Refractive indices and order parameters of some tolane-based nematic liquid crystals M.S. Zakerhamidi a, ⁎, Z. Ebrahimi a,b , H. Tajalli a,b , A. Ghanadzadeh a,c , M. Moghadam c , A. Ranjkesh c a Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz, Iran b Faculty of Physics, University of Tabriz, Tabriz, Iran c Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran abstract article info Article history: Received 21 July 2010 Received in revised form 29 August 2010 Accepted 30 August 2010 Available online 6 September 2010 Keywords: Order parameter Refractive indices Tolane-based liquid crystals Isothiocyanatotolanes Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic phase in order to determine their refractive indices and order parameters using the Vuks method. These factors characterize anisotropic behaviors of selected liquid crystals that are essential for their applications and improvement. The temperature dependence of refractive indices, birefringence and order parameters in anisotropic nematic phase, have been investigated for these four nematic liquid crystals. All the compounds have positive and large optical anisotropies that increase with decreasing temperature. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Characterization of liquid crystals (LCs) properties is one of the most interesting research topics in development and improvement of these materials for particular applications. Many researchers have studied properties of different classes of liquid crystals [1–8]. Importance of research on liquid crystal synthesis and characterization can be assigned to their widespread application in display technology, as well as to their gradually extending applications into electro-optical devices. In order to select an appropriate liquid crystal for a specific purpose various properties of the LC such as order parameter, dielectric constant, dielectric anisotropy, birefringence and optical transmittance should be characterized. Knowledge of order parameter and refractive indices of liquid crystals and their temperature dependencies is practically important [9]. Molecular polarizabilities and their anisotropies are unique molecular properties of liquid crystal substances [10,11]. These molecular parameters can be used for estimation of order parameter, which is another essential parameter of a liquid crystal compound [12–15]. The refractive indices of a liquid crystal are mainly determined by the molecular structure, operating wavelength, and temperature. Various theoretical models [16–19] have been developed for formulating the wavelength and temperature dependencies of LC refractive indices. According to deGennes any of the bulk tensorial properties like refractive indices can be used to determine macro- scopic order parameter (Q) [12–14] in liquid crystals: Q = n e -n o Δn 0 ð1Þ where Δn is the birefringence. n e and n o are extraordinary and ordinary component of refractive indices. The microscopic order parameter (S) was introduced by Tsvetkov [20,21]. S = 1 2 〈3 cos 2 θ-1〉 ð2Þ where θ is the angle between the optic axis and long molecular axis of a molecule. Due to the great importance of order parameter in understanding various physical properties of the mesogens, several methods have been developed for the determination of order parameter. Among those methods, optical methods are most commonly used [22–25]. It is generally accepted that the data obtained by the optical measurements is one of the most accurate, precise and convenient results one can acquire. Furthermore, optical methods provide optical anisotropy data that are important for application of liquid crystals in display devices [26]. In this work, we have studied four high birefringence nematic liquid crystals with positive dielectric anisotropy and high chemical and thermal stability. It comprises determination of the birefringence, ordinary and extraordinary refractive indices, macroscopic and microscopic order parameters and temperature dependencies of these parameters. Journal of Molecular Liquids 157 (2010) 119–124 ⁎ Corresponding author. Tel.: + 98 411 3393003; fax: + 98 411 3347050. E-mail address: Zakerhamidi@tabrizu.ac.ir (M.S. Zakerhamidi). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.08.015 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq