Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.
Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
J Chem Phys. 2018 Jul 21;149(3):034103. doi: 10.1063/1.5038558.
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 T 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.
自由基对重组反应对施加的低磁场和强磁场都很敏感。弱磁场的施加会降低单重态自由基对的单重态产率，而强磁场的施加则会增加单重态产率。强磁场效应源于能量守恒：当磁场强度强于两个自由基中各超精细场的总和时，S→T 跃迁在能量上被禁止，从而减少了单重态向三重态转化的途径。弱磁场效应源于对称性破缺：施加弱磁场会消除零场本征态之间的简并，并增加单重态向三重态转化的途径。然而，该效应的细节更为复杂，以前并未得到适当解释。在这里，我们对含有单个质子的自由基对和其中一个自由基包含大量超精细耦合核自旋的自由基对中的弱磁场效应进行了完整的分析。我们发现，在两种情况下，当磁场打开时发生的新跃迁都是 S 和 T 之间的跃迁，而不是以前声称的 S 和 T 之间的跃迁。然后，我们通过使用它与半经典自旋动力学模拟相结合，说明了这一结果，以解释在光激发的类胡萝卜素-卟啉-富勒烯自由基对的时间依赖性存活概率的磁场效应中，随着磁场强度的增加，观察到的双相-三相-双相转变。
