Light-emitting platinum(II) complexes supported by tetradentate dianionic bis(N-heterocyclic carbene) ligands: towards robust blue electrophosphors

The synthesis, structures and photophysical properties of the charge-neutral Pt(II) complexes (1-6) and their Pd(II) (7) and Ni(II) (8) congeners supported by tetradentate dianionic bis[phenolate-(N-heterocyclic carbene)] ligands are described. The X-ray crystal structures of two solvatomorphs of 2, which has p-F substituents on the tetradentate ligand, have been determined. The photophysical properties of all the complexes were examined. In THF solutions, 1-4 display deep blue phosphorescence (lambda(max) = similar to 440-460 nm, Phi(e) - 3-18% and tau - 0.5-3.5 mu s). In solutions at room temperature, 5-8 show profoundly different luminescence properties from being virtually non-emissive (Phi(e) < 10(-3)) for 6-8 to highly emissive (Phi(e) = 15%) with much red-shifted phosphorescence (lambda(max) = similar to 530 nm) and a long emission lifetime (tau = 47.2 mu s) in the case of 5. Time-dependent density functional theory (TDDFT) calculations reveal that the tetradentate bis(phenolate-NHC) ligands in 1-4 provide a rigid scaffold for preserving a tightly bound Pt(II) in a square-planar coordination geometry in the T-1 as in the S-0 states and the blue emission is derived from the T1 state having predominant ligand (pi(Ar-O))-to-ligand (pi*(NHC)) charge transfer (LLCT) character. A switch of orbital parentage from LLCT to ligand-centred (LC) pi-pi* is responsible for the long emission lifetime and vibronically structured emission displayed by 5 when compared to that of 1-4 and 6. Both femtosecond time-resolved fluorescence (fs-TRF) and nanosecond time-resolved emission (ns-TRE) measurements were conducted on 2 and 4 to directly probe the excited-state dynamics after photoexcitation. Excellent thermal stability of the fluorine-free complex 4 and its higher emission quantum yield (relative to 1 and 3), and using 9-(4-tert-butylphenyl)-3,6-bis( triphenylsilyl)-9H-carbazole (CzSi) as host material, led to the fabrication of highly efficient deep blue OLEDs with peak current efficiency of 24 cd A(-1) and white organic light-emitting devices (WOLEDs) with peak current efficiency of 88 cd A(-1).