Photochromic Supramolecular Isomers Derived from Pb(II)-Bipyridinedicarboxylate Complexes

Photochromic metal-organic complexes (PMOCs) have received huge attention of chemists, thanks to their diverse structural characteristic and various available photo-modulate physicochemical functionalities. The organic ligand plays a crucial role in the quest of PMOCs with specific photo-responsive functionalities. The multiple coordination modes of polydentate ligands also provide possibilities for forming isomeric MOCs, which may open a new perspective on the research of PMOCs. The exploration of suitable PMOC systems is significant for the yield of isomeric PMOCs. Taking into account extant PMOCs based on polypyridines and carboxylate as electron acceptors (EAs) and donors (EDs), the covalent fusion of suitable pyridyl and carboxyl species may produce single functionalized ligands bearing ED and EA moieties for the building of novel PMOCs. In this study, the coordination assembly of bipyridinedicarboxylate (2,2'-bipyridine-4,4'-dicarboxylic acid, H₂bpdc) and Pb²⁺ ions generate two isomeric MOCs, [Pb(bpdc)]·H₂O (1 and 2), which have the same chemical compositions with main discrepancies in the coordination mode of bpdc^2- ligands. As expected, supramolecular isomers 1 and 2 exhibited different photochromic performance, thanks to the distinct microscopic functional structural units. A schematic encryption and anti-counterfeiting device based on complexes 1 and 2 has also been studied. Compared with the extensively studied PMOCs supported by photoactive ligands like pyridinium and naphthalimide-derivatives and PMOCs derived from mixed electron-accepting polydentate N-ligands and electron-donating ligands, our work provides a new idea for building PMOCs based on pyridinecarboxylic acid ligands.