Multicationic monomethine dyes as sensitizers in two- and three-component photoinitiating systems for multiacrylate monomers

Article abstract of DOI:10.1016/j.jphotochem.2010.06.011

This paper is focused on the description of the synthesis, spectroscopic and electrochemical properties, and the free radical photoinitiation abilities of new multi-cationic monomethine dyes. The new monomethine dyes were employed both as the visible light photoinitiators of acrylate monomer polymerization and as fluorescence probes for monitoring the progress of polymerization. A degree of polymer cure from the measurement of the changes in the probe emission intensity and position shift during the thermally initiated polymerization of monoacrylate was obtained. A distinct increase in the intensity of the dyes fluorescence was observed during polymerization when the degree of monomer conversion was gradually increasing. The second part of this work is the possibility of an application of the dyes synthesized in combination with borate anions, as photoinitiating systems. Novel multi-cationic monomethine dyes paired with n-butyltriphenylborate anions are very efficient visible light photoinitiators of free radical polymerization of acrylate monomers. Their photoinitiating abilities were compared with the photoinitiation ability of Rose Bengal derivative, e.g. typical triplet state photoinitiator. The addition of a second co-initiator to the two-component photoinitiating system (monomethine borate salts) enhances their photoinitiating abilities.

Full text of DOI:10.1016/j.jphotochem.2010.06.011

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Multicationic monomethine dyes as sensitizers in two- and three-component
photoinitiating systems for multiacrylate monomers
Janina Kabatc∗
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:
This paper is focused on the description of the synthesis, spectroscopic and electrochemical properties,
and the free radical photoinitiation abilities of new multi-cationic monomethine dyes.
The new monomethine dyes were employed both as the visible light photoinitiators of acrylate
monomer polymerization and as fluorescence probes for monitoring the progress of polymerization.
A degree of polymer cure from the measurement of the changes in the probe emission intensity and
position shift during the thermally initiated polymerization of monoacrylate was obtained. A distinct
increase in the intensity of the dyes fluorescence was observed during polymerization when the degree
of monomer conversion was gradually increasing.
Received 9 February 2010
Received in revised form 12 June 2010
Accepted 14 June 2010
Available online 19 June 2010
Keywords:
Dyes
Fluorescence probes
Initiating systems
Free radical polymerization
The second part of this work is the possibility of an application of the dyes synthesized in combination
with borate anions, as photoinitiating systems.
Novel multi-cationic monomethine dyes paired with n-butyltriphenylborate anions are very efficient
visible light photoinitiators of free radical polymerization of acrylate monomers. Their photoinitiating
abilities were compared with the photoinitiation ability of Rose Bengal derivative, e.g. typical triplet state
photoinitiator.
The addition of a second co-initiator to the two-component photoinitiating system (monomethine
borate salts) enhances their photoinitiating abilities.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
(i) its molecular structure (which governs the intensity of the light
absorbed, the absorption wavelength range, the energy or electron
Direct photoinduced polymerization reactions concern the cre-
ation of a polymer P through a chain reaction initiated by light
(Scheme 1).
Since direct formation of reactive species on the monomer by
light absorption is not an efficient route, the initiation step of the
polymerization reaction requires the presence of a photoinitiator PI
which, under light excitation, is capable of generating these reactive
spaces (Scheme 2).
Extension of the spectral sensitivity (that corresponds to the
best matching between the emission spectrum of the light source
and the absorption spectrum of the formulation) can be achieved
using photoinitiating systems (PSs): their role is absorb the irradi-
ation energy at a wavelength where PI is unable to operate and to
transfer the excitation to PI. In that case, the reaction is defined as
a sensitized photoinduced polymerization (Scheme 3).
The intrinsic reactivity of photoinitiating system which plays
an important role on the rate of photoinitiated free radical poly-
merization determines its interest an is directly connected with:
transfer reaction ability) and (ii) the efficiency of the photophysical
and photochemical processes involved in the excited states (which
determines the yield of cleavage reaction, electron transfer reac-
tions, quenching by monomer or oxygen or other additives such as,
e.g. hydrogen donors, light stabilizers, interaction with PS) [1].
Many photosensitive systems PI and PS for radical photopoly-
merization have been developed in recent years to take up the
challenge of designing organic molecules working in well-defined
conditions of laser excitations [1].
In radical photopolymerization reactions, examples include:
ketocoumarins and coumarins derivatives (e.g. xanthene dyes
such as Eosin or Rose Bengal, thioxanthene dyes); thioxanthones;
bis-acylphosphine oxides, peresters; pyrylium and thiopyrylium
salts in presence of additives such as a perester; cationic dyes
containing a borate anion; dyes/bis-imidazole derivatives/thiols;
PS/chlorotriazine/additives; metallocene derivatives (such as
titanocenes); dyes or ketones/metallocene derivatives/amines;
cyanine dyes in the presence of additives; dyes/bis-imidazoles;
miscellaneous systems such as phenoxazones, quinolinones,
phtalocyanines, squaraines, squarylium containing azulenes, novel
fluorine visible light Pis, benzopyranones, rhodamines, riboflavines
and two color sensitive systems [1–8].
∗
Tel.: +48 52 374 9064; fax: +48 52 374 9009.
E-mail address: nina@utp.edu.pl.
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.06.011

J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
75
Here, it is demonstrated also that multi-cationic monomethine
dyes can be used as fluorophores for monitoring the progress of
the methyl methacrylate (MMA) polymerization. The relationship
between the change in emission spectra of dye molecules and the
degree of monomer conversion during thermally initiated poly-
merization is presented.
2. Experimental
2.1. Materials
Scheme 1.
All substrates for synthesis of multi-cationic monomethine
dyes (2-methylbenzothiazole, methyl iodide, 4-chloroquinoline,
1,3-dibromopropane, pyridine, 2-methoxyethanol, N,N,NꢀNꢀ-
tetramethyl-1,3-propanediamine,
N,N,Nꢀ,Nꢀ-tetramethyl-1,6-
hexanediamine, 1,4-diazabicyclo[2,2,2]octane and 4,4ꢀ-bipirydyle)
and tetramethylammonium n-butyltriphenylborate (triphenyl-
boron, n-butyllithium, benzene) were purchased from Aldrich
and used without further purification. n-Butyltriphenylborate
tetramethylammonium (B2) was used as a co-initiator. 2-Ethyl-
2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) and
1-methyl-2-pyrrolidinone (MP) were purchased from Aldrich and
used as monomer and solvent, respectively.
Scheme 2.
In recent years, there have been many new developments in
the synthesis and photochemical studies of novel photoinitiator
molecules with more desirable properties, such as higher activ-
ity coupled with greater speed and a low migration rate to the
surface of the cured coating in order to reduce loss of adhesion
and to minimize toxicity. One area of importance in this regard
is the development of photoinitiators having great spectral sen-
sitivity to the visible region of the spectrum [9]. These initiators
absorb visible light with a high speed and cause the same pho-
topolymerization events described above to take place – namely a
conversion of a highly functionalized liquid acrylate monomer to
a solid polyacrylate. In practice, monomers used in fast photoini-
tiated polymerization are acrylates or modified acrylates whose
structure lends to rapid cross-linking. Some of the photoinitia-
tors have been widely discussed in the literature for they are the
backbone of the photopolymerization. In one such process, the ini-
tiators are cyanine borate ion salts, so called ion pair initiators,
which utilize single electron transfer to the excited state of the light
absorbing dye from the partner ion acting as electron donors, start-
ing the crucial first step in the initiation of the radical chain. Thus,
the cyanine dye, in its excited state, accepts an electron from the
borate anion and subsequent boranyl radical decomposition leads
to the formation of free radicals which initiate polymerization [10].
The application of different cyanine borate salts in photoinitiated
free radical polymerization was first described by Schuster [10,11]
and by our research group [12–16].
2.2. Techniques
The synthesis of multi-cationic monomethine dyes was car-
ried out using the method described by Deligeorgiev [17,18].
Tetramethylammonium n-butyltriphenylborate (B2) was synthe-
sized according the method described by Damico [19]. All final
products were identified by 1H NMR spectroscopy. The spectra
obtained were the evidence that the reaction products were of
the desired structures. The purity of synthesized compounds was
determined using thin layer chromatography and by measuring of
the melting points. The purity of the dyes was as it is required for
spectroscopic studies.
Spectroscopic measurements: UV/vis absorption spectra:
obtained using Shimadzu UV–vis Multispec – 1500 Spectropho-
tometer, and steady-state fluorescence: using a Hitachi F-4500
Spectrofluorimeter.
The reduction and oxidation potentials were measured by cyclic
voltammetry. An Electroanalitical MTM System model EA9C-4z
(Cracow, Poland), equipped with a small-volume cell was used for
the measurements. A 1-mm platinum electrode was applied as the
working electrode. A Pt wire constituted the counter electrode, and
an Ag–AgCl electrode served as the reference electrode. The sup-
porting electrolyte was 0.1 M tetrabutylammonium perchlorate in
dry acetonitrile. The solution was deoxygenated by bubbling argon
gas through the solution. The potential was swept from −1.6 to
1.6 V with the sweep rate of 500 mV/s to record the current–voltage
curve.
In this paper, the kinetic studies of the novel two- and
three-component photoinitiating systems consisting of multi-
chromphore monomethine dyes (the light absorbers) coupled with
butyltriphenylborate anions (co-initiators) are described.
2.2.1. Monitoring the free radical polymerization of monoacrylate
A
deoxygenated MMA solution, containing of ␣,␣ꢀ-azo-
bisisobutyronitrile (AIBN) as a thermal free radical polymerization
initiator (1%) and a tested probe at a concentration giving an inten-
sity of the absorption at maximum about 0.2 were placed in Pyrex
test tubes. The polymerization was initiated at 50 ◦C. The samples
were periodically (20 or 30 min) removed from the water bath and
cooled with ice to about 0 ◦C to stop the polymerization and then
were warmed to the room temperature. The fluorescence of solu-
tions was measured after the subsequent time of polymerization.
The degrees of polymerization were estimated by a gravimetrical
method after the precipitation of the polymer in ethanol [20].
Scheme 3.

76
J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
2.2.2. Monitoring of the kinetics of the free radical polymerization
of polyacrylate
-4-chloroquinolinium bromide (C1) in presence of triethy-
lamine, yielding dyes D1 and D2.
It was based on the measurements of the rate of heat evolution
during polymerization in a polymerizing sample. Photoinitiated
polymerization rate (Rp) profiles were determined by a differ-
ential 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. The 0.035 0.002 g of sample was polymerized in
open aluminium pans having the diameter of 6.6 mm. Irradiation
of the polymerization mixture was carried out using the emission
(line at 514 nm) of an argon ion argon-ion laser Air-cooled Ion Laser
Systems model 177-G01 (Spectra-Physics, USA) with intensity of
light of 20 mW/0.196 cm2. The light intensity was measured by a
Coherent Model Fieldmaster power meter.
The rate of polymerization (Rp) was calculated using the formula
(1) where dH/dt is maximal heat flow during reaction, M is the
molar mass of the monomer, m is the mass of the sample, n is the
number of double bonds per monomer molecule and ꢀHPtheor is the
theoretical enthalpy for complete conversion of acrylates’ double
bonds. ꢀHPtheor = 78.2 kJ/mol for acrylic double bonds.
5. The additional quaternization of dyes D1 and D2 with pyridine,
that leads to the formation of dyes D3 and D4.
6. The bisquaternization of [1-(3-bromopropyl)-4-[(3-methyl-
2-benzothiazolilyden)methylquinolinium
iodide]
with
N,N,Nꢀ,Nꢀ-tetramethyl-1,3-propanediamine,
N,N,Nꢀ,Nꢀ-
tetramethyl-1,6-hexanediamine, 1,4-diazabicyclo[2,2,2]octane
and 4,4ꢀ-bipirydyle, giving dyes D5–D8.
2.3.1. N-(3-bromopropyl)pyridinium bromide (A1)
N-(3-bromopropyl)pyridinium bromide (A1) was prepared
using method described by Botero [21].
1,3-Dibromopropane (30 mmol) and pyridine (10 mmol) were
heated at 45–50 ◦C in acetonitrile (60 mL) for 8 h. The solvent was
removed in vacuum. The remaining oil was crystallized in ethanol-
dichloromethane mixture (5:1).
2.3.2. 2-Methyl-3-[3-(pyridino)propyl]benzothiazolium
dibromide (B1)
The quaternization reaction of 2-methylbenzothiazole with N-
(3-bromopropyl)pyridinium bromide (A1) was carried out using
method described by Deligeorgiev [17,18].
ꢀ
ꢁ
dH
dt
M
Rp
=
(1)
nꢀHptheor
m
2-Methylbenzothiazole
(0.1 mol)
and
N-(3-
bromopropyl)pyridinium bromide (A1) were melted and the
viscous alloy was stirred at 160 ◦C for 30 min. The melt was cooled
to 60 ◦C and 30 mL of acetone was added to the reaction mixture.
The acetone layer above the solidified melt was decanted. The
semisolid salt was used without further purification in the reaction
step giving the required dye [17,18].
A polymerization solution was composed of 1 mL of 1-methyl-
2-pyrrolidinone (MP) and 9 mL of 2-ethyl-2-(hydroxymethyl)-1,3-
propanediol triacrylate (TMPTA). The photoinitiators concentration
used in experiments was 5 × 10−3 M and 1 × 10−3 M for mono-
and multi-cationic dyes, respectively. The second co-initiators con-
centration was 5 × 10−2 M. As a reference sample, a polymerizing
mixture containing cyanine iodide (dye without a co-initiator) was
used. The polymerizing mixture was not deaerated. In order to
reduce the effect of diffusion-controlled termination, the effect of
a network formation, the Norrish–Troomsdorf effect and radicals
trapping effect, the initial rates of polymerization were taken into
account for further consideration.
2.3.3. 1-(3-Bromopropyl)-4-chloroquinolinium bromide (C1)
1-(3-Bromopropyl)-4-chloroquinolinum bromide (C1) was pre-
pared using a method described by Gadjev [18]. Five grams
(0.03 mol) of 4-chloroquinoline were suspended in 7 mL of
chlorobenzene. To the suspension 6.1 mL (12.1 g, 0.06 mol) of
1,3-dibromopropane were added. The reaction mixture was
refluxed and stirred for 2 h. Then additional 4.04 g (2 mL) of 1,3-
dibromopropane were added and reflux was continued for 2 h. The
reaction mixture was cooled yielding solid product. The compound
was filtered and dried under vacuum. Yield of the crude product
10.35 g (85%) [19]. N-(3-bromopropyl)-4-chloroquinolinum bro-
mide is highly hygroscopic and requires storing in a desiccator.
The obtained product was used in the next reactions steps without
additional purification.
2.3. Synthesis
The synthetic approaches that were applied are outlined below
and are based on the reaction of 2,3-dimethylbenzothiazolium
iodide with 1-(3-bromopropyl)-4-chloroquinolinium bromide in
presence of triethylamine. The obtained dye A1 were quaternized
with pyridine and bisquaternized with N,N,Nꢀ,Nꢀ-tetramethyl-1,3-
propanediamine, N,N,Nꢀ,Nꢀ-tetramethyl-1,6-hexanediamine, 1,4-
diazabicyclo[2,2,2]octane and 4,4ꢀ-bipirydyle.
2.3.4. The synthesis of the corresponding monomethine dyes (D1,
D2)
A general route of the synthesis of multi-cationic monomethine
dyes is shown in Scheme 4.
N-methyl-2-[N-(3-bromopropyl)quinoline]-
As it is shown in Scheme 4 the synthesis of the multi-cationic
monomethine dyes undergoes via few steps:
2-methylenebenzothiazolium
N-[(3-(pyridinopropyl)]-2-[N-(3-bromopropyl)quinoline]-
2-methylenebenzothiazolium diiodide (D2) were obtained
by refluxing of 1-(3-bromopropyl)-4-chloroquinolinum
chloride (C1)
(pyridino)propyl]benzothiazolium
diiodide
(D1),
1. The quaternization reaction of pyridine with 1,3-
dibromopropane
yielding
1-(3-bromopropyl)pyridinium
(0.0035 mol)
with
2-methyl-3-[3-
(B1) and
bromide (A1).
dibromide
2. The quaternization reaction of 2-methylbenzothiazole with
1-(3-bromopropyl)pyridinium bromide or methyl iodide,
that leads to the formation following salts: 2-methyl-3-
[3-(pyridino)propyl]benzothiazolium dibromide (B1) and
2,3-dimethylbenzothiazolium iodide (B2).
2,3-dimethylbenzothiazolium iodide (B2) (0.0035 mol) in
methoxyethanol (15 mL) in the presence of triethylamine (0.97 mL,
0.007 mol) for 30 min. A triple excess of saturated aqueous potas-
sium iodide solution was added to the hot dye solution. The
precipitate formed after cooling the reaction mixture was filtered
and recrystallized from either aqueous ethanol or methanol [18].
3. The quaternization reaction of 4-chloroquinoline with
1,3-dibromopropane,
chloroquinolinium bromide (C1).
4. The condensation
giving
1-(3-bromopropyl)-4-
of
2-methyl-3-[3-
2.3.5. N-methyl-2-[N-(3-bromopropyl)quinoline]-2-methyl
enebenzothiazolium diiodide (D1)
(pyridino)propyl]benzothiazolium dibromide (B1) and 2,3-
dimethylbenzothiazolium iodide (B2) with 1-(3-bromopropyl)
D1: C21H20N2SIBr; yield 54.1%; mp 110 ◦C.

J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
77
Scheme 4.

78
J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
2.3.12. N,N,Nꢀ,Nꢀ-tetramethyl-N,Nꢀ-bis{3-[4-{(3-methyl-2-
1H NMR (DMSO-d6) ı (ppm): 1.129–1.199 (t, 2H, –CH2–),
3.045–3.208 (m, 2H, –CH2–Br), 4.026 (d, 2H, N–CH2), 4.181 (s,
3H, –CH3–N+–), 6.927 (s, 1H, –CH=), 7.744–7.918 (m, 5H, Ar),
8.253–8.295 (d, 3H, Ar), 8.400–8.442 (d, 2H, Ar).
benzothiazolilydeno)methyl]-quinolinium-1-ylo]propyl}-1,6-
hexanodiamine tetraiodide (D6)
D6:
C
52H64I4N6S2; yield 74.6%; mp. 266–268 ◦C. 1H NMR
(DMSO-d6) ı (ppm): 1.315–1.327 (m, 8H, –CH2–), 3.032 (s, 12H,
N+–(CH3)3), 3.915–3.946 (d, 8H, N+–(CH2)2), 4.028 (s, 3H, N+–CH3),
4.667 (s, 2H, N–CH2–), 4.816 (s, 2H, N–CH2–), 6.848 (s, 2H, –CH=),
7.588 (d, 3H, Ar), 7.685–7.711 (d, 4H, Ar), 7.931–7.969 (d, 4H, Ar),
8.158–8.200 (d, 3H, Ar), 8.539–8.574 (d, 3H, Ar), 8.694–8.697 (m,
3H, Ar).
2.3.6. N-(3-pyridinopropyl)-2-[N-(3-bromopropyl)quinoline]-2-
methylenebenzothiazolium diiodide (D2)
D2: C28H28N3SBrI2; yield 74.1%; mp 204 ◦C.
1H NMR (DMSO-d6) ı (ppm): 1.139–1.212 (m, 2H, –CH2-),
3.058–3.154 (2H, –CH2–), 3.410–3.655 (2H, –CH2–), 4.329 (m, 2H,
–CH2N), 4.784–5.143 (m, 4H, –CH2N+), 6.934 (1H), 7.395–7.468 (m,
2H, Ar), 7.630–7.675 (2H, pyr), 7.886–7.899 (4H, Ar), 8.099–8.134
(d, 1H, J = 7 Hz, pyr), 8.261–8.298 (d, 1H, J = 7.6 Hz, pyr), 8.439–8.826
(5H, Ar).
2.3.13. N,N,Nꢀ,Nꢀ-tetramethyl-N,Nꢀ-bis{3-[4-{(3-methyl-2-
benzothiazolilydeno)methyl]-quinolinium-1-ylo]propyl}-1,3-
propanodiamine tetraiodide (D7)
D7:
C
49H58I4N6S2; yield 80.6%; mp. 174–182 ◦C. 1H NMR
(DMSO-d6) ı (ppm): 1.315–1.327 (m, 6H, –CH2–), 3.907–3.944 (d,
8H, N+–(CH2)2), 4.667 (s, 6H, N+–CH3), 4.819 (s, 4H, N+–CH2), 6.847
(s, 2H, –CH=), 7.233–7.456 (m, 3H, Ar), 7.491–7.748 (m, 6H, Ar),
7.858–7.969 (d, 6H, Ar), 8.548–8.583 (d, 3H, Ar), 8.700–8.737 (d,
3H, Ar).
2.3.7. The synthesis of the corresponding multi-cationic
monomethine dyes (D3, D4)
The corresponding dyes D1 and D2 (0.00034 mol) and 0.08 mL
of pyridine (0.001 mol) in 3 mL of 2-methoxyethanol were refluxed
with stirring for 6 h. To the hot solution, a triple excess of saturated
aqueous potassium iodide solution was added. After cooling the
precipitated dye was filtered and dried [19].
2.3.14. N,N,NꢀNꢀ-tetramethyl-N,Nꢀ-bis-{3-[4-[(3-methyl-2-
benzothiazolideno)methyl]-quinolinium-1-ylo]propyl}-4,4’-
bipirydyle tetraiodide (D8)
2.3.8. N-(3-bromopropyl)-4-{[N-(3-pyridinopropyl)-2-
benzothiazolilydene)methyl]}quinolinium triiodide (D3)
D3: C33H33SN4I3; yield 97%; mp.192 ◦C.
D8:
C
52H48I4N6S2; yield 36.5%; mp. 148–150 ◦C. 1H NMR
(DMSO-d6) ı (ppm): 1.960–1.970 (m, 4H, –CH2–), 3.836–4.034 (d,
6H, N+–CH3), 4.440–4.500 (d, 4H, N–CH2–), 4.839 (s, 4H, N+–CH2),
6.800 (s, 2H, –CH=), 7.200–7.256 (m, 4H, Ar), 7.370–7.384 (m, 8H,
Ar), 7.863–8.037 (m, 8H, Ar), 8.196–8.233 (d, 4H, Ar), 8.597–8.781
(m, 4H, Ar).
1H NMR (DMSO-d6) ı (ppm): 1.117–1.187 (t, 2H, N–CH2–),
3.110–3.465 (m, 4H, –CH2–), 4.758–4.967 (s, 4H, N+–CH2, Pyr),
5.157–5.260 (t, 2H, –CH2–N+–), 6.849 (s, 1H, –CH=), 7.355–7.545
(m, 4H, Ar), 7.725–8.326 (m, 6H, Ar), 8.545–8.805 (dd, 6H, Ar),
9.172–9.197 (d, 4H, Ar).
3. Results and discussion
2.3.9. N-(methyl)-4-{[N-(3-pyridinopropyl)-2-
3.1. Synthesis
benzothiazolilydene)methyl]}quinolinium diiodide (D4)
D4: C26H25SN3I2; yield 34.41%; mp. 292 ◦C.
For the study eight different mono- and multi-cationic
monomethine dyes were obtained. The dyes synthesis requires
some intermediates to be prepared. 2-Methylbenzothiazole was
quaternized with methyl iodide or 1-(3-bromopropyl)pyridinium
bromide (A1) by melting together the components at around 160 ◦C
for 30 min (Scheme 4), thus giving products B1 and B2.
1H NMR (DMSO-d6) ı (ppm): 1.117–1.187 (t, 2H, –CH2–),
3.125–3.136 (t, 2H, N–CH2–), 4.157 (s, 3H, CH3–N+–), 4.758–4.967
(s, 2H, N+–CH2, Pyr), 6.749 (s, 1H, –CH=), 7.355–7.545 (m, 2H, Ar),
7.725–8.326 (m, 6H, Ar), 8.545–8.805 (dd, 6H, Ar), 9.172–9.197 (d,
3H, Ar).
4-Chloroquinoline was quaternized with 1,3-dibromopropane
for the preparation of compound C1. Dyes D1 and D2 were
prepared by condensation reaction of the quaternized 2-
methylbenzothiazolium salts B1 and B2 in the presence of
triethylamine (Scheme 4). Dyes D3 and D4 with three and two posi-
tive charges were synthesized by additional quaternization of dyes
B1 and B2 with pyridine.
2.3.10. The synthesis of the corresponding four-cationic
monomethine dyes (D5–D8)
The four-cationic monomethine dyes (D5–D8) were syn-
thesized according method described by Deligeorgiev [17].
0.002 mol
of
N-methyl-2-[N-(3-bromopropyl)quinoline]-
2-methylenebenzothiazolium diiodide (D1) and 0.001 mol
of
tetramethyl-1,6-hexanediamine,
N,N,Nꢀ,Nꢀ-tetramethyl-1,3-propanediamine,
N,N,Nꢀ,Nꢀ-
In
methyl-2-benzothiazolilydeno)methylquinolinium
were bisquaternized with
N,N,Nꢀ,Nꢀ-tetramethyl-1,3-
propanediamine,
the
next
reaction
[1-(3-bromopropyl)-4-[(3-
1,4-diazabicyclo[2,2,2]octane
iodide],
or 4,4ꢀ-bipirydyle were refluxed in 5–9 ml of 2-methoxyethanol
for 15 min to 3 h. After cooling the precipitated dye was filtered
and dried. The dye was dissolved in hot methanol and an excess
of aqueous potassium iodide solution was added to the solution.
The solution was cooled and the precipitated dye was filtered and
dried. The dye was recrystallized from methanol [17].
N,N,Nꢀ,Nꢀ-tetramethyl-1,6-hexanediamine,
1,4-diazabicyclo[2,2,2]octane and 4,4ꢀ-bipirydyle, given dyes
D5–D8 (Scheme 4).
In order to transform the monomethine dyes into the efficient
free radical polymerization initiating systems an anion exchange
from halogene to borate was necessary. The ion exchange reac-
tion was performed using the procedures given by Damico [19]
and Schuster [10]. The final products were identified by 1H NMR.
The spectra and results obtained were the evidence that the tested
salts were of the desired structures.
2.3.11. N,Nꢀ-bis{3-[4-{(3-methyl-2-
benzothiazolilydeno)methylquinolinium-1-ylo]propyl}-1,4-
diazobicyclo[2,2,2]oktane tetraiodide (D5)
D5: C48H52I4N6S2; yield 45.6%; mp. 281 ◦C. 1H NMR (DMSO-d6) ı
(ppm): 2.323–2.327 (m, 8H, –CH2–), 2.794–3.010 (2s, 6H, N+–CH3),
3.938–4.034 (m, 16H, N+–CH2, Pyr), 4.325 (dd, 4H, –CH2–N–), 4.805
(dd, 4H, N+–CH2–), 6.837–6.927 (d, 2H, –CH=), 7.200 (d, 1H, Ar),
7.370–7.411 (d, 2H, Ar), 7.456–7.770 (t, 7H, Ar), 7.775–8.061 (tt,
7H, Ar), 8.194 (m, 2H, Ar), 8.610–8.647 (m, 2H, Ar).
3.2. Spectroscopic and electrochemical properties
The benzothiazole based monomethine dyes studied display
several specific properties that are similar to the properties of other

J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
79
Fig. 2. The illustrative fluorescence spectra of selected dyes (marked in figure)
recorded in N,N-dimethylformamide (DMF) as a solvent, ꢂex = 500 nm.
Fig. 1. The illustrative electronic absorption spectra of selected dyes (marked in
figure) recorded in acetone and N,N-dimethylformamide (DMF) as a solvent, respec-
tively.
the oxidation of alkyltriphenyl borate is dissociative and this in turn
causes the electrochemical process to be irreversible.
The thermodynamically meaningful oxidation potential can be
established using an indirect, kinetic method described by Murphy
and Schuster [28]. Oxidation potentials measured electrochemi-
cally and kinetically differ by about 0.3 eV.
Therefore, the obtained electrochemical values for both
components of photoredox pairs may have only approximate ther-
modynamic meaning, yet allow us to estimate roughly the free
energy of activation (ꢀGel) for the photoinduced electron transfer
process (PET).
The measured values of the dyes reduction potentials, the
electron donor (n-butyltriphenylborate anion) oxidation potential
(Eox = 1.16 eV) and the singlet state energy of the dyes, allow us to
calculate the free energy change for the photoinduced intermolec-
ular electron transfer process with the Rehm–Weller equation [27].
The estimated data are summarized in Table 2.
monomethine dyes reported in the literature [22–26]. They have a
common structural feature namely, they possess an electron-donor
and electron-acceptor moieties located on the opposite sides of the
monomethine bond.
Figs. 1 and 2 show the illustrative electronic absorption and the
fluorescence emission spectra recorded for the selected monome-
thine dyes in N,N-dimethylformamide and acetone as a solvent,
respectively.
Table 1 collects the values of the absorption maximum positions,
the fluorescence maximum positions and Stokes shifts for all dyes
measured in DMF and acetone.
The electronic absorption spectra of all the dyes display two
absorption bands, a main broad band with a maximum about
510 nm.
The difference in energy between the absorbed and emitted
radiation is known as the Stokes shift [26]. The Stokes shift val-
ues obtained for novel multi-cationic monomethine dyes tested are
shown in Table 1. The high value of Stokes shift in both solvents
(>2000 cm−1) is observed for all dyes under study.
It is well known that the main prerequisite for a photoinduced
electron transfer (PET) reaction (vital for free radical formation)
is that the thermodynamic driving force for the electron transfer
reaction between an excited state of a dye and an electron donor
should have a negative value. The free energy of activation for the
PET (ꢀGel) process can be easily estimated on the basis of the
Rehm–Weller equation (2) [27]:
The values of ꢀGel for the tested photoinitiating systems oscil-
late in the range from −16.41 to 7.72 kJ × mol−1. The calculations
clearly show that for the tested photoredox pairs the electron trans-
fer process is thermodynamically allowed. This, in turn, allows us
Table 1
Steady state spectroscopic properties of the multi-cationic monomethine dyes
under the study.
Solvent
DMF
Dye
ꢂab [nm]
ꢂfl [nm]
Stokes shift [cm−1
]
D1
D2
D3
D4
D5
D6
D7
D8
D1
D2
D3
D4
D5
D6
D7
D8
513
509
511
513
513
511
513
510
507
505
507
507
507
506
507
502
585
585
571
578
578
577
575
578
583
581
568
574
571
572
577
571
2399
2552
2056
2192
2192
2238
2101
2307
2571
2590
2118
2302
2211
2280
2393
2407
ꢄ
ꢅ
Ze2
εa
ꢂ
ꢃ
ꢃ
ꢂ
ꢃ
ꢁGel = Eox D/D•
− Ered A•−/A − E00
−
(2)
+
ꢂ
ꢃ
where Eox D/D• is the oxidation potential of the electron donor,
+
ꢂ
Ered A•−/A is the reduction potential of the electron acceptor,
E00 is the energy of the excited state involved in electron trans-
fer reaction and Ze2/εa is the Coulombic energy associated with the
process. Because the last term is relatively small in polar or medium
polarity media, it can be neglected in the estimation of ꢀGel. The
Eox and Ered of both photoredox pair components were determined
from cyclovoltameric measurements (Table 2). The electrochemical
reduction of the dyes is reversible as it is shown in Fig. 3. However,
Acetone

80
J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
Table 2
It is evident, that as the photoinitiator concentration is
increasing, the initial rate of polymerization increases and
reaches a maximum followed by a continuous mild decrease.
For the tested photoinitiator (both mono-cationic monomethine
n-butyltriphenylborate and multi-cationic), under experimental
conditions the highest rates of polymerization were observed at the
initiator concentration of about 5 × 10−3 and 1 × 10−3 M, respec-
tively.
Reduction potential data, energy of the excited state involved in electron transfer
reaction (E00) and calculated free energies (ꢀGel) of the electron transfer reaction
between the singlet excited state of the tested dyes and electron donor and the rate
of polymerization.
Dye
Eox [eV]
Ered [eV]
ꢀGel [kJ × mol−1
]
Rp [␮mol/s]
D1
D2
D3
D4
D5
D6
D7
D8
2.18
2.34
2.36
2.33
2.36
2.37
2.35
2.33
−1.10
−1.195
−1.10
−1.05
−1.03
−1.10
−1.10
−1.12
7.72
1.45
0.14
0.20
0.40
0.015
0.060
0
−9.65
−11.S8
−16.41
−10.62
−8.69
−4.83
The highest rates of polymerization photoinitiated by multi-
cationic cyanine dyes were obtained at
a concentration of
photoinitiator of about five times lower than for its mono-cationic
parent dye. This is probably caused by a higher concentration of
co-initiator (n-butyltriphenylborate anion) in a close proximity to
the excited multi-cationic chromophore in comparison to mono-
cationic dye.
0.50
0.79
The reduction of the photoinitiated polymerization rate at
higher initiator concentration can by easily understood tak-
ing into account the decrease of the penetration depth of the
laser beam across the polymerizing formulation layer. Thus,
all kinetic measurements for multi-cationic monomethine n-
butyltriphenylborate salts were curried out at concentration equal
1 × 10−3 M.
to predicting that the tested dyes paired with borate anion should
effectively generate free radicals that can start polymerization of
acrylate monomers.
3.3. The kinetic of free radical polymerization of
2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA)
The kinetic curves obtained for the photoinitiated poly-
merization of TMPTA/MP (9:1) mixture recorded for selected
monomethine borate salts studied, under irradiation with a visible
light, are shown in Fig. 5 for illustration.
3.3.1. Two-component photoinitiating systems
The photoinitiating systems composed of multi-cationic
monomethine dye (electron acceptor) as the photosensitizer cou-
pled with n-butyltriphenylborate anion (electron donor) and for
comparison corresponding parent mono-cationic monomethine
dye paried with the same electron donor were used for the initia-
tion of free radical polymerization. The structures of photoinitiators
tested are given in Chart 1.
The relative rates of photoinitiated polymerization measured
for all the tested photoredox pairs are collected in Table 2.
It is apparent from the inspection of the initial rates of polymer-
ization that the initiation efficiency of the tested photoinitiators
depends on their structure. The highest rates of photoinitiated poly-
merization were observed for multi-cationic photoinitiators. It is
also obvious that the four-cationic photoinitiators are much more
effective in comparison to their three-, two- and mono-cationic
equivalents (see Fig. 5 and Table 2). Higher photoinitiating abilities
of multi-cationic monomethine borate salts can be attributed to
an increase of an electron donor concentration in a close proxim-
ity to an excited dye molecule caused by the presence of the extra
borate anions coupled by the covalently linked to a dye molecule
additional organic cations.
The polymerization solution consisted of 5 × 10−3 M of mono-
cationic sensitizer and 1 × 10−3 M of multi-cationic sensitizer as an
n-butyltriphenylborate salt. The polymerization process was car-
ried out with irradiation at wavelength equal 514 nm.
In order to optimize the composition of a polymerization mix-
ture, at the beginning, the influence of cyanine borate concentration
on observed rate of polymerization was determined. Fig. 4 presents
the relationship between the initial rate of polymerization (taken
as the maximum value of heat evolved during polymerization time)
and concentration of photoinitiator.
3.3.2. Three-component photoinitiating systems
Three-component photoinitiating systems, which con-
tain
a
light-absorbing species (dye), an electron donor
third component (N-
(n-butyltriphenylborate salt), and
a
alkoxypyridinum salt or 1,3,5-triazine derivative), have emerged
as efficient, visible light sensitive photoinitiators of free radical
polymerization. It was found that three-component photoini-
tiating systems are more efficient than their two-component
counterparts.
To enhance the sensitivity, three-component photoinitiators
have been recently studied. In these systems, the third compo-
nent is usually supposed to scavenge the chain-terminating radicals
that are generated by primary photochemical process or produce
an additional initiating radical [29,30].
In the reach for obtain more effective photoinitiators for
free radical polymerization of acrylate monomers, multi-cationic
monomethine dyes in the three-component photoinitiatiating sys-
tems consisting of these dyes paried with borate anion in the
presence of N-alkoxypyridinium salt or 1,3,5-triazine derivative as
a second co-initiator were also investigated through polymeriza-
tion experiments.
From the kinetic results obtained for the two-component
photoinitiating systems, it is clearly evident that multi-cationic
monomethine borate salts can be used as visible light pho-
toinitiators of acrylate monomers. However, their ability to the
Fig. 3. Cyclic voltammograms of N-(3-bromopropyl)-4-{[N-(3-pyridinepropyl)-2-
benzothiazolilydene)methyl]}quinolinium triiodide (D3) in acetonitrile.

J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
81
Chart 1.
photoinitiation of free radical polymerization is significantly lower
than those observed for symmetrical cyanine dyes [12–14,31,32].
In order to accelerate the photopolymerization process initiated by
monomethine dyes borate salts the second co-initiator was added
to the two-component photoinitiating formulation. The structures
of compounds used as a second co-initiator are presented in Chart
1.
Irradiation of three-component photoinitiating systems under
the study with visible light leads, according to our previous commu-
nications [33,34], to the formation of following radicals: alkoxy or
1,3,5-triazynyl radicals beside n-butyl radical that is formed after
first electron transfer reaction. The efficiency of these processes
depends on the composition of the photoinitiator system. Thus,
one can conclude that the overall efficiency of the photoinitiation
of free radical polymerization can be affected by: (i) the rate of pri-
mary electron transfer process, (ii) the rate of carbon – boron bond
cleavage, (iii) the rate of secondary electron transfer process, (iv)
the rate of nitrogen – oxygen bond or the rate of carbon – halo-
gen bond cleavage and finally (v) on the reactivity of free radicals
formed.
The data illustrating the kinetics of free radical polymerization
of TMPTA initiated by both two- and three-component photoiniti-
ating systems are summarized in Table 3.
Fig. 6 presents the kinetic traces recorded during an argon ion
laser photoinitiated polymerization of TMPTA/MP (9:1) mixture in
the presence of monomethine dye-borate complex and functioning
as additional co-initiator selected compounds.
As can be seen in Fig. 6, the photoinitiating system composed of
mono- and multi-cationic dyes borate salts and second co-initiator
much more efficiently initiates the free radical polymerization of

82
J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
Table 3
Measured rates (Rp) of free radical polymerization of TMPTA photoinitiated by three-
component photoinitiating systems tested.
RP [␮mol s−1
]
RP
a
Initiator
NO
T
NO
9.47
2.67
2.30
11.52
7.12
T
D1B2
D2B2
D3B2
IMB2
DfB2
D6B2
D7B2
D8B2
1.33
0.54
0.92
0.17
0.43
0.57
0.92
1.93
1.75
2.38
1.84
12.53
11.88
4.60
10.94
10.40
6.72
729.36
173.34
8.07
3.32
1.84
2.44
16.15
4.21
a
The relative rates of polymerization in comparison to the rate of polymerization
initiated by two-component photoinitiating system composed of monomethine dye
as n-butyltriphenylborate salt (DB2).
Generally, the type of applied second co-initiator in three-
component photoinitiating system has effect on the overall
efficiency of the photoinitiation. It is demonstrated that the addi-
tion of N-alkoxypyridinium salt or 1,3,5-triazine derivative to the
two-component DB2 photoinitiating system leads to an enhance-
ment of the polymerization efficiency for all dyes studied. The rate
of free radical polymerization is several times more higher in the
presence of NO or even greater in the presence of T in comparison
to the two-component photoinitiating system (Table 3 and Fig. 6).
The rate of free radical polymerization of TMPTA/MP mixture
photoinitiated by systems under study changes in the order:
Fig.
4. Rate
of
polymerization
versus
N-methyl-2-[N-
(3-bromopropyl)quinoline]-2-methylenebenzothiazolium
borate D1B2,
N-(3-bromopropyl)-4-{[N-(3-pyridinopropyl)-2-
benzothiazolilydene)methyl]}quinolinium
borate
D3B2
and
N,N,NꢀNꢀ-Tetramethyl-N,Nꢀ-bis-{3-[4-[(3-methyl-2-benzothiazolideno)methyl]-
quinolinium-1-ylo]propyl}-4,4ꢀ-bipirydyle
borate
D8B2
concentrations,
DB2/T > DB2/NO > DB2
respectively.
Presented in Fig. 6 data also clearly show that the polymerization
photoinitiation ability of DB2 in presence of a 1,3,5-triazine deriva-
tive as a second co-initiator is more efficient than this observed for
RBAX-NPF couple (typical triplet photoinitiating system) [35,36].
On the basis of the our earlier experimental results we con-
cluded that the primary photochemical reaction involves electron
TMPTA in comparison to the two-component photoinitiating sys-
tem composed of the dye and borate anion exclusively.
From the analysis of the kinetic curves in Fig. 6 and the data com-
piled in Table 3, it is seen that, the sensitivity of three-component
photoinitiating systems depends on the structure of a second co-
initiator.
Fig. 6. Kinetic curves of the TMPTA/MP (9:1) polymerizing mixture recorded during
the measurements of the flow of heat during the photoinitiated polymerization initi-
ated by mono- (D1B2), two- (D4B2) nad four-cationic monomethine borates (D5B2
and D8B2) in presence of different additional co-initiators (5.0 × 10−2 M) (marked in
figure). Borate salt concentration was 1.0 × 10−3 M Inset: Comparison of photoini-
tiation ability of RBAX-NPG (N-phenylglycine concentration was 0.01 and 0.001 M)
photoinitiating system, RBAX – Rose Bengal derivative (triplet state photoinitiator)
[35,36].
Fig. 5. The kinetic curves recorded during the measurements of the flow of heat
emitted during the photoinitiated polymerization of the TMPTA/MP (9:1) mix-
ture initiated by multi-cationic monomethine dye n-butyltriphenylborate and
mono-cationic parent photoinitiator (marked in the figure). The cyanine borate
concentration was 1 × 10−3 M, Ia = 20 mW/0.196 cm−1
.

J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
83
Fig. 7. Changes in the fluorescence spectra of D3 during the thermally initiated
polymerization of MMA recorded for conversions below the transition point of the
liquid sample into a rigid polymer matrix.
Fig. 8. Changes of the fluorescence spectra of D3 during the thermally initiated
polymerization of MMA above the point of transition of the liquid sample into a
rigid polymer matrix.
transfer from the borate anion to the excited dye followed by the
reaction of resulting dye-based radical with second co-initiator
that regenerates the original dye and simultaneously produces the
alkoxy radical or triazynyl radical which can start the polymeriza-
tion chain reaction [33,34].
because of the polymerization, probe molecules are starting to
be trapped in the rigid poly(methyl methacrylate) environment,
and as a result, the emission intensity increases. Below the time
needed for the onset of the gel effect, because the probe molecules
are relatively free, they can easily relate to the ground state. This
causes a low fluorescence intensity of the probe. However, above
the time needed for the onset of the gel effect, because the reac-
tion mixture is highly viscous, the relaxation processes become less
effective, and the emission intensity can rise to high values. In gen-
eral, the specific behavior of the probes in the transition area of
a fluid monomer to a rigid polymer can be explained by the dra-
matic increase in the viscosity, and this, in turn, rapidly reduced
the efficiency of nonradiative deactivation of the emitting state.
Changes in the probe emission intensities, measured at wave-
lengths specific for each of them, were also assumed as values
3.4. Fluorescence monitoring the free radical polymerization of
monoacrylate
The second objective of this study was to investigate the fluo-
rophore response to viscosity and polarity changes that occurred
during the thermally initiated polymerization of methylmethacry-
late (MMA). All the probes under study were quite soluble both
in the monomers and in the solid polymers. There was no indica-
tion that during the course of polymerization molecular aggregates
were formed or precipitation of the probes occurred. Figs. 7 and 8
show the fluorescence spectra of the D3 probe during the thermally
initiated polymerization of MMA for different degrees of polymer-
ization.
The changes in the fluorescence intensity observed at the flu-
orescence maximum show gradual increase below the gel point.
A distinct increase in the intensity of the probe fluorescence
was observed during MMA polymerization when a liquid poly-
mer solution was transferred into the solid polymer matrix (above
the gel point). The fluorescence intensity increase because of
changes in viscosity of the probe microenvironment that occur
during the polymerization. This behavior can be attributed to the
intramolecular charge-transfer excited-state relaxation to lower
energy charge-transfer states obtained by rotation about the single
or double bonds of the molecule [37]. In general, in such a case, the
radiative deactivation constant is not affected, but a decrease in the
nonradiative deactivation constant is related to an increase in the
environment viscosity [37,38].
Fig. 9 summarizes the changes in the fluorescence intensity
observed for D probe during thermally initiated polymerization of
MMA.
Fig. 9 shows an “S” shape of the curve versus the polymerization
time. This is a common behavior and is observed in the polymer-
ization that shows a distinct transition from a fluid to a rigid glass.
To quantify the results presented in Fig. 9, we have assumed that
the probe emission intensity increases as the polymerization prop-
agates. In other words, as the monomer consumption increases
Fig. 9. Changes in the fluorescence intensity (measured at ꢂ = 370 nm (circles) and
ꢂ = 530 nm (squares), respectively for D8 probe during the thermally initiated poly-
merization of MMA in the presence of AIBN (1%).

84
J. Kabatc / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 74–85
alkoxypyridinium or 1,3,5-triazine derivative with borate salts of
suitable dye cation two initiating radicals can be generated by one
absorbed photon.
Therefore, the mono- and multi-cationic monomethine dyes
were found to be potentially very useful as a visible light sensitizers
in the multi-component photoinitiating systems.
The studied monomethine dyes can be applied both as spectro-
scopic probes monitoring the degree of curve in their environment
and as visible light photoinitiators of free radical polymerization.
The fluorescence intensity of the tested probes, though low in the
liquid monomer, increases considerably when the monomer goes
into a rigid polymer matrix. This property makes monomethine
dyes promising fluorophores for the monitoring of the free radical
polymerization process.
Acknowledgment
This work was supported by The Ministry of Science and Higher
Education (MNiSW) (Grant No. N N204 219734).
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Fig. 10. Relationship between the probe emission intensity (probe D1, recorded at
ꢂ = 510 nm) versus the degree of monomer conversion into the polymer during the
polymerization of MMA at 50 ◦C. The inset shows linear relationship between the
fluorescence intensity and degree of polymerization recorded at ꢂ = 510 nm for D8
probe, illustrating their sensitivity (IFL = fluorescence intensity).
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Eight monomethine dyes, possessing benzothiazole ring were
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were determined. The synthesized dyes were paried with borate
anion and as photoredox pairs were used for photoinitiation of free
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