Tuning the Neutral State Color of Blue, Cyan and Green PI-Conjugated Copolymers By Jointly Playing the Acceptor Strength and the Steric Hindrance

Due to their electronic structure, most of pi-conjugated polymers present visible coloration in their neutral state, hence possess a characteristic absorbance in the 400-800nm spectrum. Upon p-doping, this absorption shifts toward longer wavelengths leading to electrochromic properties. Among the colors accessible in the visible palette, the primary cyan is somewhat particular to obtain because two absorption bands are necessary, one around 400nm and a second near 670nm. This can be obtained by synthesizing donor-acceptor (D-A) pi-conjugated copolymers. Moreover during the switching of the material, these two bands have to be concomitantly shifted toward the infrared.¹

In this presentation we will present the synthesis and characterizations of four pi-conjugated monomers having D-A-D structure (Scheme 1). The donating units are based on a fixed alkylated propylenedioxythiophenes and accepting units based on benzothiadiazole or quinoxaline units bearing variable substituents. The polymerization is conducted under mild conditions via oxidative coupling using iron III trichloride. The obtained copolymers have tunable optical properties that can be relied to the acceptor strength which control of D-A charge transfer (CT). The optical characterizations allow to extract the CIELAB colorimetric coordinates and experimental HOMO-LUMO energetic levels were obtained from electrochemical characterization. We also used computational chemistry (B3LYP and TD-B3LYP methods) to browse the structures of interesting candidates to achieve successful color control in D-A polymers using model compounds (monomers and trimers). Finally a color/structure relationships provides valuable insights on molecular design principles for this class of copolymers, putting forth the importance of jointly playing the steric hindrance factor and the electron-withdrawing/donating strength of carefully chosen substituents.