X-ray studies of the crystalline and nematic phases of 4'-(3,4,5-trifluorophenyl)-4-propylbicyclohexyl

Abstract

The crystal and molecular structures of the nematogenic compound 4'-(3,4,5- trifluorophenyl)-4-propylbicyclohexyl (3ccp-fff) were determined by direct methods using single crystal X-ray diffraction data at 293K. The compound (C21H29F3) crystallises in the triclinic system with space group P1 and Z=2. The unit cell dimensions are a=5.3715(14), b=10.559(3), c=16.891(4), =86.331(5), =85.196(6) and =81.938(5). The structure was refined by the least squares method to an R-value of 0.058 for 1398 observed reflections. The fluorine atoms are in the plane of phenyl ring. Both the cyclohexyl groups are found to be in chair conformation and they are coplanar (dihedral angle 0.8). The molecules were found to be in the most extended conformation. Results of the crystal structure analysis were compared with that obtained from molecular modelling and also with that of the related bifluorinated compound (3ccp-ff). An antiparallel imbricated mode of packing of the molecules is found in the crystalline state. Several van der Waals interactions are observed between the neighbouring molecules, suggesting evidence for existence of molecular packings in head-to tail configuration. The average fluctuation length of the molecules within the nematic phase, determined from a small-angle X-ray diffraction study, is found to be about 1.4 times more than molecular length, providing further evidence for the existence of antiparallel associations between neighbouring molecules