Efficient generation of singlet oxygen by perylene diimide photosensitizers covalently bound to conjugate polymers

Article abstract of DOI:10.1016/j.jphotochem.2019.112161

New conjugated polymers with perylene diimides (PDI) as pendant groups were synthesized and deposited on glass substrates by the spin coating. The resulting thin films were characterized by UV–vis, Raman spectroscopy, atomic force microscopy and profilometry. It was shown that PDI photosensitizers retain its photoactivity after covalent immobilization and the formed layers can be applied as efficient and environmentally stable source of singlet oxygen, ¹O₂, as tested with 1,3-diphenylisobenzofuran (DPBF) specific trap. Additionally, α-terpinene heterogeneous photooxidation was studied as the practical use of singlet oxygen generated by this novel PDI-based materials. The use of such heterogeneous source of singlet oxygen can be beneficial for the fine chemicals synthesis, due to simplified products isolation and purification step.

Full text of DOI:10.1016/j.jphotochem.2019.112161

Journal Pre-proof
Efficient generation of singlet oxygen by perylene diimide
photosensitizers covalently bound to conjugate polymers
Agata Blacha-Grzechnik, Anna Drewniak, Krzysztof Z. Walczak,
Marek Szindler, Przemyslaw Ledwon
PII:
S1010-6030(19)30972-4
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112161
JPC 112161
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
13 June 2019
1 August 2019
11 October 2019
Please cite this article as: Blacha-Grzechnik A, Drewniak A, Z. Walczak K, Szindler M,
Ledwon P, Efficient generation of singlet oxygen by perylene diimide photosensitizers
covalently bound to conjugate polymers, Journal of Photochemistry and amp; Photobiology,
A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112161
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Efficient generation of singlet oxygen by perylene diimide photosensitizers
covalently bound to conjugate polymers
a
b
b
c
Agata Blacha-Grzechnik , Anna Drewniak , Krzysztof Z. Walczak , Marek Szindler ,
Przemyslaw Ledwon*a
a
Silesian University of Technology, Department of Physical Chemistry and Technology of
Polymers, Strzody 9, 44-100 Gliwice, Poland
b
Silesian University of Technology, Department of Organic Chemistry, Bioorganic Chemistry
and Biotechnology, Krzywoustego 4, 44-100 Gliwice, Poland.
c
Silesian University of Technology, Institute of Engineering Materials and Biomaterials,
Konarskiego 18A, 44-100 Gliwice, Poland
*
Corresponding author; email: przemyslaw.ledwon@polsl.pl; tel.: +48322371305; postal
adress: Strzody 9, 44-100 Gliwice, Poland
Graphical abstract
Highlights




The photoactive polymer films were formed by spin-coating on glass substrates.
The deposited layers were active under the green light illumination
1
The immobilized perylene diimide (PDI) units retain activity towards O2 generation
The layers were applied in α-terpinene photooxidation
1

Abstract
New conjugated polymers with perylene diimides (PDI) as pendant groups were synthesized
and deposited on glass substrates by the spin coating. The resulting thin films were
characterized by UV-Vis, Raman spectroscopy and profilometry. It was shown that PDI
photosensitizers retain its photoactivity after covalent immobilization and the formed layers
1
can be applied as efficient and environmentally stable source of singlet oxygen, O
2
, as
tested with 1,3-diphenylisobenzofuna (DPBF) specific trap. Additionally, the α-terpinene
heterogeneous photooxidation was studied as the practical use of singlet oxygen generated
by this novel PDI-based materials. The use of such heterogeneous source of singlet oxygen
can be beneficial for in the fine chemicals’ synthesis, due to simplified products’ isolation
and purification step.
Keywords
singlet oxygen, perylene diimide, photochemistry, α-terpinene photooxidation,
photosensitizer, conjugated polymers
1
. Introduction
1
Singlet oxygen O
2
has been under high research interest for over 50 years as a possible
candidate for application in Photodynamic Therapy (PDT), fine chemicals’ synthesis or
wastewater treatment 1–4. This excited state of oxygen without unpaired electrons is formed
in the photosensitization process, where photosensitizer molecule is excited by light
3
molecules 5–7. Various types of compounds able
illumination and transfers its energy to O
2
to generate singlet oxygen are known: dyes, aromatic hydrocarbons, porphyrins,
phthalocyanines, conjugated polymers, transition metals complexes, semiconductors oxides,
carbon-based nanostructures and described lately - perylene diimide (PDI) derivatives 5,8–10.
The latter could be particularly interesting due to high quantum yields of singlet oxygen
photogeneration 11, high photostability 12 and broad ability to modified its structure 13
.
Singlet oxygen has higher reactivity and electrophilicity, but also lower stability and shorter
lifetime than triplet oxygen, so it has to be generated in situ into reaction mixture. High
attention is nowadays paid on developing methods of photoactive molecules’ deposition on
solid support, in order to form heterogenous photocatalysts applicable in fine chemicals’
2

1
synthesis or wastewater treatment. Though, typically lower yields of O photogeneration
2
are observed for immobilized photosensitizers, such approach possess several advantages
over homogenous photocatalysis, like ease of operation or easier product separation and
purification 5,14–17. Our group has recently shown that thin organic films of phenothiazines
formed in the process of electrochemical polymerization or electrochemical reduction of the
diazonium salts, are able to generate active singlet oxygen in the reaction mixture, leading to
the oxidation of 1,3-diphenylisobenzofuran (DPBF), phenol or α-terpinene 18–21
.
Carbazoles are one of the most common compounds used as building blocks of conjugated
polymers 22–24. The great interest in carbazole derivatives results from the low cost of the
starting material, easy modification of its structure, and its good stability. In this work
carbazole was chosen as building-block for synthesis of monomer consisting of
dibromocarbazole derivative with PDI side unit linked by hexyl groups. Three photoactive
materials were synthesized: pCB-PDI and pCC-PDI consisting of conjugated main polymeric
chain and pendant PDI units linked by hexyl groups, and unmodified pCB (Scheme 1).
Conjugated pCB polymer is known as a donor-acceptor polymer with absorption of visible
light, unlike poly(2,7-carbazole) films, that are almost transparent and do not absorb visible
radiation 25,26. Moreover, the chosen polymeric backbone and covalently attached
photosensitizer, are both highly stable in air and in various organic solvents 27,28, which
makes them attractive candidates for the heterogeneous generation of singlet oxygen in
industrially important oxidation reactions 5.
3

Scheme 1 The structures of the presented compounds
2
. Materials and methods
2
.1 General information
All chemicals and solvents were purchased from Sigma Aldrich, TCI or ChemPure unless
otherwise stated. Given yields refer to isolated compounds unless otherwise stated.
Synthesis procedure includes synthesis of monomer N-(2-n-octyldodecyl)-N'-[6-(2,7-
dibromo-9H-carbazol-9-yl)hexyl]perylene-3,4,9,10-tetracarboxylic diimide (4) and synthesis
of pCC-PDI, pCB-PDI and pCB macromolecules. Other monomers were purchased
commercially and used without purification: 2,1,3-Benzothiadiazole-4,7-diboronic acid
bis(pinacol)
ester
(>98%)
and
9-(1-Octylnonyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole (>97%) were purchased from Osilla. 2,7-Dibromo-9-(9-
heptadecyl)carbazole (>98%) were purchased from TCI. Purification step by a column
chromatography was done on silica gel packed column (70-230 mesh, 63-200 µm, 60Å). 1H
1
3
NMR and C NMR were recorded on Varian Mercury Plus 600 (600 MHz) or Varian Unity
Inova (300 MHz) spectrometers, the chemical shifts are given in ppm using TMS as an
internal standard. The mass spectra were recorded with AB Sciex 4000 QTRAP (HRMS) or
4

Shimadzu Biotech Axima Performance (MALDI-TOF MS) mass spectrometers. 1,3-
diphenylisobenzofuran (DPBF) (Acros Organics, purity >97%) in methanol (Acros Organics,
1
9
9.9%) was used as a specific O
2
quencher. α-Terpinene (TCI, purity 90%) in acetonitrile
(Sigma Aldrich, HPLC grade) was employed as a substrate in the photooxidation reaction.
2
.2 Formation and characterization of photoactive films
pCB-PDI, pCB and pCC-PDI layers were deposited on 1 cm x 1 cm borosilicate glass slides
-3
(
PGO) by spin coating (2000 rpm) from polymer solutions (6,6 mg∙cm ) in chlorobenzene
with temperature of 70 ⁰C. The thickness of the polymer films were estimated by the
Profilm3D profilometer.
UV-Vis spectra of the deposited films were collected in quartz cuvette (Hellma Analytics, 2
mm) using of Hewlett Packard 8452A UV-Vis spectrometer. Raman spectra were collected by
means of Renishaw inVia Raman Microsope (Renishaw, Inc., New Mills, UK) equipped with
diode excitation laser with the wavelength of 514 nm, 2400-lines grating and 50x objective.
All Raman spectra were subjected to smoothing and baseline subtraction procedures using
Renishaw software.
The 2D and 3D topographic 2×2µm images were performed with XE-100 Park Systems
atomic force microscope (AFM). The microscope was set to non-contact mode. The RMS and
Ra coefficient values were determined with XEI software. RMS was defined as rough mean
square parameter and Ra was defined as the arithmetic mean deviation of the profile from
the mean line.
2
.3. Procedure of singlet oxygen photogeneration
Singlet oxygen photogeneration was investigated using 0.05 mM solution of 1,3-
diphenylisobenzofuran (DPBF) in methanol 29. DPBF is known as specific singlet oxygen
3
0
1
1
quencher , reacting via 1,3-addition mechanism with O of singlet oxygen . The progress
2
of the reaction, i.e. the drop in the absorbance of DPBF at 410 nm, was monitored by means
of Hewlett Packard 8452A UV-Vis spectrometer. The quartz cuvette (Hellma Analytics, 10
mm x 4 mm) applied in the measurements was arranged as a thin layer cell possessing a
cavity of 10 mm path length and thickness of 2 mm. This cavity was created between the
cuvette wall and the freshly prepared photoactive layer deposited on glass. 532 nm diode
laser (Oxxius, LCX-532L-150-CSB-PPA model, 150 mW maximum power reduced to 50 mW)
2
was employed as an excitation light source illuminating ca. 0.5 cm of sample’s surface.
5

The singlet oxygen quantum yields for each PDI-containing layers were calculated using
31
Equation 1 and Rose Bengal as the reference (??RB = 0.80 in methanol ).
풎
휶푹푩
풊
힥풊 = 힥푹푩 ∙ 풎
∙
(Equation 1),
푹푩
휶풊
where the indexes i and RB indicate investigated PDI-polymer structure and Rose Bengal,
respectively, Φ is the quantum yield of singlet oxygen photogeneration; m is the slope of a
trend line of change in the absorbance of DPBF (at 410 nm) vs. time and α is the absorption
correction factor given by α = 1 – 10 (A is the absorbance at 532 nm)32–34.
–A
2
.4 Procedure of α-terpinene photooxidation
PCB-PDI/glass layer was then applied as a source of singlet oxygen in the oxidation of α-
3
5
terpinene in acetonitrile, resulting in the formation of ascaridole . The progress of the
reaction was monitored similarly as in the case of DPBF. The yield of photooxidation
reaction, thus the singlet oxygen generation, was determined based on the decrease in the
absorption of α-terpinene at 266 nm. The initial concentration of α-terpinene was 0.25 mM.
The concentration of α-terpinene at the given time was found applying Lambert-Beer’s law
-
1
-1
for the absorption maximum at 266 nm (εterpinene = 6.95 mM cm in acetonitrile). In order to
investigate possible re-use of the photoactive layer, the process was repeated 5 times.
3
. Results and Discussion
3
.1 Synthesis
Investigated photoactive materials were synthesized using Suzuki type coupling using
3
6
appropriate monomers (Scheme 2) .
6

Scheme 2 Synthesis of studied polymers
7

N-[6-(2,7-dibromo-9H-carbazol-9-yl)hexyl]phthalimide) 2 was obtained from 2,7-dibromo-
H-carbazole and N-(6-bromohexyl)phthalimide. Compound 2 was treated with the
9
hydrazine monohydrate in an ethanol to obtain 6-(2,7-dibromo-9H-carbazol-9-yl)hexyl-1-
amine 3. Compound 3 was acquired as a mixture of phthalhydrazide in an approximate ratio
1
13
of 1:1 (the H and C NMR spectrum of the precipitate is shown in Figure S3 and S4). The
presence of phthalhydrazide did not interfere with diimide 4 synthesis.
The synthesis of 4 was conducted in one-step reaction of condensation by reacting
equimolar ratio of the amine 3, 2-n-octyldodecylamine and 3,4,9,10-perylenetetracarboxylic
dianhydride in quinoline. Monomer 4 was purified by a column chromatography and
precipitated with methanol. Polymers pCC-PDI, pCB-PDI and pCB were synthesized by Suzuki
coupling polymerization using Pd(PPh
and purified by Soxhlet apparatus.
3
4
) as catalyst. Polymers were obtained with high yields
1H NMR spectra of pCC-PDI and pCB-PDI show broad signals in the range 8.63-7.30 and 8.64-
7
.28 respectively. Such feature is characteristic for NMR spectra of polymers containing PDI
units and results from PDI aggregation occurring at the solutions’ concentration used for
NMR spectra recording 37–42. In the range 7.29-7.12 in pCC-PDI spectrum, signals from the
end groups are clearly visible.
MALDI-TOF measurements show that pCB-PDI is oligomeric/polymeric material with rather
low molecular mass up to 4.2 kDa, while pCC-PDI and pCB are polymers with molecular mass
up to 8.3 kDa and 9.3 kDa, respectively.
3
.2 Spectroscopic and AFM characterization of the photoactive layers
8

Figure 1. UV-Vis spectra of pCB-PDI, pCC-PDI and pCB deposited on borosilicate glass
The synthesized polymers were deposited on borosilicate glass by spin-coating to form films
having a thickness of 280 nm, 190 nm and 220 nm for pCB-PDI, pCB and pCC-PDI
respectively. Figure 1 presents UV-Vis spectra of formed polymeric layers. In the case of pCB-
PDI and pCC-PDI films, absorption bands characteristic for PDI unit are observed at ca. 500
and 545 nm 43,44. Those bands are partially overlapping in pCB-PDI spectrum with pCB
absorption having maximum at ca. 475 nm. For pCC-PDI absorption coming from conjugate
polymer chain is blue-shifted to 400 nm and thus separated from PDI absorption bands.
Based on these results, 532 nm laser was selected as an illumination source for singlet
oxygen photogeneration. Such wavelength should cause excitation of PDI units in both pCB-
PDI and pCC-PDI layers, and by pCB polymeric backbone.
The structure of the formed thin films was in turn investigated by means of Raman
spectroscopy. Figure 2 presents Raman spectra recorded for deposited pCB-PDI, pCC-PDI and
pCB layers. In the case of the first two layers, the vibrations characteristic for PDI unit are
-1
-1
observed at ca. 1300 cm and 1380 cm corresponding to C-H bending and at ca. 1570/1588
cm arising from C-C/C=C stretching in perylene core 45,46. Those vibrations are absent for
-1
the unmodified pCB layer. For the polymeric films bearing benzothiadiazole groups, i.e. pCB
-
1
-1
and pCB-PDI, signals at ca. 1270 cm and 1345 cm coming from C-C stretching in
benzothiadiazole aromatic ring and C=N stretching + C=N-S deformation, respectively, are
4
7
observed . For all deposited layers characteristic vibrations for carbazole units (eg. 1626
cm , ν (C=C)) and benzene rings (eg. 1458 cm , δ (C-H)) are present 48–50, confirming
-
1
-1
expected structure of the investigated films.
9

Figure 2. Raman spectra of pCB-PDI, pCC-PDI and pCB films deposited on borosilicate
glass
The 2D and 3D topographic images of photoactive polymer thin films are shown in
Figures.3a–f. The histograms of irregularities are compared in Figures S15a-c (please see
Supporting information). The values of RMS and Ra coefficients are included in Table 1. The
deposited layers are continuous and uniform. No impurities, precipitates, or delamination
have been registered. It has been found out in the images, that repetitive aggregations of
atoms have a similar geometrical features. The presence of PDI unit in the layer resulted in a
slight increase in roughness. The RMS and R
a
parameters were respectively: for the pCB-PDI
1
.23 and 0.97 nm, for pCC-PDI 0.46 and 0.36 nm while for pure PCB are 0.34 and 0.25 nm.
The histograms obtained with AFM microscope software present the number of pixels with
different height depending on its value, that corresponds to the area occupied with the
highest and lowest points. The obtained histograms are in good coincidence with
parameters in Table 1.
1
0

a)
c)
e)
b)
d)
f)
Figure 3. AFM 2D and 3D images of the Surface topography of: pCB (a,b); pCB-pdi (c,d); pCC-
PDI (e,f)
Table 1 Summary of roughness parameters for deposited photoactive polymer thin films
Surface area
RMS
[nm]
R
[nm]
a
max. irregularity
[nm]
Sample
2
[
µm ]
PCB
4.00
4.00
4.00
0.34
1.23
0.46
0.25
4.28
5.92
3.06
pCB-PDI
pCC-PDI
0.97
0.36
1
1

3
.3 Singlet oxygen photogeneration measurements
Various specific singlet oxygen chemical traps can be applied in the indirect detection of this
ROS form 30,51. In this work, the most commonly used 1,3-diphenylisobenzofuran (DPBF) was
1
1
used as a specific O quencher . Figure 4 presents the example of set of UV-Vis spectra of
2
DPBF solution in methanol recorded during illumination of pCB-PDI layer with 532 nm laser
with power of 50 mW. The clear decrease in the DPBF’s absorption at 410 nm can be
observed indicating its reaction with singlet oxygen photogenerated by pCB-PDI photoactive
5
2
film . Importantly, the investigated layer is stable under applied conditions, since no new
band at ca. 500 nm is appearing during process. Similar results were obtained when pCC-PDI
layer was illuminated, which confirms that in both cases PDI photosensitizer retains its
photoactivity after covalent bonding to the conjugate polymer support. Additionally, since
also weak drop in DPBF absorption was observed for pCB polymer itself, it may be suspected
1
that during pCB-PDI illumination the combined O
2
photogeneration by PDI photosensitizer
and conjugated backbone occurs.
Figure 4. UV-Vis spectra of DPBF recorded during illumination of pCB-PDI/glass layer with 532
nm laser
1
2

Figure 5. Drop in the absorption of DPBF at 410 nm during illumination of PDI, pCB-
PDI/glass, pCC-PDI/glass, pCB/glass and bare borosilicate glass
Table 2. Quantum yield of singlet oxygen photogeneration calculated with respect to
Rose Bengal as a standard
Photosensitizer
PDI in solution
pCB-PDI
??
0.36 ± 0.04
0.09 ± 0.01
0.06 ± 0.01
pCC-PDI
1
The comparison of the effectiveness of DPBF’s oxidation, i.e. O photogeneration, by all
2
deposited layers is presented in Figure 5 and Table 2. As it can be seen, the highest yield of
investigated photoprocess was observed for pCB-PDI film, in which both PDI and polymeric
chain are activated by green radiation. Under applied conditions, pCC-PDI layer, in which
only PDI photosensitizer is being excited shows slightly lower effectiveness, while singlet
oxygen production by pCB polymer itself is rather poor. Autooxidation of DPBF is excluded,
since almost no drop in DPBF’s absorption is observed when unmodified borosilicate glass
5
3
slide is illuminated with green laser (Figure 5) . As expected, the quantum yield of singlet
oxygen photogeneration by deposited photoactive layers is several times lower than for PDI
1
3

photosensitizer present in the solution phase, as presented in the Table 2. However, as
mentioned, the heterogeneous approach in photocatalysis is more favorable for industrial
applications ensuring easier product purification and separation, and re-use of catalyst.
Importantly, as tested by UV-Vis and Raman spectroscopy, the deposited layers were stable
under applied process conditions and the pendant PDI group was not detached from the
polymeric backbone and released to the reaction mixture.
3
.4 Oxidation of α-terpinene
In the next step, pCB-PDI photoactive layer, showing the highest efficiency in the singlet
oxygen photogeneration, has been tested as a source of singlet oxygen in the
photooxidation of α-terpinene. This industrially important process results in the formation of
ascaridole – the commonly used anthelmintic drug. In this work, the process was conducted
in acetonitrile with 532 nm laser as an excitation source. The progress of the reaction was
3
5
monitored by UV-Vis spectroscopy as the decrease in α-terpinene absorption at 266 nm .
Figure 6. UV-Vis spectra of α-terpinene recorded during illumination of pCB-PDI/glass
layer with 532 nm laser; inset: drop in concentration of α-terpinene during
st
nd
th
illumination of pCB-PDI/glass in the 1 , 2 and 5 use glass and bare borosilicate
glass
Figure 6 shows UV-Vis spectra of α-terpinene recorded in the course of illumination of pCB-
PDI layer. The clear decrease in α-terpinene absorption at 266 nm is observed, due to its
1
17,54. Since almost no drop in the absorption of α-terpinene is observed
reaction with O
2
during illumination of bare borosilicate glass (Figure 6 inset), the self-degradation of
1
4

substrate is excluded. Similarly to methanol environment the pCB-PDI layer is not being
destructed nor dissolved in the reaction medium, i.e. acetonitrile, during the process.
The rate constant of the photooxidation of α-terpinene was determined based on the
decrease in its concentration in time. The plot of ln(cterpinene/cterpinene,initial) as a function of
time gives the straight line, which indicates pseudo first-order reaction under applied
55
conditions . It was also found that only small drop in the effectiveness was observed when
th
the pCB-PDI layer was re-used: ca. 6% decrease in the photoreaction yield for 5 use of
tested material (Figure 6 inset), which indicates that the deposited layer maintains its high
photoactivity after usage and can be re-used in the consecutive processes.
Note, that since the synthesized polymers are soluble in chloroform and/or chlorobenzene,
they can be easily deposited on solid substrates, like glass by e.g. spin coating technique. On
the other hand, due to their insolubility and high stability of such in such solvents as
acetonitrile or methanol, they can be applied as the effective source of singlet oxygen in
heterogeneous synthesis of fine chemicals. The results obtained for pCB-PDI indicate that
this type of materials consisting with conjugated polymers and PDI moieties can be used as
complementary sensitizers in the photooxidation processes. It can be expected that such
systems may find application in the photooxidation processes including light sources with
broader irradiation wwavelength range, for example white light sources. The use of common
source of light and the use of solid sensitizers is considered beneficial for industrial
processes from a practical point of view and strongly agrees with green chemistry concepts.
4
. Conclusions
In summary, different conjugated polymers with pendant PDI groups deposited on the glass
support were investigated as a source of singlet oxygen in methanol and acetonitrile under
green light illumination. PDI-based polymer films show high efficiency in 1O
2
photogeneration tested by the reaction with DPBF specific trap. The presented results show
that such materials can be effectively applied in the form of thin photoactive films in the
commercially important process of α-terpinene oxidation without introduction of additional
reagents into the reaction mixture, which is extremely beneficial for the isolation and
purification of reaction products. Moreover, the green light irradiation is beneficial
compared to higher energy irradiation often used in generation of singlet oxygen, due to
smaller possibility of side reactions occurring along with the reducing of irradiation energy.
1
5

Notes
The authors declare no competing financial interest.
Acknowledgements
The work was financed by National Science Centre, Poland under grants no.
2
015/17/D/ST5/01343 and 2016/21/D/ST5/01641. We acknowledge Karol Knop for
contributing to synthetic work.
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