Magnetic Field Effects in Fluorescence of Exciplex and Fluorophore for the Weller Schemes I and II: Similarities and Differences

In exciplex-forming donor–acceptor systems the photoexcited acceptor state can be quenched either by exciplex formation at the contact of the acceptor and the donor, followed by reversible dissociation to the radical-ion pair (Scheme I), or by distant electron transfer yielding a radical-ion pair that can reversibly recombine to produce an exciplex (Scheme II). The fluorescence of the photoexcited electron acceptor (fluorophore) and the exciplex is known to be sensitive to the magnetic field, assisting spin conversion in the resulting geminate radical-ion pair (RIP). An increase of the fluorescence yield in high magnetic fields compared to low fields is known as the magnetic field effect (MFE). In this paper theories of the MFE on both the fluorophore and the exciplex within the Schemes I and II are developed. Kinetics of the reversible reactions are described in terms of the integral encounter theory that unlike the differential encounter theory is capable of dealing with such reactions. Regions of the parameters where MFE is observable are discussed. Comparison of functional dependencies of MFE on the model parameters for the Schemes I and II is carried out. Magnitudes and dependencies of MFE on the model parameters are found to be very similar for the Schemes I and II, except the dependencies of the fluorophore and exciplex MFEs on the rate constant of exciplex production from the radical-ion pair, which appear to be opposite for the Schemes I and II.