Tuning the Electrochromic Properties of Fluorinated Benzochalcogenodiazole Based Donor-Acceptor Polymers

In this study, a series of benzochalcogenodiazole based donor acceptor monomers namely 4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (TST), 5,6-difluoro-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (TST2) and 4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzoselenadiazole (TSeT) were synthesized by Stille reaction and then directly polymerized on to Indium Tin Oxide coated glass (ITO/Glass) surface by electrochemical polymerization process. According to UV-Vis measurements, a 20 nm blue shift was detected at low energy band after fluorination of TST (TST2). Besides, a 40 nm red shift was observed by changing S to Se on the benzochalcogenodiazole moiety (TSeT). Thus, the optical band gaps were varied as 2.32, 2.45 and 2.16 eV for TST, TST2 and TSeT, respectively. According to AFM results, rougher surface was observed in the PTSeT thin film than that of PTST and PTST2. Thin films of electrodeposited polymers onto ITO/glass surface displayed ambipolar multi-electrochromic behavior both anodic and cathodic region. The ambipolar charge distribution was also observed in the theoretical DFT calculations. When the electrochromic performance of the polymer films was compared to each other, it was seen that the PTST2 has a better stability and higher the percentage transmittance change (Delta T%) at near-IR regime between neutral and oxidized states.