Alkyl chain engineering in the hybrid bithiophene-3,4-ethylenedioxythiophene: Synthesis, electronic properties, and electropolymerization

• Five alkyl functionalized bithiophene–EDOT hybrid oligomers were synthesized.
• The alkyl affected the electronic properties of the oligomers and its polymers.
• The facile construction of bithiophene structure becomes a reality.
• The polymers show good stability and favorable electrochemical activity.
• The polymers are promising electrochromic materials.

Most recently, bithiophene unit has became a good choice as monomer unit for alternating π-conjugated copolymers for optoelectronic materials, because it gives good charge mobility to the corresponding polymers and serves as an appropriate donor unit in donor–acceptor polymers with narrow energy band gap. Herein, in order to get a comprehensively understanding of the structure–property relationship of the bithiophene based comonomers and copolymers, five hybrid oligomers which employed alkyl (CnH2n + 1, n = 0, 1, 6, 8, 12) functionalized bithiophene unit as the core and 3,4-ethylenedioxythiophene unit as the terminals were synthesized. Structure–property relationships of these oligomers and the corresponding polymers were analyzed by optical, electrochemical techniques, theoretical calculations, and spectroelectrochemistry. It was found that as the pendant alkyl length increased from the H to the hexyl, the electronic spectra of these oligomers were blue-shifted while the fluorescence quantum yield reduced and the oxidation peaks were positively shifted. When the number of carbon atoms in the linear alkyl chain was higher than six, these values of them became almost stable. Density functional theory calculations confirmed these trends by comparing with the dihedral angles of each oligomer because of the steric effect caused by alkyl substitution. And the steric hindrance of the alkyl substitution still remains in the polymer films although the effects are very slight. These results clearly indicated that by the simple alky chain engineering, the optoelectronic and electrochemical properties of the oligo-/polythiophenes would be tuned due to the steric hindrance of the alkyl substituent. The hybrid polymers with band gap range from 1.75 to 1.83 eV and electrochromic nature with color changing from violet and purplish in the reduced form to transparent sky blue/green upon oxidation. Combining with good stability and favorable electrochemical activity, these materials hold promise for electrochromic devices and display applications.