Azulene‐Based Macrocyclic Receptors for Recognition and Sensing of Phosphate Anions

Abstract

Herein we report the synthesis and detailed studies of the anion-binding properties of two 20-membered macrocyclic tetramide receptors: one symmetrical, containing two identical azulene-based bisamide units, the other a hybrid, containing a dipicolinic bisamide unit and an azulene-based bisamide unit. Analysis of the crystal structures of the macrocyclic receptors revealed their preference for adopting similar well-preorganized bent-sheet conformations, both as free receptors and in their complexes with anions. Studies of the optical properties of both receptors revealed abilities to selectively sense phosphate anions (H PO , HP O ), allowing for naked-eye detection of the presence of these guests in DMSO. Binding studies in solution confirmed that the receptors bind strongly to a series of anions even in highly demanding media, such as mixtures of DMSO with water or with methanol. Comparison of the anion affinity of linear analogues with that of the macrocyclic receptors evidenced the importance of macrocyclic topology. Quantitative analysis revealed that the macrocyclic receptors are selective for H PO over other anions. The affinity to H PO seen for the symmetrical receptor, containing two azulene-based subunits, is much higher than for the hybrid macrocycle containing both the azulene-based and pyridine-derived subunits. This highlights that the azulene-based building block serves efficiently as both a binding site and a structure-preorganizing motif.

Selective Fluorescence Turn‐On Sensing of Phosphate Anion in Water by Tetraphenylethylene Dimethylformamidine

“…Afterward, the intensity was not increased with further addition of phosphate. For fluorescent detection of phosphate anion, many research works are carried out in a mixed solvent of organic solvent and water or suffer from low selectivity . Fluorescent sensor for phosphate anion in water with high selectivity is still rare.…”

The Influence of Binding Site Geometry on Anion‐Binding Selectivity: A Case Study of Macrocyclic Receptors Built on the Azulene Skeleton

“…In addition to the symmetric receptors 2 and 4 , which contain two azulene‐bisamide units of the same kind (1,3‐ and 5,7‐disubstituted, respectively), we designed the hybrid receptor 5 , which contains both types of azulene units. Finally, we also designed hybrid receptors 1 and 3 , which contain either the azulene‐1,3‐bisamide or azulene‐5,7‐bisamide unit and the dipicolinic bisamide unit, previously shown to stabilize the all‐ syn conformation of tetra‐amide receptors, which is favorable for anion binding . Overall, the series of receptors 1 – 5 has allowed us to evaluate the influence of the different geometries of the building blocks on the selectivity of the macrocyclic receptors, for example, for receptor 1 versus receptor 3 , the influence of the conformation‐stabilizing dipicolinic units, for receptor 3 versus receptor 4 , and the influence of combining building blocks with dissimilar selectivity preferences, that is, the relative selectivity of receptor 5 .…”

“…As demonstrated previously, the synthesis of receptors of types 1 and 2 containing azulene‐1,3‐bisamide is straightforward, and was conducted by exploiting the standard high‐dilution protocols for the macrocyclization reaction between a suitable diamine and a diacid dichloride . In contrast, the synthesis of receptors containing azulene‐5,7‐bisamide units is challenging.…”

“…To explore the unique properties of azulene‐based building blocks for the construction of highly selective anion receptors, we have conducted systematic studies of macrocyclic receptors exploiting these units. Previously, we devised two macrocyclic receptors, 1 and 2 , containing the azulene‐1,3‐bisamide moiety that were found to be effective and selective receptors for H 2 PO 4 − anions (Scheme B) . Here, we report our comprehensive research encompassing three new macrocyclic receptors 3 – 5 containing the azulene‐5,7‐bisamide building block, exploiting its unique inherent selectivity for binding Cl − anions (Scheme C), and compare them with receptors 1 and 2 .…”

Azulene–boronate esters: colorimetric indicators for fluoride in drinking water

“…Many prior reports of azulene-based colorimetric indicators for various analytes exist, including phosphate, [27][28][29][30] acetate, 30 cyanide, 30 hydrogen peroxide, 31 silver, 32 mercury and other heavy metals. [33][34][35] (Some of these indicators are not wholly selective and also exhibit a colorimetric response to fluoride [27][28][29][30] ). Of closest relevance to our present work is a report from Eichen et al 36 which describes a fluorescent azulene-based fluoride sensor that also exhibits a colorimetric response (green to violet) in DMSO.…”