Two 3D Cd(II) Metal–Organic Frameworks Linked by Benzothiadiazole Dicarboxylates: Fantastic S@Cd₆ Cage, Benzothiadiazole Antidimmer, and Dual Emission

Abstract

On the basis of the same benzothiadiazole (BTD) ligand 2,1,3-benzothiadiazole-4,7-dicarboxylic acid ( H 2 L ), two new isomers of three-dimensional (3D) BTD-derived Cd(II) metal–organic frameworks 1 - 2 {[S@Cd 6 L 6 ]· x H 2 O} n were obtained by the different solvothermal reactions, which were structurally similar. Surprisingly, structural analyses reveal that in 1 or 2, one free sulfur atom was fixed in a Cd(II) cluster cage by strong intermolecular interaction to form the secondary building unit (SBU) S@Cd 6 . Each SBU S@Cd 6 is connected by six L 2– ligands and further extended into the 3D porous framework. In this work, the BTD antidimmer was evidenced by structural analysis and photophysical study. Furthermore, either 1 or 2 showed the uncommon dual emission, while only one emission was observed in the solution of ligand H 2 L . The dual-emission mechanism was also realized by the structural analysis and photophysical study. Interestingly, although there is slight difference in structure (regular octahedral cage in 1 and slightly distorted octahedral cage in 2 ), the changes in N 2 adsorption capability and photophysical performance between 1 and 2 are obvious, where 2 shows smaller Brunauer–Emmett–Teller surface area, broader absorption of antidimmer, and longer dual-emission lifetimes. Interestingly, either 1 or 2, the dual emission was clearly red-shifted by increasing the solvent polarity or the acidity of ambience, respectively.