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ELECTRON TRANSFER EXPERIMENTS SINCE THE LATE 1940SSince the late 1940s, the field of electron transfer processes has grown enonnously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of other kinds of chemical reactions, represents to us an intriguing history, one in which many threads have been brought together. In this lecture, some history, recent trends, and my own involvement in this research are described.The early experiments in the electron transfer field were on 'isotopic exchange reactions' (self-exchange reactions) and, later, 'cross reactions.' These experiments reflected two principal influences. One of these was the availability after the Second World War of many radioactive isotopes, which pennitted the study of a large number of isotopic exchange electron transfer reactions, such as There is a two-fold simplicity in typical self-exchange electron transfer reactions (so-called since other methods beside isotopic exchange were later used to study some of them): (1) the reaction products are identical with the reactants, thus eliminating one factor which usually influences the rate of a chemical reaction in a major way, namely the relative thennodynamic stability of the reactants and products; and (2) no chemical bonds are broken or fonned in simple electron transfer reactions. Indeed, these self-exchange reactions represent, for these combined reasons, the simplest class of reactions in chemistry. Observations stemming directly from this simplicity were to have major consequences, not only for the electron transfer field but also, to a lesser extent, for the study of other kinds of chemical reactions as well (cf. Shaik et al., ref. 2).A second factor in the growth of the electron transfer field was the introduction of new instrumentation, which pennitted the study of the rates of rapid chemical reactions. Electron transfers are frequently rather fast, compared with many reactions which undergo, instead, a breaking of chemical bonds and a fonning of new ones. Accordingly, the study ofa large body of fast electron transfer reactions became accessible with the introduction of this instrumentation. One example of the latter was the stopped-flow apparatus, pioneered for inorganic electron transfer reactions by N. Sutin. It pennitted the study of bimolecular reactions in solution in the millisecond time scale (a fast time scale at the time). Such studies led to the investigation of what has been tenned electron transfer 'cross reactions,' i.e., electron transfer reactions between two different redox systems, as in which supplemented the earlier studies of the self-exchange electron transfer reactions. A comparative study of these two types ofreaction, self-exchange and cross-reactions, stimulated by theory, was also later to have major consequences for the field and, indeed, for other areas. · Again, in the field of electrochemistry, the new post-war instrumentation in chemical laboratories led to methods which permitted ...

A unified theory of homogeneous and electrochemical electron-transfer rates is developed using statistical mechanics. The treatment is a generalization of earlier papers of this series and is concerned with seeking a fairly broad basis for the quantitative correlations among chemical and electrochemical rate constants predicted in these earlier papers. The atomic motions inside the inner coordination shell of each reactant are treated as vibrations. The motions outside are treated by the "particle description," which emphasizes the functional dependence of potential energy and free energy on molecular properties and which avoids, thereby, some unnecessary assumptions about the molecular interactions.

Since the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of other kinds of chemical reactions, presents to us an intriguing history, one in which many threads have been brought together. In this lecture, some history, recent trends, and my own involvement in this research are described.

This is the report of a DOE-sponsored workshop organized to discuss the status of our understanding of charge-transfer processes on the nanoscale and to identify research and other needs for progress in nanoscience and nanotechnology. The current status of basic electron-transfer research, both theoretical and experimental, is addressed, with emphasis on the distance-dependent measurements, and we have attempted to integrate terminology and notation of solution electron-transfer kinetics with that of conductance analysis. The interface between molecules or nanoparticles and bulk metals is examined, and new research tools that advance description and understanding of the interface are presented. The present state-of-the-art in molecular electronics efforts is summarized along with future research needs. Finally, novel strategies that exploit nanoscale architectures are presented for enhancing the efficiences of energy conversion based on photochemistry, catalysis, and electrocatalysis principles.

. Vario~s proces~es such as electron transfer reactions, redox reactions at electrodes, and electronic excitat~o~ of d1~solved wns m~~ proceed by way of intermediate states whose electrical polarization is not in equilibnum w1th ~he fie]~ ansmg ~rom th_e charges present. The usual expressions for the electrostatic free energy and_ ~or _the dtfferen~Ial equatwn satisfied by the potential assume that the polarization and the field are in eqmlibnum. _Accordmgly! these equat_ions are of but limited applicability to these processes. In the present paper equatwns are denved for varwus properties of systems having such nonequilibrium electrostatic configuration:. These properties include the fre~ energy, _energy, and entropy of the nonequilibrium system, and the spacial dependence of the electrostatiC potential. The free energy for example will be used to calculate the probability of formation of nonequilibrium states in certain pr~blems of phy~ical interest. INTRODUCTIONI N recent years processes such as electron transfer reactions, redox reactions at electrodes, and electronic excitation of ions in the solid state and in solution have been the subject of considerable interest. These processes appear to involve intermediate states having nonequilibrium electrostatic configurations in the sense that the electrical polarization may not be that which would be predicted in the usual manner from the known charge distribution. For example, when a dissolved ion absorbs light, the electron is raised to an excited state almost instantaneously. A considerably longer time is required for the solvent's dipoles to readjust themselves to this new configuration. Thus, for some brief time after the absorption act, the electrical polarization of the system will not be in equilibrium with the charge distribution. This concept served 1 as a basis of a theory of the absorption spectrum of various halide ions in solution. In a different fashion, related considerations may apply to the other processes mentioned.The results obtained in the present paper for the free energy and for other properties will later be applied to develop a quantitative theory of certain oxidationreduction reactions. A knowledge of the free energy of nonequilibrium states is of use in calculating the probability of their formation, as intermediates, in certain cases. The usual expressions given for the electrostatic free energy and energy cannot be used for this purpose. Similarly Poisson's equation for the electrostatic potential is not applicable in its commonly written form. Each of these equations assumed in their derivation that the electrical polarization at each point in the system was in equilibrium with the electric field there.In the present paper, equations for these properties of nonequilibrium states are derived. However the ' treatment of systems containing electrodes will be reserved for a later communication. Such systems involve certain additional complexities arising from the presence of charges induced in the electrodes by the medium itself. THEOR...

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Virtually all known fluorophores exhibit mysterious episodes of emission intermittency. A remarkable feature of the phenomenon is a power law distribution of onand off-times observed in colloidal semiconductor quantum dots (QDs), nanorods, nanowires and some organic dyes. For nanoparticles the resulting power law extends over an extraordinarily wide dynamic range: nine orders of magnitude in probability density and five to six orders of magnitude in time. Exponents hover about the ubiquitous value of -3/2. Dark states routinely last for tens of seconds, which are practically forever on quantum mechanical time scales. Despite such infinite states of darkness, the dots miraculously recover and start emitting again. Although the underlying mechanism responsible for this phenomenon remains a mystery and many questions persist, we argue that substantial theoretical progress has been made.