Analysis of functionalization degree of single-walled carbon nanotubes having various substituents

Article abstract of DOI:10.1021/ja308969p

Introducing substituents onto SWNT sidewalls increases their solubility and tunes their properties. Controlling the degree of functionalization is important because the addition of numerous functional groups on the sidewall degrades their intrinsic useful electronic properties. We examined the synthesis and characterization of sidewall-functionalized SWNTs in this study. The functionalized SWNTs (¹R-SWNTs-²R) were prepared in a one-pot reaction of SWNTs with alkyllithium (¹RLi) followed by alkyl bromide (²RBr). The functionalized SWNTs were characterized by the absorption and Raman spectroscopy and thermogravimetric analysis. Not only the total number of functional groups introduced on the SWNT sidewall (formula mass: ¹R = ²R) but also the ratio of ²R to ¹R in the functionalized SWNTs (formula mass: ¹R ≠ ²R) having two different substituents were clarified using the relation between results of Raman spectroscopy and thermogravimetric analysis. Results show that the degree of functionalization of ²R to ¹R in ¹R-SWNTs-²R can be well controlled by the bulkiness of the alkyl groups of ¹RLi and ²RBr. Moreover, substituent effects of reductive alkylation and reductive silylation of SWNTs via Birch reduction were investigated.

Full text of DOI:10.1021/ja308969p

Article
pubs.acs.org/JACS
Analysis of Functionalization Degree of Single-Walled Carbon
Nanotubes Having Various Substituents
,†
†
†
†
†
†
Yutaka Maeda,* Kazuma Saito, Norihisa Akamatsu, Yuriko Chiba, Seina Ohno, Yumi Okui,
†
†
‡
,§
Michio Yamada, Tadashi Hasegawa, Masahiro Kako, and Takeshi Akasaka*
†
Department of Chemistry, Tokyo Gakugei University, Tokyo 184-8501, Japan
‡
Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
§Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba, Ibaraki 305-8577, Japan
*
S Supporting Information
ABSTRACT: Introducing substituents onto SWNT sidewalls
increases their solubility and tunes their properties. Controlling
the degree of functionalization is important because the addition of
numerous functional groups on the sidewall degrades their intrinsic
useful electronic properties. We examined the synthesis and
characterization of sidewall-functionalized SWNTs in this study.
1
2
The functionalized SWNTs ( R-SWNTs- R) were prepared in a
1
one-pot reaction of SWNTs with alkyllithium ( RLi) followed by
alkyl bromide ( RBr). The functionalized SWNTs were charac-
2
terized by the absorption and Raman spectroscopy and
thermogravimetric analysis. Not only the total number of
functional groups introduced on the SWNT sidewall (formula
1
2
2
1
mass: R = R) but also the ratio of R to R in the functionalized
1
2
SWNTs (formula mass: R ≠ R) having two different substituents
were clarified using the relation between results of Raman spectroscopy and thermogravimetric analysis. Results show that the
degree of functionalization of R to R in R-SWNTs- R can be well controlled by the bulkiness of the alkyl groups of RLi and
RBr. Moreover, substituent effects of reductive alkylation and reductive silylation of SWNTs via Birch reduction were
2
1
1
2
1
2
investigated.
2
6
INTRODUCTION
using alkyllithium and alkyl halide has been reported. The two-
■
step reductive alkylation showed significant substituent effects on
Single-walled carbon nanotubes (SWNTs) have attracted broad
attention because of their outstanding mechanical and electronic
properties, which have given rise to many potential applica-
27
the degree of functionalization. The degree of functionalization
can be controlled by the substituents of alkyllithium and alkyl
halide. The controlled multiple functionalization of SWNTs is
1
−3
tions. Because of the low dispersibility of SWNTs in aqueous
and organic solvents, several dispersion procedures have been
28−32
now a challenging subject for practical applications.
4
−7
Absorption and Raman spectroscopy and thermogravimetry
TG) are currently the most powerful methods used to study the
developed.
Much attention has been devoted to the
(
introduction of functional groups onto the SWNT sidewall
because the groups affect not only the increase of solubility but
also the change of electrochemical properties of SWNTs.
1
8,33
degree of functionalization on SWNTs.
Haddon et al.
8
−11
reported details of characteristic absorption and the Raman
spectra change accompanied by an increase of the degree of
For
example, silylation of SWNTs increases field emission properties
of SWNTs and enhances the n-type properties of field effect
3
3,34
sidewall functionalization.
The degree of functionalization is
1
2−14
estimated on the basis of the intensity ratio of the D band to the
G band (D/G ratio: [D/G]). The degree of functionalization,
which is defined as the number of SWNT carbon atoms per
functional group (functional group coverage: FGC), is
transistors.
In contrast, SWNTs lose their outstanding
electronic properties by excessive functionalization because of
partial disconnection of the π-conjugated systems.
sequently, it is important to control the degree of functionaliza-
tion of the SWNT sidewall.
1
5,16
Con-
18,30
determined from the weight losses in TGA.
Substituents
having the same formula mass are required for the estimation of
FGC of functionalized SWNTs. Unfortunately, functionalization
degree is not clarified for each substituent in the multi-
Reductive alkylation of SWNTs has been conducted widely for
the introduction of functional groups onto the SWNT
1
7−25
sidewall.
This reductive alkylation reportedly proceeds
effectively. The bundles of SWNT are well exfoliated by the
repulsion of SWNT anion intermediates produced in the
reaction. Recently, two-step reductive alkylation of SWNTs
Received: August 10, 2012
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Scheme 1
functionalized SWNTs using absorption and Raman spectros-
copy and thermogravimetry, independently.
the functional groups (Mwsub) and atomic mass of carbon according to
the following equation (eq 1).
For this study, we prepared functionalized SWNTs using
butyllithium and alkyl bromide, and estimated the degree of
functionalization in two-step alkylation. A good relation between
wt%SWNTs/12.01
wt%sub/MwR
FGCexp
=
(1)
The wt%sub was determined from the mass loss from TGA at 800 °C
in N atmosphere according to the previously reported method.
[
D/G] obtained from the Raman spectra and FGCexp
1
8,30
The
determined from TGA was found in Bu-SWNTs-Bu. Moreover,
on the basis of the relation between [D/G] and FGC of Bu-
2
wt%
was determined from the mass loss of the residual materials,
SWNTs
1
2
2
1
1
R-SWMTs- R after TGA in N , at 800 °C in air atmosphere.
2
SWNTs-Bu, the functionalization ratio of R to R in R-
2
1
2
1
2
The ratios of R to R ( R/ R) of Bu-SWNTs- R were estimated
2
2
1
2
SWNTs- R ( R ≠ Bu) was revealed by analyses of R-SWNTs- R
having different substituents. In addition, substituent effects on
the reductive alkylation and silylation of SWNTs via Birch
reduction were investigated.
according to the following equation (eq 2). Wt% and wt%SWNTs in Bu-
SWNTs- R were obtained using TGA. Calculated FGC (FGC ) was
estimated from [D/G]514.5 nm of Bu-SWNTs- R using the relation
between [D/G]514.5 and FGCexp of Bu-SWNTs-Bu as shown in Figure 4.
sub
2
cal
2
wt%Sub·FGC ·12.01
wt%SWNTs(Mw − Mw R
Mw
1R
2R
cal
χ =
= (Mw
EXPERIMENTAL SECTION
■
1R
2
)
− Mw R)
2
SWNTs (HiPco SWNTs) were obtained from Unidym Inc.: SWNTs
Lot No. P2150) were used for the two-step alkylation; SWNTs (Lot
No. R0513) were used for the reductive alkylation and silylation of
SWNTs via Birch reduction. The solution-phase visible-near-infrared
Mwsub = χ·Mw1R + (1 − χ)·Mw2R
2
(
(1
R ≠ R)
(
vis−NIR) absorption spectra were recorded using a spectrophotom-
1
− χ
χ
2
1
eter (V-670; Jasco Corp.) with a Pyrex cell of 10 mm path length. Raman
spectra were measured using a spectrophotometer (LabRAM HR-800;
Horiba Ltd.) with excitation wavelengths of 514.5 and 633 nm. Scanning
electron microscope (SEM) observation was conducted using a field-
emission electron microscope (15 kV accelerating voltage, 20 mA beam
current, S4500; Hitachi Ltd.). TGA was performed at a heating rate of 10
( R/ R) =
(2)
Reductive Alkylation and Silylation of SWNTs via Birch
Reduction. In a heat-dried, 50-mL, three-necked flask, were placed 8
mg of HiPco SWNTs, 4,4′-di-tert-butylbiphenyl (DTBP) (160 mg, 0.6
mmol), and 32 mL of anhydrous tetrahydrofuran. The mixture was
sonicated and dispersed under argon atomsphere. The dispersion was
added to lithium (138.8 mg, 6 mmol) in a heat-dried, 100 mL, three-
necked flask under argon atmosphere. The mixture was sonicated for 60
min. The color of the mixture became green, indicating formation of the
DTBP anion radical from DTBP. Alkyl bromide (2, 6 mmol) was added
to the reaction mixture. The mixture was sonicated for 30 min and then
quenched with ethanol. The reaction mixture was washed with 1.0 M
hydrochloric acid and water until the pH value remained neutral. The
suspended black solid was collected by filtration using a membrane filter
°
1
C/min and a nitrogen flow rate of 400 mL/min or an air flow rate of
00 mL/min (Thermo plus TG8120; Rigaku). Ultrasound irradiation
was performed using a bath sonicator (B2510J-MT ultrasonic cleaner;
Branson).
Two-Step Alkylation of SWNTs. Into a 100-mL heat-dried, three-
necked, round-bottom flask were placed 8.0 mg of purified HiPco
SWNTs and 80 mL of anhydrous benzene; this mixture was then
sonicated for 30 min under argon. To this dispersion was added 3.2 mL
of 1.6 M solution of butyllithium (1, 5.1 mmol) in pentane. After the
+
resulting suspension of an alkylated SWNTs anion (Bu-SWNTs-Li )
was stirred for 30 min, it was sonicated for 30 min. Then butyl bromide
2, 10.2 mmol) was added to the mixture. The mixture was stirred for 1 h
(
PTFE, 0.1 mm) and washed with dichloromethane, methanol, and
tetrahydrofuran. The solid was dried under vacuum at 50 °C. For
comparison of absorption spectra under the same conditions, SWNTs
and functionalized SWNTs were dispersed in tetrahydrofuran
containing 1.0 M propylamine and used for spectral analysis.
Propylamine is used as a dispersant reagent for SWNTs because of
the low dispersibility of the starting SWNTs.
Synthetic Procedure of (3-Bromopropoxy)-tert-butyldime-
thylsilane. A mixture of 3-bromopropanol (2.6 mL, 18 mmol),
imidazole (3.7 g, 54 mmol), and tert-butyldimethylsilylchloride
(TBDMS-Cl) (4.7 g, 31 mmol) in anhydrous THF (50 mL) was
stirred at room temperature for 24 h. The solvent was removed under
reduced pressure and the residue was diluted with water and extracted
three times with dichloromethane. The organic layers were combined,
washed with brine, dried, and evaporated to obtain (3-bromopropoxy)-
tert-butyldimethylsilane in quantitative yield.
(
and then quenched by addition of 20 mL of dry ethanol. The reaction
mixture was washed with water and diluted in 1.0 M hydrochloric acid
until the pH value remained neutral. The suspended black solid was
collected by filtration using a membrane filter (PTFE, 1.0 mm) and
washed with tetrahydrofuran, methanol, and dichloromethane using the
dispersion−filtration process. The solid was dried under vacuum at 50
C. SWNTs and functionalized SWNTs were dispersed in tetrahy-
drofuran containing 1.0 M propylamine. The dispersion liquid was
used for absorption spectra measurements.
D band intensity reflects not only the defect density on the sidewalls
of SWNTs but also the amount of impurity particles. HiPco SWNTs
were used in this study in order to greatly reduce the influence of
impurity particles, such as amorphous carbon. SEM images and TGA of
the HiPco SWNTs used did not show any obvious impurity. The
information of chiral selectivity in the reaction can be evaluated due to
wide chirality distribution of HiPco SWNTs. There is an additional
advantage because it can be compared with the results reported
previously.
°
35
1
(3-Bromopropoxy)-tert-butyldimethylsilane: H NMR (400 MHz,
CDCl ): δ = 0.06 (s, 6H, Si−CH ); 0.89 (s, 9H, C−CH ); 2.03 (m, 2H,
3
3
3
CH ); 3.51 (t, J = 6.4 Hz 2H, Br−CH ); 3.73 (t, J = 5.7 Hz, 2H, −CH −
2
2
2
1
3
O). C NMR (100 MHz, CDCl ): δ = −5.3 (Si−CH ); 18.4 (C−Me );
3
3
3
26.0 (C−CH ); 30.8 (−CH −); 35.6 (Br−CH ); 60.5 (−CH −O).
3
2
2
2
FGCexp was estimated from the wt% of the functional groups (wt%sub)
(3-Bromo-2,2-dimethylpropyloxy)-tert-butyldimethylsilane: 1H
NMR (400 MHz, CDCl ): δ = 0.00 (s, 6H, Si−CH ); 0.85 (s, 9H,
1
2
and SWNTs (wt%SWNTs) in R-SWNTs- R and the molecular weights of
3
3
B
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
2
n
i
Figure 1. Absorption spectra of Bu-SWNTs-Bu in THF solution containing propylamine normalized at 628 nm. SWNTs (black), R: Bu (red), Bu
s
t
(
purple), Bu (green), and Bu (blue).
2
n
2
i
Figure 2. Raman spectra of Bu-SWNTs-Bu normalized at the G band with excitation wavelengths 514.5 nm. SWNTs (black); R = Bu (red); R = Bu
2
s
2
t
(
purple); R = Bu (green); and R = Bu (blue).
C−CH ); 0.92 (s, 6H, C−CH ); 3.30 (s, 2H, Br−CH ); 3.32 (s, 2H,
SWNTs-Bu was confirmed using absorption and Raman spectra.
The intensities of the characteristic absorptions in the absorption
spectra of Bu-SWNTs-Bu decreased remarkably (Figure 1),
which indicates that sidewall functionalization occurred. Figure 2
shows the Raman spectra of Bu-SWNTs-Bu measured at 514.5
3
3
2
13
−
CH −O). C NMR (100 MHz, CDCl ): δ= −5.4 (Si−CH ); 18.2
2
3
3
(
(
C−Me ); 23.0 (−C−(CH ) ); 25.9 (C−(CH ) ); 36.9 (−C−
3
3
2
3 3
CH ) ); 43.3 (Br−CH ); 67.9 (−CH −O).
3
2
2
2
RESULTS AND DISCUSSION
■
1
n
nm excitation. The order of [D/G]
Bu-SWNTs- Bu < Bu-SWNTs- Bu < Bu-SWNTs- Bu, as
in R-SWNTs- Bu is:
14.5 nm
5
n
Functionalization of SWNTs was achieved using the reaction
n
n
s
t
n
1
2
with alkyllithium ( RLi, 1) followed by alkyl bromide ( RBr, 2)
27
t
under argon according to the literature. (Scheme 1) Bu-
presented in Table 1. The highest [D/G]514.5 nm value of Bu-
C
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Table 1. D/G Ratio ([D/G]514.5 nm) and TGA-Determined wt
%
of Substituent (wt% ) and Functional Group Coverage
sub
(
FGC ) of Bu-SWNTs-Bu
exp
1R-SWNTs-2R
[D/G]514.5 nm
wt% of substituent
FGCexp
n
n
n
n
s
n
Bu-SWNTs- Bu
0.64
0.60
0.58
0.75
0.70
0.42
0.36
0.28
0.71
0.44
0.35
0.19
15.9
13.4
13.2
17.2
15.7
12.1
11.2
9.90
16.0
12.4
11.2
9.10
25.1
30.8
31.2
22.9
25.6
34.6
37.7
43.1
25.0
33.6
37.8
47.6
i
Bu-SWNTs- Bu
s
Bu-SWNTs- Bu
t
Bu-SWNTs- Bu
n
Bu-SWNTs- Bu
s
s
s
t
i
Bu-SWNTs- Bu
s
Bu-SWNTs- Bu
t
Bu-SWNTs- Bu
n
Bu-SWNTs- Bu
t
t
t
i
Bu-SWNTs- Bu
s
Bu-SWNTs- Bu
t
Bu-SWNTs- Bu
1
2
Figure 5. Raman spectra of R-SWNTs- R normalized at the G band
2
n
with excitation wavelengths 514.5 nm. SWNTs (black); R = octyl
red); R = dodecyl (green), octadecyl (blue); R = octyl (purple).
2
n
n
2
s
(
Table 2. D/G Ratio ([D/G]
), wt% of Substituent (wt
14.5 nm
5
2
1
2
1
1
2
%sub), FGC , and Ratio of R to R ( R/ R) of R-SWNTs- R
cal
wt% of
substituent
1R-SWNTs-2R
[D/G]514.5 nm
FGCcal R/ R)
2
1
n
n
n
n
t
n
Bu-SWNTs- octyl
0.59
0.55
0.55
0.50
0.66
0.39
0.59
0.43
19.5
18.0
21.4
22.2
20.6
13.3
23.0
14.5
29.2
30.9
31.0
32.8
26.6
37.5
29.2
35.8
0.86
0.79
0.65
0.39
0.85
0.26
0.74
0.09
s
t
Bu-SWNTs- octyl
Figure 3. TG curves of Bu-SWNTs-Bu from 100 to 800 °C at a heating
n
Bu-SWNTs- dodecyl
rate of 10 °C/min in N atmosphere (black). TG curves of the residual
2
t
n
Bu-SWNTs- octadecyl
materials, Bu-SWNTs-Bu after TGA in N atmosphere, from 100 to 800
C at a heating rate of 10 °C/min in air atmosphere (red).
2
n
°
Bu-SWNTs- octyl
t
t
t
s
Bu-SWNTs- octyl
n
Bu-SWNT- dodecyl
n
Bu-SWNTs- octadecyl
degree of functionalization is decreased with an increase of steric
1
2
1
2
hindrance of R and R, and affected by R stronger than R. The
n
t
unexpectedly high reactivity of Bu-SWNTs- Bu might be
attributable to the steric hindrance of the Bu radical, which
might prevent dimerization and disproportionation of Bu radical
and react with Bu-SWNTs radical having curvature.
t
t
n
Figure 4. [D/G]514.5 nm of Bu-SWNTs-Bu as a function of FGCexp
.
Analysis of the number of functional groups on the SWNTs
was performed on the basis of TG under heating to 800 °C at a
rate of 10 °C/min (Figure 3 and Figures S8−S10 in Supporting
Information). From the observed weight loss, FGC of Bu-
n
t
SWNTs- Bu indicates that the reactivity of BuLi to SWNTs,
t
producing the Bu-SWNTs- intermediate, is higher than those of
other BuLi. It is particularly interesting that the order of [D/
SWNTs-Bu (FGC ) was estimated as shown in Table 1. Both
exp
t
2
s
2
n
G]514.5 nm2 in Bu-SWNTs- R, Bu-SWNTs- R, and Bu-
[D/G]514.5 nm and FGCexp imply the degree of functional of
SWNTs. Therefore, FGCexp is expected to be consistent with
[D/G]514.5 nm. In fact, [D/G]514.5 nm and FGCexp show a linear
relation, as portrayed in Figure 4.
1
t
1
s
1
SWNTs- R is R-SWNTs- Bu < R-SWNTs- Bu < R-
i
1
n
SWNTs- Bu < R-SWNTs- Bu. The exception is the order of
n
t
the value of Bu-SWNTs- Bu. We can safely conclude that the
D
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Scheme 2
Figure 6. Absorption spectra of SWNTs and SWNTs-R. From top to
t
bottom at 1151 nm: SWNTs (black), SWNTs- Bu (purple), SWNTs-
i
neopentyl (aqua), SWNTs-Ph (pink), SWNTs- Pr (orange), SWNTs-
n
CH Ph (brown), SWNTs- Bu (blue), SWNTs-CH CH Ph (yellow),
and SWNTs- octyl (red).
2
2
2
n
1
2
When the formula masses of R and R are equal, the presence
1
of carbon atoms of SWNTs for one functional group on R-
2
SWNTs- R can be estimated using TGA, but no information
2
1
1
2
about the ratio of the number of R to that of R in R-SWNTs- R
1
2
is obtainable. When the formula masses of R and R are not
equal, the ratio of R to R of R-SWNTs- R ( R ≠ R) can be
estimated from comparison of the TGA weight loss and [D/G]
value in R-SWNTs- R ( R ≠ R) and that in Bu-SWNTs-Bu. To
ascertain the ratio of R to R, functionalized SWNTs ( R = Bu,
R ≠ Bu) were prepared, and wt%’s of the functional groups, [D/
G]514.5 nm, on Bu-SWNTs- R were evaluated. The degree of
2
1
1
2
1
2
Figure 7. Raman spectra of SWNTs and SWNTs-R normalized at the G
−1
band. From top to bottom at 268 cm : SWNTs, SWNTs-benzyl,
1
2
1
2
t
i
SWNTs- Bu, SWNTs-neopentyl, SWNTs-Ph, SWNTs- Pr, SWNTs-
2
1
1
n
n
phenethyl, SWNTs- Bu, and SWNTs- octyl. From top to bottom at the
D band: SWNTs- Bu, SWNTs- octyl, SWNTs-phenethyl, SWNTs- Pr,
SWNTs-neopentyl, SWNTs-benzyl, SWNTs-Ph, SWNTs- Bu, and
2
n
n
i
t
2
2
SWNTs. Excitation wavelengths: (a) 514.5 nm. (b) 633 nm.
functionalization determined by [D/G]514.5 nm in Bu-SWNTs- R
decreased along with the increase of the alkyl chain length and
2
Table 3. D/G Ratios ([D/G]514.5 nm and [D/G]633 nm) of
SWNTs-R
bulkiness of R (Figure 5). Consequently, the calculated FGC
2
2
(
FGC ) in Bu-SWNTs- R ( R ≠ Bu) was estimated from the
cal
linear relation between [D/G]
of Bu-SWNTs- R ( R ≠ Bu). The ratios of
R to R ( R/ R) of Bu-SWNTs- R were estimated using eq 2
from wt%sub and wt%SWNTs in Bu-SWNTs- R and FGC . (Table
) It is particularly noteworthy that the R/ R in Bu-SWNTs- R
and FGC in Bu-SWNTs-
SWNTs-R
[D/G]514 nm
[D/G]633 nm
5
14.5 nm
exp
2
2
Bu and [D/G]
n
5
14.5 nm
SWNTs- Bu
0.79
0.73
0.71
0.42
0.36
0.39
0.36
0.25
0.39
0.28
0.77
0.67
0.64
0.27
0.22
0.24
0.19
0.11
0.24
0.15
2
1
2
1
2
n
SWNTs- octyl
2
cal
SWNTs-phenethyl
2
1
2
2
i
SWNTs- Pr
shows the same tendency as the degree of functionalization
determined using Raman spectroscopy. The degree of
SWNTs-benzyl
SWNTs-neopentyl
SWNTs-Ph
2
1
functionalization and the R/ R are strongly influenced
2
1
respectively by the alkyl groups of R and R.
t
SWNTs- Bu
The substituent effects on the functionalization degree in the
reductive alkylation of SWNTs via Birch reduction were studied.
Alkylation of SWNTs was conducted according to the reported
SWNTs-SiMe3
SWNTs-SiEt3
2
0
methods. (Scheme 2) The intensity of the characteristic
absorptions in the absorption spectra of SWNTs-Bu decreased
remarkably (Figure 6), which indicates that sidewall function-
alization took place. The relative functionalization degree of
SWNTs-R estimated from the decrease of S bands is the
i
SWNTs- Pr, SWNTs-benzyl, SWNTs-neopentyl, SWNTs-Ph >
t
SWNTs- Bu. This order is consistent with that in the alkylation of
t
+
27
Bu-SWNTs-Li with alkyl bromide. Hirsch et al. studied the
+
selectivity in the reaction of SWNTs-Na with 1-iodobutane in
1
1
n
n
n
following: SWNTs- Bu, SWNTs- octyl, SWNTs- phenethyl >
ammonia at −78 °C. They reported that SWNTs with small
E
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 8. Relative RBM peak intensity of SWNTs-R toward that of pristine SWNTs. (○): semiconducting SWNTs; (■): metallic SWNTs.
Scheme 3
Figure 9. Absorption spectra of SWNTs-SiR3 in THF solution
containing propylamine normalized at 628 nm. SWNTs (black); R =
Et (red); and R = Me (blue).
36
diameters were more reactive than those with larger diameters.
The diameters (d ) of SWNTs used were estimated as 1.09, 1.00,
t
37
0
.89, 0.81, and 0.78 nm from the equation S = 0.962/d . The
absorbance of SWNTs- Pr, SWNTs-benzyl, SWNTs-neopentyl,
and SWNTs- Bu is markedly reduced in smaller-diameter
11 t
i
t
SWNTs compared to larger-diameter SWNTs.
Figure 7 shows the Raman spectra of SWNTs-R. Table 3
shows D/G ratios for the functionalized SWNTs determined
from the Raman spectra measured at 514.5 and 633 nm
excitations. The relative degree of functionalization of SWNTs
Figure 10. Raman spectra of SWNTs and SWNTs-SiR normalized at
the G band. SWNTs (black); R = Et (red); and R = Me (blue).
Excitation wavelengths: (a) 514.5 nm. (b) 633 nm.
3
n
estimated from the D/G ratio is the following: SWNTs- Bu,
and 1.22, 1.09, 0.93, 0.90, 0.82, and 0.78 nm (633 nm), from the
n
n
i
−1
−1
−1
SWNTs- octyl, SWNTs- phenethyl > SWNTs- Pr, SWNTs-
equation wRBM = Adt + B (A = 223.5 cm and B = 12.5 cm )
t
38−40
benzyl, SWNTs-neopentyl, SWNTs-Ph > SWNTs- Bu, which
where wRBM is the wavenumber of RBM.
The amount of the
is consistent with that estimated from the absorption spectra. To
clarify the diameter selectivity in the reaction of SWNTs-Li with
decrease of relative peak intensity of SWNTs with small
diameters was larger than those of large diameters, which also
suggests that SWNTs with small diameters are considerably
more reactive than SWNTs with large diameters. Figure 8 shows
+
1
, the diameters of the SWNTs were estimated. The SWNT
diameters were 1.27, 0.95 0.89, 0.87, and 0.75 nm (514.5 nm)
F
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
n
i
t
the relative RBM peak intensity of SWNTs-R (R = Bu, Pr, Bu)
toward that of SWNTs as a function of SWNT diameter. It is
clearly shown that the reductive alkylation of SWNTs took place
in not only diameter but also band-structure selectivity. This
result is consistent with the results previously reported by Hirsch
introduced into the multifunctionalization of SWNTs for
practical applications.
In addition, the degrees of functionalization in the reductive
+
alkylation and silylation of SWNTs-Li via Birch reduction were
respectively affected by the bulkiness of alkyl halides and silyl
chloride. The hydroxyl group was also introduced on the SWNT
sidewall using a protecting group. It is noteworthy that the degree
of functionalization of SWNTs, which affects their electronic
properties, can be tuned simply by the bulkiness of the alkyl
groups in the alkyl halides and silyl chlorides.
3
6
et al. Spectroscopic analysis of SWNTs-R revealed that the
steric hindrance of alkyl groups and the stability of radical
intermediates determine the degree of functionalization.
Recently, reductive functionalization of SWNTs with carbonyl
compounds was conducted because the functional groups on
SWNTs produced from these compounds can be reactive parts
2
4,25,41,42
for secondary functionalization.
The secondary func-
ASSOCIATED CONTENT
■
tionalization of SWNTs-R is expected to be a useful reaction for
the practical use of SWNTs. Therefore, we studied the reaction
of SWNTs with (3-bromopropyloxy)-tert-butyldimethylsilane
and (3-bromo-2,2-dimethylpropyloxy)-tert-butyldimethylsilane.
These compounds are useful for reductive alkylation with
SWNTs, and a hydroxy group can be used for secondary
functionalization after removal of the protecting group, the tert-
butyldimethylsilyl (TBDMS) group. The reductive alkylation of
SWNTs with (3-bromopropyloxy)-tert-butyldimethylsilane and
*
S
Supporting Information
Raman spectra, absorption spectra, TGA data, and SEM images
of functionaliaed SWNTs. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
akasaka@tara.tsukuba.ac.jp
(
3-bromo-2,2-dimethylpropyloxy)-tert-butyldimethylsilane fol-
Notes
lowed by the deprotection reaction using hydrochloric acid
gave the SWNTs-(CH ) OH and SWNTs-CH C-
The authors declare no competing financial interest.
2
3
2
(
CH ) CH OH, respectively. The D/G ratio at the excitation
3 2 2
ACKNOWLEDGMENTS
■
wavelengths of 514.5 and 633 nm of SWNTs-R were determined.
This work was supported in part by a Grant-in-Aid for Scientific
Research on Innovation Areas (No. 20108001, “pi-Space”), a
Grant-in-Aid for Scientific Research (A)(No. 20245006), Grant-
in-Aid for Young Scientists (B)(No. 23750035), Nanotechnol-
ogy Support Project from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
(
SWNTs-(CH ) OH: D/G
= 0.50, D/G633 = 0.41; SWNTs-
CH C(CH ) CH OH: D/G = 0.24, D/G633 = 0.12). The
514.5
2
3
514.5
2
3 2
2
functionalization degree of SWNTs-(CH ) OH and SWNTs-
CH C(CH ) CH OH are respectively lower than those of
2
3
2
3 2
2
n
SWNTs- Bu and SWNTs-neopentyl. These results indicate that
introduction of the functional groups having the protecting
group by the reductive alkylation was achieved successfully and
controlled by the bulkiness of the substituents.
The effect of substituents’ bulkiness on the efficiency of the
reductive silylation of SWNTs via Birch reduction was also
studied because introduction of silyl groups reportedly tunes the
REFERENCES
(1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603.
(2) Bethune, D. S.; Kiang, C. H.; De Vries, M. S.; Gorman, G.; Savoy,
R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605.
3) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.;
Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105.
4) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.;
Tour, J. M. Chem. Commun. 2001, 193.
5) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006,
06, 1105.
6) Ausman, K. D.; Piner, R.; Lourie, O.; Ruoff, R. S.; Korobov, M. J.
Phys. Chem. B 2000, 104, 8911.
(7) Karousis, N.; Tagmatarchis, N.; Tasis, D. Chem. Rev. 2010, 110,
5366.
■
12−14
Reactions of SWNTs-Li+
(
electronic properties of SWNTs.
with trimethylchlorosilane and triethylchlorosilane were con-
ducted (see Scheme 3). The characteristic absorptions in the
absorption spectra of SWNTs-SiMe3 and SWNTs-SiEt3
decreased, which indicates that sidewall functionalization
occurred (Figure 9). Figure 10 shows Raman spectra of
SWNTs-SiMe and SWNTs-SiEt , and the D/G ratios of the
functionalized SWNTs as determined from Raman spectroscopy
are listed in Table 3. The SWNTs-SiMe showed a degree of
functionalization higher than those of SWNTs-SiEt3 and
SWNTs- Bu, which indicates that steric hindrance in the
(
(
1
(
3
3
3
(8) Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952.
t
(
(
(
9) Balasubramanian, K.; Burghard, M. Small 2005, 1, 180.
10) Hirsch, A.; Vostrowsky, O. Top. Curr. Chem. 2005, 245, 193.
11) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes,
M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002.
12) Maeda, Y.; Sato, Y.; Kako, M.; Wakahara, T.; Akasaka, T.; Lu, J.;
chlorosilanes is an important factor for functionalization. The
degree of functionalization in the reductive silylation of SWNTs
is higher than that in the corresponding reductive alkylation.
(
Nagase, S.; Kobori, Y.; Hasegawa, T.; Motomiya, K.; Tohji, K.; Kasuya,
A.; Wang, D.; Yu, D.; Gao, Z.; Han, R.; Ye, H. Chem. Mater. 2006, 18,
4205.
CONCLUSION
■
Two-step reductive alkylation of SWNTs introduced two
functional groups on the SWNT sidewall. The good relation
between [D/G] obtained from the Raman spectra and FGCexp
determined from TGA was found in Bu-SWNTs-Bu. The degree
of functionalization of R-SWNTs- R having two different
substituents was estimated using the relation between [D/G]
(13) Kumashiro, R.; Hiroshiba, N.; Komatsu, N.; Akasaka, T.; Maeda,
Y.; Suzuki, S.; Achiba, Y.; Hatakeyama, R.; Tanigaki, K. J. Phys. Chem.
Solids 2008, 69, 1206.
1
2
(14) Chang, K.; Berber, S.; Toman
36102.
15) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.;
Hauge, R. H.; Margrave, J. L. Chem. Phys. Lett. 1998, 296, 188.
16) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L.
́
ek, D. Phys. Rev. Lett. 2008, 100,
2
(
1
2
and FGC. The bulkiness of R and R strongly affects the degree
1
2
of functionalization and the ratio of and R and R on the SWNT
sidewall. This substituent effect is useful to control the degree of
functionalization and the ratio of different functional groups
(
M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave,
J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367.
G
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
17) Chattopadhyay, J.; Sadana, A. K.; Liang, F.; Beach, J. M.; Xiao, Y.;
Hauge, R. H.; Billups, W. E. Org. Lett. 2005, 7, 4067.
18) Liang, F.; Sadana, A. K.; Peera, A.; Chattopadhyay, J.; Gu, Z.;
Hauge, R. H.; Billups, W. E. Nano Lett. 2004, 4, 1257.
19) Chattopadhyay, J.; Chakraborty, S.; Mukherjee, A.; Runtang, W.;
Engel, P. S.; Billups, W. E. J. Phys. Chem. C 2007, 111, 17928.
20) García-Gallastegui, A.; Obieta, I.; Bustero, I.; Imbuluzqueta, G.;
Arbiol, J.; Miranda, J. I.; Aizpurua, J. M. Chem. Mater. 2008, 20, 4433.
21) Mukherjee, A.; Combs, R.; Chattopadhyay, J.; Abmayr, D. W.;
Engel, P. S.; Billups, W. E. Chem. Mater. 2008, 20, 7339.
22) Syrgiannis, Z.; Hauke, F.; Rohrl, J.; Hundhausen, M.; Graupner,
R.; Elemes, Y.; Hirsch, A. Eur. J. Org. Chem. 2008, 2544.
23) Gebhardt, B.; Graupner, R.; Hauke, F.; Hirsch, A. Eur. J. Org.
Chem. 2010, 1494.
24) Gebhardt, B.; Hoffman, F.; Backes, C.; Mu
Maultzsch, J.; Thomsen, C.; Hauke, F.; Hirsch, A. J. Am. Chem. Soc.
011, 133, 19459.
25) Gebhardt, B.; Syrgiannis, Z.; Backes, C.; Graupner, R.; Hauke, F.;
Hirsch, A. J. Am. Chem. Soc. 2011, 133, 7985.
26) Roubeau, O.; Lucas, A.; Penicaud, A.; Derre,
Nanotechnol. 2007, 7, 3509.
27) Maeda, Y.; Kato, T.; Hasegawa, T.; Kako, M.; Akasaka, T.; Lu, J.;
Nagase, S. Org. Lett. 2010, 12, 996.
28) Chen, J.; Chen, S.; Zhao, X.; Kuznetsova, L. V.; Wong, S. S.;
Ojima, I. J. Am. Chem. Soc. 2008, 130, 16778.
29) Stephenson, J. J.; Hudson, J. L.; Leonard, A. D.; Prince, K. B.;
Tour, J. M. Chem. Mater. 2007, 19, 3491.
30) Brunetti, F. G.; Herrero, M. A.; Mun
Alfonsi, J.; Meneghetti, M.; Prato, M.; Vaz
008, 130, 8094.
31) Men
ard-Moyon, C.; Fabbro, C.; Prato, M.; Bianco, A. Chem.
Eur. J. 2011, 17, 3222.
32) Heister, E.; Neves, V.; Tilmacu, C.; Lipert, K.; Beltran
Coley, H. M.; Silva, S. R. P.; McFadden, J. Carbon 2009, 47, 2152.
33) Hu, H.; Zhao, B.; Hamon, M. A.; Kamaras, K.; Itkis, M. E.;
Haddon, R. C. J. Am. Chem. Soc. 2003, 125, 14893.
34) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P.
C.; Haddon, R. C. Science 1998, 282, 95.
35) Maeda, Y.; Kimura, S. I.; Hiroshima, Y.; Kanda, M.; Lian, Y.;
(
(
(
(
(
̈
(
(
̈
ller, M.; Plocke, T.;
2
(
(
́
́
A. J. Nanosci.
(
(
(
(
̃
oz, J. D. M.; Díaz-Ortiz, A.;
quez, E. J. Am. Chem. Soc.
́
2
(
́
(
́
, V. S.;
(
(
(
Wakahara, T.; Akasaka, T.; Hasegawa, T.; Tokumoto, H.; Shimizu, T.;
Kataura, H.; Miyauchi, Y.; Maruyama, S.; Kobayashi, K.; Nagase, S. J.
Phys. Chem. B 2004, 108, 18395.
(
36) Wunderlich, D.; Hauke, F.; Hirsch, A. J. Mater. Chem. 2008, 18,
493.
37) Saito, T.; Ohmori, S.; Shukla, B.; Yumura, M.; Lijima, S. Appl.
Phys. Exp. 2009, 2, 095006.
38) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R.
E.; Weisman, R. B. Science 2002, 298, 2361.
39) Kukovecz, A.; Kramberger, C.; Georgakilas, V.; Prato, M.;
Kuzmany, H. Eur. Phys. J. B 2002, 28, 223.
40) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley,
R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379.
41) Chattopadhyay, J.; De Jesus Cortez, F.; Chakraborty, S.; Slater, N.
K. H.; Billups, W. E. Chem. Mater. 2006, 18, 5864.
42) Moniruzzaman, M.; Chattopadhyay, J.; Billups, W. E.; Winey, K. I.
Nano Lett. 2007, 7, 1178.
1
(
(
(
(
(
(
H
dx.doi.org/10.1021/ja308969p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX