Resolving Femtosecond Solvent Reorganization Dynamics in an Iron Complex by Nonadiabatic Dynamics Simulations

Abstract

The ultrafast dynamical response of solute−solvent interactions plays a key role in transition metal complexes, where charge transfer states are ubiquitous. Nonetheless, there exist very few excited-state simulations of transition metal complexes in solution.Here, we carry out a nonadiabatic dynamics study of the iron complex [Fe(CN) 4 (bpy)] 2− (bpy = 2,2′-bipyridine) in explicit aqueous solution. Implicit solvation models were found inadequate for reproducing the strong solvatochromism in the absorption spectra. Instead, direct solute−solvent interactions, in the form of hydrogen bonds, are responsible for the large observed solvatochromic shift and the general dynamical behavior of the complex in water. The simulations reveal an overall intersystem crossing time scale of 0.21 ± 0.01 ps and a strong reliance of this process on nuclear motion. A charge transfer character analysis shows a branched decay mechanism from the initially excited singlet metal-to-ligand charge transfer ( 1 MLCT) states to triplet states of 3 MLCT and metalcentered ( 3 MC) character. We also find that solvent reorganization after excitation is ultrafast, on the order of 50 fs around the cyanides and slower around the bpy ligand. In contrast, the nuclear vibrational dynamics, in the form of Fe−ligand bond changes, takes place on slightly longer time scales. We demonstrate that the surprisingly fast solvent reorganizing should be observable in time-resolved X-ray solution scattering experiments, as simulated signals show strong contributions from the solute−solvent scattering cross term. Altogether, the simulations paint a comprehensive picture of the coupled and concurrent electronic, nuclear, and solvent dynamics and interactions in the first hundreds of femtoseconds after excitation.

Solvent dynamics of aqueous halides before and after photoionization

“…Even though for the atoms, a comparison with the experiment is not possible due to lack of data, the general ability of QM/MM methods to describe radical-water bonding, together with the good agreement of this method with the ions' experimental EXAFS spectra, suggests that the QM/MM-based EXAFS spectra are reliable also for the solvated-atom case. Thus, these simulations can likely provide a basis for further studies towards nonequilibrium solvent dynamics following photoionization (ideally taking into account nonadiabatic effects [117][118][119][120][121] ), which should be favored by the clearly distinctive EXAFS spectra for ions and atoms found in our simulations. Such studies may later be extended towards electron transfer processes in more complex systems, with the eventual goal of including biological activity in physiological media.…”

Solvent Dynamics of Aqueous Halides before and after Photoionization

“…Even though for the neutral atoms, a comparison with the experiment is not possible due to lack of data, the general ability of QM/MM methods to describe radical–water bonding, together with the good agreement of this method with the ions’ experimental EXAFS spectra, suggests that the QM/MM-based EXAFS spectra are reliable also for the solvated neutral atoms. Thus, these simulations can likely provide a basis for further studies toward nonequilibrium solvent dynamics following photoionization (ideally taking into account nonadiabatic effects − ), which should be favored by the clearly distinctive EXAFS spectra for ions and neutral atoms found in our simulations. Such studies may later be extended toward electron transfer processes in more complex systems, with the eventual goal of including biological activity in physiological media.…”

Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials

“…This variation of E MLCT is well documented for chargetransfer states in chromophores bearing cyano li-gands 13,16,17,39,40 and likely results from Lewis acid−base interactions between the solvent and the basic nitrogen of the bound cyanide, as recently described computationally for [Fe(bpy)(CN) 4 ] 2− in explicitly modeled H 2 O. 41 Plots of the excited-state lifetime versus other solvent parameters such as dipole moment, dielectric constant, and viscosity did not reveal any clear trends (Figure S26).…”

“…Highlevel ab initio methods including explicit solvent, combined with quantum dynamics calculations, will be needed to confirm this mechanistic hypothesis. 41 In summary, 1 is distinct from mixed polypyridyl/cyano iron(II) chromophores in that the maximum lifetime achieved is 1.25 ns in CHCl 3 , comparable to the MLCT states of stateof-the-art Fe II chromophores (e.g., Figure 1, A−C). 9−11 In contrast, the longest MLCT lifetime achieved within a polypyridyl/cyano framework is 67 ps.…”