Dynamics of a Molecular Rotor Exhibiting Local Directional Rotational Preference within Each Enantiomer

Directional internal rotation in molecular systems, generally controlled by chirality, is known to occur in natural and artificial systems driven by light or fueled chemically, but spontaneous directional molecular rotation is believed to be forbidden. We have designed a molecular rotor, whereby ferrocene and triptycene linked by a methylene bridge provide two rotational degrees of freedom. On the basis of experimental observations, in conjunction with computational data, we show that the two different modes of rotation are strongly coupled and the spatial orientation of the bistable ferrocene moiety controls the barrier to its own rotation about the triptycene axis. It is proposed that the barrier to clockwise 120° rotation across each individual triptycene blade is lower in the M-enantiomer and for counterclockwise 120° rotation, it is lower in its P-counterpart. These findings demonstrate the possibility of locally preferred thermal directional intramolecular rotation for each dynamically interconverting enantiomer.