Probing local structure of glass with orientation-dependent luminescence.

The local ordering of particles is considered an important process in glass transition. Ordering is usually observed in simulation and micrometer-sized colloid. However, clear information on local ordering at the molecular level is difficult to obtain experimentally. In this study, we prepared an easily glass-forming fluorophore with a color change owing to the intermolecular arrangement in the liquid, glass, and crystal states. The bathochromic shifts of the photoluminescence spectra indicated a change in the intermolecular orientation upon immediate cooling of the melt. The recovery of the spectra by successive heating indicated that rotation contributed to the change in the intermolecular orientation. The orientation in the glass was distinct from that during crystal growth, which was observed as a slow bathochromic shift by maintaining the temperature between the melting points of the blue- and green-luminescent crystals obtained from dichloromethane/ethanol and dichloromethane/hexane, respectively. Our results demonstrate that the anisotropic interaction between glass-forming luminophores is useful for uncovering molecular-level events in the glassy state.

Donor-acceptor random regioregular π-conjugated copolymers based on poly(3-hexylthiophene) with unsymmetrical monothienoisoindigo units.

Donor-acceptor π-conjugated random copolymers based on regioregular poly(3-hexylthiophene), rr-P3HT, with unsymmetrical monothienoisoindigo moieties were obtained by direct arylation polycondensation of 2-bromo-3-hexylthiophene with unsymmetrical monothienoisoindigo motifs under the optimized conditions [palladium-immobilized on thiol-modified silica gel with chloride counter anions, PITS-Cl (2.5 mol%), PivOH (1.0 equiv.), K2CO3 (3.0 equiv.), DMAc, 100 °C, 24 h]. Incorporation of unsymmetrical monothienoisoindigo electron-acceptor units into the polymers tuned their highest occupied and lowest unoccupied molecular orbital levels, which were close to those of the hole transport material (PEDOT) and electron transport material (PCBM), respectively, in thin-film organic solar cells. Alkyl chains of the unsymmetrical monothienoisoindigo units in the polymers tuned their macrostructural order, resulting in the observation of crystalline patterns and specific absorption peaks in thin films. An organic solar cell containing the most crystalline random copolymer showed an efficiency of 1.91%.

2,5-Dimethoxybenzene-1,4-dicarboxaldehyde: An Emissive Organic Crystal and Highly Efficient Fluorescent Waveguide.

To identify the simplest organic structure for an emitter, we focused on 2,5-dimethoxybenzene-1,4-dicarboaldehyde. This symmetric molecule has a very low molecular weight (MW=194), a single benzene unit, and consists of only three elements (H, C and O). It forms highly efficient and pure emitting crystals (λem=499 nm, ΦF =0.42, FWHM=42 nm) due to the rigid structure based on the single benzene framework and four intramolecular hydrogen bonds between electron-donating methoxy and electron-accepting aldehyde groups. This crystal acts as a good optical waveguide with pure green emission (FWHM=34 nm) and very low loss coefficient (0.00120 dB/µm).

Solvent Control over Supramolecular Gel Formation and Fluorescence for a Highly Crystalline π-Conjugated Polymer.

In π-conjugated polymers (πCPs), crystallinity and fluorescence typically exhibit a trade-off relationship. Here, we have synthesized a highly crystalline and fluorescent π-conjugated polymer with a simple alternating structure of 1,2,4,5-tetrafluorophenylene and 3,3'-dihexyl-2,2'-bithiophene units. In film, the polymer exhibited efficient red-colored fluorescence, an improved quantum yield (Φsol =13 %→Φfilm =23 %) and a crystalline structure. Interestingly, supramolecular gel formation occurred in appropriate solvents, and the macrostructure and fluorescence properties of the gel could be directly controlled by the choice of the solvent. The polymer self-assembled into a spherical form that exhibited red fluorescence in non-aromatic solvent (1,2-dichloroethane) and into a fibrous form that exhibited yellow fluorescence in aromatic solvent (mesitylene).

Mechanically Induced Shaping of Organic Single Crystals: Facile Fabrication of Fluorescent and Elastic Crystal Fibers.

Mechanically induced shaping (i.e., top-down processing) of organic single crystals is an undeveloped area of research because applying stress to such nonflexible crystalline materials generally causes them to disintegrate. Herein, we present a mechanically induced splitting phenomenon of elastic organic single crystals, and study on a facile shaping processing method of centimeter-scale elastic organic single crystal of fluorescent π-conjugated molecule into various fine crystal fibers (thickness: ≈50 µm; width: ≈150 µm; length: ≈25 mm). The fibers produced maintained their original crystal structure and properties (i.e., fluorescence efficiency and elastic flexibility).

Fluorescent organic single crystals with elastic bending flexibility: 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene derivatives.

Organic single crystals with elastic bending flexibility are rare because they are generally brittle. We report here fluorescent organic single crystals based on thiophene-tetrafluorobenzene-thiophene derivatives, mainly 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene. Three derivatives were synthesized by Pd-catalyzed cross-coupling reactions (Stille or direct arylation pathways). The crystallization of the derivatives gave large (mm- or cm-scale) crystals. Two crystals of 1,4-bis(thien-2-yl)-2,3,5,6-tetrafluorobenzene, 1, and 1,4-bis(4-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, 3, bent under applied stress and quickly recovered its original shape upon relaxation. The other crystal of 1,4-bis(5-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, 2, showed brittle breakage under applied stress (normal behavior). Fibril lamella crystal structure based on criss-cross packed slip-stacked molecular wires and its structural integrity are important factors for the design and production of next generation crystal materials with elastic bending flexibility. Furthermore, mechanical bending-relaxation resulted in reversible change of the morphology and fluorescence (mechanofluorochromism). Such bendable crystals would lead to the next generation solid-state fluorescent and/or semiconducting materials.

Effects of molecular weight on the optical and electrochemical properties of EDOT-based π-conjugated polymers.

Absolute molecular weight values obtained by gel permeation chromatography with multi angle light scattering of PEDOTF8 were approximately 65% of the relative molecular weight values obtained by gel permeation chromatography using polystyrene standards. Both light absorption and molecular weight measurements showed the effective conjugation lengths (absolute molecular weight <2600, relative molecular weight <4000, number of EDOT-F8 units: n < ca. 5 unit). The low molecular weight polymers showed higher energy absorption and fluorescence bands. Molecular weight also affected the electrochemical process of the polymer films. The high molecular weight PEDOTF8 (number average molecular weight >70000) showed a higher redox stability than the low molecular weight one. The two polymers with number average molecular weights of 70200 and 40000 retained 65% and 25% of the charge storage capacity after 100 electrochemical scans, respectively. Square-wave potential step absorptiometry measurements of the polymers with number average molecular weights of 70200 and 40000 showed that the optical contrasts remain stable after ca. 55 and three cycles, respectively. The high molecular weight polymer has a high electrochemical stability and would be a good material for optoelectronic devices.

Enzymatic Specific Production and Chemical Functionalization of Phenylpropanone Platform Monomers from Lignin.

Enzymatic catalysis is an ecofriendly strategy for the production of high-value low-molecular-weight aromatic compounds from lignin. Although well-definable aromatic monomers have been obtained from synthetic lignin-model dimers, enzymatic-selective synthesis of platform monomers from natural lignin has not been accomplished. In this study, we successfully achieved highly specific synthesis of aromatic monomers with a phenylpropane structure directly from natural lignin using a cascade reaction of ß-O-4-cleaving bacterial enzymes in one pot. Guaiacylhydroxylpropanone (GHP) and the GHP/syringylhydroxylpropanone (SHP) mixture are exclusive monomers from lignin isolated from softwood (Cryptomeria japonica) and hardwood (Eucalyptus globulus). The intermediate products in the enzymatic reactions show the capacity to accommodate highly heterologous substrates at the substrate-binding sites of the enzymes. To demonstrate the applicability of GHP as a platform chemical for bio-based industries, we chemically generate value-added GHP derivatives for bio-based polymers. Together with these chemical conversions for the valorization of lignin-derived phenylpropanone monomers, the specific and enzymatic production of the monomers directly from natural lignin is expected to provide a new stream in "white biotechnology" for sustainable biorefineries.

Elastic Organic Crystals of a Fluorescent π-Conjugated Molecule.

An elastic organic crystal of a π-conjugated molecule has been fabricated. A large fluorescent single crystal of 1,4-bis[2-(4-methylthienyl)]-2,3,5,6-tetrafluorobenzene (over 1 cm long) exhibited a fibril lamella morphology based on slip-stacked molecular wires, and it was found to be a remarkably elastic crystalline material. The straight crystal was capable of bending more than 180° under applied stress and then quickly reverted to its original shape upon relaxation. In addition, the fluorescence quantum yield of the crystal was about twice that of the compound in THF solution. Mechanical bending-relaxation resulted in reversible change of the morphology and fluorescence. This research offers a more general approach to flexible crystals as a promising new family of organic semiconducting materials.

Palladium Immobilized on Thiol-Modified Silica Gel for Effective Direct C-H Arylation.

A palladium catalyst immobilized on thiol-modified silica gel (PITS) was developed for the efficient synthesis of various π-conjugated oligomers and polymers. Direct arylation of 3,4-ethylenedioxythiophene (EDOT) gave not only the corresponding oligomeric compounds in high yields but also high-molecular-weight π-conjugated polymers in high yields. Moreover, the polycondensation of the ß-unprotected bithiophene monomer with dibromoarenes was also achieved. This solid-supported Pd catalyst was used to conduct regioselective direct arylation polycondensation, giving high-molecular-weight π-conjugated polymers in high purity (low levels of residual Pd impurities and structural ß-defects).

Raman spectroscopic study on the coordination behavior of rare earth ions in N-methylacetamide.

The coordination behavior of rare earth (Ln (3+)) ions in N-methylacetamide (NMA) solution has been investigated at room temperature by Raman spectroscopy. The behavior of the symmetric Raman Ln-Cl stretching (nu Ln-Cl) band, and amide I (nu AI), and III (nu AIII) bands of NMA with the rare earth series is discussed in conjunction with the change in the coordination structure occurring in the middle of the rare earth series. A competition for a coordination equilibria between a Cl (-) ion and an NMA molecule from the rare earth chloride-NMA complex might occur in the middle rare earth region. It is demonstrated that the change in the coordination structure of Ln (3+) ions in NMA is due to an elimination of an NMA molecule.