Quenching of the fluorescence of 3-carboxy-5,6-benzocoumarin by aromatic amines

The change of the emission properties of a coumarin derivative, 3-carboxy-5,6-benzocoumarin in the presence of five aromatic amines was studied by means of steady-state fluorescence spectroscopy. The quenching process was characterized by Stern–Volmer (SV) plots which display either the usual linear aspect (aniline, dimethyl- and diethylaniline) or an upward curvature (diphenyl- and triphenylamine). The analysis of the plots made in terms of the usual linear SV equation or of a SV modified equation including both the dynamical and static quenching allows for the estimation of the rate constant of the bimolecular quenching reaction. The use of the finite sink approximation model for the amines leading to a curve SV plot shows that the quenching was diffusion limited. The experimental data were rationalized in terms of the Marcus theory on the electron transfer reactions. The bimolecular quenching constants and the free energy change estimated by the Rehm–Weller relationship fit well the data for other coumarin–amine systems.