Phenacyl Phenothiazinium Salt as a New Broad-Wavelength-Absorbing Photoinitiator for Cationic and Free Radical Polymerizations

Article abstract of DOI:10.1002/anie.202104531

A novel broad-wavelength-absorbing photoinitiator based on phenacyl phenothiazinium hexafluroantimonate (P-PTh) possessing both phenacyl and phenothiazine chromophoric groups was reported. P-PTh absorbs light at UV, Visible and Near-IR region. Photophysical, photochemical, and computational investigations revealed that P-PTh in solution decomposes at all wavelengths by homolytic and heterolytic cleavages and generates cationic and radical species, which could efficiently initiate cationic and free radical polymerizations. It is anticipated that the photoinitiator with such wavelength flexibility may open up new pathways in curing applications of formulations of pigment systems.

Full text of DOI:10.1002/anie.202104531

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Phenacyl Phenothiazinium Salt as A New Broad-wavelength-
absorbing Photoinitiator for Cationic and Free Radical
Polymerizations
Authors: Yi Zhu, Dandan Xu, Yuchao Zhang, Yufan Zhou, Yusuf Yagci,
and Ren Liu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202104531
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10.1002/anie.202104531
Angewandte Chemie International Edition
COMMUNICATION
Phenacyl Phenothiazinium Salt as a New Broad-wavelength-
absorbing Photoinitiator for Cationic and Free Radical
Polymerizations
Yi Zhu, [a, b] Dandan Xu, [a] Yuchao Zhang, [d] Yufan Zhou, [a] Yusuf Yagci*[b, c] and Ren Liu*[a, b]
[a]
Dr. Y. Zhu, D. Xu, Y. Zhou, Prof. R. Liu
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 214122,
Wuxi, Jiangsu (P. R. China)
E-mail: liuren@jiangnan.edu.cn
[b]
[c]
Dr. Y. Zhu, Prof. R. Liu, Prof. Y. Yagci
International Research Center for Photoresponsive Molecules and Materials, Jiangnan University, 214122, Wuxi, Jiangsu (P. R. China)
Prof. Y. Yagci
Department of Chemistry, Faculty of Science and Letters, Istanbul Technical University, 34469, Maslak, Istanbul (Turkey)
E-mail: yusuf@itu.edu.tr
[d]
Y. Zhang
School of Chemistry and Chemical Engineering, Nanjing University, 210023 Nanjing, Jiangsu (P. R. China)
Abstract: A novel broad-wavelength-absorbing photoinitiator based
on phenacyl phenothiazinium hexafluroantimonate (P-PTh)
possessing both phenacyl and phenothiazine chromophoric groups
was reported. P-PTh absorbs light at UV, Visible and Near-IR region.
Photophysical, photochemical, and computational investigations
revealed that P-PTh in solution decomposes at all wavelengths by
homolytic and heterolytic cleavages and generates cationic and
radical species, which could efficiently initiate cationic and free radical
polymerizations. It is anticipated that the photoinitiator with such
wavelength flexibility may open up new pathways in curing
applications of formulations of pigment systems.
Depending on the structure of the chromophore group containing
heteroatom, the absorption of the salt can be shifted to longer
wavelengths. Previously, phenacyl salts with sulphonium,[11]
aniline,[12] pyridine,[13] carbazole,[14] and thioxathone[15] groups
were synthesized and successfully used in cationic, free radical
and step-growth[16] polymerizations. It was also shown that their
charge transfer complexes further extend their absorption
characteristics to longer wavelengths.[17]
Based on its spectral characteristics and excited state properties,
it seemed appropriate to synthesize phenacyl salt with
phenothiazine chromophore. Herein we report for the first time a
new phenacyl phenothiazinium salt (P-PTh) that absorbs light
over the UV–vis and near-IR region of the electromagnetic
spectrum with relatively high absorption coefficient and initiates
both radical and cationic photopolymerizatons at whole
wavelength range. The structure and photophysical properties of
the salt were evaluated by spectral and computational
investigations.
As stated, phenothiazine is widely used in many photoinduced
processes as a general chromophore and modified phenothiazine
derivatives show outstanding absorption and high sensitizing
efficiency.[8] As an efficient chromophore, phenacyl group has
been incorporated into various heteroatom containing molecules
to enhance spectral sensitivity.[11-15] We incorporated phenacyl
group to photosensitive phenothiazine as an additional
chromophore. P-PTh was prepared by direct phenacylation
process of N-ethyl-phenothizine (E-PTh) in one step, one pot
according to generally applied method.[16,18] (Scheme 1) However,
unlike the previous work, there are two positions on phenothiazine,
N and S, that can potentially react with 2-bromoacetophenone.
DFT computational analysis, 1H-1H COSY and NOESY spectra
were used to confirm that 2-oxo-2-phenylethyl group was
substituted on S atom of phenothiazine (Supporting Information,
Figures S4 and S5).
In recent years, photoinduced processes have received
revitalized interest offering robust and versatile synthetic routes to
form complex macromolecular structures in an energetically
favorable and environmentally friendly fashion.[1] The
developments in photoinduced electron transfer reactions (PET)[2]
have led to a wide variety of new photoinitiating systems with
tunable absorption characteristics. Different PET systems that
can be activated at UV, visible and near-IR region of the
electromagnetic spectrum have profusely been explored for chain
polymerizations[3] involving radical and ionic species and step-
growth polymerizations[4] as well as copper catalyzed azide
alkyne click[5] processes. Controlled/living radical polymerizations,
namely Photoinduced Atom Transfer Radical Polymerizations
(PhotoATRP)[6] and Radical Addition Fragmentation Transfer
(RAFT)[7] polymerization can be conducted by photochemical
means, also referred to as PET reactions, yield polymers with
various controlled architectures, narrow molecular weight
distribution, and chain-end functionalities. Among various highly
conjugated aromatic compounds, phenothiazine derivatives
appeared[8] to be the most efficient organic catalysts in metal-free
photoATRP. Detailed laser flash photolysis and spectroscopic
investigations [9] revealed that the triplet-excited phenothiazines
played a dominant role in the mechanism of metal free
photoATRP.
Phenacyl salts are a class of onium salts that can be used as
photoinitiators for cationic and free radical polymerizations.[10]
1
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10.1002/anie.202104531
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COMMUNICATION
Table 1. Photophysical properties of PTh-P.
PI
λmax
(nm)
514
λem
(nm)
572
εmax
(L mol−1 cm−1
5151
Φf
)
PTh-P
0.002
Photolysis experiments were performed to investigate the
photochemical behavior of the photoinitiator at different
wavelengths. As can be seen from Figure 2, upon irradiation at
all wavelengths, the absorbance in the near UV region increases,
but decreases in the visible and NIR region indicating
decomposition of the photoinitiatior and concomitant formation of
new photoproducts absorbing at low wavelengths. The
disappearance of the orange color of the solution upon irradiation
further confirms the decomposition. The observed behavior
clearly demonstrates that the photoinitiator provides immense
wavelength flexibility and therefore, applicable to the wide range
of pigmented formulations. Moreover, the activity at NIR region
reflects potency in deep curing applications due to the weak
photon energy and deep penetration depth of NIR light.[19]
Scheme 1. Synthesis of phenacyl phenothiazinium salt (P-PTh)
Optical absorption characteristic of the PTh-P photoinitiator was
examined by UV spectroscopy (Figure 1) and the photophysical
properties of PTh-P are displayed in Table 1. PTh-P showed high
molar absorption coefficient and very weak fluorescence
(fluorescence quantum yield: Φf ∼ 0.002). A strong red shift in the
fluorescence spectrum (Figure S3) observed indicates that
minimize energy point geometry of the excited state for PTh-P is
quite different from that of the ground state. It also explains the
reason of the observed low Φf. Internal conversion by the transfer
of electronic energy to molecular vibration contributes mainly to
the non-radiation process. Beyond our expectation, the UV−vis
spectrum of PTh-P exhibits absorption at longer wavelengths with
high molar absorption coefficients than common commercially
available photoinitiators. In addition, it has broad-band absorption
in different spectrum range, including UV, visible and near infrared
(NIR) region. Best to our knowledge, such characteristic
absorption in onium type photoinitiators has never been reported.
Figure 2. Steady-state photolysis of P-PTh (1.72 × 10−4 mol/L) in acetonitrile
after (A) LED@405 nm (100 mW/cm2), (B) LED@365 nm (100 mW/cm2), (C)
LED@850 nm (1.5 W/cm2), (D) LED@405 nm (100 mW/cm2) irradiation under
air, RhB-L base (2.34 ×10−6 mol/L) is added as an acid indicator
Figure 1. UV−vis spectrum of P-PTh (1.72 × 10−4 mol/L) in acetonitrile at room
temperature
Figure 3. Real-time NMR spectral changes at 7.2-8.2 ppm and 1.9-2.6 ppm regions during photolysis process. The photolysis study of P-PTh (0.043 mol/L) was
conducted in Acetonitrile-D3 irradiated by LED@365 nm (150 mW/cm2) under Ar at room temperature. Curves represent NMR spectrum at 0 min, 15 min, 30 min,
45 min, 60 min and 75 min with irradiation. Photolysis were conducted in an NMR tube
2
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10.1002/anie.202104531
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Upon irradiation, in addition to phenacyl radicals and Brønsted
-
super acids (H+SbF6 ) are expected to be formed by homolytic
cleavage (vide infra). In order to confirm formation of Brønsted
acid, photolysis of P-PTh was conducted in the presence of an
acid indicator, Rhodmine B lactone (RhB-L). The observed
increase of the absorption band at 550 nm corresponding to the
acid form of RhB evidently confirms the formation of Brønsted
acid[20] (Figure 2D). Since chain reactions do not occur, the acid
formation may explain the low fluorescence quantum efficiency.
The photolysis process was also monitored by NMR spectroscopy.
As can be seen from Figure 3, the signals of P-PTh disappears
and new broad signals, which may correspond to the
phenothiazine radical cations, appear with the irradiation time.
These paramagnetic species result in the signal of water in
solvent get an obvious change in chemical shift and broadening
due to the impact on spin-spin relaxation times (T2).
The ESR experiment was conducted to further confirm the
formation of radical species under light irradiation. As shown in
Figure 4, under LED@365 nm for 15 min of P-PTh in solvent, one
triplet signal (aN = 13.39 G, aH = 2.21 G) was observed, which was
assumed to correspond to radical of phenothiazine radical cation.
Spin density analysis confirmed that the phenothiazine radical
cation is a decisive of N radical with high spin density localized on
N atom.
Scheme 2. Proposed Initiation mechanism for cationic and free radical
photopolymerizations by using P-PTh
Table 2. TGA and DSC analyses of P-PTh-P/TMPTA, and P-PTh-P/EPOX
formulations (1.72 × 10-5 mol/g resin)
photoinitiator
decomposition
temperature
(°C)
onset temperature
peak temperature
(°C)
(°C)
TMPTA
100
EPOX
160
TMPTA
150
EPOX
210
P-PTh
170
The viscosity of the formulations containing P-PTh and monomers
polymerizable by free radical and cationic mechanism did not
change significantly after keeping more than 100 days in the dark
at 60 °C indicating excellent storage stability of P-PTh in the
formulations (Supporting Information, Figure S11). Excellent
thermal and storage stability indicate that P-PTh has important
potential practical application value in the field of photocuring.
The kinetics of cationic polymerization bifunctional monomer, 3,4-
Figure 4. ESR spectra of the radicals generated in P-PTh upon light exposure
and trapped by DMPO in tert-butylbenzene: a) experimental and b) simulated
spectra. c) Spin density isosurface at 0.03 a.u.
epoxycyclohexylmethyl
3,4-epoxycyclohexanecarboxylate
As outlined in Scheme 2, the photolysis mechanism of P-PTh is
based on homolytic cleavage of P-PTh[15a,16]. Homolytic cleavage
leads to the formation of phenacyl radical and phenothiazine
radical cation. The phenacyl radicals are able to initiate FRP in
the presence of monomer. These radicals can undergo
dimerization and hydrogen abstraction reactions in the absence
of monomer. The product of the latter process was identified by
(EPOX) were studied by Real-time IR in thin samples (2 mm) in
the presence of P-PTh salt using multi wavelength LED light
irradiation sources. The related profiles are displayed in Figure 5.
As can be seen, high conversions were attained in short
irradiation times at the all wavelength range studied indicating
high multi-band initiation efficiency.
GC-MS analysis (acetophenone m/z
=
120, Supporting
Information, Figure S6). As evidenced by ESR, GC-MS
(phenothiazine, m/z = 227, Supporting Information, Figure S7)
and photolysis studies in the presence of acid indicator,
phenothiazine radical cation is formed simultaneously.
Thermal and storage stability of a photoinitiator is an important
concern particularly for practical applications. The thermal
stability experiments of P-PTh were conducted by TGA and DSC
analyses in the absence and presence of monomers, respectively.
The latter provides more conclusive information as the exothermic
polymerization processes can readily be monitored (Supporting
Information, Figure S10). As can be seen from Table 2, P-PTh
has notable thermal and storage stability.
Figure 5. Kinetic profiles of EPOX in the presence of P-PTh salt under different
irradiation sources. (A) LED@365 nm (30 mW/cm2), LED@405 nm (30
mW/cm2). (B) LED@850 nm (1.5 W/cm2)
3
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10.1002/anie.202104531
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The free radical photopolymerizations of di- and tri-functional
acrylates, namely tripropylene glycol diacrylate (TPGDA) and
trimethylolpropane triacrylate (TMPTA) were studied on KBr
tablet in the presence of P-PTh using different wavelength LED
light sources. The photoiniatior shows almost similar activity with
the monochromatic LED@365 nm or LED@405 light sources for
both monomers (Figure 6). Notably, with broad-spectrum light
source, much faster polymerization with quantitative conversion
was attained.
heterolytic cleavages yielding initiating radical and ionic species
responsible for the initiation of the respective polymerization
reactions. The single component photoinitiator described here
has the advantage of having broad-wavelength absorption
characteristic particularly at NIR region and may provide new
pathway for the curing applications of formulations of various
pigmented systems.[19a, 21]. Although FRP dominates most of the
applications comprising pigmented systems, CP has also gained
importance in recent years.[22, 23]
Acknowledgements
The authors acknowledge the financial support by the National
Nature Science Foundation of China (51673086), Nature Science
Foundation of Jiangsu Province (BK20200610), Postdoctoral
Science Foundation of China (2019M661720). One of the authors
(Y.Y.) is indebted to the Local Minister of Jiangsu Province, China
for providing a visiting scientist grant through the International
Collaboration program (MOE and SAFEA, 111 Project (B13025)).
Figure 6. Kinetic profiles of double bond conversion under air in the presence
of P-PTh salt under different light irradiation sources. (A) The conversion of
double bond in TPGDA (LED@365 nm (30 mW/cm2) and LED@405 nm (30
mW/cm2)). (B) The conversion of double bond in TMPTA (LED@365 nm (30
mW/cm2), LED@405 nm (30 mW/cm2) and Broad-spectrum light (260
mW/cm2))
Conflict of interest
The authors declare no conflict of interest.
Keywords: broad-wavelength • photoinitiator •phenacyl•
phenothiazine • photopolymerization
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4
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COMMUNICATION
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10.1002/anie.202104531
Angewandte Chemie International Edition
COMMUNICATION
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We report a new single component broad-band photoinitiator absorbing light at UV, Visible and Near-IR region for the initiation of free
radical, cationic and hybrid polymerizations. This new photoinitiator may expand the curing application range of formulations of
various pigmented systems.
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This article is protected by copyright. All rights reserved.