Stable Biradicals Research Articles

Bis(9-Boraphenanthrene) and Its Stable Biradical.

Selective and site-specific boron-doping of polycyclic aromatic hydrocarbon frameworks often give rise to redox and/or photophysical properties that are not easily accessible with the analogous all-carbon systems. Herein, we report ligand-mediated control of boraphenanthrene closed- and open-shell electronic states, which has led to the first structurally characterized examples of neutral bis(9-boraphenanthrene) (2-3) and its corresponding biradical (4). Notably, compounds 2 and 3 show intramolecular charge transfer absorption from the 9-boraphenanthrene units to p-quinodimethane, exhibiting dual (red-shifted) emission in solution due to excited state conjugation enhancement (ESCE). Moreover, while boron-centered monoradicals are ubiquitous, biradical 4 represents a rare type of open-shell singlet compound with 95% biradical character, among the highest of any reported boron-based polycyclic species with two radical sites.

Stepwise photoinduced transformation of fused aziridines via stable biradicals and azomethine ylides

Some fused aziridines are highly sensitive photochromes due to the presence of sterically strained three-membered ring. The study of photoinduced transformations of several such compounds in solutions, crystalline state, and polymeric and liquid crystal matrices using UV–vis and ESR spectrometry in combination with quantum chemical calculations revealed stepwise character of the transformations. The first deep-colored product was proven to be a new type of stable triplet biradical species while the second one was poor-colored azomethine ylide which had been previously considered as the species responsible for photochromism in crystalline state.

Dynamic Nuclear Polarization Signal Enhancement with High-Affinity Biradical Tags.

Dynamic nuclear polarization is an emerging technique for sensitizing solid-state NMR experiments by transferring polarization from electrons to nuclei. Stable biradicals, the polarization source for the cross effect mechanism, are typically codissolved at millimolar concentrations with proteins of interest. Here we describe the high-affinity biradical tag TMP-T, created by covalently linking trimethoprim, a nanomolar affinity ligand of dihydrofolate reductase (DHFR), to the biradical polarizing agent TOTAPOL. With TMP-T bound to DHFR, large enhancements of the protein spectrum are observed, comparable to when TOTAPOL is codissolved with the protein. In contrast to TOTAPOL, the tight binding TMP-T can be added stoichiometrically at radical concentrations orders of magnitude lower than in previously described preparations. Benefits of the reduced radical concentration include reduced spectral bleaching, reduced chemical perturbation of the sample, and the ability to selectively enhance signals for the protein of interest.

Three-Pulse ELDOR Theory Revisited

The current theory of three-pulse electron double resonance (PELDOR) has been generalized to the case, when paramagnetic particles (spin labels) in pairs or groups have the electron paramagnetic resonance (EPR) spectra, which overlap essentially or coincide. The PELDOR signal modulation induced by the dipole–dipole interaction between paramagnetic spin ½ particles in pairs embedded in disordered systems has been analyzed comprehensively. It has been shown that the PELDOR signal contains additional terms in contrast to the situation considered in the current theory, when the EPR spectra of the spin labels in the pairs do not overlap. In disordered systems, the pairs of spin labels have the characteristic dipolar interaction frequency. According to the current theory for pairs of spin labels, the PELDOR signal reveals the modulation with this characteristic frequency. The additional terms, which are obtained in this work, do not change the modulation frequency of the PELDOR signal for pairs of spin labels. However, these additional terms should be taken into account when analyzing the amplitude of the PELDOR signal and the amplitude of the modulation of the PELDOR signal. The consistent approach to treating the PELDOR data for the groups containing three or more spin labels has been outlined on the basis of the results for pairs of spin labels. It has been also analyzed how the spin flips and molecular motion or molecular isomerization can affect the manifestation of the interaction between the spin labels in PELDOR experiments. PELDOR experiments for the stable biradicals (biradicals I containing 1-oxyl-2,2,5,5-tetramethylpyrroline-3-yl spin labels and biradicals II containing 3-imidazoline spin labels) have been performed. The results have been interpreted within the theory developed in this work.

Pulsed electron spin nutation spectroscopy for weakly exchange-coupled multi-spin molecular systems with nuclear hyperfine couplings: a general approach to bi- and triradicals and determination of their spin dipolar and exchange interactions

Weakly exchange-coupled biradicals have attracted much attention in terms of their dynamic nuclear polarisation application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits in quantum information processing/quantum-computing technology. Analogues multi-partite molecular systems are important in entering a new phase of the relevant fields. Many stable organic biradicals known so far have nitrogen nuclei at their electron spin sites, where singly occupied molecular orbitals are dominating and large hyperfine couplings occur. A salient feature of such weakly exchange-coupled molecular systems in terms of electronic spin structures is underlain by small zero-field splitting (ZFS) parameters comparable with nuclear hyperfine and/or exchange interactions. Pulse-based electron spin nutation (ESN) spectroscopy of weakly exchange-coupled biradicals, applicable to oriented or non-oriented media, has proven to be a useful and facile approach to the determination of ZFS parameters, which reflect relatively short distances between unpaired electron spins. In the present study, we first treat two-dimensional single-crystal ESN spectroscopy (Q-band) of a 15N-labelled weakly exchange-coupled biradical, showing the nuclear hyperfine effects on the ESN phenomena from both the experimental and theoretical side. ESN spectroscopy is transition moment spectroscopy, in which the nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be treated. The results provide a testing ground for the simplified but general approach to the ESN analysis. In this study, we have invoked single-crystal electron-electron double resonance measurements on a typical biradical well incorporated in a diamagnetic host lattice and checked the accuracy of our ESN analysis for the spin dipolar tensor and exchange interaction. Next, we extend the general approach to analogues multi-partite molecular systems such as stable organic triradical, in which the exchange interaction can be governed by a significant amount of the delocalisation of three unpaired spins over the molecular frame of a triangular structure. The triangular structure maintains π-conjugation in which each spin-bearing nitroxide at the vertex participates and the exchange interaction is greatly controlled by the dihedral angle between the π-conjugation plane and nitroxide moiety at the vertex. In this context, the ZFS parameters do not correspond to spin distances (1.0 nm) in a straightforward manner, but reflect a salient electronic structure associated with both the π-electron network and the symmetry property of the triradical under study. Thus, both the D-value and exchange interaction J have been controlled in this study. In order to interpret the experimental ZFS parameters and exchange interaction, which is three-order of magnitude reduced in the present poly(methyl methacrylate) polymer matrix compared with that in the crystal, sophisticated quantum chemical calculations of the ZFS tensor and exchange interaction were carried out and reproduced the experimental values, concluding that the present triradical of the triangular structure undergoes significant twisting at the nitroxide sites in the polymer matrix. In this study, we observed electron spin resonance forbidden transitions between the Ms-manifolds belonging to the spin-doublet ground state and spin-quartet excited state. The observation enables us to derive the magnitude of the exchange coupling.

Organic Single Molecular Structures for Light Induced Spin-Pump Devices

We present theoretical results on molecular structures for realistic spin-pump applications. Taking advantage of the electron spin resonance concept, we find that interesting candidates constitute triplet biradicals with two strongly spatially and energetically separated singly occupied molecular orbitals (SOMOs). Building on earlier reported stable biradicals, particularly bis(nitronyl nitroxide) based biradicals, we employ density functional theory to design a selection of potential molecular spin-pumps which should be persistent at ambient conditions. We estimate that our proposed molecular structures will operate as spin-pumps using harmonic magnetic fields in the MHz regime and optical fields in the infrared to visible light regime.

Conversion of a Singlet Silylene to a stable Biradical

Silicon becomes colored: Stable biradicals were prepared from an N-heterocyclic carbene stabilized SiCl2 and a cyclic alkyl(amino)carbene, and characterized as two polymorphs. The deep-blue crystals of one polymorph are stable upon exposure to air for about a week, while the solution in THF decomposes rapidly when exposed to air. In a side reaction, the different carbene species react with each other under CH activation and CC bond formation in the presence of the biradical.

ChemInform Abstract: MO Analysis of Photocycloaddition Reactions of 2-Pyrones.

The skeleton of the 2-pyrones (1) is a simple one composed of 2,4-pentadien-5-olides, but has many possibilities for photocycloadditions. To elucidate the peri-, site-, and regioselective photoadditions of the triplet 2-pyrones with some ethylenes, the factors were analyzed by PM3-CI-UHF methods, etc. The regioselective [4+2]cycloadducts (3) and site-selective [2+2]cycloadducts (4) across the C5–C6 double bonds of the 2-pyrones with electron-deficient ethylenes were deduced to be formed by their preferable HSOMO–LUMO interactions and via more stable biradicals by the C6-Cβ bondings. On the other hand, the other kinds of regioselective [4+2] (6) and [2+2]adducts (5) across the C3–C4 double bonds of the 2-pyrones with an electron-donating ethylene were inferred to mainly come from LSOMO–HOMO interactions and via biradicals by C3-Cβ bondings. Selectivities between 3 and 4, and 5 and 6 were also inferred to be caused by SOMOs of the biradicals.

ChemInform Abstract: Selective Synthesis of a Novel Family of Oligopyridine Based Imino- Nitroxide Biradicals Catalysed by Selenium Dioxide.

Abstract Selective synthesis of bis- N -hydroxyimidazolidines based on pyridines, 2,2′-bipyridines, 1,10-phenanthroline, pyridazine and 1,8-naphthyridine from the corresponding dialdehydes and N,N′ -dihydroxy-2,3-diamino-2,3-dimethylbutane in the presence of catalytic amounts of SeO 2 . Mild oxidation afforded the corresponding stable imino-nitroxide biradicals 4 to 11 .

Does Double Electron Capture Lead to the Formation of Biradicals? An ECD-SORI-CID Study on Lacticin 481

We studied lacticin 481, a small lantibiotic with three lanthionine bridges, by electron capture dissociation (ECD) in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Following electron capture, very little fragmentation was observed, but species formed by nondissociative single and multiple electron capture were abundant. Ions formed by double electron capture were subjected to sustained off resonance irradiation collision induced dissociation (SORI-CID) to determine whether stable biradicals were formed. In the SORI-CID spectra of the ions formed by double electron capture, some, but minor, H* radical loss was observed, which was not observed at all for regularly protonated ions. A small part of the ions formed by double electron capture are thus long-lived biradicals. Apart from the observed H* loss, the SORI-CID spectra of ions that captured two electrons was similar to that of regularly protonated ions and quite different from the SORI-CID spectra of radical ions formed by single electron capture. This implies that recombination of the two radical sites is the dominant process in biradical lacticin 481 ions, at least on the time scale of our SORI-CID experiments.

Reductive cleavage of dichalcogenide bonds

Electrochemical reduction of 1,2-di(2,4,6-triphenylpyridinium-l)ethane, 1,4-di(2,4,6-triphenylpyridinium-1)butane, 1,6-di(2,4,6-triphenylpyridinium-l)hexane, 1,4-di(N-methyl-4,6-diphenylpyridinium-2)benzene, and 2,7-di(N-methyl-4,6-diphenylpyridinium-2)fluorene in solutions of DMF and MeCN in the potential range from−0.8 to −1.7 V (SCE) was studied by CV. The successive formation of stable radical cations, biradicals, and dianions was investigated by semiempirical MO calculations at the PM3 level.

Monocyclic biradicals in the imidazolidine series

The reaction of 1-hydroxy-3-imidazoline 3-oxides with alkyl- or phenyliithium followed by oxidation gives stable biradicals with two nitroxyl groups in the imidazolidine ring. Anisotropic ESR spectra of these biradicals are discussed.

Triplet Biradicals that are Stable in Crystals

Abstract Three trans-syn- 3,3′-diary1-2,2′-biindenyl-idene-1,1′-diones (2a-c) showed sharp photochromic phenomena in their crystalline forms. By exposure to sunlight, 2a-c were converted to stable biradicals (3a-c). Although N,N′-dimethyl indigo (6) has an almost planar structure, 6 in its 1:1 inclusion crystal with the host 1,1,6,6-tetrapheny1-2,4-hexadiyne-1,6-diol (7) was found to have a twisted structure of dihedral angle 42.6° between the two rings.

MO Analysis of Photocycloaddition Reactions of 2-Pyrones

Abstract The skeleton of the 2-pyrones (1) is a simple one composed of 2,4-pentadien-5-olides, but has many possibilities for photocycloadditions. To elucidate the peri-, site-, and regioselective photoadditions of the triplet 2-pyrones with some ethylenes, the factors were analyzed by PM3-CI-UHF methods, etc. The regioselective [4+2]cycloadducts (3) and site-selective [2+2]cycloadducts (4) across the C5–C6 double bonds of the 2-pyrones with electron-deficient ethylenes were deduced to be formed by their preferable HSOMO–LUMO interactions and via more stable biradicals by the C6-Cβ bondings. On the other hand, the other kinds of regioselective [4+2] (6) and [2+2]adducts (5) across the C3–C4 double bonds of the 2-pyrones with an electron-donating ethylene were inferred to mainly come from LSOMO–HOMO interactions and via biradicals by C3-Cβ bondings. Selectivities between 3 and 4, and 5 and 6 were also inferred to be caused by SOMOs of the biradicals.

Palladium(II) chloride compounds with stable biradicals of imidazoline

The first coordination compounds of stable nitroxyl biradical imidazolines have been synthesized. From the spectral data obtained, the paramagnetic ligand 4,4′-(ethyleneiminomethyl)di-2, 2,5,5-tetramethyl-3-imidazoline-1-oxyl acts as a tetradentate bridge in a binuclear coordination compound with Pd(II), while the azine ligand 4-formyl-2,2,S,5-tetramethyl-3-imidazoline-1-oxyl acts as a bidentate bridge.

ChemInform Abstract: A Stable Biradical in the Benzimidazoline Series Containing Two Cation‐Radical Fragments with Strong Exchange Interaction.

ChemInformVolume 19, Issue 19 Heterocyclic Compounds ChemInform Abstract: A Stable Biradical in the Benzimidazoline Series Containing Two Cation-Radical Fragments with Strong Exchange Interaction. A. S. MORKOVNIK, A. S. MORKOVNIK NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this authorA. N. SUSLOV, A. N. SUSLOV NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this authorE. P. IVAKHNENKO, E. P. IVAKHNENKO NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this authorB. A. TERTOV, B. A. TERTOV NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this author A. S. MORKOVNIK, A. S. MORKOVNIK NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this authorA. N. SUSLOV, A. N. SUSLOV NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this authorE. P. IVAKHNENKO, E. P. IVAKHNENKO NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this authorB. A. TERTOV, B. A. TERTOV NII fiz. i org. khim. Rostovskogo gos. univ. im. Suslova, Rostov-na-Donu 344104Search for more papers by this author First published: May 10, 1988 https://doi.org/10.1002/chin.198819174AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume19, Issue19May 10, 1988 RelatedInformation

Stable biradical in the benzimidazoline series, containing two cation-radical fragments with strong exchange interactions

A large number of uncharged stable biradicals are known [i]; there have been practically no reports, however, of biradicals containing two cation-radical fragments [bi(cationradicals)]. The only reported examples of radicals of this type have been polymeric viologens containing two or more unpaired electrons, in which the cation-radical functional groups are composed of nonaromatic y,y-dipyridylidene units [2]~ The exchange interaction between the unpaired electrons in this class of radicals is negligible; thus, their EPR spectra do not differ from the spectra of the corresponding viologen monoradicals. Viologenic biand poly(cation-radicals) are formed upon reduction of polypyridinium salts [2]. It is assumed that two-electron oxidation of compounds of the type R-X-R, in which the R groups are more readily ionized than the bridging group X, should provide a more general method for the preparation of hi(cation-radicals). We succeeded in carrying out this type of reaction in the case of 1,4-di(l,2,3-trimethylbenzimidazolinyl-2)benzene (I), * which contains two benzimidazoline functional groups linked by a nonconjugating p-phenylene bridge.

Quenching of triplet states of organic molecules by nitroxyl biradicals

1. During quenching of triplet molecules of aromatic hydrocarbons, retinal, and cyanine dye by stable nitroxyl biradicals, the contribution of processes analogous to triplet-triplet interactions is insignificant and does not appreciably influence the value of the quenching rate constant. 2. The rate of quenching of the triplet states by radicals containing benzene rings in their structure is, in general, higher than that by the corresponding radicals without benzene rings. This is probably due to the formation of π-complexes between the triplet molecules and the radicals.

Deactivation of electron-excited states of stable nitroxyl biradicals

A study was made of the deactivation of the electron-excited states of pyrene and a ruthenium trisbipyridyl complex by stable nitroxyl mono- and biradicals. The deactivation rate constants were measured for nine radicals.

Influence of electron spin exchange on the interaction of stable nitroxyl radicals with triplet states of cyanine dyes

1. The efficiency of the quenching of the triplet states of cyanine dyes by stable nitroxyl biradicals rises with an increase in the spin exchange between the paramagnetic centers of the biradicals. 2. Pronounced spin exchange between the paramagnetic centers of a biradical probably leads to a change in the mechanism of the quenching of triplet states by the biradicals.