Subtle ligand effects in oxidative photocatalysis with iridium complexes: Application to photopolymerization

Article abstract of DOI:10.1002/chem.201101445

Iridium(III) complexes were designed and evaluated as efficient photoinitiators of polymerization reactions in combination with iodonium salts and silanes. Mechanistically, these reactions were shown to proceed through oxidative photoredox catalysis, generating aryl and silyl radicals under very soft irradiation conditions (blue LED, xenon lamp, and even sunlight). These radicals can initiate the free radical polymerization of acrylates or can be oxidized during the catalytic cycle to promote the ring-opening polymerization of epoxy monomers. Remarkably, both the (photo)chemical reactivity and the practical efficiency are dramatically affected by the ligands. In addition, the central role played by the oxidation ability of the excited state of the photocatalyst is discussed.

Full text of DOI:10.1002/chem.201101445

FULL PAPER
DOI: 10.1002/chem.201101445
Subtle Ligand Effects in Oxidative Photocatalysis with Iridium Complexes:
Application to Photopolymerization
Jacques Lalevꢀe,*[a] Mathieu Peter,[a] Frꢀdꢀric Dumur,[b] Didier Gigmes,*[b]
Nicolas Blanchard,[c] Mohamad-Ali Tehfe,[a] Fabrice Morlet-Savary,[a] and
Jean Pierre Fouassier[d]
Abstract: Iridium
(III) complexes were
LED, xenon lamp, and even sunlight).
These radicals can initiate the free rad-
ical polymerization of acrylates or can
be oxidized during the catalytic cycle
to promote the ring-opening polymeri-
zation of epoxy monomers. Remarka-
bly, both the (photo)chemical reactivity
and the practical efficiency are dramat-
ically affected by the ligands. In addi-
tion, the central role played by the oxi-
dation ability of the excited state of the
photocatalyst is discussed.
designed and evaluated as efficient
photoinitiators of polymerization reac-
tions in combination with iodonium
salts and silanes. Mechanistically, these
reactions were shown to proceed
through oxidative photoredox catalysis,
generating aryl and silyl radicals under
very soft irradiation conditions (blue
Keywords: iridium · photochemis-
try · photoredox catalysis · ring-
opening polymerization · silyl radi-
cals
Introduction
dtb-bpy=4,4’-di-tert-butyl-2,2’-dipyridyl).[1–3] These strong
metal-centered reductants (e.g., [Ir(ppy)2A(dtb-bpy)], [Ru-
(bpy)3]+) or oxidants (e.g., [Ir(ppy)3]+, [Ru(bpy)3]3+) can ef-
G
CHTUNGTRENNUNG
Photoredox catalysis recently emerged as a powerful ap-
proach in the synthetic community for the activation of dif-
ferent chemical bonds, owing to the smooth generation of
free radical species under soft irradiation conditions.[1–3] As
far as the mechanism is concerned, it is assumed that the ex-
cited state of the photocatalyst (PC) reacts with an oxidizing
or reducing agent by an electron-transfer reaction (amine,
methyl viologen, enamine, etc.), thereby generating a strong
reductant or oxidant metal complex; for example, [Ru-
E
R
ACHTUNGTRENNUNG
C
ficiently promote radical reactions; for example, CF3 radi-
cals can be generated by a single-electron-transfer from [Ir-
ACHUTNGREN(NUG ppy)2ACHTUNTGREN(NUNG dtb-bpy)] to CF3I. Very interestingly, the PCs are
characterized by an intense visible-light absorption and can
be used with practical visible light sources, such as fluores-
cent or LED bulbs, or even ambient sunlight.
The use of organometallic compounds as photoinitiating
systems is well documented in the literature and received a
revival of interest[4] in the last years. Recently, the photore-
dox catalysis approach has been extended to photopolymeri-
zation applications.[5] An efficient oxidative cycle has been
proposed for the formation of silyl radicals and silylium ions
(Scheme 1).[5] 1) The PC excited state reacts with an iodoni-
um salt to generate the strong oxidant metal complex (PC+)
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
G
CHTUNGTRENNUNG
[a] Prof. J. Lalevꢀe, Dr. M. Peter, Dr. M.-A. Tehfe, Dr. F. Morlet-Savary
Institut de Science des Matꢀriaux de Mulhouse
IS2M, LRC CNRS 7228–ENSCMu-UHA
C
C
and phenyl radicals (Ph ). 2) Silyl radicals R3Si are generat-
ꢀ
C
ed by a hydrogen abstraction process from Ph /R3Si H. 3)
15, rue Jean Starcky, 68057 Mulhouse Cedex (France)
E-mail: j.lalevee@uha.fr
[b] Dr. F. Dumur, Dr. D. Gigmes
UMR 6264 Laboratoire Chimie Provence, Equipe CROPS
Universitꢀ de Provence, Avenue Escadrille Normandie-Niemen
Case 542, Marseille 13397, Cedex 20 (France)
E-mail: didier.gigmes@univ-provence.fr
[c] Dr. N. Blanchard
University of Haute Alsace, ENSCMu
Laboratory of Organic and Bioorganic Chemistry
3 rue Alfred Werner, 68093 Mulhouse Cedex (France)
[d] Prof. J. P. Fouassier
University of Haute Alsace, ENSCMu
4 Rue des Frꢁres Lumiꢁre
68200 Mulhouse (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101445.
Scheme 1. Proposed oxidative cycle for the formation of silyl radicals and
silylium ions.
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The formation of the silylium ion R3Si+ (R3Si+ can initiate
Results and Discussion
the ring-opening polymerization of epoxides:[6] R3Si++M!
+
ꢀ
R3Si M , with M=epoxy) is easily obtained by oxidation
PCs Ir1–Ir6 in photoinitiating systems for the ring-opening
polymerization: The ring-opening photopolymerization of
(3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxy-
late (EPOX) under air in the presence of Ir1–Ir6/Ph2I+ is
quite slow using blue LED bulb irradiation. Even upon
xenon lamp exposure, a rather low final conversion is
reached: <25% after 180 s of irradiation (Figure S1 in the
Supporting Information).
+
C
of R3Si by PC ; moreover, a PC regeneration is observed.
The oxidation of the silyl by Ph2I+ can also be expected.[5]
Albeit good to excellent polymerization profiles were ob-
tained with homoleptic complexes [Ru
(bpy)3]2+ and [Ir-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
erties represents an exciting challenge. More specifically, it
would be worthwhile 1) to tune the absorption of the initiat-
ing system for a better matching with the irradiation device
and 2) to enhance the chemical reactivity of the initiating
systems for polymerization under very soft irradiations con-
ditions (sunlight, household fluorescence, or blue LED
bulbs). Herein, six new PCs, Ir1–Ir6, are presented for pho-
topolymerization processes. The underlying chemical mecha-
nisms are investigated by ESR and luminescence experi-
ments.
In contrast, excellent polymerization profiles are obtained
using the different three-component systems (PC/tris(trime-
thylsilyl)silane/Ph2I+) under both blue LED bulb irradiation
and xenon lamp exposure (Figure 1 and Figure S1 in the
Supporting Information). The formation of the polyether
network is easily characterized by its absorption band at
1080 cmꢀ1 and the epoxy conversion can be followed at
790 cmꢀ1 (Figure 1A). A high Si H consumption is noted
ꢀ
(close to 50% for the different initiating systems) in full
agreement with the formation of silyl radicals through the
hydrogen abstraction reaction (Scheme 1, reaction 3). For
Ir5/tris(trimethylsilyl)silane/Ph2I+, the stability of the formu-
lation is not good enough (<1 day) and the results obtained
with Ir5 will not be discussed further. The best system (Ir1)
exhibits a higher polymerization initiating ability than that
found with other classical systems based on phosphine oxide
or titanocene.[6]
The ligand on the Ir center strongly affects the initiating
ability of these three-component systems with a reactivity
following the order: Ir1>[IrACHTNUGTRENUNG(ppy)3]>Ir3>Ir2>Ir4>Ir6
(Figure 1B). This behavior is found for xenon lamp and
blue LED bulb irradiation (see also the polymerization
rates for xenon lamp irradiation in Table S1 in the Support-
ing Information). These results highlight the interest of an
appropriate selection of the Ir derivatives to improve the
polymerization process. For the best initiating system (Ir1/
tris(trimethylsilyl)silane/Ph2I+), the polymerization under
sunlight is a very efficient process (Figure S2 in the Support-
ing Information), with a conversion >75% for 600 s of irra-
diation.
C
C
The formation of Ph and (TMS)3Si (Scheme 1) for these
three-component systems can be also highly desirable to ini-
tiate radical polymerization processes. The radical addition
onto the monomer will be in competition with the oxidation
process 3 (Scheme 1). The polymerization profile of trime-
thylolpropane triacrylate is found excellent for Ir1/tris(tri-
methylsilyl)silane/Ph2I+ as photoinitiating system, showing
an interesting dual radical/cationic initiating character (Fig-
ure 1C).
The ligands were selected to generate different light-ab-
sorption properties, redox potentials, and excited-state reac-
tivity. It is also worth noting that only few data are available
for the selected compounds and that they have been deter-
mined in this work.
Role of the ligands for the PC reactivity: The visible absorp-
tion spectra of the different PCs are depicted in Figure S3A
in the Supporting Information. At 462 nm, which represents
the maximal emission wavelength for the blue LED bulb,
the extinction coefficients are 2100, 300, 650, 1000, 7200,
Interestingly, the selected homoleptic and heteroleptic IrIII
derivatives exhibit triplet emitter properties. The polymer
films obtained through the new proposed approach are also
characterized by original photoluminescence properties.
and 1100mꢀ1 cmꢀ1 for Ir1, Ir2, Ir3, Ir4, Ir6, and [Ir
ACTHUNTRGNE(UGN ppy)3], re-
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Ligand Effects in Oxidative Photocatalysis
FULL PAPER
cribed to the formation of [Ir1]+ and [Ir2]+ in a way similar
to that reported for [IrACHTNUTGRNEUNG
(ppy)3].[5,7] This is in full agreement
with the fast oxidative quenching of the different Ir deriva-
tives by Ph2I+ (k1 =0.4–9ꢃ109 mꢀ1 sꢀ1, Table 1 and Figure 2).
The luminescent excited states are strongly quenched by O2
(Table 1), that is, their interaction rate constants with Ph2I+
(k1) were determined under argon. Upon the blue LED
bulb exposure of PC/Ph2I+ solutions, Ph radicals are detect-
C
ed by ESR spin-trapping experiments with phenyl-N-tert-bu-
tylnitrone (PBN, Figure 3). All these results demonstrate
Table 1. Oxidation potentials (Eox), excited state energy levels (ET), lu-
minescence lifetimes under argon (t), rate constants for the interaction
of the luminescent state with Ph2I+ and O2 in acetonitrile, and DG for re-
action 1 for the different investigated Ir complexes. The error bar for the
rate constants (k (Ph2I+) and k (O2)) is 10%.
[c]
k (Ph2I+)
[mꢀ1 sꢀ1
k (O2)
t [ns] Eox ([V] vs. ET
DG[d]
[eV]
]
[mꢀ1 sꢀ1
]
SCE)
[eV]
[Ir
N
8.7ꢃ109
9.1ꢃ109
8.9ꢃ109
9.0ꢃ109
8.8ꢃ109
8.5ꢃ109
1300
280
1600
890
0.77[a]
0.76
1.10
1.30
1.27
2.5
ꢀ1.53
ꢀ1.29
ꢀ1.45
ꢀ1.1
Ir1
Ir2
Ir3
Ir4
Ir6
2.25
2.75
2.6
2.6
2.6
830
ꢀ1.13
ꢀ1.07
760[b] 1.33
[a] From reference [9]. [b] The long-lived transient is observed at 550 nm.
[c] Evaluated from the luminescence band edge (see Figure S3 in the
Supporting Information for the different luminescence spectra). [d] DG=
EoxꢀEredꢀET ; calculated from the classical Rehm–Weller equation, in
which Eox, Ered, and ET are the oxidation potential of the donor, the re-
duction potential of the acceptor, and the triplet state energy; Ered
=
ꢀ0.2 V was used for Ph2I+.[5]
Figure 1. A) IR spectra recorded during the photopolymerization of
EPOX under air in the presence of Ir1/tris(trimethylsilyl)silane/Ph2I+
(0.2%/3%/2% w/w); blue LED bulb irradiation at t=0, 30, 60 and 180 s.
B) Polymerization profiles of EPOX (conversion vs. time) under air upon
blue LED bulb irradiation in the presence of a) Ir1/tris(trimethylsilyl)si-
lane/Ph2I+ (0.2%/3%/2% w/w); b) [Ir
ACTHNUTRGNEU(GN ppy)3]/tris(trimethylsilyl)silane/
Ph2I+ (0.2%/3%/2% w/w); c) Ir3/tris(trimethylsilyl)silane/Ph2I+ (0.2%/
3%/2% w/w); d) Ir2/tris(trimethylsilyl)silane/Ph2I+ (0.2%/3%/2% w/w);
e) Ir4/tris(trimethylsilyl)silane/Ph2I+ (0.2%/3%/2% w/w); f) Ir6/tris(tri-
methylsilyl)silane/Ph2I+ (0.2%/3%/2% w/w). C) Polymerization profiles
of trimethylolpropane triacrylate upon xenon lamp irradiation (l>
390 nm) in the presence of 1) Ir1 (0.2% w/w) and 2) Ir1/tris(trimethylsi-
lyl)silane/Ph2I+ (0.2%/3%/2% w/w); laminated conditions.
spectively. These PCs exhibit an interesting tunable absorp-
tion and allow a recovering of the photons up to 650 nm.
Upon the blue LED bulb exposure, a bleaching of PC/
Ph2I+ is found (e.g., Figure S3C in the Supporting Informa-
tion for Ir1). The bleaching of Ir1 and Ir2 is accompanied by
the buildup of a new absorption band (lmax ꢁ560 nm), as-
Figure 2. A) Emission decay curves of Ir4 monitored at 550 nm for differ-
ent [Ph2I+] (0–0.018m); degassed (argon) acetonitrile; excitation at
355 nm. B) Emission decay curves of Ir1 monitored at 570 nm: a) de-
gassed (argon) and b) under air; acetonitrile; excitation at 355 nm. a.u.=
arbitrary units.
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J. Lalevꢀe, D. Gigmes et al.
Photoluminescence of the polymer films: Interestingly, the
different iridium derivatives are involved in the photoinitiat-
ing systems as part of a catalytic cycle (see Scheme 1); these
complexes acting as photocatalysts are thus regenerated
after use and incorporated at their initial oxidation state
into the polymers, hence providing the photoluminescence
properties to the polymer film. Since such compounds are
already known for their luminescence properties, for exam-
ple, triplet emitters for OLED applications,[9,11] the polymer
films obtained through the new presented approach are
characterized by photoluminescence properties. The emis-
sion wavelengths can be tuned by an appropriate choice of
the ligand (from 480 to 620 nm, see Figure S3B in the Sup-
porting Information). Remarkably, the luminescence of the
selected compounds does not decrease during the polymeri-
zation process (Figure S4 in the Supporting Information);
this is in agreement with the photocatalyst behavior of the
Ir complexes, that is, these compounds are regenerated in
the photoinitiating system. Interestingly, the luminescence
quantum yield probably increases in the polyether network,
since the photoluminescence of the film also increases
during the polymerization process (Figure S4 in the Support-
ing Information). These unique results highlight the dual be-
havior of the photocatalyst: 1) The photocatalyst is necessa-
ry in the photoinitiating system for the formation of the
polymer network and 2) it is incorporated in the formed
polymer at its initial oxidation state by regeneration in situ
of the catalyst during the catalytic process (see Scheme 1)
and provides the desired photoluminescence properties to
the polymer film.
Figure 3. ESR spectra obtained after blue LED bulb irradiation of Ir1/
Ph2I+ (in tert-butylbenzene/acetonitrile, phenyl-N-tert-butylnitrone
(PBN) is used as spin-trap, [Ph2I+]=0.011m): experimental 1) and simu-
lated 2) spectra (hyperfine coupling constants for the radical adduct: aN =
14.4 G and aH =2.3 G, in agreement with the known data for the Ph· spin
adduct).[8]
that reaction 1 occurs for the new PCs (Scheme 1). For Ir
derivatives/tris(trimethylsilyl)silane/Ph2I+, the formation of
the silyl radicals is also well shown by ESR spin-trapping ex-
periments with phenyl-N-tert-butylnitrone (hyperfine cou-
pling constants: aN ꢁ15.0 G, aH ꢁ5.6 G, in agreement with
the known data for this structure[8]). Photolysis experiments
show that the Ir derivatives are now regenerated. All these
results clearly show that the mechanism presented in
Scheme 1 for [Ir
The differences observed in PC activity (Ir1>[Ir
Ir3>Ir2>Ir4>Ir6) can be ascribed to 1) the absorption
properties (for 462 nm: Ir6>Ir1>[Ir(ppy)3]>Ir4>Ir3>Ir2,
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
see the absorption coefficients above) and 2) the reactivity
Conclusion
of the luminescent state with Ph2I+ to initiate the catalytic
cycle (Scheme 1, reaction 1, Ir1>Ir2>[Ir
(ppy)3]>Ir3ꢁ
Selected Ir complexes are presented as new photocatalysts.
Interestingly, the selected ligands strongly affect the PC abil-
ity. For the oxidative cycle, the key parameters are the ab-
sorption properties and the oxidation ability of the excited
states (*Ir) by Ph2I+. Ir1 is proposed as a new promising PC
with an enhanced reactivity. The ability of these PCs for
other polymerization initiating systems based on reductive
cycles will be presented in forthcoming work. The use of Ir1
for organic synthesis applications is also highly desirable
and the luminescence properties of the formed polymers for
OLED applications deserve to be investigated in future
work.
Ir4>Ir6). The rate constants k1 are found related to the oxi-
dation potentials (Eox) of Ir complexes (Table 1); for exam-
ple, the rate constants decrease in the series Ir1>Ir3>Ir6 in
agreement with their oxidation ability (Eox =0.76, 1.3, and
1.33 V for Ir1, Ir3, and Ir6, respectively). The free energy
changes for reaction 1 (Scheme 1), calculated from the clas-
sical Rehm–Weller equation (Table 1), support the electron
transfer process.[10] For the iridium complexes characterized
by DG<ꢀ1.29 eV (Ir1, Ir2, and [Ir
ACHTUNGRTEN(NUNG ppy)3]), the reaction is
diffusion controlled (k1 >5ꢃ109 mꢀ1 sꢀ1). For the other com-
plexes, DG is less favorable and accordingly the associated
rate constants decrease.
Ir1 exhibits both very good light absorption properties (>
[IrACHTUNGTRENNUNG(ppy)3]) and the highest k1, thus leading to the best per-
Experimental Section
formance. For Ir6, despite an excellent absorption, this com-
plex is characterized by the lowest reactivity for reaction 1.
For Ir2–Ir4, different intermediate situations occur and the
PC ability is a compromise between the absorption and the
excited state reactivity. The optical yields and the excited
state life times can also probably affect the efficiency of the
initiation.
IridiumACTHUNRTGNEUNG(III) complexes: The six metal complexes Ir1–Ir6 were prepared
according to the different procedures outlined in the Supporting Informa-
tion. Typically, the synthetic procedure for the new complexes Ir1, Ir4,
Ir5, and Ir6 included two steps. In a first step, the chloro-bridged dimers
[Ir2Cl
(2,4-difluorophenyl)pyridine) were synthesized according to the Nonoya-
ma route starting from ppy, dfppy, and iridium
(III) chloride.[12] Then, in a
second step, all complexes were obtained by a bridged-splitting reaction
2ACHTUNGTRENGU(N ppy)4] (ppy=2-phenylpyridine) and [Ir2Cl2ACHTUNGTERN(NUGN dfppy)4] (dfppy=2-
AHCTUNGTRENNUNG
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Ligand Effects in Oxidative Photocatalysis
FULL PAPER
of the dimers with two equivalents of the ancillary ligand. In particular,
the dimeric cleavage of [Ir2Cl2ACHTUNGTRENNG(U ppy)4] and [Ir2Cl2HCATUTGNREN(NUGN dfppy)4] to form the
(0.1m) as a supporting electrolyte (Voltamaster 6 Radiometer; the work-
ing electrode was a platinum disk and the reference electrode a saturated
calomel electrode (SCE)). Ferrocene was used as a standard and the po-
tentials determined from the half-peak potential were referred to the re-
versible, formal potential of this compound (+0.44 V/SCE).
neutral complexes Ir1 and Ir2 was performed in the presence of Ag+ to
remove Clꢀ. Depending on the charge of the complex, two distinct purifi-
cation procedures were carried out. Purification of the neutral complexes
(Ir1 and Ir2) was obtained by column chromatography on silica gel, fol-
lowed by the precipitation of the two complexes during evaporation of
the chromatography fractions. When charged, the different complexes
(Ir3 and Ir4) were isolated by metathesis with a fivefold excess of an
aqueous solution of sodium hexafluorophosphate from a solution of the
complex in methanol. All complexes were stable under air and the struc-
tures of the complexes were established by 1H, 13C, and, where applica-
ble, 31P or 19F NMR spectroscopy. Complexes were also characterized by
HRMS. Purities higher than 90% for the six complexes were determined
Acknowledgements
This work was supported by the “Agence Nationale de la Recherche”
(grant ANR 2010-BLAN-0802). J.L. thanks the Institut Universitaire de
France IUF for the support.
1
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monomers, respectively.
For the free-radical-promoted cationic polymerization (FRPCP), the two-
and three-component photoinitiating systems were based (except other-
wise stated) on Ir1–Ir6/Ph2I+ (0.2%/2% w/w) and Ir1–Ir6/TTMSS/Ph2I+
(0.2%/3%/2% w/w). The EPOX films (25 mm thick) deposited on a BaF2
pellet were irradiated under air inside the IR spectrometer cavity. The
evolution of the epoxy group content at about 790 cmꢀ1 was continuously
followed by real-time FTIR spectroscopy (Nexus 870, Nicolet), as report-
ed in reference [5]. For free-radical polymerization (FRP), the two- and
three-component photoinitiating systems were based on Ir1–Ir6/Ph2I+
(0.2%/2% w/w) and Ir1–Ir6/TTMSS/Ph2I+ (0.2%/3%/2% w/w). The
evolution of the double bond content was continuously followed by real
time FTIR spectroscopy (see above) at about 1630 cmꢀ1. The different ir-
radiation sources are presented in the Supporting Information.
ESR experiments: ESR spin-trapping (ESR-ST) experiments were car-
ried out using a X-Band EMX-plus spectrometer (Bruker Biospin). The
radicals were produced at room temperature under LED bulb exposure
and trapped by phenyl-N-tert-butylnitrone (PBN), according to a proce-
dure described in detail in reference [5].
Luminescence lifetimes: The luminescence lifetimes were determined
with an Edinburgh LP900 laser flash photolysis spectrometer in emission
mode. The emission wavelengths were selected from the associated lumi-
nescence spectra recorded in acetonitrile (FP-750, JASCO; spectra given
in the Supporting Information).
Received: May 11, 2011
Revised: September 2, 2011
Redox potentials: The redox potentials were measured in acetonitrile by
cyclic voltammetry with tetrabutylammonium hexafluorophosphate
Published online: November 28, 2011
Chem. Eur. J. 2011, 17, 15027 – 15031
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15031