title: Hydrogen-bond strengthening upon photoinduced electron transfer in ruthenium-anthraquinone dyads interacting with hexafluoroisopropanol or water.
abstract: Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three p-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor-bridge-acceptor molecules is found to accelerate intramolecular Ru(bpy)(3)(2+)-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor. Similarly, the rates for intramolecular electron transfer are significantly higher in acetonitrile/water mixtures than in dry acetonitrile. In dichloromethane, an increase in the association constant between hexafluoroisopropanol and anthraquinone by more than 1 order of magnitude following quinone reduction points to a significant strengthening of the hydrogen bonds between the hydroxyl group of hexafluoroisopropanol and the anthraquinone carbonyl functions. The photoinduced intramolecular long-range electron transfer process thus appears to be followed by proton motion; hence the overall photoinduced reaction may be considered a variant of stepwise proton-coupled electron transfer (PCET) in which substantial proton density (rather than a full proton) is transferred after the electron transfer has occurred.
authors.affiliation: Georg-August-Universität , Institut für Anorganische Chemie, Göttingen, Germany.
authors.forename: Jihane
authors.initials: J
authors.lastname: Hankache
authors.forename: David
authors.initials: D
authors.lastname: Hanss
authors.forename: Oliver S
authors.initials: OS
authors.lastname: Wenger
mesh_terms: D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives / Q000737:chemistry*; D000880:Anthraquinones / Q000737:chemistry*; D004579:Electron Transport; D006860:Hydrogen Bonding; D007700:Kinetics; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D055668:Photochemical Processes; D020005:Propanols / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*; D006845:Hydrocarbons, Fluorinated
chemical_list: D015082:2,2'-Dipyridyl; D000880:Anthraquinones; D009942:Organometallic Compounds; D020005:Propanols; D012428:Ruthenium; D014867:Water; C001337:hexafluoroisopropanol; D006845:Hydrocarbons, Fluorinated