Abstract:
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.
