ABSTRACT
Compared to the large number of liquid crystal dimers consisting of two rod-like mesogens, the numberofdiscoticdimersisverylow.Thevariousstructuralpossibilitiesfordiscoticdimersare shownschematicallyinFigure3.1.Dimersinwhichtwoidenticaldiscoticmesogensareconnected viaa¨exiblespacer(Figure3.1a)arethemostwidelysynthesizedandstudieddiscoticdimers. Afewexamplesofsymmetricdiscoticdimerswitharigidspacerinsteadofa¨exiblespacerare also known. It is generally difƒcult to link two different discotic mesogens and, therefore, only a fewexamplesofunsymmetricaldimers(Figure3.1b)arerealized.Dimersinwhichtwodiscogens areconnectedlaterallyorlinearlytoacalamiticmoleculearedepictedinFigure3.1candd,respectively.Cyclicdimers(Figure3.1e)inwhichtwomesogenicunitsareconnectedtoeachothervia morethanonespacerarealsoknown.Anumberofdouble-deckerdimers(Figure3.1f)inwhich two discotic mesogens are linked via a metal or any other atom have been documented. In additiontothesepossibilities,severalothercombinations,suchasrod-diskdimers,inwhicharod-like mesogenisconnectedtoadiscoticmesogen,phasmids,orpolycatenarmesogensthatƒllthegap betweenrod-likeanddisk-likeliquidcrystalsexhibitingnematic,smectic,andcolumnarphases,are alsoknown.However,thischaptercoversonlydimericliquidcrystalshavingtwodiscoticmesogens. Other architectures, though they form columnar phases, have not been treated here. Discotic coresthathavebeenexploredtocreatedimersarebenzene,scylloinositol,pyranosesugars,anthraquinone,triphenylene,cyclotetraveratrylene,phthalocyanine,porphyrin,andhexabenzocoronene (HBC). Discotic dimers derived from these cores are discussed in the following sections.
