Ultrafast photochemistry of aqueous iron(III) complexes

Abstract

The ultrafast photochemical dynamics of aqueous iron(III) solutions were measured utilizing ultrafast pump probe spectroscopy. Aqueous solutions of iron(III) were prepared at low pH (<4.5) and low iron(III) concentration (<5 mM) to allow for small aquairon(III) complexes and ferrihydrite to be studied. Small monomeric and dimeric aquairon(III) complexes were studied to elucidate the mechanisms involved in the formation of OH ° after UV excitation which were previously known to generate OH ° in vastly different quantities. Upon excitation of Fe 3+, a proton is released from a coordinated water molecule to generate FeOH 2+ in less than 200 fs. The newly generated FeOH 2+ can then undergo numerous recombination pathways to regenerate the Fe 3+. Approximately 10% of the excited Fe 3+ undergoes photoreduction and subsequent release of OH ° and Fe 2+ within 20 ps. Exciting FeOH 2+, results in homolysis to form Fe 2+ and OH ° with a wavelength dependent yield with a lifetime of 20 ps. Fe 2(OH) 2 4+ does not appear to generate significant quantities of OH ° however, the dimer is photostable in comparison to Fe 3+ and FeOH 2+. To further the understanding of the primary kinetics of iron(III) in aqueous solutions, ferrihydrite nanoparticles were studied. Ferrihydrite exhibits similar dynamics to hematite in which electrons are excited into the conduction band of ferrihydrite. The electrons can then relax to the bottom of the conduction band within 390 fs before undergoing various recombination process. This limits the amount of iron(III) converted into iron(II) in ferrihydrite. All iron(III) systems studied show unique kinetics after excitation that elucidate the mechanisms behind the generation of OH °.