Discrete Self-Assembly and Functionality of Guest Molecules in an Organic Framework

In this study, the fundamental issue of “how various functional molecules can be homogeneously and densely arrayed in a solid material” is addressed using discrete self-assembly of guest molecules with an orthogonal architecture (<b>1</b>) comprising hydrogen bonds; this method has become a new paradigm for constructing functional materials. The serendipitous finding of cocrystallization in <b>1</b> was prompted by its unexpectedly tight but transmutable inclusion of guest molecules. Notably, the self-assembly of <b>1</b> with imidazole molecules produced a water-durable, heat-resistant, one-dimensional (1D) anhydrous proton-conducting channel with proton-transfer sites in very close proximity (N–N distance, 2.481 Å). The close sites lead to faster proton transfer when compared with pure imidazole; this advantage via the close sites is attributable to wobbling rather than rotational motion. These results suggest that discrete self-assembly can enable the development of new design concepts for functional materials

Synthesis and Property of Semicrystalline Anion Exchange Membrane with Well-Defined Ion Channel Structure

A new poly­(3,4′-alkylbithiophene)-based anion exchange membrane was designed and synthesized. A well-defined crystalline bilayer lamellae structure with ion channel was constructed by self-organization of the polymer and maintained even in a hydrated state because of the semicrystalline nature of the materials. As a result, swelling of the membrane was effectively suppressed, and higher anion conductivity was obtained compared with a control polymer without ion channel. The synthesized polymer is a promising model system for further understanding of the relationship between solid-state structures and anion-conducting membrane properties

Quantum Chemical Studies on Electron-Accepting Overcrowded Ethylene with a Polarizable Skeleton

We report the quantum chemical studies on the neutral and radical anion forms of an electron-accepting overcrowded ethylene (OCE1) featuring a highly polarizable skeleton based on the density functional theory (DFT) approach using the M06-2X hybrid functional. Calculated results indicate that OCE1 (bis­{4<i>H</i>,8<i>H</i>-4-(dicyanomethylene)­benzo­[1,2-<i>c</i>:4,5-<i>c</i>′]­bis­[1,2,5]­thiadiazol-8-ylidene}) shows conformational behaviors and energetics similar to those of bianthrone (OCE2), a typical thermochromic overcrowded ethylene. Neutral OCE1 and its radical anion have antifolded (afOCE1) and twisted (tOCE1) isomers on their potential energy surfaces. The calculated isomerization barrier heights of OCE1 and its radical anion are considerably low, indicating that its conformation is susceptible to interactions with surrounding molecules. While two afOCE1 molecules can form a simple π-stacked dimer, tOCE1 tends to be converted to afOCE1 when the two tOCE1 molecules come close together, indicating the instability of tOCE1 in the homogeneous OCE1 solid state. The thermochromic behavior difference between OCE1 and OCE2 in solution is closely associated with the considerably small energy difference between the afOCE1 and the tOCE1 as compared with OCE2. The properties of OCE1 are also compared with other typical electron-accepting overcrowded ethylenes in terms of electronic structure, energetics, and conformational behaviors

Dynamic or Nondynamic? Helical Trajectory in Hexabenzocoronene Nanotubes Biased by a Detachable Chiral Auxiliary

When ether vapor was allowed to diffuse into a CH₂Cl₂ solution of an enantiomer of a hexa-<i>peri</i>-hexabenzocoronene (HBC) derivative carrying a chiral (BINAP)­Pt­(II)-appended coordination metallacycle (HBC^Py_{[(<i>R</i>)‑Pt]} or HBC^Py_{[(<i>S</i>)‑Pt]}), screw-sense-selective assembly took place to give optically active nanotubes (NT^Py_{[(<i>R</i>)‑Pt]} or NT^Py_{[(<i>S</i>)‑Pt]}) with helical chirality, which were enriched in either left-handed (<i>M</i>)-NT^Py_{[(<i>R</i>)‑Pt]} or right-handed (<i>P</i>)-NT^Py_{[(<i>S</i>)‑Pt]}, depending on the absolute configuration of the (BINAP)­Pt­(II) pendant. When MeOH was used instead of ether for the vapor-diffusion-induced assembly, nanocoils formed along with the nanotubes. As determined by scanning electron microscopy, the diastereomeric excess of the nanocoils was 60% (80:20 diastereomeric ratio). Removal of the (BINAP)­Pt­(II) pendants from NT^Py_{[(<i>R</i>)‑Pt]} or NT^Py_{[(<i>S</i>)‑Pt]} with ethylenediamine yielded metal-free nanotubes (NT^Py) that remained optically active even upon heating without any change in the circular dichroism spectral profile. No helical inversion took place when NT^Py derived from HBC^Py_{[(<i>R</i>)‑Pt]} or HBC^Py_{[(<i>S</i>)‑Pt]} was allowed to complex with (BINAP)­Pt­(II) with an absolute configuration opposite to the original one

Appearance of Different Conductance States in Monomolecular Films of Ferrocene-Decorated Triptycene-Based Tripods

Ferrocene (Fc) is a widely used building block of molecular rectifiers in the context of molecular electronics. Here, we studied the molecular organization and charge transport properties of the Fc-substituted, triptycene-based tripodes (Fc-Trip), assembled on Au(111) in the self-assembled monolayer-like fashion. The most intriguing property of this system is the occurrence of two distinctly different conductance states, high and low (HCS and LCS, respectively), which can be accessed dynamically by either asymmetric or symmetric bias sweeping in molecular junctions featuring bottom Au (substrate) and top EGaIn electrodes. For the asymmetric sweeping mode, the difference between these states results in an effective rectification ratio (RR) of ∼400–600 at such a small bias as 0.1–0.2 V, which is in contrast to other Fc-based molecular rectifiers showing high RR at a bias of 1.0 V and higher. Following a literature model, the observed behavior was explained by bias-induced, nonreversible oxidation of the Fc groups in combination with conformational changes in the molecular film. The above results show that redox groups in ME systems, including metallocenes in particular, can exhibit a complex behavior that can only be observed by the variation of the sweeping mode and monitoring of individual sweeps

Helix Sense-Selective Supramolecular Polymerization Seeded by a One-Handed Helical Polymeric Assembly

Helix sense-selective supramolecular polymerization was achieved using a one-handed helical nanotubular polymeric assembly as a seed. First, bipyridine (BPY)-appended achiral hexabenzocoronene (^BPYHBC) was copolymerized noncovalently with chiral ^BPYHBC_<i>S</i> (or ^BPYHBC_<i>R</i>) at a molar ratio of 9:1, which, via the sergeants-and-soldiers effect, afforded a <i>P</i>-helical (or <i>M</i>-helical) nanotube, which was then treated with Cu²⁺ to transform into structurally robust ^(BPY)CuNT_(<i>P</i>) (or ^(BPY)CuNT_(<i>M</i>)) with a Cu²⁺/BPY coordination polymer shell. Helical seeds ^(BPY)CuNT_(<i>P</i>) and ^(BPY)CuNT_(<i>M</i>) brought about the controlled assembly of fluorinated chiral FHBC_<i>S</i> and FHBC_<i>R</i> as well as achiral FHBC to yield one-handed helical nanotubular supramolecular block copolymers, in which the helical senses of the newly formed block segments were solely determined by those of the helical seeds employed. Noteworthy, FHBC_<i>S</i> and FHBC_<i>R</i> alone without the helical seeds form ill-defined agglomerates. Attempted supramolecular polymerization of a racemic mixture of FHBC_<i>S</i> and FHBC_<i>R</i> from ^(BPY)CuNT_(<i>P</i>) (or ^(BPY)CuNT_(<i>M</i>)) resulted in its chiral separation, affording <i>P</i>-helical (or <i>M</i>-helical) diastereomeric block segments composed of FHBC_<i>S</i> and FHBC_<i>R</i> with different thermodynamic properties

Mesoscopic Structural Aspects of Ca²⁺-Triggered Polymer Chain Folding of a Tetraphenylethene-Appended Poly(acrylic acid) in Relation to Its Aggregation-Induced Emission Behavior

We recently reported that tetraphenylethene-appended poly­(acrylic acid) derivatives [PAA-TPE_<i>x</i> (<i>x</i> = 0.01–0.05)] provide a fluorescent Ca²⁺ sensor, where aggregation-induced emission (AIE) of the TPE pendants occurs in conjunction with Ca²⁺-triggered polymer-chain folding. On the basis of dynamic and static light-scattering data, here we discuss the hydrodynamic radius and molar mass of PAA-TPE_0.01 in the presence of Ca²⁺, Mg²⁺, or Na⁺ at various concentrations and elucidate the origin of the Ca²⁺ selectivity. In contrast to Na⁺, which exclusively triggered nonfluorescent interpolymer aggregation of PAA-TPE_0.01, Ca²⁺ and Mg²⁺ induced polymer-chain folding associated with AIE from the TPE pendants. Importantly, Ca²⁺ caused polymer-chain folding more effectively than Mg²⁺. Consequently, polymer aggregates formed in the presence of Ca²⁺ possessed a much higher inner density than those formed in the presence of Mg²⁺, leading to a more pronounced AIE behavior and, in turn, the Ca²⁺ ion selectivity over Mg²⁺

Effect of Acceptor Lamination on Photocarrier Dynamics in Hole Transporting Hexabenzocoronene Nanotubular Self-Assembly

Measurements of transient photoconductance under an external magnetic field were used to investigate photocarrier dynamics in low-dimensional hexabenzocoronene (HBC) self-assemblies, which are a promising material group for highly efficient solar cells achieved by bottom-up technology, and to clarify the effect of lamination with electron acceptor layer on the surfaces of HBC nanotubes. In an HBC column without an acceptor, the carrier generation yield from a geminate electron–hole (e-h) pair is dependent on the external electric and magnetic fields. The time dependence of the magnetic field effect on geminate e-h pair dynamics in the HBC column structure was analyzed to estimate the recombination rate constants of the singlet and triplet e-h pairs (<i>k</i>_r^S and <i>k</i>_r^T), which were 1.5 × 10⁸ and 1.2 × 10⁸ s^–1 respectively. The same kinetic analysis with consideration of the electric field effect on the photocarrier generation yield provided an electric field dependent dissociation rate constant in the range of 10⁷–10⁸ s^–1 in the HBC column structure. However, neither electric nor magnetic field effects on the carrier generation process were observed in acceptor-appended HBC nanotubes. The disappearance of the external field effects in acceptor-appended HBC indicates that the geminate recombination is reduced substantially by a well-organized donor/acceptor heterojunction with an interval of a few nanometers due to some σ-bonds. However, efficient nongeminate recombination with a ratio of <i>k</i>_r^S:<i>k</i>_r^T = 1.0:0.8 in the acceptor-appended HBC nanotubes was also elucidated by the incident photon density and magnetic field effects, which is an inherent nature in materials with high carrier density

Photoconductivity of Self-Assembled Hexabenzocoronene Nanotube: Insight into the Charge Carrier Mobilities on Local and Long-Range Scales

We performed kinetic study on the photoconductivity of coassembled nanotubes consisting of an amphiphilic hexa-<i>peri</i>-benzocoronene (HBC) and a HBC derivative appended with an electron-accepting trinitrofluorenone (TNF) moiety. The transient signals measured by flash-photolysis time-resolved microwave conductivity (FP-TRMC) were analyzed with a modified analytical solution of a first- and second-order differential equation. On the basis of charge-carrier generation efficiency independently determined by transient absorption spectroscopy (TAS), we simultaneously assessed the local and long-range charge carrier mobilities of the coassembled nanotubes. The former was found to be 3 cm² V^–1 s^–1 for the coassemblies but dropped to 0.7 cm² V^–1 s^–1 at HBC-TNF = 100% due to lowering of a structural integrity of π-stacked HBC arrays. In contrast, the latter was <1.5 × 10^–4 cm² V^–1 s^–1 and monotonically decreased when the HBC-TNF content was increased. Discussion would be of help in the better understanding of the electrical property of self-assembled organic nanoelectronics

Supramolecular Assemblies of Ferrocene-Hinged Naphthalene­diimides: Multiple Conformational Changes in Film States

We design a new naphthalenediimide (NDI) π-system, <b>NDI–Fc–NDI</b>, having a ferrocene linker as a hinge unit and long alkyl chains as supramolecular assembling units. The NDI units are “directionally flexible” in concert with the pivoting motion of the ferrocene unit with a small rotational barrier. The NDI units rotate around the ferrocene unit faster than the NMR time scale in solution at room temperature. UV–vis absorption, synchrotron X-ray diffraction, and atomic force microscope studies reveal that <b>NDI–Fc–NDI</b> forms a fibrous supramolecular assembly in solution (methylcyclohexane and highly concentrated chloroform) and film states, wherein the NDI units are in the slipped-stack conformation. The <b>NDI–Fc–NDI</b> supramolecular assembly in the film state exhibits multiple phase transitions associated with conformational changes at different temperatures, which are confirmed by differential scanning calorimetry, polarized optical microscopy, and temperature-dependent X-ray diffraction. Such thermal transitions of <b>NDI–Fc–NDI</b> films also induce changes in the optical and electronic properties as revealed by UV–vis absorption and photoelectron yield spectroscopies, respectively. The thermal behaviors of <b>NDI–Fc–NDI</b>, realized by the unique molecular design, are considerably different from the reference compounds such as an NDI dimer connected with a flexible 1,4-butylene linker. These results provide us with a plausible strategy to propagate the molecular dynamics of the π-system into macroscopic properties in film states; the key factors are (i) the supramolecular alignment of molecular switching units and (ii) the directional motion of the switching units perpendicular to the supramolecular axis