New Chromogenic and Fluorescent Probes for Anion Detection:  Formation of a [2 + 2] Supramolecular Complex on Addition of Fluoride with Positive Homotropic Cooperativity

Abstract

Two new chromogenic and fluorescent probes for anions have been designed, synthesized, and characterized. These probes contain multiple hydrogen bonding donors including hydrazine, hydrazone, and hydroxyl functional groups for potential anion interacting sites. Despite the possible flexible structural framework due to the presence of sp3 carbon linkage, X-ray structure analysis of probe 2 displayed an essentially planar conformation in the solid state owing to strong crystal packing interactions comprising a combination of favorable pi-pi stacking effect and hydrogen bonding to cocrystallized CH3OH molecules. Both probes 1 and 2 display orange color in DMSO solution and show fairly weak fluorescence at room temperature. Binding studies revealed that both probes 1 and 2 show noticeable colorimetric and fluorescent responses only to F-, OAc-, and H2PO4- among the nine anions tested (F-, Cl-, Br-, I-, OAc-, H2PO4-, HSO4-, ClO4-, and NO3-). The general trend of the sensitivity to anions follows the order of F- > OAc- > H2PO4- > Cl- > Br- approximately I- approximately HSO4- approximately ClO4- approximately NO3-. A 1:2 (probe to anion) binding stoichiometry was found for probe 1 with OAc- and H2PO4- and probe 2 with F-, OAc-, and H2PO4-. The binding isotherm of probe 1 to F- was found to be complicated with apparent multiple equilibria occurring in solution. The formation of an aggregated supramolecular complex upon addition of fluoride is proposed to rationalize the observed optical responses and is supported by ESI mass spectrometry and pulsed-field gradient NMR spectroscopy. Data analysis suggests that the binding of probe 1 to F- shows positive homotropic cooperativity.

Functionalized Quinoxaline for Chromogenic and Fluorogenic Anion Sensing

“…Further towards the development of highly sensitive chromogenic and fluorescent anion sensors with polarized −NH functions, Sun and co‐workers established that quinoxaline‐based receptors 35 a and 35 b (Figure ) featuring the combination of hydrazine, hydrazone, imine, and hydroxyl functions can serve as efficient optical sensors for F − , AcO − , and H 2 PO 4 − in DMSO solutions . The involvement of hydroxyl groups in anion binding resulted in stronger binding affinity of 35 b than 35 a , as established from the 1 H NMR titration experiments in [D 6 ]DMSO, and energy‐minimized molecular models derived from semi‐empirical MOPAC/AM1 method.…”

“…Furthert owards the development of highly sensitivec hromogenic and fluorescent anion sensors with polarized ÀNH functions, Sun and co-workerse stablished that quinoxaline-based receptors 35 a and 35 b (Figure 28) featuring the combination of hydrazine, hydrazone, imine, andh ydroxylf unctions can serve as efficient opticals ensors forF À ,A cO À ,a nd H 2 PO 4 À in DMSO solutions. [61] The involvement of hydroxyl groups in anion binding resulted in stronger binding affinity of 35 b than 35 a,a se stablished from the 1 HNMR titration experiments in [D 6 ]DMSO,a nd energy-minimized molecular modelsd erived from semi-empirical MOPAC/AM1 method. Owing to their structuralf lexibility,r eceptors 35 a and 35 b could adopt ac onformation to accommodate strongly interacting anionss uch as F À ,A cO À ,a nd H 2 PO 4 À and to compensatet he energy required to "twist"t he structuralf ramework by forming multiple hydrogen bonds.T wo potential anion binding pockets within the receptor structure result in a1 :2 binding stoichiometry with F À , AcO À ,and H 2 PO 4 À in case of 35 a;w hereas, for 35 b,a1:2binding stoichiometry was observed only with AcO À and H 2 PO 4 À , and the binding isotherm with F À was found to be complicated with multiple equilibria occurring in solution.T he formation of an aggregated [2+ +2] supramolecular complex has been proposed to rationalize the observed absorption ande missive responses of 35 b upon addition of F À (both titrations pectra ex- hibited two distinct conversion steps with increasing concentration of F À anion), and is also supported by electrospray ionization (ESI) mass spectrometry and pulsed-field gradient NMR spectroscopy.T he association of af irst F À aniont of orm ad imeric aggregate via ac ombination of multiple hydrazine/hydrazone/imine/hydroxyl anion hydrogen bonding and p-p stacking interactions allows for the associationo fasecond F À anion within the preorganized hydrogen bondingp ocket and could, therefore, exert homotropic cooperativity effect.…”

Photochemistry of 1‐ and 2‐Naphthols and Their Water Clusters: The Role of 1ππ*(La) Mediated Hydrogen Transfer to Carbon Atoms

“…Specifically, there exists a strong dependence on the isomer and solvent, as well as on the size and geometry of the cluster. For a long time, the excited state dynamics of water or ammonia clusters of 1 and 2 have been interpreted within the framework of excited state proton transfer (ESPT)—a concept well established in condensed phase photochemistry and exploited in different applications including construction of white‐light‐emitting devices, sensing, development of photoswitches, and photocages, photolithography, photo‐biolabeling of proteins, and DNA alkylation …”

Quantification of Synthetic Amino-Nitroquinoxaline Dyes: An Approach Using Image Analysis

“…3,4 In addition, quinoxalines are well established as antagonists 5,6 and found interacting with DNA. 7 On the other hand, several papers are centered on the optical properties of quinoxaline derivatives and their applications as chemosensors for cations 8,9 and anions, 10,11 as well as functional materials such as organic light-emitting diodes (OLED), [12][13][14] liquid crystals 15,16 and solar cells. 17,18 Due to these latter relevant applications based on color properties of organic compounds, we found reactions of 2,3-dichloro-6,7-dinitrioquinoxaline (1) with aliphatic amines emerging as an interesting approach for obtainment of synthetic quinoxaline dyes especially by the mild conditions of aromatic nucleophilic substitution in highly activated substrates and the intense color found in aromatic compounds bearing both amino and nitro groups.…”