Simultaneous discrimination of diameter, handedness, and metallicity of single-walled carbon nanotubes with chiral diporphyrin nanocalipers

Article abstract of DOI:10.1021/ja312519s

We have been developing the methodology to discriminate the handedness and diameter of single-walled carbon nanotubes (SWNTs) through molecular recognition using chiral diporphyrin nanotweezers. Although relatively small diameters of SWNTs (<1.0 nm) were recognized well, nanotweezers were not able to form stable complexes with the SWNTs having the diameters >1.0 nm. In this context, we designed chiral diporphyrin with a much larger cavity, namely, "nanocalipers". The feature of the newly designed host molecule is: (1) long spacer with more than 1.4 nm consisting of three aromatic moieties; (2) nearly parallel orientation of the two porphyrins; (3) restricted conformation by biaryl linkages of the porphyrin-carbazole and carbazole-anthracene; (4) strong interaction of two porphyrins and anthracene with the surface of a SWNT through π-π stacking; and (5) stereogenic centers at the periphery of porphyrins discriminating helicity of SWNTs. As expected, we obtained optically active SWNTs with >1.0 nm in diameter and, unexpectedly, enriched metallic SWNTs over semiconducting ones. The optically active metallic SWNTs are identified for the first time, in addition to the optically active semiconducting SWNTs with such large diameters. The nanocalipers are found to recognize the diameter, handedness, and metallicity of SWNTs simultaneously.

Full text of DOI:10.1021/ja312519s

Article
pubs.acs.org/JACS
Simultaneous Discrimination of Diameter, Handedness, and
Metallicity of Single-Walled Carbon Nanotubes with Chiral
Diporphyrin Nanocalipers
Gang Liu,† Feng Wang,†,⊥ Songpol Chaunchaiyakul,§ Yukie Saito,‡ Ajoy K. Bauri,∥ Takahide Kimura,†
Yuji Kuwahara,§ and Naoki Komatsu*,†
†Department of Chemistry, Shiga University of Medical Science, Seta, Otsu 520-2192, Japan
§Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka
565-0871, Japan
‡Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
∥Bio-Organic Division, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India
S
* Supporting Information
ABSTRACT: We have been developing the methodology to
discriminate the handedness and diameter of single-walled
carbon nanotubes (SWNTs) through molecular recognition
using chiral diporphyrin nanotweezers. Although relatively
small diameters of SWNTs (<1.0 nm) were recognized well,
nanotweezers were not able to form stable complexes with the
SWNTs having the diameters >1.0 nm. In this context, we
designed chiral diporphyrin with a much larger cavity, namely,
“nanocalipers”. The feature of the newly designed host
molecule is: (1) long spacer with more than 1.4 nm consisting
of three aromatic moieties; (2) nearly parallel orientation of the two porphyrins; (3) restricted conformation by biaryl linkages of
the porphyrin−carbazole and carbazole−anthracene; (4) strong interaction of two porphyrins and anthracene with the surface of
a SWNT through π−π stacking; and (5) stereogenic centers at the periphery of porphyrins discriminating helicity of SWNTs. As
expected, we obtained optically active SWNTs with >1.0 nm in diameter and, unexpectedly, enriched metallic SWNTs over
semiconducting ones. The optically active metallic SWNTs are identified for the first time, in addition to the optically active
semiconducting SWNTs with such large diameters. The nanocalipers are found to recognize the diameter, handedness, and
metallicity of SWNTs simultaneously.
right-handed SWNTs preferentially through molecular recog-
nition.10,11 In the latter case, chiral fluorene−binaphthol
copolymers also discriminated metallicity of SWNTs,12
resulting in enrichment of optically active semiconducting
SWNTs.11
In our previous works following the first report of the
optically active SWNTs,6 it was found that the chiral
nanotweezers discriminate diameter and/or (n,m), in addition
to handedness, of SWNTs according to the size and shape of
the cleft made by two porphyrins and a spacer in between
(Figure 1a,b).4,13−18 Therefore, we have rationally designed
several chiral nanotweezers to improve the selectivity toward
(n,m) and handedness and realized single species enrichment,
namely, enantiomers of (6,5)-SWNTs through complexation
with 1 (Figure 1a).16 It was rationalized that the nanotweezers
with smaller dihedral angle made by two porphyrin units
accepted smaller diameter of SWNTs deeply in the cleft,
INTRODUCTION
■
Single-walled carbon nanotubes (SWNTs) have been attracting
extensive interest due to their unique physical and chemical
properties. However, the properties of SWNTs are known to
strictly correlate with their structures,1,2 encouraging separation
of SWNTs according to their structures, such as diameter,
length, roll-up index or (n,m), and handedness, and physical
properties, such as metallicity.3,4 Accordingly, several kinds of
separation methods have been developed so far, such as
electrophoresis, chromatography, density gradient ultracentri-
fugation (DGU), selective solubilization, and selective reac-
tion.3,4 As for optical resolution or handedness-based
separation, preferential disperse5 of either left- or right-handed
SWNTs was achieved by use of “chiral diporphyrin nano-
tweezers” to give optically active SWNTs in 2007.6 DGU using
sodium cholate as chiral surfactant was also applied successfully
to sort (n,m)-enantiomers, uncovering several kinds of optically
active (n,m)-SWNTs.7−9 Quite recently, chiral supramolecule
and copolymers based on flavin mononucleotide and fluorene−
binaphthol, respectively, were found to extract either left- or
Received: December 21, 2012
© XXXX American Chemical Society
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Figure 1. Computer-generated molecular modeling of the complex structures of (6,5)-, (9,4)-, and (9,7)-SWNTs with chiral diporphyrin
nanotweezers 1 (a) and 2 (b) and nanocalipers 3 (c), respectively. The line is drawn to clarify the depth of SWNT accepted in the cleft of the host
molecules. The two direction arrows and the values above the SWNTs indicate the diameters of these SWNTs.
Scheme 1. Synthesis of Chiral Diporphyrin Nanocalipers (R)- and (S)-3
enabling precise differentiation of the diameter. In sorting of
SWNTs with larger diameters, however, there has been
limitation in selectivity by use of nanotweezers, because a
larger dihedral angle, or shallow cavity, is needed to accept
SWNTs with large diameters, decreasing the selectivity toward
the diameter (Figure 1b).15 It was concluded in our previous
studies that the nanotweezers with the longer spacer and
narrower angle exhibits better (n,m) selectivity than those with
the shorter spacer and wider angle.16 Based on this theory,
“chiral diporphyrin nanocalipers” have been newly designed as
shown in Figure 1c and Scheme 1. The points of this molecular
design are: (1) much longer spacer (>1.4 nm) connecting three
aromatic moieties directly; (2) nearly parallel orientation of the
two porphyrins using two carbazoles at the edges of the spacer;
(3) restricted conformation by biaryl linkages of the
porphyrin−carbazole and carbazole−anthracene; (4) strong
interaction of two porphyrins and anthracene with the surface
of a SWNT through π−π stacking; and (5) stereogenic centers
at the periphery of porphyrins discriminating helicity of
SWNTs. As we expected, optically active SWNTs with >1.0
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nm in diameter were dominant after the extraction of SWNTs
(HiPco) with the chiral nanocalipers 3. Unexpectedly, we found
that the nanocalipers recognize metallicity of SWNTs leading
to enrichment of metallic SWNTs in the extract. The chiral
nanocalipers are concluded to have multifunctions to recognize
structural and physical properties of SWNTs, namely, helicity,
diameter, and metallicity.
RESULTS AND DISCUSSION
■
Synthesis of Chiral Diporphyrin Nanocalipers. The
chiral diporphyrin nanocalipers, (R)- and (S)-3, are synthesized
as shown in Scheme 1. The long spacer consisting of three
aromatics (carbazole−anthracene−carbazole) was prepared by
Suzuki−Miyaura coupling of boronated carbazoles with
dibromoanthracene. The two carbazoles bear long alkyl chains
at the N-positions to increase solubility in organic solvents.17
The three aromatic rings in the coupling product are
conformationally restricted at the biaryl linkages, although
they can rotate under ambient conditions.19,20 Two carbazoles,
perpendicular to anthracene, align almost in parallel in the most
stable conformational isomers, enabling the parallel alignment
of two porphyrins. The coupling product is considered to
consist of the two conformational isomers in almost equal
amounts, because the energy difference between the two
conceivable isomers shown in Scheme 1 is calculated to be very
small (<0.1 kJ mol−1 by molecular mechanics). After
bromination of the spacer, two chiral porphyrins were coupled
through the Suzuki−Miyaura coupling reaction. The product
was identified as chiral diporphyrin nanocalipers by absorption,
Figure 2. Fluorescence spectra of (R)-3 in methanol and the extract.
These solutions exhibited the same intensity (1.0 in Figure S6) of the
absorption at 419 nm corresponding to the porphyrin Soret band.
emission (∼600 nm) lights, this phenomenon can be
interpreted by quenching of the excited porphyrins by
SWNTs accommodated in the cavity of the nanocalipers (see
the Supporting Information in detail), implying existence of the
SWNT complex of 3. After extraction of SWNTs, upward shift
of the baseline and broad peak from 700 to 900 nm (ES
22
region shown in Figure 6) were observed in the absorption
spectra shown in Figure 3a, supporting the above conclusion of
existence of SWNTs in the extract. In addition, many strings,
which seem to be individualized without bundling, are observed
in the scanning transmission electron microscopy (STEM) and
atomic force microscopy (AFM) of the extract as shown in
Figures S1 and S2, respectively. It can be interpreted that each
string of SWNT is individualized through complexation with
nanocalipers not only in the solution phase (extract) but also in
the solid phase after evaporating the solvent.
1
circular dichroism (CD), and H and 13C NMR spectroscopies
(Figure 3a,b, and the Supporting Information). The nano-
calipers should contain two conformational isomers, syn and
anti forms. The anti-conformation is calculated to be more
stable than the syn-one with a very small energy difference (0.16
kJ mol−1 by molecular mechanics). Since the aromatic rings can
rotate as mentioned above, the syn-conformations of nano-
calipers, (R)- and (S)-3, in Scheme 1 should be taken upon
complexation with the SWNTs shown in Figure 1c.
Extraction of SWNTs with Chiral Diporphyrin Nano-
calipers. First, 76-CoMoCAT (SWeNT SG-76 purchased
from Sigma-Aldrich) was employed for the extraction; the
SWNTs were bath sonicated in methanol in the presence of
(R)-3, and the resulting black suspension was centrifuged.
However, no SWNTs were extracted in the supernatant. The
diameter of 76-CoMoCAT ranging from 0.7 to 1.1 nm was
thought to be smaller than the size of the cavity in the
nanocalipers, making the complex less stable. Second, we used
SWNTs synthesized by arc discharge method (Honjo Chemical
Co.), because the diameter (around 1.4 nm) is larger than that
of 76-CoMoCAT. We failed to extract once again, probably
because of too large diameters and/or tight bundling. Finally,
HiPco (Carbon Nanotechnologies Inc.) was adopted due to
larger diameter range, resulting in extraction of SWNTs with
nanocalipers in methanol after high-speed centrifugation for
two days (see the Experimental Section). The greenish extract
was subjected to UV−vis−NIR, CD, and fluorescence spec-
troscopy measurements to confirm the existence of the complex
between SWNTs and 3, as depicted in Figure 1c.
A broad peak appeared around 450 nm as a shoulder of the
Soret band in the absorption spectra (Figure 3a), which may be
caused by red shift of the Soret band through complexation. In
that region, enhanced split CDs are observed in Figure 3b.
Since fixation of the two porphyrin units in the diporphyrin
complex is known to enhance the intensity of the split Cotton
effects,17,18,21 the split CDs are considered to be originated
from the SWNT complexes of the nanocalipers, and hence the
broad peak around 450 nm should correspond to the complex.
Furthermore, the shoulder in the Soret band was deconvoluted
into two peaks as shown in Figure 3c. These wavelengths are
matched with those of the two deconvoluted peaks of the split
CD in the extract (436 and 450 nm). The energy difference is
known as Davydov splitting and is caused by exciton
coupling.22 The split Cotton effects were observed not only
in the extract but also in the solutions of (R)- and (S)-3. The
symmetrical positive and negative split CDs of (R)- and (S)-3
and their SWNT complexes resulted from the exciton coupling
of the two porphyrins located in the asymmetrical relation-
ship.21,23,24 In these exciton couplings, contribution from the
SWNT excitons could be ignored, because the excitation at 450
nm generates a much lower number of excitons at SWNTs than
that at porphyrins, because absorption due to SWNTs is much
less than that of porphyrins at 450 nm as suggested by Figures
3a and 6. Since the positive and negative exciton-coupled CDs
are reported to correspond to a clockwise and anticlockwise
relationship between the two porphyrins,22,23 (R)- and (S)-
nanocalipers, exhibiting positive and negative split CDs, are
suggested to adopt a clockwise and anticlockwise relationship,
In the fluorescence spectroscopy (Figure 2), the fluorescence
intensity of the extract is much less than that of the
nanocalipers at the excitation of 419 nm after normalization
of the absorption at Soret band. Although SWNTs in the
extract absorb and/or scatter the excitation (419 nm) and
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Figure 3. Absorption (a) and CD (b) spectra of (R)- and (S)-3 before and after extraction. CD spectra of nanocalipers and nanocalipers−SWNT
complexes were normalized at the absorbance of 419 and 450 nm (shoulder), respectively. (c) Deconvoluted (dotted lines) and experimentally
obtained (solid lines) absortion and CD peaks of the extract from 380 to 500 nm after subtracting baseline. (d) Conformations of the nanocalipers
before and after complexation suggested from the signs of the exciton couplings in the CD spectra. The substituents on the aromatic rings are
omitted for clarity.
respectively, before and after complexation, as shown in Figure
3d. These asymmetric conformations are considered to be
induced by the stereogenic centers at the periphery of the
porphyrins. Based on these conformations of the nanocalipers,
molecular modeling calculations (molecular mechanics) show
that (R)- and (S)-3 form more stable complexes with (P)- and
(M)-(9,7)-SWNTs than (M)- and (P)-ones,4,15 respectively,
which will be mentioned below (Figure 8).
Complex Structure. Fluorescence quenching (Figure 2),
red shift of the Soret band in the absorption spectrum (Figure
3a,c), and enhanced split Cotton effect in the CD spectra
(Figure 3b,c) indicate the existence of the complex between
SWNTs and the nanocalipers. In order to obtain direct
evidence of the complex, we measured transmission electron
microscopy (TEM, Figure 4a), AFM (Figure 4b), and scanning
tunneling microscopy (STM, Figure 4c) of the SWNT extract
with nanocalipers.
In the TEM image (Figure 4a), we could see something
wrapping around the SWNTs. This is confirmed in the z-profile
at AFM (Figure 4b). The height difference is observed even in
the one string of SWNT. Relatively dark parts of the SWNT
string (A−B and E−F in Figure 4b) exhibit 1.04−1.37 nm
height (1.17 nm on average), which may correspond to the
diameters of the bare SWNTs. On the other hand, the height of
1.90−2.34 nm (2.12 nm on average) is observed at the brighter
part in the strings (C−D and G−H in Figure 4b). The average
height difference between adjacent brighter and darker parts is
calculated to be 0.97 nm. Since almost the same height
difference was observed in the AFM of the SWNTs extracted
with the nanotweezers,17 we can conclude that the brighter and
darker parts (2.12 and 1.17 nm in average height, respectively)
in Figure 4b correspond to the individualized SWNTs coated
and uncoated with nanocalipers, respectively. A closer view of
the SWNT complex with nanocalipers was obtained by high
resolution STM shown in Figure 4c. The 0.56 nm height
difference between darker and brighter parts shown in the z-
profile of I−J indicates the existence of the nanocalipers
complexed with the SWNT. The two brightest parts, indicated
in the red arrows in Figure 4c, and the less bright part in
between may correspond to the two carbazoles and the
anthracene, respectively, implying that the spacer of the
nanocalipers is on the top of the SWNT. The discrepancy in
the above height differences determined by AFM (0.97 nm)
and STM (0.56 nm) might be attributed to the different
orientation of the nanocalipers around the SWNT on the mica
and HOPG substrates in the AFM and STM, respectively. The
conceivable orientation of the complexes on the substrates is
illustrated in Figure S3.
Selectivity toward Diameter and Metallicity of
SWNTs. After concentration of the supernatants including
the complex, the resulting solids were washed with THF and
pyridine several times to thoroughly remove the nanocalipers
from the complex. The washed samples were subjected to
Raman spectroscopy (Figure 5) to determine the diameter
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Remarkable diameter shift through the extraction is
suggested in the radial breathing mode (RBM) at the excitation
of 785 nm (Figure 5a and Table 1). SWNTs with relatively
small diameters, (9,4), (10,2), and (8,6) with 0.88−0.97 nm,
are the major components in the HiPco (59% based on Raman
intensity). After the extraction, the abundance of these SWNTs
decreases significantly to 8%, and instead SWNTs with larger
diameters ranging from 1.05 to 1.17 nm increase from 30% to
82%. The abundance of (11,3) with 1.01 nm in diameter is
almost the same before and after the extraction. These results
based on the Raman spectra (Figure 5a) suggest that the
nanocalipers recognize the diameter of SWNTs and form more
stable complexes with SWNTs with relatively larger diameters
(1.05−1.17 nm) than those with diameters <1.0 nm.
Although only semiconducting SWNTs were detected at the
excitation of 785 nm, metallic SWNTs were also found in the
RBM at the excitation of 633 nm as shown in Figure 5b. Drastic
change in the spectra has been also seen in this excitation
wavelength through the extraction (Figure 5b,d). In terms of
diameter selectivity, SWNTs with relatively larger diameters
(1.07−1.22 nm) significantly increase from 49% to 93%, and
accordingly the abundance of the smaller diameters decreases
from 51% to 7% (Table 1). A similar range of diameters (1.07−
1.22 nm) to that shown in Figure 5a (1.05−1.17 nm) is
enriched in Figure 5b, suggesting the diameter selectivity
toward the relatively larger diameters of SWNTs mentioned
above. From the viewpoint of metallicity, metallic SWNTs,
(13,4), (12,3), and (13,1), are enriched significantly, which is
supported by the more pronounced Breit−Wigner−Fano
(BWF) feature of the extracted SWNTs in the G-band region
shown in Figure 5d.25 Since all the SWNTs with larger
diameters excited at 633 nm are metallic, the selectivity toward
metallicity is ambiguous in Figure 5b.
At the excitation of 488 nm, metallic SWNTs with smaller
diameters, (7,7) and (8,5), can be found in Figure 5c. In the
semiconducting region <230 cm−1, a larger diameter is
preferred to a smaller one, as in the case of the excitation of
785 and 633 nm. While the peaks corresponding to (12,2)- and
(13,0)-SWNTs with smaller diameters (1.03−1.04 nm) almost
disappear after the extraction, the peak intensity of (13,3)
increases from 62% to 90%. Although the metallic SWNTs
detected at this excitation wavelength, (7,7) and (8,5), possess
smaller diameters (0.90−0.96 nm), this intensity slightly
increases from 7% to 9%, suggesting that the nanocalipers
have preference toward metallic SWNTs in addition to larger
diameters of 1.07−1.22 nm on the basis of the results in the
Raman spectra (Figure 5).
Figure 4. TEM (a), AFM (b), and STM (c) images of the SWNT
complexes. z-Profiles along A−B, C−D, E−F, and G−H in the AFM
image (b), and I−J in the STM image (c) are also added next to these
images. Red arrows in (c) indicate the two brightest parts.
The absorption spectra shown in Figure 6 support the above
conclusion of the selectivity toward larger diameters and
metallic SWNTs. Since almost the same spectra were obtained
in the SWNTs extracted with (R)- and (S)-3, we used results of
(R)-3 in Figure 6. In both ES and ES regions, the major
range enriched through the extraction with the nanocalipers.
After the solid was dispersed in D2O in the presence of achiral
detergent, sodium dodecylbenzenesulfonate (SDBS), the
resulting dispersions were analyzed with absorption and CD
spectroscopies (Figures 6 and 7, respectively). It is worth
noting that the intensity of the photoluminescence (PL) of the
D2O dispersion of the extracted SWNTs was too low to
determine the abundance of the semiconducting SWNTs,
although we tried several times.
Raman spectra of SWNTs (HiPco) before and after
extraction were measured at the excitation of 785, 633, and
488 nm (Figure 5), and the intensity of the observed peaks is
compared as shown in Table 1. Since almost the same spectra
were obtained in the SWNTs extracted with (R)- and (S)-3, we
used results of (R)-3 in Figure 5 and Table 1.
11
22
peaks with relatively large intensity shift from the shorter
wavelength to the longer one, indicating that SWNTs with
larger diameters are enriched through the extraction. The
absorbance in the EM region and the degree of the baseline
11
upward shift of the extracted SWNTs increased as compared
with those of the HiPco SWNTs, suggesting the enrichment of
the metallic SWNTs.26−28 The change in the relative
abundance of (n,m)-SWNTs through the extraction is
summarized in Table 2. Regardless of the metallicity, SWNTs
with diameters of 1.05−1.22 nm, from (10,5) to (13,4) in
Table 2, increase in relative abundance after the extraction,
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Figure 5. Raman spectra of SWNTs (HiPco) before and after the extraction with (R)-3 in the region of RBM at the excitation of 785 (a), 633 (b),
and 488 nm (c), and in the G band at the excitation of 633 nm (d). Green and pink regions indicate semiconducting (S) and metallic (M),
respectively. The spectra were normalized at 233 (a), 220 (b), and 203 cm−1 (c), and the G band (d).
though that of (13,1) cannot be determined correctly because
of the very steep absorption change in this region. On the
contrary, the abundance of SWNTs with diameters smaller than
1.05 nm, from (12,2) to (8,3) in Table 2, relatively decrease
except for metallic SWNTs, (9,6), (7,7), and (8,5). This shows
that the nanocalipers recognize not only the diameters but also
metallicity of SWNTs.
ment of metallic or semiconducting SWNTs has been observed
in the separation through molecular recognition. This indicates
that these molecules did not recognize the metallicity of
SWNTs. The nanocalipers are the first host molecule to
recognize the metallicity of SWNTs in our studies. The parallel
alignment of the two porphyrins in the nanocalipers and/or
interaction of the anthracene with the SWNT surface (Figure
1c) might have a special effect to recognize the metallicity,
which was not realized by the gable-type diporphyrins and
monoporphyrin. A recognition of the metallicity of SWNTs
with porphyrin was reported in 2004 by use of free-base
tetraphenylporphyrin (TPP) derivative.30 However, semi-
conducting SWNTs were preferentially extracted rather than
metallic ones. Although we cannot interpret these opposite
preference of nanocalipers and TPP derivative for metallic and
semiconducting SWNTs, respectively, we would like to point
out the structural difference between these porphyrins: TPP has
four phenyl rings to prevent the porphyrin rings from sufficient
direct interaction with the SWNT surface, while our porphyrin
can face the SWNT surface, as shown in Figures 1 and 8, due to
more flexible and less bulky substituents at the periphery of the
porphyrins. In addition, the porphyrins in the nanocalipers
include Zn as a central metal, while the TPP is a free base.
Metallo- and free-base porphyrins have a difference in the
structural rigidity and electrical property. These structural
difference might cause the difference in the preference for the
metallicity of SWNTs.
In order to make clear the preference for the metallic
SWNTs, the baseline is subtracted in the ES22 and EM11 regions
of the absorption spectra, and the abundance of the metallic
and semiconducting SWNTs is compared, as shown in Figure
S4a. It is found that the metallic SWNTs are significantly
enriched through the extraction. In addition, the abundance of
the metallic SWNTs in the supernatant was monitored at the
duration of the centrifugation, 9, 25, and 45 h, as shown in
Figure S4b. A significant increase in the abundance of the
metallic SWNTs is confirmed at the 25 h centrifugation.
Furthermore, the purple color of the D2O dispersion of the
extracted SWNTs shows the high concentration of the metallic
SWNTs (Figure S5).26,27 The enrichment of the metallic
SWNTs may be a reason why no clear PL was observed in the
PL spectra of the extracted SWNTs, as mentioned above. That
is, the extracted SWNTs are considered to form small bundles
after tip sonication followed by centrifugation, because of the
tight bundling of the extracted SWNTs. Since the content of
the metallic SWNTs is relatively large, most of the small
bundles may contain metallic SWNTs to quench the PL from
the semiconducting SWNTs included in the bundles. In
Optical Enrichment of SWNTs. The optical enrichment of
the extracted SWNTs was confirmed by CD spectra, as shown
in Figure 7. The SWNTs extracted with (R)- and (S)-3 exhibit
symmetrical CD peaks, indicating that the extracted SWNTs
are optically active. Prominent peaks are indentified, as shown
addition, the metallic SWNTs exhibit no PL at the excitation
1
of the wavelength of ES
.
22
In our previous studies using the gable-type diporphyrins18
and dipyrene20 and monoporphyrin,29 no significant enrich-
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Table 1. Comparison in Raman Intensity of (n,m)-SWNTs before and after the Extraction with Nanocalipers in the RBM
Raman intensity before Raman intensity after
a
b
c
d
d
excitation wavelength (nm)
roll-up index
(13,3)
diameter (nm)
Raman shift (cm−1
)
separation (%)
separation (%)
1.170
1.111, 1.103
1.050
206 (204)
16
38
(10,6), (9,7)
(10,5)
216 (214, 216)
226−227 (226)
4
30
22
22
82
10
785
(11,3)
1.014
233−234 (233)
11
11
10
10
(8,6)
(9,4)
0.966
0.916
0.884
246 (244)
259 (256)
267 (264)
6
1
1
6
17
36
59
49
8
(10,2)
(13,4)
1.222
197 (196)
26
23
64
29
93
(12,3), (13,1)
1.091, 1.074
219−220 (218, 221)
633
(10,3)
(7,6)
(7,5)
(8,3)
0.936
0.895
0.829
0.782
253 (251)
257−258 (261)
284 (281)
15
27
7
3
3
1
0
51
62
7
296−297 (297)
2
(13,3)
(10,6)
1.170
1.111
203 (204)
217 (214)
62
90
90
−
−
488
(12,2), (13,0)
1.041, 1.032
228 (227, 229)
31
31
7
1
1
9
(7,7)
(8,5)
0.963
0.902
243−246 (244)
258−261 (260)
2
5
3
6
a
b
c
Italic font (n,m) indicates metallic SWNTs, while regular font (n,m) is semiconducting SWNTs. Refs 31 and 32. Raman shifts in parentheses are
the reported ones.33 Raman intensity was determined in the Raman spectra shown in Figure 5a−c. Value in the right column is the subtotal in each
d
section.
(Figure 7). These assigned SWNTs exhibiting prominent CDs
are the SWNTs enriched in their abundance through the
extraction except for (8,7)-SWNTs (Table 2).
Although many optically active SWNTs have been identified
so far by CD, all of them are semiconducting SWNTs with
diameters <1.0 nm.4,8,15 In contrast, all the (n,m)-SWNTs
exhibiting prominent CDs in Figure 7 have diameters >1.0 nm,
owing to the preference of the nanocalipers for SWNTs with
relatively large diameters as described above. This indicates that
all of them are newly identified optically active SWNTs. In
addition, optically active metallic SWNTs are identified for the
first time thanks to the significant enrichment of the matallic
SWNTs through the extraction with the nanocalipers
mentioned above.
In order to correlate the helicity of a SWNT (M or P) with
the sign (positive or negative) in CD, we carried out molecular
Figure 6. Absorption spectra of D2O dispersions of SWNTs in the
presence of SDBS before and after extraction with (R)-3. Green and
modeling calculations (molecular mechanics) for the 1:1
complexes of (R)- and (S)-3 with (P)- and (M)-(9,7)-
SWNTs. The energy-minimized structures are shown in Figure
8, where the conformations of (R)- and (S)-3 are clockwise and
anticlockwise, respectively, determined by the CDs of the
complexes mentioned above (Figure 3b,d). Although the
energy difference is not so large (0.72 kJ mol−1), (R)-3/(P)-
pink regions indicate semiconducting (S) and metallic (M),
respectively.
in Figure 7. Since no clear and symmetrical CD is observed in
the wavelength of 300−400 nm and 900−1100 nm, the
semiconducting SWNTs having absorption in these ranges
(Table 2) are not considered for the assignment of the CDs
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Table 2. Comparison in Absorbance of (n,m)-SWNTs before and after the Extraction with Nanocalipers
a
b
c
c
d
c
e
roll-up index
diameter (nm)
ES (nm)
ES (nm)
ES (nm)
EM (nm)
relative abundance
11
22
33
11
(13,4)
(13,3)
(12,4)
(10,6)
(9,7)
1.222
1.170
1.145
1.111
1.103
1.091
1.074
1.050
1.041
1.038
1.032
1.032
1.014
0.966
0.963
0.936
0.916
0.902
0.895
0.884
0.829
0.782
644 (640)
+
+
+
+
+
+
− (1497)
767 (764)
862 (867)
767 (758)
799 (799)
1345 (1342)
1379 (1380)
1322 (1323)
444 (439)
408 (406)
(12,3)
(13,1)
(10,5)
(12,2)
(9,6)
595 (598)
609 (600)
1260 (1250)
1379 (1376)
799 (795)
673 (683)
− (379)
444 (454)
+
−
+
551 (551)
(8,7)
1260 (1267)
1379 (1384)
1200 (1197)
1172 (1172)
723 (732)
673 (672)
799 (798)
723 (722)
408 (404)
444 (456)
− (384)
−
−
(13,0)
(11,3)
(8,6)
− (370)
−
+
(7,7)
504 (505)
504 (506)
(10,3)
(9,4)
1260 (1250)
1108 (1101)
632 (628)
723 (724)
−
−
+
− (362)
(8,5)
(7,6)
1108 (1122)
1053 (1053)
1023 (1023)
949 (952)
644 (650)
727 (735)
644 (647)
673 (661)
− (371)
− (372)
− (337)
− (351)
−
−
−
−
(10,2)
(7,5)
(8,3)
a
b
c
Italic font (n, m) indicates metallic SWNTs, while regular font (n,m) is semiconducting SWNTs. Refs 31 and 32. Wavelengths are determined in
the absorption spectra shown in Figure 6. Wavelengths in parentheses are the reported ones.33 The peaks in the range of wavelength shorter than
d
e
400 nm were not seen behind high and/or steep background and are indicated as “−”. Ref 34. Signs of “+”, “−”, and “ ” indicate that the
abundance of the given (n,m)-SWNTs relatively increase, decrease, and cannot be determined, respectively.
(9,7) and (S)-3/(M)-(9,7) are more stable than (R)-3/(M)-
(9,7) and (S)-3/(P)-(9,7), implying that (P)-(9,7)-SWNTs
exhibit positive and negative CDs in the regions of ES and
33
ES22, respectively, and (M)-(9,7)-SWNTs do opposite signs, as
shown in Figure 7.
CONCLUSION
■
Based on the previous research on the chiral nanotweezers, we
rationally designed the chiral diporphyrin nanocalipers and
employed them for the SWNT separation. In the molecular
design, the two chiral porphyrin units align in parallel through
the spacer consisting of three aromatic sequence, carbazole−
anthracene−carbazole, with >1.4 nm in length. The much
deeper and larger cavity of the nanocalipers than nanotweezers
Figure 7. CD spectra of SWNTs extracted with (R)- and (S)-3 and
absorption spectra of SWNTs extracted with (S)-3. CD spectra are
enabled to accommodate SWNTs with larger diameters (>1.0
normalized at the absorption at 551 nm. The CD spectra at 300−500
nm) and to discriminate the handedness of SWNTs, as we
intended in the molecular design. An additional property of
nanocalipers, which has not been observed in the nanotweezers,
and 500−1100 nm were taken using different spectrophotometers.
Green and pink regions indicate semiconducting (S) and metallic (M),
respectively.
Figure 8. Computer-generated molecular modeling of the complex structures of 1:1 complexes of (R)- and (S)-3 with (P)- and (M)-(9,7)-SWNTs.
The clockwise and anticlockwise conformations shown in Figure 3d are adopted for (R)- and (S)-3, respectively.
H
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Article
Figure 9. Summary in discrimination of diameter, handedness, and metallicity of SWNTs with chiral diporphyrin nanoclipers 3.
is to discriminate the metallicity of SWNTs. These three
functions to recognize diameter, handedness, and metallicity of
SWNTs have been found in the nanocalipers, as summarized in
Figure 9. The SWNTs with diameters >1.05 nm are enriched in
their abundance regardless of the metallicity. Even if the
diameters are <1.05 nm, metallic SWNTs are enriched slightly
through the extraction with nanocalipers. As for the optical
enrichment, the CDs of the semiconducting SWNTs with
diameters >1.0 nm are identified for the first time, such as
(9,7)- and (8,7)-SWNTs. In addition, optically active metallic
SWNTs, such as (9,6)-, (12,3)-, and (13,4)-SWNTs, are also
found for the first time.
AUTHOR INFORMATION
■
Corresponding Author
nkomatsu@belle.shiga-med.ac.jp
Present Address
⊥Key Laboratory for Green Chemical Process of Ministry of
Education, Wuhan Institute of Technology, Wuhan 430073,
China.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Prof. Yasushi Kawai (Nagahama Institute of
Bio-Science and Technology) and Prof. Hiroshi Imahori, Dr.
Tomokazu Umeyama, and Dr. Tatsuya Murakami (Kyoto
University) for allowing us to use CD spectropolarimeter and
PL spectrophotometer, respectively. The authors are grateful to
Prof. Kazunari Matsuda (Kyoto University) and Prof. Lorenzo
EXPERIMENTAL SECTION
■
Extraction of HiPco-SWNTs with Nanocalipers. Chiral nano-
calipers (R)- or (S)-3 (10 mg) and HiPco-SWNTs (10 mg) in
methanol (40 mL) were bath sonicated at 20 °C for 36 h. After
centrifugation of the resulting suspension at 50 400g for 45 h, the
supernatant was subjected to UV−vis−NIR, fluorescence, CD, TEM,
AFM, STM, and STEM measurements. After concentration of the
supernatant, the residual solid was washed with THF and pyridine
several times until the porphyrin Soret band disappeared in the UV−
vis spectra of the washings. The thoroughly washed SWNTs (0.65 mg)
were analyzed with Raman spectroscopy at excitation of 488, 633, and
785 nm. The solid sample was dispersed in D2O (16 mL) in the
presence of SDBS (10 mg mL−1) by bath sonication at 20 °C for 10 h
and then tip sonication for 3 h at 0 °C. After centrifugation at 136 000g
for 40 min, the upper layer (about 85%) of the supernatant was
poured up and subjected to UV−vis−NIR and CD measurements. For
the dispersion of the raw HiPco-SWNTs, tip sonication was carried
out for 40 min under the same conditions followed by centrifugation at
543 000g for 60 min.
̀
Stella (Universita di Roma Tor Vergata) for helpful suggestions
about exciton coupling between SWNTs and porphyrin and
fluorescence quenching of the complex, respectively. This work
was financially supported by Grant-in-Aid for Scientific
Research (S) (no. 24221009), Grant-in-Aid for Scientific
Research on Priority Areas (no. 22016005), Grant-in-Aid for
Scientific Research (B) (no. 23350062), the Kurata Memorial
Hitachi Science and Technology Foundation, Adaptable and
Seamless Technology Transfer Program through Target-driven
R (JST), and Industrial Technology Research Grant Program in
2005 from New Energy and Industrial Technology Develop-
ment Organization (NEDO) of Japan.
ASSOCIATED CONTENT
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