Metal-Organic Frameworks (MOFs) have attracted a tremendous research interest because of their significant potential for practical applications in areas such as gas storage and separation, drug delivery, sensing, catalysis, etc.¹ The crystalline nature of these materials allows them to be characterized via single – crystal X-ray diffraction, which provides valuable insight of their structural features. MOFs with fine-tuned properties can be prepared through a process called post synthesis modification. PSM allows the introduction/exchange of functional groups of a MOF and is preferable to proceed in a single-crystal-to-single-crystal (SCSC) fashion because with this way direct structural information can be provided for the achieved structural modifications via single crystal x-ray crystallography. Several types of SCSC transformations have been reported which include insertion/exchange of organic ligands, exchange of lattice solvent molecules or terminally ligated molecules, transmetallations, metalation of the framework, etc.²

We shall report two families of trivalent rare earth (RE³⁺) MOFs based on a hexanuclear (RE³⁺)₆ SBU and their exchanged analogues. The first one involves 8-connected 2-D MOFs based on an angular dicarboxylic ligand 4,4'-(hydroxymethylene)dibenzoic acid (H₂BCPM), UCY-17(RE). A series of exchanged analogues UCY-17(Tb)/L produced from linker installation SCSC reactions of UCY-17(Tb) with selected dicarboxylic ligands shall also be discussed. The SCSC installation of the dicarboxylic ligands resulted not only to the turn-on of the thermometric properties of these materials but also to a variety of different thermometric performances.³ The second family of compounds with the general formula ((CH₃)₂NH₂)₂[Y₆(μ₃-ΟΗ)₈(bpydc)₆] is based on the linear dicarboxylic ligand H₂bpydc= [2,2'-bipyridine]-5,5'-dicarboxylic acid. Its subsequent metalation with transition metal ions was achieved giving rise to a series of exchanged analogues with various metal ions. Gas sorption measurements of the metalated analogues reveal lower Brunauer - Emmett Teller (BET) surface areas consistent with the complexation of metal ions to the accessible nitrogen atoms of the bpydc^2- ligand whereas the CO₂ uptake of the metalated analogues is increased. Furthermore, gas sensing studies of the pristine and metalated compounds revealed a variety of different gas sensing capabilities. Thus, SCSC transformation reactions allowed not only the targeted modification of the structures of the two MOFs but also the modulation of their temperature and gas sensing properties.