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

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.