Photophysics of Pyrenyl Acrylic Acid and Its Methyl Ester. A

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Photophysics of Pyrenyl Acrylic Acid and Its Methyl Ester. A Spectroscopic Method to Monitor Polymerization and Surface Properties S. Pankasem, M. Biscoglio, and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received August 27, 1999. In Final Form: January 28, 2000 This paper reports photophysical studies of pyrenylacrylic acid (PAA) and its methyl ester (PAM), along with studies utilizing these fluorescent probes to monitor polymerization rates. Due to the conjugation of the chromophore and the COOH group, the PAA fluorescence band is a mixture of π-π* and n-π* states and changes its structure and spectral position in solvents of varying polarity, from hydrocarbon solvents to aromatic hydrocarbons and polar solvents. PAA also shows two stages of protonation in aqueous solution: protonation on the negative carboxyl group to form the neutral acid, PCOOH, followed by protonation of the carbonyl oxygen to give the protonated cation, PCOOH2+. These effects are absent in PAM, and only the interplay of the double bond and the pyrene chromophore is observed. Later studies show that fluorescence probing with PAA can also be used to distinguish vicinal and geminal OH groups on silica gel surfaces, to comment on micelle-water interfaces, and to monitor polymerization of monomers. These probes are convenient for introducing fluorescent chromophores into polyelectrolytes and polymer systems.

Introduction Fluorescent probes have long been used to investigate polymerization rates. Changes in fluorescence yields,1 polarization,2 diffusion coefficients,3 and the efficiency of intramolecular and intermolecular excimer formation4-8 have been related to changes in the viscosity that occur during the polymerization events and, as such, are used to monitor the extent of curing in certain applications. Most methods utilize selected configurations of the probe molecule, for example the use of twisted intramolecular charge transfer, to monitor the polymerization rate. These methods require a significant change in the viscosity of the system, which only occurs when the degree of polymerization is very high. In other words, most of these methods are sensitive to later stages of polymerization. Hence, it is the purpose of this work to monitor the early stages of polymerization, especially events that occur immediately following initiation. This study reports on alternative fluorescent probes, pyrenylacrylic acid (PAA) and its methyl ester (PAM), to monitor polymerization. Here, use is made of the concept of copolymerization of a monomer and the probe molecule, where the concentration of the probe is much smaller than that of the monomer. Pyrenyl acrylic acid and its methyl ester were chosen as probes because their fluorescence * To whom correspondence should be addressed. (1) Valdes-Aguilera, O.; Pathak, C. P.; Neckers, D. C. Macromolecules 1990, 23, 689. (2) Ebeid, E. M.; El-Gamal, G.; Morsi, S. E. Photochem. Photobiol. 1986, 44, 547. (3) Hirayama, S.; Lampert, R. A.; Phillips, D. J. Chem. Soc., Faraday Trans. 1985, 81, 371. (4) Paczkowski, J.; Neckers, D. C. Macromolecules 1991, 24, 3073. Paczkowski, J.; Neckers, D. C. Macromolecules 1992, 25, 548. (5) Wang, F. W.; Lowery, R. E.; Grant, W. H. Polymer 1984, 25, 690. (6) Wang, F. W.; Lowery, R. E.; Fanconi, B. M. Polymer 1986, 27, 1529. (7) Paczkowski, J.; Neckers, D. C. Macromolecules 1992, 25, 548. (8) Warman, J.; Abellen, R.; Verney, H. J.; Verhoeven, J. W.; Hofstraat, J. W. J. Phys. Chem. B 1997, 101, 4913.

properties show marked changes after polymerization.9 The photophysical properties of the two pyrene probes are also established. The unique photophysical properties of pyrene and its derivatives in different media enables this chromophore to play an important role in the characterization of microenvironments.10-13 Pyrene fluorescence exhibits a change in the relative intensities of its vibrational bands which depends on the polarity of the solvent.14 A variety of pyrene derivatives such as pyrenecarboxaldehyde and aminopyrene display mixing of their π-π* and n-π* states, which results in a red shift of their fluorescence bands with increasing polarity.15,16 In the latter class, vinyl polyenes may be good candidates as probes due to conjugation of the chromophore and the vinyl double bond. Vinyl polyenes are also useful as precursors or monomers to obtain specific polymers with fluorophores attached to the polymer backbones.17 Few studies have concentrated directly on the photophysical and photochemical properties of these compounds.18,19 However, earlier work9 showed that use of PAA was an excellent method of tagging polyelectrolytes with a fluorescent probe by copolymerization. (9) Chu, D.; Thomas, J. K. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1991; Vol. 3, p 49. (10) Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph Series 181; American Chemical Society: Washington, DC, 1984. (11) Thomas, J. K. Chem. Rev. 1993, 93, 301. (12) Thomas, J. K. J. Phys. Chem. 1987, 91, 267. (13) Krasnansky, R.; Thomas, J. K. Langmuir 1994, 10, 4551. (14) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (15) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176. (16) Hite, P.; Krasnansky, R.; Thomas, J. J. Phys. Chem. 1986, 90, 5795. (17) For examples: (a) Nowakowska, M.; Gullet, J. E. Macromolecules 1991, 24, 474. (b) Sowash, G. G.; Webber, S. E. Macromolecules 1988, 21, 1608. (c) Chu, D.; Thomas, J. K. Macromolecules 1984, 117, 2142. (18) Yamaguchi, K.; Oh, S.; Shirota, Y. Chem. Lett. 1986, 9, 1445. (19) Ebeid, E. M.; El-Gamal, M. A.; Morsi, S. E. Photochem. Photobiol. 1986, 44, 547.

10.1021/la9911638 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/15/2000

Photophysics of Pyrenyl Acrylic Acid

Figure 1.

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NMR spectrum at 300 MHz of pyrenylacrylic acid in DMSO-d6 (number of transients accumulated ) 4).

Pyrenylacrylic acid (PAA) was chosen in this study because the molecule consists of a pyrene chromophore, a vinyl double bond, and a carboxylic acid group. The present study shows that these three functionalities all contribute to PAA photophysical and photochemical properties which vary with solvent type. Also, changes of the photophysical properties of this compound with a variety of solvents illustrate some applications of the probe to micellar systems and silica gel surfaces. The methyl ester of PAA, denoted by PAM, was also studied in order to elucidate the photophysics of this system in the absence of protonation. Preliminary studies show that the two probes also comment on the nature of the surfaces of micelles and porous structures. An additional feature of interest is the current concern for coatings, in such diverse industries as protective coatings (paint) and electronics. Pyrenyl acrylic acid and its methyl ester provide an opportunity to coat surfaces (by polymerization of the monomer or by spin coating of the polymer) with a polymer with unique photophysical properties. The unique fluorescent properties of PAA and PMA can provide a route to describe the nature of thin polymer films. Experimental Section Materials. Pyrenylacrylic acid (PAA) was synthesized from the reaction of pyrene carboxyaldehyde and malonic acid with a small amount of pyridine.20 The product was then recrystallized (20) Rade, F. Elsevier’s Encyclopedia of Organic Chemistry; Elsevier: London, 1951; p 442.

three times from methanol. PAM was subsequently produced by the Fischer esterification of the acid in methanol. Methanol (Aldrich, spectrograde 99.5+%), cyclohexane (Aldrich, HPLC grade 99.9+%), benzene (Aldrich, HPLC grade 99.9+%), acetonitrile (Aldrich, HLPC grade 99.9+%), sodium dodecyl sulfate (Aldrich, 98%), cetyltrimethylammonium bromide (Aldrich, 98%), sodium hydroxide (Fisher, 99.1%), hydrochloride acid (Fisher, certified ACS), and DMSO-d6 (Aldrich, 99.9%+) were used as received. Acrylic acid (Aldrich, 99%) was purified with an inhibitor remover to remove hydroquinone monoethyl ether before use. Azobis(isobutyronitrile) (AIBN, Aldrich 98%) was recrystallized from methanol. Methods. Absorption spectra were measured with a Cary 13 spectrophotometer, and steady-state fluorescence was measured with a SLM spectrofluorometer, model SPF 500C equipped with a thermostatically controlled sample holder. All the spectra were taken at an excitation wavelength of 360 nm, with band-passes of 10 and 1 nm for the excitation and the emission monochromators, respectively. Time-resolved fluorescence experiments were performed with a ND-YAG synchronously pumped, modelocked, and cavity-dumped dye laser (Continuum, PY61-10), using the third harmonic 355 nm with 1 mJ/pulse and a fwhm of 35 ps. A Hamamatsu R 1664 U multichannel plate photomultiplier and a Tektronix 7912 AD 500-MHz waveform digitizer were used to detect the emitted light and capture the electrical signal, respectively. NMR. The purified PAA and PMA exhibited melting points of 279-281 and 144-145 °C, respectively, as given in the literature.20 Figure 1 illustrates an NMR spectrum of 1 × 10-2 M PAA in DMSO-d6, taken on a Varian Unity Plus 300 NMR. This spectrum clearly shows that the material is of high purity and is all in the trans form.

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Figure 2. (a) Steady-state fluorescence spectra of 3 × 10-5 M PAA in cyclohexane, benzene, methanol, acetonitrile, and water (pH 7) with excitation at 360 nm. (b) Steady-state fluorescence spectra of 3 × 10-5 M PAM in cyclohexane, benzene, methanol, acetonitrile, and water (pH 7) with excitation at 360 nm. IR. IR spectra were taken on a Perkin-Elmer 1420 ratio recording infrared spectrophotometer. Polymerization Studies. Samples with the probe, the monomer, and the initiator (AIBN) were purged with nitrogen for approximately 15 min, and then the polymerization was initiated in a water bath at 60 °C. Periodically, a sample was removed and cooled to 0 °C in an ice bath to quench the reaction. After the sample was warmed up to room temperature, a known amount of pentane was added to precipitate the polymer from the unreacted acrylic acid. The polymer sample was then dried to a constant weight, and the percentage of conversion to monomer was calculated from the weight of the polymer. Independent IR measurements taken at 988 cm-1 to monitor the double-bond content of the system were in agreement with the gravimetric data.

Results and Discussion Fluorescence Spectra of PAA and PAM. Before embarking on the application of the two probes, it is essential to establish their photophysics in various media. Most pyrene derivatives tend to exhibit altered fluorescence spectra in different solvents, either by a change in the relative intensity of vibrational bands or by a shift in the positions of the emitting bands. The probes PAA and PAM are no exception. Parts a and b of Figure 2 feature the fluorescence spectra of PAA and PAM in various solvents. It is noted that the fluorescence spectra in nonpolar solvents such as cyclohexane are similar for PAA and PAM and exhibit structured bands similar to that of pyrene, with peaks at 407, 432, and 458 and a shoulder at 490 nm. The 0-0 vibrational fluorescence band for most pyrene derivatives normally appears at around 375 nm, but for PAA and PAM the 0-0 band appears at 387 nm. The marked red shift (12 nm) is due to conjugation of the pyrene ring and the vinyl double bond. Although this effect may stabilize both the ground and excited states of PAA, the bathochromic shift of the fluorescence band indicates that the excited state is stabilized to a greater extent than

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the ground state, an effect which occurs with most vinyl derivatives of polyaromatic compounds.21 In aromatic hydrocarbon solvents such as benzene and toluene, the whole fluorescence band shifts even farther to the red, although the vibrational structure is maintained. Here, stabilization of the excited state is greater than that observed in cyclohexane, suggesting that the probe must experience interaction with benzene, an event that does not occur in cyclohexane. The most reasonable explanation is aromatic interaction between the aromatic solvents and the aromatic moiety of the acid. In methanol, interaction between the acid PAA and the solvent, as seen with benzene, does not take place. Therefore, the fluorescence band appears in a position similar to that of the band for PAA in cyclohexane. However, the resolution of this fluorescence band is not as well defined as that in cyclohexane solution, and the peak at 432 becomes more intense. This is a consequence of a strong interaction between methanol and the acid group, PAA. Basically, the emitting state is a mixture of π-π* and n-π* states, and interaction between a polar solvent and the acid emphasizes the π-π* state contribution to the emitting state, resulting in the fluorescence band becoming broader and shifting to the red. The effect is more pronounced in acetonitrile and aqueous solution (Figure 2), where an even stronger interaction between the acid group and the solvent is involved and only one broad band appears. A pronounced effect is also observed in PAM (Figure 2b), so it is instructive to compare the present data to earlier studies with pyrenecarboxaldehyde, aminopyrene, and pyrenecarboxylic acid. Pyrene derivatives with carbonyl groups such as pyrenecarboxaldehyde have low-lying π-π* and n-π* states, which lie close to each other.22 Observation of the n-π* state is obscured due to overlap with the more intense π-π* band. Interaction between these two states takes place, resulting in a loss of spectral structure and a red spectral shift of the π-π* band. The lowest excited state is then a mixture of both states, with contributions depending on the energies of the contributing states and the geometric arrangement of both chromophores. Brederek23 suggested that polar solvents change the order of the energy levels, lowering the fluorescent π-π* state below the n-π* state. This is observed in aminopyrene and pyrenecarboxaldehyde. In these derivatives the conversion of the active photostate for π-π* and n-π* character to π-π* is observed by a continuous shift of the fluorescence peak maximum with increasing solvent polarity. This is clearly seen with PAM in Figure 3, where the polarity of the solvent is continuously varied with dioxane-water mixtures. In this study, a moderately intense and structured fluorescence band of PAA in cyclohexane is obtained, suggesting that π-π* is the main contributor to the emitting state. Nevertheless, the contribution or overlap by the n-π* state cannot be completely disregarded. Furthermore, acid derivatives of pyrene, such as pyrenecarboxylic acid, also exhibit a structured band similar to that for pyrene even in polar solvents such as methanol. As the polarity of the solvent increases, the π-π* state becomes more dominant, which leads to a broader red-shifted fluorescence band. The spectroscopy of PAA in water is best interpreted by reference to similar studies with pyrenecarboxylic acid. (21) Berlman, I. B. Handbood of Fluoresence Spectra of Aromatic Molecules; Academic Press: New York, 1971. (22) Hirayama, S. Rev. Phys. Chem. Jpn. 1972, 42, 49. (23) Brederek, K.; Fo¨rster, T. H.; Oesterlin, H. G. Molecular Luminescence of Organic and Inorganic Materials; Kallmann, H., Spruch, G. M., Eds.; Wiley: New York, 1962; p 164.

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Figure 3. Steady-state fluorescence spectra of 3 × 10-5 M PAM in dioxane-water mixtures with excitation at 360 nm. Figure 5. Steady-state fluorescence spectra of 1 × 10-5 M pyreneacrylic acid in 10% acrylic acid with 2.4 nM of AIBN at 60 °C: (a) before polymerization; (b) after 15 min of polymerization.

Figure 4. Steady-state fluorescence spectra of 3 × 10-5 M PAA in water at various pH values: (a) 7; (b) 4.5; (c) 4 (× 10); (d) 3 (× 10); (e) 1 (× 10); (f) 1 (× 100) (with excitation at 360 nm).

Previous studies showed that the anion of pyrenecarboxylic acid (RCOO-) exhibits a fluorescence band which resembles that of pyrene.24 At 77 K, the protonic forms (RCOOH and RCOOH2+) also exhibit similar structured fluorescence bands which are red shifted with respect to those for the anionic species. However, at room temperature, the pyrenecarboxylic acid spectra in aqueous solutions of low pH exhibit a structureless band with a maximum at 420 nm, which was attributed to the fluorescence band of RCOOH2+. There is no effect of pH (3 to 11) on the fluorescence spectrum of PAM in water or in methanol. Hence, the prior pH effects on PAA are attributed to the COOH group. Excited PAA gives rise to fluorescence spectra which are somewhat different from those of pyrenecarboxylic acid. Figure 4 shows the fluorescence spectra of PAA in aqueous solutions at various pH values. At pH g 5, the spectra exhibit a broad band with a maximum at 450 nm. When the pH decreases to 4.5, the spectrum becomes broader, and the band maximum shifts to 480 nm; in addition, the fluorescence intensity decreases significantly. The fluorescence spectrum of PAM is very similar to that of PAA in water at pH 4 with a λmax of 480 nm. However, at pH > 5 the fluorescence maximum of PAM (unlike that of PAA) resides at 480 nm. This is indicative of the fact that at pH > 5 the observed fluorescence of PAA is due to that of the anion, a situation that does not occur with the ester PAM. At pH < 4.5, the band maximum of PAA shifts to 500 nm. From these results, it is clear that the band at 450 (24) Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1988, 92, 2997.

nm is characteristic of the anion of the acid, PCOO-, whereas the band at 500 nm is characteristic of the neutral acid form, PCOOH. The fluorescence band at 480 nm which is observed at pH 4.5 is essentially a combination of the anion band (450 nm) and the acid band (500 nm). At pH < 2, another broad fluorescence band arises with a maximum at 530 nm. By comparison with studies of pyrenecarboxylic acid,24 this is assigned to PCOOH2+*. Summary of the Photophysics. Both PAA and PAM, due to the conjugation of the various chromophores in these molecules, show photophysical properties that are markedly solvent dependent. These properties may be used to monitor other systems, for example polymerization, micellar surfaces, and SiO2 surfaces. Polymerization of PAA. Figure 5 shows the fluorescence spectra of the probe (1 × 10-5 M) in 10% acrylic acid aqueous solution in the presence of the initiator AIBN (2.4 mM) before and after polymerization. Before polymerization (curve a), the spectrum exhibits a broad band with a maximum at about 500 nm, which is a typical spectrum for the probe in polar solvents at low pH, as described above. The effect of the vinyl double bond is to extend the conjugation, and the broad fluorescence band at 500 nm indicates that the π-π* state is the emitting entity. Polymerization or conversion of the vinyl double band to a single bond of the probe molecule not only shuts down the extension of the conjugation of the pyrene ring but also prohibits the nonbonding state of the carbonyl group from being involved in the overall photophysics of pyrene. As a result, the system should yield a spectrum similar to that of pyrene. Curve b of Figure 5 shows the spectrum of the system after 15 min of polymerization, where about 65% of the probe was copolymerized with the monomer. It can be seen that the spectrum exhibits a new structured band with maxima at 380, 400, and 420 nm, indicative of a pyrene derivative. Figure 6 shows changes of the monomer concentration and the probe fluorescence intensity versus time with various initial concentrations of monomer. The curves represent the change of the probe’s fluorescence intensity while the discrete symbols represent the percentage of monomer left, which was determined from the gravimetric method, as described earlier. The rate of double-bond conversion of the monomer (PAA) matches well with the rate of the probe fluorescence intensity decrease. This indicates that the rate of removal of the probe and the decrease in monomer are correlated. As illustrated in Figure 6, the rate of polymerization starts to decrease (as the slope of the curve decreases)

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Figure 6. Loss in fluorescence intensity monitored at 500 nm during polymerization of 1 × 10-5 M pyreneacrylic acid in various acrylic acid solutions at 60 °C: 10% (top); 20% (middle); 40% (bottom). The percentage of acrylic acid left, calculated from gravimetric analysis, is plotted for 10% (4), 20% (×), and 40% (3).

with time. In a radical polymerization, the probability that reaction will occur during an encounter is relatively high and increases with increasing duration of an encounter. When the system reaches sufficiently high viscosity and the mobility of the molecule is highly suppressed, the reaction rate becomes equivalent to the number of encounters. Thus, the reaction becomes diffusion controlled and its rate constant decreases sharply under such viscous conditions. While the kinetic constants start to decrease relatively early, the change in kinetics becomes very obvious at the moment when the polymerizing system gels and loses macroscopic fluidity. Effect of Monomer Concentration. In this study, the concentrations of the monomer used were 10, 20, and 40%. From Figure 6, the rate of polymerization is determined from the slope of each curve. At early stages of copolymerization the viscosity of a sample has little effect on the kinetics of polymerization. The rate of polymerization of a sample of 20% AA is about twice that of the 10% AA sample, which is expected. However, when the concentration of monomer increases to 40%, only a slight increase was found compared to that of 20% AA, which is explained by the increased viscosity of the medium. Effect of Probe Concentration. The copolymerization rate is markedly affected at high probe concentrations: the greater the probe concentration, the slower the rate at which the monomer polymerizes. In this situation, the probe may react with AIBN (the initiator) and interfere with the kinetic scheme of the monomer. As a result, polymerization of the monomer slows down. However, it is found that, at probe concentrations < 2 × 10-5 M, the copolymerization rate is independent of probe concentration. This also indicates that the fluorescence change of the probe can be used to determine the polymerization rate of the monomer. Copolymerization with Methyl Acrylate. Figure 7 shows the fluorescence intensity changes of the probe in pure methyl acrylate and the IR measurements which were performed to measure the amount of methyl acrylate. Polymerization takes place slowly at the beginning and then becomes autocatalytic, due to the Trommsdorff effect.25 Again, the data from IR measurements match well the fluorescence data. (25) Trommsdorff, E.; Kohle, H.; Lagally, P. Makromol. Chem. 1948, 1, 169.

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Figure 7. Loss in fluorescence intensity monitored at 500 nm during polymerization of 2 × 10-6 M pyrene acrylic acid in methyl acrylate at 60 °C with 2.4 mM AIBN (line) and % methyl acrylate left (b) with time. Table 1. Fluorescence and Lifetimes of PAA in Various Solvents solvent

fluorescence maxima (nm)

cyclohexane methanol n-propanol n-butanol benzene toluene acetonitrile water (pH 7) SDS Triton X-100 CTAB SiO2 (Fisher) (vicinal OH) SiO2 (MCB) (geminal OH)

407, 432, 458 409, 428 400, 423, 450 402, 424, 450 430, 450 427, 449 450 458 475 409, 428 409, 428 470 495

lifetime (ns) 3.98 4.54 7.58 6.84 3.14 2.79 2.70 3.03 1.73 5.46 3.36

Micelles. In aqueous solution, above a critical concentration, surfactants such as sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB, usually form micellar aggregates. These micelles solubilize a wide variety of hydrophobic and hydrophilic substances. Depending on their molecular structure, the solubilized molecules tend to reside, on an average, either toward the micellar interior core or at the micellar surface, the picture being a dynamic one. For example, NMR and UV spectrometric studies have shown that aromatic compounds with hydrophilic groups such as aromatic aldehydes, ketones, and alcohols are solubilized with their hydrophobic moiety toward the micellar core and with the hydrophilic group protruding into, or anchored at, the micellar surface or double layer. Anionic Surfactant. Both PAA and PAM fluorescence strongly in SDS solution, and relevant data are given in Table 1. The fluorescence spectrum of PAA exhibits a broad band with a peak maximum at 475 nm, which is reminiscent of PAA in the free acid form in a polar environment similar to water. In principle, PAA is solubilized in the micelle with the acid group anchored at the micellar surface, where it experiences a pH < 7 due to the H+ attracted to the negatively charged micelle surface. Similar studies with PAM (Table 1) indicate that this probe is in a polar environment that is similar to water. Both probes indicate a location that is at the micelle-water interface. Nonionic Surfactant. Trition X-100 was used as a model for nonionic surfactants. The fluorescence spectral maxima of PAA in 5 mM Triton X-100 (cmc ) 3 × 10-4 M) are given in Table 1. The spectra exhibit structured bands with peak

Photophysics of Pyrenyl Acrylic Acid

maxima at 409 and 428 nM similar to those found in pure methanol solution, indicating interaction of PAA with the micellar form of the surfactant. This indicates that PAA is mainly present as the neutral acid. As mentioned in the previous section, anionic surfactants such as SDS, due to the negatively charged surface, lower the pKa of the acid compared to what happened in aqueous solution, while, in Triton X-100 solutions, PAA exists mainly in the neutral acid form in a polar environment such as the ethylene oxide mantle of the surfactant. This is confirmed by the fluorescence maximum for PAA (Table 1). Cationic Surfactant. CTAB was used as an example of cationic surfactants. The fluorescence spectra maxima of PAA in CTAB are given in Table 1. The spectra indicate maxima similar to those in Triton X-100 and indicate that the PAA is mainly present in the free acid form but in a nonaqueous environment. The neutral PAM fluorescence data (Table 1) also show that this probe is in a polar environment at the micelle-water interface. Silica Gel Surfaces. In the previous studies13,16,24 aminopyrene and pyrenecarboxylic acid were used successfully to distinguish two types of OH groups, vicinal and geminal, on silica gel surfaces. Two different silica gel surfaces with different relative amounts of these two OH groups were used: Fisher silica, which mainly contains vicinal OHs, and MCB silica, which contains geminal OHs. The fluorescence spectra of PAA at silica gel/cyclohexane readily distinguish between the two surfaces (Table 1). The spectrum of PAA on a Fisher silica surface exhibits a broad band with a maximum at 470 nm and approaches that of the free acid. By comparison with the behavior of PAA in hydroxylated polar solvents such as water, it is concluded that PAA is present in the free acid and protonated form on MCB silica (λmax ) 495 nm) while on Fisher silica gel it is present mainly in the anionic form. The fluorescence band in Fisher silica is broad, and it is possible that some of the PAA might be present in the

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protonated form, which has an emission band close to that of the free acid form. These results, indicating a more acidic surface for the geminal OH groups, are in good agreement with the previous studies on aminopyrene and pyrenecarboxylic acid. Conclusion These studies show that pyreneacrylic acid and its methyl ester are useful probes of their environment, exhibiting a variety of changes in their photophysical properties, especially fluorescence, in different solvents. These changes are caused by mixing of the n-π* and π-π* states, extension of the aromatic conjugation, and protonation and deprotonation. A general summary can be drawn from the following: 1. Although the emitting state is described as a mixture of the n-π* and the π-π* states, evidence from its vibrational structure suggests that the major contribution is from the π-π* state. 2. Interaction between aromatic solvents and the aromatic moiety of PAA stabilizes the excited state of PAA to a greater extent than the ground state of PAA, which leads to a red shift of its fluorescence band. 3. The pKa and proton dissociation and association rate constants of both the ground and excited states of various PAA species can be determined from fluorescence spectra and quenching studies. 4. Spectral information gained from the solvent effect on PAA fluorescence can be applied to polymerizing systems, micellar systems, and silica gel surfaces. 5. These probes are convenient for fluorescence tagging of polyelectrolytes and polymer systems. Acknowledgment. The authors thank the Bayer Foundation, NSF (CHE96-10187-002), Dr. Douglas Weiss, and 3M Corporation for the support of this work. LA9911638