Influence of Coulombic Interaction on the Interfacial Self-Assembly of Discotic Liquid Crystal Amphiphiles: A Combined Experimental and Computer Simulation Study

Abstract

Self-assembly of amphiphilic molecules largely depends on the structure and electronic properties of the polar head groups. An important class of amphiphiles with technological applications comprises the discotic liquid crystal (DLC) amphiphiles. Here, we report remarkable differences in the self-assembly properties of two similar discotic amphiphiles with dissimilar polar head groups, viz., imidazole-tethered with hexaalkoxytriphenylene (neutral-ImTp) and imidazolium-tethered with hexaalkoxytriphenylene (ionic-ImTp). Surface manometry reveals that the ionic-ImTp exhibits a larger limiting area, higher collapse pressure, and smaller compressional elastic modulus at the air–water interface as compared to the neutral-ImTp system. At the air–solid interface, ionic-ImTp can be transferred only up to a bilayer structure with undulated morphology, whereas the neutral-ImTp exhibits smooth morphology and higher transfer efficiency. These results are explained by density functional theory (DFT) calculations and molecular dynamics (MD) simulations, which elucidated that the Coulombic interaction is the dominant factor that controls the organization of these molecules. DFT calculations predicted a T-shaped π-stacking geometry for the ionic-ImTp and a parallel-displaced stacking geometry for the neutral-ImTp. MD simulation predicted the orientation of molecules and their strength of hydrogen bonding. Understanding the intermolecular interactions governing self-assembly is important to engineer molecular packing that controls the charge transport in DLC-based organic electronics.