Solid-state luminescence of Au-Cu-alkynyl complexes induced by metallophilicity-driven aggregation

A new series of homoleptic alkynyl complexes, [{Au2Cu2(C2R)4}n] (R=C3H7O (1), C6H11O (2), C9H19O (3), C13H11O (4)), were obtained from Au(SC4H8)Cl, Cu(NCMe)4PF6, and the corresponding alkyne in the presence of a base (NEt3). Complexes 1-4 aggregate upon crystallization into polymeric chains through extensive metallophilic interactions. The cluster that contains fluorenolyl functionalities, C13H9O (5), crystallizes in its molecular form as a disolvate, [Au2Cu2(C2C13H9O)4]·2THF. The substitution of weakly bound THF molecules with pyridine molecules leads to the complex [Au2Cu2(C2C13H9O)4]·2py (6), thus giving two polymorphs in the solid state. Such structural diversity is established through metal-chain and hydrogen-bond formation, which depends on the stereochemical characteristics of the organic ligands. More interestingly, this solid-state structural arrangement affords good emission properties, such as intensity and spectroscopic profile, which are otherwise very weakly emissive in solution. Metallophilic aggregation of the {Au2Cu2} cluster units, as observed in the crystals, results in dramatic enhancement of the room-temperature phosphorescence, thereby reaching a maximum quantum efficiency of 95% (4). A theoretical approach further indicates a synergistic effect of the array of the metal chain upon aggregation, which greatly enhances the spin-orbit coupling and, hence, the phosphorescence, thereby opening up a new direction in the field of aggregate-enhanced emission.