Photoinduced spin crossover in a Fe-picolylamine complex: A far-infrared study on single crystals

Far-infrared spectroscopy has been performed on [Fe(2-picolylamine)₃]Cl₂EtOH (Fe-pic) single crystals to probe changes in the molecular vibrations upon the photoinduced and temperature-induced spin crossovers. Synchrotron radiation has been used as the far-infrared source to overcome the strong absorption and the small sizes of the samples. Absorption lines due to FeN₆ cluster vibrations, observed below 4000.3em0excm^-1, show strong intensity variations upon the crossover due to the compression/expansion of FeN₆ between high-spin and low-spin states. However, they remain almost unchanged between the photo- and temperature-induced high-spin states. This is in sharp contrast to the lines at 500--7000.3em0excm^-1 due to intramolecular vibrations of the picolylamine ligands, which show marked variations between the two high-spin states. It is concluded that the most important microscopic difference between the two high-spin states arises from the ligands, which are likely to reflect different states of intermolecular bonding.