Heteroleptic cuprous complexes of a diimine MePBO ligand and their structure influence on phosphorescent color: Syntheses, structure characterizations, properties and TD‐DFT calculations

Abstract

AbstractThree new heteroleptic [Cu(NN)P2]+ type cuprous complexes 1–3 were designed and synthesized by utilizing a diimine ligand 5‐methyl‐2‐(2′‐pyridyl)‐benzoxazole (MePBO) and different phosphine ligands PPh3 (1), m‐Tol3P (2) and POP (3), (PPh3=triphenylphosphine, m‐Tol3P=tris(3‐methylphenyl)phosphine, POP=bis[2‐(diphenylphosphino)phenyl]ether), respectively. All complexes were characterized by single‐crystal X‐ray diffraction, spectroscopic analysis (IR, UV‐Vis), elemental analysis, and photoluminescence study. Single‐crystal X‐ray diffraction revealed that complexes 1–3 all adopt discrete cation complex structure with a tetrahedral CuN2P2 coordination geometry with diverse P−Cu−P angles. Their UV‐Vis absorption spectra exhibit a blue‐shift sequence under the enlarging of P−Cu−P angle from 3 to 2 then to 1. The PL emission peak wavelengths of 1–3 also present similar blue‐shift sequence (3→1/2). Their microsecond PL lifetime indicates that their PL behavior belongs to phosphorescence. TD‐DFT calculation and wavefunction analysis illuminate that the S1 and T1 states of 1–3 should all be assigned as (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 MePBO ligand. Therefore, as P−Cu−P angle increases (lower HOMO), the energy of S1 and T1 states also increase, following the change of PL color. Additionally, the steric hindrance from substituents of phosphine ligand, as well as extra strong intra‐molecular π−π stacking interactions should effectively inhibit nonradiative decay so that an abnormal PL emission blue‐shift is observed from 1 to 2.