Abstract
This review summarizes the results on the aromaticity of a series of synthesized and hypothetical neutral heterocirculene molecules and their double charged ions. The aromaticity of heterocirculenes is a direct reflection of their electronic structure responsible for the specific optoelectronic and photophysical properties. We show how the presence of a heteroatom in the outer macrocycle affects the aromaticity of hetero[8]circulenes. In addition, we also describe the change in aromaticity and strain energy for a series of the “lower” (n < 8) and “higher” (n > 8) hetero[n]circulenes. It was demonstrated that the loss of planarity with increased strain leads to an increased antiaromaticity of the lower hetero[n]circulenes, whereas higher hetero[n]circulenes demonstrate a more pronounced aromatic nature because of the small departure from planarity of each heteroarene ring in hetero[n]circulene molecule. Finally, we discuss the aromatic nature of the first examples of π-extended hetero[8]circulenes.
Highlights
Aromaticity is one of the central concepts in chemistry [1], which implies a set of special features inherent to a number of cyclic conjugated molecules to one degree or another
This review presents the current state of aromaticity research regarding a series of partially and fully heteroannelated circulenes with different heteroatoms in the outer macrocycle
Radenković et al [45] came to a similar conclusion when calculating the ring cu rent densities for a series of tetraoxa[8]circulenes. They found the presence of two type of ring currents in tetraoxa[8]circulene molecules: the paratropic ring current induced b the internal octatetraene ring, and the diatropic ones induced by the outer system of th furan and benzene rings [45]
Summary
Aromaticity is one of the central concepts in chemistry [1], which implies a set of special features (criteria) inherent to a number of cyclic conjugated molecules to one degree or another. Aromatic molecules are characterized by a reduced molecular energy due to the specific cyclic conjugation of the π-electron systems In this regard, the topological graph-theoretical energetic indices including topological resonance energy (TRE), bond resonance energy (BRE) and circuit resonance energy (CRE) were developed, which made it possible to unify the graph theories of aromatic stabilization and ring current diamagnetism [49]. The BRE values of the C−C bonds in the octatetraene ring are significantly large and negative, pointing to local antiaromaticity of the octatetraene ring in the composition of quasicirculenes This fact can be explained by the fact that the octatetraene ring does not satisfy Hückel’s rule of aromaticity. For molecules of the quasicirculenes 1, 2, 5−7, the benzene and heterocyclic rings exhibit an aromatic nature, as evidenced by the positive CRE values (Table 2). Ref. [51] Reproduced witohupteerrmsyissstieomn firnomcluKdeersimtheet aalr.o, JmouartnicalboefnMzeonleecuanladr Mheotdeerloinagre; npeubrliinshgesd. by Springer, 2018
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