Thioxanthone-diphenyl anthracene: Visible light photoinitiator

Article abstract of DOI:10.1021/ma202168m

7,12-Diphenyl-14H-naphto[2,3-b]thioxanthen-14-one (TX-DPA) was synthesized and characterized as a potential visible light photoinitiator for radical polymerization. TX-DPA has an excellent absorption character in the visible region and the photophysical p

Full text of DOI:10.1021/ma202168m

Article
pubs.acs.org/Macromolecules
Thioxanthone−Diphenyl Anthracene: Visible Light Photoinitiator
Demet Karaca Balta, Gokhan Temel, Gokce Goksu, Nuket Ocal, and Nergis Arsu*
Department of Chemistry, Yildiz Technical University, Davutpasa Campus, Istanbul 34210, Turkey
ABSTRACT: 7,12-Diphenyl-14H-naphto[2,3-b]thioxanthen-14-one (TX-DPA) was
synthesized and characterized as a potential visible light photoinitiator for radical
polymerization. TX-DPA has an excellent absorption character in the visible region and
the photophysical properties of TX-DPA were investigated by fluorescence and laser
flash photolysis studies and the photopolymerization of methyl methacrylate in air
atmosphere helped to understand the initiation mechanism of TX-DPA. Moreover, it is
3
possible that the production of singlet oxygen from the quenching of TX-DPA by
molecular oxygen resulted in the reaction of singlet oxygen with TX-DPA to form an
endoperoxide: decomposition of endoperoxide leads to initiating radicals.
of the one-component structure of the photoinitiator, it can
INTRODUCTION
■
serve as both a triplet photosensitizer and as a hydrogen donor
during photopolymerization. Thus, these photoinitiators do not
require an additional co-initiator.
An alternative approach concerns the attachment of both
chromophoric and hydrogen donating groups into polymer
chains.29−35 This way the odor and toxicity problems perceived
with the conventional photoinitiators and amine hydrogen
donors have been overcome.
We have also developed a novel thioxanthone based
photoinitiator possessing an anthracene group, thioxanthone−
anthracene36 (TX-A, Chart 1), which exhibits completely
different photochemical properties compared to conventional
TX type photoinitiators.
In recent years, many attempts have been made to synthesize
photoinitiators that operate in the visible region of the
spectrum. The reason for this is that visible light is cheap,
safe and possesses high penetration ability in the presence of
UV absorbing monomers, pigments and substrates.1−7
Thioxanthone (TX) and its derivatives are some of the most
widely used type II photoinitiators in various UV curing
applications because of their excellent light absorption
characteristics.8 In most cases, the photoinitiating free radicals
are generated by hydrogen abstraction of triplet excited states
of TX from hydrogen donors such as amines or thiols (see
Scheme 1).9−11
Scheme 1. Photoinitiated Free Radical Polymerization Using
TX as Photoinitiator
Chart 1. Structure of TX-A
In contrast to TX type photoinitiators, TX-A is an efficient
photoinitiator for free radical polymerization of both acrylic and
styrenic type monomers in the presence of oxygen without an
additional hydrogen donor.36−38 Moreover, for other TX type
photoinitiators oxygen inhibits the polymerization because of
the quenching of the excited triplet TX with oxygen. In the case
of TX-A, molecular oxygen is essential for the initiation process.
The initiation mechanism involves the formation of singlet
oxygen by energy transfer from triplet TX-A39 (Scheme 2).
However, low molecular-weight amines, particularly when
used at high concentrations, have several intrinsic disadvantages
such as odor, toxicity12 and migration in UV-curing technology
and cause a decrease in the pendulum hardness of the cured
film.13 One way to overcome these problems is to chemically
incorporate the hydrogen donating sites into TX chromo-
phores.14−16
Amine,17−20 alkyl amino,21,22 acid,23−27 or thiol28 linked
photoinitiators are seen in the literature and they may find
various applications in the Radiation Curing Industry because
of the great advantages of their one-component nature. Because
Received: September 26, 2011
Revised: November 29, 2011
Published: December 16, 2011
© 2011 American Chemical Society
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Scheme 2. Photoinitiation Mechanism of TX-A in the
Presence of Oxygen
Chart 3
There is another possible application of using an oil soluble
initiator in water; complexation of TX-A with β-CD was
achieved and photopolymerization of acrylamide with TX-A/β-
CD was performed.40
Our aim is to synthesize new photoinitiators which have
absorption in the visible region and operate in oxygen
atmosphere. We achieved the synthesis of 7,12-diphenyl-14H-
naphto[2,3-b]thioxanthen-14-one (TX-DPA). 1H NMR,
1H−1H COSY, elemental analysis, and GC−MS were used
for the characterization of TX-DPA. Photophysical properties
were determined by the use of UV−vis, fluorescence and laser
flash photolysis, and photoinitiated polymerization of methyl
methacrylate was performed with TX-DPA by using a xenon
lamp as the irradiation source.
L.mol−1.cm−1, respectively. These high molar absorptivity
values would be beneficial for a photoinitiator in the visible
region (Figure 2).
A photodecomposition study of TX-DPA was performed in
CHCl3 and this solution was exposed to UV light at certain
periods of time and the photodecomposition of TX-DPA was
followed by detecting UV-spectral changes (Figure 2).
At the end of the irradiation (270 s) while the typical
absorption bands of the diphenylanthracene moiety from DPA
completely disappeared, the weak absorption of the TX group
at 380 nm was still detectable.
Fluorescence measurements were conducted in order to
clarify the mechanistic details. As can be seen from Figure 3, the
excitation spectrum of TX-DPA is very similar to its absorption
spectrum.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis of TX-
DPA was achieved by using a similar method to that which was
described in our previous publication (see Experimental
Section, Chart 2).36 Since there are more than one ring closure
The emission spectrum represents the characteristics of the
TX moiety rather than DPA. The intersection of the emission
and excitation spectra at 464 nm, allowed us to calculate the
singlet excited state energy of TX-DPA, as ca. 257.8 kJ/mol.
The fluorescence quantum yield, ϕf, was estimated to be 0.45
by using 9,10-diphenylanthracene as the standard. (ϕf =
0.95).41,42 From the calculations, the fluorescence quantum
yields (ϕ(TX‑DPA) = 0.45) of anthracene chromophore were
more dominant than the TX [ϕ(TX) = 0.07]. This high
fluorescence quantum yield reduced the triplet yield, the state
from which initiating radicals are generated.
Chart 2. Synthesis of TX-DPA
Fluorescence quenching studies of TX-DPA were performed
by adding MDEA at different concentrations and the changes in
the fluorescence emissions were recorded for the TX-DPA/
ethanol solution. The Stern−Volmer representation (i.e., I0/I vs
[Q]) is shown in Figure 4. It can be observed that the least-
squares fit of the experimental results produces a straight line
with reasonable point scattering. The linearity of the
corresponding plot was always excellent (R2 > 0.90), and no
deviation was detected at high quencher concentrations. The
Stern−Volmer quenching constant, Ksv, obtained from the
slope is 0.86 M−1.
Laser Flash Photolysis. To investigate the properties of
the triplet state and its reactivities, laser flash photolysis
experiments were performed. Figure 5 shows the transient
absorption spectra of TX-DPA directly after irradiation with
laser pulses at 355 nm. Three transient absorption maxima were
observed at 400, 490, and 590 nm (see Figure 5a) which
decayed in a first-order kinetic with a lifetime of 2.6 μs (Figure
5b). The peak at 590 nm was assigned to the triplet−triplet
absorption of TX-DPA based on similarities with the triplet−
triplet spectra of TX43 and TX derivatives.28 Recently we
possibilities, due to the aromatic groups on the DPA (see
Experimental Section, Chart 3), the COSY NMR method was
employed to define the structure of the product in addition to
1
using H NMR spectroscopy.
1H−1H COSY was employed to confirm the structure of the
1
compound. Figure 1 shows the H−1H COSY spectrum, the
peak at 8.03 ppm as doublet is coupled with the H15 proton of
the phenyl group as expected, while the peak at 7.62 ppm as
doublet is coupled with the proton H18. The H4 proton is
determined by 1H−1H COSY as doublet and is clearly coupled
with H3.
Absorption and Fluorescence. TX-DPA has excellent
absorption properties in the visible region of the electro-
magnetic spectrum and the absorption characteristics of TX-
DPA were found to be very similar to the parent compounds,
both thioxanthone and 9, 10-diphenylanthracene. The molar
absorptivities of TX-DPA where the wavelength values were
383 and 450 nm were calculated as 10633 and 4347
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1
Figure 1. H−1H COSY spectrum of TX-DPA in CDCl3.
Figure 2. Photobleaching of TX-DPA [4.10 × 10−5 M] in CHCl3
during 0−270 s in air atmosphere with polychromatic light (unfiltered
light from a medium pressure mercury lamp).
Figure 3. Fluorescence excitation (__) and emission (--) spectra of
TX-DPA in ethanol; λexc = 390 nm.
From the previous studies36,39 related with anthracene
attached thioxanthone (TX-A), polymerization experiments in
the presence and absence of oxygen clearly confirmed the
crucial role of oxygen for successful polymerization as the
efficient initiation of TX-A can be attributed to the formation of
endoperoxide by the reaction of TX-A with singlet oxygen.
Photopolymerization. A similar way was followed to
determine the initiation mechanism of TX-DPA, and polymer-
submitted LFP studies of TX-A, and the triplet lifetime of TX-A
was calculated as 28 μs.39
Notably, all transients were quenched by oxygen and were
tentatively assigned to the triplet state. Quenching experiment
of transients with MDEA were also achieved and the rate
constant was found as ∼2.72 × 109 M−1 s−1 (see Figure 6).
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Figure 6. Reaction of MDEA with triplet TX-DPA in acetonitrile
solutions 23 °C. Dependence of the pseudo-first-order rate constant of
the decay of the optical absorption at 400 nm on the concentration of
MDEA after laser excitation (355 nm).
Figure 4. Stern−Volmer plot of the quenching of TX-DPA [1.0 ×
10−5 mol L−1] by MDEA in ethanol (λexc = 390 nm). I0 = fluorescence
intensity of TX-DPA, I = fluorescence intensity in the presence of
MDEA.
effect of the amine appears to be due to the formation of α-
aminoalkyl radicals (see Table 1).
ization experiments of methyl methacrylate with TX-DPA in
DMF in the prescence and absence of oxygen atmosphere were
performed. Additionally, a tertiary alkyl amine namely N-
methyldiethanolamine (MDEA) was also added to the same
formulation to see the inhibition effect of oxygen beside the
synergistic effect of the amine. A xenon lamp source was used
as the irradiation source, since TX-DPA has excellent
absorption in the visible region.
Table 1. Photoinitiated Polymerization of Methyl
Methacrylate [4.68 mol L−1] in DMF with TX-DPA in the
Absence and Presence of MDEA in/under a Xenon Lamp
System
[TX-DPA]
(mol L−1
[MDEA]
%
convn
Mn × 10−4
At the very low initiator concentration level (5 × 10−5 M),
polymerization was not observed in air atmosphere, but when
MDEA was added to this formulation in air atmosphere, the
conversion percentage was found to be 3.50. When the initiator
concentration increased to 1 × 10−4 M, some polymer
formation was observed but the conversion percentage was
still low compared to the formulation containing MDEA. By
further increasing the initiator concentration to 5 × 10−4 M, the
beneficial effect of oxygen for the formation of endoperoxide
appeared and the conversion percentage was 5.3 in air
atmosphere, 3.8 in the presence of MDEA, and 2.9 in nitrogen
atmosphere. When the obtained results in the prescence of
amine and nitrogen atmophere were compared, the beneficial
a
)
(mol L−1
)
(g mol−1
)
Mw/Mn
5 × 10−5
5 × 10−5
1 × 10−4
1 × 10−4
5 × 10−4
5 × 10−4
5 × 10−4
1 × 10−3
1 × 10−3
-
0.0
3.5
0.7
4.5
5.3
3.8
2.9
1.2
1.0
-
-
5 × 10−2
2.51
3.50
2.30
4.33
2.04
3.22
2.39
3.65
1.39
1.40
1.32
2.09
1.25
1.44
1.24
1.48
-
5 × 10−2
-
5 × 10−2
b
-
5 × 10−2
-
a
b
tirr = 60 min. Under nitrogen atmosphere I0(UV‑A) = 175 W·m−2
Figure 5. (a) Transient optical absorption spectrum recorded 1.15 μs, 2.27 and 1.4 μs following laser excitation (355 nm, 5 ns) of TX-DPA [2.4 ×
10−5 M] in argon saturated acetonitrile solution at 23 °C; .(b) Kinetic traces of the transient optical absorption spectrum at 400 nm recorded 1.15,
2.27, and 1.4 μs following laser excitation (355 nm; 5 ns) of TX-DPA in argon-saturated acetonitrile solution at 23 °C.
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For comparison, photopolymerization experiments using
either TX or DPA themselves or their combinations in the
presence and absence of the hydrogen donor MDEA were
performed (Tables 2 and 3).
Table 2. Photoinitiated Polymerization of Methyl
Methacrylate [4.68 mol L−1] in DMF with DPA in the
Absence and Presence of MDEA in/under a Xenon Lamp
System
a
[DPA] (mol L−1
)
[MDEA] (mol L−1
)
% convn
5 × 10−5
5 × 10−5
1 × 10−4
1 × 10−4
5 × 10−4
5 × 10−4
1 × 10−3
1 × 10−3
-
0.0
3.8
0.0
4.6
0.3
6.7
0.6
7.9
5 × 10−2
-
5 × 10−2
-
5 × 10−2
-
Figure 7. Absorption spectra of TX-DPA (―) and poly(methyl
methacrylate) (---) obtained ([PI]: 5 × 10−4 M) in THF.
5 × 10−2
a
tirr = 60 min.
singlet oxygen formed from the triplet excited state in the
presence of oxygen as illustrated in Scheme 3. The excitation of
this intermediate eventually leads to the formation of peroxy
and/or alkoxy radicals capable of initiating free radical
polymerization of MMA.
Table 3. Photoinitiated Polymerization of Methyl
Methacrylate [4.68 mol·L−1] in DMF in the Prescence of
9,10-Diphenylanthracene (DPA) and Thioxanthone (TX)
in/under a Xenon Lamp System
In the presence of MDEA, the possibility of the formation of
endoperoxide is getting lower since oxygen in the medium is
scavenged by MDEA. Therefore, the TX part plays a more
dominant role and so a typical type II initiation mechanism
appears (Scheme 4).
a
[DPA] (mol L−1
)
[TX] (mol L−1
)
[MDEA] (mol L−1
)
% convn
5 × 10−4
5 × 10−4
5 × 10−4
5 × 10−4
-
0.6
2.3
5 × 10−2
a
tirr = 60 min.
CONCLUSION
Anthracene derivatives are known to form instable
■
endoperoxides upon irradiation.44 The singlet oxygen (1O2)
addition rate constant was reported in the literature45,46 as 5.4
× 105 M−1 s−1 for anthracene and 3.0 × 106 M−1 s−1 for
diphenylanthracene. The singlet oxygen rate constants and
stabilities of the endoperoxides increase as more electron-
pushing substituents are added to the anthracene ring. The
bimolecular quenching rate constant of singlet oxygen by TX-A
was found to be similar to the reported value for anthracene
(k1O2 = 5.4 × 105 M−1 s−1).45 These endoperoxides decompose
(thermally and photochemically) through radical intermedi-
ates,47−49 which could initiate the polymerization of the
monomers. The endoperoxide of diphenylanthracene-TX is
probably formed very easily due to the stability of TX-DPA,
and possibly the formation of endoperoxide is not occurring
efficiently in room temperature to produce initiating alkoxy
and/or peroxy radicals. This could be an explanation of the low
polymer yield in the presence of TX-DPA when comparing the
results obtained with TX-A as the initiator.36
In conclusion, diphenylanthracene (TX-DPA) was successfully
synthesized and was found to have excellent optical properties
in the visible region of the electromagnetic spectrum. TX-DPA
initiated polymerization of methyl methacrylate in the presence
and absence of oxygen. The initiation mechanism in the
presence of oxygen possibly explains the involvement of the
formation of singlet oxygen by energy transfer from triplet TX-
DPA and resulted in alkoxy or peroxy radicals after photo-
excitation of TX-DPA-O2 initiated free radical polymerization.
High molar absorptivity in the visible region may possibly lead
to using this photoinitiator in long wavelength region
applications in the presence of oxygen atmosphere.
EXPERIMENTAL SECTION
■
Materials. 9,10-Diphenylanthracene (DPA, 97%, Aldrich) thio-
salicylic acid (98.5%, Fluka), glacial acetic acid (Aldrich), N-
methyldiethanolamine (MDEA, 99%, Aldrich), sulfuric acid (H2SO4,
98%, Merck), ethanol (HPLC grade, Aldrich), acetonitrile (HPLC
grade, Aldrich), chloroform (CHCl3, ≥99.8%, Fluka), N,N′-dimethyl-
formamide (DMF, +99%, Aldrich) and thioxanthone (TX, ≥98.5%,
Fluka) were used as received. Methyl methacrylate (MMA, ≥99%,
Fluka) was washed with 5% aqueous NaOH solution, dried over
CaCl2, and distilled over CaH2 in vacuo. Tetrahydrofuran (99.8%, J.T.
Baker) was dried and distilled over LiAlH4.
Synthesis of 7,12-Diphenyl-14H-naphto[2,3-b]thioxanthen-
14-one (TX-DPA). A heated solution of 9,10-diphenylanthracene (2
× 10−3 mol) in glacial acetic acid (15 mL) was added slowly to the
thiosalycylic acid (2 × 10−3 mol) under stirring followed by the
addition of 5 mL concentrated sulphuric acid. After the addition, the
reaction mixture was refluxed at 120 °C for 5 h after which it was left
to stand at room temperature overnight. The resulting mixture was
poured carefully under stirring into a 10-fold excess of boiling water.
The mixture was stirred under boiling for an additional 5 min. The
In air atmosphere, the initiating ability of DPA for the MMA
polymerization in the presence and absence of TX was seen as
proof of the formation of endoperoxide.
For the characterization of the polymer, UV spectroscopy
was used. In Figure 7, the absorption spectra of TX-DPA and
the poly(methyl methacrylate) obtained from the irradiation
with xenon lamp in the absence of MDEA are shown. The UV
absorption spectra of purified polymer produced from initiation
with a high initiator concentration (5 × 10−4 M) showed
absorbance at 300 nm, which indicates TX chromophores
bound slightly to polymers.
A feasible mechanism for the initiation process involves the
formation of an endoperoxide by the reaction of TX-DPA with
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Scheme 3. Photoinduced Radical Generation Mechanism of TX-DPA Photoinitiator in the Prescence of Oxygen
Scheme 4. Photoinduced radical generation mechanism of TX-DPA photoinitiator with N-methyldiethanolamine
solution was cooled and filtered. The residue product was purified by
column chromatography over silica gel eluting with chloroform (Chart
2). MA (C33H20OS): 464 g mol−1; yield: 52%; mp 263−266 °C.
1H NMR (500 MHz) in CDCl3: δ 8.03 (d, J = 8.78 Hz, 1H, H13),
7.62 (d, J = 8.78 Hz, 1H, H6), 7.61 (dd, J = 8.78 ; Hz, 1H, H3), 7.55−
7.48 (m, 4H, H1, H2, H8, H11), 7.42−7.38 (m, 5H, H15, H15, H16,H16,
H17), 7.38−7.28 (m, 4H, H18, H18, H19, H19), 7.21−7.15 (m, 3H, H9,
H10, H20), 7.08 (d, J = 8.78 Hz, 1H, H4) ppm.
IR(KBr): ν (cm−1) aromatic (C−H: 3053), ketone (CO: 1634),
Anal. Calcd for C33H20OS (464.12 g mol−1): C, 85.31; H, 4.34; S,
6.90. Found, C, 85.29; H, 4.35; S, 6.90.
Photopolymerization. Appropriate solutions of the monomer
and TX-DPA in DMF in the absence and presence of N-
methyldiethanolamine (MDEA) were irradiated in a photoreactor
consisting of a 400 W medium pressure mercury lamp with a water
cooling system and simultaneously irradiated in a photoreactor
(CC: 1590).
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consisting of a 400 W xenon lamp with a water cooling system, in an
air atmosphere. Polymers were obtained after precipitation in
methanol and drying in vacuo. Conversions were calculated for all
samples gravimetrically.
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Analyses. GPC analyses of the polymer were performed at room
temperature with a setup consisting of a pump (Agilent 1100) and
three columns (Zorbax PSM); THF was used as the eluent (flow rate
0.3 mL min−1), and detection was carried out with the aid of an
Agilent 1100s differential refractometer. The number-average molec-
ular weights were determined using Polymer Laboratories polystyrene
standards. UV−vis spectra were taken on an Agilent 8453. IR spectra
were recorded on an ATI Unicam Mattson 1000 FT/IR-3
spectrophotometer on a KBr disk. Fluorescence spectra were recorded
on a Jobin Yvon−Horiba Fluoromax-P.
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AUTHOR INFORMATION
■
Corresponding Author
*Telephone: + 90 212 383 4186. Fax: +90 212 383 4134. E-
mail: narsu@yildiz.edu.tr.
(34) Balta, D. K.; Cetiner, N.; Temel, G.; Turgut, Z.; Arsu, N. J.
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