A Crystalline Supramolecular Rotor Functioned by Dual Ultrasmall Polar Rotators

As an extended model of conventional molecular rotors,a conceived construction of novel crystalline molecular rotor that simultaneously contains two discrete polar rotators is presented here.The supramolecular self-assembly of 18-crown-6 host and two rotator-containing ion-pair guests affords a three-in-one cocrystal,(2-NH_(3)-iBuOH)(18-crown-6)[ZnBr_(3)(H_(2)O)],in which the hydroxyl group and aqua ligand both function as ultrasmall polar rotators.On the basis of the variable-temperature single-crystal X-ray diffraction,variable-temperature/frequency dielectric response,density functional theory calculations,and molecular dynamics simulations,it is found that such dual polar rotators experience a gradually enhanced rotation with increasing temperature,and more importantly,could be controlled by a reversible polar-to-polar structural phase transition,i.e.,from a“single-(polar rotator)”state at low-temperature phase to a“mixed-dual-(polar rotator)”state in the vicinity of transition,and to an unusual“synchronized-dual-(polar rotator)”state at high-temperature phase.