Deciphering the interactions between liquid crystals and chemically functionalized surfaces: Role of hydrogen bonding on orientations of liquid crystals

Abstract

This paper reports an investigation of the influence of interfacial hydrogen bonding on the orientations of liquid crystals at surfaces. The orientations of 10 liquid crystals were examined on carboxylic acid- or methyl-terminated self-assembled monolayers (SAMs) supported on obliquely deposited gold films. The mesogens comprising the liquid crystals each possessed different hydrogen bond acceptors. The azimuthal orientations (in-plane orientation of director relative to the direction of the deposition of gold films) of the liquid crystals were found to depend on the nature of the hydrogen bond acceptors in the mesogens when the SAMs presented carboxylic acid groups but were independent of the hydrogen bond acceptors when the SAMs presented methyl groups. Polarization modulated infrared reflection–absorption spectroscopy (PM-IRRAS) revealed that liquid crystals containing nitrogen atoms form hydrogen bonds with the carboxylic acid groups of the SAMs, and thereby disrupt the hydrogen bonds amongst carboxylic acid groups within the SAMs. Furthermore, liquid crystals that disrupt the interfacial hydrogen bonds within the SAMs assume an azimuthal orientation that is orthogonal to the orientation of liquid crystals that do not disrupt the hydrogen bonds within the SAMs. We conclude that the azimuthal orientations of the liquid crystals reflect the relative strength of hydrogen bonding between the mesogens and the COOH-terminated SAMs.