Studies on self-assembled porphyrin-fullerene and porphyrin-carbon nanotube donor-acceptor conjugates

Abstract

The research presented in this dissertation deals with the syntheses, characterization, electrochemical and photophysical studies of porphyrin-fullerene or porphyrin-carbon nanotube based donor-acceptor conjugates. The first chapter provides an introduction to the research presented in this thesis. A summary of primary events that occur in natural photosynthetic systems, the necessity and importance of the artificial photosynthetic mimics for light energy harvesting, the importance of non-covalent porphyrin-fullerene donor-acceptor models over covalent ones and some recent developments in model biomimetic systems are presented. Additionally, the importance of the nano-sized materials, solubilization of carbon nanotubes via chemical functionalization and utilization of these nanomaterials in building donor-acceptor nanohybrids is discussed. The second chapter discusses the syntheses and characterization of the control compounds and physical methods employed to monitor the photochemical pathways occurring in the studied donor-acceptor conjugates. The third chapter focuses on the design and syntheses of rigid, noncovalent “two-point” bound porphyrin-fullerene dyads assembled via crown etherammonium cation complexation and axial coordination or ∏-∏ interactions. The effect of axial ligation or ∏-∏ interactions in addition to the crown etherammonium cation binding, on photochemical charge stabilization in these supramolecuar dyads is systematically investigated. The fourth chapter deals with the photoinduced electron-transfer processes occurring in supramolecular triads comprised of cis and trans functionalized bis-18-crown-6 porphyrin self assembled with fullerene functionalized with pyridine or alkyl ammonium cation entities. The 1:2 stoichiometric supramolecular porphyrin:fullerene conjugates are obtained by utilizing either “two-point” binding methodology involving metalligand coordination and alkyl ammonium cation-crown ether binding or the latter type of binding solely. Variation of metal ion in the porphyrin (zinc or magnesium) cavity results in the free energy changes of charge separation and charge recombination. Employing higher number of the acceptor entities improves the electron-transfer rates, with kCS being 2-3 orders of magnitude higher than the kCR, thus indicating charge stabilization in these conjugates. The fifth chapter discusses the role of a secondary electron donor in generating longlived charge separated states in supramolecular conjugates formed via crown ether-ammonium cation binding. The sixth chapter presents the application of the present type of conjugates. Here, potassium ion-induced switching of intrato intermolecular electron transfer in crown ether appended porphyrin-fullerene donor acceptor conjugates is demonstrated. Investigations in the seventh chapter involve solubilization of single wall carbon nanotubes by ∏-stacking with pyrene functionalized imidazole or phenyl moieties and subsequent utilization of these nanotubes to build supramolecular donor-acceptor nanohybrids with donors such as porphyrin or napthalocyanine The compounds described in this dissertation were synthesized and characterized by proton NMR and ESI-Mass spectroscopy. Binding constants pertaining to the formation of various complexes were obtained by using UV-visible, fluorescence and ¹H NMR spectral data. Density functional theory (DFT) calculations were performed to gain insight into the structural aspects and orientation of the donor-acceptor groups in these supramolecular complexes. Electrochemical studies were performed to obtain free energy changes for charge separation and charge recombination. Steady state and time resolved fluorescence emission studies in addition to transient absorption studies were employed to obtain charge separation and charge recombination rates and lifetimes of photo-induced electron transfer.