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Second Harmonic Generation (SHG) was used to observe the reversible formation of chiral molecular aggregates at the air-water interface. The aggregates were formed by the chiral bridged binaphthol derivative 1( + ) under lateral compression in a Langmuir trough. The S polarized output SHG intensity was measured as a function of the input polarization angle of the fundamental beam during several compression cycles of the molecular film. The chirality of the supramolecular aggregates was established from the non vanishing SHG intensities collected for the p and s input polarization angles. It is demonstrated that the formation of the chiral domains with compression is a reversible process.
© 2014 Optical Society of America
1. Introduction
Chirality plays an important role in the interaction between light and matter. Chiral media composed of chiral molecules indeed possess specific properties known as chiroptical properties. Two main features can be identified, namely optical activity (OA) and circular dichroism (CD), and they have both been extensively discussed in the literature over the past few decades for their potential applications in molecular recognition [1–3], asymmetric catalysis [4–7] or enantioselective separation [7–10]. Moreover, it has been recently shown that chiral isotropic media can contribute to the design of new technologies in the future. An example is the class of optical planar waveguides called chirowaveguides that have been recently proposed [11]. For this application, it is however necessary to realize thin chiral molecular films. Different techniques are though available like spin- or dip-coating, Langmuir-Blodgett layer transfer or Pulsed Laser Ablation (PLD). Nevertheless, many questions remain, and in particular, the birth of chirality in large molecular ensembles still remains to be clarified. For example, supramolecular chirality can appear in molecular aggregates made from chiral or achiral molecules as a symmetry breaking phenomenon. This feature has in particular been shown at different interfaces for achiral amphiphilic compounds: Langmuir-Blodgett or Langmuir-Schaeffer films [12, 13], metal surfaces [14], liquid-solid [15] or air-water interfaces [16–18]. At the air-water interface, one simple way to form these chiral aggregates is to use lateral compression in a Langmuir trough. The technique is known to be suitable for the realization of molecular films and supramolecular chiral structures at two or three dimensions [19]. To characterize these films though, few techniques with surface chirality sensitivity are available. Techniques like scanning tunneling microscopy (STM) [20], scanning force microscopy (SFM) [21] or circular dichroism [13] cannot indeed be readily applied to Langmuir films. Brewster Angle Microscopy (BAM) can also be used to investigate chiral aggregation at the air-water interface [22]. This technique is nevertheless limited to aggregates with micrometer average diameters. One way to probe a monolayer film at the air-water interface at the microscopic level is to use nonlinear optics and in particular the simplest of the second order ones, namely Second Harmonic Generation (SHG). SHG has proven in the past to be a powerful tool to investigate molecular monolayers at liquid interfaces [17, 23]. Indeed, the technique, based on the conversion of two photons at the fundamental frequency ω into one photon at the harmonic frequency 2ω, is inherently surface sensitive at the interface between two centrosymmetric media like liquids. No Second Harmonic (SH) light can effectively be generated in the bulk of media possessing inversion symmetry like gases and liquids within the electric dipole approximation. At the interface between two such media, the centrosymmetry is broken and SH light can be produced. Hence, the approach is non-invasive and can be used to investigate both the structure and the dynamics at such surfaces or interfaces [24–27]. Its combination with a Langmuir trough allows furthermore the realization of the nonlinear optical studies with a precise control on the average surface density of the amphiphilic compounds spread out at the liquid surface [28].
The objective of this work is to investigate the quadratic nonlinear optical properties of molecular films formed by the chiral bridged binaphthol 1( + ) molecule [29]. The chirality property of the film can thus occur at two levels: at the molecular level from the chiral 1( + ) compound itself and at the supramolecular level in 1( + ) molecular aggregates. Here, we focus on the supramolecular chirality of the molecular film, showing that it stems from the compression induced formation of molecular aggregates and that it is a reversible process.
2. Materials and methods
2.1 Chemical compounds and films preparation
The structure of the 1 molecule, a binaphthol derivative, is presented in Fig. 1.In this work, only the 1( + ) enantiomer was used. This chiral molecule is in particular characterized by a high optical rotation power [29]. Besides, 1( + ) does not present any absorption bands at wavelengths longer than 400 nm. The details of its chemical synthesis and characterization can be found elsewhere [29].
The first step in forming 1( + ) molecular films is to prepare chloroform solutions. A syringe was then used to spread droplets of this solution at the air-water interface formed in a Langmuir trough (NimaTechnology, model 601). The sub-phase was composed of ultra pure water (Millipore 18 MΩ.cm) at a constant temperature of 17°C. The trough was fitted with a Wilhelmy plate to record the surface pressure during film compression.
2.2 Optical setup
The light source of the setup is a femtosecond Ti-sapphire oscillator laser characterized by pulses with duration of about 70 fs at a repetition rate of 80 MHz (Spectra-Physics, model Tsunami). The fundamental beam set to a wavelength of 840 nm and an averaged power of about 800 mW was focused by a lens with a 10 cm focal length onto the liquid interface in the Langmuir trough. A rotating half-wave plate was used to control the input polarization angle γ of the linearly polarized fundamental beam. A low-pass filter was used to remove any unwanted harmonic light generated prior to the interface. The laser beam was reflected on the interface with an incidence angle set to a value of 70° corresponding to an optimum incidence angle for the SHG intensity in reflection. Downstream of the trough, a 10 cm focal length lens was used to collect the SH signal and was associated with a high pass filter to separate it from the fundamental laser beam. The output polarization angle Г was selected thanks to an analyzer placed in front of the detection system. The following three angles were used: Г = 0 and Г = π/2 corresponding respectively to the P-out and S-out polarizations and the intermediate angle of π/4 corresponding to an equal mix of the S- and P-polarization intensities. The latter one was used to fully determine the relative phase between the three susceptibility tensor elements of the neat air-water interface. The SH light detection was performed through a spectrometer (Spex, 500M) associated with a water-cooled back-illuminated CCD camera (Andor, DU440).
3. Results and discussions
3.1 Bare air-water interface
In order to define a reference for the SHG measurements performed in the presence of the 1( + ) films, the study for the neat air-water interface was performed beforehand. To characterize this interface, standard polarization angle-resolved SHG intensity measurements were realized. These measurements were obtained by rotating the input polarization angle of the linearly polarized fundamental beam from 0 to 2π and collecting the SH intensity for three different output polarizations, namely the S-out, P-out and 45-out polarizations. A typical polarization plot is shown in Fig. 2 or the neat air-water interface.
Fig. 2 SHG intensity as a function of the input polarization angle for the S-, P- and 45-output polarizations obtained for the neat air-water interface. The solid lines correspond to the fit of the experimental data using Eq. (1).
These plots can be analyzed using the standard form of the SHG intensity in the electric dipole approximation as a function of the input polarization angle γ [30, 31]:
In the previous equation, Г is the output polarization angle. The ai, i = 1..5 parameters are five coefficients depending on the linear optical indices of air and water and the incidence angle. Only three non-vanishing and independent elements of the susceptibility tensor at the level of the electric dipole approximation are necessary to describe the isotropic air-water interface. These components, namely are associated with the isotropic and achiral interface. The ratios between these three elements are found to be = 0.37 and = 0.09 and are in perfect agreement with previously reported data [32,33]. It is important to note that the 45-out polarized component is useful to remove the sign ambiguity between the and elements.
3.2 1( + ) films at the air-water interface
The optical measurements were then performed with a 1( + ) monolayer spread at the air-water interface. The film was formed following the procedure described previously. After the deposition of the chiral molecules onto the air-water interface and after the evaporation of the solvent (approximately 10 min. later), several successive compression and decompression cycles were realized at the slow speed of 5 cm2/min. The maximum surface pressure was limited to 10 mN/m corresponding to a compression regime far from the film collapse [34, 35]. In Fig. 3, several compressions and decompression cycles are presented. Due to its molecular structure, the 1( + ) molecule does not possess a strong amphiphilic character. As a result, a non-negligible fraction of the 1( + ) molecules spread at the interface may transfer into the bulk volume of the water phase, as monomers or aggregates. Hence, areas per molecule extracted from the isotherms are therefore unreliable and largely underestimated as compared to those obtained for a truly amphiphilic binaphtyl acid [36]. However, the reproducibility of the isotherms demonstrates within the surface pressure range used that the system behaves as expected for a hydrophobic monolayer. For the first isotherm, the sharp slope increase occurs around 70 cm2 and this value shifts to about 66 cm2 for the following compression cycles and staying sensibly the same afterwards. This difference between the first isotherm and the following ones is due to the molecular organization at the air-water interface after the initial spreading. The packing of the film is thus efficient and due to the molecular aggregation at the interface. However, this molecular aggregation is reversible as demonstrated by the similarity of the successive isotherms.
Fig. 3 Successive compressions recorded at 17°C for a 1( + ) monolayers at the air-water interface. The black line corresponds to the first compression with a slope rupture around 70 cm2. The following compressions are represented by the dashed lines. The slope rupture is shifted to about 66 cm2.
Two different methods can be used to study the 1( + ) molecular films and bring out the chirality property. The first one consists in opening and closing the Langmuir trough barriers while recording the SHG intensity for a defined input and output polarization configuration. The second one consists in measuring the polarization resolved SHG intensity plots for a well defined surface pressure.
In order to observe the chirality of the 1( + ) molecular films, the second method was used. However, in this work, only the S-out polarization plots were measured. Indeed, it has been demonstrated in the past that the P-out polarization plots are less sensitive to chirality than the S-out ones [16]. The symmetry breaking induced by chirality of the initially isotropic interface is reflected in the interface susceptibility tensor. In the present case, the number of independent non-vanishing components of the susceptibility tensor increases. For the binaphthol derivative molecules, an intrinsic molecular chirality is expected at the electric dipole approximation due to the excitonic coupling between the two naphthol groups [37,38]. On the other hand, it has been shown in the past that, to account for supramolecular chirality, it is necessary to go beyond the electric dipole approximation and introduce magnetic dipole contributions as well [16,17]. In this case, Eq. (1) becomes invalid and the S-out intensity as a function of the input polarization angle γ is now given by [16]:
In this equation, the electric dipole chiral component was introduced as well as the chiral and non-chiral elements of the susceptibility tensor for completeness. Also, at the level of the magnetic dipole approximation, we have omitted the susceptibility tensor. The magnetic dipole approximation only operates at the fundamental frequency. At the harmonic one, 1( + ) electric dipole resonances start to play a non-negligible role and the susceptibility tensor is not expected to play any role.Starting with Eq. (2), it is now possible to bring out the intrinsic chirality of the monolayer arising from the 1( + ) molecular chirality, removing magnetic dipole components . The expected theoretical curve is shown on the Fig. 4 for a theoretical ratio . The plot is characterized by two different maximum values, thereby reducing the fourfold symmetry down to a twofold one. At the same time, the minimum S-out intensity remains at zero. Experimentally however, it is not simple to observe this molecular chirality for the 1( + ) compound. It is indeed necessary to work at very low surface pressures, conditions for which SH intensities are rather low, especially for the S-out configuration. Hence, observing unequal SH intensities between two successive maximum remains challenging. A higher sensitivity of the experimental set-up is required and this work is currently in progress in our laboratory.
Fig. 4 Theoretical S-Out SHG intensity as a function of the input polarization angle expected for a monolayer of chiral molecules at the air-water interface. The tensor elements were removed two bring out the intrinsic chirality.
Experimentally, plots of the S-out SHG intensity as a function of the input polarization angle similar to that shown in Fig. 4 were not observed. On the opposite, at the low surface pressure of 1 mN/m, the observed S-out SHG intensity was rather different, see Fig. 5.This plot is indeed no more exhibiting the expected twofold symmetry with four unequal peaks. Moreover at the input polarization angles γ = 0, γ = 90°, γ = 180° and γ = 270°, the SHG intensity no longer vanishes as compared to the theoretical S-out SHG intensity plots obtained in the presence of intrinsic chirality. The plot observed in Fig. 5 confirms the necessity to introduce higher orders beyond the electric dipole approximation, in particular the chiral electric dipole and magnetic dipole components of the surface tensor. It is again emphasized that the components used in Eq. (2) are the dominant ones considering the proximity of the electronic resonance of the 1( + ) molecules to the 400 nm wavelength for the SHG light. On Fig. 5, the SHG intensity collected for the 0° input polarization angle cannot however allow for the unambiguous determination of the origin of chirality. Indeed, according to Eq. (2), both the chiral electric dipole and the chiral magnetic dipole tensor elements contribute. Hence, at γ = 0, both the molecular intrinsic and the supramolecular chiral contributions contribute, namely respectively and . However, for the γ = 90° S-out polarization configuration, this determination is possible since only the magnetic dipole element contributes, see Eq. (2). Consequently, considering the S-out SHG intensity graph shown in Fig. 5, it is concluded that the supramolecular chirality of the molecular films with a magnetic dipole origin is the main contribution to the observed SHG intensity [38]. This contribution arises from molecular 1( + ) aggregation formed during films compression at the air-water interface. Obviously, the intrinsic chirality of the 1( + ) molecules does contribute too but its magnitude with respect to the magnetic dipole one cannot be assessed with the present data. It is expected to be much weaker. Experimentally, the plot in Fig. 5 exhibits two unequal peaks as opposed to the expected equal intensity peaks observed in other systems and in agreement with Eq. (2) [16]. This difference may arise from a slight defocusing of the optical conditions due to the water evaporation and the domain dynamics undergoing under the laser beam during the course of the experiment.
Fig. 5 S-out SHG intensity as a function of the input polarization angle obtained for a 1( + ) monolayer formed at the air-water interface for a surface pressure of 1 mN/m. The gray dashed line corresponds to the fit of the experimental data using Eq. (2).
The γ = 90° S-out polarization configuration is thus useful to bring out the chirality origin of the 1( + ) monolayer film compressed at the air-water interface. In order to get further insights into the property of the monolayer film, the SHG intensity was then recorded for the fixed input polarization configuration of γ = 90° and the S-out polarization configuration whereas the film was compressed and relaxed successively for several cycles. Figure 6 reports the isotherms and the square root of the SHG intensity as a function of these compression/decompression cycles. From Eq. (2), the square root of the SHG intensity for this polarization configuration is proportional to the modulus of the susceptibility tensor element. The experiment was realized in the low surface pressure regime with a maximum surface pressure of 10 mN/m to prevent the film collapse. The evolution of the susceptibility tensor with the surface pressure shows distinct regimes. At the beginning of a compression cycle, the tensor modulus is vanishing in agreement with the expectation of an isotropic interface. This regime corresponds to extremely low surface pressures, i.e. below 0.6 mN/m, where the SHG intensity barely differs from that of the neat air-water interface. As soon as the surface pressure starts to increase, the susceptibility tensor element significantly increases indicating the appearance of the supramolecular chirality. The tensor element value remains large during the remaining part of the compression cycle and the decompression part as the barriers open up again. When the surface pressure reaches again a value below 0.6 mN/m, the tensor element vanishes again back to its initial value. Interestingly though, the magnitude of the tensor element greatly fluctuates. This is attributed to the large inhomogeneity of the chiral domains formed at the air-water interface. Also, surface diffusion and convection participates to these fluctuations of the recorded SHG intensity as pointed out for other hydrophobic monolayers [39]. Figure 6 clearly shows the reversible property of the 1( + ) chiral aggregates. The successive isotherm cycles are indeed reflected into the SHG intensity cycles. These results also show that the aggregation energy is rather weak. The forced lateral compression induces their formation but as the barriers open, thermal energy rapidly re-disperse them.
Fig. 6 Time evolution of the surface pressure (black dashed line) and the susceptibility tensor element modulus (red continuous line) for successive isotherm cycles for γ = 90° and an S-out polarization configuration.
4. Conclusion
In conclusion, the present work shows how the nonlinear optical technique of SHG can reveal fine details occurring at the air-water interface. Here, the supramolecular chirality of the 1 ( + ) system under lateral compression is observed and shown to be reversible, due to the weak energy of the aggregation process. The analysis of the SHG intensity as a function of the input and output polarization configuration also allows for the unambiguous determination of the magnetic dipole origin of this supramolecular chirality. Further work is nevertheless required to observe the intrinsic molecular chirality of the 1( + ) compound in the monolayer films.
Acknowledgments
The authors thank the Centre for Nano-Optics (NanOpTec) of the University Claude Bernard Lyon 1 and the financial support of the French National Agency for Research under project DYNACHIR ANR-12-BS04-0018-01 for a PhD studentship (AB).
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