Langmuir 1996, 12, 5909-5914
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Spectroscopic Study of Nonamphiphilic Carbazole 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 May 28, 1996. In Final Form: September 3, 1996X Mono- and multilayers of carbazole mixed with stearic acid (SA) have been prepared by the LangmuitBlodgett (LB) technique. Surface pressure studies at the air-water interface show that pure carbazole does not form a stable film and collapses at a low surface pressure of about 12 mN/m only. However, carbazole mixed with SA forms excellent films that may be easily transferred onto solid supports. The surface pressure versus area per molecule isotherm studies show that the average area per molecule initially increases and then decreases with increasing mole fraction of carbazole. These results indicate that the carbazole moieties are accommodated in the SA matrix at low concentrations but with increasing concentration are very likely squeezed out of the air-water interface to remain sandwiched in between the fatty acid chains and on the surface of the monolayer. The positive deviation of the observed area per molecule from the ideal curve corresponding to the additivity rule suggests a repulsive interaction between the components in the mixed films. Spectroscopic studies of the LB films reveal a broadening and red shift of the absorption spectra compared to the solution absorption spectrum that confirms the formation of organized aggregates of carbazole in the mixed LB films. Fluorescence studies of the mixed LB films reveal new bands in the low-energy region of the fluorescence emission spectrum. Increasing the concentration of carbazole in the mixed films results in the enhancement of the intensity of these low-energy emission bands and quenching of the high energy bands that may be attributed to aggregation-induced reabsorption effects indicating the formation of crystallites in the LB films. A comparative study of the spectroscopic characteristics of these aggregates formed in the LB films and in the binary solvent mixtures of ethanol and water reveals identical spectra that confirm formation of crystallites in both the systems.
Introduction Carbazole and its derivatives, owing to their excellent photoconducting properties, have found extensive industrial and commercial applications especially in the manufacture of organic photoconductors,1 electroluminescent devices,2 and xerographic machines.3 Carbazole and its derivatives have also been of great interest to the spectroscopist, owing to their unique photophysical properties like an intense well characterized fluorescence spectrum, large quantum yield4 of fluorescence (φ ) 0.38), long fluorescence lifetime4 (τ ) 16 ns), and ability to form excimers both in the singlet and the triplet states. Several workers have extensively studied the spectroscopic5 and photoconductive1 properties of carbazole and its derivatives chemically tagged to amphiphilic and polymeric chains or doped in polymers at ordinary or low temperatures. Utilizing the close similarity between the naturally existing biomembranes and ultrathin LangmuirBlodgett (LB) films,6 energy7 and electron8 transfer * Present address: Centre de Recherche en Photobiophysique, Universite´ du Que´bec a` Trois-Rivie´res, Trios-Rivie´res, Quebec, G9A 5H7, Canada. Fax: +1-819-376-5057. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) (a) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals; Clarendon Press and Oxford University Press: Oxford and New York, 1982. (b) Klopffer, W. In Electronic Properties of Polymers; Mort, J., Pfister, G., Eds.; Wiley: New York, 1982. (c) Simon, J.; Andre, J. J. Molecular Semiconductors; Springer-Verlag: Weinheim, 1985. (2) Bisberg, J.; Curming, W. J.; Gaerdiana, R. A.; Hutchinson, K. D.; Ingwell, R. I.; Kolb, E. S.; Mehta, P. G.; Mimms, R. A.; Peterson, L. P. Macromolecules 1995, 28, 386. (3) Law, K. Y. Chem. Rev. 1993, 93, 449. (4) (a) Berlaman, 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. (5) (a) Bukhart, R. D. In Optical Techniques to Characterize Polymer Systems; Bassler, H., Eds.; Elsevier: New York, 1989; p 227. (b) Guillet, J. E. In Polymer Photophysics and Photochemistry; Cambridge University Press: Cambridge, 1985.
S0743-7463(96)00514-8 CCC: $12.00
processes between amphiphilic carbazoyl derivatives and other chromophoric groups have been studied in an effort to understand better the energy7 and electron8 transport properties in real photosynthetic reaction centers. Mimicking such energy and electron transfer reaction processes occurring in real photosynthetic membranes is deemed to be the basis of constructing future generation nanodimensional optoelectronic read-write memory units,9 and hence such studies are considered important. Combining the excellent optical and electronic properties of carbazole and the possibility of tailoring them using the LB technique, various amphiphilic derivatives of carbazole have been synthesized and studied in the LB films.7,8 Synthesis of these amphiphilic carbazole derivatives and their purification is however extremely difficult,8d and the yield of the final products is often too low to find (6) (a) Kuhn, H.; Mobius, D.; Bucher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B. W., Eds; Wiley: New York, 1972, Vol. 1, Part 3B, p 577. (b) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuit-Blodgett Films to Self Assemblies; Academic: New York, 1991. (c) Tredgold, R. H. Order in Thin Films; Cambridge University Press: Cambridge, U.K., 1994. (d) Petty, M. C. Langmuit-Blodgett Films: An Introduction; Cambridge University Press: Cambridge, U.K., 1996. (7) (a) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516. (b) Tamai, N.; Matsuo, H.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1992, 96, 6550. (c) Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1987, 91, 841. (d) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1987, 91, 3572. (e) Yamazaki, I.; Tamai, N.; Yamazaki, T.; Murakami, A.; Mimuro, M.; Fujita, Y. J. Phys. Chem. 1988, 92, 5035. (f) Yamazaki, I.; Ohta, N.; Yoshinari, S.; Yamazaki, T. In Microchemistry, Spectrosocopy and Chemistry in Small Domains; Masuhara, H., DeSchryver, F. C., Kitamura, N., Tamai, N., Eds.; NorthHolland: New York, 1994. (8) (a) Yatsue, T.; Matsuda, M.; Miyashita, T. J. Phys. Chem. 1992, 96, 10125. (b) Miyashita, T.; Yatsue, T.; Matsuda, M. J. Phys. Chem. 1991, 95, 2448. (c) Yatsue, T.; Miyashita, T. J. Phys. Chem. 1995, 99, 16047. (d) Miyashita, T.; Yatsue, T.; Matsuda, M. J. Phys. Chem. 1991, 95, 2448. (9) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932.
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any practical large scale device-oriented applications. To circumvent these difficulties, we have tried to explore the possibilities of incorporating nonamphiphilic carbazole in LB films mixed with a fatty acid10 and studying their spectroscopic characteristics, as such techniques are unique in providing information on the orientation and aggregation of the molecules assembled in the LB films. Interestingly, nonamphiphilic carbazole molecules incorporated in LB films have never been reported. This work demonstrates that nonamphiphilic carbazole molecules may be readily incorporated in LB films mixed with a fatty acid. Detailed studies on the behavior of the carbazole mixed stearic acid (SA) films at the air-water interface indicate a repulsive interaction between the SA and carbazole moieties, resulting in the formation of aggregates. A comparative study of the absorption spectra of carbazole in ethanol and in the mixed films with SA shows that the LB film absorption spectra are broadened and red shifted with respect to the spectra in solution that confirm formation of J-aggregates. Fluorescence emission studies of the mixed LB films show little shift of the 0-0 band but quenching of the high-energy bands and enhancement of the low-energy bands, indicating aggregation-induced reabsorption effects. Surprisingly, however, spectroscopic studies failed to reveal the existence of excimers despite the high degree of organization and aggregation of the carbazole moieties in the LB films. One possible explanation seems to be an unfavorable overlap configuration of the carbazole moieties in the LB films.
Dutta
Figure 1. Surface pressure versus area per molecule isotherms of mixed films of carbazole and SA at different mole fractions of carbazole (a) 0; (b) 0.18; (c) 0.33; (d) 0.57; (e) 0.75; (f) 0.82; (g) 1.0.
Experimental Section Carbazole was purchased from Aldrich Chemical Co., Milwaukee, WI, and extensively zone refined by about 200 passes. The purity of the zone-refined product was checked by comparing the absorption and emission spectra reported elsewhere.4a Stearic acid (SA) purchased from Sigma Chemical Co., St. Louis, MO, was used without further purification. All solvents used were of spectroscopic grade and obtained from E. Merck, Germany. Chloroform used was distilled prior to use and stored in dark-colored bottles in a refrigerator. A commercially available automated alternate layer Langmuit-Blodgett Joyce-Loebl trough 1V obtained from Joyce-Loebl Inc., Newcastle upon Tyne, U.K., was used for depositing mono- and multilayers. A filter paper Wilhelmy balance was used to measure the surface pressure at the air-water interface and was also interfaced to a computer that controlled the motion of the moving barrier, maintaining constant surface pressure at the air-water interface with an accuracy of (0.1 mN/m. The subphase used was triple-distilled water deionized by a Milli-Q plus water purification system and had a resistivity of about 18.2 MΩ cm. The pH of the water subphase was 6.3, in equilibrium with atmospheric carbon dioxide. Mixed films of carbazole and SA at the air-water interface were formed by spreading a dichloromethane solution of carbazole and SA mixed in a predetermined ratio at the airwater interface and compressing with a low speed of 3 × 10-3 nm2 mol-1 s-1 after allowing sufficient time for the volatile solvents to have evaporated. A Joyce-Loebl closed circuit refrigeration system maintained a constant temperature of the subphase at 20 °C. The Π-A isotherms were recorded by a filter paper Wilhelmy plate interfaced to a computer. Data acquisition and its analysis were achieved by software supplied by JoyceLoebl Inc. The isotherms recorded were the average of four runs, and each isotherm was found to be reproducible well within an experimental error of about 10%. Fluorescent grade quartz slides (4 cm × 2 cm × 0.1 cm) were used for the deposition of the LB mono- and multilayers. The slides were kept immersed in chromic trioxide solution overnight, then boiled in concentrated nitric acid to remove by oxidation all traces of organic matter, and washed with deionized water. The slides were subsequently sonicated in chloroform and dried in a vacuum furnace, where they were stored till use. Y-type deposition was performed by allowing the slides to move in a vertical plane very slowly at a rate of 5 mm/min, and a drying time of 15 min was allowed
Figure 2. Plot of the average area per molecule of the mixed films of carbazole and SA versus the mole fraction of carbazole at different surface pressures: (open circles) 20 mN/m; (open triangles) 25 mN/m; (open squares) 30 mN/m. between two consecutive dips. The average transfer ratio of the film was obtained from the ratio of the decrease in the area per molecule at the air-water interface to the product of the surface area of the substrate and the number of layers deposited. The transfer ratio was found to be 0.91. The absorption spectra of the solution and the LB films were recorded on a Shimadzu 2010 UVPC absorption spectrophotometer, and emission was recorded on a Perkin-Elmer MPF-44A spectrofluorimeter. To reduce the effects of reabsorption and scattering, front face excitation was provided by placing the LB films in special holders such that the film remained at an angle of 45° to the source and the detector. Narrow band pass filters were used to minimize the effects of scattering.
Results and Discussion Behavior of Carbazole at the Air-Water Interface Studied by the Surface Pressure versus Area per Molecule Isotherms. A solution of carbazole in chloroform (2 × 10-3 M) was spread at the air-water interface and then compressed slowly at a rate of 3 × 10-3 nm 2 mol-1 s-1, after allowing sufficient time for the volatile solvents to have evaporated. It was observed that the film of pure carbazole collapsed at a low surface pressure of 5 mN/m only, indicating the formation of crystals at the air-water interface. Mixing carbazole with SA resulted in highly stable and compressible monolayers that could be easily transferred onto solid substrates with a good transfer ratio. Figure 1 shows the Π-A isotherms of pure and mixed films of carbazole mixed with SA at the airwater interface. The isotherms typically resemble the
Nonamphiphilic Carbazole Assembled in LB Films
Langmuir, Vol. 12, No. 24, 1996 5911
Figure 3. Plot of the average area per molecule of the mixed films of carbazole and SA versus the mole fraction of carbazole at two different surface pressures: 5 and 10 mN/m, respectively. The dotted line corresponds to the ideality curve.
isotherms of molecules which form aggregates at the airwater interface and are pushed together as a result of compression of the monolayer at the air-water interface.6c It is interesting to observe that the average area per molecule of the mixed film initially increases with increasing molar concentration of carbazole at low concentrations of carbazole in the mixture but decreases at higher concentrations of carbazole. One possible explanation seems to be that, at low concentrations of carbazole, the carbazole moieties find the SA matrix an excellent microphase in which they are readily accommodated. However, at higher concentrations of carbazole, phase separation between the carbazole and SA moieties occurs that originates from the immiscibility of the components owing to their differences in molecular structure and physical and chemical properties which generate aggregates and multilayers. The formation of aggregates and multilayers at the air-water interface is manifested as a decrease in the average area per molecule. However other explanations are also plausible. While one possibility could be that the carbazole moieties sandwiched between the SA chains and squeezed into the SA matrix and out on the air-water interface, an alternative possibility could be the loss of carbazole molecules through precipitation into the water subphase. To confirm whether or not the carbazole molecules at the air-water interface are lost through precipitation into the bulk of the subphase, small aliquots of water from just below the air-water interface were sucked out and the fluorescence from the water samples was recorded. It was confirmed from the failure to detect any fluorescence that carbazole moieties were not lost through submerging below the air-water interface but were very likely pushed up in between the SA chains so as not to occupy any area at the air-water interface. Moreover, an increase in the absorbance and emission intensities with increasing mole fraction of carbazole in the mixture at different surface pressures (figure not shown) confirmed the incorporation of the carbazole moieties in the LB films. These results support the fact that very likely the carbazole aggregates were forced out of the air-water interface and remain lodged in between the SA chains partially and also partially squeezed out on the surface of the layer. In this context, it may be mentioned that scanning electron micrographs (SEMs)10i,j,16f,17 of the surfaces of the mixed films of polyaromatic hydrocarbons (PAHs) and fatty acids transferred onto substrates have provided evidence of crystallites which confirms that the PAH molecules are indeed squeezed out on the surface of the floating layers. Figure 3 shows the plot of the average area per molecule at the air-water interface versus the mole fraction of carbazole at two different surface pressures of 5 and 10
mN/m, respectively. The dotted line shown in the figure corresponds to an ideal curve given by the equation11a-d
A12 ) A1N1 + A2N2 which assumes the absence of all possible interactions between the components in the mixed films. A1 and A2 correspond to the average areas per molecule of the pure components while N1 and N2 correspond to the mole fractions of the pure components in the mixture. A positive or a negative deviation of the observed curve from the ideality relation signifies a repulsive or an attractive type of interaction between the components of the mixture. In this mixed system of carbazole and SA, the observed positive deviation from the ideality relation confirms a repulsive interaction between the components that is manifested as a demixing and phase separation of the components. Such characteristics are consistent with the behavior of other polyaromatic hydrocarbon (PAH) molecules.11,12 Although the interactions involved at the airwater interface are complex and a complete theory is still lacking, a quantitative explanation however seems possible. While the interaction between the PAH molecules and the water surface primarily determines the orientation and alignments of the molecules at the air-water interface, the SA-SA, carbazole-carbazole, and carbazoleSA interactions largely determine the mixing-demixing of the components in the mixed film. The Van der Waals interactions between the carbazole-carbazole and SASA molecules tend to form crystallites of pure SA and carbazole while the weaker carbazole-SA interactions tend to promote the miscibility of the components. The formation of pure crystallites of SA and carbazole in the mixed LB films demonstrates a phase separation of the (10) (a) Kuhn, H.; Mobius, D. Angew. Chem. 1971, 83, 672. (b) Kuhn, H.; Mobius, D. Angew. Chem., Int. Ed. Engl. 1972, 10, 620. (c) Kuhn, H. Thin Solid Films 1983, 99, 1. (d) Kuhn, H. Thin Solid Films 1989, 178, 1. (e) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53. (f) Jones, R.; Tredgold, R. H.; Hodge, P. Thin Solid Films 1983, 99, 25. (g) Schoeler, U.; Tews, K. H.; Kuhn, H. J. Chem. Phys. 1974, 61, 5009. (h) Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, K. Thin Solid Films 1988, 160, 87. (i) Dutta, A. K.; Misra, T. N. Opt. Mater. 1993, 3, 35. (j) Dutta, A. K. Solid State Commun. 1996, 97, 785. (11) (a) Adamson, A. W. Physical Chemistry of Surfaces; WileyInterscience, New York, 1990. (b) Gaines, G. L., Jr. J. Colloid Interface Sci. 1966, 21, 315. (c) Gaines, G. L., Jr.; Tweet, A. G. J. Chem. Phys. 1964, 41, 538. (d) Gaines, G. L., Jr. J. Chem. Phys. 1978, 69, 924. (e) Ito, H.; Morton, T. H.; Vodyanoy, V. Thin Solid Films 1989, 180, 180. (f) Vodyanoy, V.; Bluestone, G. L. Biochim. Biophys. Acta 1990, 284, 1047. (12) (a) Pagano, R. E.; Gershfeld, N. L. J. Phys. Chem. 1972, 76, 1238. (b) Alsina, M. A.; Mestres, C.; Valencia, G.; Anton Garcia, J. M.; Reig, F. Colloids Surf. 1988/1989, 151. (c) Angelova, A. Thin Solid Films 1994, 243, 394.
5912 Langmuir, Vol. 12, No. 24, 1996
Dutta
Figure 4. Absorption spectra of carbazole in chloroform (2 × 10-3 M) shown by the dashed line, LB films mixed with SA in the molar ratio 1:10 shown by a continuous line, and PMMA films doped with carbazole (8 × 10-3 M) shown by a thick broken line.
components, indicating a repulsive interaction between the components. Moreover, as the strength of interaction between the components constituting the mixture is an inverse function of the distance of separation between the components, mixing-demixing of the components is therefore expected to depend on the molar composition of the mixture as well as the surface pressure. Indeed, such observations have been made by several workers13 and the formation of crystallites has been confirmed for amphiphilic as well as nonamphiphilic molecules using different techniques.14 Spectroscopic Studies of Carbazole in Solution and in LB Films Mixed with SA. Figure 4 shows the absorption spectra of carbazole in chloroform and in the LB films mixed with SA. The solution absorption spectrum shows two sets of bands, one in the 250-300 nm region corresponding to the S2-S0 (1La-1A) transition that is directed parallel to the long axis of the molecule and one in the 300-350 nm region representing the S1-S0 (1Lb-1A) transition that is directed parallel to the short axis of the molecule. The LB film absorption spectrum of carbazole mixed with SA appears diffuse, broadened, and red shifted compared to the solution absorption spectrum that indicates the formation of aggregates in the LB layers. It is worthy of mention that, in the LB films, the S1-S0 transition bands are more intense compared to the S2-S0 transition bands. This feature indicates that the carbazole moieties are preferentially oriented in the LB films. Considering the direction of the wave propagation vector to be normal to the plane of the film and the electric field vector to be either perpendicular or parallel to the dipping direction of the film, it seems likely from the large absorbance of the S1-S0 bands that the corresponding 1L -1A transition must be almost parallel to the dipping b direction. The low absorbance of the S2-S0 band implies that very likely the long axis of the molecule corresponding to the S2-S0 transition is directed almost parallel to the normal to the plane of the film. To confirm that the observed changes did not originate from the rigidification of the matrix, the absorption spectrum of carbazole dispersed in a poly(methyl methacrylate) (PMMA) matrix was also recorded, as shown in Figure 4. The absorption (13) (a) Angelova, A.; Van der Auweraer, M.; Ionov, R.; Vollhardt, D.; DeSchryver, F. C. Langmuir 1995, 11, 1367. (b) Pal, P.; Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1994, 10, 2339. (14) (a) Weiss, R. M.; McConnel, M. Nature 1984, 310, 47. (b) Losche, M.; Sackmann, E.; Mohwald, H. Ber. Bunsen-Ges Phys. Chem. 1984, 87, 848. (c) Berg, B. Nature 1991, 322, 350. (d) Losche, M.; Rabe, J.; Fischer, A.; Rucha, B. U.; Knoll, W.; Mohwald, H. Thin Solid Films 1984, 117, 269. (e) Majewski, J.; Margulius, L.; Jacquemain, D.; Leveillu, F.; Bohm, C.; Arad, T.; Talmon, Y.; Lahav, M.; Leiserowitz, L. Science 1993, 261, 899. (f) Majewski, J.; Margulius, L.; Weissbuch, I.; PopovitzBiro, R.; Arad, T.; Talmon, Y.; Lahav, M.; Leiserowitz, L. Adv. Mater. 1995, 7, 26. (g) Honig, D.; Mobius, D. Thin Solid Films 1992, 210/211, 64. (h) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (i) Siegel, S.; Honig, D.; Vollhardt, D. Mobius, D. J. Phys. Chem. 1992, 96, 8157.
Figure 5. Emission spectrum of carbazole in ethanol (dashed line). Emission spectra of a mixed LB film of carbazole and SA at difference molar ratios of carbazole:SA: (open diamonds) 1:50; (filled diamonds) 1:20; (continuous line) 1:1.
spectrum of carbazole dispersed in the PMMA matrix was found to be identical to the solution absorption spectrum, which confirmed that the large changes observed in the case of the LB film absorption spectrum arose from the organization of the carbazole chromophores in the LB films. The large red shifts observed in the absorption spectrum of the LB films indicate strong exciton coupling between the carbazole moieties in the LB films. According to the exciton theory by McRae and Kasha,15 the resultant exciton level produced as a result of a dipole-dipole interaction may be located either above or below the monomer exciton band by ∆E, where ∆E ) (2M2)/r3 (1 3 cos2 θ). Here M corresponds to the dipole moment vector, r to the distance of separation between the centers of the dipoles and θ to the angle between the dipole moment vector and the line joining the centers of the dipoles. It is evident from the mathematical equation that for 0° e θ e 54.7° the exciton band is energetically lowered, resulting in a red shift, while for 54.7° e θ e 90° the corresponding exciton band, is energetically raised above the monomer exciton band causing a blue shift. While red-shifted bands are referred to as the J-aggregates,16a-c blue-shifted bands are referred to as H-aggregates.16a-c At the magic angle θ ) 54.7° a zero shift is observed independent of r and the corresponding aggregates are referred to as the I-aggregates.16d-f Although small shifts are expected as a result of the differences in refractive index, the large shifts observed in this case suggest the formation of organized J-aggregates. Concentration Dependence of Fluorescence Emission from the LB Films of Carbazole Mixed with SA. Figure 5 shows the emission spectra of carbazole in ethanol and in the mixed films of carbazole and SA at different molar compositions. In ethanol, the fluorescence emission spectrum consists of a 0-0 band located at 347 nm and vibronic bands at 359 and 378 nm, respectively, which is in excellent agreement with reported data. A comparative (15) (a) McRae, E. G.; Kasha, M. In Physical Processes in Radiation Biology; Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic Press, New York and London, 1964; p 23. (b) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (c) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 37. (16) (a) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (b) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (c) West, W.; Caroll, B. M. J. Chem. Phys. 1951, 19, 417. (d) Van der Auweraer, M.; Verschuere, B.; DeSchryver, F. C. Langmuir 1988, 4, 583. (e) Miyata, A.; Heard, D.; Unuma, Y.; Higashigaki, Y. Thin Solid Films 1992, 210/211, 175. (f) Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1996, 12, 459.
Nonamphiphilic Carbazole Assembled in LB Films
study of the LB film and the solution emission spectrum shows large changes. While the 0-0 bands at 346 nm and the vibronic bands at 359 and 378 nm in the LB films appear to be slightly shifted with respect to the solution emission bands, new bands are observed to appear in the 380-480 nm region. Although the bands in the 340-380 nm region may be readily assigned to the S1-S0 transition in carbazole, the origin of the bands in the 380-480 nm region is not readily explicable. It was observed that, with increasing molar ratio of carbazole in the LB films, the fluorescence bands in the 340-380 nm region decrease while the emission bands in the 380-480 nm region intensify. These features indicate strong aggregationinduced reabsorption effects17 that signify formation of crystallites in the LB films. In fact the gradual decrease of the higher energy emission bands and their subsequent disappearance altogether with enormous enhancement of the low-energy bands confirm aggregation of the carbazole moieties in the LB films. In this context, it is perhaps reasonable to compare the photophysical properties of amphiphilic carbazole moieties assembled in LB films studied by various workers with our own results. Yamazaki et al.7 have observed that, in addition to a structured monomeric emission with the 0-0 band at about 347 nm, a broad band with its maximum at about 420 nm is observed that may be assigned to a complete overlap sandwich excimer. Carbazole moieties attached to polymeric chains and incorporated in the LB films also show similar emission spectra.8 Polyvinylcarbazole (PVCz) however does not exhibit a structured monomeric fluorescence but instead shows a very broad band in the 350-550 nm region that may be resolved into two overlapping broad bands with their maxima located at about 370 and 420 nm, respectively.18 While the band at 370 nm has been assigned to a partial excimer band with the benzene rings of two adjacent carbazole moieties eclipsed partly, the band at 420 nm is attributed to a sandwich type excimer where the carbazole moieities are in a fully eclipsed geometry.18 As mentioned above, partial excimers originate from a partial overlap of the aromatic rings. In fact several overlap configurations are possible that may form different partial excimeric emission states. The extent of overlap is determined by the extent of geometric, energetic, and thermodynamic constraints offered by the system. Externally applied pressure provides excellent possibilities of altering the overlap geometry of the molecules that may cause large changes in the emission spectra of the excimers. Johnson and Offen18b have demonstrated that, on increasing the externally applied pressure on PVCz from 1 to 30 kbar, the emission peak of the excimer band shifts from 380 to 430 nm, which is attributed to changes in the overlap (17) (a) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 4365. (b) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 12844. (c) Dutta, A. K. J. Phys. Chem. 1995, 99, 14758. (18) (a) Zachariasse, K. A.; Duveneck, G.; Kuhnle, W. Chem. Phys. Lett. 1985, 113, 337. (b) Johnson, P. C.; Offen, H. W. J. Chem. Phys. 1971, 55, 2945. (c) Itaya, A.; Okamoto, K.; Kusabayashi, S. Bull. Chem. Soc. Jpn. 1976, 49, 2082. (d) Nozue, Y.; Hisamune, T.; Goto, T.; Tsurata, H. J. Phys. Chem. Soc. Jpn. 1984, 53, 2818. (e) DeSchryver, F. C.; Moens, L.; Van der Auweraer, M.; Boens, N.; Monnerie, L. Macromolecules 1982, 15, 64. (f) DeSchryver, F. C.; Vandendriessche, J.; Troppet, S.; Demeyer, K.; Boens, N. Macromolecules 1982, 15, 406. (g) Collart, P.; Toppet, S.; Zhou, Q. F.; DeSchryver, F. C. Macromolecules 1985, 18, 1026. (h) Johnson, G. E. J. Chem. Phys. 1975, 62, 4697. (i) Hoyle, C. E.; Nemzek, T. L.; Mar, A.; Guillet, J. E. Macromolecules 1978, 11, 429. (j) Ghiggino, K. P.; Wright, R. D.; Phillips, D. Eur. Polym. J. 1978, 14, 567. (k) Sakai, H.; Itaya, A.; Masuhara, H.; Sasaki, K.; Kawata, S. Polymer 1996, 37, 31. (l) Ohta, N.; Okazaki, S.; Yoshinari, S.; Yamazaki, I. Thin Solid Films 1995, 258, 305. (m) Johnson, G. E. J. Chem. Phys. 1974, 78, 1512. (19) (a) Zhen, Z.; Tung, C. J. Photochem. Photobiol.; A: Chem. 1992, 68, 247. (b) Ruban, A. V.; Horton, P.; Young, A. J. J. Photochem. Phobiol.; A: Chem. 1993, 21, 229.
Langmuir, Vol. 12, No. 24, 1996 5913
Figure 6. Emission spectrum of carbazole in ethanol (dashed line). Emission spectra of carbazole in a binary solvent mixture of ethanol and water at different volume fractions of water in the mixture: (short dashes) 0; (long dashes) 0.6; (open circles) 0.8; and (filled circles) 0.90.
geometry of the carbazole moieties brought about by pressure. Our experimental studies on carbazole assembled in LB films mixed with SA however do not give any indication of the formation of partial or full excimers. One possible explanation seems to be an unfavorable overlap of the carbazolyl moieties in the aggregates formed that results in the failure of the aggregates to produce an excimer emission. These results however highlight the fact that the organization of the nonamphiphilic carbazole moieties in LB films is different from that of their amphiphilic counterparts. Fluorescence Study of Aggregates of Carbazole in Binary Solvent Mixtures of Ethanol and Water. Binary solvent mixtures provide an unique and alternative method of producing organized aggregates. By varying the composition of the binary mixture, the size and extent of aggregation of the carbazole moieties may be altered. To understand better the process of aggregation and its role in modifying the spectral characteristics, we have used a binary solvent mixture of ethanol and water. Figure 6 shows the emission spectra of carbazole at different compositions of a binary solvent mixture of ethanol and water. For all compositions of the binary solvent mixture below the specific composition corresponding to 0.76 volume fraction of water in the mixture, the emission spectrum corresponds to that in pure ethanol. The small observed shifts possibly originate from the small differences in refractive indexes brought about by the changes in the composition of the binary mixture. However, above the specific composition corresponding to 0.76 volume fraction of water in the binary mixture, new bands at about 400 and 420 nm appear. On increasing the volume fraction of water further, the high-energy bands appear to be quenched and the low-energy bands are enhanced, which indicate reabsorption effects. Interestingly, the partial or full excimeric emission is never observed independent of the concentration of carbazole in the binary mixture of ethanol and water or the volume fraction of water in the mixture. It is evident that although organized aggregates are formed, the dominant spectral effects correspond to reabsorption effects. One rational explanation for the lack of excimer fluorescence seems to be an unfavorable overlap configuration of the carbazolyl moieties in these aggregates. Excitation Spectra of Carbazole in Solution, Binary Solvent Mixtures and in the LB Films Mixed with SA. Figure 7 shows the excitation spectra of carbazole in ethanol, LB films, and binary solvent mixtures. It is observed that the 0-0 band of carbazole
5914 Langmuir, Vol. 12, No. 24, 1996
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were found to be identical, that these bands were not spurious and were of vibronic origin. These results further confirmed aggregation-induced reabsorption effects.
Figure 7. Excitation spectrum of carbazole in ethanol (dashed line) monitored at the emission band at 380 nm. Excitation spectra of the LB films with emission monitored at 420 nm: (open diamonds) 1:50; (continuous line) 1:1. Excitation spectra of carbazole in a binary solvent mixture of ethanol and water at different volume compositions of the mixture with emission monitored at 415 nm: (open circles) 0.8; (filled circles) 0.9.
in pure ethanol is located at 360 nm while in the binary solvent mixtures the 0-0 band is located at 370 nm. However, the intensity distribution of the excitation spectra is observed to change with increasing volume fraction of water, which reflects the changing interaction between the carbazole molecules in their aggregates formed in the binary solvent mixture. The excitation spectra of the LB film of carbazole mixed with SA show the 0-0 band at 376 nm, which is in excellent agreement with the solution excitation spectrum. A comparative study of the excitation spectra of the mixed LB films at two different molar ratios (1:50 and 1:1) showed completely different intensity distributions despite a good agreement in their 0-0 band positions, which reflects the interaction between the carbazole moieties and their microenvironment. Monitoring the emission at different low-energy bands both in the binary solvent mixture and in the LB films, it was confirmed from their excitation spectra, which
Conclusions Our studies show that nonamphiphilic carbazole mixed with SA forms excellent floating layers at the air-water interface that are easily transferred onto solid substrates as LB films. Surface pressure versus area per molecule studies at the air-water interface reveal the existence of a repulsive interaction between the carbazole and SA molecules. A comparative study of the absorption spectra of carbazole in the LB films and in solution reveals that the S1-S0 transition bands are more intense than the S2-S0 bands in the case of the LB films as compared to that in solution. In addition, the absorption bands in the LB films are red shifted and broadened relative to those in solution, which indicates the formation of organized aggregates in the LB films. A comparative study of the emission spectra of carbazole in solution, in the LB films and in the binary solvent mixture reveals aggregationinduced reabsorption effects that are supported by absorption studies and confirm the formation of crystallites in the LB films. The close resemblance of the emission spectra of the aggregates formed in the LB films and in the ethanol-water binary solvent mixture suggests that the aggregates formed in the two systems are very likely similar. Sharply in contrast to the spectroscopic characteristics of the amphiphilic carbazolyl derivatives assembled in the LB films, an excimer-like emission is never observed, which may be attributed to a failure of the carbazole moieties to attain a configuration that is favorable to the formation of excimers. Excitation spectra of carbazole in the LB films and in the binary solvent mixtures are observed to be similar, which confirms aggregate formation in these films. Acknowledgment. The author would like to thank Prof. T. N. Misra, Department of Spectroscopy, Indian Association for the Cultivation of Science, Calcutta, India, for providing all possible experimental facilities and for encouragement and interest in this work. LA9605145
