Excited State Electronic Structure and Dynamics in Diblock pi-Conjugated Oligomers

The excited state electronic structure and ultrafast energy transfer in two diblock oligomers F3-TBT and F3-mP-TBT was explored (F3 = tri-(9,9-dioctylfluorene), TBT = 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, mP = meta-phenylene). These diblock oligomers feature strongly coupled, pi-conjugated F3 and TBT segments that have different bandgaps. In F3-TBT, the two conjugated chromophores are directly linked whereas in F3-mP-TBT the chromophores are spaced by a meta-linked phenylene unit. The oligomers excited state dynamics were probed by steady-state UV-visible absorption, fluorescence and excitation wavelength dependent ultrafast time resolved transient absorption (TA) spectroscopy. Density functional theory (DFT) calculations were applied to provide insight concerning the electronic structure of the oligomers. The diblock oligomers feature distinct absorption bands due to transitions mainly localized on the F3 (high energy) and TBT (low energy) segments. Time-dependent DFT calculations support the assignments of the electronic transitions. Ultrafast TA spectroscopy with excitation corresponding to the low energy TBT unit reveals that the initial excitation is localized on TBT and does not vary with time. By contrast, excitation corresponding to high energy F3 produces an excited state that evolves rapidly (200 300 fs) into the lower energy state that is localized on the TBT chromophore. The ultrafast process is attributed to relaxation of an excited state that is predominantly localized on F3 into a lower energy state that is localized on TBT. The extent of delocalization of the initial state and the rate of relaxation depends on the extent of electronic coupling between the F3 and TBT segments.