Linear Dichroism of Zn(II) - American Chemical Society

that the apparent CD spectra observed by the CLM-CD method were mainly due ... Finally, the orientation angle of the interfacial aggregate in the CLM ...
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Langmuir 2008, 24, 4722-4728

Linear Dichroism of Zn(II)-Tetrapyridylporphine Aggregates Formed at the Toluene/Water Interface Hideaki Takechi, Kenta Adachi,† Hideaki Monjushiro, and Hitoshi Watarai* Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka, 560-0043, Japan ReceiVed September 8, 2007. In Final Form: January 8, 2008 The apparent circular dichroism (CD) and the linear dichroism (LD) spectra of the aggregates of achiral zinc(II)-5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP), formed at the toluene/water interface in a centrifugal liquid membrane (CLM) cell, were investigated by comparison with the microscopic CD and LD spectra of a single interfacial aggregate of ZnTPyP about 100 µm in length, measured by a microscope-spectropolarimeter. The interfacial ZnTPyP aggregate showed two types of flat trapezoidal shapes, one had a seedlike core at an edge (type I) and another a needlelike core at an edge (type II). The microscopic CD and LD spectra were observed by varying the angle between the parallel axis of the trapezoidal aggregate and the perpendicular axis of a polarized light for LD. The plot of the CD intensity against the LD intensity for a single aggregate, observed at a given wavelength, showed a rotated elliptical shape with a long axis through the origin, when the orientation angle was changed. From these results, it was concluded that the apparent CD spectra observed by the CLM-CD method were mainly due to the large linear dichroism of the aggregate. Both type I and type II structures showed two transition dipole moments, parallel and perpendicular to the long axis of the structure, but suggesting a more developed J-aggregate in type II structure. AFM measurements showed that the interfacial ZnTPyP aggregate had a multilayer structure, in which the unit monolayer thickness was 1.58 ( 0.23 nm. Finally, the orientation angle of the interfacial aggregate in the CLM cell was estimated as 41°-44° to the rotating axis of the cell.

1. Introduction Self-aggregation and self-organization are natural processes caused mainly by noncovalent interactions, such as van der Waals, hydrogen bonding, hydrophilic/hydrophobic, electrostatic, donor and acceptor interactions, and coordination bonds.1 As a “bottomup” strategy, self-aggregation and self-organization are showing ever increasing importance in chemistry, material science, life science, and nanotechnology. Now, from natural and artificial self-assemblies, a wide variety of nanometer- or micrometersized structures and assemblies has been produced.2-9 Advanced functions of self-assemblies may meet requirements of many objectives in industrial technology, such as sensor, electronics, and electromechanical devices.10-16 Recently, the importance of molecular chirality in selfassembled materials has been extensively recognized. In living * Corresponding author. E-mail: [email protected]. † Present address: Research and Development Department, Moresco, Kobe, Hyogo, 650-0047, Japan. (2) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (3) Zhang, L. F.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (4) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (5) Gao, P. X.; Ding, Y.; Mai, W.; Hughes, W. L.; Lao, C. S.; Wang, Z. L. Science 2005, 309, 1700. (6) Kong, X. Y.; Ding, Y.; Yang, R. S.; Wang, Z. L. Science 2004, 303, 1348. (7) Kato, T. Science 2002, 295, 2414. (8) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401. (9) Jin, S. H.; Yu, G. G.; Han, P. L.; Li, J. W.; Li, J. J. Am. Chem. Soc. 2005, 127, 17090. (10) Li, Y. L.; et al. J. Am. Chem. Soc. 2005, 127, 1120. (11) Drain, C. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5178. (12) Du, C.; Falini, G.; Fermani, S.; Abbott, C.; Moradian-Oldak, J. Science 2005, 307, 1450. (13) Kurth, D. G.; Lehmann, P.; Schu¨tte, M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 5704. (14) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (15) Winfree, E.; Liu, F.; Wenzler, L. A.; Seeman, N. C. Nature 1998, 394, 539. (16) Vriezema, D. M.; Aragones, M. C.; Elemans, J. A. A. W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Chem. ReV. 2005, 105, 1445. (17) Co¨lfen, H.; Mann, S. Angew. Chem. Int. Ed. 2003, 42, 2350.

organisms, optically active macromolecules, such as proteins, nucleic acids, and polysaccharides, are universally involved in the life processes.17,18 These macromolecules possess specific conformational and highly ordered structure associated with their chiral properties. In some cases, biological supramolecules exhibit a distinct helical or twisted structure, which is a conspicuous sign of the optical chirality of the supramolecular aggregate.19-21 A variety of artificial self-assembled chiral structures have been synthesized from chiral molecules with noncovalent interactions, such as helical fiber and twisted ribbons of sugars,22 helicenes,23 helical metal complexes,24-26 or block copolymers.27-29 Optical rotary dispersion (ORD) and circular dichroism (CD) are now widely employed for characterizing and quantifying natural and synthetic chiral systems. Using a conventional circular dichroism spectropolarimeter, CD spectra have been measured not only for the isotropic samples but also for various anisotropic samples, such as solid, liquid crystal, film, membrane, micelles, and gel. However, the CD spectra of anisotropic samples were more or less accompanied by artifacts due to the optical (18) Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737. (19) Straley, J. P. Phys. ReV. A 1976, 14, 1835. (20) Engelman, D. M.; Steitz, T. A. Cell 1981, 23, 411. (21) Eisenberg, D.; Weiss, R. M.; Terwilliger, T. C. Nature 1982, 299, 371. (22) Hildebrand, P. W.; Rother, K.; Goede, A.; Preissner, R.; Fro¨mmel, C. Biophys. J. 2005, 88, 1970. (23) Fuhrhop, J. H.; Helfrich, W. Chem. ReV. 1993, 93, 1565. (24) Nuckolls, C.; Katz, T. J.; Verbiest, T.; Elshocht, S. V.; Kuball, H. G.; Kiesewalter, S.; Lovinger, A. J.; Persoons, A. J. Am. Chem. Soc. 1998, 120, 8656. (25) Shinoda, S.; Okazaki, T.; Player, T. N.; Misaki, H.; Hori, K.; Tsukube, H. J. Org. Chem. 2005, 70, 1835. (26) Wu, Z.; Yang, G.; Chen, Q.; Liu, J.; Yang, S.; Ma, J. S. Inorg. Chem. Commun. 2004, 7, 249. (27) Kawamoto, T.; Hammes, B. S.; Haggerty, B.; Yap, G. P. A.; Rheingold, A. L.; Borovik, A. S. J. Am. Chem. Soc. 1996, 118, 285. (28) Morino, K.; Oobo, M.; Yashima, E. Macromolecules 2005, 38, 3461. (29) Goto, H.; Zhang, H. Q.; Yashima, E. J. Am. Chem. Soc. 2003, 125, 2516. (30) Nonokawa, R.; Yashima, E. J. Am. Chem. Soc. 2003, 125, 1278.

10.1021/la7027862 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/01/2008

LD of Zn(II)-Tetrapyridylporphine Aggregates

anisotropies of the samples, such as linear birefringence (LB) and linear dichroism (LD).30 Therefore, it is necessary to give a lot of care to such chiral artifacts in CD measurements, unless the samples are completely randomly oriented systems such as solutions. Interfacial nanochemistry is growing as a new field in relation to analytical, separation, synthetic, and molecular simulation sciences.31 Determination of the structure of adsorbed species at the liquid-liquid interface is highly important in the study of kinetic mechanisms of solvent extraction and enzymatic reaction mechanisms at biomembranes.32 At the liquid-liquid interface, it is expected that a molecular aggregate, which is not formed in the bulk phase, can be formed easily, because the interfacial concentration of a surface active monomer becomes much higher than that in the bulk phase. The adsorption and aggregation behaviors of molecules were investigated by means of various methods.33-37 Recently, we investigated the chiral aggregates of porphyrins and phthalocyanine formed at liquid/liquid interfaces by circular dichroism measurements combined with a centrifugal liquid membrane (CLM) technique or second-harmonic generation spectrometry (SHG).38-40 It has been suggested by a CLMCD measurement that the interfacial self-aggregate of zinc(II)5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP) shows an apparent circular dichroism.41 In the present study, the cause of the apparent circular dichroism of ZnTPyP aggregates reported previously41 was investigated in detail from the comparison between the CD and LD spectra measured by the CLM method and those measured by a new microscopic spectropolarimeter technique. The present method, which examines the correlation between the CD and LD spectra, is thought to be very useful as a criterion of the genuine optical chirality, since a simple linear relationship will be observed in the case of an achiral and anisotropic system. Furthermore, the characteristic structure of the interfacial self-aggregate of ZnTPyP was examined by optical microscope and AFM measurements. In the present study, the orientation analysis of the interfacial aggregates in the CLM cell has also been carried out for the first time. 2. Experimental Section 2.1. Materials. ZnTPyP (Figure 1) and sodium perchlorate (GR grade) were purchased from Aldrich. Toluene and chloroform (GR grade) were purchased from Nacalai Tesque Inc. (Kyoto, Japan) and used as received. Toluene was a poor solvent for ZnTPyP. Therefore, toluene including 5% chloroform was used as the organic phase solvent. Chloroform was also used to prepare the stock solution of ZnTPyP. Water was distilled and deionized by a Milli-Q system (Millipore). (31) Kuroda, R.; Harada, T.; Shindo, Y. ReV. Sci. Instrum. 2001, 72, 3802. (32) Watarai, H.; Teramae, N.; Sawada, T., Eds. Interfacial Nanochemistry: Molecular Science and Engineering at Liquid-Liquid Interfaces (Nanostructure Science and Technology); Plenum: New York, 2005. (33) Volkov, A. G., Ed. Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications; Marcel Dekker: New York, 1997. (34) Fujiwara, K.; Monjushiro, H.; Watarai, H. ReV. Sci. Instrum. 2005, 76, 023111. (35) Yulizar, Y.; Monjushiro, H.; Watarai, H. J. Colloid Interface Sci. 2004, 275, 560. (36) Steel, W. H.; Damkaci, F.; Nolan, R.; Walker, R. A. J. Am. Chem. Soc. 2002, 124, 4824. (37) Nagatani, H.; Samec, Z.; Brevet, P. F.; Fermin, D. J.; Girault, H. H. J. Phys. Chem. B 2003, 107, 786. (38) Moriya, Y.; Nakata, S.; Morimoto, H.; Ogawa, N. Anal. Sci. 2004, 20, 1533. (39) Adachi, K.; Chayama, K.; Watarai, H. Langmuir 2006, 22, 1630. (40) Watarai, H.; Wada, S.; Fujiwara, K. Tsinghua Sci. Technol. 2006, 11, 228. (41) Fujiwara, K.; Monjushiro, H.; Watarai, H. Chem. Phys. Lett. 2004, 394, 349. (42) Matsumoto, Y.; Watarai, H. SolVent Extr. Res. DeV. Jpn. 2006, 13, 207.

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Figure 1. Molecular structure of ZnTPyP. The size of ZnTPyP is the reported size of TPyP, which is a ZnTPyP analogue.42 The red arrows show the transition dipole moment of the molecule.

2.2. UV-Vis Absorption, Linear Dichroism, and Circular Dichroism Spectral Measurements by the Centrifugal Liquid Membrane Technique. The formation of the self-aggregate of ZnTPyP at the toluene/water interface was directly observed using the CLM-Abs, CLM-LD, and CLM-CD methods,43,44 in which the CLM cell was installed in the CD spectropolarimeter (J-820E, JASCO). The apparatus for the CLM-Abs and CLM-CD measurements were essentially the same as the one reported previously,44 but the CLM-LD method was introduced in the present sturdy. A cylindrical cell, whose height and outer diameter were 3.4 and 1.9 cm, respectively, was placed horizontally in the sample chamber of the polarimeter and rotated at 7000 rpm by a speed-controlled electric motor (NE-22E, Nakanishi Inc.). At first, a blank spectrum was measured by introducing 0.500 mL of an aqueous solution of 0.1 M NaClO4 (pH 5.5) and 0.380 mL of toluene with a microsyringe into the cylindrical cell. Then, 0.020 mL of a chloroform solution of ZnTPyP was added and the interfacial reaction was initiated. The sum of the spectra of the bulk phase and the interface was measured by this method. The calculated values of the thickness of the organic and aqueous phase were 0.20 and 0.25 mm, respectively. The interfacial area, Si, between two phases was 20 cm2. 2.3. Microscopic UV-Vis Absorption, Linear Dichroism, and Circular Dichroism Measurements of a Single Interfacial Aggregate. We employed a home-built microscopic device, which was installed in a sample chamber of the CD spectropolarimeter (J-820, JASCO, Japan). The details of this new method will be published elsewhere. Briefly, the circularly polarized beam or the linear polarized beam from a light source of the polarimeter was condensed to the sample on a cover glass, which was vertically fixed on a position-orientation controllable stage, through an object lens (20×), and the penetrated beam was expanded again by an another objective lens (20×) and then detected by a frequency-controlled photomultiplier measurement technique to obtain CD, LD, and UVvis spectra of a minute region (60 µm × 20 µm) of the sample. The measurement region and the rotation angle of the sample were checked by CCD images through the same objective lens. The blank spectra were taken of a region of the cover glass without sample. The aggregate that formed at the liquid/liquid interface was transferred onto a cover glass by a simple lifting method. 2.4. AFM Measurement. The AFM images of the ZnTPyP aggregates transferred from the interface onto a cover glass after CLM measurement were measured by an atomic force microscope (SPI3800N/SPA400; SII NanoTechnology Inc., Tokyo, Japan) with a silicon cantilever, using the tapping mode. AFM images were presented in the height mode without any image processing except flattening. 2.5. Other Apparatus. The pH value of the aqueous phase was measured by a F-14 pH meter (HORIBA) equipped with a 6366(43) Sharma, C. V. K.; Broker, G. A.; Szulczewski, G. J.; Rogers, R. D. Chem. Commun. 2000, 1023. (44) Nagatani, H.; Watarai, H. Anal. Chem. 1998, 70, 2860. (45) Wada, S.; Fujiwara, K.; Monjushiro, H.; Watarai, H. Anal. Sci. 2004, 20, 1489.

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10D glass electrode. To observe the image of ZnTPyP aggregates at the liquid/liquid interface by a conventional optical microscope, a flat liquid/liquid interface was required under the objective. For this purpose, we used a two-phase glass cell, as reported previously, which was constructed by an outer glass cylinder cell (41 mm i.d. and 8.0 mm height) and an inner glass cylinder (36 mm i.d. and 5.0 mm in height).45

3. Results and Discussion 3.1. Interfacial Self-Aggregation of ZnTPyP at the Toluene/ Water System. The interfacial self-aggregation of ZnTPyP at the toluene/water system was directly measured using centrifugal liquid membrane-absorption spectrometry (CLM-Abs),43 centrifugal liquid membrane-circular dichroism spectroscopy (CLM-CD),44 and centrifugal liquid membrane-linear dichroism spectroscopy (CLM-LD). The preliminary study on the process of the interfacial self-aggregation of ZnTPyP has been reported previously.46 We observed that ZnTPyP was highly interfacially active and formed self-aggregates at the liquid-liquid interface by the bonding between zinc and the pyridyl group.46 The previous study of CLM measurements showed that ZnTPyP monomer had a peak at 423 nm that gradually decreased with time, while a new peak at 454 nm assigned to ZnTPyP aggregate appeared and reached an equilibrium after about 8 min under the conditions of 7.5 × 10-6 M ZnTPyP in the organic phase (toluene/chloroform ) 95/5, v/v) and 0.1 M NaClO4 in the aqueous phase. In the present study, CLM spectra of ZnTPyP aggregates were measured 30 min later after the start of the reaction. CLM-Abs, CLM-CD, and CLM-LD spectra of ZnTPyP aggregates are shown in Figure 2. The CLM-Abs spectrum showed a new peak at 454 nm, which was never observed in ZnTPyP monomer solution, thus assignable to ZnTPyP aggregates formed at the liquid/liquid interface. Though the CD spectrum of ZnTPyP monomer solution showed no optical chirality, the CLM-CD spectrum gave an apparent optical chirality, which should be due to the interfacial ZnTPyP aggregates.41 In the present study, the observed CLM-CD spectrum of the aggregate looked like a mirror image of the CLM-LD spectrum, though the intensity was smaller than that of the CLM-LD, as expected from the difference in definitions between CD and LD. Therefore, it was suggested that the circular dichroism reported previously was mainly due to the large linear dichroism. To discuss more about the cause of the apparent CD spectra in detail, we measured CD and LD spectra of an individual ZnTPyP aggregate by applying the new microscopic CD measurement device. 3.2. Linear Dichroism Spectra of a Single ZnTPyP Aggregate. ZnTPyP aggregates formed at the interface of CLM cell were rather small in size, only a few micrometers when observed after the CLM measurements. Therefore, it was difficult to measure the microscopic CD or LD spectrum of an individual ZnTPyP aggregate by defining the orientation angle of the single aggregate. To make larger ZnTPyP aggregates, we used a twophase glass cell, whose liquid/liquid interface was flat and whose specific interface area was 1.0 cm-1. Figure 3 is the optical microscopic images of ZnTPyP aggregates formed at the liquid/ liquid interface of the two-phase glass cell under the initial concentration of [ZnTPyP]ini ) 4.0 × 10-6 M. The interfacial aggregates had mainly flat and trapezoidal structures, different from the ZnTPyP crystals formed in a bulk solution that had thicker trapezoidal structures. Each semitransparent ZnTPyP aggregate at the interface had a black seed at the edge of the aggregate or a dark needle in an edge of the aggregate. We called (46) Yamamoto, S.; Watarai, H. Langmuir 2006, 22, 6562. (47) Takechi, H.; Watarai, H. SolVent Extr. Res. DeV. Jpn. 2007, 14, 133.

Figure 2. (a) Linear dichroism (LD) spectra, (b) circular dichroism (CD) spectra, and (c) absorption spectra of ZnTPyP aggregates in the toluene/water system measured by the CLM method (black line) and ZnTPyP monomer solution in a 1 mm cell (red line). The concentration of ZnTPyP in the organic phase (toluene/chloroform ) 95/ 5, v/v) in a CLM cell was 7.5 × 10-6 M (solid line) and the concentration of ZnTPyP solution (toluene/chloroform ) 95/5, v/v) in a 1 mm cell was 8.9 × 10-7 M (dotted line).

them a seed-type (type I) structure and a needle-type (type II) structure, respectively. To find any difference between type I and type II aggregates, CD and LD spectra of a single ZnTPyP aggregate were measured by the microspectropolarimeter. ZnTPyP aggregate was transferred from the liquid/liquid interface on a cover glass and the glass plate was fixed on the movable and rotatable stage to measure CD and LD spectra by the microspectropolarimeter with various angles of θ, which was defined as the angle between the long axis of the vertical rectangular beam and the long axis of the ZnTPyP aggregate (Figure 4a). Measurements were done for the type I ZnTPyP aggregate (Figure 4b) and the type II ZnTPyP aggregate (Figure 4d). In Figure 4b, many small seeds were observed around the type I aggregate, but the type I aggregate showed very often a fanlike or stacked-card-like structure. The observed micro-LD, micro-CD, and micro-Abs spectra of an individual structure of type I and type II aggregates are shown in Figure 4, parts c and e, respectively. The absorption spectra of the type I aggregates had two maxima at about 420 and 450 nm, though the 420 nm peak was smaller than the 450 nm peak. While in the absorption spectrum of type II aggregate, the intensities at the maxima at 420 and 460 nm were almost the same. Miyake et al. reported that ZnTPyP monolayer at the air/water interface had a similar absorption spectrum showing a smaller maximum at 422 nm and a larger maximum at 466 nm, suggesting an overlapped aggregate

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at various angles of θ for type I and type II aggregates was very similar, though the maximum wavelengths were different between type I and type II. The dependence of micro-LD and apparent micro-CD intensities on the angle θ at 449.5 nm was examined for a type I aggregate (Figure 6a). Although a LD maximum was noted at θ ) 90° and LD minima at θ ) 0° and 180°, a CD maximum was observed at θ ) 106° and CD minimum at θ ) 16°. The phase of the apparent CD intensity curve was shifted 16° from that of the LD intensity curve. The phase difference at 449.5 nm of a type II aggregate (Figure 4d) was 23°. Then, these phase differences may depend on the structural difference between the type I and type II aggregates. Figure 6b shows the correlation between micro-LD and microCD intensities of type I aggregate at 449.5 nm. The θ values were changed from 0° to 180° by selecting properly oriented aggregate. This figure shows a linear correlation between LD and CD intensities through the origin, though it has an elliptical deviation, confirming a significant relationship between the LD and CD of ZnTPyP aggregate. Similar correlation between LD and CD intensities was also observed for type II aggregate (Figure 4d), although the figure was not shown. It was reported that when a sample had macroscopic anisotropies such as linear birefringence (LB), circular birefringence (CB), and linear dichroism (LD), these anisotropies affect to the apparent circular dichroism, CDapp, through the following equation

CDapp ) CD + LD cos 2β0 sin κ + 1/6[CB LD LB Figure 3. Typical optical microscopic images of self-aggregates of ZnTPyP at the liquid-liquid interface. Each structure has a seedlike core at the center (type I) or a needlelike core at an edge (type II). The concentration of ZnTPyP in the organic phase (toluene/ chloroform ) 95/5, v/v) was 4.0 × 10-6 M in the two-phase system. The concentration of NaClO4 in the aqueous phase was 0.1 M (pH 5.5).

structure,47 but they did not discuss the possibility of the binding interaction between the pyridyl nitrogen and the Zn(II) ion. In the micro-LD and micro-CD spectra, the difference between type I and type II is more remarkable. The maximum at 450 nm in type I shifted to 470 nm in type II aggregate, as noted from Figure 4c,e, suggesting more extended J-aggregation in type II. Thus, it was noted that the extent of the J-aggregation of ZnTPyP molecules in the aggregate depended on the structures of the dark portion in the aggregate: seed-type and needle-type. The LD intensity is defined by the absorbance at a horizontally polarized light minus the absorbance at a vertically polarized light. If a ZnTPyP aggregate has only one transition dipole parallel to the long axis of a trapezoidal aggregate, the LD intensity should have a maximum at θ ) 0° or 90° and zero at θ ) 45°. However, the observed LD spectra were different from this simple prediction, as shown in Figure 4c,e. In the LD spectra of type I aggregate (Figure 4c), the observed LD intensities were in the order of LD(θ)90°) > LD(θ)45°) > LD(θ)0°) at 450 and 410 nm. However, at 430 nm, the order was completely reversed. This result indicates that the electric transition dipole moments corresponding to the maxima at 450 and 410 nm are parallel to the long axis of the aggregate and that corresponding to 430 nm is perpendicular to the long axis (Figure 5) in the type I structure. On the other hand, the transition dipole moments of type II aggregate at 470, 440, and 400 nm are parallel to the long axis and that at 430 nm is perpendicular to the long axis. 3.3. Correlation between Micro-LD and Micro-CD Spectra. In Figure 4c,e, the shape of the micro-LD and micro-CD spectra (48) Qian, D. J.; Nakamura, C.; Miyake, J. Langmuir 2000, 16, 9615.

CD LB2 + (1/2 ln 10)2(CD3 + CD LD2)] (1) where κ is the static birefringence of the modulator oriented at the angle β0 to the axis system of the induced birefringence.48 In the present study, it was thought that the second term of eq 1 was most important, because a large LD signal was observed for ZnTPyP aggregate. The slope of Figure 6b, 0.032 ( 0.005 at 449.5 nm, corresponds to the value of cos 2β0 sin κ in eq 1, which showed small dependence on wavelength (0.034 ( 0.004 at 429.0 nm and 0.028 ( 0.004 at 471.0 nm). Davidsson et al. reported that the value for cos 2β0 sin κ should be 0.02-0.1.48 Our result was well within the reported region for the value. The correlation between the apparent CD and LD in Figure 6b was not completely linear. The phase difference between CD and LD discussed above might be a cause. This phase difference might be due to the optical property of an aggregate and instrumental condition. Kuroda et al. reported that a sample with macroscopic anisotropy, like a crystal, e.g., R-Ni(H2O)6SO4, had not only a large LD signal but also a large LB signal.30 If the ZnTPyP aggregate belongs to the case, the linear birefringence (LB) in the third term of eq 1 may have some contribution to the apparent CD of ZnTPyP aggregate. After all, it is thought that the apparent CD of ZnTPyP aggregate arises from mainly the second term of eq 1, including LD, which means that the interfacial ZnTPyP aggregate is essentially optically achiral. 3.4. Nanostructure of ZnTPyP Aggregate. We tried to get further structural information on the interfacial self-aggregate of ZnTPyP, but the aggregates were too small to measure X-ray diffraction patterns. Instead, the aggregates were transferred onto a cover glass and observed by AFM. Figure 7a shows the AFM image of ZnTPyP aggregate, which is a part of the corner of the rectangular structure assignable to a type II aggregate. The thickness of the flat region of the aggregate was about 10 nm, and that of the needlelike region of the edge was about 40 nm. Such a very thin structure of ZnTPyP aggregates might be ascribable to a few nanometer thickness of the liquid/liquid (49) Davidsson, Å.; Norden, B.; Seth, S. Chem. Phys. Lett. 1980, 70, 313.

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Figure 4. (a) CCD image of a ZnTPyP aggregate taken by a microscopic device. Here, θ is the angle between the polarized light and the long axis of a ZnTPyP aggregate. CCD images of a type I ZnTPyP aggregate, which has a seedlike core (b), and a type II ZnTPyP aggregate, which has a needlelike core (d). LD, CD, and absorption spectra of the type I ZnTPyP aggregate (c) and the type II ZnTPyP aggregate (e) were measured by a microscopic spectropolarimeter. These spectra were normalized at the absorption maximum. CD and LD spectra were measured at various angles, and θ values are noted in the figure.

interfacial region.49 The needlelike portion at an edge of the type II aggregate observed by an optical microscope was confirmed to be thicker than the flat region. The flat region of the aggregate had nanosteps, as shown in Figure 7b. The Gaussian analysis of the observed steps was shown in Figure 7c. The peaks of the Gaussian fit appeared at 1.58 ( 0.23, 3.08 ( 0.13, and 4.60 ( 0.37 nm, giving the constant interval of about 1.6 nm. Therefore, the thickness of the unit step was obtained as 1.58 ( 0.23 nm (50) Watarai, H.; Gotoh, M.; Gotoh, N. Bull. Chem. Soc. Jpn. 1997, 70, 957.

from the first Gaussian peak. Interestingly, the unit thickness is almost the same size as the ZnTPyP molecule presented in Figure 1. This indicates that the unit step is a monolayer of ZnTPyP molecules, in which the ZnTPyP molecules stand perpendicularly and assemble by bonding between the pyridyl nitrogen and the zinc(II) ion.46 3.5. Orientation of ZnTPyP Aggregate in CLM Cell. From the above discussion on the micro-LD and micro-CD spectra, it was noted that the apparent circular dichroism of ZnTPyP

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Figure 5. Illustrations of ZnTPyP aggregate that had a seed (type I) and a needle (type II) at the edge of the aggregates. Red arrows show the directions of electric transition dipole moments in ZnTPyP aggregate suggested by micro-LD measurements.

Figure 7. (a) The AFM image of the interfacial self-aggregate of ZnTPyP (type II) after a CLM measurement ([ZnTPyP]ini ) 2.5 × 10-6 M), transferred on a cover glass. (b) 3D image of the flat region of the ZnTPyP aggregate. (c) Distribution of the thickness of the step of the flat region of ZnTPyP aggregate.

axis in the CLM cell (Figure 8). Then, the intensity of the CLMCD and CLM-LD spectra at a given wavelength can be represented as Figure 6. (a) CD and LD intensities of the type I ZnTPyP aggregate (Figure 4b) at 449.5 nm measured by the microscopic spectropolarimeter, when the angle between the polarized vertical light and the long axis of a ZnTPyP aggregate, θ, was changed. CD and LD values were normalized at the absorption maximum. Lines were fitted by a cosine curve. (b) Correlation between CD and LD intensities at various θ angles. Lines were the best fit of the second term of eq 1.

aggregate mainly arose from the linear dichroism. Here, we will discuss the difference in sign between the CLM-LD and the CLM-CD spectra, in which the sign of CLM-LD spectrum is reversed versus the CLM-CD spectrum (Figure 2a,b). We will postulate that the type I aggregates, whose microscopic spectra are same with those of Figure 4c with the angle dependence given by Figure 6a, have oriented with angle θ from the vertical

ICLM-CD ) ICD(θ) + ICD(180°-θ)

(2)

ICLM-LD ) ILD(θ) + ILD(180°-θ)

(3)

because in the CLM method the sum of the spectra of a front interfacial aggregate and a behind turned-over interfacial aggregate can be measured as illustrated in Figure 8. From the above relations, the angle dependences of CLM-CD and CLMLD intensities of type I aggregate at 449.5 nm were calculated as shown in Figure 9. From Figure 9, the CLM-LD intensity was positive in the range 46° < θ < 134° and negative in the range 134° < θ < 46°. On the other hand, the sum of ICD(θ) and ICD(180°-θ) at 449.5 nm became positive in the region 49° < θ < 131° and negative for 131° < θ < 49°. In this case, the ICLM-LD is positive and ICLM-CD is negative in the region of 46° < θ < 49° or 131° < θ < 134°. Thus, the observed signs of negative CLM-CD intensity and positive CLM-LD intensity at

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4. Conclusion

Figure 8. Illustration of ZnTPyP aggregates oriented with angle θ in the liquid/liquid interface of the CLM cell. The aggregates were exaggerated to show their orientation at the interface.

Figure 9. Angle dependence of CLM-CD and CLM-LD intensities of type I aggregate at 449.5 nm calculated by eqs 2 and 3. The inset is the enlarged one around 40°-50°.

449.5 nm could be explained by using those of micro-CD and micro-LD in the region of 46° < θ < 49°. This results means that the long axis of the aggregate at the interface in the CLM cell have orientated at 41-44° to the rotating axis of the CLM cell. The CLM-CD and CLM-LD spectra were thought to be composed of those of type I and type II from the observed positive or negative maxima at 400, 410, 430, 440, 450, and 470 nm.

The LD and CD measurements of the interfacial ZnTPyP aggregates using the CLM method and microscopic method revealed that the apparent CD of the aggregate was mainly due to the large linear dichroism.41 From the comparison between the CLM-CD and microscopic spectropolarimetry measurements, it was indicated that the interfacial aggregates formed in the CLM cell were orientated at 41°-44° to the rotating axis of the CLM cell. The individual ZnTPyP aggregate had two types of structure, a seed-type (type I) and a needle-type (type II), which showed the characteristic LD and CD spectra. Both type I and type II ZnTPyP aggregates had electric transition dipole moments of two directions, parallel and perpendicular to the long axis of the ZnTPyP aggregate. J-coupling of ZnTPyP molecular transition dipole moments seemed more pronounced in the type II aggregate along with the long axis of the trapezoidal structure. The thickness of unit monolayer of ZnTPyP aggregate was estimated as 1.58 ( 0.23 nm from the AFM measurements. From the present results, the molecular image of the microstructure of ZnTPyP aggregate was thought to be a stacked monolayer of ZnTPyP molecules, which was composed by the coordination bond between zinc(II) and the pyridyl nitrogen and the van der Waals stacking between porphyrin rings.8,50 The molecular origin of the differences between type I and type II aggregates is still obscure. However, it is plausible that ZnTPyP aggregate at the liquid-liquid interface of water and toluene had some different structures, because Goldberg et al. reported that the different structures of ZnTPyP crystals were produced depending on the solvent molecules included, such as water, methanol, aniline, and o-chlorophenol, by X-ray diffraction measurements.50,51 We have found in the present study at least two types of ZnTPyP aggregates from the differences in the microscopic LD and CD spectra. Also, it was demonstrated that the correlation between the apparent CD and LD is a useful criterion of a truly chiral conformation of the molecular aggregate formed at the liquid-liquid interface. Acknowledgment. This study was supported by the Grantin-Aid for Scientific Research (S) (No. 16105002) and in part by “Special Coordination Funds for Promoting Science and Technology: Yuragi Project” of the Ministry of Education, Culture, Sports, Science and Technology, Japan. LA7027862 (51) Krupitsky, H.; Stein, Z.; Goldberg, I.; Strouse, C. E. J. Inclusion Phenom. 1994, 18, 177. (52) George, S.; Goldberg, I. Acta Crystallogr. 2005, E61, m1441.