Electron transfer in the quenching of protonated triplet methylene blue by ground-state molecules of the dye
A Q-switched pulsed ruby laser emitting at 694.3 nm was used in an investigation by means of flash photolysis-kinetic spectrophotometry of the mechanism of quenching of the monoprotonated lowest triplet state of methylene blue, /sup 3/MBH/sup 2 +/, by the ground state of the dye, MB/sup +/. Quenching in 0.01 N acid is accompanied by electron transfer to give the half-oxidized and half-reduced ion radicals, MB/sup 2 +/. and MBH/sup +/.. The absorption spectrum of MB/sup 2 +/. has been characterized in several media from 360 to 600 nm. The rate constant for quenching, k/sub q/, varies with solvent, ionic strength, and nature of anions with values around 1 x 10/sup 8/ M/sup -1/ s/sup -1/ in water, aqueous CH/sub 3/CN, and aqueous EtOH. The efficiency of net electron transfer in quenching, F/sub 1/(= k/sub et//k/sub q/), varies with solvent but is independent of the ionic strength or the nature of the anions. F/sub 1/ varies inversely with polarity of the solvent from 0.055 in water to 0.48 in 90% (vol/vol) aqueous CH/sub 3/CN. On the basis of analogy to the behavior of a number of other quenchers and the observed linear variation of the function ln ((1/F/sub 1/)-1) with Kosower's polarity parameter Z, it is suggested that reversible electron transfer is the only significant mechanism of quenching of /sup 3/MBH/sup 2 +/ by MB/sup +/(S/sub 0/). Both MBH/sup +/. and MB/sup 2 +/. decay by second-order processes in solvents containing 75% (vol/vol) or less of organic component but the specific rates are different for the two species in most media. It is suggested that in the latter media both cross reaction of MBH/sup +/. with MB/sup 2 +/. and biomolecular reaction of two molecules of the same radical occur. This study shows that ground-state quenching can significantly reduce the sunlight engineering efficency of photogalvanic conversion in systems incorporating relatively concentrated dyes.
- Research Organization:
- Boston Univ., MA
- OSTI ID:
- 6417835
- Journal Information:
- J. Phys. Chem.; (United States), Vol. 85:7
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
METHYLENE BLUE
SCINTILLATION QUENCHING
ABSORPTION SPECTRA
CHEMICAL REACTION KINETICS
ELECTRON TRANSFER
GROUND STATES
PHOTOLYSIS
SPECTROPHOTOMETRY
TRIPLETS
AMINES
ANTI-INFECTIVE AGENTS
ANTIMICROBIAL AGENTS
AZINES
CHEMICAL REACTIONS
CHLORIDES
CHLORINE COMPOUNDS
DECOMPOSITION
DRUGS
ENERGY LEVELS
HALIDES
HALOGEN COMPOUNDS
HETEROCYCLIC COMPOUNDS
KINETICS
MULTIPLETS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANIC SULFUR COMPOUNDS
PHENOTHIAZINES
PHOTOCHEMICAL REACTIONS
REACTION KINETICS
SPECTRA
400500* - Photochemistry