Proton coupled electron transfer reaction of phenols with excited state ruthenium(II) – polypyridyl complexes

Abstract

The reaction of phenols with the excited state, *[Ru(bpy)3]2+ (E0 = 0.76 V) and *[Ru(H2dcbpy)3]2+, (dcbpy = 4,4′‐dicarboxy‐2,2′‐bipyridine) (E0 = 1.55 V vs. SCE) complexes in CH3CN has been studied by luminescence quenching technique and the quenching is dynamic. The formation of phenoxyl radical as a transient is confirmed by its characteristic absorption at 400 nm. The kq value is highly sensitive to the change of pH of the medium and ΔG0 of the reaction. Based on the treatment of kq data in terms of energetics of the reaction and pH of the medium, proton coupled electron transfer (PCET) mechanism has been proposed for the reaction. Copyright © 2010 John Wiley & Sons, Ltd.

Photoluminescence electron transfer quenching of ruthenium(II)-polypyridyl complexes with biologically important phenolate ions in aqueous acetonitrile solution

“…The PET reactions of ruthenium(II)-polypyridyl complexes with several ortho-, meta-and para-substituted phenolate ions are highly influenced by the change of structure of the ligands of the complex as well as the substrates. The decrease in the quenching rate constant (k q ) value with increase in the bulkiness of the ligand as well as the quencher has been explained in terms of an increase in the electron transfer distance [12][13][14][15]. The rate of ET from a donor molecule to an acceptor in a solvent is controlled by several factors and the most important of them are the free energy change (ΔG 0 ) of the reaction, the reorganization energy (λ) and the electron transfer distance (d) between the donor and acceptor [12][13][14][15]34].…”

“…Many efforts have been made in recent years to design and synthesize molecules to mimic the important light-driven process, photosystem II [27][28][29]. Realizing the importance of Ru(II)-polypyridyl complexes as model photosensitizers and phenols as electron donors in photosystem II, several researchers have investigated the photoinduced electron transfer (PET) reactions of ruthenium(II)-polypyridyl complexes with several phenols in homogeneous medium and the formation of phenoxyl radical as the transient has been established using time-resolved techniques [12][13][14][15][30][31][32][33][34]. The PET reactions of ruthenium(II)-polypyridyl complexes with several ortho-, meta-and para-substituted phenolate ions are highly influenced by the change of structure of the ligands of the complex as well as the substrates.…”

“…The quenching studies are performed in the presence of NaOH, since most of the phenols do not quench *[Ru(NN) 3 ] 2+ in neutral medium [14,15]. As far as phenols are concerned they are in the undissociated form at low pH, i.e., pH < pK a , but they are in the form of phenolate ions at high pH, i.e., pH > pK a .…”

“…The generation of phenoxyl radical involves either H-atom or electron abstraction from phenol and phenolate ion, respectively. The study of the kinetic and thermodynamic aspects of electron transfer (ET) to generate phenoxyl radicals bearing bulky groups in the ortho-and para-positions may help to understand the different biological roles of phenols [12][13][14][15].The oxidation potential of a polyphenol provides an estimate of the energy required to donate an electron; the lower the oxidation potential, lesser the energy required to donate an electron, hence it undergoes oxidation easily. The mechanism of oxidation of polyphenols and their stability in solution depend on pH [16,17].…”

“…The generation of phenoxyl radical involves either H-atom or electron abstraction from phenol and phenolate ion, respectively. The study of the kinetic and thermodynamic aspects of electron transfer (ET) to generate phenoxyl radicals bearing bulky groups in the ortho-and para-positions may help to understand the different biological roles of phenols [12][13][14][15].…”