Research & Results
Adv. Photon. Res. 2023, 2200357.
“Solvent-Dependent Growth of Rigid Styrylstilbene Dicarboxylic Acid Microcrystals as Bent Waveguides and Microlasers”
Organic single-crystalline structures can effectively generate and confine photons, which can be potential building blocks for the optical circuits. While designing of organic molecules can tune the photoluminescence (PL) property, it is challenging to achieve efficient laser emission and light resonance in different forms of molecular aggregates. Herein, a dicarboxylated carbon-bridged oligophenylenevinylene (COPV2-(COOH)2) is newly synthesized. The THF/water solution of COPV2-(COOH)2 exhibits green-color PL with quantum yield >0.90 over the range of THF/water = 100/0–40/60. COPV2-(COOH)2 forms either bent microrods (MR) or microsheets (MS) depending on the crystallization solvents. Significantly, PL resonance and lasing are only observed in MS, indicating that MS has better light confinement property than MR. MS features Q-factor as high as 1.8 × 103 with the lasing threshold of 222 μJ cm−2. Finite-difference time-domain (FDTD)-simulated electric field distributions in MR and MS validate experimental observations.
Laser Photon. Rev. 2023, 2200874.
“Pneumatically Tunable Droplet Microlaser”
An ideally flexible laser may function in unison with minute fluctuations in nature. Lasers made solely from liquids are promising toward this end, but they are intrinsically unstable and have been inapplicable to steady operation under ambient conditions unless they are enclosed in a tailored container or a matrix to prevent the evaporation of the liquid. Here, a simple methodology is reported to form a self-standing spherical microlaser that is composed fully of liquid and operates steadily even under atmosphere. The robustness and spherical morphology of the droplets are achieved by using ionic liquid as the liquid medium and gently casting the droplets on a substrate covered with hydrophobic nanoparticles to enhance the metastability of the contact angle. The resulting droplets are highly robust and work as efficient long-lasting laser oscillators. The lasing wavelength is sensitively shifted when the droplets are subjected to a faint breeze or moisture, which is associated with the deformation of the droplet. The morphological and optical responses of the droplet under gas convection are consistently supported by aerodynamic and electromagnetic simulations. The droplets are readily scalable with an inkjet printer without the need for any further treatments.
Chem. Commun. 2023, 59, 1477–1480
“Degradable Optical Resonator as in situ Micro-probes for Microscopy-based Observation of Enzymatic Hydrolysis”
Optical resonators work as precise physical and chemical sensors. Here, we assemble a whispering gallery mode resonator from a natural polymer, fibroin protein, and successfully observe its catalytic degradation reaction as a spectral shift. This methodology will contribute to the precise in situ observation of biological reactions by optical microscopy.
Adv. Opt. Mater. 2022, 2202563
“Micrometer-Scale Optical Web Made of Spider Dragline Fibers with Optical Gate Operations”
Spider dragline silk fibers produced by spiders are a masterpiece structural protein that is durable even in the wild conditions. Dragline silk fibers are known to have excellent mechanical toughness and flexibility and further possess optical transparency, which are highly promising as textiles, sensors, and optical devices. Here, we show that the dragline silk microfibers act as a good optical waveguiding properties with the optical loss coefficient as small as 0.03 decibel per micrometer. The light transportation in the dragline silk fiber is performed by fluorescence energy transfer between two microspheres located on a suspended dragline silk microfiber. By utilizing a micromanipulation equipment, we fabricate micrometer-scale spider web-like structures. The experimentally observed optical waveguiding properties of the weaves matches well with simulation results using simple mathematical models. Furthermore, optical logic gate operations are demonstrated using photoswitchable microsphere resonators attached on the dragline fibers.
Adv. Opt. Mater. 2022, 2202134
“Optical Control of Aggregation Induced Emission Shift by Photoisomerizable Precipitant in a Liquid Droplet Microresonator”
Emission-switchable fluorophores often include stimuli-responsive units in their molecular structures. This strategy works well, but the applicable compounds are limited to the derivatives of several kinds of photochromic molecules such as diarylethene, azobenzene, and spiropyran. Here, this work presents a simple methodology based on a photoresponsive precipitant for achieving color-switchable photoluminescence. A luminescent dye, cyano-substituted oligo(phenylenevinylene) (COPV), features both twisted intramolecular charge transfer and aggregation-induced emission shift properties, leading to the change in the luminescence color from green to red upon precipitation. The COPV, together with photoisomerizable precipitant azobenzene (C6), is doped into spherical droplets of epoxy resin (ER) in a liquid state. The photoisomerization of C6 induced by UV irradiation and heating alternatively precipitates out and dissolves COPV in ER and changes the photoluminescence color, while maintaining the optical microresonator properties. This study will open a promising way for assembling/disassembling novel emission color-switchable systems.
Chem. Commun. 2022, 58, 11887–11897
“Functions and fundamentals of porous molecular crystals sustained by labile bonds”
Organic molecules favour dense packing so that they can maximise the enthalpic gain upon solidification. Multidentate organic molecules that can form reticular bonding networks have been considered essential to overcome this tendency and assemble the molecules in a porous manner. Meanwhile, contrary to this understanding, a few organic molecules have been found to form porous molecular crystals by simply stacking with each other via van der Waals forces or analogous very weak noncovalent interactions. Although the porous molecular crystals were relatively rare in the 2000s due to the difficulty in the synthesis, their number has increased in the last decade, and their functional uniqueness has been unveiled eventually. This article reviews the recent advances in such functional porous molecular crystals. Particularly, thermal stability, processability, structural dynamicity, reactivity, and self-healing ability are highlighted. In addition, fundamental principles behind their functionalities, including the history, energetics, and the effect of crystallization solvent, are also reviewed.
Science 2022, 377, 673-677
“Synchronous assembly of chiral skeletal single-crystalline microvessels”
Skeletal or concave polyhedral crystals appear in a variety of synthetic processes and natural environments. However, their morphology, size, and orientation are difficult to control because of their highly kinetic growth character. We report a methodology to achieve synchronous, uniaxial, and stepwise growth of micrometer-scale skeletal single crystals from planar-chiral double-decker molecules. Upon drop-casting of a heated ethanol solution onto a quartz substrate, the molecules spontaneously assemble into standing vessel-shaped single crystals uniaxially and synchronously over the wide area of the substrate, with small size polydispersity. The crystal edge is active even after consumption of the molecules and resumes stereoselective growth with successive feeding. The resultant morphology can be packed into polycyclic aromatic hydrocarbon–like microarchitectures and behaves as a microscopic container.
Polymer J. 2022, 55, 547–553.
“Hydrothermal crosslinking of poly(fluorenylamine) with styryl side chains to produce insoluble fluorescent microparticles”
Fluorescent microbeads are emerging as optical probes for biological applications. However, insoluble crosslinked microbeads are difficult to synthesize. The existing methodology, in which polymerization occurs within dispersed micelles, is only applicable to a limited number of polymeric compounds. Herein, we report the hydrothermal synthesis of insoluble fluorescent microbeads. The newly designed and synthesized fluorenylamine-based polymers contained two styryl groups and self-assembled into spherical microparticles upon emulsification. In contrast to microparticles heated under atmospheric conditions, the microparticles became insoluble upon hydrothermal treatment and maintained their spherical morphology. Microparticles that contained a mixture of the thermosetting polymer with polystyrene resulted in an enhanced fluorescence quantum yield and a fluorescence color that could be adjusted by the mixing ratio from red to green and blue.
ACS Appl. Polym. Mater. 2022, 4, 1065-1070.
“Nanoporous Fluorescent Microresonators for Non-wired Sensing of Volatile Organic Compounds down to the ppb Level”
Luminescent resonators have been exploited as promising non-wired sensors for volatile organic compounds (VOCs). Nonetheless, their sensitivity is insufficient to detect dilute VOCs down to ppb concentration ranges due to the little physisorption quantity at the surface. Here we report a nanoporous fluorescent resonator that exhibits an enhanced optical response to VOC molecules by accumulative physisorption in the pores and, thus, detects VOCs with concentration down to ppb ranges. A polymer with intrinsic microporosity is allowed to assemble into spherical microparticles (MSPIM-1) that exhibit whispering gallery mode resonance. The resonance peak undergoes spectral shift upon exposure to a model gaseous compound (pyridine) with excellent sensitivity (0.40 nm ppm–1) and LOD (470 ppb) without losing its linearity down to 800 ppb. Adsorption of pyridine in the pores and underlying molecular mechanism can be quantitatively evaluated based on the change in refractive index. MSPIM-1 preferentially detects aromatic hydrocarbons as well as pyridine while is less responsive to highly polar or non-polar chemicals.
Adv. Opt. Mater. 2021, 2101808.
“Mechanically Flexible and Optically Tunable Organic Crystal Resonator”
An organic single-crystal optical resonator that maintains its optical performance even upon mechanical bending remains a challenging target. Here, the first flexible organic crystal (FOC) that works as a tunable optical resonator as a function of bending angle is developed. A fluorescent-conjugated molecule, cyano-substituted oligo(phenylenevinylene)s, is allowed to crystallize slowly, yielding a discrete rod-shape microcrystal (FOCCOPV). FOCCOPV not only works as an e& cient optical waveguide along its longitudinal direction with the optical loss coefficient of 0.249 dB μm−1 but also works as a Fabry-Pérot type optical resonator along its lateral direction. The resonance wavelength is finely tunable toward the shorter wavelength region by the mechanical bending of FOCCOPV plausibly due to the Poisson effect, namely, the contraction of the lateral dimension of the crystal by the longitudinal strain, along with the change in the refractive index. The mechanical bending and associated spectral shift is repeated for many cycles without deterioration in their mechanical and optical properties.
Commun. Chem 2021, 4, 122.
“Solvophobicity-directed assembly of microporous molecular crystals”
Dense packing is a universal tendency of organic molecules in the solid state. Typical porous crystals utilize reticular strong intermolecular bonding networks to overcome this principle. Here, we report a solvophobicity-based methodology for assembling discrete molecules into a porous form and succeed in synthesizing isostructural porous polymorphs of an amphiphilic aromatic molecule Py6Mes. A computational analysis of the crystal structure reveals the major contribution of dispersion interaction as the driving force for assembling Py6Mes into a columnar stacking while the columns are sterically salient and form nanopores between them. The porous packing is facilitated particularly in solvents with weak dispersion interaction due to the solvophobic effect. Conversely, solvents with strong dispersion interaction intercalate between Py6Mes due to the solvophilic effect and provide non-porous inclusion crystals. The solvophobicity-directed polymorphism is further corroborated by the polymorphs of Py6Mes-analogues, m-Py6Mes and Ph6Mes.
ACS Appl. Polym. Mater. 2022, 4, 1065-1070.
“Polymer Optical Microcavity Sensor for Volatile Organic Compounds with Distinct Selectivity toward Aromatic Hydrocarbons”
A whispering-gallery mode (WGM) optical resonance sensor for volatile organic compounds (VOCs) is developed from polystyrene (PS) microspheres doped with fluorescent β-cyano-appended oligo(p-phenylenevinylene) (β-COPV). The β-COPV-doped PS microspheres (MSCOPV) are formed by the miniemulsion method in a binary solvent. MSCOPV expand upon permeation of VOCs into the PS matrix and exhibit a spectral shift of the WGM resonance peak. The permeation efficiency is highly dependent on the chemical affinity between the analyte and the polymer matrix, with exceptionally high selectivity toward aromatic hydrocarbons such as benzene, toluene, and xylenes (BTXs). The high selectivity and sensitivity of MSCOPV are in clear contrast to those of conventional WGM sensors that just detect VOCs nonpreferentially through adsorption onto the surface.
Adv. Funct. Mater. 2021, 2103685.
“Photochemically Switchable Interconnected Microcavities for All-Organic Optical Logic Gate”
Optical microcavities confine molecular luminescence and transfer it to a far longer distance than the conventional Förster resonant energy transfer process. Such cavity-mediated energy transfer is advantageous for use in optical circuitry. However, to realize all-organic optical circuits, optical gate operation with organic materials is indispensable. Here, all-organic optical gates consisting of polymer whispering gallery mode (WGM) resonators that work as the optical source, drain, and gate, which are interconnected with polymer microfiber, are demonstrated. Photoirradiation of the source sphere, as an optical input, triggers the blue fluorescence that transmits to the gate sphere through the fiber. The fiber interconnection enhances both the light confinement efficiency in the individual spheres and the light transmission efficiency between distant spheres. The gate sphere contains photoisomerizable fluorescent dye that converts, in its closed state, the blue emission into green light, which is again transmitted to the drain sphere through the fiber and lets the sphere emit red light as an output. This optical cascade is switched on and off upon photoisomerization of the dye in the gate sphere. Furthermore, an energy cascade equipped with two gate spheres works as an OR-type logic gate, demonstrating potential utility for the future all-organic and all-optical integrated devices.
Mater. Chem. Front. 2021, 5, 5653-5657.
“Silk fibroin microspheres as optical resonators for wide range humidity sensing and biodegradable lasers”
Biopolymers, which are in definition organic macromolecules produced by living matter, are promising materials in terms of biodegradability and sustainability. However, in the field of optical sensing, most of the devices or their elements still rely on the synthetic inorganic or organic compounds. In this study, we successfully transform pristine silk fibroin (SF), a naturally abundant and eco-friendly material, into discrete, well-defined microspherical particles that function as an active optical resonator for precise moisture sensing over a wide humidity range. The SF self-assembles into microspherical particles upon water-in-oil emulsification. When being photoexcited, the SF microspheres, doped with fluorescent ionic dye, display whispering gallery mode (WGM) resonant luminescence. Because of the excellent hygroscopic property, the SF microspheres expand in size as increasing the surrounding humidity, and the WGM resonant peaks exhibit a red shift with a linear relation to the surrounding humidity over a wide humidity range up to 95 percent relative humidity (%RH) with the sensitivity as high as 187 pm/%RH. Furthermore, the SF microsphere works as a laser resonator upon femto-second pumping. The fully natural microspheres as presented in this work will serve as sustainable and environmentally friendly optical resonators and sensors.
J. Am. Chem. Soc. 2021, 143, 8772-8779.
“Robust Angular Anisotropy of Circularly Polarized Luminescence from a Single Twisted-bipolar Polymeric Microsphere”
It has long been surmised that the circular polarization of luminescence (CPL) emitted by a chiral molecule or a molecular assembly should vary with the direction in which the photon is emitted. Despite its potential utility, this anisotropic CPL has not yet been demonstrated at the level of single molecules or supramolecular assemblies. Here we show that conjugated polymers bearing chiral side chains self-assemble into solid microspheres with a twisted bipolar interior, which are formed via liquid–liquid phase separation and subsequent condensation into a cholesteric lyotropic liquid crystalline mesophase. The resultant microspheres, when dispersed in methanol, exhibit CPL with a glum value as high as 0.23. The microspheres are mechanically robust enough to be handled with a microneedle under ambient conditions, allowing comprehensive examination of the angular anisotropy of CPL. The single microsphere is found to exhibit distinct angularly anisotropic birefringence and CPL with glum up to ∼0.5 in the equatorial plane, which is 2.5-fold greater than that along the polar axis. Such optically anisotropic solid materials are important for the application to next-generation microlight-emitting and visualizing devices as well as for fundamental optics studies of chiral light–matter interaction.
Polymers 2021, 13, 269/1-9.
“Fluorescence switchable conjugated polymer microdisk arrays by cosolvent vapor annealing”
Depositing minute light emitters into a regular array is a basic but essential technique in display technology. However, conventional lithographic methodologies involve multistep and energy-consuming processes. Here, we develop a facile method in which organic and polymeric fluorescent dyes spontaneously aggregate to form a patterned microarray. We find that a thin film of fluorescent π-conjugated polymer transforms into micrometer-sized aggregates when exposed to binary organic vapor at ambient temperature. The arrayed microaggregates can be formed over the whole substrate surface when using a quartz substrate that is prepatterned with regular hydrophilic boxes and hydrophobic grids. The resultant microarray is applicable to optical memories and displays when photoswitchable fluorophores are doped into the polymer matrix.
Adv. Mater. 2020, 33, 2006061.
“Fast Response Organic Supramolecular Transistors Utilizing In‐Situ π‐Ion Gels”
Abstract: Despite their remarkable charge carrier mobility when forming well‐ordered fibers, supramolecular transistors often suffer from poor processability that hinders device integration, resulting in disappointing transconductance and output currents. Here, a new class of supramolecular transistors, π‐ion gel transistors (PIGTs), is presented. An in situ π‐ion gel, which is an unprecedented composite of semiconducting nanofibers and an enclosed ionic liquid, is directly employed as an active material and internal capacitor. In comparison to other supramolecular transistors, a PIGT displays a high transconductance (133 µS) and output current (139 µA at −6 V), while retaining a high charge‐carrier mobility (4.2 × 10^−2 cm^2 V^−1 s^−1) and on/off ratio (3.7 × 10^4). Importantly, the unique device configuration and the high ionic conductivity associated with the distinct nanosegregation enables the fastest response among accumulation‐mode electrochemical‐based transistors (＜20 µs). Considering the advantages of the absence of dielectric layers and the facile fabrication process, PIGT has great potential to be utilized in printed flexible devices. The device platform is widely applicable to various supramolecular assemblies, shedding light on the interdisciplinary research of supramolecular chemistry and organic electronics.
Mater. Chem. Front. 2020, 5, 799-803.
“Highly sensitive humidity sensor based on aggregation-induced emission luminogen-appended hygroscopic polymer microresonator”
Abstract: Self-assembled microspherical aggregates that work as whispering gallery mode (WGM) optical resonators are synthesized from a novel hygroscopic polymer covalently appended with an aggregation-induced emission luminogen. The resultant microspheres can absorb moisture from the surronding atmosphere. Accordingly, the WGM resonant peaks shift due to the change in the diameter of the microsphere. The sensitivity of the peak shift is as high as 255 picometre per percent relative humidity, which is the highest among non-tapered type optical humidity sensors. The responsivity toward moisture is kept virtually intact at least through five cycles of hydration and dehydration processes.
Comms. Chem. 2020, 3:113.
“Sigmoidally hydrochromic molecular porous crystal with rotatable dendrons”
Abstract: Vapochromic behaviour of porous crystals is beneficial for facile and rapid detection of gaseous molecules without electricity. Toward this end, tailored molecular designs have been established for metal–organic, covalent-bonded and hydrogen-bonded frameworks. Here, we explore the hydrochromic chemistry of a van der Waals (VDW) porous crystal. The VDW porous crystal VPC-1 is formed from a novel aromatic dendrimer having a dibenzophenazine core and multibranched carbazole dendrons. Although the constituent molecules are connected via VDW forces, VPC-1 maintains its structural integrity even after desolvation. VPC-1 exhibits reversible colour changes upon uptake/release of water molecules due to the charge transfer character of the constituent dendrimer. Detailed structural analyses reveal that the outermost carbazole units alone are mobile in the crystal and twist simultaneously in response to water vapour. Thermodynamic analysis suggests that the sigmoidal water sorption is induced by the affinity alternation of the pore surface from hydrophobic to hydrophilic.
Chem. Commun. 2020, 56, 6937-6940.
“Facile light-initiated radical generation from 4-substituted pyridine under ambient conditions”
Abstract: Photochemical reactions that generate stable radical species in ambient conditions find unique applications in the materials science. Here we present a facile photogeneration of a stable radical species from a 4-substituted pyridine derivative in the presence of water and air at room temperature. The radical generation reaction accompanies visible colour change into green and is repeatable for multiple times.
Mater. Horiz. 2020, 7, 1801-1808.
“Optical microresonator arrays of fluorescence-switchable diarylethenes with unreplicable spectral fingerprints”
Abstract: High-security identification requires authentication that is hard to counterfeit and replicate. For anti-counterfeiting data storage and rewritable memory devices, chromic materials are adoptable, where the dichromatic colours can be switched by external stimuli. If further individual information is embedded in each pixel, a much higher-level security system beyond the zero/one data array will be realized. For this purpose, a fine whispering gallery mode (WGM) fingerprint pattern from a microresonator is applicable. Here we propose that photoswitchable optical microresonators made of a fluorescent photochromic organic material function as anti-counterfeiting, rewritable optical memories. The WGM photoluminescence of the resultant microspheres can be switched on and off repeatedly by irradiation with ultraviolet and visible light. The shape of the microresonator varies from a sphere to an oblate ellipsoid and hemisphere, depending on the self-assembly process, and the WGM spectral pattern depends sensitively on the morphology of the resonators. Furthermore, surface self-assembly on a hydrophobic/hydrophilic micropatterned substrate affords a highly integrated array of microresonators as dense as millions of pixels per square centimetre. The spectral fingerprints of all pixels are different from one another; therefore, the photoswitchable microarrays are applicable as an ultimate anti-counterfeiting system which is hard to replicate.
Angew. Chem. Int. Ed. 2020, 59, 12674-12679.
“Single‐Crystalline Optical Microcavities from Luminescent Dendrimers”
Abstract: Microcrystallites are promising minute mirrorless laser sources. A variety of luminescent organic compounds have been exploited along this line, but dendrimers have been inapplicable owing to their fragility and extremely poor crystallinity. Now, a dendrimer family that overcomes these difficulties is presented. First‐, second‐, and third‐generation carbazole (Cz) dendrimers with a carbon‐bridged oligo(phenylenevinylene) (COPV2) core (Gn COPV2, n=1–3) assemble to form microcrystals. The COPV2 cores align uni/bidirectionally in the crystals while the Cz units in G2‐ and G3COPV2 align omnidirectionally. The dendrons work as light‐harvesting antennas that absorb non‐polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. Furthermore, these crystals act as laser resonators, where the lasing thresholds are strongly coupled with the crystal morphology and the orientation of COPV2, which is in contrast with the conventional amorphous dendrimers.
ACS Appl. Energy. Mater. 2019, 7, 13156-13160.
“Excellent Oxygen Reduction Reaction Performance in Self-Assembled Amyloid-β/Platinum Nanoparticle Hybrids with Effective Platinum-Nitrogen Bond Formation”
Abstract: Development of highly efficient catalysts for electrochemical oxygen reduction reactions (ORR) is crucial for energy applications such as metal-air batteries and fuel cells. Here, we show an enhanced electrocatalytic activity of a new functional material composed of Pt nanoparticles (PtNPs) and self-assembled β-sheet peptide (βP). The PtNP/βP hybrids, under an optimized assembly condition, display higher ORR electrocatalytic activity than a commercially available benchmark Pt/C electrocatalyst in terms of the onset potential and reaction kinetics. Moreover, the PtNP/βP hybrids show one order of magnitude higher ORR mass activity than previously reported peptide-based ORR electrocatalysts. The superb ORR activity with high durability is derived from the well-dispersed PtNPs on βP, where 50% of the amine group on the side chain bound with Pt to form Pt–N bond that functions as active sites for the catalytic reaction. This work opens new avenues for efficient ORR electrocatalyst using self-assembled peptides.
ACS Appl. Poly. Mater. 2019, 1, 2240-2248.
“Conjugated Copolymers of Poly(arylenevinylene)s: Synthesis by Ring-Opening Metathesis Polymerization, Film Morphology, and Resonant Luminescence from Microspheres”
Abstract: A series of copolymers of poly(arylenevinylene)s have been prepared by ring-opening metathesis polymerization (ROMP) of cyclic conjugated monomers composed of cyclopentadithiophene, bis(thienyl)benzothiadiazole, or bithiophene using a second-generation Grubbs catalyst and the sequential monomer addition technique. The spectroscopic, electrochemical, and morphological properties of these polymers are tailored by changing the chemical structures and fractions of the monomers. The polymers self-assemble to form microspheres (diameter: 2–5 μm), which act as whispering-gallery-mode optical resonators.
Chem. Lett. 2019, 48, 607-610.
“Modulation of Whispering Gallery Modes from Fluorescent Copolymer Microsphere Resonators by Protonation/Deprotonation”
Abstract: We demonstrate a modulation of whispering gallery mode (WGM) photoluminescence (PL) from self-assembled microsphere of fluorene-terpyridine alternating copolymer by protonation/deprotonation. Upon subsequent addition of acid and base, the PL color of the microspheres can be modulated between blue and green repeatedly, accompanying the shift of the WGM PL peaks.
ACS Appl. Polym. Mater. 2019, 1, 118-123.
“Significant Enhancement of Hole Transport Ability in Conjugated Polymer/Fullerene Bulk Heterojunction Microspheres”
Abstract: Bulk heterojunction (BHJ) strategy requires morphology of wide area donor-acceptor interfaces with high charge carrier mobilities through the bicontinuous charge transporting layers. Here, we report formation of well-defined bulk heterojunction (BHJ) microspheres from regiorandom poly(hexylthiophene) (rra-PHT)/phenyl-C61-butyric acid methyl ester (PCBM) mixture by a vapor diffusion method. By electrochemical oxidation, the BHJ microsphere exhibits enhanced generation of PHT cation species due to increased hole-transport property in comparison with a solution-cast film derived from rra-PHT/PCBM mixture without microsphere morphology. Photoconductivity and electrochemical stability of the microsphere are comparable or even higher than a cast film of irregular aggregates of regioregular P3HT.
Angew. Chem. Int. Ed. 2019, 57, 17019-17022.
“Dipole Switchable Poly(para‐phenyleneethynylene)s: Ferroelectric Conjugated Polymers”
Abstract: Dipolar 2,3‐difluorobenzene‐units are introduced into the backbone of a poly(para‐phenylene¬ethynylene) (PPE) to generate a ferroelectric conjugated polymer. The structural features of the partially fluorinated PPE allow for the generation of a remanent polarization in the solid state: The difluorinated benzene rings behave as molecular rotors at high temperature, while at room temperature, stacking of such rings clamps down the ring orientation. The molecular dipoles can still be oriented by moderate external electrical fields at room temperature, and this PPE is then a ferroelectric. The concept should be transferable to other poly(aryleneethynylene)s and novel conjugated ferroelectric conjugated polymers will be accessible using this concept.
Adv. Funct. Mater. 2019, 29, 1805824.
“Carbazole-Dibenzofuran Dyads as Metal-Free Single-Component White-Color Photoemitters”
Abstract: White-color light emitters from single organic molecule without heavy metals are valuable for practical applications in organic light-emitting devices (OLED). In this study, carbazole (Cz)-dibenzofuran (DBF) donor-acceptor dyads are designed for white-color light emitters. Originally, these molecules show photoluminescence (PL) in near ultraviolet region. However, upon successive ultraviolet (UV) irradiation, white-color PL appears, comprising dual-color phosphorescence from the amorphous and crystalline state of the dyad. A continuous UV irradiation makes the twisting angle between the Cz and DBF planes flatten through the triplet-excited state, which proceeds crystallization. Thermal annealing and UV irradiation can switch the blue- and white-color phosphorescence from the dyad. Furthermore, charge injection generates white-color electroluminescence. The materials with PL color modulation ability by UV-light irradiation and heating can be applicable as light- and thermo sensors.
J. Phys. Chem. C 2019, 123, 5061-5066.
“Polychromatic Photoluminescence of Polymorph Boron Dipyrromethene Crystals and Heterostructures”
Abstract: Micrometer-size BODIPY crystalline rods were grown from solution. Fluorescence microscopy images reveal that each rod displays characteristic visible light emission of different color. In a particular case, optical heterostructures with discrete, differently colored sections are observed within a single microrod. Micro-photoluminescence (µ-PL) spectra of green and red rods at room temperature show multiple contributions, indicating the presence of micro domains. Temperature dependent µ-PL measurements further confirm this, as red emission decreases and green emission increases at lower temperatures. These observations are discussed as a result of crystalline polymorphism, leading to a local variation of the HOMO-LUMO energy difference. An Arrhenius plot quantifies the hopping barrier for the charge carriers to reach the low emission energy (red) regions. A line scan of a single rod further supports that micro domains of green and orange-red emitting crystal phases are present in a single rod. Time-resolved microwave conductivity studies clarify that micro-domain-free green rods display two orders of magnitude longer photocarrier lifetime and 5-fold higher photoconductivity than the red rods with many small band gap regions.
Chem. Asian J. 2019, 14, 1637-1641.
“Energy Transfer-Assisted Whispering Gallery Mode Lasing in Conjugated Polymer/Europium Hybrid Microsphere Resonators”
Abstract: Lanthanide metal complexes display luminescence with narrow bandwidth. Here, we present coupling of the luminescence from europium ion (Eu3+) with whispering gallery modes (WGMs) in conjugated polymer microsphere resonators. Self‐assembly of fluorene‐terpyridine alternating copolymer, coordinated by Eu3+ (F8tpy–Eu3+), forms well‐defined microspheres with the average diameter of 3.2 µm. Upon focused laser excitation, a microsphere of F8tpy copolymer displays WGM photoluminescence (PL) at a wide spectral range from 420 to 680 nm. In contrast, F8tpy–Eu3+ hybrid microspheres exhibit sharp WGM PL at a narrow spectral range of 615–630 nm, which is characteristic of a luminescence from Eu3+. The PL behavior indicates that photoinduced energy transfer from F8tpy to Eu3+ occurs efficiently. Furthermore, the intensity of the PL peak increases nonlinearly upon strong pumping, indicating that a lasing action appears with the threshold of 1.85 mJ cm–2. These results will pave the way to developing microlasers and photonic devices from soft organic materials.
Science 2018, 361, 1242-1246.
“Self-assembly of lattices with high structural complexity from a geometrically simple molecule”
Abstract: Here we report an anomalous porous molecular crystal built of C–H···N-bonded double-layered roof-floor components and wall components of a segregatively interdigitated architecture. This complicated porous structure consists of only one type of fully aromatic multijoint molecule carrying three identical dipyridylphenyl wedges. Despite its high symmetry, this molecule accomplishes difficult tasks by using two of its three wedges for roof-floor formation and using its other wedge for wall formation. Although a C–H···N bond is extremely labile, the porous crystal maintains its porosity until thermal breakdown of the C–H···N bonds at 202°C occurs, affording a nonporous polymorph. Though this nonporous crystal survives even at 325°C, it can retrieve the parent porosity under acetonitrile vapor. These findings show how one can translate simplicity into ultrahigh complexity.
Nano Lett. 2018, 18, 4396-4402.
“π-Electronic Co-crystal Microcavities with Selective Vibronic-Mode Light Amplification: Toward Förster Resonance Energy Transfer Lasing”
Abstract: π-conjugated organic microcrystals often act as optical resonators in which the generated photons in the crystal are confined by the reflection at the crystalline facets and interfere to gain lasing action. Here, we fabricate microcrystals from a mixture of carbon-bridged oligo-para-phenylenevinylenes (COPVs) with energy-donor (D) and energy-acceptor (A) characters. Upon weak excitation of the single D−A co-crystal, Förster resonant energy transfer (FRET) takes place, exhibiting spontaneous emission from A. In contrast, upon strong pumping, stimulated emission occurs before FRET, generating lasing action from D. Lasing occurs with single- and dual-vibronic levels, and the lasing wavelength can be modulated by the doping amount of A. Time-resolved spectroscopic studies reveal that the rate constant of lasing is more than 20 times greater than that of FRET. Furthermore, microcrystals, vertically grown on a Ag-coated substrate, reduce the lasing threshold by one-fourth. This study proposes possible directions toward organic solid FRET lasers with microcrystalline resonators.
Chem. Commun. 2018, 54, 2534-2537.
“A fluorescent microporous crystalline dendrimer discriminates vapour molecules”
Abstract: A self-assembled crystalline microporous dendrimer framework (MDF) exhibits novel turn-on and ratiometric fluorescence upon exposure to solvent vapours. The donor-acceptor character, combined with the large surface area (> 650 m2 g–1), allows the MDF to discriminate vapours of volatile solvents with turn-on and color change of photoluminescence.
Mater. Chem. Front. 2018, 2, 270-274.
“FRET-mediated near infrared whispering gallery modes: studies on the relevance of intracavity energy transfer with Q-factor”
Abstract: Near infrared (NIR) optical microsphere resonators are prepared by coassembly of energy-donating and accepting conjugated polymers. In the microspheres, fluorescence resonance energy transfer occurs, leading to sharp and periodic photoluminescence from whispering gallery modes in the NIR region with Q-factors as high as 600.
J. Phys. Chem. Lett. 2017, 8, 4580-4586.
“From Linear to Foldamer and Assembly: Hierarchical Transformation of a Coplanar Conjugated Polymer into a Microsphere”
Abstract: Despite the coplanar structure, a conjugated alternating copolymer forms amorphous, well-defined microspheres without π-stacked crystalline domains. Here, we gain insights into the mechanism of how the coplanar conjugated polymer forms amorphous microspheres by means of spectroscopic studies on the assembly/disassembly processes. The difference of the spectral profiles of photoabsorption and photoluminescence with varying the solvent/nonsolvent composition clarifies that stepwise assembly takes place through the microsphere formation;  intrapolymer linear-to-folding transformation upon diffusion of polar nonsolvent and  interpolymer assembly of the foldamers upon further addition of the nonsolvent to form microspheres. As shown in various biopolymers such as proteins and DNA, such stepwise folding and assembly behaviors of conjugated polymers from primary to secondary and tertiary structure open a new way to create transformable functional materials.
Adv. Opt. Mater. 2017, 5, 1700123/1-7.
“Low-Threshold Whispering Gallery Mode Lasing from Self-Assembled Microspheres of Single-Sort Conjugated Polymers”
Abstract: Low-threshold organic microlasers are demanded for advanced optical applications such as nano/micrometer scale memory, sensing, and communication tools, and further valuable for future electrically-driven laser applications. In this paper, we demonstrate that various highly-fluorescent conjugated polymers self-assemble to form single-component microspheres that exhibit, upon femtosecond pumping to a single microsphere, whispering gallery mode (WGM) lasing with blue, green, and red emission colors. In particular, the microsphere consisting of polyfluorene shows the lowest threshold fluence as low as 1.5 μJ cm^–2 and high photostability against successive pumping of > 10^5 pulse. The threshold fluence was further reduced by one fourth (0.37 μJ cm^–2) by mounting the microspheres on a Ag-coated substrate, where a mirror effect of the Ag layer enhances efficiency of the PL confinement with a minor effect of plasmonic near field. Considering the intrinsic charge injection and transport properties, π-conjugated polymer microsphere resonators will be possible materials for electrically pumped WGM luminescence.
ACS Appl. Mater. Interfaces 2017, 9, 9996-10002.
“Peptide Cross-linkers: Immobilization of Platinum Nanoparticles Highly Dispersed on Graphene Oxide Nanosheets with Enhanced Photocatalytic Activities”
Abstract: For exerting potential catalytic and photocatalytic activities of metal nanoparticles (MNPs), immobilization of MNPs on a support medium in highly dispersed state is desired. In this article, we demonstrated that surfactant-free platinum nanoparticles (PtNPs) were efficiently immobilized on graphene oxide (GO) nanosheets in a highly dispersed state by utilizing oligopeptide β-sheets as a cross-linker. The fluorenyl-substituted peptides were designed to form β-sheets, where metal-binding thiol groups and protonated and positively charged amino groups are integrated on the opposite sides of the surface of a β-sheet, which efficiently bridge PtNPs and GO nanosheet. In comparison to PtNP/GO composite without the peptide linker, the PtNP/peptide/GO ternary complex exhibited excellent photocatalytic dye degradation activity via electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. Furthermore, the ternary complex showed photoinduced hydrogen evolution upon visible light irradiation using a hole scavenger. This research provides a new methodology for the development of photocatalytic materials by a bottom-up strategy on the basis of self-assembling features of biomolecules.
Chem. Lett. 2017, 46, 423-425.
“Controlled Self-Assembly of Oligopeptides Bearing Electron Donor and Acceptor Units on the Side Chains to Form β-Sheets with Selective π-Stacking Configuration”
Abstract: We describe the β-sheet formation from heptapeptides bearing one electron-donating free-base porphyrin and two electron-accepting naphthalenediimides on the side chains. The porphyrin units in the resultant β-sheet fibers form J- or H-type aggregates, depending on the protonated state of the porphyrin unit in the assembling process.
Polym. J. 2016, 48, 1045-1050.
“Spherical resonators from π-conjugated polymers” （Focus Review）
Abstract: Optical resonators play an important role in optical devices; they confine, sharpen, and amplify light. Self-assembled polymer resonators are advantageous for their simple, low-cost, and low-energy fabrication processes. In this Focus Review, we show recent advances in the self-assembly of π-conjugated polymers to form microspheres that act as optical resonators. The key factors in sphere assembly are low crystallinity of the polymers and slow diffusion of polar nonsolvent during the assembling process. Focused laser excitation of a single isolated microsphere excites whispering gallery modes (WGMs), in which sharp and periodic lines are observed in the photoluminescence (PL) spectrum; these are characterized as transverse electric and magnetic modes. Efficient intrasphere energy transfer occurs in the case of microspheres composed of energy-donating and -accepting polymer blends. Furthermore, WGM-mediated long-range intersphere PL propagation and subsequent color conversion were observed in the case of linearly coupled microspheres. Conjugated polymer microspheres are beneficial as optical resonators in terms of their high refractive indices, high absorptivity, and high PL efficiency, which are achieved by using simple fabrication processes.
ACS Nano 2016, 10, 7058-7063.
“Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade”
Abstract: Color-tunable resonant photoluminescence (PL) was attained from polystyrene microspheres doped with a single polymorphic fluorescent dye, boron-dipyrrin (BODIPY) 1. The color of the resonant PL depends on the assembling morphology of 1 in the microspheres, which can be selectively controlled from green to red by the initial concentration of 1 in the preparation process of the microspheres. Studies on intersphere PL propagation with multicoupled microspheres, prepared by micromanipulation technique, revealed that multistep photon transfer takes place through the microspheres, accompanying energy transfer cascade with stepwise PL color change. The intersphere energy transfer cascade is direction selective, where energy donor-to-acceptor down conversion direction is only allowed. Such cavity-mediated long-distance and multistep energy transfer will be advantageous for polymer photonics device application.
RSC Adv. 2016, 6, 52854-52857.
“Self-assembled polycarbazole microspheres as single-component, white-colour resonant photoemitters”
Abstract: Self-assembled polycarbazole (PCz) microspheres exhibit whispering gallery mode photoluminescence (PL), where resonant PL lines appear at the whole visible spectral range. The ultra-wide-range PL results from a partial oxidation of PCz upon strong photoexcitation. The single component micro-photoemitters, preparable with simple and inexpensive process, is applicable as full-colour, narrow-width light emission sources.
Chem. Lett. 2016, 45, 1024-1026.
“Enwrapping Conjugated Polymer Microspheres with Graphene Oxide Nanosheets”
Abstract: Water-dispersible conjugated polymer microspheres were obtained by enwrapping with graphene oxide (GO) nanosheets. Simply mixing the polymer microspheres and GO in water results in an exclusive formation of GO-wrapped microspheres. Photoluminescence (PL) spectra of the GO-wrapped single microsphere show whispering gallery modes, in which the PL lines are broadened in comparison with bare microspheres without GO. The broadening is attributed to scattering and reabsorption of the confined PL.
ACS Nano 2016, 10, 5543-5549.
“Conjugated Polymer Blend Microspheres for Efficient, Long-Range Light Energy Transfer”
Abstract: Highly luminescent π-conjugated polymeric microspheres were fabricated through self-assembly of energy-donating and energy-accepting polymers and their blends. To avoid macroscopic phase separation, the nucleation time and growth rate of each polymer in the solution were properly adjusted. Photoluminescence (PL) studies showed that efficient donor-to-acceptor energy transfer takes place inside the microspheres, revealing that two polymers are well-blended in the microspheres. Focused laser irradiation to a single microsphere excites whispering gallery modes (WGMs), where PL generated inside the sphere is confined and resonates. The wavelengths of the PL lines are finely tuned by changing the blending ratio, accompanying the systematic yellow-to-red color change. Furthermore, when several microspheres are coupled linearly, the confined PL propagates the microspheres through the contact point, and a cascade-like process converts the PL color while maintaining the WGM characteristics. The self-assembly strategy for the formation of polymeric nano- to microstructures with highly miscible polymer blends will be advantageous for optoelectronic and photonic device applications.
Thin Solid Films 2016, 603, 408-412.
“Enhancement of grain size and crystallinity of thin layers of pentacene grown under magnetic field”
Abstract: Field-effect mobilities (µ) of pentacene films, prepared by a thermal deposition under a magnetic field (H-field), were largely enhanced, in comparison with that prepared without an H-field. Under a perpendicular H-field with respect to the substrate surface, the crystallinity of the edge-on pentacene orientation is enhanced, resulting in the 9-fold enhancement of µ. Furthermore, under parallel H-field with respect to the substrate surface, µ of the pentacene films were 23-fold greater than that prepared without the H-field. The surface morphology studies by atomic force microscopy of the ultra thin films of pentacene clarified that the grain size of the pentacene at the interface with the substrate is larger for films under parallel H-field than that prepared without an H-field. The simple and effective method for enhancing the semiconducting properties of the organic thin films gives high technological impact in its application to organic electronics.
Sci. Rep. 2016, 6, 19635/1-6.
“Optically induced mode splitting in self-assembled, high quality-factor conjugated polymer microcavities”
Abstract: We investigate the whispering gallery modes (WGMs) of self-assembled single microspheres. They consist of a recently developed highly fluorescent π-conjugated copolymer and exhibit excellent optical properties with Q-factors up to 10^4. Under continuous laser irradiation, we observe a splitting of the highly degenerate spherical WGMs into a multiplet of lines. Comparison with the calculated spectral response of a weakly distorted sphere shows that the optical excitation induces a change of the optical path length in the microcavity so that it resembles a prolate spheroid. The separation of the lines is given by the ellipticity and the azimuthal mode number. Measurements in various gaseous environments suggest that the distortion is caused by light induced oxidation of the polymer. Our findings show that photooxidation can be a beneficial mechanism for in-situ tuning of optically active polymer structures.
J. Mater. Chem. A 2015, 3, 17612-17619.
“Cysteine-containing oligopeptide β-sheets as redispersants for agglomerated metal nanoparticles”
Abstract: Oligopeptide β-sheets comprising fluorenyl methoxy carbonyl (Fmoc) group on its N-terminus and five amino acid residues of cysteine, lysine and valine displays redispersive properties of agglomerated metal nanoparticles (MNPs, M = Au, Cu, Pt and Pd). The ligand-free MNPs prepared by laser ablation technique in liquid keep high dispersion state due to the inherent surface charges delivered by anionic species present in solution but may agglomerate after the preparation which depends on concentration or salinity. We show how the agglomerated MNPs can be returned to the dispersive state by adding the Fmoc-oligopeptide β-sheets in methanol, which is characterized by photoabsorption spectroscopy and transmission electron microscopy. Systematic studies varying the concentration, the amino acid sequences and secondary structures of a series of the oligopeptides clarify that the β-sheet structure is essential for the redispersion of the MNPs, where metal-binding thiol groups are integrated on one side and positively charged amino groups are located on the other side of the β-sheet. A possible mechanism for the redispersion may be that the agglomerated MNPs are subsequently enwrapped by the flexible β-sheets and gradually separated due to the reconstruction of peptide β-sheets under the assembly/disassembly equilibrium.
Macromolecules 2015, 48, 3928-3933.
“Whispering Gallery Resonance from Self-Assembled Microspheres of Highly Fluorescent Isolated Conjugated Polymers”
Abstract: Self-assembly of highly fluorescent isolated conjugated polymers (ICPs), comprising alternating phenylene moieties with an insulating cyclic side chain and different arylene moieties, was comprehensively studied. Two out of nine ICPs were identified to form well-defined spheres of 1–6 μm diameter. The degree of twisting of the main chains was found to be an important structural factor enabling formation of spheres, for which dihedral angles >50° between the neighboring arylene moieties were required. A single microsphere with high sphericity exhibited whispering gallery mode (WGM) photoemission upon excitation with a focused laser. In this emission, sharp and periodic emission lines were superimposed on a broad photoemission spectrum. The WGM spectral profiles were very sensitive to the integrity of the spherical geometries and surface smoothness, which depends on the self-assembling condition as well as the structure of the polymer backbone. Microspherical optical resonators consisting of such highly fluorescent conjugated polymers are novel. They also present advantages in that (i) there is no need for a light waveguide and fluorescent-dye doping, (ii) its high refractive index is beneficial for light confinement, and (iii) the fabrication process is simple, not requiring sophisticated, costly microfabrication technology.
Macromolecules 2015, 48, 2570-2575.
“Colloidal Crystallization and Ionic Liquid Induced Partial β-Phase Transformation of Poly(vinylidene fluoride) Nanoparticles”
Abstract: Colloidal crystallization of poly(vinylidene fluoride) (PVDF) nanoparticles (NPs) and its β-phase transformation were studied. The pristine PVDF NPs with an average diameter of 230 nm consist of 46% α-phase and 54% amorphous PVDF. The PVDF NPs were assembled on a quartz substrate by means of vertical deposition method from a tetrahydrofuran dispersion of PVDF NPs with a few volume percentage of n-alkane. The resultant colloidal thin films displayed a pale-greenish structural color with the selective reflection at around 550 nm wavelength due to closely packed PVDF NPs. The colloidal thin films were immersed into an acetonitrile solution containing 2 wt % ionic liquid, subsequently air-dried, and thermally annealed at 140 °C, just below the melting point of the PVDF–IL blends. After annealing, the PVDF NPs partially transformed into its β-phase with the volume percentages of α-, β-, and amorphous phases of 22, 32, and 46%, respectively. The postannealed colloidal films still maintained the face-centered-cubic assembling structure of PVDF NPs, thus displaying the greenish structural color and selective reflection.
Chem. Commun. 2015, 51, 1206-1209.
“Translation of Assembling Trajectory by Preorganisation: A Study of the Magnetic Properties of 1D Polymeric Unpaired Electrons Immobilised on a Discrete Nanoscopic Scaffold”
Abstract: A nitronyl nitroxide (NN)-appended hexabenzocoronene (HBCNN), when allowed to coassemble with bis(hexafluoroacetylacetonato)cobalt(II), forms a coaxial nanotubular architecture featuring NN-Co(II) coordinated copolymer chains immobilised on the outer and inner nanotube surfaces. Upon lowering the temperature, this nanotube has enhanced magnetic susceptibility below 10 K.
Asian J. Org. Chem. 2014, 3, 1182-1188.
“Charge-Separated Fmoc-Peptide β-Sheets: Sequence-Secondary Structure Relationship for Arranging Charged Side Chains on Both Sides”
Abstract: β-Sheet formation from fluorenylmethoxycarbonyl (Fmoc)-substituted polar oligopeptides was demonstrated, where acidic and basic side chains are located separately on either side of the β-sheet surfaces. For yielding such charge-separated β-sheets, self-assembly of 18 pentapeptides was studied, all of which contain glutamic acid (E), lysine (K), and valine (V). Fmoc-pentapeptides containing one E and one K all formed fibrillar nanostructures consisting of stacked β-sheets. On the other hand, Fmoc-pentapeptides containing two E and two K formed β-sheet fibrils only when V was located at the center and separated two EK pairs. Photoluminescence studies of these peptides in a glycine buffer containing thioflavin T revealed a clear relationship between the amino acid sequence and secondary structure, where the location of neutral V plays a pivotal role for the β-sheet formation. The β-sheet formation propensity was further supported by computer simulation studies with the TANGO algorithm.
Sci. Rep. 2014, 4, 5902/1-5.
“Self-Assembled Conjugated Polymer Spheres as Fluorescent Microresonators”
Abstract: Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2–10 μm diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.
Thin Solid Films 2014, 562, 467-470.
“Control of Molecular Orientation and Morphology in Organic Bilayer Solar Cells: Copper Phthalocyanine on Gold Nanodots”
Abstract: Molecular orientation, morphology of donor (D)/acceptor (A) interface and photoabsorptivity in organic bilayer solar cells were controlled using Au nanodots with ~20 nm diameter inserted between the bottom electrode and the organic layer. Copper phthalocyanine (CuPc) molecules deposited onto the Au nanodot-coated electrode were mostly oriented face-on with large surface roughness, which is beneficial for photoabsorption, charge separation and transport. Furthermore, Au nanodots exhibit blue-shifted plasmon bands so that CuPc absorbs light more efficiently than that on thin Au layer. Bilayer C60/CuPc solar cells containing Au nanodots exhibited 1.4 times higher photoelectric conversion efficiency than those without Au nanodots. Factors for the enhanced efficiency are (i) improvement of the optical absorption characteristics by face-on orientation of CuPc and (ii) increase of the D/A heterointerface area. In addition, the shift of the plasmon absorption band of Au by the formation of nanodots makes absorption of the CuPc layer much more efficiently, resulting in better photovoltaic output.
Polym. Chem. 2014, 5, 3583-3587.
“Tetramethylbithiophene in π-Conjugated Alternating Copolymers as Effective Structural Component for the Formation of Spherical Assemblies”
Abstract: π-Conjugated alternating copolymers containing a tetramethylbithiophene unit show a strong tendency to form well-defined, sub- to several-micrometer-sized spheres. The twisted bithiophene unit inhibits interchain stacking and anisotropic crystal growth of these copolymers, leading to the formation of structurally isotropic spheres by means of a slow diffusion of nonsolvent into a solution of the copolymers. These micrometer-sized spheres display extremely long photocarrier lifetimes (~ 10^-3 s) in comparison with cast films from the solutions of the polymers and those of the irregular aggregates (＜ 10^-6 s).
Appl. Phys. Lett. 2013, 103, 043301/1-4.
“Magnetic-field-induced enhancement of crystallinity and field-effect mobilities in phthalocyanine thin films”
Abstract: Organic semiconductor thin films were fabricated by thermal deposition of free-base and metal phthalocyanines under a static magnetic field. A vertical magnetic field enhanced the crystallinity of the edge-on orientation of the phthalocyanine discs, whereas a horizontal magnetic field had a minimal effect on the crystallinity. The major factor for the orientation change is attributed to the diamagnetic anisotropies of p -electrons in the phthalocyanine macrocycles. Field-effect transistors of phthalocyanine films fabricated under a vertical magnetic field exhibited better hole mobilities and on-current values with smaller threshold voltages than those of phthalocyanine films fabricated without a magnetic field.
J. Am. Chem. Soc. 2013, 135, 870-876.
“Spherical Assemblies from π-Conjugated Alternating Copolymers: Toward Optoelectronic Colloidal Crystals”
Abstract: Self-assembly of conducting polymers, which are often used as photoabsorbing, charge-transporting, and photoemission layers of organic photovoltaic and light-emitting devices, were comprehensively studied by means of slow precipitation from polymer solutions upon addition of a vapor of nonsolvents. Polymers such as polyfluorene and polythiophene having a single monomer component hardly formed defined and discrete objects but only gave ill-defined aggregates. In contrast, alternating copolymers typically having both fluorene and thiophene components in their repeating unit self-assembled into well-shaped spheres with diameters ranging from several hundreds of nanometers to several micrometers. Such clear differences in terms of the assembling geometries derive from the rigidity and crystallinity of the polymers, where the copolymers possess large steric hindrance on their backbone that reduces planarity of the polymers and inhibits anisotropic crystal growth, leading to the formation of structurally isotropic spheres. Changing the assembling parameters can systematically control diameter and deviation of the spheres. Furthermore, photocarrier lifetimes of the spheres were markedly enhanced by more than three orders of magnitude in comparison with those of cast films from their solutions. This research gives a useful guide for preparation of colloidal crystals from π-conjugated polymers toward their optoelectronic applications.
Angew. Chem. Int. Ed. 2012, 51, 8490-8494.
“Discotic Ionic Liquid Crystals of Triphenylene as Dispersants for Orienting Single-Walled Carbon Nanotubes”
Abstract: Triphenylene-based discotic ionic liquid crystals (ILCs) with six imidazolium ion pendants can disperse pristine single-walled carbon nanotubes (SWNTs) 2–3 orders of magnitude better than reported LCs. When ILC is columnarly assembled, doping with SWNTs results in homeotropic columnar orientation up to a macroscopic length scale. Combination of shear and annealing treatments gives rise to three different states in terms of the orientations of the LC columns and SWNTs, where the anisotropy of electrical conduction is determined predominantly by whether SWNTs are unidirectionally oriented or not.
J. Am. Chem. Soc. 2012, 134, 2524–2527.
“Segregated and Alternately Stacked Donor/Acceptor Nanodomains in Tubular Morphology Tailored with Zinc Porphyrin-C60 Amphiphilic Dyads: Clear Geometrical Effects on Photoconduction”
Abstract: Amphiphilic zinc porphyrin (PZn; electron donor, D)–fullerene (C60; electron acceptor, A) dyads 2 and 3, bearing an identical hydrophilic wedge with triethylene glycol chains but different linkers between the PZn and C60 units, self-assemble into nanotubes with essentially different dimensional and geometrical features from one another. The nanotube from dyad 2 with an ester linker consists of a bilayer wall formed with coaxially segregated D and A nanodomains along the tube axis (coaxial D–A heterojunction), thereby displaying explicit photoconductivity with ambipolar carrier transport properties. In contrast, the nanotube from dyad 3 with a rigid arylacetylene linker consists of a monolayer wall with an alternate geometry of D/A stacking, resulting in poor photoconducting outputs. Such a geometrical difference also significantly affects the photovoltaic properties.
Sci. Technol. Adv. Mater. 2012, 51, 13, 033001/1-15. （Review）
“Programmed Self-Assembly of Large π-Conjugated Molecules into Electroactive One-Dimensional Nanostructures”
Abstract: Electroactive one-dimensional (1D) nano-objects possess inherent unidirectional charge and energy transport capabilities along with anisotropic absorption and emission of light, which are of great advantage for the development of nanometer-scale electronics and optoelectronics. In particular, molecular nanowires formed by self-assembly of π-conjugated molecules attract increasing attention for application in supramolecular electronics. This review introduces recent topics related to electroactive molecular nanowires. The nanowires are classified into four categories with respect to the electronic states of the constituent molecules: electron donors, acceptors, donor–acceptor pairs and miscellaneous molecules that display interesting electronic properties. Although many challenges still remain for practical use, state-of-the-art 1D supramolecular nanomaterials have already brought significant advances to both fundamental chemical sciences and technological applications.
Bull. Chem. Soc. Jpn. 2011, 84, 17-25. （Account）
“Electroactive Nanotubes from π-Conjugated Discotic Molecules”
Abstract: Self-assembly of π-conjugated molecules is attractive for construction of well-defined, nanometer-scale electroactive materials. This account describes our developments on self-assembled nanotubes from Gemini-shaped hexa-peri-hexabenzocoronenes (HBCs). At first, detailed molecular arrangement in the nanotube is presented, which is perfectly revealed by a synchrotron radiation X-ray diffraction analysis of a macroscopic fiber consisting of highly aligned HBC nanotubes. Next, electroconductive properties of the HBC nanotubes are investigated. By means of direct current and noncontact methods, anisotropic charge-transport properties in the nanotubes are confirmed. The effect of the surface oligoether chains on intertubular conduction is also examined by field-effect transistor measurements. Finally, optoelectronic applications are developed by constructing newly designed nanotubes. These nanotubes possess a coaxial configuration, where an electron-donating graphitic bilayer of π-stacked HBC arrays is laminated by an electron accepting molecular layer. Due to the molecular-layer donor/acceptor heterojunction, the nanotubes exhibit remarkable photoconduction and photovoltaic outputs. Furthermore, the optoelectronic properties are modulated by changing the density of electron acceptors on the nanotube surfaces by coassembly of multiple components or utilizing photochromism. These results will advance to electronic and optoelectronic applications of supramolecular nanomaterials.