ENERGY MIGRATION IN RARE EARTH CHELATES: ROLE OF THE TRIPLET STATE.

Abstract

The mechanism of electronic energy transfer in rare earth chelates involves two steps, following absorption of light by the ligand; 1) transfer of energy from ligand to an upper level of the rare earth ion, and 2) subsequent conversion of energy from the upper ion level to the emitting level of the ion. Two mechanisms have been invoked to explain transfer from ligand to ion. One considers transfer to occur from the singlet state of the ligand to a high-lying level of the ion. The other interprets this transfer as occurring from the lowest triplet state of the ligand to the rare earth ion. Recent evidence is reviewed that supports the latter view. Studies in this laboratory have shown that the fluorescence yield of europic ion decreases as progressively higher ion levels are excited. Thus energy is not transferred from upper ion levels to the emitting level with unit efficiency. This work has now been extended to terbium compounds. The results show that here too the efficiency of energy transfer between ion levels is less than unity. The results indicate that, as well as transferring energy to the ion, the triplet state may also be involved in energy dissipation. To support this, evidence is presented that the triplet state may effectively quench fluorescence of the ion if it closely matches an ion level, especially the emitting level.