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基于宽带飞秒光谱学和计算的溶液中反式和顺式偶氮苯的光异构化动力学及途径

Photoisomerization dynamics and pathways of trans- and cis-azobenzene in solution from broadband femtosecond spectroscopies and calculations.

作者信息

Quick M, Dobryakov A L, Gerecke M, Richter C, Berndt F, Ioffe I N, Granovsky A A, Mahrwald R, Ernsting N P, Kovalenko S A

机构信息

Department of Chemistry, Humboldt-Universität zu Berlin , Brook-Taylor-Str. 2, D-12489 Berlin, Germany.

出版信息

J Phys Chem B. 2014 Jul 24;118(29):8756-71. doi: 10.1021/jp504999f. Epub 2014 Jul 11.

Abstract

The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S(1)) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S(1t )(L) and subsequent transition to a dark intermediate S(1t)(D) over an 8 kJ/mol barrier. The existence of stationary points S(1t)(L) and S(1t)(D) is confirmed by quantum-chemical calculations. The third time corresponds to S(1t) (D) → S0 relaxation to the ground state via an S1/S0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S(1t)(D) is consistent with the hula twist rather than with the inversion mechanism. For the cis-totrans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S1/S0 conical intersections and relaxation paths for the cis-totrans and trans-to-cis reaction. New results are also obtained with ππ*(Sn) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an Sn/S1 intersection, resulting in a mixed cis/trans S1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.

摘要

通过实验和计算研究了溶液中偶氮苯的光异构化。通过宽带瞬态吸收、荧光和受激拉曼光谱描述了反式到顺式和顺式到反式的动力学过程。瞬态吸收不仅扩展到覆盖紫外区域的nπ带,还覆盖了ππ带。异构化产率用于定量比较不同激发条件下反式和顺式瞬态光谱。对于nπ*(S(1))激发下的反式到顺式路径,其演化过程有0.3皮秒、3皮秒和16皮秒三个阶段。前两个时间反映了布居弛豫到局部最小值S(1t)(L),随后越过8千焦/摩尔的势垒跃迁到暗中间体S(1t)(D)。量子化学计算证实了驻点S(1t)(L)和S(1t)(D)的存在。第三个时间对应于S(1t)(D)通过跨越12千焦/摩尔势垒的S1/S0锥形交叉点弛豫到基态S0。因此,16皮秒的时间常数归因于异构化过程,而不是振动冷却,这与当前观点相反,与Lednev等人之前的解释一致(《物理化学杂志》,1996年,第100卷,第13338页)。长寿命中间体S(1t)(D)的衰减与呼啦圈扭转机制一致,而不是与反转机制一致。对于nπ激发后的顺式到反式反应,信号衰减强烈非指数化,时间为0.1皮秒和1皮秒。后者(1皮秒)比反式异构体的16皮秒衰减短得多,这意味着顺式到反式和反式到顺式反应的S1/S0锥形交叉点和弛豫路径不同。通过ππ(Sn)激发也获得了新的结果。因此,对于反式偶氮苯,50%的布居弛豫到一个在nπ*激发下无法到达的S1区域。对于顺式偶氮苯,高达30%的激发物种通过Sn/S1交叉点异构化为反式,导致顺式/反式S1布居混合。偶氮苯的异构化动力学不显示粘度依赖性,这对扭转机制提出了质疑,并暗示了呼啦圈扭转异构化机制。

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