet formation of 6-azauridine and singlet oxygen sensitization with UV light irradiation

Excited state characteristics of 6-azauridine (6AUd), which is known as a medicine against psoriasis and neoplastic, were investigated with laser plash photolysis, time-resolved thermal lensing, and near IR single photon counting method. The triplet-triplet absorption spectrum of 6AUd was observed for the first time. The formation quantum yield of excited triplet 6AUd ((ISC)-I-Phi) was estimated by acetone triplet sensitization and actinometry with benzophenone to be 1.00 +/- 0.07 (248 nm excitation) and 0.78 +/- 0.05 (308 nm excitation). This excitation wavelength effect could be explained by intersystem crossing (ISC) to the excited triplet manifolds occurring during the relaxation on the potential energy surface (PES) of the S-1(n pi*) state and be in competition with internal conversion to the S-0 state after the relaxation to the minimum of the S-1(n pi*) state. 6AUd had a lower (ISC)-I-Phi value than 6-azauracil (6AU) with the 308 nm excitation ((ISC)-I-Phi = 0.93 +/- 0.04 for 6AU). The nucleoside has more vibrational modes than 6AU, and therefore the ribose would accelerate intramolecular vibrational energy redistribution and the relaxation to the minimum of the PES of the S-1(n pi*) state. Sensitized singlet oxygen formation of 6AUd was also detected in the O-2-saturated condition with quantum yields of 0.49 +/- 0.01 with the 248 nm excitation, indicating the high phototoxity of 6AUd.