Mary D. Archer

Intramolecular photochemical electron transfer. Part 5.—Solvent dependence of electron transfer in a porphyrin–amide–quinone molecule

The compound PAQ, which consists of a tetra-arylporphine attached to methyl-p-benzoquinone via a single amide linkage, exhibits light-induced intramolecular electron transfer from the porphyrin excited-singlet state to the quinone at a rate which is strongly solvent-dependent. The rate constants are found to correlate well with the semiclassical Marcus theory of electron transfer, provided that the solvent effect on both the Gibbs energy change, ΔG°, for the electron-transfer reaction 1P*AQ → P˙+ AQ˙– and the Marcus reorganisation energy, λ, are considered. The ΔG° values are obtained from direct measurement of redox potentials in each solvent with various work-term corrections for Coulombic interaction in P˙+AQ˙–, and λ is calculated from the optical and dielectric properties of each solvent. For fourteen solvents, reasonable agreement with Marcus theory is found using this approach on uncorrected ΔG° values and those corrected with a solvent-dependent work term; a solvent-independent correction is not successful. For two solvent mixtures (acetonitrile–benzonitrile and acetonitrile–chloroform), excellent agreement with Marcus theory is found using uncorrected ΔG° values and those corrected with a solvent-dependent work term. We have found that Wellers method for calculating ΔG° in various solvents from a single measurement in a reference solvent gives a poor correlation with Marcus theory, primarily because of a poor prediction of the solvent dependence of ΔG°.

Achievable boundary conditions in potentiostatic and galvanostatic hydrogen permeation through palladium and nickel foils

Two popular electrochemical methods for the investigation of the permeability of metal membranes to atomic hydrogen are critically discussed. In the potentiostatic (P) method, hydrogen is generated at constant potential at the entrance face; in the galvanostatic (G) method, it is generated at constant current. In both, the concentration at the exit face of the membrane is zero. The boundary condition at the entrance face usually taken to correspond with these experiments is either that the surface concentration is constant (the C case) or that the flux of hydrogen entering the membrane is constant (the F case). It is pointed out that the widespread assumptions that use of the P technique guarantees the C boundary condition, and that use of the G technique guarantees the F condition, are incorrect. The boundary condition actually established depends on the relative rates of the various steps involved in hydrogen evolution at the entrance face and its diffusion through the membrane. Experimental work on ca. 25 μm thick nickel and palladium, which supports this contention, is described. The F boundary condition is readily established by the G experiment on palladium, but the C condition cannot be established by the P experiment. The converse is true for nickel. These differences are explained in terms of the greater solubility and diffusivity of hydrogen in palladium as compared with nickel. An extended potentiostatic experiment, termed the Pf experiment, is described. In the Pf experiment, all the potentiostatically generated hydrogen enters the membrane. The currents passing at both faces of the membrane are measured during permeation, and also as they decay after the potential of the entrance face is switched to that of the exit face, causing hydrogen to diffuse out of both sides of the membrane. The Pf experiment is shown to work well with thin palladium membranes, and to provide crosschecks on the diffusivity of hydrogen. The diffusion coefficient of hydrogen in nickel is sensitive to the thermal history of the metal. Decay transients give some evidence for the existence of hydrogen traps in both nickel and palladium. The potential of the entrance face during G experiments on either metal is not related to the surface concentration of hydrogen by the Nernst equation. It is concluded that a full analysis of the permeation transients obtained by P or G experiments should be made to establish the boundary conditions actually created by the experimental procedure. Some previously published permeation work is critically examined in the light of this conclusion.

CHEMICAL CONVERSION AND STORAGE OF SOLAR ENERGY - AN OVERVIEW

ABSTRACT This article surveys methods by which solar energy may drive useful chemical reactions. Photochemical processes are those in which the absorption of solar photons in a molecule produces excited states, or alternatively in a semiconductor raises electrons from the valence band to the conduction band. Chemical reactions then may occur, either to store some of the excitation energy as chemical energy or to catalyze a useful chemical reaction. Basic concepts are briefly reviewed, and some remarks on efficiency limits are presented. Applications include: molecular energy storage reactions; photochemical redox reactions, such as the photodecomposition of water to hydrogen and oxygen; photosynthesis and attempts to mimic this natural process; photoelectrochemical cells, configured either to generate electricity or to produce a fuel, such as hydrogen; and microheterogeneous systems, which act as powerful photocatalysts. Finally, high-temperature thermochemcial systems are introduced; these can either act as a heat-storage system or as a fuel-generation system.

Spectroscopic and Electrochemical Studies of Photochemical Electron Transfer in Linked Donor-Acceptor Molecules

Some covalently-linked porphyrin-quinone (PQ) molecules of restricted molecular geometry are described. The excited state P*Q, Where P* is the porphyrin S1 state, probably undergoes intramolecular electron transfer to produce the charge-separated state P†Q. The forward rate constant et f at 295 K for this electron transfer, measured from fluorescence quenching in PAQ (Scheme 1) compared with its hydroquinone analogue PAQH2, lies in tne range (1 − 230) × 107 s− and is strongly solvent dependent. The electrochemically measured energy U± of P†Q with respect to the ground state PQ is 1.35 – 1.41 eV and is also somewhat solvent dependent. Rationalization of the solvent dependence of k et f and of U± is attempted both in terms of the Marcus theory of electron transfer and in terms of the Onsager reaction field theory; the latter appears to be more successful.