Construction of triptycene molecular rotors with intermeshing arrangement and low rotational barrier

Abstract

Abstract Molecular rotors are one of the building blocks of molecular machines and they are nano-sized with mechanically rotating moieties. Among them, intermeshing triptycenes with a gear-like skeleton allow 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 axes to 8.1 Å are required. However, with conventional methods, because of the restrictions on bond lengths and atomic radii, 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 combined it with amines of 2 different steric factors (normal-amylamine [nAmA] and guanidine [Gu]), which allowed us to prepare organic salts with varying arrangements of triptycenes. In the organic salt with the planar amine (Gu), the crystal structure was close to the ideal intermeshing arrangement of the triptycene and the distance between their axes was 7.7 Å. The T1ρ 13C spin-lattice relaxation time using solid-state nuclear magnetic resonance (NMR) demonstrated that triptycene rotates efficiently at 24 kHz at 313 K with a low rotational barrier (10.9 kcal/mol) compared with non-intermeshing structures.