Self-assembly of gelator molecules in liquid crystals studied by ESR

Thermotropic liquid crystal trans-4-heptylcyclohexanecarboxylic acid (HCCA) doped with 4-oxo-2,2,6,6,-tetramethyl-1-piperidinyloxy spin probe (Tempone) is investigated by electron spin resonance (ESR) spectroscopy in the presence of chiral bisoxalamide gelator 1 during both cooling and heating cycles. In the temperature range 295–383K, 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.