J. Phys. Chem. 1987, 91, 865-869
865
Excited-State Quenching in Reversed Micelle Solutions: The Role of Hydrophobic Effects and Solute-Solute Interactlons in Pyrene Fluorescence Quenching‘ Carol A. Backer and David G. Whitten* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: June 6, 1986) A study of pyrene photophysics and fluorescence quenching in a water/oil microemulsion consisting largely of reversed micelles is reported. Quenching by both hydrophobic and hydrophilic reagents cosolubilized in Aerosol OT-heptane-water (Aerosol OT = AOT = sodium bis(2-ethylhexyl)sulfosuccinate) has been investigated. Thus the hydrophobic quencher 2,5-dimethyl-2,4-hexadiene quenches the pyrene singlet chiefly in the hydrocarbon phase in a process which shows an apparent rate decrease as the water pool concentration increases. An opposite effect is observed for Cu2+ as a quencher, while the organic cation methylviologen2+shows little sensitivity to water or surfactant concentration. Iodide exhibits virtually no quenching effect in all the solutions studied. The results are interpreted in terms of a distribution of pyrene between the hydrocarbon and the hydrocarbon-water pool interface with distribution for the various quenchers dependent on their charge and/or hydrophobicity. For pyrene a constancy of the 11/13 vibronic ratio of fluorescence in both quenched and unquenched samples indicates rapid equilibration between the various phases or solubilization sites.
Introduction Probably the most outstanding characteristic of microheterogeneous media formed by admixture of water and various surfactants or detergents is their ability to solubilize a diverse array of solutes varying from inorganic ions to hydrocarbons. In recent investigations micelles have been perhaps the most widely used as a medium for study of thermal and photochemical reactions; a number of investigations have demonstrated enhanced or “unusual” reactivity which can be attributed in many cases to concentration of reactants in a relatively polar “interface region”.24 While reactivity in ionic micellar media has been observed to be retarded or enhanced, depending upon the specific reagents or reaction type involved, there have been relatively few demonstrations of clear effects due to compartmentalization of solutes except for exclusion of like-charged ions from reactivity with water insoluble reagent^.^^ In contrast, bilayer vesicles have been shown to provide a rich array of solubilization sites for a variety of different reagents and selective reactivity attributable to the different sites is frequently observed;6 however, the attractiveness of vesicles as a reaction medium is somewhat limited due to their reduced ability to generally solubilize substances (compared to micelles) and their lack of long-term stability in many cases. The latter problem has been solved in many cases by polymerization of small and large vesicles either prior to or after incorporation of solute^.^^* Microemulsions, particularly water/oil or reversed micelles, have also been shown to be capable of solubilizing a variety of solutes ranging from very hydrophobic organic compounds to metal salts. A reasonably well-studied medium both in terms of its microstructure and its employment as a reaction medium is the Aerosol-OT (sodium bis(2-ethylhexy1)sulfosuccinate, A0T)-water-hydrocarbon reversed m i ~ e l l e ; ~ -at’ ~ low-to-moderate water concentrations (w L 10-12) this medium (1) Photochemical Reactions in Organized Assemblies. 51. Part 50 is Shin, D. M.; Schanze, K. S.; Otruba, J. P.; Brown, P. E.; Whitten, D. G., to be published. (2) Menger, F. M.; Doll, D. W. J . Am. Chem. SOC.1984, 106, 1109. (3) Zachariasse, K.; Van Phuc, N.; Kozanklewicz, B. J . Phys. Chem. 1981, 85. 2676. (4) Menger, F. M. Acc. Chem. Res. 1979, 12, 111. ( 5 ) Whitten, D. G.; Russell, J. C.; Schmehl, R. H. Tetrahedron 1982, 38, 2455. (6) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (7) Fendler, J. H. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 1947. (8) Nome, F.; Reed, W.; Politi, M.; Tundo, P.; Fendler, J. H. J . Am. Chem. SOC.1984, 106, 8086. (9) Fletcher, P. D. I.; Robinson, B. H. Ber. Eunsenges. Phys. Chem. 1981, 85, 863. (IO) Luisi, P. L. Angew. Chem., Int. Ed. Engl. 1985, 24, 439. (1 1) Kotlarchyk, M.; Huang, J. S.; Chen, S . H. J . Phys. Chem. 1985,89, 4382. (12) Martin, C. A,; Magid, L. J. J . Phys. Chem. 1981, 85, 3938. (13) Rodgers, M. A,; Lee, P. C. J . Phys. Chem. 1984, 88, 3480. i~
0022-3654/87/2091-0865$01.50/0
has been shown to consist of small droplets containing water, the surfactant, and counterions in an oil ~ 0 n t i n u u m . I ~Interesting properties established in a number of investigations include the presence of both “interfacial” and normal water in the water pools of the reversed micelle and the persistence of a relatively normal liquid hydrocarbon region even when the concentration of water is quite high.14J5 The results of previous studies suggest that, depending upon their hydrophobicity or charge, AOT reversed micelle-solubilized reagents can occupy sites that are specifically associated with the hydrocarbon, water pool-hydrocarbon interface, or inner water p00l.l~ A number of investigations have indicated that communication or reaction between reagents solubilized in different pseudophases can occur to a limited extent which can be controlled or modified in several case^.^^^^^ The present paper reports a study of pyrene photophysics and fluorescence quenching in reversed micelle solutions consisting of heptane-Aerosol OT-water in which the water/surfactant ratio is held constant and small but the total surfactant and water concentrations are varied. Over the domain of surfactant-water concentrations explored the medium can be shown to consist of relatively small reversed micelles which do not vary a great deal in size; however, while the microscopic properties of the medium might be expected to be relatively constant, its macroscopic properties such as viscosity undergo pronounced changes over the region explored. The studies with pyrene reported in this paper complement and yet contrast with earlier investigationsIs with pyrene, surfactant pyrenes, and other aromatic hydrocarbons in reversed micelles where fluorescence quenching by varying reagents has been studied. In the present investigations we have found that a wide variety of reagents ranging from quite hydrophilic to hydrophobic quench the pyrene excited singlet rapidly and efficiently. These findings, together with a determination of the reversed micelle “solvent” dependence of the pyrene fluorescence and excimer formation, present a picture indicating that the long-lived pyrene singlet samples a number of microenvironments during the excited-state lifetime which encompass a wide range of polarity and enable free encounters with a variety of different solutes.
Experimental Section Materials. Pyrene was purchased from Aldrich and was purified, by recrystallization from benzene, passed through an alumina (14) Wong, M.; Thomas, J. K.; Gratzel, M. J . Am. Chem. Soc. 1976, 98, 2391. (15) Wong, M.; Thomas, J. K.; Nowak, T. J . Am. Chem. SOC.1977, 99, 4730. (16) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems, in press. (17) Atik, S. A,; Thomas, J. K. J . Am. Chem. Soc. 1981, 103, 3543. (18) Pileni, M. P.; Furois, J. M.; Hickel, B. In Surfactants in Solutions; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 1471.
0 !987 American Chemical Society
Backer and Whitten
866 The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 TABLE I: Characterization of the AOT Reversed Micelle ~~
~
wt %
macrovissolution water AOT heptane cosity," CP 6.3 91.1 0.5 heptane; [H20] = 1 M; w = l o b 2.6 5.1 6.3 88.6 0.6 heptane; [H20] = 2 M; w = 20 60.4 3.0 heptane; H20] = 5 M; w = 10 11.4 28.2 31.8 72.4 heptane; [H,O] = 10 M; w = 10 19.6 48.6 'Viscosities measured at 25 [H2Ol/[AOTl'
OC
with an Ostwald viscometer. b o =
column using benzene as eluent, and then recrystallized twice from ethanol. n-Heptane and hexane (EM Science, OmniSolv spectrophotometric grade) were distilled from sodium metal. The quenchers sodium iodide (I-) and copper sulfate (Cuz+) were obtained from Baker and were dried in an oven at 150 OF until use. Methylviologen (MVZf,Aldrich) was stored in a desiccator until use. 2,5-Dimethyl-2,4-hexadiene (DHD, Aldrich) was freshly distilled prior to each quenching experiment. The water used was deionized by passing through a Millipore ion-exchange and filtration system. Aerosol OT (AOT, Aldrich) was purified according to the procedure outlined by MartinI2 with toluene being substituted for benzene and hexenes in place of ligroine. The purified AOT was stored under vacuum in a desiccator over P2O5 until use. Solutions. All solutions were 1.0 X lo-' M in pyrene unless otherwise noted. The appropriate volume of a hexane solution 1.0 X lo4 M in pyrene was added to a flask and then the solvent evaporated under a stream of dry nitrogen to prepare the working solutions. Reversed Micelles. AOT was weighed in an Erlenmeyer flask. Heptane was then added to the flask and the mixture was bath sonicated until a clear solution was obtained. The AOT/heptane mixture was then quantitatively transferred to a volumetric flask coated with pyrene (vide supra) and the appropriate amount of water was then added. Heptane was added to the mark and the AOT solution was bath sonicated 10 min. It was necessary to warm the mixtures containing higher surfactant concentration in order to obtain transparent solutions. Reversed micelle solutions containing 1 M AOT or greater required stirring for 1-2 days in order to obtain clear solutions. Quenchers were weighed in a flask and the reversed micelle solution was added. The quencher/AOT solutions were inverted several times and bath sonicated for 5 min. Samples were degassed in fluorescence cells under heptane-saturated dry argon for 16 min. Solutions containing the quencher 2,5-dimethyl-2,4-hexadienewere degassed by freezepump-thaw cycles to minimize loss of quencher. Instrumentation. Steady-state fluorescence spectra were recorded on a Spex 11 1-CM fluorimeter with a 1-nm band-pass. Fluorescence lifetimes were measured on a PRA single proton counter and the data analyzed on a PDP-11 computer. Particle sizes were measured by dynamic light scattering using a Malvern PCS 100 spectrophotometer.
Results Table I lists the composition of the reversed micelles used in the present study. While there has been considerable investigation of the structure and solvent properties of the heptane-AOT-water reversed micelle at lower surfactant concentration^,^-^^^^^ there has been relatively little work carried out at the higher surfactant concentrations investigated in the present study. Although, as listed in Table I, the surfactant becomes the major component by weight with 10 M H20-1 M AOT and the macroscopic viscosity increases enormously, the mixture remains a single-phase clear liquid and a particle size determination via light scattering indicates no detectable change in size of the reversed micelles with or without added pyrene or quencher. Phase diagrams measured for other similar hydrocarbon-AOT-water mixtures suggest that the concentrations used in this study are all well within the region (19) Wong, M.; Thomas, J. K. Micellizarion, Solubilization, and Microemulsions; Plenum: New York, 1977; p 641.
TABLE 11: Sensitivity of Pyrene Fluorescence to Water Pool Concentration
solution" heptane heptane; [H20] = 1 M; w = I O heptane; [H,O] = 2 M ; o = 20 heptane; [H20]= 5 M ; w = 10 heptane; [H20]= 10 M; w = 10 heptane; [H,O] = 15 M; o = 15
0.64 0.66 0.65 0.76 0.90 0.94
378 354 313 292 263 260
f 57 f 92 f 49 f 63 f 91 f 25
458 f 10 428 f 23 433 338 f 57 318
a Solutions contained 10 r M pyrene. *Solutions were degassed in fluorescence cells under heptane-saturated dry argon for 16 min.
