Shotaro Hayashi

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 1u2005cm 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.

Chloride-promoted Pd-catalyzed direct C–H arylation for highly efficient phosphine-free synthesis of π-conjugated polymers

We have successfully demonstrated a chloride-promoted Pd-catalyzed phosphine-free direct C–H arylation for highly efficient synthesis of high molecular weight π-conjugated polymers. Among halide ions, chloride ions played a crucial role in highly efficient C–H activation.

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.

Direct arylation of fluoroarenes toward linear, bent-shaped and branched π-conjugated polymers: polycondensation post-polymerization approaches

A direct arylation polycondensation of fluoroarenes with 2,7-dibromo-9,9-dioctylfluorene, F8, was achieved in a low polar solvent, toluene. The polycondensation of 1,2,4,5-tetrafluorobenzene, M1, or 2,2′,3,3′,4,4′,5,5′-octafluorobiphenyl, M1′, with F8 in toluene under our optimal conditions (2.5 mol% of palladium acetate, 5.0 mol% of tBu2MeHBF4, 2.0 equiv. of acetic acid, 3.0 equiv. of K2CO3, 0.6 M, 120 °C, 24 h) gave high molecular weight linear π-conjugated polymers (Mn > 60u2006000 and 40u2006000, respectively). Carboxylate additives promoted the direct arylation polycondensation. On the other hand, the direct arylation polycondensation of M1 with F8 gave insoluble products under the same conditions when DMAc was used instead of toluene. The network structure was probably formed by side reactions on the polymer chain. Thus, toluene is a useful solvent for the direct arylation polycondensation of fluoroarenes. In contrast with the linear (through) conjugated polymer synthesis, the direct arylation polycondensation of 1,2,3,5-tetrafluorobenzene, M2, with F8 gave bent-shaped (cross) conjugated polymers (Mn = 7300). The direct arylation polycondensation of 1,3,5-trifluorobenzene, M3 with F8 (monomer ratio 1u2006:u20061) gave branched polymers having many unreacted C–H of trifluorobenzene inner and end units (Mn = 2900, DB = 53%). The degree of branching, DB, of the polymer was 53%, but the Mn value of the polymer was low (2900). Ozawas conditions in toluene were also effective for synthesizing a high molecular weight linear polymer from F8 and M1. In contrast with our polycondensation, however, the polycondensation of F8 with M2 or M3 did not proceed well under Ozawas conditions. Our polycondensation method is applicable to the synthesis of various fluoroarene-based polymers. The direct arylation post-polymerization (polymer reaction) to the remaining C–H bonds of the low-molecular weight branched polymer with F8 afforded a branched π-conjugated polymer with a higher molecular weight (Mn = 10u2006600, DB = 50%). This is the first example of direct arylation post-polymerization between “polymer and polymer” or “polymer and bromoarenes”.

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: ≈50u2005μm; width: ≈150u2005μm; length: ≈25u2005mm). The fibers produced maintained their original crystal structure and properties (i.e., fluorescence efficiency and elastic flexibility).