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 (10(3) to 1.8 × 10(9) 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.

Molecular dynamics of a binary mixture of twist-bend nematic liquid crystal dimers studied by dielectric spectroscopy

“…3 there is only one mode, in both nematic phases the dielectric 278 permittivity shows three relaxations, as in other nonsymmetric 279 dimers [11,28,29] and in particular as in FFO9OCB [30]. (inverse process).…”

“…It was found 27 that these compounds, of which 1 ,7 -bis(4-cyanobiphenyl- 28 4 -yl) heptane, CB7CB, is the most studied, show a first 29 order nematic-nematic phase transition. The low-temperature 30 nematic phase is the so-called twist-bend nematic, N TB .…”

“…Although the precise 351 temperature dependence is intricate, the usual trend [11,28,29] 352 is that the strength of the lower-frequency mode (m 1L ) 353 increases while that of m 1H decreases when the temperature is 354 reduced as found for FFO9OCB [30]. Despite the similarity of …”

“…We have successfully employed such an approach to 75 interpret the static and dynamic dielectric behavior of various 76 nematic dimers, ranging from the symmetric dimers CB7CB or 77 CB9CB [1,6] to the highly nonsymmetric CBOnO.Py dimeric 78 family [28,29]. However, until now all dimers showing a 79 stable N TB phase and studied by dielectric spectroscopy have 80 had identical terminal groups, and so the main motivation of 81 this paper is to analyze the molecular orientational dynamics 82 in the N TB phase when the dimers are nonsymmetric.…”

“…What is interesting 390 is the general trend: ε 1H decreases while ε 1L tends to 391 saturate. It is important to recall that, even if the alignment were 392 perfect, the precise analysis of the temperature dependence 393 would be complicated since it involves both the temperature 394 evolution of the population of the different conformers and 395 the relative value of the dipoles in the terminal mesogenic 396 units [28][29][30]34]. The frequency dependence of the modes can 397 be determined over a broad temperature range, and as can be 398 observed in Fig.…”