Theoretical investigation of multi-spin excited states of anthracene radical-linked -conjugated spin systems by computational chemistry

Multi-spin excited states of chromophore radical-linked pi-conjugated spin systems are investigated by molecular orbital calculations based on density functional theory (DFT). The investigated systems consist of an anthracene photosensitive unit leading to a triplet-excited-state (S = 1), pi-conjugated linker to propagate spin exchange-coupling, and stable organic radical with a doublet-ground-state (S = 1/2). The intramolecular exchange coupling (JDQ), g value, and fine-structure interaction of their excited states depended on the pi-conjugation network (pi-topology), type of radical, and molecular structure of the pi-linker (length and dihedral angle). The exchange interaction was dependent on the pi-topology and the type of radical species. A decrease in the dihedral angle between the anthracene moiety and phenyl linker in the photo-excited state led to larger exchange coupling. With an increase in the pi-linker length (r), the magnitude of the exchange coupling gradually decreased in the photoexcited states according to JDQ = JEx0 exp(-beta r), similar to the ground-state exchange. The g values of the quartet (Q) state depended only on the radical type (independent of the linker). Conversely, the fine-structure interaction of the Q state was independent of the radical type and depended on both the linker length and the dihedral angle. Exchange interactions, g-values, and fine-structure splitting in the multi-spin excited states of anthracene radical-linked pi-conjugated spin systems are calculated by DFT method. Dependence on radical species, pi-topology, and linker are revealed.