THE PHOTOINDUCED DISSOCIATION OF NITRIC-OXIDE FROM NITROSYL METALLOPORPHYRINS

Nitric oxide, once thought of only as an environmental pollutant, has gained recognition as a biologically pertinent entity. It is known to form relatively stable complexes with metalloporphyrins and previous s tudies have revealed that many of these complexes release nitric oxide , to varying degrees subsequent to photon absorption in the molecular pi-system. The results of studies probing the ultrafast dynamics of ph otoinduced deligation in two transition metalloporphyrin-nitrosyl comp lexes, (TPPFeNO)-N-II and (TPPCoNO)-N-II, in addition to results of a study based on energy transfer lead to the conclusion that the differe nce in the denitrosylation yields is the result of energy partitioning in the upper excited states of the porphyrin. The energy study yielde d the energies of the metal centered states, believed to be of CT(pi,d (Z2)) nature, and of the localized porphyrin triplet states. The CT st ates in the two complexes were found to lie at similar energies; howev er, the localized porphyrin triplet state energies were found to be di fferent by 1500 cm(-1) This difference in energies of the respective t riplet states facilitates efficient intersystem crossing in the excite d state deactivation of (TPPFeNO)-N-II, which has the higher triplet s tate energy, but does not allow any competitive triplet formation in ( TPPCoNO)-N-II. The direct excitation studies of (TPPFeNO)-N-II reveale d the formation of the intersystem crossing product which was then see n to relax back to the ground state without the loss of NO. The direct excitation studies of (TPPCoNO)-N-II yielded no evidence for intersys tem crossing in the deactivation of the electronically excited singlet state, so that all of the initially deposited energy resulted in NO l oss. The results indicate that the differences in the photoinduced den itrosylation yields in protein-free nitrosylmetalloporphyrin complexes are a direct manifestation of energy partitioning in the upper excite d state deactivation pathway and so only those excited states that rel axed via the CT state result in loss of NO. Spectral and dynamic chara cterization of the other excited stated involved in deactivation of th ese complexes is discussed.