Synthesis and properties of donor acceptor molecular dyads

Abstract

This thesis is concerned with the design, preparation and structural characterisation of several molecular dyads. The molecular dyads were synthesized for the study of energy/electron/charge transfer processes to advance our understanding of artificial photosynthesis. We were hoping to enhance our ability in mimicking natural photosynthesis through our discoveries. Chapter 1 introduces the field of artificial photosynthesis. The working principles and the highest efficiency dye sensitized solar cells (DSSCs) to date are highlighted. Molecular units working as photo-active switches for fabrication of light-driven molecular scale machines are emphasized. Finally, previous work on the acridinium-, naphthalimde-, viologen- and 1,1'-bi-2-naphthol-based derivatives are reviewed. Chapter 2 is the experimental section, it describes the techniques used for characterisation, and details of synthetic procedures used to obtain molecular dyads discussed in chapters 3 to 6. Chapter 3 discusses the synthesis and properties of expanded acridinium dyads, and their application as sensitizers for DSSCs. Coupling of N,N-dimethylaniline and an expanded acridinium unit afforded a purple dyad, DMA. The charge transfer property of DMA was studied by solvent polarity dependent UV-Visible absorption spectroscopy. The fluorescence of DMA was almost totally quenched by the charge transfer state. The quenching of fluorescence was also found in BODAC, which couples a bodipy unit with the expanded acridinium unit through an acetylene bridge. No long-lived species were found from a transient absorption study of DMA and a decay model was proposed to explain the relaxation process. We demonstrated for the first time that acridinium based dyads can be used as sensitizers for DSSCs by testing results of two dyads on p-and/ or n-type semiconductors. Chapter 4 demonstrates the synthesis of donor-acceptor systems based on naphthalimide and phthalimide derivatives for a solid state charge transfer fluorescence study. The fluorescence images of the dyads were obtained through confocal microscopy. UV-Vis absorption spectra also revealed their charge transfer properties. X-ray crystal structures of several compounds were obtained. The existence of two polymorphs of one dyad enabled us to compare their solid state fluorescence properties more closely. The polymorphs displayed different properties such as colours, melting points and the emission lifetimes. Chapter 5 concerns the synthesis of a disulfide-strapped methyl viologen derivative DSV, which can work as a molecular switch. The viologen derivative is capable of storing up to four electrons as revealed by reversible reduction peaks for a cyclic voltammogram. DSV is one of the easiest to reduce viologen derivatives to date. Molecular orbital calculations were used to model the “spring opening” of the disulfide-strapped six membered ring. Chapter 6 continues our further work into bridging units for donor acceptor assemblies and focuses on the synthesis of 1,1'-bi-2-naphthol based molecular dyads. The length of alkyl chain to link the diol group was varied to control the conformation of the chiral bridges. The aim was to study chirality effects on energy/electron transfer processes. Different synthetic routes were tried to achieve the target molecular dyads. Primary photophysical properties and results on attaching the dyad to fullerene are presented.