Cucurbit[n]uril based supramolecular assemblies: tunable physico-chemical properties and their prospects

This Feature Article provides an account of the recent work by our research group (with some reference to relevant work of other groups) on the spectacular physico-chemical properties of cucurbituril-based supramolecular assemblies of chromophoric dyes having technological and biological importance. Simultaneous association of multiple hosts or guests either by cooperative binding or competitive displacement is an applied strategy to construct novel functional assemblies with predesignated characteristics. Here, effort has been made to briefly discuss the diverse photophysical characteristics of several host-guest complexes and the dynamic responses of such molecular assemblies towards external stimulants like metal ions. The detailed experimentation on these assemblies project their applications in the controlled uptake and release of drugs, photofunctional devices, aqueous-based dye lasers, and molecular architectures.     

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)₃(THF)₃] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P2₁/c space group with a slight compression of the unit cell from 3396.4(2) ų to 3373.2(4) ų along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT²⁻ ligands with an end-bound {Ca²⁺(THF)₃} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT²⁻ ligands act in a η⁸-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)₃(THF)₃] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)₃(THF)₃] in contrast to [ErKCa(COT)₃(THF)₃], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations.

A new class of heterotrimetallic COT-based self-assemblies accommodates metals from groups I–III in three different oxidation states and enables tuning of electronic and magnetic properties.     

Solid-state tubular assemblies of thiolactones: synthesis and structural characterization

The synthesis of cyclic thiolactones, 2,5,8-trithiacyclododecane-1,9-dione (4), 2,5,8,14,17,20-hexathiacyclotetracosane-1,9,13,21-tetraone (5), 2,5,8-trithiacyclotetradecane-1,9-dione (6) and 2,5,8,16,19,22-hexathiacyclooctacosane-1,9,15,23-tetraone (7) was achieved by tin-template reaction of 2,2-dibutyl-2-stanna-1,3,6-trithiacyclooctane (1) with corresponding diacyl chlorides. The structures of 12-, 14-, 24- and 28-membered ring systems of 4, 6, 5, and 7, respectively, were investigated by X-ray structure analysis. These investigations revealed that, in the solid-state, thiolactones 4 and 7 form tubular assemblies. However, the crystal structure of 5 forms layered packing dominated by CH?O hydrogen bonds whereas 6 forms three-dimensional network via CH?O hydrogen bonds and van der Waals interactions.     

Lanthanide-mediated supramolecular cages and host-guest interactions

The structure and thermodynamic properties of lanthanide complexes with a new tripodal ligand L2 have been elucidated using different physicochemical methods. At stoichiometric ratios, the tetrahedral three-dimensional complexes with lanthanide cations are formed in acetonitrile with good stabilities. Despite minor structural changes comparing to previously investigated tripodal ligands, the resulting assembly exhibits different features revealed with the crystal structure of [Eu(4)L2(4)](OH)(ClO(4))(11) (orthorhombic, Pbcn). Interestingly, the highly charged edifice contains an inner cage encapsulating a perchlorate anion. Such lanthanide mediated cage-like assemblies are rare, and may be of interest for different sensing applications. Indeed, the anionic guest can be exchanged with different anions. The related host-guest equilibria were investigated with NMR techniques. Various aspects of these reactions are qualitatively discussed.     

Photoinduced structural transformations in mixed monolayer assemblies

In mixed monolayers with stearic acid at the air-water interface, paranaric acid undergoes photochemical reaction, which, in turn, triggers marked structural change in the monolayer assembly. Changes in the fluid-expanded and fluid-compressed regimes are qualitatively different and are interpreted in terms of a simple model.     

Hydrodispersible copolyesters: Relationship between chemical structure and physico-chemical properties

Anionic copolyesters with sodium sulfonate functions and short polyethylene oxyde blocks are used for polyester filament sizing. These polymers were prepared using a three stages melting process at high temperature combining ester interchange, direct esterification, polycondensation and etherification reactions. A study on the relationship between chemical structure and physico-chemical properties (glass transition temperature, solubility behaviour, hydrolytical and thermal stability) shows that the control of the ether bridges content is important.     

Mesoporous MgO: Synthesis,physico-chemical,and catalytic properties

Mesoporous MgO was obtained via the hydrothermal synthesis using both ionogenic and non-ionogenic surfactants as structure-directing templates. The materials prepared were characterized by SEM, BET-N₂, XRD, and TG-DTA techniques. MgO particles are spherical 20-μm aggregates of primary oxide particles well shaped as rectangular parallelepipeds. Magnesium oxide samples have the specific surface area of 290–400 m²/g and pore sizes of 3.3–4.1 nm. Their mesoporous structure remained unchanged after calcination up to 350°C. Catalytic activity of mesoporous MgO was studied in acetone condensation reaction.     

d-Xylose and l-Arabinose-based surfactants: Synthesis,reactivity and physico-chemical properties

The valorization of d-xylose and l-arabinose towards the formation of non-ionic surfactants is here described. Several processes are employed involving telomerization, glycosylation and phosphorylation reactions. Various structures of surfactants are given with respectively their amphiphilic properties.     

QSAR modeling and design of cationic antimicrobial peptides based on structural properties of amino acids

Drug resistance to existing antibiotics poses alarming threats to global public health, which inspires heightened interests in searching for new antibiotics, including antimicrobial peptides (AMPs). Accurate prediction of antibacterial activities of AMPs may expedite novel AMP design and reduce the costs and efforts involved in laboratory screening. In the present study, a novel quantitative prediction method of AMP was established by quantitative structure-activity relationship (QSAR) modeling based on the physicochemical properties of amino acids. The indices of these physicochemical properties were used to define AMP. The structural variables were optimized by stepwise regression (STR). Three series of AMPs from the QSAR model were constructed by multiple linear regressions (MLR). These QSAR models showed good performance in reliability and predictability. The normalized regression coefficients of the QSAR model and the contribution of amino acids at each position of AMP may determine the suitableness of a particular residue at any given position. QSAR models constructed by STR-MLR should prove to be useful tools in peptide design with respect to the calculation, explanation, good and reliable performance, and definition of physiochemical properties.     

Non-covalent assemblies of negatively charged boronated porphyrins with different cationic moieties

A unique approach to non-covalent electron and energy transfer is described that is based on the formation of salt bridges between oppositely charged porphyrin units. A new class of electrostatically linked dimeric and pentameric porphyrins was synthesized by interaction of novel anionic boron containing porphyrins such as 5-(benzamidodecahydro-closo-dodecaborate)-10,15,20-triphenylporphyrin (N1) and meso-tetrakis-benzamidodecahydro-closo-dodecaborate)porphyrin (N2) and a variety of cationic meso-tetraarylporphyrin units. A bipyridine linked dimer (N1 · bpy · N1) was also prepared by employing N,N′-dimethyl-4,4′-bipyridinium (bpy) as a spacer between two mono-anionic species. A quinone-porphyrin dyad was also prepared for electron or energy transfer demonstration. All the synthesized assemblies were characterized by NMR, IR, UV-Vis, and mass spectroscopy. Significant spectral changes occurred in the absorption spectra of these non-covalent porphyrin assemblies compared to those of the reference monomers, indicating the presence of electronic interaction between the adjacent porphyrin units. Resonance light scattering was also used to study the formation of these assemblies in solution.     

Lanthanide-mediated phosphoester hydrolysis and phosphate elimination from phosphopeptides

Lanthanide ions can mediate both phosphomonoester hydrolysis and beta-elimination of inorganic phosphate from polypeptide substrates under near-physiological conditions of pH, temperature, and salt.     

Polyhydroxyl-based surfactants and their physico-chemical properties and applications

This contribution deals with the physical-chemical properties of surfactants carrying hydroxyl groups in the polar part. Topics discussed include surface and colloid properties, micelles (both single and multi-component), general phase behaviour and microemulsions. A general conclusion is that our understanding of polyhydroxy surfactants is increasing, but that further work is needed to unravel certain aspects of this important class of surfactants. Examples of such aspects are the origin of the liquid-liquid phase separation and the adsorption to solid surfaces of polyhydroxy surfactants.     

Impact of alkali treatment on physico-chemical,thermal, structural and tensile properties of Carica papaya bark fibers

This study aims to examine the effect of sodium hydroxide (NaOH) treatment on the physico-chemical properties, structure, thermal, tensile and surface topography of Carica papaya fibers (CPFs). The surface of raw CPFs was modified by soaking with 5% NaOH solution for 15, 30, 45, 60, 75 and 90?min. The results of thermo-gravimetric analysis revealed that the optimum treatment time for alkali treatment was 60?min. It was found that the alkali treatment improved the properties of the CPFs. The results of TGA, FT-IR, XRD and AFM suggest that the treated CPF is a suitable alternative as reinforcement in polymer composites.     

Organized molecular assemblies: Creation and investigation of their functional properties

One of the most important problems of modern chemistry, both preparative and theoretical one, is the creation of organized atomic - molecular assemblies i.e., atoms and molecules bound in a definite manner and interacting with each other, as a jointly, they become able to behave like an assembly, a single mechanism, to perform some functions, for example, to recognize separate molecules of environment (receptors), to transform an electromagnetic signal to electric field (photosynthetic), to memorize (memory cell), catalyze function and others. The chemistry of preceding period had been developing as the science of properties and transformations of individual compounds and substances. At present the progress in the notion of the nature of chemical bonds, a new level of understanding of the chemical reactions mechanism, the knowledge of the regularities of nonstationary states in reactive media and the data on the at processes interfaces have determined the up to day problem of organized molecular assemblies with a required functionality. Thermal oxidation of silicon crystals which is the key process in the technology of integral circuits production can be represented, in its main features, by the mechanism of semiconductor dissolving in the condensed phase. A characteristic feature of silicon oxidation is that the silicon activation energy of the limiting stage is lower than that of the formation of gas - phase monoxide, difference being the energy of siloxane net reorganization in silicon dioxide. The analysis shows that within the row of possible systems semiconductor - dielectric, it is silicon - silicon dioxide that is distinguished by a maximum self - organizing effect. This is one of the main reasons why only silicon could be chosen among large number of semiconductor substances for use in the technology of integral circuits. A molecular system of amphiphilic long - chain molecules, obtained according to the Langmuir - Blodgett method is an example of simplest assembly on the basis of which the elements of gas sensors can be prepared, as well as the systems of quantum dots, the objects of the low - dimension physics. We have investigated assemblies constructed from multimolecular compositions using the Langmuir - Blodgett technique and thin - layer systems based on glassy and amorphous silica films with fragments of organic molecules introduced into the matrix. Molecular engineering is a new level of the modern preparative chemistry; moreover, it is a system of concepts introducing a new culture in technology. In this sense, it is most important to realize the evolution principle in the strategy of science research.     

Precipitation and physico-chemical properties of a drug substance

The useful!ness of thermoanalytical techniques in early preformulation studies of drug substances is demonstrated with a substituted benzamide derivative precipitated from solvents containing various amounts of water. Slow ageing of the crystalline products registered as changes in the bonding of water could be registered with TA-techniques only.     

Investigation of physico-chemical properties and structural parameters of diene-vinyl aromatic thermoplastic elastomers as a polymeric base of adhesive compositions

Adhesion and mechanical properties of vinyl aromatic thermoplastic elastomers on the Russian market were studied. Significant differences between the materials were shown that are due to differences in the chemical natures and structural parameters of block copolymers.     

Os(bipy)(CN)4]2- and its relatives as components of polynuclear assemblies: structural and photophysical properties

A series of diimine-tetracyanoosmate anions [Os(diimine)(CN)4]2- [diimine=2,2'-bipyridine (bipy), 2,2'-bipyrimidine (bpym), 1,10-phenanthroline (phen), and 4,4'-tBu2-2,2'-bipyridine (tBu2bpy)] were prepared and isolated as their Na+ salts (water soluble) or PPN+ salts (soluble in organic solvents). Several examples were crystallographically characterized; the Na+ salts form a range of 1D, 2D, or 3D infinite coordination polymers via coordination of the cyanide groups to Na+ cations in either an end-on or a side-on manner. The [Os(diimine)(CN)4]2- anions are solvatochromic, showing three MLCT absorptions, which are considerably blue-shifted in water compared to organic solvents, in the same way as is well-known for the analogous [Ru(diimine)(CN)4]2- anions. Luminescence in the red region of the spectrum is very weak but (following the expected solvatochromic behavior) is higher energy and more intense in water. However, by exploiting the effect of metallochromism (ref 4), the emission from [Os(tBu2bpy)(CN)4]2- in MeCN can be very substantially boosted in energy, intensity, and lifetime in the presence of Lewis-acidic metal cations (Na+, Ba2+, Zn2+), which, in a relatively noncompetitive solvent, coordinate to the cyanide groups of [Os(tBu2bpy)(CN)4]2-. This has an effect similar in principle to hydrogen bonding of the cyanides to delta+ protons of water, but very much stronger, such that in the presence of Zn2+ ions in MeCN the 1MLCT and 3MLCT absorptions are blue-shifted by ca. 7000 cm(-1), and the luminescence moves from 970 nm (vanishingly weak) to 610 nm with a lifetime of 120 ns (dominant component). Thus, the binding of metal cations to the cyanides provides a mechanism to incorporate [Os(diimine)(CN)4]2- complexes into polynuclear assemblies and simultaneously increases their 3MLCT energy and lifetime to an extent that makes them comparable to much-stronger luminophores such as Ru(II)-polypyridines.     

Nature and properties of ionomer assemblies. II

The principle subject in the current paper is to summarize and characterize the ionomers based on polymers and copolymers such as polystyrene (PSt), polyisoprene (PIP), polybutadiene (PB), poly(styrene-b-isobutylene-b-styrene) (PSt-PIB-PSt), poly(butadiene-styrene) (PB-PSt), poly(ethylene terephthalate) (PET), poly(butylene adipate) (PBA), poly(butylene succinate) (PBSi), poly(dimethylcarbosiloxanes), polyurethane, etc. The self-assembly of ionomers, models concerning ionomer morphologies, physical and rheological properties of ionomer phase and percolation behavior of ionomers were discussed. The ionomer phase materials and dispersions have been characterized by differential scanning calorimetry (DSC), small-angle X-ray catering (SAXS), small-angle neutron scattering (SANS), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), etc. The wide range of compositions, molecular architectures, and morphologies present in ionomeric disperse systems are of great interest. The research is particularly devoted to the potential application of these materials and an understanding of the fundamental principles of the ionomers. They are extremely complex systems, sensitive to changes in structure and composition, and therefore not easily amenable to modeling and to the derivation of general patterns of behavior. The reviewed data indicate that a large number of parameters are important in influencing multiplet formation and clustering in random ionomers. Among these are the ion content, size of the polyion and counterion, dielectric constant of the host, T(g) of the polymer, rigidity or persistence length of the backbone, position of the ion pair relative to the backbone, steric constraints, amount and nature of added additive (plasticizer), thermal history, etc.     

Transitions of organized assemblies in mixed systems of cationic bolaamphiphile and anionic conventional surfactants

The transition from vesicles to tubelike structures has been studied in mixed systems of cationic bolaamphiphile BPHTAB [biphenyl-4,4'-bis(oxyhexamethylenetrimethylammonium bromide)] and its oppositely charged conventional surfactants with transmission electron microscopy (TEM) and dynamic light scattering (DLS). This transition can be attributed to the fact that tube-like structures are more stable aggregates than vesicles because of the special molecular packing in the aggregates of the mixed systems. The effects of temperature and salt addition on this transition have also been investigated, and the rate of the transition was found to be strongly dependent on temperature. Addition of the appropriate amount of NaBr will accelerate the transition from vesicles to tube-like structures, but the vesicles will transform into micelles at higher salt concentration. Moreover, the micelle-vesicle transition can be realized by addition of n-octanol in the mixed system of BPHTAB/sodium caprate (SC) at higher salt concentrations.