Nature of the Alternate CF2CH2 Chain: Study Based on the

Study on the Aggregation and Deaggregation Behaviors of Phosphatidylcholines. Guo-Zhen Ji , Ya-Qiu Long , Xi-Kui Jiang. Chinese Journal of Chemistry 2...
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Langmuir 1997, 13, 3603-3609

3603

Nature of the Alternate CF2CH2 Chain: Study Based on the Measurement and Comparison of the CAgC’s of Aggregators with Alternate Chains and with Hydrocarbon Chains Michael Yu Zhu,* Thomas Xin-Yu Zhang, June Hong Zeng, and Xi-Kui Jiang* Shanghai Institute of Organic Chemistry, 354 Feng-Lin Lu, Shanghai 200032, China Received December 27, 1996X The saponification rate constants of p-nitrophenyl esters of carboxylic acids with the hydrocarbon chain and the alternate CF2CH2 chain, i.e., H(CH2CH2)(m-1)/2COOC6H4NO2 (C-m) and Cl(CF2CH2)(m-1)/2COOC6H4NO2 (AL-m), in which m stands for the number of chain-carbon atoms and is equal to 3, 5, 7, 9, 11, and 13, have been measured in the 20:80 (v/v) (Φ ) 0.20) buffered dioxane-H2O binary solvent. Meanwhile, the fluorescence spectra of 2-(R-naphthyl)ethyl esters of carboxylic acids with the hydrocarbon chain and the alternate CF2CH2 chain, i.e., H(CH2CH2)(m-1)/2COOCH2CH2C10H7 (Fl-C-m) and Cl(CF2CH2)(m-1)/2COOCH2CH2 C10H7 (Fl-AL-m) (m ) 3, 5, 7, 9, 11, and 13) have been measured in the Φ ) 0.20 dioxane-H2O solvent system. Critical aggregate concentration (CAgC) values derived from the aforesaid kinetic and spectral measurements revealed that the alternate chain possesses a smaller tendency toward aggregation than the hydrocarbon chain does. Therefore, the alternate CF2CH2 chain seems to possess a minute degree of polarity. It has been shown that even neutral organic molecules with a threecarbon chain could show some aggregating tendencies, and our results imply that simple aggregates are formed by a stepwise process.

Hydrophobic-lipophilic interactions (HLI), together with nature’s other forces, create simple aggregates (Ag’s), micelles, vesicles, and living cells from organic molecules in solvents with SAgP (solvent aggregating power).1,2 Neutral organic molecules which form Ag’s are called aggregators (Agr’s).2f Ag’s may serve as one of the simplest models for HLI study. Over the last 12 years, a great deal of investigations have been done on the aggregation and self-coiling behaviors of Agr’s with the hydrocarbon chain.2 Fluorocarbon molecules seem to possess more pronounced hydrophobic character in comparison with hydrocarbon molecules.3,4 One might expect that in aquiorgano solvents HLI could also effectively push fluorocarbon chains toward Ag formation. In fact, Tung’s recent results have revealed that the aggregating tendency of the fluorocarbon chain is greater than that of the hydrocarbon chain.4 The nature of another type of carbon chain, i.e., the alternate CF2CH2 chain, has not received much attention. It is characterized by alternating CF2 and CH2 groups (or building blocks) and will be simply referred to as the “alternate” chain [(CF2-CH2)n] in this paper. The agX

Abstract published in Advance ACS Abstracts, June 15, 1997.

(1) Tanford, C. The Hydrophobic Effects, 2nd ed.; Wiley: New York, 1980. (2) (a) Jiang, X. K. Acc. Chem. Res. 1988, 21, 362-367. (b) Jiang, X. K. Pure Appl. Chem. 1994, 66, 1621-1628. (c) Jiang, X. K. J. Chin. Chem. Soc. 1995, 42, 623-626. (d) Jiang, X. K.; Sun, S. X. Prog. Nat. Sci. 1995, 5, 527-535. (e) Tung, Z. H.; Xu, C. B. In Photo-chemistry and Photophysics; Rabek, J. F., Ed.; CRC Press Inc.: Boca Raton, FL, 1990; Vol. 4, Chapter 3. (f) Jiang, X. K.; Ji, G. Z.; Zhang, J. T. Langmuir 1994, 10, 122-125. (3) (a) Kissa, E. Fluorinated Surfactants, Synthesis‚Properties‚ Applications; Maecel Dekker Inc.: New York, 1993. (b) Mukerjee, P. Colloids Surf., A 1994, 84, 1-10. (c) Ravey, J. C.; Stebe, M. J. Colloids Surf., A 1994, 84, 11-31. (d) Giulieri, F.; Krafft, M.-P. Colloids Surf., A 1994, 84, 121-127. (e) Zhang, Y. X.; Da, A. H.; Butler, G. B.; HogenEsch, T. E. J. Polym. Sci., Part A 1992, 30, 1383-1391. (f) Asakawa, T.; Mouri, M.; Miyagishi, S.; Nishida, M. Langmuir 1985, 5, 343-348. (g) Kalyanasundaram, K. Langmuir 1988, 4, 942-945. (h) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162-171. (i) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165-175. (j) Mukerjee, P.; Gumkowski, M. J.; Chan, C. C.; Sharma, R. J. Phys. Chem. 1990, 94, 8832-8835. (k) Shinoda, K.; Nomura, J. J. Phys. Chem. 1980, 84, 365-369. (l) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 24682470. (m) Mathis, G.; Leempoel, P.; Ravey, J.-C.; Selve, C.; Delpuech, J. J. J. Am. Chem. Soc. 1984, 106, 6162-6171.

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gregating tendency of the alternate chain to form simple Ag’s was completely unknown. It might have been speculated that since structurally it lies in between the fluorocarbon and the hydrocarbon chain, its aggregating tendency might also lie somewhere in between those of the two well-known types of chains. However, it might also be expected that the alternate chain should possess a minute degree of alternating polarity because of the electronegativity of the fluorine atoms, as shown in the following picture. If this picture were realistic, then the alternate chain might possess a smaller inherent aggregating tendency than either of the two classical chains. The present work aims to find a preliminary answer to this interesting question.

In order to study the aggregating tendency of the alternate chain and compare it with that of the hydrocarbon chain, kinetic probes (C-m and AL-m) and fluorescence probes (FL-C-m and Fl-AL-m) of the following structures were used. Full names of these ester probes are given in the Experimental Section. In these structures, m always represents the number of carbon atoms of the acyl moiety of these esters.

(4) (a) Tung, C. H.; Ji, H. F. J. Chem. Soc., Faraday Trans. 1995, 91, 2761-2765. (b) Tung, C. H.; Ji, H. F. J. Phys. Chem. 1995, 99, 83118316.

© 1997 American Chemical Society

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Zhu et al.

Table 1. Σf a and ∆fCl%b Values of Compounds C-m, AL-m, Fl-C-m, and Fl-AL-m C-3

C-5

C-7

C-9

C-11

C-13

Σf a

1.863

2.901

3.939

4.977

6.015

7.051

AL-3

AL-5

AL-7

AL-9

AL-11

AL-13

Σf a

2.446

3.335

4.204

5.083

5.962

6.841

Fl-C-3

Fl-C-5

Fl-C-7

Al-C-9

Fl-C-11

Fl-C-13

Σf

a

4.120

5.158

6.196

7.234

8.272

9.310

Table 2. CAgC Values (10-5 M) of C-m, AL-m, Fl-C-m, and Fl-AL-ma in the Φ ) 0.20 DX-H2O Solvent System C-7

C-9

C-11

C-13

CAgCb

6.3 ( 0.5

2.0 ( 0.1

0.52 ( 0.04

(0.050)d

AL-7

AL-9

AL-11

AL-13

CAgCb

(7.4)d

(5.8)d

1.8 ( 0.1

0.84 ( 0.09

Fl-C-7

Al-C-9

Fl-C-11

Fl-C-13

5.6 ( 0.6

0.46 ( 0.06

0.10 ( 0.02

0.054 ( 0.006

CAgCc

Fl-AL-3 Fl-AL-5 Fl-AL-7 Fl-AL-9 Fl-AL-11 Fl-AL-13 4.703 Σf a ∆fCl%b 45.8

5.582 29.7

6.461 17.4

7.340 9.6

8.219 5.0

9.098 2.6

a

Σf values were calculated from Rekker fragmental constants listed in ref 6. b ∆fCl values for AL-m and Fl-AL-m: ∆fCl% ) ∆fCl/ [∆fCl + Σf(CF2CH2)(m-1)/2], see text.

Unfortunately, probe molecules with HCF2 instead of ClCF2 end groups, i.e., H(CF2CH2)(m-1)/2CO2C6H4NO2 and H(CF2CH2)(m-1)/2CO2CH2CH2C10H7, could not be readily prepared, and we were forced to compare probe molecules with different end atoms, i.e., ω-Cl in AL-m’s and Fl-AL-m’s against ω-H in C-m’s and Fl-C-m’s. Otherwise, our probe molecules differ only by the nature of the chain, i.e., [(CF2CH2)](m-1)/2 versus [(CH2CH2)](m-1)/2. The abovementioned problem or difficulty caused by this ω-endgroup difference, however, does not seem to be irremediable, because our approach consists of trying to reveal the nature of the alternate chain by comparing the aggregating tendencies of the following pairs, i.e., C-m/ AL-m and Fl-C-m/Fl-AL-m, with m ) 9, 11, and 13, and the legitimacy of this approach may be shown by the following arguments. It has been amply demonstrated that, for the same type of aggregators, Rekker’s Σf values closely parallel their aggregating tendencies (cf. Table 1).2b,5 Therefore the above-mentioned ω-end-group difference may be approximately expressed in terms of the ∆fCl value, where ∆fCl ) Σf(CF2Cl) - Σf(CF2H) ) 1.284 0.542 ) 0.742,6 and the relative importance of this ω-endgroup effect in probe molecules AL-m and Fl-AL-m of different chain lengths may be expressed by the value of ∆fCl%, where ∆fCl% ) ∆fCl/Σf(CF2CH2)(m-1)/2. Since the Σf value for each CF2CH2 segment is 0.879, the value of Σf (CF2CH2)(m-1)/2 can be easily calculated by the expression Σf(CF2CH2)(m-1)/2 ) 0.879(m - 1)/2. The ∆fCl% values for the AL-m and Fl-AL-m probes with different chain lengths (m ) 3, 5, 7, 9, 11, and 13) are summarized in the last row of Table 1. It is clearly seen that the ω-end-group effect is large (ca. 46%) for the short-chained probes (AL-3 and Fl-AL-3), but it quickly becomes smaller when the chain lengthens (ca. 30% at m ) 5 and 17% at m ) 7). By the time the chain consists of more than nine carbons, the ∆fCl% value falls below 10% (2.6% at m ) 13). Consequently, it would be relatively safe to assume that the differences in aggregating tendencies (CAgC values) between the following pairs, namely, C-m/AL-m and Fl-C-m/Fl-AL-m, with m ) 9, 11, and 13, can basically reveal the difference in the inherent nature of the alternate (5) Jiang, X. K.; Ji, G. Z.; Tu, B.; Zhang, X. Y.; Shi, J. L.; Chen, X. J. Am. Chem. Soc. 1995, 117, 12679-12682. (6) Rekker’s f values or hydrophobic fragmental constants are calculated from octanol-water partition coefficients. The total hydrophobicity of an organic molecule can be roughly estimated from a summation of the f values of all the “fragments” of that molecule. For illustration, the parenthesized f values are given by Rekker for the following fragments: CH3 (0.701), CH2 (0.519), C (0.155), H (0.182), CF2 (0.360), Cl (0.057), COO (-1.251), OOC (-0.962), C6H4 (1.658), C10H7 (3.113), NO2 (-0.053). See: (a) Rekker, R. F. The Hydrophobic Fragmental Constants; Elsevier: Amsterdam, 1977; Vol. 1. (b) Rekker, R. F.; de Kort, H. M. Eur. J. Med. Chem. 1979, 14, 479-488.

CAgCc

Fl-AL-7

Fl-AL-9

Fl-AL-11

Fl-AL-13

2.4 ( 0.5

1.2 ( 0.3

0.58 ( 0.06

0.49 ( 0.07

a m is the number of chain carbon atoms. b Measured by the kinetic method at 35 °C; an aqueous solution of Na2CO3 (0.20 M), NaHCO3 (0.067 M), and NaCl (0.34 M) was used as buffer solution. c Measured by the fluorescence method at 25 °C. d Roughly estimated value.

Figure 1. Plot of log kob vs logarithm of initial concentration of C-m in the Φ ) 0.20 buffered DX-H2O system at 35 °C.

and the hydrocarbon chain, because under these circumstances, the effect of the alternate chain is probably more than an order of magnitude larger than the ω-end-group effect. In fact, all our data are consistent with the aforesaid proposition. On the basis of this approach, CAgC values of all our probes were measured and summarized in Table 2, in which it was noted (note d) that three of the CAgC values (for C-13, AL-7, and AL-9) were roughly assessed values. In recent years, CAgC has been firmly established as the most reliable indicator of the aggregating tendency of an Agr.2,5,7 The best methods used for the evaluation of CAgC are those of plotting log kob against the logarithm of the initial concentration of the ester ([Agr]i), where kob is the observed saponification rate constant of the ester Agr, as illustrated by Figures 1 and 2,7,8 and those of plotting log (Ie/Im) or Ie/Im against the concentration of the Agr-probe species ([Agr]), where Ie and Im are the relative fluorescence intensities of the monomer and the excimer of the fluorescence probe, as illustrated by Figures 3 and (7) Zhang, J. T.; Nie, J.; Ji, G. Z.; Jiang, X. K. Langmuir 1994, 10, 2814-2816. (8) (a) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1968, 90, 1875-1878. (b) Guthrie, J. P. J. Chem. Soc., Chem. Commun. 1972, 897-899. (c) Guthrie, J. P. Can. J. Chem. 1973, 51, 3494-3498. (d) Murakami, Y.; Aoyama, Y.; Kida, M. J. Chem. Soc., Perkin Trans. 2 1977, 1947-1952. (e) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q. J. Am. Chem. Soc. 1984, 106, 3839-3843. (f) Jiang, X. K.; Li, X. Y.; Huang, B. Z. Proc. Ind. Acad. Sci. (Chem. Sci.) 1987, 90, 409-422. (g) Jiang, X. K.; Li, X. Y.; Huang, B. Z. Proc. Ind. Acad. Sci. (Chem. Sci.) 1987, 90, 423-434. (h) Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J. Chem. 1991, 9, 448-452. (i) Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J. Chem. 1991, 9, 453-457.

Nature of the Alternate CF2CH2 Chain

Figure 2. Plot of log kob vs logarithm of initial concentration of AL-m in the Φ ) 0.20 buffered DX-H2O system at 35 °C.

Langmuir, Vol. 13, No. 14, 1997 3605

Figure 5. Fluorescence spectra of different concentrations of Fl-AL-3 in water at 25 °C.

Results and Discussion

Figure 3. Plot of log Ie/Im vs concentration of Fl-C-m in the Φ ) 0.20 DX-H2O system at 25 °C, where Ie and Im are the relative fluorescence intensities of monomer and excimer, respectively.

Figure 4. Plot of log Ie/Im vs concentration of Fl-AL-m in the Φ ) 0.20 DX-H2O system at 25 °C, where Ie and Im are relative fluorescence intensities of monomer and excimer, respectively.

4.7,9,10 The two methods are therefore selected for the present study, with the target probes C-m and AL-m serving as substrates for kinetic measurements and with Fl-C-m and Fl-AL-m serving as the target probes for fluorescence measurements. (9) Jiang, X. K.; Hui, Y. Z.; Fei, Z. X. J. Am. Chem. Soc. 1987, 109, 5862-5864. (10) Zhang, X. Y.; Zhu, Y.; Ji, G. Z.; Jiang, X. K. Chin. Chem. Lett. 1996, 7, 759-762.

1. Comparison of Aggregating Tendencies of the Alternate Chain and the Hydrocarbon Chain by the CAgC Values of Compounds with an Alternate Chain or a Hydrocarbon Chain. CAgC values have been extensively and successfully used for the assessment of the aggregation tendencies of Agr’s.2,5,7,8 A smaller CAgC value signifies a greater aggregating tendency. Table 2 lists the CAgC values of the compounds containing either an alternate chain or a hydrocarbon chain. The CAgC values of C-m and Fl-C-m show that a longer hydrocarbon chain has a larger aggregating tendency. Similarly, the CAgC values of AL-m and Fl-AL-m show that a longer alternate chain possesses a larger aggregating tendency. In other words, as expected for the chain-length effect, the order of aggregating tendencies for C-m is C-13 > C-11 > C-9 > C-7, that for Fl-C-m is FL-C-13 > Fl-C-11 > Fl-C-9 > Fl-C-7, that for AL-m is AL-13 > AL-11 > AL-9 > AL-7, and that for Fl-AL-m is Fl-AL-13 > Fl-AL-11 > Fl-AL-9 > Fl-AL-7. By comparing the CAgC values of compounds AL-m (m ) 9, 11, 13) with those of compounds C-m of equal chain length, it is easily seen that the CAgC values of AL-m’s are larger than those of their hydrocarbon counterparts. The difference between the corresponding CAgC values increases with increasing chain length. For example, at a chain length of 9 carbons, the CAgC value of AL-9 is about three times that of C-9; at a chain length of 13 carbons, the CAgC value of AL-13 is about 17 times that of C-13. Similar results are obtained by comparing the CAgC values obtained from fluorescence measurements; i.e., compounds Fl-AL-m (m ) 9, 11, 13) have larger CAgC values than the corresponding compounds Fl-C-m have. Again, the difference increases with increasing chain length; e.g., the CAgC of Fl-AL-9 is about three times that of Fl-C-9, whereas, the CAgC of Fl-AL-13 is about nine times that of Fl-C-13. These differences in CAgC between C-m and AL-m and between Fl-C-m and Fl-AL-m are clearly shown both in Figures 6 and 7. The above-mentioned results conclusively demonstrate that aggregating tendencies of compounds possessing an alternate chain are smaller than those of the corresponding compounds possessing a hydrocarbon chain when the chain length is equal to or greater than 9 carbons. According to our previously mentioned analysis, when m is equal to 9, 11, and 13, it is realistic to consider that the difference between the CAgC values of C-m and AL-m or those of Fl-C-m and Fl-AL-m can basically reveal the difference in the inherent nature of the alternate and the

3606 Langmuir, Vol. 13, No. 14, 1997

Zhu et al.

Figure 6. Chain-length dependence of log CAgC values of C-m and AL-m on m, the number of chain-carbon atoms.

Figure 7. Chain-length dependence of log CAgC values of Fl-C-m and Fl-AL-m on m, the number of chain-carbon atoms.

hydrocarbon chain; therefore, it can be concluded that the aggregating tendency of the alternate chain is less than that of the hydrocarbon chain. It appears that if we extrapolate the straight lines in Figures 6 and 7, we would observe a reverse of the CAgC order. In other words, the orders of aggregating tendencies would become AL-3 > C-3, AL-5 > C-5, Fl-AL-3 > Fl-C-3, Fl-AL-5 > Fl-C-5, and Fl-AL-7 > Fl-C-7. Very likely, this is a consequence of the ω-end-group effect of the chlorine atom discussed in the Introduction. 2. Free Energy Difference per CF2CH2 or CH2CH2 Group between the Aggregate State and the Monomeric State in the Φ ) 0.20 DX-H2O System. Figures 6 and 7 show that the log CAgC values listed in Table 2 are linearly related to chain lengths of either the alternate chain or the hydrocarbon chain. The correlation equations are

is the Boltzmann constant, T is the absolute temperature, N is Avogadro’s number, and v is the free volume of one molecular.

C-m: log CAgC ) -0.345m - 1.673, r ) 0.985, n ) 4 (1) Fl-C-m: log CAgC ) -0.336m - 2.105, r ) 0.965, n ) 4 (2) AL-m: log CAgC ) -0.168m - 2.873, r ) 0.975, n ) 4 (3) Fl-AL-m: log CAgC ) -0.120m - 3.828, r ) 0.972, n ) 4 (4) These equations suggest that the relationship of CAgC with carbon chainlength (m) can be expressed by eq 5. It is worthy to note that the B values (-0.35 and -0.34) for C-m and Fl-C-m, both of which contain a hydrocarbon chain, are approximately the same, while the B values (-0.17 and -0.12) for AL-m and Fl-AL-m, both of which contain an alternate chain, are rather close to each other. Therefore, a question emerges: what is the significance of the B value?

log CAgC ) -Bm + constant

(5)

It is well-known that the effect of chain length on the critical micelle concentration (cmc) of nonionic surfactants can be derived from statistical thermodynamics and follows eq 6,11 in which m′ is the number of carbon atoms constituting a hydrocarbon chain, ω′ is the change of free energy per methylene group in the micelle formation, k (11) Shinoda, K. Colloidal Surfactants; Academic Press: New York, 1963; Chapter 1.

ln cmc ) ln

1000 m′ω′ m′ω′ -1)+ constant Nv kT kT (6)

Although simple aggregates are not typical nonionic micelles, both of them are assemblages formed by HLI. By using the statistical thermodynamics treatment that is analogous to that derived by Shinoda11 for critical micelle concentration (cmc), then CAgC might also be expressed as a function of carbon chain length, i.e., by eq 7. In other words, we might be able to replace m′ (eq 6) by q, and ω′ (eq 6) by ω, where q is the number of CH2CH2 or CF2CH2 groups in the chain and ω represents the change of free energy per CH2CH2 or CF2CH2 group between the aggregate state and the monomeric state and m is the number of carbon atoms in the chain (i.e., including the carbonyl carbon), as we defined in the Introduction. Therefore q is equal to (m - 1)/2. C and D are constants here. The former is related to the particular Agr series, and the latter is related to the salt additive and its concentration.11 Clearly, the coefficient B of eq 5 becomes ω/(2kT × 2.303) in eq 7.

ln CAgC ) -

qω ω m-1 +C+D)+C+D) kT kT 2 mω + constant (7) 2kT

Apparently, eq 7 can rationalize the dependence of the CAgC value on the number of chain-carbon atoms. It can also rationalize the closeness of the B values obtained from eqs 1 and 2 for compounds possessing a hydrocarbon chain and that of those B values obtained from eqs 3 and 4 for compounds possessing an alternate chain. Obviously, a larger B value signifies a larger ω or a stronger aggregating tendency for that carbon chain. By using the B values in eqs 1 and 2, the ω value, i.e., the free energy difference per CH2CH2 group between the aggregate and the singly dispersed state in the Φ ) 0.20 DX-H2O system, may be roughly estimated to be 1.57kT, and by using the B values in eqs 3 and 4, the free energy difference per CF2CH2 group between the aggregate and the singly dispersed state in the Φ ) 0.20 DX-H2O system may be approximately estimated to be 0.66kT, which is definitely smaller than the 1.57kT value for the CH2CH2 group. These results also indicate that the alternate chain

Nature of the Alternate CF2CH2 Chain Scheme 1

has a weaker aggregating tendency than the hydrocarbon chain does. The smaller aggregating tendency of the alternate chain apparently reveals that it may possess minute polarity. In other words, the hydrophobicity and lipophilicity of the alternate chain are weaker than those of the two other classical types of chains, i.e., perfluorocarbon chain and hydrocarbon chains, because of this minute polarity. 3. How Are the Simple Ag’s Formed? An incidental but interesting result of this study is an observation which seems to be of relevance to the mechanism of the formation process of the simple Ag’s. Intuitively many chemists would visualize a stepwise process for Ag formation, as shown by Scheme 1,12 in which S represents an Agr molecule, N is the average aggregation number, and τ(N) standards for the half-life of the N-mer. This scheme suggests that the half-life of the encounter pair (2-mer), or τ(2),13 is greatly lengthened by HLI, so that 3-mers, 4-mers, ..., N-mers, etc. (all of them are designated as n-mers) can form. Obviously, under specific conditions (temperature, SAgP, [Agr], etc.), the magnitude of τ(2) is of basic relevance to all the other τ(n)’s and to the concentrations of the n-mers. However, direct experimental evidence for this common-sense scheme is difficult to come by. For instance, the fact that a reliable CAgC is derived from a relatively sharp break-point of a log kob vs log [Agr]i plot does not seem to fully reconcile with such a scheme; i.e., it seems that a more gradual transition from the horizontal line to the slanting line in such plots could also be visualized.7,8f-h,14 In particular, previously we have more or less accepted the “conventional wisdom” that only compounds with seven or eight carbon chains will form simple Ag’s.2 The question of why the shorter chains, say three to five carbon chains, will not form smaller Ag’s readily is not an easy one to answer. Now we see the gradual transitions of the horizontal lines to the slanting lines in Figure 1, for C-3 and C-5, and in Figure 2 for AL-3, AL-5, and AL-7. They seem to speak in favor of the “common-sense” process depicted in Scheme 1. The reason why previously these “gradual transition” curves had not been carefully observed could be that previous workers had not cared to work in the “uninteresting” higher concentration regions ([Agr]i > 1 × 10-4 M). The fact that aggregators possessing a carbon chain with less than seven carbon atoms also have some aggregating tendency in solvents with SAgP can be further demonstrated by comparing the fluorescence behaviors of Fl-C-m and Fl-AL-m at 25 °C in both the Φ ) 0.20 and the Φ ) 0.15 DX-H2O systems. These compounds have fluorescence λmax at 337 nm (naphthalene ring) and 400 nm (excimer). The latter was used to monitor excimer formation, which gives direct evidence to aggregate formation.7,9 It was found that in the Φ ) 0.20 DX-H2O system the excimer peak of Fl-C-m (m ) 7, 9, 11, and 13) appears at 400 nm if their concentrations are beyond a certain value, i.e., CAgC. Similarly, excimer formation was also observed from compounds Fl-AL-m (m ) 7, 9, (12) Jiang, X. K.; Zhang, J. T. Aggregation and Self-Coiling Behavior of Organic Molecules; Shanghai Press of Science and Technology, Shanghai, 1996. (13) Adamson, A. W. A Textbook of Physical Chemistry, 2nd ed.; Academic Press Inc.: New York, 1979; p 612. (14) Zhang, J. T.; Nie, J.; Sun, S. X.; Ji, G. Z.; Jiang, X. K. Chin. J. Chem. 1994, 12, 179-182.

Langmuir, Vol. 13, No. 14, 1997 3607

11, and 13) (see Figures 3 and 4). However, excimer formation at 400 nm was not observed from either Fl-AL-3 and Fl-AL-5 or Fl-C-3 and Fl-C-5. Obviously, this could be a consequence of the fact that the SAgP was not high enough in the Φ ) 0.20 DX-H2O system. Thus, by reducing the Φ value to Φ e 0.15, excimer formation at 400 nm from either Fl-AL-3 and Fl-AL-5 or Fl-C-3 and Fl-C-5 can then be observed. This is illustrated by the fluorescence spectra of Fl-AL-3 in water, as shown in Figure 5, in which the 400 nm fluorescence was clearly observed at higher concentrations of Fl-AL-3 (see curves d and e). All the above-mentioned results convincingly support our conclusion that short-chain compounds may also possess an aggregating tendency, even though this tendency is much smaller than those for compounds with longer chains. Conclusion Our studies of aggregation behaviors of esters with the alternate CF2CH2 chain and the hydrocarbon chain indicate that, in solvents with sufficient SAgP, the building-up process for Ag-formation is gradual and stepwise. The aggregating tendency of the alternate chain is smaller than that of the hydrocarbon chain, most likely a consequence of the presence of minute polarity in the alternate chain. Experimental Section Apparatus. Melting and boiling points were not corrected. Mass spectra were obtained by using a HP 5989A spectrometer at an ionization potential of 70 eV. 1H NMR spectra were taken on a Varian EM 360L or EM 390L spectrometer with TMS as the internal standard. 19F NMR spectra were obtained by using a Varian EM 360L spectrometer with CF3COOH as the external standard. Infrared spectra were recorded by using a Shimadzu IR 440 spectrometer. Telomerization of Vinylidene Fluoride with Carbon Tetrachloride as the Telogen.15 Into a 2000 mL stainless steel autoclave equipped with an electronic magnetic stirrer and containing 600 mL (6.24 mol) of carbon tetrachloride with 2.0 g (0.0083 mol) of benzoyl peroxide and cooled to -75 °C in liquid nitrogen, 200 g (3.13 mol) of vinylidene fluoride was condensed in vacuo. After the mixture was warmed to room temperature, the autoclave was heated at a temperature of 100 °C with stirring for 6 h. The pressure dropped from 28 to 18 kg/cm2 during this time interval. After the unreacted vinylidene fluoride was vented, the remaining liquid reaction products were washed with 100 mL of 0.1 M aqueous NaHCO3 and 0.1 M NaCl, and dried with MgSO4. The above-mentioned procedure for telomerization was repeated several times, and the liquid products thus obtained were combined. By means of fractional distillation, the following telomers with chain lengths from 3 to 13 carbons were separated. ClCF2CH2CCl3. Colorless liquid, bp 46 °C/4 Torr (lit.15 133.5 °C/760 Torr). 1H NMR (CCl4) δ 3.80 (t, 2H, J ) 12Hz). 19F NMR (CF3COOH) δ -25.7 (2F). Cl(CF2CH2)2CCl3. Colorless liquid, bp 56 °C/3 Torr (lit.15 42 °C/2 Torr). 1H NMR (CCl4) δ 3.62-2.90 (m, 4H, J ) 12Hz). 19F NMR (CF3COOH) δ -27.7 (2F), 12.7 (2F). Cl(CF2CH2)3CCl3. Colorless liquid, bp 72-74 °C/2 Torr (lit.15 68 °C/1-4 Torr). 1H NMR (CCl4) δ 3.60-2.49 (m, 6H, J ) 12Hz). 19F NMR (CF COOH) δ -27.2 (2F), 10.6 (2F), 11.9 (2F). 3 Cl(CF2CH2)4CCl3. White crystals, bp 74 °C/0.4 Torr (lit.15 102 °C/2 Torr). 1H NMR (CCl4) δ 3.58-2.42 (m, 8H, J ) 12Hz). 19F NMR (CF COOH) δ -27.1 (2F), 10.3(2F), 11.6 (4F). 3 Cl(CF2CH2)5CCl3. White waxy solid, bp 104-110 °C/0.3 Torr (lit.15 112-115 °C/0.5-0.7 Torr). 1H NMR (CCl4) δ 3.61-2.42 (m, 10H, J ) 12Hz). 19F NMR (CF3COOH) δ -26.9 (2F), 10.4 (2F), 11.6 (6F). Cl(CF2CH2)6CCl3. White waxy solid, bp 133-141 °C/0.5 Torr (lit.15 132-136 °C/0.7 Torr). 1H NMR (CCl4) δ 3.62-2.40 (m, 12H, J ) 12Hz). 19F NMR (CF3COOH) δ -26.9 (2F), 10.6 (2F), 11.6 (8F). (15) Chen, Q. Y.; Ma, Z. Z.; Jiang, X. K.; Zhang, Y. F.; Jia, S. M. Acta Chim. Sin. 1980, 38, 175-183.

3608 Langmuir, Vol. 13, No. 14, 1997 Preparation of [Cl(CF2CH2)(m-1)/2COOH] from the Telomers [Cl(CF2CH2)(m-1)/2CCl3]. (1) Hydrolysis of ClCF2CH2CCl3.15 In a 250 mL flask equipped with a magnetic stirrer and a reflux condenser, 30 g (0.14 mol) of ClCF2CH2CCl3, 40 mL of sulfuric acid, and 7 mL of 20% fuming sulfuric acid were added. After the reaction mixtures were stirred for 48 h at 90 °C, 5 mL of 50% fuming sulfuric acid was added, and stirring was continued for another 48 h. Then the reaction product was allowed to cool and was poured into 300 mL of ice-water, extracted with ether, and dried with Na2SO4. By fractional distillation, 8 g (yield 40%) of ClCF2CH2COOH with a boiling point of 50 °C/3 Torr was obtained. ClCF2CH2COOH. Colorless liquid, bp 50 °C/3 Torr (lit.15 71 °C/9 Torr). 1H NMR (CDCl3) δ 3.30 (t, 2H, J ) 15Hz). 19F NMR (CF3COOH) δ -27.0 (2F). IR (KBr) 2920-3200, 1720, 1340 cm-1. Anal. Calcd for C3H3ClF2O2: C, 24.91; H, 2.08. Found: C, 24.57; H, 2.18. (2) Hydrolysis of Cl(CF2CH2)2CCl3. In a 250 mL flask equipped with a magnetic stirrer and a reflux condenser, 20 g (0.07 mol) of Cl(CF2CH2)2CCl3 and 40 mL of fuming nitric acid were added. The reaction mixtures were stirred for 36 h at 100 °C and then poured into 200 mL of ice-water. The reaction product was extracted with ether and dried with Na2SO4. By fractional distillation, 9 g (yield 61%) of Cl(CF2CH2)2COOH with a boiling point of 60 °C/0.3 Torr was obtained. Cl(CF2CH2)2COOH. Colorless liquid, bp 60 °C/0.3 Torr (lit.15 81 °C/4 Torr). 1H NMR (CDCl3) δ 10.10 (s, 1H), 3.60-3.03 (m, 4H, J ) 15Hz). 19F NMR (CF3COOH) δ -26.6 (2F), 11.2 (2F). IR (KBr) 2920-3200, 1730, 1380 cm-1. Anal. Calcd for C5H5ClF4O2: C, 28.85; H, 2.42. Found: C, 28.22; H, 1.89. (3) Hydrolysis of Cl(CF2CH2)(m-1)/2CCl3 (m ) 7, 9, 11, 13).15 Typical run: In a 100 mL flask equipped with a magnetic stirrer and a reflux condenser, 20 g (0.05 mol) of Cl(CF2CH2)4CCl3 and 40 mL of fuming nitric acid were added. The reaction mixtures were stirred for 8 h at 100 °C till the solution became homogeneous; then they were poured into 200 mL of ice-water, filtered, and recrystallized from trichloromethane to give 14.5 g (yield 88%) of Cl(CF2CH2)4COOH. The other acid, Cl(CF2CH2)3COOH, was obtained with the same yield and was recrystallized from trichloromethane; Cl(CF2CH2)5COOH and Cl(CF2CH2)6COOH were obtained in yields of 90% and 87% and were recrystallized from trichloromethane-methanol. Cl(CF2CH2)3COOH. White crystals, mp 68-70 °C (lit.15 7072 °C). 1H NMR (CDCl3) δ 3.3-2.8 (m, 6H, J ) 15Hz). 19F NMR (CF3COOH) δ -26.8 (2F), 10.6 (2F), 13.6 (2F). IR (KBr) 29203200, 1740, 1340, 1240, 700 cm-1. Anal. Calcd for C7H7ClF6O2: C, 30.83; H, 2.57. Found: C, 30.54; H, 2.50. Cl(CF2CH2)4COOH. White crystals, mp 93-94 °C (lit.15 9799 °C). 1H NMR (CDCl3) δ 3.3-2.6 (m, 8H, J ) 15Hz). 19F NMR (CF3COOH) δ -26.9 (2F), 10.6 (2F), 12.8 (4F). IR (KBr) 29403200, 1699, 1374, 1252 cm-1. Anal. Calcd for C9H9ClF8O2: C, 32.10; H, 2.67. Found: C, 32.29; H, 2.65. Cl(CF2CH2)5COOH. White powder, mp 110-111 °C (lit.15 116-117 °C). 1H NMR (acetone-d6) δ 3.6-2.8 (m, 10H, J ) 15Hz). 19F NMR (CF COOH) δ -26.6 (2F), 13.1 (2F), 14.7 (4F), 16.6 3 (2F). IR (KBr) 2980-3200, 1737, 1344, 1195 cm-1. Anal. Calcd for C11H11ClF10O2: C, 32.96; H, 2.75. Found: C, 33.16; H, 2.69. Cl(CF2CH2)6COOH. White powder, mp 120-122 °C (lit.15 122-124 °C). 1H NMR (acetone-d6) δ 3.3-2.7 (m, 12H, J ) 15Hz). 19F NMR (CF3COOH) δ -25.9 (2F), 13.1 (2F), 14.6 (6F), 16.6 (2F). IR (KBr) 2980-3300, 1742, 1337, 1167 cm-1. Anal. Calcd for C13H13ClF12O2: C, 33.58; H, 2.80. Found: C, 33.22; H, 2.67. Preparation of Carboxylic Esters Possessing an Alternate Chain. The carboxylic acids with the alternate chain were converted to the corresponding acid chlorides by reaction with thionyl chloride and subsequently to the 4-nitrophenyl esters by reaction with 4-nitrophenol (for compounds AL-m, m ) 7, 9, 11, and 13). The 2-(R-naphthyl)ethyl esters were obtained by reaction of the corresponding acid chlorides with 2-(R-naphthyl)ethanol in the presence of magnesium powder in anhydrous benzene at 80 °C (for compounds Fl-AL-m, m ) 7, 9, 11, and 13). Compounds AL-3 and AL-5 were obtained by reaction of the corresponding acids with 4-nitrophenol and dicyclohexylcarbodiimide (DCC) in absolute ether. Compounds FL-AL-3 and FL-AL-5 were obtained by reaction of the corresponding acids with 2-(R-naphthyl)ethanol in the presence of concentrated

Zhu et al. sulfuric acid in benzene at reflux temperature. The hydrocarbon carboxylic esters, i.e., C-m and Fl-C-m, were synthesized by similar procedures. These esters were purified on a silica gel column with petroleum-dichloromethane as eluent, and their purity was further established by elemental analysis. Compounds AL-m, Fl-AL-m, and Fl-C-m are new. Their physical and analytical data are reported below. AL-3 (p-nitrophenyl 3-chloro-3,3-difluoropropanoate). White crystals, mp 42-44 °C. 1H NMR (CDCl3) δ 8.2-7.2 (dd, AA′BB′, 4H), 3.6 (t, 2H). 19F NMR (CF3COOH) δ -26.6 (2F). MS m/z 266 (MH+), 184, 139, 127, 109, 99, 91, 63. Anal. Calcd for C9H6ClF2NO4: C, 40.68; H, 2.26; N, 5.27. Found: C, 40.60; H, 2.13; N, 5.25. AL-5 (p-nitrophenyl 5-chloro-3,3,5,5-tetrafluorovaleroate). White crystals, mp 34-36 °C. 1H NMR (CDCl3) δ 8.2-7.3 (dd, AA′BB′, 4H), 3.4 (m, 4H). 19F NMR (CF3COOH) δ -26.4 (2F), 9.5 (2F). MS m/z 330 (MH+), 274, 232, 171, 139, 129, 109, 91, 63. Anal. Calcd for C11H8ClF4NO4: C, 40.06; H, 2.43; N, 4.25. Found: C, 39.98; H, 2.28; N, 4.21. AL-7 (p-nitrophenyl 7-chloro-3,3,5,5,7,7-hexafluoroheptanoate). White crystals, mp 38-40 °C. 1H NMR (CDCl3) δ 8.4-7.3 (dd, AA′BB′, 4H), 3.5-2.3 (m, 6H). 19F NMR (CF3COOH) δ -29.5 (2F), 9.5 (2F), 14.0 (2F). IR (KBr) 1759, 1540, 1350, 1258, 1200 cm-1. MS m/z 394 (MH+), 318, 235, 193, 139, 109, 91, 64. Anal. Calcd for C13H10ClF6NO4: C, 39.64; H, 2.54; N, 3.56. Found: C, 39.59; H, 2.46; N, 3.29. AL-9 (p-nitrophenyl 9-chloro-3,3,5,5,7,7,9,9-octafluorononanoate). White crystals, mp 61-62 °C. 1H NMR (CDCl3) δ 8.4-7.3 (dd, AA′BB′, 4H), 3.5-2.3 (m, 8H). 19F NMR (CF3COOH) δ -28.7 (2F), 10.7 (2F), 13.3 (4F). IR (KBr) 1761, 1540, 1350, 1224, 1210 cm-1. MS m/z 458 (MH+), 360, 299, 279, 257, 237, 139, 64. Anal. Calcd for C15H12ClF8NO4: C, 39.34; H, 2.62; N, 3.06. Found: C, 39.19; H, 2.23; N, 2.89. AL-11 (p-nitrophenyl 11-chloro-3,3,5,5,7,7,9,9,11,11-decafluoroundecanoate). White crystals, mp 82-83 °C. 1H NMR (CDCl3) δ 8.3-7.3 (dd, AA′BB′, 4H), 3.5-2.6 (m, 10H). 19F NMR (CF3COOH) δ -26.9 (2F), 10.0 (2F), 12.2 (6F). IR (KBr) 1761, 1530, 1410, 1350, 1266 cm-1. MS m/z 522 (MH+), 446, 363, 343, 141, 64. Anal. Calcd for C17H14ClF10NO4: C, 39.12; H, 2.68; N, 2.68. Found: C, 39.11; H, 2.68; N, 2.56. AL-13 (p-nitrophenyl 13-chloro-3,3,5,5,7,7,9,9,11,11,13,13dodecafluorotridecanoate). White crystals, mp 87-88 °C. 1H NMR (CDCl3) δ 8.3-7.3 (dd, AA′BB′, 4H), 3.5-2.6 (m, 12H). 19F NMR (CF3COOH) δ -26.8 (2F), 10.0(2F), 12.0(8F). IR (KBr) 1761, 1530, 1410, 1350, 1268 cm-1. MS m/z 586 (MH+), 488, 427, 407, 387, 199, 139, 91, 64. Anal. Calcd for C19H16ClF12NO4: C, 38.94; H, 2.73; N, 2.39. Found: C, 39.60; H, 2.75; N, 2.30. Fl-AL-3 [2-(R-naphthyl)ethyl 3-chloro-3,3-difluoropropanoate]. Colorless liquid. 1H NMR (CDCl3) δ 8.0-7.2 (m, 7H), 4.4 (t, 2H), 3.4-3.1 (t, 4H). 19F NMR (CF3COOH) δ -26.2 (2F). IR (KBr) 1770, 1200, 800. MS m/z 298 (M+), 154, 141, 115, 64 cm-1. Anal. Calcd for C15H13ClF2O2: C, 60.30; H, 4.36. Found: C, 60.51; H, 4.41. Fl-AL-5 [2-(R-naphthyl)ethyl 5-chloro-3,3,5,5-tetrafluorovaleroate]. White crystals, mp 56-57 °C. 1H NMR (CDCl3) δ 8.17.3 (m, 7H), 4.4 (t, 2H), 3.4-2.9 (m, 6H). 19F NMR (CF3COOH) δ -28.2 (2F), 10.6 (2F). IR (KBr) 1740, 1200, 800 cm-1. MS m/z 362 (M+), 154, 141, 115, 64. Anal. Calcd for C17H15ClF4O2: C, 56.28; H, 4.14. Found: C, 56.36; H, 3.87. Fl-AL-7 [2-(R-naphthyl)ethyl 7-chloro-3,3,5,5,7,7-hexafluoroheptanoate]. White crystals, mp 64-65 °C. 1H NMR (CDCl3) δ 8.2-7.3 (m, 7H), 4.5 (t, 2H), 3.5 (t, 2H), 3.3-2.8 (m, 6H). 19F NMR (CF3COOH) δ -27.5 (2F), 10.3 (2F), 13.3 (2F). IR (KBr) 1730, 1200, 780 cm-1. MS m/z 426 (M+), 235, 193, 154, 141, 115, 64. Anal. Calcd for C19H17ClF6O2: C, 53.45; H, 3.99. Found: C, 53.53; H, 3.96. Fl-AL-9 [2-(R-naphthyl)ethyl 9-chloro-3,3,5,5,7,7,9,9-octafluorononanoate]. White crystals, mp 91-93 °C. 1H NMR (CDCl3) δ 8.3-7.3 (m, 7H), 4.6 (t, 2H), 3.7-2.6 (m, 10H). 19F NMR (CF3COOH) δ -27.4 (2F), 10.3 (2F), 11.8 (4F). IR (KBr) 1730, 1200, 780 cm-1. MS m/z 490 (M+), 299, 154, 141, 115. Anal. Calcd for C21H19ClF8O2: C, 50.96; H, 3.87. Found: C, 51.03; H, 3.67. Fl-AL-11 [2-(R-naphthyl)ethyl 11-chloro-3,3,5,5,7,7, 9,9,11,11-decafluoroundecanoate]. White crystals, mp 108-109 °C. 1H NMR (CDCl3) δ 8.3-7.3 (m, 7H), 4.6 (t, 2H), 3.7-2.6 (m, 12H). 19F NMR (CF COOH) δ -26.9 (2F), 10.5 (2F), 12.5 (6F). IR (KBr) 3

Nature of the Alternate CF2CH2 Chain 1720, 1200, 760 cm-1. MS m/z 554 (M+), 499, 154, 141, 115, 64. Anal. Calcd for C23H21ClF10O2: C, 49.78; H, 3.79. Found: C, 49.60; H, 3.76. Fl-AL-13 [2-(naphthyl)ethyl 13-chloro-3,3,5,5,7,7,9,9,11,11,13,13-dodecafluorotridecanoate]. White crystals, mp 118-119 °C. 1H NMR (CDCl3) δ 8.1-7.2 (m, 7H), 4.5 (t, 2H), 3.4 (t, 2H), 3.2-2.5 (m, 12H). 19F NMR (CF3COOH) δ -27.6 (2F), 10.2 (2F), 11.4 (8F). IR (KBr) 1740, 1200, 780 cm-1. MS m/z 618 (M+), 563, 407, 154, 141, 115, 64. Anal. Calcd for C25H23ClF12O2‚H2O: C, 47.13; H, 3.93. Found: C, 46.96; H, 3.41. Fl-C-3 [2-(R-naphthyl)ethyl propanoate]. Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 8.4-7.6 (m, 7H), 4.8-4.6 (t, 2H), 3.8-3.6 (t, 2H), 2.9-2.5 (q, 2H), 1.5-1.3 (t, 3H). MS m/z 228 (M+), 154, 141, 115, 57. Anal. Calcd for C15H16O2: C, 78.92; H, 7.06. Found: C, 79.06; H, 7.22. Fl-C-5 [2-(R-naphthyl)ethyl valeroate]. Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 8.4-7.6 (m, 7H), 4.8-4.6 (t, 2H), 3.8-3.6 (t, 2H), 2.7-2.5 (t, 2H), 2.0-1.4 (m, 4H), 1.3-1.1 (t, 3H). MS m/z 256 (M+), 154, 141, 115, 85, 57. Anal. Calcd for C17H20O2: C, 79.65; H, 7.86. Found: C, 79.62; H, 8.01. Fl-C-7 [2-(R-naphthyl)ethyl heptanoate]. Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 8.4-7.6 (m, 7H), 4.8-4.6 (t, 2H), 3.8-3.6 (t, 2H), 2.6-2.4 (t, 2H), 2.1-1.7 (m, 2H), 2.5 (m, 6H), 1.13 (t, 3H). MS m/z 284 (M+), 154, 141, 115, 57, 43. Anal. Calcd for C19H24O2: C, 80.24; H, 8.51. Found: C, 80.07; H, 8.52. Fl-C-9 [2-(R-naphthyl)ethyl nonanoate]. Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 8.4-7.6 (m, 7H), 4.8-4.6 (t, 2H), 3.8-3.6 (t, 2H), 2.6-2.4 (t, 2H), 2.0-1.7 (m, 2H), 1.5 (m, 10H), 1.2-1.0 (t, 3H). MS m/z 312 (M+), 154, 141, 115, 57, 43. Anal. Calcd for C21H28O2: C, 80.73; H, 9.03. Found: C, 80.56; H, 9.04. Fl-C-11 [2-(R-naphthyl)ethyl undecanoate]. Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 8.4-7.6 (m, 7H), 4.8-4.6 (t, 2H), 3.8-3.6 (t, 2H), 2.6-2.4 (t, 2H), 2.0-1.7 (m, 2H), 1.5 (m, 14H), 1.2-1.0 (t, 3H). MS m/z 340 (M+), 309, 269, 154, 141, 115, 57, 43. Anal. Calcd for C23H32O2: C, 81.13; H, 9.47. Found: C, 81.21; H, 9.47. Fl-C-13 [2-(R-naphthyl)ethyl tridecanoate]. Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 8.4-7.6 (m, 7H), 4.8-4.6 (t, 2H), 3.8-3.6 (t, 2H), 2.7-2.5 (t, 2H), 2.0-1.8 (m, 2H), 1.6 (m, 18H), 1.3-1.1 (t, 3H). MS m/z 368 (M+), 309, 269, 154, 141, 115, 57, 43. Anal. Calcd for C25H36O2: C, 81.47; H, 9.84. Found: C, 81.35; H, 10.08. The purity of all known compounds, i.e., C-m,16-19 prepared in this laboratory had also been established by elemental analysis. The physical and spectral data obtained in this laboratory for these compounds are summarized below. C-3 (p-nitrophenyl propanoate). White crystals, mp 58-60 °C (lit.16 62-64 °C). 1H NMR (CDCl3) δ 7.8-6.8 (dd, AA′BB′, 4H), 2.6-2.3 (t, 2H), 1.3-1.1 (t, 3H). IR (KBr) 1750, 1530, 1350, 1200 cm-1. MS m/z 196 (MH+), 168, 140, 123, 57. C-5 (p-nitrophenyl valeroate).16 Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 7.9-6.8 (dd, AA′BB′, 4H), 2.6-2.4 (t, 2H), 1.9-1.2 (m, 4H), 1.0-0.8 (t, 3H). IR (KBr) 1767, 1530, 1350, 1210 cm-1. MS m/z 224 (MH+), 168, 140, 123, 85, 57. (16) Detar, D. F.; Delahunty, C. J. Am. Chem. Soc. 1983, 105, 27342739. (17) Ueno, A.; Suzuki, I.; Hino, Y.; Suzuki, A.; Osa, T. Chem. Lett. 1985, 159-162. (18) Okahata, Y.; Ando, R.; Kunitake, T. Bull. Chem. Soc. Jpn. 1979, 52, 3647-3653. (19) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q.; Wang, S. J. Acta Chim. Sin. 1985, 43, 983-987.

Langmuir, Vol. 13, No. 14, 1997 3609 C-7 (p-nitrophenyl heptanoate).17 Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 7.9-6.8 (dd, AA′BB′, 4H), 2.72.4 (t, 2H), 1.8-1.1 (m, 8H), 0.85 (t, 3H). IR (KBr) 1770, 1530, 1350, 1210 cm-1. MS m/z 252 (MH+), 180, 140, 123, 113, 85, 57, 43. C-9 (p-nitrophenyl nonanoate).18 Colorless or slightly yellowish liquid. 1H NMR (CDCl3) δ 7.9-6.8 (dd, AA′BB′, 4H), 2.6-2.4 (t, 2H), 1.8-1.1 (m, 12H), 0.85 (t, 3H). IR (KBr) 1768, 1525, 1350, 1210 cm-1. MS m/z 280 (MH+), 180, 141, 123, 71, 57, 43. C-11 (p-nitrophenyl undecanoate). White crystals, mp 31-33 °C (lit.19 32-33 °C). 1H NMR (CDCl3) δ 7.9-6.8 (dd, AA′BB′, 4H), 2.6-2.4 (t, 2H), 1.8-1.1 (m, 16H), 1.0-0.8 (t, 3H). IR (KBr) 1759, 1530, 1350, 1220 cm-1. MS m/z 308 (MH+), 169, 95, 85, 71, 57, 43. C-13 (p-nitrophenyl tridecanoate). White crystals, mp 43-44 °C (lit.19 43-44 °C). 1H NMR (CDCl3) δ 7.9-6.8 (dd, AA′BB′, 4H), 2.6-2.3 (t, 2H), 1.6-1.1 (m, 20H), 1.0-0.8 (t, 3H). IR (KBr) 1758, 1525, 1350, 1220 cm-1. MS m/z 336 (MH+), 197, 169, 123, 95, 85, 71, 57, 43. Kinetics. Kinetic measurements were performed on a PerkinElmer 559 recording spectrophotometer equipped with a thermostated cell holder. A. R. grade 1,4-dioxane was refluxed with sodium wire for 10 h and distilled. Water was twice distilled. Sodium carbonate, sodium bicarbonate and sodium chloride used to prepare the buffer solution were A. R. grade. An aqueous solution of Na2CO3 (0.02 M), NaHCO3 (0.067 M), and NaCl (0.34 M) with a pH of 10.67 at 15 °C was used as the buffer solution; it was mixed with dioxane in a 4:1 volume proportion (Φ ) 0.2). The pH value of the Φ ) 0.2 solution was 11.31 at 15 °C. A 1.0 cm cell was filled with 3.00 mL of the solution and thermally equilibrated for 10 min. All kinetic experiments were performed at 35.0 ( 0.2 °C. A dioxane solution of the substrate (e30 µL) was injected into the cell with a microsyringe. The increase of absorbance of p-nitrophenoxide ion at 405 nm was then traced as a function of time. Pseudo-first-order rate constants (kob) were obtained in the normal manner in all cases. For esters with m ) 3 and m ) 5, the absorbance at 440 nm was traced. The ester AL-3 contains an alternate chain with a chain length of only three carbons; it hydrolyzed much faster than the other esters, and there were negative deviations of kob after 70% conversion of this ester (only within 1 min). Therefore, the pseudo-firstorder rate constants of AL-3 were obtained by measuring the average rate constants within 10 s after it was injected into the solution. All of the rate constants are accurate to within (5%; for compounds C-3 and AL-3, they are accurate to within (10%. Fluorescence Measurement. Fluorescence spectra of Fl-C-m and Fl-AL-m were measured on a Perkin-Elmer LS-50 Luminescence Spectrometer, by using the excitation wavelength of 285 nm. All fluorescence measurements were performed at 25 °C. A 1.0 cm cell was filled with 2.50 mL of the Φ ) 0.2 DX-H2O solution. A dioxane solution of the fluorescence probe was repeatedly injected into the cell with a microsyringe until the solution became turbid. The fluorescence intensities of monomers (Im, λmax ) 336 nm) and excimers (Ie, λmax ) 400 nm) of the solutions were recorded after each injection. The total volume that was injected into the cell was less than 25 µL. From the plot of log(Ie/Im) vs probe concentration, the CAgC value can be obtained. Fluorescence measurements are accurate to within (10%.

Acknowledgment. We are grateful to the National Natural Science Foundation of China for financial support. LA962129L