INTRAMOLECULAR QUENCHING OF EXCITED SINGLET-STATES IN A SERIES OF FLUORESCAMINE-DERIVATIZED NITROXIDES

Steady-state and time-resolved absorption and fluorescence measurement s were used to probe the mechanism(s) of excited singlet state quenchi ng by stable nitroxyl radicals in a series of fluorescamine-derivatize d nitroxides. Rate constants for intramolecular quenching (k(q)) acqui red from fluorescence lifetime measurements were very high, ranging fr om similar to 0.3 x 10(10) s(-1) to greater than or equal to 5 x 10(10 ) s(-1), and showed little dependence on solvent polarity. The k(q)'s did not track the values of the Dexter and Forster spectral overlap in tegrals, thus indicating that energy transfer, through either mechanis m, cannot account for the quenching. Quenching by electron transfer al so appears unlikely, owing to the dependence of k(q) on solvent polari ty and the complete lack of radical ion generation in highly polar sol vents. Time-resolved absorption measurements revealed the presence of a very weak transient centered at similar to 470 nm in the diamagnetic reference compounds that was not observed in the paramagnetic nitroxi des; this transient was tentatively assigned to the excited triplet st ate of the fluorescamine moiety. The rapid singlet state quenching in this series of compounds thus appears to arise from enhanced internal conversion.