Synthesis, Structure, and Photophysical Properties of Two Four-Coordinate Cu^I–NHC Complexes with Efficient Delayed Fluorescence

Two luminescent cationic heteroleptic four-coordinate Cu^I complexes supported by N-heterocyclic carbene ligand and diphosphine ligand were successfully prepared and characterized. These complexes adopt typical distorted tetrahedral configuration and have high stability in solid state. Quantum chemical calculations show carbene units have contributions to both highest occupied molecular orbitals and lowest unoccupied molecular orbitals of these Cu^I–NHC complexes, the lowest-lying singlet and triplet excitations (S₀ → S₁ and S₀ → T₁) of [Cu­(Pyim)­(POP)]­(PF₆) are dominated by metal-to-ligand charge transfer (MLCT) transition, while the S₀ → S₁ and S₀ → T₁ excitations of [Cu­(Qbim)­(POP)]­(PF₆) are mainly MLCT and ligand-centered transitions, respectively. These Cu^I–NHC complexes show efficient long-lifetime emissions (λ_em = 520 nm, τ = 79.8 μs, Φ = 0.56 for [Cu­(Pyim)­(POP)]­(PF₆), λ_em = 570 nm, τ = 31.97 μs (78.99%) and 252.2 μs (21.01%), Φ = 0.35 for [Cu­(Qbim)­(POP)]­(PF₆)) in solid state at room temperature, which are confirmed as delayed fluorescence by investigating the emissions at 77 K.