Computational Design of a Family of Light-Driven Rotary Molecular Motors with Improved Quantum Efficiency

Two
new light-driven molecular rotary motors based on the N-alkylated indanylidene benzopyrrole frameworks are proposed
and studied using quantum chemical calculations and nonadiabatic molecular
dynamics simulations. These new motors perform pure axial rotation,
and the photochemical steps of the rotary cycle are dominated by the
fast bond-length-alternation motion that enables ultrafast access
to the S1/S0 intersection. The new motors are
predicted to display a quantum efficiency higher than that of the
currently available synthetic all-hydrocarbon motors. Remarkably,
the quantum efficiency is not governed by the topography (peaked versus
sloped) of the minimum-energy conical intersection, whereas the S1 decay time depends on the topography as well as on the energy
of the intersection relative to the S1 minimum. It is the
axial chirality (helicity), rather than the point chirality, that
controls the sense of rotation of the motor.