supramolecular chirality at the air/water interface [invited]

Supramolecular chirality at the air/water

interface [Invited] Emmanuel Benichou,

1,* Arnaud Derouet,

1 Isabelle Russier-Antoine,

1 Christian Jonin,

1

Noëlle Lascoux,1 Minghua Liu,

2 and Pierre-François Brevet

1

1Laboratoire de Spectrométrie Ionique et Moléculaire, Université Claude Bernard Lyon 1- CNRS (UMR 5579),

Bâtiment Alfred Kastler, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne cedex, France 2Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences Beijing

100080, China

*[email protected]

Abstract: 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

OCIS codes: (190.2620) Harmonic generation and mixing; (240.4350) Nonlinear optics at

surfaces; (190.4710) Optical nonlinearities in organic materials.

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1. Introduction

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 [3], liquid/solid [4] 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)

[7]. This molecular system is well-known to form chiral Langmuir-Schaeffer films [8]. 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 [21]. 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 [7]. 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 (~3x104

M) 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 [7] showing that TARC18 monolayer on the pure water

surface is in a condensed state.


Fig. 1. Isotherm recorded at 20°C of a TARC18 monolayer at the air/water interface. Insert:

TARC18 molecular structure.

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.

Fig. 2. Schematics of the experimental setup: Half-wave plate (WP), low and high pass filters

(RG, BG), mirrors (M1, M2, M3, M4), fused-silica 10 cm focal length lenses (L1, L2), fused-silica 5 cm focal length lens (L3), Langmuir trough (LT), and analyzer (half-wave plate and

polarizer cube).


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 [5]. 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]:

2

1 sin(2 )eee

s xxzI a (1)

where a1 is a constant coefficient depending on the geometrical configuration and the optical

indices of water and air at 800 nm and 400 nm. The quantity eee

xxz is the component of the

quadratic susceptibility tensor in the electric dipole approximation corresponding to incidents

fundamental field components along the x and the z axes where z corresponds to the interface

normal and a fundamental field polarized along the x axis. Equation (1) was established in the

case of an isotropic and achiral interface within the vC symmetry. In that case, the

susceptibility tensor possesses only 7 non-vanishing elements, three of them being

independent, namely eee

zzz , eee

zxx and eee

xxz . 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.

Fig. 3. S-Out SH intensity plots as a function of the input polarisation angle for a TARC18

monolayer at the air-water interface for two surface pressures of the isotherm shown in Fig. 1: at the beginning of the isotherm (approx. 0 mN/m, open circles) and near the film collapse (25

mN/m, filled squares). The dash and the solid-line curves correspond respectively to fits to the

experimental data with Eqs. (1) and (2).


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 [8]. 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 C, a symmetry with no mirror planes. This lowering of the symmetry

increases the number of independent non vanishing components of the susceptibility tensor

[31] and, if the electric dipole approximation is still valid, the new element eee

xyz 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 [5]:

2

2

1 7sin(2 ) cos ( )eee eee

s xxz xyzI a a (2)

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 [5]:

2

2 2( ) sin 2 cos sin71 10 11 9 8

eee eem eem eee eem eemI a a a a a axxz xyz xzy xyz xzx xxzS (3)

where eem is the susceptibility tensor at the level of the magnetic dipole approximation. In

this equation, the presence of additional terms permits to reproduce the experimental data

adequately. More particularly, the component eem

xxz which is a pure chiral element, is the only

source of the non-vanishing intensity observed for the polarization angle γ = π/2. Therefore,

by using 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 [5]. 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 eem

xxz 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.

Fig. 4. Evolution of the S-Out SHG intensity for the three polarization angles γ = π/4, π/2 and

3π/4 during three successive compression cycles of a TARC18 film at the air/water interface.

The evolution of the surface pressure (dashed curve) is also reported in this figure.

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.

Fig. 5. Evolution of the S-Out intensities for three polarization angles γ = π/4, π/2 and 3π/4

during a full compression of a TARC18 film at the air/water interface. The evolution of the

surface pressure (dashed curve) is also reported in this figure. Insert: zoom of the S-Out intensity evolution at low surface pressure.

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.

Fig. 6. Evolution of the S-Out intensities for three polarization angles γ = π/4, π/2 and 3π/4

during compression followed by a constant pressure regime at a surface pressure of 20 mN/m.

The evolution of the surface pressure (dashed curve) is also reported in this figure.

4. Conclusion

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.

Acknowledgments

The authors thank the Centre for Nano-Optics NANOPTEC of the Université Claude Bernard

Lyon 1 for support.


supramolecular chirality at the air/water interface [invited]

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