Jahn–Teller Distortion Enables Enhanced Piezoelectric Energy Harvesting Properties of a Metal-Pyrazolylborate Complex

Molecular piezoelectrics have garnered significant attention in energy harvesting and sensing fields due to their high intrinsic piezoelectricity, low elastic properties, and excellent solution processability. Recent efforts have primarily focused on rationally tuning the piezoelectric performance of these materials through the molecular predesign of organic components. However, the regulation of piezoelectric properties via the central metal ion has remained relatively underexplored. Herein, we report the mechanical properties and the piezoelectric output performances of three isostructural molecular piezoelectrics, [PhB(pz)₃]₂TM (PhB = phenylboronic acid, pz = pyrazole, TM = Fe(II), <b>1-Fe</b>; Co, <b>2-Co</b>; Ni, <b>3-Ni</b>), via first-principles calculations and piezoelectric experiments. Compared with <b>1-Fe</b> and <b>3-Ni</b>, <b>2-Co</b> shows an enhanced structural distortions arising from Jahn-Teller effect, the unequal occupancy of degenerate orbitals of 3<i>d</i>⁷ and enhanced polarization degree. The composite films of <b>2-Co</b> dispersed in poly(butyleneadipate-<i>co</i>-terephthalate) (PBAT) are fabricated as piezoelectric generators (PEGs) and display enhanced performances with open-circuit voltages (<i>V</i>_oc) of 3.5 V, and short-circuit current (<i>I</i>_sc) of 1.4 μA. Moreover, <b>2-Co</b> PEG can successfully detect human motion sensing including fingertip taping, finger bending, and walking. This work emphasizes the importance of the Jahn-Teller effect in molecular piezoelectrics and provides a strong tool to design and engineer the piezoelectric performance.