Throwing in a Monkey Wrench to Test and Determine Geared Motion in the Dynamics of a Crystalline One-Dimensional (1D) Columnar Rotor Array

Crystals of molecular rotor 1 with a central 1,4-phenylene rotator linked to two molecules of the steroid mestranol were prepared with 1%, 5%, 20%, and up to 40% of the analogous 2, which contains a larger 2,3-difluorophenylene rotator and effectively acts as a monkey wrench that affects the rotation of the host. The packing motif of the desired P3₂ crystal form consists of 1D columns of nested rotors arranged in helical arrays with the central aromatic rotators disordered over two sites related by 85° rotation about their 1,4-axes. Rotational dynamics measured by quadrupolar echo ²H NMR line shape analysis were analyzed in terms of a process model that involves degenerate 180° jumps in the fast exchange regime combined with a highly correlated and entropically demanding jump of 85° between the two dynamically disordered sites. While the enthalpic and entropic barriers for the 180° jump estimated from ²H T₁ measurements were ΔH^⧧ = 2.7 ± 0.1 kcal mol^–1 and ΔS^⧧ = −5.0 ± 0.5 cal mol^–1 K^–1, respectively, the corresponding parameters for the slower 85° jumps, determined by line shape analysis, were ΔH^⧧ = 2.2 kcal mol^–1 and ΔS^⧧ = −23 cal mol^–1 K^–1. Increasing amounts of the larger molecular rotor 2 in the solid solution results in significant dynamic perturbations as the guest, acting as a monkey wrench, reaches values of one out of every five molecular rotors in the chain.