MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY†

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV²⁺) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH⁺). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH⁺ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φ_s of AOH⁺ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φ_s is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH⁺ is quenched by MV²⁺ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 10¹⁰M^?1 s^?1. The quenching of the AOH⁺ excited singlet state by MV²⁺ almost certainly returns the AOH⁺ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 10⁸M^?1s^?1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV²⁺, flash irradiation of AOH⁺ does result in the reversible formation of the semireduced MV? which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH⁺ with MV²⁺. The initial quantum yield for formation of MV? (φ_MV:)₀ was found to be constant at 0.10 ± 0.05 for [MV²⁺] from 5 × 10^?5 to 1.0 × 10^?3 with [AOH⁺] = 8 × 10^?6M. Previous workers had found that (φ_MV:)₀ appears to decrease with decreasing [AOH⁺]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH⁺ by trace amounts of oxygen. When EDTA is added to a mixture of AOH ⁺ and MV²⁺ at pH 7, the photochemical formation of MV? becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV? exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH⁺ to produce the semi-reduced AOH- which then reacts with MV²⁺ to produce MV?. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO₂ was added to the optimal mixture [EDTA] = 3.4 × 10^?2M; [MV²⁺] = 5 × 10^?4M; [AOH⁺] = 4 × 10^?5M; pH6 H₂ gas was produced at a rate of 0.2μmol H₂h^?1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.     

Homolytic reactions of polyfluoroaromatic compounds. Part XII. Aryldehydrogenation and aryldefluorination in polyfluorobenzenes

The thermal decomposition of benzoyl peroxide in a range of polyfluorobenzenes at 80° gives biaryls arising from both the displacement of fluorine and hydrogen. Phenyldefluorination predominated only in the attack of 1,2,3,4-tetrafluorobenzene and of pentafluorobenzene, but was evident even in the attack of $\text{o}$-difluorobenzene. The relative rates of displacement, as shown by the isomer distributions, were generally well reflected by the application of partial rate factors found in the phenylation of fluorobenzene; however, phenyldefluorination was not so easily amenable to this treatment, a possible consequence of complex formation between radical precursor and substrate.     

Size dependent melting mechanisms of iron nanoclusters

Molecular dynamics simulations were used to study the change in the mechanism of iron cluster melting with increasing cluster size. Melting of smaller clusters (e.g., Fe₅₅ and Fe₁₀₀) occurs over a large temperature interval where the phase of the cluster repeatedly oscillates between liquid and solid. In contrast, larger clusters (e.g., Fe₃₀₀) have sharper melting points with surface melting preceding bulk melting. The importance of the simulation time, the force field and the definition of cluster melting is also discussed.     

FLASH PHOTOLYSIS-ELECTRON SPIN RESONANCE: A KINETIC STUDY OF ENDOGENOUS LIGHT-INDUCED FREE RADICALS IN REACTION CENTER PREPARATIONS FROM RHODOPSEUDOMONAS SPHEROIDES*

Abstract— Using the technique of flash photolysis-electron spin resonance, we have detected, by means of a kinetic analysis, a rapidly decaying signal in reaction center preparations from the R26 blue-green mutant of Rhodopseudomonas spheroides. This signal, which we designate Signal B3, is essentially the same as that seen previously in iron-free preparations. Signal B3 decays at 20°C with a 1/ e time of ˜ 3 ms and exhibits an activation energy of 5 ± 1 kcal mol^-1 over the temperature range 0–30°C. Extraction with isooctane completely eliminates Signal B3, whereas readdition of exogenous ubiquinone-30 completely restores the signal. o -Phenanthroline has no effect on Signal B3. We discuss these results in terms of a model in which the primary acceptor is an iron-ubiquinone complex with excess ubiquinone serving as a secondary electron acceptor pool.     

Homolytic reactions of polyfluoroaromatic compounds IX. Abstraction reactions of some p-substituted tetra-fluorophenyl radicals

The decomposition of pentafluoroaniline, 2,3,5,6-tetrafluoroaniline and its 4-bromo and 4-methoxy analogues by pentyl nitrite gives the corresponding aryl radicals which, when generated in the presence of chloroform, carbon tetrachloride, or bromotrichloromethane, remove atoms from these solvents to give substituted benzenes. The ease of removal of X from XCCl₃ decreases in the order X = Br,H > Cl.The yield of substituted benzene product depends upon the p-substituent of the polyfluoroaniline and is greatest for tetrafluoro-p-anisidine.     

The electron-impact-induced fragmentation of acridine

Studies of both high and low resolution spectra, and of metastable decompositions occurring in both the first and second field-free regions of the mass spectrometer have led to a postulated scheme for the fragmentation of acridine under electron-impact. There is no specific loss of label from either [9-²H₁]acridine or [4,5-²H₂]acridine in any fragmentation, nor is there any total scrambling of label in either molecular ion prior to loss of HCN. There is certainly some degree of scrambling preceding HCN loss from [M]⁺˙ at 70 eV, but this does not involve the 9-H to any detectable extent. There is no strong evidence for the acridine molecular ion having the same structure as that of four other C₁₃H₉N isomers.