Synthesis of Functionalized Azobiphenyl- and Azoterphenyl- Ditopic Linkers: Modular Building Blocks for Photoresponsive Smart Materials

Modular synthesis of structurally diverse functionalized azobiphenyls and azoterphenyls for the realization of optically switchable materials has been described. The corresponding synthesis of azobiphenyls and azoterphenyls by stepwise Mills/Suzuki-Miyaura cross-coupling reaction, proceeds with high yields and provides facile access to a library of functionalized building blocks. The synthetic methods described herein allow combining several distinct functional groups within a single unit, each intended for a specific task, such as 1) the −N=N− azobenzene core as a photoswitchable moiety, 2) aryls and heteroaryls, functionalized with carboxylic acids or pyridine at its peripheries, as coordinating moieties and 3) varying substitution, size and length of the backbone for adaptability to specific applications. These specifically designed azobiphenyls and azoterphenyls provide modular bricks, potentially useful for the assembly of a variety of polymers, molecular containers and coordination networks, offering a high degree of molecular functionality. Once integrated into materials, the azobenzene system, as a side group on the organic linker backbone, can be exploited for remotely controlling the structural, mechanical or physical properties, thus being applicable for a broad variety of ‘smart’ applications.     

Enantioselective formation of a dynamic hydrogen-bonded assembly based on the chiral memory concept

In this paper, we report the enantioselective formation of a dynamic noncovalent double rosette assembly 1a(3).(CYA)(6) composed of three 2-pyridylcalix[4]arene dimelamines (1a) and six butylcyanuric acid molecules (BuCYA). The six 2-pyridyl functionalities of the assembly interact stereoselectively with chiral dicarboxylic acids 3a-e via two-point hydrogen-bonding interactions. One of the two enantiomeric assemblies (P- or M-) 1a(3).(CYA)(6) is formed in excess as the result of the complexation of the chiral diacids, resulting in formation of optically active assemblies. The complexations with dibenzoly tartaric acids D-3a and L-3a (3 equivalent), respectively, leading to the formation of diastereomeric assemblies (P)-1a(3).(BuCYA)(6).(D-3a)(3) and (M)-1a(3).(BuCYA)(6).(L-3a)(3) with 90% diastereomeric excess. The diastereomeric excess in (M)-1a(3).(BuCYA)(6).(L-3a)(3) is "memorized" when L-3a is removed by precipitation with ethlylenediamine (EDA). The assembly (M)-1a(3).(BuCYA)(6) is still optically active (90% enantiomeric excess), although none of its individual components are chiral. (M)-1a(3).(BuCYA)(6) has a high kinetic stability toward racemization (E(a) = 119 kJ mol(-)(1), half-life of (M)-1a(3).(BuCYA)(6) is ca. 1 week at 20 degrees C).     

含手性末端基偶氮聚合物膜的全息光栅研究

偶氮苯侧链型高分子由于其含有偶氮苯基团 ,在光作用下会发生可逆的顺反异构过程 ,具有光致取向特性 ,在光学处理、衍射光学、投影显示、光开关等许多方面具有潜在的应用性[1] .近 1 0多年来国内外学者对此类化合物进行了广泛的研究 ,已有文献报道可利用Ａｒ+激光束在偶氮苯聚合物薄膜上直接“写入”表面凸起光栅 ,并且通过原子力显微镜观测到光栅起伏 .这种光栅很稳定 ,并可以利用光学方法“擦去” .偶氮苯聚合物上述独有的性质引起了许多学者的兴趣[2 ,3] .另一方面 ,由于旋光性聚合物在光学物理性质上的优势 ,我们已将手性基团引入偶氮苯…     

Photonic control of surface anchoring on solid colloids dispersed in liquid crystals

The anchoring of liquid-crystal (LC) mesogens to the surfaces of colloids is an important factor in determining intercolloidal interactions and the symmetry of the ensuing colloidal assembly in nematic colloids. The dynamic control of surface anchoring could therefore provide a handle to tune the colloidal organization and resulting properties in these systems. In this article, we report our results on the study of thermotropic nematic LC (E7) dispersions of silica and glass microcolloids bearing photosensitive surface azobenzene groups. By the photoinduced modulation of the colloidal-LC interfacial properties, due to the trans-cis isomerization of azobenzene units, we tune the anchoring on silica colloids from homeotropic (trans-azobenzene) to homogeneous planar (cis-azobenzene) reversibly. In tune with the change in surface anchoring, the interparticle interactions were also dictated by dipolar and quadrupolar symmetries for homeotropic and homogeneous planar anchoring, respectively. In our experiments, we find that, in addition to the isomerization state of the surface-bound azobenzene units, the nature of the colloid plays a crucial role in determining the anchoring state obtained on applying photostimuli. We also study the LC anchoring on colloids as a function of the azobenzene surface density and find that beyond a threshold value the anchoring properties remain invariant.     

Helical assembly of azobenzene-conjugated carbohydrate hydrogelators with specific affinity for lectins

Carbohydrate-mediated interactions are involved in various biological processes via specific molecular assembly and recognition. Such interactions are enhanced by multivalent effects of the sugar moieties, and thus supramolecular sugar-assembly, i.e., spontaneous association of glycoamphiphiles, is a promising approach to tailor glycocluster formation. In this study, novel sugar-decorated nanofibers were successfully prepared by self-assembly of low molecular weight hydrogelators composed of azobenzene and disaccharide lactones. Circular dichroism measurement of the as-prepared hydrogels indicated that the azobenzene amphiphile containing a lactose moiety possessed (R)-chirality, while the maltose-azobenzene conjugate exhibited (S)-chirality, even though the cellobiose-conjugated azobenzene existed in an achiral form. This suggests that the chiral orientation of the chromophoric azobenzene depended on both the glycosidic linkages and the steric arrangement of hydroxyl groups in the conjugated carbohydrates. Lectin-binding and cell adhesion assays revealed that the nonreducing ends of the conjugated sugar moieties were exposed on the surfaces of self-assembled nanofibrous hydrogels, allowing them to be effectively recognized by the corresponding lectins. In addition, photoisomerization of azobenzene under ultraviolet irradiation induced the sol-gel transitions of the hydrogels. These results demonstrate that the reversibly transformed fibrous glycohydrogels show potential for application as carbohydrate-decorated scaffolds for cell culture engineering.     

Synthesis, resolution, and applications of 1,16-dihydroxytetraphenylene as a novel building block in molecular recognition and assembly

This paper concerns the synthesis of 1,16-dihydroxytetraphenylene (DHTP) (2) by employing a novel NBS bromination route. (+/-)-DHTP 2 was successfully resolved into its optical antipodes and converted to (+/-)-1,16-bis(diphenylphosphino)tetraphenylene (BPTP) (26), whose platinum complex BPTP-PtCl(2) (27) was also obtained. As a hydrogen bond donor, racemic and optically active DHTP 2 was allowed to assemble with 4,4'-bipyridine to form single crystals of good quality. X-ray Diffraction studies of these crystals revealed that the crystallographic packing of the hydrogen bonded complex between (+/-)-2 and 4,4'-bipyridine was different from the one formed from (S)-2 and 4,4'-bipyridine. It was found that an infinite zigzag chain with alternate chirality was formed in the assembly of (+/-)-2 and 4,4'-bipyridine, while (S)-2 and 4,4'-bipyridine failed to show the same assembly pattern. The reason (+/-)-2 formed an alternate and zigzag chain with 4,4'-bipyridine was most likely due to the inherent stability of this supramolecular assembly. The chiral recognition between 2 and optically active BINAP under the direction of platinum(II) has also been examined. (1)H and (31)P NMR spectroscopic studies demonstrated that there was an obvious discrimination of 2 between the enantiomers of BINAP-PtCO(3).     

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(Ml) was synthesized. Polymer (PMl)possessing optical phase conjugated response was obtained by homopolymerzation of the optically active monomer (Ml)using free radical polymerization. The polymer was very soluble in common solvents and good optical quality films could beeasily fabricated by spin coating. The optical phase conjugated responses of the polymer PM1 were measured by degeneratefour-wave mixing (DFWM). In comparison with polymer containing no chiral group, it was found from the preliminarymeasurement of photoisomeric change that optical phase conjugated response of the PM1 in the long-range order hexagonalsymmetry microstructure could be easily controlled by choosing the appropriate polarization direction of the irradiatingbeams (514.5 nm) and the irradiating number, presumably due to the chiral group in the PM1 molecular structure. For thecase of the polymer investigated here, a chiral group side chain was introduced to increase optical phase conjugated responseintensity with different polarization directions of the irradiating beams, which aims originally at searching for a new photo-ative material.     

A Hierarchical MFI Zeolite with a Two‐Dimensional Square Mesostructure

A conceptual design and synthesis of ordered mesoporous zeolites is a challenging research subject in material science. Several seminal articles report that one‐dimensional (1D) mesostructured lamellar zeolites are possibly directed by sheet‐assembly of surfactants, which collapse after removal of intercalated surfactants. However, except for one example of two‐dimensional (2D) hexagonal mesoporous zeolite, no other zeolites with ordered 2D or three‐dimensional (3D) mesostructures have been reported. An ordered 2D mesoporous zeolite can be templated by a cylindrical assembly unit with specific interactions in the hydrophobic part. A template molecule with azobenzene in the hydrophobic tail and diquaternary ammonium in the hydrophilic head group directs hierarchical MFI zeolite with a 2D square mesostructure. The material has an elongated octahedral morphology, and quaternary, ordered, straight, square channels framed by MFI thin sheets expanded along the a–c planes and joined with 90° rotations. The structural matching between the cylindrical assembly unit and zeolite framework is crucial for mesostructure construction.     

Hydrolysis of activated esters catalyzed by L-histidine graft copolymers

A graft copolymer of L -histidine on polyethylenimine has been demonstrated to be an efficient catalyst for the hydrolysis of activated phenyl esters in aqueous systems. A lack of any significant cooperative effects between neighboring imidazoles or amines has been shown by a study of the catalytic solvolysis of p-nitrophenyl acetate at varying pH. A similar study with 4-acetoxy-3-nitrobenzoic acid has displayed a bell-shaped pH-rate profile indicative of electrostatic interactions between the catalyst and substrate. Hydrophobic interactions were also evident from an investigation of the hydrolytic rates of a series of 4-acyloxy-3-nitrobenzoic acids. In an attempt to demonstrate stereospecific catalyzed reactions, the hydrolyses of optically active esters catalyzed by the optically active copolymer were studied.     

主链型聚氨酯光动力高分子的合成与表征

合成了一种主链含推拉电子型偶氮苯基团的聚氨酯光动力高分子 .对聚合物的结构、热性能及光学性能进行了表征 .此聚合物为主链含假芪型偶氮生色团的无定形高分子 .用 4 88nmAr+ 激光对聚合物薄膜进行光加工 ,得到了规则的正弦表面起伏光栅 ,其起伏深度大于 2 0 0nm .光栅的一级衍射效率可达 2 4 % .     

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.     

Reversible photoregulation of binding of alpha-chymotrypsin to a gold surface

An ability to optically modulate the interactions of surfaces with functional biomolecules provides an important basis for generating new technologies including reversible biosensors, advanced medical implants, and biomolecular computers. Here we report the first example of reversible photoregulation of binding of a protease to a functional surface. A modular approach is presented with a surface-bound inhibitor containing a photoisomerizable azobenzene core to which is attached (i) appropriate protease binding functionality and (ii) a tether for surface attachment. The principle is demonstrated for alpha-chymotrypsin using a phenylalanine-based trifluoromethylketone inhibitor containing an azobenzene core and an alkyne-functionalized ethylene glycol tether, which is attached to the surface using click chemistry. UV/vis irradiation of the functional surface leads to a significant, reversible change in the amount of alpha-chymotrypsin that attaches to the surface, as measured by surface plasmon resonance.     

Dual modulation of a molecular switch with exceptional chiroptical properties

The ability to modulate the chiroptical properties of optically active molecules induced by external stimuli such as light, heat, and electrical fields allows for the design and development of molecular switches, memory devices, sensors, and photonic devices. A helical o-terphenyl compound functionalized with photoresponsive azobenzene and electroactive imide groups is designed as a dual-mode chiroptical molecular switch. Its exceptional optical activity (e.g., [alpha]436 = -9500) can be changed and modulated through photoisomerization of the azobenzene moiety using UV and visible light. Reversible modulation by electrochemical means was also achieved through the redox reaction occurring at the imide group. Large chiroptical read-out signals were observed during the redox cycles as indicated by the molar ellipticity values as high as 285,000 deg.cm2.dmol-1. Exceptionally high optical activity and large responses to both light and electrical bias make this chiral molecule suitable for the development of new molecular switches, sensors, and other optical devices.     

Ultrafast Light‐Driven Substrate Expulsion from the Active Site of a Photoswitchable Catalyst

The photoswitchable piperidine general base catalyst is a prototype structure for light control of catalysis. Its azobenzene moiety moves sterically shielding groups to either protect or expose the active site, thereby changing the basicity and hydrogen‐bonding affinity of the compound. The reversible switching dynamics of the catalyst is probed in the infrared spectral range by monitoring hydrogen bond (HB) formation between its active site and methanol (MeOH) as HB donor. Steady‐state infrared (IR) and ultrafast IR and UV/Vis spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within a few picoseconds. Thus, the force generated by the azobenzene moiety even in the final phase of its isomerization is sufficient to break a strong HB within 3 ps and to shut down access to the active site.     

Liquid crystal polymers containing permanent dipole azobenzene chromophores

Three photochromic monomers containing a permanent dipole photochromic azobenzene group spaced from the methacryloyl moiety by a polymethylene segment were synthesized in three steps starting from 4-cyanoaniline. The monomers were homopolymerized and copolymerized with an optically active monomer, (-)-menthyl methacrylate, in the presence of AIBN as a radical initiator. Structurally similar amphiphilic polymers were obtained by homopolymerization of analogous methacrylates in which oligo(oxyethylene) segments replaced the polymethylene spacers. The polymeric materials, having a molar content of photochromic units between 5 and 100% and molecular weights of about 15,000, were characterized by thermal analysis, X-ray diffraction, 1D and 2D NMR. Mobility and photochromic properties in solution were also investigated.     

Self-assembly of bridged silsesquioxanes incorporated with conjugated organic functionalities

Self-assembly of bridged silsesquioxanes with a chemical structure of (RO)₃Si-R′-Si(OR)₃ represents an efficient approach to design and to fabricate functional organic/inorganic nanocomposites. The desired functionalities are mainly incorporated into the R′ with R of a hydrolysable alkoxide group such as CH₃ or CH₃CH₂. This feature article discusses two typical assembly approaches: self-directed assembly aiming at ordered solid materials and surfactant-directed assembly aiming at periodic mesoporous organosilica, with emphasis on the bridged silsesquioxanes in which conjugated functional organic groups are incorporated. The conjugated moieties are shown to be critical to the resulting assembly structure, morphology, and property. Self-assembly of three bridged silsesquioxanes based on polydiacetylene, perylene, and porphyrin has been detailed, respectively.     

Specific thermoresponsive behaviours exhibited by optically active and inactive phenylalanine modified hyperbranched polyethylenimines in water

Optically active and inactive hyperbranched polymers with specific thermoresponsive behaviours in water were reported.Through two steps hyperbranched polyethylenimine (HPEI) polymers terminated with different amount of D-phenylalanine (D-Phe),L-phenylalanine (L-Phe) or DL-phenylalanine (DL-Phe) were prepared and characterized.The analyses on the solution properties by turbidimetry,dynamic light scattering,fluorescence probe and 1H-NMR demonstrated that all the polymers exhibited specific thermoresponsive behaviours in water,including:(1) In the dilute polymer concentration region,increasing the polymer concentration led to the increase of phase transition temperature;(2) The optically inactive thermoresponsive hyperbranched polymers showed a higher cloud-point temperature (Tcp) than their corresponding optically active ones in a relatively higher polymer concentration;(3) At the same polymer concentration the hydrophobic groups of the optically inactive HPEI-DL-Phe formed more perfect hydrophobic domain than those of the optically active HPEI-L-Phe and HPEI-D-Phe.     

Spectroscopic studies of poly(vinylamine) containing optically active thymine derivatives as grafted pendants

The optical properties of poly(vinylamine) containing optically active (+)- or (?)-2-(thymin-1-yl) propionyl groups as grafted pendants (PT) and the related monomer (MT) and dimer models (DT) were investigated by UV, circular dichroism (CD), and NMR spectroscopy. Highly syndiotactic PT has a smaller hypochromicity versus MT and a larger optical rotation than the less syndiotactic PT in various solutions. These results are attributed to an interaction between the configurational arrangement of thymines, the conformation of the polymers, and base stacking between thymines. The interactions of these polymers with poly(adenylic acid) (polyA) were also studied and the results compared with other vinyl-type nucleic acid model polymers. The isomers of the optically active dimer models [prepared from meso and (dl)-2,4-diaminopentane] were separated. The CD spectra of (+)-D(?)T in CHCl₃ and trifluoroethanol (TFE) displayed extremely solvent-dependent exciton coupling of the π–π^* (B_2u) transition of the base chromophore, which was not observed in the other models or polymers, except the meso-type dimer model (meso)-D-(?)T. This coupling decreased with increasing solvent dielectric constant, while UV hypochromicity increased. This behavior as well as the 360-MHz NMR spectra suggest that (+)-D(?)T exists in an extended form in solvents of low dielectric constant and gradually assumes a stacked conformation as the dielectric constant increases.     

Exploring London Dispersion and Solvent Interactions at Alkyl–Alkyl Interfaces Using Azobenzene Switches

Interactions on the molecular level control structure as well as function. Especially interfaces between innocent alkyl groups are hardly studied although they are of great importance in larger systems. Herein, London dispersion in conjunction with solvent interactions between linear alkyl chains was examined with an azobenzene‐based experimental setup. Alkyl chains in all meta positions of the azobenzene core were systematically elongated, and the change in rate for the thermally induced Z→E isomerization in n‐decane was determined. The stability of the Z‐isomer increased with longer chains and reached a maximum for n‐butyl groups. Further elongation led to faster isomerization. The origin of the intramolecular interactions was elaborated by various techniques, including ¹H NOESY NMR spectroscopy. The results indicate that there are additional long‐range interactions between n‐alkyl chains with the opposite phenyl core in the Z‐state. These interactions are most likely dominated by attractive London dispersion. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra‐ as well as an intermolecular setting.     

Synthesis of chiral azobenzene derivatives and the performance in photochemical control of blue phase liquid crystal

A series of chiral azobenzene compounds with branched terminal were synthesised, and the photosensitive performances were investigated accordingly. It was found that the photochemical properties were mainly affected by the trans–cis configuration of azobenzene and the linked position between the azobenzene and chiral centre. The para-type azobenzenes showed general photochemical decrease in helical twisting power (HTP), but the meta-type ones appeared interestingly showed photochemical increase in HTP. This work provided an effective method for designing molecules to control blue phase (BP) including adjusting colours, inducing BP and extending BP range, which was promising in the applications of optically addressable devices.