Unexpected correlation between electronic coupling and excited state redox properties in PET

Fluorescence quenching by photoinduced electron transfer (PET) of polyaromatic acceptors by aliphatic amines was studied in acetonitrile. Sorting the quenching rate constants kQ according to the different amines led to monotonic curves of log kQvs. ΔGet (the Gibbs energy change), which could be fitted in some cases by Marcus' law. However, detailed examination of the fitting parameters led to the conclusion that this approach misinterpreted the data. By contrast, sorting the data according to the acceptors showed that kQ rose rapidly as the exergonicity increased, and reached a plateau located under the limiting diffusion rate constant. This was ascribed to weak and variable electronic coupling between the various accepting orbitals and the donating n orbital of the homogeneous set of aliphatic amines. Moreover, when the plateau values log(kmaxet) were plotted against the excited reduction potential of the acceptors, a good correlation was revealed. This underlined the crucial role of the accepting orbital topology towards a homogeneous set of donors.