Anisotropic Hyperfine Coupling Research Articles

Stabilization of a flavoprotein for solid-state photo-CIDNP MAS NMR at room temperature by embedding in a glassy sugar matrix

Hyperpolarization via the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect can be detected in frozen solutions of electron transfer proteins generating a radical-pair upon illumination. The effect has been observed in various natural photosynthetic reaction centers and in light-oxygen-voltage (LOV) sensing domains incorporating a flavin mononucleotide (FMN) as chromophore. In LOV domains, where a highly conserved cysteine is mutated to a flavin to interrupt its natural photochemistry, a radical-pair is generated by electron transfer from a nearby tryptophan to the photoexcited triplet state of FMN. During the photocycle, both the LOV domain and the chromophore are photochemically degraded, e.g., by the formation of singlet oxygen. This limits the time for collection of hyperpolarized nuclear magnetic resonance (NMR) data. We show that embedding of the protein into a trehalose sugar glass matrix stabilizes the protein for 13C solid-state photo-CIDNP NMR experiments which can be conducted at room temperature in a powder sample. Additionally, this preparation allows for incorporation of high amounts of protein further boosting the intensity of the detected signals from FMN and tryptophan at natural abundance. Signal assignment is aided by quantum chemical calculations of absolute shieldings. The underlying mechanism for the surprising absorption-only signal pattern is not yet understood. Comparison to calculated isotropic hyperfine couplings imply that the enhancement is not due to the classical radical-pair mechanism (RPM). Analysis of the anisotropic hyperfine couplings associated with solid-state photo-CIDNP mechanisms also show no simple correlation, suggesting a more complex underlying mechanism.

A comparative analysis of powder ENDOR spectra of aromatic and aliphatic radicals by exact and 1st order simulation

Exact and perturbation methods were employed in simulations of powder ENDOR spectra to obtain the anisotropic hyperfine ( hfc ) and nuclear quadrupole ( nqc ) coupling constants of certain organic radicals of interest in fundamental research and in applications in radiation research, surface chemistry and biophysics. The principal hfc values of the ring protons and methyl substituents for several aromatic cations trapped in disordered matrices might be more accurate than those previously reported using regular EPR. Only one of the earlier assignments of the naphthalene cation spectrum was in acceptable agreement with the simulations. The proton couplings at the β-position of alkyl radicals were deduced by simulations while the spectrum due to α-couplings with appreciable anisotropy was weak. Accurate simulation of the 14 N (I = 1) spectra in bio-radicals was obtained by adjustment of the relative orientation of the principal hfc and nuclear quadrupole coupling ( nqc ) tensors as well as the principal values. Adjustment of the excitation width parameter employed in the software was also required in a few cases to improve the agreement with the experimental spectra. The hfc patterns due to matrix nuclei ( 1 H and 7 Li) around the radicals of X-irradiated samples were simulated to elucidate the nature of the trapping sites in materials used for EPR dosimetry. ENDOR simulation programs known to us are presented in an Appendix. The performance of the ENDORF2 program used in previous works was examined by comparison with exact treatment. Input and output examples, source and executable codes used in this work can be downloaded at https://github.com/EndorF2/Simulation .

Site-Dependent Local Spin Susceptibility and Low-Energy Excitation in a Weyl Semimetal WTe2

Site-dependent local spin susceptibility is investigated with $^{125}$Te nuclear magnetic resonance in a Weyl semimetal WTe$_2$. The nuclear spin-lattice relaxation rate $1/T_1T$ shows a dependence of the square of temperature $T$ at high temperatures, followed by a constant behavior below 50 K. The temperature dependence features Weyl fermions appearing around the linearly crossing bands. The Knight shift $K$ scales to the square root of $1/T_1T$, corroborating a predominant spin contribution in low-lying excitation. The observed dependence of $K$ and $1/T_1T$ on the four Te sites shows the site-dependent electron correlation and density of states. The angular profile of the NMR spectrum gives the anisotropic hyperfine coupling tensor, consistent with $5p$ hole occupations on Te sites.

Impact of Chlorine Substitution on Electron Spin Relaxation of a Trityl Radical.

A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13C at the central carbon (13C1-PTMTC) was characterized in phosphate buffered saline solution (pH = 7.2) (PBS) at ambient temperature. Samples immobilized in 1:1 PBS:glycerol or in 9:1 trehalose:sucrose were studied as a function of temperature. Isotope enrichment at C1 creates a trityl that can be used to accurately measure microscopic viscosity. Understanding of the impact of the 13C hyperfine interaction on electron spin relaxation is important for application of this trityl in oximetry and distance measurements. The anisotropic 13C1 hyperfine couplings (Ax = Ay = 24 ± 2 MHz, Az = 200 ± 1 MHz) are larger than for the related 13C1-perdeuterated Finland trityl (13C1-dFT) and the g anisotropy (gx = 2.0013, gy = 2.0016, gz = 2.0042) is slightly larger than for 13C1-dFT. The tumbling correlation times (τR) for 13C1-PTMTC are 0.20 ± 0.02 ns in PBS and 0.40 ± 0.05 ns in 3:1 PBS:glycerol, which are shorter than for 13C1-dFT in the same solutions. T1 for 13C1-PTMTC is 3.5 ± 0.5 μs in PBS and 5.3 ± 0.4 μs in 3:1 PBS:glycerol, which are shorter than for 13C1-dFT due to faster tumbling, larger anisotropy of the 13C1 hyperfine, and about 30% larger contribution from the local mode. In immobilized samples T1 for 13C1-PTMTC is similar to that for 13C1-dFT and other trityls without chlorine or 13C1 substituents, indicating that the 13C1 and Cl substituents on the phenyl rings have little impact on T1. The temperature dependence of T1 was modeled with contributions from the direct, Raman, and local mode processes. Broadening of CW linewidths of about 0.6 G in fluid solution and about 2 G in rigid lattice is attributed to unresolved 35,37Cl hyperfine couplings.

Exploring the Role of the Central Carbide of the Nitrogenase Active-Site FeMo-cofactor through Targeted 13C Labeling and ENDOR Spectroscopy.

Mo-dependent nitrogenase is a major contributor to global biological N2 reduction, which sustains life on Earth. Its multi-metallic active-site FeMo-cofactor (Fe7MoS9C-homocitrate) contains a carbide (C4-) centered within a trigonal prismatic CFe6 core resembling the structural motif of the iron carbide, cementite. The role of the carbide in FeMo-cofactor binding and activation of substrates and inhibitors is unknown. To explore this role, the carbide has been in effect selectively enriched with 13C, which enables its detailed examination by ENDOR/ESEEM spectroscopies. 13C-carbide ENDOR of the S = 3/2 resting state (E0) is remarkable, with an extremely small isotropic hyperfine coupling constant, Ca = +0.86 MHz. Turnover under high CO partial pressure generates the S = 1/2 hi-CO state, with two CO molecules bound to FeMo-cofactor. This conversion surprisingly leaves the small magnitude of the 13C carbide isotropic hyperfine-coupling constant essentially unchanged, Ca = -1.30 MHz. This indicates that both the E0 and hi-CO states exhibit an exchange-coupling scheme with nearly cancelling contributions to Ca from three spin-up and three spin-down carbide-bound Fe ions. In contrast, the anisotropic hyperfine coupling constant undergoes a symmetry change upon conversion of E0 to hi-CO that may be associated with bonding and coordination changes at Fe ions. In combination with the negligible difference between CFe6 core structures of E0 and hi-CO, these results suggest that in CO-inhibited hi-CO the dominant role of the FeMo-cofactor carbide is to maintain the core structure, rather than to facilitate inhibitor binding through changes in Fe-carbide covalency or stretching/breaking of carbide-Fe bonds.

Performance of the DLPNO-CCSD and recent DFT methods in the calculation of isotropic and dipolar contributions to 14N hyperfine coupling constants of nitroxide radicals.

In the present study, the performance of a set of density functionals: BP86, PBE, OLYP, BEEF, PBEpow, TPSS, SCAN, PBEGXPBE, M06L, MN15L, B3LYP, PBE0, mPW1PW, B97, BHandHLYP, mPW1PW, B98, TPSS0, PBE1KCIS, SCAN0, M06, M06-2X, MN15, CAM-B3LYP, ωB97x, B2PLYP, and the B3LYP/N07D and PBE/N07D schemes in the calculation of the 14N anisotropic hyperfine coupling (HFC) constants of a set of 23 nitroxide radicals is evaluated. The results are compared with those obtained with the DLPNO-CCSD method and experimental HFC values. Harmonic contribution to the 14N HFC vibrational correction was calculated at the revPBE0/def2-TZVPP level and included in the evaluation. With the vibrational correction, the DLPNO-CCSD method yielded HFC values in good agreement with the experiment (mean absolute deviation (MAD) = 0.3G for the dipole-dipole contribution and MAD = 0.8G for the contact coupling contribution). The best DFT results are obtained using the M06 functional with MAD = 0.2G for the dipole-dipole contribution and MAD = 0.7G for the contact coupling contribution. In general, vibrational correction significantly improved most DFT functionals' performance but did not change its overall ranking.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH2PO4 crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH2PO4 and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K19H40P14O56, is constructed to model the STH. The ωB97XD functional with the 6−31+G* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO4 unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO4 units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one 31P nucleus and equal, but smaller, hyperfine interactions with two 1H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.

Hole spin echo envelope modulations

Hole spins in semiconductor quantum dots or bound to acceptor impurities show promise as potential qubits, partly because of their weak and anisotropic hyperfine couplings to proximal nuclear spins. Since the hyperfine coupling is weak, it can be difficult to measure. However, an anisotropic hyperfine coupling can give rise to a substantial spin-echo envelope modulation that can be Fourier-analyzed to accurately reveal the hyperfine tensor. Here, we give a general theoretical analysis for hole-spin-echo envelope modulation (HSEEM), and apply this analysis to the specific case of a boron-acceptor hole spin in silicon. For boron acceptor spins in unstrained silicon, both the hyperfine and Zeeman Hamiltonians are approximately isotropic leading to negligible envelope modulations. In contrast, in strained silicon, where light-hole spin qubits can be energetically isolated, we find the hyperfine Hamiltonian and $g$-tensor are sufficiently anisotropic to give spin-echo-envelope modulations. We show that there is an optimal magnetic-field orientation that maximizes the visibility of envelope modulations in this case. Based on microscopic estimates of the hyperfine coupling, we find that the maximum modulation depth can be substantial, reaching $\sim 10\%$, at a moderate laboratory magnetic field, $B\lesssim 200\,\mathrm{mT}$.

Performance of DFT methods in the calculation of isotropic and dipolar contributions to 14N hyperfine coupling constants of nitroxide radicals.

In the present study, we tested the widely used density functionals BP86, PBE, OLYP, TPSS, M06-L, B3LYP, PBE0, mPW1PW, B97, BHandHLYP, TPSS0, M06, M06-2X, CAM-B3LYP, ωB97x, and B2PLYP with the cc-pCVQZ basis set in calculations on a set of 23 nitroxide radicals with well-resolved 14N anisotropic hyperfine coupling (HFC) constants. The results were compared with those obtained using the B3LYP/N07D and PBE/N07D methods. The convergence of the HFC values to the complete basis set limit is briefly discussed. The best results were obtained using the M06/COSMO method, with a mean absolute deviation (MAD) of 0.4G for the dipole-dipole contribution and MAD = 0.6G for the contact coupling contribution (as compared to 1.1G and 1.0G, respectively, for the B3LYP/N07D/COSMO method and 1.7G and 0.5G, respectively, for the B3LYP/N07D method). The majority of the functionals yielded satisfactory results for the dipole-dipole contribution, but only the M06 functional yielded similar errors for both the dipole-dipole and isotropic contributions. The RIJCOSX and RI approximations introduced errors equal to or smaller than 0.01G.

Orientation Selective 2D-SIFTER Experiments at X-Band Frequencies

Frequency-correlated 2D SIFTER with broadband pulses at X-band frequencies can be used to determine the inter-spin distance and relative orientation of nitroxide moieties in macromolecules when the flexibility of the spin-labels is restricted. At X-band frequencies the EPR spectrum of nitroxides is governed by the strongly anisotropic nitrogen hyperfine coupling. For rigid spin-labels, where the orientation of the inter-connecting vector R correlates to the relative orientations of the nitroxide labels, the dipolar oscillation frequency varies over the EPR spectral line shape. Broadband shaped pulses allow excitation of the complete nitroxide EPR spectra. In this case, Fourier transform of the echo signal gives both fast and direct access to the orientation dependent dipole coupling. This allows determination of not only the inter-spin distance R, but also their mutual orientation. Here, we show the application of the frequency-correlated 2D SIFTER experiment with broadband pulses to a bis-nitroxide model compound and to a double stranded DNA sample. In both molecules, there is restricted internal mobility of the two spin-labels. The experimental results are compared to orientation selective pulsed electron double resonance (PELDOR) experiments and simulations based on a simple geometrical model or MD simulations describing the conformational flexibility of the molecules. Fourier transformation of the SIFTER echo signal yields orientation selective dipolar time traces over the complete EPR-spectral range. This leads to an improved frequency resolution and either to a reduced experimental measurement time or a larger span of frequency offsets measured compared to orientation selective PELDOR experiments. The experimental potential and limitations of the 2D SIFTER method for samples containing rigid spin-labels will be discussed.

Magnetic field dependence of triplet-state ONP: theoretical analysis in terms of level anti-crossings

ABSTRACTA theoretical approach to Optical Nuclear Polarisation (ONP) is described, which is based on the analysis of Level Anti-Crossings (LACs) in triplet states. Here we consider ONP formed in molecular crystals doped with suitable guest molecules and ONP generated in diamond crystals containing negatively charged nitrogen-vacancy (NV–) centres. In both cases, electron spin polarisation of triplet states generated by light excitation is transferred to nuclei giving rise to ONP. Polarisation transfer is most efficient at LACs; for this reason, we consider in detail crossings of electron–nuclear energy levels and the role of different perturbation terms (coming from isotropic and anisotropic hyperfine coupling, zero-field splitting and sample orientation), which turn these crossings into LACs and give rise to ONP. Analytical results are supported by numerical calculations of the ONP field dependences. Thus, the outlined LAC analysis is a useful approach for interpreting the ONP magnetic field dependence.

Modulated magnetic structure of Fe3PO7 as seen by Fe57 Mössbauer spectroscopy

The paper reports new results of the 57Fe M\"ossbauer measurements on Fe3PO4O3 powder sample recorded at various temperatures including the point of magnetic phase transition TN ~ 163K. The spectra measured above TN consist of quadrupole doublet with high quadrupole splitting of D300K ~ 1.10 mm/s, emphasizing that Fe3+ ions are located in crystal positions with a strong electric field gradient (EFG). In order to predict the sign and orientation of the main components of the EFG tensor we calculated monopole lattice contributions to the EFG. In the temperature range T < TN, the experimental spectra were fitted assuming that the electric hyperfine interactions are modulated when the Fe3+ spin (S) rotates with respect to the EFG axis and emergence of spatial anisotropy of the hyperfine field Hhf = S\~AI at 57Fe nuclei. These data were analyzed to estimate the components of the anisotropic hyperfine coupling tensor (\~A). The large anharmonicity parameter, m ~ 0.94, of the spiral spin structure results from easy-axis anisotropy in the plane of the iron spin rotation. The temperature evolution of the hyperfine field Hhf(T) was described by Bean-Rodbell model that takes into account that the exchange magnetic interactions are strong function of the lattice spacing. The obtained M\"ossbauer data are in qualitative agreement with previous neutron diffraction data for a modulated helical magnetic structure in strongly frustrated Fe3PO4O3.

Anti-oxidant behavior of functionalized chalcone-a combined quantum chemical and crystallographic structural investigation

Compound (2E)-3-(methoxyphenyl)-1-(4-methylphenyl) prop-2-en-1-one (Ch) was synthesized by the Claisen-Schmidt condensation reaction between para-methylacetophenone and para-methoxybenzaldehyde under basic condition. The structure of the molecule was elucidated using X-ray diffraction. Compound (Ch) demonstrated higher antioxidant activities in the DPPH test and H2O2 assay (IC50 = 12.23 ± 0.53 and 15.62 ± 0.98) than with the standard ascorbic acid (IC50 = 17.32 ± 0.44 and 19.07 ± 0.35). An evaluation of the atomic and molecular properties of ascorbic acid and Ch were computed based on their antioxidant activities. The molecular properties give insight into possible reasons for the enhanced antioxidant properties of Ch compared to ascorbic acid. The atomic properties provide further insight into chemical changes of the atoms of the compounds. Such changes include electronic shifting of the compounds electrophilic and/or nucleophilic states which highlight chemical moieties which characterize the antioxidant activity but do not directly relate to a variation in their antioxidant activities. The results obtained reflect oxygen atoms having significant nucleophilic interactions of each of the compounds. This was characterized by higher Fukui indices, isotropic and anisotropic hyperfine and orbital coupling stability energy.

Ratcheting rotation or speedy spinning: EPR and dynamics of Sc3C2@C80.

Besides their technological applications, endohedral fullerenes provide ideal conditions for investigating molecular dynamics in restricted geometries. A representative of this class of systems, Sc3C2@C80 displays complex intramolecular dynamics. The motion of the 45Sc trimer has a remarkable effect on its electron paramagnetic resonance (EPR) spectrum, which changes from a symmetric 22-peak pattern at high temperature to a single broad lineshape at low temperature. The scandium trimer consists of two equivalent and one inequivalent metal atom, due to the carbon dimer rocking through the Sc3 triangle. We demonstrate through first-principles molecular dynamics (MD), EPR parameter tensor averaging, and spectral modelling that, at high temperatures, three-dimensional movement of the enclosed Sc3C2 moiety takes place, which renders the metal centers equivalent and their magnetic parameters effectively isotropic. In contrast, at low temperatures the dynamics becomes restricted to two dimensions within the equatorial belt of the Ih symmetric C80 host fullerene. This restores the inequivalence of the scandium centers and causes their anisotropic hyperfine couplings to broaden the experimental spectrum.

The Quantum Dynamical Basis of a Classical Kinetic Scheme Describing Coherent and Incoherent Regimes of Radical Pair Recombination

Abstract In recent work from this group (J. H. Klein et al. J. Am. Chem. Soc. 2015, 137, 11011), the magnetic field dependent charge recombination kinetics in donor/Ir-complex/acceptor triads has been determined with outstanding accuracy and reproducibility. The field-dependent kinetics has been analyzed in terms of a classical reaction scheme including the field-independent rate parameters of singlet recombination (rate constant k S) and S/T0 mixing (rate constant k ST0) and the field-dependent rate constant k±(B) connecting central and outer Zeeman levels. In the present work, the extraction of k± from the experimental data is more precisely defined and the appearance of a “coherent” and “incoherent” regime of spin motion in a double log plot of k± vs. B is confirmed. The experimental decay curves have been reproduced by a full quantum dynamical model based on the stochastic Liouville equation, which was solved numerically, taking into account isotropic hyperfine coupling with five nuclear spins (1 N on donor radical, 4 H on acceptor radical) and anisotropic hyperfine coupling with the nitrogen nucleus at the donor radical. The results of the quantum calculations serve as a rigorous basis of interpreting the classical parameter k±. Furthermore, it is demonstrated that the incoherent part of spin motion is essential for a full understanding of the charge recombination kinetics even in the “coherent” regime.

The magnetic structure on the ground state of the equilateral triangular spin tube

The ground state of the frustrated equilateral triangular spin tube CsCrF4 is still hidden behind a veil though NMR spectrum broaden into 2 T at low temperature. In order to investigate the spin structure in an ordered state by 19F-NMR, we have determined the anisotropic hyperfine coupling tensors for each three fluorine sites in the paramagnetic state. The measurement field was raised up to 10 T to achieve highest resolution. The preliminary analysis using the obtained hyperfine tensors has shown that the archetypal 120°-type structure in ab-plane does not accord with the NMR spectra of ordered state.

Electron paramagnetic resonance study of gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2)

ABSTRACTIn this study, gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2) single crystals were investigated and the structure of the free radical occurring by γ-irradiation was determined by using Electron paramagnetic resonance (EPR) technique. C9H10O2 single crystals were produced by slow evaporation with distilled water solution at room temperature. These single crystals were irradiated by 60Co-gamma-ray source with a dose speed of 0.981 kGy/h at room temperature for 72 h. The EPR spectra of irradiated C9H10O2 single crystals were recorded between 125 and 400 K temperatures in steps of 10° intervals at different orientations in magnetic field with an X-band EPR spectrometer. The spectra were found to be slightly anisotropic and temperature independent; taking into consideration the chemical structure with experimental data we found only one free radical structure . The radical structure was simulated using the Bruker software WinEPR-Simfonia and the experimental g-value with anisotropic hyperfine coupling constants of radical were calculated as follows: g = 2.0124, = 26.51G, = 5.45G.

A [3Fe-3S](3+) cluster with exclusively μ-sulfide donors.

A [3Fe-3(μ-S)](3+) cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with an S = 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

π-Radical to σ-Radical Tautomerization in One-Electron-Oxidized 1-Methylcytosine and Its Analogs.

In this work, iminyl σ-radical formation in several one-electron-oxidized cytosine analogs, including 1-MeC, cidofovir, 2'-deoxycytidine (dCyd), and 2'-deoxycytidine 5'-monophosphate (5'-dCMP), were investigated in homogeneous, aqueous (D2O or H2O) glassy solutions at low temperatures by employing electron spin resonance (ESR) spectroscopy. Upon employing density functional theory (DFT) (DFT/B3LYP/6-31G* method), the calculated hyperfine coupling constant (HFCC) values of iminyl σ-radical agree quite well with the experimentally observed ones, thus confirming its assignment. ESR and DFT studies show that the cytosine iminyl σ-radical is a tautomer of the deprotonated cytosine π-cation radical [cytosine π-aminyl radical, C(N4-H)(•)]. Employing 1-MeC samples at various pHs ranging from ca. 8 to 11, ESR studies show that the tautomeric equilibrium between C(N4-H)(•) and the iminyl σ-radical at low temperature is too slow to be established without added base. ESR and DFT studies agree that, in the iminyl σ-radical, the unpaired spin is localized on the exocyclic nitrogen (N4) in an in-plane pure p-orbital. This gives rise to an anisotropic nitrogen hyperfine coupling (Azz = 40 G) from N4 and a near isotropic β-nitrogen coupling of 9.7 G from the cytosine ring nitrogen at N3. Iminyl σ-radical should exist in its N3-protonated form, as the N3-protonated iminyl σ-radical is stabilized in solution by over 30 kcal/mol (ΔG = -32 kcal/mol) over its conjugate base, the N3-deprotonated form. This is the first observation of an isotropic β-hyperfine ring nitrogen coupling in an N-centered DNA radical. Our theoretical calculations predict that the cytosine iminyl σ-radical can be formed in double-stranded DNA by a radiation-induced ionization-deprotonation process that is only 10 kcal/mol above the lowest energy path.

Complete Monitoring of Coherent and Incoherent Spin Flip Domains in the Recombination of Charge-Separated States of Donor-Iridium Complex-Acceptor Triads.

The spin chemistry of photoinduced charge-separated (CS) states of three triads comprising one or two triarylamine donors, a cyclometalated iridium complex sensitizer and a naphthalene diimide (NDI) acceptor, was investigated by transient absorption spectroscopy in the ns-μs time regime. Strong magnetic-field effects (MFE) were observed for two triads with a phenylene bridge between iridium complex sensitizer and NDI acceptor. For these triads, the lifetimes of the CS states increased from 0.6 μs at zero field to 40 μs at about 2 T. Substituting the phenylene by a biphenyl bridge causes the lifetime of the CS state at zero field to increase by more than 2 orders of magnitude (τ = 79 μs) and the MFE to disappear almost completely. The kinetic MFE was analyzed in the framework of a generalized Hayashi-Nagakura scheme describing coherent (S, T0 ↔ T±) as well as incoherent (S, T0 ⇌ T±) processes by a single rate constant k±. The magnetic-field dependence of k± of the triads with phenylene bridge spans 2 orders of magnitude and exhibits a biphasic behavior characterized by a superposition of two Lorentzians. This biphasic MFE is observed for the first time and is clearly attributable to the coherent (B < 10 mT) and incoherent (10 mT < B < 2 T) domains of spin motion induced by isotropic and anisotropic hyperfine coupling. The parameters of both domains are well understood in terms of the structural properties of the two triads, including the effect of electron hopping in the triad with two donor moieties. The kinetic model also accounts for the reduction of the MFE on reducing the rate constant of charge recombination in the triad with the biphenyl bridge.