Covalent Triazine Framework as an Efficient Photocatalyst for Regeneration of NAD(P)H and Selective Oxidation of Organic Sulfide

Article abstract of DOI:10.1111/php.13504

Covalent triazine frameworks (CTFs), belonging to the super-family of covalent organic frameworks, have attracted significant attention as a new type of photosensitizer due to the superb light-harvesting ability and efficient charge transfer originating f

Full text of DOI:10.1111/php.13504

Photochemistry and Photobiology, 20**, **: *–*
Research Article
Covalent Triazine Framework as an Efficient Photocatalyst for
Regeneration of NAD(P)H and Selective Oxidation of Organic Sulfide
Surabhi Chaubey1, Rajesh K. Yadav1* , Santosh K. Tripathi2, Bal Chandra Yadav3, Sudhir N. Singh4
and Tae Wu Kim5*
1Department of Chemistry and Environmental Science, Madan Mohan Malaviya University of Technology, Gorakhpur, India
2Defence Materials Stores and Research & Development Establishment (DMSRDE), Kanpur, India
3Nanomaterials and Sensors Research Laboratory, Department of Physics, Babasaheb Bhimrao Ambedkar University,
Lucknow, India
4Department of Humanities & Management Science, Madan Mohan Malaviya University of Technology, Gorakhpur, India
5Department of Chemistry, Mokpo National University, Muan-gun, Jeollanam-do, Republic of Korea
Received 8 April 2021, revised 20 July 2021, accepted 10 August 2021, DOI: 10.1111/php.13504
ABSTRACT
adenine dinucleotide (NADH) cofactor or its phosphorylated
form (NADPH) has been fascinated for the various applications
such as in the synthesis of alcohol, and the photoreduction of
CO2 (3,4). To do so, the artificial photocatalytic system which is
based on the natural photosynthetic system could be an efficient
method.
Covalent triazine frameworks (CTFs), belonging to the
super-family of covalent organic frameworks, have attracted
significant attention as a new type of photosensitizer due to
the superb light-harvesting ability and efficient charge trans-
fer originating from the large surface area. However, the
wide optical band gap in CTFs, which is larger than 3.0 eV,
hinders the efficient light harvesting in the visible range. To
overcome this limitation, we developed the new type CTFs
photocatalyst based on the donor–acceptor conjugation
In natural plants, light stimuli can be successfully converted
to the chemical energy stored in the solar chemicals through the
multiple transfers of photoexcited electrons (5). In natural photo-
synthetic system, photosystem I (PS-I) is an integral membrane
protein complex that uses solar light energy to mediate the elec-
tron transfer from plastocyanin (Pc) to ferredoxin (Fd). The
photo-generated electrons ultimately arrive at the ferredoxin–
NADP+ reductase (FNR) enzyme where the reduction from NAD
(P)+ to NAD(P)H occurs and results in storing solar energy as a
type of chemical bond (5). Recently, many researchers have
actively implanted such photoinduced sequential electron trans-
fers in natural photosynthetic system to the artificial photocataly-
sis for the regeneration of NAD(P)H cofactor (2–4,6). The
various photocatalysts including organic semiconductor (7–9),
carbon-based low-dimensional materials (10–13), inorganic semi-
conductor (14–16) and so on have been employed for the regen-
eration of redox-active cofactors. Some researchers also
developed a metal oxide-based nanosheet as a photocatalyst
backbone for the photocatalytic applications. For example, TiO2-
based nanosheets with the huge number of Ti3+ sites were
employed for the photocatalytic reduction of CO2 to CH4 (17).
However, such types of materials have some limitations includ-
ing fast recombination of photoinduced electron–hole pairs and
narrow spectral window in view of photon absorption (18),
which is detrimental to improve the solar energy conversion effi-
ciency. To overcome this limitation, many experimental efforts
have been focused on the uses of polymer networks or reticular
structures such as covalent organic frameworks (COFs) because
the optoelectronic properties of COFs are readily tuned by the
chemical modifications of building blocks such as by introducing
external stimulus-responsive materials (19–24). Recently, COFs,
are used as catalysts, due to their highly convenient skeleton
structures, structural flexibility and diversity. In addition, some
scheme
by
using
melamine
(M)
and
2,6-
diaminoanthraquinone (AQ) as monomeric units. The
melamine-2,6-diaminoanthraquinone-based covalent triazine
frameworks (M-AQ-CTFs) photocatalyst shows the excellent
light-harvesting capacity with high molar extinction coeffi-
cient, and the suitable optical band gap involving the internal
charge transfer character. Combination of M-AQ-CTFs and
artificial photosynthetic system including the organometallic
rhodium complex, acting as an electron mediator, exhibited
the excellent photocatalytic efficiency for the regeneration of
the nicotinamide cofactors such as NAD(P)H. In addition,
this photocatalyst showed the high photocatalytic efficiency
for the metal-free aerobic oxidation of sulfide. This study
demonstrates the high potential of CTFs photocatalyst with
the donor–acceptor conjugated scheme can be actively used
for artificial photosynthesis.
INTRODUCTION
Solar energy is a green and sustainable energy source and has
attained more attention in the area of photocatalysis (1–4).
Developments of efficient and sustainable technologies for the
utilization of solar energy have been implemented by many
researchers. As one of promising ways, the conversion of solar
energy into the reduced solar chemicals such as nicotinamide
*Corresponding authors email: rajeshkr_yadav2003@yahoo.co.in (Rajesh K.
Yadav); twkim@mokpo.ac.kr (Tae Wu Kim)
© 2021 American Society for Photobiology
1

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Surabhi Chaubey et al.
distilled water followed by acetone. The resultant products were dried in
the oven during overnight at 70°C. All the steps involved in synthetic
procedure are summarized in Scheme 1.
researchers used COFs in different fields for example in hetero-
geneous catalysis, in electrochromic (EC) devices, as an elec-
trode for rechargeable batteries and as light-emitting materials
(25–29).
Photocatalytic reactions for the regenerations of NAD(P)H cofactors.
The photo-driven regeneration of NAD(P)H cofactors were conducted by
the irradiation of solar light (k > 420 nm) on the quartz cuvette reactor.
The reaction mixture contains 248 lL of NAD(P)+ with the
concentration of 0.4 mM, 124 lL of organometallic rhodium complex
with the concentration of 0.2 mM, 310 lL of ascorbic acid (AsA) with
the concentration of 0.1 M, and 31 lL of M-AQ-CTFs photocatalyst with
the concentration of 10 lM. All the components were dissolved in 0.1 M
sodium phosphate buffer with pH 7.0. All the reactions were performed
under the nitrogen atmosphere at the room temperature. The
photocatalytic efficiencies were estimated by tracking the change in the
absorbance at 340 nm using the UVÀvisible spectrometer.
Among the various types of COFs, the covalent triazine
frameworks (CTFs) belong to the super-family of COFs which
have triazine units as a constituent in their polymeric structure
(30). Moreover, CTFs enhance the charge transport and exciton
separation, which are favored conditions for catalysis (31). CTF
nanocomposites have been widely used in the practical applica-
tions of catalytic reactions (32–34), gas storages (35,36) and
energy storages (37) owing to the high chemical and thermal sta-
bility originating from the presence of aromatic C=N linkers in
the triazine unit (38). Jiang et al. reported that CTF-0 with the
conjugation between triazine and heteroatom-incorporated ben-
zene units has an optical band gap around 3.0 eV and can be
served as the photocatalyst for hydrogen production (39). Never-
theless, to improve the photocatalytic efficiency, CTFs with the
lower optical band gap to cover the visible spectral range are
desired.
In this work, we reported the new type of CTFs based on the
donor–acceptor conjugation scheme and then demonstrated the
photocatalytic performance of newly designed CTFs acting as
the primary light-harvesting module for the regeneration of NAD
(P)H cofactors. The two-dimensional (2D) CTFs were synthe-
sized by the poly-condensation between melamine molecules (M)
and 2,6-diaminoanthraquinone (AQ) as building units. The cova-
lent conjugation between two units with the different electron
affinities in M-AQ-CTFs facilitates the lowering optical band
gap relative to the CTF-0, and superb chemical stability com-
monly observed in other CTFs. Nowadays, organometallic rho-
dium complex-based catalysis for cofactor regeneration has
fascinated a big attention by the researchers (40). The artificial
photosynthetic module including M-AQ-CTFs, acting as the pri-
mary photosensitizer, and the electron mediator of rhodium com-
plex showed the excellent charge separations through the
efficient exciton splitting in M-AQ-CTFs and exhibited the
regeneration yields of 73.1% for NADH and 86.0% for NADPH.
In addition, we demonstrated that M-AQ-CTFs can be used as
the photocatalyst for the oxidation reactions, which have great
importance in the fields of agro-chemical and pharmaceutical
industries (41,42). The synthesized COFs photocatalyst in the
aerobic condition catalyzed the oxidation of sulfide to sulfoxide
with the high yield of 99% and the superb selectivity of 99%.
Our results give a prospect on the potential of M-AQ-CTFs as
the highly efficient and eco-friendly CTFs photocatalysts facili-
tating the direct utilization of solar energy.
Selective oxidation of dibenzyl sulfide to dibenzyl sulfoxide. In a dried
Schlenk tube, the dibenzyl sulfide (10 mM) and M-AQ-CTFs
photocatalyst (5 mg) were mixed in 4 mL of methanol under the oxygen-
purged condition. After that, the reaction mixture was stirred at room
temperature under the irradiation of solar light (k > 420 nm) and the
oxygen atmosphere for 8 h. The completion of the reaction was checked
by the thin layer chromatography. After that, the reaction mixture was
washed with the high-concentration solution of NaCl dissolved in the
distilled water. The aqueous medium was further re-extracted with ethyl
acetate and then were dried and concentrated by rotary evaporation. After
that, the obtained residual from the rotary evaporation was purified by
the silica gel column chromatography. The production yield of dibenzyl
sulfoxide was 99%, and the formation of final product was characterized
by 1H NMR and mass spectroscopy.
RESULTS AND DISCUSSION
Development of an artificial photosynthetic system
In natural plants, solar energy is captured on the photosynthetic
pigments such as chlorophyll and then the photo-generated elec-
trons undergo the sequential electron transfers from the photosys-
tem II to the final catalytic center, as shown in Scheme 2a. The
reaction mechanism in the natural photosynthetic system was
implanted in the artificial photosynthetic system for the regenera-
tion of valuable NAD(P)H cofactors, shown in Scheme 2b (43).
We designed and developed the artificial photosynthetic system
that utilizes the solar energy for the production of NAD(P)H
cofactors by using the highly efficient photosensitizer. In this
scheme, the anthraquinone-based covalent triazine frameworks
(M-AQ-CTFs) acts as a primary photosensitizer to generate the
photoinduced electrons, which are transferred to Rh-complex
([Cp*Rh(bpy)Cl]Cl) acting as an electron mediator (44). In the
reduced Rh-complex, the photoinduced electrons are used for the
regenerations of NAD(P)H cofactors.
Characterization of covalent triazine framework
photocatalyst
MATERIALS AND METHODS
To synthesize the M-AQ-CTFs photosensitizer, we used the
direct poly-condensation of melamine (M) and 2, 6-
diaminoanthraquinone (AQ) units (Scheme 1). As shown in Fig-
Materials. Melamine, 2,6-diaminoanthraquinone, ferrous sulfate
(FeSO4Á7H2O), potassium permanganate (KMnO4), NAD+, NADP+,
ascorbic acid (AsA), dibenzyl sulfide, (pen-tamethylcyclopentadienyl)
rhodium(III) dichloride dimer, 2, 2’-bipyridyl and all the solvents were
purchased from Sigma-Aldrich and were used without further
purifications.
Synthesis of covalent triazine framework photocatalyst. Melamine
(1.00 g) and 2,6-diaminoanthraquinone (2.83 g) were dissolved in 50 mL
of DCM. Thereafter, the well mixed solid mixture of FeSO4Á7H2O
(1.102 g) and KMnO4 (0.313 g) was added into the above solution. The
reaction mixture was refluxed for 16 h at 35°C. After the completion of
reaction, the reacted solution was cool down. After that, the reaction
mixture was filtered and was washed with the copious amount of
ꢀ
ure S1, the structure of M-AQ-CTFs has the pore size of ~28 A,
which is supported by the theoretically modeled structure. Pow-
der X-ray diffraction (PXRD) measurements were performed by
X-ray diffractometer. Among the Bragg peaks shown in Fig-
ure S2, the prominent peaks around 15.8° and 26.9° can be
assigned to (200) and (001) facets, respectively (45). The crys-
talline nature of M-AQ-CTFs with the porous structure can be
supported by the observed Bragg patterns in the PXRD data.
The results from the thermogravimetric analysis (TGA) show that

Photochemistry and Photobiology
3
Scheme 1. Synthesis of melamine-2,6-diaminoanthraquinone (M-AQ) based covalent triazine frameworks (M-AQ-CTFs).
the M-AQ-CTFs composite has the superb thermal stability with-
out significant decomposition up to 350°C shown in Figure S3.
This high thermal stability is likely to mainly originate from the
reticular structure of M-AQ-CTFs that is confirmed by the PXRD
measurement. In addition, we performed the nuclear magnetic
resonance (NMR) measurements for the structural characteriza-
tion. Solid-state cross-polarized magic angle spinning 13C NMR
spectroscopy was implemented by using 400 MHz solid-state
NMR spectrometer (Avance III HD Bruker) at KBSI Western
Seoul center. For the NMR measurement, we used the experi-
mental parameters with the delay time of 3 s, and the basic fre-
quency of 100.613 MHz. The powder sample was packed in
4 mm zirconia rotors and was subjected to a spinning speed of
14 kHz. In the measured data of M-AQ-CTFs shown in Fig-
ure S4, the formation of conjugation between melamine and 2,6-
diaminoanthraquinone exhibited the clear resonance peaks around
181, 165, 156, 136, 128, 123 and 120 ppm. Especially, the peaks
of 136 ppm can be assigned to the carbons in benzene, cova-
lently conjugated with nitrogen (N-C = C). The peaks around
165 ppm mainly originate from the carbons in the triazine moi-
eties covalently linked with the ─N = N─ groups. The results
show that the covalent ligations between melamine and 2,6-
diaminoanthraquinone are well conserved. We also measured the
Fourier transform infrared (FTIR) spectra of AQ monomer and
M-AQ-CTFs composite. According to the previous studies of
AQ-based covalent organic frameworks (COFs) (46), the promi-
nent peak at 1292.6 cmÀ1 in M-AQ-CTFs can be assigned as the
stretching mode of ─C─N─. Based on the IR features of azo-
compounds (47), the peak at 1567.9 cmÀ1 in M-AQ-CTFs is
clearly observed and corresponds to the stretching mode of
─N = N─. This IR band, corresponding to the group of
─N = N─, is absent in the spectrum of AQ monomer. In addi-
tion, M-AQ-CTFs shows the peak at 841.8 cmÀ1, which is
attributed to the characteristic mode of triazine unit (18). The
results from the FTIR spectroscopy further support the successful
formation of covalent conjugation between the building blocks in
M-AQ-CTFs.
The morphological structure of AQ and M-AQ-CTFs compos-
ite was investigated by using the field emission scanning electron
microscopy (FESEM) shown in Figure S6. The FESEM image
of AQ monomer shows the typical agglomeration morphology,
which commonly observed in the AQ composite (48). In con-
trast, the FESEM image of M-AQ-CTFs clearly shows the rod-
like morphology, which is similar with the morphology of azine
COFs with the hexagonal pores (49). The change of morphology
from the agglomeration in AQ monomer to rod shape indicates
the formation of well-organized periodic structure in M-AQ-
CTFs, resulting in the high chemical stability. Furthermore, the
zeta potential values of AQ monomer and M-AQ-CTFs were
found to be À0.703 and À4.25 mV, respectively (Figure S7).
Compared with the AQ monomer, the more negative value of
the zeta potential value in the M-AQ-CTFs is likely to originate
from the high chemical stability due to the formation of
─N = N─ bond (50). Meanwhile, the distributions of particle
sizes of AQ monomer and M-AQ-CTFs composite were investi-
gated by using dynamic light scattering (DLS), as shown in Fig-
ure S8. The M-AQ-CTFs has the average size with the value of
450 nm, which is smaller than the average size of AQ monomer
with 1400 nm. The size distribution of M-AQ-CTFs composite
shows the highly homogeneous crystalline state in terms of dis-
tribution of particle size relative to that in AQ monomer. The
homogeneous distribution of M-AQ-CTFs facilitates the efficient
intermolecular charge separation between M-AQ-CTFs composite
and electron mediator during the artificial photosynthetic system
(50,51). We note that the absolute size of the synthesized com-
posite is highly dependent on the reaction time for the growth of
nanocomposite. In our study, we only focused on making the
microcrystals that have the uniform distribution of particle size
because those microcrystals will be pertinent to facilitate the
through-space electron transfer on the surface of photocatalyst.

4
Surabhi Chaubey et al.
Scheme 2. Pictorial representation of an artificial photosynthetic system mimicking the natural system. (a) Regeneration of NADPH cofactor in the natu-
ral photosynthesis with the Z-scheme nature. (b) Regeneration of NAD(P)H cofactors in the artificial photosynthesis where ascorbic acid (AsA) acts as a
sacrificial agent, ET stands for through-space electron transfer, ADPR stands for adenosine diphosphate ribose, and ADPR1 stands for adenosine diphos-
phate ribose with one extra phosphate group.
To estimate the optoelectronic properties of the newly synthe-
sized CTFs, we measured the UV-visible spectra of AQ mono-
mer and M-AQ-CTFs composite, as shown in Fig. 1. The M-
AQ-CTFs shows the broad absorption spectrum in the visible
region compared with that of AQ monomer (52). Importantly,
the covalent conjugations of M and AQ units induce the promi-
nent increase of absorption coefficient in the spectral window
<400 nm that facilitates the efficient light-harvesting ability. We
performed the quantum calculation with time-dependent density
functional theory (TD-DFT) in order to investigate the optical
characteristics in the M-AQ-CTFs composite. The computational
details are described in the supporting information (SI). Fig-
ure S9 shows the molecular orbitals and its energetics including
highly occupied molecular orbital (HOMO) and lowest

Photochemistry and Photobiology
5
Table 1. Comparison for the photocatalytic activities of AQ monomer
and M-AQ-CTFs in terms of the regeneration yield of 1,4-NADH cofac-
tor. After the reaction time of 150 min, the M-AQ-CTFs shows the high
catalytic yield with 73.1%, which is much higher than that in the AQ
monomer.
Yield of NADH (%)
Photocatalytic activity*
Dark reaction
Time (min)
M-AQ-CTFs
AQ
0
30
60
90
120
150
0.00
0.00
9.64
40.5
67.5
73.1
0.00
0.00
4.82
9.24
17.6
24.9
Light activation
*Experimental condition: The reaction medium contains 10 lM photocat-
alyst, 0.4 mM NAD+/NADP+, 0.2 mM rhodium complex and 0.1 M ascor-
bic acid (AsA) dissolved in 0.1 M sodium phosphate buffer (NaH2PO4–
Na2HPO4, pH ~7.0) under the nitrogen-purged atmosphere at room tem-
perature.
Figure 1. UV-visible absorption spectra of AQ (red line) and M-AQ-
CTFs (blue line).
unoccupied molecular orbital (LUMO). The energy difference
between HOMO and LUMO states is 3.387 eV. The HOMO-to-
LUMO transition accompanies the spatial dislocations of electron
densities from melamine moieties to adjacent AQ moieties,
which implies the existence of intramolecular charge transferred
(ICT) state. The results from the TD-DFT calculations show that
the ICT character in M-AQ-CTFs composite, which is absent in
the AQ monomer, facilitates the efficient splitting of photoin-
duced charge carriers, so-called exciton.
condition. We noted that the regeneration yields with AQ mono-
mer showed 24.9% for NADH and 29.7% for NADPH. This
observation implies that M-AQ-CTFs photocatalyst has the high
catalytic efficiency relative to the AQ monomer owing to the
efficient charge transfer originating from the covalent conjugation
between two different building units. Meanwhile, to check the
stability of the M-AQ-CTFs photocatalyst during the regenera-
tion of NAD(P)H, the recycling experiments were performed by
re-using the same photocatalyst for five successive runs (i.e. four
recycles) under the same reaction conditions. As shown in Fig-
ure S12, the photocatalytic efficiency was kept to be almost con-
stant during the recycling experiment, indicating the high
chemical stability of M-AQ-CTFs.
Regeneration of 1,4-NAD(P)H using the artificial
photosynthetic system
This study focused on the regeneration of enzymatically active
1,4-NAD(P)H cofactors from its oxidized forms NAD(P)+. As
shown in Figure S10, the direct electrochemical reduction of
NAD(P)+ is likely to cause the unselective protonation as well as
the radical coupling, which can generate the various types of
NAD isomers, including both the enzymatically active and inac-
tive forms (53). To avoid the regeneration of unwanted products
such as the inactive isomers, we employed the organometallic
rhodium complex acting as an electron mediator, leading to the
regeneration of enzymatically active isomers under the irradiation
of solar light (53). The photochemical regenerations of 1,4-NAD
(P)H cofactors were performed at the ambient temperature in an
inert atmosphere under artificial solar light illumination
(k > 420 nm). First, the reaction was initiated in the absence of
solar light for about 30 min, so-called the dark incubation.
According to the Tables 1 and 2, the dark condition could not
produce any reduced form of 1,4-NAD(P)H from its oxidized
forms of NAD(P)+. In the presence of solar light, the increased
production yield was observed as a function of reaction time. To
quantitatively estimate the production yield of 1,4-NAD(P)H, we
monitored the absorbance at 340 nm in the UV-visible spectrum
as shown in Figure S11. For the calculation of catalytic effi-
ciency, it was considered that the molar extinction coefficient of
Reaction mechanism during the regeneration of NAD(P)H
cofactors
A plausible mechanism for the regeneration of NAD(P)H cofac-
tors is described in Scheme 3. In an aqueous buffer medium, the
cationic form of Rh-complex, named as A, can be readily hydro-
lyzed and form the water-coordinated complex [Cp*Rh(bpy)
Table 2. Comparison for the photocatalytic activities of AQ monomer
and M-AQ-CTFs in terms of the regeneration yield of 1,4-NADPH cofac-
tor. After the reaction time of 150 min, the M-AQ-CTFs shows the high
catalytic yield with 86.0%, which is much higher than that in the AQ
monomer.
Yield of NADH (%)
Photocatalytic activity*
Dark reaction
Time (min)
M-AQ-CTFs
AQ
0
30
60
90
120
150
0.00
0.00
25.7
51.4
73.9
86.0
0.00
0.00
6.43
13.6
17.6
29.7
Light activation
À1
NAD(P)H at 340 nm is 6.22 mM cmÀ1 (53). Based on the cal-
*Experimental condition: The reaction medium contains 10 lM photocat-
alyst, 0.4 mM NAD+/NADP+, 0.2 mM rhodium complex, and 0.1 M
ascorbic acid (AsA) dissolved in 0.1 M sodium phosphate buffer
(NaH2PO4–Na2HPO4, pH ~7.0) under the nitrogen-purged atmosphere at
room temperature.
culation of catalytic efficiency, the regeneration yields with the
reaction time of 2 h are 73.1% for NADH and 86.0% for
NADPH. As a control experiment, we tested the catalytic perfor-
mance by employing the AQ monomer in the exact same

6
Surabhi Chaubey et al.
Scheme 3. Plausible mechanism for the regeneration of NAD(P)H cofactors using M-AQ-CTFs photocatalyst.
(H2O)]2+, named as B. The B intermediate consequently reacts
with the formate (HCOOÀ), which is produced from an ascorbic
acid (AsA) acting as a hydride source, via the b-hydride elimina-
tion process (54). This reaction produces the rhodium hydride
complex of [Cp*Rh(bpy)(H)]+, named as C, and leads to the loss
of CO2. The M-AQ-CTFs photocatalyst participates in donating
the external electrons to the rhodium hydride complex, generat-
ing the reduced intermediate, named as D. At this stage, NAD
(P)+ can be coordinated with the D intermediate, facilitating the
transfer of hydride to form the region-selective NAD(P)H
cofactors (55).
which indicates the photo-generation of 1O2 from 3O2 (56,57).
Based on this catalytic test, we applied the M-AQ-CTFs photo-
catalyst for the selective oxidation of dibenzyl sulfide to dibenzyl
sulfoxide. The compound of dibenzyl sulfoxide produced from
the photocatalysis was characterized by the 1H NMR spec-
troscopy and mass spectrometry shown in Figures S15 and S16.
The details are described in the SI. To find out the optimal cat-
alytic condition, we screened various reaction conditions summa-
rized in Table 3. The solvents of EtOH, CH3CN, DMSO and
DMF showed the relatively low catalytic yields whereas CH3OH
exhibited the highest yield and selectivity with ~99% under the
irradiation of solar light for 8 h. This result suggests that the
chemical properties of solvents may play a significant role in the
photocatalytic oxidation of dibenzyl sulfide. Based on the previ-
ous study (58), it was reported that the increase in the polarity of
solvent brings about the improved catalytic efficiency for the
photocatalytic oxidation of thioanisole because the physical
Photocatalytic oxidation of dibenzyl sulfide
We also tested the M-AQ-CTFs photocatalyst as a quencher to
generate singlet oxygen (1O2) from triplet oxygen (3O2). The
UV–visible spectra of M-AQ-CTFs photocatalyst dissolved in
DMF were recorded during the irradiation of solar light as a
function of reaction time as shown in Figure S13. The decrease
in the absorbance was observed around the near-UV region,
1
quenching of O2 is more efficient in the low polar solvents. As
a control experiment, we tested the photo-oxidation performance
by employing the AQ monomer in the solvent of CH3OH. From

Photochemistry and Photobiology
7
Table 3. Optimizationof the photocatalytic oxidation of dibenzyl sulphide.
Entry
Photocatalyst
Solvent
Yield (%)
Selectivity (%)
1
2
3
4
5
6
M-AQ-CTFs
M-AQ-CTFs
M-AQ-CTFs
M-AQ-CTFs
M-AQ-CTFs
AQ
EtOH
62
34
26
99
7
65
39
29
99
8
CH3CN
DMSO
CH3OH
DMF
CH3OH
28
32
this experiment, we determined 28% yield and 32% selectivity
of the product. To check the reusability and stability of the M-
AQ-CTFs photocatalyst during the photocatalytic oxidation, we
also performed the additional experiments by re-using the same
photocatalyst for five successive runs (i.e. four recycles) under
the same reaction conditions. As shown in Figure S14, the
results suggest that the production yield of dibenzyl sulfoxide is
almost constant, which confirms that M-AQ-CTFs photocatalyst
is highly stable during the recycling.
The photocatalytic oxidation of dibenzyl sulfide to dibenzyl sul-
foxide generally follows two main mechanisms; (1) the oxidation
of singlet oxygen through the energy transfer and (2) the radical
pathway through electron transfer. Initially, M-AQ-CTFs photo-
catalyst, named as PC, absorbs a photon from the solar light,
forming the singlet excited state, named as PC*. Subsequently,
PC* directly converts the triplet oxygen to the singlet state. The
photo-generated singlet oxygen (1O2) can be reduced by the lone
pair electron of dibenzyl sulfide, named as A. This reduction can
form the dibenzyl sulfide radical cation, named as B, as well as
the superoxide radical anion. The B intermediate can react with
the superoxide radical anion, generating the C intermediate. The
C intermediate further reacts with another molecule of A and
finally forms the product, named as D (59).
Reaction mechanism during the photocatalytic oxidation
The reaction mechanism during the selective oxidation of diben-
zyl sulfide to dibenzyl sulfoxide is summarized in Scheme 4.
Scheme 4. Proposed mechanism during the photocatalytic oxidation of dibenzyl sulfide to dibenzyl sulfoxide.

8
Surabhi Chaubey et al.
Figure S13. Linear absorption spectra recorded for the photo-
catalytic oxygen quenching. For this measurement, the M-AQ-
CTFs photocatalyst was dissolved in DMF solution.
Figure S14. Reusability test of M-AQ-CTFs photocatalyst
during the photocatalytic transformation of dibenzyl sulfide to
dibenzyl sulfoxide.
CONCLUSION
In summary, we synthesized a highly efficient, inexpensive and
environmentally friendly M-AQ-CTFs photocatalyst. The synthe-
sized M-AQ-CTFs photocatalyst are used for the regeneration of
nicotinamide cofactors, 1,4-NAD(P)H. By using M-AQ-CTFs,
we achieved the catalytic yield of 73.1% and 86.0% for the pro-
ductions of 1,4-NADH and 1,4-NADPH, respectively. In addi-
tion, by using the separate experiment, we also demonstrated the
ability of the M-AQ-CTFs photocatalyst for the sulfoxidation
reaction. M-AQ-CTFs photocatalyst was able to oxidize the
organic sulfides (dibenzyl sulfide) to the sulfoxides (dibenzyl sul-
foxide) with the yield and selectivity of 99%. It was expected
that the high photocatalytic efficiency in M-AQ-CTFs photocata-
lyst originates from the excellent light-harvesting ability and the
efficient charge migration in the interfacial region. The compre-
hensive study such as the measurement with the time-resolved
spectroscopy (60) will be required to unveil the electronic
dynamics in the photocatalytic reaction, and thus, the detailed
information about the migrations of photo-generated charge carri-
ers will allow us to design the more efficient catalytic system.
Figure S15. 1H NMR spectrum of dibenzyl sulfoxide pro-
duced from the photocatalytic reaction.
Figure S16. Mass spectrum of dibenzyl sulfoxide produced
from the photocatalytic reaction.
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SUPPORTING INFORMATION
Additional supporting information may be found online in the
Supporting Information section at the end of the article:
Figure S1. Theoretically modeled structure of M-AQ-CTFs
composite.
Figure S2. Powder XRD patterns of M-AQ-CTFs composite.
Figure S3. Results from the thermogravimetric analysis
(TGA) for the compounds of melamine and M-AQ-CTFs.
Figure S4. Results from solid-state CP/MAS 13C-NMR of
M-AQ-CTFs.
Figure S5. FTIR spectra of AQ (red line) and M-AQ-CTFs
(blue line).
Figure S6. Field emission scanning electron microscopic (FE-
SEM) studies of (a) M-AQ-CTFs composite and (b) AQ mono-
mer.
Figure S7. Zeta potential graphs of (a) AQ monomer with the
value of -0.703 mV and (b) M-AQ-CTFs composite with the
value of -4.25 mV.
Figure S8. Particle size distribution of (a) AQ monomer and
(b) M-AQ-CTFs composite.
Figure S9. Energetics of molecular orbitals (MOs).
Figure S10. Productions of enzymatically active/inactive
NAD(P)H monomers and enzymatically inactive NAD(P)H
dimers during the electrochemical reductions of NAD(P)+.
Figure S11. Linear absorption spectra recorded for the regen-
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