No abstract

In order to study the role of the direction of the connecting groups in bent-core mesogens we synthesized two series of ten possible achiral isomeric five-ring bent-core compounds in which all aromatic rings are connected by ester groups and each of which possesses the same length of the terminal chains (octyloxy or dodecyloxy, respectively). The structure of the isomers is distinguished by the direction of at least one ester group, only. The mesophase behaviour of the compounds has been studied by polarizing microscopy, differential scanning calorimetry, X-ray experiments and electro-optical measurements. We have found that in spite of the minor structural differences a variety of mesophases occur (SmCP A , Col rec , Col ob ) whereby the clearing temperatures vary from 121 to 193 uC (octyloxy isomers) and 112 to 189 uC (dodecyloxy isomers). Depending on the direction of the ester groups some of these isomers show interesting properties, such as field-induced inversion of chirality in SmCP A and columnar phases, the field-induced enhancement of the clearing temperature, a second-order phase transition Col ob A SmCP A or the reversible field-induced phase transition Col ob A SmCP F . The unexpectedly strong influence of the direction of the connecting groups is discussed on the base of theoretical calculations and molecular dynamics simulation on isolated molecules.

Systematic studies on symmetric twin molecules composed of two bent-core mesogenic units are reported for the first time. It is shown that the formation of mesophases mainly depends on the chemical structure of the spacer. A sufficiently flexible spacer containing dimethylsiloxane units allows a decoupling of the mesogenic moieties; the related compounds exhibit a ferroelectric SmCP phase proved by electro-optical measurements. On the basis of X-ray data, a plausible structure model is presented. The connection of two bent-core mesogenic units by means of a tetraethylene glycol spacer results in dimers, which are able to form columnar phases. For twins containing a simple hydrocarbon chain as spacer, in most cases liquid-crystalline behavior could not be detected. These general tendencies remain valid if the bent-core unit is changed, e.g., by increasing the number of aromatic rings, by lateral substitution, and by inversion of the direction of one of the connecting groups between the aromatic rings, respectively. The mesophase behavior of the new dimers is compared with that of the corresponding single-unit compounds as well as a related dendrimer and a polymer. It is of interest that the antiferroelectric switching behavior found for the nonchiral “monomeric” compounds is changed to a ferroelectric behavior for the dimeric compounds.

Electrophilic nitration, formylation and acetylation of the title heterocycle 1 were studied. Monosubstitution proceeds preferentially in position 2 and to a lesser extent in position 4. Lowering the reaction temperature substantially increases regioselectivity of nitration and acetylation. Substitution reactions of the 2-substituted derivatives of 1 led to the corresponding 2,7-disubstituted derivatives as major products. Formation of other regioisomers was also observed. Metallation of 1 with butyllithium preferably proceeds in position 1; subsequent reaction with N,N-dimethylformamide, carbon dioxide or iodine gives rise to corresponding 1-substituted derivatives. Long-chain 2,7-disubstitued derivatives exhibit liquid-crystalline properties.