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The generation of strong oxidizing and reducing equivalents during tne photoexcitation process is very important to solar energy conversion, phototherapy of cancer, and several oasic processes in biochemical reactions. While most photochemical reactions involving porpnyrins involve the long-lived singlet and triplet states, it has been shown that photochemical reactions can pe catalyzed by iron (III) porphyrins 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 promoted at wavelengths shorter than 500 nm. We nave 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 psec pulses, aria probed uy time-delayed continuum from the same pulse. We have observed an initial state characterized uy oleacning of the original Soret at 405 and Q at 572 nm during the passage of the excitation pulse. A new Soret at 42u appears and continues to grow in intensity while shifting to 418 nm during tne first 10U psec. Tne Q banu bleaching 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 ooserved. Tne final spectra (100 psec) are compatible with the disproportionation of the starting dimer into tne 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 mucn of tne initial photoproduct is a porphrincentered radical.
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