Carbon dots as photocatalysts for organic synthesis: Metal-free methylene-oxygen-bond photocleavage

Article abstract of DOI:10.1039/c9gc03811f

We report for the first time that irradiation of four different citric acid-derived carbon dots (CDs), in the absence of any other redox mediators, promotes an organic reaction. In this proof-of-concept study methylene-oxygen bond reductive photocleavage in N-methyl-4-picolinium esters is demonstrated. Cyclic voltammetry and UV-Vis spectra of the CDs and of the esters indicate that photocleavage reactivity correlates with the redox properties and the relative energies expressed in the Fermi scale. A photo-fragmentation mechanism is proposed. This study offers a new possibility to employ inexpensive and readily available CDs to promote photo-organic reactions.

Full text of DOI:10.1039/c9gc03811f

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DOI: 10.1039/C9GC03811F
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Carbon Dots as Photocatalysts for Organic Synthesis: Metal-Free
Methylene-oxygen-bond Photocleavage
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Simone Cailotto,a ‡ Matteo Negrato,a ‡ Salvatore Daniele,a Rafael Luque,a,b Maurizio Selva,a
Emanuele Amadio,a* Alvise Perosaa*
We report for the first time that irradiation of four different citric CDs with fumarate reductase promotes photoenzymatic
acid-derived carbon dots (CDs), in the absence of any other redox reduction of fumarate to succinate in a semi-biological
[6]
mediators, promotes an organic reaction. In this proof-of-concept photosynthesis. Direct photo-organic chemistry using only
study methylene-oxygen bond reductive photocleavage in N- organic sensitizers akin to CDs is instead still an open field for
methyl-4-picolinium esters is demonstrated. Cyclic voltammetry research. Flavins have been recognized as promising
and UV-Vis spectra of the CDs and of the esters indicate that photoredox catalysts, albeit mainly for oxidation reactions;[7]
photocleavage reactivity correlates with the redox properties and while graphitic carbon nitride (g-C3N4)[8] has attracted interest
the relative energies expressed in the Fermi scale. A photo- as
a
non-metallic photocatalyst for example for H2O2
[10]
fragmentation mechanism is proposed. This study offers a new synthesis[9], as a polymerization photoinitiator
and as a
[11]
possibility to employ inexpensive and readily available CDs to metal-free photosensitizer for dithiol-ene click reactions.
promote photo-organic reactions.
However, up to date the only example of direct photo-organic
chemistry using only CDs as photosensitizers seems to be a
metal-free photocatalytic oxidation of benzylic alcohols using
near infrared radiation. [12]
Due to their ability to transfer electrons upon irradiation,
photoluminescent carbon nanoparticles, referred to herein as
carbon dots (CDs) have been used as photosensitizers for water
splitting, CO2 reduction and degradation of pollutants.[1] CDs are
mainly carbonaceous materials, typically 2–10 nm in diameter,
constituted of amorphous organic oligomers together with
graphitic nanocrystalline regions. They often display strong
photoluminescence and good optical absorption in the UV and
near-visible regions, making them candidates for photocatalytic
applications, usually in conjunction with other photosensitizers.
For example, photocatalysis with CDs had been explored in the
presence of redox mediators such as nanocomposites or
enzymes, that were necessary to shuttle the electrons from the
CDs to the substrate. For example, CDs/SiO2, CDs/Au, Ag or Cu
nanocomposites were developed for the oxidation of cis-
cyclooctene,[2] alkanes[3] and for the reductions of nitro-
benzenes[4] or quinones[5]. In another instance, the coupling of
Our interests in green organic syntheses, [13] led us to explore
light-driven metal-free photocatalytic reactions promoted by
CDs with the aim of demonstrating that these nanomaterials
can directly promote photo-organic reactions. To this end, we
recently described the synthesis and full characterization a set
of four amorphous and graphitic CDs derived from citric acid:
two purely organic, a-CDs and g-CDs, and two others as
nitrogen-doped systems, a-N-CDs and g-N-CDs.[14] These
materials are readily obtainable in gram-scale quantity with
only minimal isolation and purification procedures (see SI for
full synthetic details), they are inexpensive, safe, easily
disposable and are constituted by aggregates of fluorophores,
heavier aggregates/oligomers and extended graphitic carbon
cores.
Our previous study showed that these CDs are able to transfer
an electron to methyl viologen; it also demonstrated that the
carbon-nitrogen source, the synthetic method and the resulting
structural properties strongly affect the photo-reactivity of the
CDs. Based on this work, the four CDs are here explored for a
photo-organic reaction: the reductive methylene-oxygen bond
photocleavage of N-Methyl-4-picolinium esters. This reaction
was chosen not only as a proof-of-concept transformation, but
also for its synthetic relevance since picolinium esters are used
as photolabile protecting groups for the carboxyl group.[15] So
[a]
[b]
[‡]
Department of Molecular Sciences and Nanosystems, Università Ca’ Foscari
Venezia, Via Torino 155, 30172 Venezia Mestre, Italy
E-mail: emanuele.amadio@unive.it, alvise@unive.it
Departamento de Quı́mica Organica, Universidad de Córdoba,
Edif. Marie Curie, Ctra Nnal IV-A Km 396, E-14014
Córdoba, Spain.
These two authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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far, such photo-deprotection has been carried out with Results from Table 1 clearly indicated that each of the four
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[16]
[Ru(II)(bpy)3]2+
or with organic dyes (such as N,N,N′,N′- different types of CDs could selectively phDoOtoI:c1l0e.a10v3e9t/hCe9GeCs0te38r111aF
tetramethylbenzidine, 9-methylcarbazole, pyrene, and with conversions between 15 and 73% (Table 1, entries 1-4 and
[17]
triphenylamine, pyrromethene and coumarin)
via
a
Figure S2-5). The most active photocatalyst was the a-N-CDs
photoinduced electron transfer pathway, rather than by direct that was also the one shown to most easily reduce methyl
photolysis. We here demonstrate that conversion of viologen.[14] Control experiments conducted in the absence of
methylpicolinium esters into the corresponding carboxylic acids either CDs or light showed negligible activity (Table 1, entries 5-
can be also carried out using solely a-CDs, g-CDs, a-N-CDs or g- 6 Figure S6-7) thus indicating that the combination of both a
N-CDs as the photo-reductant. Cyclic voltammetry was further light source and CDs were required for the reaction to take
used to measure the redox activity of these CDs, allowing to place. Interestingly, the photodeprotection proceeded also
correlate the energy levels of the nanomaterials, and the without EDTA as sacrificial electron source, albeit with lower
relative band gap, with the photo-reactivity towards conversion (49%; Table 1, entry 7 and Figure S8). This
methylene-oxygen bond cleavage.
observation indicated that CDs themselves could transfer an
The photocleavage efficiency of the citric acid-derived a- electron upon irradiation and that this photo-electron transfer
CDs, g-CDs, a-N-CDs, g-N-CDs[14] was first tested in the reduction ability is therefore an intrinsic property of these nanomaterials.
of the picolinium ester of octanoic acid (1a), selected as the This was further verified by conducting the reaction in the
model substrate (Table 1). The reaction was carried out in ACN- absence of EDTA and by increasing the CDs loading from 20
d3/D2O (6/4) under nitrogen with EDTA as sacrificial electron mg/mL to 100 mg/mL, whereby photocleavage reached 90%
source, by irradiating the sample with UV light at 365 nm for conversion with 100% CH2-O cleavage selectivity (Table 1, entry
48 hours. The cleavage was monitored by NMR (see SI section 8 and Figure S9). These EDTA-free conditions are ideally suited
for further details and Figure S1), by measuring the conversion for synthetic scale-up in view of having cleaner mixtures and
of 1a, and the formation of N-methyl-4-methylpyridinium (2) avoiding waste-generating work-up and purification protocols.
and octanoic acid (3a) derived from CH2-O cleavage, as well as As a control experiment, the reaction carried out with 100
any formation of methyl pyridinium carbinol (4) potentially mg/mL of a-N-CDs and EDTA showed quantitative conversion
derived from competitive ester hydrolysis.
and specificity for CH2-O cleavage already after 28 h (Table 1,
entry 11 and Figure S12).
Table 1. CDs promoted CH2-O bond photocleavage of 1a
For comparison, the same photodeprotection reaction was also
performed using Ru(bpy)3Cl2 (Table 1, entry 9 and Figure S10),
the best-known metal photosensitizer for the deprotection of
these methylpicolinium esters.[17] Due to the maximum
absorption wavelength at 455 nm, the Ru catalysed reaction
was performed under visible light (See SI for further details),
conditions where a-N-CDs still exhibited some reactivity (7%
conversion and high C-O cleavage selectivity; Table 1, entry 10).
Gratifyingly, the photodeprotection with the Ru system was
comparable with the one with the CDs in terms of conversion,
selectivity and reaction rates (see Figure S11). The fact that
these carbon-based nanomaterials are re-useable as already
demonstrated,[14] that they are simple to make and available in
gram-quantity at low cost, makes them in our view remarkably
competitive compared to semiconductor- or metal-containing
systems.
h
CDs
O
O
OH
+
O
nC7
+
N+
N+
HO
nC7
ACN-d3/D2O
N+
-
-
ClO4
ClO4
-
ClO4
1a
2
3a
4
Entry
Sensitizer
(mg/mL)
EDTA Conv
(%)
Yield
2/3a/4
(%)
CH2-O
Cleavage
Selectivity
(%) a
1
2
3
4
a-CDs (20)
YES
YES
YES
YES
YES
YES
NO
NO
YES
YES
YES
15
25
52
73
15/15/0
25/25/0
49/52/3
73/73/0
>99
g-N-CDs (20)
g-CDs (20)
>99
94
a-N-CDs (20)
>99
5
6b
7c
8d
9e
10f
11g
None
0
0
--
--
--
--
After optimizing the photocleavage conditions, the scope of
the reaction was extended to other N-methyl-4-picolinium
esters 1a-e, using the most active a-N-CDs, both in the presence
and absence of EDTA (Scheme 1). The reactions were monitored
by NMR and GC-MS (see SI section for further details).
a-N-CDs (20)
a-N-CDs (20)
49
90
99
7
48/49/1
90/90/0
96/99/3
7/7/0
98
a-N-CDs (100)
Ru(bpy)3Cl2
>99
>99
>99
>99
a-N-CDs (20)
a-N-CDs (100)
99
99/99/0
Standard conditions: [CDs] = 20 mg/mL, substrate (0.06 M), EDTA (0.1 M) in ACN-
d3/D2O (6/4) at pD = 7, h = 365 nm, 48 h at R.T. under N2. a C-O cleavage selectivity
determined by considering the amount of 2. All reactions carried out under
standard conditions except b) in the absence of light, c) without EDTA, d) without
EDTA and with 100 mg/ml of a-N-CDs. e) Ru(bpy)3Cl2 5 % mol, substrate (0.06 M),
EDTA (0.1 M) in ACN-d3/D2O (6/4) at pD = 7, halogen lamp, 48 h at R.T. under N2. f)
under standard conditions but with halogen lamp irradiation g) with 100 mg/ml of
a-N-CDs and EDTA (0.1 M).
2 | J. Name., 2012, 00, 1-3
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Scheme 1. Reductive CH2-O photocleavage of N-Methyl-4-picolinium esters
1a-e promoted by a-N-CDs. Scope of the substrate.
electrochemical energy gaps that best correspondVtioewthAreticolepOtniclinael
ones are shown in Table 2 and Figure 1.
DOI: 10.1039/C9GC03811F
h
Table 2. Selected Eonset reduction potentialsa and estimated energy levels for a-
CDs, g-CDs, a-N-CDs, g-N-CDs and 1a-e
CDs
O
O
O
R
+
N+
HO
R
ACN-d3/D2O
EDTA
N+
Eonset,ox
(V)
vs
Eonset,red
(V)
vs
Fermi
Fermi
Energy
gapd
-
ClO4
-
ClO4
Energy ox
Energy red
1
2
3
c
c
Ag|AgCl b Ag|AgCl b
(eV)
-5.9
(eV)
-3.0
(eV)
2.9
O
O
O
1.27
0.52
0.56
1.12
-1.60
-1.83
-1.87
-1.94
-0.97
-0.94
-0.89
-0.93
-0.65
g-CDs
a-CDs
g-N-CDs
a-N-CDs
1a
HO
nC7
OH
Boc
HO
-5.1
-5.2
-5.7
-2.8
-2.7
-2.7
-3,6
-3,7
-3,7
-3,7
-4,0
2.3
2.5
3.0
HN
3a
3b
3c
73% Conv., >99% Sel.b
a 90% Conv., >99% Sel.b
70% Conv., >99% Sel.b
a 43% Conv., >99% Sel.b
45% Conv., >99% Sel.b
a 60% Conv., >99% Sel.b
O
O
1b
HO
HO
NO2
1c
3e
3d
20% Conv., >99% Sel.b
a 88% Conv., >99% Sel.b
20% Conv., 57% Sel.b
a 68% Conv., 11% Sel.b
1d
1e
Standard conditions: a-N-CDs (20 mg/mL), 1a-e (0.06 M), EDTA (0.1 M) in ACN-
a Uncertainty of Eonset is ± 0.01 V. b Cyclic voltammetry measurements: 10 mM for
1a-e and 25 mg/mL for CDs in DMFaq. scan rate 100mV/s, 0.1
tetrabutylammonium perchlorate, glassy carbon working electrode, saturated
Ag|AgCl|KCl reference, platinum counter electrode, and referred to Fc+|Fc as
internal standard. c See SI section for the energy levels calculations. d Calculated by
CV, see SI section.
d3/D2O (6/4) at pD = 7, hv = 365 nm, 48 h at R.T. under N2. a Reactions carried out
,
M
b
under standard conditions with 100 mg/mL of a-N-CDs and without EDTA. C-O
cleavage selectivity determined by considering the amount of 2.
High methylene-oxygen cleavage selectivity (>99%) was
observed for benzoic 1b, phenylalaninate 1c and phenylacetic
1d esters, both with and without EDTA as sacrificial electron The
CVs of a-CDs, g-CDs, a-N-CDs, and g-N-CDs are
source. Phenylacetic methylpicolinium ester 1b yielded the representative of complex electrochemical behaviours,
desired CH2-O photocleavage product 3b in 70% yield in the highlighting how the syntheses and composition of the CDs
presence of EDTA (Figure S16), and only 43% in its absence influenced the electrochemical, structural, as well as optical and
(Figure S17). The opposite behaviour was observed for photoredox properties.[14] In particular, the electron transfer
phenylalaninate 1c (45% and 60%, with or without EDTA, processes, characterizing the CDs employed here, could depend
respectively, Figure S19-S20) and 1d (20% and 88%, with or on the type, density and location of active sites produced during
without EDTA respectively, Figure S22-S23). The p-nitrobenzoic their synthesis.[14b] Nonetheless, the presence of nitrogen
ester 1e showed low CH2-O photocleavage selectivity (ranging simplified the CV curves that showed fewer redox processes in
from 11 to 57%, Figure S25-S26). Overall, the photocleavage both the positive and negative potential regions. In general, a-
reactivity of the picolinium esters followed the order 1a > 1b > N-CDs exhibited the most negative potential (Eonset red = -1.94 vs
1c > 1d > 1e.
The different photocleavage activity of the CDs prompted us
to investigate their electrochemical properties as well as those
of the substrates by cyclic voltammetry (CV, full details in SI).
The cyclic voltammograms of the a-CDs, g-CDs, a-N-CDs, and g-
Ag|AgCl) while g-CDs possessed the highest positive potential
(Eonset ox = 1.27 vs Ag|AgCl). This suggested that these CDs have
the strongest reducing or oxidizing potential among the
investigated CDs, respectively.
N-CDs, exhibited several irreversible redox processes. The scans
g-N-CDs
a-N-CDs
-2.0
-3.0
-4.0
-5.0
-6.0
a-CDs
g-CDs
in the positive and negative potential regions are shown in
Figure S30. Each observed process involves an electron transfer
(from the electrode to the CDs or vice versa) which causes
reduction (in the negative potential window) or oxidation (in
the positive potential window) of the different surface moieties
present in the CDs. Several waves were observed which
correspond to different energy levels, expressed in the Fermi
1a
1b
1c
1d
1e
2.5 eV
2.3 eV
3.0 eV
2.9 eV
scale.[18]
respective positive (Eonset,ox
T
he energy values were estimated by using the
and negative (Eonset,red onset
)
)
potentials of the CV responses (Table S1). The largest energy
gap is associated to the maximum oxidation or reduction
strength of the CDs. The maximum energy gap of each CD was
also extrapolated from the UV-vis spectra (Figure S33 and Table
S3) and compared with those calculated by CV. The largest
CDs photoactivity
C-O bond fragmentation
Figure 1. Estimated Fermi energy levels of the CDs and of the esters 1a-e.
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CV was also used to investigate the redox properties of the quencher (E0 ½ EDTA/ EDTA˙+ = 0.57 0.82 vs SCE[15]), thus increasing
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N-methylpicolinium esters 1a-e. Each compound showed the reduction potential and extendingDOthI:e10.l1i0fe3t9i/mC9eGCo0f38t1h1eF
several irreversible reduction peaks in the negative potential CDs*.[20] The electron injected into the methylpicolinium ester
region between -0.65 and -1.46 V vs Ag|AgCl (see Figure S31) forms a pyridinium radical which undergoes reductive C-O bond
which could be attributed to the reduction of the N- cleavage to release a carboxylate along with a N-methyl-4-
methylpicolinium, C=O moiety or of the nitro group in 1e.[15c, 17c, methyl pyridinium radical. In the final step, hydrogen
19]
Eonset,red – which for the substrates corresponds to the LUMO abstraction and abstraction of a proton from either the
energy levels – evaluated considering the first observed deuterated solvent (deuterated products have been observed,
reduction peak (Table S2, Table 2 and Figure 1), decreased in see Supporting info Figure S13-14) or radical EDTA˙+ - or its
the order 1a > 1b > 1c > 1d > 1e which correlates with the decomposition products[21] yields the desired N-methyl-4-
electron-withdrawing ability of the substituent (i.e., 1e > 1d > 1c methyl pyridinium and the carboxylic acid.
> 1b > 1a) and thus, with the stability of the radical formed upon
Scheme 2. Proposed CH2-O bond photocleavage mechanism with CDs
injection of an electron.
The combined CV data of the CDs and N-methylpicolinium
esters indicate that there is a correlation between the redox
properties and the photoreactivity. Based on these data, all the
CDs have sufficiently negative potentials to act as reductants
and to inject an electron into the substrates, at least into the
chemical moiety involved in the less negative reduction process.
Among the investigated CDs, the a-NCDs possessed the highest
Conclusions
energy gap and the highest energy gaps of the CDs decreased in
the same order as their photocleavage activity: a-N-CDs > g-CDs
> g-N-CDs > a-CDs. A greater energy gap of the CDs – attributed
In summary, we have provided a proof-of-concept for the use
of inexpensive and readily available bio-based CDs as sensitizers
to the presence of free molecular-like fluorophores that ensure
to trigger a novel photosynthetic protocol for the deprotection
a high PET efficiency of the CDs – implies a greater energy of the
of N-methyl-4-picolinium esters. The origin of photo-electron
excited electron generated upon irradiation, as already
transfer is not fully understood yet and cannot be attributed to
observed previously.[14] This fact justifies the higher
photocleavage efficiency towards the N-methylpicolinium
any of the single different moieties of the CDs. Nonetheless,
these complex systems promote a very efficient reductive
esters. Similar considerations explain the reactivity of
methylene-oxygen photocleavage and enable a versatile,
substrates 1a-e: the energy of the LUMO reflects the energy of
inexpensive, robust and highly selective photodeprotection of a
the radical anion formed by photoinduced single electron
series of esters of carboxylic- and amino-acids representing a
transfer from CDs into the methylpicolinium ester. A higher
greener alternative to the currently used metal complexes and
LUMO energy of the methylpicolinium ester implies that the
organic dyes. We have also demonstrated how the structure
transfer of the electron requires more energy and thus that the
and electrochemical properties of CDs can influence the
ester is less prone to be reduced. However, in this case, once
photocleavage efficiency. In general, the presence of nitrogen
the electron is transferred from the CDs, its radical is more
in the amorphous structure of CDs of CDs makes the reduction
reactive. Conversely, easier reduction implies a more stable
potential of the CDs more negative, while the energy gaps
radical ion and a slower methylene-oxygen bond cleavage. In
increase, thereby improving photogeneration of reactive
the present case, the octanoic ester 1a, has a higher energy
LUMO and is more difficult to reduce, but once it is reduced it
readily undergoes photocleavage. On the other hand, the p-
nitrobenzoic ester 1e is easily reduced, and thus it is less prone
electrons and photoreduction efficiency. We have also shown
how the reduction potential of the substrates determines ease
of photocleavage.
to undergo the bond-scission process.
The results discussed above allowed to propose a CH2-O
Acknowledgements
photocleavage mechanism (Scheme 2). In an initial step, the
Mr. Stefano Perin and Mr. Andrea Turino are acknowledged for
CDs and the methylpicolinium esters 1a-1e approach one-
preliminary experimental assistance. E.A. acknowledges
another by electrostatic interactions forming a ground-state
Regione Veneto POR - FSE 2014-2020 (Grant no. 2120-31-2216-
supramolecular complex. Formation of such a complex was
2016) for a post-doctoral scholarship.
previously confirmed by the static quenching occurring
between the a-CDs, g-CDs, a-N-CDs, and g-N-CDs and methyl
viologen, observed by PL quenching data and ζ-potential
Conflicts of interest
There are no conflicts to declare.
measurements.[14] In the second step, the CDs are promoted to
the excited state (CDs*) by irradiation, followed by single
electron transfer to the methylpicolinium ester via an exergonic
(Eonset red= -1.60 -1.94 V vs Ag|AgCl for CDs and Eonset red = - 0.65
References
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4 | J. Name., 2012, 00, 1-3
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