The oligomer approach: An effective strategy to assess phenylene vinylene systems as organic heterogeneous photocatalysts in the degradation of aqueous indigo carmine dye

Article abstract of DOI:10.1016/j.jphotochem.2020.112980

Four oligo-phenylenevinylenes (OPVs) were synthesized by the Mizoroki-Heck cross-coupling reaction to provide a deeper understanding of the mechanism and fate of phenylenevinylene systems when applied to the heterogeneous photocatalytic degradation of indigo carmine (IC) dye in aqueous media. OPVs displayed visible light absorption near 500 nm and appreciable emission properties. The stability, mechanism, and photodegradation activity of the OPV systems over aqueous indigo carmine were investigated using radical scavengers, a singlet oxygen (¹O₂) trap, and ESI-IT-MS. It was confirmed that superoxide radical (O₂?^?), ¹O₂, and direct oxidation are responsible for dye degradation. Hydroxide radical formation, under neutral pH conditions, does not occur, and it was corroborated by the HOMO and LUMO levels of the photocatalysts. Additionally, the use of oxalic acid as an electron sacrificial donor was demonstrated as an effective approach to enhance the OPVs photocatalytic activity. However, a significant decrease in activity during the first three irradiation cycles was observed, indicating that the enhancement in photo- and chemical stability is required for further dye-contaminated water treatment applications.

Full text of DOI:10.1016/j.jphotochem.2020.112980

Journal Pre-proof
The Oligomer Approach: An Effective Strategy to Assess Phenylene
Vinylene Systems as Organic Heterogeneous Photocatalysts in the
Degradation of Aqueous Indigo Carmine Dye
´
Mauricio Acelas, Cesar A. Sierra
PII:
S1010-6030(20)30777-2
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112980
JPC 112980
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
26 May 2020
29 September 2020
11 October 2020
Please cite this article as: Acelas M, Sierra CA, The Oligomer Approach: An Effective
Strategy to Assess Phenylene Vinylene Systems as Organic Heterogeneous Photocatalysts
in the Degradation of Aqueous Indigo Carmine Dye, Journal of Photochemistry and amp;
Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112980
This is a PDF file of an article that has undergone enhancements after acceptance, such as
the addition of a cover page and metadata, and formatting for readability, but it is not yet the
definitive version of record. This version will undergo additional copyediting, typesetting and
review before it is published in its final form, but we are providing this version to give early
visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal
pertain.
© 2020 Published by Elsevier.

The Oligomer Approach: An Effective Strategy to Assess Phenylene Vinylene Systems as
Organic Heterogeneous Photocatalysts in the Degradation of Aqueous Indigo Carmine Dye
Mauricio Acelas and César A. Sierra*
Grupo de Investigación en Macromoléculas. Departamento de Química, Universidad Nacional de
Colombia, Bogotá 111321, Colombia.
Graphical abstract
Highlights
 Oligo phenylenevinylenes (OPVs) were employed as heterogeneous
photocatalysts for the first time in the degradation of aqueous indigo carmine dye.
 Degradation of IC-dye proceeds via generation of superoxide radicals, singlet
oxygen, and direct oxidation at the HOMO level of the OPV.
 The HOMO levels of OPV systems indicate OH· radical formation via direct
water oxidation is not favorable.
 OPV degradation products were identified, employing ESI-IT experiments.
 The use of oxalic acid as a sacrificial electron donor for OPVs provided superior
discoloration efficiency compared to TiO2.
1

Abstract
Four oligo-phenylenevinylenes (OPVs) were synthesized by the Mizoroki-Heck cross-coupling
reaction to provide a deeper understanding of the mechanism and fate of phenylenevinylene
systems when applied to the heterogeneous photocatalytic degradation of indigo carmine (IC)
dye in aqueous media. OPVs displayed visible light absorption near 500 nm and appreciable
emission properties. The stability, mechanism, and photodegradation activity of the OPV
systems over aqueous indigo carmine were investigated using radical scavengers, a singlet
oxygen (1O2) trap, and ESI-IT-MS. It was confirmed that superoxide radical (O2·-), 1O2, and
direct oxidation are responsible for dye degradation. Hydroxide radical formation, under neutral
pH conditions, does not occur, and it was corroborated by the HOMO and LUMO levels of the
photocatalysts. Additionally, the use of oxalic acid as an electron sacrificial donor was
demonstrated as an effective approach to enhance the OPVs photocatalytic activity. However, a
significant decrease in activity during the first three irradiation cycles was observed, indicating
that the enhancement in photo- and chemical stability is required for further dye-contaminated
water treatment applications.
Keywords: Oligomer approach; oligo phenylenevinylenes; organic heterogeneous photocatalyst; indigo
carmine; photodegradation.
1. INTRODUCTION
Organic dyes are widely employed in the global textile industry for the manufacture of denim
fabrics and other materials, including paper and plastics, which are incorporated in daily-use
2

apparel.1,2 Especial attention has been placed on indigo carmine, an indigoid-type compound
discharged into industrial wastewaters, due to their well-known toxicity and high impact over
aquatic environments, since this compound reduces sunlight penetration, favoring
eutrophication.3 Although indigo carmine (IC) can be used for medical diagnostic purposes,4
many health concerns regarding skin and eye irritation, carcinogenicity and reproductive toxicity
have been established.5 Considering this set of negative effects, the removal or degradation of IC
has become a major concern for the scientific community.
Several strategies have been explored to address this problem. Physical processes such as
adsorption based on low-cost natural materials, including chitin, chitosan, and hen feathers, have
been investigated.6,7 However, further treatments are required to remove IC from the solid
adsorbent, increasing the operative costs of dye removal. Alternatively, biological,8
electrochemical,9 and advanced oxidation processes have been employed as a feasible choice for
IC degradation.10,11 Out of all these treatment options, photocatalytic degradation has emerged as
the method of choice for environmental remediation for soluble organic pollutants including IC,
due to the low-cost scaling to industrial level.4,12,13
Similarly, over the last decades inorganic TiO2-based materials have been subject of study for
mineralization of contaminants via heterogeneous photocatalysis.14 However, their high bandgap
values represent a severe limitation considering that the incident solar spectrum contains a high
percentage of visible light.15 On the other hand, organic compounds and polymers arise as a
metal-free approach for photocatalytic applications offering easily tunable properties and
lowering the radiative requirements taking advantage of a wide range of synthetic pathways that
are available for their construction.16
Examples of heterogeneous organic and polymeric photocatalysts for pollutants degradation in
aqueous media have been described before, some displaying distinct π-conjugation structures
embedded in hetero-aromatic rings and alternated double bonds.17–19 Thus, phenylenevinylenes
(PV) as conjugated systems serve as promising molecular models for heterogeneous
photocatalysis, as demonstrated by several applications involving two-photon excitation singlet
oxygen production,20 Orange G dye degradation21 and light-promoted oxidative hydroxylation of
3

-
arylboronic acids.22 In all this, the participation of radical reactive species such as OH· and O2·
have been described. However, detailed studies on photocatalytic properties, for example, the
determination of HOMO-LUMO levels by redox behavior in poly-PVs has not been extendedly
performed, considering that direct study of high molecular weight poly-conjugated systems is
often restricted by low solubility and heterogeneity.
As consequence, the oligomer approach is presented as a useful and expedite tactic to perform
extensive characterization -including optoelectronic-, predict reactivity, and provide key
information to understand the photocatalytic degradation process.23,24 Oligo-phenylenevinylenes
(OPVs), as model π-conjugated systems, can be easily prepared by the Mizoroki-Heck reaction
in a highly stereoselective fashion,25 where structural and chemical diversity around the
conjugated scaffold offers a wide range of controllable properties for light absorption and
photocatalytic applications.26,27 Although small molecules and oligomers are challenging models
for heterogeneous photocatalysis due to photostability requirements,28 it has been demonstrated
that long-term stability and recyclability can be achieved.29
Herein we examine the synthesis, optoelectronic properties, stability, and heterogeneous
photocatalytic performance of four OPVs for the treatment of aqueous IC. Also, the radical
oxygen species and their role in the degradation mechanism are discussed as the basis for the
future development of novel photostable poly-PV-based heterogeneous materials.
2. EXPERIMENTAL SECTION
2.1. Materials
All starting materials were purchased from Merck, Alfa Aesar, and Sigma-Aldrich and used as
received. Indigo carmine (C.I. 73015) for clinical diagnosis (>98%) was supplied from Panreac.
TiO2 Degussa P25 Aeroxide® (<45 microns, SBET 35-65 m2/g) was obtained from Acros
Organics. Silica gel 60 (Macherey-Nagel GmbH & Co., 0.063-0.200 mm, SBET 480-540 m2/g)
was employed for photocatalyst dispersion, and it was also used for column chromatography.
Thin-layer chromatography (TLC) PF254 aluminum sheets (Merck) were used to monitor the
reaction progress. Oxygen sensitive reactions were carried out under atmospheres of dry argon or
4

nitrogen. Distilled, deionized (MilliQ purification systems) water was used for photocatalytic
and ESI-IT MS experiments.
2.2. Synthesis
OPVs were prepared according to a previously described procedure.26,30
4,4'-((1E,1'E)-(2,5-Dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl)) dibenzoic acid, OPV 1.
Yellow solid. m.p. 232-234 °C; Recrystallized from DMF, yield 0.337 g (79%) from 4-
vinylbenzoic acid (0.250 g, 1.68 mmol), 1,4-diiodo-2,5-dimethoxybenzene (0.327 g, 0.84 mmol),
Pd(dba)2 (19.3 mg, 0.03 mmol), triphenylphosphite (46.9 mg, 0.15 mmol) and triethylamine
(0.340 g, 3.36 mmol). IR (ATR, cm-1): 3055, 2939, 1678, 1600, 1292, 1207, 1176, 952. 1H-NMR
(400 MHz, DMSO-d6) δ(ppm): 3.92 (s, 6H), 7.39 (s, 2H), 7.44 (d, J=16.5 Hz, 2H), 7.57 (d,
J=16.5 Hz, 2H), 7.69 (d, J=8.4 Hz, 4H), 7.95 (d, J=8.4 Hz, 4H), 12.89 (bs, COOH). 13C-NMR
(100 MHz, DMSO-d6) δ(ppm): 56.2, 109.6, 125.0, 125.9, 126.3, 128.3, 129.4, 129.8, 141.8,
151.3, 167.0. MALDI-MS Calc. for C26H22O6 430.141; found [M]+ 430.57. Anal. Calcd. for
C26H22O6: C, 72.55; H, 5.15%. Found: C, 72.26; H, 5.23%.
Complete experimental details and spectroscopic characterization of all compounds, including
precursors, intermediates, and final products can be found in the supporting information.
2.3. Techniques
A Stuart SMP10 melting point instrument was employed to obtain the uncorrected melting
points. FT-IR spectra were acquired on a Shimadzu IR Prestige–21 adapted to an ATR module.
Mass spectra were collected in an UltraFlextreme MALDI-TOF-MS (Bruker) with a solid laser
source of Nd:YAG and 1000 Hz acquisition frequency from m/z 0 – 1000 in positive ion mode
employing the LDI method at a 25% laser fluency. An Amazon X (Bruker) ESI-IT operating in
alternate positive/negative ion modes at 300°C and 4500V in the capillary was also used for MS
data collection. Nitrogen was used as nebulizer gas with an influx of 8 L/min and 30 psi. MeCN
(HPLC grade) was used for sample dilution. Elemental analyses were carried out on a Flash 2000
Thermo Scientific CHNS analyzer. 1H and 13C NMR spectra were obtained from a Ultrashield-
400 spectrometer (Bruker, 400 MHz for 1H-NMR and 100 MHz for 13C-NMR), using CDCl3,
5

and DMSO-d6 as solvents. Coupling constant values (J) and chemical shifts are reported in Hz
and in ppm (δ), respectively. Signals were designated as follows: s, singlet; d, doublet; dd,
doublet of doublets; t, triplet; td, triplet of doublets; q, quartet; m, multiplet; br, broad. The
absorption spectra were recorded using a Thermo Scientific Evolution 300 UV-Vis
spectrophotometer. The emission spectra were measured using a PTI QuantaMaster™ 40
spectrofluorometer adapted to an integrating sphere. The reference method described in literature
to obtain the relative fluorescence quantum yields Φf was employed using perylene as standard.31
The onset value of the absorption spectra was used to calculate the optical band gap for OPV
solutions.32 Diffuse reflectance measurements were performed on an Ocean Optics spectrometer
adapted to a reflection probe. Cyclic voltammograms were obtained on a Gamry interface 1010E
potentiostat. 0.1 M OPVs samples were prepared with 0.05 M tetra‐ n‐ butylammonium
hexafluorophosphate in anhydrous acetonitrile. Measurements were made employing a glassy
carbon working electrode, a platinum wire as a counter electrode, an Ag/Ag+ reference electrode,
and a scan rate of 50 mV/s. The glassy carbon electrode was polished between each scan.
2.4. Photocatalytic IC degradation
The photocatalysts were prepared by the wet impregnation method by dissolving 5 mg of the
corresponding OPV in 10 mL of chloroform. Then, silica gel (500 mg) was added, keeping the
mixture under a constant flow of air at 50 °C for complete solvent evaporation. The IC
degradation reactions were performed on an LED-based panel type photoreactor equipped with a
3W LED array as irradiation source within the UVA and visible portion of the spectrum (350-
450 nm) (Fig S10). In a typical dye degradation experiment, 20 mg of catalyst was suspended on
3 mL of 50 μM aqueous IC solution on the UV-vis cuvette to facilitate direct monitoring of the
color intensity. The mixture is then left for 15 min to ensure the adsorption equilibrium is
reached. The temperature was maintained constant at 20 °C using circulated water and, the
experiments were performed under a low-stirring regime (100 rpm).
3. RESULTS AND DISCUSSION
3.1. Synthetic strategy and optoelectronic characterization
6

The synthesis of the OPV scaffold was successfully achieved via the Mizoroki-Heck reaction in
moderate to good yields according to the optimized protocol previously described by our
research group.26 Amino and hydroxy OPVs 2 and 3 were obtained by subsequent hydrolysis and
reduction reactions of acetoxy and nitro OPV derivates, respectively.33 Structural
characterization by 1H-NMR indicated an E-double bond geometry based on the 3J coupling
constant values around 16.5 Hz in all OPV derivatives. Also, MALDI-MS experimental results
were in accordance with the expected molecular weight values in all cases.
Scheme 1. Synthesis of OPVs 1-4 via Mizoroki-Heck reaction.
Absorption and emission spectra of OPVs were acquired in MeCN solution (Figure 1), in solid-
state, and adsorbed over SiO2 (named as SiO2-OPV, Figure 2) in order to obtain relevant
optoelectronic characterization (Table 1). It must be disclosed that SiO2 was chosen as an inert
support for the evaluation of OPVs as photocatalysts to increase surface area, promoting mass
transfer in the photocatalytic process by improving the surface wetness, and providing
stabilization.34 This strategy also contributes to the catalyst reuse, allowing the easy separation
from the reaction products and solvent.
7

All OPVs displayed a double absorption band due to the π → π* transition expected for the
OPV conjugated backbone and, an additional n → π* transition originated by the -OMe group
bonded to the central aryl ring (Figure 1). Also, a double band emission is observed only for
OPVs containing -OH and -NH2 as electron-donating groups (EDG) on the flanking aryl rings,
indicating a significant modification of the vibronic states in these molecules. Interestingly, the
λem value undergoes a bathochromic shift in OPVs 1 and 4 as consequence of the presence of the
electron-withdrawing groups COOH and CHO, respectively.
Figure 1. Normalized absorption (black) and emission (blue) spectra for OPVs 1-4 in MeCN
solution.
The optical ΔE values for OPVs indicate that photocatalytic activity can be achieved even under
blue visible light irradiation. Furthermore, the low to moderate fluorescence quantum yields (Φf)
suggests deficient radiative charge recombination, implying that photocatalysis and related
surface reactions might be favored as an alternative relaxation pathway.35
8

Regarding solid-state optoelectronic properties (Figure 2), diffuse reflectance spectra revealed a
broad absorption for all OPVs and SiO2-OPVs starting in the visible region. The Kubelka-Munk
function was calculated from the reflectance spectra to estimate the ΔE values for all the
materials.
Figure 2. Diffuse reflectance spectra of solid OPVs (a) and OPVs adsorbed in SiO2 (b),
Kubelka-Munk function of solid OPVs (c) and OPVs adsorbed in SiO2 (d) and solid-state
emission spectra for solid OPVs (e) and OPVs adsorbed in SiO2 (f).
From Figure 2, it is clear that the distinctive chemical nature of the substituents around the OPV
scaffold is responsible for the differences in ΔE values exhibited by the OPVs (solid and
adsorbed in SiO2). However, no clear trends were observed regarding the electron-withdrawing
or electron-donating character of the functional groups. On the other hand, the shift of the ΔE to
9

lower energies in solid OPVs, compared to the values in MeCN solution and dispersion in SiO2,
might be the result of solid-state non-covalent interactions such as ππ stacking, where the
HOMO-LUMO energy gap can decrease due to orbital overlapping of the oligomers (Table 1).36
At the same time, all OPVs exhibited moderate fluorescence emission when excited under 370
nm light. It is worth noting that all the λem values are redshifted when compared to the spectra
acquired in MeCN solution, serving as evidence for the existence of the above mentioned ππ
stacking interactions. Moreover, the dispersion of OPVs on silica led to a blueshift in the
emission maxima compared to the pure solid OPV samples. This phenomenon has already been
described and can be attributed to a restricted molecular reorientation caused by the adsorption
interactions with the SiO2 surface, causing the radiative relaxation to occur from a higher energy
level.37
Table 1. Optoelectronic properties of OPVs 1-4 in MeCN solution, solid state and adsorbed in
SiO2.
OPV
1
2
3
4
λmax (nm)
λem (nm)
ΔE (eV)
Φf*
407
475
2.67
0.25
2.41
540
392
442
2.83
0.58
2.70
483
403
454
2.71
0.10
2.35
574
413
500
2.59
0.21
2.32
568
MeCN solution
ΔE (eV)
λem (nm)
Solid state
ΔE (eV)
2.69
513
3.15
477
2.51
501
2.47
547
Adsorbed on
λem (nm)
SiO2
*Perylene was used as standard
3.2. Photostability of OPV systems
Preliminary assessment of OPVs stability was performed by direct irradiation of the 1x10-5 M
MeCN solutions in the quartz cuvette with the LED panel source (350-450 nm) (Fig S10) while
monitoring the absorbance decrease of each compound at the λmax value. The experiments were
carried out in both aerobic and anaerobic (N2) atmospheres, as depicted in Figure 3.
10

Figure 3. Absorption decay profile at λmax for OPVs 1-4 in MeCN solution.
For experiments under nitrogen, a decrease in the rate of discoloration was observed, suggesting
that oxygen plays an important role in molecular photodegradation for all OPV materials.
Moreover, the presence of electron-withdrawing groups EWG (OPVs 1 and 4) in the flanking
aryl rings of the OPV moiety seems to significantly delay the OPV photodegradation process
compared to OPVs baring EDG (OPVs 2 and 3).
Complete disappearance of the main visible absorption band for OPVs 1 and 4, was achieved
after 20 minutes of irradiation, yielding a colorless solution. Upon examination of the UV-Vis
spectra for these solutions, an absorption band at 306 nm was observed, suggesting the presence
of less-conjugated photoproducts or fragments. Oddly, OPVs 2 and 3 decreased their λmax
absorbance in the visible region to a constant, but much lower value after only 200 seconds of
irradiation along with the notorious formation of bands at 270 and 348 nm for OPV 2, and one
band at 312 nm for OPV 3 (see Fig. S11). This outcome clearly indicates that the initial OPVs 2
11

and 3 are being rapidly transformed into other visible light-absorbing conjugated products as
result of the electronic effect exerted by -OH and -NH2 groups in the OPV scaffold.
In all cases, the main absorption bands were seen to decrease upon irradiation. In contrast, new
absorptions appeared in the UV region, creating isosbestic points for OPVs 2, 3 and 4 as
evidence of a clean interconversion towards photoproducts (see Fig S11-12). However, for OPV
1 a distortion in the isosbestic point is observed, suggesting different simultaneous
photodegradation pathways. Moreover, the isosbestic points for the spectra sets under aerobic
and anaerobic conditions do not match, except for OPV 4. This result is probably due to the
formation of other products with different absorption profiles, once again indicating different
photodegradation pathways.
1H-NMR experiments confirmed the complete disappearance of the OPV peaks after irradiation,
yielding a complex mixture of aromatic and deshielded aliphatic signals, also demonstrating the
formation of multiple products. Additionally, the NMR spectra for experiments performed under
aerobic and anaerobic conditions are not identical, confirming different photodegradation
pathways (see Fig. S14).
ESI-IT-MS experiments were performed as an attempt to identify possible photodegradation
products. For OPV 1, irradiated under open atmosphere, ions at 339, 325, 311, and 297 m/z
confirm OPV double bond cleavage reactions mediated by reactive oxygen species (ROS), with
the formation of carbonyl and analogous compounds38 along with the characteristic
fragmentation of the methoxy group (Figure 4). MS spectrum for OPV 1 irradiated under N2
atmosphere provided similar peaks with a significantly decreased intensity, making the
identification of other photodegradation products very challenging.
12

Figure 4. ESI (-)-IT mass spectrum of OPV 1 after irradiation under open atmosphere and
structure of the identified degradation products.
Interestingly, for OPV 3, positive ion ESI-MS confirmed the formation of a unique OPV
derivative, possibly a nitroso compound (Figure 5). This photoinduced transformation explains
the remaining visible-light absorption observed in Figure 3. Also, the MS spectrum of the
solution irradiated under N2 shows unnumbered low-intensity peaks; in consequence, we were
unable to provide chemical identification of additional degradation products under these
conditions. A very similar chemical behavior was observed for OPVs 2 and 4, where the number
of different products generated after irradiation, for both aerobic and anaerobic experiments,
prevented us from obtaining an accurate description of the photodegradation process.
Figure 5. ESI (+)-IT mass spectra of OPV 3 before and after irradiation under aerobic, and
anaerobic conditions.
13

3.3. Photocatalytic performance of OPVs
First, to evaluate the photostability of aqueous IC, a 50 μM solution was directly irradiated with
the LED panel (350-450 nm, 3W). No change in the dye absorption profile was observed after 60
minutes, indicating that IC is stable under the employed irradiation conditions (Figure 6a,
control). An additional control experiment was performed, adding silica gel to the IC solution,
displaying no significant dye-adsorption (Figure 6a, SiO2).
Figure 6. SiO2-OPVs induced photodegradation of IC-dye (a) and, dye degradation profile at
λmax employing SiO2-OPV 1 as photocatalyst (b).
As expected, only when SiO2-OPV systems were employed in the reaction, a decrease in the
concentration of dye was observed confirming the photocatalytic ability arises exclusively from
the OPVs. Noteworthy, SiO2-OPV 1 exhibited the best photocatalytic performance among all the
evaluated compounds, reaching a 98% dye removal within 60 minutes, yielding a colorless
solution (Figure 6a, SiO2-OPV 1). The time-dependent absorption spectrum of IC-dye in the
presence of SiO2-OPV 1 under irradiation is depicted in Figure 6b. The maximum absorption
band for IC around 610 nm gradually decreases with irradiation following pseudo-first-order
kinetics with a kap value of 0.041 min-1 suggesting that the dye undergoes a degradation process
that affects the molecular integrity of the chromophoric fragment.
To determine the chemical species involved in the photodegradation, benzoquinone (BZ, 2 mM)
and potassium iodide (KI, 2 mM) were employed as scavengers for superoxide radical (O2·-) and
holes (h+), respectively.39 A decrease in the efficiency of the process using SiO2-OPV 1 was
14

observed for both experiments, especially for BZ, suggesting that the process comprises
superoxide radical as the main active species responsible for IC degradation (Figure 7).
Hydroxyl radical (OH·) scavengers40 (2 mM); isopropanol (i-PrOH), ethanol (EtOH), and
methanol (MeOH) were also employed, unexpectedly achieving an improvement in the
degradation process. This observation clearly indicates that these substances act as sacrificial
electron donors, thus enhancing the electron/hole separation efficiency and promoting the
electron transfer from OPV 1 LUMO to dissolved oxygen molecules.41
Figure 7. Scavengers effect on IC degradation with SiO2-OPV 1 after 45 minutes of reaction.
Control: SiO2-OPV 1 without addition of any scavenger.
To validate this hypothesis regarding the regeneration of the OPV HOMO level, an additional
experiment was performed employing oxalic acid (2 mM), a well-known and very efficient
sacrificial electron donor in photocatalysis.42 A remarkable decrease in the reaction time was
observed, achieving 97% degradation in only 5 minutes of irradiation (Figure 8). The efficiency
of this OPV photocatalytic system in the IC discoloration is superior to that of TiO2 (Degussa
P25, 0.2 mg/3 mL), where a 93% of degradation was achieved after 20 minutes of irradiation.
Moreover, the OPV degradation follows an apparent zeroth-order reaction (kap=0.22 mol·L-
1·min-1). This indicates that the rate is now limited by a constant oxygen mass transfer from the
solution to the photocatalyst surface, as a result of a significant increase in the regeneration of
the HOMO level for OPV 1 by means of oxalic acid oxidation.43
15

Figure 8. Color degradation profile of IC in the presence of SiO2-OPV 1 and oxalic acid (2 mM)
as sacrificial electron donor. The degradation profile of IC employing Degussa P25 is also
presented for comparative purposes.
Considering that singlet oxygen is a very reactive intermediate, the production of which was
previously demonstrated by analog OPV structures,20,44 its generation, was initially evaluated
employing sodium azide as a scavenger (Figure 7). However, an unexpected increase in the
degradation efficiency suggested that 1O2 is not the main active specie involved in the reaction.
Moreover, it simultaneously prevented us from satisfactory establish it as one of the
photogenerated products.45 This particular phenomenon encouraged us to use 2-furoic acid as an
alternative singlet oxygen probe.46,47 The method consists of a [2+4] cycloaddition reaction
between the furan moiety and singlet oxygen, generating an endoperoxide that decomposes into
5-hydroxy-5H-furan-2-one (Scheme 2).48
16

Scheme 2. Reaction of 2-furoic acid with singlet oxygen.
The reaction can be followed by UV-vis using the furoic acid absorption maximum at 246 nm.
After 30 minutes of irradiation of a furoic acid solution (1.0 mM) in a 3:1 water-methanol
mixture, and in the presence of OPV 1, a slight decrease (11%) in the absorption was achieved,
suggesting that singlet oxygen is being generated in small amounts (Figure 9).
Figure 9. Degradation of furoic acid mediated by singlet oxygen in the presence of SiO2-OPV 1.
To provide additional insight into the degradation pathway of IC, negative mode ESI-MS was
employed as a characterization technique. Initial analysis of pure IC was performed for
comparative purposes showing ions at 210, 421, and 443 m/z associated with [IC-2Na]2-, [IC+H-
2Na]- and [IC-Na]-, respectively (Figure 10). The colorless solution obtained after 60 minutes of
irradiation established isatin-5-sulfonic acid (226 m/z) as the major IC degradation product.
Tandem MS experiments confirmed the identity of this compound. Also, fragments at 216 and
17

244 m/z were identified as 4-amino-3-carboxybenzenesulfonate and 4-amino-3-
(carboxycarbonyl) benzenesulfonate, respectively.49,50 No IC characteristic ions remained after
the irradiation cycle indicating total dye-degradation.
Figure 10. Negative mode ESI-MS spectra of IC and its photo-irradiated solutions in the
presence of SiO2-OPV 1 and structure of most abundant ions observed in MS spectra.
A similar MS profile was obtained for the solution where oxalic acid was employed as a
sacrificial electron donor for the photocatalytic process (with just 5 minutes of irradiation), being
the isatin-5-sulfonic acid (226 m/z) the primary degradation product as well. Although oxalic
acid may also modify the pH of the IC solution, promoting the superoxide chemical
transformation into hydroxide radicals, no hydroxylated products were observed on the MS
spectra for any of the experiments. Thus, the photocatalyzed formation of OH· radicals and their
role in the IC degradation under the employed experimental conditions can be disregarded.
The photocatalytic activity of SiO2-OPV 1 was also tested in the absence of oxygen (Figure 11).
The degradation profile under nitrogen atmosphere indicated that IC maintains a constant
discoloration rate, attributed to a direct interaction of the dye with the excited molecular state of
the OPV.
18

Figure 11. Effect of oxygen in the degradation profile of IC for SiO2-OPV 1.
To accurately determine the nature of the process, ESI-IT MS was employed to establish the
structure of the products after irradiation of the IC solution in the presence of SiO2-OPV 1 under
inert atmosphere (Figure 12). Interestingly, isatin-5-sulfonic acid (226 m/z) was again detected as
the major photodegradation product along with a compound at 202 m/z with considerable
intensity, that has not been previously detected in the experiments that were carried out under
open atmosphere. This degradation product corresponds to the ion formed after continuous
oxidative degradation of isatin-5-sulfonic acid (Figure 12) as it has been previously reported by
Combariza et al.50 This outcome suggests that direct and continuous oxidation of IC and its
subsequent degradation products by OPV 1 HOMO is contributing to the overall observed dye
degradation. Also, the absence of reduction products in the MS spectrum indicates that direct
electron transfer from OPV 1 to the IC-dye does not occur. Nevertheless, the MS spectrum
includes ions that belong to OPV 1 and its degradation products, indicating desorption and
migration of the photocatalyst to the aqueous phase is occurring, making the covalent
functionalization of SiO2 with the OPV moiety a plausible strategy to prevent this phenomenon
and eventually help to maintain the molecular integrity in the conjugated scaffold.
19

Figure 12. ESI (-)-IT mass spectrum of IC irradiated under inert atmosphere for SiO2-OPV 1 and
proposed direct oxidative degradation pathway for isatin-5-sulfonic. Marked peaks belong to
desorbed OPV 1 (■) and OPV 1 degradation products (▼).
To further corroborate the absence of OH· radicals, cyclic voltammetry was employed to
estimate the OPVs HOMO-LUMO energy levels (see Fig. S16). The onset reduction (Ered) and
oxidation (Eox) peak potentials (Table 2) were determined directly from the electrochemistry
experiments and correlated to ferrocene.51
Table 2. Electrochemical data obtained for OPVs 1-4.
EHOMO
(V vs NHE)
1.81
ELUMO
(V vs NHE)
-0.59
OPV
Eox (V)
Ered (V)
1
2
3
4
1.31
0.83
0.68
1.18
-1.09
-1.82
-1.80
-1.30
1.33
-1.32
1.18
-1.30
1.68
-0.80
20

A marked influence in the HOMO level is observed when EWG are placed around the OPV
backbone (OPV 1 and 4), changing the frontier orbital potentials to more positive values.
Noteworthy, all EHOMO levels fall in the range of 1.18 to 1.81 eV (vs. NHE). Therefore, the
formation of OH· radicals is not possible, considering that the oxidation of water at pH=7 occurs
at 2.31 eV (vs. NHE).52 This finding indicates that these OPV materials are unsuitable for
mineralization of aqueous organic pollutants under the employed conditions (Figure 13).53
Figure 13. Relative HOMO-LUMO levels alignment for OPVs (vs. NHE). The LUMO position
was estimated by adding the experimental optical ΔE value.
Likewise, the presence of EWG in the flanking aryl rings of the OPV (OPV 1 and 4) causes the
LUMO level to shift to more positive potential values, compared to EDG. All ELUMO levels are
located at lower potentials compared to the reduction of oxygen (-0.18 eV vs. NHE). Hence an
effective transformation of O2 into superoxide radicals can be achieved in all cases. However, the
proximity of the oxygen reduction potential and the LUMO level for OPV 1, implies that the
electron transfer rate is the main factor for determining the observed IC degradation efficiency.
Interestingly, the HOMO-LUMO levels for IC, that were previously calculated employing a
DFT/B3LYP/SDD basis set,54 indicate that electron transfer from the dye to OPV 1 and OPV 4 is
21

favored, corroborating the experimental observation where oxidation of IC can occur even under
anaerobic conditions (Figure 11). This additional direct oxidative pathway clearly explains why
SiO2-OPV 1 resulted in the best photocatalytic system for degrading indigo carmine.
Moreover, these results help to understand better the previous experimental observation where
KI was employed as a scavenger for holes (h+). If effective hole regeneration takes place, the
recombination rate of charge carriers decreases, prompting the electron transfer from the LUMO
level of the excited OPV 1 to O2. This increases the dye degradation performance via superoxide
and singlet oxygen generation, as observed for alcohols and oxalic acid. In addition, since the
release of OH· radicals at the HOMO level by direct oxidation of water does not occur, as
previously demonstrated, the observed decrease in the IC degradation efficiency when KI is
added to the mixture (Figure 7) cannot be attributed to the suppression of such radical species,
-
but to the generation of oxidation products in the photocatalyst surface such as IO3 and I2.
Species that could directly oxidize the OPV skeleton, decreasing its photocatalytic efficiency.39
With all this information in hand, a schematic degradation pathway under aerobic conditions for
indigo carmine is proposed (Figure 14). After irradiation and generation of the OPV excited
state, the photogenerated electron in the LUMO is transferred to oxygen, creating the superoxide
radical. At the same time, oxidation of IC can occur due to direct interaction with the HOMO.
On the other hand, the OPV can also generate singlet oxygen by either electron transfer or by
acting as a photosensitizer transferring energy from the triplet state.55 Both chemical oxygen
species, superoxide radical and singlet oxygen, react with the IC causing cleavage of the double
bond. Finally, the addition of oxalic acid in solution rapidly regenerates the half-vacant HOMO
via oxidation, making the OPV readily available for another irradiation cycle.
22

Figure 14. Photocatalytic schematic pathway for IC degradation in the presence of SiO2-OPV 1
and oxalic acid as sacrificial electron donor.
In addition to the photocatalytic performance, the reusability of SiO2-OPV 1 was evaluated by
recycling experiments employing the best reaction conditions found in this study (OPV 1 and
oxalic acid, 2 mM). As shown in Figure 15, the dye degradation percentage is dramatically
lowered after each irradiation cycle reaching only 20% for the third test. Although oxalic acid
addition reduces the time exposure of the OPV to the photogenerated superoxide radical and
singlet oxygen, the catalyst activity is strongly compromised due to molecular degradation.
By the results, it is evident that further stability improvements are required to enhance the
efficiency of the photocatalyst after several irradiation cycles. Therefore, strategies such as
chemical immobilization on solid supports and polymerization are proposed as rational
approaches to overcome this issue. Notwithstanding, OPVs proved to be useful and interesting
models for heterogeneous photocatalytic applications allowing easy and complete elucidation of
the dye-degradation process.
23

Figure 15. Reusability of SiO2-OPV 1 in the degradation of 50 μM IC aqueous solution.
4. CONCLUSION
Oligo phenylenevinylenes, prepared by the Mizoroki-Heck coupling reaction and adsorbed on
silica gel, were evaluated for the first time as heterogeneous photocatalysts in the degradation of
indigo carmine dye, as the basis to provide a deeper understanding of the photochemical
behavior for phenylenevinylene systems. The stability experiments indicated that ROS induce
cleavage of the OPV conjugated structure. However, this feature can be improved by the
insertion of EWG around the OPV scaffold.
It was demonstrated that the degradation of IC-dye proceeds via generation of superoxide
radicals, singlet oxygen, and direct oxidation at the HOMO level of the OPV. Moreover, the
addition of oxalic acid as a sacrificial electron donor provides a drastic improvement in the time
required for the dye photodegradation by rapid HOMO regeneration, achieving superior
discoloration efficiency compared to TiO2.
The HOMO energetic levels, estimated by cyclic voltammetry, indicated that the formation of
hydroxyl radicals does not occur as corroborated by the organic products observed in the ESI-
24

MS spectra and radical trapping assays. Thus, these materials are unsuitable for the
mineralization of other organic pollutants under the employed conditions. However, the
reusability tests, along with the identification of OPV double bond-cleavage fragments, and the
use of sacrificial electron donors, served as the basis for ongoing research aimed to develop other
phenylenevinylene systems, with significant stability enhancement for dye-contaminated and
other water treatment applications.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Conflicts of interest
There are no conflicts to declare
Acknowledgments
The authors acknowledge financial support from Colciencias (project Nº FP44842-025-2017),
DIB-UNAL (project with code HERMES 36990), the Colombian Ministry of Science -
Bicentenario Grant Program and, the UNAL Teaching Assistant Scholarship Program. We also
wish to thank Laboratorio de Espectrometria de Masas – Parque Tecnológico Guatiguará –
Universidad Industrial de Santander for their support in MS data collection.
25

References
(1) Othman, I.; Mohamed, R. M.; Ibrahem, F. M. Study of Photocatalytic Oxidation of Indigo
Carmine Dye on Mn-Supported TiO2. J. Photochem. Photobiol. A Chem. 2007, 189 (1),
80–85. https://doi.org/10.1016/j.jphotochem.2007.01.010.
(2) Li, H. X.; Xu, B.; Tang, L.; Zhang, J. H.; Mao, Z. G. Reductive Decolorization of Indigo
Carmine Dye with Bacillus Sp. MZS10. Int. Biodeterior. Biodegrad. 2015, 103, 30–37.
https://doi.org/10.1016/j.ibiod.2015.04.007.
(3) Vautier, M.; Guillard, C.; Herrmann, J. M. Photocatalytic Degradation of Dyes in Water:
Case Study of Indigo and of Indigo Carmine. J. Catal. 2001, 201 (1), 46–59.
https://doi.org/10.1006/jcat.2001.3232.
(4) Barka, N.; Assabbane, A.; Nounah, A.; Ichou, Y. A. Photocatalytic Degradation of Indigo
Carmine in Aqueous Solution by TiO2-Coated Non-Woven Fibres. J. Hazard. Mater.
2008, 152 (3), 1054–1059. https://doi.org/10.1016/j.jhazmat.2007.07.080.
(5) Mittal, A.; Mittal, J.; Kurup, L. Batch and Bulk Removal of Hazardous Dye, Indigo
Carmine from Wastewater through Adsorption. J. Hazard. Mater. 2006, 137 (1), 591–602.
https://doi.org/10.1016/j.jhazmat.2006.02.047.
(6) Mittal, A.; Mittal, J.; Kurup, L. Utilization of Hen Feathers for the Adsorption of Indigo
Carmine from Simulated Effluents. J. Environ. Prot. Sci 2007, 1, 92–100.
(7) Prado, A. G. S.; Torres, J. D.; Faria, E. A.; Dias, S. C. L. Comparative Adsorption Studies
of Indigo Carmine Dye on Chitin and Chitosan. J. Colloid Interface Sci. 2004, 277 (1),
43–47. https://doi.org/10.1016/j.jcis.2004.04.056.
(8) Wang, J.; Lu, L.; Feng, F. Improving the Indigo Carmine Decolorization Ability of a
Bacillus Amyloliquefaciens Laccase by Site-Directed Mutagenesis. Catalysts 2017, 7 (9),
275. https://doi.org/10.3390/catal7090275.
(9) Palma-Goyes, R. E.; Silva-Agredo, J.; González, I.; Torres-Palma, R. A. Comparative
Degradation of Indigo Carmine by Electrochemical Oxidation and Advanced Oxidation
26

Processes. Electrochim. Acta 2014, 140, 427–433.
https://doi.org/10.1016/j.electacta.2014.06.096.
(10) Flox, C.; Ammar, S.; Arias, C.; Brillas, E.; Vargas-Zavala, A. V.; Abdelhedi, R. Electro-
Fenton and Photoelectro-Fenton Degradation of Indigo Carmine in Acidic Aqueous
Medium. Appl. Catal. B Environ. 2006, 67 (1–2), 93–104.
https://doi.org/10.1016/j.apcatb.2006.04.020.
(11) Hammami, S.; Oturana, M. A.; Oturana, N.; Bellakhal, N.; Dachraoui, M. Comparative
Mineralization of Textile Dye Indigo by Photo-Fenton Process and Anodic Oxidation
Using Boron-Doped Diamond Anode. Desalin. Water Treat. 2012, 45 (1–3), 297–304.
https://doi.org/10.1080/19443994.2012.692059.
(12) Subramani, A. K.; Byrappa, K.; Ananda, S.; Lokanatha Rai, K. M.; Ranganathaiah, C.;
Yoshimura, M. Photocatalytic Degradation of Indigo Carmine Dye Using TiO2
Impregnated Activated Carbon. Bull. Mater. Sci. 2007, 30 (1), 37–41.
https://doi.org/10.1007/s12034-007-0007-8.
(13) Sood, S.; Kumar, S.; Umar, A.; Kaur, A.; Mehta, S. K.; Kansal, S. K. TiO 2 Quantum
Dots for the Photocatalytic Degradation of Indigo Carmine Dye. J. Alloys Compd. 2015,
650, 193–198. https://doi.org/10.1016/j.jallcom.2015.07.164.
(14) Huo, P.; Kumar, P.; Liu, B. The Mechanism of Adsorption, Diffusion, and Photocatalytic
Reaction of Organic Molecules on TiO2 Revealed by Means of On-Site Scanning
Tunneling Microscopy Observations. Catalysts 2018, 8 (12), 616.
https://doi.org/10.3390/catal8120616.
(15) Bora, L. V.; Mewada, R. K. Visible/Solar Light Active Photocatalysts for Organic
Effluent Treatment: Fundamentals, Mechanisms and Parametric Review. Renew. Sustain.
Energy Rev. 2017, 76 (April), 1393–1421. https://doi.org/10.1016/j.rser.2017.01.130.
(16) Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis. Chem. Rev. 2016, 116
(17), 10075–10166. https://doi.org/10.1021/acs.chemrev.6b00057.
(17) Lima, C. S.; Batista, K. A.; García Rodríguez, A.; Souza, J. R.; Fernandes, K. F.
27

Photodecomposition and Color Removal of a Real Sample of Textile Wastewater Using
Heterogeneous Photocatalysis with Polypyrrole. Sol. Energy 2015, 114, 105–113.
https://doi.org/10.1016/j.solener.2015.01.038.
(18) Nowakowska, M.; Szczubiałka, K. Photoactive Polymeric and Hybrid Systems for
Photocatalytic Degradation of Water Pollutants. Polym. Degrad. Stab. 2017, 145, 120–
141. https://doi.org/10.1016/j.polymdegradstab.2017.05.021.
(19) Marin, M. L.; Santos-Juanes, L.; Arques, A.; Amat, A. M.; Miranda, M. A. Organic
Photocatalysts for the Oxidation of Pollutants and Model Compounds. Chem. Rev. 2012,
112 (3), 1710–1750. https://doi.org/10.1021/cr2000543.
(20) Nielsen, C. B.; Johnsen, M.; Arnbjerg, J.; Pittelkow, M.; McIlroy, S. P.; Ogilby, P. R.;
Jørgensen, M. Synthesis and Characterization of Water-Soluble Phenylene-Vinylene-
Based Singlet Oxygen Sensitizers for Two-Photon Excitation. J. Org. Chem. 2005, 70
(18), 7065–7079. https://doi.org/10.1021/jo050507y.
(21) Muktha, B.; Madras, G.; Guru Row, T. N.; Scherf, U.; Patil, S. Conjugated Polymers for
Photocatalysis. J. Phys. Chem. B 2007, 111 (28), 7994–7998.
https://doi.org/10.1021/jp071096n.
(22) Wang, Z. J.; Li, R.; Landfester, K.; Zhang, K. A. I. Porous Conjugated Polymer via Metal-
Free Synthesis for Visible Light-Promoted Oxidative Hydroxylation of Arylboronic
Acids. Polymer (Guildf). 2017, 126, 291–295.
https://doi.org/10.1016/j.polymer.2017.04.052.
(23) Diederich, F.; Martin, R. E. Linear Monodisperse p -Conjugated Oligomersꢀ: Angew.
Chem. Int. Ed. 1999, 38, 1350–1377. https://doi.org/10.1002/(SICI)1521-
3773(19990517)38:10<1350::AID-ANIE1350>3.0.CO;2-6.
(24) Cárdenas, J. C.; Ochoa-Puentes, C.; Sierra, C. A. Phenylenevinylene Systems: The
Oligomer Approach. In Conducting Polymers; 2016; p Ch. 10.
(25) Cárdenas, J. C.; Ochoa-Puentes, C.; Gutiérrez-Puebla, E.; Sierra, C. A. Synthesis, Crystal
Structure Determination and Photoluminescence Properties of a Pure Anti Trans-Trans
28

Phenylenevinylene Derivative. Synth. Met. 2016, 215, 194–199.
https://doi.org/10.1016/j.synthmet.2016.02.021.
(26) Estrada, S. E.; Ochoa-Puentes, C.; Sierra, C. A. Phenylenevinylene Oligomers by
Mizoroki-Heck Cross Coupling Reaction. Structural and Optoelectronic Characterization.
J. Mol. Struct. 2017, 1133, 448–457. https://doi.org/10.1016/j.molstruc.2016.12.032.
(27) Estrada-Flórez, S. E.; Moncada, F. S.; Lanterna, A. E.; Sierra, C. A.; Scaiano, J. C.
Spectroscopic and Time-Dependent DFT Study of the Photophysical Properties of
Substituted 1,4-Distyrylbenzenes. J. Phys. Chem. A 2019, 123 (30), 6496–6505.
https://doi.org/10.1021/acs.jpca.9b04492.
(28) Morales-Vidal, M.; Boj, P. G.; Villalvilla, J. M.; Quintana, J. A.; Yan, Q.; Lin, N. T.; Zhu,
X.; Ruangsupapichat, N.; Casado, J.; Tsuji, H.; et al. Carbon-Bridged Oligo(p-
Phenylenevinylene)s for Photostable and Broadly Tunable, Solution-Processable Thin
Film Organic Lasers. Nat. Commun. 2015, 6, 1–8. https://doi.org/10.1038/ncomms9458.
(29) Nath, K.; Chandra, M.; Pradhan, D.; Biradha, K. Supramolecular Organic Photocatalyst
Containing a Cubanelike Water Cluster and Donor − Acceptor Stacks: Hydrogen
Evolution and Dye Degradation under Visible Light. ACS Appl. Mater. Interfaces 2018,
10, 29417–29424. https://doi.org/10.1021/acsami.8b07437.
(30) Acelas, M.; Sierra, A. F.; Sierra, C. A. Exploring the Nitro Group Reduction in Low-
Solubility Oligo-Phenylenevinylene Systems: Rapid Synthesis of Amino Derivatives.
Synth. Commun. 2020. https://doi.org/10.1080/00397911.2020.1731828.
(31) Brouwer, A. M. Standards for Photoluminescence Quantum Yield Measurements in
Solution ( IUPAC Technical Report ). Pure Appl. Chem 2011, 83 (12), 2213–2228.
https://doi.org/10.1351/PAC-REP-10-09-31.
(32) Zhou, B. Y.; He, Q.; Yang, Y.; Zhong, H.; He, C.; Sang, G.; Liu, W.; Yang, C.
Binaphthyl-Containing Green- and Red-Emitting Molecules for Solution-Processable
Organic Light-Emitting Diodes. Adv. Funct. Mater. 2008, 18, 3299–3306.
https://doi.org/10.1002/adfm.200800375.
29