π-Electronic ion pairs: building blocks for supramolecular nanoarchitectonics viaiπ-iπ interactions

The pairing of charged pi-electronic systems and their ordered arrangement have been achieved by i pi-i pi interactions that are derived from synergetically worked electrostatic and dispersion forces. Charged pi-electronic systems that provide ion pairs as building blocks for assemblies have been prepared by diverse strategies for introducing charge in the core pi-electronic systems. One method to prepare charged pi-electronic systems is the use of covalent bonding that makes pi-electronic ions and valence-mismatched metal complexes as well as protonated and deprotonated states. Noncovalent ion complexation is another method used to create pi-electronic ions, particularly for anion binding, producing negatively charged pi-electronic systems. Charged pi-electronic systems afford various ion pairs, consisting of both cationic and anionic pi-systems, depending on their combinations. Geometries and electronic states of the constituents in pi-electronic ion pairs affect the photophysical properties and assembling modes. Recent progress in pi-electronic ion pairs has revealed intriguing characteristics, including the transformation into radical pairs through electron transfer and the magnetic properties influenced by the countercations. Furthermore, the assembly states exhibit diversity as observed in crystals and soft materials including liquid-crystal mesophases. While the chemistry of ion pairs (salts) is well-established, the field of pi-electronic ion pairs is relatively new; however, it holds great promise for future applications in novel materials and devices.