Structure Of Guest Molecules Research Articles

Supramolecular Chirogenesis in Bis- and Multi-Porphyrin Systems: Recent Developments and Further Perspectives

Supramolecular chirogenesis is a fundamentally novel interdisciplinary field of research being a smart combination of supramolecular/nano chemistry and chiral science. Supramolecular chirogenesis deals with the phenomenon of “symmetry breaking” in achiral multi- or unimolecular host systems by chiral guests (and vice versa), and is achieved by the rational application of specific non-covalent interactions via a chirality information transfer mechanism. Recently, we have discovered a novel and very efficient supramolecular chirogenic system based upon an achiral ethane-bridged bis(metallo- or free base octaethylporphyrin).1-6 Upon interaction with the corresponding chiral guests, there are ligand induced structural deformations in the achiral host, resulting in a asymmetric orientation of the two porphyrin units and subsequent pronounced and easily detectable spectral changes monitored by various spectroscopies. Particularly, through-space exciton coupling between the porphyrin electronic transitions induces bisignate Cotton effects in the circular dichroism (CD) spectra. Significantly, the sign of the observed CD couplet is unequivocally correlated with the absolute configuration of the chiral ligand used, allowing us to develop an absolute configuration sensor for enantiopure amines, alcohols, acids. This system possesses a high degree of chiroptical activity, which allows the differentiation of one of the smallest homologous elements of organic chemistry, i.e. the methylene subunit, such as between methyl and ethyl groups attached to the asymmetric carbon; further, it is able to sense a remote chiral center at a position beta to the binding group. It was also found that this system is highly sensitive to various internal and external controlling factors such as the structure of guest molecules, stoichiometry, number of binding sites, solvent, temperature and phase transition. Further developments and prospects toward new chirogenic systems on the basis of heterometallic bis- and multi-porphyrin structures will be discussed. V. Borovkov, Symmetry 2014, 6, 256.V. Borovkov, Symmetry 2010, 2, 184.V. Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189.G. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1V. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.

Complex formation of cyclodextrins with some pharmacologically active 1,2,4-thiadiazole derivatives

Previously synthesized 1,2,4-thiadiazole derivatives displayed an activity in the treatment of Alzheimer’s disease; however, these compounds are poorly soluble in aqueous solutions. Inclusion complex formation with cyclodextrins is proposed for the development of water-soluble dosage forms of 1,2,4-thiadiazoles under study. To this end, binding of 1,2,4-thiadiazole derivatives with α-, β- and γ-cyclodextrins as well as with modified β-cyclodextrins was investigated by isothermal titration calorimetry, 1H NMR and UV–Vis spectroscopy in aqueous buffered solution (pH = 7.4) at 298.15 K. The detailed analysis of the thermodynamic parameters of complex formation is presented. The structure of 1,2,4-thiadiazole/cyclodextrin complexes was evaluated on the basis of 2D 1NMR (ROESY) experiments. The influence of structural factor (dimensions of macrocyclic cavity, modification of the host external surface, structure of guest molecules) on the thermodynamics and binding mode was discussed.

Selective binding behaviors of p-sulfonatocalixarenes in aqueous solution

The complex structures, binding abilities, molecular selectivities, and thermodynamic origin of p-sulfonatocalixarenes upon complexation with kinds of guests are outlined in this review article, including inorganic cations, organic ammonium cations, pyridiniums and viologens, neutral organic molecules, dye molecules, and others. Calorimetric and spectroscopic investigations afford the complex stability constants, thermodynamic parameters and binding manners of the inclusion complexation of p-sulfonatocalixarenes with guest molecules. The π-stacking, hydrophobic and charge interactions are the main driving-forces during the course of the host–guest inclusion complexation. The molecular binding abilities and selectivities are influenced by not only the frameworks of calixarene cavities, structures of guest molecules, and their binding manners but also the conditions of solutions (mainly pH), which are discussed from the correlation between the structural features and molecular-recognition abilities. Moreover, the further applications and potentials of p-sulfonatocalixarenes are briefly described.

Dielectric relaxation of polar aromatics in unoriented and oriented linear low-density polyethylene

The dielectric behavior of some polar aromatics dissolved in nonpolar unoriented and stretched linear low-density polyethylene was investigated within the temperature region between 150 and 350 K. The measurements were carried out in the frequency range 1 kHz to 10 MHz. The maximum temperatures and the half widths of the loss tangent peaks depend upon the shape and the polar structure of guest molecules. Stretching the samples induced a shift of the loss tangent to higher temperatures, decreased the height, and increased the width of tan δ peak. The activation energy is also influenced by the type of guest molecules and orientation of polymer matrix.

Syntheses of intercalation compounds of tetraamine in α-Zr(HPO4)2·H2O and the structures of guest molecules by solid state NMR

Conformational change of N,N′-Bis-(3-aminopropyl)-1,3-propanediamine (3P4A) intercalated in α-zirconium phosphate (α-ZrP) is reported. It is demonstrated that two kinds of intercalation compounds with different interlayer spacings, i.e., d=2.0 nm and d=1.7 nm, can be isolated by reacting at ca. 333 K and at ca. 298 K in aqueous suspension of 3P4A and α-ZrP, respectively. The lineshape analysis of 13C CP/MAS NMR spectra suggests that the conformation of 3P4A assumes the ‘straight’ form in the higher temperature product while it assumes the ‘bent’ form in the lower temperature product. According to this model, 31P MAS NMR spectra can be consistently assigned. It is found that 31P chemical shift values strongly depend on the bonding state of the amine groups in 3P4A molecule to the phosphate groups of α-ZrP. It is also shown that removing and/or controlling the amount of water molecules in the intercalation compounds are very important for the detailed assignment of the complicated 31P resonances.

Time-resolved observation of photochemical phase transition in polymer liquid crystals

Liquid crystal (LC) polymers [CH 2 CH(COOR)] n (R= (CH 2 ) 3 OPhCOOPhOCH 3 , (CH 2 ) 3 PhPhCN) doped with azobenzene derivatived and LC copolymers [CH 2 CH(COOR)] x [CH 2 CH(COOR')] y (R= (CH 2 ) 3 OPhCOOPhOCH 3 ) were studied. Using a nanosecond pulsed laser, it was found that the relaxation time required for the nematic (N) to isotropic (I) phase transition was in a time range of 10 (copolymer)-100 ms (homopolymer doped with azobenzene) at T/T NN = 0.999, which was comparable to that of low molecular weight LC. Effects of temperature, laser power and structure of guest molecule were discussed