Molecular Switching via Multiplicity-Exclusive E/Z Photoisomerization Pathways.
Molecular Switching via Multiplicity-Exclusive E/Z Photoisomerization Pathways.
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DOI：
10.1021/jacs.5b07348
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发表时间：
2015-08
影响因子：
15
通讯作者：
Jiawang Zhou;Xin Guo;H. Katz;A. Bragg
中科院分区：
文献类型：
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作者：
Jiawang Zhou;Xin Guo;H. Katz;A. Bragg
正向异构化途径和保留异构化途径的互斥性为根据分子结构或外部刺激可预测地控制光开关材料的响应提供了希望。在这里，我们用超快暂态吸收光谱表征了可见光触发开关1，2-二硫代-1，2-二氰基乙烯(4TCE)在氯苯中的E/Z光异构化反应机理。我们观察到开关机制完全是通过不同自旋多重数的电子流形发生的：在40ps的时间尺度上，反式到顺式的异构化只通过单重态流形内的电子驰豫发生；相反，在440 nm以上没有观察到顺式到反式的异构化，而是在更高的能量(例如420 nm)下分别通过两个快速的自旋多重态过程在∼2ps和0.4 ns的时间尺度上发生进入和离开三重态流形的过程。在cis-4TCE中观察到的超快ISC与相关的噻吩基齐聚物的光诱导动力学一致。顺式到反式异构化通过三重态敏化的能量转移有效地发生，而反式到顺式异构化则不是，这支持了对这些异构化反应背后的光物理途径的解释。量子化学计算表明，T1势能面在从顺式Franck-Condon区(θ=175°)到跨基态势垒区几何构型的中心乙烯二面角(θ)坐标上是无障碍的；此外，T1和S1表面有很大的自旋-轨道耦合。总而言之，我们证明了4TCE的E/Z光开关是通过多重性排除的途径进行操作的，这使得通过改变分子结构或物理环境来操纵自旋-轨道耦合来定制开关性能成为可能。
Mutual exclusivity in the nature of forward and reserve isomerization pathways holds promise for predictably controlling responses of photoswitchable materials according to molecular structure or external stimuli. Herein we have characterized the E/Z photoisomerization mechanisms of the visible-light-triggered switch 1,2-dithienyl-1,2-dicyanoethene (4TCE) in chlorobenzene with ultrafast transient absorption spectroscopy. We observe that switching mechanisms occur exclusively by relaxation through electronic manifolds of different spin multiplicity: trans-to-cis isomerization only occurs via electronic relaxation within the singlet manifold on a time scale of 40 ps; in contrast, cis-to-trans isomerization is not observed above 440 nm, but occurs via two rapid ISC processes into and out of the triplet manifold on time scales of ∼2 ps and 0.4 ns, respectively, when excited at higher energies (e.g., 420 nm). Observation of ultrafast ISC in cis-4TCE is consistent with photoinduced dynamics of related thiophene-based oligomers. Interpretation of the photophysical pathways underlying these isomerization reactions is supported by the observation that cis-to-trans isomerization occurs efficiently via triplet-sensitized energy transfer, whereas trans-to-cis isomerization does not. Quantum-chemical calculations reveal that the T1 potential energy surface is barrierless along the coordinate of the central ethylene dihedral angle (θ) from the cis Franck-Condon region (θ = 175°) to geometries that are within the region of the trans ground-state well; furthermore, the T1 and S1 surfaces cross with a substantial spin-orbital coupling. In total, we demonstrate that E/Z photoswitching of 4TCE operates by multiplicity-exclusive pathways, enabling additional means for tailoring switch performance by manipulating spin-orbit couplings through variations in molecular structure or physical environment.