Interfacial packing and optical organization in <scp>Langmuir–Schaefer</scp> fullerene/polythiophene blend films

Abstract Langmuir–Schaefer (LS) films based on conjugated polymers and fullerene derivatives have attracted considerable interest due to the strong influence of molecular organization on their optical and electronic properties. In this work, LS films based on the fullerene derivative oligo[(phenyl‐C61‐butyric acid methyl ester)‐ alt ‐2,5‐bis(octyloxybenzene)] (OPCBMMB), the polythiophenes poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) and poly(3‐octylthiophene‐2,5‐diyl) (P3OT) and their respective blend systems were investigated through surface pressure–area ( π – A ) isotherms and UV–visible absorption spectroscopy in order to evaluate the relationship between interfacial packing and optical organization. The π – A isotherms demonstrated that all systems are capable of forming organized Langmuir monolayers at the air–water interface, exhibiting liquid‐condensed behavior during compression. The blend monolayers showed shifts in the isotherms and changes in the mean molecular area in comparison to the pristine materials, indicating modifications in the intermolecular packing induced by the incorporation of the polythiophenes into the fullerene‐based systems. Differences between the P3HT‐ and P3OT‐based films suggest that the alkyl side‐chain length influences the molecular arrangement of the monolayers. The UV–visible absorption spectra revealed broadened absorption bands and redshifts in the LS films relative to the corresponding solutions, indicating intermolecular interactions and structural rearrangements during film formation. In the blend systems, the optical differences between P3HT and P3OT became less pronounced after the incorporation of OPCBMMB, suggesting that the fullerene derivative influences the packing and optical response of the polythiophenes within the mixed films. Overall, the combined analysis of the interfacial and optical results suggests that the molecular arrangement established during Langmuir monolayer compression may directly influence the optical organization of the transferred LS films. © 2026 Society of Chemical Industry.