Molecular Hyperpolarisability as a Screening Descriptor for Second-Order Nonlinear Optics in Ferroelectric Nematic Liquid Crystals

Abstract

Ferroelectric nematic liquid crystals combine fluidity with macroscopic polar order. Although the archetypal NF material RM734 exhibits large nonlinear optical coefficients, most known ferroelectric nematics were designed without consideration of optical nonlinearity. Here, we assess whether electronic-structure calculations can be used to identify promising nonlinear optical candidates within known polar liquid-crystal materials. Frequency-dependent molecular hyperpolarisability tensors were calculated using a range of DFT methods and basis sets, then converted to macroscopic d-coefficients using an oriented-gas model with empirical and values. Calculated values were benchmarked against available experimental d33, d15, and d13/d31 coefficients for representative materials. We find that absolute values depend strongly on method and bulk-parameter assumptions, whereas relative trends are more useful for screening. Explicit conformer averaging does not consistently improve agreement with experiment. Applying the best-performing single-conformer protocols to a broader polar liquid-crystal dataset identifies candidate materials with enhanced predicted nonlinear optical response and reveals simple design rules based on donor-acceptor asymmetry, conjugation length, and linker choice.