Nonlinear optical properties of an azobenzene polymer

The nonlinear optical properties of an azobenzene polymer azol2-MO were investigated by a Z-scan technique. The polymer was synthesized by assembling the liquid-crystalline polymer azol2 with methyl orange. The nonlinear refrac- tive index （1.39×10^-15 cm2/W） and the nonlinear absorption coefficient （0.11 cm/GW） of azol2-MO were determined with 532-nm picosecond laser pulses at the irradiance of 92.40 GW/cm2. When compared to the nonlinear properties of azol2 and methyl orange, azol2-MO possesses the advantages of its two constituents and shows larger nonlinear optical properties.

PHOTOINDUCED BIREFRINGENCE AND NUMERICAL SOLUTION OF A NEW DYNAMIC MODEL IN AN AMORPHOUS COPOLYMER CONTAINING AZOBENZENE GROUPS

Photoinduced birefringence is investigated in a new amorphous copolymer containing azobenzene groups. The levels of birefringence signal are found to depend an the polarization angle between the pump beam and the probe beam, and on the ellipticity of the pump beam. Both the growth and decay processes of the birefringence signal can be described by known biexponential equations. The rate constants and the amplitudes associated with the growth process of the photoinduced birefringence are observed to display a linear dependence with the pump beam intensity. A new dynamic model of the photoinduced birefringence is presented taking into account the contributions of both the bans and cis isomers of azobenzene groups and the local polymer segments. The numerical treatment of this model shows good agreement with the experimental data in the whole writing-erasing processes of the photoinduced birefringence conducted in our polymer samples.

Photorheological fluids of azobenzene polymers for lubrication regulation

Fluid viscosity is ubiquitous property and is of practical importance in intelligent fluids,industrial lubrication,and pipeline fluid transportation.Recently,there has been a surging interest in viscosity regulation.Here,we have developed a group of photorheological fluids by utilizing azobenzene polymers with a light-induced microstructure transformation.In this work,a photosensitive polymer with 4,4'-bis-hydroxyazobenzene as the main chain was designed and synthesized as a pivotal functional material.The sufficiently large structural difference under ultraviolet and near-infrared light makes it possible to regulate the viscosity of a polyethylene glycol solution.The viscosity of the photosensitive rheological fluids under ultraviolet light radiation is found to be up to 45.1%higher than that under near-infrared light radiation.To explore this intelligent lubricating technology,the friction regulation of ceramic sliding bearings was investigated utilizing photosensitive rheological fluids.Reversible friction regulation with a ratio of up to 3.77 has been achieved by the alternative irradiation of near-infrared and ultraviolet light,which can be attributed to the differences in mechanical properties and molecular structures under ultraviolet and near-infrared light according to both simulations and experiments.Such photorheological fluids will have promising applications in controllable lubrication,intelligent rheological fluids,and photosensitive dampers.

OPTICAL PHASE CONJUGATION RESPONSE OF PHOTOINDUCED POLYMER FILMS CONTAINING AZOBENZENE MOIETIES WITH CHIRAL GROUP

An optically active monomer containing azobenzene moieties with chiral group (s-2-methyl-butyl), 4-[2-(methacryloyloxy)ethyloxy] -4'-(s-2-methyl-1-butyloxycarbonyl) azobenzene (M1) was synthesized. Polymer (PM1) possessing optical phase conjugated response was obtained by homopolymerization of the optically active monomer (M1) using free radical polymerization. The polymer was very soluble in common solvents and good optical quality films could be easily fabricated by spin coating. The optical phase conjugated responses of the polymer PM1 were measured by degenerate four-wave mixing (DFWM). In comparison with polymer containing no chiral group, it was found from the preliminary measurement of photoisomeric change that optical phase conjugated response of the PM1 in the long-range order hexagonal symmetry microstructure could be easily controlled by choosing the appropriate polarization direction of the irradiating beams (514.5 nm) and the irradiating number, presumably due to the chiral group in the PM1 molecular structure. For the case of the polymer investigated here, a chiral group side chain was introduced to increase optical phase conjugated response intensity with different polarization directions of the irradiating beams, which aims originally at searching for a new photoactive material.

DESIGN AND STUDY OF NEW AZOBENZENE LIQUID CRYSTAL/POLYMER MATERIALS

Discussion is presented on the use of the photoisomerization of azobenzene chromophore in the design andpreparation of novel functional materials. The two systems reviewed are azobenzene polymer-stabilized liquid crystals andazobenzene elastomers. In the first case, a polymer network containing azobenzene moieties is used to optically induce andstabilize a long-range liquid crystal orientation without the need of treating the surfaces of the substrates. This optical andrubbing-free approach was applied to nematic and ferroelectric liquid crystals. In the second case, an azobenzene side-chainliquid crystalline polymer is grafted onto a styrene-butadiene-styrene triblock copolymer to yield a photoactive thermoplasticelastomer. Coupled mechanical and optical effects make possible the formation of dimaction gratings that may be useful formechanically tunable optical devices.

Induction and modulation of supramolecular chirality in side-chain azobenzene polymers through the covalent chiral domino effect

Induction,transfer,and modulation of intriguing supramolecular chirality were achieved in a chiral domino-type polymer system using a chiralα-end azobenzene(Azo)moiety as one chiral terminus and achiral Azo repeating units as building blocks.The Azo polymers prepared by atom transfer radical polymerization were imparted with well-controlled chain lengths and a chiral moiety at a well-defined position.One chiral terminus can effectively dictate the helical orientation of the achiral Azo stacks in the aggregation state.The spacer length between the chiral residue and the achiral repeating units dominates the preferred handedness of the side-chain Azo stacks.For instance,the R-configuration moiety with a short alkyl chain(with 0,2,and 3 carbons)induces right-handed supramolecular chirality,whereas those with long alkyl chains(with 4,5,and 6 carbons)induce the opposite helical orientation of the Azo units.Moreover,the chiral regulation of polymer aggregates is successfully achieved using the unique photoisomerization transition of the Azo chromophore and the heat-assisted reassembly approach.Chiral induction and chiral-to-achiral communication are further verified via theoretical simulations.

PHOTOINDUCED HOLOGRAPHIC PHASE GRATINGS BURIED IN AZOBENZENE SIDE-CHAIN POLYMER FILMS WITH A CHIRAL GROUP

An optically active polymer (PM1) containing azobenzene moieties with a chiral group (s-2-methyl-butyl) wassynthesized by homopolymerization of monomer, 4-[2-(methacryloyloxy)ethyloxy-4'-(s-2-methyl-1-butyl oxycarbonyl)azobenzene, using the free radical polymerization method. The polymer dissolved in tetrahydrofuran (THF) could be easilyprocessed into high optical quality films. The optical anisotropy of the polymer films was investigated by polarizing opticalmicroscopy (POM). The experimental results showed that irradiation with a circularly polarized beam could align theorientation of the molecules in the polymer films. Moreover, the holographic phase gratings of photo-induced polymer filmswere detected by atomic force microscopy (AFM) and POM. In comparison with polymer containing no chiral group, it wasfound from the preliminary measurement of the photo-induced holographic phase gratings that PM1 containing a chiralgroup could form holographic phase gratings buried in the films.

Photopatterning via photofluidization of azobenzene polymers

In current photo-based patterning techniques,an image is projected onto a photosensitive material to generate a pattern in the area where the light is focused.Thus,the size,shape,and periodicity of the pattern are determined by the features on the photomask or projected images,and the materials themselves generally do not play an active role in changing the features.In contrast,azobenzene polymers offer a unique type of photopatterning platform,where photoisomerization of the azobenzene groups can induce substantial material movements at the molecular,micro-,and macroscales.Stable surface relief patterns can be generated by exposure to interference light beams.Thus,periodic nano-and microstructures can be fabricated with both two-and three-dimensional spatial control over a large area in a remarkably simple way.Polarized light can be used to guide the flow of solid azobenzene polymers along the direction of light polarization via an unusual solid-to-liquid transition,allowing for the fabrication of complex structures using light.This review summarizes the recent progress in advanced manufacturing using azobenzene polymers.This includes a brief introduction of the intriguing optical behaviors of azobenzene polymers,followed by discussions of the recent developments and successful applications of azobenzene polymers,especially in micro-and nanofabrication.

POLYMER SCAFFOLDS BEARING AZOBENZENE-POTENTIAL FOR OPTICAL INFORMATION STORAGE

The fundamental optical storage mechanism of the laser light eddressable azobenzene moiety is briefly introduced.A modular and flexible synthesis design furnishes polyester matrices covalently integrating cyanoazobenzene in regularlyspaced side chains. Thin films of these materials are particularly well suited for holographic storape. Notable figures of meritsof liquid crystalline polyesters are response time to blue-green laser light of the order of nanoseconds, storage capacityexpressed as 5000 lines/mm, and high, permanent (almost nine years) diffraction efficiency of the order of 50% or greater,and erasability, The implications of the main chain nature for polyester morphology and for the permanency of the inducedanisotropy are discussed, The design and methods of preparation of other significantly different polymer scaffolds supportingcyanoazobenzene are elaborated. Oligopeptides always result in amorphous materials, whereas copolymethacrylates anddendritic or hyperbranched polyesters provide some materials that exhibit liquid crystallinity. However, none of these scaffolds affords materials that result in permanent anisotropy when exposed to interfering laser light.

SYNTHESIS AND PROPERTIES OF A NEW AZOBENZENE SIDE-CHAIN POLYMER CONTAINING A TEMPO RADICAL

To allow anisotropies of optical properties in a magnetic field, nitroxide radical is introduced into the ortho-position of the phenylene ring in the side chain. A new azobenzene side-chain polymer (TEMPO-PAZ) containing TEMPO radical was synthesized. The polymer has a good solubility in organic solvents. The ESR spectrum of the polymer indicated three absorption lines characteristic of TEMPO radical. The optical phase conjugated responses (I-4) of the polymer films were investigated by degenerate four-wave mixing (DFWM). The experimental results showed that optical phase conjugated response of the TEMPO-PAZ could be easily controlled by choosing the appropriate direction of magnetic field presumably due to the nitroxide radical in the TEMPO-PAZ molecular structure. For the polymer investigated here, the nitroxide radical was introduced to increase optical phase conjugated response intensity in a magnetic field, aiming originally at searching for a new photo-active organic magnetic multifunctional materials.

Deformation of amorphous azobenzene-containing polymer films induced by polarized light

Photomechanical response of amorphous polymer films containing azobenzene chromophores in side chains is studied. By invoking the trans-cis isomerization mech- anism, the steady-state deformation of the film induced by uniform illumination of linearly polarized light is obtained analytically. The deformation turns out to be of entropic origin,produced to compensate the entropy decrease due to photo-induced redistribution of azobenzene chromophores normal to the polarization direction. The predicted elongation direction of the film is consistent with previous experimental observations.

PHOTOINDUCED ALIGNMENT OF OPTICALLY ACTIVE POLYMER CONTAINING A TEMPO RADICAL END GROUP

A new azobenzene side-chain polymer (TEMPO-PAZ) containing TEMPO (4-hydroxy-2, 2, 6, 6-tetramethyl-piperidinooxy) radical end group was synthesized by free radical copolymerization. Photoinduced alignment was studied onthe polymer films at room temperature with linearly polarized light of 514.5 nm, The experimental results showed that themagnetic response intensity of the TEMPO-PAZ could be easily controlled by choosing the appropriate polarized lightirradiating times, presumably due to the nitroxide radical in the TEMPO-PAZ molecular structure. For the polymerinvestigated here, the photoinduced alignment technique was introduced to increase the magnetic response intensity ofpolymer under irradiation, aiming originally at searching for a new photo-active organic magnetic multifunctional materials.On the other hand, experimental results also showed that the TEMPO-PAZ can be used as a material for optical image storage.

Poly(4,4＇-bis(thien-2-yl)azobenzene-co-3-hexylthiophene)： Synthesis and Optical/Electrochemical Properties

A conjugated polymer of poly（4,4＇-bis（thien-2-yl）azobenzene-co-3-hexylthiophene）（PTAB-H）, bearing azo group in main chain and soluble in THF, chloroform, was synthesized through FeCl3 oxidation copolymerization with 3-hexylthiophene, and the monomer of 4,4＇-bis（thien-2-yl）azobenzene was synthesized by Sandmeyer reaction and Suzuki reaction. The UV-Vis spectra of PTAB-H reveal that azo group in the main chain can extend the absorption and lower the band gap. The fluorescent spectra, photoluminescence（PL） spectra, cyclic voltammetry（CV） of PTAB-H were recorded.The gel permeation chromatography（GPC）, thermogravimetry analysis（TGA）, and differential scanning calorimetry（DSC） measurements indicated that the molecular weight Mn, Mw and polydispersity indices（PDI） of the polymer are 10876 g/mol, 22338 g/mol and 2.05 respectively and the temperature of 5% weight-loss is 224.6 ℃. No glass transition and crystallization melting peak was observed in the temperature range from 30 ℃ to 400 ℃.

Synthesis of an Azobenzene-containing Main-chain Crystalline Polymer and Photodeformation Behaviors of Its Supramolecular Hydrogen-bonded Fibers

The synthesis of a new azobenzene(azo)-containing main-chain crystalline polymer with reactive secondary amino groups in its backbone and photodeformation behaviors of its supramolecular hydrogen-bonded fibers are described. This main-chain azo polymer(namely Azo-MP6) was prepared via first the synthesis of a diacrylate-type azo monomer and its subsequent Michael addition copolymerization with trans-1,4-cyclohexanediamine under a mild reaction condition. Azo-MP6 was found to have a linear main-chain chemical structure instead of a branched one, as verified by comparing its ~1H-NMR spectrum with that of the azo polymer prepared via the polymer analogous reaction of AzoMP6 with acetic anhydride. The thermal stability, phase transition behavior, and photoresponsivity of Azo-MP6 were characterized with TGA,DSC, POM, XRD, and UV-Vis spectroscopy. The experimental results revealed that it had good thermal stability, low glass transition temperature,broad crystalline phase temperature range, and highly reversible photoresponsivity. Physically crosslinked supramolecular hydrogen-bonded fibers with good mechanical properties and a high alignment order of azo mesogens were readily fabricated from Azo-MP6 by using the simple melt spinning method, and they could show "reversible" photoinduced bending under the same UV light irradiation and good anti-fatigue properties.

Coherent Random Lasing Realized in Polymer Vesicles

We have demonstrated the realization of a coherent vesicle random lasing(VRL)from the dye doped azobenzene polymer vesicles self-assembled in the tetrahydrofuran-water system,which contains a double-walled structure:a hydrophilic and hydrophobic part.The effect of the dye and azobenzene polymer concentration on the threshold of random laser has been researched.The threshold of random laser decreases with an increase in the concentration of the pyrromethene 597(PM597)laser and azobenzene polymer.Moreover,the scattering of small size group vesicles is attributed to providing a loop to boost the coherent random laser through the Fourier transform analysis.Due to the vesicles having the similar structure with the cell,the generation of coherent random lasers from vesicles expand random lasers to the biomedicine filed.