Impact of Solvent Polarity on the Photoinduced Dynamics of a Push-Pull Molecular Motor

Light-driven rotary molecular motors convert light energy into unidirectional rotational movement. In overcrowded alkene-based molecular motors, rotary motion is accomplished through consecutive cis-trans photoisomerization reactions and thermal helix inversion steps. To date, a complete understanding of the photoisomerization reactions of overcrowded alkene motors has not been achieved yet. In this work, we use quantum chemical calculations and quantum mechanics/molecular mechanics (QM/MM) nonadiabatic dynamics simulations to investigate the photoinduced dynamics of a push-pull alkene-based molecular motor in two different solvents: cyclohexane and methanol. We show that, while in both solvents the main photorelaxation pathway of our investigated push-pull motor involves two different excited-state minima, in polar methanol the photorelaxation dynamics is much faster than in nonpolar cyclohexane because of two main effects: (i) a lowering of the energy barrier between the excited-state minima, and (ii) a reduction of the energy gap with the ground state at the largely twisted dark minimum, where the excited-state decay takes place. Both effects can be attributed to solvent-polarity stabilization of the charge-transfer excited state along the photorelaxation pathway. In line with the experimental findings, our simulations also indicate that in methanol the accelerated photoinduced dynamics goes along with a faster fluorescence decay and a large reduction in the forward photoisomerization yield of our investigated motor.