Tuning Molecular Motors with tert-Butyl and Fluorinated tert-Butyl Groups

The effect of modifying substituents in the rotor group of five second generation molecular motors is estimated by theoretical calculations. The rotational speed is estimated by calculating the rate limiting step, the thermal helix inversion, and the competing backward transition using harmonic transition state theory with energy and atomic forces obtained from density functional theory. First, a methyl group at the stereogenic center is replaced with a tert-butyl (tBu) group, and the rotational speed is found to increase due to reduced lifetime of the metastable state. For two of the rotors, comparison can be made with experimental measurements, and the calculated half-life is in close agreement. Second, the effect of substituting the nine hydrogen atoms in the tBu group with fluorine atoms is studied, and this is found to increase the rotational rate further without significantly altering the molecular structure. The excitation wavelength of both the stable and metastable states is calculated, and the separation of the absorption peaks is found to increase by the tBu substitution and even more so by the fluorinated tBu substitution, up to 40 nm. These findings can help to develop a strategy for designing molecular motors with a rotational speed that best fits a given application.