Polycatenar mesogens which consist of a rod-like core and two half disc-shaped moieties can be considered as intermediates between rod-like and disc-like mesogens. ['s21 Depending on the length and the structure of the rod-like core and on the number and length of the terminal chains polycatenar mesogens exhibit nematic, lamellar, columnar or cubic mesophases. ['] Double swallow-tailed corn pound^[^-^^ with long-chain branches are special cases of tetracatenar mesogens, where the branches are not directly attached at the terminal rings. This paper deals with new double swallow-tailed compounds whereby the middle part A of the core and the length of the terminal chains are varied in different ways. It is shown that the mesophase behavior is influenced by the structure of the middle part A as well as by the length of terminal alkyl chains. Table 1 shows the transition temperatures ["C] and the phase behavior of the investigated substances I-XII, the structure of which is only distinguished by the middle part A of the core. As known from calamitic liquid crystals, the clearing temperature significantly increases with increasing length of the rigid core. This is obvious by the comparison of the compounds I-III. However, for the compounds II, XI, and XU the middle parts A exhibits a rod-like structure of nearly comparable length. Therefore, the relationship between chemical structure and mesophase behavior can be discussed.The insertion of two methylene groups between the aromatic rings of the biphenyl system in II extends the core to yield XI, but also increases the flexibility. These parameters compensate each other, the clearing points are nearly identical. However, the biphenyl derivative II exhibits only a columnar phase, whereas the more flexible compoundXI exhibits an unusual polymorphism including a re-entrant smectic C phase, (Sere). Comparing the middle parts of XI and XU, the amide group only produces columnar phases at higher temperature. The partial double-bond properties of the amide group as well as the possibility of additional intermolecular hydrogen bonding should influence the mesophase stability.A comparison of compounds II and M or XI and X, respectively, shows that lateral substituents clearly depress the transition temperatures. The compounds IV, V, VI do not exhibit any mesophase or only exhibit monotropic mesophases at relatively low temperature because the linking groups within the middle part A (-S-, -SO*-, -CO-, -0-) give rise to an angular geometry of the core. This very strong depression of the anisotropic behavior is unexpected for six-ring compounds, because five-ring mesogens having a strong angled structure but no terminal branches have been described in the literature"' to show stable mesophases. In the double-swallow-tailed substances under discussion the additional steric influence of both terminal double chains must be taken into account.With increasing length of the core of the double-swallowtailed compounds the tendency to form columnar mesophases increases (see compounds I...
The result of optical X-ray, DSC and magnetic susceptibility studies of new Co(I1) metallomesogens are reported. The di-(5-substituted-salicylidene)ethylenediaminato cobalt(I1) complexes exhibit broad range SA phases. A comparison with corresponding copper(I1) and nickel(I1) complexes leads to conclusions about how the properties are influenced by the metal atom. Cobalt(I1) complexes of this kind form different modifications which are either active or inactive against oxygen. The conversion process (inactiveactive) is monitored by time dependent X-ray diffractometric measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.