A tris(triphenylphosphine)ruthenium(II) complex as a UV photoinitiator for free-radical polymerization and in situ silver nanoparticle formation in cationic films

Article abstract of DOI:10.1021/ma4019872

The characterization and the photochemical investigation of a Ru ^II complex (Ru(PPh₃)₃Cl₂) having phosphine ligands are reported. DFT calculations and ESR spin trapping experiments revealed for the first time that the photodecomposition of the complex is governed by a homolytic cleavage of the P-(C₆H ₅) bond generating phenyl radicals Ph^? which are able to initiate the free radical polymerization of acrylate monomers. The addition of a H-donor HD plays a key role in the cationic photopolymerization of epoxides: (i) the reaction efficiency is enhanced using [Ru(PPh ₃)₃Cl₂]/HD and (ii) the in situ formation of Ag nanoparticles is observed in the presence of Ru(PPh₃) ₃Cl₂/HD/AgSbF₆ according to the following reactions: Ph^?/DH hydrogen abstraction yielding a D ^? radical and oxidation of D^? by the silver salt.

Full text of DOI:10.1021/ma4019872

Article
pubs.acs.org/Macromolecules
A Tris(triphenylphosphine)ruthenium(II) Complex as a UV
Photoinitiator for Free-Radical Polymerization and in Situ Silver
Nanoparticle Formation in Cationic Films
Davy-Louis Versace,†,* Javier Cerezo Bastida,‡ Cedric Lorenzini,† Christine Cachet-Vivier,†
́
Estelle Renard,† Valerie Langlois,† Jean-Pierre Malval,§ Jean-Pierre Fouassier,⊥,# and Jacques Lalevee§,*
́
́
†Institut de Chimie et des Mater
94320 Thiais, France
́
iaux Paris-Est, UMR 7182, Universite
́
Paris-Est Cret
́
eil Val de Marne (UPEC), 2-8 rue Henri Dunant,
‡Departamento di Quimica Fisica, Universidad de Murcia, 30100 Murcia, Spain
§Institut de Science des Mater
́
iaux de Mulhouse, UMR CNRS-UHA 7361, 15 rue Starcky - 68057 Mulhouse, France
⊥ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France
S
* Supporting Information
ABSTRACT: The characterization and the photochemical investigation
of a RuII complex (Ru(PPh3)3Cl2) having phosphine ligands are reported.
DFT calculations and ESR spin trapping experiments revealed for the first
time that the photodecomposition of the complex is governed by a
homolytic cleavage of the P−(C6H5) bond generating phenyl radicals Ph•
which are able to initiate the free radical polymerization of acrylate
monomers. The addition of a H-donor HD plays a key role in the cationic
photopolymerization of epoxides: (i) the reaction efficiency is enhanced
using [Ru(PPh3)3Cl2]/HD and (ii) the in situ formation of Ag
nanoparticles is observed in the presence of Ru(PPh3)3Cl2/HD/AgSbF6
according to the following reactions: Ph•/DH hydrogen abstraction yielding a D• radical and oxidation of D• by the silver salt.
necessary for getting a faster and a better controlled
polymerization.
INTRODUCTION
■
In the past years, there has been a growing interest in the use of
ruthenium RuII complexes containing phosphines in both
organic transformations and polymer synthesis. Such complexes
are of considerable interest as they find applications in classical
catalytic processes, e.g., aerobic oxidation,1 hydrogenation,2
isomerization,3 reductive elimination4 or decarbonylation,5 and
metathesis activity;6 more recently, they have been applied as
catalysts in metal-catalyzed living radical polymerization
reactions.7 They are active toward a large variety of monomers,
their versatility steming from the flexible modification of their
properties through the introduction of various ligands.
Moreover, they present a high stability in alcoholic solvents8
due to the low oxophilicity of the ruthenium atom, thereby
allowing the synthesis of polymer architectures including
functionalized random, block and star polymers in polar
solvents (see for example in ref 9 their use in the living
polymerization of MMA and HEMA in conjunction with a
series of bulky fluoroalcohols at 0 °C or the synthesis of
syndiotactic-b-atactic poly(HEMA) by changing both the
nature of the solvent and the temperature). The widely used
complex RuCl2(PPh3)3 suffers, however, from two severe
limitations: a relatively high catalyst concentration is needed
and rather harsh reaction conditions (temperature between 100
and 120 °C) are often used;10 moreover, the addition of
cocatalyst (like a aluminum-derived compound)11 is often
Some efforts have been devoted so far to design efficient
ruthenium catalysts which can be activated under light
activation. Indeed, compared to a thermal activation, a
photochemical route provides numerous advantages,12 e.g.,
the use of mild experimental conditions. For example, the
photopolymerization of acrylamide using RuII-catalysts has
been described in the 80s (see review in ref 13) and the
photoactivated metathesis polymerizations where RuII-based
complexes are used as precatalysts have been reported.14 More
recently, a new photoredox catalysis approach involving
ruthenium (or iridium, zinc) complexes and various additives
and working through oxidation or reduction cycles was
proposed to initiate the free radical polymerization FRP of
acrylates, the cationic polymerization CP of epoxides or the free
radical promoted cationic photopolymerization FRPCP of N-
vinylcarbazole, epoxides or renewable cationic monomers
under soft irradiation conditions in the visible wavelength
range.15 In the same way, photoactivated catalytic systems
(based on cyclometalated RuII complexes containing strongly
coordinating bidentate ligands) in conjunction with traditional
Received: September 25, 2013
Revised: November 7, 2013
Published: November 19, 2013
© 2013 American Chemical Society
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Scheme 1. Structure of the Monomers and Photoinitiator Used in This Study
(3 g, 13.4 mmol) was then added slowly to a solution of m-
chloroperbenzoic acid (7 g, 40.6 mmol) in 12 mL of methylene
chloride. The reaction was stirred at a temperature of 20 °C for 72 h.
The solution was then filtrated and washed with a solution of 10%
sodium bisulfate followed by saturated sodium bicarbonate and
distillated water. The organic layer was then dried over magnesium
alkyl bromides have been also developed for the free radical
polymerization of hydrophobic monomers,16 i.e., methyl
methacrylate, styrene, and n-butyl acrylate.
In the present paper, we will explore the in situ formation of
silver nanoparticles in a curable neat epoxide matrix using a
ruthenium complex as a photoinitiator (PI) incorporated into a
suitable photoinitiating system (PIS). This has never been
checked although a lot of organic PI and PIS (based on
ketones, e.g., benzoin ethers, acyl germanes, benzophenones, or
thioxanthones; hydrocarbons, e.g., pyrene or anthracene
derivatives; and dyes, e.g., eosin) have been tested for the
production of metal NPs (see, e.g., in ref 17). The well-known
compound RuCl2(PPh3)3 is selected here as an example:
compared to other RuII based complexes, it should work as a
one-component PI, a cleavage of the ligands being expected.
TD-DFT calculations, UV spectroscopy, steady state photol-
ysis, electron spin resonance spin-trapping experiments, TEM
measurements and EDX spectroscopy will allow a complete
description of the absorption properties of RuCl2(PPh3)3, the
generation of radicals, the mechanisms involved in the initiation
step of the FRP of (meth)acrylates (PEGDA and EBDMA) and
the CP/FRPCP of a renewable diepoxide monomer (diallyl
isosorbide ether, IDE) as well as the formation of Ag NPs in
solution and in film.
1
sulfate. The yield of the reaction is 60%. H and 13C NMR spectra of
the synthesized epoxy monomer were displayed in the Supporting
Information part (Figures S1 and S2).
Photopolymerization Procedures. For the free radical photo-
polymerization, Ru(PPh3)3Cl2 (1 wt % with respect to the acrylate
monomer) was dissolved into acrylate monomer formulation (PEGDA
or EBDMA, 1 g) containing 1 mL of THF or ACN. For free-radical
promoted cationic polymerization of the epoxy monomer (IDE, 1 g),
four photoinitiating systems have been tested: (1) Ru(PPh3)3Cl2 (1 wt
% with respect to IDE)/ACN (0.5 mL), (2) Ru(PPh3)3Cl2/AgSbF6
(1/3 wt % with respect to IDE)/ACN (0.5 mL), (3) Ru(PPh3)3Cl2 (1
wt % with respect to IDE)/THF (0.5 mL), and (4) Ru(PPh3)3Cl2/
AgSbF6 (1/3 wt % with respect to IDE)/THF (0.5 mL). Kinetics of
photopolymerization were followed by real time Fourier transform
infrared spectroscopy (RT-FTIR) using a Thermo-Nicolet 6700
instrument. The liquid samples were applied to a BaF2 chips by
means of calibrated wire-wound applicator. The thickness of the UV-
curable film was evaluated at 4 μm. The RT-FTIR analyses were
carried out under laminated conditions: a polypropylene film was laid
on the top of the photosensitive layer to prevent oxygen diffusion.
Samples were irradiated at room temperature, by means of a
Lightningcure LC8 (L8251) from Hamamatsu, equipped with a
mercury−xenon lamp (200 W) coupled with a flexible light guide. The
end of the guide was placed at a distance of 6 cm. The maximum UV
light intensity at the sample position was evaluated to be 150 mW/
cm2. The photopolymerization was monitored by the disappearance of
the carbon double bonds of the acrylate derivative monomers at 1636
cm−1 (for poly(ethylene glycol) (400) diacrylate and ethoxylated
bisphenol A dimethacrylate) and the epoxide group18 at 1247 cm−1
(for isosorbide diglycidyl ether).
EXPERIMENTAL SECTION
■
Chemical Compounds. All reagents and solvents were purchased
from Aldrich and used as received without any further purification. 1H
and 13C NMR spectra were recorded at room temperature in 5 mm
1
o.d tubes on a Bruker Avance 400 spectrometer: H (400 MHz) and
13C (100 MHz). The 1H chemical shifts were referenced to the solvent
peak CDCl3 (7.26 ppm), and the 13C chemical shifts were referenced
to the solvent peak CDCl3 (77.0 ppm). Isosorbide diglycidyl ether
(IDE) were prepared following the literature procedure and obtained
in similar yield. Tris(triphenylphosphine)ruthenium(II) dichloride
(97%, Ru(PPh3)3Cl2) and silver hexafluoroantimonate (98%, AgSbF6)
were obtained from Sigma-Aldrich. Poly(ethylene glycol) (400)
diacrylate (PEGDA, SR 344) and ethoxylated bisphenol A
dimethacrylate (EBDMA, SR348) were kindly provided by Sartomer,
Arkema group. Isosorbide diglycidyl ether was selected as a
representative epoxy monomer (Scheme 1).
UV−Vis Spectroscopy. UV spectra were recorded on a Varian
spectrophotometer (Cary 50 bio).
ESR Experiments. ESR spin-trapping experiments were carried out
using an X-Band EMX spectrometer (Bruker Biospin). The radicals
were generated at room temperature using polychromatic light
irradiation (Xe−Hg lamp; Hamamatsu, L8252, 150 W) and trapped
by phenyl-N-tert-butylnitrone (PBN) according to a procedure
described in detail in ref 19. The ESR spectra simulations were
generated using the PEST WINSIM program. All of the samples were
prepared in a 6 mm quartz cylindrical tube and dissolved in tert-
butylbenzene as an inert solvent.
Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP)
experiments were carried out using a Q-switched nanosecond Nd/
YAG laser at λexc = 355 nm (9 ns pulses; energy reduced down to 10
mJ; minilite Continuum) and the analyzing system consisted of a
Synthesis of Isosorbide Diglycidyl Ether (IDE). Diallyl
isosorbide ether was prepared by the Williamson reaction. Isosorbide
(3 g, 20.4 mmol) was dissolved in a 50% aqueous NaOH (6g, 107.2
mmol) solution. Allyl bromide (10 mL, 115.7 mmol) was used as an
alkylating agent in the presence of 4.6% of tetrabutylammonium
bromide (TBAB, 300 mg, 0.94 mmol) with respect to isosorbide. The
reaction was stopped after 4 h of heating at 65 °C and the mixture was
extracted with methylene chloride. Freshly prepared diallyl isosorbide
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pulsed xenon lamp, a monochromator, a fast photomultiplier and a
transient digitizer20 (Luzchem LFP 212).
Redox Potentials. The electrochemical studies were carried out in
a three-electrode cell using an AUTOLAB PT30 potentiostat
(METROHM). The working electrode was a platinum disk (2 mm
diameter) which was carefully polished with an abrasive paper before
measurements. The counter-electrode was a gold wire and the
reference one a saturated calomel electrode (SCE). Acetonitrile was
distilled and used as solvent, and tetrabutylammonium tetrafluor-
oborate (NBu4BF4) as supporting electrolyte with a concentration of
0.1 mol. L−1. Ruthenium complex, silver salt or triphenylphosphine
were added to this electrolytic solution for obtaining a 10−2 mol L−1
concentration. NBu4BF4 was dried at 100 °C before use. All the
solutions were deoxygenated by bubbling N2 gas.
Transmission Electron Microscopy. observations were con-
ducted on a Tecnai F20 ST microscope (field-emission gun operated
at 3.8 kV extraction voltage) operating at an acceleration voltage of
200 kV and equipped with an X-ray energy-dispersive spectrometer
(XEDS from Oxford Instruments) for chemical analysis. The samples
were prepared as follows: a 15 μL drop of acetonitrile nanoparticle
suspension was deposited on a carbon-coated 400 mesh copper grid.
DFT Calculations. The structure of tris(triphenylphosphine)-
ruthenium was initially modeled using the experimental X-ray
structural parameters given by LaPlaca et al.21 and subsequently
optimized by DFT methods, using the B3LYP functional along with
the Stuttgart−Dresden basis set and pseudopotentials for ruthenium,
additionally including an extra f polarization function with a coefficient
of 0.96, the Ahlrichs triple-ξ, pVTZ, for carbon, chlorine, and
phosphorus, and Ahlrichs double-ξ, pVDZ, for hydrogen. Such
combination of basis set and functional has been previously proven
to be useful in the calculation of other ruthenium complexes. The
effect of the solvent, i.e., acetonitrile, was included through the
conductor-like PCM algorithm (CPCM). A time-dependent (TD)
calculation was performed on the optimized structure at TD-B3LYP
level using the same basis set as for the optimization and the CPCM
algorithm to account for solvent (acetonitrile) effects. The most
relevant transitions were analyzed by computing the natural transition
orbitals (NTO), which offers a compact orbital representation for
electronic transitions and reveals very useful to characterize the
transitions when more that one monoexcitation is relevant for the
transition using canonical orbitals, as was actually the case.
Figure 1. (A) Representation of Ru(PPh3)3Cl2 optimized at B3LYP/
LANL2DZ level and (B) comparison of the experimental (dashed
line) and the theoretical (solid line) spectra of tris-
(triphenylphosphine)ruthenium dichloride (Ru(PPh3)3Cl2). The
theoretical spectrum of Ru(PPh3)3Cl2, optimized at B3LYP/
LANL2DZ level and computed at TD-B3LYP/LANL2DZ levels,
includes the solvent effect (acetonitrile).
RESULTS AND DISCUSSION
■
1. Absorption Properties of [Ru(PPh3)3Cl2] Complex.
The fully optimized geometry of the complex ruthenium is
displayed in Figure 1A. As expected, the metal complex exhibits
a distorted square pyramidal geometry in agreement with the
crystallographic structure.21 Although the global trends are well
reproduced, this calculated DFT structure displays slightly
larger bond lengths as compared with those reported in the X-
ray structure (∼0.1 Å for Ru−P and Ru−Cl). For instance, the
apical Ru−P bond was found to be around 0.2 Å shorter than
the basal Ru−P distances, while the two Ru−Cl bonds are of
similar length and shorter than the basal Ru−P. The distorted
geometry of a pentacoordinated complex can be further
characterized through the τ parameter which is defined as the
difference between the two largest X-Metal-Y angle in degrees
divided by 60 and whose value ranges from 1 for a trigonal
pyramidal (TBP) to 0 for a square pyramidal geometry. The
crystallographic and DFT computed values of τ are 0.01 and
0.02, respectively, with P−Ru−P and Cl−Ru−Cl angles also
similar between X-ray and DFT, being 156.4 and 157.2 (X-ray),
and 157.4 and 158.8 (DFT). The ability of the DFT methods
to reproduce the experimental geometry was nevertheless
observed and, interestingly, the DFT level leads to a structure
closer to the crystallographic one, further validating our
procedure.
Regarding the spectroscopic properties, the UV−visible
absorption spectrum obtained in acetonitrile shows weakly
intense bands at around 355 and 385 nm and a more intense
one below 300 nm (Figure 1B). The vertical transitions
calculated by the TD-DFT method nicely match the
experimental ones and, therefore, allow a confident assignment
of these spectroscopic bands as shown in Figure 1B. Natural
transition orbitals22 (NTOs) offer the most compact view of an
excited state as a single orbital transition from the ground state,
and therefore have been computed to facilitate the spectro-
scopic assignment using the usual descriptions based on orbital
transitions. Note that, in the nomenclature of NTOs, the
transition is as follows: hole → particle. For instance, the
contribution of each fragment can be measured through the
contribution of the corresponding atomic orbitals to the NTO.
By computing the contributions at the hole and at the particle
NTO, one has an idea about how the electronic distribution
can be changed upon the transition. The contributions of each
relevant transition are shown in Table 1. This information,
together with the representation of the corresponding NTO
(Figure 2), can be used to identify the nature of each state.
According to Table 1 and Figure 2, the assignment of the
three main transitions can be ascribed as follow:
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Table 1. Contribution (%) of the Atomic Orbitals for each
Fragment to Hole and Particule NTOs
observed, i.e., a d → d transition from the dxz orbital to
the dz orbital.
a
2
It is worth noticing that a weak spectroscopic band at around
615 nm appears and is associated with an electronic transition
that displays an important metal to ligand charge transfer
(MLCT) character. Concretely, the transition goes from a
metal-centered orbital, dxz, to a hybrid molecular orbital mixing
the dz orbital on the metal with an antibonding orbital on the
apical triphenylphosphine ligand (Figure S3, see Supporting
Information).
2. Photochemical Properties of [Ru(PPh3)3Cl2] Com-
plex. The steady state photolysis of the Ru(PPh3)3Cl2 complex
was carried out in acetonitrile (Figure 3). A very fast
photobleaching occurs as revealed by the decrease of the
absorption band at 355 nm.
transition at 380
nm
transition at 357
nm
transition at 288
nm
fragment
hole
particle
hole
particle
hole
particle
metal Ru
14.2
0.3
15.7
5.6
4.4
5.3
12.5
3.2
3.8
4.9
13.9
3.9
chlorine 1
chlorine 2
phosphine 1
phosphine 2
phosphine 3
2
0.6
5.3
4.0
3.7
4
3.7
34.0
24.2
26.7
13.0
29.6
30.8
10.0
38.8
37.4
35.9
22.5
22.3
10.5
39.1
37.7
12.7
32.8
33.0
a
“Phosphine 1” is ascribed to the apical one.
Figure 3. Photolysis of Ru(PPh3)3Cl2 in acetonitrile. [Ru(PPh3)3Cl2]
= 1.13 × 10−3 mol L−1. Irradiation = Hg−Xe lamp; I0 = 150 mW cm−2.
Key: () before irradiation, (- - -) after 300 s of irradiation.
Figure 4 displays the experimental and calculated ESR-ST
spectra recorded after the UV irradiation (200−400 nm) of
Ru(PPh3)Cl2 under argon saturated tert-butylbenzene (Figure
4A) or toluene (Figure 4B). The presence of a phenyl radical
•
(C6H5 ) resulting from the homolytic P-(C6H5) single bond
photocleavage (Scheme 2) is clearly supported (Figure 4A; hfc
of the PBN radical adduct: aN = 14.1G and aH = 2.1G in
agreement with known values23). The phenyl reacts with
toluene and yields a benzyl radical: both kinds of PBN radical
adducts are observed in Figure 4B: (i) aN = 13.6G and aH = 2G
for the phenyl and (ii) aN = 14.3 G and aH = 2.9 G for the
benzyl. Surprisingly, the expected phosphorus−centered radical
Ru(PPh3)2(•PPh2)Cl2) (Scheme 2) is not trapped by PBN.
This could be explained by (i) the short lifetime of the
phosphorus-centered radical24 (<100 ns) or/and (ii) the
unfavorable addition of this crowded radical to PBN (indeed,
the unpaired electron in the SOMO of Ru(PPh3)2(•PPh2)Cl2)
is mainly delocalized over a π orbital of the PPh2 ligand and the
empty dz2 orbital of the ruthenium complex as shown in Figure
5). No transient absorption and bleaching are observed in laser
flash photolysis experiments suggesting a relatively low
dissociation quantum yield.
3. Redox Properties of [Ru(PPh3)3Cl2] Complex. The
comparison of the cyclic voltammograms of Ru(PPh3)Cl2 and a
free triphenylphosphine (Figure 6) reveals that the anodic and
cathodic peaks respectively located at 1.48 and 1.25 V/SCE are
specific of the RuII complex. The cyclic voltamogramm of Ph3P
in ACN shows three successive oxidation waves whose
identification has been previously reported.25 The first
oxidation wave (E ∼ 0.50 V vs SCE) corresponds to the
Figure 2. Natural transition orbitals computed at TD-DFT level for
the transitions at (A) 380, (B) 355, and (C) 288 nm including the
solvent effect (acetonitrile).
• Transition at 380 nm: In this case, Table 1 indicates a
significant charge transfer from the apical phosphine
toward basal ones, thus leading a transition with a
remarkable Interligand charge transfert (ILCT charac-
ter).
• Transition at 355 nm: It can be considered as an
interligand charge transfer, as the electronic density over
the basal ligands in the initial orbital is transferred to the
apical ligand in the final one. Figure 2 indicates that this
charge transfer arises along the transition from dxz to
2
2
dx −y in the metal.
• Transition at 288 nm: This transition does not involve a
significant charge displacement as it remains in the base
of the pyramid, but a change of the metal orbital is
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Figure 5. SOMO of the radical ruthenium complex (Ru-
(PPh3)2(•PPh2)Cl2) after the homolytic cleavage of one of the apical
P−Ph bonds, computed at DFT level.
Figure 4. ESR spin trapping experimental (1) and simulated (2)
spectra obtained during irradiation of Ru(PPh3)3Cl2 under argon (A)
in tert-butylbenzene and (B) in tert-butylbenzene/toluene. Xenon−
mercury lamp exposure. PBN: 0.05 M.
Scheme 2. Mechanistic approach to [Ru(PPh3)3Cl2]
Complex Photolysis under Argon Saturated Atmosphere in
(A) tert-Butylbenzene and (B) Toluene
Figure 6. Cyclic voltammograms of [Ru(PPh3)Cl2] complex (solid
line) and triphenylphosphine (dash line) in acetonitrile.
a
phine oxide (Ph3PO) and a proton which sequentially reacts
with a Ph3P molecule to generate Ph3PH+. It should noted that
these multiples oxidation waves are also observed for the
ruthenium complex and do not affect the positions of anodic
and cathodic peaks of the metal.
4. Photoinduced Generation of Silver Nanoparticles.
Figure 7 shows the typical evolution of the absorption spectrum
of Ru(PPh3)3Cl2 mixed with an excess of silver salt (AgSbF6) in
oxygen-free acetonitrile under UV irradiation. The rapid
increase (within ∼15 s) of a large band with a maximum
located at 450 nm can be safely assigned to the known26 silver
surface plasmon band. The Ag formation can be explained as
resulting from two reactions. First, an electron transfer between
the excited state of Ru(PPh3)3Cl2 and AgSbF6 is favorable.
Indeed, according to the Rehm−Weller equation, the ΔG for
this electron transfer is negative (ΔG = −1.7 eV using ERed
(AgSbF6) = +0.21 V/SCE and EOx (Ru(PPh3)3Cl2) = +0.5 V/
SCE; the triplet state energy level of Ru(PPh3)3Cl2 is not
known but is probably close to 2 eV as evaluated from the red-
shifted absorption (see above) at 615 nm). Second, the
phosphinyl radical generated during the photolysis of Ru-
(PPh3)3Cl2 (Scheme 2) is oxidized by AgSbF6: this process is
similar to the reduction of an onium salt by a phosphinoyl
radical as described in.27 The addition of a H-donor solvent
such as THF (Figures 7C and 7D) accelerates the formation of
the silver nanoparticles28 according to reactions 1 and2.EDX
and TEM experiments confirm the formation of Ag NPs. The
EDX spectroscopy carried out on the Ru(PPh3)3Cl2/AgSbF6
acetonitrile solution (Figure 8) reveals an intense silver peak at
a
The UV light intensity is 180 mW/cm2. Radicals are trapped by PBN.
oxidation of triphenylphosphine to triphenylphosphonium
radical whereas the other waves correspond to the oxidation
of secondary products stemming from a reaction between this
generated radical cation with residual water in the solvent. As
proposed by Ohmori et al,25b a very fast redox reaction occurs
between +•PPh3 with residual water yielding triphenylphos-
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Figure 7. (A) Evolution of the absorption spectra during the irradiation of Ru(PPh3)3Cl2 (3 × 10−4 mol L−1)/AgSbF6 (1.3 × 10−3 mol L−1) in
acetonitrile. Inset: evolution of the absorbance change at 450 nm with the irradiation time. (B) Sample before and after 15 s of irradiation. (C)
Evolution of the absorption spectra during the irradiation of Ru(PPh3)3Cl2 (3 × 10−4 mol L−1)/AgSbF6 (1.3 × 10−3 mol L−1)/THF (2 mL). (D)
Evolution of the Ag NPs formation (absorbance at 450 nm) in the absence (1) and presence of THF (2). All the solutions are degassed with argon
prior to irradiation. Hg−Xe lamp; I0 = 150 mW cm−2.
species according to the ESR spin-trapping results and the very
high ki (of phenyl radicals) to acrylates.12 It should be pointed
out that during the free-radical photopolymerization of the
acrylate monomers, an induction period is observed. Indeed,
during the photoinitiating process, the remaining oxygen
molecules of the photopolymerized solution react with radical
species to generate inert peroxyls ROO• that are not reactive
toward the acrylate double bonds and cannot initiate any
polymerization reaction12 (induction period). When all the
oxygen molecules are consumed, the photopolymerization can
start.
6. Photopolymerization of Epoxy Matrix with [Ru-
(PPh3)3Cl2] complex/AgSbF6. The irradiation of a diepoxide
in laminate using Ru(PPh3)3Cl2 (1 wt %) in the presence of
ACN (0.5 mL) or THF (0.5 mL) does not lead to any efficient
polymerization (Figure 10, parts A-1 and B-1; final conversion,
10% after 300 s of irradiation). The addition of AgSbF6 (3 wt
%) to Ru(PPh3)3Cl2/ACN (Figure 10A-2) has no influence on
the final conversion. Interestingly, the use of Ru(PPh3)3Cl2/
AgSbF6/THF (Figure 10B-2) leads to a significant improve-
Figure 8. (A) TEM image of the photoinduced silver nanoparticles
solution and (B) EDX spectra acquired in STEM. Xenon−mercury
lamp exposure (15 s, argon atmosphere).
ment of the cationic polymerization (the final conversion
3 eV, thus indicating the presence of substantial amounts of
Ag(0) in the analyzed area. The TEM image of the
increases from 10% to 50% and exhibits a brown coloration
thereby confirming the presence of Ag NPs). This suggests that
photoinduced silver nanoparticles solution is also shown in
Figure 8.
5. Photopolymerization of Acrylate Matrixes with
[Ru(PPh3)3Cl2] Complex. The photopolymerization profiles
the in situ reduction of the silver salt to Ag mainly occurs
according to eq1 and eq2; the main initiating cationic species is
the THF cation.
of a diacrylate (PEGDA) and a dimethacrylate monomers
(EBDMA) in laminate upon UV light exposure in the presence
of Ru(PPh3)3Cl2 are displayed in Figure 9 (to increase the
solubility of the Ru complex, a small amount (1 mL) of THF or
ACN was introduced). Final conversions are almost 100% after
50 s of irradiation for PEGDA but only 70% after 300s for
EBDMA. The phenyl radicals are obviously the initiating
CONCLUSION
■
In the present paper, we have shown that, under UV light
activation, Ru(PPh3)3Cl2 exhibits a interesting photocleavage
ability to generate phenyl radicals being able to (i) initiate a
(meth)acrylate polymerization and (ii) react with THF. This
last reaction is particularly important when using a Ru-
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Figure 9. Photopolymerization profiles. A) Poly(ethylene glycol) (400) diacrylate (PEGDA) in the presence of Ru(PPh3)3Cl2 (1 wt %). B)
Ethoxylated bisphenol A dimethacrylate (EBDMA) in the presence of (1) Ru(PPh3)3Cl2 (1 wt %) and (2) absence of Ru(PPh3)3Cl2. In laminate.
Polychromatic Xe−Hg lamp irradiation; I0 = 150 mW cm−2.
Figure 10. Photopolymerization profiles of a diepoxide in the presence of (A) (1) Ru(PPh3)3Cl2 (1 wt %)/ACN (0.5 mL) and (2) Ru(PPh3)3Cl2 (1
wt %)/AgSbF6 (3 wt %)/ACN (0.5 mL) and (B) (1) Ru(PPh3)3Cl2 (1 wt %)/THF (0.5 mL) and (2) Ru(PPh3)3Cl2 (1 wt %)/AgSbF6 (3 wt
%)/THF (0.5 mL). In laminate. Polychromatic Xe−Hg lamp irradiation; I0 = 150 mW cm−2.
(PPh3)3Cl2/silver salt/THF photoinitiating system as it allows
(i) the efficient polymerization of a cationic matrix and (ii) the
concomitant in situ production of Ag(0) NPs. This approach
evidence the role of the phosphine ligands in the ruthenium
complex series toward photopolymerization processes and
should broaden the application portfolio of such compounds. It
could also be the starting point of further studies for developing
biobased antibacterial coatings.
ACKNOWLEDGMENTS
■
The authors would like to thank the CNRS Institute for
financial support.
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S
* Supporting Information
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Corresponding Author
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Notes
The authors declare no competing financial interest.
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