4f energies in an organic-rare earth guest-host system: the rare earth tris-8-hydroxyquinolines

The systematics of the energy transfer from an organic ligand to the rare earth ion 4f states, the so-called antenna effect, has been investigated for a series of rare-earth tris-8-hydroxyquinolines. Of the potential luminescent 4f–4f transitions in these materials only the infra-red emitting peaks of Er and Yb are observed. We show that energy transfer from the organic host to the lanthanide ion can proceed either via direct transfer from the triplet excited state, or from the singlet excited state via an internal chemiluminescent redox reaction. Emission from the visible transitions of the rare-earth ions is shown to be energetically unfavourable, as is the capture of individual holes during electrical injection.