Mesomorphic properties of cyanobiphenyl dimers with a substituted central malonate unit: overruling effect of fluorination

Two series of C2-substituted dimers carrying a central C2-hexyl- or C2-nonafluorohexyl malonate and two cyanobiphenyls attached via alkoxy spacers of various lengths (n = 6–12) were synthesised, and the mesomorphic properties were studied by differential scanning calorimetry, polarising optical microscopyand X-ray diffraction (wide-angle and small-angle X-ray scattering). The C2-substituent dimers together with the spacer lengths had a pronounced influence on the phase behaviour. In case of C2-hexyl-substituted dimers, shorter spacers favoured N phases (n = 6–9), while longer spacers favoured SmA phases (n ≥ 9). In addition, decreased clearing temperatures were observed. In contrast, C2-nonafluorohexyl-substituted dimers suppressed N phases and displayed only SmA phases. Moreover, clearing temperatures and phase stabilities considerably increased. Based on the experimentally determined layer thickness, a packing model for the SmA phases was proposed, where C2-substituted dimers displaced mesophases with molecules in a Y-conformation. As a result, less stable mesophases for C2-hexyl-substituted dimers due to weakened intermolecular interactions and more stable mesophases for C2-nonafluorohexyl-substituted dimers due to the fluorophobic effect and nanosegregation were obtained.本研究合成了两类C2取代二聚体（C2-substituted dimers），其分子以C2-己基或C2-全氟己基丙二酸酯为中心结构单元，通过长度各异（n=6~12）的烷氧基间隔基（alkoxy spacers）连接两个氰基联苯端基；并采用差示扫描量热法（differential scanning calorimetry）、偏振光学显微镜法（polarising optical microscopy）以及X射线衍射法（含广角与小角X射线散射，wide-angle and small-angle X-ray scattering）表征了其介晶性能（mesomorphic properties）。C2取代基与间隔基长度均对相行为（phase behaviour）具有显著调控作用：对于C2-己基取代二聚体，较短的间隔基（n=6~9）更易促成向列相（Nematic phase，简称N相）的形成，而较长的间隔基（n≥9）则倾向于生成近晶A相（Smectic A phase，简称SmA相），同时观测到清亮点温度（clearing temperature）出现下降；与之形成鲜明对比的是，C2-全氟己基取代二聚体完全抑制了向列相的生成，仅能观察到近晶A相，且其清亮点温度与相稳定性均得到显著提升。基于实验测得的层间距，本研究提出了近晶A相的分子堆积模型：呈Y型构象的C2取代二聚体构成了该介晶相的分子排布模式。最终，C2-己基取代二聚体因分子间相互作用减弱，其介晶相稳定性更低；而C2-全氟己基取代二聚体则因氟疏效应（fluorophobic effect）与纳米相分离（nanosegregation），获得了稳定性更高的介晶相。