Ultrafast Energy Transfer in Fully Conjugated Thiophene-Benzothiadiazole Capped Poly(Phenylene Ethynylene) Molecular Wires

Energy transfer was explored in a series of poly(phenylene ethynylene) (PPE) polymers with increasing lengths that were end-capped with thiophene-benzothiadiazole (TBT) groups to form fully conjugated donor-acceptor polymers. For the polymers in solution, ultrafast transient absorption and time-resolved fluorescence spectroscopy reveal the majority of energy transfer from the PPE backbone to the TBT end-caps occurs rapidly, with subpicosecond and picosecond kinetics, and the rates are independent of the size of the polymer. The quantum efficiency for energy transfer was inversely proportional to the length of the polymers. Despite the highly conjugated structure, intrachain exciton hopping along the PPE backbone is significantly slower than the rate of radiative decay. The presence of an additional deleterious decay pathway for PPE excited state was also uncovered for the TBT-capped polymers. The photophysical phenomena are viewed in the context of the positioning of molecular orbitals, conjugation, and torsional effects, as determined by density functional theory calculations. The results of the photophysical characterization of this structurally well-defined system provide critical insight into the design of highly conjugated light-harvesting systems and their behavior as "molecular wires."