Concealed antiaromaticity

Numerous articles in the recent literature report molecules that are claimed to be antiaromatic, as they feature a formal 4n π-electron system. The purported antiaromaticity often serves well as an explanation for some of the observed properties, but it neglects the actual local aromaticity of the molecules, which often feature multiple subunits with 4n+2 π-electrons besides the formal 4n π-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 π-electron system but should be reserved for truly antiaromatic molecules. To reconcile the different viewpoints, the term and concept of concealed antiaromaticity is introduced here. The concept accepts that many of the molecules claimed to be antiaromatic are not truly antiaromatic. However, it acknowledges that this does not prevent those molecules from exhibiting behaviour under certain conditions that would normally be expected for antiaromatic molecules, due to the formal, concealed 4n π-electron system that characterizes these molecules. Based on the conditions under which the molecules can behave like antiaromatic molecules, three types of concealed antiaromaticity are distinguished here: concealed antiaromaticity revealable in redox reactions (Type I-CA), upon excitation (Type II-CA), and in intermolecular interactions (Type III-CA). The concept of concealed antiaromaticity will enable the conscious, rational design of molecules that show the desirable properties of antiaromatic molecules while avoiding or diminishing the undesirable properties, namely the low stability of truly antiaromatic molecules. This will yield molecules and materials with highly interesting properties for a broad range of applications, from organic electronics to supramolecular chemistry.