Molecular and crystal structure of 4-alkoxybenzoic acids: Design of the mesogenic phase

• Published in: Crystallography reports Vol. 54; no. 5; pp. 862 - 879
• Main Authors: Kuz’mina, L. G., Kucherepa, N. S., Pestov, S. M., Kochetov, A. N., Rukk, N. S., Syrbu, S. A.
• Format: Journal Article
• Language: English
• Published: Dordrecht SP MAIK Nauka/Interperiodica 01.09.2009
• Subjects: Crystallography and Scattering Methods; Physics; Physics and Astronomy; Structure of Organic Compounds; 61.64.Hg
• ISSN: 1063-7745; 1562-689X
• DOI: 10.1134/S1063774509050204

The crystal and molecular structures of 4-alkoxybenzoic acids C n H 2 n + 1 -O-C 6 H 4 -COOH, n = 4–12, which are nematic ( n = 4–6) or smectic-nematic ( n = 7–12) mesogenes, are studied. No direct relationship between the molecular geometry and mesogenic properties is found. All the crystal structures contain dimers formed by pairs of O…H-O hydrogen bonds between carboxyl groups. Crystals belonging to the group of smectic-nematic liquid crystals (SNLC) are built of separate regions, namely, loosely packed aliphatic regions and closely packed aromatic regions with significant π-stacking interactions. Loose aliphatic regions occur in crystals belonging to the group of nematic liquid crystals (NLC); however, they are surrounded by dense aromatic regions and do not run throughout the crystal. In NLC, π-stacking interactions are far weaker than in SNLC. At the early stage of melting of SNLC crystals, structurized associates due to both π-stacking interactions and hydrogen bonds are retained in the melt (smectic phase), whereas, upon further heating, only hydrogen-bonded associates are left (nematic phase). In the course of melting of NLC, only hydrogen-bonded dimers are retained in the melt (nematic phase).