Photoluminescent nickel(II) carbene complexes with ligand-to-ligand charge-transfer excited states

Abstract

While nickel(II) complexes have been widely used as catalysts for carbon-carbon coupling reactions, the exploration of their photophysical and photochemical properties is still in the infancy. Here, a series of square-planar Ni(II) complexes [(diNHC)NiX2] bearing chelating benzimidazole-based bis(N-heterocyclic carbene) ligands and varying anionic coligands (1, X = Cl; 2, X = Br; 3, X = I) are synthesized and structurally characterized. In solid state, both 1 and 2 exhibit orange-red photoluminescence under ambient conditions. The photophysical and electrochemical measurements along with density functional theory (DFT) calculations reveal that the low-energy emissions can be attributed to singlet excited states with ligand-to-ligand charge-transfer (LLCT) character. This work suggests that strong-field N-heterocyclic carbene ligands play a crucial role to achieve the luminescence of Ni(II) complexes.