A Spectroscopic Study of Nonamphiphilic Pyrene Assembled in

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A Spectroscopic Study of Nonamphiphilic Pyrene Assembled in Langmuir-Blodgett Films: Formation of Aggregates A. K. Dutta* and T. N. Misra Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India

A. J. Pal Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received August 29, 1994. In Final Form: September 18, 1995X Nonamphiphilic pyrene molecules mixed with a fatty acid have been incorporated in Langmuir-Blodgett (LB) films. The surface pressure versus area per molecule isotherms show that the average area per molecule at first increases and then decreases with increasing mole fraction of pyrene in mixtures of pyrene and stearic acid (SA), which indicates that the pyrene molecules are very likely squeezed out of the airwater interface in between the fatty acid chains and are on the surface of the monolayer. A positive deviation from the additivity rule has been observed in the plot of the average area per molecule versus the mole fraction of pyrene, which suggests a repulsive type of interaction between the pyrene and the fatty acid molecules. Spectroscopic studies reveal the formation of I-aggregates. The emission spectra of mixed films of pyrene and stearic acid show a single broad band at 420 nm which has been assigned to a partial excimeric emission band originating from an incomplete or partial overlap of the pyrene molecules. Such an incomplete or partial overlap along with its small area per molecule at the air-water interface add credence to the thesis of formation of I-aggregates of pyrene molecules in mixed LB films. The possible aggregation has further been verified by spectroscopic studies of pyrene in ethanol-water mixtures. The partial excimer to monomer intensity ratio is seen to increase sharply with increasing volume fraction of water in the binary mixture of ethanol and water above a critical composition of the mixture corresponding to a volume fraction of 0.75 for water, suggesting the onset of formation of aggregates. The discussion on the excitation spectra of pyrene in LB films and in ethanol-water mixtures also supports the propositions of the formation of aggregates. Finally, scanning electron microscopy has been employed to reveal the existence of microcrystalline aggregates in the LB films.

Introduction Pyrene is perhaps the most extensively studied member of the polycyclic aromatic hydrocarbon (PAH) family owing to its interesting photophysical properties like a long excited state lifetime, a high quantum yield of fluorescence, and a pronounced ability to form excimers.1a-c In ordinary organic solvents pyrene exhibits a structured monomer fluorescence with its 0-0 band at about 370 nm. With increasing concentration, the pyrene monomer fluorescence eventually decreases and a red-shifted broad structureless fluorescence with its maximum at about 470 nm appears. This emission band is due to an excimer produced as a result of the collisional association between an excited and a ground state pyrene molecule.1b,c The absorption spectral profiles for a very dilute (monomers only) solution and a concentrated (monomer and excimer) solution of pyrene are identical.1b,c The steady-state emission spectrum of monomeric pyrene shows five distinct vibrational bands; the intensity ratio of the first and the third vibronic band is found to be extremely sensitive to the solvent characteristics.2 Extensive studies in solution2 and in supercritical fluids3a-c have revealed that the sensitivity * To whom all correspondence should be addressed. Present address: Dr. A. K. Dutta, Photonics Chemistry Section, Department of Optical Materials, Osaka National Research Institute, AIST, Ikeda City, Osaka 563, Japan. Fax: +81-727-51-9628. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) (a) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 1965. (b) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (c) Winnik, F. M. Chem Rev. 1993, 93, 587.

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of the pyrene monomer fluorescence emission arises as a result of the dipole-induced dipole interaction between solute and solvent. Recently, Karpovich and Blanchard3d have shown that the solvent sensitivity of the monomer fluorescence of pyrene may be attributed to the vibronic coupling between the weakly allowed first excited and the strongly allowed second excited states. Owing to such interesting photophysical properties, especially its strong interaction with the microenvironment in which it is located, pyrene molecules have been employed as a molecular probe to examine the microenvironment in various systems like supercritical fluids,3a-c silica gels,4 zeolites,5 clays,6 micelles,7 microparticles,8 (2) (a) Ham, J. S. J. Chem. Phys. 1953, 21, 756. (b) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560. (c) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 17. (d) Kalyansundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (3) (a) Brennecke, J. F.; Tomasko, D. L.; Peshkin, J. Ind. Eng. Chem. Res. 1990, 29, 1682. (b) Brennecke, J. F.; Tomasko, D. L.; Eckert, C. A. J. Phys. Chem. 1990, 94, 7692. (c) Zagrobelny, J.; Betts, T. A.; Bright, F. V. J. Am. Chem. Soc. 1992, 114, 5249. (d) Karpovich, D. S.; Blanchard, G. J. J. Phys. Chem. 1995, 99, 3951. (4) (a) Avinir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariasse, K. A. J. Phys. Chem. 1985, 89, 3521. (b) Levitz, P.; Van Damme, H.; Keracis, P. J. Phys. Chem. 1984, 88, 2228. (c) Mao, Y.; Thomas, J. K. Langmuir 1992, 8, 2501. (5) (a) Turro, N. J.; Baretz, B. H. J. Photochem. 1984, 24, 201. (b) Suib, S. L.; Kostapapas, A. J. Am. Chem. Soc. 1984, 106, 7705. (6) Dellaguardia, R. A.; Thomas, J. K. J. Phys. Chem. 1983, 87, 3550. (7) (a) Kalyansundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FL, 1986. (b) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730. (8) (a) Misawa, H.; Kitamura, N.; Masuhara, H. J. Am. Chem. Soc. 1991, 113, 7859. (b) Nakatani, K.; Misawa, H.; Saaki, K.; Kitamura, N.; Masuhara, H. J. Phys. Chem. 1993, 97, 1701.

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vesicles,9 membranes,10 and Langmuir-Blodgett (LB)11,12 films. Although fatty acid derivatives of PAH molecules incorporated in LB films have been extensively studied, little effort has been made to study nonamphiphilic PAH molecules incorporated in LB films. Recently, Wistus et al.13 have reported on the behavior of pyrene mixed with lipids at the air-water interface using in-situ steady-state and time-resolved fluorescence techniques. The results showed some very interesting features never observed before. For example, the Langmuir films at the air-water interface only showed monomer fluorescence and the well-known excimer band at 470 nm was not observed even at high concentrations of pyrene in mixed monolayers with the fatty acid. While freshly prepared mixed LB films of pyrene and eicosanoic acid showed a broad band emission with its maximum at about 420 nm, the structured monomer fluorescence was, however, totally absent. The films left to age for a few days showed remarkable changes. The broad band emission with its maximum at 420 nm was found to be totally absent, and the monomer fluorescence was restored. The authors have explained this observation in terms of redistribution of the pyrene aggregates into monomeric entities via diffusion of the pyrene moieties in the LB films. Since we failed to reproduce these results we thought that a systematic study was probably necessary. Recently, Kozarec et al.14 have also reported their failure to reproduce the results obtained by Wistus et al.13 In this paper we report on the behavior of pure pyrene and pyrene mixed with fatty acids at the air-water interface. The surface pressure (π) versus area per molecule (A) isotherms as well as the morphological studies of such films indicate the formation of organized aggregates in the LB films. Spectroscopic studies of the aggregates in the LB films and in binary solvent mixtures reveal the formation of partial excimers that seem to be possibly created as a result of the partial overlap between the aromatic rings of pyrene. Our results suggest the formation of organized aggregates in both the LB films and the binary solvent mixtures. Experiment Pyrene purchased from Aldrich Chemical Co., Milwaukee, WI, was vacuum sublimed and then extensively zone refined (200 passes) before use. Stearic acid (SA) was purchased from Sigma Chemical Co., St. Louis, MO, and used as received. The purity of the zone-refined pyrene samples used was tested by absorption and emission spectroscopy. A commercially available LangmuirBlodgett (LB) alternate layer trough made of poly(tetrafluoroethylene) manufactured by Joyce-Loebl, Newcastle upon Tyne, UK, Joyce-Loebl Model IV, was used for the deposition of the (9) (a) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279. (b) Daems, D.; Van der Zegel, M.; Boens, N.; De Schryver, F. C. Eur. Biophys. J. 1985, 12, 97. (c) Tokumura, T.; Hikida, T. J. Photochem. Photobiol. A 1993, 72, 69. (10) Jenkins, R. M.; Weiss, R. G. Langmuir 1990, 6, 1408. (11) (a) Itaya, A.; Masuhara, H.; Taniguchi, Y.; Imazaki, S. Langmuir 1989, 5, 1407. (b) Itaya, A.; Kawamura, T.; Masuhara, H.; Taniguchi, Y. Thin Solid Films 1990, 185, 307. (c) Loughran, T.; Hatlee, M. D.; Patterson, L. K.; Kozak, J. J. J. Chem. Phys. 1980, 72, 5791. (d) Grieser, F.; Thistlethwaite, P. J.; Urquhart, R. S. Chem. Phys. Lett. 1987, 141, 108. (12) (a) Yamazaki, T.; Tamai, N.; Yamazaki, I. Chem. Phys. Lett. 1987, 124, 326. (b) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1987, 91, 3572. (c) Bohorquez, M.; Patterson, L. K. J. Phys. Chem. 1988, 92, 1835. (d) Subramanian, R.; Patterson, L. K. J. Phys. Chem. 1985, 89, 1202. (e) Vaidyanathan, S.; Patterson, L. K.; Mobius, D.; Gruniger, H.-R. J. Phys. Chem. 1985, 89, 491. (f) Bohorquez, M.; Patterson, L. K. Langmuir 1993, 9, 2097. (g) Caruso, F.; Grieser, F.; Thistlethwaite, P. J.; Almgren, M. Langmuir 1993, 9, 3142. (h) Caruso, F.; Grieser, F.; Murphy, A.; Thistlethwaite, P. J.; Urquhart, R.; Almgren, M.; Wistus, E. J. Am. Chem. Soc. 1991, 113, 4838. (13) Wistus, E.; Mukhtar, E.; Almgren, M.; Lindquist, S. E. Langmuir 1992, 8, 1366. (14) Kozarec, Z.; Ahuja, R. C.; Mobius, D. Langmuir 1995, 11, 568.

Dutta et al. mono- and multilayers on solid substrates. A filter paper Wilhelmy plate attached to a microbalance measured the surface pressure with an accuracy of (0.1 mN/m. The Wilhelmy plate balance was also interfaced to a microcomputer that monitored the movement of the compressing barrier maintaining constant pressure at the air-water interface with an accuracy of (0.1 mN/m. Triply distilled water was then deionized using a Milli-Q water purification system and had a resistivity of about 18.2 MΩ‚cm and a pH of 6.4 in equilibrium with atmospheric carbon dioxide; this was used as the subphase. The temperature of the trough was maintained constant at 20 °C by a Joyce-Loebl water refrigerating system that allowed us to maintain a constant temperature of the subphase. Chloroform solutions of pyrene and SA mixed in different predetermined molar ratios were used as spreading solutions for isotherm measurements. 100 µL of the solution was spread on the water surface, and after allowing sufficient time for the solvents to evaporate, the floating layer was compressed at a rate of 6 × 10-3 nm2 mol-1 s-1. The isotherm data was acquired by an IBM-PC, and the data was processed using the software provided by Joyce-Loebl. The isotherms were reproducible within an error of (0.02 nm2/molecule. Each isotherm was obtained by averaging three to four runs. The transfer ratio was measured by calculating the ratio of the actual decrease in the subphase area to the actual area on the slides coated by the floating layer multiplied by the number of layers. For Y-type deposition on quartz slides the transfer ratio was about 0.92 ( 0.02. Fluorescent grade quartz slides were thoroughly cleaned by chromic trioxide and boiling nitric acid to remove all traces of organic matter and then rinsed with deionized water, sonicated in chloroform, and finally dried in a hot air oven. Deposition of the LB mono- and multilayers was achieved by allowing the slides to move through the floating layer vertically with a speed of 5 mm/min. A drying time of 20 min was allowed before the next subsequent dip. The absorption and emission spectra of pyrene in solution and in the LB films were recorded on a Shimadzu 2101 PC UV-vis spectrophotometer and a PerkinElmer MPF-44A spectrofluorimeter, respectively. To minimize the effects of reabsorption, front face illumination of the films was achieved by placing the films in suitable holders such that the angle between the film and the incident beam was 45°. Emission was collected in a conventional right angled geometry, i.e., the angle between the incident and emission beams was 90°. To reduce the effects of scattering narrow band-pass (5 nm) filters were used.

Results and Discussion When a solution of pure pyrene in chloroform (1 × 10-3 M) was spread at the air-water interface and compressed slowly, it was observed that it does not give rise to a compact monolayer but instead formed microcrystals which were pushed together in the compression process. On relaxing the surface pressure, it was found that the large clusters formed during compression broke into smaller ones but did not disintegrate completely. Repeated attempts to transfer the floating layer of pure pyrene onto quartz and glass slides failed. Identical results have been reported for other nonamphiphilic PAH molecules, namely, chrysene15 and p-terphenyl.16 However, on mixing pyrene with SA a stable and compressible floating layer was obtained which could be transferred onto solid substrates with a high transfer ratio. Plots of the area per molecule of the mixed film versus time at constant surface pressures of 15, 20, and 25 mN/m (figures not shown) were found to be straight lines parallel to the time axis that indicated that the mixed floating layers were highly stable at the air-water interface. Figure 1 shows the surface pressure (π) versus area per molecule (A) isotherms of pure pyrene and the mixed films of pyrene (15) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 4365. (16) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 12844.

Nonamphiphilic Pyrene Assembled in LB Films

Figure 1. Surface pressure versus area per molecule isotherms of pyrene and SA mixtures at the air-water interface. The mole fraction of pyrene in the mixtures is (a) 0, (b) 0.08, (c) 0.20, (d) 0.4, (e) 0.64, (f) 0.8, and (g) 1.0.

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of samples were collected from different regions of the trough at different surface pressures, and the emission of the water samples was examined. No emission corresponding to the characteristics of pyrene was observed, suggesting very likely that pyrene molecules have not precipitated out of the air-water interface but are located in the stearic acid matrix of the floating layer. A plot of the absorbance versus the mole fraction of pyrene in mixed LB films for a fixed number of layers (figure not shown) yielded a straight line which indicated that the pyrene molecules were indeed present in the LB layers. It seems likely that the pyrene molecules are pushed out of the air-water interface in between the stearic acid chains and probably also are on the surface of the LB films. In fact, similar results have been obtained for other PAH molecules.15-18 Figure 2 shows a plot of the average area per molecule versus the mole fraction of pyrene in the mixed films of pyrene and SA at the air-water interface corresponding to different surface pressures. It is evident from this figure that the area per molecule at first increases with increasing mole fraction of pyrene in the mixture, attains a maximum, and then decreases. For a two-component system of noninteracting molecules the average area per molecule is given by the relation19-22

A12 ) N1A1 + N2A2

Figure 2. Plot of the average area per molecule of pyrene and SA mixtures versus the mole fraction of pyrene at different surface pressures: (a) 5 mN/m, (b) 10 mN/m, (c) 15 mN/m, and (d) 20 mN/m. The average areas per molecule were obtained from the surface pressure versus area per molecule isotherms. The dashed lines represent the ideal curve suggested by the additivity rule.

with SA at the air-water interface. Each curve corresponds to a particular composition of the pyrene and SA mixture. A closer look at Figure 1 reveals that with increasing mole fraction of pyrene in the mixture the area per molecule of the mixed film of pyrene and SA initially increases, reaches a maximum, and then decreases. This feature is also evident from Figure 2, discussed later in this section. This seems to suggest that with increasing mole fraction of pyrene in the mixture the pyrene molecules are lost from the air-water interface. When the impossible proposition of the pyrene molecules being lost through such processes as evaporation is ruled out owing to the high boiling point of pyrene, loss of the molecules as a result of the formation of microcrystals at the air-water interface and subsequent precipitation into the bulk of the subphase due to the high hydrophobicity and low solubility of pyrene molecules in water seems quite plausible. Another possibility is that the pyrene molecules may be located within the SA matrix so as not to claim any area at the air-water interface. This is indeed possible if the pyrene molecules are located above the airwater interface sandwiched in between adjacent chains of SA. To check the possibility of the pyrene molecules being precipitated into the bulk of the water subphase, an additional experiment was performed. A bent pipette was introduced from behind the compressing belts, and a small amount of water just below the floating layer was sucked out following a standard procedure.12 A very large number

(1)

where A12 is the average area per molecule of the mixed system, N1, A1, N2, and A2 correspond to the mole fraction and area per molecule of the first and second components, respectively. The dotted line in Figure 2 corresponds to the ideal curve dictated by the above equation. The experimental curve shows a positive deviation from the ideal behavior indicating that, at all compositions of the mixture, the experimentally obtained area per molecule of the mixed system was greater than the sum of the areas of the pure components, which suggests the existence of a repulsive interaction between the SA and the pyrene molecules. In fact a similar repulsive interaction has been reported for chrysene,15 p-terphenyl,16 valinomycin,23 and biphenyl derivatives24 mixed with fatty acids. Spectroscopic Studies of Pyrene in Solution in Mixed LB Films with SA Figure 3 shows the absorption spectra of pyrene in ethanol and in LB films mixed with SA. The absorption spectrum of pyrene in ethanol consists of sharp and intense bands in the 200-350 nm spectral region and is in good agreement with that reported in the literature.1a,25 The (17) Dutta, A. K.; Misra, T. N. Opt. Mater. 1994, 3, 35. (18) Warren, J. G.; Cresswell, J. P.; Petty, M. C.; Lloyds, J. P.; Vitukhnovsky, A.; Sluch, M. I. Thin Solid Films 1989, 179, 515. (19) (a) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (b) Gaines, G. L., Jr. J. Colloid Interface Sci. 1966, 21, 314. (c) Gaines, G. L., Jr.; Bellamy, W. D.; Tweet, A. G. J. Chem. Phys. 1964, 41, 538. (d) Tweet, A. G.; Gaines, G. L. Jr.; Bellamy, W. D. J. Chem. Phys. 1964, 40, 2596. (e) Gaines, G. L., Jr. J. Chem. Phys. 1978, 69, 924. (20) Vialallonga, F. Biochem. Biophys. Acta 1968, 163, 290. (21) Ito, H.; Morton, T. H.; Vodyanoy, V. Thin Solid Films 1989, 180, 180. (22) Vodyanoy, V.; Bluestone, G. L.; Longmuir, G. L. Biochim. Biophys. Acta 1990, 284, 1047. (23) Pathirana, S.; Neely, W. C.; Myers, L. J.; Vodyanoy, V. Langmuir 1992, 8, 1984. (24) Hall, R. A.; Thistlewaite, P. J.; Greiser, F. Langmuir 1993, 9, 2128. (25) Jaffe, H. H.; Orchin, M. Theory and Applications of UV Spectroscopy; John Wiley & Sons, Inc.: New York, 1962.

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Figure 3. The normalized absorption spectra of pyrene (1 × 10-5 M) in ethanol shown by the continuous line and 10 layers of the mixed LB film of pyrene and SA (mole fraction of pyrene 0.3) shown by the dashed line.

intense set of bands in the 200-300 and 300-350 nm regions in solution corresponds to the well-identified 1Bb and 1La states of the pyrene molecule.1,32 The absorption spectra of the LB films of pyrene mixed with SA also show a similar set of bands but with a different intensity distribution and a small blue shift of about 2 nm. Such a blue shift is not readily explicable. Several workers have reported a small red shift and broadening of the absorption profile in LB films and have attributed such broadening and red shift to the formation of pyrene aggregates in the LB films.1c,26 Several other factors may also contribute to such shifts in the band positions in the absorption spectrum. It seems likely that the small blue shift may originate from the difference in polarity and refractive index between ethanol and SA, ethanol being more polar than SA. Recent studies3 have demonstrated that the photophysical properties of PAH molecules show (26) (a) Taniguchi, Y.; Mitsuya, M.; Tamai, N.; Yamazaki, I.; Masuhara, H. Chem. Phys. Lett. 1986, 132, 516. (b) Yamazaki, T.; Tamai, N.; Yamazaki, I. Chem. Phys. Lett. 1986, 124, 326. (c) Yamazaki, I.; Winnik, F. M.; Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987, 91, 4213. (d) Tsuji, I.; Fukuda, T.; Miyamoto, T.; Ito, S.; Yamamoto, M. Langmuir 1992, 8, 936. (27) (a) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology; Augenstien, L., Mason, R., Rosenberg, B., Eds.; Academic Press, New York, London, 1964; p 23. (b) Kasha, M. Spectroscopy of the Excited State; Bartolo, B. D., Eds; NATO Advanced Study Institute Series 12; New York, 1976; p 337. (c) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (28) (a) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (b) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (c) West, W.; Caroll, B. M. J. Chem. Phys. 1951, 19, 417. (29) (a) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. C. Langmuir 1988, 4, 583. (b) Miyata, A.; Heard, D.; Unuma, Y.; Higashigaki, Y. Thin Solid Films 1992, 210/211, 175. (30) (a) Zhen, Z.; Tung, C. J. Photochem. Photobiol., A: Chem. 1992, 68, 247. (b) Weinberger, R.; Clinelove, L. J. Spectrochim. Acta 1984, 40A, 49. (c) Wang, Y. M.; Kamat, P. V.; Patterson, L. K. J. Phys. Chem. 1993, 97, 8793. (d) Ruban, A. V.; Horton, P.; Young, A. J. J. Photochem. Photobiol. A 1993, 21, 229. (e) Tiddy, G. J. T.; Mateer, D. L.; Omerod, A. P.; Harrison, W. J.; Edwards, D. J. Langmuir 1995, 11, 390. (31) (a) Kitamura, T.; Takahashi, Y.; Yamanaka, T.; Uchida, K. J. Lumin. 1991, 48-49, 373. (b) Kitamura, T.; Takahashi, Y.; Yamanaka, T.; Uchida, K. Chem. Phys. Lett. 1990, 172, 29. (c) Avinir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariasse, K. A. J. Phys. Chem. 1985, 89, 3521. (d) Zilberstien, T.; Bromberg, A.; Berkovic, G. J. Photochem. Photobiol., A 1994, 77, 69. (32) (a) Itaya, A.; Kawamura, T.; Masuhara, H.; Taniguchi, Y. Chem. Lett. 1986, 1541. (b) Kunjappu, J. T.; Somasundaran, P. Langmuir 1995, 11, 428. (c) Ananthapadmanabham, K. P.; Somasundaran, P. J. Colloid Interface Sci. 1988, 122, 104. (d) Goddard, E. D.; Smith, S. R.; Kao, S. R. J. Colloid Interface Sci. 1966, 21, 320. (33) (a) Avinir, D. J. Am. Chem. Soc. 1987, 109, 2931. (b) Levitz, P.; Drake, J. M.; Klafter, J. J. Chem. Phys. 1988, 89, 5224. (c) Levitz, P.; Drake, J. M.; Klafter, Chem. Phys. Lett. 1988, 148, 557 (d) Drake, J. M.; Levitz, P.; Sinha, S. K.; Klafter, J. Chem. Phys. 1988, 128, 199.

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considerable changes due to the corresponding changes in the dielectric constant of the microenvironment in which the probe molecule is located. The fact that the shift in this case arose as a result of the polarity of the solvent was confirmed by comparing the absorption spectrum of pyrene in the LB films to that in a nonpolar solvent like cyclohexane. The shift was found to be almost negligible demonstrating that the observed shift did not originate from the aggregation of the pyrene moieties in the LB films. Although this may mean that pyrene moieties do not exist as aggregates in the LB films, the π-A isotherm measurements, emission and excitation spectra, and morphological studies, as discussed in a later section, do not support such a conclusion. Indeed a “zero” shift as a result of dipole-dipole interaction is possible and may be accounted for in terms of the exciton model. According to the exciton model27 the dipole-dipole interaction between two dipoles may result in the creation of an exciton band which may be located above or below the monomeric band. The change in energy brought about by such an interaction is expressed mathematically by the relation

∆E ) 2M2(1 - 3 cos2 Θ)/r3 where M is the dipole moment, Θ is the angle made by the dipole with the r vector, and r is the length of the vector joining the centers of the two dipoles. For 0 < Θ < 54.7° the exciton band is located energetically below the monomeric state, which gives rise to a red shift in the absorption spectra. On the other hand for 54.7° < Θ < 90° the exciton band is located above the monomer band and this gives rise to a blue shift in the absorption spectrum. At the magic angle of 54.7°, no shift is observed irrespective of the magnitude of r. When the alignment of the dipole moments in the aggregates is such that 0 < Θ < 54.7°, the aggregates are referred to as J-type aggregates.28 In the extreme case, i.e., Θ ) 0, the arrangement is a head to tail type of configuration. However when the alignment of the dipole moments in the aggregates is such that 54.7° < Θ < 90°, the aggregates are referred to as H-type aggregates,28 which in the extreme case, i.e., Θ ) 90°, corresponds to a deck of cards type packing. Aggregates corresponding to Θ having values in between 54.7° and 60° are referred to as the intermediate or I-type aggregates.29 The formation of such I-aggregates has been reported for octadecylrhodamines and spiropyrans.29 As will be shown below the presence of the partial excimers strongly supports the existence of I-aggregates in the pyrene-SA mixtures. Figure 4 shows the emission spectra of pyrene in ethanol and in LB films mixed with SA at room temperature. At low concentrations, the emission spectrum in ethanol consists of the well-known sharp and intense vibrational bands of the pyrene monomer in the spectral region 370420 nm with its 0-0 band at 372.5 nm. With increasing concentration of pyrene a broad and structureless band appears in the long wavelength region with its peak at 470 nm as shown in Figure 4, corresponding to the much studied excimer fluorescence band of pyrene originating from a face to face overlap of the pyrene moieties with the average center to center distance of separation equal to 3 Å. The fluorescence spectrum of the LB film of pyrene mixed with SA is found to be quite different from the emission spectrum in ethanol. It shows a single broad and structureless band with its maxima at about 420 nm, which is different from the well-known excimer band at 470 nm. Interestingly, unlike in solution no new features were observed in the emission spectra of the mixed films

Nonamphiphilic Pyrene Assembled in LB Films

Figure 4. Fluorescence emission spectra of pyrene in ethanol (1 × 10-5 M) shown by the dashed line, emission spectra of a concentrated solution of pyrene in ethanol (1 x10-4 M) shown by the dashed and dotted line, and 10 layers of the pyrene SA mixture (mole fraction 0.3) LB film at room temperature shown by a continuous line. Excitation was provided at 300 nm.

with increasing mole fraction of pyrene. Wistus et al.12 have reported a similar emission band for pyrene assembled in LB films mixed with eicosanoic acid. Moreover, this emission spectrum was reported to change with time and, after a sufficiently long period of time, the monomer fluorescence of pyrene can be observed. Such a behavior has been attributed to the breaking up of the pyrene clusters and redistribution of the pyrene moieties as monomeric species in the LB films. However, we have failed to reproduce such observations made by Wistus et al.12 The LB films, in our conditions, showed no change even after several weeks. The emission band of pyrene in the LB films at 420 nm may be assigned to a higher energy excimer species, the configuration of which corresponds to a partial overlap of the pyrene moieties with one of the aromatic rings overlapping. The existence of such a band has been reported in time-resolved1c,26 experiments. In the 0-50 ps time window, the partial excimer at 420 nm dominates over the 470 nm excimer. After this time interval both the excimers appear, while only the 470 nm excimer band survives in the longer time domain. The above observations clearly indicate that the packing of the pyrene molecules in the aggregates formed in the LB films is completely different from that formed in ethanol. This result demonstrates that the interaction of molecules with their environment plays a dominant role in affecting the self-assembling forces that are believed to be responsible for self-organization of molecules into specific crystallographic configurations that are characteristic of the specific molecules. Spectroscopic Studies of Pyrene in Ethanol-Water Mixtures As mentioned earlier, aggregation of pyrene molecules seems to strongly affect the photophysical properties of the molecule. In an attempt to investigate the role of aggregation and self-assembly in modifying its photophysical properties, we have opted for an alternate wellestablished elegant method of producing aggregates.30 It consists of adding water to a solution of pyrene in ethanol to produce aggregates by enhancing the hydrophobic

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Figure 5. Normalized fluorescence emission spectra of pyrene in pure ethanol (1 × 10-5 M) and in ethanol-water mixtures at different volume fractions of water 0.5, 0.7, 0.8, and 0.9 as indicated in the figures. The emission intensities were normalized to be equal to the strongest band. Excitation was provided at 300 nm.

Figure 6. Ratio of the partial excimer to the monomer emission intensities (IE/IM) versus the volume fraction of water (Φ).

interaction between the pyrene molecules and the medium and, at the same time, to lower its solubility in the mixture. Figure 5 shows the emission spectra of pyrene in pure ethanol and in the ethanol-water mixture containing different volume fractions of water. In pure ethanol, a highly structured emission band system corresponding to the monomer fluorescence of pyrene appears while a broad band with its fluorescence maxima at 450 nm is observed in an ethanol-water mixture for the higher volume fraction of water. Although the appearance of this band seems to be inexplicable, it is quite possible that such a band may originate from a partial excimer of pyrene. It seems plausible that, as a result of self-assembly, the pyrene molecules are oriented in such a manner that a partial overlap is favored instead of the usual sandwich type overlap possible in crystals. In fact the formation of such organized clusters is possible and has been reported very recently for polyenes30d and cyanine dyes.30e The effects due to the formation of clusters may be perhaps best understood if we plot the ratio of the partial excimer intensity to the monomer intensity (IE/IM) versus the volume fraction of water in the ethanol-water mixture as shown in Figure 6. It was found that the IE/IM ratio shows a sharp change at a volume fraction (Φ ) 0.75 + 0.03) of water in the ethanol-water mixtures. The corresponding concentration may be defined as a critical aggregation concentration30a for pyrene in ethanol-water mixtures. The existence of a similar critical concentration has been observed for chrysene15 and p-terphenyl.16 An increase in the IE/IM ratio implies a relative increase in

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Figure 7. The fluorescence excitation spectra of pyrene in an ethanol-water mixture with the volume fraction of water being 0.9 monitored at 379 nm (- - -) and at 450 nm partial excimer emission (- ‚ -). The continuous line represents the excitation spectrum of 10 layers of pyrene-SA mixture in the LB films monitored at the partial excimer fluorescence (420 nm) at room temperature.

the population of the excimeric species compared to the monomeric species. Such an increase in the IE/IM ratio may result from the formation of excimer species or efficient energy transfer from the monomeric species to the excimeric species. Since excimeric states in pyrene molecules are generated as a result of the close approach of the pyrene moieities on the order of 3 Å the manifestation of excimer emission in pyrene confirms aggregation.1c It is worthy of mention here that, while in ethanol solution the formation of excimer with its band maximum at 470 nm suggests a face to face overlap of the pyrene monomers, in ethanol-water mixtures the formation of excimer emission with its band maximum at 450 nm suggests a different geometrical configuration of the pyrene molecules in the aggregates formed. In the LB films no monomer fluorescence emission is observed; only the partial excimer band with its maximum at 420 nm is observed, which implies efficient energy transfer from the monomeric species of pyrene to the partial excimeric species. One possibility for such an efficient energy transport process could be a favorable and compact packing of the pyrene moieties in the LB films. As mentioned in an earlier section it seems likely that due to the different microenvironment in the LB films the self-assembling forces between the pyrene molecules that tend to aggregate the molecules are so affected that a partial overlap of the pyrene rings is preferred to a complete overlap configuration possible in ordinary crystalline pyrene and aggregates in pure ethanol. The fact that the excimers formed in pure ethanol, in ethanol-water mixtures, and in LB films are different and located at 470, 450, and 420 nm clearly establishes the role of the microenvironment in affecting the self-assembling processes that give rise to the aggregates. Excitation Spectra Figure 7 shows the excitation spectra of pyrene monitored at its fluorescence maxima in ethanol, ethanolwater mixtures, and LB films mixed with SA. The excitation spectra of pyrene in ethanol with the emission monitored at the monomer maxima band at 377 nm as well as the excimer maxima at 470 nm are found to be

Dutta et al.

identical and give rise to a structured spectrum in the 200-400 nm region which is in good agreement with the reported data.1c The excitation spectra of the emission monitored at 450 nm corresponding to the partial excimer maximum in the ethanol-water mixtures sho distinct broad bands in the 200-400 nm region. The 0-0 band of the excitation spectra in the ethanol-water mixtures is located at 385 nm, which is red-shifted by about 8 nm relative to the 0-0 band at 377 nm in pure ethanol. The broadening and red shift of the excitation spectra provide compelling evidence of preassociation of the pyrene moieties1c in the ground state which is justified in view of the strongly hydrophobic nature of the pyrene molecules which form aggregates. It may be relevant to mention that pyrene incorporated in systems representing restricted geometries1c,31 has shown similar red shifts and changes in the shape of the spectral profile of the excitation spectra compared to that in solution that has been attributed to the preassociation of the pyrene molecules in such systems. Furthermore, the shape and intensity distribution of the bands in the excitation spectra is dependent on the concentration of pyrene, the pH, and the geometrical details of the microenvironment in which it is located. The excitation spectrum of the LB films of pyrene and SA monitored at the excimer fluorescence maximum at 420 nm shows a very broad band spectra in the 250-400 nm region with the 0-0 band located at 395 nm. Such a broad and diffuse structure of the band spectra associated with a red shift of the 0-0 band in the LB films seems justified and suggests aggregation in the LB films. Dissolution of the LB films in ethanol reproduced the emission and excitation spectra in ethanol, confirming that the LB emission was genuine and did not originate from some artifact or impurities. A comparative study of the excitation spectra shown in Figure 7 clearly demonstrates that the aggregated species formed in the three different systems are completely different, which is supported by the difference in the emission spectra as discussed earlier. Since the excitation spectra provide information on the ground states of the molecules, the large differences in the excitation spectra in the three systems reveal that the interaction between the pyrene moieties in the ground state is different and is plausibly brought about by the difference in the structures of the organized aggregates of pyrene attained in the three different systems brought about by the interactions between the molecules and their microenvironment. Scanning Electron Microscopy Figure 8 shows a scanning electron micrograph of pyrene assembled in LB films mixed with SA. The clusters with sharp and distinct edges correspond to the three dimensional (3D) pyrene aggregates formed in the LB films. The smooth background corresponds to the SA matrix. In fact, the formation of aggregates as seen in the scanning electron micrograph seems to be well-justified by the π-A isotherms as well as the spectroscopic results. As mentioned earlier, it seems likely that the dissimilar physical and chemical properties of the pyrene and SA molecules and the strong interaction between similar molecules result in phase separation of the components that leads to the formation of minute aggregates which act as nucleating sites for the molecules to self-assemble into crystalline domains. Brewster angle microscopy (BAM),34 transmission electron microscopy (TEM),35 and electron diffraction36 studies have confirmed the formation of crystalline domains in the LB films. The formation of (34) (a) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (b) Siegel, S.; Honig, D.; Volhardt, D.; Mobius, D. J. Phys. Chem. 1992, 96, 8157. (c) Retter, U.; Volhardt, D. Langmuir 1993, 9, 2478.

Nonamphiphilic Pyrene Assembled in LB Films

Figure 8. A scanning electron micrograph of 10 layers of the pyrene-SA mixture (mole fraction of pyrene is 0.3) LB films at room temperature.

distinct crystalline domains of pyrene as evidenced from the scanning electron micrograph (SEM) provides compelling visual evidence of aggregation of the nonamphiphilic molecules in the LB films that also supports the conclusions drawn from spectroscopic studies. Conclusions In conclusion, this study shows that pure pyrene molecules can be easily incorporated in LB films mixed with SA. The π-A isotherm studies indicate a repulsive type interaction between pyrene and SA molecules. It is likely that the strong repulsive interaction between the pyrene and SA molecules and the strong attraction (35) (a) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. (b) Gavish, M.; Popovitz-Biro, R.; Lahav, M.; Leisorowitz, L. Science 1990, 250, 973. (c) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9. (36) (a) Kirstein, S.; Mohwald, H. Chem. Phys. Lett 1992, 189, 408. (b) Kirstein, S.; Steitz, R.; Garbella, R.; Mohwald, H. J. Chem. Phys. 1995, 103, 818. (c) Kirstein, S.; Mohwald, H. J. Chem. Phys. 1995, 103, 826.

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between the similar components of the mixed film result in a phase separation of the components into distinct domains that self-assemble into microcrystalline aggregates. The negligible shift in the absorption spectrum of the LB films of pyrene compared to that in solution suggests the formation of I-aggregates in the LB films. The appearance of the new band at 420 nm in the LB films of pyrene suggests the formation of partial excimers produced as a result of a partial overlap of the pyrene rings. In addition, the fluorescence studies of pyrene in ethanol-water binary mixtures reveal the formation of another partial excimer with its maximum at 450 nm clearly demonstrating that different partial excimers are possible that depend on the extent of overlap of the pyrene moieties and the arrangement of the pyrene moieties in the aggregates that is largely dependent on the microenvironment in which the pyrene moieties are located. Moreover, these findings demonstrate that the selfassembling forces that are responsible for the formation of aggregates are largely affected by the interaction of the molecules with their microenvironment that results in large changes in the organization of the molecules in the aggregates that are reflected as large changes in the spectra. It is worthwhile to mention here that these partial excimers detected in time-resolved studies are readily manifested in the steady state. A comparative study of the excitation spectra of the emissions monitored at the excimer maximum in the ethanol-water mixtures and LB films showed large changes with respect to the excimer in ethanol. The large differences observed in the intensity distribution and position of the 0-0 bands suggest different interactions and configuration of the ground states of the pyrene molecules that are dependent on the microenvironment in which the molecules are located. Scanning electron micrographs confirm the existence of organized clusters of pyrene in the LB films. Acknowledgment. The authors express their thanks to the Department of Science and Technology, Government of India, for financial support. LA940678Q