Photocyclization\nand Photoisomerization Mechanisms\nof an Indolylfulgide Derivative in Acetonitrile Solution by Using\nthe QM (MS-CASPT2)/MM Method

The indolylfulgide systems have been extensively investigated\ndue\nto their potential applications as photochromic materials. In this\nwork, the photoinduced ring-closure/opening and isomerization reactions\nof a photochromic indolylfulgide in vacuum and acetonitrile solvent\nhave been investigated by means of MS-CASPT2//CASSCF and QM(MS-CASPT2)//CASSCF/MM.\nThe deactivation mechanisms of indolylfulgide have been proposed based\non the optimized structures in the S<sub>0</sub> and S<sub>1</sub> states, S<sub>1</sub>/S<sub>0</sub> conical intersections, and the\ncalculated minimum-energy paths. After excitation into the first singlet\nexcited-state, which is spectroscopically bright in the Franck–Condon\npoint of the E, the photoprocesses proceed toward a nearby S<sub>1</sub> minimum. Then, two possible nonadiabatic relaxation paths exist\nto repopulate the ground state. In the ring closure reaction, the\nS<sub>1</sub> E isomer evolves directly into one S<sub>1</sub>/S<sub>0</sub> conical intersection and decays to the ground state with\nbifurcation toward C or E. In the E → Z tautomerization pathway,\nthe excited system can deactivate to the S<sub>0</sub> state via a\ndistinct conical intersection. The minimum-energy paths of the indolylfulgide\nrevealed that the ring closure reaction in the solvent is more facile\nto take place than the E → Z isomerization after irradiation\nof the same E. Furthermore, for the ring opening reaction from the\nC side, there exists an energy barrier (11.1 kcal/mol) in the S<sub>1</sub> state before arriving at the conical intersection. The computational\nresults showed that the solvent has some influence on the system compared\nwith that in the gas phase. The present work could contribute to comprehending\nthe photoreactions of indolylfulgide and its derivatives in solution.