Yellow–green luminescence of four-coordinate copper(I) complexes bearing N–heterocyclic carbene (NHC) ligands: Synthesis, photophysical and computational studies

Abstract

The synthesis and photophysical investigation of a series of six copper(I) complexes bearing benzimidazolylidene–type N–heterocyclic carbene (NHC) ligands with different groups are reported. Their photophysical properties can be tuned by adjusting the groups at the R1/R2 positions of the pyridine ring. The lowest-lying metal–to–ligand (MLCT) absorption bands at 325–437 nm can be observed for all the complexes. Upon excitation with UV light all the complexes emit bright yellow-green light (λem = 540–573 nm) with higher emission quantum yields of 21.1–55.6% and excited state lifetimes on the microsecond scale (τ = 30.6–62.1 μs) in PMMA films. The substituents (R2 = COCH3, F, Br, CH3, or OCH3) on R2–position of the pyridine ring were shown to have effects on the emission wavelengths of these complexes, while the introduction of R1 substituents (R1 = CH3) shows negligible effects on the emission wavelength but allow the enhancement of photoluminescence quantum yield (Φ) of the complexes. Additionally, the electronic properties, absorption and emission profiles of these complexes were further explored using DFT/TDDFT methods.