Pressure-Driven Proton Transfer Accompanied by Pronounced Piezochromism in a Schiff-Base Crystal

Stimuli-responsive molecular crystals that exhibit abrupt color changes are commonly observed in Schiff-base organic compounds when exposed to temperature or light irradiation; these alterations are often driven by proton transfer. However, reversible switching involving combined structural and optical changes under pressure remains underexplored, because of the limited understanding of the charge transfer processes and lattice-charge coupling involved. Here, we design a new Schiff-base crystal, 2,5-dichlorosalicylideneaniline-benzonitrile, which undergoes a reversible piezochromic transition from pale yellow to orange-red at ∼0.65 GPa, accompanied by a unit-cell volume collapse of ∼2.7%. High-pressure single-crystal X-ray diffraction data reveal that the enol form undergoes planarization, reducing the dihedral angle between the salicylidene and phenyl rings from 40.15° to nearly 0°. High-pressure infrared spectroscopy and density-functional theory calculations support pressure-induced intramolecular proton transfer from the hydroxyl (−OH) to the imine (−NH) group, ultimately driving enol→cis-keto tautomerization. Moreover, the transformation causes a large band gap reduction of ∼0.70 eV and a photoluminescence peak shift of ∼10 nm. This work establishes an interplay of pressure-induced intramolecular proton transfer, molecular planarization, and large color changes in a new Schiff-base crystal, thereby providing a blueprint for designing piezo-responsive chromic switches operable under extreme environments.