Comparison of bulk and thin film structures of the liquid crystal 28OBC: measurements and simulations

Abstract

Freely-suspended liquid crystalline films of ethyl 4'-n-octyloxybiphenyl-4-carboxylate (28OBC) were prepared and transferred onto different substrates which enable detailed structural characterization. The structures of these thin film assemblies, which are only accessible in this way, were determined and compared with the crystal and molecular structure of 28OBC as formed by crystallization from toluene solution. The compound crystallizes in the monoclinic space group P21/c, a = 11.168(1) A, b = 7.595(2) A, c = 49.106(1) A, beta = 94.01(1) degree, Z = 8. The two symmetrically independent molecules of the asymmetric unit have been used as starting geometries for semi-empirical MO calculations. The difference between the experimentally observed and the optimized molecular structures is interpreted as the influence of the crystal field. The structures of the crystalline and E film phases have been investigated by SAXR and TED and the former has been found to be different from the bulk structure. The structural relationships between the different phases are discussed and a suggestion for the crystalline film structure is given as deduced from simulations of electron diffraction patterns.