Unraveling the monomer conjugation length effect on the optoelectronic performances of thiophene-EDOT hybrid electrochromic polymers

The combination of 3,4-ethylenedioxyphene (EDOT) unit with other units can make the resultant hybrid conjugated monomer/polymer have some excellent optoelectronic properties. Herein, in this work, a series of novel thiophene-EDOT hybrid monomers (EDOT-T, EDOT-2T, and EDOT-3T) with simple oligothiophene units as the core and EDOT unit as the end groups were designed and synthesized by Stille coupling reaction, and the corresponding hybrid electrochromic polymer films (P(EDOT-T), P(EDOT-2T), and P(EDOT-3T)) were successfully obtained by potential cycling. Subsequently, the electrochemical properties and electrochromic properties of these three hybrid polymers were systematically studied in ACN-Bu₄NPF₆ system. The photophysical study shows that the increase of conjugated length of monomer will lead to the stepwise red-shifted absorption and fluorescence spectra, decreased quantum yield, and increased Stokes shift. Due to the stronger electron donating ability of the EDOT unit, all the hybrid monomers show a lower initial oxidation potential (≤0.6 V), which is beneficial to the preparation of high-quality electrochromic polymer films with fewer defects. The results show that as-formed polymers have good redox activity and excellent redox stability. Due to the increase of conjugation length, the obtained P(EDOT-3T) film has good electrochromic properties: the highest optical contrast is 44.3 %, the coloration efficiency is above 120 C⁻¹ cm², and the fastest response time is 0.58 s, indicating that P (EDOT-3T) is a very promising electrochromic material. This work also successfully proved that changing the different initial monomers was an effective tool to construct electroactive polymers with very significant different physicochemical and electrochromic properties.