Synthesis, Electronic Properties, and Reactivity of Phospholes and 1,1′-Biphospholes Bearing 2-or 3-Thienyl C-Substituents

Two series of phospholes and 1,1'-biphospholes bearing either 2- or 3-thienyl substituents at the C atoms are prepared by using the Fagan-Nugent route. Their optical (UV/Vis absorption, fluorescence spectra) and electrochemical properties are systematically evaluated. Of particular interest, the first ever reported 3-thienyl-substituted phospholes exhibit higher LUMO levels than their 2-thienyl analogues, and show accordingly different physical properties. This study also reveals that the 1,1'-biphospholes exhibit sigma-pi conjugation. The phosphole and 1,1'-biphosphole derivatives bearing 3-thienyl substituents are characterized by X-ray diffraction study. The structure-property relationship established following the experimental data are fully supported by theoretical studies including time-dependent(TD)-DFT spectra. A photocyclization reaction performed on the thioxo- and oxo-phospholes having 3-thienyl substituents affords a novel ring-fused phosphole-thiophene derivative, which was characterized by an X-ray diffraction study. The structure and electronic properties of this novel dithienophosphole are discussed based on experimental and theoretical data.