On the origin of chloride-induced emission enhancement in ortho substituted squaramides

Abstract There is significant current interest in the development of fluorescent sensors for the physiologically relevant chloride ion. In this study we report a new squaramide-based chloride sensor, derived from ethyl ortho aminobenzoate, whose emission intensity increases by nearly 40% upon chloride binding. The intramolecularly hydrogen bonded squaramide exhibited a blue-shifted excitation spectrum relative to its absorption spectrum, and its emission displayed excitation wavelength dependence. These unusual characteristics were attributed to the existence of two ground state conformers for the unbound squaramide; an intramolecular hydrogen bonded conformer which undergoes non-radiative decay and a non-hydrogen bonded non-planar conformer from which emission occurs. Using computational investigations we identified two nearly degenerate excited states arising from two different charge transfer pathways with the squaramide nitrogen atom as the donor and either the cyclobutene dione ring or the ortho substituted N-phenyl ring functioning as acceptors. In the unbound squaramide, this competition between the two charge transfer pathways decreases fluorescence intensity. Chloride binding, suppresses charge transfer into the cyclobutene dione ring thereby enhancing emission intensity. Our study contributes to an increased understanding of the excited states of squaramides and could facilitate their use in the development of new sensors.