Three-component coordination networks based on [Ru(phen)(CN)4]2− anions, near-infrared luminescent lanthanide(iii) cations, and ancillary oligopyridine ligands: structures and photophysical properties

Abstract

A series of cyanide-bridged coordination networks has been prepared which contain [Ru(phen)(CN)₄]²⁻ anions, Ln(III) cations, and additional oligopyridine ligands (1,10-phenanthroline, 2,2′:6′,2‴-terpyridine or 2,2′-bipyrimidine) which coordinate to the Ln(III) centres. Five structural types have been identified and examples of each type of structure are described: these are hexanuclear Ru₄Ln₂ clusters; two-dimensional Ru–Ln sheets with a honeycomb pattern of edge-linked Ru₆Ln₆ hexagons; one-dimensional chains consisting of two parallel cross-linked strands in a ladder-like arrangement; simple single-stranded chains of alternating Ru/Ln components; and a one-dimensional ‘chain of squares’ in which Ru₂Ln₂ squares are linked by bipyrimidine bridging ligands which connect to the Ln(III) corners of adjacent squares in the sequence. The ³MLCT luminescence characteristic of the [Ru(phen)(CN)₄]²⁻ units is quenched in those networks containing Ln(III) which have low-lying near-infrared luminescent f–f states [Pr(III), Nd(III), Er(III), Yb(III)], with sensitised Ln(III)-based near-IR luminescence generated by d → f energy-transfer. The rate of d → f energy-transfer, and hence the degree of quenching of the ³MLCT luminescence from the [Ru(phen)(CN)₄]²⁻ units, depends on the availability of f–f levels of an appropriate energy on the Ln(III) centre, with Nd(III) (with a high density of low-lying f–f states) being the most effective energy-acceptor and Yb(III) (with a single low-lying f–f state) being the least effective. Rates of d → f energy-transfer to different Ln(III) centres could be determined from both the residual (partially quenched) lifetimes of the ³MLCT luminescence, and—in the case of the Yb(III) networks—by a rise-time for the sensitised near-IR luminescence. The presence of the ‘blocking’ polypyridyl ligands, which reduced the number of cyanide and water ligands that would otherwise coordinate to the Ln(III) centres, resulted in increases in the Ln(III)-based emission lifetimes compared to networks where these blocking ligands were not used.