Heteroleptic [Cu(NN)P2]+‐type cuprous complexes and their structural modulation on phosphorescent color: Synthesis, structural characterization, properties, and theoretical calculations

Abstract

Four new heteroleptic [Cu(NN)P2]+‐type cuprous complexes—1‐TPP, 2‐POP, 3‐Xantphos, and 4‐DPPF—were designed and synthesized using a diimine ligand 2‐(2′‐pyridyl)benzoxazole (2‐PBO) and different phosphine ligands (TPP, triphenylphosphine; POP, bis[2‐(diphenylphosphino)phenyl]ether; Xantphos, 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene; DPPF, 1,1′‐bis(diphenylphosphino)‐ferrocene). All complexes were characterized using single‐crystal X‐ray diffraction, spectroscopic analysis (infrared, UV–Vis.), elemental analysis, and photoluminescence (PL). Single‐crystal X‐ray diffraction revealed complexes 1–4 as isolated cation complex structures with a tetrahedral CuN2P2 coordination geometry and diverse P–Cu–P angles. Their UV–Vis. absorption spectra exhibited a blue‐shift sequence in wavelength with an enlarged P–Cu–P angle from 4 to 2 then to 3 and then to 1. The PL emission peaks of 1–3 also exhibited a similar blue‐shift sequence (2 → 3 → 1). Their PL lifetime in microseconds (~7.5, 5.1, and 4.7 μs for 1, 2, and 3, respectively) indicated that their PL behavior represents phosphorescence. Time‐dependent density functional theory (TD‐DFT) calculation and wavefunction analysis revealed that S1 and T1 states of 1–3 should be assigned as metal–ligand and ligand–ligand charge‐transfer (ML + L'L)CT states. Their UV–Vis. absorption and phosphorescence should be attributed to the charge transfer from the P–Cu–P segment to the 2‐PBO ligand. Therefore, as the P–Cu–P angle increased (lower HOMO), the energy of S1 and T1 states also increased, following the change of PL color.