Büchner-type ring expansion of aromatic main-group biradicaloids toward phosphorus radical-derived NIR-II photothermal materials

The Büchner reaction enables the construction of functionalized cycloheptatrienes and aromatic tropylium cations, and has recently been extended to the ring expansion of arenes with low-valent main-group systems. Although stable 6π-aromatic biradicaloids typically exhibit characteristic biradical reactivity, we leverage the aromaticity of dicarbondiphosphides (1) to achieve an unprecedented Büchner-type ring expansion, yielding air-stable aromatic 1,3-diphospholium cations (3). Multimodal characterization (NMR, UV-vis, SC-XRD, computational studies) confirms the 6π-aromaticity of 3. Reduction of 3 yields stable 7π-electron neutral radicals (4), which contrast with conventional five-membered neutral radicals possessing 5π-electron systems. These radicals (4a: <i>λ</i> _max = 1113.2 nm; 4b: <i>λ</i> _max = 1330.7 nm) exhibit strong second near-infrared biowindow (NIR-II, 1000-1350 nm) absorptions, and shown promising application potential as the first phosphorus radical-derived organic NIR-II photothermal materials.