Fluorescence quenching by the stable free radical di-t-butylnitroxide

Bimolecular rate constants (kQ) were measured for the fluorescence quenching of a series of aromatic hydrocarbons by di-t-butylnitroxide (DTBN). In methylcyclohexane, measured values of kQ were close to the diffusion limit with only a 6.5-fold variation through the hydrocarbon series. A study of the viscosity dependence of the quenching reaction, in the Smoluchowski manner, suggests an interaction distance of 4–6 Å. A comparison is made of the efficiency of the DTBN and O2 fluorescence quenching reactions. Possible mechanisms are discussed.