Intramolecular quenching of excited singlet states by stable nitroxyl radicals
- Woods Hole Oceanographic Institution, MA (USA)
- Oregon State Univ., Corvallis (USA)
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{sub q}) are high (10{sup 8}-10{sup 10} s{sup {minus}1}) and decrease significantly with increasing nitroxide to fluorophore distance-however, relatively high rates of quenching (>10{sup 8} s{sup {minus}1}) are observed over distances as great as 12 {angstrom}; (3) Foerster energy transfer does not contribute significantly to the 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} 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.
- OSTI ID:
- 5478539
- Journal Information:
- Journal of the American Chemical Society; (United States), Journal Name: Journal of the American Chemical Society; (United States) Vol. 112:20; ISSN 0002-7863; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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