Spontaneous Formation of Artificial Vesicles in Organic Media through

Article pubs.acs.org/Macromolecules

Spontaneous Formation of Artificial Vesicles in Organic Media through Hydrogen-Bonding Interactions Sabareesh K. P. Velu,† Minhao Yan,† Kuo-Pi Tseng,‡ Ken-Tsung Wong,‡ Dario M. Bassani,§ and Pierre Terech†,* †

SPrAM, UMR CEA/CNRS/UJF-Grenoble, INAC, 17 rue des Martyrs, 38054-Grenoble, France Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, 10617-Taipei, Taiwan § CNRS, ISM UMR 5255, Université Bordeaux 1, 351 Cours de la Libération, 33400-Talence, France ‡

ABSTRACT: The unusual spontaneous formation of submicrometer-sized vesicles from a small, nonamphiphilic bis-biuret difluorene derivative upon dissolution of the solid in an anhydrous organic solvent was investigated using multiple scattering techniques. Timeresolved light scattering (TLS) measurements confirm that the self-assembly process is driven by hydrogen-bonding interactions, leading to the formation of vesicles at a critical concentration ∼1 × 10−4 M in tetrahydrofuran as determined by absorbance and surface tension measurements. Results from cryogenic-scanning electron microscopy (cryo-SEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) experiments are consistent with the existence of vesicle-like aggregates in solution. DLS studies indicate a broad distribution of aggregates with a mean hydrodynamic radius ⟨RH⟩ = 303 nm (polydispersity =0.49). SAXS profiles show two decay regimes (low-Q decay, very large aggregates; large-Q decay, smaller species). The analysis models the large aggregates as vesicles (hollow spheres) with a mean external radius Ro = 750 nm and an internal radius Ri = 720 nm while the smaller aggregates have a mean radius R = 2.2 nm. The results obtained by cryo-SEM show spherical aggregates of vesicles size in the range ca. 100 nm to 1 μm. Transmission electron microscopy (TEM) micrographs evidence the presence of aggregates whose morphology is compatible with budding and pearling processes as possible mechanisms for the formation of vesicles.

■

INTRODUCTION Natural phospholipids spontaneously self-assemble in water to form ordered mesophases, which can fold into vesicles and thus become one of the most prevalent structures in nature where they serve to support, contain, separate, and protect cellular and sub−cellular components. Numerous artificial systems mimicking such behavior have been designed based on synthetic amphiphiles or amphiphilic block copolymers.1,2 Besides their use for encapsulation, such small well-defined aggregates can serve to compartmentalize molecular sensors,3 catalysts,4 or to hierarchically organize chemical potential gradients5−7 or other functionalites. Electro- and optically active vesicles are also appealing for the design of components in molecular electronic applications as their size bridges the gap between top-down and bottom-up manufacturing. Recent examples include block copolymers containing a conjugated electro-active central unit (polythiophene or polyphenylene-vinylene) forming vesicles in water.8−10 The emissive nature of these aggregates makes them interesting as point light sources whose emission spectrum can be tailored through the combination of tuning the bandgap of the material and the use of dopants. However, for applications in organic LED (OLED) devices, their preparation as aqueous suspensions can be a drawback. Likewise, the use of long alkyl chains yields aggregates in which the aromatic π-conjugated electro-active units are buried deep within an inert alkane matrix, which may reduce charge transport. © 2013 American Chemical Society

We recently reported a new approach to forming luminescent vesicle-like aggregates based on small hydrophobic molecules that does not rely on the use of charged head groups or long hydrocarbon chains.11 It is based on the combination of a rigid aromatic chromophore appended with antipodal hydrogen-bonding (H−B) biuret units to induce its aggregation into hollow spheres with submicrometer dimensions (typ. 200−500 nm). Surprisingly, the formation of the vesicles is not affected by the presence (or absence) of water, or variations in the structure of the central rigid core. Among the compounds tested, the bisfluorene derivative 1 (Figure 1) is particularly interesting as it is found to be a strong blue-light emitter that can serve as a basis for the design of luminescent vesicle-like aggregates of almost any visible color (including white) through the introduction of red and green emissive dopants. Preliminary results suggested that aggregates are present in dilute solution (1 × 10−4 M in THF), although positive identification of the vesicles (SEM, TEM, and AFM microscopy) could only be accomplished on drop-cast samples following solvent evaporation. A variety of hydrogen-bonded self-assembled oligofluorenes have been reported12−17 but the spontaneous formation of vesicles in organic solvents is rare. Received: December 18, 2012 Revised: January 24, 2013 Published: February 6, 2013 1591

dx.doi.org/10.1021/ma302595g | Macromolecules 2013, 46, 1591−1598

Macromolecules

Article

Figure 1. (A) Chemical structure of 1 and (B) space-filling model obtained from semiemipirical PM3 molecular modeling. and trans) was used as an index matching fluid to suppress the reflection from the cell walls. The autocorrelation functions were measured at scattering angles (30° to 130° with 10° interval) and the duration of each measurement was about 3 min. Time resolved light scattering measurements were carried out by collecting scattering light at scattering angle 90° using 1 mm entrance aperture of the photodiode. The scattered light was collected for every second continuously. All the measurements were performed at T = 25 °C ± 0.1 °C. Small angle X-ray scattering experiments used the ID02 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) with a wavelength λ = 0.995 Å and a sample-to-detector distance of 1.13 m. The range of the scattering vector Q was from ca. 0.005 to 0.28 Å−1 where |Q| = (4π sin θ)/λ, λ being the X-ray wavelength and θ half the scattering angle. The samples were kept in glass capillaries of 1 mm diameter and 0.1 mm wall thickness. Transmission electron microscopy images were obtained on samples prepared by drop-casting a THF solution of 1 (10−4 M) onto a 200 mesh copper grid coated with Formvar film stabilized with vacuum-evaporated carbon and air-dried. The samples were examined using Hitachi H-7650 (operating at 75 kV) or JEOL JEM-2100 (operating at 200 kV) electron microscopes. Cryo-scanning electron microscopy samples were prepared by rapidly freezing through immersion in liquid nitrogen one drop of a freshly prepared aqueous solution of 1 (obtained by rapidly mixing 1 mL of a 1 × 10−4 M solution of 1 in THF with 3 mL of water) or 1 drop of a THF solution of 1 (1 × 10−4 M). After introduction of the sample into the microscope chamber (JEOL 6700F equipped with a Gatan ALTO cryogenic sample chamber), the surface of the frozen sample was scraped with a scalpel to expose the frozen aggregates. The samples were then metal-coated and imaged.

Although spontaneous vesicle formation in water is common for many natural phospholipids and some small molecule- or polymer-based surfactants, their direct genesis upon dissolution of a solid sample in an anhydrous organic solvent is quite unexpected. Understanding the mechanism for this process, and whether vesicle-like aggregates are present in dilute solutions of 1 in THF or whether they form during the solvent evaporation process, is necessary to elucidate the structural requirements for vesicle formation and for the development of new mesoscopic materials for organic electronic devices. In this work, we investigate the aggregation of 1 in THF solutions using UV spectroscopy, tensiometric, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) techniques and provide evidence for the presence and the hollow sphere morphology of aggregates present in THF solutions of 1 using cryogenic scanning electron microscopy (cryo-SEM). A mechanism for the formation of vesicles is proposed based on budding and tabulation,18 and pearling instabilities models.19

■

EXPERIENTAL SECTION

The synthesis of compound 1 was reported previously.20 Tetrahydrofuran (Sigma-Aldrich) was used as received or dried by refluxing over Na/benzopheneone and distilled immediately prior to use. Solutions of 1 in THF were prepared by adding appropriate volume of THF to a vial containing a solid sample of 1 to produce various concentrations in the range between 8.68 × 10−6 M and 1.11 × 10−3 M. Dissolution was achieved by manually shaking the vial. Electronic absorption spectra were obtained on a Hewlett-Packard 8452A diode array spectrophotometer in the wavelength (λ) range 190−820 nm using a 1 mm path length quartz cuvette. Surface tension experiments were performed using a Sigma702 force tensiometer and Wilhelmy plate method. The measurements were carried out at room temperature (T = 25 °C ± 0.1 °C) using a thermostated glass vessel. Dynamic light scattering experiments were performed using a Brookhaven BI-200SM motorized goniometer and BI-9000AT digital correlator. The light source used was a He−Ne laser (λ = 632.8 nm, 35 mW). The incident laser beam was vertically polarized and the scattering beam was collected using an avalanche photodiode with a 100 μm entrance aperture. Decahydronaphthalene (98%, mixture of cis

■

RESULTS AND DISCUSSION On the basis of the crystal structure of related aryl−biuret derivatives, we expect the H−B group in 1 to adopt a cyclic conformation (Figure 1) due to the formation of an intramolecular H−B. The compound is not intrinsically amphiphilic, though aggregation through H−B should be favored in nonprotic solvents. Previous work established that the self-assembly of 1 compound spontaneously leads to the deposition of vesicle-like aggregates when drop-cast from dilute (10−4 M) THF solutions. The formation of these aggregates 1592


Macromolecules

Article

was found to be independent of the nature of the substrate and to the presence or absence of traces of water. The critical concentration of 1 below which no vesicle-like aggregates are formed (CAC) was thus determined using UV−visible absorbance spectroscopy21 and surface tension measurements.22 The UV−visible absorbance spectrum of 1 in THF (Figure 2) is composed of two bands at λmax = 218 and 342 nm. The

Figure 2. Electronic absorption spectrum of compound 1 in THF (5.55 × 10−4 M, 1 mm path length cuvette).

molar absorbance coefficient, ε at 218 nm is 37 840 M−1 cm−1 and 45 050 M−1 cm−1 at 342 nm.23 This spectrum also evidences the beginning of light scattering by the aggregates that are disrupted upon addition of 10% DMSO to break the H−B interactions (no other discernible modifications of the absorption spectrum take place). This can be used to determine the critical aggregate concentration (CAC) by plotting the absorbance of solutions of 1 at 342 nm vs concentration and to monitor the deviation from the Lambert−Beer law (Figure 3A). Two separate linear regions are easily identified at low and high concentrations. The intersection of the two straight lines indicates the CAC to be ca. 1.07 × 10−4 M. A nearly identical value (0.98 × 10−4 M) is obtained from the plot of surface tension vs [1] over a similar concentration range (Figure 3B). The UV−visible absorbance and the surface tension measurements confirm the presence of aggregates at [1] > CAC. To verify whether these aggregates are similar in morphology to the vesicles observed upon drop drop-casting, cryo-SEM measurements were carried out according to two different procedures for sample preparation. First, 1 mL of a 5.55 × 10−4 M solution of 1 in THF was rapidly diluted with 3 mL of water to provide a favorable medium for cryo-fracture. The frozen sample is introduced in the sample chamber and scraped to expose the aggregates and then metal coated. Parts A and B of Figure 4 show the cryo-SEM micrographs of cryofractured 1 in THF/water solution. The cryo-SEM micrographs show the presence of numerous spherical aggregates whose size ranges from ca. 100 nm to 1 μm. Both the size and shape of the aggregates are similar in to those observed by SEM and TEM upon drop-casting. To further verify that the aggregates are not formed upon aqueous dilution of the THF solution, a sample of the latter was directly used for sample preparation. Although the aggregates are somewhat harder to see due to the frozen solvent’s structuring, the presence of spherical aggregates in the THF (a few are highlighted by red circles. See Figure 4C) unequivocally provides evidence for their existence in solution.

Figure 3. Critical aggregate concentration measured by (A) UV− visible absorbance (plotted at 342 nm) and (B) surface tension versus concentration.

The aggregates observed in the cryo-SEM experiments are similar in size and morphology to those previously observed by AFM and TEM on samples prepared by drop-casting.11 This confirms that the vesicle-like aggregates are present in solution and not only formed upon evaporation of the solvent. The size of the aggregates in THF was monitored using DLS24 for concentrations of 1 between 2.5 × 10−4 M (slightly above the CAC) and 6.5 × 10−4 M. Above this concentration, the scattering signal is rapid with multiple scattering. The normalized time-averaged intensity−intensity autocorrelation function g(2)(Q,τ) is measured at a given scattering wave vector, Q = (4πn/λ)sin(θ/2) [where, n is the refractive index of the medium (n = 1.404 for THF at 25 °C), λ is the wavelength of the incident laser beam, and θ is the scattering angle] is given by expression 1:24 g(2)(Q , τ ) =

⟨I(Q , t + τ )I(Q , t )⟩ ⟨I(Q , t )⟩2

(1)

(2)

The time averaged g (Q,τ) is related to the time-averaged electric field auto correlation function, g(1)(Q,τ) through Siegert relation:25 g(2)(Q , τ ) = 1 + |βg(1)(Q , τ )|2

(2)

The coherence factor β (≤1) depends on the experimental setup. The coherence factor, β is found to be ∼0.7 for our experimental setup obtained using a dilute suspension of calibrated latex spheres (Duke Scientific Cooperation, diameter 200 nm). Figure 5 shows the normalized time-averaged 1593


Macromolecules

Article

intensity−intensity autocorrelation function measured at scattering angle 90° for vesicles prepared at [1] = 5.55 × 10−4 M. For monodisperse systems, g(2)(Q,τ) can be fitted to a single exponential function (monomodal distribution) with a characteristic decay rate (Γ = DQ2), where D is the diffusion coefficient of the scatterers. For polydisperse systems, the mean characteristic decay rate (⟨Γ⟩ = ⟨D⟩Q2) can be obtained by using the CONTIN regularization function26,27 (nonmonomodal distribution). In our case, the self-assembled vesicles are found to be polydisperse (see Figure 4A) and the normalized g(2)(Q,τ) is analyzed using CONTIN (solid line in Figure 5). Knowing ⟨D⟩, the mean hydrodynamic diameter ⟨dH⟩ is calculated using the Stoke−Einstein relationship.28 The CONTIN size histogram is shown in inset of Figure 5 and indicates a rather broad distribution of size population. The measured mean hydrodynamic diameter ⟨dH⟩ of the selfassembled vesicles is 606.16 nm (mean hydrodynamic radius ⟨RH⟩ = ⟨dH⟩/2 = 303.08 nm) and the polydispersity index is ca. 0.49. In contrast to systems based on ionic amphiphiles, in which electrostatic forces are important in controlling interaggregate interactions, aggregates formed from neutral molecules using H−B interactions in organic media may experience a strong tendency to further agglomerate. This would lead to a continuous evolution in the size of the aggregates over time, and could be detrimental to their use in the formulation of stable inks for printed electronic devices. To study the interactions between aggregates of 1, the normalized intensity−intensity autocorrelation functions were measured at various scattering angles (30° to 130° with 10° intervals). Figure 6A shows the normalized intensity−intensity autocorrelation functions plotted against Q2τ, where it can be seen that the correlation functions measured at various angles collapse into a single characteristic time. This is typical of particles that undergo simple Brownian motion with negligible interactions, suggesting that the interactions between the spherical vesiclelike aggregates are small. Similarly, the average gamma ⟨Γ⟩ is evaluated by CONTIN analysis for each g(2)(Q,τ) measured at various scattering wave vectors and they are plotted as a function of Q2 as shown in Figure 6B (blue circles). The solid blue line is the linear fit. Notice that the intercept of the line is zero revealing again the particles undergo simple Brownian motion. In agreement with this, we do not observe variations in the size distribution of the aggregates as determined by DLS over a period of two months (Figure 6B, data: red circles and red line is linear fit). This behavior is consistent with the selfassembly of 1 in a structure that does not expose vacant H−B sites on the outer surface of the aggregates, as this would almost certainly result in the observation of strong interactions between the aggregates. The size of the aggregates in THF was also studied using small-angle X-ray scattering. The suspension of 1 in THF appears to scatter according to an intensity profile versus Q showing two decay regimes (Figure 7). From low to large-Q, a first fast decay I ∼ Q‑2 is followed by a much flatter profile and finally a sharp decay. Such a scattering suggests the existence of two very different morphologies. Very large aggregates are responsible for the I ∼ Q‑2 low-Q decay and a much smaller finite species is responsible for the plateau region followed by a sharp decay. Following the above electron microscopy observations and DLS measurements, it is reasonable to assign the low-Q component of the scattering to large vesicles while the large-Q one could be due to micellar-like species. To further

Figure 4. Cryo-SEM micrographs of compound 1 in THF/water (A, B) and in THF (C). Red circles show small vesicles.

Figure 5. Normalized time-averaged intensity−intensity autocorrelation function at scattering angle 90° for 1 concentration 5.55 × 10−4 M in THF solution. Solid line is the best fit of the CONTIN function.

1594


Macromolecules

Article

characterize the self-assembled nature of the aggregates, the temperature is increased from 20 to 55 °C: Figure 7 shows that the fraction of the micellar-like component increases. This can be interpreted as a disintegration of a small fraction of vesicular species into smaller aggregates. It can be assumed that due to the small concentration (10 mM), the scattering of the solution is not affected by interferences between the scatterers. Thereby, the scattered intensity reduces to the form-factor scattering function IF(Q) for hollow spheres (expression 3)29 is IF(Q ) ≅ {3Ro3[sin(QRo) − QRocos(QRo)]/(QRo)3 2

− 3R i 3[sin(QR i) − QR i cos(QR i)]/(QR i)3 }

(3)

where Ro is the external radius of the vesicle (hollow sphere) while Ri is the internal radius and thus (Ro − Ri) is the thickness of the interfacial membrane. Figure 8A considers that the size of the vesicles is large compared to the experiment Q-domain so that their curved

Figure 6. (A) Normalized time-averaged intensity−intensity autocorrelation function at various scattering angles plotted as a function of Q2τ for 1 (5.55 × 10−4 M) in THF solution. (B) Average γ plotted against Q2 for 1 (5.55 × 10−4 M) in THF solution (blue circles) and after 60 days (red circles). Solid lines are the linear fits.

Figure 8. SAXS profiles of 1 (10 mM in THF). (A) Indicative low-Q decays following expression 4 for membranes having a transverse dimension of 3 nm (curve 1) or 10 nm (curve 2) are shown. (B) Indicative large-Q decays following expression 3 for spherical micelles (Ro = 2.2 nm, Ri = 0 nm).

Figure 7. SAXS profiles of 1 (10 mM in THF) at two temperatures (20 and 55 °C). An indicative slope of −2 is indicated in the low-Q domain.

1595


Macromolecules

Article

interfaces appear as flat membranes whose scattering reduces to expression 4: IF(Q ) ≅ Δρ2 /Q 2[sin(Q (Ro − R i)/2)/(Q (Ro − R i)/2)]2 (4)

Figure 8A also shows that the low-Q asymptotic behavior (I ∼ Q−2) is correctly observed while the transverse dimension cannot be distinguished from the experimental data since this later is truncated by the second component of the system. Two transverse dimensions (3 and 10 nm) are used to demonstrate the issue: the intensity decay of the membranes due to the finiteness of their cross-section occurs in the Q-range of the scattering from the smaller species (see Figure 8B). Figure 8B is an indicative fit showing that small scatterers can be responsible for the large-Q decaying scattering component. Nonunivocally and very approximately, a typical correlation distance of ca. 4.4 nm can be considered. The molecule of 1 has four phenyl lateral grafts that generate a small shift when a sideby-side aggregation process is modeled. If the number n of monomers becomes large, a curvature is generated whose consequence on a 2D process can be the vesicular formation. The mechanism does not exclude the formation of a multilayered thickness. The electron contrast of 1 aggregates in THF is mainly due to the hydrophobic and unsaturated part of the molecule which has an average size of ca. 2.2 nm. This is supporting the SAXS data for the existence of single-walled small micelles along with larger vesicles. While Figure 8A shows the extraction of the wall thickness (Ro − Ri) of the vesicles using expression 4, Figure 9A shows the best fit option offered by expression 3 (only the low-angle ca. Q−2 component has to be considered). Using the approximate approach, the values Ro = 750 nm and Ri = 720 nm can be extracted which are well within the area of the maximum of the Gaussian describing the distribution of distances as probed by DLS (Figure 9B). The wall thickness (ca. 30 nm) that can be roughly estimated from SAXS measurements indicates that multilamellar of 1 molecules are involved. AFM, SEM, and TEM also suggest that the shell is very thin (≤10 nm).11 To confirm that H−B interactions are responsible for aggregation, time-resolved light scattering was used to monitor the presence of aggregates before and after the addition of a cosolvent (DMSO) capable of disrupting H−B. The scattered light collected at 90° from a sample of 1 in THF (5.55 × 10−4 M) is shown in Figure 10. The variation in intensity upon addition of 50 μL (5%) of DMSO is very fast and indicative of a breakup of the aggregates as expected for the disruption of the H−B network. This, along with our previous observation that a compound analogous to 1 but without the biuret units does not aggregate in THF20 strongly supports the conclusion that H−B interactions are principally responsible for the self-assembly properties of 1. As mentioned in the introduction, the spontaneous formation of vesicles in THF is particularly intriguing. The DLS, SAXS, and cryo-SEM experiments support the conclusion that the vesicles are directly formed upon dissolution of 1 in THF at concentrations above the CAC. The formation of vesicle-like aggregates therefore proceeds without requiring specific processes that would accompany solvent evaporation under ambient conditions such as a strong concentration gradient or water condensation at the air/solution interface. To understand how the vesicles are formed, we must first consider how molecules of 1 may interact in a hydrophobic environ-

Figure 9. Aggregates of 1 (10 mM in THF). (A) Red line is the theoretical SAXS profile following expression 3 for Ro = 750 nm and Ri =720 nm. (B) Gaussian distribution of the distances probed by DLS.

Figure 10. Time-resolved light scattering measurements collected from a solution of 1 (5.55 × 10−4 M) in THF. The addition of 5% DMSO (indicated by red arrow) is followed by a slow decrease in the signal.

ment. Examination of the solid-state structure of the precursor phenylbiuret derivative30 evidence the formation of H−B ribbons that, in the case of a compound containing two biuret units (such as 1), would lead to the formation of extended sheet-like structures. These are known to be subject to various 1596


Macromolecules

Article

reminiscent of vesicles. Given that the aromatic core in 1 is sterically encumbered to avoid π-stacking of the bis-fluorene moiety, which would otherwise lead to a bathochromic shift of the emission spectrum due to excimer formation (not observed), and that short fluorene oligomers are highly soluble in THF, it is very likely that the principal driving force toward aggregation results from H−B interaction between the biuret units. This is particularly promising for the targeted design of photo- and electro-active vesicle-like aggregates as no additional stabilization from hydrophobic interactions (from e.g. long hydrocarbon chains) or π-stacking between aromatic units (which would alter the electronic and photophysical properties of the material) are required. Investigation of the aggregation properties of other bis-biuret rigid core compounds is ongoing and will provide more information on the generality of this approach.

instabilities in solution, which may eventually lead to the formation of vesicle-like aggregates. Along these lines, TEM micrographs of vesicles prepared by drop-casting showed the presence of possible intermediate structures resulting from pearling instabilities (Figure 11A) and budding (Figure 11B).

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the Agence National de la Recherche (Grant no ANR-09-BLAN-0387 OLEV) and the National Science Council (Taiwan) is gratefully acknowledged. S.K.P.V acknowledges the CEA-Eurotalents program. The authors are grateful to the European Synchrotron Radiation Facility (ESRF, Grenoble, France) for all technical support during the scattering experiments and Drs T. Narayanan and P. Boeseke are deeply acknowledged. Jose Galvez is thanked for his technical help.

Figure 11. TEM micrographs of vesicles by drop-cast method showing pearling of vesicles (A) and budding of vesicles (B). (C) Possible origin of such structures from delamination of a molecular solid composed of H−B sheets, followed by folding of the sheets into large vesicles or tubules.

These suggest that the mechanism for the formation of vesicles could be via pearling instabilities and/or budding models.18,19 The presence of regions in which the vesicles are very uniform in size and connected by a thin fibril as well as regions in which the vesicles are very polydisperse in size could imply that the two mechanisms are in competition and not mutually exclusive, as summarized schematically in Figure 11C. Furthermore, if H−B sheets are indeed formed through the assembly of biuret ribbons, then the latter would necessarily lie in the plane of the surface of the vesicle and no H−B sites would be exposed to the hydrophobic solvent. Such a structure would minimize the overall energy of the architecture, while reducing the probability of strong H−B interactions between aggregates that would lead to agglomeration of the vesicles over long times.

■

ABBREVIATIONS DLS, dynamic light scattering; TLS, time-resolved light scattering; SAXS, small-angle X-ray scattering; cryo-SEM, cryogenic scanning electron microscopy; CAC, critical aggregation concentration

■

REFERENCES

(1) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47 (42), 8002−8018. (2) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105 (4), 1491−1546. (3) Zhang, X.; Rehm, S.; Safont-Sempere, M. M.; Würthner, F. Nat. Chem. 2009, 1 (8), 623−629. (4) Bhattacharya, S.; Kumari, N. Coord. Chem. Rev. 2009, 253 (17− 18), 2133−2149. (5) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2000, 34 (1), 40−48. (6) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1997, 385 (6613), 239−241. (7) Steinberg-Yfrach, G.; Rigaud, J.-L.; Durantini, E. N.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1998, 392 (6675), 479−482. (8) Hoeben, F. J. M.; Shklyarevskiy, I. O.; Pouderoijen, M. J.; Engelkamp, H.; Schenning, A. P. H. J.; Christianen, P. C. M.; Maan, J. C.; Meijer, E. W. Angew. Chem., Int. Ed. 2006, 45 (8), 1232−1236. (9) Manyuhina, O. V.; Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.; Fasolino, A.; Katsnelson, M. I.; Schenning, A. P. H. J.; Meijer, E. W.; Henze, O.; Kilbinger, A. F. M.; Feast, W. J.; Maan, J. C. Phys. Rev. Lett. 2007, 98 (14), 146101. (10) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.; Schenning, A. P. H. J.; Meijer, E. W.; Henze, O.; Kilbinger, A. F. M.;

■

CONCLUSIONS A combination of dynamic light scattering, small-angle X-ray scattering techniques and cryo-SEM supports the conclusion that polydisperse vesicle-like aggregates are present in anhydrous THF solutions of 1 when the concentration is above ca. 1 × 10−4 M. From SAXS measurements, the existence of smaller aggregates alongside the larger, vesicle-like aggregates can be deduced. Once formed, the vesicles are stable over extended periods of time and do not further agglomerate or break up unless a cosolvent capable of interfering with the H−B interactions is added. Structures whose morphology suggests that budding and pearling instabilities may by responsible for the formation of vesicles from sheets were observed in TEM micrographs. From these results, we conclude that the presence of two self-complementary molecular recognition sites on the opposite ends of a rigid core promotes the formation of H−B sheets, which then give rise to hollow sphere aggregates 1597


Macromolecules

Article

Feast, W. J.; Del Guerzo, A.; Desvergne, J.-P.; Maan, J. C. J. Am. Chem. Soc. 2005, 127 (4), 1112−1113. (11) Tseng, K.-P.; Fang, F.-C.; Shyue, J.-J.; Wong, K.-T.; Raffy, G.; Del Guerzo, A.; Bassani, D. M. Angew. Chem. 2011, 123 (31), 7170− 7174. (12) Abbel, R.; Grenier, C.; Pouderoijen, M. J.; Stouwdam, J. W.; Leclère, P. E. L. G.; Sijbesma, R. P.; Meijer, E. W.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2008, 131 (2), 833−843. (13) Abbel, R.; van der Weegen, R.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Commun. 2009, 0 (13), 1697−1699. (14) Abbel, R.; van der Weegen, R.; Pisula, W.; Surin, M.; Leclère, P.; Lazzaroni, R.; Meijer, E. W.; Schenning, A. P. H. J. Chem.Eur. J. 2009, 15 (38), 9737−9746. (15) Balan, B.; Vijayakumar, C.; Ogi, S.; Takeuchi, M. J. Mater. Chem. 2012, 22 (22), 11224−11234. (16) Dudek, S. P.; Pouderoijen, M.; Abbel, R.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127 (33), 11763−11768. (17) Vijayakumar, C.; Sugiyasu, K.; Takeuchi, M. Chem. Sci. 2011, 2 (2), 291−294. (18) Tsafrir, I.; Caspi, Y.; Guedeau-Boudeville, M.-A.; Arzi, T.; Stavans, J. Phys. Rev. Lett. 2003, 91 (13), 138102. (19) Tsafrir, I.; Sagi, D.; Arzi, T.; Guedeau-Boudeville, M.-A.; Frette, V.; Kandel, D.; Stavans, J. Phys. Rev. Lett. 2001, 86 (6), 1138−1141. (20) Fang, F.-C.; Chu, C.-C.; Huang, C.-H.; Raffy, G.; Guerzo, A. D.; Wong, K.-T.; Bassani, D. M. Chem. Commun. 2008, 0 (47), 6369− 6371. (21) Ysambertt, F.; Vejar, F.; Paredes, J.; Salager, J. L. Colloids Surf. A 1998, 137 (1−3), 189−196. (22) Lin, S.-Y.; Lin, Y.-Y.; Chen, E.-M.; Hsu, C.-T.; Kwan, C.-C. Langmuir 1999, 15 (13), 4370−4376. (23) Pace, C. N.; Vajdos, F.; Fee, L.; Grimsley, G.; Gray, T. Protein Sci. 1995, 4 (11), 2411−2423. (24) Berne, B. J.; Pecora, R. Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics; Dover Publications, Inc.: New York, 2000. (25) Siegret, A. J. F. On the fluctuations in signals returned by many independently moving scatterers. MIT Radiation Laboratory Report No. 465; MIT: Cambridge, MA, 1943. (26) Provencher, S. W. Comput. Phys. Commun. 1982, 27 (3), 213− 227. (27) Provencher, S. W. Comput. Phys. Commun. 1982, 27 (3), 229− 242. (28) Einstein, A. Investigations on the Theory of the Brownian Movement; Dover Publication, Inc.: New York, 1956. (29) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley Publications, Inc.: New York, 1955. (30) Kuo, M.-C.; Chen, H.-F.; Shyue, J.-J.; Bassani, D. M.; Wong, K.T. Chem. Commun. 2012, 48 (65), 8051−8053.

1598


Spontaneous Formation of Artificial Vesicles in Organic Media through

Recommend Documents