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Radical pair recombination reactions are known to be sensitive to the application of both low and high magnetic fields. The application of a weak magnetic field reduces the singlet yield of a singlet-born radical pair, whereas the application of a strong magnetic field increases the singlet yield. The high field effect arises from energy conservation: when the magnetic field is stronger than the sum of the hyperfine fields in the two radicals, S → T± transitions become energetically forbidden, thereby reducing the number of pathways for singlet to triplet interconversion. The low field effect arises from symmetry breaking: the application of a weak magnetic field lifts degeneracies among the zero field eigenstates and increases the number of pathways for singlet to triplet interconversion. However, the details of this effect are more subtle and have not previously been properly explained. Here we present a complete analysis of the low field effect in a radical pair containing a single proton and in a radical pair in which one of the radicals contains a large number of hyperfine-coupled nuclear spins. We find that the new transitions that occur when the field is switched on are between S and T0 in both cases, and not between S and T± as has previously been claimed. We then illustrate this result by using it in conjunction with semiclassical spin dynamics simulations to account for the observation of a biphasic-triphasic-biphasic transition with increasing magnetic field strength in the magnetic field effect on the time-dependent survival probability of a photoexcited carotenoid-porphyrin-fullerene radical pair. [ABSTRACT FROM AUTHOR]

The standard quantum mechanical expressions for the singlet and triplet survival probabilities and product yields of a radical pair recombination reaction involve a trace over the states in a combined electronic and nuclear spin Hilbert space. If this trace is evaluated deterministically, by performing a separate time-dependent wavepacket calculation for each initial state in the Hilbert space, the computational effort scales as O(Z² log Z), where Z is the total number of nuclear spin states. Here we show that the trace can also be evaluated stochastically, by exploiting the properties of spin coherent states. This results in a computational effort of O(MZ log Z), where M is the number of Monte Carlo samples needed for convergence. Example calculations on a strongly coupled radical pair with Z > 106 show that the singlet yield can be converged to graphical accuracy using just M = 200 samples, resulting in a speed up by a factor of > 5000 over a standard deterministic calculation. We expect that this factor will greatly facilitate future quantum mechanical simulations of a wide variety of radical pairs of interest in chemistry and biology. [ABSTRACT FROM AUTHOR]

The relations have been analytically derived for recombination yields of radical pairs with different precursor states and by different reaction channels (singlet and/or triplet) in arbitrary magnetic and microwave fields. Recombination of two particles from a radical pair mostly with anisotropic reactivity is assumed to take place in a narrow reaction zone, and relative translation motion including rotation of reactants may be an arbitrary stochastic process. In addition, the approach may take into account the time-dependent spin Hamiltonian. The novel method for the calculation of recombination yields is suggested, and general expressions for recombination yields are derived for strong singlet-triplet dephasing. Some of the relations and expressions are the basis of the results given in the literature and their generalization to the case of non-model reacting systems that does not use simplification of reactants structure and reactants motion in solutions. [ABSTRACT FROM AUTHOR]

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 kS) and S/T0 mixing (rate constant kST0) 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. [ABSTRACT FROM AUTHOR]

In the present work, the reaction operator for radical pairs of the main equation of spin chemistry has been modified on the basis of an exactly solvable model. The model contains both the reversible and irreversible transitions. For this model, expression for the reaction operator is more complex due to the expanded basis of spin states. The reaction operator of a phenomenological equation is a particular case of a more general approach and only in the limiting case of the completely irreversible recombination process, these operators coincide. [ABSTRACT FROM AUTHOR]

Abstract: Solution deposited bulk heterojunction organic solar cells are viewed as one of the most promising alternative energy sources because of their ease of processing and their potential to be produced using large scale techniques such as roll-to-roll, newspaper style, coating. Since organic materials have a relatively low dielectric constant the dissociation of an excited electron–hole pair into free collectable charge carriers is inefficient in many cases. Often the excited electron–hole pairs recombine back to the ground state in a process known as geminate recombination before they ever fully dissociate into free charge carriers. Even after dissociation, free holes and electrons can encounter each other once more and subsequently recombine back to the ground state in a process known as nongeminate recombination. In both cases the incident photon energy is lost and fewer carriers are collected at the electrodes. Hence, charge carrier recombination is one of the key loss mechanisms in organic solar cells. In this review the latest on geminate and nongeminate recombination is discussed. [Copyright &y& Elsevier]

Highlights: [•] We propose use of radical triads for verification of spin-chemistry master equation. [•] Spin-selective reaction of radicals B and C modifies spin state of radical pair AB. [•] Various forms of the master equation predict various spin state populations of AB. [Copyright &y& Elsevier]