Fine Tuning of the Rotational Speed of Light-Driven, Second-Generation Molecular Motors by Fluorine Substitution

The elementary steps in the rotation of several second-generation molecular motors are analyzed by finding the minimum energy path between the metastable and stable states and evaluating the transition rate within harmonic transition state theory based on energetics obtained from density functional theory. Comparison with published experimental data shows remarkably good agreement and demonstrates the predictive capability of this approach. While previous measurements by Feringa and co-workers have shown that a replacement of the hydrogen atom at the stereogenic center by a fluorine atom can slow down the rate-limiting thermal helix inversion (THI) step by raising the energy of the transition state, even to the extent that the backreaction in the ground state becomes preferred in some cases, we find that a replacement of a CH₃ group by CF₃ at the same site accelerates the THI by elevating the energy of the metastable state without affecting the transition state significantly. Since these two fluorine substitutions have an opposite effect on the rate of the THI, the combination of both can provide ways to fine-tune the rotational speed of molecular motors.