Synthesis of Benzo[1,2-b:4,3-b']dithiophene/triphenylamine Copolymers: Effects of Substituents on Optical and EL Properties

Benzo[1,2-b:4,3-b']dithiophene/triphenylamine copolymers P-2eh (with no methyl substituents) and P-Me and P-2Me (with methyl substituents at triphenylamine sites) were synthesized by Stille coupling reactions. In the ground state, the methyl substituents at the ortho positions to the BDT unit on the triphenylamine (TPA) moiety in P-Me and P-2Me caused twisting of the polymer structure and clearly limited conjugation of the polymer backbones. However, in their excited states, all of these copolymers were shown to have planar structures, and almost the same emission maxima were observed. This result indicates that, in the excited states, π-conjugation of the polymer backbone results in the adoption of a more stable planar structure rather than the twisted structure observed in the ground state. The maximum EL efficiencies of devices based on P-Me and P-2Me were about three times higher than that of P-2eh due to restricted π-stacking/aggregation of the conjugated copolymers in the solid state and improvements in thermal and electrochemical stability.