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Ion Pairs and C 60 : Simultaneous Guests in Supramolecular Nanotubes

2009; Wiley; Volume: 16; Issue: 1 Linguagem: Inglês

10.1002/chem.200902662

1521-3765

Emiliano Tamanini, G. Dan Pantoş, Jeremy K. M. Sanders,

Synthesis and Properties of Aromatic Compounds

Unlikely bedfellows: Naphthalenediimide-based supramolecular nanotubes act as size-selective receptors for ion pairs, as demonstrated by competition experiments with C60 (see picture). Fullerene forms mixed complexes inside the cavity of the nanotubes in the presence of the anion and ammonium ions of the appropriate size. We have recently reported that supramolecular nanotubes constituted of amino acid-functionalised naphthalenediimides (NDIs)1 act as receptors for fullerenes,2 condensed aromatic systems and quaternary ammonium ions.3 We showed that C60 forms a closed-packed one-dimensional array inside the nanotube and that the ion pair complexation ability of the nanotubes is size selective. We now report the remarkable ability of the supramolecular nanotubes to form mixed fullerene-ion pair inclusion complexes. The formation of the mixed complexes is directly dependent on the size of the ion pair component. These unusual results highlight the versatile nature of NDI nanotubes as selective receptors in apolar solvents. We have previously shown that NDI nanotubes can readily take up C60 in chloroform, increasing its solubility up to 16 times.2 A sharp colour change from pale yellow to brown is observed upon complexation of C60, as well as an upfield shift of the fullerene 13C NMR signal. The chirality of the nanotube is sensed by the guest as shown by a weak but significant Cotton effect at 593 and 660 nm (C60 absorbance). We envisaged that, through these spectroscopic responses, this system could be used as a platform for studying the relative binding affinities of fullerenes and ion pairs for the NDI nanotube. Measurement of conventional binding constants is not practical due to the unknown and probably undefined stoichiometry of the interaction and, in the case of C60, complications arising from low solubility (see the Supporting Information). Colorimetric studies were carried out to test the ability of different ion pairs to displace C60 from a fullerene-saturated solution of 1 (Scheme 1) in chloroform.5 The addition of one equivalent of 2⋅Cl to a 10 mM solution of C60⊂1 results in a cloudy suspension due to the precipitation of solid C60. Once the solid has settled, a clear colour change from brown to light orange can be observed in the supernatant. Addition of bromide and iodide salts of the same ammonium ion (2⋅Br and 2⋅I respectively) did not produce the same colour change and no precipitation of C60 was observed (Figure 1). UV/Vis spectroscopy confirms that the intensity of the bands at 593 and 660 nm decreases dramatically upon addition of one equivalent of 2⋅Cl to a solution of C60⊂1 in chloroform (see the Supporting Information). On the other hand, the UV/Vis spectrum of the C60⊂1 solution remains unchanged upon addition of 2⋅Br or 2⋅I. The induced CD signals (ICD) observed on the C60 absorbance bands (at 593 and 660 nm) are also affected by the presence of the ion pairs in solution (Figure 2). The intensity of the negative ICD bands is dramatically decreased upon addition of 2⋅Cl to a C60⊂1 solution, while it remains essentially unchanged in the presence of the larger ion pairs, 2⋅Br or 2⋅I. These observations, together with the significant precipitation of a black solid observed only when 2⋅Cl is added to C60⊂1, are clear indications that C60 is readily displaced from the cavity of the nanotube in the presence of an appropriate competing guest. Consequently this suggests that 2⋅Br or 2⋅I interact less strongly with the NDI nanotube than 2⋅Cl and C60 itself, as they are not capable of displacing C60 from the cavity of the host. Lysine-functionalised NDI (1) and dimensions of the ammonium ions (2 and 3) and anions (in Å).4 Schematic representation of the self-assembled nanotube and the competition experiment with C60 and ion pairs. Picture: photographical comparison of a solution of 1 + C60 in the presence of different ion pairs. CD spectra of a 10 mM solution of 1 + C60 in CHCl3 in the presence of one equivalent of different ion pairs. Spectra recorded at 21 °C. The size of the cation also affects the binding affinity of the ion pair: a control experiment with tetrabutylammonium chloride (3⋅Cl) shows that in the presence of a bigger cation (Scheme 1) the decrease in intensity of the bands a 593 and 660 nm is smaller than that observed for 2⋅Cl. These observations were mirrored in the CD spectra, where the ICD observed on the C60 absorbances is reduced in the presence of 3⋅Cl to a lower extent than in the presence of 2⋅Cl (Figure 2). From this observation it is possible to propose a scale of decreasing binding affinity between the NDI nanotubes and the ion pairs tested: 2⋅Cl> 3⋅Cl≫ 2⋅Br ≈ 2⋅I. The closed-packed one-dimensional array of C60 inside the cavity of the nanotube also generates an intense absorption band at 452 nm. This band has previously been attributed to fullerene-fullerene interactions;6 it is therefore a characteristic spectroscopic signature for fullerenes that are in close contact (Figure 3).2 The same band would not be observed in the case of isolated C60 molecules complexed by smaller NDI oligomers. As shown in Figure 3 the addition of increasing amounts of 2⋅Cl to a C60⊂1 solution results in a rapid decrease of the intensity of the band at 452 nm. This indicates that the closed-packed C60 array is partially disrupted in the presence of the appropriate competing guest. This observation suggests the formation of a mixed complex ion-pair/C60⊂1, where the ion pair is intercalating between the fullerenes. UV/Vis spectra of a 10 mM solution of 1 + C60 in CHCl3 in the presence of increasing amounts of 2⋅Cl. Spectra recorded at 21 °C. Chiroptical studies in the UV region of the spectrum (from 280 to 400 nm) show that the nanotubes remain intact in the presence of one equivalent of the ion pairs tested. The persistence of the nanotube in solution is monitored by the presence of a strong induced Cotton effect band at 383 nm (and other minor bands between 300 and 378 nm),1 which are the result of the fixed geometrical relationship between successive NDI units in the helical nanotube.7 Computational studies on a model system based on the solid-state structure of the NDI nanotubes showed that the shape of the CD spectrum observed in solution is strictly dependent on the orientation of the NDI units with respect to each other. Small changes of the angle between consecutive NDI units in the nanotubular structure result in significant changes of the predicted CD spectrum.7 The addition of ammonium ions to a 0.7 mM solution of 1 in chloroform results in negligible changes on the intensity of the ICD band at 383 nm and it does not alter the overall appearance of the spectrum (see the Supporting Information). This indicates that the overall structure of the nanotubes is unchanged upon complexation of the ion pairs. This is not surprising since the nanotube structure is conserved upon complexation of C60 and extended aromatic systems.3 The structure of the nanotubes remains intact (as shown by CD and NMR) in the presence of up to the 7.5 equivalents of MeOH per NDI, indicating its stability in the presence of small amounts of hydrogen-bond competitors.3 The NDI nanotube inclusion complexes have characteristic NMR signatures. The presence or absence of an upfield shift in the 1H NMR spectrum of 1 upon addition of a potential guest molecule is a diagnostic and reliable way to assess the formation of a host-guest complex in solution. In this case, the magnitude of the chemical shift recorded for the NDI α protons in the presence of the four different ion pairs tested in this study follow the same trend as observed for the C60 competition experiments followed by UV/Vis spectroscopy. A Δδ of 0.08 ppm for 2⋅Cl, 0.03 ppm for 3⋅Cl and about 0.02 ppm for 2⋅Br and 2⋅I is obtained when one equivalent of ion pair is added to a 10 mM solution of C60⊂1 in CDCl3. 1H NMR titrations using 1 and 2⋅Cl, 2⋅Br and 2⋅I confirm this chemical shift difference over a range of concentrations of ion pairs (see the Supporting Information). This highlights the size-discrimination ability of the NDI nanotubes in complexing ion pairs. The 13C NMR spectrum of C60 is strongly influenced by the presence of the NDI nanotubes. An upfield shift of more than 1.4 ppm was observed for the 13C signal of C60 upon inclusion in the nanotube cavity, indicating a shielding effect due to the proximity of other aromatic units (Figure 4).2 Upon addition of increasing amounts of 2⋅Cl (0.5 and 1 equivalent) to a 10 mM solution of C60⊂1 in CDCl3 a small but significant downfield shift, towards the uncomplexed position, of the fullerene signal is observed. This is also accompanied by a decrease of the intensity of the 13C signal indicating that C60 is released in solution in the presence of the competing ion pair. As a result of the poor solubility of C60 in chloroform, the released fullerene quickly precipitates out of solution making it difficult to detect the presence of free C60 by 13C NMR spectroscopy. The presence of a C60 signal even after the addition of one equivalent of ion pair indicates that not all the C60 has been displaced from the nanotube's cavity. This is possible only if a new mixed host-and-two-guests complex has formed, in which the fullerene and ion pairs are simultaneously complexed inside the tubular receptor. This is supported by the decrease in intensity observed for the C60 signal, which is a direct measure of the amount of C60 complexed. The downfield shift, towards the uncomplexed position, observed for the same peak is also indicative of the disruption of the close packed one-dimensional array of C60. This allows us to propose that the initial upfield shift in the C60 resonance is due to the shielding produced by a combination of the NDI cores and the neighbouring C60 molecules. In the mixed complex the influence of the neighbouring C60 molecules is reduced, accounting for the downfield shift observed for the C60 resonance. The presence of the ion pair inside the tubular cavity is confirmed by the expected upfield shift observed in the 1H NMR spectrum, for the cation's benzyl protons (see the Supporting Information). 13C NMR spectra of a saturated solution of C60 and of a 10 mM solution of 1 + C60 in the presence of different amounts of 2⋅Cl. 1024 scans were acquired for each 13C spectrum of the complexes, and 11 000 scans were acquired for a saturated solution of C60 in CDCl3. The reverse experiments in which C60 was added to solutions of 2⋅Cl⊂1 led to the formation of the same nanotube-fullerene-ion pair mixed complex. UV and NMR experiments confirm that C60 partially displaces ion pairs from the nanotube's cavity even when the host is saturated with the ion pair guest (see the Supporting Information). This is indicative of the dynamic nature of the systems and the propensity of these nanotubes to form mixed complexes. In conclusion we have demonstrated through competition experiments with C60 that NDI-based helical nanotubes act as selective receptors for ion pairs. The selectivity is based on the relative size of the ion pairs and is a direct measure for the association strength between the nanotubes and ion pairs. Virtually all the receptors currently available for binding ion pairs display specific ion binding motifs such as crown ethers, amides and ureas, calixarenes and calixpyrroles8 or combinations of these to generate heterotopic receptors that are able to bind simultaneously multiple ions.9 We believe that the unexpected results reported above show that the NDI nanotubes represent an unprecedented and unique example of a self-assembled non-specific receptor for ion pairs. Furthermore, we have shown that the nanotubes have a tendency to form mixed complexes in which both ion pairs and C60 are present simultaneously in the nanotubes' cavity. This leads us to suggest that the nanotubes have the potential to act as nanoscale reactors in which the effective molarity of the reactants is increased dramatically by complexation within the confined nanotube cavity. We thank EPSRC (E.T. and J.K.M.S.) and Pembroke College, Cambridge (G.D.P.) for funding. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.