AN X-RAY DIFFRACTION STUDY OF TRIPHENYL BASED DISCOTIC LIQUID CRYSTALS, T. Blanton, H.P. Chen, J. Mastrangelo, S.H. Chen, pp. 18-23

Liquid crystalline materials comprised of a cyclic ring core and pendant triphenyl groups have been prepared. Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were utilized to identify the presence of the mesophases. X-ray diffraction (XRD) techniques confirmed that these liquid crystalline materials were discotic. The hexagonal nature of these triphenyl-based discotics was confirmed with the observance of weak (110), (200) and, in some cases, (210) diffraction peaks. Crystallite size measurements using the Scherrer technique determined that the correlation lengths were the result of about 14 to 18 stacked discotic units, depending upon the mesophase composition. INTRODUCTION Single-component liquid crystals (LCs) of disc-shaped molecules, known as discotic liquid crystals (DLCs), were discovered over 20 years ago by Chandrasekhar and coworkers [1]. The compounds investigated were benzene-hexa-n-alkanoates. Though first synthesized in 1937 [2], the mesogenic character of these materials was not realized until 1977 [1]. Based on X-ray diffraction data, a structure was proposed in which the benzene-hexa-n-alkanoate molecules were considered to be discs stacked in columns that form a planar hexagonal arrangement. However, the spacing between the discs in each column was found to be irregular. This structure has translational periodicity in two-dimensions (hexagonal arrangement) and liquid-like disorder in the third dimension (columnar arrangement) [1]. A structure of this type is similar to that of hexagonal phases observed for soaps and other lyotropic systems [3]. Soon after Chandrasekhar’s discovery of DLCs, hexa-alkoxy and hexa-alkanoyloxy triphenylenes were synthesized [4,5] and again it was established by X-ray diffraction that the mesophases of these compounds were DLCs [6]. Molecular engineering of liquid crystalline systems allows for combining molecular architecture with a desired functionality. Although discotic mesogens are generally comprised of a simple flat, or nearly flat, core surrounded by four to eight long-chain hydrocarbons, more complex DLCs have been synthesized. In one example alkylated hexa-peri-hexabenzocoronenes with 13 benzene rings in the core were fabricated [7]. These compounds were found to be DLCs, exhibited melting points above 700 oC, and were highly insoluble in all known solvents. Star-like heptamers comprised of a triphenyl core and a hexagonal arrangement of triphenylene units in which the six peripheral units are covalently linked via a flexible -(CH2)nspacer (n = 6 – 11) showed an increase in the clearing temperature of up to 30o when compared to a hexa-alkoxy derivative made up of a triphenyl core and six C5H11 chains [8]. Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 1 18