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 Å2; volume: 219 Å3) 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 Å2; volume: 361 Å3; 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 2H 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.