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

Three new heteroleptic [Cu(NN)P-2](+) 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-Tol(3)P (2) and POP (3), (PPh3=triphenylphosphine, m-Tol(3)P=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 S-1 and T-1 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 S-1 and T-1 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 pi-pi stacking interactions should effectively inhibit nonradiative decay so that an abnormal PL emission blue-shift is observed from 1 to 2.