Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution
Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution
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高薄荷醇水杨酸酯在气相和乙腈溶液中激发态衰变机理的理论研究
DOI：
10.1021/acs.jpca.1c07108
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发表时间：
2022
期刊：
影响因子：
--
通讯作者：
Cui Ganglong
中科院分区：
文献类型：
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作者：
Chang Xue-Ping;Zhang Teng-Shuo;Cui Ganglong
本文采用CASPT2//CASSCF和QM（CASPT2//CASSCF）/MM方法研究了水杨酸高薄荷酯（HMS）在真空和乙腈溶液中的光化学反应机理。结果表明，在这两种情况下，激发态弛豫主要涉及光谱上"亮"的S1（1 π π *）态和较低的T1和T2态.在主要的弛豫途径中，光激发的S1酮系统首先经历基本上无势垒的激发态分子内质子转移（ESIPT），以产生S1烯醇最小值，在该最小值附近，有利的S1/S0圆锥相交使系统衰减到S0状态，然后通过反向基态分子内质子转移（GSIPT）重新填充初始的S0酮物种。在次系统中，酮区的S1/T2/T1三态交叉使T1态通过直接和T2介导的系统间交叉（ISC）过程被填充。在T1状态下，发生ESIPT，随后在烯醇区域的T1/S0交叉点附近发生ISC，到达S0状态，最后回到S0酮物质。此外，在T1酮最小值附近的T1/S0交叉点也可以帮助系统衰变为S0酮物质。然而，在这些T1/S0交叉点处，T1和S0之间的小自旋轨道耦合使得ISC到S0态非常缓慢，并使系统在T1态中被捕获一段时间。本工作不仅合理化了HMS的超快激发态衰减动力学，而且合理化了其在77 K的低磷光量子产率。
Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically “bright” S1(1ππ*) state and the lower-lying T1and T2states. In the major relaxation pathway, the photoexcited S1keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S1enol minimum, near which a favorable S1/S0conical intersection decays the system to the S0state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S0keto species. In the minor one, an S1/T2/T1three-state intersection in the keto region makes the T1state populated via direct and T2-mediated intersystem crossing (ISC) processes. In the T1state, an ESIPT occurs, which is followed by ISC near a T1/S0crossing point in the enol region to the S0state and finally back to the S0keto species. In addition, a T1/S0crossing point near the T1keto minimum can also help the system decay to the S0keto species. However, small spin–orbit couplings between T1and S0at these T1/S0crossing points make ISC to the S0state very slow and make the system trapped in the T1state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.