Azobenzene-functionalized Poly Research Articles

Cephalopod-Inspired Chemical-Gated Hydrogel Actuation Systems for Information 3D-Encoding Display.

Cephalopods evolve the acetylcholine-gated actuation control function of their skin muscles, which enables their dynamic/static multimode display capacities for achieving perfectly spatial control over the colors/patterns on every inch of skin. Reproduction of artificial analogs that exhibit similar multimodal display is essential to reach advanced information three-dimensional (3D) encoding with higher security than the classic 2D-encoding strategy, but remains underdeveloped. The core difficulty is how to replicate such chemical-gated actuation control function into artificial soft actuating systems. Herein, this work proposes to develop azobenzene-functionalized poly(acrylamide) (PAAm) hydrogel systems, whose upper critical solution temperature (UCST) type actuation responsiveness can be intelligently programmed or even gated by the addition of hydrophilic α-cyclodextrin (α-CD) molecules for reversible association with pendant azobenzene moieties via supramolecular host-guest interactions. By employing such α-CD-gated hydrogel actuator as an analogue of cephalopods' skin muscle, biomimetic mechanically modulated multicolor fluorescent display systems are designed, which demonstrate a conceptually new α-CD-gated "thermal stimulation-hydrogel actuation-fluorescence output" display mechanism. Consequently, high-security 3D-encoding information carriers with an unprecedented combination of single-input multiple-output, dynamic/static dual-mode and spatially controlled display capacities are achieved. This bioinspired strategy brings functional-integrated features for artificial display systems and opens previously unidentified avenues for information security.

Controlling the LCST-Phase Transition in Azobenzene-Functionalized Poly (N-Isopropylacrlyamide) Hydrogels by Light

Poly(N-isopropylacrylamide) PNIPAAm hydrogels were modified with a new azobenzene-containing co-monomer. In this work, light responsiveness as an additional functionality, is conceptualized to induce two phase transitions in the same material, which can be controlled by light. For a hydrogel with merely 2.5 mol% of this co-monomer, the lower critical solution transition temperature (LCST) was lowered by 12 °C (to 20 °C) compared to PNIPAAm (LCST at 32 °C), as analyzed by differential scanning calorimetry (DSC). The untreated unimodal endotherm split into a bimodal peak upon irradiation with UV-light, giving a second onset due to the switched (Z) isomer-rich regions, LCST*H2.5%-(Z) = 26 °C. On irradiation with 450 nm, leading to the reverse (Z) to (E) isomerization, the endotherm was also reversible. Thus, a photo-switchable hydrogel whose LCST and structure are tunable with the hydrophobicity-hydrophilicity of the (E) and (Z) isomeric state of azobenzene was obtained. The influence of the increase in the mol% of azoacrylate on the LCST was evaluated via DSC, in combination with NMR studies, UV-vis spectroscopy and control experiments with linear polymers. The large light-driven modulation of the LCST adds bistability in thermoresponsive hydrogels, which may open diverse applications in the field of soft robotics actuators.

Photopatterned azo poly(amide imide) layers as aligning substrates of holographic liquid crystal diffraction gratings for beam steering applications

Custom synthesized “T-type” azobenzene-functionalized poly(amide imide) allows for effective fabrication of a tunable liquid crystal photonic device for light beam steering.

A NIR-triggered gatekeeper of supramolecular conjugated unimicelles with two-photon absorption for controlled drug release.

A near-infrared (NIR)-sensitive gated assembly of supramolecular conjugated unimicelles based on robust host-guest recognition between a β-cyclodextrin-grafted hyperbranched conjugated polymer and azobenzene-functionalized poly(ethylene glycol) was constructed. Utilized as a drug carrier, these unimicelles exhibited controlled drug release through the NIR-triggered photoisomerization of azobenzene in cancer cells via a two-photon excited fluorescence resonance energy transfer (TP-FRET) approach, leading to efficient cancer therapy.

Photoinduced birefringence of azobenzene polymer at blue excitation wavelengths

The photoinduced birefringence resulting from trans–cis–trans isomerizations has been measured in a thin film of azobenzene-functionalized poly(esterimide) at three excitation wavelengths. The excitation wavelengths of 388 and 420 nm have been located at exactly opposite sides of the trans azobenzene absorption band. A third irradiation wavelength was shifted to 438 nm. Distinct saturation levels and growth dynamics of photoinduced birefringence have been observed under the 388- and 420-nm illumination, while very similar characteristics were recorded at 420 and 438 nm. Differences in the obtained birefringence signals have been explained considering the cis isomer absorption and photoisomerization pathways.

Synthesis, characterization and photoresponsive behaviour of a series of azobenzene-containing poly(ether sulfone)s with high glass transition temperatures

To improve the stability of photoinduced birefringence, a series of azobenzene-functionalized poly(ether sulfone)s (azo-PES) with high glass transition temperature ( Tg) were successfully synthesized via Ullmann coupling between PES with bromine side groups and azo compound 4-((4-methoxyphenyl)diazenyl)phenol. Their chemical structures and properties were characterized by means of ultraviolet–visible and proton nuclear magnetic resonance spectroscopies. The results of differential scanning calorimetry and thermogravimetric analysis suggested that the polymers had high Tgs and good thermal stability. Upon irradiation with a 532 nm neodymium-doped yttrium aluminium garnet (Nd:YAG) laser beam, these azo polymers presented large photoinduced birefringence with a remnant value up to 94%, indicating good stability of the photoinduced orientation. Furthermore, multiple writing/erasing experiments indicate that the azo polymers have potential applications in reversible optical storage.

Synthesis and characterization of azobenzene-functionalized poly(styrene)-b-poly(vinyl acetate) via the combination of RAFT and “click” chemistry

Here, we described a strategy for preparing well-defined block copolymers, poly(styrene)-b-poly(vinyl acetate) (PS-b-PVAc), containing middle azobenzene moiety via the combination of the reversible addition-fragmentation chain transfer (RAFT) polymerization and “click” chemistry. Firstly, a novel RAFT agent containing α-alkyne and azobenzene chromophore in R group, 2-(3-ethynylphenylazophenoxycarbonyl)prop-2-yl-9H-carbazole-9-carbodithioate (EACDT), was synthesized and used to mediate the RAFT polymerization of styrene (St). Well-defined α-alkyne end-functionalized poly(styrene) (PS) was obtained. Secondly, the RAFT polymerization of vinyl acetate (VAc) was conducted using functionalized RAFT reagent with ω-azide structure in Z group, O-(2-azidoethyl) S-benzyl dithiocarbonate (AEBDC). Well-defined ω-azide end-functionalized poly(vinyl acetate) (PVAc) was obtained. Afterwards, the resulting α-alkyne terminated PS was coupled by “click” chemistry with the azide terminated PVAc. The block copolymer, PS-b-PVAc, was obtained with tailored structures. The products from each step were characterized and confirmed by GPC, 1H NMR, IR and differential scanning calorimetry (DSC) examination. Kinetics of the trans-cis-trans isomerization from azobenzene chromophore in PS-b-PVAc and PS were investigated in CHCl3 solutions.

Synthesis and characterization of novel photoactive poly(arylene ether ketone) with azobenzene pendants

A novel azobenzene-functionalized poly(arylene ether ketone) (azo-PAEK) with azo moieties on the pendants was prepared by post-esterification reaction of acid-containing poly(arylene ether ketone) (acid-PAEK) with 4-((4-nitrophenyl)diazenyl)phenol (NAP). DSC and TGA measurements indicate the azo-PAEK has high glass transition temperature of 152 °C and good thermal stability with 5% weight loss at 319 °C. Irradiated by 360 nm UV light, the azo-PAEK shows a significant photoisomerization effect. Under the illumination of linearly polarized laser beam, significant surface-relief-gratings (SRGs) with the surface modulation depth of 37 nm were fabricated rapidly on the azo-PAEK film.

On the Inscription of Period and Half-Period Surface Relief Gratings in Azobenzene-Functionalized Polymers

Laser-light-induced surface relief grating inscription was carried out in the newly synthesized azobenzene-functionalized poly(amide-imide)s having the same main- and side-chain structures but different substituents in the azobenzene groups. The gratings were inscribed employing the two-wave mixing technique with linearly polarized laser beams. Three different polarization configurations were used: s-s, p-p, and s-p. The relatively deep surface relief gratings of period Lambda were formed for the case of s-s and p-p polarizations, whereas the s-p inscription resulted in the half-period grating (Lambda/2) with the weak surface modulation. The origin of the formation of Lambda/2 structure for s-p configuration results from the interference between zeroth- and first-order beams scattered on the polarization refractive index grating and having the same polarization. On the basis of this idea, we presented the simple kinetic model predicting and modeling the half-period grating formation with its temporal evolution. The proposed model is consistent with the experimental findings.

Preparation and nonlinear optical studies of a novel thermal stable polymer containing azo chromophores in the side chain

A novel bisfluoro NLO chromophore monomer, 4-[(4-cyanophenyl)diazenyl]phenyl-2,6-difluorobenzoate was synthesized via a two-step synthetic procedure. Azobenzene-functionalized poly(aryl ether) (azo-PAE) was prepared via a nucleophilic aromatic substitution polycondensation based on this monomer. The obtained polymer was characterized and evaluated by FT-IR, UV–vis, 1H NMR, DSC and TGA. The third-order NLO coefficient was measured by Z-scan technique. The results showed the azo-PAE has high glass transition temperature, good thermal stability and large third-order nonlinear susceptibility.

Photoinduced Holographic Gratings in Azobenzene-Functionalized Poly(amideimide)s

We present the diffraction grating recording in a series of poly(amideimide)s functionalized with methyl-substituted azobenzene chromophore as side group and correlate the results with the molecular structure. Poly(amideimide)s, differing in their backbone structure and in position in which azobenzene group is attached to polymer chain, were synthesized and characterized by the spectroscopic methods. Diffraction gratings were fabricated using degenerate two-wave mixing technique with cw Ar+ laser working at λ=514.5 nm and monitored by light diffraction of low power He-Ne probe beam λ=632.8 nm in a transmission mode. Two qualitatively different configurations of the recording beam polarizations were used s-s and s-p. The dynamics of diffraction grating inscription was described by mono- or biexponential growth functions. The surface relief grating formation was also examined in poly(amideimide)s. They were studied by atomic force microscope and revealed the strong dependence on the writing beams polarization. Additionally the thermal stability of the photoinduced surface gratings was checked.

Blends of Poly(methacrylate) Block Copolymers with Photoaddressable Segments

The paper presents the synthesis of azobenzene-functionalized block copolymers based on a poly(methyl methacrylate) (PMMA) segment and an azobenzene-functionalized poly(hydroxyethyl methacrylate) segment, and a basic study of blending these block copolymers with homopolymers is given. Two diblock copolymers, prepared via different routes, were synthesized by a living anionic polymerization followed by a polymer analogous reaction to attach the azobenzene side groups. Self-assembly of the block copolymers resulted in phase-separated morphologies on the nanometer scale. The photoaddressable azobenzene segments are dispersed in the PMMA matrix and locally confined. Special focus is given to the preparation and characterization of block copolymer blends with PMMA homopolymer in order to dilute the phase-separated azobenzene morphology and reduce the optical density while maintaining the confinement. The block copolymer blends were characterized with respect to their morphology and initial holographic experiments were performed.

Solvent effects on phototriggered conformational changes in an azobenzene-functionalized poly( p-phenylene vinylene)

Poly(2-hexyloxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylene vinylene) (HPA-10-PPV) is an azobenzene-functionalized poly( p-phenylene vinylene) derivative that previously has been shown to undergo changes in the position and shape of its fluorescence spectrum in response to trans → cis-azobenzene photoisomerization. This effect was initially observed in a binary dichloromethane-methanol solution and was attributed to a phototriggered expansion of polymer coil dimensions. In this report we explore the mechanism by which azobenzene isomerization induces this conformational change and the solvent properties that govern the magnitude of the change. Phototriggered fluorescence shifts are observed in mixtures of dichloromethane and each of 12 cosolvents, 7 protic and 5 aprotic. The shifts are largest in protic solvents but do not correlate with any single solvent parameter or combination of parameters. Smaller shifts are observed in mixtures with aprotic cosolvents, and the magnitude of these shifts correlates with solvent polarity/polarizability. The conformational changes responsible for the fluorescence shifts are attributed to a change in polymer–solvent interactions induced by the difference between properties of the trans- and cis-azobenzene isomers. Isomerization to the more polar cis-azobenzene promotes an increase in polymer–solvent interactions. These results suggest a means by which the magnitude of phototriggered fluorescence changes could be increased in future derivatives.

Phototriggered Fluorescence Color Changes in Azobenzene-Functionalized Conjugated Polymers

We report fluorescence studies of phototriggered changes in spectral position and shape for two azobenzene-functionalized poly(p-phenylenevinylene) derivatives, poly(2-methoxy-5-(4-phenylazophenyl-4'-(1,10-dioxydecyl))-1,4-phenylenevinylene) (MPA-10-PPV) and poly(2-hexyloxy-5-(4-phenylazophenyl-4'-(1,10-dioxydecyl))-1,4-phenylenevinylene) (HPA-10-PPV). Upon trans --> cis azobenzene photoisomerization, small (ca. 1 nm) blue shifts in spectral position are observed for MPA-10-PPV in 100% toluene, a good solvent for this polymer. These shifts are reversed upon visible irradiation and can be cycled many times. To probe the dependence of these shifts on initial polymer conformation, a dichloromethane-methanol cosolvent study was performed in which the solvent quality was decreased incrementally to induce a reduction in polymer coil dimensions. Unirradiated dichloromethane solutions of both MPA-10-PPV and HPA-10-PPV showed a red shift and reduction in quantum yield with increasing methanol concentration as expected based on literature results for other poly(p-phenylenevinylene) derivatives. These changes have been attributed to a dramatic conformational collapse by others and occur for these azo polymers over the 30-60% (v/v) methanol range. While little or no light-induced spectral shifting was observed at low (<or=20%) and high (>or=70%) methanol concentrations, significant spectral shifts were observed for both polymers upon azobenzene photoisomerization in solutions with 30-60% methanol, the same range over which the polymer undergoes collapse to a highly coiled state. The largest shifts are visible to the eye, with a 65:35 (v/v) dichloromethane-methanol solution of HPA-10-PPV showing yellow-orange fluorescence when the azobenzenes are trans, green fluorescence when they are cis, and yellow-orange again after the azobenzenes are returned to the trans state. We attribute these color changes to a reversible UV-phototriggered expansion of polymer coil dimensions that occurs as a result of trans --> cis azobenzene side chain isomerization and provide temperature data to support this conclusion.

Synthesis, Fluorescence and Photoisomerization Studies of Azobenzene‐Functionalized Poly(alkyl aryl ether) Dendrimers

A series of azobenzene-functionalized poly(alkyl aryl ether) dendrimers have been synthesized and their photochemical and photophysical properties in solution and as thin films have been investigated. Although the photochemical behavior of the azodendrimers in solution indicated that the azobenzene units behave independently, very similar to the constituent monomer azobenzene unit, the properties of thin solid films of the dendrimers were distinctly different. The azodendrimers, AzoG1, AzoG2, and AzoG3 were observed to form stable supercooled glasses, which showed long-wavelength absorption and red emission characteristics of J-aggregates of the azobenzene chromophores. Reversible photoinduced isomerization of the azodendrimers in the glassy state is described.

Photostimulated Phase Separation Encapsulation

Polymer capsules were prepared by photostimulated precipitation of azobenzene-functionalized poly(styrene-alt-maleimide) copolymers dissolved in an oil phase and dispersed in a continuous aqueous phase. The oil phase is selected to be a near ϑ-solvent for the copolymers, such that the increasing polarity of the polymer due to the photochemical trans-to-cis isomerization induces polymer phase separation and migration to the interface. The resulting polymer walls are permanent even during storage in the dark or irradiation with visible light.