Magnetic field effects (MFEs) on reactions of biradical radical ion pairs (BRIPs, S = 3/2 and 1/2) were investigated for the electron transfer (ET) reactions of the triplet state of 10-methylphenothiazine (3MPTZ*) (S = 1) and electron acceptors linked with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) (A n −R·) (S = 1/2) in various solvents by means of a nanosecond laser photolysis technique. In 2-propanol, the yield of free ions dramatically increased with increasing magnetic field (B) from 0 to 2 T, but slightly decreased from 2 to 10 T. The magnitude of the MFEs was much larger than those observed in ET reactions of 3MPTZ* with acceptors without TEMPO (S = 0). To interpret the MFEs observed in the reactions with A n −R·, we considered the following two factors. (i) In the quenching of 3MPTZ* by A n −R·, not only the ET reaction but also triplet−doublet (T−D) quenching takes place. The T−D quenching efficiency decreases with increasing B, resulting in a higher yield of BRIPs ([MPTZ•+ A n •-−R·]). However, this factor was found to have only a small contribution to the observed MFEs on the free ion yield. (ii) The ET reactions generate 4,2[MPTZ•+ A n •-−R·] in quartet (Q) and doublet (D) states, which decay through either the spin-selective back ET from the D states or the separation to free ions. At B = 0 T, the spin conversion between the Q and D states is efficient. The increase in the free ion yield with increasing B from 0 to 2 T can be attributed to the spin relaxation from the Q± 3/2 states to the Q± 1/2 and D± 1/2 states due to the dipole−dipole interaction of 3(A n •-−R·). The slight decrease in the free ion yield with increasing B from 2 to 10 T can be attributed to acceleration of the relaxation induced by the anisotropic Zeeman interaction and/or enhancement of the Q± 1/2 -D± 1/2 conversion induced by the difference in the isotropic g-factor between MPTZ•+ and A n •-−R·. Effects of solvents and additives on the present MFEs were also studied.

Magnetic field effects on the photochemical electron-transfer reactions of 10-methylphenothiazine with 1,4dicyanobenzene and tetrafluoro-1,4-dicyanobenzene are investigated by a nanosecond laser photolysis technique. For these reactions, the recombination of the geminate radical ion pairs as well as the yields and decay rates of the escaped radical ions showed clear magnetic field dependence in nonviscous homogeneous solutions. The mechanisms of the magnetic field effects are ascribed to the hyperfine coupling and ∆g mechanisms at low and high magnetic fields, respectively. The smaller magnetic field effects on the reaction of the fluorinated derivative than those of 1,4-dicyanobenzene are ascribed to its smaller back-electron-transfer rate because its free-energy change is smaller than that of the nonsubstituted one.

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Photoexcitation of zinc porphyrin−bridge−naphthalenediimide (ZP−B−NI) dyads, 1 and 2, generated the short- and long-distance charge-separated (CS) states, [ZP•+−B−NI•-], through the intramolecular electron-transfer from excited ZP to NI in solvents of various polarity. The energy level of [ZP•+−B−NI•-] was either higher (in benzene and 1,4-dioxane) or lower (in solvents of higher polarity) than that of 3ZP*−B−NI. When generated in the singlet spin state, the short-distance CS state derived from 1 rapidly (109−1010 s-1) decayed through the charge recombination (CR) leading to the ground state. On the other hand, when generated from the triplet excited state of 1, the decay of the CS state was much slower and showed magnetic field effects attributable to the level-crossing mechanism. For the long-distance CS state derived from 2, the decay dynamics and its magnetic field dependence exhibited quite different features. To examine the effects of a neighboring additional radical on the decay dynamics of these CS states, three-spin CS states [ZP•+−B−NI•-−R•] were generated from 1R• and 2R•, in which 2,2,6,6-tetramethyl-1-piperidinoxy radical (R•) was connected to the NI part in 1 and 2, respectively. The decay rate of [ZP•+−B−NI•-−R•] derived from 1R • was much faster than that of [ZP•+−B−NI•-] derived from 1. For the CS state generated from 2R•, the initial decay could be retarded compared to the CS state from 2 through the equilibration between the doublet and quartet spin states. The observed effects of R• on the decay processes of the CS states are attributed to the alteration of the energy gap between the states with different spin multiplicities and to the efficient conversion between the doublet and quartet spin states induced by the dipole−dipole interaction between NI•- and R•.