Stereoelectronic control of oxidation potentials of 3,7-bis(diarylamino)phenothiazines

Abstract

We found a unique stereoelectronic-substituent effect on the oxidation potentials of bis(diarylamino)phenothiazines through large conformational changes during the oxidation process.

Triplet Diradical‐Cation Salts Consisting of the Phenothiazine Radical Cation and a Nitronyl Nitroxide

“…The N−C(sp 2 ) and S−C(sp 2 ) bond lengths in the radical cation are shorter than those in the neutral species, similarly to previously reported PTZ radical‐cation species . These tendencies (planarization of the PTZ moiety and decreased bond lengths) are commonly observed in the π‐electronic system of PTZ radical cations .…”

“…The first oxidation potential (−0.02 V) was assigned to the one‐electron oxidation of the PTZ moiety, and is the same as that of DAA‐PTZ‐I ( E ox =−0.02 and +0.41 V); however, it is considerably lower than that of N ‐phenylphenothiazine ( E ox =0.26 V) . The coplanar structure of the π plane of PTZ + and the nodal plane of the sp 2 nitrogen atom in DAA is responsible for the low oxidation potential of the DAA‐PTZ systems . The second oxidation potential (+0.40 V) of DAA‐PTZ‐NN was assigned to the formation of ( DAA‐PTZ ) 2+ ‐NN by analogy with the second oxidation potential (+0.41 V) of DAA‐PTZ‐I .…”

“…Thus, in summary, the first oxidation (at −0.02 V) of DAA‐PTZ‐NN selectively produces ( DAA‐PTZ ) + ‐NN . In fact, it was possible to isolate ( DAA‐PTZ ) + ‐NN⋅ MBr 4 − (M=Ga, Fe) by chemical oxidation with the corresponding thianthrenium radical cation ( TA + ⋅ MBr 4 − , Figure ) . The radical‐cation salts ( DAA‐PTZ ) + ‐NN⋅ MBr 4 − were characterized by X‐ray diffraction (see below), MS, EPR spectroscopy, and elemental analysis.…”

“…The molecular structure of ( DAA‐PTZ ) + ‐NN⋅ GaBr 4 − is shown in Figure b (see Figure S2b and Table S1 for ( DAA‐PTZ ) + ‐NN⋅ FeBr 4 − ). The structures of the ( DAA‐PTZ ) + ‐NN moiety are more planar than that of DAA‐PTZ‐NN , the dihedral angles A/B, A/C, and B/D being smaller in the diradical cations (see Table S1) …”

“…[13] The coplanar structure of the p plane of PTZ + and the nodal plane of the sp 2 nitrogen atom in DAA is responsible for the low oxidation potential of the DAA-PTZ systems. [14] The second oxidation potential( + 0.40 V) of DAA-PTZ-NN was assigned to the formation of (DAA-PTZ) 2 + + -NN by analogy with the second oxidation potential (+ 0.41 V) of DAA-PTZ-I.T he third oxidation potential( + 0.54 V) was attributed to the oxidation process (DAA-PTZ) 2 + + -NN! (DAA-PTZ) 2 + + -NN + + ,w hicho ccurs at ac onsiderably higherp otentialthan the oxidationpotential of NN-Ph (E ox =+ 0.37 V), [15] because of the strongly electron-withdrawing nature of the (DAA-PTZ) 2 + + moiety in (DAA-PTZ) 2 + + -NN.T hus, in summary,t he first oxidation (at À0.02 V) of DAA-PTZ-NN selectively produces (DAA-PTZ) + + -NN.I nf act, it was possible to isolate (DAA-PTZ) + + -NN·MBr 4 À (M = Ga, Fe) by chemical oxidationw ith the corresponding thianthrenium radical cation (TA + + ·MBr 4 À , Figure 1).…”