Probing Conformational Variation in Luminescent Dinuclear Gold(I) N‐Heterocyclic Carbene Complexes

Abstract

A series of six dinuclear AuI complexes bearing arene‐bridged bidentate imidazolylidene (N‐heterocyclic carbene, NHC) ligands have been synthesized and characterized. The ligand arene linkers (benzene, pyridine, pyrazine) and NHC‐wingtip R‐groups (methyl, benzyl) were modified to investigate the influence of these groups on the preference for two distinct conformational isomeric forms. These two conformations differ in their Au···Au distances; an open form with an Au···Au separation of ca. 7 Å and a twisted conformation with a short Au···Au distance of ca. 3 Å. Five of the complexes were structurally characterized using X‐ray crystallography. Photophysical studies on the complex with a pyridyl linker group and benzyl wingtip groups showed that the twisted conformation resulted in a low‐energy (LE) photoluminescence emission that is associated with the intermolecular aurophilic interaction. In contrast, no room temperature (r.t.) photoluminescence was observed for the open conformation in the solid state, while a high‐energy (HE) emission was observed at 77 K. The pyridyl linked complexes were non‐emissive in solution at room temp., but upon freezing to form a glassy matrix at 77 K, dual HE and LE emissions were observed that closely match the solid‐state spectra for open and twisted complexes, respectively. These observations, in combination with a thorough theoretical investigation, provide important insights into the conformational behavior of complexes of this type. The use of low temperature frozen glasses to trap individual components of conformationally diverse samples for photoluminescence investigations represents a novel approach to study the dynamic and conformational behavior of dinuclear AuI complexes in solution.