Highly Luminescent Octanuclear AuI–CuI Clusters Adopting Two Structural Motifs: The Effect of Aliphatic Alkynyl Ligands

Reactions of the homoleptic (AuC2R)n precursors with stoichiometric amount of diphosphine ligand PPh2C6H4PPh2 (P^P) and Cu+ ions lead to an assembly of a new family of bimetallic clusters [Au6Cu2(C2R)6(P^P)2]2+ (type I; R=9-fluorenolyl (1), diphenylmethanolyl (2), 2,6-dimethyl-4-heptanolyl (3), 1-cyclohexanolyl (4), Cy (5), tBu (6)). In the case of R=1-cyclohexanolyl, a structurally different complex [Au6Cu2(C2C6H11O)6(P^P)3]2+ (7, type II) could be obtained by treatment of 4 with one equivalent of the diphosphine, while for R=isopropanolyl only the latter type of cluster [Au6Cu2(C2C3H7O)6(P^P)3]2+ (8) was detected. Steric bulkiness of the alkynyl ligands and O⋅⋅⋅HO hydrogen bonding are suggested to play an important role in stabilizing the type I and type II cluster structural motif, respectively. All the complexes exhibit intense photoluminescence in solution with emission parameters that depending on the geometrical arrangement of the octanuclear metal core. The clusters 1–4 and 6 show single emission band in a blue region (469–488 nm) with maximum quantum yield of 94 % (4), while structurally different 7 and 8 emit yellow-orange (590 nm) with unity quantum efficiency. The theoretical DFT calculations of the electronic structures have been carried out to demonstrate that the metal-centered triplet emission within the heterometallic core plays a key role for the observed phosphorescence.