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

Abstract

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 Δ H⧧ = 2.7 ± 0.1 kcal mol-1 and Δ S⧧ = -5.0 ± 0.5 cal mol-1 K-1, respectively, the corresponding parameters for the slower 85° jumps, determined by line shape analysis, were Δ H⧧ = 2.2 kcal mol-1 and Δ S⧧ = -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.