Photochromic and Nonphotochromic Luminescent Supramolecular Isomers Based on Carboxylate-Functionalized Bipyridinium Ligand: (4,4)-Net versus Interpenetrated (6,3)-Net

Photochromic and nonphotochromic luminescent bipyridinium-based supramolecular isomers [Cd­(CPBPY)­(BDC)­(H2O)]n (1) and {[Cd­(CPBPY)­(BDC)]·H2O}n (2) (CPBPY = N-(3-carboxyphenyl)-4,4′-bipyridinium, BDC= terephthalate) have been successfully obtained by solvothermal reactions at 100 °C via tuning stoichiometric ratios of starting reagents. Isomer 1 features (4,4)-topological layer constructed by edge-shared Cd2 SBUs and BDC linkers attached by N-pendent CPBPY groups. Isomer 2 has (6,3)-topological layers with Cd atoms as nodes and BDC and double CPBPY as linkers, which are 4-fold interpenetrated into 3D network. Although both 1 and 2 contain bipyridinium ligands, only isomer 1 possesses reversible photochromic behavior with quick-switchable luminescence in the solid state. Compound 2 does not show photochromic behavior even after exposure to UV light for more than 2 h. Photochromism process of 1 originates from photostimulated reduction of CPBPY ligands to generate CPBPY•– radicals after irradiation, confirmed by EPR spectra. Careful check on structure reveals that the offset π–π stacking interaction between the pyridine ring of CPBPY and benzene ring of BDC with inter-ring shortest C···C distance of 3.214 Å in 1 is responsible for electron transfer to form the CPBPY•– radicals. The speculation is further supported by DFT calculation of frontier orbital matching of electron donor and acceptor. HOMO and HOMO–2 orbitals of BDC involve the carbon atoms of benzene ring while LUMO and LUMO+1 orbitals of CPBPY involve the carbon atoms of pyridinium ring. Importantly, the photoinduced formed CPBPY•– radicals in 1 have a long-lived lifetime (at least six months in air and room temperature condition), which is mainly attributed to the close packing mode.