Alignment and Chirality of Porphyrin J Aggregates Formed at the

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Alignment and Chirality of Porphyrin J Aggregates Formed at the Liquid−Liquid Interface of a Centrifugal Liquid Membrane Cell Hideaki Takechi,†,∥ Adolf Canillas,*,‡ Josep M. Ribó,§ and Hitoshi Watarai*,† †

Institute for NanoScience Design, Osaka University, 1-3 Machikaneyama-machi, Toyonaka, Osaka 560-8531, Japan Department of Applied Physics and Optics Science, Institut de Nanociència i Nanotecnologia (IN2UB) and §Department of Organic Chemistry, Institute of Cosmos Science (IEEC-UB), University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain

‡

S Supporting Information *

ABSTRACT: Previous direct observations of the J aggregates of diprotonated 5,10,15,20-tetraphenyl-21H,23H-porphine (H4TPP2+) formed at the dodecane−water interface in a centrifugal liquid membrane (CLM) cell using conventional CD spectroscopy have shown the existence of circular dichroic signals with a bisignated shape whose sign depends on the rotation direction of the cell. Herein we demonstrate that the determination of the optical Mueller matrix with a twophotoelastic modulator generalized ellipsometer (2-MGE), working in transmission mode, along with the assumption of a two superimposed twin Mueller matrices model (two opposite interfaces in the cylindrical rotation cell) allows us to infer the CD spectra due to overlapping linear polarizations from J aggregates at the front and back liquid−liquid interfaces in the rotating cell. The rotation direction dependence of the CD spectra was thoroughly interpreted by the present model considering the sign and the magnitude of the orientation angle between the front and back J aggregates. The present analysis should help us to understand the optical chirality due to the structural changes in supramolecular and macromolecular systems under flow shear forces as well as changes in birefringence.

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INTRODUCTION

In chemical and biological sciences, the chirality of molecules and molecular aggregates is an essential issue, especially when achiral molecules yield chiral supramolecular aggregates. Optical activity (i.e., circular dichroism (CD) and/or circular birefringence (CB)) is the physical signature of chemical chirality. However, in supramolecular species such physical measurements may be hampered by the presence of anisotropies, which contribute to linear dichroic and birefringence signals. These anisotropies originate from the organization of the material, and combinations of their linear contributions may lead to circular contributions.1−3 Although these anisotropies are not a common issue in physics, where the objective is to realize beams with specific polarizations, they are a challenge in supramolecular chemistry where the objective is to use spectroscopic measurements to evaluate the chirality of samples with high anisotropies and unknown orientations. Therefore, more sophisticated instruments are being developed to study the circular polarization properties in anisotropic samples.4,5 To elucidate the optical polarization properties, the present work investigates the formation of insoluble diprotonated porphyrin J aggregates under shear forces at the liquid−liquid interface of a rotating cell (centrifugal liquid membrane (CLM) cell; Figure.1).6 A two photoelastic modulator generalized ellipsometer (2-MGE)7,8 in transmission mode allowed 15 © 2012 American Chemical Society

Figure 1. Schematic illustration of the CLM cell installed in a CD spectropolarimeter or 2-MGE instrument. Axes and rotation directions are defined.

Special Issue: Interfacial Nanoarchitectonics Received: November 7, 2012 Revised: December 25, 2012 Published: December 26, 2012 7249

dx.doi.org/10.1021/la304437g | Langmuir 2013, 29, 7249−7256

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the true CD character of the detected signals for some systems.23,26−28 However, in most other systems the nature of the dichroic signal remains unexplained. Because the chromophores around the vortex generate a multilayered arrangement, several of the authors speculate that the detected circular dichroic signal is an artifact. From conventional CD spectroscopy, a pure mechano-chiral effect has been reported to occur at the interface where the sign of the bisignated circular dichroic signal depends on the direction of rotatation of the CLM cell.29 Herein, 2-MGE measurements are employed to determine the effect of the shear forces generated at the liquid−liquid interface in the CLM cell on the CD signal. It is revealed that the observed CD signals result from the superimposed helical alignment of the tilted J aggregates formed at the front and back liquid−liquid interfaces in the CLM cell.

elements of the normalized Mueller matrix to be observed directly. Then the optical polarization properties were inferred using previous methods9−12 and by considering the possible interface arrangements in the CLM cell. Understanding the linear and circular polarizations in these samples is also interesting because of previous reports on the influence of shear flow on porphyrin J aggregates. It has been reported that for amphiphilic porphyrin (diprotonated 5(phenyl)-10,15,20-tris(4-sulfonatophenyl)porphine) the chiral shear forces of the stirring vortex can select the chiral sign of the natural optical activity of its supramolecular J aggregates;13−15 this has been demonstrated by CD spectroscopy either because the aggregates in the solution (small round particles) do not exhibit interfering linear dichroic contributions3 or because Mueller matrix methods can differentiate between true CD signals and circular polarization originating from the overlapping linear polarization contributions.16 Such a mechano-chiral effect has also been reported for other J aggregates17,18 and in Langmuir systems.19,20 Additionally, 2MGE has been used to demonstrate optical activity16 as well as the presence of linear dichroism (LD) in the corresponding solid film.17 In the case of Langmuir films,19,20 analysis using Mueller matrix spectroscopic methods is not possible using current polarimetric techniques. Therefore, CD signals were observed using conventional CD spectroscopy of the corresponding Langmuir−Blodgett films,20 and Brewster angle microscopy showed the formation of chiral structures.19 However, we believe that it is reasonable to assume that Langmuir systems show optical activity. It has also been reported in different supramolecular species that huge circular dichroic signals under the shear forces of vortex stirring can be reversibly generated.21−26 2-MGE shows ⎛ m00 ⎜m 10 M =⎜ ⎜ m20 ⎜ ⎝ m30

m01 m11 m21 m31

m02 m12 m22 m32

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EXPERIMENTAL SECTION

Chemicals. 5,10,15,20-Tetraphenyl-21H,23H-porphine (H2TPP) was purchased from Aldrich (USA). Dodecane (GR) was purchased from Sigma (USA). Sulfuric acid (GR) was purchased from Merck (USA). Milli-Q water (type-II quality) was used to prepare the samples. Mueller Matrix Measurement at the Liquid−Liquid Interface. The Mueller matrix is a 4 × 4 real matrix that describes changes in light polarization as it interacts with an optical element. Thus, the Mueller matrix of a sample contains all of the information about its polarization properties; that is, the matrix includes chiral information such as CD and circular birefringence (CB) as well as anisotropic information such as linear dichroism (LD) and linear birefringence (LB). For a homogeneous sample with small chiral and linear effects, the Mueller matrix can be expressed as1,2

m03 ⎞ m13 ⎟ ⎟ m23 ⎟ ⎟ m33 ⎠

⎛ ⎞ 1 1 2 2 − LD − LD′ CD + (LBLD′ − LB′LD)⎟ ⎜ 1 + 2 (LD′ + LD ) 2 ⎜ ⎟ 1 1 2 2 ⎜ ⎟ − LD 1 + (LD − LB′ ) CB + (LBLB′ + LDLD′) LB′ ⎜ ⎟ 2 2 ⎟ ≈ e − 2k ⎜ 1 1 − LD′ − CB + (LBLB′ 1 + (LD′2 − LB2) − LB ⎜ ⎟ 2 2 ⎜ ⎟ + LDLD′) ⎜ ⎟ ⎜ ⎟ 1 1 2 2 ⎜CD − (LBLD′ − LB′LD) ⎟ − LB′ LB 1 + (LB − LB′ ) ⎝ ⎠ 2 2 Here, LD′ and LB′ refer to 45° linear dichroism and 45° linear birefringence, respectively. Table S1 in the Supporting Information defines CD, CB, LD, LB, LD′, and LB′.2,3,9−11 CD and CB are twice as large as the ellipticity and optical rotation angle, which are popular definitions in chemistry. The inversion of the Mueller−Jones matrices has been employed as a methodology to extract individual polarization characteristics. This methodology has been used to determine CD, CB, LD, LB, LD′, and LB from the experimental Mueller matrix.10,11 The results reported here correspond to low depolarizing samples and have polarization factor values of at least 0.95. Values of 1 and 0 correspond to the complete absence of depolarization and total depolarization, respectively.10,11

(1)

A 2-MGE was used to measure the Mueller matrices of the CLM system.7,8 This ellipsometer uses two polarizer−photoelastic modulator (PEM) pairs: one as a polarization-state generator (PSG) and the other as a polarization-state analyzer (PSA). For a single configuration, eight independent parameters, which correspond to eight different elements of the Mueller matrix, are measured. Changing the azimuthal orientations of the PSG and PSA allows complete normalized Mueller matrices to be measured. The data analysis method is reported elsewhere.10,11 The formation of interfacial aggregates at the dodecane−water interface was directly observed using a CLM cell installed in the 2MGE instrument.30 Figure 1 shows the CLM cell and defines the axes and angle of polarized light in the measurements. The x and y- axes are 7250


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Figure 2. Experimental Mueller matrices of the H4TPP2+ J aggregates in a CLM cell under CW and CCW rotations. [H2TPP]org = 1.2 × 10−4 M, [H2SO4]aq = 4 M, and t = 10 min.

Figure 3. Observed CD, CB, LD, LB, LD′, and LB′ spectra of H4TPP2+ J aggregates in a CLM cell for CW and CCW rotations. Spectra are from the Mueller matrices in Figure 2 determined by the analytical inversion method.10,11 [H2TPP]org = 1.2 × 10−4 M in dodecane, [H2SO4]aq = 4 M, and t = 10 min. the horizontal and perpendicular directions, respectively, and the z axis is the direction of light propagation. The ±45° axes are defined as rotating clockwise in the x−y plane with respect to the light source as shown in Figure 1. In the CLM measurements, a cylindrical glass cell, which had a 1.9 cm diameter and 3.4 cm length, was attached to an electric motor. This cell was placed horizontal with respect to the laboratory axes (i.e., the x

axis of the 2-MGE) and was rotated in the clockwise (CW) or counterclockwise (CCW) direction at 7000 rpm by a speed-controlled electric motor (NE-22E, Nakanishi Inc., Japan). To realize reproducible spectra, it is important to reduce the vibration of the cell; consequently, the CLM was set symmetrically with the motor. At first, 0.250 mL of a 4 M sulfuric acid aqueous solution was introduced into the cylindrical cell through a sample injection hole by 7251


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we have plotted in Figure 4 the ΔCD values against the LD intensities at 472 nm for both rotation directions. As can be

a transferpette. Then, 0.200 mL of H2TPP dodecane solution was added quickly near the injection hole via a microsyringe to initiate the interfacial reaction. Reproducible results were obtained by a quick injection in the bottom of the CLM cell. The sum of the spectra of the water−oil and oil−water interfaces and the bulk liquid membrane phases was measured by this method. The calculated thicknesses of the organic and aqueous phases were 0.10 and 0.12 mm, respectively, and the total interfacial area between the two phases was 20 cm2.

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RESULTS AND DISCUSSION Mueller Matrix of H 4 TPP 2+ Aggregates at the Dodecane−Water Interface. The Mueller matrix of H4TPP2+ aggregates at the dodecane−water interface in the CLM cell was measured with the 2-MGE under the CW or CCW rotation condition. The CD signals were strongest at ca. 10 min after the injection of the H2TPP solution and then gradually decreased. Hence, we measured the Mueller matrix 10 min after the porphyrin injection. This procedure corresponds to an experimental situation in which the circular movement determines the vertical orientation (y axis in Figure 1) of J aggregates, and additional diffusion from the bottom to the top of the CLM (along the x axis in Figure 1) is assumed. Similar to previous reports for other J aggregates, the injection procedure may cause a torque that “dislocates” the J aggregate into a chiral shape.18 Figure 2 shows characteristic examples of the experimental Mueller matrix obtained under CW and CCW conditions. The spectra of m03 and m30 elements have similar shapes, suggesting that the J aggregates of H4TPP2+ in the rotation cell show optical activity. The measured Mueller matrices are inverted into CD, CB, LD, LB, LD′, and LB′ spectra by the analytical inversion method.10,11 The observed CD spectra show a bisignated shape with maxima at 468 and 476 nm. The CD and CB are consistent with the Kramers−Kronig relationship. Figure 3 shows that the CD spectra of the interfacial aggregates under CW rotation have the typical shape for a positive Cotton effect (negative maximum at the shorter wavelength and positive maximum at the longer wavelength). Under CCW rotation, the opposite bisignated CD spectra are observed (Figure 3). The bisignated CD spectra are characteristic of porphyrin J aggregates as a result of the degenerate electronic transitions of the porphyrin system.31,32 Such an effect has also been reported29 but with less-intense CD spectra of opposite sign compared to those in this work. The sign of the LD and LD′ spectra indicate the orientation direction of the J aggregates. The extent of aggregation (size and shape) and the velocity and position of the injection may affect this sign. Moreover, these factors may lead to different diffusion directions and shapes. In the present measurements, the injection procedure almost always gave a negative LD and a very low LD′ (Figure 3), indicating a vertical alignment of the J aggregates. To increase the sensitivity of the 2-MGE measurements that, at present, are lower than in a conventional CD spectrometer, the concentration of H2TPP considered in the present measurements was increased 10-fold with respect to that in previously reported experiments.29 For each experimental condition, experiments were repeated more than 10 times, and the reproducibility of the shape of CD spectra exceeds 80%. The intensity of the CD signal, defined as ΔCD = CD476 nm − CD468 nm, is scarcely reproducible and seems to be correlated with the corresponding LD intensity. To investigate the relationship between the intensities of the CD and LD spectra,

Figure 4. Correlation between ΔCD of H4TPP2+ J aggregates measured by the CLM method under CW and CCW rotations. ΔCD = CD476 nm − CD468 nm (the difference in the two CD peaks (476 and 468 nm)). The LD intensity has its maximum value at 472 nm. [H2TPP]org = 1.2 × 10−4 M, [H2SO4]aq = 4 M, and t = 10 min.

inferred from the figure, there is a negative proportionality between ΔCD and LD for the CW rotation and a positive proportionality for the CCW rotation. In summary, the sign of the observed bisignated CD spectra depends on the alignment of the J aggregates. The negative LD signals and the low values of LD′ (Figure 3) indicate that the J aggregates at the interface are oriented mainly toward the vertical axis (y axis in Figure 1). However, the experimental situation corresponds to a sample possessing two interfaces. Consequently, the vertical alignment inferred from the LD and LD′ spectra may also be due to a fixed and different alignment of the J aggregates at both interfaces: for an angle +θ from the vertical axis on the front side of the rotating cell interface, it will correspond to an angle of −θ at the interface on the back side. Although the front and back J aggregates at the two interfaces are separated by about 2 cm, when traversed by a light beam, the two interfaces may exhibit a two-layer chiral arrangement (i.e., a nonuniform anisotropic sample). Consequently, the assumption implied in the matrix (eq 1 in the Experimental Section) may not hold true for this arrangement. This may lead to an artifactual CD when using the approximation in eq 1. There are four plausible cases for the overlapped helical alignments, depending on the alignment of the J aggregates. Each of the four cases shown in Figure 5 corresponds to a quadrant in Figure 4. In Figure 5, the angle θ is positive for a clockwise rotation viewed from the incident light side. Next, the experimental data is used to determine if the optical activity can be generated by the linear polarization combinations of the supramolecular architecture. The likely alignment is shown in the first column of Figure 5, which is observed in the negative LD spectra (Figure 3) for two reasons. First, the injection point and strong flow shear of the circular movements exert force upon the growing J aggregates. Second, the J aggregate excitonic absorption is aligned with the longer axis of J aggregates (mostly long particles).23,28 Superimposed Mueller Matrix Model. The measured Mueller matrix, Mexp, of the superimposed interfacial J aggregates in a rotating CLM cell can be calculated from the matrix product, Mcalc, as M calc = M 2M1

(2)

where M1 and M2 refer to the Mueller matrix of the front and back interfacial J aggregates in the rotating cell, respectively. Because the front interface is reversed with respect to the back 7252


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Figure 5. Schematic representation of four possible cases for helical combinations of the interfacial J aggregates of H4TPP2+ at the front and back interfaces of the CLM cell viewed from the incident light side. (Left) CW and CCW cell rotation under low diffusion conditions (LD < 0). (Right) CW and CCW cell rotation under high diffusion conditions (LD > 0). The angle θ is positive for a clockwise rotation viewed from the incident light side. We assign θ, which can be positive or negative, to the orientation of the front interface and −θ to the orientation of the back interface.

Figure 6. Experimental and simulated Mueller matrices of a CW rotating sample. Calculated matrices correspond to the best fit obtained assuming the superimposed Mueller matrix model at θ = −22.5° (upper left in Figure 5).

interface as the cell is rotated, we can reasonably assume that M1 and M2 can be determined from a unique matrix, M0, and the corresponding matrix rotation. When the front aggregate in the interface of the CLM cell has a positive angle θ (clockwise rotation from a reference axis, x or y, viewed from the incident light side), the back aggregate will have a negative angle θ (Figure 5). Consequently, the twisted angle of the principal transition moments of the superimposed J aggregates is 2θ. Thus, M1 and M2 can be expressed as

M1 ≡ R( −θ ) M 0R(θ )

(3)

M 2 ≡ R(θ ) M 0R( −θ )

(4)

⎛1 0 0 ⎜ θ θ) 0 cos(2 ) sin(2 ⎜ R(θ ) = ⎜ ⎜⎜ 0 −sin(2θ ) cos(2θ ) ⎝0 0 0 7253

0⎞ ⎟ 0⎟ ⎟ 0⎟ ⎟ 1⎠

(5)


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Figure 7. LD0 and LB0 values of matrix M0 (left). Simulated and observed CD and CB spectra obtained from the analytical inversion method10,11 of the data in Figure 6 (right).

where R(−θ) M0R(θ) is the conventional matrix rotation of M0 by an angle θ. Furthermore, it is assumed that M0 contains only anisotropic parameters LD and LB ⎛ 1 −LD0 0 0 ⎞ ⎟ ⎜ 1 0 0 ⎟ ⎜−LD0 M0 ≡ ⎜ ⎟ 0 1 −LB0 ⎟ ⎜ 0 ⎟ ⎜ 0 LB0 1 ⎠ ⎝ 0

In the previous report, CW rotation generates positive LD spectra and negative apparent CD spectra.29 This situation corresponds to CW rotation and LD > 0 in Figure 5, where the diffusion effect is predominant and the aggregates tend to orient horizontally. This situation may be related to how the H2TPP solution is injected or the H2TPP concentration. In the previous measurement, prior to the injection of 100 μL of a H2TPP dodecane solution, 300 μL of dodecane solvent was introduced into a 4 M sulfuric solution in the CLM cell. In contrast, the present study directly injects 200 μL of an H2TPP solution in dodecane into a 4 M sulfuric aqueous solution in the CLM cell. Additionally, direct injection reduces the diffusion of H2TPP and orients the H4TPP2+ aggregates in the rotating direction as a result of the shear force produced from the slower rotation of the organic phase.29 Furthermore, the concentration of H2TPP in the present study is almost 10 times higher, thus the aggregates become larger and are readily aligned in the direction of rotation. The previous results of the positive LD spectra and negative ΔCD spectra observed by a conventional CD apparatus under the condition of CW rotation were observed once by the present 2-MGE measurement as shown in a single plot at the bottom right of Figure 4. It can be explained by the configuration that the aggregates are tilting in the front and back interfaces at θ = 24 and −24° from the horizontal x axis, respectively, suggesting left-handed alignment. All of these facts support superimposed helical alignment mechanisms of the J aggregates in the rotation cell. It is noteworthy that for two identical layers with only LB and LD the calculated CD and CB are maximized when the two layers are twisted at a 45° angle. At lower angles, the observed CD should decrease. In summary, the superimposed and twisted combination of two Mueller matrices with some degree of orientation and both LD and LB generate remarkable CD spectra. (The combination of Mueller matrices with only LD or LB cannot produce such a CD signal.) The CD sign and the twist direction in the exciton coupling of porphyrin molecules33−36 are similar to that of the present situation. The present superimposed Mueller matrix model does not include parameters related to the distance between the two J aggregates. Therefore, the superimposed

(6)

We employed the solver in Microsoft Excel to fit Mcalc to Mexp using eqs 2−6. The spectral dependence of the linear anisotropies, LB0 and LD0, and angle θ are used as free parameters. Figure 6 shows the best fit obtained for the CW rotation, which corresponds to an angle of θ = −22.5° from the y axis. Figure 7 shows the LB0 and LD0 values of matrix M0 as well as the simulated and observed values of the CD and CB spectra. Thus, the superimposed Mueller matrix model where only the anisotropic parameters are twisted by 2θ can simulate the observed CD spectra of oriented J aggregates at the liquid− liquid interface inside a rotating cell. CW rotation and LD < 0 correspond to the negative sign of angle θ in Figure 5. Therefore, the observed CD spectra in the present study originate from superimposed interfacial J aggregates at the front and back interfaces in the CLM cell, which form right-handed helical alignments where the twists equal the cell diameter when viewed from the light source side. As can be inferred from the above fitting procedure, when the absolute value of θ between the superimposed aggregates is small, the calculated CD spectrum is also small. In particular, when θ is zero, the CD signal is also zero. In the present experiment, the observed CD intensity of the J aggregates and the LD intensity decreased gradually beginning about 10 min after injection. This agrees with the artifactual character of the CD signal due to a combination of linear polarizations. The decrease of the LD intensity with time may be originated from a simple loss of alignment or the evolution of the particle shape toward fewer elongated particles. 7254


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(5) Kuroda, R. Harada, T. In Comprehensive Chiroptical Spectroscopy; Berova, N., Polavarupu, P. L., Nakanishi, K., Woody, R. W., Eds.; John Wiley & Sons: Hoboken, NJ, 2012; Vol. 1, pp 91−114. (6) Nagatani, H.; Watarai, H. Direct spectrophotometric measurement of demetalation kinetics of 5,10,15,20tetraphenylporphyrinatozinc(ii) at the liquid−liquid interface by a centrifugal liquid membrane method. Anal. Chem. 1998, 70, 2860. (7) Jellison, G. E.; Modine, F. A. Two-modulator generalized ellipsometry: theory. Appl. Opt. 1997, 36, 8190. (8) Jellison, G. E.; Modine, F. A. Two-modulator generalized ellipsometry: experiment and calibration. Appl. Opt. 1997, 36, 8184. (9) Arteaga, O.; Canillas, A. Pseudopolar decomposition of the Jones and Mueller-Jones exponential polarization matrices. J. Opt. Soc. Am. A 2009, 26, 783. (10) Arteaga, O.; Canillas, A. Analytic inversion of the Mueller-Jones polarization matrices for homogeneous media. Opt. Lett. 2010, 35, 559. (11) Arteaga, O.; Canillas, A. Analytic inversion of the Mueller− Jones polarization matrices for homogeneous media: erratum. Opt. Lett. 2010, 35, 3525. (12) Arteaga, O.; Canillas, A. Measurement of the optical activity of anisotropic samples by transmission Mueller matrix ellipsometry. EPJ Web Conf. 2010, 5, 03001. (13) Ribo, J. M.; Crusats, J.; Sagues, F.; Claret, J.; Rubires, R. Chiral sign induction by vortices during the formation of mesophases in stirred solutions. Science 2001, 292, 2063. (14) Escudero, C.; Crusats, J.; Diez-Perez, I.; El-Hachemi, Z.; Ribo, J. M. Folding and hydrodynamic forces in j-aggregates of 5-phenyl10,15,20-tris-(4-sulfophenyl)porphyrin. Angew. Chem., Int. Ed. 2006, 45, 8032. (15) Micali, N.; Engelkamp, H.; van Ree, J. P. G.; Christianen, P. C. M.; Monsù Scolaro, L.; Maan, J. C. Selection of supramolecular chirality by application of rotational and magnetic forces. Nat. Chem. 2012, 4, 201. (16) El-Hachemi, Z.; Arteaga, O.; Canillas, A.; Crusats, J.; Escudero, C.; Kuroda, R.; Harada, T.; Rosa, M.; Ribo, J. M. On the mechanochiral effect of vortical flows on the dichroic spectra of 5-phenyl10,15,20-tris(4-sulfonatophenyl)porphyrin J-aggregates. Chem.Eur. J. 2008, 14, 6438. (17) Yamaguchi, T.; Kimura, T.; Matsuda, H.; Aida, T. Macroscopic spinning chirality memorized in spin-coated films of spatially designed dendritic zinc porphyrin J-aggregates. Angew. Chem., Int. Ed. 2004, 43, 6350. (18) El-Hachemi, Z.; Arteaga, O.; Canillas, A.; Crusats, J.; Llorens, J.; Ribo, J. M. Chirality generated by flows in pseudocyanine dye Jaggregates: Revisiting 40 years old reports. Chirality 2011, 23, 585. (19) Petit-Garrido, N.; Ignés-Mullol, J.; Claret, J.; Sagues, F. Chiral selection by interfacial shearing of self-assembled achiral molecules. Phys. Rev. Lett. 2009, 103, 237802 (4). (20) Chen, P.; Ma, X.; Hu, K.; Rong, Y.; Minghua, L. Left or right? The direction of compression-generated vortex-like flow selects the macroscopic chirality of interfacial molecular assemblies. Chem.Eur. J. 2011, 17, 12108. (21) Tsuda, A.; Alam, M. A.; Harada, T.; Yamaguchi, T.; Ishii, N.; Aida, T. Spectroscopic visualization of vortex flows using dyecontaining nanofibers. Angew. Chem. 2007, 46, 8198. (22) Wolffs, M.; George, S. J.; Tomovi, Z.; Meskers, S. C. J.; Schenning, A. P. H.; Meijer, E. W. Macroscopic origin of circular dichroism effects by alignment of self-assembled fibers in solution. Angew. Chem., Int. Ed. 2007, 46, 8203. (23) Arteaga, O.; Escudero, C.; Oncins, G.; El-Hachemi, Z.; Llorens, J.; Crusats, J.; Canillas, A.; Ribo, J. M. Reversible mechanical induction of optical activity in solutions of soft-matter nanophases. Chem. Asian J. 2009, 4, 1687. (24) D’Urso, A.; Randazzo, R.; Lo Faro, L.; Purrello, R. Vortexes and nanoscale chirality. Angew Chem., Int. Ed. 2010, 49, 108. (25) Tsujimoto, Y.; Ie, M.; Ando, Y.; Yamamoto, T.; Tsuda, A. Spectroscopic visualization of right- and left-handed helical alignments of DNA in chiral vortex flows. Bull. Chem. Soc. Jpn. 2011, 84, 1031.

Mueller matrices model may be applied to truly chiral molecular systems as well as macroscopic helical aligned systems of molecular aggregates.

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CONCLUSIONS CLM can align supramolecular species formed at the liquid− liquid interface. Because of the formation of a noninteracting bilayer, CLM can generate new optical responses. For supramolecular systems, Mueller matrix polarimetry using 2MGE may distinguish between circular polarization due to natural optical activity and that originating from combinations of linear polarization. Previously, we demonstrated the presence of natural optical activity in other supramolecular systems26−28 as well as the same J aggregates, but the optical activity was induced by chiral dopants.37 Herein we analyze the perfect case of CD originating from the superimposed linear J aggregates of achiral porphyrin molecules formed at the liquid−liquid interfaces without mutual chemical interactions. The results herein show that the determination of the full Mueller matrix using several photoelastic modulators is a powerful tool for analyzing and interpreting the effect of flow on the birefringence of supra- and macromolecular systems. Finally, it might be summarized that there are three categories in the observation of CD spectra that are due to (i) the intrinsic molecular chirality, (ii) the chiral aggregation of achiral molecules induced with a chiral molecule or a chiral force, and (iii) the helical overlapping of the linearly polarized achiral molecular aggregates, which was just analyzed in the present study.

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ASSOCIATED CONTENT

S Supporting Information *

Notations and conventions in anisotropic spectroscopy. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Present Address ∥

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the JSPS International Training Program (ITP) and a Grant-in-Aid for Scientific Research (A) (no. 2450220) of the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the Spanish Government (AYA2009-13920-C01 and -C02).

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REFERENCES

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Alignment and Chirality of Porphyrin J Aggregates Formed at the

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