Rational Design of Hemicyanine Langmuir Monolayers by Cation-Induced Preorganization of Their Structure for Sensory Response Enhancement

Abstract

This study takes a novel approach to the enhancement of receptor properties of thin-film sensors based on hemicyanine dyes with dithia-aza-crown-ionophoric moiety. By means of in situ UV-vis and X-ray reflectivity (XRR) measurements, it was revealed that the introduction of up to 0.25 mmol of Hg under a preliminarily compressed monolayer, formed on pure water, does not lead to cation binding. This is due to the formation of "head-to-tail" aggregates (H-type), in which ionophoric group is blocked by the neighboring molecule. However, the presence of barium cations in the subphase under the forming Langmuir monolayer of the mentioned compound causes codirectional (head-to-head) orientation of chromoionophore fragments. This provides preorganization of a monolayer structure that facilitates the binding of complementary mercury cations, even in a compressed state: asymmetric sandwich complexes containing two dye molecules coordinate a Hg cation between them. This complex structure was confirmed by molecular modeling based on the electron density distribution calculated from XRR measurement data. Such preorganization of supramolecular ensembles induced by cations, which do not participate in the complex formation with macroheterocyclic receptors, may have applications in fields where strict control of molecular orientation at the interface is required, such as nanoelectronics, sensorics, catalysis, etc.

Switching Ion Binding Selectivity of Thiacalix[4]arene Monocrowns at Liquid–Liquid and 2D-Confined Interfaces

“…On the contrary, crown-ether cavity is more accessible for solvent molecules in type-I receptors 2a-c and the effect of residual CHCl3 solvent on ion-monolayer interactions could be remarkable. A comparison of the π-A isotherm of the monolayer of compound 2c over the pure water subphase with those for the monolayers formed over the AgNO3 aqua solution subphase and ones under which AgNO3 was added after CHCl3 evaporation confirmed the role of organic solvent in ion interactions and showed that the monolayer expansion Previous experiments with crown-ether hemicyanine dyes confirmed that effective "oxygen crown-ether-metal ion" interactions occur only at initial stage of monolayer formation before CHCl 3 evaporation [50][51][52], and this interaction can alter the molecular arrangement in the monolayer. According to this model, the influence of cation on monolayer formation correlates with ion-receptor complementarity.…”

“…These experiments demonstrate that the assembly of oxyethylene units into crown-4-6 macrocycle on 1,3-alternate topology are crucial for effective Ca 2+ uptake at the air-water interface. Previous experiments with crown-ether hemicyanine dyes confirmed that effective "oxygen crown-ether-metal ion" interactions occur only at initial stage of monolayer formation before CHCl3 evaporation [50][51][52], and this interaction can alter the molecular arrangement in the monolayer. According to this model, the influence of cation on monolayer formation correlates with ion-receptor complementarity.…”

Thiacalixarenes with Sulfur Functionalities at Lower Rim: Heavy Metal Ion Binding in Solution and 2D-Confined Space

“…A good example of a solid-state metal-coordination framework was obtained for the complex of CuI and bisthiacrown receptor IV, which displayed photoluminescence switching [10]. To date, the receptor ability of hydrophobic thioether units remains relatively unexplored at the air-water interface, where Langmuir monolayers are produced, and only a few compounds have been reported [11,12]. Given the outstanding binding characteristics of thioether units preorganized on a calixarene macrocycle, as well as unique coordination features with soft metal ions, understanding the interaction of these ligands with metal ions in confined space is highly demanding for sensor design and related applications.…”

Subnanomolar Detection of Mercury Cations in Water by an Interfacial Fluorescent Sensor Achieved by Ultrathin Film Structure Optimization

“…An exception is a series of papers by the authors of this article investigating the structural transformations in monolayers of amphiphilic crown-substituted hemicyanine chromoionophores and their effect on the receptor function of the planar system. − In refs − we have described in detail the spectral changes recorded during the interaction of the compound HCS (Scheme ) with mercury cations in solutions and obtained absorption and fluorescence spectra indicating the interaction of precompressed Langmuir monolayers of this compound with mercury cations (Hg 2+ ) at the air/water interface. Based on these data, calibration curves were constructed to quantify the concentration of mercury cations in the aqueous subphase beneath the precompressed monolayer using a ratiometric approach.…”

“…Based on these data, calibration curves were constructed to quantify the concentration of mercury cations in the aqueous subphase beneath the precompressed monolayer using a ratiometric approach. Using the method of X-ray reflectometry in combination with the method of standing X-ray waves, it was found − that the preorganization of the Langmuir HCS monolayer with ″inert″ barium cations in the subphase allows the formation of a supramolecular architecture with improved receptor properties with respect to mercury cations. The mechanism of such preorganization has been revealed.…”