Piezoelectricity from Dopant-Induced Structural Distortions in Molecular Crystals Revealed by Raman Spectroscopy

High Resolution Image Download MS PowerPoint Slide Crystal engineering offers a route to functional molecular solids through controlled noncovalent interactions. In centrosymmetric crystals, piezoelectricity can be induced by stereoselective doping, where chiral additives are incorporated in a biased orientation that lowers inversion symmetry and generates net polarization. However, the microscopic origin of this polarization, specifically whether it arises primarily from dipole mismatch between the dopant and the host molecule it replaces, or from dopant-induced lattice distortion, remains generally unclear. Here, we disentangle these two contributions by incorporating four chiral N -acetyl- L -amino acids into the centrosymmetric crystal N -acetyl- DL -valine and examining the resulting materials using piezoelectric measurements, density functional theory (DFT), and low-frequency Raman spectroscopy that reveals concentration-dependent symmetry-breaking phenomena not resolved by conventional X-ray diffraction. We show that in the systems studied, macroscopic piezoelectricity correlates directly with the magnitude and orientation of local lattice distortions, whereas dipole mismatch along the polar axis plays a secondary role. These results establish a direct structure–function relation in doped molecular crystals and provide predictive design principles for engineering electromechanical response in molecular crystals through biased local distortions.