Giant SHG Switching in a Molecular Crystal via Pressure-Driven Conformational Isomerization

Second-harmonic generation (SHG) switching materials are of great significance for advanced optoelectronic applications, and their design is often guided by the "anionic group theory", which states that the nonlinear optical (NLO) effect of a material originates from microscopic polar groups and their geometric superposition. Therefore, external stimulus-triggered structural transformations are recognized as an effective strategy to achieve SHG switching. However, pressure-induced conformational isomerization is rarely reported in crystalline solids, which limits the development of pressure-responsive SHG switches. Herein, pressure-driven conformational isomerization is reported in a crown ether-based molecular crystal, [APIm·18-crown-6][TFSI]·H₂O (APIm = N-(3-aminopropyl)imidazolium, TFSI = bis(trifluoromethanesulfonyl)imide). A giant 206-fold enhancement of the SHG signal at 0.4 GPa is attributed to the trans-to-cis conformational transition of partial TFSI anions, accompanied by dynamic disorder suppression of the TFSI anions. At the molecular level, the cis conformers contribute diffraction intensity at previously forbidden hkl planes, while the parent phase maintains the macroscopic symmetry, resulting in a mixed-phase coexistence within the single crystal. Notably, it is extremely rare for an incomplete phase transition to induce such a dramatic SHG enhancement. This work enriches the understanding of the high-pressure effect in molecular crystals and thereby offers a new design strategy for exceptional optoelectronic materials.