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

Amphiphilicity of Tetraazaporphyrins Containing Four Terminal Carboxylic Acid and Four Alkyl Groups Promotes Face-On Orientation in Langmuir Films

Control over the orientation of polycyclic aromatic dyes in thin films is paramount to tailoring their optical, electronic, and mechanical properties. Their supramolecular assembly in films is tuned here by converting the macrocyclic dyes to large amphiphiles. Two octaalkythio-substituted tetraazaporphyrins (TAPs) with one 5-carboxypentyl and one pentyl or dodecyl chain per pyrrole ring were synthesized as statistical mixtures of four regioisomers. The unsymmetrically substituted maleodinitrile precursors were prepared in good yield with a flow reactor. Neither the tetraester precursor TAPs nor the tetraacid TAPs show mesomorphism, and both belong to the small class of porphyrin derivatives that display isotropic liquid phases at or close to room temperature. Interfacial properties of the two amphiphilic tetraacids were probed using Langmuir films on aqueous subphases at different pH values, and Langmuir-Blodgett films were transferred onto mica substrates. The tetraacid with the longer dodecyl chains forms inhomogeneous films comprising a combination of monolayer, stacked macrocycles with interdigitated chains, and 3D structures, with the latter favored at higher subphase pH and higher surface pressures. In contrast, films of the tetraacid with the shorter pentyl chains yielded relatively homogeneous monolayers. The combination of atomic force microscopy imaging and packing correlations elucidated by grazing incidence X-ray diffraction suggests that these form spider-like conformations, with the macrocycles close packed and oriented parallel to the substrate (face-on). This difference in molecular packing is attributed to a possible intramolecular mixing of 5-carboxypentyl and pentyl chains that gives a better match between the footprints of the macrocycle, acid groups, and alkyl groups. The longer dodecyl chains are too large for filling the space between 5-carboxypentyl chains (mixing) and too small for filling the footprint of a TAP macrocycle. We demonstrate that by judicious tailoring of the chain length and subphase conditions, a desirable homogeneous film of face-on oriented macrocycles can be formed.

Structure affinity of the Langmuir monolayer and the corresponding Langmuir-Blodgett film revealed by X-ray techniques

The possibility of reproducing the structural organization and functional abilities of a Langmuir monolayer in a film formed from it is one of the fundamental problems of ultrathin film science. This work is devoted to the comparison of monolayer and Langmuir-Blodgett (LB) film characteristics using the example of 2D systems based on the dithia-aza-crown substituted hemicyanine dye HCS. As was shown earlier, the investigated systems are promising for the preparation of selective sensors and extractors for mercury ions in aqueous solutions with a subnanomolar sensitivity threshold. Therefore, the study of the analyte binding mechanism by such a film is of great importance. The study carried out using an ultra-highly brilliant X-ray source (ESRF) allows the application of highly sensitive techniques such as X-ray reflectometry (XRR) and X-ray standing wave (XSW). Comparison of the electron density depth profile of the HCS Langmuir monolayer at the air/water interface and the HCS film transferred to a silicon substrate shows the preservation of the film structure and its functional features. The XSW measurements in turn reveal the similarities in the fine structure of preorganized Langmuir monolayers and Langmuir-Blodgett films of HCS. The integration of X-ray techniques with molecular modeling methods allowed us to show that the crown-ether groups of HCS molecules in the pre-organized monolayer and in the corresponding LB film lie on the surface of water or silicon, and the bound mercury ion is located above the crown-ether, partially binding to the nitrogen atom. The latter loses conjugation to the chromophore group, thereby altering the UV-vis spectrum and providing a response signal. The revealed mechanism of imprinting preorganization allows the proposed approach to be extended to other crown-substituted amphiphilic dyes to significantly enhance the sensory response.

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.

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

Sulfur-containing groups preorganized on macrocyclic scaffolds are well suited for liquid-phase complexation of soft metal ions; however, their binding potential was not extensively studied at the air-water interface, and the effect of thioether topology on metal ion binding mechanisms under various conditions was not considered. Herein, we report the interface receptor characteristics of topologically varied thiacalixarene thioethers (linear bis-(methylthio)ethoxy derivative L², O₂S₂-thiacrown-ether L³, and O₂S₂-bridged thiacalixtube L⁴). The study was conducted in bulk liquid phase and Langmuir monolayers. For all compounds, the highest liquid-phase extraction selectivity was revealed for Ag⁺ and Hg²⁺ ions vs. other soft metal ions. In thioether L² and thiacalixtube L⁴, metal ion binding was evidenced by a blue shift of the band at 303 nm (for Ag⁺ species) and the appearance of ligand-to-metal charge transfer bands at 330-340 nm (for Hg²⁺ species). Theoretical calculations for thioether L² and its Ag and Hg complexes are consistent with experimental data of UV/Vis, nuclear magnetic resonance (NMR) spectroscopy, and single-crystal X-ray diffractometry of Ag-thioether L² complexes and Hg-thiacalixtube L⁴ complex for the case of coordination around the metal center involving two alkyl sulfide groups (Hg²⁺) or sulfur atoms on the lower rim and bridging unit (Ag⁺). In thiacrown L³, Ag and Hg binding by alkyl sulfide groups was suggested from changes in NMR spectra upon the addition of corresponding salts. In spite of the low ability of the thioethers to form stable Langmuir monolayers on deionized water, one might argue that the monolayers significantly expand in the presence of Hg salts in the water subphase. Hg²⁺ ion uptake by the Langmuir-Blodgett (LB) films of ligand L³ was proved by X-ray photoelectron spectroscopy (XPS). Together, these results demonstrate the potential of sulfide groups on the calixarene platform as receptor unit towards Hg²⁺ ions, which could be useful in the development of Hg²⁺-selective water purification systems or thin-film sensor devices.

Supramolecular tuning of thioflavin-T aggregation hosted by polystyrene sulfonate

Tunable and controllable emission is an extremely desirable feature for advanced functional materials that finds usage in optoelectronic utilization, fluorescence probing/sensing, drug-delivery monitoring, etc. In the present contribution, we have employed a macrocyclic host molecule, sulfobutyl ether-β-cyclodextrin (SBE-β-CD), as a tuning agent for an intensely emissive aggregate assembly of a molecular rotor dye, thioflavin-T (ThT), in the presence of an anionic polyelectrolyte, polystyrene sulfonate (PSS). The macrocyclic host breaks the PSS templated ThT aggregates and leads to encapsulation of released ThT molecules, tailoring the emission response of the system in terms of intensity and wavelength. Utilizing the established selectivity of the cyclodextrin-adamantane system, reverse control of this tunable emission has been further achieved. The controllable fluorescence system has been extensively investigated using ground-state absorption, steady-state and time-resolved emission spectroscopy. This kind of supramolecular tailoring of self-assembled aggregate emission has enormous potential in the field of fluorescence sensors and probes, and imaging and tracking in biological systems.

Supramolecular Control on the Optical Properties of a Dye-Polyelectrolyte Assembly

Control of fluorescent molecular assemblies is an exciting area of research with large potential for various important applications, such as, fluorescence sensing/probing, cell imaging and monitoring drug-delivery. In the present contribution, we have demonstrated control on the extent of aggregation of a dye-polyelectrolyte assembly using a macrocyclic host molecule, sulfobutylether-β-cyclodextrin (SBE-β-CD). Initially, a cationic molecular rotor based organic dye, Auramine-O (AuO), undergoes aggregation in the presence of an anionic polyelectrolyte, polystyrene sulfonate (PSS), and displays a broad intense new emission band along with large variation in its absorption features and excited-state lifetime. A manipulation of the monomer-aggregate equilibrium of the dye-polyelectrolyte assembly has been achieved by introducing a cyclodextrin based supramolecular host, SBE-β-CD, which leads to relocation of AuO molecules from polyelectrolyte (PSS) to supramolecular host cavity, owing to the formation of a host-guest complex between AuO and SBE-β-CD. A reversible control on this manipulation of monomer-aggregate equilibrium is further achieved by introducing a competitive guest for the host cavity i. e., 1-Adamantanol. Thus, we have demonstrated an interesting control on the dye-polyelectrolyte aggregate assembly using a supramolecular host molecule which open up exciting possibilities to construct responsive materials using a repertoire of various host-specific guest molecules.