Alkaline Earth Metal Ion/Dihydroxy-Terephthalate MOFs: Structural Diversity and Unusual Luminescent Properties

Alkaline earth (group 2) metal ion organic frameworks (AEMOFs) represent an important subcategory of MOFs with interesting structures and physical properties. Five MOFs, namely, [Mg-2(H(2)dhtp)(2)(mu-H2O)(NMP)(4)] (AEMOF-2), [Mg-2(H(2)dhtp)(1.5)(DMAc)(4)]Cl center dot DMAc (AEMOF-3), [Ca(H(2)dhtp)(DMAc)(2)] (AEMOF-4), [Sr-3(H(2)dhtp)(3)(DMAc)(6)]center dot H2O (AEMOF-5), and [Ba(H2dhtp)(DMAc)] (AEMOF-6) (H4dhtp = 2,5-dihydroxy-terepthalic acid; DMAc = N,Ndimethylacetamide; NMP = N-methylpyrrolidone), are presented herein. The reported MOFs display structural variety with diverse topologies and new structural features. Interestingly, AEMOF-6 is the first example of a Ba2+-H(2)dhtp(2-) MOF, and AEMOF-5 is only the second known Sr2+-H(2)dhtp(2-) MOF. Detailed photoluminescence studies revealed alkaline earth metal ion-dependent fluorescence properties of the materials, with the heavier alkaline earth metal ions exhibiting red-shifted emission with respect to the lighter ions at room temperature. A bathochromic shift of the emission was observed for the MOFs (mostly for AEMOF-3 and AEMOF-4) at 77 K as a result of excited state proton transfer (ESIPT), which involves an intramolecular proton transfer from a hydroxyl to an adjacent carboxylic group of the H(2)dhtp(2-) ligand. Remarkably, AEMOF-6 displays rare yellow fluorescence at room temperature, which is attractive for solid state lighting applications. To probe whether the alkaline earth metal ions are responsible for the unusual luminescence properties of the reported MOFs, the potential energy surfaces (PESs) of the ground, S-0, and lowest energy excited singlet, SD states of model complexes along the intrarnolecular proton transfer coordinate were calculated by DFT and TD-DFT methods.