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Recent Advances in Free Radicals, Radical Ions and Radical Pairs

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A special issue of International Journal of Molecular Sciences (ISSN 1422-0067). This special issue belongs to the section "Physical Chemistry and Chemical Physics".

Deadline for manuscript submissions: closed (30 November 2023) | Viewed by 18660

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Special Issue Editors

Dr. Alexandra V. Yurkovskaya

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Guest Editor

International Tomography Center, Siberian Branch of the Russian Academy of Science, Novosibirsk 630090, Russia
Interests: photochemistry; photophysics; NMR; CIDNP; hyperpolarization; coherency; spin chemistry; free radicals; parahydrogen

Dr. Dmitri V. Stass

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Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia
Interests: radical pairs; X-ray induced luminescence; magnetic field effects; ESR spectroscopy

Special Issue Information

Dear Colleagues,

It has long been known that the presence of an unpaired electron imparts very special properties to a molecule that with proper understanding can be used to great advantage. This Special Issue of the International Journal of Molecular Sciences was conceived to provide a first-hand timely overview of the advances in using and understanding the spin and charge in molecular systems and new trends in this field that have emerged over the last two decades. Synthetic approaches have been developed that allow using the once delicate and fragile radicals as conventional off-the-shelf synthons that can go intact through multistep chemical transformations. Highly “persistent” radicals that can survive at elevated temperatures or in highly reductive native biofluids are now available and can be used as advanced spin probes, while even conventional spin probes and labels enjoy wider and wider application in the study of advanced materials. Certain stable closely-coupled multispin organic systems with sizeable ferromagnetic exchange have at last been obtained that push quantitative quantum chemistry to the edge, driving further refinement in the accuracy and predictive power of calculations. At the other extreme of stability are the elusive radicals and radical ions that are the acting agents in certain biological damage and repair processes, mediating a wide class of chemical reactions, or determining the ultimate fate of irradiated materials by channeling the deposited energy down either safe or damaging reaction paths. Certain unusual “degenerate” reactions with nominally no changes to reagents then become possible. The creation of radicals in pairs, as is often the case, opens a whole new world of quantum coherence and entanglement phenomena in spin-correlated radical pairs, with curious reaction patterns and non-thermal distribution of reaction products. Radical ion pairs in donor-acceptor systems open the way for hitherto impossible recombination exciplexes with highly tunable optical emission properties. A recent and crucially important realization is the possibility of converting “electron spin polarization” from radical pairs into polarization of nuclear spins in target molecules, boosting the potential sensitivity of NMR and MRI techniques by orders of magnitude. A further dimension to the picture is added by the sensitivity of processes involving pairs of electron spins to applied magnetic fields, opening the possibility of external control, or more modestly, of probing the reaction and the reactants. These are only some of the topics that we invite you to address in your tentative contributions to this Special Issue, and we are certainly open to further suggestions on recent advances in spin-related chemistry, biology, and material science.

Dr. Alexandra V. Yurkovskaya
Dr. Dmitri V. Stass
Guest Editors

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Keywords

persistent radicals
spin probes for material science
coupled organic multispin systems
quantum chemistry of free radicals
biologically relevant radicals and radical ions
matrix-stabilized radicals and radical ions
radical-controlled energy channeling
spin-correlated radical pairs and beyond
radical-derived nuclear spin hyperpolarization
magnetic field sensitivity of radical pairs

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Published Papers (10 papers)

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Research

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15 pages, 2144 KiB

Open AccessArticle

Shining a Spotlight on Methyl Groups: Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of 5-Deazariboflavin and Its Nor Analogs

by Sabrina Panter, Audrey Ayekoi, Jannis Tesche, Jing Chen, Boris Illarionov, Adelbert Bacher, Markus Fischer and Stefan Weber

Int. J. Mol. Sci. 2024, 25(2), 848; https://doi.org/10.3390/ijms25020848 - 10 Jan 2024

Viewed by 1205

Abstract

5-Deazaflavins are analogs of naturally occurring flavin cofactors. They serve as substitutes for natural flavin cofactors to investigate and modify the reaction pathways of flavoproteins. Demethylated 5-deazaflavins are potential candidates for artificial cofactors, allowing us to fine-tune the reaction kinetics and absorption characteristics of flavoproteins. In this contribution, demethylated 5-deazariboflavin radicals are investigated (1) to assess the influence of the methyl groups on the electronic structure of the 5-deazaflavin radical and (2) to explore their photophysical properties with regard to their potential as artificial cofactors. We determined the proton hyperfine structure of demethylated 5-deazariboflavins using photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy, as well as density functional theory (DFT). To provide context, we compare our findings to a study of flavin mononucleotide (FMN) derivatives. We found a significant influence of the methylation pattern on the absorption properties, as well as on the proton hyperfine coupling ratios of the xylene moiety, which appears to be solvent-dependent. This effect is enhanced by the replacement of N5 by C5-H in 5-deazaflavin derivatives compared to their respective flavin counterparts. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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24 pages, 3540 KiB

Open AccessArticle

Marcus Cross-Relationship Probed by Time-Resolved CIDNP

by Maksim P. Geniman, Olga B. Morozova, Nikita N. Lukzen, Günter Grampp and Alexandra V. Yurkovskaya

Int. J. Mol. Sci. 2023, 24(18), 13860; https://doi.org/10.3390/ijms241813860 - 8 Sep 2023

Viewed by 954

Abstract

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5′-monophosphate anion GMP(-H)⁻ with the neutral radical GMP(-H)^•, of the N-acetyl tyrosine anion N-AcTyrO⁻ with a neutral radical N-AcTyrO^•, and of the tyrosine anion TyrO⁻ with a neutral radical TyrO^• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2′-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)^•/TyrO⁻ was determined at T = 25 °C. Rate constants of the GMP(-H)^• radical reduction reactions with TyrO⁻ and N-AcTyrO⁻ anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)⁻/GMP(-H)^•, N-AcTyrO⁻/N-AcTyrO^•, and TyrO⁻/TyrO^• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and β-protons of TyrO^• and N-AcTyrO^•, the 8-proton of GMP(-H)^•, and the 3,4-protons of DPH^• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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16 pages, 10457 KiB

Open AccessArticle

Preparation of Xanthene-TEMPO Dyads: Synthesis and Study of the Radical Enhanced Intersystem Crossing

by Wenhui Zhu, Yanran Wu, Yiyan Zhang, Andrey A. Sukhanov, Yuqi Chu, Xue Zhang, Jianzhang Zhao and Violeta K. Voronkova

Int. J. Mol. Sci. 2023, 24(13), 11220; https://doi.org/10.3390/ijms241311220 - 7 Jul 2023

Viewed by 1578

Abstract

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C–N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin–spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 μs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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18 pages, 8401 KiB

Open AccessArticle

Anisotropic and Coherent Control of Radical Pairs by Optimized RF Fields

by Akihiro Tateno, Kenta Masuzawa, Hiroki Nagashima and Kiminori Maeda

Int. J. Mol. Sci. 2023, 24(11), 9700; https://doi.org/10.3390/ijms24119700 - 2 Jun 2023

Cited by 1 | Viewed by 1689

Abstract

Radical pair kinetics is determined by the coherent and incoherent spin dynamics of spin pair and spin-selective chemical reactions. In a previous paper, reaction control and nuclear spin state selection by designed radiofrequency (RF) magnetic resonance was proposed. Here, we present two novel types of reaction control calculated by the local optimization method. One is anisotropic reaction control and the other is coherent path control. In both cases, the weighting parameters for the target states play an important role in the optimizing of the RF field. In the anisotropic control of radical pairs, the weighting parameters play an important role in the selection of the sub-ensemble. In coherent control, one can set the parameters for the intermediate states, and it is possible to specify the path to reach a final state by adjusting the weighting parameters. The global optimization of the weighting parameters for coherent control has been studied. These manifest calculations show the possibility of controlling the chemical reactions of radical pair intermediates in different ways. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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27 pages, 3769 KiB

Open AccessArticle

Recombination of X-ray-Generated Radical Ion Pairs in Alkane Solution Assembles Optically Inaccessible Exciplexes from a Series of Perfluorinated para-Oligophenylenes with N,N-Dimethylaniline

by Pavel V. Nikul’shin, Roman G. Fedunov, Leonid V. Kuibida, Alexander M. Maksimov, Evgeni M. Glebov and Dmitri V. Stass

Int. J. Mol. Sci. 2023, 24(8), 7568; https://doi.org/10.3390/ijms24087568 - 20 Apr 2023

Cited by 1 | Viewed by 1552

Abstract

We demonstrate that a series of perfluorinated para-oligophenylenes C₆F₅-(C₆F₄)_n-C₆F₅ (n = 1–3) produce exciplexes with N,N-dimethylaniline (DMA) in degassed X-irradiated n-dodecane solutions. The optical characterization of the compounds shows that their short fluorescence lifetimes (ca. 1.2 ns) and UV-Vis absorption spectra, overlapping with the spectrum of DMA with molar absorption coefficients of 2.7–4.6 × 10⁴ M⁻¹cm⁻¹, preclude the standard photochemical exciplex formation pathway via selective optical generation of the local excited state of the donor and its bulk quenching by the acceptor. However, under X-rays, the efficient assembly of such exciplexes proceeds via the recombination of radical ion pairs, which delivers the two partners close to each other and ensures a sufficient energy deposition. The exciplex emission is completely quenched by the equilibration of the solution with air, providing a lower bound of exciplex emission lifetime of ca. 200 ns. The recombination nature of the exciplexes is confirmed by the magnetic field sensitivity of the exciplex emission band inherited from the magnetic field sensitivity from the recombination of spin-correlated radical ion pairs. Exciplex formation in such systems is further supported by DFT calculations. These first exciplexes from fully fluorinated compounds show the largest known red shift of the exciplex emission from the local emission band, suggesting the potential of perfluoro compounds for optimizing optical emitters. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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15 pages, 1595 KiB

Open AccessArticle

Magnetic Field Effect in Bimolecular Rate Constant of Radical Recombination

by Alexander B. Doktorov and Nikita N. Lukzen

Int. J. Mol. Sci. 2023, 24(8), 7555; https://doi.org/10.3390/ijms24087555 - 20 Apr 2023

Cited by 2 | Viewed by 1920

Abstract

The influence of magnetic fields on chemical reactions, including biological ones, has been and still is a topical subject in the field of scientific research. Experimentally discovered and theoretically substantiated magnetic and spin effects in chemical radical reactions form the basis of research in the field of spin chemistry. In the present work, the effect of a magnetic field on the rate constant of the bimolecular spin-selective recombination of radicals in the bulk of a solution is considered theoretically for the first time, taking into account the hyperfine interaction of radical spins with their magnetic nuclei. In addition, the paramagnetic relaxation of unpaired spins of the radicals and the non-equality of their g-factors that also influence the recombination process are taken into account. It is found that the reaction rate constant can vary in magnetic field from a few to half a dozen percent, depending on the relative diffusion coefficient of radicals, which is determined by the solution viscosity. It is shown that the consideration of hyperfine interactions gives rise to the presence of resonances in the dependence of the rate constant on the magnetic field. The magnitudes of the magnetic fields of these resonances are determined by the hyperfine coupling constants and difference in the g-factors of the recombining radicals. Analytical expressions for the reaction rate constant of the bulk recombination for magnetic fields larger than hfi (hyperfine interaction) constants are obtained. In general, it is shown for the first time that accounting for hyperfine interactions of radical spins with magnetic nuclei significantly affects the dependence of the reaction rate constant of the bulk radical recombination on the magnetic field. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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10 pages, 1440 KiB

Open AccessArticle

Fast Recombination of Charge-Transfer State in Organic Photovoltaic Composite of P3HT and Semiconducting Carbon Nanotubes Is the Reason for Its Poor Photovoltaic Performance

by Mikhail N. Uvarov, Elena S. Kobeleva, Konstantin M. Degtyarenko, Vladimir A. Zinovyev, Alexander A. Popov, Evgeny A. Mostovich and Leonid V. Kulik

Int. J. Mol. Sci. 2023, 24(4), 4098; https://doi.org/10.3390/ijms24044098 - 17 Feb 2023

Cited by 1 | Viewed by 1654

Abstract

Although the photovoltaic performance of the composite of poly-3-hexylthiophene (P3HT) with semiconducting single-walled carbon nanotubes (s-SWCNT) is promising, the short-circuit current density j_SC is much lower than that for typical polymer/fullerene composites. Out-of-phase electron spin echo (ESE) technique with laser excitation of the P3HT/s-SWCNT composite was used to clarify the origin of the poor photogeneration of free charges. The appearance of out-of-phase ESE signal is a solid proof that the charge-transfer state of P3HT⁺/s-SWCNT⁻ is formed upon photoexcitation and the electron spins of P3HT⁺ and s-SWCNT⁻ are correlated. No out-of-phase ESE signal was detected in the same experiment with pristine P3HT film. The out-of-phase ESE envelope modulation trace for P3HT/s-SWCNT composite was close to that for the polymer/fullerene photovoltaic composite PCDTBT/PC₇₀BM, which implies a similar distance of initial charge separation in the range 2–4 nm. However, out-of-phase ESE signal decay with delay after laser flash increase for P3HT/s-SWCNT composite was much faster, with a characteristic time of 10 µs at 30 K. This points to the higher geminate recombination rate for the P3HT/s-SWCNT composite, which may be one of the reasons for the relatively poor photovoltaic performance of this system. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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21 pages, 2957 KiB

Open AccessArticle

Understanding the Exchange Interaction between Paramagnetic Metal Ions and Radical Ligands: DFT and Ab Initio Study on Semiquinonato Cu(II) Complexes

by Aleksandra Ziółkowska and Maciej Witwicki

Int. J. Mol. Sci. 2023, 24(4), 4001; https://doi.org/10.3390/ijms24044001 - 16 Feb 2023

Cited by 1 | Viewed by 2692

Abstract

The exchange coupling, represented by the J parameter, is of tremendous importance in understanding the reactivity and magnetic behavior of open-shell molecular systems. In the past, it was the subject of theoretical investigations, but these studies are mostly limited to the interaction between metallic centers. The exchange coupling between paramagnetic metal ions and radical ligands has hitherto received scant attention in theoretical studies, and thus the understanding of the factors governing this interaction is lacking. In this paper, we use DFT, CASSCF, CASSCF/NEVPT2, and DDCI3 methods to provide insight into exchange interaction in semiquinonato copper(II) complexes. Our primary objective is to identify structural features that affect this magnetic interaction. We demonstrate that the magnetic character of Cu(II)-semiquinone complexes are mainly determined by the relative position of the semiquinone ligand to the Cu(II) ion. The results can support the experimental interpretation of magnetic data for similar systems and can be used for the in-silico design of magnetic complexes with radical ligands. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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Review

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28 pages, 442 KiB

Open AccessReview

Astrochemically Relevant Radicals and Radical–Molecule Complexes: A New Insight from Matrix Isolation

by Vladimir I. Feldman

Int. J. Mol. Sci. 2023, 24(19), 14510; https://doi.org/10.3390/ijms241914510 - 25 Sep 2023

Cited by 4 | Viewed by 1330

Abstract

The reactive open-shell species play a very important role in the radiation-induced molecular evolution occurring in the cold areas of space and presumably leading to the formation of biologically relevant molecules. This review presents an insight into the mechanism of such processes coming from matrix isolation studies with a main focus on the experimental and theoretical studies performed in the author’s laboratory during the past decade. The radicals and radical cations produced from astrochemically relevant molecules were characterized by Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopy. Small organic radicals containing C, O, and N atoms are considered in view of their possible role in the formation of complex organic molecules (COMs) in space, and a comparison with earlier results is given. In addition, the radical–molecule complexes generated from isolated intermolecular complexes in matrices are discussed in connection with their model significance as the building blocks for COMs formed under the conditions of extremely restricted molecular mobility at cryogenic temperatures. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

19 pages, 4263 KiB

Open AccessReview

Spin Dynamics of Flavoproteins

by Jörg Matysik, Luca Gerhards, Tobias Theiss, Lisa Timmermann, Patrick Kurle-Tucholski, Guzel Musabirova, Ruonan Qin, Frank Ortmann, Ilia A. Solov’yov and Tanja Gulder

Int. J. Mol. Sci. 2023, 24(9), 8218; https://doi.org/10.3390/ijms24098218 - 4 May 2023

Cited by 6 | Viewed by 2926

Abstract

This short review reports the surprising phenomenon of nuclear hyperpolarization occurring in chemical reactions, which is called CIDNP (chemically induced dynamic nuclear polarization) or photo-CIDNP if the chemical reaction is light-driven. The phenomenon occurs in both liquid and solid-state, and electron transfer systems, often carrying flavins as electron acceptors, are involved. Here, we explain the physical and chemical properties of flavins, their occurrence in spin-correlated radical pairs (SCRP) and the possible involvement of flavin-carrying SCRPs in animal magneto-reception at earth’s magnetic field. Full article

(This article belongs to the Special Issue Recent Advances in Free Radicals, Radical Ions and Radical Pairs)

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