Synthesis and properties of fully conjugated porphyrin arrays for light harvesting : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand
This thesis presents the synthesis of porphyrin arrays for light-harvesting applications using Wittig chemistry, which allows the construction of covalently bound systems that are conjugated, stable and easy to characterize. This was achieved using a dendrimer strategy utilizing tetraarylporphyrins as building blocks, monofunctionalized with either aldehyde or phosphonium salt groups at the β-pyrrolic position, and benzenes, polyfunctionalized with either aldehyde or phosphonium salt groups; stepwise control of the addition of each porphyrin moiety was thus obtained. In this way, different porphyrins in different metallated states were arranged in a determinate geometrical relationship, which is of great importance in the investigations on electron/energy transfers. Arrays containing up to five metalloporphyrin units (two kinds of porphyrins coordinating two different metals) were synthesized and characterized. The unexpected chromatography behaviour and 1H-NMR spectra of a Zn porphyrin functionalized with a 1,3-bis(methyl(diethylphosphonate) benzene were the reason for an investigation, which uncovered, mainly with the use of NMR spectrometry, the first case of intramolecular coordination between the Zn centre and a phosphonate group of the same porphyrin. The dynamic nature of this coordination was characterized and chemical-physical parameters for Zn porphyrin/phosphonate binding were determined. In order to establish the photophysical properties of our conjugated arrays, we synthesized a series of dyads containing Zn and free-base tetraphenylporphyrins (TPPs) connected through variable length phenylenevinylene-type bridges; along with this series, the preparation of the Zn and free-base homometallic homologue dyads and two series of monomers carrying the conjugated linker were realized. Collaboration with IFOS-CNR in Bologna, Italy was established in order to investigate the intramolecular photophysics of those systems, which involve efficient intramolecular energy transfer from the Zn to the free-base porphyrin. Finally, dyads composed of Fe(III) and Zn porphyrin were prepared as part of a project in collaboration with the University of Pennsylvania for the investigation of new artificial photosynthetic systems. Two series of dimers were prepared in order to obtain incorporation in both the classes of hydrophobic and hydrophilic proteins. TPPs were used for the making of the hydrophobic dyads while hydrophilicity was achieved by employing tetraester porphyrin derivatives, which can be quantitatively hydrolyzed to afford the correspondent water soluble acids. A new monosubstituted porphyrin was also synthesized and incorporated in the arrays to minimize steric hindrance inside the protein binding sites.