Gao Yuan-Jun, Chang Xue-Ping, Liu Xiang-Yang, Li Quan-Song, Cui Ganglong, Thiel Walter
Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University Beijing 100875, China.
School of Chemistry and Chemical Engineering, Beijing Institute of Technology , Beijing 100081, China.
J Phys Chem A. 2017 Apr 6;121(13):2572-2579. doi: 10.1021/acs.jpca.7b00197. Epub 2017 Mar 28.
The photophysical properties of tetraphenylethene (TPE) compounds may differ widely depending on the substitution pattern, for example, with regard to the fluorescence quantum yield ϕ and the propensity to exhibit aggregation-induced emission (AIE). We report combined electronic structure calculations and nonadiabatic dynamics simulations to study the excited-state decay mechanisms of two TPE derivatives with four methyl substituents, either in the meta position (TPE-4mM, ϕ = 0.1%) or in the ortho position (TPE-4oM, ϕ = 64.3%). In both cases, two excited-state decay pathways may be relevant, namely, photoisomerization around the central ethylenic double bond and photocyclization involving two adjacent phenyl rings. In TPE-4mM, the barrierless S cyclization is favored; it is responsible for the ultralow fluorescence quantum yield observed experimentally. In TPE-4oM, both the S photocyclization and photoisomerization paths are blocked by non-negligible barriers, and fluorescence is thus feasible. Nonadiabatic dynamics simulations with more than 1000 surface hopping trajectories show ultrafast cyclization upon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactive during the 1 ps simulations. We discuss the chances for spectroscopic detection of the postulated cyclic photoproduct of TPE-4mM and the relevance of our findings for the AIE process.
四苯乙烯（TPE）化合物的光物理性质可能会因取代模式而有很大差异，例如，在荧光量子产率ϕ以及表现出聚集诱导发光（AIE）的倾向方面。我们报告了结合电子结构计算和非绝热动力学模拟，以研究两种具有四个甲基取代基的TPE衍生物的激发态衰变机制，甲基取代基分别在间位（TPE - 4mM，ϕ = 0.1%）或邻位（TPE - 4oM，ϕ = 64.3%）。在这两种情况下，两种激发态衰变途径可能是相关的，即围绕中心乙烯双键的光异构化和涉及两个相邻苯环的光环化。在TPE - 4mM中，无障碍的S光环化是有利的；这导致了实验中观察到的超低荧光量子产率。在TPE - 4oM中，S光环化和光异构化路径都被不可忽略的势垒所阻挡，因此荧光是可行的。超过1000条表面跳跃轨迹的非绝热动力学模拟表明，TPE - 4mM光激发后会发生超快光环化，而在1 ps模拟期间TPE - 4oM保持无反应。我们讨论了对TPE - 4mM假定的环状光产物进行光谱检测的可能性以及我们的发现与AIE过程的相关性。
