INTRODUCTION

As soft electrical materials, the anisotropic carrier mobility of discotic liquid crystals (DLCs) along their π-π columns is promising very much as the components of various organic semi-conducting materials. The π-π stacked aromatic cores act as "conducting wires", whilst the peripheral chains act as "insulators". This allows quasi one-dimensional carrier mobility, which is a desirable state for various semi-conducting applications, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) or organic photovoltaic cells (OPVs). For those applications, one dimensional charge transport in a macro-length scale is essential, so the arrangement control of DLCs is a crucial issue.

The scope of this philosophy doctoral thesis is the preparation, characterization, and application of novel soft electrical materials based on triphenylene (TP)-cored DLCs. The focus is mainly about the control of their arrangement in a certain direction in a macro-length scale for anisotropic electrical material development. The research includes 1) the hybridization of DLCs with carbon nanotubes (CNTs), and 2) the formation of aligned double layers composed DLCs. Throughout these investigations, this research will suggest new strategies for DLCs towards anisotropic electrical material development.

SECTION I

A variety of soft electrical conductors have been developed by doping CNTs into organic and polymeric materials, where better electrical properties are realized by dispersion of a larger amount of CNTs. Recently, LC materials attract increasing attention for hybridization with CNTs, as LC materials have the potential to orient CNTs for anisotropic electrical conduction. However, the loading levels of CNTs are only as small as 0.01 wt%. Very recently, TP derivative DLCs also have been reported to orient CNTs. However, due to their rather low miscibility with pristine CNTs, the use of CNTs covalently modified with TP was essential.

Herein the section 1, TP derivatives bearing six imidazolium ion pendants (TPIa and TPIb, Figure 1a) serve as excellent dispersants for pristine single-walled CNTs (SWNTs). The resultant composite materials can maintain their LC properties up to the SWNT content of about 8 wt%, which is 2-3 orders of magnitude greater than those reported previously. Of further interest, the ILC composites, when sheared, display anisotropic conducting properties, as SWNTs are oriented along the shear direction.

In 2003, our group reported that imidazolium ion-based ionic liquids, when being ground with SWNTs, are transformed into physical gels (bucky gels), where SWNTs are highly dispersed by a π-cation/π-electronic interaction and eventually form a 3D network structure associated with an interionic interaction of ionic liquids. As reported previously, TPIa and TPIb utilized for the present study assemble into hexagonal columnar (134-188 ℃ on cooling; Figure 1b) and cubic (221-188 ℃ on cooling; Figure 1b) mesophases, respectively, over a wide temperature range, including room temperature.

As a typical example of the hybridization of TPIs with SWNTs, pristine HiPco SWNTs were added at 150 °C to TPIa (isotropic melt) with a SWNT content of 5 wt%, and the mixture was ground with a pestle for 30 min, whereupon it turned to a viscous black paste (Figure 1c). As observed by optical microscopy and cross-section transmission electron microscopy, only a very small amount of SWNT agglomerates was detected in the black paste. Similar black pastes resulted when the SWNT contents employed were in a range of 3-15 wt%.

In the course of the above studies, it was serendipitously discovered that the DLC columns of TPIa, upon being doped with SWNTs, align homeotropically with respect to the substrate surface. For a clear demonstration of this phenomenon, SWNT-doped TPIa was sandwiched by glass plates and allowed to assemble into a LC mesophase by slow cooling (1 °C min-1) from its isotropic melt. Polarized optical microscopy (POM) of TPIa alone at 25 °C displayed a birefringence texture in the entire view. In contrast, POM of the TPIa/SWNT (5 wt%) composite exhibited a dark-field image entirely. Shearing treatment reconfirmed the dark image is attributed to the homeotropic alignment.

During the shear treatment of the TPI/SWNT composites, it was uncovered that SWNTs align along the shearing direction. Polarized absorption spectroscopy of the composite showed that the spectral intensity changes with an applied angle between the polarizing and shear directions. Since SWNT bears a transition dipole along its longer axis, these results clearly indicate that SWNTs in the sheared composite predominantly align along the shear direction.

Of particular interest, the orientation of SWNTs is a dominant factor for charge-carrier transport properties of the TPIa/SWNT composite (Figure 2). For example, when TPIa doped with 1 wt% SWNTs was sandwiched by ITO electrodes, the DC conductivity across the film (s) at 25 °C in State 1 (circles) was nearly two orders of magnitude smaller than that in State 3 (squares). In contrast, State 1 and State 2 (rhomboids) were comparable to one another in DC conductivity, indicating a negligibly small contribution of the LC columns to the observed conduction profile. Similar tendencies have been found in the samples doped 3 or 5 wt% SWNTs.

In summary, imidazolium ion-appended LC TP derivatives are the excellent LC dispersants for pristine SWNTs. Dispersed SWNTs, though randomly oriented in TPIa, give rise to a homeotropic orientation of the LC columns up to a macroscopic length scale. Combination of shear and annealing treatments can give 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 oriented or not.

SECTION II

Homeotropic alignment of DLCs is particularly attracting much attention from researchers because it can maximize anisotropic properties of DLCs along their columns. However, little has been known about the mechanism of the formation of homeotropic alignment. Although many efforts to achieve homeotropic alignment have been made, general strategy has not been established yet. In particular, bilayer homeotropic alignment was remained a great challenge.

In-situ photopolymerization can be a key breakthrough in this challenge. Kato et al. reported that homeotropic alignment could be fixed without structural disturbance even after polymerization. It is expected that polymerized DLCs are chemically and thermally stable enough for the deposition of another layer of DLCs on it. The polymerized homeotropic layer might act as a template for the second layer if both layers have structural similarities. In the section 2, a novel strategy for the formation of homeotropically aligned bilayers are discussed, which is based on the in-situ polymerization of TP-cored DLCs with polymerizable groups.

A spontaneous homeotropic alignment of TP derivatives is relatively reported a lot, especially when modified with certain flexible chains with ether or ester spacers. The polymerizable TP-cored molecule was synthesized by attaching five hexyl and one hept-4,6-dienyl groups to the TP core via ether spacers, where the conjugated diene terminus of the latter works as a polymerizable group (TPDa; Figure 3a). To form cross-linked network, the monofunctional monomer TPDa should be copolymerized with a multifunctional monomer. Therefore, another TP-cored molecule possessing hept-4,6-dienyl groups was also synthesized (TPDb; Figure 3a). A bilayer homeotropic alignment will be achieved by means of in-situ polymerization as shown in the process in Figure 3c. After obtaining homeotropic alignment of DLCs, the alignment will be fixed by polymerization. Then another layer of DLCs will be deposited to achieve bilayer homeotropic alignment.

As expected, the mixture of TPDa and TPDb (with a molar ratio of 17:2; denoted as TPDc) showed spontaneous homeotropic alignment during the LC phase on a cooling process (69-33 ℃). The in-situ crosslinking of the homeotropically aligned mixture was carried at 55 ℃ by irradiating UV/visible light under nitrogen gas flow. Through the in-situ crosslinking process, homeotropically aligned hexagonal columnar structure was hardly perturbed, as confirmed by X-ray diffraction analysis and POM. This structural anisotropy was well converted into an electronic anisotropy, confirmed by TRMC. The polymerized homeotropic layer was tolerant of mechanical shearing and the treatment with organic solvents, suitable for the deposition of the second layer on it. Since this thermal and chemical stabilization after the crosslinking, the deposit of another layer on the first layer is allowed.

For the second layer, charge-transfer (CT) complexes of TPDc with acceptor molecules were chosen, so that both layers possess similar structures but are of different electrical properties. Three acceptors were employed here, 2,4,7-trinitro-9-fluorenone (TNF), 1,2,4,5-tetracyanobenzene (TCNB) and N,N'-(1-hexyl)-1,4,5,8-naphthaleneteβtracarboxydiamide (HNP) as shown in Figure 3b. The CT complexes lost their homeotropic alignment as showing the clear conical textures during POM observation (Figure 4a, b and c). However, when they are deposited on the first layer that consists of TP-cored DLCs only, the tendency towards homeotropic alignment was returned (Figure 4d, e and f). It is attributed to the inducing effect from the first layer because of similar structures.

Figure 1. a) Molecular structures of TPIa and TPIb. b) Schematic representations of the LC assemblies with hexagonal columnar and cubic geometries. c) Pictures of a TPIa/SWNT composite.

Figure 2. a) Processings and orientational characteristics of States 1-3. TPIa/SWNT composites were sheared (State 1), shortly annealed for 5 min at 150 °C (State 2), and then annealed for 1 h at 150 °C (State 3). b) Plots of conductivities across the film (s) of States 1-3 at 25 °C of TPIa films doped with 0, 1, 3, and 5 wt% SWNTs, sandwiched by ITO electrodes with a separation of 12.5 μm.

Figure 3. a) Molecular structures of liquid crystals TPDa and TPDb. b) Molecular structures of small acceptors of TNF, TCNB and HNP. c) A schematic strategy to achieve bilayer homeotropic alignment. The polymerization of the first layer is precedent to the deposition of the second layer.

Figure 4. POM micrographs of CT complex single layers of TPDc-TNF (a), TPDc-TCNB (b) and TPDc-HNP (c) respectively, spun coated on single glass substrates. POM micrographs of double layers consisting of TPDc-TNF (d), TPDc-TCNB (e) and TPDc-HNP (f), deposited on the polymerized TPDc. The insets of (d, e and f) are the conoscopic images. Scale bars: 100 μm.

有機半導体は、既存のシリコン半導体に匹敵する高い電気伝導度を示すと理論予想されているうえ、シリコン半導体では困難な軽量性・柔軟性電子デバイスの開発を可能とし、さらにシリコン半導体に比べはるかに低コストかつ低環境負荷プロセスで製造できるなどの特徴を有し、次世代電子材料として高い注目を集めている。さらに、有機化学・超分子化学的な手法を駆使し、電気的特性を分子レベルで精密に制御できる点もまた、他材料と比べた際の有機半導体の最大の利点である。しかしながら現時点では、有機半導体の電気伝導度は無機半導体と比べて極めて低いことが、深刻な課題となっている。最も有効な打開策として、従来の有機半導体材料では全く制御されていない「分子一つ一つの配向」を制御することで、キャリアー移動を一方向に制限することが挙げられる。このような分子配向制御を実現するためには、構造秩序性と動的特性とを併せ持つ液晶材料が、他材料(結晶性材料・アモルファス性材料など)に比べて有利である。本論文では、カラム状集合構造を取り易い液晶として、トリフェニレン分子を中心骨格とする円盤形液晶を用い、その側鎖末端に2種類の官能基を導入することにより、液晶カラムの構造や性質(配向挙動・他材料との親和性・構造維持能)のチューニングを試みている。これを通じ、巨視的な異方性をもつ有機半導体材料の開発を目的とした研究について述べている。

序論では、まず、他材料と比べた際の有機半導体の特徴・長所について述べている。その後、液晶、とりわけ円盤形液晶の歴史・一般的性質・重要な過去の研究について解説している。ここでは、液晶の分子設計・化学的/物理的特性・液晶分子集合体の構造・その解析法が詳しく述べている。続いて、液晶集合体を巨視的に一方向に揃えることが、有機太陽電池や有機発光ダイオードなどの電子デバイスの性能を向上させる上で極めて重要なポイントであることが、事例とともに説得力のある形で主張されている。最後には、トリフェニレンを用いた初期の研究から最新のものまでを紹介し、本研究の目的に対しこの分子を選択した妥当性が明確に述べられている。

第一章では、トリフェニレン円盤形液晶と単層カーボンナノチューブとの複合材料について述べている。相溶性の乏しい二つの物質の親和性を向上させるため、イミダゾリウム塩型イオン液体が単層カーボンナノチューブとの混合によりゲルを形成するという過去の報告に基づき、イミダゾリウム塩を液晶の側鎖の末端に導入した。その結果、過去の全ての報告を大幅に上回る非常に高い比率で、カーボンナノチューブと液晶と混合させることに成功した。その値は他の液晶材料に比べて100倍以上という破格の混合比率である。さらに、カーボンナノチューブの混合により、液晶カラムが基板に対し自発的に垂直配向するという、前例のない現象を見いだした。液晶カラムの配向方向は、剪断応力を印加することで、垂直から水平へと簡単に変えることができる。興味深い点は、剪断応力の印加は、液晶カラムのみならず、系中のカーボンナノチューブの水平配列を誘起するということである。すなわち、本複合材料中では、分子の配向方向を自在かつ可逆的に制御することができる。さらに興味深いことに、それぞれの配向状態で電気伝導度を測定した結果、その異方性が明確に確認された。このように、極めて簡便な操作により、電気伝導の方向性を任意に制御できる有機半導体材料の開発に成功した。

第二章では、液晶の分子配向の光重合によって固定化することで、垂直配向した異種二層の有機半導体材料の開発について述べている。光重合能をもつ液晶として、共役ジエン基を側鎖の末端に導入したトリフェニレン誘導体を新規に合成した。この化合物は、溶融状態から液晶温度範囲へ徐冷することにより、自発的に垂直配向する。少量の光ラジカル重合開始剤存在下、垂直配向状態にて可視光照射することにより、液晶分子のラジカル重合が効率よく進行し、カラムの垂直配向を反永久的に保持した高分子が得られる。この高分子は溶媒に対して不溶であるため、スピンコーティングにより、この上に別の液晶を集積することができる。本論文では二層目の液晶として、一層目と同じトリフェニレン液晶分子に対し、様々な電子アクセプター分子を添加することで得られる電荷移動錯体を用いている。ただし、単独では自発的垂直配向能をもつこのトリフェニレン液晶分子が、電荷移動錯体を形成することより、その能力を失ってしまうことが分かった。しかしながら、予想外かつ幸いなことに、この電荷移動錯体(二層目)を垂直配向した高分子(一層目)の上に集積した場合、両層の構成ユニットが類似の分子構造を持つために、一層目が二層目の「エピタキシャル」成長を促し、電荷移動錯体(二層目)も垂直配向することが明らかとなった。一連の操作で得られた二層の垂直配向材料を用い、正孔移動度を測定したところ、分子配向が制御されているサンプルとそうでないものとで、実に15倍もの差が確認された。電荷キャリアーを電極に対し垂直に輸送する構造は、有機太陽電池や有機発光ダイオードにおける理想状態であるが、その構築は極めて困難であり、一般的手法はこれまでに皆無であった。本研究により、この構造を構築する初めての一般法が提案されたことになる。

以上、本論文では、トリフェニレン中心の円盤形液晶の分子方向を制御することにより、巨視的な異方性をもつ電子材料の開発及び、その電子性能評価について報告している。ここで提案・実証された概念は、基礎科学的に重要な意味を持つとともに、実用を志向した有機半導体材料の開発に対しても大きく貢献すると見込まれる。

よって本論文は博士(工学)の学位請求論文として合格と認められる。