Spectroscopic Study of Nonamphiphilic 9-Phenylcarbazole

Langmuir 1997, 13, 6731-6736

6731

Spectroscopic Study of Nonamphiphilic 9-Phenylcarbazole Assembled in Langmuir-Blodgett Films Krishanu Ray and T. N. Misra* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received June 27, 1997. In Final Form: September 24, 1997X The monolayer characteristics of nonamphiphilic 9-phenylcarbazole (PCB) mixed with stearic acid (SA) have been studied at the air-water interface. Miscibility studies as well as the Gibbs free energy measurements indicate a repulsive type of interaction between the two component molecules in the mixture that results in the formation of aggregates of PCB. Spectroscopic studies reveal that PCB in mixed LB films is highly fluorescent and forms organized aggregates. The high-energy phosphorescence emission bands in an ethanol-glass matrix are quenched whereas the low-energy bands are manifested in the LB films. This has been attributed to aggregation-induced reabsorption, which is unusual for singlet-triplet transitions. The large difference in phosphorescence lifetime in two different microenvironments supports the formation of small microcrystalline domains in LB films at 77 K. Scanning electron micrographs clearly demonstrate the existence of such aggregates in the films, and the size of the aggregates is in the range 0.1-4 µm.

Introduction 1

The Langmuir-Blodgett (LB) technique offers an unique method to fabricate ultrathin organized films where the optical and electronic properties may be manipulated with ease. These features make the LB technique unique and versatile in fabricating various optoelectronic devices and sensors. For the past several decades extensive studies have been performed on LB films, as they are expected to open up new aspects of chemical, physical, and biological processes in this uniquely ordered environment.2 Fluorescent probes, dyes, and polymers incorporated in restricted geometries show marked changes in their electronic states which are manifested in their electronic spectra that reflect the interaction between the probe and its local environment. In addition, incorporating suitable molecules in LB films, one may obtain valuable information regarding the spectra-structure-property correlation for such molecules assembled in constrained media or restricted geometries. Carbazole and its derivatives constitute a class of aromatic amines of relatively low ionization potential which readily forms charge transfer (CT) complexes with compounds functioning as electron acceptors.3 The photophysical and the photochemical properties of carbazole4,5 are an area of active research due to its role as a chromophore in polymeric systems such as poly(Nvinylcarbazole),6 which exhibits photoinduced discharge. Bichromophoric carbazole derivatives exhibit a broad redshifted emission in polar solvent due to the formation of twisted intramolecular charge transfer (TICT) states * Corresponding author. Phone: +91 33 4734971. Fax: + 91 33 4732805. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Ulman, A. An introduction to Ultrathin Organic Films: From Langmuir-Blodgett Films to Self-Assembly; Academic Press: New York, 1991. (2) (a) Kuhn, H. Thin Solid Films 1989, 178, 1. (b) Ringsdorf, H.; Schmidt G.; Schneider, J. Thin Solid Films 1987, 152, 207. (c) Tamai, N.; Matsuo, H.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1992, 96, 6550. (3) Bigelow, R. W.; Johnson, G. E. J. Chem. Phys. 1977, 66, 4861. (4) Pinkham, C. A.; Wait, S. C. J. Mol. Spectrosc. 1968, 27, 326. (5) Bree, A.; Zwarich, R. J. Chem. Phys. 1969, 49, 3355. (6) Sakai, H.; Itaya, A.; Masuhara, H.; Saasaki, K.; Kawata, S. Chem. Phys. Lett. 1993, 208, 283.

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where charge transfer takes place from the carbazole moiety to the substituents.7 Two-dimensional singlet and triplet energy migration in LB films containing amphiphilic carbazole has been investigated by several workers.8 However only limited effort has been put forth to investigate the spectroscopic properties of nonamphiphilic carbazole9a and its derivatives in well defined two-dimensional LB films mimicking biomembranes. Recent studies9-11 suggest that the spectroscopic and aggregating properties of nonamphiphilic molecules mixed with fatty acids assembled in LB films are quite similar to those of their amphiphilic counterparts which are difficult to synthesize and are expensive. Molecular aggregation of guest molecules in LB monolayer films,2c,8a formation of aggregates, and island structures in LB films have already been reported.9a,d-f,10,11 In this paper we report the behavior of nonamphiphilic 9-phenylcarbazole (PCB) in a mixed monolayer with stearic acid (SA) at the air-water interface and the photophysical properties of the PCB molecules in the restricted geometries of the LB films. Experimental Section 9-Phenylcarbazole, referred to as PCB, was purchased from Aldrich Chemical Co., Milwaukee, and was used without further purification. The purity of the PCB samples was checked by absorption and emission spectroscopy. Stearic acid, purchased from Sigma Chemical Co., St. Louis, was used as received. All solvents used were of spectroscopic grade. A commercially available Langmuir-Blodgett (LB) deposition trough (JoyceLoebl model 4 manufactured by Joyce-Loebl Inc. U.K.) was used for deposition of mono- and multilayers. Triple-distilled water obtained with a Milli-Q-plus water purification system with a (7) Rettig, W.; Zander, M. Chem. Phys. Lett. 1982, 87, 229. (8) (a) Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1987, 91, 841. (b) Hisada, K.; Ito, S.; Yamamoto, M. Langmuir 1996, 12, 3682. (9) (a) Dutta, A. K. Langmuir 1996, 12, 5909. (b) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53. (c) Jones, R.; Tredgold, R. H.; Hodge, P. Thin Solid Films 1983, 99, 25. (d) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 4365. (e) Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1996, 12, 459. (f) Wistus, E.; Mukhtar, E.; Almgren, M.; Lindquist, S. E. Langmuir 1992, 8, 1366. (g) Ulman, A.; Scarnige, R. P. Langmuir 1992, 8, 894. (10) (a) Kuhn, H. Thin Solid Films 1983, 99, 1. (b) Kuhn, H. J. Photochem. 1979, 10, 111. (c) Kuhn, H. Pure Appl. Chem. 1981, 53, 2105. (11) (a) Flament, C.; Gallet, F. Thin Solid Films 1994, 244, 1026. (b) Tsuruk, V. V.; Reneker, D. H.; Bliznyuk, V. N.; Kirstein, S.; Mohwald, H. Thin Solid Films 1994, 244, 763.

© 1997 American Chemical Society

6732 Langmuir, Vol. 13, No. 25, 1997

Figure 1. Surface pressure versus area per molecule isotherms of mixed films of PCB and SA at different mole fractions of PCB: (a) 0.0; (b) 0.05; (c) 0.1; (d) 0.4; (e) 0.6; (f) 0.8; (g) 1.0. Inset shows the structure of the PCB molecule. resistivity of 18.2 MΩ‚cm was used as the subphase. The pH of the subphase was 6.4 in equilibrium with atmospheric carbon dioxide, and the temperature was 23 °C. The surface pressure at the air-water interface was measured by a Wilhelmy plate attached to a microbalance, the output being fed to an IBM PC controlling the film compression barrier. The surface pressure versus area per molecule isotherm measurements were achieved by spreading about 100 µL of a chloroform solution of SA and PCB mixed in a predetermined ratio on the pure water subphase in the LB trough. After the solvents were allowed to evaporate, the film at the air-water interface was compressed very slowly at the rate of 2 × 10-3 nm2 molecule-1 s-1. The area per molecule was calculated by dividing the total surface area occupied by the monolayer under the specified pressure by the total number of molecules in the monolayer. Fluorescence grade quartz slides cleaned by the usual procedure1 were used for spectroscopic measurements of LB films. Mono- and multilayers of the mixed films were deposited on solid substrates by moving the slides vertically through the floating monolayer at a rate of 5 mm/min. Y-type depositions of mono- and multilayers were obtained on quartz slides at a constant surface pressure of 25 mN/m. Examination of the films under an optical microscope revealed that the films were highly uniform and homogeneous. The transfer ratio was calculated from the ratio of the total decrease of the surface of the film at the air-water interface and the total surface area of the substrate covered by the film multiplied by the number of layers deposited on the substrate. The transfer ratio was found to be 0.92 ( 0.02 on quartz slides. Absorption and emission spectra were recorded with a Shimadzu 2101 UVvis absorption spectrophotometer and a Hitachi F-4500 spectrofluorimeter. Scanning electron micrographs were recorded on a Hitachi S-415A (Japan) scanning electron microscope.

Results and Discussion Monolayer Characteristics of Mixed PCB and SA at the Air-Water Interface. To study the behavior of pure PCB at the air-water interface, a 100 µL chloroform solution of PCB (1 × 10-3 M) was spread at the water subphase. After the solvents were allowed to evaporate for 30 min, the barriers were compressed very slowly at a rate of 2 × 10-3 nm2 molecule-1 s-1. It was observed that the surface pressure rises up to 15 mN/m only. The calculated area per molecule is 0.02 nm2. This low area per molecule for pure PCB indicates that the molecules do not lie flat at the water interface but form aggregates/ clusters. At 10 mN/m surface pressure, all attempts to transfer the monolayer of pure PCB from the air-water interface onto the solid substrates failed. However when PCB was mixed with SA, a highly stable compressible floating monolayer was obtained which could be transferred onto a suitable substrate with a transfer ratio of 0.92 ( 0.02. Figure 1 shows the surface pressure (π) versus area per molecule (A) isotherms of pure PCB mixed with SA at different molar ratios on a pure water surface at 23 °C. The π-A isotherms are reminiscent of the fatty acid isotherms with a distinctly steep region at high pressure indicating a solid condensed phase. However the most

Ray and Misra

Figure 2. Plot of area per molecule of the mixed monolayer versus the mole fraction of PCB at the air-water interface at different surface pressures.

interesting feature of the isotherms is that the area per molecule of the mixed film initially increases with increasing mole fraction of PCB molecules in the SA matrix up to a mole fraction of 0.1. On further increase of the PCB mole fraction, it decreases. Such characteristics have been observed in other nonamphiphilic molecules at the air-water interface.9a,d,e The π-A isotherms at the air-water interface provide information not only on the orientation of the molecules but also on the interactions between the constituents of the mixed monolayer. Several workers12-15 have calculated the average area per molecule for a mixed twocomponent system using the additivity expression

A12 ) N1A1 + N2A2

(1)

where N1, A1 and N2, A2 are the mole fractions and the area per molecule at zero pressure of the first and second components, respectively. A12 is the average area per molecule at zero surface pressure of the mixed monolayer of the two component systems. Figure 2 shows the plot of the average area per molecule versus mole fraction of PCB in mixed films at the air-water interface at different constant surface pressures. The dotted line (Figure 2) represents the ideal curve for noninteracting two component systems calculated from the above equation. It is interesting to note (Figure 2) that the area per molecule of the mixed monolayer at the air-water interface does not change linearly with the PCB concentration at different constant surface pressures. This observation may arise due to two possible reasons: (i) PCB molecules occupy such locations that do not contribute to the area at the air-water interface; (ii) PCB molecules may submerge below the air-water interface. The area per molecule versus time curve (figure not shown) is parallel to the time axis, which negates the possibility of PCB molecules submerging below the interface. Another auxiliary experiment was performed to confirm the latter result. A small bent pipet was introduced from behind the compression belts, a small amount of water just below the floating layer was sucked out, and the emission was studied. No emission was observed which corresponds to PCB molecules, suggesting that the PCB molecules were not submerged at the air-water interface. It, therefore, (12) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (13) (a) Costin, I. S.; Barnes, G. T. Colloid Interface Sci. 1975, 51, 106. (b) Galvez Ruiz, M. J.; Cabrerizo-Vilchez, M. A. Colloid Polym. Sci. 1991, 269, 77. (14) Pal, P.; Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1994, 10, 2339. (15) (a) Gaines, G. L., Jr. J. Collid Interface Sci. 1966, 21, 315. (b) Vialallonga, F. Biochim. Biophys. Acta 1968, 163, 290. (c) Ito, H.; Morton, T. H.; Vodyanoy, V. Thin Solid Films 1989, 180, 180. (d) Vodyanoy, V.; Bluestone, G. L.; Longmuir, G. L. Biochim. Biophys. Acta 1990, 284, 1047.

9-Phenylcarbazole Assembled in LB Films

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Figure 3. Plots of Gibbs free energy versus the mole fraction of PCB in a mixed PCB-SA monolayer at different ranges of surface pressure: (O) 0-5 mN/m; (0) 0-10 mN/m; (2) 0-15 mN/m. The ideal curve calculated from eq 4 is shown by (4).

seems likely that PCB molecules form clusters or aggregates and remain sandwiched between the fatty acid chains, which are the characteristics of the mixed nonamphiphilic-fatty acid monolayers.9d-e,16,17 The interactions between the two different types of molecules at the air-water interface are extremely complex; however, a plausible explanation can be forwarded. Three types of interactions, namely SA-SA, PCB-PCB, and PCB-SA, determine the spatial distribution of the molecules in the monolayer. The strong attractive interaction between similar kinds of molecules (SA-SA and PCB-PCB) favors the formation of two- and/or three-dimensional aggregates9f,18 whereas the weaker SA-PCB interaction tends to make the component miscible, which tends to produce a homogeneous distribution of the constituent moieties in the monolayer. Information on the miscibility of the components in the mixture and the aggregation of components may be obtained by measuring the Gibbs free energy from the π-A isotherms. The excess free energy of mixing (GME) has been calculated from the following equations12,14,15

GME )

∫0π(ASAPCB - NPCBAPCB - NSAASA) dπ

(2)

GMI ) RTNPCB(ln NPCB) + RTNSA(ln NSA)

(3)

GM ) GME + GMI

(4)

where GM ) the free energy of mixing, GMI ) the free energy of mixing for an ideal system, R ) the universal gas constant, π ) the surface pressure obtained from isotherms, and T ) the temperature in kelvin. The plot of the Gibbs free energy of mixing (GM) versus the mole fraction of PCB in the mixed monolayer at different surface pressures is shown in Figure 3. It is evident from the figure that GM departs significantly from ideality where GME ) 0. This indicates immiscibility of the components of the mixture. This immiscibility and aggregation of the components at all compositions of the mixture may originate from a completely different physical (16) (a) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 12844. (b) Dutta, A. K. J. Phys. Chem. 1995, 99, 14758. (17) Warren, J. G.; Cresswell, J. P.; Petty, M. C.; Lloyd, J. P.; Vitukhnovsky, A.; Sluch, M. I. Thin Solid Films 1989, 179, 515. (18) (a) Angelova, A.; Van der Auweraer, M.; Ionov, R.; Vollhardt, D.; De Schryver, F. C. Langmuir 1995, 11, 3167. (b) Overbeck, G. A.; Honig, D.; Mobius, D. Langmuir 1993, 9, 555. (c) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (d) Honig, D.; Mobius, D. Thin Solid Films 1992, 210/211, 64. (e) Honig, D.; Overbeck, G. A.; Mobius, D. Adv. Mater. 1992, 4, 419.

Figure 4. Absorption spectra of PCB in ethanol (- - -), in a binary mixture of ethanol and water (0.8 mole fraction) (- - -), and in LB films mixed with SA (s).

and chemical nature of the two components. Since the existence of some specific interaction between PCB and SA molecules seems improbable, it is likely that the dissimilar nature of the PCB and SA molecules may result in a phase separation of the components as clusters or aggregates. Spectroscopic Studies of PCB Molecules in Solution and in LB Films. Figure 4 shows the absorption spectra of PCB in ethanol solution (1 × 10-5 M), an ethanol-water binary mixture, and LB films mixed with stearic acid at room temperature. The absorption spectrum shows three distinct band systems in the regions 300-360, 250-300, and 210-250 nm in increasing order of intensity. These bands are reminiscent of the absorption bands of pure carbazole.19 The long wavelength band corresponds to the 1Lb r 1A transition and is polarized along the short axis in carbazole. In PCB, this transition will be along a direction close to the molecular axis containing the NC6H5 group and perpendicular to the axis corresponding to the long molecular axis of carbazole.4 The absorption band in the 250-300 nm region corresponds to the 1La r 1A transition of carbazole, which is polarized along the long axis of the carbazole moiety. All these transitions are of π* r π nature. The two lowenergy band systems are characterized by a sharp (0-0) band which is the most intense transition in the system. This is the characteristic of electronically allowed transitions where there is no appreciable change in the equilibrium nuclear configuration between the ground and excited states. Molecular orbital calculation suggests that for carbazole moieties the oscillator strength is 0.098 and 0.424 for the lowest and the second lowest singlet-singlet transitions, respectively.4 The electronic state symmetries of the first four excited states of PCB are A1, B2, A1, and B2, respectively, whereas those of the carbazoles are A1, B2, B2, and A1.20 Dutta9a has reported that in a carbazoleSA mixed LB film the S1-S0 transition (300-350 nm region) is very intense but the S2-S0 transition (250-300 nm) is very weak, suggesting specific orientation of the carbazole moiety in the LB films. In the absorbtion setup the electric vector is either perpendicular or parallel to the dipping direction of film. For carbazole-SA LB films,9a the large absorbance value of the S1-S0 band suggests that the 1Lb r 1A transition must be almost parallel to the dipping direction. The second transition in the 250-300 nm region is intense in our PCB-SA LB film. This evidently arises from the orientational difference between PCB and carbazole in the LB film where both the long and (19) Jaffe, H. H.; Orchin, M. Theory and Application of UV Spectroscopy; Wiley: New York, 1962; p 355. (20) Johnson, G. E. J. Phys. Chem. 1974, 78, 1512.

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Figure 5. Emission spectra of PCB in ethanol solution (a) at 1 × 10-5 M (s) and (b) at 77 K (- - -).

Figure 6. Emission spectra of PCB (a) in monolayer LB films (- - -), (b) in 30-layer LB films (s), and (c) in microcrystals (- - -) at room temperature.

short molecular axes of PCB make significant projections on the film surface. A comparative study of the absorption spectra of PCB in ethanol solution and in the LB film shows a broadening and red shift of all three transitions in the LB films, which indicates aggregation. Indeed, a small shift may originate from differences in the refractive index (n) and the dielectric constant (). However the shift observed in the LB films of PCB, we believe, originates from aggregation of PCB in the LB films.21 To test the thesis of aggregation, we have considered another established method of producing aggregates.22 We have used an ethanol-water mixture as the solvent. The presence of water in the mixture promotes hydrophobic interaction, which results in the molecular association of the aromatics. The absorption spectrum of PCB in an ethanol-water mixture (volume fraction of water 0.8) shows a similar type of shift and broadening as in the LB films. Figure 5 shows the emission spectra of PCB in ethanol solution at room temperature and at 77 K. The solution emission spectra (1 × 10-5 M) show a good correspondence of the 0-0 band of the absorption spectra at 340 nm. The emission spectra of PCB in ethanol solution (1 × 10-4 M) at 77 K show sharp intense bands at 346 and 361 and a shoulder at 379 nm. In addition a structured spectrum in the 400-520 nm region at this temperature is observed. This long wavelength emission arises from the triplet state of PCB, as demonstrated by its long lifetime. Figure 6 shows the emission spectra of PCB in LB films mixed with SA at room temperature. The emission spectra of mixed monolayer LB films of PCB-SA (1:1 molar ratio) (21) (a) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (b) Kasha, M. Radiat. Res. 1963, 20, 55. (22) (a) Weinberger, R.; Clinelove, L. J. Spectrochim. Acta 1984, 40A, 49. (b) Wang, Y. M.; Kamat, P. V.; Patterson, L. K. J. Phys. Chem. 1993, 97, 8793.

Ray and Misra

Figure 7. Emission spectra of PCB in binary solvent mixtures of ethanol and water at different volume fractions of water in the mixture: (a) 0.5 (- - -); (b) 0.9 (s).

show bands at 348 and 364 and a shoulder at 383 nm. The emission spectrum of PCB in a mixed LB monolayer is red shifted by about 6 nm and is somewhat broadened. This indicates formation of aggregates of the molecules even in the monolayer, as was imperative from the π-A isotherms. With an increasing number of layers, the emission spectrum changes. For 30 layers of a PCB-SA mixed LB film the emission spectrum at room temperature shows a strong band at 368 nm with a weak shoulder at 357 and 387 nm. The fluorescence spectrum of PCB microcrystals has also been recorded for comparison. The remarkable similarity between the emission spectra of 30 layers of mixed PCB-SA in the LB film and PCB microcrystals suggests the formation of microcrystalline domains in the LB films. The quenching of the highenergy bands and the manifestation of the low-energy bands in the emission spectra of PCB in the multilayer LB films suggest an aggregation-induced reabsorption effect which has been observed for other nonamphiphilic molecules incorporated in mixed fatty acid LB films.9a,16 Figure 7 shows the emission spectra of PCB at different volume fractions of water in a binary mixture of ethanol and water. A comparative study of the emission spectrum of PCB in a binary mixture and that in ethanol solution shows broadening and a small red shift. On increasing the volume fraction of water, the 0-0 band shifts to the longer wavelength side and decreases in intensity but the lower energy vibrational bands are enhanced in intensity. When the volume fraction of water is increased to 0.9, the 0-0 emission band shifts to 353 nm, as was observed in multilayer LB films. The next vibronic band shifts to 367 nm, and the intensities of the lower energy bands at 389 and 412 nm are enhanced. These observations support the thesis of aggregation-induced reabsorption in the fluorescence spectra of PCB incorporated in the mixed LB films and binary solvents. Similar observation has been reported for other aromatic hydrocarbons in ethanol-water mixture.22,23 Figure 8 shows the excitation spectra of PCB in ethanol, an ethanol-water mixture, LB films, and microcrystals obtained by monitoring the band maxima of the fluorescence emission. The excitation spectra of PCB in ethanol solution at room temperature (1 × 10-5 M) show the 0-0 band at 340 nm that is in good agreement with the 1Lb r 1A band in the solution absorption spectrum. The 1L -1A a transition in the 275-295 nm region of the solution excitation spectrum is also in good agreement with the absorption spectrum. The excitation spectrum for the binary mixture also closely resembles the absorption (23) (a) Wakayama, N. I. Spectrochim. Acta 1988, 44A, 355. (b) Ruban, A.; Horton, P.; Young. A. J. J. Photochem. Photobiol. B 1993, 21, 229.

9-Phenylcarbazole Assembled in LB Films

Figure 8. Excitation spectra of PCB in (a) ethanol solution (- - -), (b) a binary mixture with a volume fraction of water of 0.8 (-‚-‚-), (c) microcrystals at room temperature (- - -), and (d) 30-layer mixed LB films at 77 K (s).

Figure 9. Delayed emission spectra of PCB in ethanol (- - -) and LB films (s).

spectrum. Monitoring the long wavelength emission bands at 389 and 412 nm in binary solvent mixtures (volume fraction of water 0.9) shows that the excitation spectrum clearly resembles the well identified excitation spectrum of the ethanol solution, confirming the aggregation-induced reabsorption effects in the binary mixture. In the case of multilayered LB films at 77 K, monitored at the emission maxima, the excitation profile broadens and shifts to the lower energy side, which further supports the formation of aggregates in the LB film. Both the 1Lb and 1La states of the PCB-SA mixed LB film are red shifted and broadened compared to the solution spectrum. The excitation spectrum for PCB microcrystals has also been recorded for comparison. The excitation spectra of multilayered LB films at 77 K and PCB microcrystals at room temperature are similar, which further confirms the formation of microcrystalline domains in multilayered LB films. The difference in the intensity distribution and the shape of the excitation spectra reflects the interaction between the PCB molecules and their microenviroment. Figure 9 shows the delayed emission spectra of PCB in an ethanol glass matrix and in a LB film at 77 K recorded through a chopper. In ethanol the phosphorescence spectrum shows a structured band system with its 0-0 band at 409 nm. Monitoring at the phosphorescence emission maxima gives a phosphorescence lifetime of 7.8 s. The lowest triplet state, simply by virtue of its long lifetime alone, can be identified as the (π,π*) state.24 The delayed emission spectrum of PCB-SA mixed LB films is also shown in Figure 9. Two distinct band systems are observed. A broad but weak emission spectrum with λmax around 365 nm and another intense band system in the (24) Lower, S. K.; El-sayed, M. A. Chem. Rev. 1966, 66, 199.

Langmuir, Vol. 13, No. 25, 1997 6735

Figure 10. Scanning electron micrographs of PCB mixed with SA in a LB film (10 layers) at a molar ratio of 1:1.

405-550 nm spectral region are observed in the LB film. The long wavelength band system can readily be assigned to the phosphorescence emission of PCB. The much less intense band system in the 350-400 nm spectral region observed in case of LB films corresponds to the delayed fluorescence of PCB crystallites. Monitoring at 465 nm, the phosphorescence lifetime of PCB in the LB films is 0.124 s. The log-log plot of the delayed fluorescence and phosphorescence intensity of a PCB-SA mixed LB film yielded a straight line with a slope of unity. This suggests that the mechanism for delayed fluorescence involves direct back transfer of the excitation from the low-energy triplet state to the high-energy singlet state due to thermal activation and subsequent emission from the singlet state. This process is referred to as E-type delayed fluorescence. Comparing the phosphorescence spectra of PCB in an ethanol-glass matrix and LB films mixed with SA, it is interesting to note that for LB film the high-energy band in the 400-440 nm spectral region is considerably quenched whereas the low-energy bands at 465 and 495 nm are enhanced, considerably. The steady decrease in the intensity of the high-energy phosphorescence emission bands in LB films is possibly due to the reabsorption effect. The considerable lowering of phosphorescence lifetime in LB films compared to ethanol-glass matrices indicates an efficient deactivation process which is known to increase in aggregates and crystals due to the availability of the exciton band. Figure 10 shows the scanning electron micrographs of a PCB-SA mixed LB film at a molar ratio of 1:1. The micrographs reveal a heterogeneous, grainy morphology, indicating aggregation or clusterization. The smooth background describes the homogeneous SA matrix. As has already been pointed out the aggregates or clusters occur due to the totally different chemical and physical nature of the two molecules and their immiscibility in each other. The sizes of the aggregates were found to be in the range 0.1-4 µm. By employing different methods, namely transmission electron microscopy (TEM),25 electron diffraction,26 and Brewster angle microscopy (BAM),18c-e the formation of such aggregates and crystalline domains in the LB films has been reported. Our SEM study on LB films seems to be well justified by spectroscopic results. (25) (a) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. (b) Gavish, M.; Popovitz-Biro, R.; Lahav, M.; Leisoriwitz, L. Science 1990, 250, 973. (c) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9. (26) (a) Kirstein, S.; Mohwald, H. Chem. Phys. Lett. 1992, 189, 408. (b) Kirstein, S.; Seitz, R.; Garbella, R.; Mohwald, R.; Mohwald, H. J. Chem. Phys. 1995, 103, 818.

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Conclusions Nonamphiphilic PCB molecules mixed with SA form excellent compressible floating monolayers at the airwater interface that may be easily transferred onto the glass/quartz substrates as LB films with a high transfer ratio. Isotherm studies and Gibbs free energy measurements suggest a repulsive type of interaction between the PCB and the SA molecules at the air-water interface that results in aggregation of the PCB molecules in the SA matrix. Spectroscopic studies reveal the existence of

Ray and Misra

such aggregates in LB films and binary ethanol-water mixtures. The reabsorption effect is dominant in the emission spectra of the aggregates and the crystallites, and the magnitude and extent of reabsorption depend on the microenvironment and the aggregate size. Lowering of the phosphorescence lifetime supports the formation of aggregates/microcrystalline domains in mixed LB films. Scanning electron micrographs add to the visual evidence of such aggregates in mixed LB films. LA970689U