The electronic excited
states of the iron(II) complex [Fe
II
(tpy)(pyz-NHC)]
2+
[tpy = 2,2′:6′,2″-terpyridine; pyz-NHC
= 1,1′-bis(2,6-diisopropylphenyl)pyrazinyldiimidazolium-2,2′-diylidene]
and their relaxation pathways have been theoretically investigated.
To this purpose, trajectory surface-hopping simulations within a linear
vibronic coupling model including a 244-dimensional potential energy
surface (PES) with 20 singlet and 20 triplet coupled states have been
used. The simulations show that, after excitation to the lowest-energy
absorption band of predominant metal-to-ligand charge-transfer character
involving the tpy ligand, almost 80% of the population undergoes intersystem
crossing to the triplet manifold in about 50 fs, while the remaining
20% decays through internal conversion to the electronic ground state
in about 300 fs. The population transferred to the triplet states
is found to deactivate into two different regions of the PESs, one
where the static dipole moment is small and shows increased metal-centered
character and another with a large static dipole moment, where the
electron density is transferred from the tpy to pyz-NHC ligand. Coherent
oscillations of 400 fs are observed between these two sets of triplet
populations, until the mixture equilibrates to a ratio of 60:40. Finally,
the importance of selecting suitable normal modes is highlighted—a
choice that can be far from straightforward in transition-metal complexes
with hundreds of degrees of freedom.