Fine Tuning of the Rotational Rate of Light-Driven, Second Generation Molecular Motors by Fluorine Substitutions

TL;DR

This study uses density functional theory and harmonic transition state theory to predict how fluorine substitutions affect the relaxation times of second-generation molecular motors, aligning well with experimental data.

Contribution

It introduces a theoretical approach to accurately predict and tune the rotational speed of molecular motors through specific fluorine substitutions.

Findings

Fluorine substitution at certain sites shortens relaxation time.

Replacement of CH₃ with CF₃ increases relaxation time.

The combined substitutions allow precise control of motor rotation speed.

Abstract

The relaxation time of several second generation molecular motors is analysed by calculating 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 the theoretical approach. While previous measurements by Feringa and coworkers [Chem.\,Eur.\,J.\,(2017) 23, 6643] have shown that a replacement of the stereogenic hydrogen by a fluorine atom increases the relaxation time because of destabilization of the transition state for the thermal helix inversion, we find that a replacement of CH$_3$ by a CF$_3$ group at the same site shortens the relaxation time because of elevated energy of the metastable state without a significant shift in the transition state energy. Since these two fluorine substitutions have an opposite effect on the relaxation time, the two combined can provide a way to fine tune the rotational speed of a molecular motor.