Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Abstract

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" S 1 ( 1 ππ*) state and the lower-lying T 1 and T 2 states. In the major relaxation pathway, the photoexcited S 1 keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S 1 enol minimum, near which a favorable S 1 /S 0 conical intersection decays the system to the S 0 state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S 0 keto species. In the minor one, an S 1 /T 2 /T 1 three-state intersection in the keto region makes the T 1 state populated via direct and T 2 -mediated intersystem crossing (ISC) processes. In the T 1 state, an ESIPT occurs, which is followed by ISC near a T 1 /S 0 crossing point in the enol region to the S 0 state and finally back to the S 0 keto species. In addition, a T 1 /S 0 crossing point near the T 1 keto minimum can also help the system decay to the S 0 keto species. However, small spin−orbit couplings between T 1 and S 0 at these T 1 /S 0 crossing points make ISC to the S 0 state very slow and make the system trapped in the T 1 state 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.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution

“…[63][64][65] The final MD snapshot that deletes the frozen methanol molecules is utilized as the starting structure for the following QM/MM calculations and this strategy has been used in many similar works before. [35][36][37][66][67][68][69][70] Fig. 1 shows our used QM/MM model.…”

“…75,76 The 6-31G* basis set [77][78][79] is employed for all the computations, which has been utilized in many similar works and has been demonstrated to provide accurate description of excited-state structures and energetics. [30][31][32][33][34][35][36][37][45][46][47][48] Vertical excitation energies and dipole moments at the Franck-Condon point and spin-orbit coupling matrix elements with the atomic mean-field approximation are calculated at the QM(CASPT2)/MM level. [80][81][82] All QM/MM calculations are carried out using the MOLCAS8.0 package interfaced with the TINKER6.3.2 package.…”

“…[83][84][85][86][87] This QM/MM approach was recently shown to provide an accurate description for excited-state structures and energetics in organic and protein systems. [35][36][37][66][67][68][69][70] Results and discussion S 0 minima and vertical excitation energies At the QM(CASSCF)/MM level, we optimize the S 0 equilibrium structures for the normal and proton transfer tautomers, which are denoted as S0-ENOL and S0-KETO, respectively, in Fig. 3.…”

“…This barrierless ground-state reverse process has also been found in previous works, for example in salicylic acid, methyl salicylate, GFP chromophores, indigo, etc. [36][37][38][45][46][47][48][49][88][89][90][91][92][93]…”

“…Also, note that a similar singletmediated ESIPT mechanism is also widely found in many similar compounds such as in salicylic acid, homosalate, GFP Chromophore, indigo etc. [36][37][38][45][46][47][48][49][88][89][90][91][92] The minor ones are related the ISC processes. At S2-PP-ENOL, the existence of the S 2 ( 1 pp*)/S 1 ( 1 np*)/T 2 ( 3 np*) three-state intersection region helps the S 2 system hop to the S 1 and T 2 state via S 2 -S 1 IC and S 2 -T 2 ISC (spin-orbit coupling: 5.8 cm À1 ) processes.…”