Photochromism And Luminescence Of Bipyridinium-based Coordination Polymers

In recent years,metal-organic coordination polymers have been an expanding research area for their wide applications in gas absorption/separation,catalysis,drug delivery and photochemistry,thus the rational design and synthesis of multifunctional metal-organic coordination polymers have become a very hot research topic in the field of materials science.The design and synthesis of multifunctional materials based on photoactive molecules through molecular assembly has been widely developed for their potential application in optical data storage,optical switches,displays and so on.In this thesis,the bipyridinium ligand was utilized as the mian functional building block to synthesize seven novel coordination polymers together with multidentate carboxylic organic ligands though solvothermal synthesis method,and their structures and properties have been characterized by X-ray single-crystal/powder diffraction studies,IR,UV-Vis,elemental analysis,thermogravimetric analysis,electron spin resonance and luminescent spectra.In this work,a series of strategies are employed to tune the photochromic properties of bipyridinium-based coordination polymers in crystalline states.The innovative features of my present work are described as follows:(1)We choose the 1,3-benzenedicarboxylic acid(m-H2BDC),1,2-benzenedicarboxylic acid(o-H2BDC),1,4-benzenedicarboxylic acid(p-H2BDC)and 5-methylisophthalic acid(H2mip)as the auxiliary ligands to construct four coordination polymers,compounds 2-1,2-2,2-3 and 2-4,via the molecular self-assembly with the bipyridinium ligand BpyenBr2-The key factors impacting on the photosensitivity and the stability of the free radicals were studied from the following aspects:the distance between donor and acceptor,the dihedral angle between the two adjacent pyridine rings and the ?-? stacking interaction between bipyridinium ligand.(2)We have synthesized three isomorphous compounds(2-5,2-6 and 2-7)based on the bipyridinium ligand by introducing different halogen ions(Cl-,Br-and I-),and analyzed the kinetics of the photochemical reactions of the three compounds.The kinetic calculation results based on the UV-vis diffuse reflectance spectra indicate that the variation of halogen atoms can influence the photochromic behaviors.The photoluminescence properties of three compounds were investigated in the solid state at room temperature.Compounds 2-5 and 2-6 can produce luminescence while compound 2-7 is non-luminescent due to the strong heavy atom quenching effect of iodine ions.The excitation wavelengths of compounds 2-5 and 2-6 are located in the visible spectral region,which provides us a platform to study the photo-switchable luminescence properties.It is interesting to note that compounds 2-5 and 2-6 are the first two examples showing photo-modulated luminescence switching featuring a non-destructive readout capability based on transition metals.