Structural effects on the photodissociation of alkoxyamines

Article abstract of DOI:10.1039/c0ob01207f

The search for photosensitive alkoxyamines represents a huge challenge. The key parameters governing the cleavage process remain unknown. The dissociation process of light sensitive alkoxyamines is studied as a function of their chemical structures. The p

Full text of DOI:10.1039/c0ob01207f

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PAPER
Structural effects on the photodissociation of alkoxyamines†‡
Davy-Louis Versace,a Yohann Guillaneuf,b Denis Bertin,b Jean Pierre Fouassier,a Jacques Laleve´e*a and
Didier Gigmes*b
Received 19th December 2010, Accepted 2nd February 2011
DOI: 10.1039/c0ob01207f
The search for photosensitive alkoxyamines represents a huge challenge. The key parameters governing
the cleavage process remain unknown. The dissociation process of light sensitive alkoxyamines is
studied as a function of their chemical structures. The photochemical properties of 6 selected
compounds are investigated by ESR and laser flash photolysis. It is found that (i) the selectivity of the
cleavable N–O vs. C–O bond and (ii) the efficiency of the nitroxide formation are strongly related to the
alkoxyamine structure. The distance between the chromophore and the aminoxy group is a key
parameter for an efficient pathway of the radical generation as displayed by the photopolymerization
ability of these alkoxyamines.
obviously introduced on the alkoxyamine structure as classical
alkoxyamines used in thermal processes usually exhibit light
Introduction
Alkoxyamines are characterized by a widespread use in different
absorption at short wavelengths (l < 250 nm) which are also
absorbed by the solvent or the vessel.7
fields i.e. in organic synthesis, ref. 1 and references therein,
the design of fluorescence probes2 or in the development of
controlled polymerization processes such as the Nitroxide Me-
diated Polymerization (NMP)3 and more recently in Nitroxide
Mediated Photopolymerization (NMP2).4–10 Usually, nitroxides
can be generated by thermolysis of alkoxyamines (at temperature
typically ~ 80–130 ◦C) through a O–C bond cleavage process
(Scheme 1). The BDE(O–C) is typically in the 80–130 kJ mol-1
range depending on both the alkyl and nitroxide fragments.
Albeit that the thermolysis process of alkoxyamines has been the
subject of numerous studies in the literature,3 few photosensitive
alkoxyamines are known.4–10 The key parameters governing the
cleavage process remain unknown: improved knowledge of the
factors governing this reaction and nitroxide production still
remains highly desirable. New light sensitive alkoxyamines were
recently proposed, as mediators in the NMP2 process.8–10 The first
step corresponds to the photodecomposition of the alkoxyamine
with formation of an initiating radical and a nitroxide (Scheme 1).
As in NMP, the reversible reaction between the polymeric radical
and the nitroxide allows control of the polymerization reaction
through a significant decrease in the proportion of irreversible
self-terminating reactions. Typically, the chromophore group must
be located on the nitroxide moiety to ensure reversible cleavage of
the macroalkoxyamine during the photopolymerization process. It
has to be mentioned that, the use of non-absorbing alkoxyamines
in conjunction with a thermal initiator (azobisisobutylonitrile)
and an iodonium salt was recently reported in photoliving poly-
merization processes11a,11b however the mechanism still remains
unclear.11c
To get new insights into these crucial points, our first NMP2
experiments were promising and encouraging,8,10 the photochem-
ical behavior of six alkoxyamines is investigated here (Scheme 2).
Two new alkoxyamines (5 and 6) are synthesized and compared
to three previously presented compounds 1–3.8–10 These two new
compounds are not expected to show NMP2 behavior because
the chromophore resides in a different moiety to the nitroxide
and so would separate as the polymerization takes place. These
compounds are designed here for a better understanding of
the cleavage process and more particularly to investigate the
Scheme 1 Decomposition of alkoxyamines.
Photosensitive alkoxyamines can be highly worthwhile (being
able to decompose under light irradiation at RT) i.e. for NMP2 or
organic synthesis applications. Such photosensitive compounds,
however, have been the subject of relatively few studies.4–6
A
chromophore with good light absorption properties must be
aDe´partement de Photochimie, CNRS, Universite´ de Haute Alsace, EN-
SCMu, 3 rue Alfred Werner, 68093, Mulhouse, Cedex, France
bLaboratoire Chimie Provence, Faculte´ des Sciences et Techniques, Uni-
versite´s Aix-Marseille I, II, III-CNRS UMR 6264, Equipe CROPS,
campus Saint Je´roˆme Case 542, 13397, Marseille, Cedex 20, France.
E-mail: didier.gigmes@univ-provence.fr, j.lalevee@uha.fr
† This work is dedicated to the memory of Athel Beckwith, a teacher and
scientist from whom we learned how to study chemistry by example. His
pioneering advances in radical chemistry laid the foundation for much of
the current radical clock methodology.
‡ Electronic supplementary information (ESI) available: The different
optimized geometries are given for the representative structures 4–6. See
DOI: 10.1039/c0ob01207f
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Scheme 2 Investigated alkoxyamines.
behavior of alkoxyamines exhibiting separated chromophore–
aminoxy functions. Compound 4 was selected for sake of compar-
ison (its photochemical properties have already been investigated
by laser flash photolysis without ESR investigation5). All the
alkoxyamines have a benzophenone chromophore; the structure
of the leaving nitroxide as well as its position on the alkoxyamine
varies. In the present paper, we investigate and discuss the effects of
the alkoxyamine structure on the efficiency of nitroxide generation,
by means of laser flash photolysis, ESR and molecular orbital
(MO) calculations.
and the 4-acryloylbenzophenone were prepared as described in
refs. 12a and 12b, respectively.
Synthesis of alkoxyamine 5
A solution of BlocBuilder MA or MAMA-SG1 (from Arkema)
(4.03 g, 10.58 mmol) and N-acrylamidobenzophenone (2.47 g,
9.83 mmol) in THF was introduced in a Schlenk tube equipped
with a rotaflo, deoxygenated by nitrogen bubbling and heated
at 100 ◦C for 1 h under stirring. The reaction mixture was
then concentrated under reduced pressure. The yellowish oil
was dissolved in dichloromethane and extracted with sodium
hydroxide solution (5 wt%). The aqueous phase was then acidified
with concentrated aqueous hydrochloride until pH 1, and ex-
tracted with dichloromethane. After drying on magnesium sulfate,
filtration and evaporation of dichloromethane, the mixture was
precipitated in cold pentane to obtain the alkoxyamine as a white
powder.
Experimental part and computational procedure
Materials
Ebecryl 605, an oligomer/monomer formulation based on 75/25
w/w epoxyacrylate/tripropyleneglycoldiacrylate was purchased
from Cytec at the highest purity level available and used without
further purification. 2,2-Dimethoxy-2-phenylacetophenone (I651
or Irgacure 651 or Esacure KB1 or DMPA) which behaves
as a free-radical photoinitiator of acrylate polymerization was
provided by Ciba Specialty Chemicals. The alkoxyamine 4 was
prepared as described in [ref. 5]. The N-acrylamidobenzophenone
1
Yield: 60%. H NMR (300.13 MHz, CDCl3): 7.92–7.39 (m,
9H), 5.54 (dd, 1H), 4.30–3.93 (m, 4H), 3.45 (d, J = 27.6 Hz,
1H), 2.39–2.17 (m, 2H), 1.44–0.90 (m, 30H). 31P NMR (121.49
MHz, CDCl3): 25.43. ESI-HRMS: calcd for C33H49N2O8P [M+H]+
633.3299, found 633.3283.
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Synthesis of alkoxyamine 6
in DMPA (the reference standard) or in alkoxyamines were used.
The monomer formulation was applied on a BaF2 pellet with a bar
coater to form a 25 mm thick liquid film layer. The progress of the
polymerization was continuously monitored through the decrease
of the 1630 cm-1 band corresponding to the C C bond of the
monomer acrylate group.15
A solution of BlocBuilder MA (5.66 g, 14.85 mmol) and 4-
acryloylbenzophenone (3.12 g, 12.37 mmol) in THF was intro-
duced into a Schlenk tube equipped with a rotaflo, deoxygenated
by nitrogen bubbling and heated at 100 ◦C for 1 h under stirring.
The reaction mixture was then concentrated under reduced
pressure. The yellowish oil was dissolved in dichloromethane
and extracted with sodium hydroxide solution (5 wt%). The
aqueous phase was then acidified with concentrated aqueous
hydrochloride until pH 1, and extracted with dichloromethane.
After drying on magnesium sulfate, filtration and evaporation of
dichloromethane, the mixture was precipitated in cold pentane to
obtain the alkoxyamine as a white powder.
Computational procedure
Molecular orbital calculations were carried out with the Gaussian
03 suite of programs.16 The different alkoxyamine structures were
fully optimized in the Density Functional Theory framework (at
B3LYP/6-31G* level). The Bond Dissociation Energies (BDE)
were calculated as the energetic difference between the radicals
and the starting molecules.
1
Yield: 55%. H NMR (300.13 MHz, CDCl3): 7.91–7.38 (m,
9H), 4.90 (dd, 1H), 4.37–3.98 (m, 4H), 3.50 (d, J = 26.2 Hz,
1H), 2.64–2.45 (m, 2H), 1.54–0.95 (m, 30H). 31P NMR (121.49
MHz, CDCl3): 23.27. ESI-HRMS: calcd for C33H48NO9P [M+H]+
634.3139, found 634.3143.
Results and Discussion
1
Synthesis of alkoxyamines
Functionalized alkoxyamines are usually prepared by multi-step
synthesis. Recently, Gigmes et al.12c proposed a straightforward
method to obtain functionalized alkoxyamines in one step.
They developed a clean intermolecular radical 1,2 addition of
alkoxyamines on activated olefins, which generate a “second gen-
eration” functionalized alkoxyamine (Scheme 3). The challenge is
to avoid multiple addition and then to circumvent any undesired
polymerization, the thermal stability of the obtained alkoxyamines
has to be higher than the starting material.
Laser Flash Photolysis (LFP)
The nanosecond laser flash photolysis set up working at 355 nm
is based on a nanosecond Nd:YAG laser (Powerlite 9010, Con-
tinuum) operating at 10 Hz with a 7 mJ/pulse energy.13 The
analyzing system (LP900, Edinburgh Instruments) used a 450 W
pulsed xenon arc lamp, a Czerny–Turner monochromator, a fast
photomultiplier and a transient digitizer (TDS 340, Tektronix).
The sample was contained in a 1 cm cell. Measurements were
done at room temperature.
Following this strategy alkoxyamines 5 and 6 were efficiently
prepared from the 1,2 radical addition of MAMA-SG1, at 100 ◦C
in deoxygenated THF solutions, onto benzophenone derivatives
(Scheme 3). In each case, monomer conversion was above 90%
and the corresponding alkoxyamines were obtained with a yield
close to 60% after purification. The success of these reactions is
strongly related to the lower BDE of the NO–C bond in MAMA-
SG1 compared to that in 5 or 6. Indeed, compared to 5 and 6, for
MAMA-SG1 the 1-carboxy-1-methy ethyl moiety introduces both
a destabilization of the ground state of the starting alkoxyamine,
and a higher stabilization of the generated alkyl radical, which
results in lower NO–C BDE.
Electron Spin Resonance (ESR)
Electron Spin Resonance (ESR) experiments were carried out
using a X-Band spectrometer (MS 200 Magnettech). The radicals
were generated using a polychromatic light irradiation (Xe–Hg
lamp; Hamamatsu, L8252, 150 W; l > 310 nm). The ESR spectra
simulations were done with the PEST WINSIM program.14 All of
the samples were prepared in a 6 mm quartz cylindrical tube and
dissolved in tert-butylbenzene as the inert solvent.
Polymerization experiments
The photopolymerization profiles were followed by Real-Time
FTIR (Nexus 870, Nicolet). All the experiments were carried out
at room temperature in laminate using the polychromatic light
emitted by a Xe–Hg lamp (Hamamatsu, L8252, 150 W, light
intensity at the sample: 22 mW cm-2). Concentrations of 0.5 wt%
2
Light absorption properties of 1–6
The absorption spectra of 4–6 are shown in Fig. 1. The introduc-
tion of the benzophenone moiety on the alkoxyamines leads to
good absorption properties in the 300–400 nm range. The weak
Scheme 3 Intermolecular radical 1,2 addition of MAMA-SG1 onto benzophenone derivatives.
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Scheme 4 Competitive dissociation processes for 1 under light irradiation.
is only related to the formation of nitroxides. Peroxyl radicals are
not observed in these experiments.
Two main cleavage processes can be expected in alkoxyamines
(Scheme 4). In 1–3, the formation of nitroxides under light
irradiation is clearly demonstrated.8-10 For 1, a competitive N–
O cleavage process has also been found and is supported by
the observation of aminyl radicals. For 2 and 3, the C–O
cleavage is highly selective and quantitative as the final nitroxide
concentration, after irradiation and complete photolysis, is equal
to the initial alkoxyamine concentration.
In 4, the nitroxide could also be detected (thanks to the
observation of a strong ESR signal: g ~ 2.0065 and aN = 15.4
G) as shown in Fig. 2; no other bond cleavage was noted as only
nitroxides were observed albeit that the associated signal decreases
for long irradiation time (t > 350 s). This can be ascribed to the
quenching of the triplet state of 4 by the formed nitroxides. Indeed,
such a process is efficient for long lived triplet states (see e.g. ref.
7). The formation of nitroxides is more difficult in 4 compared
with 2–3. Indeed, for a similar light absorption, the increase in
nitroxide concentration is much slower for 4 i.e. at a 400 s time
scale for 4 compared to 1–10 s for 2–3 (Fig. 2).
In 6, the starting ESR signal (aP = 46.2 G; aN = 13.6 G) at time
t = 0 is assigned to the stable and known SG1 radical19 that may
be present in the starting sample due to the known higher thermal
lability of the SG1-based alkoxyamines.7 The ESR signal does
not increase with irradiation time which indicates a low nitroxide
initiation ability (Fig. 2C). A similar behavior is observed for 5.
As in 4, no evidence for N–O cleavage can be provided in 5 and 6.
Fig. 1 UV-visible absorption spectra of 4, 5 and 6 in acetonitrile.
absorption band (or the shoulder for some compounds) usually
observed at about 330 nm is ascribed to the np* transition of the
benzophenone chromophore; the molar extinction coefficients are
typically in the 100–500 M-1cm-1 range. A pp* transition located
between 250 and 310 nm also associated with the benzophenone
structure is observed. For 1–3, the UV absorption properties are
reported8–10 with similar trends for the np* and pp* transitions.
These good absorption properties of 1–6 are in contrast to those
of typical alkoxyamines used for thermal processes which do not
exhibit any absorption at l > 280 nm.7
3
Cleavage process: ESR results
4
Excited state processes: a LFP investigation
The observation of nitroxides generated under light irradiation
requires specific selected conditions to get a rapid conversion
of the alkyl radicals into unreactive species and to avoid their
recombination with nitroxides. Different trapping agents such as
other nitroxides, hydrogen donors or oxygen have already been
used to prevent this back-reaction.17–18 For easy handling and
rapid measurements, the use of oxygen is chosen. In this case, the
carbon-centered radicals (resulting from C–O bond dissociation)
are trapped by oxygen to yield peroxyl radicals, thereby preventing
the recombination between the nitroxide and the carbon-centered
radical. As a consequence, the intensity increase of the ESR signal
Upon laser excitation of 4, 5 and 6, transient absorptions at
about 550, 480 and 520 nm are observed (Fig. 3). These species
are strongly quenched by camphorquinone (k > 107 M-1s-1) and
oxygen (k > 109 M-1s-1), which are well known triplet energy
acceptors (Fig. 3D). Therefore, these transients can be confidently
ascribed in any case to a triplet state located on the benzophenone
moiety. The lifetime of this transient (Table 1) is quite short (0.9 ms
for 4) but it increases for 5, 6 (2.2–2.5 ms). The reduced lifetime
of 4 compared to that of the benzophenone triplet state is in
agreement with previous data on this compound5, and is also
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Fig. 3 Transient absorption spectra of (A) 4, (B) 5, (C) 6 in acetonitrile at
time t = 0 after laser excitation at 355 nm. (D) Transient decay at 460 nm
upon addition of camphorquinone to 5 under argon.
confirms the lack of formation of nitroxides in ESR experiments.
The dissociation process for this reaction is probably rather hard.
The same holds true in 1–3 where triplet states have been already
observed in LFP experiments (maximum absorption at 525 nm).
The lifetimes were further reduced (200 ns - 1.5 ms)8–10 than for
the benzophenone triplet state (~5 ms under similar experimental
conditions), supporting the cleavage processes observed by ESR.
5
Molecular orbital calculations
At a first sight, the radical production efficiency is connected
with (i) the amount of light absorbed by the chromophore
and (ii) the cleavage quantum yield which is governed by the
intersystem crossing quantum yield, the energy transfer to the
bond being broken and the bond breaking ability. As stated in4 for
photosensitized decomposition of TEMPO-based alkoxyamines
(R-TEMPO when R is a benzyl group), the cleavage of the C–
O bond in 1–6, is seen to result from an intramolecular energy
transfer from the chromophore triplet state to the dissociative
triplet surface associated with the C–O bond: the more reduced
the distance, the more efficient the transfer. Considering the bond
dissociation energies BDE (for N–O and C–O) and the triplet
energy levels (ET) calculated for 1–6 (Table 1), the dissociation
process is found highly exothermic (DHdiss = BDE - ET ~ -80
to -160 kJ mol-1) for all the investigated compounds. As a
consequence, in a qualitative approach, the production efficiency
of the nitroxides in 1–3 can be related to the energy transfer
efficiency i.e. to the close vicinity between the chromophore
and the aminoxy function. In fact, only one (for 1) or two
Fig. 2 (A) ESR spectra obtained under light irradiation of 4 in
tert-butylbenzene in aerated conditions. The increase of the ESR spectrum
is ascribed to the nitroxide radical. Light intensity = 44 mW cm-2. (B)
Double integration of the ESR signal ascribed to the nitroxide vs. the
irradiation time in 2 and 4 for similar light absorptions; light intensity =
44 mW cm-2. (C) ESR spectra for 6 in tert-butylbenzene: --- (before
irradiation) and (at 60 s of irradiation); light intensity = 220 mW cm-2.
In aerated conditions.
consistent with the cleavage from the triplet state, which is in line
with the nitroxides observed by ESR experiments. For 5–6, the
triplet state lifetimes are higher than those found for 4, which
Table 1 Parameters characterizing the cleavage processes for the investigated alkoxyamines (molecular orbital calculations at B3LYP/6-31G* level)
a
BDE (C–O) (kJ mol-1)
BDE (N–O) (kJ mol-1)
ET (kJ mol-1)
d (A)
t (triplet state) in ms
˚
Alkoxyamines
1
2
3
4
5
6
122
118
80
115
87
143
187
143
146
134
155
265
265
266
258
264
267
1.44
2.49
2.36
6.51
3.47
4.17
1.5
0.39
0.02
0.9
2.2
2.5
105
a distance determined by molecular orbital calculations (see also Scheme 5).
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Scheme 5 Representation of the distance between the chromophore and the aminoxy groups (left) and geometry for 6 optimized at UB3LYP/6-31G*
level (right) (d, determined by molecular orbital calculations are shown in Table 1).
(for 2–3) bonds are located between these two moieties. For 4–
6, six (for 4) or four (for 5–6) bonds separate the two fragments
which is in agreement with their lower nitroxide production. For
5–6, the cleavage process is difficult. The nature of the spacer used
also affects the cleavage process i.e. 4 exhibits a higher nitroxide
formation than 5–6 despite the longer distance. For a more refined
analysis of the structure effect on the dissociation process, an
analogous series of compounds must be investigated as has been
done for other photoinitiators.20
To take into account the real structure of the molecules
and not only the number of bonds, the distance between the
chromophore and the aminoxy function was evaluated from the
relaxed geometries determined by molecular orbital calculations
(Scheme 5). The results provided in Table 1 show that a trend
similar to that found above for the number of bonds is obtained.
The flexibility of the linking group is probably also an important
factor as thermal activation can change the distance between
the two functions. The linking groups in (4–6) can also slightly
affect the chromophore properties depending on their chemical
nature (ester, amide): the investigation of an analogous series is
still needed for more in depth studies.
For these very large structures, the obtained BDEs can be
probably slightly affected by the configuration i.e. different local
dips will coexist and can be thermally populated. The calculated
BDEs resulting from full optimization procedures will be used for
sake of comparison. For 2–4, the C–O bond is weaker than N–O
with the difference in respective BDEs higher than 50 kJ mol-1.
This is in agreement with the selective C–O cleavage process for
these compounds. For 5–6, the same difference is noted but the
energy transfer is less favorable (longer d values compared to 1–3)
so that the yield in nitroxide is low. For 1, BDE(N–O) is quite
close to BDE(C–O) in agreement with the formation of stabilized
aromatic aminyl radicals: this results in the observed competitive
N–O and C–O bond cleavages.8
Fig. 4 Conversion vs. irradiation time curves for the photopolymerization
of Ebecryl 605 in laminated conditions. Photoinitiating system (0.5 wt%):
DMPA; 4, 5 and 6. Polychromatic light intensity: 22 mW cm-2.
better when 4 is used instead of 5–6 (Fig. 4), this is in agreement
with the enhanced dissociation process found for this structure.
For 5–6, the poorest Rps are noted, as expected, from the rather
difficult cleavage process described above. These Rp values roughly
correlate with the radical production in 1–6.
Conclusions
In the present paper, the effect of the alkoxyamine structure on
the photodissociation process was investigated. It is found that (i)
when the p system of the chromophore is directly linked to the
aminoxy function, both N–O and C–O cleavage occurs, which is
in agreement with the formation of stabilized aminyl radicals in
1 (ii) when the chromophore and the N–O containing moiety
are separated, distance becomes an important parameter: a 2
bond separation length appears the best compromise between
efficient nitroxide formation (2–3 vs. 4–6) and selective C–O
cleavage reaction (2–3 vs. 1). The synthesis of new alkoxyamines
based on other high energy chromophores exhibited good to
excellent absorption in the UV-near visible wavelength range, the
use of other spacers for the benzophenone chromophore will be
examined in forthcoming studies.
6
Photoinitaitor ability
The investigated alkoxyamines generated both carbon centered
radicals and nitroxides, the photoinitiating abilities were investi-
gated in an acrylate matrix. The results obtained using 4–6 are
shown in Fig. 4. For all these compounds, the polymerization rate
Rp is lower than that of the reference photoinitiator (DMPA).
With 1–3, the Rps were found to be significantly higher than
those determined here for 4–6.8–10 This is related to the higher
cleavage efficiency for these structures (1–3) (see above) leading
to the formation of the initiating carbon centered radicals. Rp is
Acknowledgements
This work was supported by the “Agence Nationale de la
Recherche” ANR under Grant NMP2 07-JCJC0143.
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12 (a) N. S. Allen, D. Mallon, A. Timms, A. W. Green and F. Catalina,
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14 D. R. Duling, J. Magn. Reson., Ser. B, 1994, 104, 105.
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