Carlos M. Previtali

The Excited-State Interaction of Resazurin and Resorufin with Aminesin Aqueous Solutions. Photophysics and Photochemical Reaction¶

Abstract The photophysics and photochemical behavior of the phenoxazin-3-one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N-oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (ΦF = 0.75). At lower pH the fluorescence is strongly reduced. The N-oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N-oxide in the presence of amines occurs from the triplet state.

Photoinitiated vinyl polymerization by safranine T/triethanolamine in aqueous solution

The photolysis of safranine T in the presence of triethanolamine has been used as a source of free radicals in the polymerization of 2-hydroxyethyl methacrylate in aqueous solution. The polymerization rate was measured at several amine and dye concentrations. Flash photolysis studies of safranine in basic aqueous solutions indicate that the photoinitiation proceeds by the amine radicals formed in the interaction of the unprotonated form of the dye triplet with the hydroxyl amine. Primary radical termination does not play a significant role.

FLUORESCENCE QUENCHING OF INDOLIC COMPOUNDS IN REVERSE MICELLES OF AOT

Abstract— The fluorescence properties of indole, 1,2‐dimethylindole and tryptophan have been determined in sodium bis(2‐ethylhexyl)sulfosuccinate inverse micelles in heptane. Fluorescence quenching studies were done using CCl4 and acrylamide as quenchers localized in the organic and micellar pseudophase respectively. The Stern‐Volmer plots show departure from linearity and are dependent on the emission wavelength. The results were interpreted using a three pseudophases model. 1,2‐Dimethylindole is predominantly solubilized in the n‐heptane; however a small interaction with the micelles is apparent at high detergent concentration. Indole is distributed between the organic and the interfacial microphase with a partition constant of 34 ± 3, independent of the water content. Tryptophan is entirely localized at the interface. However the quenching experiments show that tryptophan molecules are not uniformly distributed in the interfacial pseudophase.

Experimental study of the excited-state properties and photostability of the mycosporine-like amino acid palythine in aqueous solution

Characterization of the excited states of the mycosporine-like amino acid palythine (lambda(max) = 320 nm) in aqueous solutions was achieved experimentally. The low value for the photodegradation quantum yield, (1.2 +/- 0.2) x 10(-5), confirms that palythine is highly photostable in air saturated-aqueous solutions. Laser flash photolysis of acetone in the presence of palythine allowed for the observation of a transient spectrum which is consistent with the triplet-triplet absorption of palythine. Kinetic treatment of the transient signals yields a lifetime of the triplet state of ca. 9 micros and a triplet energy around 330 kJ mol(-1). The photoacoustic calorimetry results are consistent with non-radiative decay as the major fate of excited palythine. A comparison of the photodegradation quantum yields and photophysical properties of palythine with those previously determined for the other mycosporine-like amino acids, shinorine and porphyra-334, suggests that geometrical isomerization around the C=N bond may contribute to the rapid deactivation of this group of molecules.

THE INTERACTION OF THE EXCITED STATES OF SAFRANINE T WITH ALIPHATIC AMINES IN ORGANIC SOLVENTS

Abstract—

Comparison of photoinduced electron transfer to singlet and triplet states of safranine T

Abstract The reactivities of the singlet and triplet excited states of safranine T with several substituted amines and phenols were determined by static fluorescence quenching and laser flash photolysis. The results are interpreted in terms of the Rehm and Weller formulation for electron transfer quenching processes. All the experimental points fall near the theoretical curve predicted by Marcus theory, suggesting that there is no fundamental difference between the reactivities which can be ascribed to the multiplicities of the excited states.

Excimer emission of pyrene derivatives induced by ionic detergents below the critical micelle concentration

Abstract The fluorescence of three ionic pyrene derivatives in the presence of CTAC and SDS was investigated at detergent concentrations below the critical micelle concentration (CMC). The characteristic excimer emission was observed for surfactant concentrations more than one order of magnitude lower than the CMC. Lifetime measurements show that excimer formation must be via a static mechanism. This is explained by the formation of mixed premicellar aggregates between the surfactants and the probes. The quenching of the excimer emission by ionic (TI + , S 2 O 2- 3 ) and neutral (N,N-dimethylaniline, fumaronitrile) species was studied. The quenching is only effective when the ions are of opposite charge to that of the detergent. The quenching by neutral species can be explained by a static mechanism which involves the association of the quencher with the aggregates.

Photodechlorination of trichlorobenzenes mediated by a naphthalene—triethylamine exciplex

Abstract The fluorescence quenching of the naphthalene—triethylamine exciplex by trichlorobenzenes was studied in cyclohexane and ethyl acetate solutions. In ethyl acetate the quenching is accompanied by the production of hydrogen chloride. A mechanism is proposed to explain the experimental results in which naphthalene behaves like a photocatalyst and the exciplex transfers an electron to the trichlorobenzenes. The rate constants for fluorescence quenching and the quantum yields of hydrogen chloride formation are reported.

Photophysical studies of indole alkanoic acids and tryptamine in reverse micelles of sodium dioctyl sulfosuccinate

Abstract— The fluorescence spectra and emission lifetimes of several 3‐alkanoic indoles of different chain length and tryptamine (TA) were studied in sodium dioctyl sulfosuccinate (AOT)/heptane reverse micelles over a wide range of water/AOT ratio (R ‐ 5 to 44). Fluorescence quenching experiments were done using carbon tetrachloride and acrylamide as quenchers. Experiments with TA were carried out using water at pH 3 in order to assure its protonation. Under these conditions, the results indicate that the indole moiety of TA remains at the micellar interface over all the range considered. Furthermore, the results can be interpreted assuming for the TA population a single microenvironment whose properties remain almost invariant when R increases from 11 to 44. The studies employing the 3‐alkanoic indoles were carried out at pH 10. Under these conditions, the anions are progressively displaced to the water pool when the R value increases. This displacement is determined by the length of the side alkyl chain of the 3‐indole derivatives. For these compounds, the quenching experiments indicate that, even at low R values, the excited indole moieties are distributed among different microenvironments.

The reductive quenching of excited Ru(bpy)32+ by aromatic amines: solvent and structural effects

Abstract The luminescence quenching of excited Ru(bpy) 3 2+ by 1-naphthylamine, 2-naphthylamine, N , N -dimethylaniline and diphenylamine was studied in methanol, ethanol, 2-propanol, acetonitrile, propionitrile and acetone. The bimolecular rate constants for the electron transfer process from the aromatic amines to the excited ruthenium complex were measured as a function of temperature. Activation enthalpies and entropies were determined for each amine in all the solvents. The results were analysed in the framework of Marcus theory for electron transfer reactions. In the alcoholic solvents the process seems to follow the predictions of the theory if an outer-sphere mechanism is assumed. However, for the aprotic solvents the observations are not in agreement with the expected behaviour. In these solvents the rate constants for a given free energy change are dependent on the structure of the aromatic moiety and are much lower for the naphthylamines.