Excited state proton transfer dye with an emission quantum yield up to 60% upon Zn2+ coordination

Abstract

Intramolecular excited state proton transfer (ESIPT) dyes are appealing for metal ion sensing because of their large apparent Stokes-shift, environmental sensitivity, and turn-on fluorescence capabilities. Here we introduce 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole (2) as a new ESIPT sensing motif. The 2,5-bis(2-pyridylimino)pyrrole portion of 2 readily coordinates Hg2+, Zn2+, Co2+, Cu2+, and Ni2+ ions and results in a visible red-shift in absorption. While coordination of Hg2+, Co2+, Ni2+, and Cu2+ completely quenches emission, coordination to Zn2+ increases the ESIPT emission quantum yield from 25% to 60%. Unlike traditional ESIPT sensors, the metal ion binding site of 2 is intrinsic to the chromophore but is not the ESIPT site, thus enabling the selective modification of each functionality. As proof of concept, we use bromobutyrate as an alcohol protecting group whose cleavage by hydrazine results in turn-on fluorescence response. Collectively these results demonstrate the 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole motif is sensitive to several stimuli (i.e. metal ion, base, solvent, and N2H4) and is a promising scaffolding for sensing applications.