Formation of Highly Oriented Domains of a Thiacarbocyanine Dye in a

Formation of Highly Oriented Domains of a Thiacarbocyanine Dye in a Monolayer at the .... Surface Pressure Driven Supramolecular Architectures from Mi...
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Langmuir 2002, 18, 1641-1648

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Formation of Highly Oriented Domains of a Thiacarbocyanine Dye in a Monolayer at the Air-Water Interface Nadia Vranken, Mark Van der Auweraer,* and Frans C. De Schryver Laboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, KULeuven, Celestijnenlaan 200F, B3001 Heverlee, Belgium

Hugo Lavoie and Christian Salesse GREIB, De´ partement de Chimie-Biologie, Universite´ du Que´ bec a` Trois-Rivie` res, Trois-Rivie` res, Que´ bec, Canada, G9A 5H7, and Centre de recherche en sciences et inge´ nierie des macromole´ cules, Universite´ Laval, Ste-Foy, Que´ bec, Canada, G1K 7P4 Received July 25, 2001. In Final Form: November 26, 2001 Polarized fluorescence microscopy was used to investigate the organization of domains of J-aggregates formed upon adsorption of the thiacarbocyanine dye THIATS (3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine) onto a Langmuir film of the oppositely charged amphiphile dioctadecyldimethylammonium bromide at the air-water interface. When combined with the measurement of the spectral properties of this film at the air-water interface, it was possible to propose a model for the orientation and packing of the aggregated dye molecules in these domains. The aggregates of THIATS formed circular domains with an average diameter of approximately 35 µm. Linear polarization of the excitation light beam led to fluorescence from two opposing quadrants of these circular domains. Upon rotation of the polarizer by 90°, the two other opposing quadrants became fluorescent whereas the fluorescence from the former fluorescent quadrants was extinguished. The orientation of the overall transition moment of these aggregates was found to be radial with relation to the large circular domains. Therefore, we proposed a model of radial growth of aggregated THIATS molecules, from a nucleation site into circular domains where the dye molecules adopt a brickstone arrangement.

Introduction For several decades, it has been known that increasing the concentration of some cyanine dyes in an aqueous solution results in the observation of a new narrow, intense, and red-shifted absorption and emission band, due to the formation of J-aggregates.1,2 Their spectral characteristics are important for the spectral sensitization of silver halides.3-5 Kasha and McRae related the spectral properties of dye aggregates to the molecular packing of the chromophores using a point dipole approximation for the chromophores.6 Kuhn and co-workers refined this model by using an extended dipole approximation.7,8 This model allows one to get a qualitative picture of the packing of dye molecules in an aggregate based upon its spectral properties. A number of techniques are available to confirm these models or to visualize them; for example, X-ray diffraction allows one to obtain the packing parameters of a crystallized sample, but it does not provide real time visualization.9,10 Electron microscopy gives * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 016 32 79 90. (1) Jelley, E. E. Nature 1936, 138, 1009. (2) Scheibe, G. Angew. Chem. 1939, 52, 633. (3) Wo¨rz, O.; Scheibe, G. Z. Naturforch. 1969, 14b, 381. (4) Maskasky, J. E. Langmuir 1991, 7, 407. (5) The Theory of the Photographic Process, 4th ed.; James, T. H., Ed.; Macmillan: New York, 1977. (6) Kasha, M.; McRae, E. G. Physical Processes in Radiation Biology; Academic Press: New York, 1964. (7) Kuhn, H.; Czikkely, V.; Fo¨rsterling, H. D. Chem. Phys. Lett. 1970, 6 (1), 11. (8) Kuhn, H.; Czikkely, V.; Fo¨rsterling, H. D. Chem. Phys. Lett. 1970, 6 (1), 207. (9) Pomerantz, M. Thin Solid Films 1987, 152, 165.

information on the film morphology but cannot relate this to the photophysical properties of the film.11,12 More direct information on the molecular packing of J-aggregates of cyanine dyes adsorbed on a Langmuir film was obtained by electron diffraction.13-15 Atomic force microscopy has also been used to obtain topography information of thin films of J-aggregates of a merocyanine dye.16 Fluorescence microscopy has proven to be an excellent tool for the investigation of film morphology, based on the fluorescence properties of the film.11,12,17 This allowed Kirstein and Mo¨hwald to image successfully domains of aggregated dye molecules that were adsorbed to an oppositely charged Langmuir film at the air-water interface.13-15,18 In this contribution, polarized fluorescence microscopy is used to investigate the structure of aggregates formed by a thiacarbocyanine dye adsorbed onto a layer of an oppositely charged amphiphile at the air-water interface. Using this technique, we have proposed a possible model for the long-range orientation of the dye molecules in the J-aggregates formed upon adsorption. (10) Barton, S. W.; Thomas, B. N.; Flom, E. B.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988, 89 (4), 2257. (11) Fisher, A.; Lo¨she, M.; Mo¨hwald, H.; Sackmann, E. J. Phys., Lett. 1984, 45, L785. (12) Duschl, C.; Kemper, D.; Frey, W.; Meller, P.; Ringsdorf, H.; Knoll, W. J. Phys. Chem. 1989, 93, 4587. (13) Kirstein, S.; Steitz, R.; Grabella, R.; Mo¨hwald, H. J. Chem. Phys. 1995, 103, 818. (14) Kirstein, S.; Mo¨hwald, H. Chem. Phys. Lett. 1992, 189, 408. (15) Kirstein, S.; Bliznyuk, K.; Mo¨hwald, H. Physica A 1993, 200, 759. (16) Wolthaus, L.; Schaper, A.; Mo¨bius, D. Chem. Phys. Lett. 1994, 225, 322. (17) Knobler, C. M. Science 1990, 249, 870. (18) Kirstein, S.; Mo¨hwald, H.; Schimomura, M. Chem. Phys. Lett. 1989, 154 (4), 303.

10.1021/la011164b CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

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Experimental Section The dye 3,3′-disulfopropyl-5,5′-dichloro-9-ethyl-thiacarbocyanine (THIATS) was a gift from Agfa. The amphiphile dioctadecyldimethylammonium bromide (DODAB) was purchased from Sigma-Aldrich (purity 95%) and was used as received. The organic solvents used were of spectroscopic quality. The ultrapure water (Milli-Q), used for all experiments and all cleaning steps, was obtained by using a filter system with several filtration steps (Millipore, catalog no. CFOF 012 05) to remove ions, organic materials, and small particles. The Millipore filtered water had a resistivity of 18.2 MΩ cm, a pH of 5.5, and a surface tension of 71 mN/m. All experiments were performed at 20 °C and 50% air humidity. A commercially available trough (KSV Instruments; size: 150 mm wide, 528 mm long) was used for the measurement of the surface pressure isotherms. To obtain the isotherms of DODAB for this study, the trough was thermostated at a temperature of 20.5 ( 0.1 °C. After 10 min evaporation time of the spreading solvent, the film was compressed at a speed of 3 mm/min (∼2 Å2/molecule min). The design of the trough and instrumentation for the in situ spectroscopic measurements at the air-water interface have been described in detail elsewhere.19 A 10-6 M solution of THIATS in pure water was used as the subphase. DODAB was spread on this subphase and compressed at a rate of 2 Å2/molecule min. The surface pressure is measured with a Langmuir balance. Spectra were recorded as a function of time after compression of the DODAB layer to 30 mN/m, starting 5 min after stopping compression. A spectrum was then recorded every 15 min until the intensity of the J-band did not increase anymore. Polarized fluorescence spectra were taken by placing a polarizer just after the excitation monochromator and another one just before the emission monochromator. The two polarizers were rotated so that both the incident and the fluorescent light beams are either s-polarized (perpendicular to the plane of incident light and to the surface normal) or p-polarized (parallel to the plane of incident light and to the surface normal). The design of the trough and instrumentation for the in situ fluorescence microscopy experiments have been described elsewhere.19,20 Fluorescence micrographs were recorded during the compression of the DODAB layer. The “M-filter set” that was used to detect monomer fluorescence during the experiments was a Nikon G-2A set which included an excitation filter at 535 nm with a bandwidth of 50 nm, a 580 nm dichroic mirror, and a 590 nm high-pass emission filter. The “J-filter set” that was used for the detection of fluorescence from the J-aggregates during the experiments consisted of an excitation filter at 610 nm with a bandwidth of 20 nm, a dichroic mirror at 630 nm, and an emission filter at 670 nm with a bandwidth of 40 nm (Omega Optical, Brattleboro, VT). A photographic polarizer (Optex) was placed in a holder that can be positioned in the excitation pathway of the light beam to obtain linearly polarized light. By placing another polarizer, with known orientation, just above the illuminated area of the trough, it was possible to determine the absolute polarization direction of the excitation light beam, by the total extinction of the light. Two perpendicular orientations of the polarization of the incident light beam were used to study the effect of the polarization of the incident light on the fluorescence of the THIATS/DODAB film at the air-water interface.

Results and Discussion Influence of THIATS in the Subphase on the Isotherm of DODAB. The isotherm of DODAB on pure water (Figure 1) closely resembles the one presented by Eriksson et al.,21 but it is rather different from the one reported by Kirstein et al.18 on pure water at the same temperature. The area of DODAB at the onset of the (19) Gallant, J.; Lavoie, H.; Tessier, A.; Munger, G.; Leblanc, R. H.; Salesse, C. Langmuir 1998, 14, 3954. (20) Vranken, N.; Lavoie, H.; Be´langer, P.; Van der Auweraer, M.; Salesse, C.; De Schryver, F. C. Langmuir 2000, 16, 9518. (21) Eriksson, L. G. T.; Claesson, P. M.; Ohnishi, S.; Hato, M. Thin Solid Films 1997, 300, 240.

Figure 1. Plot of the π-A isotherms of DODAB, recorded at 20.5 ( 0.1 °C: (thin line) isotherm of DODAB on a water subphase; (thick line) isotherm of DODAB on a subphase containing 10-6 M THIATS.

surface pressure isotherm measured by Eriksson et al.,21 by Kirstein et al.,18 and in the present study are ∼122, ∼105, and 132 Å2/molecule, respectively. At 40 mN/m, the molecular area measured by Kirstein et al.18 is smaller (43 Å2/molecule) than that of Eriksson et al.21 (54 Å2/ molecule) and that presented in Figure 1 (58 Å2/molecule). The values for area at collapse presented by Eriksson et al.21 (53 Å2/molecule) and in Figure 1 (52 Å2/molecule) are very similar. A clear collapse cannot be distinguished in the isotherm of Kirstein et al.18 Moreover, in agreement with our data, Eriksson et al.21 and also Kirstein et al.18 observed a plateau in the isotherm of DODAB which they attributed to a phase transition using Brewster angle and fluorescence microscopy, respectively. The plateau in the isotherm of Kirstein et al.18 is situated between ∼72 and ∼53 Å2/molecule, compared to ∼79 and ∼63 Å2/molecule for Eriksson et al.21 and ∼77.5 and ∼65.5 Å2/molecule in our experiments. The onset of the plateau observed by Kirstein et al.18 is situated at a surface pressure of ∼15 mN/m compared to ∼18 mN/m for Eriksson et al.21 and 23 mN/m in our experiments (Figure 1). This discrepancy could be explained by the strong effect of temperature on the position and width of the plateau of DODAB. Indeed, Eriksson et al.21 investigated the temperature dependence of the π-A isotherm of DODAB, and they have shown that the plateau can no longer be observed at temperatures higher than 23 °C.21 Thus, a small change in temperature can lead to a large difference in the position and extension of this plateau. Furthermore, Kirstein et al.18 incorporated a dye probe in the amphiphile film at the air-water interface in order to visualize domain formation. Although the concentration of this probe was low, it could result in the observed differences. More in general, any impurity could influence the shape of the isotherm when the amount is large enough. However, neither for our sample of DOBAB nor for any of the available papers are detailed data on possible impurities available. Previous to Eriksson et al.,21 others have reported on the π-A isotherm of DODAB, on a pure water subphase.22-24 However, these isotherms were recorded at temperatures above 20 °C and did not reveal the plateau region as described by Eriksson et al.,21 Kirstein et al.,18 and our results. Thus, no comparison will be made with those isotherms. The influence of THIATS on the film of DODAB was then investigated by comparing the isotherm of DODAB (22) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D. Thin Solid Films 1994, 242, 195. (23) Vogel, V.; Mo¨bius, D. J. Colloid Interface Sci. 1988, 126, 408. (24) Claesson, P.; Carmona-Ribiero, A. M.; Kurihara, K. J. Phys. Chem. 1989, 93, 917.

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Figure 2. (a) Absorption spectra of a film of THIATS adsorbed onto a compressed layer of DODAB (0) 5 min, (3) 20 min, (O) 35 min, (4) 50 min, and ([) 65 min after compression of the DODAB layer to 30 mN/m. Inset: absorbance at 657 nm as a function of time. (b) Emission spectra of the film of THIATS adsorbed onto a compressed layer of DODAB (0) 5 min, (3) 20 min, (O) 35 min, (4) 50 min, and ([) 65 min after compression of the DODAB layer to 30 mN/m. Inset: fluorescence at 658 nm as a function of time. The excitation wavelength was 620 nm.

on a pure water subphase with that on a 10-6 M THIATS subphase (see Figure 1). The same amount of DODAB was spread, and both films were compressed at the same compression speed. It can be seen that the shape of these isotherms is very different. Indeed, the onset of surface pressure of DODAB on THIATS is much larger (∼195 Å2/molecule) than that on pure water (∼132 Å2/molecule). Moreover, a plateau between ∼155 and ∼90 Å2/molecule can be observed in the isotherm of DODAB in the presence of THIATS, at a surface pressure πc of 4 ( 1 mN/m, which is much lower than the plateau in the isotherm of DODAB on pure water (23 mN/m, Figure 1). The molecular area of DODAB at the onset of the isotherm, when THIATS is present in the subphase, closely resembles the area of a side-on oriented dye molecule, taking into account the molecular dimensions of the analogue dye TDC, determined by Asanuma.25 A fully edge-on orientation for the dye molecule would, according to their structural parameters, correspond to an area of 76 Å2. The transition to the solid state for DODAB on a THIATS subphase is in good agreement with this value. At higher surface pressures, the molecular area of DODAB is somewhat less than that value. Although the isotherm only reflects the molecular area for the amphiphile molecules, the onset of the isotherm as well as the phase transition to the solid state correspond with cross sections for a side-on and an edge-on orientation of the dye molecules. This would suggest the adsorption of one dye molecule per DODAB molecule. From the observed domain formation (see further), we can deduce that the surface density is different at the domains and between the domains. If on the average we would have one adsorbed THIATS per DODAB, this means that we have an excess of THIATS at the domains, resulting in a decreased electrostatic driving force for adsorption and eventually in a halt for the growing of the domains. Our results agree with the observations of Kirstein et al.18 showing an increase in molecular area and a reduction of the main phase transition πc of DODAB in the presence of another thiacarbocyanine dye. They ascribed this behavior to a partial dye insertion in the monolayer. The increase in molecular area and the reduction of πc, observed over a major part of the π-A isotherm, are, however, larger in our experiments (63 Å2/molecule and 19 mN/m) than what was observed by Kirstein et al.18 (∼25 Å2/molecule and 7 mN/m). Kirstein et al.18 followed a totally different procedure when collecting the isotherm, so we should proceed with caution when comparing our results with

theirs. After compressing to an area of 80 Å2/molecule, they stopped the barrier movement for 70 min, before resuming the compression. During this time, the surface pressure at the plateau region dropped by ∼5 mN/m. The film of DODAB collapses at 60 mN/m on the THIATS subphase compared to 51 mN/m on pure water. Because of the reproducibility of the collapse points for both isotherms within 1 mN/m, these results indicate that introducing the dye into the subphase has a stabilizing effect on the monolayer of DODAB, as shown by the increase in collapse pressure by almost 10 mN/m in the presence of THIATS. Furthermore, the slope of the isotherm of DODAB in the presence of THIATS is much steeper than that of DODAB on pure water indicating that a more condensed film is formed in the presence of THIATS. The “bump” at the beginning of the plateau region was very reproducible which indicates that it is no artifact in the isotherm of DODAB on a THIATS-containing subphase. In fact, this feature has already been observed without further discussion by Flament and Gallet,26 for an isotherm of a long-chain oxacarbocyanine dye. It might be the result of a competition between surface pressure relaxation during adsorption of the dye molecules from the subphase and a surface pressure increase due to the continuous compression of the DODAB film. If this is true, then there should be a dependence on the compression speed. Since this bump was in no way affecting the other experiments, no further investigations of this matter were done. Spectral Characteristics of a Film of THIATS Adsorbed onto a DODAB Layer. Absorption spectra of a film of THIATS adsorbed onto a compressed layer of DODAB are shown in Figure 2a. The data from the absorption spectra are listed in Table 1. The monomer absorption band has a maximum at 554 nm, and its bandwidth at two-thirds of the maximum is 1000 ( 20 cm-1 (spectra not shown).20 A new narrow, red-shifted absorption band can already be observed after 5 min adsorption of THIATS. The appearance of this new band is clearly due to J-aggregate formation of these dye molecules caused by electrostatic interactions with the oppositely charged layer of amphiphiles.27 Initially, the maximum of the J-band is situated at 654 nm, but it (25) Asanuma, H.; Ogawa, K.; Fukunaga, H.; Tani, T.; Tanaka, J. Proceedings of the ICPS ’98 International Congress on Imaging Science, Antwerpen, Belgium, 1998; Vol. 1, p 178. (26) Flament, C.; Gallet, F. Thin Solid Films 1994, 244, 1026.

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Table 1. Spectral Data of a Film of THIATS Adsorbed to a Compressed Layer of DODAB adsorption time (min)

λabs (nm)

maxabs (au)a

FW2/3Mabs (cm-1)

λem (nm)

maxem (au)a

FW2/3Mem (cm-1)

Neff

5 20 35 50 65

654 657 657 657 657

0.29 0.66 0.76 0.93 1.00

350 280 260 260 280 290 ( 40

653 656 657 658 658

0.30 0.50 0.65 1.00 0.96

250 250 230 220 220 230 ( 20

8 13 15 15 13 13 ( 3

a

The intensities are normalized to 1.

reaches its most red-shifted absorption maximum of 657 nm after 20 min adsorption. The evolution of the absorbance at 657 nm is plotted as a function of the adsorption time at 30 mN/m (see inset of Figure 2a). This figure shows that dye molecules aggregate immediately after compression to 30 mN/m. Indeed, it can be seen that the absorbance of the J-band increases rapidly during the first 20 min of adsorption. After longer periods of adsorption (35 min), the aggregation rate decreases until a maximum absorbance is reached after 65 min. Although the π-A curves indicate that dye molecules are present at the surface as soon as compression of the film is started, the formation of J-aggregates still requires several minutes after reaching a surface pressure of 30 mN/m. This could be due to the fact that the original density of the adsorbed dyes is too low to form J-aggregates. To get a full packed monolayer of J-aggregates, the diffusion of new THIATS molecules to the surface is necessary. To obtain 1.3 × 1012 molecules/mm2 (the density in the crystal plane according to Asanuma25), a layer with 2 mm thickness has to be depleted of the dye in a 10-6 M solution. If the adsorption is diffusion controlled, this will take 1500 s when assuming diffusion coefficient of 10-5 cm2/s. Hence, in a 10-6 M solution formation of a complete monolayer can take 2025 min or more.28 At the beginning of the experiment (see spectrum after 5 min adsorption, Figure 2a), the width of the J-band at two-thirds of the maximum is larger (350 cm-1) than the average bandwidth of 290 ( 40 cm-1 calculated from all spectra, suggesting an effective coherence length of 8 molecules. The average value for the effective coherence length is found to be 13 ( 3 using eq 1:

Neff )

[

]

FW2/3M(M) FW2/3M(J)

2

(1)

The effective coherence length (Neff) is the number of effectively coupled dye molecules in the aggregate, over which the exciton is delocalized.29 It is limited due to thermal effects (exciton phonon coupling and homogeneous band broadening), impurities, and inhomogeneous band broadening. At room temperature, kT amounts to 200 cm-1, which corresponds to 70% of the average bandwidth. Hence, at room temperature the coherence length is determined to a large extent by thermal effects rather than by diagonal disorder.30-33 (27) Van der Auweraer, M.; Vranken, N.; Grim, K.; Hungerford, G.; De Schryver, F. C.; Vitukhnovskhy, A.; Varnavsky, O. Proceedings of Excon ’96: 2nd International Conference on Excitonic Processes in Condensed Matter, Bad Schandau, Germany, 1996; p 178. (28) Gerischer, H.; Tobias, C. W. In Advances in Electrochemistry and Electrochemical Engineering; Willig, F., Ed.; Wiley-Interscience: New York 1982; Vol. 12, pp 1-111. (29) Knapp, E. W. Chem. Phys. 1984, 85, 73. (30) Malyshev, V.; Moreno, P. Phys. Rev. B 1995, 51, 14587. (31) . Fidder, H.; Knoester, J.; Wiersma, D. A. J. Chem. Phys. 1991, 95, 7880. (32) Malyshev, V. A.; Rodriguez, A.; Dominguez-Adame, F. Phys. Rev. B 1999, 90, 14140. (33) Van der Auweraer, M.; Scheblykin, I. Chem. Phys. 2001, 275, 285.

Figure 3. Polarized emission spectra of a film of THIATS adsorbed onto a compressed layer of DODAB, taken after 40 min adsorption: spectrum recorded with s-polarized light, both in the excitation and emission path (0), and spectrum recorded with p-polarized light, both in the excitation and emission path (O).

The observation, 5 min after the onset of adsorption of THIATS (see Table 1), of a broader bandwidth (350 versus 290 cm-1) and a blue shift of the absorption maximum (654 versus 657 nm), compared to the spectrum of the completely relaxed film, indicates that the J-aggregates have not yet reached their equilibrium size or packing at this point of the experiment (see also the fluorescence microscopy data). The emission spectra of the film of THIATS adsorbed onto a compressed DODAB layer are shown in Figure 2b. A red shift of a few nanometers can be observed after longer adsorption times of THIATS, similar to the wavelength shift observed for the absorption spectra (Table 1). An emission maximum of 658 nm is reached after 50 min adsorption. Polarized fluorescence spectra of the film show that the emission spectrum measured with spolarized light has a higher intensity than the spectrum taken with p-polarized light (Figure 3). This indicates that the dye molecules are preferentially oriented with their long axis, with collinear transition dipole moment, parallel to the air-water interface. The evolution of the emission at 658 nm as a function of the adsorption time (see inset of Figure 2b) parallels that of the absorption spectra (compare insets of parts a and b of Figure 2) which further supports the rapid formation of J-aggregates at the beginning of the adsorption of THIATS followed by equilibration. Here, we should remark that despite a highly similar chemical structure (meso-ethyl instead of meso-methyl substituent and 5,5′-Cl substituents instead of 5,5′-H) the absorbed THIATS molecules show absorption and emission spectra differing strongly from those observed in the work of Kirstein and Mo¨hwald.18 While the spectra of the latter suggest a herringbone packing,15 those of THIATS suggest a brickstone packing.34-36 This preference of the meso-methyl substituted dyes for a herringbone packing was also found by scanning microscopy experiments37 and electron diffraction of a dye layer adsorbed to a Langmuir

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Figure 4. Fluorescence micrographs of the THIATS/DODAB film: micrograph a is recorded at the beginning of domain formation, i.e., at the beginning of the plateau region in the isotherm; micrograph b is recorded at the end of the plateau region, at a smaller area per molecule than the former micrograph; and micrograph c is recorded at a surface pressure of about 15 mN/m, at the steeper part of the isotherm past the plateau region.

film of DODAB.13-15 Furthermore, while the thermodynamic stable aggregates of THIATS in an aqueous solution have a brickstone packing and absorption and emission maxima between 620 and 625 nm, other experimental conditions lead to a herringbone packing35,38 or a brickstone packing absorbing and emitting between 645 and 660 nm.20,35,36 Polarized Fluorescence Microscopy of THIATS Adsorbed onto a DODAB Layer. Observation of an Organized Layer of Dye Molecules upon Adsorption onto DODAB. Fluorescence micrographs were recorded at the air-water interface on a 10-6 M THIATS subphase in two different conditions: (1) during the compression of a layer of DODAB and (2) as a function of time after compression of the DODAB layer to a surface pressure of 30 mN/m. Immediately after spreading of the DODAB layer, no fluorescence of the film could be detected from the focal point of the microscope, neither with the M-filter set nor with the J-filter set, even if we waited for 30 min before starting compression of the film. Here, one should bear in mind that the fluorescence quantum yield of monomer dye molecules in water is ∼10-3.33,39,40 Furthermore, taking into account the 10-6 M concentration, the intensity of this monomer fluorescence is probably too low to detect. Due to the large molecular area of DODAB, leading to a small number of adsorption sites as well as a small surface potential,18 the surface concentration of dye molecules will remain too low to allow the formation of J-aggregates. When the plateau region is reached in the isotherm of DODAB, that is, at ∼4 mN/m, the growth of circular, fluorescent domains can be observed using the J-filter set (Figure 4, micrograph a). From this point and onward, all fluorescence micrographs were recorded using the J-filter set. The film is however still very mobile, as indicated by the continuous movement of the fluorescent domains. These domains are ∼20 µm in diameter. At the end of the (34) Van der Auweraer, M.; Hungerford, G.; Vranken, N.; Gretchikhine, A.; De Schryver, F. C.; Scheblykin, I.; Varnavsky, O.; Vitukhnovskhy, A. Proceedings ICPS ’98 International Congress on Imaging Science, Antwerp, Belgium, 1998; Vol 1, p 174. (35) Drobizhev, M. A.; Saposhnikov, M. N.; Varnavsky, O. P.; Van der Auweraer, M.; Vitukhnovskhy, A. G. Chem. Phys. 1996, 211, 455. (36) Gretchikhine, A.; Schweitzer, G.; Van der Auweraer, M.; De Keyzer, R.; Vandenbroucke, D.; De Schryver, F. C. J. Appl. Phys. 1999, 85, 1283. (37) De Rover, G. (Agfa-Gevaert N.V., Belgium) New Techniques for Investigating Photosensitive Organic Molecules; KULeuven, Leuven, Belgium; Sept 27, 2001. (38) Scheblykin, I. G.; Drobhizev, M. A.; Varnavsky, O. P.; Verbouwe, W.; De Backer, S.; Vitukhnovskhy, A. G.; Van der Auweraer, M. Chem. Phys. Lett. 1998, 282, 250. (39) Noukakis, D.; Toppet, S.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1995, 99, 11860. (40) Vranken, N.; Jordens, S.; De Belder, G.; Lor, M.; Rousseau, E.; Schweitzer, G.; Toppet, S.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. A 2001, 105, 10196.

plateau region in the isotherm (Figure 4, micrograph b), the fluorescent domains are very irregular in size and the film is still very mobile. When the film has been compressed beyond the plateau region and the first steep part of the isotherm is reached, the size distribution becomes much smaller and the domains become more closely packed. Further compression resulted in an even more densely packed film (Figure 4, micrograph c). In some experiments, the compression was stopped at a surface pressure of 15 mN/m and fluorescence micrographs were acquired at different time intervals to investigate the evolution of the fluorescence characteristics. During the period that the barriers were stopped, the surface pressure did not decrease, most probably because the film is nearly in a condensed state. The experiments indicated no changes of the film characteristics during a 5-10 min time scale. Apparently, the domains do not seem to grow after stopping the compression and no new domains develop in contrast with the data of Kirstein et al.18 Indeed, on stopping compression, they observed the appearance of rod-shaped domains attached to the initial fluorescent domains. The different shapes can probably be related to a different molecular packing reflected by different spectroscopic properties.15,18,20 This different packing could lead to a different growth of the domains. A number of circular domains have reached a diameter of ∼35 µm and do not further increase in size, but several domains still have a smaller diameter (Figure 4, micrograph c). These smaller domains continue to grow upon further compression. Although many circular domains are in close proximity, the film has not yet reached a monolayer coverage made up of the closest packing of the domains (Figure 5, micrograph a). When a linear polarizer is placed in the pathway of the excitation light beam, the circular domains appear to be made up of four quadrants, of which two opposing quadrants are strongly fluorescent and the two other ones are not fluorescent (Figure 5, micrograph b). When the polarization direction is rotated by 90°, the other two quadrants become fluorescent, whereas the fluorescence of the former fluorescent quadrants is extinguished (Figure 5, micrograph c). This polarization effect clearly indicates a long-range order in the domains. Kirstein et al.18 also observed the formation of nearly circular domains upon first compression of DODAB in the presence of a thiacarbocyanine. However, they found that these domains were not polarized in contrast with our data. One should keep in mind that these domains are made up of differently packed dye molecules (see previous section). Furthermore, the data of Kirstein and Mo¨hwald were obtained for a Langmuir film with a different history of compression and relaxation.18 The initially observed circular domains developed in a rather similar way as in our experiments

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Figure 5. Fluorescence micrographs of the THIATS/DODAB film, recorded when the compression was stopped at a surface pressure of 15 mN/m. Micrograph a is recorded with nonpolarized light, micrograph b is recorded with linearly polarized light, and micrograph c is taken with linearly polarized light that is perpendicular to the former polarization direction. The orientation of the linear polarization is for both micrographs coinciding with the orientation of the two fluorescent quadrants.

Figure 6. Fluorescence micrographs of the THIATS/DODAB film: micrograph a is recorded immediately after compression of the film to a surface pressure of 30 mN/m; micrograph b is recorded 60 min after compression of the film to a surface pressure of 30 mN/m; and micrograph c is recorded at an even higher surface pressure (∼45 mN/m), using a larger magnification.

upon compression of the amphiphile for which the isotherm initially also revealed a plateaulike region. However, after the initial compression, Kirstein and Mo¨hwald stopped compressing for 70 min which resulted in a recrystallization of the circular domains into elongated domains, for which then a polarization effect (the domains were polarized along the long axis of the domains) was found.15 When the DODAB film was compressed to a surface pressure of 30 mN/m (Figure 6, micrograph a), the film is made up of very closely packed circular domains although the border between the domains is difficult to distinguish. Nonfluorescent dark areas can still be observed in this micrograph. At this surface pressure, the area per molecule is ∼65 Å2/molecule, which is somewhat smaller than an edge-on orientation of the dye molecules (again assuming a 1/1 THIATS/DODAB ratio).25 Approximately 60 min after compression to 30 mN/m, it can be seen that the film was more homogeneously fluorescent and that the number of nonfluorescent dark areas has decreased to a large extent (Figure 6, micrograph b). Furthermore, the domain boundaries could only be seen with the largest magnification (Figure 6, micrograph c). The patterns observed in micrographs b and c in Figure 6 suggest that the light beam is initially already partially polarized (see also Figure 4, micrograph a), since no polarizer was used to record these micrographs. When the film is imaged by placing the polarizers in the pathway of the light beam (Figure 7), the circular domains reveal the same polarization effect as was observed in micrographs b and c in Figure 5. An average diameter for the circular domains was then determined from 8 micrographs (a typical micrograph is shown in Figure 7), with linearly polarized light (10 domains per micrograph were measured). These values were very consistent and resulted in an average diameter for the fully grown domains of 35 ( 3 µm. This method could not be used previously (for micrographs recorded at lower surface pressures) to determine an average domain size, since at that time there was still a much larger distribution in domain size.

Figure 7. Fluorescence micrograph of the THIATS/DODAB film, recorded immediately after compression of the film to a surface pressure of 30 mN/m. The domains were visualized using polarized light as it allowed the boundaries between the domains to be properly distinguished.

The present observations can be compared with those reported in an earlier paper20 where spectral properties and film morphology of THIATS and TDC adsorbed at the air-water interface onto a compressed Langmuir film of an oppositely charged amphiphile are presented. A different experimental approach was used for those experiments. The fluorescence micrographs were recorded after the injection of THIATS underneath a compressed layer of DODAB at 30 mN/m.20 We could observe the formation of a monolayer that is made up of very regularly packed circular, fluorescent domains of ∼1-2 µm in diameter20 which are thus much smaller than those shown in Figure 4. This difference can readily be explained by the fact that the DODAB film was first spread and compressed before injecting the THIATS molecules into the subphase20 whereas in the present work the THIATS molecules are already present in the subphase when the DODAB film is spread. This should allow THIATS molecules to organize simultaneously with the DODAB

Highly Oriented Domains of a Thiacarbocyanine Dye

molecules during compression. This corresponds to the electron diffraction work of Kirstein and co-workers13,15 suggesting a tuning of the lipid lattice by the packing of the adsorbed dye. For the cationic dye TDC, on the other hand, no domain formation was observed in the earlier experiments;20 the film revealed a homogeneous fluorescence. The cooperative effect demonstrated for the formation of J-aggregates6 suggests that THIATS molecules will preferentially stay adsorbed at the edge of domains where other THIATS molecules have already adsorbed and aggregated rather than to other sites of the DODAB layer. In this respect, the formation of the domains can be considered as a crystallization process starting from a limited number of nucleation sites. Actually, the spectroscopy of other N,N′sulfopropyl or N,N′-sulfobutyl substituted thiacarbocyanines at the interface between water and a DODAB film indicates that J-aggregate formation is a necessity, although not always sufficient to observe a stable adsorbed dye layer.41 The difference between previous experiments20 and the current ones suggests also that THIATS does not adsorb on preformed DODAB domains, because in this case we should find the same THIATS domains as those presented in the present work (with a size of around 35 µm), when DODAB is compressed before adding THIATS. Although there is no direct evidence that 1/1 THIATS/ DOBAB complexes are formed, the area per molecule at the onset of the π-A isotherm is a good indication to suggest that the THIATS molecules adsorb in a first step side-on. As the area per molecule decreases, upon compression, the THIATS molecules evolve into an edge-on orientation. This edge-on orientation was also observed by Kirstein and co-workers for similar dyes.14 This favors the formation of J-aggregates, which has a stabilizing effect on the domains that are formed during the phase transition (as shown by a decrease of the surface pressure). Upon further compression where the area per DODAB molecule is no longer in agreement with the molecular area of THIATS in an edge-on position, the domains end up with an excess positive charge density. Long-range electrostatic interactions will then limit the further growth of the domains. Although it was not possible to directly visualize a regularly stacked pattern for the aggregated dyes at the molecular level (due to the restricted resolution), the observed polarization effect proves that the packing of the dye molecules inside the domains is very regular over a macroscopic range detectable by fluorescence microscopy. From these observations, a model for the stacking of the aggregated dye molecules was proposed (see next section). The Model Proposed for the Organization of the Dye Molecules in the Domains. The micrographs presented in Figures 4-6 indicate that the fluorescent domains are circular (1); upon excitation with linearly polarized light, there is a partial fluorescence coming from two opposing quadrants of the domains (2); upon rotation of the polarizer by 90°, the two other quadrants become fluorescent (3); and the fluorescence from the quadrants is more intense in the center than at the edges of the quadrants (4). The fluorescent domains have an average diameter of 35 µm, but it is known that coherent aggregate formation is restricted by defects to a much smaller coherence length (between 5 and 20 molecules at room temperature).20,30-32,34 On the other hand, it was found that under a broad range of experimental conditions34,42-46 incoherent energy transfer can occur over at least several thousands of molecules. Therefore, the polarization effect observed with the J-filter (41) Vranken, N. Ph.D. Thesis, KULeuven, Leuven, Belgium, 2001.

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Figure 8. Illustration of the proposed model for the stacking of the aggregated dye molecules, where the dye molecules are packed in a brickstone arrangement and they grow radially into circular domains: (gray) dye molecules parallel to the polarization of the excitation light beam; (black) dye molecules perpendicular to the polarization of the excitation light beam.

set indicates that over this length scale the average orientation of the transition dipoles of the aggregates with a coherence size of Neff does not randomize. Actually, to explain the observed pattern, using linear polarized light, the average orientation of the transition dipoles must remain the same over several microns and result in a long-range order, on a micrometer scale, within the domains. The red shift of the absorption spectrum suggests that the aggregates observed in the present study should have a brickstone arrangement according to Kuhn.7,8,33 The determination of the absolute polarization (i.e., with respect to the laboratory coordinate system) of the polarized excitation light beam indicates that the transition dipoles of the aggregates have a radial rather than a tangential orientation in the domains. Therefore, we suggest for this film a radial growth of aggregated THIATS molecules, from a nucleation site into circular domains while the dye molecules adopt a brickstone arrangement. This model is schematically depicted in Figure 8. Near the nucleation site of the circular domains, a large density of defects can be expected due to mismatch in the packing, but the defects should be less important at the edges of the domains. For the moment, it is unclear why the domains arrive at this circular structure, with a radial orientation of the dye molecules. However, once a small circular aggregate is formed, the brickstone packing, which leads to an optimal balance of π-stacking, dipole-dipole, dipolequadrupole, or quadrupole-quadrupole interactions and repulsion,47-49 can only be continued by adsorption of (42) Scheblykin, I. G.; Varnavsky, O. P.; Bataiev, M. M.; Sliusarenko, O.; Van der Auweraer, M.; Vitukhnovskhy, A. G. Chem. Phys. Lett. 1998, 298, 341. (43) Paillotin, G.; Swenberg, C. E.; Breton, J.; Geacintov, N. E. Biophys. J. 1979, 25, 513. (44) Sundstro¨m, V.; Gillbro, T.; Gadonas, R. A.; Piskarskas, A. J. Chem. Phys. 1988, 89, 2754. (45) Brumbaugh, D. V.; Muenter, A. A. J. Lumin. 1984, 31&32, 783. (46) Tsubomura, T.; Sakurai, O.; Morita, M. J. Lumin. 1990, 45, 263.

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radially oriented molecules. When the domains continue to grow, the increasing divergence of the radii will prompt the formation of dislocations where extra radii are inserted. Such dislocations can easily be formed in a 2D system.15,50,51 Those packing defects lead apparently not to the formation of a large number of trap sites with a lower relaxed singlet excited state. A more detailed model, especially on the formation of the nucleation sites, would at this point be mainly speculation. A paper is in preparation where experiments were performed on a deposited film of THIATS adsorbed onto a DODAB layer with a much better spatial and spectral resolution.52 Da¨hne and co-workers53-55 also observed a pronounced polarization effect with a film of a streptopolymethine dye prepared using the thin layer aggregation (TLA) method. After incubation of this amorphous layer where a film thickness of 80 nm was obtained, they observed the formation of nuclei that grew radially to large spherulites with diameters of several millimeters. In their micrographs, recorded in transmission using an optical microscope, the spherulites exhibited two differently colored layer regions when a linear polarizer was put in the pathway of the light beam. They proposed a model for the self-organization of the dye molecules during the TLA process. Despite the major differences between their film and our experiments (different film thickness, sample preparation method, packing of the dye molecules, etc.), these results suggest that it is reasonable to use a similar model, the radial growth of herringbone (a brickstone in our experiments) packed molecules into spherulite (2D (47) Kitaigorodsky, A. J. Chim. Phys. 1961, 63, 9. (48) Kitaigorodsky, A. Molecular Crystals and Molecules; Academic Press: New York, 1973. (49) Silinsh, A. E. Organic Molecular Crystals; American Institute of Physics/AIP Press: New York, 1994; pp 25-29. (50) Kosterlitz, J. M.; Thouless, D. J. J Phys. C 1973, 6, 399. (51) Nelson, D. R.; Halpern, B. I. Phys. Rev. B 1979, 19, 2457. (52) Vranken, N.; Foubert, P.; Ko¨hn, F.; Gronheid, R.; Scheblykin, I. G.; Van der Auweraer, M.; De Schryver, F. C. Manuscript in preparation. (53) Da¨hne, L. J. Am. Chem. Soc. 1995, 117, 12855. (54) Da¨hne, L.; Biller, E.; Baumga¨rtel, H. Angew. Chem., Int. Ed. Engl. 1998, 37 (5), 646. (55) Da¨hne, L.; Biller, E. Adv. Mater. 1998, 10 (3), 241.

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circular domains in our experiments) structures, for the formation of the domains of THIATS adsorbed onto DODAB at the air-water interface. Conclusions In this work, polarized fluorescence microscopy was used to investigate the fluorescence polarization effect of J-aggregates formed by adsorption of the thiacarbocyanine dye THIATS onto a layer of DODAB at the air-water interface. Combined with the spectral data of this film at the air-water interface, it was possible to propose a model for the packing and orientation of the aggregated dye molecules in the observed domains. The aggregates of THIATS form circular domains with an average size of 35 µm. Upon excitation of the film with linearly polarized light, two opposing quadrants of the circular domains fluoresce, whereas the other two quadrants are not fluorescent. Upon rotation of the polarization by 90°, the other two quadrants become fluorescent whereas the fluorescence from the former quadrants is extinguished. These observations indicate that the orientation of the transition dipoles of the aggregated dye molecules, persisting over several micrometers, is radial and results in a radial growth of the aggregated dye molecules into the circular domains. The group of Da¨hne53-55 made similar observations when they investigated a film of a streptopolymethine dye using the TLA method. Acknowledgment. N. Vranken thanks the “Vlaams instituut voor de bevordering van het wetenschappelijk en technologisch onderzoek” (IWT). The authors gratefully acknowledge the continuing support from DWTC (Belgium) through Grant IUAP-IV-11, the F.W.O.-Vlaanderen, the Nationale Loterij, the research Council of the K.U.Leuven through GOA 2001/2, the bilateral collaboration Flanders-Que´bec and the European Union through COST D14. The authors are grateful to Agfa N.V. for the gift of the dye. The authors are indebted to the Natural Sciences and Engineering Research Council of Canada, the “Fonds pour la formation de Chercheurs et l’Aide a` la Recherche” (FCAR), and FRSQ for financial support. LA011164B