Direct Observation of Sequential Electron and Proton Transfer in Excited-State ET/PT Reactions

Abstract

The prospect of a photoexcited chromophore undergoing a reaction that involves transfer of an electron and a proton within an encounter complex with an appropriate acceptor is examined in this work, employing [(bpy)2Ru­(4,4′-dhbpy)]2+ (4,4′-dhbpy = 4,4′-dihydroxy-2,2′-bipyridine, [Ru­(II)­OH]2+) as a chromophore and MQ+ (N-methyl-4,4′-bipyridinium) as an acceptor. The use of an aprotic solvent (CH3CN) allowed unambiguous evaluation of the photoproducts emerging from the encounter complex by using transient absorption spectroscopy in the visible region. While the overall photoproducts reflect transfer of an electron and a proton from [Ru­(II)­OH]2+ to MQ+, detailed analysis suggests that the process occurs via excited-state electron transfer (k q = 7 × 107 M–1 s–1), followed by proton transfer to MQ0 from [Ru­(II)­OH]2+ in solution. The proton transfer process follows pseudo-first order kinetics in the Ru complex and has overall second-order rate constants that are near the diffusion limit. The results are discussed in terms of the free energies for the reactions involved.

Platinum@Hexaniobate Nanopeapods: A Directed Photocatalytic Architecture for Dye-Sensitized Semiconductor H2 Production under Visible Light Irradiation

“…While continuing studies aim to more precisely elucidate the mechanism for photocatalytic water splitting by Pt@HNB NPPs, our observations were consistent with the expected results from the reductive quenching mechanism within this photosystem. This system has been well-characterized in previous studies published by our group, which include measuring the quantum yield of one-electron-reduced species of Ru dyes as well as the different photoactivities of various catalysts including MoS complexes supported by dithiocarbamate ligands, tungsten carbonyl complexes, and transition-metal hydride catalysts. − The reductive quenching mechanism was verified through comprehensive spectroscopic analysis including transient absorbance and time-resolved luminescence spectroscopy …”

“…38−41 The reductive quenching mechanism was verified through comprehensive spectroscopic analysis including transient absorbance and time-resolved luminescence spectroscopy. 42 We propose a reductive quenching mechanism consisting of two main phases: generation of the strong reducing agent Rubpy 1+ (or Rudcby 1+ ), followed by charge injection and electron transfer through the semiconductor to the Pt NPs for H 2 evolution (Scheme 2). The proposed reaction series is shown below, in which the ground, excited, and reduced states of the chromophores are represented as Ru 2+ , *Ru 2+ , and Ru 1+ , respectively.…”

“…DeLaive et al utilized one-electron reducing agents and hydrophobic Ru­(bpy) chromophores to successfully reduce water using heterogeneous photocatalysts via a reductive quenching mechanism with good quantum yield and high turnover number. , This mechanism has served as the basis for advanced reductive quenching photosystems such as that of Shan and Schmehl, which couples photoinduced electron transfer to secondary irreversible electron-transfer processes to generate strongly reducing chromophore equivalents in non-aqueous solution . This process increases quantum efficiency by effectively obviating the unproductive back electron-transfer reactions, as demonstrated by multiple studies on homogeneous catalysts such as MoS complexes, tungsten carbonyl complexes, and transition-metal hydrides. − …”

Multiple molecular logic gate arrays in one system of (2-(2′-pyridyl)imidazole)Ru(ii) complexes and trimeric cyclophanes in water

“…those for atomic counterparts. 27 In the case of polypyridineRu(II) lumophores, [28][29][30] those responding to H + , [31][32][33][34][35][36][37][38][39][40][41] Na + , [41][42][43] Cl À , 44,45 Ni 2+ , [46][47][48] and even electrons 49,50 outnumber those switching with glucose, 51 H 2 PO 4 À , 52 MoO 4 À . 53 Cases involving O 2 -induced quenching 54,55 have not been included in this group because no denable receptor exists.…”

Photo-induced Electron Transfer or Proton-Coupled Electron Transfer in Methylbipyridine/Phenol Complexes: A Time-Dependent Density Functional Theory Investigation

“…The importance of proton-coupled electron transfer (PCET) is substantially realized in many biophysical and catalytic processes. − Concerted electron and proton transfer is essential to avoid high energy intermediates. − However, the signatures of the PCET process are not too different from the conventional charge transfer (CT) or electron transfer (ET) . The similarity of features between PCET and ET often creates confusion to track down the actual nature of the process.…”