Highly efficient green organic light-emitting diodes containing luminescent tetrahedral copper(i) complexes

Abstract

A series of highly emissive sublimable copper(I) complexes with tetrahedral geometries, Cu(dppb)(pz₂Bph₂) 1, Cu(dppb-F)(pz₂Bph₂) 2, and Cu(dppb-CF₃)(pz₂Bph₂) 3 [dppb = 1,2-bis(diphenylphosphino)benzene, dppb-F = 1,2-bis[bis(3,5-difluorophenyl)phosphino]benzene, and dppb-CF₃ = 1,2-bis[bis[3,5-bis(trifluoromethyl)phenyl]phosphino]benzene, pz₂Bph₂⁻ = diphenyl-bis(pyrazol-1-yl)borate], were synthesized and investigated as luminescent guest molecules in prototype organic light-emitting diodes. Thermogravimetric analysis of 1–3 under vacuum revealed that introduction of F or CF₃ substituents to the meta positions of the four peripheral phenyl groups in the dppb skeleton increased the ability of the copper(I) complexes to be sublimed. 1–3 exhibited strong green emission in amorphous films at 293 K with maximum emission wavelengths of 523–544 nm, quantum yields of 0.50–0.68, and decay times of 3.6–8.2 μs. Molecular orbital calculations indicated that the origin of green phosphorescence from 1–3 was a mixture of σ → π* and π → π* transitions. Conventional bottom-emitting devices with three-layer structures containing 1–3 produced bright green luminescence with maximum external quantum efficiencies of 11.9, 16.0, and 17.7% for 1, 2 and 3, respectively.