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 Ti II ‐mediated cyclization of the corresponding dialkynylated bithiophene derivatives as a key step. Each σ 3 ‐phosphorus center of the benzo[ c ]phosphole subunits was readily transformed into σ 4 ‐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 S N Ar 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 σ 3 ‐P derivatives and σ 4 ‐P‐AuCl complexes were found to emit fluorescence in the orange–red region, and the σ 4 ‐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 Alq 3 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.