Electron transfer reactions of excited dyes with metal complexes. Progress report, March 1, 1978-February 28, 1979. [Iron-methylene blue]

An intense absorption band of /sup 3/MBH/sup 2 +/ at 700nm was characterized in several media and its pK/sub A/ was measured to be 7.17 +- .1 in water. Flash photolysis show that semiethylene blue, MBH/sup +/, formed by quenching /sup 3/MBH/sup 2 +/ with Fe/sub II/(H/sub 2/O)/sub 6//sup 2 +/, decays by disproportionation at a diffusion-controlled rate. Rates of decay of the photostationary state of solutions of MB/sup +/ and Fe/sup II/(H/sub 2/O)/sub 6//sup 2 +/ in acidic media were measured using crossed beams to yield specific rates of oxidation of leucomethylene blue (MBH/sub 3//sup 2 +/) by MB/sup +/ (synproportionation) and by Fe(III). Laser flash-photolysis shows that quenching of MB/sup +/(S/sub 1/) by Fe(H/sub 2/O)/sub 6//sup 2 +/ can result in electron transfer to give MBH/sup +/. Profound differences between processes initiated by quenching triplet methylene blue with Fe(H/sub 2/O)/sub 6//sup 2 +/ and with stable coordination complexes of Fe(II) were found. Quenchers included (Fe/sup II/(CN)/sub 6/)/sup 4 -/, (Fe/sup II/(CN)/sub 4/(bpy)/sub 2/)/sup 2 -/, (Fe/sup II/(CN)/sub 2/(bpy))/sup 0/, and (Fe/sup II/(bpy)/sub 3/)/sup 2 +/. Measurements were made in aqueous and aqueous-alcoholic solutions at pH 2, 4.4, and 8.2. Quenching of /sup 3/MBH/sup 2 +/ or /sup 3/MB/sup +/ by a stable complex of Fe(II) is diffusion-controlled and 10/sup 2/ to 10/sup 3/ times the rate at which Fe(H/sub 2/O)/sub 6//sup 2 +/ quenches /sup 3/MBH/sup 2 +/. Net electron transfer accounts for less than 1/3 of total quenching by complexed Fe(II). In contrast, quenching by Fe(H/sub 2/O)/sub 6//sup 2 +/ goes essentially entirely with net electron transfer. It is concluded that quenching of triplet methylene blue by complexes proceeds via electron transfer which is reversible in the encounter complex. The MBH/sup +/ which results from net electron transfer from complexes of Fe(II) decays essentially entirely to MB/sup +/ via oxidation by complexed Fe(III); no disproportionation has been observed

Photochemical Determinants of the Efficiency of Photogalvanic Conversion of Solar Energy

A photogalvanic cell is a battery in which the cell solution absorbs light directly to generate species which, upon back reaction through an external circuit with the aid of suitable electrodes, produces electric power; photoactivation of the electrodes is not involved. The charge-carrying species have storage capacity if they are long-lived and can be prevented from engaging in degradative back reactions in bulk solution. The efficiency of a photogalvanic cell for the conversion of photon energy into electrical energy is determined by photochemical and electrochemical factors. Among the latter are the choice of electrode materials and the kinetics of electron transfer at the heterogeneous surfaces. The photochemical determinants of the efficiency of photogalvanic cell operations are examined: the absorption spectral characteristics of the cell solution, the efficiency of formation of separated charge carriers, and the lifetimes of the carriers toward back electron transfer. Modulation of bulk solution dynamics can be achieved by variation of the solution medium. The photochemical determinants are discussed with particular reference to the use of thionine or RU(bpy)2/3/sup +/ as the light absorbing species. 76 references

Previously unreported intense absorption band and the pK/sub A/ of protonated triplet methylene blue

Excitation by a Q-switched giant ruby laser (1.2 joule output at 694 nm, approx. 50 nsec flash) of 2-10 ..mu..M solutions of methylene blue in water, 30% ethanol in water or 50 v/v% water - CH/sub 3/CN at pH values in the range 2.0 - 9.3 converted the dye essentially completely to its T/sub 1/ state. The absorption spectrum of T/sub 1/ dye was measured in different media at pH 2.0 and 8.2 by kinetic spectrophotometry. Previously reported T-T absorption in the violet in acidic and alkaline solutions and in the near infrared in alkaline solution was confirmed. Values found for these absorptions in the present work with 30% ethanol in water as solvent are lambda/sub max/ approx. 370 nm, epsilon/sub max/ approx. 13,200 M/sup -1/ cm/sup -1/ at pH 2 and lambda/sub max/ approx. 420 nm, epsilon/sub max/ approx. 9,000 M/sup -1/ cm/sup -1/, lambda/sub max/ approx. 840 nm, epsilon/sub max/ approx. 20,000 M/sup -1/ cm/sup -1/ at pH 8.2. Long-wavelength T-T absorption in acidic solution is reported here for the first time: lambda/sub max/ approx. 680 nm, epsilon/sub max/ approx. 19,000 M/sup -1/ cm/sup -1/ in 30% ethanol in water at pH 2. Observation of a pH-independent isobestic point approx. 720 nm confirms that the long-wavelength absorptions are due to different protonated states of the same species, MB/sup +/(T/sub 1/) and MBH/sup 2 +/(T/sub 1/). The pK/sub A/ of MBH/sup 2 +/(T/sub 1/) in water was determined from the dependence on pH of absorption at 700 and 825 nm to be 7.1/sub 4/ +- .1 and from the kinetics of decay of triplet absorption to be 7.2. The specific rate of protonation of MB/sup +/(T/sub 1/) by H/sub 2/PO/sub 4//sup -/ in water at pH 4.4 was found to be 4.5 +- .4 x 10/sup 8/ M/sup -1/ sec/sup -1/