Structure–Property Relationship of Ester-Functionalized and Hydrolyzed ProDOT Polymers for Electrochromic Applications

This work investigates how post-synthetic hydrolysis of the ester-containing 2-ethylhexyl ester (EHE) side chains of PProDOT(EHE) affects the surface composition, electronic structure, and electrochromic properties of the hydroxylated derivative PProDOT(OH). Structural and surface analyses confirmed that hydrolysis modifies the side-chain functionality while preserving the ProDOT-based conjugated backbone. XPS analysis showed changes in the sulfur chemical environment after hydrolysis, including a decrease in neutral thiophene sulfur and an increase in the high-binding-energy sulfur component, which may be related to surface oxidation, residual post-treatment species, or fitting-related effects. Tauc analysis showed that the optical band gap decreases from 1.64 to 1.43 eV after hydrolysis. UPS analysis revealed a decrease in the work function and photoemission onset energy from 4.70 and 1.20 eV to 4.31 and 0.88 eV, respectively, with HOMO/LUMO levels shifting from −5.90/−4.26 eV to −5.19/−3.76 eV. These changes indicate easier oxidation of the hydrolyzed polymer. Spectroelectrochemical measurements at −2.0/+2.0 V showed reversible electrochromic behavior, with decreased visible π–π* absorption and increased long-wavelength absorption upon oxidation. Cyclic voltammetry at 25–500 mV s-1 confirmed the retained redox activity of both polymers, although PProDOT-OH showed a lower current response. At the device level, hydrolysis accelerates switching, with Tc/Tb values of 0.28/0.25 s compared with 0.60/0.35 s for PProDOT-EHE but decreases optical contrast and coloration efficiency from 61.99% and 872.2 cm2 C-1 to 43.41% and 599 cm2 C-1, respectively. Therefore, hydrolysis provides an effective route for tuning the balance between switching speed, optical contrast, and electronic structure in ProDOT-based electrochromic polymers.