Dimer-parity dependent odd-even effects in photoinduced transitions to cholesteric and twist grain boundary SmC* mesophases: PBG characteristics

We describe the first investigations on the influence of flexible spacer parity of the guest photoactive liquid crystalline dimer on the photonic bandgap (PBG) features of the cholesteric (Ch) and twist grain boundary smectic C* (TGBC*) phases of the host molecule. Both these phases exhibit PBG structure, with the former having it in one and the latter in three dimensions respectively. The concentration of the photoactive dimer is very small in the host liquid crystal (HLC) system that exhibits isotropic (Iso)-* Ch-* TGBC* sequence. The photoisomerization experiment brings out the feature that the pitch of the Ch phase blue-shifts, with the magnitude being more for the even dimer system compared to the odd one. This effect is accompanied by a reduction in the width of the photonic bandgap, again the effect is more pronounced for the even parity mixture compared to the odd one. The two-dimensional periodicity of the SmC* helix identified with the grid pattern in the TGBC* phase shows the memory effect upon photoisomerization. X-ray diffraction measurements reveal that the odd dimer system shows a larger layer spacing than the even dimer one. Upon UV illumination for one of the materials, layer spacing shows an increase. The investigations discussed in this article reveal that, apart from the actinic light, the spacer and parity of the photoactive dimer play a significant role in controlling the PBG properties in all three directions.