New core-pyrene π structure organophotocatalysts usable as highly efficient photoinitiators

Article abstract of DOI:10.3762/bjoc.9.101

Eleven di- and trifunctional compounds based on a core-pyrene π structure (Co-Py) were synthesized and investigated for the formation of free radicals. The application of two- and three-component photoinitiating systems (different Co-Pys with the addition of iodonium or sulfonium salts, alkyl halide or amine) was investigated in detail for cationic and radical photopolymerization reactions under near-UV-vis light. The proposed compounds can behave as new photocatalysts. Successful results in terms of rates of polymerization and final conversions were obtained. The strong MO coupling between the six different cores and the pyrene moiety was studied by DFT calculations. The different chemical intermediates are characterized by ESR and laser flash photolysis experiments. The mechanisms involved in the initiation step are discussed, and relationships between the core structure, the Co-Py absorption property, and the polymerization ability are tentatively proposed.

Full text of DOI:10.3762/bjoc.9.101

New core-pyrene π structure organophotocatalysts
usable as highly efficient photoinitiators
Sofia Telitel1, Frédéric Dumur2, Thomas Faury2, Bernadette Graff1,
Mohamad-Ali Tehfe1, Didier Gigmes*2, Jean-Pierre Fouassier3
and Jacques Lalevée*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 877–890.
1Institut de Science des Matériaux de Mulhouse IS2M – UMR 7361 –
UHA; 15, rue Jean Starcky, F-68057 Mulhouse Cedex, France,
2Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, UMR
7273, F-13397 Marseille, France and 3ENSCMu-
UHA, 3 rue Alfred Werner, F-68093 Mulhouse Cedex, France
doi:10.3762/bjoc.9.101
Received: 02 March 2013
Accepted: 12 April 2013
Published: 07 May 2013
Email:
This article is part of the Thematic Series "Organic free radical chemistry".
Guest Editor: C. Stephenson
Didier Gigmes* - didier.gigmes@univ-amu.fr; Jacques Lalevée* -
jacques.lalevee@uha.fr
* Corresponding author
© 2013 Telitel et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
cationic photopolymerization; free-radical-promoted cationic
photopolymerization; photocatalysts; photoinitiators; radical
photopolymerization
Abstract
Eleven di- and trifunctional compounds based on a core-pyrene π structure (Co_Py) were synthesized and investigated for the for-
mation of free radicals. The application of two- and three-component photoinitiating systems (different Co_Pys with the addition of
iodonium or sulfonium salts, alkyl halide or amine) was investigated in detail for cationic and radical photopolymerization reac-
tions under near-UV–vis light. The proposed compounds can behave as new photocatalysts. Successful results in terms of rates of
polymerization and final conversions were obtained. The strong MO coupling between the six different cores and the pyrene moiety
was studied by DFT calculations. The different chemical intermediates are characterized by ESR and laser flash photolysis experi-
ments. The mechanisms involved in the initiation step are discussed, and relationships between the core structure, the Co_Py
absorption property, and the polymerization ability are tentatively proposed.
Introduction
Free radical sources are encountered in various areas such as (FRP), where the PIs work as either cleavable type I PIs or
organic chemistry, biochemistry and polymer chemistry. In the uncleavable type II PIs in dependence of couples formed by the
field of polymer photochemistry applied to photopolymeriza- PI and hydrogen or electron donors (r1 in Scheme 1). Secondly,
tion reactions, they are referred to as photoinitiators (PI) [1]. PIs may be used in free-radical-promoted cationic polymeriza-
These PIs are usable in two scenarios, both of which are light tion (FRPCP), in which the produced radical R• is oxidized by
induced. Firstly, they are usable in free radical polymerization an onium salt, e.g., Ar2I+, to form Ar2I• and a cation R+ suit-
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Beilstein J. Org. Chem. 2013, 9, 877–890.
The photopolymerization reactions in radiation curing technolo-
gies are often carried out under high-intensity light sources
(>1000 mW cm−2). Applications in this area and in other fields
may require to avoid the use of UV rays delivered by Hg lamps
or to irradiate with visible light (400–700 nm) and low photon
fluxes (~2–10 mW cm−2). Recent works have aimed at working
under soft irradiation conditions (near-UV–vis light, low light
intensity), which allows the use of Xe lamps, laser diodes, solar
radiation, household halogen lamps, fluorescent bulbs and blue,
Scheme 1: Typical reactions for photoinitiated cationic polymerization.
able for the ROP reaction. The Ar2I• species is readily decom- green and white LEDs (see a recent review in [1]).
posed into ArI and Ar• (r2 in Scheme 1). There is also an usual
cationic polymerization (CP; r3 in Scheme 1) [1,2]. When the New high-performance PIs and PICs are continuously being
PIs exhibit a catalytic behavior analogous to a photocatalyst developed [1-10] through the design of novel structures (e.g.,
(PC) in organic chemistry, they are designed as photoinitiator novel chromophores, novel cleavable bonds) and the modifica-
catalysts (PIC). PIs as well as PICs are based on pure organic, tion ofexisting structures by using the classical introduction of
that is, metal free, or organometallic compounds. In the pres- electron-donating or -withdrawing substituents, conjugated
ence of additives, they constitute a photoinitiating system (PIS). groups or moieties, and more complex changes on the skeleton
(see typical recent examples in [11-41]). To meet the challenge
Among others, two points of interest in the photopolymeriza- of designing efficient PIs and PICs under soft irradiation, we
tion area are (i) the reactivity and efficiency of the PIs and PICs have recently started the search for suitable multifunctional
and (ii) the irradiation conditions. Indeed, the absorption prop- arrangements exhibiting a strong coupling of the molecular
erties (wavelengths λ, molar extinction coefficients ε), the orbitals MOs [42,43]. Improved absorption properties (red-
excited-state processes, the reactivity of the produced radicals, shifted λ, higher ε) while keeping a high reactivity have been
and the characteristics of the light sources (emission spectra, already achieved. Examples of these investigated architectures
available luminous power density) govern the polymerization (Scheme 2) [42-44] are linked PI–PI units (e.g., PI = pyrene) or
efficiency (rate of polymerization, final monomer conversion). PI-PI’ (e.g., PI = pyrene and PI’ = 2,2’-dimethoxy-2-phenylace-
Scheme 2: Examples of previously investigated architectures.
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Beilstein J. Org. Chem. 2013, 9, 877–890.
tophenone) and PI moieties (e.g., PI = benzophenone, thioxan- ized truxene (Py_5), several substituted phenyl rings (Py_2,
thone, 2,2’-dimethoxy-2-phenylacetophenone, pyrene) linked to Py_3, Py_4, Py_9, Py_10) two functionalized triazines (Py_7,
a trifunctional core (truxene, triazine, benzene); the same could Py_12), a triphenylamine (Py_6), a carbazole (Py_11) and a
be expected by using a difunctional core.
benzothiadiazole (Py_8) (Scheme 3). The idea is to get a high
absorption around 380–410 nm where Xe–Hg lamps, Xe lamps,
In the present paper, we consider a series of eleven di- and LED, laser diodes and even household halogen lamps are
trifunctional core-pyrene π compounds Co_Py, where the usable. The large choice of Co_Pys should allow for studying
pyrene moiety is used as a PI. The cores include a functional- the effect of the core on the MO coupling and the resulting
Scheme 3: Investigated Co_Pys.
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Scheme 4: Other chemical compounds.
absorption properties. The activity of these Co_Py compounds of Co_Pys to behave as new organophotocatalysts is also
in the FRP of acrylates and the FRPCP (and eventually the discussed.
cationic polymerization CP) of epoxides through a ring-opening
Results and Discussion
polymerization (ROP) under exposure to the near-UV–vis light
Light absorption, redox properties and MO
calculations
delivered by a Hg–Xe lamp (~30 mW cm−2) and the visible
light of a halogen lamp (soft irradiation conditions;
~10 mW cm−2) is investigated. The Co_Pys are used in
combination with additives: iodonium or sulfonium salts for
ROP and amine or/and alkyl halide for FRP. The monomers
investigated are given in Scheme 4. The mechanisms are
analyzed by electron spin resonance (ESR), steady-state photo-
lysis, fluorescence and laser flash photolysis (LFP). The ability
The UV absorption spectra of the different pyrene derivatives
are depicted in Figure 1. The solvents were selected for good
solubility of the Co_Pys. Absorption maxima are located at
~350 nm for Py_2, Py_3, Py_4, Py_5, Py_12, Py_8, Py_9,
Py_10, Py_11, ~370 nm for Py_6, Py_7, and 330 nm for Py_1
(Table 1). This corresponds to a red shift of the absorption for
Figure 1: UV–vis absorption spectra of the investigated compounds: (A) In acetonitrile for Py_1 and acetonitrile/toluene (50/50) for the other mole-
cules, (B) in toluene.
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Table 1: Properties of the pyrene derivatives in acetonitrile (*) and toluene. Light absorption and emission properties (experimental λmax,abs,exp and
calculated λmax,abs,calc maximum absorption wavelengths, emission wavelengths λmax,em), excited-state energy levels (ES), oxidation potentials (Eox)
and free-energy changes ΔG (see text) [1].
λmax,abs,exp
λmax,abs,calca
λmax,em
Eox
ES
ΔGPBr
ΔGIod
(nm)
(nm)
(nm)
(V/SCE)
(eV)
(eV)
(eV)
Py_1*
Py_2
Py_3
Py_4
Py_5
Py_6
334
344
351
351
353
365
327 (f = 0.27)
334 (f = 0.41)
356 (f = 0.50)
373 (f = 0.3)
387 (f = 1.16)
393
380
393
423
404
441
1.33
0.8
0.9
0.6
0.7
0.9
3.44
3.45
3.36
3.21
3.24
3.04
−1.33
−1.87
−1.68
−1.83
−1.76
−1.36
−1.9
−2.45
−2.26
−2.41
−2.34
−1.94
344 (f = 0.52);
403 (f = 0.74)
Py_7
367
344
346
347
346
346
344 (f = 0.68)
365 (f = 0.39)
336 (f = 0.44)
376 (f = 0.51)
370 (f = 0.4)
377 (f = 1.00)
440
530
428
425
409
397
–
3.02
2.6
–
–
Py_8
0.7
0.8
0.9
−
−1.12
−1.62
−1.52
–
−1.7
−2.2
−2.1
–
Py_9
3.2
Py_10
Py_11
Py_12
3.2
3.22
3.22
0.6
−1.84
−2.42
aMolecular orbital MO calculations (using the time-dependent density functional theory at B3LYP/6-31G* level on the relaxed geometries calculated at
UB3LYP/6-31G* level); f = oscillator strength.
Py_2 to Py_12 compared to the reference compound Py_1. At couplings of the MOs lead to red-shifted transitions compared
365 nm, the maximum emission wavelength of the Hg–Xe to Py_1 and enhanced absorption coefficients (Figure 1), e.g.,
lamp, the molar extinction coefficients εmax of the different for Py_3, Py_5, Py_6, Py_8, Py_11 and Py_12. A strong
Co_Pys follow the order Py_5 > Py_7 > Py_11 > Py_9 > Py_6 delocalization of the HOMOs and LUMOs is observed in line
> Py_12 > Py_3 > Py_10 > Py_8 > Py_4 > Py_1 = Py_2. The with their better absorption properties. For Py_2, the coupling is
highest εmax is obtained for Py_5 (ε353 = 13 × 104 M−1 cm−1, weak and the absorption properties are moderately affected
ε346 = 6 × 104 M−1 cm−1 versus ε334 = 6 × 104 M−1 cm−1 for compared to Py_1.
Py_5, Py_9 and Py_1, respectively). Py_4 exhibits low ε
values, and thus a low efficiency can be expected for this com- Photochemical reactivity
pound.
Fluorescence experiments
A strong quenching of the Co_Py singlet states by PBr, amine
The oxidation potentials of the Co_Pys (Table 1) range from (MDEA), Iod and the sulfonium salt TH is found. Py_3 is
1.33 V (Py_1) to 0.6 V (Py_4, Py_12). The singlet-state energy selected for this mechanistic study (Figure 3) because of the
is also affected by the core of the structure, i.e., 2.6 eV for Py_8 high reactivity of this compound in photopolymerization experi-
versus 3.44 eV for the reference Py_1. The free-energy change ments (see below). The rate constants in that case are 1.5 ×
ΔG of the Co_Py/phenacyl bromide PBr (or the iodonium salt, 1010, 9 × 109, 1 × 1010 and 3.7 × 109 M−1 s−1, respectively. The
Iod) interaction is very favorable (ΔG << 0). Py_2 exhibits the 1Py_3/PBr, 1Py_3/Iod and 1Py_3/TH interactions are diffusion-
most favorable ΔGs: −2.45 eV (Iod) and −1.87 eV (PBr).
controlled in full agreement with the highly favorable ΔGs (Ta-
ble 1; the reduction potential for TH is −1.3 V [1]; the calcu-
MO calculations show that the calculated and experimental lated ΔG is −1.16 eV). They correspond to an efficient electron
values (Table 1) are approximately the same, e.g., a difference transfer followed by the usual fast generation of radicals from
of 5 to 10 nm is observed for Py_2 and Py_3. The oscillator the radical anion species (r5–r7 in Scheme 5) or an electron/
strengths (noted f in Table 1) are higher for the new proposed proton transfer with the amine (r8 in Scheme 5).
structures than for Py_1 in agreement with their better light
absorption properties (Figure 1). A strong coupling of the π ESR spin trapping experiments
MOs of the core with those of the pyrene substituents is shown In ESR spin trapping experiments upon UV light exposure, the
supporting a clear ππ* character of the HOMO–LUMO tran- interaction of the Co_Pys with the iodonium salt Iod
sition, as evidenced in Figure 2 for the investigated derivatives. (Figure 4A) or the sulfonium salt TH (Figure 1 in Supporting
For the various π-core/pyrene substituent combinations, Information File 1) leads to an aryl radical Ar• (e.g., hyperfine
different coupling strengths can be observed (Figure 2). Strong coupling constants hfcs of the PBN adduct: aN = 14.2 G and aH
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Beilstein J. Org. Chem. 2013, 9, 877–890.
Figure 2: HOMO–LUMO orbitals for Py_2, Py_3, Py_5, Py_6, Py_8, Py_11 and Py_12 involved in the π–π* transition (calculated at UB3LYP/6-31G*
level; for Py_5 and Py_11, the alkyl chains have been simplified to reduce the computational cost).
Scheme 5: Photochemical processes for the different Co_Pys.
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The short S1 lifetime (e.g., ~55 ns under N2 for Py_3) and the
diffusion controlled interactions of S1 with Iod, PBr or MDEA
(see above) prevent a significant production of T1 in Co_Py/Iod
(PBr or MDEA). Therefore, a triplet-state pathway can be
neglected in the investigated systems.
Thermal processes
For some Co_Py/Iod couples, a slow thermal redox process can
also occur. Indeed, in ESR–ST experiments, aryl radicals are
generated without irradiation within 24 h (no free radical was
observed in Py_9 or Iod alone). For example, in Py_9/Iod, the
same radicals as in r5 are produced (Figure 5; hfccs: aN = 14.2
G and aH = 2.2 G for the PBN radical adduct). Peroxyl radicals
are also observed, as aryls easily add to O2 (aN = 13.3 G and aH
= 1.6 G for the PBN radical adduct, in agreement with [45]). As
in r5 in Scheme 5, a cation radical Py_9•+ is concomitantly
formed. This thermal process is slow as Eox(Py_9) > Ered(Iod)
(Table 1). The formation of radical cations can be worthwhile to
initiate cationic polymerization at room temperature (see
Figure 3: Fluorescence quenching of 1Py_3 by the phenacylbromide
(PBr) in acetonitrile/toluene (50/50). Insert: Stern–Volmer plot (the
concentrations used are given in this plot).
= 2.2 G in agreement with known data [22-25,45]) in line with below).
(r5). In the same way, phenacyl radicals and α-aminoalkyl radi-
cals (r7 and r8 in Scheme 5) are observed in 1Py_3/PBr and
1Py_3/EDB (aN = 14.6 G, aH = 4.5 G and aN = 14.3 G, aH = 2.5
G, respectively; Figure 4B and Figure 4C; ethyldimethylamino-
benzoate (EDB) instead of MDEA is used to avoid a high
polarity of the sample preventing an ESR investigation). The
Py_3/Iod or the Py_3/TH couple generates a cation radical
(Py_3•+) that can initiate a cationic polymerization whereas the
phenacyl and aminoalkyl radicals formed in Py_3/PBr or Py_3/
MDEA are susceptible to radical polymerization.
Laser flash photolysis
Upon laser excitation of Py_3 at 355 nm, a triplet state T1 char-
acterized by a maximum absorption at ~420 nm and a rather
Figure 5: ESR-spin trapping spectra of Py_9/Iod in tert-butylbenzene
(storage at rt for 24 h); (a) experimental and (b) simulated spectra.
long lifetime is formed (t > 4 μs) (Figure S2 in Supporting
Information File 1). This T1 state is similar to that of Py_1 [31].
Figure 4: ESR spectra obtained upon irradiation of (A) Py_3/Iod, (B) Py_3/PBr and (C) Py_3/EDB in tert-butylbenzene. Phenyl-N-tert-butylnitrone
(PBN) is used as a spin trap; (a) experimental and (b) simulated ESR spectra.
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Beilstein J. Org. Chem. 2013, 9, 877–890.
Figure 6: Photopolymerization profiles of EPOX upon Xe–Hg lamp irradiation (λ > 340 nm) under air for different photoinitiating systems: (A) (a) Iod
(2% w/w); (b) Py_3/Iod (1%/2% w/w); (c) Py_1/Iod (1%/2% w/w); (B) (a) TH (2% w/w); (b) Py_3/TH (0.2%/2% w/w); (c) Py_8/TH (0.2%/2% w/w); (d)
Py_9/TH (0.2%/2% w/w); (e) Py_10/TH (0.2%/2% w/w); (f) Py_11/TH (0.2%/2% w/w).
Ability of the different pyrene structures in
photopolymerization reactions
Cationic photopolymerization (CP)
The ring-opening polymerization profiles of EPOX upon expo-
sure to the Xe–Hg lamp under air using two-component PISs
are shown in Figure 6, Figure 7 and Figure 8. A comparison of
the photopolymerization profiles using the Py_1 (to Py_12)/Iod
couples (Figures 6–8) shows that the fastest polymerization
rates (Rp) and the highest final conversion (conv) are obtained
with Py_11/Iod and Py_3/Iod. No polymerization is observed in
the presence of Iod alone Figure 6A and Figure 8A, curves a.)
Py_3/Iod leads to a conv ~90% (Figure 6A, curve b) compared
to Py_1/Iod (conv ~50%; Figure 6A, curve c). Almost all the
new proposed structures are better than the starting structure
Py_1 (Figure 7, curve b). This highlights the interest of the
present approach associated with the modification of the Py
Figure 7: Photopolymerization profiles of EPOX upon a Xe–Hg lamp
chromophore. CP can also be initiated in the presence of the
sulfonium salt TH, e.g., with Py_3, Py_8, Py_9, Py_10 and
Py_11 (Figure 6B). The conversions reach 80% and 75% with
Py_3/TH and Py_11/TH, respectively (after 200 s light expo-
sure). EPOX-Si can also be easily polymerized (Figure 8).
Py_3/Iod (Figure 8A) is still efficient (conv ~50%).
irradiation (λ > 340 nm) under air for different photoinitiating systems:
(a) Iod (2% w/w); (b) Py_1/Iod (0.2%/2% w/w); (c) Py_3/Iod (0.2%/2%
w/w); (d) Py_4/Iod (0.2%/2% w/w); (e) Py_5/Iod (0.2%/2% w/w); (f)
Py_6/Iod (0.2%/2% w/w); (g) Py_7/Iod (0.2%/2% w/w); (h) Py_8/Iod
(0.2%/2% w/w); (i) Py_9/Iod (0.2%/2% w/w); (j) Py_10/Iod (0.2%/2%
w/w) ; (k) Py_11/Iod (0.2%/2% w/w).
A cationic photopolymerization profile of EPOX under very
In these systems, the Co_Py•+ generated in r5,r6 (Scheme 5) can soft irradiation (halogen lamp exposure) is represented in
initiate the CP process. The relative efficiency of these Co_Py/ Figure 8B. In the presence of Iod, several Co_Pys exhibiting an
Iod (or TH) combinations will be affected by different parame- acceptable absorption at λ > 380 nm lead to efficient polymer-
ters: (i) their relative light absorption properties, (ii) the ization reactions. For example, conv = 40 and 55% within 200 s
quantum yields of radical or radical cation formation in the with the Py_7/Iod and Py_11/Iod couples.
singlet state (the singlet state is predominant, see above), (iii)
the rate constants for reactions r5 and r6 and (iv) the ability of The three-component PIS (Py_11/Iod/NVK) (Figure 8B, curve
Co_Py•+ to initiate a ring-opening polymerization process. 5) allows an increase of the EPOX conversion up to 75%. A
Structure–reactivity relationships for the different derivatives consumption of the NVK double bond is observed (Figure S3 in
can hardly be extracted. This is probably ascribed to a strong Supporting Information File 1). As known in other systems, this
interplay between (i) to (iv).
is accounted for by the addition of Ar• to the NVK double bond
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Beilstein J. Org. Chem. 2013, 9, 877–890.
Figure 8: (A) Photopolymerization profiles of EPOX-Si upon a Xe–Hg lamp irradiation (λ > 340 nm) under air for different photoinitiating systems: (A)
(1) Iod (2% w/w); (2) Py_3/Iod (1%/2% w/w); (B) Photopolymerization profiles of EPOX under air upon halogen-lamp irradiation in the presence of (1)
Py_6/Iod (0.2%/2% w/w); (2) Py_7/Iod (0.2%/2% w/w); (3) Py_10/Iod (0.2%/2% w/w); (4) Py_11/Iod (0.2%/2% w/w); (5) Py_11/Iod/NVK (0.2%/2%/
2% w/w).
(formation of an Ar–NVK• radical) (r9 in Scheme 5). This elec- Scheme 5). For the other Co_Py, a good thermal stability is
tron rich radical is easily oxidized (r10 in Scheme 5) by Iod, the found. This dual behavior of some Co_Pys, which are able to
resulting cation Ar–NVK+ being able to initiate the cationic initiate both a thermal and a photochemical cationic polymer-
polymerization.
ization, can be very useful to initiate the polymerization in the
shadow areas.
Thermal polymerization at room temperature
Remarkably, the thermal polymerization of EPOX can also be Free radical photopolymerization (FRP)
initiated by different Co_Py/Iod (1%/2% w/w) systems. This Typical free radical polymerization profiles of TMPTA upon
reaction proceeds smoothly at rt but is completed after 24 h, the Xe–Hg lamp exposure in laminate by using the Co_Py/
e.g., with Py_9, Py_6, Py_10 or Py_11/Iod, the final conver- MDEA (or PBr) two-component PISs are represented in
sion is >55%. As supported by the ESR experiments (see Figure 9. In the presence of Co_Py alone, FRP occurs but the
above), the presence of, e.g., Py_9•+ explains the thermal initia- inhibition time is high and the final conversion is low (50 s;
tion of the cationic polymerization of the epoxide (r11 in 35% with Py_3). The addition of PBr (Figure 9A) or MDEA
Figure 9: Photopolymerization profiles of TMPTA upon Xe–Hg lamp irradiation (λ > 340 nm) in laminate for different photoinitiating systems: (A) (a)
PBr (2% w/w); (b) Py_2 (0.2% w/w); (c) Py_3 (0.2% w/w); (d) Py_5 (0.2% w/w); (e) Py_2/PBr (0.2%/2% w/w); (f) Py_3/PBr (0.2%/2% w/w); (g) Py_5/
PBr (0.2%/2% w/w); (h) Py_3/MDEA/PBr (0.2%/2%/2% w/w); (B) (a) MDEA (2% w/w); (b) Py_2 (0.2% w/w); (c) Py_3 (0.2% w/w); (d) Py_5 (0.2% w/
w); (e) Py_2/MDEA (0.2%/2% w/w); (f) Py_3/MDEA (0.2%/2% w/w); (g) Py_5/MDEA (0.2%/2% w/w); (h) Py_3/MDEA/PBr (0.2%/2%/2% w/w).
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Beilstein J. Org. Chem. 2013, 9, 877–890.
Figure 10: Photolysis of (A) the Py_3/PBr couple, (B) the Py_3/Iod couple, and (C) the Py_3/MDEA couple; Xe–Hg lamp irradiation. In acetonitrile/
toluene (50/50); from t = 0 to 5 min.
(Figure 9B) shortens the inhibition time and increases the final Py_3/Iod, Py_3/MDEA couples highlights the consumption of
conversion to ~55%. For the Co_Py/amine or Co_Py/PBr two- the pyrene moiety under light exposure (Figure 10). In Py_3/
component systems, the radicals generated in r7 and r8 in PBr and Py_3/MDEA, isosbestic points are present at 330 nm
Scheme 5 can initiate the FRP process. Due to its absorption in (Figure 10A) and 315 nm (Figure 10B), respectively, suggesting
the UV, PBr alone initiates the acrylate photopolymerization stoichiometric reactions and no other side reactions.
(conv = 45%; Figure 9A, curve a). No striking improvement of
the final conversion is observed when using the Py_3/MDEA/ The addition of NVK to Py_11/Iod shows that the photolysis is
PBr three-component PIS instead of Py_3/PBr (~55%; faster with Py_11/Iod than with Py_11/Iod/NVK couples
Figure 9A, curve f versus h), but both Rp and conv are better (Figure 11A versus Figure 11B). This difference highlights that
when comparing to Py_3/MDEA.
in the presence of NVK, Py_11 is regenerated according to an
oxidation cycle (Scheme 6). Therefore, Py_11 behaves as a new
photocatalyst in agreement with (r12 in Scheme 5).
Photocatalyst behavior of the Co_Pys
The photocatalytic behavior of the Co_Pys is investigated in the
most interesting compounds reported above for the photopoly- In the Co_Py/PBr/MDEA three-component system, reactions
merization reactions. The steady-state photolysis of Py_3/PBr, (r7, r8) are probably competitive (e.g., k[MDEA] ~ k[PBr]
Figure 11: Photolysis of (A) the Py_11/Iod and (B) the Py_11/Iod/NVK couple. Halogen lamp irradiation. In acetonitrile/toluene (50/50); from t = 0 to 5
min.
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Beilstein J. Org. Chem. 2013, 9, 877–890.
Conclusion
In this paper, the core-pyrene π structures Co_Pys appear as
interesting photoinitiators. Successful cationic and radical
photopolymerization reactions were carried out under near-
UV–vis irradiation. A photocatalytic behavior is noted, that is,
an oxidation cycle for Co_Py/Iod/NVK and a partial oxidation
cycle for Co_Py/MDEA/PBr. These experimentally and
theoretically investigated di- and trifunctional compounds
outline the interest in a suitable strong MO coupling. This could
open the way to an a priori choice of absorption spectra driven
by the theoretical design of chemical compounds. Other
proposals will be provided in the future.
Experimental
Co_Pys
Scheme 6: The oxidative cycle.
The synthesized compounds are presented in Scheme 3. The
procedures are presented in detail in Supporting Information
using Py_3). As a consequence, both a reduction and an oxi- File 1. 1H and 13C NMR spectra and HRMS mass spectral
dation cycle could simultaneously proceed (Scheme 7). analysis are also given. Py_1 was purchased from Aldrich. 1,4-
However, the ESR and polymerization results (see above) (i) Di(pyren-1-yl)benzene (Py_4) was prepared by adapting a
suggest that the contribution of the reduction cycle should be literature procedure [48]. Py_12 was synthesized as previously
weak and (ii) confirm that the regeneration in the oxidation reported [49]. The syntheses of Py_2 [50], Py_3 [51], Py_6
cycle has a low efficiency. This might be due to a MDEA•+ → [52] and Py_7 [53] were already reported under reaction condi-
MDEA–H• process (in r8 in Scheme 5) less efficient than PBr•− tions different to the ones reported in this article and in lower
→ phenacyl radical (in r7 in Scheme 5). Nevertheless, such an yields. The results obtained in our work agree with the previ-
oxidation cycle that can contribute here to some extent appears ously reported characterization of these products.
as one of the rare examples observed in photocatalyst (PC)/PBr/
amine systems, because most of them work according to a Other chemical compounds
reduction cycle [30,46] (see other examples in [47]; in a related The [methyl-4-phenyl (methyl-1-ethyl)-4-phenyl] iodonium
system, still unpublished, based on another PC, we have demon- tetrakis(pentafluorophenyl)borate [54,55] (Iod; PI 2074 from
strated the true occurrence of a unique oxidation cycle).
Bluestar Silicones - France) was used as the iodonium salt
(Scheme 4). Phenacyl bromide (PBr), N-methyldiethanolamine
(MDEA) and N-vinylcarbazole (NVK) were obtained from
Aldrich and (4-hydroxyethoxyphenyl) thianthrenium hexafluo-
rophosphate (TH; recrystallized form of Esacure 1187) from
Lamberti Spa. The (epoxycyclohexylethyl)methylsiloxane-
dimethylsiloxane copolymer (EPOX-Si) was obtained from
Bluestar Silicones-France (Silcolease UV POLY 200);
trimethylolpropane triacrylate TMPTA and (3,4-epoxycyclo-
hexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX or
UVACURE 1500) were provided by Cytec.
Photopolymerization procedures
TMPTA was irradiated at room temperature under lamination
conditions: the formulation (25 μm thick) is sandwiched
between two polypropylene films. The weight percentages for
the photoinitiators are given in the figure captions. The evolu-
tion of the acrylate content was continuously followed by real-
time FTIR spectroscopy (FTIR NEXUS 870) at ~1620 cm−1.
The formulations (25 μm thick) based on the cationic monomers
(EPOX, EPOX-Si) deposited on a BaF2 pellet were irradiated
Scheme 7: Oxidation versus reduction cycles.
887

Beilstein J. Org. Chem. 2013, 9, 877–890.
under air inside the IR spectrometer cavity at room temperature
[56,57]. The evolution of the epoxy content was also continu-
ously followed by real-time FTIR spectroscopy (FTIR NEXUS
870). A Xe–Hg lamp (Hamamatsu, L8252, 150 W, filtered light
at λ > 340 nm; intensity ~ 30 mW/cm²) and a halogen lamp
(intensity ~10 mW/cm²; the emission spectrum is given in [43])
were used as the irradiation sources.
Supporting Information
Supporting Information File 1
Experimental procedures, characterization data, and
additional spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-101-S1.pdf]
ESR experiments
ESR spin-trapping (ESR-ST) experiments were carried out by
using a X-Band ESR spectrometer (EMX-plus from Bruker
Biospin or MS400 from Magnettech). The radicals were
produced at RT under Xe–Hg lamp exposure and trapped by
phenyl-N-tert-butylnitrone (PBN) according to a procedure
described in detail in [58,59].
Acknowledgements
This work was partly supported by the “Agence Nationale de la
Recherche” grant ANR 2010-BLAN-0802. JL thanks the
Institut Universitaire de France for the financial support.
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