Synthesis and characterization of p-phenylenediamine derivatives bearing an electron-acceptor unit

Two series of aniline oligomers bearing fused heterocycles as an electron-acceptor unit were synthesized. They consist of aniline or its derivatives as an electron donor and benzothiadiazole (BT) or quinoxaline (QX) as an electron-acceptor unit. Benzothiadiazoles 1-3 were synthesized by palladium-catalyzed amination. Quinoxalines 4-6 were prepared by palladium-catalyzed amination or transformation from the benzothiadiazoles. These compounds showed a HOMO-LUMO gap smaller than those of their analogues such as thiophene-substituted BT/QXs. Cyclic voltammetry revealed that the electrochemical behavior is dependent on the position of the acceptor heterocycle. Chemical oxidation with Ag2O afforded the corresponding 1,4-quinonediimine derivatives as an E,E-isomer, stereoselectively. As for the BT pentamer analogues 2 and 3, the first oxidation selectively occurred at the amino group adjacent to the benzothiadiazole unit, giving the regiospecific half-oxidized derivatives. Furthermore, the fully oxidized derivative 24 was isolated and characterized.