White-light-emitting edge-functionalized graphene quantum dots

Article abstract of DOI:10.1002/anie.201311248

Graphene quantum dots (GQDs) have received considerable attention for their potential applications in the development of novel optoelectronic materials. In the generation of optoelectronic devices, the development of GQDs that are regulated in terms of their size and dimensions and are unoxidized at the sp² surfaces is desired. GQDs functionalized with bulky Frechet's dendritic wedges at the GQD periphery were synthesized. The single-layered, size-regulated structures of the dendronized GQDs were revealed by atomic force microscopy. The edge-functionalization of the GQDs led to white-light emission, which is an uncommon feature. In the dotlight: Graphene quantum dots (GQDs) functionalized with bulky Frechet's dendritic wedges at the GQD periphery were synthesized. The single-layered, size-regulated structures of the dendronized GQDs were revealed by atomic force microscopy. The edge-functionalization of the GQDs led to white-light emission, which is an uncommon feature.

Full text of DOI:10.1002/anie.201311248

Angewandte
Chemie
DOI: 10.1002/anie.201311248
Graphene Quantum Dots
White-Light-Emitting Edge-Functionalized Graphene Quantum
Dots**
Ryo Sekiya, Yuichiro Uemura, Hideki Murakami, and Takeharu Haino*
Abstract: Graphene quantum dots (GQDs) have received
considerable attention for their potential applications in the
development of novel optoelectronic materials. In the gener-
ation of optoelectronic devices, the development of GQDs that
are regulated in terms of their size and dimensions and are
unoxidized at the sp2 surfaces is desired. GQDs functionalized
with bulky Frꢀchetꢁs dendritic wedges at the GQD periphery
were synthesized. The single-layered, size-regulated structures
of the dendronized GQDs were revealed by atomic force
microscopy. The edge-functionalization of the GQDs led to
white-light emission, which is an uncommon feature.
and dimensions of the conjugated aromatic surface. The
oxidative-cutting method as reported by Hummers is typically
applied to obtain GQDs and it leads to oxidation at the sp2
surfaces as well as the periphery.[8] To recover the smooth sp2
surfaces, hydrazine reduction frequently follows. However,
regulating the size and dimensions of reduced GQDs remains
problematic, and furthermore, oxygen-containing function-
alities sometimes continue to be found on the sp2 surface.
These functionalities represent major drawbacks for the
production of electro- and photochemically uniform GQDs.
The development of GQDs that are regulated in terms of
their size and dimensions and are unoxidized at the sp2
surfaces is therefore desired.
Carbon is a ubiquitous and important element in nature and
forms millions of organic compounds in conjunction with
other elements. Its various allotropes include graphite,
diamond, fullerenes, and carbon nanotubes. Since the discov-
eries of fullerenes[1] and carbon nanotubes,[2] these two
allotropes have been extensively studied for electronic
applications.[3] However, obtaining chemically pure forms of
fullerenes and carbon nanotubes on a gram scale is not
economical, and production difficulties limit the practical
development of unique carbon-based materials from these
sources. By contrast, graphene,[4] a single planar sheet of sp2-
bonded carbon atoms, can be obtained from commercially
available graphite. The unique chemical and physical proper-
ties of graphene make it a promising material for versatile
applications in nanotechnology.[5] Among graphene deriva-
tives, graphene quantum dots (GQDs) have received consid-
erable attention because of their electronic and optical
properties, which originate from the quantum-size effect.[6]
Optical bio-imaging and photovoltaic devices can be potential
applications resulting from the electronic and optical proper-
ties of GQDs.[7] These properties are influenced by the size
The post-synthesis modification of GQDs is another
potential strategy to regulate their chemical, electronic, and
photophysical properties. Graphene surfaces and peripheries
can be modified by chemical reactions. However, GQDs are
mainly soluble in water and in polar solvents, thus imposing
a limit on the range of chemical modifications. The post-
synthesis modification of GQDs through mild chemical
reactions would be welcomed as a technique for tuning the
aforementioned properties. The development of a method
that fulfills these requirements for GQD synthesis is a sig-
nificant challenge in graphene chemistry. Herein, we report
the synthesis of size-regulated GQDs through oxidative-
cutting and post-synthesis functionalization with bulky
Frꢀchetꢁs dendritic wedges through Huisgen cycloaddition
(Scheme 1).[9] Detailed structural characterizations of the
periphery and the sp2 surfaces of the GQDs were also
performed through spectroscopic and microscopic studies.
The photochemical modulation of the GQDs was achieved
through the post-synthesis modification of the GQD periph-
eries. Finally, functionalizing the edges of GQDs in this
manner resulted in white-light emission, an uncommon effect.
Hummers method[8] is frequently used for graphene
synthesis but the potent reaction conditions do not favor
a smooth sp2 surface. Among the possible exfoliation
conditions of graphite that were carefully investigated,
oxidative cutting of commercially available graphite by
using a mixture of concentrated H2SO4 and HNO3 (3:1, v/v)
at 1208C for 24 h gave the highest quality water-soluble
GQDs (GQD-1) on a gram scale.[10] X-ray photoelectron
spectroscopy (XPS) was informative for gaining structural
insights into GQD-1. The XPS spectrum of GQD-1 was
deconvoluted into three components with binding energies of
[*] Prof. Dr. R. Sekiya, Y. Uemura, Prof. Dr. T. Haino
Department of Chemistry, Graduate School of Science
Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526 (Japan)
E-mail: haino@hiroshima-u.ac.jp
Prof. Dr. H. Murakami
Research Institute for Nanodevice and Biosystems
Hiroshima University
1-4-2 Kagamiyama, Higashi-Hiroshima, 739-8530 (Japan)
[**] This work was supported by Grants-in-Aid for Scientific Research (B)
(No. 24350060) and Challenging Exploratory Research (No.
23655105) of JSPS, as well as Grants-in-Aid for Scientific Research
on Innovative Areas, “Stimuli-responsive Chemical Species for
Creation of Functional Molecules”, and “New Polymeric Materials
Based on Element-Blocks” (Nos. 25109529, 25102532).
=
ꢀ
=
284.6, 286.4, and 288.5 eV, attributable to C C, C O, and C
O species, respectively. Graphene oxide, prepared by the
=
traditional oxidative cutting method, shows intense C C and
C O bands.[11] By contrast, GQD-1 predominantly displayed
ꢀ
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311248.
2
=
the C C band. These results suggested that the sp graphitic
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Scheme 1. Preparation of GQD-2 and the introduction of dendritic wedges by copper-catalyzed Huisgen cycloaddition. DMF=N,N-dimethylform-
amide, DMAP=4-dimethylaminopyridine.
carbon network is maintained. The 13C NMR spectrum of
GQD-1, measured in D2O, provided supporting evidence that
the sp2 graphitic carbon network for GQD-1 was not oxidized.
Only sp2 carbon and carbonyl carbon resonances were
observed, with no signals present in the aliphatic region. By
contrast, solid-state 13C MAS NMR spectra of graphene
oxides prepared by Hummers method indicated oxidized sp3
carbon atoms with resonances at d = 59.1 and 69.0 ppm.[12]
Accordingly, no sp3-hybridized carbon atoms bearing epoxide
or alcohol functionalities were formed on the surface of
GQD-1.
The edge-functionalization of GQD-1 with 4-
propynyloxybenzyl groups is outlined in Scheme 1. GQD-
1 was treated with oxalyl chloride under reflux conditions for
3 days. The reaction mixture was then subjected to an
imidation reaction with benzylamine 4 at 1008C for 3 days.
Subsequent chromatographic purification of the mixture on
silica gel afforded GQD-2 as a brown powder. The introduc-
1
tion of the benzylamino group was confirmed by H NMR
and IR spectroscopy (Figure 1a). The IR absorption spec-
trum[13] of GQD-1 showed broad absorption bands corre-
ꢀ
sponding to hydrogen-bonded O H groups (3700–
2300 cmꢀ1). These bands were absent in the GQD-2 spectrum,
Figure 1. a) IR absorption spectra of GQD-1, GQD-2, M1, M2, and M3.
b) Characteristic edge structures of graphene.
and new bands were observed at 3286 and 2119 cmꢀ1,
ꢁ ꢀ
ꢀ ꢁ ꢀ
attributable to C H and C C absorptions, respectively,
thus suggesting that most of the carboxylic acid groups at the
GQD periphery had reacted with benzylamine 4.
3214, 1695, and 1643 cmꢀ1, which are attributable to N H,
ꢀ
ꢁ ꢀ
=
Armchair and zigzag arrangements are typical GQD edge
structures (Figure 1b). To determine the edge structures of
GQD-2, model compounds M1, M2, and M3 were synthe-
sized. The five-membered cyclic imide M1 is formed only with
the armchair arrangement in the region corresponding to the
peripheral carbon atoms of GQD-2, whereas the zigzag
arrangement is observed with the six-membered cyclic imide
M2. Carboxylic acid M3 is a possible product when the
imidation reaction is not completed. Their IR spectra are
presented in Figure 1a. M3 generated sharp bands at 3360,
C H, and two C O groups, respectively. In the IR spectrum
ꢀ
of GQD-2, the N H absorption band was absent, while the
ꢁ ꢀ
=
C H and C O absorptions were shifted. Therefore, the
presence of a 2-carbamoylbenzoic acid structure was not
detected. The two possible cyclic imide structures represented
=
by M1 and M2 displayed characteristic differences in their C
O absorptions. The weak and strong carbonyl bands of M1 at
1771 and 1717 cmꢀ1 are characteristic of five-membered cyclic
imides, whereas those of M2 appeared at 1695 and
1656 cmꢀ1.[14] GQD-2 exhibited weak and strong carbonyl
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bands at 1765 and 1712 cmꢀ1, corresponding to those of M1,
thus indicating that the periphery of GQD-2 was principally
functionalized with the phthalimide structure. Therefore, the
peripheral carbon atoms of GQD-2 most likely adopt the
armchair arrangement.
GQD-2 was soluble in the common organic solvents ethyl
acetate, acetone, 1,2-dichloroethane, dichloromethane, tetra-
hydrofuran, and acetonitrile. The evident lipophilicity of
GQD-2 is a result of the edge-functionalization of GQD-
1 with 4-propynyloxybenzyl groups. Solutions of GQD-2 in
these solvents were stable; no precipitation was observed
when the solutions were left standing for more than a week. In
general, graphene sheets often precipitate through self-
aggregation. The high solution stability of GQD-2 suggests
that the 4-propynyloxybenzyl moieties likely provide steric
protection for the sp2 carbon surface, thereby preventing self-
aggregation driven by p–p stacking interactions.
Graphene quantum dots have aromatic surfaces that are
large, flat, and flexible. Under certain conditions, these large
surfaces are bent, folded, and twisted. Such structural
distortions are expected to modulate the electronic properties
of GQDs. The introduction of bulky substituents at the
periphery of GQDs should generate a steric requirement for
the expansion of the aromatic surface so that effective
conjugation is extended. Therefore, we introduced dendritic
wedges at the periphery of GQD-2. Dendrons 5a–c, which
possess an azide group, were incorporated onto GQD-2
through copper-catalyzed Huisgen cycloaddition (Scheme
1).[9a,c,15] The reaction of GQD-2 with 5a–c was monitored by
1H NMR and IR spectrosopy. Figure 2 displays partial IR
3b, and 3c, thus suggesting that the dendritic wedges were
successfully incorporated at the GQD peripheries. The
introduction of the dendritic wedges greatly improved the
solubility of GQD-3a–c, which were even more soluble in
ethyl acetate, acetone, 1,2-dichloroethane, dichloromethane,
tetrahydrofuran, benzene, and chlorobenzene.
Atomic force microscopy (AFM) measurements of the
GQDs provided detailed information about their sizes and
dimensions. The statistical size distribution of (20 ꢂ 1) nm for
GQD-2 indicates that oxidative cutting was effective in
producing GQDs with a uniform size. It was estimated from
TG/TGA analysis that approximately 60–70 4-propynyloxy-
benzylamine groups had been introduced at the GQD
periphery. The incorporated dendritic wedges were directly
visualized by AFM measurements. The AFM images of
GQD-2 show trapezoidal morphologies with a height of
0.40 nm, approximately the size of an aromatic ring edge,
although many agglomerated morphologies were observed
(Figure 3a,c,d). The GQDs featuring dendritic wedges gave
rise to unique morphologies; GQD-3b displayed a trapezoidal
shape surrounded by higher walls (Figure 3b,e,f). The cross
section revealed a center height of 0.48 nm, which is
consistent with the thickness of a single-layered GQD, as
well as two peaks heights of 0.64 nm, which confirmed that
the dendritic wedges are successfully incorporated around the
edges of the GQDs. GQD-3b was observed to be chiefly
single-layered with a uniform diameter of (21 ꢂ 2) nm, thus
Figure 2. IR absorption spectra of GQD-3a–c.
absorption spectra for GQD-3a–c. Complete conversion of
the triple bonds to triazole rings was evidenced by the
ꢁ ꢀ
disappearance of the C H absorption band. GQD-2
smoothly reacted with 5a over 48 h to give GQD-3a, whereas
the reactions with 5b and 5c required more than 120 h, thus
implying that the dendrons might generate severe steric
congestion at the periphery of GQD-2 to reduce the reactivity
of 5b and 5c. The incorporation of the dendritic wedges was
Figure 3. AFM images (topography) of a) GQD-2 (0.9 mmꢀ0.9 mm)
and b) GQD-3b (0.9 mmꢀ0.9 mm) on mica disks. c,f) Height profiles
of the cross sections of the white lines in (a) and (b), and magnified
images of (d) GQD-2 and (f) GQD-3b. g) Calculated structure of
GDQ-3c.
ꢀ
confirmed by IR spectroscopy. Intense aromatic C C absorp-
tions of GQD-3a–c were located at approximately 1600 cmꢀ1.
ꢀ
The ratios of the aromatic C C absorptions in proportion to
=
the C O absorptions increased along the series of GQD-3a,
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strongly suggesting that the introduced dendritic wedges
hamper its agglomeration in solution.
546, 546, and 539 nm, respectively. The red-shifts of the
emission maxima for GQD-3a–c were observed even upon
excitation at wavelengths ranging from 360 to 480 nm. The
absolute photoluminescence quantum yields of GQD-2 and
GQD-3a–c in dichloromethane with excitation wavelength at
360 nm were approximately 1–2% and did not change at
other excitation wavelengths. These observations imply that
the excited states of GQD-3a–c are more stable than those of
GQD-2. Although the precise mechanism of excited-state
stabilization through the incorporation of dendritic wedges
remains uncertain, the peripheral dendritic wedges of GQD-
3a–c create significant steric interactions that restrict stack-
ing, twisting, bending, and folding of the GQD surface. As
a result, the effective conjugation of the aromatic surface of
GQD-3a–c could be extended through steric effects and may
result in the observed excited state stabilization.
The emission behaviors correspond to the optical images.
Commission Internationale de l’Eclairage (CIE) 1931 chro-
maticity diagrams were employed to characterize the emis-
sion profiles of GQD-2 and GQD-3a–c. The CIE 1931
chromaticity coordinates for GQD-2 and GQD-3a–c when
excited at 360 nm and for GQD-2 when excited at 380 nm
were calculated, and a series of digital photographs of the
corresponding samples in dichloromethane solutions under
360 and 380 nm light are presented (Figure 4e), along with
the chromaticity coordinates in the CIE 1931 diagram (Fig-
ure 4 f). GQD-3a–c exhibited white-light emission, an
uncommon feature, upon excitation at 360 nm, while GQD-
2 displayed bluish-white emission; however, excitation at
380 nm gave rise to white emission. The CIE 1931 chroma-
ticity coordinates changed with an increase in the sizes of the
dendritic wedges [e.g., GQD-2 (x = 0.223, y = 0.214)!GQD-
3c (x = 0.324, y = 0.383)], thus indicating that the photo-
luminescence behavior of GQD-2 is tunable through intro-
ducing organic functional groups at the periphery of GQD-2.
In conclusion, we synthesized white-light-emitting edge-
functionalized graphene quantum dots GQD-2 and GQD-
3a–c through the reaction of edge-oxidized GQDs (GQD-1)
with 4-propynyloxybenzylamine (4). 13C NMR spectroscopy
revealed that most regions of the sp2 surfaces of GQD-1 were
not oxidized. The lipophilicity of GQD-2 permits a range of
chemical modifications in organic solvents for the develop-
ment of optoelectronically functionalized carbon-based mate-
rials. Indeed, the optical properties of GQD-2 were tuned by
functionalizing the edges with dendritic wedges, with the
resulting GQDs demonstrating white-light emission. Because
these lipophilic GQDs are chemically accessible, they hold
promise for application in carbon-based materials that may
become a source of unique optoelectronic devices.
To visualize the plausible structure of edge-functionalized
GQDs, molecular mechanics calculations were carried out
with MacroModel V.9.1 using the OPLS2005 force field.[16]
The resultant structure of a GQD with third generation
dendron wedges is illustrated in Figure 3g. The dendritic
wedges provide significant steric congestion that interferes
with self-aggregation through p–p stacking interactions.
Bending, twisting, and folding of the GQD surface are most
likely prevented, thus implying that effective aromatic con-
jugation can be extended as a result of steric effects. The
optoelectronic properties of the GQDs should be responsive
to the structures of the wedges at the periphery.
The UV/Vis absorption bands of GQD-2 and GQD-3a–c
in dichloromethane appeared within the visible region (Fig-
ure 4a–d). The incorporation of dendritic wedges onto GQDs
likely has a limited influence on the ground-state electronic
structures, but the emission properties of the GQDs are quite
noteworthy. Photoluminescence studies with GQD-2 and
GQD-3a–c revealed that the emission maxima depended on
the excitation wavelengths.[17] Upon excitation at 360 nm,
GQD-2 exhibited an emission maximum at 411 nm, while the
emission maxima of GQD-3a, 3b, and 3c were red-shifted to
Received: December 28, 2013
Published online: && &&, &&&&
Keywords: graphene · nanostructures · nanotechnology ·
quantum dots · white-light emission
Figure 4. UV/Vis absorption and fluorescence spectra of a) GQD-2,
b) GQD-3a, c) GQD-3b, and d) GQD-3c in CH2Cl2. e) Optical images
of GQD-2 (360 and 380 nm), GQD-3a (360 nm), GQD-3b (360 nm),
and GQD-3c (360 nm). f) The CIE 1931 chromaticity diagram of GQD-
2 at 360 nm (x=0.223, y=0.214), GQD-2 at 380 nm (x=0.240,
y=0.265), GQD-3a (x=0.336, y=0.394), GQD-3b (x=0.340,
y=0.392), and GQD-3c (x=0.324, y=0.383).
.
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Communications
Graphene Quantum Dots
In the dotlight: Graphene quantum dots
(GQDs) functionalized with bulky Frꢁ-
chet’s dendritic wedges at the GQD
periphery were synthesized. The single-
layered, size-regulated structures of the
dendronized GQDs were revealed by
atomic force microscopy. The edge-func-
tionalization of the GQDs led to white-
light emission, which is an uncommon
feature.
R. Sekiya, Y. Uemura, H. Murakami,
T. Haino*
&&&& — &&&&
White-Light-Emitting Edge-
Functionalized Graphene Quantum Dots
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!