Photoelectron Detachment and Solvated Electron Dynamics of the Cobalt(III) and Iron(III) Oxalato Complexes

Abstract

The photochemical redox reaction kinetics of [M(III)(C(2)O(4))(3)](3-) [M = Co, Fe] complexes have been reexamined and studied by time-resolved spectroscopy. The redox mechanisms of the two systems, Co and Fe, were found to be similar to each other, and solvated electrons were observed immediately after 266/267 nm photoexcitation. A reaction mechanism is proposed that involves photoelectron detachment as a primary process. The charge-transfer bands for both complexes, which had been attributed to ligand to metal charge-transfer (LMCT) transitions previously, are reassigned to charge transfer to solvent (CTTS) transitions.

Photochemistry and electron-transfer mechanism of transition metal oxalato complexes excited in the charge transfer band

“…Solvated electrons were observed only by a two-photon process using 400-nm photons or a one-photon process using 267-nm excitation (26). This reaction path has also been observed for trisoxalato cobaltate (III) with similar photon energy selectivity (27). These results suggest that the strong absorption in the CT band is at least partially caused by the charge transfer from the trisoxalato metalate complex to the solvent.…”

“…A 400-to 800-nm transient was generated by one-photon, 267-nm, excitation of trisoxalato cobaltate (III) and is assigned to solvated electrons (27) for the following reasons. (i) It has the shape, width, spectral range, and absorption maximum (41,42 molecule by a nanosecond concentration-dependent reaction 10 (see Table 2).…”

“…It is also to be noted that our time-resolved optical studies were aimed only at the cage intermediates (25) and not at the study of the expected out-of-cage photochemical intermediates that were observed later by time-resolved x-ray diffraction (24). Recently, we reported on the photochemistry of ferrioxalate in water by means of ultrafast transient optical and EXAFS spectroscopy and density functional theory (DFT)/unrestricted Hartree-Fock (UHF) calculations (26)(27)(28)(29). Previously, we reported that excitation of ferrioxalate with either 267/266 nm or 400/355 nm pulses results predominantly in Fe-O bond dissociation, concurrent with photoelectron detachment followed by electron solvation as a side reaction, rather than intramolecular ET (26,27,29).…”

“…Recently, we reported on the photochemistry of ferrioxalate in water by means of ultrafast transient optical and EXAFS spectroscopy and density functional theory (DFT)/unrestricted Hartree-Fock (UHF) calculations (26)(27)(28)(29). Previously, we reported that excitation of ferrioxalate with either 267/266 nm or 400/355 nm pulses results predominantly in Fe-O bond dissociation, concurrent with photoelectron detachment followed by electron solvation as a side reaction, rather than intramolecular ET (26,27,29). Direct intramolecular ET may take place with much lower efficiency.…”

Ultrafast time resolved x-ray diffraction, extended x-ray absorption fine structure and x-ray absorption near edge structure

“…16 Electron transfer in iron and cobalt liquid complexes, in solution, has also been studied using time resolved EXAFS with subpicosecond and sub-Angstrom resolution. 17,18 These studies determined that the mechanism of the electron transfer processes, initiated by photo-excitation in the charge transfer band, can be either intra or intermolecular depending on the structure of the molecule and degree of freedom of rotation of the ligand. In addition, several electron and x-ray ultrafast time resolved experiments have reported on the transient changes in the lattice structure of single crystals after excitation with an fs optical pulse.…”