Quantum-classical simulations reveal the photoisomerization mechanism of a prototypical first-generation rotary motor

Light-driven rotary motors convert the energy of absorbed light into unidirectional rotational motion and are key components in the design of molecular machines. The archetypal class of light-driven rotary motors is chiral overcrowded alkenes, where the rotational movement is achieved through a cycle consisting of four consecutive steps: two cis-trans photoisomerization reactions and two thermal helix inversion steps. While the thermal steps of the rotation cycle have been rather well understood by now, our understanding of the photoisomerization reactions of overcrowded alkene-based motors still misses key points that would allow for explaining striking differences in operation efficiency of the known systems. Here, we employ quantum-chemical calculations and nonadiabatic molecular dynamics simulations to investigate the excited-state decay and photoisomerization mechanisms in a prototypical alkene-based first-generation rotary motor, considering the excitation of all its four ground-state isomers. We show that the initially excited bright state undergoes an ultrafast relaxation to multiple excited-state minima separated by low energy barriers and reveal a slow, picosecond timescale decay to the ground state, which only occurs from a largely twisted dark excited-state minimum, far from any conical-intersection point. Additionally, we identify the origin of the high yields of forward photoisomerization in our investigated motor, attributing them to the favorable topography of the ground-state potential energy surface controlled by the conformation of the central cyclopentene rings.