Multimetallic Supramolecular Complexes: Synthesis, Characterization, Photophysical Studies and Applications in Solar Energy Utilization and Photodynamic Therapy

Abstract

This thesis describes the study of a series of multimetallic supramolecules containing varied metals and ligands, synthesized by a building block method and characterized by mass spectrometry, electronic absorption spectroscopy, and electrochemistry. Incorporating different functional units into complex systems allowed these multimetallic supramolecules to perform various light activated tasks including DNA cleavage and hydrogen generation from water. The complex [({(bpy)₂Os(dpp)}₂Ru)₂(dpq)](PF₆)₁₂ and [{(bpy)₂M(dpp)}₂Ru(BL)PtCl₂](PF₆)₆ were synthesized (M = Os^II or Ru^II; BL = dpp or dpq; bpy = 2,2^'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline). The building blocks displayed varied electrochemical properties upon complexation. The bridging ligands dpp and dpq display their reduction potentials shifted to less negative values when they changed from monochelating to bischelating. The electronic absorption spectra of the multimetallic systems displayed transitions of each contributing chromophore, with overlapping metal to ligand charge transfer (MLCT) transitions in visible region of spectrum. Spectroelectrochemistry revealed the nature of MLCTs and helped to identify fingerprint features of complex supramolecules. Photophysical measurements include emission spectroscopy with quantum yield measurements and emission lifetime measurements. Photophysical data provided detailed information to aid in developing an understanding of excited state properties of these complexes. Supported by the electrochemical data and spectroelectrochemistry, the hexametallic complex was suggested to have a HOMO localizing in the peripheral Os and a LUMO localizing in the central dpq, separating by a Ru energy barrier. This research systematically investigated photophysical properties of some building blocks and the mixed-metal, mixed-ligand supramolecules constructed by a variety of building blocks coupling light absorbing subunits to a reactive Pt metal center. Preliminary studies suggested [{(bpy)₂Ru(dpp)}₂Ru(dpq)PtCl₂](PF₆)₆ was a photocatalyst for H2 production from water in the presence of a sacrificial electron donor. The complex [{(bpy)₂Ru(dpp)}₂Ru(dpq)PtCl₂](PF₆)₆ had been studied for its catalytic ability in generating hydrogen and was found to have 34 product turnovers after 3 h photolysis. Photolysis and gel electrophoresis revealed that the tetrametallic complexes were able to bind to and then photocleave DNA through an oxygen mediated mechanism. The independence of ionic strength variation when [{(bpy)₂Ru(dpp)}₂Ru(dpp)PtCl₂](PF₆)₆ interacted with DNA, suggested the covalent interaction nature of the complex. These results suggest future work on understanding the excited state properties of supramolecular complexes is suggested. The designs of future photocatalysts for hydrogen production from water and anticancer photodynamic therapy drugs are also proposed.