Over the past few decades, many studies have been conducted on -conjugated molecules, which have unique electronic properties based on an effective -electron delocalization. Since -conjugated molecules with rigid and planar structures, in general, have p-orbitals overhanging perpendicular to the molecular plane, stacking of two or more molecular planes can affect the molecular orbitals themselves, leading to significant changes in physical properties such as absorption properties. Therefore, when two -conjugated units are close to a separation distance of 3.4 Å (the sum of van der Waals (vdW) radii of carbon atoms) or less by a stacked structure that can induce an interaction between their molecular orbitals by forming new orbitals through linear combination of the components, it is expected that they could exhibit unprecedented properties that are not seen with monomeric -conjugated units. There have been several reports that the stacking of aromatic compounds can modify their intrinsic properties such as conductive/energy transfer properties and absorption/emission properties due to the formation of H-or J-aggregates. Recently, a unique property of three-dimensional (3D) aromaticity based on stacked anti-aromatic molecules has been reported by Shinokubo et al (Figure ). A similar 3D (anti)aromaticity was also predicted by closely stacked aromatic units, and thus the stacking of planar -systems is key for the development of molecules with novel functions. Cationic -conjugated molecules often exhibit a long-wavelength absorption with a large molar extinction coefficient due to the narrow HOMO-LUMO gap based on the effective delocalization of both charge and -electrons. In addition, since well-designed organic cations can interconvert with neutral species upon electron transfer (Figure ), cationic -conjugated molecules are promising candidates for functional organic materials such as chromic materials and organic semiconductors.