Colorimetric and Luminescent Sensors for Chloride: Hydrogen Bonding vs Deprotonation

Abstract

The synthesis and photophysical properties of four squaramide based fluorescent anion sensors (1À4) are described. These luminescent compounds showed selectivity for Cl À over various other anions with concomitant changes in both their UV/visible and fluorescence properties upon Cl À addition, attributed to initial H-bonding followed by NH deprotonation in the presence of excess Cl À , signaled by a color change. The nature of the electron withdrawing aryl substituents is directly related to the H-bonding ability/acidity of the squaramide protons and can be used to tune the deprotonation behavior.Colorimetric and luminescent sensors provide a visual method for detection of a wide range of chemical species. Substantial effort has recently been invested in the synthesis of such chemosensors for detection of anions in many diverse areas including biology, industry, and the environment. 1À3 In particular, the design of charge neutral sensors capable of binding anions in competitive media, where the spectroscopic properties arising from the sensors, e.g. a color change, are modulated by addition of the external analyte has become a popular method of sensing chemical species. 4 Anion recognition has been successfully achieved through the use of numerous neutral receptors including imidazoles,

Anion Receptor Chemistry

“…Elmes et al 4 reported anion binding and spectroscopic sensing properties of squaramide-based anion receptors 4-6 ( Figure 3), which showed the highly unusual behavior of deprotonation by the relatively weakly basic anion chloride in DMSO. Titration of 4-6 with chloride led to dramatic changes in the color of the solution and quenching of the excimer emission at 530 nm.…”

“…Calix [4]arene is an ideal scaffold for constructing multivalent anion receptors for high-affinity anion binding and sensing. 5 Gaeta et al 6 developed two anion receptors, 8 and 9 (Figure 3), in which two squaramide moieties were attached to the upper rim of the calix [4]arene scaffold.…”

“…5 Gaeta et al 6 developed two anion receptors, 8 and 9 (Figure 3), in which two squaramide moieties were attached to the upper rim of the calix [4]arene scaffold. Interestingly, spherical anions including chloride and bromide were found to induce dimerization of both 8 and 9, forming 2:1 calixarene-anion complexes as demonstrated by Job plots and electrospray ionization mass spectrometry (ESI-MS) data.…”

“…Chang et al 15 have reported hexapyrrolic calix [4]pyrrole isomers 37 and 38 (Figure 10), which interact with anions via different types of interactions, leading to different anion-binding affinities. Compounds 37 (trans-isomer) and 38 (cis-isomer) differ in the orientation of two pyrrole side arms with respect to the calixpyrrole macrocycle.…”

Hydrogen-Bonded Homoleptic Fluoride–Diarylurea Complexes: Structure, Reactivity, and Coordinating Power

“…27b, 28 In a direct comparison, transmembrane transport is more efficient for chloride or bicarbonate anions with squaramides than with either analogous ureas or thioureas. 29 The structures of three fluoride ion complexes with squaramides 5a-c (6a-c) were determined by single-crystal X-ray diffraction.…”