Early Stages of Formation of Branched Host−Guest Supramolecular

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Early Stages of Formation of Branched Host-Guest Supramolecular Polymers Luciano Galantini,*,†,‡ Aida Jover,§ Claudia Leggio,†,‡ Francisco Meijide,§ Nicolae Viorel Pavel,†,‡ Victor Hugo Soto Tellini,§ Jose´ Va´zquez Tato,§ and Cristina Tortolini† Dipartimento di Chimica and Research center SOFT-INFM-CNR, Sapienza UniVersita` di Roma, P.le A. Moro 5, 00185 Roma, Italy, and Departamento de Quı´mica Fı´sica, Facultad de Ciencias, UniVersidad de Santiago de Compostela, AVda. Alfonso X El Sabio s/n, 27002 Lugo, Spain ReceiVed: April 22, 2008

A structural characterization of host-guest supramolecular copolymers, formed by an adamantane dimer and two β-cyclodextrin trimers in aqueous solution, has been carried out by combining small angle X-ray scattering and light scattering experiments. A shape-reconstruction method was applied to the SAXS data to obtain relatively high-resolution conformation information, and a correlation with the experimental dynamic light scattering results was performed, by estimating the hydrodynamic radii of the reconstructed shape through a shell model method. When applied on the solutions of the trimers, the analysis provides a globular reconstructed shape with a hydrodynamic radius in agreement with the experimental one. For the polymers, elongated structures were inferred which grow both in length and in cross section by increasing the concentration. Depending on the β-cyclodextrin trimer employed in the polymer preparation, polymerization degrees ranging between roughly 7 and 14 or 9 and 22 were obtained in the concentration range 4.00-10.0 or 3.10-6.60 mM of the trimer (6.00-15.0 or 4.65-9.90 mM of the dimer). Aggregation schemes were proposed accounting for the formation of hyperbranched, linear, and network like polymers. The experimental results are not far from those expected on the basis of the aggregation in hyperbranched structure, for which the growth of elongated aggregates can be predicted in the early stages of the polymerization. However, the coexistence of the other structures, in particular of the linear one, cannot be ruled out. Introduction Obtaining sophisticated structure by assembling molecules through noncovalent interactions represents a fundamental topic in supramolecular chemistry.1,2 In this field, a lot of work has been focused on the preparation of supramolecular polymers, which are generated by a directional and reversible connection of monomers. For this purpose, different intermolecular interactions have been used as hydrogen bonding,3–5 nucleobase pair interactions,6 interprotein heme heme pocket binding,7 and host-guest complexation of crown ether/organic salt8–11 or cyclodextrin/hydrophobic molecule systems.12–30 In particular, during the past few years, data on several host-guest linear supramolecular polymers, exploiting the hosting properties of cyclodextrins, have been published. Polymers have been obtained either by mixing unimers carrying complementary units, i.e., host and guest sites,17–24 or complementary monomers having two interacting host or guest moieties.25–29 Recently, moreover, branched polymers have been prepared by mixing tritopic host and ditopic guest derivatives.24,29,30 Although very few papers have been published on this subject, these structures seem to be very interesting either because they are particularly new or because of the applicative importance of molecules with similar structure (dendrimers). In the light of this interest, in this work, we reported preparation and characterization of two of these polymers. The structure of the polymers was studied by combining small angle X-ray scattering (SAXS) and static and dynamic light * Corresponding author. Telephone: (+39)-06-49913687. Fax: (+39)06-490631. E-mail: [email protected]. † Dipartimento di Chimica, Universita ` di Roma “La Sapienza”. ‡ Research center SOFT-INFM-CNR, Universita ` di Roma “La Sapienza”. § Universidad de Santiago de Compostela.

scattering (SLS and DLS, respectively) measurements. The supramolecular polymers were prepared by mixing the two trimers βCD3A and βCD3B, and the adamantane dimer Ad2, of Figure 1, thus exploiting the highly favorable interaction between the adamantyl group and the β-cyclodextrin (βCD) cavity.19,27,29,31,32 Besides the conventional analysis of the scattering data, the 3D reconstruction of the electronic density distribution was performed from the SAXS spectra, to visualize the structure of the scattering particles. Moreover, the hydrodynamic radii were estimated, from the shape of the electronic density distribution, and compared with the experimental ones. The interpretation procedure was first checked on the single βCD trimers and finally used to study the supramolecular aggregates. Experimental Section Materials. Syntheses of Ad2 and βCD3B have been reported elsewhere.29 The βCD3A synthesis is reported as Supporting Information. The solutions of the polymers Ad2-βCD3A and Ad2βCD3B were prepared by mixing Ad2 with βCD3A and βCD3B, respectively, at molar ratios of 3:2 (dimer:trimer). A 8 wt% of water was determined by thermogravimetric measurements, performed with a Netzsch STA 409 PC Luxx simultaneous thermal analyzer, on the solid samples of βCD3A and βCD3B. This water fraction was considered in the preparation of the solutions. All the samples were prepared in 150 mM sodium azide in order to prevent mold growing and to guarantee some ionic concentration. SLS and DLS Measurements. A Brookhaven instrument constituted by a BI-2030AT digital correlator with 136 channels and a BI-200SM goniometer was used. The light source was a Uniphase solid-state laser system model 4601 operating at 532

10.1021/jp803496q CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

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Figure 1. Structures of the precursors.

TABLE 1: Some Parameters Estimated by Light Scattering and SAXS and Formula Weight (M) of the βCD Trimersa

βCD3A βCD3B

Rapp (Å)

MSLS (kg mol-1)

Rg (Å)

MSAXS (kg mol-1)

Rhydro (Å)

M (kg mol-1)

16.5 16.5

3.4 3.4

11.1 11.2

3.5 3.7

17.1 16.7

3.539 3.564

a The estimated standard deviations are within 0.3 Å (Rapp, Rg) and 0.3 kg mol-1 (MSAXS and MSLS).

nm. Dust was eliminated by means of a Brookhaven ultrafiltration unit (BIUU1) for flow-through cells, the volume of the flow cell being about 1.0 cm3. Nuclepore filters with a pore size of 0.1 µm were used. The samples were placed in the cell for at least 30 min prior the measurement to allow for thermal equilibration. Their temperature was kept constant within 0.5 °C by a circulating water bath. In the DLS experiments, the intensity-intensity autocorrelation function was measured, at a particular value of the scattering vector q, and related to the normalized electric field autocorrelation function g1(q,τ) by the Siegert relation. Therefore, g1(q,τ) was analyzed through the cumulant expansion, and the so-called apparent diffusion coefficient Dapp was obtained from the first cumulant. From the Dapp value an apparent hydrodynamic radius Rapp was calculated by the Stokes-Einstein equation. As a check, an analysis by CONTIN of g1(q,τ) was also performed for verifying multimodal distributions. In the SLS measurements, an apparent molecular weight MSLS was estimated from the excess Rayleigh ratio ∆R by means of the equation

cK 1 ) ∆R MSLS where c is the solute concentration (g mL-1) and K is a constant that depends on the solvent refractive index, the solution refractive index increment and the laser wavelength. The refractive index measurements were performed by an ATAGO differential refractometer model DD7. The observed excess Rayleigh ratios and the apparent diffusion coefficients did not

depend on the exchanged wave vector in the scattering angle range 30-150° under our experimental conditions; therefore, only the results at 90° were analyzed. SAXS. SAXS measurements were carried out in a thermostated (25.0 ( 0.1 °C) quartz capillary of 1 mm by using a Kratky compact camera, containing a slit collimation system, equipped with a NaI scintillation counter. Ni-filtered Cu KR radiation (λ) 1.5418 Å) was used. Scattering curves were recorded within the range 0.012 e q e 0.5 Å-1. The moving slit method was employed to measure the intensity of the primary beam. The collimated scattering intensities were put on an absolute scale, subtracted for the solvent and the capillary contributions and then expressed in electron units, e.u. (electrons2 Å-3) per cm primary-beam length.33,34 In terms of total scattering cross section of an ensemble of particles, one e.u. corresponds to 7.94056 × 10-2 cm-1.35 The indirect Fourier transform method developed in the ITP program was used for interpreting the spectra.36 From the desmeared I(q) curve the zero angle intensity I(0) was obtained and a molecular weight MSAXS was inferred by

I(0) ) cMSAXS∆F2 where ∆F is the electron density difference between the particle and the solvent. For each sample the ∆F value was estimated on the basis of the molecular volumes of ref 37. Moreover, by neglecting the particle interaction, the electron pair distribution function p(r) was inferred according with the equation

I(q) )

dr ∫0∞ p(r) sin(qr) qr

The p(r) function is strongly dependent on the shape and size of the scattering particles and vanishes at the maximum electron pair distance within the particle. Furthermore, it allows the determination of the electronic radius of gyration Rg.36 For rod like particles, the scattered intensity can be expressed in terms of a function Ic(q), which is related only to the cross section I(q) ) Lπ/qIc(q), where L is the length of the rod. From

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the innermost part of the Ic(q) curve it is possible to obtain the radius of gyration of the cross section, Rc, as34

(

Ic(q) ) Ic(0) exp -

q2Rc2 2

)

Shape reconstructions were performed by using the program GA_STRUCT,38 which provided, for each analyzed SAXS spectrum, a consensus envelope, reproducing the 3D shape of the scattering particle. For the trimers, a q range of 0.02-0.3 Å-1 was employed, and a number of 1000 spheres was used for the consensus envelope. For the polymers, a population of 4000 spheres was used, and the minimum q value was changed as a function of the particle maximum distance. Results and Discussion Monomers. The behavior of Ad2 in aqueous solution and the effect of the complexation with βCD have been reported in a previous paper.29 In particular, SAXS and light scattering measurements demonstrated that Ad2 molecules self-assemble into big aggregates, which are disrupted in the presence of βCD, because of the host-guest interaction between βCD and the adamantyl group, leading to the formation of the Ad2(βCD)2 complex. Some light scattering and SAXS results, on the solutions of the two βCD trimers, are reported in Table 1. In both cases, the molecular weights, estimated with the two techniques, are very similar to the value obtained from their formulas, thus pointing out that no self-assembly is given by the host trimers. Similar Rg and Rapp values are also obtained, both consistent with the monomeric form. Quite globular 3D reconstructed images (Figure 2) were inferred from the SAXS curve, as expected for a rough representation of the structure of the trimers. It is important to notice that, in the trimer size range, GA_STRUCT provides only a qualitative reproduction of the shape. As a matter of fact, for this program the shape reconstruction is mainly based on the interpretation of the I(q) values up to q ) 0.3 nm-1, and, in the case of the trimers, the I(q) curves are significantly different from zero even beyond this limit. To compare SAXS and DLS results the hydrodynamic radii (Rhydro) were estimated for the reconstructed shapes by using the program HYDROPRO, based on a shell model.39–41 A good agreement with the experimental values (Rapp) was obtained (Table 1), although, as mentioned above, it is expected that the consensus envelope is unable to represent the details of the molecules. Polymers. Some SAXS and light scattering parameters of the polymeric solutions are reported in Figure 3 and in Table 2. Although particle interaction can affect the reported parameters, the remarkable increase of the measured molecular weight and radii unambiguously points out that a significant polymer growth is observed with increasing concentration, especially for the Ad2-βCD3B polymer. This behavior is different from the one shown by the solution of linear homologous polymers, where a constant polymerization degree was observed as a function of concentration.26,27,29 The strongly asymmetric shape of the p(r) functions, extracted from the SAXS spectra, clearly shows that anisotropic aggregates are formed. This result is confirmed by the shape reconstruction images showing elongated distribution of the particle electron densities (Figures 4 and 5). Obviously, a particle interaction effect is expected to bias these results. However, this effect is negligible for the most diluted samples whose SAXS spectra can be exclusively related to the single particle

Figure 2. SAXS spectra, p(r) functions and 3D reconstructed images of the β-cyclodextrin trimers.

Figure 3. Parameters obtained by SAXS (open symbols) and light scattering (full symbols) for Ad2-βCD3A (squares) and Ad2-βCD3B (circles) polymeric solutions as a function of concentration.

structure. The particle interaction effect, should increase and sensitively affect the p(r) curves, by increasing the particle volume fraction. Actually, it can be noticed that our curves preserve the same peculiar shape at all the concentrations and only show a variation of the maximum distance value, which parallels the polymer growth. This suggests that the spectra, and the extracted structural information, are poorly affected by particle interactions for all the studied samples.

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TABLE 2: Parameters of Figure 3 together with the Polymerization Degree, N (the Repetitive Unit Being Constituted by 1 βCD3 and 1.5 Ad2 Molecules); the Hydrodynamic Radii Estimated for the GA_STRUCT Consensus Envelopes by HYDROPRO, Rhydro; the Volumes Estimated from ref 37, Vm, GA_STRUCT, VG, and GA_STRUCT after Rescaling, VGRE, of the Supramolecular Polymersa

Ad2-βCD3A Ad2-βCD3B

c (g L-1)

Rapp (Å)

Rg (Å)

Rc (Å)

MSAXS (kg mol-1)

MSLS (kg mol-1)

17.8 29.8 44.7 14.9 19.4 31.5

40.0 44.2 55.5 41.0 71.0 128.0

47.1 60.2 65.1 49.6 77.9

10.9 12.8 14.7 13.8 22.2

33.9 51.4 59.5 40.5 99.1

31.5 39.9 49.0 37.6 92.2 176.2

N

Vm (nm3)

VG (nm3)

VGRE (nm3)

Rhydro (Å)

7.6 11.6 13.4 9.1 22.2

40.1 60.7 70.3 48.6 118.5

87.9 151.6 203.0 119.5 425.0

83.4 128.8 203.0 104.0 395.2

41.9 55.6 56.1 46.7 75.5

a The estimated standard deviations are within 0.3 Å (Rapp, Rg, Rc), 0.3 kg mol-1 (MSAXS, MSLS), 0.9 (N) and 5 nm3 (Vm). Some parameters of the Ad2-βCD3B polymer at the highest concentration were not derivable from the SAXS spectrum, being the maximum size of the polymer larger than the SAXS detectable limit size.

Figure 5. SAXS spectra, p(r) functions and 3D reconstructed images of the Ad2-βCD3B supramolecular polymer at different concentrations.

Figure 4. SAXS spectra, p(r) functions and 3D reconstructed images of the Ad2-βCD3A supramolecular polymer at different concentrations.

From the ln(I(q) · q) vs q2 plot a linear region was recognized in the SAXS spectra, which allowed the determination of the cross section gyration radii (Rc) of the aggregates. As reported in Table 2, a significant increase of the Rc values accompanies the polymer growth. The molecular weights, inferred from the extrapolated forward scattering intensities of the SAXS spectra, were used to estimate the polymerization degree N by assuming a ratio of 3:2 (dimer:

trimer) for the stoichiometry of the polymer. Therefore, the volume of the aggregates (Vm) was estimated on the basis of the atomic volumes.37 All of these parameters are reported in Table 2 together with the volumes provided by GA_STRUCT for the 3D reconstructed shape (VG). It is known that, when scattering spectra of systems of particles with different structures are investigated, the recostructed shape provided by GA_STRUCT represents a superposition of all of the particle shapes in the sample. This means that the volume of the reconstructed shape is significantly larger than the single particle volume and that the hydrodynamic radii calculated for the consensus envelope by HYDROPRO are overestimated.42 In fact, we observe that on the average Rhydro is slightly greater than Rapp, the difference being very small, which suggests that a low

8540 J. Phys. Chem. B, Vol. 112, No. 29, 2008 conformational polydispersity can be hypothesized for our polymers (Table 2). In other terms, it means that the polymers are quite rigid. Conversely, VG values significantly larger than the Vm values are observed, which indicates that the aggregate structure is largely water permeated. The consequence of the previous considerations is that, when a system of conformationally polydisperse particles is investigated, a reliable Rhydro value must be estimated on a reduced consensus envelope. We observed, in particular, that a value in good agreement with the experimental hydrodynamic radius is obtained, if the consensus envelope is reduced up to the volume of the single particle, keeping invariant its shape. In this framework, we searched the agreement between experimental hydrodynamic radii and those estimated for the reconstructed shape, by reducing the volume of the consensus envelope to an agreement volume VGRE. Actually, despite the volume reduction, we observe that the consensus envelope volumes remain larger than the Vm values (Table 2). Reasonably, the difference between the two values is due to the hydration; therefore, a hydration volume VH ) VGRE - Vm, and a hydration number per repetitive unit nh ) VH/(N · VH2O) (VH2O ) 30 Å3 being the single hydration water molecule volume) can be estimated for each polymer. When applied to the most diluted samples, these concepts allow the determination of nh values equal to 209 ( 19 and 204 ( 19 for the Ad2-βCD3A and Ad2-βCD3B polymers, respectively, thus showing a quite consistent hydration of the supramolecular aggregates. This conclusion is in agreement with the large number of hydration water molecules observed in crystal structure of cyclodextrin based supramolecular polymers.19 Because this discussion is based on the apparent values of hydrodynamic radii, we decided not to extend it to more concentrated samples. The reason is that, in general, at high concentration, the Rapp values can be sensitively affected by interactions. Both direct and hydrodynamic interaction must be considered, as a matter of fact, although we observed that the effect of direct interactions is negligible on the SAXS q range, it could be significant in the light scattering q region and could affect the DLS data, as well as the hydrodynamic interactions. As mentioned in the Introduction, the data so far reported in the literature for the systems investigated in this paper suggest that they form branched supramolecular aggregates. Actually, the results of this work show that aggregates with an elongated shape are formed and seem not to fit a branched supramolecular aggregation. The picture describing some possible aggregation schemes is shown in Figure 6. The aggregation model which is expected to be obeyed, to explain the formation of hyperbranched structure, implies that the monomers are joined as in the scheme a. In this framework, it must be stressed however that, under the hypothesis that the aggregates are generated by a completely random connection among the monomers, the formation of elongated structures is not astonishing in the early stages of the aggregation. As a matter of fact, if we try to build the aggregate by joining the repetitive units in all the possible arrangements, it is easy to demonstrate that only linear structure can be formed up to an aggregation number of 3. The presence of ramified arrangements starts with the polymer of 4 repetitive units, where a ratio linear:ramified isomers of 4:1 is expected, and grows with the polymerization degree. This model justifies the formation of elongated polymers, with an increasing cross section, in the early steps of the growth, in agreement with our results, and predicts the generation of structures with recognizable branches only at large aggregation numbers. The latter are not observed in our samples, although for the largest aggregate, whose spectrum can be interpreted by GA_STRUCT

Galantini et al.

Figure 6. Polymerization schemes of βCD trimers (βCD3) with the Ad2 dimer.

(Ad2-βCD3B 19.3 g L-1 solution), the 3D reconstructed image starts to deviate from the worm like shape and to show consistent inhomogeneities of the cross section along the elongation direction. To complete the interpretation framework, the possibility that the linear and network structures of Figure 6, schemes b and c, are also formed, must be considered. Being more ordered than those of scheme a, reasonably these arrangements are less favored on the basis of statistic considerations. However, these structures could be stable, being characterized by a large number of host-guest linkages and, therefore, by a low number of free host and guest groups. This is particularly true for the linear arrangement, where the lowest ratio between the unsaturated host and guest groups and the aggregation number is realized. In view of these considerations, it is possible that both the linear and network structures are present with a significant fraction in solution. In particular, a consistent amount of the linear polymer is expected which could contribute to the elongated shape of the 3D reconstructed images. It is important to remember that the observed growth with concentration is not obvious for solutions of linear supramolecular polymers. As a matter of fact, very often, small oligomers are formed that do not show any concentration dependence. On the basis of the explanations so far proposed, the formation of closed structures or coiled conformation, trapping the terminal host, are responsible for this behavior, and the growth is observed only when rigid polymers are prepared that are unable to adopt small stable cyclic or shrunk conformations.26,27 It has been observed in particular that small not growing oligomers are formed when linear polymers are prepared by mixing Ad2 and βCD dimers. With respect to the polymer formed in that case (ditopic host and guest monomers) the linear polymer of Figure 6b shows an increase of the average number of the host-guest linkages between adjacent monomers, which could be responsible of the polymer growth. An increase of the polymer rigidity could be the cause. Actually, both the network like and the linear aggregation alone are unable to explain the experimental results. As a matter

Early Stages of Polymer Formation of fact, they suggest a growth which is bidimensional, in the first case, and linear with a constant cross section, in the second one, and both clash with the observed growth, where an increase of both the length and the cross section of elongated structures occurs. On the contrary, as previously mentioned, the experimental results are not far from those predicted on the basis of the aggregation in branched structures (Figure 6a), in the early stages of the polymerization. However, the agreement between the experimental results and the prediction becomes questionable for the largest aggregation number, where still well defined branches in the 3D reconstructed shapes are not observed. Reasonably, the best explanation of the experimental data is obtained if the coexistence of two or more of the structures of Figure 6 is hypothesized. For example, the coexistence of linear and dendritic structures (Figure 6, schemes a and b, respectively) could explain either the marked anisotropy of the aggregate shape at all the concentration or the sensitive concentration induced growth of the aggregate cross section. It is important to remark, finally, that the polymers reported in Figure 6 represent simplified and limit models. Reasonably, hybrid structures are formed in solution where the three limit arrangements are realized in different regions. In any case, it is expected that they evolve to more isotropic and branched structures when very big aggregates are formed. Conclusions A detailed structural study was carried out, by combining SAXS and light scattering experiments, on host-guest supramolecular copolymers formed by adamantane dimers and β-cyclodextrin trimers in aqueous solution. A shape reconstruction method was applied to the SAXS data to obtain relatively high resolution conformation information. A correlation with the experimental dynamic light scattering results was performed, by estimating the hydrodynamic radii of the reconstructed shape by a shell model method. From a preliminary analysis of the trimer solutions, globular shapes were obtained with hydrodynamic radii in agreement with the experimental ones. For the polymers, elongated structures were inferred. A growth is observed by increasing the concentration involving an increase of both the length and the thickness of the aggregates. Different aggregation models (or basic structures that probably coexist in solution) have been proposed to explain the experimental results. Acknowledgment. We thank Prof. W. T. Heller for graciously supplying the GA_STRUCT code and Dr. Alessandro Latini for the thermogravimetric measurements. We also thank for the Spain-Italy Integrated Action grants. The authors from USC thank the Ministerio de Ciencia y Tecnologı´a (Project MAT2004-04606)andXuntadeGalicia(PGIDIT05PXIC26201PN) for financial support. The authors from Sapienza Università di Roma thank the MIUR financial supports (PRIN Project 2006 039789-001). Supporting Information Available: Synthesis of βCD3A. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ciferri, A. Macromol. Rapid Commun. 2002, 23, 511.

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