Dynamics of light-induced excited state quenching of ferrioxalate complexes by peroxides. Fast kinetics events and interaction with toxic pollutants

Field Evidence of Fe-Mediated Photochemical Degradation of Oxalate and Subsequent Sulfate Formation Observed by Single Particle Mass Spectrometry

In this work, we deployed a single particle aerosol mass spectrometer (SPAMS) at a suburban coastal site in Hong Kong from February 04 to April 17, 2013 to study individual oxalate particles and a monitor for aerosols and gases in ambient air (MARGA) to track the bulk oxalate concentrations in particle matter smaller than 2.5 μm in diameter (PM_2.5). A shallow dip in the bulk oxalate concentration was consistently observed before 10:00 am in the morning throughout the observation campaign, corresponding to a 20% decrease in the oxalate concentration on average during the decay process. Such a decrease in PM oxalate was found to be coincident with a decrease in Fe-containing oxalate particles, providing persuasive evidence of Fe-mediated photochemical degradation of oxalate. Oxalate mixed with Fe and Fe_NaK particles, from industry sources, were identified as the dominant factors for oxalate decay in the early morning. We further found an increase of sulfate intensity by a factor of 1.6 on these individual Fe-containing particles during the oxalate decomposition process, suggesting a facilitation of sulfur oxidation. This is the first report on the oxalate-Fe decomposition process with individual particle level information and provides unique evidence to advance our current understanding of oxalate and Fe cycling. The present work also indicates the importance of anthropogenic sourced iron in oxalate-Fe photochemical processing. In addition, V-containing oxalate particles, from ship emissions, also showed evidence of morning photodegradation and need further attention since current models rarely consider photochemical processing of oxalate_V particles.

Tear Down the Fluorescent Curtain: A New Fluorescence Suppression Method for Raman Microspectroscopic Analyses

The near exponential proliferation of published Raman microspectroscopic applications over the last decade bears witness to the strengths and versatility of this technology. However, laser-induced fluorescence often severely impedes its application to biological samples. Here we report a new approach for near complete elimination of laser-induced background fluorescence in highly pigmented biological specimens (e.g., microalgae) enabling interrogation by Raman microspectroscopy. Our simple chemiphotobleaching method combines mild hydrogen peroxide oxidation with broad spectrum visible light irradiation of the entire specimen. This treatment permits observing intracellular distributions of macromolecular pools, isotopic tracers, and even viral propagation within cells previously not amenable to Raman microspectroscopic examination. Our approach demonstrates the potential for confocal Raman microspectroscopy becoming an indispensable tool to obtain spatially-resolved data on the chemical composition of highly fluorescent biological samples from individual cells to environmental samples.

Time-resolved Fourier-transform infrared spectroscopy reveals the hidden bimolecular process of the ferrioxalate actinometer

Step-scan FTIR-spectroscopy reveals the bimolecular reaction in the ferrioxalate photochemistry, which builds the molecular-level foundation of the Hatchard–Parker actinometer.

Photochromism in oxalatoniobates

Addition of 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) or 2-aminopyridine (2-NH₂-py) to aqueous solutions of (NH₄)[NbO(C₂O₄)₂(H₂O)₂]·3H₂O (Nb-Ox) yields tris-oxalate complexes (bpyH₂)(bpyH)[NbO(C₂O₄)₃]·2H₂O (1), (phenH)₃[NbO(C₂O₄)₃]·3H₂O (2), and (2-NH₂-pyH)₃[NbO(C₂O₄)₃]·2H₂O (3), which were characterised by XRD, IR and EA. Bipyridinium salt 1 demonstrates remarkable photoactivity even under irradiation by daylight. The nature of the photoactivity was studied by diffuse reflectance (DR) spectroscopy, ESR and quantum-chemical calculations.

Femtosecond infrared spectroscopy reveals the primary events of the ferrioxalate actinometer

Ultrafast vibrational spectroscopy unravels the primary processes of Hatchard and Parker's ferrioxalate actinometer – a widely used tool in the photochemistry community.

Distinct effects of oxalate versus malonate on the iron redox chemistry: Implications for the photo-Fenton reaction

The dicarboxylic acids oxalate (Oxal) and malonate (Mal) are frequently detected as the final low-molecular-weight organic acids during oxidative degradation of aromatic compounds. Here a distinct effect of Oxal versus Mal on iron-based photocatalytic technologies was reported by testing the degradation efficiency of the dye rhodamine B. The rates of dye degradation in irradiated Fe(III) solutions depended on Fe(III/II) speciation, photoreactivities of Fe complexes and reactivities of Fe(II) complexes with H2O2. Photolysis of the Fe(III)-oxalato complex was favorable due to the formation of O2-, HO2 and OH for oxidizing the dye; however, an excess of H2O2 could quench the excited state of ferrioxalate, decreasing the degradation efficiency. In contrast, activities of UV/Fe(III) in the presence of Mal were significantly diminished because Fe(III)-Mal complexes, with much lower quantum yield of Fe(II) from photoreduction, dominated Fe(III) speciation. The results provide data for an understanding of the mechanism of iron redox (photo)chemistry mediated by diacids, which will aid in selecting appropriate Fe ligands, screening photo-Fenton conditions and designing UV/Fe(III) treatability.

Ultrafast photophysical processes for Fe(iii)-carboxylates

Recent works devoted to the investigation of ultrafast processes for several environmentally important Fe(iii) carboxylates were observed and discussed.

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

The photoredox reaction of trisoxalato cobaltate (III) has been studied by means of ultrafast extended x-ray absorption fine structure and optical transient spectroscopy after excitation in the charge-transfer band with 267-nm femtosecond pulses. The Co-O transient bond length changes and the optical spectra and kinetics have been measured and compared with those of ferrioxalate. Data presented here strongly suggest that both of these metal oxalato complexes operate under similar photoredox reaction mechanisms where the primary reaction involves the dissociation of a metal-oxygen bond. These results also indicate that excitation in the charge-transfer band is not a sufficient condition for the intramolecular electron transfer to be the dominant photochemistry reaction mechanism.

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

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.