Three-State Switching of an Fe Spin Crossover Complex

Isolated tetramers of the spin crossover compound Fe(H2B-(pyrazole)(pyridylpyrazole))2were investigated on Ag(111) at cryogenictemperature (5 K) with a scanning tunneling microscope (STM) in ultrahighvacuum. In addition to spin states obtained by electron-induced spincrossover, a third state of the complexes is discovered with different current-voltage characteristics and intramolecular image contrast. It is stable over alimited range of voltages beyond which it decays into the low- and high-spinstates of the Fe complex. The state may be prepared in a deterministicmanner that enables measurements of its decay dynamics at various currentsand voltages. We interpret this state as being due to a negatively charged Fecomplex in its high-spin state, which is isoelectronic to the analogous Cocompound.