Elucidating the Transition Kernel and Anharmonic Coupling in the Spin‐crossover Process of a [Fe ^III (qsal) ₂ ] CH ₃ OSO ₃ Complex

ABSTRACT A spin‐crossover (SCO) process involves a change in the spin‐state, affecting the spatial distribution of electron density through spin‐orbit coupling. SCO can be understood as the interplay of anharmonically coupled vibrational modes that collectively drive the system across curve‐crossings. However, these modes are difficult to identify due to challenges in simulating open‐shell systems. Here, we combine ultrafast broadband transient absorption spectroscopy in single crystals with multireference excited‐state dynamical simulations to reveal the SCO mechanism in an Fe(III) complex. We identify the key doorway modes that direct the system across the curve‐crossing region to form the high‐spin state. The pronounced anharmonicity and reactive forces at SCO curve crossings provide a strong driving force for these displaced modes, leading to phase‐delayed, coherent non‐impulsive vibrational energy transfer. This study leads to unprecedented direct visualization of the SCO dynamics, revealing how the transition kernel and low‐dimensional pathways emerge from the strongly anharmonic crossing regions of the potential energy surfaces. A detailed understanding of these SCO processes is crucial for the development of advanced materials with applications ranging from high‐speed memory storage to light‐harvesting devices.