Towards responsive lanthanide probes for detection of reactive oxygen species

Abstract

The lanthanide emissive state can be quenched by electron/charge transfer from electron rich donors, resulting in a reduction of both emissive intensity and lifetime. The quenching of luminescence by electron rich donors was further studied in an attempt to derive a feasible mechanism for the quenching process. Additional studies were performed on urate, ascorbate and series of catechols, involving ionic strength variation and extended range Stem-Volmer plots. As a result of these and further studies, a model was proposed for the quenching process; based upon the formation of a long-lived 'exciplex'.In the proposal to synthesise complexes incorporating an intramolecular quenching moiety, two complexes have been synthesised. These complexes incorporate a tetraazatriphenylene chromophore and a methylene-protected catechol positioned trans in the cyclen ring system. Lifetime studies on these complexes show a reduction of 35% for the Eu system (from 1.08 ms to 0.69 ms), and 70% for the Tb system (1.46 ms to 0.47 ms) with respect to analogous complexes. The difference in susceptibility of europium (III) and terbium (III) to quenching affords a promising basis for the development of a ratiometric probe. A two-electron photoreduction was also unexpectedly observed for the Tb complex, where the bipyridyl ring imdergoes irreversible reduction, confirmed by solvent deuteration and MSMS studies.