Synthesis and chiroptical properties of liquid crystalline copolymers containing azobenzene chromophores

A series of new liquid crystalline copolymers, poly[((S)‐2‐methyl‐1‐butyl methacrylate)‐co‐(6‐(4‐(4‐cyanophenylazo)phenoxy)hexyl methacrylate)], with different contents of chiral units of 17, 36, 54 and 78 mol% were synthesized. The structures and properties of the copolymers were characterized and evaluated using infrared, ¹H NMR and UV spectroscopy, differential scanning calorimetry, gel permeation chromatography and circular dichroism (CD). The CD results suggested that absorptions of azobenzene chromophores were observed in films of copolymer containing 17, 36 or 54 mol% chiral units, but not in the film of copolymer containing 78 mol% chiral units. Also, CD values of the copolymeric films decreased with increasing chiral content. After irradiation with linear polarized light at 442 nm, CD values were changed in all the copolymeric films, and the CD values increased with decreasing chiral content in a nonlinear way, while the photoinduced change of chirality of the copolymers increased in a linear way with decreasing chiral content. The results are discussed in terms of interactions between structures and chiroptical properties. Copyright © 2007 Society of Chemical Industry     

Standardization of photoresponsive LB films fabricated from simple azobenzene derivative

It is believed that azobenzene amphiphiles tend to be strongly H-aggregated during pure Langmuir-Blodgett (LB) film manipulation and therefore they are unable to photoisomerize. However, we present in this article the manner for destabilizing such aggregation by means of a simple physical treatment process, obtaining finally well photoresponsive amphiphilic azobenzene in the pure LB films. Such LB films are highly suitable for regulating the orientation and photoreactivity of other surface-in touch systems such as nematic liquid crystals and photoreactive monomers to be polymerized. Therefore, we investigated the manipulation of LB films of the azobenzene derivative, 11-(4-(4-hexyl-phenylazo)-phenoxy)-undecanoic acid using different monomeric isomers, by temperature and monolayer transfer methods. The LB films were then exposed to heat treatment process and finally to photophysical processes.     

Aggregation behavior of dabsylated poly(ε-L-lysine) in aqueous DMSO solution

Summary Side chain amino groups of microbial Poly(ε-L-lysine) (ε-PL) were chemically modified with an amino labeling reagent, 4-dimethylaminoazobenzene-4’-sulfonyl chloride (dabsyl chloride) to give dabsylated ε-PL (Dabs-ε-PL). Aggregation behavior of Dabs-ε-PL in aqueous DMSO solution was studied through UV-Vis and CD spectra. In DMSO solution, Dabs-ε-PL shows a symmetric visible absorption at 450 nm, corresponding to the azobenzene chromophore in the monomeric state. Upon addition of water to the DMSO solution, the λ_max is shifted to around 400 nm. CD spectra were detected in the region of the blue shifted band: a pair of CD signals of opposite sign appears with a crossover point related to the shifted λ_max. Similar trends were also observed for UV-Vis and CD spectra of dabsylated oligo(ε-L-lysine)mers. These spectroscopic results indicate that side chain azo chromophores of Dabs-ε-PL form H-aggregates and are disposed according to a chiral arrangement with right handedness in aqueous DMSO solution.     

Electrochemical and photoelectrochemical response of electrodes coated with LB films of an azopolymer

This paper describes the electrochemical and photoelectrochemical behaviour of electrodes coated with Langmuir-Blodgett (LB) films of an azopolymer. The coating material used is a pyridine azopolymer (PAzPy) obtained by free radical polymerization of 6-[2-(4-pyridyazo)phenoxy]hexyl methacrylate (AzPy). Cyclic voltammetry experiments of LB films deposited at several transference surface pressures were performed to analyze the effect of the molecular packing on the electrochemical response. The influence of the pH of the electrolytic solution was also considered. AFM images have helped in the interpretation of the molecular architecture influence on the redox activity of the films. From the experimental results it was concluded that PAzPy is situated with the nitrogen from the pyridine group close to the ITO electrode surface in the LB films, which allows a direct electron transfer between the electrode surface and the azobenzene group leading to a quick electrochemical response of the films. The azobenzene electrochemical activity and the kinetics of the process are also highly dependent on the proton transfer process between the electrolytic solution and the azobenzene unit. The efficiency of the proton transfer process is determined by the pH of the electrolytic solution as well as by the molecular architecture of the film. The results presented in this paper show that, under optimal conditions, both the percentage of electroactive azobenzene chromophores and the standard heterogeneous rate constant of electron transfer are higher for PAzPy arranged in LB films compared with the values so far reported for azobenzenes of lower molecular weight.     

Synthesis and characterization of photoactive chiral copolymers of (S)-N-(1-phenyl ethyl) methacrylamide and disperse red 1 methacrylate

In this article, we report the synthesis and characterization of chiral copolymers containing a photoactive azobenzene pendant group. The chiral copolymers of (S)-N-(1-phenyl ethyl) methacrylamide (S-NPEMAM) and disperse red 1 methacrylate (DR1MA) with two different feed compositions were synthesized by free-radical polymerization with 2,2′-azobisisobutyronitrile as an initiator. The purified chiral copolymers, poly[(S)-N-(1-phenyl ethyl) methacrylamide-co-disperse red 1 methacrylate], were characterized by Fourier transform infrared spectroscopy, NMR, ultraviolet–visible spectroscopy, and gel permeation chromatography analyses. The photoluminescence and chiroptical properties of the copolymers were evaluated. Induced circular dichroism (CD) was observed in the CD spectra, which arose because of the exciton splitting originated by the cooperative interactions between the pendant azobenzene chromophore of DR1MA and the benzene ring of the chiral monomer. The glass-transition temperatures of the chiral copolymers were observed to be above 117°C, and the chiral copolymers were thermally stable up to 250°C. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Chirality delivery through multiple and helical H-bonding from chiral coordination complex to its supramolecular architecture

The path of the chirality delivery in the crystalline and chiral nucleotide–Co(II) complex, [Co(GMP)(H₂O)₅]·3H₂O (GMP = guanosine-5′-monophosphate), has been studied based on X-ray single crystal diffraction analysis, liquid- and solid-state circular dichroism (CD) spectroscopy. The multiple and helical H-bonding is a distinctive way of chirality delivery from the discrete molecules to three-dimensional supramolecular architecture.     

A study of photooptical processes in photosensitive cholesteric azobenzene-containing polymer mixture under an action of the polarized and nonpolarized light

New liquid crystalline cholesteric mixture consisting of azobenzene-containing side-chain copolymer and the chiral low-molar-mass dopant (5%) was prepared and its properties were studied. It was shown that freshly prepared spin-coated thin films are optically isotropic. However, the films possess a noticeable circular dichroism with a maxima corresponding to the absorbance of the nonchiral azobenzene fragments. This phenomenon is an evidence for the existence of the helical supramolecular structure elements in the mixture films. An annealing of the films at temperatures above the glass transition leads to the strong increase in the circular dichroism due to formation of the perfect cholesteric phase. UV light irradiation results in the E-Z isomerization of azobenzene groups and the significant drop in circular dichroism values (almost to zero) both for the fresh and the annealed films that is associated with the transition from cholesteric to the fully isotropic state. An action of polarized UV and visible light leads to the photoorientation of azobenzene groups perpendicular to the electric vector of the polarized light. Kinetic features of the photoorientation processes in cholesteric phase in comparison with nematic systems were studied. It has been experimentally found that the helical order to some extent prevents the photoorientation of the side groups. Upon UV irradiation of the thick, planarly-oriented films of the mixture at temperatures higher than glass transition, the selective light reflection maximum is shifted to the long-wavelength spectral region. All photoinduced phenomena occurring in the mixture are thermally reversible and annealing of the films at temperatures above glass transition results in the back recovery to the cholesteric nonoriented state and to the initial helix pitch value.     

Chirality amplification of syndiotactic polystyrene induced circular dichroism chiral film in δ form upon annealing

The induced circular dichroism (ICD) syndiotactic polystyrene (sPS) chiral films in sPS δ phase were prepared and the chirality of the ICD sPS films upon annealing at different temperature was investigated. The sPS δ crystals in the ICD films transform into γ and then into α crystals as those of achiral sPS δ crystals during annealing at different temperature. The CD signal intensity at λ of 200 nm remains unchanged during the δ to γ crystalline transformation, but enhanced tremendously during the γ to α crystalline transformation. The amplification CD signal attributes to the closer package of the phenol rings in sPS α phase than that in sPS δ and γ phases. The lamellae grown at a temperature above the γ to α transition follow the chiral aggregation structure, which also contribute to the amplification of the CD signal.     

Flow Effects in Supramolecular Chirality

There are an increasing number of reports on the emergence of circular dichroic signals when solutions of J-aggregates are vortex-stirred. Some authors claim that the transfer of chirality from the macroscopic stirring vortex occurs at the level of electronic transitions. Others argue that the signal observed is not due to true circular dichroism (CD), but rather that it is an artifact due to combinations of linear polarization contributions. For both interpretations it is necessary to assume that the alignment by effect of flows dominates over the Brownian dynamics. Therefore, the topic belongs to supramolecular chemistry because the alignment in current flows can only occur for relatively large particles with an appropriate shape. The phenomenon is detected by CD spectroscopy, which itself is also the main hindrance, even controversial, in its study and interpretation. However, modern Mueller matrix polarimetry can solve some of the problems. The phenomenon has been explained using different scenarios; however, in some of the systems the experimental evidence only supports the model in which deformation (folding/torsion) of elastic soft-matter yields true CD. Here we discuss the role of the gradient of shear rates in the deformation of nano particles of soft matter to give chiral supramolecular structures, as the first step to explaining the phenomenon.     

Control of polymerization processes of 10,12-pentacosadiynoic acid LB films

Studies have been carried out on KrF excimer laser light (EX), X-ray or electron beam (EB) induced polymerization of 10,12-pentacosadiynoic acid (PDA) Langmuir-Blodgett (LB) films in relation to molecular density or molecular arrangement of the films using X-ray diffraction analysis, infrared (IR) spectroscopy and Raman spectroscopy. The molecular arrangement or density of the PDA LB films was controlled by subphase conditions when the films were built up, such as pH, temperature of a subphase or salt concentration in the subphase. Polymerization sensitivity of the PDA LB film was affected by the arrangement or molecular density. On low density (A type) films the polymerization occurred by irradiation with EX, X-ray or EB, but on high density (B type) films the polymerization occurred only when the irradiation was carried out by high energy beams such as X-ray or EB. Decomposition of polymerized films was observed further by excessive irradiation of EX or X-ray, but not on the B type films. It was revealed by X-ray diffraction analysis that in the A type film, the PDA molecules bent to a larger extent than those in the B type film and the polymerization proceeded topochemically, that is, the thickness decreased little after EB irradiation in a helium atmosphere. On the other hand, in the B type film, the thickness decreased by about 10% as a result of EB irradiation. By IR reflection-absorption (RA) and Raman measurements, it was confirmed that conjugated diacetylenic bonds disappeared and conjugated double and new conjugated triple bonds appeared after high energy beam irradiations. These results support the supposition that 1,4-polymerization, i.e. polydiacetylene type polymerization, occurs easily in the A type film and 1,2- or 3,4-polymerization, i.e. polyacetylene type polymerization, occurs in the B type film, and that the polymerized A type film was decomposed at the polydiacetylenic bond when the irradiation continued further. It was also shown that the polyacetylene type polymer was obtained only when the B type film was irradiated with the high energy beam.     

Four chiral Ag(I) complexes with dual chiral components: Effect of positional isomerism,luminescence and SHG response

Four chiral Ag(I) complexes with dual chiral components have been synthesized by using a series of (1R,2R)-N¹,N²-bis(pyridinylmethyl)cyclohexane-1,2-diamine ligands with different N-positions of pyridyl rings. The circular dichroism (CD) spectra and second-harmonic generation (SHG) efficiency measurements confirmed that they are of structural chirality in the bulk samples. The luminescence properties indicated that they may have potential applications as optical materials. The formation of discrete mononuclear and binuclear complexes and one-dimensional chains may be attributed to positional isomerism of the ligands.     

Sugar-Induced Chiral Orientation of Boronic-Acid-Appended Amphiphile Clusters Formed in the Dipalmitoylphosphatidylcholine (DPPC) Membrane

A boronic-acid-appended amphiphile bearing an azobenzene chromophore in the chain center ( 3 ) was synthesized. Although 3 could not form the membrane-like, ordered aggregate by itself, it formed a phase-separated aggregate in a dipalmitoylphosphatidylcholine (DPPC) matrix membrane. When saccharides were added, the boronic acid group reversibly formed the saccharide complexes, and 3 in the DPPC matrix membrane became CD-active with the appearance of exciton-coupling bands. Comparison of the saccharide absolute configuration with the CD intensity established that the saccharide possessing the OH group (as 3-OH, 4-OH, and 5-CH₂OH) in the same side as the cis-1,2-diol gives the strong CD band. Judging from the structure of 3 –saccharide complexes, these “same-side” OH groups can enjoy intermolecular hydrogen-bonding interactions, which eventually induce the chiral orientation of azobenzene chromophores. This is a novel membrane system useful to read out the information stored in the saccharide structure and to create novel membrane structures by added saccharides.     

Preparation and characterization of novel optically active polyurethanes containing 1,1′‐binaphthol

Novel optically active polyurethanes (BPUs) based on chiral 1,1′‐binaphthol were synthesized via direct hydrogen transfer addition polymerization. The polymers were analyzed by FTIR, ¹H NMR, DSC‐TGA, CD spectra.The results showed that the specific rotation [α]²⁵_D were ?78.0° and +54.6° for the S‐BPU and R‐BPU respectively, and these polymers showed better thermal stability. The circular dichroism spectra of the chiral polymers were almost identical except that they gave opposite signals at each wavelength, and the infrared emissivity values of the S‐BPU and R‐BPU were 0.618 and 0.682, they displayed low infrared emissivity. Meantime the polymers implanted with PEG group exhibit better solubility, however thermal stability reduced to some extent. Some properties of the new optically active polyurethanes were reported. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Crystal structure and chirality of adenosine-5′-diphosphate coordination complex

The first neutral coordination complex of adenosine-5′-diphosphate (ADP) with transition metal Cu(II) ion, [Cu(phen)(H₂ADP)(NO₃)]·H₂O (ADP–Cu) (ADP = adenosine-5′-diphosphate, phen = 1, 10-phenanthroline monohydrate) has been synthesized and investigated by X-ray single-crystal diffraction method. Based on its crystal structure and supramolecular assembly analysis, the molecular and supramolecular chirality of ADP–Cu was studied comprehensively by liquid- and solid-state circular dichroism (CD) spectroscopy. It is the first example of the study about the chirality and supramolecular chirality for polyphosphate nucleotide coordination complex.     

Engineering Achiral Liquid Crystalline Polymers for Chiral Self-Recovery

Flexible construction of permanently stored supramolecular chirality with stimulus-responsiveness remains a big challenge. Herein, we describe an efficient method to realize the transfer and storage of chirality in intrinsically achiral films of a side-chain polymeric liquid crystal system by combining chiral doping and cross-linking strategy. Even the helical structure was destroyed by UV light irradiation, the memorized chiral information in the covalent network enabled complete self-recovery of the original chiral superstructure. These results allowed the building of a novel chiroptical switch without any additional chiral source in multiple types of liquid crystal polymers, which may be one of the competitive candidates for use in stimulus-responsive chiro-optical devices.     

Chirality Emergence in Thin Solid Films of Amino Acids by Polarized Light from Synchrotron Radiation and Free Electron Laser

One of the most attractive hypothesis for the origin of homochirality in terrestrial bioorganic compounds is that a kind of “chiral impulse” as an asymmetric excitation source induced asymmetric reactions on the surfaces of such materials such as meteorites or interstellar dusts prior to the existence of terrestrial life (Cosmic Scenario). To experimentally introduce chiral structure into racemic films of amino acids (alanine, phenylalanine, isovaline, etc.), we irradiated them with linearly polarized light (LPL) from synchrotron radiation and circularly polarized light (CPL) from a free electron laser. After the irradiation, we evaluated optical anisotropy by measuring the circular dichroism (CD) spectra and verified that new Cotton peaks appeared at almost the same peak position as those of the corresponding non-racemic amino acid films. With LPL irradiation, two-dimensional anisotropic structure expressed as linear dichroism and/or linear birefringence was introduced into the racemic films. With CPL irradiation, the signs of the Cotton peaks exhibit symmetrical structure corresponding to the direction of CPL rotation. This indicates that some kinds of chiral structure were introduced into the racemic film. The CD spectra after CPL irradiation suggest the chiral structure should be derived from not only preferential photolysis but also from photolysis-induced molecular structural change. These results suggest that circularly polarized light sources in space could be associated with the origin of terrestrial homochirality; that is, they would be effective asymmetric exciting sources introducing chiral structures into bio-organic molecules or complex organic compounds.     

Synthesis and conformational study of novel,stable, helical poly(N-propargylamides) containing dipole azobenzene chromophores in the side chains

A series of novel N-propargylamides carrying dipole azobenzene chromophores were synthesized and polymerized with [Rh(nbd)Cl]₂ catalyst to obtain cis-transoid poly(N-propargylamides). The solubility, thermal stability and conformation of these polymers were also studied. The introduction of valeryl group increased the solubility of this kind of poly(N-propargylamides). All the polymers exhibited a good, thermal stability. CD and UV–Vis spectra showed that poly(N-propargylamide) (Poly(V_2b)) took a tight helical structure with predominantly one-handed screw sense. Poly(V_2b) exhibited a good stable helical structure at various temperatures (5–60 °C). It also maintained good helicity in polar solvent. The improved helix stability could be attributed to the enhanced hydrogen bonding strength and steric repulsion between the newly designed azobenzene groups. Furthermore, the electrostatic repulsion between the dipole azobenzene chromophores in the side chains improved the helix stability as well. The backbone of Poly(V_2b) still kept helical sense after the isomerization of trans-azobenzene to the cis form under UV irradiation.     

Induction of chirality into a fully sulfonated poly(methoxyaniline) via acid-base interactions with chiral amines

A wide range of chiral amines and amino alcohols associate with poly(2-methoxyaniline-5-sulfonic acid) (PMAS) in aqueous solution, from which optically active PMAS·(amine) films can be cast. The chiral induction is believed to be initiated by acid-base interactions with “free” sulfonic acid groups on the PMAS chains. Chiral amine:PMAS dimer molar ratios as low as 1:4 give PMAS·(amine) films with similar optical activity to those cast from 1:1 molar mixtures, indicating that only one in four of the “free” sulfonate groups on the PMAS chains need to be electrostatically bound by chiral ammonium ions to achieve optimal chiral induction. Circular dichroism studies show that the enantiomeric amines (R)-(+)- and (S)-(−)-1-phenylethylamine induce the opposite helical hands for the supermolecular assemblies of PMAS chains. However, there is no clear correlation between the sign of the CD signals for the PMAS·(amine) films and the configuration of structurally diverse amines.     

Comparison of hydrogen bonding for chiral and racemic 1-monostearoylglycerols in solvent

Comparison of the hydrogen bonding of racemic RS-1-monostearoylglycerol and chiral S-1-monostearoylglycerol in benzene was carried out through the NMR measurement. In addition, the concentration dependence of a chirality of S-1-monostearoylglycerol in hexane was studied though UV and circular dichroism (CD) measurements. The chemical shifts of OH protons of the S- and RS-1- monostearoylglycerols indicated that the hydrogen bonding between the R- and S-form molecules of RS-1-monostearoylglycerol is stronger than that between the S- and S-form molecules of S-1-monostearoylglycerol in the low concentration region and that the difference in the strength of hydrogen-bonding between them becomes small in the high concentration region. The chirality of the S-1-monostearoyglycerol in hexane decreased with increasing its concentration. This suggests that the association of chiral S-1-monoacylglycerol arising from its increasing concentration reduces the chirality of S-1-monostearoylglycerol.     

Photo-reversible Reaction in Polymer Langmuir-Blodgett Films

Preparation of photoreactive Langmuir-Blodgett (LB) films was carried out using polymers/oligomers prepared from long-chain dialkyl esters of di-and tetra-olefins and having cyclobutane rings in the main chain. These polymers/oligomers formed stable monolayers on a water surface when mixed with arachidic acid. These monolayers could be deposited onto a substrate successfully forming Y-type films. The oligomer LB films were polymerized on irradiation. The polymer LB films showed a photo-reversible process between polymers and oligomers, depending on wavelength of the irradiating light. On the basis of spectral data and molecular weight change, this behaviour was found to be caused by cleavage and formation of cyclobutane rings.