Design and optoelectronic profiling of a semicrystalline thiophene–phenylene copolymer for advanced organic electronics

The copolymer poly[1,4‐bis(thienyl)‐2,5‐bis(2‐butoxyethoxy)benzene] (PTBT‐3OBu) was synthesized by monomer functionalization (alkylation and bromination) followed by Pd‐catalyzed Stille polycondensation with 5,5′‐bis(tributylstannyl)‐2,2′‐bithiophene. Unlike previously reported thienylene–phenylene copolymers based on shorter repeat units, PTBT‐3OBu incorporates an additional thiophene unit through a distannylated bithiophene comonomer, generating an extended thiophene–phenylene–thiophene conjugated backbone. This structural design was employed to investigate the impact of backbone extension on intermolecular aggregation and solid‐state optical response. The copolymer was characterized using thermogravimetric analysis, gel permeation chromatography, NMR spectroscopy, X‐ray diffraction (XRD), and electrochemical analyses. PTBT‐3OBu exhibited excellent solubility in organic solvents and thermal stability up to 350 °C. XRD revealed a predominantly amorphous pattern with a diffuse halo at 2 θ ≈ 22.7° and a weaker reflection at 2 θ ≈ 5.8°, corresponding to lamellar stacking and a crystalline fraction of approximately 27%. Optical spectra showed pronounced red shifts in both absorption and emission upon film formation, indicating enhanced interchain coupling and partial molecular ordering. Electron–phonon coupling analysis was performed based on Fermi's Golden Rule and molecular exciton scope. Cyclic voltammetry evidenced reversible redox behavior and an electrochemical band gap of 2.16 eV. The synergy between backbone planarity, semicrystalline morphology, and efficient π – π interactions endows PTBT‐3OBu with promising optoelectronic properties, making it a valuable candidate for use in organic photovoltaic and electrochromic devices.