Structural Studies of 4‘-n-Alkoxy-3‘-nitrobiphenyl-4-carboxylic Acids by Infrared Spectroscopic Analysis

Abstract

Infrared (IR) spectra were measured for three 4‘-n-alkoxy-3‘-nitrobiphenyl-4-carboxylic acid homologues [ANBC-n, where the carbon number of the alkoxy group (n) is 14, 16, and 22] as a function of temperature. The phase sequences for ANBC-14, -16, and -22 are crystal (C)−smectic C (SC)−smectic A (SA)−“structured” liquid (I1)−isotropic liquid (I2), crystal 2 (C2)−crystal 1 (C1)−SC−cubic (D)−SA−I1−I2, and C−SC−D−I1−I2, respectively. The molar fraction of dimeric COOH groups was determined by using the absorbances of two bands at ∼1696 and ∼1734 cm-1, which are assigned to the dimer and monomer carbonyl stretching vibrations, respectively. In the crystalline phase all the COOH groups form hydrogen-bonded dimers, and the dissociation of the dimers into monomers occurs above the melting point. It is noted that about 85% of ANBC molecules are still in the dimer state in the D phase, and the D phase transforms into SA/I1 phase, when the fraction of dimeric COOH is ∼0.74 mol %, for both n = 16 and 22. The temperature dependence of the CH2 stretching bands for these two compounds suggests that the alkoxy chains are rather linear-shaped to hold some local order in the SC phase, and the conformational structure becomes liquidlike above the SC−D phase transition temperature. On the basis of the present IR results, we deduce a possible structure inside micelles in the D phase and the molecular state in the I1 phase.