Spin‐Crossover Confinement in Mesoporous Matrices Enables Tunable Porosity for Molecular Sensing

Bistable spin crossover (SCO) complexes are promising functional materials for microscale and nanoscale sensing applications due to their ability to reversibly switch physical properties in response to external stimuli such as temperature or molecular adsorption. Nevertheless, their practical integration into responsive devices is often limited by the poor accessibility of active sites in typically nonporous solids. Herein, we report a hierarchically structured hybrid platform in which mesoporous silica networks are supported within cellulose‐paper microfibers and subsequently loaded with the SCO complex [Fe(II)(Trz) 3 ]BF 4 (Trz = 1,2,4‐triazole). This multiscale architecture establishes a continuous structural connection from the microscopic paper scaffold down to the molecular level, enabling confinement‐driven coupling between architecture and bistability. We show that confinement within the mesoporous framework not only modulates the thermodynamic behavior of the spin transition but also promotes cooperative interactions between the SCO material and the porous matrix, where molecular self‐assembly increases accessible surface area. The confined SCO domains exhibit guest‐responsive spin transitions, as molecular uptake induces measurable shifts in the transition temperature. These combined effects reveal a cooperative mechanism linking nanoscale confinement and microscale hierarchy, providing a versatile pathway toward flexible, paper‐based materials for responsive and sensing technologies.