Coupling Molecular Rotation and Symmetry to Control Solid-State Chiroptical Properties in Crystalline Chiral N-Heterocyclic Carbene Au(I) Complexes

Abstract

Rotational dynamics in molecular crystals influence not only internal structures but also bulk properties such as photophysical behavior. In this work, we present novel crystalline chiral binuclear N-heterocyclic carbene (NHC) Au(I) complexes, 1-R and 1-S, which display a distinct axially chiral conformation with C₂-symmetry, derived from non-equivalent orientations of phenyl groups on the NHC ligands. These phenyl moieties undergo two distinct types of rapid rotational motion, as revealed by variable-temperature solid-state ²H NMR studies. Such dynamic motions promote structural symmetrization within the crystal, shifting from C₂-symmetry toward a more D₂-like symmetry. This symmetry evolution significantly affects the chiroptical properties of the crystals. Both experimental measurements and TD-DFT calculations confirm that such motion modulates chiroptical properties, leading to temperature-dependent changes in emission intensity and the luminescence dissymmetry factor (g_lum). These results highlight dynamic molecular rotation as a powerful tool for tuning symmetry and chiroptical responses in crystalline materials, offering new design principles for solid-state chiral systems.

Keywords

amphidynamic crystals / artificial molecular rotor / circularly polarized luminescence / NHC gold(I) complexes) / solid-state molecular dynamics / symmetry

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