Application of the novel dynamic thermo-optical analysis for identification of the sequence of mesophases in thermotropic liquid crystal

Identification of mesophases using a polarising microscope is vital in the area of research and development of the liquid crystal (LC) materials, however, sometimes it is very difficult to distinguish one texture from another. Here, we propose a novel method allowing for the examination of thermotropic phase sequence in polymorphic LCs called dynamic thermo-optical analysis (DTOA). In the DTOA technique the LC subjected to the cooling process is simultaneously influenced by the electric field varying sinusoidally with time. It results in the dynamic changes in the light intensity passing through the layer of such LC observed under a polarised microscope and recorded by a charge-coupled device camera. The magnitude of the changes uniquely depends on the type of the mesophase. The subsequent numerical analysis of the recorded movie (performed with a help of the image-processing software) reveals the changes in the amplitude of the average intensity of the images of the respective textures, i.e. nematic, SmA, SmC, SmI, thereby allowing for their clear identification. Furthermore, the DTOA allows to distinguish easily the SmA from the SmC phase. The azobenzene-based LC known of rich sequence of mesophases has been used here to demonstrate the principle of the DTOA method.     

Synthesis and mesophase behaviour of branched azobenzene-based supramolecular hydrogen-bonded liquid crystals

Five types of new pyridine-based azo compounds, namely, 4-(4′-pyridylazo)-4′‘-alkoxybenzoates or 4-(4′-pyridylazo)-4′’-substituted benzoates with straight or branch terminal were prepared as hydrogen bond (H-bond) acceptors to build mesogens. After self-assembled with different carboxylic acids, the resulted H-bond complexes were confirmed for the stability of H bond by fourier transform infrared spectroscopy (FTIR) spectroscopy, and investigated for the mesophase behaviour by differential scanning calorimetry and polarised optical microscopy. The influence of terminal chains in azo accepter or acid donor and the structure of acid donors on the mesogenic behaviour were discussed in detail.     

Oxidative coupling of hydroxy- or aminoazobenzenes with secondary phosphine chalcogenides: Towards new media-responsive molecular switches

One or two chalcogenophosphinate groups were introduced to the azobenzene scaffold via the oxidative cross-coupling reaction of 4-amino-, 4-hydroxy- and 4,4′-dihydroxyazobenzenes with secondary phosphine chalcogenides using the CCl₄/Et₃N system under mild conditions in 41–95% yield. Cis-trans photoisomerization of the phosphorylated azobenzenes was reversibly controlled by alternating UV/Vis light irradiation. The chalcogenophosphinate group imparts the properties of media-responsive molecular photoswitches to the synthesized azobenzenes.     

新型弯曲形双偶氮苯类液晶合成及光致异构性能研究

弯曲形偶氮苯液晶由于其偶氮键独特的光致可逆异构化性能,已成为光电子信息材料研究的热门课题,但光响应速度慢已成为制约其发展的关键因素.目前报道的弯曲型偶氮类液晶化合物的偶氮键都距离中心核较远的位置,光致异构的响应时间较长,大都在分钟以上,不利于光敏器件应用研究.本工作试图以2-甲基-1,3-间苯二胺为中心核,将偶氮键紧挨中心核两边,末端为直链烷基,设计合成了一系列新型弯曲形双偶氮苯类液晶化合物,以期缩短光响应时间.通过IR, 1H NMR, 13C NMR和ICP-MS光谱鉴定这些化合物的分子结构,经差示扫描量热仪(DSC)和偏光显微镜(POM)测定其液晶相变温度和相态织构;并通过测定2-甲基-1,3-双(4-((4-庚基苯基)酯基)-1-(E)-偶氮苯基)苯(2c)的紫外-可见光的吸收光谱研究其光致异构化性能,通过UV-Vis光谱仪和偏光显微镜(POM)测定其液晶化合物以及掺杂向列相液晶材料的光致异构现象和响应时间.实验结果表明,所有设计合成的弯曲形双偶氮苯类液晶化合物均具有近晶相相态,且相态温度范围较宽,当近晶相态化合物2c掺杂到向列相混合液晶中时,其光致异构响应时间为2~3 s,在日光下液晶态恢复时间为3~4s,在乙酸乙酯稀溶液中时10s可达到光稳态.说明这类弯曲形双偶氮液晶化合物具有较快的光致异构响应速度.     

RA-IR Studies on the Structure of Mercapto-Ended Azobenzene Derivatives in Self-Assembled Monolayers

Self-Assembled Monolayers (SAMs) were prepared from mercapto-ended azobenzene derivatives with the structure of n-C_nH_2n+1, AzoO(CH₂)_mSH (n=4,6,8,10,12; m=3,5). the structure of these SAMs was thoroughly studied with grazing-angle incident reflection absorption FT1R technique and wettability measurement. the results suggested that the plane of Azobenzene system of the assembling molecules in the SAMs lies on its back with an approximate angle of 22° included between the substrate surface normal and the Azobenzene plane. Tail alkyl groups (n-C_nH_2n+1) in these assembling molecules were considered to be in an all-trans conformation, as if they were in a crystalline-like environment. and the C-C-C plane of these all-trans tail alkyl groups, while n≤8, lies also on its back with an angle about 70° between its plane and the substrate surface normal. the conformation of these head groups (-O(CH₂)_mSH) in SAMs are disturbed by many structural factors. While m=3 or 5, the head group chain was proposed to remain in a gauche conformation to fit the upright orientation of the Azobenzene plane. the packing density was investigated by measuring the contact angle of water on these SAMs. the results showed that the self-assembled monolayer films are perfectly packed and the coverage density might be improved with increasing the length of both tail and head alkyl chains.     

Synthesis and mesomorphic properties of nonsymmetric liquid crystalline dimers containing azobenzene groups

Four novel nonsymmetric dimers containing azobenzene mesogenic groups were synthesized. The nonsymmetric dimers compounds namely, ethyl 4-[(4-{4-(4-((4-nitrophenyl)diazenyl)phenoxy)alkyloxy}phenyl)diazenyl]benzoate were obtained from the alkylation of ethyl 4-[(4-(4-bromoalkyloxy)phenyl)diazenyl]benzoate with 4-[(4-nitrophenyl)diazenyl]phenol. The mesomorphic properties of the compounds were determined by DSC and polarizing optical microscopy. The first member of the series was nonliquid crystalline while all other homologues display nematic and smectic A phases. The trans-azobenzene groups of the dimers display a high-intensity π–π* transition at about 365 nm and a low-intensity n–π* transition at around 465 nm, therefore, photochromism can be achieved by the introduction of the azo linkage to the dimeric liquid crystalline molecules.     

偶氮类有机材料光致双折射的测量

研究了2种偶氮聚合物薄膜的光致双折射效应,并探讨了泵浦光偏振态和光强对光致双折射的影响.采用远离共振区的He-Ne光(633nm)作为探测光,用Ar+激光(488nm)作为泵浦光,通过测量相关参量得到了样品的光致双折射值.实验结果表明:改变泵浦光的偏振态可以控制光致双折射值和探测光的透过信号强度,在0°～45°范围内,泵浦光光强存在最佳值,此时偶氮薄膜实验样品具有最大的双折射值.     

不同亲疏水比例的光响应性嵌段共聚物的合成及溶液组装行为研究

以末端带有二硫代酯基团的聚合物PhC（=S）-S-PEO作为大分子链转移剂,利用可逆加成-断裂链转移自由基聚合法（RAFT）合成一系列亲疏水比例不同的嵌段共聚物PEO_n-b-PMeA6AB2_m,并采用核磁共振（¹H NMR）和凝胶渗透色谱（GPC）对产物进行了表征. 运用场发射透射电子显微镜（FETEM）、扫描电子显微镜（SEM）、场发射扫描电子显微镜（FESEM）、光学显微镜、粒径分析仪和荧光光谱研究了聚集体的形貌、粒径及其在光刺激下的响应行为. 结果表明,聚集体形态为胶束,包含大复合胶束. 随着自组装初始浓度的降低、转速的增加,聚集体尺寸降低,分布趋于均一;当亲疏水比例为2.3时,球形复合胶束中出现了花生型、棒状结构. 包覆有尼罗红的聚集体在紫外/可见光的循环照射下,部分尼罗红被释放. 共聚物BCP-S3-0.82形成的自组装体系在紫外光照射下,尺寸较大的复合胶束发生一定程度的不可逆的解离.