title: Excited state characteristics of 6-azauracil in acetonitrile: drastically different relaxation mechanism from uracil.
abstract: Excited-state dynamics of 6-azauracil (6-AU) and sensitized singlet oxygen formation in acetonitrile solution with UV irradiation were investigated for the first time. In the transient absorption measurement, the 248 nm laser photolysis gave a relatively intense absorption band at 320 nm (= 1100 +/- 100 dm(3) mol(-1) cm(-1)) and a broadband in the 500-700 nm region due to triplet 6-AU. The triplet 6-AU, decaying with the rate constant of (5.3 +/- 0.2) x 10(6) s(-1) in Ar saturated acetonitrile, was quenched by molecular oxygen with the rate constant of (2.5 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1). The formation quantum yield of excited triplet 6-AU was estimated to be unity by acetone triplet sensitization and actinometry with benzophenone. The time-resolved thermal lensing signal of 6-AU was also observed by 248 nm laser excitation. In the presence of molecular oxygen, the sensitization from triplet 6-AU gave rise to formation of singlet oxygen O(2) ((1)Delta(g)) with a quantum yield of 0.63 +/- 0.03. Drastically different excited-state dynamics of aza-substituted uracil from normal uracil were clarified, and the mechanism for the enhancement of intersystem crossing by aza-substitution is discussed.
authors.affiliation: Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kobayashi
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Harada
authors.forename: Tadashi
authors.initials: T
authors.lastname: Suzuki
authors.forename: Teijiro
authors.initials: T
authors.lastname: Ichimura
mesh_terms: D000096:Acetone / Q000737:chemistry; D000097:Acetonitriles / Q000737:chemistry* / Q000528:radiation effects; D007700:Kinetics; D016877:Oxidants / Q000737:chemistry / Q000528:radiation effects; D010782:Photolysis / Q000528:radiation effects; D011789:Quantum Theory; D026082:Singlet Oxygen / Q000737:chemistry / Q000528:radiation effects; D014466:Ultraviolet Rays; D014498:Uracil / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000528:radiation effects
chemical_list: D000097:Acetonitriles; D016877:Oxidants; D000096:Acetone; D026082:Singlet Oxygen; D014498:Uracil; C100248:azauracil; C032159:acetonitrile