Photochemistry of 5-methylphenazinium salts in aqueous solution. 2. Optical flash photolysis and fluorescence results and a proposed mechanism

The photochemistry of 5-methylphenazinium (MP/sup +/) salts in aqueous solutions for pH 2 to 7 has been studied by using fluorescence, optical flash photolysis, and electron paramagnetic resonance (EPR) techniques. A complete mechanism has been proposed which accounts for all of the known observations on this reaction. The fluorescence yield is approx. 1% and independent of pH for pH > 4 but drops sharply for pH < 3 presumably because of the formation of the nonfluorescent protonated excited state MPH/sup 2 +/(S/sub 1/). This state also appears to be photochemically inert as the quantum yield of formation of MPH/sup +/.also drops for pH < 3. The triplet state MP/sup +/(T/sub 1/) has likely been detected by its triplet-triplet absorption and is found to have a lifetime in solution of approx. 0.5 ms. The triplet state is quenched efficiently by oxygen; howeverk oxygen does not quench the photochemistry; hence, it is concluded that photochemistry occurs via the excited n..pi..* singlet state MP/sup +/(S/sub 1/). The novel feature of the proposed mechanism is that the primary photochemical step is proposed to involve the addition of water to MP/sup +/(S/sub 1/) with the concomitant loss of a proton to form an adduct X.more » X is a strong oxidizing agent and appears to react as an OH radical addition reagent in its reactions with MP/sup +/, formate, and acetate. In the absence of added reagents, X reacts with MP/sup +/ to initiate a complex series of reactions. Most of the rate constants and some equilibrium constants have been determined for these reactions and are summarized. It is possible that this dye may be useful as a photochemical oxidizing agent in the sensitized photochemical decomposition of water.« less