Nitric oxide deligation from nitrosyl complexes of two transition metal porphyrins: A photokinetic investigation

The results of an investigation of the ultrafast dynamics of photoinduced deligation in two transition metalloporphyrin-nitrosyl complexes, TPPFe(II)NO and TPPCo(II)NO, in conjuction with the results of an energy transfer study lead to the conclusion that the difference in the denitrosylation yields (∅(NO) = 0.5 for TPPFe(II)NO and ∅(NO) = 1.0 for TPPCo(II)NO is the result of energy partitioning in the upper excited states of the porphyrin. The energy transfer study yielded the energies of the metal centered states, believed to be of CT(π,d(z)²) nature, and of the localized porphyrin triplet states. The CT states in the two complexes were found to lie at similar energies (TPPFe(II)NO 8650 cm^-1 and TPPCo(II)NO 8900 cm^-1); however, the localized porphyrin triplet states were found to be at 16200 cm^-1 in TPPFe(II)NO and 14700 cm^-1 in TPPCo(II)NO. This difference in energies of the respective triplet states facilitates efficient intersystem crossing in the excited state deactivation of TPPFe(II)NO, but does not allow any triplet formation in TPPCo(II)NO. The direct excitation studies revealed that intersystem crossing in TPPFe¹¹NO occurs with a rate constant of 7.3 x 10¹¹ s^-1 to yield a localized porphyrin triplet state that absorbs maximally at 450 nm. This state then relaxes back to the ground state without the loss of NO. Only those excited states that relax via the CT state result in loss of NO. The direct excitation studies yielded no evidence for intersystem crossing in the deactivation of the electronically excited singlet state of TPPCo(II)NO, hence all of the energy deposited in the initial photoexcitation step results in NO loss. The lifetimes and spectral characteristics of the other excited states involved in deactivation of these transition metalloporphyrin-nitrosyl complexes will be discussed.