Photoinduced self-structured surface pattern on a molecular azo glass film: structure-property relationship and wavelength correlation

In this study, three series of star-shaped molecular azo glasses were synthesized, and self-structured surface pattern formation on the azo compound films was studied by laser irradiation at different wavelengths. The molecular azo glasses were synthesized from three core precursors (Tr-AN, Tr-35AN, Tr-H35AN), which were prepared by ring-opening reactions between 1,3,5-triglycidyl isocyanurate and corresponding aniline derivatives. The star-shaped azo compounds were obtained through azo-coupling reactions between the core precursors and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile, and 4-nitroaniline, respectively. By using the two-step reaction scheme, three series of azo compounds with different structures were obtained. The core precursors and azo compounds were characterized by using (1)H NMR, FT-IR, UV-vis, mass spectrometry, and thermal analyses. The self-structured surface pattern formation on films of the azo compounds was studied by irradiating the azo compound films with a normal-incident laser beam at different wavelengths (488, 532, and 589 nm). The results show that the photoinduced surface pattern formation behavior is closely related to the structure of the azo compounds, excitation wavelength, and light polarization conditions. The absorption band position of the π-π* transition is mainly determined by the electron-withdrawing groups on the azo chromophores. When the excitation wavelength is between λ(max) and the band tail at the longer wavelength side, the self-structured surface patterns can be more efficiently induced to form on the films. The 3,5-dimethyl substitution on azo chromophores inhibits the surface pattern formation for certain excitation wavelengths. Increasing molecular interaction also shows an effect of restraining the surface pattern formation. The irradiations with linearly and circularly polarized light cause significant differences in the alignment manner of the pillarlike structures and their saturated height.     

Epoxy-based azo polymers with high chromophore density: Synthesis,characterization and photoinduced birefringence

Three epoxy-based azo polymers(PEP-AZ-Cl,PEP-AZ-CN and PEP-AZ-NT) with high chromophore density were synthesized by using post-polymerization azo-coupling reactions between epoxy-based precursor polymer(PEP-AN) and diazonium salts of 4-chloroaniline,4-aminobenzonitrile and 4-nitroaniline,respectively.The structures and properties of the azo polymers were characterized by using ~1H-NMR,FT-IR,UV-Vis and thermal analyses.The photoinduced birefringence of the azo polymers was studied by irradiating spin-coated films of the polymers with laser beam at three different wavelengths(488,532,and 589 nm).The results indicate that the photoinduced birefringence of the azo polymers is related with the electron-withdrawing group on azo chromophores and the excitation wavelength.The excitation wavelength that can cause the efficient responses is determined by the absorption band positions of the azo chromophores,which are mainly affected by the electron-withdrawing group on the chromophores.Therefore,the azo polymers containing chromophores with different electron-withdrawing groups show different responsive behavior to the irradiation light at different wavelengths.When irradiated with 488 nm light,PEP-AZ-Cl shows the shortest time to reach the saturated birefringence but with the lowest saturation birefringence level compared with the other two azo polymers.When irradiated with 532 nm light,PEP-AZ-CN shows the shortest time to reach the saturated birefringence.When irradiated with 532 and 589 nm light,PEP-AZ-NT shows the highest saturation birefringence level.     

Effect of crosslinking on the photoinduced orientation of azo groups in thin polymer films

A series of amorphous copolymers containing disperse red 1 and crosslinkable acrylic groups were prepared. The crosslinked polymers were prepared in thin films by thermal polymerization of the acrylic groups in the copolymers. The orientation induced by irradiation with a linearly polarized laser was measured as birefringence at several temperatures, and the effect of crosslinking on the photoinduced orientation was investigated. Crosslinking enhanced the stability of the photoinduced birefringence. In particular, crosslinking helped to maintain the birefringence both at high temperatures and after the linearly polarized laser was turned off. The birefringence dynamics was analyzed with biexponential curve fitting. Crosslinking influenced not only the birefringence levels but also its rate of growth. The growth rate of the photoinduced birefringence decreased by crosslinking, whereas the relaxation was not significantly affected. Although crosslinking restrained the mobility of the azo chromophores, a certain fraction could orient or move randomly even in highly crosslinked polymer networks. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1686–1696, 2001     

含三氟甲基偶氮生色团环氧树脂类聚合物合成和光致表面自结构研究

利用后重氮偶合反应,将4-(三氟甲基)苯胺的重氮盐与两种具有高苯胺残基密度的环氧树脂类前体聚合物在极性有机溶剂中反应,制备了两种具有高生色团密度的环氧树脂类偶氮聚合物PEP-AZ-CF3和PEP-35AZ-CF3.利用1H-NMR、FTIR、UV-Vis和DSC等分析方法对2种偶氮聚合物结构、热性能及吸收光谱性能进行了分析及表征.研究了在不同波长的激光(488、532及589 nm)照射下,2种偶氮聚合物薄膜光致表面自结构的形成.研究结果表明,入射激光的波长对偶氮聚合物表面自结构现象的形成有明显影响.仅在波长488 nm的偏振激光(线偏振光或圆偏振光)照射下,2种偶氮聚合物薄膜表面能观察到自结构现象.圆偏光比线偏光更有利于诱导表面起伏结构的形成,但形成的表面结构不具备长程取向有序性.聚合物生色团中偶氮键邻位的取代甲基对表面自结构的形成起到阻碍作用.     

Photoinduced optical anisotropy in azobenzene methacrylate block copolymers: Influence of molecular weight and irradiation conditions

The photoinduced anisotropy in a series of azomethacrylate block copolymers with different molecular weights and azo contents has been investigated under several irradiation conditions. Depending on molecular weight and composition, different microstructures (disordered, lamellar, spherical) appear, due to block microsegregation. Measurements of birefringence (Δn) induced with linearly polarised 488 nm light show that the highest (and stable) Δn_N values (birefringence normalised to the azo content) are achieved in copolymers with a lamellar structure. Lower Δn_N are obtained in the copolymers in which azobenzene units segregate to nano spheres and the lowest (and less stable) Δn_N values, appear in disordered systems not showing any defined microstructure. Besides, higher Δn_N is obtained in the copolymers with larger molecular weight of the poly (methyl methacrylate) and the azo polymer blocks, both in the microspheres segregated polymers as well as in those without a clear microstructure. This behaviour is consistent with that of the photoinduced order of azobenzene units obtained from dichroism measurements. Irradiation temperature (from 30 to 90 °C) and light power (from 100 to 500 mW/cm²) also influence the photoinduced response. Photoinduced Δn_N growth rate is faster when both temperature and irradiation power increase. Furthermore, birefringence is only induced at temperatures up to 90 °C, the maximum value being obtained at about 70 °C. No clear dependence of the final Δn_N value with light power has been found.     

Influence of chromophore concentration and network structure on the photo‐orientation properties of crosslinked epoxy‐based azopolymers

An azo prepolymer (TAZ) was synthesized by reaction between Disperse Orange‐3 (DO3) and diglycidyl ether of bisphenol A (DGEBA). Selected amounts of TAZ were blended with DGEBA and cured with an aliphatic diamine, either meta‐xilylenediamine (MXDA) or polyetheramines (Jeffamine D series). The photoinduced anisotropy and optical storage properties in two series of crosslinked epoxy‐based azo polymers with different architectures and chromophore contents (from 3 to 24 in weight) have been investigated. Measurements of the birefringence (Δn) induced with linearly polarized 488 nm light show that the Δn values increases with DO3 weight fraction. Networks with the same chromophore concentration but different backbones exhibit similar levels of induced anisotropy under the same irradiation conditions. The remaining birefringence and relaxation times are influenced by the molecular weight between crosslinks of networks. Higher is reflected in shorter relaxation times and lower remaining birefringence. In terms of the level of induced dichroism measured on relaxed films, it was found an agreement with the remaining anisotropy. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1004–1014, 2009     

官能化度对环氧树脂类偶氮聚合物光致取向的影响

利用光诱导双折射和偏振红外光谱法研究了偶氮官能化度对环氧树脂类偶氮聚合物BP-AZ-CA的光致取向行为的影响,重点研究了偶氮官能化度对偶氮生色团和聚合物主链的光致取向速度及饱和取向程度的影响规律.结果表明,随着偶氮官能化度的增加,偶氮生色团和聚合物主链的光致取向速度均降低,但二者的饱和取向程度增加.体系中氢键相互作用的增强是导致BP-AZ-CA的光致取向行为随偶氮官能化度增加而变化的原因之一.     

两种偶氮分子玻璃材料的光致取向特性

利用光诱导双折射和偏振红外光谱法研究了2种无定形树枝状偶氮分子材料G1-AZ-CN和G2-AZ-CN的光致取向行为, 对其偶氮生色团及非光响应性星形核的光致取向速度、 取向松弛速度和饱和取向程度进行了系统研究. 结果表明, G2-AZ-CN中偶氮生色团和星形核的光致取向速度快于G1-AZ-CN, 但其取向松弛速度慢于G1-AZ-CN, 这与G2-AZ-CN具有更高的偶氮生色团密度有关. G1-AZ-CN具有较高的饱和取向程度及较好的取向稳定性. 与具有相同类型偶氮生色团的偶氮聚合物BP-AZ-CN相比, G1-AZ-CN和G2-AZ-CN中偶氮生色团表现出较慢的取向速度. 其取向行为与其特殊的星形分子结构有关.     

表面起伏光栅形成速率的影响规律Ⅱ.偶氮生色团上甲基取代基的影响

设计并合成了偶氮生色团上含有不同数目甲基取代基的环氧树脂类偶氮高分子 .研究了偶氮生色团上甲基取代对表面起伏光栅形成速率的影响规律 .实验结果表明 ,在偶氮生色团上引入甲基取代基使光栅形成的速率明显减慢 ,引入甲基数目越多 ,光栅形成速率就越慢 .     

Surface-relief-grating formation behavior of two hyperbranched azo polymers

In this work,surface-relief-grating formation behavior was studied by using two hyperbranched azo polymers.The hyperbranched polymers containing pseudo-stilbene type azo chromophores throughout the hyperbranched structure were synthesized by step-growth polycondensation of AB₂ type monomers.The AB₂ monomer,4-（4’-（bis（2-chloroethyl）amino）-phenylazo） benzoic acid（BAA）,was prepared through azo-coupling reaction between N,N’-bis（2-chloroethyl）aniline and 4- aminobenzenic acid.The another AB₂ monomer,4-（4’-（bis（2-chloroethyl）amino）phenylazo）-3-nitro-benzoic acid（BANA）, was prepared through azo-coupling reaction between N,N-bis（2-chloroethyl）aniline and 3-nitro-4-aminobenzcnic acid.The hyperbranched polymers（PBAA and PBANA） were prepared through nucleophilic substitution reaction of BAA and BANA, respectively.The polymers synthesized were characterized by using spectroscopic methods and thermal analysis.The photoinduced dichroism and photo-induced surface-relief-grating（SRG） formation of the hyperbranched polymers were investigated upon irradiation with Ar⁺ laser at 488 nm.PBAA shows typical photoinduced dichroism SRG formation behavior.On the contrary,PBANA does not show the photoresponsive properties.The results indicate that the nitro at the ortho position of azo group of PBANA shows the effect of inhibiting the photoinduced variations.The effect can be attributed to the blockage of the trans-cis isomerization of the azobenzene moieties by the steric hindrance.     

Photofabrication of surface relief gratings from azobenzene containing perfluorocyclobutane aryl ether polymer

An azobenzene based perfluorocyclobutane (PFCB) aryl ether polymer, poly[4‐(phenylazo)triphenylamine‐1,2‐hexafluorocyclobutyl ether], was synthesized by using a palladium catalyzed amination reaction and the 2π + 2π cyclodimerization of 4‐[(trifluorovinyl)oxy]bromobenzene. This polymer was designed and synthesized to permit azo chromophores to be incorporated, thus introducing photoinduced dynamic behavior into the PFCB containing polymer structure. The polymer exhibited a T_g of about 122 °C, and the maximum absorption of the azo chromophores was 407 nm in the film state. A thin film was prepared through a spin coating process, and the rapid growth of diffraction efficiency was then achieved by irradiation with an interference beam. The efficient formation of holographic surface relief gratings was also verified from AFM images, which show regularly defined and aligned grating structures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3525–3532, 2005     

Synthesis, photochromic properties, and complex formation with metal ions of hydrazones based on a spiropyran of oxaindane series

A series of hydrazones based on a new spiropyran oxaindane series was synthesized. The compounds were found to possess photochromism and are characterized by a high rate of spontaneous decoloration of their photoinduced merocyanine form. Dark and photoinduced formation of complex compounds in the reaction of metal ions with molecules of the hydrazones in solution was observed.     

Methacrylic azopolymers for holographic storage: A comparison among different polymer types

The photoinduced anisotropy and volume holographic storage in a series of polymers with different architectures and azo contents of 7% and 20% in weight have been investigated. Measurements of the birefringence (Δn) induced with linearly polarised 488 nm light show that for polymers with an azo content of 20%, the highest and more stable Δn values are achieved in copolymers with a di-block architecture. The Δn induced in polymers with 7 wt% azo content is not stable. Volume holographic polarisation gratings have been stored in the polymers. Only in the case of the di-block copolymer with an azo content of 20% in weight, the stored gratings are stable. A diffraction efficiency higher than 90% and an angular selectivity of 4° have been achieved in gratings with a period of 2 μm. It can be concluded that the di-block copolymer with 20 wt% azo content is the most suitable, among the studied copolymers, as volume holographic storage material.     

A DFT study on photoinduced surface catalytic coupling reactions on nanostructured silver: selective formation of azobenzene derivatives from para-substituted nitrobenzene and aniline

We propose that aromatic nitro and amine compounds undergo photochemical reductive and oxidative coupling, respectively, to specifically produce azobenzene derivatives which exhibit characteristic Raman signals related to the azo group. A photoinduced charge transfer model is presented to explain the transformations observed in para-substituted ArNO(2) and ArNH(2) on nanostructured silver due to the surface plasmon resonance effect. Theoretical calculations show that the initial reaction takes place through excitation of an electron from the filled level of silver to the lowest unoccupied molecular orbital (LUMO) of an adsorbed ArNO(2) molecule, and from the highest occupied molecular orbital (HOMO) of an adsorbed ArNH(2) molecule to the unoccupied level of silver, during irradiation with visible light. The para-substituted ArNO(2)(-)˙ and ArNH(2)(+)˙ surface species react further to produce the azobenzene derivatives. Our results may provide a new strategy for the syntheses of aromatic azo dyes from aromatic nitro and amine compounds based on the use of nanostructured silver as a catalyst.     

Photofabrication of two-dimensional quasi-crystal patterns on UV-curable molecular azo glass films

In this work, two-dimensional surface quasi-crystal patterns were developed by using a novel azobenzene-containing amorphous material (IAC-4), which was newly synthesized for the application. IAC-4 contains a core of isosorbide moiety and two push-pull type azo chromophores as the inner part. The periphery of IAC-4 is functionalized with four cinnamate groups, which can undergo [2+2] photocycloaddition reaction upon UV light irradiation. The molecular design can allow IAC-4 to readily form surface relief structures upon Ar+ laser irradiation, and the formed structures can be further stabilized through a photo-cross-linking reaction induced by UV light irradiation. On the basis of the material, two-dimensional (2D) quasi-crystal structures with different rotation symmetries were successfully fabricated on the IAC-4 films by using the dual-beam multiple exposure technique. In contrast to the approach using photoresist, the quasi-crystal structures were fabricated through the photoinduced mass migration, and no subsequent wet-etch or dry-etch step was required in the process. The quasi-crystal structures with rotation symmetry as high as 60-fold could be feasibly fabricated through this approach. The surface patterns and fabrication method can be potentially applied in areas such as optics, communications, and security inspection.     

Diazo liquid crystals for potential infrared applications

Nine series of liquid crystals containing an electron-donating group at one end of a conjugated system and an electron-withdrawing group at the other end have been synthesized. The electron-donating group is 4-n-alkylpiperazine and the electron-withdrawing group is the nitro group; and the conjugated system is diphenyldiazene with zero, one or two substituents on the phenyl ring not containing the nitro group. The substituents are-F,-Cl, and-CH₃. Most of the compounds synthesized are nematogenic and exhibit rather broad liquid crystalline ranges. The effects of the lateral substituents on the optical absorption and phase transition temperatures are correlated with their nature and position of substitution. Dielectric anisotropy and birefringence measurements were made using mixtures containing 10 wt % of synthesized liquid crystals dissolved in commercial mixture E7.     

Synthesis and characterization of 7, 16‐bis(phenylazo)tetraaza[14]annulenes

The diazo coupling reaction between a tetraaza[14]annulene ( 1 ) and a series of 4‐substituted phenyl‐diazonium tetrafluoroborates yielded the corresponding 7, 16‐disubstituted products. Mass spectra indicate the presence of molecular ion peaks that substantiate the 7,16‐disubstituted products ( 4 ); the lack of olefinic proton signals corresponding to the 7,16‐ position in the ¹H nmr spectra of 4 also show that diazo coupling has taken place. Analysis of signals corresponding to the methyl groups of 4 in their ¹H and ¹³C nmr spectra indicate that the imine‐bis‐hydrazone form is present for compounds that do not contain p‐methoxyphenyl groups. However, analysis of methyl signals in the nmr specta of compounds 4 containing p‐methoxyphenyl groups show that the bis‐azo form and the imine‐bis‐hydrazone form are present through tautomerism. Complexation with nickel(II) ion induces the formation of the bis‐ azo structure.     

Silicon‐containing azo polymers with paired mesogens and their applications to optical memory media

We synthesized a novel photoresponsive monomer, silicon‐containing azo monomer with paired mesogens in the side chain, by reacting 3‐methacryloxypropylmethyldichlorosilane with 2‐[2‐(4‐cyano‐azobenzene‐4′‐oxy)ethylene‐oxy]ethyl alcohol, a mesogenic group. Corresponding homopolymer and copolymers with methyl methacrylate were generated via radical polymerization with AIBN as a radical initiator. Investigations of their thermal properties and optical textures confirmed the monomer and the homopolymer have smectic structures. Homo‐ and copolymer films showed high potential as reversible data recording media via photoinduced alignment of azobenzene groups with irradiation of a linearly or circularly polarized light. Out of all the samples, the copolymer films with high azo dye contents showed the best resolution in the recorded data as well as the fastest response to a pump beam due to large optical birefringence induced in a write‐in process. Strong dependence of the stability of the data stored in the films on the glass transition temperature of the polymers was also observed. In addition, high‐quality holographic grating patterns were inscribed even on the copolymer film with azo molar content of only 7.0% using a modified two‐wave mixing technique. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6734–6745, 2008     

Photoresponsive peptide and polypeptide systems. VIII. Synthesis and reversible photochromism of azo aromatic poly(L-ornithine)

Poly(L -ornithine) having azo aromatic side chain was synthesized by the water-soluble carbodiimide procedure. The photochemical properties of the azo polypeptide poly[N^δ-p-(phenylazo)benzoyl-L -ornithine] (PPABLO) was investigated by absorption and circular dichroism spectroscopy in hexafluoro-2-propanol (HFIP). The photochromism of the dichroic bands in the visible and ultraviolet wavelength regions was found to be reversible as a function of irradiation time at different wavelengths due to the photostationary state (about 70% trans)–cis photoisomerization of the azo aromatic moieties. The reversible photoinduced solubility change was also studied. On irradiation PPABLO is soluble under ultraviolet light (cis) and precipitate under visible light (70% trans) in HFIP–water. A discussion was presented that includes results on azo aromatic poly(L -lysine).     

Diazo liquid crystals for potential infrared applications

Nine series of liquid crystals containing an electron-donating group at one end of a conjugated system and an electron-withdrawing group at the other end have been synthesized. The electron-donating group is 4-n-alkylpiperazine and the electron-withdrawing group is the nitro group; and the conjugated system is diphenyldiazene with zero, one or two substituents on the phenyl ring not containing the nitro group. The substituents are-F,-Cl, and-CH₃. Most of the compounds synthesized are nematogenic and exhibit rather broad liquid crystalline ranges. The effects of the lateral substituents on the optical absorption and phase transition temperatures are correlated with their nature and position of substitution. Dielectric anisotropy and birefringence measurements were made using mixtures containing 10 wt % of synthesized liquid crystals dissolved in commercial mixture E7.