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

Abstract

The study of π-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 π-conjugated oligomers consisting of furan (F) and thiophene (T) subunits to afford homomeric (Pyr2F3 and Pyr2T3) and heteromeric (Pyr2F2T and Pyr2T2F) 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 (Pyr2F3 and Pyr2F2T) gave rise to larger solution-state quantum yields and optical band gaps. Oligomers possessing a central furan subunit (Pyr2F3 and Pyr2T2F), on the other hand, were found to be nearly nonemissive in the solid state, which is attributed to nonradiative decay likely caused by π–π stacking interactions. Unlike the Pyr2T2F hybrid oligomer, Pyr2F2T 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 Pyr2F3 and Pyr2F2T while those of Pyr2T2F and Pyr2T3 are the lowest among the oligomers considered here (i.e., Pyr2F3 > Pyr2F2T > Pyr2T2F > Pyr2T3). These results suggest that hybrid furan–thiophene oligomers—like that of Pyr2F2T—could serve as viable building blocks for optoelectronic devices, while possessing the positive attributes of both individual heterocycles in a synergistic manner.