This review covers the advances made in the synthesis of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands. The presence of a high field strength ligand such as an NHC in the complexes gives rise to high energy emissions, and consequently, to the desired blue colour needed for OLED applications. Furthermore, the great versatility of NHC ligands for structural modifications, together with the use of other ancillary ligands in the complex, provides numerous possibilities for the synthesis of phosphorescent materials, with emission colours over the entire visible spectra and potential future applications in fields such as photochemical water-splitting, chemosensors, dye-sensitised solar cells, oxygen sensors, and medicine.

This tutorial review covers the recent advances made in the study of gold complexes containing N-heterocyclic carbene ligands with biological properties.

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The optical properties of [Au 2 Ag 2 (C 6 F 5 ) 4 (OCMe 2 ) 2 ] n (1) have been studied in the solid state at room temperature and at 77 K and in acetone solution (5 × 10 -4 M). The crystal structure of 1, analyzed by X-ray diffraction, consists of polymeric chains formed by repetition of Au 2 Ag 2 moieties linked through short gold-gold interactions. The emission profile observed for 1 in dilute acetone solution (5 × 10 -4 M) is assignable to pentafluorophenyl localized ππ* excited states or from π-MMCT transitions, and in the solid-state arises from metal-centered (dσ*) 1 (pσ) 1 or (dδ*) 1 (pσ) 1 excited states. When the absorption and emission spectra of compound 1 in acetone are registered at different concentrations, they display a band that does not obey the Lambert-Beer law. This deviation is consistent with molecular aggregation in solution through gold-gold interactions, and a clear correlation between the emission wavelength and the structure of 1 in the solid state and in solution is shown. DFT calculations accord with the observed experimental behavior and show the nature of the orbitals involved in each transition.

A new series of isostructural, brilliantly luminescent gold-silver complexes having the formula [Au3(mu3-E)Ag(PPh2py)3](BF4)2 where E = O, S, Se and Ph2Ppy = 2-diphenylphosphinopyridine has been synthesized and characterized. The structural core of these complexes is a Au3Ag metallophilically linked tetrahedron with a group-16 atom functioning as a mu3-ligand capping the three gold atoms. In the solid state, pairs of clusters are joined by two unsupported aurophilic interactions. The emission energy changes strikingly in going from O (blue) to S (yellow) and Se (orange). The luminescence from the E = O system is the first to be reported for a gold(I) oxo system. Additionally, the luminescent 4-methylpyridyl analogue with E = S has been prepared and structurally characterized. For E = S, Se, the change in emission energy with mu3-bridging atom provides a sound basis for an LMMCT assignment of the excited state while lifetime measurements support its spin-forbidden nature. Frozen glass measurements indicate the presence of a higher-energy emitting state for these systems, and for the E = O system, either LMMCT or metal-centered cluster-based emission can be proposed.

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Chalcogenide-centred gold complexes are an important class of compounds in which a central chalcogen is surrounded by several gold atoms or gold and other metals. They have special characteristics such as unusual geometries, electron deficiency and properties such as luminescence or non-linear optical properties. The best known species are the trinuclear [E(AuPR3)3]+, 'oxonium' type species, that have high synthetic applicability, not only in other chalcogen-centred species, but in many other organometallic derivatives. The aurophilic interactions play an important role in the stability, preference for a particular geometry and luminescence properties in this type of derivatives (critical review, 117 references).

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