First principles prediction of wavelength-dependent isomerization quantum yields of a second-generation molecular nanomotor.

Second-generation molecular nanomotors are gaining popularity within the biomedical field and intense research is being conducted to increase their efficiency for light-induced ultrafast photoisomerization. A key requirement for designing efficient molecular nanomotors is ensuring unidirectional rotation during isomerization and thermal helix inversion. Here, we used non-adiabatic trajectory surface hopping molecular dynamics based on TDDFT to study the excited state dynamics of the stable M- and metastable P-conformers of a second-generation Feringa-type molecular nanomotor. From the trajectories, we computed quantum yields for clockwise and anti-clockwise photoisomerization. Results show that the helicity of the initial structure dictates the direction of the isomerization: 52% of the trajectories starting from M-conformers isomerize clockwise, whereas 23% of the trajectories starting from P-conformers isomerize anti-clockwise. The quantum yield for clockwise isomerization can be maximized by excitation in the center of the absorption spectrum (350-400 nm). In this region, the M-conformer exhibits its maximum absorption and maximum (clockwise) isomerization quantum yield, whereas the P-conformer shows negligible excitation probability and (anti-clockwise) isomerization quantum yield. Moreover, we also find several trajectories, where thermal helix inversion occurs in the hot ground state, directly after excited state relaxation. While helix inversion was observed to proceed in both directions, its net effect favors unidirectional rotation. We further report excited state lifetimes and details about the structural dynamics.