Magnetic field effect in homogeneous medium for triplet born radical ions : a way for assessment of inter-radical distance in intermolecular photoinduced electron transfer

Abstract

Photoinduced electron transfer (PET) reaction involves excitation of either donor or acceptor prior to electron transfer. The PET reaction might occur in the singlet or in the triplet electronic state of the chromophore. These reactions could be influenced by the presence of internal or external magnetic field since radical ions with free electrons are generated as intermediates. Magnetic field effect (MFE) in homogeneous medium could be observed on exciplex luminescence, however the change is very small as the rate of recombination is very high owing to the singlet spin correlation of the geminate radical ion pair (RIP). On the other hand for the triplet born RIPs the detection of MFE needs confinement of the triplet species because at ambient temperature the lifetime of the solvent cages containing spin correlated RIP in homogeneous solvent is 10 -10 s whereas the rate of intersystem crossing is 10 -8 s. It is possible to confine the triplet born radicals by using organized assemblies like micelles, reverse micelles, etc., highly viscous solvents at low temperature or long chain biradicals which help to reduce fast escape and thus retain the spin-correlation between the partners of the geminate RIP. MFE is a composite of diffusion dynamics, spin dynamics leading to intersystem crossing and recombination of free ion formation. The effect is optimized only when the inter-radical distance through diffusion becomes sufficient to make exchange interaction between free electrons negligible retaining the original spin-correlation in the solvent cage. In this review we would like to highlight a few systems where MFE for the triplet born radicals could be observed even in homogeneous media due to some specific interactions other than covalent linking, which also helps to assess the inter-radical distance in such intermolecular PET, which is very rare.