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‐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 φs of AOH+ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φs 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† (φMV:)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 (φMV:)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.

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