Ortho - closo or ortho-nido -carborane-diphosphanes have been selected to prepare the heteroleptic cationic or neutral [Cu(N^N){(PPh 2 ) 2 C 2 B 10 H 10 }]PF 6 ( 1 ) and [Cu(N^N){(PPh 2 ) 2 C 2 B 9 H 10 }] ( 2 ) [N^N = 2-(4-thiazolyl)benzimidazole], respectively. Complexes 1 and 2 display very different emissive behavior. Neutral complex 2 exhibits TADF (time activated delayed fluorescence) which has been studied both as powder and PMMA composite with similar Δ E (S 1 – T 1 ), τ(T 1 ), and τ(S 1 ) in both phases. Cationic complex 1 displays a much lower quantum yield than 2 and does not show TADF, but it exhibits a significant thermochromic luminescence, and its emission is very dependent on the medium. Theoretical studies show that metal–ligand (M–diphosphane) to ligand (L′, diimine) transitions, MLL′CT, are responsible of the transitions which originate the emissive properties, but with very different contribution of the copper center, carborane cluster, and diphosphane phenyl rings for 1 and 2 .

Trigonal planar coordination at gold allows phosphorescent 3 MC transitions, not expected for luminescent mononuclear linear gold complexes. The careful selection and nature of the ancillary ligands bonded to gold, being fluorophore groups or not emissive ligands may cause a distortion of ideal trigonal planar coordination favoring different origins for the emissions. Consequently, intraligand (IL) or charge transfer (CT) transitions may be present and besides the expected phosphorescent MC transitions, fluorescence or thermally activated delayed fluorescence (TADF) may be the origin of the luminescence. In addition of the advantages of triplet population and deactivation via phosphorescent or delayed fluorescence processes, these systems are very versatile and may show interesting properties such as mechanochromism and vapochromism and, consequently, have revealed as promising candidates for optical applications. The objective of this review is to provide an overview of the reported luminescent three-coordinated gold systems and to get insight into their emissive properties.

The Front Cover shows different ligands with phosphorus, nitrogen, carbon, or other donor atoms, used as building blocks for the construction of three‐coordinated gold(I) complexes. These ligands may or may not be emissive, but their selection, together with different distortions from the ideal geometry around the gold atom, leads to luminescent three‐coordinated complexes with different emission energies. Triplet harvesting, reported for some of these complexes, is desirable for building devices used, for instance, in mobile phones. In addition, changes in molecular disposition in the solid phase upon exposure to different solvents or VOCs allow tuning the emissive properties. The authors thank the GEQO group of the Spanish Royal Chemical Society (RSEQ) for their kind invitation to contribute to the RSEQ‐GEQO Prize Winners Special Collection. Credit (cover background, images of the mobile phone and TV screen): Pixabay. More information can be found in the Minireview by A. Alconchel, O. Crespo, and M. C. Gimeno.

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Cationic [Cu­(P^P)­(Htbz)]­PF6 [P^P = xantphos, dpephos; Htbz = 2-(4-thiazolyl)­benzimidazole] and the corresponding neutral complexes [Cu­(P^P)­(tbz)], obtained through deprotonation of the diimine ligand, have been synthesized with the aim of analyzing the role of the diphosphane and Htbz deprotonation in the emissive properties of these complexes. For the study of the diphosphane effect, the luminescence properties of these compounds have been compared with those of the reported analogous derivatives with Htbz and carborane diphosphanes. Complexes [Cu­(P^P)­(Htbz)]­PF6 (P^P = xantphos, dpephos) and [Cu­(dpephos)­(tbz)] display thermally activated delayed fluorescence, which has been studied, revealing a ΔE(S1–T1) between 658 and 455 cm–1. Theoretical calculations indicate different origins for the absorptions, leading to the observed emissions.