Ferroelectric liquid crystals induced by dopants with axially chiral 2,2′-spirobiindan-1,1′-dione cores: substituent effect on the polarization power

Abstract

Dopants with axially chiral 2,2′-spirobiindan-1,1′-dione cores induce a spontaneous polarization P_S in the SmC liquid crystal phase that varies with the structure of the liquid crystal host. This host dependence of the polarization power δ_p was rationalized based on an equilibrium between two conformations of opposite polarities (P and C₂) that conform to the binding site of the Boulder model. In order to test this conformational distribution model, we modified the structure of the dopant (R)-5,5′-diheptyloxy-2,2′-spirobiindan-1,1′-dione ((R)-2-H) by introducing a substituent at the 6-position of one indanone segment to give the series (R)-2-X in enantiomerically pure form, where X = Cl, Me, CN and NO₂. The effect of each substituent on δ_p was measured in four different liquid crystal hosts PhP1, PhB, NCB76 and DFT. The results reported herein show that the substituent X has a significant effect on δ_p in each of the four hosts, but suggest that the P/C₂ model in a passive dopant mode cannot account for the observed trends in δ_p and that the effect of chirality transfer on rotational and/or conformational distribution must be taken into account. Nevertheless, the results show that δ_p can be tuned over a wide dynamic range—from +1019 nC cm⁻² to −293 nC cm⁻²—by varying the substituent X.