Organization of an Amphiphilic Azobenzene Derivative in

Theoretical Organization Models: φ = 0 Model 1 (2b), and φ = θ Model 2 (2c). .... As an example, with θ = 60° corresponding to the area is equal ...
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J. Phys. Chem. B 2002, 106, 2583-2591

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Organization of an Amphiphilic Azobenzene Derivative in Monolayers at the Air-Water Interface Jose´ -Marı´a Pedrosa, Marı´a Teresa Martı´n Romero, and Luis Camacho* Departamento de Quı´mica Fı´sica y Termodina´ mica Aplicada, Facultad de Ciencias, UniVersidad de Co´ rdoba, Campus de Rabanales, Ed. Marie Curie, E-14014 Co´ rdoba, Spain

Dietmar Mo1 bius† Max-Planck-Institut fu¨ r biophyikalische Chemie, D-37070 Go¨ ttingen, Germany ReceiVed: July 9, 2001; In Final Form: NoVember 28, 2001

The organization of an amphiphilic azobenzene derivative, 4-octyl-4′-(5-carboxy- pentamethyleneoxy) azobenzene (8A5), in monolayers at the air-water interface is strongly controlled by association phenomena due to intermolecular interactions of the azobenzene moiety. The monolayer behavior has been investigated by measuring surface pressure-area and surface potential-area isotherms as well as by Brewster angle microscopy (BAM) and reflection spectroscopy. In particular, dimers are formed that reorient from a flat or slightly tilted orientation to an orientation nearly normal to the water surface and forming H-aggregates upon monolayer compression. The BAM images clearly show domains with long range tilt orientational order. The observed shifts of the reflection maximum as well as the intensities of the band are accounted for by a model of molecular association and reorientation. The intermolecular interactions are strongly modified by variation of the subphase pH controlling the dissociation of the hydrophilic carboxyl group. On a subphase pH ) 9.4, flat lying monomers are observed at low surface pressure and dimers form upon compression, however, no domains are detected by BAM in this case.

Introduction Monolayers and Langmuir-Blodgett (LB) films containing azobenzene moieties are very promising photocromic materials for optical information storage, light switching devices, non linear optical devices, and liquid crystal.1-4 The expectation that this type of molecule might be a molecular conductor that could be used in an appropriate orientation as a component of nanoscaled devices with vectorial electron transfer is one of reasons of the great interest in its study. The organization of these molecules at the air-water interface by forming insoluble monolayers is the first step in the construction of artifical machines of molecular dimensions.5,6 Such organized monolayers are assembled on solid substrates using the Langmuir-Blodgett (LB) technique. Monolayer systems that are capable of energy conversion or of functioning as organic light emitting diodes require components with π -electron systems oriented more or less normal to the layer plane for the vectorial transfer of electrons. Such a molecular orientation is difficult to achieve,7 and has often been claimed on the basis of measured surface pressure/area isotherms. However, the observed average area per molecule determined in this way very often does not represent the cross-sectional area of the upright standing molecule, but is rather due to stacking of two or more flat lying molecules. The most reliable way to evaluate molecular orientation in monolayers at the air-water interface as well as in monolayer systems assembled on solids is the measurement of spectra (in the UV-Vis or IR regions) with polarized radiation incident * To whom correspondence should be addressed. E-mail: [email protected]. † E-mail: [email protected].

on the system under well-defined angle.8-12 In several cases, an orientation of the optical transition moment being parallel to the long axis of the molecule normal to the layer plane has been clearly determined.13-15 An alternative possibility of detecting such an orientation using unpolarized light under normal incidence exists in the case of molecules having two transition moments with different orientations in the molecular frame like aromatic molecules. Upon reorientation of such molecules during monolayer compression, the ratio of the intensities of such bands changes and can be analyzed in order to determine the average angle of the molecule with respect to the normal surface.16 Both types of techniques have been used in the case of amphiphilic azobenzene derivatives forming stable monolayers at the air-water interface that may be transferred to solid substrates.11,12 Such azobenzene derivatives show molecular association leading the formation of H-aggregates characterized by a shift of the absorption band to shorter waves with respect to the monomer absorption band. This phenomenon has been used to investigate the type of organization of azobenzene moieties and other chromophores in self-assemblies systems, monolayers, and LB films.17-26 In this work, by using reflection spectroscopy as well as Brewster angle microscopy (BAM) in addititon to the usual monolayer techniques, we have investigated the monolayer behavior of the azobenzene derivative 8A5 (structure shown in Scheme 1) that has been described in the literature,11,12 and some quantitative calculations about the arrangement of the chromophores according to the extended dipole model,27,28 as well as on the orientation of the molecule from the analysis of the reflection spectra measured with unpolarized light16 have been done. Furthermore, in an attempt to control the molecular

10.1021/jp012584c CCC: $22.00 © 2002 American Chemical Society Published on Web 02/19/2002

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SCHEME 1: Chemical Structure of the Amphiphilic Azobenzene Derviative 8A5.

association observed before, we used an aqueous subphase of pH ) 9.4 leading to an increase of the dissociation of the carboxyl headgroups which should cause an increase in molecular repulsion in the absence of heavy metal ions and thereby reduce the tendency of association. Experimental Section Materials. All chemicals were analytical grade and were used as received. The azobenzene-containing long-chain fatty acid used in this study (8A5, Scheme 1) was a high-purity product of Dojindo Laboratories. Pure chloroform as well as methanol was obtained from Baker Chemicals (Germany), and a mixture of chloroform:methanol ) 3:1, v/v, was used as spreading solvent. Ultrapure water from a Millipore Milli-Q-Plus system was used throughout. NaHCO3 and Na2CO3 were purchased from Merck and used as supplied. Methods. Monolayers of the azobenzene 8A5 were prepared on pure water at pH 5.7, as well as on water buffered with 4 × 10-4 M NaHCO3 and 5 × 10-5 M Na2CO3 to pH 8.5, and with 4.5 × 10-2 M NaHCO3 and 5 × 10-3 M Na2CO3 to pH 9.4. After evaporation of the organic solvent, the monolayer was compressed on a rectangular trough provided with a filter paper Wilhelmy plate29 with a compression velocity of 0.1 nm2 molecule-1 min-1. This device was used for measurement of surface pressure (π)/area (A) and surface potential (∆V)/area isotherms, and for reflection spectroscopy at normal incidence. The difference in reflectivity, ∆R, of the monolayer-covered water surface and the bare water surface is determined. Details of the reflection spectrometer have been described elsewhere.30 In the text, ∆Vnorm ) ∆V‚A corresponds to the experimental ∆V multiplied by the surface area, A (nm2/8A5 molecule) taken from the isotherm in order to normalize to the same surface density. In analogy, the reflection spectra were normalized ∆Rnorm ) ∆R‚A. Finally, Brewster Angle Microscopy (BAM) has been used to gain additional information on the molecular organization of 8A5 azobenzene in monolayers at the air-water interface by using different aqueous subphases. Details of this technique have been described elsewhere.31 A modified BAM2 of NFT, Go¨ttingen, as well as a MiniBAM of NFT, Go¨ttingen, have been used in this study. The images shown here, have been taken with the modified BAM2. Results and Discussion Isotherms and Brewster Angle Microscopy. The surface pressure/area (π/A) and surface potential/area (∆V/A) isotherms of the amphiphilic azobenzene derivative 8A5 are shown in Figure 1a and 1b, respectively, for the subphases water (solid lines) and aqueous bicarbonate buffer, pH ) 8.5 (dashed lines)

Figure 1. Surface pressure/area (a) and surface potential/area (b) isotherms of the azobenzene derivative 8A5 on water (solid lines) and bicarbonate buffer, pH ) 8.5 (dashed lines), as well as pH ) 9.4 (dotted lines); further, in (c) ∆Vnorm is plotted against area for water (solid) and the aqueous buffers pH ) 8.5 (dashed) and pH ) 9.4 (dotted line), respectively.

as well as pH ) 9.4 (dotted lines), respectively. The π/A isotherms on the three subphases clearly differ from each other. On water, the surface pressure starts rising at an area of A ) 0.42 nm2 and gradually increases upon compression to about π ) 4 mN/m, A ) 0.36 nm2, where the monolayer obviously starts undergoing a phase transition as indicated by the following nearly horizontal part of the isotherm. This part ends at an area of A ) 0.32 nm2, where a change in slope may be interpreted as the beginning of another phase transition ending at about A ) 0.295 nm2. Upon further compression, the surface pressure rises strongly for areas A < 0.295 nm2 to nearly 30 mN/m when collapse of the monolayer occurs. This isotherm is similar to that reported in,11 although in this work the π-A curve is displaced a little to the higher surface side from that measured previously, and also the discontinuity for 0.32 nm2 > A > 0.295 nm2 is not visible in that report. In analogy to the phenomena observed with long chain fatty acids,32,33 the nearly horizontal

Amphiphilic Azobenzene Derivative in Monolayers part of the isotherm may be attributed to the coexistence region of a transition from the liquid expanded (LE) to a liquid crystalline phase with tilted molecules. We attribute the little discontinuity in the range of 0.32 nm2 > A > 0.295 nm2 to another phase transition. This interpretation is supported by the observed surface potential. The change of the surface potential (see Figure 1b, solid line) in the range between the area A ) 0.36 nm2 to A ) 0.32 nm2 indicates a compression without change of the apparent dipole moment of the molecules. In Figure 1c, where ∆Vnorm is plotted against A, the quantity ∆Vnorm is proportional to the normal component of the dipole moment per molecule. It contains contributions from the hydrophobic end groups, the azobenzene moiety, and from the headgroup region including the electric double layer. The observed behavior is similar to that found, e.g., in the case of monolayers of monomethyl-octadecanedioate34 or of dipalmitoyl-phosphatidylcholin.35 The linear change of ∆Vnorm in the range of the transition is typical for the gradual change in apparent molecular dipole moment in the course of a first-order phase transition.34 In the range 0.32 nm2 > A > 0.295 nm2, again, a linear change of ∆Vnorm with a slope different of that in the preceding phase transition upon compression is found after a sudden jump. Upon further compression, another part with a linear decrease of ∆Vnorm is seen in Figure 1c. These observations show that the monolayer behavior is rather complex, and these results do not provide sufficient information for modeling structural changes occurring during monolayer compression in these transition regions. The reflectivity of the monolayer in the BAM is determined by the layer thickness and the refractive index. Monolayer phases with tilted molecules often show long range tilt orientational order giving rise to an optical anisotropy in the plane. In the region of coexistence of two phases, domains may form that are sufficiently large to be visible in the Brewster angle microscope (BAM).36-38 The domains generally differ in brightness due to different azimuth of the molecular tilt. Indeed, extended fluctuating domains of different brightness are observed in monolayers of 8A5 on water, as shown in Figure 2a for an area of A ) 0.43 nm2 (π ) 0 mN/m). The aromatic part and the alkyl tail have a different tilt to account for the different area requirements. The refractive index of the chromophoric part should be larger than that of the hydrocarbon fraction of the molecules. Consequently, the brightness of the domains in the BAM images as well as the optical anisotropy in the plane are due to the organization of the azobenzene moiety of 8A5. From the BAM images, we conclude that formation of domains with long range order of the chromophores, i.e., a liquid crystalline phase (LC), occurs already before compression (π ) 0 mN/m). In the range of the nearly horizontal part of the isotherm, the domains are growing, however, the morphology is practically unchanged, see Figure 2b with A ) 0.36 nm2, π ) 4 mN/m. The brightness changes without showing sharp domain borders indicate gradual changes of the azimuth and/or variations of the polar tilt angle. A drastic change is observed in the course of the phase transition as observed at A ) 0.31 nm2, π ) 5 mN/m (see Figure 2c). Now, domains with well defined borders are characteristic for the morphology, and the brightness is modified by turning the analyzer (not shown here), which is typical for an optical anisotropy in the plane due to molecular tilt. When the monolayer is compressed to the steeply increasing part of the isotherm, A ) 0.28 nm2, π ) 15 mN/m, the domains have grown and the contrast is reduced. The π/A and ∆V/A isotherms of 8A5 monolayers on bicarbonate buffer, pH ) 8.5, are shown in Figure 1a and b,

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Figure 2. Brewster angle microscope (BAM) images of monolayers of 8A5 at different areas A per molecule on water a-d, and on bicarbonate buffer, pH ) 8.5, e-h; area A and surface pressure π are indicated in the images.

dashed lines, respectively. The behavior at pH ) 8.5 clearly differs from that on water. Upon compression, the surface pressure starts rising at about A ) 0.40 nm2, and the rise of surface pressure occurs at somewhat smaller area as compared to the isotherm on water, crossing that isotherm when the phase transition sets in on water upon compression. The bending of the isotherm at an area of about A ) 0.32 nm2 may indicate a phase transition, followed by a discontinuity at A ) 0.305 nm2 and a nearly horizontal part ending at about A ) 0.28 nm2 and a surface pressure of π ) 14 mN/m. At this point, the isotherms gets steeper, merging with that measured on water, and collapse occurs at about 25 mN/m corresponding to an area of A ) 0.27 nm2. The surface potential-area isotherm provides supporting information concerning possible phase transitions. As seen in particular in Figure 1c, dashed line, the quantity ∆Vnorm is practically constant in the range 0.40 nm2 > A > 0.32 nm2. As compared to the isotherm measured on pure water, this phase exists now up to the higher surface pressure of about π ) 12 mN/m. Upon compression, two phase transitions occur as characterized by the different linear sections in the plot of ∆Vnorm against area in Figure 1c, dashed line.

2586 J. Phys. Chem. B, Vol. 106, No. 10, 2002 This interpretation is in coincidence with the evidence obtained from BAM images taken in the course of monolayer compression (Figure 2e to 2h). Monolayer morphology in the range of the initial surface pressure rise (Figure 2e with A ) 0.34 nm2, π ) 6 mN/m) is similar to that on water at in the region A > 0.36 nm2. Upon further compression, an entirely new morphology is found, see Figure 2f with A ) 0.31 nm2, π ) 12 mN/m. Extended bright stripes separated by dark zones dominate the image. The brightness gradually changes across the stripes. Such a stripe texture is typical for liquid crystalline phases, and the particular brightness distribution has been attributed unambiguously to a variation of molecular tilt in such domains in the case of monolayers of octadecanoic acid.33 Therefore, the section of the isotherm for 0.32 nm2 > A > 0.305 nm2 clearly is a particular phase as has been deduced from the isotherm measurements (see above). At smaller area, the stripes transform to extended domains, see Figures 2g (A ) 0.29 nm2, π ) 13 mN/m) and 2h (A ) 0.275 nm2, π ) 16 mN/m). The concept of controlling molecular association in monolayers of 8A5 by varying the subphase pH is obviously correct, although the effects are not so prominent as may have been anticipated. However, it should be kept in mind, that at pH ) 8.5, only a fraction of the carboxyl groups may be dissociated. Therefore, a further increase of the pH should more drastically change the monolayer behavior of 8A5. This is indeed observed in the π/A isotherm, see Figure 1a, dotted line (aqueous bicarbonate buffer subphase, pH ) 9.4). The surface pressure starts rising upon compression already at an area of A ) 1.2 nm2 (this part is not shown in Figure 1a) and the isotherm shows no apparent phase transition until collapse occurs at about π ) 25 mN/m. Therefore, the monolayer seems to exist throughout the whole range of the area in Figure 1a in the liquid expanded state. This view is strongly supported by the BAM observations. The images taken in the whole range (not shown here) are totally featureless indicating the absence of domains of liquid-crystalline or other phase more condensed than liquid expanded of lateral dimensions of more than a few microns (lateral resolution of the BAM used here). However, the plot of ∆Vnorm against area (Figure 1c, dotted line) shows a slight rise of ∆Vnorm in the range 0.8 nm2 > A > 0.47 nm2 upon compression, and a subsequent linear decrease in the range 0.47 nm2 > A > 0.28 nm2. This may indicate molecular reorientation and/or association without formation of a new phase. Reflection Spectroscopy Reflection spectra ∆R of monolayers of 8A5 on water measured at different surface pressures are shown in Figure 3a, and plots of the corresponding product ∆R‚A ) ∆Rnorm are shown in Figure 3b. The quantity ∆Rnorm, representing the reflection normalized to the surface density of 8A5, shows more clearly changes of orientation and/or association than the directly measured spectra. Also represented in Figure 3a is the spectrum of 8A5 in solution (dot-dot-dashed line). The main band with maximum at 351 nm in solution is attributed to a π-π* transition, whereas the long wavelength band of low intensity is due to a n-π* transition. At a surface pressure π ) 0, A ) 0.45 nm2, the reflection spectrum shows a broad band with maximum at λ ) 339 nm and a band with much lower intensity between 400 and 500 nm. Compared with the spectrum measured in solution having the absorption maximum at λ ) 351 nm, a shift to shorter waves is observed. Upon compression to π ) 4 mN/m, A ) 0.36 nm2, the intensity of the band increases slightly, and an increasing

Pedrosa et al.

Figure 3. Reflection spectra ∆R of monolayers of 8A5 on water taken at different areas A (given at the curves in nm2 per molecule) (a), and plot of the corresponding values of ∆Rnorm ) ∆R × A against wavelength λ (b). For the evaluation in the following Section, the spectrum measured in solution (solvent: chloroform:methanol ) 3:1, v/v; concentration: 7.5 × 10-5M, length of the light path: 1 cm) is plotted in (a), dot-dot-dashed curve.

shift to shorter wavelengths is observed. Further compression causes a reduction of the intensity and additional shift to shorter waves. Such a shift of the strong band to shorter wavelengths has been reported by Kawai et al.11 and has been attributed to increasing association of the azobenzene chromophores. However, these authors did not report a decrease at higher surface pressures found here on water subphase. The plots of the corresponding ∆Rnorm shown in Figure 3b clearly demonstrate a reduction of the reflection normalized to the same surface density. On bicarbonate buffer, pH ) 8.5, as subphase, the spectroscopic changes upon compression, see Figure 4a, are similar to those on water. At π ) 0 mN/m, A ) 0.45 nm2, the reflection band is a little higher and extends more to longer waves. This may indicate a reduced association as compared to the water subphase. Upon compression, the band increases a bit in intensity and is shifted to shorter waves. Comparing ∆Rnorm (Figure 4b) with that obtained on water (Figure 3b) a smaller value is found at π ) 25 mN/m, A ) 0.27 nm2, than in the corresponding case on water. The situation should be quite different for monolayers on bicarbonate buffer, pH ) 9.4 because a much larger fraction of dissociated carboxyl groups may be expected in this case, giving rise to a reduction of the attractive intermolecular forces. This has already been expressed in the π/A and ∆V/A isotherms discussed above. Indeed, the reflection spectrum measured at large area, A ) 0.79 nm2 (see Figure 5a) has the same shape including the full width half-maximum and wavelength of the maximum, λmax ≈ 351 nm, as the spectrum in organic solution (Figure 3a, dotted line). This clearly indicates the absence of associates of 8A5 in the monolayer under these conditions in contrast to the behavior on water and on the bicarbonate buffer

Amphiphilic Azobenzene Derivative in Monolayers

J. Phys. Chem. B, Vol. 106, No. 10, 2002 2587 SCHEME 2: Graphic Definition of the Polar Tilt Angle, θ (2a), and the Slip Angle, O (2a). Theoretical Organization Models: O ) 0 Model 1 (2b), and O ) θ Model 2 (2c).

Figure 4. Reflection spectra ∆R of monolayers of 8A5 on bicarbonate buffer, pH ) 8.5, taken at different areas A (given at the curves in nm2 per molecule) (a), and plot of the corresponding values of ∆Rnorm against wavelength λ (b).

(Figure 5b), although it is smaller than that observed for the other subphases at the same area. Analysis of Molecular Orientation and Aggregation

Figure 5. Reflection spectra ∆R of monolayers of 8A5 on bicarbonate buffer, pH ) 9.4, taken at different areas A (given at the curves in nm2 per molecule) (a), and plot of the corresponding values of ∆Rnorm against wavelength λ (b).

of pH ) 8.5. Upon compression to an area of A ) 0.45 nm2 (π ) 4.5 mN/m), the intensity of the band increases without change of the shape. At smaller areas a shift to shorter waves is observed (see Figure 5a). Indeed, ∆Rnorm decreases under compression

When the surface pressure increases two phenomena take place in the reflection band: first, ∆Rnorm decreases that should be related to a decreasing of the polar tilt angle of the chromophores, and second, shift of the maximum wavelength to shorter waves that is attributed to the formation of H aggregates of the azobenzene chromophores.11 In the last case, the shift is a function of the aggregation number and of the tilt between chromophores. Therefore, two diferent tilt angles with influence on the spectroscopic behavior of the azobenzene molecules should be defined. We denominate: polar angle, θ, as the angle between the azobenzene transition moment and the normal to the air-water interface, and slip angle, φ, as the complementary angle between the long chromophore axis and the line connecting the chromophore centers of neighbor molecules. Both angles are represented in Scheme 2a, where for simplicity the alkyl chains are not drawn. The area per 8A5 molecule obtained by measuring the π/A isotherms (Figure 1a) indicates that the molecule is not lying flat on the aqueous substrate, provided that there is no stacking normal to the water surface because the area of the azobenzene chrompohore as determined from molecular models is about Achr ) 0.75 nm2 in a flat-lying orientation. If the minimum area required by the hydrocarbon substitutent as well as the carboxyl group, 0.20 nm2 each part, is taken into account the total minimum area of a flat lying molecule is Aflat ) 1.15 nm2. The mimimum area of the azobenzene chromophore is A0 ) 0.245 nm2 corresponding to an orientation of the transition moment normal to the water surface. Assuming that the hydrocarbon part of the molecule can easily adapt to any orientation of the

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chromophore and does not require additional area in this situation, the area per molecule is determined by the chromophore. Further, if we assume stacking of the chromophores when tilted by the polar angle θ, the area is given by

A ) A0/cosθ

(1)

This equation is not valid for the case of flat-lying molecules. From this simple model, we obtain the tilt angle θ at the various areas per molecule for which reflection spectra have been measured. Independent information on the molecular tilt is obtained from the analysis of the pressure dependence of the reflection spectra. Molecular orientation and association can be investigated more directly by spectroscopy rather than by the monolayer techniques used above providing some preliminary information. This approach has already been taken for transferred monolayers on solid support by the group of Hans Kuhn in the visible range8,29 and by Takenaka and collaborators using infrared ATR techniques.39,40 The average orientation of transition moments in monolayers on water has been evaluated from measurements of ∆R with polarized light incident under various well-defined angles.9 However, in this work, we determined the molecular orientation of the amphiphilic azobenzene from the reflection spectra measured with unpolarized light under normal incidence.16 The used method, described below, is based on the comparison of the reflection spectrum with that obtained in solution. The orientation of, e.g., hydrocarbon chains as well as headgroup organization in monolayers at the air-water interface has been determined by a variety of techniques, e.g., IR41 or nonlinear optical methods.42-45 However, the methods used here are most adequeate for investigation the orientation and association of chromophores that is the object of this study. No evaluation of chormophore association and orientation in monolayers on aqueous subphases has been done in earlier studies with 8A5.11 The solution spectrum, see Figure 3a, dot-dot-dashed line, clearly shows the band of the azobenzene in the monomeric E-form (trans) with maximum at 351 nm as well as the band of the n-π* transition in the range between 400 and 520 nm. An integration of the absorption in the range between 280 and 545 nm yields the oscillator strength f ) 0.523. The reflection ∆R observed under normal incidence of light, for low values (e.g., < 0.01), is given in a reasonable approximation by30

∆R ) 2.303 × 103ΓforientxRi

(2)

where Γ is the surface concentration in mol cm-2, Ri ) 0.02, the reflectivity of the interface air-water at normal incidence,  the extinction coefficient given as L mol-1 cm-1, and forient is a numerical factor that takes into account the different average orientation of the azobenzene transition moment in solution as compared to the monolayer at the air-water interface. In solution, where the orientation is at random, the absorption must be proportional to a factor 2/3, because only two of the three components of the transition moment of the azobenzene group are interacting with the incident unpolarized radiation. However, at the air-water interface, and for a statistical distribution of the azobenzene transition moment in the layer plane, this factor is 1, since all the transition moments interact with the incident unpolarized radiation. Therefore, in this case forient ) 1/(2/3) ) 3/2. For a general case, and with a statistical

Figure 6. Plot of the apparent oscillator strength fapp determined according to eq 6 against area A. The circles refer to two sets of spectra measured at various surface pressures on water, the squares refer to those measured on aqueous bicarbonate buffer, pH ) 8.5, and the triangles refer to those on bicarbonate buffer, pH ) 9.4. The solid line represents the theoretical dependence of fapp on A according to eq 7 with the parameters f ) 0.523, and A0 ) 0,245 nm2. The right scale refers to the corresponding tilt angles.

distribution of the transition moments around the surface normal, the orientation factor is

3 forient ) sin2 θ 2

(3)

In any case, the eq 3 is applicable only under the condition of a homogeneous distribution of the transition moments around the surface normal. The oscillator strength is defined as46

f)

402.303mec0 2

N Ae

∫band dυ ) 1.44 × 10-19∫band dυ

(4)

where 0 is the permittivity of vacuum, me the electron mass, e the elementary charge, c0 the speed of light in a vacuum and NA the Avogadro constant. In eq 4, the numerical factor 1.44 × 10-19 is expressed in mol L-1 cm s. From equations 2 and 4 is possible to define an apparent oscillator strength determined from the measured reflection spectra as

fapp ) f × forient )

1.44 × 10-19

∫band ∆Rdυ

2.303 × 103ΓxRi

(5)

If we take in account the relationship between the surface concentration, Γ, in mol cm-2, and the area per molecule, A, in nm2 (Γ ) 1014/ANA) and we introduce the value Ri ) 0.02,9 we obtain

fapp ) f × forient ) 2.6 × 10-12

∫band A∆Rdυ ) 2.6 × 10-12∫band ∆Rnormdυ

(6)

where the numeric factor 2.6 × 10-12 is expressed in nm-2 s. Thus, fapp is obtained by the integration of the normalized reflection band in the range between 280 and 545 nm. The values of fapp evaluated from two different series of reflection spectra on the three different subphases are plotted using different markers against the area A in Figure 6. The tilt angle θ can be calculated for any fapp by using eq 3 and fapp ) f × forient with f ) 0.523. The θ values thus obtained are plotting at the right axis in Figure 6.

Amphiphilic Azobenzene Derivative in Monolayers

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Figure 7. Values of the wavelengths of the maximum, λmax, observed in the reflection spectra of the monolayers of 8A5 on different aqueous subphases plotted against area A; circles: water; squares: bicarbonate buffer, pH ) 8.5; triangles: bicarbonate buffer, pH ) 9.4. The solid lines are polynomial fits.

Figure 8. Reciprocal of the aggregation number Nagg of 8A5 as calculated from the values of λmax observed on the subphases water and bicarbonate buffer pH ) 8.5, respectively, according to the theoretical treatment given in the Appendix plotted against area A. The straight line is a least-squares fit.

The results shown in Figure 6 allow to conclude that the decrease of ∆Rnorm with decreasing area A is due to a decrease of the polar tilt angle, θ, of the chromophores. The fapp value, and consequently the polar tilt angle of the chromophores, only depends on the area A but not on the composition of the subphase. According to eq 1 and combining eqs 3 and 4 a simple relation between fapp and area A can be achieved

3 fapp ) f × forient ) f 2

x () 1-

A0 A

2

(7)

The solid curve shown in Figure 6 represents this function for f ) 0.523 and using the value A0 ) 0.245 nm2. The experimental data determined from the reflection spectra measured on the three different subphases at various surface pressures coincide very well with the theoretical dependence. Therefore, the simple determination of the tilt angle from the measured area per molecule is supported by the spectroscopic measurements. The values of λmax observed on the three different aqueous subphases are plotted in Figure 7 against the area A. Here, the circles refer to water, the squares to pH ) 8.5, and the triangles to pH ) 9.4. It is obvious from this plot, that the monolayer behavior on pH ) 9.4 is quite different from that on the other subphases with monomers existing for A > 0.5 nm2 and increasing formation of dimers and/or H aggregates with decreasing area A. On the other subphases, associates exist even at a surface pressure of π ) 0 mN/m, A ) 0.45 nm2 as indicated by the λmax ) 340 nm (circles and squares), as observed by Kawai et al.11 The increasing shift with increasing surface pressure may be attributed to an increase of the number Nagg of chromophores associated in the aggregate, and/or to the decrease of the slip angle, φ, between chromophores. However, the shift is independent of the θ angle. The shift is calculated using the extended dipole model28 instead of the point dipole approximation. Calculations of the frequency shift for the aggregation may be accomplished presuming either a point-dipole approximation to coupling interactions or an extended-dipole approximation. These two approximations were derived and compared for several relative geometries in the seminal papers by Kuhn.27,28 In contrast to the point-dipole method, the extended-dipole approximation considers the Coulombic interactions of the partial charges with both distal and proximal neighbors. For

Figure 9. Plot of the calculated wavelength λN of the absorption maximum of the aggregate against aggregation number N for different values of slip angle φ. The dotted lines represent the λmax of the monomer (351 nm), the dimer (340 nm) and of the aggregate with Nagg ≈ ∞(317 nm), for φ ) 0°, respectively.

this reason, the extended-dipole approximation is more accurate in determining interaction energies for the small intermolecular separation distances appropriate to organic monolayer systems. In the limit of large separation distance, both approximations become identical.47 In the Appendix, we obtain the shift as a function of Nagg and φ (Figure 9), and two extreme situations will be analyzed. In model 1 (see Scheme 2b), the slip angle φ ) 0 for any θ value is supposed. According to model 2 (see Scheme 2c), φ ) θ, like occurs during the domino effect. Intermediate situations are possible, although the comparison between our experimental results and these two extreme situations allows us to discriminate the type of preferential tilt of the azobenzene molecules. According with model 1, φ ) 0, the value of λmax ) 340 nm is obtained for the dimer, Nagg ) 2 (see Figure 9). A λmax ) 340 nm is obtained for subphases water and bicarbonate buffer, pH ) 8.5, at π ≈ 0 mN/m, suggesting that under these conditions dimers are formed, and φ ) 0 for any surface pressure. By comparison of the data shown in Figures 6 and 7 with the theoretical predictions shown in Figure 9, the model 2 may be discarded. As an example, with θ ) 60° corresponding to the area is equal to 0.40 nm2 (data from Figure 6) and

2590 J. Phys. Chem. B, Vol. 106, No. 10, 2002 according to model 2, i.e., θ ) φ, a value of λN g 349 nm (Figure 9) should be obtained for any aggregation number. However, at such area value (A ) 0.40 nm2) λmax ≈ 335 nm (Figure 7) is found for subphases water and aqueous bicarbonate buffer, pH ) 8.5. Therefore, meaningful aggregation numbers have been obtained with model 1 only. In Figure 8, the values of the reciprocal of the aggregation number Nagg according to model 1 are plotted against area A. The circles refer to measurements on water, the squares to those on bicarbonate buffer, pH ) 8.5. As expected, the reciprocal of the number Nagg decreases with monolayer compression. From Figure 8, aggregation numbers of more than 5 are evaluated for compression to areas below A ) 0.32 nm2. The straight line is a least-squares fit to the evaluated points shown here without theoretial interpretation. It is worth pointing to the fact, that the reflection spectrum measured on the bicarbonate buffer, pH ) 9.4, at A ) 0.30 nm2 (λm ) 340 nm) is practically identical with that obtained both on water and on bicarbonate buffer, pH ) 8.5, at an area of A ) 0.45 nm2 (λm ) 340 nm), respectively. This strongly suggests that the same type of organization (dimer) is present under these conditions, although the polar tilt angle is quite different, i.e., θ ≈ 52° in the case of pH ) 9.4, and θ ≈ 70° for water and pH ) 8.5. This observation favors model 1 assuming no slip between neighbor molecules. On the other hand, and although the organization of the azo group is similar in the above cases (see the reflection spectra), the BAM images of those monolayers are different. In fact, on water and bicarbonate buffer at pH ) 8.5 (Figure 2), extended fluctuating domains (liquid crystalline phase, LC) are observed. The fluctuating property of those domains indicates the fast formation and disappearance of molecular aggregates with long range order. However, the reflection spectra obtained in these situations show a small aggregation of the azo group: dimer at A ) 0.45 nm2 (π ) 0 mN/m) and almost trimer at A ) 0.36 nm2 (π ) 4 mN/m) (see Figure 8). This behavior could be due to the tendency of association of the alkyl chains of molecules driving somehow the azo-group interactions. In the case of the bicarbonate buffer subphase at pH ) 9.4, the carboxyl groups of 8A5 are totally dissociated, thus a strong repulsion of the headgroups is expected, and thereby, no LC phase and domains are observed by BAM. The formation of dimers (A ≈ 0.30 nm2, see Figure 8) requires under these conditions the application of high surface pressures (π ) 20 mN/m, see Figure 1), to achieve an orientated approach of the azo groups is allowed. Furthermore, the subphase ion interactions on the headgroups inhibit the chormophore aggregation as evidenced by the reflection spectra. Conclusion The organization azobenzene derivative 8A5 in monolayers at the air-water interface depends on the subphase composition and surface pressure. On the aqueous bicarbonate buffer, pH ) 9.4, flat lying monomers of 8A5 are dominant until the monolayer is compressed to about 5 mN/m when dimer formation sets in and the molecules start to become tilted reaching a tilt angle of about 65° with respect to the surface normal. On the subphases where a small fraction of the carboxyl groups only is ionized, i.e., water and bicarbonate buffer, pH ) 8.5, respectively, 8A5 forms dimers even at very low surface pressure. Upon compression, these dimers become tilted and associate to H aggregates as indicated by the shift of the reflection spectrum to shorter waves. The aggregation number as evaluated using the extended dipole model, increases upon

Pedrosa et al. compression and reaches values > 5 before monolayer collapse. These results demonstrate that orientation as well as molecular association of the azobenzene derivative 8A5 in monolayers may be controlled by varying the composition of the aqueous subphase. Appendix In this work, we have assumed an organization of stacked dipoles as shown in Scheme 2c where the geometrical centers of the dipoles are spaced at distance r and slipped by the angle φ. In any case, this assumption is independent of the value of the polar tilt angle, θ. Given this structure, the average interaction energy of a stacked aggregate with N dipoles, is as follows

∆EN ) ∆Emon + 2J12

2(N - 1) 2(N - 2) + 2J13 + N N 2(N - 3) + ‚‚‚ (A-1) 2J14 N

If N f ∞, this equation may be written as ∞

∆E∞ ) ∆Emon + 4

J1j ∑ j)2

(A-2)

where

J1j )

[

2 1 1 1 1 + - D a1j b1j c1j d1j

]

(A-3)

D ) 2.5 is the dielectric constant, and a1j, b1j, c1j, and d1j are the distances between ends of the dipoles positive-positive, negative-negative, positive-negative, and negative-positive, respectively. In the extended dipole approximation, the molecules are replaced by dipoles of a fixed length (l), charge ().28 The coordinates of the positive and negative ends of each dipole, i, are designated as li+ and li-, and l ) |li+ - li-|. In this way

a1j ) b1j ) |l1+ - lj+| ) |l1- - lj-| ) c1j ) |l1+ - lj-| )

x

[

x

[

(j - 1)r (A-4) cos(φ)

l2cos(φ)2 + lsin (φ) +

]

(j - 1)r 2 cos(φ) (A-5)

and

d1j ) |l1- - lj+| )

l2cos(φ)2 +

]

2 (j - 1)r - l sin (φ) cos(φ) (A-6)

If we assume that the monolayer of 8A5 arrives at infinite aggregation number under high surface pressure, λ∞ ) 317 nm, and using the value λmon ) 351 nm for the monomer, it follows from eq A-2 that ∞

J1j ) ∑ j)2

(

hc107 1 4

λ∞

-

1 λmon

)

) 1.517 × 10-13erg

(A-7)

The value M ) l ) 2.62 × 10-29 C m for the transition dipole has been obtained by integrating the absorption band in solution. With this value and assuming that r ) 0.35 nm as well as φ ) 0°, the values l ) 1.027 nm and  ) 0.159 e are determined by

Amphiphilic Azobenzene Derivative in Monolayers

J. Phys. Chem. B, Vol. 106, No. 10, 2002 2591

comparing eqs A-3 and A-7. With these values of l and , we obtain λN (maximum wavelength) for any aggregation number N and slip angle φ. From equation (A-1)

λmonhc107

λN )

N

hc10 + 4λmon 7

J1j ∑ j)2

(N + 1 - j)

(A-8)

N

Figure 9 shows a plot of λN against N for different values of φ. For N ) 2 (dimer), λmax ) λ2 ) 340 nm is found for φ ) 0. Acknowledgment. The authors wish to express their gratitude to Spain’s DCICYT in the framework of projects BQU2011792 and HA1999-0075, and German’s DAAD for financial support of this research. We thank Mr. W. Zeiss for his unestimable technical assistance. References and Notes (1) Roberts, G. G. Langmuir-Blodgett Films; Plenum: New York, 1990. (2) Ullman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (3) Polymeropouplous, E. E.; Mo¨bius, D.; Kuhn, H. J. Chem. Phys. 1978, 68, 3918. (4) Blinov, L. M.; Dubinin, N. V.; Mikhnev, L. V.; Yudin, S. G. Thin Solid Films 1984, 120, 161. (5) Kuhn, H. Pure Appl. Chem. 1965, 11, 345. (6) Kuhn, H. On possible ways of assembling simple organized systems of molecules. In Structure Chemistry and Molecular Biology; Rich, A., Davidson, N., Eds.; Freemann, W. H.: San Francisco, London, 1968; p 566. (7) Kuhn, H.; Mo¨bius, D. Monolayer Assemblies. In InVestigations of Surfaces and Interfaces; Rossiter, B. W., Baetzold, R. C., Ed.; John Wiley & Sons: New York, 1993; Vol. IXB.; p 375. (8) Bu¨cher, H.; Drexhage, K. H.; Fleck, M.; Kuhn, H.; Mo¨bius, D.; Scha¨fer, F. P.; Sondermann, J.; Sperling, W.; Tillmann, P.; Wiegand, J. Mol. Cryst. 1967, 2, 199. (9) Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966. (10) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (11) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. (12) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (13) Schoeler, U.; Tews, K. H.; Kuhn, H. J. Chem. Phys. 1974, 61, 5009. (14) Heesemann, J. J. Am. Chem. Soc. 1980, 102, 2167.

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