Study of the Light-Induced Spin Crossover Process of the [Fe-II(bpy)(3)](2+) Complex

Ab initio calculations have been performed on [Fe-II(bpy)(3)](2+) (bpy=bipyridine) to establish the variation of the energy of the electronic states relevant to light-induced excited-state spin trapping as a function of the Fe-ligand distance. Light-induced spin crossover takes place after excitation into the singlet metal-to-ligand charge-transfer (MLCT) band. We found that the corresponding electronic states have their energy minimum in the same region as the low-spin (LS) state and that the energy dependence of the triplet MLCT states are nearly identical to the (MLCT)-M-1 states. The high-spin (HS) state is found to cross the MLCT band near the equilibrium geometry of the MLCT states. These findings give additional support to the hypothesis of a fast singlet triplet interconversion in the MLCT manifold, followed by a (MLCT)-M-3-HS (T-5(2)) conversion accompanied by an elongation of the Fe-N distance.