Phosphole P-Oxide-Containing π-Electron Materials: Synthesis and Applications in Fluorescence Imaging

Phosphole P-oxide is a useful building block for π-conjugated materials due to its nonaromatic and electron-accepting character. We have synthesized a series of ring-fused derivatives of phosphole P-oxide based on the intramolecular nucleophilic cyclization of appropriate alkyne precursors or radical phosphanylations. Some of the thus obtained compounds exhibited intriguing fluorescence properties and were applied to fluorescence imaging. A donor-acceptor-type benzo[b]phosphole P-oxide with a (diphenylamino)phenyl group exhibited large solvatochromism in its fluorescence spectra, and could hence be used as a staining agent for lipid droplets. C-Naphox and PB430, which consist of fully ring-fused π-conjugated ladder-type scaffolds, exhibited outstanding photostability and their absorption and emission properties were suitable for super-resolution STED imaging. Moreover, using PB430-conjugated antibodies, we carried out a 3-D reconstruction of the STED images and developed a photostability-based multicolor STED imaging technique.