From Stilbenes to carbo‐Stilbenes: an Encouraging Prospect

Abstract

AbstractBeyond 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.