Self-Assembled Nanotubes and Helical Tapes from Diacetylene

ARTICLE pubs.acs.org/Langmuir

Self-Assembled Nanotubes and Helical Tapes from Diacetylene Nonionic Amphiphiles. Structural Studies before and after Polymerization Aur
elia Perino,† Marc Schmutz,‡ St
ephane Meunier,‡ Philippe J. M
esini,*,‡ and Alain Wagner† †

Laboratory of Functional ChemoSystems, UMR 7199, Facult
e de Pharmacie, Universit
e de Strasbourg, 74 route du Rhin, 67401 Illkirch, France ‡ Institut Charles Sadron, 3 rue du Loess, BP 84047, 67034 Strasbourg Cedex, France ABSTRACT: We synthesized new amphiphiles comprised of a single diacetylenic chain and an oligoethylenoxide polar chain linked by an amide bond. In aqueous medium, they are not soluble at room temperature but form weak gels. Electron microscopy studies have shown that they self-assemble into helical tapes or nanotubes with lengths of several micrometers, and inner and outer diameters of 50 ( 1 and 59 ( 1 nm, respectively. The wall has a thickness of 10 ( 1 nm for both kinds of objects and has an amphiphile bilayer structure. The hydrophobic chains are ordered, and the amide groups are linked with each other by H-bonds. The dissociation of the tubes is a first-order transition with an enthalpy of ca. 40 kJ mol 1. The nanotubes were photopolymerized to yield purple solutions consisting of helical tapes and almost flat ribbons. The polymers exhibit irreversible thermochromism upon heating.

iacetylenic lipids were the first compounds discovered to form helical tapes or tubes.1 Since then, more molecules able to form self-assembled nanotubes have been discovered with various chemical structures.2 The assemblies are driven by noncovalent bonds such as H-bonds, van der Waals, or solvophobic interactions. The resulting objects can find applications due to their unique shape and their high aspect ratio. They have been used for instance as scaffolds for cell growth,3 as supports for 2-D crystallization of proteins,4 6 or as templates to form silica nanotubes or mesopores in organic matrices.7 Because their self-assembly is based on noncovalent bonds, the formation of these nanotubes is thermoreversible and therefore depends on temperature or concentration. Yet for some applications it is necessary to rely on structures independent from the concentration. For this reason, there has been a growing interest to develop self-assembled tubes that can be locked by polymerization.8 11 The diacetylenic amphiphiles are appropriate for this purpose because their topochemical polymerization proceeds without drastic reorganization of the self-assembly as it has been proved very early.1 However, only a few thermal studies12 have been performed on those polymerized tubes. The first examples of molecules forming tubes were phosphatidylcholines bearing two diacetylenic chains.1,13 Efforts have been directed toward simpler structures, more easily synthesized, especially with a single acetylenic chain as the hydrophobic part and sugar14 16 or ammonium as the polar part.17 However, there are no rules to design such molecules, and, most often, a slight chemical modification of the parent molecule annihilates the formation of tubes. If predictability is lacking it is mainly because the structure at the

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r 2011 American Chemical Society

atomic resolution is known only in a few cases where the tubes have been successfully ordered.18,19 Yet generally, their limited size gives diffuse low-intensity signals on selected area diffraction, and scattering experiments on solutions give powder patterns. The helical tapes and nanotubes have triggered a lot of theoretical research by physicists to understand the underlying of their selfassembly.20 27 The proposed models derive the helical and tubular shape from the chirality of the constituent molecules that induces the intrinsic bending of the self-assembled layers. They cannot explain the few examples of nonchiral molecules forming nanotubes.17,28 These models also rely on a phenomenological description of the elastic energy but do not take into account the local packing or interactions between the molecules and cannot identify the molecular parameters that govern the formation of the tubes. In this context, the discovery of new tube forming compounds, especially nonchiral ones, is of interest and may help to identify the chemical interactions involved in the self-assemblies. Herein, we report a new nonchiral amphiphile (1, Scheme 1) able to form nanotubes in aqueous medium. This amphiphile has a single diacetylenic chain as the hydrophobic part and an oligoethylenoxide chain as the hydrophilic part. The structure and properties of the self-assemblies have been investigated by electron microscopy, DSC, and FTIR to establish which interactions are involved in the tube formation. The nanotubes were Received: June 8, 2011 Revised: July 31, 2011 Published: September 08, 2011 12149

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Langmuir Scheme 1. Chemical Structure of the Amphiphiles 1 4

Scheme 2. Synthesis of Amphiphile 1

then photopolymerized, and the resulting self-assemblies were studied.

’ EXPERIMENTAL SECTION Syntheses. The synthesis of the amphiphiles 2, 3, and 4 will be described elsewhere. Amphiphile 1 was synthesized according to Scheme 2. Compound 5 was synthesized according to a published procedure29 as well as hexaethyleneglycol monoamine.30,31 Heptacosa-12,14-diynoic Acid (6). A solution of acid 5 (240 mg; 0.54 mmol; 1 equiv) in THF (4.3 mL) was mixed with a solution of HCl in acetic acid (0.1% of concentrated acid; 4.3 mL). The mixture was stirred under microwave for 21 min at 150 °C. Water (5 mL) was added, and the aqueous layer was extracted with EtOAc (3  5 mL). Combined organic layers were dried (Na2SO4) and concentrated under vacuum. The residue was chromatographed (SiO2, CHCl3 eluent) to give 6 as a white solid in a quantitative yield (215 mg): mp 68.5 °C. 1H NMR (300 MHz, CDCl3): δ = 2.32 (t, J = 7.5 Hz, 2H, CH2COOH), 2.22 (t, J = 6.9 Hz, 4H, CH2 CtC), 1.63 1.57 (m, 2H, CH2CH2COOH), 1.52 1.24 (m, 34H), 0.86 ppm (t, J = 6.8 Hz, 3H, CH3). 13C NMR (75 MHz, CDCl3): δ = 179.9 (COOH), 77.8 (C12H25 CtC CtC), 77.7 (C12H25 Ct C CtC), 65.5 (CtC CtC), 34.2 (CH2COOH), 32.1, 30.0, 29.9, 29.8, 29.7, 29.6, 29.6, 29.4, 29.3, 29.3, 29.1, 29.0, 28.6, 24.9, 22.9, 19.4 (CH2 CtC), 14.3 ppm (CH3). FTIR (solid): ν = 2918 (νas CH2), 2848 (νs CH2), 1721 (ν CO), 1467 (δ CH2), 727 cm 1. HRMS: m/z 401.3432 (M H+, C27H46O2 requires 401.3498). N-(17-Hydroxy-3,6,9,12,15-pentaoxaheptadec-1-yl)heptacosa-12,14-diynamide (1). In a solution of acid 6 (215 mg; 0.53

mmol; 1 equiv) in anhydrous DMF (4 mL) were added EDC (153.5 mg; 0.80 mmol; 1.5 equiv), HOBt (108.5 mg; 0.80 mmol; 1.5 equiv), a solution of hexaethyleneglycol monamine (181 mg; 0.64 mmol; 1.2 equiv), and DIPEA (140 μL; 0.80 mmol; 1.5 equiv) in anhydrous DMF (1.5 mL). The solution was stirred 18 h at room temperature, diluted with a saturated solution of NH4Cl, and extracted with DCM (3  10 mL). The combined organic layers were washed with a saturated solution of NH4Cl (20 mL) and dried over Na2SO4. Chromatography of the residue (SiO2, MeOH CHCl3: 3/97 (v/v) eluent) yielded pure 1 as a white solid (248 mg, 70%): mp 62.3 °C. 1H NMR (400 MHz, CDCl3): δ = 6.27 (br s, 1H, NH), 3.49 3.68 (m, 22H, CH2O), 3.38 3.40 (m, 2H, CH2NH), 3.06 (br s, 1H, OH), 2.18 (t, J = 6.8 Hz, 4H,

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CH2 CtC), 2.12 (t, J = 7.6 Hz, 2H, CH2COOH), 1.58 1.20 (m, 36H), 0.83 (t, J = 6.4 Hz, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ = 173.5 (CONH), 77.7 (C12H25 CtC CtC), 77.6 (C12H25 Ct C CtC), 72.7 70.9, 70.8, 70.7, 70.7, 70.6, 70.6, 70.5, 70.4, 70.3, 70.1 (CH2O), 65.4 (CtC CtC), 61.8 (CH2OH), 39.1 (HN CH2), 36.8 (CH2COOH), 32.1, 29.8, 29.8, 29.7, 29.6, 29.6, 29.6, 29.5, 29.5, 29.2, 29.0, 28.5, 25.9, 22.8, 19.3 (CH2 CtC), 19.3 (CH2 CtC), 14.3 ppm (CH3). FTIR (solid): ν = 3301 (ν NH), 2916 (νas CH2), 2847 (νs CH2), 1738 (amide I), 1555 (amide II), 1463 (ν CH2), 1348, 1244, 1110 cm 1. HRMS m/z 666.5255 (M + H+, C39H71NO7 requires 666.5309). FT-IR. Spectra were recorded on a Bruker Vertex 70 spectrophotometer equipped with a thermostatic cell holder and a temperaturecontrolling unit (Specac West 6100+). The gels or solutions were placed in a CaF2 cell with an optical path of 0.1 mm. The temperature was increased of 1 °C before each spectrum (rate 1 °C/min), and the measurement was performed after the temperature was stabilized (about one-half a minute). The samples were visually checked after each heating phase to ensure that no loss of solvent had occurred. The spectra were recorded at different temperatures, were corrected from CO2 and water vapor, and compensated for D2O. They were processed and fitted with Igor (Wavemetrics, Inc.). The maxima of the peaks were found with the built-in algorithm using second derivatives. For the curves with higher signal/noise ratio, the maxima were found by fitting the peaks with a Gaussian curve. Both calculation techniques gave the same results within an error of 0.1 cm 1. UV. Spectra were recorded on a Cary/Varian 500 spectrometer equipped with a Linkam stage heating cell. The solutions to be measured were put in quartz cells. The cells were heated by increments of 1 °C (rate of 1 °C/min), the temperature was allowed to stabilize 30 s before each measurement, and the corresponding spectrum was recorded during each temperature plateau. DSC. Thermograms were recorded with a microcalorimeter DSC III from Setaram. The heating and cooling rates were set at 0.1 °C/min. Electron Microscopy Negative Staining. A droplet of the solution to be studied was cast on a 400 mesh copper grid with a carbon film. After a few seconds of adsorption, the excess solvent was blotted with a small piece of Whatmann paper (No. 4 or 5). A droplet of a solution of uranyl acetate (2%) was laid on the grid and soaked again. The grid was observed with a Philips CM 120 electron microscope operating at 100 kV.

Electron Microscopy

Cryogenic Preparation (cryo-TEM).

A 400 mesh lacey carbon film copper grid was rendered hydrophilic by a mild glow discharge. A 5 μL aliquot of the studied solution was deposited and left to adsorb for 2 min. Excess of sample solution was removed with a piece of filter paper (Whatmann 2 or 5) to leave a thin film. The grid was rapidly dipped into liquid nitrogen-cooled ethane and stored in liquid nitrogen until observation. The grid was transferred in a Gatan 626 cryo holder and observed with a FEI Tecnai G2 at 200 kV equipped with a FEI eagle 2k ssCCD camera. Distances were measured with Analysis software (SIS-Olympus, M€unster, Germany). More than 200 measurements were done and averaged. The uncertainty was taken equal to the standard deviation. Polymerization. Solutions of 1 in water (1 mg/mL) were irradiated in a thermostatic homemade cabinet with a 1000 W UV Hg lamp, under Ar at 10 °C during 15 min. The rate conversion was measured as follows: A solution of the polymer prepared as described above was precipitated in THF (10 times the volume of the solution), and the resulting suspension was stirred 12 h under argon and in the dark. The suspension was centrifuged, and the supernatant was evaporated and diluted in a known volume of THF to reach a concentration of about 1.5 mg/mL. 200 μL aliquots of this solution were injected on chromatography setup composed of a pump (Shimadzu LC20AD) operating at a flow of 1 mL/min, 4 columns PLgel (granulometry 5 μ), 12150

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Figure 1. Thermogram of a gel of 1 in water (1 wt %). Scan rate 0.1 °C/min. 50, 100, 500, 1000 Å (separation area: 100 20 000 g/mol), a differential refractometer from Shimadzu (RID10A), and a UV detector from Shimadzu (SPD 10 Avp). The pure starting materials, which is soluble in THF, was injected separately to identify the elution times and to calculate the molar extinction at 239 nm (ε) and refraction increment (dn/dc). The supernatant showed one main peak corresponding to the monomer and traces of oligomers (DPn = 10) not detectable by RI but only by UV. The results obtained with a PEO calibration were in good agreement for both detections. The amounts of the monomer in the supernatant were measured from the areas from both the RI and the UV traces, and a conversion rate of 71 ( 5% was found.

Figure 2. Top: VT-FTIR spectra of a gel of 1 in water (1 wt %); amide I and II bands 25 38 °C. Bottom: Frequency and intensity of the amide I band versus temperature.

’ RESULTS AND DISCUSSION Associating Properties of 1. The structure of amphiphile 1 studied in this Article is shown in Scheme 1. It bears a diacetylene chain comprised of 25 carbon atoms, and hexaethylenglycol as the hydrophilic part. This compound was synthesized readily at the gram scale by a malonic synthesis as described in the Experimental Section. It is not soluble at 25 °C in water, for concentrations above 1 mg/mL. Suspensions at this concentration dissolve above 36 °C and slowly precipitate when cooled back to room temperature. By comparison, amphiphiles 2 and 3 solubilize and form spherical micelles under the same conditions, and 4 is unsoluble in water even under heating. The solubilities can be explained by the length of the hydrophilic chains, which modifies the hydrophobic/hydrophilic balance. However, compound 3 has the same structure as 1 except that the hydrophobic chain comprises 23 carbon atoms instead of 25, and this slight difference leads to surprisingly different associating behavior. Indeed, a suspension of 1 at concentrations higher than 1 wt % dissolves upon heating and forms an opaque gel when it is cooled back at 25 °C. When the gel is heated, it melts again at 32 34 °C, giving a clear solution. This thermoreversibility encountered for all gelators32,33 is the signature at the macroscopic level of association through noncovalent bonds, and the melting of the gel corresponds to the dissociation of these bonds. The gel-to-sol transition was studied by DSC with 1% wt gels. The thermograms (Figure 1) show a sharp endothermic peak at 33.0 °C (onset 32.2 °C), indicating a first-order transition, the gel-to-sol transition, with an enthalpy of 42 kJ mol 1. Cooling the same melted gel yields an exothermic transition, with a slight hysteresis (onset 29.5 °C; max 29.0 °C) and with the same heat absolute value, within the experimental uncertainty. The nature of the interactions involved in gel formation was explored by variable temperature FTIR, a technique that has proven to be powerful to investigate gels of self-assembled amides.34 36 Spectra of a 1% gel in D2O were measured at temperatures from

Figure 3. Top: VT-FTIR spectra of a gel of 1 in water (1 wt %) in the CH stretching from 25 to 38 °C. Bottom: Frequencies of the νas and νs CH2 bands versus temperature.

25 to 38 °C. The spectrum at 25 °C shows a band at 1634 cm 1 identified as the amide I band. Its frequency is characteristic of amide groups H-bonded with each other. As the temperature increases, the intensity of this band slightly decreases. The spectrum also shows a broad peak between 1400 and 1500 cm 1, which comprises the CH2 bending and the amide II bands. The frequency of the former also shows that the amides are H-bonded. As the temperature increases, the intensity of the amide I decreases abruptly between 32 and 34 °C, and the maximum of the band shifts to 1628 cm 1 (Figure 2, bottom). This change coincides with gel-to-sol transition and attests a transformation of the H-bonds involving the amides. The amide I band above 34 °C is deconvoluted into two peaks; the first one is the above-mentioned maximum at 1628 cm 1, and the second one is at 1647 cm 1. The lower frequency is consistent with a stronger H-bond than in the gel state and is attributed to the amide group H-bonded with water.37 The second peak has been observed for other amide bearing compounds and can be interpreted as an amide forming H-bonds with two water molecules.38 In the CH stretching area (Figure 3), the symmetric and antisymmetric CH2 stretching bands (νs CH2 and νas CH2) are 12151

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Scheme 3. Array of the Amphiphiles in the Wall of the Tubes

Figure 4. Top: Negative staining TEM of 1 (0.1% aqueous solution); arrow, helical defects. Bottom: Cryo-TEM of 1 (0.05% aqueous solution); arrow, self-assembled nanotubes; arrowhead, helical tapes. Inset: Magnification of the tube showing the triple sheet structure of the walls.

found at 2848 and 2918 cm 1. These values are close to those obtained for crystalline paraffins and indicate that the chains are well ordered and in an all-trans conformation. It has been shown by the same FTIR measurements that similar diacetylenic molecules have no ordered alkyl chains in other nanostructures such as micelles39 or thin films.40 As the temperature increases, the frequencies of these bands exhibit a sharp increase from 2848 to 2854 cm 1 and from 2915 to 2924 cm 1 between 32 and 33 °C. These shifts characterize the presence of many gauche defects in the alkyl chains. Hence, the first-order transition observed from the DSC experiments is concomitant with an evolution of the alkyl chains to a disordered state. These studies show that the self-assembly of tubes involves the packing between the alkyl chains and that H-bonds are crucial for the self-assembly into nanotubes. The structure of the aggregates was investigated by electron microscopy. The self-assemblies in 0.1% wt solutions were first studied by negative staining after their adsorption on carboncoated grids. The micrographs (Figure 4, top) show mixtures of nanotubes and helical tapes that are several micrometers long. Some tubes display helical defects (arrow) that show their relationship with helical tapes. The apparent diameter of the tubes is about 70 nm but is overestimated due to the adsorption of the objects on the substrate and their subsequent deformation. We also studied the self-assemblies by cryo-TEM. This technique has two advantages: the observed contrast is due solely to the chemical nature of the objects; and it preserves the objects in their native environment by a deep freeze of the water to yield amorphous ice. The micrographs (Figure 4, bottom) show the same helical tapes and tubes as in the previous preparation, which confirms that the objects observed by the previous technique are not due to concentration variations (during adsorption and blotting steps) or reorganization of the objects because of the substrate. The external diameters of the tubes and the helical

Figure 5. DSC of solution of polymerized 1 (0.5% in water, scan rate 0.1 °C/min). (A) First heating and cooling cycle. (B) Second cycle.

tapes are 59 ( 1 nm and 50 ( 1 nm, respectively. Sections of the tubes and tapes are clearly visible (Figure 4, inset) and exhibit a triple sheet structure, with a bright layer between two darker layers. The thickness of the inner layer is 4 ( 1 nm, and that of the outer layer is 3 ( 1 nm. The staining pattern ascertains the composition of the layers: the brighter inner sheet corresponds to a lower scattering density and is made of the alkyl parts, whereas the darker outer layers correspond to the oligoethylenoxide chains. Therefore, the self-assembly is a bilayer with the alkyl-diyne chains in the center (Scheme 3). The length of the hydrophobic chain is 3.2 nm, and that of the hydrophilic chain is 2.3 nm. To account for the thickness of the inner sheet, the hydrophobic part has to be tilted of 40° to match the measured thickness of the inner layer. This hypothesis is reasonable because, as shown below, the self-assembled amphiphile is polymerizable and it has been proved that this polymerization requires a tilt angle of about 45°.41,42 Moreover, many structures, determined by crystallography, have shown similar tilt, due to the interaction between the diyne groups.42 Study of the Polymerized Tubes. When suspensions of 1 are subjected to UV-irradiation, they turn into purple solutions, which shows that polymerization takes place. The conversion is not complete and stops at ca. 70%, as determined by SEC, which shows that the self-assemblies may not offer ideal polymerization conditions. Such limited conversion rates have been observed for 12152

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Figure 6. VT-FTIR spectra of solutions of the polymer in D2O (0.5%) from 27 to 80 °C: amide I and II region.

the polymerization of diacetylenic amphiphiles in micelles.39 We also have noticed that X-rays induce polymerization of 1. Solutions of freshly photopolymerized species at 5 mg/mL were heated to 75 °C and cooled backed to room temperature while their thermal behavior was measured by DSC. During the heating phase, a broad endothermic peak is observed with an onset at 27 °C and with a maximum at 34.9 °C (Figure 5). The corresponding enthalpy, 23.0 kJ mol 1, is less than the one measured during the gel-to-sol transition of the monomer solutions, which shows that less interactions are involved in this transition than in the melting of the monomer gels. Upon the first cooling step, one observes an exothermic peak at 26.1 °C, with the same area (22.6 kJ mol 1) as the endothermic one, but sharper. The second heating step yields a different thermogram with an exothermic peak at 31.5 °C, corresponding to an enthalpy of 28.6 kJ mol 1, much sharper than in the first cycle (onset: 29.4 °C). This difference between the first and second cycles shows an irreversible transformation of the solution upon heating. The solutions contain about 30% of remaining monomers, and the observed peaks involve the dissociation and reassociation of the monomer, but can account only for part of the observed enthalpy. After the first heating ramp, the thermograms show only single peaks. It proves that the monomer and the polymer do not segregate and suggests that part of the monomer associate with the polymer in the same phase during the cooling step. In our studies, the polymerization of the nanotubes does not provide any gain in thermal stability, which can be due to the limited conversion rate. It has been shown for other nanotubes self-assembled from diacetylenic amphiphiles that the polymerization was incomplete in solution and did not increase the thermal stability, whereas a second irradiation in dry state yielded tubes that do not show any thermal transition.12 The IR spectra of the polymerized tube solutions were measured during the first heating step; they show strong similarity with those of the monomer. The stretching CH2 bands show the same frequencies as for the monomer and the same increase between 31 and 37 °C, which shows that the first-order transition observed in DSC coincides with the apparition of disorder in the alkyl chains. The amide I and II bands (Figure 6) are found at frequencies of 1632 and 1450 cm 1, respectively, and show a steep decrease between 27 and 33 °C. Yet the amide I transforms into a band that can be deconvoluted into two peaks that differ from those of the pure monomer: a major one at 1641 cm 1 and a shoulder at 1626 cm 1. These values suggest that most of the amide groups are H-bonded with each other and a few of them with the solvent. The transition can correspond to a dissociation of the self-assemblies, mainly into polymer chains where the amide groups are close. Therefore, even after disassembly, amide groups tend to have H-bonds with neighboring

Figure 7. Cryo-TEM of polymerized 1 (0.05% aqueous solution). White arrow: Nanotubes. Large black arrow: Helical tapes. Arrowhead: Ribbons.

Figure 8. VT-UV spectra of solutions of the polymer (0.1% aqueous solution) from 25 to 90 °C.

amides rather than with water. The transition can also correspond to a reorganization of the aggregates without complete dissolution of the polymer chain and to rearrangement of the H-bonds between the amides. The structure of the polymers was also investigated by electron microscopy using a cryogenic preparation. The micrographs (Figure 7) show tubes and helical tapes with the same dimensions as the objects formed by the monomers before polymerization. However, their proportion is much lower than before polymerization. The samples exhibit a large proportion of ribbons that are most often flat and are slightly twisted (Figure 7, white arrowhead). The polymer remains self-assembled, but tends to rearrange into flat ribbons. The electronic spectrum of the purple polymer at room temperature (Figure 8) exhibits three peaks at 502, 542, and 631 nm. The first two bands are the major ones, whereas the peak at 631 nm is less intense. Polydiaecetylenes obtained in other systems show a blue color corresponding to a much higher intensity of the peak at higher wavelength.43,44 This blue phase corresponds to higher effective conjugation length in the ene yne backbone with few nonplanar defects and to rod conformation in solution.45 48 The purple color obtained for polymerized 1 suggests that the backbone of the self-assembled polymer is subjected to constraints shortening the conjugation length leading to more defects. Alternatively, the purple color may reveal that this constraint limits the degree of polymerization and leads to shorter degrees of polymerization.48 When they are heated above 37 °C, the purple solutions of polymer become red. The change is irreversible: the solution 12153

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Figure 9. Intensities (A) and wavelengths (B) of the three UV absorption bands as a function of the temperature.

Figure 10. Wavelength (b and O) and intensity (2 and 4) of the second band. Plain symbols, heating phase; open symbols, cooling phase.

remains red when it is cooled back. We measured the UV spectra of a solution of freshly prepared polymer (1 mg/mL) while it was heated from 25 to 90 °C and cooled back to room temperature. When the temperature increases from 27 to 40 °C, the first two intensities of the bands at 502 and 542 nm increase, while the last band decreases and totally disappears above 45 °C (Figure 9A). The inflection point for the curves in this range is at 34 °C, which is close both to the peak observed by DSC and to the order disorder transition observed by FTIR. The frequencies of the first two bands vary slightly in this temperature range, while the shifts of the last band decrease from 631 to 617 nm. For temperatures above 40 °C, the intensities bands at 502 and 542 nm further evolve, especially with a parallel and sharp increase above 76 °C. The maxima of the peaks at 502 and 542 (Figure 9B) show a very slight evolution. The peak at 631 nm shifts toward the blue. The variation parallels that of the intensity and is also centered around 34 °C. Solutions of the polymer have been heated at 45 and 85 °C for 20 min, allowed to cool at room temperature, and observed by electron microscopy. The micrographs of the first solution showed mainly the flat ribbons observed in Figure 7, and very rarely helical tapes, which can be due to the reassembly of the unpolymerized monomers. However, the drastic decrease of the proportion of nanotubes confirms the irreversibility observed in DSC and in UV. When heated at 85 °C and cooled at room temperature, the solutions showed also mainly flat ribbons but with smaller width. The thermochromic transition of the polymer observed during heating corresponds to a reduction of the conjugation length of the ene yne backbone mainly because of torsions of the backbone leading to nonplanar defects.45,47 When the polymer is formed, its conformation is imposed by the geometry of the selfassembly, which is locked by the H-bonds between the amides and the packing of the alkyl chains. When the polymer is heated, the H-bonds between the amides are destroyed, which results in

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the dissociation or rearrangement of the chains. This reorganization favors the relaxation of the polymer and its evolution to a conformation with more nonplanar defects. Other systems where interacting H-bonding side groups govern the electronic transitions of polydiacetylenes have been described.44,46 Upon cooling, the peak at 630 nm does not reappear. The shifts and amplitudes of the peaks display an irreversible evolution (Figure 10). IR spectra show that the H-bonds between the amide groups re-form (not shown) like in the freshly polymerized solutions. However, because TEM shows that mainly flat ribbons form after cooling, it is probable that in these ribbons, the polymer is in a conformation that is close to its conformation in the hot solution, above the transition.

’ CONCLUSION In the present study, we have prepared a new amphiphile consisting of a single diacetylenic chain and an oligoethylenoxide hydrophilic linked by an amide group. We have shown that it self-assembles in water to form nanotubes or helical tapes. It represents a unique example of nonionic and nonchiral diacetylenic able to form nanotubes. The walls of the tubes are bilayers of the amphiphiles, where the alkyl chains are ordered and the amide bonds are connected with each other through H-bonds, as shown by FTIR. The self-assemblies can be photopolymerized to yield purple solutions. A minor part of the polymerized aggregates retain the tubular or helical shape, but many form loosely twisted ribbons. The polymerized assemblies undergo a thermochromic transition that can be detected by DSC and corresponds to a reorganization of H-bond between the amide groups. The comparison of a few analogues shows that the formation of tubes depends on a subtle hydrophobic hydrophilic balance. The use of an oligoethylenoxide allows one to tune precisely the length of the amphiphile part, and new analogues will be further studied. ’ AUTHOR INFORMATION Corresponding Author

*Phone: +33 388 414 070. Fax: +33 388 414 099. E-mail: [email protected].

’ ACKNOWLEDGMENT The International Center for Frontier Research in Chemistry (Strasbourg) is acknowledged for financial support. A.P. thanks ANR for financial support. We thank M. Seemann (Institut de Chimie, Strasbourg) for the VT-UV spectra. We thank O. Gavat, C. Foussat, and A. Rameau for the SEC experiments. ’ REFERENCES (1) Yager, P.; Schoen, P. E. Mol. Cryst. Liq. Cryst. 1984, 106, 371–81. (2) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005, 105, 1401–1443. (3) Vauthey, S.; Santoso, S.; Gong, H.; Watson, N.; Zhang, S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5355–5360. (4) Wilson-Kubalek, E. M.; Brown, R. E.; Celia, H.; Milligan, R. A. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 8040–8045. (5) Ringler, P.; Muller, W.; Ringsdorf, H.; Brisson, A. Europ. Chem. J. 1997, 3, 620–625. (6) Huetz, P.; van Neuren, S.; Ringler, P.; Kremer, F.; van Breemen, J. F. L.; Wagenaar, A.; Engberts, J. B. F. N.; Fraaije, J. G. E. M.; Brisson, A. Chem. Phys. Lipids 1997, 89, 15–30. 12154

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