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Natural and engineered systems often use functional or structural gradients to control the directional transport of matter.聽 Gradients within MOFs would allow for selective and directional transport of specific analytes to specific regions of a MOF crystal.聽 A ligand exchange process has been implemented to systematically tune the pore space within a class of 3-D mesoporous metal-organic frameworks (MOFs). 聽The mechanism of ligand exchange is investigated and revealed to proceed from the crystal periphery to the crystal core within single MOF crystals. 聽Upon halting the ligand exchange reaction at different time points, new single crystalline mesoporous MOFs bearing a porosity gradient have been realized, in which the pore dimensions gradually decrease when proceeding from the crystal periphery to the core.聽These methods point toward the construction of functionality gradients that can be used to shuttle molecules to specific locations within the crystal.