The
aromaticity and electronic properties of acetylene-bridged
hexadehydrotristhiopheno[12]annulenes (HDTAs) were revisited
using a combined experimental and theoretical approach. Moreover,
we attempted the synthesis of the butadiyne-bridged octadehydrobisthiopheno[12]annulenes
(ODTAs). While the formation of ODTAs was
indicated by NMR spectroscopy, mass spectrometry, and UV–vis
absorption measurements, our attempts to isolate ODTAs were unsuccessful on account of its instability. Instead, their
structure and energetic properties were predicted using DFT calculations. HDTA isomers in which the position where the thiophene rings
are fused to the 12-membered ring differs (b- vs c-position) show distinct differences in their HOMO–LUMO
energy gaps (E
Gap). ODTAs
also show large E
Gap differences depending
on the fusion position of the thiophene rings. The diene character
of the thiophene ring significantly changes the electronic properties;
i.e., E
Gap differences of >1 eV were
observed
between the isomers of both HDTAs and ODTAs. A theoretical evaluation of HDTAs and ODTAs revealed significant variation in the local aromaticity/antiaromaticity
between the b- and c-isomers. The
antiaromatic character of the 12-membered ring is attenuated for the b-isomers, whereas it is decreased substantially for the c-isomers. The results of this study are useful for a detailed
understanding of the fundamental aspects of dehydrothiopheno[12]annulenes.
Linear π-conjugated polycyclic compounds, BBCTs, containing linearly annulated 5-, 4-, 6-, 4-, and 5-membered rings were produced via copper-mediated double intramolecular coupling reactions.
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