On the mechanism of d-f energy-transfer in RuII/LnIII and OsII/LnIII dyads: Dexter-type energy-transfer over a distance of 20 Å

We have used time-resolved luminescence methods to study rates of photoinduced energy transfer (PEnT) from [M(biPY)(3)](2+) (M=Ru, Os) chromophores to Ln(III) ions with low-energy f-f states (Ln=Yb, Nd, Er) in d-f dyads in which the metal fragments are separated by a saturated -CH2CH2- spacer, a P-C6H4 spacer, or a p-(C6H4)(2) spacer. ne finding that clef PEnT is much faster across a conjugated P-C6H4 spacer than it is across a shorter CH2CH2 spacer points unequivocally to Dexter-type energy transfer, involving electronic coupling mediated by the bridging ligand orbitals (superexchange) as the dominant mechanism. Comparison of the distance dependence of the Ru -> Nd energy-transfer rate across different conjugated spacers [P-C6H4 or P-(C6H4)(2) groups] is also consistent with this mechanism. Observation of Ru -> Nd PEnT (as demonstrated by partial quenching of the Ru-II-based (MLCT)-M-3 emission (MLCT= metal-to-ligand charge transfer), and the growth of sensitised Nd-III-based emission at 1050nm) over approximately 20 angstrom by an exchange mechanism is a departure from the normal situation with lanthanides, in which long-range energy transfer often involves through-space Coulombic mechanisms.