Quenching of excited triplet states by Cr (III) and Co (III) B-diketonate complexes

Bimolecular rate constants for the quenching of up to 15 excited triplet aromatic hydrocarbons by eight S-diketonate complexes of Cr(III) and four of Co(III) have been obtained using nanosecond laser flash photolysis. The bimolecular rate constants have been plotted against donor energy and for those quenchers whose rate constants follow the available spectroscopic energy levels in the quencher, electronic energy transfer is established to be the mechanism of quenching. As the electrochemical reduction potential of the quencher is reduced by the introduction of trifluoromethyl groups into the S-diketonate complexes, an increase in quenching efficiency is observed which may be explained by modifying the mechanism of quenching to include electron transfer. Theoretical models have been developed in order to predict quenching constants for the different mechanistic pathways. For the quenching of excited triplet states by y-substituted chromium (III) complexes, rate constants greater than those predicted by the Debye Equation, once spin statistics have been introduced, were obtained. Diffusion theories were examined and a Spernol-Wirtz type treatment was employed,since this takes into account microfiction between solute and solvent molecules of differing molecular radii. Using this diffusion theory allowed all of the results presented to be interpreted within the framework of the theoretical models. The effect of geometrical isomerism on the quenching efficency of a chromium(III) complex was investigated and it was found that the aisisomers is a more efficient electronic energy transfer quencher than the trans form, especially when energy is being accepted by the quartet state. The general nature of the approach developed in the thesis was examined by measuring the quenching rate constants for four Co(III) a-diketonate complexes.