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 (E_ox¹ = +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.