Anisochronous Dynamics in a Crystalline Array of Steroidal Molecular Rotors: Evidence of Correlated Motion within 1D Helical Domains

We describe the solid-state dynamics of a molecular rotator (2) consisting of a p-phenylene rotor flanked by two ethynyl steroidal moieties that act as a stator. Single-crystal X-ray diffraction analysis of polymorph I revealed a packing motif containing 1D columns of nested rotors arranged in helical arrays (space group P3(2)) with the central phenylenes disordered over two sites related by an 85 degrees rotation about their 1,4-axes. Unexpected line shapes in quadrupolar-echo H-2 NMR measurements between 155 and 296 K for the same polymorph with a deuterated phenylene isotopologue (2-d(4)) were simulated by trajectories involving fast (>10(8) s(-1)) 180 degrees rotation (twofold flips) in each of the two rotationally disordered sites and slower exchange (2 x 10(4) to 1.5 x 10(6) s(-1)) between them. A negative activation entropy and a low enthalpic barrier for the slower 85 degrees exchange are interpreted in terms of highly correlated processes within the 1D helical domains.