New insights into mechanistic photoisomerization of ethylene-bridged azobenzene from ab initio multiple spawning simulation

Ethylene-bridged azobenzene (br-AB) has aroused broad interests due to its unique photoswitching properties. Numerous dynamical simulations have been performed for the br-AB photoisomerization, which focused mainly on the conformational effect and the funnel role of minimum-energy conical intersection (MECI) on the mechanism. In the present work, we use the "full quantum" ab initio multiple spawning method to simulate the br-AB photoisomerization, which provides new insights into the mechanism. Upon irradiation of br-AB to the first excited singlet state (S-1), most of the excess energies are trapped in the azo-moiety. Since the intramolecular vibrational energy redistribution is slower than the S1 relaxation processes, the nonadiabatic transition from S-1 to the ground state (S-0) occurs in the vicinity of high-energy crossing seam and even the largest probabilities of the S-1 -> S-0 transition are not distributed in the MECI regions. Once decaying to the S-0 state through the high-energy region, the subsequent isomerization and re-formation of the initial isomer are ultrafast processes in the S-0 state. It is the nonergodic behavior of the S-1 and S-0 dynamics that is mainly responsible for the unique photoswitching properties of the ethylene-bridged azobenzene, which will be discussed in detail. Published by AIP Publishing.