Formation of smectic and columnar liquid crystalline phases by cyclotriveratrylene (CTV) and cyclotetraveratrylene (CTTV) derivatives incorporating calamitic structural units
作者:Ralph Lunkwitz、Carsten Tschierske、Siegmar Diele
DOI:10.1039/a702034a
日期:——
Novel liquid crystalline oligomesogens, which consist of five, six or eight calamitic 4-cyanobiphenyl, 2-alkyl-5-phenyl-1,3,4-thiadiazole or 5-octyl-2-phenylpyrimidine units covalently linked by spacers of different length with a cyclotribenzylene central core (CTV derivatives, 2,3,7,8,12,13-hexa-substituted 10,15-dihydro-5H-tribenzo[a,d,g]cyclononenes) or a cyclotetrabenzylene central core (CTTV derivatives, 2,3,6,7,10,11,14,15-octa-substituted 5,10,15,20-tetrahydrotetrabenzo[a,d,g,j]cyclododecenes) have been synthesized. These compounds were investigated by polarizing microscopy, differential scanning calorimetry and some of them were also studied by X-ray diffraction. Many of the CTV derivatives show liquid crystalline properties. The cyanobiphenyl derivatives incorporating long spacer units have enantiotropic SA phases which can easily be supercooled. The thiadiazole derivatives and the pyrimidine derivatives have liquid crystalline phases only if the spacer units are rather short. Thiadiazole derivatives with an odd number of connecting atoms between the CTV core and the calamitic mesogens form SA phases, whereas those with even-numbered spacers display columnar mesophases. We propose that the columnar mesophases observed for these oligomesogens do not result from their ability to adopt a nearly disc-like shape. Instead, they represent ribbon phases which result from a steric frustration caused by the different space filling of the central cone-like core and the rod-like mesogenic groups. A chevron-like (banana-like) average shape of the molecules with even-numbered spacers is assumed to be responsible for the formation of the ribbon phases.Also a CTTV derivative with eight appended phenylthiadiazole mesogens displays a columnar mesophase. A nematic phase was found for the corresponding cyanobiphenyl derivative, whereas the pyrimidine derivative is not liquid crystalline.
新型的液晶寡聚介质,由五个、六个或八个 calamitic 4-氰基联苯、2-烷基-5-苯基-1,3,4-噻二唑或5-辛基-2-苯基嘧啶单元通过不同长度的间隔基与一个环三苯基中心核(CTV衍生物,2,3,7,8,12,13-六取代的10,15-二氢-5H-三苯并[a,d,g]环壬烯)或一个环四苯基中心核(CTTV衍生物,2,3,6,7,10,11,14,15-八取代的5,10,15,20-四氢四苯并[a,d,g,j]环十二碳四烯)共价连接而成,已被合成。这些化合物通过偏光显微镜、差示扫描量热法进行研究,其中一些还通过X射线衍射进行了研究。许多CTV衍生物显示出液晶性质。含长间隔基单元的氰基联苯衍生物具有固-液同质相(SA相),可以很容易地过冷。噻二唑衍生物和嘧啶衍生物只有在间隔基单元相当短时才具有液晶相。噻二唑衍生物在CTV核与calamitic介质之间具有奇数连接原子时形成SA相,而具有偶数间隔基的噻二唑衍生物则显示柱状液晶相。我们提出,这些寡聚介质观察到的柱状液晶相并非因其能形成近似圆盘状的形态,而是代表带状相,这是由中心锥状核和棒状介质基团的不同空间填充引起的立体障碍造成的。假定具有偶数间隔基的分子呈现类似曲折(香蕉状)的平均形状,是形成带状相的原因。此外,一个具有八个附加苯基噻二唑介质的CTTV衍生物显示柱状液晶相。相应的氰基联苯衍生物发现有向列相,而嘧啶衍生物则不具有液晶性。