Control of Photo-electron-transfer Induced Radical Production by Micellar Cages, Heavy-atom Substituents and Magnetic Fields

Intramicellar radical pair formation and recombination kinetics in the electron transfer quenching of the thionine triplet by aniline and various monohalogenated anilines have been studied by microsecond 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 S 1 which is not sensitive to the hyperfine or spin--orbit coupling parameters of the anilinetype radical. Long lived radicals are formed by radical escape from the micelles occurring with a rate constant in the order of2 x 10 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 of spin--orbit coupling exhibited by the halogen substituent. The magneticfield 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 photochemical 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 and is hence of great importance, if one is interested to utilize the radicals for further chemical transformations, e.g. for the purpose ofchemical storage of solar energy or information or for chemical synthesis. One of the basic principles, which has been implicitly confirmed in many investigations, is the conservation of electron spin in the electron transfer step. In fact, spin conservation is quite general in fast chemical reactions and in connection with the production or recombination of radical pairs it is the basic requirement of the mechanisms explaining chemical polarization of nuclear and electronic spins (CIDNP, CIDEP)I,2 and of various magnetic-field effects in chemical reactions (for reviews cf. Refs 36). Thus, when excited singlets react with closed-shell electron donors or acceptors, radical pairs in an overall singlet state are formed, i.e. the radicals originate with antiparallel spin alignment. Correspondingly, when starting from an electronically excited triplet state the radicals will originate with parallel spin alignment. As a consequence of the rule of spin conservation in electron transfer, direct recombination of triplet radical pairs to form diamagnetic ground state products is spin forbidden. Due to this fact the free radical yields observed in excited triplet state reactions are generally much higher than for the corresponding singlet state reactions. 7\ 0 Of course, the principle of electron spin conservation is an idealization and the rule can be relaxed due to several types of perturbation mechanisms, which are also responsible for the magnetic polarization effects mentioned above. During the last years we have been especially interested in the mechanisms and rules governing "spinforbidden" electron back transfer following electron transfer reactions with excited triplet states. A number of general conclusions has been obtained from our studies8,11-1 3 of the reaction of the thionine triplet CTH+) with aniline and its monohalogenated derivatives, where the dye triplet acts as an electron acceptor. The general mechanistic aspects may be described in terms of Scheme I, where A stands for electron acceptor and D for electron donor and the charges correspond to the systems we have studied. According to Scheme I a radical pair like triplet exciplex has to be considered as the primary product of the electron ® H2N'(J(1St)?,NH 2 Thionine TH+ ~ N~.o et