Influence of hydrogen bond on the mesomorphic behaviour in urethane based liquid crystalline compounds: Experimental and computer simulation study

We present a combined experimental and theoretical study on the novel hydrogen-bonded liquid crystalline complex (UR-LC11) exhibiting both nematic and smectic phases upon cooling. The complex was prepared by mixing 2-(2-methoxyethoxy)ethylbutyl carbamate (UR) as H-bond acceptor with calamitic mesogen 4′-((11-hydroxyundecyl)oxy)-[1,1′-biphenyl]-4‑carbonitrile (LC11) as H-bond donor. The complex was characterized by FTIR technique and its liquid crystalline properties were studied by differential scanning calorimetry (DSC) and polarized optical microscope (POM). The experimental IR spectra were compared with theoretically obtained IR spectra by Density Functional Theory (DFT) to suggest the structure of hydrogen-bonded liquid crystal (LC). The molecular dynamics (MD) simulations were performed to understand the impact of hydrogen bonding on the mesomorphic behaviour of the complex and the temperature dependency of the transitions between the mesophases. We determined that UR-LC11 is a stable H-bond acceptor/donor type complex and a single H-bond forms between the carbonyl oxygen atom of the amide moiety of UR and the hydrogen atom of the terminal hydroxyl group of the LC11. Although LC11 is present only in nematic liquid crystalline form, the new complex displayed both nematic and smectic phases during cooling. The reason for the two distinctive LC phases was explained by the presence of hydrogen bond interactions, which provides structural flexibility. Besides, H-bond maintains uniaxial rod shape of the molecule to promote self-assembly behaviour and induces positional ordering in the smectic phase. The enhancement in the self-assembly of the H-bonded chains in the complex is reflected in the increased ΔH_fusion values. Due to the intermolecular π-π interactions of the phenyl rings and the formation of strong dipoles on the backbone, especially at the cyanobiphenyl end of the chains, the long-range directional order of the dipoles along their long axes are preserved at elevated temperatures and nematic to isotropic phase transition is observed at around 370 K both experimentally and theoretically.