Pure blue phosphorescent platinum(ii) emitters supported by NHC-based pincer type ligands with unitary emission quantum yields

Abstract

A series of phosphorescent Pt(II) complexes containing a bis-N-heterocyclic carbene (NHC)-pincer-type ligand and a monodentate ancillary ligand, [Pt(C_NHC^C_Ar^C_NHC)L] (where L = chloride, 3,5-bis(trifluoromethyl)pyrazolate (Pz), cyanide or isocyanoborates (CNBPh₃ and CNB(C₆F₅)₃), were prepared and structurally characterized by X-ray crystallography. These complexes show intense blue phosphorescence from 441 to 457 nm with emission quantum yields of up to 0.87 in degassed CH₂Cl₂ solutions and to unity in PMMA films. The ancillary ligand L plays a pivotal role in regulating the nonradiative decay rate and propensity of the Pt(II) complex for intermolecular interactions, thereby modulating the emission profile and emission quantum yield in different media. Analysis of the temperature-dependent emission lifetimes revealed that the metal-centred (³MC) excited states of the chloride (1) and pyrazolate (8) complexes lie 2330–2730 cm⁻¹ above the emissive excited states. An efficient pure blue OLED with isocyanoborate complex 11 attained a maximum external quantum efficiency (EQE) and current efficiency (CE) of 22.0% and 25.2 cd A⁻¹, respectively. The pure blue electroluminescence of this device was characterized with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.11), revealing an outstanding blue index of 229.1. A high-performance phosphor-sensitized fluorescent blue OLED with a maximum EQE of 15.9% and blue index of 121.7 was realized with 11 and 2,5,8,11-tetra-tert-butylperylene (TPBe) as co-dopants.