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 2 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.

Distinct Fold-Mode Formation of Crystalline Cu(I) Helical Coordination Polymers with Alternation of the Solid-State Emission Using Shape of the Counter Anions.

One-dimensional cationic coordination polymers have been a promising platform for designing solid-state physical properties through diverse coordination geometries. In particular, the folding mode of the coordination polymers that form a helical structure directly determines the metal-centered coordination environment. Herein, we report N-heterocyclic carbene (NHC) Cu(I) cationic coordination polymers with pyrazine as the linker, which construct a 4-fold or 3-fold helical column in luminescent crystals using octahedral anions (SbF6- and PF6-) or a tetrahedral anion (BF4-), respectively. Single-crystal XRD studies revealed that the folding modes depend on the structural shape of the counteranions, which form H-F interactions between ligands and anions. Indeed, the folding mode change from 4-fold to 3-fold by including a different shape of the counteranions, resulting in red-shifted emission from approximately 580 to 687 nm, which is difficult to modulate in the solid state.

Encapsulating N-Heterocyclic Carbene Binuclear Transition-Metal Complexes as a New Platform for Molecular Rotation in Crystalline Solid-State.

In crystalline solids, molecules generally have limited mobility due to their densely packed environment. However, structural information at the molecular level may be used to design amphidynamic crystals with rotating elements linked to rigid, lattice-forming parts, which may lead to molecular rotary motions and changes in conformation that determine the physical properties of the solid-state materials. Here, we report a novel design of emissive crystalline molecular rotors with a central pyrazine rotator connected by implanted transition metals (Cu or Au) to a readily accessible enclosure formed by two N-heterocyclic carbenes (NHC) in discrete binuclear complexes. The activation energies for the rotation could be tuned by changing the implanted metal. Exchanging Cu to Au resulted in an ∼4.0 kcal/mol reduction in the rotational energy barrier as a result of lower steric demand by elongation of the axle with the noble metal, and a stronger electronic stabilization in the rotational transition state by enhancement of the d-π* interactions between the metal centers and the pyrazine rotator. The Cu(I) rotor complex showed a greater electronic delocalization than the Au(I) rotor complex, causing a red-shifted solid-state emission. Molecular rotation-induced emission quenching was observed in both crystals. The enclosing NHC rotors are easy to prepare, and their rotational motion should be less dependent on packing structures, which are often crucial for many previously documented amphidynamic molecular crystals. The platform from the encapsulating NHC cationic metal complexes and the metal-centered rotation-axis provide a promising scaffold for a novel design of crystalline molecular rotors, including manipulation of rotary dynamics and solid-state emission.