A series of pure-blue-light emitting Cu(<scp>i</scp>) complexes with thermally activated delayed fluorescence: structural, photophysical, and computational studies

Abstract

Four mononuclear Cu(i)-halide complexes containing phosphines and pyridine ligands with strong electron donor substituents, [CuCl(PPh₃)₂(4-NMe₂py)] (1), [CuI(PPh₃)₂(4-NH₂py)] (2), [CuI(POP)(4-NH₂py)] (3), and [CuI(POP)(4-NMe₂py)]·0.5(Et₂O) (4), (PPh₃ = triphenylphosphine, 4-NMe₂py = 4-(dimethylamino)pyridine, POP = bis[(2-diphenyl-phosphino)phenyl]ether, 4-NH₂py = 4-aminopyridine, Et₂O = diethyl ether) were synthesized and studied with regard to their structural, photophysical properties and theoretical calculations. The complexes exhibit pure blue thermally activated delayed fluorescence (λ_max = 442 (1), 436 (2), 464 (3), and 448 nm (4)) in crystalline at room temperature. Emission lifetime analyses and density functional theory (DFT) calculations show that the blue-light emission at room temperature is the singlet (metal + halide)-to-ligand charge transfer state, (¹(M + X)LCT), while that at 77 K is the state of ³(M + X)LCT transition character, owing to the small singlet-triplet energy gaps (ΔE = 660-1680 cm^-1). X-ray diffraction structure analysis, photophysical studies and theoretical calculations suggest that the much larger torsion angle between the N-heterocyclic rings and N-Cu-X planes of complex 3 than that of 1, 2 and 4 might causes the bathochromic shift of luminescence, although these complexes containing similar heterocycle ligands.