Abstract In this work, phenanthrene centered donor-acceptor monomer; 5,10-bis(4-hexyl-2-thienyl)phenantro[9,10-c][1,2,5]thiodiazole; MES16 was synthesized and then its electrochromic application of corresponding polymer was carried out. 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (TST) was also synthesized as a standard molecule to determine the effect of the planar phenanthrene center on electrochromic performance. Electrochemical properties were elucidated by cyclic voltammetry and then HOMO-LUMO band gaps were calculated by using their oxidation and reduction onset potentials. Finally, electrochromic properties of poly-(MES16) and poly-(TST) were investigated via spectro-electrochemical measurements. DFT calculations exhibit that poly-(MES16) has a high planarity and an intramolecular charge transfer from the polymer backbone-donor to thiadiazole-acceptor subunit. Because of high solid state π−π interactions of the poly-(MES16), a good quality film was obtained corresponding to self-assembly property planar phenantrene center. Owing to this property, electrochromic performance of poly-(MES16) such as response time, stability and ΔT% were better than the standard poly-(TST).