Molecular Ferroelectric [(NMe₂)₃PCH₂Cl]PbI₃ Showing Two-Step Switching Second-Order Nonlinear Optics

Molecular ferroelectrics have garnered significant attention owing to their environmental compatibility, facile synthesis, lightweight nature, and structural versatility. In this study, we present a novel one-dimensional (1D) organic-inorganic hybrid perovskite molecular ferroelectric, [(NMe₂)₃PCH₂Cl]PbI₃ (<b>1</b>), which undergoes two successive reversible phase transitions. This compound demonstrates distinct spontaneous polarization in ferroelectric domains, along with two-step switching in second-harmonic generation (SHG) and dielectric responses. Variable-temperature single-crystal X-ray diffraction reveals a symmetry-breaking transition in <b>1</b>, shifting from the centrosymmetric <i>P</i>6₃/<i>mmc</i> space group to the polar noncentrosymmetric <i>P</i>6₃ ferroelectric phase. This transition is primarily driven by an order-disorder transformation of the [(NMe₂)₃PCH₂Cl]⁺ cation. Furthermore, compound <b>1</b> exhibits promising semiconductor characteristic with a bandgap of 2.70 eV. This material's unique high-low-off SHG-switching behavior and robust polarization switching (<i>P</i>_s = 0.3 μC·cm^-2) position it as a multifunctional platform for next-generation applications in multistate memory devices, programmable optical modulators, and temperature-responsive sensors, addressing critical limitations of conventional molecular ferroelectrics in stability and functional diversity.