Giant Crystalline Molecular Rotors that Operate in the Solid State.

Molecular motion in the solid state is typically precluded by the highly dense environment, and only molecules with a limited range of sizes show such dynamics. Here, we demonstrate the solid-state rotational motion of two giant molecules, i.e., triptycene and pentiptycene, by encapsulating a bulky N-heterocyclic carbene (NHC) Au(I) complex in the crystalline media. To date, triptycene is the largest molecule (surface area: 245 Å ; volume: 219 Å ) for which rotation has been reported in the solid state, with the largest rotational diameter among reported solid-state molecular rotors (9.5 Å). However, the pentiptycene rotator that is the subject of this study (surface area: 392 Å ; volume: 361 Å ; rotational diameter: 13.0 Å) surpasses this record. Single-crystal X-ray diffraction analyses of both the developed rotors revealed that these possess sufficient free volume around the rotator. The molecular motion in the solid state was confirmed using variable-temperature solid-state H spin-echo NMR studies. The triptycene rotor exhibited three-fold rotation, while temperature-dependent changes of the rotational angle were observed for the pentiptycene rotor.