A long wavelength colourimetric chemosensor for fluoride

Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour

The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO−, SO42− HPO42−, and F−. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl−; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2.

Macrocyclic squaramides as ion pair receptors and fluorescent sensors selective towards sulfates

Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, an anion receptor R3, and a fluorescent sensor R4 using a combination of particular members of simple libraries consisting of synthesized diamines and methyl squarates, respectively. The receptors were investigated in terms of anion and ion pair binding using the ¹H NMR titration method in DMSO-d₆. We found that the major contribution to the anion binding comes from the interaction with the squaramide protons rather than with the amide functions of the receptors. The receptors demonstrated the highest affinity towards benzoates and sulfates over the anions tested, and in the case of sulfate binding more complex equilibria in solution were observed. Unlike the anion receptor R3, the ion pair receptor R1 was found to recognize anions in an enhanced manner with the assistance of sodium or potassium cations. Tethering of a simple fluorophore in close proximity to the amide function of receptor R4 resulted in an optical ion pair sensor selective towards sulfates. DFT calculations carried out for the 1 : 1 complexes of R3 with the anions helped clarify this selectivity, showing more effective participation of tetrahedral sulfate anions in binding with the amide function than in the case of benzoates or chlorides.

Fused Nickel(II) Porphyrins-Sensing of Toxic Anions and Selected Metal Ions Through Supramolecular Interactions

Ni(II) porphyrins having fused -NH group were synthesized and characterized by various spectroscopic techniques. The fused porphyrins 1 and 2 were used to detect species of opposite polarity. 1 was used to sense toxic anions viz. cyanide and fluoride ions whereas 2 was used for detecting some selective metal ions including toxic mercury(II) ions. 1 is having acidic -NH proton, which detects anions via hydrogen bonding interactions followed by anion-induced deprotonation. On the other hand, 2 senses the metal species via weak charge transfer interactions from oxygen atom of the formyl group to the added metal ions. The limit of detection was calculated in case of 1 as 2.13 and 3.15 ppm for cyanide and fluoride ions, respectively. Similarly, the detection limit was found to be 0.930, 2.231, and 0.718 ppm for Cu(II), Fe(III), and Hg(II) ions, respectively, for probe 2. The probes were recovered and reused for several cycles.

Squaramide-Naphthalimide Conjugates as "Turn-On" Fluorescent Sensors for Bromide Through an Aggregation-Disaggregation Approach

The syntheses of two new squaramide-naphthalimide conjugates (SQ1 and SQ2) are reported where both compounds have been shown to act as selective fluorescence "turn on" probes for bromide in aqueous DMSO solution through a disaggregation induced response. SQ1 and SQ2 displayed a large degree of self-aggregation in aqueous solution that is disrupted at increased temperature as studied by ¹H NMR and Scanning Electron Microscopy (SEM). Moreover, the fluorescence behavior of both receptors was shown to be highly dependent upon the aggregation state and increasing temperature gave rise to a significant increase in fluorescence intensity. Moreover, this disaggregation induced emission (DIE) response was exploited for the selective recognition of certain halides, where the receptors gave rise to distinct responses related to the interaction of the various halide anions with the receptors. Addition of F^- rendered both compounds non-emissive; thought to be due to a deprotonation event while, surprisingly, Br^- resulted in a dramatic 500-600% fluorescence enhancement thought to be due to a disruption of compound aggregation and allowing the monomeric receptors to dominate in solution. Furthermore, optical sensing parameters such as limits of detection and binding constant of probes were also measured toward the various halides (F^-, Cl^-, Br^-, and I^-) where both SQ1 and SQ2 were found to sense halides with adequate sensitivity to measure μM levels of halide contamination. Finally, initial studies in a human cell line were also conducted where it was observed that both compounds are capable of being taken up by HeLa cells, exhibiting intracellular fluorescence as measured by both confocal microscopy and flow cytometry. Finally, using flow cytometry we were also able to show that cells treated with NaBr exhibited a demonstrable spectroscopic response when treated with either SQ1 or SQ2.