Influence of Connectivity and Porosity on Ligand-Based Luminescence in Zinc Metal−Organic Frameworks

Applications of metal−organic frameworks (MOFs) require close correlation between their structure
and function. We describe the preparation and characterization of two zinc MOFs based on a flexible and
emissive linker molecule, stilbene, which retains its luminescence within these solid materials. Reaction of
trans-4,4‘-stilbene dicarboxylic acid and zinc nitrate in N,N-dimethylformamide (DMF) yielded a dense 2-D
network, 1, featuring zinc in both octahedral and tetrahedral coordination environments connected by trans-stilbene links. Similar reaction in N,N-diethylformamide (DEF) at higher temperatures resulted in a porous,
3-D framework structure, 2. This framework consists of two interpenetrating cubic lattices, each featuring
basic zinc carboxylate vertices joined by trans-stilbene, analogous to the isoreticular MOF (IRMOF) series.
We demonstrate that the optical properties of both 1 and 2 correlate with the local ligand environments
observed in the crystal structures. Steady-state and time-resolved spectroscopic measurements reveal
that the stilbene linkers in the dense structure 1 exhibit a small degree of interchromophore coupling. In
contrast, the stilbenoid units in 2 display very little interaction in this low-density 3-D framework, with excitation
and emission spectra characteristic of monomeric stilbenes, similar to the dicarboxylic acid in dilute solution.
In both cases, the rigidity of the stilbene linker increases upon coordination to the inorganic units through
inhibition of torsion about the central ethylene bond, resulting in luminescent crystals with increased emission
lifetimes compared to solutions of trans-stilbene. The emission spectrum of 2 is found to depend on the
nature of the incorporated solvent molecules, suggesting use of this or related materials in sensor
applications.