NHC stabilized two-coordinated mono- and bimetallic Cu(I) complexes: Ligand-directed emission and solvatochromism

This study presents the synthesis and photophysical investigation of two-coordinated mono- and bimetallic Cu(I) complexes stabilized by sterically demanding N-heterocyclic carbene (NHC=1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) and substituted 2-phenylbenzimidazole (PhBz) ligands. Monometallic complexes, [Cu(NHC)(MePhBz)]PF₆ (1), [Cu(NHC)(PhBz)]PF₆ (2), [Cu(NHC)(CNPhBz)]PF₆ (3), [Cu(NHC)(NO₂PhBz)]PF₆ (4), and bimetallic complexes [(Cu(NHC))₂(MePhBz)]PF₆ (5), [(Cu(NHC))₂(PhBz)]PF₆ (6), [(Cu(NHC))₂(CNPhBz)]PF₆ (7) and [(Cu(NHC))₂(NO₂PhBz)]PF₆ (8) were synthesized and thoroughly characterized. To the best of our knowledge, this constitutes the first report of bimetallic linear two-coordinated Cu(I) complexes. These complexes display tunable emission across the UV–visible range, with nanosecond lifetimes (0.28–2.70 ns). Introduction of electron-withdrawing substituents such as cyano (CN) and nitro (NO₂) groups on the PhBz ligand framework modulates the electronic structure by stabilizing the LUMO, resulting in progressively narrowed HOMO–LUMO energy gaps and red-shifted absorption/emission profiles. Additionally, complexes 3, 4, 7 and 8 exhibit positive solvatochromic behaviour across solvents of varying polarity with emission shifts of 1209 to 2661 cm⁻¹. The observed bathochromic shift with increasing solvent polarity is attributed to a solvent-induced decrease in the HOMO–LUMO energy gap.