Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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More than Expected: Overall Initiation Efficiencies of Mono‑, Bis‑, and Tetraacylgermane Radical Initiators Philipp Jöckle,†,‡,§ Philipp W. Kamm,†,‡,§ Iris Lamparth,∥ Norbert Moszner,∥ Andreas-Neil Unterreiner,*,‡ and Christopher Barner-Kowollik*,†,§
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Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18, 76131 Karlsruhe, Germany ‡ Molekulare Physikalische Chemie, Institut für Physikalische Chemie, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia ∥ Ivoclar Vivadent AG, Bendererstrasse 2, 9494 Schaan, Liechtenstein S Supporting Information *
ABSTRACT: We introduce a quantitative comparison of the overall initiation efficiency for a library of eight mono-, bis-, and tetraacylgermane visible light photoinitiators with p-methoxy and fluorine substitution patterns. Specifically, cocktail experiments were carried out, fusing pulsed laser polymerization (PLP) of methyl methacrylate (MMA) with subsequent high-resolution electrospray mass spectrometry (ESI-MS) analysis. The overall initiation efficiency reflects all critical events leading to macromolecular growth, i.e., from light absorption and cleavage of the initiator to radical initiation. Importantly, we show that the obtained overall initiation efficiencies are self-consistent when going from lower to higher substituted systems, evidencing the validity of the derived overall initiation efficiencies within the error margins of the experiment. Remarkably, the comparison of mono-, bis-, and tetraacylgermanes reveals a nonstoichiometric increase of 40−90% in the overall initiation efficiency with increasing number of benzoyl moieties per initiator (size dependence), e.g., a maximum value of 7.6 ± 1.8 for tetrabenzoylgermane versus benzoyltrimethylgermane. In addition, the size dependence observed for acylgermanes scales with the nπ* extinction coefficients with the number of benzoyl moieties of the respective photoinitiator. Finally, with increasing system size of the acylgermanes, a more complex channel branchingas suggested from time-dependent density functional theory (TDDFT) calculationsresults in substitution-dependent intersystem crossing (ISC) and cleavage quantum yields.
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INTRODUCTION Visible-light-induced free-radical polymerization (FRP) is farreaching in scientific as well as in industrial research, e.g., in advanced curing technologies,1,2 dental fillings,3,4 and tissue engineering.5,6 A key advantage of visible-light activation is the access to higher curing depths as well as bioorthogonality. The efficiency of a photo-FRP critically depends on the reactivity of the radicals formed after cleavage of an initiator, which, in turn, depends on the absorption behavior at a certain wavelength. Thus, the design of photoinitiators with a sufficient reactivity at excitation wavelengths above 400 nm is highly desired as well as challenging. Acylgermanes are promising visible-light-driven photoinitiators with a broad variety of derivatives, which enables the fundamental investigation of structure−property correlations. The first germanium-based systems were presented by Mochida et al. in 1983.7 Liska and co-workers developed efficient benzoyltrimethylgermane and bisbenzoyldiethylgermane photoinitiators with a significant red shift.8 Recently, the Stueger group synthesized tetraacylgermane © XXXX American Chemical Society
derivatives with excellent initiation properties at excitation wavelengths up to 470 nm.9 It was shown in various analytic studies that the photocleavage of acylgermanes proceeds via a Norrish type I reaction (α-cleavage),10−12 which is a common mechanism for many efficient photoinitiator systems, e.g., acetophenones,13,14 hydroxyalkylphenones,15,16 acylphosphines,17−19 and benzoins.20−22 In general, Norrish type I reactions can proceed from excited singlet and triplet states of ketone-based systems.23 Which pathway is favored strongly depends on the potential energy surfaces. α-Cleavage of acylgermanes predominantly occurs from excited triplet states, which require a sufficient intersystem crossing (ISC).10−12,24 An efficient ISC depends on a strong intersection between excited singlet and triplet states. It has been found for benzoinbased photoinitiators that excitation of nπ* bands with low Received: November 9, 2018 Revised: December 5, 2018
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DOI: 10.1021/acs.macromol.8b02404 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Overview of the Investigated Mono- (A, j = 1), Bis- (B, j = 2), and Tetraacylgermanes (C, j = 4) XjGa
extinction coefficients leads to reactive triplet population, while strong ππ* transitions eventually result in unfavorable channel branching without sufficient cleavage pathways.21,15 Thus, the extinction coefficient ε(λ) of an initiatoras a measure of solely singlet state transitionsdoes not provide sufficient insight into the reactivity of a system. However, the initiation ability should still scale with the extinction coefficient of an initiator if the correct excitation wavelength within the nπ* band is chosen. In addition to ISC and the cleavage quantum yield of a photoinitiator, the reactivity toward the monomer as well as in-cage interactions of the formed radicals contributes to an efficient initiation of a photo-FRP, defining the overall initiation efficiency.27,22 For an in-depth investigation into which effects are critical for the development of efficient acylgermane-based photoinitiators with visible light absorption behavior, mainly two approaches were pursued in the past: on the one hand, a systematic variation of substitution pattern combined with an in-depth investigation of their influence on the initiation behavior24,12 and, on the other hand, the extension of the system size from mono- over bis- to tetraacylgermanes.9,25 The first approach includes electron-donating (+M-effect) substituents in the para-position of benzoyl moieties, leading to a hypsochromic shift resulting in high efficiencies, while bathochromic acylgermanes, bearing electron-withdrawing (−M) substituents, display only a weak initiation behavior due to unfavored channel branching in the excited states.12,24 In the second approach, the number of cleavable benzoyl groups is increased from mono- and bis-8 to tetraacylgermanes, which is accompanied by a broadening of the absorption spectrum up to 470 nm.9 Bis- and tetraacylgermanes undergo complete multiple cleavages,26 which should lead to a stoichiometric increase in initiation efficiency of 2:1 for photopolymerizations initiated by tetra- compared to bis- as well as bis- compared to monoacylgermanes and of 4:1 for tetra- compared to monoacylgermanes. In this study, we investigate and quantify the influence of the number of benzoyl moieties (system size dependence) on the overall initiation efficiency for a library of mono-, bis-, and tetraacylgermanes (Scheme 1). For a comprehensive comparison, a database of overall initiation eff iciencies m is determined in cocktail experiments via PLP-SEC-ESI-MS, a methodology established by our group.27,13,12,22 The overall initiation efficiency m is a relative measure determined for pairs of initiators and encompasses all critical events during a photopolymerization, i.e., from light absorption and cleavage of the initiator to radical initiation with the monomer. Thus, m is highly dependent on the nature of the corresponding photoinitiator pairs and describes not only the initiation efficiency of a radical toward a monomer. As stated above, the absorption spectrum and the excited states of a photoinitiator are crucial for an efficient photoinduced FRP. Hence, the absorption behavior of the investigated mono-, bis-, and tetraacylgermanes is discussed on the basis of the recorded UV−vis spectra and selected time-dependent density functional theory (TDDFT) calculations.
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a
X indicates the chemical nature of j benzoyl moieties (e.g., pfluorobenzoyl FB, p-methoxybenzoyl MB, or benzoyl B). A stoichiometric behavior of the overall initiation efficiency is expected, which scales with the number of radicals provided by an initiator. PLP-SEC-ESI-MS cocktail experiments are a powerful tool to give insights into structure−reactivity correlations of an initiator. (Sigma-Aldrich). Sodium trifluoroacetate (NaTFA, Sigma-Aldrich), tetrahydrofuran (THF, Scharlau, multisolvent GPC grade, 250 ppm BHT), and methanol (MeOH, Roth, HPLC ultra gradient grade) for SEC and ESI-MS measurements were applied as received. Acetonitrile (ACN, Fisher Chemical, HPLC grade) for UV/vis spectroscopy was applied as received. Pulsed Laser Polymerization (PLP) Experiments. For PLP cocktail sample preparation two Ge photoinitiators were solved in a defined molar ratio with a total concentration of 5 mmol L−1 in methyl methacrylate (MMA) in bulk. All samples were freed from oxygen by nitrogen purging for 5 min in a photovial with a sample volume of 5 mL. The samples were placed into an in-house built sample holder for laser irradiation at ambient temperature. PLP was performed by irradiation with a tunable laser system (Tunable Laser System SpitLight EVO 200 OPO, Innolas, 270−670 nm) at ∼320 μJ/ pulse, 90000 pulses, repetition rate of 100 Hz, and the respective excitation wavelength of each initiator. A detailed description of the sample holder and the tunable laser system is given in the Supporting Information and in our earlier publication.28 SEC-ESI-MS. Hyphenated size exclusion mass spectra were recorded with a Q exactive (Orbitrap) mass spectrometer (Thermo Fisher Scientific, San Jose, CA) equipped with an HESI II probe coupled to an UltiMate 3000 UHPLC System (Dionex, Sunnyvale, CA), composed of a pump (LPG 3400SD), an autosampler (WPS 3000TSL), and a thermostated column department (TCC 3000SD). For separation, a precolumn (Mesopore 50 × 4.6 mm2) at operating temperature of 30 °C is coupled to two mixed bed size exclusion chromatography columns (Polymer Laboratories, Mesopore 250 × 5.6 mm2, particle diameter 3 μm). THF was used as an eluent at a flow rate of 0.30 mL min−1, which is conducted through a RI detector (RefractoMax520, ERC, Japan) with 0.27 mL min−1. A 0.1 mmol L−1 sodium iodide methanol solution was added at 0.02 mL min−1 by a
EXPERIMENTAL SECTION
Materials. All mono-, bis-, and tetraacylgermanes8,9 were provided by Ivoclar Vivadent (for synthesis details and NMR data, refer to the Supporting Information). Methyl methacrylate (MMA, SigmaAldrich, 99%, stabilized with
