Tuning Excited State Electron Transfer in Fe Tetracyano-Polypyridyl Complexes

Abstract

We have investigated photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)${}_4$(2,2'-bipyridine)]${}^{2-}$ (1), [Fe(CN)${}_4$(2,3-bis(2-pyridyl)pyrazine)]${}^{2-}$ (2) and [Fe(CN)${}_4$(2,2'-bipyrimidine)]${}^{2-}$ (3) complexes in various solvents with static and time-resolved UV-visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering. We observe ${}^3$MLCT lifetimes from 180 fs to 67 ps over a wide range of MLCT energies in different solvents by utilizing the strong solvatochromism of the complexes. Intramolecular electron transfer lifetimes governing ${}^3$MLCT relaxation increase monotonically and (super)exponentially as the ${}^3$MLCT energy is decreased in 1 and 2 by changing the solvent. This behavior can be described with non-adiabatic classical Marcus electron transfer dynamics along the indirect ${}^3$MLCT->${}^3$MC pathway, where the ${}^3$MC is the lowest energy metal-centered (MC) excited state. In contrast, the ${}^3$MLCT lifetime in 3 changes non-monotonically and exhibits a maximum. This qualitatively different behaviour results from direct electron transfer from the ${}^3$MLCT to the electronic ground state (GS). This pathway involves nuclear tunnelling for the high-frequency polypyridyl skeleton mode ($\hslash \omega$ = 1530 cm${}^{-1}$), which is more displaced for 3 than for either 1 or 2, therefore making the direct pathway significantly more efficient in 3. To our knowledge, this is the first observation of an efficient ${}^3$MLCT->GS relaxation pathway in an Fe polypyridyl complex. Our study suggests that further extending the MLCT state lifetime requires (1) lowering the ${}^3$MLCT state energy with respect to the ${}^3$MC state and (2) suppressing the intramolecular distortion of the electron-accepting ligand in the ${}^3$MLCT excited state to suppress the rate of direct ${}^3$MLCT->GS electron transfer.