J. Am. Chem. SOC.1994,116, 281-289
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Investigation of the Lowest Electronic States of Osmium(11) Tetratolylporphyrins: Photophysics of Metalloporphyrin (d,r*) Charge Transfer States Steve Gentemann,t Jennifer Albaneze,* Ramon Garcia-Ferrer,?Spencer Knapp,* Joseph A. Potenza,* Harvey J. Schugar,* and Dewey Holten'J Contribution from the Departments of Chemistry, Washington University, St. Louis, Missouri 631 30, and Rutgers University, New Brunswick, New Jersey 08903 Received June I , 1993. Revised Manuscript Received October 14, 1993'
Abstract: The nature of the lowest excited states of a series of osmium(I1) porphyrins has been investigated using static and time-resolved emission spectroscopy and ultrafast transient absorption measurements. It is found that a metalto-ring 3(d,?r*)charge transfer excited state is the lowest excited state of the OS~~P(CO)(L) and 0st1P(L)2 complexes, where the porphyrin macrocycle (P) is either tetratolyl- or octaethylporphyrin and the axial ligand L is a u-donor such as pyridine. Previous studies on OsTTP(CO)(py) had led to the assignment of the lowest excited state as the 3(1r,**) state of the porphyrin ring. The results on the 0 s " porphyrins can be contrasted with those found previously for the analogous Ru" porphyrins, in which the lowest excited state was found to switch from 3(9r,7r*) in the RuP(CO)(L) complexes to 3(d,7r*)in the RuP(L)2 compounds. The studies further establish the electronic origin of the red-region features in the absorption spectra of metalloporphyrin 3(~,7r*)and 3(d,i*) excited states. The combined results demonstrate the critical role played by *-accepting and u-donating axial ligands in controlling the nature of the lowest excited states, the electronic properties, and excited state dynamics of the biologically-relevantd6 metalloporphyrins.
Introduction The elucidation of the relationships among molecular composition and structure, electronic properties, and ground- and excited-state reactivity is a prime motivation for studies of the chemistry and spectroscopy of tetrapyrrole complexes in vivo and in vitro. Thenumber and type of the axial ligandscoordinated to the central metal ion are often important factors in these relationships. This is particularly true for transition metal porphyrins having a d6 electronic configuration, for which the filled d, (dxz,dyz)orbitals have energies comparable to the top HOMOSof the porphyrin ring.1-3 In these complexes axial back bonding between the metal d, orbitals and the empty ?r* orbitals of a *-accepting ligand can competewith equatorial back bonding between the d, orbitals and the empty ee(7r*) LUMOs of the porphyrin rnacrocy~le.~-~ This competition can have profound effects on the electronic structure, vibrational properties, and photophysical behavior of the c ~ m p l e x e s . ~These - ~ ~ properties are further modified by the presence and nature of a second axial ligand that is trans to the *-accepting ligand. t Washington University. t Rutgers University.
* Abstract published in Aduance ACS Abstracts, December 1,
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There has been considerable interest in exploring such relationships in simple iron porphyrins as models for the chemistry of the biologically significant hemeprotein complexes.1.2Jk However, it is not possible to explore in depth the consequences of axial ligation on the photophysics of the iron porphyrins since the fate of these photoexcited molecules is ultrafast (typically
