engleski

ESR study of gelator self-assembly in a liquid crystal

In this study, the complex system of thermotropic liquid crystal (LC) trans-4-heptylcyclohexanecarboxylic acid, HCCA, and chiral bisoxalamide gelator, 1, was studied (Scheme 1). The temperature induced phase transition/self-assembly of liquid crystalline/gelator molecules was investigated by electron spin resonance (ESR) spectroscopy. Since neither HCCA nor gelator molecules are paramagnetic, the external reporter group in terms of the 4-oxo-2, 2, 6, 6, - tetramethyl-1-piperidinyloxy spin probe (Tempone) was incorporated in the system under investigation. The advantage of the approach relies on the sensitivity of the method to detect changes in the local properties while reporting about dynamics on the time scale ranging from 10-11 s to 10-8 s.1 In the temperature range 295– 383 K, where HCCA displays isotropic, nematic, smectic B and crystalline phases, the impact of 1 self-organization was detected via (non) homogeneous partitioning of the spin probe in the environments varying in the polarity, an effect dependent on the gelator concentration. In particular, the evidence of the onset of the gelator network self-assembly in the nematic phase was detected by ESR at higher temperatures than the ones reported so far by other experimental techniques. Additionally, the spectral analysis points to the switching of the polarity in the vicinity of the spin probe when the transfer of chirality from 1 to HCCA upon cooling of the sample from isotropic to chiral nematic phase appears and when the event of LC gelation results in the achiral nematic phase during chiral gel fibers formation. When the gelation proceeds in the smectic phase, the melting of the gelator network is studied in the nematic phase during the heating cycle. Furthermore, the event of HCCA crystallization is shown to be strongly affected by the presence of 1 as well. The experimental evidence is provided that gelator network confines the HCCA into the domains within the bulk crystalline matrix where the local molecular dynamics are still not frozen. Therefore, we propose that non-homogeneous polarity profile of molecular organization/packing within LC gels could be determinable for the physical properties of various LC gel phases.

ESR; gel; LC

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