A novel Eu(III)-based luminescent chemosensor: determining pH in a highly acidic environment

The design, synthesis and photophysical evaluation of the Eu(III)-based chemosensor [Eu·1], is described. The sensors show large (ca. 300 fold) pH dependent luminescence enhancements for the Eu(III) lanthanide emission, i.e. it is ‘switched on’ in highly acidic media, between pH ca. 1.5 and 3.5 with a pKa of 2.4 (±0.1) when excited at 350 nm. The fluorescence spectra of [Eu·1], consisting of the quinoline emission, is also pH dependent, where the fluorescence is ‘switched on’ at 458 nm, in the pH region of 3–6, but the emission is quenched between pH 1.5 and 3. Both pH-intensity profiles display an ‘off–on–off’ bell-shape behaviour similar to that seen for many enzymatic pH dependent processes.