Light-Driven Control of Polar Order in Ferroelectric Nematic Liquid Crystals.

Ferroelectric nematic (NF) liquid crystals are polar fluids that combine long-range orientational order with spontaneous macroscopic polarization and exceptionally large dielectric permittivity. While these properties make NF materials attractive for responsive soft devices, incorporating additional functionality remains challenging because polar order is highly sensitive to molecular mismatch. Here we introduce a molecular design strategy that enables optical control of NF materials using azobenzene-based mesogens structurally matched to the archetypal NF compound DIO. Two derivatives are themselves ferroelectric nematogens, demonstrating that photoisomerizable mesogens can intrinsically sustain and modulate NF polar order, with large and reversible light-induced changes in polarization and dielectric permittivity. The same structural matching principle also enables non-NF azobenzenes to co-assemble with DIO at concentrations up to 20 wt.% while preserving ferroelectric nematic order. In selected blends, light irradiation drives a reversible transition from the ferroelectric nematic state to a low-permittivity state consistent with antiferroelectric smectic order. Together, light and temperature provide complementary handles to navigate this phase landscape of competing ferroelectric and antiferroelectric order. These results provide a molecular design strategy for programming collective polar order in ferroelectric nematic fluids using light-responsive mesogens.