Raman investigation of the low‐temperature phase transition in a liquid crystalline system: terephthalidine‐bis‐heptylaniline (TB7A)

Abstract

Abstract Raman spectra of terephthalidine‐bis‐heptylaniline (TB7A) were recorded in the region 1125–1225 cm −1 from room temperature to 10 K at 18 different temperatures. The splitting of the ∼1165 cm −1 band, and the variation of the ratio of integrated intensities of the original band at ∼1165 cm −1 to the new component at 1170/1171 cm −1 reveals a solid modification. The analysis of the temperature dependence of the linewidth was made using Rakov's relation. A logarithmic plot as a function of 1/ T gives two straight lines with an abrupt discontinuity, which clearly exhibits a phase transition at about 210 K. The temperature dependence of the linewidth of the ∼1165 cm −1 band was used to characterize this transition. A relatively large variation (∼3 cm −1 ) of the linewidth on going from 250 to 10 K in TB7A having an odd ( n = 7) number of carbon atoms in the alkyl tail compared with those observed (∼1 cm −1 ) in other homologues having an even ( n = 2 or 10) number of carbon atoms in the alkyl tail is a clear demonstration of the even–odd effect. Copyright © 2002 John Wiley & Sons, Ltd.