Excited states of protoporphyrin IX dimethyl ester: reaction on the triplet with carotenoids

The triplet and singlet excited states of protoporphyrin IX dimethyl ester (PPIXDME) have been identified by means of pulse radiolysis experiments in benzene solution. The absorption spectrum, extinction coefficients, energy level and rate of oxygen quenching of the triplet state have been measured. Triplet energy transfer from PPIXDME to a group of polyenes of differing conjugated double-bond chain length has been studied. Pariser–Parr–Pople m.o. calculations have been performed to find the variation in the energy of the lowest triplet excited state with the number of polyene conjugated double bonds. The quenching data are compared with similar previous results for chlorophyll triplet; they are discussed in connection with the role of protoporphyrin IX in the light-sensitive human disease erythropoietic porphyria and the treatment of this disease by induced carotenemia.