Formation of charge-transfer complexes in ionic crystals composed of 1,3-bis(dicyanomethylidene)indan anion and viologens bearing alkyl chains.

The relationship between charge-transfer (CT) properties and the molecular arrangement formed from π-electronic ion pairs remains unclear because of the limited variety of π-electron anions. This study addressed this issue by synthesising a series of ion pair assemblies composed of viologen dications with diverse alkyl chains as π-electron cations and 1,3-bis(dicyanomethilidene)indan anion (CMI-) as a stable π-electron anion. We obtained seven ionic crystals and identified their assembled structures using single-crystal X-ray analysis. These structures are categorized into three types: "columnar", "slipped columnar" and "independent". The CT properties were characterised using UV-Vis absorption spectroscopy, which revealed that the CT absorption bands were dependent on the alkyl chain length. This intriguing variation in the CT transitions can be explained by the differences in the type of assembled structure.

Gel permeation chromatography process for highly oriented Cs3Cu2I5 nanocrystal film.

The emergence of green materials has attracted considerable attention in the field of optoelectronics. Copper-based lead-free metal halide (with a near-unity quantum yield) obtained from Cs3Cu2I5 nanocrystals (NCs) can exhibit blue emission with a wavelength of 440 nm and provide outstanding stability for various applications. However, in practical applications, colloidal dispersion purity and film quality are inadequate toward a high-performance device. In this study, antisolvent-free gel permeation chromatography is used to purify Cs3Cu2I5 NCs. The purified Cs3Cu2I5 NCs exhibit a high photoluminescent quantum yield and provide a highly oriented single-crystal film. Density functional theory calculation results indicate that the iodide-rich surface in the NCs makes them highly stable. In addition, it has been demonstrated for the first time that the mixture of polymethyl methacrylate (PMMA) and Cs3Cu2I5 NCs has waterproofing capabilities. The composite film consisting of Cs3Cu2I5 NCs and PMMA can survive in water for several days. This result opens up more possibilities for the application of these green material.

Two-Step Crystallization for Low-Oxidation Tin-Based Perovskite Light-Emitting Diodes.

Metal halide perovskites attract significant attention because of their excellent optoelectronic and semiconducting properties. However, there are environmental concerns related to the toxicity of the lead metal that is mainly used in these perovskites. PEA2SnI4 perovskite is a potential candidate for lead-free perovskites because of its pure red emission. Although, undesired Sn4+ oxidation results in the deterioration of PEA2SnI4 perovskite. We demonstrate the two-step crystallization of PEA2SnI4 through the (i) reprecipitation and (ii) recrystallization processes. A film prepared using this method exhibits narrowed emission, with a full width at half-maximum from 30.0 to 26.1 nm, because of its homogeneous emission. Moreover, the Sn4+ content of two-step-crystallized PEA2SnI4 films is five times lower than that of a control film. Diffusion-ordered spectroscopy analysis indicates that the two-step precursor exhibits a smaller hydrodynamic radius crystal seed, which enhances crystallization during spin coating. The resulting two-step crystallized PEA2SnI4-based light-emitting diode (LED) exhibits a maximum external quantum efficiency (EQE) of 0.4% with an average of 0.2%, which is two times greater than that of the control device. This two-step approach may be generalized to synthesize other lead-free materials.

Crystal Structures and Optical Properties of Cyanine Dyes Depending on Various Counter Anions.

In this study, cyanine cations with various counter anions were prepared as examples of ionic materials constructed using charged π-conjugated systems. A series of ion pairs was obtained by anion exchange reactions using iodide salts of carbocyanine dyes. The optical properties were measured by UV/vis absorption and fluorescence spectroscopy; measurements performed in CHCl3 (less-polar solvent) were altered by the influence of the counter anions. The packing structures of nine crystals were determined by single-crystal X-ray analysis. Moreover, the locations of the anions relative to the cations were stabilized by hydrogen bonding and categorized into two types. In addition, delocalization of the negative charge of the anions on cyanine cations was explained by density functional theory calculations. Furthermore, it was concluded that the stack formation of cyanine cations depends on the size and structure of the anions.

Photoisomerization-induced patterning of ion-pairing materials based on anionic azobenzene and its complex with a fluorescent π-electronic system.

Large mass transport driven by the difference in the photoisomerization-induced surface tension was demonstrated in ion pairs of anionic azobenzene and a cationic polymer. This material motion enabled fluorescence patterning using a trace amount of photoisomerized azobenzenes in complex form with a π-electronic system.

Optical property control of π-electronic systems bearing Lewis pairs by ion coordination.

π-Electronic systems bearing Lewis pairs were synthesized and their optical responses to added ions were investigated. The tuning of the optical properties was demonstrated by the addition of various ion pairs, and these behaviours were elucidated by theoretical calculations.

Ion-Pairing Assemblies of Porphyrin-AuIII Complexes in Combination with π-Electronic Receptor-Anion Complexes.

Anion complexes of anion-responsive π-electronic molecules can behave as pseudo π-electronic anions providing various ion pairs in combination with countercations. In this study, single crystals of ion-pairing assemblies comprising porphyrin-AuIII complexes and Cl- complexes of dipyrrolyldiketone BF2 complexes were prepared from 1:1 mixtures of anion receptors and the Cl- salts of cationic porphyrins in solution. In the solid state, the ion pairs formed characteristic assemblies, depending on the substituents of the anion receptors and porphyrin-AuIII complexes. Theoretical calculations on the ion pairs revealed that the stacking structures are stabilized by compensating positive and negative charges as well as π-π interactions.

Switching of Two-Photon Optical Properties by Anion Binding of Pyrrole-Based Boron Diketonates through Conformation Change.

Two-photon absorption (TPA) dyes with intense fluorescence can be used to detect small chemical species and as sensors and bioimaging probes for specific analytes. Various TPA dyes responding to a number of external stimuli have been reported. Among them, biologically important anionic species have not been used as agents to control TPA properties because their direct electronic influences on the transition dipole moments of dyes are typically small. In this study, dipyrrolyldiketone BF2 complexes substituted with π-extended units exhibited efficient TPA properties that could be regulated by conformation changes induced by anion binding. The TPA intensity decreased to 1/5 of the original intensity upon anion binding, which was much larger than that observed for one-photon absorption. Anion detection was achieved by a change in the emission intensity of spatially resolved spots of two-photon-excited fluorescence (TPEF) in the sample. Experimental and theoretical studies were performed to understand the mechanism of the TPA property control and showed that the drastic changes in the transition dipole moments upon conformation changes between the straight and bending forms of the π-electronic systems caused the TPA and TPEF intensities drop.

Concerted Photoluminescence of Electrochemically Self-Assembled CuSCN/Stilbazolium Dye Hybrid Thin Films.

Hybrid thin films of crystalline CuSCN and 4-(N,N-dimethylamino)-4'-(N'-methyl)stilbazolium (DAS) in three distinctively different nanostructures were obtained by electrochemical self-assembly from a single pot containing all the chemical ingredients. Their optical properties for UV-vis-NIR absorption, photoluminescence (PL), and PL excitation spectra were examined between 77 and 298 K, in comparison with solution and solid powder of DAS tosylate (DAST). Unlike all other dyes we tested before, PL of DAS was not quenched but rather enhanced when hybridized with CuSCN. DAST exhibited a strong exciton-phonon coupling to weaken, broaden, and red shift PL at room temperature, so that it inversely is strongly enhanced, sharpened, and blue-shifted at 77 K. The PL of the same dye in the hybrid thin film, however, shows a slight red shift and only a moderate enhancement at reduced temperatures due to strong exciton stabilization in dielectric environment of CuSCN and concerted PL by energy transfer from CuSCN to DAS luminophore, making it a unique nearly temperature-independent luminescent material.

Photo-responsive dimension-controlled ion-pairing assemblies based on anion complexes of π-electronic systems.

Negatively charged π-electronic systems prepared by the complexation of anion-responsive dipyrrolyldiketone BF2 complexes with an azobenzene bearing an alkanoate and an aliphatic chain provided unique dimension-controlled assemblies, such as a photo-responsive supramolecular gel.

Ion-pairing assemblies based on π-extended dipyrrolylquinoxalines.

α-Aryl-substituted dipyrrolylquinoxalines were synthesized and were converted to deprotonated forms stabilized by intramolecular hydrogen bonding, providing π-electronic anions, which act as building units of ion-pairing assemblies in combination with countercations.

Photo-Responsive Soft Ionic Crystals: Ion-Pairing Assemblies of Azobenzene Carboxylates.

This report delineates the design and synthesis of negatively charged azobenzene derivatives that form photo-responsive ion-pairing assemblies. The azobenzene carboxylates possessing aliphatic chains were prepared as photo-responsive anions that promote the formation of ion-pairing dimension-controlled assemblies, including mesophases, when used in conjunction with a tetrabutylammonium (TBA) cation. The photo-responsive properties of the ion pairs and the precursory carboxylic acids in the bulk state were examined by polarized optical microscopy (POM) and X-ray diffraction (XRD), demonstrating that liquid crystal (LC)-liquid and crystal-liquid phase transitions occurred, depending on the number and lengths of the aliphatic chains of each assembly. An ion pair exhibited photo-induced crystal-crystal phase transitions upon switching between two irradiation wavelengths (365/436 nm).

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

Pyrrole-4-aryl-substituted dipyrrolyldiketone BF2 complexes as anion-responsive π-electronic molecules were synthesized via a 3,5-dimethylpyrrole precursor. Mesophases were observed in derivatives that possessed long alkyl chains on the pyrrole-4-aryl groups along with their anion complexes as ion-pairing assemblies in combination with appropriate cations.

Cooperatively Interlocked [2+1]-Type π-System-Anion Complexes.

As fascinating examples of closely connected multiple π-electronic systems involving noncovalent interactions, the cooperatively interlocked [2+1]-type anion complexes of well-designed π-electronic molecules are reported. The anion-responsive π-electronic molecules investigated in this study are the boron complexes of dipyrrolyldiketones bearing arylethynyl moieties at the pyrrole α-positions. Diverse substituents were introduced at the terminal aryl moieties, thus controlling the cooperativity to form [2+1]-type complexes. Theoretical studies showed that this cooperativity was induced by effective interactions between the introduced arylethynyl moieties. The investigations of the anion-binding processes showed that the first and second bindings to the guest anion were energetically almost equivalent, indicating the fundamental and important features of the electronic states of the halide anions. Furthermore, the interlocked anion complexes, whose geometries were distorted from the orthogonal arrangements, had the racemic states of the chiral conformations, either of which was induced by ion pairing with an appropriate chiral π-electronic cation, resulting in the chiroptical property as shown by circular dichroism.

Ion-Pairing Crystal Polymorphs of Interlocked [2 + 1]-Type Receptor-Anion Complexes.

The formation of a solid-state totally charge-segregated assembly (polymorph A) of negatively charged layers comprising [2 + 1]-type Cl(-) complexes of an arylethynyl-substituted dipyrrolyldiketone boron complex and positively charged layers of tetrabutylammonium (TBA) cations has already been reported. The formation of two new crystalline polymorphs (polymorphs B and C), in addition to polymorph A, is reported in this study. Both polymorphs B and C formed charge-by-charge assemblies, and the dihedral angles between two receptor units in the interlocked complexes depended on the geometries of TBA cations and the resulting packing structures. Two nonorthogonally arranged planes induced P- and M-form chiral geometries, providing diverse arrangements of chiral species according to crystal polymorphs. Furthermore, the stabilities of the three polymorphs were examined by interfragment interaction energies, which were calculated by ab initio electronic structure calculations using the fragment molecular orbital (FMO) method.

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.

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.

Carboxylate-Driven Supramolecular Assemblies of Protonated meso-Aryl-Substituted Dipyrrolylpyrazoles.

Dipyrrolylpyrazole (dpp) derivatives possessing an aryl ring at the pyrazole 4-position were synthesized. Upon protonation, modified dpp derivatives formed a variety of assembled structures through complexation with carboxylates, as observed by single-crystal X-ray and synchrotron XRD analyses. In particular, the complexation of protonated dpp species possessing long alkyl chains with dicarboxylates resulted in highly ordered assembled structures, the packing modes of which as lamellar structures were controlled by the lengths of the spacer units between two carboxylate moieties. The charge-carrier transporting properties of the solid materials were also controlled by bound anions, including dicarboxylates.

Diastereoselective cyclization of an aminobenzoic acid derivative and chiroptical properties of triple-stranded helical bis(phenylethynyl)benzene.

The diastereoselective cyclization of 2,5-dibromo-4-hexylaminobenzoic acid was achieved by the microwave-assisted condensation using SiCl4. Moreover, the triple-stranded helical structure of bis(phenylethynyl)benzene units embedded in the cyclic tri(benzamide) scaffold was obtained by a Sonogashira-Hagihara coupling reaction. Two optically active enantiomers that do not racemize even at the elevated temperature were separated by chiral HPLC. The chiral helical topology was revealed by the spectroscopic data and theoretical calculation.

Helicity Induction in Three π-Conjugated Chromophores by Planar Chirality of Calixamide.

Shall we twist? Three-dimensional arrangement of π-conjugated chromophores with triple-stranded helicity was achieved by using the planar chirality of m-calix[3]amide. Based on spectroscopic data and theoretical calculations, the dynamic and preferred helical characters of bithiophene units embedded in the tubular molecule were elucidated, and the absolute configuration was determined.