Phenyltrialkylborates as co-initiators with cyanine dyes in visible light polymerization of acrylates

Article abstract of DOI:10.1016/j.polymer.2011.03.035

Photoredox pairs consisting of selenocarbocyanine dye cations and phenyltrialkylborate anions were employed as the novel, effective visible-wavelength initiators of the radical polymerization of acrylic monomer. The influence of the sensitizers and electr

Full text of DOI:10.1016/j.polymer.2011.03.035

Polymer 52 (2011) 2110e2119
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Polymer
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Phenyltrialkylborates as co-initiators with cyanine dyes in visible light
polymerization of acrylates
,
a
b
Beata Je˛drzejewska a , Marek Pietrzak , Zbigniew Rafinski
*
ꢀ
a University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
Nicolaus Copernicus University, Faculty of Chemistry, Gagarina 7, 87-100 Torun, Poland
b
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Photoredox pairs consisting of selenocarbocyanine dye cations and phenyltrialkylborate anions were
employed as the novel, effective visible-wavelength initiators of the radical polymerization of acrylic
monomer. The influence of the sensitizers and electron donor structure on the photopolymerization
kinetics of multiacrylate monomer was investigated by photo-DSC. It was found that the polymerization
rate and the final conversion degree were dependent on both dye and borate structure. The kinetic
studies of the free radical polymerization revealed an increase in the polymerization rate with a decrease
of the borate oxidation potentials which was additionally reflected by the linear relationship between the
Hammett constant and rate of polymerization. The efficiency of these initiators was discussed on the
basis of the free energy change for electron transfer from an excited cyanine dye cation to a borate anion.
Received 30 December 2010
Received in revised form
15 March 2011
Accepted 20 March 2011
Available online 26 March 2011
Keywords:
Cyanine borate
Radical polymerization
Photolysis
The
DGel values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions
and one selenocarbocyanine dye cation. The relationship between the rate of polymerization and the free
energy of activation for electron transfer reaction gives the dependence predicted by the classical theory
of electron transfer. The photoreduction of cyanine phenyltrialkylborate complex was studied using
nanosecond laser flash photolysis. The dye triplet was found to be quenched by the electron donors via
an electron transfer process. Rate constants (kq) for the quenching of the excited states were high and
approached diffusion-controlled limits and were found to depend on the borate structure.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
with appropriate substitution. For example, upon absorption of light,
benzoin and derivatives, benzil ketals, acetophenones, aminoalkyl
Although photopolymerization is a rather old technique for the
curing of acrylate-based formulations, numerous new applications
are added regularly. Especially, visible light curing is still a challenge
due tothe high demands inmodern applicationssuch asdentalfilling
materials [1], reprography (photoresists, printing plates, integrated
circuits), laser-induced 3D curing, holographic recordings, and
nanoscale micromechanics. Therefore, important components such
as monomers and photoinitiators (PIs) [1e4] have to be adapted.
A photoinitiator or photoinitiator system can be considered to
be a molecule or combination of molecules which, when exposed to
irradiation, initiate polymerization at a much faster rate than would
occur in their absence. Most photoinitiation processes involve
a uni- or bimolecular process [5].
phenones, O-acyl-
acyphosphine oxides all spontaneously undergo
a
-oximino ketones,
a
-hydroxyalkyl ketones, and
“a-cleavage”,
generatingfreeradicals. Due to relativelylowerabsorbance, theother
cleavage initiator-based azo, peroxy, and halogenated compounds
are not widely used as photoinitiators. Each photoinitiator has
advantages and disadvantages, and the selection of a photoinitiator
very much depends on the requirements of a particular application.
The generation of free radicals from type II photoinitiators and
curing rates is generally slower than the generation of free radicals
from type I photoinitiators, which are based on unimolecular
formation of radicals. These systems are, therefore, more sensitive
to quenching of excited triplet states of the photoinitiators, which
are the reactive precursors of light-induced chemical changes for
carbonyl compounds. Indeed, quenching by monomers with low
triplet energy (e.g., styrene or N-vinylcarbazole) or by oxygen is
often observed and may lead to relatively low curing rates when
type I photoinitiators are employed [7].
Photoinitiators for radical polymerization are classified as
cleavage (type I) and H-abstraction type (type II) initiators [4e7]. The
majority of type I photoinitiators are aromatic carbonyl compounds
Visible photoinitiation systems often involve dyes as light-
absorbing chromophores. Numerous photoinitiated free radical
polymerizations using dyes have been described and reviewed by
* Corresponding author. Tel.: þ4852 374 9034; fax: þ4852 374 9009.
E-mail address: beata@utp.edu.pl (B. Je˛drzejewska).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.03.035

B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
2111
several authors [2,7e16]. Initiating radicals are typically generated
by photoinduced electron transfer and involve a co-initiator that
participates in the electron transfer process. Energy transfer to
monomers is not thermodynamically favorable in these systems
due to the low excitation energies of dyes. Depending on the nature
of the dye involved, namely photoreducible or photooxidizable
dyes, two distinct mechanisms need to be considered. These are
electron transfer from the co-initiator to the excited, photo-
reducible dye and electron transfer from the excited, photo-
oxidizable dye to the co-initiator (Scheme 1). These systems include
aromatic ketoneeamine combinations, dyeeco-initiators pairs, as
well as certain organometallic photoinitiators [6].
The excited-state electron transfer process often plays a key role
in visible light-induced free radical polymerizations [1e6]. This
process leads to the formation of free radicals that are effective
initiators of polymerizations. The efficiency of the initiating radical
formation depends on two main factors. The first factor is the
precise nature of the reacting partners taking part in the electron
transfer process. The second is the efficiency of any subsequent
fragmentations yielding secondary radicals and radical ions [8e16]
that are often much more effective in initiating polymerizations
than the primary radicals. Therefore, a detailed knowledge of such
primary and secondary photochemical mechanisms is critical for
the design of practical systems used in initiating polymerizations.
In the past decade, there have been several extensive studies on
the photooxidation of borate-containing organic compounds and
the subsequent fragmentation reactions of the photon-generated
borate-centered radical anions. These studies are of general interest
because of their potential application in photopolymerization
[9,10,17e22], and their importance as intermediates in organic
synthesis [23e26]. In the area of polymerization, for example, it
was recently shown for a series of phenyltrialkylborate salts, which
type of radicals would be the most effective co-initiators of poly-
merizations [17e20].
Upon oxidation, phenyltrialkylborate anions have been shown to
be very useful as free radical sources. Paired with an appropriate
cationic chromophore, the tetraorganylborates are used as co-initi-
ators in visible light photoinitiator systems. The mechanism of
photoinitiationwitha cyanineeboratecomplex involves alkyl radical
formation as a result of photoinduced electron transfer from a borate
anion to the singlet excited state of the cyanine dye cation, followed
by the addition of an alkyl radical formed to the carbonecarbon
double bond of the acrylate [9,10]. The relationship between the rate
ofpolymerization andthefreeenergyof theactivationof theelectron
transfer reaction shows the dependence predicted by the classical
theory of electron transfer phenomena [6].
Phenyltrialkylborates have been found to have better reactivity
in comparison to traditionally used alkyltriphenylborate salts. The
significant increase in the efficiency of the photoinitiation of free
radical polymerization is a result of the generation of more than
one radical after the absorption of one photon [9].
In this paper, the polymerization kinetics of trimethylolpropane
triacrylate (TMPTA) in the presence of several pairs of borate
initiators is compared. Each pair of the borate salts contained the
same light-absorbing chromophore as the cation and either
different phenyltrialkylborate or butyltriphenylborate as the
counterion. The borates studies are shown in Scheme 4.
2. Experimental
2.1. Materials
Monomer, 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate
(TMPTA), all chemicals and solvents were purchased from Aldrich
Chemical Co. and were used without further purification. As sensitizers
selenocarbocyanine dyes were used. The 3,30-dimethylthiacar bocya-
nine iodide (S4), 5,7-diiodo-3-butoxy-6-fluorone (DIBF), campherchi-
none (CQ) and titanocene (Ti(C6F5)2(C5H4)2; T) were applied as light
absorber for comparison. As co-initiators the following compounds
were tested: tetramethylammonium phenyltriethylborate (B6), tetra-
methylammonium p-methylphenyltriet-hylborate (B61), tetramethy-
lammonium p-methoxyphenyltriethyl-borate (B62), tetramethylamm
oniump-phenoxyphenyltriethyl-borate (B63), tetramethylammonium
p-chlorophenyltriethylbo-rate (B64), tetramethylammonium p-cya-
nophenyltriethylborate (B65), tetramethylammonium n-butyltriph
enylborate (B2), N-phenylglycine (NPG) and ethyl 4-N,N-dimethyla-
minobenzoate (ENB).
2.2. Techniques
2.2.1. Spectral measurements
Absorption (the concentration of the dye 1.0 ꢀ 10ꢁ5 mol/L) and
emission spectra (the concentration of the dye 3.0 ꢀ 10ꢁ6 mol/L) in
N,N-dimethylformamide (DMF), acetonitrile (AcCN), and tetrahy-
drofurane (THF) were recorded at room temperature using a Shi-
madzu UVevis Multispec-1501 spectrophotometer and a Hitachi
F-4500 spectrofluorimeter, respectively.
The nanosecond laser flash photolysis experiments were per-
formed using a LKS.60 Laser Flash Photolysis apparatus (Applied
Photophisics). Laser irradiation at 355 nm from the third harmonic
of the Q-switched Nd:YAG laser from a Lambda Phisik/model LPY
150 operating at 65 mJ/pulse (pulse width about 4e5 ns) was used
for the excitation. Transient absorbances at preselected wave-
lengths were monitored by a detection system consisting of
a monochromator, a photomultiplier tube (Hamamatsu R955) and
a pulsed xenon lamp (150 W) as a monitoring source. The signal
from the photomultiplier was processed by a Helwett-Packard/
Agilent an Agilent Infiniium 54810A digital storage oscilloscope
and an Acorn compatible computer.
2.2.2. Electrochemical measurements
The reduction potentials of the dyes were measured by cyclic
voltammetry using an Electroanalytical Cypress System Model
CS-1090. The typical three-electrode setup was employed for
electrochemical measurements. The electrolyte was 0.1 M ter-
trabutylammonium perchlorate, which was purged with argon
prior to a measurement. Platinum 1-mm electrode was applied as
working electrode and platinum and Ag/AgCl were used as auxil-
iary and reference electrodes, respectively.
2.2.3. Polymerization measurements
Scheme 1. Schematic presentation of both the photoreducible and the photo-
oxidizable sensitization of free radical polymerization. Note: kd is the rate constant that
The kinetics of free radical polymerization were studied using
a polymerization solution composed of 1 mL of 1-methyl-2-pyr-
rolidinone (MP) and 9 mL of 2-ethyl-2-(hydroxymethyl)-1,3-pro-
panediol triacrylate (TMPTA). The seleno- and thiocarbocyanine
represents the rate of diffusive encounters between reactants, k denotes the rate of
ꢁd
separation of the reactants after collision, kel is the first order rate constant of electron
transfer. The reverse step is designated by the rate constant kꢁel, and finally kr denotes
the rate of return electron transfer.
borate salts (photoinitiator) concentration was 0.0015 M.
A

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B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
reference formulation contained seleno- and thiocarbocyanine
iodide or bromide (dye without an electron donor) instead of
seleno- and thiocarbocyanine borate salt (photoinitiator). The
measurements were carried out at an ambient temperature and the
polymerizing mixture was not deaerated before curing.
The kinetics of free radical polymerization were measured based
on the measurements of the rate of the heat evolution during
polymerization in thin film cured sample (0.035 ꢂ 0.002 g). The
measurements were performed by measuring photo-
polymerization exotherms using photo-DSC apparatus constructed
on the basis of a TA Instruments DSC 2010 Differential Scanning
Calorimeter. Irradiation of the polymerization mixture was carried
out using the emission (line at 514 nm) of an argon-ion laser Model
Melles Griot 43 series with intensity of light of 100 mW/cm2. The
light intensity was measured by a Coherent Model Fieldmaster
power meter.
water and poured into 100 mL of saturated aqueous N(CH3)4Cl. The
white precipitate which formed was filtered, washed with water,
dried and recrystallized from ethanol.
The detailed synthetic procedures as well as the basis of
analytical results and spectral evidences of the tetramethy-
lammonium phenyltrialkylborates the readers can find in our
previous paper [29].
2.3.2. Synthesis of selenocarbocyanine dyes
The methodology for the synthesis of the trimethine cyanine
dyes is illustrated in Scheme 3. The classical orthoester approach to
carbocyanines was applied. It involves the condensation, under
basic conditions, of orthoesters with quaternary heterocyclic salts
substituted with an activated methyl group [30].
General procedure: A solution of 1.72 mmol of quaternary
heterocyclic salts, 0.76 g (0.86 mL, 5.16 mmol) of triethyl ortho-
formate and 7 mL of dry pyridine was refluxed for 2 h. After cooling
to room temperature, diethyl ether was added and the resulting
The rate of polymerization (Rp) was calculated using the Equa-
tion (1) where dH/dt is maximal heat flow during reaction and
theor
D
Hp
is the theoretical enthalpy for complete conversion of
theor
acrylates’ double bonds.
D
Hp
for acrylic double bond is equal
78.2 kJ/mol [27].
S; Se
X
ꢀ
ꢁ
dH
dt
1
Rp
¼
(1)
D
Hptheor
CH
N
3
The reaction heat liberated in the polymerization was directly
N
proportional to the number of vinyl groups reacting in the system.
By integrating the area under the exothermic peak, the conversion
of the vinyl groups (C) or the extent of reaction could be determined
according to Equation. (2).
Br
CH CH CH Br
2
2
2
C H I
2
5
D
Ht$M
Ethanol
C ¼
(2)
n
D
Hptheor
m
where
DHt is the reaction heat evolved at time t, M is the molar
X
X
N
CH
mass of the monomer, m is the mass of the sample and n is the
number of double bonds per monomer molecule.
The quantum yield of polymerization Fp was defined as the
number of polymerized double bonds per absorbed photon [28].
CH
3
3
I
N
CH
CH
CH
N
C H
2
2
2
2
Br
5
Br
2.3. Synthesis
2.3.1. Synthesis of phenyltrialkylborate salts
Simple synthetic procedures were developed to prepare suitably
substituted stable phenyltrialkylborate tetramethylammonium
salts [18,19]. The synthetic approaches that were applied are out-
lined below and are based on the addition of alkyl or phenyllithium
reagents to a triorganylborane. A general route for the synthesis of
tetramethylammonium phenyltrialkylborate is shown in Scheme 2.
Typical procedure: To a solution of triethylborane in hexane
(15 mmol, 15 mL, 1 M), anhydrous diethyl ether (25 mL) was added.
To the vigorously stirred solution, under nitrogen, p-substituted
phenyllithium (16.6 mmol) was added dropwise at ꢁ85 ꢃC over
10 min. The mixture was allowed to warm slowly to room
temperature and stirred for a further 2 h. The solvent was removed
under reduced pressure, the residue was dissolved in 10 mL of
OC H
2
5
CH
Pyridine H C O
5
2
OC H
2
5
X
N
X
CH CH CH
I
N
C H
2
C H
2
5
5
X
N
CH
CH
CH
N
X
N
CH CH CH
Br
CH
CH
CH
N
2
2
1) Li or LiC H
4
9
2
2
2
2
R
B
R
2) B(R )
1 3
1
1
_
R
Br
R
R
1
2
2
Br
Br
3) N(CH ) Cl
3 4
+
N(CH )
3 4
Scheme 2. A general route for the synthesis of B6 borate series.
Scheme 3. Synthesis of trimethine cyanine dyes.

B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
2113
mixture allowed to complete precipitation. The crystalline product
obtained was collected by filtration under reduced pressure,
washed with diethyl ether and recrystallized from dry methanol or
dimethylformamide/diethyl ether system [30].
2ꢀ CHar), 7.20 (s, 2H; 2ꢀ CHar), 7.69 (t, J ¼ 12.2 Hz, 1H; CH), 7.89 (d,
J ¼ 8.8 Hz, 2H; 2ꢀ CHar) ppm.
13C NMR (50.3 MHz, DMSO-d6):
d
¼ 34.6 (2ꢀ CH3), 55.9
(2ꢀOCH3), 100.6 (2ꢀ CH), 103.7 (CH), 112.4 (2ꢀ CH), 115.6 (2ꢀC),
126.7 (2ꢀ CH), 144.5 (2ꢀC), 149.3 (2ꢀC), 159.9 (2ꢀ CH), 170.4 (2ꢀC)
ppm.
2.3.2.1. 3-(3-(Pyridinium-1-yl)propyl)-2-((1E,3Z)-3-(3-(3-(pyridinium-
1-yl)propyl)benzo[d] [1,3]selenazol-2(3H)ylidene)prop-1-enyl)benzo[d]
[1,3]selenazol-3-ium bromide (Se1).
2.3.2.4. 3-Methyl-2-((1E,3Z)-3-(3-methylbenzo[d] [1,3]selenazol-2
(3H)-ylidene)prop-1-enyl)benzo[d]
(Se4).
[1,3]selenazol-3-ium
iodide
Se
Se
N
N
Br
Se
Se
N
Br
N
N
N
I
Br
The title compound was obtained by crystallization from dry
DMF/Et2O. Yield 37%.
The title compound was obtained by crystallization from MeOH.
Yield 62%.
1H NMR (200 MHz, DMSO-d6):
1H NMR (200 MHz, DMSO-d6):
d
¼ 3.87 (s, 6H; 2ꢀ CH3), 6.48 (d,
d
¼ 2.42 (m, 4H; 2ꢀ CH2), 4.47
J ¼ 12.8 Hz, 2H; 2ꢀ CH), 7.00 (m, 2H; 2ꢀ CHar), 7.28 (m, 2H; 2ꢀ
CHar), 7.44 (m, 2H; 2ꢀ CHar), 7.69 (t, J ¼ 12.8 Hz, 1H; CH), 7.82 (d,
J ¼ 8.8 Hz, 2H; 2ꢀ CHar) ppm.
(m, 4H; 2ꢀ CH2), 5.11 (m, 4H; 2ꢀ CH2), 7.35 (t, J ¼ 7.6 Hz, 2H; 2ꢀ
CH), 7.53e7.63 (m, 4H), 7.82 (d, J ¼ 8.2 Hz, 2H; 2ꢀ CHar), 7.95
(t, J ¼ 11.8 Hz, 1H; CH), 8.10 (d, J ¼ 8.2 Hz, 2H; 2ꢀ CHar), 8.19
(t, J ¼ 6.8 Hz, 4H; 4ꢀCHar), 8.62 (t, J ¼ 7.6 Hz, 2H; 2ꢀ CHar), 9.41
(d, J ¼ 6.2 Hz, 4H, 4ꢀCHar) ppm.
13C NMR (50.3 MHz, DMSO-d6):
d
¼ 33.5 (2ꢀ CH3), 98.8 (CH),
112.7 (2ꢀ CH), 123.4 (2ꢀC), 124.7 (2ꢀ CH), 127.5 (2ꢀ CH), 129.0 (2ꢀ
CH), 139.8 (2ꢀC), 146.0 (2ꢀ CH), 159.8 (2ꢀC) ppm.
13C NMR (50.3 MHz, DMSO-d6):
d
¼ 29.3 (2ꢀ CH2), 44.2 (2ꢀ
CH2), 57.2 (2ꢀ CH2), 105.0 (CH), 114.9 (2ꢀ CH), 125.1 (2ꢀ CH), 125.4
(2ꢀC), 126.6 (2ꢀ CH), 127.9 (2ꢀ CH), 128.1 (4ꢀCH), 142.3 (2ꢀC),
144.9 (4ꢀCH), 145.7 (2ꢀ CH), 151.0 (2ꢀC), 168.6 (2ꢀC) ppm.
2.3.2.5. 2-((1E,3Z)-3-(3-Ethylbenzo[d]thiazol-2(3H)-ylidene)prop-1-
enyl)-3-methylbenzo[d] [1,3]selenazol-3-ium iodide (Se6).
2.3.2.2. 3-Ethyl-2-((1E,3Z)-3-(3-ethylbenzo[d] [1,3]selenazol-2(3H)-
ylidene)prop-1-enyl)benzo[d] [1,3]selenazol-3-ium ethyl sulfate
(Se2).
Se
S
N
N
I
Se
Se
The title compound was obtained by crystallization from dry
DMF/Et2O. Yield 18%.
N
N
1H NMR (200 MHz, DMSO-d6):
d
¼ 1.33 (t, J ¼ 6.6 Hz, 3H; CH3),
SO3
O
3.78 (s, 3H; CH3), 4.37 (q, J ¼ 6.6 Hz, 2H; CH2), 6.60e6.70 (m, 2H),
7.28e7.79 (m, 7H), 7.98e8.06 (m, 2H) ppm.
The title compound was obtained by crystallization from dry
DMF/Et2O. Yield 78%.
13C NMR (50.3 MHz, DMSO-d6):
d
¼ 12.7 (CH3), 34.5 (CH3), 41.6
1H NMR (200 MHz, DMSO-d6):
d
¼ 1.09 (t, J ¼ 7.2 Hz, 3H; CH3),
(CH2), 98.7 (CH), 99.6 (CH), 102.7 (CH), 113.5 (CH), 114.9 (CH), 123.2
(CH), 124.8 (CH), 125.4 (CH), 126.3 (CH), 127.9 (CH), 128.2 (CH), 140.
8 (C), 143.4 (C), 146.4 (C), 148.5 (C), 164.5 (C), 169.7 (C) ppm.
1.33 (t, J ¼ 7.0 Hz, 6H; 2ꢀ CH3), 3.74 (q, J ¼ 7.2 Hz, 2H; CH2), 4.35
(q, J ¼ 7.4 Hz, 4H; 2ꢀ CH2), 6.75 (d, J ¼ 12.6 Hz, 2H; 2ꢀ CH), 7.34
(t, J ¼ 7.4 Hz, 2H; 2ꢀ CHar), 7.54 (t, J ¼ 7.4 Hz, 2H; 2ꢀ CHar), 7.70
(d, J ¼ 8.0 Hz, 2H; 2ꢀ CHar), 7.78 (t, J ¼ 12.6 Hz, 1H; CH), 8.06 (d,
J ¼ 8.0 Hz, 2H; 2ꢀ CHar) ppm.
2.3.2.6. 3-Methyl-2-((1E,3Z)-3-(3-methylbenzo[d]thiazol-2(3H)-yli-
dene)prop-1-enyl)benzo[d]thiazol-3-ium iodide (S4).
13C NMR (50.3 MHz, DMSO-d6):
d
¼ 12.6 (2ꢀ CH3), 15.2 (CH3),
42.3 (2ꢀ CH2), 61.2 (CH2), 103.2 (CH), 114.7 (2ꢀ CH), 125.0 (2ꢀ CH),
125.4 (2ꢀC), 126.4 (2ꢀ CH), 128.0 (2ꢀ CH), 142.2 (2ꢀC), 150.6 (2ꢀ
CH), 168.8 (2ꢀC) ppm.
S
S
N
N
I
2.3.2.3. 6-Methoxy-2-((1E,3Z)-3-(6-methoxy-3-methylbenzo[d] [1,3]
selenazol-2(3H)-ylidene)prop-1-enyl)-3-methylbenzo[d] [1,3]selena-
zol-3-ium iodide (Se3).
The title compound was obtained by crystallization from dry
MeOH. Yield 46%.
1H NMR (200 MHz, DMSO-d6):
d
¼ 3.81 (s, 6H; 2ꢀ CH3), 6.52 (d,
J ¼ 12.8 Hz, 2H; 2ꢀ CH), 6.94 (d, J ¼ 7.2 Hz, 2H; 2ꢀ CHar), 7.56 (t,
J ¼ 7.0 Hz, 2H; 2ꢀ CHar), 7.72 (m, 3H; 3ꢀCHar), 7.97 (d, J ¼ 7.0 Hz, 2H;
2ꢀ CHar), ppm.
OMe
MeO
Se
Se
13C NMR (50.3 MHz, DMSO-d6):
d
¼ 33.5 (CH3), 98.9 (CH), 113.6
N
N
(2ꢀ CH), 123.0 (2ꢀ CH), 124.9 (2ꢀC), 125.2 (2ꢀ CH), 128.0 (2ꢀ CH),
I
141.8 (2ꢀC), 146.2 (2ꢀ CH), 165.0 (2ꢀ C) ppm.
The title compound was obtained by crystallization from dry
DMF/Et2O. Yield 40%.
For the analysis of the photoinitiating properties of novel pho-
toredox pairs, the selected combinations of n-butyltriphenylborate
(B2) or phenyltrialkylborate (B6, B61, B62, B63, B64 and B65) anion
and mono- and tricationic cyanine dyes, were used.
1H NMR (200 MHz, DMSO-d6):
6H; 2ꢀ CH3), 6.64 (d, J ¼ 12.2 Hz, 2H; 2ꢀ CH), 6.94 (d, J ¼ 8.8 Hz, 2H;
d
¼ 3.75 (s, 6H; 2ꢀ CH3), 3.83 (s,

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B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
The difference in energy between the absorbed and emitted
Se1
Se4
S4
1.0
0.8
0.6
0.4
0.2
0.0
radiation is known as the Stokes’ shift. The Stokes’ shift values for
novel cyanine dyes are collected in Table 1. The absorption and
emission spectra of dye under the study show small Stokes’ shift.
The fluorescence spectra are roughly a mirror image of the
absorption spectra (Fig. 2). This indicates that the structure of the
dyes upon excitation is unchanged.
3.2. Kinetics study of multifunctional acrylates polymerization
The photoinitiating systems composed of mono- and tricationic
cyanine dyes (electron acceptor) coupled with phenyl-
trialkylborates (electron donor) were used for the initiation of free
radical polymerization of multifunctional acrylate monomer. The
structures of the photoinitiators are given in Scheme 4.
450
500
550
600
650
The polymerization solution consisted of 1.5 ꢀ 10ꢁ3 M of
sensitizer as a phenyltrialkylborate salt. The polymerization process
was initiated by irradiation at 514 nm. At this wavelength only the
sensitizer absorbs the light.
Wavelength (nm)
Fig. 1. Normalized absorption spectra of selected cyanine dyes (marked in Figure) in
acetonitrile.
In order to optimize the photoinitiation mixture composition, at
the beginning, the cyanine borate concentration effect on the rate
of polymerization was determined. It is well known that the pho-
toinitiator concentration plays a key role in photopolymerization.
In conventional UV/Vis photopolymerization, Rp increases when
more initiator is used, however it decreases rapidly if too much
initiator is added. This effect is rendered to the “inter filter effect”
and becomes more significant for photoinitiators with a high molar
3. Results and discussion
3.1. Spectroscopic and photophysical properties of cyanine
derivatives
In our studies, polymerization of TMPTA photoinitiated by
mono- and tricationic trimethine cyanine borate salts was per-
formed under irradiation at 514 nm, e.g. at the wavelengths where
the light is absorbed by cyanine cation (Fig. 1).
extinction coefficient (for tested dyes
3
is about 8 ꢀ 105 Mꢁ1 cmꢁ1).
Fig. 3 presents the relationship between the rate of polymerization
and concentration of photoinitiator.
It is evident that as the photoinitiator concentration increases,
the rate of polymerization increases and reaches a maximum fol-
lowed by continuous mild decrease. For the tested photoinitiators,
under experimental conditions, the highest rate of polymerization
was achieved at initiator concentrations of about 1.5 ꢀ 10ꢁ3 M. The
reduction of the photoinitiated polymerization rate at higher
initiator concentration (for applied technique of polymerization
rate measurement) can be easily understood taking into account
the decrease of the penetration depth of the laser beam across the
polymerizing formulation layer. Thus, all kinetic measurements for
cyanine borate salts were carried out at a concentration equal to
1.5 ꢀ 10ꢁ3 M.
Dithiacarbocyanine and diselenocarbocyanine dyes are charac-
terized by an intense absorption band in the orange spectral region,
with the more intense peaks (emax ¼ 8 ꢀ 105 Mꢁ1 cmꢁ1) located for
dithiacarbocyanine (S4) at about 550e560 nm and the second, less
intense, blue-shifted by about 6000 cmꢁ1 (Fig. 1). For dis-
elenocarbocyanines the absorption bands are shifted to the red area
of visible light of about 20 nm. The spectral data are listed in Table 1.
It is evident that for tested cyanine dyes only relatively small
solvent dependence of their absorption and emission spectra is
observed. Generally, we observed a blue shift in the absorption with
an increase of solvents polarity and a positive solvatochromism in
the emission.
The kinetic curves obtained for the photoinitiated polymeriza-
tion of TMPTAeMP (9:1) mixture recorded for selected cyanine
borate salts, under irradiation with a visible light laser, are shown in
Table 1
Spectral properties of cyanine dyes under the study in THF, DMF and AcCN.
Ab
max
Fl
max
Solvent
Dye
l
ðnmÞ
l
ðnmÞ
Stokes
Ab. in THF
shifts (cmꢁ1
)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Ab. in DMF
FL. in THF
FL. in DMF
Tetrahydrofurane (THF)
Se1
Se2
Se3
Se4
Se6
S4
Se1
Se2
Se3
Se4
Se6
S4
Se1
Se2
Se3
Se4
Se6
S4
586
578
593
582
568
563
583
577
585
580
567
562
577
568
586
572
559
554
600
597
615
600
583
579
601
596
617
602
586
580
595
588
613
593
579
572
398.2
550.6
603.2
515.5
453.0
490.8
513.7
552.5
886.6
630.1
571.8
552.2
524.3
598.8
751.6
619.1
617.9
568.0
N,N-dimethylformamide
(DMF)
Acetonitrile (AcCN)
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 2. Normalized absorption and fluorescence spectra of Se3 dye in THF and DMF.

B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
2115
ELECTRON
DONOR
PET
Se
N
CH
CH
CH
N
Se
R
R
1
CH CH CH
1
CHROMOPHORES
Se
Se
N
CH CH CH
PET
N
R
N
R
CH
CH
CH
N
2
2
2
2
2
2
ELECTRON
DONOR
PET
PET
ELECTRON
DONOR
ELECTRON
DONOR
ELECTRON DONORS:
B
B
B
B
B
B
B6
B61
CH
OCH
OC H
6 5
B64 Cl
B65
CN
B62
B63
3
3
Scheme 4. Conceptual presentation of an artificial increase of an electron donor concentration in close neighborhood to the excited chromophore for trimmeric cyanine dyes in
comparison to monomeric ones.
Fig. 4 for illustration. The rates of photoinitiated polymerization
measured for all the tested photoredox pairs are collected in
Table 2.
It is apparent from the inspection of the rates of polymerization
that the efficiency of the tested photoinitiators depends on both the
dye and the borate structure. Among monocationic cyanine borates
the highest rates of photoinitiated polymerization were observed
for 6,60-dimethoxy-N,N0-dimethylselenocarbocyanine phenyl-
triethylborate (Se3B6) used as initiator.
For the comparison of the photoinitiating efficiency, Fig. 5
presents the kinetic curves observed for free radical polymeriza-
tion of TMPTA/MP mixture initiated by different photoinitiating
systems.
that the photoinitiator system composed of CQ e NPG couple
initiates the free radical polymerization with the highest rate.
Additionally, it is evident that the photoinitiating ability of trica-
tionic carbocyanine borate salt (Se1B2) is lower than that observed
for the well known visible light photoinitiators.
The differences in photoinitiating abilities of tested systems
result from the efficiency of free radical formation. For the tertiary
aromatic amines, the electron transfer between an excited acceptor
and an electron donor is followed by a proton transfer from the
electron donor radical cation to the dye radical anion, which yields
a neutral radical initiating polymerization and the reduced radical
of the dye [6]. The use of NPG as electron donor gives even more
complex processes following the electron transfer that yield the
Data presented in Fig. 5 show that the structure of both an
electron donor and an electron acceptor in the photoinitiating
systems plays an important role in polymerization reaction and has
influence on the overall rate of polymerization process. It was found
150
Se3B6
Se2B6
160
120
80
40
0
Se2B6
Se6B6
Se4B6
Se9B6
120
90
60
30
0
Se2B2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (min.)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Time (min.)
Fig. 4. Family of kinetic curves recorded during the measurements of the heat flow
emitted during the photoinitiated polymerization of the TMPTAeMP (9:1) mixture
initiated by cyanine dyeseborates complexes marked in the Figure. The dye concen-
tration was 1.5 ꢀ 10ꢁ3 M, Ia ¼ 100 mW/cm2. The applied photoinitiators possess
identical borates. Inset: Comparison of the photoinitiation ability of cyanine borate
salts. The photoinitiators possess the same chromophore and either n-butyl-
triphenylborate or phenyltriethylborate as an electron donor.
Fig. 3. Rate of polymerization vs. photoinitiator concentration.

2116
B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
Table 2
The reduction potentials of the cyanine dyes, the oxidation potential of the tetramethylammonium borate salts, the singlet state energy (E00) of the dyes, the rates (Rp) and the
quantum yields (Fp) of free radical polymerization of TMPTA/MP (9/1) mixture for tested photoredox couples.
E00 (eV)a
Rp
(m
mol/s)b
Fp
No
Eox (V)
D
Gel (eV)c
Rp
(m
mol/s)c
Fp
b
c
No
Ered (V)
Se1B
Se4B
Se1B
Se4B
Se1B
Se4B
Se1
Se2
Se3
Se4
Se6
S4
ꢁ1.072
ꢁ1.282
ꢁ1.158
ꢁ1.142
ꢁ1.154
ꢁ1.184
2.09
2.11
2.05
2.09
2.15
2.17
3.90
1.64
2.06
1.30
1.29
1.36
45.36
19.13
23.96
15.13
15.06
15.78
B6
0.888
0.708
0.618
0.684
0.890
0.954
ꢁ0.253
ꢁ0.311
ꢁ0.385
ꢁ0.269
ꢁ0.127
ꢁ0.063
ꢁ0.187
ꢁ0.245
ꢁ0.319
ꢁ0.203
ꢁ0.061
0.003
1.57
1.78
2.08
1.49
1.38
1.00
0.65
0.56
0.47
0.59
0.42
0.18
18.29
20.71
24.32
17.36
14.82
11.63
7.60
6.53
5.43
6.92
4.85
2.07
B61
B62
B63
B64
B65
a
Calculated for THF.
b
c
Calculated for cyanine dyes paired with phenyltriethylborate (series Se1-6B6 and S4B6).
Calculated for a series of borates (B6-B65) paired with either Se1 or Se4 cyanine dyes (series Se1B6-65 and Se4B6-65).
free radicals being a result of N-phenylglycine radical cation
decomposition [6]. Very complex is also the potochemistry of the
titanocene photoinitiators [6,31].
by the covalent bonding of an extra organic cation that is not a part
of the chromophore, which can form an ion pair with the electron
donor anion.
The experimental results suggest that the intermolecular elec-
tron transfer process may be the limiting step in the photoinitiated
polymerization.
Fig. 6 presents kinetic curves, representing monomer conver-
sion as a function of time, obtained by photo-DSC for mono- and
tricationic photoinitiators. After an induction period during which
polymerization inhibitors are consumed, the polymerization starts
causing an increase of medium microviscosity. With the progress of
polymerization, reactive functional groups are consumed and
polymerization stops as indicated by the plateau on the kinetic
profile (Fig. 6).
It was found that the degree of monomer conversion and the
quantum yield of photopolymerization depend on the photo-
initiator pair’s structure. From the data obtained during the
measurements, it was also deduced that the conversion of mono-
mer double bonds rises from about 10% to 22%, while the quantum
yields of photopolymerization changes from 10 to 48 for the light
absorber concentration equals 0.0015 M for mono- (Se4) and tri-
cationic (Se1) photoinitiators, respectively. Generally, the photo-
initiator that couple three electron donors in one molecule (Se1)
exhibits better photoinitiating ability in comparison to the model
monocationic cyanine dye (Se4).
From literature it is well known that the distance between the
electron donor and the electron acceptor is quite important for
the photoinduced electron transfer reaction, especially when the
process occurs in the short-lived singlet excited state. In the case of
cyanine borate salts the process is efficient in non-polar solvents in
which the electron transfer occurs between components of a tight-
ion pair [9,10]. Our study reveals that a degree of dissociation of
such pairs depends on both the structure and concentration of the
solute in a complex fashion [20e22,32e34]. However, in simple
approximation, a degree of dissociation decreases as the concen-
tration of solute increases (or the concentration of non-dissociated
salt is enhanced when the concentration of the solute increases).
That is why, the simplest way to increase the efficiency of the
polymerization process is the adding of an extra amount of co-
initiator (an electron donor) into the polymerization mixture.
Traditionally, such a situation is achieved by the attachment of an
organic cation that can form an ion pair with a borate anion to an
absorbing chromophore [35e37]. In cationic dyes, this is possible
The higher photoinitiating abilities of three-cationic cyanine
borate salts can be attributed to an increase of an electron donor
concentration in a close proximity to an excited dye molecule
25
0.6
0.5
0.4
0.3
0.2
0.1
0.0
C
A
Se
Se
CH CH CH
N
N
CH2
CH2
CH2
N
CH2
CH2
CH2
N
20
15
10
5
B
D
E
Se
Se
CH CH CH
N
N
CH
CH
3
3
0
0
50
100
150
Time (s)
1
2
3
4
5
6
7
8
Time (min.)
Fig. 5. Family of kinetic curves recorded during the measurements of the heat flow
emitted during the photoinitiated polymerization of the TMPTAeMP (9:1) mixture
initiated by: A) T (0.001 M); B) DIBF (0.001 M) e NPG (0.001 M); C) CQ (0.01 M) e NPG
(0.001 M); D) CQ (0.01 M) e ENB (0.001 M) and E) Se1B2 (0.001 M). Irradiation of the
polymerization mixture was carried out using the emission (line at 457 nm or 556 nm)
of DDPS lasers (Shanghai Dream Lasers Technology) with intensity of light of 25 mW/
cm2.
Fig. 6. Family of kinetic curves recorded during the measurements of the heat flow
emitted during the photoinitiated polymerization of the TMPTAeMP (9:1) mixture
initiated by cyanine dyes e n-butyltriphenylborate complexes. The dye concentration
was 1.5 ꢀ 10ꢁ3 M, Ia ¼ 100 mW/cm2. The applied dyes possess various chromophores
and identical borate (B2).

B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
2117
caused by the presence of the extra borate anions coupled by the
Se
Se
covalently linked to dye molecule additional organic cations.
The efficiency of radical polymerization initiated via intermo-
lecular electron transfer process (PET) is dependent on the nature
of the dye and the electron donor. As can be seen in Fig. 4 (inset),
the phenyltriethylborate salt reacted faster than the corresponding
n-butyltriphenylborate at the same initiator concentration and
irradiation conditions.
CH CH CH
180
150
120
90
N
N
CH2
CH2
CH2
N
CH2
CH2
CH2
N
B62
From Schuster and his co-workers [9,10] reports, it is well known
that the irradiation of carbocyanine alkyltriphenylborate salts with
visible light absorbed by cyanine leads to the formation of a locally
excited singlet state of the dye. The singlet excited cyanine in the ion
pair is capable of oxidizing the borate anion to the boranyl radical.
The boranyl radical undergoes very fast carboneboron bond
cleavage to generate a free alkyl radical. The rate of bond cleavage
depends directly on the stability of the alkyl radical formed. When
the stabilized alkyl radicals are formed, carboneboron bond cleavage
is faster than the back electron transfer reaction that regenerates the
cyanine borate ion pair. The free alkyl radicals formed by the irra-
diation e electron transfer e bond cleavage sequence may be used in
meaningful chemical processes such as the initiation of polymeri-
zation. What is more, Neckers’ et al. studies [19] revealed that, in the
case of phenyltrialkylborates, the second product formed after an
electron transfer process, phenyldialkylborane, is also capable of
initiating a chain radical reaction. It can react either with the oxygen
centered radical product of addition of the alkyl radical to the vinyl
bond of monomers or with the excited state of the sensitizer [38,39]
forming additional radicals.
Thus, the overall efficiency of the photoinitiation of radical
polymerization can be affected by: (i) the rate of the primary
electron transfer process, (ii) the rate of carboneboron bond
cleavage, (iii) the rate of secondary processes and finally (iv) the
reactivity of free radicals formed.
Because photoinitiator performance remarkably affects the
overall performance of polymerization systems, a significant effort
to optimize this important component of the polymerization
system is underway.
B61
B6
B63
B64
B65
60
30
0
1.0
1.2
1.4
1.6
Time (min.)
1.8
2.0
2.2
Fig. 7. Family of kinetic curves recorded during the measurements of the heat flow
emitted during the photoinitiated polymerization of the TMPTAeMP (9:1) mixture
initiated by cyanine dyeseborates complexes marked in the Figure. The dye concen-
tration was 1.5 ꢀ 10ꢁ3 M, Ia ¼ 100 mW/cm2. The applied dyes possess identical
chromophore and different borates.
4-cyanophenyltriethylborate (B65), which bears a strong electron
withdrawing group, has an oxidation potential of about 0.95 V.
The effect of the borate oxidation potential on photoinitiating
abilities can be explained on the basis of the RehmeWeller [40]
Equation (3).
ꢂ
ꢃ
ꢂ
. ꢃ
Ze2
D
Gel ¼ Eox
D
ꢁ Ered Aꢄꢁ
A
ꢁ
ꢁ E00
a
(3)
ꢄþ
=
D
3
where:
DGel is the free energy of activation for the photoinduced
electron transfer (PET) process, Eox(D/Dꢄþ) is the oxidation potential
of the electron donor, Ered(Aꢄꢁ/A) is the reduction potential of the
electron acceptor, E00 is the excited state energy of the electron
The efficiency of radical polymerization photoinitiated by the
cyanine dyes under study coupled with different co-initiators was
evaluated by monitoring the rate of heat evolution during the
radical polymerization of multiacrylate.
Fig. 7 presents the family of kinetic curves recorded during an
argon-ion laser initiated polymerization of a solution composed of
1 mL of 1-methyl-2-pyrrolidinone (MP), 9 mL of 2-ethyl-2-
accepting molecule, and Ze2/
e
a is the Coulombic energy, which is
the free energy gained by bringing the radical ions formed to an
encounter distance “a” in a solvent with dielectric constant . This
term, for products of an analyzed reaction, is considered negligible
with respect to the overall magnitude of the Gel in the present
system. The Eox and Ered of both photoredox pair components were
determined from cyclovoltametric measurements (Table 2). The
electrochemical reduction of the dyes is reversible. However, the
oxidation of phenyltrialkylborate is dissociative and this in turn
causes the electrochemical process to be irreversible as is shown in
Fig. 8.
The thermodynamically meaningful oxidation potential can be
established using an indirect, kinetic method described by Murphy
and Schuster [41]. However, the peak potentials obtained by either
cyclic or square wave voltammetry usually follow the same trend as
those measured by kinetic methods. Oxidation potentials measured
electrochemically and kinetically differ by about 0.3 eV. Therefore,
these can be used for the comparison of the borates.
The measured oxidation potential of the borate, the reduction
potentials of the dyes, the singlet state energy of the dyes and the
free energy change for the photoinduced intermolecular electron
transfer process are compiled in Table 2 together with the poly-
merization results.
e
D
(hydroxymethyl)-1,3-propanediol
triacrylate
(TMPTA)
and
different photoinitiating systems with a sensitizer and a co-initiator
concentration equal 1.5 ꢀ 10ꢁ3 M.
From the inspection of the kinetic curves presented in Fig. 7 and
data collected in Table 2, it is seen that the initiation ability depends
on of the borate anion in the photoinitiators. In the photoinitiating
systems shown in Fig. 7, the chromophore structure is the same.
This causes that the reduction potential of cyanine dye cannot affect
the initiation efficiency. Since the borate salts are efficient electron
donor in the photoredox pairs, the difference in their reactivity in
the system investigated is related to substantial differences in the
oxidation potentials of the borates. The tetramethylammonium
n-butyltriphenylborate has an oxidation potential of about 1.2 V.
This is higher than that of tetramethylammonium phenyl-
triethylborates, which are in the range of 0.95e0.62 V, meaning
that phenyltriethylborate salts are much more reductive than
n-butyltriphenylborate salt. What is more, substitution on the
benzene ring strongly influences the reduction properties of tet-
ramethylammonium phenyltriethylborates. 4-Methoxyphenyl-
triethylborate (B62), which bears a strong electron releasing
substituent, has an oxidation potential of about 0.62 V, while the
The values of DGel for tested photoinitiating systems oscillate in
the range from 0.003 eV to ꢁ0.385 eV. The calculations show that
for the tested photoredox pairs the electron transfer process is
thermodynamically allowed.

2118
B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
Fig. 9. Relationship between free energy of activation of electron transfer process and
either Hammett constant or rate of polymerization initiated by the series of phenyl-
triethylborates and Se1 dye.
Fig. 8. Cyclic voltammperograms for selected tetramethylammonium phenyl-
trialkylborates under the study in acetonitrile. Solution contained tetra-n-buty-
lammonium perchlorate as the supporting electrolyte and was deoxygenated prior to
analysis by purging with argon for 15 min.
constants (kq) for the quenching of the excited state of the dye. The
triplet decay rates and transient absorption spectra of Se3 were
measured in acetonitrile (MeCN). In the case of Se3 when no
electron transfer is expected from the iodide counteranion to the
excited triplet state of cyanine cation, the triplet transient absorp-
tion spectrum was obtained. The transient absorption spectrum of
Se3, showing the triplet decay, obtained in acetonitrile at a different
time after the pulse, is shown in Fig. 10, inset.
Fig. 9 presents the relationship between the free energy of the
activation of electron transfer process and either the Hammett
constant or the rate of polymerization initiated by the series of
phenyltriethylborates and Se1 dye.
Presented in Fig. 9 data show a linear relationship between the
free energy of the activation of electron transfer process and
Hammett constants of the substituents presented in the benzene
At the wavelengths above 600 nm we observed positive
absorption which decays with a lifetime of about 40e50
ms,
ring of the borate salts. This indicates that
DGel increases as the
depending on the exact quality of the removal of oxygen from the
solution. By analogy to the other works described in literature
[43,44], we assign it to the triplet absorption of the cyanine moiety.
The triplet state of Se3 was found to be quenched by tetrame-
thylammonium phenyltrialkylborate salts with a concomitant
observation of absorptions at 630 nm. Rate constants for quenching
were determined by measuring the effects of the additive on the
lifetimes of the dye triplet state. Linear SterneVolmer plots were
observed (Fig. 11).
Hammett constant increases. Since the alkyl radicals formed from
borate anion after an electron transfer are the same for all borate
salt series, the difference in their reactivity in the system investi-
gated is related to substantial differences in the oxidation poten-
tials of the borates.
A linear relationship is also observed between the free energy of
the activation of electron transfer process. According to Fig. 9, the rate
of free radical polymerization initiated by the series of cyanine
borates increases as the driving force of the electron transfer reaction
increases. This type of behavior is predicted by the classical theory of
photoinduced electron transfer [42]. This suggests that at the
beginningof the TMPTA polymerization, the electron transfer process
may be the rate determining step in the radical formation process.
As was previously reported for the alkyltriphenylborates [9,10],
the rates of alkyl radical formation, as the result of the boranyl
radical degradation, are directly related to the known stabilities of
the alkyl radicals formed. Since the decay of the boranyl radical is
found to be very fast and irreversible, the rate of back electron
transfer (kꢁel) is negligible [9,10]. Therefore, the efficiencies of alkyl
radical formation and, hence, the initiation of polymerization
depend on the rate of electron transfer from the borate anion to the
excited state of the sensitizer.
The quenching rate constants obtained by linear fitting of the
data in Fig. 11 are 2.32 ꢀ 109 Mꢁ1
s
ꢁ1, 2.48 ꢀ 109 Mꢁ1 sꢁ1 and
1.29 ꢀ 109 Mꢁ1 sꢁ1 for B6, B62 and B65, respectively. The quenching
rate is affected by the substituents on the benzene ring in the
In an attempt to measure the intramolecular electron transfer,
a nanosecond flash photolysis experiment was conducted. The
occurrence of the electron transfer between the cyanine cation and
borate ions leads to the quenching and simultaneous shortening of
the excited state lifetime of the cyanine chromophore. If the elec-
tron transfer is the only quenching process of cyanine triplet, the
electron transfer rate constants are equal to those of triplet
quenching. The quenching rate constant of the triplet state of the
cyanine dye can be estimated based on a SterneVolmer analysis.
A nanosecond time-resolved transient absorption spectroscopy
with excitation at 355 nm using the third harmonics of
a Q-switched Nd:YAG laser was used to determine the rate
Fig. 10. Transient absorption kinetic curve observed at 630 nm at 2 ms after the pulse
for 6,60-dimethoxy-N,N0-dimethylselenocarbocyanine (Se3). Inset: Transient absorp-
tion spectra of Se3 in acetonitrile recorded at a different time after the pulse assigned
to the triplet state.

B. Je˛drzejewska et al. / Polymer 52 (2011) 2110e2119
2119
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Fig. 11. Concentration dependence of the triplet decay rate of the Se3 in MeCN.
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B65 salt bearing a strong electron withdrawing group. Rate
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Cyanine borate salts are shown to be effective photoinitiators for
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used as an electron acceptor and the borate anion used as an
electron donor. The inspection of the rates of polymerization
showed that the tricationic cyanine salts initiated polymerization
more effectively than monocationic equivalents. The comparative
analysis of the borate salts as co-initiators with cyanine dyes shows
that the number of alkyl groups per boron determines reactivity.
The reactivity of borate anion is determined by the Hammett
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Acknowledgment
This work was supported by The Ministry of Science and Higher
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