Coordination Environment-Controlled Photoinduced Electron Transfer Quenching in Luminescent Europium Complexes

The quenching of sensitized Eu­(III) luminescence by photoinduced
electron transfer from the excited light-harvesting antenna to Eu­(III)
was investigated. A series of complexes incorporating different metal
binding sites and thus having varying Eu­(III)/Eu­(II) reduction potentials
were prepared. The complexes were fully characterized using a combination
of single-crystal X-ray crystallography and paramagnetic 1H NMR spectroscopy, the results of which support the structural similarity
of the complexes. The redox and photophysical behavior of the Eu­(III)
center and the light-harvesting antenna were studied using cyclic
voltammetry and steady-state and time-resolved emission spectroscopy
on the nanosecond and millisecond time scales. The contribution of
photoinduced electron transfer to the overall reduction of the Eu­(III)
luminescence quantum yield was found to be comparable and, in many
cases, larger than the quenching caused by well-established processes
such as coupling to X–H oscillators. These results suggest
that the elimination or mitigation of photoinduced electron transfer
could substantially improve the emissive properties of the widely
used Eu­(III)-based emitters.