Magnetic field effects on exciplex-forming systems: the effect on the locally excited fluorophore and its dependence on free energy

Abstract

This study addresses magnetic field effects in exciplex forming donor-acceptor systems. For moderately exergonic systems, the exciplex and the locally excited fluorophore emission are found to be magneto-sensitive. A previously introduced model attributing this finding to excited state reversibility is confirmed. Systems characterised by a free energy of charge separation up to approximately -0.35 eV are found to exhibit a magnetic field effect on the fluorophore. A simple three-state model of the exciplex is introduced, which uses the reaction distance and the asymmetric electron transfer reaction coordinate as pertinent variables. Comparing the experimental emission band shapes with those predicted by the model, a semi-quantitative picture of the formation of the magnetic field effect is developed based on energy hypersurfaces. The model can also be applied to estimate the indirect contribution of the exchange interaction, even if the perturbative approach fails. The energetic parameters that are essential for the formation of large magnetic field effects on the exciplex are discussed.

Magnetic Field-Sensitive Radical Pair Dynamics in Polymethylene Ether-Bridged Donor–Acceptor Systems

“…The vibronic transitions with Gaussian band shape of the exciplex were extracted by employing a model as described in refs. 8,45 The fluorescence spectrum of the locally excited fluorophore was directly accessible from the spectrum in the absence of the quencher moiety.…”

“…For the N , N -dimethylaniline (DMA)/9,10-dimethylanthracene (DMAnt)-based system studied here, J is predicted to be negative (following the convention as expressed in eq 2 below), that is, the scenario as shown in Figure 2b is likely relevant. 8 …”

Diffusion affected magnetic field effect in exciplex fluorescence

“…Indeed, the MFE on the exciplex fluorescence was found to crucially depend on dielectric constant. 5,13,14 The effect is negligibly small in both regions of high and low dielectric permittivity, being considerable only in between. Such a behavior is in full accord with theoretical calculations.…”

“…1-14 Much attention was paid to the MFEs on fluorescence intensity in electron transfer quenching. 2,5,13,14 In such reactions, an exciplex composed from a pair of partially charged donor, D +δ , and acceptor of electron, A −δ , can be produced. Exciplex formation can proceed in two ways known as the Weller's Scheme I and Scheme II.…”

Spectral and thermodynamic properties for the exciplexes of N-alkyl carbazoles with dicyanobenzenes in THF

“…Such non-emitting exciplexes have properties similar to the contact ion pairs. Exciplexes show substantial magnetic field effect due to mixing of singlet and triplet radical ion pairs (in case of exciplex contact ion pairs [9][10][11][12].…”