Second Harmonic Generation (SHG) was used to study the non-linear optical properties of a two-dimensional film formed by the achiral amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18) at the air-water interface. The S-polarized SHG intensity was measured as a function of the incident fundamental wave polarization angle during the monolayer compression. The method was applied to follow the emergence of chirality during the film compression and at constant surface pressure. The formation of molecular aggregates revealing supramolecular chirality was then demonstrated. It was shown furthermore that the origin of chirality was dominated by the magnetic contributions.
©2011 Optical Society of America
Chirality is an important issue in chemistry, biology or material science. This symmetry property can be observed at different scales, from single molecule to supramolecular systems. Recently, it has been shown that achiral amphiphilic molecules can form chiral molecular aggregates at different interfaces: Langmuir-Blodgett or Langmuir-Schaeffer films [1,2], metal surfaces , liquid/solid  and air/water interfaces [1,5,6]. At the latter air/water interface, the exact mechanism inducing chirality, spontaneous or compression-induced formation of chiral aggregates is still an open question. To give further insights into the origin of this induced chirality, we report an example of chiral aggregation at the air/water interface by using the amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18) . This molecular system is well-known to form chiral Langmuir-Schaeffer films . One question still open is to know whether this chirality was already present at the liquid interface, before the transfer on the solid substrate, or not. To resolve that issue, the chirality of a Langmuir monolayer at different surface densities was measured directly at the air/water interface. Very few techniques can perform such optical measurements in situ at the air/water interface. Indeed, chiral sensitive tools such as scanning tunneling microscopy (STM) [9–11], scanning force microscopy (SFM) [12,13] or circular dichroism (CD) [2,7] are intensively used to probe chirality of surfaces but they cannot be applied to Langmuir monolayer. On the other hand, the surface Second Harmonic Generation (SHG) has proven in the past to be a powerful surface sensitive tool. This technique, based on the conversion of two photons at a fundamental frequency ω into one photon at the harmonic frequency 2ω, is surface sensitive at interfaces between two centrosymmetric media. Indeed, within the electric dipole approximation, no Second Harmonic (SH) light can be generated in the bulk of media possessing inversion symmetry like gases and liquids. As a result, SH light can only be produced at the interface between those two media where the inversion symmetry is broken. Hence, the approach is non invasive and can be used to investigate both the structure and the dynamics at such surfaces and interfaces [14–19]. Its combination with a Langmuir trough allows furthermore nonlinear optical studies with a precise control of the average surface density of the amphiphilic compounds spread out at the liquid surface [5,20].
To reveal the chiral properties of an interfacial molecular film, SHG measurements can be performed in several different ways. The first one consists in illuminating the sample with a linearly polarized beam, usually p-polarized, and measuring the polarization rotation due to the presence of chiral structures. This method is similar to the linear Optical Rotation Dispersion technique (ORD) and is named ORD-SHG in the nonlinear regime . The second one is the measurement of the SHG intensity for left and right circularly polarized fundamental beams. This method, named CD-SHG, is equivalent in the nonlinear regime to the linear Circular Dichroism technique (CD) [22–25]. The third one is SHG-linear dichroism (LD-SHG) and has no analogue in the linear regime. It consists in measuring the difference in the SHG intensity collected for a + 45° and a −45° linearly polarized fundamental beam [26–28]. In the present work, a similar study is investigated where the fundamental beam is linearly polarized with its angle of polarization varied from 0° to 360° in order to increase the sensitivity of the experimental set-up.
In this article, we study the nonlinear properties of a two-dimensional film formed by the achiral amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18). In the first part, after a brief description of the experimental apparatus and of the film fabrication, we focus on the SHG technique resolved in polarization to evidence supramolecular chirality. In the second part, we show that it is possible to apply the technique to follow the emergence of chirality not only during the film compression but also at constant surface pressure, by maintaining the mechanical constraints on the film.
2. Materials and method
The TARC18 molecules were synthesized following the procedure given in a previous publication . The molecular structure of the compound is shown in Fig. 1 . The compound exhibits a long hydrophobic alkyl chain and an efficient chromophore for SHG with delocalized π-electrons in an unsymmetric environment. The monolayers were prepared using a standard Langmuir trough with a maximum surface area of 100 cm2 (Nima Technology, model 601). The trough was associated to a Wilhelmy plate in order to record the pressure-area isotherms during the film compression at constant temperature. All experiments were carried out at room temperature. Ultra pure water (Millipore 18 MΩ.cm) was used as the sub-phase. A solution of TARC18 in chloroform was prepared (~3x10−4M) and 80 µL of this solution was spread out at the air/water interface. After the evaporation of the solvent, about 10-15 minutes later, the isotherms were recorded with a barrier speed of 10 cm2/min. A typical isotherm is plotted in Fig. 1. This isotherm was similar to the one already published in the past for this molecular system  showing that TARC18 monolayer on the pure water surface is in a condensed state.
The SHG setup was then developed around the air/water interface of the Langmuir trough, see Fig. 2 . Briefly, it consisted in a femtosecond Ti-sapphire oscillator laser source providing pulses with duration of about 70 fs at a repetition rate of 80 MHz (Spectra-Physics, model Tsunami). After passing through a low-pass filter to remove any unwanted harmonic light generated prior to the interface, the fundamental beam set to a wavelength of 800 nm and an averaged power of about 1 W was focused by a lens with a 10 cm focal length onto the air/water interface. The incidence angle was set at a value of 70° corresponding to an optimum incidence angle for the SHG intensity in reflection from an air/water interface. The SH light was collected by a 10 cm focal length lens and separated from its fundamental counterpart by a high-pass filter. The SH light was detected with a water-cooled back-illuminated CCD camera (Andor, model DU440) placed after a spectrometer (Jobin-Yvon, model Spex500M). The fundamental input beam was linearly polarized and the input polarization angle γ was selected with a rotating half-wave plate. The angle γ = 0 corresponds to a p-polarized fundamental beam and γ = π/2 to an s-polarized fundamental beam. An analyzer, placed in front of the spectrometer, was used to separate the S- and P-polarized SH intensities.
3. Results and discussions
3.1. Evidence for chirality in the molecular films
In order to demonstrate the appearance of chirality in the monolayer, we performed polarization angle-resolved SHG measurements. As seen previously on another molecular system, it has been shown that it is possible to characterize the molecular aggregation at the air/water interface by recording polarization plots [5,6]. The P-polarized and S-polarized SHG output intensities, also named the P-Out and S-Out intensities, were recorded as a function of the input polarization angle γ ranging from 0 to 2π. In this work, only the S-Out plots are displayed, the P-Out ones being less sensitive to chirality as reported previously . Initially, for the first measurements, the SH intensities were recorded for a molecular film at low surface density. In this condition, at a molecular area of 54 Å2/molecule, the surface pressure was still negligible, see Fig. 1. The S-Out polarization plot recorded is shown in Fig. 3 . This plot exhibits a four-lobe pattern and can be analyzed using the standard form of the SH intensity in the electric dipole approximation as a function of the input polarization angle γ [29,30]:Equation (1) was established in the case of an isotropic and achiral interface within the symmetry. In that case, the susceptibility tensor possesses only 7 non-vanishing elements, three of them being independent, namely, and. Therefore, in this low surface density regime, the typical four lobes pattern of the S-Out plot indicates that the molecular film spread at the interface was achiral.
Polarization plots were then recorded at a higher surface density, near the collapse of the film. A typical S-Out polarization plot obtained at a surface pressure of 25 mN/m is given in Fig. 3. This plot clearly shows strong modifications as compared to the previous one recorded at low density. In particular, the SH intensities recorded for the incident polarization angles π/2 and 3π /2 initially vanishing take values of the same order of magnitude as those of the maxima. As previously published, these strong deformations can only be explained by the presence of chiral compounds at the interface. These compounds are molecular aggregates of the initial achiral compounds as it is expected for TARC18 molecules which form chiral supramolecular structures in Langmuir-Blodgett films . Hence, it appears that, in this regime of high average surface densities, it is no longer possible to fit the experimental data with the Eq. (1). The appearance of chirality leads to a modification of the surface symmetry which becomes now, a symmetry with no mirror planes. This lowering of the symmetry increases the number of independent non vanishing components of the susceptibility tensor  and, if the electric dipole approximation is still valid, the new element has to be added in the expression of the S-Out intensity. It yields the following modification of the SH intensity in the S-out polarization configuration :
However, if the introduction of this component in the SH intensity can explain the non-vanishing value of the intensity at the polarization angle γ = 0, it will never explain the non-vanishing value of the SH intensity at the polarization angles γ = π/2. The modification of the polarization plots cannot be accounted for with surface anisotropy. It was therefore necessary to go beyond the electric dipole approximation and introduce the magnetic contributions too. The SH intensity in the S-Out polarization state then becomes :Eq. (3), it was possible to fit correctly the experimental data presented in Fig. 3. The analysis of the SHG polarization plots at this higher average density indicates clearly that electric and magnetic dipole contributions in the susceptibility tensors are both necessary to correctly analyze the data. Finally, when supramolecular chirality is observed, the SH intensity fluctuates more strongly as illustrated in Fig. 3. This behavior can be interpreted by the diffusion motion of domains at the liquid interface. The presence of such domains characterized as enantiomeric domains, has also been briefly addressed in the previous publication reporting the case of DiA films . Similarly, it is also expected the existence of such enantiomeric polydomains in the TARC18 films, a more complete study including the sign evolution of the susceptibility tensor elements being necessary to conclude on that point.
3.2. Monitoring of the chirality during the film compression
The previous section has shown that the SH intensity in the S-Out polarization is a very sensitive configuration to determine the emergence of chirality in a molecular film formed at the air/water interface. Using this configuration, we also monitored the emergence of chirality during the film compression. To follow this appearance, we measured the SH intensity only at particular input polarization angles of the polarization plot presented in Fig. 3. For instance, the measurement of the difference in SH intensities for the polarization angles γ = π/4 and γ = -π/4 (3π/4 or 135°) can be used to demonstrate SHG Linear Dichroism. On the other hand, the input polarization angle γ = π/2 is also particularly interesting because it corresponds to the element, a pure chiral term in the susceptibility tensor in the magnetic dipole approximation. For these reasons, we performed the monitoring of the SH intensity at the three particular angles of polarization γ = π/4, π/2 and 3π/4 during the compression of molecular film.
Two experiments were realized. The first one consisted in the study the nonlinear optical properties of the molecular film at low compression. We checked out the reproducibility of the measurements by recording the SH intensity during several successive compression cycles. The film was formed following the procedure described in the previous section. After deposition of the molecules onto the air/water interface, three successive slow compression and decompression cycles were realized in the low surface pressure regime. In these experiments, a maximum of 15 mN/m for the surface pressure was reached, corresponding to a regime far from the collapse conditions of the film. The evolution of the SH intensity for the input polarization angles γ = π/4, π/2 and 3π/4 during these compression cycles is presented in Fig. 4 . This figure shows that the SH signals for γ = π/4 and γ = 3π/4 are very similar in intensity. These intensities increase with the surface pressure and the general trend is maintained during the three compression cycles. However, the signal evolution was clearly different during the first compression cycle, performed after the molecule deposition. This behavior demonstrates the occurrence of reorganization in the packing of the molecules at the interface under the closing of the barriers. For the following compression cycles, the SH intensity increased more regularly as a function of the surface pressure. This figure also shows that the SH intensity for γ = π/2 is equal to zero whatever the surface pressure. Therefore, this graph clearly demonstrates that chirality was not observed during these successive compression cycles at low surface pressure. Hence, chirality only occurred at higher compression states of the film.
In a second experiment, compression cycles of the molecular film were performed until the collapse state of the film was reached. Figure 5 presents typical evolution of the SH intensity recorded simultaneously to the surface pressure. As observed previously in Fig. 4, the intensities at γ = π/4 and 3π/4 initially follow the increase of the surface pressure, see inset of Fig. 5. At the same time, the intensity for γ = π/2 is equal to zero, indicating that chirality is absent at the start of the isotherm. Then, the intensity increases near the collapse of the film, for a surface pressure equal to approximately 22 - 25 mN/m. This non-vanishing value for the intensity is a clear demonstration of the appearance of chirality. Furthermore, the observation of chirality in this polarization configuration indicates that is origin is of magnetic dipole nature. However, at this point, we cannot conclude whether chirality has a pure magnetic dipole origin due to the fact that differences in the SHG intensity were also observed for the input polarization angles γ = π/4 and 3π/4. As seen in Eq. (3), these two configuration angles involve chiral elements at the level of both the electric and the magnetic dipole approximation. However, a comparison of the SH intensities for γ = π/4 and π/2 at the highest compression states shows that, even if chirality arises in the film from the coupling of the electric and magnetic fields at the fundamental frequency, the magnetic dipole contribution is the dominant one. Once the supramolecular chirality had appeared at the interface, the SH intensity was strongly enhanced. As a result, the ratio of the SH intensities at high and low surface densities is about 100, a ratio well beyond that of the square of the surface densities which is about 4. We conclude that strong modifications of the molecular hyperpolarizability in these molecular aggregates must be taking place in these compression regimes.
3.3. Appearance of chirality at constant surface pressure
Thereby, by monitoring the appearance of chirality during the isotherm of the TARC18 film, we have shown that it was probably linked to the formation of supramolecular aggregates at high compression states of the film. Its observation near the film collapse yielded an enhancement of the SH intensity. The question is now to see whether this chirality of supramolecular origin could be obtained in conditions far away of the collapse state of the film. Since this chirality seems to be compression-induced, we performed experiments at constant surface pressure. The principle of these measurements consisted in depositing the TARC18 molecules onto the air/water interface and compressing the film until the surface pressure reached a value of 20 mN/m. Once this maximum pressure was obtained, this surface pressure was maintained with the barriers. The SH intensity was then recorded under the mechanical constraint of the barriers for the three angles of polarization γ = π/4, π/2 and 3π/4, see Fig. 6 . As observed previously, during the compression and a first period where the pressure was maintained, no evidence for supramolecular chirality was observed at the interface. However, after about 500 s, the SH intensity started to fluctuate strongly. The intensity collected for γ = π /2 was hence no more equal to zero, a clear evidence for chirality of magnetic dipole origin. Once this phenomenon occurred, i.e. once the supramolecular aggregates were formed, the process was irreversible. This could be demonstrated with opening the barriers and recording an isotherm. The latter was clearly different from the one given in Fig. 1. At this stage of the study, we suggest that these fluctuations of the SH intensity arise from the diffusion of the molecular aggregates under the laser spot. For this reason, the recorded fluctuations were not reproducible in time from one measurement to another. These fluctuations were always observed to appear, whatever the initial surface pressure reached. Hence, supramolecular chirality was also observed for a surface pressure of 10 mN/m (data not shown). These measurements show that it was possible to observe and follow the emergence of chirality by applying a mechanical constraint on an initially achiral molecular film at the air/water interface. It is interesting to note that this set of experiments where the surface pressure is maintained is the classical way to form Langmuir-Blodgett films, the deposition onto solid substrate being generally performed at constant pressure. These results indicate therefore that the formation of chiral aggregates may be initiated before the deposition, depending on the time lapse before deposition itself.
In summary, we have studied the nonlinear optical properties of molecular films formed at the air/water interface of a Langmuir trough. These films were composed by the amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18) which is an achiral molecule. This study demonstrates that supramolecular chirality appeared at high surface densities. The analysis of the SHG intensity polarization plots at high surface densities indicated that both electric and magnetic dipole contributions were necessary to correctly interpret the data. Near the film collapse, the magnetic contribution became the dominant one. Finally, the present report shows that the SHG technique was well adapted to follow in situ the emergence of chirality in a molecular film by monitoring the S-Out SHG intensity. This supramolecular chirality was observed during the compression of the monolayer or during the application of a constant mechanical constraint with the barriers fixed in the trough.
The authors thank the Centre for Nano-Optics NANOPTEC of the Université Claude Bernard Lyon 1 for support.
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