Synthesis of 1,4-Diethynyl- and 1,1,4,4-Tetraethynylbutatrienes

From Stilbenes to carbo-Stilbenes: an Encouraging Prospect

Beyond previously described carbo-naphthalene and carbo-biphenyl, a novel type of bis-carbo-benzenic molecules is envisaged from the stilbene parent. The synthesis, structure, spectroscopic and electrochemical properties of two such carbo-stilbenes are described at complementary experimental and computational DFT levels. In the selected targets, the bare skeletal carbo-mer of carbo-stilbene is decorated by 8 or 10 phenyl groups, 0 or 2 tert-butyl groups, and 2 n-octyl chains, the later substituents being introduced to compensate anticipated solubility issues. As in the parent stilbene series, isomers of the phenylated carbo-stilbenes are characterized. The cis- and trans-isomers are, however, formed in almost equal amounts and could not be separated by either chromatography or crystallization. Nevertheless, due to a slow interconversion at the NMR time scale (up to 55 °C) the 1H NMR signals of both isomers of the two carbo-stilbenes could be tentatively assigned. The calculated structure of the cis-isomer exhibits a helical shape, consistent with the observed magnetic shielding of phenyl p-CH nuclei residing inside the shielding cone of the facing C18 ring. The presence of the two isomers in solution also gives rise to quite broad UV-vis absorption spectra with main bands at ca 460, 560 and 710 nm, and a significant bathochromic shift for the decaphenylated carbo-stilbene vs the di-tert-butyl-octaphenylated counterpart. Square wave voltammograms do not show any resolution of the two isomers, giving a reversible reduction wave at -0.65 or -0.58 V/SCE, and an irreversible oxidation peak at 1.11 V/SCE, those values being classical for most carbo-benzene derivatives. Calculated NICS values (NICS(1)=-12.5±0.2 ppm) also indicate that the aromatic nature of the C18 rings is not markedly affected by the dialkynylbutatriene (DAB) connector between them.

First example of ring carbomer of 1,4-cyclohexadiene

While a series of carbo-mers of 1,3-cyclohexadienes was reported through the use of a specifically developed synthetic strategy, no example of their 1,4-regioisomers was known. Inspired by the methodology elaborated for the preparation of the 1,3-isomers, the synthesis of the first example of carbo-mer of 1,4-cyclohexadiene is presented. Comparison of physico-chemical properties of this first representative with those of the recently described 1,4-regioisomer, especially UV-vis absorption properties, is also addressed.

“Carbo-aromaticity” and novel carbo-aromatic compounds

Recent advances in experimental and theoretical studies ofcarbo-benzene derivatives, along with the proposition of a generalization of the definition of aromaticity to the two-membered π-rings of triple bonds, suggest relevance for the notion of “carbo-aromaticity”.

Synthesis and properties of long [n]cumulenes (n ≥ 5)

This review highlights the synthesis and reactivity of longer [n]cumulenes (n ≥ 5) and summarizes their electronic and structural properties.

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

Our group started a research program in acetylene chemistry in 1987; since then, an intense research effort led to a fascinating journey into acetylenic scaffolding, aimed at exploring conjugative and optoelectronic properties of acetylenic chromophores. This journey included the generation of a unique molecular construction kit for acetylenic scaffolding, consisting of (E)-1,2-diethynylethenes [DEEs, (E)-hex-3-ene-1,5-diynes], tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-1,5-diynes), chiral 1,3-diethynylallenes (DEAs, hepta-3,4-diene-1,6-diynes), 1,4-di and 1,1,4,4-tetraethynylbutatrienes, chiral trialkynylmethanes, and 1,1,2,2-tetraethynylethanes. These building modules were subsequently applied to the synthesis of carbon-rich architectures extending into one, two, and three dimensions. They include multinanometer-long monodisperse oligomers as models for infinite acetylenic polymers, molecular switches, perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes, and an octamethoxy-substituted expanded cubane with a central C56 core. Donor-substituted cyanoethynylethenes (CEEs) and 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) were introduced as new push-pull chromophores featuring intense intramolecular charge-transfer (CT) interactions. Dendritic multivalent CT chromophores were constructed using atom-economic, "click"-like reactions, and these systems were shown to behave as "molecular batteries", featuring exceptional electron uptake and storage capacity. The research finally led to the development of an unprecedented cascade reaction for the preparation of dendritic and oligomeric donor-acceptor (D-A) molecules. New [AB]-type oligomers become accessible in domino reactions involving repetitive sequences of [2+2] cycloadditions of tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF) to polyynes, followed by retro-electrocyclizations.

Property tuning in charge-transfer chromophores by systematic modulation of the spacer between donor and acceptor

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.

Butatrienes as Extended Alkenes: Barriers to Internal Rotation and Substitution Effects on the Stabilities of the Ground States and Transition States

The barriers to internal rotation of methylated, ethynylated, and vinylated butatrienes and alkenes were calculated at the CASPT2/6-31G(d)//B3LYP/6-31G(d) level. Calculated butatriene rotational barriers are lower than those of analogous alkenes, but there is a larger variance in rotational barrier for alkenes than for butatrienes. The barriers to rotation were analyzed by isodesmic equations designed to estimate the substituent effects in the ground (GS) and transition (TS) states individually. The GSs of both series are stabilized to roughly the same extent. In contrast, the TSs of butatrienes are more stabilized overall than those of alkenes. Much of the stabilization in the TS of butatrienes comes from the internal triple bond and not from the substituent. Estimation of the substituent stabilization alone reveals the TSs of ethylenes to be more stabilized by substitution than butatrienes.

Donor-Substituted Cyanoethynylethenes: π-Conjugation and Band-Gap Tuning in Strong Charge-Transfer Chromophores

An extensive series of silyl-protected cyanoethynylethenes (CEEs) and N,N-dimethylanilino donor-substituted CEEs have been synthesized. More extended chromophores were constructed by selective silyl deprotection and subsequent oxidative acetylenic coupling. The strong electron-accepting nature of the CEEs was revealed by a combination of 13C NMR spectroscopic and electrochemistry measurements. Donor-substituted CEEs display strong intramolecular charge-transfer (CT) character, resulting in intense, bathochromically shifted CT bands in the UV/Vis spectrum. Their structural diversity establishes them as suitable models for the study of pi-conjugation and band gap tuning in strong charge-transfer chromophores. The extent of pi-conjugation in the donor-substituted CEEs was investigated by a combination of ground-state techniques, such as X-ray crystallography, electrochemistry, B3 LYP calculations, and NMR spectroscopy. The comparison of these ground-state results with the features observed in the UV/Vis spectra reveals that-contrary to expectations-more extensive pi-conjugation can lead to larger band gaps in molecules with strong donor and acceptor moieties.