Water‐Assisted Concerted Proton‐Electron Transfer at Co(II)‐Aquo Sites in Polyoxotungstates With Photogenerated Ru^III(bpy)₃³⁺ Oxidant

Abstract

The cobalt substituted polyoxotungstate [Co6(H2O)2(α‐B‐PW9O34)2(PW6O26)]17− (Co6) displays fast electron transfer (ET) kinetics to photogenerated RuIII(bpy)33+, 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one‐electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)−OH2 and Co6(II)−OH, respectively, whose speciation in aqueous solution is associated to a pKa of 7.6), and generates a Co(III)−OH moiety (Co6(III)−OH), as proven by transient absorption spectroscopy; (ii) at pH>pKa, the Co6(II)−OH→RuIII(bpy)33+ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH <pKa, the process involves Co6(II)−OH2→Co6(III)−OH transformation and proceeds via a multiple‐site, concerted proton electron transfer (CPET) where water assists the transfer of the proton, as proven by the absence of effect of buffer base concentrations on the rate of the ET and by a H/D kinetic isotope in a range of 1.2–1.4. The reactivity of water is ascribed to its organization on the surface of the polyanionic scaffold through hydrogen bond networking involving the Co(II)−OH2 group.

Enhancing Oxygenic Photosynthesis by Cross-Linked Perylenebisimide “Quantasomes”

“…The resulting QS-TEG unlock features the expected spectral signature, similar CLCA response, and quantitative oxygen production ( Figures S27–S29, Tables S15 and S16 , FE O2 > 95%), confirming that decoration of the PBI scaffold with TEG residues, with or without cross-linking, can leverage the quantasome hydration and facilitate water oxidation, under light-assisted or dark electrocatalytic conditions ( Figures S30–S32 and Tables S17–S19 ). 25 , 26 Formation of TEG-templated hydration shells has been detected by Raman microscopy of water exposed photoanodes ( Figure 3 , Supporting Information section 1.2 ), 27 showing a diffuse water distribution (band area 3100–3600 cm –1 ) and a specific effect of the TEG residues in structuring “ordered water” via H-bonding, (band area < 3350 cm –1 , green and red traces in the blue zone Figure 3 A, Figures S33–S35 ) with respect to “disordered (bulk) water” observed for the TEG-free QS (band area >3350 cm –1 , blue trace in the pink zone, Figure 3 A). 27 The added value of TEG cross-linkers stems from the improved QS-TEG lock film stability under a 1 h photoelectrolysis at > 8 sun irradiance, whereby the unlocked structure reports a major photocurrent loss (73% vs 56% Figure S36, Table S1 ).…”

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

“…In order to better understand the photocatalytic performance of the IrO 2 ⊂NS−Ru system, we performed flash photolysis experiments in presence of Na 2 S 2 O 8 . These experiments allow to gain kinetic information on the hole scavenging reaction between the oxidized photosentitizer and the catalyst, that is the key reaction in Equation (3) [9,10,13,16–19] . Figure 8 shows the kinetics of the bleaching and recovery of the MLCT absorption at 450 nm for the IrO 2 ⊂NS‐RuCh system and for the NS‐RuCh in the presence of 5×10 −5 M of “free” IrO 2 , which testifies the hole scavenging process.…”

“…These experiments allow to gain kinetic information on the hole scavenging reaction between the oxidized photosentitizer and the catalyst, that is the key reaction in Equation (3). [9,10,13,[16][17][18][19] Figure 8 shows the kinetics of the bleaching and recovery of the MLCT absorption at 450 nm for the IrO 2 �NS-RuCh system and for the NS-RuCh in the presence of 5 × 10 À 5 M of "free" IrO 2 , which testifies the hole scavenging process. The bleaching of the MLCT band is produced upon excitation of the Ru-based chromophore followed by very fast oxidative electron transfer by 0.01 M persulfate anions, so generating the Ru III species, which may undergo hole scavenging by the IrO 2 catalyst, synthetically incorporated in IrO 2 �NS-RuCh or present in solution in the case of NS-RuCh.…”

Sequential proton coupled electron transfer events from a tetraruthenium polyoxometalate in photochemical water oxidation

“…49 encounter complex, eqn (2), followed by ET to form the successor complex {[Ru(bpy) 3 ] 2+ $Ru 4 POM ox }, eqn (3), and product diffusion, eqn (4); assuming a steady state condition for the encounter and successor complexes the equation of the rate constant k I can be expressed by eqn (5). 36 [Ru(bpy…”

“…This journal is © The Royal Society of Chemistry 2024 Sustainable Energy Fuels, 2024, 8, 1944-1952 | 1947 hydrophilic surface of polyoxometalates may favour its role in assisting the transfer of protons during oxidation of Ru 4 POM. 36 Finally, superimposable kinetic traces and unchanged tting parameters were obtained when registering the experiment in deuterated medium (Fig. S12 in ESI, † obtained by laser ash photolysis of 100 mM Ru(bpy) 3 Cl 2 , 5 mM Na 2 S 2 O 8 , 2.5 mM Na 10 Ru 4 POM, 0.1 M Na 2 SO 4 in 100 mM deuterated phosphate buffer at pD 7.0), indicating a negligible H/D isotopic effect in the multiple ET dynamics.…”