Equatorial–Axial Conformational Dynamics Enabling Thermoresponsive SHG Switch in a Homochiral Hybrid Compound

Abstract

Switchable second-harmonic-generation (SHG) materials are promising for sensing, imaging, and signal modulation applications. In this work, we designed and synthesized a hybrid salt, (R-3-hydroxypiperidium)₂[Fe(CN)₅(NO)] (1), by strategically introducing homochirality to drive a unique equatorial–axial conformation-switch mechanism. At room temperature, 1 displays a measured direct band gap of 3.07 eV and SHG signals approximately 0.67 times that of the referential KH₂PO₄ (KDP), with density functional theory (DFT) calculation yielding a d₃₅ coefficient of 0.424 pm/V with a band gap of 2.17 eV. Even more striking is its remarkable SHG switching behavior, exhibiting an on/off contrast of nearly 100 triggered by an iso-space-group phase transition at temperatures up to 372 K. This transition, despite involving a change in the molecular conformation, preserves the overall space-group symmetry due to the chiral nature of the material, thus bypassing the typical symmetry-breaking observed in conventional phase transitions. Such anomalous switching was further elucidated through the supercell model with DFT calculations, allowing in-depth investigations of the underlying order–disorder transitions. The key mechanism of the structural transition is attributed to the reversible switching of hydroxyl groups between equatorial and axial positions, accompanied by distinctive thermal expansions. Additionally, analysis of the first hyperpolarizability of the conformational isomers reveals the molecular-level origins behind the dramatic changes in the SHG behavior. Our findings provide a foundational understanding of the relationship between molecular dynamics and SHG modulation, offering valuable guidelines for designing advanced nonlinear optical materials.