Macrocyclic Tetraethynylethene Molecular Scaffolding: Perethynylated aromatic dodecadehydro[18]annulenes, antiaromatic octadehydro[12]annulenes, and expanded radialenes

Abstract

Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) molecular scaffolding provided access to novel macrocyclic nanometer-sized C-rich molecules with unusual structural and electronic properties. Starting from cis-bisdeprotected cis -bis(trialkylsilyl)-protected tetraethynylethenes, the per(silylethyny1)ated octadehydro[ 12lannul-enes 1 and 2 and the corresponding dodecadehydro[l8]annulenes 4 and 5 were prepared by oxidative Huy coupling.X-Ray crystal-structure analyses of (i-Pr)$i-protected 2 and Me,Si-protected 4 showed that both annulene perimeters are perfectly planar. Electronic absorption spectral comparisons provided strong evidence that the macro rings in the deep-purple-colored 1 and 2 are antiaromatic (4n n-electrons), whereas those in yellow 4 and 5 are aromatic ((4n + 2) n-electrons). Although unstable in solution, the antiaromatic compound 2 gave high-melting crystals in which the individual octadehydro[l2]annulene chromophores are isolated and stabilized in a matrix-type environment formed by the bulky (i-Pr)$i groups. Electrochemical studies demonstrated that the antiaromatic octadehydro[ 12lannulene 2 undergoes two stepwise one-electron reductions more readily than the aromatic chromophore 5. This redox behavior is best explained by the formation of an aromatic (4n + 2) n-electron dianion from 2, whereas 5 loses its aromaticity upon reduction. The Me$i derivative 4 was deprotected with borax in MeOH/THF to give the highly unstable hexaethynyl-dodecadehydro[ 1 Blannulene 6, a C,oH6 isomer and macrocyclic precursor to a two-dimensional all-C-network. Deprotection of 2 did not give isolable amounts of tetraethynyl-octadehydro[l2]annulene 3 due to the extreme instability of the latter. Starting from dirneric and trimeric acyclic tetraethynylethene oligomers, a series of expanded radialenes were obtained. They possess large C-cores with silylethynyl-protected peripheral valences and can be viewed as persilylated C,, (7), C,, (S), and C60 (9) isomers. These expanded C-sheets are high-melting, highly stable, soluble materials which were readily characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry. Due to inefficient macrocyclic cross-conjugation and/or non-planarity, the extent of rr-electron delocalization in 7-9 is limited to the longest linearly conjugated n-electron fragment. In agreement with these properties, all three expanded radialenes exhibited similar redox behavior; they are difficult to oxidize but undergo several reversible one-electron reductions in similar potential ranges. Presumably, the reduced n-electron delocalization is also at the origin of the particularly high stability of 7-9.

Spectroscopic properties of macrocyclic oligo(phenyldiacetylenes)-II. Synthesis and theoretical study of diacetylenic dehydrobenzoannulene derivatives with weak electron-donor and -acceptor groups

“…Regarding their highly conjugated structure, these compounds have intensively been studied [8][9][10][11][12][13][14] as promising candidates for innovative electronic and photonic materials [15]. The donor/ acceptor dehydrobenzo [18]annulenes attracted special interest given that this kind of push-pull systems is good candidates for use in non-linear optics [16,17].…”

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].…”

Functional acetylenic molecular architecture

“…Both annulene perimeters are perfectly planar according to X-ray crystal structure analysis [14]. The buta-1,3-diynediyl moieties in 5a/b are considerably bent with CϵC-C angles as low as 164.5Њ, whereas those *Lecture presented at the 9th International Symposium on Novel Aromatic Compounds (ISNA-9), Hong Kong, China, 2-7 August 1998, pp.…”