Quantum yield micellar solutions

Pertinent data concerning the photoreactions are summarized in Table 17. Quantum yields are for loss of starting ketone and are relative to irradiation of valerophenone in t-butyl alcohol. Both the values of d> (rel) and t/c from the micelles resemble more closely those from the t-butyl alcohol solution than from the nonpolar benzene. Coupled with the data of Winkle et al. [306], these results indicate that the alkanophenones reside primarily within the hydrophobic interiors of the micelles, but that the BRs migrate to water-enriched environments at or near the nebulous micellar surfaces. As such, even the template effect on the BRs is nonselective and probably allows all of the conformational changes which occur in a polar isotropic environment. 

TcU le I Relative Yields (based on ketone consiamed) and Quantum Yields for the Photo Products of 1 in Homogeneous and Micellar Solutions. 

Since the quantum yield for disappearance of ketone (triplet ketone nor the primary radical pair PhCH2CO CH2Ph is scavenged. As stated above, the proposed mechanism for the photolysis of DBK in micellar solution is illustrated in Fig. 5 13,21). The micellar environment inhibits the diffusion of radicals to the bulk aqueous phase the radical pair s distance maximum separation is maintained to a few tens of angstroms or less. The amount of escape being reduced, the radicals can then undergo more efficient intersystem crossing and recombination. 

The photohydrolysis of 3,5-dinitroanisole (74) to 3,5-dinitrophenoxide ion (75) is catalyzed by hydroxide ion and studied in both aqueous and micellar systems (Scheme XXIX)74). The quantum yield for the reaction decreased in tetradecyltri-methylammonium chloride (TTAC1) micelles compared to aqueous solution. The results are explained by the decrease triplet lifetime of 3,5-dinitroanisole in the nonpolar micellar environment compared to the aqueous phase. 

Fig. 9-3. The observed magnetic field dependence of the a value for the photodecomposition of DBK in a HDTCL micellar solution at room temperature. It is possible to measure the a value by two different methods (o) mass spectral analysis and (x) quantum yield measurements. (Reproduced from Ref. [2] by permission from The...

Vesicles are ordered fluids or liquid crystals, a fact which reflects well in those photoreactivities that are particular to vesicle solutions. Fatty acid derivatives 24 and 25, for example, show low quantum yields of fluorescence ((j)f) and high quantum yields of cis-trans isomerization (< )c) as well as short fluorescence life times (tf) in both methylcyclohexane and micellar SDS solutions (Table 1). In DPPC vesicles, on the other hand, cis-trans isomerizations are cumbersome and much slower, and the fluorescence yield and lifetime rise considerably (Table 1). For those stilbene derivatives which are embedded in the middle of a fatty acid backbone, isomerization is virtually eliminated in the low-temperature or gel phase of the bilayer. The vesicle thus plays the role of stabilizing trans configurations which fit into the frozen oligomethylene chain matrix. 

The effect of humic materials on the photolytic micellar system was evaluated in DR s photodegradation. DR solubilized within Tween 80 micellar solution with or without humic materials was determined. In order to calculate the quantum yield, the molar absorptivity of DR was determined by spectrophotometry. The determination of the quantum yield and reaction rates was examined through a pseudo first-order decay rate expression. Quenching and catalytic effects resulting from the humic substances were examined through Stem-Volmer analysis. A reaction mechanism of photolytic decay of DR solubilized within surfactant micelles in the presence of various amount of humic materials was proposed for this purpose. The effect of high and low concentration of humic materials has been accounted for by a designed model. 

The low quantum yield of the 3MLCT emission for [Pt(ClANAN)L]C104 36 (L=Ph2PCH2NHPh) is attributable to photoinduced electron transfer (PET) quenching by the amine component [38]. However, the emission intensity varies over a wide pH range in aqueous and micellar solutions presumably due to the extent of PET suppression through protonation at the amine moiety. As the amine receptor becomes protonated at low pH, quenching of the excited state is precluded. Hence the emission intensity is noticeably en-... 

Quantiim yields have been determined for displacement of alkoxy groups in 3-alkoxynitrobenzenes by hydroxide ion in aqueous and micelle solutions to give 3-nitrophenol. The quantvun yields are higher in the micellar medium. The photolysis of 3-nitrophenol itself has been studied in aqueous solution.The quantum yields for disappearance are very low (in the order of 10 ) and numerous products are formed which include resorcinol, nitroresorcinol and nitrocatechols. 

The principles of micellar enhancement including solubilization and solute organization, altering the local microenvironment and changing the light-emitting pathways that affect the quantum yield and reaction rate have already been discussed... 

The rates or efficiencies of bimolecular photoreactions such as excimer formation, photodimerization, and photoaddition can be strongly affected by using micellar solvents when the reactants associate with the micelles When low concentrations of reactants are solubilized in solutions of high concentrations of micelles, the reactants will be separated by association to different micelles and the reaction will be inhibited. Under opposite conditions high local concentrations will cause an increase of quantum yields compared to homogeneous solutions of equal analytical concentration. Thus in micellar solution the efficiency of bimolecular... 

Figure 9. Quantum yields of photodimeiization of 9-methylanthiacene in micellar solutions of CTAB (O) and CTAC (V) as a function of mean occupation numbers z. Straight lines drawn according to Eq. 51 using — 0.003 M, kiko — 0.011 and 0.016, k /ko = 730 and 1080 for CTAB and CTAC, respectively.

The observed enhancement in emission intensity in micellar systems must derive from an increase in either solute molar absorptivity at the exciting wavelength and/or quantum yield, compared with that in bulk solvent alone. The rate constants for deactivation of die excited states by radiationless processes are also significantly reduced when the... 

In the general case one obtains rather complicated and inconvenient equations for the relationships of observed fluorescence quantum yields and fluorescence kinetics and the rate constants of the reactions and exchange by reactants and products between micellar and volume phases and concentration of the product C. The degree of sfolubilization of the initial compound A can be taken into account similiarly as in the case of the irreversible reactions of Eqs. (2) and (22). If one can neglect some interphacial exchange processes, very simple equations similar to the case of reactions in homogeneous solutions is obtained ... 

Photoprotolytic reactions in micellar solutions were systematically studied by the authors of the present review with coworkers [15, 16, 66, 120-122]. Effective dissociation rate constants ki for a set of hydroxyaromatic compounds in anionic, cationic and uncharged micelles were determined from the dependence of fluorescence quantum yields on the concentration of micelles [Eq. (22)] or on the concentration of acid [Eq. (49)]. The latter method was also used for determining the effective equilibrium constants and effective rate constants of the reverse protonation reaction [Eq. (49)]. The data obtained are presented in Table 3. 

For the most common exciplex-forming systems, like pyrene-dimethylaniline, only a quenching is observed in the micellar phase, but no exciplex emission [25,26]. The interaction of the excited pyrene with dimethylaniline is controlled by diffusion [123] with every collision resulting in quenching, which was also shown by Malaga et al. [25,26] to yield pyrene anion and dimethylaniline radical cation. The quantum yield of radical ions is slightly greater in cationic CTAB micelles than in acetonitrile solutions, in neutral Brij 35 micelles a little less and in anionic SDS micelles considerably less than in acetonitrile. 

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