Photocyclization and Photoisomerization Mechanisms of an Indolylfulgide Derivative in Acetonitrile Solution by Using the QM (MS-CASPT2)/MM Method

The indolylfulgide systems have been extensively investigated due to their potential applications as photochromic materials. In this work, the photoinduced ring-closure/opening and isomerization reactions of a photochromic indolylfulgide in vacuum and acetonitrile solvent have been investigated by means of MS-CASPT2//CASSCF and QM(MS-CASPT2)//CASSCF/MM. The deactivation mechanisms of indolylfulgide have been proposed based on the optimized structures in the S₀ and S₁ states, S₁/S₀ conical intersections, and the calculated minimum-energy paths. After excitation into the first singlet excited-state, which is spectroscopically bright in the Franck-Condon point of the E, the photoprocesses proceed toward a nearby S₁ minimum. Then, two possible nonadiabatic relaxation paths exist to repopulate the ground state. In the ring closure reaction, the S₁ E isomer evolves directly into one S₁/S₀ conical intersection and decays to the ground state with bifurcation toward C or E. In the E → Z tautomerization pathway, the excited system can deactivate to the S₀ state via a distinct conical intersection. The minimum-energy paths of the indolylfulgide revealed that the ring closure reaction in the solvent is more facile to take place than the E → Z isomerization after irradiation of the same E. Furthermore, for the ring opening reaction from the C side, there exists an energy barrier (11.1 kcal/mol) in the S₁ state before arriving at the conical intersection. The computational results showed that the solvent has some influence on the system compared with that in the gas phase. The present work could contribute to comprehending the photoreactions of indolylfulgide and its derivatives in solution.