Difluorinated benzothiadiazole based donor-acceptor electrochromic polymers with tunable optoelectronic properties by varying the thiophene donor units

In this study, two difluorinated D-A polymers, P(FF-Th) and P(FF-EDOT), were prepared through electrochemical deposition method. The two polymers originated from difluorinated D-π-A-π-D monomers were synthesized via Stille coupling, using thiophene and EDOT as donors. Subsequently, several analytical techniques were employed to evaluate the optoelectronic and electrochromic properties of the two difluorinated D-A polymers. Furthermore, to reveal the intrinsic mechanisms influencing their performance, the impact of the thiophene donor units' structure on the electrochromic properties of the synthesized polymers was thoroughly explored. Contrary to FF-Th, FF-EDOT features a lower initial oxidation potential, thereby easing the synthesis of superior-quality difluorinated polymers with a diminished polymerization potential. In addition, the difluorinated P(FF-EDOT) films retain an impressive 77.4 % of their redox activity even after 1000 cycles. The fluorescence spectra and UV of FF-EDOT show a redshift after introducing the EDOT unit. Studies using electrochemical and spectroelectrochemical methods indicate that both polymers have hole-doping and electron-doping characteristics. Notably, P(FF-EDOT) has a smaller optical band gap, lower oxidation potential, and better dynamic stability. The optical band gap of these two newly developed difluorinated D-A polymers can be adjusted, with their electrochromic properties being significantly influenced by the differing electron-donating capacities of their thiophene-based donor units.