Quantum dynamics of the ultrafast heme-carbon monoxide photolysis and spin-crossover

The photochemistry of the heme complex with carbon monoxide (heme-CO) is known to exhibit ultrafast CO photolysis and a spin-crossover (SCO) transition within less than 100 femtoseconds. However, no unambiguous experimental evidence is available to date as to the spin state from which photolysis occurs. Here we present a first-principles quantum dynamical treatment of the ultrafast photochemistry of the heme-CO complex upon absorption to the lowest porphyrin state. Our simulations, for 179 electronic states and twelve vibrational modes, show that CO dissociation is complete within 15 fs, while Fe is still in the singlet manifold. The photolysis is dominated by vibrations inducing Jahn-Teller symmetry breaking that allow for a transition from porphyrin to metal-ligand charge-transfer states. Concomitant large amplitude nuclear motions allow for energy reorganization that makes the CO dissociation irreversible. On a slightly slower time scale as compared with CO photolysis, spin-orbit coupling induces a singlet-triplet transition in 70 fs and a triplet-quintet transition in 300 fs. The final quintet specie is entropically trapped in the excited state band. These findings illustrate the central role of ligand vibronic couplings in modulating the photodynamics of organometallic spin-crossover complexes.