Synthesis of novel oxidizable polymerization sensitizers based on the dithiinoquinoxaline skeleton

Article abstract of DOI:10.1016/j.dyepig.2011.09.006

Novel dyes, based on the dithiinoquinoxaline skeleton, were synthesized and characterized using ¹H NMR spectroscopy and chemical ionization mass spectroscopy. Their spectral properties, such as absorption, emission spectra and quantum yield of fluorescence, were also measured. Electron donating properties of the title compounds were estimated on the basis of DFT calculations. The studied dyes were used as oxidizable sensitizers for 2,4,6-tris(trichloromethyl)-1,3,5-triazine (Tz). The dye/Tz photoredox pairs were found to be effective visible-wavelength initiators of free radical polymerization. The ability of these systems to act as photoinitiators strongly depended upon the free energy change of the photoinduced electron transfer from the excited dyes to Tz. It has been shown that the intermolecular electron transfer is the limiting step in the photopolymerization initiated by these studied initiator systems.

Full text of DOI:10.1016/j.dyepig.2011.09.006

Dyes and Pigments 92 (2012) 1300e1307
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of novel oxidizable polymerization sensitizers based
on the dithiinoquinoxaline skeleton
*
Rados1aw Podsiad1y , Jolanta Soko1owska
Institute of Polymer and Dye Technology, Technical University of Lodz, 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
Novel dyes, based on the dithiinoquinoxaline skeleton, were synthesized and characterized using 1H
NMR spectroscopy and chemical ionization mass spectroscopy. Their spectral properties, such as
absorption, emission spectra and quantum yield of fluorescence, were also measured. Electron donating
properties of the title compounds were estimated on the basis of DFT calculations. The studied dyes were
used as oxidizable sensitizers for 2,4,6-tris(trichloromethyl)-1,3,5-triazine (Tz). The dye/Tz photoredox
pairs were found to be effective visible-wavelength initiators of free radical polymerization. The ability of
these systems to act as photoinitiators strongly depended upon the free energy change of the photo-
induced electron transfer from the excited dyes to Tz. It has been shown that the intermolecular electron
transfer is the limiting step in the photopolymerization initiated by these studied initiator systems.
Ó 2011 Elsevier Ltd. All rights reserved.
Article history:
Received 30 May 2011
Received in revised form
6 September 2011
Accepted 9 September 2011
Available online 17 September 2011
Keywords:
Dithiinoquinoxaline dyes
Synthesis
Oxidizable sensitization
Free radical polymerization
Photoinduced electron transfer
DFT calculations
1. Introduction
recording and nanoscale micromechanics [2]. The most important
component of such photopolymerized systems is the photoinitiators
Industrial applications of photocuring are extremely varied and
include metal coatings (automotive varnishes), the production of
printed circuit boards and the generation of three-dimensional
models [1]. Radical chain addition polymerization is one of the
simplest and most widely used methods of polymer formation. In
the industrial applications of this method, the polymerizable
formulations are based on mono- and multifunctional (meth)
acrylate, unsaturated polyester, and acrylated polyurethanes.
Many highly efficient UV initiators are commercially available
[2]. These initiators differ in their mode of generation of reactive
free radicals. During photolysis, type I initiators undergo a bond
cleavage as shown in Scheme 1 for benzoin. Alternatively, the
triplet state of type II initiators reacts with a H-donor compound
such as tertiary amines, thiols, ethers and alcohols to yield the
initiating radicals (Scheme 2).
which should have efficient absorbance at long visible wavelengths.
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 radicals are typically gener-
ated by a photoinduced electron 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 component systems [3e12]. Depending on the
nature of the dye involved, two distinct sensitizations need to be
considered: the electron transfer from the co-initiator to the
excited, photoreducible dye [3e7] and the electron transfer from
the excited, photo-oxidizable dye to the co-initiator (Scheme 3)
[8e12]. Then the initiating radicals are formed from cleavage of the
oxidized or reduced form of the co-initiator.
In photo-oxidizable sensitization, the commonly used co-
initiators are N-alkoxypyridinium salts and the derivatives of
2,4,6-tris(trichloromethyl)-1,3,5-triazine. These compounds yield
alkoxy radicals [8e12] and halogenmethyl radicals [6,7] which
initiate polymerization. However, not all dyes are able to initiate
photopolymerization with the aid of Tz [7].
At present, systems in which polymerization is sensitive to visible
light are used in conjunction 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,
photoresists, printing plates, highly pigmented coatings, holographic
As mentioned above, in oxidizable sensitization dyes undergo
the photooxidation and they should have a low ionization potential.
It is well known that 1,4-dithiin and its dibenzo-homolog, thian-
threne, are characterized by a notably low first oxidation potential
* Corresponding author. Fax: þ48 42 636 25 96.
E-mail address: radoslaw.podsiadly@p.lodz.pl (R. Podsiad1y).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.006

R. Podsiadły, J. Sokołowska / Dyes and Pigments 92 (2012) 1300e1307
1301
suspended in water (10 ml). To this suspension a saturated aqueous
KOH solution was added until the precipitate was completely dis-
solved. Then 1 M hydrochloric acid was added with stirring to adjust
the pH of the solution to pH z 1. The resulting dark-brown precip-
itate was filtered, washed with water and dried to yield 3b (3.502 g,
92%). The 3a quinoxaline-2,3-dithiol was synthesized, in a similar
fashion, from 2,3-dichloroquinoxaline starting material. 3b was used
in to the next step of the synthesis without purification.
O
O
.
UV
Ar
OH
Ar
.
+
Ar
Ar
OH
Scheme 1. Type I initiator.
due to the p-donor ability of the sulfur atom [13]. The thianthrene
2.2.3. Synthesis of [1,4]dithiino[2,3-b:5,6-b0]diquinoxaline 4a
2,3-Dichloroquinoxaline 1a (0.995 g, 0.005 mol), quinoxaline-
2,3-dithiol 3a (0.971 g, 0.005 mol) and triethylamine (1 ml) were
refluxed in dimethylformamide (15 ml) for 5 h. After cooling, the
resulting precipitate was filtered, washed with water and ethanol
to yield 4a (1.202 g, 75%, m.p. > 360 ꢀC).
radical cation is readily formed through oxidation either homoge-
neously in solution [14e16] or electrochemically [17]. Moreover,
polyheterocyclic systems with fused 1,4-dithiine units are electron
donors with moderate oxidation potentials [18]. Therefore, the
main goals of this study were to synthesize sensitizers based on the
dithiinoquinoxaline skeleton (Scheme 4) and to evaluate these
dyes’ spectroscopic, photophysical, and electron donating proper-
ties. This paper also reports the application of these dyes as visible-
light sensitizers for 2,4,6-tris(trichloromethyl)-1,3,5-triazine (Tz).
Finally, experiments demonstrated that these new photo-oxidiz-
able sensitization systems could be used as visible-light photo-
initiators for the free radical polymerization of acrylate monomers.
2.2.4. Synthesis of 3-chloropyrido[200,300:5,6]pyrazino[20,30:5,6]
[1,4]dithiino[2,3-b]quinoxaline 6b
2,3,7-Trichloropyridipyrazine 5b (1.172 g, 0.005 mol), quinoxa-
line-2,3-dithiol 3a (0.971 g, 0.005 mol) and triethylamine (1 ml)
were refluxed in dimethylformamide (30 ml) for 5 h. After cooling,
the resulting precipitate was filtered, washed with water and
ethanol, dried and recrystallized from chloroform. The product 6b
(1.415 g, m.p. 339e341 ꢀC) was obtained with an 80% yield. The
other dyes were synthesized in the same manner from the appro-
priate dichloropyridopyrazines 5 and quinoxaline-2,3-dithiols 3.
Table 1 presents the yield, melting point, elemental analysis, 1H
NMR and CI MS data for these dyes.
2. Experimental
2.1. General
Trimethylolpropane triacrylate (TMPTA),1-methyl-2-pyrrolidone
(MP) and the necessary synthesis reagents were purchased from
SigmaeAldrich (Poznan, Poland). Tz was purchased from Wako-
Chemicals. The dyes 4ae4b and 6ae6f were identified and charac-
terized via 1H NMR spectroscopy [Bruker Avance DPX 250, CDCl3,
2.3. Photochemical experiments
drop of CF3COOH, TMS internal standard,
d (ppm)]. Their purity
All photochemical experiments were carried out in a Rayonet
Reactor RPR 200 (Southern New England Ultraviolet Co, USA)
equipped with eight lamps emitting light at 419 nm. The illumi-
nation intensity was measured using uranyl oxalate actinometry
[21].
was confirmed using TLC [Merck Silica gel 60, solvent: 3:1 (v/v)
toluene/pyridine]. The chemical ionization mass spectra were
recorded on a Finnigan MAT 94 spectrometer with isobutane appli-
cation. 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.
The fluorescence quantum yield of the dye (FDYE) was calculated
from the following equation:
GDYE
GST
$
$
h2DYE
h2ST
2.2. Synthesis
FDYE
¼
FST
(1)
2.2.1. Synthesis of 6,7-dimethyl-2,3-diisothiouronium quinoxaline
dihydrochloride 2b [19]
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 absor-
bance, and is the refractive index of the solvent. Rhodamine 101 in
F
6,7-Dimethyl-2,3-dichloroquinoxaline (4.7 g, 0.021 mol) and
thiourea (3.48 g 0.046 mol) were refluxed in ethanol (55 ml, 96%)
for 5 h. After cooling, the resulting goldish-orange precipitate was
filtered and washed with 5 ml of ethanol to yield 2b (4.59 g, 58%).
2b was used in the next step of the synthesis without purification.
The 2a diisothiouronium quinoxaline dihydrochloride was
synthesized, in a similar fashion, from 2,3-dichloroquinoxaline
starting material.
h
ethanol was used as the standard (FST ¼ 1.0 [22]).
In all the polymerization experiments, a cut-off filter was used
to eliminate wavelengths shorter than 400 nm. Free radical pho-
topolymerization reactions were conducted in a solvent mixture of
1 ml of MP and 4 ml of TMPTA. The concentration of dye and Tz
were maintained at 50 mM and 1 mM, respectively. The solutions
were irradiated for 180 s. The rate of polymerization (Rp) was
calculated from Eq. (2):
2.2.2. Synthesis of 6,7-dimethylquinoxaline-2,3-dithiol 3b [19]
6,7-dimethyl-2,3-diisothiouronium quinoxaline dihydrochloride
2b (6.495 g, 0.0036 mol) was suspended in water (24 ml) and
a solution of KOH (3.2 g, 0.09 mol) in water (13 ml) was added. The
resulting orange precipitate was filtered, washed with water and
Rp ¼ Qs$M=n$
D
Hp$m
(2)
In this expression, Qs is heat flow per second during the reaction,
m is the mass of the monomer in the sample, M is the molar mass of
the monomer, n is the number of double bonds per monomer, and
H
O
3
DHp is the theoretical enthalpy for complete polymerization of
O
O
UV
R
H
.
acrylate double bonds (20.6 kcal/mol) [23]. The heat flow was
measured with a PT 401 temperature sensor (Elmetron, Poland),
immersed in the sample. A polymerizing mixture containing the
dye without a co-initiator was used as a reference sample.
+
R
Ar
Ar
.
Ar
Ar
Ar
Ar
Scheme 2. Type II initiator.

1302
R. Podsiadły, J. Sokołowska / Dyes and Pigments 92 (2012) 1300e1307
R
_
.
N
*
N
R
N
R
N
Cl3C
N
N
_
CCl3
.
N
N
N
+
Dye
Cl
+
+
.
Cl3C
Cl3C
CCl3
CCl2
hν
Dye
Dye
+ M
Polymer
+ M
+
-
PF6
N
OMe
.
.
+
OMe
Dye
+
+
.
N
N
OMe
Scheme 3. Photo-oxidizable sensitization.
2.4. Quantum chemical calculations
method on the gas-phase optimized geometry of the neutral
molecule and its radical cation. The dielectric constant of acetoni-
The geometries of all species were optimized by the B3LYP
density functional method [24,25] as implemented in the Gaussian
03 suite of programs [26]. The optimized structures were charac-
terized by harmonic frequency analysis as local minima (all
frequencies real). These calculations were performed for the
ground states using a standard 6-31G(d) basis set. The gas-phase
vertical ionization potentials (IPVgas) were calculated from the
differences in total energy between the neutral molecule and its
radical cation at the optimized geometry of the neutral molecule.
Relative energies were obtained only by single point calculations at
the B3LYP/6-31þG(d) level on optimized structures of the ground
states and of cation radicals in neutral geometry. The adiabatic
ionization potentials (IPgas) were obtained in the same way, but
using the total energy obtained from the optimized geometry of the
radical cation. Bulk solvent effects were studied by performing self-
consistent reaction field (SCRF) calculation by using the PCM
trile (
3
¼ 36.64) was used throughout. In MeCN the ionization
potentials IPMeCN were calculated by subtracting the total energies
of the neutral molecule and radical cation obtained from single
point IEFPCM calculation.
3. Results and discussion
3.1. Synthesis and spectroscopic characterization of dyes
The basis of the design of the studied dyes was literature reports
that concerned the formation of a thianthrene radical cation
[14e17]. Due to continuing interest in the applications of oxidizable
sensitizers, we prepared dyes based on the dithiinoquinoxaline
skeleton (4, 6). To the best of our knowledge, derivatives 6 are so far
not known.
Compounds 4 and 6, with fused 1,4-dithiine units, were
synthesized by a three stage synthesis shown in Scheme 5. Our
route to dyes 4 starts with 2,3-dichloroquinoxalines (1a, 1b), which
react with thiourea in refluxing ethanol (96%) to give the corre-
sponding derivatives of 2,3-diisothiouronium quinoxaline dihy-
drochloride (2a, 2b) in 60% yield [19]. In the next step,
quinoxalines-2,3-dithiols (3a, 3b) are obtained by subsequent
treatment of 2 with KOH and HCl [19]. Finally, the desired dyes (4a
and 4b) were synthesized by refluxing 3a or 3b with 2,3-dichlor-
oquinoxaline (1a) in dimethylformamide with triethylamine. Dyes
6 were synthesized in the same manner starting from the appro-
priate 2,3-dichloropyridopirazines (5ae5c), which were obtained
according to procedure described in Ref. [11].
The crude dyes were purified by recrystallization from chloro-
form until a constant molar extinction coefficient and TLC purity
were obtained. Dyes 4 and 6 were synthesized in excellent yield
(69e80%), and their chemical structures were verified by 1H NMR
and CI mass spectra (Table 1). The spectroscopic properties
(absorption and fluorescence) of dyes 4 and 6 are presented in
Table 2.
Dyes 4 and 6 have absorption bands located at approximately
390 nm, and emission bands located at approximately 480 nm
characterized by a w64e98 nm Stokes shift. These values indicate
that the geometry of the singlet excited state does not differ greatly
from the geometry of the ground state. The electronic absorption
(UVevis) and emission spectra of dye 6f are presented in Fig. 1. The
spectra of the other dyes are presented in Figs. 1Se7S (Supple-
mentary data). For all dyes tested, the absorption and fluorescence
X
N
N
N
N
S
S
R
R
Y
R
H
X
Y
4a
CH
H
H
4b CH3 CH
6a
6b
6c
H
H
H
N
N
N
N
N
N
H
Cl
Br
H
6d CH3
6e CH3
6f CH3
Cl
Br
Scheme 4. Structure of the studied dyes.

R. Podsiadły, J. Sokołowska / Dyes and Pigments 92 (2012) 1300e1307
1303
Table 1
Melting point, yield, elemental analysis, 1H NMR and CI mass spectra of dyes 4 and 6.
Dye
M.p.
[ꢀC]
Yield
[%]
Elemental analysis
MS CI
m/z
1H NMR
J [Hz]; d [ppm]
4a
>360
350 [20]
75
76
41
80
79
39
72
80
Anal. Calcd for C16H8N4S2
C, 59.98; H, 2.52; N, 17.49; S, 20.02
Found:
C, 59.4; H, 2.5; N, 17.6; S, 19.9
Anal. Calcd for C18H12N4S2
C, 62.05; H, 3.47; N, 16.08; S, 18.40
Found:
C, 62.0; H, 3.4, N, 16.1; S, 18.3
Anal. Calcd for C15H7N5S2
C, 56.06; H, 2.20; N, 21.79; S, 19.95
Found:
C, 56.1; H, 2.1, N, 21.7; S, 20.0
Anal. Calcd for C15H6N5S2Cl
C, 50.63; H, 1.70; N, 19.68; S, 18.02
Found:
C, 50.7; H, 1.6, N, 20.0; S, 17.9
Anal. Calcd for C15H6N5S2Br
C, 45.01; H, 1.51; N, 17.50; S, 16.02
Found:
C, 44.8; H, 1.5, N, 17.6; S, 16.1
Anal. Calcd for C17H11N5S2
C, 58.43; H, 3.17; N, 20.04; S, 18.35
Found:
C, 58.5; H, 3.2, N, 20.1; S, 18.3
Anal. Calcd for C17H10N5S2Cl
C, 53.19; H, 2.63; N, 18.24; S, 16.71
Found:
C, 53.3; H, 2.7, N, 18.1; S, 16.5
Anal. Calcd for C17H10N5 S2Br
C, 47.67; H, 2.35; N, 16.35; S, 14.97
Found:
M ¼ 320.4
7.93e7.97 (m, 4H), 8.16e8.20 (m, 4H)
320.1 [M]þ
321.1 [MþH]þ, 322.1 [Mþ2H]þ
4b
6a
6b
6c
6d
6e
6f
>360
M ¼ 348.45
2.61 (s, 6H), 7.94e7.96 (m, 2H), 8.62
(s, 2H), 9.24 (s, 2H)
348.1 [M]þ
>360
M ¼ 321.38
7.80e7.89 (m, 4H), 7.98e8.10 (m, 3H)
321.1 [M]þ
339e342
>360
M ¼ 355.83
7.85e7.97 (m, 2H), 8.04e8.11
(m, 3H), 9.13 (s, 1H)
355.9 [M]þ
M ¼ 400.28
7.84e7.91 (m, 2H), 8.01e8.09
(m, 3H), 9.18 (s, 1H)
399.9 [M]þ, 400.9 [MþH]þ,
>360
M ¼ 349.44
2.63 (s, 6H), 7.05e7.13 (m, 3H),
7.91 (s, 2H)
349.1 [M]þ
>360
M ¼ 383.88
384.1 [M]þ
2.52 (s, 6H), 7.76e7.83 (m, 3H),
8.56e8.57 (m, 1H)
386.1 [Mþ2H]þ
330e332
M ¼ 428.33
2.63 (s, 6H), 7.05e7.09 (m, 3H),
7.91e7.96 (m, 1H)
428.1 [M]þ, 430.0 [Mþ2H]þ
C, 47.7; H, 2.4, N, 16.4; S, 15.0
spectra were nearly mirror images of each other, with overlapping
bands corresponding to the 0/0 transition.
It is evident that the presence of additional nitrogen in dyes 6a
and 6d caused a small hypsochromic effect (6e7 nm) in comparison
to dyes 4a and 4b, respectively. The position of the absorption band
depended on the character of a dye’s halogen group. For dyes 6a
and 6d, the absorption bands were located at 380 nm and 395 nm,
respectively. The presence of a chlorine substituent in dyes 6b and
6e caused a red-shift in the absorption band compared to the cor-
responding unchlorinated dye. A larger bathochromic effect was
NH2
R
R
N
Cl
Cl
R
R
N
N
R
R
N
N
S
S
NH
x 2 HCl
NH
NH2
SH
SH
ii)
i)
N
2
3
1a R = H
1b R = Me
R
N
N
N
S
iii)
1a
3
+
R
N
S
4a R = H
4b R = Me
X
R
N
N
N
S
X
N
N
Cl
Cl
iii)
3
+
R
N
S
N
N
5a X = H
5b X = Cl
5c X = Br
6
Scheme 5. i) Thiourea, 96% EtOH; ii) KOH, HCl; iii) DMF, Et3N.

1304
R. Podsiadły, J. Sokołowska / Dyes and Pigments 92 (2012) 1300e1307
Table 2
ranges from 0.050 to 0.073. As can be seen from these data, adding
heavy atoms such as Br to the studied fluorescent system caused
a decrease of Ffl. The molecule’s triplet state stabilization results in
increased efficiency of the intersystem crossing process that causes
this effect.
Spectroscopic and photophysical parameters of the examined dyes.
[dm3
lmax
3
b [dm3
lfl
[nm]
Ffl
Stokes E00a
shift
[nm]
a
a
b
a
a
Dye lmax
3
[nm] molꢁ1 cmꢁ1
]
[nm] molꢁ1 cmꢁ1
]
[kJ molꢁ1
]
4a 386 20,100
389 24,000
391 31,600
389 22,300
404 28,800
406 21,200
393 17,000
400 18,900
404 22,300
456 0.073 70
466 0.070 74
455 0.065 75
484 0.054 84
486 0.050 90
459 0.067 75
500 0.060 98
502 0.053 96
280.0
278.0
278.4
265.3
278.0
275.7
264.3
260.1
4b 388 19,800
6a 380 20,400
6b 400 24,000
6c 403 22,200
6d 395 17,200
6e 402 17,000
3.2. Sensitized free radical photopolymerization
The spectroscopic studies revealed that the dyes 4 and 6 have an
absorption bands around 400 nm and could be applied as visible
sensitizers for the collection of light >400 nm. The well known
mechanism of the dye-sensitized photodecomposition of Tz [6] is
presented in Scheme 3. Irradiation of these two component
systems leads to electron transfer from the excited sensitizer (Dye*)
to the Tz. The initiating halogenmethyl radicals are formed from
cleavage of the radical anion of Tz. The electron transfer from the
excited dyes to the Tz is thermodynamically allowed if the free
6f
406 26,400
a
In 1-methyl-2-pyrrolidone.
In THF.
b
energy (DGet), calculated from the RehmeWeller equation (Eq. (3))
[27], is negative.
ꢀ
ꢁ
D
Get kJ molꢁ1 ¼ 97½EoxðS=SꢂþÞ ꢁ EredðAꢂꢁ=AÞꢃ ꢁ E00ðSÞ
ꢁ Z1Z2=
3 r12
(3)
In this equation (Eq. (3)), Eox (S/Sꢂþ) and Ered (Aꢂꢁ/A) are the
oxidation potential of the dye and the reduction potential of the Tz,
respectively. The term E00(S) is the singlet excited state energy of
the dye. The last term represents the Coulombic energy necessary
to form an ion pair with charges Z1 and Z2 in a medium of dielectric
constant
taking into account the overall value of
In order to be able to calculate Get, the oxidation potentials
3
for a distance r12. This factor can very often be ignored
DGet.
D
(Eox) of the studied dyes 4 and 6 should be measured. Unfortu-
nately, in solvents which are commonly used in electrochemical
experiments (CH3CN, DMF) the solubility of the studied dyes is too
low. Therefore, the oxidation potentials of these compounds were
estimated on the basis of DFTcalculations. There are several ways to
estimate the oxidation potentials. The first, and the simplest one,
involves the calculation of the energy of highest occupied molec-
ular orbital (EHOMO) of a neutral compound. The second way to
obtain the oxidation potentials involves the calculation of ioniza-
tion potentials in the gas-phase or in the solvent. In order to select
the suitable way to estimate the unknown oxidation potential of
dyes 4 and 6, we calculate the EHOMO, IPVgas, IPgas and IPMeCN of
Fig. 1. Normalized absorption (black) and emission (gray) spectra of dye 6f (3
in MP.
m
M)
observed for dyes 6c and 6f. In these dyes, the absorption bands
were located at approximately 403 nm. Additionally, the presence
of two methyl substituents had a small bathochromic effect
(3e12 nm) on the position of these molecules’ absorption bands.
The fluorescence quantum yields (Ffl) of the studied dyes are
presented in Table 2. These dyes have a low value of Ffl, which
O
S
O
O
S
8
S
9
7
OMe
O
OMe
MeO
OMe MeO
MeO
MeO
O
S
S
11
OMe
O
S
10
O
O
O
O
S
S
O
O
S
O
13
12
Scheme 6. Dibenzodioxin (7), thianthrene (8), phenoxathiine (9), 2,3,7,8-tetramethoxythianthrene (10), 2,3,7,8-tetramethoxyphenoxathiine (11), 2,3;7,8-bis(ethylenedioxy)
thianthrene (12), 2,3;7,8-bis(ethylenedioxy)phenoxathiine (13).

R. Podsiadły, J. Sokołowska / Dyes and Pigments 92 (2012) 1300e1307
1305
Table 3
Table 4
Get, kJ molꢁ1), rate of
ox
1=2
Energy of the HOMO orbitals (eV), ionization potentials (eV) for compounds 7e13
and their experimental oxidation potentials (V).
Estimated oxidation potential (E , V), thermodynamic data (D
photopolymerization (Rp, m
mol sꢁ1) and inhibition time (tinh, s).
a
ox
ox
E
ꢁEHOMO
IPVgas
IPgas
IPMeCN
Dye
E
DGet
Rp
tinh
1=2
1=2
7
8
9
10
11
12
13
1.394a
1.234a
1.201a
1.01b
0.92b
0.86b
0.77b
0.19620
0.20950
0.20105
0.19141
0.18297
0.19382
0.18474
R ¼ 0.705
7.35
7.57
7.41
6.90
6.73
6.97
6.78
0.853
7.00
6.97
6.88
6.31
6.13
6.25
6.07
0.955
5.24
5.29
5.17
5.00
4.80
5.06
4.85
0.846
4a
4b
6a
6b
6c
6d
6e
6f
1.48
1.37
1.57
1.61
1.60
1.45
1.50
1.49
ꢁ42.3
ꢁ51.0
ꢁ32.0
ꢁ15.0
ꢁ28.7
ꢁ41.0
ꢁ24.7
ꢁ21.5
38.2
19.1
32.0
26.5
53.7
27.2
31.6
24.3
28
57
35
54
25
42
37
53
a
a
In CH3CN calculated as E1o=x2ðvs SCEÞ ¼ E ðvs Fc=FcþÞ þ 0:40; E ðvs Fc=FcþÞ
Tz Ered ¼ ꢁ0.97 in CH3CN versus SCE from Ref. [7].
ox
ox
1=2
1=2
from Ref. [28].
b
In CH3CN versus SCE from Ref. [29].
trimethylolpropane triacrylate monomer. From a practical point of
view, the most important property of the initiator system is the
ability to initiate the photopolymerization of monomers in an air
atmosphere. Therefore, in our investigation, the efficiency of the
polymerization initiated by the studied photoredox systems was
measured in an air equilibrium composition. The polymerization
rate of TMPTA was measured indirectly from the reaction’s heat
flow during irradiation (Fig. 3 and Fig. 8S in Supplementary data).
The overall polymerization results are summarized in Table 4.
As can be seen from these data, all the studied dye/Tz photo-
systems are able to initiate the polymerization process in an air
equilibrium composition. However, the free radical polymerization
initiated by the tested photoredox pairs showed a significant
inhibition time (Fig. 3 and Table 4). This effect should be related to
the molecular oxygen dissolved in the composition. The oxygen
both quenched the triplet state of the dye and reacted with the
radical species generated by the dye/Tz systems. After consumption
of the oxygen, the initiating radicals reacted with the monomer. As
seen in Table 4, the most efficient photoredox pair, 6c/Tz, had the
shortest inhibition time (Fig. 4).
structurally related compounds (Scheme 6) for which redox
potentials are available in the literature [28,29].
Table 3 compiles the calculated EHOMO, IPVgas, IPgas and IPMeCN
for these selected compounds together with the experimentally
ox
determined reversible redox potentials (E ). The best linear
1=2
correlation (r ¼ 0.95) was found between the IPgas and E1o=x2 (Fig. 2).
This relationship are given by Eq. (4)
ox
E
¼ ðꢁ2:35 ꢄ 0:47Þ þ ð0:52 ꢄ 0:07ÞIPgas
(4)
1=2
ox
where E
is the reversible oxidation potential versus SCE and IPgas
1=2
is the adiabatic ionization potentials. The Eq. (4) made it possible to
estimate the Eox of dyes 4 and 6. These data are presented in Table 4.
The results indicate that dyes with two methyl groups (4b, 6de6f)
are more readily oxidized than unsubstituted dyes (4a, 6ae6c).
Once the Eox of dyes 4 and 6 had been estimated, the DGet could
be calculated using the E00(S) of the studied dyes (see Table 2) and
the reduction potential of Tz (ꢁ0.97 in CH3CN versus SCE [7]). The
calculated thermodynamic parameters listed, in Table 4, indicated
that all the tested dye/Tz systems posses a favorable thermody-
Moreover, the calculated polymerization rate (Rp) presented in
Table 4, indicated that the efficiency of polymerization strongly
depended upon the structure of the dye employed. It was apparent
that dyes 6c and 4a significantly accelerated the triacrylate pho-
topolymerization in comparison to the other dyes studied. It is well
known [30] that the Rp for polymerization initiated via intermo-
lecular electron transfer process can be described by Eq. (5).
namic driving force upon exposure to light (ꢁ
D
Get > 15 kJ molꢁ1).
This means that the excited-state photoelectron transfer is quite
facile.
Finally, the dye/Tz systems were examined for potential appli-
cations as initiators for the free radical polymerization of the
Fig. 2. Oxidation potentials of the compounds 7e13 as
(R ¼ 0.95).
a
function of the IPgas
Fig. 3. Kinetic curves of TMPTA photopolymerization recorded under an air atmo-
sphere for Tz and dyes 4b (-), 6b (C), 6c (B) and 6e (:).

1306
R. Podsiadły, J. Sokołowska / Dyes and Pigments 92 (2012) 1300e1307
spectroscopy. These new dyes, when combined with 2,4,6-tris(tri-
chloromethyl)-1,3,5-triazine (Tz), may have practical applications as
visible-light photoinitiators of free radical polymerization. The
ability of dye/Tz systems to act as a photoinitiator strongly depended
upon the free energy change of the intermolecular photoinduced
electron transfer from the excited dyes 4 and 6 to the co-initiator. The
limiting step in the photoinitiated polymerization of TMPTA by dye/
Tz systems is an electron transfer within the photoredox pair.
Appendix. Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.dyepig.2011.09.006.
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