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, [Mg2(H2dhtp)2(μ-H2O)­(NMP)4] (AEMOF-2), [Mg2(H2dhtp)1.5(DMAc)4]­Cl·DMAc (AEMOF-3), [Ca­(H2dhtp)­(DMAc)2] (AEMOF-4), [Sr3(H2dhtp)3(DMAc)6]·H2O (AEMOF-5), and [Ba­(H2dhtp)­(DMAc)] (AEMOF-6) (H4dhtp = 2,5-dihydroxy-terepthalic acid; DMAc = N,N-dimethylacetamide; 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+–H2dhtp2– MOF, and AEMOF-5 is
only the second known Sr2+–H2dhtp2– 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 H2dhtp2– 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, S0, and lowest energy excited singlet,
S1, states of model complexes along the intramolecular
proton transfer coordinate were calculated by DFT and TD-DFT methods.