The role of intermolecular interactions in stabilizing the structure of the nematic twist-bend phase

TL;DR

This study investigates how intermolecular interactions, especially hydrogen bonds, influence the stabilization of the nematic twist-bend (NTB) phase through experimental IR spectroscopy and DFT simulations, revealing electronic and geometric factors involved.

Contribution

It provides new experimental and theoretical insights into the electronic and geometric factors stabilizing the NTB phase, emphasizing the role of hydrogen bonding and molecular electronic structure.

Findings

Significant change in transition dipole moments at the NTB transition

DFT simulations support experimental IR spectroscopy results

Hydrogen bonds contribute to NTB phase stabilization

Abstract

The relationship between the molecular structure and the formation of the NTB phase is still at an early stage of development. This is mainly related to molecular geometry, while the correlation between the NTB phase and the electronic structure is ambiguous. To explore the electronic effect on properties and stabilization of the NTB phase we investigated 2,3-difluoro-4,4-dipentyl-p-terphenyl dimers (DTC5Cn). We used IR polarized spectroscopy, which can at least in principle, bring information about the ordering in NTB phase. All dimers show a significant drop of the average value of the transition dipole moment d{\mu}/dQ for parallel dipoles at the transition to the NTB phase, and an increase for perpendicular dipoles, despite its remaining unchanged for the monomer. These results coincide well with DFT simulations of vibrational dipole derivatives for molecules assembled in pseudo-layers of the NTB phase. The DFT calculations were used to determine the geometric and electronic properties of the hydrogen bonded complexes. We have provided experimental and theoretical evidence of stabilization of the NTB phase by arrays of multiple hydrogen bonds (XF...HX, X-benzene ring).