Wide-bandgap molecular ferroelectric with thermally switchable dielectric and NLO responses from non-chiral design

Abstract

Molecular ferroelectrics that combine switchable dielectric and nonlinear optical (NLO) properties hold great promise for next-generation flexible and wearable optoelectronic devices. However, their development is often hindered by the reliance on expensive chiral precursors and limited thin-film processability. In this study, we report the rational design of a flexible organic molecular ferroelectric, chloroethyltrimethylammonium fluoroborate ([CETMA][BF₄]), using low-cost non-chiral precursors and a fluoroborate anion strategy. This compound features a wide bandgap of 4.73 eV and undergoes a thermally reversible first-order phase transition at 352 K (ΔS = 21.83 J mol⁻¹ K⁻¹), enabling dual switching of both dielectric and NLO responses. The transition involves symmetry breaking between the polar phase Cmc2₁ (SHG response: 0.5×KDP; dielectric constant ε′ = 5) and the centrosymmetric phase P6₃/mmc (SHG: 0; ε′ = 10), accompanied by broad optical transparency across the 300–2500 nm range. The material exhibits a notable spontaneous polarization (Pₛ) of 17.57 μC/cm² and demonstrates a superior combination of wide bandgap and high transparency, outperforming many classical ferroelectric materials. This work provides a feasible design strategy for developing high-performance metal-free ferroelectrics with enhanced integrability and functionality for modern optoelectronic applications.