Simulation of Ultrafast Excited-State Dynamics in Fe(II) Complexes: Assessment of Electronic Structure Descriptions

The assessment of electronic structure descriptions utilized in the simulation of ultrafast excited-state dynamics of Fe(II) complexes is presented. Herein, we evaluate the performance of the RPBE, OPBE, BLYP, B3LYP, B3LYP*, CAM-B3LYP, and LC-BLYP (time-dependent) density functional theory (DFT/TD-DFT) methods in full-dimensional trajectory surface hopping (TSH) simulations carried out on linear vibronic coupling (LVC) potentials. We exploit the existence of time-resolved X-ray emission spectroscopy (XES) data for the [Fe(bmip)2]2+ and [Fe(terpy)2]2+ prototypes for dynamics between metal-to-ligand charge transfer (MLCT) and metal-centered (MC) states, which serve as a reference to benchmark the calculations (bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, terpy = 2,2’:6’,2''-terpyridine). The results show that the simulated ultrafast population dynamics between MLCT and MC states with various spin multiplicilities (singlet, triplet, quintet) highly depend on the utilized DFT/TD-DFT method with the percentage of exact (Hartree-Fock) exchange being the governing factor. Importantly, B3LYP* is the only DFT/TD-DFT method reproducing all important aspects of the experimentally resolved dynamics for both complexes, signalling an optimal balance in the description of MLCT-MC energetics. This work demonstrates the power of combining TSH/LVC dynamics simulations with time-resolved experimental reference data to benchmark full-dimensional potential energy surfaces.