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

Abstract

The influence of diarylamino (Ar2N-) substituents on the oxidation potential of 3,7-bis(diarylamino)phenothiazines (Ar2N)2-PTZ (1a-f, a: carbazolyl; b: dihydrodibenzoazepinyl; c: dibenzoazepinyl; d: diphenylamino; e: phenothiazinyl; and f: phenoxazinyl) is investigated, where the Ar2N-substituent sequence a→f is aligned in the increasing order of their electron-donating ability. Interestingly, a different sequence of electron-donating ability for Ar2N-substituents was observed for the oxidation potentials of (Ar2N)2-PTZ: 1a (Eox1 = +0.35 V vs. Fc/Fc+) > 1f (+0.30 V) > 1e (+0.15 V) > 1d (-0.05 V) > 1c (-0.19 V) > 1b (-0.22 V). The observed sequence can be explained by the stereoelectronic effect of the Ar2N-substituents to stabilize (Ar2N)2-PTZ+. Clear-cut examples are observed in the crystal structure of 1c+ and 1e+, for which coplanar conformation is observed between the PTZ+-plane and the planes of the sp2-hybridized nitrogen atoms in Ar2N-substituents through a large conformational change during the oxidation process of (Ar2N)2-PTZ.