Comparative electrooptic study of new orthoconic liquid crystals with fluorinated alkoxy terminal chains A. SPADŁO *1 , E. OTÓN 1 , R. DĄBROWSKI 2 , M. ŻUROWSKA 2 , J.M. OTÓN 1 , and N. BENNIS 1 1 ETSI Telecomunicación, Universidad Politécnica de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain 2 Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland Electrooptical properties of several new orthoconic antiferroelectric liquid crystal mixtures with partially fluorinated alkoxyalkoxy terminal chains have been investigated in order to select the best mixture for display applications. Electrooptical studies have been performed on these orthoconic materials aiming at evaluating their static and dynamic per- formance under passive multiplexing conditions. A number of parameters have been evaluated, static and dynamic contrast, driving scheme for passive multiplexing, rise and fall response times, dynamic range, and dynamic greyscale. Keywords: orthoconic antiferroelectric liquid crystals, electrooptical response. 1. Introduction Antiferroelectric liquid crystals (AFLCs) are promising can- didates for many display and photonic applications because of their particular electrooptical response. Unfortunately, the contrast of AFLC displays is usually very poor. Ortho- conic antiferroelectric liquid crystals (OAFLCs) having 45° tilt angle in the SmC* A phase are proposed as an alternative of regular antiferroelectrics because of their excellent opti- cal properties as a perfect static dark state and consequently a very high contrast. These properties make OAFLCs attrac- tive for display applications when the surface stabilized structure is achieved (SSOAFLCs), which is largely determined by their helical pitch [1–3]. In previous work, a number of chiral homologous series of three ring esters having a perfluoroalkanoyloxy alkoxy unit in the terminal chain were prepared (see main STRUC- TURE 1) and some broad temperature range orthoconic mixtures (e.g., W193B) were formulated. However, these exhibited a very short helical pitch at room temperature, usually less than 1 μm. In this case, surface stabilized struc- tures are only possible to achieve if the material is confined into very thin cells (~0.8 μm) [4,5] where n = 1–7 and m = 2–7. New OAFLC compounds containing partially fluori- nated alkoxy terminal chains and having a methylene spacer (CH 2 ) m with m = 2–7 (STRUCTURE 2) have been pre- pared. The new mixtures exhibit long pitch at room temper- ature. Several broad temperature range orthoconic mixtures based on these compounds have been formulated. where n = 3, X 1 = H, X 2 = F or X 1 = F, X 2 = H or X 1 =X 2 =F or X 1 = X 2 = H. An efficient preparation of the series has been presented elsewhere [6,7]. The F 2n+1 C n CH 2 O– group and the length of methylene spacer (i.e., the value of the index m) strongly in- fluence the temperature range of all phases, their stability and the whole sequence. The investigated series included all the spacer values m = 2–7. All the compounds showed a broad temperature range chiral anticlinic SmC A * phase. The length of methylene spacer m also influences strongly the length and temperature dependence of the pitch. An in- version of temperature dependence is observed for the lon- gest m’s. Indeed, for the highest values of m, the pitch de- creases with temperature, while for the lowest values of m, the pitch increases with temperature. Fluorosubstitution in the benzene ring does not change the kind of observed phases but it influences their thermal stability and strongly modifies the temperature dependence of the pitch [8,9]. 2. Experimental Several mixtures containing new partially fluorinated alko- xyalkoxy chain compounds were chosen as orthoconic AFLCs with a broad temperature range for ferroelectric and Opto-Electron. Rev., 18, no. 2, 2010 A. Spadło OPTO-ELECTRONICS REVIEW 18(2), 219–222 DOI: 10.2478/s11772-010-0010-2 * e-mail: aniaspadlo@tfo.upm.es Unauthenticated Download Date | 7/24/18 11:50 PM