Organic Functional Free Radicals

This special column presents a snapshot of recent progress of various organic free radicals including the design, synthesis, characterization, properties and applications. We hope that it can help readers stimulate interest to explore novel persistent/stable organic free radicals and promote the research of organic radical based multi-functional materials.     

Design magnetischer Materialien: Chemie der Magnete

The field of molecular based magnetism is an active area of research directed toward the design of new magnetic materials. The idea is to introduce molecular strategies in magneto-chemistry. This can open completely new synthetic routes to materials with previously unknown physical properties. Spin carriers used within this approach range from purely organic radicals to metal complexes and organometallic compounds. The design of new magnetic materials with tailor-made properties requires a detailed knowledge about the interactions between possible spin carriers and the strategies necessary to achieve interactions in all three dimensions. The latter is closely related to the field of crystal engineering. Starting from introductory remarks to magnetochemistry the underlaying concepts for the design of magnetic materials on the basis of molecular compounds as well as new developments and possible applications are described.     

分子纳米磁体的研究进展及应用

分子磁学主要研究无机配合物以及有机自由基的电子结构和磁性之间的关系。近些年发展起来的分子纳米磁体可以在单分子尺度上实现磁双稳态,独立作为一个磁功能单元,可能突破尺寸对传统磁性材料的制约,有望实现超高密度磁存储。分子纳米磁体中清晰的量子态也为量子退相干研究提供了化学调控的手段,这将为量子计算机提供物质基础。本文简要介绍了分子纳米磁体的概念和特征,并对研究进展进行了简要综述。     

Self-assembly of carboxylic substituted PTM radicals: From weak ferromagnetic interactions to robust porous magnets

An overview of the work that have been developed over the last six years in our group on the use of polychlorotriphenylmetyl radicals (PTM) functionalized by carboxylic groups to access to purely organic/molecular magnetic materials is reported. From the seminal work on the monocarboxylic PTM (Section 2), of great importance to determine both the ability of these molecules to form intermolecular H-bonds and the nature of the intermolecular interactions mediated through the resulting supramolecular motifs, we will move to the self-assembly of PTM radicals functionalized with two and three carboxylic groups (Section 3). In those cases, the self-assembly of the paramagnetic units yield robust and porous magnetic structures, associating in some cases magnetic ordering to the latest remarkable characteristics. The last part of the review will present the latest results obtained with the idea to increase both the structural and magnetic dimensionality in purely organic PTM-based materials using a PTM radical functionalized by six carboxylic groups (Section 4). New trends and challenges for this research line, concerning the design and synthesis of new PTM radicals, as well as the obtaining of PTM based sensors or multifunctional materials will be presented in the concluding section (Section 5).     

Recent progress toward the exploitation of organic radical compounds with photo-responsive magnetic properties

Recent studies on the exploitation of novel organic radical compounds that exhibit photo-responsive properties by illumination are described in this tutorial review. Those so far reported consist of a variety of photo-responsive pi-electron moieties such as azobenzene, diarylethene, biindenylidene, terphenoquinone, anthracene, naphthopyran, arylimine, or hexaarylbiimidazoles, bearing stable organic radicals or generating organic radicals as spin centres for magnetic properties. The switching behaviours of their magnetic properties evoked by the stimuli of irradiation are discussed by taking the associated structural changes into consideration.     

Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations

Molecular organic radicals have been intensively studied in the last decades, due to their interesting optical, magnetic and redox properties. Here we report the synthesis and characterisation of persistent organic radicals from one-electron oxidation of redox-active azines (RAAs), composed of two guanidinyl or related groups. By connecting two different groups together, asymmetric compounds result. In this way a series of compounds with varying redox potential is obtained that could be oxidised reversibly to the mono- and the dicationic charge states. The accessible redox states were fully determined by chemical redox reactions. The standard Gibbs free energy change for disproportionation of the radical monocation into the dication and the neutral molecule in solution, estimated from cyclovoltammetric measurements, varies between 43 and 71 kJ mol⁻¹. While the neutral RAAs absorb predominately UV light, the radical monocations display strong absorptions covering almost the entire visible region and extending for some compounds into the NIR region. A detailed analysis of this highly reversible electrochromism is presented, and the fast switching characteristics are demonstrated in an electrochromic test device.     

Theoretical study of the stable radicals galvinoxyl, azagalvinoxyl and wurster's blue perchlorate in the solid state

Calculations on crystalline organic radicals were performed to establish the ground states of these materials. These calculations show that the radicals may interact, depending on their orientation in the crystal structure. For galvinxoyl, a second structure is proposed which is similar to that of azagalvinoxyl, in which the radicals form pairs. This structure accounts for the anomalous magnetic properties of galvinoxyl at low temperatures.     

Triphenylamine (TPA) radical cations and related macrocycles

Triphenylamine (TPA) derivatives and their radical cation counterparts have successfully demonstrated a great potential for applications in a wide range of fields including organic redox catalysis, organic semiconductors, magnetic materials, etc., mainly because of their excellent redox activity. The stability of TPA radical cation has significant effect on the properties of the TPA-based functional materials, especially in relation to their electronic properties. Considering the instability of parent TPA radical cation, many efforts have been devoted to the development of stable TPA radical cations and related materials. Among them, TPA radical cation-based macrocycles have attracted particular attention because their large delocalized structures can stabilize the TPA radicals, thus endow them with outstanding redox behaviors, multiple resonance structures, and wide application in various optoelectronic devices. In this review, we give a brief introduction of organic radicals and the documented stable TPA radicals. Subsequently, a number of TPA radical cation-based macrocycles are comprehensively surveyed. It is expected that this minireview will not only summarize the recent development of TPA radical cations and their macrocycles, but also shed new light on the prospect of the design of more sophisticated radical cation-based architectures and related materials.     

Tailored Polarizing Hybrid Solids with Nitroxide Radicals Localized in Mesostructured Silica Walls

Hyperpolarization by dynamic nuclear polarization relies on the microwave irradiation of paramagnetic radicals dispersed in molecular glasses to enhance the nuclear magnetic resonance (NMR ) signals of target molecules. However, magnetic or chemical interactions between the radicals and the target molecules can lead to attenuation of the NMR signal through paramagnetic quenching and/or radical decomposition. Here we describe polarizing materials incorporating nitroxide radicals within the walls of the solids to minimize interactions between the radicals and the solute. These materials can hyperpolarize pure pyruvic acid, a particularly important substrate of clinical interest, while nitroxide radicals cannot be used, even when incorporated in the pores of silica, because of reactions between pyruvic acid and the radicals. The properties of these materials can be engineered by tuning the composition of the wall by introducing organic functionalities.     

Photoinduced magnetization in molecular compounds

In this review, recent studies on photoinduced magnetization in molecular compounds are introduced. The magnetic materials presented here range from Prussian blue analogues to purely organic radicals. Various photomagnetic phenomena observed in those materials are briefly described. These are, for example, photoinduced magnetization, photoinduced demagnetization, photoinduced spin cluster, photoinduced magnetic-pole inversion, photoinduced spin flipping, light-induced excited spin state trapping, photoinduced valence tautomerism, etc. It is thought that these can be used for future high-density photo-recording media and optical switches in molecular devices.     

Magnetic and conduction properties in 1D organic radical materials: an ab initio inspection for a challenging quest

The chemical control of magnetic and conduction properties for organic radicals is mainly based on t, the resonance integral, and U, the on-site repulsion, used in the Hubbard model. A qualitative analysis based on the competition between the kinetic and the Coulomb contribution, and the expression of the magnetic exchange coupling suggests that U should be roughly 800 cm(-1) while the resonance integral |t| should be 200 cm(-1) to reach bifunctionality. Ab initio wavefunction-based calculations allowed us to quantitatively measure those quantities for several organic materials considered as 1D systems starting from their reported crystal structures. The extraction of t and U parameters from the exchange coupling constants between neighbouring radicals allowed us to anticipate a possible metallic behaviour. Finally, the impact of chemical changes in the constitutive units is measured to rationalize the macroscopic behaviour modifications. It is shown that the intriguing regime characterized by simultaneous itinerant and localized electrons might be achieved by molecular engineering.     

Old materials with new tricks: multifunctional open-framework materials

The literature on open-framework materials has shown numerous examples of porous solids with additional structural, chemical, or physical properties. These materials show promise for applications ranging from sensing, catalysis and separation to multifunctional materials. This critical review provides an up-to-date survey to this new generation of multifunctional open-framework solids. For this, a detailed revision of the different examples so far reported will be presented, classified into five different sections: magnetic, chiral, conducting, optical, and labile open-frameworks for sensing applications. (413 references.)     

Post-Synthetic Modification of Metal-Organic Frameworks Bearing Phenazine Radical Cations for aza-Diels-Alder Reactions

Metal-organic frameworks (MOFs) consisting of organic radicals are of great interest because they have exhibited unique and intriguing optical, electronic, magnetic, and chemo-catalytic properties, and thus have demonstrated great potential applications in optical, electronic, and magnetic devices, and as catalysts. However, the preparation of MOFs bearing stable organic radicals is very challenging because most organic radicals are highly reactive and difficult to incorporate into the framework of MOFs. Herein we reported a post-synthetic modification strategy to prepare a novel MOF containing phenazine radical cations, which was used as heterogeneous catalyst for aza-Diels-Alder reaction. The zinc-based metal-organic framework Zn₂(PHZ)₂(dabco) ( N ) was successfully synthesized from 5,10-di(4-benzoic acid)-5,10-dihydrophenazine (PHZ), triethylene diamine (dabco) with Zn(NO₃)₂ ⋅ 6H₂O by solvothermal method. The as-synthesized MOF N was partially oxidized by AgSbF₆ to form MOF R containing ∼10% phenazine radical cation species. The resultant MOF R was found to keep the original crystal type of N and very persistent under ambient conditions. Consequently, MOF R was successfully employed in radical cation-catalyzed aza-Diels-Alder reactions with various imine substrates at room temperature with high reaction conversion. Moreover, heterogeneous catalyst MOF R was reusable up to five times without much loss of catalytic activity, demonstrating its excellent stability and recyclability. Therefore, the post-synthetic modification developed in this work is expected to become a versatile strategy to prepare radical-based MOFs for the application of heterogeneous catalysts in organic synthesis.     

Discovering Intrinsic Magnetism of Silk Fibroins

Silk fibroin (SF) is an interesting biomaterials with intriguing mechanical, optical, piezoelectric, and biocompatible properties. The present study reports the first magnetic experiments to show that organic radicals in the building blocks of SF induce permanent magnetization in these materials. A saturation magnetization of 2.8 × 10⁻³ emu g⁻¹ and non-zero remnant magnetization of 2.2 × 10⁻⁴ emu g⁻¹ with a coercive field of 65 Oe is observed in multiple SFs. Tightly bound β-sheets of polyglycine-alanine motifs provides sufficient steric hinderance to organic radicals and protects them from attack of reactive species. Moreover, it also shows that silk's ferromagnetic character can be enhanced by simple mechanical stretching and heating. This work demonstrates a new paradigm of magnetic proteins and opens a route toward the biomimetic discovery of iron-free magnetic proteins.     

On the Importance of Thermal Effects and Crystalline Disorder in the Magnetism of Benzotriazinyl‐Derived Organic Radicals

Recent experiments suggest that benzotriazinyl‐derived radicals are promising building blocks for the design of new functional materials. Herein, a detailed computational study of the main structural and magnetic features of two prototypes of this family of radicals is presented. By means of several computational techniques within the DFT framework, this work unveils the key importance of the thermal contraction of the crystal to quantitatively predict the magnetism of the studied compounds. In this sense, for the first time in the context of molecular magnetism, we propose to use variable‐cell geometry optimizations as an efficient alternative to obtain an estimation of low‐temperature crystal structures. The crucial role of crystalline disorder in defining the structure present at low temperature, and thus, the magnetic response, is revealed. Altogether, these are important elements for the rational design of future materials of this family of compounds.     

Recent Advances of Stable Phenoxyl Diradicals

Organic radicals with unpaired electrons have shown great semiconducting properties with potential applications in the field of organic photodetectors, organic light-emitting diodes and organic spintronics. A major problem for limiting radicals from laboratory research to practical applications is the relatively low chemical and physical stabilities. Therefore, the right selection of radical core is the key to meaningful scientific research. Phenoxyl radical is one of the few stable radicals with spin distribution properties. Moreover, phenoxyl diradicals provide extract stability due to multiple resonance structures. Due to the long-distance spin distribution, which makes phenoxyl diradicals show interesting electronic and magnetic properties. In this review, we summarize the progress of phenoxyl diradicals in recent years in terms of syntheses, properties and future perspective.     

Melamine–Barbiturate Supramolecular Assembly as a pH-Dependent Organic Radical Trap Material

In the last two decades, a large number of self-assembled materials were synthesized and they have already found their way into large-scale industry and science. Hydrogen-bond-based supramolecular adducts are found to have unique properties and to be perfect host structures for trapping target molecules or ions. Such chemical systems are believed to resemble living matter and can substitute a living cell in a number of cases. Herein, a report on an organic material based on supramolecular assembly of barbituric acid and melamine is presented. Surprisingly, the structure is found to host and stabilize radicals under mild conditions allowing its use for biological applications. The number of free radicals is found to be easily tuned by changing the pH of the environment and it increases when exposed to light up to a saturation level. We describe a preparation method as well as stability properties of melamine–barbiturate self-assembly, potentiometric titration, and hydrogen ions adsorption data and EPR spectra concerning the composite.     

邻、间、对-二甲氧基亚甲基苯及衍生物双自由基体系的自旋耦 合规律

采用量子化学abinitio法对具有甲氧基的碳、氧双自由基邻、间、对二甲氧基亚甲基苯及衍生物体系基态自旋耦合规律进行研究,得到非平面共轭体系中自由基之间磁性耦合的拓朴规则:共轭体系中,两个自由基之间以偶数个碳原子耦合,则有效交换积分J~i~j<0,体系具有低自旋基态;两个自由基之间以奇数个碳原子耦合,则J~i~j>0,体系具有高自旋基态。自由基性质对自旋耦合的影响较大,正离子自由基间磁性耦合能力较强,这些结论为有机磁性材料的分子设计与实验合成提供了理论依据。     

The polyoxometalates as precursors in molecular materials

The potentialities of the use of polyoxometalates as precursors for conducting and/or magnetic molecular materials are illustrated with a few examples of their chemical and electrochemical assemblies with organic donors derived from TTF, nitronyl nitroxide radicals and metallocenium radical cations.