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

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 determin ed 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 P2(1)/c, a = 11.168(1)Angstrom, b = 7.595(2) Angstr om, c = 49.106(1) Angstrom, beta = 94.01(1)degrees, Z = 8. The two symmetri cally independent molecules of the asymmetric unit have been used as starti ng geometries for semi-empirical MO calculations. The difference between th e experimentally observed and the optimized molecular structures is interpr eted as the influence of the crystal field. The structures of the crystalli ne 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 rela tionships between the different phases are discussed and a suggestion for t he crystalline film structure is given as deduced from simulations of elect ron diffraction patterns.