Kohsuke Kawabata

Dynamically Controllable Emission of Polymer Nanofibers: Electrofluorescence Chromism and Polarized Emission of Polycarbazole Derivatives

Electrochemical polymerization of a series of N-alkyl-2,7-di(2-thienyl)carbazoles in acetonitrile was performed to obtain conjugated polymers with fluorescence. Scanning electron and atomic force microscopies revealed that the surface morphology of the polymer films significantly depends on the alkyl chain lengths of the polymers. Particularly, a homopolymer bearing hexyl groups and copolymers with an average alkyl chain length of six carbon atoms show nanofiber morphology. The polymer nanofibers were stacked on a substrate electrode. The fluorescence of the polymer nanofiber film was tunable with application of voltage, with good repeatability. The X-ray diffraction pattern of the fibers showed the structural order. The polymer nanofibers thus prepared showed an electrochemically driven change in polarized photoluminescence.

Synthesis and optical properties of 1,1-binaphthyl–thiophene alternating copolymers with main chain chirality

A series of axially chiral 1,1-binaphthyl–thiophene copolymers were synthesized by Migita–Kosugi–Stille polycondensation and electrochemical polymerization. The polymers exhibit circular dichroism not only in the ultraviolet region but also in the visible region, both in solution and film states. Optical and electrochemical properties of the polymers are in good agreement with theoretical results obtained by density functional theory calculations. Although effective conjugation of the polymers was cleaved at the orthogonally oriented two naphthalene rings in the 1,1-binaphthyl unit, the electrochemically synthesized polymer film shows good electrochemical redox behavior with repeatable changes in optical absorption and circular dichroism. The changes in circular dichroism may reflect a change in the dihedral angle of the binaphthyl unit caused by reorientation of conjugated main chains between oxidized and neutral states.

Light driven asymmetric polymerization: an approach for tele-control reaction

Chiroptical activity-tunable electro-polymerization is carried out in liquid crystal (LC) electrolyte solution under irradiation by UV or visible (Vis) light. The LC electrolyte prepared in this study undergoes photo-induced phase transition through trans–cisisomerization of an azobenzene-based chiral photo-isomerization dopant in the LC electrolyte solution. The trans–cisisomerization of the dopant induced by UV light changes the structural chirality of the cholesteric LC electrolyte solution, which allows chiroptical activity tuning under the polymerization conditions. The results demonstrate realization of light-commandable asymmetric polymerization through change in a chiral environment for the electro-polymerization reaction. Note that the chiral polymer thus synthesized shows redox (electrochemical doping/dedoping)-driven change in chiroptical activity. This method can be used to induce light-driven asymmetric chemical reactions.

Uniaxially ordered conjugated polymer film prepared by electrochemical polymerization in a nematic liquid crystal with rubbing orientation method showing redox-driven tunable dichroism

We demonstrate electrochemical polymerization of an aromatic conjugated monomer in a macroscopically aligned nematic liquid crystal. This method is a combination of employment of liquid crystal as the electrolyte solution, a rubbing technique, and electropolymerization. The resulting polymer film shows linear dichroism and anisotropy. Polarizing optical microscopy reveals that the polymer film is uniformly ordered on a macroscopic scale. These results indicate that the conjugated main chains grow along a director of the aligned nematic liquid crystal solvent.

Synthesis and characterisation of thieno[3,2-b]thiophene-based linear shaped liquid crystals

A series of thieno[3,2-b]thiophene-based liquid crystalline molecules has been synthesised. To investigate the effects of alkyl substituent groups and structure of mesogenic cores, eight kinds of thieno[3,2-b]thiophene-based molecules containing different alkyl substituent groups and mesogenic cores have been synthesised. These molecules were characterised by differential scanning calorimetry, polarising optical microscopy, ultraviolet–visible absorption spectroscopy, fluorescence spectroscopy, cyclic voltammetry and polarised fluorescence spectroscopy. The results indicated that the thieno[3,2-b]thiophene-based donor-acceptor-donor (D-A-D) type molecules showed the presence of a smectic A phase. Changes in the length of alkyl substituents and mesogenic cores affected the phase transitions, optical and electrochemical properties of the molecules.

Cholesteric liquid crystal inductive asymmetric polymerisation of thiophene monomers

The thermotropic properties of cholesteric liquid crystal as a polymerisation medium for the synthesis of chiral polymers from achiral monomers were clarified. The chiral reaction medium was successfully employed to synthesise chiral polymers from thiophenes as achiral monomer. The polythiophenes thus prepared exhibit intense circular dichroism imparted by the physical chiral continuum of the reaction field. The chiral aggregate of the present polymers displays structural chirality, with optical activity derived not from the chirality of the individual main‐chain but from the chiral architecture. The chiral polymers do not exhibit a thermotropic cholesteric liquid crystal phase but nematic liquid crystal character due to the release of the chiral architecture upon heating.

Synthesis of Isothianaphthene (ITN) and 3,4-Ethylenedioxy-Thiophene (EDOT)-Based Low-Bandgap Liquid Crystalline Conjugated Polymers

Copolymers, consisting of isothianaphthene and phenylene derivatives with liquid crystal groups, were synthesized via Migita-Kosugi-Stille polycondensation reaction. IR absorption, UV-vis optical absorption, and PL spectroscopy measurements were carried out. Thermotropic liquid crystallinity of the polymers with bandgap of ~2.5 eV was confirmed.

Synthesis and characterization of a novel donor–acceptor–donor chiral inducer and its application in electrochemical polymerization

A novel chiral inducer having a donor–acceptor–donor structure was synthesized. Its rigid structure allows an increase of miscibility and helical twisting power for the host liquid crystal. The donor–acceptor–donor structure allows visible light absorption. Electrochemical polymerization of an achiral monomer was performed in a cholesteric liquid crystal induced by the inducer. The chiral inducer molecules alone showed right-handed helical aggregation in the solid state, while the inducer produces a left-handed helical arrangement of cholesteric liquid crystals in a nematic liquid crystal. The resultant conjugated polymer films showed circular dichroism and left-handed helical aggregation. Film surfaces displayed a periodic convex–concave structure that was transcribed from the fingerprint texture of the cholesteric liquid crystal electrolytes during the polymerization process.

Intramolecular charge transfer (ICT) of a chiroptically active conjugated polymer showing green colour

A donor–acceptor type achiral monomer 4,7-bis(2,3-dihydrothieno[3,4-b]-1,4-dioxin-5-yl)-2,1,3-benzothiadiazole was synthesised and electrochemically polymerised in a cholesteric liquid crystalline (CLC) medium. This film exhibits a fingerprint pattern under the polarised optical microscopic observation. A possible mechanism of the fingerprint structure formation through a procedure of electrochemical polymerisation in a CLC medium is presented. The preparation of chiral polymer films even from achiral monomers is demonstrated. The film shows green colour in the reduced state and blue in the oxidised state. Changes in colour and CD signals of the film are repeatable with electrochemical oxidation and reduction. The presence of radical cations in the chiral environment, referred to as chiralions, distributed along chiral polymer chains is proposed.

Mechanical orientation in thermotropic liquid crystal state and magnetic orientation in solvent evaporation process via lyotropic liquid crystal state of an amphotropic low-bandgap liquid-crystalline π-conjugated polymer

Molecular orientations by shear stress and magnetic field were carried out to obtain a uniaxial, low-bandgap, amphotropic liquid-crystalline (LC) π–conjugated polymer. Orientation of a thick polymer film shows orientation along the shear stress, but a thin sample produced by a spatula drawing on a glass substrate produces LC band structure with a chevron pattern. Furthermore, we report the orientation of a lyotropic LC polymer under magnetic field.