Intramolecular Quenching of Excited Singlet States by Stable Nitroxyl Radicals

摘要

Absorbance and steady-state and time-resolved fluorescence measurements wereemployed to examine the mechanism(s) of excited singlet state quenching by nitroxides in a series of nitroxide-fluorophore adducts. This work establishes the following: (1) the absorption and emission that is observed from the adducts arises from the locally excited singlet of the fluorophore, not from charge recombination; (2) rate constants for intramolecular quenching by the nitroxides and decrease significantly with increasing nitroxide to fluorophore distance-however, relatively high rates of quenching are observed over distances as great as 12 A; (3) Forster energy transfer does not contribute significantly to be quenching due to the low values for the spectral overlap integrals; (4) the k sub q's do not increase proportionally to the solvent-dependent increases in the Dexter overlap integral, indicating that energy transfer by the Dexter mechanism is not responsible for the quenching; (5) the values of k sub q's show no obvious correlation with the calculated free energies for photoinduced electron transfer, suggesting that this quenching pathway is also unimportant; (6) for hematoporphyrin-nitroxide adducts, which contain a fluorophore whose singlet energy is below that of the first excited state energy of the nitroxide (thus precluding energy transfer), significant rates of quenching are still observed; (7) for compounds with similar nitroxide-fluorophore distance, an approximately linear correlation is observed between the k sub q's of the paramagnetic compounds and the nonradiative rate constants of the diamagnetic reference compounds, suggesting that the nitroxide moiety catalyses a preexisting nonradiative pathway in the fluorophore. (ttl)