Excited-state Redox Potentials Research Articles

Excited-state redox potentials and the Z scheme of photosynthesis

The Z scheme for electron flow in oxygen-evolving photosynthetic organisms was first proposed 25 years ago by R. Hill and F. BendalP. Their original diagram is shown in Fig. 1. The basic idea, that two photochemical systems are arranged in series so that two photons act sequentially to drive electrons from the very poor electron donor 1-I20 to the weak electron acceptor NADP, is now embraced by essentially every worker in the field. This scheme rightly stands as the central integrating construct for all work on electron transport in oxygenevolving photosynthetic organisms. Introductory biochemistry texts invariably contain a version of the scheme, usually with an explanation of some of the classic experiments (such as the red drop and enhancement effects) that led to the acceptance of the idea of a sequence of two photochemical steps, each with a somewhat different wavelength optimum. The Z scheme is really a synthesis of thermodynamic and kinetic data. The midpoint redox potentials of the various electron carriers are now usually plotted vertically so that more reducing compounds are above more oxidizing ones. This ensures that thermodynamically favorable reactions are 'downhill' on the diagram. The horizontal axis corresponds to a reaction sequence and the kinetically important reactions are connected by arrows. Unfortunately, many authors are extremely careless about constructing their Z schemes, so much so that the understanding of what is in reality a simple concept is greatly impeded. Several Z schemes in the literature have P~0, the reaction centerchlorophyll of photosystem II, at an E m (oxidation-reduction midpoint potential) of +450 mV, and show electrons flowing uphill to it from 1420 against a

Synthetic control of excited states. Nonchromophoric ligand variations in polypyridyl complexes of osmium(II)

Two themes are explored with regard to the properties of the metal to ligand charge-transfer (MLCT) excited states of Os(II). For a series of Os(II) complexes it is shown that the MLCT excited states undergo facile oxidative or reductive quenching. Excited-state redox potentials have been estimated by both kinetic quenching and spectroscopic techniques for excited-state oxidative couples and excited-state reductive couples. The second theme, the manipulation of excited-state properties by synthetic changes, follows from a consideration of these factors that dictate excited-state redox potentials. It is shown that in the series (phen)OsL/sub 4//sup 2 +/ (L = pyridine, CH/sub 3/CN, PR/sub 3/, AsR/sub 3/, ... and phen = 1,10-phenanthroline) where the metal-ligand basis for the MLCT chromophore remains the same and variations are made in the nonchromophoric ligand, emission energies, excited-state redox potentials, and radiative and nonradiative rate constants all vary systematically with the potential of the ground-state Os(III/II) couple. The results show that it is possible through synthetic changes to control excited-state properties in a systematical way. 37 references, 6 figures, 5 tables.

Estimation of excited-state redox potentials by electron-transfer quenching. Application of electron-transfer theory to excited-state redox processes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEstimation of excited-state redox potentials by electron-transfer quenching. Application of electron-transfer theory to excited-state redox processesC. R. Bock, J. A. Connor, A. R. Gutierrez, Thomas J. Meyer, D. G. Whitten, B. P. Sullivan, and J. K. NagleCite this: J. Am. Chem. Soc. 1979, 101, 17, 4815–4824Publication Date (Print):August 1, 1979Publication History Published online1 May 2002Published inissue 1 August 1979https://doi.org/10.1021/ja00511a007RIGHTS & PERMISSIONSArticle Views4854Altmetric-Citations399LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

THE ENERGETICS OF ELECTRON‐TRANSFER REACTIONS OF CHLOROPHYLL AND OTHER COMPOUNDS*

Abstract— Quite often the primary photochemical reaction of an excited state molecule is transfer of an electron to or from another molecule in its ground state. Rates of such reactions are closely dependent on differences between ground and excited state redox potentials of the reagents. The solvent also plays an important role in stabilizing ion pairs formed by the electron transfer. This Review discusses experimental data relating rates to electrochemical energy parameters in the context of a scheme which portrays the energy and electron transactions in a unified manner. Three consequences of reaction of a singlet excited state are distinguished: (S1) quenching without detectable products, (S2) exciplex fluorescence, (S3) transient radical ion production, and energetically necessary conditions are derived for each. Similarly, four kinds of reactions involving the triplet state are distinguished, which depend on the relation between the energy of the triplet state and that of the ion pair states: (TI) rapid quenching, (T2) slow quenching, (T3) accelerated intersystem crossing and (T4) generation by reaction between radical ions of like spin. The last may be followed by electrochemiluminescence. Classes of compounds for which data are available include chlorophylls, porphyrins and a few other molecules of biological interest, aromatic hydrocarbons and their derivatives, heterocyclic systems, carbonyl compounds, dyes, and complexes of Ru and U. A Table compiling median or selected values of ground and excited state electrochemical potentials of chlorophylls, some porphyrins, and a few other compounds is presented.

Generalised Förster cycle. Thermodynamic and extrathermodynamic relationships between proton transfer, electron transfer and electronic excitation

The thermodynamic quantities characterizing one electron reduction, protonation and electronic excitation, are mutually related in the ground and excited states by 6 thermodynamic (or approximate thermodynamic) cycles. The system of cycles is used to predict unknown values, and its validity may be extended to other compounds by means of extra thermodynamic (e.g., Hammett-type) relations. Examples of known data concerning pK and pK* values of protolytic equilibria for the oxidized (Ox) and reduced (R) species are evaluated, tabulated and discussed in a search for a correlation between the changes of pK on excitation (ΔpK*) and on reduction (ΔpKe). Both values are thermodynamically independent but a general common trend is empirically observed in several groups of systems. Some rules are derived for the excited state redox potentials and their dependence on pH, which may be useful for photochemistry.