Tuning the Luminescent Properties of Pt(II) Acetylide Complexes through Varying the Electronic Properties of N-Heterocyclic Carbene Ligands

Abstract

This Article reports the synthesis, structural characterization, electrochemistry, and photophysical investigations of five groups of luminescent Pt(II) alkynyl complexes bearing N-heterocyclic carbene (NHC) ligands with varying electronic properties. Complexes of the type [Pt(pmdb)(C≡CR)2] 1a-c, [Pt(pm2tz)(C≡CR)2] 2a-d, [Pt(pm3tz)(C≡CR)2] 3a-c, [Pt(ppim)(C≡CR)2] 4(a, b, e), and [Pt(ppbim)(C≡CR)2] 5(a, b, e), where pmdb =1,1'-dipentyl-3,3'-methylene-dibenzimidazoline-2,2'-diylidene, pm2tz = 1,1'-dipentyl-3,3'-methylene-di-1,2,4-triazoline-5,5'-diylidene, pm3tz = 1,1'-dipentyl-3,3'-methylene-di-1,3,4-triazoline-5,5'-diylidene, ppim = 3-pentyl-1-picolylimidazoline-2-ylidene, and ppbim = 3-pentyl-1-picolylbenzimidazoline-2-ylidene, and R = 4-C6H4F, C6H5, 4-C6H4OMe, SiMe3, and 4-C6H4N(C6H5)2, were prepared, and the consequences of the electronic properties of the NHC ligands on the phosphorescent emission efficiencies were studied. Moreover, the emission quantum efficiencies of the previously reported complexes [Pt(pmim)(C≡CR)2] where pmim = 1,1'-dipentyl-3,3'-methylene-diimidazoline-2,2'-diylidene and R = 4-C6H4F 6a, C6H5 6b, and 4-C6H4OMe 6c were also recorded in neat solid and in 10 wt % PMMA film. The square planar coordination geometry with the alkynyl ligands in cis configuration was corroborated for selected complexes by single crystal X-ray diffraction studies. The observed moderate difference in emission efficiencies of the bis-carbene complexes 6a-c, 1a-c, 2a-c, and 3a-c in conjunction with the decreasing electron-donating nature of the NHC ligands, pmim > pmdb > pm2tz ≈ pm3tz, can be attributed to the slight modification of the triplet emission parentage among the different complexes. The quantum efficiencies of complexes 4(a, b) and 5(a, b) bearing pyridyl-NHC ligand were significantly low in comparison to the bis-carbene complexes owing to the significant change in the charge transfer character of the triplet manifold. Complexes 4e and 5e bearing diarylamine phenylacetylenes display high ϕem of 27% and 33% in 10 wt % PMMA film, respectively.

TD-DFT Benchmark on Inorganic Pt(II) and Ir(III) Complexes

“…NLO, luminescence). [36][37][38][39][40][41][42][43][44][45][46][47][48] These considerations prompted us to start a comprehensive investigation with the aim of getting further insights into the spectroscopic behaviour of such compounds and of proposing reliable computational protocols, which could be profitably used also for other transition metal compounds. We have compared different functionals and basis sets trying to define the methods of choice by comparison to experimental results reported in previous investigations.…”

Pt(II) Phosphors Featuring Both Dicarbene and Functional Biazolate Chelates: Synthesis, Luminescent Properties, and Applications in Organic Light-Emitting Diodes

“…Finally, all of these carbene complexes 1−6 showed bright emission ranging from blue to violet in the solid state, which is in good agreement with the higher emission energy observed in many Pt(II) carbene complexes reported in the literature. [36][37][38][39][40]82,83 Theoretical Calculation. (Time-dependent) density functional theory (DFT/TD-DFT) calculations were carried out to study the detailed electronic and photophysical properties of Pt(II) complexes 1a, 5a, and 1−6.…”

Isolation and Crystallographic Identification of Photoactive PtII Tris‐ and Bis(N‐methylbenzimidazole‐NHC) Complexes

“…The molar absorption coefficients ( ε ) for the intense absorption bands of complexes 1 – 3 are in the range of 1.5–6.0 × 10 4 m –1 cm –1 . Based on spectroscopic studies on the Pt II complexes with hybrid NHC ligands[2a], [3g], [3j], [4c] and those containing bis(NHC) ligands[2b], [9b], [10h] as well as the tetra‐NHC Pt II complexes, the absorption bands at ca. 280 nm and the weak shoulder bands at ca.…”

“…Platinum(II) N‐heterocyclic carbene (NHC) complexes have been a subject of active investigation as stable and efficient blue‐emitting materials in view of the strong σ‐donating property of NHC ligands which, apart from giving a strong Pt–C bond, results in large HOMO–LUMO energy gaps for high‐energy blue emission, and high‐lying non‐radiative d‐d excited states for favourable quantum efficiency . Most of these luminescent Pt II complexes contain chelating hybrid NHC ligands, such as NHC based C ^ C* cyclometallated ligands,[2a], C ^ N pyridyl or pyrozolate functionalized NHC ligands, C ^ N ^ C pincer type pyridyl‐NHCs, C ^ C* ^ C pincer type cyclometalled‐NHCs, O ^ C ^ O type tridentate bis‐aryloxide‐NHC ligands and O ^ C ^ C ^ O type tetradentate bis[phenolate‐(NHC)] ligands. [2b], Monodentate NHCs and bidentate bis(NHC) ligands have also been used as ancillary ligands in luminescent Pt II bis(acetylide)[4a], and cyclometallated complexes or as a linker between metal and an organic chromophore to form mono or di‐NHC Pt II complexes.…”

“…Most of these luminescent Pt II complexes contain chelating hybrid NHC ligands, such as NHC based C ^ C* cyclometallated ligands,[2a], C ^ N pyridyl or pyrozolate functionalized NHC ligands, C ^ N ^ C pincer type pyridyl‐NHCs, C ^ C* ^ C pincer type cyclometalled‐NHCs, O ^ C ^ O type tridentate bis‐aryloxide‐NHC ligands and O ^ C ^ C ^ O type tetradentate bis[phenolate‐(NHC)] ligands. [2b], Monodentate NHCs and bidentate bis(NHC) ligands have also been used as ancillary ligands in luminescent Pt II bis(acetylide)[4a], and cyclometallated complexes or as a linker between metal and an organic chromophore to form mono or di‐NHC Pt II complexes. Homoleptic and heteroleptic tetra‐NHC Pt II complexes also emit in the near UV or blue region .…”