Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 6689−6694
Modulating Chirality-Selective Photoluminescence of Single-Walled Carbon Nanotubes by Ionic Liquids Marlius Castillo,† Christine Pho,‡ Anton V. Naumov,*,‡ and Sergei V. Dzyuba*,† †
Department of Chemistry and Biochemistry, Texas Christian University, Fort Worth, Texas 76129, United States Department of Physics and Astronomy, Texas Christian University, Fort Worth, Texas 76129, United States
‡
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S Supporting Information *
ABSTRACT: The chirality-selective near-infrared emission of surfactant-stabilized single-wall carbon nanotubes could be controlled by simply varying the anion of the commonly used 1-butyl-3-methylimidazolium ionic liquids. This result advances the notion of the designer solvent ability of ionic liquids and provides opportunities for modulating the properties of nanomaterials.
onic liquids (ILs) are fluid materials that are composed entirely of ions, and they have found numerous applications in a variety of fields of science and engineering.1−3 The physical properties of ILs can be tuned by straightforward manipulations of the cation’s or anion’s structures,4−6 thus facilitating the use of ILs as designer, customizable media. This feature uniquely distinguishes ILs from conventional molecular solvents. For example, the identities of the IL cation and anion were shown to have a significant impact on several organic reactions.7−11 Furthermore, a number of studies have demonstrated the ability of ILs to control conformational preferences as well as supramolecular assemblies of various small molecules.12−21 Notably, ILs were also shown to be viable media for a variety of systems involving nanomaterials.22 Carbon-based nanomaterials, such as single-wall carbon nanotubes (SWCNTs), have received considerable attention due to their numerous applications, especially those related to sensing and electronic devices.23 SWCNTs are a collection of structurally related materials, which differ in regard to certain parameters such as diameters and chiral angles. These parameters control the spectroscopic and electronic properties of SWCNTs. The combination of SWCNTs and ILs, especially considering the tunable physical properties of ILs, should allow for a broad range of promising device applications.24−27 Notably, solubilization or homogeneous dispersion of SWCNTs into ILs is required to take full advantage of SWCNT-IL-based processes and devices. ILs were recently shown to be viable media for dispersing SWCNTs by simply milling the SWCNTs and ILs with various additives.28−32 The functionalization of SWCNTs with imidazolium-type moieties was also shown to be a viable strategy for the solubilization of SWCNT in IL media;33,34 however, this approach alters the
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structure of SWCNTs and hence the optical and electronic properties of individual SWCNTs, which are critical for a number of applications. Surprisingly, in all of the aforementioned studies, the effect of ILs on the chirality-selective photoluminescence of SWCNTs was not investigated. Thus we speculated that if the ensemble properties of SWCNTs could be controlled by simply adjusting the structure/nature of the ILs wherein SWCNTs are dispersed, then various novel and potentially useful applications could emerge. Toward this goal, we set out to investigate the effects of ILs on the near-IR fluorescence of semiconducting SWCNTs because such studies should allow for the efficient assessment of the emissive chirality distribution as well as relative stability of SWCNTs in ILs. To achieve a more facile and practical approach for screening the effect of ILs on the photoluminescence properties of SWCNTs, we utilized a stable, wellcharacterized aqueous surfactant-stabilized suspension of SWCNTs to be added to 1-butyl-3-methyl-imidazoliumbased, water-miscible ILs (Figure 1A). These ILs were selected due to the ease of preparation and their widespread utility. In this work, SWCNTs were dispersed in water via noncovalent interactions of DSPE-PEG (1,2-distearoyl-snglycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-5000] (ammonium salt)) surfactant producing stable (i.e., no precipitation was observed over several months) aqueous suspensions at a final SWCNT concentration of 175 μg/mL, as assessed by absorption spectroscopy (Figure S1). Received: September 5, 2018 Accepted: November 6, 2018 Published: November 6, 2018 6689
DOI: 10.1021/acs.jpclett.8b02734 J. Phys. Chem. Lett. 2018, 9, 6689−6694
Letter
The Journal of Physical Chemistry Letters
Figure 1. Structures of ILs (A) and emission spectra of SWCNTs (B) in ILs. Conditions: λex = 660 nm. Insets show zoomed-in spectra of several ILs. See the text and the SI for additional details.
be not suitable for quantitatively evaluating the effects of ILs on SWCNTs. The variations in the fluorescence intensities of SWCNTs in different ILs could be explained, at least in part, by their aggregation state because the formation of SWCNT aggregates facilitates charge-transfer quenching.38,39 Arguably, the chirality-specific quenching could also arise from differences in the way the surfactant interacts with different SWCNT chiralities in various ILs, which should lead to charge transfer from the ILs and, as a result, SWCNT emission quenching. Less efficient surfactant configuration on select SWCNT chiralities could be expected to lower the stability of those SWCNTs in suspension, leading to chirality-specific aggregation. To probe the role of aggregation in this process, we have performed initial investigations on the time-dependent emission of SWCNTs in ILs (Figure S3). These preliminary results indicated that although the emission intensities of SWCNTs in ILs decreased over time, there were virtually no changes in the shape or positions of the emission bands, as judged by the overlaid normalized spectra (Figure S3). This observation demonstrated that the distribution of specific chiralities was largely time-independent. Overall, it is plausible that the observed changes in SWCNT emission spectra in various ILs could be attributed to chirality-specific charge-transfer quenching induced or due to the preferential aggregation of nanotubes of certain chiralities. To investigate this possibility, 2D excitation/emission plots, commonly used for chirality assignments of emissive semiconducting SWCNT species,40−42 were recorded (Figures S4− S11), and the abundances of the emissive chiralities were determined using the manufacturer-provided software (Figure
Several studies indicated that the behavior of solutes in neat ILs (or predominately IL-based solvent systems) could be drastically different from that observed in mixed-solvent systems (i.e., mixtures of IL−molecular solvent or when ILs were used as additives).12,16,35,36 Therefore, to assess the effect of predominantly IL-based media on the spectroscopic properties of SWCNTs, the amount of water was kept to a minimum. Sufficient SWCNT emission intensity was reached when the aqueous stock solution of SWCNTs was added to neat ILs at a concentration of 2% (v/v) (final concentration of SWCNTs in ILs = 3.5 μg/mL), and the fluorescence of SWCNTs was recorded in the near-IR region (Figure 1B). The fluorescence spectra of the SWCNTs varied significantly depending on the identity of the ILs, exhibiting changes in both intensity and shape (Figure 1B). The emission of SWCNTs in [C4-mim]Br was similar to that observed in water, whereas in all other ILs the emission intensity was suppressed (Figure 1B). However, in [C4-mim]OAc and [C4-mim]BF4 ILs, the spectral features of SWCNTs showed notable variations in the shape and relative intensities of specific emission bands, whereas SWCNT spectra in NO3-, SCN-, CF3CO2-, and CF3SO3-containing ILs were similar (Figure 1B, insets). In addition, the absorption spectra of SWCNTs were also acquired (Figure S2) to assess the changes in absorption transitions. It was noted that the absorbance peaks of SWCNTs in [C4-mim]CF3SO3 and [C4-mim]BF4, for example, were broadened (Figure S2), and their spectral background was increased, as compared with that observed in water, pointing toward potential SWCNT aggregation in ILs.37 However, in general, because of the fairly similar, broad and featureless characteristics, the absorption spectra appeared to 6690
DOI: 10.1021/acs.jpclett.8b02734 J. Phys. Chem. Lett. 2018, 9, 6689−6694
Letter
The Journal of Physical Chemistry Letters
there are no theoretical protocols that allow for the facile and convenient accuracy assessment for the determination of SWCNT chirality distributions from emission spectra recorded at a number of excitation wavelengths. Thus the approach disclosed in this publication provides an advantageous assessment tool to verify fitting-derived chirality distributions. In general, the relationship between the amount of specific emissive chiralities and the nature of the ILs’ anions appeared to be complex (Figure S12). However, a few trends could be noted. Specifically, in water, (7,6) and (8,7) chiralities were the most emissive, followed by (8,6) (Figure 2, Figure S12, and Table S1). In [C4-mim]Br, SWCNT emission was mostly similar to that observed in water (Figure 2, Figure S5, and Table S1). On the contrary, in [C4-mim]CF3CO2, a drastically different spectral signature of SWCNTs was observed, as the emissive contributions were more spread out over the chirality range (Figure S12, Table S1). At the same time, [C4mim]CF3CO2, [C4-mim]CH3CO2, and [C4-mim]SCN yielded distributions of emissive SWCNT species that were more similar to those found in water but with increased contributions from larger diameter species (Figure S12). The contrasting effects of [C4-mim]NO3 and [C4-mim]BF4 are noteworthy (Figure 3, Figures S10 and S11, and Table S1) because these particular ILs have previously shown different abilities in regard to modulating intramolecular as well as intermolecular interactions of various small-molecule solutes.12,15−17 Specifically, a number of chiralities, such as (6,5), (7,5), (7,6), (10,2), and (13,5) chiralities were most prominent in [C4-mim]NO3 while being virtually absent in [C4-mim]BF4 (Figure 3 and Figure S12). On the contrary, (9,7), (9,8), (11,6), and (13,2) chiralities in [C4-mim]BF4 were more abundant than in [C4-mim]NO3 (Figure S12). Almost the same abundances of (8,4), (8,6), (8,7), (11,3), and (11,6) chiralities of SWCNTs were observed in these two ILs. Because SWCNTs appeared to be the least emissive in [C4mim]BF4, which is a widely used IL in conjunction with SWCNTs,27,32,43−45 we explored pathways for enhancing the emission of SWCNTs in this IL (Figure 4). We found that by
2, Table S1). To address accuracy estimates, standard deviations (SDs) of the chirality fitting were determined
Figure 2. Distribution of some representative SWCNTs’ emissive chirality abundances in ILs and water. Percentages are the averages of three independent fits (±SD); see Table S1 for complete emissive chirality distributions.
using the following procedure (because the provided software does not provide accuracy estimates): The best visual fit between the experimental and theoretical spectra was obtained over three independent evaluations, and the average values (±SD) for each SWCNT chirality were calculated (Table S1). In general, the SD was within 10% of the average values, whereas in a few instances, larger percent errors were noted (Table S1). However, the accuracy of the chirality assessment using this approach appears to be more than sufficient for exemplification and quantification of the effect of ILs (or potentially other types of media) on the relative amounts of emissive chiralities of SWCNTs. It is also of note that currently
Figure 3. Excitation−emission SWCNT fluorescence maps with emissive chiralities assigned as a result of spectral fitting in [C4-mim]BF4 (A) and [C4-mim]NO3 (B). 6691
DOI: 10.1021/acs.jpclett.8b02734 J. Phys. Chem. Lett. 2018, 9, 6689−6694
Letter
The Journal of Physical Chemistry Letters
Figure 4. Effect of water on the emission (A) and on the distribution of several representative emissive chiralities (B) of SWCNTs in [C4mim]BF4−water mixtures. Conditions: λex = 660 nm. (A) Inset shows the offset spectra of SWCNTs with concentration of water
