College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China.
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China.
Phys Chem Chem Phys. 2022 Jun 1;24(21):13293-13304. doi: 10.1039/d2cp01263d.
Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the ππ*, nπ*, ππ*, nπ*, and S states, we have identified several feasible excited-state relaxation pathways for the initially populated S(ππ*) state to decay to the initial enol isomer' S state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ππ* ESIPT process to generate the ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S → S internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S(ππ*) minimum, an S(ππ*)/S(nπ*)/T(nπ*) three-state intersection region helps the S system branch into the T state through a S → S → T or S → T → T process. Once it has reached the T state, the system may relax to the S state direct ISC or subsequent nearly barrierless ππ* ESIPT to yield the T keto tautomer and ISC. The resultant S keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T/S crossing points, the ISC to S state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.
在此，我们采用了 QM(CASPT2//CASSCF)/MM 方法来探索氧苯酮（OB）在甲醇溶液中的光物理和光化学机制。基于优化后的最低能量、锥形交叉点和交叉点，以及与激发态分子内质子转移（ESIPT）和激发态衰减途径相关的最小能量反应路径，我们确定了几种可行的激发态松弛途径，最初占据的 S(ππ*)态可以通过这些途径衰减到初始烯醇异构体的 S 态。主要途径是单重态介导的和拉伸-扭转耦合的 ESIPT 途径，其中系统首先经历一个基本上没有势垒的ππ* ESIPT 过程，生成ππ酮物种，最后通过随后的羰基拉伸-扭转促进 S → S 内部转换（IC）和反向基态分子内质子转移（GSIPT）过程实现其基态恢复。次要途径与系间窜越（ISC）过程有关。在 S(ππ)最低点，S(ππ*)/S(nπ*)/T(nπ*)三态交叉区域有助于 S 系统通过 S → S → T 或 S → T → T 过程分支到 T 态。一旦到达 T 态，系统可能会通过直接 ISC 或随后的几乎无势垒的ππ* ESIPT 弛豫到 T 酮互变异构体和 ISC。生成的 S 酮物种显著经历反向 GSIPT，只有一小部分生成反式酮形式，其弛豫速度较慢。然而，由于 T/S 交叉点的自旋轨道耦合较小，ISC 到 S 态的发生非常缓慢。本工作不仅解释了 OB 的超快激发态衰减动力学，还解释了其在低温下的磷光发射。
