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

Area of Science:

• Molecular chemistry
• Theoretical chemistry
• Materials science

Background:

• Second-generation molecular motors are crucial for nanoscale applications.
• Understanding substituent effects on rotor dynamics is key to optimizing motor performance.

Purpose of the Study:

• To theoretically investigate how modifying substituents in the rotor group affects the rotational speed of molecular motors.
• To explore the impact of tert-butyl (tBu) and fluorinated tBu groups on motor kinetics and photophysical properties.

Main Methods:

• Density functional theory (DFT) was used to calculate energy and atomic forces.
• Harmonic transition state theory (HTST) estimated the rate-limiting step (thermal helix inversion) and backward transitions.
• Calculations were compared with experimental half-life measurements for validation.

Main Results:

• Replacing a methyl group with a tert-butyl (tBu) group increased rotational speed by reducing the metastable state lifetime.
• Fluorinating the tBu group further accelerated the rotational rate without significant structural changes.
• Substitution increased the separation of absorption peaks for stable and metastable states by up to 40 nm.

Conclusions:

• Substituent modification, particularly with tBu and fluorinated tBu groups, offers a viable strategy to enhance molecular motor speed.
• These findings provide a basis for designing molecular motors with tailored rotational velocities for specific applications.
• The observed changes in photophysical properties suggest potential for light-driven control and tuning.