THE PHOTOINDUCED DISSOCIATION OF NITRIC OXIDE FROM NITROSYLMETALLOPORPHYRINS

• Published in: Progress in reaction kinetics and mechanism 1998-01, Vol.23 (1), p.91-115
• Main Authors: Morlino, Elisabeth A, Rodgers, Michael A J
• Format: Article
• Language: English
• Subjects: Deactivation; Dynamics; Energy management; Excitation; Excitation spectra; Nitric oxide; Partitioning; Porphyrins
• ISSN: 1468-6783; 1471-406X
• DOI: 10.3184/007967498103165004

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 studies 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 photoinduced deligation in two transition metalloporphyrin-nitrosyl complexes, TPPFeIINO and TPPCoIINO, in addition to results of a study based on energy transfer lead to the conclusion that the difference in the denitrosylation yields is the result of energy partitioning in the upper excited states of the porphyrin. The energy study yielded the energies of the metal centered states, believed to be of CT(pi,dz2) nature, and of the localized porphyrin triplet states. The CT states in the two complexes were found to lie at similar energies; however, the localized porphyrin triplet state energies were found to be different by 1500 cm­1. This difference in energies of the respective triplet states facilitates efficient intersystem crossing in the excited state deactivation of TPPFeIINO, which has the higher triplet state energy, but does not allow any competitive triplet formation in TPPCoIINO. The direct excitation studies of TPPFeIINO revealed the formation of the intersystem crossing product which was then seen to relax back to the ground state without the loss of NO. The direct excitation studies of TPPCoIINO yielded no evidence for intersystem crossing in the deactivation of the electronically excited singlet state, so that all of the initially deposited energy resulted in NO loss. The results indicate that the differences in the photoinduced denitrosylation yields in protein-free nitrosylmetalloporphyrin complexes are a direct manifestation of energy partitioning in the upper excited state deactivation pathway and so only those excited states that relaxed via the CT state result in loss of NO. Spectral and dynamic characterization of the other excited stated involved in deactivation of these complexes is discussed.