π-Electronic Ion Pairs That Form Charge-Segregated Assemblies

π-Electronic systems form various assemblies through noncovalent interactions and exhibit unique electronic and photophysical properties that arise from π-electronic states and molecular arrangements. Introducing charge into these π-electronic systems generates charged species capable of forming ion-pairing assemblies via ⁱπ-ⁱπ interactions, primarily governed by electrostatic and dispersion forces, resulting in novel functionalities derived from charged species. Typically, π-electronic ion pairs tend to form charge-by-charge assemblies, in which cations and anions alternately stack via electrostatic attractive forces. In contrast, forming charge-segregated assemblies that can exhibit semiconducting properties is challenging because a stacked arrangement of identically charged species is required to form columnar structures. Therefore, strategies to overcome electrostatic repulsion are highly desirable for the formation of such assemblies. Recent advances in molecular design have enabled the stacking of identically charged species by controlling assembly through charge delocalization, extending π-frameworks, and precise tuning of noncovalent interactions. This review summarizes recent progress in the design principles of charged π-electronic systems for charge-segregated assemblies and showcases their potential in modulating electronic, photophysical, and electric conductive properties.