Original Article in 2016

The inhibition of solution-processed 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) crystallization by mixing additives was successfully achieved for hole injection layers (HILs) in organic light-emitting devices (OLEDs). Among various additives, the film mixed with pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN) showed both transparent appearance and high solvent resistances to an upper layer coating solvent. We applied the HAT-CN mixed with PPDN for an HIL in an OLED including solution-processed four-layer stack. The device showed lower driving voltage and higher luminescent efficiency than a device containing conventional HIL, poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS).

π-Conjugated polymers capped with a boronic acid end-functional group were synthesized by chain-growth Suzuki–Miyaura polycondensation. The obtained polymers were reacted with a π-stacked-polymer scaffold to synthesize graft polymers. The spectroscopic and optical characterization of the obtained graft polymers in the solid state revealed that the original π-conjugated polymers were stacked in a single polymer chain. Direct observations of the obtained graft polymers showed that they self-assembled into spherical or fiber structures depending on the introduced polymers, whereas no ordered structures were formed by the same polymers before the polymer reaction.

Intrinsic birefringence, Δn₀, is a material constant for the orientational birefringence of polymers, and has been measured by many researchers with various methods. However, the reported values of the same polymer sometimes disagree among the researchers because determination of the orientation degree is somehow complicated. Alternatively, a new method using the modified stress-optical rule (MSOR) was proposed for Δn₀ measurement. In this study, literature data of the Δn₀ for various polymers are assessed and the reliability of Δn₀ estimated via the MSOR is discussed.

Highly luminescent donor–acceptor molecules based on a phenothiazine donor unit coupled with a xanthone or benzophenone acceptor unit were developed. While both molecules are almost non-luminescent in pure tetrahydrofuran (THF) solution, a strong yellow delayed fluorescence was observed upon their aggregation in THF/water mixtures and in neat films. This result demonstrates the unique aggregation-induced delayed fluorescence (AIDF) characteristics of these molecules. Organic light-emitting diodes (OLEDs) using these AIDF materials as a non-doped emission layer achieved high external electroluminescence quantum efficiencies of up to 11%, which far exceeds the theoretical maximum for conventional fluorescent OLEDs.

Three types of A₁–D–A₂ conjugated polymers were synthesized based on thiophene as an electron-rich unit and two similarly structured electron-deficient moieties of benzothiadiazole (BT) and benzotriazole (BTz). The weaker acceptor, BTz, with three alkyl (or alkyloxy) substituents took on a solubilizing role without deteriorating the chain planarity. The absence of alkyl substituents on thiophene induced a deep valence band. Increasing the number of fluorine substituents on BT decreased the frontier orbitals and enhanced the interchain packing. The chain planarity and crystalline interchain organization were facilitated via intra- and/or interchain non-covalent S···O and S···F coulombic interactions. The best power conversion efficiency of 7% was measured for BTzDT2FBT:PC₇₁BM.

Double acceptor donor–acceptor alternating conjugated copolymers comprising thienopyrroledione, benzothiadiazole, two cyclopentadithiophene groups and their analogs with thiophene or bithiophene units have been synthesized by combinations of direct arylation, Suzuki and Stille coupling and oxidative polymerization. These polymers were used to fabricate bulk heterojunction organic photovoltaic (OPV) devices with fullerene derivatives. The active layer comprising the double acceptor polymer with fullerene derivatives showed a subtractive color resulting from wide-range visible light absorption, which is advantageous for practical OPV applications.

The rheological properties of the suspensions of thermo-responsive poly(N-isopropylacrylamide) microgels are investigated in the swollen and collapsed states using creep and oscillatory measurements. The results from the long-time creep tests reveal that in the collapsed state with attractive interparticle interactions, the suspensions with a particle volume fraction of approximately 0.5 become viscoelastic liquids with high viscosities due to the formation of a network-like aggregation of particles. However, the suspensions with the same volume fraction in the swollen state exhibit a Newtonian fluid behavior with low viscosities.

Direct arylation polycondensation using Pd(PCy₃)₂ as a precatalyst in N,N-diethylpropanamide as a reaction solvent afforded three types of the bithiazole-based conjugated polymers that have been difficult to synthesize under previously reported conditions using N,N-dimethylacetamide. The obtained polymers exhibited aggregation properties and high crystallinity. Control of aggregation was a key factor for increasing the power conversion efficiency of the organic photovoltaic devices fabricated from these polymers.

We have prepared nanoporous polystyrene (PS) samples using a solvent extraction method starting from a solid sample with a mixture of high and low molecular weights. The resulting nanoporous samples are characterized using ellipsometry, atomic force microscopy and electron microscopy. This method can potentially be applied to many different polymer materials.

Surface carboxylation of cellulose nanowhiskers (CNWs) was performed using mPEG2000-TEMPO, a 2,2,6,6-tetramethyl-1-piperidinyloxy radical moiety grafted to monomethoxy poly(ethylene glycol) 2000. After the oxidation step, the used mPEG2000-TEMPO was recovered by extraction with dichloromethane and reused for further oxidation cycles. mPEG2000-TEMPO could be used for the surface oxidation of CNWs for at least three repeated cycles, although the degree of oxidation in the second and third cycles was reduced to 60% of that in the first cycle. The recovery of mPEG2000-TEMPO after every cycle was not complete (~80%), thus suggesting an irreversible adsorption of mPEG2000-TEMPO on the surface of the CCNWs.

Poly(N-methylethylene imine) (PMEI) has a wide solubility similar to poly(ethylene oxide) and a high protonation capacity similar to poly(ethylene imine). In addition, PMEI forms aggregates with negatively charged macromolecules, such as polystyrene latex particles, and acts as either a flocculant or a dispersant depending on the environment. An aqueous solution containing the as-synthesized PMEI (weight-average molecular weight: 26 kDa) exhibited an LCST of 116 °C, and a toluene solution exhibited a UCST of 36 °C. Therefore, PMEI is expected to be used as a highly hydrophilic polycation for various applications.

Novel zinc bis(allyldithiocarbamate) (Zn(ADTC)₂) complexes, prepared from allylamines, carbon disulfide and zinc nitrate, were applied to the synthesis of highly refractive materials. Transparent and flexible films could be prepared by the thiol-ene reaction of Zn(ADTC)₂ derivatives with a tetrathiol in the presence of a diacrylate and Irgacure 651. Refractive indices of the films increased with the feed ratios of the Zn(ADTC)₂ derivatives. The calculated n_D values of the components originating from the two Zn(ADTC)₂ were 1.71 and 1.67.

Free-standing calcite thin films containing a few wt% of organic polymer were obtained through repeated cycles of layer-by-layer deposition of the vaterite particles stabilized with an octacarboxy-terminated T₈-caged silsesquioxane, followed by a phase transition to calcite, after a poly(diallyldimethylammonium chloride) (PDDA) layer or a multilayer of PDDA with poly(sodium 4-styrenesulfonate) were coated on the surface of the calcite thin films. Poly(allylamine hydrochloride) and poly(acrylic acid) were also applied for preparing calcite thin films on a glass substrate.

This paper describes the synthesis of hyperbranched poly(siloxysilane) grafted poly(N-isopropylacrylamide) (HBPS-g-PNIPAM), and its application to thermoresponsive cell culture onto glass substrate. PNIPAM branches were growth from the termini of HBPS via reversible addition–fragmentation chain transfer (RAFT) polymerization. The obtained HBPS-g-PNIPAM was casted from a tetrahydrofuran (THF) solution onto glass slides to fabricate a thermoresponsive surface for cell culture. Mouse 3T3 fibroblast cells were seeded and cultured at 37 °C for 4 days and detachment was possible by cooling the temperature to 20 °C for 15 min.

Herein, we demonstrate a new approach to control a structural parameter of polythiophene: the dihedral angle of the backbone. To achieve this, we exploited the unique polymerization mechanism of cyclopolymerization. We designed and synthesized gemini thiophene monomers in which two thiophene monomers are tethered by an alkylene strap and we processed the monomers by using the catalyst-transfer polycondensation method. We found that the dihedral angle in the polythiophene is dictated by the size of the macrocycle formed, thus allowing for the control of the photophysical properties of polythiophenes.

Five asymmetric diacetylene carbamates that possessed a 9-phenyl-9H-carbazol-3-yl (PhCz) group as an aromatic donor that was directly attached to the diacetylene moiety were designed and synthesized. Upon UV irradiation, the highest conversion to the corresponding polydiacetylene was obtained for the benzyl carbamate derivative, and its regular 1,4-addition was confirmed by formation of a greenish-blue polymer that exhibited characteristic excitonic absorption bands. In addition, a small ionization potential of 5.2 eV was observed for this polydiacetylene.

The mean-square radius of gyration 〈S²〉 is determined for poly(di-tert-butyl fumarate) (PDtBF) for a range of weight-average number of repeat units n_w from 382 to 1520 in tetrahydrofuran at 30.0 °C. The data are analyzed on the basis of the Kratky–Porod (KP) wormlike chain with excluded volume (EV). It is shown that the average chain dimension of PDtBF is significantly larger than that of poly(diisopropyl fumarate) (PDiPF) because of a remarkable difference in chain stiffness between the two polymers.

Fullerenes (C₆₀ and C₇₀) and water-soluble poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) were mixed by physical means to prepare water-soluble fullerene/PMPC complexes. PMPC can solubilize large amounts of fullerenes in water. The complexes can generate singlet oxygen upon visible light irradiation.

The film stabilities of ω-N-(3-(dimethylamino)propyl)propylamide-terminated polystyrene (PS-N) and conventional polystyrene (PS-H) on SiO_x substrates were examined. Whereas a 20-nm-thick PS-H film was dewet at 423 K, a comparable PS-N film was stable because of the interaction between the chain end and the substrate surface. This indicates that end functionalization affects chain mobility with a relatively large scale at the solid interface.

A novel hyperbranched poly(ether nitrile) (HBPEN) and its copolymer were synthesized as supports for palladium nanoparticles (PdNPs). Small PdNPs (3.0±0.8 nm) were obtained by the reduction of HBPEN-Pd²⁺ complex with NaBH₄. A comparison of HBPEN-, HBPEN copolymer- and linear poly(ether nitrile)-supported PdNPs suggested that the branched architecture in HBPEN can help prevent the aggregation of PdNPs generated in the early stages of reduction.