Electrochemical studies of covalently linked porphyrin-quinones and related molecules
The work described in this thesis is mainly concerned with the accurate measurement of redox potentials for a series of covalently linked porphyrin-quinone molecules (P-Q), in non-aqueous media. These compounds (synthesised by Prof. J.R. Bolton and co-workers of the University of Western Ontario, Canada) are of interest as models for the primary photochemical step in the photosynthetic process. The redox potentials were measured by cyclic voltammetry and differential pulse voltammetry. These electrochemical redox potentials were used to estimate the energy of the excited charge-separated state (P.+-Q.-), and confirm that this state is likely to be photochemically produced from the excited singlet state of P-Q rather than the triplet state. As a prelude and postscript to the studies of the P-Q compounds, the electrochemical behaviour of several related but simpler compounds, both porphyrins and quinones, is described. This includes an invesigation of the solvent dependence of quinone reduction potentials in non-aqueous media. A simple linear equation, based on Swain's A+B solvent parameters, is proposed as a means of predicting quinone reduction potentials in different solvents. Finally, the electrode kinetics of some simple metallorporphyrin oxidations are studied using an a.c. technique. A non-linear fitting routine was used to optimise the rate constant values obtained.