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J. Phys. Chem. C 2007, 111, 6194-6198
Uncoiling Process of Helical Molecular Fibrillar Structures Studied by AFM Ming Wang,†,‡ Yan-Lian Yang,§ Ke Deng,§ and Chen Wang*,§ Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of China, National Center for Nanoscience and Technology, Beijing, 100080, People’s Republic of China, and Graduate School of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China ReceiVed: NoVember 20, 2006; In Final Form: March 5, 2007
We observed the helical structures formed by achiral CuNPcOC4 molecules, and the uncoiling process of the helical fibrillar aggregates is examined by variable temperature AFM. It is identified that the measured pitch remained nearly unchanged for the temperature range of 30-65 °C, indicative of stacking stability of achiral phthalocyanines in the fibrillar structures. In contrast, the diameters of the fibrillar structures display discernible variations with slightly increased width, followed by appreciable reduction at around 55 °C, suggesting that the untwisting and dissociation processes occurred to the helical fibers.
Introduction Hierarchical aggregation processes of supramolecular association have received much attention for a wide variety of systems. The assembling behavior of discotic molecules and π-conjugated polymers leading to helical and supercoil structures1-8 is keen to the controlled growth of low dimensional molecular architectures.4,9 Efforts have been made to investigate the mechanism of the helix formation.4,10-12 Temperaturedependent optical methods, including ultraviolet and visible (UV-vis) absorption, fluorescence, and circular dichroism (CD) spectra have been used to reveal the kinetics of the aggregation processes of helical fibrillar structures for small molecules.10,11 It was proposed that the high-temperature regime is associated with the nucleation of small molecules, which leads to the formation of helical filaments. These helical filaments are subsequently assembled into helical fibers at low-temperature regimes.4,10-12 The experimental and theoretical studies have enriched the knowledge of helical aggregation process, while it is still challenging to explore the assembling thermodynamics by real time investigations. Capability of high-resolution imaging of atomic force microscopy (AFM) equipped with the variable temperature sample stage enables the in situ investigation of fibril formation and dissociation processes. Few studies have been reported that helix assembling structure could be formed by achiral small molecules without chiral templates.13-15 In this paper, chiral fibrillar aggregates are observed for copper(II) 5,9,14,18,23,27,32,36-octabutoxy-2,3naphthalocyanine (CuNPcOC4) molecules with no stereocenters, and the molecular structure of CuNPcOC4 is shown in Chart 1. The uncoiling process of the helical fibrillar aggregates is examined by variable temperature AFM and UV-vis spectra. Experimental Section The CuNPcOC4 sample was purchased from Sigma-Aldrich and used as received. Silicon substrates (100) doped with phosphorus were rinsed by acetone, methanol, and ultrapure * Corresponding author. Tel: (86)-10-62652700. Fax: (86)-10-62562871. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. § National Center for Nanoscience and Technology.
Figure 1. Typical AFM images for fibrillar structures of CuNPcOC4 on Si(100) with left-handed (a, topography) and right-handed (b, phase) configurations, and the image sizes are (a) 692 nm × 692 nm and (b) 256 nm × 256 nm. The inset in panel a shows the high-resolution AFM phase image with the scan size of 315 nm × 315 nm.
CHART 1: Molecular Structure of CuNPcOC4
water (18.2 MΩ·cm, Millipore) and then soaked in RCA-1 cleaner (H2O:NH4OH:H2O2 ) 5:1:1, in v/v) for 20 min, followed by an HF (2%) dipping for 10 min. Then the Si substrates were rinsed by ultrapure water and blown dry with nitrogen gas. The 10-6 M CuNPcOC4 solutions in toluene are dropped directly on Si(100) surface, and the samples were kept in a closed small container for volatilization slowly in solvent vapor. A Nanoscope IIIA and a Dimension 3100 scanning probe microscope systems (Veeco Metrology Group, USA) were used for in situ investigations of fibrillar structural changes at variable temperatures on Si(100) surface in ambient conditions. The temperature-dependent AFM observations provide direct evidence of the dissociation of the helical fibers at elevated temperature. Absorption measurements were performed to
10.1021/jp067692j CCC: $37.00 © 2007 American Chemical Society Published on Web 04/12/2007
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Figure 2. AFM images for left-handed helical structures of CuNPcOC4 on Si(100) with diameters of (a) 17, (b) 52, and (c) 187 nm. The section analysis along the helical axes was given for panels a and b. The regular repeating units of helical pitch in panel c were labeled by green short lines and the secondary structures were indicated by green arrow. The image sizes are (a) 200 nm × 200 nm, (b) 327 nm × 327 nm, and (c) 1.856 µm × 1.856 µm.
Figure 3. Statistics on pitch (a) and helix angle (b) of CuNPcOC4 helical fibers with different diameters.
SCHEME 1: Schematic Illustrations of the Uncoiling Process with Rising Temperature and the Pink Arrows Illustrate the Helix Diameters
investigate the temperature-dependent interactions between CuNPcOC4 molecules using Perkin-Elmer Lambda 950 UVvis spectrometer. Results and Discussion Chiral fibrillar structures can be achieved by the selfassembly of achiral CuNPcOC4 molecules from solution to solid surfaces. The same results can be obtained both on Si(100) and on mica (Figure S1) using identical sample preparation method. Considering the similar observations of fiber formation on graphite surface by the recent report,10 the supramolecular
fibrils could be formed mainly due to the intermolecular interactions, and the effect of substrate surfaces on the final fibril structures could be negligible. Typical left-handed and right-handed fibrillar structures of CuNPcOC4 aggregates on silicon surface are illustrated in Figures 1a and 1b, respectively. For the convenience of discussions, only left-handed assembling structures are presented in the following. The helical aggregates with diameter ranging from 17 to 187 nm were demonstrated in Figure 2. The repeating units along fibril axis and helical direction can be recognized. For the more thick fibril with 187 nm diameter (Figure 2c), the
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Figure 4. (a) UV-vis spectra for CuNPcOC4 in toluene (black solid line) and in toluene/methanol mixed solvent (red dashed line). (b) UVvis spectra for CuNPcOC4 in toluene/methanol mixed solvent at variable temperatures ranging from 35 to 95 °C. Absorption intensity for the aggregates around 756 nm decreases (V) while that for the monomers around 825 nm increases (v) at elevated temperature.
clearly resolved grooves on the fibril surface demonstrate the regular repeating units of helix pitch with a periodicity of ca. 150 ( 9 nm. The secondary structures within one pitch are also discernible with concave-shaped grooves, which could be ascribed to the hierarchical fibrillar structures containing elemental helixes. The measured results for the pitch and helix angle (helical direction relative to helix axes) of the helix fibril with different diameters are summarized in Figure 3, which indicates the characteristic behavior for pitch and helix angles with variable helix fibril diameters. The aggregation behavior of the fibrillar aggregation of CuNPcOC4 was examined by UV-vis absorption spectroscopy. The black solid line in Figure 4a illustrates the absorption spectrum of CuNPcOC4 solution in toluene, which shows the Q-band in the range of 750-850 nm. In a mixture of toluene and methanol (red dashed line in Figure 4a), the absorption band around 850 nm blue-shifts to 825 nm, and the other band around 750 nm simultaneously red-shifts to 756 nm. The solutionpolarity-induced spectra variations suggest an H-type stacking mode (face to face) between phthalocyanines.16 Considering the
Wang et al. periodical groove exterior with fine concave surfaces, the fibrillar structures could be the twisted helixes around each other and the helixes within the helical fibers might be “spiral staircase-like” structure with a staggering angle between neighboring phthalocyanines.4 The variable temperature UVvis spectra of CuNPcOC4 illustrated that the absorption intensity for the aggregates around 756 nm decreases while that for the monomers around 825 nm increases at elevated temperature (Figure 4b). This indicated the dissociation occurred with the temperature rising. High-resolution AFM images have been utilized to characterize the chirality and the helical parameters of the fibrillar structures for π-conjugated systems.17,18 By using AFM to characterize in situ annealed CuNPcOC4 helical fibers, the evolving structural characteristics of the uncoiling process could be studied. The whole process of in situ annealed CuNPcOC4 fibril is illustrated in Figure 5 with a temperature step about 5 °C. The changes in helix angle can be identified and the clusters adhered to the fibril surface suggest the breakage of the helix structures at the elevated temperatures. The uncoiling process could also be revealed by the explicit variations of the helix aggregations in the accompanied statistics on pitch, helix angle and diameter of the fibrils with the rising temperature (as shown in Figures 6a-c). The pitch of CuNPcOC4 helical fibers remains constant during heating and the helix angle begins to drop distinctly at around 60 °C. The diameter (statistics from topography images) undergoes a slightly broadening before the temperature reaches around 55 °C, and a discernible narrowing process above 55 °C. The schematically illustrated uncoiling process of the CuNPcOC4 helical fibers is proposed in Scheme 1. It can be deduced that the helixes within the fibril are untwisted with the increasing temperature to exhibit a diameter increase from D1 to D2. The diameter decrease from D2 to D3 at even higher temperature (above 55 °C) can be attributed to the dissociation of some elemental helixes away from the main strand of the helical fibers. The apparent diameter changes with the temperature variation indicated the relatively stable fibrillar structures at moderately elevated temperatures, whereas the uncoiling process should occur at high temperatures owing to the following discussions. With the increasing of the temperature, the interaction between fibrils should be weakened and in the same time thermal fluctuation of molecules should increase. These two factors should conceptually lead to an increase of diameters of the fibers, in contrast to the observation of the reduced helix diameters at the high end of temperature range in this study. It is therefore plausible to presume that the some elemental helixes are detached away from the main strand of the helical fibers at high temperatures. However, due to the disturbances of the scanning AFM tip and the thermal drift of adsorbates at elevated temperatures, those detached elemental fibrils could not be observed by AFM at the vicinity of the main helical fiber. The partially uncoiled helical structure could possibly undergo readjustment of twisting to maximize the van der Waals interactions leading to the observed decrease of helix angle (Figure 6b).1 This is also the reason for the constant pitch during the dissociation process. The observations in the variable temperature UV-vis spectra of CuNPcOC4 in Figure 4b of the decreasing absorption magnitude corresponding to aggregates in comparison with that of the monomers at elevated temperatures are also supportive to the proposed dissociation process of the aggregates observed by AFM. This in situ investigation of AFM phase images at variable temperatures provide direct evidence to the presumed
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Figure 5. AFM images of the in situ annealed CuNPcOC4 fibrils on Si(100) at 32 (a-c), 35 (d), 40 (e), 45 (f), 50 (g), 55 (h), 60 (i), 65 (j), 70 (k), and 75 °C (l). The tracking domain (c-l) is the zoomed part of panel b. All the AFM images except for (a, height mode) are shown in phase mode. The image sizes are (a, b) 4.39 µm × 4.39 µm and (c-l) 1.79 µm × 1.79 µm.
Figure 6. The statistics for the changes of three parameters, pitch (a), helix angle (b), and diameter (c), of CuNPcOC4 fibrillar structure with temperature.
thermodynamic process of helical nucleation to helical growth, which should be useful for better understanding and possible controlling of the self-assembling process.
In conclusion, helical structures formed by achiral CuNPcOC4 molecules were observed and the uncoiling process of the helical fibrillar aggregates is examined by variable tem-
6198 J. Phys. Chem. C, Vol. 111, No. 17, 2007 perature AFM. It is identified that the measured pitch remained nearly unchanged within the temperature range of 30-65 °C, indicative of stacking stability of achiral phthalocyanines. In contrast, the diameters of the fibrillar structures display discernible variation with slightly increased width followed by appreciable reduction at around 55 °C, indicating the untwisting and dissociation of helical fibers. This result could assist the studies on the molecular aggregation process. Acknowledgment. The authors gratefully acknowledge the National Natural Science Foundation of China (Grant Numbers 20473097, 90406019, 90406024, and 20673029) for financial support. The Chinese Academy of Sciences (CAS) and Ministry of Science and Technology (MOST) are gratefully acknowledged for financial support. Supporting Information Available: The AFM images of CuNPcOC4 fibrillar structures on mica are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kimura, M.; Muto, T.; Takimoto, H.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Langmuir 2000, 16, 2078. (2) Adachi, K.; Chayama, K.; Watarai, H. Langmuir 2006, 22, 1630. (3) Kimura, M.; Kuroda, T.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Langmuir 2003, 19, 4825.
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