Ligand-field excited states of hexacyanochromate and hexacyanocobaltate as sensitisers for near-infrared luminescence from Nd(III) and Yb(III) in cyanide-bridged d-f assemblies

Crystallisation of [Co(CN)6]3- or [Cr(CN)6]3- with Ln(III) salts (Ln = Nd, Gd, Yb) from aqueous dmf afforded the cyanide-bridged d/f systems [Ln(dmf)4(H2O)3(µ-CN)Co(CN)5] (Co-Ln, discrete dinuclear species) and {[Cr(CN)4(µ-CN)2Ln(H2O)2(dmf)4]} (Cr-Ln, infinite cyanide-bridged chains with alternating Cr and Ln centres). With Ln = Gd the characteristic long-lived phosphorescence from d-d excited states of the [M(CN)6]3- units was apparent in the red region of the spectrum, with lifetimes of the order of 1 µs, since the heavy atom effect of the Gd(III) promotes inter-system crossing at the [M(CN)6]3- units to generate the phosphorescent spin-forbidden excited states. With Ln = Yb or Nd however, the d-block luminescence was completely quenched due to fast (108 s-1) energy-transfer to the Ln(III) centre, resulting in the characteristic sensitised emission from Yb(III) and Nd(III) in the near-IR region. For both Co-Nd and Co-Yb, calculations based on spectroscopic overlap between emission of the donor (Co) and absorption of the acceptor (Ln) suggest that the Dexter energy-transfer mechanism is responsible for the complete quenching that we observe.