Elucidating the Transition Kernel and Anharmonic Coupling in the Spin-crossover Process of a [Fe

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.