Dielectric and Thermodynamic Study on the Liquid Crystal Dimer α-(4-Cyanobiphenyl-4′-oxy)-ω-(1-pyreniminebenzylidene-4′-oxy)undecane (CBO11O·Py)

Abstract

Broadband dielectric spectroscopy (103 to 1.8 × 109 Hz) and specific heat measurements have been performed on the odd nonsymmetric liquid crystal dimer α-(4-cyanobiphenyl-4′-oxy)-ω-(1-pyreniminebenzylidene-4′-oxy)undecane (CBO11O·Py), as a function of temperature. The mesogenic behavior is restricted to a nematic mesophase which can be supercooled down to its corresponding glassy state if the cooling rate is fast enough (no less than 15 K·min–1). Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the nematic mesophase as well as in the isotropic phase and across the isotropic-to-nematic phase transition. Two orientations (parallel and perpendicular) of the molecular director with regard to the probe electric field have been investigated. In the nematic mesophase, the dielectric anisotropy is revealed to be positive. Measurements of the parallel component of the dielectric permittivity are well explained by means of the molecular theory of dielectric relaxation in nematic dimers ( J. Chem. Phys. 2004, 121 (16), 8079). The dimer is seen as a mixture of cis and trans conformers, and the model allows us to estimate their relative populations at each temperature. The main molecular motions are interpreted by the model as independent end-overend rotations of each terminal semirigid unit of the dimer. The nematic-to-isotropic phase transition has been exhaustively studied from the accurate evolution of the specific-heat and the static dielectric permittivity data. It has been concluded that the transition is first order in nature and follows the tricritical hypothesis. As a consequence, the nematic mesophase has been characterized as uniaxial despite the biaxiality and flexibility of the dimer molecule.