Crystal Structures and Spectroscopic Properties of Metal–Organic Frameworks Based on Rigid Ligands with Flexible Functional Groups

Two rigid linear ligands with alkoxy functional groups (L1 = 4,4′-(2,5-dimethoxy-1,4-phenylene) dipyridine; L2 = 4,4′-(2,5-diethoxy-1,4-phenylene) dipyridine) incorporating carboxyl-containing auxiliary ligands (isophthalic acid = H₂IPA; terephthalic acid = H₂TPA; biphenyl-4,4′-dicarboxylate = H₂BPDC) have been adopted to build a series of complexes with M­(II) (M = Zn, Co, Cd) under solvothermal conditions. The formula of these complexes are {[Zn­(L1)­(IPA)]}_n (1), {[Zn­(L1)­(TPA)]·DMF}_n (2), {[Co­(L1)­(TPA)­(H₂O)₂]·2DMF}_n (3), {[Cd­(L1)­(TPA)­(H₂O)₂]·2DMF}_n (4), and {[Co­(L2)­(BPDC)]·0.5H₂O}_n (5). Five complexes have been characterized by elemental analysis, infrared spectroscopy, powder X-ray diffraction and thermogravimetry measurements. Topological analyses reveal that complex 2 is a 6-connected pcu net with point symbol {4¹²·6³}, while complex 5 is a 6-connected rob net with point symbol {4⁸·6⁸·8}, the other complexes 1, 3, and 4 can be simplified as 4-connected sql nets with point symbol {4⁴.6²}. Complexes 1, 3, and 4 are 2D layer motifs, 2 and 5 are both 2-fold interpenetrating 3D frameworks. The optical absorption spectra of 3 and 5 indicate the nature of semiconductivity. The strong fluorescence emissions and long emission lifetimes of 1, 2, and 4 display that they are promising phosphorescent materials.