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 P3(2) 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 degrees rotation about their 1,4 -axes. Rotational dynamics measured by quadrupolar echo H-2 NMR line shape analysis were analyzed in terms of a process model that involves degenerate 180 degrees jumps in the fast exchange regime combined with a highly correlated and entropically demanding jump of 85 degrees between the two dynamically disordered sites. While the enthalpic and entropic barriers for the 180 degrees jump estimated from H-2 T-1 measurements were Delta H-double dagger = 2.7 +/- 0.1 kcal mol(-1) and Delta S-double dagger = -5.0 +/- 0.5 cal mol(-1) K-1, respectively, the corresponding parameters for the slower 85 degrees jumps, determined by line shape analysis, were Delta H-double dagger = 2.2 kcal mol(-1) and Delta S-double dagger = -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.