The quenching of terbium emission in the sensitized complex Tb3+-cs124-DTPA by nitroxide radical TEMPO derivatives in aqueous solutions has been studied with time-averaged and time-resolved methods. The time-resolved results show more quenching than the time-averaged values, opposite to the behavior expected for static quenching. A rapid exchange model with a slightly fluorescent fluorophore/quencher complex is proposed. Due to the long time scale of Tb3+ emission, dynamical averaging must be considered in the interpretation of experiments. The rapid exchange limit is shown to be consistent with the present results. The utility of these observations in the design of sensors that are not limited by a background level is noted.
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