Donor‐Substituted Perethynylated Dehydroannulenes and Radiaannulenes: Acetylenic Carbon Sheets Featuring Intense Intramolecular Charge Transfer
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In this article, we report the preparation of unprecedented π-conjugated macrocycles (Fig. 1) by acetylenic scaffolding using modular tetraethynylethene (TEE, 3,4-diethynylhex-3-ene-1,5-diyne) building blocks. A novel photochemical access to (Z)-bisdeprotected TEEs (Scheme 1) enabled the synthesis of the anilino-substituted perethynylated octadehydro[12]- (5) and dodecadehydro[18]annulenes (6) (Scheme 2). Following the serendipitous discovery of perethynylated radiaannulenes (Scheme 3) that can be viewed as hybrids between perethynylated dehydroannulenes and expanded radialenes, two series of monocyclic (7–9; Scheme 6) and bicyclic (10 and 11; Scheme 7) representatives were prepared. Substantial strain in the macrocyclic perimeter of radiaannulene 7 was revealed by X-ray crystal-structure analysis (Fig. 2). Nevertheless, mono- and bicyclic radiaannulenes are stable at room temperature in air for months. The opto-electronic properties of both dehydroannulenes and radiaannulenes are substantially enhanced by the introduction of the peripheral anilino donor groups that undergo strong intramolecular charge-transfer interactions with the electron-accepting all-C cores. As a result, the UV/VIS spectra feature intense, bathochromically shifted charge-transfer bands that disappear upon protonation of the anilino moieties and are fully recovered upon neutralization (Figs. 4–9). A comparison between anilino-substituted perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes revealed that the efficiency of the intramolecular charge-transfer interaction strongly depends on the structure of the electron-accepting all-C perimeter. Electrochemical investigations (Table) demonstrated that the radiaannulenes are particularly powerful electron acceptors. Thus, bicyclic radiaannulene 11, which possesses eight peripheral 3,5-di(tert-butyl)phenyl substituents, is reversibly reduced at −0.83 V in THF (vs. Fc+/Fc), making it a better electron acceptor than buckminsterfullerene C60 under comparable conditions.