Circularly Polarized Luminescence Control through Molecular Rotation in Crystalline Chiral Binuclear N-Heterocyclic Carbene Au(I) Complexes

The rotational motion within molecular crystals can affect not only their internal structure and symmetry, but also often results in the alteration of bulk properties such as thermal expansion, macroscopic motion, change in electromagnetic, optical, and photophysical properties. In this work, we demonstrate how the thermally activated molecular rotation within the crystals of chiral binuclear N-Heterocyclic carbene (NHC) Au(I) complexes can modulate their circularly polarized luminescence (CPL). The enantiomeric pair of chiral binuclear NHC Au(I) complexes, 1-R and 1-S, exhibit a distinct C2-symmetric conformation, derived from non-equivalent orientations of phenyl moieties at the chiral NHC ligands. The variable-temperature solid-state 2H NMR studies revealed that these non-equivalent phenyl moieties undergo fast rotational motion above 273K, potentially promoting symmetrization of molecular structures in the crystal. The latter, in combination with the inherent chirality of 1-R and 1-S complexes, results in the emergence of the temperature-dependent CPL behavior in the crystalline state with a noticeable increase of luminescence intensity and dissymmetry factors upon cooling from 333K to 213K (|ΔICPL|: + ~300%; |glum|: + ~50%). Such thermo-responsive behavior can be attributed not only to thermal rotation-induced luminescence quenching but also to the potential effect of phenyls rotation on the symmetry of the complexes.