pH Dependence of the energy transfer mechanism in a phenanthridine-appended ytterbium complex

Intramolecular energy transfer from a non-protonated phenanthridine chromophore to ytterbium occurs rapidly despite negligible overlap of the triplet state of phenanthridine with the absorption band of the metal centre. This can be explained by a sequential electron transfer and back electron transfer, or redox, mechanism. When the phenanthridine is protonated, this energy transfer pathway ceases to be thermodynamically feasible. Under these circumstances, energy transfer to the metal becomes rate determining and is mediated by the phenanthridinium triplet state, despite its near-zero spectral overlap with the metal-centred absorption band.