title: The methyl- and aza-substituent effects on nonradiative decay mechanisms of uracil in water: a transient absorption study in the UV region.
abstract: The nonradiative decay dynamics of photo-excited uracil (Ura) and its derivatives, i.e., thymine (5-methyluracil, Thy), 6-methyluracil (6-MU) and 6-azauracil (6-AU) in water, has been studied using a femtosecond transient absorption method. The molecules are populated in the lowest (1)ππ* state by a pump pulse at 266 nm, and a broadband continuum in the deep UV region is then employed as the probe. The extension of the continuous UV probe down to 250 nm enables us to investigate comprehensively the population dynamics of the ground states for those molecules and to uncover the substituent effects on nonradiative decay dynamics of uracil. Vibrational cooling in the ground states of Ura, Thy and 6-MU has been directly observed for the first time, providing solid evidence of the ultrafast (1)ππ* → S0 decay. In combination with the ground state bleaching signals, it is consolidated that their lowest (1)ππ* state decays via two parallel pathways, i.e., (1)ππ* → S0 and (1)ππ* → (1)nπ*. Moreover, the contribution of the (1)ππ* → (1)nπ* channel is found to be much smaller for Thy or 6-MU than for Ura. Different from methyl-substitution, the initial (1)ππ* state of the aza-substituent 6-AU decays primarily to the (1)nπ* state, while the (1)ππ* → S0 channel can be negligible. Our study provides a comprehensive understanding of the substituent effects on the excited-state dynamics of uracil in water.
authors.affiliation: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China. xjliu@wipm.ac.cn and University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
authors.forename: XinZhong
authors.initials: X
authors.lastname: Hua
authors.affiliation: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China. xjliu@wipm.ac.cn.
authors.forename: LinQiang
authors.initials: L
authors.lastname: Hua
authors.affiliation: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China. xjliu@wipm.ac.cn.
authors.forename: XiaoJun
authors.initials: X
authors.lastname: Liu
mesh_terms: D065638:Absorption, Radiation; D017785:Photons; D013050:Spectrometry, Fluorescence; D013941:Thymine / Q000737:chemistry; D014466:Ultraviolet Rays; D014498:Uracil / Q000031:analogs & derivatives / Q000737:chemistry*; D014867:Water
chemical_list: D014867:Water; D014498:Uracil; C008378:6-methyluracil; C100248:azauracil; D013941:Thymine