Modulating Exciton Dynamics Through Fluorescent Side Group Incorporation in Benzodithiophene-Benzotriazole-Isoindigo Terpolymers

In this work, we investigated the incorporation of a fluorescent side group, fluorescein octyl ester (FOE), in benzodithiophene-based donor–acceptor terpolymers as a strategy to modulate excited-state behavior. Three FOE-containing terpolymers (P2-iIa-c), obtained at different polymerization times, were systematically evaluated against an analogous material without the fluorescent pendant unit (P1-iI). Thermal analysis revealed good thermal stability and an increase in glass transition temperature upon FOE incorporation, suggesting restricted segmental mobility and increased conformational constraints within the conjugated backbone. Optical characterization showed distinct absorption spectra with reaction time and shorter fluorescence lifetimes for the FOE-containing materials, consistent with the presence of additional excited-state deactivation pathways and intramolecular energy transfer processes within the terpolymer backbone. An approximate estimation of energy transfer efficiencies (≈60–65%) suggested that such processes may be operative within the system. Cyclic voltammetry measurements showed only minor variations in HOMO and LUMO energy levels between P1-iI and P2-iIa-c series, indicating that the conjugated backbone predominantly determined the frontier orbital energies despite side chain modification. Furthermore, photocurrent measurements from the bilayer device configuration exhibited a systematic increase in photocurrent for the FOE-containing material, supporting the role of excitonic modulation, rather than significant changes in interfacial energetic alignment. These results suggest that fluorescent side chain incorporation provides an effective strategy for regulating exciton dynamics while maintaining the electronic structure of the donor–acceptor terpolymer.