Probing ultrafast dynamics in photoexcited pyrrole: timescales for pi sigma* mediated H-atom elimination.
Probing ultrafast dynamics in photoexcited pyrrole: timescales for pi sigma* mediated H-atom elimination.
复制标题
探测光激发吡咯中的超快动力学：pi sigma* 介导的 H 原子消除的时间尺度。
作者：
G. Roberts;Craig Williams;Hui Yu;A. Chatterley;J. D. Young;S. Ullrich;V. Stavros
杂芳族紫外线发色团吡咯作为亚基存在于许多重要的生物分子中：它存在于血红素中，血红蛋白的非蛋白质成分中，以及氨基酸色氨酸中。到目前为止，已经有几个实验研究在时间域和频域中询问了吡咯的激发态动力学。在这项工作中，我们的具体目标是揭示在250-200 nm的激发波长范围内从吡咯消除氢原子动力学的任何差异，这包括：(I)直接激发到(正式电偶极禁止)1(1)pigma*(1A2)态；以及(Ii)初始光激发到更高能量的1pii*(1b2)态。这是通过在气相中结合使用超快时间分辨离子产额和时间分辨速度图离子成像技术来实现的。在250 nm处直接激发到1(1)Pisigma*(1A2)，我们观察到N-H键裂变的单个时间常数为126+/-28fs。我们将其指定为隧穿垂直Franck-Condon区1(1)Pisigma*(1A2)表面的准束缚3S里德堡分量，然后通过1(1)Pisigma*/S(0)锥形交点的非绝热耦合与H原子一起产生基态(C4H4N(X))的吡咯基自由基。在238 nm处，观察到1(1)Pisigma*(1A2)表面发生直接激发和N-H解离，其时间常数为46+/-22fs。在200 nm处初始布居1pipi*(1b2)态时，在52+/-12fs内发生了一个快速的1pipi*(1b2)-gt；1(1)pisigma*(1a2)-->；N-H裂变过程。除了200 nm处的超快N-H键断裂，我们还观察到了由激发电子态弛豫而形成的振动热S(0)基态物种的统计单分子氢原子消除的开始，其时间常数为1.0+/-0.4 ns。在吡咯-D_1上的类似测量表明，这些统计氢原子只通过C-H键断裂而释放。
The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250-200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1(1)pisigma* (1A2) state; and (ii) initial photoexcitation to the higher energy 1 pipi* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1(1)pisigma* (1A2) at 250 nm, we observe a single time-constant of 126 +/- 28 fs for N-H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1(1)pisigma* (1A2) surface in the vertical Franck-Condon region, followed by non-adiabatic coupling through a 1(1)pisigma*/S(0) conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N(X)) together with H-atoms. At 238 nm, direct excitation to, and N-H dissociation along, the 1(1)pisigma* (1A2) surface is observed to occur with a time-constant of 46 +/- 22 fs. Upon initial population of the 1pipi* (1B2) state at 200 nm, a rapid 1pipi* (1B2) --> 1(1)pisigma* (1A2) --> N-H fission process takes place within 52 +/- 12 fs. In addition to ultrafast N-H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S(0) ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 +/- 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C-H bond cleavage.