“Mechanically Docked” Metallodendrimers about Single-Walled

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“Mechanically Docked” Metallodendrimers about Single-Walled Carbon Nanotubes Harsh Chaturvedi, Andrea N. Giordano, Mahn-Jong Kim, Frederick M. MacDonnell, Sarah S. Subaran, and Jordan C. Poler* Department of Chemistry and Center for Optoelectronics and Optical Communications, UniVersity of North Carolina at Charlotte, 9201 UniVersity City BouleVard, Charlotte, North Carolina 28223-0001 ReceiVed: March 12, 2009; ReVised Manuscript ReceiVed: May 20, 2009

We report experimental support for “mechanical docking” of an optically active rigid molecule, ruthenium metallodendrimer (decamer), to single-walled carbon nanotubes (SWNTs) with some diameter selectivity. Binding of these rigid metallodendrimers onto dispersed single-walled carbon nanotubes has been studied by Raman and UV-vis-near-infrared (NIR) absorption spectroscopy. Atomic force and scanning electron micrographs indicate strong binding of these molecules specifically to the ends of nanotubes and are consistent with the spectroscopic evidence for diameter selectivity presented here. Near-IR absorption and radial breathing mode Raman spectra of these “end-functionalized” SWNTs show preferential diameter-selective separation, along with a red shift of the optical transitions. A concentration-dependent red shift is observed for the NIR absorption for decamer-bound nanotubes. This indicates a strong interaction between selective SWNTs and the decamer, forming a mechanically docked supramolecular complex. Spectral shifts and intensity variations in specific radial breathing mode bands of the functionalized SWNTs are complementary to our observations of the UV-vis-NIR absorption spectra and atomic force microscopy data. Introduction Single-walled carbon nanotubes (SWNTs) have been shown to have significant potential as a material for applications in optoelectronics and photovoltaics.1,2 The presence of delocalized π electrons in nanotubes makes them interesting materials for applications involving charge transfer and charge transport. Significant research efforts are being applied toward controlled modification of the surface by adding desired functionality to the nanotubes. Various functional groups, nanoparticles, and polymers have been added noncovalently3-5 or covalently6,7 to the SWNTs. Further advancements and novel approaches are required for adding desired functionality onto the nanotubes with more electronic and spatial specificity. Our aim is to functionalize SWNTs with rigid molecular systems that are large enough to “mechanically dock” 8 around the nanotubes. With this approach, it may be possible to connect nanotubes together into morphologically stable 3D assemblies with the desired functionality. A conceptual illustration of these proposed structures is shown in Figure 1. The geometry of the assembly should be affected by the size of the binding site and the diameter of the nanoparticle. Further functionalization (oxidation or amination) of the peripheral phenanthroline ligands of the metallodendrimer can enhance integration of these supramolecular systems into composite materials. The supramolecular systems9 under study absorb strongly in the UV and provide a rigid stable architecture for the desired nanostructures. Typically, semirigid π-conjugated polymers10 or flexible polymer wrapping leads to enhanced dispersion limits for SWNTs. The mechanical rigidity of the metallodendrimers studied here provides structural advantages over polymer wrapping and nucleic acid wrapping, which are too flexible to sustain a morphologically rigid assembly.11 Effects of this binding mechanism on the electrical, vibrational, and structural * To whom correspondence should be addressed. E-mail: jcpoler@ uncc.edu.

Figure 1. Conceptual model of supramolecular “mechanical docking” of a rigid ruthenium metallodendrimer wrapped around SWNTs of various chiralities: (9, 5) dt ) 0.96 nm, (11, 0) dt ) 0.86 nm, and (9, 1) dt ) 0.75 nm. Plan view A illustrates proposed binding sites. Support for binding within the five-centered pocket is described below (arrows indicate the width of the opening of the pocket). Tube placements are based on preliminary density functional theory studies of smaller molecular analogues (see Figure 6). Edge view B conceptually illustrates how multiple SWNTs can be connected into morphologically stable 3D structural elements. This concept is central to our goal of directed self-assembly of nanoparticle devices from dispersions.

properties of the nanotubes are reported here. Characteristic and significant changes in the near-infrared (NIR) absorption spectrum and the Raman radial breathing mode (RBM) of the nanotubes bound with these metallodendrimers are observed. We have shown ruthenium metallodendrimer binds strongly and specifically to the ends of SWNTs.12 The SWNTs’ ends were uniformly functionalized with this large (5.8 nm wide, 0.9 nm thick), optically active (MLCT, ε441 ) 225 000 M-1 cm-1; π f π*, ε375 ) 257 600 M-1 cm-1),13 and mechanically rigid supramolecular complex. Using enantiomerically pure [Ru(diimine)3]2+ units, MacDonnell has synthesized large rigid metallodendrimers.14,15 Dendritic polymers are often grown by covalently attaching the next generation of molecular units through bonds which have significant free rotation, therefore providing nonrigid topology. Most dendritic polymers are globular in nature and thus do not enable a spatially controlled, well-directed rigid 3D nanoassembly with SWNTs. We are using the [Λ6∆3Λ-Ru10]20+[PF6-]20 metallodendrimer, referred to as

10.1021/jp902229v CCC: $40.75  2009 American Chemical Society Published on Web 06/09/2009

“Mechanically Docked” Metallodendrimers about SWNTs

Figure 2. The molecular model of a canted metallodendrimer bound to the small-diameter (11, 0) semiconducting SWNT (A) maximizes π-π stacking interactions of the π orbitals on the tetrapyridophenazine bridging ligands with the π orbitals on the surface of the SWNT. This model is consistent with the average tube diameter measured by AFM. A representative AFM image (500 nm wide) of a decamer molecule bound to the end of an SWNT is shown in (C), with the AFM crosssection through the bound supramolecular complex shown in (B).

“decamer” to indicate the 10 metal centers. A ball and stick molecular model of this compound is shown in Figure 1. The synthesis, characterization, and chemical structure for this decamer have been described elsewhere.16 The 2D NMR spectroscopy of this decamer is consistent with a planar, highly symmetric, and mechanically rigid molecule.17 These results are also consistent with our DFT geometry optimization of the decamer. In Figures 1 and 2A we illustrate the decamer mechanically docked around SWNTs of various diameters. Our atomic force microscopy (AFM) studies provide direct evidence of the decamer bound specifically to the ends of the SWNTs. A typical AFM image is shown in Figure 2C. A typical cross-section over a bound complex is shown in Figure 2B. These data indicate a height of 3.4 ( 0.7 nm where the decamer is bound to an SWNT with a narrow diameter range of 0.84 ( 0.03 nm.12 There are several possible binding mechanisms for this supramolecular complex. The results presented below support a mechanical docking of the SWNTs into an endoreceptor9 pocket which consists of five ruthenium coordination complexes. This five-centered host presents the guest with significant π-stacking interactions from the planar surfaces of the tetrapyridophenazine (tpphz) bridging ligands connecting the metal centers. The internal dimensions of this pocket are ideal for binding small nanoparticles and SWNTs. The opening of the five-centered pocket, shown in Figure 1A (arrows), is not wide enough at its entrance to slip over the midsection of any but the narrowest SWNTs. Using van der Waals atomic radii and the DFT geometry optimized structure shown, the calculated opening of the pocket is 0.90 nm. This is not wide enough to slip over an SWNT. A (9, 1) SWNT, with a tube diameter dt ) 0.75 nm, has a van der Waals diameter too large to slip through the opening of the pocket. Therefore, we would not expect the metallodendrimer to wrap around the SWNT near its midsection using the five-centered binding site we propose. Our previous AFM studies almost always detected the decamer bound to the very ends of the tubes and only rarely at the tubes’ midsections.12 The plan view (Figure 1A) of the decamer wrapped around a (9, 5) SWNT with diameter dt ) 0.96 nm, an (11, 0) SWNT with dt ) 0.86 nm, and a (9, 1) SWNT with dt ) 0.75 nm illustrates how the three five-centered binding sites on the metallodendrimer can interact with the ends of the SWNTs through various intermolecular interactions. Our AFM and spectroscopic results support that the metallodendrimer prefers to bind to semiconducting SWNTs with diameters similar to that of the (11, 0) SWNT shown in Figure 2. Binding of decamer to the ends of the tube is driven by more than simple Coulombic attraction. We have made the same

J. Phys. Chem. C, Vol. 113, No. 26, 2009 11255 measurements described below using Ru(phenanthroline)32+ as the coagulant. The metallodendrimer shown above is made by bridging these monovalent complexes together by replacing one of the phenanthroline ligands with tpphz. Therefore, the surface charge density on the decamer is slightly smaller than on the Ru(phenanthroline)32+. We do not observe binding of the Ru(phenanthroline)32+ complex to SWNTs.13 Therefore, ionic interactions are not a major contributor to the stability of the supramolecular complexes described here. We believe the fivecentered pocket (Figure 1A, upper right) slips over the end of an SWNT. The π-π stacking and mechanical docking of the decamer around the nanotube form a very stable supramolecular system. The endoreceptor cannot fit over SWNTs that are too large. For smaller diameter nanotubes, which do not “fill” the five-centered pocket, it is more plausible that the equilibrium geometry of the decamer is canted. The angle of the decamer to nanotube axis, illustrated in Figure 2A, has been chosen to maximize the π-π stacking interactions within the pocket and minimize the van der Waals repulsive interactions of the protruding ligands. Preliminary density functional theory (DFT) geometry optimization of a smaller metallodendrimer (Ru dimer) with a smaller nanotube (10, 0) indicates a significant π-π stacking binding energy and wrapping of the tpphz bridging ligand around the SWNT (BLYP functional with all electron relativistic treatment and DNP numerical basis set, DMol3). The distance between the wall of the (11, 0) tube and the tpphz ligand varies from 0.31 to 0.60 nm. These distances are consistent π-π stacking distances in similar structures such as graphite where dπ-π ) 0.34 nm. This type of interaction is consistent with other studies of metalloporphyrins wrapped around SWNTs.18 In this paper, we report Raman spectroscopy and UV-vis-NIR spectroscopy results, which further clarify our understanding of the nature of this binding mechanism. Characteristic changes in the electronic, vibrational, and structural properties of SWNTs due to strong “mechanical docking” of the decamer around the SWNT are consistent with our spectroscopic observations. These supramolecular interactions can lead to directed self-assembly of SWNT structures in ways similar to those of the molecular model shown in Figure 1A. By controlling the type of bridging ligands and the coordinating metal (Ru versus Si) and its stereochemistry, we should be able to direct the self-assembly of SWNTs with tube-tube distances in the range of 1-3 nm. Understanding controlled aggregation of nanotubes and nanowires from dispersions can aid the development of novel, composite high-strength materials. Experimental Section SWNTs (Carbon Nanotechnologies Inc., HiPco grade P) were dispersed into N,N-dimethylformamide (DMF) (Aldrich, asreceived) with about 500 ppm water by ultrasonication for 45 min at 10 W and then filtered through glass wool to remove larger bundles as described previously.12,13 Ultrasonication of the nanotubes in DMF results in unbundled SWNT dispersions, consistent with previous studies.19-21 Isolated SWNT dispersions were found to be stable for weeks at a concentration of 16 mg/ L, consistent with other studies in solvents with a polaritypolarizability similar to that of DMF.22 Variations of the length of the dispersion stability time of nanotubes in DMF may be related to the amount of water dissolved in the DMF. Care must be taken to keep water out of the DMF dispersions. The overtones of the infrared transitions in H2O are significant at 1425 nm. This band has led to confusion regarding SWNT optical transition assignments in the NIR.19,21 The NIR absorption from H2O at 1425 nm is 0.087 AU per percentage (w/w)

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of H2O in DMF. For our dispersions, the water peak is ∼100 times smaller than the SWNT S11 transition peaks. Leaving these dispersions exposed to air will introduce a large water error in the NIR. The concentration of our nanotube solutions is quantified by their absorption at 1000 nm. This region of the spectra is used to monitor the SWNT concentration because it does not have any characteristic sharp electronic van Hove singularity (vHs) transitions. Characteristic vHs absorptions in SWNTs are used to monitor their dispersion state.21 We conclude that, when the S11 vHs band is well resolved and the ratio of the peak height to the background π-plasmon absorption is greater than 0.5, the dispersion is mostly debundled. This ratio for our dispersions varies from 0.5 to >1 and is consistent with the same ratio for SWNTs unbundled by surfactants in H2O23,24 and other amide solvents.25 The SWNTs are functionalized with decamer by adding aliquots of the coagulant to the nanotube dispersion and storing the mixtures in the dark for at least 10 days. For this study, decamer-functionalized SWNTs (dSWNTs) were prepared from various concentrations (8.5, 3.4, 1.15, and 0.085 nM) of added decamer. The heterogeneous solutions were centrifuged, and the aggregated floc was carefully extracted from the solution using a pipet. The separated floc was redispersed into 200 µL of DMF. Detailed experimental procedures describing optimization of control parameters such as the centrifugation force (100 g) and time (5 min) have been reported previously.13 Dilute solutions of decamer are stable. Care must be taken when using more concentrated (>10-6 M) solutions due to possible degradation of the metallodendrimer. We check the integrity of the decamer using the ratio of the π f π* transition to the MLCT transition. The compound is stable for years in the solid state stored in the dark. The supernatant and the floc were characterized using UV-vis-NIR absorption spectroscopy. Solution-phase absorption spectra were recorded on a Cary UV-5000 spectrophotometer using a quartz cell with a path length of 10 mm. All spectra are baseline corrected and normalized. Raman spectra were collected from samples prepared by dropcasting a 20 µL solution of pristine SWNTs or decamerfunctionalized SWNTs on a clean glass slide. Raman spectra were acquired on a Reinshaw micro-Raman spectrophotometer using a near-IR diode (785 nm) laser with an excitation energy of 1.51 eV. The decamer does not have any Raman-active modes in the RBM region, and DMF only has a weak mode at 227 cm-1. DMF is evaporated from the sample overnight. AFM images were acquired from a Digital Instruments Dimension 3100 with a Nanoscope IV controller and numerous probes to ensure consistency and preclude scanning artifacts that are systematic to this technique. AFM images were acquired in contact mode with the probe force optimized to reduce nanotube displacement from the scanning tip. The scanner height calibration is often checked using the atomic plane step heights of epitaxial Si. AFM images were stable upon multiple scanning taken in different scan directions. The d-SWNT was extracted with acetonitrile to remove weakly bound ruthenium complex, redispersed into DMF, and then drop-cast onto an oxide-coated silicon wafer. Control experiments using NaCl or ruthenium monomer as a coagulant do not result in bright spots at the ends of the SWNTs. Adhesion forces common to ambient AFM measurements can result in underestimating the SWNT height due to tube compression; probe forces were minimized to reduce tube compression while maintaining stable contact mode images. Data from 14 samples were analyzed, and the tube diameter and decamer height were determined by analyzing a cross-

Chaturvedi et al. section of the AFM data. To determine tube diameters, the cross section was taken perpendicular to the tube axis; the average height and (95% confidence interval are reported. The height over the decamer bound to the end of the tube is measured along the tube axis, over the decamer and back down to the substrate. DFT-level calculations were completed using Materials Studio 4.3 (Accelrys) using a gradient-corrected BLYP functional, treating all core electrons fully and with relativistic effects with a self-consistent field tolerance of 10-6 Ha and geometry optimization tolerance of 10-5 Ha and (maximum atom displacement of 0.005 Å or a maximum force gradient of 0.002 Ha/Å). All optimizations were treated with a conductor-like screening model with a dielectric constant of 36.7 to simulate the DMF solvent. Results and Discussion Using AFM and scanning electron microscopy (SEM), we have shown preferential binding of the rigid ruthenium decamer to the ends of SWNTs.12 From our AFM images of the decameraggregated floc, we analyzed the diameters of the deposited SWNTs. A statistical sampling of only the tubes that were isolated and had decamer bound to their ends (sample size 14) was used to find the average tube diameter ( 95% confidence interval. SWNTs bound to decamer had a dt ) 0.84 ( 0.03 nm (0.77 nm minimum and 0.94 nm maximum tube diameter). This diameter is consistent with an (11, 0) semiconducting SWNT with dt ) 0.86 nm. It is possible that the actual tube diameter is slightly larger since contact AFM measurements will underestimate the tube’s height. We illustrate a possible binding configuration of a canted decamer to an (11, 0) SWNT in Figure 2A. The angle of the decamer was chosen to optimize π-π stacking interactions, consistent with the dimer system shown in Figure 6. The distance from the bottom of the (11, 0) nanotube to the top of the canted decamer is 3.6 nm (Figure 2A). A representative AFM image of decamer bound to an isolated SWNT is shown in Figure 2C. A cross section along the fast scan direction over the decamer (indicated by the arrow in Figure 2C) is shown in Figure 2B. From the same statistical sample described above, the average height of the decamer bound to a nanotube is 3.4 ( 0.7 nm (1.8 nm minimum and 5.8 nm maximum height). Our model in Figure 2 is consistent with these data. We have shown some AFM data that indicate the decamer is binding to multiple SWNTs and connecting shorter tubes along the length of a longer tube.12 While direct probe techniques such as AFM are consistent with our mechanical docking model, they do not inform us about the details of the binding mechanism. Spectroscopic characterization of d-SWNTs helps to further elucidate these supramolecular interactions. In Figure 3A we compare the NIR spectra of pristine SWNTs with supernatant and with floc solutions of d-SWNTs. After the SWNTs were aggregated with a 1.15 nM solution of decamer, the floc was collected and redispersed by ultrasonication in dry DMF. It is very important to monitor the water content of the DMF because of the H2O overtone absorption bands with a strong peak at 1425 nm. This water transition is within the spectral region of the vHs absorption for the first semiconducting SWNT interband transition, S11. Variations in the NIR peak intensity and position can be associated with variations in SWNT distributions within the dispersion. The peak intensities of transitions in the NIR S11 region of Figure 3A are sensitive to the concentration of SWNTs with a specific tube diameter and chirality.26 Using the tight binding approximation, the optical transition wavelength should

“Mechanically Docked” Metallodendrimers about SWNTs

Figure 3. Baseline-normalized NIR absorption spectra of SWNTs dispersed in DMF (A) and RBM region of Raman spectra of deposited SWNTs (B). Strong vHs aborptions in the NIR S11 band indicate welldispersed pristine isolated SWNTs in DMF (dashed black line). NIR spectra of SWNTs, aggregated with decamer, are shown in (A) where the remaining supernatant spectra are shown as a solid black line and the redispersed floc is shown as a solid pink line. The relative intensity of peak II (A) decreases in the supernatant and increases in the floc solution. Red-shifted peaks in the S11 band for floc spectra are consistent with charge transfer onto the nanotubes. While the RBM Raman spectra (B) of deposited pristine SWNTs (solid black line) are easily reproduced, there is more variability in the RBM Raman spectra from deposited decamer-aggregated floc (solid pink line). The relative intensity of peak I in (B) is enhanced in the spectra from the floc.

vary linearly with the semiconductor tube diameter according to the relation λ11 ) hcdt/2aC-Cγ0, where the C-C bond distance (aC-C) is measured to be 0.142 nm, γ0 ) 2.7 eV is the interaction energy,27 and dt is the tube diameter (nm). Each peak indicated in Figure 3 was fit to a Lorentzian. All spectra were baseline corrected and normalized to the most intense peak for comparison. The relative increase in intensity of peak II in Figure 3A for the d-SWNTs in the floc (λ11 ) 1340 nm) comes at the expense of the relative decrease in intensity of the same transition found in the d-SWNTs left in the supernatant.

J. Phys. Chem. C, Vol. 113, No. 26, 2009 11257 Nanotubes at this diameter are preferentially aggregating out of solution because of the presence of the decamer coagulant. Control studies using NaBr as an ionic coagulant do not show any diameter selectivity. The peak position (ωRBM) in the RBM Raman spectra of SWNTs can also be correlated with the tube diameter by the expression ωRBM ) A/dt + B, where B is a constant representing enhanced environmental interaction due to tube bundling or surface adsorption. These theoretical predictions have been correlated with experimental observations.28,29 Representative RBM Raman spectra are shown in Figure 3B. Regardless of the deposition density, pristine SWNTs (thick black line) yield similar Raman spectra. The spectral intensity from the deposited d-SWNT floc has more variability from sample to sample and even spot to spot (pink). This variability is expected due to the inhomogeneous nature of the sample and the aggregation process. Clearly the nanotubes associated with peak V in Figure 3B are enhanced in the d-SWNT floc as compared to the pristine solution to which the decamer coagulant was added. The peak intensity for Raman spectra is sensitive to perturbations of the oscillator strength and resonance with the excitation source, so care must be taken when associating the peak height with the nanotube concentration.29 The 785 nm excitation source is close to resonance with the Raman RBM of these nanotubes. It has been shown that the intensity of peak V in Figure 3B is sensitive to the aggregation state of the nanotubes and this effect is convoluted with any concentration-dependent effects as observed in our NIR data.30 It was proposed by Heller et al. that the resonance enhancement of the RBM peak at 265 cm-1 is due to increased alignment of the SWNTs. This is also consistent with our model illustrated in Figure 1A. The absolute Raman intensity of the d-SWNTs is only about twice as high as the absolute Raman intensity from the pristine deposited SWNTs. Therefore, the resonance effect does not dominate our interpretation. Future studies need to include single-nanotube Raman studies of d-SWNTs correlated with AFM. Often NIR or RBM Raman spectra are associated with a tube diameter scale. These scales are based on theory and often adjusted to correlate the average nanotube diameter from the spectral assignment with the average nanotube diameter from some independent technique, such as scanning tunneling microscopy (STM), AFM, transmission electron microscopy (TEM), or X-ray diffraction (XRD).31,32 In Figure 4 we plot both the NIR and RBM Raman spectra on the same tube diameter scale. The spectra from the d-SWNT floc are shown in pink. The diameter scale for the Raman spectra is ωRBM ) (223.5 cm-1 nm)/dt + 12.5 cm-1. We chose this relationship from the literature26 since it is highly referenced and used to described samples like ours. Using this relationship, the most prominent RBM transition, peak V in Figure 2B at ω ) 267 cm-1, corresponds to a tube diameter of dt ) 0.878 nm. Some other fitting relationships of this form are (A ) 223.75, B ) 0 f dt ) 0.838 nm),33 (A ) 218.3, B ) 15.9 f dt ) 0.869 nm),29 and (A ) 248, B ) 0 f dt ) 0.929 nm).34 NIR transition wavelengths are converted to tube diameters using the tight binding approximation model described above. Since we are mostly concerned with small-diameter semiconducting SWNTs, we have included a correction for Coulomb effects that increase the observed transition energy above that predicted by the tight binding approximation.35 This correction yields a transition energy that is proportional to dt-1.3. Using this relationship and the independently measured constants listed above, we have dt ) [(6.184 × 10-4)λ11]1/1.3. This relationship places peak II of Figure 3A at dt ) 0.86 nm. Without any

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Chaturvedi et al.

Figure 4. Combined NIR and Raman spectra showing selectivity of the decamer docking around a small range of SWNT diameters. The NIR wavelength was converted to diameter using the tight binding approximation corrected for Coulomb interaction effects. The decameraggregated floc (thick pink line) NIR spectrum shows a higher intensity at dt ) 0.86 nm as compared to the supernatant spectrum (solid black line). The RBM Raman spectrum of the deposited floc (thin pink line) also shows enhanced intensity at dt ) 0.87 nm as compared to the deposited pristine SWNT spectrum (dashed black line). The average diameter and (95% confidence interval of decamer-bound SWNTs determined by AFM is indicated by the vertical line and open brackets, respectively.

Coulomb correction peak II would correspond to dt ) 0.89 nm. We have not used any arbitrary shifting or scaling factors when assigning tube diameters in Figure 4. We observe a relative increase of peak II in the NIR spectra of the floc solution, and we observe a relative increase of peak V in the Raman spectra of the deposited floc solution. Both of these peaks correspond to the same diameter SWNTs. We have annotated Figure 4 with the average nanotube diameter ( 95% confidence interval obtained from the deposited floc of dSWNTs from AFM data. These data are consistent with our model of mechanical docking of decamer about SWNTs with diameter selectivity. To be clear, as we increase the concentration of decamer, the concentration of all SWNTs in solution decreases. These data are consistent with a preferential aggregation of SWNTs associated with peak II in the NIR such as the (11, 0) SWNTs. All tubes will eventually coagulate from solution as we approach the critical coagulation concentration (ccc) of decamer (>20 nM in dry DMF). Figure 5 shows the S11 vHs interband transitions acquired from solution-phase NIR spectroscopy. Peaks I-IV are assigned the same as those described in Figure 3A and are baseline adjusted and normalized to peak I. After addition of decamer, the floc is carefully removed using centrifugation and a micropipet. The floc is then redispersed for 45 min under 10 W of ultrasonication in pure, dry DMF. Spectra from the remaining supernatant solution are shown in Figure 5A. As the concentration of decamer is increased, the relative intensity of peak II decreases preferentially. Spectra of the redispersed floc are shown in Figure 5B. There was not enough floc from the lowest (0.085 nM) concentration of decamer to accurately measure the SWNTs above the small water contamination background. As the concentration of decamer increases from

Figure 5. NIR spectra of d-SWNT solutions, baseline adjusted at 1032 nm and normalized to peak I. A spectrum of pristine SWNTs dispersed in dry DMF is shown in (A) as a reference. The concentration of decamer is increased, 0.085, 1.15, 3.4, and 8.5 nM, but kept well below the ccc for this system. After centrifugation the floc is removed and redispersed. The NIR spectra for the supernatant are shown in (A), and the spectra for the floc are shown in (B). Smaller diameter nanotubes leave the supernatant relative to larger diameter tubes. SWNTs characterized by peak II are preferentially aggregated from the supernatant and are found in the floc. The peak positions in the supernatant are independent of the decamer concentration, while these peaks are red-shifted in the floc, indicative of charge transfer (doping).

1.15 to 8.5 nM, the relative intensity of the same peak II increases, accounting for the SWNTs that aggregated out of the supernatant. According to our mechanical docking model, the decamer should bind to an SWNT and destabilize the particle’s electrical double layer repulsion, and the supramolecular complex should aggregate from solution. Therefore, SWNTs left in the supernatant should not be strongly interacting with decamer. In fact, after we completely aggregate all of the SWNTs from dispersion, due to high ionic strength disruption of the tube’s electrical double layer, we cannot detect any decamer in the supernatant, and most of the ruthenium is accounted for in the floc.13 Charge transfer interactions effectively dope the nanotubes, which results in downshifted Raman transitions when electron density is donated into the SWNT.36 Raman modes of the decamerbound SWNT are downshifted relative to those of the pristine SWNTs. The pristine SWNTs exhibit Raman modes at 1593.4

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TABLE 1: S11 Red Shift versus Decamer Concentration of the Supernatant and Floc peak I

peak II

peak III

concn (nM) supernatant floc supernatant floc supernatant floc 0.085 1.15 3.40 8.50

0 0 0 0

n/a +10 +6 0

0 0 0 0

n/a +20 +8 +5

0 0 0 0

n/a +20 +13 +8

cm-1 (G+), 1565.0 cm-1 (G-), and 1296.9 cm-1 (D) (spectra not shown). The decamer-bound SWNTs exhibit Raman modes at 1589.9 cm-1 (G+), 1562.3 cm-1 (G-), and 1293.0 cm-1 (D) (spectra not shown). These spectral downshifts of 3.5 cm-1 (G+), 2.8 cm-1 (G-), and 3.9 cm-1 (D) are consistent with electron donation from the ruthenium coordination complex into the SWNT upon binding. We also expect that, as the interaction of decamer and SWNT increases, there should be red-shifting of the vHs transitions of the SWNTs due to charge transfer effects.37,38 For coordination complexes like the metallodendrimer under study here, we expect more charge transfer with the SWNT when the complex is more strongly bound to the tube. This hypothesis is supported by photon-enhanced aggregation of d-SWNTs from optical excitation of the MLCT band in the decamer.39 We find that all of the NIR peaks in the S11 band of the d-SWNT floc are red-shifted. Moreover, none of the peak positions of the d-SWNT supernatant are affected at any concentration of decamer. This is consistent with our model that the metallodendrimer is strongly bound to the nanotube as the supramolecular complex aggregates from solution. π-π stacking is likely the most significant interaction for our supramolecular complex. It has been shown that daunomycin also interacts strongly with SWNTs through π-π stacking and the S11 band on the functionalized tubes red-shifted by 22 nm.40 Optical transitions from the higher energy SWNT vHs bands S22 (550-900 nm) and M11 (400-550 nm) are not affected by the addition of decamer as expected.41 Interestingly, the red shift in the d-SWNT floc spectra, Figure 4B, is largest (+20 nm for peak II) for the smallest concentration of decamer added. Table 1 lists spectral shifts for the supernatant and floc solutions as a function of the decamer concentration for peaks I-III. Since the decamer is irreversibly bound to the nanotubes, it will donate the most charge density to the tubes in which it is most tightly docked. These supramolecular complexes will come out of solution first and result in the largest spectral shifts. NIR data suggest that the smaller diameter tubes are aggregating before the larger tubes. Moreover, the narrow SWNTs exhibit a larger red shift at low decamer concentration. As the decamer concentration goes up, nanotubes of all diameters continue to aggregate from solution. These larger tubes are less effectively bound to the nanotubes, and consequently, there is a smaller spectral red shift. We see a similar response in the π f π* transition of the tpphz bridging ligand upon mechanical docking of the decamer to the SWNT. The UV absorption of the tpphz π f π* transition for the free decamer in DMF is at 375 nm.13 Since the concentration of decamer added to aggregate the SWNTs is so small, it is challenging to obtain a UV-vis spectrum. The tpphz transition of the decamer, left in the supernatant after the d-SWNT is removed, is red-shifted by 2 nm (spectra not shown), indicating a weak interaction. After the d-SWNTs aggregate out of dispersion, they are collected and redispersed into DMF. The tpphz transition from the mechanically docked complex is red-shifted by 6 nm. These spectral shifts are consistent with

Figure 6. DFT geometry optimization of a dimer bound to a (10, 0) SWNT. In the initial structure the tpphz bridging ligand is planar. In the final structure the bridging ligand is distorted so as to wrap around the SWNT. The LUMO, shown in (A), exists on the tube and stretches out toward the dimer. The electrostatic potential is color mapped onto the total electron density (isosurface 0.017 e/Å3) (B) where red is +0.3 C and blue is +0.13 C.

other charge transfer studies using daunomycin (red-shifted by 9 nm)40 or tin(IV) porphyrins (red-shifted by ∼10 nm)42 either physisorbed or covalently linked to SWNTs, respectively. In each of these studies it is claimed that the SWNT is the electron acceptor. Modeling of the ruthenium metallodendrimer bound to an SWNT is computationally expensive; the model in Figure 2A has 832 atoms. Since we believe the π-π stacking between the tpphz bridge and the SWNT surface is central to these interactions, we have modeled a much smaller bimetallic ruthenium complex bound to a (10, 0) SWNT as shown in Figure 6 (number of atoms 312). The structures shown in Figure 6 were geometry optimized using DFT. The bimetallic ruthenium dimer was first optimized by itself. The tpphz bridging ligand was initially planar, and the ruthenium centers were both octahedral. The (10, 0) tube was placed near the dimer such that it was just at van der Waals contact at the narrowest point between the phenanthroline ligands. The tpphz bridge is distorted in the final optimized geometry of the bound complex. It appears that the dimer is wrapping around the nanotube. The mechanism for charge transfer is not clear for these ground-state systems. The lowest unoccupied molecular orbital (LUMO; 0.02 e/Å3) is shown in Figure 6A. This orbital is asymmetric and extending from the tube toward the dimer, but there is no overlap with a filled molecular orbital on the dimer (except at energies that would require optical excitation). There is significant overlap of electron density between the SWNT and the dimer as

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illustrated in Figure 6B. We modeled the amount of charge transferred to or from the SWNT by comparing the optimized structure shown to that where the tube was left 6.0 nm away from the dimer. Mullikan population analysis43 was used, and the net charge on every atom in the nanotube model was summed (20 H and 100 C atoms). All calculations used the conductor-like screening model44 with a dielectric constant of 36.7 (to simulate DMF). The calculated charges were very sensitive to the screening model and the atom orbital cutoff lengths that we set. These charges are not assumed to be realistic and are used for comparison only. The charge on the tube when it is far from the dimer is +0.428 e and the charge on the tube in the structure shown in Figure 5B is +0.345 e, indicating that electron density went from the dimer to the tube, which is consistent with the literature.40 A more accurate method to determine atomic charge is by fitting these charges to the electrostatic potential. This method is not useful for our model since the dimer has a net +4 charge and the SWNT lies within the molecule’s electrostatic potential. Additional time-dependent and excited-state calculations are required to more fully elaborate on the charge transfer mechanism. Conclusions Experimental evidence for mechanical docking of a rigid dendrimer around an SWNT has been presented. We have shown that the metallodendrimer binds strongly to SWNTs whereas other small ruthenium salts do not. The metallodendrimer is observed at the ends of the SWNTs and not bound along the tube wall. Previous studies have shown that extraction in acetonitrile does not remove the metallodendrimer from the SWNTs’ ends. The proposed binding site on the metallodendrimer is a five-centered pocket which is only accessible to the ends of the SWNTs since the opening to this pocket is too narrow. Moreover, the pocket preferentially binds to SWNTs of a specific diameter. Diameter-specific binding supports a mechanical docking mechanism. The diameter dependence on the binding affects the resultant enhanced aggregation rates of nanotubes from dispersion, indicating a charge transfer process. Additionally, these supramolecular complexes exhibit spectroscopic effects consistent with charge transfer from the ruthenium complex into the nanotube. The S11 bands and Raman bands of the SWNTs shift upon metallodendrimer binding. These shifts are consistent with electron transfer onto the SWNT. The smaller [Ru(phenanthroline)3]2+ is also a good charge transfer reagent, but we do not observe any spectral shifts of the SWNTs when they are mixed into the dispersion. This implies that the metallodendrimer is bound more tightly to the SWNT, such as wrapped around the nanotube. Moreover, the tpphz absorption red shifts when the dendrimer is bound to the SWNT. Further studies of this system using nanotube dispersions of constant-diameter tubes and metallodendrimers with modified binding pockets are required to fully elucidate this binding mechanism. Computational studies are in progress and are needed to determine the charge transfer mechanism in the dark and under optical illumination. This mechanical docking mechanism shows promise toward developing 3D architectures capable of light-harvesting and sensing applications. Acknowledgment. This research was supported in part by an award from the Research Corp., the NSF (Grant Nos. 0404193 (J.C.P.) and CHE-0518649 (F.M.M.)), the Robert A. Welch Foundation (Grant No. Y-1301 (F.M.M.)), and significantly the ARL (Grant No. W911NF-05-2-0053 (H.C.)). Acknowledgement is made to the donors of the Petroleum

Chaturvedi et al. Research Fund, administered by the American Chemical Society, for partial support of this research. We also thank Dr. Cliff Carlin for his help and expertise on the properties of the ruthenium complexes and various technical support and Dr. Tom Dubois for numerous discussions regarding charge transfer. Supporting Information Available: Details of the DFT calculations using DMol3 by Accelrys. This information is available free of charge via the Internet at http://pubs.acs. org/. References and Notes (1) Guldi, D. M. Angew. Chem., Int. Ed. 2006, 45, 695. (2) Dresselhaus, M.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer-Verlag: New York, 2001. (3) Chen, R.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (4) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem. Lett. 2002, 31, 638. (5) Zhu, J.; Yudasaka, M.; Zhang, M.; Iijima, S. J. Phys. Chem. B 2004, 108, 11317–20. (6) Stevens, J. L.; Huang, A. Y.; Peng, H.; Chiang, I. W.; Khabashesku, V. N.; Margrave, J. L. Nano Lett. 2003, 3, 331. (7) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Balbinot, D.; Hartnagel, U.; Tagmatarchis, N.; Prato, M. J. Am. Chem. Soc. 2005, 127, 9830. (8) Leung, K. C.-F.; Arico, F.; Cantrill, S. J.; Stoddart, J. F. J. Am. Chem. Soc. 2005, 127, 5808. (9) Lehn, J.-M. Supramolecular Chemistry Concepts and PerspectiVes; VCH: Weinheim, Germany, 1995. (10) Yi, W. H.; Malkovskiy, A.; Chu, Q. H.; Sokolov, A. P.; Colon, M. L.; Meador, M.; Pang, Y. J. Phys. Chem. B 2008, 112, 12263. (11) Star, A.; Stoddart, J. F. Macromolecules 2002, 35, 7516. (12) Chaturvedi, H.; Poler, J. C. J. Phys. Chem. B 2006, 110, 22387. (13) Giordano, A. N.; Chaturvedi, H.; Poler, J. C. J. Phys. Chem. C 2007, 111, 11583. (14) MacDonnell, F. M.; Bodige, S. Inorg. Chem. 1996, 35, 5758. (15) Ali, M. M.; MacDonnell, F. M. J. Am. Chem. Soc. 2003, 125, 2014. (16) MacDonnell, F. M.; Kim, M. J.; Wouters, K. K.; Konduri, R. Coord. Chem. ReV. 2003, 242, 47. (17) Kim, M.; MacDonnell, F.; Gimon-Kinsel, M.; Du Bois, T.; Asgharian, N.; Griener, J. Angew. Chem., Int. Ed. 2000, 39, 615. (18) Chichak, K. S.; Star, A.; Altoe´, M.; Virginia, P.; Stoddart, J. F Small 2005, 1, 452. (19) Ausman, K. D.; Piner, R.; Lourie, O.; Ruoff, R. S.; Korobov, M. J. Phys. Chem. B 2000, 104, 8911. (20) Furtado, C. A.; Kim, U. J.; Gutierrez, H. R.; Pan, L.; Dickey, E. C.; Eklund, P. C. J. Am. Chem. Soc. 2004, 126, 6095. (21) Krupke, R.; Hennrich, F.; Hampe, O.; Kappes, M. M. J. Phys. Chem. B 2003, 107, 5667. (22) Bergin, S. D.; Nicolosi, V.; Streich, P. V.; Giordani, S.; Sun, Z.; Windle, A. H.; Ryan, P.; Niraj, N. P. P.; Wang, Z.-T. T.; Carpenter, L.; Blau, W. J.; Boland, J. J.; Hamilton, J. P.; Coleman, J. N. AdV. Mater. 2008, 20, 1876. (23) Ryabenko, A. G.; Dorofeeva, T. V.; Zvereva, G. I. Carbon 2004, 42, 1523. (24) Yamamoto, T.; Miyauchi, Y.; Motoyanagi, J.; Fukushima, T.; Aida, T.; Kato, M.; Maruyama, S. Jpn. J. Appl. Phys. 2008, 47, 2000. (25) Giordani, S.; Bergin, S. D.; Nicolosi, V.; Lebedkin, S.; Kappes, M. M.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. B 2006, 110, 15708. (26) Bachilo, S.; Strano, M.; Kittrell, C.; Hauge, R.; Smalley, R.; Weisman, R. Science 2002, 298, 2361. (27) Wildoer, J. W. G.; Venema, L. C.; Rinzler, A.; Smalley, R.; Dekker, C. Nature 1998, 391, 59. (28) Jannik, C. M.; Matthieu, P.; Thierry, M.; Alain, M.; Anita, N.; Georg, S. D.; Siegmar, R.; Jean-Louis, S. Phys. ReV. Lett. 2005, 95, 217401. (29) Jorio, A.; Fantini, C.; Pimenta, M. A.; Capaz, R. B.; Ge, G. S.; Dresselhaus, G.; Dresselhaus, M. S.; Jiang, J.; Kobayashi, N.; Gruneis, A.; Saito, R. Phys. ReV. B 2005, 71, 075401. (30) Heller, D. A.; Barone, P. W.; Swanson, J. P.; Mayrhofer, R. M.; Strano, M. S. J. Phys. Chem. B 2004, 108, 6905. (31) Bandow, S.; Asaka, S.; Saito, Y.; Rao, A. M.; Grigorian, L.; Richter, E.; Eklund, P. C. Phys. ReV. Lett. 1998, 80, 3779. (32) Chiang, I. W.; Brinson, B. E.; Smalley, R. E.; Margrave, J. L.; Hauge, R. H. J. Phys. Chem. B 2001, 105, 1157. (33) Rao, A. M.; Bandow, S.; Richter, E.; Eklund, P. C. Thin Solid Films 1998, 331, 141.

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