Kazuto Takaishi

Non-volatile organic memory with sub-millimetre bending radius

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 μm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

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.

Non-destructive erasable molecular switches and memory using light-driven twisting motions

Novel types of chiroptical switches and memory with non-destructive readout that are entirely optically controlled for molecular devices in solution and neat films.

Planar chirality of twisted trans-azobenzene structure induced by chiral transfer from binaphthyls.

The absolute configuration of a binaphthyl-azobenzene dyad 2b, which has a chiral axis and a chiral plane, was determined by comparing the experimental circular dichroism (CD) spectra with the theoretical CD spectra calculated by the time-dependent (TD)-DFT method. The CD signals of the trans-azobenzene moiety indicated that the two benzene rings of this moiety are twisted unidirectionally. It is suggested that these dyads with shorter linkers may be suitable for use as chiroptical switches.

Oligonaphthofurans: Fan-Shaped and Three-Dimensional π-Compounds

Using a bottom-up method, we prepared a series of oligonaphthofurans composed of alternating naphthalene rings and furan rings. The largest compound (compound 25) contained 8 naphthalene units and 7 furan units. DFT calculations revealed that these compounds were fan-shaped molecules and each naphthalene ring was oriented in an alternate mountain-valley fold conformation because of steric repulsion by the hydrogens at the peri-positions. We investigated the optical properties that derived from their fan-shaped and mountain-valley sequences. As the number of aromatic rings of the oligonaphthofurans increased, the peaks of the longest wavelength absorptions in the UV-vis spectra (HOMO-LUMO energy gap) of these compounds steadily red-shifted, although the shapes of spectra were not sustained because of the decreasing molar absorption coefficients (εs) of their λ(max). We compared our results with those reported for other types of oligoaromatic compounds such as acenes 1, ethene-bridged p-phenylenes 2, rylenes 3, oligofurans 4, and oligonaphthalenes 5. The slopes of the plots between the transition energies (HOMO-LUMO energy gap) of the oligoaromatic compounds and the reciprocal of the number of aromatic rings indicated that the efficiency of π conjugation of the oligonaphthofurans was comparable with that of linear and rigid acenes and rylenes. The higher-order compounds 22 and 25 aggregated even under high dilution conditions (~10(-6) M).

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.

Photoinversion of cisoid/transoid binaphthyls.

Axially chiral binaphthyl-azobenzene cyclic dyads in which the two moieties are connected by two linkers of different lengths were synthesized. In the case of benzylated-binaphthyl analogue 2b, photoirradiation resulted in a dramatic change of the CD spectrum and optical rotation. Experimental and theoretical analyses indicated that the dihedral angle of the two naphthalene rings is strongly coupled to the azobenzene photoisomerization; cis-azobenzene induces a transoid-binaphthyl structure, while trans-azobenzene induces a cisoid-binaphthyl structure.

Ambipolar organic field-effect transistors based on solution-processed single crystal microwires of a quinoidal oligothiophene derivative

A simple and versatile solution-processing method based on molecular self-assembly is used to fabricate organic single crystal microwires of a low bandgap quinoidal oligothiophene derivative. Individual single crystal microwire transistors present well-balanced ambipolar behaviour with hole and electron mobilities as high as 0.4 and 0.5 cm(2) V(-1) s(-1), respectively.

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.