Issue 6, 2013

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

Abstract

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 (λmax = ∼440–460 nm, Φe = 3–18% and τ = 0.5–3.5 μs). In solutions at room temperature, 5–8 show profoundly different luminescence properties from being virtually non-emissive (Φe < 10−3) for 6–8 to highly emissive (Φe = 15%) with much red-shifted phosphorescence (λmax = ∼530 nm) and a long emission lifetime (τ = 47.2 μs) 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 T1 as in the S0 states and the blue emission is derived from the T1 state having predominant ligandAr–O)-to-ligand (π*NHC) charge transfer (LLCT) character. A switch of orbital parentage from LLCT to ligand-centred (LC) π–π* 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.

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

Supplementary files

Article information

Article type
Edge Article
Submitted
25 Oct 2012
Accepted
18 Mar 2013
First published
04 Apr 2013

Chem. Sci., 2013,4, 2630-2644

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

K. Li, G. Cheng, C. Ma, X. Guan, W. Kwok, Y. Chen, W. Lu and C. Che, Chem. Sci., 2013, 4, 2630 DOI: 10.1039/C3SC21822H

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