Compression Induced Chirality in Dense Molecular Films at the Air

12958

J. Phys. Chem. C 2008, 112, 12958–12965

Compression Induced Chirality in Dense Molecular Films at the Air-Water Interface Probed by Second Harmonic Generation G. Martin-Gassin, E. Benichou,* G. Bachelier, I. Russier-Antoine, Ch. Jonin, and P. F. Brevet Laboratoire de Spectrome´trie Ionique et Mole´culaire, UniVersite´ Claude Bernard Lyon 1-CNRS (UMR 5579), Baˆtiment Alfred Kastler, 43 BouleVard du 11 NoVembre 1918, 69622 Villeurbanne cedex, France ReceiVed: March 31, 2008; ReVised Manuscript ReceiVed: May 20, 2008

Surface second harmonic generation was used to study the nonlinear optical properties of a two-dimensional film of 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) formed at the air-water interface in a Langmuir trough. The second harmonic intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization angles during the monolayer compression. Below a critical average density of 0.55 nmol/cm2, the film is in a liquid-expanded state and the DiA molecules are strongly tilted on the interface. Above this critical average surface density, the film reveals chirality arising from the formation of molecular aggregates at the interface. It is demonstrated through a full analysis of the data that the origin of this chiral property of the film arises from the coupling of the electric and magnetic fields at the fundamental frequency. I. Introduction Surface second harmonic generation (SHG) has proven in the past to be a powerful surface sensitive technique.1,2 Indeed, the technique, based on the conversion of two photons at a fundamental frequency ω into one photon at the harmonic frequency Ω ) 2ω, is surface sensitive at interfaces between two centrosymmetric media because of the cancelation of the generation process in the two adjacent bulk phases in the electric dipole approximation. Hence, the technique is noninvasive and has been used to investigate both the structure and the dynamics at such surfaces and interfaces. Thus, its combination with a Langmuir trough allows nonlinear optical studies with a precise control of the average surface density of amphiphilic compounds spread at the liquid surface. In the past, monolayers made from nonchiral materials have been investigated and their chiral properties reported,3-5 chirality in these monolayers arising from the presence of molecular aggregates such as J-aggregates in the film. One example of such monolayers is that of a film of porphyrins, although the monolayer formed by the LangmuirBlodgett technique was transferred on a solid substrate prior to the optical studies.6 This procedure, where the film is transferred onto a solid substrate at a fixed compression by the LangmuirBlodgett technique, raises questions about the origin of chirality. Molecular aggregation may indeed stem from the solvent drying and the film aging. Hence, the aggregation process may not be controlled by an external mean and, rather, only results from the molecular structure of the compounds themselves. Chirality has though also been observed in situ for films of porphyrins spread at the liquid-liquid interface although without the complete control of the surface density of the prophyrins.7 This latter study was conducted by SHG, proving that the technique is adequate to investigate chirality in thin molecular layers.8-12 It remains, however, to achieve a better control of the molecular surface density of the films while still inducing chirality starting from nonchiral compounds. * Corresponding author. Tel: +33 472 431 914. Fax: +33 472 445 871. E-mail: [email protected].

To reveal the chiral properties of an interfacial molecular film, the SHG measurements can be performed in several different ways. The first one consists of 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 an analogue to the linear optical rotation dispersion technique (ORD) and is called ORD-SHG in the nonlinear regime.13 The second one is the measurement of the SHG intensity for left and right circularly polarized fundamental beams. This method is an analogue of the linear circular dichroism technique (CD) and is called CD-SHG in the nonlinear regime.10,11,14,15 The third one is SHG-linear dichroism (LD-SHG) and has no analogue in the linear regime. It consists of measuring the difference in the SHG intensity collected for a +45° and a -45° linearly polarized fundamental beam.12,16,17 In the present work, a similar study is performed where the fundamental beam is linearly polarized but its angle is varied from 0° to 360° to increase the sensitivity of the experimental setup. The dye used in these experiments is the stilbazolium dye 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA), which is an achiral cationic amphiphilic dye with a strong quadratic hyperpolarizability. This molecular system has already been studied by SHG18,19 and UV-visible absorption spectroscopy.20 These previous studies have revealed the presence of H-aggregates in Langmuir-Blodgett films. H-aggregates formation is well-known for stilbazolium dyes and this aggregation is characterized by a blue-shift of the electronic transition resonance in the aggregates relative to that in the monomer. For these reasons, DiA is an adequate model compound, being both achiral and forming aggregates, to investigate the possibility of an external control of chirality in a molecular film. In this work, the polarization angle-resolved SHG measurements were performed on a monolayer of DiA spread at the air-water interface in a Langmuir trough. The optical measurements were obtained during the monolayer compression, the latter becoming effectively the control parameter to induced chirality in the film. It is also the purpose of this work to give an insight on the origin of this chirality. In the first section, the experimental setup is described. In the second section the experimental results on

10.1021/jp802752h CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

Compression Induced Chirality in Molecular Films

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12959

Figure 1. (a) DiA structure. (b) Schematics of DiA at the air-water interface with the three Euler angle required to orient the molecule.

Figure 2. Schematics of the experimental setup. Half-wave plate (λ/2), low and high pass filters (F1, F2), mirrors (M1, M2), fused-silica 10 cm focal length lenses (L1, L2), Langmuir trough with two motorized barriers (B), and analyzer (A).

the linear absorption spectroscopy experiments on the molecular film during the compression are reported followed by the SHG measurements at low compression when no chirality is observed. The molecular orientation of the dye at the interface is extracted following standard procedures. The last section concerns the SHG measurements performed at the highest densities where strong deformations of the polarization curves are the signature of the appearance of chirality in the molecular film. II. Experimental Section The stilbazolium dye 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) was purchased from Molecular Probes and was used without further purification. The structure of this compound is shown in Figure 1a. The dye Langmuir monolayers were prepared using a standard Langmuir trough with a maximum surface area of 100 cm2 (Nima Technology, model 601). The trough was fitted with a Wilhelmy plate to record the pressure-area isotherms during the film compression at constant temperature. All experiments were carried out at room temperature but the trough was held at 15 °C to reduce the evaporation rate and the subsequent laser beam defocalization. Ultra pure water (Millipore 18 MΩ cm) was used as the subphase. Typically, 200 µL of a 100 µM solution of DiA in methanol was spread onto the air-water interface. After a few minutes allowing for solvent evaporation, the monolayer was slowly compressed at a rate of 5 cm2/min. Figure 1b is a schematics of the orientation of the DiA molecules at the

air-water interface. The polar charged head points toward the water phase but in absence of surface SHG spectra, unavailable to our experimental setup, the extent of the molecular hydration state cannot be discussed. The SHG setup is given in Figure 2. It is based on a femtosecond Ti-sapphire oscillator laser source providing pulses with duration of 170 fs at a repetition rate of 76 MHz (Coherent, model Mira). 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 500 mW 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. The SH light was collected by a lens with a 10 cm focal length and separated from its fundamental counterpart by a high-pass filter and a monochromator positioned on the SH wavelength. The SH light was detected with a cooled photomultiplier tube the output current pulses of which were counted with a photon counter. The fundamental beam was chopped at 97 Hz to enable a gated photon counting mode allowing automatic subtraction of the noise level. 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 beam and γ ) π/2 to an s-polarized beam. An analyzer, placed in front of the monochromator, was used to separate the S and P-polarized SH intensities.

12960 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Martin-Gassin et al.

Figure 3. Isotherm recorded at 15 °C of a DiA monolayer at the air-water interface. Roman numerals show the surface pressure where the incident polarization dependent surface SHG measurements were performed.

Figure 5. Output SSHG intensity as a function of the input polarization angle for a DiA monolayer for an average surface density of 0.2 nmol/ cm2. (a) S-out intensity (circles, experimental data; solid, fitted curve), (b) P-out intensity (triangles, experimental data; line, fitted curve).

Figure 4. UV-visible absorbance spectra of the DiA monolayer during the compression. The vertical arrow refers to the different spectra with average surface densities equal to 0.25, 0.5, 1 and 1.2 nmol/cm2. The dashed line is a spectrum recorded for a 10 µM solution of DiA in methanol. Inset: first electronic resonance peak position as a function of the average surface density (the line is guide to the eyes).

III. Results and Discussion A. Pressure-Area Isotherms. A typical isotherm is presented in Figure 3. At a low average surface density, the surface pressure is weak and the system is in an expanded liquid state. In this regime, the interactions between the DiA dye molecules are weak and the dye is relatively free to diffuse at the surface. Convection may play a nonnegligible role in this regime too.21 As the average surface density increases, the surface pressure rises up to a maximum value of about 52 mN/m. This pressure increase corresponds to a transition to the condensed-liquid regime where the interactions between the molecules dominate. The isotherm presents a rather smooth behavior at all average surface densities indicating that the film remain liquid-like at all time, probably because of the double alkyl chain of the DiA molecule which introduces disorder in the monolayer. During the compression, molecular aggregates are formed at the air/ water interface as reported previously.22 Beyond an average surface density of 2.2 nmol/cm2, a SHG signal fall is observed attributed to an optical collapse (not shown on figure). All optical measurements presented in this work were performed far from this extreme density as seen by the arrows indicating the average

surface densities where the polarization dependence curves of the SH harmonic light were recorded. All optical measurements entailed both a linear absorption spectroscopy and a nonlinear surface SHG measurement. B. Absorbance Measurements. The optical absorption spectra were obtained in situ using a compact UV-visible spectrometer (Ocean Optics, USB 1533) in transmission. The nonpolarized light of a deuterium-halogen lamp was brought to the air-water interface through an optical fiber at normal incidence and was collected in transmission through a quartz window located at the bottom of the Langmuir trough by a second optical fiber hooked to the spectrometer. UV-visible absorption spectra were recorded during the film compression and are presented in Figure 4 for several average densities, namely 0.25, 0.5, 1 and 1.2 nmol/cm2. For comparison, the absorption spectrum of DiA in methanol at the low concentration of 10 µM is also given. Two peaks are observed at 480 and 280 nm, corresponding respectively to the first electronic transition S0 f S1 and to the second electronic transition S0 f S2 of the monomer. As the average surface density is increased upon compression, the film absorbance also increases as illustrated by the vertical arrow in Figure 4 and a blue shift accompanied by a small broadening of the 480 nm peak is also observed. The absorbance increase is expected as the number of molecules under the light spot increases too. The blue shift of the 480 nm resonance is monitored as a function of the average surface density in the Figure 4 inset. Its behavior is attributed to the aggregation of DiA at the interface. Stilbazolium dyes are well-known to form H-aggregates characterized by a blue shift of the resonance peak which can reach 350 nm depending on the conditions.20 C. SHG at Low Average Surface Densities. Surface SHG measurements were then performed for several compressions of the monolayer for the average surface densities pointed out


J. Phys. Chem. C, Vol. 112, No. 33, 2008 12961 ai (i ) 1-5) coefficients are given in Tables 1 and 2. Assuming in-plane isotropy at the interface, the three nonvanishing and eee eee independent susceptibility tensor components χeee XXZ, χZXX and χZZZ are deduced from a fitting procedure of the polarization curves given in Figure 5. Only relative magnitudes are obtained because relative intensities are measured and the absolute phase is unavailable to these measurements in absence of a local oscillator. The relative phase between the three components of the susceptibility tensor is assumed identical.25,26 Setting the eee eee eee element χZZZ to unity, the fitting procedure yields χXXZ /χZZZ ) eee eee 1.9 ( 0.1 and χZXX /χZZZ ) 1.6 ( 0.1. These values are not significantly modified with an increase of the molecular density in the low average density regime below 0.5 nmol/cm2. The eee eee elements χXXZ and χZXX take rather close values, indicating that the Kleinman rule is nearly fullfilled. This is expected for a compound where a single molecular hyperpolarizability component dominates.27,28 Indeed, the macroscopic tensor χ5eee is related to the microscopic quadratic hyperpolarisability tensor β5eee through

with roman numerals in Figure 3. At each average density, the P-polarization and S-polarization SHG output intensities were recorded as a function of the input polarization angle γ, the latter ranging from 0° to 360°. Initially, for the first measurement, the average surface density is low, about 0.2 nmol/cm2, and the surface pressure is very weak; see arrow I in Figure 3. Hence, the film is in the expanded liquid phase where the interactions between the dye molecules are rather weak. The Sout and P-out SHG signals are plotted respectively in Figure 5a,b as a function of the input polarization angle. From these curves, it is possible to extract an average value for the three components of the second-order susceptibility tensor and from there, infer information at the molecular level. Surface SHG at the interface between two centrosymmetric media is described within the electric dipole approximation using the three-layers model. A sheet of nonlinear polarization lying in a slab between the two linear media is described by the following nonlinear polarization:23,24

P (r b,2ω) ) ε0χ5 eee:Em(r b,ω) Em(r b,ω)

f(2)

f

f

(1)

where b r is the position vector in the three-dimensional laboratory frame (O, X, Y, Z) and χ5eee is the pure electric dipole susceptibility tensor that does not vanish at the interface. In this geometrical configuration, the normal direction to the interface is taken along the laboratory Z axis. b Em(r b,ω) is the electric field within the slab at the interface at the fundamental frequency. The output S- and P-polarized harmonic intensities as a function of the incident polarization angle write as eee IS(Ω) ) K|a1χXXZ sin 2γ|2I2

χ5 eee )

NS 5 5eee 〈T 〉β ε0

(3)

5〉 stands for the orientational average of the transformawhere 〈T tion tensor 5 T from the molecular reference frame (O, x, y, z) to the laboratory frame (O, X, Y, Z) and NS is the average surface 5〉 is density of molecules. The frame transformation tensor 〈T defined with the help of the three Euler angles (θ, ψ, φ), following Figure 1b. The angles (ψ, φ) are assumed to take random values in the present case owing to the in-plane isotropy of the interface and the weak interactions between the aromatic cycles of the dye and the interface at room temperature. The number of independent nonzero elements of the quadratic hyperpolarizability tensor can be reduced assuming a C2V molecular symmetry for DiA. Only four elements remain in this

(2a)

eee eee eee + a3χZXX + a4χZZZ ) cos2 γ + IP(Ω) ) K|(a2χXXZ eee sin2 γ|2I2 (2b) a5χZXX

where K is a constant depending on the experimental geometry and absolute constants and I is the fundamental intensity. All

TABLE 1: Nonvanishing Second-Order Susceptibility Tensor Components for C∞W and C∞ Symmetries susceptibility tensor

isotropic and achiral C∞V symmetry

isotropic and chiral C∞ symmetry

χ5eee

eee χZZZ ;

χ5eem

eem eem eem eem eem eem χXYZ ) -χYXZ ; χXZY ) -χYZX ; χZXY ) -χZYX

eee χZXX

)

eee χZYY ;

eee χXXZ

)

eee χXZX

)

eee χYYZ

)

eee χYZY

eee eee eee χZZZ ; χZXX ) χZYY ; eee -χYZX eem eem χXYZ ) -χYXZ

eee eee eee eee eee eee eee χXXZ ) χXZX ) χYYZ ) χYZY ; χXYZ ) χXZY ) -χYXZ )

TABLE 2: Isotropic Chiral and Achiral ai Coefficients Calculated in the Electric Dipole and the Magnetic Dipole Approximationsa coefficients electric dipole approximation

magnetic dipole approximation

fundamental b eω and harmonic b eΩ vectors fundamental b bω vector

isotropic and achiral a1 ) e˜Yω e˜Zω e˜YΩ a2 ) -2e˜Zω e˜ωX e˜Ω X a3 ) e˜ωX e˜ωX e˜ZΩ a4 ) e˜Zω e˜Zω e˜ZΩ a5 ) e˜Yω e˜Yω e˜ZΩ a10 ) 1/2e˜ωX b˜Zωe˜YΩ a11 ) 1/2e˜Zωb˜ωX e˜YΩ a17 ) -e˜Zωb˜Yωe˜Ω X a18 ) e˜ωX b˜Yωe˜ZΩ a19 ) e˜Yωb˜Zωe˜Ω X a20 ) e˜Yω b˜ωX e˜ZΩ p p e˜ωX ) t1m (rm2 - 1) cos θmω s s e˜Yω ) t1m (rm2 + 1) p p e˜Zω ) t1m (rm2 + 1) sin θmω s s b˜ωX ) εm1/2t1m (rm2 - 1) cos θmω p p b˜Yω ) εm1/2t1m (rm2 + 1) s s b˜Zω ) εm1/2t1m (rm2 + 1) sin θmω

isotropic and chiral additional elements a6 ) -e˜Yω e˜Zω e˜Ω X a7 ) -2e˜ωX e˜Zω e˜YΩ

a8 ) -e˜Yω b˜Zω e˜YΩ a9 ) e˜Zω b˜Yω e˜YΩ a12 ) 1/2e˜ωX b˜Zω e˜Ω X a13 ) 1/2eZω b˜ωX e˜Ω X a14 ) -1/2e˜Zω b˜Zω e˜ZΩ a15 ) 1/2e˜Yω b˜Yω e˜ZΩ a16 ) -1/2e˜ωX b˜ωX e˜ZΩ p P Ω e˜Ω X ) (Rm2 - 1)Tm1 cos θm s S e˜YΩ ) (Rm2 + 1)Tm1 p P e˜ZΩ ) (Rm2 + 1)Tm1 sin θmΩ

a s/p rij and tijs/p are the Fresnel coefficients for reflection and transmission at the fundamental frequency ω across the interface from medium i to medium j in the case of an s- and a p-polarized wave, respectively. Rijs/p and Tijs/p are the Fresnel coefficients at the harmonic frequency Ω. θmω and θmΩ are respectively the incidence and the reflection angles in the interfacial slab.


Martin-Gassin et al.

eee eee eee case, namely βeee xxz ) βxzx , βzxx and βzzz . Considering the structure eee eee of the molecules, the elements βzzz and βzxx are expected to dominate with βeee being by far the largest because the transition zzz dipole moment corresponding to the closest transition to the harmonic wavelength, namely the S0 f S1 transition, lies along the molecular z axis where the charge transfer occurs. From these assumptions, it is possible to define the orientational parameter D as1,24

D)

eee eee eee - χZXX + χXXZ χZZZ 〈cos3 θ〉 ) eee 〈cos θ〉 χ - χeee + 3χeee ZZZ

ZXX

(4)

XXZ

and the ratio R of the two main molecular hyperpolarizability elements

R)

βeee zxx eee βzzz

)

eee eee 2(χZXX - χXXZ ) eee eee χZZZ + 2χXXZ

assuming that the electric dipole approximation is still valid. The additional coefficients a6 and a7 are given in Table 2 and the additional term in the S-out SHG intensity proportional to cos2 γ has been introduced. This term can account for the intensity difference observed in the S-polarized plots at 135° and 315°. However, it will never account for the nonvanishing values of the intensity at the input polarization angles of 90° and 270°. It is therefore necessary to go beyond the electric dipole approximation and introduce a generalized nonlinear polarization including the first-order magnetic dipole contribution. Equation 1 is then modified according to

(5)

Experimentally, D ) 0.25 ( 0.01 and R ) -0.09 ( 0.01. The latter ratio clearly underlines the uniaxial character of the nonlinear optical properties of DiA, its negative sign arising eee from the proximity of the values of the two elements χXXZ and eee χZXX. With the parameter D, it is possible to infer that the set of accessible angles between the long axis of the molecule and the surface normal is confined to rather large angular values, irrespective of the width of the distribution used. The minimum angle θ0 is obtained using a Dirac distribution function, leading to D ) cos2 θ0 and θ0 ) 61°. For larger distributions, the angle is found larger. Hence, it is concluded that in this domain of low average surface densities, the DiA molecules are strongly tilted toward the water surface. D. SHG at High Average Surface Densities. Surface SHG measurements were then performed at higher average surface densities, from 0.6 to 1.3 nmol/cm2. At each compression step indicated on the isotherm by the roman numerals, the SHG output intensity was recorded in both the P-polarization (Γ ) 0) and the S-polarization state (Γ ) π/2) as a function of the input polarization angle γ. The P-polarization curves are not presented because the plots are very similar to the one shown in Figure 5b. The S-polarization plots are given in Figure 6. They exhibit a strong evolution as a function of compression. For example, the SHG intensity no longer vanishes at high average densities for the incident polarization angles of 90° and 270° as opposed to what is observed at low densities. Besides, the SHG intensity is maximal for the incident polarization angles of 135° and 315° with lower values at 45° and 225° whereas it retained an identical value at these four angles at lower densities. At the highest densities (see cases VII and VIII), a flip between the peak intensities at 135° and 45° is observed: the SHG intensity recorded at 45° becomes more intense than that recorded at 135°. Clearly, all these plots cannot be fitted anymore with eq 2a. The latter expression imposes vanishing intensities at 0°, 90°, 180° and 270° and four equal maxima at angles of 45°, 135°, 225° and 315°. This is clearly at odds with the experimental observations. This behavior cannot be accounted for with surface anisotropy too. In particular, the difference in SH intensities for the polarization angles γ ) +45° and γ ) -45° underlines the appearance of SHG-linear dichroism (LD-SHG), a phenomenon only observed for chiral compounds. Hence it appears that the compression in a regime of rather high average surface densities leads to chirality. The surface symmetry is now of C∞ symmetry instead of C∞V, a symmetry with no mirror planes. This lowering of the symmetry increases the number of independent nonvanishing tensor components of the susceptibility χ5eee as shown in Table 1,

P (r b,2ω) ) ε0[χ5 eee:Emω(r b,ω) Emω(r b, ω) +

f(2)

f

f

b,ω) Bmω(r b,ω)] (6) χ5 eem:Eωm(r f

f

where the superscript “m” refers to an electric-magnetic dipole coupling process at the fundamental frequency. From a general point of view, a term corresponding to the electric quadrupole contribution should be included in eq 6 as well. In fact, one usually writes all terms collectively in a single generalized magnetic contribution.10,12,29 Using this nonlinear polarization expression above, the SHG intensities are given by eee eem eem eee IS(Ω) ) K|(a1χXXZ + a10χXYZ + a11χXZY ) sin 2γ + (a7χXYZ + eem eem ) cos2 γ + a8χXXZ sin2 γ|2I2 (7a) a9χXZX eee eee eee eem + a3χZXX + a4χZZZ + a17χXZZ + Ip(Ω) ) K|(a2χXXZ eem eee eem eem +a20χZXY ) sin2 γ + ) cos2 γ + (a5χZXX + a19χXZX a18χZXY eee eem eem eem + a12χXXZ + a13χXZX + a14χZZZ + (a15 + (a6χXYZ eem ) sin2 γ|2I2 (7b) a16)χZXX

for both S and P polarizations. All coefficients ai are given in Table 2. In eq 7a, all chiral terms appear in the coefficients containing cos2 γ and sin2 γ and the achiral terms in terms containing sin2 γ. These expressions are in agreement with the one derived by Persoons and co-workers.30 Equation 7a is now able to adjust the S-out harmonic intensity polar plots given in Figure 6 remarkably well at all densities. From this fitting procedure, the susceptibility tensor components for the tensors χ5eeeand χ5eem have been extracted as a function of the molecular density. These elements were normalized to the achiral element eee χZZZ previously determined at low compression. The achiral eee eee element χXXZ and the chiral element χXYZ are given in Figure 7. eem The χ5 tensor elements are plotted in Figure 8. The two achiral eem eem elements χXYZ and χXZY were undistinguishable from the fitting eem procedure and thus only their sum χeem XYZ + χXZY was determined. eem eem In the same figure, the two chiral elements χXXZ and χXZX are also reported. All electric dipole elements are taken as real quantities whereas all magnetic dipole elements are taken as imaginary quantities following previous works.25,26 Figures 7 and 8 clearly show that chirality appears above a threshold surface density that can be fixed at a value of 0.55 nmol/cm2. Below this average density, the dipolar magnetic elements vanish and the polar plots for the S-polarized SHG intensity can be eee described with the achiral element χXXZ only. The evolution of this element with compression is strongly correlated to the surface pressure evolution as presented in the isotherm of Figure 3. For low surface pressures, the χeee XXZ element remains constant. Beyond the threshold value of 0.55 nmol/cm2 corresponding to a molecular area of about 0.3 nm2/molecule, this element strongly increases. This increase is expected to be linear with the average surface density according to eq 3 as long as no


J. Phys. Chem. C, Vol. 112, No. 33, 2008 12963

Figure 6. S-out intensity plots as a function of the input polarization angle for a DiA monolayer at the air-water interface for different values of the average surface density: 0.2 (I), 0.6 (II), 0.7 (III), 0.8 (IV), 0.9 (V), 1 (VI), 1.1 (VII) and 1.3 (VIII) nmol/cm2. The solid line is a fit to the data.

molecular reorientation or molecular interactions occurs within the film. Because the linear behavior is not observed in the present set of experiments, it is expected that molecular reorientation or molecular interactions occur. The latter is certainly at work because the presence of aggregation has been confirmed through UV-visible absorption spectroscopy. Above the threshold surface density, the chiral element χeem XXZ accounting for the inequality between the S-out polarized SH intensities Is(γ)90°) and Is(γ)0°) becomes nonnegligible. This element increases slowly with compression for average surface densities below 1 nmol/cm2. Its increase though indicates that the film chirality is mainly of magnetic dipole origin. Because this

component χeem XXZ, which is defined as a purely imaginary complex number, will never explain the LD-SHG observed, i.e., the inequality Is(γ)45°) + Is(γ)135°), a second element must be introduced. However, the required chiral elements scaling the contribution in cos2 γ cannot be unambiguously chosen between the electric dipole element, namely χeee XYZ, and the magnetic dipole eem one, namely χXZX . However, in all case, these two elements eem remain relatively small as compared to the element χXXZ . To give an insight into the origin of this induced chirality at the microscopic level, several molecular models have been compared. In the past, limiting the discussion to classical models only, three different models have been proposed to describe


Figure 7. Normalized electric dipole susceptibility tensor elements eee eee χXXZ (achiral) and χXYZ (chiral) as a function of the average surface density. The lines are guides to the eyes. The normalization was done eee with the element χZZZ obtained at a density of 0.2 nmol/cm2.

Martin-Gassin et al. the magnetic dipole of another molecule. It is therefore suggested that the observed induced chirality at high compression arises in the molecular aggregates thus formed through magnetic dipole transitions. Compact stacking of the DiA molecules where the counterion will likely play an important role is therefore at the origin of helix-like electron delocalization in the aggregate.29 The question on the exact molecular arrangement in these aggregates remains open for debate. Finally, it is observed that beyond an average surface density of 1 nmol/cm2, all chiral elements become negative, revealing a phase change of π. At this point, it is important to emphasize again that the set of parameters extracted from the fitting procedure is not unique due to the fact that only intensities are measured. Another set of values is also possible where both chiral and achiral elements become negative, for instance both eem eem eee χXYZ + χXZY and χXYZ simultaneously. However, it is interesting to notice that a change of enantiomers would lead to the phase flip observed. Such a change could then be tentatively explained by a rapid change of chiral domain of the film under the laser spot during compression. These domains would be formed by the segregation of the two types of aggregates. Such an observation has already been made in two-dimensional films formed from a racemic mixture of the two enantiomers.33 IV. Conclusions

Figure 8. Normalized magnetic dipole susceptibility tensor elements eem eem eem eem χXYZ +χXZY (achiral), χXXZ (chiral) and χXZX (chiral) as a function of the average surface density. The normalization was done with the element eee χZZZ obtained at a density of 0.2 nmol/cm2.

chirality in second harmonic generation measurements. The first one entails the coupling of two electric dipole oscillators and yields only three nonvanishing independent components for χ5eee eee eem eem and χ5eem: χXYZ that is a chiral component and χXZY and χZXY 29 that are achiral components. This model cannot describe properly the present system of DiA molecules at the air-water interface because no chiral element of the tensor χ5eem appears. A second model introduces magnetic contributions to yield chirality.29 In this model, the chiral microscopic system contains two electrons oscillating along two helices having a common axis. In this model, only magnetic components are chiral, all electric components being achiral. Three nonzero elements of eem eem eem the χ5eem tensor appears, namely χXXZ ) χZXX and χZZZ . This model can in principle explain the nonvanishing values of the intensity at the input polarization angle of γ ) 90° but will never yield the observation of LD-SHG. The third model, that of a single electron on a helix, however, introduces all the tensor elements required to described correctly the experimental intensities recorded during this work.31 In this work, we do not consider the electromagnetic coupling mechanism introduced to describe linear optical activity in chiral compounds.32 In this mechanism, the electric dipole of one molecule is coupled to

This study demonstrates that chirality can be induced by compression at high average surface densities from achiral molecules in a two-dimensional film at the air-water interface. SHG then proves to be well adapted to follow these changes. At low compressions, the film did not exhibit chirality and the DiA molecules, on average, are rather tilted toward the interface. At higher compressions, the combined measurement of the UV-visible absorption spectra confirmed that DiA aggregation occurs but the exact structure of the aggregates remains unclear. The analysis of the SHG polarization plots at high average densities indicates that electric and magnetic dipole contributions in the susceptibility tensors are both necessary to correctly analyze the data but that the magnetic dipole contribution is the dominant one. The value of the tensor elements were deduced as a function of compression, showing that the magnetic dipole contribution eventually reach of the same order of magnitude as that of the electric dipole contribution. The origin of chirality at the microscopic level appears to stem from an electronic delocalization within the aggregate similar to that of an electron on a helix. References and Notes (1) Corn, R. M.; Higgins, D. A. Chem. ReV. 1994, 94, 107–125. (2) Shen, Y. R. The principles of Nonlinear Optics; Wiley: New York, 1984. (3) Peterson, I. R.; Kenn, R. M.; Goudot, A.; Fontaine, P.; Rondelez, F.; Bouwman, W. G.; Kjaer, K. Phys. ReV. E 1996, 53, 667–673. (4) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Nature 1994, 368, 440–443. (5) Yuan, J.; Liu, M. J. Am. Chem. Soc. 2002, 125, 5051. (6) Zhang, L.; Lu, Q.; Liu, M. H. J. Phys. Chem. B 2003, 107, 2565– 2569. (7) Fujiwara, K.; Monjushiro, H.; Watarai, H. Chem. Phys. Lett. 2004, 394, 349–353. (8) Fischer, P.; Hache, F. Chirality 2005, 17, 421–437. (9) Sioncke, S.; Verbiest, T.; Persoons, A. Mater. Sci. Eng. 2003, R 42, 115–155. (10) Kauranen, M.; Verbiest, T.; Maki, J. J.; Persoons, A. J. Chem. Phys. 1994, 101, 8193–8199. (11) Petralli-mallow, T.; Wong, T. M.; Byers, J. D.; Yee, H. I.; Hicks, J. M. J. Phys. Chem. 1993, 97, 1383–1388. (12) Schanne-Klein, M. C.; Hache, F.; Roy, A.; Flytzanis, C.; Payrastre, C. J. Chem. Phys. 1998, 108, 9436–9443.

Compression Induced Chirality in Molecular Films (13) Byers, J. D.; Yee, H. I.; Hicks, J. M. J. Chem. Phys. 1994, 101, 6233–6241. (14) Hicks, J. M.; Petrallimallow, T.; Byers, J. D. Faraday Discuss. 1994, 341–357. (15) Pena, A. M.; Boulesteix, T.; Dartigalongue, T.; Schanne-Klein, M. C. J. Am. Chem. Soc. 2005, 127, 10314–10322. (16) Maki, J. J.; Verbiest, T.; Kauranen, M.; VanElshocht, S.; Persoons, A. J. Chem. Phys. 1996, 105, 767–772. (17) Verbiest, T.; Kauranen, M.; Maki, J. J.; Teerenstra, M. N.; Schouten, A. J.; Nolte, R. J. M.; Persoons, A. J. Chem. Phys. 1995, 103, 8296–8298. (18) Lupo, D.; Prass, W.; Scheunemann, U.; Laschewsky, A.; Ringsdorf, H.; Ledoux, I. J. Opt. Soc. Am. B 1988, 5, 300–308. (19) Bubeck, C.; Laschewsky, A.; Lupo, D.; Neher, D.; Ottenbreit, P.; Paulus, W.; Prass, W.; Ringsdorf, H.; Wegner, G. AdV. Mater. 1991, 3, 54–58. (20) Xu, Z.; Lu, W. Y.; Bohn, P. W. J. Phys. Chem. 1995, 99, 7154– 7159. (21) Martin-Gassin, G.; El Harfouch, Y.; Benichou, E.; Bachelier, G.; Russier-Antoine, I.; Jonin, C.; Roux, S.; Tillement, O.; Brevet, P. F. J. Phys.: Condens. Matter 2008, 20, 055228. (22) Evans, C. E.; Bohn, P. W. J. Am. Chem. Soc. 1993, 115, 3306.

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12965 (23) Brevet, P. F. J. Chem. Soc., Faraday Trans. 1996, 92, 4547–4554. (24) Brevet, P. F. Surface Second Harmonic Generation; Presses Polytechniques et Universitaires Romandes: Lausanne, 1997. (25) Pershan, P. S. Phys. ReV. 1963, 137, 919. (26) Adler, E. Phys. ReV. 1964, 134, A728. (27) Revillod, G.; Russier-Antoine, I.; Benichou, E.; Jonin, C.; Brevet, P. F. Nonlinear Opt. Quantum Opt. 2006, 35, 135. (28) Revillod, G.; Duboisset, J.; Russier-Antoine, I.; Benichou, E.; Bachelier, G.; Jonin, C.; Brevet, P. F. J. Phys. Chem. C 2008, 112, 2716. (29) Hache, F.; Mesnil, H.; Schanne-Klein, M. C. J. Chem. Phys. 2001, 115, 6707–6715. (30) Maki, J. J.; Kauranen, M.; Persoons, A. Phys. ReV. B 1995, 51, 1425–1434. (31) Maki, J. J.; Persoons, A. J. Chem. Phys. 1996, 104, 9340–9348. (32) Eliel, E. L.; Wilen, H. W. Sterochemistry of organic compounds; Wiley: New York, 1994. (33) Nassoy, P.; Goldmann, M.; Bouloussa, O.; Rondelez, F. Phys. ReV. Lett. 1995, 75, 457–460.

JP802752H

Compression Induced Chirality in Dense Molecular Films at the Air

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