Supramolecular hydrogels and discrete structures based on metal coordination and hydrogen bonding

Abstract

The complexation of thiolated nucleosides and nucleobases with a range of group 11 metal ions (Au(I), Ag(I), Cu(I/II), was observed to lead to the formation of hydrogels by simple inversion tests. Atomic force and electron microscopy of the xerogels showed, in many cases, the presence of fibres with lengths in the micrometre range and above; this provided evidence for the formation of coordination polymers. This thesis is concerned with the preparation, characterisation and investigation of the physical properties, mainly photoluminescence and conductivity, of these coordination polymers. 2’-Deoxy-6-thioguanosine was successfully synthesised and characterized by UV, IR, Mass, 1H-NMR, and 13C-NMR spectroscopy. The formation of coordination polymers upon reaction of Au(I), Ag(I), and Cu(II) ions with 2’-deoxy-6-thioguanosine and 6-thioguanosine in aqueous media produced hydrogels with up to 97% water+methanol by mass. The Au-6-thioguanosine gel was studied in more detail because of the interest in gold thiolate polymers and the novel properties observed for Au(I)-6-thioguanosine. Fluorescence spectroscopy observed a strong yellow emission (λmax = 606 nm) which is not present in 6-thioguanosine nor in the Au(I) solutions. The optical absorption spectrum of the coordination polymer showed a band at (λmax = 360 nm) assigned to the HOMO-LUMO transition located mainly on the S-Au-S…chain of the polymer. An induced CD band associated with the Au-S chain and an enhancement of the CD signal at shorter wavelengths, for transitions associated with the ligand, suggested the polymer has a helical structure. Further evidence was provided by analysis of the X-ray scattering pattern of the xerogel and atomic force microscopy of single fibres deposited on silicon chips. The observation of long Au(I)-6-thioguanosine fibres and strong photoluminescence suggests some delocalisation of the states associated with the Au-S chains and the possibility of electronic conductivity. This was demonstrated upon oxidative doping of Au(I)-6-thioguanosine xerogels coated over platinum microband electrodes. Treatment with iodine vapour or ([Br(C6H4)3N]SbCl6) in anhydrous acetonitrile were found to result in linear current-voltage characteristics. The temperature dependence of the conductance showed Arrhenius behaviour (over range of temperature 223 to 323 K) with an activation energy of 94 kJ mol-1. V More complex structures based on Au(I)-6-thioguanosine polymers were observed upon synthesising the polymer in the presence of duplex DNA (from calf thymus). The templating of the polymer on DNA produces long, regular, and uniform fibres with a beads-on-a-string morphology. An unusual self-assembly of Au(I)-6-thioguanosine at flat Si surfaces upon simple drop-casting, with slow evaporation was also observed. AFM images of these films showed the formation of well-defined layers, but with each layer comprising long ribbons exceeding the maximum length of the AFM scan (~15 micrometres). Ag(I)-containing hydrogels formed by reactions with 6-thioguanine, 6-mercaptopurine, and 2-thiocytosine were prepared and characterised by AFM, TEM, XPS, X-Ray, FTIR, UV-Vis, and fluorescent spectroscopy. Au(I) 2-thiocytosine was also prepared, but this produced a discrete complex rather than a gel. AFM of the Ag(I) xerogels showed the formation of very long fibres and this was confirmed by TEM images. FTIR and X-ray diffraction studies suggested the metal coordination occurred via S atoms in all three gels. The Au(I) 2-thiocytosine was found to have strong luminescence (λmax 622 nm). A new Ag(I):6-thioethero nucleoside complex was prepared containing 6-methylmercaptopurine riboside (6-MMPR) which was shown by single crystal X-ray diffraction to unexpectedly feature coordination via N7 rather than the thioether sulfur atom. Cu(II) & Co(II):6-methylmercaptopurine(6-MMP) were also synthesised as new discrete complexes. The metals binding was studied by single crystal X-ray diffraction which showed that the binding sites were N3& N9 for Cu(II) ions, and N9 for Co(II) ion in the complexes.