Photoisomerization Dynamics and Pathways of trans- and cis-Azobenzene in Solution from Broadband Femtosecond Spectroscopies and Calculations
Photoisomerization Dynamics and Pathways of trans- and cis-Azobenzene in Solution from Broadband Femtosecond Spectroscopies and Calculations
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DOI：
10.1021/jp504999f
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发表时间：
2014-07-24
影响因子：
3.3
通讯作者：
Kovalenko, S. A.
中科院分区：
文献类型：
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作者：
Quick, M.;Dobryakov, A. L.;Kovalenko, S. A.
对偶氮苯溶液的光异构化反应进行了实验和计算研究。通过宽带瞬态吸收、荧光和受激拉曼光谱描述反式到顺式和顺式到反式动力学。瞬态吸收扩展到不仅覆盖n π * 带，而且在紫外线的π π * 带。异构化产率被用于在不同激发下的反式和顺式瞬态光谱的定量比较。对于trans-to-cis路径，在n pi*（S-1）激发下，演化以0.3、3和16 ps发展。前两个时间反映了布居弛豫到局部最小值S-1 t（L），随后在8 kJ/mol势垒上过渡到暗中间值S-1 t（D）。量子化学计算证实了稳定点S-1 t（L）和S-1 t（D）的存在。第三个时间对应于S-1 t（D）-> S-0弛豫到基态，通过12 kJ/mol势垒上的S-1/S-0圆锥相交。因此，16 Ps时间常数归因于异构化过程而不是振动冷却，这与当前的观点相反，并且与Lednev等人先前的解释（J.Phys.Chem.1996，100，13338）一致。长寿命的中间体S-1 t（D）的衰变与呼拉扭而不是反转机制一致。对于顺式到反式反应后n π * 激发，信号衰减是强烈的非指数，与0.1和1 ps。后者（1 ps）比反式异构体的16 ps衰减短得多，这意味着不同的S-1/S-0锥形交叉点和弛豫路径的顺式反式和反式顺式反应。在π π *（S-n）激发下也得到了新的结果。因此，对于反式偶氮苯，50%的人口松弛到S-1区域，这是不可访问的n π * 激发下。对于顺式偶氮苯，高达3096的激发物种异构化到反式通过S-n/S-1交叉，导致在混合的顺式/反式S-1人口。偶氮苯的异构化动力学没有表现出粘度依赖性，提出了质疑的扭转机制，并建议呼拉扭异构化机制。
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 n pi* band but also the pi pi* 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 n pi*(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-lt(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) -> S-0 relaxation to the ground state via an S-1/S-0 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-to-trans reaction following n pi* 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 S-1/S-0 conical intersections and relaxation paths for the cis-to-trans and trans-to-cis reaction. New results are also obtained with pi pi*(S-n) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S-1 region, which is not accessible under n pi* excitation. For cis-azobenzene, up to 3096 of the excited species isomerize to trans via an S-n/S-1 intersection, resulting in a mixed cis/trans S-1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.