Hydrogen-Bonded Multifunctional Supramolecular Copolymers in

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Hydrogen-Bonded Multifunctional Supramolecular Copolymers in Water Yunjie Xiang, Emilie Moulin, Eric Buhler, Mounir Maaloum, Gad Fuks, and Nicolas Giuseppone Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01093 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on June 23, 2015

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Hydrogen-Bonded Multifunctional Supramolecular Copolymers in Water Yunjie Xiang,a Emilie Moulin,a,* Eric Buhler,b Mounir Maaloum,a Gad Fuks a and Nicolas Giuseppone a,* a

SAMS research group – University of Strasbourg – Institut Charles Sadron, CNRS, 23 rue du

Loess, BP 84087, 67034 Strasbourg Cedex 2, France, Homepage: http://sams.ics-cnrs.unistra.fr/ b

Matière et Systèmes Complexes (MSC) Laboratory, University of Paris Diderot – Paris VII,

UMR 7057, Bâtiment Condorcet, 75205 Paris Cedex 13, France.

ABSTRACT. We have investigated the self-assembly in water of molecules having a single hydrophobic bis-urea domain linked to different hydrophilic functional side chains, i.e. bioactive peptidic residues and fluorescent cyanine dyes. By using a combination of spectroscopy, scattering, and microscopy techniques, we show that each one of these molecules can individually produce well-defined nanostructures such as twisted ribbons, 2D plates, or branched fibers. Interestingly, when mixing these monomers of different functionalities in an equimolar ratio, supramolecular copolymers are preferred to narcissistic segregation. Radiation scattering and imaging techniques demonstrate that one of the molecular units dictates the formation of a preferential nanostructure, and optical spectroscopies reveal the alternated nature of the copolymerization process. This work illustrates how social self-sorting in H-bond supramolecular polymers can give a straightforward access to multifunctional supramolecular copolymers.

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INTRODUCTION Block or alternated covalent copolymers are very attractive macromolecules to produce functional nanostructures and materials with a wide range of applications from biomedicine to organic electronics.1,2 To access such copolymers, the installation of lateral substituents with orthogonal reactivity on the main chain is often a prerequisite for further post-polymerization modifications.3 Thus, introducing reactive orthogonal side chains is the purpose of advanced synthetic protocols which are actively developed.4,5,6,7 However, to date, only very few examples of copolymers bearing highly functional units have been described.8,9,10 In parallel, supramolecular chemistry has been extensively developed towards functional systems and materials.11,12,13 In particular, supramolecular polymers have been shown to complement and extend structures that can be achieved with classical covalent polymers by displaying processability and recycling or self-healing properties that conventional polymers usually do not possess, owing to the lack of reversibility of their covalent linkages. Indeed, strong and directional non-covalent interactions impart high degrees of internal order while providing a dynamic constitution to supramolecular polymers. These properties make them valuable for applications in the fields of biomedicine, sensing, catalysis, and organic electronics.11,14,15,16 Among all non-covalent interactions, hydrogen-bonded supramolecular polymers have been shown to yield various structures ranging from finite-sized self-assemblies to three-dimensional (3D) networks.17,18,19 Furthermore, the combination of several hydrogen-bonding motifs has been very successful to produce self-assembled materials with advanced properties.20,21 In particular, the

mechanical

and

physical

properties

of

thermoplastic

elastomers

based

on

co(polyetherbisureas) could be greatly affected when dyes (even in catalytic amounts) with matching or non-matching spacers between the urea motifs were incorporated into the


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polymer.22,23,24,25 This approach of combining different molecules with identical recognition motifs, i.e. bis-ureido groups, into supramolecular copolymers was also extended to low molecular weight compounds.26,27,28,29,30,31 In most cases, catalytic quantities of fluorescent probes were mixed with a non-fluorescent bis-urea molecule thus forming a mixed supramolecular polymer whose dynamics was studied using fluorescent spectroscopy. For instance, using bolaamphiphiles which form stable micellar aggregates in water, the dynamic nature of the selfassembly process was observed when two micellar solutions with a same spacer length were mixed whereas coexistence of both self-assemblies occurred for micelles with non-matching spacers.31 Such events can be defined as social and narcissistic self-sorting respectively, i.e. affinity for others in one case and affinity for itself in the other one.32,33,34 In all examples, bis-urea cores were decorated with either alkyl chains or PEG units thus forming nanostructures in organic solvents or aqueous solutions respectively and, the incorporation of low quantities of bisurea-based fluorophores in these supramolecular polymers did not induce changes of their morphologies. However, equimolar mixtures of bis-urea molecules with side chains presenting very different structural and functional properties have, to the best of our knowledge, never been studied. Although one could expect that the mixed combination of such highly functional monomers would preferentially produce segregated nanostructures resulting from the incompatibility of the different building blocks, we have however envisioned the possible access to supramolecular copolymers bearing all the functions inherent to the parent monomers thanks to a social self-sorting process. In this latter case, the resulting multifunctional supramolecular alternated copolymers would be much easier to synthesize than their covalent counterparts and should display additional dynamics properties.


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In the present study, we have designed and synthesized water soluble hydrogen bond bolaphile monomers decorated on both sides of their hydrophobic bis-urea core by hydrophilic functional side chains, namely peptidic or fluorescent residues (Figure 1). With such molecules, we expected to direct supramolecular interactions mainly by their strongly associating bis-urea motifs. The resulting supramolecular polymers have been first investigated in water thanks to scattering techniques, microscopies, and optical spectroscopies. We have then studied, at thermodynamic equilibrium, mixtures made of peptidic and fluorescent monomers in an equimolar ratio. Our results show that such bicomponent mixtures lead mostly to alternated functional copolymers whose morphology is driven by only one of the two mixed molecular units.

Figure 1. Molecular structures of the four bis-urea based molecules considered in this study.


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RESULTS AND DISCUSSION Synthesis of the molecular units. Four different 1,6-bis-urea monomers incorporating either peptide chains or fluorescent cyanine derivatives as lateral residues have been successfully synthesized and fully characterized (Figure 1 and SI). Peptidic monomers BU-6AA and BU-7AA were obtained in two steps from resin-supported hexa- and hepta-aminoacids sequences, respectively (Scheme 1a). The AlaAlaAlaGluGluGlu sequence was chosen because it has already been reported as a model of bioactive peptide.35,36 For BU-7AA, the hexanoyl linker attached to the peptidic sequence was expected to stabilize further the self-assembly in water by providing a supplementary hydrophobic pocket to the hydrogen bonds between bis-urea. To our knowledge, this “on-resin” double coupling hasnever been reported for bis-urea based molecules and represents a powerful strategy to reach such peptidic compounds.37,38 Cyanine-based monomers BU-Cy3 and BU-Cy5 have been synthesized in one step from already known cyanine derivatives39 4 and 5 and the 1,6bis-urea precursor 1 (Scheme 1b). All four compounds displayed good solubility in water, thus allowing the study of their behavior in this solvent.


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a)

b)

Scheme 1. Synthetic routes for a) BU-6AA/BU-7AA and b) BU-Cy3/BU-Cy5. Their full characterizations are reported in the Supporting Information (SI). Characterization of the self-assemblies made of a single molecular unit. FT-IR experiments were performed for all monomers on lyophilized samples40 obtained from 10-3M solutions in water in order to evaluate the propensity of each monomer to form selfassemblies (Figure S2a,b). For BU-6AA, BU-7AA and BU-Cy3, the presence of characteristic bands around 3310 and 1635 cm-1 is in agreement with the formation of hydrogen bonds between urea motifs.41,42 For BU-Cy5, the presence of a broad band at ~3400 cm-1 and of a sharp one at 1637 cm-1 also indicates the formation of hydrogen bonds between urea groups together with free


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N-H stretching arising from the amide moiety on the cyanine core. Circular dichroism (CD) experiments were also performed on all peptidic monomers to determine the influence of the peptidic residues on the self-assembly (Figure S3a). The absence of a minimum at around 220 nm, which would be characteristic of β-sheet structures, suggests that BU-6AA and BU-7AA predominantly give rise to random coils, in agreement with the literature.40 Viscosimetry experiments were also carried out on freshly prepared solutions obtained by simply dissolving the title compound in water at room temperature (Figure S3b). A higher viscosity was observed in all cases when increasing the concentration, but viscosity properties of the different solutions were strongly affected by the nature of the side chains. The strong viscosity increase above a sample-dependent critical concentration was attributed to the formation of networks in the semidilute regime and suggested different growth processes and/or the formation of various selfassembled structures for the three compounds. For instance, the presence of the flexible and hydrophobic hexanoyl linker between the bis-urea core and the hexapeptide impacts the relative viscosity of BU-7AA compared to BU-6AA, indicating that these two molecules give rise to different kinds of aggregates. This was confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM) showing a characteristic structural behavior for each derivative (Figure 2). BU-6AA forms very unusual 2D plates with side length of several hundreds of nanometers resulting from a combination of hydrogen bonding interactions between the urea units and strong hydrophobic interactions due to the central hexyl backbone and alanine residues (Figures 2a and S4). For BU-7AA, which differs from BU-6AA by an aliphatic and hydrophobic C6 linker between the peptidic side chains and the bis-urea core, 2D structures were absent and a network of long ramified fibers was observed with a mesh size of the network ranging from 100 to 300 nm (Figures 2b and S5). Finally, BU-Cy3 and BU-Cy5 yield to networks


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of intertwined filaments and long hollow cylinders made from twisted ribbons in a hierarchical organization (Figures 2c-d, S6 and S7). Indeed, for BU-Cy3, intertwined fibers are observed at a monomer concentration of 1 mM, whereas isolated long hollow cylinders resulting from twisted ribbons are imaged at lower concentrations, with a diameter of ~4.7 nm and lengths up to micrometric sizes (Figures 2c (inset) and S6b).

Figure 2. Atomic force microscopy (AFM) images of a) BU-6AA, b) BU-7AA, c) BU-Cy3, and d) BU-Cy5 prepared by drop-casting on mica of a solution in D2O with an initial concentration of


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1 mM, (inset in c) shows an isolated long hollow cylinder made of a single twisted ribbon of BUCy3 and obtained at low concentration). One drawback of microscopy experiments in self-assembled systems is that, in most cases, concentrated networks (as observed in Figure 2) can result from solvent evaporation and/or surface-mediated aggregation during the samples deposition. To characterize in more details the structure and conformation of the individual (isolated) self-assemblies in the dilute concentration range, we thus turned to static light (SLS) and small-angle neutron scattering (SANS), which are powerful complementary techniques to determine sizes, structural parameters, and possibly the molecular packing of objects in solution over the range of 1-300 nm. Figure 3 shows variations in scattered intensity obtained by coupling SLS low-q data and SANS measurements as a function of the scattering wave-vector q for each of the four systems at 1 mM. For both techniques (SLS and SANS), the scattered intensity was normalized by the corresponding contrast term. Importantly, after solubilization in water at 1 mM, all systems appear as monophasic limpid solutions with low viscosity and, for all molecules, different spectra were recorded, confirming the formation of different structures. For BU-6AA, the scattering curve (Figure 3a) displays the onset of a Guinier plateau in the SLS low-q range associated with the finite size and mass of the scattered objects. This Guinier regime is followed by the scattering vector regime q>RG-1 (RG is the radius of gyration) in which I(q)~q-df, where df is the so-called fractal dimension of the particles. In the log-log representation, one obtains a linear decay over two orders of magnitude of the q-regime (from 10-3 to 10-1 Å-1) with a slope df = 2 that is characteristic of 2D-objects with smooth surfaces, such as aggregates of polymer sheets in two dimensions. To the best of our knowledge, this finding represents the


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first example of such objects based on supramolecular polymers, and confirms the previous microscopy experiments (Figures 2a and S4). a)

103

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Figure 3. SANS coupling with SLS scattering intensities as a function of scattering vector q for a) BU-6AA, b) BU-7AA, c) BU-Cy3, and d) BU-Cy5. For both techniques, the scattered intensity has been normalized by the corresponding contrast terms. The linear correlation between 1/I(q) and q2 is represented in the insets. The red line represents the fit of the high q data by a Guinier expression for the form factor of the section (Figures 3c-d, see eq 2). These experiments were recorded at room temperature using 1 mM solutions in D2O. The low-q data corresponding to large spatial scales can be fitted by a Guinier expression (1/I(q) = 1/I(0) (1+q2RG2/3)), giving the gyration radius RG = 290 nm and the number of


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monomers in a plate N = 16590. DLS experiments also give the hydrodynamic radius RH = 265 nm and the polydispersity index PDI=k2/k12=0.23 for the plates (k1 and k2 are the first and second cumulants, respectively, and were determined using the cumulant analysis, see SI section 4); a PDI value indicating polydisperse objects. For a 2D-plate with sides a and b, the radius of gyration can be written: RG2 = 1/12 (a2+b2). Assuming that a~b, we found an average side length of about 710 nm, corresponding to few aggregated plates (as shown in Figure S4). For BU-7AA, which differs from BU-6AA by an aliphatic and hydrophobic C6 linker between the peptidic side chains and the bis-urea core, a pattern characteristic of ramified filaments with df comprised between 2 and 3 is observed and also correlates with AFM images. Namely, the onset of the Guinier regime at low q is followed by a power law regime with exponent between 2 and -3 (Figure 3b). At higher q, I(q) is controlled by smaller distances than the persistence length over which filaments are rod-like, and a q-1-dependence, which is typical for locally rod structures, is observed. Neglecting Virial effects, the low-q analysis gives ξ = 306 nm, where ξ is the correlation length and represents the mesh size of the network for concentrations larger than the overlap concentration, or provides the radius of gyration of the filaments RG = ξ√3 in the dilute regime. DLS gives ξH=217 nm and PDI=0.21, a value consistent with polydisperse objects and observed in other surfactant or polymeric self-assembly systems. For BU-Cy3 and BU-Cy5, the two spectra are markedly similar (Figures 3c-d), suggesting that morphologically related structures are present in solution. For BU-Cy3, the scattering profile exhibits: (i) a Guinier regime at low-q followed by a power law with df close to 3 characteristic of large aggregates such as large fibers, as observed by AFM and TEM (Figures 2c and S6), with RG = 175 nm and MW = 26 106 Da; (ii) an intermediate regime in which the q-dependence of I(q) is described by a power law with an exponent equal to -1 and characteristic of rigid-rod-like


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behavior (df = 1) followed at high q by (iii) another Guinier regime associated with the crosssection of the individual rods. For BU-Cy5, low-q SLS experiments could not be performed due to the total absorption of the laser beam (λ = 632.8 nm) by the sample, precluding any scattering of the sample and, in the intermediate regime, df is closer to 1.5 a value still consistent with unidirectional objects such as rods with few ramifications. Above 2 x 10-1 Å-1, for both molecules, some oscillations might be observed. The whole scattering sequence is consistent with hollow rigid cylinders that self-aggregate to form larger elongated polydisperse structures with RG/RH = 175/120 ~ 1.5 and PDI~0.2 (RH is the hydrodynamic radius obtained using DLS) such as intertwined fibers as observed using microscopy techniques (Figures 2d and S7). In the intermediate q regime, the scattering curve of BU-Cy3 can be fitted satisfactorily using a rigidrod model by means of the form factor43,44,45 derived for rigid rod particles given by:

=

(1)

where L is the contour length of the rod (hollow cylinders). The high q data can be fitted by a Guinier expression for the form factor of the rods section: . =

−

(2)

where rc is the radius of gyration of the cross-section of the rods. By fitting these two equations to the experimental data, one can determine the linear mass density (µ, mass per unit length) of the rods, the cross-sectional area A, and rc. From Figure 3c, fits at intermediate and high q provided µ = 420 ± 40 g.mol-1.Å-1, A = 442 ± 40 Å2 and rc = 19 ± 1 Å for BU-Cy3. For hollow cylinders, the cross-sectional area is given by = −

(3)

whereas the radius gyration of the section is equal to:


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=

√

+

(4)

Where R and Ri are the external and internal radii of the section of hollow cylinders, respectively. Furthermore, the thickness of twisted ribbons forming hollow cylinders is thus given by e = R-Ri. Using equations 3 and 4, we calculated that R = 20.8 Å and Ri = 17 Å for BUCy3, and R = 23.5 Å and Ri = 16.7 Å for BU-Cy5 and thus that the thickness of the ribbon made of BU-Cy5 is slightly larger (e = 6.8 Å) than the one made of BU-Cy3 (e = 3.8 Å).46 Considering the linear mass density of BU-Cy3, SANS experiments suggest that the ribbon is made of a single layer of molecules whereas, for BU-Cy5, the larger thickness of the ribbon would indicate that the ribbon is composed of a bilayer. All scattering data thus confirm the formation of unique structures for all monomers, which was also demonstrated by microscopy experiments. In addition, owing to the presence of fluorescent cyanine dyes on BU-Cy3 and BU-Cy5, the optical properties of both compounds were recorded as they are of significant interest for the study of mixtures with BU-6AA or BU-7AA (see next section). Noteworthy, the study of optical properties of cyanine dyes involved in supramolecular polymers has not been carried on so far. For both cyanine-containing molecules, compared to methanolic solutions which consist of isolated single molecules (Figures S1a and c), the maximum of absorption is shifted to lower wavelengths (Figure 4a-b), which denotes the formation of H-aggregates and suggests a face-toface arrangement of the cyanine dyes.47 Additionally, a maximum of fluorescence was observed at 10-5 M in water and complete fluorescence quenching occurred at 10-3 M (Figure 4c-d). Compared to BU-Cy3 in methanol at a same concentration, the lower fluorescence of BU-Cy3 in water due to the formation of H-aggregates is in agreement with the Kasha-theory and the Davydov splitting (Figure S1b).48,49 A similar behavior is also observed for BU-Cy5 (Figure S1d).


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a)

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Figure 4. a) Comparison of the UV-Vis spectra of a 10-4 M solution of BU-Cy3 (black) and of a 1:1 mixture of BU-Cy3/BU-6AA (red) and BU-Cy3/BU-7AA (blue) in D2O; b) of the UV-Vis spectra of a 10-4 M solution of BU-Cy5 (black) and of a 1:1 mixture of BU-Cy5/BU-6AA (red) and BU-Cy5/BU-7AA (blue) in D2O; c) Comparison of the fluorescence spectra of a 10-4 M solution of BU-Cy3 (black) and of a 1:1 mixture of BU-Cy3/BU-6AA (red) and BU-Cy3/BU7AA (blue) in D2O; d) Comparison of the fluorescence spectra of a 10-4 M solution of BU-Cy5 (black) and of a 1:1 mixture of BU-Cy5/BU-6AA (red) and BU-Cy5/BU-7AA (blue) in D2O. All these experiments were recorded at room temperature. Overall, these results show that all bis-urea single molecules form stable supramolecular polymers in water with unique structures, strongly depending on the lateral side chains of the bisurea core. In the following, the fluorescent nature of both BU-Cy3 and BU-Cy5 self-assemblies


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will be instrumental to study bi-component mixtures of bis-urea molecular units incorporating different side chains. Characterization and self-sorting properties of mixtures of two molecular units with different side chains. Considering that all bis-urea molecular units form unique self-assembled structures in water, we then investigated their self-sorting properties by mixing two aqueous solutions of compounds with different side chains, namely BU-Cy3 was mixed either with BU-6AA or BU-7AA and the same was done for BU-Cy5. All experiments were performed for 1:1 mixtures of monomers after at least one hour of equilibration. We did not observe structural changes between samples prepared with different order of mixing, or equilibration time, neither after heating the solutions (70 °C for 2 hours and cooling down to room temperature), showing that these systems were studied at thermodynamic equilibrium. FT-IR experiments were performed for all mixtures on lyophilized samples40 obtained from 10-3M solutions in water in order to confirm the formation of self-assembled structures via hydrogen bonding of the urea moieties (Figure S2c,d). For all mixtures, the presence of characteristic bands around 3310 and 1635 cm-1 is in agreement with the formation of hydrogen bonds between urea motifs. Based on the optical properties of BUCy3 and BU-Cy5 supramolecular polymers, we then recorded UV-Vis and fluorescence spectra of their mixture with peptidic monomers (Figure 4). Mixtures with BU-Cy3 at 10-4 M display almost identical absorption spectra to pure BU-Cy3 polymer with a maximum absorption at ~510 nm in both cases, which is hypsochromically shifted compared to free BU-Cy3 monomers (Figure 4a). Differences in intensities between BU-Cy3 and the mixtures can be attributed to the lower concentration of dyes in the mixtures for a same total concentration of bis-urea molecules. Such behavior was also observed for mixtures with BU-Cy5 at 10-4 M (Figure 4b). In this case,


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the maximum of absorption observed on UV-Vis spectra was hypsochromically shifted from 591 nm to 560 nm compared to pure BU-Cy5 polymer, suggesting a different organization of the dyes in the mixed supramolecular copolymer. We then investigated the fluorescence properties of the different mixtures. In all cases, at 10-4 M, fluorescence of the mixtures remained as a broad band with a high intensity whereas the fluorescence of either BU-Cy3 or BU-Cy5 was almost completely quenched at this concentration (Figure 4c-d). This absence of fluorescence quenching for the mixtures suggests that the BU-6AA and BU-7AA monomers most likely intercalate within assemblies formed by bisurea-cyanine units with a statistical distribution in the supramolecular alternated copolymers via social self-sorting. The formation of fluorescent Haggregates suggests that neighboring cyanine dyes are slightly twisted due to the intercalation of peptidic monomers.47,50 SANS coupling with SLS experiments on dilute mixtures were then performed to determine the precise structure of these self-assemblies and to confirm the formation of supramolecular copolymers. Importantly, for mixtures with BU-Cy3, the spectra are markedly similar to the one recorded for BU-Cy3 alone, suggesting that morphologically identical structures are present in solution (Figure 5a). The structural parameters of the hollow cylinders obtained using a crosssection analysis in the high q range are collected in Table 1.


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Table 1. Structural parameters obtained from fitting data presented in Figures 3 and 5 to a high-q Guinier expression for the form factor of the section of the hollow cylinders.[a] Samples

rc (Å) A (Å2) R (Å) Ri (Å) e (Å)

BU-Cy3

19

442

20.8

17

3.8

BU-CY3/BU-6AA 21.3

394

22.7

19.8

2.9

BU-Cy3/BU-7AA

21.1

364

22.4

19.7

2.7

BU-Cy5

20.4

861

23.5

16.7

6.8

BU-Cy5/BU-6AA

20

494

22

18

4

BU-Cy5/BU-7AA

20.3

496

22.2

18.3

3.9

rc : radius of gyration of the cross-section, A : cross-sectional area, R : external radius, Ri : internal radius and, e : thickness of the cylinders wall. [a] According to the optimization of the fitting procedures, the calculation of the contrast and the measurement of the monomer density (see Supporting Information), we estimate an absolute error within 10% on the characteristic structural parameters of Table 1.


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a)

105 104 103 -1

-1

I (cm )

102 101 -3

100 10-1 10-2 10-3

-1.5 BU-Cy3 BU-Cy3/BU-6AA BU-Cy3/BU-7AA fit with cross-section model

10-4 10-4

10-3

10-2

10-1

100

q (Å-1)

b)

104 103

-3

-1.5

102 101 -1

I (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 10-1 10-2 10-3

BU-Cy5 BU-Cy5/BU-6AA BU-Cy5/BU-7AA fit with a cross-section model

-4

10

10-3

10-2

10-1

100

-1

q (Å )

Figure 5. a) SANS coupling with SLS scattering intensities as a function of scattering vector q for BU-Cy3 and 1:1 mixtures of BU-Cy3/BU-6AA and BU-Cy3/BU-7AA. For clarity, each of the curves has been shifted by one log unit along the y-axis. The red line represents the fit of the high q data by a Guinier expression for the form factor of the section (see eq 2); b) SANS scattering intensities as a function of scattering vector q for BU-Cy5/BU-6AA and BU-Cy5/BU7AA. For clarity, each of the curves has been shifted by one log unit along the y-axis. The red line represents the fit of the high q data by a Guinier expression for the form factor of the section (see eq 2). These experiments were recorded at room temperature using 1 mM solutions in D2O. At large scales and neglecting the Virial effects, the Guinier plot of 1/I as a function of q2 in the low-q range provide us with an apparent gyration radius (Rg, app) of 151 nm and an apparent


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molecular weight (Mw, app) of 15.27 × 106 Da for the fibrillar aggregates, which corresponds to an aggregation number Nagg of 9956 for the 1:1 mixture of BU-Cy3/BU-6AA. For the 1:1 mixture of BU-Cy3/BU-7AA in D2O, Rg, app was determined to be 137 nm and Mw, app was calculated to be as high as 14.25 × 106 Da, which corresponds to an aggregation number Nagg of 8652. Additional DLS measurements provided an apparent hydrodynamic radius RH,

app

of 52 and 58 nm after

cumulant analysis for the 1:1 mixtures of BU-Cy3 with BU-6AA and BU-7AA, respectively. A PDI of 0.22 is found for both systems. Importantly, Rg/RH ratio lies between 2 and 3, which suggests the formation of very elongated structures. Overall, these spectra indicate that the shape of the self-assemblies is dictated by the cyanine monomer. Compared to BU-Cy3 monomer alone, the global scattering spectrum is very similar and the characteristic sizes Rg and RH are slightly lower, which result in a lower degree of polymerization. For the mixture with BU-6AA, the absence of a q-2 domain in the intermediate q range suggests that these peptidic monomers have been incorporated within the BU-Cy3 polymer. In particular, the diameter of a single cylinder (~5 nm, Table 1) is almost identical for both samples (BU-Cy3 and BU-Cy3/BU-6AA), but shorter lengths are observed for the mixtures (several hundred nanometers), which is in agreement with a lower aggregation number. A similar conclusion can be reached for the mixture with BU-7AA, as no intermediate q regime in which the q-dependence of the data can be described by a power law with the exponent between -2 and -3 is observed. Importantly, AFM and TEM experiments proved to be in good agreement with scattering results (Figures 6a-b and S8). For instance, for the 1:1 mixture of BU-Cy3/BU-6AA (Figures 6a and S8a,c), compared to AFM and TEM images of BU-6AA (Figures 2a and S4), complete disappearance of the sheet-like structures is observed and the resulting morphology is very similar to the one observed for BU-Cy3 although with a lower density of intertwined ribbons


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(Figures 2c and S6). For the 1:1 mixture of BU-Cy3/BU-7AA (Figure 6b and S8b,d), compared to BU-7AA, AFM and TEM images indicate the presence of less rigid fibers, but it remains difficult to reach a conclusion regarding the preferential morphology of the co-self-assemblies. However, TEM images indicate the presence of fibers that are almost identical to the ones observed for BU-Cy3 and BU-Cy3/BU-6AA. All these images are in excellent agreement with scattering data that show identical spectra for BU-Cy3 and its mixtures.

Figure 6. a-b) Atomic force microscopy (AFM) images of 1:1 mixtures of a) BU-Cy3/BU-6AA, b) BU-Cy3/BU-7AA, c) BU-Cy5/BU-6AA and d) BU-Cy5/BU-7AA. Images were obtained by drop-casting a solution in D2O with an initial concentration of 1 mM.


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For mixtures with BU-Cy5, although light scattering experiments could not be performed due to the total absorption of the laser beam by the sample, SANS experiments showed a global behavior that is very similar to the one observed for BU-Cy5 alone (Figure 5b). This was confirmed by AFM and TEM imaging which show, for all mixtures with BU-Cy5, fiber-like morphologies with diameters matching the one observed for BU-Cy5 alone and in agreement with precise values obtained by SANS (Figures 6c-d, S9 and Table 1). Although the absence of light scattering data for BU-Cy5 mixtures preclude the determination of the molecular weight and the aggregation number of the supramolecular copolymers, AFM and TEM experiments reveal that the length of these mixed objects adsorbed on the surface (several hundred nanometers) is much shorter than the one of the parent BU-Cy5 supramolecular polymer (several micrometers). Overall, for all mixtures, scattering experiments indicate the formation of mixed copolymers with structures similar to the parent cyanine polymer (BU-Cy3 or BU-Cy5) but with lower degree of polymerization, which could also be evidenced by AFM or TEM imaging. The co-selfassembly of these hydrogen-bonding molecules having different side chains induces social selfsorting, resulting in the preferential formation of fibers with a alternated distribution of the monomers as demonstrated by fluorescence experiments.

CONCLUSIONS We have designed and synthesized four water soluble functional monomers consisting in a 1,6bis-urea hexyl core appended by peptidic residues or fluorescent cyanines as lateral side chains. These molecules are among the most functional bis-urea molecules reported so far. Their selfassemblies in water have been fully characterized by microscopies and scattering techniques,


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revealing the crucial role of bis-urea in the structuring process. Side chains were also shown to affect the long range ordering of the well-defined supramolecular polymers ranging from twodimensional sheets to ramified fibers and hollow cylinders (twisted ribbons). We have then studied the behavior of these molecules in bicomponent mixtures and we have shown that organized co-self-assemblies can be obtained from equimolar mixtures of monomers with different side chains. Under thermodynamic control, and despite very different lateral residues, a statistical distribution of alternated monomers in mixed fibers are produced as demonstrated by combined fluorescent measurements, scattering experiments, and microscopies. Furthermore, the long range organization of the supramolecular structures is preferentially driven by only one of the two mixed molecular units (namely the one incorporating the cyanine rather than the peptidic one). This work is an interesting example of social self-sorting in supramolecular polymers made of equimolar bicomponent systems which lead to the formation of multifunctional supramolecular copolymers (incorporating two types of units for biorecognition and fluorescence). It also represents an original example of morphology selection that can accommodate the presence of two different functional monomers which individually give rise to fully different nanostructures, an interesting aspect for systems chemistry.51,52,53 Beyond, the study presented in this article shows that multifunctional multicomponent supramolecular materials can become complementary to covalent copolymers with additional dynamic properties and ease of synthesis by simple mixing.


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EXPERIMENTAL SECTION Characterization of the different self-assemblies. All self-assemblies made of a single molecular unit were prepared simply by dissolving this molecule in deionized water at room temperature. Self-assemblies made of two components were obtained by mixing two solutions of both compounds in water at the same concentration and in a 1:1 ratio (v/v). UV-Vis spectra were recorded using a Varian Cary 5000 apparatus with quartz glass cuvettes of 1 mm optical path under ambient conditions, unless otherwise stated. Fluorescence

experiments

were

recorded

on

a

FluoroMax-4

(Horiba

Jobin-Yvon)

spectrofluorometer with quartz glass cuvettes of 1 cm optical path under ambient conditions. Small angle neutron scattering (SANS) experiments were carried out on the Pace spectrometer in the Léon Brillouin Laboratory at Saclay (LLB, France). Three sets of sample-to-detector distances and wavelengths were chosen (D = 0.9 m, λ = 5 ± 0.5 Å; D = 4.7 m, λ = 5± 0.5 Å and D = 4.7 m, λ = 13± 0.5 Å) so that the following q-ranges were respectively available: 4.95×10-2 ≤ q (Å-1) ≤ 5×10-1, 8.3×10-3 ≤ q (Å-1) ≤ 8.8×10-2, and 3.18×10-3 ≤ q (Å-1) ≤ 3.38×10-2. In a second run we used D = 1 m, λ = 6 ± 0.5 Å; D = 3 m, λ = 10± 1 Å and D = 4.7 m, λ = 12± 0.5 Å so that the following q-ranges were respectively available: 3.66×10-2 ≤ q (Å-1) ≤ 3.7×10-1, 6.2×10-3 ≤ q (Å-1) ≤ 6.5×10-2, and 3.44×10-3 ≤ q (Å-1) ≤ 3.7×10-2. Measured intensities were calibrated to absolute values (cm-1) using normalization by the attenuated direct beam classical method. Standard procedures to correct the data for the transmission, detector efficiency, and backgrounds (solvent, empty cell, electronic, and neutronic background) were carried out. Light scattering experiments were performed on a 3D DLS spectrometer (LS Instruments, Fribourg, Swiss) equipped with a 25mW HeNe laser (JDS uniphase) operating at λ=632.8 nm, a two channel multiple tau correlator (1088 channels in autocorrelation), a variable-angle detection


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system, and a temperature-controlled index matching vat (LS Instruments). The scattering spectrum was measured using two single mode fibre detections and two high sensitivity APD detectors (Perkin Elmer, model SPCM-AQR-13-FC). Solutions were filtered through 0.2 µm PTFE Millipore filter into the cylindrical scattering cell. In static light scattering, the scattered intensity was measured over the following q-range: 4.6×10-4 ≤ q (Å-1) ≤ 2.43×10-3. Atomic force microscopy (AFM) images were obtained by scanning the samples using a Nanoscope 8 (Bruker) operated in Peak-Force tapping mode. Samples were prepared by drop-casting 1 mM solutions on mica surfaces. TEM experiments were performed using a CM12 Philips microscope equipped with a MVIII (SoftImaging System) CCD camera. Samples were analyzed in Bright Field Mode with a LaB6 cathode and 120 kV tension. Samples were prepared by dropping solutions at ~10-4 M on a carbon-coated copper grid. The drop was left to adsorb for 30 seconds on the grid and the remaining solution was absorbed with a filter paper. AUTHOR INFORMATION Corresponding Authors * [email protected], * [email protected] ACKNOWLEDGMENT The research leading to these results has received funding from the European Research Council under the European Community's Seventh Framework Program (FP7/2007-2013) / ERC Starting Grant agreement n°257099 (N.G.). We wish to thank the Centre National de la Recherche Scientifique (CNRS), the COST action (CM 1304), the international center for Frontier Research in Chemistry (icFRC), the Laboratory of Excellence for Complex System Chemistry (LabEx CSC), the University of Strasbourg (UdS), the University of Paris Diderot


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(Sorbonne Paris Cité) and the Institut Universitaire de France (IUF) for financial supports. This work was also supported by a doctoral fellowship of the Chinese Scholarship Council (Y.X.). We also thank the Laboratoire Léon Brilloin (LLB, CEA, Saclay, France) for beamtimes allocation and M. Heinrich (University of Strasbourg) for assistance with CD experiments.

SUPPORTING INFORMATION Synthetic protocols for BU-6AA, BU-7AA, BU-Cy3 and BU-Cy5 and their corresponding 1H NMR spectra, characterization of all synthesized products, FT-IR, CD and viscosimetry experiments, supplementary AFM and TEM imaging, supplementary UV-Vis-NIR and fluorescence spectroscopy experiments, additional data on scattering experiments (SANS, SLS and DLS). This information is available free of charge via the Internet at http://pubs.acs.org.

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