Two-dimensional networks of an azobenzene derivative: bi-pyridine mediation and photo regulation

Two-dimensional photosensitive supramolecular assemblies based on an azobenzene derivative and bi-pyridine are built up and investigated using scanning tunneling microscopy (STM). In order to probe the photo-induced self-assembled behavior of these two molecules, irradiation experiments with different wavelengths are designed and performed. Our STM results show that the constructed H-bonded networks can be reversibly regulated under irradiation with UV light and visible light.     

Photoresponsive peptide and polypeptide systems: part II. Photochromism of poly(l-ornithine) containing various azo-contents in side chains

Poly(L-ornithine)s having various azo-contents in the side chains were synthesized by the water-soluble carbodiimide procedure. The photochemical properties of the polypeptides poly[N^δ-p-(phenylazo)benzoyl-L-ornithine] (PPABLO) containing 3–77 mol% azobenzene were investigated by absorption and circular dichroism spectroscopy in hexafluoro-2-propanol (HFIP) or water, and in HFIP-water or methanol-water solvent mixtures. The photochromism of the dichroic bands of the PPABLOs containing 20–77 mol% azobenzene in the visible and ultraviolet wavelength regions was found to be mostly reversible as a function of irradiation time at different wavelengths due to the photostationary state (above 80% trans-cis photoisomerization) of the azo aromatic moieties. The PPABLO containing 3.2 mol% azobenzene in water exhibited conformational changes from random coil to helix by the addition of methanol or sodium dodecyl sulphate (SDS). The photo-induced conformational change was observed in HFIP-water-SDS solvent mixtures, while no conformational change was seen in water and HFIP-water solvent mixtures.     

On-off switching properties of one-dimensional photonic crystals consisting of azo-functionalized polymer liquid crystals having different methylene spacers and polyvinyl alcohol

In this paper, photoresponsive behavior of multi-bilayered films having precisely controlled layer thickness prepared by stacking an azo-functionalized polymer liquid crystal, PMAzXAc, and polyvinyl alcohol alternatively, PVA, is described. The multi-bilayered films were found to reflect a light of specific wavelength depending on the layer thickness and refractive index, and showed the reversible change in the reflection intensity by irradiation with visible and UV lights. The change in the reflection intensity was brought about by change in the molecular orientation of PMAzXAc between an out-of-plane orientation and a photo-induced isotropic state, and was strongly dependent on the number of methylene spacer of PMAzXAc linking the azobenzene side group with the acrylate polymer main chain. PMAz6Ac with hexa-methylene spacer showed the largest change in the reflection intensity, while smaller change in the reflection intensity was observed for PMAzXAc having shorter or longer methylene spacer than 6. The effect of the methylene spacers on the photochemical change in the molecular orientation of azobenzene chromophores in the multi-bilayered films will be discussed.     

Photocontrolled adsorption chromatography for lysozyme using azoaromatic polymer

Photoresponsive polymeric adsorbents containing an azobenzene moiety were prepared, and adsorption behavior of lysozyme onto the adsorbents was studied. In the dark the amount of lysozyme adsorbed increased with increasing hydrophobicity of the surface of the adsorbents. Therefore, it is suggested that hydrophobic interaction between lysozyme and the adsorbents plays an important role in their adsorption behavior. On irradiation with UV light, the amount of lysozyme adsorbed decreased. This result is due to increased polarity of the surface of the adsorbent induced by UV irradiation and the resultant reduction in hydrophobic interaction with lysozyme. When a column chromatography of lysozyme was carried out using the photoresponsive polymeric adsorbent as a packing material, lysozyme was eluted by photoirradiation in a system using water as the single solvent.     

Stimuli-responsive polymers. 10. Photo-regulation of optical rotations in azobenzene modified poly(ester-amide)s containing highly structured,atropisomeric backbone geometries

The chiral atropisomeric monomer R-(+)-2-amino-1,1'-binaphthalen-2-ol provides convenient access to a number of azobenzene modified poly(ester-amide)s having highly-ordered main-chain arrangements of their constituent monomer groups. When evaluated in dilute solution environments, each of the polymers that contain the azobenzene stimuliphore undergo photo-induced oscillations in optical rotatory power when stimulated by multiple UV light–visible light illumination cycles. Polymer behavior is strongly influenced by the particular sequence of monomer units positioned along the chiral ester-amide backbone. Periodic variants that are fashioned from R-binaphthylene, azobenzene and para-phenylene segments exhibit particularly interesting response profiles. In certain cases, their light-modulated chiroptical outputs alternate between two discrete states, fluctuating between positive and negative specific rotations, or reversibly interconverting between optically active and optically inactive endpoints. This chiroptical switching behavior is greatly influenced by the choice of solvent, providing an added means for tuning the binary outputs afforded by these structurally well-defined materials.     

Separation of proteins by polymeric adsorbents containing azobenzene moiety as a ligand

Polymeric adsorbents containing an azobenzene moiety as a ligand were prepared, and the photoinduced adsorption behavior of proteins to the adsorbent was studied. This method regulates by light the adsorption/desorption of proteins in hydrophobic chromatography. In the dark, the amount of protein adsorbed onto the adsorbent increased with increasing hydrophobicity of either adsorbent or protein. On irradiation with UV light, the amount of protein adsorbed decreased. Such a photoinduced change of the adsorption of protein was considered to be caused by the change of the hydrophobic interaction between the adsorbent and the protein due to the photoisomerization of the azobenzene moiety accompanying the polarity change of the adsorbent. It is also found that the desorption of protein was dependent on the balance of the hydrophobicity between the adsorbents and proteins. When column chromatography was carried out, the proteins were adsorbed in the dark and could be eluted after photoirradiation, with water as the single solvent. Furthermore, mixture of proteins could be separated by using a hydrophobic gradient column which was constituted by two polymeric adsorbents having different hydrophobicity.     

The effect of the molecular structure of a cationic azo dye on the photoinduced intercalation of phenol in a montmorillonite

The effect of the molecular structure of a cationic azo dye on the photoinduced intercalation of phenol into the azo dye-montmorillonites was reported. Two types of cationic azo dyes were used; one has (2-hydroxyethyl)dimethylammonium group (AZ(OH)⁺), and the other has trimethylammonium group (AZ(CH₃)⁺). Phenol was intercalated into both cation exchanged azo dye-montmorillonites (Kunipia F) by mechanical mixing without solvent. By the UV irradiation, the basal spacings increased further, and subsequent visible light irradiation led to decrease the basal spacings, indicating the intercalation and the deintercalation of phenol by the UV and visible light, respectively. The amounts of the phenol intercalated both chemically and photochemically varied depending on the azobenzene cations, showing the interactions between the cationic head group and phenol.     

Synthesis of photoisomeric azobenzene monomers and model compound effect on electric–optical properties in PDLC films

A series of azobenzene monomers and related model compounds with various side‐chain lengths were synthesized. The electrooptical properties of a polymer‐dispersed liquid crystal (PDLC) were verified by side‐chain methoxy azobenzene in various chain lengths (n = 3, 6, 11). The properties under various voltages were measured and the effect of extra voltage on the transmittance of PDLC was researched as well. The experiment demonstrated the validity of employing these side chain methoxy azobenzene materials in electrooptical devices. The azobenzene model compound showed better electric–optical and thermal–optical properties, having a higher contrast ratio (CR = 689) and a lower saturation voltage (4.7 V/μm). All the azobenzene molecules can be photoisomerized through UV light irradiation, following the mechanism of isomerization. The reversible photo and heat isomerization property was studied. The cis‐azobenzene that was transformed from the trans‐azobenzene irradiated by UV light can decrease the clearing point of the liquid crystal phase. We used this unique characteristic to record image patterns and it worked successfully. We synthesized the azobenzene monomers can stabilize the PDLC and their relative model compounds with various alkyl chain lengths even got better electric–optical effects. We found that azobenzene monomer shows different behaviors in the electric–optical property from its relative model compound. The difference between the systems were explained using a proposed model. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 721–732, 2005     

Preparation of nano‐ and microparticles through self‐assembly of azobenzene‐pendent ionomers

The particle formation of azobenzene‐containing ionomers, through self‐assembly, in aqueous media (THF/H₂O solvent) was studied. The ionomers were synthesized by copolymerization between azobenzene‐pendent methacrylate and methacrylic acid. It was revealed by UV–vis spectra and light scattering measurements that the extent of H‐aggregation of the azobenzene units first decreased, and then increased with increasing volume fraction of H₂O of the solvent. The H₂O fraction at which the extent of H‐aggregation began to increase became lower, when the copolymers contained more azobenzene units. Colloidal particles were prepared by slow addition of various concentrations of aqueous NaOH to the copolymer THF solutions. The hydrodynamic diameters of the particles obtained by the procedures were several hundreds of nanometer. When the azobenzene unit content in the copolymer was smaller, the diameters of the particles became smaller. When the colloidal particles dispersions were cast on a carbon sheet and dried, the particles aggregated and formed larger spherical particles, with diameters of several micrometers. The size of the particles obtained by the drying process became smaller, when higher concentrations of aqueous NaOH solutions were used. Therefore, the particle sizes were controlled by the azobenzene units content in the copolymers and the concentration of aqueous NaOH solutions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3913–3918, 2006     

Photo-induced change in wettability and binding ability of azoaromatic polymers

Photochromic polymers containing p-phenylazoacrylanilide were synthesized and the photo-induced hydrophobicity change on the surface of the polymer film was studied by measuring water wettability. The wettability increased with UV irradiation and returned to the original value with visible light irradiation. This photo-induced reversible change in wettability was considered to be caused by the large change of dipole moment across the azo bonds due to the isomerization of azo dye from trans to cis form. Furthermore, the photoresponsive polymeric adsorbent prepared from the azo polymer having adsorption behavior of a low molecular weight compound was studied. The binding ability of this polymeric adsorbent was decreased by UV irradiation. These results were attributed to the decreased hydrophobic interaction between the adsorbent and the adsorbate.     

掺杂纳米二氧化钛研究进展

TiO_2作为一种重要的光催化剂,在太阳能光解水,污水处理等方面有着重要的应用前景。但由于TiO_2禁带宽度为3.2eV,只在紫外线光照射时表现出高的催化活性,而紫外光只占太阳光的5%左右,另外光生电子和空穴容易复合,降低量子产率,从而限制了TiO_2光催化剂的广泛应用。大量研究实验证明,通过掺杂可有效提高TiO2的光催化活性,拓宽其光响应范围。     

Light‐triggered reversible solubility of α‐cyclodextrin and azobenzene moiety complexes in PDMAA‐co‐PAPA via molecular recognition

Photoresponsive polymer with azobenzene pendant group (PDMAA‐co‐PAPA) was synthesized by radical polymerization of N,N‐dimethylacrylamide (DMAA) and N‐4‐phenylazophenyl acrylamide (PAPA), and the characterization of the inclusion complexes of the PDMAA‐co‐PAPA with α‐cyclodextrin (α‐CD) were performed by FTIR, GPC, ¹H NMR, 2D NOESY, and UV–vis spectroscopy. It was found that the solubility of PDMAA‐co‐PAPA and α‐CD inclusion complexes in aqueous solution showed tunable property, which could be triggered by alternating UV–vis light irradiation at a certain temperature due to the effect of molecular recognition of α‐CD with azobenzene moiety in the polymer. After UV irradiation, the lower critical solution temperature (LCST) of the polymer aqueous solution increased slightly without α‐CD while the LCST decreased sharply at presence of α‐CD. Furthermore, UV spectroscopy showed that the photoisomerization of the polymer solution went on rapidly and reversibly, and 2D NOESY data suggested that the inclusion complexation of α‐CD with trans azobenzene moiety and the decomplexation with cis azobenzene resulted in reversible solubility behavior when objected to UV and Vis light irradiation alternately. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A novel hyperbranched polyester functionalized with azo chromophore: synthesis and photoresponsive properties

Summary A hyperbranched polymer containing azo chromophores on the periphery has been synthesized from transesterification reaction between a hyperbranched polyester bearing peripheral hydroxyl groups and ethyl 4-{4'-[N, Ndi(hydroxyethyl)aminobutoxy]phenylazo}benzoate. The hyperbranched polyester was obtained by adding the AB₂ monomers step by step to a multifunctional core molecule. Narrow molecular weight distribution, with a polydispersity index as low as 1.1, was achieved by this method. The azobenzene- functionalized hyperbranched polymer as well as the corresponding azo monomer showed typical photoresponsive behavior of an azobenzene type molecule. The trans azobenzene units of the hyperbranched polymer could be reversibly switched by UV irradiation to the cis form that recovered slowly to the trans form in the dark. However, significantly less conversion from the trans isomer to the cis isomer was observed for the azobenzenefunctionalized hyperbranched polymer at the photostationary states under the irradiation of UV light, which could be attributed to the increased steric hindrance in the hyperbranched polymer. Received: 1 April 2002 /Revised version: 10 July 2002/ Accepted: 15 July 2002     

Synthesis and fluorescence studies of dual-responsive nanoparticles based on amphiphilic azobenzene-contained poly (monomethyl itaconate)

Here photo- and pH-responsive behavior of polymer micelles formed by synthesized azo-modified poly (monomethyl itaconate)s (PMMI-Azo) with appropriate amounts of azobenzene side groups (DS_Azo) were reported. The PMMI-Azo polymer with DS_azo?=?20 was used for the preparation of micelles using dialysis method. Self-assembly behavior of the PMMI-Azo was studied by transmission electron microscopy (TEM), fluorescence spectroscopy, dynamic light scattering (DLS) and UV–Vis spectrophotometry techniques. It was found that the PMMI-Azo polymers self-assembled into spherical micelles with core-shell structures. TEM images and DLS analysis of the PMMI-Azo-20 micelles showed regular spherical micelles which increased in size and polydispersity index after UV irradiation, caused by isomerization. It was also found that with increasing the polymer concentration, the fluorescence intensity decreased obviously which could be attributed to the interactions between the closely located pyrene and azobenzene in the micelles core. Experimental results suggested that the micellization-enhanced fluorescence was caused by a slowdown in the rate of the trans-to-cis photoisomerization. Furthermore, it was found that the fluorescence intensity of the aqueous micellar solutions was sensitive to stimulus such as pH change and UV light irradiation, as a result of alteration in aqueous micellar association. The results clearly indicated the role of UV irradiation and pH in the state of the polymer micellar association which is responsible for fluorescence intensity.     

Photoresponse gas permeability of azobenzene‐functionalized glassy polymer films

The gas permeability of membranes composed of a polymer functionalized with azobenzene chromophores (azopolymer) was investigated under different conditions of light irradiation. A series of measurements with He gas showed that the permeability of the membranes increased by 5–10% upon irradiation with blue light, which caused cyclic photoisomerization of the azobenzene chromophores. In contrast, no significant change in gas permeability and no photoisomerization of the azobenzene chromophores were observed upon irradiation with red light. Moreover, using a digital video microscope, we observed that azopolymer irradiated with blue light spread out from localized particulate domains and thinly covered the surface of the base membrane. Together with the results of the photoresponse permeability measurements, this observation strongly suggested the existence of photoinduced plasticization as discussed in the study of photoinduced surface relief formation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2068–2072, 2003     

Surface hydrophilic–hydrophobic property on transparent mesoporous silica thin films containing chromium oxide single-site photocatalyst

Using a sol–gel/spin-coating method with organic template, the transparent mesoporous silica thin films containing chromium oxide moieties can be prepared on quartz plate. The spectroscopic characterization has revealed that these thin films contain isolated and tetrahedrally coordinated chromium oxide moieties (single-site photocatalyst) in their frameworks. Even before UV light irradiation these Cr-containing mesoporous silica (CrMS) thin films have demonstrated a hydrophilic surface property more strongly than Cr-containing non-porous silica films and mesoporous silica thin films without chromium oxide. Furthermore, the Cr-containing mesoporous silica thin films showed the super-hydrophilic property not only under UV light irradiation but also even under visible light irradiation from a fluorescent lamp. In the presence of ethylene gas, the polyethylene was successfully formed on the surface of the CrMS thin film under UV light irradiation. After the formation of polyethylene the surface property of the CrMS thin film was converted into hydrophobic while keeping its transparency. The isolated and tetrahedrally coordinated chromium oxide moieties are responsible for these photo-induced surface reactions.     

The use of 1H NMR and UV–vis measurements for quantitative determination of trans/cis isomerization of a photo‐responsive monomer and its copolymer

Photo‐induced trans–cis isomerization of trans‐4‐methacryloyloxyazobenzene and its copolymer with methylmethacrylate, synthesized in this work, were investigated in a polar protic (CDCl₃) and a polar aprotic (DMSO‐d₆) solvent, using ¹H‐NMR_. The results were cross‐correlated with trans–cis isomerization calculated from UV–visible spectra. The kinetics of the photoisomerization reaction of the monomer species under UV light irradiation is described by a simple first order exchange between the trans and cis forms of the monomer. The behavior was found to be similar for both solvents. The cis‐to‐trans reversion in the absence of irradiation is about 3% of the back reaction under irradiation. For the polymer in solution, the behavior was more complicated and is described by the sum of two equilibria, each of first order exchange kinetics of the trans form with two different types of cis isomer in the polymer; a first type of isomerization similar in behavior to that of the monomer and the second type much faster in cis/trans exchange rate. This bikinetic behavior is in agreement with that of the azobenzene moiety in PMMA as a solid phase mixture. The relative proportion of the two rates comprising the bikinetic effect is solvent‐dependent. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1103–1112, 2006     

Investigation of optical modulated conductance effects based on a graphene oxide-azobenzene hybrid

An azobenzene (AZO) chromophore was covalently attached to graphene oxide (GO) through an amide linkage. The microstructure of the GO-AZO hybrid was characterized by microscopic and X-ray diffraction methods. An internal short-range ordered crystalline structure similar to graphite was observed. Spectroscopic evidence testified the strong electronic interactions between the AZO and GO in this GO-AZO hybrid system. Upon ultraviolet (UV) irradiation, the AZO moieties bonding on GO underwent a reversible trans-cis photoisomerization behavior. An optical modulated conductance of the GO-AZO film induced by the isomerization of the AZO chromophores was also monitored. The current showed a twofold increase after irradiation of UV light for 20 min.     

Photo-Orientation at the Interface between Thermotropic Nematic Liquid Crystal and Water Caused by Azobenzene Polymer Additives

Photoinduced orientational transitions in a thermotropic nematic liquid crystal caused by modulation of the boundary conditions at the water surface were observed and investigated. The initially degenerated planar anchoring of liquid crystal at the water surface becomes homeotropic under near-UV irradiation which induces photoizomerization of comb-shaped azobenzene polymer additives adsorbed from the liquid crystal to its interface with water. After irradiation, the photoinduced orientational structures spontaneously relax to the initial ones. It was shown that the orientational sensitivity to light irradiation strongly depends on the polymerization degree of azobenzene additives.     

Synthesis and physical properties of liquid‐crystalline copolymers with azobenzene segments

To investigate the effects of photoisomerizable azobenzene segments on the liquid‐crystalline characteristics and thermal properties of polymers, a series of liquid‐crystalline homopolymers and copolymers with azobenzene segments was synthesized. The azobenzene contents of the copolymers were estimated with elemental analysis. The photoisomerization of the azobenzene derivatives was studied with ultraviolet–visible (UV–vis) spectroscopy. The UV–vis absorption of the copolymers was found to be parallel with the content of the azobenzene segments. UV irradiation was found to cause a decrease in the copolymer transmittance around 355 nm due to the photoinduced isomerization from entgegen (E) to zusammen (Z). The phase‐transition temperatures and molecular weights of the polymers were investigated with differential scanning calorimetry and gel permeation chromatography, respectively. The variation in the phase‐transition temperature of the homopolymers before and after UV (365 nm) irradiation was investigated. The bended Z structure was found to disturb the order of the orientation of liquid crystals and to lower the phase‐transition temperature. The appearance of the polymer film was changed from opaque to clear after sufficient UV irradiation. The image recording of the polymer films was achieved after UV irradiation through a mask with pictures. The stability and reliability of the Nematic‐Isotropic phase transition of the homopolymers was evaluated with repeated cycles of 365‐nm UV irradiation and heating at 130°C. After the recycle phase transition was repeated nine times, no significant decay in the response and transmittance could be found. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 2006