Resonance Raman Optical Activity Spectra of ... - ACS Publications

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Resonance Raman Optical Activity Spectra of Single-Walled Carbon Nanotube Enantiomers Martin Magg,† Yara Kadria-Vili,‡ Patric Oulevey,† R. Bruce Weisman,*,‡ and Thomas Bürgi*,† †

Département de chimie physique, Université de Genève, Quai Ernest-Ansermet, CH-1211 Genève 4, Suisse Department of Chemistry, Rice University, 6100 Main Street, Houston, Texas 77005, United States

‡

S Supporting Information *

ABSTRACT: We present experimental Raman optical activity (ROA) spectra of enantioenriched single-walled carbon nanotubes (SWCNTs). Enantiomeric samples of (6,5) SWCNTs were prepared using nonlinear density gradient ultracentrifugation (DGU). Upon excitation at 2.33 eV, remarkably strong G-band signals are obtained due to strong resonance enhancement with the E22S transition of (6,5) SWCNTs. Enhancement allows measuring the vibrational optical activity (VOA) at unusually low concentrations. The obtained results are in good agreement with the single-excited-state theory (SES). To our knowledge, these are the first experimental VOA spectra of SWCNTs.

U

the dependence of radial breathing mode (RBM) frequency on nanotube diameter.18,19 However, the weakness of the ROA effect often requires the use of highly concentrated or even neat samples.20 Recently, substantial enhancement of ROA signals was reported using silica-coated silver plasmonic particles (surface enhanced ROA).21 Alternatively, ROA can be enhanced by strong electron−phonon coupling when the incident or Raman scattered photons match electronic transitions in the sample. This technique is known as resonance ROA, or RROA.22,23 In the scattered circular polarized (SCP) scheme, resonance Raman (RR) and RROA intensities are given by IRR = IUR + IUL and ISCP−RROA = IUR − IUL , where the superscript denotes unpolarized (U) incident laser light and subscript (R or L) denotes a circular component of the scattered light.24 RROA theory has been formulated in the single excited state limit (SES) by Nafie.25 In this limit, all RROA signals are predicted to mirror their parent Raman signals and their sign is determined by the resonant electronic transition in ECD, which is outlined in eq 1, where Deg is the dipole oscillator strength, Reg is the rotational strength and gECD is the anisotropy factor of the resonant ECD transition.25 Though Deg describes the electric-dipole electric-dipole interaction relevant to absorption spectroscopy, Reg describes the electric-dipole magnetic-dipole interaction, which is nonzero only for chiral molecules.

nique physical properties and novel potential applications have made single-walled carbon nanotubes (SWCNTs) the subject of many studies by basic and applied researchers. Asproduced SWCNT samples contain a variety of well-defined structural species. Each of these is identified by a pair of integer indices, (n,m), and has a specific diameter, chiral (roll-up) angle, and electronic structure.1,2 All structural species except for n = m (armchair) and m = 0 (zigzag) are chiral, existing as two mirrorimage helicities (enantiomers) with absolute configurations labeled as P or M.3 Although the dependence of SWCNT absorption, photoluminescence, and Raman spectra on (n,m) structure have been extensively studied, relatively little attention has so far been paid to spectral differences between enantiomers. This is largely due to the challenge of preparing enantiomerically enriched or sorted samples. However, progress in separation methods such as density gradient ultracentrifugation (DGU),4−6 two phase aqueous extraction,7,8 gel chromatography,9,10 and molecular recognition11 now enables the preparation of SWCNT samples that are enriched not only in (n,m) but also in enantiomeric form. In particular, nonlinear DGU has been used to separate enantiomers of a series of chiral semiconducting SWCNTs, including (6,5).5 The optical activity of chiral SWCNTs has been the focus of a few experimental and theoretical studies.11−15 Weisman and co-workers investigated the electronic circular dichroism (ECD) of six semiconducting SWCNTs.13 It was reported that the optical activity of the E22S transition seems inversely dependent on the nanotube diameter but not correlated with the roll-up angle. Comparison between experimental ECD spectra of (6,5) SWCNTs and simulations based on time-dependent perturbation theory allowed assignment of their absolute configurations.14,16 To our knowledge, no experimental reports and only limited theoretical predictions have been published on the vibrational optical activity (VOA) of SWCNTs.17 This is unfortunate because Raman optical activity (ROA) spectra have the potential to provide structure-resolved enantiomeric information through © XXXX American Chemical Society

Δ=

IRU − ILU IRU

+

ILU

=−

Deg R eg 2

(Deg )

=−

gECD 2

(1)

In this Letter, we report a study of enantio-enriched (6,5) SWCNTs samples that were prepared in a single step by nonlinear DGU with an iodixanol density gradient and a Received: November 23, 2015 Accepted: December 28, 2015

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The Journal of Physical Chemistry Letters

Figure 1. Characterization of (+)-(6,5) and (−)-(6,5) SWCNT samples by optical spectroscopy. (a) UV/vis absorption spectra. (b) ECD spectra. The prefixes (+) and (−) are assigned from the sign of the E22S transition. (c) Excitation−emission plots of (+)-(6,5) (left), and (−)-(6,5) SWCNTs (right). Note logarithmic intensity scales.

surfactant mixture of sodium dodecyl sulfate (SDS) and chiral sodium cholate (SC). The excitation−emission plots in Figure 1c show a strong peak at 570 nm excitation and 980 nm emission, indicating high enrichment of (6,5) SWCNTs. Selective sorting for the (6,5) SWCNT species is additionally supported by UV/ vis spectra (Figure 1a), which show just two main absorption bands. These are assigned to the E11S and E22S transitions of (6,5) SWCNTs. We estimated the carbon atom concentration of (6,5) SWCNTs in the enantiomeric samples as approximately 0.22 mM, using the measured absorbance at the E11S transition and a recently published molar absorptivity for (6,5) SWCNTs of ε = 6700 M−1cm−1.26 Figure 1b shows ECD spectra measured in bulk suspensions between 400 and 900 nm. The two spectra are nearly mirror images, which demonstrates that both collected (6,5) SWCNT fractions are enantiomerically enriched. The intense ECD feature at 635 nm, which is seen only weakly in UV/vis absorption, is an example of an interband cross-transition, allowed only for excitation polarized perpendicular to the nanotube axis.15,14 RR spectra taken from bulk suspensions of (−)-(6,5) and (+)-(6,5) SWCNTs are shown in Figure 2. The two spectra closely resemble each other except for differences in the RBM features (discussed in Supporting Information section S2) and around 1300−1700 cm−1. The latter likely represents an artifact from our background removal procedure (Supporting Information section S3). For the tangential G-band, we find three individual peaks: G+ at 1589 cm−1, G− at 1526 cm−1, and an intermediate mode, E2, at 1546 cm−1.27 G+ and G− are the two

Figure 2. RR of (6,5) SWCNT suspensions. Each spectrum was acquired in 240 min using 48 mW of laser excitation at 2.33 eV (532 nm). Spectra have been normalized with regard to accumulation time and laser power. The emissive background was removed using a polynomial function as described in the Supporting Information. The inset is an expanded view of the G-band features for (−)-(6,5), showing three components (fitted by Lorentzian functions, green curves).

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left circular polarized Raman scattering, whereas ECD is defined as the left minus right circular polarized absorption coefficients, εL − εR.25 For the E22S transition we obtained gECD values of −1.4 × 10−3 and 9.7 × 10−4 for (−)-(6,5) and (+)-(6,5) SWCNT samples. Lower gECD magnitude for the (+)-enantiomer compared to the (−)-enantiomer correlate with lower Δ values (Supporting Information section S4). Different gECD and Δ values result from a different enantiomeric purity and, in case of Δ, low signal-to-noise for some bands in the RROA spectra. Δ values for G-band signals for (−)-(6,5) and (+)-(6,5) SWCNTs (Δ(G−) = 5.5 × 10−4 and −4.5 × 10−4, Δ(G+) = 7.3 × 10−4 and −4.5 × 10−4) are in good agreement with SES theory and eq 1. Higher Δ values are found for RBM, IMF and iTOLA (Supporting Information Table S1). The quality of spectra (signal-to-noise ratio) influences the exact determination of Δ, which, becomes problematic for low intensity bands like the RBM and IMF. For the iTOLA band, Δ has been determined to be 1.1 × 10−3 and −1.0 × 10−3. In this case, higher Δ values may also indicate the involvement of more than one electronic state and thus the breakdown of the SES limit. Although only weakly seen in the RR spectrum (Figure 2), the RBM of (6,5) SWCNTs is observed in the SCP-RROA spectrum at 309 cm−1, confirming the enantio-separation of the (6,5) species (Figure 3). Separation relies on enantioselective interactions between the nanotube and the chiral surfactant SC. However, no contribution of SC can be identified in SCP-RROA spectra. In fact, this is due to the small ROA signal of SC, as illustrated in Figure S5. Within a 10 wt % solution of SC (0.24 M), the SCP-ROA signal is of order 10−5 weaker than the signal from enantio-separated SWCNTs despite the far smaller carbon atom concentration of SWCNTs compared to chiral SC. This illustrates the strong enhancement of Raman and ROA signals in the resonance regime. In conclusion, we have measured the first Raman optical activity spectra of SWCNTs. Samples enriched in both (6,5) enantiomers were prepared using nonlinear DGU sorting. Nearresonance of Raman-scattered photons with the E22S absorption band provided strong enhancement of Raman optical activity signals as well as Raman signals. This resonance enhancement enables the measurement of ROA spectra at remarkably low sample concentrations.

most intense peaks in the RR spectra. In contrast, the (6,5) RBM feature at 309 cm−1 has only weak intensity, consistent with previously reported Raman spectra of DGU-separated (6,5) SWCNTs excited at 514 nm.28 We propose two different explanations for the large G-band and comparatively small RBM signals. Large G-band signals in RR spectra of (6,5) SWCNT may be explained through a near resonance of the Stokes-shifted photons at ω(G+) = 2.13 eV (581 nm) and ω(G−) = 2.14 eV (579 nm) with the E22S transition, which is found at 2.17 eV (570 nm). By contrast, neither the incident photon at 2.33 eV (532 nm) nor the Stokes-shifted ω(RBM) photon at 2.29 eV (541 nm) is well matched to E22S, accounting for the relatively small resonance enhancement of the RBM band.29,30 Intensity for RBM also shows a dependence on (n,m) and (6,5) SWCNTs are expected to show a small Raman cross section for the RBM.31,32 We would like to point out two bands at 1050 and 1060 cm−1 in the region of so-called intermediate frequency modes (IMF). IMFs are thought to arise from combinations of optical and acoustic phonons and are thus second order Raman processes.33,34 An additional combination mode found at 1937 cm−1 (iTOLA) is assigned as a combination mode of an in-plane transverse optical phonon and a longitudinal acoustic phonon.35 We could not observe the G′-band (near 2600 cm−1), which is the most prominent higher order Raman feature, due to the limitation of our spectrograph. The influence of the two surfactants, SDS and chiral SC, on the Raman spectra of (+)-(6,5) and (−)-(6,5) SWCNTs was examined and shown to have only minor impact on the presented spectra (see Supporting Information Figure S4). SCP-RROA spectra are shown in Figure 3 for (+)-(6,5) and (−)-(6,5) SWCNTs. Evidence of the resonance character includes: (i) strong coupling to the resonant E22S transition in the ECD spectra and (ii) a constant value for Δ over the entire spectrum. It should be noted that the sign of SCP-RROA is reversed compared to the resonant ECD transition for the same enantiomer simply because ROA is defined as the right minus the

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EXPERIMENTAL METHODS Preparation of (6,5) Enantiomer Samples. SWCNTs produced in the Rice University HiPco reactor (batches 188.4 and 195.1) were used to prepare (6,5) enantiomers following the nonlinear DGU method.5 Initially, 5 mg of raw SWCNTs were suspended in 10 mL of 2% (w/v) aqueous sodium cholate (SC, [a]D20 + 31°, c = 0.5 in H2O (Chem-Impex, International)) by 1 h of bath sonication (Sharpertek model Stamina XP) followed by 30 min of tip sonication with a 3 mm probe at 7 W power (Misonix model Microson XL). The suspension was then centrifuged for 1 h at 13 300g to remove most nanotube bundles and residual iron catalyst. The supernatant was collected and mixed with 60% (w/ v) aqueous iodixanol (Sigma-Aldrich OptiPrep) and sodium dodecyl sulfate (SDS) to obtain a SWCNT sample with 2% SC, 0.2% SDS, and 25% iodixanol. To prepare the density gradient, stock solutions of 30% iodixanol and DI water, each also containing 0.7% SC and 0.175% SDS, were mixed to get a series of solutions with the following iodixanol concentrations (w/v) and volumes: 785 μL of 15%, 725 μL of 17.5%, 660 μL of 20%, 660 μL of 22.5%, 420 μL of 27.5%, 500 μL of 30%. These solutions were pipetted into a 13 mm diameter, 5 mL Polyclear open top ultracentrifuge tube

Figure 3. SCP-RROA of (6,5) SWCNT suspensions. Each spectrum was acquired in 240 min using 48 mW of laser excitation at 2.33 eV (532 nm). Spectra have been normalized with regard to accumulation time and laser power. Inset shows an expansion of the (6,5) RBM and IMF features at 309 cm−1 1050 cm−1. 223

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The Journal of Physical Chemistry Letters (Seton Scientific model 7022) starting with the highest concentration sample. The filled centrifuged tube was then set at a 51° angle from horizontal for 50 min to allow diffusional smoothing of the gradient, after which 880 μL of the prepared SWCNT sample described above was injected into the centrifuge tube close to the position of matching iodixanol concentration. The filled tube was then centrifuged at 250 000g for 22 h at 20 °C. Following centrifugation, two distinct pink layers formed at different depths in the tube (see Figure S1). We mapped in situ near-IR fluorescence spectra as a function of depth using a model NS3 NanoSpectralyzer (Applied NanoFluorescence). This fluorescence mapping showed the presence of two spatially separated (6,5) bands, which were extracted with a custom built precision fractionator.36 The extracted fractions obtained in this way from a total of eight centrifuge tubes were combined to give ca. 1.23 and 0.8 mL of solutions representing the upper and the lower bands, respectively. Based on circular dichroism measurements, the upper and the lower fractions were found to give positive and negative CD signals at the E22S electronic transition of (6,5) SWCNTs. Absorption Measurements. Absorption spectra from 400 to 1400 nm were measured relative to a reference solution of aqueous surfactants and iodixanol using a prototype model NS2 NanoSpectralyzer (Applied NanoFluorescence). Circular Dichroism Measurements. CD spectra of the enantiomeric (6,5) fractions in 1 cm quartz cuvettes were measured relative to the reference solution using a Jasco model J-815 CD spectrometer over the range of 325 to 900 nm. 2D Excitation−Emission Mapping. Excitation−emission maps for the sorted samples were measured using a Spex Fluorolog 3-211 (Horiba J-Y) spectrofluorometer equipped with a liquid nitrogen cooled single channel InGaAs detector. The scanned emission range was 850 to 1600 nm with 5 nm steps, whereas the excitation range covered 450 to 850 nm in 5 nm steps. Excitation and emission spectral slit widths were both set to 7 nm. Preparations for RROA Measurements. RROA measurements were made on samples of (+)-(6,5) and (−)-(6,5) SWCNTs prepared as described above, using a homemade Teflon cell with a sample volume of 35 μL. The SWCNT suspension was carefully transferred into the cell with a microliter syringe to avoid the formation of air bubbles. To prepare dispersions of CoMoCAT SWCNTs, 5 mg of CoMoCAT SWCNTs (Sigma-Aldrich) were placed together with 10 mL of 2% w/v SDS or 2% w/v SC surfactant solution into a bath ultrasonicator for 1 h. The resulting suspension was centrifuged twice at 13 500 rpm (20 172g) and the supernatant was extracted for RROA measurements (Supporting Information S4 and S5). SCP-ROA Instrumentation. Our SCP-ROA instrument is the ROA spectrometer built by Hug et al. and described previously.37 Raman Microscopy. Raman measurements were made using a confocal LABRAM microscope (Horiba Scientific) with a He− Ne laser source coupled with an Olympus BX51 microscope.

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Raman intensity ratios, SCP-ROA spectra of the chiral SC surfactant. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.B.W. and Y.K.V. are grateful to the National Science Foundation (Grant CHE-1409698) and the Welch Foundation (Grant C-0807) for support. We would like to thank R. Moritz (Department of Earth Sciences, University of Geneva) for use of their Raman microscope.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02612. Photos and emission spectra of the enantiomeric fractions, analysis of the RBM features of (6,5) SWCNT samples prepared with nonlinear DGU, raw Raman spectral data, evaluation of surfactant effects, calculation of ROA-to224

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