  Recent experiments reported that spin-state transitions were realized by
applying bias voltages. But these bias-induced spin state transitions (BISSTs)
are not fully understood, especially the mechanism. It is well known that the
metal-to-ligand charge transfer excitation (MLCT) and the metal-centered
excitation (MC) activated by light radiation can induce the transition from low
spin (LS) to high spin (HS) and that from HS to LS. Moreover, electronic
excitations are accessible by inelastic cotunneling in molecular junctions with
bias voltages applied. Based on these two experimental facts, we propose the
MLCT basically leads to the BISST from LS to HS, and the MC results in the
BISST from HS to LS. The rationality of the mechanism is demonstrated by
comparing first-principles results and experimental observations. The
calculated voltage threshold for activating the MLCT (MC) is close to the
experimental voltage for observing the BISST from LS to HS (from HS to LS). The
activation of MLCT (MC) depends on the bias polarity, which can explain the
bias-polarity dependence of BISST in the experiment. Our study is important for
further design of molecular spintronic devices working on spin state
transition.
