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 3MLCT 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 3MLCT relaxation increase monotonically and (super)exponentially as the 3MLCT 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 3MLCT->3MC pathway, where the 3MC is the lowest energy metal-centered (MC) excited state. In contrast, the 3MLCT lifetime in 3 changes non-monotonically and exhibits a maximum. This qualitatively different behaviour results from direct electron transfer from the 3MLCT to the electronic ground state (GS). This pathway involves nuclear tunnelling for the high-frequency polypyridyl skeleton mode (ω = 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 3MLCT->GS relaxation pathway in an Fe polypyridyl complex. Our study suggests that further extending the MLCT state lifetime requires (1) lowering the 3MLCT state energy with respect to the 3MC state and (2) suppressing the intramolecular distortion of the electron-accepting ligand in the 3MLCT excited state to suppress the rate of direct 3MLCT->GS electron transfer.
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