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Manipulation of Nanoscale Materials
An Introduction to Nanoarchitectonics
By Katsuhiko Ariga
The Royal Society of ChemistryCopyright © 2012 The Royal Society of Chemistry
All rights reserved.
Introduction: Nanoarchitechtonics for Materials Innovation
One of the most distinct differences between human beings as compared with other living creatures could be the fact that we can create various kinds of tools, apparatus, and machines. They can been seen commonly in our everyday activities. Tweezers, cloths, televisions, computers, and houses have been all developed by humans and they cannot be evolved in natural spontaneous processes. In their production, we have two key factors: (i) selection of materials (such as woods and metals) and (ii) method of making (construction and manufacturing). We have basically used raw materials, which can be obtained from surroundings, for manufacturing throughout most of our history. However, deep understanding of nanoscale materials in recent researches has changed this common situation drastically.
Rapid progresses in nanoscience and nanotechnology have created several fantastic ways to observe nanoscale objects such as atoms, molecules, clusters, and assemblies. Such progress will open new avenues in materials production, possibly by constructing even raw materials from nanoscale units. Therefore, we do not always have to select naturally-occurring substances that can be created with our 21st-century technologies. We can design functional materials through manipulation of the nanoscale, like carpenters construct houses and buildings. This novel methodology can be named nanoarchitectonics from nano + architecture + nics. This terminology was originally invented by Dr Masakazu Aono at the National Institute for Materials Science (NIMS) and the World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA).
It is necessary to differentiate two key terms, nanotechnology and nanoarchitectonics. Nanotechnology has become a popular word these days. Even non-scientists may frequently use this word in daily conversation. Nanotechnology (and/or nanoscience) deals with nanoscale of nano-related objects. On the basis of scale, it can be regarded as advanced form of microtechnology, which can be activated by considerable developments of microfabrications. However, nanotechnology and microtechnology are somewhat different. Most of the phenomena at the microscale (10-6 m range and the related scales) can be estimated from macroscopic phenomena of the same materials. In contrast, various unexpected properties becomes apparent for materials at the nanoscale (10-9 m range and the related scales). The latter features can be seen in various quantum phenomena and lots of abnormalities. Fullerene, carbon nanotubes, and graphene are zero-, one- and two-dimensional objects, respectively, where some dimensions are degenerated into nanoscales. Their properties cannot be deduced from those observed for macroscopic carbon materials such as carbon ash and diamonds. The unexpected phenomena found for nanocarbons actually create a novel scientific field.
With the emerging possibilities in nanotechnology, we anticipate lots of innovations in materials science. However, innovations, inventions, and dreams do not always come true. Most of the research in nanotechnology fields are oriented for observation and analyses on nanoscaled systems and materials. It may be said that major parts of nanotechnology still remain nanoscience. We need a breakthrough. It requires a novel concept, nanoarchitechtonics, which can bridge fundamental science in nanotechnology and practically useful materials through the application of knowledge and techniques in nanotechnology to explore useful materials, including atomic and molecular manipulation, chemical nanofabrication, self- and field-controlled organization, and theoretical modeling. In this methodology, controlled manipulation of atom/molecular-level nanostructures leads to creation of unexplored functional materials and systems.
Understanding novel concepts is always difficult. One of the best ways to promote the world of nanoarchitectonics would be highlighting the varied frontier research within this concept. From this point of view, this book includes current active research on nanoarchitectonics including (i) materials development in supramolecular chemistry, self organized polymers, in organic substances such as clay and mesoporous materials, catalysts design, and bio-related materials and (ii) systems innovations seen in biosensors, drug delivery systems, bio-related separations, lithographic techniques, and solar cell design. Of course, these topics cannot cover all the targets in nanoarchitectonics, but they will surely stimulate reader's minds to realize nanoarchitectonics for material manipulations and material production.CHAPTER 2
Supramolecular Materials Nanoarchitechtonics
KATSUHIKO ARIGA, GARY J. RICHARDS, MASAAKI AKAMATSU, HIRONORI IZAWA AND JONATHAN P. HILL
As seen in recent developments of portable computers and cellular phones, higher functions can be incorporated into very compact machines. The use of portable devices allows us to freely communicate information which can drastically change our lifestyles. Developments of portable, energy-efficient devices can impact on our societies in various ways. For example, the use of portable devices and ease of transfer of information and data between them can impart various freedoms to our lifestyles. As data and information communication between remote locations simplifies, problems such as over-population and traffic congestion within certain areas can be reduced. This in turn, can help reduce consumption of energy sources and the production of unnecessary environmental pollutants. Such innovations are based on the great success of microfabrication technologies which has led to the miniaturization of various systems.
Following on from the successes of microfabrication technologies, nanotechnology looks set to become the central subject of both scientific and technological research. Even though nanotechnology bears technology in its name, its technological aspect remains immature. Most results in nanotechnology research are observation-based and analysis-oriented. Although interesting scientific phenomena have been discovered, further development is required for more practical applications. For technological developments, production of useful materials and systems based on our knowledge of nanotechnology is necessary. We have to construct functional materials from nanostructures as architectures. This will be conducive to the creation of new functionalities that may be exhibited by nanoscale structural units through their mutual interactions even though these functionalities are not properties of the isolated units. Aono proposed a new term "nanoarchitectonics" to express this innovation of nanotechnology. Materials nanoarchitectonics is a technology system aimed at arranging nanoscale structural units, which are groups of atoms, molecules or nanoscale functional components.
Application of materials nanoarchitectonics to organic soft matters and organic/inorganic hybrids requires control of molecular interactions. The field of control of molecular interaction and synthesis of functional molecular complexes is categorized as supramolecular chemistry. Therefore, two concepts: materials nanoarchitectonics and supramolecular chemistry, share a common interest. The combined concept, supramolecular materials nanoarchitectonics can be a useful methodology for the construction of organic and/or hybrid functional materials based on nanostructure control.
In this chapter, we introduce several topics of our research concerning supramolecular materials nanoarchitectonics. These topics are roughly categorized into (i) molecular-level complexes and arrangement, (ii) self-assembly for functional structures, (iii) intentional assembly for functional structures, and (iv) bridging between the molecular and macro scales.
2.2 Molecular-level Complex and Arrangement
An initial step for supramolecular materials nanoarchitectonics is to make molecular complexes in designed and controlled ways. In this process, a rational pairing of molecules becomes a key step and is generally called molecular recognition. Molecular recognition is the most fundamental and important concept in supramolecular materials nanoarchitectonics, because architecting any supramolecular materials involves selective molecular combination. The main concept associated with molecular recognition is based on the "lock and key" principle. Although this concept is well established, development of molecular recognition systems continues to this day.
Among the various types of molecular recognition, chiral recognition has been paid special attention because of the practical importance of such recognition for certain kinds of molecules including drugs and toxins is widely recognized. The enantiomeric excess (ee) is a critical parameter both as a determinant of the efficacy of chiral therapeutic agents, and as an indicator of success of organic asymmetric reactions. Recently, we have reported a technique for the determination of the ee in chiral carboxylic acids by using achiral porphine macrocycles as nonchiral–chiral solvating agents (Figure 2.1). This system is unique in that chiral sensing is due to the fast exchange of analyte molecules at a protonated meso-substituted porphyrin dication rather than the formation of diastereomeric complexes with a chiral solvating reagent. This figure schematically illustrates the translation of chiral information from the analyte to the achiral host. A porphyrin dication binds two chiral guests in fast exchange equilibrium and each face of the macrocycle acts independently because of its saddle-shaped structure. The chiral guest bound between the opposing pyrrole groups induces non-equivalency in the adjacent pyrrolic proton signals, and anisochronicity of their resonances as a result of the preferred shielding direction. The (R)-/(S)-chiral environment induces different shielding in the place of these protons. The method is notable for the achirality of the reagent, its ease of implementation, and its application to pharmaceuticals because many drugs contain a carboxylic function in the vicinity of a chiral moiety. Exploitation of this phenomenon will lead to a new class of chiral solvating agents with unique applications.
Molecular complex systems can be used as mimics of information processing systems. Certain kinds of molecules are promising candidates for incorporation into electronic or optical devices because of their synthetic flexibility, processability and small size. One of the fundamental challenges in the development of molecular information processing systems is to prepare molecular memory and logic functions. In such systems, it is desirable for data to be stored in binary form, based on changes in optical or electronic properties, and toggled using an external stimulus, such as light, temperature, chemical concentration, voltage or a magnetic field. In particular, changes in optical absorbance and/or fluorescence can be detected as photonic output(s) and these may contain more information than simple electronic outputs. As illustrated in Figure 2.2, we have developed a nonvolatile-type memory system based on fluorescence switching and based on subtle structural modifications of dye molecules. The memory system can be operated primarily by using fluoride anions as specific writing signals. The nonvolatile memory system based on porphyrin derivatives can be achieved as follows. Fluoride anions induce the oxidative conversion from the porphyrin derivative with subsequent complexation of fluoride anion (memory writing). However, removal of the fluoride anion does not cause any change in the structure of the oxidized state, although a dissociation of the complex occurs (memory retention). Finally, the oxidized molecule is reduced to the original state by using ascorbic acid (memory erasing). These processes demonstrated fluoride-writable memory systems using simple porphyrin derivatives based on the relative oxidisability of these porphyrins in the presence of fluoride anions. In this case, OFF and ON states exhibited large differences in fluorescence emission intensity. This characteristic has great potential as a read-out signal for memory systems.
We also developed mechanical deformation of porphyrin macrocycles triggered by a photoredox reaction, which could be used as photo-driven molecular machines in future developments. As illustrated in Figure 2.3, 5,15-bis(3,5-di-tert -butyl-4-hydroxyphenyl)-substituted porphyrins can be operated as reversible photochemical molecular switches. This porphyrin undergoes photoinduced aerial oxidation to porphodimethene with drastic variations of both electronic and conformational structures. The oxidized form undergoes a photoinduced reduction, reverting to the original form in the presence of a sacrificial electron donor such as triethylamine. Forward and reverse processes are governed by different chemical mechanisms, leading to better locking of the redox states. The oxidation process entails loss of aromaticity and an extension of conjugation over a greater number of atoms, and the oxidation reaction is coupled with a profound structural change due to steric and electronic requirements. This chimeric porphyrin-hydroquinone hybrid combines the photonic functionality of porphyrins with the redox activity of hydroquinones. Such molecular design indicates a simple and versatile method for producing photoredox macrocyclic compounds, which could lead to a new class of advanced functional materials suitable for bottom-up fabrication of molecular devices and machines.
Molecular complex formation can be used for regulation of physical events between chromophores. This heterotropic allosteric regulation can be a good mimic of certain processes in nature. For example, nature uses chloride anions as a cofactor of the oxygen-evolving complex. The supramolecular oligochromophoric model system shown in Figure 2.4 contains sites for binding of a reagent species and an anionic species. The bis-porphyrin-substituted oxoporphyrinogen has two different binding sites, in which one site (composed of two porphinatozinc(II) units) is capable of binding bis(4-pyridyl)-substituted guests through coordination to the central zinc cations and the other site (composed of two pyrrole-type amine groups of the oxoporphyrinogen unit) interacts with anionic species through hydrogen bonding. This system demonstrated anion-complexation-induced enhancement of the charge separation in a supramolecular complex to produce singlet charge-separated states and also stabilization of the triplet charge-separated states in an oligochromophoric molecule possessing exclusive binding sites for both a guest electron acceptor and an anionic cofactor species. The oxoporphyrinogen framework allows for the selective positioning of substituents and permits electrochemical control of its redox potentials in the presence of inorganic anions.
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