2,2-Dipicolylamino substituted 2-(2′-hydroxybenzofuranyl) benzoxazole (HBBO) derivative: Towards ratiometric sensing of divalent zinc cations

Abstract

A dual-emissive fluorophore, HBBODipic, exhibiting an Excited-State Intramolecular Proton Transfer (ESIPT) process is described herein. This dye incorporates an aromatic amine group on the benzofuran side which is functionalized by two picolyl moieties, enabling the observation of a dual fluorescence emission in most solvents. This attractive dual emission is related to the modulation of the pKa* by electronic substitution and has already been applied for generating panchromatic white-emission. In this contribution, we extend the applicability of these HBBO-based dual emitters to the ratiometric detection of substrates, taking advantage of the highly environment-sensitive emission profile of the dye, the E*/K* intensity ratio being strongly tunable. The presence of a dipicolylamino Zn2+-sensitive chelate on the HBBO π-conjugated core enables a drastic change in the emission spectra upon addition of increasing equivalents of zinc perchlorate. The E*/K* dual emission slowly shifts to a single K* band, evidencing a decrease of electron density on the benzofuranol site, owing to the chelation of the Zn2+ cation at the dipicolylamino site. The results are rationalized using theoretical calculations.