The π-hole is an electron-deficient region formed at the center of an aromatic π-plane, characterized by a positive electrostatic potential (ESP) and a positive quadrupole moment (Qzz). This inversion of the charge distribution relative to conventional aromatic π-systems arises when electron-withdrawing elements such as fluorine or nitrogen are incorporated into aromatic frameworks, generating electrophilic π-surfaces capable of selectively recognizing electron-rich π-systems. The concept of the π-hole was first identified in arene–perfluoroarene interactions and has since evolved into a fundamental design principle in crystal engineering. In this review, the π-hole is not treated merely as a specific type of intermolecular interaction, but is positioned as a unifying electrostatic design principle within hole chemistry that connects solid-state molecular organization and host–guest recognition. Accordingly, the electronic origin, design strategies, and solid-state manifestations of π-hole···π interactions in both organic and metal–organic systems are systematically summarized, with particular emphasis on how quadrupole inversion governs molecular assembly in the solid state.
The first part of the review focuses on π-hole-driven co-crystallization phenomena, while the latter part highlights nonporous adaptive crystals (NACs) that reversibly encapsulate aromatic hydrocarbons through electronically programmed hole sites within the crystal lattice. Particular attention is devoted to perfluorinated Cu(II) complexes that lack permanent voids, which represent archetypal molecular crystals in which guest inclusion arises not from persistent porosity but from adaptive electrostatic reorganization of the crystal structure. As a result, molecular recognition and guest encapsulation are driven purely by electrostatic complementarity, without reliance on preformed pores. Furthermore, π-hole design enables discrimination among nonpolar molecules with nearly identical size and shape, such as benzene/cyclohexene/cyclohexane, benzene/trifluorobenzene/hexafluorobenzene, and CO2/C2H2. These examples demonstrate that subtle differences in quadrupole character can be translated into practical molecular separations that are inaccessible to conventional adsorbents. Finally, recent advances reveal that π-hole–induced charge inversion also provides a rational basis for solid-state color modulation and sensor design based on intermolecular charge transfer. Collectively, π-hole chemistry bridges fundamental electrostatic theory with practical applications in selective adsorption, molecular recognition, and responsive materials.
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