Perovskites as new radical photoinitiators for radical and cationic polymerizations

Tetrahedron 72 (2016) 7686e7690

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Perovskites as new radical photoinitiators for radical and cationic polymerizations
de
ric Dumur c, *, Bernard Raveau d, Fabrice Morlet-Savary a, Haifaa Mokbel a, b, Fre
gat e, Didier Gigmes c, Joumana Toufaily b, Tayssir Hamieh b, Corine Simonnet-Je
e a, * Jean Pierre Fouassier a, Jacques Laleve Institut de Science des Mat
eriaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France Laboratoire de Mat
eriaux, Catalyse, Environnement et M
ethodes analytiques (MCEMA-CHAMSI), EDST, Universit
e Libanaise, Campus Hariri, Hadath, Beyrouth, Lebanon c Aix-Marseille Universit
e, CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, 13397 Marseille, France d Laboratoire CRISMAT, CNRS-ENSICAEN, UMR 6508, 6 Bd Mar
echal Juin, Cedex 4, Caen 14050, France e Universit
e de Versailles Saint Quentin, Institut Lavoisier de Versailles, ILV, UMR CNRS 8180, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 January 2016 Received in revised form 5 March 2016 Accepted 16 March 2016 Available online 24 March 2016

Four perovskites (LaTiO3, LaCrO3, La0.6Sr0.4MnO3 and MAPbI3) are proposed here as new photoinitiators (e.g., free radical generators) in combination with iodonium salt and optionally another additive (N-vinylcarbazoledNVK) to initiate both radical and cationic photopolymerization reactions. The proposed systems are efficient phenyl radical generators under polychromatic light sources such as a halogen lamp or a XeeHg lamp. The interest of this approach is the ability to work with very stable inorganic structures as photoinitiators. To the best of our knowledge, this is the first time that perovskites are incorporated in photoinitiating systems. Photochemical mechanisms will be proposed as sustained by electron spin resonance spin trapping (ESR-ST) experiments. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Perovskites Free radical initiators Polymerization Cationic polymerization Photoinitiator Polychromatic light

1. Introduction The development of new free radical initiators or initiating systems upon low light intensity or soft irradiation conditions is actually a great challenge for both the synthetic organic and polymer chemists. Particularly, free radical polymerization (FRP) as well as cationic polymerization (CP) can be easily achieved under exposure to light sources. But, one key factor in photopolymerization reactions is concerned with the photoinitiating system (PIS), which allows the starting resin formulation to absorb the light and to create reactive species that are able to initiate the polymerization reaction. A PIS can consist in a combination of two or threecomponent systems e.g., photoinitiator PI/iodonium salt or PI/ iodonium salt/additive. The development of PIs is an attractive research field (see e.g., in1,2). In the frame of our recent works devoted to the design of new PIs through the search of novel architectures,

* Corresponding authors. E-mail addresses: [email protected]
e). (F. Dumur), [email protected] (J. Lalev e http://dx.doi.org/10.1016/j.tet.2016.03.057 0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

we look for new PIs based on inorganic structures and select here perovskites. Quite few inorganic photoinitiators were proposed previously. [1,2 and references therein] But, as interesting feature, these inorganic compounds can be characterized by a remarkable thermal and chemical stability, rendering them appealing candidates for the design of stable photopolymerizable formulations. Concerning perovskites, the general chemical formula of these inorganic structures is typically ABX3, where A and B are cations of dissimilar sizes and X is an anion binding to both. If this general formula is relatively simple, it hides an exceptional diversity of atomic arrangements but also of photophysical properties. Perovskites can notably be divided into two main families, namely oxide perovskites and halide perovskites. If the first family of oxide-based perovskites was actively studied for its ferro-electric, magnetic, piezoelectric, magnetoresistive and superconductive properties,3 the second one has become the rising star of the photovoltaics world, by allowing a five-fold enhancement of the power conversion efficiency in only six years.4 Precisely, a breakthrough in terms of photon to electron conversion was achieved with hybrid lead halide perovskites (APbX3 with X¼I, Br, Cl) where A stands for

H. Mokbel et al. / Tetrahedron 72 (2016) 7686e7690

a primary amine. These hybrid organic-inorganic perovskites were undoubtedly the most widely studied for the design of solar cells and this recent technology has sparked a revival of interest for inorganic structures known since decades.5 With regards to the numerous optoelectronics applications in which perovskites have been involved according to their dopability, electrical conductivity and redox stability, we have therefore identified oxide and halidebased perovskites as potential PIs, these species being ensured to display known electrochemical properties6 that can be valuable in photoinitiation and free radical generation. In this work, one organometal halide perovskite (MAPbI3, where MA¼CH3NH3) which is a well-known visible light photosensitizer for photovoltaic cells7 and three lanthanum-based oxide-based perovskites (LaCrO3, LaTiO3 and La0.6Sr0.4MnO3) previously studied for their transport, magnetic and magnetoresistance properties,

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The so-prepared samples were analyzed by powder X-ray diffraction (PXRD), using CuKa radiation with a Panalytical X’pertPro diffractometer. The PXRD patterns of these three samples, given as Supplementary data (Fig. S1 in Supplementary data) attest of the purity of these perovskites, which exhibit various crystal symmetries. 2.2. Other chemical compounds Diphenyliodonium hexafluorophosphate (Iod or Ph2Iþ) and Nvinylcarbazole (NVK) were obtained from Aldrich and used with the best purity available (Scheme 1). (3,4-epoxycyclohexane) methyl 3,4-epoxycyclohexylcarboxylate (EPOX; Uvacure 1500) and trimethylol-propane triacrylate (TMPTA) were obtained from Allnex (Scheme 1).

Scheme 1. Monomers and additives.

their photocatalytic activity and piezoelectric applications8 were examined for the photoinitiation of the polymerization of acrylates and epoxides. To the best of our knowledge, perovskites have never been used as PIs (the unique example of polymerization photoinitiated with perovskites concerned the use of unsaturated organic cations, namely the amino-diacetylene cations9). The four perovskites are incorporated into several PISs (containing an iodonium salt and optionally N-vinylcarbazole NVK) and exposed to a polychromatic light (XeeHg lamp and a halogen lamp) for the FRP of acrylates and the CP of epoxides. The photoinitiation ability of the perovskite-based PISs is studied using real time FTIR spectroscopy. Electron spin resonance (ESR) spin trapping experiments allow an analysis of the formed free radical species.

2. Experimental section 2.1. Preparation of the different perovskites MAPbI3 was synthesized according to Ref. 4e. The perovskites LaCrO3 and La0.6Sr0.4MnO3 (called LSMO) were synthesized from the oxides La2O3 and Cr2O3 and from the oxides La2O3, MnO2 and carbonate SrCO3, respectively. The commercial precursors were mixed in agate mortar in stoichiometric proportions, i.e., La:Sr molar ratio equal to 1 for LaCrO3 and La:Sr:Mn molar ratio equal to 6:4:1 for La0.6Sr0.4MnO3. The mixtures were intimately ground and first heated in air at temperatures ranging from 900  C to 1000  C for 12 h. The resulting products were ground again and pressed in the form of bars and heated at 1300  C in air for 12 h. Additional heating at 1400  C for 10 h was necessary in the case of the manganite La0.6Sr0.4MnO3. The perovskite LaTiO3 was prepared from a mixture of the oxides La2O3 and TiO2 with metallic titanium in the molar ratio 10:15:5. The mixture was intimately ground and pressed in the form of bars. It was then heated in evacuated quartz tube up to 900  C and maintained at this temperature for 8 h, then the temperature was raised up to 1000  C and maintained during 48 h. The sample was finally furnace cooled.

2.3. Irradiation sources Two polychromatic lights were used: i) XeeHg lamp (LC8 Hamamatsu; incident light intensity w100 mW/cm2; in the 370e800 nm range), ii) halogen lamp (Fiber-Lite, DC-950; incident light intensity w12 mW/cm2, in the 370e800 nm range). The emission spectra of these light sources were given in Ref. 10. 2.4. Free radical photopolymerization experiments The TMPTA films (25 mm thick) deposited on a BaF2 pellet were irradiated in laminate. A filter is used to avoid a direct excitation of the monomer and a self-initiation of acrylate monomers upon UV light for l<300 nm; accordingly as blank, no polymerization occurs for TMPTA alone. The evolution of the double bond content was continuously followed by real time FTIR spectroscopy (NEXUS 870 FTIR) at about 1630 cm1 as in.10 TMPTA is a trifunctional monomer and the conversion that is mentioned does not correspond to the monomer conversion but to the conversion of the polymerizable acrylate functions. 2.5. Cationic polymerization (CP) The photosensitive formulations (25 mm thick) were deposited on a BaF2 pellet and irradiated under air. The evolution of the epoxy group contents were continuously followed by real time FTIR spectroscopy (NEXUS 870 FTIR) at about 790 cm1 as in.10 2.6. Electron spin resonance (ESR) spin trapping (ST) experiments ESR-ST experiment was carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at room temperature upon the light irradiation under N2 and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure11,12 described elsewhere in detail. PBN has been selected as spin trap agent; PBN being stable enough upon light irradiation. The ESR spectra simulations were carried out with the WINSIM software.

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3. Results and discussion

70

3.1. Absorption spectra of perovskites

60

The absorption spectrum of MAPbI3 in acetonitrile (ACN) is illustrated in Fig. 1; but it should be noted that MAPbI3 exhibits a poor solubility in ACN. The absorption maxima of MAPbI3 were mainly located in the near UV range at w300e450 nm allowing an efficient covering of the emission spectra of the XeeHg lamp, and also the halogen lamp.

50

3

2,0 1,8 1,6

Conversion (%)

2 40

1

30 20 10 0 0

1,4

20

40

60

80

100

120

Time (s)

O.D.

1,2

Fig. 2. Photopolymerization profiles of TMPTA (acrylate function conversion vs time) in laminate upon the XeeHg lamp (Iw100 mW/cm2) irradiation in the presence of (1) LaSrMnO3/Ph2Iþ/NVK (0.5%/1%/2% w/w/w); (2) MAPbI3/Ph2Iþ/NVK (0.5%/1%/2% w/w/ w) and (3) LaCrO3/Ph2Iþ/NVK (0.5%/1%/2% w/w/w).

1,0 0,8 0,6 0,4 0,2 0,0 300

350

400

450

500

550

600

(nm) Fig. 1. UVevisible absorption spectra of MAPbI3 in acetonitrile.

On the opposite, LaTiO3, LaCrO3, and LaSrMnO3 were not really soluble in organic solvents which rules out the recording of any absorption spectra in solutions. Few reports on the UVevisible spectra of these compounds as powders suggest that LaCrO3 and LaTiO3 exhibit a light absorption in the UV near visible wavelength region13,14 suitable for a good overlapping with the halogen lamp and the XeeHg lamp emission spectra.

LaTiO3/Iod/NVK system was even better: higher Final Conversions (FCs) (w57% after 120 s of irradiation; XeeHg lamp) and higher rates of photopolymerization (Rps) (Fig. 3A, curve 2 vs curve 1). It means that the addition of NVK plays a significant role. Tack-free coatings can be observed after photopolymerization which demonstrate the excellent ability of this system to initiate the FRP of TMPTA. LaTiO3/Iod/NVK also worked upon the halogen lamp irradiation (Fig. 3B), but with much lower FC and Rp (FC¼w30% and 40% for LaTiO3/Iod and LaTiO3/Iod/NVK, respectively, after 800s of irradiation). Under the XeeHg lamp, the LaTiO3/Iod/NVK system worked better than under the halogen lamp: this is obviously ascribed to the much higher light intensity delivered by the XeeHg lamp (w100 mW/cm2 vs 12 mW/cm2 for the halogen lamp). The efficiency of the perovskite/Iod/NVK system upon the XeeHg lamp irradiation decreases in the series: LSMO
3.2. Free radical photopolymerization of acrylates (TMPTA)

3.3. Cationic photopolymerization of diepoxides (EPOX)

The FRPs of TMPTA in laminate in the presence of the perovskite/Iod two-component or the perovskite/Iod/NVK threecomponent systems were carried out using a polychromatic light (halogen lamp and XeeHg lamp). Typical conversions time profiles are given in Fig. 2. The MAPbI3 (LaCrO3 or LSMO)/Iod systems are not efficient (Final Conversions (FC) <15%). Upon addition of NVK, the photopolymerization efficiencies were improved. LaCrO3 exhibited a higher photoinitiated ability than the other perovskites based systems: higher FCs (w60% after 10s of irradiation with the XeeHg lamp) and higher rates of photopolymerization Rp were obtained (Fig. 2, curve 3 vs curve 1 and curve 2). The MAPbI3 or LSMO/Iod/ NVK systems led to slightly lower FCs (w50% and w35%, with MAPbI3 and LSMO respectively). Using Iod/NVK, a FCw30% was obtained in the selected conditions showing that the presence of the perovskite improves the final conversion. Tack-free coatings were easily obtained when using the perovskite/Iod/NVK mixture. These results highlight the role of NVK in the photopolymerization process (see the chemical mechanisms below). Interestingly, the performance of the LaTiO3/Iod twocomponent system upon the XeeHg lamp irradiation is good (FCw33%, Fig. 3A, curve 1). Noticeably, the efficiency of the

The LaTiO3 based systems were also efficient to initiate the cationic photopolymerization of EPOX under air upon a XeeHg lamp irradiation. When using LaTiO3/Iod, the photoinitiation ability were relatively low (FC¼w47%; slow Rps with significant inhibition times are observed). Remarkably, the LaTiO3/Iod/NVK system ensured an excellent ability (Fig. 4, curve 2 vs curve 1) with a higher FCw78% accompanied with an improvement of Rp. In this case, the addition of the NVK reduced the inhibition time. Tack-free coatings were obtained. The formation of the polyether network (as characterized by its absorption band at 1080 cm1) was also clearly shown. On the opposite, in the presence of the other perovskites (LSMO, MAPbI3 and LaCrO3), no cationic polymerization occurred. A remaining challenge in the present work is to incorporate various structures of perovskites in PISs for FRP as well as for CP. Indeed, a major drawback of these compounds is their low solubility in organic solvents or monomers. 3.4. Proposed chemical mechanisms When exposed to a XeeHg lamp, the perovskite goes from the ground state to an excited state (r1); the oxidation of the perovskite by Iod (r2a) leads to perovskitesþ and Ph2I (subsequently to

H. Mokbel et al. / Tetrahedron 72 (2016) 7686e7690

A

60

2

55

B

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2

40 35

45 40 35

1

30 25 20 15 10

Conversion (%)

Conversion (%)

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5

1

30 25 20 15 10 5 0

0 0

200

400

600

0

800

200

400

600

800

Time (s)

Time (s)

Fig. 3. Photopolymerization profiles of TMPTA (acrylate function conversion vs time) in laminate upon (A) XeeHg lamp (Iw100 mW/cm2) irradiation in the presence of: (1) LaTiO3/ Ph2Iþ(0.5%/1% w/w); (2) LaTiO3/Ph2Iþ/NVK (0.5%/1%/1% w/w/w); and (B) halogen lamp (Iw12 mW/cm2) irradiation in the presence of (1): LaTiO3/Ph2Iþ(0.5%/1% w/w); (2) LaTiO3/ Ph2Iþ/NVK (0.5%/1%/1% w/w/w).

90 80

2

a

Conversion (%)

70 60 50

1

40

b

30 20 10 0 0

200

400

600

Time (s)

800

Fig. 4. Photopolymerization profiles of EPOX (epoxy function conversion vs time) under air upon XeeHg lamp (Iw100 mW/cm2) irradiation in the presence of: (1) LaTiO3/Ph2Iþ (0.5%/1% w/w); (2) LaTiO3/Ph2Iþ/NVK (0.5%/1%/1% w/w/w).

a phenyl radical Ph originating from the extremely fast cleavage shown in Eq. (r2b),1). Indeed, the formation of Ph is supported by ESR-ST experiments (Fig. 5): the hyperfine coupling constants hfc (aN¼14.3 G; aH¼2.2 G) agree with the known data for the PBN/Ph adduct.11

3440

3460



(r1)

perovskite þ Ph2 Iþ /perovskite þ þPh2 I 





Ph2 I /PheI þ Ph



(r2a)

(r2b)

As usually, these phenyl radicals (Ph) are easily converted into NVK radicals (NVK) by addition to the NVK double bond (r3).15,16 Also, as previously discussed in other related systems, these NVK can be useful for radical polymerization in presence of TMPTA but can also be oxidized15e18 to PheNVKþ cations (r4) that will initiate the cationic polymerization.15e18

3500

3520

3540

B (G) Fig. 5. ESR-Spin trapping spectrum of MAPbI3/Iod; [Iod]¼0.01 M; in toluene; XeeHg lamp exposure; under N2; experimental (a) and simulated (b) spectra. Phenyl-N-tertbutylnitrone (PBN) is used as spin trap.





Ph þ NVK/PheNVK

(r3) 

PheNVK þ Ph2 Iþ /PheNVKþ þ PheI þ Ph 



perovskite/ perovskite ðhnÞ

3480

(r4)

4. Conclusion Inorganic compounds such as perovskites (LSMO, MAPbI3, LaTiO3, LaCrO3) can operate as free radical photoinitiators when combined with an iodonium salt (and NVK) in two- and threecomponent photoinitiating systems for the radical polymerization of TMPTA in laminate but also remarkably for the cationic polymerization of EPOX under air. Tack-free coatings are obtained. The LaTiO3/Iod/NVK system exhibits quite good initiation ability upon the XeeHg lamp exposure. The NVK additive plays a crucial role in the photopolymerization reaction. The interest of the present approach is the possibility to work with stable inorganic structures.

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This study opens up a new route for the design of new photoinitiators and a progress is expected through a better knowledge of the physicochemical properties of these series of compounds. The properties of the synthesized polymer networks with in situ embedded perovskites will be studied in future works. Acknowledgements The authors thank the Agence Nationale de la recherche (ANR) for the grant PHOTOREDOX and the grant FASTPRINTING. Supplementary data

6. 7.

8.

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