High magnetic field effects on biradical lifetimes: Evaluation of magnetic field dependence and chain length dependence using calculated g and hyperfine tensors

The g and hyperfine tensors of radicals were theoretically estimated by quantum chemical calculations, and compared with experimentally obtained ones. From the comparison, it was found that the theoretically calculated tensors can reproduce the experimental ones with accuracy. By using those tensors, therefore, high magnetic field dependences of the lifetimes of biradicals, which were generated by a photo-induced electron transfer in α-cyclodextrin inclusion complexes of chain-linked molecules of phenothiazine-(CH ₂)_n-viologen (PhnV²⁺, n = 12, 10, and 8) and carbazole-(CH₂)₁₂-viologen (Cz12V²⁺), were quantitatively analyzed on the basis of the relaxation mechanism of the generated bi-cationic biradicals (Ph^•+nV^•+ and Cz^•+ 12V^•+). The analysis conclusively showed that two extremely small correlation times of each radical motion were still necessary for reversals characteristic of the high magnetic field dependences, and also that the order of the obtained correlation time was Cz ^•+ > V^•+ > Ph^•+. The order in the correlation time was discussed while referring to the rigidity of the radical frameworks.