Anion-responsive dipyrrolyldiketone and phenalenyl units covalently linked by a spiroboronium unit form π-electronic cations with orthogonally arranged π-systems. Conformations of the anion-binding unit are dependent on the coexisting counteranions in the solution and solid states. Orthogonally arranged π-electronic cations afford solid-state ion-pairing assemblies via double i π- i π interactions at the dipyrrolyldiketone anion complex and the phenalenyl units. Transient absorption spectra revealed electron transfer from anion-binding π-units to orthogonally arranged π-electronic cations, which could be modulated by the coexisting counteranions. Orthogonally arranged π-electronic cations exhibit pressure-responsive photophysical properties.

In this study, anion-responsive π-conjugated macrocycles were synthesized to demonstrate anion-binding and ion-pairing properties along with the ordered structures. Ion-pairing charge-by-charge assembly of a [1+2]-type complex of a macrocycle as a pseudo π-electronic anion and a countercation was revealed by single-crystal X-ray analysis. Further, two-dimensional (2D) arrays of the macrocycles bearing alkoxy chains, exhibiting anion-driven disordered structures, were constructed on a highly oriented pyrolytic graphite (HOPG) substrate as observed by scanning tunneling microscopy (STM).

Deprotonated meso-hydroxyporphyrin Au-III complex, acting as a zwitterion, was synthesized. In the solid state, the zwitterionic porphyrin formed a stacked columnar structure through dipole-dipole and pi-pi interactions. Energy decomposition analysis revealed that favorable dispersion forces were the primary contributors to the interactions between stacked zwitterionic porphyrin units. Furthermore, the solution-state absorption spectra of the zwitterionic porphyrin were influenced by the polarized structure, whereas the solid-state absorption spectra were associated with the packing arrangement.

A quinonoidal dipyrrolyldiketone catecholate-boron complex, with two pyrrole-quinonemethide moieties bridged by a six-membered cross-conjugated unit, was synthesized to modulate the diradical character of the dianion formed upon deprotonation. The dianionic species exhibited near-infrared absorption and electron spin resonance (ESR) signals, confirming the diradical properties. Variable-temperature (VT) ESR spectra suggest the thermal excitation from the ground-state singlet diradical to the triplet diradical, providing singlet-triplet energy gaps modulated by coexisting cations and bridging boron moieties.

Dipyrromethane dimers linked via a boron-bridged 1,3-propanedione moiety exhibited efficient anion-binding abilities. Trifluoromethyl and pentafluorophenyl moieties substituted at the meso positions of dipyrromethane induced chirality in the π-electronic systems, whose conformations were controlled by anion binding. Anion complexes of the oligopyrrole-based chiral π-electronic systems exhibited anion-dependent chiroptical properties, as seen in circular dichroism.

Stacking-frustrated ion-pairing assemblies comprising bulky charged π-electronic systems hinder efficient stacking of π-planes, owing to spatial constraints. In this study, porphyrin AuIII complexes bearing bulky aryl groups induce a spatial barrier that significantly separates the charged π-planes. With weakly coordinating anions, stacking frustration arises between the positively charged porphyrin π-planes, whereas π-electronic anions cause cation-anion stacking frustration. These results highlight the importance of both steric and electronic factors in the π-plane organization.

Independently stacked positively and negatively charged π-electronic systems in charge-segregated columnar structures are desired for electronic properties derived from their electron-deficient and -rich assembling states, respectively. An expanded π-electronic cation, benzoporphyrin AuIII complex, was synthesized as the component of ion pairs in combination with counteranions. In contrast to benzoporphyrin, which is known for its insolubility in organic solvents, the ion pairs with bulky anions in this study are soluble in common organic solvents. The ion pairs formed charge-segregated assemblies as two pseudo-polymorphs of single-crystal and less-crystalline (LeC) states based on the stacking of the benzoporphyrin AuIII complex. XRD and solid-state NMR measurements, along with molecular dynamics (MD) simulation, revealed that the LeC states were formed by a less-ordered arrangement of constituting ions induced by bulky counteranions. The electric conductivity properties were observed in the single-crystal and LeC charge-segregated assemblies.

Two pentamethine dyes with nearly identical structures but slightly different dipole moments were prepared as ion pairs. The ion pairs provided charge-segregated assemblies stabilized by dipole-dipole interactions between the positively charged π-electronic systems. The stacking structure of the bromo-substituted pentamethine cation was more stabilized by a larger dipole moment, as suggested by energy decomposition analysis. Depending on the packing arrangements, highly electric conductive properties were observed owing to charge-segregated structures, as also correlated with the theoretically estimated transfer integrals.

A porphyrin AuIII complex comprising 5,10-penatafluorophenyl units was synthesized as a dipolar π-electronic cation. The π-electronic cation was combined with anions, including a bulky borate anion and planar π-electronic anions. The dipolar π-electronic cation formed crystal-state ion-pairing assemblies based on antiparallel stacking structures, which were stabilized by interionic dipole-dipole interactions.

The solid-state ion-pairing assemblies in the form of anion complexes with tetraalkylammonium cations exhibited enhanced phosphorescence owing to the dispersion of emissive hydrogen-bonding 1D-chain Pt II complexes by aliphatic cations.

Metal complexation and peripheral modifications of thiaporphyrins have been investigated for preparing polarized π-electronic cations with anion-dependent ion-pairing assembling modes, including charge-segregated structures exhibiting electric conductive properties.

Naphthylisoquinoline-appended dipyrrolyldiketone Pt-II complexes as helical pi-electronic systems were synthesized. The Pt-II complexes, showing chiroptical properties in solution, exhibited anion-binding behaviour, resulting in the formation of anion complexes as pseudo pi-electronic anions. In combination with the pi-electronic cation, the receptor-anion complexes formed charge-by-charge ion-pairing assemblies, with the narcissistic self-sorting columnar structures comprising either of the enantiomers, in the crystal state.

Arylethynyl-substituted dipyrrolyldiketone BF2 complexes as anion-responsive pi-electronic molecules exhibited characteristic electronic properties derived from conformation changes upon anion binding, which caused an increase in UV/vis absorption and associated two-photon absorption. The anion complexes showed expanded planar regions assisted by intramolecular interactions, resulting in charge-by-charge ion-pairing assemblies in the solid state. Arylethynyl-substituted dipyrrolyldiketone BF2 complexes as anion-responsive pi-electronic molecules exhibited characteristic electronic properties derived from the conformation changes upon anion binding. The anion complexes showed expanded planar regions, affording charge-by-charge ion-pairing assemblies in the solid state. image

Ion pairs of N-(2,6-dimethylphenyl)-substituted triazatriangulenium (TATA+) cation with various counteranions were synthesized to investigate the interactions for the bulky cation. Single-crystal X-ray analysis of the TATA+ ion pairs revealed solid-state ion-pairing assemblies without stacking at the cationic π-planes. The TATA+ cation showed counteranion-dependent assembly structures, with smaller counteranions located at the top of TATA+ and bulkier counteranions displaced from the TATA+ plane to interact with the surrounding TATA+.

Norcorrole derivatives with 3,4,5-trialkoxyphenyl moieties at the meso positions were synthesized to form various stacking assemblies in single crystals and thermotropic liquid crystals (LCs) depending on aliphatic chain lengths. Triple-decker stacking structures were formed via the interactions between the antiaromatic systems formed for the butoxy and dodecyloxy derivatives in the single-crystal and LC states, respectively. In particular, the LC state exhibited discotic columnar structures comprising triple deckers to exhibit high electric conductivity, as supported by molecular dynamics simulations.

Pt-II complexes of pi-extended dipyrrolyldiketones were synthesized as anion-responsive pi-electronic molecules. The dipyrrolyldiketone Pt-II complexes exhibited red-shifted absorption and photoluminescence properties. In the solid state, [1 + 1]-type anion complexes formed charge-by-charge ion-pairing assemblies when combined with countercations. Detailed theoretical studies of the packing structures revealed favorable interactions between the planar anion complexes and pi-electronic cations. [Graphics] .

A variety of naphthalenediolates were orthogonally introduced to the boron unit of dipyrrolyldiketone boron complexes, exhibiting electronic properties that depended on the substituting positions of the naphthyl moieties. Combining the anion complexes with countercations resulted in the formation of ion-pairing assemblies with supporting stacking interactions of the naphthyl units.

The pairing of charged π-electronic systems and their ordered arrangement have been achieved by iπ-iπ interactions that are derived from synergetically worked electrostatic and dispersion forces. Charged π-electronic systems that provide ion pairs as building blocks for assemblies have been prepared by diverse strategies for introducing charge in the core π-electronic systems. One method to prepare charged π-electronic systems is the use of covalent bonding that makes π-electronic ions and valence-mismatched metal complexes as well as protonated and deprotonated states. Noncovalent ion complexation is another method used to create π-electronic ions, particularly for anion binding, producing negatively charged π-electronic systems. Charged π-electronic systems afford various ion pairs, consisting of both cationic and anionic π-systems, depending on their combinations. Geometries and electronic states of the constituents in π-electronic ion pairs affect the photophysical properties and assembling modes. Recent progress in π-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 π-electronic ion pairs is relatively new; however, it holds great promise for future applications in novel materials and devices.