Current Organic Chemistry - Volume 9, Issue 16, 2005

This is the first issue of Current Organic Chemistry dedicated to "stimuli-induced functions". Functional materials currently cover a diverse range of applications. It is ideal to activate these functions at any time and in any environment. To realize this reality, I am convinced that it is necessary to create systems with induced-function that respond to a certain stimuli, e.g. light, heat, or physical force. When people wish to apply the function of the material, they simply input the appropriate stimuli. It is relatively easy to modify the characteristics or add functionality to organic compounds because there are many synthetic techniques already in place. In this issue, I have tried to include a variety of approaches to attaining "stimuli-induced functions. Since the discovery of Tanaka's gel, research into "Polymer gels" has progressed rapidly as an example of a stimuli-induced functional polymer. Because various stimuli such as pH, temperature, and an electric field can induce changes in the gels, many researchers have been studying these gels intensively. The first contribution of this issue, by Yoshida (The University of Tokyo, Tokyo, Japan), reviews the design and recent applications of stimuli-induced gels, and also describes his interesting selfoscillating gels, which are promising as bio-mimetic artificial muscles. The second paper by Minagawa (The University of Tokushima, Tokushima, Japan) and Koyama (Yamagata University, Yonezawa, Japan) describes electrorheological (ER), magnetorheological (MR) and electromagnetorheological (EMR) materials. Such materials include a number of compounds in the organic and inorganic areas. In this paper, they emphasize organic polymers, of which rheological functions are induced by electric and/or magnetic fields. The preparation of the polymers, their field-induced functions and their applications are described. The molecular solenoid is one proposal to provide a new organic material with electric and magnetic properties, both of which are intimately connected with each other; one property should induce the other. A helical and π-conjugated polymer would be a potential candidate for such a functional material, based on the well-defined chiral structure. In the third paper, Nishide and Iwasaki (Waseda University, Tokyo, Japan) focus on helicene derivatives and their interesting optical properties that arise from the stiff structures and molecular ordering. They also describe novel stiff helical ladder polymers, the poly(thiaheterohelicene)s. A material can be useful as a sensor if chemical bonding of a specific species to the material induces an electron transfer reaction. The fourth review by Yuasa and Oyaizu (Tokyo University of Science, Noda, Japan) is a summary of detecting and sensing of reactive oxygen species, which are found in excess in the body as a result of cancer, brain and myocardial infarction, and other diseases. They describe some biosensors including all-synthetic sensors of reactive oxygen species, of which the needle-type sensor is expected to be applicable to certain sites in the living body. I would like to thank all authors for their efforts in making this first issue of "stimuli-induced functions" interesting and informative. It was a great pleasure to be involved in this issue of Current Organic Chemistry as a Guest Editor and I hope you will enjoy reading the papers as much as I have.

Over about the last two decades, many kinds of stimuli-responsive polymer gels in response to the change in their surroundings such as solvent composition, temperature, pH, and supply of electric field, etc., have been developed. They have attracted much attention as intelligent (or smart, biomimetic) materials which have sensor, processor and actuator functions. Applications to actuator (artificial muscle), biosensor, drug delivery systems, purification or separation systems, etc. are extensively studied. In this review, design of stimuli-responsive gels and their application to bio- or biomimetic materials will be discussed as well as our recent studies on novel self-oscillating gels.

Various materials that show reversible changes in the rheological properties in response to external stimuli, especially electric and/or magnetic fields, have attracted attention because of their possible applications to devices of transformation of electric/magnetic stimulus into mechanical (passive) force. These materials are called electrorheological (ER), magnetorheological (MR), and electromagnetorheological (EMR) materials. The field-responsive properties are regarded as 'rheological functions' induced by the field, which characterize these materials. The ER/MR materials can be classified into some types, i.e. suspensions, homogeneous or heterogeneous liquids, elastomers/gels, etc. according to the rheological characteristics in the absence and presence of the electric/magnetic fields. For each type, the field-induced rheological properties are briefly summarized, followed by some topics on the material preparation, stimuli-induced functions, and/or practical applications are exemplified. The materials chemistry, especially for organic/polymer materials, are mainly focused on here.

Helical polymers and macromolecules have precisely ordered stereostructures, and their potential applications include chiral separation and sensing based on molecular recognition and liquid crystalline formation by molecular ordering. Among them, πconjugated polymers with a helical structure have often exhibited unique electro- and magnetoresponsibilies. In this review, we describe the helical πconjugated macromolecules as electro- and magneto-responsible materials including our recent studies. In Chapter 2, the recent studies of helical polymeric molecules with πconjugated structures are reviewed. We focus on "helicene" derivatives, which consist of fused-benzene and/or thiophene rings, in Chapter 3 and summarize their syntheses, characteristics, and optical properties based on the stiff structures and molecular ordering. Helical ladder polymers, "poly(thiaheterohelicene)s" comprised of fused-benzothiophene rings, are discussed in Chapter 4 among their stiff helical structure and πconjugation. The combined functions of electro- and magnetoproperties of the chiral polymer are described using the example of poly(thiaheterohelicene)s, such as electron transmission along the helical main-chain and a molecule model of a molecular solenoid, in the final section.

Reactive oxygen species (ROS) such as superoxide anion radical (O2-. ) play an essential role on normal cellular growth and homeostasis. However, excess ROS generated by perturbing O2-. homeostasis under various conditions of oxidative stress induce high radical toxicity, resulting in many diseases such as cancer, brain and myocardial infarction, and inflammation. Quantitative analysis of O2-. by a convenient method is a subject of intense research, since most of ROS are derived from O2-. . In situ real-time measurement of O2-. is very important to understand the relevance of ROS to many diseases. Recent progress in electrochemical sensors for the facile detection of O2-. , including biosensors utilizing a variety of metalloproteins as sensing elements for O2-. and very recently developed all-synthetic sensors with a high selectivity for O2-. detection, is reviewed. Emphasis is placed on the possibility of the all-synthetic sensor for convenient in vivo measurement of ROS.