Persistent Radical Anions Research Articles

Silyl Radicals as Single-Electron Reductants: α-Aminoalkyl Radical Formation via a Photocatalytic Oxidatively Initiated Radical Chain Process.

The α-amino-radical constitutes a versatile reactive intermediate that has been used to great effect in the synthesis of complex amine-containing products. Here, we report the development of a multicomponent photocatalytic platform enabling access to all-alkyl α-amino-radicals, exploiting the oxidative formation of silyl-radicals from commercially available tris(trimethylsilyl)silane. A key design element of the new process involves the role of silyl-radicals in generating α-amino-radicals from iminium ions as part of an oxidatively initiated photocatalytic radical chain process. This distinct activation mode is showcased by engaging the ensuing radicals in cross-radical coupling with persistent arene radical anions, enabling the arylation of in situ-generated all-alkyl iminium ions to furnish alkyl-substituted benzylamines.

A persistent radical anion naphthalenediimide-based hybrid material for near-infrared photothermal conversion and photocatalytic oxidative organic transformations

A naphthalenediimide-based hybrid material was designed and prepared, which exhibits NIR photothermal conversion and promotes the photocatalytic hydroxylation of arylboronic acids and C-3 thiocyanation of indoles.

Persistent radical anion of perylene dianhydride: an emerging metal-free photocatalyst for near-infrared photocontrolled RAFT polymerization

Persistent radical anion of perylene dianhydride: an emerging metal-free photocatalyst for near-infrared photocontrolled RAFT polymerization

Persistent Radical Anions: Efficient Catalysts for Iodine-Mediated Reversible-Deactivation Radical Polymerization Driven by NIR LED Light

Persistent Radical Anions: Efficient Catalysts for Iodine-Mediated Reversible-Deactivation Radical Polymerization Driven by NIR LED Light

Photoredox Product Selectivity Controlled by Persistent Radical Stability.

The use of photoredox catalysis for the synthesis of small organic molecules relies on harnessing and converting the energy in visible light to drive reactions. Specifically, photon energy is used to generate radical ion species that can be harnessed through subsequent reaction steps to form a desired product. Cyanoarenes are widely used as arylating agents in photoredox catalysis because of their stability as persistent radical anions. However, there are marked, unexplained variations in product yields when using different cyanoarenes. In this study, the quantum yield and product yield of an α-aminoarylation photoredox reaction between five cyanoarene coupling partners and N-phenylpyrrolidine were characterized. Significant discrepancies in cyanoarene consumption and product yield suggested a chemically irreversible, unproductive pathway in the reaction. Analysis of the side products in the reaction demonstrated the formation of species consistent with radical anion fragmentation. Electrochemical and computational methods were used to study the fragmentation of the different cyanoarenes and revealed a correlation between product yield and cyanoarene radical anion stability. Kinetic modeling of the reaction demonstrates that cross-coupling selectivity between N-phenylpyrrolidine and the cyanoarene is controlled by the same phenomenon present in the persistent radical effect.

Visible-light-driven three-component reductive 1,2-diarylation of alkenes

The intermolecular alkene 1,2-diarylation provides straightforward access to polyarylalkanes but suffers from substrate limitation, requiring directing auxiliaries or prefunctionalized aryl components. Herein, we report the development of a photocatalytic strategy that enables a three-component 1,2-diarylation of alkenes under mild reductive conditions using simple tertiary alkylamine as the terminal reductant. Readily accessible aryl iodides and cyanides serve as the C(sp2) electrophiles to simultaneously install two different aryl moieties across the olefins with high regioselectivity. This protocol exhibits a broad substrate scope for all of aryl, heteroatom-substituted, and even unactivated alkenes and can be easily scaled up to the gram-scale. Mechanistic studies reveal a halogen-atom transfer (XAT) of aryl iodides to transient aryl radicals while a single-electron transfer (SET) of aryl cyanides to persistent aryl radical anions, both allowing for a regiocontrollable radical relay sequence.

Direct Hydroxylarylation of Benzylic Carbons (sp3/sp2/sp) via Radical-Radical Cross-Coupling Powered by Paired Electrolysis.

Diaryl alcohol moieties are widespread in pharmaceuticals. Existing methods for the synthesis of diaryl alcohols require the use of pre-functionalized benzylic alcohols, aromatic aldehydes, or ketones as starting materials. Herein, the first convergent paired electrochemical approach to the direct hydroxylarylation of unactivated benzylic carbons (sp3/sp2/sp) is proposed. This protocol features direct functionalization of unactivated benzylic C(sp3)-H bonds and benzylic sp2/sp-carbons, mild conditions (open air, room temperature), an environmentally friendly procedure (without any external catalyst/mediator/additive), and direct access to sterically hindered alcohols from inexpensive and readily available alkyl/alkenyl/alkynylbenzenes. Mechanistic studies, including divided-cell experiments, isotope labeling, radical trapping, electron paramagnetic resonance, reaction kinetics, and cyclic voltammetry, strongly support the proposed radical-radical cross-coupling between transient ketyl radicals and persistent radical anions. Gram-scale synthesis and diversification of drug derivatives have visualized the tremendous potential of this protocol for practical applications.

A persistent radical anion derived from a propeller-shaped perylene bisimide-carbazole pentad.

Stabilizing reactive radical ions promises outstanding performances in photocatalysis, organic optoelectronics and photothermal therapies, but it remains a challenge. In this contribution, we firstly report a persistent radical anion (PBI˙--4Cz) derived from a propeller-shaped electron-deficient perylene bisimide-based pentad (PBI-4Cz). Detailed investigations confirm that PBI˙--4Cz could intactly exist under inert conditions, and its lifetime is sufficiently prolonged up to more than one week under ambient atmosphere. Such exceptional stability is ascribed to the synergistic effect of the high electron-affinity and structural shielding originating from the compact spatial arrangement of PBI-4Cz. This work contributes to rational design and appropriate chemical construction of stable open-shell species.

Reductive Arylation of Aliphatic and Aromatic Aldehydes with Cyanoarenes by Electrolysis for the Synthesis of Alcohols.

An electroreductive arylation reaction of aliphatic and aromatic aldehydes as well as ketones with electro-deficient (hetero)arenes is described. A variety of cyano(hetero)arenes and carbonyl compounds, especially aliphatic aldehydes, have been examined, providing secondary and tertiary alcohols in moderate to good yields. Mechanistic studies, including cyclic voltammetry (CV), electron paramagnetic resonance (EPR), and divided-cell experiments, support the generation of aliphatic ketyl radicals and persistent heteroaryl radical anions via cathodic reduction followed by radical-radical cross-coupling.

Raman Spectra of Persistent Radical Anions from Benzophenone, Fluorenone, 2,2'-Bipyridyl, 4,4'-Di-tert-butyl-2,2'-dipyridyl, and Anthracene: Excellent Agreement between DFT and Experiment for Highly Delocalized Radical Systems.

We report detailed Raman spectra for the neutral and radical anion forms of benzophenone, fluorenone, 2,2'-bipyridyl, 4,4'-di-tert-butyl-2,2'-dipyridyl, and anthracene. Density functional theory (DFT) predictions for the Raman spectra of these molecules give additional insight into the assignment of each vibrational mode. While the use of DFT has been problematic in quantifying the thermochemistry of highly delocalized radicals, we find that DFT-predicted spectra using the popular B3LYP functional are in excellent agreement with the observed Raman spectra. In the case of the two bipyridyl compounds, the Raman spectra allowed us to conclude that the cis form of the radical anion complexed to a sodium cation was the preferred configuration. Benzophenone and fluorenone radical anions gave a significantly weakened C═O bond stretching vibrational frequency as expected from the population of an antibonding π* orbital. For benzophenone, the C═O vibration dropped from 1659 to 1403 cm-1 upon reduction. Similarly, fluorenone showed a C═O vibration observed at 1719 cm-1 for the neutral form that decreased to 1522 cm-1 for the radical anion. The structurally rigid anthracene showed relatively smaller Raman band shifts upon single-electron reduction as the π* orbital is more equally delocalized on the entire structure. In total, we correlated 65 DFT-predicted vibrational modes for the neutral molecules with an overall error of 7.1 cm-1 (root-mean-square errors (RMSEs)) and 67 DFT-predicted vibrational modes for radical anions with an overall error of 9.9 cm-1. These comparisons between theory and experiment are another example to demonstrate the power of DFT in predicting the identity and geometry of molecules using Raman spectroscopy.

Persistent radical anions in panchromatic D-A hybrid heterostructures induced by anion-π interactions

The combination of N, N′-dipyridyl-tetrachloroperylene diimide (4Cl-DPPDI) buliding blocks with lead iodides in the presence of hydrogen iodides led to a novel D-A hybrid heterostructure with segregated iodoplumbate anions and 2-D H-bonded 4Cl-DPPDI networks as electron donors and acceptors. Due to the compact anion-π interactions as well as small energy gaps between the donors and acceptors, persistent and stable radical anions are generated in such a unique complex triggered by natural-light irradiation, which thus exhibits a panchromatic absorption.

Synthesis, structure, redox activity and luminescence of sterically crowded 6,8-di-(tert-butyl)-3H-phenoxazin-3-one

A sterically crowded 6,8-di-(tert-butyl)-3H-phenoxazin-3-one 3 was prepared by coupling 3,5-di-(tert-butyl)-o-benzoquinone with p-aminophenol. The molecular structure of 3 was established with the aid of X-ray crystallography and the redox activity studied using cyclic voltammetry and ESR spectroscopy. The reduction of 3 with a potassium mirror led to the formation of persistent radical anion 3a and dianion 3b which were characterized by their absorption and luminescence spectra.

N-Heteroacenes and N-Heteroarenes as N-Nanocarbon Segments.

N-Heteroacenes and N-heteroarenes are the heterocyclic congeners of the acenes and arenes, in which one or several perimeter C-H bonds have been substituted by pyridine-type nitrogen atoms. They are formally segments out of N-doped nanographenes. Position and number of the nitrogens vary greatly, making N-heteroacenes and N-heteroarenes define a vast class of N-nanographene segments; they display modular electronic and structural properties. The nitrogen atoms in the perimeter lead to finely tunable frontier molecular orbital positions and therefore improved electron affinity and higher oxidative stability but conversely also require and allow different synthetic approaches than those reported for the synthesis of their hydrocarbon and nanographene analogues. The chemistry of N-heteroarenes, despite being known for more than a century, has made significant progress in the last years and established these materials both as powerful n-channel semiconductors in thin film transistors and as useful emitters in organic light emitting diodes (OLEDs) and in photovoltaic devices. The electronegative nitrogen atoms impart a deep LUMO into the azaacenes and azaarenes, improve electron injection, and enable powerful electron transport but also charge separation in bulk-heterojunction type organic photovoltaic (OPV) devices. At the same time, azaacenes and azaarenes are fundamentally exciting materials that push the limits of structure and stability, constantly displaying novel topologies and structures as variations of a simple leitmotif; we expect a bright future for esthetically pleasing yet highly functional N-heterocyclic species. Firstly, we discuss novel structures and structural elements that have evolved during the lastyears in N-heteroacene and N-heteroarene chemistry and delineate their properties. An important aspect is the oligomerization or better multimerization of azaacene and azaarene units into novel and surprising topologies, in which multiple azaarenes or azaacenes are stitched together. Examples are tetrahedral assemblies of tetraazapentacenes but also cyclic tetramers of different types of azaacenes and linearly bent, S-shaped, formally dimeric species. An exciting aspect of the exploration of the structural manifold of azaacenes is their electronic interaction in such assemblies and their solid-state microstructure. A further aspect of this work is the increase in size of the azaacenes and concepts that allow stabilization of the larger congeners. The attachment of four benzo units to the azaacene core is a powerful concept that stabilizes tetraazaheptacenes and should also be useful to achieve persistent tetraazanonacenes. Secondly, we describe the success of N-heteroacenes and N-heteroarenes in organic electronic devices; specifically, the use of symmetrical halogenated tetraazapentacenes as superb n-channel transistor materials with air stable and persistent radical anions as charge carriers; we discuss the structural reason for their success. Use of azaacenes and azaarenes is not restricted to transistors, but they are also applied in bulk heterojunction photovoltaic devices and in brightly emitting OLEDs. Azaacenes and azaarenes are attractive segments out of hetero-nanographenes and objects of study, starting from fundamental structural and topological questions, ranging to powerful applications in organic electronics. The general interest in azaacenes is witnessed by the constantly increasing number of groups who discover and work on these materials as novel functional and flexible species.

A Zn4 L6 Capsule with Enhanced Catalytic C-C Bond Formation Activity upon C60 Binding.

A redox-switchable self-assembled ZnII 4 L6 cage was synthesized that contains naphthalenediimide (NDI) motifs. Its reduction lent these NDI panels persistent radical anion character. The redox activity of this cage allows it to act as a catalyst for the oxidative coupling of different tetraaryl borates to give biaryls. The catalytic activity of the cage was enhanced following its binding of C60 , which implies a mechanism that does not involve encapsulation of the substrate.

Persistent radical anions in the series of peri-arylenes: broadband light absorption until far in the NIR and purely organic magnetism

Stable radicals in organic conjugated molecules are of great interest due to their magnetic signals and broad optical absorptions. In this paper, we report on naphthalene, benzoperylene, perylene, terrylene, and quaterrylene carboximides, reduced under controlled conditions, where stable metal-free solid salts of radical anions could be obtained forming darkly colored solutions with line-rich UV/Vis/NIR spectra and exhibiting special magnetic properties. The most bathochromic shift of the absorption maxima extend from 760 until 1700 nm. Persistent paramagnetic properties of the solids were observed and temperature-dependent susceptibilities are measured.Graphical abstract

Visible-light-triggered generation of persistent radical anions from perylenediimides: A substituent effect and potential application in photocatalytic reduction of Ag+

Three perylenediimide derivatives were reduced to their persistent radical anions in N-methyl-2-pyrrolidone (NMP) under ambient conditions by a visible light photoinduced electron transfer (PET). UV–vis and electron paramagnetic resonance (EPR) measurements were carried out to confirm the formation of radicals. The relationships between molecular structure, electrochemical property, phototransformation kinetics, and air stability of radical anions were investigated by the introduction of pyrimidine rings at the core position and fluoroalkylation substituents at the imide position. This study reveals that the electron-deficient pyrimidine rings and electron-withdrawing fluoroalkylation substituents can effectively enhance the phototransformation rate and the radical stability at ambient conditions. The effects of solvents and illuminance levels on the photoinduced reduction of perylenediimides were also studied. A photocatalytic reduction of Ag+ was successfully carried out using the perylenediimide with pyrimidine and fluoroalkylation substituents as catalyst upon irradiation with visible light.

Persistent radical anion polymers based on naphthalenediimide and a vinylene spacer.

Persistent n-doped conjugated polymers were achieved by doping the electron accepting PDNDIV and PFNDIV polymers with ionic (TBACN) or neutral (TDAE) dopants. The great electron affinities, as indicated by the low LUMO levels of PDNDIV (−4.09 eV) and PFNDIV (−4.27 eV), facilitated the chemical reduction from either TBACN or TDAE. The low-lying LUMOs of the neutral polymers PDNDIV and PFNDIV were achieved by incorporation of vinylene spacers between the electron poor NDI units to increase the conjugation length without the use of an electron donor, and this was lowered further by an electron-withdrawing fluorinated N-substituent on the NDI moiety. The polymer radical anions were found to persist for several days under ambient conditions by EPR spectroscopy. A distinguishing and noteworthy feature of these polymers is that they can be consecutively reduced by up to four electrons in acetonitrile. Conductivity measurements demonstrate the prospective impact of PDNDIV and PFNDIV for organic electronics.

1,2,5]Selenadiazolo[3,4‐b]pyrazines: Synthesis from 3,4‐Diamino‐1,2,5‐selena­diazole and Generation of Persistent Radical Anions

AbstractPreviously unknown 3,4‐diamino‐1,2,5‐selenadiazole (6) was prepared by hydrolysis of [1,2,5]selenadiazolo[3,4‐c][1,2,5]selenadiazole (7b) and used in synthesis of novel 1,2,5‐selenadiazolo[3,4‐b]pyrazines by the Koerner–Hinsberg reaction covering the parent compound 4 and its substituted derivatives 5a–g. The compounds synthesized were characterized by solution and solid‐state 77Se NMR, and 6, 5‐Ph and 5,6‐Me2[1,2,5]selenadiazolo[3,4‐b]pyrazines (5a, g, respectively) by X‐ray diffraction. Electrochemical reduction of 5,6‐R2[1,2,5]selenadiazolo[3,4‐b]pyrazines 5c, g (R = Ph, Me) into novel persistent radical anions (RAs) was studied by cyclic voltammetry and the RAs [5c]– and [5g]– were characterized by EPR spectroscopy combined with DFT calculations.

Core-Modified Naphthalenediimides Generate Persistent Radical Anion and Cation: New Panchromatic NIR Probes

The generation of the first persistent radical cation of naphthalenediimide with Cu(2+)/Fe(3+) under ambient conditions is reported. An alternate anionic trigger generates a persistent radical anion within the same motif. Steric protection and H-bonding enhances the half-life of radical cation by 290-fold. The radical anion and cation have orthogonal spin density, panchromatic and NIR optical bands, which can be applied as attractive multichannel probes.

Preparation of New O-Alkyl Naphthalenecarbothioates and Alkyl Naphthalenecarbodithioates, and EPR-Spectroscopic Study of Their Radical Anions1 , 2

O-Alkyl naphthalenecarbothioates and bis-carbothioates are prepared by the reaction of the corresponding esters by use of Lawesson's reagent. The related naphthalenecarbodithioates are obtained (a) from methylnaphthalenes via bromination with NBS, subsequent methoxide-promoted reaction with sulfur, and finally alkylation of the carbodithioate salts with alkyl halides or (b) lithiation of bromonaphthalenes, reaction with carbon disulfide, and alkylation. The thiono- and dithioesters were transformed into persistent radical anions by in situ electroreduction. Spin density distributions were determined by EPR spectroscopy and MO calculations.