The methyl- and aza-substituent effects on nonradiative decay mechanisms of uracil in water: a transient absorption study in the UV region

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 1ππ* 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 1ππ* → S0 decay. In combination with the ground state bleaching signals, it is consolidated that their lowest 1ππ* state decays via two parallel pathways, i.e., 1ππ* → S0 and 1ππ* → 1nπ*. Moreover, the contribution of the 1ππ* → 1nπ* channel is found to be much smaller for Thy or 6-MU than for Ura. Different from methyl-substitution, the initial 1ππ* state of the aza-substituent 6-AU decays primarily to the 1nπ* state, while the 1ππ* → S0 channel can be negligible. Our study provides a comprehensive understanding of the substituent effects on the excited-state dynamics of uracil in water.