Synthesis, structure, and fluorescence properties of a calcium-based metal–organic framework

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H2NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H3O+)2[Ca(NDC)(C2H5O)(OH)]}4·1.1H2O. This MOF possesses a new structure composed of calcium clusters and H2NDC linker anions and shows a unique fluorescence property; it exhibits a fluorescence peak at 395 nm (λex = 350 nm) at room temperature, which is blue-shifted compared with that exhibited by the free H2NDC ligand. One of the possible mechanisms for this fluorescence is likely attributable to a ligand-to-metal charge transfer (LMCT) transition and is the first example of a calcium-based MOF exhibiting blue-shifted fluorescence due to LMCT.