[Ru(bipy)3]2+ and [Os(bipy)3]2+chromophores as sensitisers for near-infrared luminescence from Yb(iii) and Nd(iii) in d/f dyads: contributions from Förster, Dexter, and redox-based energy-transfer mechanisms

Abstract

The complexes RuL and OsL contain [M(bipy)₃]²⁺ chromophores with a pendant aza-18-crown-6 macrocycle for binding of lanthanide(III) ions. The photophysical properties of the adducts RuL·Ln and OsL·Ln, prepared by addition of excess Ln(NO₃)₃ (Ln = Nd, Yb) to solutions of RuL and OsL in MeCN, were examined using time-resolved and steady-state luminescence methods. Whereas RuL does not act as an energy-donor to Yb(III), it will transfer energy to (and generate sensitised near-infrared luminescence from) Nd(III) with a Ru(II)→Nd(III) energy-transfer rate constant of 6.8 × 10⁶ s⁻¹. In contrast, OsL is quenched by both Yb(III) and Nd(III), but with faster energy-transfer to Yb(III) (2.6 × 10⁷ s⁻¹) than to Nd(III) (1.4 × 10⁷ s⁻¹). Thus d → f energy transfer is in both cases faster for Os(II) than for Ru(II), but the relative ability of Nd(III) and Yb(III) to act as energy-acceptors is inverted from RuL·Ln to OsL·Ln. Reasons for this are discussed with reference to contributions from the Förster and Dexter mechanism for energy-transfer in RuL·Nd and OsL·Nd, using calculated spectroscopic overlap integrals coupled with molecular modelling to estimate inter-chromophore separations. The particular effectiveness of Os(II) → Yb(III) energy-transfer in OsL·Yb is explained in terms of the Horrocks redox mechanism involving an initial *Os(II) → Yb(III) photoinduced electron transfer step generating an Os(III)/Yb(II) state, which is shown to be marginally favourable for OsL·Yb, but not for RuL·Yb in which the [Ru(bipy)₃]²⁺ unit is a poorer excited-state electron-donor by about 0.1 eV.