Eosin triplet

In the delayed emission spectrum of eosin in glycerol or ethanol two bands are present, the relative intensities of which are strongly temperature-dependent (see Fig. 12). The visible band at 1.8 has a contour identical with that of the fluorescence band. It no doubt corresponds to the visible phosphorescence observed by Boudin.26 To interpret the results it was assumed that this band of delayed fluorescence was produced by thermal activation of the eosin triplet to the upper singlet level followed by radiative transition from there to the ground state. The far red band was assumed to correspond to the direct transition from the triplet level to the ground state and was therefore called phosphorescence. To determine the relationship between the intensities of the two bands we write the equations for the formation and consumption of triplet molecules as follows ... 

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

Irradiation in the presence of MDEA completely inhibits the formation of products. The amine quenches the fluorescence of Eosin with a rate constant of 8 x 108 M-1s-1 and quenches the Eosin triplet with a rate two orders of magnitude lower. A summary of rate constants for the decay of the triplet is presented in Table 8. In addition to the reactions shown in Scheme 3, with Am = (V-methyl diethanolamine, the rate constants for reaction of PDO with Eosin triplet and semioxidized Eosin radical in aqueous solution (Eqs. 19 and 20) are included in the table. 

Diphenyliodonium, at a concentration of 0.05 M, does not quench the Eosin triplet. It decreases the fluorescence intensity by a factor of 20 and shifts the fluorescence wavelength 10 nm toward the red. The calculated rate constant for singlet quenching is 5 times larger than for a diffusion controlled... 

Excitation of dendrimer-encapsulated eosin results in sensitization of the dioxygen emission via eosin triplet excited state. The behavior of fluorescein and Rose Bengal was qualitatively similar to that of eosin, whereas 5-(and-6)-carboxynaphthofluorescin was not encapsulated. 

Triplet quantum yields for eosin and some other dyes have been determinded by Soep et al. The system to be studied is exposed to a dye laser pulse (with tunable wavelength) and the transient absorb-tion changes in the system due to triplet population are recorded with the light from a dc xenon lamp. 

Phosphorescence measurements of eosin in ethanol place the 0-0 transition of 3Si 3S0 at 7150 A.98 Recent determinations of phosphorescence spectra of xanthene dyes in EPA at 77°K show these transitions to occur in the region of 7000 A 94,103 (Table I). The triplet energies of these dyes, 3Bi, are thus about 40 kcal/mole and the energy separations between the excited singlet and triplet states, AE = Elai — 3si, are about 12-13 kcal/mole. 

Triplet Lifetimes and Triplet Formation Efficiencies of Eosin Di-Aniona... 

Approximate values of v and r for proflavine hydrochloride are shown in Table III. As judged by both quantities, triplet quenching processes appear to have largely disappeared at a temperature as high as —58 °C. This is in marked contrast to eosin (see Table II). The value of... 

The delayed fluorescence produced by triplet-triplet quenching is to be sharply differentiated from that observed with eosin or proflavine hydrochloride. The latter type has the same lifetime as the triplet and its intensity is proportional to the first power of the rate of light absorption. It is produced by thermal activation of molecules from the triplet level to the excited singlet level and can occur with any substance for which... 

Triplet-Singlet Phosphorescence eosin, proflavine hydrochloride, phenanthrene. 

An Arrhenius type plot of EDas a function of temperature will give the value of the energy gap A Est. The value thus obtained has been found to agree with the singlet-triplet energy difference 42 kJ (10 kcal) mol- obtained by direct spectroscopic methods for eosin in glycerol. 

Several derivatives of Eosin have been prepared and employed to study biological systems. Their main applications are as singlet energy acceptors and as triplet probes [93-97] to measure the rotational mobility of virus particles [98] and proteins in membranes and in solution. Examples of proteins studied using Eosin derivatives include myosin [99,100], band 3 protein [101,102], pyruvate dehydrogenase [103,104], and Sarcoplasmic... 

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

Photosensitized electron injection in colloidal TiOz has been reported by Moser and Gratzel as part of a scheme to photoreduce water [155], Moser et al. [156] and Rosetti and Brus [157] saw the formation of semioxidized Eosin by monitoring its absorption in the visible and by Raman spectroscopy. No oxidation of the triplet of Eosin adsorbed in Ti02 is observed, the lifetime... 

We have performed laser flash photolysis experiments in ethyl acetate/20% methanol under conditions in which triplet-triplet annihilation is not important. We achieve these conditions by using Eosin concentrations higher than 1 x 10 5 M and low pulse energies in order to obtain clean exponential decays for the triplet. Determination of the triplet lifetime at different Eosin and amine concentrations allows us to obtain the rate constants collected in Table 5. A summary of our observations is as follows ... 

When triphenylamine 3 x 10 3 M is used to quench the triplet only the semireduced Eosin radical is formed and its absorption decays to zero within one millisecond. 

TABLE 5 Triplet Decay Rate Constants for Eosin in Ethyl Acetate/20% Methanol"... 

Steady-state irradiation of Eosin and PDO leads to the formation of ethyl benzoate, benzil, and benzoyloxycarboxylate. Energy transfer from Eosin to PDO is unfavorable and the decomposition of PDO is most likely sensitized by electron transfer. In addition, at the concentration of PDO employed the fluorescence of Eosin is not significantly quenched and it is concluded that the excited state involved is the triplet of Eosin. 

Fouassier and Chesneau [219] is not consistent with the experimental observations. From the values of the rate constants of triplet decay presented in Table 8, and taking into account that k3/k2 = 0.23 (as determined by Kasche and Lindqvist), we calculate the quantum yield of D + under the polymerization conditions. For Eosin (3 x 10 5 M) and MDEA (0.1 M) the yield of semioxidized Eosin radical is 4 x 10 3 M in the presence or in the absence of 2 x 10 3 M PDO. From the values for the quantum yield of photopolymerization and the molecular weight in the absence of PDO we calculate a quantum yield of initiation between 0.086 and 0.17, the actual value depending on the mode of termination. Therefore, we conclude that formation of a-amino radicals according to Scheme 10 represents only a minor contribution to the quantum yield of initiation observed in the presence of PDO. 

There are a large number of spectroscopic reports involving Erythrosin, particularly from Russian laboratories. The near-IR absorption spectra of both Eosin and Erythrosin triplets and their dianion radical cations have been reported [236], the pK s of both species measured [237], and the influence on radiative and radiationless transitions by external heavy atoms such as iodide reported [238]. The nonradiative decay of Erythrosin in water at room temperature has also been studied by photoacoustic spectroscopy [239],... 

Another less-common form of unimolecular decay is also temperature dependent and results in the phenomenon known as is-type delayed fluorescence.183 For example, the lowest excited singlet and triplet states of eosin are quite close together in energy, so that excited... 

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