Spin trapping and flipping in FeCO through relativistic electron dynamicsElectronic supplementary information (ESI) available. See DOI: 10.1039/c8cp06583g

Transition metal compounds are very versatile, and their characteristics can differ profoundly depending on their electronic structure. Compounds in which a spin transition from a low-spin to a high-spin state can be achieved through means of an optical excitation are particularly intriguing, as a controlled spin-flip opens promising avenues in areas such as sensing, information technology, molecular switches and energy technology. The fundamental mechanisms in spin crossover and spin transitions remain unanswered, due to the complexity of electronic structure and interplay of relativistic effects. Presented here is a new approach that allows the first direct study of spin flip dynamics through a mapping of spin-mixed to spin-pure states. The method is applied to FeCO and addresses the spin-flip dynamics during a spin transition. Wave packets that combine different spin states are generated through optical excitation and relevant mechanisms in optically triggered spin transitions are discussed. Electron dynamics of spin-state conversion compounds. Excited triplet and quintet states are significantly spin-mixed - transitions can be induced easily: "channels" that enable spin flipping. The lowest-lying quintet state acts as a "sink": exhibits weak coupling.