Effect of Methylation on the Photodynamical Behavior of Arylazoimidazoles: New Insight from Theoretical ab Initio Potential Energy Calculations and Molecular Dynamics Simulations.

Arylazoimidazoles are a series of azobenzene derivatives possessing the ability to undergo photoinduced trans-cis isomerization. Their isomerization quantum yields are found to be dependent on the excitation wavelength and chemical substituents. The current work investigated the ultrafast nonadiabatic decay behaviors of three arylazoimidazoles (Pai-H, Tai-H, and Tai-Me) after being photoexcited to the S1 and S2 states by means of high-level ab initio potential energy calculations and on-the-fly surface hopping dynamical simulations in gas phase to explore the effect of the methylation. The results found that the Pai-H with no methylation substituents only decay along a NNC bending reaction pathway for both the S1 and S2 states. The Tai-H with a methylation substituent on the six-membered ring can decay along both the NNC bending and twisting motion pathways for the S1 and S2 states. The Tai-Me has methylation substituents on both the six- and five-membered rings prefers to decay by a twisting motion in the S1 state, while a NNC bending channel is activated following excitation to the S2 state. The position and numbers of methylation substituents has important influence on the dynamical behaviors of arylazoimidazoles. The current work provides fundamental knowledge of the arylazoimidazoles and will be helpful for advanced and further exploration and application.