Volume 49 Issue 1, January 2017

The state-of-the-art atomic force microscopy allows us to observe inner structures of single molecules. Such observation is quite beneficial to study single and self-assembled molecules on surface as well as on-surface chemical reaction. This focus review describes recent technical improvement and achievements in the field of on-surface molecule studied with high-resolution atomic force microscopy.

A series of dioxocycloalkene-annelated thiophenes was designed to utilize as noble acceptor units in donor–acceptor (D–A) copolymers, and D–A copolymers containing these acceptor units were systematically developed for the application to organic photovoltaics (OPVs). Photophysical and electrochemical properties of these copolymers as well as their photovoltaic performance under the combination with [6,6]-phenyl-C_x-butyric acid methyl ester (x=61 or 71) were summarized. These systematic investigations provide important clues for the development of high-performance D–A-type OPV materials.

Liquid crystalline organic semiconductors are good candidates for organic transistors because the materials have properties that are beneficial compared with those of polymer materials. Liquid crystalline materials show good solution processability for the fabrication of uniform crystalline thin films. The novel liquid crystalline material 2-phenyl-7-decyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) exhibits a highly ordered liquid crystal phase, and has good solution processability and high thermal durability to use for creating transistor devices with high mobility, over 10 cm² Vs⁻¹, following thermal annealing.

In this focused review, we provide an overview of our recent work on the design and synthesis of a new type of side-chain liquid crystal π-conjugated polymeric system associating regioregular poly(3-alkylthiophene) backbones with laterally pending π-conjugated mesogenic groups. Several polymeric architectures have been prepared based on different polymer backbone structures and various side groups, providing important insight into the relationships among the polymeric architecture, nano- and macroscale organization and charge transport properties in such self-organized multi-lamellar macromolecular systems.

Stretchable electronics are an attractive means for producing mechanically robust devices with enhanced utility and can enable novel applications such as conformal solar cells, electronic skin and wearable electronics. Approaches to stretchable organic electronics, specifically polymer solar cell active layer materials, via a molecular design strategy are presented. Further discussion into polymer blends and engineering approaches to producing other types of stretchable electronics is given. The relationships between mechanical and electrical properties of all these materials are discussed and suggestions are given for future areas of research.

Developing renewable polymeric materials for electronics applications is crucial toward next generation of green electronic industry. Current progress on the renewable materials used as the passive and active components in electronics is reviewed, where the correlation between the relationships of chemical structures, morphology and device characteristics is featured. Furthermore, their future perspectives are provided for foreseeable paths in the development of green electronics.

Perylene tetracarboxylic bisimide (PTCBI) derivatives bearing oligosiloxane moieties were synthesized. At room temperature, the PTCBI derivatives bearing oligosiloxane chains exhibit a nanosegregated columnar phase in which crystal-like π-stacks are surrounded by a liquid-like mantle. These PTCBI derivatives are soluble in various organic solvents, and thin films can be produced by a spin-coating method. The PTCBI derivatives bearing polymerizable cyclotetrasiloxane rings show the liquid crystalline (LC) phases at room temperature. The spin-coated films of the LC PTCBI derivatives with cyclotetrasiloxane rings can be polymerized and insolubilized by exposure to the vapors of trifluoromethanesulfonic acid.

Liquid crystals (LCs) have recently gained significant importance in organic photovoltaics (PVs). Power-conversion efficiency up to about 10% has reached in solar cells incorporating LCs. This review presents an overview of the developments in the field of organic PVs with LCs. Comprehensive details of LCs used in bilayer solar cells, bulk heterojunction solar cells and dye-sensitized solar cells have been given. An outlook into the future of this newly emerging, fascinating and exciting field of self-organizing supramolecular LC PV research is provided.

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.

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.

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.

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.

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).

Hydrophilic tetraethylene glycol moieties are incorporated into the electron-accepting block of a donor–acceptor block copolymer (BCP) to afford two novel amphiphilic materials based on the P3HT-b-PFTBT scaffold. These amphiphilic BCPs are designed to provide enhanced morphological control as organic photovoltaic active layer materials. The chemical disparity introduced by the hydrophilic side chains enabled the design of a purification system capable of isolating the BCP product, an advantage over previously reported systems.

Alternating current bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT) was performed in the presence of benzoquinone in tetrabutylammonium perchlorate/acetonitrile. The obtained poly(3,4-ethylenedioxythiophene) (PEDOT) was deposited at both ends of bipolar electrode (BPE) gold wires in a linear fiber form (φ=ca. 30 μm) owing to limited accessibility of the EDOT monomer to the active site of BPEs in a micro-space. The linear growth of the PEDOT microfibers was demonstrated to be useful for application in circuit wiring.

To investigate the effect of the branching position of the alkyl side chain of semiconducting polymers, we synthesized two series of semiconducting polymers based on thienothiophene-2,5-dione (PTTD4Ts) and quinacridone (PQA2Ts). The polymer thin films exhibited small but clear differences in their optical absorption spectra and X-ray diffraction patterns, suggesting that the intermolecular interaction in the solid state varied as a function of the branching position. Although PTTD4Ts did not exhibit a clear correlation between the branching position and charge carrier mobility, the mobility of PQA2Ts gradually increased as the branching position moved away from the backbone.

The results of the X-ray diffraction suggest that the d_π-spacing distance for poly(diethylhexyloxy benzo dithiophene-dioctyloxy dithiophene dibenzophenazine (PBDTAFQ) (d_π=3.89Å) was smaller than poly(diethylhexyloxy benzo dithiophene-dioctyloxy dithiophene diphenyl Qu) (PBDTAQ) (d_π=4.36 Å) because of fused-phenyl ring derivative. As a result, PBDTAFQ displayed a higher absorbance at the UV–vis absorption spectra in the film state. And, as a consequence, PBDTAFQ showed an improved short current density (J_SC) in organic photovoltaic cells. The PBDTAFQ:PC₇₀BM blend devices that were fabricated exhibited a power conversion efficiency of 3.4%.

Regioisomeric introduction of barbituric acid and aliphatic tail units onto anthracene core resulted in distinctly different π–π stacking arrangements upon aggregation in nonpolar solvent, resulting in the formation of supramolecular polymers with either non-helical or helically twisted fibrous morphologies. The difference in the topology of these fibrous nanoaggregates is attributed to the conformational difference of anthracene chromophores with respect to the plane of hydrogen-bonded supermacrocycle.

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.

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.

The molecular orientation of polymer-stabilized nematic liquid crystals (PSLCs) doped with oligothiophene can be easily manipulated using irradiation with linearly polarized light above a threshold intensity. The photoresponsive behavior depended on the initial molecular orientation, as controlled by the surface treatment of PSLC cells. The threshold light intensity for molecular reorientation was successfully reduced by 30% by optimizing the silane coupler concentration because of the decrease in surface anchoring. Thus, weak surface anchoring leads to a reduced threshold light intensity for molecular reorientation.

Alkylated dinaphtho[2,3-b:2',3'-d]furan (DNF–V) derivatives are systematically investigated in view of the impact of the substitution position as well as length of alkyl side chains. For both alkylated DNF–VV and DNF–VW derivatives, the solubilities as well as phase-transition temperatures increase as the shorter alkyl chains are attached. Especially, alkylated DNF–VV simultaneously gain the high solubility over 1 wt% and highly stabilized crystal phase up to 199 °C. Furthermore, DNF–V derivatives exhibit deep-blue emission in solid state with the quantum efficiency ranging from 17 to 51%.