尿嘧啶激发态动力学溶剂效应的飞秒瞬态吸收光谱研究

Solvent effect on ultrafast decay of uracil studied by femtosecond transient absorption spectroscopy

尿嘧啶是构成RNA链最基本的结构单元之一,其光辐射下的光物理和光化学过程与光致癌变等过程紧密关联,因而受到广泛的关注.本文采用飞秒瞬态吸收光谱技术,研究了在紫外光辐射下,溶剂效应对尿嘧啶激发态超快动力学的影响.实验利用一束264 nm的激发光将尿嘧啶激发到(1)^(π,π^(*))态(即S_(2)态).然后,利用覆盖范围为280—360 nm的紫外超连续谱作为探测光,实现对尿嘧啶激发态和基态动力学的同步探测.测量发现,尿嘧啶在乙腈溶剂中显现了两个动力学衰减过程,即9.8 ps(在300 nm处)和>1000 ps,分别对应于振动冷却过程和三重态的衰减过程.这一观察与水溶液中的结果差异很大,因为在水溶液中基本没有测量到三重态的布居.通过对一系列溶液展开测量,发现尿嘧啶的超快动力学过程依赖于溶剂形成氢健能力的大小.越容易形成氢键的溶剂,尿嘧啶越不容易布居到三重态.

As one of the building blocks in RNA chain,uracil and its derivatives have attracted a great deal of interest since its ultrafast dynamics is closely related to mutagenic and carcinogenic effects.In this study,the solvent effect on the ultrafast decay of excited uracil is studied by femtosecond transient absorption spectroscopy in the UV region.The uracil molecule is populated to the(1)^(π,π^(*))state(i.e.S_(2)state)with a pump pulse at 264 nm.Broad-band white light continuum in the UV region from 280 to 360 nm is used as the probe.With a detail analysis of the measured transient spectra,two decay time constants,i.e.9.8 ps and>1000 ps,are directly obtained at 300 nm in the solvent of acetonitrile.Compared with our previous experiments,where no obvious triplet population is observed in water,triplet population is found to play an important role in acetonitrile.A comparison of excited-state dynamics among different solvents is also carried out.It reveals that the decay from the(1)^(n,π)state(i.e.,Sstate)to the Tstate shows a clear dependence on the H bonding of the solvents.With stronger H bonding,the(1)^(n,π)excited state decays faster and has less chance to transfer to the triplet state.These results suggest that only when the(1)^(n,π)state has excess vibrational energy can it transit to the triplet state through the intersystem crossing process.With this new information obtained in the present measurement,the decay dynamics of uracil on the Sexcited state can be further understood.