Organic Redox Systems

Synthesis, Properties, and Applications

Tohru Nishinaga

• Publisher: Wiley
• Year: 2015
• Print ISBN: 9781118858745
• eBook ISBN: 9781118858721
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Providing a thorough overview of leading research from internationally-recognized contributing authors, this book describes methods for the preparation and application of redox systems for organic electronic materials like transistors, photovoltaics, and batteries.

• Covers bond formation and cleavage, supramolecular systems, molecular design, and synthesis and properties
• Addresses preparative methods, unique structural features, physical properties, and material applications of redox active p-conjugated systems
• Offers a useful guide for both academic and industrial chemists involved with organic electronic materials
• Focuses on the transition-metal-free redox systems composed of organic and organo main group compounds

1
INTRODUCTION: BASIC CONCEPTS AND A BRIEF HISTORY OF ORGANIC REDOX SYSTEMS

Tohru Nishinaga

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo, Japan

1.1 REDOX REACTION OF ORGANIC MOLECULES

Redox is a portmanteau word of “reduction” and “oxidation.” Originally, oxidation meant a chemical reaction in which oxygen combines with another substance, after Antoine Lavoisier, late in the eighteenth century, called a product of the reaction an oxide [1]. The term “reduction” had been used long before the introduction of the term “oxidation” in the smelting to produce iron from ore and coke [1]. In the contemporary definition recommended by IUPAC [2], oxidation is a reaction that satisfies criteria 1 “the complete, net removal of one or more electrons from a molecular entity” and 2 “an increase in the oxidation number of any atom within any substrate” and meets in many cases criterion 3 “gain of oxygen and/or loss of hydrogen of an organic substrate.” Conversely, reduction is the reverse process of oxidation.

For transition metals, a direct one-electron transfer related to the aforementioned criterion 1 is common due to their relatively lower ionization energy in comparison with main group elements [3] and low reactivity of the unpaired d-electrons. In contrast, the mechanisms of common organic redox reactions do not involve a direct one-electron transfer [4], and reactions based on the criterion 3 are typical. For example, oxidation of primary alcohol (RCH₂OH) to aldehyde (RHC═O) with Cr(VI)O₃ proceeds via chromic ester intermediate (RCH₂O₃Cr(VI)OH), and proton and HOCr(IV)O₂⁻ are eliminated from the intermediate [5] (Scheme 1.1a). In this reaction, the total number of electrons in the outer shell decreases from 14 at the C─O moiety to 12 at the C═O moiety, that is, two-electron oxidation, while the formal oxidation number of Cr changes from +6 to +4, that is, two-electron reduction. Similarly, reduction of carbonyl group to alcohol with NaBH₄ in ethanol formally proceeds via nucleophilic attack of a pair of electrons in hydride to electron-deficient carbonyl carbon [5] (Scheme 1.1b). Thus, formally, a pair of two electrons moves together in typical organic redox reactions as known in other organic reactions such as substitutions.

SCHEME 1.1 (a) Oxidation of alcohol to aldehyde with Cr(VI) and (b) hydride reduction of aldehyde to alcohol.

On the other hand, one-electron oxidation or reduction of a neutral or ionic molecule (Scheme 1.2) gives generally highly reactive ion radicals or radicals, and follow-up reactions such as radical coupling and deprotonation are prone to take place [6]. Nevertheless, some organic molecules give persistent species after one-electron transfer at ambient temperature [7, 8]. Simple π-extension and substituents of resonance electron donating R₂N─, RO─, RS─ or withdrawing N≡C─, C═O groups cause delocalization of spin and charge density, which reduces the reactivity of the reactive center. As the other thermodynamic stabilization, aromatization after electron transfer plays an important role for certain molecules. An appropriate steric protection is also an effective strategy for protecting a reactive radical center [9]. As a result of these effects, they can be reversibly regenerated by the reverse electron transfer. This book deals with organic π-electron systems and related organo main group compounds that show such reversible one-electron transfer.

SCHEME 1.2 One-electron oxidation and reduction of neutral and ionic molecules.

1.2 REDOX POTENTIAL IN NONAQUEOUS SOLVENTS

Redox potential is the important measure for redox systems, by which one can predict how easily one-electron oxidation or reduction takes place with other redox reagents. For the measurement of redox potential, cyclic voltammetry is usually the first choice, because not only the redox potential but also the stability of the species generated after electron transfer can be observed. Several types of reference electrodes are used to measure redox potentials. The standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE), which is deter...

Table of contents

• COVER
• TITLE PAGE
• TABLE OF CONTENTS
• LIST OF CONTRIBUTORS
• PREFACE
• 1 INTRODUCTION: BASIC CONCEPTS AND A BRIEF HISTORY OF ORGANIC REDOX SYSTEMS
• 2 REDOX-MEDIATED REVERSIBLE σ-BOND FORMATION/CLEAVAGE
• 3 REDOX-CONTROLLED INTRAMOLECULAR MOTIONS TRIGGERED BY π-DIMERIZATION AND PIMERIZATION PROCESSES
• 4 TETRATHIAFULVALENE: A REDOX UNIT FOR FUNCTIONAL MATERIALS AND A BUILDING BLOCK FOR SUPRAMOLECULAR SELF-ASSEMBLY
• 5 ROBUST AROMATIC CATION RADICALS AS REDOX TUNABLE OXIDANTS
• 6 AIR-STABLE REDOX-ACTIVE NEUTRAL RADICALS: TOPOLOGICAL SYMMETRY CONTROL OF ELECTRONIC-SPIN, MULTICENTERED CHEMICAL BONDING, AND ORGANIC BATTERY APPLICATION
• 7 TRIARYLAMINE-BASED ORGANIC MIXED-VALENCE COMPOUNDS: THE ROLE OF THE BRIDGE
• 8 MAGNETIC PROPERTIES OF MULTIRADICALS BASED ON TRIARYLAMINE RADICAL CATIONS
• 9 OPEN-SHELL π-CONJUGATED HYDROCARBONS
• 10 INDENOFLUORENES AND RELATED STRUCTURES
• 11 THIENOACENES
• 12 CATIONIC OLIGOTHIOPHENES: p-DOPED POLYTHIOPHENE MODELS AND APPLICATIONS
• 13 ELECTRON-DEFICIENT CONJUGATED HETEROAROMATICS
• 14 OLIGOFURANS
• 15 OLIGOPYRROLES AND RELATED COMPOUNDS
• 16 PHOSPHOLES AND RELATED COMPOUNDS: SYNTHESES, REDOX PROPERTIES, AND APPLICATIONS TO ORGANIC ELECTRONIC DEVICES
• 17 ELECTROCHEMICAL BEHAVIOR AND REDOX CHEMISTRY OF BOROLES
• 18 ISOLATION AND CRYSTALLIZATION OF RADICAL CATIONS BY WEAKLY COORDINATING ANIONS
• 19 HEAVIER GROUP 14 ELEMENT REDOX SYSTEMS
• 20 π-ELECTRON REDOX SYSTEMS OF HEAVIER GROUP 15 ELEMENTS
• INDEX
• END USER LICENSE AGREEMENT