Porphyrins for surface modification : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand
The controlled synthesis of a variety of benzoic acid porphyrins ranging from monomers to arrays for the modification of TiO2 and GaAs semiconductors, and sulfur functionalised porphyrin monomers for attachment to GaAs and Au surfaces was achieved. A semi-quantitative study of the photosensitisation of TiO2 by the porphyrin acids was carried out. The syntheses of β-styryl linked porphyrin benzoic acids and some meso-substituted benzoic acid porphyrins was successfully carried out employing Wittig chemistry and classical porphyrin-forming condensation reactions with appropriate formyl methyl esters. Hydrolysis of the resulting porphyrin esters provided a facile and reliable acid synthesis, particularly where multi-step reactions were necessary. It was also demonstrated that acid functionality on porphyrins could be generated from aldehydes via esters, even though direct oxidation of the aldehydes to acids could not be achieved. The syntheses of "dipole" and "collinear" diporphyrins were achieved, providing two different porphyrin light harvesting arrays for evaluation on semiconductor surfaces. As a result of the synthesis of a new linear diporphyrin Building Block C, an alternative pathway to the controlled syntheses of mixed-metal and mixed-porphyrin arrays was achieved. This provided an alternative strategy for the controlled placement of three different metals into three different porphyrins of a linear triporphyrin, pentaporphyrin and a larger star-shaped nonaporphyrin. The exploitation of the stepwise controlled synthesis of the triporphyrin systems was expanded to include mixed-porphyrin systems synthesised with a unique tetraester porphyrin phosphonium salt. This phosphonium salt afforded mixed-diporphyrin and mixed-triporphyrin arrays, which were hydrolysed to give "sticky" mixed-diporphyrin and triporphyrin acid arrays. An alternative milder and higher yielding stepwise Wittig method was developed for the synthesis of a star shaped TXP pentaporphyrin. This new method involving milder base conditions gives advantages over the traditional acid catalysed approach developed in these laboratories. It is now possible to build these arrays in a stepwise manner with acid labile metals present in the porphyrin moieties. Access to the controlled synthesis of "sticky" mixed-pentaporphyrin arrays was then achieved using this new methodology. With the synthesis of a variety of unique benzoic acid functionalised porphyrin monomers and multiporphyrin array systems, evaluation of their performance in the dye-sensitised TiO2 Grätzel cell was carried out. The development of a reliable solar cell testing apparatus and procedures required to assess the solar cell performance of these chromophores is presented. None of the conditions employed have been optimised, but insights into the significance of the porphyrin photoelectrochemical cell variables has been obtained. It was determined that the porphyrin acids are better photosensitisers than their salts, and that Zn(II) metalloporphyrins performed best as dyes. The results also suggest that Cu(II) metalloporphyrins are worth pursuing in future where long term stability of the chromophores is required in solar cells. It was also found that adsorption solvent choice, electrolyte composition and dye concentrations are all critical to cell performance, and should all be optimised in future studies. The tetraaryl β-substituted monoporphyrin acids were found to have a significant advantage over the multiporphyrin arrays and other monoporphyrins synthesised and examined in this work. A variety of new disulfide porphyrins and some new 2- and 3-thienylporphyrins were successfully synthesised. A new class of terthiophene-appended porphyrins was also synthesised. Using a combination of Wittig chemistry and classical condensation reactions, β-substituted, and bis- and tetra-meso-porphyrin variants were synthesised and characterised. Both the bis- and tetraterthienylpoprhyrins were isolated as mixtures of atropisomers.