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Photocontrollable J-Aggregation of a DiarylethenePhthalocyanine Hybrid and Its Aggregation-Stabilized Photochromic Behavior Jiaxiang Yi,† Zihui Chen,*,†,‡ Junhui Xiang,§ and Fushi Zhang*,† †
The Key Lab of Organic Photoelectrons & Molecular Engineering of Ministry of Education, Department of Chemistry, and Department of Physics & Tsinghua-Foxconn Nanotechnology Research Centre, Tsinghua University, Beijing 100084, PR China § College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing, China ‡
bS Supporting Information ABSTRACT: The photocontrollable J-aggregation of a diarylethenephthalocyanine hybrid (T-ZnPc) and its aggregation-stabilized photochromic behavior were investigated by various techniques. T-ZnPc initially exhibited slight J-aggregation tendency in solvents such as chloroform and toluene through conformational planarization effect, but formed much stronger J-aggregates upon the illumination of 254 nm UV light. In darkness, the UV-irradiated solutions gradually returned to their initial state. These phenomena can be explained by the pronounced change in molecular planarity accompanying the reversible isomerization of the diarylethene units of T-ZnPc. Besides, we have found that the thermal stability of the closed-ring diarylethene isomers in molecularly dispersed T-ZnPc is much poorer than that in aggregates. As long as the aggregates were broken, they converted to corresponding open-ring form instantly. This study provided an example of fully photocontrollable aggregation of phthalocyanines and paved a new way for improving the stability of the photochromic systems.
’ INTRODUCTION Stimuli-responsive supramolecular systems are being intensively investigated for drug delivery,1 biosensors2 and various intelligent nanodevices.3 Among organics, phthalocyanines (Pcs) and related derivatives have emerged as ideal building blocks as a result of their high versatility and exceptional thermal and chemical stability, as well as their intriguing electronic and optical properties.4 It is well-established that Pcs could form two types of one-dimensional aggregates, namely, face-to-face H-aggregates and head-to-tail J-aggregates, depending on the orientation of the induced transition dipoles of their constituent monomers.5 The properties of H-aggregates and J-aggregates are remarkably different. For J-aggregates, the optical properties of the organic dyes are desirably maximized, which are much more advantageous for various technological applications such as spectral sensitization, optical storage and nonlinear optics than H-aggregates.6 However, while extensive studies have been devoted to the field of Pc assemblies, the methods to assemble the J-aggregates are still quite rare. To date, most reported J-aggregates of Pcs were fabricated either by utilizing the intermolecular coordination7 or by interfacial engineering.8 For example, Kobuke et al.7a synthesized two imidazolyl-appended metal Pcs. They found that these compounds could self-assemble to J-type fluorescent dimer with an extremely large self-association constant of 1012 M1, based on the complementary coordination of the imidazolyl group from one Pc to the central Zn2þ or Mg2þ ion in a second Pc molecule. Adachi and co-workers8a reported a Pd(II)-induced interfacial J-aggregation of Pc derivatives at the liquidliquid interface and r 2011 American Chemical Society
suggested that the spatial arrangement could be tuned by the size of the peripheral substituents. Isogo9 prepared an antimony(III) Pc and observed that it could J-aggregate in nonaqueous solution. However, the corresponding mechanism is as yet unclear. Light is fast, clean, and controllable both spatially and temporally. As a kind of stimuli-responsive functional supramolecular system, photocontrollable J-aggregation of Pc is very appealing but so far is very limited. The only example to our knowledge is an R-phenylazophenoxy-substituted zinc Pc, in which the lightinduced transcis isomerization of azobenzene could greatly improve the J-aggregation tendency of this compound.10 Recently, we unexpectedly discovered that the thiophenedecorated Pc derivative T-ZnPc (Scheme 1), which possesses typical “picket-fence” optimal geometry, could J-aggregate through a conformational planarization effect.11 On closer inspection of Scheme 1, it is clear that Zn-TPc could be regarded as a hybrid composed of one zinc phthalocyanine (ZnPc) and four diarylethene (DTE) units. It is well-known that DTE derivatives undergo reversible isomerization between a relatively loose twisted open-ring isomer and a stiff coplanar closed-ring isomer, upon the alternative irradiation of suitable UV and visible light.12 This pronounced change in the electronic properties and conformational flexibility has been exploited to photocontrol the formation/dissociation of the self-assembled structures by Irie,13 Feringa,14 and other groups.15 Therefore, if the “conformational Received: April 1, 2011 Revised: May 27, 2011 Published: June 13, 2011 8061
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Langmuir Scheme 1. Molecular Structure of T-ZnPc and Photoisomerization of Its DTE Moieties
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Table 1. Molar Extinction Coefficients of T-ZnPc in Chloroform (c = 5.0 106 mol L1) ε/104 L mol1 cm1 (wavelength/nm)
Figure 1. The UVvis spectral changes of 5.0 106 mol L1 T-ZnPc in chloroform under the 254 nm UV light.
planarization mechanism” is correct for the J-aggregation behavior of T-ZnPc, we infer that the aggregation tendency of T-ZnPc will be inevitably enhanced when its DTE moieties are phototransformed to closed-ring isomers. With this in mind, we demonstrated here the solvent-dependent photocontrollable aggregation of T-ZnPc by various techniques. Moreover, we found that the thermal stability of the closed-ring DTE units in molecularly dispersed T-ZnPc is very poor indeed, whereas it can be greatly improved in the aggregates. As long as the aggregates are broken, they will instantly convert to open-ring isomers.
’ RESULTS AND DISCUSSIONS The Photocontrollable Aggregation of T-ZnPc in Chloroform. T-ZnPc was prepared according to the literature method11
and its photocontrollable J-aggregation behavior was first investigated by UVvis spectroscopy. Figure 1 showed the spectral changes of 5.0 106 mol L1 T-ZnPc in chloroform solution upon the illumination of 254 nm UV light. The corresponding molar extinction coefficients are summarized in Table 1. Initially, T-ZnPc showed a typical Q-absorption at 700 nm and a broad red-shifted band around 750, which arose from its weak J-aggregation ability in chloroform. With the illumination of 254 UV light, drastic spectral changes took place, and the photostationary state (PSS) was reached after only 20 s. The Q-band absorption at 700 nm decreased from 0.88 to 0.28 and the peak position redshifted to 706 nm. Meanwhile, the absorption at 750 nm increased from 0.056 to 0.159. Four isosbestic points at 328, 397, 590, and 716 nm were observed, suggesting the formation of new species.
initial state
PSS
5.64 (360)
3.80 (360)
2.98 (630)
1.18 (630)
3.68 (673) 17.64 (700)
6.20 (706)
1.12 (750)
3.18 (750)
Accompanying the spectral changes, the color of solution changed to yellow from green. Keeping the PSS solution in darkness for 1 day at room temperature restored the original spectrum. Such a switching process can be repeated several times without detectable degradation. Irradiation of the PSS solution with 600800 nm visible light also resulted in the decrease of the 750 nm absorption and the increase of the 700 nm Q-absorption, yet the initial spectrum cannot be fully regenerated because of the moderate photodecomposition of T-ZnPc. Generally, there are four possibilities for the UV-induced spectral changes in Figure 1: (1) photo-oxidation of the Pc ring, (2) the protonation of ZnPc due to photolysis of chloroform upon the UV irradiation (chloroform is able to photolyze to hydrogen chloride), (3) isolated photochromis, and (4) the enhanced aggregation tendency of T-ZnPc as a result of the ringclosure reaction of its DTE moieties. Aforementioned switching process ruled out the first two possibilities, which are thermodynamically irreversible. In order to clarify the latter alternatives, a series of experiments were carried out. Figure 2a showed the 1H NMR spectra of T-ZnPc solutions in chloroform-d before and after 254 UV light irradiation. Without 254 nm UV light illumination, T-ZnPc in chloroform exhibited a very complex 1H NMR spectrum due to its weak J-aggregation. Upon UV light irradiation, the peaks at 9.44, 9.18, 8.80, 8.45, 6.69, and 5.86 ppm dropped significantly, and the signal at 7.09 ppm disappeared [Figure 2a (below)], which can be readily explained by the increased shielding effect as a result of the enhanced aggregation of T-ZnPc. In addition, the 1H NMR spectral changes shown in Figure 2a further eliminated the possibility of a protonation mechanism, because for a typical dynamic protonationdeprotonation process, the changes in equilibrium can only change the relative intensity of signals. Dynamic laser scattering (DLS) is usually used to determine the size distribution profile of tiny particles in solution. The size of a single T-ZnPc molecule is 2 nm (estimated from its optimal structure). Before UV irradiation, the average aggregation number of Zn-TPc in 1.0 104 mol L1 chloroform solution is about 1.78,11 which means that the initial J-aggregates of Zn-TPc are too small to be detected. We have found that for the PSS of T-ZnPc in chloroform, the absorptions at 706 and 750 nm were found to follow the BeerLambert law in the concentrationdependent experiments (Supporting Information, Figure S1). Therefore, if the spectral changes in Figure 1 were attributed to a photoenhanced aggregation phenomenon, the average aggregation number would increase significantly and the size of aggregates might be large enough to give certain signals. DLS experimental results were consistent with this prediction. Before UV irradiation, there is no signal (Supporting Information, Figure S3). However, after illumination with 254 nm UV light to the PSS, 8062
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Figure 2. (a) 1H NMR spectra of T-ZnPc (c = 1.5 103 mol L1) in chloroform-d before (top) and after (below) 254 nm UV irradiation. (b) DLS result of T-ZnPc (1 104 mol L1) in chloroform after UV irradiation. (c) TEM photographs of T-ZnPc before (left) and after (right) UV irradiation. The samples were prepared from its chloroform solution with 1 105 M concentration. (d) The UVvis spectra of PSS species in chloroform and upon the addition of 20% (v/v) THF solvents.
as shown in Figure 2b, signals that range from 100 to 250 nm were observed from DLS measurement. Transmission electron microscopy (TEM) provided another direct evidence for understanding the aggregation behaviors of T-ZnPc in chloroform before and after UV light irradiation.
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Shown in Figure 2c (left) is the TEM image of a sample prepared from a chloroform solution of T-ZnPc without UV light illumination. It can be seen clearly that DTE-ZnPc was homogeneously dispersed on the microgrid. This homogeneity was radically changed after 254 nm UV illumination. As seen in Figure 2c (right), a large amount of intercrossed lines were observed. The whole image showed clear inhomogeneity, which can only be attributed to the photocontrollable aggregation behavior of T-ZnPc. Solvent substitution experiments provided additional evidence. Solvents such as THF or pyridine are able to coordinate with the central metal of ZnPcs, thereby disrupting their aggregates.16 When a large amount of THF (20% v/v) was added to the PSS solution of T-ZnPc in chloroform or the PSS species was simply dissolved in THF, as shown in Figure 2d, the broad absorption around 750 nm completely diminished, and a typical UVvis absorption spectrum of monomeric Pc was obtained. Taken together, it was clear now that the drastic spectral changes of T-ZnPc in chloroform under UV light (Figure.1) were the result of enhanced aggregation, which was accompanied by the ring-closure reaction of DTE moieties. Considering that solvents always play an important role in molecular self-assembly, we thus further studied the photocontrollable aggregation behavior of T-ZnPc in different solvents. Solvent-Dependent Photocontrollable Aggregation of T-ZnPc. Toluene can disrupt the intermolecular ππ interaction, a disadvantage of the aggregation of Pcs.17 Figure 3a showed the spectral changes of T-ZnPc in toluene (c = 5.0 106 mol L1) under the 254 nm UV light. Before UV irradiation, the J-aggregation of our molecule in toluene is a bit weaker than that in chloroform. Upon the irradiation with 254 nm light, the absorption band at 700 nm gradually decreased. Meanwhile, the absorption at 750 nm slowly increased and reached its maximum after 4 min. Compared with the PSS obtained in chloroform, as shown in the inset of Figure 3a, the aggregation tendency of the PSS in toluene is apparently weaker. In 20:1 chloroformethyl acetate (abbreviated as C/E) mixture, we have found that the aggregation ability of T-ZnPc could be modulated in a larger range than that in pure chloroform. It can be seen from Figure 3b that the initial spectrum of T-ZnPc in 20:1 C/E mixture is typical of monomeric Pcs, suggesting that this compound was molecularly dispersed in the presence of ethyl acetate before UV irradiation. Nevertheless, upon the 254 nm UV light irradiation for 40 s, PSS was achieved. At that moment, both of the absorptions at 700 and 750 nm (inset of Figure.3b) were close to those observed in pure chloroform. T-ZnPc did not show any photocontrollable aggregation behavior in coordinating solvents such as THF, pyridine, and DMF. As shown in Figure 3c, instead of observable photochromic reaction, T-ZnPc in THF underwent only slight degradation upon prolonged UV illumination. Tian and co-workers reported a series of bisthienylethenebased tetraazoporphyrin and Pc hybrids as near-infrared fluorescence switches.18 These compounds did not show any J-aggregation and photocontrollable aggregation behavior, suggesting that the intact Pc core is very important to the self-assembly behavior of T-ZnPc. It seems that the Zn2þ ion also plays its role. When Zn2þ in T-ZnPc was changed to Ni2þ or Cu2þ, such photocontrollable aggregation could not be observed any more. Mechanism Accounting for the Photocontrollabe Aggregation of T-ZnPc. Thanks to the well-designed structure of T-ZnPc and its J-aggregation behavior induced by conformational planarization, we can simply attribute the photocontrollable 8063
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Figure 4. Computer-optimized conformation of Zn-TPc with different closed-ring isomers in PM3 mode: (a) one closed ring, (b) two closed rings at the ortho-positions, (c) two closed rings at the para-positions, (d) three closed rings.
Figure 3. The UVvis spectral changes of 5.0 106 mol L1 T-ZnPc in (a) toluene, (b) in the 20:1 chloroformethyl acetate (C/E), and (c) in THF under 254 nm UV light. The insets are the UVvis spectra of photostationary states obtained in different solvents upon the UV illumination.
aggregation behavior of T-ZnPc to the variation in molecular geometry induced by the reversible isomerization of DTE units. On the basis of chemical structure analysis, it was found that no more than three open-ring DTE units in a single T-ZnPc molecule could be converted to closed-ring isomers, even with extended irradiation with 254 nm UV light. We calculated the optimal conformations of four possible products with one to three closedring isomers in the PM3 model, respectively, and the results are shown in Figure 4. When the DTE unit is in open-ring configuration, the corresponding thiophene rings are tilted from the Pc core of T-ZnPc, and the dihedral angles between each thiophene ring and the Pc plane are about 60°. However, this situation was changed when the open-ring DTE converted to the closed-ring form. The corresponding thiophene rings became largely coplanar with the Pc core, forming an extensive π-conjugation system. Both the changed geometric structure and the newly formed conjugation system were highly helpful for the intermolecular ππ stacking of T-ZnPc, which in turn increased its aggregation ability. While the open-ring DTE unit in T-ZnPc could also form the coplanar conformers through the rotation of thiophene rings, the advantages of the closed-ring isomer are self-evident. First,
the generation of coplanar configurations from the open-ring DTE is an energy-consuming process; second, the coplanar conformers obtained from open-ring DTE cannot be as perfect as T-ZnPc bearing closed-ring DTE isomers. Therefore, accompanying with UV-induced ring-closure reactions of DTE moieties, the aggregation ability of T-ZnPc was drastically increased. As we noted earlier, not all the open-ring DTE units in T-ZnPc could be converted to closed-ring isomers, which precludes parallel faceto-face intermolecular interactions. Besides, in Figure 1, the absorption bands of aggregates remain unchanged during the whole UVirradiation process. It was thus reasonable to conclude that Zn-TPc also formed J-aggregates in chloroform upon the irradiation with 254 nm UV light. The Aggregation-Stabilized Photochromic Behavior of T-ZnPc. After considering the mechanism accounting for the photocontrollable aggregation behavior of T-ZnPc, a question naturally appeared: on average, how many open-ring DTE in a single T-ZnPc molecule were converted to closed-ring isomers when the PSS was reached under 254 nm UV light? To answer this question, the monomeric T-ZnPc bearing ring-closed DTE isomers should be obtained. THF and pyridine could efficiently break the J-aggregates of T-ZnPc by coordinating with its central zinc ion. Therefore, at the first step, the PSS of T-ZnPc obtained from chloroform solution (Figure 1) was distilled under reduced pressure. Then the yellow residue (J-aggregates of T-ZnPc) was dissolved in THF, which generated green solution immediately. As shown in Figure 5a (red and dash line), the UVvis spectrum of this solution was typical of Pc monomers, indicating that the aggregates of T-ZnPc have been destroyed. Generally, the photochromic reaction of DTE is accompanied by a remarkable change in electronic structure. However, we surprisingly found that the UVvis spectrum obtained from the above-mentioned procedures was almost the same as that obtained from T-ZnPc in THF without UV irradiation (dark and solid line in Figure 5a). In order to gain more insights, we added a few drops of pyridine-d5 to the PSS of T-ZnPc in chloroform-d (CDCl3) solution to probe its 1H NMR signals. For comparison, we also presented the 1H NMR spectrum of T-ZnPc in 5:1 mixed solvent of chloroform-d and pyridine-d5 without UV irradiation. It can be seen from Figure 5b that the peaks belonged to Pc were the same in both cases. We further added a small amount T-ZnPc solid to the first sample and did not find any additional peaks. Subsequently, we performed HPLC analyses with acetonitrile as the mobile phase (Figure 5c). A monitoring wavelength at 8064
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Figure 6. Interconversion of T-ZnPc monomers and J-aggregates. Different colors were used to describe molecular isomerizations.
strong J-aggregates accompanying the ring-closure reaction of its DTE units. The HPLC analysis of this sample exhibited a broad, irregular trace, from 27 to 28 min. After adding 5% THF to the second sample to make the solvent nearly the same as that of the first one, the J-aggregates were completely disrupted. The corresponding retention time was almost exactly the same as that of the monomeric T-ZnPc with open-ring DTE units before UV illumination. The experimental results depicted in Figure 5 implied that upon the addition of THF or pyridine, the J-aggregates of ZnTPc in PSS solutions did not generate the monomers with closed-ring DTE isomers but with four open-ring DTE units. Remember that, when kept in darkness, it took 1 day for the PSS chloroform solution of T-ZnPc to return to its initial state. The thermal stability of the closed-ring DTE isomers in molecularly dispersed T-ZnPc seems much poorer than that in aggregates. As long as the aggregates were disrupted, they converted to the open-ring units in very short time.
Figure 5. (a) The UVvis spectra of T-ZnPc (Black and solid) and its PSS species obtained from chloroform upon 254 nm UV irradiation (red and dash) in THF. c = 5.0 106 mol L1. (b) Partial 1H NMR spectra of T-ZnPc with different treatments: (top) directly dissolved in CDCl3/ pyridine-d5 and unirradiated and (bottom) dissolved in CDCl3 and irradiated, then adding pyridine-d5. The signals marked with asterisks are due to solvents. (c) HPLC analysis at 700 nm with acetonitrile as mobile phase. Corresponding samples: T-ZnPc in 20:1:1 chloroform/ethyl acetate/THF mixture (c = 5.0 106 mol L1) before UV irradiation (black) and in 20:1 C/E mixture (c = 4.8 106 mol L1) after UV irradiation for 40 s and upon adding 5% (v/v) THF to the UV-irradiated sample (blue).
700 nm was used. The first sample was the molecularly dispersed T-ZnPc in 20:1:1 chloroform/ethyl acetate/THF mixture (c = 5.0 106 mol L1) without 254 nm UV light irradiation. For this sample, T-ZnPc was eluted out after 26.3 min. The second sample was the PSS of T-ZnPc in 20:1 C/E mixture (c = 4.8 106 mol L1) under 254 nm light, in which T-ZnPc formed
’ CONCLUSION We have shown that the photochromic reaction of DTE can be used to reversibly modulate the J-aggregation ability of Pc through careful design of the photoresponsive structure. Upon the illumination with 254 nm UV light, T-ZnPc formed much stronger aggregates in some solvents such as chloroform and toluene, as a result of the pronounced enhancement in the molecular planarity accompanying the ring-closure reaction of its DTE units. In darkness, the UV-irradiated solutions gradually returned to the initial state. As far as we know, this study provided an example of the first fully photocontrollable aggregation of Pc. In fact, such experimental results were predictable on the basis of the possible J-aggregation mechanism of T-ZnPc (conformational planarization effect), which, in turn, further confirmed the reasonability of this hypothesis. Moreover, we have found that the closed-ring DTE isomers in molecularly dispersed T-ZnPc are much more thermally unstable than that in aggregates. As long as the aggregates were broken, they converted to the corresponding open-ring form immediately (Figure 6). On one hand, it may explain why the photochromic reaction of T-ZnPc was not observed in coordinating solvents such as THF and pyridine; on the other hand, it made it difficult for us to figure out that how many open-ring DTE units in a single T-ZnPc molecule on average were converted to closed-ring isomers when the PSS was reached. This aggregation-stabilized photochromic behavior is quite rare but very logical, which may pave a new way for enhancing the stability of the photochromic systems. 8065
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’ ASSOCIATED CONTENT
bS
Supporting Information. The experimental methods, concentration-dependent absorption of T-ZnPc under 254 nm UV light, the IR measurements, and the DLS result of T-ZnPc in chloroform before UV irradiation, and the quantum calculation of T-ZnPc. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (Z.H.C.) and zhangfs@ mail.tsinghua.edu.cn (F.S.Z.). Tel. 86-10-62782596.
’ ACKNOWLEDGMENT This work was supported by National Natural Science General Foundation of China (20773077, 21073105), the Postdoctoral Science Funding (20090460297), and the National Key Fundamental Research Program (2007CB808000).
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