Transient Structures and Kinetics of the Ferrioxalate Redox Reaction Studied by Time-Resolved EXAFS, Optical Spectroscopy, and DFT

Abstract

The photoredox reaction transients of ferrioxalate in water have been studied by means of time-resolved EXAFS and ultrafast optical transient spectroscopy. The transient spectra and kinetics have been measured from the femtosecond to millisecond range, and the Fe-O bond lengths of the ferrioxalate redox reaction transients have been determined with 2 ps time resolution and 0.04 A accuracy. These data in conjunction with quantum-chemistry DFT and UHF calculations were used to formulate a mechanism for the Fe(III) to Fe(II) redox reaction where dissociation precedes electron transfer. In addition, radical scavenging experiments support the mechanism proposed.

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

“…Direct intramolecular ET may take place with much lower efficiency. 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. In the case of photodissociation, which we found to be the dominant reaction, we did not observe a significant difference in the kinetics and mechanism by exciting ferrioxalate either in the CT band, with 267-nm femtosecond (fs) pulses or the crossing point of the CT and LF band, with 400-nm fs pulses (26). These data indicate that excitation in the CT band does not necessarily yield intramolecular ET but rather is in competition with other reaction paths such as dissociation.…”

“…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.…”

“…In addition, we have performed DFT (B3LYP/6-31G), quantum chemical, and Hartree-Fock (H-F/6-31G) calculations that provide supporting information that has helped us to elucidate the mechanism of the photoredox reaction of trisoxalato cobaltate (III) in aqueous solution. The experimental results observed previously (26,29) for ferrioxalate are also considered and compared with the trisoxalato cobaltate(III) to deduce a rather general mechanism of the photochemistry and ET of metal trisoxalato complexes. pulses; 0.6 ps, 6.6-8.6 KeV x-ray pulses were used as the x-ray continuum probe pulses for time-solved transient structure EXAFS experiments.…”

Synthesis and characterization of some new ruthenium (II) complexes as photosensitizers in dye-sensitized solar cells

“…The reduction of oxidized sensitizer molecules occurs in the ms or µs range, which in by several orders of magnitude slower than injection rate. Also, the transient state formed by ] was found to reached its maximum intensity 2.6 ps after excitation then remains constant for at least 4ns [40]. Upon comparison of the times mentioned above, it is clear that the ferrioxalate complex will play an enhancement effect of the performance of the DSSCs if there is a good interaction between it and the sensitizer.…”