Tuning spin-crossover behavior via guest-induced effects: spin-state-dependent host-guest interaction in an azobenzene-loaded 3D Hofmann-type framework.

The pursuit of stimuli-responsive smart materials drives innovation in molecular science. Spin-crossover (SCO) compounds, with their reversible switching of magnetic, optical, and electronic properties, stand out as prime candidates. A central challenge for their application is the precise and controllable modulation of SCO characteristics. Utilizing host-guest chemistry in porous frameworks to impart "chemical pressure" is a promising strategy. Here, we report a novel three-dimensional Hofmann-type SCO metal-organic framework, {Fe(pdmh)[Pt(CN)4]} (1) (pdmh = 2-(4-pyridinylmethylene)hydrazide). Single-crystal X-ray diffraction confirms a temperature-driven SCO transition. Magnetic measurements of the pristine framework reveal a cooperative first-order SCO with a thermal hysteresis of ∼20 K. Remarkably, the inclusion of azobenzene guest molecules (1·Azobenzene), whose successful loading is confirmed by infrared spectroscopy, drastically modulates the SCO properties: it shifts the rapid, single-step transition to a higher temperature range (∼250 K on cooling, ∼240 K on heating) and reduces the hysteresis width to ∼10 K. Density functional theory calculations uncover a spin-state-dependent host-guest interaction via π-π stacking, which is significantly stronger in the low-spin state. This enhanced interaction exerts stabilizing chemical pressure on the Fe(II) centers, elucidating a clear mechanism for guest-mediated tuning. Given the photoresponsive nature of azobenzene, this work paves a novel avenue toward the development of optically controlled molecular materials.