Combining Polycarboxylate and Bipyridyl-like Ligands in the Design of Luminescent Zinc and Cadmium Based Metal-Organic Frameworks

Detailed structural characterization and photoluminescence properties of four new metal organic frameworks (MOFs) based on zinc(II) or cadmium(II) metal ions, di-. or tricarboxylic aromatic ligands, and bipyridyl-like elongated ancillary linkers, namely, {[Zn-2(mu(4)-bdc)(2)(mu-pbptz)]center dot 2DMF center dot 3H(2)O}(n) (1), {[Cd(mu(3)-bdc)(mu-pbptz)]center dot 3DMF}(n) (2), {[Cd-3(mu(5)-btc)(2)(mu-pbptz)]center dot 2DMF}(n) (3), and {[Zn-2(mu-dhbdc)(2)(mu-pbptz) (DME)(4)]center dot 2DMF center dot H2O}(n) (4) (where bdc = benzene-1,4-dicarboxylato, btc = benzene-1,3,5-tricarboXylato, dhbdc = 2,5-dthydroxobenzene-1,4-dicarboxylato, pbptz = 3,6-bis(4-pyridy1)1,2,4,5-tetrazine, DMF = N,N-dimethylformaniide) are reported. The occurrence of large accessible volumes and structural and topological diversity are a constant for crystal structures of these compounds, which is a result of the connectivity established among the metal-carboxylato building units. formed in each case. Three-dimensional (3D) pcu frameworks of compounds 1 and 2 are built from the linkage of dimeric cores (established by the coordination of dicarboxylato bdc ligands) into two-dimensional networks that are further joined together by ancillary ligands, whereas the novel jcr7 topological 3D framework is achieved in 3 owing to the presence of the tricarboxylia btc ligand. Two-dimensional layers are generated in 4 given the bidentate coordination of both dhbdc and pbptz ligands. Interestingly, most crystal structures (3D frameworks of 1, 2, and 3) exhibit open architectures containing large solvent-occupied void systems that account for high relative void volumes. A deep analysis of the photophysical properties has been also accomplished for all compounds, confirming an overall blue emission under UV excitation in the steady state. Compound 3 is characterized with a strong phosphorescent emission that lasts a few seconds and is observed by the naked eye, which constitutes an infrequent photoluminescent behavior for metal-organic materials.