Construction of Triptycene Molecular Rotors with Intermeshing Arrangement and Low Rotational Barrier

Abstract

Molecular rotors are one of the building blocks of molecular machines and are nano-sized machines with mechanically rotating moieties. Among them, intermeshing triptycenes with a gear-like skeleton allows the construction of a molecular rotor that transmits rotational motion. For triptycenes to mesh with each other without loss of rotation, intermeshing them in parallel and adjusting the distance between their axis to 8.1 Å are required. However, with conventional methods, because of the restrictions on bond lengths and atomic radius, achieving an ideal arrangement in which the triptycenes mesh in parallel at 8.1 Å has been difficult. In this work, we synthesized disulfonic acid containing a triptycene as a rotator, and combining it with amines of two different steric factors (normal-amylamine (nAmA) and guanidine (Gu)) allowed us to prepare organic salts with varying arrangements of triptycene. In the organic salt with the planar amine (Gu), the crystal structure was close to the ideal intermeshing arrangement of triptycene, and the distance between their axis was 7.7 Å. The T1ρ  13C spin-lattice relaxation time by solid-state NMR demonstrated that triptycene rotates efficiently at 24 kHz at 313 K with a low rotational barrier (10.9 kcal/mol) compared to non-intermeshing structures.