Tunable aromaticity in bicalicenes

Abstract

The unusual aromatic stability of cyclic bicalicene has been suggested to come from a tetraionic structure, where positive and negative charges are located on the cyclopropene and cyclopentadiene rings, respectively. Energetic, magnetic, geometric and electron delocalization analysis performed on a series of bicalicene derivatives, incorporating different electron donating and withdrawing groups, and electrically perturbed bicalicene structures provide additional proof of the role played by this tetraionic structure in the aromatic stability of bicalicene. In this work the aromatic stabilization is chemically and electrically tuned, enhancing or disrupting the electron delocalization and aromatic stability of the cyclopropene and cyclopentadiene rings by increasing or decreasing their corresponding charges. It is shown how the electron delocalization within these rings is similar to that of cyclopropene cation and cyclopentadiene anion for a perfect polarization of one electron.

Aromaticity Induced by Electric Field: The Case of Polycalicenes

“…Similar findings have been demonstrated for the ground states of bicyclic tricalicene and tricyclic tetracalicene, for which several different electronically excited biradical structures participate . Recently, it has been shown that π-electron delocalization in bicalicene can be tuned both by the substitution of hydrogen atoms with electron withdrawing or electron donating groups and by application of external electric perturbation …”

“…These observations are in line with the results obtained for bicalicene by Mandado and Ramos-Berdullas. 17 Changes in HOMA due to the change in field force are monotonic and can be expressed as a polynomial regression (|R| = 0.999, see Figure 7).…”

“…16 Recently, it has been shown that π-electron delocalization in bicalicene can be tuned both by the substitution of hydrogen atoms with electron withdrawing or electron donating groups and by application of external electric perturbation. 17 A computational investigation of some polycalicenes as novel nonbenzenoid aromatic molecules has been carried out. 18 They are built up by varying the number of calicene subunits and may possess various bonding motifs (e.g., head-to-tail or head-tohead).…”

Tuning Aromaticity ofpara‐Substituted Benzene Derivatives with an External Electric Field

“…It can be observed that the external electric field applied to a para ‐substituted benzene derivative may significantly change the aromaticity of the benzene ring (or, in other words, its aromaticity can be tuned by the presence of the external electric field). Similar aromaticity tuning has been described for a calicene molecule (tuning with an external electric field) and for bicalicene derivatives (tuning with an external electric field or with substituents) . For para ‐substituted benzene derivatives, it is important to point out that the aromaticity tuning is most effective when benzene is substituted with one electron‐donating and one electron‐withdrawing substituent, since in such a case the external electric field, depending on its direction, either reinforces or counteracts the action of both substituents cooperating together in the sense of their substituent effect.…”

“…Similara romaticity tuning has been described for ac alicene molecule [14] (tuning with an external electric field) and for bicalicene derivatives (tuning with an external electric field or with substituents). [15] For para-substitutedb enzene derivatives, it is important to point out that the aromaticity tuning is most effective when benzene is substituted with one electron-donatinga nd one electron-withdrawing substituent, sincei ns uch ac ase the external electric field, depending on its direction, either reinforces or counteracts the action of both substituents cooperating together in the sense of their substituent effect. For substituents of the same type (two electron-withdrawing or two electron-accepting groups), the electric field reinforces the action of one substituent, simultaneouslyc ounteractingt he action of the other one.…”