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Langmuir 2006, 22, 7600-7604
Study of Internal Structure of meso-Tetrakis (4-Sulfonatophenyl) Porphine J-Aggregates in Solution by Fluorescence Microscope Imaging in a Magnetic Field Yasutaka Kitahama, Yasuyuki Kimura, and Ken Takazawa* Tsukuba Magnet Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan ReceiVed April 26, 2006. In Final Form: June 19, 2006 To determine the internal molecular arrangement of organic dye aggregates, a technique for observing the fluorescence microscope image of a solution consisting of dye aggregates in a magnetic field was developed. Using this technique, the fluorescence image of meso-tetrakis (4-sulfonatophenyl) porphine (TPPS) J-aggregates in a solution in a magnetic field of 10 T was observed. It was observed that individual rod-shaped TPPS aggregates (4-20 µm in length) were aligned parallel to the applied field. The polarized absorption spectra of the sample solution were also measured in the fields of up to 10 T. The spectra show the magnetic field dependence of the J-band intensity, reflecting the magnetic alignment of the aggregates. On the basis of the magnetic and optical properties obtained by the experiments, it was proposed that TPPS J-aggregates have a tube-like structure and are constructed from one-dimensional molecular arrays that are stacked parallel to the long axis of the tube.
Introduction Molecular aggregates self-assembled from organic dye molecules in a solution have attracted considerable interest because of their characteristic optical properties such as intense narrow absorption bands (J- and H-bands), strong nonlinear optical response, and fast energy transfer within the aggregates.1 These properties are due to the strong coupling between the transition dipoles of constituent molecules that leads to delocalized excitonic states over a few to several molecules upon optical excitation. Therefore, the spatial arrangement of the transition dipoles, namely, the internal molecular arrangement within the aggregate, is closely related to the optical properties. For example, an aggregate exhibits a red-shifted absorption band (J-band) with respect to monomer absorption when it has a brickwork-like internal molecular arrangement, whereas it exhibits a blue-shifted absorption band (H-band) when it has a ladder-like arrangement.1-3 Therefore, determination of the internal molecular arrangement of aggregates is essential to understand their optical properties and apply these aggregates to molecular devices by utilizing their optical properties. The morphology and internal molecular arrangement of the dye aggregates have been studied by various techniques such as polarized fluorescence microscopy,4-7 cryo-TEM,8,9 and X-ray diffraction.10-12 Recently, it has been demonstrated that a * Corresponding author. E-mail:
[email protected]. (1) Kobayashi, T., Ed. J-Aggregates; World Scientific: Singapore, 1996. (2) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (3) Kasha, M. In Spectroscopy of the Excited State; Plenum Press: New York, 1976; p 337. (4) Higgins, D. A.; Reid, P. J.; Barbara, P. F. J. Phys. Chem. 1996, 100, 1174. (5) Higgins, D. A.; Kerimo, J.; Vanden Bout, D. A.; Barbara, P. F. J. Am. Chem. Soc. 1996, 118, 4049. (6) Vacha, M.; Takei, S.; Hashizume, K.; Sakakibara, Y.; Tani, T. Chem. Phys. Lett. 2000, 331, 387. (7) Vacha, M.; Saeki, M.; Isobe, O.; Hashizume, K.; Tani, T. J. Chem. Phys. 2001, 115, 4973. (8) von Berlepsch, H.; Bo¨ttcher, C. J. Phys. Chem. B 2001, 106, 3146. (9) von Berlepsch, H.; Bo¨ttcher, C.; Da¨hne, L. J. Phys. Chem. B 2000, 104, 8792. (10) Yoshioka, H.; Nakatsu, K. Chem. Phys. Lett. 1971, 11, 255. (11) Harrison, W.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310.
magnetic field is a useful tool to investigate the internal structure of the aggregates in solution, which is their native environment.13-15 Since dye aggregates have a highly ordered internal molecular arrangement, they generally exhibit an anisotropic diamagnetic susceptibility. Because of the anisotropic diamagnetic susceptibility, the aggregates in a magnetic field tend to orient in a specific direction with respect to the applied field so that the extra energy from the magnetic field is minimized. Therefore, aggregates in solution can be aligned by applying a magnetic field, which is sufficiently strong to overcome the thermal fluctuation. Since the absorption spectra of such aligned aggregates exhibit a polarization dependence, the internal molecular arrangement can be estimated from the relative orientation between the absorption polarization and the direction of the applied magnetic field. In this study, we developed a technique to observe the fluorescence microscope image of the dye aggregates in solution in a magnetic field. By utilizing this technique, we can directly observe the magnetic alignment of individual micron-sized dye aggregates. Using this technique, we demonstrate the determination of the internal structure of the meso-tetrakis (4-sulfonatophenyl) porphine (TPPS) J-aggregate, which is reported to have a hierarchical high-order internal structure.16-19 The optical properties and morphology of the TPPS aggregate have been extensively studied because the TPPS aggregate is a good model system for studying the energy transfer in a light(12) Harrison, W.; Mateer, D. L.; Tiddy, G. J. T. Faraday Discuss. 1996, 104, 139. (13) Shklyarevskiy, I. O.; Christianen, P. C. M.; Aret, E.; Meekes, H.; Vlieg, E.; Deroover, G.; Callant, P.; van Meervelt, L.; Maan, J. C. J. Phys. Chem. B 2004, 108, 16386. (14) Shklyarevskiy, I. O.; Boamfa, M. I.; Christianen, P. C. M.; Touhari, F.; van Kempen, H.; Deroover, G.; Callant, P.; Maan, J. C. J. Chem. Phys. 2002, 116, 8407. (15) Christianen, P. C. M.; Shklyarevskiy, I. O.; Boamfa, M. I.; Maan, J. C. Phys. B 2004, 346-347, 255. (16) Rotomskis, R.; Augulis, R.; Snitka, V.; Valiokas, R.; Liedberg, B. J. Phys. Chem. B 2004, 108, 2833. (17) Schwab, A. D.; Smith, D. E.; Rich, C. S.; Young, E. R.; Smith, W. F.; de Paula, J. C. J. Phys. Chem. B 2003, 107, 11339. (18) Gandini, S. C. M.; Gelamo, E. L.; Itri, R.; Tabak, M. Biophys. J. 2003, 85, 1259. (19) Snitka, V.; Rackaitis, M.; Rodaite R. Sens. Actuators, B 2005, 109, 159.
10.1021/la061127q CCC: $33.50 © 2006 American Chemical Society Published on Web 07/27/2006
Internal Structure of TPPS J-Aggregates
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Figure 2. Setup for the fluorescence microscope imaging of the sample solution in a magnetic field.
Figure 1. (A) Diacid form of TPPS molecule. (B) Schematic representation of the diacid TPPS molecule. The spheres represent the negatively charged sulfonato groups, and the square disk represents the doubly positively charged porphyrin and phenyl groups. The two orthogonal arrows represent the transition moments. (C) The linear array of the TPPS molecule. The arrows represent the transition moments of the J- and H-bands. (D) Schematic representation of the nanorod observed on a substrate. (E) Bundle of nanorods. (F) Nanorod in solution. (G) Schematic representation of the linear array shown in panel C. The red surface represents the porphyrin ring plane of the constituent TPPS molecules. (Note that hierarchical structures other than those shown in panels A-G were also proposed. For example, see refs 18 and 19.)
harvesting antenna complex in a natural photosynthetic system, which also utilizes the energy transfer between circularly arranged porphyrin-based pigments such as chlorophylls.20 Under the acidic conditions, a porphyrin ring of TPPS has a positively charged diacid form (Figure 1A,B) and negatively charged sulfonato groups associated with the porphyrin ring. As a result, the molecules stack in a staircase manner and form a one-dimensional linear array (Figure 1C).16,17,21-30 The transition dipoles that are aligned parallel to the long axis of the linear array couple in a head-to-tail manner, resulting in a J-band that is polarized parallel to the long axis (Figure 1C). On the other hand, the dipoles (20) Kuhlbrandt, W. Nature 1995, 374, 497. (21) Koti, A. S. R.; Taneja, J.; Periasamy, N. Chem. Phys. Lett. 2003, 375, 171. (22) Micali, N.; Mallamace, F.; Romeo, A.; Purrello, R.; Scolaro, L. M. J. Phys. Chem. B 2000, 104, 5897. (23) Maiti, N. C.; Ravikanth, M.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. 1995, 99, 17192. (24) Misawa, K.; Kobayashi, T. Tech. Digest IQEC 1998, 99. (25) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1993, 99, 4128. (26) Okada, S.; Segawa, H. J. Am. Chem. Soc. 2003, 125, 2792. (27) Collings, P. J.; Gibbs, E. J.; Starr, T. E.; Vafek, O.; Yee, C.; Pomerance, L. A.; Pasternack, R. F. J. Phys. Chem. B 1999, 103, 8474. (28) Castriciano, M. A.; Romeo, A.; Villari, V.; Micali, N.; Scolaro, L. M. J. Phys. Chem. B 2003, 107, 8765. (29) Rubires, R.; Farrere, J.-A.; Ribo, J. M. Chem.sEur. J. 2001, 7, 436. (30) Micali, N.; Romeo, A.; Lauceri, R.; Purrello, R.; Mallamace, F.; Scolaro, L. M. J. Phys. Chem. B 2000, 104, 9416.
aligned perpendicular to the long axis couple in a face-to-face manner, resulting in an H-band that is polarized perpendicular to the long axis (Figure 1C). The width of a TPPS molecule is reported to be ∼2 nm; 29 thus, the width of the linear array is also estimated to be ∼2 nm. Spectroscopic studies have suggested that a linear array consists of 6-32 molecules.21-23 Rotomskis et al.16 and Schwab et al.17 have reported that the atomic force microscope (AFM) images of TPPS aggregates transferred on a substrate showed two types of structures with different geometries: a nanosized rod (nanorod) (Figure 1D) and a bundle of the nanorods (Figure 1E). Since the height of the nanorod (∼4 nm) is equal to the thickness of a bilayer of molecules, it was suggested that the nanorod has a tube-like structure with a monolayer wall in the solution and is flattened on the substrate (Figure 1D,F).16 The size of the nanorod is greater than that of the linear array, suggesting that the nanorod is constructed from the linear array building blocks (Figure 1G). The AFM image of the bundle showed that the bundle is constructed from nanorods that are stacked parallel to each other along the long axis of the bundle (Figure 1E).16 The length and width of the bundle depends on the number of nanorods within the bundle and varies depending on the concentration of the solution and aging time.17 The length of the bundle is reported to reach the millimeter range when the concentration of the solution is high.16 To investigate the internal structure of TPPS nanorods, namely, the arrangement of the linear arrays inside the nanorods, we observed the magnetic alignment of individual bundles of the nanorods in solution in a magnetic field by fluorescence microscopy. We also observed the polarized absorption spectra of the magnetically aligned TPPS aggregates. We found that the magnetic and optical properties of TPPS aggregates can be well explained by assuming that the TPPS nanorods have a tube-like structure, constructed from the linear array building blocks that are aligned parallel to the long axis of the tube. Experimental Procedures The TPPS was obtained from Frontier Scientific and was used as obtained. The sample solution was prepared by the addition of 1 mL of HCl (6 M) to 5 mL of a TPPS aqueous solution (2.0 µM), resulting in an acidic solution of pH ≈ 1. The sample solution was aged for 18 h before measurement. Figure 2 shows the experimental setup for the fluorescence microscope imaging of the sample solution in a magnetic field. A superconducting magnet (Japan Magnet Technology, maximum field: 10 T and bore diameter: 100 mm) was used. The output of an argon-ion laser (Coherent, Innova Sabre DBW-DP 15/3) at 488 nm was passed through a spatial filter and then directed to a microscope objective (Olympus, LMPlanFL, 20×, NA ) 0.4), which was mounted in a bore, by using a dichroic mirror and a prism. The optical axis of the objective was perpendicular to the field direction. A microscope slide glass with a round well with a diameter of 15 mm and a depth of 0.6 mm was used as the sample holder. The dip was filled with the sample solution and covered with a microscope
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Figure 3. (A) Fluorescence microscope image of the solution consisting of TPPS J-aggregates observed at 0 T. (B) Fluorescence microscope image of TPPS aggregates observed at 10 T. The exposition time for both images is 60 ms. cover glass. The sample was mounted in the bore of the magnet. The fluorescence from the sample was collected by the same objective and recorded by an image-intensified CCD camera (Hamamatsu, C8780-02) through an imaging lens. Laser light reflected by the sample surface was eliminated by a long-pass filter (cutoff wavelength: 650 nm) placed before the CCD camera. The polarized absorption spectra of the sample solution were also measured by using the superconducting magnet. A glass cell with an optical path length of 3 mm was mounted in the bore of the magnet. Unpolarized light from a tungsten-halogen lamp was guided to the sample using an optical fiber. The light transmitted through the sample was passed through a film polarizer and coupled with another optical fiber that guided the light to a spectrometer (OceanOptics, USB2000), which was placed outside the bore.
Results Figure 3A shows the fluorescence microscope image of the sample solution in the absence of a magnetic field. Randomly oriented rod-shaped aggregates were observed in the image. The video-rate acquisition of these images exhibited the rotational and translational motions of the aggregates. Figure 3B shows the fluorescence image of the same sample measured in a magnetic field of 10 T. This image shows that the aggregates are aligned parallel to the field direction. The video-rate measurement exhibited only the translational motion of these aggregates, confirming the magnetic alignment parallel to the field. Since the focus plane is also parallel to the magnetic field, the magnetically aligned aggregates are entirely in-focus, whereas the randomly oriented aggregates could be partly out-of-focus when they tilt from the focus plane. Therefore, the average length of the aggregates observed in the field is longer than that observed in the absence of the field, as can be seen from Figure 3A,B. The lengths of the aggregates can be measured from the image obtained in the field and were found to be 4-20 µm. The lengths agree with those of the bundle of nanorods observed on the substrate by AFM,16,17 indicating that these aggregates are the bundles. Single isolated nanorods (0.1-3 µm in length by the AFM observation16,17) were not observed probably due to their fast translational motion. Figure 4A shows the polarized absorption spectra of the sample solution measured at 0 and 10 T for the parallel and perpendicular
Figure 4. (A) Polarized absorption spectra of the solution consisting of TPPS J-aggregates measured at 0 and 10 T for parallel polarization (||) and perpendicular (⊥) polarization to the applied magnetic field direction. (B) Intensity of the J-band at 489 nm as a function of the magnetic field for the parallel (||) and perpendicular (⊥) polarizations.
polarizations with respect to the applied magnetic field. The spectra consist of four absorption bands. The bands at 705 and 489 nm are the J-bands of the aggregates. The two bands at 435 and 420 nm are the Soret band (the S2-S0 transition) of monomers and the H-band of the aggregates, respectively.25,26,31 The spectra show that the intensities of the J-bands for parallel polarization increase by applying the magnetic field, whereas the intensities for perpendicular polarization decrease. The H-band exhibits an opposite field dependence to the J-band, indicating that the H-band is polarized perpendicular to the J-band. Figure 4B shows the intensity of the J-band at 489 nm as a function of the applied magnetic field. For the parallel (perpendicular) polarization with regard to the magnetic field, the intensity increases (decreases) rapidly up to ∼2 T. Above ∼2 T, the intensity increases (decreases) gradually up to 10 T. The rapid increase below ∼2 T indicates that the magnetic alignment of large aggregates (i.e., the bundles of nanorods) occurs in the low fields near 0 T and that the alignment is complete at the magnetic field of ∼2 T. The increase in the J-band intensity for parallel polarization indicates that the J-band is polarized parallel to the applied field direction. The fluorescence image in the field showed that the bundles were aligned parallel to the field direction as shown in Figure 3B. Thus, we can conclude that the J-band is polarized parallel to the long axis of the bundle. The gradual increase in the J-band intensity up to 10 T is probably due to the magnetic alignment of small aggregates such as single nanorods and linear arrays and may reflect the broad size distribution of such small aggregates. (31) Pasternack, R. F.; Huber, P. R.; Boyd, P.; Engasser, G.; Francesconi, L.; Gibbs, E.; Fasella, P.; Venturo, G. C.; de C. Hinds, L. J. Am. Chem. Soc. 1971, 94, 4511.
Internal Structure of TPPS J-Aggregates
Figure 5. (A) Magnetic and optical properties of the bundle of nanorods. The bundle is aligned parallel to the magnetic field, and its J-band is polarized parallel to the bundle. (B) Magnetic and optical properties of the nanorod. The nanorod is aligned parallel to the field, and the J-band is polarized parallel to the nanorod.
Discussion The experimental results obtained by fluorescence imaging and the polarized absorption spectra in the magnetic fields are summarized as follows: the bundle of nanorods is aligned parallel to the field direction, and the J-band is polarized parallel to the long axis of the bundle (Figure 5A). Since the bundle is constructed from nanorods that are simply stacked parallel to each other,16 the isolated nanorods should have the same magnetic and polarization properties as those of the bundle. The nanorod in the field is aligned parallel to the field direction, and the J-band of the nanorod is polarized parallel to the long axis of the rod (Figure 5B). To determine the internal structure of the nanorods based on these experimental results, we first considered the diamagnetic susceptibility of single TPPS molecules. For a molecule consisting of a π-conjugated ring, the diamagnetic susceptibility dominantly originated from a diamagnetic ring current, which was induced by an external magnetic field. Thus, the diamagnetic susceptibility of the π-conjugate ring is strongly anisotropic, and the anisotropy is expressed by |χ⊥| . |χ||, where χ⊥ and χ| are the diamagnetic susceptibilities along the axes perpendicular and parallel to the ring plane, respectively (note χ⊥,| < 0). Thus, the ring in the magnetic field tends to orient such that the ring plane is parallel to the field (the ⊥ axis is perpendicular to the field) to minimize the magnetic energy given by E ) -χB2. In the case of the TPPS molecule, the diamagnetic susceptibility originated from a porphyrin ring and four phenyl rings. Therefore, to determine the anisotropic diamagnetic susceptibility of the TPPS molecule, the relative orientation (torsion angle) between the porphyrin ring and the phenyl rings should be considered. A molecular modeling calculation of the TPPS molecule within the linear array showed that the four phenyl rings are considerably twisted with respect to the porphyrin ring28 (the value of the torsion angle is not given in the reference). X-ray crystallography of tetraphenylporphyrin (TPP) crystals also showed that the phenyl groups are twisted by 60-80° due to steric hindrances between the porphyrin and the phenyl rings.32,33 Therefore, it is reasonable to assume that the phenyl rings of TPPS are also twisted by 60-80° due to the steric hindrance. However, the |∆χ| () |χ⊥ - χ||) of the porphyrin ring (∆χ ) -1.0 × 10-27 cm3)34 is approximately 10 times larger than that of benzene ring (∆χ ) -1.0 × 10-28 cm3).35 Therefore, the axis of the largest diamagnetic susceptibility (Z axis) of the TPPS molecule is perpendicular to the porphyrin ring plane even if the phenyl rings are perpendicular (32) Silvers, S. J.; Tukinsky, A. J. Am. Chem. Soc. 1967, 89, 3331. (33) Hamor, M. J.; Hamor, T. A.; Hoard, J. L. J. Am. Chem. Soc. 1964, 86, 1938. (34) Bothner-By, A. A.; Gayathri, C.; van Zijl, P. C. M.; Maclean, C.; Lai, J.; Smith, K. M. Magn. Reson. Chem. 1985, 23, 935. (35) Maret, G.; Dransfeld, K. In Strong and Ultrastrong Magnetic Fields and Their Applications; Springer-Verlag: Berlin, 1985; p 143.
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Figure 6. (A) Diamagnetic susceptibility of the TPPS molecule. The axis of the largest diamagnetic susceptibility (Z axis) is perpendicular to the porphyrin ring plane. (B) The diamagnetic susceptibility of the linear array. The Z axis is perpendicular to the porphyrin ring plane of the constituent TPPS (red surface).
to the porphyrin ring. Consequently, TPPS in a magnetic field tends to orient such that the Z axis is perpendicular to the field direction (Figure 6A). It should be noted that this discussion does not imply that individual TPPS molecules orient in such directions in the magnetic field of 10 T. The |∆E| () |∆χ|B2) for single molecules in the magnetic field of 10 T is generally much smaller than the thermal energy at room temperature (|∆E| , kT); thus, the magnetic field effect on the molecular orientation is not observable for single molecules due to the thermal fluctuation. On the basis of the previous considerations, we can determine the anisotropic diamagnetic susceptibility of the linear array. Since TPPS molecules are arranged in a staircase manner in the linear array, the anisotropic diamagnetic susceptibility of the linear array is expressed as |χΖ| . |χX | ≈ |χY|, where Z, X, and Y represent the axes perpendicular to the porphyrin ring, parallel to the long axis of the linear array, and parallel to the short axis of the linear array, respectively (Figure 6B). Therefore, the linear arrays in the field orient with the Z axis perpendicular to the field direction (Figure 6B). It should be noted that the linear array is not aligned in a specific direction with respect to the magnetic field because the diamagnetic susceptibilities along the X and Y axes are nearly equal (χX ≈ χY). The linear array can rotate around the Z axis, which is perpendicular to the field, without a change in the magnetic energy. To determine the internal structure of the nanorod, which is constructed from the linear array, it is important that the nanorod is aligned parallel to the magnetic field while the linear array is not aligned in one direction. If the nanorod is formed by simply stacking the linear arrays parallel to each other, the nanorod is not aligned in one direction similarly to the linear array (Figure 7A). Therefore, it is suggested that the nanorod is not simply a stack of linear arrays but has a high-order internal structure. By assuming that the rod has a tube-like structure as suggested by Rotomskis et al.,16 the magnetic alignment of the nanorod can be explained. Figure 7B,C shows two tube-like structures that can be aligned parallel to the field direction. The Z axes of the linear arrays in the tubes orient tangentially (Figure 7B) and radially (Figure 7C). In both the structures, the Z axes of all the constituent linear arrays are perpendicular to the field direction when the tubes are aligned parallel to the field direction, leading to the magnetic alignment parallel to the field. Moreover, in both the structures, the J-bands of the linear arrays are polarized parallel to the long axis of the tubes as observed by our experiments. On the other hand, the transition dipoles of the H-bands orient radially and tangentially in the tube shown in Figure 7B,C, respectively. Therefore, the H-bands are polarized perpendicular to the tubes in both the structures, and thus, the magnetic field dependence of the H-band intensity, which is observed to be opposite to that of the J-band, also supports the structures.
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Figure 7. (A) Simple parallel stack of linear arrays is not aligned in one direction in the magnetic field. The short arrows represent the Z axes of the linear arrays. (B) The tube structure with tangentially oriented Z axes of the linear arrays (short arrow) exhibits a magnetic alignment parallel to the field. (C) The tube structure with radially oriented Z axes of the linear arrays (short arrow) exhibits a magnetic alignment parallel to the field. (D) Tube structure constructed from the rings. This tube can be aligned parallel to the field, but its J-band is polarized perpendicular to the tube.
Figure 7D shows the tube structure suggested by Rotomskis et al.16 In this tube, the linear arrays first form rings, and the rings are then stacked into a tube. This structure is supported by the experimental result that the width and height of the nanorods observed on the substrate are independent of the concentration of the sample solution.16 When the linear arrays first form a ring
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with a fixed diameter, the diameter of the nanorod is determined only by the ring diameter and is independent of the concentration. Since the Z axes of the TPPS molecules orient radially in the tube, this tube also can be aligned parallel to the field direction. However, the J-band of this tube is polarized perpendicular to the long axis of the tube, as shown in Figure 7D. Therefore, the polarization direction of this tube does not agree with the experimental observation. From the previous discussion, we propose structures shown in Figure 7B,C as the possible structures of the TPPS nanorod. By considering the π-π interaction between the porphyrins, the structure shown in Figure 7B may be the most likely structure because porphyrins in the neighboring linear arrays are arranged in a face-to-face manner. In conclusion, we observed the fluorescence microscope image of the TPPS J-aggregates in solution in a magnetic field of 10 T. It was observed that the rod-shaped TPPS J-aggregates are aligned parallel to the applied magnetic field. The length of the observed rods indicates that they are the bundles of nanorods. Since the bundles are constructed from nanorods that are simply stacked parallel to each other, it was concluded that the nanorods also tend to align parallel to the magnetic field. We also observed the polarized absorption spectra of the TPPS aggregates in solution in magnetic fields of up to 10 T. The spectra show that the intensity of the J-band increases (decreases) with an increase in the magnetic field for the polarization parallel (perpendicular) to the field direction. On the basis of our experimental observations and the previously reported geometry of nanorods on a surface, we proposed that the TPPS J-aggregates (nanorods) have a tubelike structure and are constructed from the linear array building blocks that are stacked parallel to the long axis of the tube. Although the structures we proposed well elucidate the observed magnetic and optical properties of TPPS J-aggregates, further studies on the formation mechanism of such tube-like structures are necessary to explain the even width of the nanorods regardless of the sample concentration. LA061127Q
