Impact of solvent polarity on the photoinduced dynamics of a push–pull molecular motor

What is it about?

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 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 in 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.

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Photo by MARIOLA GROBELSKA on Unsplash

Photo by MARIOLA GROBELSKA on Unsplash

Why is it important?

Our study reveals, for the first time, the excited-state dynamics of a push–pull Feringa’s first-generation rotary motor in solution and provides a detailed interpretation of the spectroscopic observations, thereby contributing to uncovering how solvent polarity can impact the photochemical behavior of light-driven rotary molecular motors. Understanding such effects is important because changing the medium polarity represents a possible way of fine-tuning the photochemical behavior of light-driven rotary motors.