Search

Dynamic CD and (super 1)H NMR spectroscopies are applied for studying the phenomenon of chirality inversion in the supramolecular system consisting of bis(Zn porphyrin) and (R)-amine upon stepwise titration with (S)-amine. Dynamic approach using CD and (super 1)H NMR spectroscopy can serve as an indispensable technique for monitoring of the structural changes of bis(Zn porphyrin) leading to the formation of the chiral screw structures.

Research describes the synthesis and characterization of two new heteroatom-bridged bis(benzo-12-crown-4ehter)s and diselenides together with their silver-selectivities behavior and structure-selectivity relationship in the silver-ion-selective electrodes in terms of steric and electronic effects of the nitro groups.

Research is presented describing the used of temperature on bis(zinc prohyrin) during supramolecular chirality induction.

A synthesis of the optically active octahedral ruthenium complexes has been carried out and a new concept has been realized. The chiroptical properties of these complexes are discussed.

Research has been conducted on the synthesis of Ru(tris(pp)) complexes. Results indicate that the complexes contain novel ligand architecture.

A study concerning bis(beta-cyclodextrin)s synthesized from beta-cyclodextrin is presented. Experiments include the investigation of the inclusion complexation behavior.

Alcohol-containing nonpolar solvents have been found to be the venue for temperature-dependent, syn-anti conformational switching in bis- and monozinc ethane-bridged porphyrin dimers. The dimers are believed to exhibit a syn conformation in the absence of alcohol, but a decrease in temperature occurs in the presence of alcohol. This temperature decrease results in the gradual shift of the conformational equilibrium toward the anti conformer.

Mono(6-(p-methoxyphenylseleno)-6-deoxy)-beta-cyclodextrin was synthesized and characterized by elemental analysis and mass, FT-IR and 1H NMR spectroscopy. The circular dichroism spectral titration method was used to determine the stability constants of the inclusion complexation of the novel cyclodextrin derivative and mono(6-(p-tolylseleno)-6-deoxyl)-beta-cyclodextrin with a series acyclic and cyclic alcohols in a phosphate buffer solution (pH7.20) at 25 degrees centigrade.

Research was conducted to examine the synthesis and the molecular recognition behavior of L-tryptophan-modified beta-cyclodextrin. Fluorescence and circular dichroism spectrometry and fluorescence lifetime measurement were performed with and without different alcohols as guest molecules. Results suggest that the modified beta-cyclodextrin can recognize minimal structural differences in the guest alcohols analyzed based on their size/shape, hydrophobicity and rigidity.

The complexation thermodynamics of cyclodextrins have been investigated utilizing various experimental methods with a large number of guest molecules. However, the accuracy of a large percentage of the thermodynamic data need to be validated. Some of factors which may affect the correlation between the thermodynamic quantities, and the structural and electronic characteristics of the guest molecules include hydrophobicity, the shape-matching and steric effects upon host-guest interaction, and the hydrogen-bonding capability of an aromatic hydroxyl group.

Research was conducted to examine the molecular design of crown ethers. Calorimetric titrations were carried out in anhydrous acetonitrile at 25 degrees C to yield the complex stability constants (Ks) and the thermodynamic parameters for the stoichiometric 1:1 complexation of light lanthanoid(III) nitrates with N-benzylaza-16crown-5 and 15,15-dimethyl-16-crown-5. Results suggest that the introduction of a nondonating sidearm to C- and N-pivot 16-crown-5 substantially changes not only the binding constant but also the relative cation selectivity.

A thermodynamic experiment was conducted to evaluate the complexation of alkali metal ions with N-phenyl-aza-15-crown-5, N-octylaza-15-crown-5, N-benzylaza-15-crown-5, N-benzylaza-18-crown-6, N-benzylaza-21-crown-7, and N-benzylaza-24-crown-8 in methanol at 25 degrees centigrade. Results show that N-substituents are important in the complexation of alkali metal ions with a nondonating side-chain.

A series of [Beta]-cyclodextrin derivatives, carrying pyridinio (4-6), phosphonyl (7, 8), seleno (9-11), m- and p-picolinyl (12, 13), o-chloroanilino (16), 8-quinolyl (17), furfuryl (18), and 9-fluorenyl (19) moieties in the side chain, were newly synthesized, and their complexation behavior was assessed and discussed thermodynamically, using L-tryptophan and a few naphthalene derivatives as representative guests. Calorimetric titrations have been performed at 25.0 [degrees] C in buffered aqueous solution (pH 7.20) to give the complex stability constants and thermodynamic parameters for the 1:1 inclusion complexation of these guests with the native and modified [Alpha]-, [Beta]-, and/or [Gamma]-cyclodextrins (1-20). All of the chemical modifications to the primary side of cyclodextrins examined led to significant changes in complex stability and thermodynamic parameters, which are elucidated in terms of the conformational, electrostatic, hydrogen-bonding, and hydration effects. Thermodynamically, the inclusion complexation is mainly enthalpy-driven with a negative or minor positive entropic contribution, which in some cases determines the complex stability. The induced circular dichroism spectral analyses of these cyclodextrin derivatives indicated that the aromatic moiety in modified [Beta]-cyclodextrins (4-6, 9-19) only shallowly penetrates into the hydrophobic cavity of [Beta]-cyclodextrin, while the phenyl phosphate and fluorenyl moieties in 7 and 20 are embedded into the hydrophobic cavity of [Beta]-cyclodextrin because of the longer linking chain. Using all the thermodynamic data for a wide variety of cyclodextrin derivatives obtained in this and previous studies, the entropy changes (T[Delta]S) were plotted against the enthalpy changes ([Delta]H) to give an excellent linear relationship. The slope ([Alpha]) of 1.02 and an intercept (T[Delta][S.sub.0]) of 4.3 of the regression line indicate substantial conformational changes and extensive desolvation caused upon complexation, respectively.

The synthesis and characterization of beta-cyclodextrins bearing m-toluidinyl or (9-fluorenyl)alkenyl moeity are described. The inclusion complexation properties of the compounds with L/D-amino acids were determined by fluorescence spectroscopy. The results showed that the modified cyclodextrins can recognize the size, shape and chirality of he amino acids. These properties were discussed in terms of the size/shape-fit relationship between the host cavity and guest molecules.

Enantiodifferentiating photoisomerizations of (Z,Z)-1,3-cyclooctadiene (1ZZ) to the chiral E,Z-isomer 1EZ were conducted at different temperatures in the presence of some benzenepoly-, naphthalene(di)-, and anthracene-carboxylates. Of these chiral sensitizers, (-)-menthyl benzenehexacarboxylate produced the highest enantiomeric excesses (ee). On the other hand, the employment of polar solvents greatly reduced the product's ee, indicating intervention of a radical ionic rather than exciplex intermediate in these solvents.

Steric effects, hydrophobic interactions and hydrogen bonding are the factors determining the interacting character of alpha- and beta- cyclodextrins with phenethylamine, ephedrines, and related substances. For the reactions of the chiral isomers of ephedrines and pseudoephedrine with the cyclodextrins, a profile of the standard molar enthalpy vs the standard molar entropy is linear. The 1:1 binding reactions of the cyclodextrins with the model compounds result in differences in the thermodynamic values in phosphate and acidic buffers due to the presence of a hydrophobic CH3 group.

2,3-Diphenyloxirane, 1,2-diphenyl- and 1,2-bis(4-methoxyphenyl)cyclopropanes undergo enantiodifferentiating photoisomerizations in polar and nonpolar solvents in the presence of chiral electron-accepting photosensitizers. The study shows that the products' optical purities (op), which may reach 10.4%, vary radically with solvent polarity, with polar and nonpolar solvents at times giving antipodal products. Analysis suggests the enantiodifferentiation taking place as the racemic substrate quenches the excited chiral sensitizer.

Triplex formation and high steric congestion provide a temperature-dependent route to photoproducts with high optical purities (op). The enantiocontrolled isomerization of cyclooctene from (Z) to (E) is achieved by sensitization with chiral benzene(poly)carboxylates. The bulky carboxylates with their electron-donating groups are able to provide up to 68% op at -89 degrees Celsius and up to 50% op at ambient temperatures. The role of entropy in enantiodifferentiation is traced to structural dynamics of the exciplex.

The inclusion complexation of naphtalenesulfonates and naphtaleneacetate with alpha-, beta- and gamma-cyclodextrins is investigated. A series of naphtalene derivatives possessing hydropphilic sulfonate or carboxylate groups at different positions is subjected to calorimetric titrations in a buffered aqueous solution. Results reveal that the inclusion compensation mostly enthalpy-driven with a minor or major positive entropic contribution.

The absorption anisotropy (g) factors, while limiting the optical yield (op) in direct asymmetric photoinduction by circularly polarized light (CPL), offers other ways of raising op's. The anomalous two-photon laser excitation, high g transitions of ketones and other compounds, the Cotton effect and careful choice of CPL wavelength and bandwidth are promising routes towards high g factors. Other possible techniques include the use of chemically bonded chiral auxiliaries, chiral complexing agents, tandem photochemical/ thermal reactions and the rotation switching phenomenon at the equipodal temperature.

Chiral polyalkyl benzenepolycarboxylates serve well as enantiodifferentiating singlet sensitizers in the photoisomerization of cyclooctene. The photostudy shows that the sensitizer forms a singlet exciplex, within which the favored reaction takes place via the rotational relaxation of 2-cyclooctene from planar to twisted singlet. Quenching by (E)-cyclooctene is not so favored but is controlled more by the diffusion rate. The unequal response to temperature variations also enhances enantiodifferentiation. The results open new photochemical routes with optical yields as high as 53%.