Journal of Physics and Chemistry of Solids 68 (2007) 523–529 Shifting of transition temperature of ferroelectric liquid crystals due to addition of dye-An optical and dielectric study A.K. Srivastava, A.K. Misra, P.B. Chand, R. Manohar à , J.P. Shukla Physics Department, Lucknow University, Lucknow 226007, India Received 19 August 2006; received in revised form 11 December 2006; accepted 5 January 2007 Abstract In order to study the shifting of phase transition temperature of ferroelectric liquid crystals due to addition of dye molecules, we have investigated two ferroelectric liquid crystal materials (Felix 16/030 and Felix 16/100) and their five mixtures with Anthraquinone dye (1%, 2%, 3%, 4% and 5% wt/wt). The phase transition scheme has been investigated and analyzed by results obtained from the optical transmittance and the dielectric permittivity study with variation of temperature in the range of 30 to 100 1C. Both the samples clearly show the shifting of phase transition temperature with dye concentration, especially the SmC*–SmA phase transition temperature. It is also clear from the study that SmC*–SmA phase transition phenomenon also becomes stronger with the addition of dye molecules. A theoretical explanation has also been given for shifting of phase transition temperature. The amount of shift in transition temperature agrees well as obtained from optical and dielectric studies. r 2007 Elsevier Ltd. All rights reserved. Keywords: D. Phase transition 1. Introduction The field of ferroelectric liquid crystals has advanced very rapidly after the discovery of ferroelectricity in some liquid crystals, which made it one of the most attractive fields of research. Ferroelectricity in liquid crystals is the combined property of chirality in the molecule, the tilt angle of the director and smectic-layered structure of the material. The ferroelectric liquid crystals have been explored by a large number of researchers for their use in electro-optical devices [1–3]. Most of the studies on ferroelectric liquid crystals are concentrated around di- electric and optical properties of liquid crystals as these properties are critical for their application in different devices [4]. Addition of small amount of dye changes almost all the properties of ferroelectric liquid crystals drastically [5,6]. Some researchers have studied effect of addition of dye on the electro-optical behavior of ferro- electric liquid crystals [7,8]. Effect of dye doping on dielectric relaxation behavior of ferroelectric liquid crystals has also been reported by some groups [9,10] as dielectric study provides vital information regarding the molecular interaction of dye and ferroelectric liquid crystal molecules. One more property that changes with the addition of dye is the phase transition temperature of the material. However no serious effort has been made to study this aspect by any group. Therefore we have concentrated our investigations on change in the transition temperature of ferroelectric liquid crystal materials by the addition of dye, specially SmC*–SmA phase transition. To arrive at a conclusive result we have investigated two ferroelectric liquid crystals (Felix 16/030 and Felix 16/100) and their five mixtures with anthraquinone dye (1%, 2%, 3%, 4% and 5% wt/wt). We have used two techniques to determine the change in phase transition temperature of pure liquid crystals and dye doped liquid crystals i.e. optical transmittance and di- electric study with the variation of temperature. We have also given a theoretical approach to explain this change in transition temperature of ferroelectric liquid crystals due to the addition of dye. ARTICLE IN PRESS www.elsevier.com/locate/jpcs 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.01.010 à Corresponding author. Tel.: +91 9415023080. E-mail address: rajlu1@rediffmail.com (R. Manohar).