MAGNETIC-FIELD EFFECT IN THE RECOMBINATION REACTION OF RADICAL-ION PAIRS - DEPENDENCE ON SOLVENT DIELECTRIC-CONSTANT

The influence of external magnetic fields on the reaction yield of rec ombining radical ion pairs of polymethylene-linked acceptor/donor syst ems (pyrene/N,N-dimethylaniline) is investigated in solvents of varyin g dielectric constant (epsilon = 7-37). Different bulk DKs are also ad justed with binary solvent mixtures. Similar to nonlinked radical ion pairs of the same type, the highest magnetic field effects (MFEs) on t he yields of products measured (exciplexes and pyrenyl triplets) are f ound in solvents of medium DK (epsilon approximate to 20). In such lin ked systems, only geminate recombination is necessarily possible, whil e homogeneous recombination does not occur. We conclude, therefore, th at earlier theoretical models for the explanation of the so-called ''m edium-DK MFE maximum'' in free-radical ion pair systems must be consid ered obsolete, These earlier models are mainly based on the competitio n between geminate and homogeneous recombination pathways. Based upon support from all present experimental results, an alternative model is proposed which correlates the radical ion pair lifetime with the achi evable magnitude of the MFE.