Electronic Communication in Heterometallated Porphyrin Oligomers

This thesis explores the design and synthesis of heterometallated oligo-porphyrin nanostructures and presents an investigation into the photophysical and electronic properties of this class of compounds. This introductory chapter presents a literature review of heterometallic porphyrin arrays, followed by an introduction to some of the techniques used to investigate the structures presented throughout this thesis.

The affinity of copper(II) porphyrins for pyridine ligands is extremely weak, but oligo-pyridine templates can be used to direct the synthesis of Cu-containing cyclic porphyrin oligomers when they also contain zinc porphyrins. Here we describe the synthesis of a heterometallated 10-porphyrin nanoring prepared from a Zn/Zn/Cu/Zn/Zn pentamer. The macrocycle has copper porphyrins at two specific positions across the diameter of the ring and zinc porphyrins at the other sites. The Zn/Zn/Cu/Zn/Zn linear porphyrin pentamer binds strongly to a penta-pyridyl template, despite the weakness of the Cu–N interaction, due to of the chelate cooperativity of the neighbouring Zn–N coordinations. The stabilities of a family of four linear porphyrin pentamer complexes were determined by UV-Vis-NIR titrations and analysed using a chemical double-mutant cycle. The results show that the binding energy of the copper centre to an axial pyridine ligand is −6.2 kJ mol⁻¹ when the entropy cost of bringing together the two molecules has already been paid by pyridine-zinc interactions. The development of template-directed approaches in the synthesis of nanorings with combinations of different metals around the ring opens up many possibilities for controlling the photophysical behaviour of these supramolecular systems and for probing their conformations by EPR.

The exchange interaction (J) between two spin centres is a convenient measure of through bond electronic communication. This chapter will describe the investigation of quantum interference phenomena in a bis-copper six-porphyrin nanoring (c-P6_Cu2·T6) by using electron paramagnetic resonance spectroscopy via measurement of the exchange coupling between the copper centres. Using an analytical expression accounting for both dipolar and exchange coupling to simulate the time traces obtained in a double electron-electron resonance experiment, we demonstrate that J can be quantified to high precision even in the presence of significant through-space coupling (D). We show that the exchange coupling between two spin centres is increased by a factor of 4.5 in the ring structure with two parallel coupling pathways as compared to an otherwise identical system with just one coupling path. This observation is a signature of constructive quantum interference.

The synthesis of three isomers of bis-copper 6-porphyrin nanorings is described. The structures differ only in the relative positions of the copper porphyrins within the ring. Generally, J coupling is expected to decay exponentially as a function of through-bond distance. However, DFT calculations performed by Dr. Martin Peeks suggest that the coupling between the two copper centres in a 6-porphyrin nanoring does not solely depend on the distance between the ions, indicating potential interference phenomena. Experimental evidence to support these calculations would provide a relatively simple molecular system with behaviour resembling that of a hypothetical molecular interferometer. This chapter only describes the synthesis and characterisation of the rings. EPR measurements are currently underway.

The preparation of conjugated organic systems with strong π-orbital overlap between the molecular components is both challenging and laborious. This chapter describes the template-directed synthesis and characterisation of a π-conjugated 14-porphyrin nanoball. The bicyclic structure consists of a 6- and 10-porphyrin nanoring intersecting at two points. UV-Vis-NIR titrations were performed to investigate the binding behaviour of the templates inside the cavity of the ball. In addition, photoluminescence (PL) upconversion spectroscopy revealed strong coupling between the two rings. By removing the templates, the flexibility in the system increases which results in a decrease in the PL anisotropy. This indicates that excitations may migrate between the two ring components of the nanoball.

The synthesis of heterometallated zinc and magnesium porphyrin oligomers allowed selective demetallation to be used as a synthetic tool to prepare a 10-porphyrin nanoball. The T6 template and a rationally designed 10-legged T10 template were used to investigate the binding properties of the D_4h symmetric structure by UV-Vis-NIR.