The electron spin resonance study on primary photochemical reaction of hematoporphyrin derivative.

The rate of oxygen consumption and the yield of free radical anion of hematoporphyrin derivative (HPD) in aqueous solutions of HPD and pyrocatechol were measured by the probe 2,2,6,6-tetramethyl-4-piperidone-1-oxyl. It has been found that both singlet oxygen and free radical mechanisms exist simultaneously in primary photochemical reactions, and there is a competition between both mechanisms. When the oxygen concentration in solutions comes down to 12-14% of the stanting level, the predominant mechanism can be changed from the singlet oxygen to the free radical. Whether HPD exists in aggregation state is very important to photosensitization mechanisms. In the presence of the aggregation state of HPD, the predominant mechanism is the free radical, and photosensitization effects of HPD are all the better.