Fig. 1.

Photodynamics of photolysis and spin-crossover. Quantum photodynamics of heme–CO complex during the first 0.5 ps, with initial conditions averaged over 10 molecular dynamics snapshots. a Evolution of diabatic populations for states ¹Q (magenta), ¹MLCT (orange), triplet band (green) and quintet band (blue). The ¹Q population rapidly decays giving rise to ¹MLCT population dominating by 75 fs, at which point the triplet population increases. The quintet population builds up more slowly, and evolves into the dominant state at around 350 fs. b Schematic representation of the reaction mechanism and interpretation in terms of time constants. Upon initial excitation to the Q-band, the metal-to-ligand charge transfer (MLCT) state is populated in ∼25 fs. In a second step, the system relaxes to the triplet (∼75 fs) and to the lowest quintet state (∼430 fs). Black arrows indicate the direction of the electron transfer and the main nuclear motions. c Evolution of the Fe–C(O) distance (magenta, left axis) and the Fe out-of-plane distance (black, right axis). Large amplitude motions are observed with a period of oscillation of 40 fs. The amplitude of oscillation is initially 0.9 Å and converges towards a value of 2.2 Å. At this distance, the CO is essentially photolyzed. The standard deviation of these geometric values is shown as a shaded area. In the inset, the Fourier transform of the Fe–C(O) oscillations is shown (in cm⁻¹)