Electronic Communication in Heterometallated Porphyrin Oligomers

Abstract

This thesis presents the synthesis and characterisation of a range of heterometallated porphyrin oligomers and other novel 3D π-conjugated porphyrin nanostructures. Subsequently, their physical organic properties were evaluated which revealed some fascinating electronic properties. Chapter 1 summarises some of the work done in the Anderson group on porphyrin nanostructures and reviews the literature regarding heterometallated porphyrin oligomers. In addition it introduces the main concepts and techniques used in the remainder of the thesis. In Chapter 2 the stabilities of a family of four linear porphyrin pentamer complexes are determined by UV-vis-NIR titrations and analysed using chemical double-mutant cycles which reveal that the binding energy of the copper centre to an axial pyridine ligand is –6.2 kJ mol–1. Subsequently, the Zn-Zn-Cu-Zn-Zn pentamer is used in the synthesis of a heterometallated 10-porphyrin nanoring. Chapter 3 will describe the investigation of quantum interference phenomena in a bis-copper six-porphyrin nanoring by using EPR spectroscopy. We show that the exchange coupling between two spin centres is increased by a factor 4.5 in the ring structure with two parallel coupling pathways as compared to an otherwise identical system with just one coupling path. In Chapter 4 the syntheses of three isomers of the bis-copper 6-porphyrin nanoring are described. DFT calculations have indicated potential destructive interference phenomena in one of the isomers which would allow for the formation of a molecular system with behaviour resembling that of a hypothetical molecular interferometer. Chapter 5 reports on the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This bicyclic structure consists of two intersecting nanorings of 6 and 10 porphyrin units. Fluorescence up-conversion spectroscopy experiments demonstrate that electronic excitation delocalises over the whole 3D π-system within 0.3 ps if the nanoball is bound to its templates or within 5 ps if the nanoball is empty. In Chapter 6 the synthesis and characterisation of a D4h symmetric analogue of the porphyrin nanoball is described. The structure consists of ten porphyrin units arranged as two perpendicular 6-porphyrin nanorings intersecting at two porphyrins. In the synthesis, a combination of magnesium and zinc porphyrins are used which allows for the introduction of a selective demetallation method crucial for accessing this novel structure.