Exciton structure and dynamics in π-conjugated molecular wires

Exciton and charge transport through π-conjugated oligomers and polymers is critical to the performance of organic electronic materials and essential for understanding carrier transport mechanisms on the nanoscale. This study applies photophysical methods to explore exciton structure and transport in π-conjugated molecules consisting of a diblock oligomer with terfluorene (F3) and tetrathiophene (T4) segments that are capped on either end with a boron dipyrromethene (BDP) unit. Two trichromophores were the focus of the investigation, F3T4BDP and T4F3BDP, where the sequence of the segments in the molecules is indicated by the acronyms. The molecules were probed by using steady-state and time-resolved absorption and fluorescence spectroscopy. Comparison of the absorption spectra of the trichromophores with model compounds (F3, T4, BDP) reveals that they exhibit absorption bands that can be assigned to transitions localized on the chromophore elements. Steady-state and time resolved fluorescence spectroscopy shows that excitation of the trichromophores leads to efficient transport of the excitation energy (exciton) to the BDP chromophore. By using femtosecond transient absorption (TA) spectroscopy it is shown that in F3T4BDP where the segments are arranged to allow vectorial exciton transfer (F3 → T4 → BDP) regardless of excitation wavelength exciton transfer to the BDP chromophore occurs in <1.5 ps. By contrast in T4F3BDP the energy landscape is more complex, and femtosecond TA results reveal that there is a bifurcation of the exciton between the T4 and BDP units, and the partitioning ratio depends on the excitation wavelength.