Donor‐Substituted Perethynylated Dehydroannulenes and Radiaannulenes: Acetylenic Carbon Sheets Featuring Intense Intramolecular Charge Transfer

Abstract

In this article, we report the preparation of unprecedented p-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 À 0.83 V in THF (vs. Fc / Fc), making it a better electron acceptor than buckminsterfullerene C 60 under comparable conditions.

Conjugation and optoelectronic properties of acetylenic scaffolds and charge-transfer chromophores

“…6) [63]. In contrast to the oxidative couplings of cis-bisdeprotected TEEs, as described above [48][49][50][51], the formation of cyclic dimers was not observed. Apparently, annellation of the five-membered TTF rings prevents the large distortion of the inner C(sp)-C=C bond angles that is necessary for formation of the strained cyclic dimer, as revealed by X-ray crystallographic analysis of per(silylethynyl)ated octadehydro [12]annulene [48,49].…”

“…Over the past decade, we applied TEE building blocks to the construction of novel families of acetylenic macrocycles such as perethynylated dehydroannulenes [48][49][50][51], expanded radialenes [49,[52][53][54], and radiaannulenes combining the structural features of both dehydroannulenes and expanded radialenes [51,55]. With their extended π-chromophores, a number of representatives feature interesting optoelectronic properties, such as high third-order optical nonlinearities [53,54].…”

“…5) [48,49]. Later, the terminally N,N-dimethylanilino (DMA)-substituted derivatives 9b and 10b were synthesized [50,51]. Three beneficial effects were obtained upon introduction of the peripheral N,N-dialkylanilino electron-donating groups:…”

Advanced opto-electronics materials by fullerene and acetylene scaffolding

“…A recently developed simple photochemical route now enabled us to prepare and study derivatives 6 and 7, bearing lateral DMA substituents (Fig. 5) [69,70].…”

“…By combining the structural features of expanded dehydroannulenes and expanded radialenes, a new class of planar, highly conjugated all-carbon macrocycles (11a-c, 12a,b), which we christened "radiaannulenes", was recently prepared (Fig. 7) [67,70]. Compound 11a was characterized by X-ray crystallography as a planar, substantially strained compound.…”