Concealed antiaromaticity

Abstract

The literature reports numerous molecules claimed to be antiaromatic because of a formal 4n pi-electron system. However, this neglects the actual local aromaticity of the molecules, which often feature multiple subunits with [4n+2] pi-electrons besides the formal 4n pi-electron system. This has led to considerable criticism from those who believe that the term antiaromatic should not be used for any molecule with a formal 4n pi-electron system but should be reserved for truly antiaromatic molecules. To reconcile the different viewpoints, the concept of concealed antiaromaticity is introduced here. Concealed antiaromaticity acknowledges that many molecules claimed to be antiaromatic are not truly antiaromatic, but they can exhibit behaviour under certain conditions that would normally be expected for antiaromatic molecules. Three types of concealed antiaromaticity are distinguished based on the conditions under which the molecules can behave like antiaromatic molecules: concealed antiaromaticity revealable in redox reactions (Type I-CA), upon photoexcitation (Type II-CA), and in intermolecular interactions (Type III-CA). The concept of concealed antiaromaticity will enable the rational design of molecules that show the desirable properties of antiaromatic molecules under the different conditions, with broad applications from organic electronics to supramolecular architectures, while avoiding the low stability of truly antiaromatic molecules.

Electronic Structure of Metalloporphenes, Antiaromatic Analogues of Graphene

“…Metalloporphenes such as ZnP are composed of both aromatic circuits (pyrrole and benzene, shown in blue in Figure ) with delocalized π-bonds and antiaromatic circuits (cylooctatetraene, orange) with distinctly localized bonds . This leads to mixed aromaticity (or concealed antiaromaticity), suggesting that both types of circuits need to be described with a similar level of accuracy to properly capture their electronic structure. All previous investigations of ZnP − used pure density functional theory (DFT) to optimize its geometry, predicting a D 4h (square) unit cell with a single minimum.…”

Multicenter interactions and ligand field effects in platinum(ii) tripyrrindione radicals

“…We considered the possibility that the intermonomer bonding in the oxidized dimers might lie in so-called double pancake bonding, 45 which has been invoked for antiaromatic or diradicaloid compounds such as norcorrole. 46,47 However, B3LYP-D3 calculations on both oxidized monomers and dimers revealed substantial singlet-triplet gaps >0.7 eV, so these species do not really qualify as diradicaloids.…”

Functionalisation of conjugated macrocycles with type I and II concealed antiaromaticity via cross-coupling reactions

“…Paracyclophanetetraene (PCT, Scheme 1 left) and its derivatives are among the most interesting macrocycles in this regard, [2][3][4][5][6][7] as they feature concealed antiaromaticity. 8 Despite the formal macrocyclic system of 24 (4n) π-electrons along the perimeter of the molecule (Scheme 1 left, bold bonds), PCT and its derivatives are not antiaromatic but dominated by the local aromaticity of the four phenylene subunits with 6 (4n + 2) π-electrons, which obey Hückel's rule and conceal the antiaromaticity of the macrocyclic conjugated system. Nevertheless, the macrocycles can still exhibit behaviour under certain conditions that would normally be expected for antiaromatic molecules.…”

“…Such behaviour, defined as type I concealed antiaromaticity, was shown to be particularly interesting for use as battery electrode materials, as switching between different aromatic states can enable excellent redox properties. 4 PCT and its derivatives are also considered to feature type II concealed antiaromaticity, 8 meaning that the macrocycles can become globally aromatic in the lowest singlet and/or triplet excited states, where the electron counting rules are reversed according to what is known as Baird's rule. [9][10][11] In these states, molecules with 4n π-electrons in a conjugated system can be expected to be aromatic and with 4n + 2 π-electrons to be antiaromatic.…”

“…PCT and its derivatives are also considered to feature type II concealed antiaromaticity, 8 meaning that the macrocycles can become globally aromatic in the lowest singlet and/or triplet excited states, where the electron counting rules are reversed according to what is known as Baird's rule. 9–11 In these states, molecules with 4 n π-electrons in a conjugated system can be expected to be aromatic and with 4 n + 2 π-electrons to be antiaromatic.…”