Visualization of Light‐Induced Rotational Movement in Crystal: Exploring the Mechanism of Viologen‐Containing Molecular Machines

Constructing effective light-driven crystal molecular machines and exploring their mechanisms of movement are vital for the development of molecular machines. These explorations afford an Mn2+ metal-organic framework (MOF)crystal, marking the first instance of a viologen-containing crystalline MOF capable of exhibiting reversible light-driven rotational motion. Structural analysis indicates that the rotation of the bipyridine units initiates with a transformation in the coordination configuration of the manganese ion, which subsequently alters the dihedral angle of the bipyridine from -6.7° to -46.8°. This rotation is facilitated through a single bond and involves a transition state with the dihedral angle approaching 0° during the radical phase of the viologen. This result not only elucidates the mechanism by which photo-induced electron transfer propels the controlled rotation of biaryl groups within crystalline MOFs but also highlights the great potential of viologen-containing crystalline MOFs as molecular machines.