Intramolecular quenching of excited singlet states by stable nitroxyl radicals

Article abstract of DOI:10.1021/ja00176a038

Absorbance and steady-state and time-resolved fluorescence measurements were employed 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 energies of the fluorophores are unaffected by the presence of the nitroxide substituent(s), and the residual 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 (k) are high (10⁸-10¹⁰s^-1) and decrease significantly with increasing nitroxide to fluorophore distance-however, relatively high rates of quenching (>10⁸ s^-1) are observed over distances as great as 12 ?; (3) F?rster energy transfer does not contribute significantly to the quenching due to the low values for the spectral overlap integrals; (4) the k_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_q 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_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. These results indicate that the quenching arises through electron exchange which causes relaxation of the (local) singlet state to the triplet and/or ground state of the fluorophore.