Photodismutation Of Fe(III)-Oxo Tetraphenylporphyrin
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The generation of strong oxidizing and reducing equivalents during the photoexcitation process is very important to solar energy conversion, phototherapy of cancer, and several basic processes in biochemical reactions. While most photochemical reactions involving porphyrins involve the long-lived singlet and triplet States, it has been shown that photochemical reactions can be catalyzed by iron (III) porpnyrins which have a very short excited-state lifetime. It is known that in the presence of small amounts of pyridine photocatalytic oxidation of olefins is prornoted at wavelengths shorter than 500 nm. We have investigated the oxygen-bridged dimeric iron (III) tetraphenylporpnyrin (TPP) pnotoexcitation on the picosecond time scale. The excited States were generated with 530 or 355 nm, 25 p sec pulses, and probed by time-delayed continuum from the same pulse. We have observed an initial state characterized by bleacning of the original Soret at 405 and Q at 572 nm during the passage of the excitation pulse. A new Soret at 418 appears and continues to grow in intensity while shifting to 418 nm during the first 100 psec. The Q Dana bleacning at 572 does not change during this time. After 100 psec the spectrum decays to the initial state in 200-400 psec. The kinetics and spectra were independent of excitation wavelength. There was no difference between pyridine and benzene as solvents. No irreversiole changes were observed. The final spectra (100 psec) are compatible with the disproportionation of the starting dimer into the Fe (II) and Fe (IV) = 0 TPP monomers, generation of a porphyrin-centered radical species, or the simple pnotodissociation into an ion-pair. The absorbance increases over the 600-700 nm range indicates that much of the initial photoproduct is a porphrin-centered radical. 1988 SPIE.
