No abstract

Seminal findings in the liquid-crystal properties of bent-core molecules, such as the observation of ferroelectricity and spontaneous breaking of chiral symmetry in smectic phases composed of achiral molecules, [1][2][3] have broad implications for the general field of soft condensed matter. However, their practical applications [4][5][6][7][8][9] are limited because they appear only at high temperatures (>70 8C) owing to the bends in the molecules, which lead to locking into layered smectic structures. To overcome this difficulty, one needs to prohibit the locking mechanism either by molecular design or creating mixtures, or by combining both. Mixtures of bent-core and rod-shaped molecules have shown interesting properties, such as enhancement of the chirality in cholesterics, [10,11] induction of antiferroelectric order in smectics, [12,13] or a complete miscibility of smectic bent-core and nematic rod-shape substances. [14,15] Although there are few examples in which bent-core materials do not crystallize at room temperature, [16,17] they are glassy and could not be switched at room temperature. Here we show that by mixing suitable bent and rod-shape molecules one can form fluid liquid crystals at room temperature that change birefringence color at electric fields, thus opening up a path towards possible practical applications.We have studied binary mixtures of several rod-shape and bent-core molecules, but will focus only on those that showed complete miscibility, and provided an electrically switchable smectic phase at convenient temperature ranges. The rodshape compound 4-n-octyloxyphenyl 4-n-hexyloxybenzoate (6OO8) [18] is one of the simplest liquid-crystal materials exhibiting nematic and tilted smectic (SmC) mesophases. The bent-core components 4-chloro-1,3-phenylene bis[4-(10-decenyloxy)benzoyloxy] benzoate (ClPbis10BB) [19] and 4,6-dichloro-1,3-phenylene-bis[4 0 -(9-decen-1-yloxy)-1,1 0 -biphenyl]4-carboxylate (10DClPBBC), [20] contain one and two chlorine atoms on their central rings in the 4 and 4,6 positions, respectively. This enhances their ability to form a nematic phase at relatively low, although still elevated temperatures. [19][20][21] In addition, they have double bonds on their terminal groups, which further decreased the nematic phase range compared to those with unsaturated chains. [22] In ClPbis10BB, which has been the subject of several studies, [23][24][25] the arms are relatively flexible because the outer benzene rings are separated by ester groups, whereas in 10DClPBBC the aromatic rings of the arms are directly linked, making them much more rigid. Differential scanning calorimetry (DSC) and textural observations show complete miscibility with 6OO8 for both bent-core compounds. All mixtures have a nematic (N) phase below the isotropic (I), and in the intermediate concentration range a smectic (Sm) phase below the nematic. The I-N transition enthalpies linearly decrease from about 3 J g À1 to 0.7 J g À1 from 100% to 0% 6OO8 content, whereas the enthalpy at the transition to the induc...