Excited state electronic structure and dynamics in diblock π-conjugated oligomers

Abstract

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, π-conjugated F3 and TBT segments that have different bandgaps. F3-TBT is a fully π-conjugated chromophore in which the two conjugated segments are directly linked. By contrast, in F3-mP-TBT the chromophores are separated by a meta-linked phenylene unit. The objective of this study was to discern whether the Franck-Condon excited state produced by selective excitation of the F3 moiety in fully conjugated F3-TBT gives rise to a localized or fully delocalized excited state. 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, 3.5 eV) and TBT (low energy, 2.75 eV) segments. 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, in both oligomers 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 observation of an F3-localized excited state is unexpected for F3-TBT, given that the molecule is fully π-conjugated.