Cation-Controlled Excimer Packing in Langmuir-Blodgett Films of Hemicyanine Amphiphilic Chromoionophores

Supramolecular Control of Photonic Response and Sensing of Nitricoxide using Iron(III) Corrole Monolayers and Their Stacks

In this work, we assemble amphiphilic iron(III) corroles at air-water interfaces into well-defined quasi-two-dimensional molecular monolayers and theirs stacks for sensing of nitric oxide (NO). For this purpose, we use the Langmuir-Blodgett (LB) technique, which allows varying the packing density of iron(III) corroles anchored to the aqueous subphase via one molecular side. The stacks of ten down to three molecular monolayers on the front and back sides of the substrates are sufficiently optically dense to detect NO binding to the layers photometrically. This sensing with few layers demonstrates the potential for electronic detection, where very thin surface functionalizations enable efficient electronic communication between the layer and the (semi)conductor. Despite increasing optical densities, the spectral responses to NO exposure become smaller with increasing packing density until the collapse point of the monolayers is reached. This demonstrates that the highest molecular efficiency for binding and detection of NO is achieved at the smallest packing densities. This finding is relevant to all molecular sensor films with axial binding of analytes to the sensor molecules and demonstrates the advantage of sensor molecule assembly into monolayers on water-air interfaces using the LB technique.

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

The objectives of this work were to develop and extend the previously proposed approaches to control the structure of nanoscale planar systems based on modular fluorophore compounds capable of efficient analyte binding and to optimize the architecture of ultrathin films formed from them to create a thin-film sensor element for mercury cations. The possibility of applying the ratiometric approach to the fluorescence measurements to obtain a quantitative analytical signal was shown. It was found that films with the Langmuir-Schaefer film architecture, in which the receptor crown ether groups of the fluoroionophore are oriented toward the studied solution, allow the quantitative determination of mercury cations in water at concentrations below the threshold limit value and are especially effective in the analyte concentration range of 10^-10-10^-5 M. High selectivity of the obtained thin-film sensitive elements with respect to mercury cations and the possibility of regeneration of such elements after quantitative determination of mercury cations in aqueous solutions are demonstrated.

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

Understanding the interaction of ions with organic receptors in confined space is of fundamental importance and could advance nanoelectronics and sensor design. In this work, metal ion complexation of conformationally varied thiacalix[4]monocrowns bearing lower-rim hydroxy (type I), dodecyloxy (type II), or methoxy (type III) fragments was evaluated. At the liquid–liquid interface, alkylated thiacalixcrowns-5(6) selectively extract alkali metal ions according to the induced-fit concept, whereas crown-4 receptors were ineffective due to distortion of the crown-ether cavity, as predicted by quantum-chemical calculations. In type-I ligands, alkali-metal ion extraction by the solvent-accessible crown-ether cavity was prevented, which resulted in competitive Ag⁺ extraction by sulfide bridges. Surprisingly, amphiphilic type-I/II conjugates moderately extracted other metal ions, which was attributed to calixarene aggregation in salt aqueous phase and supported by dynamic light scattering measurements. Cation–monolayer interactions at the air–water interface were monitored by surface pressure/potential measurements and UV/visible reflection–absorption spectroscopy. Topology-varied selectivity was evidenced, towards Sr²⁺ (crown-4), K⁺ (crown-5), and Ag⁺ (crown-6) in type-I receptors and Na⁺ (crown-4), Ca²⁺ (crown-5), and Cs⁺ (crown-6) in type-II receptors. Nuclear magnetic resonance and electronic absorption spectroscopy revealed exocyclic coordination in type-I ligands and cation–π interactions in type-II ligands.

Supramolecular control of the structure and receptor properties of an amphiphilic hemicyanine chromoionophore monolayer at the air/water interface

Designing sensors for toxic compounds such as mercury salts in aqueous solutions still remains one of the most pressing tasks of modern chemical research, since many existing systems do not show enough sensitivity and/or response. In this regard, the opportunities offered by supramolecular approaches can be used to improve both these characteristics by creating a new self-organized smart system. Herein, we show that barium cations, that according to the data of X-ray standing waves do not bind directly to the ionophore molecules in the monolayers at the air/water interface, could be used to efficiently preorganize such molecules to achieve supramolecular architecture. We demonstrate that such preorganization ensures both low analyte detection threshold and high fluorescent response. We reveal the interrelation of the monolayer structure and receptor characteristics of a sensory system and show that such cation-induced preorganization in Langmuir monolayers of a hemicyanine dithia-aza-crown-substituted chromoionophore inhibits the formation of non-fluorescent aggregates with low receptor function, and allows the quantitative detection of mercury cations using a ratiometric fluorometric approach.

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

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 Hg²⁺ 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 Hg²⁺ 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.

Inorganic mixed phase templated by a fatty acid monolayer at the air-water interface: the Mn and Mg case

We studied by means of Grazing Incidence X-ray Diffraction (GIXD) coupled with X-ray fluorescence spectroscopy the structure of a behenic acid monolayer spread at the surface of Mg²⁺/Mn²⁺ mixed aqueous solutions. For the pure Mg²⁺ and Mn²⁺ aqueous solutions, the cations induce at the surface different 2D lattice superstructures of the organic monolayer. These superstructures correspond to an inorganic organized monolayer anchored to the hydrophilic group of the ordered behenic acid monolayer. Among the various diffraction peaks, we focused on those characteristics of the behenic acid oblique cell. As the Mg²⁺ mole fraction x increases in the Mg²⁺/Mn²⁺ mixed subphase, a continuous evolution of the oblique cell parameters is observed indicating the insertion of Mg²⁺ cations in the Mn²⁺ ordered monolayer. Then, a further increase leads to the appearance of a coexistence between two oblique surface phases. The cell parameters of both phases evolve continuously along the x range of the transition until a single Mg-rich ordered phase is detected. However, although the intensities of the peaks in the coexistence region are in agreement with a first-order phase transition, the cell parameters evolve simultaneously. Considering a thermodynamics analysis, this evidences that, apart from the concentration, another unidentified intensive parameter is varying. We suggest that it is the ionic strength, which appears to be strongly related to the concentrations.

Ink-Free Reversible Optical Writing in Monolayers by Polymerization of a Trifunctional Monomer: Toward Rewritable "Molecular Paper"

A Langmuir-Blodgett film consisting of a dense array of trifunctional monomers bearing three 1,8-diazaanthracene units is polymerized at an air/water interface or after transfer on solid substrates. The transfer does not affect the excimer fluorescence of the film, indicating that the monomers' packing with their diazaanthracene units stacked face-to-face is retained-a prerequisite for successful polymerization. The monomer film can be polymerized in confined areas on solid substrates by UV irradiation with a confocal microscope laser. The underlying chemistry of the polymerization, a [4+4]-cycloaddition of the diazaanthracene units, leads to disappearance of the fluorescence in the irradiated regions which enables writing into the monolayer on a µm scale-thus the term "molecular paper." The reaction can be reversed by heating which leads to a recovery of the fluorescence and to erasing of the writing. Alternative pathways for this phenomenon are discussed and control experiments are conducted to rule them out.

Photo-switching of a non-ionic azobenzene amphiphile in Langmuir and Langmuir–Blodgett films

The concept of programmable and reconfigurable soft matter has emerged in science in the last few decades and can be realized by photoisomerization of azobenzene derivatives.