The light-induced spin transition of tetranuclear spin crossover complex [Fe4(CN)4(bpy)4(tpa)2](PF6)4

We report on the light induced spin transition in the tetranuclear spin crossover complex [Fe4(CN)4(bpy)4(tpa)2](PF6)4. The photo-conversion occurs at the specific site (Fe2) of four FeII ions. The red light irradiation (1.79 eV) gives rise to full conversion of Fe2 into the high spin state from the low spin state. The green light irradiation (2.33 eV) can convert only the half of Fe2 into the high spin state, though the photo-conversion rate in the beginning is much higher than that with the red light. We present a simple model in which the photo-conversion kinetics is controlled by a large background absorption due to remaining three FeII ions (Fe1, Fe3 and Fe4).