Molecular Dipole Optimized Hybrid Metal Halide for Piezoelectric Ultrasound Monitoring of Aquatic Organisms

Piezoelectric hybrid metal halides are promising materials for energy conversion and sensing, yet their design has largely relied on empirical approaches. Here, we report the design and synthesis of a zero-dimensional piezoelectric hybrid metal halide, (HP-MeV)SnCl₆ (HP-MeV = N-(3-hydroxypropyl)-N'-methyl-4,4'-bipyridinium), via amplification of the dipole moment of the organic amine cation. Substituting MeV with HP-MeV increases the molecular dipole from 3.17 to 12.07 debye, transforming centrosymmetric (MeV)SnCl₆ into a noncentrosymmetric structure. Density functional theory reveals that (HP-MeV)SnCl₆ exhibits low elastic moduli, high piezoelectric strain coefficients, and reduced acoustic impedance. Composite devices with (HP-MeV)SnCl₆/polydimethylsiloxane demonstrate efficient energy harvesting and sensitive human motion sensing. Notably, these devices achieve >90% fidelity in underwater ultrasound detection, enabling ultrasound monitoring of aquatic biological activity. This work establishes a rational molecular engineering route for piezoelectric hybrid metal halides and expands their potential for biological and environmental sensing applications.