Radical-Pair Dynamics in the Photoreduction of Anthraquinone in Sodium Dodecyl Sulfate Micellar Solution Detected by Pulse-Mode Product-Yield-Detected Electron Spin Resonance: Temperature and Salt Dependence

A large number of papers have been published on the magnetic field effect (MFE) in various kinds of chemical which were detected mainly by the transient absorption meth0d~9~ and product analy~is.~,~ The radical pair model of CIDNP9$Io has been proposed for the mechanism of this magnetic field dependence and was confirmed by several techniques. For the photoreduction of quinones in sodium dodecyl sulfate (SDS) micellar solutions, we found that the ESR transitions of both the SDS radical and the semiquinone radical affect the yield of a product that is produced only from the SDS radical (i.e. spin adduct).” This is direct evidence of the RP model for this kind of magnetic field dependence. As another evidence of the RP model, Closs et a1.I2 and McLauchlan et al.I3 have explained a new-type of CIDEP found by Sakaguchi et al.I4 for a similar system with the interaction in the RP. Because with our method the ESR spectrum of the RP is traced by the change of the final-product yield, we call this method product-yield-detected ESR,l5%l6 or PYESR in short. Since these spin-dependent phenomena are deeply concerned with the relation between the chemical bond and the spin correlation of the two unpaired electrons from which a new bond is created,I7 these are undoubtedly an important theme of the basic chemistry. In the PYESR technique, we usually use the spin trapping meth~d’~.’~ to separate the escape product as a spin adduct from the cage product, and also to determine the yield by ESR with which in-situ observation is easy. Since PYESR is also possible by using the HPLC method,20 other techniques such as NMR should be potentially applicable. As a similar technique to detect the ESR of a spin pair,21 the optically-detected ESR technique has been well-known in solid-state and subsequently applied to solution chemi~try.~~-~~ These studies have been successful in elucidating the structure of ionic species in the irradiated sy~tems,~~,~~,~~ the reaction mechanism of the photosynthetic system,27 and the dynamics of an ion pair in a photolytically ionized system.28 The main difference with our method is that we observe the yield of a final product and the