New two- and three-cationic polymethine dyes. Synthesis, properties and application

Article abstract of DOI:10.1016/j.dyepig.2014.06.016

Three new two- and three-cationic monochromophoric polymethine dyes based on the 2-methylbenzothiazole were synthesized and applied as visible light initiators for acrylic monomer polymerization. In the photoinitiating system, the polymethine dye cation a

Full text of DOI:10.1016/j.dyepig.2014.06.016

Dyes and Pigments 112 (2015) 24e33
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New two- and three-cationic polymethine dyes. Synthesis, properties
and application
*
Janina Kabatc , Katarzyna Jurek
University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2014
Received in revised form
Three new two- and three-cationic monochromophoric polymethine dyes based on the 2-
methylbenzothiazole were synthesized and applied as visible light initiators for acrylic monomer
polymerization. In the photoinitiating system, the polymethine dye cation acts as an electron acceptor
whereas borate anion is an electron donor.
The actual photoinitiators can only operate in a restricted part of the visible light region; as a
consequence, a several photoinitiators are usually necessary to harvest all the emitted visible photons. In
13 June 2014
Accepted 14 June 2014
Available online 20 June 2014
the present paper, new dyes based on a donor-p-acceptor structure (2-methylbenzothiazole derivatives)
Keywords:
Polymethine dye
are incorporated into visible light sensitive photoinitiating systems of triacrylate polymerization. They
exhibit an unusual and remarkable broad absorption lying from the blue to the red. When employed in
the presence of organoborate salts, these dyes can efficiently initiate the radical photopolymerization of
acrylates under exposure at 514 nm.
2-Methylbenzothiazole derivative
Spectroscopic properties
Kinetics
Polymerization
Photoinitiators
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
light harvesting compounds or multicolor photoinitiators [1,4e52].
Nevertheless, there is still a place for the continuous disclosure of
Visible light induced photopolymerization reactions occur in the
presence of a colored molecule incorporated into an absorbing
photoinitiating systems PIS and being able to act as a photoinitiator
PI or a photosensitizer PS [1e3].
Free radical polymerization can be easily achieved both under
high and low intensity light sources. Many dyes incorporated in
mono- and multi-component photoinitiating systems have been
already proposed [1,2].
new photoinitiating systems (PIS). Following our work in this area
[17,53] we now explore the possibility of using new molecules, N-
methylpicolinium ester derivatives, that could exhibit a broad
visible absorption and act as photosensitizers for free radical
polymerization operating in the visible light region. New sensi-
tizers have symmetrical (thiacarbocyanine dye), and unsymmetri-
cal (hemicyanine dyes) structure. As known, the hemicyanine dye
molecules consist of unsymmetrically substituted D-p-A arrange-
Improving the performance attained or meeting new or prom-
ising possibilities of applications requires a continuous search for
new structures.
ments bearing electron donor (D) and electron acceptor (A) func-
tionalities at each end of planar conjugated spacer.
The pushepull effect which results from the intramolecular
donor/acceptor interaction and a favorable orientation of the
charge delocalization in the axis of the chromophore is character-
ized by a strong absorption band detected in the visible region
(named intramolecular charge transfer band ICT) [1]. While the
structure of the third type of dye under study is symmetrical and it
belongs to carbocyanine dyes. Many different carbocyanine dyes
have been studied as photosensitizers in the visible light photo-
initiating systems for the radical polymerization of acrylates
Looking at the literature, it is quite amazing; despite the fact that
more than 50 years of research have led to the possible use of a
huge number of photoinitiators, photosensitizers and photo-
initiating systems, the most recent analysis of the current work in
this area shows tremendous and successful efforts deployed by
numerous groups for innovative proposals of new dye structures as
exemplified by the recent introduction of colored substituted ke-
tones, modified organometallic derivatives, various series of dyes,
[12,15e19]. Commonly, an organoborate salt was used as a co-
initiator is such systems. On the other hand, many types of com-
pounds have been added to such photoinitiating systems in order
to improve the photoinitiating efficiency, for example: N-alkox-
ypyridinium salts [15,54], 1,3,5-triazine derivatives or N-
*
Corresponding author. Tel.: þ48 52 374 9112; fax: þ48 52 374 9005.
E-mail address: nina@utp.edu.pl (J. Kabatc).
http://dx.doi.org/10.1016/j.dyepig.2014.06.016
143-7208/© 2014 Elsevier Ltd. All rights reserved.
0

J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
25
methylpicolinium ester derivatives [12,16]. In our previous papers
12,17], it was shown that N-methylpicolinium derivatives can act
2.2. Synthesis of different photoinitiators
[
as an efficient second co-initiator for free radical polymerization
induced by cyanine dye, a popular photosensitizer. The second co-
initiator strongly affects the rates of cyanine/borate-induced poly-
merization. It was found that the second electron transfer process
from the dye-based radical to the N-methylpicolinium derivative
enhances the rate of photopolymerization. Generally, by a combi-
nation of N-methylpicolinium derivative and cyanine borate salt,
two radicals can be generated per one absorbed photon, thus
enhancing the overall efficiency of polymerization.
The investigated dyes (NS1, NS2 and S2) shown in Scheme 1
were prepared in multistep reactions presented in Scheme 2.
Generally, dyes (NS1, NS2, S2) were prepared by Knoevenagel
condensation of the appropriate 2-methylbenzothiazole quater-
nary salt with the corresponding aldehyde and triethyl ortho-
formate, respectively.
The synthesis of pyridin-4-ylmethyl 2,2-diphenylacetate was
carried out based on the method described by Sunderarajan [55].
n-Butyltriphenylborate tetramethylammonium salt (B2), sec-
butyltriphenylborate tetramethylammonium salt (B3), tert-butyl-
triphenylborate tetramethylammonium salt (B4), tetrakis (n-
butylborate) tetramethylammonium salt (B5) were synthesized
based on the method described by Damico [56].
Our actual search for new initiating systems for visible light
prompts us to develop new dyes based on the D-p-A structure.
They are based on the difunctional structures where the benzo-
thiazole moiety is linked to N-alkyl methylpicolinium ester group
through a carbonecarbon bond. Therefore, present paper is focused
on the synthesis and application of the visible light bifunctional
two-component photoinitiating systems composed of sensitizer
with pyridin-4-ylmethyl 2-phenylacetate and pyridin-4-ylmethyl
All compounds were prepared with analytical purity (>98%)
1
13
which was checked by proton ( H) and carbon ( C) nuclear mag-
netic resonance (NMR) spectroscopy, IR spectroscopy and
elemental analysis. The spectra obtained were the evidence that
the reaction products were of the desired structure.
2
,2-diphenylacetate groups attached to a nitrogen atom in a het-
erocyclic ring (Dyes NS1, NS2, S2) and borate salts as the co-
initiators (B2, B3, B4, B5) in the photopolymerization reaction of
triacrylate (TMPTA) (Scheme 1).
2
.2.1. Synthesis of pyridin-4-ylmethyl 2,2-diphenylacetate
3.4 g (16.4 mmol) of N,N'-dicyclohexylcarbodiimide (DCC) was
added to a solution of 3.84 g (16.4 mmol) of diphenylacetic acid and
1.67 g (14.8 mmol) of 4-pyridylcarbinol in 50 mL of dichloro-
methane. The reaction mixture was stirred at room temperature for
about 24 h. The urea precipitate was filtered off, and the filtrate was
2
. Experimental section
2.1. Materials
washed with (2 ꢀ 25 mL) NaHCO
3
and (2 ꢀ 25 mL) water. The
The substrates used for the synthesis of dyes (2-
organic layer were collected and dried over MgSO
4
, and the solvent
methylbenzothiazole, 4-pirydylcarbinol, diiodopropane, N,N'-dicy-
cloheksacarbodiimide, diphenylacetic acid, p-(N,N-dimethylamino)
benzaldehyde, triethyl orthoformate), 2-ethyl-2-(hydroxymethyl)-
was evaporated yielding required compound [56].
ꢁ
Yield: 3 g, (58.8%), m.p. 59e61 C.
1
6 2
H NMR (DMSO-d ), d (ppm): 5.22 (s, 2H, eCH ); 5.36 (s, 1H,
1
(
,3-propanediol triacrylate (TMPTA) and 1-methyl-2-pyrrolidinone
MP) were purchased from Aldrich (Poland) and were used without
further purification.
eCH); 7.20e7.23 (d, J ¼ 6 Hz, 2H, Ar); 7.23e7.35 (m, 10H, Ar);
13
8.49e8.52 (d, J ¼ 6 Hz, 2H, Ar). C NMR (DMSO-d
6
), d (ppm): 39.5;
47.50; 55.59; 64.37; 121.07; 128.44; 138.63; 144.78; 149.53; 171.54.
Scheme 1. Structures of sensitizers, co-initiators and monomer.

26
J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
Scheme 2. The synthesis of new photoinitiators.

J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
27
Anal Calcd for: C20
H
17NO
2
: C, 79.21; H, 5.61; N, 4.62. Found: C,
and 0.25 g (0.76 mmol) of p-(N,N-dimethylamino)benzaldehyde
were suspended in 10 mL of methanol, and a few drops of piperi-
dine were added. Immediately, the solution turned deep red. The
solution was refluxed for 6 h. After cooling dark red solid was
filtered off, crystallized form ethanol and dried on the air.
7
9.05; H, 5.59; N, 4.68.
2.2.2. Synthesis of N-(3-iodopropyl)-2-methylbenzothiazole iodide
(
Quaternary salt (I))
5 mL (38.475 g,120 mmol) of diiodopropane was added to 3 mL
3.519 g, 23.6 mmol) of 2-methylbenzothiazole in 20 mL of toluene.
ꢁ
1
Yield: 0.16 g (69.80%), m.p. 215e217 C.
1
(
H NMR (DMSO-d
); 2.87 (s, 2H, eCH
4H, NeCH
6
),
d
(ppm): 1.63 (m, 2H, CH ); 2.24e2.45 (d,
2
The reaction mixture was refluxed for 24 h. The precipitate was
filtered off and dried on the air.
2H, eCH
2
2
); 3.03 (s, 6H, eN(CH ) ); 4.53e4.56 (d,
3 2
2
); 6.75e6.82 (m, 5H, Ar); 6.82e6.88 (d, J ¼ 12 Hz, 1H,
ꢁ
Yield: 9.54 g (94.24%), m.p. 190e192 C.
eCH ¼ ); 6.86e7.72 (d, J ¼ 8 Hz, 4H, Ar); 7.76e7.80 (d, J ¼ 8 Hz, 4H,
1
H NMR (DMSO-d
); 3.24e3.31 (d, 2H, eCH
.76 (d, J ¼ 8.8 Hz, 2H, Ar); 8.31 (d, J ¼ 8.6 Hz, 2H, Ar). C NMR
DMSO-d ), (ppm): 2.35; 8.07; 17.28; 28.41; 30.48; 31.53; 31.53;
6.66; 49.79; 57.37; 116.51; 116.69; 124.63; 127.97; 129.23; 177.52.;
6
),
d
(ppm): 2.32e2.51 (m, 2H, eCH
2
); 3.21 (s,
Ar); 7.78e7.84 (d, J ¼ 12 Hz, 1H, eCH ¼ ); 8.06e8.09 (d, J ¼ 6 Hz, 2H,
13
3
7
(
4
H, eCH
3
2
e); 4.69e4.76 (m, 2H, NeCH
2
e);
Pyr); 8.28e8.31 (d, J ¼ 6 Hz, 2H, Pyr). C NMR (DMSO-d
6
), d (ppm):
13
1.61; 27.65; 87.41; 105.44; 111.92; 115.47; 121.30; 123.08; 123.89;
ꢂ1
6
d
127.84; 128.32; 128.85; 129.38; 132.89; 171.38.; IR (KBr)
n
(cm ):
3002; 2902; 2810; 1615 (C]O); 1572; 1462; 1443; 1161; 1072
(CeO). Anal Calcd for: C34 SO : C, 50.81; H, 4.86; N, 5.23.
Found: C, 50.71; H, 4.97; N, 5.12.
ꢂ1
IR (KBr),
n
(cm ): 3065; 2947; 1829; 1575; 1516; 1232; 955; 494
13NSI : C, 29.66; H, 2.92; N, 3.14. Found:
H N
35 3
2 2
I
(
CeI). Anal Calcd for: C11
H
2
C, 29.71; H, 2.99; N, 3.18.
2
.2.6. Synthesis of dye (NS2). The condensation of 3-(3-(4-((2,2-
2
.2.3. Synthesis of 2-methyl-3-(3-(4-((2-phenylacetoxy)methyl)
pyridin-1-ium-1-yl)propyl)benzo-[d]thiazol-3-ium diiodide
Quaternary salt (II))
.02 g (0.02 mol) of pyridin-4-ylmethyl 2-phenylacetate was
diphenylacetoxy)methyl)pyridin-1-ium-1-yl)propyl)-2-
methylbenzo[d]thiazol-3-ium diiodide with p-(N,N-dimethylamino)
benzaldehyde
(
1
0.57 g (0.76 mmol) of 3-(3-(4-((2,2-diphenylacetoxy)methyl)
dissolved in 35 mL of methanol and next the 3.0 g of N-(3-
iodopropyl)-2-methylbenzothiazole iodide in 60 mL of methanol
was added. The reaction mixture was stirred and refluxed for 8 h.
The reaction mixture was concentrated in vacuum and the violet
residue was crystallized from ethanol.
pyridin-1-ium-1-yl)propyl)-2-methylbenzo[d]thiazol-3-ium diio-
dide and 0.135 g (0.76 mmol) of p-(N,N-dimethylamino)benzalde-
hyde were suspended in 10 mL of methanol, and a few drops of
piperidine were added. Immediately, the solution turned deep red.
The solution was refluxed for 12 h. Addition of saturated solution of
potassium iodide in methanol precipitated a dark red solid which
was filtered off, washed several times with water and dried on the
air.
ꢁ
Yield: 2.06 g (68.21%), m.p. 199e200 C.
1
H NMR (DMSO-d
); 3.31e3.44 (m, 2H, eCH
.73e4.81 (m, 4H, NeCH ); 7.10e7.61 (m, 9H, Ar); 7.75e7.93 (t, 2H,
CH
); 8.28e8.32 (d, J ¼ 8 Hz, 1H, Ar); 8.41e8.45 (d, J ¼ 8 Hz, 1H, Ar).
C NMR (DMSO-d ), (ppm): 2.30; 16.98; 17.21; 30.45; 31.50;
6
),
d
(ppm): 2.50 (s, 2H, eCH
2
e); 3.16 (s, 3H,
eCH
4
3
2
e); 3.49e3.55 (m, 2H, eCH
2
e);
2
ꢁ
Yield: 0.70 g (88.90%), m.p. 204e205 C.
2
1
H NMR (DMSO-d
); 2.24 (s, 2H, eCH
.65 (s, 2H, NeCH ); 4.55 (m, 2H, NeCH
6
),
d
(ppm): 1.58e1.63 (m, 2H, eCH
); 2.49e3.31 (m, 8H, eCH , eN(CH
); 6.68e6.86 (t, 5H, Ar);
2
); 1.89 (s,
13
6
d
2
3
H, eCH
2
2
2
3 2
)
);
4
6.90; 49.76; 57.34; 116.49; 116.67; 124.54; 124.60; 127.96; 129.20;
ꢂ1
2
2
129.28.; IR (KBr)
n
(cm ): 3067; 2997; 2721; 1726 (C]O); 1578;
7
.29e7.64 (d, 5H, Ar); 7.64e7.72 (2d, J ¼ 16 Hz, 2H, eCH ¼ );
26 2 2 2
1549; 1234; 1148 (CeO). Anal Calcd for: C25H N SO I : C, 44.64; H,
1
3
7.78e7.81 (d, J ¼ 6 Hz, 2H, Pyr); 8.31e8.34 (d, J ¼ 6 Hz, 2H, Pyr).
C
3
.87; N, 4.17. Found: C, 44.71; H, 3.89; N, 4.28.
6
NMR (DMSO-d ), d (ppm): 2.28; 3.21; 20.10; 20.62; 24.28; 31.87;
3
2.31; 46.56; 47.29; 51.32; 87.41; 93.95; 105.38; 111.76; 111.89;
2
1
.2.4. Synthesis of 3-(3-(4-((2,2-diphenylacetoxy)methyl)pyridin-
-ium-1-yl)propyl)-2-methyl-benzo[d]thiazol-3-ium diiodide
ꢂ1
1
2
15.45; 115.62; 121.28; 132.88.; IR (KBr)
n
(cm ): 3002; 2902;
810; 1614 (C]O); 1572; 1462; 1443; 1185; 1037 (CeO). Anal Calcd
SO : C, 55.57; H, 4.74; N, 4.63. Found: C, 55.60; H,
(
Quaternary salt (III))
.7 g (5 mmol) of pyridin-4-ylmethyl 2,2-diphenylacetate was
for: C42
H
43
N
3
2 2
I
0
4.69; N, 4.64.
dissolved in 20 mL of methanol and next the 1.5 g of N-(3-
iodopropyl)-2-methylbenzothiazole iodide in 40 mL of methanol
was added. The reaction mixture was stirred and refluxed for
2.2.7. Synthesis of dye (S2). The condensation of 3-(3-(4-((2,2-
diphenylacetoxy)methyl)pyridin-1-ium-1-yl)propyl)-2-
methylbenzo[d]thiazol-3-ium diiodide with triethyl orthoformate
0.57 g, (0.76 mmol) of 3-(3-(4-((2,2-diphenylacetoxy)methyl)
pyridin-1-ium-1-yl)propyl)-2-methylbenzo[d]thiazol-3-ium diio-
dide was dissolved in 10 mL of acetic acid anhydride. After cooling,
0.3 mL of triethyl orthoformate was added to the mixture. Next, the
reaction mixture was refluxed for 24 h. The precipitated violet solid
was filtered off, washed several times with water and dried on the
air.
8
h and then was concentrated in vacuum. The residue was crys-
tallized from ethanol.
Yield: 1.14 g (52%), m.p. 195e196 C.
ꢁ
1
H NMR (DMSO-d
d, 6H, CH ); 3.37e3.44 (s, 3H, CH
.38e5.51 (m, 2H, Ar); 7.26e7.30 (d, J ¼ 8 Hz, 4H, benzothiazole);
.58e7.62 (d, J ¼ 8 Hz, 2H, Ar); 7.76e7.90 (m, 5H, Ar); 8.30e8.46 (m,
6
),
d
(ppm): 2.31e2.50 (m, 2H, CH
2
); 3.20e3.23
(
5
7
5
4
2
3
); 4.68e4.75 (t, 2H, eCH
2
e);
H, Ar). 13C NMR (DMSO-d
), d (ppm): 17.11; 17.37; 22.43; 30.48;
6
4.70; 46.95; 49.82; 55.34; 57.30; 64.00; 116.5; 117.00; 124.00;
ꢂ1
ꢁ
125.5; 127.12; 128.00; 129.00; 142.5; 145.5.; IR (KBr)
n
(cm ):
Yield: 0.54 g (94.06%), m.p. 189e190 C.
H NMR (DMSO-d
1
3
067; 2997; 2866; 1634 (C]O); 1576; 1517; 1233; 1021 (CeO). Anal
SO : C, 49.73; H, 4.01; N, 3.74. Found: C, 49.71;
6 2
), d (ppm): 2.11 (s, 2H, eCH eC]O); 2.25 (s,
Calcd for: C31
H, 3.97; N, 3.78.
H
30
N
2
2
I
2
2H, eCH
2H, eCH
eCH
2
eOe); 2.29 (d, 2H, eCH
2
); 2.39 (m, 2H, eCHe); 2.58 (d,
þ
2
eOe); 2.60e3.32 (m, 8H, N eCH
2
e); 3.94e4.00 (t, 2H,
þ
2
e); 4.17e4.64 (t, 6H, N eCH
2
e); 7.08e7.15 (t, 2H, eCHe);
2
(
.2.5. Synthesis of dye (NS1). The condensation of 2-methyl-3-(3-
4-((2-phenylacetoxy)methyl)pyridin-1-ium-1-yl)propyl)benzo[d]
thiazol-3-ium diiodide with p-(N,N-dimethylamino)benzaldehyde
.46 g (0.685 mmol) of 2-methyl-3-(3-(4-((2-phenylacetoxy)
methyl)pyridin-1-ium-1-yl)propyl)benzo[d]thiazol-3-ium diiodide
7.24e7.39 (m, 10H, Ar, 2H, ¼CHe); 7.43e7.72 (m, 10H, Ar,
ꢂ1
2H, ¼CHe); 7.76e7.79 (d, J ¼ 6 Hz, 4H, Pyr).; IR (KBr)
n
(cm ):
3068; 2951; 2861; 1680 (C]O); 1572; 1485; 1185; 1037 (CeO). Anal
Calcd for: C63 : C, 54.86; H, 4.14; N, 4.06. Found: C,
54.71; H, 4.07; N, 4.16.
0
57 4 2 4 3
H N S O I

28
J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
2
2
.3. Techniques
dH=dt
DH0
R
p
¼
(2)
.3.1. Spectroscopic measurements
Ultravioletevisible (UVeVis) electronic absorption spectra were
3
. Results and discussion
obtained using a Shimadzu UVeVis Multispec-1500 Spectropho-
tometer (Japan) and steady-state fluorescence was performed using
1
13
3.1. Absorption properties of new dyes (NS1, NS2 and S2)
a Hitachi F-4500 Spectrofluorimeter (Japan). The H and C NMR
spectra were recorded with the use of a Varian Gemini 200 spec-
Absorption spectra always reflect molecular interactions, espe-
trometer operating at 200 MHz. Dimethylsulfoxide (DMSO-d
used as a solvent and tetramethylsilane as internal standard.
The IR spectra of the synthesized compounds were recorded
6
) was
cially these between two or more chromophores. If there are mo-
lecular aggregates in the system, than the absorption spectroscopic
method is almost the most powerful technique for identifying them
using
a
Bruker Spectrometer Vector 22, in the range
ꢂ1
[
57]. We are not observing the absorption shift when the concen-
4
00e4500 cm , by KBr pellet technique.
tration of the solution changes. That is to say, in the solution, the
aggregation can be ignored. The ground state electronic absorption
spectra of the proposed 2-methylbenzothiazole derivatives allow a
large and efficient matching with the light source emission spectra
The elemental analysis was made with a Vario MACRO 11.45-
000, Elementar Analyser Systeme GmbH (Germany), operating
with the software VARIOEL 5.14.4.22.
0
(
argon laser at 514 nm) (Fig. 1).
2
.3.2. Reduction and oxidation potentials of dyes
The reduction and oxidation potentials of dyes were measured
Remarkably, high molar extinction coefficients are determined
ꢂ1
ꢂ1
ꢂ1
ꢂ1
(about 70 000 M cm for NS1, 110 000 M cm for NS2 and
by cyclic voltammetry. An Electroanalytical MTM System model
EA9C-4z (Poland), equipped with a small-volume cell, was used for
the measurements. A 1 mm platinum electrode was applied as the
working electrode. A platinum wire constituted the counter elec-
trode, and a silver/silver chloride (Ag/AgCl) electrode served as a
reference. The supporting electrolyte was 0.1 M tetrabutylammo-
nium perchlorate in dry acetonitrile. The solution was deoxygen-
ated by bubbling argon gas through the solution. The potential was
swept from ꢂ1.8 to 1.8 V at a rate of 500 mV/s to record the cur-
rentevoltage curve.
ꢂ1
ꢂ1
1
20 000 M cm for S2, respectively) compared to absorption
only in the UV range for 2-methylbenzothiazole, p-(N,N-dimethy-
lamino)benzaldehyde, N-methylpicolinium ester moieties. It is well
known, that in a case of unsymmetrical dyes (NS1, NS2) the lowest
energy transition involves strongly delocalized HOMO and LUMO
orbitals for both dyes. For these dyes, the HOMO and LUMO orbitals
are mainly localized on amine and benzothiazole fragment,
respectively. This highlights a charge transfer character in agree-
ment with the pushepull character of this structure. As it was
mentioned above, in these dyes, two nitrogen containing moieties
are linked by a polymethine bridge possessing two or three
numbers of carbons, which allows resonance delocalization of a
positive charge between the two nitrogens. The electronic excita-
tion of unsymmetrical polymethine dyes gives broad absorption
band in the range from 450 to 600 nm (see Figs. 1 and 2 and data in
2.3.3. Polymerization
p
Photoinitiated polymerization rate (R ) profiles were deter-
mined by differential scanning calorimetry (DSC) under isothermal
conditions at room temperature using a photo-DSC apparatus
constructed on the basis of a TA Instruments DSC 2010 Differential
Scanning Calorimeter (USA). The sample (0.035 ± 0.002 g) was
polymerized in open aluminum pans with a diameter of 6.6 mm.
Irradiation of the polymerization mixture was carried out using the
visible emission (514 nm) of the air-cooled Ion Laser System model
0
Table 1) corresponding to the S / CT transition attributed to an
intramolecular charge transfer (ICT) involving the electron lone
pair of the amino nitrogen and the cationic benzothiazolium ni-
trogen moiety [58,59]. The inspection of the absorption spectra
presented in Fig. 1 and data in Table 1 shows that the position of the
CT absorption band does not depend on the molecular structure of
unsymmetrical dye (NS1 and NS2).
In order to support the presence of this charge transfer char-
acter, their solvatochromic behavior was investigated in different
polarities solvents (Fig. 2).
177-G01 (Spectra-Physics, USA). The average power of irradiation
2
was 15 W/0.196 cm at 514 nm. The light intensity was measured by
a Coherent Model Fieldmaster power meter (Germany).
The polymerization solution was composed of 1 mL of 1-methyl-
-pyrrolidinone (MP) and 9 mL of 2-ethyl-2-(hydroxymethyl)-1,3-
2
propanediol triacrylate (TMPTA). The composition of photo-
initiating systems was: dye halide/tetramethylammonium borate
salt. Both components concentration used in the experiments was
In solvents tested the maximum absorption position of dyes
(NS1 and NS2) (Table 1) differs only few nm and this indicates that
ꢂ3
5
ꢀ 10 M. As a reference sample, a polymerizing mixture con-
taining monomer and dye halide (sensitizer without a co-initiator)
was used. The polymerizing mixture was not deareated. In order to
reduce the effect of diffusion-controlled termination, the effect of a
network formation, the NorrisheTroomsdorff effect and the radi-
cals trapping effect, the initial rates of polymerization were taken
into account for further consideration.
The degree of monomer conversion into polymer determined
after a specific irradiation time was also presented here. The con-
version percentages were obtained integrating the area under the
exothermic peak
DH
t
C% ¼ DH ꢀ 100
(1)
0
where
D
H
t
is the reaction heat evolved at time t and
D
H
0
is the
heat evolved assuming total conversion. Polymerization rates were
calculated using
Fig. 1. Electronic absorption spectra of dyes studied in acetonitrile recorded at 293 K.

J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
29
the conceptual presentation of the energy gaps between the ground
state energy and the lowest excited states for hemicyanine dyes
(three different fluorescence states reached after the twist of a single
bond and called the intramolecular change transfer states) [67].
3.2. Free radical photopolymerization
The photoinitiating systems composed of unsymmetrical and
symmetrical polymethine dyes (NS1, NS2 and S2) as the photo-
sensitizer with various co-initiators (borate salts) (structures given
in Scheme 1) were used for the initiation of free radical polymeri-
zation of triacrylate (TMPTA). The polymerization solution con-
ꢂ3
sisted of
5
ꢀ
10
M
of sensitizer (as
a
halide) and
ꢂ3
alkyltriphenylborate salts with concentration 5 ꢀ 10
M. The
polymerization process was initiated by irradiation at 514 nm. As it
was shown above, at this wavelength only the sensitizer absorbs
the light.
Fig. 2. Influence of the solvent polarity on the electronic absorption spectra of dye NS1
recorded at 293 K.
The best typical conversionetime profiles of 2-ethyl-2-
hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) under irra-
diation with argon ion laser at 514 nm are shown in Figs. 4e6.
A relatively low final conversion is obtained when using S2B2,
S2B3 and S2B5 (Fig. 6) whereas a better polymerization profiles is
reached for following photoinitiating systems: NS2B4 and NS2B5
the electronic transition should be similar for both unsymmetrical
compounds (the same electron donor moiety).
Indeed, pushepull molecules represent a particularly inter-
esting class of molecules that shows outstanding transition energy
shift by changing the solvent polarity/polarizability [1,60]. Results
are summarized in Table 1.
(
(
final conversion ~25%, Fig. 5). From results obtained it is known,
Over the years, several empirical polarity scales were developed
to investigate the solvatochromism, and different empirical solvent
polarity scale, for example: DimrotheReichardt's [61], Kam-
leteTaft's [62], LipperteMataga's [63], Bakhshiev's [64], Kawskie-
ChammaeViallet's [65], McRae's and Suppan's scales [66]. An
increase of the Stokes shift with the polarity of the solvent is not
observed for dyes NS1 and NS2. This irregular behavior was evi-
denced for the dyes under study and on correlation could be
established in first approximation (data in Table 1). While plotting
the Stoke's shift values against the DimrotheReichardt solvent
parametres or the empirical KamleteTaft parameters, no reason-
able linear correlations could be obtained for the tested dyes. On
the other hand, LipperteMataga's, Bakhshiev's, KawskieCham-
maeViallet's, McRae's and Suppan's polarity scales all based on the
both the dielectric constant and the refractive index of the solvents
gave remarkable correlations for dyes NS1 and NS2 (Fig. 3). In
particular, the strong negative slopes obtained for Lip-
perteMataga's and KawskieChammaeViallet's equations are
indicative of significant charge redistribution upon photoexcitation.
As anticipated, the larges magnitude of the Stokes shift was
observed in the most polar solvents and this variation is indicative
of a major change in the dipole moment value referring to a ge-
ometry of the excited state different to that of the ground state.
In general, there are a blue shift of the absorption band for dyes
NS1 and NS2 and the red shift of fluorescence maxima with an in-
crease of solvent polarity, observed. This demonstrates the forma-
tion of an intramolecular charge transfer state. This is supported by
that symmetrical carbocyanine dye (S2) is not found to be an effi-
cient sensitizer for free radical polymerization of acrylate tested.
The reactivity of sensitizers studied will be discussed below.
3.3. Chemical mechanism
When exposedtothe argonlaserat514 nm, averyfastoxidation of
borate salt (B2, B3, B4 or B5) by sensitizer occurs (Eq. (3)). This pro-
cess leads to sensitizer radical, triphenylborate (or tri-n-butylborate)
ꢃ
and butyl radical Bu (originating from the subsequent very fast car-
boneboron bond cleavage in boranyl radical) formation (Eq. (4)) [12].
It was found that irradiation of photoinitiating systems studied
with the visible light caused in a very fast bleaching of sensitizer. It
is a result of the recombination of both dye-based radical and alkyl
radical (Eq. (5)). This reaction competes with the initiation process.
The sensitizer-based radical and butyl radical may also undergo an
electron transfer process which led to regeneration of the sensitizer
(Eq. (6))
hn
1
Sen! Sen
ꢂ ET
ꢃ
ꢃ
1
Sen þ BuBðPhÞ !Sen þ BuBðPhÞ
(3)
3
3
or
ꢃ
ꢃ
1
ꢂ
Sen þ Bu B /Sen þ Bu B
4
4
Table 1
Absorption and emission maxima (nm) obtained experimentally by UVeVIS spectroscopy and fluorescence spectroscopy for dyes NS1, NS2, S2.
Solvent
Dielectric
constant (
Refractive
index (n)
l
ab [nm]
l
em [nm]
Stokes shift
l
ab [nm]
l
em [nm]
Stokes shift
l
ab [nm]
l
em [nm]
Stokes shift
ꢂ1
ꢂ1
ꢂ1
[cm ]
3 )
(NS1)
(NS1)
[cm
]
(NS2)
(NS2)
[cm
]
(S2)
(S1)
Water
DMSO
DMF
Acetonitrile
Methanol
Acetone
THF
78.39
46.7
37
37.5
32.7
20.6
7.4
1.33
520
532
529
529
531
530
539
531
600
613
610
601
599
604
604
599
2564
2483
2510
2264
2137
2311
1996
2137
513
529
527
527
528
527
533
525
600
613
607
604
600
605
603
592
2826
2590
2500
2419
2272
2446
2177
2156
*
*
*
*
1.479
1.4305
1.344
1.328
1.359
1.407
1.372
572
574
*
578
*
604
596
*
615
*
926
640
1040
Ethyl Acetate
6.02
*
*
*Insoluble.

30
J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
Fig. 3. LipperteMataga's (A), Bakhshiev's (B), KawskieChammaeViallet's (C), McRae's (D) and Suppan's (E) solvatochromic correlations plots for dyes NS1 and NS2 in different
polarities solvents.
Fig. 4. (Left) The family of kinetic curves recorded during the measurements of the flow of heat for the photoinitiated polymerization of the TMPTA/MP (9/1) mixture initiated by
ꢂ3
ꢂ2
dye NS1 and n-butyltriphenylborate salt (B2) and sec-butyltriphenylborate salt (B3). The concentration of photoinitiating systems was equal 5 ꢀ 10 M, I
a
¼ 76 mW ꢀ cm . (Right)
Relationship between the degree of double bond (eCH]CHe) conversion of trimetylolpropane triacrylate and irradiation time. The type of photoinitiating system is marked in the
Figure.

J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
31
Fig. 5. (Left) The family of kinetic curves recorded during the measurements of the flow of heat for the photoinitiated polymerization of the TMPTA/MP (9/1) mixture initiated by
dye NS2 and n-butyltriphenylborate salt (B2), sec-butyltriphenylborate salt (B3), tert-butyltriphenylborate salt (B4) and tetra-n-butylborate salt (B5). The concentration of pho-
ꢂ3
ꢂ2
toinitiating systems was equal 5 ꢀ 10 M, I
a
¼ 76 mW ꢀ cm . (Right) Relationship between the degree of double bond (eCH]CHe) conversion of trimetylolpropane triacrylate
and irradiation time. The type of photoinitiating system is marked in the Figure.
Fig. 6. (Left) The family of kinetic curves recorded during the measurements of the flow of heat for the photoinitiated polymerization of the TMPTA/MP (9/1) mixture initiated by
systems composed of dye S2 and n-butyltriphenylborate salt (B2), sec-butyltriphenylborate salt (B3), tert-butyltriphenylborate salt (B4) and tetra-n-butylborate salt (B5). The
ꢂ3
ꢂ2
concentration of photoinitiating systems was equal 5 ꢀ 10 M, I
a
¼ 76 mW ꢀ cm . (Right) Relationship between the degree of double bond (eCH]CHe) conversion of tri-
metylolpropane triacrylate and irradiation time. The type of photoinitiating system is marked in the Figure.
ꢃ
ꢃ
reactants and can be calculated from the RehmeWeller equation
Eq. (7)) [69].
BuBðPhÞ /Bu þ ðPhÞ B
(4)
3
3
(
ꢃ
ꢃ
Bu B /Bu þ ðBuÞ B
4
3
ꢃ
ꢃ
Z Z
1 2
DGel ¼ EoxðE=E Þ ꢂ EredðDye=Dye Þ ꢂ E00ðDyeÞ ꢂ
3 r
12
(7)
ꢃ
ꢃ
Sen þ Bu /Sen ꢂ Buðbleaching productÞ
(5)
(6)
The value of DGel was calculated using the oxidation potential of
borate salts (Eox ¼ 1.16 eV, 0.965 eV, 0.916 eV and 0.651 eV for B2,
ꢃ
ꢃ
ET
Sen þ Bu !Sen þ Buꢂ
þ
In photoinitiating systems studied, three different butyl radicals
are formed: (n-butyl, sec-butyl and tert-butyl). On the basis of the
previously described results the experimentally determined
electron-transfer efficiency for n-butyl radical is about 1, but for
sec-butyl is 0.78 and for tert-butyl is about 0.83 [68]. Therefore the
photoinitiating ability of photoredox systems may depend on the
reactivity or quantum yield of radical formation. Based on the ki-
netic results, one can conclude that the radical structure does not
influence on the photoinitiating ability of PIS. So, it may be ex-
pected that other factors influence on the effectiveness of initiating
of polymerization process.
Table 2
Electrochemical properties, thermodynamic and kinetic parameters of dyes (NS1),
(NS2) and (S2).
Dye
E
00
E
red
D
G
el[eV] Rp
Monomer
Polymerization
[eV] [eV]
[m
mol/s] conversion quantum yield
[%]
NS1
NS1B2
NS1B3
NS1B4
NS1B5
2.22 ꢂ1.450
0.39
0.195
0.146
0.41
0.79
0
4.13
9.4
0
4.80
9.30
0
ꢂ0.119
0
0
0
NS2
2.25 ꢂ1.236
2.11 ꢂ1.418
The lower photoinitiating ability can be explained by a back
electron transfer in reaction (Eq. (3)) regenerating the starting
compounds or photobleaching process (Eq. (5)). Such a deactivation
pathway decreases the yield of butyl radical and sensitizer-based
radical in line with the very low efficiency of S2 based systems in
polymerization experiments.
NS2B2
NS2B3
NS2B4
NS2B5
0.146
ꢂ0.049
ꢂ0.098
ꢂ0.363
0.37
0.75
1.41
1.57
6
4.35
8.85
16.50
18.50
16
22
24
S2
S2B2
S2B3
S2B4
S2B5
0.468
0.273
0.224
0.06
0.031
0.73
1.8
0.9
1.9
0.69
0.36
0.90
8.55
el for the 1Sen/borate salt electron
DG
The free energy change
transfer reaction is favorable according to the redox properties of
ꢂ0.041
0.077
12.12

32
J. Kabatc, K. Jurek / Dyes and Pigments 112 (2015) 24e33
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