Symmetry Breaking Induced by H/F Substitution in Molecular Ferroelectrics for Flexible Wearable Sensors

Molecular ferroelectric materials exhibit structural tunability and high flexibility, demonstrating significant potential in flexible electronics and wearable sensing applications. Herein, we use nonpolar piperidinium tetrafluoroborate PipBF₄ (1, Pip⁺ = piperidine) as the precursor. We designed and synthesized an organic-inorganic hybrid ferroelectric, (4,4-DFPD)BF₄ (2, 4,4-DFPD⁺ = 4,4-difluoropiperidinium), via H/F substitution strategy. Single-crystal structural analysis indicates that the introduction of fluorine atoms successfully induces symmetry breaking in the crystal structure, causing it to crystallize at room temperature in the ferroelectric space group Pca2₁. At the Curie temperature, the (4,4-DFPD)BF₄ transitions from an ordered ferroelectric state to a disordered paraelectric state. This (4,4-DFPD)BF₄ exhibits a phase transition temperature (T_c = 398 K), spontaneous polarization, and piezoelectric properties (d₃₃ ≈ 28 pC/N). Based on the piezoelectric properties of (4,4-DFPD)BF₄, we fabricated flexible composite films of (4,4-DFPD)BF₄/PBAT PBAT = poly(butylene adipate-co-terephthalate). These films achieve mechanical-to-electrical energy conversion without additional polarization treatment, delivering an output voltage up to 4 V at the optimal composition. This flexible sensor can stably monitor various physiological signals ranging from large-scale human joint movements to minute vocal cord vibrations. It exhibits high durability, exceeding 2000 cycles.