Comparative Study on the Structural, Optical, and Electrochemical Properties of Bithiophene-Fused Benzo[c]phospholes

Three types of bithiophene‐fused benzo[c]phospholes were successfully prepared by TiII‐mediated cyclization of the corresponding dialkynylated bithiophene derivatives as a key step. Each σ3‐phosphorus center of the benzo[c]phosphole subunits was readily transformed into σ4‐phosphorus center by Au coordination or oxygenation. In addition, the bithiophene subunit was functionalized at the α,α′‐carbon atoms by Pd‐catalyzed cross‐coupling reactions with heteroarylmetals and by an SNAr reaction with hexafluorobenzene. The experimentally observed results (NMR spectroscopy, X‐ray analysis, UV/Vis absorption/fluorescence spectroscopy, and cyclic/differential‐pulse voltammetry) have revealed that the structural, optical, and electrochemical properties of the bithiophene‐fused benzo[c]phospholes vary considerably depending on the π‐conjugation modes at the bithiophene subunits and the substituents of the heterocyclopentadiene components. The appropriately ring‐annulated σ3‐P derivatives and σ4‐P‐AuCl complexes were found to emit fluorescence in the orange–red region, and the σ4‐P‐oxo derivatives proved to undergo reversible one‐electron reduction at −1.4 to −1.8 V (vs ferrocene/ferrocenium). These results indicate that the bithiophene‐fused benzo[c]phospholes possess narrow HOMO–LUMO gaps and low‐lying LUMOs, which was confirmed by density functional theory calculations of their model compounds. The time‐of‐flight measurement of an ITO/benzo[c]phosphole/Al device showed that the electron mobility in the P‐oxo derivative is one‐order higher than that in Alq3 at low electric fields. The present study demonstrates that the arene‐fused benzo[c]phosphole skeleton could be a highly promising platform for the construction of a new class of phosphole‐based optoelectrochemical materials. New P,S‐hybrids: The design, preparation (see scheme), and characterization of three types of bithiophene‐fused benzo[c]phospholes are described. The structural, optical, and electrochemical properties of these compounds vary considerably depending on the π‐conjugation modes at the bithiophene subunits and the substituents of the heterole components.