Photosensitized cis-trans isomerization in aqueous solution. pH effect on the efficiency of triplet-triplet energy transfer to maleic acid

Aromatic ketones, made water soluble by the introduction of ionic substituents, photosensitize cis reversible transfer isomerization in the maleic-fumaric acid system. The photostationary-state cis/trans ratio depends on the triplet energy of the sensitizer and the pH of the medium. All sensitizers studied show a pH effect on the cis/trans ratio which varies from 2.8 to 1.2 and is independent of whether the sensitizer is negatively or positively charged. Parallel quenching studies show the major part of this stems from a pH effect on the relative rates of energy transfer from the sensitizer to cis and trans acids. A smaller pH effect, in the opposite direction, is found with the relative rates of decay of the triplet to ground-state cis and trans isomers. Direct photoisomerization was also found to show this same pH effect in the decay of the lowest excited singlet to ground-state molecules. A rationalization is presented. 22 references, 4 figures, 3 tables