Characterization of Furan- and Thiophene-containing Bispyridyl Oligomers via Spectroscopic, Electrochemical, and TD-DFT Methods

The study of pi-conjugated oligomers has garnered significant interest because of their use in organic optoelectronic devices, such as organic light-emitting diodes or organic field-effect transistors. Herein, we varied the inner heterocyclic units of pyridyl (Pyr)-capped pi-conjugated oligomers consisting of furan (F) and thiophene (T) subunits to afford homomeric (Pyr(2)F(3) and Pyr(2)T(3)) and heteromeric (Pyr(2)F(2)T and Pyr(2)T(2)F) molecules as applicable semiconducting building blocks. The oligomers were synthesized, and their solution- and solid-state spectroscopic properties were characterized. Compared to their thiophene congeners, oligomers with furans directly attached to the pyridyl moieties (Pyr(2)F(3) and Pyr(2)F(2)T) gave rise to larger solution-state quantum yields and optical band gaps. Oligomers possessing a central furan subunit (Pyr(2)F(3) and Pyr(2)T(2)F), on the other hand, were found to be nearly nonemissive in the solid state, which is attributed to nonradiative decay likely caused by pi-pi stacking interactions. Unlike the Pyr(2)T(2)F hybrid oligomer, Pyr(2)F(2)T exhibited not only a comparatively high solution-state quantum yield (7%) but also the brightest solid-state quantum yield emission (5%) and photostability (98%) when evaluated under ambient conditions. Density functional theory (DFT) computations support these trends, indicating that the largest HOMO-LUMO energy gaps and optical band gaps are possessed by Pyr(2)F(3) and Pyr(2)F(2)T while those of Pyr(2)T(2)F and Pyr(2)T(3) are the lowest among the oligomers considered here (i.e., Pyr(2)F(3) > Pyr(2)F(2)T > Pyr(2)T(2)F > Pyr(2)T(3)). These results suggest that hybrid furan-thiophene oligomers-like that of Pyr(2)F(2)T-could serve as viable building blocks for optoelectronic devices, while possessing the positive attributes of both individual heterocycles in a synergistic manner.