Conformation Changes and Luminescent Properties of Au-Ln (Ln = Nd, Eu, Er, Yb) Arrays with 5-Ethynyl-2,2′-Bipyridine

Reaction of polymeric gold(I) acetylide species (bpyC CAu)(n) (bpyC CH = 5-ethynyl-2,2'-bipyridine) with diphosphine ligands Ph2P(CH2)(n)PPh2 (n = 2-6) or 1,1'-bis(diphenylphosphino)-ferrocene (dppf) in dichloromethane induces isolation of binuclear gold(I) complexes (bpyC CAu)(2){mu-Ph2P(CH2)(n)PPh2} or (bpyC CAu)(2)(mu-dppf). Complexation of Ln(hfac)(3) (hfac = hexafluoroaceyacetonate, Ln = Nd, Eu, Er, Yb) subunits to the binuclear gold(l) complexes through 2,2'-bipyridyl chelation gives the corresponding Au(4)Ln(4) or Au(2)Ln(2) heteropolynuclear complexes. Noticeably, upon formation of the Au(4)Ln(4) arrays by complexation of (bpyC CAu)(2)(mu-Ph2P(CH2)(4)PPh2) (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 Au(2)Ln(2) Subunits. In contrast, cis-conformation in (bpyC CAu)(2)(mu-dppf) (6) stabilized by Au-Au interactions is reversed to the trans-oriented form upon formation of Au(2)Ln(2) arrays by introducing Ln(hfaC)3 units through 2,2-bipyridyl chelation. The binuclear gold(l) complexes show bright blue luminescence featured by ligand-centered pi -> pi* (C Cbpy) states together with low-energy emission at 500-540 nm, associated with (3)(pi ->pi*) excited states, mixed probably with some characteristic from (Au-Au) -> (C Cbpy) (MMLCT)-M-3 transition. For Au(4)Ln(4) or Au(2)Ln(2) 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.