Supramolecular control of synthesis and electronic structure of porphyrin oligomers

Thesis

The work described in this thesis demonstrates the use of supramolecular chemistry in the template-directed synthesis of porphyrin nanorings and as a tool to control conformation and topology of π-conjugated porphyrin oligomers. Particular emphasis is placed on changes to the electronic structure of these oligomers depending on their conformation.

Chapter 1 gives an overview of π-conjugated porphyrin oligomers and conjugated macrocycles in general, followed by an introduction into supramolecular cooperativity and small angle X-ray scattering in solution.

Chapter 2 describes advances in the synthesis, solution structure elucidation and optoelectronic properties of a fully conjugated cyclic porphyrin hexamer. The high rigidity and symmetry of this nanoring as well as its bent π-system lead to a significant decrease in its HOMO-LUMO gap. The resulting near-infrared emission was exploited in the fabrication of light emitting diodes, demonstrating the use of a bent topology for minimizing aggregation in thin films.

The synthesis of a [12]porphyrin nanoring using a hexadentate template is presented in Chapter 3. The concept of Vernier templating is introduced as a general strategy for the synthesis of large monodisperse macrocycles. The nanoring is characterized and its cooperative binding to two template molecules is studied. In Chapter 4 a bidentate ligand is used in the cooperative formation of a sandwich complex beween two nanorings.

Chapter 5 provides an analysis of the rigidity of the butadiyne linked porphyrin oligomers used throughout this thesis. A linear chain is significantly more flexible in solution than on a surface where it is confined within two dimensions. Analysis of the persistence length indicates that a porphyrin nanoring with more than around 20 units would not be significantly strained.

The supramolecular binding of linear porphyrin oligomers to carbon nanotubes is analyzed in Chapter 6. The binding strength increases sharply with oligomer length and binding is strongest with (8,6) and (7,5) tubes. The energy level alignment in these porphyrin CNT complexes appears to be favorable for an application in photovoltaics.

In Chapter 7 bisamidine-carboxylate salt bridges together with zinc-nitrogen coordination chemistry are used to access a variety of topologies in assemblies based on carboxylic acid functionalized porphyrin dimers.

Authors: Sprafke, JK

• Publication date: 2011
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: porphyrin; self-assembly; pi-conjugation; SAXS; supramolecular cooperativity; OLED; template
• Subjects: Synthetic organic chemistry; Nanomaterials; Organic chemistry; Supramolecular chemistry