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

Abstract

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.