1. Introduction

Studies of carbon clusters and their self-organization have been attracted much attention in these decades due to their potential application in the field of material and biological science. From the view point of synthetic chemistry, fullerene and carbon nanotube are widely studied because of the reactive π electron rich surface, which can be easily modified by chemical reactions. Chemical modification of carbon clusters bestows them functionality for application use. The modification can be observed by electron microscopic analyses even for single molecular level, which is due to the high stability of carbon cluster under electron beam. In this work, I will report the preparation, characterization and properties of chemically modified fullerene vesicles, which are assembly of anionic fullerene amphiphiles. The chemically modified vesicles display highly hydrophobic environment even in water to serve as a nanoscopic scaffold for non covalently anchoring organic molecules.

2. Preparation, Characterization and Properties of Fullerene Vesicles with Fluorous Surface

Potassium salt of highly fluorinated fullerene 1 was synthesized via pentaaddition reaction of C60. The fullerene 1 features a nonpolar/polar/nonpolar ternary motif, which is far different from the widely accepted amphiphilic molecules, featured as a polar head/nonpolar tail for constructing of assembled structure in water. The unique amphiphilic fullerene 1 was found to dissolve in water to form vesicles with an average diameter of 36nm determined by dynamic light scattering (DLS) measurement. The membrane structure of fullerene vesicle was confirmed by DLS and static light scattering (SLS) measurement. The SLS measurement shows the vesicle has a hollow spherical shape with interdigitated monolayer membrane structure with its nonpolar fluorous chains facing to the aqueous environment (Figure 1a). Unlike lipid vesicles that easily loose their structural integrity when removed from aqueous solution, the vesicle of 1 is very robust and retains the spherical shape even on a solid substrate under high vacuum (Figure 1b).

SLS study of the vesicle of 1 showed the vesicle has a room on its fluorous surface, which can be utilized for molecular accommodation. The vesicle solution was found to dissolve C8F18, which is completely insoluble in water. The solubilization of C8F18 in aqueous solution of fluorous vesicle of 1 was studied by 19F NMR (Figure 2). One broad peak at δ-86.2ppm due to the terminal CF3 group (signal i, Figure 2a) was observed for the mixture of C8F18 in fluorous vesicle solution. At 80 °C, broad signals due to the three CF2 groups appeared (signals ii, iii, iv, Figure 2b). From the integrated area of the CF3 signal, the concentration of C8F18 in the vesicle solution to be approximately 1.9 g/L (i.e., 3.2 molecules of C8F18 per molecule of 1) and the solubility is 1,000,000 times larger than that in water.

3. Effect of Surface Modification of Vesicle on the Membrane Property and Vesicle-covered Substrate

Formation of fluorous vesicle of 1 shows the high stability of fullerene vesicle, even after chemical modification and open the way for further study of the substituent effect on vesicle. Fullerene amphiphiles bearing different linear substituents at para positions of phenyl groups (2-6) were designed and synthesized to study the effect of (i) chain length and (ii) polarity on structure and property of the vesicles. Introduction of hydrophobic alkyl chains in different length (2-5) or hydrophilic diethylene oxide chain (6) did not hamper the formation of fullerene vesicle. DLS measurements of the aqueous solution of vesicle of 2-6 showed the sizes of the vesicles were controlled in the range of 20-45nm regardless of the side chain components, without any separatory methods (Figure 3).

Properties of surface-modified vesicles are highly affected by the substituents both on solid substrates and in solution. The effect of substituents on vesicle surface was observed in wettability of vesicle-coated substrates. Figure 4 shows water contact angle of vesicle-covered mica surface. Contact angle increases according to the length of alkyl substituents (2<3<4<5) or hydrophobicity of substituents with the similar chain length (6<3<1). This result shows the bulk property of a substrate can be controlled by modification of fullerene molecules. In solution, water permeability through fullerene membrane was measured by 17O NMR method. The permeability of fullerene membranes 2-5 (Figure 5a), which have alkyl chains on its surface in different length, showed the water permeation through the membrane was suppressed by long alkyl chains, especially the membrane made of fullerene 5 was found to be the least water-permeable membrane. As for the comparison between vesicle 1, 3 and 6, which have similar chain length in different polarity, water permeability decrease as the polarity of side chain decrease. Thermodynamic study of water permeability showed the substituents on vesicle affect the packing structure of fullerene moiety in the membrane. These tendency of water permeability of fullerene vesicle is well corresponding with the tendency of water contact angle of vesicle coated surface (Figure 4). The surface modification of fullerene vesicle affects both the repellency of bulk water and permeation of water molecules.

4. Highly Watertight Fullerene Vesicles on Solid Surfaces

The fullerene vesicle shows water-tightness also on a solid substrate. Fullerene vesicle of 1 showed strong contrast inside under scanning transmission electron microscopy (STEM) measurement and the contrast did not change after 10 minutes of electron irradiation (Figure 6a). This strong contrast indicates water is encapsulated in the vesicle, which forms via noncovarent interaction, under 10-5 Pa. Interestingly, decrease of inner contrast of the vesicle was observed by the addition of 1% C8F18 (Figure 6b). After 2 minutes of exposure to electron beam, only the shell structure of the vesicle was observed. These results indicate the addition of fluorous molecule loosen the membrane structure to release the inner water. Fullerene vesicle of 1 was found to keep water inside under high vacuo and the water can be released by the addition of fluorous molecules.

5. Transmission Electron Microscopic Study of Crystal Growth on Single Organic Molecule

Transmission electron microscopic (TEM) study of Y-shaped molecule attached on nanohorn (Y-NH) made it possible to visualize two fundamental properties of organic molecules (Scheme 1). One is the C-C bond rotation. From the TEM image of Y-NH, I estimate the rotation speed is 4.8 rpm, and the speed did not change by cooling the sample to 4 K. Another interesting image was obtained by the addition of tribromide 7 to Y-NH. Nanometer-sized crystalline objects were observed on NH. The TEM image of the nanocrystal was reproducible by the simulation image of tribromide molecules stacking on Y-shaped molecule on NH.

6. Conclusion

Water-soluble self-assembly with hydrophobic surface was prepared and the characteristic properties were studied. The properties of fullerene vesicles were changed according to the bulkiness or hydrophobicity of substituents, which were observed in contact angle of water and water permeation. Vesicle of 1 shows fluorous environment in water to accommodate fluorous molecules on surface and the membrane property was also changed to release the inner contents under STEM condition. TEM can be also utilized for the study of single organic molecule and crystal growth. From the TEM studies, the speed of C-C bond rotation and initial state of crystal growth were visualized for the first time.

Figure 1. (a) Schematic illustration of the vesicle from fluorous fullerene amphiphile 1 and (b) SEM image of the vesicles on an ITO surface.

Figure 2. Solubilization of C8F18 on the surface of vesicle of 1 in water. 19F NMR chart of (a) C8F18 in a D2O solution of vesicle of 1 at 25 °C, (b) at 80 °C, (c) neat C8F18 at 25 °C.

Figure 3. Diameters of fullerene vesicles of 1-6 in water measured by DLS.

Figure 4. Contact angle of water on vesiclecovered mica surface.

Figure 5. Permeability coefficients of water through (a) fullerene membrane 2-5 and (d) fullerene membrane 1, 3, 6 measured at 20℃.

Figure 6. STEM images of (a) vesicle of 1 and (b) vesicle of 1 containing 1% of C8F18. Inner content was rapidly lost under electron irradiation for sample b. Scale bars represent 50nm.

Scheme 1. Formation of nanocrystal on a single organic molecule on nanohorn.

本論文は六章から構成されており,化学修飾された炭素ナノクラスターを用いた溶液中および電子顕微鏡下での研究について論じている.

第一章では,研究背景として,炭素ナノクラスターの重要性と歴史的背景が述べられている.炭素ナノクラスターの化学修飾法の合成例,分子集合体の化学,および電子顕微鏡による研究について概説することで,本研究の意義を明確にしている.

第二章では,パーフルオロアルキル鎖を表面に持つフラーレン二重膜ベシクルの調製,構造同定および性質について述べられている.既存の両親媒性分子が親水―疎水構造を有しているのに対し,本研究で用いたフラーレン分子は疎水―親水―疎水という全く新しい構造モチーフを有するが故に調製されるベシクルはこれまでにない,疎水的な表面を有する水溶性ベシクルである.この特異な構造を有するベシクルは光散乱法および各種顕微鏡によって構造同定されている.また,表面の疎水的性質をいかし,フルオラス分子の吸着による水溶化および固体基板表面の撥水化を達成している.本研究は両親媒性分子の新たな構造モチーフの呈示という基礎的な側面から,バルク表面の撥水化など応用面においても有用なものである.

第三章では,第二章で提示された疎水―親水―疎水構造を有する両親媒性分子のベシクル形成を一般化すべく,同様の構造モチーフを有するフラーレン化合物でのベシクル形成について述べられている.フラーレン上に長さの異なるアルキル鎖を導入しベシクルを形成させることに成功しており,疎水―親水―疎水構造が両親媒性分子の新たなモチーフとなることを示している.また,ベシクル表面の置換基の性質は疎水表面上での安定性,バルク状態での撥水性および水溶液中での水の透過性に反映されることが明らかとなった.本研究は疎水―親水―疎水構造を有する両親媒性分子の一般性を示したのに加え,表面置換基の変換によりベシクルの性質制御が可能になったという重要な知見を与えている.

第四章では,第二章で見いだしたフルオラスベシクルについて電子顕微鏡観察下で水を保持していることを明らかにした.通常の二重膜ベシクルとことなり,フルオラスベシクルが透過型電子顕微鏡観察において強いコントラストを伴って観測されることに着目し,EDS測定によってベシクル内部に水が保持されていることを証明した.また,フルオラス分子の添加により,内包された水が徐々に抜けて行く様子の可視化にも成功した.自己集合体であるベシクルが高真空下で水を保持できると言う,常識を覆す発見であり,新たな反応場としての利用,および,数十ナノメートルの水の材料的利用など,本研究によってもたらされる知見は大きい.

第五章では,カーボンナノホーン上に有機分子を結合させ,これを単分子結晶核として結晶を成長させ,高分解能電子顕微鏡によってその初期過程について研究を行っている.結晶成長と言うきわめて一般的な現象について,これまで分子レベルでの知見は全くと言っていいほど得られていなかったが,本研究によって有機一分子レベルでの結晶成長について初めて知見を与えている.これにより,結晶成長における臨界サイズを初めて決定したのみならず,結晶核の数を分子レベルで変化させることにより,バルクサイズの結晶サイズを制御することに成功している.本研究により与えられた成果は,結晶多形が重要である製薬および工業界に大きな知見を与えると考えられる.

第六章は本研究の総括である.第二章から第五章の結果を分子集合体の化学,電子顕微鏡の化学および結晶成長の化学における意義を述べるとともに,今後の展望について述べている.

なお、本論文第二~五章は中村栄一博士,原野幸治博士,第二,三章は磯部寛之博士との共同研究であるが,研究計画および検討の主体は論文提出者であり,論文提出者の寄与が十分であると認められる.

本研究は分子集合体の化学において40年間常識とされてきた親水―疎水構造を有する両親媒性分子に加え,疎水―親水―疎水構造を有する両親媒性分子もベシクル構造を形成することを見いだし,その特異な性質について多くの知見を与えた.また電子顕微鏡を駆使することで,これまで不可能であった有機単分子レベルでの結晶成長について重要な知見をもたらした.したがって,本論文は博士(理学)の学位論文として価値のあるものと認める.