Beyond Linker Length: Three-Dimensional Linker Geometry Governs Flexibility in Zn-Triazolate MOFs

Flexible metal-organic frameworks undergo cooperative structural transformations that underpin applications in molecular separations, sensing, and gas storage; however, the molecular origins of framework flexibility remain poorly understood. Here, we investigate the role of linker geometry in directing the structural response of a family of isoreticular Zn-triazolate pillared-layer MOFs incorporating cubane (cdc), bicyclo[1.1.1]pentane (pdc), and bicyclo[2.2.2]octane (boct) dicarboxylate linkers. Variable-temperature and in situ CO2 powder X-ray diffraction, variable-temperature single-crystal X-ray diffraction, gas adsorption measurements, and machine-learning-enabled atomistic simulations reveal fundamentally different mechanical responses despite the frameworks possessing closely related architectures. The cdc framework exhibits reversible cooperative breathing, the pdc analogue undergoes pathway-dependent structural reorganization consistent with kinetic trapping following cooling of the activated framework to 77 K, whereas the boct framework accommodates deformation through continuous elastic distortion without discrete phase transitions. Computational analysis demonstrates that these contrasting behaviors arise primarily from the manner in which linker geometry governs strain accommodation within the flexible Zn-triazolate sheets, rather than from the deformation of the linker itself. These results establish structure-mechanics relationships across this family of flexible MOFs and demonstrate that subtle changes in molecular geometry provide a powerful strategy for programming cooperative structural behavior.