Fluorescence-detected magnetic resonance in organic systems: A pair-density matrix formalism approach

A theoretical approach, based on a pair-density matrix formalism, is developed for the study of the dynamic triplet-pair annihilation in organic materials. Stochastic Liouville equation, taking into account effects of static and dynamic magnetic fields, with superoperators representing coherent evolution, spin-independent annihilation rate, spin-dependent recombination, and diffusion of triplet exciton pairs, is used. This approach is applied to analyze fluorescence detected magnetic resonance (FDMR) spectra of one-dimensional and two-dimensional exciton motion systems. The nearest-neighbor and the long-range mutual annihilations of triplets are taken in account, and the ${S}_{0}{Q}_{0}$ mixing, in triplet-pair states, is pointed out. The long-range annihilation rate $\ensuremath{\lambda}$ and the triplet effective decay rate $\ensuremath{\beta}$ are determined from the best fit with experimental FDMR spectra.