Visible-light-driven photocatalytic activity of ZnO/g-C3N4 heterojunction for the green synthesis of biologically interest small molecules of thiazolidinones

Article abstract of DOI:10.1016/j.jphotochem.2020.112786

Designing a photocatalytic system with complementary properties including high surface area, high loading, extraordinary electronic properties, and easy separation for increases photocatalytic performance has remained a challenge in photocatalytic applications. Herein, an environmentally benign approach was developed to fabricate graphitic carbon nitride (g-C₃N₄) decorated with nanorods zinc oxide (ZnO). The photocatalytic activity of ZnO decorated on the g-C₃N₄ surface was developed in the synthesis of 1,3-thiazolidin-4-ones and bis-thiazolidinones under very mild and sustainable reaction conditions. The reaction can be carried out by utilizing visible light without the requirement of any additive and other external sources of energy. It was found that the proposed photocatalyst is a more facile, recyclable, large-scale application and provides some new insights into the stabilization of semiconductors for a variety of applications.

Full text of DOI:10.1016/j.jphotochem.2020.112786

Journal Pre-proof
Visible-light-driven photocatalytic activity of ZnO/g-C3N4 heterojunction
for the green synthesis of biologically interest small molecules of
thiazolidinones
Robabeh Mohammadi (Investigation) (Conceptualization)
(Methodology), Hassan Alamgholiloo (Investigation)
(Conceptualization) (Methodology) (Writing - original draft), Behnam
Gholipour (Investigation) (Conceptualization) (Methodology),
Sadegh Rostamnia (Supervision) (Writing - review and editing)
(Funding acquisition), Samad Khaksar (Writing - review and
editing), Mustafa Farajzadeh (Writing - review and editing),
Mohammadreza Shokouhimehr (Supervision) (Writing - review and
editing) (Formal analysis)
PII:
S1010-6030(20)30585-2
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112786
JPC 112786
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
7 May 2020
14 July 2020
15 July 2020
Please cite this article as: Mohammadi R, Alamgholiloo H, Gholipour B, Rostamnia S,
Khaksar S, Farajzadeh M, Shokouhimehr M, Visible-light-driven photocatalytic activity of
ZnO/g-C3N4 heterojunction for the green synthesis of biologically interest small molecules of
thiazolidinones, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112786

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Visible-light-driven photocatalytic activity of ZnO/g-C3N4 heterojunction for the green
synthesis of biologically interest small molecules of thiazolidinones
a
a
a,b
Robabeh Mohammadi, a Hassan Alamgholiloo, Behnam Gholipour, Sadegh Rostamnia,
*
Samad Khaksar, c Mustafa Farajzadeh, a Mohammadreza Shokouhimehr, d,*
a Organic and Nano Group (ONG), Department of Chemistry, Faculty of Science, University of
Maragheh, PO Box 55181-83111, Maragheh, Iran Tel:+98(421) 2276066; Fax: +98(421) 2276066;
b Organic and Nano Group (ONG), Department of Chemistry, Iran University of Science and Technology
(IUST), PO Box 16846-13114, Tehran, Iran
Email: srostamnia@gmail.com;rostamnia@maragheh.ac.ir
c School of Science and Technology, The University of Georgia, Tbilisi, Georgia
d Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul
National University, Seoul 08826, Republic of Korea. Email: mrsh2@snu.ac.kr
Graphical abstract
1

Highlight
 Fabrication of ZnO/g-C3N4 photocatalysts with exceptional activity is reported.
 A mild and green strategy for the synthesis of 1,3-thiazolidin-4-ones.
 40ꢀW domestic bulb is used because of nontoxicity and cost-effectiveness.
 Operational simplicity, durability, and large-scale application.
ABSTRACT:
Designing a photocatalytic system with complementary properties including high surface area,
high loading, extraordinary electronic properties, and easy separation for increases photocatalytic
performance has remained a challenge in photocatalytic applications. Herein, an environmentally
benign approach was developed to fabricate graphitic carbon nitride (g-C3N4) decorated with
nanorods zinc oxide (ZnO). The photocatalytic activity of ZnO decorated on the g-C3N4 surface
was developed in the synthesis of 1,3-thiazolidin-4-ones and bis-thiazolidinones under very mild
and sustainable reaction conditions. The reaction can be carried out by utilizing visible light
without the requirement of any additive and other external sources of energy. It was found that the
proposed photocatalyst is a more facile, recyclable, large-scale application and provides some new
insights into the stabilization of semiconductors for a variety of applications.
Keywords: Biologically interest small molecules;1,3-Thiazolidin-4-ones; Bis-thiazolidinones;
ZnO/g-C3N4; Visible light; Photocatalyst.
1- Introduction
Graphitic carbon nitride (g-C3N4) as a polymeric semiconductor has become a new research
hotspot and increased interest in the arena of solar energy and environmental remediation [1, 2].
2

Additionally, these polymeric nanostructures due to semi-conductivity, chemical robustness,
excellent physicochemical stability, and earth-abundant nature can be excellent candidates for
visible-light mediated photocatalytic processes [3]. Due to these properties, numerous applications
of these polymeric semiconductors in different fields have been explored including
nanoelectronics, [4] energy storage materials, [5] biosensors, [6] supercapacitors, [7]
photovoltaics, [8] photocatalytic, [9-15] and catalytic [16]. Incorporation of metal oxides in g-
C3N4 nanosheets as semiconductor materials has been explored with TiO2 [17], Fe2O3, [18] SnO2,
[19] CdS, [20] BiVO4, [21] Ag3PO4, [22] NiS, [23] ZrO2, [24] and ZnO [25] in photocatalytic
arena. Among metal oxides, ZnO with bandgap energy 3.37 eV [26] has been widely immobilized
on solid materials for catalytic/photocatalytic aims [27-29]. Due to its specific structure,
supporting ZnO on g-C3N4 sheets could provide strong interaction between support and metal
oxide for increase activity photocatalytic.
Thiazolidinones have been sought after by pharmacologists as an important compound in
medicinal chemistry. It is also employed as the inhibition of numerous targets such as HCV NS3
protease [30], β-lactamase [31], PMT1 mannosyl transferase [32], PRL-3 and JSP-1 phosphatases
[33]. Several methods have been reported for the synthesis of thiazolidinone-based heterocyclics [34-
38] as shown in Scheme 1 and previous methodologies comprised of the current study. However,
they suffer from multistep processes, harsh conditions, low yields, and long reaction times.
Siddiqui and co-workers reported an efficient and atom economic one-pot protocol for the
synthesis of 1,3-thiazolidin-4-ones under visible-light illumination [39]. Our group's recent
success is applying the catalytic application nanostructures such as HFIP/SBA-15, [40] β-
cyclodextrin, [41] γ-Fe2O3, [42] and κ-carrageenan/Fe3O4 [43] for the synthesis of thiazolidinones
skeletons. Thus, the development of new synthetic methods remains an attractive goal.
3

Generally, there is an immediate need for developing efficient and sustainable protocols for all
biological molecules under milder conditions. Based on the above considerations and previous
studies, [44-46] we report a simple and efficient method for the synthesis of ZnO/g-C3N4 and its
catalytic potential towards the preparation of thiazolidinones under visible light irradiation and
milder reaction conditions. To the best of our knowledge, the synthesis of thiazolidinone
compounds under visible light irradiation and milder reaction conditions by photocatalysts has not
been reported thus far.
2- Experimental
2.1. Materials and Methods
The materials and solvents used in the current work were obtained from commercial sources
without further purification. The FT-IR spectra were performed using a Perkin Elmer-Spectrum
Two with ATR probe. The morphologies of the samples were taken on FE-SEM, Zeiss-SIGMA
1
VP. The TEM images were recorded on Zeiss-EM 900. The H and 13C NMR spectra were
recorded on a Bruker AVANCE NMR spectrometers at 400 and 100 MHz, respectively.
2.2. Synthesis of g-C3N4
The nanosheets g-C3N4 was fabricated by thermal polymerization of melamine by terms of our
previous report [47] with some modifications. In a typical synthesis, 5.0 gr of melamine was put
o
in an alumina crucible with a cover and heated for 550 ℃ for 4 hours at a rate of 5 C/min. The
obtained yellow precipitate was cooled slowly at a rate of 1 oC/min to ambient condition.
4

2.3. Synthesis of ZnO/g-C3N4
0.5 g of g-C3N4 was dispersed in 50 mL of DI-H2O under ultrasonic irradiation for 30 min to obtain
g-C3N4 nanosheets. Then, 5.0 mL aqueous dispersion of g-C3N4 with 5.0 mL aqueous 0.1 M of
ZnSO4.7H2O was stirred for 30 min at room temperature. For growth ZnO nanorods on the g-C3N4
surface, the reaction mixture was kept at 373 K for six hours in the reflux condition. The obtained
white solid was purified with MeOH for several times and at dried room temperature. The loading
of 15 wt % of ZnO on the surface of g-C3N4 was confirmed by AAS.
2.4. General Procedure for the synthesis of 1,3-thiazolidin-4-ones.
The model reaction, the mixture of aniline (1 mmol), benzaldehyde (1 mmol), thioglycolic acid (1
mmol), PhMe (5 mL) as a solvent in 100 mg of ZnO/g-C3N4 catalyst in a 20 mL Schlenk tube was
added. The reaction mixture was exposed to visible light irradiation using 40 W domestic bulb for
12 hours. After the reaction is complete, the organic phase was extracted with ethyl acetate/diethyl
ether (9:1) and then dried over sodium sulfate (Na2SO4), and finally, the main product identifies
with 1H and 13C-NMR spectroscopy.
2.5. Data NMR.
After compounds are known and are previously reported in the literature [48].
1
2,3-diphenylthiazolidin-4-one: (Table 3, 4a): White solid, Yield = 97%. H NMR (400 MHz,
DMSO-d6), δH (ppm) = 3.87 (dd, J = 15.8 Hz & 1.6 Hz 1H), 6.13 (s, 1H), 7.11-7.38 (m, 10H), 13C
NMR (100 MHz, DMSO-d6): δC (ppm) = 33.53, 65.37, 125.61, 126.93, 127.06, 128.89, 128.91,
129.11, 137.29, 139.50, 170.94.
3-(4-hydroxyphenyl)-2-phenylthiazolidin-4-one: (Table 3, 4b): White solid, Yield = 68%. 1H NMR
(400 MHz, DMSO-d6), δH (ppm) = 3.91-4.07 (dd, J = 15.8 Hz & 1.6 Hz 1H), 5.94 (s, 1H), 6.36-
7.11 (m, 9H) 13C NMR (100 MHz, DMSO-d6): δC (ppm) = 34.08, 64.89, 116.13, 123.83, 127.00,
127.23, 128.61, 136.75, 139.92, 155.57, 170.33.
5

1
2-phenyl-3-(p-tolyl)thiazolidin-4-one: (Table 3, 4c): White solid, Yield = 78%. H NMR (400
MHz, DMSO-d6), δH (ppm) = 2.31 (s, 3H), 4.03-4.07 (dd, J = 14.8 & 4.2 Hz, 1H), 6.08 (s, 1H),
13
7.04-7.56 (m, 9H), C NMR (100 MHz, DMSO-d6): δC (ppm) = 20.65, 34.18, 65.02, 121.36,
127.02, 127.54, 128.83, 129.77, 130.85, 137.41, 140.11, 170.75.
1
2-phenyl-3-(pyridin-4-yl) thiazolidin-4-one: (Table 3, 4d): White solid, Yield = 97%. H NMR
(400 MHz, DMSO-d6), δH (ppm) = 3.91-4.11 (dd, J = 16.3 & 2.4 Hz, 1H), 6.72 (s, 1H), 7.25-7.49
13
(m, 7H), 8.46-8.48 (m, 2H), C NMR (100 MHz, DMSO-d6): δC (ppm)= 33.16, 62.48, 117.82,
125.59, 129.02, 129.39, 140.15, 145.17, 150.70, 171.76.
3-(2-aminoethyl)-2-(p-tolyl)thiazolidin-4-one: (Table 3, 4e): White solid, Yield = 57%. 1H NMR
(400 MHz, CDCl3), δH (ppm) = 2.39 (s, 3H), 2.55 (t, J = 4.8 Hz, 2H), 2.74-2.89 (m, 1H), 3.62-3.79
(m, 1H), 3.49-3.57 (dd, J = 12.2 Hz & 1.6 Hz 1H), 5.57 (s, 1H), 7.17-7.31 (m, 4H). 13C NMR (100
MHz, CDCl3): δC (ppm)= 21.27, 32.23, 41.03, 46.69, 63.67, 127.19, 129.84, 135.85, 139.47,
171.39.
1
3-(p-tolyl)-2-(p-tolylamino)thiazolidin-4-one: (Table 3, 4f): White solid, Yield = 85%. H NMR
(400 MHz, DMSO-d6), δH (ppm) = 2.26 (s, 3H), 3.60-3.69 (dd, J = 16.6 Hz & 2.6 Hz, 1H), 7.13
(d, J = 10.9 Hz, 2H), 7.48 (d, J = 11.2 Hz 2H). 13C NMR (100 MHz, DMSO-d6): δC (ppm)= 20.93,
43.65, 119.75, 129.63, 132.99, 136.74, 166.91.
3-(4-chlorophenyl)-2-phenylthiazolidin-4-one: (Table 3, 4g): White solid, Yield= 88%. 1H NMR
(400 MHz, CDCL3), δH (ppm) = 3.70-3.74 (dd, J = 14.8 Hz & 1.4 Hz, 1H), 6.11 (s, 1H), 7.35-7.63
(m, 9H). 13C NMR (100 MHz, DMSO): δC (ppm) = 43.65, 121.25, 127.58, 129.16, 138.17, 167.32.
3-benzyl-2-phenylthiazolidin-4-one: (Table 3, 4h): White solid, Yield= 93%. 1H NMR (400 MHz,
DMSO-d6), δH (ppm) = 3.35-3.38 (dd, J = 10.28 Hz & 1.2 Hz, 1H), 3.95-4.01 (m, 2H), 5.57 (s,
1H), 7.09-7.38 (m, 10H). 13C NMR (100 MHz, DMSO-d6): δC (ppm)= 32.2, 46.11, 62.29, 127.43,
127.94, 129.15, 129.09, 129.27, 129.39, 136.23, 140.36, 171.37.
3-(4-chlorophenyl)-2-phenylthiazolidin-4-one: (Table 4, 4i): White solid, Yield = 95%. 1H NMR
(400 MHz, DMSO-d6), δH (ppm) = 3.81-3.92 (dd, J = 16.4 Hz, 1H), 6.61 (s, 1H), 7.14-7.48 (m,
14H). 13C NMR (100 MHz, DMSO-d6): δC (ppm) = 34.27, 65.11, 123.26, 126.42, 127.38, 129.11,
139.04, 139.57, 170.70.
6

2,2' (1,4-phenylene)bis(3-(p-tolyl)thiazolidin-4-one): (Table 4, 4j): White solid, Yield = 94%. 1H
NMR (400 MHz, DMSO-d6), δH (ppm) = 2.22 (s, 3H), 3.80-3.92 (dd, J = 16.4 Hz, 1H), 6.39 (s,
1H), 7.04-7.21 (m, 8H), 7.29 (s, 4H). 13C NMR (100 MHz, DMSO-d6): δC (ppm)= 20.98, 33.01,
63.54, 125.90, 127.73, 129.66, 135.40, 136.31, 140.96, 170.83.
2,2'-(1,2-phenylene)bis(3-(p-tolyl)thiazolidin-4-one): (Table 4, 4k): White solid, Yield = 85%. 1H
NMR (400 MHz, DMSO-d6), δH (ppm) = 2.26 (s, 3H) 4.01-4.08 (dd, J = 14.2 Hz & 4.7 Hz, 1H),
13
7.11-7.49 (m, 12H), 10.10 (s, 1H). C NMR (100 MHz, DMSO-d6): δC (ppm)= 20.93, 43.65,
119.75, 129.63, 132.99, 136.73, 166.92.
3- Results and Discussion
In the current study, we synthesized ZnO/g-C3N4 as an excellent semiconductor under the thermal
polymerization of melamine. Meanwhile, by embedding ZnO metal oxide in the g-C3N4 surface,
the photocatalytic properties of these nanosheets are well increased. The salient advantages of the
proposed photocatalyst are demonstrated in the stabilization of ZnO and synergistic effect with
visible-light illumination for enhancing the photocatalytic activity in the synthesis of thiazolidinones
under very mild and sustainable reaction conditions.
The chemical groups of the ZnO/g-C3N4 were analyzed by FT-IR spectra. As depicted in Figure
2a, several absorption bands in 1242, 1326, 1424, and 1580 cm−1 can be ascribed to aromatic C–
N stretching [49]. The absorption band assigned to the C=N stretching at 1644 cm-1 is observed [50].
Also, the peak at 812 cm-1 indicates the out-of-plane bending vibration of triazine units [51]. The
signal appears in 491 and 2178 cm−1 are attributed to Zn
̶ O mode which suggesting the successful
preparation of ZnO/g-C3N4 [52]. Furthermore, the phase structure of the ZnO/g-C3N4 was
characterized by powder XRD, shown in Figure 2b. The XRD pattern of composite material
indicates the diffraction peaks at 31.91, 34.59, 36.44, 47.67, 56.63, 62.90, 66.56, 68.19 and 69.26°
which were attributed to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal
7

planes of hexagonal Wurtzite structure of ZnO (JCPDS Card No. 36–1451) [53]. The presence of
g-C3N4 diffraction peak at 2θ of 27.60° confirms (002) lattice plane of g-C3N4, also appears which
demonstrate the existence of ZnO on g-C3N4 surface.
The surface morphology of the obtained nanocomposite was investigated by FE-SEM analysis,
and corresponding images at different magnifications (2μm to 500 nm) are depicted in Figure 3.
The irregular and randomly sheets with micron-sized comprised over the entire surface of the
sample Figure 3a. The magnified images from the FE-SEM (scale of 1 μm and 500 nm) depict
the formation of planar sheets having a few stacked layers of g-C3N4 (Figure 3b, c). The FE-SEM
analysis demonstrates that the interaction of ZnSO4 with triazine units led to the formation of the
ZnO/g-C3N4 with irregular nanorods. The SEM-mapping images in Figure 5d proved the
existence of all constituent elements (i.e., C, N, O, Zn) in the ZnO/g-C3N4 structure.
Further topography of the ZnO/g-C3N4 composite was investigated using TEM analysis as
depicted in Figure 4a, b. TEM images of the ZnO/g-C3N4 indicating embed and distribution ZnO
nanorods among the g-C3N4 surface, which results in forming a ZnO/g-C3N4 heterojunction.
Furthermore, the STEM micrograph can provide more detailed information that morphology
ZnO/g-C3N4 composite. As shown in the dark-field STEM image confirms the nanorods of ZnO
well anchoring on wrinkled sheets of g-C3N4 (Figure 4). The collected data from energy-dispersive
X-ray spectroscopy (EDS) analysis further reveals that all the elements (i.e., C, N, O, and Zn) in
structures of ZnO/g-C3N4, which exhibit purity and the perfect combination of metal oxide on g-
C3N4 sheets Figure 4g.
The importance of the multi-component reaction for forming structural chiral thiazolidinones in
the medicinal arena and materials science has led to wide research by chemists [54-57]. For
8

exploring catalytic activity synthetic conditions were initially employed for the synthesis of
thiazolidinone scaffolds using benzaldehyde, aniline, and thioglycolic acid (TGA). The reaction
in the presence of the ZnO/g-C3N4 catalyst and 40 W domestic bulb lead to the formation of 1,3-
thiazolidin-4-ones with 97% yield (Scheme 2).
Control experiments indicated in absence ZnO/g-C3N4 catalyst negligible conversion (Table 1,
entry 1). When the amount of the catalyst increased from 50 mg to 100 mg, the yield reaction was
also increased from 68 to 97% (Table 1, entries 2 and 3). For the exploration of the reaction
progress, we screened different solvents in optimized conditions. Solvents polar such as EtOH,
DMF, and H2O, the reaction underwent a sluggish conversion (Table 1, entries 4-6). Toluene as a
nonpolar solvent improved the reaction yield up to 97% suggesting that PhMe can be an excellent
solvent in such a reaction (Table 1, entry 3). The desired result with PhMe solvent encouraged to
use nonpolar solvents such as cyclohexane and n-hexane. The results indicated that these solvents
had a moderate effect on reaction progress (Table 1, entries 7 and 8). The activity of the ZnO/g-C3N4
composite was enhanced under irradiation of visible light and the proposed catalyst produces a large amount
of product in comparison with a high-speed stirrer (HSS), Microwave, and ultrasonic irradiation (Table 1,
entries 10-12). Despite decrease time reaction over Microwave and ultrasonic irradiation, based on
the importance irradiation of visible light in photochemical processes and energy issues, we
continued our studies in under 40 W domestic bulb. On the other hand, the increase in bulb power
from 40 W to 100 W had no significant effect on reaction progress (Table 1, entry 13). Moreover,
pristine g-C3N4 sheets and ZnO results in a low product yield (<49%) under optimized conditions
(Table 1, entries 13 and 14). When ZnO anchored on g-C3N4 sheets yield reaction increase to 97%
which demonstrates that the synergetic effect of metal oxide in the ZnO/g-C3N4 structure (Table
1, entry 3).
9

The catalytic activity of ZnO/g-C3N4 was also compared with that of homogeneous and
heterogeneous catalysts and results present in Table 2. To highlight the advantages of using this
proposed catalyst, it was compared with semiconductor-based photocatalysts. The reaction in the
presence of 100 mg of ZnO/g-C3N4 gave a >97% conversion after 60 min under visible light
irradiation. The other semiconductor-based photocatalysts such as P25, ZnO, CdS exhibited poorer
catalytic activity in this transformation (Table 2, entries 1-3). The catalytic performance of ZnO/g-
C3N4 was also compared with that of homogeneous catalysts including ZnCl2, Zn(NO3)2, ZnSO4,
and Zn(OAc)2 in optimize condition reaction. The results of the reaction demonstrated that ZnSO4
used for preparing ZnO/g-C3N4, higher catalytic activity in comparison with other homogeneous
catalysts (71% yield), but lower than that of ZnO/g-C3N4 (Table 2, entry 6).
To further verify the generality of the ZnO/g-C3N4 catalyst in the synthesis of thiazolidinones, we
explored different substituted substrates, using various types of aldehydes and amines (Table 3).
Treatment of benzaldehyde with a series of amines substituents furnished the corresponding
rhodanine in good conversion (4a, 4c, 4d, and 4h). However, ethylene diamine as aliphatic amine
gave a lower yield (57%, 4e).
Among the 1,3-thiazolidin-4-one derivatives, bis-thiazolidinones indicate a broad spectrum of
biological activities and diverse pharmacological properties [58, 59]. In a continuation of these
efforts, we expanded the synthesis of bis-thiazolidinones by selecting various dialdehydes. This
reaction in the presence of the proposed catalyst and 40 W domestic bulb lead to the formation of
10

three series of symmetrical bis-thiazolidinone derivatives in superb yields (Table 4). The results
demonstrated 1,4-aryl linked bis-thiazolidinone derivatives (4i and 4j) formed in better yields as
compared to the 1,2-disubstituted derivatives (4k). The low yield of the 4k derivative as compared
to 4j and 4i derivatives could be attributed to the steric repulsion formed at the adjacent position.
Differential rates of the reaction for substrates with the varying electronic environment would offer
a handle for the selective production of the 1,3-thiazolidin-4-ones. Deploying an equimolar
mixture of benzaldehyde (1a), amine derivatives (2b-d), TGA (3) in optimized conditions are used
for the chemoselectivity study (Scheme 3). These results demonstrate that the 1,3-thiazolidin-4-
ones production is dependent on the better nucleophilicity of the amine nitrogen and electrophilic
nature of the carbonyl carbon.
In addition to catalytic activity, a plausible mechanism for synthesis of 1,3-thiazolidin-4-ones is
shown in Scheme 4. Firstly, the aldehyde has to protonated in the presence of ZnO/g-C3N4, and in
the next step, condensation reaction of amine and aldehyde forming imine as intermediate (A).
The TGA under visible light (40 W bulb) became thiol-substituted acyl radical via homolysis of
CO ̶ OH bond. Then, nucleophilic attack of TGA on Intermediate (A) generated intermediate (B)
via remove water. Finally, it intermediate (B) proceeded for cyclization followed by dehydration
to giving the final product. The proposed catalyst with having ZnO as Lewis acid and tri-s-triazine
framework connected by ZnO makes this nanostructure to excellent bifunctional photocatalyst for
protonation aldehyde. The photonic effect on the catalyst promoted the formation of 1,3-
thiazolidin-4-ones with high conversion. Recently, Kaur and co-worker [60] explored photonic
11

effect Cu2ONPs/g-C3N4 in the synthesis of aminoindolizines, pyrrolo[1, 2-a]quinoline, and
Ynones. Besides, the reaction was carried out under dark conditions but low conversion (38%>)
of 1,3-thiazolidin-4-ones was obtained in 12 h.
In addition to catalytic activity, and stability, reusability is a key factor for heterogeneous catalysts.
To examine the reusability, the ZnO/g-C3N4 was isolated from the reaction mixture and reused in
the next cycle. It can be observed in Table 5 that the proposed catalyst recycled reactions up to
four successive runs. The ICP-AES analysis of the multi-component reaction demonstrates that
negligible ZnO leaching occurred. Also, the FE-SEM and TEM images of the recovered proposed
catalyst after the 4th run affirmed the overall structural integrity of the material which remains
intact after the catalytic experiment, demonstrating that the ZnO/g-C3N4 is stable under visible
light irradiation (Figure 5). Ultimately, a large-scale reaction was indicated with the 5 mmol scale
reaction under optimizing condition with 91% yield.
4. Conclusion
In conclusion, we presented an ingenious strategy to the synthesis of thiazolidinones under visible-
light illumination. We have deployed a preparation of g-C3N4 by thermal polymerization as
excellent support for anchoring and stabilization of ZnO nanorods. The proposed methodology is
efficient, simple, high scale, and easily recyclable for the synthesis of thiazolidinone scaffolds at
room temperature. Moreover, modifying these nanosheets with the various semiconductors can be
12

developed to prepare versatile heterogeneous photocatalysts for organic transformations under
visible-light illumination and other photocatalytic applications.
Notes
The authors declare no competing financial interest.
Competing interests
The author(s) declare no competing interests.
CRediT authorship contribution statement
Robabeh Mohammadi: Investigation, Conceptualization, Methodology,
H. Alamgholiloo: Investigation, Conceptualization, Methodology, Writing - Original Draft
S. Rostamnia: Supervision, Writing - Review & Editing, Funding acquisition
B. Gholipour: Investigation, Conceptualization, Methodology,
S. Khaksar: Review & Editing
M. Farajzadeh: Review & Editing
M. Shokouhimehr: Supervision, Review & Editing, Formal analysis
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18

Figure 1. Synthetic pathway of ZnO/g-C3N4 composite.
b)
a)
ZnO/g-C3N4
g-C3N4
N-H
Triazine
units
JCPDS Card No. 36-1451
ZnO
ZnO/g-C3N4
C=N
C-N
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
20
30
40
50
2 (deg)
60
70
80
Figure 2. FT-IR spectrum (a) and XRD pattern of ZnO/g-C3N4 composite (b).
19

Figure 3. FE-SEM images of ZnO/g-C3N4 in different magnifications (a, b, c), SEM mapping of ZnO/g-
C3N4 composites.
20

Figure 4. TEM images of ZnO/g-C3N4 in different magnifications (a, b) STEM image (c) and EDS
analysis (d) of ZnO/g-C3N4 composites.
Figure 5. FE-SEM and TEM images after the 4th recycling of the ZnO/g-C3N4 catalyst.
21

Scheme 1. General strategy for the synthesis of thiazolidin-4-ones and comparison with
previously reported methods.
Scheme 2. Synthesis of 1,3-thiazolidin-4-ones catalyzed by the ZnO/g-C3N4 catalyst.
22

Scheme 3. The influence of the electronic and steric effect of the amine for selective 1,3-thiazolidin-4-
ones production during intermolecular competition.
Scheme 4. Plausible mechanism for the formation of 1,3-thiazolidin-4-ones.
Table 1: Optimization of the reaction condition in the formation of rhodanine. a
23

Entry Cat.
Solvent
PhMe
Time (min) Condition Reaction Yield (%)b
1
-
360
90
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
Stirrer d
17>
68c
97
51
39
74
69
57
52
65
89
95
98
49
46
2
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
ZnO/g-C3N4
g-C3N4
PhMe
3
PhMe
60
4
EtOH
60
5
DMF
60
6
water
60
7
Cyclohexane
n-hexane
Solvent-free
PhMe
60
8
60
9
150
360
30
10
11
12
13
14
15
PhMe
Microwave
Ultrasonic
100 W bulb
40 W bulb e
40 W bulb
PhMe
20
PhMe
90
PhMe
60
ZnO
PhMe
60
a Reaction conditions: benzaldehyde (1 mmol), aniline (1 mmol), TGA (1 mmol), PhMe (5 mL) in 100 mg of
catalyst.
b Isolated yield of rhodanine.
c 50 mg of catalyst was applied.
d Progress of reaction with a high-speed stirrer in PhMe solvent under reflux condition.
e Progress of reaction with an ultrasonic generator (50-60 Hz/305 W) in PhMe solvent at the ambient condition.
24

Table 2. The comparison of the homogeneous and heterogeneous catalysts during the rhodanine
formation. a
Entry Cat
Time (min) Condition Reaction Yield (%)b
1
2
3
4
5
6
7
P25
60
60
60
60
60
60
60
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
40 W bulb
79
46
52
43
35
71
59
ZnO
CdS
ZnCl2
Zn(NO3)2
ZnSO4
Zn(OAc)2
a Aldehyde (1 mmol) was treated with amine (1 mmol) and TGA (1 mmol) in presence
of homogeneous and heterogeneous catalysts (100 mg) in PhMe (5 mL) under visible
light irradiation.
Table 3. Substrates scope for the synthesis of 1,3-thiazolidin-4-ones with the
variation of the amine and aldehyde components.
25

a Reaction condition: 1a (1 mmol), 2a (1 mmol), 3a (1 mmol), and catalyst (100 mg)
under 40 W bulb. The yields were isolated after column chromatography.
Table 4. Substrates scope for the synthesis of bis-thiazolidinones with the
variation of the dialdehyde and amine components.
a Reaction condition: dialdehyde 1 (1 mmol), 2a (2 mmol), 3a (2 mmol), and catalyst
(100 mg) under 40 W bulb. The yields were isolated after column chromatography.
Table 5. The catalyst recovery and reuse during the formation of rhodanine. a
Run
Scale
ZnO/g-C3N4
Leaching
(ppm)
1.2
Yield
(%)
96
(mmol) used (g) recovery(%)
1st
5
0.50
0.48
0.45
0.42
96
91
84
53
2nd
3rd
4th
4.81
4.56
4.21
1.3
93
1.9
92
2.4
89
a
Reaction conditions: benzaldehyde (1 eq.), aniline (1 eq.), thioglycolic acid
(1 eq.), PhMe (5mL per 1 mmol), 40 W bulb in 1 h.
b Isolated yield of rhodanine.
26