In addition to the characteristics that make gold a unique element, such as the lowest electrochemical potential of all metals, its high electronegativity, or the fact that it may have a mononegative oxidation state, perhaps the most intriguing characteristic is the tendency of many gold complexes to aggregate into oligomers or supramolecular assemblies through metal-metal interactions. This fact, far from being a chemical curiosity, has become an object of study because of the associated properties. One of the most studied properties and that which involves many groups all over the world is the luminescence displayed by many gold-containing complexes. In fact, after the discovery of the luminescence of the three-coordinate complex [AuCl(PPh 3 ) 2 ] by Dori and coworkers in 1970 [1], the number of groups that have studied this topic has increased rapidly in recent years, together with the number of synthesized luminescent gold molecules. At the same time, the large number of studies performed in this area has provided knowledge of the conditions that gold complexes require to display luminescence. For instance, luminescence can originate from ligands, assigned to certain geometries around the gold atom or from the presence of metal-metal interactions in the complexes. More specifically, it can be produced by transitions between orbitals of the metal center exclusively, in orbitals of the ligands, usually among p orbitals, or in transitions involving both metal and ligands, where these can act as donors or acceptors of electronic density (charge transfer transitions). In gold (I), with a d 10 closed shell configuration, the ground state is 1 S 0 , while the excited states are 3 D 2 , 3 D 1 , 3 D 0 and 1 D 0 (see Figure 6.1) [2].Of all these, the only permitted electronic transition is 1 D 0 1 S 0 , while the rest are forbidden according to the spin rule, leading to phosphorescent emissions. However, these are precisely the ones responsible for the emissions found in many luminescent gold complexes, appearing in a range between 500 and 700 nm (665 nm in the gaseous ion).
j347Another very important factor that should be considered is the expected large spin-orbit effect promoted by a heavy atom such as gold. This effect produces a relaxation of the spin rule and makes orbital forbiddenness instead of the spin rule the key factor for determining whether a gold complex will show luminescence. For instance, phosphorescence is generally not observed for linear molecules (where the ligands are not involved in the transitions), since the mixing of s, p z and d z 2 of gold form a HOMO of S þ symmetry, while the LUMO orbital is formed mainly from p x and p y leading to a P symmetry, consequently, there is no orbital forbiddenness in the transition and phosphorescence is not observed [3]. Another possible coordination environment at gold is the tetrahedral one. In that case, the HOMO and the LUMO are 1 T 2 in symmetry and the transition is allowed according to Laportes rule; therefore, phosphorescence is not observed in thes...