Metal-organic　frameworks　(MOFs)　provide　an　ideal　platform　for　ion　exchange　due　to　their　high　porosity　and　structural　designability;　however,　developing　MOFs　that　have　the　essential　characteristics　for　ion　exchange　remains　a　challenge.　These　crucial　features　include　fast　kinetics,　selectivity,　and　stability.　We　present　two　anionic　isomers,　DGIST-2　(2D)　and　DGIST-3　(3D),　comprising　distinctly　arranged　5-(1,8-naphthalimido)-isophthalate　ligands　and　In3+　cations.　Interestingly,　in　protic　solvents,　DGIST-2　transforms　into　a　hydrolytically　stable　crystalline　phase,　DGIST-2＇.　DGIST-2＇　and　DGIST-3　exhibit　rapid　Cs+　adsorption　kinetics,　as　well　as　high　Cs+　affinity　in　the　presence　of　competing　cations.　The　mechanism　for　rapid　and　selective　sorption　is　explored　based　on　the　results　of　single-crystal　X-ray　diffraction　analysis　of　Cs+-incorporated　DGIST-3.　In　Cs+-containing　solutions,　the　loosely　incorporated　dimethylammonium　countercation　of　the　anionic　framework　is　replaced　by　Cs+,　which　is　held　in　the　hydrophobic　cavity　by　supramolecular　ion-ion　and　cation-pi　interactions.