Using Inclusion Complexes with Cyclodextrins To Explore the

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Using Inclusion Complexes with Cyclodextrins to Explore the Aggregation Behavior of a Ruthenium Metallosurfactant Nerea Iza, Andrés Guerrero-Martínez, Gloria Tardajos, Maria José Ortiz, Eduardo Palao, Teresa Montoro, Aurel Radulescu, Cecile A. Dreiss, and Gustavo González-Gaitano Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504929x • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Using Inclusion Complexes with Cyclodextrins to Explore the Aggregation Behavior of a Ruthenium Metallosurfactant Nerea Izaa, Andrés Guerrero-Martíneza, Gloria Tardajos*,a, María José Ortizb, Eduardo Palaob, Teresa Montoroc, Aurel Radulescud, Cécile A. Dreisse, Gustavo González-Gaitano*,f a Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain b Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain c EUIT Forestales, Universidad Politécnica de Madrid, 28040 Madrid, Spain d Jülich Center for Neutron Science, JCNS Outstation at MLZ, Forschungszentrum Jülich GmbH, Lichtenbergstraße 1,85747 Garching, Germany e Institute of Pharmaceutical Science, King’s College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, UK f Departamento de Química y Edafología. Facultad de Ciencias, Universidad de Navarra, 31080, Pamplona, Spain

ABSTRACT The aggregation behaviour of a chiral metallosurfactant, bis(2,2′-bipyridine)(4,4′-ditridecyl-2,2′bipyridine)ruthenium(II) dichloride (Ru24C13), synthesized as a racemic mixture was characterized by small-angle neutron scattering, light scattering, NMR and electronic spectroscopies. The analysis of the SANS data indicates that micelles are prolate ellipsoids over the range of concentrations studied, with a relatively low aggregation number, and the micellization takes place gradually with increasing concentration. The presence of cyclodextrins (β-CD and γ-CD) induces the break-up of the micelles and helps to establish that micellization occurs at a very slow exchange rate compared to the NMR time scale. The open structure of this metallosurfactant enables the formation of very stable complexes of 3:1 stoichiometry, in which one CD threads one of the hydrocarbon tails and two CDs the other, in close contact with the polar head. The complex formed with β-CD, more stable than the one formed with the wider γCD, is capable of resolving the ∆ and Λ enantiomers at high CD/surfactant molar ratios. The chiral recognition is possible due to the very specific interactions taking place when the β-CD

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covers - via its secondary rim - part of the diimine moiety connected to the hydrophobic tails. A SANS model comprising a binary mixture of hard spheres (complex + micelles) was successfully used to study quantitatively the effect of the CDs on the aggregation of the surfactant.

INTRODUCTION In contrast to traditional cationic surfactants, metallosurfactants are surface-active molecules that integrate a polar transition-metal complex as the headgroup of the amphiphilic structure1. These novel surfactants have gained attention because of their interesting applications in diverse subject areas such as colloidal templating 2 , development of luminescent devices 3 , or optoelectronics 4 . In particular, complexes bearing the [Ru(bipy)3]2+ (where bipy = 2,2´bipyridine) moiety as building block are of wide interest due to their characteristic photophysical properties5. Thus, they offer significant advantages such as long-lived luminescent excited states, high chemical and photochemical stability and tunability of the excited-state energies 6 . Moreover, they can be employed as energy donor or acceptor units in energy transfer processes within colloidal systems7. Therefore, their upgrading to surface-active molecules can improve their applications in aqueous media, such as alternative to organic dyes as luminescent probes in the analysis of micelles and proteins 8 . However, the unusual architectures of [Ru(bipy)3]2+ metallosurfactants, which contains one alkylated bipyridine ligand accounting for the hydrophobic region, while the other two bipyridines are unsubstituted, imparting

suitable

counter-ions solubility in water to the bulky ruthenium compound, lead to highly complex micellization processes9. Indeed, the formation of aggregates with different morphologies has been reported, depending for instance on the position in which the bypyridine ligand is substituted or on the variation in alkyl chain number and length 10 . Additionally, when the metallosurfactant possesses several long linear chains, the formation of inverted micelles has also been observed in organic solvents 11 . All these features make the understanding of


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metallosurfactants micellisation quite difficult in comparison with conventional ionic surfactants. Microencapsulating effect of cyclodextrins (CDs) as organic hosts on the inherent properties of surfactants as guest molecules have been studied to understand micellisation processes12. These macrocycles are cyclic oligosaccharides formed by units (six, seven, or eight) of α-D-(+)glucopyranose named α, β, or γ−CD, respectively. They have the shape of a hollow truncated cone, with a hydrophobic cavity and two hydrophilic rims in which the primary and secondary OH groups are inserted (SI. Scheme 1). There are several factors that influence the complexation of CDs, such as the relief of conformational strain of the macrocycle, the release of high-energy water from the cavity, hydrophobic interactions, and induction and dispersion forces 13 . Therefore, these macrocycles are prime host candidates to understand the processes involved in the micellisation of surfactants due to the competition between colloidal aggregation and supramolecular complexation. In addition, CDs can bind with different affinity to molecular enantiomers and have thus been investigated from the viewpoint of their chromatographic recognition properties through different types of spectroscopies, such as UV-vis-NIR absorption and fluorescence, and in particular nuclear magnetic resonance (NMR)14. Due to the molecular recognition capacity, CDs are most popular as chiral stationary phases in HPLC and CE 15 . Within this context, the chemical synthesis of [Ru(bipy)3]2+ metallosurfactants lead to mixtures of octahedral ∆ and Λ enantiomers16, which have shown different amphiphilic properties upon micellisation with respect to the racemic mixture 17. Therefore, CDs may offer an interesting handle to investigate [Ru(bipy)3]2+ metallosurfactants enantiomers through supramolecular complexation. Although the formation of complexes between metallosurfactants and CDs has been demonstrated recently18, the chiral recognition of such enantiomers and its relation with the micellisation process has not been clarified to date. In this study, we have focused on a chiral metallosurfactant, bis(2,2′-bipyridine)(4,4′-ditridecyl2,2′-bipyridine)ruthenium(II) dichloride (Ru24C13), synthesized as a racemic mixture, and fully


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investigated its micellisation properties and the effect that the presence of cyclodextrins (β-CD and γ-CD) have on its aggregation behaviour by scattering and spectroscopic techniques. The ability of native CDs to shift the cmc of the metallosurfactant is envisaged as a method to reveal the time scale on which micellisation is taking place. In addition, the chiral nature of these macrocycles permits the enantiomeric resolution of the racemic, by taking advantage of the very specific interactions occurring when the inclusion complex forms. MATERIALS AND METHODS Synthesis of bis(2,2′-bipyridine)(4,4′-ditridecyl-2,2′-bipyridine)ruthenium(II) dichloride (Ru24C13) The first step consisted in the synthesis of the 4,4´-ditridecyl-2,2´-bipyridine ligand, following the procedure described by Bowers et al10. Following this, the complex precursor, cis[RuCl2(bipy)2], was heated under reflux in 2-propanol and water with the alkylated ligand obtained in the first step, according to the standard method described by Yousif and Seddon19. The resulting product was purified twice by column chromatography eluting the first column with 2-propanol and the second with 1:1 isopropanol and water. In each case, the red band was collected and evaporated to dryness. The final product was dried in an oven at 80 °C to give a red-orange crystalline solid (SI. Scheme 1). The resulting compound was characterised by mass spectrometry (MS), high resolution mass spectrometry (HRMS) and FAB (fast atom bombardment) in a Thermo Fisher MAT 95 XP. MS m/z (%): 520.4 (M+ -(bipy)2Ru, 91 %); 365.3 (100 %); 156.1 (65 %). FAB m/z (%): 934.5 (M+, 60 %); 611.7 (100 %); 613.7 (54 %); 593.8 (56 %). Due to the extremely rapid and easy fragmentation of the compound, HRMS gives the mass of the fragment (4,4-ditridecyl-2,2-bipyridine). HRMS (EI) calculated for C36H60N2 520.4756; found 520.4753. Fluorescence and UV-vis studies. Steady state fluorescence spectra were recorded using an AMINCO Bowman Series 2 spectrometer, with 4.0 nm bandwidth for excitation and emission,


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by using either 1 or 0.1cm quartz cells. The temperature of the cuvette was controlled by recirculating water from a water bath at 25±0.1 ºC. UV absorption spectra were registered using an UVICON XL (Bio-Tex Instruments) spectrometer. The excitation and emission slits were fixed at 2 nm by using 1, 0.5, or 0.1cm quartz cells, depending on the concentration range utilised. All the solutions were freshly prepared. Nuclear Magnetic Resonance. Monodimensional 1H-NMR and 2D-ROESY experiments spectra were recorded at 298 K on a Bruker AVIII 700 spectrometer. Samples were prepared in D2O (Carlo Erba > 99.90% in D), using the residual HDO signal as the reference20. Dynamic light scattering (DLS) DLS measurements were performed at a scattering angle of 90º using a DynaPro-MS/X photon correlation spectrometer. The size distributions were obtained from the intensity autocorrelation function by regularization analysis with DynaLS 2.0 software. The temperature of the samples was controlled with a built-in Peltier in the cell compartment and set at 25.0 ± 0.1 ºC unless otherwise stated. Prior to the measurements, the samples were filtered with 0.02 µm Anotop syringe filters onto glass cuvettes washed with deionized water and dried with compressed air to remove dust. Small-Angle Neutron Scattering Small-angle neutron scattering (SANS) experiments were performed on the instrument KWS-2 at the Jülich Centre for Neutron Science (JCNS), Münich, Germany. Incidental wavelengths of 4.97 Å were used with detector distances of 1.7, 7.6 and 19.6 m to cover a q-range from 0.007 to 0.3 Å-1. The temperature was controlled with a Peltier system with an accuracy of 0.1ºC. Solutions of the surfactants were prepared by mass and measured in 1 mL quartz cells (Hellma) with a path length of 2 mm, using D2O as the solvent (Aldrich > 99.9% in D, and Armar chemicals > 99.8% in D). Data analysis of the SANS curves was performed with Sasview 2.2.1 (ref. 21) by using different models, as described in the Results section.


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RESULTS AND DISCUSSION Aggregation of the metallosurfactant The absorption spectrum of a 5.9×10-6 M water solution of Ru24C13 shows three clear maxima at approximately 243, 286 and 458 nm, the latter as a result of the overlap of two bands (SI. Fig. 1A). The absorption band with a maximum at 458 nm is due to the spin-allowed singlet metal-toligand charge transfer (1MLCT) transition of the complex22. The bands at the UV are mostly π-π transitions involving the bipy ligands. No significant shifts in the position of the bands are detected by varying the concentration of the metallosurfactant within the range between 1×10-7 and 1×10-3 M. However, a slight loss of structure is perceived in the visible zone of the spectrum when increasing the concentration (Fig. 1A, insert), which can be attributed to aggregation. By plotting the absorbance at 458 nm as a function of concentration of Ru24C13, a change in the slope appears around 5×10-6 M, which corresponds to the critical micelle (cmc) of the metallosurfactant (SI, Fig. 1B). This allows the estimation of the molar absorptivity of the surfactant in its monomer and micelle form as ε = (8.8 ± 0.15) ×103 L·mol-1·cm-1 and ε = (1.192 ± 0.008) ×104 L·mol-1·cm-1, respectively. The study of the fluorescence of Ru24C13 corroborates the results obtained by UV-vis spectroscopy and provides complementary information on the aggregation process. Figs. 1A and 1B show the emission spectra and position of the maximum in emission as a function of the concentration of surfactant. This intense emission, which is characteristic of the triplet MLCT (3MLCT) transition in [Ru(bipy)3]2+ complexes7 shifts towards blue with the concentration, reaching λmax a minimum around 5×10-6 M. The emission occurs at longer wavelengths as the concentration is low, and the λmax is close to the value of the Ru(bipy)32+ (ref. 7), indicating that the surfactant is in its monomer form and the minimum ascribed to the cmc. Above this value the emission spectrum shifts towards longer wavelengths (red shift), while the shape of the band is 6 ACS Paragon Plus Environment

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conserved over the whole range of concentrations studied. The highest shift observed is 15 nm at [Ru24C13] = 7.6×10-4 M. A red-shift can be interpreted as a better stabilization of the excited state of the cationic head bearing the bypiridine moieties, which would occur when the micelles are fully formed. Structure of Ru24C13 micelles In spite of having two tails, which would tend to favour structures with low curvature such as cylinders or bilayers, the head group is also quite bulky, and

tris(bipyridine)ruthenium

derivative metallosurfactants are in fact known to form spherical and ellipsoidal micelles10,17, depending on which position of the bypyridine ligand the alkyl chains are substituted. SANS were used here to elucidate the shape and internal structure of the micelles. The neutron scattering curves of the metallosurfactant have been recorded at three different concentrations (1, 2.5 and 4 mM), shown in SI. Fig. 2 with the whole range of q. The middle-to-high q regions (Fig.2) present typical features of interacting, spherical particles. At low q however the very high scattering intensity suggests the presence of large aggregates, which scale with surfactant concentration. In order to gain more insight into these aggregates, we performed DLS experiments on a 1 mM solution of the metallosurfactant in D2O, filtered through a 0.45 µm filter. The distribution of hydrodynamic radii (SI. Fig. 3) shows two relaxation modes, a fast one ascribed to the micelles, and a slower one, corresponding to particles of ca. 146 nm radius and highly polydisperse, which corroborates the presence of unspecific aggregation. Both modes are very different in size but comparable in intensity, which indicates that the volume fraction of the aggregates is very small compared to that of the micelles, an important point to consider in fitting the SANS curves. As a rough estimation, if we consider that both the micelles and aggregates are spheres of nearly the same density, the ratio of the scattered intensity assigned to each mode is proportional to31


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Im w = m I agg wagg

 Rm  R  agg

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   

3

Eq.1

where I is the measured intensity, w the mass fraction, R the hydrodynamic radius of the scattering objects, and the indices m and agg correspond to the micelles and the aggregates, respectively. By substituting the parameters obtained from the DLS analysis, the mass fraction of the aggregates is about (4×10-6%) of that of the micelles. This value may be higher if the aggregates are less compact (the exponent in Eq. 1 would then be lower than 3), but, overall, the volume fraction of the aggregates is likely to be negligible compared to the micelles. We have simulated SANS curves under these conditions for a system consisting of polydisperse spheres (PI=2.1) of 146 nm and scattering length density 1.02×10-6 Å-2 (calculated value for the surfactant), interacting with a hard sphere (excluded volume) potential. The simulated curves are compared to the experimental one for a 4 mM concentration of the surfactant (Fig. 2A). The scattering from the aggregates, because of their large size, quickly reaches background level and is only detectable above 0.01 Å-1 for volume fractions over 1×10-5 (0.001%), a value much higher than the one estimated by DLS. Hence, in the following, we restrict our analysis of the scattering curves to the q-region above q = 0.015 Å-1. The other main feature of the SANS curves is the presence of a peak around 0.03 Å-1, whose intensity increases with concentration, due to strong Coulombic interactions between the micelles. For fitting purposes, this implies the need of including an interparticle structure factor, S(q), describing the electrostatic interactions. An appropriate choice for this may be the Hayter-Penfold model for macroions, commonly use to model ionic micelles or charged particles in solution 23 . The model uses a Mean Spherical Approximation (MSA) closure relation to provide an analytical solution to the Ornstein-Zernike equation, by introducing a screened Coulomb potential. The main fitting parameters include the surface charge of the macroion and the Debye-Hückel screening length, κ-1, which is related to the ionic strength and describes the decay of the electrostatic potential. The ionic strength 8 ACS Paragon Plus Environment

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depends on the number of free ions in solution, which in turn is connected to the bound fraction of counterions at the surface of the macroion. This usually implies the determination of κ by iteration with respect to the overall micelle charge for a specific concentration, the fit being quite sensitive to the concentration of surfactant, particularly under dilute conditions. We have considered here instead an interparticle structure factor based on a square well potential (SW),24 which appears to be a good compromise between a more robust S(q) and a reasonable number of fitting parameters for our system. This structure factor is also deduced from the MSA closure relation and, as it is defined, may account for either attractive or repulsive interactions, depending on the sign of the well-depth (expressed in terms of kT units), while its width (in 2R units, R being the radius of the interacting spheres) is an indirect measure of the screening of the charges on the micellar surface. For the shape factor, P(q), the most obvious option for a micelle is usually a sphere or core-shell spherical model (CSS). Initial fits with a simple sphere model, assuming an average sld (scattering length density) of the micelles equal to that of the surfactant (1.02×10-6 Å-2), and setting free the volume fraction, radius and background, provides good fits, giving an average micellar radius of 22.2 Å and volume fractions that correlate well with the experimental concentrations. These values were used as an input for the more refined core-shell model, where the fitting parameters were therefore set as the background, volume fraction, and structure factor parameters (well-depth and well-width). Neutron scattering length densities were set to the calculated values for the surfactant tail and head. For the head, the sld is 2.14×10-6 Å-2 (one methylene group is taken as part of the head, with density10 1.2 g·cm-3), while the shell is 3.91×10-7 (density 0.8 g·cm-3). Very good fits were obtained with this model. The results of the analysis are shown in Table 1. Table 1. Fitted parameters for the different models used in SANS. CSS-SW model

c (mM)

core radius, R (Å)

shell thickness, t, (Å)

well-widtha

well-depth

WW (2R)

WD (kT)

volume fraction, φv


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1.0

12.8

8.5

5.93

-0.40

0.00064

2.5

12.8

9.7

4.32

-0.78

0.00170

4.0

13.4

9.3

3.78

-1.08

0.00279

equatorial / polar radius (Å)

equatorial core / polar core radius (Å)

1.0

17.0 / 29.5

8.5 / 21.0

5.93

-0.29

0.00078

2.5

20.5 / 26.2

10.9 / 16.5

4.31

-0.74

0.00176

4.0

20.2 / 28.1

10.9 / 18.7

3.77

-1.01

0.00296

CSE-SW model

a

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the outermost radius (shell + core) is used as the effective radius for S(q)

Over the range of concentrations studied here, there does not seem to be any trend in the size of either the core or shell thickness with concentration, with average values of 13.0 and 9.2 Å, respectively. Bowers et al.10 also observed that the size of double chained metallosurfactants of tris(bipyridine)ruthenium micelles (substituted in position 5 of the bipy) did not depend on the concentration in the range 0.1 to 5 mM. For a fully extended alkyl chain, the size of the core can be estimated by using Tanford formula l(Å) = 1.540 + 1.265n. This gives for our surfactant a maximum chain length of 16.7 Å. The smaller value obtained by SANS is in good agreement with this estimation, in that it implies a certain degree of interdigitation and flexibility of the alkyl chains in the micellar core. For the shell thickness, the average value of 9.2 Å obtained from the fits seems reasonable for the bulky head group of the Ru24C13. The total micellar radius is therefore 22.2 Å. We have also obtained the size of the micelles in H2O by DLS on a 9.8×10-4 M solution of Ru24C13, resulting in a hydrodynamic radius of 21 Å, in good agreement with the results obtained by SANS. As for the temperature, it does not seem to modify the dimensions of the aggregates, with mean radii of 21, 21, 20 and 23 Å at 15, 25, 35 and 45ºC, respectively.


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The aggregation number can be determined from the ratio of the core volume to the individual volume occupied by a surfactant tail. For gemini surfactants, the volume contribution of a methylene group in the micelle has been reported to depend slightly on the length of the alkyl chain25 , however the overall volume of the hydrophobic moiety can be estimated to a good approximation with Tanford formula V(Å3) = 27.4 + 26.9n, considering two alkyl chains, which gives a value of 646.6 Å3, or 389 cm3·mol-1. With these values, an aggregation number of 14 is obtained. This result is in good agreement with the value of 12-13 monomers per micelle obtained by light scattering for the same surfactant 26 . Finally, the micellar volume fractions returned by the fits, φv, correlate very well with the experimental concentrations (assuming an overall density of 1). Regarding the interaction parameters of the square-well potential (SW), the well-width diminishes with concentration, while the well-depth shows an opposite trend. These results are reasonable, considering the enhanced screening of micellar charge with ionic strength, which is proportional to the concentration of the surfactant. For a charged macroion, the Coulomb potential is defined as the product of an exponential decay function of the distance r and a term related to the charge and size of the micelle and the ionic strength as23:

D 2 zm2 e −κ ( r − D ) 2 ρN A2 e 2 I U (r ) = , with κ = πε 0ε (2 + κD) r ε 0εRT

Eq.2

where zm is the micelle charge, D the particle diameter, ε the solvent permittivity and κ the Debye-Hückel screening parameter, which includes the density of the macroion. Thus, in dilute conditions, an increase in micellar concentration extends U(r) toward larger distances (higher Debye length, or lower κ), but reduces it at a given distance. In these expressions, for a given micellar size (D) and charge (zm), increasing κ (screening) produces a steeper fall of the exponential function, which is reflected by a shorter range of the electrostatic interactions between the micelles. In the SW potential approximation, this is reflected by a wider, less deep


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well. Conversely, higher ionic strength must reduce the width and increase the depth of the well, which is precisely the trend observed in our experiments, and does seem to validate the choice of a SW model to describe the interactions. In the following step, we attempted to refine the model further by considering a core-shell ellipsoid (CSE) form factor. Bowers et al. provided evidence of a dependence of the micelle shape on the tail length for double-tailed metallosurfactants substituted in position 5 of the bipy: for Ru25C12, the preferred micellar shape was found to be ellipsoidal, while for 15 and 19 carbon atoms it became spherical17. In our case, with a 13 carbon tail, it is therefore sensible to test also the ellipsoidal model. The fit was performed individually for each concentration, imposing as a constraint that the shell thickness does not depend on the direction of the ellipsoid and is equal to the value obtained with the CS model, for consistency and to reduce the number of floating parameters. Good fits were obtained with this model (Fig. 2B) and the resulting parameters are collected in Table 1. The outer radii of the spheroid are the sum of the corresponding core radius and shell thickness (equatorial and polar). The major axes of the ellipsoid obtained from the fits are 27.9 and 19.2 Å on average. These correspond to a prolate ellipsoid with an axial ratio X = rmin / rmaj = 0.69. For the Ru25Cn series, Bowers et al.10 obtained best fits by setting X = 0.5 for n = 12 and X = 1 (i.e, spherical micelles) for longer chain surfactants (n = 15). Our surfactant Ru24C13 with 13 carbons has an intermediate chain length and therefore the value of 0.69 is in good agreement with this previous study, suggesting slightly distorted spheres. The size of the micelles according to the CSS model (Rc + t) are comprised between the corresponding equatorial and axial values for the ellipsoids, as expected. The fitted volume fraction is in perfect agreement with the values obtained with the core-shell model. The concentration of micelles in solution is cM = c/Nagg – cmc. By considering that the cmc is lower than 1×10-5 mol dm-3, the volume fraction can be approximated by: φv ≈

cM VM cV = m 1000N agg 1000

Eq.3 12

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where c is the surfactant concentration in molar units, VM and Vm are the molar volume of the micelle and that of the surfactant in its micellar form, respectively, expressed in cm3⋅mol-1. By substituting the fitted volume fractions in eq. 3, we obtain a mean Vm of 742 cm3·mol-1. Additional information on the aggregation process can be deduced by NMR. Fig. 3 shows the 1D proton spectra of Ru24C13 with its corresponding signal assignation which was done with the help of the COSY and HSQC spectra27. Given the low cmc, even at the first concentration measured the spectrum reflects the situation of a totally micellized surfactant, with broad signals that do not allow the resolution of the multiplets. A change in shape of the broad signal corresponding to the methylene groups of the tail is observed, indicating that some methylene groups in that signal are shifting with the concentration This is clearly visible in SI. Fig. 4, which shows the variations in the chemical shifts of selected signals of the tails (-CH2-) and the head (H3c and H33’). The protons of the head facing outwards (H33’) scarcely change, whereas -CH2- and those of the head closer to the tails (H3c) move significantly in opposite directions. A possible interpretation of these findings is that micellisation takes place in a stepwise manner. The fact that the external H33’ protons do not shift but the inner ones do suggests that changes with concentration are taking place inside the micelle. As in this metallosurfactant the substitution is on the 4,4’ positions of the bipy, its structure is more open (the two tails are quite far apart), thus interdigitation of the hydrophobic tails in the micellar aggregates is likely. Thus, micelles would start forming at concentrations above 5×10-6 M, becoming progressively more compact. As the SANS experiments were performed at higher concentrations, due to the detection limit of this technique, this effect was not detected, as micelles would have reached practically a constant size within the range where the scattering experiments were done. Nonetheless, considering the CS model, the overall size increased slightly from 21.3 Å at 1 mM to 22.7 Å at 4 mM, which could indeed suggest that the aggregation is taking place in a step-wise manner over a wide range of concentrations.


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Effects of cyclodextrins on the aggregation of Ru24C13 Spectroscopy. Cyclodextrins normally modify the spectral properties of guests, for example by increasing or quenching the fluorescence - in the case of luminescent guests - or producing changes in the absorbance spectrum or shifts of the bands 28 . In the case of Ru24C13 in the presence of β-CD, the changes observed in the emission and absorption are not significant for concentrations below the cmc, in contrast to the case of the surfactant when it is micellized. Fig. 4A shows the absorption spectrum of the ternary system above the cmc (7.6×10-4 M). The appearance of an isosbestic point at 475 nm is indicative of an equilibrium between two different absorbing species. The fluorescence spectrum upon addition of β-CD (Fig. 4B) resembles that of the Ru24C13 at concentrations lower than the cmc, i.e blue-shifted, and the emission shifts progressively towards lower wavelengths when increasing the β-CD content (Fig. 4B). Above this concentration, no further changes either in absorption or emission are observed. This observation is consistent with the breaking of the micelles, with the corresponding shift of the cmc, due to the formation of a complex between the Ru24C13 and the macrocycle, with a quantum yield higher than Ru24C13 has in the micelle. The blue-shift, together with the better resolved structure, seems to indicate that the environment of the metallosurfactant is less polar than in the micelle. A much more detailed description of the interactions between the metallosurfactant and the CDs can be achieved by NMR. Fig. 5 shows the 1H NMR spectra at different mole fractions of β-CD. The CD produces important effects on the signals. Focusing on the protons of the macrocycle (Fig. 5B), the signals of the H3 and H5 protons, i.e., those inside the cavity, undergo upfield shifts (-0.048 and -0.66 ppm, respectively, at a 2:1 ratio), while the outer protons H2 and H4 move significantly less (0.001 and 0.008 ppm), and so does H1 (-0.005 ppm, not shown), confirming the formation of an inclusion complex. No duplicity of signals of the CD occurs, indicating that the equilibrium of complex formation has a fast exchange rate compared to the


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NMR time scale. The stoichiometry of the complex can be obtained from Job’s method, by normalizing the variations of the chemical shifts of the host by its mole fraction and plotting them against XCD (Fig. 6). By considering the protons that undergo more changes upon inclusion, the graph shows a maximum (or minimum) at XCD = 0.75, indicative of a 3:1 stoichiometry (three CDs per monomer). Similar results are obtained with γ-CD (Fig. 6), where the inner H3 and H5 are the most affected protons, with overall chemical shifts however lower than with βCD. H1 and H6 change in the same direction as H3 and H5, while H2 and H4 do it in opposite direction. ROESY experiments provide a better picture of the topology of the complexes. In this technique, cross-peaks may be observed in a bidimensional spectrum when internuclear distances are less than 3-4 Å. Fig. 7 shows a zoomed-in view of the ROESY spectrum at a 3:1 molar ratio (4.6×103

M of surfactant). Obvious NOEs appear between all the protons the surfactant tail and H5 and

H3 (the latter overlapped with H6) (Fig. 7A), which confirms that the macrocycle threads the hydrocarbon chain. On the other hand, there are also clear cross-peaks between the outer protons of the CD (H2 and H4) and those of the tail of the metallosurfactant, indicating a close contact between the exterior of the CD and the central part of the hydrocarbon tail. It is worth noting also that no cross-peaks are detected between the aromatic protons of the cationic head and the CD, with the sole exception of three small NOEs between H3 and H3cad, H5cad and H6cad, i.e., the protons of the bipy where the two hydrocarbon tails are united (Fig. 7B). No interactions are observed between the CD and the other aromatic protons of the cationic head. These data taken together point to a complex in which three CDs are bound to both tails of the monomer, one of them attached by its secondary rim to the inner part of the cationic head. As each tail comprises 13 methylene groups, it is possible to have two CDs threaded on the same chain but it is not possible, due to steric hindrance, to fit more than one on the other tail, which accounts for the 3:1 stoichiometry observed. As for the wider macrocycle, γ-CD, the same inclusion mode can be


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deduced from the ROESY spectrum (SI Fig. 5). In this case, the smaller variations in H3 and H5 chemical shifts suggest a lower affinity for the metallosurfactant. A closer inspection of the signals from Ru24C13 reveals some interesting results. By focusing on the zone corresponding to the aliphatic protons (Fig. 5A), the signal of the intermediate methylene groups, which appears as a broad peak around 1.10 ppm, is split into two or more signals and the terminal CH3 is resolved in its multiplet structure at high CD/surfactant ratios, while a new signal arises at 0.99 ppm. Likewise, the resonances at 1.47 and 2.63 ppm corresponding to the Ha and Hb protons vanish at high ratios, and new signals appear at 1.72 and 2.83 ppm. In order to observe the effect of the β-CD on the aromatic protons, a new set of solutions keeping the MS concentration constant (4.64 mM) were prepared for several molar ratios Ru24C13/CD. The aromatic region of these spectra is plotted in Fig. 5C.The appearance of additional signals and enhanced resolution of others also occurs systematically in the protons of the bipy heterocycles. This behavior is striking, as no duplicity of signals is detected with the protons of either β- or γ-CDs, which is a clear indication of the fast exchange rate in the reaction. The only possible explanation is that the micellization reaction must have a very slow exchange rate compared to the NMR time scale. As the cmc is rather low, this effect is only observed in the spectrum beyond the stoichiometric ratio of the complex, when virtually all the monomer is in its complexed form. As long as there is some free surfactant in the solution (above 5×10-6 M), it micellises, causing the broad signals, characteristic of the aggregated monomer. It is thus clear that the use of CDs can be advantageously utilized to gain precious information on the dynamics of micellization of a surfactant with a low cmc preventing a direct observation of the NMR spectrum. This “slow” kinetics of micellisation has been observed in more conventional gemini cationic surfactants with ethylene (12–2–12), propylene (12–3–12), and butylene (12–4–12) spacer groups.29 Most interestingly, the new signal of H3cad that appears at 8.4 ppm - necessarily a singlet, as there are no neighbor protons to couple with (SI. Scheme 1) - changes progressively


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into a doublet at high amounts of β-CD, up to the total extinction of the signal of the micellized Ru24C13 (8.16 ppm) at high molar ratios. Under these conditions, there are no longer micelles but complexed monomer. As the synthetized surfactant is a racemic, the apparition of the doublet has to be attributed to the resolution of the ∆ and Λ enantiomers caused by the chiral nature of the CDs, as it has been reported for some Ru(II) trisdiimine compounds with substituted CDs30. Evidently, the binding constants with any of the two chiral forms of the Ru24C13 must be extremely close, giving two signals with the same intensity that look like a doublet, but have different chemical shifts when complexed, due to the slightly different magnetic environment, thus making them appear at different positions of the spectrum. As a difference to the other aromatic protons, only H3cc can act as a “probe” of the different enantiomers, as it is the only one in close contact with the inner H3 of the cyclodextrin, according to the ROESY spectrum shown in Fig. 7A. It is worth noting that this chiral resolution is not observed with γ-CD (see SI. Fig.6). Either more excess of CD is necessary to shift the equilibrium enough to produce the doublet, or the H3cad does not interact so strongly with H3, most likely due to the wider size of the cavity of the macrocycle, or both. SANS studies. The addition of either β-CD or γ-CD to a 4 mM solution of the surfactant produces in all cases important changes in the shape of the SANS curves. For instance, with γ-CD, the intensity of the micellar interaction peak is gradually reduced with increasing CD/Ru24C13 molar ratio, the peak disappearing completely at a 6:1 ratio (Fig. 8). These changes in the scattering pattern indicate a decrease in the number of micelles in solution, confirming that the addition of CD breaks up the micelles. An interesting point to note is that the capacity of the two CDs to break-up the micelles is different. At a 3:1 β-CD/Ru24C13 molar ratio the micelles have practically disappeared, while to obtain the exact matching shape of the scattering curve with γCD a 10:1 γ-CD/Ru24C13 is needed. The scattering at low q due to large aggregates does not disappear; instead, it increases with CD concentration. The most likely explanation is that it is 17 ACS Paragon Plus Environment

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due to the self-aggregation of the native cyclodextrins in scarce but large and polydisperse aggregates, typically larger than 100 nm in hydrodynamic radius, which has been reported by several authors31. As explained earlier, due to their large size, the effect on the scattering curves may be neglected beyond a given q value, limited to the medium-to-high q range. Indeed, SANS curves of the two cyclodextrins in solution at 9 mM β- and γ-CD, similar to the one used with the surfactant, show a high scattering at low q, consistent with the presence of large CD aggregates (SI. Fig. 7). Knowing the molar volume of each CD, it is possible to estimate the size of the macrocycle from the SANS curves by assuming a spherical shape. The analysis was performed above q = 0.025 Å-1 by introducing a hard-sphere structure (HS) factor and setting as constraints the neutron scattering length densities and the volume fraction of the CD, calculated from the concentration and the molar volumes of the CDs (704.0 and 801.2 cm3mol-1 for β- and γ-CD, respectively32,33). The radii obtained are 6.4 (β-CD) and 6.6 Å (γ-CD), in agreement with the reported values of hydrodynamic spheres from DLS data34. A rigorous data analysis on the mixed CD + surfactant systems presents a number of difficulties: both the Ru24C13and the CD are prone to unspecific aggregation detected in the low q region; the micellar fraction, as CD is gradually added, is an unknown parameter; inclusion complexes of varying stoichiometry are present (according to the binding constants for each CD), and strong charge interactions are present, which gradually decrease as the micelles are broken-up. Therefore, some reasonable assumptions are needed to model the behaviour of these systems, while keeping the number of fitting parameters to a sensible level. First, we can neglect the effect of large aggregates in the scattering curves by restricting the analysis to the medium-tohigh q region - a series of preliminary fits show that appropriate results can be obtained for q values above 0.025 Å-1. Second, we can simplify the system by considering only two types of structures: the micelles (assimilated to plain spheres) and the CD/surfactant complexes, which we also assimilate to spheres. We therefore use a simple model of a binary mixture of hard


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spheres (BHS), which includes hard-sphere interactions between all the particles. The overall size of the micelles is set to 22.2 Å (obtained above). For the complex, the scattering length density was set to 1.44×10-6 Å-2, a volume-averaged value between the scattering length densities of the surfactant and the cyclodextrin (1.89×10-6 Å-2), assuming a 1:1 complex. Although the stoichiometry may vary, this would not produce a significant difference on the sld of the complex, since both values (CD and surfactant) are very close. The fitted curves are shown as solid lines in Fig. 8, and the resulting parameters are compiled in Table 2. Table 2. Fitted parameters for the BHS model applied to the mixtures of Ru24C13 and CD Molar ratio CD/ Ru24C13

β-CD

γ-CD

complex radius

micellar volume fraction

complex volume fraction

% of complex

Rcx (Å)

φv

φv

1:1

7.8

0.00191

0.00238

52

3:1

6.8

0.00093

0.00722

88

3:1

7.7

0.00183

0.00238

57

6:1

6.6

0.00124

0.01129

90

10:1

6.6

0.00091

0.00903

91

χ

The trend in the fitted volume fractions when the amount of CD increases is in agreement with the shift of the equilibrium from a micellar solution to the formation of a surfactant/CD complex and concomitant break-up of the micelles. It is clear that, at equivalent molar ratio, β-CD is more efficient in forming a complex with the metallosurfactant than γ-CD. As for the size of the complex, crudely approximated to a sphere, it can be best estimated at intermediate molar ratios: the complex is slightly larger than the CD alone, by about 1.5 Å. This size is reasonable, given the fact that the complex involves the inclusion of part of the surfactant tail by the CD, which thus does not contribute to any additional volume. At higher molar ratios, the size of the complex is closer to a single CD molecule; this is also quite reasonable, by considering that at higher


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molar ratios, an excess of CD is present in solution, which will thus contribute - with its smaller size – to a lower average size value (complex and unbound CD). Using the NMR data, the binding constants could in principle be calculated according to multivariable non-linear squares fitting of the measured chemical shifts of the CD, provided that the exchange rate for the complex formation is fast 35 . However, a 3:1 stoichiometry is a challenge when considering the large number of parameters to be estimated (three binding constants plus the chemical shifts of the complexes of the three stoichiometries), even if considering all the protons of the CD to increase the number of fitting points. It is possible, however, to obtain a global binding constant according to the reaction: S + 3CD ⇔ S:CD3, with Kg = [S:CD3]/[S][CD]3, as a measure of the overall affinity of Ru24C13 for the macrocycle36. This is based on the evidence that the Job’s plots display a well-defined peak at 0.75. According to this, the area of the signal of H3cad at 8.16 ppm can be taken as proportional to the concentration of micelles, and that appearing at 8.4 ppm (which eventually transforms in a doublet in the case of the β-CD) as proportional to the concentration of the complex with the highest stoichiometry. Knowing these concentrations, the global binding constants can be obtained straightforwardly. We obtain logKg = 11.0 ± 0.7 for β-CD and logKg = 9.2 ± 0.4 for γ-CD. These values agree with the trend observed by SANS for a higher percentage of complex formed with β-CD compared to γ-CD. On the other hand, the average value for the binding constant of a single step (inclusion of one CD onto one of the tails), taken as the geometric average, would be of 4.6×103 and 1.1×103 M-1, for β-CD and γ-CD, respectively, which are typical values of binding constants for complexes of single-tailed surfactant and cyclodextrins37. CONCLUSIONS In summary, we have synthesized and fully characterised the aggregation behaviour of a ruthenium-based metallosurfactant in its racemic form, [bis(2,2′-bipyridine)(4,4′-ditridecyl-2,2′bipyridine)ruthenium(II) dichloride, and the complexes it forms with β-CD and γ-CD. According 20 ACS Paragon Plus Environment

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to our NMR results, micellization takes place in a stepwise manner and has a very slow exchange rate compared to the NMR time scale. Small-angle neutron scattering has been used to determine the micelles shape and size, revealing prolate ellipsoids with an axial ratio X = rmin / rmaj = 0.69 and an aggregation number of 14, at concentrations above 1 mM. Due to the relatively open structure of this surfactant both β-CD and γ-CD form complexes of 3:1 stoichiometry in which one macrocycle is located on one of the hydrocarbon tails and two CDs on the other, in close contact with the cationic head. The presence of either CDs induces micellar break-up as the monomer becomes complexed, as deduced from the resolution of the fine structure of the spectrum corresponding to the monomer at high CD/surfactant molar ratios, with the β-CD complex being about two orders of magnitude more stable than the γ-CD complex. For the first time, a calculation approach by SANS using a model of a binary mixture of hard spheres including hard-sphere interactions between all the particles was introduced successfully to study quantitatively the effect of CDs on the aggregation of the surfactant. In addition, β-CD was shown to resolve the ∆ and Λ enantiomers of the metallosurfactant above a threshold concentration of the oligosaccharide, this being due to the very specific interaction between this CD, through its wider rim, and the aromatic protons of the bipy nucleus where the hydrophobic tails are inserted. This ability, derived from the chiral nature of β-CD, can be used advantageously to understand better how the enantiomeric resolution of racemic mixtures of organometallic compounds, particularly trisdiimine complexes, takes place, as well as in the design of chiral stationary phases in HPLC and in capillary electrophoresis.. ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support provided by JCNS at the Heinz MaierLeibnitz Zentrum (MLZ), Garching, Germany and to MINECO (Spain, project CTQ201018564). A.G.-M. acknowledges receipt of a Ramón y Cajal Fellowship from the Spanish MINECO.


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FIGURES

Figure 1. A. Fluorescence spectra of Ru24C13 as a function of concentration. B. position of the maximum of fluorescence (λex = 460 nm). Insert: low concentration expanded.


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Figure 2. A. Simulation of SANS curves for polydisperse spheres of 183 nm at different volume fractions. B. SANS curves with their corresponding fits (solid lines) to the CSE-SW model (see text).


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Figure 3. Zoomed out views (aromatic and aliphatic) of the 1H-NMR spectra of Ru24C13 at different concentrations

Figure 4. A. Absorption spectra of Ru24C13 at 7.6×10-4 M in the presence of β-CD 2 mM. B. Normalized emission spectra of Ru24C13 7.6×10-4 M at increasing concentrations of CD showing a blue-shift


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H3

H6

H5

H2

H4

X=1.00

X=0.86

X=0.73

X=0.60

X=0.46

X=0.33

X=0.20

X=0.07

3.95

3.85

3.75 3.65 chemical shift (ppm)

3.55

Figure 5. 1H NMR spectra of Ru24C13 and β-CD mixtures at different mole fractions of CD for the aliphatic region (A), and that corresponding to the β-CD resonances (B). C. Detail of the aromatic zone of the spectrum for different surfactant/β-CD molar ratios (where [Ru24C13] = 4.64 mM)


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Figure 6. Job’s plots for the protons of the CD for complexes of the metallosurfactant with β-CD (A) and γ-CD (B)

3.45

3.45

3.55

3.55

3.60

3.60

3.65

3.65

3.70

3.70

3.75

3.75

3.80

3.80

ppm

3.50

3.50

ppm

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3.85 2.5

2.0

1.5 ppm

1.0

0.5

8.6

8.4

8.2

8.0 7.8 ppm

7.6

7.4

7.2

7.0

Figure 7. Zoomed in views of the ROESY spectrum of the mixtures of Ru24C13 with β-CD (3:1 molar ratio, 4.6×10-3 M of surfactant). A. Aliphatic protons (tail). B. Aromatic protons (head)


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Figure 8. A. SANS curves for mixtures of Ru24C13 with γ-CD at different CD/surfactant molar ratios (4 mM). B. Fits to the model described in the text (solid lines)

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Mahato, P.; Saha, S.; Choudhury, S.; Das, A. Solvent-dependent aggregation behavior of a new Ru(II)-polypyridyl based metallosurfactant. Chemical Communications 2011, 47, 1107411076.

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Bowers, J.; Amos, K.E.; Bruce, D.W.; Heenan, R.K. Surface and Aggregation Behavior of Aqueous Solutions of Ru(II) Metallosurfactants: 4. Effect of Chain Number and Orientation on the Aggregation of [Ru(bipy)2(bipy’)][Cl]2 Complexes. Langmuir 2005, 21, 5696-5706.

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http://www.sasview.org/, developed by the DANSE project under NSF award DMR-0520547

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GRAPHICAL ABSTRACT (TOC)

“Using Inclusion Complexes with Cyclodextrins to Explore the Aggregation Behavior of a Ruthenium Metallosurfactant” Nerea Iza, Andrés Guerrero-Martínez, Gloria Tardajos, María José Ortiz, Eduardo Palao, Teresa Montoro, Aurel Radulescu, Cécile A. Dreiss, Gustavo González-Gaitano


Using Inclusion Complexes with Cyclodextrins To Explore the

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