N-substituted quinoxalinobenzothiazine/iodonium salt systems as visible photoinitiators for hybrid polymerization

Article abstract of DOI:10.1016/j.dyepig.2013.01.021

Sensitizers based on the 12-substituted 12H-quinoxalino-[2,3-b][1,4]- benzothiazine skeleton were synthesized and characterized using ¹H NMR spectroscopy, mass spectrometry and elemental analysis. Their electrochemical and spectral properties,

Full text of DOI:10.1016/j.dyepig.2013.01.021

Dyes and Pigments 97 (2013) 462e468
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
N-substituted quinoxalinobenzothiazine/iodonium salt systems as visible
photoinitiators for hybrid polymerization
*
Rados1aw Podsiad1y , Rafa1 Strzelczyk
Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Sensitizers based on the 12-substituted 12H-quinoxalino-[2,3-b][1,4]-benzothiazine skeleton were syn-
thesized and characterized using 1H NMR spectroscopy, mass spectrometry and elemental analysis. Their
electrochemical and spectral properties, including absorption and emission spectra, fluorescence
quantum yield and singlet lifetime were also measured. These benzothiazines combine with commer-
cially available diphenyliodonium hexafluorophosphate to create novel visible initiator systems of free
radical/cationic hybrid polymerization of glycidyl methacrylate. The efficiency of these initiator systems
is discussed on the basis of the free energy change for electron transfer from the benzothiazines to the
iodonium compound. During the photopolymerization photobleaching of the benzothiazines was
observed; a mechanism to account for this bleaching is proposed.
Received 11 October 2012
Received in revised form
25 January 2013
Accepted 28 January 2013
Available online 6 February 2013
Keywords:
Synthesis
12-Phenyl-12H-quinoxalino-[2,3-b][1,4]-
benzothiazine
Ó 2013 Elsevier Ltd. All rights reserved.
12-Benzoyl-12H-quinoxalino-[2,3-b][1,4]-
benzothiazine
12-Acetyl-12H-quinoxalino-[2,3-b][1,4]-
benzothiazine
Hybrid photopolymerization
Photobleaching
1. Introduction
component systems [3e12]. Depending on the nature of the dye
involved, two distinct sensitizations need to be considered: the
Photopolymerization is of great practical interest due to its
applicability for curing coatings and printing inks and for resist
technology [1,2]. Initially, photoinitiating systems that were sen-
sitive to ultraviolet light were used. However, at present, systems in
which polymerization is sensitive to visible light are used in con-
junction with visible emitting light sources, such as lasers and LEDs.
These systems are of particular interest because of their use in
many special applications, such as dental filing materials, photo-
resists, printing plates, highly pigmented coatings, holographic
recording and nanoscale micromechanics [3]. One way to create
a system in which polymerization can be photoinduced by visible
light is to use a dye. In such dye based photoinitiator systems, the
initiating species are typically generated by a photoinduced elec-
tron transfer. These systems involve a second component, an
appropriate co-initiator that participates in the electron transfer
process. Many attempts have been made to develop such two
electron transfer from the co-initiator to the excited, photo-
reducible dye [4e7] and the electron transfer from the excited,
photo-oxidizable dye to the co-initiator (Scheme 1) [8e12]. Then
the initiating radicals are formed from cleavage of the oxidized or
reduced form of the co-initiator. In photo-reducible sensitization,
the commonly used co-initiators are N-phenylglycine [4,5], phe-
nylthioacetic acid [5e7], amine [8,10] and alkyltriphenylborate
[11,12], whereas in photo-oxidizable sensitization, the commonly
used co-initiators are N-alkoxypyridinium salts [13,14] and the
derivatives of 2,4,6-tris(trichloromethyl)-1,3,5-triazine [15e17]. For
example, in photo-reducible sensitization the sulfur centered rad-
ical zwitterions from phenylthioacetic acid decayed rapidly into
CCH2eSeC6H5 and CO2. In photo-oxidizable sensitization single-
electron transfer to an N-alkoxypyridinium results in NeO bond
cleavage and the formation of an alkoxy radical. These dye based
photoinitiator systems have been mainly applied to initiate free
radical polymerization.
The second important application of oxidizable sensitization is
the cationic photopolymerization of epoxide monomers [18e21]. In
this case the dyes have been combined with diphenyliodonium
salts and photoinduced electron transfer forms several cationic
* Corresponding author. Tel.: þ48 42 631 32 37; fax: þ48 42 636 25 96.
E-mail
addresses:
radoslaw.podsiadly@p.lodz.pl,
radekpod@p.lodz.pl
(R. Podsiad1y).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.021

R. Podsiadły, R. Strzelczyk / Dyes and Pigments 97 (2013) 462e468
463
O
I+
+ PhSCH2COOH
N
N
S
.
Dye
.
-
+
.
+ PhSCH2COOH
PhSCH2
-
O
- H+, - CO2
PF6
(a)
O
N
X
hν
*
Dye
Dye
2, X = Ph; 3, X = COPh, 4, X = Ac
Scheme 3. Structures of the compounds used in this study.
(b)
.
.
.
+
OMe
Dye + Py-OMe
- Py
+ Py-OMe+
spectroscopy [Bruker Avance DPX 250, CDCl3, TMS standard,
(ppm)]. Their purity was confirmed using TLC. The chemical
Scheme 1. (a) Photo-reducible and (b) photo-oxidizable sensitization.
d
ionization mass spectra were recorded on a Finnigan MAT 95
spectrometer with isobutane application. Electrochemical experi-
ments were measured as described in Ref. [30]. The absorption and
steady-state fluorescence spectra were recorded using a JASCO V-
670 spectrophotometer (Jasco, Japan) and a Lumina fluorescence
spectrometer (Thermo Scientific, USA), respectively. All photo-
chemical experiments were carried out in a Rayonet Reactor RPR
200 (The Southern New England Ultraviolet Co, USA) equipped
with lamps emitting light at 419 nm. A specific spectral band was
isolated using band-pass filters at 419 ꢀ 10 nm, and illumination
intensities were measured using uranyl oxalate actinometry [31].
species (Scheme 2), which can also initiate the cationic polymer-
ization: the radical cation (DyeCþ), formed after electron transfer
to an onium salt, the strong protic acid formed from the reaction
between the DyeCþ and solvent or monomer; and the carbocation
of the monomer (Mþ), which is formed during the oxidation of the
monomer radical (MC) by the iodonium salt [22].
In recent years, cationic and free radical polymerizations have
been combined to form hybrid polymerization systems. The com-
bination of these polymerization methods has led to the prepara-
tion of unusual hybrid polymer structures such as multi-block [23],
graft [24] and random [25] copolymers and interpenetrating pol-
ymer networks [26,27]. Nearly all of the reports of hybrid photo-
polymerization have used separate radical and cationic
photoinitiators with one or two UV light sources. There are only
two published reports that describe the application of two com-
ponent visible initiator systems which were applied to initiate
a dual curing. In these report, dye derivatives of quinoxaline [28]
and 3,30-carbonylbis(7-diethylaminocoumarin) [29] were com-
bined with either triarylsulfonium [28] or diphenyliodonium [29]
salts, respectively. The photoinduced electron transfer between
dyes and onium salt gives an unstable onium radical, which rapidly
decomposes to generate an aryl radical to induce radical poly-
merization. The dye radical cation, or the proton generated from it,
induces cationic polymerization. The main goal of the current study
is the synthesis of N-substituted benzothiazines (2e4), the evalu-
ation of their spectroscopic, photophysical, electrochemical prop-
erties and their application in a hybrid polymerization as visible
photoinitiators of glycidyl methacrylate (GMA) in the presence of
diphenyliodonium hexafluorophosphate (Ph2IPF6). The structures
of the studied quinoxalinobenzothiazines, iodonium salt and
monomer are presented in Scheme 3.
2.2. Synthesis
2.2.1. Synthesis of 12-phenyl-12H-quinoxalino-[2,3-b][1,4]-
benzothiazine (2)
12H-Quinoxalino-[2,3-b][1,4]-benzothiazine 1 (10 g, 0.04 mol),
Na2CO3 (4.22 g, 0.04 mol) and some copper shavings were heated
under reflux in iodobenzene (50 mL) until disappearance of 1. The
progress of the reaction was monitored by TLC [Merck Silica gel 60,
solvent: 4:1 (v/v) toluene/ethyl acetate]. After 10 h unreacted
iodobenzene was distilled off under reduced pressure. Absolute
alcohol was then added, the mixture heated and filtered after dis-
solution of the product. The filtrate was cooled giving yellowish
crystals of the product 2 (12.3 g, 94%), m.p. 202 ꢁC, 1H NMR
(350 MHz, CDCl3)
d
¼ 6.10e6.14 (m, 1H), 6.84e6.89 (m, 2H), 7.02e
7.07 (m, 1H), 7.29e7.40 (m, 5H), 7.50e7.54 (m, 1H), 7.58e7.61 (m,
3H); MS (CI): m/z 327.0 (Mþ), 328.0 (M þ Hþ), 329.0 (M þ 2Hþ);
Elemental analysis: found C 73.5; H 3.8, N 12.9; S 9.7% C20H13N3S
required C 73.37; H 4.00; N 12.83; S 9.79%.
2.2.2. Synthesis of 12-benzoyl-12H-quinoxalino-[2,3-b][1,4]-
benzothiazine (3) [32]
2. Experimental
Benzoyl chloride (1.65 mL, 0.0096 mol) was added dropwise
during 30 min to a stirred suspension of 12H-quinoxalino-[2,3-b]
[1,4]-benzothiazine 1 (1 g, 0.004 mol) in dry pyridine (8 mL) at
35 ꢁC. The mixture was further heated at 90 ꢁC for 3 h. After cooling,
the mixture was poured into 10% aqueous HCl (40 mL). The pre-
cipitate was filtered, washed with NaHCO3 (10% aq., 25 mL) of, dried
and recrystallized from ethanol. The product 3 (1.19 g) was
obtained with an 84% yield. M.p. 200 ꢁC (194 ꢁC [32]); 1H NMR
2.1. General
The hybrid monomer glycidyl methacrylate (GMA), dipheny-
liodonium hexafluorophosphate, and the necessary synthesis re-
agents were purchased from SigmaeAldrich (Poznan, Poland). The
starting compound 12H-quinoxalino-[2,3-b][1,4]-benzothiazine (1)
was synthesized according to the procedure described in Ref. [30].
The dyes 2e4 were identified and characterized via 1H NMR
(350 MHz, CDCl3)
d
¼ 7.06e7.26 (m, 4H), 7.31e7.39 (m, 3H), 7.44e
7.61 (m, 4H), 7.85e7.89 (m, 1H), 8.10e8.14 (dd, J ¼ 1.36, 1H); MS
(CI): m/z 356.1 (M þ Hþ); Elemental analysis: found C 71.1; H 3.6, N
11.7; O 4.6; S 9.0% C21H13N3OS required C 70.97; H 3.69; N 11.82; O
4.50; S 9.02%.
electron
.
transfer
.
.
+
+
Dye*
+ Ph2I
Dye
+ Ph2I
2.2.3. Synthesis of 12-acethyl-12H-quinoxalino-[2,3-b][1,4]-
benzothiazine (4)
The synthesis was conducted using the modified procedure [32].
12H-Quinoxalino-[2,3-b][1,4]-benzothiazine 1 (1 g, 0.004 mol) and
sodium acetate (5.5 g, 0.067 mol) were heated under reflux in acetic
anhydride (100 mL). The reaction was monitored by TLC [Merck
.
.
+
Dye+
.
PhI + Ph
+
M
H
Dye H +
M
+ Ph2I+
H +
+ + Ph2I
M
Scheme 2. Formation of cationic species during photo-oxidizable sensitization.

464
R. Podsiadły, R. Strzelczyk / Dyes and Pigments 97 (2013) 462e468
transparent polypropylene film at a thickness of 30
mm using pol-
N
N
S
N
ypropylene film as a spacer. Another 30 m thick polypropylene
m
i)
film was laminated onto top of the sample. The photosensitized
hybrid polymerization of the GMA was monitored using Fourier
transform real-time infrared spectroscopy (FT-RTIR). Photocured
samples were analyzed with an IR Nicolet (Thermo Scientific, USA)
spectrophotometer with a resolution of 4 cmꢂ1 in the absorbance
mode. All photopolymerizations were conducted using 419 nm
light (2 ꢃ 1015 quant sꢂ1). Epoxide group and methacrylic bond
conversions were determined by monitoring the peak area at
909 cmꢂ1 and 1637 cmꢂ1, respectively. From the FTIR spectra, the
conversion of each functional group was calculated using the fol-
lowing equation:
2
S
N
N
N
N
N
S
ii)
N
H
3
O
O
1
AX;t AST;0
AX;0 AST;t
Conversion ¼ 1 ꢂ
$
(2)
N
N
S
N
where A0 and At are the initial peak area of functional group and at
a given irradiation time. The subscripts ST and X denote the internal
reference ester carbonyl peak area at 1730 cmꢂ1 and functional
group, respectively. The kinetic parameter, Rp/M0, was determined
from the initial slopes of the irradiation timeeconversion curves
according to Eq. (3):
iii)
CH3
4
Scheme 4. i) Iodobenzene, Cu, reflux; ii) benzoyl chloride, pyridine; iii) acetic anhy-
dride, reflux.
ꢀ
ꢁ.
Rp=M0
¼
½conversionꢄt ꢂ ½conversionꢄt
ðt2 ꢂ t1Þ
(3)
2
1
where Rp and M0 are the rate of polymerization and the initial
monomer concentration, respectively. The conversions are deter-
mined from the curves at irradiation time t1 and t2.
Silica gel 60, solvent: 7:3 (v/v) benzene/acetone]. After dis-
appearance of 1 the precipitate was filtered and dissolved in hot
acetone. The sodium acetate was filtered and the acetone was
evaporated in vacuum. The product 4 (0.80 g) was obtained with
a 68% yield. M.p. 195e196 ꢁC (196 [32]); 1H NMR (350 MHz, CDCl3)
2.4. Coating formulation, curing procedure and film property test
d
¼ 2.43 (s, 3H), 7.29e7.37 (m, 1H), 7.41e7.54 (m, 2H), 7.74e7.90 (m,
3H), 8.02e8.13 (m, 2H); MS (CI): m/z 294.0 (M þ Hþ); Elemental
analysis: found C 65.7; H 3.7, N 14.2; O 5.5; S 10.8% C16H11N3OS C,
65.51; H, 3.78; N, 14.32; O, 5.45; S, 10.93%.
The GMA were poured onto a glass plate to form a layer ca. 3 mm
thick, and then the samples were exposed to visible radiation for a
given time. The concentration of dye and Ph2IPF6 was 0.33 mM and
28 mM, respectively. The photopolymerizations were conducted in
air using 419 nm light with an intensity of 3.2 ꢃ 1017 quant sꢂ1
.
2.3. Photochemical experiments
Pencil hardness was conducted to evaluate the surface and curing
properties of the cured films according to the standard ASTM
methods D3363-74. The gel content was determined on the cured
films by measuring the weight loss after 24 h extraction in 2-
butanone at room temperature according to the standard test
method ASTM D2765-84. Shore durometer HPE II (Bareiss Prüf-
gerätebau GmbH) was employed to measure the hardness of GMA
cured film. Five measurements were made in each sample, and the
results presented are an average of these measurements.
The fluorescence quantum yield of the dye (FDYE) was calculated
from the following equation:
GDYE
GST
$
$
h2DYE
h2ST
FDYE
¼
FST
(1)
where the subscripts ST and DYE denote standard and test
respectively, is the fluorescence quantum yield, G is the gradient
from the plot of integrated fluorescence intensity versus absorb-
ance, and is the refractive index of the solvent. Rhodamine 101 in
F
The DSC experiments were carried out using a Mettler Toledo
Stare TGA/DSC1 (Leicester, UK) unit, under nitrogen. Samples (ca.
5 mg) were placed in aluminum pans and were cooled down
to ꢂ150 ꢁC and then heated to 250 ꢁC at constant heating rate of
h
ethanol was used as the standard (FST ¼ 1.0 [33]).
Time-resolved fluorescence was measured using
a
time-
10 ꢁC minꢂ1
.
correlated single photon counting system, Edinburgh Analytical
Instruments FL 900CDT. Fluorescence lifetimes were calculated
from intensity decay analysis with goodness of fit judged in terms
of residual distributions using software supplied by Edinburgh
Instruments.
3. Results and discussion
3.1. Synthesis and spectroscopic characterization of benzothiazines
Singlet quenching constants were obtained from fluorescence
quenching experiments, and the fluorescence spectra of the solu-
tions of the dyes (6e10 mM) in ethanol, which contained various
amounts of Ph2IPF6, were measured at room temperature in the air
by excitation at lmax. The relative fluorescence intensities (I0/I)
The basis of the design of these novel oxidizable initiator sys-
tems were literature reports that concerned the oxidation of N-
substituted phenothiazines to the stable radical cations [34e36]
and the ability of these radical cations to initiate the cationic pol-
ymerization [37]. Due to continuing interest in the applications of
oxidizable sensitizers in photopolymerization, the compounds
based on the N-substituted 12H-quinoxalino-[2,3-b][1,4]-benzo-
thiazine (2e4) were synthesized according to stage shown in
were determined by measuring the heights of the peaks at lem
.
In photopolymerization studies the concentrations of the ben-
zothiazine and iodonium salt were 1 mM and 85 mM, respectively.
The mixture for photopolymerization was deposited onto

R. Podsiadły, R. Strzelczyk / Dyes and Pigments 97 (2013) 462e468
465
Table 2
Oxidation potential (V) of benzothiazines, thermodynamic data (kJ molꢂ1), Sterne
Volmer (Mꢂ1) and singlet quenching (Mꢂ1 sꢂ1) constants for benzothiazines/Ph2IPF6
photoredox pair.
kq ꢃ 1010
a
Dye
Eox
DGet
Ksv
2
3
4
1.44
1.62
1.71
ꢂ103.3
ꢂ108.4
ꢂ99.6
132.9
e
2.91
e
20.4
0.79
a
In DMF versus SCE.
A larger hypsochromic effect was observed for 3 and 4. In these
compounds, the absorption bands were located at approximately
370 nm.
The fluorescence quantum yields (Ffl) of benzothiazines are
presented in Table 1. As can be seen from these data, the Ffl of these
compounds range from 0.02 to 1.0 with dyes 2 having the highest
values of Ffl
.
3.2. Sensitized hybrid photopolymerization
The studied benzothiazines 2-4 were synthesized in order to
obtain the sensitizers which are able to work via an electron
transfer process. To predict the potential applicability of these
compounds as electron sensitizers, the free energy change of the
electron transfer (DGet) between the oxidizing agent (Ph2IPF6) and
benzothiazines must be known. The electron transfer from these
excited benzothiazines to the iodonium salt is thermodynamically
favorable if the free energy (
DGet), calculated from the Rehme
Weller equation (Eq. (4)) [38] is negative.
Fig. 1. Normalized absorption (black) and emission (gray) spectra of dye 2 (12
and dye 4 (10 M, B) in MeCN.
mM, A)
ꢀ
ꢁ
h
ꢀ
ꢁ
ꢀ
ꢁi
m
D
Get kJ molꢂ1 ¼ 97 Eox D=Dꢅþ ꢂ Ered Aꢅꢂ=A ꢂ E00ðDÞ
ꢂ Z1Z2=εr12ð4Þ
Scheme 4. Our route to dyes 2 starts with 1, which react with
iodobenzene to give the corresponding derivatives of 12-phenyl-
12H-quinoxalino-[2,3-b][1,4]-benzothiazine (2) in 94% yield. Com-
pounds 3 and 4 are obtained by reaction of appropriate benzo-
thiazine (1) with benzoyl chloride or acetic anhydride, respectively.
The crude dyes were purified by recrystallization from ethanol
or acetone until a constant molar extinction coefficient and TLC
purity were obtained. Dyes 2e4 were synthesized in excellent yield
(68e94%), and their chemical structures were verified by 1H NMR
spectroscopy, mass spectrometry and elemental analysis.
The electronic absorption (UVeVIS) and emission spectra of
benzothiazines 2 and 4 are presented in Fig. 1. For all studied
compounds, the absorption and the fluorescence spectra were
nearly mirror images of each other with overlapping bands corre-
sponding to the 0 / 0 transition.
In this equation, Eox (D/Dꢅþ) and E00(D) are the oxidation potential
of the electron donor (benzothiazines) and the singlet excited state
energy of the electron donor, respectively. Ered (Aꢅꢂ/A) is the
reduction potential of Ph2IPF6. The last term represents the Cou-
lombic energy necessary to form an ion pair with charges Z1 and Z2
in a medium of dielectric constant ε for a distance r12. The Cou-
lombic energy coefficient (Z1Z2/εr12) was omitted in these calcula-
tions because a neutral radical of the onium compound was formed
(Z ¼ 0) in the electron transfer process.
In order to be able to calculate DGet, the oxidation potentials of
the benzothiazines 2e4 were measured in separate experiments.
The obtained results (Table 2) indicate that the location of the
oxidation peak depends on the structure of the benzothiazines. N-
phenyl substituted benzothiazines 2 are more readily oxidized in
comparison with the N-benzoylated 3 and N-acetylated analog 4.
The spectroscopic properties (absorption and fluorescence) of
compounds 2e4 are presented in Table 1. It is evident that the
position of the absorption band depended on the character of
a compound’s X group. In comparison with 12H-quinoxalino-[2,3-
b][1,4]-benzothiazine (1) [30], the presence of phenyl in 2 caused
a small blue shift of the longest wavelengths absorption band.
Once the Eox of benzothiazines had been measured, the DGet could
be calculated using equation (4). The calculated thermodynamic
parameters listed in Table 3 indicated that all the tested benzothia-
zines/Ph2IPF6 systems posses a favorable thermodynamic driving
Table 1
Spectroscopic and photophysical parameters of the compounds 1e4.
εa
lmax
εb
lfl
Ffl
SS
[nm]
sb
[ns]
E00b
a
b
b
b
Dye
lmax
[nm]
[dm3 molꢂ1 cmꢂ1
]
[nm]
[dm3 molꢂ1 cmꢂ1
]
[nm]
[kJ molꢂ1
]
1
2
3
4
425
416
373
371
7400
9400
9200
9800
416
413
370
367
530c
500
499
497
0.22
1.0
0.02
0.45
109
87
129
130
4.6c
4.57
4.33
2.56
253.5c
262.4
284.9
284.9
8200
9800
9750
a
In 1-methyl-2-pyrrolidone.
In CH3CN.
In EtOH from Ref. [30].
b
c

466
R. Podsiadły, R. Strzelczyk / Dyes and Pigments 97 (2013) 462e468
force upon exposure to light (eD
Get > 99 kJ molꢂ1). Moreover, the
fluorescence of benzothiazines 2 and 4 were effectively quenched by
diphenyliodonium salt. The example of the fluorescence spectra of
dye 2 recorded in EtOH containing various amounts of Ph2IPF6 are
presented in Fig. 2. The absence of any new peak in the emission
spectra excludes any exciplex formation. The bimolecular quenching
constant kq was calculated from Eq. (5) using the fluorescence life-
time of the benzothiazines (s0) without any quencher (Table 1).
I0=I ¼ 1 þ kqs0½Onꢄ
(5)
The calculated singlet quenching constants (kq) are summarized in
Table 2. These values are close to the diffusion-controlled limit
(kq w 1 ꢃ109 ꢂ 1 ꢃ1010
M
ꢂ1 sꢂ1). Dye 2 has the highest value of kq,
which also has the lowest oxidation potential. This suggests that
fluorescent quenching proceeds via electron transfer.
Finally, the benzothiazine/Ph2IPF6 two component systems
were examined for their usefulness as photoinitiators in the free
radical/cationic hybrid photopolymerization of GMA, which con-
tains a methacrylate double bond and an epoxide ring. From
a practical point of view, the most important property of the ini-
tiator system is the ability to initiate the photopolymerization of
monomers in the presence of air. Therefore, in our investigation,
the efficiency of the polymerization initiated by the studied pho-
toredox systems was measured in an air equilibrium composition.
FTIR spectroscopy was used to measure the conversion of meth-
acrylate double bonds and epoxide groups as a function of irradi-
ation time. Conversions were determined by monitoring the peak
area at characteristic wavenumber (909 cmꢂ1 for the epoxide group
[39] and 1637 cmꢂ1 for methacrylate bonds [39]) and calculating
the ratio of this area relative to that of the ester carbonyl peak at
1720 cmꢂ1. Fig. 3 shows an example of an FTIR spectra recorded
before and after irradiation of the GMA initiated by a 2/Ph2IPF6
photoredox pair. The reduction in the absorption of the character-
istic peak on visible irradiation shows that both epoxy groups as
well as the double bonds underwent polymerization. Moreover, the
increase of the absorption at 3450e3550 cmꢂ1 attributable to OH
stretching also confirms that the epoxy rings of GMA underwent
ring-opening polymerization. The kinetic parameters Rp/M0 were
determined from the slopes of the initial portion of the conversion
of the epoxide ring and methacrylate double bonds versus time
curves (Fig. 4) and are presented in Table 3.
Fig. 3. IR absorption spectra of GMA recorded before (black) and after (red) 6000 s of
the irradiation in the presence of 2/Ph2IPF6. Characteristic peaks in IR spectrum of
GMA: internal reference band at 1720 cmꢂ1, epoxide ring at 909 cmꢂ1 and C]C double
bond at 1637 cmꢂ1. Inset: IR absorption region of OH stretching. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version
of this article.)
From the foregoing data, it is evident that systems composed with
benzothiazines 2e4 and diphenyliodonium hexafluorophosphate
are able to simultaneously initiate cationic and radical polymer-
ization. In all of the photosystems studied here, a higher conversion
of methacrylate double bond compared to conversion of the epoxide
ring was observed. Similar effect was observed in hybrid photo-
polymerization of 3,4-epoxycyclohexylmethyl methacrylate ini-
tiated by 2,2-dimethoxy-2-phenylacetophenone (radical initiator)
and diaryliodonium hexafluoroantimonate (cationic initiators) [40].
These low conversions of epoxide can be explained by difference in
reactivity of double bond and epoxide ring. The epoxide group react
more slowly than double bonds. Moreover, the mobility of the cati-
onic reactive centers was restricted due to vitrification resulting from
the conversion of C]C double bonds [40].
The effectiveness of the studied two component systems de-
pends on the applied benzothiazines. The compounds 2 significantly
accelerated the polymerization of GMA compared to dyes 3 and 4.
However, it is evident that the hybrid polymerization initiated by
the 3/Ph2IPF6 photoredox pairs showed a significant inhibition time
(Fig. 4 and Table 3). Among the studied benzothiazines, the
12-benzoyl analog has the lowest quantum yield of fluorescence
(see Table 1). It is possible that the electron transfer between this
compound and the iodonium salt occurs via the triplet state.
Therefore, the molecular oxygen dissolved in the composition
quenched the triplet state of the benzothiazine 3. After consumption
of the oxygen, the initiating species are formed via electron transfer.
In the photopolymerization initiated by a dye photoredox
pair, the most important properties are high initiation reactivity
and photobleaching of the dye. The photobleaching of benzothia-
zines 2e4 was measured in GMA in the presence of air. An example of
the absorptionspectra of thecombinationofdye 2 andPh2IPF6 before
and during irradiation in GMA are shown in Fig. 5. The same pho-
tochemical behaviour was observed for benzothiazines 3 and 4.
Decay of the dye absorption at 415 nm is accompanied by the growth
of the band at 370 nm. It is well known that phenothiazine de-
rivatives are easily oxidized to yield stable radical cations [37]. A low
nucleophilic anion, such as BF4ꢂ or PFꢂ6 , stabilized the phenothiazine
radical cation. It is also known, that the phenothiazine radical cation
undergoes one electron oxidation by the iodonium compound to
form the phenothiazine dication, which absorbs in the UV region
Fig. 2. Fluorescence quenching of dye 2 by Ph2IPF6 in EtOH. Inset: SterneVolmer plot
of fluorescence quenching of dye 2 (10 mM) by Ph2IPF6 in EtOH.

R. Podsiadły, R. Strzelczyk / Dyes and Pigments 97 (2013) 462e468
467
Table 4
Conversion (%) of epoxy group and methacrylate bond of GMA obtained for pho-
topolymerization of freshly prepared or stored formulation and properties of the
VIS-cured films obtained in the presence of the presented photoredox pairs.b
Dye Epoxy
Methacrylate Gel
contentc (ꢁC)
Tg,c DSC Hardness Pencil
shore Dc hardnessc
Conva Convb Conva Convb
(%)
2
3
4
45
30
20
28
20
15
80
60
76
70
53
69
96.0
94.6
95.4
61
56
58
86
82
84
5H
8B
H
a
b
c
Freshly prepared formulation; light intensity 1.6 ꢃ 1017 quant sꢂ1
.
Formulation stored by seven days; light intensity 1.6 ꢃ 1017 quant sꢂ1
.
Irradiated for 15 min with light intensity 3.2 ꢃ 1017 quant sꢂ1; properties were
measured after 72 h.
To analyze the properties of the photocured film, the GMA were
photopolymerized in a photochemical reactor with a higher radi-
ation intensity (3.2 ꢃ 1017 quant sꢂ1) for 15 min. All coatings were
stored in the dark for 72 h to ensure that no additional photo-
chemical reactions occur in formulation. The resulting film was
300 mm thick with a gel content above 94% (Table 4) indicating the
formation of a highly crosslinked network.
The thermal characterization of the films obtained upon the pho-
topolymerization of GMA using dye/iodonium photoredox pair is
reported in Table 4. The thermogram of this film shows one glass
transition temperature around 58 ꢁC. All coatings were tested by both
pencil hardness and Shore hardness. Pencil hardness is a generally
accepted method to measure the resistance of a coating (also known as
its pencil hardness). It is determined as the hardest pencil grade that
does not mark the coating when pressed firmlyagainstitata45ꢁ angle.
The results of these tests are presented in Table 4. The me-
chanical properties of the cured GMA were found to depend on the
dye that was employed as the sensitizer. The coating with the best
properties was obtained with aid of sensitizers 2. This dye dem-
onstrates a highest Ffl. Therefore one can conclude and at a depth of
Fig. 4. Conversion of acrylate (B) and epoxy (A) groups during photopolymerization of
GMA initiated by Ph2IPF6 and 2 (C), 3 (,) or 4 (-).
[41,42]. Therefore, it is possible that during photolysis in GMA, the
studied N-substituted benzothiazines may work as two electron
transfer sensitizers. First, they are oxidized by Ph2IPF6 to radical
cations, which are then oxidized to a dication (Scheme 5). A similar
mechanism of oxidation was proposed for photobleaching of ben-
zothiazine 1 [41].
1 mm, where O2 can quench the radical and sensitizers’ triplet state,
electron transfer reaction between iodonium salt and dye 2 may
occur via a singlet state. This is particularly apparent on the top
layer of the coatings. The lowest pencil hardness has coating
obtained with aid of dyes 3 which have the lowest Ffl. While the top
layer of the film was polymerized via cationic mechanism the dif-
fusion of molecular oxygen was inhibited. Therefore, at deeper
layers, the radicals initiate the polymerization of GMA. The overall
The calculated photobleaching quantum yields are presented in
Table 4. These data clearly indicate that in these oxidizable sensitiza-
tion systems the oxidation potential of benzothiazines determine the
photoreactivity of these compounds. Among the studied benzothia-
zines, dye 2 shows the highest photobleaching quantum yield and this
compound is the most promising photosensitizer for VIS dual curing.
In industry, both the physical properties of coatings and stability
of the formulation must be tailored to the ultimate application.
Therefore, the polymerizations of GMA were carried out for freshly
prepared formulation and after 7 days of storage. The comparison
of epoxy group and double bond conversion these two formulation
are presented in Table 4. It is evident from these data that the ini-
tiator systems composed with 12-substituted 12H-quinoxalino-
[2,3-b][1,4]-benzothiazine dye and Ph2IPF6 leads to a slow degra-
dation and should be prepared just before application.
Table 3
Conversion (%) of epoxy group and methacrylate bond of GMA, and photobleaching
quantum yield (mmol quantꢂ1) of compounds 2e4.
Epoxy
Methacrylate
tinh Conv
Fbl
tinh
Conv
Rp/M0
Rp/M0
2
3
4
0
1200
0
39
19
34
1.37
0.30
1.94
0
50
25
39
1.82
0.47
1.94
9.3
4.5
2.8
Fig. 5. Electronic absorption spectra obtained upon photolysis of the dye 2 (66.7 mM)/
Ph2IPF6 (5.67 mM) system in GMA under air atmosphere. Arrows indicate changes in
the spectra during irradiation.
1800
0

468
R. Podsiadły, R. Strzelczyk / Dyes and Pigments 97 (2013) 462e468
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Scheme 5. Photobleaching of studied benzothiazines.
hardness of the coatings, measured as Shore hardness, is a result of
these two polymerizations.
4. Conclusions
12-substituted
12H-quinoxalino-[2,3-b][1,4]-benzothiazines
were successfully synthesized and characterized using 1H NMR
spectroscopy, CI mass spectrometry and elemental analysis. These
compounds, when combined with diphenyliodonium hexa-
fluorophosphate, may be applied as electron transfer photosensi-
tizers. These two component systems were employed as initiators
for visible-light initiated free radical and cationic photo-
polymerization of an acrylate/epoxide hybrid system. The FT-IR
data revealed that these photoredox pairs are able to simulta-
neously initiate both cationic and radical polymerization in the
presence of air. The kinetic data and photobleaching quantum yield
indicated, that when combined with iodonium salts, benzothiazine
2 may have practical applications as a visible-light hybrid initiator.
Acknowledgments
This work was supported by the grant of Rector of Lodz Uni-
versity of Technology.
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