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A series of first-generation light-driven molecular motors with rigid substituents of varying length was synthesized to act as "molecular stirrers". Their rotary motion was studied by (1)H NMR and UV-vis absorption spectroscopy in a variety of solvents with different polarity and viscosity. Quantitative analyses of kinetic and thermodynamic parameters show that the rotary speed is affected by the rigidity of the substituents and the length of the rigid substituents and that the differences in speed are governed by entropy effects. Most pronounced is the effect of solvent viscosity on the rotary motion when long, rigid substituents are present. The α values obtained by the free volume model, supported by DFT calculations, demonstrate that during the rotary process of the motor, as the rigid substituent becomes longer, an increased rearranging volume is needed, which leads to enhanced solvent displacement and retardation of the motor.
Chen et al. (Thu,) studied this question.