Cholesteric mixture containing a chiral azobenzene-based dopant: material with reversible photoswitching of the pitch of the helix

A new low molar mass chiral-photochromic dopant was synthesized. It contains a menthyl fragment as the chiral group and an azobenzene group, capable of E - Z photoisomerization, as the photochromic component. The substance obtained was used as a chiral dopant in mixtures with a comb-shaped cholesteric acrylic copolymer with menthyl-containing chiral side groups and phenyl benzoate nematogenic side groups. Such mixtures form a cholesteric mesophase. The chiral dopant led to an additional twisting of the cholesteric helix, i.e. to a shift of the selective light reflection peak to a shorter wavelength region of the spectrum. The initial copolymer gave selective light reflection in the spectral range 1200-1400 nm; the mixture containing 3.5 mol % of chiral-photochromic dopant reflects light with λ_max~ 850 nm. The action of light with λ_ir~ 440 nm results in E - Z isomerization of the azo-group of the chiral dopant and in a shift of the selective light reflection peak to the long wavelength region of the spectrum (amplitude of shift = 30 nm). This is explained by a lower helical twisting power of the Z-isomer of the chiral dopant. This process is thermally reversible: annealing of irradiated films leads to a back shift of the selective light reflection peak to the short wavelength region of the spectrum due to Z - E isomerization. Kinetic features of the direct and backward processes of isomerization were studied: it was shown, that mixtures of the chiralphotochromic azobenzene-containing dopant with cholesteric polymers give new possibilities for the creation of polymer materials with a reversibly regulated helical supramolecular structure which determines their optical properties.     

Light-controlled supramolecular helicity of a liquid crystalline phase using a helical polymer functionalized with a single chiroptical molecular switch

Control over the preferred helical sense of a poly(n-hexyl isocyanate) (PHIC) by using a single light-driven molecular motor, covalently attached at the polymer's terminus, has been accomplished in solution via a combination of photochemical and thermal isomerizations. Here, we report that after redesigning the photochromic unit to a chiroptical molecular switch, of which the two states are thermally stable but photochemically bistable, the chiral induction to the polymer's backbone is significantly improved and the handedness of the helical polymer is addressable by irradiation with two different wavelengths of light. Moreover, we show that the chiral information is transmitted, via the macromolecular level of the polyisocyanate, to the supramolecular level of a lyotropic cholesteric liquid crystalline phase consisting of these stiff, rodlike polymers. This allows the magnitude and sign of the supramolecular helical pitch of the liquid crystal film to be fully controlled by light.     

A comparative study of photo-optical behaviour of photosensitive chiral copolymers with cholesteric mesophases induced in nematogenic and smectogenic matrices

The photo-optical behaviour of two series of chiral photochromic acrylic copolymers with a chiral nematic phase has been studied. These copolymers contain identical chiral photochromic units, but have different structures of the phenyl benzoate mesogenic side groups which are responsible for the development of LC phases. This approach allowed us to examine specific features of the photo-optical behaviour of the copolymers as a function of the nature of the LC 'matrix' in which the cholesteric phase was induced. The action of UV irradiation was shown to lead to the E-Z isomerization of the chiral side groups and, as a consequence, to untwisting of the cholesteric helix of the copolymers. For copolymers of both series, the effective quantum yields of this photochemical process were calculated. In the case of copolymers in which the cholesteric mesophase is induced in a smectogenic matrix, the corresponding values of the quantum yield are lower and depend slightly on temperature. A plausible explanation of the above phenomena is suggested.     

Photo-optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer-liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E-Z isomerization under UV irradiation was used. A merocianine-type substance was selected as a dichroic dye. Introduction of a dye-doped cholesteric mixture with a helical pitch higher than ∼300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal-polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Unexpected photoinduced phenomena in chiral–photochromic cholesteric copolymers with a triplet sensitizer.

For the first time the possibility of energy transfer from a triplet photosensitizer to chiral–photochromic fragments in photoactive cholesteric systems was demonstrated. For this purpose we prepared mixtures containing chiral–photochromic cholesteric copolymers with a triplet sensitizer—acrydine orange (AO). Chiral–photochromic groups in copolymers contain a C=C bond capable for undergoing E–Z isomerization during UV irradiation. All polymer mixtures form a chiral nematic phase displaying selective light reflection with _max~650–1,000 nm depending on the structure and concentration of the chiral groups. Irradiation of mixtures by visible light (>450 nm) leads to a shift of the selective light reflection peak to a long-wavelength spectral region. This effect is associated with a decrease of anisometry of chiral–photochromic fragments in copolymers during their E–Z isomerization. It is important to emphasize, that chiral–photochromic side groups of copolymers do not absorb visible light themselves; therefore, the previously mentioned changes can be explained by the energy transfer from photoexcited AO molecules in the triplet state to isomerizable fragments. The study of the kinetics of this process revealed a rather unexpected phenomenon: after the first 60–80-min irradiation, the helix pitch of the supramolecular structure of the mixtures increases, but after successive irradiation the helix pitch decreases. The possible explanations of this phenomenon were suggested. It was demonstrated that these mixtures may be used for irreversible recording of optical information.     

Photoregulation of polymer conformation by photochromic moieties. II. Cationic and neutral moieties on an anionic polymer

Abstract— Photoregulation of the conformation of poly (methacrylic acid) (PMA) in the range 4·5 < pH > 6·5 has been achieved by means of the cationic photochromic ligand, p-phenyl-azophenyl trimethyl ammonium iodide (PTA), as well as by means of copolymerization of methacrylic acid (MA) with the neutral photochromic co-monomer, p-phenyl azomethacrylanilid (PM). At degrees of ionization α, corresponding to the conformational unfolding of PMA, the pK_app is 0·2 units higher when the ligand PTA is trans than when PTA is in the photostationary 80%“cis” state. Equilibrium dialysis shows that the binding of PTA is independent of the cis/trans ratio at high α (after the conformational unfolding), but at lower αtrans-PTA binds positively and cis-PTA negatively. In the transition region, the presence of non-polar trans-azo ligand is therefore held responsible for a shift of the conformational equilibrium toward the more compact, folded form, which is harder to ionize than the unfolded form. Because of the relation pK_app# pH + log [(1 –α)/α], the degree of ionization of PMA, and thus the polymer conformation, as reflected in the reduced viscosity, can be photochemically regulated in pH buffers. At pH # 5·5 (α# 0·20) a maximal relative shift of Δα/α= 20 per cent was found, with a corresponding relative shift in reduced viscosity of 20·5 per cent. In the photochromic copolymer a pK_app switch of similar magnitude was found. In this case the azo-moiety is permanently affixed to the PMA so that the photosensitivity of the pK_app in the range of the conformational unfolding is due to a shift in the conformational equilibrium induced by the presence of non-polar trans-azo and polar “cis” azo, respectively. In both cases only about 1 photochromic moiety per 100 monomer units is capable of appreciably shifting the conformational manifold of PMA and/or the degree of ionization in constant pH buffers, provided one stays close to the unfolding transition. Mention is made of the implications for photoregulation of the permeability of, and the potential across. model membranes made of photochromic PMA molecules.     

Photosensitive chiral polyisocyanates

The dynamic polyisocyanate helix amplifies changes in the conformation of its side groups. Thereby it acts like a fast responding switch for optical properties. Here we show how the photoisomerization of chiral azo side groups can be used to induce large changes of chirooptical properties. These changes can be detected by CD measurements or by ORD measurements far from the absorption region. Large changes of the optical rotation can be induced reversibly in a multicycle process.     

Photosensitive cholesteric polymers with azobenzene-containing chiral groups and mixtures of cholesteric copolymer with chiral-photochromic dopants

New chiral photochromic cholesteric comb-shaped acrylic copolymers and low molecular mass dopants containing azobenzene photosensitive fragments and chiral groups based on menthol and menthone were synthesized. For the copolymers and their mixtures with low molecular mass dopants, the phase behaviour and optical properties were studied. Under irradiation with UV and visible light, the untwisting of cholesteric helix takes place, and the selective light reflection maximum is shifted to the long wavelength spectral region. This shift is related to the E-Z isomerization of the azobenzene chiral groups. For the copolymers and mixtures of the cholesteric polymer with the menthyl-containing dopant, this process is thermally reversible. The specific features of the kinetics of the forward and the reverse thermal processes were characterized. It was demonstrated, that the copolymers and mixtures of the cholesteric copolymer with the menthyl-containing dopant may be used for coloured reversible recording of optical information. For such materials, their resistance with respect to the repeated 'recording-erasing' cycles was tested, and the fatigue resistance was shown to be rather high.     

Photo‐optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer–liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E–Z isomerization under UV irradiation was used. A merocianine‐type substance was selected as a dichroic dye. Introduction of a dye‐doped cholesteric mixture with a helical pitch higher than ～300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal–polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Gel formation and photoactive properties of azobenzene-containing polymer in liquid crystal mixture

For the first time, a photochromic azobenzene-containing liquid crystalline (LC) acrylic polymer was used for gelation of low-molar-mass nematic mixture (LMNM). Dissolution of LC polymer in amount of only 2.5 wt.% in LMNM at 120°C (isotropic state) followed by cooling down results in formation of the solid-like photochromic LC gel. Gelation is associated with a phase separation and formation of microsized LC polymer domains, which form a physical “network” containing encapsulated nematic host. Textural changes of mixture during gel formation were analyzed, and absorbance spectra were measured. A special attention was paid to the kinetic study of photoinduced E-Z and Z-E isomerization of azobenzene side groups of polymer in gel. It was shown that ultraviolet (UV)-irradiation and E-Z isomerization processes are accompanied by disruption of H-aggregates of azobenzene moieties and partial dissolution of polymer.     

Photochromism and electrochemistry of a dithienylcyclopentene electroactive polymer

A bifunctional substituted dithienylcyclopentene photochromic switch bearing electropolymerisable methoxystyryl units, which enable immobilization of the photochromic unit on conducting substrates, is reported. The spectroscopic, electrochemical, and photochemical properties of a monomer in solution are compared with those of the polymer formed through oxidative electropolymerization. The electroactive polymer films prepared on gold, platinum, glassy carbon, and indium titanium oxide (ITO) electrodes were characterized by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The thickness of the films formed is found to be limited to several monolayer equivalents. The photochromic properties and stability of the polymer films have been investigated by UV/vis spectroscopy, electrochemistry, and XPS. Although the films are electrochemically and photochemically stable, their mechanical stability with respect to adhesion to the electrode was found to be sensitive to both the solvent and the electrode material employed, with more apolar solvents, glassy carbon, and ITO electrodes providing good adhesion of the polymer film. The polymer film is formed consistently as a thin film and can be switched both optically and electrochemically between the open and closed state of the photochromic dithienylethene moiety.     

Large Thermally Irreversible Photoinduced Shift of Selective Light Reflection in Hydrazone-Containing Cholesteric Polymer Systems

Stimuli responsive liquid crystalline polymers are a unique class of so-called “smart” materials demonstrating various types of mesomorphic structures easily controlled by external fields, including light. In the present work we synthesized and studied a comb-shaped hydrazone-containing copolyacrylate exhibited cholesteric liquid crystalline properties with the pitch length of the helix being tuned under irradiation with light. In the cholesteric phase selective light reflection in the near IR spectral range (1650 nm) was measured and a large blue shift of the reflection peak from 1650 nm to 500 nm was found under blue light (428 or 457 nm) irradiation. This shift is related to the Z-E isomerization of photochromic hydrazone-containing groups and it is photochemically reversible. The improved and faster photo-optical response was found after copolymer doping with 10 wt % of low-molar-mass liquid crystal. It is noteworthy that both, the E and Z isomers of hydrazone photochromic group are thermally stable that enable to achieve a pure photoinduced switch without any dark relaxation at any temperatures. The large photoinduced shift of the selective light reflection, together with thermal bistability, makes such systems promising for applications in photonics.     

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer

Abstract  A novel chiral–photochromic side-chain polyacrylate with azobenzene fragments in the side groups has been synthesised. It was shown that the polymer forms a smectic phase and a cholesteric supramolecular helical structure with selective light reflection in IR spectral range. Thin spin-coated films of the polymer were prepared and their photooptical and chirooptical properties were studied in detail. It was found that UV irradiation of the films led to E–Z isomerization of the azobenzene moieties with high conversion, which is dependent on thermal prehistory of the films. Subsequent action of visible light results in partial recovery of the E-isomer content, whereas annealing leads to the full back conversion. Circular dichroism (CD) measurements revealed formation of the helical supramolecular structure even in the initial spin-coated polymer films. The E–Z isomerization induces complete disruption of helical order in non-annealed films of the polymer, whereas in the smectic phase of the annealed film only a significant decrease in CD values was found. In addition, the photoorientation phenomena induced by polarized light were studied. It was shown that polarized light induces linear dichroism in the films provided by azobenzene group orientation and the dichroism is stable at room temperature for a prolonged time. These combined chirooptical and photooptical features of this novel polymer enable one to consider this multifunctional compound as a promising material for photonics and for optical applications. Graphical abstract  

A comparative study of photo-optical behaviour of photosensitive chiral copolymers with cholesteric mesophases induced in nematogenic and smectogenic matrices

The photo-optical behaviour of two series of chiral photochromic acrylic copolymers with a chiral nematic phase has been studied. These copolymers contain identical chiral photochromic units, but have different structures of the phenyl benzoate mesogenic side groups which are responsible for the development of LC phases. This approach allowed us to examine specific features of the photo-optical behaviour of the copolymers as a function of the nature of the LC 'matrix' in which the cholesteric phase was induced. The action of UV irradiation was shown to lead to the E-Z isomerization of the chiral side groups and, as a consequence, to untwisting of the cholesteric helix of the copolymers. For copolymers of both series, the effective quantum yields of this photochemical process were calculated. In the case of copolymers in which the cholesteric mesophase is induced in a smectogenic matrix, the corresponding values of the quantum yield are lower and depend slightly on temperature. A plausible explanation of the above phenomena is suggested.     

The effect of the steric hindrance on metallic cation conduction in polymer electrolytes

2,6-Di-t-butylphenol and oligo(ethylene oxide) bound covalently to polyisocyanate were synthesized and characterized. The ionic conductivities of their Li, Na, and K phenolates were studied at various temperatures. The conductivities were in the range of 10^?7?10^?5 S/cm at 30°C. The conductivity of Na and K salts was approximately 10² greater than that of the Li salts. The t-butyl groups serve to dissociate K and Na ions from the phenoxide. The cations, therefore, are more mobile as a result increasing the conductivity. The temperature dependence of ionic conductivity suggests that the migration of ions is controlled by segmental motion of the polymer, shown by linear curves obtained in Vogel–Tammann–Fulchere plots. The polyisocyanate backbone is a rather stiff structure, however, a flexible oligo(ethylene oxide) side chain forms complexes with metal ion. Since the ion transport is associated with the local movement of polymer segments, the rigidity of the polymer backbone does not have much influence on the ion mobility.     

Photoinduced surface relief gratings in high-Tg main-chain azoaromatic polymer films

We have synthesized two classes of polyureas with mono- and bisazoaromatic groups in their main chains via reactions between isophorone diisocyanate and the corresponding diamines. Holographic gratings were fabricated on azoaromatic polyurea films prepared by spin-coating from solutions. The effect of high glass transition temperature and dipole moment of azo groups on the formation of gratings was investigated. Although the two polymers have relatively high glass transition temperatures (197 and 236°C), chromophore alignment was induced by laser beam irradiation at modest light intensities. Regularly spaced surface relief gratings on the polymer film were also recorded upon exposure to an interference pattern of two polarized argon laser beams. Erasure could be achieved by heating above T_g or by exposure to one of the beams in a manner similar to low-T_g side-chain azo polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 283–289, 1998     

Photo-orientation phenomena in photosensitive chiral nematic copolymers

Photo-orientational phenomena have been studied for two comb-shaped cholesteric copolyacrylates containing azobenzene side groups. Copolymer ^I contains nematogenic phenyl benzoate groups and photosensitive chiral menthyl-containing azobenzene side groups. Copolymer ^II is composed of nematogenic phenyl benzoate groups, photosensitive cyanoazobenzene groups and chiral photochromic benzylidene- ^p -methan-3-one fragments. Under the action of polarized Ar⁺ laser light (488 nm), orientation of the side groups of the copolymers takes place, and this orientation is perpendicular to the vector of the electric field of the incident light. This process shows a co-operative character; that is, it involves both photosensitive azobenzene and phenyl benzoate groups. The kinetics of growth of the photoinduced orientational order parameter were studied as a function of film thickness, incident light intensity, and preliminary UV irradiation. For the planar oriented films of the copolymers, irradiation with polarized light leads to the development of photoinduced birefringence δ ⁿ ; maximum values of δ ⁿ reach 0.01. The photo-optical properties of copolymers ^I and ^II are compared. Such materials may be used for ‘dual’ data recording by varying the helix pitch, selective light reflection maximum, and photoinduced birefringence or linear dichroism.     

Self-assembled second order nonlinear optical multilayer azo polymer

An epoxy based polymer with nonlinear optical azo chromophores was designed to contain anionic groups to induce water solubility and self assembly. Using this polyanion with a polycation, multilayers were prepared on a glass substrate by alternating adsorption from dilute aqueous solutions. The azo chromophores in the confined layer of the polyanion in the multilayer films self-assemble into a noncentrosymmetric alignment and demonstrate second order optical nonlinearity (d₃₃ = 19 pm/V).     

Homo‐double helix formation of an optically active conjugated polymer bearing carboxy groups and amplification of the helicity upon complexation with achiral and chiral amines

An optically active, m‐terphenyl‐based π‐conjugated polymer bearing carboxy groups was synthesized by the copolymerization of the diethynyl monomer bearing a carboxy group with (S,S)‐2,5‐bis(2‐methylbutoxy)‐1,4‐dibromobenzene using Sonogashira reaction. The copolymer showed a weak circular dichroism (CD) in the main‐chain chromophore region due to a homo‐double helix formation with an excess helical handedness biased by the chiral alkoxy substituents through self‐association. However, upon complexation with achiral amines, such as piperidine, the CD intensity of the polymer significantly increased resulting in the formation of a greater excess one‐handed homo‐double helix via hydrogen‐bonded inclusion complexation with the achiral amines between each strand, leading to the amplification of the helicity. A preferred‐handed homo‐double helix was also induced in the polymer in the presence of nonracemic amines. The effect of the achiral and chiral amines on the homo‐double helix formation was investigated by comparing the CD spectra of the polymer to those of its model dimer. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 990–999     

Editorial

Abstract

Asymmetric syntheses of optically active polymethacrylate, polyacrylate, polyacrylamide, and polyisocyanate with helical conformation and their chiral recognition abilities are described. 1-Phenyldibenzosuberyl methacrylate (PDBSMA) gave a purely onehanded-helical, optically active polymer ([α]₃₆₅ +1670 ～ +1780º) with almost perfectly isotactic structure by anionic polymerization using optically active initiators. Radical polymerizations of PDBSMA using chiral initiators, chain transfer agents, and additives also afforded optically active polymers with a prevailing onehanded helicity. Triphenylmethyl acrylate yielded an optically active, helical polymer ([α]₃₆₅ +102º) having a dyad isotacticity of 70% using an optically active anionic initiator. Although the polyacrylate demonstrated chiral recognition ability as a chiral stationary phase for HPLC, the ability was low mainly because of the low degree of one-handedness. N-(3-Chlorophenyl)-N-phenylacrylamide gave an optically active, helical polymer ([α]_365–343º) in the asymmetric anionic polymerization; the polymer had a dyad tacticity of 77%. Optically active polyisocyanates with a predominantly one-handed helical conformation were prepared in homo-and co-polymerization of optically active phenyl isocyanate derivative. These polyisocyanates showed the ability to discriminate enantiomers in solution.