Structural Features of a Family of Coumarin–Enamine Fluorescent Chemodosimeters for Ion Pairs

Abstract

A family of coumarin–enamine chemodosimeters is evaluated for their potential use as fluorescent molecular probes for multiple analytes [cadmium­(II), cobalt­(II), copper­(II), iron­(II), nickel­(II), lead­(II), and zinc­(II)], as their chloride and acetate salts. These fluorophores displayed excellent optical spectroscopic modulation when exposed to ion pairs with different Lewis acidic and basic properties in dimethyl sulfoxide (DMSO). The chemodosimeters were designed to undergo excited-state intramolecular proton transfer (ESIPT), which leads to significant Stokes shifts (ca. 225 nm) and lower-energy fluorescence emission (ca. 575 nm). A more basic anion, e.g., acetate, inhibited the ESIPT mechanism by deprotonation of the enol, producing a binding pocket (N^O– chelate) that can coordinate to an appropriate metal ion. Coordination of the metal ions enhances the fluorescent intensity via the chelation-enhanced fluorescence emission mechanism. Subjecting the spectroscopic data to linear discriminant analysis provided insights into the source of these systems’ markedly different behavior toward ion pairs, despite the subtle structural differences in the organic framework. These compounds are examples of versatile, low-molecular-weight, dual-channel fluorescent sensors for ion-pair recognition. This study paves the way for using these probes as practical components of a sensing array for different metal ions and their respective anions.

Back to the Drawing Board: A Unifying First-Principle Model for Correlating Sample UV–Vis Absorption and Fluorescence Emission

“…UV–vis absorption and fluorescence spectroscopies are the two most used methods in chemistry, biology, and materials sciences. − Understanding the relationship between the fluorophore light absorption and fluorescence emission is important not only for a fundamental understanding of materials photophysical properties but also for reliable fluorescence analysis. While the origins of light absorption and fluorescence emission have become common knowledge for most fluorescent materials, the interplay among the sample light absorption and emission can be highly complicated in many practical fluorescence applications. − Mechanistically, fluorophore light absorption must precede its fluorescence emission.…”

4-Methyl-7-((2-((5-methyl-1,3,4-thiadiazol-2-yl)thio)ethyl)thio)-coumarin

“…Coumarin, or 2H-chromen-2-one, is a natural bicyclic organic compound isolated from a variety of plants. Its derivatives have found broad applications such as fluorescent probes [1][2][3][4], food additives [5][6][7], in polymer chemistry [8][9][10][11], and so on. Numerous representatives have displayed variable bioactivity profiles [12][13][14][15][16][17][18], including anticancer [19][20][21][22][23][24], anti inflammatory [25][26][27], anti-tuberculosis [28,29], antimicrobial [30], and many others.…”

A molecular chemodosimeter to probe “closed shell” ions in kidney cells

“…Previously, our group has shown that the anion can deprotonate the enol proton that is involved in the Resonance-Assisted Hydrogen Bonding interaction (RAHB) to form the bidentate binding motif (N^O − ), which drastically changed the optical response upon the addition of metal acetate salts in DMSO, by inhibiting the ESIPT mechanism and producing a CHEF signal upon the binding of metal ions. 10…”

“…Moreover, the acetate ion was initially used as the counter ion as we have previously shown that the metal acetate could deprotonate the OH group on the enol to form the chelating motif to coordinate with the metal ions. 10 The initial UV-Vis band is seen at 331 nm for QHC2 , assigned to the n–π* transition (the π–π* band is seen at 261 nm), with a calculated molar absorbance coefficient of 13 909 mol dm −3 cm −1 (Fig. S34†).…”