Azobenzene isomerization in polymer co-crystalline phases

Three different co-crystalline phases of syndiotactic polystyrene (s-PS) with azobenzene have been prepared and structurally characterized. The triclinic δ-clathrate phase is characterized by a styrene-monomeric-unit/guest ratio (SU/G) equal to 8/1 while the δ-intercalate phase is characterized by SU/lG = 4/1. The orthorhombic ε-clathrate phase allows stacking of a variable number of guest molecules per monomer unit with SU/G ≥ 9/1. Isomerization reactions of the azobenzene guest molecules (both photo-induced trans → cis and spontaneous cis → trans) have been studied by UV–Visible and FTIR measurements. Repeated isomerization cycles produce progressive desorption from the polymer film of azobenzene molecules being simply dissolved in s-PS amorphous phases, while poorly affect the content of azobenzene molecules being guest of s-PS co-crystalline phases. Polarized FTIR spectra of axially oriented films indicate that isomerizations occur without expulsion of azobenzene guest molecules from the polymer co-crystalline phases. The whole set of data indicate that films exhibiting co-crystalline phases of azobenzene with s-PS are suitable for producing self-erasing photo-isomerization patterns with molecular size marks.     

Photoresponsive Crown Ethers. Part 11. Azobenzene-Pillared Cylindrical Macrocycle As a Photoresponsive Receptor

A cylindrical ionophore (1) in which two diaza-crown ethers are linked by two azobenzene pillars serves as a photoresponsive host molecule. In alkaline solution containing K⁺, (1) forms a cascade complex with dianionic guest molecules, but photoirradiated (1) which has a shortened cylinder cannot bind the guest molecules so effectively as (1). (1) solubilised in acidic aqueous solution by protonation of four tertiary nitrogens provides a hydrophobic cavity to bind several anionic azo dyes. The association is also suppressed by photoirradiation. The results indicate that parallel array of the azobenzene segments in (1) provides a host cavity and the photoinduced structural change is readily reflected by the association tendency. This is the first example for the photoresponsive inclusion complex.     

A Novel Alkoxysilyl Azobenzene Dye Photosensitizer with Alkylamino Group for Dye-Sensitized Solar Cells

A newly designed alkoxysilyl azobenzene dye with alkylamino group, 4-diethoxyphenylsilyl-4′-dimethylaminoazobenzene, was synthesized and examined as the photosensitizer for a dye-sensitized solar cell (DSSC). The novel azobenzene dye exhibited a strong absorption band in the visible region, and the solar cell using the dye as the sensitizer showed an incident monochromatic photon-to-current conversion efficiency (IPCE) value of 66 % at 440 nm and a light-to-electric energy conversion efficiency of 2.2 % under simulated sunlight irradiation of AM-1.5G one sun condition. The energy conversion efficiency obtained here is the highest value among those reported so far for azobenzene-sensitized solar cells, indicating the potential of alkoxysilyl dyes as photosensitizers for DSSCs.     

Polymeric dispersions of model azobenzene dyes

New host-guest systems were prepared by using poly(vinyl acetate) and different ethylene-vinyl acetate copolymers as polymeric hosts because of their good compatibility with polar molecules. Two symmetric azobenzene diesters, that is 4,4′-dicarboxyethylazobenzene and 4,4′-di(2-ethylhexyloxycarbonyl)azobenzene were selected as guests. Both solution casting and melt processing were adopted for the preparation of dispersion films that were thoroughly characterized by thermal analysis, UV-vis and FT-IR spectroscopy, and by polarized optical microscopy. The reported results indicate that solution casting afforded heterogeneous dispersions of dye microcrystals at guest concentrations larger than 0.3% whereas homogeneous colored films were obtained at lower dye contents. On the other hand, both melt processing and the presence of branched diester groups favored the dye dispersion within the polymer matrix. At 0.1% dye content, the adopted preparation technique did not appreciably affect the film properties. After 4-8 fold stretching, the host-guest films were analyzed by polarized light spectroscopy. Some of the investigated films displayed interesting polarization efficiency, potentially suited for the preparation of thin film polarizers.     

All‐optical switching modulation in poly(vinyl carbazole)—based azobenzene blends

We report on the all‐optical switching polymer blends based on photoconductive poly(vinyl carbazole) (PVK) and azobenzene dyes. The optical switching effects of the blends were measured by using a 514 nm pump beam and a 632.8 nm probe beam. The blends' photoinduced reversible and repeatable change in the transmittance of the probe beam is attributed to the photoinduced anisotropy of the azobenzene dyes. The influencing factors of the optical switching effect, such as the modulation frequency and pump beam intensity, were studied experimentally.     

Recent advances and potential applications of the photo‐induced macroscopic response of self‐assembled azopolymer in aqueous media

This mini‐review highlights issues related to the photo‐triggered macroscopic response of azopolymer in supramolecular hydrogel and polymer vesicles. Cyclodextrin (CD) can form an inclusion complex with trans‐azobenzene while cis‐azobenzene is expelled out of the CD's cavity. This photo‐reversible supramolecular host ? guest interaction is used to trigger sol ? gel transitions and to induce macroscopic assembly. When substituted azobenzene is located at chain ends of linear polyethylene glycol, monodisperse vesicles are formed exhibiting a photo‐induced pulsating behavior. The macroscopic phase transition and assembly or pulsating behavior of azopolymers can be expected to have potential applications as smart biomedical materials. © 2014 Society of Chemical Industry     

Bistable light scattering effects in a (side chain liquid crystalline polymer)/(low molecular weight liquid crystal) composite and in a ferroelectric liquid crystalline polymer

The electro-optical effects and aggregation states of liquid crystalline polymer (LCP)/ low molecular weight liquid crystal (LC) composite and ferroelectric liquid crystalline copolymer (FLCP) have been investigated. The nematic LCP was observed to be miscible with the nematic LC over wide ranges of concentration and temperature. The binary mixture showed an induced smectic phase in the range of 80/20-20/80 mol%. The electro-optical effects of the LCP/LC composite in an induced smectic phase could be classified into the turbid (light- scattering) and the transparent states upon application of AC and DC electric fields, respectively. The transient scattering mode was obtained by repeated voltage polarity reversal in the chiral smectic C phase of FLCP. The reversible transparent-opaque (light scattering) change was observed in the chiral smectic C state upon application of DC and AC electric fields, respectively. Both transparent and light-scattering states of the LCP/LC composite and the FLCP could be maintained, even after the electric field had been turned off (memory effect). The bistable effects of LCP/LC composite and FLCP are opposite under the same conditions. A novel type of electro-optical effect on light scattering was obtained for liquid crystalline polymer in the smectic states.     

Effect of Methyl Group on Acyclic Serinol Scaffold for Tethering Dyes on the DNA Duplex Stability

Acyclic serinol derivatives are useful scaffolds for tethering dyes within DNA duplexes. Here we synthesised an inverse l ‐threoninol (il ‐threoninol) scaffold and compared its effect on DNA duplex stability to other acyclic artificial nucleic acid scaffolds that are based on d ‐threoninol, l ‐threoninol, and serinol. When planar trans‐azobenzene was incorporated into the DNA duplex through a single bulge‐like motif (the wedge), the il ‐threoninol scaffold stabilised the duplex most efficiently. When scaffolds were incorporated in complementary positions (dimer motif) or in three adjacent positions (cluster motif), d ‐threoninol was the most stabilising. CD spectra indicated that the effect of scaffold on the duplex stability was closely related to the winding induced by each scaffold. When trans‐azobenzene was photo‐isomerised to non‐planar cis‐azobenzene, il ‐threoninol destabilised the duplex most strongly, irrespective of the number of artificial residues incorporated. The properties of the il ‐threoninol scaffold make it a useful tether for dyes or other functionalities.     

Light-Tunable Solvent Drying in Photo-Responsive Solution Coatings

Solvent drying rate of azobenzene/acetone solution coating was tunable by linearly polarized UV light. The light exposure promotes the photo-induced conformational change of azobenzene from its trans to its cis form, as well as the molecular alignment perpendicular to a polarization direction. Drying experiments of the photo-responsive solutions revealed that angled irradiation of s-polarized light showed a peculiar angle-dependent drying behavior, while no dependence was observed for the p-polarized light irradiation. The photo-induced solvent drying was observed in a particular range of azobenzene weight fractions and UV light intensities, suggesting a promising way to tune solvent drying rates by light.     

Azobenzene disperse dyes-past development and future prospects

The development of azobenzene disperse dyes is briefly reviewed. Some factors affecting the current application of these dye types are examined and future developments are considered.     

The aniline-to-azobenzene oxidation reaction on monolayer graphene or graphene oxide surfaces fabricated by benzoic acid

The oxidation of aniline to azobenzene was conducted in the presence of either monolayer graphene (EG) or graphene-oxide-like surface, such as GOx, under ultra-high vacuum conditions maintaining a 365-nm UV light exposure to enhance the oxidation reaction. The surface-bound products were investigated using micro Raman spectroscopy, high-resolution photoemission spectroscopy, and work function measurements. The oxygen carriers present on the GOx surfaces, but not on the EG surfaces, acted as reaction reagents to facilitate the oxidation reaction from aniline to azobenzene. Increasing the aniline concentration at 300 K confirmed that the exchange ratio from the aniline to the azobenzene was enhanced, as determined by the intensity ratio between the aniline- and azobenzene-induced N 1 s core-level spectra. The work function changed dramatically as the aniline concentration increased, indicating that the aniline on the GOx surface conveyed n-type doping characteristics at a low coverage level. A higher aniline concentration increased the p-type doping character by increasing the azobenzene concentration on the GOx surface. A comparison of the oxidation reactivity of aniline molecules on the EG or GOx surfaces revealed the role of the oxygen carriers on the GOx surfaces in the context of catalytic oxidation.     

Theory of optical anisotropy and spontaneous polarization in the tetragonal phase of BaTiO3

The microscopic mechanism of optical anisotropy in the tetragonal phase of BaTiO₃ is discussed taking account of the dipole-dipole effect due to the ionic and electronic polarizations of the crystal and the spontaneous Kerr effect. The last effect is a cause of the strong local field acting on the constituent ions. For the spontaneous polarization we use only the first effect. In this molecular model, only the Ti ions situated at the center of the lattice are assumed to have the spontaneous shift. It is found that the spontaneous polarization and the birefringence are in good agreement with the experimental data.     

偶氮苯聚合物（PGMAA-20）膜光栅结构的分子取向机制

设计合成及表征了一种含偶氮苯聚合物(PGMAA-20)材料．利用两束S偏振光作为泵浦光在PGMAA-20膜上刻写光栅并观察光栅的衍射信号；然后再用相衬显微镜直接观察偶氮苯薄膜上的光栅结构，并以分子取向机制为基础对光栅形成和消除过程提出了新解释，这种观点被实验观察结果进一步证实．     

Block copolymer aggregates with photo-responsive switches: Towards a controllable supramolecular container

Herein, we present a concept of combining host-guest chemistry with block copolymer self-assembly to fabricate an inner cross-linking block copolymer aggregate with photo-responsive switches on the basis of the reversible interaction between azobenzene and β-cyclodextrin, which can serve as a controllable supramolecular container to load and release guest molecules reversibly. The inner cross-link makes the block copolymer aggregates exhibit good stability, and the aggregates can keep their spherical morphologies during photo-irradiation treatment. When the switches are in on-state, cyclodextrins can bind with hydrophobic pyrene molecules; and when the switches are in off-state, pyrene molecules will get away from the cyclodextrins. The photo-controllable switches embedded in the aggregates endow this new supramolecular container with the capability to load and release guest molecules reversibly without structure disruption. It is anticipated that this line of work may open an avenue for fabricating new polymeric containers which can be used for controllable molecular transfer and catalysis.     

Electrochemical and photochemical reduction of a series of azobenzene dyes in protic and aprotic solvents

The electrochemical and photochemical reduction in ethanol of a series of azobenzene dyes containing tertiary amine groups has been investigated. It has been shown that the two mechanisms are of parallel importance and an explanation suggested using quantum chemical calculations (AM1, DFT). The solvation of these dyes in protic (methanol) and aprotic (dimethylformamide) solvents has also been studied and the effect of solvation on their electrochemical and photochemical reduction is discussed.     

Enhancing piezoelectric response in (002)-Oriented TaxAl(1−x)N films by magnetron-sputtering composition-tunable AlTa alloys

Aluminum nitride (AlN) films suffer from poor piezoelectric response due to the local hexagonal symmetry of wurtzite crystal structure. The fundamental mechanism of improving piezoelectric response in Ta-doped AlN thin films has not been fully understood. Here, we report that composition-tunable AlTa alloys, instead of Al and Ta individual sputtering targets, can be employed to deposit (002)-oriented Ta_xAl_(1−x)N films by magnetron-sputtering, where Ta-doping ratio is tunable by changing Al:Ta composition ratio of the AlTa alloys. Importantly, Ta-doping enhances spontaneous polarization to improve piezoelectricity significantly, as suggested by a threefold increase of the effective piezoelectric coefficients (d_33,eff) upon doping 7 at.% Ta. A better alignment of spontaneous polarization for stronger piezoelectric response originates from the reduction in point defects and crystalline disorder in the reduced-symmetrical wurtzite structure by doping Ta in the Ta_xAl_(1−x)N. This study provides significant insights in improving piezoelectric response in AlN for the development of high-performance film-bulk-acoustic-resonators.     

Stilbene and Azobenzene Diazonium Compounds as dye reagents for crown ethers

The change of light absorption of six coloured diazonium compounds ( 1 , 2a , 2b , 3 , 4 , 5 ) on complexation with five 18-crown-6 derivatives ( 6 – 10 ) in 1,2-dichloroethane was investigated. In the benzene, stilbene and azobenzene series complexation produces only hypsochromic shifts of the intramolecular ct-band of the diazonium ions up to 140 nm. The extend of the shift is effected by ion pairing. The ct-transition energy of the complexes diazonium ion is independent on the present anion in contrast to the uncomplexed compounds. The colour changes are not caused by host to guest charge transfer or E/Z-isomerisation of the stilbene and azobenzene derivatives. A quantitative explanation of the colour changes is given basing on an electrostatic interaction model.     

Photostability of a range of azobenzene dyes and their benzothiazolyl analogues in the presence of air

Irradiation of a range of azobenzene dyes and their benzothiazolyl analogues with visible light in the presence of air has been shown to give rise to the conversion of a triplet oxygen to the singlet state. As proof of the presence of a singlet oxygen, its reaction with tetraphenylcyclopentadienone was studied. The formation of the oxidation product cis-α,α'-dibenzoylstilbene by a singlet oxygen has been confirmed by chemical ionisation mass spectrometry, both in ethyl benzoate solution and on polyester fabric. Inhibition of photodegradation of the dyes by a singlet oxygen quencher (DABCO) has also been demonstrated.     

New Intermediates and Dyes for Synthetic-polymer Fibres Derivatives of 2-Chloro-4-nitro-4‘-(N-β-hydroxyethyl-N- β-cyanoethylamino)azobenzene

Condensation of 2-chloro-4-nitro-4^t-(N-(β-hydroxyethyl-N-β-cy anoethylamino)azobenzene with various acid chlorides, chloroformates, isocyanates, isothiocyanates and reactive halogeno compounds gives dyes of excellent fastness to light and sublimation on polyester fibres.     

Synthesis of temporarily solubilised azo disperse dyes containing a β‐sulphatoethylsulphonyl group and dispersant‐free dyeing of polyethylene terephthalate fabric

Disperse dyes containing a β‐sulphatoethylsulphonyl group have temporary solubility and can be applied for dispersant‐free dyeing of hydrophobic fibre. Six novel temporarily solubilised azo disperse dyes having a β‐sulphatoethylsulphonyl group in their structures were synthesised, and their dyeing properties on polyester were investigated. As a dye intermediate, a diazo component having dibromo groups was prepared, and 4‐diethylamino‐4′‐(2‐sulphatoethylsulphonyl‐4,6‐dibromo)azobenzene dyes were prepared by a diazo‐coupling reaction. Then, the dyes containing dicyano groups were prepared by cyanation of corresponding dyes with dibromo groups. The absorption maxima of the dyes were affected by the substituents in the diazo and coupling component rings and varied from 434 to 616 nm in dimethylformamide. Polyethylene terephthalate woven fabric could be dyed with the synthesised temporarily solubilised dyes without using any dispersants. Dyebath pH affected the K/S value at maximum absorption as well as percentage exhaustion on polyethylene terephthalate fabric, and the optimum pH was 5. The dyes gave brownish orange, red, purple, and greenish blue hues on polyethylene terephthalate fabrics, and colour build‐up was good. Wash fastness was good to excellent, rubbing fastness was moderate to excellent, and light fastness was poor to moderate.