J-Resonance Line Shape of Magnetic Field-Affected Reaction Yield Spectrum from Charge Recombination in a Linked Donor–Acceptor Dyad

Abstract

Magnetic field effects (MFEs) allow detailed insight into spin conversion processes of radical pairs that are formed, for example, in all charge separation processes, and are supposed to play the key role in avian navigation. In this work, the MFE of charge recombination in the charge-separated state of a rigid donor–bridge–acceptor dyad was analyzed by a classical and a quantum theoretical model and represents a paradigm case of understanding spin chemistry with unprecedented detail. The MFE is represented by magnetic field-affected reaction yield (MARY) spectra that exhibit a sharp resonance, resulting from S/T level crossing as the Zeeman splitting equals twice the exchange interaction. Although in the classical kinetic model, the spin conversion processes between the four singlet and triplet substates are shown for the first time to obey an identical generalized energy dependence, quantum theory proves that the MARY resonance line is composed of relaxation, coherent hyperfine induced spin mixing, and S/T dephasing contributions.

Radical pair intersystem crossing: Quantum dynamics or incoherent kinetics?

“…Higher order truncations of the kernel in Eq. (7) and approximate resummations of these higher order terms could in principle be used to obtain master equations valid beyond the perturbative limit, 16,20 however the resulting rate constants would have a significantly more complex functional form than that proposed by Steiner et al [Eq. (1)].…”

“…1 (B). [3][4][5][6][7][8] One particular model proposed recently by Steiner et al employs the following functional form for the spin state interconversion rates, [5][6][7] k nm = k hfc 1 + ( n − m ) 2 /γ 2 hfc + k rel 1 + ( n − m ) 2 /γ 2 rel + k 0 , (1) in which k hfc , k rel , k 0 , γ hfc and γ rel are free parameters and n is the energy of the coupled electronic spin state |n = |S , |T + , |T 0 , or |T − in the absence of hyperfine interactions. 9 Here the first term represents the isotropic hyperfine contribution to the interconversion and the second represents the spin relaxation contribution.…”

“…This ansatz has been used successfully to interpret the magnetic field effects on radical pair survival probabilities in several sets of experiments. [5][6][7] A similar expression for the hyperfine mediated intersystem crossing rate has previously been arrived at by applying the steady-state approximation to the coherences in a simple two-state model of the radical pair spin states. [10][11][12][13] At a glance, the coherent quantum dynamics approach and the kinetic approach appear to be fundamentally different.…”

Electron spin relaxation in radical pairs: Beyond the Redfield approximation

“…It should be noted that this version of Redfield theory explicitly includes the effect of asymmetric radical pair recombination (k S k T ) on the relaxation processes, although to the best of our knowledge, asymmetric recombination is normally ignored in evaluating this superoperator. 23,24,26 It can be easily verified that our R RF reduces to the standard Redfield superoperator in the case of symmetric recombination (k S = k T ).…”

On the magnetosensitivity of lipid peroxidation: two- versus three-radical dynamics

“…In diradicals in solution, i.e., for averaged electron-electron dipolar interactions but relatively constant exchange interaction, characteristic MFEs can result from the S/T AE -crossing. 70,71 For immobilized radical pairs, similar degeneracies of the electronic terms can ensue as a function of the dipolar coupling and the external magnetic field. 72 Here, the exchange coupling could compensate for the non-zero electronelectron dipolar interaction, thereby boosting MFEs and their anisotropies at low magnetic fields.…”