The role of photoinduced electron transfer in the quenching of sensitized Europium emission

Lanthanide (Ln) complexes equipped with a light-harvesting antenna can undergo numerous processes upon light excitation of the chromophore. Some of these, e.g., energy transfer to the Ln are beneficial for Ln luminescence, while others, e.g., interactions with O-H oscillators, are detrimental. Eu(III) is the most reducible of the Ln(III) ions, with a reduction potential of E(Ln(II)/Ln(III)) = -0.35 V vs NHE for the hydrated ions. This means that the Eu(II) redox state is relatively easily accessible by reduction with a range of excited state aromatics, such as are present in luminescent Ln complexes carrying a light harvesting antenna. Electron transfer almost always results in diminished Eu emission quantum yields, and is therefore undesirable; however, it is usually not taken into consideration during complex design. Here, we evaluate the role of Eu complex design for electron transfer. We also discuss strategies for diminishing this quenching pathway, including lowering the reducing power of the antenna excited state, and increasing the stability of the +3 oxidation state of Eu. Finally, we review applications of Eu(II) and photoexcited Eu(II). (C) 2018 Elsevier B.V. All rights reserved.