π-Expansion-Directed Modulation of Spin–Fluorescence Coupling in Spin-Crossover Hofmann-Type Frameworks

The coupling between spin crossover (SCO) and luminescence provides a direct, noninvasive optical readout of spin-state switching, which is pivotal for the development of advanced molecular sensors, memory devices, and opto-spintronic applications. Herein, we report two two-dimensional Hofmann-type coordination polymers, i.e., {Fe₂(PYNA)₄[Ag(CN)₂]₄} (<b>1</b>) and {Fe₃(PYAN)₆[Ag(CN)₂]₆}·<i>o</i>-DCB (<b>2</b>, <i>o</i>-DCB = <i>ortho</i>-dichlorobenzene), based on two extended π-conjugated ligands, 4-(2-naphthalenyl)pyridine (PYNA) and 4-(2-anthracenyl)pyridine (PYAN), respectively. Divergent SCO behaviors were observed for the two compounds, as confirmed by temperature-dependent magnetic susceptibility measurements, structural analyses, and differential scanning calorimetry measurements. In addition, light-induced excited spin-state trapping effects were found in the two SCO compounds, and bidirectional photoswitching of spin states can be realized reversibly. Crucially, a comparative variable-temperature fluorescence (FL) study highlights a π-expansion-driven contrast: SCO-FL coupling is operative in <b>1</b> but absent in <b>2</b>. Theoretical calculations directly account for the distinct energy transfer pathways in <b>1</b> and <b>2</b>, rationalizing their contrasting outcomes in achieving SCO-FL coupling. This study provides important support for the precise modulation of the spin-optical coupling properties through ligand engineering.