Tunable optoelectronic performances of aminated benzothiadiazole-based D-A-D conjugated electrochromic polymers

Abstract

Introducing heteroatom groups into benzothiadiazole (BT) unit has proven to be a very effective way to improve the optoelectronic properties of conjugate polymer due to its comprehensive advantages of lowering the HOMO or LUMO energy level, and increasing the interaction force on the polymer backbone. However, at present, little attention was paid to introducing amino groups into the BT unit. Herein, in this work, three D-A-D type aminated monomers (Th–NH₂-BT, Th–2NH₂-BT, and EDOT-2NH₂-BT) with aminated benzothiadiazole as the acceptor unit and thiophene or 3,4-ethylenedioxythiophene (EDOT) as the donor unit were successfully synthesized by Stille coupling reaction, and their corresponding aminated polymers (P(Th–NH₂-BT), P(Th–2NH₂-BT), and P(EDOT-2NH₂-BT)) were facilely achieved by electrochemical polymerization method. The optoelectronic properties of the aminated monomers and their D-A polymers were carefully analyzed, and the electrochemical and electrochromic properties of the resultant aminated polymers were further studied. The finally results show that the absorption and fluorescence spectra of Th–2NH₂-BT and EDOT-2NH₂-BT are both blue-shifted compared to Th–NH₂-BT, and the corresponding optical bandgap are gradually increased (2.41 eV–2.65 eV). EDOT-2NH₂-BT has a relatively lower oxidation potential (0.73 V) and P(EDOT-2NH₂-BT) also has a wide potential window and better redox stability, which was very beneficial for improving its resultant D-A aminated polymers’ optoelectronic properties. As a result, as-formed P(EDOT-2NH₂-BT) shown obvious color change from green in the neutral state to gray in the oxidized state with favorable electrochromic performances, with a high optical contrast (ΔT = 17.17 % at 860 nm) and high coloration efficiency (CE = 223 C⁻¹ cm² at 860 nm), so it is one of the promising materials that can be used in electrochromic devices.