Supramolecular Soft Matter E-Book

Takashi Nakanishi

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 939

Veröffentlichungsjahr: 2011

Beschreibung

The pivotal text that bridges the gap between fundamentals and applications of soft matter in organic electronics

Covering an expanding and highly coveted subject area, Supramolecular Soft Matter enlists the services of leading researchers to help readers understand and manipulate the electronic properties of supramolecular soft materials for use in organic opto-electronic devices, such as photovoltaics and field effect transistors, some of the most desired materials for energy conservation. Rather than offering a compilation of current trends in supramolecular soft matter, this book bridges the gap between fundamentals and applications of soft matter in organic electronics in an effort to open new directions in research for applying supramolecular assembly into organic materials while also focusing on the morphological functions originating from the materials' self-assembled architectures. This unique approach distinguishes Supramolecular Soft Matter as a valuable resource for learning to identify concepts that hold promise for the successful development of organic/polymeric electronics for use in real-world applications. Supramolecular Soft Matter:

• Combines important topics to help supramolecular chemists and organic electronics researchers work together

• Covers an interdisciplinary field of prime importance to government-supported R&D research

• Discusses the concepts and perspectives in a dynamic field to aid in the successful development of organic electronics

• Includes applications for energy conservation like photovoltaics and field effect transistors

Teeming with applicable information on both molecular design and synthesis, as well as the development of smart molecular assemblies for organic electronic systems, Supramolecular Soft Matter provides more practical in-depth coverage of this rapidly evolving technology than any other book in its field.

Leseprobe

Table of Contents

Title Page

Copyright

Preface

Contributors

Part I: Supramolecular Objects Towards Multi-task Organic Materials

Chapter 1: Supramolecular Materialization of Fullerene Assemblies

1.1 Introduction

1.2 Hydrophobic–Amphiphilic Concept

1.3 Supramolecular Assemblies of C60-Bearing Aliphatic Chains

1.4 Functions Originated from Three-Dimensional Flakelike Microparticles

1.5 Photoconductive Soft Materials

1.6 Conclusions

References

Chapter 2: Tuning Amphiphilicity of Building Blocks for Controlled Self-Assembly and Disassembly: A Way for Fabrication of Functional Supramolecular Materials

2.1 Introduction

2.2 Irreversible Methods to Tune the Amphiphilicity of Building Blocks

2.3 Reversible Stimuli-Responsive Methods

2.4 Supramolecular Methods

2.5 Conclusion and Outlook

2.6 Acknowledgments

References

Chapter 3: Organic–Inorganic Supramolecular Materials

3.1 Introduction

3.2 Film-Type Supramolecular Hybrids

3.3 Endo-Type Mesoporous Supramolecular Hybrids

3.4 Exo-Type Mesoporous Supramolecular Hybrids

3.5 Conclusions

3.6 Acknowledgment

References

Part II: Stimuli Responsive Dye Organized Soft Materials

Chapter 4: Functional Materials from Supramolecular Azobenzene Dye Architectures

4.1 Introduction

4.2 Azobenzene Dyes for Functional Materials

4.3 Strategies for the Production of Functional Supramolecular Materials

4.4 Ionic Self-Assembly

4.5 Hydrogen-Bonded Polymeric Assemblies

4.6 Summary and Conclusions

References

Chapter 5: Stimuli-Responsive Supramolecular Dye Assemblies

5.1 Introduction

5.2 Supramolecular Dye Assemblies with Stimuli-Responsive Optical Properties

5.3 Supramolecular Dye Assemblies with Stimuli-Responsive Nanostructures

5.4 Conclusions

References

Chapter 6: Anion-Responsive Supramolecular Dye Chemistry

6.1 Introduction

6.2 Hydrogen-Bonding-Based Anion-Responsive Supramolecular Gels

6.3 Metal-Coordinated Gels Responsive to Anions

6.4 Pyrrole-Based, Anion-Responsive π-Conjugated Molecules that form Supramolecular Assemblies

6.5 Charge-By-Charge Assemblies from Anion-Responsive Supramolecular Gels

6.6 Conclusions

References

Part III: Dimension Controlled Organic Frameworks

Chapter 7: Polymeric Frameworks: Toward Porous Semiconductors

7.1 Introduction

7.2 General Synthetic and Analytical Methods for Porous Polymers

7.3 Porous π-Conjugated Polymers

7.4 Porous Graphitic Carbon Nitride Semiconductors

7.5 Conclusion

References

Chapter 8: Two-Dimensional Semiconductive π-Electronic Frameworks

8.1 Introduction

8.2 Two-Dimensional Polymers on Metal Surfaces

8.3 Two-Dimensional Polymers with Covalent Organic Frameworks

8.4 Conclusions

References

Chapter 9: Polymer-Friendly Metal–Organic Frameworks

9.1 Introduction

9.2 Characteristic Features of MOFs

9.3 Polymer Synthesis in One-Dimensional Channels of MOFs

9.4 Polymer Synthesis in Higher Dimensional Channels of MOFs

9.5 Polymer–MOF Composites

9.6 Summary

References

Part IV: Recent Trends of Organic Radical Materials

Chapter 10: Multidimensional Supramolecular Organizations Based on Polychlorotriphenyl-methyl Radicals

10.1 Introduction

10.2 Zero Dimensional (0-D) Supramolecular Organizations

10.3 One Dimensional (1-D) Supramolecular Organizations

10.4 Two-Dimensional (2-D) Supramolecular Organizations

10.5 Three-Dimensional (3-D) Supramolecular Organizations

10.6 Conclusions

References

Chapter 11: Photoswitching Property of Diarylethenes in Molecular Magnetism and Electronics

11.1 Introduction

11.2 Molecular Magnetism

11.3 Intramolecular Magnetic Interaction

11.4 Photochromic Spin Coupler

11.5 Diarylethene as a Photoswitch

11.6 Photoswitching of Magnetic Interaction

11.7 Reversed Photoswitching Using Bis(2-Thienyl)Ethene

11.8 Photoswitching Using array of Photochromic Molecules

11.9 Switching on Aryl Group

11.10 Noble Metal Nanoparticle

11.11 Photoreaction on Metal Nanoparticles

11.12 Conductance Photoswitching of Diarylethene–Gold Nanoparticle Network

11.13 Conductance Switching of Diarylethene–Gold Nanoparticle Network by Oxidization

11.14 Conclusions

References

Part V: Organogels and Polymer Assembly

Chapter 12: Self-Oscillating Polymer Gels

12.1 Introduction

12.2 Design of the Self-Oscillating Gel

12.3 Self-Oscillating Behaviors of the Gel

12.4 Design of the Biomimetic Micro-/Nanoactuator using Self-Oscillating Polymer and Gel

References

Chapter 13: Self-Assembly of Conjugated Polymers and their Application to Biosensors

13.1 Introduction

13.2 Background

13.3 Self-Assembly of Conjugated Polymers

13.4 Applications of Self-Assembly of Conjugated Polymers to Biosensors

13.5 Outlook and Future Work

13.6 Acknowledgments

References

Part VI: Supramolecular Liquid Crystals

Chapter 14: Advanced Systems of Supramolecular Liquid Crystals

14.1 Introduction

14.2 Design of Materials Structures

14.3 Design of Materials Functions

14.4 Summary and Outlook

References

Chapter 15: Supramolecular and Dendritic Liquid Crystals

15.1 Introduction

15.2 Liquid-Crystalline Dendrimers

15.3 Multipedes

15.4 Summary

References

Chapter 16: Photoresponsive Chiral Liquid Crystals

16.1 Introduction

16.2 Photoresponsive Liquid Crystals

16.3 Photoresponsive Chiral Liquid Crystals

16.4 Summary

References

Chapter 17: Liquid Crystals Toward Soft Organic Semiconductors

17.1 Introduction

17.2 Smectics and Discotics as Anisotropic Organic Semiconductors

17.3 Summary

References

Part VII: Supramolecular Composites Based on Carbon Nanotubes

Chapter 18: CNT/Polymer Composite Materials

18.1 Introduction

18.2 Strategy for Carbon Nanotube Solubilization and Functionalization

18.3 Redox Reaction and Determination of Electronic States of Carbon Nanotubes

18.4 DNA/Carbon Nanotube Hybrids

18.5 Curable Monomers and Nanoimprinting

18.6 Conductive Nanotube Honeycomb Film

18.7 Nanotube/Polymer Gel near IR-Responsive Materials

18.8 Electrocatalyst for Fuel Cell Using Soluble CNTs

18.9 Concluding Remarks

References

Chapter 19: Interaction of Carbon Nanotubes and Small Molecules

19.1 Introduction

19.2 Interaction of Polyaromatics with CNTs

19.3 Interaction of Extended π–Systems with CNTs

19.4 Interaction of Conjugated Macrocycles with CNTs

19.5 Interaction of Ionic Liquids with CNTs

19.6 Interaction of Metal Ion Complexes with CNTs

19.7 Interaction of Surfactants with CNTs

19.8 Interaction of Functional Dyes with CNTs

19.9 Composites of Liquid Crystals and CNTs

19.10 Conclusions

References

Chapter 20: The Tuning of CNT Devices Using Self-Assembling Organic and Biological Molecules

20.1 Introduction

20.2 Tuning SWCNT Devices Using SAMs Composed of Organic and Biological Molecules

20.3 Functional SWNT Devices Coated with Self-Assembled Organic or Biological Molecules

20.4 Conclusions

References

Part VIII: Optoelectronics Based on Supramolecular Assemblies

Chapter 21: Mimicking Photosynthesis with Fullerene-Based Systems

21.1 Introduction

21.2 Fullerenes in Electron Transfer

21.3 Fullerenes in Solar Cells

21.4 Summary

21.5 Acknowledgments

References

Chapter 22: Recent Trends in Supramolecular Photovoltaic Systems

22.1 Introduction

22.2 Principles of Supramolecular Organic Photovoltaic Devices

22.3 Self-Assembly Based on Hydrophobic Interactions

22.4 Self-Assembly Based on H-Bonding

22.5 Supramolecular Hybrid Solar Cells

22.6 Conclusion

References

Part IX: Future Perspective in Supramolecular Soft Materials

Commentary 1: What will be The Rosetta Stone for the Next-Generation Supramolecular Chemistry?

Commentary 2: Supramolecular Chemistry in Materials Science

References

Index

Color Plates

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Library of Congress Cataloging-in-Publication Data:

Supramolecular soft matter : applications in materials and organic electronics / edited by Takashi Nakanishi.

p. cm.

ISBN 978-0-470-55974-1 (hardback)

1. Supramolecular electrochemistry. 2. Molecular structure..

3. Molecular biology. I. Nakanishi, Takashi.

QD880.S87 2011

541'.226–dc22

2011010584

Preface

Soft matter including organic assemblies and supramolecular objects are of high potential, but still seminal, because of their unique characteristics such as flexibility, self- and hierarchical-organization, self-repairing, self-healing, and stimuli-responsiveness. All their features are highly desired in practical applications in wet-processed electronic systems such as organic semiconducting devices, which are difficult to achieve by inorganic hard matter and organic crystals. Utilization of weak intermolecular forces for the construction of new supramolecular objects with controlled dimensionality and their implementation in practical applications is a major theme in contemporary chemistry, nanoscience, nanotechnology, and materials science. Moreover, fine-tuning of the interaction between π-conjugated molecules can enable the development of supramolecular materials with attractive electronic properties such as semiconductivity, redox activity, magnetism, and photoresponse. Better understanding and manipulation of these electronic properties together with the aforementioned characteristics of soft matter are necessary for the development of new generations of organic/polymeric soft materials toward organic electronic devices such as photovoltaics and field-effect transistors, representing some of the most desired materials for saving energy sources in the future. Under these conditions, the utilization of various performances achieved from supramolecular assemblies composed of dyes, organic frameworks, organic radicals, gels, liquid crystals, conjugated polymers, and nanocarbon clusters (fullerenes and carbon nanotubes) are promising approaches to open the door for ideal interdisciplinary treatment of organic or polymeric soft materials. Critical to the realization of the supramolecular materialization in organic electronic systems is the imaginative molecular design and synthesis as well as the development of smart molecular assemblies that are exemplified by researches highlighted in this book.

This book, Supramolecular Soft Matter: Applications in Materials and Organic Electronics, has a selection of subjects that do not aim to offer a plain compilation of current trends in supramolecular soft matter; it rather attempts to identify concepts that I believe hold promise for successful development of organic electronics with tremendous prospects. Hopefully, this book will stimulate all scientists working in broad research fields such as organic, polymer or supramolecular chemistry, nanomaterials, surface science, optics, photophysics, and materials science, among others, and will provide a useful guideline to direct research in supramolecular chemistry toward electronic soft materials. In addition, the chapters in this book will inspire the current and future generations of chemists to create ever more useful and diverse soft materials, because most chemists still have not understood what kind of organization of materials is suitable for bringing out the intrinsic features of organic materials.

Takashi Nakanishi

National Institute for Materials Science, Tsukuba, Japan

Contributors

Takuzo Aida

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Ayyappanpillai Ajayaghosh

Photosciences and Photonics Group, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Trivandrum 695 019, India

Katsuhiko Ariga

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Japan Science and Technology Agency (JST), Core Research of Evolutional Science and Technology (CREST), Japan

Hidehiko Asanuma

Department of Interfaces, Max Planck Institute of Colloids and Interfaces, Germany

Sukumaran Santhosh Babu

National Institute for Materials Science (NIMS), Japan

Dario M. Bassani

Institut des Sciences Moléculaires, Université Bordeaux 1, CNRS, 351, Cours de la Libération, 33405 Talence, France

David Bilby

Department of Materials Science and Engineering, University of Michigan, 2098 H.H. Dow, 2300 Hayward St., Ann Arbor, MI 48109, USA

Michael J. Bojdys

Technische Universität Berlin, Englische Straβe 20, 10587 Berlin, Germany

Suresh Das

Photosciences and Photonic Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR), Thiruvananthapuram 695 019, Kerala, India

Juan Luis Delgado

Depatamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040, Madrid, Spain

IMDEA-Nanociencia, Facultad de Ciencias, Spain

Xuesong Ding

Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan

Charl F. J. Faul

School of Chemistry, University of Bristol, Bristol BS8 1TS, UK

Bristol Centre for Nanoscience and Quantum Information, University of Bristol, Bristol BS8 1FD, UK

Tsuyohiko Fujigaya

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan

John W. Goodby

Department of Chemistry, University of York, Heslington, York YO10 5DD, UK

Dirk M. Guldi

Lehrstuhl für Physikalische Chemie I, Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraβe 3, 91058 Erlangen, Germany

Jia Guo

Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, Japan

Kenji Higashiguchi

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Jonathan P. Hill

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Japan Science and Technology Agency (JST), Core Research of Evolutional Science and Technology (CREST), Japan

Qingmin Ji

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Japan Science and Technology Agency (JST), Core Research of Evolutional Science and Technology (CREST), Japan

Donglin Jiang

Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan

Takashi Kato

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Jinsang Kim

Department of Materials Science and Engineering, University of Michigan, 2098 H.H. Dow, 2300 Hayward St., Ann Arbor, MI 48109, USA

Ju-Jin Kim

Department of Physics, Chonbul National University, Korea

Dirk G. Kurth

Chemische Technologie der Materialsynthese, Universität Würzburg, Röntgenring 11, 97070 Würzburg, Germany

Jeong-O. Lee

NanoBio Fusion Research Center, Korea Research Institute of Chemical Technology, Daejeon 305-343, Korea

Hiromitsu Maeda

College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan

Japan Science and Technology Agency (JST), Japan

Nazario Martín

Depatamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040, Madrid, Spain

IMDEA-Nanociencia, Facultad de Ciencias, Spain

Marta Mas-Torrent

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and Centro de Investigación Biomédica en Red en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Spain

Kenji Matsuda

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Veronica Mugnaini

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and Centro de Investigación Biomédica en Red en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Spain

Takashi Nakanishi

National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan

Naotoshi Nakashima

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan

Japan Science and Technology Agency (JST), CREST, Japan

Imma Ratera

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and Centro de Investigación Biomédica en Red en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Campus Universitari de la Universtitat Autonoma de Barcelona, Bellaterra, Spain

Concepció Rovira

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and Centro de Investigación Biomédica en Red en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Campus Universitari de la Universtitat Autonoma de Barcelona, Bellaterra, Spain

Isabel Saez

Department of Chemistry, University of York, Heslington, York YO10 5DD, Great Britain, UK

Yo Shimizu

National Institute of Advanced Industrial Science and Technology (AIST), Kansai Center, Midorigaoka, Ikeda, Osaka 563-8577, Japan

Sampath Srinivasan

Photosciences and Photonics Group, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Trivandrum 695 019, India

Yasuhiko Tanaka

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan

Arne Thomas

Institute of Chemistry: Functional Materials, Technische Universität Berlin, Englische Straβe 20, 10587 Berlin, Germany

Takashi Uemura

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Jaume Veciana

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and Centro de Investigación Biomédica en Red en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Campus Universitari de la Universtitat Autonoma de Barcelona, Bellaterra, Spain

Ratheesh K. Vijayaraghavan

Photosciences and Photonic Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR), Thiruvananthapuram 695019, Kerala, India

Jens Weber

Max Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Science Park Golm, D-14424 Potsdam, Germany

Huaping Xu

Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China

Shiki Yagai

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Takuma Yasuda

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Ryo Yoshida

Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Xi Zhang

Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China

Part I

SUPRAMOLECULAR OBJECTS TOWARDS MULTI-TASK ORGANIC MATERIALS

Chapter 1

Supramolecular Materialization of Fullerene Assemblies

Sukumaran S. Babu, Hidehiko Asanuma, and Takashi Nakanishi

National Institute for Materials Science, Sengen, Tsukuba, Japan

Max Planck Institute of Colloids and Interfaces, Potsdam, Germany

1.1 Introduction

Self-organization of molecules using various supramolecular interactions has gained much attention in past decades because a large number of versatile assemblies have been created with excellent and tunable properties [1–3]. Hence, it is particularly important to study the various aspects of supramolecular chemistry, including soft functional assemblies [4]. The crucial and deciding factors while assembling molecules in a regular pattern are satisfied by the incorporation of suitable functional moieties [4]. It enables the molecules to recognize and self-organize in a programmed manner to gather amended properties, which is not attainable as monomers [5]. The reversibility of functional properties through monomer–aggregate transition is always an inspiration for scientists to design new functional assemblies. The challenging task is the control over the self-organization, whereby morphologies with defined size and shape are to be tuned. The incorporation of various hydrogen bonding units, ionic groups, chiral directing moieties, and solubility-controlling alkyl/glycol chains to various π-conjugated, commonly planar, chromophores such as oligo(p-phenylenevinylene)s (OPVs), perylene- or merocyanine-type dyes, porphyrins, hexabenzocoronene (HBC), and dendritic type mesogens enabled precise control over assembly formation [4].

Meijer et al. have shown that the incorporation of various self-recognition as well as hydrogen bonding units and various alkyl side chains resulted in functional OPV assemblies [6]. Interestingly, these assemblies acted as a good medium for energy or electron transfer studies [7]. In addition, this resulted in the formation of assemblies with exciting supramorphologies [8]. Reports from the group of Ajayaghosh showed that the OPV scaffold is adequate for facile energy transfer mediated by tunable organogel medium with enhanced emission [9]. Organogelation of a series of OPVs with different end functional groups, which are donors (D) and acceptors (A), enabled tuning of the excited state properties, resulting in white-light-emitting organogels [10]. Würthner et al. have contributed much in the area of self-organization of perylene- and merocyanine-based dyes [11]. The extensive studies have shown that different self-assembled dyes with near-infrared (NIR) absorption features are potential candidates in creating supramolecular assemblies and applications in organic photovoltaic devices [12]. Aida and coworkers have studied the self-assemblies and functional properties of HBC [13]- and porphyrin [14]-based systems. Self-organization of amphiphilic HBC molecules has led to HBC nanotubes, which may find direct application in organic devices. Recently, the incorporation of various functional chromophores in the liquid crystalline assemblies have also been extensively studied [15].

Apart from the flat planar π-systems, self-organization of spherical π-systems such as fullerenes (C60 and C70) have also been vastly studied in the past decades [16]. The abundant and unique optoelectronic properties of the curved π-surface of C60 has been utilized for the sensible design of inexpensive, lightweight, and durable organic photovoltaic devices [17]. The functionalization of fullerenes, reactive owing to its enhanced curvature, using different synthetic protocols resulted in a large number of derivatives with exceptionally good electron transfer and self-assembly properties [18]. The research area of fullerene self-assembly has formulated new dimensions through molecular design. Here, we focus on the new concept of hydrophobic amphiphilicity, which gained much attention recently [19].

1.2 Hydrophobic–Amphiphilic Concept

The concept of molecular amphiphilicity is one of the widely exploited strategies in the area of interface science because of the versatile features of self-assembly [20]. Amphiphilic molecules consist of hydrophobic (hydrocarbon moiety) and hydrophilic units (charged anionic/cationic groups or uncharged polar groups), which can exhibit a large number of supramolecular architectures, such as micelles, vesicles, lamellae, tubular arrangements, as well as various cubic phases, depending on the relative balance between hydrophobic and hydrophilic interactions and on the solvophobic conditions [21–23]. The concept of amphiphilicity has been studied deeply in surfactants, detergents, and oils, and has great implications toward chemistry, biochemistry, biophysics, and colloid science [24]. The structural features of an amphiphile enable it to aggregate in an aqueous environment through aggregation of the hydrophobic apolar tails, which are protected from water by polar heads.

The research interest regarding self-assembly behaviors of amphiphilic fullerenes has been significantly increased. Reports from the group of Nakamura have shown that a potassium salt of pentaphenyl C60 (1) (Scheme 1.1) forms vesicles in aqueous conditions [25]. Zhang et al. reported that another amphiphile, octahydroxy C60 (2) (Scheme 1.1), forms spherical aggregates in water with a hydrodynamic radius Rh of about 100 nm [26]. Tour and coworkers investigated the self-assembly of C60-N,N-dimethylpyrrolidinium iodide (3) (Scheme 1.1) leading to the formation of 1-D nanorods and vesicles under various experimental conditions [27]. Later, Shiga et al. studied the self-assembly of 3 in binary liquid mixtures of toluene and iodomethane, which resulted in the formation of nanosheets and precipitation as nanofibers in toluene/dimethyl sulfoxide (DMSO) mixture [28]. Interestingly, the matted nanosheets, several micrometers in length and about 100 nm in thickness, were formed from a large number of nanorods of 20-nm diameter. Another report from the group of Prato showed that C60 derivative with a short aliphatic chain containing an ammonium cation and a counter anion, (4), forms well-ordered nanorod-like aggregates in water [29]. All the above examples are less extended in shape and morphology.

Scheme 1.1 Chemical structures of potassium salts of pentaphenyl fullerene (1), C60-N,N-dimethylpyrrolidinium iodide (2), octahydroxy fullerene (3), fullerene containing ammonium cation and BF4− counter anion (4), C60 derivatives consisting of three alkyl chains with an amide and three ester groups (5–7), C60-didodecyloxybenzene dyad (8), fulleropyrrolidine functionalized with 3,4,5-alkyloxyphenyl groups (9, 10), 3,4,5-semiperfluoroalkyl (11), and diacetylene (12) groups.

In this context, a concept of “hydrophobic amphiphilicity” will be considered [19]. The structural modification of an amphiphile that relies on hydrophobic–hydrophobic balance has added a new dimension to amphiphilicity. The relative balance between two hydrophobic interactions, namely, π–π (C60) and van der Waals (alkyl chains), enabled delivery of diverse nano and microscopic architectures (supramolecular polymorphism) [30], which are never observed in the case of conventional amphiphilic fullerenes. A clear evidence of solvophobicity is exhibited by this C60-based hydrophobic amphiphiles, leading to supramolecular assemblies with tunable properties and morphology.

1.3 Supramolecular Assemblies of C60-Bearing Aliphatic Chains

Nakashima and coworkers have reported C60 derivatives that bear three alkyl chains with an amide and three ester connectors (5–7, Scheme 1.1) [31–33]. These molecules lack the high hydrophilic nature of the conventional C60 amphiphiles and hence are soluble in various organic solvents. More interestingly, although the molecules are mainly composed of hydrophobic C60 and aliphatic chains, they were capable of forming Langmuir films on water, which behaves similar to lipid biomembranes [33]. Patnaik and coworkers reported the self-assembly of a partially ground-state charge-separated nonpolar–polar–nonpolar fullerene(C60)-didodecyloxybenzene dyad (8) (Scheme 1.1) to form micrometer-sized rod/sheet-like aggregates and in-plane bilayer vesicles with a head-to-head C60 packing conformation [34, 35].

In addition, a series of alkylated C60 derivatives synthesized in our laboratory have exhibited interesting assembly phenomena when compared to the other alkylated C60 derivatives [36]. The relative balance of two hydrophobic interactions derived from C60 (less hydrophobic) and alkyl chains (more hydrophobic) [19] has resulted in dimensionally controlled supramolecular architectures with interesting assembly properties. Detailed assembly studies have revealed that this multi(alkyloxy)-phenyl substituted fulleropyrrolidines show supramolecular polymorphism in different organic solvents.

1.3.1 Hierarchical Supramorphology

The self-assembly of fulleropyrrolidine functionalized with a 3,4,5-tri(hexadecyloxy)phenyl group (9) exhibited supramolecular polymorphism: formation of different well-defined self-organized superstructures in various solvents and experimental conditions (Fig. 1.1) [30]. It is remarkable that a delicate balance between the hydrophobic interaction of chemically different C60 and saturated hydrocarbon chains drives the molecules to nanostructures with various interesting morphologies. The C60 composed of carbon atoms with sp2 hybrid orbitals exhibits high affinity toward aromatic solvents (benzene, toluene, and xylenes), whereas the saturated hydrocarbon part consists of sp3 hybridized carbons that have a higher affinity to alkanes than the aromatic compounds. This preferential affinity of the aromatic and aliphatic moieties toward different solvents is the basis of the unusual amphiphilicity observed in the molecular assemblies of C60 derivatives that bear aliphatic chains.

Figure 1.1 (a) The proposed structural model of bilayer assembly with interdigitated alkyl chains. (b) AFM image of disc-shaped assemblies of 9 formed in 1,4-dioxane as a precursor of the flower-shaped supramolecular assembly (h). SEM images of supramolecular assemblies of 9, (c) spherical, (d) fibrous, (e) conical, (f) left-handed, and (g) right-handed spiral objects obtained from various solvent conditions. (i) Schematic representation of the formation mechanism of the flower-shaped supramolecular assembly. Source: Reprinted with permission [30, 37]

Self-assembled supramolecular objects were prepared by evaporation to dryness of a 1 mL chloroform solution of 9 ([9]=1.0 mM) followed by the addition of 1 mL of solvents with different polarities. The field emission scanning electron microscopic (FE-SEM) images of the light brown mixtures from the respective solvents after heating at 60–70°C for 2 h indicated that 9 self-assembles into hierarchically ordered nano- and micro-superstructures [30]. It is assumed that the self-organization starts with a bilayer structure of self-assembled interdigitated bilayer of 9, in which aromatic C60 layers in the upper and lower parts are separated by an aliphatic chain layer (Fig. 1.1a). Interestingly, when 1,4-dioxane is used for self-assembly, 2-D self-organized single bilayer discs (Fig. 1.1b) with diameters of 0.2–1.5 μm were obtained. In the case of 2-propanol/toluene system, 9 forms spherical aggregates with an average diameter of 250 nm (Fig. 1.1c). Fibers with partially twisted tapes appeared in 1-propanol (Fig. 1.1d), whereas conical objects with a diameter of 60 nm and perforated at the cone apex were developed from H2O-tetrahydrofuran (THF) mixture (1:1) (Fig. 1.1e). In addition, left-handed spiral microstructures were obtained from 2-(R)-butanol (Fig. 1.1f), whereas 2-(S)-butanol provided right-handed spiral objects (Fig. 1.1g) of 3 to 6 μm diameter. The hierarchical organization of 9 on cooling down the 1,4-dioxane solution from 60 to 20°C, followed by cooling to 5°C, resulted in microscopic flowerlike superstructures (Fig. 1.1h) 3–10 μm in size, with crumpled-sheet-like or flakelike nanostructures of several tens of nanometers in thickness [37]. Figure 1.1i explains the transformation (shape shift) mechanism from assembled molecular bilayer discs to microscopic flower-shaped superstructures of the alkyl-conjugated C60-derivative 9. In order to understand the mechanism and intermediate assembly structures of the flowerlike objects in detail, a homogeneous 1,4-dioxane solution of 9 was cooled rapidly from 60 to 5°C. Interestingly, preformed disc objects were loosely rolled up at the edges (Fig. 1.1i). The rolling distortions at every quarter of the disc resulted in square-shaped objects having four corners with conical shapes developed by the encounter of the rolled or folded edges on the discs. As rolling up proceeds continuously, spatial congestions at the four corners lead to crumpling, bending, stretching, and fracture of the discs. Immediately after these transformations, the bilayer growth at the edges continues which fixes the spatial conformation of the crumpled sheets, and finally leads to the formation of flower-shaped superstructures (Fig. 1.1i) [37].

In order to characterize the self-organized objects, various optical, morphological, and analytical experimental techniques have been utilized. At first, the assemblies were observed through an optical microscope to confirm the formation and further the nano-micrometer-sized structures. A more magnified analysis was carried out by scanning electron microscopy (SEM). To get a clear approximation of the organization at the molecular level, X-ray diffraction (XRD) and transmission electron microscopy (TEM and cryo-TEM) experiments were performed. In our case, the bimolecular layer assembles through alkyl chain interdigitation. Analytical experiments such as differential scanning calorimetry (DSC) to understand the thermal phase transitions and stability of the assembly and optical measurements using UV-vis spectroscopy to monitor the C60–C60 interaction and FT-IR to record the alkyl chain conformation have been commonly used. The surface topography of the resulted assembly structures were observed through atomic force microscope (AFM) and occasionally by scanning tunneling microscope (STM) imaging [38–40].

1.3.2 Antiwetting Architectures

Antiwetting feature is a fundamentally important phenomenon. The tendency of water molecules to exclude or move away from nonpolar molecules leads to marginal segregation between water and nonpolar substances. The crucial deciding factor is the surface roughness of the nonpolar surface on which water is spread. A surface with micro- and nanostructured roughness generates superhydrophobicity with a water contact angle (CA) greater than 150°; the air is trapped between the surface and water droplets [41]. The development of functional assemblies of C60 with antiwetting properties is of considerable significance in current research interest because of the applications in durable organic devices. In this context, the well-defined three-dimensional (3-D) fractal architectures of alkyl-conjugated C60 find useful applications using the morphological features. Self-organization of a C60 derivative 10 (Scheme 1.1) with 3,4,5-tri(eicosyloxy)phenyl group in 1,4-dioxane solution led to spontaneous formation of micrometer-sized globular objects with wrinkled nanoflake structures at the outer surface (Fig. 1.2a) [42]. The dewetting ability of the globular objects was investigated by measuring the static water contact angle of the superhydrophobic surface obtained from 10. Interestingly, in the case of globular objects, a water contact angle of 152° (inset of Fig. 1.2a) was observed, whereas a simple spin-coated film prepared from a homogeneous chloroform solution of 10 exhibited a static water contact angle of about 103°. The unique geometry of the “nano”-flakelike “micro” particles possesses tiny pockets at the surface that entrap air inside, exhibit two-tier roughness, and enhance the surface hydrophobicity. One of the advantages of these superhydrophobic systems is the reusability; the prepared thin films can readily be recovered by dissolving them in chloroform and reused.

Figure 1.2 SEM images of flower-shaped supramolecular assemblies of 10 from (a) 1,4-dioxane and (b) n-dodecane, (c) 11 from diethoxyethane, (d) 12 from THF/MeOH mixture. Insets show the photographs of a water droplet on the surface with static water contact angles (a) 152°, (b) 164°, (c) 148°, and (d) 146°. (e) Schematic representation of the photo-cross-linking process in the bilayer structural subunit of 12. Source: Reprinted with permission [42–44]

It is a crucial question whether the C60 or the alkyl tails of 10 is exposed to the outer surface. In order to understand this, nano-flakelike microparticles of 10 have been prepared from the nonpolar solvent n-dodecane (Fig. 1.2b), which exhibited a static water contact angle of 164° (inset of Fig. 1.2b). This contact angle value is higher than that of the assembly prepared from the polar solvent 1,4-dioxane (152°, Fig. 1.2a), indicating that in the case of assemblies in a polar solvent, such as 1,4-dioxane, C60 moieties are exposed to the outer surface. The hydrocarbon tails are more hydrophobic than the moderately hydrophobic C60 moiety [19, 33], and hence, owing to the mutual affinity, the outer surface components of the assembly of 10 from n-alkane solvents are presumably composed of the hydrocarbon tails [43]. This observation was further confirmed by the static water contact angle of the fractal-shaped microparticles (∼148°) (Fig. 1.2c) of the fluoroalkyl conjugated C60 derivative (11, Scheme 1.1) assembled in a polar solvent, that is, diethoxyethane. The C60 moieties are exposed to the outer surface of the nanoflaked microparticles prepared from polar solvent conditions.

Even though the self-assembled objects exhibit an interesting surface morphology with water repellent properties, the robustness of the assemblies still remains a challenge. This may limit the potential use of these and most of the self-assembled organic objects for practical applications. In order to overcome this difficulty, C60 equipped with alkyl chains containing photo cross-linker (diacetylene) was synthesised (12, Scheme 1.1) [44]. UV light irradiation of the flakelike microparticles of 12 (Fig. 1.2d) resulted in the polymerization of both diacetylene and C60 moieties (Fig. 1.2e) and enabled getting chemically and mechanically robust assemblies (>29-fold compared to the nonpolymerized one) with antiwetting property (inset of Fig. 1.2d).

1.4 Functions Originated from Three-Dimensional Flakelike Microparticles

One of the challenging tasks in the case of supramolecular architectures created by the self-assembly approach is to find out suitable applications. As a new strategy, reports from our group have shown how best the assemblies can be effectively utilized. Two unique approaches have been implemented to diversify the applications.

1.4.1 Supramolecular Molding Method

The remarkable and reliable method to develop well-designed hard matter using molecularly assembled supramolecular soft matter is by metallization. Interestingly, supramolecular architectures with structural and morphological diversity can deliver a broad range of high-definition templates for the design and synthesis of unusual functional inorganic nanoarchitectures. In this direction, transcription of C60-based microparticles having flakelike outer surface features obtained through self-assembly into various metals was demonstrated [45]. The sputtering of the desired metal (Au, Pt, Ti, and Ni) directly onto a thin film of the supramolecular assemblies, followed by the removal of the C60 template using organic solvents, enabled successfully transferring the self-assembled features directly on to the metal surfaces (Fig. 1.3a). The advantage of this transcription method is the possibility of recovering and reusing the template, making the entire process sustainable. In order to understand the features of the nanostructured metal surfaces, it has been applied in surface-enhanced Raman experiments. For example, surface-enhanced Raman scattering (SERS) of the resulting metal Au nanoflake exhibited an enhancement factor on an order of 105. This study has revealed a simple method for the transcription of the morphological features of a self-assembled object directly to different active metal surfaces, which has useful applications [46].

1.4.2 Thermal Indicator for NIR-Induced Local Heating of Carbon Nanotube

A promising strategy for assessing temperature rise during photothermal conversion of single-walled carbon nanotube (SWCNT) on NIR irradiation [47] was demonstrated by microparticles obtained by the coassembly of 10 and SWCNT (Fig. 1.3b) [48]. The flake-shaped microparticles of 10-SWCNT were prepared by heating 1,4-dioxane solution of 10 and SWCNT to 70°C with ultrasonication followed by cooling to room temperature. On irradiation with NIR laser of lower intensity (50 mW), the microparticles started to deform (Fig. 1.3b), and when the laser intensity was increased to 90 mW, they were destroyed immediately. The flake-shaped surfaces of 10-SWCNT assemblies have a mesomorphic-to-isotropic transition (melting point) at 191.8°C. Interestingly, microdisc (9-SWCNT, 2–8 μm) and flake-shaped microparticles (13-SWCNT, 2–4 μm) with melting points of 217.2 and 223.0°C, respectively, were also deformed on NIR laser illumination. This study demonstrates that NIR irradiation of C60-SWCNT assembly can reach a local heating temperature in excess of around 220°C. The advantage of this system is the possibility of in situ visualization of deformation during photothermal conversion by means of an optical microscope and the tuning of assembly melting point by selecting an appropriate C60 derivative with suitable length or number of alkyl chains. More importantly, considering that SWCNTs are widely used in biology for local heating with operations conducted around body temperature, our results serve as a reminder that NIR irradiation of carbon nanotubes can induce an extreme temperature rise.

Figure 1.3 (a) Schematic representation for the preparation of metal nanoflake surfaces by using supramolecular architectures of 10 as templates. (b) Scheme of the coassembly of 10-SWCNT for evaluating photothermal conversion of SWCNT; SEM images before (left) and after (right) illumination by an NIR laser (50 mW). Source: Reprinted with permission [45, 48]

1.5 Photoconductive Soft Materials

The covalent functionalization of C60 has resulted in the formation of various functional assemblies, including organogels and liquid crystals (LCs). Nakamura et al. have reported organogels of alkyl-conjugated C60 derivative (14, Scheme 1.2) and developed self-assembled nanowires of C60 using the Langmuir Blodgett method [49]. The control over solubility of C60 obtained by derivatization using an l-glutamide moiety (15–17, Scheme 1.2) has led to the formation of organogels, especially in mixed organic solvents [50]. In recent years, there has been considerable interest in the design and synthesis of C60-based LC assemblies because of the unique properties resulting from the ordering of different LCs. Deschenaux [51] and Felder-Flesch [52] are the leading contributors in the area of C60-based LCs. The reports from their groups have demonstrated how synthetic design strategies can be utilized to develop plenty of C60-based LCs and to impart anisotropic liquid crystalline character to completely isotropic fullerenes. One of the disadvantages of these systems is that the content of C60 part in the liquid crystalline C60 derivatives is relatively low, and hence the optoelectronic properties corresponding to C60 part will be limited.

1.5.1 C60-Rich Thermotropic Liquid Crystals

Since C60-based LCs find direct applications in organic photovoltaic devices because of their predetermined, controllable organization at the molecular level, it is extremely important to select the necessary anisotropic building blocks along with C60 to attain the preferred ordering. Recently, alkylated C60 derivatives, that is, 9, 10, 13 (Schemes 1.1 and 1.6) with a high C60 content (up to 50%) and high carrier mobility in the highly ordered mesophase were reported [53]. Polarized optical microscopic (POM) studies of 10 showed a birefringent optical texture (Fig. 1.4a) comparable to smectic phases with a fluid nature in the wide temperature range between 62 and 193°C. An unusual long-range ordered lamellar mesophase in which molecules are assumed to arrange their long axis, on average, perpendicular to the plane of the layers with the C60 moieties in a head-to-head configuration was confirmed by the presence of multiple Bragg peaks in the LC state (Fig. 1.4b) [54]. The mesomorphic fullerenes retain reversible electrochemistry as cast films and exhibit electron mobility of ∼3×10−3 cm2/Vs, which is the largest photoconductivity value for C60-containing LCs. The dense packing of fullerenes carrying the charges is responsible for the high electron mobility observed in the mesophase.

Scheme 1.2 Chemical structures of fulleropyrrolidine functionalized with 3,4-alkyloxyphenyl groups (13), 3,4,5-tris(dodecyloxy)benzamide-linked C60 organogelator (14), C60 gelator with l-glutamide moiety (15–17), fulleropyrrolidine functionalized with 2,4,6-alkyloxyphenyl groups (18–20).

1.5.2 Room Temperature Fullerene Liquids

The advantages of synthetic organic chemistry have been extended to design some room temperature liquid fullerenes. The controlled aggregation of fullerenes at the molecular level by attaching a 2,4,6-tri(alkyloxy)phenyl group to the fulleropyrrolidine (18–20, Scheme 1.2) resulted in a new kind of nanocarbon fluid matter (Fig. 1.4c) [55]. The substitution pattern and length of alkyl chains were carefully chosen to serve as an effective steric stabilizer, preventing C60 aggregation. The lack of perfect molecular ordering in the liquid material is evidenced by very broad peaks in the XRD pattern of the liquid form. The higher loss modulus (G″) value than the storage modulus (G′) has confirmed the liquid character of these derivatives at room temperature, and the viscosity of liquid fullerenes can be effectively controlled by changing the alkyl chain length (Fig. 1.4d). An important feature is that liquids with higher alkyl chains show lower viscosity, which is a completely opposite trend to that seen in alkanes. The liquid C60 compounds retain the characteristic electrochemical features of C60 and carrier mobility of ∼3×10−2 cm2/Vs (20 at 20°C). These features make it an extremely attractive novel carbon material for future applications because of the absence of structural defects and quite high C60 content.

Figure 1.4 (a) Polarized optical micrographic texture of mesophase of 10 at 190°C on cooling from the isotropic phase at a rate of 0.1°C/min and (b) XRD patterns of 10 at 185°C. (c) Photographs of the liquid fullerenes (18–20) at room temperature with (d) the corresponding viscosity values. Source: Reprinted with permission [53, 55]

1.6 Conclusions

The rational design of covalently functionalized C60 derivatives has enlightened the self-assembly of fullerenes. Hence the remarkable achievement in crafting supramorphologies with molecular-level precision using the simple spherical π–π interaction of fullerenes has gathered much attention. The important criterion in the design of C60 derivatives to obtain functional assemblies with a highly ordered C60 arrangement and having a high C60 content has been realized by alkyl-conjugated fullerenes. In this way, a concept of hydrophobic amphiphilicity through the delicate balance between hydrophobic interactions of the aromatic and aliphatic moieties has been established. The self-organized supramolecular objects exhibited superhydrophobicity, and further robustness of these structures were enhanced through polymerization of the diacetylene unit. Self-assembled microparticles have been used to transfer the nanomorphology to different metal surfaces, enabling the development of metal nanoflakes as highly sensitive SERS surfaces. The incorporation of carbon nanotubes into self-assembly enabled monitoring the temperature rise during photothermal conversion on NIR irradiation. In addition to superhydrophobic characteristics, these molecules revealed the presence of mesophases, with the largest electron mobility reported for C60-containing LCs. The synthetic manipulation of the alkylated fullerenes has ended up with new fluid nanocarbon materials with good photoconductivity. Recent developments in our group have delivered new C60 derivatives with better crystallinity and higher C60 content. Photoconductive flowerlike supramolecular architectures were developed via self-assembly of a C60 derivative (C60 content of 84%) bearing a pyridine substituent [56]. Arene–perfluoroarene interaction has been used to develop transparent millimeter-sized flat crystalline C60 sheets with anisotropic photoconductivity through 1:1 coassembly of phenyl- and perfluorophenyl-substituted fullerenes [57]. We believe that the control over the morphology of π-conjugated compounds with a high π-core content would be used to diversify their variety of applications in organic electronics.

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Chapter 2

Tuning Amphiphilicity of Building Blocks for Controlled Self-Assembly and Disassembly: A Way for Fabrication of Functional Supramolecular Materials

Huaping Xu and Xi Zhang

Tsinghua University, Department of Chemistry, Beijing, China

2.1 Introduction

Supramolecular chemistry aims at developing highly complex chemical systems from components interacting by means of noncovalent intermolecular forces. As initiated by Lehn, the field was and is the basis for most of the essential biochemical processes of life. It has grown over 20 years into a major domain of modern teaching, research, and technology. Self-assembly is one of the key issues in supramolecular science and refers to the autonomous organization of components into patterns or structures. In nature, the components of cells and organisms self-assemble spontaneously, leading to hierarchically organized structures that allow life to exist. In science, the self-assembly of small molecules into large functional nanostructures has led to the construction of supramolecular systems with defined dimensions, both in solution and on solid substrates, which display unique properties through collective interactions, much like natural systems [1, 2].

Self-assembly processes are mainly classified as static self-assembly and dynamic self-assembly. While the current understanding of self-assembly comes from the examination of static systems, many mechanistic details of dynamic self-assembly and disassembly are poorly understood [3, 4]. Although studies on self-assembly have been pursued for more than three decades, making components and letting them assemble “correctly” is still far from being routine [5]. The control of the subtle balance between weak interactions (e.g., hydrogen bonding, polar attractions, van der Waals forces, hydrophilic–hydrophobic interactions, and charge-transfer interaction) and the use of cooperative effects to influence self-assembly and disassembly processes in order to generate artificial functional systems thus remains a great challenge [6].

Amphiphilicity is one of the molecular bases for self-assembly. An amphiphile, a molecule that contains both hydrophilic and hydrophobic parts, can self-assemble in solution or at interface to form diversified molecular assemblies, such as micelles, reversed micelles, lyotropic mesophase, monolayers, and vesicles [7, 8]. The hydrophilic part of the amphiphile is preferentially immersed in water, while the hydrophobic part preferentially resides in air or in the nonpolar solvent. The amphiphiles are aggregated to form different molecular assemblies by the repelling and coordinating actions between the hydrophilic and hydrophobic parts and the surrounding environment [9]. If we can tune the amphiphilicity of the building blocks, we can control the process of the self-assembly to some extent. The tuning of the amphiphilicity of the building blocks, including small surfactants and amphiphilic copolymers, can be used for controlling self-assembly and disassembly [10].

There is no doubt that molecular chemistry remains an important tool for creating new matter and materials. However, self-assembly provides an alternative route in this regard, which involves controlling spatiotemporal structures for the design and fabrication of functional supramolecular assemblies and materials [1–6]. The self-assembly of amphiphiles has shown significant importance in many research fields, for example, as building blocks for the fabrication of novel organic nanotubes or nanofibers toward electric and medical devices [11–15], candidates for drug delivery [16–19], templates for processing well-defined materials [20–22], nano- or microreactors for carrying out reactions in aqueous solutions and even for artificial enzyme mimicking [23–27], and stabilizer for incorporating emulsion used for cleaning and green organic reactions [28, 29]. Moreover, the introduction of stimuli responsiveness into the building blocks can lead to the fabrication of smart supramolecular materials that are responsive to external stimuli, such as light irradiation, heat, pH change, and so on.

This chapter discusses different methods for tuning the amphiphilicity of building blocks for controlled self-assembly and disassembly, as shown in Fig. 2.1. It discusses irreversible methods that convert amphiphilic building blocks to either hydrophilic or hydrophobic by chemical approaches. On transforming to nonamphiphilic, the molecular assemblies formed by the amphiphilic building blocks can be collapsed irreversibly. There are also methods that can be used to tune the amphiphilicity reversibly. In this respect, reversible stimuli-responsive and supramolecular chemical methods are involved. The alternation between the hydrophilic and hydrophobic parameters allows for the reversible self-assembly and disassembly of the building blocks.

Figure 2.1 Schematic illustration of the irreversible ((a) UV irradiation [30], (b) oxidation [31], (c) redox switch [32], (d) pH variation [33]) and reversible ((e) redox switch [34], (f) reversible combination with carbon dioxide [35], and (g) and (h) photo-irradiation [36]) methods for tuning the amphiphilicity of building blocks.

2.2 Irreversible Methods to Tune the Amphiphilicity of Building Blocks

Normally, there are two kinds of irreversible reactions used to tune the molecular amphiphilicity: the in situ polarity variation of the special groups on the building blocks and the detachment of the labile groups from the building blocks. In both cases, the polarity variation induced by those irreversible reactions can change the molecular amphiphilicity concomitantly, which will endow the building blocks with new properties for application in materials science. In this section, we discuss how to use photochemical, oxidation–reduction, and pH-stimuli reactions for tuning the amphiphilicity.

2.2.1 Photo-Irradiated Irreversible Methods

Without the need for additional substances, light is one of the most desirable stimuli for tuning the molecular amphiphilicity, thus providing methods for clean and rapid control of critical micelle concentrations (CMCs), surface tension, aggregation behavior, and types of aggregates. Ringsdorf et al. employed two different photoreactions to tune the amphiphilicity of the building blocks, which strongly influenced the stability of the liposomal structure formed by the amphiphiles [30]. One is to tune the amphiphilicity through in situ photoreaction on the molecular skeleton. The amphiphile bearing a photosensitive head of pyridinioamidates can form liposomes by self-assembly in water. The photosensitive positively charged pyridinioamidate on the surface of the liposome was triggered to form neutral diazepin on UV light irradiation. The transformation of the head groups from polar to nonpolar induced the liposome to be metastable and even to collapse. This work opened an avenue to tune the amphiphilicity of the self-assembling systems, such as micelles and liposomes, surface wettability, and so on. The other photostimuli method to vary the amphiphilicity is the detachment of photolabile groups from the amphiphiles. As shown in Fig. 2.1a, the hydrophilic head of the amphiphilic 3,5-dialkoxybenzylammonium salt was detached through UV light irradiation. Therefore, the liposome formed by such amphiphiles was deformed significantly [37, 38]. It should be pointed out that this line of research provides a series of photodegradative surfactants for application in soft lithography and separation technology [39, 40].

2.2.2 Redox Response

Redox responsive polymers have attracted wide interest for their promising applications in controllable encapsulation and delivery in physiological environments, where the redox process is constantly and widely present. It has been reported that tumor cells exhibit a more oxidative atmosphere intracellularly than healthy cells. As an unconventional irreversible method, oxidation reactions were also well developed to tune molecular amphiphilicity for its potential use in drug delivery in the oxidative environment of extracellular fluids, physiologically and pathophysiologically. Usually, it is easy to understand that the oxidative reaction, with the oxygen atom involved, can enhance the polarity of targeted molecules. One simple example is the surface treatment by oxygen plasma bombardment: the surface hydrophilicity can be improved significantly by the oxidation of surface molecules. If a similar oxidation reaction is introduced to change the polarity of some neutral or charged groups on the amphiphile, it is hoped that the hydrophilic–lipophilic balance of the amphiphile will be destroyed.

Recently, Hubbell and his coworkers designed and synthesized an ABA-type triblock copolymer as the candidate amphiphile to control molecular self-assembly by oxidation reaction. The ABA-type triblock copolymer has hydrophilic poly(ethylene glycol) (PEG) as A-part and hydrophobic poly(propylene sulfide) (PPS) as B-part, in which the PPS segment is responsive to oxidative chemicals, such as H2O2