FLUORESCENCE QUENCHING OF PYRENE BY WEAK ELECTRON-DONORS AND ACCEPTORS IN POLAR MEDIA - THE ROLE OF EXCIPLEXES IN THE FORMATION OF TRIPLET-STATES AND RADICAL IONS

The kinetics of formation and decay of triplet species and radical ions, die transient products of fluorescence quenching of pyrene by weak electron donor and acceptors in polar media was studied using a technique of microsecond flash photolysis. The observed dependences of quantum yields for triplets and radical ions on quencher concentration disagree with the model of direct electron transfer in the excited state, but are consistent with the model of exciplex formation in a polar medium. It was shown that the formation of triplet fluorophore occurs predominantly via two pathways: by intersystem crossing from a singlet excited state of fluorophore and from a singlet exciplex. The free radical ions are formed from the exciplex due to its dissociation accompanied by the complete charge transfer from the donor to the acceptor.