Emergence of Coupled Rotor Dynamics in Metal–Organic Frameworks via Tuned Steric Interactions

The organic components
in metal–organic frameworks (MOFs)
are unique: they are embedded in a crystalline lattice, yet, as they
are separated from each other by tunable free space, a large variety
of dynamic behavior can emerge. These rotational dynamics of the organic
linkers are especially important due to their influence over properties
such as gas adsorption and kinetics of guest release. To fully exploit
linker rotation, such as in the form of molecular machines, it is
necessary to engineer correlated linker dynamics to achieve their
cooperative functional motion. Here, we show that for MIL-53, a topology
with closely spaced rotors, the phenylene functionalization allows
researchers to tune the rotors’ steric environment, shifting
linker rotation from completely static to rapid motions at frequencies
above 100 MHz. For steric interactions that start to inhibit independent
rotor motion, we identify for the first time the emergence of coupled
rotation modes in linker dynamics. These findings pave the way for
function-specific engineering of gear-like cooperative motion in MOFs.