Structural studies of 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids by infrared spectroscopic analysis

Infrared (IR) spectra were measured for three 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxy 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 (S-C)-smectic A(S-A)-''structured'' liquid (I-1)-isotropic liquid (I-2) crystal 2 (C-2)-crystal 1 (C-1)-S-C-cubic (D)-S-A-I-1-I-2, and C-S-C-D-I-1-I-2, respectively. The molar fraction of dimeric COOH groups was determined by using the absorbances of two bands at similar to 1696 and similar to 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 stale in the D phase, and the D phase transforms into S-A/I-1 phase, when the fraction of dimeric COOH is similar to 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 S-C phase, and the conformational structure becomes liquidlike above the S-C-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 I-1 phase.