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This book covers the most recent developments in the field of PCET reactions, from the theoretical and experimental points of view.
The handbook comprehensively covers the field of inorganic photochemistry from the fundamentals to the main applications. The first section of the book describes the historical development of inorganic photochemistry, along with the fundamentals related to this multidisciplinary scientific field. The main experimental techniques employed in state-of-art studies are described in detail in the second section followed by a third section including theoretical investigations in the field. In the next three sections, the photophysical and photochemical properties of coordination compounds, supramolecular systems and inorganic semiconductors are summarized by experts on these materials. Finally, the application of photoactive inorganic compounds in key sectors of our society is highlighted. The sections cover applications in bioimaging and sensing, drug delivery and cancer therapy, solar energy conversion to electricity and fuels, organic synthesis, environmental remediation and optoelectronics among others. The chapters provide a concise overview of the main achievements in the recent years and highlight the challenges for future research. This handbook offers a unique compilation for practitioners of inorganic photochemistry in both industry and academia.
(Cont.) Comparative photophysical studies have shown that changes of the polarization within the protonic interface or the coupling between the redox sites and the supramolecular bridge dramatically attenuates the kinetics of charge transport along such paths. The area of major focus has centered on the development of discrete molecular systems capable of driving water -- oxygen interconversion via PCET. These studies have taken advantage of Pacman and Hangman porphyrin architectures to drive oxygen activation. Both Pacman and Hangman motiefs are well suited to facilite oxygen reduction and oxidation chemistry. Comparative reactivity studies for the generation of H20 from oxygen reveals that it is necessary to dictate the delivery of protons and electrons to activated oxygen species in order to efficiently drive drive 0-0 bond activation.
The coupling of proton and electron transfers in concerted or sequential processes is of central importance to many biochemical and catalytic reactions. In this context, proton-coupled electron transfer reactions are typically described electrochemically, whereby the transfer of a proton is coordinated to a change in the oxidation state of the constituent donor/acceptor pairs. In the excited-state, intermolecular proton transfers are often facilitated through a redistribution of electron-density (charge-transfer) along the proton-transfer reaction coordinate without a corresponding change in the oxidation state of the donor/acceptor pair. Distinguishing charge-transfer from full electron-transfer reactions along the excited-state potential energy surface has received increased attention as advancements in engineering allow for the interrogation of the fastest molecular events. This dissertation seeks primarily to examine the confluence of charge-transfer, electron-transfer, and proton-transfer reactions which occur adiabatically in the excited-states of N, N-dimethyl-3-arylpropan-1-ammonium salts in solution. For these compounds, an excited-state intermolecular proton-transfer to the solvent is accompanied by intramolecular charge-transfer and the formation of either an emissive exciplex or a transient solvent-separated ion pair. The various electronic configurations have been interrogated through an array of spectroscopic techniques in order to more thoroughly understand the convergence of thermodynamic and kinetic factors affecting the proposed mechanism. In this regard, a range of temperatures, solvents, counterions, and lumophores have been explored. In addition, the ground-state equilibrium has been investigated through targeted theoretical calculations and experiments. The summation of these experiments provide unique insights into a class of novel exciplex-mediated proton-coupled charge-transfer reactions.
Two new model systems for the study of orthogonal proton-coupled electron transfer (PCET) have been developed. The first model system is based on Ru"(HzO)(tpy)(bpy) (tpy = 2,2';6',2"terpyridine, bpy = 2,2'-bipyridine) where methyl viologen (MV2 ) electron acceptors were appended to the ruthenium aqua complex through the bpy. Picosecond transient absorption measurements show that electron transfer from the excited state of the ruthenium complex to MV2+ occurs with [tau] -200 ps. Experiments performed in water and buffered solution at pH = 7 show no evidence of the loss of proton from the aqua ligand to the bulk solvent or to the phosphate buffer. A minor kinetic isotope effect for the rate of charge separation was found with kH/kD = 1.8 + 0.1 ps. Preliminary synthetic attempts of coupling the ET event to the PT event was accomplished by appending xanthene "Hangman" scaffolds to the 4' position of the tpy. The feasibility of modifying the xanthene scaffold to accommodate various hanging groups has been demonstrated. The second model system is based on Re'(CO)3(phen)(pyr) (phen = 1,10phenanthroline, pyr = pyridine) where tyrosine was appended to the rhenium complex through the axial pyr ligand. Unlike previous Re'(CO)3(bpy)(CN) (CN = cyanide) systems, substitution to the more rigid phen extended the lifetime of the excited state of the rhenium complex to 3.0 ls, which allowed PCET to occur from the tyrosine to the rhenium metal center and hydrogenbonded base in dichloromethane. This was inferred from substantial emission quenching of the rhenium-tyrosine complex through the titration of base (base = pyridine, imidazole, 2,4,6trimethylpyridine). Equilibrium constants measuring the extent of formation of the [rheniumtyrosine---base]+ species were found to correlate with the strength of the base based on aqueous pKa values.
Various aspects of electron and proton transfer in chemistry and biology are described in this volume. The joint presentation was chosen for two reasons. Rapid electron and proton transfer govern cellular energetics in both the most primitive and higher organisms with photosynthetic and heterotrophic lifestyles. Further, biology has become the area where the various disciplines of science, which were previously diversified, are once again converging. The book begins with a survey of physicochemical principles of electron transfer in the gas and solid phase, with thermodynamic and photochemical driving force. Inner and outer sphere mechanisms and the coupling of electron transfer to nuclear rearrangements are reviewed. These principles are applied to construct artificial photosynthesis, leading to biological electron transfer involving proteins with transition metal and/or organic redox centres. The tuning of the free energy profile on the reaction trajectory through the protein by single amino acids or by the larger ensemble that determines the electrostatic properties of the reaction path is one major issue.Another one is the transformation of one-electron to paired-electron steps with protection against hazardous radical intermediates. The diversity of electron transport systems is represented in various chapters with emphasis on photosynthesis, respiration and nitrogenases. The book will be of interest to scientists in chemistry, physics and the life sciences.