New low-bandgap donor–acceptor (D–A) functional copolymers, CDTDBT and CEHTDBT, composed of a 3,3′-didodecyl-2,2′-bithiophene donor and 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole (DT) or 4,6-bis(3′-(2-ethylhexyl)thiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole (EHT) acceptor structural units, respectively, were synthesized by Stille coupling of the corresponding comonomers. The influence of decyls or branched 2-ethylhexyls chains attached to the thiophenes of the 4,6-bis(3′-alkylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole on the photophysical, thermochromic, electrochemical, and spectroelectrochemical properties are reported and discussed in comparison to a series of D–A copolymers with various donor units and the corresponding dithiophenylthienothiadiazole (DT or EHT) acceptor units. The new copolymers, CDTDBT and CEHTDBT, exhibit interesting electrochromic behaviours with fast response times, which reach full optical contrast in 0.3 s. These new copolymers have the potential to be used for optical switching.