Comparative Studies of Magnetic Field Effect on Radical Pairs Photogenerated by Electron Transfer from Biphenyl to Derivatives of Phenyl Pyrilium Salt and to Corresponding Thio Salts

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We have generated radical pairs (RPs) in SDS micellar medium by electron transfer from the ground state of biphenyl (BP) to various alkyl-substituted phenyl pyrilium cations (PP + ) and corresponding thio-phenyl pyrilium cations (SPP + ) and have compared the dynamics of RP recombination in magnetic fields varying from 0 to 5 T. A comparison of PP • /BP •+ decay curves with SPP • /BP •+ decay curves leads to the conclusion that both 1 RPs and 3 RPs are produced in the former case, the temporal behavior at early times being dominated by field-insensitive 1 RP and at later times by highly field-sensitive 3 RPs. In the case of the SPP • /BP •+ system, on the other hand, 3 RPs dominate the decay throughout but the field sensitivity is reduced. The large MFE in O-analogues is presumably caused by reduction of micellar escape rate while the reduced MFE for the S-analogues has been ascribed to spin−orbit coupling (SOC).