Role of long-range interaction in photoinduced spin-state transitions in spin-crossover complexes

We explain theoretically the peculiarities of photoinduced low-spin (LS)-->high-spin (HS) transitions in spin-crossover complexes, such as the threshold behavior in the excitation light intensity for phase conversion and the existence of the incubation period. Our investigation is based on a model where a single complex is described by the two electronic states and the breathing oscillation mode with the long-range interaction among the complexes. As the spin transition processes, we take into account the LS-->HS photoexcitation process and the HS-->LS nonradiative decay process. The rate for the latter process is sensitive to the LS fraction in the crystal because the potential barrier for the HS-->LS decay is dependent on the LS fraction through the long-range interaction among the complexes; the lifetime of the metastable HS state becomes longer as the LS fraction decreases. Such sensitivity leads to the nonlinear temporal evolution for the LS fraction, which coincides with the experimental results.