ChemInform Abstract: Control of Photo‐Electron‐Transfer Induced Radical Production by Micellar Cages, Heavy‐Atom Substituents and Magnetic Fields.

Abstract

Intramicellar radical pair formation and recombination kinetics in the electron transfer quench­ ing of the thionine triplet by aniline and various monohalogenated anilines have been studied by micro­ second and nanosecond laser flash spectroscopy in reversed micellar solution of COBA in benzene. Clear kinetic evidence of the micellar cage effect is provided by a comparative spectro-kinetical study in homogeneous aqueous and reversed micellar solution. In zero magnetic field the radical pairs which originate with a triplet spin alignment recombine in the waterpools of the micelles with a rate constant of about 3 x 10 6 S - 1 which is not sensitive to the hyperfine or spin--orbit coupling parameters of the aniline­ type radical. Long lived radicals are formed by radical escape from the micelles occurring with a rate constant in the order of2 x 10 6 S-l and being insensitive to an external magnetic field. Intramicellar radical pair recombination is slowed down by an external magnetic field. A maximum effect (measured at I T) of a factor of 3 is observed for non-halogenated anilines. Halogen substitution attenuates this magnetic-field effect depending on the strength ofspin--orbit coupling exhibited by the halogen substituent. The magnetic­ field effect is interpreted in terms of the radical pair mechanism with special emphasis on the role of spin relaxation. Suppression of the magnetic-field effect by halogen substituents is attributed to the spin-rotational relaxation mechanism which is independent of a magnetic field. A heavy-atom substituent effect is also borne out in the primary yield of radical pairs which is decreased in the same way as in homogeneous solution. This is attributed to the role of a triplet exciplex formed as a precursor of the radical pair, where heavy-atom substituents cause very efficient radiationless decay to the ground state. A magnetic-field effect typical for the triplet mechanism in the exciplex has been detectable with 4-iodoaniline as quencher. Among the various mechanistic pathways of photo­ chemical radical production photo-induced electron transfer is a most important one, especially if the generation of radical ions is concerned. The radical pairs produced in this way are generally higher in energy than the corresponding ground state unreacted donor-acceptor pairs and therefore may be stabilized by fast reverse electron transfer before the radicals can separate. The efficiency of reverse electron transfer vs radical separation determines the yield of free radicals