Direct Observation of Charge Transfer and Magnetism in Fe₄Co₄ Cyanide-Bridged Molecular Cubes

We have studied the zero-dimensional cubane molecular correspondent of a Prussian blue analogue Cs-Fe₄Co₄ at low temperature and high magnetic field by means of L-edge X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We probe the magnetic and electronic structures of Fe and Co separately upon light irradiation, which allows us to observe directly the electron transfer coupled to a spin transition phenomenon within the molecular cubes and to investigate the nature of the metastable photoexcited state. The magnetic moments in the photoexcited state are found to be M = 1.3μ_B ( M_spin = 0.59μ_B with large orbital moment, M_orbit = 0.74μ_B) for low-spin Fe^III and M = 1.5μ_B ( M_spin = 1.08μ_B with orbital moment, M_orbit = 0.41μ_B) for high-spin Co^II at 2 K and 6.8 T. From our results, we evidence that a strong antiferromagnetic coupling between the metal ions can be ruled out.