Molecular organization in polar nematic phases: a combined FTIR spectroscopy and molecular simulation approach

The emergence of polar nematic phases, such as the ferroelectric (N_F) and antiferroelectric (N_X) phases, has opened new frontiers in soft matter science. However, the relationship between molecular structure, intermolecular interactions, and the resulting macroscopic order remains poorly understood. This work investigates the molecular organization in the polar nematic phases of the nJK (n = 1-5) homologous series using a combined approach of polarized Fourier-transform infrared (FTIR) spectroscopy, density functional theory (DFT) calculations, and atomistic molecular dynamics (MD) simulations. A central finding is the inadequacy of single-molecule DFT models to describe the system. While DFT accurately predicts vibrational band positions, it fails to replicate their experimental intensities, suggesting that intermolecular interactions are dominant. This is further corroborated by the anomalous temperature dependence of the average absorbance for parallel and perpendicular vibrations, which deviates significantly from simple density effects. Atomistic MD simulations of a multi-molecule system provide insight into this behavior, revealing the formation of locally ordered nanodomains driven by strong short-range correlations. Experimentally, we quantified the primary nematic order parameter (S), which follows a typical progression in the N phase but shows anomalous changes at the N-N_X and N_X-N_F transitions, ultimately saturating at a high value of S ≈ 0.7 in the ferroelectric state. The difference between orthogonal absorbance components for perpendicular vibrations reveals a significant biaxiality order parameter (C) that increases upon cooling and approaches 0.6 in the N_F phase. This study demonstrates that the unique properties of polar nematics are governed by specific molecular self-assembly and establishes phase biaxiality as a key feature of these systems.