Orientational Binding of Substituted Naphthoate Counterions to the

Department of Chemistry, Saint Mary's College, Moraga, California 94575, and ... Biochemistry, San Francisco State University, San Francisco, Californ...
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J . Phys. Chem. 1991, 95, 480-488

480

Orientational Binding of Substituted Naphthoate Counterions to the TetradecyltrtmeZhylammonlum Bromide Micellar Interface Steven J. Bachofer,*.t Ursula Simonis,t and Thomas A. Nowicki Department of Chemistry, Saint Mary's College, Moraga, California 94575, and Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, California 941 32 (Received: May 3, 1990)

The orientational binding of four naphthoate anions to the tetradecyltrimethylammonium bromide (TTAB) micellar interface has been studied by recording surface tension and 'H NMR data. The critical micelle concentration (cmc) values for 1:l stoichiometric mixtures of TTAB and sodium 1-naphthoate and sodium 2-naphthoate are 0.62 and 0.33 mM, respectively. The fluorescence measurements for TTAB/ I-naphthoate 1:l mixtures also gave a cmc value of 0.65 mM. The TTAB/IOH-2-naphthoate and TTABf3-OH-2-naphthoate mixtures have cmc values of 0.10 and 0.17 mM for a 2:l stoichiometry. The organic counterions all stabilize the formation of the micellar aggregates. The orientational assignmentsof all the aromatic anions were determined from NMR chemical shift data. The changes in chemical shift with increasing mole fraction of anion indicate a change in micelle structure from the globular to prolate or rodlike, which is also clearly observable for samples of I :1 stoichiometry since they exhibit viscoelasticity at dilute concentrations. The aromatic induced chemical shifts of the alkyl chain methylene protons demonstrate the deep penetration into the palisade layer by these anions. The nuclear Overhauser spectroscopic data (NOESY) provide further support for the conclusion of deeper penetration since an intermolecular NOE is observed between the counterion and the alkyl chain. The critical assignment of the anions' orientation with respect to the micellar interface elucidates the subtle control of the aggregate packing constraints to define the aggregate shape.

Introduction The structure of micellar aggregates is of interest in the numerous industrial applications of amphiphilic surfactants. Recent studies of cationic micellar systems have demonstrated that the addition of aromatic anions can induce shape selective phase transition^.'-^ The theoretical treatises on micellization propose that the micellar aggregate shape is defined by the aggregate volume to surface area ratio.6-'0 The rheological phenomenon viscoelasticity has been observed at low concentration in a few specific systems involving cationic surfactants (such as cetyltrimethylammonium and cetylpyridinium cations) which undergo sphere-to-rod phase t r a n s i t i ~ n s . ~ - ~It~has ~ l been demonstrated for these viscoelastic systems that the formation of prolate, rodlike micellar aggregates is important.'-5 The theoretical treatises of lsraelachvili et aL6 and Gelbart et a1.12 highlight the importance of the packing constraints. Our research endeavors to clarify the packing constraints involved in micelle formation by establishing orientations and the degree of penetration of various isomeric naphthoate anions at the tetradecyltrimethylammonium micellar interface. We have studied four naphthoate anion salts, with the structures labeled 1 to 4, interacting with tetradecyltrimethylammonium 1

2

4

3

bromide (TTAB) by recording surface tension and 'H NMR data to elucidate the orientation of the anions to yield mixed micelles. The term mixed micelles is used to emphasize the strong binding of the naphthoate anions with the cationic tetradecyltrimethyl-

ammonium micellar interface and the pronounced changes in the aggregate structures with these counterions present. We here report both the critical micelle concentration (cmc) values and IH NMR data for the two structurally distinct I-naphthoate and 2-naphthoate anions and two different hydroxynaphthoate anions forming mixed micelles with TTAB. The naphthoate anions were chosen in the present study, because earlier studies from this group on the substituted benzoate binding at the cetyltrimethylammonium bromide (CTAB) interface indicated that the naphthoate anions would be useful structural probes for cationic micelle^.'^ In addition, Underwood and Anacker demonstrated with their cmc data that the naphthoate anions have a stronger affinity than the benzoate anions for the similar decyltrimethylammonium ion micelle^.'^ The naphthoic acids are also known to induce viscoelastic phases with CTAB, but the nature of the interaction has not been previously associated with mixed micelle formation. I The counterion binding of the various naphthoates for these cationic micellar systems is observed in the cmc and 'HNMR measurements as will be discussed below. The decrease in the cmc values for the cationic surfactant-anion system in comparison to TTAB alone (which has only the inorganic bromide counterion) indicates that mixed micelle formation occurs. The NMR data elucidate the orientation of the anions and the penetration into the palisade layer from the observed chemical shift changes of both the aromatic ring and the surfactant methylene proton^.^,'^ Fluorescence data involving other aromatic compounds binding ( I ) Gravsholt, S.J. Colloid Interface Sci. 1976, 57, 575. (2) Ulmius, J.; Wennerstrom, H.; Johansson, L.-B.; Lindblom, G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232. (3) Iyer, R. M.; Rao, U. R. K.; Manohar, C.; Valauikar, B. S. J. Phys. Chem. 1987, 91, 3286. (4) Anet, Frank A. L. J. Am. Chem. Soc. 1986, 108, 7102. (5) Broxton, T. J.; Christie, J. R.; Chung, R. P.-T. J. Org. Chem. 1988, 53, 3081. (6) Israelachvili, J. N.; Mitchell, D. J.; Ninham, 8. W. J. Chem. SOC., Faraday Trans. 2 1976, 72, 1525. (7) Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1973, 71, 580. (8) Jansson, M.; Stilbs, P. J. Phys. Chem. 1987, 9 / , 113. (9) Scamehorn, J. F.; Rathman, J. F. J. Phys. Chem. 1984, 88, 5807. (IO) Tanford, C. R. J. Phys. Chem. 1974, 78, 2469. ( 1 I ) Hoffmann, H.; Rehage, H.; Reizlein, K.; Thurn, H. Macro and Microemulsions: Theory and Applicafions; Shah, D. 0.. Ed.; ACS Symposium Series 272; American Chemical Society: Washington, DC, 1985; p 41. (12) Gelbart, W. M.; Ben-Shaul, A,; Rorman, D. H.; Hartland, G.V. J. Phys. Chem. 1986, 90, 5277. (13) Bachofer, S. J.; Turbitt, R. M. J. Colloid Inferface Sci. 1990, 135, 325 .-.

'Saint Mary's College. *San Francisco State University.

0022-3654/91/2095-0480$02.50/0

(14) Underwood, A. L.; Anacker, E. W. J. Phys. Chem. 1984,88,2390. ( I 5) Nash, T. J. Colloid Interface Sci. 1958, 13, 134.

0 1991 American Chemical Society

Binding of Naphthoate Ions to the TTAB Interface to surfactant interfaces have demonstrated that these compounds situate near the micellar The cmc and N M R data in this report will demonstrate the specificity of the micellar interface and the appearance of the rheological effect which can be understood with respect to the formation of the prolate mixed micelles. Experimental Section Materials and Methods. The stoichiometric mixtures of tetradecyltrimethylammonium bromide (TTAB) and X-naphthoate were prepared by adding an aqueous basic solution of the respective sodium naphthoates to the micellar TTAB solutions. The bulk pH of all micellar TTAB/X-naphthoate solutions was adjusted to be within the range of pH 4-7.5 with addition of dilute HC1 if necessary. The TTAB (cationic surfactant, Sigma) was used as received, and the sharp breakpoint of the surface tension versus the logarithm of the surfactant concentration data indicates the high purity of the material. The sodium naphthoate stock solutions were prepared with typically twice the stoichiometric amount of NaOH. The 1-naphthoic acid, 2-naphthoic acid, 1-OH-2naphthoic acid, and 3-OH-2-naphthoic acid were also used as received from Aldrich Chemical Co. The 2,7-dichlorofluorescein was used as received from Eastman Organic Chemicals. All the mixed micellar NMR samples were -10 mM in TTAB in D 2 0 (Aldrich, 99.8 atom %). The concentrations of TTAB stock solutions were determined by tne Epton two-phase titration method as reported elseThe empirical observation of the appearance or nonappearance of viscoelasticity was determined on samples visually containing 10 mM TTAB, and the respective substituted naphthoate anion concentration was either 5 or 10 mM. Physical Chemical Measurements. Surface tension measurements were obtained with a Du Nouy tensiometer (Cenco tensiometer No. 926) using a 5.996-cm-circumference platinum ring. The surface tension was plotted versus logarithm of the cationic surfactant concentration, and the cmc values were interpreted from the intersection of the two linear portions of data.24 The determination of cmc values in the presence of a fluorescence probe provided concurrent values to the surface tension measurements for the experiments with the anion 1 -naphthoate.20-21 The fluorescence intensity measurements of 2,7-dichlorofluorescein were recorded at a constant probe concentration of 1.25 pg/mL in the presence of various concentrations of the surfactant up to and above the cmc. The change in the probe's fluorescence intensity indicating the onset of micellization was measured with a Sequoia-Turner digital fluorimeter (Model 450) using a 490-nm excitation filter and a 51 5-nm emission filter.20*2' N M R Measurements. The ' H N M R data were obtained on either a GE QE 300-MHz or a GE G N 300-MHz spectrometer, which are equipped with 1280 computer systems, and an AM 400-MHz Bruker spectrometer which is equipped with an Aspect 3000 computer system, using typical pulse widths of 3.0 and 5.0 ps, respectively, corresponding to a flip angle of about 30° (90° pulse angle of 9.0 and 13.4 ps). Typically, one-dimensional ' H NMR spectra were recorded with a sweep width of 6000 Hz, an acquisition time of 1.36 s, and a relaxation delay of 1 s on the 300-MHz spectrometers. A total of 128 FIDs (16K time domain) were acquired, signal averaged, and Fourier transformed to yield spectra with a digital resolution of 0.7 Hz/point. The stoichiometric ratios of TTAB to the X-naphthoate anions were confirmed from the integrated peak intensities and were found acceptable. 'H NMR chemical shifts of a 20 mM TTAB (micellar solution) (16) Folkhart, €3. D. J. Colloid Interface Sci. 1957, 12, 557. (17) Turro, N. J.; Geiger, M. W.; Hautala, R. R.; Schore, N. E. Micelliration, Solubilizaton, and Micrwmulsions; Mittal, K. L., Ed.; Plenum: New York. 1977; Vol. I , p 75. (18) Warr, G . G.;Evans, D. F. Langmuir 1988, 4, 217. (19) Roelants, E.; DeSchryver, F. C. Langmuir 1987, 3, 209. (20) Harkins, W. D.; Corrin, M. L. J. Am. Chem. Soc. 1947, 69, 679. (21) Rujimethabhas, M.; Wilairat, P. J . Chem. Educ. 1978, 55, 342. (22) Reid, V. W.; Longman, G . F.; Heinerth, E. Tenside 1967, 4, 292. (23) Smith, W. €3. J. Soc. Cosmet. Chem. 1963, 14, 513. (24) Harkins, W. D.; Jordan, H. F. J. Am. Chem. SOC.1930, 52, 1751.

The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 481 TABLE I: Data for TTAB/X-Naphthoate Mixed Micelles cmc, X-naphthoate

1-naphthoate" 1-naphthoateb 2-naphthoate" 1-OH-2-naphthoate" 3-OH-2-naphthoate"

x = 0.16~5~x 1.15 1.23

0.16 0.15 0.27

mM

= 0.333 0.8 1 0.9 1 0.43 0.10 0.17

x = 0.50 0.62 0.65 0.33 c c

"cmc values determined from surface tension measurements. cmc values determined from fluorescence intensity measurements. Insoluble. d~ = mole fraction.

were determined relative to an external standard DSS reference sample with DSS being at 0.0 ppm. The resonance singlet at 3.17 ppm was assigned to the head group methyl protons of TTAB by the relative peak intensity and was used as an external reference for all other N M R samples. Selective homonuclear decoupling experiments were applied to assign the aromatic protons on the I-naphthoate, 2-naphthoate, and the 1-OH-2-naphthoate anions. The chemical shift correlated spectroscopic (COSY) experiments were recorded on the TTAB/2-naphthoate (10:2 and 10:4 stoichiometric ratio) and TTAB/ 1-OH-2-naphthoate (10:2 and 10:3 stoichiometric ratio) samples and on the respective naphthoate anions alone in D20.25 Quadrature detection was applied, and typical spectral parameters for the obtained COSY spectra are listed below. A sweep width of 2404 Hz in F2 and *I202 Hz in Fl were used. The size of F2 was 512 w; 128 FIDs were taken in F I with 16 scans each. The recycle delay was set to 1.5 s to give a total measuring time of 1.28 h. A nonshifted sine bell was used to process the two-dimensional data set. The data were zero-filled once in F , to give a final matrix of 256 w X 256 w with a corresponding digital resolution of 18 Hz/point. Two-dimensional cross-relaxation correlation and chemical exchange correlation spectra (2D NOE, NOESY) were recorded on the G N 300-MHz spectrometer using the standard 9O0-t,90°-t,-900 (NOESY) 2D pulse sequence with a composite 1 80° pulse incremented through the mixing period, t,. During the evolution time, t , , between the first two pulses the various magnetization components are frequency labeled. During the mixing period, t,, between the second and third pulses, crossrelaxation leads to exchange of magnetization between the nearby nuclei through mutual dipolar interactions. The composite 1 80° pulse in combination with symmetrization of the final data suppressed cross peaks due to scalar coupling after smearing them along the F , d i m e n s i ~ n . ~Quadrature ~,~~ detection was utilized with a sweep width of 2604 Hz in F2 and *I302 Hz in F , and a mixing time of 400 ms. The size in F2 was 51 2 w [w = digital points]. A total of 128 FIDs were taken in F , with a 128 scans each. The recycle delay was set to 1.5 s to give a total measuring time of 6.82 h. Phase cycling was applied to generate pure absorption mode spectra. The data were zero-filled once in F Ito give a final matrix of 256 w X 256 w. Results Four naphthoate anions, labeled 1 to 4, were used in this study. Viscoelasticity is observed for 1: 1 stoichiometric mixtures of tetradecyltrimethylammonium bromide (TTAB) and sodium 2-naphthoate at a concentration of 10 mM in each material, but not with the I-naphthoate present. The rheological effect is observable for TTAB in an equimolar ratio of the sodium 1naphthoate at 20 mM concentration in each reactant. With the two sodium hydroxynaphthoates as the anion, the samples with 1: 1 stoichiometry (TTAB/X-naphthoate) yielded an insoluble material with the pH near neutral; however, viscoelasticity is (25) Simonis, U.; Walker, F. Ann; Lee, P. L.;Hanquet, B. J.; Meyerhoff, D. J.; Scheidt, W. R. J. Am. Chem. SOC.1987, 109, 2659. (26) (a) Kumar, A,; Ernst, R. R.; Wuthrich, K. Biochem. Biophys. Res. Commun. 1980.95, 1. (b) Macura, S.; Wuthrich, K.;Ernst, R. R. J. Magn. Reson. 1982, 46, 269. (27) (a) Jeener, J.; Meier, B. H.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. (b) Kumar, A.; Wider, G.; Macura, S.; Ernst, R. R.; Wuthrich, K.J . Magn. Reson. 1984, 56, 207.

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TABLE 11: NMR Chemical Shift Data for TTAB/X-Naphthoate Mixed Micelles (Aromatic Region) chemical shifts, ppm anion I-naphthoate (std) I -naphthoate (IO: 1 )

HI

A80 I-naphthoate (10:2) 580

2-naphthoate (std) 2-naphthoate (1O:l) A6" 2-naphthoate (10:2)

Ah" 2-naphthoate (10:4) A&" I -OH-2-naphthoate (std) I-OH-2-naphthoate ( 1 O : l ) 56" I-OH-2-naphthoate (10:2) A6" I-OH-2-naphthoate (l0:3)

H2 7.60 7.87 0.27 7.82 0.25

8.42 8.55 0.13 8.51 0.09 8.47 0.05

A80 3-OH-2-naphthoate (std) 3-OH-2-naphthoate ( 1 O : l ) A6" 3-OH-2-naphthoate (10:2) A6a a

H3 7.60 7.52 -0.08 7.46 -0.14 7.98 8.1 1 0.13 8.07 0.09 8.03 0.05 7.80 7.88 0.08 7.84 0.04 7.80 0.00

8.24 8.45 0.21 8.41 0.17

H4 7.95 7.75 -0.20 7.70 -0.25 7.95 7.82 -0.13 7.74 -0.21 7.64 -0.3 1 7.38 7.23 -0.15 7.16 -0.22 7.06 -0.32 7.16 7.05 -0.1 1 7.01 -0.15

H5 7.95 7.75 -0.20 7.68 -0.27 7.95 7.79 -0.16 7.71 -0.24 7.64 -0.31 7.88 7.69 -0.19 7.62 -0.26 7.57 -0.3 1 7.70 7.52 -0.18 7.48 -0.22

H6

H7

7.60 7.45 -0.15 7.39 -0.21 7.63 7.42 -0.21 7.36 -0.27 7.30 -0.33 7.65 7.37 -0.28 7.33 -0.32 7.29 -0.36 7.32 7.20 -0.12 7.15 -0.17

7.60 7.62 0.02 7.56 -0.04 7.61 7.49 -0.12 7.41 -0.20 7.34 -0.27 7.54 7.37 -0.17 7.33 -0.21 7.29 -0.25 7.48 7.28 -0.20 7.24 -0.24

Hs 8.20 8.74 0.54 8.70 0.50 8.05 8.1 1 0.06 8.04 -0.01 7.98 -0.07 8.28 8.30 0.02 8.27 -0.01 8.20 -0.08 7.86 7.92 0.06 7.77 -0.09

A8 = chemical shift differences between resonances of the naphthoates in the micelle and the naphthoate alone in D 2 0 .

TABLE 111: NMR Chemical Shift DATA for TTAB/X-Naphthoate Mixed Micelles (Methylene Region) chemical shifts, ppm samples

TTAB (std) I-naphthoate ( 1 O : l ) 160 I-naphthoate (10:2) 560

2-naphthoate ( 1 O : l ) A60 2-naphthoate (10:2) A80 2-naphthoate (10:4) Aha I-OH-2-naphthoate ( 1 O : l ) A60 I-OH-2-naphthoate (10:2) 560 I-OH-2-naphthoate (l0:3) 560

3-OH-2-naphthoate ( 1 O : l ) 580

3-OH-2-naphthoate (10:2) 58"

(u-CH~ 3.42 3.30 -0.12 3.20 -0.22 3.31 -0.1 1 3.21 -0.21 3.10 -0.32 3.30 -0.12 3.23 -0.19 3.18 -0.24 3.30 -0.12 3.21 -0.21

P-CH2 1.81 1.70 -0.1 1 1.58 -0.23 1.71 -0.10 1.57 -0.24 1.12 -0.69 1.68 -0.13 1.58 -0.23 1.51 -0.30 1.67 -0.14 1.57 -0.24

YJ - CH2 1.40 1.28 -0. I O 1.18 -0.22 1.29 -0.1 1 1.14 -0.26 0.96 -0.44 1.26, 1.23 -0.14, -0.17 1.22, 1 . 1 1 -0.18, -0.29 1.03 -0.37 1.24, 1.21 -0.16, -0.19 1.12 -0.28

u-CH~ 0.93 0.92 -0.01 0.94 0.01 0.93 0.00 0.94 0.01 1

.oo

0.07 0.92 -0.01 0.94 0.01 0.96 0.32 0.90 0.03 0.93 0.00

N(CH313 3.17 3.12 -0.06 3.06 -0.1 1 3.13 -0.04 3.07 -0.10 3.00 -0.17 3.13 -0.04 3.10 -0.07 3.08 -0.09 3.1 I -0.06 3.08 -0.09

A6 = chemical shift differences between the resonances of the surfactant alone and surfactant in the presence of various stoichiometric amounts of aromatic anion.

observed for micellar solutions with 103 stoichiometry. All of the samples with added naphthoate anions demonstrate a decrease in the cmc value relative to TTAB, as shown in Table I, which indicates that the anions stabilize micelle formation. ' H NMR chemical shift data demonstrate that the aromatic anions penetrate the TTAB interface, which will be. discussed later in detail.2-5J3 The orientation of the anion is assigned by concluding that the aromatic protons embedded into the micellar interior shift upfield and protons nearer to the charged interface shift downfield relative to the anion alone in D20, which are listed The naphthoate anion was assumed to be in Table strongly bound to the interface at the ratios studied although it is known that there is fast chemical exchange between the micelle and the bulk solution. With the addition of the naphthoate anions approaching 1 :1 stoichiometric mixtures, both the aromatic anion ll.2-5*13328

(28) Bacalodu, R.; Bunton, C. A.; Cerichelli, G . ; Ortega, F. J . Phys.

Chem. 1989, 93. 1490.

and surfactant resonances demonstrate simultaneous line broadening which is indicative of prolate micelle formation and 10 mM TTAB/2-naphthoate stoichiometric mixtures do exhibit viscoela~ticity.~.~J~ The cmc values for 1:l stoichiometric mixtures of TTAB/lnaphthoate and TTAB/2-naphthoate evaluated from surface tension measurements are 0.62 and 0.33 mM, respectively, which are listed in Table I. Fluorescence intensity measurements yield a cmc value of 0.65 mM for TTAB/l-naphthoate ( I : ] stoichiometry). The larger magnitude of the lowering of the cmc value by the 2-naphthoate anion relative to the 1-naphthoate anion for the TTAB/2indicates the more negative ACmicelliration naphthoate. Both naphthoate anions lower the cmc of TTAB from 3.3 mM, so the aromatic anions must be apart of the mixed micellar aggregates. ' H N M R data for a micellar TTAB solution alone clearly distinguish the a-CH2at 3.42 ppm, the head group methyl at 3.17 ppm, P-CH2at 1.8 1 ppm, 7,G-CH2at 1.40 ppm, and the terminal

Binding of Naphthoate Ions to the TTAB Interface

The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 483

C

8.5 8. 0 7 . 5 PPM 3 2 1 PPM Proton resonances for TTAB/I-naphthoate (a) the aromatic resonances of the 1-naphthoateanion alone in D20and the aliphatic region for a micellar samde of TTAB alone. The added naDhthoate anion is varied with a constant concentration, [TTAB] = 10 m M . For the other recorded spectra, (b) 1 m M I-naphthoate and (c) 2 m M I-n’aphthoate. Figure 1.

w-CH3.protons at 0.928 ppm, which are listed in Table 111 and shown in Figure la (methylene region). The assignment of various methylene resonances is in agreement with similar studies of the CTAB micellar solution^.^^'^*^"^ The chemical shift values for the 1-naphthoate anion in the micellar interface compared to the chemical shift values for the I-naphthoate anion alone in D 2 0 demonstrate that the 1naphthoatc binds with a specific orientation with respect to the interface. The resonances of the aromatic ring for the anion alone in D 2 0 were assigned in comparison to the literature values of 1 -NO2-naphthalene and 1 -methylnaphth~ate.’~.~’The most downfield shifted resonance at 8.20 ppm was assigned as H8. The aromatic region of the TTAB/ I-naphthoate sample (1O:l stoichiometric ratio) exhibits a first-order spectrum as shown in Figure 1b. The most downfield doublet at 8.74 ppm is assigned as H8. This assignment is justified knowing that a perishift occurs which has been well-established for naphthalene derivatives with electron-withdrawing substituents on the 1- p o s i t i ~ n . ~Homonuclear Z~~ decoupling experiments show that irradiation of the H8 proton collapses the triplet at 7.62 ppm into a doublet. This triplet is therefore assigned to be H7. Irradiation of H7 affects H8 and the triplet at 7.45 ppm, which collapses into a doublet. Furthermore, upon irradiation of the two overlapping doublets at 7.75 ppm, the signal at 7.45 ppm collapses into a doublet. Consequently, this triplet can be assigned to H, and one of the overlapping doublets must be assigned to H5. The protons on the aromatic ring bearing the carboxylate group were assigned in a similar fashion, and H4 was found to be one of the overlapping doublets at 7.75 ppm. The transfer of the anion into the micellar interface is observed in the chemical shift changes for the aromatic protons, and the most significant changes are observed for the overlapping doublets (H4 and H,) which shift upfield by 0.2 ppm and the doublet H8 which shifts downfield by 0.53 ppm as shown in Figure 1 and Table 11. The large downfield shift of H8 indicates that it must be located close to the charged interface. The upfield chemical shift for H4 and H5and the magnetic equivalence of their positions define the (29) (30) (31) (32) (33) 3175.

Stilbs, P. J . Colloid Interface Sei. 1983, 94, 463. Eriksson, J. C.; Gillberg, G . Acta Chem. Scand. 1966, 20, 2019. Olsson, U.; Soderman, 0.; Guering, P. J . Phys. Chem. 1986,90,5223. Lucchini, V.: Wells, P. R. Org. Mugn. Reson. 1976, 8, 137. Yukawa, Y . ;Tsuno,Y.; Shimizy, N. Bull. Chem. Soc. Jpn. 1971,44,

orientation of the anion with the carboxylate group pointing out toward the aqueous/micellar interface and the aromatic rings slicing in between the alkyl chains in the palisade layer. We are utilizing Rosen’s definition of the palisade layer, which includes the region between the hydrophilic groups and the first few carbon atoms of the hydrophobic groups that comprise the outer core of the micelle interi~r.’~The other aromatic protons demonstrate consistent chemical shift changes with respect to the charged interface, as listed in Table 11. The comparison of the micellar TTAB solution alone to the methylene proton resonances of the TTAB with the I-naphthoate anion embedded in the interface clearly demonstrate an aromatic induced chemical shift in which the methylene protons are shifted upfield, as shown in Figure 1 and Table III.8929With increasing mole fraction of the anion, an even larger upfield shift of the methylene protons is observed which is consistent with the anion penetrating into the interface as shown in Table 111. A similar shift of -0.22 ppm of the y,6-CH2 resonances is also observed in studies with the benzoate anion binding to the CTAB ir~terface.~*’~ The chemical shifts of the methylene protons are consistent with the chemical shifts of the aromatic protons and support an orientation of the anion with the exocyclic carbon bond to the carboxylate group pointing out to the aqueous/micellar interface with the aromatic ring embedded in the palisade layer of the micelle. The IH N M R data of TTAB/2-naphthoate demonstrate that the anion is also oriented relative to the charged interface. Again, the chemical shift changes of the assigned proton resonances of the 2-naphthoate were used to determine the orientation of the anion with respect to the micellar interface. The protons of the 2-naphthoate anion alone in D 2 0 were assigned by homonuclear decoupling measurements on a 10 mM sample as shown in Table 11. The assignments of protons H3 and HBare reversed from the reported values for 2-methylnaphthoate, but differences in chemical shifts are not larger than 0.07 ppm. However, one possible explanation for this discrepancy in the assignments might be that the spectrum of the methyl ester was recorded in CDC13 and not in D,O, which may lead to differences in solvation and hence different chemical shifts. (34) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989; pp 173-174.

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d

C

b

a 3 2 1 PPM 8. 5 8.0 7 . 5 PPM Figure 2. Proton rcsonances for TTAB/Z-naphthoate anion (a) the aromatic res0nance.s of 2-naphthoate anion alone in D20and the aliphatic region for a micellar sample of TTAB alone. The added naphthoate anion is varied and the TTAB concentration held constant ([TTAB] = IO mM). (b) 1 m M 2-naphthoate. ( c ) 2 m M 2-naphthoate, (d) 4 m M 2-naphthoate.

The aromatic region of the TTAB/2-naphthoate (10:2) spectrum shows a singlet at 8.51 ppm, which is assigned as H I , two overlapping doublets at 8.05 ppm assigned as H3and H8, another set of two overlapping doublets at 7.73 ppm assigned as H4 and HS,and two overlapping triplets at 7.39 ppm assigned as H6 and H,,listed in Table 11 and shown in Figure 2. This assignment was established with a COSY experiment. An increasing upfield shift of the aromatic resonances is again observed with increasing mole fraction of the anion present at constant TTAB concentration as seen in Table 11. This supports the conclusion that the 2naphthoate penetrates deeper into the palisade layer upon more complete charge neutralization of the interface. As already discussed for the I-naphthoate anion, the methylene protons of TTAB are shifted by the aromatic ring current of the 2-naphthoate anion as shown in Table 111. The @-CHIshift upfield by 0.1 ppm for the sample with 1O:l stoichiometry. The y,6-CH2 resonances shift upfield from 1.40 to 1.29 ppm, which is slightly larger than observed for the I-naphthoate with TTAB noted above. At a stoichiometry of 10:2 and 10:4, an even larger upfield shift is observed for these protons. In the 10:4 sample, the y,G-CH2 resonances shift so much that they overlap the w-methyl resonance as shown in Figure 2. The broadening of the resonances at higher anion concentration is indicative of prolate micelle formation which is also observable for the aromatic The NOESY experiments performed on the samples with l0:4 and 10:2 stoichiometric ratios show cross peaks that can be interpreted as intramolecular NOE effects of the head group methyl protons to both the y,6-CH2 and 0-CH, protons, which have merged in the 10:4 sample into the resonance of the other methylene protons as shown in Figure 3. The assignment of the /3-CH2 was confirmed by a COSY experiment recorded on the 10:2 samplc. The @CH2 resonance here is resolved from the other methylene protons and shows scalar coupling to both the a-CH,

and y,6-CH2 protons. An NOE effect interpreted as intramolecular NOE between the a-CH2and the y,G-CH2 is also observed for the 10:2 sample in another NOESY spectrum, in addition to the head group methyl NOE effects noted for the 10:4 sample. The most striking feature of the NOESY 10:4 spectrum is that each of the aromatic protons exhibits an intermolecular NOE effect to the protons of the head group methyls as shown in Figure 3. The occurrence of cross peaks from H4, H5, H6, and H7 to the &CH2 and y,6-CH2 protons demonstrates the anion penetration into the palisade layer and excludes the possibility that the aromatic anions are only surface absorbed species. The observed NOE effects are consistent with the changes in chemical shifts and strongly support the conclusion that the anions are embedded into the micellar interior. The cmc values for the hydroxyl-substituted naphthoates are smaller than for the 1- and 2-naphthoate mixed micelles, which are listed in Table I. This is indicative of the greater stabilization for the formation of the mixed micelle. The proton NMR spectra were again helpful in addressing the question of the orientation of the anion with respect to the micellar interface. The assignment of the aromatic protons are based on COSY experiments and chemical shift changes that are observed for the anion in the micelle compared to the anion in D20solution as shown in Figures 4 and 5 and Table 11. Based on the COSY data, the assignment of H,, H,, and H5 is unambiguous. A strong scalar coupling is observed for H3 and H4, and in addition on the naphthoate anion alone in D 2 0 (COSY spectrum, not shown), a long-range coupling is observed for H8 to H4 and H5 to H4. The long-range coupling distinguishes H3 from H4. If the I-OH-2-naphthoate binds with a similar orientation to the interface as the 2-naphthoate anion, then it can be assumed that H3 points toward the interface and H, is embedded in the micellar interior. Consequently, H3 is predicted to shift downfield and H, to shift upfield, and this is

The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 485

Binding of Naphthoate Ions to the TTAB Interface

region are consistent with the conclusion that the anion is penetrating the interface more deeply. The cmc values for the TTAB/3-OH-2-naphthoate are similar in magnitude to the values for TTAB/ I-OH-2-naphthoate mixed micelle, which are listed in Table I. The aromatic region for the 10:2 sample of TTAB/3-OH-2-naphthoate exhibits a first-order spectrum of two singlets, assigned as H I and H,, two doublets, assigned as H5 and Hg, and two triplets, assigned as H6 and H7. The change in chemical shift data supports the orientation of this anion with the carboxylate group pointed nearly perpendicular to the interface and the hydroxyl group toward the interface as listed in Table 11. The aromatic region of 3-OH-2-naphthoate is assigned by analogy to the three other naphthoate anions since homonuclear decoupling experiments would not provide a unique assignment of the proton resonances. The chemical shift data of the methylene region for TTAB/3-OH-2-naphthoate demonstrate the relative peak intensity changes similar to the TTAB/ I-OH2-naphthoate. The two hydroxy-substituted naphthoate anions appear to more deeply penetrate the TTAB interface than the two naphthoate anions with the observed intensity changes in the methylene proton spectral regions.

0

0 ’

0

6

4

2

PPM

spectrum of the TTAB/2-naphthoate (10:4 mM) sample. The spectrum contains cross peaks interpreted as originating from intramolecular NOEs and intermolecular NOEs. The intermolecular NOEs demonstrate the naphthoate anion is penetrating into the micellar interior since the cross peaks from the aromatic region are to both the head group methyl protons and the methylene protons of the surfactant alkyl chain which are a number of bonds down the chain from the head groups. The asterisk (b) on the one-dimensional spectrum denotes an impurity. Figure 3. NOESY

what is observed in the COSY spectrum in Figure 5. The aromatic ring protons must be embedded in the palisade layer, and the carboxylate group must point nearly perpendicular to the aqueous/micellar interface. With increasing mole fraction of the anion, a slight upfield shift of the aromatic protons is observable as shown in Figure 4. This shift is in agreement with the conclusion that the anion is penetrating deeper into the micellar interior upon increasing mole fraction of the added counterion. The resonances in the methylene region for the 10:3 sample of TTAB/ I-OH-2-naphthoate at 3.1 8, 1 S 5 , 1.03, and 3.08 ppm are assigned as a-CH2,P-CH2, y,S-CH2, and head group methyl protons, respectively, as shown in Figure 4. These assignments are deduced from the COSY experiment and the methylene protons again show an aromatic induced upfield chemical shift which is indicative of the degree of penetration. The deeper degree of penetration of the anion can be addressed by observing which set of methylene protons are shifted most upfield of the methylene protons exhibiting an aromatic induced chemical shift change with increasing mole fraction of aromatic anion. If the conclusion of deeper penetration is correct, then the largest incremental upfield shift of the methylene groups affected should be demonstrated by the methylene groups further away from the N-(CH3)3+ head group. The chemical shift values listed in Table 111 of the y,6-CH2 protons do show a pronounced upfield shift, and the head group methyl protons show the smallest upfield shift upon increasing the mole fraction of the anion. The y,6-CH2 protons are resonating at 1.26 and 1.23 ppm for the 1O:l ratio, at 1.22 and 1.1 1 ppm for the 10:2 ratio, and superimposed at 1.03 ppm for the 10:3 ratio samples and show the largest upfield shift. Furthermore, the relative peak intensity of the y,6-CH2 resonance increases to a total number of six protons, indicating that the methylene protons are now being shifted along with the y and 6 protons in the 10:2 sample. The relative peak intensity increases further in the 10:3 stoichiometric ratio sample to a total of eight protons, indicating penetration to the {-position, the sixth carbon from the head group -N(CH3)3+ on the alkyl chain. The proton chemical shift data from the aromatic and the methylene

Discussion The two hydroxynaphthoate and two naphthoate anions demonstrate that the counterion binding at the TTAB interface occurs with a specific orientation for each anion. The hydroxy groups being water soluble enhance the orientation of the carboxylate group to be fixed nearly perpendicular with respect to the micellar interface. The chemical shift changes for the protons of the aromatic region upon transfer from the aqueous bulk to the micellar environment allow us to assign the orientation for all naphthoate anions studied. An increasing degree of penetration is observed in the upfield shift of aromatic protons upon increasing mole fraction of the organic anion, which is shown in Table 11. The conclusion of increasing degree of penetration is also supported by three other experimental values: the upfield shift of the y,6CHI protons changing with increasing mole fraction of added anions, the increased integrated peak intensities of y,6-CH2 protons, and the observed NOESY intermolecular cross peaks, which are shown in Table 111 and Figure 3. The observed upfield chemical shift of protons H6 and H7 for all three anions with the carboxylate group at the 2-position demonstrate that these two protons are embedded in the micellar interior. This assignment is also consistent with the NMR studies of Bunton et al. for the 2-naphthalenesulfonate anion and methyl-2-naphthalenesulfonateester binding to CTAB which demonstrate a large upfield chemical shift change for H6 and H7.28 The one- and two-dimensional proton NMR studies presented in this paper provide unambiguous assignments of the aromatic and aliphatic resonances which allow us to select an orientational assignment with the exocyclic carbon bond linkage normal to the micellar interface as shown in Figure 6. The chemical shift change versus mole fraction data support the conclusion of the anion orientation with the exocyclic carbon bond linkage normal to the micellar interface. The orientational assignments of the various 2-naphthoate anions in this paper allow us to suggest that the observed difference in the chemical shift changes for H I and H4 in Bunton’s work may be due to the orientation of the 2naphthalenesulfonate anion at the interface, in which H I and H4 are at different distances away from the charged interface similar to our oriented 2-naphthoate anion. The observed chemical shift changes for H I and H3 in the 2-naphthoate anion are comparable to those observed by Bunton et al. for the methyl-2naphthalenesulfonate ester. This is also consistent with the interpretation of the exocyclic bond to the sulfonate group being oriented normal to the aqueous/micellar interface. Stigter’s data and calculations show these two head groups (carboxylate and sulfonate) protrude almost the same distance from the micelle interface when they are the head groups of simple ionic surfactants with similar alkyl chain lengths.35 Further support for the sim(35) Stigter, D. J . Phys. Chem. 1974, 78, 2480.

486

The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991

8. 0

7.0

7.5

Bachofer et al.

PPM

3

2

1

PPM

Figure 4. Specific orientational binding to the surfactant interface of the I-OH-2-naphthoateanion: (a) aromatic region of the aqueous solution standard of sodium I-OH-2-naphthoatein D20and the aliphatic region for a micellar sample of TTAB alone; (b) the I-OH-2-naphthoateanion in the presence of IO m M TTAB in a IO:] stoichiometricratio: (c) the 1-OH-2-naphthoateanion in the presence of IO mM TTAB in a 10:2 stoichiometricratio; (d) the I-OH-2-naphthoate anion in the presence of IO mM TTAB in a 10:3 stoichiometric ratio.

0

0

Figure 6. Proposed orientation of the 2-naphthoate anion with respect to the TTAB/2-naphthoate aqueous/micellar interface.

0

6

4

2

PPM

Figure 5. COSY spectrum of TTAB/I-OH-2-naphthoate (l0:3 stoichiometric ratio). The scalar couplings of the proton resonances of the

alkyl chain and of the aromatic anion provide data for assigning the anion orientation.

ilarity of the 2-naphthalenesulfonate and 2-naphthoate is that CTAB/2-naphthalenesulfonate mixed micelles (formed by 1 :1 stoichiometric mixtures) have been observed to form prolate aggregate structures as reported by light scattering measurements.36

The assignments of the protons for the various naphthoate anions were evaluated for a number of mole,fractions of added anion at constant TTAB concentration, so the trends in the chemical shift changes are easily observable. This fact is not trivial since the COSY data establish that the H3 and H5protons for the 1-OH-2-naphthoate anion pass through each other's chemical shift range upon anion binding to the micellar interface from the bulk aqueous solution. The TTAB/ 1-OH-2-naphthoate COSY spectrum also allows the unambiguous assignment of the methylene linkages along the alkyl chain which are affected by the aromatic induced chemical shifts as shown in Figure 5 . The large upfield shift of the y,6-CH2 protons results in the conclusion of deeper penetration by the anion upon increasing mole fraction. ( 3 6 ) Brown, W.; Johnasson, 5888.

K.;Almgren, M. J . Phys. Chem. 1989, 93,

Binding of Naphthoate Ions to the TTAB Interface

The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 481

The NOESY data are also supportive of deeper penetration the lower curvature prolate aggregate structure. However, this at higher mole fraction of added aromatic anion. An intermofact does not exclude that TTAB/salicylate mixed micelles can lecular NOE effect is observed from the protons of the counterion also form the prolate structures after the concentration of to the interior methylene protons of the micelle. We do realize TTAB/salicylate is sufficient to allow self-aggregation to occur. that these measurements will not distinguish any ion pair of the The cmc value changes are indicative of the naphthoate alkyl chain surfactant and the aromatic anion undergoing chemical counterion binding stabilizing the interface to form globular mixed exchange. The appearance of cross peaks between the aromatic micelles at low mole fraction (less than 0.166) and prolate at higher ring protons and the numerous alkyl chain methylene protons was mole fraction (greater than 0.33). Gelbart’s recent work indicates detected only in samples with high mole fraction of organic anion that changing the head group charge repulsion contribution to the free energy of micellization can define the shape of the region where the concentration of micellar bound anions is sufficently which predominates in the aggregate.I2 The magnitude of the high to be detected. The additional NOE cross peaks of the H6 ’H NMR chemical shift changes of both the aromatic anions and and H7protons to the inner palisade layer methylene protons is also consistent with the deeper binding of the anion into the the surfactant methylene protons supports the various degrees of palisade layer. stabilization of the interface (the changes in the AGmlcellization) by The chemical shift changes of the methylene protons on the the different anions. The decrease in both the electrostatic resurfactant are induced by the magnetic anisotropy of the oriented pulsion of the head groups and the favorable packing of the anions aromatic rings penetrating into the palisade layer of the micelle. supports the conclusion of deeper penetration by the naphthoate Aromatic induced ring current shifts have been observed in studies anions which increases the aggregate volume to surface area ratio of monomeric tetrasulfonatotetraphenylporphyrins(TPP) binding to prescribe the sphere-to-rod phase transition. to the interior of cationic and nonionic micellar aggregates as The orientational assignments supported by the one- and two-dimensional proton NMR data of the four naphthoate anions reported by Kadish et aL3’ The heme compounds studied by Medhi et al. also demonstrate an observable aromatic induced are with the carboxylate group pointed nearly perpendicular to the micellar interface, which agrees with other N M R chemical chemical shift change due to the large porphyrin ring current.3s shift data that has been obtained for the CTAB/X-benzoate mixed The depth of penetration by the various naphthoate anions is larger than for the benzoate anions that we have studied previously Our data are also in agreement with the studies of salicylate binding to the CTAB interface!*45 The only reported at the CTAB interface.13 The hydrophobicity of the naphthoate anions is higher than the benzoate anions. A previous conclusion study known to the authors of an anion assigned with a tilted that the hydrophobicity of both cations and anions in these mixed orientation at a cationic micellar interface is CTAB/m-OHben~oate.~ The assignment of the m-hydroxybenzoate anion with micelles must play a significant role in the thermodynamics of self-aggregation is rather s t r a i g h t f ~ r w a r d . ~ + Increasing ~ ~ * ~ the a hydrophilic group oriented to the micellar interior would most hydrophobicity of the anions does indeed increase the counterion likely lead to a less favorable AGmicelii~ti,,nthan all of the presently studied anions which should all have their hydrophilic groups binding constant as demonstrated by Stilbs, but it is not the only oriented to the aqueous/micellar interface. driving force for determining the aggregate shape.s The hydroxynaphthoate anions appear to pack as efficiently The packing constraints are significant in defining the aggregate as salicylate to promote the prolate micellar structure. The upfield shape, since TTAB interacting with salicylate anions also show shift changes with increasing mole fraction of naphthoate a viscoelastic effect at IO mM in each r e a ~ t a n t . ~ * ” J The ~ . ~ ~ * ~chemical ~ anion indicate the deeper penetration of the interface by the anions salicylate anion does not show the pronounced penetration into and may also indicate the change in micelle aggregate structure. the palisade layer like the naphthoate anions, since the 7,G-CH2 The resonances in the NMR spectra of the 2:l TTAB/hydroxyprotons of TTAB do not shift upfield as much with the salicylate naphthoate have broadened to a point that the individual methcompared to the naphthoates at the same mole fraction.43 The ylene protons cannot be resolved and the aggregate structure is large decrease in the cmc values for the TTAB/X-naphthoate concluded to be prolate, r ~ d l i k e . ~The ’ 1-OH-2-naphthoate and mixed micelle in comparison to TTAB indicates the four 3-OH-2-naphthoate would most likely have very similar packing naphthoate anions do stabilize aggregate formation consistent with Stilbs’ counterion binding constant data for various hydrophobic constraints to the salicylate discussed above. The conclusion for a prolate, rodlike micellar aggregate structure is also supported anions. The naphthoate anions are observed to penetrate the by the recent SANS and shear viscosity data of Hoffmann et al. interface sufficiently to promote sphere-to-rod phase transition at low concentration^.^*^^ The packing constraints can be estimated on dilute TTAB/aromatic anion mixed micelle^.^^^^' The shear by calculating for a near-zero curvature interface the surface area viscosity measurements clearly define the formation of a prolate per TTAB/ 1 -naphthoate and TTAB/Znaphthoate pair, applying micellar aggregate with 3-OH-Znaphthoate as the anion forming the mixed micelle.41 Our NMR and cmc data clarify the structural the Gibbs adsorption isotherm to our surface tension data. The constraint of these anions to stabilize the micellar interface and calculations for the surface area per TTAB/ 1-naphthoate and then the possibility to stabilize a prolate micelle structure in dilute TTAB/2-naphthoate pair yield values equal to 59 f 10 and 57 solutions. f IO A2,respectively. The calculated values do indicate that these anions pack efficiently at the interface with low or near-zero We will continue to study these systems by changing both the pH and electrolyte concentrations. The application of NMR and curvature. The calculated head group surface area for the ion-selective electrode techniques in tandem will allow us to more TTAB/salicylate pair is 57 f IO A2, which is approximately the clearly discern the pH and electrolyte concentration effects on same value considering the experimental error and indicates that the counterion binding equilibria, especially since the prolate the salicylate anion packs as efficiently at a near-zero curvature TTAB/naphthoate mixed micelles appear less sensitive than interface as the more hydrophobic 1-naphthoate and 2-naphthoate. CTAB/benzoate mixed micelles to pH and ionic strength The recorded cmc value for TTAB/salicylate is higher than the changes.”,& The evaluation of the different head group area and TTAB/na~hthoates.~~ Our data set indicates that the naphthoate electrostatic charge repulsion contributions to micellization can anions penetrate deeper into the micellar interface and promote be further clarified upon determining the counterion binding equilibria. (37) Kadish, K. M.; Maiya, G. B.; Araulb, C.; Guilard, R. Inorg. Chem. 1989, 28. 2125. (38) Medhi, 0. K.; Mazumdar, S.;Mitra, S. Inorg. Chem. 1989,24,3243. (39) Tiddy, G. J . T.; Barker, C. A.; Saul, D.; Wheeler, B. A,; Willis, E. J . Chem. Soc.. Faraday Trans. I 1974, 70, 163. (40) Tiddv. G. J . T.:Saul. D.: Wheeler. B. A,; Wheeler. P. A,: Willis, E. J . Chem. So;., Faraday Trans. I 1974, 70, 154. (41) Kalus, J.; Hoffmann, H.; Ibel, K. Colloid Polym. Sci. 1989, 267,818. (42) Ohlendrof, D.; Interthal, W.; Hoffmann, H . Rheol. Acta 1986, 25, 468. (43) Bachofer, S.J.; Nowicki, T.A . Unpublished results.

Acknowledgment. The authors thank Drs. F. Ann Walker, Clayton Radke, and John Correia for the positive encouragement (44) Lindman, B.; Lindblom, G.; Mandell, L. J . Colloid Interface Sci. 1973, 42, 400. (45) Hirata, H.; Shikata, T.; Kotaka, T. Langmuir 1988, 4, 354. (46) Underwood, A . L.; Anacker, E. W. J . Colloid Interface Sci. 1985, 106, 89.

488

J . Phys. Chem. 1991, 95,488-492

and helpful suggestions throughout this research. The authors graciously acknowledge the Research Corporation for funding this research. The access to the high-field NMR spectrometers provided by Dr. Clayton Radke (Department of Chemical Engineering at University of California, Berkeley) and Dr. F. Ann Walker (Department of Chemistry at San Francisco State University) was instrumental to a more complete understanding

of these viscoelastic systems and is gratefully acknowledged. The Department of Chemistry, San Francisco State University, also acknowledges grants from the National Institutes of Health (RR 02684) and the National Science Foundation (DMB-8516065) for purchase of the NMR spectrometers. The financial support of Saint Mary's College is acknowledged for providing an additional stipend for T.A.N.

Energy- and Electron-Transfer Shuttling by a Soluble, Bifunctional Redox Polymer Janet N. Younathan,+ Wayne E. Jones, Jr., and Thomas J. Meyer* Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27599-3290 (Received: May 7, 1990)

A soluble, bifunctional polymer ( [PS-An28,SPTZI,S]) based on derivatized polystyrene was prepared in which there are both energy-transfer acceptors (modified anthracene, An) and electron-transfer donors (derivatized phenothiazene, PTZ). The polymer was successfully incorporated into a photochemical electron-transfer sequence based on [Ru(bpy)J2' (bpy is 2,2'-bipyridine) in which separate oxidative and reductive equivalents were generated in solution. In the sequence, sensitized occurred by diffusion and formation of the triplet excited state of the polymer-bound anthryl sites ( [PS-3An*An27.5PTZl,s]) energy transfer from [Ru(bpy)J2+* following visible excitation of [Ru(bpy)J2+. In the presence of the oxidative quencher and monomeric paraquat ( PQ2+),a series of electron-transfer steps led,ultimately, to the appearance of [PS-An2s,sPTZ+PT~,5] PQ' in solution. The recombination rate constant between PQ+ and [PS-An2,,sPTZ+PTZo,s]was reduced by a factor of 7 relative to back electron transfer between the unbound, IO-methylphenothiazene cation (10-MePTZ') and ?Q+.

-

SCHEME I

Introduction

In reactions 1-3, visible excitation followed by a sequence of bimolecular electron-transfer events leads to the conversion of light into transiently stored oxidative and reductive equivalents (bpy is 2,2'-bipyridine; PQ2+ is paraquat, IO-MePTZ is 10-methylphenothiazene).' Ru(bpy)F Ru(bpy)p' Ru(bpy),3t

+ PO2+

+ 10-MePTZ

hv

Ru(bpy)$'

(1)

Ru(bpy),& + PO'

(2)

RU(bpy)? + 10-MePTZ'

(3)

(9-MeAn)'

0022-3654/91/2095-0488$02.50/0

[PS-PTZs]

relay between the light absorbing sensitizer and paraquat, eqs 5 and 6. Prompt recombination within the solvent cage between + 9-AnC0;

319-AnC0.'i

Present address: Eastman Kcdak Company, Rochester, NY 14650.

9;MeAn'

[PS-PTZ~PT~]

RU(bpy)T'

Attempts have been made to maximize yields and inhibit back electron transfer for such reactions by selective modification of the reaction microenvironement. This has involved the utilization of micelles, colloids, microemulsions, or polyelectrolytes.2 Another strategy has involved anchoring the Ru(I1) sensitizer and/or viologen quencher to macromolecules. This can reduce the rate of diffusional encounter but, at the same time, decreases the rate of back electron t r a n ~ f e r . ~ - ' ~ An inherent limitation in the [ R ~ ( b p y ) ~ l ~ + / PsensitizerQ~+ quencher combination is the relatively low cage escape yield associated with eq 2. In homogeneous solutions containing [Ru(bpy)J2+* and enough PQ2+to quench the excited state with near unit efficiency, PQ+ yields are 0.25 or l ~ w e r . ' ~ - 'The ~ separation efficiency can be even lower with polymer-bound reagents,4-6,8,9,1 1-13 The results of studies by Johansen, Mau, and Sasse2OS2'have demonstrated how separation efficiencies can be enhanced by incorporating 9-anthracene-carboxylate (9-AnC02-) as an energy

+

I

9-MeAn,

t

Po2+

___t

Ru(bpY)$ 9-AnC02'

t

'(9-AnCO;)'

(5)

+ PQ'

(6)

COi (9-AnCOf) ~

~

~~~

(1) (a) Young, R. C.; Meyer, T. J.; Whitten, D. G . J . Am. Chem. Soc. 1975, 97,4781. (b) Meyer, T. J. Isr. J . Chem. 1977, I S , 200. (c) Bock, J. A.; Connor, A. R.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G . ; Sullivan, B. P.; Nagel, J. K. J. Am. Chem. Soc. 1979, I O / , 4815. (d) Sutin, N.; Creutz, C. Pure Appl. Chem. 1980,52,2717. (e) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159 and references therein. (2) For example, see: (a) Rabani, J.; Sasson, R. E. J. Photochem. 1985, -29, 7. (b) Gratzel, M., Ed. Energy Resources through Photochemistry and Cafalysis;Academic: New York, 1983. (c) Kalyanasundaram, K. Phorochemistry in Microheterogeneous Systems; Academic: New York, 1987. (d)

McLendon, G., et al. In Photochemical Energy Conversion, Proc. Int. Conf. Photochem. Convers. Solar Energy Storage, Elsevier: New York, 1989, 47-59. (e) Rabani, J. In Photoinduced Electron Transfer, Part B; Fox, M. A., Chanon, M., Eds.; Elsevier: New York, 1988; p 642. (3) Kaneko, M.; Nakamura, H. Macromolecules 1987, 20, 2265. (4) Kaneko, M.; Hou, X.-H.; Yamada, A. Bull. Chem. Soc. Jpn. 1987,60, 2523. (5) Kaneko, M.; Nakamura, H. Makromol. Chem. 1987, 188, 201 1. (6) Hou, X.-H.; Kaneko, M.; Yamada, A. J . Polym. Sci., Polym. Chem. Ed. 1986, 24, 2749.

0 1991 American Chemical Society