Article pubs.acs.org/Macromolecules
Photosensitized Formation of Phosphorus-Centered Radicals: Application to the Design of Photoinitiating Systems Jacques Lalevée,*,† Fabrice Morlet-Savary,† Mohamad Ali Tehfe,† Bernadette Graff,† and Jean Pierre Fouassier‡ †
Institut de Science des Matériaux de Mulhouse IS2M, LRC CNRS 7228, ENSCMu-UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ University of Haute Alsace-ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse, France ABSTRACT: The processes involved in the photosensitized decomposition of phosphorus-containing compounds in the presence of a photoinitiator PI were investigated and characterized by laser flash photolysis, electron spin resonance, and molecular modeling (density functional theory calculations). They lie on (i) a photoinitiator/phosphorus-containing compound electron transfer from phosphinites a, b and phosphites c, leading to a phosphoniumyl radical ion R3P•+ or a photoinitiator/phosphorus-containing compound hydrogen abstraction reaction from labile P−H bonds in phosphine oxides d, phosphonates e, and trialkylphosphine salts f, g and (ii) the subsequent production of phosphorus centered radicals P• in a−e and a radical ion P•+ in f, g. The interaction rate constants for the key processes (photoinitiator/phosphorus-containing compound, alkoxyl radical/phosphorus-containing compound, and peroxyl radical/ phosphorus-containing compound), the associated transient absorption, and electron spin resonance−spin trapping spectra were determined. The recorded acrylate polymerization profiles show that the photoinitiator/phosphorus-containing compound (a− g) systems are often efficient. Compared to reference systems, they can lead, in some cases (with benzophenone or a thiopyrylium salt as photoinitiator), to similar or even better polymerization rates. The cationic photopolymerization of epoxides was also feasible with f, g, thereby demonstrating that photoinitiator/f (or g) couples behave as dual radical/cationic photoinitiating systems.
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INTRODUCTION Phosphorus-containing compounds (noted PCC below) are used in organic synthesis; phosphorus-centered radicals are often involved as intermediates in these purposes.1,2 Most of reactions are thermally initiated e.g. through redox or electrochemical reactions. Some of the rare examples of photochemically triggered reactions include the direct photolysis of arylethylphosphites (known as a photo-Arbuzov rearrangement) or the photosensitized rearrangement of phenylallylphosphites (into phosphonates) in the presence of dicyanonaphthalene. Phosphorus-centered radicals initiating species are also encountered in photopolymerization reactions: they are mainly generated from type I cleavable photoinitiators (PI) such as acylphosphine oxide derivatives (e.g., diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide, TPO, or bis(acylphosphine oxide)s, BAPO);3 acyl- or thioacylphosphonates have been also mentioned.4 The produced phosphinoyl radicals are used in (i) free radical polymerization, FRP,3a,b and (ii) free radical promoted cationic photopolymerization, FRPCP.3e,5 To the best of our knowledge, PCCs such as the compounds shown in Scheme 1 (phosphinites, phosphines, phosphonates, phosphineoxides, and phosphine salts) were never tested as coinitiators in bimolecular photoinitiating systems in polymerization reactions (FRP or cationic photopolymerization, CP) under near UV/vis light exposure. The aim of the present work is therefore (i) an assessment of the photosensitized decomposition of these PCCs, (ii) the investigation of the associated phosphorus-centered radical © 2012 American Chemical Society
Scheme 1
chemistry, and (iii) the design of new type II photoinitiating systems based on a photoinitiator PI and a PCC as a co-initiator. This works extends the search for other sources of radicals in FRP and cationic species in CP. In fact, a large variety of initiating Received: April 21, 2012 Revised: May 29, 2012 Published: June 8, 2012 5032
dx.doi.org/10.1021/ma3008064 | Macromolecules 2012, 45, 5032−5039
Macromolecules
Article
experiments were carried out using a X-Band spectrometer (MS 400 from Magnettech-Berlin; Germany) at room temperature. The interaction rate constants of the tert-butyl peroxyl (tBu-OO•) with the different PCCs were determined from the lifetime of tBu-OO• at different quencher concentrations through a classical Stern−Volmer plot. 5. Electron Spin Resonance Spin Trapping Experiments. This ESR technique (ESR-ST) is now recognized as particularly powerful for the identification of the radical centers.19 The procedure used here has been described recently in ref 20. The generated radicals were trapped by phenyl-N-tert-butyl nitrone (PBN) or 2,2-dimethyl-3,4-dihydro-2Hpyrrole-1-oxide (DMPO). tert-Butylbenzene was used as a solvent. The ESR spectrum simulations were carried out with the WINSIM software.21 6. Redox Potentials. The redox potentials were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (Voltalab 06Radiometer; the working electrode was a platinium disk and the reference a saturated calomel electrode-SCE). Ferrocene was used as a standard, and the potentials determined from the half-peak potential were referred to the reversible formal potential of this compound (+0.44 V/SCE). The free energy change ΔGet for an electron transfer reaction is calculated from the classical Rehm−Weller equation22
radicals, e.g. alkyl, benzoyl, hydroxy isopropyl, phosphinoyl, sulfonyl, aminoalkyl, thiyl radicals,5,6 and more recently sulfur,7 carbon (derived from boranes 9 ), boryl, 10 silyl, 8,11 and germanium12,13 centered radicals have been already proposed (see also a review in ref 14). The design of new structures as well as new routes to produce phosphorus radicals (and even phosphoniumyl radical ions) still remain an interesting topic.
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EXPERIMENTAL SECTION
1. The Investigated PCCs. The different PCCs shown in Scheme 1methyl diphenylphosphinite (a), ethyl diphenylphosphinite (b), dimethyltrimethylsilyl phosphite (c), diphenylphosphine oxide (d), diethyl phosphonate (e), tricyclohexylphosphine tetrafluoroborate (f), and tributylphosphine tetrafluoroborate (g)were obtained from Aldrich and used with the best purity available. The selected phosphorus-containing compounds are phosphinites (a, b), phosphites (c), phosphine oxides (d), phosphonates (e), and trialkylphosphine salts (f, g). Compared to other phosphines, these compounds do not exhibit strong unpleasant odor and can be manipulated easily in a chemical lab. Benzophenone (BP), 2-isopropylthioxanthone (ITX), camphorquinone (CQ), 2,4,6-(4-methoxyphenyl)thiopyrylium tetrafluoroborate (TP+), and Eosin-Y (Eo) were used as model PIs (Aldrich). Ethyldimethylaminobenzoate (EDB; Esacure EDB from Lamberti) and methyldiethanolamine (MDEA; Aldrich) were chosen as reference amine co-initiators. 2. Photopolymerization Experiments. The laminated films (50 μm thick) of a bulk oligomer/monomer acrylate formulation based on 75/25 w/w epoxyacrylate/tripropylene glycoldiacrylate (Ebecryl 605 from Cytec) deposited on a BaF2 pellet were either exposed to a UV irradiation (polychromatic light delivered by a Xe−Hg lamp: Hamamatsu, L8252, 150 W; cutoff filter to select λ > 300 nm) or to a visible light irradiation (xenon lamp: Hamamatsu, L8253; cutoff filter to select λ > 400 nm). These experiments were carried out both in laminated and under air conditions. Some experiments were also carried out with a low-viscosity monomer (trimethylolpropane triacrylate, TMPTA, from Cytec). The evolution of the double-bond content was continuously followed by real-time FTIR spectroscopy (Nexus 870, Nicolet).15 The Rp quantities refer to the maximum rates of the polymerization reaction and were calculated from the maximum of the first derivative of the conversion vs time curves. The reported values are expressed as Rp/[M0] (s−1) where [M0] is the initial monomer concentration. A dicycloaliphatic epoxide monomer (UVacure 1500 from Cytec) is used in cationic photopolymerization experiments. A cutoff filter allows to select λ > 300 nm (UV irradiation). The evolution of the epoxy group content is continuously followed by real-time FTIR spectroscopy; the absorbance of the epoxy group was monitored at about 790 cm−1 as in ref 13. The photoinitiating systems will be compared to ITX/ diphenyliodonium hexafluorophosphate (Ph2I+ ) in which Ph2I+ represents a well-known cationic photoinitiator. 3. Laser Flash Photolysis Experiments. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/YAG laser (λexc = 355 nm, 9 ns pulses; energy ∼10 mJ, from Minilite Continuum) and an analyzing system consisting of a pulsed xenon lamp, a monochromator, a fast photomultiplier, and a transient digitizer.16 The interaction rate constants between the tert-butoxyl radicals (tBuO•) and a−g were measured through a classical Stern−Volmer treatment using the rising time of the radical adduct at different quencher concentrations (the t-BuO• radicals were generated through the photochemical decomposition of di-tert-butyl peroxide).11a,d The interaction rate constants of the PCCs with 3BP, 3ITX, 3CQ, and 3TP+ were obtained (by Stern−Volmer treatments) from their triplet state lifetimes measured at 525, 600, 800, and 550 nm, respectively. The ketyl radical quantum yields for BP (ΦK•) were determined by a classical procedure.16,17 For the interaction with 1TP+, time-resolved fluorescence experiments were used as presented in ref 15c. 4. Kinetic Electron Spin Resonance (KSER). General kinetic ESR (KESR) procedures have been described in detail in ref 18. The ESR
ΔGet = Eox − Ered − E T + C
(1)
where Eox, Ered, ET, and C are the oxidation potential of the donor, the reduction potential of the acceptor, the triplet state energy, and the Coulombic term for the initially formed ion pair, respectively. C is neglected as usually done in polar solvents.11c 7. Density Functional Theory (DFT) Calculations. All the calculations were performed using the hybrid functional B3LYP from the Gaussian 03 suite of program.23 Reactants and products were fully optimized at the B3LYP/6-31+G* level (and frequency checked). The bond dissociation energies (BDE) were calculated as the energetic difference between the parent compounds and the radicals (P−H → P• + H•).
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RESULTS AND DISCUSSION 1. Photosensitized Decomposition of the Studied Phosphorus-Containing Compounds PCCs and Formation of the Phosphorus-Centered Radical Ions and Radicals. According to what is proposed in the PCC literature (see e.g. a review in ref 1), we could expect, in a general way, two different kinds of processes for the photosensitized decomposition of the PCCs shown in Scheme 1. The first one is based on the fragmentation of a phosphoniumyl radical ion R′2P•+−OR into a phosphinoyl radical R′2P(O)• (1b): R′2P•+−OR is generated by an electron transfer process 1a between a phosphinite R′2P−OR or a phosphite R′P[−OR]2 and a suitable photoinitiator PI playing the role of photosensitizer. Benzophenone, isopropylthioxanthone, camphorquinone, a thiopyrylium salt, and eosin should work as PI. PI + R′2 P−OR → PI•− + R′2 P•+−OR
(hν)
(1a)
R′2 P•+−OR → R′2 P(=O)• + R+
(1b)
The second one involves a hydrogen abstraction reaction from the P−H bond of a phosphine oxide R′2P(=O)−H or a phosphonate (R′O)2P(=O)−H (2a) or a phosphine salt R″3P+−H (2b) using a suitable photoinitiator PI: phosphinoyls (R′2P(=O)•) and phosphoniumyl radical ions (R″3P+•) are formed, respectively. PI + R′2 P(=O)−H → PI−H• + R′2 P(=O)•
(hν) (2a)
PI + R″3 P+−H → PI−H• + R″3 P+• 5033
(hν)
(2b)
dx.doi.org/10.1021/ma3008064 | Macromolecules 2012, 45, 5032−5039
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a. Cleavage of the Phosphoniumyl Radical Ions (in Compounds a−c). The radicals generated upon a photosensitized UV irradiation of a−c using BP were characterized by ESR-ST using PBN as a spin trap. A third hyperfine splitting constant (HFS) ap due to the phosphorus nuclei must be included to reproduce the experimental spectrum (Figure 1 and
clearly evidence the fragmentation of the phosphoniumyl radical ion R′2P•+−OR into a phosphinoyl radical R′2P(=O)• as suggested in eq 1. This fragmentation is likely fast.1 The interaction rate constants of the different PI excited states with the phosphinites (a, b) and the phosphite (c) are high in agreement with a roughly favorable electron transfer reaction (Table 1) e.g. for 3BP/a (using a reduction potential Eox = −1.79 V and a triplet state energy ET = 2.98 eV for BP),10 ΔGet = +0.09 eV. Phosphite c exhibits a higher oxidation potential (Eox > +1.8 V) than the phosphinites a and b; i.e., compound c leads to a lower interaction rate constant with 3BP (ΔGet > 0.61 eV for c). When using 3TP+ (Ered = −0.39 V; ET = 2.3 eV),15c the electron transfer (1a) is highly exergonic with a−c (ΔGet = −0.63, −0.58, and >−0.11 eV, respectively) in line with the rate constants which are diffusion controlled. This obviously also holds true in the 1TP+ singlet state as the associated energy is higher by about 0.2 eV.15c With ITX (Ered = −1.57 V; ET = 2.69 eV) or CQ (ET = 2.2 eV and Ered = −1.44 V),15c the electron transfer is noticeably less favorable (e.g., ΔGet = +0.16 and +0.21 eV for ITX/a and ITX/b, respectively). In the 3BP/a (b, c) interaction, no residual absorption is noted between 400 and 800 nm (Figure 2) ruling out the presence of the radical anion BP•− in the investigated time scale. Moreover, no ketyl radical BPH• is observed, demonstrating that no direct hydrogen abstraction occurs. The observed transient absorption at λ < 350 nm is ascribed to the phosphinoyl radicals in agreement with the spectrum of ref 26. A similar situation is found in 3ITX/a (b, c). This can demonstrate that either (i) the quantum yield for the ion formation (in phosphoniumyl radical ion) is low or (ii) a fast in-cage electron transfer reaction between PI•− and R+ (formed in 1b) occurs leading to PI and an additional radical R• (3). This second hypothesis is fully supported by the observation of a silyl radical (in the case of the irradiation of a BP/c solution) or an ethyl radical (in the case of the irradiation of a BP/b solution) in the ESR-ST experiments (Table 1).
Figure 1. ESR spectrum observed in spin trapping experiments (using phenyl-N-tert-butyl nitrone (PBN) as spin trap agent; [PBN] = 10−2 M) for Xe−Hg lamp irradiation of (A) BP/c and (B) BP/a solutions (in (A) and (B): experimental (up) and simulated (down) spectra are given; the hyperfine splitting constants for the different adducts are gathered in Table 1).
PI•− + R+ → PI + R•
(3)
In 3TP+/a (b, c), the 2,4,6-triphenylthiabenzene radical TP• (formed after the reduction of TP+) is clearly observed in LFP and ESR experiments (Figure 3). This highlights that the electron transfer (TP•/R+; eq 3) does not occur. This is probably related to the electron transfer properties of TP• which are less favorable than those of BP•−.
Table 1). The HFS found for a and b on one hand and for c on the other are very close to those reported for the phosphinoyl Ph2P(O)• and the (MeO)2P(O)• radicals, respectively (obtained elsewhere in other conditions24,25). All these results
Table 1. Rate Constants for the Interaction of the Different Photoinitiator Excited States and tBu−O• with PCCs (k, k′, k″, k‴, k′′′′);a Oxidation Potential Eox of PCCs; HFS Constants Characterizing the Phosphorus-Centered Radicals Obtained in ESR-Spin Trapping Experiments (BP/PCC) Using PBN Eox, V a b c d e f g
1.28 1.33 >1.8 >1.8 >1.8 >1.8 >1.8
k(tBu−O•),b 107 M−1 s−1 148 210 50 35
