A Metal-Free, Nonconjugated Polymer for Solar Photocatalysis

Article abstract of DOI:10.1002/chem.201605064

Heterogeneous catalysts that can absorb light over the solar range are ideal for green photocatalysis. Recently, attention has been directed towards the generation of novel solar-light photocatalysts, in particular, metal-free polymers. Herein, it is demonstrated that a metal-free, nonconjugated, anthraquinone-based copolymer (poly[1,4-diamine-9,10-dioxoanthracene-alt-(benzene-1,4-dioic acid)] (COP)) with a strong absorption in the visible region is effective as a sunlight heterogeneous photocatalyst. As a proof of concept, it has been used to mineralize 2,5-dichlorophenol (2,5-DCP) in water under air and sunlight irradiation. The photocatalytic efficiency of COP compares well with that of TiO₂-P25 when the reaction is carried out in a solar photoreactor in acid medium. Steady-state and time-resolved (absorption and emission) studies performed on COP suspended in 6:4 DMF/H₂O have provided valuable information about the COP species generated under different pH conditions. Steady-state absorption and fluorescence data are consistent with the existence of a tautomeric equilibrium between the 9,10-keto and 1,10-iminoketo quinoid forms for the anthraquinone in the ground state. Moreover, in basic media, transient absorption measurements showed the presence of two bands ascribed to the tautomeric triplet excited states, whereas only one of the triplets was observed in acid medium. A mechanism for the photocatalyzed degradation of 2,5-DCP by COP is proposed on the basis of these observations.

Full text of DOI:10.1002/chem.201605064

A Journal of
Accepted Article
Title: A Metal-free, Non-conjugated Polymer for Solar Photocatalysis
Authors: Julia Perez-Prieto, Rafael Irigoyen, Maria Gonzalez-Bejar,
Eduardo Pino, and Jose B. Proal-Nájera
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201605064
Link to VoR: http://dx.doi.org/10.1002/chem.201605064
Supported by

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A Metal-free, Non-conjugated Polymer for Solar Photocatalysis
[
a]
[b]
[b]
[a]
Rafael Irigoyen-Campuzano, María González-Béjar, Eduardo Pino, Jose B. Proal-Nájera* and
[
b]
Julia Pérez-Prieto*
[
2]
Abstract: Heterogeneous catalysts that can absorb light in the solar
range are ideal for green photocatalysis. Recently, attention has
been directed towards the generation of novel solar-light
photocatalysts, in particular metal-free polymers. We demonstrate
are ideal for green photocatalysis.
Advanced oxidation processes (AOPs) have been widely
employed to degrade highly persistent and non-biodegradable
pollutants (pesticides, herbicides, insecticides, fungicides, dyes,
[
3, 4]
here that
a
metal-free, non-conjugated, anthraquinone-based
etc.) in wastewater and contaminated environments.
degradation under optimal conditions can lead to mineralization
of organic contaminants to CO , H O, and ions (chloride,
sulphate, or nitrates, depending on their chemical nature).
This
copolymer (COP) with strong absorption in the visible is effective as
sunlight heterogeneous photocatalyst. As a proof of concept it has
been used to produce mineralization of 2,5-dichlorophenol (2,5-
DCP) in water under air and sunlight irradiation. The photocatalytic
2
2
[
4, 5]
These processes frequently involve the use of strong oxidants
like ozone or the formation of highly reactive species, such as
hydroxyl radicals formed in situ by Fenton reaction or
2
efficiency of COP compares well with that of TiO -P25 when the
reaction is carried out in a solar photoreactor in acid medium.
Steady-state and time-resolved (absorption and emission) studies
[
6]
photochemically induced by photo-Fenton,
UV/H
Undoubtedly, semiconductor photocatalysts based on
2
are being widely studied for applications in environmental
2
O
2
or
[
5]
performed on COP suspended in 6:4 DMF:H
2
O solutions have
UV/O
TiO
remediation and solar energy conversion.
inexpensive, stable to irradiation, and it is able to oxidize
3
.
provided valuable information about the COP species generated
under different pH conditions. Steady-state absorption and
fluorescence data are consistent with the existence of a tautomeric
equilibrium between the 9,10-keto and the 1,10-imino-keto quinoid
forms for the anthraquinone in the ground state. Moreover, transient
absorption measurements showed the presence in basic media of
two bands ascribed to the tautomeric triplet excited states, whereas
only one of the triplets was observed in acid medium. A mechanism
for the photocatalyzed degradation of 2,5-DCP by COP is proposed
on the basis of these observations.
[
3, 7]
2
TiO is non-toxic,
[
8]
contaminants effectively.
Most of these studies have employed anatase, which is the most
photocatalytically active crystalline structure of TiO , with the
onset of absorption around 387 nm and band gap energy (Ebg
2
)
[
9]
of ~3.2eV.
Currently, the aim is to perform photocatalysis in the visible to
benefit from effective utilization of the solar spectrum. To that
purpose, doped (e.g., with transition metal ions or noble
[
3, 10]
metals)
and undoped nanometric-sized semiconductor
[
11]
particles or other materials, such as graphene oxide , are
being used for preparing photocatalysts for air and water
purification.
Introduction
Nowadays, the scientific community is making a huge effort to
develop greener processes in the hope of a better and more
Regarding polymeric photocatalysts, conjugated polymeric
materials are organic semiconductors that offer a wide range of
[
1]
sustainable world. Undoubtedly, solar light is considered the
cheapest, greenest and most abundant reagent for catalytic
processes. Recently, attention has been directed towards the
generation of novel photocatalysts that can be developed at a
minimal cost and degrade pollutants effectively. In this context,
heterogeneous catalysts that can absorb light in the solar range
[12]
redox potentials and they often absorb visible light.
Despite
these desirable properties for photocatalysis, there are still a
limited number of organic reactions using this kind of metal-free
polymers for solar energy conversion. Among them, carbon
[13]
nitrides-based photocatalysts
have been proposed for
photosynthesis and environmental remediation; poly(p-
phenylene) has been tested in visible-light-driven pinacol
coupling of benzaldehyde; polybenzothiadiazoles have been
[
14]
[15]
[
[
a]
b]
MSc R. Irigoyen-Campuzano, Prof. J. B. Proal-Nájera.
Instituto Politécnico Nacional. Centro Interdisciplinario de
Investigación para el Desarrollo Integral Regional- Unidad Durango.
Calle Sigma 119, Fracc. 20 de Noviembre II. C.P. 34220, Durango,
Dgo., México.
used to prepare tetrahydroquinoline , to produce hydrogen
[
16]
and also for water and wastewater treatment
;
and
poly(diphenylbutadiyne) has shown efficient in the degradation
[
17]
of pollutants such as phenol or methyl orange.
E-mail: joseproal@hotmail.com
To our knowledge, there are few reports on metal-free, non-
conjugated polymers with good absorption in the visible, and
Dr. María González-Béjar, Dr. Eduardo Pino, Prof. Julia Pérez-Prieto
Instituto de Ciencia Molecular (ICMol)/Departamento de Química
Orgánica
[18]
therefore useful for solar-light-driven photocatalysis.
We demonstrate here that
a
copolymer consisting of
Universidad de Valencia
C/ Catedrático José Beltrán 2, 46980, Paterna, Valencia, Spain
E-mail: julia.perez@uv.es
anthraquinone units linked through terephthalate units (COP),
which can be easily synthesized and coated on a glass surface,
can act as an efficient solar photocatalyst for dichlorophenols
degradation. In particular, we have used supported COP for the
photodegradation of 2,5-dichlorophenol (2,5-DCP) in water and
demonstratd that COP activity is exclusively due to visible light
Supporting information for this article is given via a link at the end of
the document.
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absorption. The degradation has been scaled-up with a solar
that can be ascribed to the amide C=O stretching and perfectly
[
20]
reactor and its photocatalytic activity has been compared to TiO
2
matches that reported for the model compound (1,4-DBAAQ).
-
1
(
used as standard photocatalyst). Steady-state and time-
The other C=O band at 1687 cm can be attributed to the
carbonylic groups of the anthraquinone moiety, shifted to a
resolved (absorption and emission) studies carried out on COP
solutions shed light on the mechanism of this photocatalytic
process.
-
1
higher frequency (ca. 76 cm ) in comparison with that of 1,4-
DAAQ. Indeed, the N-H vibrations of the secondary amide are
-
1
observed at 3450 cm , whereas the characteristic double band
[
20, 21]
pertaining to the amine groups of 1,4-DAAQ disappeared.
Results and Discussion
1,4-DBAAQ was soluble in common organic solvents, such as
methanol, dioxane, chloroform, and DMF. However, COP
proved to be soluble in DMF, but it was hardly soluble in many
other organic solvents and it was insoluble in water (Figure S5
and Table S1 in the Supporting Information). This low solubility
Synthesis and characterization of the copolymer and a
model compound
Poly(1,4-diamine-9,10-dioxoanthracene-alt-[bencen1,4-dioic
acid]), COP, was easily prepared following a similar strategy to
[
22]
has already been reported for other anthraquinone polymers.
Gel permeation chromatography (GPC) analysis showed that
[
19, 20]
that reported for analogous polycondensations.
Thus, the
-
1
the molar mass of COP was smaller than 1000 g mol . High
resolution mass spectrometry analysis showed the molecular
mass of COP is 901.96, and corresponds to an oligomer of
reaction of theraphtaloyl chloride (CTer) with 1,4-
diaminoanthraquinone (DAAQ) in dimethylformamide (DMF) led
to a brownish-red solid, whose spectroscopic data (see below)
were consistent with the formation of COP (83% yield); see
Scheme 1 and Figure S1. Likewise, a model compound, 1,4-
degree of polymerization
5 (see further details in in the
Supporting Information and Figure S6). Thermogravimetric
analysis (TGA) of COP showed a main weight loss (50%) at ca.
dibenzoylamino-9,10-anthraquinone
(1,4-DBAAQ),
was
[
20]
3
53 °C, which is indicative of considerable thermal stability
prepared by reaction of 1,4-DAAQ with benzoyl chloride (See
Scheme 1 in the Supporting Information).
[
20]
(
Figure S7 in the Supporting Information).
Photophysical characterization of COP
COP was not water-soluble, but it proved to be soluble enough
in a (6:4) DMF:H O mixture. This made it possible to determine
2
its photophysical behavior under conditions closer to that of the
photocatalytic process (see below).
Figure 1. Left: UV-Visible absorption spectra of COP and 1,4-DBAAQ in
DMF/H
DMF/H
2
O (6:4). Right: UV-Visible spectra of COP in methanol (black line),
O (red line), and toluene (blue line).
Scheme 1. Reaction between 1,4-diamino-9,10-anthraquinone (1,4-DAAQ)
and terephthaloyl chloride (CTer) to generate poly(1,4-diamine-9,10-
dioxoanthracene-alt-[bencen1,4-dioic acid], COP. Structure of 1,4-
dibenzoylamino-9,10-anthraquinone (1,4-DBAAQ), and 2,5-dichlorophenol
2
(
2,5-DCP).
Figure 1 (left) shows the UV-Visible absorption spectrum of COP
in DMF/H O (which clearly differs from the UV-Visible spectrum
2
of the starting materials, see Figure S8 in the Supporting
Information). Both 1,4-DBAAQ and COP present a strong
absorption band in the visible which can be attributed to ππ*
1
The H-NMR spectrum of COP revealed the absence of free
monomers and evidenced the important differences in the
position and number of chemical shifts compared with those of
[
23]
charge transfer electronic transitions.
Considering that the
1
,4-DBAAQ (Figures S2 and S3 in the Supporting Information).
molecular weight determined by MALDI TOF spectrometry was
-
1
This indicated hindered rotation and, consequently, the
appearance of different chemical environments in the copolymer
backbone.
of 902.81 g mol , we estimated an extinction coefficient of COP
6
-1
-1
-1
at 525 nm of 2.5 x 10 M cm in polar and non-polar solvents.
The absorption spectrum of COP changes slightly as solvent
polarity increases; specifically the visible band becomes broader
and blue-shifted (Figure 1, right). This solvent dependence was
The Fourier transform infrared (FT-IR) spectrum of COP was
compared with those of the starting materials (Figure S4 in the
-
1
Supporting Information). It presents a weak band at 1634 cm
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not observed for 1,4-DBAAQ (Figure S9 in the Supporting
Information).
of the singlet excited states. In this case, only a band with a
maximum at 574 nm was observed.
The absorption spectrum of COP in a 6:4 DMF:H
2
O exhibits low
The emission spectrum of 1,4-DBAAQ mainly showed the longer
wavelength emission band which was considerably red-shifted
compared to that of COP, specifically at λmax ca. 620 nm, (Figure
3 and Figure S10 in the Supporting Information). In addition, the
emission of 1,4-DBAAQ was slightly dependent on the solvent,
and it blue-shifted to shorter wavelengths as the solvent polarity
decreased (Figure 3).
dependence with the pH (see Figure 2 for pH 3, 7, and 10).
It has been previously reported that 1-acylaminoanthraquinones
can undergo excited-state intramolecular proton transfer
(
ESIPT). As a consequence, two singlet excited states, the 9,10-
keto (normal form, S (N)) and the 1,10-imino-keto (tautomeric
form, S (T)) are formed (Scheme 2).
process can be controlled by varying the acyl substituent
1
[
24]
1
The extent of this
[
24, 25]
group.
2
Figure 3. Normalized emission spectra of (left) a COP solution in DMF/H O
(
6:4), methanol, and toluene; (right) normalized emission spectra of a 1,4-
DBAAQ solution in DMF/H
nm).
2
O (6:4), methanol, dioxane, and toluene (λex= 470
Figure 2. UV-Visible spectra of COP in methanol DMF/H
black line), 7 (red line), and 9.5 (blue line).
2
O (6:4) at pH 3
(
The absolute fluorescence quantum yield (Φ) of COP was
-
5
7
.8x10 , which is in accordance with the values reported for
[
24, 26]
other 1-acylaminoanthraquinones.
The low fluorescence of
anthraquinone derivatives has been attributed to fast internal
conversion due to intra- and intermolecular hydrogen bonds, as
well as to intersystem crossing to the triplet excited state.
The emission spectra of COP and 1,4-DBAAQ were also
recorded under air atmosphere, thus confirming that oxygen has
no effect on their emissive properties (See Figure S11 in the
Supporting Information).
Scheme 2. Proposed equilibrium between the normal (N) and the tautomeric
T) forms of the 1,4-diaminoanthraquinone units of 1,4-DBAAQ and COP in
the ground state (S ).
(
0
To determine if COP could undergo ESIPT, first we registered its
emission spectrum in DMF/H O (6:4), methanol, and toluene,
2
Figure 4. Left: Excitation spectra of an anthraquinone-based copolymer
and compared the results with those of 1,4-DBAAQ, measured
in the same solvents as COP and also in dioxane (Figure 3).
The copolymer exhibited a dual emission in polar solvents. The
2
solution in DMF/H O (6:4), methanol or toluene and Right: 1,4-DBAAQ
solution in DMF/H O (6:4), methanol, dioxane or toluene (λem= λem,max).
2
emission spectrum of COP in DMF/H
maximum at 570 nm (ascribed to the S
shoulder at 604 nm (ascribed to the S (T) species), whereas the
emission maximum was detected at 604 nm with a shoulder at
70 nm in methanol. Toluene was used as a non-polar solvent
to evaluate the influence of the solvent polarity on the emission
2
O (6:4) exhibited a
1
(N) species) and a
Then, excitation spectra of COP and 1,4-DBAAQ were
registered in different solvents to gain insight into the nature of
the light absorbing species, eventually leading to the dual
emission (Figure 4). Each excitation spectrum was recorded at
the wavelength with the maximum emission intensity.
Remarkably, while the excitation spectrum of COP in toluene
1
5
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was similar to its absorption spectrum, it shifted to shorter
wavelengths and exhibited structured features in polar solvents.
By contrast, the excitation spectrum of 1,4-DBAAQ in all the
solvents tested resembled that of the absorption spectrum,
though it showed some structured features at shorter
wavelengths. The excitation spectra of these compounds at
shorter wavelengths can be attributed to the normal 9,10-
The transient absorption spectrum of COP was registered in
2
DMF:H O (6:4) at different pHs. While both triplets were
detected in neutral and basic media, only the absorption of T
was observed at pH 3 (see Figure 5D for pH 3 and 10).
1
(T)
Solar photodegradation of 2,5-dichlorophenol
Chlorophenols are weak acids and can be partially dissociated
[
28]
anthraquinone form,
wavelengths can be assigned to the 1,10-quinoid tautomeric
form, S (T). These observations are consistent with the two
emissions mostly originating after excitation of S (N) and S (T),
S
0
(N), whereas those at longer
in water. Thus, 2,5-DCP (pKa2,5-DCP=7.51) is a phenol at pH 3
and pH 4, a phenolate at pH 11, and a phenol/phenolate
equilibrium at pH 7; this can be appreciated in the UV-visible
absorption spectra of 2,5-DCP shown in Figures 6 and Figure
S12 in the Supporting Information.
0
0
0
in equilibrium at the ground state (Scheme 2). In COP, the
contribution of each form changes with the solvent polarity.
2
Moreover, the emission spectra of COP in DMF/H O (6:4)
registered at different pHs indicated that this equilibrium was not
influenced by changes in the pH of the medium (see spectra
below in the quenching studies).
Laser flash photolysis experiments (laser pulse 10 ns, 15 mJ)
were performed to determine the formation of the triplet excited
state of each of the forms in equilibrium at the ground state. The
transient absorption spectrum of 1,4-DBAAQ in dioxane (Figure
5
, top) showed the formation of two broad transient absorptions
with maxima at 370 nm and 470 nm (see Figures 5A,B).
While that at 370 nm could be attributed to the triplet of the 9,10-
anthraquinone form, T
the triplet of the 1,10-quinoid, T
well with that of other 1,10-anthraquinone triplets.
lifetimes of T (N) and T (T), measured in oxygen-free dioxane
solutions, were 11.9 µs and 9.8 µs, respectively. Spectra
registered in the presence of oxygen showed that T (N) was
more sensitive to oxygen than T (T). By contrast, the transient
absorption spectrum of 1,4-DBAAQ in DMF:H O (6:4) revealed
(T) tautomeric form with a lifetime
1
(N), that at 470 nm could be assigned to
(T), whose spectrum compares
Figure 6. UV-Visible spectra of 2,5-dichlorophenol at pH 3 (black line) and 10
1
[
27]
(red line) in water.
The
1
1
0
A fixed bed photoreactor with panels oriented at an angle of 18
1
was used for the photochemical experiments to maximize the
average solar irradiation on the panels (Figure 7). In order to
have an easy separation of the solution after illumination, the
photocatalyst was sprayed on a glass surface and fixed to the
bottom surface of the reactor (See experimental section for
further details).
1
2
the presence of mainly the T
1
of ca. 4.5 µs (Figure 5C).
Figure 5. Top: transient absorption spectra of 1,4-DBAAQ in dioxane under
nitrogen (A) or oxygen (B). Bottom: transient absorption spectra of (C) 1,4-
Figure 7. Photograph and basic process design of the solar photocatalysis
DBAAQ in DMF/H
2
O 6:4 and under nitrogen and (D) COP in DMF/H
2
O (6:4)
set-up comprising: 1-L tank; an effluent recycling loop; a metallic surface to
under nitrogen at pH 3 and 10.
place the light-sensitive and –active photocatalyst (TiO
glass.
2
, left, COP, right) on
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Then, a continuous and constant flow rate (172 L/h) of reactant
solution (2,5-DCP, 20 ppm) was fluxed through the reactor; the
volume treated was 1L. All the experiments were performed in
the photoreactor under natural sunlight (see Figure S13 in the
Supporting Information). The concentration of 2,5-DCP
decreased under sunlight. The degradation kinetics were
obtained by plotting the ratio between the concentration of 2,5-
the photocatalyzed degradation of 2,5-DCP aqueous solutions at
pH 3 was performed in a photoreactor under air and visible light
irradiation (Figure S14 in the Supporting Information). Results
obtained with different glass plates differed by <5%. The trends
were assessed using the same plate. Moreover, it was very easy
to analyze the reaction mixture by taking aliquots and recording
their UV-visible spectrum at several irradiation times since the
COP remained on the support. Between each test, the
0
DCP over time (C) and the initial concentration (C ) at several
time intervals. This was evaluated by UV-visible spectroscopy
either in the absence (photolysis) or in the presence of the
photocatalyst. From these kinetic values, the photodegradation
or photocatalytic activity was calculated (e.g. the efficiency and
the average operational rate constants and lifetimes).
supported COP was thoroughly washed with H
traces were detected after rinsing).
2
O (no 2,5-DCP
Table 1. Main kinetic parameters calculated from the photocatalytic testing
for 2,5-DCP degradation under sunlight at pH 3.
The photocatalytic efficiency of COP was compared with that of
TiO
surface individually, and the photocatalytic degradation of the
,5-DCP solutions was performed in two reactors in parallel
2 2
. To that purpose, TiO and COP were supported on a glass
-
1
Phot.
k
op [min ]
τ [min]
144
Average values
TiO op=5.41x10
τ = 130
op=5.25x10
τ = 139
2
-3
-3
-3
TiO
TiO
2
2
4.80x10
4.00x10
6.01x10
6.51x10
2
k
under solar light irradiation. Photocatalytic testing experiments
were carried out in duplicate. COP showed similar photocatalytic
[
a]
-3
-3
-3
173
2
efficiency to TiO at pH 3 (33% vs 25%), (Figure 8).
COP
COP
115
COP
k
[
a]
106
[
a] Replicate.
The photodegradation was repeated and followed by HPLC-
DAD, measuring the 2,5-DCP consumption (Figure S14 in the
Supporting Information). No photoproducts were detected, which
is in accordance with the absence of new bands in the UV-
visible spectra (Figure S15 in the Supporting Information).
Photodegradation at pH 3 was 45% after 180 min (Figure 9).
Similar experiments at pH 10 showed less phenol mineralization
(
only a 10%). This is consistent with the observation of COP
2
Figure 8. Comparison of the activities of TiO (black dot) and COP (red
detachment from the plate.
square) deposited on a glass surface at pH 3 using two reactors in parallel.
Photolysis at pH 3 has been included for comparison (black empty circles).
The photocatalytic activity is based on the rate of the 2,5-dichlorophenol (20
0
ppm) degradation (C/C ) monitored by UV-Visible absorption at several time
intervals; results in duplicate.
However, COP loss from the glass substrate at pH 10 (see
control studies below) led to a less 2,5-DCP degradation under
[
29]
the otherwise same conditions.
In addition, control
experiments showed that degradation due to temperature
changes in the photoreactor during operation was not significant.
Table
1
summarizes the main operational parameters,
operational rate constant (kop), and kinetic values calculated
from the positive experimental results at acidic pHs for both
photocatalysts, TiO
2
and COP. This is in accordance with
Figure 9. Figure Ratio of 2,5-DCP (0.2 mM) degradation vs irradiation time
upon visible light irradiation of supported COP in water at pH 3 (black square)
and pH 10 (red circle) under air.
previous reports related to the degradation of chlorophenols
[
30]
using TiO
2
-P25 as the photocatalyst.
Both photocatalysts
showed similar efficiency in the acidic medium.
Comparative studies were performed in the lab, using COP
supported on a glass plate to simulate as much as possible the
fixed bed photoreactor. Photocatalyzed degradation of 2,5-DCP
aqueous solution was not observed under UVA irradiation. Then,
In addition, the amount of the organic content in water after
irradiation was also evaluated by using Chemical Oxygen
Demand (COD) analysis. These results together with the
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Chemistry - A European Journal
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absence of new photoproducts in the HPLC-DAD analyses,
agree well with mineralization of 2,5-DCP.
The emission spectrum of COP was registered at different pHs
(3, 7 and 10) and in the presence of increasing amounts of 2,5-
DCP. Figure 10 shows the higher quenching efficiency of COP
emission by the phenol in the acidic medium, as well as the
upward dependence of the emission, which indicates the
contribution of both static and dynamic quenching.
Finally, for comparative purposes, experiments were carried out
using suspended COP and carrying the process at pH 3. First,
three aerated cuvettes containing 2,5-DCP, COP, and a mixture
of both of them, respectively, were irradiated with UVA light or
visible light. Only COP is capable of absorbing visible light under
these conditions (see Figure S16 in the Supporting Information).
Both 2,5-DCP and COP solutions were stable at pH 3.0 under
illumination and in the dark. Interestingly, photodegradation was
exclusively observed for the aerated mixture of 2,5-DCP and
COP under visible light irradiation and the supported COP was
more efficient than the suspension.
Time-resolved fluorescence measurements were carried out to
corroborate the existence of both types of quenching. These
experiments evidenced not only the decrease of the initial
emission with the quencher concentration and, therefore, the
contribution of static quenching, but also that this was the main
mechanism involved in the emission quenching. Thus, the
emission spectra revealed significant interaction between COP
and 2,5-DCP in the ground state, while the UV-visible absorption
spectrum was not sensitive enough to show such interaction
(Figure S19 in the Supporting Information).
Moreover, photodegradation was inhibited at high COP
concentrations (0.5 g/L) (Figure S17 in the Supporting
Information). The optimal concentrations found for the
degradation experiments were COP (0.1 g/L) and 2,5-DCP (0.2
mM) (see Figure S18 in the Supporting Information). Attempts to
recover the suspended COP from the reaction mixture and to
obtain the adsorption isotherm in the dark were fruitless.
In addition, time-resolved absorption studies were carried out in
COP/2,5-DCP mixtures at pH 3 at which no COP detachment
from the plate was detected. The decrease in the formation of
S
1
(T) at increasing concentrations of 2,5-DCP and, consequently,
of T (T), made it more difficult to follow the potential quenching
of T (T).
1
Mechanistic studies of the photocatalyzed degradation of
1
2
,5-DCP by COP
From the above data, it can be inferred that the species involved
in the transformation of 2,5-DCP in acid medium photocatalyzed
by COP are the undissociated phenol and the triplet excited
The study of COP photophysics was carried out in the presence
of 2,5-DCP in order to gain insight into the photocatalytic
mechanism. These studies were performed in DMF/H
2
O (6:4)
1
state of the tautomeric form T (T).
solution in order to mimic the photodegradation operational
conditions as much as possible and because COP is not water-
soluble.
Scheme 3. Proposed mechanism for the photocatalyzed degradation of 2,5-
DCP in water (pH 3) by COP in sunlight.
1
The reaction of 2,5-DCP with T (T) could involve an intermediate
hydrogen bonded exciplex that would lead to the phenoxyl and
ketyl radicals via an electron transfer proton transfer (ETPT)
[
31]
mechanism (Scheme 3). Reaction of the resulting ketyl radical
with oxygen would produce a peroxy radical, which would
eventually give rise to HOO· peroxyl radicals and to the recovery
of the photocatalyst. Further oxidation of the phenoxyl radical
Figure 10. Top and bottom left: Emission spectra of COP in the absence
and in the presence of increasing amounts of 2,5-DCP in DMF/H
different pHs (3, 7 and 10). Bottom right: Stern-Volmer plots for singlet
quenching of the copolymer by (2,5-DCP) in DMF/H O at pH 3, 7 and 10 (λex
70 nm). Area under the emission.
2
O (6:4) at
[
32]
could occur by its reaction with the peroxyl radical.
2
=
4
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Conclusions
Synthesis of the organic photocatalyst: poly(1,4-diamine-9,10-
dioxoanthracene-alt-[bencen1,4-dioic acid]). In a flask containing 0.6091g
(
2
3.0 mmol) of theraphtaloyl chloride (CTer) was purged with N and
We demonstrate here for the first time that a metal-free, non-
conjugated, anthraquinone-based copolymer with strong
absorption in the visible is effective as sunlight heterogeneous
photocatalyst for the mineralization of 2,5-DCP in water. Steady-
state and time-resolved studies carried out with the copolymer in
dissolved with DMF. Simultaneously, 0.7147g (3.0 mmol) of 1,4-
diaminoanthraquinone (1,4-DAAQ) in DMF were degassed in another
flask. Then, the 1,4-DAAQ solution was added with a syringe to flask
than contained the CTer solution. The mixture was stirred for 1 h at room
temperature. Next, the reaction mixture was poured into an erlenmeyer
with 50 mL of cold water and stirred vigorously until precipitation was
observed. The mixture was filtered under vacuum and rinsed with cold
water. The brownish-red solid was dried at 100 °C for 2 h.
2
solution (DMF:H O) were very valuable to gain information about
the involved species in the degradation process. We postulate
that in acid medium the tautomeric 1,10-ketoimino triplet excited
state is the photocatalytic active species which, in the presence
of oxygen, eventually leads to the reactive oxygen species and
the phenoxyl radical. The smaller efficiency in basic media is
related with progressive detachment of the copolymer from the
glass surface in such media and, therefore, future studies will be
addressed to find the proper substrate to fix the polymer firmly to
its surface.
Solar reactor. The samples used for photocatalysis experiments were
prepared using a commercial frosted glass (6 mm thickness) and a
2
surface of 100 cm (8.0 cm x 12.5 cm).
The glass surface was washed with a commercial detergent, rinsed with
tap water followed by demonized water, and washed with ethanol to get
rid of possible organic contaminants. A dispersion of TiO
2
P25 (5 mL of
0 mg/L) or copolymer was sprinkled onto the clean glass surface at
0 °C using a commercial sprinkler (compressor and airbrush). The
(23.61 ± 0.23 mg/plate) and copolymer (21.08 ± 1.29
4
6
amount of TiO
2
Experimental Section
[
33]
mg/plate) on the glass was similar, according to Stintzing (2003). The
as-prepared impregnated surface was dried for two hours.
Materials and methods
Visual inspection of coated glass supports was positive for both
Materials. 1,4-diaminoanthraquinone (1,4-DAAQ), terephthaloyl chloride
2
photocatalysts, with TiO changing color to white and anthraquinone-
(
CTer), 2,5-dichlorophenol (2,5-DCP), dimethylformamide (DMF),
based copolymer changing to dark red, which clearly indicates that the
coating technique has been successful.
Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC•HCl),
benzoic acid (AB) and 9,10-anthracenediyl-bis(methylene)dimalonic acid
(
ABDA) were purchased from Aldrich and used without further
purification. 1-hydroxibenzotriazol (HBOt) was purchased from Fluka.
TiO Aeroxide ® P25 was obtained from Evonik Degussa. Classical silica
A
prototype solar photoreactor consisting of two identical but
independent reactors was setup. Each reactor consisted of a flat solar
panel” (See Figure 7) with the following bed dimensions: 34 cm wide
2
“
gel 60 F254 plates were used for thin layer chromatography (TLC) and
silica 60 0.040-0.063 (230-400 mesh) for column chromatography.
Solvents used for this study (dichloromethane, ethyl acetate and hexane)
were distilled and dried prior use.
and 51 cm long and placed 15 cm above the reactor metallic base (34
cm wide, 51 cm long and 21 cm height). The effluent was pumped to the
reactor inlet (Bomb BIOPRO model H-331) and then recirculated to the
sprinkler reactor (Compressor Oswald, motor Siemens 1RF3-054-4YC31,
PVC pipe): 48 cm Length, ½ inch inner diameter, 8.5 cm of perforations
in the central part right on top of the glass coated plate.
Commercial frosted glass plates (8 cm x 12.5 cm) were used as
photoinert supports for the photocatalyst used in this study: TiO
anthraquinone-based copolymer.
2
and
During the experiments, radiation measurements were carried out using
a precision Global Water’s WE300 radiation sensor (High-stability silicon
Methods and equipment
-
2
photovoltaic detector -blue enhanced-; Range: 0-1500 Wm ; spectral
range: UVA - Visible). The average solar radiation either for photolysis or
1
13
Methods. H-NMR and C-NMR spectra were recorded on an AVANCE
00 MHz Bruker spectrometer. Here NMR data are reported in ppm,
coupling constants in Hz and multiplicity as follows: singlet (s), doublet
d), triplet (t), quartet (q) or multiplet (m).
2
photocatalytic experiments was 616.6 W/m . An hourly Gaussian graph
3
2
(
Watts/m ) showing the maximum and minimum radiation received vs.
time that clearly shows the amount of irradiation over the coated glass
surfaces during the experiments carried out in the same frame time
during the day (See Figure S12).
(
Infrared spectra were obtained using
a FT-IR Brucker Tensor 27.
-
1
Measurements were collected at 8 cm resolution and 4 scans per
spectrum.
Photodegradation or photocatalytic activity testing. In all cases, the
solution pH was adjusted with H
required.
2 4
SO 0.1M and/or NaOH 0.1M when
High-resolution mass data were obtained on
a TripleTOF™ 5600
LC/MS/MS System, (AB Sciex) spectrometer in positive mode and
evaluated using PeakView™.
Comparative experiments between the photocatalytic activity of the
anthraquinone-based copolymer or TiO were conducted under the same
2
experimental conditions and time (by duplicate).
MALDI-TOF was carried out in a 5800 MALDI TOFTOF (Sciex) in
positive and negative reflectron modes (1500 shots every position) in a
mass range of 200-1500 m/z in positive mode and 200-3000 m/z in
negative.
Samples were taken every five minutes during the first half hour
(
including time=0 min). Then, samples were taken at 40, 60 and 90 min
to determine the photodegradation kinetics. To that purpose, the aliquots
were measured by UV-Visible spectroscopy to evaluate the concentration
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Chemistry - A European Journal
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of pollutant during the experiments in a HACH DR 500 spectrometer
since HPL-DD was not available nearby.
Time-resolved fluorescence experiments. These experiments were
carried out by using either a compact fluorescence lifetime spectrometer
C11367, Quantaurus-Tau with a LED pulse light source at 540 nm
(
Instrument Response factor: Ludox®;
Microbeam GL-3300 nitrogen laser/dye laser (C-440 (5x10 M/MeOH)
437 nm; 427-457 nm], 814 photomultiplier detection system
Photon Technology International) and λex=405 nm (200 channels, 7 Hz,
λem=539 and 653 nm) or a
Photocatalyst-coated glass. 7.1 mg of COP were dissolved in 5 mL of
MeOH and sonicated for 5 minutes. The borosilicate glass plate was
coated on both sides and by drop casting the COP solution and
evaporated at 70 °C for 5 min (repeated 10 times to use the entire
volume of dissolved COP, on both sides of the plate). The plate was
-
3
max
[λ :
(
integration time 50 µs, 1 Averages and 5 shots: 5). The concentration of
the samples was the same of that used for the steady-state fluorescence
experiments. The average lifetime was calculated from the bi-exponential
decays.
2
thoroughly washed with milliQ H O and dried at 60 °C for 60 min.
HPLC analyses. 2,5-DCP photodegradation was analyzed by HPLC-
DAD VWR HITACHI (Elite LaChrom) under the following conditions: a
RP 18 column, 250 mm, was used for chromatographic separations at
Laser flash photolysis experiments. The measurements were carried
out with the second harmonic (532 nm) of a Q-switched Nd:YAG laser
2
2
5°C. The mobile phase, MeOH:H O (70:30, v/v), was vacuum filtered
(
Spectra Physics QuantaRay (Indi); pulse width ca. 9 ns and 15 mJ per
with a filtering kit. The mobile phase was set at 0.8 mL/min, and the
detection wavelengths were set at 215, 250, 280 and 300 nm.
pulse). The signal from the monochromator/photomultiplier detection
system (Luzchem Research) was captured by a Tektronix TDS3032B
digitizer and transferred to a PC computer that controlled the experiment
and provided suitable processing and data storage capabilities. Basically,
solutions of the anthraquinone-based copolymer or the model compound
For tests at pH 10.0 COP detachment was observed by absorbance
measurements and staining of the solution before addition of 2,5-DCP.
1
,4-DBAAQ (3 mL) were prepared with an absorbance of 0.3 at 532 nm
Chemical Oxygen Demand (COD). This was determined using a Hanna
and irradiated with a pulsed 532 nm excitation wavelength. Kinetics were
obtained upon excitation of the corresponding solution and monitored at
the desired wavelength (e.g. 470 nm). The lifetime values were
calculated by using an exponential fitting. Several solvents and pHs were
C214 Instrument and Digester for heating.
Photocatalysis in suspension. The COP solutions used for
photocatalytic experiments were prepared at two concentrations (0.1 or
used for these studies. The solutions were degassed with N
minutes when required for the experiment.
2
for 20
0
2
.5 g/L) in DMF:H 0 at pH 3 or 10. Then, 2,5-DCP was added and prior
to illumination, each suspension was stirred in the dark for 30 min to
achieve the adsorption–desorption equilibrium. Then, samples (3 mL)
were irradiated in a quartz cuvette (1x1 cm) under air in a Luzchem
photoreactor (SOLSIM-5) at fixed intervals. Light sources tested were
UVA (λmax = 350 nm) or visible lamps (400-700 nm). At those intervals,
aliquots were taken (3 mL) and filtered through a 0.22 mm Millipore filter
and used for UV-Visible measurements.
Reactive Oxygen Species (ROS). Singlet oxygen generation was
monitored via chemical photoconsumption of 9,10-anthracenediyl-
bis(methylene)dimalonic acid (ABDA). The samples were placed in
quartz cuvettes (1x1 cm path length) and the emission was monitored in
an AB2 Series spectrofluorometer. All measurements were carried out
under air at room temperature. The induced generation of singlet oxygen
was estimated by measuring the decrease of ABDA fluorescence due to
Photophysical studies. In all cases, the solution pH was adjusted with
[34]
the formation of its endoperoxide.
Briefly, a stock solution of ABDA
2 4
H SO 0.1M and/or NaOH 0.1M when required.
1
.36 m (56 mg/ 10 mL) in DMF/water 6:4 was prepared. To a solution of
the copolymer in DMF/water (6:4) and absorbance of 0.1 at pH=3, 25 µL
of the ABDA stock solution were added (total volume of 1.5 mL in a 3mL
quartz cuvette). The emission of ABDA in the mixture was registered (λex
= 380 nm) and next, the solution was irradiated at 470 nm in order to
excite copolymer and evaluate the singlet oxygen generation. The ABDA
emission was recorded at different irradiation time intervals (1, 3, 5, 10,
UV-Visible measurements were performed using
equipped with quartz cuvettes. All samples were analyzed in milliQ water
or DMF:H O solutions.
a Uvi light XT5
2
Steady-state fluorescence. A solution (3 mL) of the anthraquinone-
based copolymer DMF/water (6:4) with an absorbance of 0.1 at 532 nm
was tested for fluorescence using an AB2, AMINCO. Bowmann Series 2
fluorometer (slits, 4 nm; 800 V; λex= 470 nm; 40% full range; scan 5
nm/s). All measurements were performed at room temperature using the
same quartz cuvette (1cm×1cm path length). Measurements were
performed under air unless otherwise indicated. Oxygen or nitrogen was
bubbled through the solution for at least 20 min when required.
-
5
1
5 and 20 min). For comparative purposes, a solution of ABDA (2x10
M) in phosphate buffer was also irradiated under the same conditions. In
addition, a control sample containing copolymer and ABDA was kept in
the dark (except when taking its fluorescence λex = 380 nm, monitoring at
4
00 nm) at different time intervals (Figure 20).
Acknowledgements
The fluorescence quantum yield was determined using an integrated
sphere coupled to an UDT instruments S370 Optometer. The sample
was finely pulverized in an agate mortar and before introducing it to the
integrating sphere, its static was removed.
J.R. Irigoyen-Campuzano thanks CONACYT for the fellowship
and CIIDIR- Durango (IPN, México) and ICMol (Universitat de
Valencia) for their support. Authors also thank the Spanish
Ministry of Economy and Competitiveness (Projects CTQ2014-
60174 and M.G.B. Ramón y Cajal contract). E. Pino thanks
DICYT-USACH-02154PL. The proteomic analysis was
performed in the proteomics facility of SCSIE University of
Valencia that belongs to ProteoRed, PRB2-ISCIII, supported by
grant PT13/0001 of the PE I+D+i 2013-2016, funded by ISCIII
Singlet state quenching experiments.
anthraquinone-based copolymer DMF/water (6:4) with an absorbance of
.1 at 532 nm for all experiments. Similar equipment conditions were
A solution (3 mL) of the
0
used that those used previously for steady-state fluorescence. Aliquots of
the organic pollutant (2,5-DCP) were added 10 µL from a stock solution
(
[2,5-DCP]=1.43 M in DMF/H
2
O 6:4) at the corresponding pH for the
experiment.
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and FEDER. We also thank the SCSIE for providing access to
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Keywords: Anthraquinone-based copolymer • heterogeneous
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Entry for the Table of Contents
Layout 1:
Entry for the Table of Contents
FULL PAPER
Text for Table of Contents
Rafael Irigoyen-Campuzano, María
González-Béjar, Eduardo Pino, Jose B.
Proal-Nájera* and Julia Pérez-Prieto*
Metal-free, non-conjugated
copolymer produces mineralization
of 2,5-dichlorophenol in water under
air and sunlight irradiation. The
polymer can be easily synthesized
and coated on a glass surface in a
solar reactor.
Page No. – Page No.
Title
A Metal-free, Non-conjugated
Polymer for Solar
Photocatalysis
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