Electrochromic p-Conjugated Copolymers Derived from Azulene, Fluorene, and Dialkyloxybenzothiadiazole

A series of random p-conjugated copolymers P1, P2, and P3 were synthesised from 1,3-dibromoazulene, 4,7-dibromo-5,6-bis(dodecyloxy)benzo-2,1,3-thiadiazole, and 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) via Suzuki coupling reactions. The copolymers P1–3 had molecular weights in the range of 17000–30900 g mol–1 with polydispersity indexes of 1.45–2.03. Thermal analysis showed that the polymers P1–3 had good thermal stability with decomposition temperatures ranging from 341 – 363°C both in air and in nitrogen. Photoluminescence studies showed that polymer P1 and P2 are weakly fluorescent with low quantum yields of 0.013 and 0.0029 for P1 borne with 30 % azulene and P2 borne with 50 % azulene in the polymer backbone, respectively. P3 borne with 70 % azulene resulted in complete quenching of fluorescence. The electrochemical band gaps for P1–3 are very close to their corresponding optical band gaps. Electrochromic study showed that three polymer thin films displayed the same colour change from yellowish green at the neutral and electrochemically-reduced state to greyish brown at the electrochemically-oxidised state. In particular, electrochromic contrasts of 17 % and 13 % for P2 and P3, respectively, were recorded in the near infrared region.