Two-dimensional acetylenic scaffolding: extended donor-substituted perethynylated dehydroannulenes

Starting from (Z)-bis(N,N-diisopropylanilino)-substituted tetraethynylethene (TEE), perethynylated octadehydro[12]- and dodecadehydro[18]annulenes were prepared by oxidative Hay coupling. The dodecadehydro[18]annulene with six peripheral N,N-diisopropylanilino substituents was characterized by X-ray crystallography. Elongation of the Z-bisdeprotected TEE by Cadiot-Chodkiewicz coupling with 1-bromo-2-(triisopropylsilyl)ethyne provided a Z-configured bis(butadiyne), which after alkyne deprotection afforded under Hay coupling conditions N,N-diisopropylanilino-substituted perethynylated hexadecadehydro[20]- and tetracosadehydro[30]annulenes. The diisopropylanilino substituents enhance the properties of these unprecedented all-carbon perimeters in several distinct ways. They ensure their solubility, increase their stability, and importantly, engage in strong intramolecular charge-transfer interactions with the electron-accepting all-carbon cores, resulting in intense, bathochromically shifted charge-transfer bands in the UV/Vis spectra. The charge-transfer character of these bands was confirmed by protonation-neutralization experiments. The redox properties of the new carbon-rich chromophores were investigated by cyclic voltammetry and rotating disk voltammetry, which indicated different redox behavior for aromatic (4n+2 pi electrons) and antiaromatic (4n pi electrons) dehydroannulenes.