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

Abstract

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.