Miguel G. Neumann

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.

The interaction of cationic dyes with anionic surfactants in the premicellar region

Abstract A simplified aggregate model has been used to calculate the emission behavior of cationic dye-surfactant systems at low concentrations. By fitting the resultant three parameter equation to the experimental data, values for the equilibrium constants for the ion-pair formation ( K O ), surfactant molecule addition to aggregates ( K S ), and dye dimerization ( K D ) reactions could be evaluated. K O and K S values were larger than those expected from electrostatic interactions, indicating a hydrophobic contribution. The K D s were 10–20 times higher than those found for dimerization in pure aqueous solutions, which can be ascribed to the screening of the electrostatic repulsion.

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.

Effect of sonication on the particle size of montmorillonite clays

This paper reports on the effect of sonication on SAz-1 and SWy-1 montmorillonite suspensions. Changes in the size of the particles of these materials and modifications of their properties have been investigated. The variation of the particle size has been analyzed by DLS (dynamic light scattering). In all cases the clay particles show a bimodal distribution. Sonication resulted in a decrease of the larger modal diameter, as well as a reduction of its volume percentage. Simultaneously, the proportion of the smallest particles increases. After 60 min of sonication, SAz-1 presented a very broad particle size distribution with a modal diameter of 283 nm. On the other hand, the SWy-1 sonicated for 60 min presents a bimodal distribution of particles at 140 and 454 nm. Changes in the properties of the clay suspensions due to sonication were evaluated spectroscopically from dye-clay interactions, using Methylene Blue. The acidic sites present in the interlamellar region, which are responsible for dye protonation, disappeared after sonication of the clay. The changes in the size of the scattering particles and the lack of acidic sites after sonication suggest that sonication induces delamination of the clay particles.

Photophysics and photoreactivity of substituted thioxanthones

The photophysics and the interaction of the excited states of substituted thioxanthones have been studied in order to be able to predict their behaviour as photoinitiators in photopolymerization systems. Electron-donor groups in position 2 in the thioxanthone ring lead to high fluorescence quantum yields, red-shifts in the emission spectra and long singlet lifetimes, explained by the separation of the nπ* and ππ* excited states induced by the higher electron density of the aromatic system. For some of the derivatives a biexponential decay due to slow relaxation of the solvent has been observed. Rate constants for the quenching of the singlet state by amines are near the diffusional limit and are not modified by substituents. On the other hand, substitution of electron-donor groups on the thioxanthone ring in position 2 decreases the triplet reactivity towards amines and alkenes. The incorporation of double bonds on the side chains of the compounds does not affect the lifetimes and reactivity of the thioxanthones.

Interaction of methyl orange with cationic micelles and its effect on dye photochemistry

Abstract The interaction of the azo dye methyl orange with cationic and anionic surfactant micelles was studied by absorption and emission spectroscopy. The dye interacts strongly with the cationic micelles cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB), but does not interact with the anionic micelle sodium dodecylsulphate (SDS). The association constants of the dye to CTAC and CTAB micelles are 5.6 × 10 4 and 5.9 × 10 5 respectively. The fluorescence quenching of pyrene by methyl orange in CTAC micellar solution occurs via non-radiative energy transfer. The variation of the relative fluorescence intensity agrees well with a model in which the dye is almost completely associated with the micellar phase. The photolysis of an aqueous solution of the dye in the presence of acetone gives rise to an irreversible photoreduction of the azo dye by the ketyl radical formed during photolysis. The presence of CTAB micelles reduces the photodecomposition rate of the dye. This effect is ascribed to the micellar cage effect which inhibits the second-order disproportionation reaction of the hydrazo compound formed.

Dependence of the thioxanthone triplet-triplet absorption spectrum with solvent polarity and aromatic ring substitution

Foram estudadas as absorcoes triplete-triplete (TT) e de transientes de tioxantonas substituidas e nao substituidas em diferentes solventes, com a finalidade de avaliar o efeito do solvente, assim como dos substituintes no anel aromatico. Os espectros determinados alguns microssegundos depois da excitacao mostram absorcoes de tres transientes principais: o estado triplete (600-650 nm), o radical cetila da tioxantona (~450 nm) e uma superposicao de ambos (~300 nm). A quantidade de radicais formados em solventes nao-hidroxilicos e muito menor do que em alcoois. O maximo da absorcao TT mostra uma boa correlacao com o parâmetro ET(30) dos solventes.

Photopolymerization of methyl methacrylate initiated by thioxanthone derivatives: photoinitiation mechanism

The photopolymerization of methyl methacrylate initiated by substituted thioxanthones in the presence of an amine has been investigated. The polymerization rates in acetonitrile were measured at several amine concentrations. The dependence of the polymerization rates with the amine concentration and the photoinitiation efficiency at a given amine concentration are highly dependent on the ketone structure. The rate constants for the quenching of thioxanthones excited states by the monomer and the amine were measured by fluorescence and laser flash photolysis, under the polymerization conditions. These data show that the reactivity of the ketone is also highly dependent on the ketone structure. The values of the quenching rate constants show that the initiation efficiency is controlled by the competition between the quenching of the excited states by the monomer and also the amine. From the latter a mechanism is proposed to simulate the dependence of the polymerization rates with amine concentration.

The influence of the photophysics of 2-substituted thioxanthones on their activity as photoinitiators

The polymerization of methyl methacrylate photoinitiated by thioxanthones (TXs) substituted in the position 2 of the chromophore ring in the presence of 2-(N,N-diethylamino)ethanol has been investigated. The photoinitiation efficiency of these systems is highly dependent on the structure of the 2-substituent. At high amine concentrations, compounds with electron-withdrawing substituents appear as more efficient photoinitiators. The photophysics of the ketones was studied in the polymerization medium. These data show that the singlet excited state of TXs is deactivated by the amine with a rate constant near the diffusional control limit. The triplet rate constants for quenching by the amine and the monomer are highly dependent on the ketone structure. Compounds with electron-withdrawing substituents in the position 2 are more reactive. These studies allow to simulate the dependence of the photoinitiation efficiency with the amine concentration and indicate that the active radicals are produced by the interaction of the ketone triplet with the amine.

A fluorescence study of the interactions between sodium alginate and surfactants.

The interactions between the polysaccharide alginate with charged ionic surfactants (anionic and cationic) in aqueous solution have been investigated using pyrene as a photophysical probe. Static fluorescence determinations have been used to obtain information about the new microenvironments arising by these interactions. Micropolarity studies using the I(1)/I(3) ratio of the vibronic bands and I(E)/I(M) ratio between the excimer and monomer emissions of pyrene shows the formation of hydrophobic domains. The interactions between the natural polyelectrolytes and the oppositely charged surfactants lead to the formation of pre-micelles at surfactant concentrations lower than the CMC of the surfactants. The aggregation process is assumed to be due to electrostatic attraction. On the other side, systems containing an anionic surfactant do not show the same behaviour at low concentrations.