New low-bandgap donor-acceptor functional copolymers, CDTDP and CDTDOP, composed of 4,6-bis-(3'dodecylthien-2'-yl)thieno[3,4-c][1,2,5]thiadiazole (DT) and 2,5-didodecyl-1,4-phenylene (DP) or 2,5didodecyloxy-1,4-phenylene (DOP) structural units, respectively, were synthesized by a rapid Suzuki coupling reaction of the corresponding comonomers in high-boiling-point solvents. The influence of alkyl and alkyloxy side-chains attached to the benzene ring on photophysical, thermochromic and electrochemical properties are reported. CDTDP exhibits absorption in the visible spectral region with long-wavelength maxima at 401-410 nm and 673-703 nm in solutions and at 461-467 nm and 827 841 nm in thin films. The absorption of CDTDOP in solution is significantly red-shifted, extending up to the near infrared region. CDTDOP shows more pronounced thermochromic changes. Both copolymers possess similar high values of electron affinity. CDTDOP with an alkyloxy side chain exhibits a lower ionization potential than that of CDTDP due to the stronger donor character of the substituted benzene unit, which narrows the bandgap value to 1.15 eV compared to that of 1.3 eV for CDTDP. Both copolymers exhibited interesting electrochromism. Spectroelectrochemical properties are shown for both oxidation and reduction. Optical switching is demonstrated and it is shown that response times depended on the side chain nature. Fast response times were detected for CDTDOP with alkyloxy side chains. Photovoltaic (PV) devices made of blends using the copolymers and a fullerene derivative, 16,61-phenyl-Cm-butyric acid methyl ester (1601PCBM), exhibited different performances. PV devices with an active layer containing CDTDP exhibited higher power efficiency. (C) 2017 Elsevier Ltd. All rights reserved.