Self-Organization of a Polymerizable Bolaamphiphile Bearing a

Mar 31, 2009 - a diacetylene group and two L-aspartic groups. We found .... (3) (a) Zhang, X.; Wang, M. F.; Wu, T.; Jiang, S. C.; Wang, Z. Q. J. Am. C...
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Self-Organization of a Polymerizable Bolaamphiphile Bearing a Diacetylene Group and L-Aspartic Acid Group Shouchun Yin,* Chao Wang, Bo Song, Senlin Chen, and Zhiqiang Wang* Key Laboratory of Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China, Received February 20, 2009. Revised Manuscript Received March 4, 2009 We report herein the self-organization of the polymerizable bolaamphiphile (noted, L-Asp-DA) that contains a diacetylene group and two L-aspartic groups. We found that L-Asp-DA can self-organize into stable fiberlike nanostructures at the mica-water interface. The micellar nanostructures of L-Asp-DA can be polymerized both in the bulk solution and in the film on UV irradiation, and the nanostructures of the L-Asp-DA micelles at the mica-water interface can be maintained after polymerization. The polymerized L-Asp-DA nanostructures can go through blue-red transition upon pH stimuli arising from the transformation of polydiacetylene skeleton. In addition, the concentration (above cmc) of L-Asp-DA has a great effect on the nanostructures. At a concentration of 3.2  10-4 mol/L, L-Asp-DA can self-organize into stable fiberlike nanostructure at the mica-water interface, while cylindrical nanostructures self-organize at a concentration of 6.0  10-4 mol/L. This work may provide a new approach for designing and fabricating molecular assemblies with controlled sizes and shapes, leading to the development of novel functional polymer nanostructure materials.

Introduction Amphiphiles containing long hydrophobic alkyl chains and hydrophilic head groups can self-organize into various micellar structures with defined size and shape both in bulk solutions and at interfaces.1 However, these micelles associated by noncovalent interactions are inherently dynamic and fluid; hence, they lack the robustness required for detailed structural characterization and material applications. In the past few years, Zhang and his co-workers have developed methods of stabilizing these micelles nanostructures.2,3 They have obtained stable nanostructures with different morphologies such as stripes, spaghettis, and discs by enhancing the intermolecular interactions among amphiphiles.2 Polymerization methods have also been used to immobilize micelles by γ-ray irradiation or by tuning the pH value.3 Recently, we have introduced a diacetylene residue into bolaamphiphiles for fabricating stable and functional nanomaterials.4 The reason for this is that diacetylene monomers can undergo topochemical polymerization upon irradiation by UV light or γ-rays to form an eneyne-alternated conjugated poly*To whom correspondence should be addressed. E-mail: (S.Y.) yinsc@ustc. edu and (Z.W.) [email protected]. (1) (a) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (b) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (c) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (d) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (e) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (2) (a) Zou, B.; Wang, L. Y.; Wu, T.; Zhao, X. Y.; Wu, L. X.; Zhang, X.; Gao, S.; Gleiche, M.; Chi, L. F.; Fuchs, H. Langmuir 2001, 17, 3682.bZou, B.; Wang, M. F.; Qiu, D. L.; Zhang, X.; Chi, L. F.; Fuchs, H. Chem. Commun. 2002, 1008. (c) Wang, M. F.; Qiu, D. L.; Zou, B.; Wu, T.; Zhang, X. Chem.;Eur. J. 2003, 9, 1876. (d) Song, B.; Wang, Z. Q.; Chen, S. L.; Zhang, X.; Fu, Y.; Smet, M.; Dehaen, W. Angew. Chem., Int. Ed. 2005, 44, 4731. (e) Song, B.; Wei, H.; Wang, Z. Q.; Zhang, X.; Smet, M.; Dehaen, W. Adv. Mater. 2007, 19, 416. (f) Song, B.; Wang, Z. Q.; Zhang, X. Pure Appl. Chem. 2006, 78, 1015. (g) Song, B.; Liu, G. Q.; Xu, R.; Yin, S. C.; Wang, Z. Q.; Zhang, X. Langmuir 2008, 24, 3734. (3) (a) Zhang, X.; Wang, M. F.; Wu, T.; Jiang, S. C.; Wang, Z. Q. J. Am. Chem. Soc. 2004, 126, 6572. (b) Liu, F.; Wang, M. F.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2006, 1610. (4) Yin, S. C.; Song, B.; Liu, G. Q.; Wang, Z. Q.; Zhang, X. Langmuir 2007, 23, 5936. (5) (a) Enkelmann, V. Adv. Polym. Sci. 1984, 63, 91. (b) Tieke, B. Adv. Polym. Sci. 1985, 71, 80.

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diacetylene,5 which are used in the fields of nonlinear optics,6 emitting materials,7 conductive materials,8 sensors,9 and photolithography.10 We found that the bolaamphiphiles with suitable spacer lengths (DPDA-10) can self-organize into ordered cylindrical nanostructures in aqueous solutions, and the cylindrical nanostructures can be maintained in dry states due to the enhancement of the intermolecular interactions among amphiphiles. Although the micelles formed by DPDA-10 can be polymerized by UV irradiation, the effective conjugated length is very low because of the steric effect of the phenyl group directly linked to the diacetylene group. As a result, the photoelectrical and blue-red transition properties of polydiacetylene are not good in the polymerized DPDA-10. To avoid the steric effect of the phenyl group and promote the topochemical polymerization, we designed and synthesized a new system that contains a methylene group between the phenyl group and the diacetylene group. However, the introduction of (6) (a) Paley, M. S.; Frazier, D. O.; Abdeldeyem, H.; Armstrong, S.; McManus, S. P. J. Am. Chem. Soc. 1995, 117, 4775. (b) Huggins, K. E.; Son, S.; Stupp, S. I. Macromolecules 1997, 30, 5305. (c) He, J. A.; Yang, K.; Kumar, J.; Tripathy, S. K.; Samuelson, L. A.; Oshikiri, T.; Katagi, H.; Kasai, H.; Okada, S.; Oikawa, H.; Nakanashi, H. J. Phys. Chem. B 1999, 103, 11050. (7) (a) Musso, G. F.; Ottonelli, M.; Alloisio, M.; Moggio, I.; Comoretto, D.; Cuniberti, C.; Dellepiane, G. J. Phys. Chem. A 1999, 103, 2857. (b) Aida, T.; Tajima, K. Angew. Chem., Int. Ed. 2001, 40, 3803. (c) Gan, H. Y.; Liu, H. B.; Li, Y. J.; Zhao, Q.; Li, Y. L.; Wang, S.; Jiu, T. G.; Wang, N.; He, X. R.; Yu, D. P.; Zhu, D. B. J. Am. Chem. Soc. 2005, 127, 12452. (8) (a) Savenije, T. J.; Warman, J. M.; Barentsen, H. M.; Dijk, M.; Zuilhof, H.; Sudhoelter, E. J. R. Macromolecules 2000, 33, 60. (b) Hoofman, R. J. O. M.; Gelinck, G. H.; Siebbeles, L. D. A.; Haas, M. P.; Warman, J. M.; Bloor, D. Macromolecules 2000, 33, 9289. (c) Takami, K.; Mizuno, J.; Megumi, A.; Saito, A.; Aono, M.; Kuwahara, Y. J. Phys. Chem. B 2004, 108, 16353. (9) (a) Okada, S.; Peng, S.; Spevak, W.; Charych, D. Acc. Chem. Res. 1998, 31, 229. (b) Kolusheva, S.; Shahal, T.; Jelinek, R. J. Am. Chem. Soc. 2000, 122, 776. (c) Lu, Y. F.; Yang, Y.; Sellinger, A.; Lu, M. C.; Huang, J. M.; Fan, H. Y.; Haddad, R.; Lopez, G.; Bums, A. R.; Sasaki, D. Y.; Shelnutt, J.; Brinker, C. J. Nature (London) 2001, 410, 913. (d) Mueller, A.; O’Brien, D. F. Chem. Rev. 2002, 102, 727. (e) Gill, I.; Ballesteros, A. Angew. Chem., Int. Ed. 2003, 42, 3264. (f) Rangin, :: M.; Basu, A. J. Am. Chem. Soc. 2004, 126, 5038. (g) Ma, G. Y.; Muller, A. M.; Bardeen, C. J.; Cheng, Q. Adv. Mater. 2006, 18, 55. (10) (a) Morigaki, K.; Baumgart, T.; Offenhaeusser, A.; Knoll, W. Angew. Chem., Int. Ed. 2001, 40, 172. (b) Morigaki, K.; Baumgart, T.; Jonas, U.; :: Offenhausser, A.; Knoll, W. Langmuir 2002, 18, 4082. (c) Kim, B. G.; Kim, S.; Seo, J.; Oh, N. K.; Zin, W. C.; Park, Y. S. Chem. Commun. 2003, 2306.

Published on Web 03/31/2009

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methylene group may decrease the interaction between the bolaamphiphiles without the rigid mesogenic group. As we know, hydrogen bonding can remarkably stabilize the nanostructure formed by amphiphiles. L-Aspartic acid is a kind of hydrophilic amino acid and contains two carboxyl groups, which may form strong intermolecular and intramolecular hydrogen bonds. Thus, we introduced the L-aspartic acid group into the bolaamphiphile to stabilize the self-organized structures. It is hoped that the L-aspartic acid group may endow stability and biocompatibilility to the supramolecular structures, and the polymerizable diacetylene group may further stabilize the micelles and offer new functional properties to the nanomaterials after UV polymerization. This work may offer a new approach for preparing molecular assemblies with controlled sizes and shapes, leading to the development of functional polymer nanometerials.

Experimental Section Materials. L-Aspartic acid dimethyl ester hydrochloride, (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), and N,N-diisopropylethylamine (DIEA) were purchased from Aldrich and used without further purification. The synthesis of 11-(4-propargyloxy)benzoylamidoundecanoic acid methyl ester (A) was depicted in the Supporting Information. Tetrahydrofuran (THF) and toluene were distilled from sodium benzophenone ketyl, and triethylamine was distilled from potassium hydroxide prior to use. Instruments and Methods. The 1H NMR spectra were recorded on a JEOL ECA-300 apparatus. Mass spectra (MS) were recorded on a LTQ LC/MS apparatus. UV-vis spectra were measured with a Hitachi U-3010 spectrometer. Fourier transform infrared spectroscopy was performed on a Bruker IFS 66v/s spectrophotometer. The samples were dropped onto the CaF2 and measured in a vacuum. Fluorescence emission spectra were recorded with a Perkin-Elmer LS 55 spectrophotometer. Raman spectra were obtained using a Renishaw Raman imaging microscope, using a 40 mW Ar+ laser (wavelength = 1064 nm). Before the Raman spectra of L-Asp-DA monomer were recorded, the L-Asp-DA aqueous solution (3.2  10-4 mol/L) was dropped on mica and then dried under reduced pressure. To obtain Raman spectra of the L-Asp-DA polymer film, a mica sheet with L-Asp-DA micelles, which was obtained just as above, was irradiated for 7.5 min with a UV fiber lamp and then placed at the Raman apparatus. To obtain Raman spectra of the L-Asp-DA polymer, the L-Asp-DA aqueous solution (3.2  10-4 mol/L) was irradiated for 35 min with a UV fiber lamp, and then, this solution was dropped on the mica. Synthesis of 1,4-Bis(4-[N-(10-carboxyl)decylmethylamino)carbonyl]phenyloxymethylene)butadiyne (B). A 1.12 g (0.3 mmol) amount of compound A in 10 mL of pyridine was added to a solution of 120 mg (1.20 mmol) of copper(I) chloride in 20 mL of pyridine, into which oxygen was bubbled. Then, the mixture was stirred overnight at room temperature. After the pyridine was removed under reduced pressure, the residue was dissolved into 100 mL of ethyl acetate and washed with 5% acetic acid until the blue color disappeared. The resultant organic layer was dried over anhydrous magnesium sulfate, filtrated, and evaporated. The residue was purified by SiO2 column chromatography using a mixture of ethyl acetate/ CH2Cl2 (3:1 v/v) as an eluent to give a pale yellow solid (350 mg). The pale yellow solid (0.5 mmol) was dissolved in a solution of THF (10 mL) and methanol (10 mL). A 1.5 mL amount of 1 mol/L NaOH aqueous solution was added, and the mixture was stirred for 2 h under Ar. The solution was acidified with 1 mol/L HCl to pH ≈ 7, and then, 1.5 mL of 1 mol/L NaOH aqueous solution was added again and stirred for 2 h under Ar. After the solution was acidified with 1 mol/L HCl to pH ≈ 3, the solvent was removed under reduced pressure. The resulting solid was washed with water three times and filtrated to give a pale Langmuir 2009, 25(16), 8968–8973

yellow solid in 27.1% yield. 1H NMR (300 MHz, DMSO-d6) δ (ppm): 8.25 (1H, t, NH), 7.77 (2H, d, J = 8.6 Hz, Ar-H ortho to CdO), 6.99 (2H, d, Ar-H meta to CdO), 5.00 (2H, s, CH2OAr), 3.19 [2H, t, J = 6.9 Hz, NHCH2(CH2)9], 2.16 [2H, t, J = 7.6 Hz, NH(CH2)9CH2], 1.46 [4H, m, NHCH2CH2(CH2)6CH2CH2], 1.35-1.13 [12H, m, NH (CH2)2(CH2)6(CH2)2].

Synthesis of 1,4-Bis(4-[N-(10-(L-aspartic acid dimethyl ester)carboxyl)decylmethylamino)carbonyl]phenyloxymethylene)butadiyne (C). A 297 mg (1.5 mol) amount of L-aspartic acid dimethyl ester hydrochloride and 360 mg (0.5 mmol) of compound B were dissolved in 30 mL of CH2Cl2. Then, 3 mL of DIEA and 663 mg (1.5 mmol) of BOP were added, and the reaction mixture was stirred at room temperature under Ar for 48 h. Thirty milliliters of 1 mol/L HCl aqueous solution was added, and the organic layer was extracted twice. The organic lay was washed with saturated NaHCO3 and water, then dried over MgSO4, and evaporated. The residue was purified by SiO2 column chromatography using a mixture of ethyl acetate/CH2Cl2 (3:1 v/v) as an eluent to give a pale yellow solid (61.8%). 1H NMR (300 MHz, CDCl3) δ (ppm): 7.74 (2H, d, J = 8.9 Hz, Ar-H ortho to CdO), 6.95 (2H, d, Ar-H meta to CdO), 6.47 (1H, d, J = 6.9 Hz, NHCH), 6.19 (1H, br, NHCOAr), 4.85 (1H, qt, NHCH), 4.79 (2H, s, CH2OAr), 3. 75 (3H, s, COOCH3), 3.69 (3H, s, CH2COOCH3), 3.42 [2H, q, J = 6.8 Hz, NHCH2(CH2)9], 2.89 (2H, dddd, J = 16.3 Hz, CH2COOCH3), 2.20 [2H, t, J = 7.9 Hz, NH(CH2)9CH2], 1.61 [4H, m, NHCH2CH2(CH2)6CH2CH2], 1.43-1.15 [12H, m, NH (CH2)2(CH2)6(CH2)2].

Synthesis of 1,4-Bis(4-[N-(10-(L-aspartic acid)carboxyl) decylmethylamino)carbonyl]phenyloxymethylene)butadiyne (L-Asp DA). A 155 mg (1 mmol) amount of compound C was dissolved in a solution of 8 mL of THF, and 4 mL of 1 mol/L NaOH aqueous solution was added dropwise in the dark. The mixture was stirred for 2 h under Ar, and the solution was acidified with 1 mol/L HCl to pH ≈ 6. Then, 4 mL of 1 mol/L NaOH aqueous solution was added again in the dark and stirred for 2 h under Ar, after which the solution was acidified with 1 mol/L HCl to pH ≈ 6. The hydrolyzation-acidification process was repeated four times, and the solvent was removed at 0 °C under reduced pressure. The resulting solid was washed with water three times and filtrated to give a pale yellow solid in 54.8% yield. 1H NMR (300 MHz, DMSO-d6) δ (ppm): 8.55 (1H, d, J = 4.1 Hz, NHCH), 7.75 (2H, d, J = 8.6 Hz, Ar-H ortho to CdO), 7.41 (2H, d, Ar-H meta to CdO), 6.97 (1H, br., NHCOAr), 4.99 (2H, s, CH2OAr), 4.47 [2H, q, J = 7.2 Hz, NHCH2(CH2)9], 3.17 [2H, t, J = 6.9 Hz, NH(CH2)9CH2], 2.48 (2H, dddd, CH2COOCH3), 2.04 [2H, t, J = 7.2 Hz, NH(CH2)9CH2], 1.44 [4H, m, NHCH2CH2(CH2)6CH2CH2], 1.35-1.08 [12H, m, NH(CH2)2(CH2)6(CH2)2]. ESI-MS: m/z 947.58 [MH]+. Preprocedure of L-Asp-DA. L-Asp-DA was purified before use every time. About 20 mg of L-Asp-DA was dissolved into 30 mL of CH3OH and then filtrated in the dark to remove the polymer solid. The pale yellow filtrate was dried at 0 °C in the dark. Critical Micelle Concentration (cmc). Beacuse the ratio I1/I3 (I1 and I3 are the intensities of peak at 373 and 384 nm in the pyrene’s emission spectra, respectively) reflects the polarity of its microenvironment, pyrene is employed as a probe to detect the cmc of L-Asp-DA. Twenty microliters of pyrene acetone solutions (0.5 mg/mL) was added into a series of the L-Asp-DA aqueous solutions with different concentrations before measurement, respectively. Atomic Force Microscopy (AFM). A commercial multimode Nanoscope IV AFM was employed to characterize the surface structure. All cantilevers were purchased from Veeco Company. Muscotive mica (Plannet GmbH, Germany) was freshly cleaved before the samples were dropped on the mica. DOI: 10.1021/la900628k

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The images were recorded by tapping in air. Every time before measurement, the L-Asp-DA monomer aqueous solution was ultrasonated for 15 min and placed for 10 min in the dark. Thirty microliters of this solution was dropped onto the mica and dried at room temperature in the dark. UV-Initiated Polymerization. The polymerization was carried out under argon by UV irradiation using a 1 W low-pressure mercury fiber lamp. The distance between the sample and the UV lamp was 10 cm. Blue-Red Change Property of L-Asp-PDA. Four milliliters of L-Asp-DA aqueous solution with a concentration of 3.2  10-4 mol/L was ultrasonated for 15 min and placed in the dark for 10 min. After the solution was irradiated for 35 min with a UV fiber lamp, the pH value of the solution was adjusted to 8, 10, 12, and 14 by adding 1 M NaOH, and the absorption spectra were also recorded at every condition, respectively. In the following, the pH value was decreased from 14 to 12, 10, 8, and 6 by stepwise adding 1 M HCl, and the absorption spectra were recorded in every step as well.

Results and Discussion Synthesis and cmc of L-Asp-DA. The bolaamphiphile bearing a polymerizable diacetylene group and two L-aspartic acid groups at both ends was designed and synthesized. The synthetic route is shown in Scheme 1. The L-aspartic acid group not only can form strong intermolecular hydrogen bonding to enhance the intermolecular interaction between the bolaamphiphiles but also has a good biocompatibility, which makes it possible to render for a stable and biocompatible supramolecular organization. The polymerizable diacetylene can further stabilize the supramolecular organization by polymerizing the diacetylene group to form the polydiacetylene polymer. Furthermore, the formed polydiacetylene polymer may endow the stable supramolecular organizations with novel properties. Because the I1/I3 value of pyrene’s emission spectra can reflect the polarity of its microenvironment, we have employed pyrene as a probe to detect the cmc of L-Asp-DA. When the concentration is below the cmc, the I1/I3 intensity rapidly decreased with an increase in the concentration. However, when the solution is higher than cmc, the I1/I3 value was stable, changing slowly, which indicated the formation of micelles. The surfactant concentration where I1/I3 dropped to a low and a nearly constant value is marked as the cmc.11 Thus, the I1/I3 value of pyrene absorption as a function of the logarithm of the concentration of LAsp-DA was plotted. As shown in Figure 1, the cmc of L-Asp-DA obtained by the fluorescence probing method is 1.6  10-4 mol/L. L-Asp-DA

(11) (a) Pietek, E. S.; Wolszczak, M.; Plonka, A.; Schlick, S. J. Am. Chem. Soc. 1998, 120, 4215. (b) Aathimanikandan, S. V.; Savariar, E. N.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 14922.

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Figure 1. Plot of pyrene I1/I3 fluorescence ratio vs the logarithm of the concentration of L-Asp-DA, showing that the cmc is about 1.6  10-4 mol/L.

Self-Organized Structures of L-Asp-DA. A concentration of 3.2  10-4 mol/L (2 cmc) was selected as the typical concentration of bulk solution when measuring the micellar structures by AFM. Thirty microliters of the solution was dropped on a freshly cleaved mica sheet. The mica sheet was kept away from light and at room temperature until it was totally dry. The fabricated nanostructure was observed by tapping mode AFM. As shown in Figure 2a,b, L-Asp-DA forms fiberlike structures on the mica surface. The aggregates are confirmed to have an average width of 20 nm and an average height of 2.7 nm. The length of the fiberlike structures reaches as long as a few micrometers. The self-organized structures of the aggregates depend on the concentration of the solution. As mentioned above, L-Asp-DA with a concentration of 3.2  10-4 mol/L forms fiberlike structures on the mica surface. In contrast, when the concentration increases to 6.0  10-4 mol/L, the aggregates are confirmed to form cylindrical structures (Figure 2c,d). The cylindrical aggregates are packed tightly. The width of the stick is about 30-60 nm, and its length is approximately 200-400 nm. Photopolymerization of L-Asp-DA in the Aqueous Solution. Because L-Asp-DA can self-organize into micellar structures in the aqueous solution, we wondered if the molecules in the aggregates can undergo the topochemical polymerization by irradiation. To address this issue, UV-vis spectroscopy was employed to follow the polymerization process. Figure 3 shows the UV-vis spectra of the photopolymerization process of the -4 L-Asp-DA aqueous solution (3.2  10 mol/L). As shown in Figure 3, there is no obvious absorption peak in the range of 400-800 nm before UV irradiation (Figure 3, solid curve). However, after being exposed to UV light for 5 min, two Langmuir 2009, 25(16), 8968–8973

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Figure 2. Ex situ AFM images of the aggregate structure of L-AspDA adsorbed on a mica sheet with different concentrations: (a and b) 3.2  10-4 mol/L and (c and d) 6.4  10-4 mol/L.

Figure 3. UV-vis absorption spectra of the L-Asp-DA solution dependence on UV light irradiation time.

distinctive peaks appear at 652 and 605 nm, corresponding to the characteristic absorbance of blue-phase polydiacetylene. After further irradiation, the two peaks are enhanced until the exposure time reaches 35 min. After that, the absorbance slightly decreases, possibly due to the UV-induced degradation. Thus, the optimized polymerization time is 35 min. After UV irradiation, the L-Asp-DA aggregates display a blue color, indicating that the effective conjugated length of the formed polydiacetylene is comparatively high.12 We have also employed Raman spectroscopy to further confirm the polymerization. As depicted in the Raman spectra in Figure 4, before UV irradiation (Figure 4a, solid), there are two weak peaks at 3070 and 1604 cm-1, corresponding to the stretching vibration of the C-H and CdC bonds of phenyl groups. There are two strong peaks at 2890 and 2260 cm-1, which is assigned to the stretching vibration of the C-H of methylene group and the stretching vibration of CtC bonds of the symmetrically substituted diacetylene group, respectively. (12) Foley, J. L.; Li, L.; Sandman, D. J. Chem. Mater. 1998, 10, 3984.

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Figure 4. (a) Raman spectra of L-Asp-DA before UV irradiation (solid). The samples were dropped onto mica and dried before recording. (b) Raman spectra of L-Asp-DA solution after UV irradiation (dash). The samples were dropped onto mica and dried before recording.

After UV irradiation (Figure 4b, dash), however, the absorption bands at 3070 and 1604 cm-1 disappear and the absorption band at 2890 cm-1 is also weakened. The changes may arise from the loss in symmetry after polymerization. Simultaneously, the characteristic absorption band at 2260 cm-1 of the diacetylene group totally disappears, and a new absorption band at 2093 cm-1 assigned to the stretching vibration of the CtC bonds in the polydiacetylene main chain appears, suggesting that polymerization has taken place.12 From the UV-vis and Raman spectra, we conclude that L-Asp-DA in the selforganized assemblies can be polymerized, which may further help to stabilize the micellar structures. Photopolymerization of L-Asp-DA in the Film. In the following, we want to know whether L-Asp-DA monomer can also be polymerized in the film by UV irradiation. The -4 L-Asp-DA aqueous solution (3.2  10 mol/L) was dropped in the dark onto the freshly cleaned quartz slide and dried in a vacuum. This procedure was repeated several times until there was sufficient L-Asp-DA on the mica sheet. Then, the quartz slide was exposed to UV irradiation for polymerization, and the UV-vis absorption of the film was measured. As shown in Figure S1 of the Supporting Information, the UV-vis spectra upon polymerization in the film are almost the same as those in aqueous solution. Before UV irradiation, there is no absorption peak in the range of 400-800 nm (Supporting Information, Figure S1, solid). After the film irradiation, the new absorption peak at 658 nm appears and reaches a maximum in 7.5 min. Simultaneously, the color of the film changes from colorless to blue during the polymerization. The polymerization in the film was also evidenced by Raman spectroscopy. From Figure S2a,b of the Supporting Information, we can see that the result corresponds to that in aqueous solution, which indicates that L-Asp-DA can also be polymerized in the film. Self-Organized Structures Can be Maintained after Polymerization. We wondered whether the assemblies could maintain their nanostructure at the surface after UV irradiation. If the nanostructures were maintained, it would furnish an attractive mean of constructing stabilized and functional nanomaterials in a mild environment due to the properties of the formed polydiacetylene. Thus, we used a UV fiber lamp to irradiate a mica sheet with preformed fiberlike micellar DOI: 10.1021/la900628k

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Figure 6. Colorimetric response of L-Asp-PDA to pH increase. The L-Asp-PDA solution was equilibrated for 20 min after each pH increase before the spectrum was recorded.

Figure 5. Morphology of the L-Asp-PDA micelles at the micawater interface before and after UV irradiation. (a and b) Before UV irradiation and (c and d) after UV irradiation.

structures of L-Asp-DA (Figure 5a,b) for 7.5 min under an argon atmosphere. After polymerization, the sample was observed by ex situ AFM. As show in Figure 5c,d, the fiberlike micellar structures at the mica-water interface are preserved after polymerization. Further measurements indicate that the dimensions remained almost unchanged after polymerization; that is, the nanostructures were maintained. We also observed the morphologies of the cylindrical micellar structures of L-Asp-DA before and after UV irradiation. We found that the cylindrical micellar structures at the mica-water interface can also be preserved after polymerization. From these results, we can conclude that the self-organized structures formed by L-Asp-DA can be maintained after polymerization. Blue-Red Transition Property of the Polymerized Nanostructures. Diacetylene monomers can go through topochemical polymerization by UV irradiation to form conjugated polydiacetylenes, which show a blue color. When external stimuli (heat, pH, pressure, temperature, etc.) are imposed upon the blue polydiacetylene polymer, the color may change from blue to red presumably by reduction of effective conjugated length. The blue-red transition property of PDA has been extensively utilized for bio- and chemosensing.9 Because the DA monomer (L-Asp-DA) can go through effective polymerization to form conjugated blue PDA polymer (L-Asp-PDA) and the DA monomer (L-Asp-DA) has the L-aspartic acid group, we wondered whether the formation of polydiacetylene has the blue-red transition property upon pH stimuli. Thus, we have measured the UV-vis spectra of the L-Asp-PDA polymer aqueous solution at the different pH values. After the L-Asp-DA aqueous solution with the concentration of 3.2  10-4 mol/L was irradiated for 35 min with a UV fiber lamp, the UV-vis spectrum was recorded. As shown in Figure 6, the aqueous solution of L-Asp-PDA aggregates possesses two absorbance peaks at 652 and 605 nm, and the color of the solution is blue. When the pH value of the solution is adjusted to 8 by adding a trace of a NaOH solution, a new peak at 572 nm appears, corresponding to the absorption of red PDA. The intensity of this peak increases gradually with the increase of 8972 DOI: 10.1021/la900628k

Figure 7. Colorimetric response of L-Asp-PDA to pH decrease. The L-Asp-PDA solution was equilibrated for 20 min after each pH decrease before the spectrum was recorded.

pH value, and the color of the solution changes from blue to red, indicating that the blue-red transition of L-Asp-PDA takes place. To the red solution is then added a trace of HCl solution to decrease the pH value, followed by the investigation of absorption changes. As shown in Figure 7, the absorbance at 572 nm decreases smoothly as the pH value decreases, accompanied with the color change of the solution. When the solution is adjusted to the original pH, the absorbance spectrum is nearly the same as that of the initial PDA solution. These results confirm that the L-Asp-PDA goes through blue-red transition upon pH stimuli. At the same time, we have observed that the characteristic absorbance of blue phase PDA fails to decrease during the bluered transition process, although at this moment the color of the solution changed from blue to red. It is presumably because of the increase of the solubility of PDA. When NaOH is added to the solution, the carboxyl head groups of L-Asp-PDA are changed into carboxyl salt, which remarkably increases the solubility of the aggregates. As a result, some insoluble PDA become soluble, and the absorbance of blue phase PDA does not decrease.

Conclusion We have designed and synthesized a polymerizable bolaamphiphile bearing a diacetylene group and two L-aspartic acid Langmuir 2009, 25(16), 8968–8973

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groups. L-Asp-DA can self-organize into stable fiberlike nanostructures at the mica-water interface. The L-Asp-DA micelles can be photopolymerized both in the bulk solution and in the film by UV irradiation. The nanostructures of the L-Asp-DA micelles at the mica-water interface can be maintained after polymerization. The polymerized L-AspPDA nanostructures can go through a blue-red transition upon pH stimuli. Thus, this work may provide a new approach for fabricating functional polymer nanostructured materials.

Langmuir 2009, 25(16), 8968–8973

Article

Acknowledgment. We thank the National Basic Research program of China (2007CB808000), the National Natural Science Foundation of China (50703022), and the Ministry of Education for financial support. Supporting Information Available: Synthetic route of A, experimental details, dependence of the UV-vis absorption spectrum of L-Asp-DA, and Raman spectra of L-Asp-DA. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la900628k

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