Tuning the Carbon Nanotube Selectivity â Optimizing Reduction Po

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Tuning the Carbon Nanotube Selectivity – Optimizing Reduction Po-tentials and Distortion Angles in Perylenediimides Peter Muenich, Christoph Schierl, Konstantin Dirian, Michel Volland, Stefan Bauroth, Leonie Wibmer, Zois Syrgiannis, Timothy Clark, Maurizio Prato, and Dirk M. Guldi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00452 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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Journal of the American Chemical Society

Tuning the Carbon Nanotube Selectivity – Optimizing Reduction Potentials and Distortion Angles in Perylenediimides £

Peter W. Münich,‡,§ Christoph Schierl,‡,§ Konstantin Dirian,§ Michel Volland,§ Stefan Bauroth,§, Leonie £ Wibmer,§ Zios Syrgiannis,# Timothy Clark, Maurizio Prato,#,¶,† Dirk M. Guldi§ § Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials, Friedrich-AlexanderUniversität Erlangen-Nürnberg, 91058 Erlangen, Germany # Center of Excellence for Nanostructured Materials, Dipartimento di Scienze Chimiche e Farmaceutiche, INSTM unit of Trieste, University of Trieste, Piazzale Europa 1, 34127 Trieste, Italy £ Computer-Chemie-Centrum, Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität ErlangenNürnberg, Nägelsbachstr. 25, 91052 Erlangen, Germany ¶ Carbon Nanobiotechnology Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia-San Sebastian (Spain) †

Basque Fdn Sci, Ikerbasque, Bilbao 48013, Spain

ABSTRACT: Different water-soluble perylenediimides (PDI) have been used to individualize and stabilize single walled carbon nanotubes (SWCNT) in aqueous media. A key feature of the PDIs is that they can be substituted at the bay positions via the addition of two and/or four bromines. This enables control over structural and electronic PDI characteristics, which prompted us to conduct comparative assays with focus on SWCNT chirality and charge transfer. Electrochemical, microscopic, and spectroscopic experiments were used to investigate the SWCNT chiral selectivity of PDIs, on the one hand, and the manipulation of reorganization energies in charge-transfer reactions by PDIs, on the other hand.

Introduction Nanocarbons play a key role in the construction of artificial electron donor-acceptor architectures.1 In such architectures, a unidirectional charge separation, which is induced by selective photoexcitation, powers the formation of charge-separated states. To be used efficiently in, for example, solar energy conversion schemes the charge-separated state should be high in energy, on the one hand, and long in lifetime, on the other.2 In the context of photoinduced electron transfer chemistry, the advent of zero dimensional fullerenes was a major milestone.3 Important is their ability to accept electrons at rather low potentials,4 while only undergoing very minor structural rearrangements.5 Subsequently, the focus shifted to one-dimensional single wall carbon nanotubes (SWCNT). In stark contrast to fullerenes, which serve mainly as electron acceptors,6 SWCNTs have been shown to act as both electron acceptor and donor.1b, 1c, 7 It is, however, their electron donating, p-type nature that is of great interest in combination with electron accepting, n-type molecular materials.8 Perylenediimides (PDI) stand out among the latter.7a, 9 For example, it has been demonstrated that PDIs assist in individualizing SWCNTs in aqueous media

by forming SWCNT/PDI electron donor-acceptor hybrids.9a, 10 The stability of these hybrids results from p-doping of SWCNTs (donation of electron density from SWCNTs to PDIs). SWCNTs are, in stark contrast to fullerenes, polydisperse:11 a typical batch contains SWCNTs of several different chiralities/diameters. Enriching and sorting SWCNTs by chirality/diameter is, however, fundamental for understanding the strength of SWCNT/PDI interactions in the ground and excited states. Notably, attempts towards sorting SWCNTs have been made,12 but with only a limited success. Examples for chirality/diameter dependent thermodynamics and kinetics of charge separation and recombination are scarce.13 We have therefore now employed three different water soluble PDIs (13, see Scheme 1), which differ in their electron acceptor strengths and their core twist angles. Of great significance is that 1-3 are SWCNT chirality/diameter selective. We present here, to the best of our knowledge for the first time, a comprehensive study of the effect of different electron acceptor strengths and core twist angles, on binding strength, excitedstate dynamics, and chiral SWCNT selectivity.

Results and Discussion

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tion. The dispersion stability is found to decrease from 3/SWCNT to 1/SWCNT and to 2/SWCNT. The short-time stability needed to characterize them fully, however, is confirmed by UV/Vis/nIR control measurements, which show no appreciable decrease in optical density due to SWCNT precipitation.

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Scheme 1. Structures of PDIs 1-3. The iodide salts were used. 0.2

Steady state experiments The stepwise introduction of two or four bromine atoms into the bay position of the perylene core of 1 gives rise to a distortion in the perylene core in 2 and 3, and to a concomitant lowering of the reduction potential.14 In recent work, the distortion angles for 2 and 3 are reported as 24.4 and 37.0°, respectively.15 In our own electrochemical investigations in water – Figure S1 – the reduction potentials were found to be as low as -350 mV (1), -150 mV (2), and -40 mV (3) versus Ag/AgCl. Because the optical fingerprints of PDIs allow their interactions with SWCNTs to be monitored in situ, we probed the absorption and fluorescence of 1-3 in D2O – Figures S2 and S3.7a, 10a Two major absorption maxima at 500 and 535 nm dominate the absorption spectrum of 1, whereas maxima at 540 and 585 nm are seen in the fluorescence spectra. Absorption and fluorescence are, however, not mirror-images of each other, neither is the excitation spectrum superimposable on the absorption spectrum. The same conclusions can be derived for 2 with 498 and 535 nm absorption maxima and 559 and 604 nm fluorescence maxima. Regardless of the concentration, 1 and 2 are present in the form of H-type aggregates.16 3, in stark contrast, whose absorption and fluorescence maxima at 501/535 and 576/625 nm, respectively, are mirror images of each other, and whose excitation and absorption spectra are a perfect match, is present as monomer.17 To distinguish between the effects of aggregation and of the bromine substituents, we probed the excited-state dynamics of 1-3 by femtosecond pump-probe measurements – Figures S4 – S6. The most significant spectral changes on photoexcitation of 1 with 387 nm laser pulses are found at 700 and 590 nm; these represent the monomer and aggregate, respectively. Multi-wavelength analyses reveal a 1.8 x 109 s-1 fast deactivation of the singlet excited-state feature in the aggregate, while in the monomers the deactivation is as slow as 2.6 x 108 s-1. 2 gives rise to a similar deactivation pattern. The only exception is that intersystem crossing (ISC) affords the corresponding triplet excited state in aggregates of 2.16b, 18 ISC was also confirmed in pump-probe experiments with monomeric 3. For 2 and 3, enhancement of the ISC rates originates either from a heavy atom effect, a twisting of the aromatic system or in case of 2 an aggregation induced ISC.16b, 19 Detailed descriptions of the preparation of 1/SWCNT, 2/SWCNT and 3/SWCNT are given in the experimental sec-

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Figure 1. UV/Vis/nIR absorption spectrum of 1/SWCNT (black), 2/SWCNT (red), and 3/SWCNT (grey) recorded in D2O.

The PDI absorption spectra show a redistribution of the relative oscillator strengths and a red shift of the maxima on complexation– Figures 1 and S7. For example, the maxima red shift from 500/518 nm to 535/560 nm goes hand-in-hand with the relative oscillator strength change from 500 > 518 nm to 535 < 560 nm for 1 and 1/SWCNT, respectively. Similar trends were found for 2 and 3 and point to effective individualization of the PDIs and to their successful immobilization onto SWCNTs.7a, 10, 13b The SWCNT absorption shows an overall broadening of the S11 features of 6,5-SWCNT, 7,5SWCNT, and 7,6-SWCNT at 1010, 1050, and 1160 nm, respectively, compared, for example, to those of SWCNTs wrapped and individualized by surfactants.11 Both the fluorescence of the PDIs and that of the SWCNTs are strongly quenched in the respective PDI/SWCNTs – Figure S8. Two likely explanations are the presence of bundles/ropes within the dispersions,20 or electronic interactions between 1, 2, and 3, and the SWCNTs.21 Careful topographical analysis of 1/SWCNT, 2/SWCNT, and 3/SWCNT by transmission electron microscopy (TEM) and atomic force microscopy (AFM) demonstrates that individualized SWCNTs are present – Figures S9 - S14. Please note that we considered for our AFM analysis only the lateral resolution. In other words, absorption broadening and fluorescence quenching originate from strong electronic coupling within 1/SWCNT, 2/SWCNT, and 3/SWCNT, rather than from bundling/roping. Closer examination of the SWCNT S11 transitions reveals no discernible selectivity for 1, since 6,5-SWCNT, 7,5-SWCNT, and 7,6-SWCNTs all coexist. In contrast, 2 and 3 show preferential complexation. For example, in 2/SWCNT, 6,5-SWCNTs clearly complex more strongly than 7,5- or 7,6-SWCNTs, whereas 3/SWCNT consists mainly of 7,5-SWCNTs, as shown by comparing the absorptions at 1050 nm with those at 1008 and 1159 nm. We hypothesize that the oscillator strength increases upon selective interaction of the PDIs with the different SWCNT chiralities.


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Figure 2. Histograms resulting from statistical Raman spectroscopy of 1/SWCNT (black), 2/SWCNT (red), and 3/SWCNT (grey) showing the peak maxima of the G-mode (top) and 2Dmode (bottom). The displayed histograms were generated from more than one thousand recorded spectra.

Statistical Raman spectroscopy at 633 nm excitation was used to complement the above – Figure 2. Firstly, the question of the electronic coupling was answered by inspecting changes in the G- and 2D- modes upon immobilization of the PDIs on SWCNTs. For the G-mode in the 1590 cm-1 range, sizable upshifts support a redistribution of charge density with upshifts to 1585 cm-1 for 1, 1588 cm-1 for 2, and 1591 cm-1 for 3. The same is seen for the 2D-mode in the 2600 cm-1 range. This mode is shifted to 2599 cm-1 for 1, to 2602 cm-1 for 2, and to 2607 cm-1 for 3. Clearly, the upshifts correlate with the electron-acceptor strength of the PDI: 1 < 2 < 3. This suggests partial oxidation of the SWCNTs and a partial reduction of the PDIs, even in the ground state. Derived mean spectra are shown in Figure S15. Secondly, we focused on the chiral selectivity, for which the radial breathing mode (RBM) in the 250 to 300 cm-1 range is indicative – Figure 3. All spectra were normalized to the RBM of 7,6-SWCNTs at around 260 cm−1. The intensity of 6,5-SWCNTs in 2/SWCNT and 7,5-SWCNTs in 3/SWCNT is increased relative to 1/SWCNT, for which no particular selectivity was observed. Thus, the results of the Raman spectroscopy are consistent with those seen in the absorption measurements.

Figure 3. Top: Averaged Raman spectra of 1/SWCNT (black) and 2/SWCNT (red) showing the RBM of 7,6, 7,5 and 6,5 SWCNTs. The spectra are normalized to the 7,6 RBM at 255 cm1 . Bottom: Averaged Raman spectra of 1/SWCNT (black) and 3/SWCNT (grey) showing the RBM of 7,6, 7,5 and 6,5 SWCNTs. The spectra are normalized to the 7,6 RBM at 255 cm-1. All spectra are recorded on silica wafers using 633 nm laser excitation.

We also monitored the fluorescence intensity of 1/SWCNT, 2/SWCNT, and 3/SWCNT as different concentrations of cetyltrimethylammoniumbromide (CTAB) were added to replace the PDIs from SWCNTs. Emphasis was placed on the fluorescence intensity, which was plotted versus the [CTAB/SWCNT] ratio, and fitted to a sigmoidal function.22

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Figure 4. Top: Emission spectra of 1/SWCNT following 660 nm excitation with increasing amounts of CTAB added. The legend indicates the [CTAB/SWCNT] ratio. Bottom: Plot of the emission recovery vs. the [CTAB]/[SWCNT] ratio in 1/SWCNT dependent on the SWCNT chirality, i.e. 6,5 (black cycles and traces), 7,5 (grey cycles and traces) and 7,6 (red cycles and traces) SWCNTs.

Figure 2. Top: Emission spectra of 3/SWCNT following 660 nm excitation with increasing amounts of CTAB added. The legend indicates the [CTAB/SWCNT] ratio. Bottom: Plot of the emission recovery vs. the [CTAB]/[SWCNT] ratio in 3/SWCNT dependent on the SWCNT chirality, i.e. 6,5 (black cycles and traces), 7,5 (grey cycles and traces) and 7,6 (red cycles and traces) SWCNTs.

By monitoring of the near-infrared fluorescence of the three most prominent types of SWCNTs in either 1/SWCNT, 2/SWCNT, or 3/SWCNT enables conclusions about the selectivity of PDI 1-3 with respect to SWCNT chirality. Nearinfrared fluorescence results for 1/SWCNT and 3/SWCNT are shown in Figures 4 and 5, respectively, and those for 1/SWCNT, 2/SWCNT, and 3/SWCNT in Figures S16-S18. Figure 4 shows that the inflection point for 1/SWCNT changes insignificantly and ranges from 8.4 for 6,5-SWCNTs and 7,5-SWCNTs to 7.9 for 7,6-SWCNTs. The differences for 2/SWCNT (Figure S19) are more subtle, with values as low as 5.7 for 7,6-SWCNTs and 6.0 for 6,5-SWCNTs, but strikingly higher for 7,5-SWCNTs with 6.7. The dissimilarities for 3/SWCNT are much stronger: in perfect agreement with absorption and Raman measurements, higher [CTAB/SWCNT] ratios were found necessary for 7,5-SWCNTs (9.7), followed by 6,5-SWCNTs (9.1) and a significantly lower value of 7.4 for 7,6-SWCNTs. Overall, the displacement of 7,6-SWCNTs is easier than of 6,5-SWCNTs and 7,5-SWCNTs regardless of the PDI. This trend is in sound agreement with theoretical works that describes SWCNT diameter and chirality dependent affinities of alkyl-chain-based surfactants and SWCNTs.23 For the same reason, the relative changes of the inflection points, rather than absolute values, should be considered.

On the other hand, displaying just a single SWCNT chirality as a function of the PDI reflects the respective binding strength. A similar picture evolves when the three different PDIs are compared with respect to one single SWCNT chirality. In general, the binding between 2 and all SWCNTs is the weakest among all PDIs, whereas 1 and 3 are different. 3 shows the strongest binding of all PDIs, while the binding strength for 1 is between that of 2 and 3. A likely rationale for the striking differences in both binding strength and chiral selectivity is the distortion in the perylene core in the following order: 1 < 2 < 3. It seems, however, that in addition to the core twist, further factors counterbalance the overall binding strength in 2/SWCNT – see molecular modeling. As such, fine-tuning the twist angle in PDIs is the key feature that determines the chiral selectivity of electronic interactions with SWCNTs.24

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Molecular modelling To provide atomistic details, density-functional theory (DFT) calculations were performed to determine the binding strength for the different PDI-SWCNT hybrids.25 All diasteriomer combinations of chiral SWCNTs with 1 (C2v-symmetry) and 2,3 (C2-symmetry) were optimized at the PBE026/631G(d,p)27 level including Grimme’s D3 dispersion correction28 and water PCM-solvation model29. The binding strength (Tables S1-S2, and Figures S20 and S21) increase from 35 kcal mol-1 to 45 kcal mol-1, while the change in charge on the PDI core (Figures S22 and 6) reflects the increasing reductive strength with higher degree of bromination in the bay positions.30 Average charges on the PDI core of -0.64, -0.66 and 0.70 electrons were found for 1/SWCNT, 2/SWCNT, and 3/SWCNT. Interestingly the changes in the PDI core charge for 2anti/SWCNT, where the bromines face the solvent environment, is higher than for 2/SWCNT hybrids. This is one reason for the lower binding energies (up to 6.5 kcal mol-1) determined for 2anti/SWCNT.

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Journal of the American Chemical Society SWCNTs, and 7,6-SWCNTs, respectively. On the same grounds, charge separation is confirmed for 6,5-SWCNTs, 7,5-SWCNTs, and 7,6-SWCNTs in 3/SWCNT (Figures 7 and S35) and 2/SWCNT (Figure S34-S35). The SWCNT centered bleaching in the near infrared is in sound agreement with the chirality-selective interactions of SWCNTs, as they are mirror images of the ground-state absorption spectra. – Figure 8.

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Figure 6: Change in electrostatic potential (∆ESP) for 3 / 7,6 – SWCNT mapped on the electron density (0.02 e-/Å3) ranging from -0.018 (blue) to +0.004 (red). ∆ESP was derived by subtraction of individual ESPs of SWCNT and pure PDI from the hybrid ESP.

The changes in binding energy of different SWCNTs with the same PDI are less pronounced and within a difference of 2 kcal mol-1. This is not surprising, bearing in mind the small variance in the geometric parameters of the SWCNT´s; the diameter (7.5, 8.2 and 8.9 Å) and the chiral angle of (32.8°, 35.3° and 32.5°) for 6,5-, 7,5- and 7,6- SWCNT, respectively. However, the subtle changes in the binding energy can be correlated to a change in binding geometry. For instance, the planar and most flexible PDI, 1 (Figure S23) is better stabilized with increasing size of the SWCNT (Figure S20), whereas the pre-shaped PDIs 2 and 3 clearly show better stabilization of smaller 6,5-SWCNT than for 1.31 As the chiral angles of 6,5 -and 7,6- SWCNTs are almost the same, we conclude that this is a pure diameter effect. Taking into account that 3 is 0.4 kcal mol-1 better stabilized on 7,5- while the corresponding 6,5-hybrid is 0.95 kcal mol-1 less stable than the 7,6-SWCNT hybrid clearly indicates the importance of two main geometric parameters. Van der Waals interactions can be optimized by matching size and by proper alignment along the carbon lattice of the SWCNT.32 In order to study this dependency on a variety of structures, a reference hybrid system was used with force-field molecular-dynamics calculations (see Figures S24-S33). There the clear relation of the correlation energy with the geometric parameters - the bay twist angle, the van der Waals distance and the orientation with respect to the carbon lattice (rotation) can be seen in the supporting information. Interestingly, the bay twist angle is smaller in all hybrids PDIs of 2 and 3 than for the pure PDIs. In total, the force-field and DFT results and binding strengths are well in line with the experimental findings. Time resolved pump-probe spectroscopy A complete separation of charges, i.e. one electron reduced PDIs and oxidized SWCNTs, upon photoexcitation is indicated by femtosecond pump-probe measurements. The transient spectra of 1/SWCNT (Figure S34), which develop immediately after 387 nm laser pulses, are characterized in the visible range by minima at 517, 560, and 665 nm and maxima at 480, 630, 715, and 858 nm. Of great importance are the maxima, since they match the fingerprint absorptions of the oneelectron reduced form of PDIs established in, for example, spectroelectrochemical assays.18a, 18b, 33 In the near-infrared range, bleaching at 1015, 1055, and 1160 nm correlates with the red-shifted ground state absorption of 6,5-SWCNTs, 7,5-

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The transient kinetics, irrespective of whether for 1/SWCNT, 2/SWCNT, or 3/SWCNT, are complex and multiexponential. A suitable model (Figure 9) for the time absorption profiles at 1000 nm (6,5-SWCNT), 1050 nm (7,5SWCNT), and 1150 nm (7,6-SWCNT) includes four species – Figure S36. Considering the strength of ground state interactions and the doping in PDI/SWCNTs – not only in this work but also in previous work – the first species is a high-lying charge transfer (CT**) state.13a, 34 It is subject to a rapid vibrational cooling (τCT**) and yields an energetically lower lying CT state (CT*) of ~ 2.0 eV. It is from this second species that the third species, which is a charge-separated state (CSS1), evolves with (τCT*). The charge-separated state either deactivates directly (τCSS1) to the ground state or indirectly (τCSS2) via the fourth species in the form of a decoupled chargeseparated state (CSS2), based on hole migration in SWCNTs.9c As an approximation to this model, we apply four sequential exponentially decaying components to fit the data at wavelengths corresponding to the respective SWCNT chiralities – Tables S3-S6 and Figure S36.



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potentials published by Tanaka and coworkers. The energies calculated by addition of the PDI reduction potential and SWCNT oxidation potential are summarized in Table S7. Figure 10 summarizes the CSS1 lifetimes as a function of driving forces for the charge recombination for all PDI/SWCNTs. At first glance, the deactivation in 1/SWCNT, 2/SWCNT, and 3/SWCNT is fast and on the same order of magnitude. A closer look reveals, however, the lifetimes for 7,5-SWCNTs are consistently shorter than those for 6,5- and 7,6-SWCNTs. This trend is, however, is disagreement with the binding strengths, where clear differences emerged for the respective PDI and SWCNT chiralities. The excited state dynamics are affected by the strength of the interactions; strong complexation disables, on the one hand, solvent perturbations and provides, on the other hand, structural rigidity. Another factor that cannot be ruled out is the contribution of structural changes that occur on reduction of the different PDIs. In principle, the same conclusions apply for CSS2 with the only notable exception that its lifetime are an order of magnitude longer than for CSS1 – Figure S37.

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Figure 9. Schematic energy diagram for 387 nm excitation into a vibrationally excited CT** state of PDI/SWCNT and subsequent excited state deactivation. CSS1 and CSS2 correspond to coupled ●●+ and uncoupled PDI /SWCNT charge-separated states, respectively.

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Figure 8. Normalized differential absorption spectra of 1/SWCNT (top, green), 2/SWCNT (middle, violet), 3/SWCNT (bottom, blue), obtained upon femtosecond second flash photolysis (λex = 387 nm) in D2O with a time delay of 5 ps, in comparison with the normalized absorption spectra of 1/SWCNT (black), 2/SWCNT (red) and 3/SWCNT (grey) recorded in D2O.

The lifetimes for τCT** (< 1 ps) are upper limits because they cannot be resolved in our experiments. Similarly, it is difficult to determine differences for 1/SWCNT, 2/SWCNT, and 3/SWCNT in terms of τCT* (~3 ps) because of the magnitude of the experimental error. Significant SWCNT chirality-selective differences were, however, observed for τCSS1 and τCSS2 – Tables S4-6. States whose energies depend on the reduction potentials of the PDIs – vide supra – and recently published oxidation potentials of the SWCNT35 are common to both processes. We restrict further discussion to SWCNT oxidation

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Figure 10. Driving force dependence of the lifetime determined for CSS1 in 1/SWCNT (black), 2/SWCNT (red) and 3/SWCNT (grey).

In conclusion, changes in the reduction potentials and the distortion angles in three different PDIs have been found to tune the SWCNT chirality/diameter selectivity. It is particularly important that this work combines studies of the selective SWCNT interactions with those on the modulation of their electronic properties using a family of molecular building


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Journal of the American Chemical Society blocks. On the one hand, we can conclude chirality selective interactions from absorption, Raman, and fluorescence spectroscopies. For example, 1 lacks any appreciable selectivity as far as binding SWCNTs is concerned. 2 and 3 interact selectively with 6,5- and 7,5-SWCNTs, respectively. Removal of 1-3 from SWCNTs further supports the notion of chiral selectivity and indicates the potential of reversible functionalization of SWCNTs. On the other hand, femtosecond pump-probe measurements provide insight into excited-state dynamics. Charge separation is followed by charge recombination, whose dynamics are bi-exponential in nature. Here, holemigration/diffusion transforms a short-lived, coupled chargeseparated state to a long-lived, decoupled one. Regardless of the nature of the charge-separated state, the corresponding lifetimes are on the same order of magnitude. We believe that our results are of great value as they outline an approach based on reversible interactions between small twisted molecules and SWCNTs to complex different SWCNTs selectively. Several obstacles exist for an initial separation process. For example, a deeper understanding of the role of SWCNT bundling and of several other equilibria that induce chirality-selective interactions is needed. The workup, including centrifugation or salting, is equally important in order to obtain truly unfunctionalized chiralitysorted samples.

ASSOCIATED CONTENT Supporting Information Additional information containing UV/Vis/nIR-absorption and emission experiments, femtosecond transient absorption data, Raman spectra, replacement titration experiments, transmission electron microscopy maps, and molecular modelling results are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT C.S. gratefully acknowledges the Fonds der Chemischen Industrie (FCI) for funding. C.S., P.W.M, K.D and L.W. acknowledge the Graduate School of Molecular Science for funding.

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