Substituent Effects Govern Packing and Nematic Phase Stability in Liquid Crystalline Monomers and Dimers

Liquid crystals (LCs) possess mechanical and optical properties with applications ranging from soft robotics to display technology. Here we show that even a single methyl substitution can markedly shift LC phase transition temperatures, an effect we capture using a molecular dynamics (MD) force field optimized with density functional theory and machine-learning-guided analysis. Despite advances in the precise synthesis of liquid crystalline materials, the microscopic origins of such substituent effects remain poorly understood, limiting rational materials design. To address this, we optimize a transferable MD force field capable of reproducing accurate conformational energetics and phase behavior. Using this, we find that methyl substitution on the LC core dramatically shifts the liquid crystal transition temperature, consistent with experimental observations. Interpretable machine-learning via Shapley additive explanations highlights specific dihedral angles whose shifts correlate with these thermal transitions. In dimeric systems built from these monomers, distinct hairpin and extended conformations emerge. The extended state exhibits a bend angle consistent with experimental signatures of modulated nematic phases. Together, these results show how substituent effects can impact LC monomer and dimer transition temperatures, providing microscopic insight to guide future materials design efforts.