Effect of Processing Conditions on the Formation of Aggregates and

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J. Phys. Chem. B 2001, 105, 462-470

Effect of Processing Conditions on the Formation of Aggregates and Phase Domains in Monolayers of the Hemicyanine Dye, 4-(4-(Dihexadecylamino)styryl)-N-methylpyridinium Iodide Amy L. Lusk and Paul W. Bohn* Department of Chemistry and Beckman Institute for AdVanced Sciences and Technology, UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue, Urbana, Illinois 61821 ReceiVed: May 22, 2000; In Final Form: September 21, 2000

Epifluorescence microscopy and tapping mode AFM measurements were used to study the morphology of Langmuir-Blodgett monolayers of the hemicyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, I, prepared under conditions of varying spreading solution concentration, compression speed, aging, subphase composition, and deposition pressure. The morphology, which is the result of coexisting liquidexpanded (LE) and liquid-condensed (LC) phases, is very sensitive to film preparation conditions but relatively insensitive to deposition pressure for a given film preparation protocol. Emission images reveal the following: domain size and shape are unaffected by deposition pressure; increasing spreading solution concentration while maintaining a constant number of molecules on the surface results in increased domain size; emission from the LC domains is more highly variegated as compression speed increases; and incorporation of I- in the subphase results in an entirely new morphology, in contrast to Cl- which only reduces domain size slightly. Far-field fluorescence from films of I is nearly constant as a function of pressure, indicating that aggregation occurs long before the compression process, perhaps even persisting from the spreading solution. This suggests that for monolayers of I, unlike monolayers of fatty acids, both aggregates and phase domains are seeded early in the Langmuir process. This is corroborated by the observation of aggregate formation at the air-water interface before compression takes place. A third phase was also observed and shown to be tens of monolayers in thickness and ca. 1 µm in lateral size by atomic force microscopy. This phase is likely associated with local film collapse and the presence of small bright regions in the epifluorescence images.

Introduction Hemicyanines have some of the largest second-order molecular hyperpolarizabilities, β, among all organic molecules. However, when fabricated into useful thin film geometries, the resulting second-order nonlinear susceptibility, χ(2) is modest at best. Formation of supermolecular aggregates, a well-known phenomenon in thin films of hemicyanine dyes, is strongly implicated in the smaller than expected χ(2) values. Nearly all previous studies of aggregation in such films have been carried out on large, > 1 mm2, sections of film producing results, which are necessarily averaged over smaller spatial structures. In a previous paper we used fluorescence microscopy to build twodimensional maps of aggregation in undiluted monolayer films of the hemicyanine dye, 4-(4-(dihexadecylamino)styryl)-Nmethylpyridinium iodide (I), illustrating the high degree of spatial and spectroscopic heterogeneity in these films.1 In the current work the effect of preparation conditions on the heterogeneity in these films is examined. Aggregation is the result of excited state interactions of chromophores that produces spatially extended electronic states with transition energies shifted relative to the monomer S0 S1 energy difference. In the generally accepted model2 two limiting cases, differing in molecular packing, are identified. In the first (J aggregate), the dispersion is such that the k ) 0 * To whom correspondence should be addressed. E-mail: bohn@ scs.uiuc.edu

aggregate state lies lowest, producing allowed transitions and a bathochromic shift of both absorption and emission. In the second limiting case (H-aggregate) blue-shifted absorption occurs to the k ) 0 end of the aggregate band, followed by rapid relaxation to a k * 0 lowest-lying state which is formally nonradiative. The magnitude of observed spectral shift correlates with the number of molecules participating in the extended electronic aggregate state, although experimental evidence for smaller incompletely formed aggregates, n-mers, coextant with the fully developed H-aggregate has only recently been obtained.3,4 The observation that hemicyanines form H aggregates5-13 instigated work from our laboratory exploring the spectral behavior of hemicyanines I and II. Substituting a methyl substituent in I for the negatively charged sulfonate group in II has a major impact on the packing of the dye. Most strikingly, the aggregation behavior is very different for the two hemicyanines, with the zwitterionic II aggregating strongly and spontaneously,14-17 while the cationic dye I forms the aggregate

10.1021/jp001867m CCC: $20.00 © 2001 American Chemical Society Published on Web 12/16/2000

Morphology of L-B Monolayers of Hemicyanine Dye state more slowly, so that it can be prepared with a distribution of aggregation states,1,18,19 resulting in spectrally heterogeneous behavior in solution, at the air-water interface (Langmuir film) and at the air-solid interface (Langmuir-Blodgett film). Dye I exhibits bands which can be assigned to monomer and smaller aggregated species, i.e., n-mers, in both fluorescence excitation and emission. The established model for the nonradiative decay of the excited aggregate state of II involves energy transfer directly to monomer, possibly through intermediate aggregate species.1,15 In addition to this spectroscopic heterogeneity, Langmuir-Blodgett (LB) films containing only dye I are also spatially heterogeneous, exhibiting coexisting liquid-expanded (LE) and liquid-condensed (LC) phases. Furthermore, the LC domains display distinct structure, being ca. 5-20 µm in lateral size and displaying a multilobed shape.1 The much richer array of structural possibilities that occur in monolayers of I is associated with a more diverse set of relaxation phenomena and makes this dye an ideal candidate for studying the relationship between spatial and spectroscopic heterogeneity. The characteristic aggregation behavior of stilbazolium-based hemicyanines allows the electronic spectra to be used as a contrast mechanism in fluorescence microscopy of LB films of I. The coexistence of multiple aggregate species each with different energy can thus be exploited for spatial mapping of film composition and aggregation state. Preferential partitioning of monomers and partially aggregated species, such as dimers, trimers, etc., into the LC phases, for example, results in longer wavelength and spatially heterogeneous emission from this phase. Much work has gone into the study of analogous phase behavior in fatty acid monolayers. Epifluorescence microscopy employing a small percentage of a fluorescent probe amphiphile, only soluble in the LE phase, reveals phase boundaries and shows that processing conditions such as deposition speed20 and counterion identity, and concentration21 are very important to phase behavior. Atomic force microscopy, which does not require the addition of a probe, likewise shows that morphology depends on counterion, deposition pressure, and compression speed.22-24 Many workers have suggested means for reducing the degree of aggregation in hemicyanine assemblies, including altering the compression process,7 diluting with fatty acids,25-28 and adding specific counterions to the subphase.5,10,29,30 In this paper a study of domain size, shape, and spectroscopic behavior is used to reveal that both aggregation state and domain structure, both spectroscopic and physical, are likely determined very early in the Langmuir process, due to the strong intermolecular interactions exhibited by these mesogenic molecules. Experimental Section Sample Preparation. The hemicyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, I, was obtained from Molecular Probes and used without further purification. Fused silica substrates (Heraeus-Amersil) were prepared by immersion in freshly prepared piranha cleaning solution (1:4 H2O2:H2SO4) followed by rinsing with copious amounts of deionized H2O. Monolayers were fabricated using a KSV Instruments (model 5000) Langmuir-Blodgett trough. Substrates were immersed into the subphase prior to spreading amphiphiles I onto the subphase. A total of 5 × 1016 (( 3 × 1016) molecules were spread from a solution of I in CHCl3 (Burdick & Jackson LC grade) onto a deionized (18 MΩ cm) H2O subphase (Millipore Corporation, model Milli-Q UV plus) at 300 K. One hour was allowed for solvent evaporation and

J. Phys. Chem. B, Vol. 105, No. 2, 2001 463 TABLE 1: Deviations from Standard Preparation and Deposition Conditions deposition condition

deviation from standard procedure

Case I Case II Case III Case IV Case V Case VI Case VII

0.05 mM spreading solution 0.1 mM spreading solution 0.2 mM spreading solution 0.2 mM aged spreading solution 5 Å2/molecule/minute compression speed 0.5 mM KCl subphase 0.5 mM KI subphase

equilibration. Monolayers were typically compressed at a rate of 1 ( 0.5 Å2/molecule/minute to 30 mN/m. To allow comparison of Langmuir-Blodgett films transferred from the same Langmuir layer at various deposition pressures, the monolayer was compressed to 15 mN/m and deposited onto a small portion (∼15 mm) of the substrate, the monolayer was then compressed sequentially to 20, 25, 30, and 35 mN/m, with the transfer being repeated at each pressure, all to the same substrate. LB monolayers were transferred in a Y-type configuration at a rate of 5 mm/min. Deviations from this standard procedure were performed to assess the importance of various deposition parameters. The conditions used are summarized in Table 1. Monolayer Characterization. Absorption spectra of Langmuir films were acquired with Xe lamp illumination directed through a fiber bundle (Fiberguide Industries) and multiply reflected through the air-water interface, then collected by another fiber bundle and imaged into a 0.2 m flat-field spectrograph (Instruments SA, model CP200) and photodiode array (Tracor Northern, Model 6500). Fluorescence measurements were obtained with a commercial dual monochromator system (Spex, model DM1B) with thermoelectrically cooled photomultiplier tube. Spectra were taken in a front-face configuration with emission collected at 23° from the excitation axis. Epifluorescence microscopy was performed on an inverted microscope (Zeiss Axiovert 100) equipped with a 150 W Hg lamp, a color camera (Sony Medical Instruments, model DCX 9000), and a thermoelectrically cooled black and white camera (Hamamatsu). The filter set (Chroma Inc.) employed in these studies consisted of a 360(40)/400LP excitation/emission pair with 360 nm center wavelength (40 nm band-pass) excitation filter and a 400 nm long-pass emission filter. Atomic force microscopy images were obtained in tapping mode on a Nanoscope (Digital Instruments) at a scan rate of 0.5 Hz. Results Absorption Spectra. Figure 1 displays absorption spectra of I in solution, Langmuir films and Langmuir-Blodgett films. The H-aggregate band, centered near 345 nm, is absent from the solution spectrum and from the on-trough spectrum of the Langmuir film acquired immediately after deposition of the spreading solution. However, if the deposited dye molecules are allowed to stand prior to being mechanically compressed, an H-aggregate characteristic absorption develops. Comparing the position of the aggregate band in the Langmuir film with that in the transferred Langmuir-Blodgett film shows that it remains unchanged as compression and transfer take place. These results are consistent with our previous work in which we used on-trough dwell (standing) time to adjust the relative densities of monomer (480 nm) and aggregate (345 nm) species in transferred films.18, 19 Far Field Fluorescence of LB Films. A set of representative far field fluorescence spectra, all taken on films deposited at

464 J. Phys. Chem. B, Vol. 105, No. 2, 2001

Figure 1. Absorption spectra for dye I under various conditions. (a) On-trough spectra of Langmuir films immediately after deposition (s) and after standing for ca. 1 h but before compression (---). (b) Spectra in MeOH solution (s) and in a Langmuir-Blodgett film prepared under condition III (---).

20 mN/m, is shown in Figure 2. No significant deviation from these spectral positions and shapes was observed over the deposition pressure range 15 mN/m e πdep e 35 mN/m. The excitation spectra (Figure 2a) show a large band centered at 340 nm and smaller overlapping bands at 432 and 457 nm that have been assigned to aggregate, trimer and dimer excitation, respectively. The emission spectrum from excitation at the aggregate wavelength (340 nm), Figure 2b, reveals a broad band centered at ca. 480 nm with a shoulder near 410 nm. Excitation at the trimer wavelength (431 nm), Figure 2c, results in an emission band at 495 nm, and excitation at the dimer wavelength (457 nm), Figure 2d, shifts the emission to 505 nm, both bands being ca. 80 nm wide. The position of the bands in the excitation spectrum remained constant under all preparation conditions (Figure 2a); however, small spectral shifts were observed for some preparation conditions in the emission spectra. The center of the broad band resulting from excitation at 360 nm shifted blue by ca. 20 nm for Case VII films and shifted ca. 20 nm red for Case IV and Case V films (Figure 2b) at moderate deposition pressures. The other emission bands remained constant (Figures 2c and 2d). Effect of Deposition Pressure on Domain Morphology. The rows in Figures 3-5 show images of sequential depositions from a single Langmuir film, prepared under the stated conditions, but compressed to increasingly greater pressure: 15, 20, 25, 30, and 35 mN/m in columns (a) - (e), respectively. It is worth noting exactly what the images report. All epifluorescence images shown here were collected by excitation at 360 nm, near the peak of the aggregate band. Therefore, the lighter regions

Lusk and Bohn of the images, which indicate high fluorescence emission, must contain aggregates. Furthermore, the emission is wavelength integrated, so there is no distinction among monomer emission and emission by small aggregates, e.g., dimers, trimers, etc. Because light emission from H-aggregates is formally forbidden by wavevector conservation, emitting areas arise by energy transfer from the excited state of the fully extended aggregate to smaller n-mers and monomer. The dark areas, therefore, contain very small densities of fully extended aggregates. Under all conditions studied, with the exception of the 0.5 mM KI subphase (Case VII), increasing pressure had little effect on the size and shape of the LC domains, although the number of LC domains per unit area increased with pressure. The increase in density is thought to occur due to the increasingly constrained area per molecule during compression, rather than through the appearance of new domains. The areal density of domains was confirmed to be inversely proportional to trough area. This interpretation is corroborated by the lack of a size distribution, which would be expected to accompany continual seeding of domains during compression, and is consistent with a mechanism wherein the phase domains are seeded early in the Langmuir process. Under several sets of conditions, notably Cases III-VI, a number of small circular bright areas appear at higher deposition pressures. These image elements are characteristically observed in LB films of I prepared under highpressure conditions, and their distinct morphology relative to the dominant LE/LC phase equilibria suggests they belong to a separate phase. Effect of Spreading Solution Concentration. Images acquired from Case I-III films, produced from 0.05, 0.1, and 0.2 mM spreading solution concentrations, respectively, are shown in Figure 3, rows 1-3. Care was taken to deposit a constant number of molecules in each run. LC domain size and shape depend only moderately on spreading solution concentrations the average LC domain size for Case I being approximately half the size of Case II and III LC domains. Additionally, Case III films contain a greater density of the small bright circular domains that appear at large πdep. The isotherms, cf. Figure 6a, are all very similar, exhibiting the characteristic behavior of similar hemicyanines. However, at a given pressure there is a monotonic shift in the isotherm to smaller ∆MMA (mean area per molecule) with increasing spreading solution concentration, as would be expected if aggregation can occur prior to compression as indicated in Figure 1. Effect of Spreading Solution Aging. Solution aging could be accompanied by two possible changes: solvent evaporation leading to higher solution concentration, and/or chemical change resulting from thermal or photochemical reactions. The spreading solution used in Case IV experiments was measured to have a concentration of 0.3 mM, only modestly larger than the nominal conditions in Case III. The top two rows of Figure 4 compare the morphology of domains obtained from the aged solutions with those from the freshly prepared amphiphile solution in Case III. Films prepared from the aged solution clearly exhibit a greater variability in morphology than those fabricated with freshly diluted spreading solutions. Especially notable is the appearance of dendritic LC domains in Case IV films. LC domains, whether circular or dendritic, in Case IV films are present as smaller features but in greater areal density than those of Case III. Case IV films are somewhat less compressible than Case III films (Figure 6b) and exhibit fewer of the small circular brightly emitting regions at comparable transfer pressures.

Morphology of L-B Monolayers of Hemicyanine Dye

J. Phys. Chem. B, Vol. 105, No. 2, 2001 465

Figure 2. Fluorescence spectra for films produced under conditions I-VII: (a) excitation spectra for emission at 570 nm; (b) emission spectra for excitation at the aggregate wavelength (360 nm); (c) emission spectra for excitation at the trimer wavelength (433 nm); (d) emission spectra for excitation at the dimer wavelength (470 nm). All spectra were acquired on films transferred at 20 mN/m.

Effect of Subphase Composition. Overall domain morphology was changed little by the addition of 0.5mM KCl to the subphase (Figure 5, row 2). A comparison of Case VI films with Case III films reveals only a moderately reduced LC domain size. However, addition of 0.5 mM KI to the subphase caused drastic changes to both domain morphology and fluorescence properties, cf. Figure 5, row 3. Case VII film morphology is dominated by the LC phase, with very little of the observable area commanded by the LE phase, even at 15 mN/m. Phase boundaries are occupied by the LE domains in the shape of rings and threads. Case VII films also exhibit reduced fluorescence yields, though the band shapes are unchanged relative to Case III films, and the position of the emission band from excitation at the aggregate wavelength (360

nm), is blue shifted by ca. 20 nm (Figure 2b). Consistent with these observations, the isotherm (Figure 6c) reveals a very different shape for I on KI than on KCl or deionized H2O subphases. For Case VII films, the microscopy accentuates this phase transition with the disappearance of the ring-shaped domain boundaries from the images taken at πdep ) 30 and 35 mN/m, panels VIId and VIIe, respectively. Effect of Compression Speed. Films prepared under Case V conditions (Figure 4, row 3) exhibit LC phase domains with greater morphological differentiation than any other conditions studied. At the lowest transfer pressure, dendritic features similar to but larger than the domains produced under Case IV conditions, are observed. At intermediate deposition pressures, the LC domains develop internal structure, emission intensity


Lusk and Bohn

Figure 3. Epifluorescence microscopy images for Case I-III films at sequential deposition pressures. All images were acquired with the UV filter set, i.e., 360 nm excitation and 400 nm long-pass emission filter. Columns are organized by deposition pressure: (a) 15 mN/m; (b) 20 mN/m; (c) 25 mN/m; (d) 30 mN/m; (e) 35 mN/m. Rows are organized by deposition condition from Table 1 as indicated by the individual image labels. The white scale bar in each figure corresponds to 50 µm.

Figure 4. Epifluorescence microscopy images for Case III-V films at sequential deposition pressures. All images were acquired with the UV filter set, i.e., 360 nm excitation and 400 nm long-pass emission filter. Columns are organized by deposition pressure: (a) 15 mN/m; (b) 20 mN/m; (c) 25 mN/m; (d) 30 mN/m; (e) 35 mN/m. Rows are organized by deposition condition from Table 1 as indicated by the individual image labels. The white scale bar in each figure corresponds to 50 µm.

varying from dark to light and back to dark proceeding from the center out to the edge of a given domain. This spatial

alternation in luminescence intensity has been seen previously in this system1 and presumably corresponds to regions, within



Figure 5. Epifluorescence microscopy images for Case III, VI, and VII films at sequential deposition pressures. All images were acquired with the UV filter set, i.e., 360 nm excitation and 400 nm long-pass emission filter. Columns are organized by deposition pressure: (a) 15 mN/m; (b) 20 mN/m; (c) 25 mN/m; (d) 30 mN/m; (e) 35 mN/m. Rows are organized by deposition condition from Table 1 as indicated by the individual image labels. The white scale bar in each figure corresponds to 50 µm.

a given LC domain, with varying aggregate excitation efficiency, energy transfer efficiency, or a combination of the two. It is interesting that while the size and shape of islands varies from island-to-island, the form of the internal structure is quite similar among islands. The compositional differences result in domain center and edges darker than the intervening intermediate light region. Furthermore, the 400 nm shoulder resulting from excitation at the aggregate wavelength (360 nm) is the largest, relative to the main emission band, of any of the conditions studied (Figure 2b). The isotherm (Figure 6b) reveals a film that is slightly less compressible than Case III films, which were prepared identically except for being compressed more slowly. As with Case III films the LC domains are less dendritic; however, these domains have a more differentiated composition. The small circular bright regions appear at lower pressures in case V films than in any of the other conditions studied. Atomic Force Microscopy. Figure 7 shows phase and height channel tapping-mode atomic force microscopy images of domains in a Case III film. Figure 7a is a phase image which shows one eight-lobed domain in full and 3 partial domains ca. 10 µm in lateral size, spaced ca. 10 µm apart. A large number of round features, ca. 1 µm in diameter, corresponding in size and shape to the brightly emitting features observed at high transfer pressure in the epifluorescence images, are distributed across the image randomly. Most are surrounded by lobed regions, as indicated in the height cross section in Figure 7c. Height profiling reveals that these features are ca. 75 nm in height (Figure 7c) at their apex. Figure 7b is an enhanced phase channel image of one full LC domain. The variation in phase retardation across the domain is striking, especially when correlated to the spatial variation in emission observed for these features in epifluorescence microscopy.

Discussion Standard treatments of LB film deposition start by assuming that the amphiphiles deposited on the subphase are noninteracting. Ideally as the molecules are compressed the film approaches the molecular packing associated with the lamellae of a bulk crystal of that compound. However, it is wellestablished that intermolecular interactions of hemicyanines dominate their spectroscopic behavior.8,9,14,15,17,19,28,30-34 The absorption spectra for I (Figure 1) demonstrate the propensity for aggregation in this system. Aggregation appears to produce a thermodynamically favored state which can be reached even in the absence of mechanical compression. The data discussed above shows that the ensemble averaged two-dimensional far-field fluorescence behavior of I is largely independent of deposition pressure but strongly dependent on the details of film preparation. This is in contrast to fatty acid or lipid monolayers, which exhibit distinct changes in twodimensional morphology as a function of both deposition pressure and preparation conditions. The current experiments illustrate that variations in spectroscopic behavior observed in films prepared under different conditions are strongly correlated with significant changes in LE/LC domain morphology. Furthermore, under a given set of preparation conditions, i.e., any of the cases from Table 1, the morphologies obtained are similar in shape and emission behavior, independent of transfer pressure, indicating that compression of the film changes inter-LC domain spacing but does not radically alter the shape of the domains. This is consistent with the observation of aggregation in uncompressed Langmuir films within an hour after deposition. During subsequent compression, the aggregate absorption band is constant in position and width, but it increases in intensity due to the increased density of molecules sampled. This strongly suggests that the aggregation state of the monolayer is deter-


Figure 6. Compilation of isotherms for sequential depositions. Due to creep associated with the intermediate transfers the abscissa is given as change in mean molecular area, ∆MMA. (a) Comparison of spreading solution concentration: Case I, 0.05 mM; Case II, 0.1 mM; Case III, 0.2 mM. (b) Comparison of film preparation: Case III, freshly prepared; Case IV, aged before spreading; Case V, freshly prepareds fast compression. (c) Comparison of different subphases: Case III, deionized H2O; Case VI, 0.2 mM KCl; Case VII, 0.2 mM KI. The horizontal artifacts indicate the positions at which the compression was interrupted for film deposition.

mined very early in its formation, independent of whether the film has been compressed or not. Intermolecular interactions may also affect the molecular packing in monolayers of I,35,36 and the isotherms shown in Figure 6 are distinctive for the general lack of sharp phase transitions. In a typical fatty acid isotherm there is sharp inflection point at which the slope of the pressure-area curve changes dramatically, indicating a transition from one phase to the next. Since this curve follows compression, the initial mean area per molecule (MMA) is greater than the final MMA, and the progression of phases is generally LE to LC to solid. The isotherms for I are very reproducible and exhibit no sharp inflection point, suggesting a coexistence of the LE and LC phases rather than a transition from one to the other. Furthermore, the two phases observed in fluorescence microscopy are

Lusk and Bohn analogous in size and shape to phases identified in fatty acid monolayers to be LC and LE domains. Their observation together in the fluorescence images, regardless of deposition pressure, is entirely consistent with an extended range of surface pressure where LE and LC phases coexist, as reflected in the isotherm shape. Although quantitative characterization of the coexistence region is beyond the scope of this study, these images and isotherms provide the first direct evidence that aggregation of I has a strong and definite effect on the twodimensional morphology of LB films of I. The spectroscopic and mechanical heterogeneity observed in the LC phase suggest that its shape is intimately linked to the aggregation properties of I. Therefore, intermolecular interactions, which drive aggregation, necessarily affect film morphology and shape of the isotherm. The observation of constant domain size and shape as a function of deposition pressure under most conditions is consistent with this smearing of phase transitions in the isotherm. Furthermore, there is a strong tendency to form more highly variegated morphologies at fast compression speeds and with aged spreading solutions. The only apparent departure from this general behavior is associated with the addition of I- to the subphase (Case VII). The Case VII isotherm (Figure 6c) shows a distinct phase transition, which correlates with a change in domain morphology in the range 25 mN/m e πdep e 30 mN/m (Figure 5, panels VIIc and VIId). Several groups have shown that I- affects the nonlinear optical activity of hemicyanine films,11,37 and two predominant explanations have been put forth. The first invokes a simple spacer effect. Large subphase concentrations of Iwould enhance the interfacial concentration of I- through mass action. The large radius of the I- counterion could then prevent molecules from approaching close enough to interact and aggregate. In the second proposed mechanism, I- affects the excited-state energies of the molecule, changing the electronaccepting properties of the pyridinium moiety. Wavelengthresolved epifluorescence emission images (not shown) of LB films of I reveal an extended LC phase that emits near 610 nm. This emission has been correlated with the existence of monomers and small pre-aggregate structures, e.g., dimers, trimers, etc.,1 thus linking structural heterogeneity with spectroscopic heterogeneity. The data, cf. Figure 5, row 3, clearly indicate that addition of I- enhances the population of the LC phase at the expense of the LE phasesbehavior that is inconsistent with the geometric effect of I- required in the first mechanism. Thus, it appears that the effect of I- is mediated through the electronic structure of the chromophores constituting the n-mers that dominate the composition of the LC phase. AFM images reveal cloverleaf shaped LC domains ca. 10 µm in lateral size. The phase image (Figure 7b) reveals several subdomains within the 10 µm domain. Because the phase channel monitors retardation or acceleration of the sinusoidally applied tip dither, changes in the phase channel reflect either changes in sample stiffness or an interaction of the tip with the sample. Because the films studied are homogeneous chemically, differential phase behavior is much more likely to result from differences in sample stiffness. Thus, the center and edges of the domain have distinctly different mechanical properties than the intermediate area, which is of comparable stiffness to the surrounding LE phase. At moderate tapping conditions, light areas (large phase shifts) indicate stiffer regions of the sample.38 The radial variation in mechanical properties could result, for example, from variations in packing of the molecules in the center and edges relative to the intermediate region.



Figure 7. Tapping-mode AFM images of Case III films transferred at 30 mN/m: (a) phase channel overview of spatial distribution; (b) enhanced phase-channel image of a single LC domain; (c) height channel cross-section of one of the tall 1 µm diameter structures visible around the periphery of the LC domain in panel (a).

There is a striking correlation between the variation in mechanical properties and the spatial variation in spectroscopic behavior within individual LC domains. Wavelength-resolved epifluorescence images show the edges and center emit orange, while the intermediate regions are characterized by a blue-shifted emission similar to that observed in the LE regions.1 Because the wavelength of emission is an accurate reporter of the local state of aggregation, these data suggest that the center and edges of the LC domains are in a different, more closely packed, state of aggregation than the intermediate area and LE phase, which have a greater mole fraction of monomer contributing to the higher energy emission and larger ∆MMA. Prior to the AFM studies, such richly patterned fluorescence had only been observed in rapidly compressed (Case IV) films. Seeing an analogous physical morphology in a film compressed more slowly confirms that the LC domains are likely not structurally homogeneous under any of the conditions studied. The presence of small circular bright features in the fluorescence images is a characteristic feature of films transferred at high deposition pressure. These domains are matched in the AFM images by similar-sized and shaped domains, that are much higher than the surrounding area (see height image, Figure 7c), strongly suggesting that the bright areas in epifluorescence are regions of the film that have collapsed locally to produce solid-like structures with different emission or light-scattering properties than the surroundings. One result of the thermodynamic stability of the aggregates is an inability to undergo molecular reorganization to accommodate stress associated with the compression process, a fact consistent with the presence of local collapse areas across the film at higher deposition

pressures. This is corroborated by epifluorescence images, which reveal that the number of local collapse regions increases with increasing deposition pressure. Fast compression (Case V) results in a greater density of local collapse regions at any pressure and their appearance at lower pressures than in films compressed more slowly. AFM images (Figure 7c), which show that these features can reach ca. 75 nm in height, clearly indicate the very different morphology in these collapsed structures than in the surrounding phase. Slight differences in the LC domain size and shape were induced by preparation conditions. For example, the domain size could be decreased by decreasing the spreading solution concentration while maintaining a constant number of molecules on the surface. The addition of Cl- to the subphase also reduced domain size. Mohwald reported that for fatty acids, monovalent ions in the subphase reduce the size and increase the density of islands due to screening effects.39 Although the isotherms for I on deionized H2O and KCl-containing subphases are very similar, it is possible that charge screening at the pyridinium end of the molecule is responsible for reduced LC domain size. Last, LC domain shape changes from cloverleaf to dendritic in films prepared from aged spreading solutions. These films were less compressible and exhibited ca. 20 nm red shift of the main aggregate emission band. This behavior suggests the aggregation state in the spreading solution was more established, perhaps approaching an equilibrium. Stronger aggregation in the spreading solution would dominate the line tension in these films and result in lower energy emission.

470 J. Phys. Chem. B, Vol. 105, No. 2, 2001 Conclusions The aggregation properties of the hemicyanine dye I are known to dominate spectroscopic behavior in solution and in LB films. Previous work in our laboratory established a link between the spectroscopic heterogeneity, typically observed in far-field fluorescence experiments, and structural heterogeneity in films deposited in the LE-LC coexistence region. The current experiments explore this structural heterogeneity in a systematic way as a function of film preparation conditions. Aggregation state and domain behavior were observed to be relatively independent of deposition pressure, strongly suggesting that the characteristic features observed in the fluorescence emission images are formed prior to compression of the film. This, of course, is consistent with previous observations of the development of aggregate spectroscopic properties in the absence of compression and of the mechanical stability of aggregates once formed.14,16-18,32,34 Striking similarities were noted in the internal structure of LC domains probed by spectroscopic (epifluorescence emission imaging) and mechanical (tapping mode AFM) measurements. In particular both types of probes revealed a radial variation in structure within the LC domains. While the variation could only be observed spectroscopically under special conditions, it was observed more generally in the mechanical response. Both types of imaging point to a radial pattern in which the center and outer edges of an LC domain are more closely packed (more LC-like), while there is an intermediate region of looser packing and altered spectroscopic behavior, which is similar to the surrounding LE phase in both mechanical and spectroscopic response. Under most preparation conditions deposition at high pressures produced a number of small bright regions in the epifluorescence images and similarly sized and shaped regions in the AFM. The AFM data showed these regions to be much higher than the surrounding phase. The similarity in physical morphology suggests the tall features in the AFM images are the bright regions from fluorescence imaging. Further, their height suggests that they are formed by local collapse of the LC phase to a solid-like structure with the observed emission properties. The most dramatic changes in morphology caused by preparation conditions were associated with the addition of Ito the subphase. These changes are unlikely to result from a simple spacing effect. Rather they can be assigned to an I--induced alteration in the electronic structure of aggregates in these films. Acknowledgment. This work was supported by the National Science Foundation through grant CHE 99-10236 and by the Department of Energy through grant DE FG0288ER13949. References and Notes (1) Lusk, A. L.; Bohn, P. W. Langmuir 2000, 16, 9131-9136. (2) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317-331.

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Effect of Processing Conditions on the Formation of Aggregates and

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