N-Alkyl Chain Induced Molecular Aggregation in Mononuclear Gold(I)-N-Heterocyclic Carbene Complexes for Blue Light Emitting Applications

Abstract

The gold(I) N-heterocyclic carbene (NHC) molecule exhibits outstanding luminescent properties along with high thermal stability due to its strong carbene gold bond. Gold(I)-NHC complexes are known to emit green to yellow color, while blue emission is limited. Herewith, we report the photoluminescence and thermal stability correlation between blue emitting mononuclear gold(I)-NHC chloride complexes, N-(9-anthracenyl)-N’(n-alkyl)imidazol-2-ylidene gold(I) chloride (alkyl=n-butyl (1), n-pentyl (2), n-hexyl (3)). These complexes were synthesized by transmetallation route and characterized by FT-IR, NMR, TGA, and single crystal X-ray diffraction analysis. The solid-state study reveals a unique molecular packing due to hydrogen bonding, aurophilic interaction, and CH⋅⋅⋅π interactions in all 1–3. In addition, the π⋅⋅⋅π stacking has been found in 1. All these complexes show good thermal stability due to the strong metal-ligand bond strength along with crystal packing. In addition, 1–3 emits a blue colour in both solution and crystalline state. The quantum yield in the crystalline state was found to be higher for 1 (22.44 %) compared to 2 (9.90 %), and 3 (14.98 %). A similar high quantum yield for blue-emitting gold(I)-NHC chloride complex is rare. This promising quantum yield for 1 can be ascribed to the crystal packing effect. The theoretical calculations were carried out to understand the electronic and structural properties of 1–3. Computational studies revealed the origin of the luminescence behaviour of complexes. The blue light emitting thin film and LED are demonstrated using 1 exhibited the emission peak at the blue region with Commission Internationale de L'Eclairage (CIE) coordinates near the National Television Standards Committee (NTSC) value.