A two-state model for the modulated B₇ liquid crystalline phase exhibited by bent-core molecules

Liquid crystals (LC) made of bent-core (BC) organic molecules have been intensively studied over the past two decades. The B₇ LC consists of smectic layers in which tilted molecules have an in-plane polar packing, the polarization vector ( P ) having a splay distortion forming stripes of tens of nanometers in width, and undulated layers giving rise to 2D rectangular or oblique lattices. The prevailing phenomenological theory attributes this structure to a strong coupling between molecular tilt in the layers and div P . Based on our recent studies on other phases exhibited by BC molecules, we propose a new model in which the physical origin of the stripes arises from a minority (~10%) of the BC molecules having less bent excited state (ES) conformers which can freely rotate about their long axes, and aggregate to form smectic C type walls, in which the tilt angle is relatively small. The more bent ground state (GS) conformers with polar packing in turn form the splayed structure between such walls to lower the free energy. The mismatch in the layer spacing between the domains with ES and GS conformers generates the structures in the B₇ phase described above. The model predicts a weak first order transition from B₇ to the uniform B₂ phase as the temperature is lowered, as experimentally observed in some compounds. We show that the observed slow increase of the stripe width in the LC to micrometer dimensions in free standing films can be attributed to changes in the physical parameters by chemical degradation and absorption of ions by the exposed polarized layers. We also describe different possible structures of the undulated layers, including a stacking of racemic pairs of SmC_aP_F layers within the stripes. Some possible methods of testing the model are also indicated.