Carbon quantum dots with photoenhanced hydrogen-bond catalytic activity in aldol condensations

Article abstract of DOI:10.1021/cs401118x

Carbon quantum dots (CQDs) were synthesized by an electrochemical etching method. The CQDs are well-dispersed with uniform size about 5 nm. FT-IR spectra suggest the presence of many hydroxyl groups on the surface of CQDs. Here, CQDs with diameter approximately 5 nm, directly used as effective heterogeneous nanocatalysts for H-bond catalysis in aldol condensations, show excellent photoenhanced catalytic ability (89% yields when 4-cyanobenzaldehyde is used). It demonstrated that aldol condensation between acetone and aromatic aldehydes resulted in higher yields with visible light irradiation than in the dark, confirming visible light is necessary for good conversion. The H-bond catalytic activities of CQDs can be significantly enhanced with visible light irradiation. The high catalytic activities of CQDs are due to highly efficient electron-accepting capabilities. Repeated catalytic experiments suggest that the CQD catalyst can be easily recycled as a heterogeneous catalyst with a long catalyst life.

Full text of DOI:10.1021/cs401118x

Research Article
pubs.acs.org/acscatalysis
Carbon Quantum Dots with Photoenhanced Hydrogen-Bond
Catalytic Activity in Aldol Condensations
Yuzhi Han, Hui Huang, Hengchao Zhang, Yang Liu,* Xiao Han, Ruihua Liu, Haitao Li,
and Zhenhui Kang*
Institute of Functional Nano & Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and
Technology, Soochow University, Suzhou, Jiangsu 215123, China
S
* Supporting Information
ABSTRACT: Carbon quantum dots (CQDs) were synthe-
sized by an electrochemical etching method. The CQDs are
well-dispersed with uniform size about 5 nm. FT-IR spectra
suggest the presence of many hydroxyl groups on the surface of
CQDs. Here, CQDs with diameter approximately 5 nm,
directly used as effective heterogeneous nanocatalysts for H-
bond catalysis in aldol condensations, show excellent photo-
enhanced catalytic ability (89% yields when 4-cyanobenzalde-
hyde is used). It demonstrated that aldol condensation
between acetone and aromatic aldehydes resulted in higher
yields with visible light irradiation than in the dark, confirming
visible light is necessary for good conversion. The H-bond
catalytic activities of CQDs can be significantly enhanced with
visible light irradiation. The high catalytic activities of CQDs are due to highly efficient electron-accepting capabilities. Repeated
catalytic experiments suggest that the CQD catalyst can be easily recycled as a heterogeneous catalyst with a long catalyst life.
KEYWORDS: carbon quantum dots, hydrogen bond catalysis, heterogeneous catalysis, aldol condensation, electron acceptor/donor
ydrogen bonding (H-bonding) plays a crucial role in
stable and biocompatible and have strong and tunable
Henzyme-catalyzed reactions by orienting the substrate photoluminescence (PL). In addition to downconverted PL,
molecules and lowering barriers to reactions.1 Recently, this
CQDs show excellent up-converted PL (UCPL), which enable
CQDs to efficiently utilize the full spectrum of sunlight.27 Due
to the novel PL and excellent photoinduced electron transfer/
kind of noncovalent interaction (H-bonding) was introduced
into many organic chemical reactions,2−9 including the Diels−
reservoir properties of CQDs, composites of CQDs and
semiconductors have been successfully prepared. CQD/semi-
conductor composites have enhanced photocatalytic activities
and are stable in visible light. Examples include CQDs/TiO2,
CQDs/SiO2, CQDs/Fe2O3, CQDs/Cu2O, and CQDs/
Ag3PO4.26−30 CQDs are considered to be good H-bonding
catalysts because of their rich photochemical properties and
functional carboxylic and hydroxyl groups.
In the current study, we used CQDs as heterogeneous
nanocatalysts for H-bond catalysis. CQDs showed good
photoenhanced catalytic abilities (89% yield when 4-cyano-
benzaldehyde is used) in the aldol condensation. A series of
catalytic experiments confirmed that the catalytic activity of
CQDs can be effectively enhanced by visible light, which may
be attributed to their photoinduced electron-accepting proper-
ties.
Alder, Aldol reaction, aza-Henry, Mannich, Michael, Morita−
Baylis−Hillman, and Strecker reactions.10−17 As a typical
organocatalysis, hydrogen bond (H-bond) catalysis uses H-
bonding interactions to accelerate organic reactions and
stabilize anionic intermediates and transition states.18 Several
organic molecules with hydroxyl groups can be used as
homogeneous H-bond catalysts.19−21 For example, Chiral
BINOL-derived Brønsted acids catalyze the enantioselective,
asymmetric Morita−Baylis−Hillman (MBH) reaction of cyclo-
hexenone with aldehydes.22 However, these catalytic reactions
produce low yields, and it is difficult to isolate the catalyst from
the reaction system. Inorganic solid nanoparticles with
abundant surface hydroxyl groups have been developed as
alternative heterogeneous catalysts for H-bonding reac-
tions.23,24 For example, aldol reactions could be catalyzed by
hydroxyl groups on the Fe(OH)3 shell of Fe3O4@Fe(OH)3
core−shell microspheres.25 However, compared with a
bioenzyme system, this artificial catalyst system faces the
challenges of low efficiency and poor controllability.
CQDs were synthesized through an electrochemical etching
method (see the experimental section in the Supporting
Carbon quantum dots (CQDs) constitute a new class of
zero-dimensional nanostructures with sizes below 10 nm.26 As a
novel class of recently discovered nanocarbons, CQDs are
Received: August 14, 2013
Revised: January 21, 2014
© XXXX American Chemical Society
781
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787

ACS Catalysis
Research Article
Information (SI)).28 A typical transmission electron micros-
copy (TEM) image of the as-synthesized CQDs is shown in
Figure 1a. The CQDs were well-dispersed and uniform with
excitation wavelengths in the same sample. Figure S3 shows the
PL spectra of the as-synthesized CQDs with different sizes: (a)
<4 nm; (b) 5 nm; and (c) >10 nm The excitation wavelength
of the black, red, blue, green, magenta, and dark yellow PL lines
are 240, 300, 360, 420, 500, and 580 nm, respectively. The PL
spectra show that the as-synthesized CQDs with different sizes
exhibit the different distribution of emission colors, and that
smaller size (<4 nm) leads to increasing amount of CQDs
emitting at shorter wavelengths.
The CQDs (5 nm) without surface hydroxyl groups were
synthesized for comparison (the detailed experiments are in
SI). Figure S4a shows the typical TEM image of hydroxyl-
group-free CQDs. The CQDs are well-dispersed and uniform
with diameter of about 5 nm. The inset in Figure S4a is the
EDS analysis of hydroxyl-group-free CQDs. No N signal was
detected, indicating that the amines do not exist in reaction
system. Elemental analyses of C, N, O, and H were further
performed with an Elementar Vario EL III elemental analyzer
(shown in Table S2). The results show that only carbon,
hydrogen, and oxygen elements were observed with no
additional elements on the CQDs. These results indicate that
the catalytic active sites are the oxygen functional groups on the
surface of CQDs. The size distribution (Figure S4b) was
calculated by measuring approximately 200 particles, revealing
that the CQDs have an average size of 5.00 0.58 nm. The
typical CQD UV−vis absorption and PL spectra were shown in
Figure S4c. The peak at 240 nm represented the typical
absorption of an aromatic π system. Furthermore, the CQDs
revealed broad absorption of sunlight in the ultraviolet and
visible region. The PL spectrum shows that the maximum
emission wavelength of the hydroxyl-group-free CQDs aqueous
solution is 504 nm. FT-IR spectrum was recorded to provide
further structural insights about the hydroxyl-group-free CQDs
(Figure S4d). Compared with Figure 1b, which shows the
presence of many hydroxyl groups on the water-soluble CQD
surface, this spectrum indicated a narrow and weak peak
centered at 3443 cm−1, suggesting a small amount of hydroxyl
groups on the surface of hydroxyl-group-free CQDs. This can
explain why hydroxyl-group-free CQDs cannot catalyze the
reaction but water-soluble CQDs can. The peaks at about 1635
cm−1 and in the range of 1000−1400 cm−1 indicated the
presence of carbonyl (CO) and C−O groups, respectively.
To quantify the hydroxyls and carboxylic acid on the surface
of the water-soluble CQDs (about 5 nm), base titration was
performed to measure the total quantity of both hydroxyls and
carboxylic acid following a standard method.31a Subsequently,
the relative content of hydroxyls versus carboxylic acid of the
water-soluble CQDs (about 10:1) was determined by
conductometric titration (for experimental details, see SI and
Figure S5).31b The detailed quantification data were shown in
Table S1.
Figure 1. (a) TEM image of CQDs (inset is HRTEM image), (b)
particle-size histogram, (c) UV−vis absorption spectrum, (d) Raman
(red line) and FT-IR (black line) spectra of 5 nm CQDs.
diameters of about 5 nm. A high-resolution TEM (HRTEM)
image of the CQDs (Figure 1a inset) shows that the lattice
spacing was around 0.212 nm, which is consistent with the
(100) facet of the graphite. The size distribution (Figure 1b)
was calculated by measuring approximately 200 particles. The
CQDs had an average size of 5.40 0.72 nm.
A typical CQD ultraviolet and visible (UV−vis) absorption
spectrum was shown in Figure 1c. The peak at 240 nm
represented the typical absorption of an aromatic π system,
which is similar to that of polycyclic aromatic hydrocarbons.
CQDs showed broad absorption of sunlight in the ultraviolet
and visible region, which suggested that CQDs may have high
photocatalytic activities in visible light. Raman and Fourier
transform infrared (FT-IR) spectra were recorded to provide
further structural insights about the as-synthesized CQDs. The
Raman spectrum of CQDs showed two characteristic peaks at
about 1330 and 1595 cm−1 (Figure 1d, red line), which
corresponded to the D-band and G-band of carbon,
respectively. The peak at 1330 cm−1 (D-band) was associated
with the vibrations of sp3-banded carbon atoms in disordered
graphite. The peak at 1595 cm−1 (G-band) corresponded to the
vibrations of sp2 carbon atoms in a two-dimensional (2D)
hexagonal lattice. The FT-IR spectrum (Figure 1d, black line)
showed a broad peak centered at 3445 cm−1, which suggested
the presence of many hydroxyl groups on the CQD surface.
The peak at about 2960 cm−1 corresponded to the C−H
stretch of polycyclic aromatic hydrocarbons. The peaks at about
1637 cm−1 and in the range of 1000−1400 cm−1 indicated the
presence of carbonyl (CO) and C−O groups, respectively.
The TEM images, HRTEM images, particle-size histograms,
UV−vis absorption, Raman (red line), and FT-IR (black line)
spectra of CQDs (<4 nm and >10 nm) were shown in Figures
S1 and S2, respectively. The energy dispersive X-ray spectros-
copy (EDS) analysis of four kinds of CQDs were shown in
Figure S6. Figure S7 provided the pictures of solid CQDs (<4
nm; 5 nm; >10 nm and hydroxyl-group-free CQDs), indicating
that these CQDs can well-dissolved in solvent.
The photoenhanced catalytic activities of CQDs (about 5
nm) were confirmed in the Aldol reaction, which is an
important strategy for C−C bond formation in organic
synthesis.31c Song and co-workers showed that the Aldol
reaction between aromatic aldehydes and ketone in solvent
(acetone, ethanol, THF, CHCl3) can proceed and the aldol
condensation products can be gained directly.25 Herein we
report aldol condensation with a high yield of the aldol
condensation product under CQDs as catalyst. In our
experiments, acetone was the solvent and reactant in the first
set of trials and reacted with a series of aromatic aldehydes at
room temperature. Detailed yields of the aromatic compounds
To further explore the optical properties of as-synthesized
CQDs, a detailed PL investigation was carried out with different
782
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787

ACS Catalysis
Research Article
are displayed in Table 1. As shown in entry 8, the aldol
condensation exhibited a very low yield in the absence of
Table 2. Aldol Condensation between 4-Cyanobenzaldehyde
and Ketones in Various Solvents Catalyzed by CQDs with or
without Visible Light Irradiation
Table 1. Room-Temperature Aldol Condensation between
Acetone and Aromatic Aldehydes in the Presence of CQDs
with or without Visible Light
b
light
dark
a
c
entry
ketone/solvent
yields (%)
yields (%)
b
b
light
dark
1
2
3
4
5
acetone/ethanol
23
32
12
67
54
trace
trace
trace
trace
trace
a
c
acetone/THF
entry
Ar-
yields (%)
yields (%)
acetone/CHCl3
1
2
3
4
5
6
7
C6H5
19
3
cyclopentanone/toluene
cyclohexanone/toluene
4−Cl-C6H4
4−Br-C6H4
68
15
13
8
63
a
Reaction conditions: 0.2 mmol 4-cyanobenzaldehyde, 1 mmol
4-NO2−C6H4
4-CN-C6H4
4-CH3−C6H4
4-CH3O−C6H4
4-CN-C6H4
4-CN-C6H4
65
ketones, 2 mL solvent, 60 mg CQDs, room temperature, 24 h.
89
18
9
b
Visible light irradiation (Xenon lamp, 300 W, λ ≥ 420 nm).
Analyzed by GC analysis.
67
c
65
8
d
8
trace
99
trace
32
e
absence of light, nearly no conversion was observed. In
contrast, high yields were simultaneously achieved with visible
light irradiation (Table 2, entries 1−5). A series of catalytic
experiments with the same reactant and different solvents
(Table 2, entries 1−3) indicated that solvents greatly influenced
the CQD-catalyzed aldol condensation. As shown in Table 2,
when ethanol (entry 1), THF (entry 2), and CHCl3 (entry 3)
were used as solvents, the highest yield when 4-cyanobenzalde-
hyde was used (32%) was obtained with THF as solvent. The
lowest yield (12%) was for CHCl3. Different ketones were also
tested with toluene as the solvent (Table 2, entries 4−5). When
cyclopentanone (entry 4) and cyclohexanone (entry 5) were
used, aldol condensations showed yields of 67% and 54%,
respectively. These data suggest that different ketones influence
the aldol condensation. In contrast, a higher yield of 89% was
obtained when acetone was the solvent and reactant (Table 1,
entry 5). Thus, in the present catalytic system, toluene was the
most suitable solvent for the aldol condensation with CQDs as
catalysts.
All of the above experimental results demonstrate that CQDs
are excellent photocatalysts for the aldol condensation.
Considering that these reactions were performed at room
temperature, it is unlikely that cationic or anionic intermediates
formed in these CQD-catalyzed reactions. However, the
hydroxyl groups on the surface of CQDs are very weak acids,
which can form H-bonds with oxygenates.32,33 Therefore, we
propose that H-bonds catalyzed the aldol condensations in the
present system.
Both reactants (aldehydes and ketones) are able to form H-
bonds in the aldol condensation with CQDs as catalysts. We
demonstrated that the hydroxyl groups on the surface of CQDs
prefer to contact aldehyde groups. In a control experiment, 0.01
mmol 4-cyanobenzaldehyde, 0.01 mmol acetone, (aldehyde/
ketone, molar ratio = 1:1), and 60 mg 5 nm CQDs were mixed
with 1 mL p-xylene and stirred for 10 min followed by
centrifugation to obtain a clear solution. The solution was
analyzed by GC to determine the relative content of 4-
cyanobenzaldehyde versus acetone in the solution. GC analysis
showed that the molar ratio of aldehyde/ketone decreased to
1:2 after mixing with 5 nm CQDs. As shown in Table S4, there
was no change in molar ratio when no catalyst was used, but the
molar ratios of aldehyde/ketone decreased to 1:1.7, 1:2.0, and
1:1.6 when <4 nm, 5 nm, and >10 nm CQDs were used,
9
a
Reaction conditions: 0.2 mmol aromatic aldehydes, 2 mL acetone, 60
mg CQDs, room temperature, 24 h. Reference experiments with and
without visible light irradiation, respectively (Xenon lamp, 300 W, λ ≥
420 nm). The reaction products were isolated through silica column
chromatography and analyzed by GC. Reference experiment without
catalyst. Heated to 50 °C, 4h.
b
c
d
e
CQDs as catalysts whether under light irradiation or not,
confirming the CQDs as catalyst is necessary for the reaction.
In general, these reactions yielded higher yields with visible
light irradiation than those produced in the dark. These data
confirm that visible light is necessary for good conversion.
When benzaldehyde (entry 1) is used, the lowest yield in visible
light (19%) with CQDs as catalyst (Table 1) was obtained.
Other aromatic aldehydes (Table 1, entries 2−7) showed
higher conversion. Among them, when 4-cyanobenzaldehyde is
used, the aldol condensation showed the highest yield (89%,
entry 5) with CQDs as catalyst. These catalytic results were
similar to the results reported by Song et al.25 It would be
attributed to the weak interaction between the para-position of
aldehyde group and the groups on the CQDs surface, which
make the reactant molecules closer to the catalytic active center.
In contrast, without visible light irradiation, yields were reduced
to 18% for 4-cyanobenzaldehyde (entry 5). When the reaction
temperature was elevated to 50 °C, a total conversion of 4-
cyanobenzaldehyde in aldol condensation was achieved in 4 h
(Table 1, entry 9). When 4-chlorobenzaldehyde (entry 2) and
4-bromobenzaldehyde (entry 3) were used, the yields reached
68% and 63%, respectively, with visible light irradiation,
whereas the yields fell to 15% and 13% in the dark. In the
present system, the reaction products were dehydrated
products, and the typical one (Table 1, entry 1) was (E)-4-
phenylbut-3-en-2-one (analyzed by gas chromatography (GC)
and H nuclear magnetic resonance spectroscopy (HNMR),
shown in Figures S8 and S9).
In the following experiments, different solvents (toluene,
ethanol, THF, and CHCl3) and ketones (cyclopentanone,
cyclohexanone, and acetone) were selected to evaluate the
catalytic abilities of CQDs (about 5 nm) for aldol
condensations with 4-cyanobenzaldehyde. The corresponding
aldol condensations shown in Table 2 were performed with and
without visible light at room temperature. On the whole, in the
783
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787

ACS Catalysis
Research Article
respectively. These data support stronger H-bonding between
4-cyanobenzaldehyde and the catalyst versus the ketone and
catalyst.
We further studied the reactions catalyzed by CQDs−OD
(surface bound with −OD but not −OH) to prove that CQDs
could improve the reactions’ conversion via surface H-bond
catalysis rather than acid catalysis. CQDs−OD were prepared
by the electrochemical etching method in heavy water (D2O).
The aldol condensation between 4-cyanobenzaldehyde and
acetone was catalyzed by CQDs−OD to produce products 1
and 2 (Table 3). If CQDs work as acid catalysts, the O−D
Table 3. H/D Exchange Results for the Aldol Condensation
Figure 2. FT-IR spectra of benzaldehyde mixed with CQDs in the
−OH stretching region (inset, enlarged graph corresponding to the
black rectangle). The red and black curves denote the solutions tested
after and before visible light irradiation, respectively. The peak
positions of the curves are marked by vertical lines.
yields (%)
a
b
entry
1
catalyst
light
75
dark
17
X (%)
c
irradiation. As shown in the red line, the peak shifted to 3383
cm−1. This is a blue-shift (Δσ = 12 cm−1, from 3371 cm−1 to
3383 cm−1), and the increase of frequency means the stretching
vibration of O−H···O was enhanced,34 suggesting that light
enhanced the interaction between the functional group and
aldehyde. So the H-bond catalytic activities of CQDs can be
significantly enhanced with visible light irradiation, which is
consistent with our experimental results shown in Tables 1 and
2. On the basis of these experimental results, we propose the
following mechanism for the enhanced photocatalytic activity
of CQD photocatalysts. CQDs can bind with aldehydes by H-
bonding to form CQD−aldehyde complexes. CQDs act as
highly efficient electron acceptors in visible light. Electrons
transfer from the O−H···O region to part of the electron
acceptor and lead to the blue-shift, suggesting that CQDs have
highly enhanced photocatalytic activity for target reactions.35,41
FT-IR spectra of 4-cyanobenzaldehyde mixed with three
different sizes CQDs and hydroxyl-group-free CQDs (5 nm)
before and after visible-light irradiation in the −OH stretching
region were shown in Figure S11. As shown in Figure S11a, the
black line is the FT-IR spectrum of 5 nm CQDs in the −OH
stretching region. Blue and red line are for the mixture of 4-
cyanobenzaldehyde and 5 nm CQDs before and after visible-
light irradiation, respectively. The red shift (black line to blue
line, Δσ = 12 cm−1) indicates the formation of hydrogen bond
after adding 4-cyanobenzaldehyde.36 Further exposing the
mixture of 4-cyanobenzaldehyde and CQDs under visible
light for 10 min and then the FT-IR spectrum (red line) was
detected. The blue shift (blue line to red line, Δσ = 21 cm−1)
proves that the H-bond catalytic activities of CQDs can be
significantly enhanced with visible light irradiation.35,41 The
similar results were obtained when >10 nm CQDs and <4 nm
CQDs were tested. Figure S11b,c shows the FT-IR spectra of 4-
cyanobenzaldehyde mixed with >10 nm CQDs and <4 nm
CQDs before and after visible-light irradiation in the −OH
stretching region, respectively. The red shifts for >10 nm CQDs
and <4 nm CQDs were 8 and 3 cm−1, respectively, when a
hydrogen bond formed. Then, the blue shift (11 cm−1 for >10
nm CQDs and 7 cm−1 for <4 nm CQDs) were detected after
visible light irradiation for 10 min. These blue shifts were
smaller than that for 5 nm CQDs (21 cm−1) due to the highly
efficient electron accept ability of 5 nm CQDs. This was
elucidated in the next experiments. The FT-IR spectra of 4-
CQDs−OD
0
a
Reaction conditions: 0.2 mmol 4-cyanobenzaldehyde, 2 mL acetone,
b
60 mg CQDs−OD, room temperature, 24 h. X = Deuterium content
in products (atom %). Based on the identical ion mass pattern of
water.
c
bonds on their surfaces will be destroyed during H/D exchange
and will form the deuterium-labeled products, D2O or HDO.
Deuterium signals were not detected in products 1 or 2 by GC-
mass spectrometry (GC-MS) analyses (see Table 3), further
proving that the reaction products were dehydrated products.
In other words, CQDs act as H-bond catalysts in the aldol
condensation. The O−D bonds form single or multiple H-
bonds (D−H···O) with aldehyde to promote the aldol
condensation, and then H-bonds open, demonstrating that
the O−D bonds on CQDs−OD surfaces have not been
destroyed. At last, CQDs−OD was separated via centrifugation
from the reaction mixture.
CQDs without surface hydroxyl groups were synthesized for
comparison. The same amounts of hydroxyl-group-free CQDs
were then used as catalysts for the same reaction in the same
conditions. Interestingly, hydroxyl-group-free CQDs were
unreactive, and no product was detected with or without
visible light irradiation. This dependence on a hydroxyl group
suggests that the ability of CQDs to mediate reactions is via
interfacial H-bond catalysis. As shown in Table S3, a series of
controlled trials proved that phenol and oxalic acid have very
low catalytic effect, indicating that weak acids almost cannot
catalyze the aldol condensation between acetone and aromatic
aldehydes.12b,32
The Table 1 results (entry 5) revealed that the conversion of
4-cyanobenzaldehyde was about 4 times higher when the
mixture was stirred under visible light versus when the reaction
was performed in the dark. We compared the FT-IR spectra of
benzaldehyde mixed with 5 nm CQDs before and after visible
light irradiation to further investigate the catalytic mechanism.
Figure 2 shows the FT-IR spectra in the O−H stretching
region. As shown in the enlarged graph (inset in Figure 2), the
peak around 3400 cm−1 corresponded to the vibrations of O−
H···O. A characteristic peak was observed at 3371 cm−1 (black
line) before visible light irradiation due to the presence of many
hydroxyl groups on the CQD surface. Then the mixture was
exposed to visible light for 10 min and was detected after
784
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787

ACS Catalysis
Research Article
cyanobenzaldehyde mixed with hydroxyl-group-free CQDs
were shown in Figure S11d, which shows no shift whether
the mixture was exposed under visible light or not, indicating
that there is no hydrogen bond formation.
Table 4. Aldol Condensation between 4-Cyanobenzaldehyde
and Ketones in Acetone Catalyzed by CQDs of Different
a
Sizes
To elucidate the photoenhanced catalytic abilities of CQDs,
the time-correlated single-photon counting (TCSPC) experi-
ments were performed to verify the photoinduced electron-
accepting properties of CQDs. The charge transfer of CQDs
was tested in the presence or absence of electron donor N, N-
diethylaniline (DEA, 0.88 V vs NHE)37 and electron acceptor
2,4-dinitrotoluene (−0.9 V vs NHE).38,39 CQDs with three
different average particle sizes (5 nm, >10 nm, and <4 nm)
were tested. As shown in Figures 3a and S12a, the
b
catalyst
CQDs
size (nm)
t (h)
yields (%)
5
>10
<4
24
24
24
89
40
25
a
Reaction conditions: 2 mL acetone, 0.2 mmol 4-cyanobenzaldehyde,
1 mmol ketones, 60 mg CQDs, room temperature, 24 h. Visible light
b
irradiation (Xenon lamp, 300 W, λ ≥ 420 nm).
in O−H bond hybridization.42 The first O−H bond shortening
effect is due to the increase in C−H bond polarization upon H-
bond formation. The second group of changes involves O−H
bond shortening effects. As shown in Scheme 1, the surface
Scheme 1. Schematic of the Photoenhanced Catalytic
Mechanism of H-Bond Catalysis by CQDs in Visible Light
Figure 3. (a) Luminescence decays (485 nm excitation, monitored
with 550 nm narrow bandpass filter) of the CQDs with DEA, (b)
Stern−Volmer plots for the quenching of luminescence quantum
yields (485 nm excitation) of the different-sized CQDs by DEA.
luminescence of CQDs was quenched by DEA and 2,4-
dinitrotoluene. These results show that photoexcited CQDs are
excellent electron donors and electron acceptors. The Stern−
Volmer plots for the quenching of luminescence by DEA are
shown for the different-sized CQDs in Figure 3b. The
quenching constants (KSV = τF kq) from linear regression
were calculated to be 18.6 (5 nm CQDs), 14.4 (>10 nm
CQDs), and 10.8 M−1 (<4 nm CQDs), respectively. The
quenching constant for 2,4-dinitrotoluene was 14.5 M−1 (5 nm
CQDs, Figure S12b). Obviously, the constant for DEA (18.6
M−1) is larger than that of 2, 4-dinitrotoluene (14.5 M−1),
indicating that the as-synthesized CQDs (5 nm) are highly
efficient electron acceptors. The Stern−Volmer plots for the
quenching of luminescence quantum yields of the hydroxyl free
CQDs by DEA and 2, 4-dinitrotoluene were shown in Figure
S13.
Next, we performed the control experiments to study the
influence of size on the photocatalytic activity of CQDs. The
CQDs with three different average diameters (5 nm, >10 nm,
and <4 nm) were used to catalyze the reaction between 4-
cyanobenzaldehyde and ketones. The reaction yields were
shown in Table 4. The highest yield of 89% was achieved for 5
nm CQDs when 4-cyanobenzaldehyde is used, whereas yields
of 40% and 25% were obtained for CQDs >10 nm and <4 nm,
respectively. Here, 5 nm CQDs had the most satisfying
photocatalytic performance for this H-bond catalysis reaction,
which should attribute to the highest electron-accepting ability
when compared with other CQDs (with size >10 nm and <4
nm).
hydroxyl groups on the surface of CQDs form single or
multiple H-bonds with 4-cyanobenzaldehyde. The CQD−
aldehyde complex can be considered to be a reaction substrate
for other molecules. H-bonds decreased the energy barrier for
the reaction in the dark, and a lower 4-cyanobenzaldehyde yield
of 18.2% was obtained. Our TCSPC experiments (Figure 3)
proved that CQDs act as highly efficient electron acceptors
with visible light irradiation, as these electron acceptors attract
electrons from the O−H···O region. This effect results in the
increase of positive charge on hydrogen and the increase of
negative charge on oxygen. The group of changes is reflective of
a decrease in effective electronegativity of the hydrogen atom
which, in excellent agreement with Bent’s rule, leads to an
increase in the s-character in the oxygen hybrid orbital forming
the O−H bond effect, thereby leading to O−H bond
shortening.42,43 In other words, this process in turn leads to a
strengthening of the O−H bond and effectively activates the
CO bond of 4-cyanobenzaldehyde and accelerates the aldol
condensation. At the same time, the enhanced O−H bonds
may further stabilize the reaction-intermediate species or
transition-state species, which promote higher yields (highest
yield of 89.4%).18,40
The rehybridization and hyperconjugation act in opposite
directions, the effect of the bond lengths of H-bonding is a
result of a balance of these two intrinsic effects. The O−H bond
lengthens when hyperconjugation is dominant, and the O−H
bond shortens when hyperconjugation is weak and the
structure of the H-bond donor allows for a significant change
Repeated catalytic experiments were performed by recycling
the CQDs catalysts five times. It was found that the yields were
almost the same in all five runs (Figure 4). When 5 nm CQDs
785
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787

ACS Catalysis
Research Article
were used as catalysts, the yields for the first and fifth cycle were
89.4% and 82.6%, respectively.
Figure 6. Kinetic plots track conversion over time for the first (red
line), second (blue line), and fifth (black line) cycle experiments with
different size CQDs as catalysts: (a) 5 nm; (b) >10 nm; and (c) <4
nm.
test results suggest that the CQD catalyst can be easily recycled
as a heterogeneous catalyst with a long catalyst life.
In summary, we have demonstrated that CQDs (about 5
nm) were efficient photocatalysts for H-bond catalysis in the
aldol condensation. CQDs showed excellent photoenhanced
catalytic abilities (89% yield when 4-cyanobenzaldehyde is
used) for H-bond catalysis with a series of aromatic aldehydes.
Control catalytic experiments confirmed that the efficient
electron-accepting properties of CQDs strengthened the O−H
bond and activated the CO bond of the 4-cyanobenzalde-
hyde, accelerating the aldol condensation. Our findings offer a
novel approach for designing and fabricating the next
generation of high-performance H-bond catalysts.
Figure 4. Recycling results for the different sizes of CQDs as catalysts
(5 nm, >10 nm, and <4 nm) after five runs.
In order to further clarify the effect of light, the following
experiments were carried out. We irradiated the mixture
(solvent, 5 nm CQDs, and aldehyde) for 6 h and then removed
the light source, and we monitored the conversion as a function
of time. As shown in Figure 5, the black line shows the
ASSOCIATED CONTENT
* Supporting Information
■
S
Preparation and characterization results of CQDs, FT-IR
spectra of benzaldehyde mix with CQDs, luminescence decays
of CQDs. This information is available free of charge via the
Internet at http://pubs.acs.org/.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhkang@suda.edu.cn.
*E-mail: yangl@suda.edu.cn.
■
Figure 5. Kinetic plots monitor the conversion as a function of time
(black line: exposed in the first 6 h and in the dark the next time; red
line: exposed in light all the time).
Author Contributions
Y.H. and H.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
conversion of the reaction, which was irradiated in the first 6 h
and then kept in the dark. Compared with the red line (exposed
in light all the time), the conversion (see black line) does not
improve obviously after removing the light source, suggesting
that the light is actually needed for photocatalytic enhancement
activity.
The kinetic plots tracking yield over time for the first,
second, and fifth cycle experiments are shown in Figure 6.
These plots unambiguously demonstrate an induction period,
which suggested that the most active catalytic species are not
initially present. The reason is the CQD catalysts were initially
not dispersed when we added to the reaction mixture. As
shown in Figure 6a, when 5 nm CQDs were used as catalyst,
the final yields in aldol condensation were achieved in about 18
h for the first (red line) and second (blue line) cycle
experiments and 22 h for the fifth (black line) cycle experiment.
Moreover, the yield rate was slightly decreased along with the
increase in cycles. For the other two kinds of catalysts (Figure
6b, >10 nm; Figure 6c, <4 nm), the similar catalytic effects were
also observed in the recycle experiments. The above stability
ACKNOWLEDGMENTS
■
This work is supported by the National Basic Research
Program of China (973 Program) (No. 2012CB825800,
2013CB932702), National Natural Science Foundation of
China (NSFC) (No. 51132006), a project funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), a Suzhou Planning Project of
Science and Technology (ZXG2012028).
REFERENCES
■
(1) Knowles, R. R.; Jacobsen, E. N. Proc. Natl. Acad. Sci. U.S.A. 2010,
107, 20678−20685.
(2) Pihko, P. M. Angew. Chem., Int. Ed. 2004, 43, 2062−2064.
(3) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2004, 43, 5138−
5175.
(4) Akiyama, T.; Itoh, J.; Yokota, K.; Fuchibe, K. Angew. Chem., Int.
Ed. 2004, 43, 1566−1568.
(5) Gondi, V. B.; Gravel, M.; Rawal, V. H. Org. Lett. 2005, 7, 5657−
5660.
786
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787

ACS Catalysis
Research Article
̌
̌ ́
va, J.; Spirko, V.; Hobza, P. Phys. Chem. Chem.
(6) Sigman, M. S.; Vachal, P.; Jacobsen, E. N. Angew. Chem., Int. Ed.
2000, 39, 1279−1281.
(36) Chocholouso
Phys. 2004, 6, 37−41.
(37) Wang, X.; Cao, L.; Lu, F. S.; Meziani, M. J.; Li, H. T.; Qi, G.;
Zhou, B.; Harruff, B. A.; Kermarrec, F.; Sun, Y.-P. Chem. Commun.
2009, 3774−3776.
(7) Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem. Soc. 2003, 125,
12672−12673.
(8) Nugent, B. M.; Yoder, R. A.; Johnston, J. N. J. Am. Chem. Soc.
2004, 126, 3418−3419.
(9) Uraguchi, D.; Terada, M. J. Am. Chem. Soc. 2004, 126, 5356−
5367.
(10) Huang, Y.; Unni, A. K.; Thadani, A. N.; Rawal, V. H. Nature
2003, 424, 146.
(38) Rehm, J. M.; Mclendon, G. L.; Fauchet, P. M. J. Am. Chem. Soc.
1996, 118, 4490−4491.
(39) Li, H. T.; Liu, R. H.; Lian, S. Y.; Liu, Y.; Huang, H.; Kang, Z. H.
Nanoscale 2013, 5, 3289−3297.
(40) Chen, X.; Brauman, J. I. J. Am. Chem. Soc. 2008, 130, 15038−
15046.
(11) Unni, A. K.; Takenaka, N.; Yamamoto, H.; Rawal, V. H. J. Am.
Chem. Soc. 2005, 127, 1336−1337.
(41) Kubota, Y.; Yamaguchi, H.; Yamada, T.; Inagaki, S.; Sugi, Y.;
Tatsumi, T. Top. Catal. 2010, 53, 492−499.
(42) Alabugin, I. V.; Manoharan, M.; Peabody, S.; Weinhold, F. J.
Am. Chem. Soc. 2003, 125, 5973−5987.
(43) Joseph, J.; Jemmis, E. D. J. Am. Chem. Soc. 2007, 129, 4620−
4632.
(12) (a) Hoashi, Y.; Okino, T.; Takemoto, Y. Angew. Chem., Int. Ed.
2005, 117, 4100−4103. (b) Zeidan, R. K.; Hwang, S.-J.; Davis, M. E.
Angew. Chem., Int. Ed. 2006, 45, 6332−6335. (c) Zeidan, R. K.; Davis,
M. E. J. Catal. 2007, 247, 379−382. (d) Solin, N.; Han, L.; Che, S.;
Terasaki, O. Catal. Commun. 2009, 10, 1386−1389.
(13) Rajaram, S.; Sigman, M. S. Org. Lett. 2005, 7, 5473−5475.
(14) Sigman, M. S.; Vachal, P.; Jacobsen, E. N. Angew. Chem., Int. Ed.
2000, 112, 1336−1338.
(15) Wenzel, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124,
12964−12965.
(16) Akiyama, T.; Itoh, J.; Yokota, K.; Fuchibe, K. Angew. Chem., Int.
Ed. 2004, 116, 1592−1594.
(17) McGilvra, J. D.; Unni, A. K.; Modi, K.; Rawal, V. H. Angew.
Chem., Int. Ed. 2006, 45, 6130−6133.
(18) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713−5743.
(19) Kelly, T. R.; Meghani, P.; Ekkundi, V. S. Tetrahedron Lett. 1990,
31, 3381−3384.
(20) Momiyama, N.; Yamamoto, H. J. Am. Chem. Soc. 2005, 127,
1080−1081.
(21) Niu, F.; Wu, J. Y.; Zhang, L. S.; Li, P.; Zhu, J. F.; Wu, Z. Y.;
Wang, C. R.; Song, W.-G. ACS Catal. 2011, 1, 1158−1161.
(22) McDougal, N. T.; Schaus, S. E. J. Am. Chem. Soc. 2003, 125,
12094−12095.
(23) Niu, F.; Liu, C. C.; Cui, Z. M.; Zhai, J.; Jiang, L.; Song, W.-G.
Chem. Commun. 2008, 44, 2803−2805.
(24) Niu, F.; Zhai, J.; Jiang, L.; Song, W.-G. Chem. Commun. 2009,
45, 4738−4740.
(25) Niu, F.; Zhang, L.; Luo, S. Z.; Song, W.-G. Chem. Commun.
2010, 46, 1109−1111.
(26) Li, H. T.; He, X. D.; Kang, Z. H.; Huang, H.; Liu, Y.; Liu, J. L.;
Lian, S. Y.; Tsang, C. H. A.; Yang, X. B.; Lee, S. T. Angew. Chem., Int.
Ed. 2010, 49, 4430−4434.
(27) Zhang, H. C.; Huang, H.; Ming, H.; Li, H. T.; Zhang, L. L.; Liu,
Y.; Kang, Z. H. J. Mater. Chem. 2012, 22, 10501−10506.
(28) Zhang, H. C.; Ming, H.; Lian, S. Y.; Huang, H.; Li, H. T.; Zhang,
L. L.; Liu, Y.; Kang, Z. H.; Lee, S. T. Dalton Trans. 2011, 40, 10822−
10825.
(29) Han, X.; Han, Y. Z.; Huang, H.; Zhang, H. C.; Zhang, X.; Liu, R.
H.; Liu, Y.; Kang, Z. H. Dalton Trans. 2013, 42, 10380−10383.
(30) Li, H. T.; Liu, R. H.; Liu, Y.; Huang, H.; Yu, H.; Ming, H.; Lian,
S. Y.; Lee, S. T.; Kang, Z. H. J. Mater. Chem. 2012, 22, 17470−17475.
(31) (a) Hohl, H.; Stumm, W. J. Colloid Interface Sci. 1976, 55, 281−
288. (b) Crocker, M.; Herold, R. H. M.; Sonnemans, M. H. W.; Emeis,
C. A.; Wilson, A. E.; Vandermoolen, J. N. J. Phys. Chem. 1993, 97,
432−439. (c) Sengoku, T.; Takemura, T.; Fukasawa, E.; Hayakawa, I.;
Kigoshi, H. Tetrahedron Lett. 2009, 50, 325−328.
(32) Zhao, L.; Sun, Z.-Z.; Ma, J. Environ. Sci. Technol. 2009, 43,
4157−4163.
(33) McKeen, J. C.; Yan, Y. S.; Davis, M. E. Chem. Mater. 2008, 20,
3791−3793.
(34) Mukhopadhyay, A.; Mukherjee, M.; Pandey, P.; Samanta, A. K.;
Bandyopadhyay, B.; Chakraborty, T. J. Phys. Chem. A 2009, 113,
3078−3087.
(35) van der Veken, B. J.; Herrebout, W. A.; Szostak, R.; Shchepkin,
D. N.; Havlas, Z.; Hobza, P. J. Am. Chem. Soc. 2001, 123, 12290−
12293.
787
dx.doi.org/10.1021/cs401118x | ACS Catal. 2014, 4, 781−787