Title:	A two-state model for the modulated B7 liquid crystalline phase exhibited by bent-core molecules
Authors:	Madhusudana, N.V.
Keywords:	bent-core molecules B7 phase layer polarization
Issue Date:	May-2020
Publisher:	The IOP Publishing Ltd.
Citation:	Journal of Physics: Condensed Matter, 2020, Vol.32, Article No.234003
Abstract:	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 SmCaPF layers within the stripes. Some possible methods of testing the model are also indicated.
Description:	Restricted Access.
URI:	http://hdl.handle.net/2289/7475
ISSN:	0953-8984 1361-648X (Online)
Alternative Location:	https://pubmed.ncbi.nlm.nih.gov/32050185/ https://doi.org/10.1088/1361-648X/ab7574
Copyright:	2020, IOP Publishing Ltd.
Appears in Collections:	Research Papers (SCM)

Items in RRI Digital Repository are protected by copyright, with all rights reserved, unless otherwise indicated.