Reaction Pathways of the Electron Transfer from Photoexcited 10-Methylphenothiazines to Electron Acceptors in Polar Solvents. Effects of Magnetic Fields and Heavy Atoms on Efficiency of Free Ion Formation

The lowest triplet excited states of (2-substituted) 10-methylphenothiazine were found to be quenched by various electron acceptors in polar solvents such as 2-propanol and acetonitrile through electron transfer (ET). The transient absorption and time-resolved EPR spectra indicated that the radical cation of the phenothiazine and radical anions of the acceptors were formed as the ET products in moderate to high yields. These free radical ions were formed via two types of intermediates, (i) a triplet contact radical ion pair (³CRIP) or a triplet exciplex (³Ex*) and (ii) a triplet solvent-separated radical ion pair (³SSRIP). In the quenching by the Br-substituted acceptors, a large fraction of ³CRIP (or ³Ex*) was deactivated to the singlet ground states due to the breakdown of the spin-forbiddance by strong spin−orbit coupling. On the other hand, ³CRIP (or ³Ex*) containing no heavy atom was mainly transformed into ³SSRIP by solvation. ³SSRIP decayed through either the separation to free ions or the triplet−singlet conversion followed by the spin-allowed backward ET to the ground states. The backward ET rates of ¹SSRIPs were estimated to be ≥10⁹ s^-1, even when the reaction fell into the deeply inverted region. In 2-propanol, the free ion yields were affected by magnetic fields. The magnitudes of magnetically induced changes strongly depended on the polarity and viscosity of solvents, suggesting that the separation rate of SSRIP should be a crucial factor determining the field dependence of the free ion yields.