New donor-acceptor stenhouse adducts as visible and near infrared light polymerization photoinitiators

Article abstract of DOI:10.3390/molecules25102317

Polymerization photoinitiators that can be activated under low light intensity and in the visible range are being pursued by both the academic and industrial communities. To efficiently harvest light and initiate a polymerization process, dyes with high molar extinction coefficients in the visible range are ideal candidates. In this field, Donor-acceptor Stenhouse Adducts (DASA) which belong to a class of recently discovered organic photochromic molecules still lack practical applications. In this work, a series of DASA-based dyes are proposed as photoinitiators for the free radical polymerization of (meth)acrylates upon exposure to a near infrared light (laser diode at 785 nm).

Full text of DOI:10.3390/molecules25102317

molecules
Article
New Donor-Acceptor Stenhouse Adducts as
Visible and Near Infrared Light
Polymerization Photoinitiators
Guillaume Noirbent 1 , Yangyang Xu 2,3, Aude-Héloise Bonardi 2,3, Sylvain Duval 4,
Didier Gigmes 1, Jacques Lalevée 2,3,* and Frédéric Dumur 1,
*
1
Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France;
guillaume.noirbent@univ-amu.fr (G.N.); didier.gigmes@univ-amu.fr (D.G.)
Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France; ahutxyy@163.com (Y.X.);
aude-heloise.bonardi@uha.fr (A.-H.B.)
2
3
4
Université de Strasbourg, F-67000 Strasbourg, France
Universit
Solide, F-59000 Lille, France; sylvain.duval@univ-lilles.fr
Correspondence: Jacques.lalevee@uha.fr (J.L.); frederic.dumur@univ-amu.fr (F.D.)
é
de Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181—UCCS—Unit
é
de Catalyse et Chimie du
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Giovanni Palmisano
Received: 24 April 2020; Accepted: 13 May 2020; Published: 15 May 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Polymerization photoinitiators that can be activated under low light intensity and in the
visible range are being pursued by both the academic and industrial communities. To efficiently
harvest light and initiate a polymerization process, dyes with high molar extinction coefficients in
the visible range are ideal candidates. In this field, Donor-acceptor Stenhouse Adducts (DASA)
which belong to a class of recently discovered organic photochromic molecules still lack practical
applications. In this work, a series of DASA-based dyes are proposed as photoinitiators for the
free radical polymerization of (meth)acrylates upon exposure to a near infrared light (laser diode at
785 nm).
Keywords: photoinitiators; photopolymerization; visible light; DASA; Stenhouse adducts; push-pull;
free radical polymerization; cationic polymerization
1. Introduction
During the past decades, push-pull dyes composed of an electron donor connected to an electron
acceptor by means of a conjugated or a non-conjugated spacer have become a thriving research area,
impelled by their potential applications ranging from non-linear optical (NLO) applications [
1], organic
photovoltaics (OPVs) [ ], organic field effect transistors (OFETs) [ ], photopolymerization [4–
2
3
11] or
photocatalytic degradation of dyes [12]. As interesting features, the optical properties of the dyes
can be finely tuned not only by adjusting the strength of the electron-withdrawing and releasing
abilities of both the donors and the acceptors used to create the chromophores but also the length of
the spacer used to connect the two partners [13]. By elongating the π-conjugated spacer introduced
between the electron donors and acceptors, the molar extinction coefficients of the dyes can be
significantly increased while jointly inducing a redshift of the absorption spectrum. Specifically,
when exposed to light irradiation and irrespective of the wavelength used to excite the molecules,
no color change occurs for this first class of dyes, no isomerization or structural rearrangement
occurring upon photoexcitation. Besides, in some cases, a photodegradation of the molecule can
nonetheless be observed, resulting from a low photochemical stability of the dye. Conversely, for a
second class of dyes named photochroms or photoswitches, a totally different behavior is observed.
Notably, the dyes can undergo a complete discoloration upon photoexcitation with the appropriate
Molecules 2020, 25, 2317; doi:10.3390/molecules25102317
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wavelength [14]. As specificity, the color change is reversible and can be triggered by exciting the
molecules at a second irradiation wavelength carefully selected. In this field, the most popular
photoswitches include azobenzenes [15], diarylethenes [16], dihydroazulenes [17] or spiropyrans [18].
For these dyes, the stable form is a colorless state and use of a high energy (UV) light is required to
produce the colored form.
In 2014, a new class of photochrome dyes was developed by Read de Alaniz and coworkers,
exhibiting an opposite behavior [19]. Hence, these highly colored molecules, i.e., the Donor-acceptor
Stenhouse Adducts (DASA) can be discolored upon photoexcitation with visible light, constituting
an original class of negative molecular photoswitches [20]. When kept in the dark, the colored form
can be restored by isomerization, the cyclized form being converted into a linear nonpolar form (see
Scheme 1). However, DASA are not the only negative photochromes to be reported in the literature
and bistable hydrazones have been discovered concomitantly to DASA [21–33].
Scheme 1. General structures of the open/closed forms of the Donor–acceptor Stenhouse adducts
(DASA) based on Meldrum’s acid.
Since 2014, several applications have been found for these molecular photoswitches such as the
development of light-responsive drug delivery systems [34,35], liquid crystals [36], or colorimetric
sensors [37 38]. To the best of our knowledge, DASA have never been reported as polymerization
,
photoinitiators, despite the fact their strong absorption in the visible region makes these dyes ideal
candidates for such an application. Indeed, compared to the traditional UV photoinitiators, highly
colored molecules constitute appealing candidates for the elaboration of visible light photoinitiating
systems which are currently actively coveted for their significant advantages including irradiation
safety, curing depth and equipment cost [39–43]. Notably, by developing dyes with high molar
extinction coefficients, the photoinitiator content can be drastically reduced while maintaining an
efficient production of radicals. Another point governing the reactivity of photoinitiators is also the
light penetration inside the photocurable resin. Indeed, if the light penetration is limited to a few
hundreds of micrometers at 400 nm, this latter can reach 5 cm at 800 nm, clearly evidencing the
benefits to develop photoinitiating systems activable in the near-infrared region [44]. To end, DASA are
chromophores well-known to exhibit high molar extinction coefficients [45
photocurable resin can be drastically reduced, addressing the photoinitiator extractability issue [48
Facing these different considerations, in this work, a series of twenty-four push-pull adducts
PP24 which represent precursors for the design of DASA photoswitches have been examined
–48], so their content in the
–
52].
PP1
–
as potential precursors for the synthesis of Stenhouse adducts (see Figure 1). Parallel to this, several
DASA photoswitches (DASA1–DASA13) have been prepared and examined as polymerization
photoinitiators activable in the near infrared region. Interestingly, during the course of our
investigations, some unexpected reactions occurred during the synthesis of DASA (see Figure 2)
and the cyclization products PP25 and PP26 obtained during these reactions have been identified and
also tested as polymerization photoinitiators.

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Figure 1. Chemical structures of PP1–PP24 used as precursors for the design of Donor–acceptor
Stenhouse adducts (DASA) photoswitches.
Figure 2. Cont.

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Figure 2. Chemical structures of DASA 1–DASA 13, PP25 and PP26.
2. Results and Discussion
2.1. Synthesis of PP1–PP24
PP1
–
PP24 were prepared by a Knoevenagel reaction combining four electron donors D1
–D4
with six different electrons acceptors EA1
–
EA3 EA5 EA6. Considering that piperidine can initiate an
,
–
undesired nucleophilic addition on the push-pull dyes subsequent to their syntheses, pyridine was
selected as the base to catalyze the Knoevenagel reaction (see Scheme 2). Synthetic procedures are
detailed in the Supplementary Materials.
Scheme 2. Cont.

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Scheme 2. Synthetic route to PP1–PP5, PP7–PP9, PP13–PP21, PP23–PP24.
All dyes, except for PP6, that could not be synthesized, were obtained in good yields
ranging from 79% for PP24 to 97% for PP16 (see Table 1).
Indeed, instead of PP6,
[1,20-biindenylidene]-10,3,30(2H)-trione, also named bindone and resulting from the self-condensation
of indane-1,3-dione, was isolated in 89% yield as the sole reaction product (see Scheme 3).
Scheme 3. Synthetic route to PP10–PP12 and PP22.

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This is directly related to the low reactivity of pyrrole-2-carboxaldehyde D3, favoring the
dimerization of indane-1,3-dione EA2. Concerning the synthesis of PP10 PP12 and PP22 with EA4
–
,
a completely different synthetic route had to be used due the remarkable stability of the EA4 anion
under basic conditions. To overcome this drawback, the Knoevenagel reactions were carried out in
acetic anhydride and PP10
–PP12 and PP22 could be isolated with reaction yields ranging from 82% to
88% yield (see Scheme 2 and Table 1).
Table 1. Reaction yields obtained for the synthesis of PP1–PP24 by Knoevenagel reactions.
Dyes
Reaction Yield
Dyes
PP1
85
PP2
92
PP3
89
PP4
72
PP5
84
PP6
-
PP7
83
PP8
91
PP9
88
PP10
83
PP11
82
PP12
88
PP13
89
PP14
82
PP15
90
PP16
97
Reaction Yield
Dyes
PP17
86
PP18
85
PP19
87
PP20
87
PP21
84
PP22
81
PP23
88
PP24
79
Reaction Yield
2.2. Synthesis of DASA1–DASA13 and Cyclized Structures PP25 and PP26
For the synthesis of the Stenhouse adducts, five different amines were investigated,
namely, diethylamine (A1), piperidine (A2), diethanolamine (A3), dibenzylamine (A4), and
N-methyl-benzylamine (A5). When examining the nucleophilic addition of these five amines A1
PP1 PP20, no reaction was detected for any of the dyes bearing thiophenes or pyrroles as heterocycles.
This is directly related to an aromaticity increase of the order furan < pyrrole < thiophene, impeding
the ring-opening reaction of pyrrole and thiophene by a nucleophile [53 55]. A similar behavior
was also observed with PP17 PP22, despite the presence of the furyl group. In this case, no ring
–A5 to
–
–
–
opening reaction could occur with the different amines due to the elongation of the spacer introduced
between the furyl donor and the acceptors. As a result of this, the electron-withdrawing ability of the
different acceptors was insufficient to promote the ring-opening reaction. Conversely, as expected, the
nucleophilic addition of the different amines to PP1, PP4, PP7 and PP13 could proceed, promoting the
synthesis of the expected Stenhouse adducts in high yields (see Schemes 4–6).
Notably, reaction yields ranging between 38% for DASA10 and 96% for DASA6 could be obtained.
It should be noted that several of the Stenhouse adducts could not be obtained and our results are
consistent with those previously reported in the literature [19]. One exception was noted, that of
DASA1. Indeed, DASA1 could be prepared in 85% yield whereas in previous investigations [19] its
synthesis was reported as being impossible. When using the same reaction conditions used with PP7
and diethylamine, Read de Alaniz and coworkers reported a nucleophilic attack of piperidine on the
cyano groups of PP7, producing the cyclization product CP1 in 96% yield as the only reaction product
according to the mechanism presented in Scheme 6.

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Scheme 4. Synthetic route to DASA1–DASA10. Investigated amines were: diethylamine (A1),
piperidine (A2), diethanolamine (A3), dibenzylamine (A4) and N-methylbenzylamine (A5).

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Scheme 5. Synthetic route to DASA10–DASA13.
Scheme 6. Mechanisms involved in the synthesis of cyclization product (CP1) and DASA1.
It should be noticed that the nucleophilic addition of secondary amines onto the cyano groups
of push-pull dyes has been recently reported in the literature as a side-reaction occurring during the
synthesis of push-pull dyes comprising EA3 or EA4 as the electron acceptors [56–60]. The preferential
nucleophilic addition of diethylamine on cyano groups rather than onto the electrodeficient furyl group
was confirmed during the synthesis of PP25 starting from PP7 and diethylamine (A1). PP25 could be
isolated in pure form in 89% yield. Unexpectedly, a more complex reaction occurred when PP10 was
reacted with N-methylbenzylamine (A5) as the nucleophile. As shown in Scheme 7, the nucleophilic
addition of compound A5 occurred on the furyl side, as classically observed during the synthesis

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of the Stenhouse adduct. By an addition/elimination mechanism, a ring-opening of the furyl group
can occur. Following a proton transfer, an iminium cation is formed. By a 6 electrocyclization, a
π
six-membered ring can then be obtained. Finally, a nucleophilic attack on the iminium cation furnishes
PP26 as a polycyclic molecule. It should be mentioned that such a cascade reaction is unprecedented
in the literature (See Scheme 7).
Scheme 7. Mechanisms supporting the formation of PP26.
The chemical structure of PP26 was confirmed by proton (1H) and carbon-13 nuclear magnetic
resonance (13C-NMR) analyses(Avance 400 spectrometer, Bruker, Billerica, MA, USA), as detailed in
the Figure 3. Notably, in the 2.0–5.5 ppm region, the four aliphatic protons in the cycles of PP26 as well
as the methyl group of N-methylbenzylamine can be detected in the 1H-NMR spectrum. The presence
of aliphatic protons in the chemical structure of PP26 was confirmed by 13C-NMR analyses, where six
carbons could be detected between 20 and 70 ppm.

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Figure 3. Proton nuclear magnetic resonance (1H-NMR) spectrum (
resonance (13C NMR) spectrum (b) of cyclized compound PP26.
a) and carbon-13 nuclear magnetic
Finally, by slow evaporation of the deuterated solvent in an NMR tube, crystals of PP26
could be obtained and the crystal structure was resolved by X-ray crystallography (see Figure 4).
It should be noted that only one of the possible diastereoisomers has been found in the crystals,
despite the analysis of several crystals to confirm this point. Based on the crystal structure,
the relative stereochemistry of PP26 could be determined as (4R,5R,7S)-10-(dicyanomethylene)-5-
(methyl-(phenyl)amino)-8-oxo-6,7,8,10-tetrahydro-5H-4b,7-methanobenzo[a]azulene-11,11-dicarbonitrile
(see Figure 4). Configurations of the asymmetric carbons can also be found in the Checkcif. As also
evidenced in the 1H- and 13C-NMR spectra of PP26 (see Figure 3, and details concerning the crystal
structure of PP26 can be found in the Supplementary Materials), only slight traces of diastereoisomers
could be detected on the baseline, at 7.15 and 8.62 ppm, respectively.

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Figure 4. Crystal structure of PP26.
2.3. Photopolymerization Experiments
A benchmarked methacrylate monomer blend, “Mix-MA” was used here, the composition of which
is presented in Figure 5. This blend has been prepared with 33.3 wt% of (hydroxypropyl)methacrylate
(HPMA), 33.3 wt% of 1,4-butanediol dimethacrylate (1,4-BDMA) and 33.3 wt% of a urethane
dimethacrylate monomer (UDMA), obtained from Sigma Aldrich (St. Louis, MO, USA). This benchmark
blend has an adapted viscosity for polymerization under air [44,61–64].
Figure 5. Benchmark methacrylate monomer blend Mix-MA.
It has to be noticed that the 13 DASA dyes DASA1–DASA13 examined in this work are Stenhouse
adducts meaning that these structures can discolor upon irradiation in the visible range. However,
the efficiency of the discoloration process is directly related not only to the excitation wavelength,
but also the intensity of the light source [19]. In the present case, 785 nm was selected as the wavelength
for photopolymerization due to the weak absorption of the different Stenhouse adducts in this region,
ensuring slow cyclization kinetics with regards to the photopolymerization timescale. The optical

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density at 785 nm for these 1.4 mm samples is always < 0.1 to ensure a good light depth penetration (any
inner filter effect must be avoided for polymerization in depth). Indeed, these dyes are characterized
by low extinction coefficients at 785 nm (<20 M−1 cm−1) that ensure such low absorption at 785 nm.
The absorption of DASA dyes is mainly centered in the 500–600 nm range (see Figure 5) with extremely
high molar extinction coefficients. However, polymerization cannot be efficiently initiated in this
latter spectral range due to the low efficiency of photothermal processes in the 500–600 nm range (less
efficient than in the near-infrared (NIR) region, see Figure 6).
Figure 6. UV-vis absorption spectra of DASA12 (1); DASA11 (2) and DASA9 (3) in acetonitrile.
Additionally, all formulations were kept in the dark prior to photopolymerization to ensure the
different dyes would be in their colored forms. To perform the irradiation at 785 nm, a NIR laser diode
with a tunable irradiance from 0 W to 2.55 W/cm2 was purchased from Changchun New Industries
(CNI, Changchun, China) and used here. The kinetics of photopolymerization of the photosensitive
resins were followed through the double bond C=C conversion vs. time upon NIR irradiation (see
Figure 7). The peak followed by Real Time Fourier Transform Infrared (RT-FTIR) spectroscopy (using a
Jasco 4600 instrument, Jasco, Tokyo, Japan) is located at 6100–6220 cm−1 as presented in [37]; this peak
corresponds to the first overtone of the C-H vibrations of the =CH2 acrylate group.
Figure 7. Chemical structures of additives.
The NIR photoinitiating systems are composed of (NIR) dyes (DASA1–DASA13, PP25 or
PP26) combined with an iodonium salt (Ar2I+.PF6), a phosphine (4-dppba) and a thermal initiator
(here BlocBuilder MA) (see Figure 7) to take advantage of the photoinitiating system (NIR
dye/iodonium/phosphine) associated with the photothermal system (NIR dye/BlocBuilder MA) as
presented for other NIR dyes in [64].
The expected mechanisms are given in r1–r5. The free radicals are generated through two different
mechanisms: (1) a photochemical pathway through the interaction of the dye with the iodonium salt
i.e., the photoreduction of the iodonium salt by the excited state of the dye leads to the formation of aryl
radicals that are very efficient to initiate polymerization processes (r1) and (2) a photothermal effect i.e.,

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the dye is able to convert NIR light into heat that decomposes the thermal initiator (r2–r3). For the
activation 1 and 2, the phosphine is used to overcome the oxygen inhibition as all the polymerization
processes were carried out under air through r4–r5 i.e., the phosphine is able to convert non-initiating
peroxyls (ROO•) to reactive alkoxyl radicals (RO•) [40]. All these chemical mechanisms can also
be depicted by Figure 8. Overall, by combining a photochemical and a thermal polymerization
process, four distinct types of radicals can potentially be formed, namely, Ar•, HOOC-C•(Me)2 as
the primary radical sources and to a less extent, the radicals resulting from the decomposition of the
non-initiating peroxyls by 4-dppba i.e., PhO• and HOOC-CO•(Me)2. It has to be noticed that the
autooxydation of triarylphosphine by peroxy radicals has been extensively studied by Buckler and
coworkers in 1962. In his work, he suggested the oxidation of the triarylphosphine to occur via a radical
attack on the heteroatom, resulting in a valence expansion producing a short-lived phosphoranyl
radical. By mean of a β-scission, a phosphine oxide can be formed due to the weak O-O bond [65,66].
More recently, exact role of phosphines in photopolymerization processes has been examined by the
group of Lalevée et al. [67].
dye → *dye (hν)
*dye + Ar2I+ → dye•+ + Ar• + ArI
*dye → dye + ∆
Blocbuilder + ∆ → free radicals
ROO• + PAr3 → ROO-P•Ar3
ROO-P•Ar3 → RO• + O=PAr3
(r1)
(r2)
(r3)
(r4)
(r5)
Figure 8. Overview of the chemical mechanism.
Interestingly, twelve of the investigated structures exhibit significant polymerization processes
upon NIR light at 785 nm under air (final methacrylate function conversion (FC) > 20%) (see
Figure 9). For these photopolymerization reactions, no clear bleaching is observed. For the other
dyes (DASA6
20%. Interestingly for DASA1
,
DASA8 and DASA13), rather poor polymerization processes are observed with FC <<
DASA2 and DASA7, fast and efficient polymerization processes are
,
observed despite a rather long lag time of ~200 s ascribed to the oxygen inhibition (i.e., oxygen must be
consumed before the radical polymerization can start). This excellent reactivity shows that these DASA
dyes can work through combined approach (photochemical and photothermal) to obtain very high
final methacrylate function conversion (>80%) in less than 350 s. In reality, only the combined mode

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gathering photochemical and photothermal activations is really efficient. For the control experiments
(only photochemical or photothermal mode), only low conversions are observed (<20%). This shows
that only the combined mode leads to a high yield in initiating radicals to overcome the oxygen
inhibition. Such a behavior has been found in [64] for other dyes. In this work, only dyes active in this
combined mode are efficient.
Figure 9. Real Time Fourier Transform Infrared (RT-FTIR) photopolymerization kinetics (C=C
conversion vs. irradiation time; 1.4 mm thick sample) under air upon exposure to a Laser Diode@785nm;
2.5 W/cm2; for Mix-MA in the presence of dye/iodonium/4-dppba/BlocBuilder MA(0.1/2/2/2% w/w);
the irradiation starts after 15 s for: (A) (1) DASA1; (2) DASA2; (3) DASA3; (4) DASA4; (5) DASA5;
(6) DASA12; (7) DASA10; (8) DASA9; (9) DASA11; (B) (10) DASA7; (11) PP25; (12) PP26.
3. Conclusions
In this work, a series of 24 push-pull dyes have been designed as precursors for the synthesis
of Stenhouse adducts. Unfortunately, all attempts to prepare Stenhouse adducts with thiophene and
pyrrole were unsuccessful due to the high energy of the C-N and C-S bonds. Similarly, all attempts
to induce a ring opening reaction of the pyrrole moiety in the PP19
insufficient electron-withdrawing ability of the different acceptors upon elongation of the
–
PP24 failed as a result of an
-conjugated
π
spacer. Finally, 13 DASA adducts could however be prepared and the preliminary tests revealed
most of these dyes to be promising candidates as photoinitiators of polymerization for the free radical
polymerization (FRP) of acrylates upon excitation at 785 nm. The main parameter governing the
efficiency of the dye is its ability to be active both in photothermal and photochemical modes (combined
mode). Indeed, only this combined mode is really efficient. Parallel to this, an unexpected cyclization
product issued from a cascade mechanism was obtained, resulting from the nucleophilic attack of
diethylamine on the furyl group of PP10. In light of these preliminary results, no clear trend could be
deduced concerning the structure-performance relationship due to the paucity of structures examined.
Future works concerning the Stenhouse adducts will consist in examining a broader range of structures
in order to establish the full guidelines for the design of highly efficient photoinitiators.
Supplementary Materials: The following are available online, Synthetic procedures of all dyes, Figure S1.
Configuration of the asymmetric carbons: C3 (R), C5 (R), C20 (S); Table S1. Crystal data and structure refinement
for compounds PP26 (CCDC 1998738).
Author Contributions: Conceptualization, G.N., F.D. and J.L.; methodology, G.N., F.D. and J.L.; validation, G.N.,
Y.X., A.-H.B., S.D., D.G., J.L. and F.D.; X-ray analyses, S.D.; investigation, G.N., F.D. and J.L.; resources, G.N., Y.X.,
A.-H.B., S.D., D.G., J.L. and F.D.; data curation, G.N., Y.X., A.-H.B., S.D., D.G., J.L. and F.D.; writing—original draft
preparation, G.N., Y.X., A.-H.B., S.D., D.G., J.L. and F.D.; writing—review and editing, G.N., Y.X., A.-H.B., S.D.,
D.G., J.L. and F.D.; All authors have read and agreed to the published version of the manuscript.
Funding: The Agence Nationale de la Recherche (ANR agency) is acknowledged for funding through the PhD
grant of Guillaume Noirbent (ANR-17-CE08-0054 VISICAT project). Aix Marseille University and the Centre
National de la Recherche Scientifique (CNRS) are acknowledged for permanent fundings.

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Acknowledgments: Aix Marseille University, the Centre National de la Recherche Scientifique (CNRS) and the
Agence Nationale de la Recherche (ANR agency) are acknowledged for fundings.
Conflicts of Interest: The authors declare no conflict of interest.
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