Luminescent Complexes of Platinum, Iridium, and Coinage Metals Containing N-Heterocyclic Carbene Ligands: Design, Structural Diversity, and Photophysical Properties

Abstract

The employment of N-heterocyclic carbenes (NHCs) to design luminescent metal compounds has been the focus of recent intense investigations because of the strong σ-donor properties, which bring stability to the whole system and tend to push the d−d dark states so high in energy that they are rendered thermally inaccessible, thereby generating highly emissive complexes for useful applications such as organic light-emitting diodes (OLEDs), or featuring chiroptical properties, a field that is still in its infancy. Among the NHC complexes, those containing organic chromophores such as naphthalimide, pyrene, and carbazole exhibit rich emission behavior and thus have attracted extensive interest in the past five years, especially carbene coinage metal complexes with carbazolate ligands. In this review, the design strategies of NHC-based luminescent platinum and iridium complexes with large spin−orbit-coupling (SOC) are described first. Subsequent paragraphs illustrate the recent advances of luminescent coinage metal complexes with nucleophilic-and electrophilic-based carbenes based on silver, gold, and copper metal complexes that have the ability to display rich excited state emissions in particular via thermally activated delayed fluorescence (TADF). The luminescence mechanism and excited state dynamics are also described. We then summarize the advance of NHC−metal complexes in the aforementioned fields in recent years. Finally, we propose the development trend of this fast-growing field of luminescent NHC− metal complexes.

Engineering of Cyano Functionalized Benzo[d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes

“…To overcome these drawbacks, N-heterocyclic carbene (NHC) cyclometalates, with much stronger Ir–C dative bonding interaction, have been tested as chelates for potential blue phosphors. − However, due to the much higher lying LUMO of the NHC chelates, the parent complex m -Ir­(pmb) 3 exhibited an emission peak wavelength in the near ultraviolet region at 380 nm, which is too high in energy to give efficient blue emission. Numerous works were next conducted with an aim of reducing emission energy gap, which included enlargement of π-conjugation at the N -phenyl cyclometalate, such as employment of a dibenzo­[ b , d ]­furan-4-yl fragment and attachment of an electron-withdrawing group at the parent imidazolylidene or benzoimidazolylidene, in giving pyridoimidazolylidene , and pyrazinoimidazolylidene , entities (Scheme ).…”

Synthesis and Photophysical Properties of Monometallic C^C* Platinum(II) Formamidinate Complexes using Sterically Demanding Ligands

“…[18][19][20][21][22][23] Among diverse motifs C^C* cyclometalating ligands based on N-heterocyclic carbenes (NHCs) have been receiving large recognition in the literature as an alternative to the well-known C^N cyclometalating ligands in luminescent transition metal complexes. [24][25][26][27][28] Like the cyclometalating ligand, the ancillary ligand has a strong influence on the photoluminescent properties of the metal complex. Ligands forming six-membered metallacycles like β-diketonates- [29][30][31] or bis(pyrazolyl)-type [32][33][34][35][36][37] ligands are some of the most utilized motifs for luminescent platinum complexes together with a continuously growing class of tetradentate ligands.…”

Theoretical investigation of the influence of heterocycles on the radiative and non-radiative decay processes of iridium(iii) complexes

“…N-heterocyclic moieties, as the scaffold materials for incorporating chelating ligands into Ir(III), have sparked significant interest because the corresponding Ir(III) complexes exhibit excellent performances and chemical stabilities. [18][19][20][21] Recently, Lee et al chose phenylpyridine-ligand-based and phenylimidazole-based Ir(III) complexes as CT complex-forming materials. White OLEDs could be constructed by doping these Ir(III) complexes into the host material, which exhibited both blue emission and yellow emission with a quantum efficiency of 13.7%.…”