Ru(bpy)3 2+ (bpy ≡ bipyridine) and Ru(o-phen)3 2+ (o-phen ≡ o-phenanthroline) have been adsorbed to a membrane layer made of TiO2 nanocrystallites from aqueous solutions at pH 2.5 by means of pretreatment of the surface with Nafion, sodium dodecyl sulfate (SDS) or sodium dodecyl benzyl sulfate (SDBS). Pulsed laser-induced emission and absorbance changes have been studied. The time profiles provide information concerning environmental effects (charge and hydrophobic interactions) on the rates and yields of electron injection from the excited dyes to the TiO2 membrane and subsequent electron recapture by Ru(III). The differences between the rates of electron injection and recapture, the multiexponential nature of these reactions, and the differences between specific photosensitizers and binders are discussed in light of the semiconducting properties of the TiO2 nanocrystallites and the hydrophobic and ionic interactions between the photosensitizers and the binders. Oxidation of iodide ions by Ru(III) was also studied. Iodide ions react efficiently with Ru(III) despite the negative charge of the binders, indicating that most of the charge is neutralized by the surface charge of TiO2 and by the Ru(III) ions. Quantum yields for net electron injection were determined from the initial (extrapolated) bleaching. In most systems observed bleaching corresponds to 30−80% of the absorbed photons. At relatively high laser pulse intensities, emission measurements show that bimolecular (and apparent higher order) processes take place, involving fast triplet annihilation. A detailed mechanism provides quantitative kinetic treatment of the data. Comparison of results in dry and wet layers indicates that energy migration is responsible for the enhanced triplet annihilation.

1937rings, respectively. This suggestion is consistent with the observed Si/Al ratio for ferrierite. The present calculations further indicate that diagonal pairing is favored relative to the meta or ortho pairings in accordance with electrostatic principles and the aluminum avoidance rule.2. As observed in our previous work on mordenite and ZSM-5, the analysis of the formal atomic charges distributions in the clusters indicates the highly covalent nature of aluminosilicate frameworks. The spreading of the negative charge; associated with the presence of Al, indicates that the anionic framework behaves as weak but a soft base.The results presented in this paper complement our earlier findings16J7 and confirm the general principles which have been delineated to describe the behavior and understand the chemistry of zeolite structures. Acknowledgment.Various polyviologen polyelectrolytes which may participate in photoinitiated electron-transfer reactions have been investigated as quenchers. A spectral study was made of the reduced polyviologen radicals on reducing the polyviologen species using the 2-propanol radical. Some of the mono radicals produced, possessing absorption spectra similar to that of the methylviologen radical cation, are found to produce multiradical species in the time scale of seconds and minutes. The nature of the final reduced products and the types of reactions generating them are discussed. The abilities of the polyviologens to quench the emission of the lowest excited states of the two photosensitizers, R~(bpy),(cN)~ and Ru(bpy)?+, were investigated together with the quantum yields of electron transfer and the rates of their back-reactions by using the laser flash photolysis technique. The highest yield of electron transfer was found for the Ru(bpy),2+-poly(o-xylylviologen) system where the quantum yield of photoinitiated electron transfer was determined to be 0.57. The results were compared to those obtained previously with methylviologen and other polymeric viologen systems. The higher than expected rates of quenching and back-reactions were attributed to hydrophobic interactions between the bipyridine groupings of the photosensitizer and quencher which may overcome the repulsive Coulombic forces between them.

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The mechanism of the reactions taking place in a photoredox system which exhibits extremely long lifetimes of redox products was investigated using photochemical, electrochemical, and radiation chemical methods. The system contains a tris(2,2'-bipyridine)ruthenium(II) derivative anchored to a positive poly-3,3-ionene polyelectrolyte (P3,3-Ru(bp~)~~+) as a photosensitizer, 4-methoxy-N,N-dimethylaniline (4-MeODMA) as a quencher and a N,N,N',N'-tetramethyl-p-phenylenediamine derivative covalently bound to another poly-3,3-ionene polyelectrolyte (P3,3-TMPD) as a secondary donor. Synthesis and characterization of these species are described in this work. The lowest emitting excited state of P3,3-R~(bpy)~*+ was found to possess an emission maximum at 618 nm and a lifetime of 502 ns. Stern-Volmer plots indicate that it is quenched by 4-MeODMA with a rate constant of (1.1 f 0.1) X lo9 M-' s-'. The yield and back reaction of the electron-transfer products of this process were followed using laser flash photolysis while the electron transfer scavenging of the 4-MeODMA'+ product by P3,3-TMPD was studied using the pulse radiolysis method. The lifetimes of the two ultimate photoredox products, P3,3-Ru(bpy),+ and P3,3-TMPD+, were found to be in the time range of minutes under our experimental conditions corresponding to a factor of inhibition of their back reaction of more than 5 orders of magnitude as compared to the corresponding reaction of their monomer analogues in acetonitrile solution. This reaction appears to w u r via reoxidation of a small fraction of 4-MeODMA. IntroductionThe goal of storing solar energy as stable chemical fuels has been sought by photochemists for several years. Many such systems are designed based on photosensitized electron-transfer processes of the following type.