Chain-Growth Synthesis of Extensively Cross-Conjugated Polyenes via Strained [3]Cumulenes

For years, polyenes with an extensively cross-conjugated backbone have fascinated chemists by their unique opto-electronic properties and reactivities, but so far only short oligomers (≤ 12 vinylene units) are available through multi-step assembly of vinylic building blocks. Here, we report a single-step chain-growth approach that streamlines the synthesis of cross-conjugated polyenes with up to 86 consecutive vinylene units averaged per chain and well-defined end groups. This method highlights a key interaction between an organocopper species with a strained [3]cumulene, unlocking a previously unknown 2,3-polymerization pathway. The resulting polyenes display decent two-photon-absorption capacity without compromising visible transparency, facilitating visible range two-photon lithography with sub-diffraction-limit resolution.