Radical Pair Research Articles

Ion Pairs of Charged Porphyrins: Ordered Arrangement and Radical-Pair Formation

π-Electronic ions with appropriate geometries and peripheral substituents provide assemblies through the interactions between charged building subunits, resulting in fascinating electronic properties. Structures and properties of the assemblies can be controlled by the combined positively and negatively charged species in the assemblies.[1,2] In fact, π-electronic ion pairs comprising porphyrin-based π-electronic anions[3] have exhibited characteristic assembling modes via i π– i π interactions and resulting electronic properties such as solid-state absorption, which was correlated with the arrangement of constituent charged π-systems, and photoinduced electron transfer.[4] On the other hand, ion pairs of porphyrin–AuIII complexes as π-electronic cations, prepared with the combination of various anions including π-electronic anions, formed assemblies as crystals and thermotropic liquid crystals, whose ionic components were highly organized by i π– i π interactions (mainly electrostatic and dispersion forces).[5] Among various combinations of these porphyrin cations and anions, the “activated” ion pair of meso-EWG (electron-withdrawing group)-substituted cation and meso-EDG (electron-donating group)-substituted anion exhibited the electron transfer in the steady state according to solvent polarity, resulting in the production of the radical pair. The ESR in frozen toluene revealed the formation of a heterodiradical in a close stacking structure by the antiferromagnetic dipolar interaction and temperature-dependent spin transfer behavior.[6,7][1] Recent reviews: (a) Haketa, Y. et al. Mol. Syst. Des. Eng. 2020, 5, 757; (b) Yamasumi, K. et al. Bull. Chem. Soc. Jpn. 2021, 94, 2252. [2] Recent reports on pyrrole-based π-electronic molecules: (a) Watanabe, Y. et al. Chem. Eur. J. 2020, 26, 6767; (b) Haketa, Y. et al. J. Am. Chem. Soc. 2020, 142, 16420; (c) Kuno, A. et al. Chem. Eur. J. 2021, 27, 10068; (d) Fujita, M. et al. Chem. Commun. 2022, 58, 9870. [3] (a) Sasano, Y. et al. Dalton Trans. 2017, 46, 8924; (b) Sasano, Y. et al. Chem. Eur. J. 2019, 25, 6712. [4] Sasano, Y.; Tanaka, H. et al. Chem. Sci. 2021, 12, 9645. [5] (a) Haketa, H. et al. iScience 2019, 14, 241; (b) Tanaka, H. et al. Chem. Asian J. 2019, 14, 2129. [6] Tanaka, H. et al. J. Am. Chem. Soc. 2022, 144, 21710. [7] The details of excited-state radical pairs: Tanaka, H. et al. to be submitted.

(Invited) Non-Polymer Organic Solar Cells: Microscopic Phonon Control to Suppress Non-Radiative Voltage Loss Via Charge-Separated State

Recent remarkable developments on non-fullerene solar cells have reached photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this respect, development of the small donor molecule is essential. Although several methods have been applied to elucidate the elementary processes of charge carriers in BHJ-OSCs, mechanistic details of non-radiative voltage loss are poorly understood originating from interfacial heterogeneous electron-hole pairs.A recent time-resolved electron paramagnetic resonance (TREPR) study suggested an impact of the recombination process generating the triplet excitons in non-fullerene acceptor solar cells,[1] while geometries and motions of charge-separated (CS) states were elucidated in details by electron spin polarization (ESP).[2-4] Furthermore, although effects of low-frequency nuclear motions (phonon) on the photocarrier generations have been discussed in several literatures,[2,5,6] microscopic origin of the phonon playing a role on the OPV performance is unclear including impact of the phonon assist on primary recombination processes. Here we systematically investigated mechanisms of solar cell performance with diketopyrrolopyrrole (DPP)–tetrabenzoporphyrin (BP) conjugates of C4-DPP–H2BP and C4-DPP–ZnBP where C4 represents butyl group substituted at the DPP unit as small p-type molecules in non-polymer organic solar cells (OSC) with an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester. We clarified microscopic origins of the photocarrier caused by phonon-assisted 1D electron-hole dissociations at donor:acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking, ensuring carrier transport through face-on molecular conformations to suppress non-radiative voltage loss via capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. While disordered lattice motions from the π-π stackings via zinc ligation are essential to enhance the entropy for charge-dissociations at the interface, too much ordered crystallinity causes backscattering phonon to reduce OSC performances.

Kinetics of Photoreactive 4,4'-Dimethylbenzophenone Nanocrystals: Relative Contributions to Triplet Decay from Intermolecular H-Atom Transfer and Reductive Charge Transfer Quenching.

Crystals of 4,4'-dimethylbenzophenone (DMBP) are known to react by intermolecular H atom transfer, followed by radical pair recombination. To determine the contribution of the H atom transfer reaction to the deactivation of the triplet ketone, transient absorption spectra and kinetics were obtained using aqueous nanocrystalline suspensions. Single-exponential lifetimes of ca. 1185 ns with no deuterium isotope effect and inefficient product formation suggest that the reaction does not contribute significantly to the kinetics of triplet decay. By contrast, the observed lifetime is consistent with previous observations with p,p'-disubstituted benzophenones that undergo an efficient self-quenching process by a reductive charge transfer mechanism.

Chiral-induced spin selectivity in photo-induced electron transfer: Investigating charge and spin dynamics in a master equation framework.

Investigating the role of chiral-induced spin selectivity in the generation of spin correlated radical pairs in a photoexcited donor-chiral bridge-acceptor system is fundamental to exploit it in quantum technologies. This requires a minimal master equation description of both charge separation and recombination through a chiral bridge. To achieve this without adding complexity and entering in the microscopic origin of the phenomenon, we investigate the implications of spin-polarizing reaction operators to the master equation. The explicit inclusion of coherent evolution yields non-trivial behaviors in the charge and spin dynamics of the system. Finally, we apply this master equation to a setup comprising a molecular qubit attached to the donor-bridge-acceptor molecule, enabling qubit initialization, control, and read-out. Promising results are found by simulating this sequence of operations assuming realistic parameters and achievable experimental conditions.

Cryptochromes in mammals: a magnetoreception misconception?

Cryptochromes are flavoproteins related to photolyases that are widespread throughout the plant and animal kingdom. They govern blue light-dependent growth in plants, control circadian rhythms in a light-dependent manner in invertebrates, and play a central part in the circadian clock in vertebrates. In addition, cryptochromes might function as receptors that allow animals to sense the Earth's magnetic field. As cryptochromes are also present in mammals including humans, the possibility of a magnetosensitive protein is exciting. Here we attempt to provide a concise overview of cryptochromes in mammals. We briefly review their canonical role in the circadian rhythm from the molecular level to physiology, behaviour and diseases. We then discuss their disputed light sensitivity and proposed role in the magnetic sense in mammals, providing three mechanistic hypotheses. Specifically, mammalian cryptochromes could form light-induced radical pairs in particular cellular milieus, act as magnetoreceptors in darkness, or as secondary players in a magnetoreception signalling cascade. Future research can test these hypotheses to investigate if the role of mammalian cryptochromes extends beyond the circadian clock.

No evidence for magnetic field effects on the behaviour of Drosophila

Migratory songbirds have the remarkable ability to extract directional information from the Earth’s magnetic field1,2. The exact mechanism of this light-dependent magnetic compass sense, however, is not fully understood. The most promising hypothesis focuses on the quantum spin dynamics of transient radical pairs formed in cryptochrome proteins in the retina3–5. Frustratingly, much of the supporting evidence for this theory is circumstantial, largely because of the extreme challenges posed by genetic modification of wild birds. Drosophila has therefore been recruited as a model organism, and several influential reports of cryptochrome-mediated magnetic field effects on fly behaviour have been widely interpreted as support for a radical pair-based mechanism in birds6–23. Here we report the results of an extensive study testing magnetic field effects on 97,658 flies moving in a two-arm maze and on 10,960 flies performing the spontaneous escape behaviour known as negative geotaxis. Under meticulously controlled conditions and with vast sample sizes, we have been unable to find evidence for magnetically sensitive behaviour in Drosophila. Moreover, after reassessment of the statistical approaches and sample sizes used in the studies that we tried to replicate, we suggest that many—if not all—of the original results were false positives. Our findings therefore cast considerable doubt on the existence of magnetic sensing in Drosophila and thus strongly suggest that night-migratory songbirds remain the organism of choice for elucidating the mechanism of light-dependent magnetoreception.

Reactivity Tracking of an Enzyme Progress Coordinate.

The reactivity of individual solvent-coupled protein configurations is used to track and resolve the progress coordinate for the core reaction sequence of substrate radical rearrangement and hydrogen atom transfer in the ethanolamine ammonia-lyase (EAL) enzyme from Salmonella enterica. The first-order decay of the substrate radical intermediate is the monitored reaction. Heterogeneous confinement from sucrose hydrates in the mesophase solvent surrounding the cryotrapped protein introduces distributed kinetics in the non-native decay of the substrate radical pair capture substate, which arise from an ensemble of configurational microstates. Reaction rates increase by >103-fold across the distribution to approach that for the native enabled substate for radical rearrangement, which reacts with monotonic kinetics. The native progress coordinate thus involves a collapse of the configuration space to generate optimized reactivity. Reactivity tracking reveals fundamental features of solvent-protein-reaction configurational coupling and leads to a model that refines the ensemble paradigm of enzyme catalysis for strongly adiabatic chemical steps.

Chemical Nuclear Polarization during the Photoreduction of Anthraquinones

The mechanism and products of the photoreaction of 2-isopropyl-9,10-anthraquinone (1) with ethanol have been established. An analysis of the signals of chemical nuclear polarization (CNP) showed that during the formation of CNP, only the mechanism of singlet-triplet transitions in radical pairs takes place without the participation of a triplet mechanism. In the study of the phase of multiplet and integral effects, the elementary acts of photoreaction were determined, and the structure of the anthrasemiquinone radical participating in the reaction was established.

Photoproduct formation in coenzyme B12-dependent CarH photoreceptor via a triplet pathway.

CarH is a cobalamin-based photoreceptor which has attracted significant interest due to its complex mechanism involving its organometallic coenzyme-B12 chromophore. While several experimental and computational studies have sought to understand CarH's mechanism of action, there are still many aspects of the mechanism which remain unclear. While light is needed to activate the Co-C5' bond, it is not entirely clear whether reaction pathway involves singlet or triplet diradical states. A recent experimental study implicated triplet pathway and importance of intersystem crossing (ISC) as a viable mechanistic route for photoproduct formation in CarH. Herein, a combined quantum mechanics/molecular mechanics approach (QM/MM) was used to explore the involvement of triplet states in CarH. Two possibilities were explored. The first possibility involved photo-induced homolytic cleavage of the Co-C5' where the radical pair (RP) would deactivate to a triplet state (T0) on the ground state potential energy surface (PES). However, a pathway for the formation of the photoproduct, 4',5'-anhydroadenosine (anhAdo), on the triplet ground state PES was not energetically feasible. The second possibility involved exploring a manifold of low-lying triplet excited states computed using TD-DFT within the QM/MM framework. Viable crossings of triplet excited states with singlet excited states were identified using semiclassical Landau-Zener theory and the effectiveness of spin-orbit coupling by El-Sayed rules. Several candidates along both the Co-NIm potential energy curve (PEC) and Co-C5'/Co-NIm PES were identified, which appear to corroborate experimental findings and implicate the possible role of triplet states in CarH.

Frustrated radical pairs in selective functionalization of inert aliphatic C‒H bonds

Frustrated radical pairs in selective functionalization of inert aliphatic C‒H bonds

Magnetic stimulation of chemical reactions in coal

Purpose. To identify a mechanism in terms of which a signal of weak magnetic field is transformed into response of some components of nanostructure of a carbonaceous matter with further formation of chemical bonds. Methodology. Physical and mathematical modelling procedures of elementary chemical acts have been applied. Regularities of quantum mechanics and dynamics of chemical bonds were also used in addition to a magnetic isotope theory, a diffusion theory of recombination of radical pairs taking into consideration triplet-singlet transit, and nuclear-spin selectivity of chemical reactions. Findings. The physical mechanism of the magnetic scenario of interradial reactions is considered from the viewpoint of their energy stimulation through magnetic fields, i.e. the idea has been implemented according to which the number of radical pairs, able to be recombined into stable molecules, increases significantly if the weak magnetic field exercises certain influence. In addition to stimulation of interradial reactions, the magnetic field impact on organic coal mass-radicals results in stabilization of carbonaceous structures with regular atomic arrangement (being two-dimensional matrices, chains etc.) and their increase. Originality. A physical model of structural and phase changes in coal, depending upon the effect by external weak magnetic field, has been developed. It has been shown that the weak magnetic field signal, being incomparably smaller energetically to compare with the energy of thermal molecular motion, can initiate triplet-singlet transitions, i. e. transform radicals into reactive state. A mechanism has been proposed to form chemical bonds between movable radicals and non-complete bonds of atoms at the surfaces of solid phases of carbonaceous matter. Practical value. Implementation of the obtained results, concerning magnetic coal processing, may be connected with the development of a new procedure to avoid explosive conditions in coal seams. Progress in the field of magnetic processing should involve studies concerning calculation and selection of coal processing parameters (i.e. magnetic field density, frequency, energy, and a processing period) to control efficiently the chemical reactions in the carbonaceous matter. A substantiation degree of coal processing will influence greatly both efficiency of chemical processes and expediency of practical use of the results.

Long-Lived Charge-Separated State in Naphthalimide-Phenothiazine Compact Electron Donor-Acceptor Dyads: Effect of Molecular Conformation Restriction and Solvent Polarity.

To study the charge separation (CS) and long-lived CS state, we prepared a series of dyads based on naphthalimide (NI, electron acceptor) and phenothiazine (PTZ, electron donor), with an intervening phenyl linker attached on the N-position of both moieties. The purpose is to exploit the electron spin control effect to prolong the CS-state lifetime by formation of the 3CS state, instead of the ordinary 1CS state, the spin-correlated radical pair (SCRP), or the free ion pairs. The electronic coupling magnitude is tuned by conformational restriction exerted by the methyl groups on the phenyl linker. Differently from the previously reported NI-PTZ analogues containing long and flexible linkers, we observed a significant CS emission band centered at ca. 600 nm and thermally activated delayed fluorescence (TADF) with a lifetime of 13.8 ns (population ratio: 42%)/321.6 μs (56%). Nanosecond transient absorption spectroscopy indicates that in cyclohexane (CHX), only the 3NI* state was observed (lifetime τ = 274.7 μs), in acetonitrile (ACN), only the CS state was observed (τ = 1.4 μs), whereas in a solvent with intermediate polarity, such as toluene (TOL), both the 3NI* (shorter-lived) and the CS states were observed. Observation of the long-lived CS state in ACN, yet lack of TADF, confirms the spin-vibronic coupling theoretical model of TADF. Femtosecond transient absorption spectroscopy indicates that charge separation occurs in both nonpolar and polar solvents, with time constants ranging from less than 1 ps in ACN to ca. 60 ps in CHX. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of the 3NI* and CS states for the dyads upon photoexcitation. The electron spin-spin dipole interaction magnitude of the radical anion and cation of the CS state is intermediate between that of a typical SCRP and a 3CS state, suggesting that the long CS-state lifetime is partially due to the electron spin control effect.

Pyglotaran: a lego-like Python framework for global and target analysis of time-resolved spectra

The dynamics of molecular systems can be studied with time-resolved spectroscopy combined with model-based analysis. A Python framework for global and target analysis of time-resolved spectra is introduced with the help of three case studies. The first study, concerning broadband absorption of intersystem crossing in 4-thiothymidine, demonstrates the framework's ability to resolve vibrational wavepackets with a time resolution of ≈10 fs using damped oscillations and their associated spectra and phases. Thereby, a parametric description of the “coherent artifact” is crucial. The second study addresses multichromophoric systems composed of two perylene bisimide chromophores. Here, pyglotaran's guidance spectra and lego-like model composition enable the integration of spectral and kinetic properties of the parent chromophores, revealing a loss process, the undesired production of a radical pair, that reduces the light harvesting efficiency. In the third, time-resolved emission case study of whole photosynthetic cells, a megacomplex containing ≈500 chromophores of five different types is described by a combination of the kinetic models for its elements. As direct fitting of the data by theoretical simulation is unfeasible, our global and target analysis methodology provides a useful ‘middle ground’ where the theoretical description and the fit of the experimental data can meet. The pyglotaran framework enables the lego-like creation of kinetic models through its modular design and seamless integration with the rich Python ecosystem, particularly Jupyter notebooks. With extensive documentation and a robust validation framework, pyglotaran ensures accessibility and reliability for researchers, serving as an invaluable tool for understanding complex molecular systems.Graphical abstract

Disproportionation and dimerisation of kynurenic acid under UV light

In this work, we report on the photo-induced oxidation of kynurenic acid (KNA) by the triplet-excited KNA in a neutral aqueous solution. This reaction is confirmed by two different time-resolved methods: optical spectroscopy and chemically induced dynamic nuclear polarization (CIDNP). The disproportionation reaction proceeds with the rate constant of (6.0 ± 1.2) × 107 M−1s−1 via the mechanism of H-atom transfer to the photoexcited KNA in the triplet state, yielding the reduced and oxidized forms of KNA, KNAH2− and KNA(-H)−, respectively. The formation of these radicals is confirmed by a good proportionality relationship between the calculated proton hyperfine coupling constants of both radicals and the signal intensities in 1H CIDNP spectra detected after the recombination of the geminate radical pairs formed immediately after the laser pulse. The oxidized forms of KNA recombine yielding dimeric KNA-KNA products crosslinked via different positions within aromatic systems. The oxidized form of KNA, KNA(-H)−, could be of high biological interest due to its ability to recombine with other electron-deficient radicals, including tryptophan and tyrosine radicals, unravelling a radical pathway for the covalent attachment of KNA to proteins in living tissues.

Frustrated radical pairs facilitate C–H activation

Frustrated radical pairs facilitate C–H activation

Induction of chirality in the charge transfer complex photochemistry

Electron donor and acceptor undergo association in solution to form a weak molecular complex, namely a charge transfer complex. The formation of the complex is apparent by its new low-energy absorption band. Upon photoexcitation of this charge transfer band, an intracomplex electron transfer is facilitated to form a contact ion radical pair. The theoretical basis for the complex formation and the subsequent photoinduced electron transfer process has been well established already half a century ago. Nevertheless, the charge transfer complex photochemistry is now in a renaissance, due to its synthetic utility of its radical ion pair. By intentionally sidestepped unproductive back electron transfer process, a desired product is effectively obtained with or without external reagent and/or catalyst. In addition, the photoexcitation of the complex does not require any additional photocatalyst and often conceivable with the visible light. Accordingly, various donor/acceptor pairs and synthetic applications have been emerged as novel photoreaction systems in the last two decades. Some have also addressed the stereoselective transformations. This mini review highlights the recent progress of the asymmetric photoreaction, in particular in synthetic applications, based on the photoexcitation of the charge transfer complex, in comparison with photoredox catalysis.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry.

The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Nanosecond time-resolved infrared spectroscopy for the study of electron transfer in photosystem I.

Microsecond time-resolved step-scan FTIR difference spectroscopy was used to study photosystem I (PSI) from Thermosynechococcus vestitus BP-1 (T. vestitus, formerly known as T. elongatus) at 77K. In addition,photoaccumulated (P700+-P700) FTIR difference spectra were obtained at both 77 and 293K. The FTIR difference spectra are presented here for the first time. To extend upon these FTIR studies nanosecond time-resolved infrared difference spectroscopy was also used to study PSI from T. vestitus at 296K. Nanosecond infrared spectroscopy has never been used to study PSI samples at physiological temperatures, and here it is shown that such an approach has great value as it allows a direct probe of electron transfer down both branches in PSI. In PSI at 296K, the infrared flash-induced absorption changes indicate electron transfer down the B- and A-branches is characterized by time constants of 33 and 364ns, respectively, in good agreement with visible spectroscopy studies. These time constants are associated with forward electron transfer from A1- to FX on the B- and A-branches, respectively. At several infrared wavelengths flash-induced absorption changes at 296K recover in tens to hundreds of milliseconds. The dominant decay phase is characterized by a lifetime of 128ms. These millisecond changes are assigned to radical pair recombination reactions, with the changes being associated primarily with P700+ rereduction. This conclusion follows from the observation that the millisecond infrared spectrum is very similar to the photoaccumulated (P700+-P700) FTIR difference spectrum.

Regioselective aliphatic C-H functionalization using frustrated radical pairs.

Frustrated Lewis pairs (FLPs) are well documented for the activation of small molecules such as dihydrogen and carbon dioxide1-4. Although canonical FLP chemistry is heterolytic in nature, recent work has shown that certain FLPs can undergo single-electron transfer to afford radical pairs5. Owing to steric encumbrance and/or weak bonding association, these radicals do not annihilate one another, and they have thus been named frustrated radical pairs (FRPs). Notable preliminary results suggest that FRPs may be useful reagents in chemical synthesis6-8, although their applications remain limited. Here we demonstrate that the functionalization of C(sp3)-H bonds can be accomplished using a class of FRPs generated from disilazide donors and an N-oxoammonium acceptor. Together, these species undergo single-electron transfer to generate a transient and persistent radical pair capable of cleaving unactivated C-H bonds to furnish aminoxylated products. By tuning the structure of the donor, it is possible to control regioselectivity and tailor reactivity towards tertiary, secondary or primary C-H bonds. Mechanistic studies lend strong support for the formation and involvement of radical pairs in the target reaction.

Upper bound for broadband radiofrequency field disruption of magnetic compass orientation in night-migratory songbirds

Night-migratory songbirds have a light-dependent magnetic compass sense, the mechanism of which is thought to depend on the photochemical formation of radical pairs in cryptochrome (Cry) proteins located in the retina. The finding that weak radiofrequency (RF) electromagnetic fields can prevent birds from orienting in the Earth's magnetic field has been regarded as a diagnostic test for this mechanism and as a potential source of information on the identities of the radicals. The maximum frequency that could cause such disorientation has been predicted to lie between 120 and 220 MHz for a flavin-tryptophan radical pair in Cry. Here we show that the magnetic orientation capabilities of Eurasian blackcaps (Sylvia atricapilla) are not affected by RF noise in the frequency bands 140 to 150 MHz and 235 to 245 MHz. From a consideration of its internal magnetic interactions, we argue that RF field effects on a flavin-containing radical-pair sensor should be approximately independent of frequency up to 116 MHz and that birds' sensitivity to RF disorientation should fall by about two orders of magnitude when the frequency exceeds 116 MHz. Taken together with our earlier finding that 75 to 85 MHz RF fields disrupt the magnetic orientation of blackcaps, these results provide compelling evidence that the magnetic compass of migratory birds operates by a radical pair mechanism.