Cluster phases of 4-cyanoresorcinol derived hockey-stick liquid crystals

Abstract

Here, we report the synthesis and investigations of hockey-stick liquid crystalline materials derived from 4-cyanoresorcinol as the central core unit. The effect of distinct linking groups (NN, COO, OOC) and the effect of inverting the direction of the cyano group on the resorcinol core with respect to the different side arms on the liquid crystalline properties were investigated. The self-assembly was characterized by polarising optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), electro-optical and second harmonic generation (SHG) experiments and dielectric spectroscopy. It is found that depending on the alkyl chain length, nematic and smectic liquid crystalline phases were observed. All nematic phases exhibited by the compounds with the CN group adjacent to the shorter rod-like arm of the mesogen are N_CybA phases composed of large non-tilted (SmA-type) cybotactic clusters, while inverting the direction of the cyano group leads to nematic phases composed of smaller cybotactic clusters (N_CybC) with tilted-smectic correlations. The former compounds form additional non-tilted (SmA) and tilted smectic (SmC) composed of polar domains with a strong polar response. There is a transition from synclinic to anticlinic molecular arrangement (SmC_s and SmC_a phases) with decreasing temperature and mirror symmetry breaking in the SmC_s phases. The synclinic–anticlinic transition is associated with a reduction of the polar domain size, being opposite to observations made for related bent-core compounds and is attributed to the combined effects of nano-segregation and entropy.