5216 J. Am. Chem. Soc. zyxwvu 1988, zyxwvu 110, 5276-5280 Picosecond Absorption Studies on the Excited State of (p-Oxo)bis[ (tetraphenylporphinato)iron( 111) zy ] C. R. Guest, K. D. Straub,+ J. A. Hutchinson, and P. M. Rentzepis* Contribution from the Department of Chemistry, UniEersity of California, Iruine, California 9271 7. Received October 19, 1987 Abstract: We have measured the kinetics and spectra of the intermediates of zyxwv (p-oxo)bis[(tetraphenylporphinato)iron(III)] in various solvents. The spectra of the intermediate species were measured at various intervals of time from -100 to 4.3 ns after excitation with a 532- or 355-nm 25-ps pulse. The intermediate state, possibly a cation radical, was assigned to a monomer that forms the photodissociated pair TPPFe"' + TPPFe"I-0- and yields a small amount of disproportionation reaction products, Fe"TPP and TPPFeIV=O. Iron porphyrins have been the subject of intensive investigations over the past years. The partially filled d orbital system of iron porphyrins lie in an energy range that spans from below the highest filled zyxwvutsrqp r orbital of the tetrapyrrole system to above the lowest unfilled r* orbital.' The addition of spin-spin and spin-orbit interactions of the d electrons give numerous sets of available energy levels, which are easily perturbed by axial ligation, por- phyrin ring substituents, and local field gradients in the envi- ronment.2 This wide range of electronic energy configurations can easily be seen in the rich absorption spectra of iron porphyrins and hemoprotein^.^,^ The decay of excited states in iron porphyrinsS are to a very large extent radiationless with lifetimes in the picosecond and subpicosecond r a ~ ~ g e . ~ - ~ In spite of these short excited-state lifetimes, some iron porphyrins are known to undergo photo- chemistry. In the presence of short-chain alcohols, (proto- porphyrin)iron(III) chloride and (deuteroporphyrin)iron(III) chloride can give photocatalytic oxidation of aliphatic alcohols when irradiated at 355 nm.loJ1 In addition, the (tetraphenyl- porphyrin)iron( 111) complex can also photocatalyze hydrocarbon hydroxylation through a peroxyl radical chain mechanism.12 Ligand ejection from hemes is a well-studied process. CO and O2 photodissociation from ferrous hemoproteins has been exten- sively studied on the nanosecond and picosecond time ~ c a l e . ' ~ - ' ~ More recently the picosecond events of NO photodissociation from ferric hemoproteins16 have been examined. Other nitrogenous ligands such as piperidine and pyridine are also known to undergo photodissociation from iron porphyrin^.^^'^ The required transformations in the excited state are not known in detail, but many of the studies on photodissociation from the ferrous state indicate the existence of an intermediate with a lifetime of - 15-40 ps.899J3~14 Similar transients have been observed in ferrous por- phyrins that have no ligands or in which there is no evidence of ligand diss~ciation.*-~ Much less is known of the initial events in the photochemistry of the ferric porphyrins. The recent de- scription" of the photochemical oxidation of triphenylphosphine by (p-oxo)Fe"'TPP, [Fe"'TPPI20, has indicated the possibility of an intramolecular redox reaction from the excited state in these compounds. There appears to be a disproportionation of [TPP Fe"'],O as follows: TPPFe"'-O-Fe"'TPP - TPPFe" + TPPFe'V=O (1) in an internal redox reaction, which can as an alternative give TPPFelll-O-FelllTPP - TPPFe" + TPP'Fe"'=O (2) in which the porphyrin is oxidized to a .rr cation radical and is strongly antiferromagnetically coupled to the Fe(II1). It is interesting to note that while the photochemical trans- formations take place within 100 ns,19 the excited-state lifetime of most ferric porphyrins is less than 1 ps.s*6 We have examined the picosecond evolution of the photoexcited p-oxo dimer 'Permanent address: Veterans Administration Medical Center and University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205. 0002-7863/88/1510-5276$01.50/0 [ Fe"'TPP],O by means of picosecond absorption spectroscopy in order to better understand the mechanism of the energy dissipation of the excited state in iron porphyrins. Experimental Section Chemicals. Iron(II1) tetraphenylporphyrin chloride was obtained from Strem Chemicals. All solid chemicals were reagent grade, and solvents were either reagent or spectroscopic grade. The p-oxo-bridged FeII'TTP dimer was prepared by a modification of the Dolphin et al. method.*O Approximately 10 mg of TTPFe'I'CI in 30 mL of methylene chloride was shaken with 30 mL of 2 M NaOH for 10 min. The aqueous phase was discarded, and the procedure was repeated two more times. The meth- ylene chloride phase was then washed three times with approximately 50 mL of H20. The methylene chloride was then evaporated under nitrogen at 40 OC, and the p-oxo compound was taken up in a small amount of benzene. The compound was chromatographed on a 2.5 X 25 cm silicic acid column. The silicic acid had been dried at 120 OC for 6 h and was poured into the column as a benzene slurry. The pure [Fet1'TPPl2O was eluted with benzene. ~ ~~ (1) Loew, G. H. In Iron Porphyrins: Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; Part 1, pp 1-87. (2) Scheidt, W. R.; Gouterman, M. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; Part 1, pp 89-139. (3) Adar, F. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 111, Chapter 2, p 167. (4) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 111, Chapter 1, p 1. (5) Huppert, D.; Straub, K. D.; Rentzepis, P. M. Proc. Natl. Acad. Sci. U.S.A. 1977, zyxwvu 74, 4139-4143. (6) Adar, F.; Gouterman, M.; aronowitz, S. J. Phys. Chem. 1976, 80, 2 184-21 9 1. (7) Cornelius, P. A,; Steele, A. W.; Chernoff, D. A,; Hochstrasser, R. M. Chem. Phys. Lett. 1981, 82, 9-14. (8) Straub, K. D.; Rentzepis, P. M. In Porphyrins, Excited States and Dynamics; Gouterman, M., Rentzepis, P. M., Straub, K. D., Eds.; American Chemical Society Symposium Series 321; American Chemical Society: Washington, DC, 1986; pp 168-181. (9) Straub, K. D.; Rentzepis, P. M., In Advances zyxw in Chemical Reaction Dynamics; Rentzepis, P. M., Capellas, C., Eds.; D. Reidel: Boston, MA, 1986; (10) Bartocci, C.; Scandala, F.; Ferri, A,; Carassiti, V. J. Am. Chem. SOC. (I 1) Bizet, C.; Morliere, P.; Brault, D.; Delgado, 0.; Bazin, M.; santus, (12) Hendrickson, D. N.; Kinnard, M. G.; Suslick, K. S. J. Am. Chem. (13) Noe, L. J.; Eisert, W. G.; Rentzepis, P. M. Proc. Natl. Acad. Sci. (14) Reynolds, A. H.; Rand, S. D.; Rentzepis, P. M. Proc. Natl. Acad. Sci. (15) Reynolds, A. H.; Rentzepis, P. M. Biophys. J. 1982, 38, 15-18. (16) Cornelius, P. A.; Hochstrasser, R. M.; Steele, A. W. J. Mol. Biol. (17) Dixon, D. W.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1985, 107, (18) Richman, R. M.; Peterson, M. W. J. Am. Chem. SOC. 1982, 104, (19) Peterson, M. W.; Rivers, D. S.; Richman, R. M. J. Am. Chem. SOC. (20) Dolphin, D.; Sams, J. R.; Tsin, T. B.; Wong, K. L. J. Am. Chem. SOC. pp 165-170. 1980, 102, 7067-7072. R. Photochem. Photobiol. 1981, 34, 315-321. 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