MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS-II. pH DEPENDENCE OF ELECTRON ABSTRACTION RATE CONSTANT OF THE DYES IN THEIR TRIPLET STATE

Abstract— The pH dependence of the apparent reactivity of thiazine dyes in their triplet states has been studied in aqueous solutions, using as electron donor HY^-3, the trianionic species of ethylene diamine tetraacetic acid (EDTA), in the pH range 4–8. The pH dependence is found to be related to a change in the degree of protonation of the triplet excited dye. The apparent reactivity and lifetime of two differently protonated forms of thionine, azur B and methylene blue were determined by classical and dye-laser flash techniques, making it possible to evaluate the rate constant for electron abstraction of these molecules in their triplet states. It is found that: (a) protonation on the ring nitrogen increases the electron-abstraction rate constant of the triplet-state species about twenty-fold, and (b) methylation on the side amino groups decreases it.     

TIN CHLORIN-ISOBACTERIOCHLORIN: EVIDENCE FOR A REVERSIBLE PHOTOREDOX PROCESS

Abstract— It is shown that the photoreduction of tin tetraphenylporphyrin using a variety of reducing agents does not stop at the tetraphenylchlorin stage but proceeds to the isobacteriochlorin derivative. However, this latter compound can be photooxidized in the presence of oxygen to yield exclusively tin chlorin which in turn can again be photoreduced to the isobacteriochlorin. The process appears to be photoreversible at this stage. These reactions have been studied in homogeneous and micellar solutions.     

Oriented interaction between naphthalene and alkane molecules: Theoretical interpretation of spectral shifts of ππ* bands of aromatic hydrocarbons in alkane solvents

Several features of the spectral shifts of ππ^* bands for aromatic hydrocarbons dissolved in n-alkane solvents have been explained by the existence of preferred relative orientations between the aromatic hydrocarbon and the n-alkane molecules. Theoretical evaluation of the interaction energy between naphthalene and n-pentane or n-octane actually display two preferred orientations. Moreover, these orientations are identical with those proposed, on steric grounds, for explaining the adsorption of alkanes on graphitic basal planes as proposed by Groszek.     

pH DEPENDENCE OF SINGLET OXYGEN PRODUCTION IN AQUEOUS SOLUTIONS USING THIAZINE DYES AS PHOTOSENSITIZERS

Abstract— The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of ¹O₂* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the p K 's of the triplet dyes, by evaluating the yields of ¹O₂* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.     

Laser flash photolysis of iodine-iodide mixture in hydroxylic solvent. Evidence for the existence of the radical anion I

The laser flash photolysis of iodine-iodide mixtures in alcoholic solvents produces transient species absorbing in the red and near infrared region with maxima at 740 and 590 nm. The 740 nm absorption band has already been attributed to the well known I

ion. In this work the 590 nm absorption band is assigned to the I

radical anion which presumably could be formed by the equilibrium reaction: I

+ I₃^- ? I

. The variations of the intensities of the 590 and 740 nm transient absorption bands as a function of I₃^? concentration, temperature and solvent, support this assignment. The kinetics of formation of I

and I

has been studied in solutions of tertiary butanol. An analysis of the experimental results shows that the I

ion could be formed via a mechanism involving an I

intermediate.     

Dye laser action at 330 nm using benzoxazole: A new class of lasing dyes

Dye laser action has been observed without dispersion elements at 330 nm in phenyl-benzoxazole, the shortest wavelength observed so far in these experimental conditions. Quantum efficiency of laser emission point to benzoxazole as a new class of dye lasers.     

ÉTUDE EN SPECTROSCOPIE PAR ÉCLAIR DES COLORANTS THIAZINIQUES EN SOLUTION AQUEUSE

Abstract— p K values of ionisation equilibrium of thiazines dyes in their triplet state have been measured in aqueous solutions by spectroscopy. The triplet-triplet absorption bands, in the red part of the spectrum, are given for thionine, azurs and methylene blue. It is shown that, in the pH range 4–9, the equilibria investigated correspond to a second protonation of thiazines dyes that occurs in the triplet state:
³TH⁺+ H⁺→³TH₂²⁺
designating by ³TH⁺ the thiazine cation similar to the species stable in neutral aqueous solutions.