Transient EPR of radical pairs in photosynthetic reaction centers: prediction of quantum beats

Direct observation of chirality-induced spin selectivity in electron donor-acceptor molecules

The role of chirality in determining the spin dynamics of photoinduced electron transfer in donor-acceptor molecules remains an open question. Although chirality-induced spin selectivity (CISS) has been demonstrated in molecules bound to substrates, experimental information about whether this process influences spin dynamics in the molecules themselves is lacking. Here we used time-resolved electron paramagnetic resonance spectroscopy to show that CISS strongly influences the spin dynamics of isolated covalent donor-chiral bridge-acceptor (D-Bχ-A) molecules in which selective photoexcitation of D is followed by two rapid, sequential electron-transfer events to yield D•+-Bχ-A•-. Exploiting this phenomenon affords the possibility of using chiral molecular building blocks to control electron spin states in quantum information applications.

Computational tools for the simulation and analysis of spin-polarized EPR spectra

The EPR spectra of paramagnetic species induced by photoexcitation typically exhibit enhanced absorptive and emissive features resulting from sublevel populations that differ from thermal equilibrium. The populations and the resulting spin polarization of the spectra are dictated by the selectivity of the photophysical process generating the observed state. Simulation of the spin-polarized EPR spectra is crucial in the characterization of both the dynamics of formation of the photoexcited state as well as its electronic and structural properties. EasySpin, the simulation toolbox for EPR spectroscopy, now includes extended support for the simulation of the EPR spectra of spin-polarized states of arbitrary spin multiplicity and formed by a variety of different mechanisms, including photoexcited triplet states populated by intersystem crossing, charge recombination or spin polarization transfer, spin-correlated radical pairs created by photoinduced electron transfer, triplet pairs formed by singlet fission and multiplet states arising from photoexcitation in systems containing chromophores and stable radicals. In this paper, we highlight EasySpin's capabilities for the simulation of spin-polarized EPR spectra on the basis of illustrative examples from the literature in a variety of fields ranging across chemistry, biology, material science and quantum information science.

A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Direct detection of spin polarization in photoinduced charge transfer through a chiral bridge

It is well assessed that the charge transport through a chiral potential barrier can result in spin-polarized charges. The possibility of driving this process through visible photons holds tremendous potential for several aspects of quantum information science, e.g., the optical control and readout of qubits. In this context, the direct observation of this phenomenon via spin-sensitive spectroscopies is of utmost importance to establish future guidelines to control photo-driven spin selectivity in chiral structures. Here, we provide direct proof that time-resolved electron paramagnetic resonance (EPR) can be used to detect long-lived spin polarization generated by photoinduced charge transfer through a chiral bridge. We propose a system comprising CdSe quantum dots (QDs), as a donor, and C₆₀, as an acceptor, covalently linked through a saturated oligopeptide helical bridge (χ) with a rigid structure of ∼10 Å. Time-resolved EPR spectroscopy shows that the charge transfer in our system results in a C₆₀ radical anion, whose spin polarization maximum is observed at longer times with respect to that of the photogenerated C₆₀ triplet state. Notably, the theoretical modelling of the EPR spectra reveals that the observed features may be compatible with chirality-induced spin selectivity, but the electronic features of the QD do not allow the unambiguous identification of the CISS effect. Nevertheless, we identify which parameters need optimization for unambiguous detection and quantification of the phenomenon. This work lays the basis for the optical generation and direct manipulation of spin polarization induced by chirality.

Our work provides a first attempt to directly detect the spin polarisation of Chiral-Induced Spin Selectivity (CISS) effect by studying the photoinduced electron transfer in a CdSe Quantum Dot-chiral bridge-fullerene derivative (QD–χ–C₆₀) system.

Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science

More than a half century ago, the NMR spectra of diamagnetic products resulting from radical pair reactions were observed to have strongly enhanced absorptive and emissive resonances. At the same time, photogenerated radical pairs were discovered to exhibit unusual electron paramagnetic resonance spectra that also had such resonances. These non-Boltzmann, spin-polarized spectra were observed in both chemical systems as well as in photosynthetic reaction center proteins following photodriven charge separation. Subsequent studies of these phenomena led to a variety of chemical electron donor-acceptor model systems that provided a broad understanding of the spin dynamics responsible for these spectra. When the distance between the two radicals is restricted, these observations result from the formation of spin-correlated radical pairs (SCRPs) in which the spin-spin exchange and dipolar interactions between the two unpaired spins play an important role in the spin dynamics. Early on, it was recognized that SCRPs photogenerated by ultrafast electron transfer are entangled spin pairs created in a well-defined spin state. These SCRPs can serve as spin qubit pairs (SQPs), whose spin dynamics can be manipulated to study a wide variety of quantum phenomena intrinsic to the field of quantum information science. This Perspective highlights the role of SCRPs as SQPs, gives examples of possible quantum manipulations using SQPs, and provides some thoughts on future directions.

Assessing the Nature of Chiral-Induced Spin Selectivity by Magnetic Resonance

Understanding chiral-induced spin selectivity (CISS), resulting from charge transport through helical systems, has recently inspired many experimental and theoretical efforts but is still the object of intense debate. In order to assess the nature of CISS, we propose to focus on electron-transfer processes occurring at the single-molecule level. We design simple magnetic resonance experiments, exploiting a qubit as a highly sensitive and coherent magnetic sensor, to provide clear signatures of the acceptor polarization. Moreover, we show that information could even be obtained from time-resolved electron paramagnetic resonance experiments on a randomly oriented solution of molecules. The proposed experiments will unveil the role of chiral linkers in electron transfer and could also be exploited for quantum computing applications.

Quantum effects in biology: golden rule in enzymes, olfaction, photosynthesis and magnetodetection

Despite certain quantum concepts, such as superposition states, entanglement, 'spooky action at a distance' and tunnelling through insulating walls, being somewhat counterintuitive, they are no doubt extremely useful constructs in theoretical and experimental physics. More uncertain, however, is whether or not these concepts are fundamental to biology and living processes. Of course, at the fundamental level all things are quantum, because all things are built from the quantized states and rules that govern atoms. But when does the quantum mechanical toolkit become the best tool for the job? This review looks at four areas of 'quantum effects in biology'. These are biosystems that are very diverse in detail but possess some commonality. They are all (i) effects in biology: rates of a signal (or information) that can be calculated from a form of the 'golden rule' and (ii) they are all protein-pigment (or ligand) complex systems. It is shown, beginning with the rate equation, that all these systems may contain some degree of quantumeffect, and where experimental evidence is available, it is explored to determine how the quantum analysis aids in understanding of the process.

What you get out of high-time resolution electron paramagnetic resonance: example from photosynthetic bacteria

The primary energy conversion steps of natural photosynthesis proceed via light-induced radical ion pairs as short-lived intermediates. Time-resolved electron paramagnetic resonance (EPR) experiments of photosynthetic reaction centers monitor the key charge separated state between the oxidized primary electron donor and reduced quinone acceptor, e.g., P(+)(865)Q(-)(A) of purple photosynthetic bacteria. The time-resolved EPR spectra of P(+)(865)Q(-)(A) are indicative of a spin-correlated radical pair that is created from the excited singlet state of P(865) in an ultra-fast photochemical reaction. Importantly, the spin-correlated radical pair nature of the charge separated state is a common feature of all photosynthetic reaction centers, which gives rise to several interesting spin phenomena such as quantum oscillations, observed at short delay times after optical excitation. In this review, we describe details of the quantum oscillation phenomenon and present a complete analysis of the data obtained from the charge separated state of purple bacteria, P(+)(865)Q(-)(A). The analysis and simulation of the quantum oscillations yield the three-dimensional structure of P(+)(865)Q(-)(A) in the photosynthetic membrane on a nanosecond time scale after light-induced charge separation. Comparison with crystallographic data reveals that the position of Q(-)(A) is essentially the same as in the X-ray structure. However, the head group of Q(-)(A) has undergone a 60° rotation in the ring plane relative to its orientation in the crystal structure. The results are discussed within the framework of the previously suggested conformational gating mechanism for electron transfer from Q(-)(A) to the secondary quinone acceptor Q(B).

High Time Resolution Multifrequency EPR of Radical Pair Intermediates in Photosynthetic Reaction Centers: Structure Determination on a Nanosecond Time Scale

The primary steps of photosynthesis proceed via light-induced radical pairs as short-lived intermediates. In this paper we discuss novel coherence phenomena which can be observed for these species. It is demonstrated that the analysis of quantum beat oscillations in combination with multifrequency EPR and a magnetically oriented sample represents a powerful structural tool. We expect that this is of general interest, since the detailed structure of radical pair intermediates can be determined on a nanosecond time scale.

Structure of the charge separated state P865(+)Q(A)- in the photosynthetic reaction centers of Rhodobacter sphaeroides by quantum beat oscillations and high-field electron paramagnetic resonance: evidence for light-induced Q(A)- reorientation

The structure of the secondary radical pair, P865(+)Q(A)-, in fully deuterated and Zn-substituted reaction centers (RCs) of the purple bacterium Rhodobacter sphaeroides R-26 has been determined by high-time resolution and high-field electron paramagnetic resonance (EPR). A computer analysis of quantum beat oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the orientation of the various magnetic tensors of P865(+)Q(A)- with respect to a magnetic reference frame. The orientation of the g-tensor of P865(+) in an external reference system is adapted from a single-crystal W-band (95 GHz) EPR study [Klette, R.; Törring, J. T.; Plato, M.; Möbius, K.; Bönigk, B.; Lubitz, W. J. Phys. Chem. 1993, 97, 2015-2020]. Thus, we obtain the three-dimensional structure of the charge separated state P865(+)Q(A)- on a nanosecond time scale after light-induced charge separation. Comparison with crystallographic data reveals that the position of the quinone is essentially the same as that in the X-ray structure. However, the head group of Q(A)- has undergone a 60 degrees rotation in the ring plane relative to its orientation in the crystal structure. Analysis suggests that the two different QA conformations are functionally relevant states which control the electron-transfer kinetics from Q(A)- to the secondary quinone acceptor QB. It appears that the rate-limiting step of this reaction is a reorientation of Q(A)- in its binding pocket upon light-induced reduction. The new kinetic model accounts for striking observations by Kleinfeld et al. who reported that electron transfer from Q(A)- to QB proceeds in RCs cooled to cryogenic temperature under illumination but does not proceed in RCs cooled in the dark [Kleinfeld, D.; Okamura, M. Y.; Feher, G. Biochemistry 1984, 23, 5780-5786].

Long-range distance determinations in biomacromolecules by EPR spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy provides a variety of tools to study structures and structural changes of large biomolecules or complexes thereof. In order to unravel secondary structure elements, domain arrangements or complex formation, continuous wave and pulsed EPR methods capable of measuring the magnetic dipole coupling between two unpaired electrons can be used to obtain long-range distance constraints on the nanometer scale. Such methods yield reliably and precisely distances of up to 80 A, can be applied to biomolecules in aqueous buffer solutions or membranes, and are not size limited. They can be applied either at cryogenic or physiological temperatures and down to amounts of a few nanomoles. Spin centers may be metal ions, metal clusters, cofactor radicals, amino acid radicals, or spin labels. In this review, we discuss the advantages and limitations of the different EPR spectroscopic methods, briefly describe their theoretical background, and summarize important biological applications. The main focus of this article will be on pulsed EPR methods like pulsed electron-electron double resonance (PELDOR) and their applications to spin-labeled biosystems.

Structural organization in photosynthetic proteins as studied by high-field EPR of spin-correlated radical pair states

We demonstrate the potential of high-field (HF) time-resolved electron paramagnetic resonance (EPR) spectroscopy to reveal unique information about electron transfer processes and the structure of photosynthetic systems. The lineshapes and electron spin polarization (ESP) of spin-correlated radical pair (SCRP) spectra recorded with HF-EPR are very sensitive to the magnetic parameters, interactions, and geometry of the radicals in the pair. This sensitivity facilitates an analysis of more sophisticated models and methods to reveal the important relationship between structural organization and light-induced electron transfer of the photosynthetic proteins. In this review, we report on a new time-resolved HF and multi-frequency EPR approach developed in the Freiburg laboratory in cooperation with the Argonne Photosynthesis group. The method is designed to probe the geometric structure of charge separated states in the photosynthetic membrane. First, we discuss the magneto-orientation of photosynthetic cyanobacteria as revealed by time-resolved HF-EPR of SCRPs. Then, we demonstrate how the three-dimensional structure of the SCRP P700(+)A1 from photosystem I of oxygenic photosynthesis and its arrangement in the membrane is obtained from application of multi-frequency including time-resolved HF-EPR techniques.

Electron transfer pathways and protein response to charge separation in photosynthetic reaction centers: time-resolved high-field ENDOR of the spin-correlated radical pair P865(+)QA(-)

Recently we reported the first observation of time-resolved (TR) high-frequency (HF) electron nuclear double resonance (ENDOR) of the transient charge separated state P865(+)Q(-)A in purple photosynthetic bacterial reaction centers (RC) (Poluektov, O. G., et al. J. Am. Chem. Soc. 2004, 126, 1644-1645). The high resolution and orientational selectivity of HF ENDOR allows us to directly probe protein environments by spectrally selecting specific nuclei in isotopically labeled samples. A new phenomenon associated with the spin correlated radical pair (SCRP) nature of P865(+)Q(-)A was observed. The TR-HF ENDOR spectra of protein nuclei (protons) surrounding deuterated QA(-) exhibit a derivative-like, complicated line shape, which differs considerably from the HF ENDOR spectrum of the protein nuclei surrounding thermally equilibrated QA(-). Here, a theoretical analysis of these observations is presented that shows that the positions and amplitudes of ENDOR lines contain information on hyperfine interactions (HFI) of a particular nucleus (a proton of the protein) with both correlated electron spins. Thus, spin density delocalization in the protein environment between the SCRP donor and acceptor molecules can be revealed via HF ENDOR. Novel approaches for acquiring and analyzing SCRP ENDOR that simplify interpretation of the spectra are discussed. Furthermore, we report here that the positions of the ENDOR lines of the SCRP shift with an increase in the time after laser flash, which initiates electron transfer. These shifts provide direct spectroscopic evidence of reorganization of the protein environment to accommodate the donor-acceptor charge-separated state P865(+)QA(-).

Transient radical pairs studied by time-resolved EPR

Photogenerated short-lived radical pairs (RP) are common in biological photoprocesses such as photosynthesis and enzymatic DNA repair. They can be favorably probed by time-resolved electron paramagnetic resonance (EPR) methods with adequate time resolution. Two EPR techniques have proven to be particularly useful to extract information on the working states of photoinduced biological processes that is only difficult or sometimes even impossible to obtain by other types of spectroscopy. Firstly, transient EPR yields crucial information on the chemical nature and the geometry of the individual RP halves in a doublet-spin pair generated by a short laser pulse. This time-resolved method is applicable in all magnetic field/microwave frequency regimes that are used for continuous-wave EPR, and is nowadays routinely utilized with a time resolution reaching about 10 ns. Secondly, a pulsed EPR method named out-of-phase electron spin echo envelope modulation (OOP-ESEEM) is increasingly becoming popular. By this pulsed technique, the mutual spin-spin interaction between the RP halves in a doublet-spin pair manifests itself as an echo modulation detected as a function of the microwave-pulse spacing of a two-pulse echo sequence subsequent to a laser pulse. From the dipolar coupling, the distance between the radicals is readily derived. Since the spin-spin interaction parameters are typically not observable by transient EPR, the two techniques complement each other favorably. Both EPR methods have recently been applied to a variety of light-induced RPs in photobiology. This review summarizes the results obtained from such studies in the fields of plant and bacterial photosynthesis and DNA repair mediated by the enzyme DNA photolyase.

Removal of PsaF alters forward electron transfer in photosystem I: evidence for fast reoxidation of QK-A in subunit deletion mutants of Synechococcus sp. PCC 7002

Recent studies of point mutations in photosystem I have suggested that the two kinetic phases of phylloquinone reoxidation represent electron transfer in the two branches of cofactors. This interpretation implies that changes in the relative amplitudes of the two kinetic phases represent a change in the extent of electron transfer in the two branches. Using time-resolved electron paramagnetic resonance (EPR), this issue is investigated in subunit deletion mutants of Synechococcus sp. PCC 7002. The spin-polarized EPR signals of P(700)(+)A(1)(-) and P(700)(+)FeS(-), both at room temperature and in frozen solution, are altered by deletion of PsaF and/or PsaE, and the differences from the wild type are much more pronounced in PS I complexes isolated from the mutants using Triton X-100 rather than n-dodecyl beta-d-maltopyranoside. The changes in the transient EPR data for the mutant complexes are consistent with a significant fraction of reaction centers showing (i) faster electron transfer from A(1)(-) to F(X), (ii) slower forward electron transfer from A(0)(-) to A(1), and (iii) slightly altered quinone hyperfine couplings, possibly as a result of a change in the hydrogen bonding. The fraction of fast electron transfer and its dependence on the isolation procedure are estimated approximately from simulations of the room temperature EPR data. The results are discussed in terms of possible models for the electron transfer. It is suggested that the detergent-induced fraction of fast electron transfer is most likely due to alteration of the environment of the quinone in the PsaA branch of cofactors and is not the result of a change in the directionality of electron transfer.

ENDOR of Spin-Correlated Radical Pairs in Photosynthesis at High Magnetic Field: A Tool for Mapping Electron Transfer Pathways

A new phenomenon has been detected in the time-resolved electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pairs in photosynthetic reaction center proteins. The observed effects result from both increased resolution and orientational selectivity provided by high magnetic field EPR and are manifest as specific, derivative-type lines in the ENDOR spectrum. Importantly, the positions and amplitudes of these lines contain information on the interaction of a particular nucleus with both correlated electron spins. Thus, spin density delocalization in the protein environment between the donor and acceptor in the SCRP can be revealed via SCRP ENDOR, providing a unique opportunity to probe the electron-transfer pathways in natural and artificial photosynthetic assemblies.

Pulsed EPR spectroscopy on short-lived intermediates in Photosystem I

The application of pulsed electron paramagnetic resonance spectroscopy on short-lived intermediates in Photosystem I is reviewed. The spin polarization in light-induced radical pairs gives rise to a phase shifted 'out-of-phase' electron spin echo signal. This echo signal shows a prominent modulation of its intensity as a function of the spacing between the two microwave pulses. Its modulation frequency is determined by the electron-electron spin couplings within the radical pair. Thereby, the measurement of the dipolar coupling gives direct information about the spin-spin distance and can therefore be used to determine cofactor distances with high precision. Application of this technique to the radical pair P(*+)(700)A(*-)(1) in Photosystem I is discussed. Moreover, if oriented samples (e.g. single crystals) are used, the angular dependence of the dipolar coupling can be used to derive the orientation of the axis connecting donor and acceptor with respect to an external (crystal) axes system. Using out-of-phase electron spin echo envelope modulation spectroscopy, the localization of the secondary acceptor quinone A(1) has become possible.

A new approach to determining the geometry of weakly coupled radical pairs from their electron spin polarization patterns

Analytical expressions for the spin polarized EPR lineshapes of weakly coupled radical pairs (RPs) are derived as functions of the angles between the anisotropic g-tensors of the radicals and the vector describing the dipolar coupling. It is shown that with a singlet precursor the EPR signal of the RP can be written as a linear function of the dipolar coupling. Under these conditions, the calculated powder spectrum can be expressed as a linear combination of four powder spectra, which are independent of the geometry of the RP. To reproduce the experimental spectra the optimal set of coefficients can be found by least-squares fitting. The advantage of this approach is that the four powder spectra must only be calculated once. This treatment shows very clearly the restrictions placed on the information obtainable from such spectra. Most importantly, a unique set of angles can only be obtained if the absolute amplitude of the spectrum is known. In general, the calculated spectrum is related to the experimental spectrum by an unknown, arbitrary scaling factor. In this case, sets of angles consistent with the data are obtained. Possible strategies for obtaining unique geometric information are discussed and demonstrated with the experimental data for the state P+*(865)Q-*(A) in Zn-substituted bacterial reaction centres.

Structure of the P700(+ )A1(-) radical pair intermediate in photosystem I by high time resolution multifrequency electron paramagnetic resonance: analysis of quantum beat oscillations

The geometry of the secondary radical pair P700(+)A1(-), in photosystem I (PSI) from the deuterated and 15N-substituted cyanobacterium Synechococcus lividus, has been determined by high time resolution electron paramagnetic resonance (EPR), performed at three different microwave frequencies. Structural information is extracted from light-induced quantum beats observed in the transverse magnetization of P700(+)A1(-) at early times after laser excitation. A computer analysis of the two-dimensional Q-band experiment provides the orientation of the various magnetic tensors of with respect to a magnetic reference frame. The orientation of the cofactors of the primary donor in the g-tensor system of is then evaluated by analyzing time-dependent X-band EPR spectra, extracted from a two-dimensional data set. Finally, the cofactor arrangement of P700(+)A1(-) in the photosynthetic membrane is deduced from angular-dependent W-band spectra, observed for a magnetically aligned sample. Thus, the orientation of the g-tensor of P700(+) with respect to a chlorophyll based reference system could be determined. The angle between the g1(z) axis and the chlorophyll plane normal is found to be 29 +/- 7 degrees, while the g1(y) axis lies in the chlorophyll plane. In addition, a complete structural model for the reduced quinone acceptor, A1(-), is evaluated. In this model, the quinone plane of is found to be inclined by 68 +/- 7 degrees relative to the membrane plane, while the P700(+)-A1(-) axis makes an angle of 35 +/- 6 degrees with the membrane normal. All of these values refer to the charge separated state, observed at low temperatures, where forward electron transfer to the iron-sulfur centers is partially blocked. Preliminary room temperature studies of P700(+)A1(-), employing X-band quantum beat oscillations, indicate a different orientation of A1(-) in its binding pocket. A comparison with crystallographic data provides information on the electron-transfer pathway in PSI. It appears that quantum beats represent excellent structural probes for the short-lived intermediates in the primary energy conversion steps of photosynthesis.

Nutation-Frequency Correlated EPR Spectroscopy: The PEANUT Experiment

The phase-inverted echo-amplitude detected nutation (PEANUT) experiment for the measurement of transient electron spin nutation frequencies is introduced. In this new pulse sequence, nutation refocused to a rotary echo is detected via the amplitude modulation of a primary electron spin echo. Since detection and excitation of the nutation are fully separated in time, experiments at very high nutation frequencies become feasible. The nutation frequencies which are proportional to the transition moment of an EPR transition can be used to label individual EPR lines by an additional parameter. Using a two-dimensional PEANUT experiment which correlates the nutation frequencies with the resonance fields, the interpretation of complicated field-swept EPR spectra can considerably be simplified. A theoretical description of the experiment is given and the inner working of the approach is described. The predicted features of PEANUT spectra are verified experimentally and examples of applications to both ordered and disordered systems are given. Copyright 1998 Academic Press. Copyright 1998 Academic Press

NEW EPR METHODS FOR INVESTIGATING PHOTOPROCESSES WITH PARAMAGNETIC INTERMEDIATES

▪ Abstract  Some of the significant advances in time-resolved multifrequency electron paramagnetic resonance (EPR) methods are reviewed, with the explicit focus on studies of light-driven processes and photoreactions in real time. Prominent examples are excited state electron transfer reactions with transient charge-separated radical pairs playing a central role. Paramagnetic intermediates and products are key functional states; thus EPR is the method of choice for their characterization. Photogenerated spin polarization and coherences as process-inherent features add the practical advantage of compensation in the trade-off between sensitivity and time resolution. Additionally, they provide detailed structural and dynamic information on the photoreactive system. Significance and specificity of the results achieved for charge separation in photosynthetic reaction centers and donor-acceptor model complexes indicate highly promising perspectives in photochemical research.