Fluorescence Quenching Via Charge Transfer: The Perylene-N,N-Dimethylaniline System

Abstract

The mechanism of fluorescence quenching in a system where there was evidence that quenching proceeds via a charge‐transfer complex has been investigated using the direct observation of fluorescence decay as a tool to explore the kinetics of the quenching process. The system studied was perylene in its fluorescent state quenched by amines in a variety of solvents with dielectric constants ranging from 2.3 to 37. In solvents with dielectric constant greater than 12, the fluorescence decay is exponential and no CT emission is seen. In low‐dielectric media (ε = 2.3 to 4) the decay of the perylene emission consisted of a two‐component exponential decay and CT emission was observed as well. The results obtained can be explained on the basis of the mechanism of Leonhardt and Weller, in which the excited‐state charge‐transfer complex, once formed from the excited molecule and quencher, can decompose into its components in a feedback step, emit CT fluorescence, or undergo one or more nonradiative processes. In media of high dielectric constant, the importance of the nonradiative decay increases compared to the other two paths. From an analysis of the decay curves the rate constant for the formation of the charge‐transfer complex from excited perylene and N , N ‐dimethylaniline was evaluated over the full dielectric constant range. Its magnitude suggests a diffusion‐controlled reaction. The rate constant and activation energy for the feedback step were also obtained.