Copolymers based on fluorene dyads mixed to 3,4-ethylenedioxythiophene units: Optical properties of random versus regular structures

We report on the synthesis of two copolymers containing both ethylenedioxyhiophene (EDOT) derivatives and dihexylfluorene (DHF) units. In one hand, a set of copolymers was obtained by dehalogenating Yamamoto coupling from a feed solution of both monomers. This route leads to materials with a random composition of each monomer. The substitution by a tetradecyloxy chains on the dioxyethylene ring ensures a good solubility of the copolymer when increasing the proportion of the EDOT units in the structure. On the other hand, the second polymer was synthesized by Suzuki cross-coupling, the resulting structure being a fully alternated of one EDOT and two DHF. All polymers were characterised by steric exclusion chromatography, indicating in all cases that small polymeric chains are obtained. FT-IR and 1H NMR spectroscopy were investigated in order to solve the structure and to determine the EDOT/fluorene ratio of the random materials. The electronic and optical properties were investigated by UV-vis spectroscopy and Photoluminescence experiments. The role of EDOT units on the electronic structure of the copolymers has been established; mainly the electron-rich EDOT ring increases the HOMO energetic level. This observation is confirmed by cyclic voltamperometry experiments. This trend is also observed within a regular polymeric sequence. While the optical properties are finely controlled in this case, the photoluminescent properties are drastically hindered by the π-stacking that results from the highly ordered material during the cast process. These results confirm our choice to built electroluminescent devices with the random copolymers as active layers. Green light emission is obtained with moderate emission.