Effects of the lateral substituent on the cubic phase formation of two analogous compounds, 4ʹ-n-hexadecyloxy-3ʹ-cyanobiphenyl-4-carboxylic acid (ACBC-16) and its 3ʹ-nitro compound (ANBC-16)

Abstract

Two cubic (Cub) phase forming compounds, 4ʹ-n-hexadecyloxy-3ʹ-cyanobiphenyl-4-carboxylic acid (ACBC-16) and its 3′-nitro analogue (ANBC-16) were studied by infrared (IR) spectroscopy. In the temperature region of the Cub phase, the molar fraction of hydrogen-bonded COOH groups estimated for ACBC-16 was by ≈0.05 at maximum larger than that for ANBC-16 and the aromatic ring C═C stretching (ν(C═C)ring) band frequency of ACBC-16 was by 3 cm−1 lower than that of ANBC-16. The quantum chemical calculation at DFT/B3LYP level, on the one hand, showed no meaningful difference in the stabilisation energy for dimerisation and the ν(C═C)ring band frequency between the respective model compounds. These results can be ascribed to the different steric effects of the side groups; the CN group would make possible the close contact of neighbouring phenyl rings while the bulky NO2 group would not, giving slightly more stabilised dimerisation of ACBC-16 in the Cub phase than in ANBC-16.