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Langmuir 1998, 14, 3036-3040
Spectroscopic Studies of Nonamphiphilic Coronene Assembled in Langmuir-Blodgett Films: Aggregation-Induced Reabsorption Effects A. K. Dutta† Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received October 9, 1997. In Final Form: March 13, 1998 This paper reports the incoroporation of large flat molecules of coronene in Langmuir-Blodgett (LB) films mixed with stearic acid (SA). The spectroscopic characteristics of these films have been investigated here. An outstanding feature of this study is the appearance of the S1-S0 absorption band at 420 nm in the LB films. Interestingly, in solution this band is never observed owing to this transition being a electricdipole forbidden arising from the D6h symmetry of the molecule. Spectroscopic studies of these mixed LB films of coronene and SA indicate the formation of aggregates. The spectroscopic characteristics of the aggregates formed in the LB films were found to be identical to those formed in a binary solvent mixture of ethanol and water that confirm the formation of microcrystallites in the mixed LB films and binary solvent mixtures. A comparative study of the fluorescence excitation spectra of the aggregates formed in the binary solvent mixtures and in the LB films reveals the hidden S0-S1 transition in LB films in contrast to that in solution where the S0-S1 transition is not manifested. Furthermore, the excitation spectra of the aggregates formed in different systems were found to be different suggesting different packing configuration of the molecules in the aggregates formed in different systems.
Introduction In recent years, considerable attention has been focused on the study of photofunctional groups or chromophores incorporated in ultrathin supramolecular assemblies as these studies are deemed important in the design of ultrafast, miniaturized, optoelectronic and photonic devices.1 Fabrication of ultrathin monomolecular thick films by the Langmuir-Blodgett (LB) technique is a simple and unique method that provides flexibility in controlling the spatial distribution as well as the orientation of the molecules in these films.2 These features allow tailoring of the optical and electronic properties of the films that is of vital importance in the design of photonic devices.1,2 Although considerable efforts have been made to study amphiphilic molecules assembled in LB films, few studies on nonamphiphilic molecules are available despite the fact that they may be incorporated in LB films when mixed with a fatty acid. Using nonamphiphilic molecules in place of their amphiphilic counterparts is a definite advantage as large scale industrial applications are easy and costeffective and frequently encountered difficulties in chemical synthesis are eliminated. Coronene is a well-known member belonging to the family of condensed polyaromatic hydrocarbon macrocyclic † Present address: Dr. Ashim Kumar Dutta, Departement de Chimie-Biologie, Universite du Quebec a` Trois-Rivieres, C.P. 500, Trois-Rivieres, Quebec, Canada G9A 5H7. Tel +1-819-376-5077 ext 3340. Fax +1-819-376-5057.
(1) (a) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Isrealachvili, J.; McCarthy, T. J.; Murray, R.; Pease, F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 392. (b) Prasad, P. N.; Willams, D. J. Nonlinear Optical Effects in Molecules and Polymers: John Wiley and Sons. Inc.: New York, 1991. (c) Nolte, D. D. Photorefractive Effects and Materials; Kluwer Academic Publishers: London, 1995. (d) Burghard, M.; Fischer, C. M.; Schmeizer, M.; Roth, S.; Hanack, M.; Gopel, W. Chem. Mater. 1995, 7, 2104. (2) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett Films to Self-Assemblies: Academic: New York, 1991. (b) Gaines, G. L., Jr. Insoluble Monolayers at the Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966. (c) Mobius, D. Acc. Chem. Res. 1981, 14, 63.
ring systems and is characterized by an intense structured fluorescence (φf ) 0.33) in the green region of the visible spectrum, a reasonably long fluorescence lifetime (τf ) 55 ns) that makes it an excellent fluorescent molecular probe as demonstrated in several studies.3 Moreover, the rigid and planar structure of coronene with D6h symmetry makes it a unique test system for modeling and calculating the molecular structure of other similar planar molecules using the Hu¨ckel or ab initio molecular orbital quantum theoretical calculations.4 Coronene is also known to form monoclinic crystals where the molecules are stacked along the b-axis with an interplanar separation of 0.34 nm that is of special interest as exction migration in such systems is one-dimensional.5 Interestingly, coronene undergoes solid state carbonization forming highly anisotropic coke without attaining an intermediate mesophasic state.6 This coke so obtained contains graphite layers that are oriented as in the original crystal ensuring high electrical conductivity of the films making them attractive for thin film gas sensors. Detailed studies have demonstrated that coronene molecules deposited on MoS2 substrates lay flat on the substrate and scanning tunneling microscopic (3) (a) Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: London, 1970. (b) Ham, N. S.; Ruedenberg, K. J. Chem. Phys. 1956, 25, 1. (c) Ham, N. S.; Ruedenberg, K. J. Chem. Phys. 1956, 25, 13. (d) Nakatani, S.; Nakamura, T.; Mizuno, K.; Matsui, A. H. J. Lumin. 1994, 58, 343. (4) (a) Ohno, K.; Sinohara, H. J. Mol. Struct. 1995, 352/353, 475. (b) Orlandi, G.; Zerbetto, F. Chem. Phys. 1988, 123, 175. (c) Gutman, I.; Cyvin, J. S. Introduction to the Theory of Benzoid Hydrocarbons; Springer-Verlag: New York, 1989. (d) Gutman, I., Cyvin, J. S., Eds. Advances in the Theory of Benzoid Hydrocarbons. Top. Current Chem. 1990, 153, 1. (e) Herndon, W. C.; Nowak, P. C.; Connor, D. A.; Lin, P. J. Am. Chem. Soc. 1992, 114, 41. (5) (a) Robertson, J. M.; White, J. G. J. Chem. Soc. 1945, 607. (b) Matsui, A. H.; Mizuno, K. J. Phys. 1993, B242. (6) (a) Goddard, R.; Haenel, M. W.; Herndon, W. C.; Kruger, C.; Zander, M. J. Am. Chem. Soc. 1995, 117, 31. (b) Zander, M. Proceedings of the 5th International Carbon Conference (Carbon 92), Essen, FRG June 22-26, 1992, pp 11-13. (c) Naarmann, H. J. Polym. Sci. Polym. Symp. 1993, 75, 53. (d) Mycielski, W.; Staryga, E.; Kasica, H., Lipinski, A. Thin Solid Films 1994, 238, 266.
S0743-7463(97)01103-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998
Nonamphiphilic Coronene Assembled in LB Films
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(STM) studies have confirmed epitaxial deposition of the molecules into stacked structures.7 These features as well as the rigid, extended, planar structure of the coronene molecule representing a graphite fragment and its enormous propensity to be stacked into a one-dimensional (1D) solid facilitating soliton conduction make coronene attractive for designing opto- and microelectronic devices. In this paper, we report the spectroscopic properties of the LB films of coronene and stearic acid (SA) transferred on to quartz substrates. Interest in nonamphiphilic coronene stems from our previous and continuing efforts to study such nonamphiphilic polyaromatic compounds in LB films and to understand better the role of aggregation and its manifestation in their spectroscopic characteristics. Experimental Section Coronene was purchased from Aldrich Chemical Co., Milwakee, WI, and recrystallized six times from benzene after careful vacuum sublimation. SA was obtained from Sigma Chemical Co., Inc., St. Louis, MO, and was used as received. A commercially available LB trough, Joyce-Loebl IV manufactured by JoyceLoebl, Inc., Newcastle upon Tyne, U.K., was used for studying the behavior of pure and mixed films of coronene and SA at the air-water interface. A filter paper Wilhelmy balance was used for the measurement of surface pressures at the air-water interface with an accuracy of (0.1 mN/m. The surface pressure versus area per molecule isotherm data was acquired by a computer and processed using software provided by Joyce-Loebl, Inc. This computer also controlled the movements of the moving barrier at the air-water interface with an accuracy of (0.1 mN/ m. Triple distilled water deionized by a Milli-Q plus water purification system having a pH of 5.6 in equilibrium with atmospheric carbon dioxide and resistivity equal to 18.2 MΩ cm was used as the subphase. Monolayers of a mixed film of coronene and SA were formed at the air-water interface by spreading about 100 µL of a chloroform solution of coronene and SA mixed in a predetermined ratio. After allowing 20 min for the solvents to have evaporated, the film formed at the air-water interface was compressed continuously at a rate of 3 × 10-3 nm2 mol-1 s-1. Mixed films of coronene and SA were transferred on to quartz substrates by vertically dipping a slide through the monolayer at a low speed of 1 mm/min. A drying time of about 10 min was allowed between sucessive dipping cycles, and the film transfer ratio for Y-type deposition of the mixed films was estimated to be 0.93. The transfer ratio of the films was calculated from the ratio of the decrease in the area of the mixed film at the airwater interface to the area of the substrate coated with the mixed monolayer. Absorption and emission spectra were recorded on a Shimadzu 2010 UVPC and a Perkin-Elmer MPF-44A spectrofluorometer, respectively. For solutions, standard cuvets were used and for LB films deposited on quartz slides special holders that held the film at an angle of 45° to the source and the photomultiplier was used. Narrow band-pass filters (5 nm) were used for excitation to reduce the effects of scattering. Fluorescence microscopic studies of the transferred LB films were performed on a Biorad confocal microscope using a heliumcadmium laser that provided excitation at 388 nm. Photographs were recorded on a Nikon camera coupled to the microscope.
Results and Discussion Spectroscopic Studies of Coronene in Solution and in the Langmuir-Blodgett Films. Figure 1 shows the absorption spectrum of coronene in ethanol (1 × 10-3 M) and 10 layers of a mixed LB film of coronene with SA (molar ratio 1:10). In ethanol, the absorption spectrum of coronene in the 250-500 nm region consists of two distinct band systems, one in the 280-340 nm region (7) (a) England, C. D.; Collins, G. E.; Schuerlein, T. J.; Armstrong, N. R. Langmuir 1994, 10, 2148. (b) Zimmerman, U.; Karl, N. Surf. Sci. 1992, 268, 296. (c) McKinnon, A. W.; Welland, M. E.; Warren, S. J. D. Thin Solid Films 1995, 257, 63.
Figure 1. Absorption spectrum of coronene in chloroform (4 × 10-3 M) shown by a dashed line and in mixed LB films with SA (10 layers, molar ratio 1:20) deposited on quartz substrates at a surface pressure of 25 mN/m and shown by a continuous line. Inset shows the molecular structure of coronene.
corresponding to the S3-S0 transition and the other in the 330-360 nm region corresponding to the S2-S0 transition with the 0-0 band located at about 348 nm, which is in excellent agreement with reported data.7 Interestingly, the lowest transtion S1-S0 is absent, being an electric-dipole-forbidden transition arising from the D6h symmetry of the coronene molecule.8 The LB film absorption spectral profile in the 250-500 nm as shown in Figure 1 is extremely broad and diffuse and bears little similarity with the absorption spectrum of coronene in solution. Dissolution of the mixed LB films in chloroform reproduced the solution absorption spectrum confirming that the observed effects did not arise from impurities or from photoproducts. The diffuse spectral profile and extreme broadening of the LB film absorption spectrum clearly indicates strong ground-state dipoledipole interaction between the coronene molecules assembled in the LB films suggesting organized aggregation of the coronene molecules. In fact, such strong interactions are expected to arise from the large extended structure of the coronene molecules. Similar results have been obtained for other large molecules, namely, pyrene, perylene, and their derivatives. Perhaps, the most interesting feature observed in the LB film absorption spectrum is the appearance of prominent bands with their peaks located at 360 and 420 nm, respectively, which plausibly correspond to the origin of the S2-S0 and S1-S0 transitions of the coronene molecule. Quantitative estimates of the strength of the various transition dipole moments obtained from the broadening of the absorption bands was calculated from the oscillator strength of the various transitions using the expression9
f ) (4.3 × 10-9)
∫ (ν) dν ) (4.3 × 10-9)max(∆ν)1/2
where (ν) corresponds to the molar extinction coefficient at the frequency ν while max is the molar extinction coefficient of the molecule corresponding to the absorption (8) (a) Ho, C. J.; Babbitt, R. J.; Topp, M. J. Phys. Chem. 1987, 91, 5599. (b) Tucker, S. A.; Acree, W. E., Jr.; Fetzer, J. C. Appl. Spectrosc. 1995, 49, 8. (c) Ohno, K.; Inokuchi, H. Chem. Phys. Lett. 1973, 23, 56. (d) Ohno, K.; Kajiwara, T.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1972, 45, 996. (e) Ohno, K. Chem. Phys. Lett. 1980, 70, 526. (f) Lamotte, M.; Merle, A. M.; Jussot-Dubien, J.; Dupuy, F. Chem. Phys. Lett. 1975, 35, 410. (g) Pitts, W. M.; Merle, A. M.; El-Sayed, M. A. Chem. Phys. Lett. 1979, 36, 437. (h) Pfister, C. Chem. Phys. 1973, 2, 181. (9) Turro, N. J. In Modern Molecular Photochemistry; The Benjamin/ Cummings Publishing Co., Inc.: London, 1936.
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band maximum (νmax) and (∆ν)1/2 is the full width at halfmaximum (fwhm) of the absorption band. The ratio of oscillator strengths for the S2-S0 and S3-S0 transitions expressed as f(S2-S0)/f(S3-S0) is 0.39 in the LB films compared to 0.15 in solution. Such a large difference may be, at first sight, attributed to the deformation of the coronene molecules. However, in view of the rigid and planar structure of coronene, such a structural deformation seems improbable. It seems likely that as a result of organized aggregation, large changes in the profile of the FranckCondon potential well occur, probably making the forbidden or weak transitions become partially allowed resulting in the manifestation of the S1-S0 absorption band in the LB films in contrast to that in solution. In this context, it must be mentioned that similar observations have been reported for small rigid molecules such as naphthalene and pyrene where the weak S1-S0 absorption bands are observed to be enhanced when these molecules are incorporated in the LB films.10 In case of the polyphenyls,11 the 1Lb transiton is weak and is reported to be hidden and is never manifested in the solution absorption spectrum. This transition is however readily observed in the LB film which is attributed to the deformation of the molecules in the LB films in compliance with the energetic, entropic, and geometric requirements.11 Given these facts, it does not seem unresonable that the observed band at 420 nm, which was totally absent in the solution absorption spectrum, is actually the S1-S0 band that becomes manifested owing to an ordered organization of the coronene molecules in the LB film. However, it is unlikely that the coronene molecule undergoes deformation owing to its highly stable and rigid molecular structure, but it is possibe that due to organized aggregation, the dipoledipole interactions between the molecules perturb the Franck-Condon profiles resulting in the forbidden transitions becoming allowed. Figure 2 shows the fluorescence emission spectrum of coronene in chloroform and in a LB film (10 layers) mixed with SA. The solution emission spectrum in the 400-550 nm spectral region is highly structured with the 0-0 band corresponding to the S1-S0 transition located at 426 nm and is in excellent agreement with reported data.8 The LB film emission spectrum appears to be broadened and red shifted with respect to the solution emission spectrum indicating organized aggregation of the coronene molecules in the LB film. To elucidate the nature of aggregates formed in the LB film, we have compared the LB film emission spectrum with the emission spectrum of coronene microcrystals. Interestingly, the emission spectrum of the coronene microcrystals was found to be identical with that obtained for the mixed LB film confirming that the broad emission band observed originated from the microcrystals formed in the LB film. In addition to this evidence of molecular aggregation of the coronene chromophores in the LB film, visual evidence of the existence of macroscopic aggregates is obtained from the fluorescence micrographs of the LB film (Figure 3) that reveals the presence of large needlelike crystals. Identical results have been reported for other polyaromatic hydrocarbons. Formation of similar crystallites both at the air-water interface and in transferred films have been observed by (10) (a) Dutta, A. K.; Ray, K.; Mandal, T. K.; Haque, Md. E.; Misra, T. N. Opt. Mater. 1995, 4, 609. (b) Mathauer, K.; Frank, C. Langmuir 1993, 9, 3002. (c) Frank, C. W.; Gashgari, M. A. Macromolecules 1979, 12, 163. (d) Winnik, F. M. Chem. Rev. 1993, 93, 587. (e) Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1996, 12, 459. (11) (a) Dutta, A. K, J. Phys. Chem. B 1997, 101, 569 (b) Dutta, A. K. J. Phys. Chem. 1995, 99, 14758. (c) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 12844.
Dutta
Figure 2. Fluorescence emission spectra of coronene in chloroform (4 × 10-3 M) shown by a dashed line, excitation wavelength λexc ) 300 nm. Fluorescence emission from 10 layers of the mixed film of coronene with SA (molar ratio 1:20) deposited on quartz substrates at a surface pressure of 25 mN/m is shown by a continuous line, excitation wavelength λexc ) 300 nm. The spectrum shown by filled triangles corresponds to the fluorescence emission spectrum of microcrystals of coronene corresponding to λexc ) 300 nm.
Figure 3. Fluorescence confocal micrograph of a mixed film of coronene and SA (10 layers, molar ratio 1:20) deposited on quartz substrates at a surface pressure of 25 mN/m. Excitation of these films was provided with the 388 nm line of an heliumcadmium laser (5 mW). (Scale bar ) 10 µm).
several authors using different experimental techniques.12 In this context, it must be mentioned that the formation of crystallites is not unexpected owing to the limited solubility of the coronene molecules in the SA microphase due to the enormous differences in their physical and chemical properties that cause phase separation of the components in the mixed film. Formation of crystals are facilitated by the fact that the SA-SA and the coronenecoronene interactions are much stronger over the SAcoronene interactions. With pressure, the phase-separated molecules form microscopic crystallites that evolve in time to form large crystals as revealed in Figure 3. (12) (a) Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1996, 12, 459. (b) Dutta, A. K. Solid State Commun. 1996, 97, 785. (c) Dutta, A. K., Misra, T. N.; Pal, A. J. Solid State Commun. 1996, 99, 767. (d) Dutta, A. K. Langmuir, in press. (e) Weiss, R. M.; McConnel, M. Nature 1984, 310, 47. (f) Berg, B. Nature 1991, 322, 350. (g) Kirstein, S.; Mohwald, H. Chem. Phys. Lett. 1992, 189, 408. (h) Kirstein, S.; Mohwald, H. J. Chem. Phys. 1995, 103, 826. (i) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (j) Honig, D.; Mobius, D. J. Phys. Chem. 1992, 96, 8157.
Nonamphiphilic Coronene Assembled in LB Films
Figure 4. Fluorescence emission spectra of coronene in binary solvent mixtures of ethanol and water corresponding to different volume fractions of water in the mixture: Vf ) 0.2, long broken line; Vf ) 0.6, dashed line; Vf ) 0.8, filled circles; Vf ) 0.9, unfilled circles. Excitation wavelength λexc ) 300 nm.
To understand the role of aggregation and its influence on the absorption and fluorescence emission characteristics, we have closely monitored the absorption and emission spectra of coronene in different compositions of a binary solvent mixture of ethanol and water. In this connection it may be mentioned that aggregation may be induced by binary solvent mixtures consisting of two different solvents, one being an excellent solvent while the other is a poor solvent for the solute. Increasing the volume fraction of water in the mixture decreases the solubility of the coronene chromophores causing crystallization of the coronene molecules in the binary solvent mixture. In fact, it has been demonstrated elegantly that organized aggregates are formed in binary solvent mixtures and the extent of aggregation may be controlled by varying the volume composition of the ethanol-water ratio.13 Figure 4 shows the emission spectra of coronene in different compositions of the binary solvent mixture of ethanol and water. It is observed that with increasing volume fraction of water (vf) in the binary solvent mixture, the intensity of the high-energy emission bands decreased and dissappeared altogether at vf g 0.9 while the intensity of the low-energy emission bands increased, which is diagnostic of aggregation-induced reabsorption effects.11,12a-d The emission spectrum of coronene in binary solvent mixtures at vf g 0.9 is found to be identical with the emission spectrum of coronene in the mixed LB films. At low volume fractions of water of vf e 0.4 the emission spectrum corresponds to that in ethanol. The excellent agreement between the emission spectrum of coronene in the LB films and in microcrystallites and the observed differences with the solution emission spectrum may be attributed to aggregationinduced reabsorption effects.11,12a-d Figure 5 shows the excitation spectra of coronene in ethanol, binary solvent mixtures, LB films, and microcrystals. On monitoring the S1-S0 emission band at 420 nm for coronene in solution, the excitation spectrum shows an intense and structured profile with the 0-0 band at 348 nm that is in good agreement with the 0-0 band corresponding to the S2-S0 transition. This feature clearly demonstrates that the S1-S0 absorption transition in solution is forbidden and the S1-S0 emission originates (13) Zhen, Z.; Tung, C. J. Photochem. Photobiol., A 1992, 68, 247.
Langmuir, Vol. 14, No. 11, 1998 3039
Figure 5. Excitation spectrum of coronene in chloroform (4 × 10-3 M) shown by a long broken line with the emission monitored at λem ) 475 nm. Excitation spectrum of the mixed LB film (10 layers, molar ratio 1:20) shown by a continuous line corresponding to emission λem ) 510 nm. Excitation spectrum of coronene in binary solvent mixture of ethanol and water Vf ) 0.8 indicated by filled circles and λem ) 510 nm. Excitation spectrum of the microcrystals of coronene indicated by triangles corresponding to emission λem ) 510 nm.
solely as a result of energy cascading from the higher singlets (Sn, Sn-1, ..., S3, S2) to S1 eventually in accordance with Kasha’s rule. The LB film excitation spectrum obtained by monitoring the emission maximum is broad and diffuse with the 0-0 band located at 435 nm, which is consistent with the emission and absorption data. These features clearly demonstrate that as a result of aggregation the forbidden transition becomes allowed as also observed in the absorption spectrum. The excitation spectrum of coronene microcrystals shows an intense broad band at about 460 nm. The large differences between the excitation spectrum of coronene in the LB films and the microcrystals clearly demonstrate that despite the similarity in the emission spectra, the organization and packing of the molecules in the microcrystals and in the LB films are different. Conclusions Briefly, this work describes the spectroscopic properties of coronene mixed with SA and deposited onto solid substrates as LB films. A comparative study of the absorption spectra of coronene in solution and in LB films demonstrates that while the absorption spectrum of coronene in solution is characterized by sharp structured bands, the LB film spectrum is broad and diffuse indicating aggregation of the coronene moieties. This seems reasonable owing to the low solubility of the coronene molecules in the SA matrix. Perhaps, the most interesting feature revealed in this work is the manifestation of an electric dipole forbidden S1-S0 absorption band located at 420 nm, in the LB films, that may be attributed to the aggregation of the coronene molecules. The S1-S0 fluorescence emission from coronene in solution is observed to be structured with the 0-0 band located at 420 nm. In the LB film, the fluorescence emission spectrum of coronene is red shifted relative to that in solution and is attributed to aggregation-induced reabsorption effects. Inducing aggregation of the coronene molecules by increasing the volume fraction of water in binary solvent mixtures of ethanol and water clearly revealed quenching of the high-energy vibronic bands and enhancement of the low-energy vibronic bands, which are diagnostic of aggregation-induced reabsorption effects. At large volume fractions of water (vf > 0.9) in the binary solvent mixture,
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the high-energy emission bands are observed to be completely supressed while the low-energy bands are enormously enhanced and are in excellent agreeement with the LB film emission spectrum that confirms the observed red shift to be due to aggregation-induced reabsorption effects. Confocal microscopy provides visual evidence of the existence of large macroscopic crystallites in the LB film. Monitoring the fluorescence emission maximum of coronene in solution, the excitation spectrum obtained failed to reveal the band corresponding to the S1-S0 transition that suggests efficient energy transfer from S3 and S2 to S1 and decay therefrom to the ground
Dutta
state eliminating the possibility of direct excitation of the S1 states through absorption. In the case of the LB films, a weak band corresponding to the S1 state is observed that suggests direct excitation of the S1 state in addition to energy cascading from the higher singlets S3 and S2 to S1. Furthermore, the excitation spectra of the crystallites formed in the LB films and in the binary solvent mixtures were found to be completely different, which suggests different packing configurations of the coronene molecules in the aggregates formed in the different systems. LA9711037
