College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, P. R. China.
Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China.
J Phys Chem A. 2021 Mar 11;125(9):1880-1891. doi: 10.1021/acs.jpca.0c10589. Epub 2021 Mar 1.
Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S, S, and T states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S state is a spectroscopically bright ππ* state in the Franck-Condon region. From the initially populated S state, there exist three nonradiative relaxation paths to repopulate the S state. In the first one, the S system (i.e., ketoB form) first undergoes an ESIPT path to generate an S tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S enol tautomer further evolves toward the nearby S/S conical intersection and then hops to the S state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S state. In the second one, the S system first hops through the S → T intersystem crossing (ISC) to the T state, which then further decays to the S state via T → S ISC at the T/S crossing point. In the third path, the T system that stems from the S → T ISC process via the S/T crossing point first takes place a T ESIPT to generate a T enol tautomer, which can further decay to the S state via T-to-S ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S state. Our present work could contribute to understanding the photophysics of MS and its derivatives.
水杨酸甲酯（MS）作为商业防晒霜中较大水杨酸酯的一个亚单位，在吸收紫外线（UV）辐射后，已被证明通过激发态分子内质子转移（ESIPT）表现出酮-烯互变异构和双荧光发射。然而，其激发态弛豫机制尚不清楚。在此，我们采用量子力学（CASPT2//CASSCF）/分子力学方法，在甲醇溶液中研究了 MS 在最低三个电子态，即 S、S 和 T 态的 ESIPT 和激发态弛豫机制。基于优化的几何和电子结构、锥形交叉点和交叉点以及与计算的线性插值笛卡尔坐标路径相结合的最小能量路径，提出了 MS 的光物理机制。S 态是 Franck-Condon 区域中光谱上明亮的ππ*态。从最初占据的 S 态出发，存在三条非辐射弛豫途径来重新填充 S 态。在第一种途径中，S 体系（即酮 B 形式）首先经历 ESIPT 途径，生成实验中表现出大斯托克斯位移的 S 互变异构体（即烯醇形式）。生成的 S 烯醇互变异构体进一步向附近的 S/S 锥形交叉点演化，然后跃迁到 S 态，随后通过反向基态分子内质子转移（GSIPT）回到初始酮 B 形式的 S 态。在第二种途径中，S 体系首先通过 S→T 系间交叉（ISC）跃迁到 T 态，然后通过 T/S 交叉点进一步通过 T→S ISC 衰变到 S 态。在第三条途径中，源自 S→T ISC 过程的 T 体系通过 S/T 交叉点首先发生 T ESIPT，生成 T 烯醇互变异构体，然后通过 T 到 S ISC 进一步衰变到 S 态。最后，发生 GSIPT 将系统返回到初始酮 B 形式的 S 态。我们的工作有助于理解 MS 及其衍生物的光物理性质。
