Photochemical and Photophysical Dynamics of the Aqueous Ferrate(VI) Ion

Abstract

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as "green" alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications.

Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

“…Although ISC is often ultrafast even for 3d transition metal complexes [44][45][46] , it can also be rather slow in the picosecond and even nanosecond range. [47][48][49][50][51][52][53] After the population of the 2 LMCT/ 2 MC state is completed, the shape of the spectrum remains basically unchanged, besides minor shifts and a small increase of the intensity of the newly formed band (2 = 9.7 ps), which we attribute to thermal relaxation and reorganization of the solvent cage (VR, vibrational relaxation) due to the substantial change of the dipole moment during the LMCT transition. Hence, the slow final decay to the quartet ground state (3 = 1.6 ns) occurs from the thermally relaxed 2 LMCT/ 2 MC state.…”

“…Furthermore, an intersystem crossing (ISC) process 43 is required to reach the long-lived 3 MLCT states, which is often fast and efficient for transition metal complexes, even for first row metals, [44][45][46] yet in certain cases also slow and inefficient ISC processes have been reported. [47][48][49][50][51][52][53] In the present study we demonstrate that a mixed pyridine/guanidine complex with manganese(IV) as earth-abundant metal ion can be excited with low-energy NIR light to give two photoactive states, namely an unconventional, luminescent and long-lived mixed doublet ligand-to-metal charge transfer/metal-centered ( 2 LMCT/ 2 MC) excited state with some spin-flip admixture at lower energy capable of oxidizing moderately challenging substrates and a short-lived 4 LMCT excited state at high energy capable of oxidizing even extremely challenging substrates.…”

Enhancing Ferrate Oxidation of Micropollutants via Inducing Fe(V)/Fe(IV) Formation Needs Caution: Increased Conversion of Bromide to Bromate

“…Besides, Fe(V) and Fe(IV) were detected during the self-decay of Fe(VI) in water. 7,13,15,16 Fe(V) and Fe(IV) were reported to be two to six orders of magnitude more reactive than Fe(VI), and their participation in micropollutants removal highly enhances the performance of the Fe(VI) oxidation process. 3,13,17 Therefore, researchers are currently focusing on applying different strategies to activate Fe(VI) in aqueous solutions by selectively generating highly reactive intermediates such as Fe(V) and Fe(IV).…”

Ten-Fold Solvent Kinetic Isotope Effect for the Nonradiative Relaxation of the Aqueous Ferrate(VI) Ion

“…The optical absorption spectrum of ferrate­(VI) (Figure S1A) was extensively studied throughout the latter half of the 20th century. − We recently reviewed that foundational work and reported the first time-resolved measurements of the excited state dynamics of the ion using a combination of optical and X-ray spectroscopies . Briefly, the ultraviolet (UV) absorption bands of ferrate­(VI) are comprised of ligand-to-metal charge transfer (LMCT) transitions, while the visible absorption band centered at 560 nm corresponds to two clusters of 3 A 2 → 3 T 1 ligand field (or metal-centered, MC) transitions .…”

“…Briefly, the ultraviolet (UV) absorption bands of ferrate­(VI) are comprised of ligand-to-metal charge transfer (LMCT) transitions, while the visible absorption band centered at 560 nm corresponds to two clusters of 3 A 2 → 3 T 1 ligand field (or metal-centered, MC) transitions . Following UV excitation into the 3 LMCT states, branching between the intersystem crossing (ISC) and internal conversion (IC) pathways occurs on the time scale of tens of ps, with only ∼15% of the population undergoing ISC to reach the 5 LMCT state from which the highly oxidizing metastable [FeO 3 OH] 2– species forms; the remainder relaxes to the ground state through the manifold of 3 MC states . Crucially for the work presented here, we also found that ground state recovery occurs nonradiatively from the 3 T 2 MC state with a time constant of 3.8 ns.…”