Ion materials comprising planar charged species

Supramolecular Assemblies of Dipyrrolyldiketone CuII Complexes

Dipyrrolyldiketones are essential building units of anion-responsive π-electronic molecules and ion-pairing assemblies. Here, we demonstrated that they form complexes with CuII characterized by planar geometries. The solid-state stacking assembled structures, as revealed by single-crystal X-ray analysis, were modulated by the substitution of pyrrole units. The rectangular shapes of the CuII complexes resulted in the formation of mesophases upon introduction of aliphatic chains.

Arylethynyl Groups That Modulate Anion-Binding and Assembling Modes of Rod- and Fan-Shaped π-Electronic Systems

The introduction of arylethynyl moieties at the pyrrole α- and β-positions of dipyrrolyldiketone BF₂ complexes as anion-responsive π-electronic molecules was investigated. The arylethynyl-substituted derivatives formed a variety of anion complexes with planar [1+1]- and interlocked [2+1]-type structures in solution and in the solid state. The derivatives with long alkyl chains in the introduced arylethynyl groups also formed mesophases in the form of ion pairs of the anion complexes and a counter cation. The geometries of the constituent anion complexes affected the packing modes of the dimension-controlled assemblies.

Arylpyrrolyldiketone Boron Complexes Exhibiting Various Anion-Binding Modes Based on Dynamic Conformation Changes

Arylpyrrolyldiketone boron complexes as anion-responsive π-electronic molecules were synthesized by Claisen condensations of acetylpyrrole and corresponding aryl esters. The synthesized π-electronic molecules exhibited anion-binding behavior with various binding modes including pyrrole-inverted and non-inverted [1+1]-type anion complexes as well as [2+1]-type complexes owing to the presence of only a single pyrrole ring. Furthermore, solid-state ion-pairing assemblies, comprising receptor-anion complexes and countercations, were constructed based on fairly planar [2+1]-type complexes.

Peripheral Modifications of meso-Hydroxyporphyrins: Formation of π-Electronic Anions and Ion-Pairing Assemblies

Phenyl groups were introduced at the β-positions proximal to the meso-hydroxy moiety in hydroxyporphyrin Ni^II complexes by oxidized BINAP Pd^II complexes. Ion-pairing assemblies of deprotonated π-electronic anions, anionic site of which was stabilized by the introduced phenyl moieties with a bulky cation, were formed. They showed charge-by-charge assemblies, assembly modes of which were modulated by the anionic building units.

Liquid Crystals Comprising π-Electronic Ions from Porphyrin-AuIII Complexes

Porphyrin–Au^III complexes were found to act as π-electronic cations, which can combine with various counteranions, including π-electronic anions. Single-crystal X-ray analyses revealed the formation of assemblies with contributions of charge-by-charge and charge-segregated assemblies, depending on the geometry and electronic state of the counteranions. Porphyrin–Au^III complexes possessing aliphatic alkyl chains formed dimension-controlled ion-pairing assemblies as thermotropic liquid crystals, whose ionic components were highly organized by π–π stacking and electrostatic interactions.

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Ion pairs based on porphyrin–Au^III complexes as π-electronic cations were prepared

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Porphyrin–Au^III complexes formed ion-pairing assemblies in combination with anions

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Aliphatic substituents in the cations provided liquid crystalline mesophases

Ion-Pairing Assemblies of π-Electronic Anions Formed by Intramolecular Hydrogen Bonding

π-Extended nitro-substituted dipyrrolylphenol derivatives were synthesized, and upon deprotonation they provided π-electronic anions stabilized by hydrogen bonding between the phenolate (phenoxide) moiety and the pyrrole NH group. Ion-pairing assemblies of the deprotonated anions were formed in the solid state and as mesophases. In the solid state, the extended π plane was found to be more suitable to form charge-by-charge assemblies in combination with tetraalkylammonium cations with highly anisotropic orientations as a result of efficient stacking. The mesophase also included a charge-by-charge assembly comprising the deprotonated anion bearing aliphatic chains, as revealed by synchrotron X-ray diffraction analysis.

Dynamic Polymorph Formation during Evaporative Crystallization from Solution: The Key Role of Liquid-Like Clusters as "Crucible" at Ambient Temperature

Understanding the polymorph phenomenon for organic crystals is essential for the development of organic solid materials. Here, the fluorescence study of the evaporative crystallization of 1,3-dipyrrol-2-yl-1,3-propanedione boron difluoride complex (1), which has three polymorphs showing different emission profiles, is reported. The droplet of 1 in 1,2-dichloroethane showed blue emission just after dropping. Solids with bluish-green emission were observed. As time elapsed, a solid with red or orange emission was observed around the droplet. Time evolution of the fluorescence spectra, observed for the first time, implied that the molten state of 1 was observed by emission of an intermediate, even at ambient temperature. These findings suggested that the liquid-like cluster incidentally forms an ordered array as the crystallites nucleate. The liquid-like cluster can be considered as the "crucible" in the nucleation of polymorphs.

Pyrrole N-H Anion Complexes

Synthetic pyrrole-based anion receptors date back to the 1990s. They have been extensively developed in the context of macrocyclic systems as expanded porphyrins and calixpyrroles, and related systems. The chemistry of open-chain pyrrolic systems is, in many respects, no less venerable. It also has more direct analogy to naturally occurring pyrrole-based anion binding motifs. However, it has not been the subject of a comprehensive review. Presented herein is a summary of efforts devoted to the creation of de novo pyrrole-based receptors, as well as the anion recognition chemistry of naturally occurring pyrrolic systems as prodigiosins and their synthetic analogues.

Dimension-controlled ion-pairing assemblies based on π-electronic charged species

This feature article summarizes the recent progress in the study of ion-pairing assemblies based on π-electronic ion pairs, including anion complexes of π-electronic molecules.

Dimension-controlled assemblies of anion-responsive π-electronic systems bearing aryl substituents with fan-shaped geometries

Pyrrole-4-aryl-substituted dipyrrolyldiketone BF₂ complexes showed fan-shaped and rod-shaped geometries for anion-free states and receptor–anion complexes, respectively, as building units of dimension-controlled assemblies.

Dipyrrolyphenol as a precursor of π-electronic anion that forms ion pairs with cations

A nitro-substituted dipyrrolylphenol was synthesized as a precursor of a π-electronic anion, whose phenolate (phenoxide) moiety upon deprotonation was stabilized by the hydrogen-bond-donating pyrrole NH, thus forming solid-state ion pairs with various cationic species.

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

β-Perfluoroalkyl-substituted pyrrole as an anion-responsive π-electronic system through a single NH moiety

A pyrrole derivative possessing two perfluorohexyl (C₆F₁₃) chains at the β-positions was synthesized. The significant electron-withdrawing properties of the β-perfluoroalkyl chains induced fascinating anion-binding behaviour.

Dipyrrolylpyrimidines as anion-responsive π-electronic systems

Dipyrrolylpyrimidines synthesized by coupling reactions exhibited anion-binding abilities with the inversion of pyrrole rings. In the solid state, anion complexes of dipyrrolylpyrimidines formed charge-by-charge columnar structures in combination with counter cations.

π-Electron Systems That Form Planar and Interlocked Anion Complexes and Their Ion-Pairing Assemblies

Interactions between designed charged species are important for the ordered arrangements of π-electron systems in assembled structures. As precursors of π-electron anion units, new arylethynyl-substituted dipyrrolyldiketone boron complexes, which showed anion-responsive behavior, were synthesized. They formed a variety of receptor-anion complexes ([1+1] and [2+1] types) in solution, and the stabilities of these complexes were discussed in terms of their thermodynamic parameters. Solid-state ion-pairing assemblies of [1+1]- and [2+1]-type complexes with countercations were also revealed by single-crystal X-ray analysis. In particular, a totally charge-segregated assembly was constructed based on negatively and positively charged layers fabricated from [2+1]-type receptor-anion complexes and tetrabutylammonium cations, respectively. Furthermore, the [1+1]-type anion complex of the receptor possessing long alkyl chains exhibited mesophases based on columnar assembled structures with contributions from charge-by-charge and charge-segregated arrangements, which exhibited charge-carrier transporting properties.

Ion-based assemblies of planar anion complexes and cationic PtII complexes

Because the metallophilicity of attractive Pt^II⋯Pt^II interactions helps in the fabrication of columnar structures, terpyridine–Pt^II complexes were used as planar counter cationic species for formation of charge-segregated assemblies using π-conjugated receptor–Cl⁻ complexes as planar anions.

Recent progress in research on stimuli-responsive circularly polarized luminescence based on π-conjugated molecules

This short review article summarizes recent progress in research on various stimuli-responsive circularly polarized luminescence (CPL) properties derived from π-conjugated molecules and related materials. As representative examples, assembled structures of chiral π-conjugated polymers and molecules showed tunable CPL resulting from the enhancement of chirality induction by aggregation. Fascinating CPL-active species, pyrrole-based anion-responsive π-conjugated molecules exhibiting CPL induced by anion binding and ion pairing, are also discussed.

Ion-based materials derived from positively and negatively charged chloride complexes of π-conjugated molecules

Oriented salts from planar charged species were prepared by combining positively and negatively charged receptor-anion complexes with similar geometries using dicationic and electronically neutral π-conjugated receptors. Phenylene- or pyrimidine-bridged bis(imidazolium) dicationic anion receptors formed monocationic receptor-Cl(-) complexes that were accompanied by a free Cl(-). This free Cl(-) was subsequently captured by pyrrole-based neutral anion receptors to form negatively charged receptor-Cl(-) complexes. The ion pair of the resulting positively and negatively charged planar receptor-Cl(-) complexes could produce a supramolecular octane gel, adopting a lamellar self-organized structure in its xerogel state. On the other hand, the solid-state ion pairs had hexagonal columnar mesophases, which formed via alternate stacking of the positively and negatively charged planar receptor-Cl(-) complexes. By use of the flash-photolysis time-resolved microwave conductivity technique, the one-dimensional charge-carrier transporting property, with a mobility of 0.05 cm(2) V(-1) s(-1), was determined for the newly prepared solid-state ion pairs.

Self-assembly and near perfect macroscopic alignment of fluorescent triangulenium salt in spin-cast thin films on PTFE

Highly fluorescent, discotic trioxatriangulenium dyes were aligned by simple spin-casting on substrates with friction transferred PTFE layers. The fluorescent crystalline thin films show near perfect macroscopic alignment on centimeter large areas directly from spin-casting. Gracing Incidence X-ray Diffraction (GIXD) unambiguously allowed the determination of a long-range order unit cell as well as its orientation with respect to the PTFE fibers. Further analysis of the X-ray data, in conjunction with polarized absorption spectroscopy, suggest a lamellar packing model with alternating layers of alkyl chains and ionic dyes oriented parallel to the substrate. This structure results in a highly anisotropic electrostatic potential around the cationic chromophore, causing significant shifts in energy and orientation of the optical transitions. Thus, the optical properties of the material are, to a large extent, controlled by the position of the otherwise inert PF6(-) counterions. The bright fluorescence from the films is also polarized parallel to the PTFE alignment layer. Doping of the thin films with fluorescent energy acceptor traps shows that efficient exciton migration takes place in the thin films. The excellent exciton transfer capabilities, in conjunction with the perfect alignment, might be of interest in future applications in solar energy harvesting or as thin film sensors.

Cation modules as building blocks forming supramolecular assemblies with planar receptor-anion complexes

Ion-based materials were fabricated through ion pairing of planar receptor-anion complexes and cation modules as negatively and positively charged building blocks, respectively. Anion receptors that could not form soft materials by themselves provided mesophases upon anion binding and subsequent ion pairing with aliphatic cation modules. The mesogenic behaviors were affected by structural modification of both the cation module and the anion receptor. Synchrotron X-ray diffraction measurements suggested the formation of columnar mesophases with contributions from charge-by-charge and charge-segregated arrangements. Flash-photolysis time-resolved microwave conductivity measurements further revealed a higher charge-carrier mobility in the assembly with a large contribution from the charge-segregated arrangement than in the charge-by-charge-based assembly.