Preventing porphyrin aggregation and controlling microenvironments using mixed polymer micelles

Inspired by the porphyrin-protein composites in the biological system, such as chlorophyll, peroxidase, neuroglobin, myoglobin, haemoglobin and cytochrome, the self-assembly method has proven to be a promising one for assembling biomimetic materials. In most biological systems, proteins have a unique structure that provides a distinctive isolated and hydrophobic macroenvironment for porphyrins, which stabilize them in the aqueous solution. Therefore, a protein matrix can optimise the functionality of porphyrins by incorporating and spacing many of them in close proximity, without sacrificing their activities due to aggregation. One of the aims of this project was to develop a simple synthetic system that could mimic this property of porphyrin containing biomolecules. Specifically, the development of a self-assembled system that could incorporate a number of porphyrins at precise locations without being aggregated was targeted.
The first step of this thesis involved the synthesis of an amphiphilic mPEG45-PCLn diblock copolymer using ring opening polymerisation. Four different degrees of polymerisation for the polycaprolactone component of mPEG45-PCLn were synthesised and characterised. Their aggregation in aqueous solution was also studied, determining a CMC for each polymer. The second step involved the synthesis of a PEG-porphyrin polymer that could co-assemble with the mPEG45-PCLn diblock copolymer chains in aqueous solution. The resulting micelles were able to hold a number of porphyrins in close proximity without being aggregated, as shown throughout the fluorescence quenching experiments. The size and morphology of the PEG-porphyrin/mPEG45-PCLn mixed micelles were measured by DLS and TEM. The degree of polymerisation of PCL in the mPEG45-PCLn diblock copolymer determined the amount of PEG-porphyrin that could be inserted (before quenching). Once synthesised, the micelles were tested as artificial light harvesting systems by encapsulating tin phthalocyanine as an acceptor unit. Although UV-vis experiments showed that the donor porphyrins were able to donate their energy to the phthalocyanine, no emission was observed. This was due to the uncontrolled aggregation of the phthalocyanines, which resulted in quenching – even at low concentration.
Although the micelles failed as light harvesting systems, they proved to be excellent catalytic/nanoreactors. Iron was inserted into a PEG-porphyrin system, which was then co-micellised with mPEG45-PCL25. The resulting micelle was utilised as an oxidation catalyst for use in water. The PEG-porphyrin/mPEG45-PCL25 micelle was tested against two azo dyes and showed excellent reactivity for the hydrophobic azo dye red oil when the hydrophobic oxidant m-CPBA was used. The result emphasised the importance of the micelle system that could control the porphyrin position and prevent aggregation.