Liquid crystals with axially chiral 3,3′-dinitro-2,2′,6,6′-tetramethylbiphenyl cores: the lateral shielding effect of bicyclo[2.2.2]octane-1-carboxylate terminal chains

Abstract

Calamitic mesogens with axially chiral 3,3′-dinitro-2,2′,6,6′-tetramethylbiphenyl cores and combinations of 4-alkoxybenzoate and 4-pentyl- or 4-alkoxybicyclo[2.2.2]octane-1-carboxylate terminal chains were synthesized and characterized. The results suggest that the bicyclo[2.2.2]octane segment is effective in shielding the unfavorable lateral bulk of the axially chiral biphenyl core and imparting mesogenic properties to the calamitic structures. Most of the compounds reported herein form a chiral nematic (N*) phase with a pitch in the submicron range that increases with temperature. Two homologues with the longest alkoxy terminal chains form a chiral smectic A (SmA*) phase. Two symmetrical derivatives with either 4-octyloxybenzoate terminal chains (3(8)) or 4-octyloxybicyclo[2.2.2]octane-1-carboxylate terminal chains (5(8)), and one unsymmetrical derivative with 4-octyloxybenzoate and 4-octyloxybicyclo[2.2.2]octane-1-carboxylate terminal chains (7(8,8)) were doped in the liquid crystal host 2-(4-butyloxyphenyl)-5-octyloxypyrimidine (PhP). Ferroelectric polarization powers (δ_p) and helical pitch induced by the three dopants in both the SmC* and N* phases were measured. The results show that the polarization power at T–T_C = −10 K decreases in the order 3(8) > 7(8,8) > 5(8), whereas the induced SmC* pitch at a constant dopant mole fraction x_d = 0.015 increases in the same order, which suggests that the shielding effect of the bicyclo[2.2.2]octane segment hinders chirality transfer in the SmC* phase.