The structural effects of ligands on the emission properties of Pt(ii) complexes and promising applications of luminescent Pt(ii) complexes in various areas are discussed.

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 (λ_max = ∼440–460 nm, Φ_e = 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 (Φ_e < 10⁻³) for 6–8 to highly emissive (Φ_e = 15%) with much red-shifted phosphorescence (λ_max = ∼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 ligand (π_Ar–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.

A new class of cyclometalated Ir(III) complexes supported by various bidentate C-deprotonated (C^N) and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized. These complexes display strong emission in deaerated solutions at room temperature with photoluminescence quantum yields up to 89% and emission lifetimes up to 96 μs. A photo-stable complex containing C-deprotonated fluorenyl-substituted C^N shows no significant decomposition even upon irradiation for over 120 h by blue LEDs (12 W). These, together with the strong absorption in the visible region and rich photo-redox properties, allow the bis-NHC Ir(III) complexes to act as good photo-catalysts for reductive C–C bond formation from C(sp³/sp²)–Br bonds cleavage using visible-light irradiation (λ > 440 nm). A water-soluble complex with a glucose-functionalized bis-NHC ligand catalysed a visible-light-driven radical cyclization for the synthesis of pyrrolidine in aqueous media. Also, the bis-NHC Ir(III) complex in combination with a cobalt catalyst can catalyse the visible-light-driven CO₂ reduction with excellent turnover numbers (>2400) and selectivity (CO over H₂ in gas phase: >95%). Additionally, this series of bis-NHC Ir(III) complexes are found to localize in and stain endoplasmic reticulum (ER) of various cell lines with high selectivity, and exhibit high cytotoxicity towards cancer cells, revealing their potential uses as bioimaging and/or anti-cancer agents.