Reversible protonation of amine-functionalized luminescent Au–Cu clusters: characterization, photophysical and theoretical studies

Abstract

Reaction of the polymeric alkynyl complexes (AuC₂C₆H₄R)_n (R = 4-NH₂ and 3-NH₂) with the diphosphine PPh₂C₆H₄PPh₂ in the presence of Cu⁺ ions gave two novel heterometallic aggregates [{Au₃Cu₂(C₂C₆H₄R)₆}Au₃(PPh₂C₆H₄PPh₂)₃](PF₆)₂ (R = 4-NH₂ (2), 3-NH₂ (3)). The compounds obtained were characterized by NMR spectroscopy and ESI-MS measurements. The solid-state structure of their 4-NMe₂ congener 1 is reported. The complexes 1–3 reversibly react with strong (HSO₃Me and HSO₃CF₃) acids to give the adducts [{Au₃Cu₂(C₂C₆H₄-R)₆*(R′SO₃H)₆}Au₃(PPh₂C₆H₄PPh₂)₃](PF₆)₂ (R = 4-NMe₂ (4), 4-NH₂ (5), 3-NH₂ (6)) with six acid molecules bound to the amine groups of the alkynyl ligands. Composition and structure of the adducts were established using ESI-MS and multinuclear (³¹P, ¹H and ¹H-¹H COSY) NMR spectroscopy. It was found that formation of these adducts results in crucial changes of luminescence characteristics of the complexes 1–3 to give substantial (ca. 100 nm) blue shift of the emission maxima and a sharp increase (about an order of magnitude) in luminescence quantum yield for 4-NR₂ substituted derivatives. In the case of 3-substituted complex 3 the effect of adduct formation is much less pronounced and leads to blue-shift of emission maximum for 30 nm accompanied with a small drop in emission quantum yield. Computational studies have been performed to provide additional insight into the structural, electronic and photophysical properties of the starting complexes and their acid adducts. Interpretation of the photophysical effects induced by the adduct formation was suggested.