Thermodynamic control of quantum defects on single-walled carbon nanotubes

Article abstract of DOI:10.1039/c9cc05623h

Single-walled carbon nanotubes with designed quantum defects are prepared and characterized. The photoluminescence (PL) of the nanotubes can be modified by thermal treatment from 1215-1224 to 1249-1268 nm. Theoretical calculations suggest that the change

Full text of DOI:10.1039/c9cc05623h

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Thermodynamic control of quantum defects on
single-walled carbon nanotubes†
Cite this: Chem. Commun., 2019,
55, 13757
Yutaka Maeda, *a Hiyori Murakoshi,a Haruto Tambo,a Pei Zhao,
b
a
c
d
Kiyonori Kuroda,a Michio Yamada,
Xiang Zhao,
Shigeru Nagase
and
Received 20th July 2019,
Accepted 21st October 2019
b
Masahiro Ehara
*
DOI: 10.1039/c9cc05623h
rsc.li/chemcomm
Single-walled carbon nanotubes with designed quantum defects stable isomer being an ortho L
adduct among their possible
À33
are prepared and characterized. The photoluminescence (PL) of the 1,2- and 1,4-addition isomers.20 Conversely, the reaction of (6,5)
nanotubes can be modified by thermal treatment from 1215–1224 SWNTs with 1,3-dibromopropane presented a new PL peak at 1215
to 1249–1268 nm. Theoretical calculations suggest that the change nm.21 The theoretical calculation of these model compounds,
in the PL spectra by thermal treatment can be explained by SWNT-(C3H6), showed that an ortho LÀ33 adduct is the most stable
isomerization from kinetic to thermodynamic products.
isomer among the possible 1,2-addition isomers. Referring to the
assignment of hydorophenylated SWNTs reported by Htoon and
Single-walled carbon nanotubes (SWNTs) have attracted attention co-workers,20 functionalized SWNTs with dibromopropane may
due to their remarkable mechanical stability and optoelectronic not be the thermodynamic product.21 Positional isomerization
properties.1,2 The ability to tailor the electronic properties of of fullerene derivatives has been studied to control their optical
SWNTs based on their diameter and helicity makes them attractive and electronic properties by changing the p electron system.22–24
materials as building blocks of electronic and optical nanodevices. Encouraged by these reports on isomerization of fullerene deriva-
Their suitability for an application depends on their band gap tives, the thermodynamic control of the PL wavelength as well as
along with their absorption and photoluminescence (PL) proper- the intensity of quantum defects on SWNTs is studied and the
ties which are observed in the near-infrared (NIR) region.3 As NIR results are presented in this paper.
photoluminescent materials, SWNTs have been used in bio-
A series of functionalized SWNTs (3a–3e and 4a–4c), depicted in
imaging, telecommunications, and sensor design applications.4–8 Scheme 1, were designed and prepared by the reductive alkylation
It has been reported that functionalization of SWNTs can control of SWNTs with dibromopropane (1a–1e) and dibromobutane
their NIR PL properties, resulting in the emergence of red-shifted derivatives (2a–2c), respectively. It has been reported that cyclo-
PL peaks with high quantum yields.9–18 Recent theoretical studies addition products of fullerenes are preferred for isomerization via a
on functionalized SWNTs using density functional theory (DFT) bond cleavage and recombination mechanism between fullerenes
revealed that the PL wavelength of functionalized SWNTs strongly and addenda.25 Recently, it has been reported that 1a and 2a
depends on the addition patterns of addenda.19,20 For example, are suitable reagents to afford cycloaddition products of SWNTs21
the reaction of (6,5) SWNTs with phenyl diazonium salt afforded a similar to functionalization of fullerenes.26 In addition, the bulki-
new PL peak at 1119 nm. The theoretical calculation of these ness of addenda and the stability of the fragments after elimination
model compounds (hydrophenylated SWNTs) showed that the strongly influenced the elimination reactions of functionalized
band gap energies depend on the addition patterns with the most SWNTs.27 Thus, cyclic addenda having methyl or phenyl groups
a Department of Chemistry, Tokyo Gakugei University, Tokyo 184-8501, Japan.
E-mail: ymaeda@u-gakugei.ac.jp
b Research Center for Computational Science, Institute for Molecular Science,
Okazaki, 444-8585, Japan. E-mail: ehara@ims.ac.jp
c Institute for Chemical Physics & Department of Chemistry, School of Science,
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong
University, Xi’an 710049, China
d Fukui Institute for Fundamental Chemistry (FIFC), Kyoto University, Sakyou-ku,
Kyoto 606-8103, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Reductive alkylation of SWNTs.
c9cc05623h
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Fig. 1 (a) Absorption, (b) Raman (561 nm excitation), (c) PL (567 nm excitation),
and (d) excitation spectra of SWNTs (the dotted line) and functionalized SWNTs
(the solid line).
on SWNTs were synthesized for isomerization experiments. Fig. 1
shows the absorption, Raman, PL, and excitation spectra of 3 and 4.
The pristine (6,5) SWNTs show a PL peak at 980 nm at an excitation
wavelength of 567 nm, which is assigned to the E11 PL of (6,5)
SWNTs. After the functionalization, new red-shifted PL peaks were
observed as dominant peaks (3a: 1215 nm,21 3b: 1215 nm, 3c:
Fig. 2 PL spectra (567 nm excitation) of the functionalized SWNTs (3a–3c
and 4a–4c) before and after thermal treatment.
1216 nm, 3d: 1224 nm, 3e: 1219 nm, 4a: 1230 nm,21 4b: 1236 nm, after thermal treatment can be observed in Fig. 2 and Fig. S12–S16
and 4c: 1240 nm). The excitation spectra of these red-shifted (ESI†). The red-shifted PL peak intensities of 3a–3c and 4a–4c
PL peaks showed characteristic peaks at approximately 567 and increased with a decrease in the functionalization degree upon
980 nm, which is in good agreement with the characteristic absorp- thermal treatment. Minor peak shifts in the red-shifted PL were
tion peaks of (6,5) SWNTs, suggesting that these PL peaks originated observed and accompanied by an increase in the peak intensity after
from functionalized (6,5) SWNTs. As described above, it has been thermal treatment (3a (400 1C): 1201 nm, 3b (300 1C): 1207 nm, 3c
proposed that the PL wavelength of functionalized SWNTs depends (350 1C): 1212 nm, 3d (200 1C): 1222 nm, 3e (200 1C): 1219 nm, 4a
on the addition patterns of addenda;19,20 therefore, the selective (350 1C): 1227 nm, 4b (200 1C): 1237 nm, and 4c (250 1C): 1231 nm).
emergence of a new red-shifted PL peak in functionalized (6,5) Similar blue shifts of PL peaks were also observed after thermal
SWNTs indicates the high regioselectivity of (6,5) SWNTs in the treatment of acyclic alkylated SWNTs, reported previously.27 Inter-
reaction. In addition to the difference in the alkyl chain length estingly, new PL peaks were observed from 3a–3d at 1249–1268 nm
between 3 and 4, the substituents (R1, R2, and R3) in 3d, 3e, 4b, and after thermal treatment (3a (500 1C): 1262 nm, 3b (300 1C): 1249 nm,
4c affected the red-shifted PL wavelength. The difference in the PL 3c (350 1C): 1268 nm, and 3d (300 1C): 1242 nm (200 1C)). Notably,
wavelength between 3a and 4a can be caused by local strain induced highly selective emergence of the PL peak at 1268 nm was achieved
by the addenda; this conclusion is based on the results of theoretical in 3c by thermal treatment at 300 1C for 6 h. The good agreement
calculations that have been reported previously.21
of the excitation spectrum (1268 nm) of 3c (300 1C, 6 h) with the
Thermal treatment of the functionalized SWNTs was conducted absorption spectrum of (6,5) SWNTs suggests that the PL originates
at a heating rate of 10 1C minÀ1 under an atmosphere of nitrogen. from the functionalized (6,5) SWNTs (Fig. S17, ESI†). After thermal
The change in the functionalization degree of 3 and 4 was estimated treatment of 3e, 4a, 4b, and 4c, new PL peaks at longer wavelength
from the relative E11 absorption peak intensities normalized at the are not observed. The emergence of new PL peaks at longer
local minimum (B775 nm) and the ratios of the D band toward the wavelengths in 3c and 3d at a relatively low temperature suggests
G band (D/G) in the Raman spectra (Fig. S1–S10, ESI†). An increase that the thermal control of the PL wavelength involves a bond
in the relative E11 absorption peak intensities and a decrease in the cleavage and a recombination process causing isomerization of
D/G ratio were observed as the temperature of thermal treatment the addenda. In the case of 3e, it is believed that the elimination
increased (Fig. S11, ESI†). Results showed that 3a and 4a were more reaction proceeded preferentially as the thermal stability of 3e is
stable than 3b–3e and 4b–4c, respectively. These results indicate too low. In radical cyclization reactions of alkenyl radicals, it is
that the methyl and phenyl groups in the cyclic addenda promote known that the six-membered ring formation yield is lower than
the elimination of the addenda. The PL spectra of 3a–e and 4a–c the five-membered ring formation yield.28 This might be one of
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the reasons for the elimination reactions of 4a to 4c to proceed
preferentially over isomerization. In a previous report, the emergence
of the corresponding red-shifted PL peak had not been observed
from acyclic alkylated SWNTs after thermal treatment.13,27
Determining the local structures of functionalized SWNTs is
necessary to reveal the origin of PL changes. However, as it is
difficult to determine the local structure of functionalized SWNTs
experimentally, DFT and time-dependent DFT calculations of model
compounds are performed (Fig. 3 and Fig. S20, ESI†).29–32 As
mentioned above, the possibility of cycloaddition of SWNTs using
3a and 4a has been reported.21 Therefore, we considered three
model structures of the corresponding 1,2-cycloadducts. These
isomers are distinguished by their abbreviations reported by Htoon
and co-workers (Fig. 4).20 It was reported that the calculated
emission energies showed a similar tendency to the calculated
absorption energies.33 Therefore, instead of emission energies,
absorption energies of the model compounds were calculated to
decrease the computational cost of geometry optimization. In the
present study, it is confirmed that absorption and emission trends
of regioisomers of SWNT-(C3H6) calculated by CAM-B3LYP/STO-3G
were the same (Table S2, ESI†). The relative energy, absorption
wavelength, and C–C bond length between SWNTs and the sub-
stituents of the model compounds for 3a–3c and 4a–4c are shown in
Fig. 5, Tables S3 and S4 (ESI†), respectively. Relative energies were
also examined using the functionals with dispersion corrections;
the effects of which are minor in the present system as shown in
Table S8 (ESI†). The increase in the C–C bond length with an
increase in the number of methyl substituents indicates the
presence of steric repulsion between the methyl group and SWNTs.
This result is consistent with the experimental results showing
that the presence of methyl groups facilitates the elimination of
addenda. In comparisons with model compounds with the same
Fig. 5 Relative energies using DFT with B3LYP/6-31G* and the calculated
absorption wavelength shift from SWNTs of functionalized SWNTs using
TD-DFT with B3LYP/3-21G.
addition patterns, the calculated absorption wavelength showed
good agreement with the experimental PL wavelengths of 3a–3c
and 4a–4c. The wavelengths of the 5-membered cyclic adducts are
shorter than those of the 6-membered cyclic adducts in the same
addition patterns, indicating that the calculated absorption wave-
length is strongly dominated by the alkyl chain length and its
addition patterns (ortho LÀ33 : 880–893 nm, ortho L87 :920–938 nm,
and ortho L27 : 1099–1130 nm).20,33 In addition, the results show
that the substituted methyl groups influence the absorption wave-
length of these cyclic addition products. The ortho LÀ33 isomers are
the most stable regardless of the addenda, suggesting that the ortho
L
À33
isomers are not the potential compounds for the products
before and after thermal treatment; this is because they have the
highest relative stability and the shortest absorption wavelength.
In order to improve our understanding of candidate isomers
for functionalized SWNTs before thermal treatment, the spin
density of the n-butylated (6,5) SWNT (nBu-SWNT) radical as a
corresponding intermediate in the reductive alkylation is cal-
culated. Results show that the carbon atom at the ortho L87
position has the highest spin density (Fig. 4, Fig. S23 and
Table S5, ESI†). Therefore, ortho L87 isomers are potential
compounds for kinetic products in the cyclization reaction.
ortho L27 isomers have the potential as thermodynamic
products after thermal treatment for 3; this is because the ortho
L27 isomers are more stable than the ortho L87 isomers, except
for SWNT-(cis-C3H6Me2), and possess longer absorption wave-
lengths than the ortho L87 isomers. Htoon et al. reported the
correction of the emission energy of functionalized SWNTs at an
infinite length E11*(N) by DFT calculations using finite-length
tube models.34 Thus, corrections of the emission energies of
SWNT-(C3H6) and SWNT-(cis-C3H4Me2) were conducted using
the 3-unit model and using the following equation, which was
proposed by Htoon and co-workers:34
Fig. 3 Partial view of the optimized structures of functionalized (6,5)
SWNTs by B3LYP/6-31G*.
E11*(N) = À0.59(E11 À E11*) + 1.25
where E11 and E11* represent the emission energies of the
pristine SWNT and each functionalized SWNT, respectively.
The corrected emission energies of functionalized SWNTs
originating from ortho L87 and ortho L27 (Table 1) showed good
agreement with the experimental values (3a: 1.03 eV, 3c: 1.02 eV,
3a(500 1C): 0.983 eV, and 3c(300 1C, 6 h): 0.978 eV), strongly
Fig. 4 Addition sites of functionalized SWNTs and the spin density of the
nBu-SWNT radical by UB3LYP/3-21G using 3-unit cells. The optimized
structure is shown in Fig. S23 (ESI†).
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Table 1 Calculated emission energies for SWNT-(C3H6) and SWNT-
(trans-C3H4Me2) after the correction
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This work was supported by JSPS KAKENHI Grant-in-Aid for
Scientific Research (B) (17H02735, 16H06511, and 16H04104) and
by the National Natural Science Foundation of China (Grant
21573172 and 21773181). The authors also acknowledge financial
support from the PhD short-term academic visiting program and
the Nanotechnology Platform Program (Molecule and Materials
Synthesis: 20191102) of the Ministry of Education, Culture, Sports,
Science, and Technology of Japan.
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There are no conflicts to declare.
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13760 | Chem. Commun., 2019, 55, 13757--13760
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