Acylstannanes: Cleavable and Highly Reactive Photoinitiators for Radical Photopolymerization at Wavelengths above 500 nm with Excellent Photobleaching Behavior

Article abstract of DOI:10.1002/anie.201804094

Within this work, a novel acylstannane-based photoinitiator (PI) is presented. Tetrakis(2,4,6-trimethylbenzoyl)stannane (1) displays outstanding properties compared to state-of-the-art acylgermane-based initiators. Most importantly, the initiator shows absorption up to 550 nm, which allows higher penetration depths, especially in highly filled photopolymers. Besides that, 1 shows extremely high photoinitiating activity towards (meth)acrylic double bonds, as well as very fast photobleaching. Furthermore, unlike many organotin compounds, 1 shows surprisingly low cytotoxicity.

Full text of DOI:10.1002/anie.201804094

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Accepted Article
Title: Acylstannanes - A New Generation of Cleavable and Highly
Reactive Photoinitiators for Curing at Wavelengths Above 500 nm
with Excellent Photobleaching Behavior
Authors: Moritz Mitterbauer, Patrick Knaack, Sergej Naumov, Marica
Markovic, Alexandr Ovsianikov, Norbert Moszner, and Robert
Liska
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804094
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Angewandte Chemie International Edition
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COMMUNICATION
Acylstannanes - A New Generation of Cleavable and Highly
Reactive Photoinitiators for Curing at Wavelengths Above 500 nm
with Excellent Photobleaching Behavior
Moritz Mitterbauer,[a,b] Patrick Knaack,[a,b] Sergej Naumov, Marica Markovic,
[c]
[d,e]
A. Ovsianikov,[d,e]
Norbert Moszner[b,f] and Robert Liska*[a,b]
Abstract: Within this work, a novel acylstannane-based photoinitiator
PI) is presented. Tetrakis(2,4,6-trimethylbenzoyl)stannane (1)
absorption is shifted to longer wavelengths due to orbital
interactions of the empty d orbitals of the heteroatom with the π*
orbital of the carbonyl group.[6] Within this communication, we
report the synthesis of tetrakis(2,4,6-trimethylbenzoyl)stannane
(1) and more importantly we demonstrate its suitability as
photoinitiating species in visible light free radical polymerization,
which is in line with a recent patent publication.[7] We show, that 1
could be the first of a new, promising generation of photoinitiators.
(
reveals outstanding properties, compared to state of the art
acylgermane-based initiators. Most importantly the initiator shows
absorption up to 550 nm, which allows higher penetration depths,
especially in highly filled photopolymers. Besides that, 1 shows
extremely high photoinitiating activity towards (meth)acrylic double
bonds, as well as very fast photobleaching. Furthermore, unlike many
organotin compounds, 1 shows surprisingly low cytotoxicity.
The new photoinitiator 1 was synthesized based on
procedures known from the analogue Si- and Ge-compounds
(Scheme 1).[5,
8]
Using the method of Marschner,[8b]
Visible light-induced photopolymerization has a wide field of
application, such as printing inks and different kinds of coatings.
Recently, its scope was further expanded by more advanced
techniques like dental curing[1] or lithography-based ceramic
manufacturing.[2] This evolution was facilitated by the
tris(trimethylsilyl)stannyl potassium could be obtained reacting
tetrakis(trimethylsilyl)stannane with potassium tert. butoxide. The
potassium species was then further reacted with 4 eq. of 2,4,6-
trimethylbenzoyl fluoride to achieve the desired product 1 in high
purity (58% yield).
[
3]
development of highly reactive acylgermane-initiators, such as
bis(4-methoxybenzoyl)diethylgermane (Ivocerin®, 2). These
photoinitiators are used to initiate radical polymerization of a large
variety of monomers upon radiation in the visible range with
wavelengths up to 490 nm. A working Type I system, which can
be cleaved using radiation with wavelengths above 500 nm and
in addition to that shows sufficient photobleaching has not been
previously reported. To achieve a bathochromic shift of the nπ*
absorption band and therefore go from classical UV Type I
photoinitiators (e.g. hydroxyalkylphenones) to systems working
under visible light irradiation, a heteroatom has to be introduced
next to the benzoyl chromophore (e.g. acylphosphine oxides,[
acylsilanes,[5] acylgermanes ). The absorption band of the nπ*
Scheme 1. Synthesis of tetrakis(2,4,6-trimethylbenzoyl)stannane (1)
4]
[3]
Tetrakis(2,4,6-trimethylbenzoyl)stannane
(1)
was
then
investigated regarding to its photochemical properties. For that
purpose, the commercial state of the art initiator Ivocerin® (2),
tetrakis(2-methylbenzoyl)germane (3) as well as Irgacure® 784[9]
[
a]
Dipl.-Ing. M. Mitterbauer, Dr. P. Knaack, Prof. Dr. R. Liska
Institute of Applied Synthetic Chemistry, TU Wien
Getreidemarkt 9, 1060 Vienna, Austria
(
4) were used as reference compounds for comparison
E-mail: robert.liska@tuwien.ac.at
(Scheme 2).
[
[
[
b]
c]
d]
Dipl.-Ing. M. Mitterbauer, Dr. P. Knaack, Prof. Dr. R. Liska
Christian Doppler Research Association
Boltzmanngasse 20, 1090 Vienna, Austria
Dr. S. Naumov
Chemical Department, Leibniz Institute of Surface Modification
Permoserstrasse 15, 04318 Leipzig, Germany
Dr. M. Markovic, Associate Prof. Dr. A. Ovsianikov
Institute of Materials Science and Technology, TU Wien
Getreidemarkt 9/308, 1060 Vienna, Austria
Dr. M. Markovic, Associate Prof. Dr. A. Ovsianikov
Austrian Cluster for Tissue Regeneration, Austria
Prof. Dr. N. Moszner
[
e]
f]
[
Scheme
2.
Reference
systems:
Ivocerin®
(2),
tetrakis(2-
Ivoclar Vivadent AG
Bendererstraße 2, 9494 Schaan, Liechtenstein
methylbenzoyl)germane (3) and Irgacure® 784 (4)
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
wavelengths for compounds 1, 2 and 3. After the absorption of
photons with a wavelength of 460 nm (2.70 eV), the first excited
1
1
singlet state S will be populated. The effective population of S
should proceed more effectively for PIs 1 and 4 due to larger
0
S - S1 transition energies than for compounds
2
and 3.
Furthermore, due to the fact, that the absorption bands of
titanocene 4 are considerably red shifted compared to the ones
1
of the other PIs, the population of S is also possible by excitation
with 522 nm (2.38 eV). After intersystem crossing (ISC) the triplet
1
1
state T * will be formed. This T * state undergoes further
relaxation through adjustment of the molecular structure to the
changes of electron distributions upon excitation. The calculations
show, that for compounds 1, 2 and 3 the first excited states S1
and T1 are formed mostly through excitations of oxygen n
electrons into the π* orbital localized on the carbonyl double bond.
These n and π* electrons are oriented perpendicular to each other
Figure 1. a) UV-VIS spectra for compounds 1, 2, 3 and 4 and b) enlarged
tailouts of nπ* transition bands
UV-VIS spectroscopy (c = 1 x 10-3 mol L-1 of 1, 2, 3 and 4 in
acetonitrile) showed, that the nπ* absorption band lies within the
visible range for all 3 compounds (Figure 1). However, for the tin-
based compound 1 the absorption band tails out at particularly
high wavelengths. It reaches out to ~550 nm, which is, when
compared to 2 (~480 nm) and 3 (~500 nm), a bathochromic shift
of at least 50 nm. This is probably caused by the orbital
interactions of the Sn d orbitals with the π* orbitals of the carbonyl
group. This specific long wavelength absorption makes 1 unique
in terms of classical Type I photoinitiators.
0
1
resulting in the low oscillator strength of the S -S transition. In the
0
1
case of compound 4 an even lower energy of the S -S transition
can be found due to excitation of π electrons from the aromatic
ring into the titanium d orbital. In the cases of compounds 2, 3 and
4
the energy of triplet relaxation is sufficient to undergo exergonic
bond scission reactions leading to the formation of reactive
radicals, which could initiate polymerizations. Moreover, for the
1
new acylstannane 1 the unrelaxed triplet T * undergoes Sn-C
bond scission already during triplet optimization, indicating a very
effective formation of reactive species.
The analysis of the formation of the reactive excited triplet
state T for compounds 1, 2, 3 and 4 and the calculated UV-VIS
1
spectra are shown below (Figure 2). The calculated spectra
For many applications of photopolymerization in the visible
range, the final material should not show any coloration after
curing. Due to the photocleavage of the Sn-CO bonds,
(
acetonitrile) are in good agreement with the experimental ones,
though the appearing bands are slightly shifted to shorter
Figure 2. Calculated UV-VIS spectra and Jablonski energy scheme of the population of the reactive triplet by excitation with 460 nm and 522 nm of (a) acylstannane
, (b) Ivocerin® 2, (c) tetrakis(2-methylbenzoyl)germane 3 and (d) titanocene 4 by two-photon excitation of studied TPA PIs (calculated in acetonitrile at the TDM06-
D3/LACVP(d,p)/PBE level of theory); f-oscillator strength
1
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Figure 3. Steady state photolysis of PIs in acetonitrile (c
under argon with 460 nm LED for (a) acylstannane 1, (b) Ivocerin 2, (c)
tetrakis(2-methylbenzoyl)germane 3 and (d) titanocene 4
0
= 1 x 10-3 mol L-1)
Figure 4. Photo-DSC conversion curves for PIs 1-4 in methacrylates using (a)
a 460 nm LED and (b) a 522 nm LED as well as in HDDA using (c) a 460 nm
LED and (d) a 522 nm LED
®
acylstannanes are expected to show fast photobleaching, as
acylgermanes do as well. To investigate the photobleaching
behavior of 1, steady state photolysis was carried out. The results
(equimolar amount of 1 used) and 4 (2 times the equimolar
amount of 1 used) in the same monomers were prepared as well.
The amounts of PI used were those related to the amount of
cleavable groups present in the structures. The conversion curves
for the experiments with the 460 nm LED show (Figure 4a,c), that
the activity of tetrakis(2,4,6-trimethylbenzoyl)stannane (1) to
initiate free radical polymerization is extremely high in both
monomer systems. Looking at the achieved results, the values
are within the range of the reference compounds 2 and 3. As
expected, 4 does not show high initiating activity upon curing at
460 nm. However, the experiments using the 522 nm LED
showed, that at that wavelength the acylstannane 1 still shows
high reactivity, while for 2 and 3 no photopolymerization occurred
(Figure 4b,c). Comparing the achieved results of 1 with those of
4, the reactivities of 1 and 4 are quite similar, however, in HDDA
the acylstannane 1 shows a higher final DBC. For PI 1 the lower
conversions reached upon irradiation with 522 nm compared to
those achieved upon irradiation with 460 nm can on one hand be
explained by the lower light intensity and the lower extinction
coefficient of 1 at 522 nm. Furthermore, it can be assumed, that
upon irradiation with 522 nm only for the tetrakis- (and maybe the
subsequently formed trisacyl) species the nπ* absorption tailout
is sufficient to cause cleavage during polymerization. The
remaining bis- and monoacyl species may not further cleave at
were then compared to those of 2, 3 and 4. Each PI was dissolved
-1
in acetonitrile (c
0
= 1 x 10-3 mol L ) and the solutions were then
irradiated with a LED (460 nm). Samples were taken in defined
intervals and UV-VIS spectra were recorded (Figure 3). As the
achieved data show, 1 showed very fast photobleaching. After 3
minutes, the nπ* transition band of 1 had disappeared. For 2 and
3, substantially longer radiation times (15 minutes) were needed
to reach the same point. Compound showed fast
4
photobleaching in solution as well, however its photoproducts
show absorption up to 650 nm. From the photolysis experiments,
the rate of photoinitiator decomposition R
d
as well as the quantum
= 0,90
yield of decomposition Φ were calculated for PIs 1-4 (Φ
d
d
for acylstannane 1) and arranged within a table in the supporting
information of this article.
To explore the photoinitiating activity of 1, photo-DSC
experiments were carried out in dimethacrylates as well as in
diacrylate formulations. This technique can be used to determine
the time until heat flow maximum is reached (tmax) and the overall
double bond conversion (DBC) as well as parameters like the time
until 95% of the overall heat flow is reached (t95) and the rate of
p
polymerization (R ). Curing was carried out using two different
5
22 nm, which is also indicated by slightly yellow coloration of the
polymeric samples (compared to colorless for 460 nm) directly
after curing. The collected values for tmax, DBC, t95 and R of all
LEDs (460 and 522 nm). The light intensities were 1 W cm-2 (460
nm) and 0.42 W cm-2 (522 nm). The prepared formulations
contained 0.1wt% of 1 either in a 1:1 mixture of urethane
p
experiments are given within the supporting information of this
article.
3
dimethacrylate (UDMA) and decanediol dimethacrylate (D MA) or
in hexanediol diacrylate (HDDA). As stabilizer, 500 ppm of
butylated hydroxytoluene (BHT) were added. For comparison,
formulations of 2 (2 times the equimolar amount of 1 used), 3
To demonstrate the potential advantages of 1 in industrial
applications such as additive manufacturing, penetration depth
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shown below (Figure 5a,b). The formulations were then cured for
2
40 s each and the resulting curing depths were then measured
®
from the obtained polymeric samples. In the case of Ivocerin (2)
a 460 nm LED was used for curing, however, for the sample
containing tetrakis(2,4,6-trimethylbenzoyl)stannane (1) a 522 nm
LED was utilized. The light intensities were adjusted to be 33.5
mW cm-2 behind the glass plate for both light sources. After curing
the samples were taken out from the mold and unreacted
formulation was washed away. The height of the resulting
polymeric samples (Figure 6) was then measured at the centre.
The sample, which contained 1 and was polymerized using the
5
22 nm LED showed a much higher curing depth (more than
Figure 5. Setup used for curing depth (a, b) and green laser curing experiments
double), proving the higher penetration depth and demonstrating
potential advantages of the acylstannane 1 in many applications.
(c)
To further test the novel photoinitiator 1 on its ability to initiate
polymerization particularly at wavelengths above 500 nm,
experiments using a green laser (532 nm) were carried out. A
similar setup to the one shown above (Figure 5c) was used,
except in this case the LEDs were replaced by a commercial
green laser pointer. A large amount of samples was polymerized
using this setup, varying the PI concentration, the filler content
(Schott® dental glass) as well as the curing time (Figure 7a-d).
Figure 6. Curing depth experiments: Resulting polymer after curing of
formulation containing (a) 0.1wt% of acylstannane 1 and 50wt% of ZrO
HDDA with a 522 nm LED and (b) a formulation containing 0.1wt% of Ivocerin
and 50wt% of ZrO in HDDA with a 460 nm LED at the same light intensity
2
in
®
2
2
The samples containing
1 show remarkable results. The
absorption of tetrakis(2,4,6-trimethylbenzoyl)stannane (1) at 532
nm is still high enough to cause cleavage upon radiation at this
wavelength. Even formulations with high filler contents (up to
70%) could be polymerized using the given setup. Furthermore,
the PI concentration could be lowered significantly, but still giving
satisfying results. The studies regarding the curing time further
showed, that with higher curing times, the achieved curing depth
could be increased (Figure 7c,d). Although with formulations
®
containing Irgacure 784 (4) slightly higher curing depths could be
achieved, the samples containing the acylstannane 1 revealed
very fast photobleaching in the crosslinked polymer (Figure 7a-c),
which is desired in many industrial applications such as e.g.
dental materials. The samples containing titanocene 4 did not
show any photobleaching (Figure 7d). The collected values for all
green laser curing experiments are given within the supporting
information of this article.
Since many organotin compounds show considerable
cytotoxicity, cytocompatibility evaluation was carried out for the
photoinitiators 1-4. For cytocompatibility testing, L929-mCherry
cells were exposed to different concentrations of initiators 1-4 and
therefore incubated with different dilutions of PI in
dimethylsulfoxide (DMSO; 0.1, 0.05, 0.025, 0.0125, 0.00625 and
Figure 7. Green laser curing experiments in UDMA:D
PI concentration (0.01 – 0.1wt% of 1, 10wt% of filler); b) variation of filler content
0 – 70wt% of filler, 0.1wt% of 1); c) variation of curing time (60 – 500 s, 0.1wt%
of 1, 50wt% of filler); d) variation of curing time (60 – 500 s, 0.1wt% of 4, 50wt%
of filler)
3
MA (1:1): a) variation of
(
0.003125 mM). Besides the evaluation of the cytotoxicity of the
experiments were carried out. Especially in recently developed
processes like lithography-based ceramic manufacturing[2] this is
of great importance. The big advantage of using light with longer
wavelengths is the potentially higher penetration depth of the light
into the material, which should lead to higher curing depths as
well. To investigate this subject, a cylindrical mold was placed on
a glass plate and filled with a formulation containing hexanediol
photoinitiator itself, also the cytocompatibility of the photolysis
products upon exposure to UV light (365 nm, 10 min) was
examined. For PI concentrations of 0.1 mM, the corresponding
amount of DMSO present (1%) is high enough to cause severe
cell death (cf. Supporting Information). However, at a PI
concentration of 0.05 mM cell survival in the DMSO control lies at
around 80%. At that concentration for all four initiators the
metabolic activity of the cells, (irradiated and not irradiated with
UV) is decreased drastically to less than 60%, where the cells
exposed to the acylstannane 1 showed the highest survival rate.
At PI concentrations of 0.025 mM only the cells exposed to
diacrylate (HDDA), 50wt% of inorganic filler (ZrO
as well as either 0.1wt% of
trimethylbenzoyl)stannane (1) or Ivocerin (2). As stabilizer, 500
2
, Lithoz GmbH)
tetrakis(2,4,6-
®
ppm of butylated hydroxytoluene (BHT) were added. The setup is
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COMMUNICATION
initiators 1 and 4 showed high metabolic activities at around 80%, Acknowledgements
however, the acylgermane 3 only showed 36% (no UV irradiation).
For the UV illuminated samples at the same concentration, the
metabolic activity was above 65% for all initiators (70% for
acylstannane 1). At lower concentrations the metabolic activity of
the cells in almost all samples was higher than 80%. The only
exception was the cells exposed to 0.0125 mM of initiator 3 (no
UV curing), for which the metabolic activity was decreased to 70%.
Summarizing the results, the new Sn-based initiator 1 surprisingly
showed higher cytocompatibility than the structurally similar
germanium compound 3 and the obtained values generally lie in
the same range as the ones achieved for compounds 2 and 4.
Furthermore, the photolysis products of 1 (upon radiation with UV)
reveal high cytocompatibility as well. All cytocompatibility data
including cell microscopy are given within the supporting
information of this article.
Financial support by the Christian Doppler Society and Ivoclar
Vivadent is kindly acknowledged.
Keywords: Photoinitiator • Long wavelength •
Photopolymerization • Visible light • Photobleaching
[
1]
a) N. Moszner, U. Salz, U. K. Fischer, R. Liska, H. Gruber, B.
Ganster, Ivoclar Vivadent A.-G., Liechtenstein . 2008, p. 17 pp,
US20080076847A1 b) N. Moszner, V. M. Rheinberger, U. Salz, H.
Gruber, R. Liska, B. Ganster, G. Ullrich, Ivoclar Vivadent A.-G.,
Liechtenstein . 2007, p. 14 pp, US20070027229A1.
[
[
2]
3]
M. Schwentenwein, J. Homa, Int. J. Appl. Ceram. Technol. 2015,
12, 1-7.
a) B. Ganster, U. K. Fischer, N. Moszner, R. Liska,
Macromolecules 2008, 41, 2394-2400; b) D. Neshchadin, A.
Rosspeintner, M. Griesser, B. Lang, S. Mosquera-Vazquez, E.
Vauthey, V. Gorelik, R. Liska, C. Hametner, B. Ganster, R. Saf, N.
Moszner, G. Gescheidt, J. Am. Chem. Soc. 2013, 135, 17314-
In conclusion, a novel stannane-based photoinitiator for
radical polymerizations in the visible range, particularly at
wavelengths above 500 nm was presented. It showed high
reactivity compared to current state of the art PIs in methacrylate-
and acrylate formulations. Unlike known germane-based
compounds, tetrakis(2,4,6-trimethylbenzoyl)stannane (1) was
able to initiate polymerization upon green light radiation (522 nm
LED, 532 nm laser) leading to higher curing depths in filled
systems, which is desirable for many industrial applications. The
acylstannane 1 further showed very fast photobleaching in
solution as well as within the polymer network. In contrast to many
organotin compounds, compound 1 and its photoproducts
showed surprisingly low cytotoxicity, especially when compared
to similar germanium compounds.
1
7321; c) N. Moszner, U. K. Fischer, B. Ganster, R. Liska, V.
Rheinberger, Dent. Mater. 2008, 24, 901-907.
[
4]
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Ganster, U. Salz, N. Moszner, R. Liska, J. Polym. Sci., Part A:
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[
5]
M. Mitterbauer, M. Haas, H. Stueger, N. Moszner, R. Liska,
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[
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9]
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2012.
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COMMUNICATION
COMMUNICATION
Within this work, a novel
acylstannane-based visible light
photoinitiator is presented.
Moritz Mitterbauer, Patrick Knaack,
Sergej Naumov, Marica Markovic,
Aleksandr Ovsianikov, Norbert
Moszner and Robert Liska*
Tetrakis(2,4,6-
trimethylbenzoyl)stannane shows
absorption up to 550 nm, which
allows higher penetration depths,
especially in highly filled
photopolymers. Besides that, it
shows extremely high photoinitiating
activity towards (meth)acrylic double
bonds, very fast photobleaching as
well as surprisingly low cytotoxicity.
Page No. – Page No.
Acylstannanes - A New Generation
of Cleavable and Highly Reactive
Photoinitiators for Curing at
Wavelengths Above 500 nm with
Excellent Photobleaching
Behavior
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