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 (Phi(ISC)) 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)(npi*) state and be in competition with internal conversion to the S(0) state after the relaxation to the minimum of the S(1)(npi*) state. 6AUd had a lower Phi(ISC) value than
6-azauracil (6AU) with the 308 nm excitation (Phi(ISC) = 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)(npi*) 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
phototoxicity of 6AUd.