α,α′-Diarylacenaphtho[1,2-c]phosphole P-Oxides: Divergent Synthesis and Application to Cathode Buffer Layers in Organic Photovoltaics

A divergent method for the synthesis of a,a'-diarylacenaphtho[1,2-c]phosphole P-oxides has been established; a,a-dibromoacenaphtho[c]phosphole P-oxide, which was prepared through a TiII-mediated cyclization of 1,8-bis(trimethylsilylethynyl)naphthalene, underwent a Stille coupling with three different kinds of aryltributylstannanes to afford the a,a'-diarylacenaphtho[c]phosphole P-oxides in moderate to good yields. X-ray crystallographic analyses and UV/Vis absorption/fluorescence measurements have revealed that the degree of p-conjugation, the packing motif, the electron-accepting ability, and the thermal stability of the acenaphtho[c]phosphole p-systems are finely tunable with the a-aryl substituents. All the P?O and P?S derivatives exhibited high stability in their electrochemically reduced state. To use this class of arene-fused phosphole p-systems as n-type semiconducting materials, we evaluated device performances of the bulk heterojunction organic photovoltaics (OPV) that consist of poly(3-hexylthiophene), an indene-C70 bisadduct, and a cathode buffer layer. The insertion of the diarylacenaphtho[c]phosphole P-oxides as the buffer layer was found to improve the power conversion efficiency of the polymer-based OPV devices.