Persistent Organic Radicals Research Articles

In Situ Generated 1‐Naphthylmethyl Radicals From Bis(1‐Naphthylmethyl)tin Dichlorides: Utilization For C‐C, C‐N, and C‐O Bond‐Forming Reactions

The in situ‐generated 1‐naphthylmethyl radicals from the thermolysis of bis(1‐naphthylmethyl)tin dichlorides combine with persistent organic radicals, 4‐hydroxy‐TEMPO or 4‐oxo‐TEMPO designs C−O bond forming products. Subsequently, the C‐N bond occurs when the 1‐naphthylmethyl radicals unite with nitric oxide (NO) under a nitrogen atmosphere.In contrast, the oxidation instead of the addition reaction predominantly happens in the 1‐naphthylmethyl radicals when nitrogen dioxide contains a high oxidation state nitrogen center, i.e., N(IV) used.The by‐product, dodecanuclear organotin cages, with twelve peripheral naphthyl units, could be isolated from the 4‐hydroxy‐TEMPO reaction. Besides, the synthesis of unsymmetrical diarylmethanes RArCH2 (R = 1‐naphthyl, 2,4,6‐Me3C6H2, 3‐ and 4‐MeC6H4, phenyl, 4‐ClC6H4; Ar = 4‐MeOC6H4, 3,4‐, 2,4‐ and 2,5‐Me2C6H3) in high yields and with good regioselectivity is reported. This new methodology involves the iodine‐mediated thermolysis of bis(arylmethyl)tin dichlorides (RCH2)2SnCl2 and an excess of an arene. This reaction involves homolytic cleavage of Sn–C bonds to give arylmethyl radicals that react with iodine in the presence of SnCl2 to give the corresponding cations, which undergo electrophilic attack on the arene. Functionalised diarylmethanes have wide applications in pharmaceutical, agrochemical, and materials sciences. Alternative synthetic approaches to this class of organic compounds that avoid using a transition‐metal catalyst or a strong Lewis acid are desirable.

Tunable Charge Transport and Spin Dynamics in Two-Dimensional Conjugated Metal-Organic Frameworks.

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have attracted increasing interest in electronics due to their (semi)conducting properties. Charge-neutral 2D c-MOFs also possess persistent organic radicals that can be viewed as spin-concentrated arrays, affording new opportunities for spintronics. However, the strong π-interaction between neighboring layers of layer-stacked 2D c-MOFs annihilates active spin centers and significantly accelerates spin relaxation, severely limiting their potential as spin qubits. Herein, we report the precise tuning of the charge transport and spin dynamics in 2D c-MOFs via the control of interlayer stacking. The introduction of bulky side groups on the conjugated ligands enables a significant dislocation of the 2D c-MOFs layers from serrated stacking to staggered stacking, thereby spatially weakening the interlayer interactions. As a consequence, the electrical conductivity of 2D c-MOFs decreases by 6 orders of magnitude, while the spin density achieves more than a 30-fold increase and the spin-lattice relaxation time (T1) is increased up to ∼60 μs, hence being superior to the reference 2D c-MOFs with compact stackings whose spin relaxation is too fast to be detected. Spin dynamics results also reveal that spinless polaron pairs or bipolarons play critical roles in the charge transport of these 2D c-MOFs. Our strategy provides a bottom-up approach for enlarging spin dynamics in 2D c-MOFs, opening up pathways for developing MOF-based spintronics.

Frontier orbital stability of nitroxyl organic radicals probed by means of inner shell resonantly enhanced valence band photoelectron spectroscopy.

We have investigated the frontier orbitals of persistent organic radicals known as nitroxyls by resonant photoelectron spectroscopy (ResPES) under inner shell excitation. By means of this site-specific technique, we were able to disentangle the different atomic contributions to the outer valence molecular orbitals and examine several core-hole relaxation pathways involving the singly occupied molecular orbital (SOMO) localized on the nitroxyl group. To interpret the ResPES intensity trends, especially the strong enhancement of the SOMO ionized state at the N K-edge, we computed the Dyson spin orbitals (DSOs) pertaining to the transitions between the core-excited initial states and several of the singly ionized valence final states. We found that the computed vertical valence ionization potentials and norms of the DSOs are reasonably reliable when based on the long-range corrected CAM-B3LYP density functional. Thanks to their unpaired electrons, nitroxyls have recently found application in technological fields implying a spin control, such as spintronics and quantum computing. The present findings on the electronic structure of nitroxyl persistent radicals furnish important hints for their implementation in technological devices and, more in general, for the synthesis of new and stable organic radicals with tailored properties.

(Invited) Electrogenerated Radical Ions: A Platform for Extreme Reactivity with Unique Selectivity

Metal-based catalysts and reagents possess unique reactivity profiles that are inaccessible using conventional organic molecules and, consequently, occupy a privileged role in organic synthesis. We are investigating how organic radical ions–typically thought of as fleeting intermediates–can be tamed and exploited to expand the reactivity of bench-stable organic molecules. Our central hypothesis is that these organic radical ions will not only mimic the unique reactivity of metallic species but will also engender selectivity profiles inaccessible using existing synthetic tactics. Herein, we will describe two new reaction platforms enabled by electrochemically-generated persistent organic radical ions. In the first system, we report a strategy to mediate single electron transfer to substrates with reduction potentials more negative than <–3.0 V vs. SCE. In the second, we will discuss a new approach to oxidatively couple of alkenes and electron-rich nucleophiles via a metastable electrophilic intermediate.

On the Track of Long-Range Electron Transfer in B-Type Dye-Decolorizing Peroxidases: Identification of a Tyrosyl Radical by Computational Prediction and Electron Paramagnetic Resonance Spectroscopy.

The catalytic activity of dye-decolorizing peroxidases (DyPs) toward bulky substrates, including anthraquinone dyes, phenolic lignin model compounds, or 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), is in strong contrast to their sterically restrictive active site. In two of the three known subfamilies (A- and C/D-type DyPs), catalytic protein radicals at surface-exposed sites, which are connected to the heme cofactor by electron transfer path(s), have been identified. So far in B-type DyPs, there has been no evidence for protein radical formation after activation by hydrogen peroxide. Interestingly, B-type Klebsiella pneumoniae dye-decolorizing peroxidase (KpDyP) displays a persistent organic radical in the resting state composed of two species that can be distinguished by W-band electron spin echo electron paramagnetic resonance (EPR) spectroscopy. Here, on the basis of a comprehensive mutational and EPR study of computationally predicted tyrosine and tryptophan variants of KpDyP, we demonstrate the formation of tyrosyl radicals (Y247 and Y92) and a radical-stabilizing Y-W dyad between Y247 and W18 in KpDyP, which are unique to enterobacterial B-type DyPs. Y247 is connected to Y92 by a hydrogen bonding network, is solvent accessible in simulations, and is involved in ABTS oxidation. This suggests the existence of long-range electron path(s) in B-type DyPs. The mechanistic and physiological relevance of the reaction mechanism of B-type DyPs is discussed.

Synthesis of Fluorogenic Arylureas and Amides and Their Interaction with Amines: A Competition between Turn-on Fluorescence and Organic Radicals on the Way to a Smart Label for Fish Freshness

We describe the synthesis of fluorogenic arylureas and amides and their interaction with primary or secondary amines under air and light in organic-aqueous mixtures to give rise to a new class of persistent organic radicals, described on the basis of their electron paramagnetic resonance (EPR), as well as UV–vis, fluorescence, NMR, and quantum mechanics calculations, and their prospective use as multi-signal reporters in a smart label for fish freshness.

A quantitative metric for organic radical stability and persistence using thermodynamic and kinetic features

Long-lived organic radicals are promising candidates for the development of high-performance energy solutions such as organic redox batteries, transistors, and light-emitting diodes. However, “stable” organic radicals that remain unreactive for an extended time and that can be stored and handled under ambient conditions are rare. A necessary but not sufficient condition for organic radical stability is the presence of thermodynamic stabilization, such as conjugation with an adjacent π-bond or lone-pair, or hyperconjugation with a σ-bond. However, thermodynamic factors alone do not result in radicals with extended lifetimes: many resonance-stabilized radicals are transient species that exist for less than a millisecond. Kinetic stabilization is also necessary for persistence, such as steric effects that inhibit radical dimerization or reaction with solvent molecules. We describe a quantitative approach to map organic radical stability, using molecular descriptors intended to capture thermodynamic and kinetic considerations. The comparison of an extensive dataset of quantum chemical calculations of organic radicals with experimentally-known stable radical species reveals a region of this feature space where long-lived radicals are located. These descriptors, based upon maximum spin density and buried volume, are combined into a single metric, the radical stability score, that outperforms thermodynamic scales based on bond dissociation enthalpies in identifying remarkably long-lived radicals. This provides an objective and accessible metric for use in future molecular design and optimization campaigns. We demonstrate this approach in identifying Pareto-optimal candidates for stable organic radicals.

Oxime radicals: generation, properties and application in organic synthesis.

N-Oxyl radicals (compounds with an N–O• fragment) represent one of the richest families of stable and persistent organic radicals with applications ranging from catalysis of selective oxidation processes and mechanistic studies to production of polymers, energy storage, magnetic materials design and spectroscopic studies of biological objects. Compared to other N-oxyl radicals, oxime radicals (or iminoxyl radicals) have been underestimated for a long time as useful intermediates for organic synthesis, despite the fact that their precursors, oximes, are extremely widespread and easily available organic compounds. Furthermore, oxime radicals are structurally exceptional. In these radicals, the N–O• fragment is connected to an organic moiety by a double bond, whereas all other classes of N-oxyl radicals contain an R2N–O• fragment with two single C–N bonds. Although oxime radicals have been known since 1964, their broad synthetic potential was not recognized until the last decade, when numerous selective reactions of oxidative cyclization, functionalization, and coupling mediated by iminoxyl radicals were discovered. This review is focused on the synthetic methods based on iminoxyl radicals developed in the last ten years and also contains some selected data on previous works regarding generation, structure, stability, and spectral properties of these N-oxyl radicals. The reactions of oxime radicals are classified into intermolecular (oxidation by oxime radicals, oxidative C–O coupling) and intramolecular. The majority of works are devoted to intramolecular reactions of oxime radicals. These reactions are classified into cyclizations involving C–H bond cleavage and cyclizations involving a double C=C bond cleavage.

Highly Stable Organic Bisradicals Protected by Mechanical Bonds.

Two new highly charged [2]catenanes-namely, mHe[2]C·6PF6 and mHo[2]C·6PF6-were synthesized by exploiting radical host-guest templation between derivatives containing BIPY•+ radical cations and the meta analogue of cyclobis(paraquat-p-phenylene). In contrast to related [2]catenanes that have been isolated as air-stable monoradicals, both mHe[2]C·6PF6 and mHo[2]C·6PF6 exist as air-stable singlet bisradicals, as evidenced by both X-ray crystallography in the solid state and EPR spectroscopy in solution. Electrochemical studies indicate that the first two reduction peaks of these two [2]catenanes are shifted significantly to more positive potentials, a feature which is responsible for their extraordinary stability in air. The mixed-valence nature of the mono- and bisradical states endows them with unique NIR absorption properties, e.g., NIR absorption bands for the mono- and bisradical states observed at ∼1800 and ∼1450 nm, respectively. These [2]catenanes are potentially useful in applications that include NIR photothermal conversion, UV-vis-NIR multiple-state electrochromic materials, and multiple-state memory devices. Our findings highlight the principle of "mechanical-bond-induced stabilization" as an efficient strategy for designing persistent organic radicals.

Tris-pyridylmethylamine (TPMA) complexes functionalized with persistent nitronyl nitroxide organic radicals.

The chance to have persistent organic radicals in combination with metals has attracted much interest since it offers the possibility of having new functional molecules with multiple open-shell elements. In this study, we report the synthesis of two tripodal tris(2-pyridyl)methylamine ligands (TPMA) functionalized with nitronyl nitroxide persistent radicals. The newly formed ligands have been used to coordinate zinc(ii), copper(ii), iron(ii) and cobalt(ii). The resulting complexes have been investigated by means of electron paramagnetic resonance (EPR), ESI-MS, FT-IR spectroscopy and X-ray diffraction. An electron reduction of the N-O radical moiety has been observed, depending on the metal used for the formation of the complex and the reaction conditions. We have observed small differences in the EPR spectra depending on the meta or para position of the radical moiety in the complex structure and some antiferromagnetic interactions between the paramagnetic M(ii) ions and the radical species.

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.

Synthesis, Physical Properties, and Reactivity of Stable, π-Conjugated, Carbon-Centered Radicals.

Recently, long-lived, organic radical species have attracted much attention from chemists and material scientists because of their unique electronic properties derived from their magnetic spin and singly occupied molecular orbitals. Most stable and persistent organic radicals are heteroatom-centered radicals, whereas carbon-centered radicals are generally very reactive and therefore have had limited applications. Because the physical properties of carbon-centered radicals depend predominantly on the topology of the π-electron array, the development of new carbon-centered radicals is key to new basic molecular skeletons that promise novel and diverse applications of spin materials. This account summarizes our recent studies on the development of novel carbon-centered radicals, including phenalenyl, fluorenyl, and triarylmethyl radicals.

Formation of persistent organic diradicals from N,N\u2032-diphenyl-3,7-diazacyclooctanes

N,N′-Diphenyl-3,7-diazacyclooctane and structurally related N,N′-diphenylbispidine derivatives react with silver(I) ions in a high-yielding C–C coupling reaction to produce dication–diradical species, with the silver ions serving a double function both as template and as an oxidant. The resulting bis(benzidino)phane derivatives are persistent organic radicals, stable for several months in solution as well as in the solid state, at room temperature and above, as well as being exposed to the atmosphere. The molecular structure features a double-decker cyclophane motif, stabilized by intramolecular π-dimerization of two delocalized benzidinium radical segments. Intermolecular π-dimers are formed in the solid state.Graphical abstract

Enhancing the Stability of Photogenerated Benzophenone Triplet Radical Pairs through Supramolecular Assembly.

Supramolecular assembly of urea-tethered benzophenone molecules results in the formation of remarkably persistent triplet radical pairs upon UV irradiation at room temperature, whereas no radicals were observed in solution. The factors that lead to emergent organic radicals are correlated with the microenvironment around the benzophenone carbonyl, types of proximal hydrogens, and the rigid supramolecular network. The absorption spectra of the linear analogues were rationalized using time-dependent density functional theory calculations on the crystal structure and in dimethyl sulfoxide, employing an implicit solvation model to describe structural and electronic solvent effects. Inspection of the natural transition orbitals for the more important excitation bands of the absorption spectra indicates that crystallization of the benzophenone-containing molecules should present a stark contrast in photophysical properties versus that in solution, which was indeed reflected by their quantum efficiencies upon solid-state assembly. Persistent organic radicals have prospective applications ranging from organic light-emitting diode technology to NMR polarizing agents.

Self-Assembly of an Organic Radical Thin Film and Its Memory Function Investigated Using a Liquid-Metal Electrode

In this work, the deposition of a persistent organic radical by thermal evaporation on Au, Pt, and graphene is performed. The impact of the deposition parameters and the nature of the substrate on the molecular organization within the deposited film are investigated. The nonplanarity of the molecule and the role of the molecule–molecule and molecule–substrate interactions are discussed. UV photoelectron spectroscopy experiments demonstrate that the radical character, and hence its magnetic and redox properties, is preserved on the three surfaces. The optimized films are electrically characterized by top-contacting the film/substrate system using a liquid metal that permits achievement of a soft contact avoiding damaging the layer. The hysteretic current versus voltage curves obtained from the electrical characterization point to the potential applicability of the studied system as an organic memory. Moreover, the demonstrated feasibility of using a liquid metal is an appealing approach toward the preparat...

Robust Organic Radical Molecular Junctions Using Acetylene Terminated Groups for C-Au Bond Formation.

Organic paramagnetic and electroactive molecules are attracting interest as core components of molecular electronic and spintronic devices. Currently, further progress is hindered by the modest stability and reproducibility of the molecule/electrode contact. We report the synthesis of a persistent organic radical bearing one and two terminal alkyne groups to form Au-C σ bonds. The formation and stability of self-assembled monolayers and the electron transport through single-molecule junctions at room temperature have been studied. The combined analysis of both systems demonstrates that this linker forms a robust covalent bond with gold and a better-defined contact when compared to traditional sulfur-based linkers. Density functional theory and quantum transport calculations support the experimental observation highlighting a reduced variability of conductance values for the C-Au based junction. Our findings advance the quest for robustness and reproducibility of devices based on electroactive molecules.

Self-Assembled, Biodegradable Magnetic Resonance Imaging Agents: Organic Radical-Functionalized Diblock Copolymers.

We report the design, synthesis, and evaluation of biodegradable amphiphilic poly(ethylene glycol)-b-polycarbonate-based diblock copolymers containing pendant persistent organic radicals (e.g., PROXYL). These paramagnetic radical-functionalized polymers self-assemble into micellar nanoparticles in aqueous media, which preferentially accumulate in tumor tissue via the enhanced permeability and retention (EPR) effect. Through T1 relaxation NMR studies, as well as magnetic resonance imaging (MRI) studies on mice, we show that these nanomaterials are effective as metal-free, biodegradable MRI contrast agents. We also demonstrate anticancer drugs can be readily loaded into the nanoparticles, conferring therapeutic delivery properties in addition to their imaging properties making these materials potential theranostic agents in the treatment of cancer.

NIR mechanochromic behaviours of a tetracyanoethylene-bridged hexa-peri-hexabenzocoronene dimer and trimer through dissociation of C–C bonds

Tetracyanoethylene-bridged hexabenzocoronene oligomers exhibit NIR mechanochromic behaviour through generation of persistent organic radicals by mechanical stimuli.

Hydrogen Bonding in Inclusion Chemistry and Porous Materials

Hydrogen bonding played an important role in the development of crystal engineering and cocrystal formation, and continues to do so. Our take on hydrogen bonding will focus on its consequences on the functional properties of a number of materials based on how hydrogen bonding directionality effects porosity design and inclusion chemistry. The design of molecular porous materials has focused on the production of intrinsic porosity within a number of interesting materials where the intrinsic voids are defined by capsules (tetrahedra in our examples) and macrocycles (2X2 metallocycles in our examples). The crystal engineering of the macrocycle packing efficiencies can result in the intrinsic voids spaces being connected into pore networks allowing for diffusion of guest species such as gases. We will show how the hydrogen bonding of these intrinsically porous molecules can result in an increased porosity through the creation of extrinsic void space.[1] The inclusion chemistry we have investigated has looked at gas and vapour sorption where we have determined the guest:framework interaction of a number of porous materials. This includes the inclusion chemistry of a number of persistent organic radicals where the hydrogen bonding and Pi-Pi stacking interactions with the flexible host material results in paramagnetic materials whereas the pure radical materials tend towards being diamagnetic.[2] We will also describe our crystallographic insights into gas:framework interactions, especially that of the hydrogen bonding of Carbon Dioxide.[3]

ChemInform Abstract: Playing with Organic Radicals as Building Blocks for Functional Molecular Materials

AbstractReview: 257 refs.