[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

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(NO(3))(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-infraredluminescence 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(II), but with faster energy-transfer to Yb(III) (2.6 x 10(7) s(-1)) than to Nd(III) (1.4 x 10(7) s(-1)). Thus d -> 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 Forster 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 center dot Yb in which the [Ru(bipy)(3)](2+) unit is a poorer excited- state electron-donor by about 0.1 eV.