Photoinduced Optical Anisotropy Research Articles

Thermally stable birefringent films from a liquid crystalline polymer having side groups containing N-benzylideneaniline

Polymethacrylate liquid crystalline polymer having side groups containing N-benzylideneaniline moiety was synthesized, and its photoinduced reorientation behavior was investigated using linearly polarized UV light. Films of these polymer exhibited axis-selective trans-cis-trans photoreorientation of the side groups upon irradiation. Annealing the film in the liquid crystalline temperature range of the polymer after irradiation greatly amplified their otherwise minimal photo-induced optical anisotropy. Due to the extended mesogenic structure of the side groups, the polymer showed significant thermal stability and a high degree of optical anisotropy after annealing.

Photopatterning of Azobenzene-Containing Liquid Crystalline Triblock Copolymers: Light-Induced Anisotropy and Photostabilization.

Photochromic liquid crystalline block copolymers (PLCBCs) are currently playing a significant role as light-responsive materials because of their light controllable features over multiple length scales. Herein, a study of the photoinduced optical anisotropy derived by the combination of orientation phenomena at molecular and supramolecular levels in a novel kind of side-chain PLCBCs with mesogenic phenyl benzoate groups and pyridine units that is hydrogen bonded with azobenzene-containing phenol is reported. Based on the polymeric architectures and composition, the supramolecular configuration self-organizes in different microphases that affect the material response to the external stimuli. Simple, 1D, polarization holograms are recorded to evaluate the photoinduced birefringence. The first step, light patterning, involves the orientation of the azobenzene units and precedes a thermal treatment that amplifies the induced anisotropy through the cooperative orientation of the mesogenic units. By selective extraction, the azobenzene units can be removed, making the material transparent to the visible light. Excellent photostability of the material birefringence is obtained, whose final value is strongly affected by the block copolymer's architecture. The versatility in the molecular design, the fine control of the photoinduced features by external parameters, and, finally, the possibility to achieve photostability make these materials of great potential for developing optical and photonic devices.

Electrooptical Effect in Polymeric Composites Containing a Heterometallic Cu/V/Mo Complex

The influence of an electric field on the transmission of linearly polarized light by polyvinylbutyral composite films doped with particles of the heterometallic complex (NH4)2{[Cu(dien)(H2O)]2[α-V2Mo6O26]}∙5H2O (dien = diethylenetriamine) was studied. The absorption became anisotropic if an electrostatic field was applied during irradiation of the composites by linearly polarized light. The observed electrooptical effect was explained by photo-induced optical anisotropy that was caused by the linearly polarized light and changed in space due to reorientation of the dipole moment of photogenerated electron–hole pairs in the composite under the action of the external electric field.

A family of azoquinoline derivatives: Effect of the substituent at azo linkage on thermal cis-trans isomerization based on an experimental and computational approach

The subject of the research is a series of azo-dyes based on the quinoline structure, which are practically omitted as materials which use photoinduced optical anisotropy for applications. The work presents the kinetics of thermal cis-trans isomerization reaction of 5-azo-8-hydroxy-2-methylquinoline and 5,5'-((1E,1′E)-[1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(2-methylquinolin-8-ol) compounds. The impact of the structure of the substituent at the azo linkage on the cis-trans isomerization kinetics was investigated experimentally and theoretically using the UV–Vis spectroscopy and the density-functional theory (DFT). The kinetics of the dark-return of the cis-to trans-isomer of azoquinolines was investigated after UV-irradiation. It was found that attaching the nitro substituent to the azoquinolines structure retards the photoisomerization process in comparison to the other substituents (i.e. F, Cl, Br, N atoms or CH3, OH, COOH, OCH3 groups). The calculated rate constants were in qualitative agreement with the experimental values. To the best of our knowledge, the cis-trans isomerization kinetics for azoquinolines has not been studied yet.

ВЛИЯНИЕ ПРИРОДЫ ЗАМЕСТИТЕЛЯ В АЗОБЕНЗОЛЬНЫХ ХРОМОФОРАХ НА ДИФРАКЦИОННУЮ ЭФФЕКТИВНОСТЬ ПОЛЯРИЗАЦИОННЫХ ГОЛОГРАММ

The films of polymeric composites containing monomers of azobenzene dyes or azobenzene lateral groups chemically bonded to the main polymer chain demonstrate photoactive properties, and can be used in electrooptical light modulators and in recording media (RM) for polarization holography. Photoinduced optical anisotropy (PIA) appears under influence of linearly polarized light caused by the processes of trans-cis-izomerization of the azobenzene groups. This process is determining for application of the considered materials as RM for polarization holography. The films of polymeric polarization sensitive media based on a copolymer of styrene with nonyl methacrylate doped with azobenzene-type dyes with different electron-acceptor substituents are obtained in this work. In the films of copolymer the holograms of the plane wave front were registered for parallel and orthogonal orientations of polarization vectors of the object and reference light beams. Their photoelectrophysical, in particular photodielectric, and information properties are investigated. It was established, that when the mutual orientation of the recording beams changes from the orthogonal to parallel orientation the diffraction efficiency of the holograms increases in the case of using azo dye with the maximum number of auxochromic substituents in the molecule. The effect of the diffraction efficiency dependence on the chromophores structure is connected with the deepening of dye color. The photo-induced optical anisotropy in the films appears due to a change of the concentration ratio of trans- and cis-isomers of azobenzene dye fragments. This conclusion is confirmed by the photodielectric measurements data, namely, by the negative sign of the photodielectric effect, observed experimentally in the investigated thin film structures. It was shown, that investigated compositions can be used as information media for polarization holography.

Monte-Carlo simulation of the birefringence kinetics in azobenzene-containing materials

The photoinduced optical anisotropy (birefringence and dichroism) in azobenzene-containing materials gives rise to a variety of applications in photonics by way of controlling the anisotropic properties at a nanoscale level. These applications require modeling the photoisomerization process kinetics. This is a complex and stochastic task that depends on many parameters. The paper presents a computationally effective Monte-Carlo simulation of recording, relaxation and erasure of the photoinduced anisotropy for a large number of 3D-oriented molecules at small angular increments between the neighboring orientations. The modeling is in good agreement with the experiment for measuring the birefringence in the PAZO polymer.

Spectral Photosensitization of Optical Anisotropy in Solid Poly(Vinyl Cinnamate) Films

The possibility and features of formation of sensitized photoinduced optical anisotropy in amorphous films of poly(vinyl cinnamate) and its derivative poly(vinyl-4-metoxicinnamate) under the action of polarized light (including light that is not absorbed by polymer macromolecules themselves) have been investigated. It is found that the effect of induced optical anisotropy is based on the transfer of electron excitation energy from donor (sensitizer) molecules to acceptor molecules and is observed in the course of phototopochemical biomolecular cyclization reaction of cinnamate fragments in polymer macromolecules. The detected photoinduced anisotropy in solid films of poly(vinyl cinnamate) and its derivative poly(vinyl-4-metoxicinnamate) ensures sensitized photo-orientation of low-molecular thermotropic liquid crystals.

Photoinduced ultrafast optical anisotropy encountered by spin-flip transition in La0.67Ca0.33MnO3

Understanding and controlling the transient optical anisotropy of strong correlation systems is of great interest in the quest for information processing and storage. Here, we report on ultrafast optical pump-probe measurements with linearly and circularly polarized laser pulses in the manganite La0.67Ca0.33MnO3 thin film. We show both the time-resolved reflectivity and the polarization state (Kerr rotation and ellipticity) of the probe-pulse at different temperatures, which are analyzed by the ultrafast intersite transition between Mn3+ and Mn4+ sites at photon energies around 1.55 eV. During the temperature-induced dynamical spectral weight transfer, a spin-flip photo-transition between spin up eg states of Mn3+ and spin down eg states of Mn4+ ions occurs and is imprinted on the optical anisotropy of the probe beam.

Photo-induced anisotropy in ZnO/PVA nanocomposites prepared by modified electrochemical method in PMA matrix

Substantial photo-induced optical anisotropy was discovered in ZnO/PVA nanocomposites under the influence of external bicolor laser illumination. Zinc oxide nanoparticles were synthesized by electrolysis of a sodium chloride aqueous medium including poly-methacrylic acid (PMA) in a cell system having a soluble zinc anode. The structural analysis of the ZnO powder samples has been carried out by X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM). The polyvinyl alcohol (PVA) embedded ZnO films obtained from the powder samples possess larger grain sizes than those in powder form. The films were prepared from the same polymer matrix but elaborated with two different PVA contents which are respectively 15% and 30%. The photoinduced anisotropy was identified by using two bicolor Er: glass laser beams incident at different angles. Substantial influence of the technological processes on the embedded nanoparticle sizes and related birefringence was explored. The process of laser induced anisotropy shows an occurrence of birefringence saturation.

Poly(amic acid)s and their poly(amide imide) counterparts containing azobenzene moieties: Characterization, imidization kinetics and photochromic properties

We report on a series of novel photochromic poly(amide imide)s and their poly(amic acid) precursors bearing azobenzene chromophores as the side groups. The chemical structures of the polymers were designed so that they exhibited an enhanced thermal stability combined with a large and stable birefringence photogenerated by light of the wavelengths belonging to a wide spectral range. The polymers possessed rigidly attached azochromophores in the content of either one or two per a repeating unit, which in the latter case differed in their structures. The imidization kinetics of the poly(amic acid)s was investigated by differential scanning calorimetry and the kinetic parameters were estimated using Ozawa and Kissinger methods. Measurements of the selected physical properties of the polymers, such as solubility, supramolecular structure, linear absorption, thermal stability, glass transition and photochromic response were performed and used for determination of the structure-property relations. The measurements of photochromic properties showed a very efficient generation of optical anisotropy upon blue and violet irradiation, for both the poly(amide imide)s containing two different chromophores in the repeating unit and for their precursors. For these poly(amide imide)s and for their precursors an exceptionally slow decrease in the photoinduced optical anisotropy in the dark was also observed.

Thermal Amplification of Photoinduced Cooperative Reorientation of Liquid Crystalline Copolymer Films Comprised of N-Benzylideneaniline and Benzoic Acid Side Groups

This paper describes photoinduced cooperative molecular reorientation and its thermally stimulated amplification in liquid crystalline copolymethacrylates (LCPs) with hexamethylene spacer groups terminated with N-benzylideneaniline (NBA) and benzoic acid (BA) in the side chain. Influence of the copolymerization ratio and annealing temperature on the molecular reorientation behavior is investigated. Both NBA and BA side groups cooperatively reorient, but the orientation order depends on the copolymerization ratio. In contrast, sufficient thermal amplification of the photoinduced optical anisotropy is achieved for all LCPs, which is controlled by adjusting the annealing temperature.

Azopolimery supramolekularne powstale na bazie wiazan wodorowych

The paper is a literature review on photoactive supramolecular polymers formed through noncovalent interactions, i.e. hydrogen bonds between azo chromophore molecules and macromolecule chains. The individual kinds of such polymers as well as the results of photoinduced optical anisotropy investigations related to them have been presented. The supramolecular azopolymers based on hydrogen bonds have been compared with (i) systems containing chromophores dispersed in polymer matrix, (ii) supramolecular systems based on ionic interactions and (iii) functionalized azopolymers. Additionally, the methods used for the identification of hydrogen bonds in supramolecular azopolymers were discussed.

Single-exposure polarization phase-shifting interferometer using an azo-polymer orientation array

We propose a single-exposure polarization phase-shifting interferometer based on a beam-splitting method using an azo-polymer orientation array, which is prepared from an ordinary azo-polymer film with photo-induced optical anisotropy. The azo-polymer orientation array gives a four-step phase shift on the interferogram, from which phase differences between the orthogonal polarized reference and object beam can be calculated. Experimental results are provided to verify the feasibility of the proposed interferometer. The azo-polymer orientation array is easy to fabricate, making this method convenient and low-cost.

Photoinduced Anisotropy in Poly(Methyl Methacrylate)/Sudan Red III Thin Films

The dynamic behavior of photoinduced optical anisotropy of amorphous PMMA/Sudan Red III thin films has been investigated using the pump-probe method. The results showed that the anisotropy increased rapidly in an exponential fashion in the first seconds of pumping, so that a photostationary state was reached as a result of the photoisomerization process cycles. The anisotropy relaxation phase decreased exponentially in a slow manner, which means the anisotropy did not disappear instantaneously when the irradiation was switched off. Evidence of the existence of the Angular Hole Burning (AHB) and the Angular Redistribution (AR) effects in the Sudan Red III molecules in the PMMA polymeric host is presented. The relative photoinduced optical anisotropy was calculated. The results showed that, at room temperature, the thermal isotropic relaxation of the photoisomer was negligible compared to the anisotropic reverse photoisomerization that occurred as soon as the pump beam was cut-off.

Influence of End Groups in Photoinduced Reorientation of Liquid Crystalline Polymer Films Based on Axis-Selective Photo-Fries Rearrangement

Photoreactive liquid crystalline polymethacrylates with phenylbenzoate side groups with different molecular terminal were synthesized and their photoresponsive behavior was investigated. All polymer films underwent axis-selective photo-Fries rearrangement and generated photoinduced optical anisotropy upon exposure to linearly polarized ultraviolet (LPUV) light. In addition, annealing the photoreacted films enhanced molecular reorientation perpendicular to polarization direction of LPUV light, and the molecular terminal groups strongly affected the reorientation behavior.

Polarization phase-shifting interferometer by rotating azo-polymer film with photo-induced optical anisotropy

A new type of phase-shifting interferometer was presented by rotating an azo-polymer film with photo-induced optical anisotropy in the light path of orthogonal polarization interferometry. By changing the angle of the optical axis of the azo-polymer film, four phase-shifted fringe patterns were obtained, from which phase difference between orthogonal polarized light could be calculated. The polarization states and the formation of the interferograms were analyzed theoretically using Jones matrix calculations and the experimental verification of the proposed method was also given. This method has a simple optical configuration which utilizes commercially available polarization devices and anisotropic azo-polymer film of low cost. The phase-shifting interferometer also demonstrates a new application of azo-polymer film.

Characterization of poly(amic acid)s and resulting polyimides bearing azobenzene moieties including investigations of thermal imidization kinetics and photoinduced anisotropy

Azobenzene-containing poly(amic acid)s and resulting polyimides were synthesized. Polymers differed in the polymer backbone structure and the position at which the azobenzene group was attached to the polymer chain. Polymers were characterized and evaluated using Fourier transform infrared, 1H NMR and UV-visible spectroscopies, X-ray diffraction, differential scanning calorimetry (DSC) and thermogravimetric analysis. Polyimides revealed glass transition temperatures in the range 160–265 °C, and thermal stability with decomposition temperatures in the range 336–444 °C. The thermal imidization kinetics of poly(amic acid)s was investigated by means of DSC. Kinetic parameters, such as the activation energy (E) and the frequency factor, were estimated using the Ozawa and Kissinger models. The values of E, determined using both methods, were in the range 142.52–200.92 kJ mol−1. The photoinduced optical anisotropy (POA) was studied in the obtained azopolymers using a holographic grating recording technique. Surface relief gratings, which appeared after light exposure, were observed using atomic force microscopy. For the first time to the best of our knowledge, photoinduced anisotropy in poly(amic acid)s was studied and compared with POA in their polyimide analogues. © 2014 Society of Chemical Industry

Temporal characteristics of polarization holographic gratings formed in a photosensitive polymeric film containing N-benzylideneaniline derivative side groups

A polarization holographic grating was recorded in a transparent thin film formed from polymethacrylate with N-benzylideneaniline (NBA) derivative side groups. We measured the real time diffraction properties. The data were analyzed based on a theoretical model that accounted for the distribution of optical anisotropy caused by molecular reorientation as well as for surface relief (SR) deformation caused by molecular motion. Optical anisotropy rapidly increased and then slowly decreased, with increasing recording time. This phenomenon was described based on photoisomerization and photocleavage reactions of the NBA side groups. SR deformation was also induced in the film by polarization holographic recording, without any subsequent processes. The photoinduced optical anisotropy and SR deformation were retained after the recording was turned off.

Study on photoinduced anisotropy of liquid crystal doped in polymer film containing coumarin group

In this article, the photoinduced anisotropy of a liquid crystal molecule doped into a polymer with photosensitive group was studied. The polymer with coumarin photosensitive group as side chain can generate a polarization axis-selective photoreaction and obtain anisotropy after being exposed at a certain wavelength of linearly polarized ultraviolet light, which occurs as [2+2] cycloaddition and form anchoring force. The thin film underwent subsequent annealing in the liquid crystal temperature range and it leads to a large optical anisotropic change in the polymer film for the self-assembly of molecule. The ultraviolet dynamic spectrum was used to evaluate the absorbance of the film, and the photoinduced optical anisotropy ΔA and order parameter S were calculated. The result shows that when the exposure energy was increased to 1908 mJ/cm2, the value of the absorbance does not change with the increase in exposure energy. When annealing temperature changed from 120 °C to 240 °C, ΔA changed from 0.07 to 0.11 and S changed from 0.02 to 0.04.

Photo-induced optical anisotropy and coherent oscillations in a quasi-two-dimensional two-orbital model for LaMnO3

Ultrafast transient optical conductivity of spin-orbital ordered LaMnO3 was investigated theoretically. A quasi-two-dimensional Hubbard Hamiltonian based on a four-site Mn cluster was established to simulate the LaMnO3 system. The time evolution of the transient optical conductivity was numerically calculated. Intensity oscillations of the optical conductivity were observed, which were interpreted as the result of the quantum interference between phonon-coupled orbiton excited states. Also, the photo-induced anisotropy was observed, which revealed the broken orbital symmetry of the optical excited state.