Theoretical Study on the Structures, Electronic Properties, and Aromaticity of Thiophene Analogues of Anti-Kekulene

Abstract

We predict the geometries, electronic properties, and aromaticity of thiophene analogues of anti-kekulene with six to nine thiophene rings 1–4, together with those of cyclobutadithiophenes (CDTs) and anti-kekulene as reference compounds, using density functional theory calculations. Investigation of the simplest reference compounds, CDTs, reveals that the local aromaticity of their thiophene rings is influenced by their fused position (b- or c-bond) to the four-membered ring (4MR). A thiophene ring fused at the b-position (b-TR) retains its aromatic character to some extent, whereas the aromatic character of one fused at the c-position is attenuated. The 4MR with two fused b-TRs retains a strong anti-aromatic character. Thiophene analogues of anti-kekulene with six to eight thiophene rings 1–3 favor bowl-shaped structures, in contrast to the planar structure of anti-kekulene, because of the shorter distances of the sulfur bridges. Compound 4, with nine thiophene rings, adopts a planar structure. The local aromaticity and anti-aromaticity of the thiophene ring and 4MR are significantly attenuated in 1–4 compared with the reference compounds, the CDTs and anti-kekulene. This can be attributed to the considerable contribution of the quinoidal electronic structure in 1–4. The present study provides new insight into the aromatic and electronic nature of systems containing cyclobutadienothiophene.

Linear π-conjugated polycyclic compounds consisting of four-, five-, and six-membered rings: benzo[1′′,2′′:3,4;4′′,5′′:3′,4′]bis(cyclobuta[1,2-c]thiophene)

“…The shared bonds show single-bond character to reduce the electron density in the 4MRs. 27,35 As the structural criterion for the local aromaticity, we performed the harmonic oscillator model of aromaticity (HOMA). 54 The HOMA value of the central benzene ring of BBCT is large (0.88, Fig.…”

“…BBCT will show moderate global aromaticity; the two fused 4MRs will attenuate the aromaticity of the central benzene ring and the fused thiophene ring at the 3,4-bond (c-type) with low electron density because its inherent diene character will attenuate the antiaromatic character of the 4MRs. 27,31–35 Finally, BBCT has a unique structural feature, i.e. , the linear annulation of 5-, 4-, 6-, 4-, and 5-membered rings.…”

“…BBCT will show moderate global aromaticity; the two fused 4MRs will attenuate the aromaticity of the central benzene ring and the fused thiophene ring at the 3,4-bond (c-type) with low electron density because its inherent diene character will attenuate the antiaromatic character of the 4MRs. 27,[31][32][33][34][35] Finally, BBCT has a unique structural feature, i.e., the linear annulation of 5-, 4-, 6-, 4-, and 5-membered rings. [36][37][38][39][40][41][42][43] In this context, we herein report the synthesis, electronic properties, crystal structures, and aromaticity of BBCT and its methylated compound BBCT-Me and compare them with the properties and structure of [3]phenylene as a reference compound, 10 for future potential application of these molecules as p-type semiconducting materials.…”

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“…Often, the computational exploration of molecules with unusual electronic properties has the potential to uncover new chemical phenomena. For example, Hashimoto and Tahara use thiophene analogues of anti-kekulene to get fresh insights into the tug-of-war between aromaticity and anti-aromaticity [33]. Changes in the electronic structure of polycyclic aromatic hydrocarbons (PAHs) upon photochemical excitation reveal a switch from aromaticity to antiaromaticity and provide an important step in consolidating varying viewpoints of electronically excited states.…”