Role of Hydrogen Bonding in Photoinduced Electron–Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand

Abstract

Electron-proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1(T), can be described by a kinetic model that accounts for the H-bonding of 1(T) to phenol, 1(T) to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis. Control experiments replacing either 1(T) by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between 1(T) and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT.

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

“…It has been pointed out that determination of the association constants in this way leads to large errors. 37 In order to improve the accuracy of the measured association constants, we made use of the temperature dependence of the enthalpy term of the association constant. Briey, kinetic measurements were conducted varying the temperature from 5 C to 25 C. K HB was extracted and t to the van't Hoff equation.…”

The Role of H-Bonding in Nonconcerted Proton-Coupled Electron Transfer: Explaining the Voltammetry of Phenylenediamines in the Presence of Weak Bases in Acetonitrile

“…This can be an intramolecular H-bond, but it also could be an intermolecular H-bond. In the latter case the bimolecular reactions forming the H-bond complex are integral to the overall reaction and need to be considered in the overall kinetic analysis. , To do this, several years ago we introduced the “wedge scheme” mechanism, Scheme b, in which the bimolecular reactions forming and breaking the H-bond intermediate with and without PT are displayed along a third axis perpendicular to the ET–PT square. , This clearly defines an alternative pathway for ET, shown by the red horizontal line, that corresponds to ET and possibly PT within the H-bond complex. It is straightforward to show that the formal electrode potential for this reaction, E °′(AHB + /AHB), falls in between that for the fully protonated redox couple (AH + /AH) and that for the fully deprotonated couple (A/A – ).…”

“…In the latter case the bimolecular reactions forming the H-bond complex are integral to the overall reaction and need to be considered in the overall kinetic analysis. 13,14 To do this, several years ago we introduced the "wedge scheme" mechanism, Scheme 1b, in which the bimolecular reactions forming and breaking the Hbond intermediate with and without PT are displayed along a third axis perpendicular to the ET−PT square. 15,16 This clearly defines an alternative pathway for ET, shown by the red horizontal line, that corresponds to ET and possibly PT within the H-bond complex.…”

Excited-State Quenching of Porphyrins by Hydrogen-Bonded Phenol-Pyridine Pair: Evidence of Proton-Coupled Electron Transfer

“…When the PCET occurs in a single reaction step without forming any intermediate, it is generally termed as concerted PCET. Such reactions play a pivotal role in a wide range of chemical and biological processes, such as oxygen production and reduction in photosynthesis, , catalytic nitrogen fixation, catalytic oxidation, and production of molecular hydrogen, in mitochondria and in fuel cells. , Since the past two decades, the PCET reactions have gained immense importance for their critical role in a variety of chemical and biological processes − and for understanding the phenomena from the fundamental point of view. − Although photophysical behaviors of organometallic systems (metal complexes) involved in PCET reactions were explored extensively by several groups, ,− photophysical behaviors of pure organic systems involved in PCET reactions have not received much attention, and the literature regarding the fluorescence emission quenching studies in ET processes accompanied by proton movement is remarkably sparse. − PCET reactions can be classified broadly in two distinct categories, according to whether the electron and proton move to the same acceptors, that is, single-site PCET, or different acceptors, that is, multiple-site PCET . In multiple-site PCET, the movement of the electron and proton occurs in two opposite directions; it is also classified as bidirectional PCET .…”