Acidichromism and Supramolecular Chirality of Tetrakis(4

Mar 12, 2008 - ... and Supramolecular Chirality of Tetrakis(4-sulfonatophenyl)porphyrin in Organized Molecular Films. Li Liu,Yuangang Li, andMinghua L...
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J. Phys. Chem. C 2008, 112, 4861-4866

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Acidichromism and Supramolecular Chirality of Tetrakis(4-sulfonatophenyl)porphyrin in Organized Molecular Films Li Liu, Yuangang Li, and Minghua Liu* Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface, and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of China ReceiVed: October 4, 2007; In Final Form: January 13, 2008

An anionic porphyrin, tetrakis(4-sulfonatophenyl)porphyrin (TPPS), was assembled into complex organized molecular films with an L-glutamic acid derivative (L-GluC18) or octadecylamine (ODA) through in situ complex formation at the air/water interface. TPPS could form J-aggregates when deposited from the pH 3.1 subphase or exist as H-dimers from the pH 5.1 subphase. The complex films showed acidichromism (i.e., reversible color changes upon alternate exposure to acidic and basic gases). Despite the initial color, the film turned into a wheat color, and TPPS aggregated into an H-dimer when exposed to ammonia gas. When treated with dry HCl gas, the film became light yellow, and the H-dimer changed into the protonated species. Upon subsequent exposure to water vapor, the film became yellow, and TPPS formed a J-aggregate in the film. Three states including the H-dimers, protonated species, and J-aggregates in the films could be switched by exposing the film to NH3, dry HCl, and subsequently water vapor, respectively. This process can be repeated many times. When TPPS formed the J-aggregate in the complex film, the film showed circular dichroism in the TPPS Soret band regardless of L-GluC18 or ODA. However, the supramolecular chirality changed and disappeared upon exposing the film to the NH3 and HCl gases. A linear and helical stacking of porphyrin in the film was proposed to explain such phenomena.

1. Introduction Recently, stimuli-responsive functional materials have been attracting a great amount of interest.1 Various stimuli such as pH, heat, and light can be applied to induce changes in materials.2 Acidichromism describes reversible color changes depending on the pH of the solution or the stimulus by acid/ base gases.3,4 Together with the other chromisms such as photochromism, electrochromism, and thermochromism,5-8 acidichromism has been investigated intensively in solution systems but relatively less in film systems.4,9 In this paper, we report the acidichromism as well as supramolecular chirality in organized molecular films containing anionic tetrakis(4-sulfonatophenyl)porphyrin (TPPS). TPPS is one of the mostly investigated water-soluble porphyrins due to its many unique properties. First, TPPS shows very interesting aggregation behavior in aqueous solution, solid films, and at the liquid-liquid interface.10-17 TPPS could essentially form highly ordered H- and/or J-aggregates, in which the porphyrin macrocyclic ring is stacked in a face-to-face or edge-to-edge arrangement.18,19 The aggregation of these two forms and their transformation are interesting topics concerning spectroscopy and functional materials. Second, although TPPS is achiral, under certain conditions, it could spontaneously form chiral assemblies. For example, Ribo` et al. have reported that under certain stirring, TPPS could form chiral assemblies.20 Furthermore, the handedness of the chirality could be controlled by the vortex direction.20 When dispersed in a cetyl trimethylammonium bromide (CTAB) micelle, TPPS also could form chiral assemblies.11 Purrello et al. realized chiral memory using * To whom correspondence should be addressed. Tel.: +86-1082615803; fax: +86-10-62569564; e-mail: [email protected].

SCHEME 1: Chemical Structures of Compounds Used in This Work

achiral porphyrins.21 We have found that through organization at the air/water interface, achiral TPPS could also be assembled into optically active assemblies.22 It was suggested that the chirality was related to the helical stacking of the macrocyclic ring. On the other hand, it was reported by Scolaro et al. that TPPS showed a clear color change upon exposure to photogenerated acidic or basic gases. This process was reversible.23 In this paper, we investigated the acidichromism and supramolecular chirality of TPPS assembled in organized molecular films and clarified the relationship among molecular aggregation, acidichromism, and supramolecular chirality. TPPS was assembled into a Langmuir-Blodgett film through the in situ adsorption of TPPS onto certain monolayers of amphiphiles. We used the Langmuir-Blodgett technique because it assures the formation of chiral assemblies from achiral TPPS. Two kinds of amphiphiles were used. One was octadecylamine, which has been reported previously to form wellorganized molecular films with TPPS and where the organized films showed supramolecular chirality.22 The other is a chiral amphiphile we synthesized to control the handedness of the

10.1021/jp709734d CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008

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Figure 1. Surface pressure-area (π-A) isotherms of the monolayers of L-GluC18 on water (a) and the aqueous subphase containing 1 × 10-5 M TPPS at pH 3.1 (b) and at pH 5.1 (c).

chirality in the supramolecular assemblies. The optical properties of the films were measured by UV-vis and CD spectra. AFM was also used to characterize the morphological changes during acidichromism. 2. Experimental Procedures 2.1. Materials. TPPS was purchased from Dojindo Laboratories as a sodium salt and used without further purification. Octadecylaminie (ODA) was purchased from Aldrich. The L-GLUTAMIC ACID DERIVATIVE (L-GluC18) was synthesized according to a method in the literature.25 The structures of amphiphiles and TPPS are shown in Scheme 1. 2.2. Assembly of Complex Multilayer Films. Amphiphiles in the chloroform solution were spread on a subphase containing 1 × 10-5 M TPPS (pH 3.1 or 5.1), and a complex film was formed in situ at the air/water interface. After 20 min for the evaporation of chloroform, the monolayer was compressed, and the surface pressure-area (π-A) isotherm was recorded with a compressing speed of 7.5 cm2/min. The complex monolayer was subsequently transferred onto a quartz substrate using a horizontal lifting method for characterization. Measurements of -π-A isotherms and the deposition of multilayer films were carried out using a computer-controlled KSV-1100 film balance system (KSV Instruments, Helsinki, Finland). A JASCO UV550 spectrophotometer was used for the UV-vis absorption measurements, and CD spectra were recorded on a JASCO J-810 CD spectrometer. In the measurement of the CD spectra of the film, the quartz plates were placed perpendicular to the light path and simultaneously rotated within the film plane to eliminate the possible birefringence and linear dichroism.26 The in situ formed monolayer was transferred onto freshly cleaved mica by a vertical lifting method, and the AFM image was recorded on a Digital Instrument Nanoscope IIIa Multimode system (Santa Barbara, CA) with a silicon cantilever using the tapping mode. Vapor of HCl, NH3, or water was introduced into the film for 10, 5, and 10 s, respectively. 3. Results 3.1. In Situ Formation of Complex Monolayers. We previously found that ODA could form a complex monolayer at the air/water interface with TPPS.21 When L-GluC18 was spread on an aqueous subphase containing TPPS, a similar complex monolayer was formed. Figure 1 shows the π-A isotherm of L-GluC18 spread on water and aqueous TPPS subphases with different pH values. On the water surface, the L-GluC18 monolayer showed an onset of the surface pressure at a molecular area of 0.5 nm2/molecule. When compressed, a condensed type monolayer was formed. The limiting molecular area, obtained by extrapolating the linear part of the isotherm

Liu et al.

Figure 2. UV-vis absorption spectra of the TPPS solution at pH 3.1 and 5.1 (a and b) and of the as-deposited film of L-GluC18 from the TPPS subphase at pH 3.1 (c) and 5.1 (d). Inset shows the Q band region for curves b and d in an expanded scale.

to zero surface pressure, was 0.36 nm2 molecule-1. This value is slightly smaller than twice the cross-section of the alkyl chain, suggesting that part of the molecules might overlap during compression. On the subphase of the pH 3.1 or 5.1 TPPS aqueous solution, the spreading monolayers show the onset of surface pressures at 1.07 and 0.94 nm2 molecule-1, respectively. Condensed regions were formed when the monolayers were compressed to a surface pressure of 20 mN/m. The extrapolating molecular area from this linear region gave a molecular area of 0.5 nm2 molecules-1, indicating the complex formation between L-GluC18 and TPPS. 3.2. Characterization of Transferred LB Films. 3.2.1. UVVis Spectra of Transferred LB Films. The complex monolayers from the TPPS subphases of different pH values were transferred onto the solid substrate, and their UV-vis spectra were measured. Figure 2 shows the UV-vis spectra of the transferred LB films in comparison to TPPS in aqueous solution. In the aqueous solution at pH 3.1, the UV-vis spectrum of TPPS shows Soret and Q bands centered at 435 and 645 nm, respectively, which correspond to the protonated porphyrin monomer. In the aqueous solution at pH 5.1, the absorption spectrum of TPPS exhibits an intense Soret band at 413 nm and four very weak Q bands at 648, 580, 552, and 515 nm, which are characteristic features of monomeric porphyrin in free base. The as-deposited L-GluC18/TPPS film from the subphase at pH 3.1 shows a red-shifted Soret band and Q band to 494 and 708 nm in comparison to TPPS in aqueous solution, suggesting the formation of J-aggregates of TPPS in the film.8 On the other hand, the L-GluC18/TPPS film transferred from the subphase at pH 5.1 shows a blue-shifted Soret band to 411 nm with peak broadening. In addition, a considerable red-shift of Q bands to 653, 595, 558, and 523 nm (inset of Figure 2) is a characteristic feature of H-dimers.27 These results indicate that the aggregation of TPPS in the film can be controlled by the pH of the subphase. Owing to the nearly coplanar orientation, protonated TPPS could be mainly aggregated into the edge-toedge J-aggregate in the complex monolayer transferred from the subphase at pH 3.1. On considering the peripheral phenyl groups in a nearly perpendicular orientation relative to the macrocycle of the porphyrin free base and the large steric hindrance, TPPS in free base only could form H-dimers in the complex monolayer transferred from the subphase of pH 5.1. 3.2.2. Acidichromism in Transferred LB Films. The complex film is sensitive to acidic or basic gases. Figure 3 shows the UV-vis spectral changes of the as-deposited films upon alternate exposure to NH3 gas and dry or moisture HCl gas. The spectral changes are largely related as to what kind of gases the film was exposed. Basically, three different states of TPPS were observed when the film was exposed to various gases. When the as-deposited film of L-GluC18/TPPS or ODA/TPPS

Tetrakis(4-sulfonatophenyl)porphyrin in Films

Figure 3. UV-vis spectral changes of complex films [L-GluC18/TPPS film from pH 3.1 subphase (A and C); ODA/TPPS film from pH 3.1 subphase (B); and L-GluC18/TPPS film from pH 5.1 subphase (D)] under various conditions. Letters a-e represent spectral changes to different stimuli. a (black): As-deposited film; b (cyan): exposing the as-deposited films to HCl gas; c (green): exposing the treated films subsequently to NH3 gas or exposing the as-deposited film directly to NH3 gas (C); d (blue): exposing the curves c or the curve a (D) to HCl gas; and e (red): exposing the line c or the curve a (D) to HCl gas and subsequently to H2O. All the films were transferred at 10 mN/ m. Bottom: illustrations of the changes of the three states of TPPS in the films upon exposure to various gases. H denotes the H-dimer; M denotes a mixture of protonated dimers and small aggregates; and J denotes the J-aggregate.

from the TPPS subphase at pH 3.1 was initially exposed to dry HCl gas, the film maintained its olivine color with the Soret band showing a growing narrowness in its peak and an increase in its intensity (curves a and b in Figure 3A,B), which suggested the progress of aggregation of TPPS.9,22 When this film was subsequently treated with NH3 gas, the color of the film turned instantaneously from olivine to wheat, and a significant change in the spectrum was observed. The Soret band showed a large blue-shift to 410 nm, and the Q band became four weak redshifted peaks in comparison to monomeric porphyrin in free base, indicating the formation of H-dimers in the film, as shown in curve c in Figure 3A,B. When this treated film was further exposed to dry HCl gas again, the film turned the wheat color into light yellow, and a broad Soret band appeared from 400 to 500 nm with its peak maximum at ca. 452 nm and the Q band of TPPS at 670 nm. The broad Soret band could be assigned to the protonated species, namely, a mixture composed of protonated dimers and smaller aggregates, considering that the redshift of the absorbance band increased with the aggregation number,9,22 as shown in curve d in Figure 3A,B. Interestingly, if we further exposed this film with the protonated TPPS species to water vapor, then the strong Soret band and Q band appeared

J. Phys. Chem. C, Vol. 112, No. 13, 2008 4863 at 488 and 701 nm, respectively. This indicated that upon treatment with water vapor, the protonated TPPS predominantly formed J-aggregates in the film and that the film turned from the light yellow to yellow.21 If the as-deposited L-GluC18/TPPS film was initially exposed to NH3 gas, the absorbance band appeared at 410 nm, and the following exposure to dry HCl gas or moisture HCl gas induced the same spectral changes as stated previously, as shown in Figure 3C. If the film was initially exposed to water vapor, no change was observed. With all these spectral changes combined, it is clear that there existed three states or three colors for TPPS in the organized molecular film. We tentatively assigned these three states as H (maximum absorption peak at 410 nm), M (around 450 nm), and J (around 490 nm), respectively. Then, all spectral changes of the films’ response to the external acid/base gases could be summarized as in the bottom of Figure 3. No matter what the initial complex films are, the same changes were observed upon exposure to gases. The L-GluC18/ TPPS film transferred from the subphase of pH 5.1 went into the cycle of acidichromism as an H-dimer. That is, the as-deposited film was in the H-state in the film. When initially exposed to dry HCl gas, it became the M-state. When it initially was exposed to HCl and subsequently to water vapor or directly to moisture HCl gas, it formed the J-state in the film. The H-state TPPS can be switched to the J-state by exposure to moisture HCl gas, while the J-state can be switched to the H-state by exposing the film to NH3 gases. The same can be applied to the H- and M-state. All the processes of acidichromism can be repeated many times. The stability and repetition in the process of acidichromism enable TPPS to be an excellent responsive material. 3.2.3. CD Spectra of Transferred LB Films. Previously, we found that through the organization at the air/water interface, TPPS could form complex films with achiral amphiphiles and that the films showed optical activity, although both of the amphiphiles and TPPS were achiral.24 It was suggested that the helical stacking during the organization at the air/water interface plays an important role in forming such macroscopic chirality. When L-GluC18 was employed as the amphiphile, similar results were obtained. That is, the complex films showed CD signals in the Soret band of TPPS, as shown in Figure 4A,B. As has been reported, when achiral ODA was applied, the sign in the CD spectra of the ODA/TPPS film was undetermined.22 However, when L-GluC18 was used, the handedness of the CD signals was determined. Within 10 experiments, the same sign CD signals were always obtained for L-GluC18/TPPS films. It should be noted that these CD spectra are free from linear dichroism (LD) by rotation of the sample within the film plane. Recently, Aida et al. and Meijer et al. observed LD in aligned nanofiber systems.28 Upon evaluating the LD signals using the LD attachment, we found that the contribution from LD could be neglected for our samples (Supporting Information). On the other hand, while we observed reversible changes in the color or UV-vis spectra when the film was exposed to acidic or basic gases, different changes in the CD spectra were observed. Figure 5 shows the CD spectra of the various films during acidichromism. The as-deposited L-GluC18/TPPS film showed a bisignate (with both positive and negative signs) CD signal centered at the absorption wavelength of the J band of 492 nm. This indicated that TPPS formed a supramolecular chirality when J-aggregates formed in the complex films, which is essentially the same as the results reported before.21 When the film was initially exposed to moisture HCl gas, the CD intensity slightly increased without a change in its peak position.

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Liu et al. the chiral arrangement of the system. The CD spectra disappeared completely in the case of the ODA/TPPS film upon exposure to NH3 gas, and subsequent exposure to moisture HCl gas could not recover the CD signal anymore. 3.2.4. AFM Studies of Surface Structure of Transferred LB Film. Figure 5 shows the AFM images of the formed complex monolayers and their changes upon exposure to different gases. Fiber structures were observed for the as-deposited film from the subphase of pH 3.1. In a large scan scale, it is clear that these fibers curled in one direction, which might be related to the chirality of L-GluC18 itself. Upon exposure to HCl and NH3 gases, slight changes in the morphology were observed. When the as-deposited film was initially exposed to HCl gas, short line protrusions were torn into dotted protrusions on the flat film region. When subsequently exposed to NH3 gas, many granular domains were observed, which was suggested to be due to the formation of NH4Cl in the film. When the film was initially exposed to moisture HCl gas, a great change was observed, in which the boundary of the fiber became clearer. When subsequently exposed to NH3 gas, the fibers became small. 4. Discussion

Figure 4. CD spectral changes of the complex multilayer films [L-GluC18/TPPS (A and B) and ODA/TPPS (C and D)] transferred from the subphase of pH 3.1 under various conditions. a (black): Asdeposited film; b (cyan): exposing the as-deposited film to moisture HCl gas (A and C); c (green): exposing the treated film subsequently to NH3 gas or exposing the as-deposited film directly to NH3 gas (B and D); d (red): exposing curve c to moisture HCl gas again; and e (blue): exposure to NH3 gas again.

Figure 5. AFM images of L-GluC18/TPPS film transferred from the pH 3.1 subphase at 10 mN/m during the acidichromism. a: As-deposited film; b: upon exposure to HCl gas; c: subsequent exposure to NH3 gas; d: upon exposure of the as-deposited film to moisture HCl gas; and e: subsequent exposure to NH3 gas. f: AFM images of L-GluC18/ TPPS film transferred from the pH 5.1 subphase at 10 mN/m. Scan areas were 2 µm × 2 µm.

However, when this film was subsequently exposed to NH3 gas, the CD spectra disappeared completely. When the film was again exposed to moisture HCl gas, although the UV-vis spectra could be recovered, no CD signal could be detected anymore. Similar changes in CD spectra were observed in the case of the ODA/TPPS film upon alternate exposure to moisture HCl gas and then to NH3 gas. When the as-deposited L-GluC18/TPPS film was initially exposed to NH3 gases, a weakened CD spectrum was obtained, as shown in Figure 4B. When this film was subsequently exposed to moisture HCl gases, however, the Cotton effect could only be partially recovered at around 492 nm in this film. The CD signals were not detectable in the film upon further exposure to basic and acidic gases. This indicated that NH3 gas destroys

It is well-known that TPPS could easily form J-aggregates in solution upon a pH change. Usually, a lower pH value of less than 2.8 is necessary for TPPS to form J-aggregates in aqueous solution.10 At the air/water interface, due to the ordered arrangement of the monolayers, even at a subphase of pH 3.1, TPPS could form J-aggregates in the monolayers. When the film was exposed to acidic or basic gases, the film experienced protonation and deprotonation in the central nitrogen of TPPS, resulting in color changes. Upon exposure to NH3 gas, the central proton was removed, and TPPS formed an H-dimer. When exposed to HCl gas, TPPS in the film was protonated again. Different from that in solution, protonated TPPS formed a mixture of protonated dimers and small aggregates in the film without water. Upon subsequent exposure to water vapor, TPPS formed J-aggregates. This indicates that water played a very important role in the formation of J-aggregates. The result is very similar to the case of the interaction between DNA and TMPyP in the film.24 Ribo` et al. also addressed the importance of water in the formation of TPPS aggregates.29 Scolaro et al. observed and discussed acidichromism in the TPPS films.23 They reported that the absorption of the Soret band shifted to 480 nm after a prolonged exposure to HCl. Here, we newly disclose that the water vapor plays an important role in the film, which could instantly transfer the film into the J-aggregates. Another new phenomenon we observed is that although the same J-aggregates were observed in the UV-vis spectra, they could be different when subjected to the CD measurements. It is obvious that the as-deposited films with TPPS in J-aggregates showed a strong supramolecular chirality, while the J-aggregates in the treated films with NH3 gas and then moisture HCl gas did not. J-Aggregates are formed due to the edge-to-edge stacking of the porphyrin macrocyclic rings. It seems that there are three possible stackings of the macrocyclic ring: linear, helical, and randomly edge-to-edge stacking, as shown in Scheme 2. All of these stackings showed the same UV-vis absorption. However, they showed significant differences in chirality. While the linearly and randomly edge-to-edge stacked aggregates showed no supramolecular chirality, the helical stacking could possibly show macroscopic chirality. The asdeposited film showed helical stacking due to symmetry breaking through the interfacial organization, and one of the

Tetrakis(4-sulfonatophenyl)porphyrin in Films SCHEME 2: Illustration of Possible Stacking of TPPS in the Filmsa

a (a) Linear stacking; (b) random stacking; and (c) helical stacking. For clarity, the amphiphiles were omitted.

enantiomeric stackings was supposed to be in excess. Thus, we observed supramolecular chirality in the as-deposited film. When such an as-deposited film initially was exposed to NH3 gas, deprotonation occurred. Such a process destroyed both helical and J-aggregation of TPPS, and thus, the chirality disappeared in the ODA/TPPS film, and the L-GluC18/TPPS film maintained only weak CD signals at 492 and 410 nm. The weak CD signals may have originated from the H-dimer of TPPS attached to the optical active GluC18. When the film composed of the H-dimer was subsequently exposed to moisture HCl gas, although protonation occurred, no helical arrangement of the macrocyclic ring occurred in the ODA/TPPS film, and a partial helical arrangement of the macrocyclic ring was recovered in the L-GluC18/TPPS film due to the handedness of L-GluC18 itself. No CD signals could be detected upon the next exposure cycle of the film. This may be due to the production of granular NH4Cl in the film, which affected the subtle arrangement of TPPS in the film. Upon exposure to HCl gas, the as-deposited film experienced small changes in the spectrum because the macrocyclic rings of TPPS already were protonated through interfacial organization. HCl gas further improved the helical structure of Jaggregates in the film, and the intensity of the supramolecular chirality increased. With subsequent exposure to NH3 gas, the chirality disappeared. Then, upon exposure to moisture HCl gas again, the chirality was not recovered, although the aggregation state was recovered according to the UV-vis spectrum. Here, the formation of the J-aggregate of TPPS through the air/water interface seemed to be vitally important in forming supramolecular chirality. The air/water interface facilitated not only J-aggregation but also the helical stacking of TPPS, which was suggested to be due to a cooperative arrangement of the molecules in a confined two-dimensional array. This viewpoint was further supported by the following experimental results. When we cast TPPS on the solid substrate, we could obtain the J-aggregate of TPPS. However, no chirality was determined. It should be further noted that the complex film from enantiomeric L-GluC18 and TPPS at pH 5.1, in which the TPPS H-dimer was formed, showed no chirality. One reason may be that the interaction between the sulfonic group of TPPS and the ammonium of L-GluC18 was not strong enough to induce the chirality of TPPS. In addition, even when this film was changed into J-aggregates by exposure to HCl gas, no induction of chirality could be obtained. This further suggests that the in situ organization of TPPS into J-aggregates is important to obtain supramolecular chirality.

J. Phys. Chem. C, Vol. 112, No. 13, 2008 4865 5. Conclusion Anionic TPPS was assembled into organized molecular films through electrostatic and π-π interactions with amphiphiles. Depending on the pH of the subphase, TPPS could be assembled as J-aggregates or H-dimers in the films. The film showed a rapid response to acid or base vapors. When the film was exposed to NH3 gas, the H-dimer TPPS was formed in the film. When subsequently exposed to HCl gas, the film changed its color to light yellow. Upon treatment with water vapor, a J-aggregate also was formed in the film, and the film was yellow. Both the J-aggregate and the H-dimer could be switched by alternate exposure to moisture HCl and NH3 gas. The asdeposited J-aggregate film showed a strong supramolecular chirality. Although the film showed a reversible color change upon exposure to acid/base vapor, the chirality could not be recovered. It was suggested that TPPS stacked in a linear or helical J-aggregate, and while the helical J-aggregate is responsible for chirality, the linear aggregate is not. Only through organization at the air/water interface can helical stacking of TPPS in the film be realized. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20533050 and 50673095), the Basic Research Development Program (2007CB808005), and the Fund of the Chinese Academy of Sciences. Supporting Information Available: LD spectra of the films. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Winnik, F. M.; Whitten, D. G. Langmuir 2007, 23, 1. (b) Special issue on stimuli-responsive materials: Langmuir 2007, 23, issue 1. (2) Garcia, A.; Marquez, M.; Cai, T.; Rosario, R.; Hu, Z.; Gust, D.; Hayes, M.; Vail, S. A.; Park, C. D. Langmuir 2007, 23, 224. (3) (a) Sun, X. D.; Fan, M. G.; Meng, X. J.; Knobbe, E. T. J. Photochem. Photobiol., A 1997, 102, 213. (b) Liang, Y.; Ming, Y.; Fan, M.; Sun, X.; Knobbe, E. T. Sci. China, Ser. B 1997, 40, 535. (c) Fan, M.; Sun, X.; Liang, Y.; Zhao, Y.; Ming, Y.; Knobbe, E. T. Mol. Cryst. Liq. Cryst. 1997, 298, 29. (4) Bamfield, P. Chromic Phenomena; RSC: Cambridge, 2001. (5) (a) Yin, M. F.; Gong, H. F.; Zhang, B. W.; Liu, M. H. Langmuir 2004, 20, 8042. (b) Bhoo, S. H.; Hirano, T.; Jeong, H. Y.; Furuya, M.; Song, P. S. J. Am. Chem. Soc. 1997, 119, 11717. (6) Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie, M. J. Am. Chem. Soc. 2005, 127, 8922. (7) Yao, J. N.; Yang, Y. A.; Loo, B. H. J. Phys. Chem. B 1998, 102, 1856. (8) Fujiwara, T.; Harada, J.; Ogawa, K. J. Phys. Chem. B 2004, 108, 4035. (9) Liu, Y. H.; Liu, M. H. New J. Chem. 2002, 26, 180. (10) Akins, D. L.; Zhu, H. R.; Guo, C. J. Phys. Chem. 1994, 98, 3612. (11) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (12) Kubat, P.; Lang, K.; Janda, P.; Anzenbacher, P. Langmuir 2005, 21, 9714. (13) Fujii, Y.; Hasegawa, Y.; Yanagida, S. Chem. Commun. (Cambridge, U.K.) 2005, 24, 3065. (14) Ogawa, T.; Tokunaga, E.; Kobayashi, T. Chem. Phys. Lett. 2005, 408, 186. (15) Kitahama, Y.; Kimura, Y.; Takazawa, K. Langmuir 2006, 22, 7600. (16) Miguel, G.; Morales, M. P.; Romero, T. M.; Munoz, E.; Richardson, T. H.; Camacho, L. Langmuir 2007, 23, 3794. (17) Fujiwara, K.; Wada, S.; Monjushiro, H.; Watarai, H. Langmuir 2006, 22, 2482. (18) (a) Fleischer, E. B.; Palmer, J. M.; Srivastava, T. S.; Chatterjee, A. J. Am. Chem. Soc. 1971, 93, 3162.(b) Pasternak, R. F.; Huber, P. R.; Boyd, P.; Engasser, G.; Francesconi, L.; Gibbs, E.; Fasella, P.; Cerio Venturo, G.; Hinds, L. de C. J. Am. Chem. Soc. 1972, 94, 4511. (c) Ribo, J. M.; Crusats, J.; Farrera, J. A.; Valero, M. L. J. Chem. Soc., Chem. Commun. 1994, 681.

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