Molecules with Type I-CA conceal their antiaromaticity in the neutral state but behave like antiaromatic molecules in the doubly reduced or oxidized state, where -just like truly antiaromatic molecules -the molecules become globally aromatic. This is because the formal 4n π-electron system transitions to a [4n+2] πelectron system upon twofold reduction or oxidation. For molecules that can conceal their antiaromaticity by conformational changes, the reduction/oxidation often goes along with a planarization of the involved π-electron system and a loss of the conformational flexibility. The planarization enhances the π-electron delocalization in the system, which is energetically favourable in the doubly charged, aromatic states. However, the molecules can also become globally aromatic if full planarization is not possible for structural reasons, as other structural changes that result in enhanced π-electron delocalization and reduced bond-length alternation can still be possible. Type I-CA is the most frequently found type of concealed antiaromaticity in the literature. There are many reports of molecules that become globally aromatic upon twofold reduction or oxidation without being truly antiaromatic in the neutral state and the examples provided here should not be seen as a complete list of molecules exhibiting such behaviour. As structurally simple examples, both cyclooctatetraene and dibenzocyclooctatetraene planarize and become aromatic upon twofold reduction (Figure ). Similarly, dibenzopentalene (Figure ) and its derivatives were shown to become globally aromatic upon reduction to the dianion or oxidation to the dication. Tetraoxa [8]circulene and the large molecular propeller based on this structure (Figure ) also switch to a globally aromatic state upon reduction. Moving on to larger formal 4n π-electron systems, paracyclophanetetraene (Figure ) and its derivatives were shown to become globally aromatic upon two-electron reduction or oxidation, although they cannot fully planarize for steric reasons. For paracyclophanetetraene, switching between a locally aromatic neutral state (with strongly concealed antiaromaticity) and a globally aromatic charged state was introduced as a platform for obtaining organic battery electrode materials. Ni(II) norcorrole (Figure ) also transitions to a globally aromatic state upon twofold reduction or oxidation, and the effect was used to obtain a bipolar battery electrode material. However, as the antiaromaticity is only weakly concealed in Ni(II) norcorroles (as discussed above), mesityl substituents were attached to increase the stability by steric protection. Larger macrocycles, such as cycloparaphenylenes (Figure ) of different size, can also reveal their concealed antiaromaticity in redox reactions. In these macrocycles, each phenylene subunit contributes 4 π-electrons to the formal macrocyclic 4n π-electron system, meaning that all cycloparaphenylenes that were studied featured Type I-CA (independent of the number of subunits). In contrast, each of the repeat units of furanyleneacetylene macrocycles contributes 6 π-electrons to the formal macrocyclic π-electron system, resulting in alternation between formal 4n and [4n+2] π-electron systems with increasing ring size. Those with a formal 4n π-electron system were more readily oxidized in cyclic voltammetry measurements, indicating Type I-CA. For the neutral macrocycles, the authors compared the properties to those of the corresponding phenylene-acetylene macrocycles, which showed that the antiaromaticity is more strongly concealed with phenylene than with the furanylene subunits. Other examples of macrocycles with Type I-CA include oligofuran macrocycles, expanded carbaisophlorinoids, heteroatom-substituted porphyrinoids of different size, and porphyrin-based nanorings. Concealed antiaromaticity revealable upon photoexcitation (Type II-CA): Molecules with Type II-CA conceal their antiaromaticity in the ground state but behave like antiaromatic molecules in the excited state, where the molecules become Baird aromatic, a behaviour normally ascribed to antiaromatic molecules. As the redox reactions of molecules with Type I-CA, the photoexcitation often goes along with a planarization of the involved π-electron system, but the molecules can also become Baird aromatic if full planarization is not possible for structural reasons. Many molecules that feature Type I-CA also feature Type II-CA, as shown for cyclooctatetraene, biphenylene, pentalene (Figure ) and their derivatives. Studies on π-expanded oxepines (Figure ) and other 8π-electron heterocycles with locally aromatic units fused with the formal 8π-electron system show that these molecules can also feature Type II-CA. Moving on to conjugated macrocycles, paracyclophanetetraene (Figure ) and its derivatives also feature Type II-CA according to computations and experimental indications. The results of computational studies of cycloparaphenylenes (Figure ) of different size and their furanylene analogues (Figure ) show that -with both types of subunits -these macrocycles can feature Type II-CA. The Baird aromaticity was more pronounced in the macrocycles with furanylene units, in which the antiaromaticity is less concealed due to the weaker local aromaticity of the subunits. The authors ascribe this to a tug-of-war between the local Hückel aromaticity of the subunits and the macrocyclic Baird aromaticity in the excited state, which can tip to the favour of macrocyclic Baird aromaticity if the antiaromaticity in the ground state is less concealed. For the macrocycles with furanylene-acetylene repeat units (with alternating formal 4n and [4n+2] π-electron systems) discussed in the section on Type I-CA, the pronounced differences in the emission properties depending on the ring size indicate Type II-CA for those macrocycles with a formal 4n π-electron system. In a host-guest complex of a fullerene and a macrocycle composed of benzene-fused pyrrolo [3,2-b]pyrrole units with Type II-CA, Baird aromaticity is believed to play a role in rendering the locally excited state at the macrocycle the lowest excited state of the complex. The photoresponsive behaviour of a borole fused with aromatic units may also be explained by Type II-CA. Concealed antiaromaticity revealable in intermolecular interactions (Type III-CA): Molecules with Type III-CA conceal their antiaromaticity as an individual molecule (in solution) but behave like antiaromatic molecules in the solid state, where energetically favourable intermolecular interactions of the 4n π-electron systems can occur. These interactions can lead to intermolecular π-electron delocalization and stacked-ring aromaticity, which are highly interesting features for organic electronic materials. In contrast to truly antiaromatic molecules, no covalent bonds between the molecules are formed as a result of the interactions. Unfortunately, however, Type III-CA is the most difficult type of concealed antiaromaticity to achieve, as the antiaromaticity should only be weakly concealed for the molecules to behave like antiaromatic molecules in intermolecular interactions, meaning that the number of aromatic subunits in relation to the number of π-electrons in the 4n πelectron system should be low and no strongly aromatic subunits should be present. This requirement needs to be carefully balanced with the stability requirements. Furthermore, full planarity of the 4n π-electron system in the solid state is important for Type III-CA, as this enables the close, energetically favourable intermolecular interactions of the π-electron systems. Ni(II) norcorrole derivatives (Figure ) can fulfil these requirements and, indeed, were shown to feature stacked-ring aromaticity when forced to stack as dimers in micellar capsules or connected via linkers. The molecular orbital interactions in these systems may be compared to the SOMO-SOMO interactions in double pancake-bonded dimers. Besides these dimer interactions, also other types of highly interesting solid-state behaviour has been reported for norcorrole derivatives, such as chain-like stacking, which resembles the staggered, chain-like stacking of organic biradicals that maximize SOMO-SOMO interactions by this type of stacking, triple-decker stacking, and onedimensional supramolecular assembly. However, as discussed above, bulky substituents can be required to sufficiently increase the stability of Ni(II) norcorrole derivatives for applications, but these hinder the close intermolecular interaction required for Type III-CA. As an alternative to attaching bulky substituents, designing molecules with a formal 4n π-electron system that fulfil the above requirements but -in contrast to Ni(II) norcorroles -are capable of conformational changes to further conceal the antiaromaticity (in addition to integrating fragments of the 4n π-electron systems into locally aromatic subunits) may yield more stable molecules with Type III-CA. In solution, such molecules are expected to avoid coplanarity of the conformationally flexible aromatic subunits to conceal the antiaromaticity. In the solid state, however, full planarization of the molecules is expected to give rise to the energetically favourable intermolecular interactions that are characteristic for 4n π-electron systems, potentially giving rise to stacked-ring aromaticity. Stable molecules with Type III-CA have been observed in the literature; the first macrocycle, [24]annulene tetroxide in cis,trans,cis,trans configuration (Figure ), was reported to be virtually planar in the solid state (Figure ), indicating that it revealed its antiaromaticity for energetically favourable intermolecular interactions. The macrocycle is a configurational isomer of the all-cis macrocycle obtained when replacing the phenylene subunits of paracyclophanetetraene (Figure ) with furanylene subunits. The furanylene subunits are less aromatic and less sterically demanding than phenylene subunits, which reduces the degree of concealment and allows for planarization in the solid state. In solution, however, [24]annulene tetroxide was reported to be highly flexible, enabling it to avoid planarity and to conceal its antiaromaticity to a large extent. The black-violet crystals of the macrocycle showed a melting point >300 °C and a staggered, chain-like face-to-face stacking (Figure bottom), resembling the stacking of methyl-substituted Ni(II) norcorrole. Besides Type III-CA, [24]annulene tetroxide also features Type I-CA according to 1 H NMR measurements of its dication. It was reported that the "highly dynamic 24 π electron system of [the macrocycle] apparently loses its flexibility when it is oxidized". Figure . Crystal structures and packing of annulenes with concealed antiaromaticity. Crystal structure and packing of (a) [24]annulene tetroxide in cis,trans,cis,trans configuration and (b) 5,6,17,18-bisdehydrotetrathia [24]annulene. The corresponding molecular structures are shown in Figure . (c) Crystal structure of 5,6,17,18-bisdehydrotetrathia [24]annulene with eight n-butyl chains attached to the thienylene units.