NITRIC-OXIDE DELIGATION FROM NITROSYL COMPLEXES OF 2 TRANSITION-METALPORPHYRINS - A PHOTOKINETIC INVESTIGATION

The results of an investigation of the ultrafast dynamics of photoindu ced deligation in two transition metalloporphyrin-nitrosyl complexes, (TPPFeNO)-N-II and (TPPCoNO)-N-II, in conjuction with the results of a n energy transfer study lead to the conclusion that the difference in the denitrosylation yields (phi(NO) = 0.5 for (TPPFeNO)-N-II and phi(N O) = 1.0 for (TPPCoNO)-N-II) is the result of energy partitioning in t he upper excited states of the porphyrin. The energy transfer study yi elded the energies of the metal centered states, believed to be of CT( pi,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 ((TPPFeNO)-N-II 8650 cm(-1) and (TPPCoNO)-N-II 8900 cm(-1)); however,, the localized porphyrin triplet states were found to be at 16200 cm(- 1) in (TPPFeNO)-N-II and 14700 cm(-1) in (TPPCoNO)-N-II. This differen ce in energies of the respective triplet states facilitates efficient intersystem crossing in the excited state deactivation of (TPPFeNO)-N- II, but does not allow any triplet formation in (TPPCoNO)-N-II. The di rect excitation studies revealed that intersystem crossing in (TPPFeNO )-N-II occurs with a rate constant of 7.3 x 10(11) s(-1) to yield a lo calized porphyrin triplet state that absorbs maximally at 450 nm. This state then relaxes back to the ground state without the loss of NO. O nly 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 st ate of (TPPCoNO)-N-II, hence all of the energy deposited in the initia l photoexcitation step results in NO loss. The lifetimes and spectral characteristics of the other excited states involved in deactivation o f these transition metalloporphyrin-nitrosyl complexes will be discuss ed.