Conformation changes and luminescent properties of Au-Ln (Ln = Nd, Eu, Er, Yb) arrays with 5-ethynyl-2,2′-bipyridine

Article abstract of DOI:10.1021/ic800733x

Reaction of polymeric gold(I) acetylide species (bpyC≡CAu) _n (bpyC≡CH = 5-ethynyl-2,2′-bipyridine) with diphosphine ligands Ph₂P(CH₂)_nPPh₂ (n = 2-6) or 1,1′-bis(diphenylphosphino)-ferrocene (dppf) in dichloromethane induces isolation of binuclear gold(I) complexes (bpyC≡CAu)₂{μ- Ph₂P(CH₂)_nPPh₂} or (bpyC≡CAu)₂(μ-dppf). Complexation of Ln(hfac)₃ (hfac = hexafluoroacetylacetonate, Ln = Nd, Eu, Er, Yb) subunits to the binuclear gold(I) complexes through 2,2′-bipyridyl chelation gives the corresponding Au₄Ln₄ or Au₂Ln₂ heteropolynuclear complexes. Noticeably, upon formation of the Au ₄Ln₄ arrays by complexation of (bpyC≡CAu) ₂(μ-Ph₂P(CH₂)₄PPh₂) (3) with Ln(hfac)₃ units, trans-conformation in 3 transforms dramatically to the cis-arranged form due to the strong driving force from ligand-unsupported Au-Au contacts between two Au₂Ln₂ subunits. In contrast, cis-conformation in (bpyC≡CAu)₂(μ- dppf) (6) stabilized by Au-Au interactions is reversed to the trans-oriented form upon formation of Au₂Ln₂ arrays by introducing Ln(hfac)₃ units through 2,2-bipyridyl chelation. The binuclear gold(I) complexes show bright blue luminescence featured by ligand-centered π → π* (C≡Cbpy) states together with low-energy emission at 500-540 nm, associated with ³(π→π*) excited states, mixed probably with some characteristic from (Au-Au) → (C≡Cbpy) ³MMLCT transition. For Au₄Ln₄ or Au ₂Ln₂ complexes, sensitized lanthanide luminescence is achieved by energy transfer from Au-acetylide chromophores with lifetimes in the sub-millisecond range for Eu^III complexes, whereas in the microsecond range for near-infrared emitting Nd^III, Er^III, and Yb^III species.