Photocontrol of supramolecular architecture in azopolymers: Achiral and chiral aggregation

The absorption spectra and circular dichroism responses upon irradiation with 488 nm circularly polarized light (CPL) have been studied on polymeric films processed from two nematic copolymers containing azobenzene chromophores. Influence of the aggregation and orientation of azobenzenes on the photoinduced chiroptical properties has been analyzed. Moreover, polymeric films and 4 μm planar cells filled with the copolymers were studied by polarized optical microscopy (POM) with the aim of investigating the change of macroscopic optical properties and textures of azopolymers upon irradiation with CPL.     

Supramolecular chirality and reversible chiroptical switching in new chiral liquid-crystal azopolymers

The synthesis of chiral liquid-crystalline polymers of well-controlled structure (linear and three-armed star-shaped) with distinct average chain lengths and low polydispersity was achieved by atom transfer radical polymerisation (ATRP) of a new optically active monomer (S)-4-[6-(2-methacryloyloxypropanoyloxy)hexyloxy)]-4'-ethoxyazobenzene [(S)-ML6A], containing the L-lactic residue of one absolute configuration in the side-chain. All the obtained polymeric samples, characterised by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarised optical microscopy (POM), exhibit a smectic A(1/2) (fully interdigitated) liquid-crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerisation degree and the macromolecular structure. The chirality originated at the molecular level by the asymmetric functionality of the L-lactic acid residue provides the polymers, in the smectic phase, of highly homogeneous conformations with a prevailing chirality related to the presence of H-aggregates having conformational dissymmetry of one prevailing screw-sense. By irradiating with circularly polarised light (CPL), it is possible to photomodulate the chiroptical properties of these intrinsically chiral polymeric thin films. Upon irradiation with left-handed CPL (l-CPL), the circular dichroism (CD) spectra of the films show enhancement of ellipticity and a net inversion of sign. The effect is reversible and the mirror image of the CD spectrum can be restored by pumping with right-handed CPL radiation (r-CPL). The results show the ability of l-CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates.     

Pathways to increase the dissymmetry in the interaction of chiral light and chiral molecules

The dissymmetric interaction between circularly polarised (CP) light and chiral molecules is central to a range of areas, from spectroscopy and imaging to next-generation photonic devices. However, the selectivity in absorption or emission of left-handed versus right-handed CP light is low for many molecular systems. In this perspective, we assess the magnitude of the measured chiroptical response for a variety of chiral systems, ranging from small molecules to large supramolecular assemblies, and highlight the challenges towards enhancing chiroptical activity. We explain the origins of low CP dissymmetry and showcase recent examples in which molecular design, and the modification of light itself, enable larger responses. Our discussion spans spatial extension of the chiral chromophore, manipulation of transition dipole moments, exploitation of forbidden transitions and creation of macroscopic chiral structures; all of which can increase the dissymmetry. Whilst the specific strategy taken to enhance the dissymmetric interaction will depend on the application of interest, these approaches offer hope for the development and advancement of all research fields that involve interactions of chiral molecules and light.

This perspective explores the dissymmetric interaction between circularly polarised (CP) light and chiral molecules. Such interactions are central to many applications from next generation displays to asymmetric photochemical synthesis.     

Photoinduced orientational ordering in the series of methacrylic azopolymers

Polymethacrylates with side azo fragments containing various end substituents and spacers are synthesized and characterized. Spatial ordering of azo fragments in the series of the synthesized azo polymers under the action of polarized excitation light is studied by the methods of null ellipsometry and polarization spectroscopy. Two types of anisotropy development during photoirradiation are found. In homologs with strong acceptor substituents of azo fragments, biaxial orientational structure can be observed; as the radiation dose is increased, this structure is transformed into a uniaxial structure with a negative order parameter and with an axis parallel to the polarization vector of the excitation light. In this structure, the maximum degree of orientational order is achieved in polymers with high concentrations of azo fragments. In homologs with donor substituents of azo fragments, the initial stages of irradiation likewise lead to the development of biaxial orientation; later, this orientation is transformed into an isotropic distribution. The mode of anisotropy development is controlled by the lifetime of photoisomers of azo fragments.     

Enantiomer migration order in chiral capillary electrophoresis

Enantiomer migration order (EMO) in chiral capillary electrophoresis (CE) represents a challenging issue, referred to in less than 20% of the articles on CE enantioseparations. This review article will (i) illustrate the actuality of the topic, (ii) discuss some technical problems related to EMO in CE enantioseparations, (iii) examine the principal differences between CE and other separation techniques from the viewpoint of enantiomer elution order, (iv) demonstrate the potential for a designed reversal of EMO in CE, and (v) emphasize the importance of studying EMO for better understanding of chiral CE as well as its more effective application. Along with CE, the results obtained by other instrumental techniques such as nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), X-ray crystallography, as well as molecular modeling calculations will be shortly discussed. Rather than referring to all published examples of the opposite migration order of enantiomers in CE, the emphasis will be on general aspects. Recently, the reversal of the EMO was described in detail in a book chapter (Chankvetadze, B., Capillary Electrophoresis in Chiral Analysis, Wiley & Sons, Chichester, UK 1997, Chapter 12) as well as in three review articles.     

Thermal decomposition of light lanthanide diglycolates

The conditions of thermal decomposition of La, Ce(III), Pr, Nd, Sm, Eu and Gd diglycolates have been studied. On heating, the diglycolates of Ce(III), Pr, Eu and Gd lose crystallization water and yield anhydrous salts, which are then transformed into oxides. The diglycolates of La, Nd and Sm are decomposed in three stages. First, the diglycolates undergo dehydration to form the anhydrous salts, which are next decomposed to Ln₂O₂CO₃. In the last step the thermal decomposition of Ln₂O₂CO₃ to Ln₂O₃ takes place, accompanied by an endothermic effect.     

Reversible operation of chiral molecular scissors by redox and UV light

Upon changing the oxidation state, a reversible open-close motion of chiral molecular scissors, composed of a redox-active ferrocene pivot and an isomerizable azobenzene strap, can be realized only by UV light.     

Thermal decompositions of light lanthanide aconitates

The conditions of thermal decomposition of Y, La, Ce(III), Pr, Nd, Sm, and Gd aconitates have been studied. On heating, the aconitate of Ce(III) loses crystallization water to yield anhydrous salt, which then is transformed in to oxide CeO₂. The aconitates of Y, Pr, Nd, Sm, Eu and Gd decompose in three stages. First, aconitates undergo dehydration to form the anhydrous salts, which next decompose to Ln₂O₂CO₃. In the last one the thermal decomposition of Ln₂O₂CO₃ to Ln₂O₃ is accompanied by endothermic effect. Dehydration of aconitate of La undergoes in two stages. The anhydrous complex decomposes to La₂O₂CO₃; this subsequently decomposes to La₂O₃.     

Synergistic regulation of intermolecular interactions to control chiral structures for chiral recognition

Understanding the regulatory mechanism of self-assembly processes is a necessity to modulate nanostructures and their properties. Herein, we have studied the mechanism of self-assembly in the C3 symmetric 1,3,5-benzentricarboxylic amino acid methyl ester enantiomers (TPE) in a mixed solvent system consisting of methanol and water. The resultant chiral structure was used for chiral recognition. The formation of chiral structures from the synergistic effect of multiple noncovalent interaction forces was confirmed by various techniques. Molecular dynamics simulations were used to characterize the time evolution of TPE structure and properties in solution. The theoretical results were consistent with the experimental results. Furthermore, the chiral structure assembled by the building blocks of TPE molecules was highly stereoselective for diamine compounds.     

Circular differential scattering of light in films of chiral polyfluorene

Circular differential transmission in thick films (1.1 mum) of poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} is investigated. The vitrified liquid crystalline film obtained after annealing shows high circular differential transmission of light in the wavelength range where the polyfluorene does not absorb (lambda > 450 nm). Using a specifically designed chiroptical setup, we show that circular selective reflection of light in which the circular polarization of the light is retained after reflection, a process which is characteristic for cholesteric liquid crystalline films, makes a negligible contribution to the differential transmission. Using an integrating sphere, we show that circular differential scattering can account for the observed circular differential transmission for lambda > 450 nm.     

Synthetic control leading to chiral compounds

A highly diastereoselective cross aldol reaction is developed using divalent tin enolates formed from stannous trifluoromethanesulfonate and carbonyl compounds. The reaction is extended to a highly enantioselective cross aldol reaction employing chiral diamines derived from (S)-proline as ligands.     

Stark control of a chiral fluoroethylene derivative

Hydrogen dissociation is an unwanted competing pathway if a torsional motion around the C═C double bond in a chiral fluoroethylene derivative, namely (4-methylcyclohexylidene) fluoromethane (4MCF), is to be achieved. We show that the excited state H-dissociation can be drastically diminished on time scales long enough to initiate a torsion around the C═C double bond using the nonresonant dynamic Stark effect. Potential energy curves, dipoles, and polarizabilities for the regarded one-dimensional reaction coordinate are calculated within the CASSCF method. The influence of the excitation and the laser control field is then simulated using wave packet dynamics.     

Thermal, pressure and light switchable spin-crossover materials

This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.     

Thermal and photochemical racemization of chiral aromatic sulfoxides via the intermediacy of sulfoxide radical cations

Efficient racemization of enantiomerically pure methyl aryl sulfoxides was obtained by N-methylquinolinium tetrafluoborate (NMQ+) sensitized photolysis and by one-electron oxidation catalyzed by tris(2,2'-bipyridyl)ruthenium(III) hexafluorophosphate.     

Separation and control of the elution order of N-t-butyloxycarbonyl amino acids D/L isomers by reversed-phase HPLC using cyclodextrins as chiral selectors for the mobile phase.

The enantiomeric resolution of N-t-butyloxycarbonyl (N-t-Boc) amino acids D/L isomers by reversed-phase HPLC was investigated using cyclodextrins (CD's) as chiral selectors for the mobile phase. The use of a low pH (pH<4) for the mobile phase enabled the enantioseparation of N-t-Boc amino acids. The opposite elution order of D/L isomers was observed when hydroxypropyl-derivatized beta-CD was used instead of native beta-CD. A computer simulation of the enantioseparation showed that the ratio of the retention factors of the chiral selector and the sample determined the elution order and the resolution. When the retention factor of the chiral selector is smaller than that of the sample, an isomer having larger complex formation constant eluted faster. However, when the chiral selector had a larger retention factor than the sample, an opposite elution order of the isomers was obtained. The large difference in the retention factors between the chiral selector and the sample led to good enantiomeric separation.     

True and false reversal of the elution order of barbiturates on a bonded cellulose-based chiral stationary phase

A set of racemic N-alkylated barbiturates were investigated for their direct enantioselective HPLC separation on a new chiral stationary phase (CSP) containing cellulose tris(3,5-dimethylphenylcarabamate) immobilized onto silica gel (Chiralpak IB) and its coated version (Chiralcel OD). They were online detected by UV and optical rotation detectors to trace the elution order of their enantiomers. Surprisingly, examples of false and true reversal of the elution order of enantiomers of barbiturates were observed and reported.     

Synthesis of chiral polyacetylenes carrying amino acids and azobenzenes and transformation of the higher order structure by photoirradiation

This study examined the synthesis of copolymers of an alanine-derived chiral N-propargylamide [N-(tert-butoxycarbonyl)-L -alanine N′-propargylamide] with an azobenzene-containing N-propargylamide [N-4-(phenylazo)benzoyl-L -alanine N′-propargylamide] and the transformation of the higher order structure of the copolymers by photoirradiation. The trans-azobenzene moiety of the copolymers isomerized into the cis one, which was accompanied by the transformation of the helical structure into a random state by UV irradiation. Visible-light irradiation induced the reisomerization of cis-azobenzene into the trans one, but it did not induce the recovery of the higher order structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4641–4647, 2004     

Chirality control in oligothiophene through chiral wrapping

[structure: see text] Mixing oligothiophenes and polysaccharides, such as amylose and schizophyllan, affords novel inclusion complexes, in which oligothiophene guests adopt twisted conformation in the chiral channel created by left- or right-handed helical wrapping with the polysaccharide host polymers, leading to optical activity.