Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

Abstract

Bright lanthanide‐based probes rely on sensitised lanthanide luminescence. The outcome of the excited state energy transfer cascade – from sensitiser chromophore to lanthanide(III) ion – should be the population of the lanthanide excited state with an efficiency equal to unity. This is rarely the case, and this is a particular problem for europium(III) due to the low energy of the Eu(II)/Eu(III) redox couple. Thus, photoinduced electron transfer quenching can be an efficient drain for the excited state energy, reducing the brightness of a probe. Here, the effect of intramolecular PeT in a lanthanide‐based probe is investigated by contrasting intramolecular processes in coumarin‐appended 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetate (DO3A) complexes with intermolecular processes induced through collisional quenching. We find that the excited energy transfer processes involving terbium(III) and europium(III) are slow in the complex when compared to collisional quenching. Further, we show that the quenching event may lead to lanthanide luminescence and suggest that the fate of a PeT quenching event may not be non‐radiative loss of the excitation energy.