Exciplex parameters: key factors in the dielectric behaviour of magnetodynamics

Photoinduced electron transfer reactions in solution produce two primary geminate radical ion pairs: contact ion pair or exciplex and solvent-separated ion pair. The magnetodynamics of radical ion pairs involves suppression of the spin-evolution between singlet and triplet states of a fraction of solvent-separated ion pairs, the partners of which undergo prior diffusion to attain the distance where exchange interaction is negligible, in the presence of an external low magnetic field of the order of the hyperfine interactions present in the system. This results in an increase in geminate recombination of the singlet solvent-separated ion pairs and enhancement in exciplex luminescence since the precursor radical ion pair is singlet. Although seemingly magnetodynamics is a diffusion-controlled phenomenon that should depend mainly on the dielectric constant of the medium (keeping viscosity almost constant), it is not true for all the exciplex systems since the nature of the magnetic field effect versus medium dielectric curves differ from each other in peak positions, peak heights and onset points. To investigate this differential nature, magnetic field effects among exciplex systems consisting of different derivatives of carbazole as electron donors are compared with a universal acceptor, 1,4-dicyanobenzene, with the pyrene-N,N-dimethylaniline exciplex system as reference. It was found that, apart from the solvent dielectric, the exciplex energy and the bulk effect of the steric constraints present on either donor or acceptor site, regulating the optimum inter-radical distance in the initially formed radical ion pair, are the key factors in controlling the magnetodynamic behaviour.