Yuya Bando

Chemical-Stimuli-Controllable Circularly Polarized Luminescence from Anion-Responsive π-Conjugated Molecules

Introduction of a BINOL-boron moiety to dipyrrolyldiketones as precursors of anion-responsive π-conjugated molecules results in the formation of a chiral environment in the form of anion-free receptors and anion-binding complexes. Conformation changes by inversion (flipping) of two pyrrole rings as a result of anion binding can control the chiroptical properties of the anion receptors. In particular, appropriate pyrrole β-substituents induce distorted receptor π-planes and, as a result, give larger circularly polarized luminescence (CPL), which can be tuned by chemical stimuli (anions). This is the first example of chemical-stimuli-responsive CPL properties.

Asymmetric Induction in the Preparation of Helical Receptor–Anion Complexes: Ion‐Pair Formation with Chiral Cations

A number of helical structures have been reported. Foldamers form helical structures in response to chemical stimuli such as neutral molecules, metal cations, and anions. The ability to prepare enantiomerically enriched helical foldamers is crucial for applying helical structures to functional materials with chiroptical properties. One strategy for preparing enantiomerically enriched helices is the direct attachment of chiral moieties to the foldamers. In addition, the introduction of a chiral guest species can also induce the preferential formation of one diastereomer of the resulting complex through specific noncovalent interactions between the guest and the host system. Electrostatic interactions between oppositely charged species can occur in the absence of specific interactions. Therefore, a challenging way to make a compound fold into an enantiomerically pure chiral structure is to use electrostatic interactions between an achiral ion and an enantiomerically pure chiral counterion. In fact, chiral anions have been used for the preparation of enantiomerically pure metal helicates. 8] Conversely, the association of chiral cations with helixforming compounds that contain receptor and anionic moieties has led to the formation of enantiomerically pure helical structures. The chiroptical properties of receptor– anion helical complexes that form through hydrogen bonding can be difficult to examine because they can undergo more facile interconversion between enantiomeric helical forms compared to metal-based helices that form through coordination bonds. This fast interconversion is not a problem when one diastereomer of an ion pair consisting of a helical receptor–anion complexes and chiral counter cations is more stable than the other because then only one enantiomeric helix structure predominates in solution. p-Conjugated molecules that form helical structures in the presence of anions include boron complexes of 1,3-dipyrrolyl1,3-propanediones. These complexes, an example being 1a (Scheme 1a), bind anions through dynamic conformational changes involving rotation of the bond between the carbonyl group and the pyrrole moiety, thus resulting in helical oligomers (e.g., 2a and 2b, Scheme 1b). 11b, c,e] These helical oligomers were observed in the solid state and were comprised of alternately stacking negatively and positively charged species, that is, oligomer–anion complexes and counter cations, respectively. Anion complexes of the receptor-containing oligomers could be formed in enantiomerically enriched state in solution through ion pairing with optically active cations. In this paper, we report the preparation of enantiomerically enriched anionic helices that form electrostatic interactions with chiral counter cations; we also describe the chiroptical properties of these helices such as their circularly polarized luminescence (CPL). 13] Chiral p-conjugated cations are suitable candidates for inducing asymmetry in helix formation owing to their ability to form interactions with p-conjugated receptor–anion complexes. Therefore, we focused on the chiral binaphthylammonium Cl and Br salts, RR·X and SS·X (X = Cl and Br) (Scheme 1c), which Ooi, Kameda, and Maruoka reported as being efficient phase-transfer catalysts in enantioselective reactions. The formation of 1:1 receptor–anion complexes in solution can be followed by analyzing electronic spectra. Upon the addition of RR·Cl (1.5 equivalents) to 2b in CH2Cl2 (1 mm) at 20 and 70 8C, the UV/Vis absorption bands associated with 2 b at 514 and 523 nm decreased and those at [*] Dr. Y. Haketa, Y. Bando, Prof. Dr. H. Maeda College of Pharmaceutical Sciences, Ritsumeikan University Kusatsu 525–8577 (Japan) E-mail: maedahir@ph.ritsumei.ac.jp

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.

Electronic and Optical Properties in the Solid‐State Molecular Assemblies of Anion‐Responsive Pyrrole‐Based π‐Conjugated Systems

On the basis of the chemistry in solution, the solid-state structures and the corresponding electronic and optical properties of dipyrrolyldiketone boron complexes as π-conjugated acyclic anion receptors have been investigated. Solid-state assemblies of the receptors exhibit anion-dependent properties that are in sharp contrast to those in the solution state. Anion complexation, along with structural differences in the pyrrole subunits such as a benzo-fused pyrrole, plays an essential role not only in the formation of the assembled modes but also in determining electronic and optical properties, as well as the charge-carrier mobilities. In addition, by anion complexation, inclusion of the counter cations into the crystals has also been found to be one of the essential factors to determine the properties.

Corannulene‐Fused Anion‐Responsive π‐Conjugated Molecules that Form Self‐Assemblies with Unique Electronic Properties

The fusion of bowl-shaped π-conjugated corannulene units to anion-responsive π-conjugated dipyrrolyldiketone-boron complexes resulted in new molecular materials with a unique self-assembly capability. The bowl-fused receptor with aliphatic tails could form both supramolecular gels and mesophases through π-stacking interactions and also exhibited anion-responsive characteristics. The presence of the π-bowl unit not only afforded enhanced self-assembly capability both in solution and in the mesophases, as evidenced by gelation experiments and phase-transition profiles, but also enhanced intrinsic charge-carrier mobility.

Chirality Induction by Formation of Assembled Structures Based on Anion-Responsive π-Conjugated Molecules

Anion-responsive π-conjugated compounds having chiral alkyl chains were synthesized. Circular dichroism (CD) and circularly polarized luminescence (CPL) were observed in the solution-state assemblies of the chiral anion receptors and those of their anion complexes as salts of a planar triazatriangulenium cation. The CD and CPL spectral patterns of the ion-pair-based assemblies were completely opposite to those of the anion-free assemblies, and this suggests that anion binding and subsequent ion pairing change the chirality of the assembly modes.

Self-sorting self-complementary assemblies of π-conjugated acyclic anion receptors

Pyrrole-based π-conjugated anion-responsive molecules bearing an anionic moiety form self-complementary dimers, which exhibit self-sorting behaviours depending on the substituted positions of anionic sites.

Chiroptical Control in Helical Receptor–Anion Complexes

Dimers of appropriately arranged anion-responsive π-conjugated moieties form helical structures by interaction with chiral anions. Terphenyl-bridged dimers of dipyrrolyldiketone boron complexes show chirality induced by binding l-amino acid anions, as observed by circular dichroism (CD) and circularly polarized luminescence (CPL). The preferred configurations of helical structures depend on the geometries of the terphenyl spacer moieties.

Formation and geometrical control of polygon-like metal-coordination assemblies

Polygon-like [2+2]- and [3+3]-type metal complexes were prepared from dipyrrin dimers connected by acute-angled spacers. The electrical conduction depends strongly on the packing alignment of the compounds, revealing the presence of effective hopping pathways for holes with relatively high mobility up to 0.11 cm(2)  V(-1)  s(-1) along the aligned axis of [3+3]-type metal-bridged assemblies. These observations correlated with the geometrical control of the π-conjugated metal complexes in the cyclic structures, which enables their ordered arrangement in the assemblies.

Ion-Pairing Assemblies Based on Pentacyano-Substituted Cyclopentadienide as a π-Electronic Anion

Pentacyanocyclopentadienide (PCCp(-) ), a stable π-electronic anion, provided various ion-pairing assemblies in combination with various cations. PCCp(-) -based assemblies exist as single crystals and mesophases owing to interionic interactions with π-electronic and aliphatic cations with a variety of geometries, substituents, and electronic structures. Single-crystal X-ray analysis revealed that PCCp(-) formed cation-dependent arrangements with contributions from charge-by-charge and charge-segregated assembly modes for ion pairs with π-electronic and aliphatic cations, respectively. Furthermore, some aliphatic cations gave dimension-controlled organized structures with PCCp(-) , as observed in the mesophases, for which synchrotron XRD analysis suggested the formation of charge-segregated modes. Noncontact evaluation of conductivity for (C12 H25 )3 MeN(+) ⋅PCCp(-) films revealed potential hole-transporting properties, yielding a local-scale hole mobility of 0.4 cm(2)  V(-1)  s(-1) at semiconductor-insulator interfaces.