Initiators based on benzaldoximes: Bimolecular and covalently bound systems

Article abstract of DOI:10.1021/ma301959n

Typical bimolecular photoinitiators (PIs) for radical polymerization of acrylates show only poor photoreactivity because of the undesired effect of back electron transfer. To overcome this limitation, PIs consisting of a benzaldoxime ester and various sensitizers based on aromatic ketones were introduced. The core of the photoinduced reactivity was established by laser flash photolysis, photo-CIDNP, and EPR experiments at short time scales. According to these results, the primarily formed iminyl radicals are not particularly active. The polymerization is predominantly initiated by C-centered radicals. Photo-DSC experiments show reactivities comparable to that of classical monomolecular type I PIs like Darocur 1173.

Full text of DOI:10.1021/ma301959n

Article
pubs.acs.org/Macromolecules
Initiators Based on Benzaldoximes: Bimolecular and Covalently
Bound Systems
Markus Griesser,† Arnulf Rosspeintner,†,⊥ Claudia Dworak,‡ Michael Hofer,‡ Gottfried Grabner,§
̈
Robert Liska,‡ and Georg Gescheidt*,†
†Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria
‡Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163/MC, 1060 Vienna, Austria
§Max F. Perutz Laboratories, University of Vienna, Campus-Vienna-Biocenter 5, 1030 Vienna, Austria
S
* Supporting Information
ABSTRACT: Typical bimolecular photoinitiators (PIs) for
radical polymerization of acrylates show only poor photo-
reactivity because of the undesired effect of back electron
transfer. To overcome this limitation, PIs consisting of a
benzaldoxime ester and various sensitizers based on aromatic
ketones were introduced. The core of the photoinduced
reactivity was established by laser flash photolysis, photo-
CIDNP, and EPR experiments at short time scales. According
to these results, the primarily formed iminyl radicals are not
particularly active. The polymerization is predominantly initiated by C-centered radicals. Photo-DSC experiments show
reactivities comparable to that of classical monomolecular type I PIs like Darocur 1173.
decarboxylation step delivers enough CO2 to displace O2 from
the curing material.9,10 Recently, a very efficient class of PIs has
been created by covalently linking benzophenone and N-
phenylglycine, thus keeping the co-initiator in close prox-
imity.8,11
INTRODUCTION
■
Photocuring is the key technique for the preparation of films
and coatings as it offers a broad and economic application
spectrum for industry.1 During the curing process, the
photoinitiator (PI) plays the decisive role. It absorbs energy
from a photon either in a direct or an indirect process,
transferring it into chemical energy. After the excitation process
a reactive radical can be formed, which is able to induce the
polymerization of a wide range of monomers. Among the
bimolecular type II PIs for radical photopolymerization,
excitable chromophores like benzophenone in combination
with tertiary amines as co-initiators are commonly applied.2
The ketone−amine interactions proved to be highly efficient
concerning radical formation due to electron transfer. The
efficiency of the photochemical process depends on the rate
constant of electron and proton transfer as well as the reactivity
of the α-amino alkyl radical toward reactive double bonds and
quenching by side reactions.3 A wide range of co-initiators have
been investigated such as different aliphatic and aromatic
amines.4,5 Thiols6 and Si−H groups7 are also described as
efficient co-initiators in the literature, but only limited storage
stability is given for such formulations.
Also, oxime esters might produce CO2 after cleavage of the
N−O bond and decarboxylation of the acyloxy radical.
Accordingly, this type of functional group in combination
with photosensitizers was considered as an alternative concept
for type II initiator systems. Several investigations on the
photolysis of O-acyl oximes in the presence of photosensitizers
can be found in the literature.12,13 Yoshida et al.14 described the
nature of the triplet states and the subsequent photochemistry
of aromatic O-acyl oximes. They postulated a requirement of
close triplet state energies of the oximes (ET = 289−305 kJ
mol−1) and their ketone-based sensitizers, displaying a π−π*
character. The excitation energies are dissipated by cleavage of
the N−O bond. Moreover, the photolysis of aldoxime esters in
presence of a sensitizer like 4-methoxyacetophenone was
investigated by McCarroll and Walton, performing EPR
measurements and radical trapping experiments, thus present-
ing a new class of radical precursors for spectroscopic studies
(Scheme 1).15
Unfortunately, the efficiency of this system is usually reduced
by back electron transfer (BET), the solvent cage effect, and
limited diffusion capability in highly viscous formulations or
water-based systems.8 Molecular oxygen from the atmosphere
easily inhibits the polymerization process. Consequently, PI
systems, which are unperturbed by oxygen are preferred
because it is unfavorable to work under a nitrogen atmosphere.
Using N-phenylglycine as co-initiator for benzophenone, the
Furthermore, such sensitized O-acyloximes were tested in a
patent,16 but unfortunately no exact data on their performance
was given. Structure related α-keto-O-acyloximes17 have found
Received: September 19, 2012
Revised: October 8, 2012
Published: November 1, 2012
© 2012 American Chemical Society
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when added0.15 M in butyl acrylate. The EPR spectra were
simulated using the Easyspin package21 for MATLAB (version 3.1.7).
The hyperfine coupling constants (hfcs) of the free radicals were
calculated using the Gaussian03 package.22 All calculations (geometry
optimizations and single point calculations) were conducted at the
B3LYP23,24 level of theory with the basis set TZVP.25
Scheme 1. Photochemistry of (Sensitized) Aldoximes
Photo-CIDNP experiments were performed on a 200 MHz Bruker
AVANCE DPX spectrometer. Irradiation was carried out using a
frequency-tripled Spectra-Physics Nd:YAG INDI laser (355 nm, ca. 40
mJ/pulse, ca. 10 ns). The following pulse sequence was used:
presaturation−laser flash−30° RF detection pulse (2.2 μs)−free
induction decay. The concentrations of the initiators were typically
0.01 M in d3-acetonitrile or d6-benzene, deaerated by bubbling argon
through the solution.
DSC photocuring experiments were carried out in HDDA with 0.12
μM of each component (∼2 wt % of 1) under a nitrogen atmosphere,
using an EXFO Omnicure 2001 UV lamp with a 200−500 nm filter
and a Netzsch DSC 204 F1 Phoenix with autosampler.
specialized industrial applications, such as color filter resists,
however, thermal stability of such compounds is always
crucial.18 Very recently, we have investigated some benzaldox-
ime esters in combination with 4-methylbenzophenone as
sensitizer and have found surprisingly high photoreactivity in
photo-DSC experiments.19
Therefore, it was of interest to investigate the initial reactions
of this photoinitiator system in more detail by laser flash
photolysis experiments, photo-CIDNP, and EPR measure-
ments. Photo-DSC experiments allow the comparison of these
new systems with established photoinitiators.
RESULTS
■
Laser Flash Photolysis. Nanosecond transient absorption
spectroscopy of solutions of benzophenone and the co-
initiating O-acyloximes 1 and 2 as well as solutions of the
covalently bound O-acyloxime 4 was used to gain information
on the early photochemical steps. In case of the co-initiating
oximes, the experiments were performed in deaerated MeCN in
equimolar (5 × 10−3 and 1 × 10−2 M) and different
concentrations of sensitizer BP (1 × 10−2 M) and oxime (1
× 10−3 M), respectively. The irradiation wavelength (355 nm)
exclusively excites benzophenone chromophores into the n, π*
state.26
Figure 1. Benzaldoxime esters and the employed sensitizer.
Upon irradiation of solutions BP/1 and BP/2, as expected,
the first transient appearing at the nanosecond time scale was
the characteristic benzophenone triplet−triplet absorption
around 520 nm. At a longer time scale, several additional
absorption bands emerged. However, the microsecond-range
lifetimes of these transients suggested that they originated from
radical-type precursors. The degradation kinetics of the
benzophenone triplet (BPT) in the conducted experiments
under variation of sensitizer and co-initiating oxime concen-
trations gave no indication of ground-state complex formation.
Regarding transients originating from BPT, no traces of
benzophenone ketyl radicals or radical anions could be
detected. Because of the lack of product species originating
from H abstraction reactions or electron transfer processes, the
sensitization presumably proceeds via an efficient energy
transfer.14,27
In contrast to the co-initiating systems BP/1 and BP/2, no
local triplet state of the benzophenone moiety was detectable in
the photolysis of covalently bound benzophenone−O-acylox-
ime 4. Here the sensitization process is successfully shifted from
a diffusion-controlled, bimolecular process to an intramolecular
energy transfer due to the spatial arrangement of the
chromophore system and the oxime moiety. As a result of
this fundamental change in the sensitization mechanism, the
triplet lifetime is shortened to an extent (τ < 5 ns) that is no
longer directly assessable by the time resolution of our laser
flash photolysis setup. Nevertheless, other transient absorption
bands on a longer time scale remained completely identical.
In regard to possible primary cleavage products in the
photolysis of the co-initiating O-acyloxime 1, a transient
subsequent to triplet states on the nanosecond time scale
obviously originates from the triplet BPT. The visible spectrum
of this transient (6) exhibited a slow but continuous progress of
EXPERIMENTAL SECTION
■
All reagents were purchased from Sigma-Aldrich and were used
without further purification. HDDA and 2-hydroxy-2-methyl-1-phenyl-
1-propanone (Darocur 1173; D1173) were received as a gift from
Ivoclar Vivadent and BASF, respectively. The solvents were dried and
purified by standard laboratory methods. The synthesis of benzaldox-
ime esters 1−4 has recently been published.19
Nanosecond transient absorption spectroscopy was carried out
using the third harmonic (355 nm) of a Q-switched Nd:YAG laser
(Spectra-Physics LAB-150) with a pulse duration of 8 ns. Transient
absorbances were measured in a right-angle setup using a cell holder
with incorporated rectangular apertures defining a reaction volume of
dimensions 0.17 cm (height), 0.32 cm (width), and 0.13 cm (depth)
within the cell. Pulse energies between 0.1 and 4 mJ/pulse were used,
the typical value for the measurement of transient spectra being 2 mJ/
pulse. Pulse energies were measured using a ballistic calorimeter
(Raycon-WEC 730). Solutions were deoxygenated by bubbling with
argon. Further details of experimental procedures have been published
previously.20
Time-resolved continuous-wave electron paramagnetic resonance
(CW TR-EPR) experiments were performed using a frequency-tripled
Nd:YAG laser (Continuum Surelite II, 20 Hz repetition rate; 355 nm;
ca. 10 mJ/pulse; ca. 10 ns), a Bruker ESP 300E X-band spectrometer
(unmodulated static magnetic field), and a LeCroy 9400 dual 125
MHz digital oscilloscope. The TR-EPR spectrum is obtained by
scanning the desired magnetic field range recording the accumulated
(usually 50−100 accumulations) EPR time responses to the incident
laser pulses at a given static magnetic field. The system is controlled
using a program developed, kindly provided and maintained by Dr. J.
T. Toerring (Berlin, Germany). Argon-saturated solutions were
pumped through a quartz tube (inner diameter 2 mm, flow ca. 2−3
mL/min) in the rectangular cavity of the EPR spectrometer. The
samples were 10 mM in benzaldoxime and benzophenone, while
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absorption in minor intensity from below 550 nm up to and
above 800 nm, resembling the absorption of the benzoyloxyl
radical obtained by photolysis of dibenzoyl peroxide.28 In case
of the oxime 2, which should produce a less stable alkoyloxyl
radical, an analogue transient to 6 could not be detected. The
degradation of the triplet BPT at 530 nm and the buildup of the
absorption of 6 at 830 nm in the photolysis of O-acyloxime 1
are illustrated in Figure 2. The kinetic congruence between
both traces is a strong indication that the benzophenone triplet
BPT is the precursor of the benzoyloxyl radical 6.
absorption spectroscopy, the covalently bound O-acyloxime 4
yields a transient, which is completely congruent to the
benzoyloxyl radical 6 but shows accelerated buildup kinetics.
Apart from the differences in sensitization, 4 follows an
analogue photodegradation chain as 1. However, in comparison
to the co-initiating oxime 1, the product ε830 × Φ(6) is
approximately halved. Thus, the spatial arrangement of the
benzophenone and oxime moieties in the structure of 4 causes
a reduction in radical quantum yield in comparison to the co-
initiating system.
It must be assumed that the benzoyloxyl radical 6 is
produced by a homolytic scission of the N−O bond upon
photolysis of 4 and 1. Consequently, this type of photo-
degradation should result in the formation of an iminyl radical.
Nevertheless, no transient absorption in the measured
wavelength range (300−840 nm) could be assigned to a
primary formed iminyl radical. Since iminyl radicals are rarely
reported in the literature as transients in laser flash photolysis, it
can be reasonably argued that this species shows no or an
insufficient absorbance for a detection in the applied analytical
setting.
Figure 2. Temporal absorbance change of BPT at 530 nm and 6 at 830
nm upon 355 nm laser flash photolysis of an equimolar solution of
benzophenone and the O-acyloxime 1 in MeCN (1 × 10−2 M).
Apart from primary photolysis products, secondary radicals
appear as subsequent transients at the nanosecond time scale in
the photolysis of all O-acyloxime PIs. These transients possess
absorptions between 350 and 500 nm and resemble adducts of
benzoyloxyl radicals to aromatic rings as described in the
literature.28,31 The degradation of the benzoyloxyl radical 6 at
830 nm and the buildup of one of these transients (T1add) at
390 nm in the photolysis of O-acyloxime 1 are illustrated in
Figure 4. The kinetic resemblance between the traces is another
strong indication that the benzoyloxyl radical 6 is the precursor
Unfortunately, 6 displays a low extinction coefficient in the
accessible wavelength range, and related values given in the
literature vary significantly.29,30 Consequently, a reliable
estimation of the quantum yield is not possible. Nevertheless,
the yield of the benzoyloxyl radical 6 was determined as the
product ε830 × Φ(6) in an equimolar solution of 1 and
benzophenone in MeCN (1 × 10−2 M). The results are shown
in Figure 3 in the form of the absorbance of BPT at 530 nm and
6 at 830 nm, versus the energy of the laser pulse.
add
of the secondary adduct T1 (see Scheme 2).
add
Figure 4. Temporal absorbance change of BPT and T1 at 390 nm
and 6 at 830 nm upon 355 nm laser flash photolysis of an equimolar
solution of benzophenone and the O-acyloxime 1 in MeCN (1 × 10−2
Figure 3. Absorption of BPT at 530 nm obtained at 20 ns and 6 at 830
nm at 50 ns after 355 nm laser flash photolysis of an equimolar
solution of benzophenone and the O-acyloxime 1 in MeCN (1 × 10−2
M) as a function of laser pulse energy.
M).
The decay of the transient 6, as shown in Figure 4, proceeded
with a first-order rate constant k1 = 4 × 106 s−1. This rate has to
be regarded as the product of a superposition of two well-
known concurrent reactions: the first-order decarboxylation
process after escape from the solvent cage and the pseudo-first-
order addition to aromatic structures of the photolysis solution.
The dependence determined for both the yield of triplet
benzophenone BPT and benzoyloxyl radical 6 is linear. The
value of the product ε830 × Φ(6) in MeCN was found to be
200 M−1 cm−1. Additionally, an important conclusion that can
be drawn from this experiment is that the formation process is
purely monophotonic.
add
The trace of T1 in Figure 4 is partially distorted due to a
spectral overlap by at least a second, similar benzoyloxyl radical
add
adduct T2 which exhibits different degradation kinetics. In
In the photo-DSC experiments19 the co-initiating oximes 1
and 2 displayed a higher activity as PIs compared to the
covalently bound O-acyloxime 4. Therefore, two different
conclusions are possible: PI activity of 4 is reduced due to (1) a
reduction of quantum yield for the cleavage or (2) a less
reactive initiating species produced. In nanosecond transient
contrast to T1add, an adduct transient which resembles T2add was
also detected in the photolysis of O-acyloxime 4. In the case of
the covalently bound O-acyloxime, the congruence between the
degradation of the benzoyloxyl radical and the buildup kinetics
add
of T2 could be more easily studied since no overlap with a
triplet spectrum occurs. In the photolysis of the co-initiating O-
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radicals formed by homolytic N−O bond cleavage of 1 can be
easily identified. The nitrogen-centered radical 5 dominates the
spectrum and is characterized by two triplets centered at 338.2
mT. The benzoyloxyl radical 6 gives rise to a broad peak at
336.8 mT, with a high g value characteristic for oxygen-centered
radicals. Additional signals stemming from phenyl radical 7 in
the center of the spectrum are shown in Figure 6. From the
Scheme 2. Cleavage Mechanism of the BP/1 System and the
Follow-up Transients (Ar = Substituted Aromatic Fragment,
Derived from the Starting Compounds) Identified by LFP
acyloxime 2, which produces an alkoyloxyl radical, the relative
add
add
absorbance of transients comparable to T1
or T2
is
significantly lower. This is the logical consequence of an
acceleration of the concurrent reaction, the decarboxylation of
the less stable alkoyloxyl radical.
Figure 6. Zoom-in on the central part of the TR-EPR spectrum shown
in Figure 5 in the absence of butyl acrylate. For better visibility of the
low intensity radical 7 signals the color map was cut at values far below
the maximum of the radical 6 signal maximum. The blue and green
spectra and time traces are cuts along the field and time axis at values
indicated by the colored dashed lines.
Magnetic Resonance. To obtain insight into the early
1
stages of polymerizations, continuous-wave TR-EPR and H
CIDNP experiments were performed. Both methods yield
complementary results. Via EPR one is able to establish the
radicals formed within the first 50 ns after irradiation,32,33
whereas using CIDNP one can gain information about the
products formed via the primary radical pair.34,35 As the general
model, the system BP/1 was investigated by both TR-EPR and
associated time traces it can be inferred that 6 produces 7 via
decarboxylation within about 500 ns. The extracted hyperfine
coupling constants (hfcs) for 7 are in agreement with published
data37 and calculations (see Figure 8). No signals stemming
from (reduced) BP or related radicals could be identified, again
indicating that BP only acts as a sensitizer.
To study the time profiles of the addition of radicals to
monomers, butyl acrylate (BA) was added to the BP/1 system.
Upon this addition (EPR spectrum in Figure 5, upper panel)
one can immediately see that the time evolution of radicals 5
and 6 is hardly affected. But a change in the signal pattern of
the central part of the spectrum can be clearly discerned
(Figure 7). In the presence of 0.1 M BA, 7 vanishes and gives
rise to a new signal, which can be assigned to the addition
product of phenyl radical 7 to BA, 7-BA. No direct hyperfine
data for the adduct 7-BA could be found in the literature, but
EPR data for the addition of phenyl radicals to methyl
acrylate38 and benzoyl radicals to BA39 as well as theoretical
calculations are in agreement with this assignment.
1
1H CIDNP. Additionally, we present H CIDNP results for
BP/2, BP/3, and 4.
TR-EPR. Upon irradiation of a mixture of BP (10 mM) and
1 (10 mM) in acetonitrile in an EPR spectrometer, employing a
355 nm pulsed light source, the time-resolved EPR spectrum
shown in Figure 5 (lower panel) is recorded. All signals appear
in absorption with similar signal intensities in the low- and
high-field portions of the spectrum, indicating that the radicals
are formed via the triplet mechanism.36 The two primary
Figure 5. TR-EPR spectrum of a solution of 1 (10 mM) and BP (10
mM) in acetonitrile in the absence (lower panel) and presence (upper
panel) of 0.1 M butyl acrylate. The upmost panel represents the
spectrum at the time delay (150 ns) indicated by the dashed red line in
the 3D spectrum, showing the nitrogen-centered radical (and its
simulation) as well as the benzoyloxyl radical.
Figure 7. Zoom-in on the central part of the TR-EPR spectrum shown
in Figure 5 in the presence of butyl acrylate (0.1 M). The 2D spectrum
and time traces are obtained by cuts along the axis at the indicated
colored dashed lines.
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Thus, we were able to establish the primary radicals formed
directly upon photolysis of 1 by TR-EPR. By the addition of BA
it could be shown that there is no reactivity of the nitrogen-
centered iminyl radical 5 toward the monomer in the observed
time scale. Additionally decarboxylation of 6 toward 7 is fast
enough that no addition of 6 to BA could be discerned from
the spectra. The addition of phenyl radicals 7 to BA is fast
enough to repress the EPR signal of 7 (Figure 7).
As the reference experiment, the mixture benzophenone BP/
benzaldoxime benzoate 1 was photolyzed inside the NMR
spectrometer. The NMR spectrum before irradiation (see
Figure 9a) shows a distinct signal for the aldoxime hydrogen at
δ = 8.70 ppm. The rest of the spectrum is made up of two
separated groups in the aromatic region the one with the higher
shift (δ = 8.09−8.15 ppm) stemming from the protons of the
benzoate moiety. The signals of BP as well as the other
aromatic hydrogens of 1 make up the rest of the spectrum.
The polarization effect of CIDNP can occur in absorption
and emission. It is caused by an interaction of the electron and
nuclear spin in the radical pair. Therefore, the determining
factors to the effect are the sign and magnitude of the radicals’
magnetic properties (g value, hyperfine, and J coupling) as well
as the initial spin state and the reaction pathway (“cage” or
“escape” product). The resulting CIDNP signals can easily be
rationalized employing Kaptein’s rules.40
Immediately after irradiation using a single Nd:YAG laser
pulse, a CIDNP spectrum was recorded (see Figure 9b). It can
be assigned as follows: The emissive peak at δ = 8.70 ppm
corresponds to the reformation of parent compound 1. A
second emissive peak can be found at δ = 8.03 ppm, which is
assigned to the Z-isomer 1i of the parent compound (E-
isomer), also generated by in “cage” recombination. The shift
difference of Δδ = 0.67 ppm is characteristic for the two
isomers.41 Of the signals in absorption, benzene 13 at δ = 7.37
ppm has the highest intensity. It is generated by the
decarboxylation of benzoyloxyl radical 6 to phenyl radical 7
and followed by hydrogen transfer from iminyl radical 5, the
most likely hydrogen donor (see Scheme 3). Benzonitril 8, the
byproduct of this reaction, is not distinguishable owing to its
aromatic hydrogens stemming from iminyl radical 5 where they
only had small hfcs, thus leading to low polarizations. The two
absorptive signals at δ = 8.55 and 8.42 ppm can be assigned to
the E- and Z-isomers of N-benzylideneaniline 11 and 12,
respectively. Those are formed from the recombination of
phenyl radical 7 and iminyl radical 5. The E-isomer displays
higher signal intensity because it is the less sterically hindered
Figure 8. Calculated hyperfine coupling constants and g factors
(B3LYP/TZVP).
Table 1. Hyperfine and g Value Data Extracted from TR-
EPR Spectra
radical
g value
hfc/mT
5
6
7
2.0017
2.0103
2.0012
2.0023
2.0025
0.99 (N,1)/8.08 (H,1)
<1
1.74 (H,2)/0.62 (H,2)/0.23 (H,1)
1.743 (H,2)/0.625 (H,2)/0.204 (H,1)
2.04 (H,1)/2.18 (H,2)/0.16 (H,2)
7 (ref 37)
7-BA
CIDNP. Initially experiments irradiating only the benzaldox-
ime esters 1, 2, and 3 were performed. Except for some small
polarized signals of parent compounds, no reaction products
could be discerned, reflecting the necessity of a photosensitizer.
1
1
Figure 9. (a) H NMR spectrum of the mixture BP/1. (b) H CIDNP spectrum obtained immediately after the laser pulse (355 nm).
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Scheme 3. Reaction Products Generated by Photolysis of the Mixture BP/1
1
1
Figure 10. (a) H NMR spectrum of the mixture BP/2. (b) H CIDNP spectrum obtained immediately after the laser pulse (355 nm).
isomer. In addition, the E-isomer is thermodynamically favored
leading to thermal isomerization of the Z-isomer.42 There are
also another few possibilities of recombination, which are
indicated in Scheme 3. The recombination of two phenyl
radicals 7 leads to biphenyl 14 which can be accommodated by
the occurrence of small signals at around δ = 7.4 ppm. The low
intensity is expected because the phenyl ring does not carry a
lot of polarization in addition to 14 being an “escape” product.
An “escape” recombination of two radicals 6 is not expected
because of the fast decarboxylation reaction. An analogous
recombination is likely for iminyl radical 5 leading to azine 9.
Radical 5 can also abstract a hydrogen (most likely from
another 5) and form benzaldimine 10. The CIDNP spectra of
1, 2, and 3 are all expected to give identical follow-up products
via radical 5. This is corroborated by identical peaks at δ = 7.89
and 8.30 ppm in all three spectra. Substances 9 and 10 are
therefore tentatively assigned to those peaks.
Benzaldoxime acetate 2 shows a similar behavior as 1, except
for the differing carbon-centered radical fragment. To avoid a
signal overlap appearing in d3-acetonitrile, the spectra of 2 were
recorded in d6-benzene. Except for solvent shifts, the reaction is
compatible in both solvents. In the NMR spectrum (Figure
10a) of the mixture BP/2 there are only two identifiable singlet
peaks, namely the signal of the aldoxime hydrogen at δ = 7.81
ppm and the singlet of the methyl end group at δ = 1.73 ppm.
The remaining signals stem from the aromatic hydrogens of
both compounds, which are not individually attributable. The
solvent signal is visible at δ = 7.16 ppm.
Upon irradiation of the solution (Nd:YAG laser - single
pulse) there are more signals in the CINDP spectrum (Figure
10b) than in the case of 1. There are signals in emission at δ =
7.81 and 7.21 ppm, which stem from recombination of the
primary radicals 5 and 15 to the parent compound 2 and its
isomer 2i, respectively (Δδ = 0.60 ppm). The acetoyl radical 15
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Scheme 4. Reaction Products Generated by Photolysis of the Mixture BP/2
resulting from triplet cleavage of 2 undergoes decarboxylation
generating methyl radical 16. This radical can then take part in
different recombination reactions. The first one is hydrogen
transfer from radical 5, which leads to methane 19 and can be
found in absorption at δ = 0.15 ppm. As for 1, benzonitrile 8 is
a byproduct but is not clearly distinguishable in the spectrum.
Another absorptive signal in the aliphatic region is found at δ =
0.79 ppm and can be assigned to the “escape” reaction of two
methyl radicals forming ethane 20. Moreover, the recombina-
tion of radicals 5 and 16 leads to the E- and Z-isomer of N-
benzylidenemethanamine, 17 and 18, respectively. The
corresponding signals are two quadruplets at δ = 8.48 and
7.89 ppm as well as two doublets at δ = 3.35 and 3.27 ppm.
Because these signals stem from coupled polarized protons,
they both show the multiplet effect A/E (first absorption, then
emission). This behavior, also described in Kaptein’s rules,40 is
easily visible for the two methyl doublets. For the two iminyl
quadruplets there is also overlying enhanced absorption; thus,
only the latter peaks are visible in emission. The two isomers
leading to polarized signals of parent 3 at δ = 8.56 ppm and of
isomer 3i at δ = 7.93 ppm (Δδ = 0.63 ppm). Also substances 9
and 10 appear in the spectrum as two polarized doublets at δ =
7.89 and 8.30 ppm. The iminyl hydrogens of products 23 and
24 stemming from recombination of radicals 5 and propen-2-yl
radical 22 after decarboxylation can be found at δ = 8.32 and
8.11 ppm, respectively. There are some additional signals visible
in this region, which are probably generated by additional
recombination reactions of 5. All the other fragmentation
products lead to compounds containing one or more double
bonds generating a number of polarized multiplets for vinyl and
aliphatic hydrogens. For those it is impossible to discern the
single components, owing to several overlapping resonances
(see Supporting Information).
The unimolecular initiator 4 shows similar behavior and
1
spectra as the bimolecular type BP/1. The H NMR spectrum
(see Figure 11a) shows one isolated resonance signal at δ =
8.80 ppm stemming from the aldoxime hydrogen. The
4
could be assigned using the J coupling constant, which is
known for the E-isomer (4J = −1.6 Hz).43 Therefore, the signals
at δ = 7.89 and 3.27 ppm belong to 17 (4J = −1.6 Hz) and the
signals at δ = 8.48 and 3.35 ppm to 18 (4J = −2.2 Hz). Two
other signals at δ = 8.31 and 7.82 ppm are assigned to
substances 9 and 10 in analogy to 1 (see above). The slightly
different shift values are caused by the different solvent (d6-
benzene vs d3-acetonitrile).
Benzaldoxime methacrylate 3 shows the same behavior as
esters 1 and 2. But, contrary to the other substances
investigated in this work, 3 features an end group including a
reactive vinyl moiety, causing additional follow-up products and
polarization effects.
The 1H NMR spectrum of BP/3 shows the aldoxime
hydrogen signal at δ = 8.56 ppm, and the protons of the
aromatic ring are concentrated in the region δ = 7.44−7.81
ppm. Further features of the NMR spectrum are two multiplets
at δ = 5.75 and 6.19 ppm assigned to the E- and Z-hydrogen of
the vinyl bond, respectively. Additionally a multiplet stemming
from the methyl group can be seen at δ = 2.01 ppm.
The 1H CIDNP spectrum shows typical features of
recombination reactions of the initial radical pair of 5 and 21,
1
1
Figure 11. (a) H NMR spectrum of 4. (b) H CIDNP spectrum
obtained immediately after the laser pulse (355 nm).
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Scheme 5. Reaction Products Generated by Photolysis of the Covalently Bound Initiator 4
remaining protons of the molecule are attached to aromatic
rings and occur as one block of signals (δ = 7.52−8.15 ppm).
Upon irradiation, the initiator 4 undergoes cleavage of the
N−O bond analogous to 1 (see Laser Flash Photolysis section).
The most prominent signal in the 1H CIDNP spectrum (Figure
11b) is benzene 13 at δ = 7.37 ppm in absorption generated via
decarboxylation of benzoyloxyl radical 6. The recombination
isomer of 4 (emissive signal at δ = 8.80 ppm), 4i, can be found
in emission at δ = 8.15 ppm. The shift difference of Δδ = 0.65
ppm is in accordance with the isomerization of the other three
molecules 1−3. The two remaining absorptive signals are
assigned to the recombination reactions of iminyl radical 27
and benzyl radical 7 after decarboxylation. In the same way as
for the bimolecular system BP/1, the two E- and Z-isomers (31
and 32, respectively) can be found at δ = 8.67 and 8.54 ppm, 31
showing the higher intensity. Furthermore, signals stemming
from biphenyl 14 can be seen at low intensity at the base of the
benzene signal. There are also products 28−30, analogous to
8−10 formed from the iminyl radical 5 stemming from 1. The
4-cyanobenzophenone 28 did not show polarization in the
CIDNP spectrum, but the peaks at δ = 8.05 and 8.42 ppm are
tentatively assigned to 29 and 30. The reaction mechanism can
be found in Scheme 5.
To study the reactivity of the generated radicals toward
monomers, CIDNP experiments were performed in the
presence of two acrylates, butyl acrylate (BA) and 3,3-
dimethyl-2-methylenebutanoate (t-BAM), analogous to the
TR-EPR measurements. The bulky tert-butyl substituent of t-
BAM hampers the polymerization process and thus does not
lead to a propagating polymer chain. This is desired, because it
reduces the number of follow-up reactions and therefore the
influence of the growing chain on the studied reactions can be
followed. The molar ratio of initiator to monomer was in the
range of 1:5−1:15.
It can be seen in Figure 12 that the addition of the monomer
BA leads to an easily identifiable effect in the CIDNP spectrum.
The benzene signal intensity is reduced by about 60%. A similar
reduction of intensity can be seen for the two products 11 and
12, which are recombination products formed after decarbox-
ylation. This decrease also correlates with the initiator/
monomer ratio. In contrast, the two emission peaks of the
Figure 12. Zoom-in of the 1H CINDP spectrum of the initiator system
BP/1 and the same with the addition of butyl acrylate (spectrum
horizontally shifted).
parent compound and its isomer are not affected by the
addition of quencher (the monomer). This reveals that the
formation of “cage” products is hardly influenced by the
polymerization process. The clearly diminished intensity of the
peaks corresponding to “escape” products 11−13 shows that
phenyl radical 7 adds to the acrylates. Consequently all
products formed at longer time scales from 7 are outcompeted
by the monomer, which is present at much higher
concentration compared to photoinitiator radicals. There was
no effect seen on the two minor peaks at δ = 7.89 and 8.30 ppm
(compare above), which are attributed to compounds 9 and 10.
Therefore, the CIDNP spectra reflect that the iminyl radical is
not reactive toward the acrylate. This behavior of the two
radicals 5 and 6 is in agreement with the TR-EPR results above.
The same experiment performed with t-BAM gave
comparable relative intensities of the different signals. There-
fore, one can expect to generalize the results for all acrylates.
Furthermore, the reaction was performed with styrene as
monomer, but the CIDNP spectra only gave very small signals
compared to noise. This is attributed to the ability of styrene to
act as a quencher of the BP triplet44 while the oxime benzoate
itself is not able to efficiently generate radicals under
irradiation.13,14
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Photoreactivity of 1 in the Presence of Sensitizers.
Preliminary experiments19 have indicated that 4-methyl
benzophenone A is an efficient sensitizer for 1. To establish
the range of suitable sensitizers, A−C in combination with 1
were tested.
radical initiation proceeds via triplet energy transfer from the
sensitizer, followed by cleavage of the oxime N−O bond. In
first instance an iminyl and a carboxylate radical are formed.
The latter radical decarboxylates to give a carbon-centered
radical, which induces polymerization. On the other hand, the
iminyl radical does not contribute to the initiation at the
observed time scale. Cis−trans photoisomerization at the C
N bond of the oxime moiety is observed in the CIDNP
experiments.
In summary, the benzaldoxime-based initiators reveal
efficient systems, which circumvent the complication of “back
electron transfer” in type II systems. The reaction principles
established here indicate pathways for the development of
advanced phototriggers.
ASSOCIATED CONTENT
* Supporting Information
■
S
NMR, CIDNP spectra, and reaction scheme of BP/3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 13. Reference initiator D1173 and sensitizers A−C. The triplet
energies of A−C are given in parentheses (in kJ mol−1).45
AUTHOR INFORMATION
Corresponding Author
*E-mail: g.gescheidt-demner@tugraz.at.
■
Photo-DSC provides insights into the overall performance of
polymerizing systems. Here the parameter tmax, the time needed
to reach the maximum heat of polymerization, comprises effects
which can be traced back to the efficiency of the photoinitiator.
A comparison of tmax values for selected sensitizer/initiator
combinations for the polymerization of HDDA (1,6-hexanediol
diacrylate) is shown in Figure 14. Sensitizer C does not reveal
Present Address
⊥Department of Physical Chemistry, University of Geneva,
Quai Ernest-Ansermet 30, 1211 Geneva, Switzerland.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support by the Austrian Science Fund FWF for this
project, P19769-N14, is gratefully acknowledged.
■
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