Spin polarization generated by reversible doublet-quartet transitions in photoexcited chromophore-radical conjugates

Light-induced spin polarization can be produced in chromophore-radical conjugates by reversible transitions between the excited trip-doublet and trip-quartet states. The precise origin of this polarization is often difficult to elucidate because different transition pathways, promoted by different interactions, can occur depending on the nature of the conjugate. Moreover, the complexity of the expressions describing the evolution of the spin state populations and polarization generated by these transitions makes it difficult to estimate the dependence of the polarization on factors such as the exchange interaction and spin-orbit coupling. Here, we present a theoretical analysis and show that by making assumptions for specific cases, simplified expressions can be obtained that provide better insight into the physical origins of the polarization.