Abstract
New 4-bromoresorcinol based bent-core molecules with peripheral fluoro substituted azobenzene wings have been synthesized and the liquid crystalline self-assembly was investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM), electro-optic studies and X-ray diffraction (XRD). A new type of optically isotropic mesophase composed of chiral domains with opposite handedness (dark conglomerate phases, DC phases) is observed, which for some homologues with medium alkyl chain length is stable down to ambient temperature. It is proposed that these DC phases are formed by helical twisted nano-domains of limited size and composed of the crystallized aromatic cores which are separated by the disordered alkyl chains. This structure is distinct from the previously known soft helical nano-filament phases (HNF phases, B4 phases) formed by extended crystalline nano-filaments and also distinct from the fluid sponge phases composed of deformed fluid layers. Comparison with related bent-core molecules having H, F, Cl, I, CH3 and CN groups in the 4-position at the resorcinol core, either with or without additional peripheral fluorines, provided information about the effects of these substituents on the tendency to form DC phases. Based on these relationships and by comparison with the minimum energy conformations obtained by DFT calculations a hypothesis is provided for the formation of DC phases depending on the molecular structure.
Highlights
The observation of spontaneous development of macroscopic chirality in so matter systems of achiral molecules is a contemporary research topic with great importance for the general understanding of spontaneous mirror symmetry breaking, and this might be useful for numerous practical applications
Conglomerates of chiral domains were found in apolar SmC phases with only local polar domains (SmCsPR[*] phases)[31,32] and in nematic phases formed by some bent-core mesogens.[33,34,35]
3.1 Mesomorphic properties of compounds BrFn depending on terminal alkyl chain length
Summary
The observation of spontaneous development of macroscopic chirality in so matter systems of achiral molecules is a contemporary research topic with great importance for the general understanding of spontaneous mirror symmetry breaking, and this might be useful for numerous practical applications This phenomenon is in recent years most o en found in liquid crystalline phases of some molecules with an extended bent aromatic core (bent-core molecules).[1,2] Helical superstructures in columns[3,4] and self-assembly in helical laments[5,6,7,8,9] represent other prominent examples of mirror symmetry breaking at the nano-scale.[10] in these 1D organizations chiral segregation occurs only locally, along the aggregates, whereas the lateral interaction between them is only weak, so that usually no macroscopic chirality could be achieved in the absence of a chiral dopant.[2] Only for achiral dibenzo[g,p]chrysenes with short peripheral chains macroscopic. DC phases could represent interesting materials for a variety of applications, such as organic semiconductors, thin- lm transistors and solar cells,[36] as thin- lm polarizers,[37] as nonlinear optical materials[38] as well as for the detection and ampli cation of chirality,[39] and eventually for separation of enantiomers and for enantioselective synthesis.[40]
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