Oligomeric Photocatalysts in Photoredox Catalysis - ACS Publications

Article pubs.acs.org/Macromolecules

Oligomeric Photocatalysts in Photoredox Catalysis: Toward High Performance and Low Migration Polymerization Photoinitiating Systems. Emel Ay,† Zaher Raad,† Olivier Dautel,‡ Frédéric Dumur,§ Guillaume Wantz,∥,⊥ Didier Gigmes,§ Jean-Pierre Fouassier, and Jacques Lalevée*,† †

Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA; 15, rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ Institut Charles Gerhardt de Montpellier, UMR 5253, ENSCM AM2N, 8 rue de l’Ecole Normale, 34296, Montpellier Cedex 05, France § Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, 13397 Marseille Cedex 20, France ∥ CNRS, IMS, UMR 5218, F-33405 Talence, France ⊥ Bordeaux INP, IMS, UMR 5218, F-33405 Talence, France S Supporting Information *

ABSTRACT: In the present paper, four fluorescent materials currently used in organic light emitting diodes (OLEDs) are presented in an original way as high performance photocatalysts usable in polymerization photoinitiating systems. Their performance is excellent in free radical polymerization, cationic polymerization but also in the synthesis of interpenetrating polymer networks (IPNs). A coherent picture of the chemical mechanisms involved in these new photocatalytic systems is provided. Remarkably, an oligomeric and copolymerizable photocatalyst (PVD2) is proposed here for the first time, i.e., both the high molecular weight of PVD2 and the presence of reactive double bonds as end groups (which could be involved in a copolymerization reaction) ensure a very low migration of the catalyst from the synthesized polymer.

■

Independently, fluorescent materials (FMs) have attracted a sustaining attention in organic light-emitting diodes (OLEDs) due to their great promise in full-color large display applications and lighting sources.51 The development of highly emissive fluorescent molecules is of special significance for OLEDs design because such emitters can be utilized as neat films contrarily to phosphors that require to be diluted in a host matrix to prevent self-quenching of formed excitons.52 Among the various organic emitters, those with intrinsic light-emitting capability and spontaneously obviating self-aggregation in the solid state are actively researched. Through a rational design consisting in the chemical linkage of bulky side groups to the chromophore core or by use of light-emitting materials displaying a three-dimensional scaffold, this critical issue could be efficiently addressed. From the photopolymerization point of view, the examination of materials previously used in organic electronics and presently examined in the context of photoinitiation ensures these molecules to be electroactive, namely, that these materials can be oxidized or reduced within a reasonable potential

INTRODUCTION

Photopolymerization reactions occur under light exposure (see e.g. in1−10). The presence of radical or cationic sources is required to start the reactions, these sources being referred as photoinitiators (PI) (one-component system) or photoinitiating systems PIS (when containing a PI and one or more additives).1−10 The formation of initiating speciesradicals R•, radical ions RC•+ or ions C+depends on the chromophores. Recently, many books (e.g., refs 1−10), reviews (e.g., refs 11−24), and original papers (e.g., refs 25−45) reported on the design of efficient PIs and PISs. The search for novel systems still remains an attractive challenge. More recently, photocatalysts were developed through an approach called photoredox catalysis for many transformations in organic chemistry but also to initiate or control polymerization processes [see in46−50 and reference therein]. The use of a catalytic cycle generating radicals and/or cations ensured a high reactivity and the access to high performance photoinitiating systems.[see examples in refs 47−50] Photocatalysts can be characterized by unique properties compared to classical photoinitiators: they are regenerated upon light irradiation (contrary to photoinitiators) and accordingly a much better reactivity associated with a higher photosensitivity can be expected. © XXXX American Chemical Society

Received: December 22, 2015 Revised: March 4, 2016

A

DOI: 10.1021/acs.macromol.5b02760 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Fluorescent Materials Investigated in This Study

Scheme 2. Chemical Pathway to PVD2

■

window. Despite these promising prospects of high photoinitiating efficiency, the extractability of initiators is another major issue, especially when small molecule-based photoinitiators are used. More precisely, migration of residues in the polymer network, potential toxicity and odor of photoinitiators can constitute a major impediment for the future usability of the resulting polymers. To satisfy the aforementioned requirements of low extractability, increase of the molecular weight so that polymeric photoinitiators are obtained can substantially reduce the amount of extractable fragments without adversely affecting the photoinitiating ability of the macroinitiator. In the present paper, two molecules (DPVBi and PVD1; see Scheme 1) previously used as emitters for OLEDs are proposed for the first time as photocatalysts of polymerization. It is the first report showing that these materials with good light absorption properties and adapted redox potentials can be used as photocatalysts. In complement of these two small moleculebased PIs, an oligomeric and copolymerizable version of PVD1 (i.e., PVD2) that is potentially less extractable has been synthesized and tested as a macrophotocatalyst and compared to poly(vinylcarbazole) (see Scheme 1); no macrophotocatalyst has been proposed so far. This property can be very useful to reduce the migration of the catalyst from the synthesized polymer network. To the best of your knowledge, PVD2 is the first oligomeric and copolymerizable photocatalyst used in photoredox catalysis for the synthesis of polymer. The polymerization initiating ability will be checked by realtime FTIR spectroscopy. The chemical mechanisms will be investigated by steady state photolysis, fluorescence, laser flash photolysis, electron spin resonance (ESR), and cyclic voltammetry experiments.

EXPERIMENTAL SECTION

The Fluorescent Materials (FM) used as PIs or PCs. They are shown in Scheme 1. DPVBi was obtained from American Dye Source, Inc. (ADS082BE) and used with the highest purity available. The synthesis of PVD1 has been already described in detail in.53 The synthesis of PVD2 will be described in detail below. Poly(9vinylcarbazole) was obtained from SigmaAldrich and used with the best purity available (Mn = 25 000 g/mol). Synthesis of Oligomeric and Copolymerizable Photocatalyst (PVD2). All the reactions were performed under nitrogen atmosphere using Schlenk tube techniques. The solvents were distilled under nitrogen over Na/benzophenone (THF), CaH2 (DMF), KOH (NEt3), and P2O5 (CH2Cl2) prior to use. Optically active trans(1R,2R)-1,2-cyclohexyldiamine was obtained enantiomerically pure from the commercial racemic cis/trans mixture according to literature.54 4-Bromophthalic imide (1) was prepared according to our previously described procedure.55 1,4-Bis(octyloxy)-2,5-divinylbenzene (2) was synthesized according to the reported procedure.56 Analytical data were in good agreement with the literature. 1H NMR spectrum was recorded on a Bruker AC 250 spectrometer and CDCl3 was used as solvent. Chemical shifts (δ), reported in parts per million, are relative to tetramethylsilane. Signal multiplicities: m (multiplet). FTIR spectrum of PVD2 was recorded on a PerkinElmer 1000 FTIR spectrometer by spin coating a solution of THF on a silicon wafer. UV−vis spectra were recorded on a Hewlett-Packard 8453 spectrophotometer. The solid state quantum yields (ϕ) were measured at room temperature using a quantum yield measurement system (Hamamatsu, Model C9920−02) with a 150-W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as the sample chamber, and a multichannel analyzer for signal detection. GPC measurements were made on a Waters device equipped with HR2 and HR3 styragel columns using THF as eluent. PVD1,53 1,55 and 256 were synthesized according to literature procedures. PVD2 was prepared by the polymerization of an B


Article

Macromolecules

Figure 1. 1H NMR spectrum of PVD2 in CDCl3. equimolar mixture of 1 and 2 using a palladium catalyzed Heck coupling (Scheme 2). PVD2: The polymerization of the chiral bromophthalimide 1 (532.2 mg, 1 mmol) and the 1,4-bis(octyloxy)-2,5-divinylbenzene 2 (386.3 mg, 1 mmol) was performed in the presence of Pd(OAc)2 (9 mg, 0.04 mmol), P(o-C6H4Me)3 (76 mg, 0.25 mmol) and triethylamine (607.1 mg, 6 mmol, 0.85 mL) in 50 mL of dry N,N-dimethylformamide (DMF). At 100 °C the coupling occurred and provided PVD2 as a yellow solid after 64 h of heating. After cooling to room temperature (∼20 °C), the polymer was separated from the low weight oligomers by a precipitation with 300 mL of cold methanol. Centrifugation gave a yellow powder with 86% of yield (650 mg). This new material PVD2 is soluble in common organic solvents (THF, CH2Cl2, Toluene, Xylene) and gel permeation chromatography (THF, polystyrene standard) analysis showed Mn = 3309, Mw = 4388, and PDI (Mw/Mn) = 1.32; 1H NMR (250 MHz, CDCl3): δ (ppm) 7.72 (m, 6H), 6.97 (s, 2H), 5.00 (m, 2H), 3.99 (m, 4H), 2.41 (m, 2H), 1.23−1.89 (m, 30H), 0.81 (m, 6H). FTIR (cm‑1): 2927 (νas CH2), 2855 (νs CH2), 1775 ((νCO), 1723 (νCO). [α]D = −428° (c = 1, CH2Cl2). UV−vis (CH2Cl2, nm): λabs_max = 425. UV−vis (film on glass, nm): λabs_max = 428. Fluo (λex = 400 nm, CH2Cl2, nm): λem_max = 508; Fluo (film on glass, nm): λem_max = 524; ΦFex425 (CH2Cl2) = 37%/fluorescein, ΦFex425 (THF)= 56%/fluorescein, ΦF (solid) = 42% (λem_max = 540 nm), ΦF (solide/PMMA)= 54% (λem_max = 540 nm). Optical bandgap =2.508 eV. Estimation of n for the Oligomeric Photocatalyst (PVD2). a. By 1 H NMR. By studying the 1H NMR spectrum of PVD2 (Figure 1), we could estimate its formula or n (Figure 2). Indeed, between 5.2 and 5.8 ppm, we can observe the signals corresponding to the remaining unreacted terminal vinyl functions. This is corresponding to 4H since there is two terminal functions composed each by one HA and one HB; HC chemical shift being located around 7.5 ppm. Those 4H (2HA+2HB) are integrating for 0.60. If an integration of 0.6 is corresponding to 4H then the massif integrating for 4.0 at 4.036 ppm represents 4 × 4/0.6 = 26.6H which corresponds to 13xCH2. Those

Figure 2. Chemical formula and molecular weights of PVD2 as a function of the polymerization degree (n).

methylene groups are the one linked to the oxygen of the central phenyl ring (H10 on the formula of PVD2). For n = 1, there are 6xCH2; for n = 2, 8 × CH2; for n = 3, 10 × CH2, for n = 4, 12 × CH2 and for n = 5, 14 × CH2. We can conclude from the NMR that n = 4 or 5. b. By GPC. GPC measurement was made on a Waters device equipped with HR2 and HR3 styragel columns using THF as eluent. GPC analysis realized on PVD2 in THF using polystyrene standards gave a molecular weight in number of Mn = 3309 g/mol, a molecular weight in weight of Mw = 4388 g/mol, and a dispersity (Mw/Mn) = 1.32. The theoretical molecular weight for n = 3 is M3 = 3414.56 g/ mol, for n = 4, M4 = 4171.54 g/mol, and for n = 5, M5 = 4928.52 g/ mol. Hence, the experimental values obtained are consistent with n between 3 and 5. C


Article

Macromolecules Scheme 3. Chemical Structures of the Additives and the Monomers

The free energy change ΔGet of the electron transfer reaction between 1FMs and Iod can be calculated from the classical Rehm− Weller equation: ΔG = Eox − Ered − ES (or ET) + C; where Eox, Ered, ES (or ET), and C are the oxidation potential of the studied photoinitiators, the reduction potential of Iod, the excited singlet state energy of the studied FMs, and the electrostatic interaction energy for the initially formed ion pair (this latter parameter is generally considered as negligible in polar solvents). ESR Spin Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at room temperature upon the LED@405 nm exposure under N2 and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure described elsewhere in detail.45 The ESR spectra simulations were carried out using the WINSIM software. Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/ YAG laser (λexc = 355 nm, 9 ns pulses; energy reduced down to 10 mJ) from Continuum (Minilite) and an analyzing system consisted of a ceramic xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer (Luzchem LFP 212).45

Other Chemical Compounds. Diphenyliodonium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), and N-tert-butyl-α-phenylnitrone (PBN) (Scheme 3) were purchased from Sigma-Aldrich and used as received without further purification. Trimethylolpropane triacrylate (TMPTA) and (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) were obtained from Allnex and used as benchmark monomers for radical and cationic photopolymerization reactions, respectively (Scheme 3). Irradiation Sources. Different light sources were used for the irradiation of the photocurable samples: LEDs centered at 395 nm (M395L3, ThorLabs; ∼45 mW cm−2); 365 nm (M365L2, ThorLabs; ∼20 mW cm−2); 405 nm (M405L2, ThorLabs; ∼110 mW cm−2) and 455 nm (M455L3, ThorLabs; ∼80 mW cm−2). Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 package. The electronic absorption spectra for the FM derivatives were calculated from the timedependent density functional theory at B3LYP/6-31G* level on the relaxed geometries calculated at this latter level; the molecular orbitals involved in these electronic transitions were also extracted. Photopolymerization Experiments. For photopolymerization experiments, the conditions are given in the Figure captions. The photosensitive formulations (25 μm thick) were deposited on a BaF2 pellet in laminate (the formulation is sandwiched between two polypropylene films to avoid the reoxygenation during the photopolymerization) or under air and irradiated with the different light sources. The evolutions of the TMPTA double bond content or the EPOX epoxy group content were continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100) at about 1630 and 790 cm−1, respectively.45 For all the presented polymerization kinetics, the irradiations start for t = 10 s. TMPTA and EPOX being multifunctional monomers, the conversions indicated in the present paper correspond to the conversion in the polymerizable functions (acrylate and epoxy functions, respectively). Migration Study. A 0.1 g sample of EPOX/TMPTA blend (50%/ 50%, w/w) in the presence of PVD2/Iod/NVK (0.5%/1%/1%, w/w/ w) was polymerized upon a LED@405 nm. The produced interpenetrating polymer network (IPNs) was immersed in 3 mL of toluene for 12 h. The amount of the extracted PVD2 was determined using UV−vis spectroscopy. For neat EPOX, the same procedure was also used. Steady State Photolysis Experiments. The studied FMs in the presence of an additive (e.g., Iod) in acetonitrile/toluene were irradiated with the LEDs, and the UV−vis spectra were recorded using a JASCO V-730 UV/vis spectrophotometer at different irradiation times. Fluorescence Experiments. The fluorescence properties of the investigated FMs in acetonitrile were studied using a JASCO FP-6200 Spectrofluorimeter. The interaction rate constants kq between the studied FM and the quencher were extracted from classical Stern− Volmer plots (I0/I = 1 + kqτ0[quencher], where I0 and I stand for the fluorescence intensity of FM in the absence and in the presence of the quencher, respectively; τ0 stands for the lifetime of the excited FM in the absence of quencher). Redox Potentials. The oxidation potentials (Eox vs SCE) of the fluorescence materials (FMs) used as photoinitiators (PIs) were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (Voltalab 6 Radiometer). The procedure has been presented in detail in.45

■

RESULTS AND DISCUSSIONS 1. Light Absorption Properties of the FMs. The ground state absorption spectra of the investigated fluorescent materials (FMs) (PVD1, PVD2, PVK and DPVBi in Scheme 1) in toluene are shown in Figure 3 and/or in Table 1. The

Figure 3. UV−visible absorption spectra of DPVBi, PVD1, and PVD2 in toluene.

absorption maxima λmax are located at 353 nm with a molar extinction coefficient εmax ∼ 60000 M−1cm−1 for DPVBi and 422 nm (εmax ∼ 41000 M−1cm−1) for PVD2. A slightly longer wavelength with a lower absorption is noted for PVD1 (λmax = 430 nm; εmax ∼ 33000 M−1cm−1). These compounds are obviously suitable for an irradiation with a LED at 395 or 405 nm (DPVBi, PVD1 and PVD2) but also at 455 nm (for PVD1 and PVD2). However, for PVK, the absorption maximum λmax is located at ∼344 nm (Figure S1 in Supporting Information) with a very low absorption for λ > 360 nm. This is in agreement with the poor performance of PVK in photopolymerization for LED with irradiation wavelength >360 nm (see below). D


Article

Macromolecules

Table 1. Maximum Absorption Wavelengths (λmax Abs.) in Toluene (or Acetonitrile/Toluene), Maximum Emission Wavelengths (λmax Em.) Determined by Fluorescence Spectroscopy, Oxidation Potentials (Eox vs. SCE), Singlet-State Energy Levels (ES, from the UV−Vis and Fluorescence Spectra), and Free-Energy Changes (ΔGet) for the FM/Iod Interactions PVD2 PVD1 DPVBi

λmax Abs. (nm) in toluene

λmax Em. (nm)

ES (eV)

Eox (V vs SCE)

ϕeta

ΔGet(1FM/Iod) (eV)

422 (424) 430 (432) 353 (350)

539 530 436

2.53 2.56 3.09

1.2 0.9 1.28

0.24 0.41 0.18

−1.13 −1.46 −1.61

ΦeT = 1FM/Iod electron transfer quantum yields in the singlet state calculated according to ΦeT = kqτ0 [additive]/(1+ kqτ0 [additive])1, ([additive]: [Iod] = 2.3 × 10−2 M in the formulations). a

the acrylate function conversions, i.e., converting 50% of these functions, corresponds thus already to a higher monomer conversion (order of magnitude: 87.5% for a trifunctional one). The DPVBi/Iod initiating system also leads to a high conversion (44% after 100 s of irradiation at 405 nm) but FC decreases when using the other LEDs (curve 2 in Figure 5, parts A−C). For a similar weight content, PVD1/Iod is much less efficient (FC < 20%, curve 1 in Figure 5). For PVK/Iod, no polymerization occurs for LED@365 nm in agreement with the lack of absorption for PVK for λ > 365 nm. 3. Cationic Photopolymerization of an Epoxide. The cationic photopolymerization of EPOX was carried out under air upon the same LEDs as before (LED@405 nm, LED@365 nm, LED@395 nm and LED@455 nm) using different FM/Iod combinations. In the presence of DPVBi/Iod or PVD2/Iod, both high Rps and high FCs are obtained (e.g., FC = ∼ 50% and 40% respectively after 400 s of irradiation with the LED@ 405 nm - Figure 6A). When irradiating at 395 nm, these two latter systems also led to good polymerization profiles (Figure 6B). Upon the LED@455 nm, a good performance is only observed with PVD2/Iod (Figure 6C; the low efficiency of DPVBi/Iod is in line with the lack of absorption of DPVBi for λ > 410 nm). The best profiles are obtained upon exposure to the LED@405 nm thanks to the higher light intensity of this LED (see the experimental part) and the good absorption of PVD2 and DPVBi at 405 nm. The addition of NVK still increased the final conversion (curve 4 in Figures 6A and 6C). As before in free radical polymerization, the PVD1/Iod system is not very suitable here for the cationic polymerization of EPOX (FC < 25%). For PVK/Iod, no polymerization occurs for LED@365 nm in agreement with the low absorption properties for PVK for λ > 365 nm (see above). 4. Interpenetrating Polymer Networks (IPN) Synthesis: Photopolymerization of TMPTA/EPOX Blends. An interpenetrating polymer network (IPN) can be fabricated through a concomitant radical/cationic photopolymerization of a TMPTA/EPOX blend (50%/50% w/w). Remarkably, this

Molecular 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, show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized on the π systems for DPVBi and PVD1, thereby clearly showing the presence of π → π* transitions in agreement with the high extinction coefficients of these materials (Figure 4). For PVD2, a similar behavior can be expected.

Figure 4. HOMO and LUMO for DPVBi and PVD1 (for PVD1, the OC8H17 groups were simplified for OC2H5 chains to reduce the computational cost).

2. Free Radical Photopolymerization of a Multifunctional Acrylate. The photopolymerization of TMPTA was carried out in laminate upon visible lights (LED@405 nm, LED@365 nm, LED@395 nm, and LED@455 nm) using the FM/Iod systems (FM stands for PVD1, PVD2, PVK or DPVBi). In the presence of PVD2/Iod, high rates of polymerization Rp and high final conversions FC are found (e.g., FC = ∼ 50%, 45% and 43% after 100 s of irradiation with the LED@405 nm, LED@395 nm and LED@455 nm, respectively; curve 1 in Figure 5, parts A−C). TMPTA being a trifunctional monomer, the conversions given correspond to

Figure 5. Photopolymerization profiles of TMPTA (acrylate function conversion vs time) in laminate, in the presence of (1) PVD1/Iod (0.5%/1% w/w), (2) DPVBi/Iod (0.5%/1% w/w), and (3) PVD2/Iod (0.5%/1%, w/w) upon exposure to (A) LED@405 nm, (B) LED@395 nm, and (C) LED@455 nm. E


Article

Macromolecules

Figure 6. Photopolymerization profiles (conversion of epoxy functions vs time) for EPOX under air, in the presence of (1) PVD1/Iod (0.5%/1% w/ w), (2) DPVBi/Iod (0.5%/1% w/w), (3) PVD2/Iod (0.5%/1% w/w), and (4) PVD2/Iod/NVK (0.5%/1%/1% w/w) upon exposure to (A) LED@ 405 nm, (B) LED@395 nm, and (C) LED@455 nm.

Figure 7. Photopolymerization profiles for the acrylate functions (of TMPTA) and the epoxy functions (of EPOX) for an EPOX/TMPTA blend (50%/50% w/w) in the presence of PVD2/Iod (0.5%/1% w/w) in laminate (A) and under air (B) upon the LED@405 nm exposure.

consumed during the photoinitiation step. Therefore, FMs are in situ incorporated in the polymers during the photopolymerization reaction (e.g., in the polyether network in the course for the cationic polymerization of EPOX). Since FMs exhibit strong luminescence properties (see Introduction for OLED), the polymer films obtained through the new presented approach can be characterized by strong photoluminescence properties (Figure 8). This is a valuable result in full agreement with the dual behavior of FMs: (1) as photocatalyst in the photoinitiating system for the formation of the polymer network and (2) as blue light emitter in the formed polymer to generate photoluminescence properties. 5.2. Migration Study of the Oligomeric and Copolymerizable Photocatalyst (PVD2). The free PVD2 in the IPNs was extracted using THF or toluene. Almost all the PVD2 amount was retained in the film prepared in laminate (≫99.5%). Indeed, much less PVD2 can be extracted from the IPNs prepared in laminate (≪0.5%) (% values related to the initial amount of PVD2 in the formulation). Both the high molecular weight of PVD2 and the presence of reactive double bonds as end groups which could be involved in a copolymerization reaction with the acrylate functions of TMPTA in IPN synthesis ensure a very low migration from the synthesized networks. To the best of our knowledge, PVD2 is the first oligomeric and copolymerizable photocatalyst used in photoredox catalysis for the synthesis of polymer. 6. Chemical Mechanisms for the FM/Iod and FM/Iod/ NVK Systems. 6.1. The FM/Iod Systems. As PVK does not exhibit significant absorption for λ > 365 nm and can not be used in PIS for wavelength higher than 350 nm, the

corresponds to a one-step hybrid cure process. Using e.g. PVD2/Iod in laminate and under air, upon exposure at 405 nm, led to significant conversions of both the acrylate and epoxide functions (Figure 7): FC = 80% and 50% in laminate (50% and 52% under air) for the acrylate and the epoxide, respectively. 5. Properties of the Synthesized Polymers. 5.1. Luminescence Properties of the Synthesized Films. Interestingly, the FMs are involved in the photoinitiating systems as part of a catalytic cycle (see belowScheme 4); these FMs are partly regenerated in the catalytic cycle and, therefore, are not fully Scheme 4. Photoredox Catalytic Cycle for the ThreeComponent FM/Iod/NVK System

F


Article

Macromolecules

Figure 8. Luminescence spectra for the polymer film obtained by photopolymerization of EPOX upon LED@405 nm using different initiating systems: (A) PVD2/Iod/NVK (0.5%/1%/1% w/w); (B) DPVBi/Iod/NVK (0.5%/1%/1% w/w).

Figure 9. Fluorescence spectra of PVD1 (A) and PVD2 (B) as a function of [Iod].

The radicals generated in these systems were also fully characterized in ESR-spin trapping experiments. Phenyl radicals (Ph•) are clearly observed in irradiated FM/Iod solutions (Figure 11; reaction 1 in Scheme 4; hyperfine coupling constants of the Ph•/PBN radical adduct: aN = 14.2 G and aH = 2.2 G in agreement with the known data for this structure observed in other systems).45 In laser flash photolysis experiments on PVD1 and PVD2, long-lived transients were monitored at λ > 450 nm (Figure 12A). A bleaching of the ground state (ΔOD < 0) occurs between 400 and 450 nm. For both compounds, these transient species are characterized by long lifetimes (50−100 μs) and exhibit an absorption maximum at ∼600 nm (Figure 12B). These transients are also strongly quenched by O2 as their lifetimes are now ∼300 ns under air (Figure 12C; rate constant: kO2 ∼ 3 × 109 M−1 s−1): therefore, they can be ascribed to triplet states (3PVD1 and 3PVD2). Their quenching by Iod is, however, low (rate constants 90%) within 5 min of irradiation, albeit in the presence of NVK, this consumption is only 30%. This result shows that PVD2 is partly regenerated in the PVD2/Iod/NVK system. Interestingly, in ESR spin trapping experiments on a PVD2/ Iod/NVK solution (upon LED irradiation), the formation of the NVK radical (Ph-NVK•) is detected (reaction 2 in Scheme

photochemical reactivity of PVK was not investigated in detail (no photolysis was found upon LED@365 nm for the PVK/Iod systemFigure S1 in the Supporting Information). The three other FMs used as photoinitiators in this study are fluorescent compounds (e.g., for PVD1, a fluorescence quantum yield of 0.7 has been reported in ref 53). The reactivity of their singlet excited state can be easily followed by fluorescence quenching experiments as shown in Figure 9. A strong fluorescence quenching was observed for all the FM/Iod systems. The Stern−Volmer coefficients are kqτ = 10, 30, and 14 M−1 for 1 DPVBi/Iod, 1PVD1/Iod, and 1PVD2/Iod, respectively. The fluorescence lifetimes of PVD1 and PVD2 are lower than 6 ns (from time-resolved experiments); the 1PVD/Iod interaction rate constants are very high (>2 × 109 M−1 s−1). These very strong interactions are in full agreement with the very favorable free energy change ΔGet of the electron transfer reaction between 1FMs and Iod (reaction 1 in Scheme 4, Table 1) calculated from the classical Rehm−Weller equation: indeed, the ΔGets for the 1DPVBi/Iod, 1PVD1/Iod, and 1PVD2/Iod interactions are highly negative (ΔG = −1.61, −1.46, and −1.13 eV, respectively; Ered(Iod) = −0.2 V; see the used parameters in Table 1). A heavy atom quenching due to the presence of I can be ruled out (no quenching was observed between PVD1 and Ph-I). When exposed to the LED@395 nm, a very fast bleaching of the PVD1/Iod, PVD2/Iod, or DPVBi/Iod solution is clearly found (Figure 10; without irradiation, no bleaching is observed). All these steady state photolysis data are in agreement with the strong 1FM/Iod interaction observed above. G


Article

Macromolecules

Figure 10. Photolysis of (A) PVD1/Iod, (B) PVD2/Iod, and (C) DPVBi/Iod in acetonitrile/toluene (50%/50%), upon irradiation with the LED@ 395 nm, [Iod] = 4.7 × 10−3 M, and (D) consumption of PVD2 followed at 450 nm for PVD2/Iod and PVD2/Iod/NVK, upon irradiation with the LED@395 nm, [Iod] = 4.7 × 10−3 M.

Figure 11. ESR-spin trapping spectra obtained for the irradiation of (A) PVD1/Iod and (B) PVD2/Iod (solvent = toluene), irradiation with the LED@405 nm; experimental (a) and simulated (b) spectra. Phenyl-N-tert-butylnitrone (PBN) is used as spin trap.

4; hyperfine coupling constants of the Ph-NVK•/PBN adduct: aN = 14.4 G and aH = 2.5 G, reference values with known data for this structure in ref 57). The Ph-NVK• radical being an easily oxidized structure, the regeneration of FM through reaction 3 (Scheme 4) is expected. This latter process is in full agreement with the high cationic polymerization initiating ability of FM/Iod/NVK; i.e., it has been shown in previous reports that Ph-NVK+ is an excellent initiating species for the ring-opening polymerization of epoxy monomers.12,57,58 6.3. Polymerization Initiating Species in FM/Iod and FM/ Iod/NVK Systems. In the radical polymerization, the aryl radicals generated in reaction 1 (Scheme 4) are the initiating

species when using the FM/Iod initiating systems. These radicals are characterized by very high addition rate constants to acrylate double bonds (k ∼ 108 M−1 s−1).7 The FM+ cation also generated in this latter reaction is likely the initiating species for the cationic polymerization of EPOX. The evolution of the electron transfer quantum yields in reaction 1 (PVD1 > PVD2 > DPVBi; Table 1) does not follow the reactivity trend observed in the polymerization experiments (PVD2 ∼ DPVBi ≫ PVD1). This suggests that a back electron transfer reaction probably affects the production of the initiating species. H


Article

Macromolecules

Figure 12. (A) Kinetics recorded at different wavelengths after laser excitation at 355 nm of PVD1 in toluene under N2. (B) Transient spectrum of PVD1 in toluene recorded at t = 10 μs under N2. (C) Kinetics observed upon irradiation of PVD2 in toluene and recorded at 600 nm (1) under N2 and (2) under air.

■

In the three-component system (FM/Iod/NVK), FM is largely regenerated and the very reactive Ph-NVK+ species generated through reactions 2−3 in Scheme 4 appears as an excellent initiating species for the cationic polymerization: accordingly a strong improvement of the cationic polymerization profiles are observed.

(J.L.) E-mail: *[email protected]. Notes

The authors declare no competing financial interest.

■ ■

■

ACKNOWLEDGMENTS The authors thank the Agence Nationale de la recherche for the grant IMPACT and the grant PHOTOREDOX.

CONCLUSION Combined with an iodonium salt (Iod) or N-vinylcarbazole (NVK), the proposed photo initiators efficiently produce highly reactive radicals and cations under LED exposures at 395 and 405 nm (and even 455 nm). Initiation of the radical polymerization of TMPTA in laminate, the cationic polymerization of EPOX under air and the synthesis of TMPTA/EPOX IPNs under air are feasible. They behave as photocatalysts. Remarkably, an oligomeric and a copolymerizable version of a photocatalyst is proposed here for the first time. This paper illustrates the fact that the search of potentially useful compounds in other research areas (here in the OLED field) can lead to the proposal of novel structures of photoinitiators, well designed for specific applications.

■

AUTHOR INFORMATION

Corresponding Author

REFERENCES

(1) Photopolymerization: Fundamentals and Applications, 1st ed.; Scranton, A. B., Bowman, C. N., Peiffer, R. W., Eds.; ACS Symposium Series 673; American Chemical Society: Washington DC, 1997. (2) Handbook of Vinyl Polymers: Radical Polymerization, Process, and Technology, 2nd ed.; Mishra, M., Yagci, Y., Eds.; CRC Press: Boca Raton, 2008. (3) Bongiovanni, R.; Sangermano, M. UV-Curing Science and Technology In Encyclopedia Of Polymer Science and Technology 2014, 1−20. (4) Sangermano, M.; Razza, N.; Crivello, J. V. Cationic UV-Curing: Technology and Applications. Macromol. Mater. Eng. 2014, 299, 775− 793. (5) Photoinitiated Polymerization, 1st ed.; Belfied, K. D., Crivello, J. V., Eds.; ACS Symposium Series 847; Amercian Chemical Society: Washington DC, 2003. (6) Macromolecular Engineering: from Precise Macromolecular Synthesis to Macroscopic Materials Properties and Applications, 1st ed.; Matyjaszewski, K., Gnanou, Y., Leibler, L., Eds.; Wiley-VCH: Weinheim, Germany, 2007; Vol. 1−4. (7) Fouassier, J.-P., Lalevée, J. Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency, 1st ed.; Wiley-VCH: Weinheim, Germany, 2012.

ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02760. Figure S1, steady state photolysis of PVK/Iod upon LED@365 nm (PDF) I


Article

Macromolecules (8) Dietliker, K. A. Compilation of Photoinitiators commercially available for UV today; Sita Technology Ltd.: Edinburgh, London, 2002. (9) Crivello, J. V., Dietliker, K. A. In Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization, 2nd ed.; Bradley, G., Ed.; John Wiley & Sons: Chichester, U.K., 1998. (10) Dyes and Chromophores in Polymer Science, 1st ed.; Lalevée, J., Fouassier, J.-P., Eds.; Wiley-ISTE: London, 2015. (11) Yagci, Y.; Jockusch, S.; Turro, N. J. Photoinitiated Polymerization: Advances, Challenges, and Opportunities. Macromolecules 2010, 43, 6245−6260. (12) Crivello, J. V. In Dyes and Chromophores in Polymer Science, 1st ed.; Lalevée, J., Fouassier, J.-P., Eds.; Wiley-ISTE: London, 2015. (13) (a) Strehmel, B., Brömme, T., Schmitz, C., Reiner, K., Ernst, S., Keil, D. In Dyes and Chromophores in Polymer Science, 1st ed.; Lalevée, J., Fouassier, J.-P., Eds.; Wiley-ISTE: London, 2015. (b) Klee, J. E., Maier, M., Fik, C. P. In Dyes and Chromophores in Polymer Science, 1st ed.; Lalevée, J., Fouassier, J.-P., Eds.; Wiley-ISTE: London, 2015. (c) Lalevée, J., Fouassier, J.-P. In Dyes and Chromophores in Polymer Science, 1st ed.; Lalevée, J., Fouassier, J.-P., Eds.; Wiley-ISTE: London, 2015. (14) Doran, S., Taskin, O. S., Tasdelen, M. A., Yagci, Y. In Dyes and Chromophores in Polymer Science, 1st ed.; Lalevée, J., Fouassier, J.-P., Eds.; Wiley-ISTE: London, 2015. (15) Tasdelen, M. A., Ç iftci, M., Uygun, M., Yagci, Y. In Progress in Controlled Radical Polymerization: Mechanisms and Techniques, 1st ed.; Matyjaszewski, K., Sumerlin, B., Tsarevsky, N., Eds.; ACS Symposium Series 1100; American Chemical Society − : Washington DC, 2012; p59. (16) Lalevée, J., Fouassier, J.-P. In Photochemistry, 1st ed.; Fasani, E., Albini, A., Eds.; RSC: Cambridge, U.K., 2014; Vol. 42, p 215. (17) Fouassier, J.-P.; Lalevée, J. Three-component photoinitiating systems: towards innovative tailor made high performance combinations. RSC Adv. 2012, 2, 2621−2629. (18) Xiao, P.; Zhang, J.; Dumur, F.; Tehfe, M. A.; Morlet-Savary, F.; Graff, B.; Gigmes, D.; Fouassier, J.-P.; Lalevée, J. Visible light sensitive photoinitiating systems: Recentprogress in cationic and radical photopolymerizationreactions under soft conditions. Prog. Polym. Sci. 2015, 41, 32−66. (19) Lalevée, J.; Xiao, P.; Telitel, S.; Lepeltier, M.; Dumur, F.; Morlet-Savary, F.; Gigmes, D.; Fouassier, J.-P. Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization. Beilstein J. Org. Chem. 2014, 10, 863−876. (20) Ligon, S. C.; Husar, B.; Wutzel, H.; Holman, R.; Liska, R. Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization. Chem. Rev. 2014, 114, 557−589. (21) Kahveci, M. U., Gorkem, A. G., Yagci, Y. In Handbook on Photochemistry and Photophysics of Polymer Materials, 1st ed.; Allen, N. S., Ed.; Wiley: Hoboken, NJ, 2010; p 421. (22) Ivan, M. G., Scaiano, J. C. In Handbook on Photochemistry and Photophysics of Polymer Materials, 1st ed.; Allen, N. S., Ed.; Wiley: Hoboken, NJ, 2010; p 479. (23) Muftuogli, A. E., Tasdelen, M. A., Yagci, Y. In Handbook on Photochemistry and Photophysics of Polymer Materials, 1st ed.; Allen, N. S., Ed.; Wiley: Hoboken, NJ, 2010; p 509. (24) Moszner, N.; Lamparth, I.; Angermann, J.; Fischer, U. K.; Zeuner, F.; Bock, T.; Liska, R.; Rheinberger, V. Synthesis of bis(3-{[2(allyloxy)ethoxy]methyl}-2,4,6 trimethylbenzoyl) (phenyl)phosphine oxide − a tailor-made photoinitiator for dental adhesives. Beilstein J. Org. Chem. 2010, 6, 1−9. (25) Neshchadin, D.; Rosspeintner, A.; Griesser, M.; Lang, B.; Mosquera-Vazquez, S.; Vauthey, E.; Gorelik, V.; Liska, R.; Hametner, C.; Ganster, B.; Saf, R.; Moszner, N.; Gescheidt, G. Acylgermanes: Photoinitiators and Sources for Ge-Centered Radicals. Insights into their Reactivity. J. Am. Chem. Soc. 2013, 135, 17314−17321. (26) Tar, H.; Sevinc Esen, D.; Aydin, M.; Ley, C.; Arsu, N.; Allonas, X. Panchromatic Type II Photoinitiator for Free Radical Polymer-

ization Based on Thioxanthone Derivative. Macromolecules 2013, 46, 3266−3272. (27) Corakci, B.; Hacioglu, S. O.; Toppare, L.; Bulut, U. Long wavelength photosensitizers in photoinitiated cationic polymerization: The effect of quinoxaline derivatives on photopolymerization. Polymer 2013, 54, 3182−3187. (28) Yang, J.; Shi, S.; Xu, F.; Nie, J. Synthesis and photopolymerization kinetics of benzophenone sesamol one-component photoinitiator. Photochem. Photobiol. Sci. 2013, 12, 323−329. (29) Pietrzak, M.; Wrzyszczyński, A. Novel sulfur-containing benzophenone derivative as radical photoinitiator for photopolymerization. J. Appl. Polym. Sci. 2011, 122, 2604−2608. (30) Kumbaraci, V.; Aydogan, B.; Talinli, N.; Yagci, Y. Naphthodioxinone-1,3-benzodioxole as photochemically masked one-component type II photoinitiator for free radical polymerization. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 2612−2618. (31) Esen, D. S.; Arsu, N.; Da Silva, J. P.; Jockusch, S.; Turro, N. J. Benzoin type photoinitiator for free radical polymerization. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1865−1871. (32) Alzahrani, A. A.; Erbse, A. A.; Bowman, C. N. Evaluation and development of novel photoinitiator complexes for photoinitiating the copper-catalyzed azide−alkyne cycloaddition reaction. Polym. Chem. 2014, 5, 1874−1882. (33) Xu, J.; Ma, G.; Wang, K.; Gu, J.; Jiang, S.; Nie, J. Synthesis and photopolymerization kinetics of oxime ester photoinitiators. J. Appl. Polym. Sci. 2012, 123, 725−731. (34) Orzeł, A.; Podsiadły, R.; Podemska, K.; Strzelczyk, R.; Kolińska, J.; Sokołowska, J. Dyes based on the 6,7-dichloro-5,8-quinolinedione skeleton as new type II photoinitiators for radical polymerisation. Color. Technol. 2014, 130, 185−190. (35) Wu, Q.; Xiong, Y.; Liang, Q.; Tang, H. Developing thioxanthone based visible photoinitiators for radical polymerization. RSC Adv. 2014, 4, 52324−52331. (36) Kabatc, J.; Jurek, K.; Czech, Z.; Kowalczyk, A. Xylene-1,4-bis[4(p-pyrrolidinostyryl)benzothiazolium borate] salt as new functional dye. Dyes Pigm. 2015, 114, 144−145. (37) Taskin, O. S.; Erel-Goktepe, I.; Khan, M. A. A.; Pispas, S.; Yagci, Y. Polystyrene-b-poly(2-vinyl phenacyl pyridinium) salts as photoinitiators for free radical and cationic polymerizations and their photoinduced molecular associations. J. Photochem. Photobiol., A 2014, 285, 30−36. (38) Sun, F.; Li, Y.; Zhang, N.; Nie, J. Initiating gradient photopolymerization and migration of a novel polymerizable polysiloxane α-hydroxy alkylphenones photoinitiator. Polymer 2014, 55, 3656−3665. (39) Dell’Erba, I. E.; Arenas, G. F.; Schroeder, W. F.; Asmussen, S. V.; Vallo, C. I. Hybrid organic−inorganic macromolecular photoinitiator system for visible-light photopolymerization. Prog. Org. Coat. 2014, 77, 1848−1853. (40) Karaca Balta, D.; Karahan, Ö .; Avci, D.; Arsu, N. Synthesis, photophysical and photochemical studies of benzophenone based novel monomeric and polymeric photoinitiators. Prog. Org. Coat. 2015, 78, 200−207. (41) Qiu, J.; Wei, J. Water-soluble and polymerizable thioxanthone photoinitiator containing imidazole. J. Appl. Polym. Sci. 2014, 131, 40659. (42) Davidson, B. J.; Gee, S. J.; Hurst, G. C.; Rucker, J. B.; Shouse, J. L.; Smith, J. B.; Cavitt, T. B. J. Appl. Polym. Sci. 2015, 132, 41883. (43) Aguirre-Soto, A.; Lim, C.-H.; Hwang, A. T.; Musgrave, C. B.; Stansbury, J. W. Visible-Light Organic Photocatalysis for Latent Radical-Initiated Polymerization via 2e−/1H+ Transfers: Initiation with Parallels to Photosynthesis. J. Am. Chem. Soc. 2014, 136, 7418− 7427. (44) Zhang, G.; Song, I. Y.; Ahn, K. H.; Park, T.; Choi, W. Free Radical Polymerization Initiated and Controlled by Visible Light Photocatalysis at Ambient Temperature. Macromolecules 2011, 44, 7594−7599. (45) Tehfe, M. A.; Lalevée, J.; Telitel, S.; Contal, E.; Dumur, F.; Gigmes, D.; Bertin, D.; Nechab, M.; Graff, B.; Morlet-Savary, F.; J


Article

Macromolecules Fouassier, J.-P. Polyaromatic Structures as Organo-Photoinitiator Catalysts for Efficient Visible Light Induced Dual Radical/Cationic Photopolymerization and Interpenetrated Polymer Networks Synthesis. Macromolecules 2012, 45, 4454−4460. (46) (a) Allen, A. E.; MacMillan, D. W. C. Synergistic catalysis: A powerful synthetic strategy for new reaction development. Chem. Sci. 2012, 3, 633−658. (b) Narayanam, J. M. R.; Stephenson, C. R. J. Visible light photoredox catalysis: applications in organic synthesis. Chem. Soc. Rev. 2011, 40, 102−113. (c) Prier, C. K. D.; Rankic, A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363. (d) Yoon, T. P.; Ischay, M. A.; Du, J. Visible light photocatalysis as a greener approach to photochemical synthesis. Nat. Chem. 2010, 2, 527−532. (47) (a) Fors, B. P.; Hawker, C. J. Control of a Living Radical Polymerization of Methacrylates by Light. Angew. Chem., Int. Ed. 2012, 51, 8850−8853. (b) Konkolewicz, D.; Schröder, K.; Buback, J.; Bernhard, S.; Matyjaszewski, K. Visible Light and Sunlight Photoinduced ATRP with ppm of Cu Catalyst. ACS Macro Lett. 2012, 1, 1219−1223. (48) (a) Ribelli, T. G.; Konkolewicz, D.; Bernhard, S.; Matyjaszewski, K. How are Radicals (Re)Generated in Photochemical ATRP. J. Am. Chem. Soc. 2014, 136, 13303−13312. (b) Anastasaki, A.; Nikolaou, V.; Zhang, Q.; Burns, J.; Samanta, S. R.; Waldron, C.; Haddleton, A. J.; McHale, R.; Fox, D.; Percec, V.; Wilson, P.; Haddleton, D. M. Copper(II)/Tertiary Amine Synergy in Photoinduced Living Radical Polymerization: Accelerated Synthesis of ω-Functional and α,ωHeterofunctional Poly(acrylates). J. Am. Chem. Soc. 2014, 136, 1141− 1149. (c) Frick, E.; Anastasaki, A.; Haddleton, D. M.; Barner-Kowollik, C. Enlightening the Mechanism of Copper Mediated PhotoRDRP via High-Resolution Mass Spectrometry. J. Am. Chem. Soc. 2015, 137, 6889−6896. (d) Fu, C.; Xu, J.; Tao, L.; Boyer, C. Combining Enzymatic Monomer Transformation with Photoinduced Electron Transfer − Reversible Addition−Fragmentation Chain Transfer for the Synthesis of Complex Multiblock Copolymers. ACS Macro Lett. 2014, 3, 633−638. (e) Xu, J.; Jung, K.; Boyer, C. Oxygen Tolerance Study of Photoinduced Electron Transfer−Reversible Addition− Fragmentation Chain Transfer (PET-RAFT) Polymerization Mediated by Ru(bpy)3Cl2. Macromolecules 2014, 47, 4217−4229. (f) Baralle, A.; Fensterbank, L.; Goddard, J.-P.; Ollivier, C. Aryl Radical Formation by Copper(I) Photocatalyzed Reduction of Diaryliodonium Salts: NMR Evidence for a CuII/CuI Mechanism. Chem. Chem. - Eur. J. 2013, 19, 10809−10813. (g) Nzulu, F.; Telitel, S.; Stoffelbach, F.; Graff, B.; Morlet-Savary, F.; Lalevée, J.; Fensterbank, L.; Goddard, J.-P.; Ollivier, C. A dinuclear gold(I) complex as a novel photoredox catalyst for light-induced atom transfer radical polymerization. Polym. Chem. 2015, 6, 4605−4611. (h) Xu, J.; Jung, K.; Atme, A.; Shanmugam, S.; Boyer, C. A Robust and Versatile Photoinduced Living Polymerization of Conjugated and Unconjugated Monomers and Its Oxygen Tolerance. J. Am. Chem. Soc. 2014, 136, 5508−5519. (i) Shanmugam, S.; Boyer, C. Stereo-, Temporal and Chemical Control through Photoactivation of Living Radical Polymerization: Synthesis of Block and Gradient Copolymers. J. Am. Chem. Soc. 2015, 137, 9988−9999. (49) Treat, N. J.; Fors, B. P.; Kramer, J. W.; Christianson, M.; Chiu, C.-Y.; Alaniz, J. R. d.; Hawker, C. J. Controlled Radical Polymerization of Acrylates Regulated by Visible Light. ACS Macro Lett. 2014, 3, 580− 584. (50) (a) Lalevée, J.; Peter, M.; Dumur, F.; Gigmes, D.; Blanchard, N.; Tehfe, M. A.; Morlet-Savary, F.; Fouassier, J.-P. Subtle Ligand Effects in Oxidative Photocatalysis with Iridium Complexes: Application to Photopolymerization. Chem. - Eur. J. 2011, 17, 15027−15031. (b) Lalevée, J.; Blanchard, N.; Tehfe, M. A.; Morlet-Savary, F.; Fouassier, J.-P. Green Bulb Light Source Induced Epoxy Cationic Polymerization under Air Using Tris(2,2′-bipyridine)ruthenium(II) and Silyl Radicals. Macromolecules 2010, 43, 10191−10195. (c) Lalevée, J.; Blanchard, N.; Tehfe, M. A.; Peter, M.; Morlet-Savary, F.; Fouassier, J.-P. Household LED irradiation under air: cationic polymerization using iridium or ruthenium complex photocatalysts. Polym. Bull. 2012, 68, 341−347.

(51) (a) Tang, C. W.; Van Slyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913−915. (b) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer. Nature 2000, 403, 750−753. (c) Wu, C.-C.; Lin, Y. T.; Wong, K. T.; Chen, R. T.; Chien, Y. Y. Efficient Organic Blue-Light-Emitting Devices with Double Confinement on Terfluorenes with Ambipolar Carrier Transport Properties. Adv. Mater. 2004, 16, 61−69. (d) Burn, P. L.; Lo, S. C.; Samuel, I. D. W. The Development of Light-Emitting Dendrimers for Displays. Adv. Mater. 2007, 19, 1675−1688. (e) Wu, K. C.; Ku, P. J.; Lin, C. S.; Shih, H. T.; Wu, F. I.; Huang, M. J.; Lin, J. J.; Chen, I. C.; Cheng, C. H. The Photophysical Properties of Dipyrenylbenzenes and Their Application as Exceedingly Efficient Blue Emitters for Electroluminescent Devices. Adv. Funct. Mater. 2008, 18, 67−75. (52) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X. Recent progress in metal−organic complexes for optoelectronic applications. Chem. Soc. Rev. 2014, 43, 3259−3302. (53) Dautel, O. J.; Wantz, G.; Almairac, R.; Flot, D.; Hirsch, L.; LerePorte, J.-P.; Parneix, J.-P.; Serein-Spirau, F.; Vignau, L.; Moreau, J. J. E. Nanostructuration of Phenylenevinylenediimide-Bridged Silsesquioxane: From Electroluminescent Molecular J-Aggregates to Photoresponsive Polymeric H-Aggregates. J. Am. Chem. Soc. 2006, 128, 4892−4901. (54) Larrow, J. F.; Jacobsen, E. N.; Gao, Y.; Hong, Y.; Nie, X.; Zepp, C. M. A Practical Method for the Large-Scale Preparation of [N,N′Bis(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediaminato(2-)]manganese(III) chloride, a Highly Enantioselective Epoxidation Catalyst. J. Org. Chem. 1994, 59, 1939−1942. (55) Dautel, O. J.; Wantz, G.; Flot, D.; Lere-Porte, J.-P.; Moreau, J. J. E.; Parneix, J.-P.; Serein-Spirau, F.; Vignau, L. Confined photoactive substructures on a chiral scaffold: the design of an electroluminescent polyimide as material for PLED**. J. Mater. Chem. 2005, 15, 4446− 4452. (56) (a) Zhang, Z.; Peng, B.; Ding, P.; Liu, B.; He, Y.; Zhou, K.; Li, Y.; Pan, C.; Zou, Y. Synthesis and photovoltaic properties of dithienyl benzotriazole based poly(phenylene vinylene)s. J. Appl. Polym. Sci. 2011, 120, 2534−2542. (b) Wantz, G.; Dautel, O. J.; Almairac, R.; Hirsch, L.; Serein-Spirau, F.; Vignau, L.; Lere-Porte, J.-P.; Parneix, J.P.; Moreau, J. J. E. Layered organic film growth by substrate temperature tuning for efficiency-enhanced OLEDs. Org. Electron. 2006, 7, 38−44. (57) Lalevée, J.; Dumur, F.; Mayer, C. R.; Gigmes, D.; Nasr, G.; Tehfe, M. A.; Telitel, S.; Morlet-Savary, F.; Graff, B.; Fouassier, J.-P. Photopolymerization of N-Vinylcarbazole Using Visible-Light Harvesting Iridium Complexes as Photoinitiators. Macromolecules 2012, 45, 4134−4141. (58) Hua, Y.; Crivello, J. V. Synergistic interaction of epoxides and Nvinylcarbazole during photoinitiated cationic polymerization. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3697−3709.

K


Oligomeric Photocatalysts in Photoredox Catalysis - ACS Publications

Recommend Documents