Synthesis, Characterization, and First Application of High Molecular

J. Phys. Chem. B 2000, 104, 2215-2223

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ARTICLES Synthesis, Characterization, and First Application of High Molecular Weight Polyacrylic Acid Derivatives Possessing Perfluorinated Side Chains and Chemically Linked Pyrene Labels Megh Raj Pokhrel and Stefan H. Bossmann* Lehrstuhl fu¨ r Umweltmesstechnik, Engler-Bunte-Institut, UniVersita¨ t Karlsruhe, D-76128 Karlsruhe, Germany ReceiVed: May 25, 1999; In Final Form: December 15, 1999

The authors describe a new and unique nanosensing system: a derivative of poly(acrylic acid) (PAA, Mw ) 1.250.000) possessing a comb-like structure. This structure is achieved by attaching perfluorinated (-OC8F17) functions and chemically linked pyrene fluorescent units to the PAA backbone. The average composition of the new materials was investigated by UV/vis absorption spectroscopy, DOC, and elementary (CHN) analysis. Gel permeation chromatography (GPC) led to the conclusion that the new polymeric materials are almost monodisperse. The change of pH from the basic to the acidic range causes a collapse of the macromolecules in an aqueous environment, and consequently the formation of preformed excimers is suppressed. These novel materials are designed for use in sensor heads for the photochemical detection of the pH and CO2 content of water solutions. Using a model experiment, we show how these characteristics can be used for the measurement principles.

Introduction In recent years, the synthesis and characterization of monodisperse polymer nanoparticles gained a profound interest,1 and today it is widely known that many nanoparticles possess very interesting physical and chemical properties,2 permitting applications which cannot be achieved by using bulk materials. Examples of future applications of functionalized polymer nanoparticles are the generation of long-term stable and highly sensitive optical sensors3 and catalysts for the industrial synthesis of fine organic chemicals,4 as well as polymer-derived AOPcatalysts.5 Furthermore, the upscaling of chemical processes occurring within nanodomains can be achieved using suitable mathematical models such as neural networks.6 There is a great demand for monodisperse polymers that possess tailor-made properties and sufficient long-term stabilities against decomposition reactions.7 Their industrial production must be inexpensive in order to gain a significant advantage in comparison to classical synthesis processes. A special emphasis has been put on the synthesis of inexpensive and durable materials which can be employed for the construction of versatile sensors. This can be done using polymers which contain partially fluorinated features. There are only a few examples of polymers possessing fluorinated side chains known from the literature.8 Jiang and co-workers made important synthetic and mechanistic contributions to the field of fluorocarbon-modified, water-soluble polymers.9,10 Their polymer materials, synthesized by several copolymerization techniques, are based either of poly-Nisopropyl-acrylamide (PNIPAM)9 or poly(acrylic acid) (PAA).10 * To whom correspondence should be addressed: Priv.-Doz. Dr. Stefan H. Bossmann, Institute of Environmental Analysis Technology at the EnglerBunte-Institute of the University of Karlsruhe, 76128 Karlsruhe, Germany. E-mail: [email protected].

However, their polymers feature only relatively small amounts of C8 fluorocarbon chains, ranging from 0.40 to 1.37 mol percent. In our work, the following synthetic paradigm was applied: Monodisperse PAA was employed as starting material. This macromolecule acts as a polyelectrolyte at high pH. If an acidic medium is chosen, the charge of the carboxylic acid functions will be neutralized and residual charges cannot be observed. The PAA macromolecule employed in this study showed a pKs value of 4.55.11 Fluorinated -C8F17 side chains have been selected to introduce a hydrophobic structure element into the PAA-based macromolecule. It was of special importance to our work to optimize the relative concentration of -C8F17 side chains in order to maximize the impact of the hydrophobic interactions on the polymer dynamics. It is the goal of our synthesis strategy to achieve a fluorine content of more than 50 wt %. The strong hydrophobic influence of the -C8F17 labels should lead to the folding of individual macromolecular chains at a low density of carboxylate functions (below pH < 4.55). At a pH higher than the pKs, the macromolecule should unfold to some extent, depending upon the labeling degree. The detection of these changes as a function of the environmental impact can be achieved using chemically attached fluorescence labels.12 The influence of the hydrophobic centers on the conformation and dynamics of the macromolecules can be elucidated with high precision.13-15 The application of the well established pyrene-fluorophor permits the detection of the polymer dynamics by means of the separate monomer and excimer emission measurement: whereas monomer emission arises from isolated pyrene units in the photochemically excited singlet state, excimer emission occurs from an excited dimer, formed by an excited and a ground-state pyrene molecule.16 These studies can be

10.1021/jp9917190 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000

2216 J. Phys. Chem. B, Vol. 104, No. 10, 2000 performed in a steady state and time-resolved manner. The comparison of both methods permits the elucidation of the photophysical mechanisms occurring within the macromolecule. Compared to dissolved pyrene labels, which have been employed for the structure elucidation of fluorocarbon-modified polymers,10 chemically attached fluorophores possess one intrinsic advantage: the chemical linker groups prevent any diffusion of the molecular probes along the polymer strands and nano- or microstructures which might have been formed. Therefore, the molecular probes cannot “escape”, and changes in the polymer structure can be detected with higher precision. In this report we describe an optimized synthetic method and the characterization by GPC-, DOC-, and CHN-analysis. Furthermore, an example of the application of these novel materials is presented here with the development of inexpensive, enduring, and reliable analytical systems for environmentally important measurements.17 Experimental Section DOC Measurements. DOC (dissolved organic carbon) measurements were performed using a total organic carbon analyzer (Dohrmann DC-190, Rosemount Analytical). Its operating components consist of an Al2O3/Pt catalyst bed (constant O2 flow of 5 mL min-1, 680 °C) and a nondispersive CO2-IR detector. Calibration of the carbon content in aqueous solution was achieved using the monopotassium salt of phthalic acid, PAA, and 1-bromo-perfluoro-octane. Elementary Analysis. CHN analysis has been performed on a C, H, N, S analyzer (LECO Instruments) in collaboration with the Department of Organic Chemistry of the University of Saarbru¨cken, Germany. Viscosity Measurements. A Haake VT5 viscometer was employed for standard measurements of the viscosity of aqueous polymer solutions. Gel Permeation Chromatography. The gel permeation chromatography (GPC) experiments were carried out employing an HP TSK-AC/4000 SW; 7.5 × 300 mm column. An aqueous solution of Na2SO4 (0.10 M-1) and Na2HPO4 (0.10 M-1) was used as eluent. The pH of 7.0 was adjusted using diluted H3PO4. The polymers were detected at a wavelength of 220 nm. Fluorescence Spectroscopy. Steady-state and time-resolved fluorescence measurements were performed using an Edinburgh Analytical Instruments (EAI-FS/FL900) single photon counting device working in the lifetime range of 500 ps to 500 µs. Our method of data analysis of the steady state and time-resolved fluorescence data was previously described in detail.18,19 The decay traces were analyzed by use of a computer program generously provided by Prof. F. C. De Schryver of the University of Leuven, Belgium, and the software provided by Edinburgh Analytical Instruments. The results from both computer programs deviated in no case by more than 5 relative percent with respect to the calculated fluorescence lifetimes and relative contributions of both components to the observed decays. The polymer concentrations in bidestilled H2O were 1.0, 0.1, and 0.01 g L-1. All measurements were performed in aerated solutions and after 24 h of equilibration time. The original pH was 3.0. The pH of the polymer solutions was adjusted to pH ) 3.8, 4.2, 5.0, 5.4, 6.0, and 6.5 using diluted sodium hydroxide solution (1.0 mol L-1). The pH was measured using a Methrohm pH 4000-II instrument. No hysteresis effects were found: the pH was measured immediately after adding diluted NaOH solution, after 1 h, and after 24 h and remained unchanged. During all measurements, the temperature was 298 ( 1 K.

Pokhrel and Bossmann Synthesis and Characterization of Hydrophobically Modified Polyacrylic Acid Macromolecules Possessing Chemically Attached Pyrene Labels. Poly(acrylic acid) of high molecular weight (Mw ) 1,250,000 g, Aldrich) and well-defined size was chosen as starting material. All solvents were purchased from Roth; all inorganic materials were purchased from Fluka. 1-Aminomethyl-pyrene (Aldrich) was attached to the carboxylic acid functions of the macromolecule by the formation of an amide bond (Figure 1). The hydrophobic units were attached by means of an ester bond employing 1-bromoperfluoro-n-octane (Elf-Atochem) (Figure 2). Note that our synthesis strategy permits the separate linkage of the 1-aminomethyl-pyrene and 1-bromo-perfluoro-n-octane units, and therefore, our novel macromolecules can be designed in a flexible manner according to the constraints of their future applications. The formation of an amide bond between poly(acrylic acid) and 1-aminomethyl-pyrene was accomplished using dicyclohexylcarbodiimide (DCC, Aldrich) as agent for the water removal in dimethylformamide (DMF).20 The hydrophobic side chains were introduced by the reaction of poly(acrylic acid) and 1-bromo-perfluoro-octane in DMF. Sodium carbonate (Na2CO3) was added in order to trap the HBr formed during synthesis. The novel polymeric materials possess an excellent long-term stability from pH ) 1 to pH ) 10. The pKs has been determined by classic acid/base titration to be 4.50 to 4.60 for the four PAA derivatives synthesized here. Snythesis Procedures. An amount (0.010 g, 3.73 × 10-5 mol) 1-aminomethyl-pyrene hydrochloride (C17H14NCl, m ) 267.7 g mol-1) was dissolved in 10 mL bidest. H2O. The pH of the solution was adjusted to 10.0 using NaOH solution (0.10 M-1). 1-Aminomethyl-pyrene was extracted employing 20 mL of diethyl ether. An amount (10.0 g) of poly(acrylic acid) (0.138 mol of carboxylic acid units) was suspended in 250 mL anhydrous DMF. An amount (0.10 g (4.85 × 10-4 mol)) of dicyclohexylcarbodiimide (DCC, C13H22N2, m ) 206.3 g mol-1) and the 20 mL of diethyl ether containing 1-aminomethyl-pyrene were added and the mixture, which was constantly purged with nitrogen, and allowed to react for 2 h at 60 °C. Then 34.4 g (6.90 × 10-2 mol), 9.82 g (1.97 × 10-2 mol), 5.29 (1.06 × 10-2 mol), 2.29 g (4.6 × 10-3 mol) 1-bromo-perfluoro-octane (C8F17Br, m ) 498.9 g mol-1) and 5.0 g Na2CO3 were added, and the reaction mixture was allowed to react for 12 h at 60 °C. The resulting polymer solution was filtered warm in order to remove the inorganic salts. After being cooled to 10 °C, 250 mL of diethyl ether was added dropwise and the polymer precipitated slowly. After being stirred for 1 h, the polymer was filtered off and dissolved again in 100 mL of DMF. Then again, 250 mL of diethyl ether was added and the polymer was collected. This procedure was repeated one additional time. The yield of the hydrophobically modified polymer ranged between 7.2 and 9.4 g (68-99%). Characterization of the Novel Macromolecules. 1. Determination of the Pyrene Content. The pyrene content of the novel polymer materials has been determined using UV/vis absorption spectroscopy ((334 nm) ) 42500 [mol cm L-1].21 2. Determination of the Perfluoro-octylester Content. The characterization of the chemically linked perfluoro-octylester groups content was performed by the measurement of the TOC (total organic carbon) of the aqueous polymer solutions, containing 1.00 g dissolved polymer per liter. Since the carbon content of these solutions is a function of the degree of perfluoro-octylester groups formed during synthesis, the com-

PAA Derivatives with Perfluorinated Side Chains

J. Phys. Chem. B, Vol. 104, No. 10, 2000 2217

Figure 1. Labeling of poly(acrylic acid) with 1-aminomethyl-pyrene.

Figure 2. Labeling of poly(acrylic acid) with 1-bromo-perfluoro-n-octane.

parison of the calibration curve (employing different mixtures of polycarboxylic acid and 1-bromo-perfluoro-octane) and the novel polymers permits the exact determination of the labeling degree. The experimental points resulting from the calibration procedure were mathematically interpreted by using a fifthdegree polynomial equation. The parameter n is defined as the quotient of the total number of carboxylic acid groups in the macromolecules divided by the number of perfluoro-octylester functions. Consequently, n ) 1 means that 100% of the carboxylic acid functions would have been transformed into

perfluoro-octylesters. Note that the experimentally observed n did not exceed 2.95. The parameter l, describing the labeling degree with 1-aminomethyl-pyrene fluorophors, was derived in the same manner. The measurements obtained from the novel polymers described here are marked with arrows in Figure 3. The method of elementary analysis served as control experiment for the DOC method for the elucidation of the average decomposition of the novel PAA/pyrene/-C8F17 macromolecules. The labeling degree of poly(acrylic acid) can be derived also from the C/H ratio. Therefore, both methods lead to exactly

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Pokhrel and Bossmann

Figure 3. Characterization of the novel macromolecules by DOC measurements (Dohrmann DC-190).

the same results within the range of experimental error. Table 1 summarizes the results obtained using both methods. Note that the highest amount of fluorine found in PAA-pyrene/151C8F17/2.85 was 51.8%. 3. Viscosity Measurements of PAA/pyrene/-C8F17 Macromolecules DissolVed in Water. A rotor viscometer was used for the determination of the viscosity of PAA/pyrene/-C8F17 macromolecules in water. Solutions of 2.0 g L-1, 1.0 g L-1, and 0.1 g L-1 PAA/pyrene/-C8F17 were slowly dissolved during 24 h of continuous stirring and then analyzed. A nonclassic viscosity behavior was observed22 as the viscosity was strongly dependent on the rotation rate of the rotor viscometer. Typical viscosities at that dissolved concentration were found in the range of 2200 to 1400 mPas. This phenomenon will be studied in detail in our future work.

Figure 4. GPC analysis of PAA-pyrene/n-C8F17/m by gel permeation chromatography (HP TSK-AC/4000 SW; 7.5 × 300).

laboratory led to the formation of highly monodisperse comblike PAA/pyrene/-C8F17 polymers. Note that a precise calibration of the GPC column could not be achieved because suitable reference materials do not exist as of yet. Since the exclusion

Results and Discussion GPC Characterization. The results from GPC are shown in Figure 4. The synthesis procedures developed in our

TABLE 1: Degree of Perfluoro-octylester- (n) and 1-aminomethyl-pyrene Labeling (l) and Molecular Weight (Mw) of the Four Newly Synthesized Hydrophobically Labeled Water Soluble Polymersa ncalc

nfound based on [ppm C]

nfound based on [% C]

nfound based on [% H]

navg

yield (%)

2 7 13 30

2.95 [(0.10] 7.90 [(0.15] 12.5 [(0.50] 30.2 [(1.0 ]

2.89 [(0.15] 7.86 [(0.25] 12.25 [(0.30] 30.35 [(0.50]

2.78 [(0.15] 8.02 [(0.25] 12.70 [(0.30] 30.85 [(0.50]

2.85 [(0.15] 7.95 [(0.25] 12.5 [(0.50] 30.5 [(0.50]

68 [(3] 88 [(3] 99 [(2] 98 [( 2]

ncalc.

lfound based on [ppm C]

Mw based on [% C]

Mn/Mw based on [% H]

PAA-pyrene/l-C8F17/n

2 7 13 30 no C8F17-labeling

151 [(15] 271 [(15] 313 [(15] 469 [(15] 798 [(35]

3,800,500 [(25,000] 2,164,316 [(21,000] 1,831,505 [(22,000] 1,488,321 [(15,000] 1,250,000 [(12,000]

1.14 1.125 1.12 1.12 1.05

PAA-pyrene/151-C8F17/2.85 PAA-pyrene/271-C8F17/7.95 PAA-pyrene/313-C8F17/12.5 PAA-pyrene/469-C8F17/30.5 PAA-pyrene/798

PAA-pyrene/l-C8F17/n

C8 fluorocarbon chains [mol %]

fluorine in polymer [wt %]

PAA-pyrene/151-C8F17/2.85 PAA-pyrene/271-C8F17/7.95 PAA-pyrene/313-C8F17/12.5 PAA-pyrene/469-C8F17/30.5 PAA-pyrene/798

35.1 12.6 8.00 3.27 0.0

51.8 32.6 24.5 12.3 0.0

a Further details are explained in the text. Here, n and l are the ratios of labeled carboxylate groups and the total number of carboxylate groups. l ) 151 means that 1 pyrene-labeled carboxylate groups exists in 151.

PAA Derivatives with Perfluorinated Side Chains

Figure 5. Series of emission spectra of polymer-attached pyrene (PAApyrene/151-C8F17/2.85, 0.010 g L-1) in aqueous solution as a function of pH (λEX ) 332 nm).

behavior of GPC columns depends on the volume of the analyzed polymer in solution,23 a precise GPC analysis of comblike materials becomes extremely difficult. A strong deviation in the perfluoro-octyl labeling degree of individual (monodisperse) PAA chains would, however, lead to a broad distribution in the GPC chromatograms. This behavior was not observed. From the size and the line shape of the GPC peaks, typical polydispersities (Mw/Mn) of 1.12 to 1.14 could be estimated. However, the macromolecular masses of the comb-like PAA/ pyrene/-C8F17 polymers can be calculated because the average elementary composition is known from CHN and DOC analyses. Those calculated macromolecular masses are presented in Table 1. Steady-State Fluorescence Measurements. In Figure 5 a typical series of emission spectra of polymer-attached pyrene in aqueous solution as a function of pH is shown (λEX ) 332 nm). pH changes trigger changes of the polymer conformation (extended at high pH due to consequent deprotonation of the polymeric carboxylic acid functions at pH > 4.55 vs folded at pH < 4.55 because of increased hydrophobic interactions). These changes lead to different amounts of excimer formation, and therefore their detection by emission spectroscopy becomes possible. The integrated monomer emission (λ ) 360-450 nm) and excimer emission (λ ) 450-650 nm) were compared as a function of pH and polymer concentration. The quotient of the relative monomer and excimer intensities (IM/IE) was plotted versus the labeling degree with perfluoro-octyl side chains (n), because the relative intensities in our experiments were varying

J. Phys. Chem. B, Vol. 104, No. 10, 2000 2219 strongly in this microheterogeneous environment due to scattering, light reflection, and possible nonlinear optical effects.24 In general, only the highly C8F17-labeled polymer PAA-pyrene/ 151-C8F17/2.95 showed a strong dependence on the experimental variables selected in our experiments. Therefore, we conclude that in this case, the hydrophobic interaction of pyrene with itself, leading to the formation of preformed excimers,28 is of a similar order of magnitude as its interaction with the C8F17 side chains. It becomes clear from Figure 6a and 6b, the polymer concentration influenced the ratio (IM/IE) (monomer intensity/excimer intensity) significantly. At high polymer concentration (1.0 g L-1) only an increase of the IM/IE ratio was detected at pH ) 6.5. There is a remarkable difference in the photophysical behavior of the PAA/pyrene/-C8F17 polymers under steady-state irradiation in dependence of the dissolved polymer concentration. A decrease of the integrated excimer emission was found at high concentration (1.0 g L-1) with increasing pH. This behavior is in qualitative agreement with the behavior reported for PAA/ pyrene in the literature.25 The excimer emission decrease has been attributed to the evolvement to an extended polymer conformation because of an increasing negative charge, due to the formation of deprotonated carboxylate groups in direct proximity to the PAA-polymer backbone. However, the prevailing mechanism in the case of our comb-like PAA/pyrene/C8F17 polymers, could as well be distinctly different in comparison to PAA/pyrene. The formation of supramolecular structures, by the interaction of individual comb-like macromolecules, could lead to extended conformations of PAA/ pyrene/-C8F17 due to the formation of micellar structures of the C8F17 labels belonging to at least two different polymer chains. These extended conformations would cause a decrease in the pyrene excimer formation. However, at low PAA/pyrene/-C8F17 concentration (0.01 g L-1) the opposite trend was observed. This trend was especially strong at the highest degree of C8F17 labeling. From a comparison of the photophysical behavior of PAA/pyrene and PAA/pyrene/-C8F17 we concluded that the presence of the C8F17 labels is responsible for the observed change in the excimer formation at high pH. Therefore it is likely that the polymer chain dynamics of PAA/pyrene/-C8F17 are very different than that of PAA/pyrene. The formation of supramolecular structures due to C8F17 interaction of different polymer chains is not possible at low polymer concentration. Therefore, the C8F17 labels attached to a single polymer backbone form micelles or similar structures. This leads to a contraction of the PAA/

Figure 6. (a and b). Quotient of the relative monomer and excimer intensities (IM/IE) of the integrated monomer emission (λ ) 360-450 nm) and excimer emission (λ ) 450-650 nm) of PAA-pyrene/151-C8F17/2.95, compared as a function of pH and polymer concentration (a: c ) 1.0 g L-1; b: 0.010 g L-1). (IM/IE) was plotted versus the labeling degree with perfluoro-octyl side chains (n). (A: pH ) 3.5; B: pH ) 4.3; C: pH ) 5.5; D: pH ) 8.0.)

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Figure 7. (a) Luminescence lifetimes of the monomer-emission (upper graph) and the corresponding relative fractions (lower graph) of PAApyrene/151-C8F17/2.85 (1.0 g L-1) in the pH range of 3.4 to 10.3. In all cases, a biexponential luminescence decay was observed. (b) Luminescence lifetimes of the eximer emission (upper graph) and the corresponding relative fractions (lower graph) of PAA-pyrene/151-C8F17/2.85 (1.0 g L-1) in the pH range of 3.4 to 10.3. In all cases, a biexponential luminescence decay was observed.

pyrene/-C8F17 polymer chains. The latter process favors the formation of excimers and thus the integrated excimer emission increases. Time-Resolved Fluorescence Measurements. The timeresolved fluorescence measurements were carried out and revealed the prevailing photophysical mechanism occurring in aqueous solutions of our novel comb-like PAA/pyrene/-C8F17 macromolecules. Monomer emission was monitored at 380 nm. Excimer emission was recorded at 600 nm.25 Whereas pyrene monomer emission arises from a single excited pyrene molecule, excimer emission occurs from an excited-state complex formed by one excited and one ground-state pyrene.26 According to their definition, excimers (excited dimers) are stable only in the photochemically excited state, whereas their ground state is dissipative. Principally, two different kinds of excimers exist: dynamic excimers, which are formed in a diffusion-controlled process within the lifetime of excited pyrene,27 and static or preformed excimers, which exist already before the absorption of a photon takes place.28 Such ground-state dimers are favored due to hydrophobic interactions, if the pyrene is located in a very hydrophilic environment.10,28 This is exactly the situation which we designed for these measurements, especially at lower densities of perfluoro-octyl side chains of the PAA polymer strands. The electronically excited state of the pyrene molecule possesses an excited-state energy of EΠ-Π* ) 321 kJ mol-1.29 Most of the pyrene derivatives differ only slightly in their energies. The observed fluorescence lifetimes for different pyrene derivatives from literature examples varied between 1.7

and 24.2 ns.30,31 The excited-state lifetime of pyrene derivatives linked to polymers and other microheterogeneous structures is very dependent on the local environment as well as the local density of excited pyrene molecules.32 The presence of hydrophobic side chains, attached to a hydrophilic polymer in a comblike manner, influences the amount of pyrene ground-statedimers in dependence to the polymer’s conformation and local mobility. This behavior can be explained as competition between the hydrophobic interactions of pyrene with another pyreneunit and of pyrene with the domains where perfluoro-octylester chains exist, which form hydrophobic clusters of similar structures such as micelles, providing their local density is sufficient.33 A pH change will result in a conformational change of the macromolecules, because the creation of negative charges at the polymer backbone (at higher pH) will lead to a stretching of individual chains due to charge repulsion.34 In this state, preformed excimers occur more likely between pyrene units, which are chemically attached to different polymer chains, because they are embedded in a more hydrophilic environment.28 It can be considered typical behavior of preformed excimers that their fluorescence lifetimes are of an order of magnitude similar to those of the corresponding monomer emission occurring from the same type of polymer-linked pyrene. At lower pH, the carboxylate charges will be annihilated by protonation, and the noncharged individual polymer chains will collapse and form hydrophobic (micro)domains.35 In this state, the interaction of pyrene with the hydrophobic domains is more likely, and consequently excimer formation is suppressed.28

PAA Derivatives with Perfluorinated Side Chains Experimental Results (1) PAA-pyrene/151-C8F17/2.85. In Figure 7a the fluorescence lifetimes observed at λem ) 370 nm (pyrene monomer emission) and λem ) 600 nm (pyrene excimer emission) are plotted as a function of pH. In the pH range from 3.4 to 10.3, a bis-exponential decay pattern prevailed for the monomer as well as the excimer emission. The first monomer fluorescence lifetime component decreases from 150 ( 5 ns at low pH to 65 ( 8 ns at high pH, whereas the second monomer fluorescence lifetime component decreases from 260 ( 5 ns at low pH to 175 ( 5 ns at high pH. A very similar experimental behavior was also found for the excimer emission. In this case, the first emission lifetime component decreases from 148 ( 3 ns to 85 ( 4 ns and the second lifetime component decreased from 200 ( 4 ns to 160 ( 15 ns if the pH changes from 3.4 to 10.3. Note that no rise terms for the excimer emission have been detected. From this experimental finding we exclude the occurrence of dynamic excimers throughout the whole pH range investigated. Furthermore, the observed changes in the fluorescence emission lifetimes do not take place over the whole pH range. Therefore they cannot be used for the construction of a pH sensor working in a broad pH extent. If the relative contributions of the two fluorescence lifetime components, arising each from the monomer and the excimer emission, are plotted versus pH (Figure 7b), the same principal information can be extracted as from Figure 7a. The relative amount of monomer lifetime component (I) decreases gradually from 62 ( 2% to 44 ( 2% during a pH change from 3.4 to 10.3. Component (II) rises correspondingly from 38 ( 2% to 56 ( 2%. In contrast to the observed lifetime changes of the excimer emission, the relative amount of excimer lifetime components remains unchanged at a value of 40 ( 5%. Correspondingly, the second lifetime component stays at 60 ( 5% throughout the whole pH range investigated. Again, a simple design of a polymer-based sensor is not possible using this information. (2) PAA-pyrene/271-C8F17/7.95; PAA-pyrene/313-C8F17/ 12.5; PAA-pyrene/469-C8F17/30.5. Electronic supplementary information is available on the monomer and excimer emission lifetimes of the four newly synthesized polymers, which are plotted versus pH (range from 3.4 to 10.3). Typical fluorescence lifetimes of the shorter monomer component are of the order of 65 ( 10 ns. The longer component possesses a lifetime of 165 ( 20 ns. In principle, the same luminescence lifetimes are found within the experimental error for both components of the excimer emission. Conclusions As it has been demonstrated by GPC, DOC, and CHN analyses, comb-like poly(acrylic acid)-pyrene-perfluoro-octylester materials possessing a very high labeling degree have been successfully synthesized. The polymers also possess a very low polydispersity (Mw/Mn ) 1.12 to 1.14). We have demonstrated that the ratio of IM/IE is strongly dependent on the pH of the solution. This behavior results from the formation of pyrene ground-state dimers due to hydrophobic interaction (in a hydrophilic environment)28 and from the competition of the hydrophobic interaction between pyrene units, on one hand, and perfluoro-octylester labels on the other.10 Among the four newly synthesized polymers, PAA-pyrene/151-C8F17/2.95 (Mw ) 3,800,500 [( 25,000]) responded most strongly to pH changes. In Scheme 1, the collapse of a single polymer chain due to hydrophobic interaction of the -C8F17 labels is shown (low concentration). The protonation of the carboxylate groups at

J. Phys. Chem. B, Vol. 104, No. 10, 2000 2221 SCHEME 1. Balance of the Hydrophobic Interaction of Pyrene/Pyrene, Pyrene/-C8F17, and -C8F17/-C8F17 Units at Low pH and Resulting (supramolecular) Structures (A: 0.01 g L-1; B: 1.0 g L-1).

low pH diminishes the effect of the charge repulsion. Note that at high concentration, a strong interaction occurs most likely between two or several PAA/pyrene/-C8F17 polymer chains. Due to the balance of the hydrophobic interaction of pyrene/ pyrene, pyrene/-C8F17, and -C8F17/-C8F17 units, a medium concentration of preformed pyrene (ground state) excimers was found in both concentration ranges. In Scheme 2, the situation at high pH is shown at low and high concentration. At low PAA/pyrene/-C8F17 concentration, a surprising increase in the pyrene excimer emission was observed. We interpret this finding as experimental evidence for the prevailing of strong interactive forces, caused by the very high amount of -C8F17 side chains in our novel polymer materials. The process of the breaking-up of individually collapsed chains at low concentration (globule-to-extended-chain transition10), which one typically sees for hydrophobically labeled poly(acrylates), is not observed here. Furthermore, the formation of pyrene ground-state excimers becomes more likely due to the hydrophilic (aqueous) environment of the dissolved polymer possessing negatively charged carboxylate groups in direct proximity to the polymer backbone. Although the existence of a globule-to-extended-chain transition was considered by Jiang and co-workers employing fluorocarbon-modified poly(acrylate) solutions possessing low labeling degrees, the most significant difference between those polymer materials10 and the comb-like polymers reported here is clearly their degree of -C8F17 labeling.We have therefore concluded that the unusually high labeling degree of our high molecular weight poly(acrylic acid) derivatives possessing perfluorinated side chains is responsible for their unusual (photo)physical behavior. At high polymer concentration, the interaction of several PAA/ pyrene/-C8F17 polymer strands is strongly favored. Under those conditions, the novel materials are most comb-like. The attractive forces responsible for the supramolecular aggregation are again hydrophobic in nature. Hence, the formation of pyrene

2222 J. Phys. Chem. B, Vol. 104, No. 10, 2000 SCHEME 2. PAA/Pyrene/-C8F17 Polymer Chains at High pH and Low Polymer Concentration (A: 0.01 g L-1): Pyrene (ground state) Excimer Formation Is Favored. PAA/Pyrene/-C8F17 Polymer Chains Form Supramolecular Aggregates at High Concentration (B: 1.0 g L-1). The Formation of Pyrene Ground State Excimers Is Suppressed Due to the Interaction of -C8F17 Side Chains of Different Polymer Strands.

ground-state excimers is suppressed compared to the situation at low concentration due to the interaction of -C8F17 side chains of different polymer strands. Acknowledgment. This work is dedicated to Prof. Dr. Dr. hc. Henri Bouas-Laurent on the occasion of his honorary doctorate received from the University of Saarland, Germany. The authors acknowledge a fellowship of Mr. Megh Raj Pokhrel, which has been provided by the DAAD (Deutscher Akademischer Austauschdienst). Further financial support from HewlettPackard is also gratefully acknowledged. This work has also been funded by the German Research Council (DFG, grant # Bo-1060/3-1). The authors thank Prof. Dr. Andre´ M. Braun for the use of his facilities at the Institute of Environmental Analysis Technology (Lehrstuhl fu¨r Umweltmesstechnik) at the EnglerBunte-Institute, University of Karlsruhe, as well as his valuable advice. We also thank Prof. Dr. H. Du¨rr for the use of his facilities at the University of Saarbru¨cken, Germany. Supporting Information Available: Figure S1, a plot of the monomer and excimer emission lifetimes vs pH, and information on the design of a fluorescence sensor for pH measurements. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Busser, G. W.; Ommen, J. G. v.; Lercher, J. A. Polym. Prepr. (Am. Chem. Soc, DiV. Polym. Chem.) 1995, 40, 72-77. Du, H.; Chen, P.;

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