Polarized infrared spectra of liquid crystalline 4-octyloxy-4′-cyanobiphenyl

Polarized infrared spectra of 4-octyloxy-4'-cyanobiphenyl (8OCB) were studied for thin layers between either cleaned or covered by rubbed PVA film KRS-5 windows over the broad temperature range going from crystalline state at 25 degrees C via Smectic A, Nematic to isotropic liquid phase above 90 degrees C. The dependence of absorbance in the range of v(C N) and gamma(C-C N) vibrations on the polarization angle shows that the angle between the projection of the preferred orientation of the long axis of molecules on KRS-5 windows and the x-polarization equals to 45 degrees C for Smectic A and Nematic phases and 55 degrees C for isotropic liquid. The plots of dichroic ratio A(parallel to)/A(perpendicular to) versus temperature express semiquantitatively changes of the order degree in phase transitions. In isotropic phase the ordering disappears only at temperatures at least 10 degrees C above the transition temperature. The analysis of IR spectra in the region of v(C N), gamma(C-C N) as well as of v(CH)(ar) vibrations was performed by means of the DFT calculations for monomer and dimeric head-to-head (HTH) and core-to-core (CTC) species. The calculations show that HTH dimers are by 5.6 kJ/mol more stable than CTC ones. Experimental data obtained in this paper and those collected in the literature are discussed in the context of acetonitrile and cyanobiphenyls spectral properties. The attention was particularly focused on interpretation of the increase of v(C N) frequency in crystalline and Smectic A phases compared to Nematic and isotropic liquid phase. It is argued that this phenomenon is connected with a dense packing of C N groups in layered structures. The discrepancy between the interpretation of the bilayer structure of the Smectic A phase and the structure of 8OCB polymorphs needs further detailed studies. (c) 2007 Elsevier B.V. All rights reserved.