Locking and Unlocking of the Molecular Spin Crossover Transition*

The electronic state of Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine) has been investigated by X-ray absorption spectroscopy (XAS). Locking in a largely low-spin-state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K, to well above room temperature, has been found in both nanometer thin films (by substrate interactions) and powder (by mixing with strongly dipolar zwitterions). The XAS clearly shows the change of iron L₂ edge spectra, typically associated with thermal induced spin crossover. Remarkably, incident X-ray fluences then restore the complex moiety to a high spin state at temperatures of 200 K to above room temperature, well above the spin crossover transition temperature for the pristine powder, and this is an activated process, as will be discussed in this presentation. Relaxation back to the initial locked state can be accomplished by heating slightly above room temperature.