Quantum oscillations in radical pairs: The spin dynamics of photoinduced radical pairs, involving an interconversion between singlet and triplet spin states, plays an important role in nature, for example, in avian magnetoreception. The spin interconversion is a truly quantum process with characteristic coherent oscillations (quantum beats) that should be reflected in the reaction kinetics. However, their experimental observation has remained challenging. Mims et al . developed an optical readout technique that can directly monitor the singlet-triplet interconversion quantum beats, as demonstrated for a photoinduced, charge-separated state of an electron donor–acceptor dyad (see the Perspective by Hore). The present work opens a new way to monitor the spin evolution in radical pairs, which will be important not only in biological physics but also in organic solar cells and other practical applications. —YS

The main concern of the present work is consideration of sterically specific liquid-phase reactions in the case where one of the reactants has several active sites. In kinematic approximation we derived compact formulas for partial rate constants of individual active sites. Reaction rates are expressed via the kinetic rate constants and convolutions of the free Green functions over reaction zones. These results are valid for arbitrary geometry of reactants and active sites. Effects of mutual influence of the active sites were studied, their simple estimate was proposed.

Previous transient absorption measurements using the magnetically affected reaction yield (MARY) technique for a series of rigidly linked electron donor/electron acceptor dyads (D-X-A) consisting of a triarylamine donor, a naphthalene diimide acceptor, and a meta-conjugated diethynylbenzene unit as a bridge had revealed the presence of electronic exchange interaction, J, in the photoexcited charge separated (CS) state. Here, we present results obtained by photochemically induced dynamic nuclear polarization (photo-CIDNP) that allows for determining the sign of J. By variation of the magnetic field from 1 mT to 9.4 T, pronounced absorptive maxima of CIDNP were detected for more than 20 ¹H nuclei disregarding the sign of their hyperfine coupling constants in the transient charge separated state, with positions of maxima close to those found by the MARY technique. Quantitative comparison of the observed CIDNP signals for various D-X-A dyads reveals an increase in the CIDNP enhancement factor with increasing population of the triplet state determined by MARY spectroscopy at zero magnetic field. For CIDNP of the methyl groups of the TAA donor dyads, we found in all studies a good linear dependence between the CIDNP signal amplitude and the initial population of the CS triplet state. The linear relationship together with the absorptive CIDNP allows us to conclude that (i) the sign of the electronic exchange interaction J_ex is positive, (ii) CIDNP is formed predominantly in the vicinity of level anticrossing between the T₊ and S electronic levels, and (iii) coherent triplet-singlet transitions are induced by hyperfine interaction and accompanied by simultaneous electron and nuclear spin flip, $\left|T_{+}\beta \right.⟩\to \left|S\alpha \right.⟩$ .

Charge recombination in the photoinduced charge separated (CS) state of a rigidly linked donor/bridge/acceptor triad with a triarylamine (TAA) donor, a 1,3-diethynyl-2,5-dimethoxy benzene bridge (OMe), and a perylenediimide (PDI) unit as an acceptor, represents a spin chemical paradigm case of a rigid radical ion pair formed with singlet spin and recombining almost exclusively to the locally excited PDI triplet state (³PDI). The magnetic field dependence of the CS state decay and ³PDI formation kinetics are investigated from 0 to 1800 mT by nanosecond laser flash spectroscopy. The time-resolved magnetic field affected reaction yields spectra of the CS state population and ³PDI population exhibit a sharp and deep resonance at 18.9 mT, indicating level crossing of the S and T₊ levels separated by an exchange interaction of J = 18.9/2 mT at zero field. The kinetics are biexponential around the resonance field and monoexponential outside that range. The monoexponential behavior can be simulated by a classical kinetic model assuming a single field dependent double Lorentzian function for the energy gap dependence of all spin conversion processes. The full field dependence of the kinetics has been simulated quantum theoretically. It has been shown that incoherent and coherent hyperfine coupling contribute to S/T₊ spin conversion at all fields and that the biexponentiality of the kinetics at resonance is due to a partitioning of the overall kinetics into 2/3 of the singlet hyperfine states exhibiting strong isotropic coupling to T₊ and 1/3 of the singlet hyperfine states that do not or only weakly couple isotropically to T₊.

Integral encounter theory is applied to investigate spin effects in radical recombination. Effects of electron spin relaxation and of S/T₀ mixing by an external magnetic field are considered. An analytical expression for the stationary rate constant is obtained from which appropriate conditions for experimental investigations are predicted.