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Langmuir 1993,9, 16-18
Dynamics of Intervesicle Transfer of Dihexadecyl Phosphate-Bound Viologens Brian C. Patterson and James K. Hurst* Department of Chemical and Biological Sciences, Oregon Graduate Institute of Science and Technology, Beaverton, Oregon 97006-1999 Received July 13,1992. I n Final Form: November 4,1992 Transfer of an amphiphilicviologen dication between anionic vesicleswas accelerated by a water-soluble anioniczinc porphyrin. On the basis of observed reactivity patterns, a novel mechanism involvingviologenzinc porphyrin ion pairing is proposed wherein the zinc porphyrin functions as a viologen "shuttle". Two distinct pathways have been identified for interparticle migration of adsorbates in micellar media from kinetic studies on interfacial electron transfer' and fluorescence quenchingS2 One pathway involves desorption to the bulk medium and subsequent readsorption to a different micelle, whereas the other involves direct intermicellar t r a n ~ f e r .Dopant ~ migration in other types of microphases has been less extensively studied; however, intervesicle transfer of aromatic compounds appears to involve desorption-readsorption processes which are ratelimited by release of the vesicle-solubilized compound^.^ We report herein an unusual desorption-readsorption pathway on vesicles that is mediated by an aqueous-phase carrier capable of ion-pairing with the migrating adsorbate. The system investigated comprises anionic dihexadecyl phosphate (DHP) small unilamellar vesicles containing amphiphilic N-methyl-N'-hexadecyl-4,4'-bipyridinium (alkylviologen, C,MV2+) ions bound at the external aqueous-organic interface. Molecular structures are given in Scheme I. We have found that both the rates of interfacial oxidative quenching of aqueous triplet-excited [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatolzinc(II) (ZnTPPS4-) by DHP vesicle-bound viologens5 and the relative amounts of one-electron-reduced viologen radical cations in their monomeric and multimeric forms6 are dependent upon the extent of viologen adsorption. Either measurement can therefore be used to assess the average distributions of the viologens on the vesicles. In a study5 designed to probe rates of viologen redistribution by monitoring 3ZnTPPS4-lifetimes, we found that lifetimes were nearly identical for (i) solutions containing a mixture of unloaded vesicles and vesicles that were highly loaded with c16MV2+and (ii) solutions containing vesicles with CuMV2+ at loadings approximately equal to the anticipated equilibrium distribution. Specifically, the lifetime measured for a mixture containing a DHP/C16MV2+ratio of 3.3 and an 8-fold excess of unloaded vesicles (equilibrium DHP/ C16MV2+ratio 30) was T = 0.16 ms compared to a value of 7 = 0.14 ms for a reference solution with DHP/C16MV2+ (1)Moroi, Y.; Braun, A. M.; Gratzel, M. J.Am. Chem. SOC.1979,101, 567. Moroi, Y.; Infelta, P. P.; Gratzel, M. J. Am. Chem. SOC.1979,101, 573. (2) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. Lett. 1979, 68, 451. Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. J.Phys. Chem. 1981,85, 1198. Atik, S. S.; Thomas, J. K. J. Am. Chem. SOC.1981,103, 3543. (3) Infelta, P. P. In Energy Resources through Photochemistry and Catalysis; Gritzel, M., Ed.; Academic Press: New York, 1983; p 49. Almgren, M.; Lofroth, J.-E.; van Sta", J. J.Phys. Chem. 1986,90,4431. (4) Almgren, M. J. Am. Chem. SOC.1980, 102, 7882 and references
cited therein. (5) Hurst, J. K.; Thompson, D. H. P.;Connolly, J. S. J.Am. Chem. SOC.
1987,109, 507. (6) Patterson, B. C.; Thompson, D. H.; Hurst, J. K. J.Am. Chem. SOC. 1988,110, 3656. Patterson, B. C.; Hurst, J. K. J.Phys. Chem. 1993,97, 454.
= 38. In contrast, the lifetime before dilution with unloaded vesicles (DHP/C16MV2+= 3.3) was 7 = 0.03 ms. The 3ZnTPPS4-lifetime remained invariant with time in the mixed vesicle suspension from 2 min after mixing, the minimum time required to make the measurements, to several hours afterward. Under the experimental conditions, 3ZnTPPS4-decay occurred primarily by oxidative quenching by the DHP-bound C16MV2+ions;5 the metalloporphyrin does not bind to the vesicle^.^ These data were interpreted to indicate that C&V !2+ equilibrium among the vesicles was achieved within a few minutes after mixing. An apparently conflicting observation was made when the C16MV2+distribution was monitored by determining the extent of aggregation upon reduction to the corresponding radical cations. The C16MV2+monomedmultimer ratios are easily determined by optical spectroscopy.8 When C16MV2+ was added from a concentrated stock solution to DHP vesicle suspensions with conventional mixing to give a DHP/C16MV2+ratio of 400,then subsequently reduced with Na2S204, about 35 % of the radical cation was monomeric. In contrast, the relative amount of monomer formed was 75 % when the vesicles were loaded to an equivalent level by flow-mixingthe solutions through a tangential 12-jet chamber. This difference apparently arose because rapid mixing minimized concentration inhomogeneities during binding, thereby yielding a more uniform distribution of the dopant over the vesicle population. Implicit in this interpretation, however, is the assumption that redistribution of C16MV2+among the vesicles was slow under these conditions. This divergent behavior can be reconciled by assuming that the ZnTPPS4-ion facilitates removal of C16MV2+from the interface, thereby shortening residence times on individual vesicles. Viologen dications strongly ion-pair to ZnTPPS4- in aqueous solutions, resulting in diagnostic bathochromic shifts in the metalloporphyrin optical absorption bands and static quenching of its photoexcited triplet ~ t a t e . ~ ~In ~ Jthe O presence of DHP vesicles, the c16Mv2+ions bound preferentially to the vesicles, so that under the conditions of these experiments only a few percent of the viologen was ion-paired to the zinc porphyrin.' This extent of ion association may be sufficient for the ZnTPPS4- ion to act as a viologen "shuttle", however, assisting C&iV2+ redistribution in the manner (7) Hurst, J. K.; Lee, L. Y.-C.; Gratzel, M. J.Am. Chem. SOC.1983,105, 7048.
(8)We have extensively studied this aspect of C,MVz+binding to DHP vesicles, from which the empirical formula 7Z monomer = (abs&absb52 - 0.59)/0.0098 was derived. The term abseo5refers to the absorbance at 605 nm, the monomer peak maximum, and abs552is the absorbance at 552 nm, the monomer-multimer isosbestic point.6 (9) Kalyanasundaram, K.; Grltzel, M. Helu. Chim. Acta 1980,63,478. (10) Rougee, M.; Ebbesen, T.; Ghetti, F.; Bensasson, R. V. J. Phys. Chem. 1982,86,4404.
0743-746319312409-0016$04.00/0 0 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No.1, 1993 17 Scheme I
Zn p * v 2 L
Table I. Redistribution of ClsMV2+to Unloaded DHP Vesicles systema
incubatnb time
% multimef
calcd % redistribn
100
5 min 40 min 2 min 5 min 5 min 15min 5 min
30 50 46 39 38 34 43 44 45
25 pM standard
100 pM standard no added ions no added ions 50 pM ZnTPPS450 pM ZnTPPS" 50 pM EDTA450 pM EDTA450 pM MB+
0 20 55 60 80
d d d
a Reactions were initiated by combining 25% 100pM C&W+ on 1mM DHP vesicles with 75% 2 mM DHP vesicles plus additional ions where indicated; medium conditions were 20 mM Tris-HC1, pH 8.0,23 "C. The time between mixing of components and reduction by dithionite ion. c The percent multimer was determined from the equation given in ref 8. Under these conditions, the spectral change at 604 nm upon 20% conversion of monomer to multimer was 0.07 absorbance unita; therefore, changes in monomer/multimer ratios of 2-3% could readily be detected. Invariant with time.
shown in Scheme I. For simplicity,fast equilibriainvolving reversible assocation of free viologen with the zinc porphyrin and DHP vesicles, e.g. ZnTPPS" -k c16Mv2+ (znTPPs*c16MV)2are omitted. Our intent is to emphasize an essentialfeature of the reaction, namely, that rate acceleration arises from biomolecular interaction of ZnTPPS4- and (ZnTPPS. c16Mv)2-with the vesicles. The initially formed porphyrin-viologen pair might undergo dissociative exchange numerous times before eventual transfer of the viologen to a second vesicle. To test these ideas, we flow-mixed C&lV2+solutions with DHP vesicle suspensions to obtain uniformly distributed vesicle-bound viologens with DHP/C16MV2+ ratios of 20 and 80; the more highly loaded vesicles contained about 20% more multimer when reduced (Table I). A portion of the more highly loaded sample was then diluted 4-fold with an unloaded vesicle suspension at the same DHP concentration, and the mixture was periodically
sampled by reduction with Na2S204 to determine the monomer-multimer composition. Changes in this composition were equated with rates of intervesicle redistribution, on the basis of the composition of the reference standards. The influence of ZnTPPS4-or other ions upon redistribution rates could be ascertained by including them in the unloaded vesicle suspensions. The results of these experiments are given in Table I. They showthat redistribution of C & W + between vesicles is normally very slow, with an apparent halflife exceeding 30 min. Upon addition of ZnTPPS4-;this process becomes much more rapid, with a halflife of 2 min. No shift was detected in the ZnTPPS4- optical spectrum; under comparable conditions, the yield of 3ZnTPPS4-was typically >90% of the yield measured for aqueous solutions of the ZnTPPS4-ion alone.' These observations established that only a few percent of the total ZnTPPS4- was ion-paired to c16Mv2+.Assuming C16MV2+ dissociation is rate limiting, the unassisted exit rate constant for C&V2+ from the vesicle was k = 4 X lo4 s-l, compared to k = 6 X l W 3 s-l when 50 pM ZnTPPS4- was present." This latter value corresponds to a second-order rate constant of kl = 100 M-1 s-l in Scheme I. No rate enhancement was found when EDTAb or N-methyl-4,4'-bipyridiniumion (MB+),a structural analog of MV+,was added to the suspensions,consistent with the conclusion that the effect is due to specific ion-pairing interactions. For CnMV2+ions with n = 1or 6, the physical (11)A reviewer has suggested an interesting comparisonwith exitratea for surfactant monomers from anionic micelles. The exit rate constant for the dodecyl sulfate anion from ita micelle is about 104 a-1.12 This reaction is consideredto be diffusioncontrolled.13 Becausethe microphase volume is larger in vesicles than micelles,the corresponding rate constant in vesicles should be 10-102-fold lower? When the surfactantand micellar headgroups are oppositely charged, the reduction in the rate constant arising from substitutionof an attractiveelectrostatic forcefor a repulsive one is expected to exceed 102-fold.14Assuming this potential is linearly dependent upon charge, the rate retardation for a divalent cation would be approximately 105-fold. Considering the cumulative effects of these factors,the diffusion-controlledexit rate constant for CleMV2+from DHP vesicles is estimated to be k N 104/105(10-102)= 10-"10-3 a-1. Given the many uncertainties of the approximation, this estimate is in reasonable agreement with the experimentally determined value of k = 4 X s-l. This comparison also suggests that ZnTPPS4- influencesthe rate primarily by reducing attractive electrostatic interactions between the viologen dication and anionic vesicle interface.
18 Langmuir, Vol. 9, No.1, 1993
characteristics of vesicle Suspensions were identical regardless of the method of mixing. These shorter-chain analogs bind less extensively to the DHP vesicles,1s so that under comparable experimental conditions 20-2596 of these viologens were free in solution and unassisted redistribution rates should be rapid. Consistent with this behavior, exit rates for surfactant ions from micelles have been shown to decrease drastically with increasing chain length.12Je The ion-pair shuttle pathway demonstrated here for CleMV2+-DHP vesicles may be quite general, (12) Anianeeon, E. A. C.; Wall,S. N.;Almgren, M.; Hoffmann, H.; Keilmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phy8. Chem. 1976,80,905.
Letter8 however, and is likely to be important for micropbeseparated systems where the adsorbate residence times are relatively long.
Acknowledgment. Funding for this research was provided by the Office of Basic Energy Sciences, US. Department of Energy, under Grant DEFG 87ER 13664. (13) Almgren, M.; Crieaer, F.; Thomaa,J. K. J. Am. Chem. SOC.1979, 101, 279. (14) Muto, Y.; Keiko, Y.; Ywhida, N.;% m i , K.; Meguro, K.; BinmaLimbale, W.; Zana, R. J. Colloid Interface Sci. 1989,130, 165. (16) Lei, Y.;H m t , J. K. J. Phys. Chem. 1991,95,7918. (16) Infelta, P. P.; Brugger, P.-A. Chem. Phy8. Lett. 1981,82,462.
