Steric and Guest-Controlled Strain Redistribution Modulates Spin Crossover and Optical Readout in BODIPY-Functionalized Hofmann-Type Frameworks

Modulating spin crossover (SCO) in Fe(II) frameworks requires control over both the ligand field and the way spin-state strain is redistributed through the lattice. Herein, we report a BODIPY-functionalized Hofmann-type framework platform in which steric design and guest-imposed mechanical constraints tune SCO through lattice compliance. 1 and 2, constructed from methyl- and phenyl-substituted pyridyl-BODIPY ligands, retain closely related [Ag(CN)2]--based connectivity. The moderate increase in ligand steric demand from 1 to 2 perturbs linker geometry, interlayer coupling, and packing environment, producing two-step SCO in 1 and a frustrated four-step transition in 2. In contrast, the anthracenyl-substituted ligand in 3 induces formation of a distinct framework containing [Ag2(CN)3]- linkers. Its higher-temperature one-step SCO therefore reflects the combined effects of ligand identity, altered cyanometallate connectivity, and framework packing rather than anthracenyl steric alone. Guest inclusion within porous 1 provides an alternative strategy: differences in guest rigidity, loading, and host-guest contact density reshape the host deformation pathway and tune the SCO profile without changing the Fe-Ag framework connectivity. The incorporation of the BODIPY chromophore induces diverse, nonmonotonic emission evolution across 1, 2, desolvated 3, and guest-loaded derivatives of 1, suggesting the unique ability of the BODIPY chromophore to probe both spin transitions and local microenvironments. These results identify steric bulk, framework connectivity, and guest-imposed constraints as coupled chemical variables for controlling strain accommodation and SCO in Hofmann-type frameworks featuring optical readouts.