Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2′-bipyridine)(CN)₄]²⁻

Abstract

The excited state dynamics of solvated [Fe(bpy)(CN)], where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)] has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile. In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)] in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)] allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated MC intermediate provides a strong foundation for identifyingMC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.

Towards Noble‐Metal‐Free Dyads: Ground and Excited State Tuning by a Cobalt Dimethylglyoxime Motif Connected to an Iron N‐Heterocyclic Carbene Photosensitizer

“…Figure ), an additional destabilization of the MC levels in relation to the ground state seems plausible. A similar solvent‐dependent effect was observed for [Fe(bipy)(CN) 4 ] 2– by Kjær et al, and could also occur for the complexes discussed here. Therefore, this observation will be investigated in the future with time resolved X‐ray studies in different solvents and complemented with excited state calculations.…”

“…Iron is considered as a sustainable and earth‐abundant element. However, iron complexes are characterized by comparatively low ligand field splitting, which results in energetically low‐lying metal centered (MC) states , . As a consequence, the metal‐to‐ligand charge‐transfer states (MLCT) are short‐lived with fast non‐radiative deactivation pathways into relatively long‐living MC states.…”

NHC-Based Iron Sensitizers for DSSCs

“…In a recent work on [Fe(2,2 -bipyridine) 2 (CN) 2 ] complexes, the relaxation of 3 MLCT into metal-centered triplet states 3 MC with several-picosecond lifetimes was revealed [32,33]. This should lead, as for the case of the 5 T 2 state in Fe(2,2 -bipyridine) 3 , to a GSB recovery retarded with respect to the 3 MLCT ESA decay.…”

Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer

“…[24] Gaffney and co-workers determined the lowest MLCT state of [Fe(bpy)(CN) 4 ] 2À in water to be energetically above metal-centered states. [25] Spin crossover from the initially photo-excited 1 MLCT to the 3 MLCT state therefore mediates femtosecond relaxation to a 3 MC state.InDMSO on the other hand, the stabilization of MLCT excitation energies results in the 3 MLCT state being the lowest valence-excited state exhibiting an extended lifetime of 19 ps. [8] Figure 1c shows the transient L 3 -edge absorption difference spectrum of [Fe(bpy)(CN) 4 ] 2À in DMSO compared to its steady-state ground state spectrum (see Supporting Information for experimental details and measurements in water).…”