An In-Depth Mechanistic Investigation of the Radical Initiation

Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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An In-Depth Mechanistic Investigation of the Radical Initiation Behavior of Monoacylgermanes Philipp Jöckle,†,‡,§ Caroline Schweigert,§ Iris Lamparth,∥ Norbert Moszner,∥ Andreas-Neil Unterreiner,*,§ and Christopher Barner-Kowollik*,†,‡,⊥ †

Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18, 76131 Karlsruhe, Germany ‡ Institut für Biologische Grenzflächen, Kalsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Molekulare Physikalische Chemie, Institut für Physikalische Chemie, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2,76131 Karlsruhe, Germany ∥ Ivoclar Vivadent AG, Bendererstrasse 2, 9494 Schaan, Liechtenstein ⊥ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia S Supporting Information *

ABSTRACT: Five para-substituted monoacyltrimethylgermane derivatives, i.e., p-fluorobenzoyltrimethylgermane (pFBG, λmax = 405 nm), p-methoxybenzoyltrimethylgermane (pMBG, λmax = 397 nm), benzoyltrimethylgermane (pHBG, λmax = 409 nm), p-cyanobenzoyltrimethylgermane (pCBG, λmax = 425 nm), and p-nitrobenzoyltrimethylgermane (pNBG, λmax = 429 nm) are investigated via a combination of pulsed laser polymerization with subsequent electrospray ionization and mass spectrometry (PLP-ESI-MS) as well as femtosecond transient absorption spectroscopy. The relative initiation efficiencies of the initiating benzoyl radical fragments of pFBG, pMBG, and pHBG are determined using PLP-ESI-MS. The para-substituted derivatives with the electron-donating groups, pFBG and pMBG, display a factor 1.5 and 1.3, respectively, superior overall initiation efficiency compared to the unsubstituted pHBG. In contrast, the derivatives pCBG and pNBG carrying electron-withdrawing groups display only weak initiation behavior at a factor 4 higher total energy of ∼112 J (∼28 J for typical PLP experiments with pMBG, pFBG, and pHBG at ∼320 J and 90 000 pulses). The differences in the initiation efficiencies are representative for two classes of monoacyltrimethylgermane initiators, i.e., efficient initiators and weak initiators, each distinct in their specific radical cleavage mechanism. The efficient initiators pMBG, pFBG, and pHBG show an ultrafast intersystem crossing within 2−4 ps after pulse irradiation and subsequent formation of benzoyl and trimethylgermyl radical fragments. In contrast, the weak initiators pCBG and pNBG relax to the ground state after photoexcitation via a dominating ultrafast internal conversion (IC) within 13 and 2 ps, respectively, disallowing effective initiation under typical PLP conditions (∼320 J/pulse with 90 000 pulses resulting in ∼28 J total energy per sample). pCBG features weak initiation behavior additionally forming methyl and p-cyanobenzoyldimethylgermyl radicals at a factor 4 higher total energy of ∼112 J. Consistent with a considerably faster IC relaxation, pNBG features a factor 10 weaker monomer conversion than pCBG.

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germanes.7 In analogy to acylphosphin-based initiators8−10 acyltrimethylgermane and bisacyldiethylgermane photoinitiators were developed with a significant red-shift and an efficient initiation behavior by Liska and co-workers.11 Further investigations have demonstrated that triplet states are crucial for a subsequent α-cleavage of mono- and bisacylgermane initiators.12,13 The most recent development in the field of

INTRODUCTION Light-induced free-radical polymerization and especially the development of visible light initiators are of critical importance for advanced curing technologies including coatings,1,2 dental fillings,3,4 and tissue engineering.5 While several classes of visible light initiators have been identified, for example oxime esters, the photochemistry of which has most recently been investigated,6 germanium-based systems are highly promising visible light initiation systems. Germanium-based initiators were initially reported by Mochida and colleagues in the 1980s, who conducted photolysis studies with hetero-substituted hydro© XXXX American Chemical Society

Received: August 8, 2017 Revised: October 10, 2017

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DOI: 10.1021/acs.macromol.7b01721 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules germanium-based initiators is a synthetic route for tetraacylgermanes, showingbased on a tetrasubstitution patternan enhanced per-mole initiation efficiency.14 Clearly, for the further improvement of germanium-based visible light photoinitiators it is mandatory to vary the substituents at the acyl moiety becauseas expected by the Hammett relation15the choice of substituents critically influences the reactivity and the bathochromic shift of the absorbance maxima. In general, the cleavage mechanism of a photoinitiator occurs via Norrish type I (α-cleavage) or Norrish type II (hydrogen abstraction) reactions from an excited triplet state. Typical type I initiators are acetophenones,16,17 hydroxyalkylphenones,18,19 acylphosphines,8−10 benzoins,20−22 oras already notedmonoacylgermanes.12,13 From earlier studies with benzoin-like photoinitiators20−22 it is known that an efficient intersystem crossing (ISC) with a subsequent α-cleavage mainly occurs by exciting transitions with nπ* character in the S1 state, which is the energetically lowest absorption band with a small extinction coefficient in the ground-state UV/vis spectrum. The excitation of higher absorption bands with strong extinction coefficients due to ππ* character leads to population of higher states with a high channel branching. Thus, a manifold of photophysical processesradiative, such as luminescence, and nonradiative, such as internal conversion (IC) or ISCare competing without reaching the correct triplet state for an efficient radical formation. In addition, an ab initio prediction of the most effective initiation wavelength is challenging, and the groundstate UV/vis spectrum does mostly not serve as a guide to predict initiation behavior.6 In a previous theoretical assessment by our team, five monoacylgermanes (Scheme 1) with variable substituents were

Scheme 2. Predicted Cleavage Mechanism for Benzoyltrimethylgermanes with Electron-Donating Substituents Methoxy (M), Fluorine (F), and Unsubstituted (H)

ionization and mass spectrometry (PLP-ESI-MS) and femtosecond (fs)-spectroscopy,20,21 as illustrated in Scheme 3. On the basis of PLP-ESI-MS studies, it is possible to clarify the cleavage mechanism of photoinitiators by end-group analysis via simulations of the isotopic patterns of the recorded polymer species. In addition, PLP-ESI-MS studies allow in socalled cocktail experiments to quantify the overall initiation efficiency of benzoyl radicals, including all photochemical events from excitation to radical formation until final initiation, as a function of their origin. As already noted in earlier studies,20,21 the behavior of an initiator is critically determined within the first picosecond after excitation. This can be rationalized by inspecting the lower right part of Scheme 3. Efficient photoinitiators require efficient population transfer from excited singlet to triplet states. For example, if ultrafast IC competes, the relevant ISC processes have to be ultrafast as well. Thus, femtosecond (fs)-pump−probe experiments provide in-depth insight into these early photophysical processes, which are crucial for a successful cleavage of an initiator by characterizing the transient behavior of the relevant excited states. Thus, the combination of fs-spectroscopy and PLP-ESI-MS studies fused with earlier theoretical investigations allows for the in-depth mechanistic assessment of the influence of substituent patterns on the behavior of monoacylgermanebased visible light photoinitiators. Critically, the understanding of structure−reactivity correlations enables the informed design of next-generation visible light driven germanium-based initiation systems.

Scheme 1. Monoacylgermanes pMBG, pFBG, pHBG, pNBG, and pCBG Investigated in the Current Study

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EXPERIMENTAL SECTION

Materials. Monoacylgermanes were provided by Ivoclar Vivadent (for experimental details refer to the Supporting Information). Methyl methacrylate (MMA, Sigma-Aldrich, 99%, stabilized) was freed from inhibitor by passing through a column of activated basic alumina (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 ESI-MS measurements were applied as received. MeOH (Roth, 99.9%) for fs-experiments was used as received. PLP-Experiments. All samples were prepared with a concentration of 5 mmol L−1 in MMA (sample volume ∼0.5 mL) and freed from oxygen by purging with nitrogen for 5 min. Subsequently, the samples were individually placed into the sample holder and irradiated at ambient temperature. The laser-induced radical polymerization was carried out with a tunable laser system (Tunable Laser System SpitLight EVO 200 OPO, Innolas, 270−670 nm, repetition rate 100 Hz) at typical PLP energies,20 ∼320 μJ/pulse with 90 000 pulses and wavelengths between 400 and 425 nm, resulting in total energies of ∼28 J. A more detailed description of the sample holder and the tunable laser system is presented in one of our earlier studies.6 Direct Infusion ESI-MS Measurements. Mass spectra were recorded on a Q Exactive (Orbitrap) mass spectrometer (Thermo

investigated by (time-dependent) density functional theory (TD-DFT) methods, providing a detailed understanding of the processes leading from photoexcitation to radical formation under the influence of electron-donating substituents such as methoxy or fluorine and electron-withdrawing substituents such as cyano or nitro.23 The theoretical investigation confirms the expectation of a high substitution pattern dependence on the cleavage probability for an α-cleavage visualized in Scheme 2; i.e., electron-donating substituents such as methoxy and fluorine improve, while electron-withdrawing groups such as cyano and nitro reduce, the cleavage efficiency. To establish an experimentally based mechanism, which can be critically compared to the theoretical assessment,23 a systematic approach is mandatory based on a combination of pulsed-laser polymerization with subsequent electrospray B


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Scheme 3. Schematic Overview of Applied Methods for Investigating Five Acyltrimethylgermanes: PLP-ESI-MS Studies To Determine Initiation Efficiencies in Cocktail Experiments and To Clarify the Formation of Radical Species Fused with Femtosecond Absorption Spectroscopy To Reach an In-Depth Understanding of the Underlying Photophysical Processes

Fisher Scientific, San Jose, CA) equipped with an HESI II probe. The calibration of the instrument was carried out in the m/z range of 74− 1822 by using premixed calibration solutions (Thermo Scientific). A constant spray voltage of 3.6 kV, a dimensionless sheath gas of 5, and a dimensionless sweep gas flow rate of 1 were used. Furthermore, a capillary temperature of 320 °C was chosen, and the S-lens RF level was set to 68.0. All direct infusion samples were prepared with a 3:2 mixture of THF and MeOH doped with NaTFA at a polymer concentration of 2 mg mL−1 measured by using a direct infusion syring pump at a flow velocity of 5 μL min−1. The high polymer concentration of 2 mg mL−1 is chosen to suppress the generation of multiple charged species. Consequently, the mass spectrometer is cleaned carefully after each experiment. SEC-ESI-MS Measurements. For recording mass spectra with hyphenated size exclusion chromatography, the above-described mass spectrometeric setup was 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). Two mixed bed size exclusion chromatography columns (Polymer Laboratories, Mesopore 250 × 5.6 mm, particle diameter 3 μm) were used for separation with a precolumn (Mesopore 50 × 4.6 mm) at operating temperature of 30 °C. THF was used for elution with a flow rate of 0.30 mL min−1. 0.27 mL min−1 of the eluent is conducted through a RI detector (RefractoMax520, ERC, Japan), and 0.03 mL min−1 was infused to the ESI-MS system after postcolumn addition of a 0.1 mM solution of sodium iodide in MeOH at 0.02 mL min−1 by a microflow HPLC syringe pump (Teledyne ISCO, Model 100DM). Instead of the described ESI conditions above, a constant spray voltage of 4.6 kV, a dimensionless sheath gas of 8, and a dimensionless sweep gas flow rate of 2 were used. The S-lens RF level were set to 62.0, and the capillary temperature to 320 °C. The samples were prepared with a polymer concentration of 2 mg mL−1, whereby an aliquot of 0.1 mL was used for injection. Steady-State Spectroscopy. UV−vis spectra were recorded with a Cary 500 (Varian) spectrometer in the wavelength range of 200−800 nm in fused silica cuvettes (Hellma) with an optical path length of 1 mm and MeOH as solvent. The measured spectra were used for sample preparation for fs-spectroscopy. Pump−Probe Femtosecond Spectroscopy. Pump−probe experiments were carried out with a femtosecond laser system

(Astrella, Coherent), which generates ultrashort laser pulses with a repetition of 1 kHz, pulse duration of 34 fs, and ∼7 mJ pulse energy at 800 nm. A fraction of the laser output is used to operate an in-housebuilt pump−probe experiment including a frequency-doubling unit (BBO-crystal) as pump possibility and a CaF2 white light continuum generation unit for probing between 350 and 700 nm. In the current study pump pulses were generated with the frequency-doubling unit (20−25% efficiency) at 400 nm with 0.4 μJ pulse energy (chopped to 500 Hz) and were focused together with the white light continuum for probing to the location of the sample. The focus diameter of the pump pulses (0.06 mm) was larger than the focus diameter of the probe pulses and optimized for temporal and spatial overlap in 1 mm fused silica cuvettes (Hellma) under magic angle (54.7°) conditions by using a λ/2-plate (tunable, Spindler and Hoyer) to exclude anisotropic effects. Transient absorption (TA) spectra were recorded at variable delay times between probe and pump pulse by detecting the probe pulses with a CCD camera (Linescan 2000, Ingenieurbüro Stresing), after spectral decomposition with a prism polychromator. Delay times between pump and probe pulses were accomplished by delaying the pump pulse relative to the probe pulse (monitoring the intensity of the probe pulse with pump pulse on: Iw) with a computer-controlled translation stage (ThorLabs). For probe-only spectra at each delay step the pump pulse was blocked with the chopper, and the probe pulse intensity (Iw/o) was recorded to finally obtain time-delayed ΔA (log(Iw/o/Iw)) values via in-house programming with the program package LabView 2012. Continuous stirring of the sample solutions with a cylindrical micro stir bar (length × radius: 3 mm × 0.1 mm) avoided local heating and ensured the replenishment of the sample at the focus. All substances for the time-resolved fs-spectroscopy sample preparation were dissolved in MeOH and concentrated to reach an optical density at 400 nm of ∼0.4 for pCBG and pNBG and ∼1.4 for pMBG, pFBG, and pHBG (cf. absorption spectra in Figures S1−S5).

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RESULTS AND DISCUSSION PLP-ESI-MS Investigations. Initially, five monoacylgermane derivatives (Scheme 1) were investigated with PLP-ESIMS to assess their photodissociation behavior andimportantlytheir ability to initiate macromolecular growth based on the individually generated radical fragments. On the basis of previous studies using benzoin-like photoinitiators20−22 and of C


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Scheme 4. Pathway of Radical Formation and Subsequent Macromolecular Species Generated by Disproportionation Events Based para-Methoxyl (R = OMe), para-Fluoro (R = F), or Unsubstituted (R = H) Benzoyl and Trimethylgermyl Radical Fragments Using MMA as Monomer

theoretical investigations23 of the monoacylgermanes, it is expected that the initiation efficiency is dependent on the substituent at the phenyl moiety. To quantify the influence of each substituent, PLP cocktail experiments were carried out at typical PLP conditions at the S1 absorption maxima of each initiator (∼320 μJ/pulse with 90 000 pulses resulting in ∼28 J total energy per sample) using pMBG (397 nm, cf. Figure S1) and pFBG (405 nm, cf. Figure S2), employing the unsubstituted pHBG (409 nm, cf. Figure S3) as reference system. The initiation ability of pNBG and pCBG was probed at high total energy conditions with a pulse-energy-dependent conversion experiment, whereby the samples are irradiated at the constant total energy of 112 J (factor of 4 higher than typical conditions) by varying the number of pulses correlated to different chosen pulse energies per sample. To obtain a deeper understanding of the initiation behavior of each initiator, detailed cleavage mechanisms are proposed on the basis of simulations of the mass spectra of the photogenerated macromolecules. Subsequently, a comprehensive assessment of the photophysical behavior of each photoinitiator by femtosecond pump−probe experiments is carried out. Accessing Initiation Efficiencies via PLP Cocktail Experiments. The overall initiation efficiency of the methoxy (pMBG), fluoro (pFBG), and unsubstituted monoacyltrimethylgermane derivative (pHBG) (Scheme 1) was quantified via PLP-ESI-MS cocktail experiments based on a method introduced earlier by our team.16,20,22 Here, two initiators are mixed in a range of predefined ratios, i.e., from ∼0.2 to ∼6 (Table S1 for more details) with MMA in bulk. Photoinitiation was achieved by irradiating the samples with a monochromatic tunable laser system at wavelengths near to each absorption maximum of the S1 states of pMBG (397 nm, Figure S1), pFBG (405 nm, Figure S2), and pHBG (409 nm, Figure S3). In general, it is to be expected that differences in the overall initiation efficiency exist due to wavelength dependence of the extinction coefficients. To assess the wavelength dependency of the initiation process, PLP-ESI-MS cocktail experiments were carried out at variable wavelengths guided by the S1 absorption maxima of each photoinitiator in one mixture. The transition in S1 is a weak absorption band with a nπ* character, from which an efficient ISC with a subsequent αcleavage can be expected.20,23 After the photoinduced polymerization, the generated poly(methyl methacrylate)s (PMMA) are assessed via high resolution direct infusion Orbitrap ESI-MS to obtain a product distribution mass spectrum of each initiator cocktail sample, containingto a large fractiondisproportionation and recombination termination products. Solely on the basis of recombination products, it is impossible to differentiate between the modes of end-group incorporation, i.e., either via a termination or initiation event. However, disproportionation products can be exploited for the determination of relative

initiation efficiencies (refer to Scheme 4), as the initiating step occurs via a radical fragment of one of both initiators. At a given chain length i, disproportionation peaks appearing in a repeat unit with a mass difference of 2 amu in the mass spectrum are labeled with X= or XH. Here, X denotes the specific initiating fragment (MB, FB, and HB), while = and H imply the nature of the ω-end of the polymer, generated by H-abstraction during the disproportionation process. To determine the initiation efficiencies via the mass spectrometric assessment of each radical fragment X, the first peak of the disproportionation pattern X= is employed. The relative peak height, hiX=, of the disproportionation product X= is a measure of the number of initiation events by the radical fragment X yet does not provide an absolute measure without calibration. By defining the ratio GiX= of the relative peak heights hiX= of two disproportionation products initiated by radical fragments originating from each initiator in a cocktail, an expression for the relative number of initiation events is obtained and the calibration problem is circumvented. In general, the ionization probability of the polymer is a function of chain length, leading to a potential mass bias during the ESI process. The mass bias can be readily visualized by plotting the mole fraction FiX= (definition in Scheme S2) of a specific disproportionation product vs the degree of polymerization, DPn. Further, it is assumed that the ionization of the polymer is predominantly carried by the backbone, thus giving equal ionization probabilities for chains with disparate end groups. In such a situation, small slopes indicate a mass bias-free ESI process (refer, for example, to the right part of Figures 1 and 2). Nevertheless, to reduce the influence of an ionization induced mass bias, a specific averaging method is applied, introduced in previous studies,20,22 to obtain the mass bias-free ratio Gm/z,0X= (definition in Scheme S2), which correlates the ratio GiX= of each repeating pattern i with the ratios Gi−1X= and Gi+1X= of its adjacent repeating patterns i − 1 and i + 1, respectively. The mass biasfree ratio Gm/z,0X= evaluated from one mass spectrum is assigned to the associated initial molar ratio of cocktail mixture; thus, all Gm/z,0X= can be plotted vs the initial molar ratios in on one graph. In an ideal experiment (without chain-length-dependent ionization processes and isobaric overlaps), a linear dependence with zero intercept is expected; therefore, the plot is analyzed via linear regression analysis. The number of disproportionation products obtained from polymerization by both photoinitiators are equal at Gm/z,0X= = 1. Thus, the overall initiation ef f iciency of two initiating fragments originating from disparate photoinitiator molecules can be defined as the reciprocal of the molar ratio at Gm/z,0X= = 1. The overall initiation efficiency is a powerful parameter for the post-mortem quantification of the influence of a substituent on the initiation process of a radical, including all photochemical events occurring since excitation of the primary initiator. D


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Figure 1. (a) Evaluation of overall initiation efficiency of pFBG (1.5 ± 0.3) vs pHBG by linear regression of all Gm/z,0FB= ratio pairs reproduced twice at 409 and 405 nm in one joint plot. (b) Example plot of the mole fraction FFB= vs the degree of polymerization DPn of pFB= at 405 nm.

Figure 2. (a) Evaluation of overall initiation efficiency of pMBG (1.30.3) vs pHBG by linear regression of all Gm/z,0MB= ratio pairs reproduced at 397, 400, and 409 nm in one joint plot. (b) Example plot of the mole fraction FMB= vs the degree of polymerization DPn of pMB= at 400 nm.

In the current study, the time-consuming evaluation methods for the determination of the mass bias-free ratio Gm/z,0X= were fully automated with the aid of an in-house developed Matlab script (detailed description in Scheme S2). The algorithm allows for the identification of every disproportionation peak within all repeat units of each photoinitiating fragment and calculates all required mean values for the determination of Gm/z,0X= = 1. By virtue of the automated approach, higher throughput is achieved resulting in better signal-to-noise ratios by accumulating and analyzing more data sets per time unit. In the following, two multiple reproduced cocktail experiments are presented: first pFBG vs pHBG (I) and subsequently pMBG vs pHBG (II). For evaluation, the disproportionation products HB = , MB=, and FB= initiated by the corresponding benzoyl radicals (refer to Scheme 4) were employed for evaluation of Gm/z,0X= as described above. For evaluation of the overall initiation efficiency at Gm/z,0X= = 1, the mass bias free ratios of all in this study reproduced experiments are jointly plotted within one graph for regression analysis. Gaussian error

propagation based on the fit error is applied to obtain the error in the overall initiation efficiency. PLP-ESI-MS Cocktail Experiment of pFBG vs pHBG. The cocktail initiation experiments with pFBG vs pHBG in MMA were carried out at 405 nm (peak of S1 band of pFBG, cf. absorption spectrum Figure S2) and 409 nm (peak of S1 band of pHBG; cf. absorption spectrum Figure S3). The experiment at 409 nm was reproduced twice with the same set of samples, while the experiment at 405 nm was reproduced with two independent sets of samples. Mass spectra and corresponding isotopic pattern simulations of pFBG and pHBG are presented in Figures S12 and S13 and the corresponding Tables S5 and S6. No significant wavelength dependence is observed between the different data sets of mass bias-free ratios Gm/z,0FB= (cf. Table S1). Thus, to determine an averaged overall initiation efficiency and its statistical error for the benzoyl radicals of pFBG vs pHBG, all Gm/z,0FB= (Table S1) of the four data sets including two different excitation wavelengths were combined in one plot and were analyzed by linear regression (refer to Figure 1a). E


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Macromolecules Plotting Gm/z,0FB= vs the initial molar ratio shows a linear behavior as expected. From linear regression results a 1.5 ± 0.3 higher overall initiation efficiency of the fluorobenzoyl radical derived from pFBG compared to the benzoyl radical of pHBG. The experiment is reproducible, and influences on the overall initiation efficiency dependent on the excitation wavelength between 405 and 409 nm are negligible. The estimated statistical error of up to 0.3 reflects scattering of the experiment due to weighing uncertainties, intrinsic mass overlaps in mass spectra, or chain length dependent ionization bias of the ESI process. Chain-length-dependent ionization bias during the ESI process for one experiment is depicted in Figure 1b, for example (for all plots in Figures S6 and S7). It shows a nearly linear increasing mole fraction FFB= with increasing degree of polymerization DPn. A linear ionization bias is a controllable problem due to the applied averaging method, which correlates one repeating i unit with its adjacent repeating units i − 1 and i + 1, as described above. Nevertheless, an error caused by an intrinsic mass near isobarism of the disproportionation products FB= and Ge=/GeH is not negligible (refer to Figure S8), which is also reflected in the nonzero y-intercept of the linear regression in Figure 1a. PLP-ESI-MS Cocktail Experiment of pMBG vs pHBG. The cocktail initiation experiments with pMBG vs pHBG in MMA were carried out at 397 nm (peak of S1 band of pMBG; cf. absorption spectrum Figure S1) and at 409 nm (peak of S1 band of pHBG; cf. absorption spectrum in Figure S3). An additional experiment was performed at 400 nm, which is close to both absorption maxima of each initiator. The mass spectrum and corresponding isotopic pattern simulation of pMBG are presented in Figure 4 and the corresponding Table 1. In line with the experiment pFBG vs pHBG, no significant

bias can be virtually excluded, and no mass bias due to superposition with other species is observed. Thus, the overall initiation efficiency of the investigated benzoyl radicals is quantitatively determined, showing that benzoyl radicals originating from monoacyltrimethylgermane derivatives with para-substituted electron-donating groups clearly display higher overall initiation efficiency than the radicals from unsubstituted pHBG, which is in line with theory.23 Conversion Studies: The Weak Initiators pCBG and pNBG. pCBG and pNBG showat the typical PLP conditions (∼320 μJ/pulse with 90 000 pulses resulting in ∼28 J total energy per sample)no photoinitiating ability at 425 and 429 nm. However, by irradiating at a factor 4 higher total energy per sample (112 J) at 425 nm for pCBG (peak of S1 band; cf. absorption spectrum Figure S4) and at 429 nm for pNBG (peak of S1 band; cf. absorption spectrum in Figure S5) in MMA, a small amount of monomer conversion is observed. The conversion determination was reproduced seven times by gravimetry at the constant total energy of 112 J, yet at different pulse energies, which is readily achieved by varying the number of pulses correlated to the chosen pulse energy (Table S3). The corresponding results are depicted in Figure 3. pCBG shows a conversion of close to 1%, while pNBG only leads to

Table 1. Atomic Mass and Resolution of the Simulated and Measured PMMA Species Initiated by pMBG Depicted in Figure 4b for Evaluation of the Quality of Isotopic Pattern Simulation in Figure 4a species

exact mass (simulated)

resolution at exact mass (simulated)

exact mass (measured)

R at exact mass (measured)

Ge= GeH GeGe MB= MBH MBGe MBMB

1337.61 1339.61 1357.55 1357.66 1359.67 1373.61 1393.66

59947 40114 57444 59935 55242 59943 59934

1334.61 1339.61 1357.56 1357.66 1359.67 1373.61 1393.66

60118 40577 59089 56480 56946 58603 56539

Figure 3. Pulse-energy-dependent conversion studies with pCBG (black) at 425 nm and pNBG (red) at 429 nm in MMA with c = 5 mmol L−1 at the constant total energy = 112 J (factor 4 higher than for pMBG, pFBG, and pHBG).

wavelength dependence of the mass bias-free ratio Gm/z,0MB= (cf. Table S2) is observed. Thus, all Gm/z,0MB= (Table S2) of the five data sets at three different wavelengths were placed in a joint plot for linear fitting to determine an averaged overall initiation efficiency and its statistical error for the benzoyl radicals of pMBG vs pHBG (refer to Figure 2a). In analogy to the cocktail experiment of pFBG, the data show a linear behavior and linear regression results in a 1.3 ± 0.3 higher overall initiation efficiency of the methoxybenzoyl radical originating from pMBG compared to the benzoyl radical originating from pHBG. Within the experimental accuracy, no excitation wavelength dependence of the overall initiation efficiency is observed between 397 and 409 nm. According to the small slopes in example Figure 2b (for all plots Figures S9−S11), a chain-length-dependent ionization

approximately 0.1%. A pulse-energy-dependent behavior is not observed within the measured range. An initiator such as pMBG hasunder typical PLP conditionsa conversion of close to 4−5%. Thus, it can be estimated that an initiator like pMBG has a 16−20% higher initiation efficiency than pCBG. In comparison, the initiation efficiency of pNBG is approximately a factor 10 poorer than pCBG. Nevertheless, even acylgermane derivatives, which were expected to be noninitiating systems, have a small ability to initiate a polymerization under highenergy PLP conditions. In conclusion, pMBG, pFBG, and pHBG can be viewed as “efficient initiators”, while pCBG and pNBG are “weak initiators”. PLP-ESI-MS Derived Initiator Cleavage Mechanism. The difference between so-called “efficient initiators” and “weak initiators”as defined abovecan be illustrated by a detailed F


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Figure 4. (a) Measured mass spectrum (black) of PLP products of MMA initiated by pMBG at 400 nm recorded by direct infusion ESI-MS and simulated (red) with disproportionation and combination species shown in (b). The associated cleavage mechanism of pMBG is depicted in (c).

Cleavage Mechanism of the Weak Initiators pCBG and pNBG. For clarification of the initiating radical fragments originating from the weak initiators pCBG and pNBG isotopic pattern simulation were carried out. In order to compare simulation and experimental data of the polymer initiated by pCBG, a size exclusion chromatography (SEC)-ESI-MS spectrum was recorded of a PMMA sample pulsed at 425 nm with a pulse energy close to 2.7 mJ and 40 900 pulses, corresponding to a total energy of 112 J (Figure 5a). Using SEC allows the separation of multiple charged polymer species from single charged which simplifies the evaluation of the mass spectra due to the intrinsic high isotopic pattern diversity of pCBG and pNBG as depicted in Figure 5. The isotopic pattern simulation of pCBG was performed with analogous simulation parameters as above, assuming a mass resolving power R of 60 000 (Gaussian-shaped peak profile), and thus, the quality of the simulation can be evaluated in Table 2. The spectrum of pCBG shows nearly twice as many peak patterns than the spectrum of pMBG. Thus, we postulate a second cleavage pathway A in addition to the expected αcleavage B (refer to Figure 5b), which includes the formation of a methyl (Me) and a cyanobenzoyldimethylgermyl (CBG) radical. Fifteen species are required to simulate the spectrum in a representative fashion. The methyl radical is distinctly verifiable because the related disproportionation products are main species of the simulation without any isobaric overlap. The verification of cyanobenzoyldimethylgermyl radical-initiated species is more complicated, as isobaric structures exist with identical molecular formulas, leading to identical peak patterns within the isotopic pattern simulation. For instance, to explain the signal at m/z = 1372.59 of CBGMe, it is possible to assume a cyanobenzoyldimethylgermyl and a methyl radical as initiating species. However, assuming a cyanobenzoyl and a trimethylgermane radical leads to an equivalent mass spectral description. When considering the formally possible presence

evaluation of the mass spectra, whose interpretation suggests the existence of two cleavage mechanisms. In the following, the simulations of the mass spectra of the polymerization products obtained via pMBG and pCBG initiation are discussed to propose cleavage mechanisms for both efficient and weak initiators, respectively. pNBG is discussed separately due to its divergent behavior. Cleavage Mechanism of Efficient Initiators: The Example of pMBG. To study the cleavage mechanism of pMBG, a PLP experiment at 400 nm with 90 000 pulses and an energy per pulse of close to 0.31 mJ (corresponds to a total energy of 27.9 J) was carried out. The recorded ESI-MS spectrum is depicted in Figure 4a in black, while the simulated spectrum is depicted in red. The software Xcalibur (Thermo Fischer) was used to simulate the ESI-MS spectrum, with the relative amount x of single charged species shown in Figure 4b. The simulations were conducted by assuming a mass resolving power R of 60 000 (Gaussian-shaped peak profile). The quality of the simulation can be evaluated by comparing the absolute masses and the mass resolving power R of the measured and the simulated peaks listed in Table 1. Seven main species were observed, whereby the dominant peaks are−as expected−associated with disproportionation products initiated by methoxybenzoyl and trimethylgermyl radicals (MB=/MBH, Ge=/GeH, Figure 4). The other signals are associated with combination products derived from both initiator fragments (MBGe, GeGe, and MBMB, Figure 4). Note that Ge radical initiated or terminated PMMA products show distinctive broad isotopic peak patterns due to the isotopic diversity of germane. pFBG and pHBG exhibit the same cleavage mechanism based on an inspection of the corresponding ESI-MS spectra shown in Figures S12 and S13. Hence, the typical α-cleavage mechanism for monoacylgermane initiators is verified, already known from earlier studies investigating monoacylgermanes.11,12 G


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Macromolecules

Figure 5. (a) Measured mass spectrum (black) of PLP products of MMA initiated by pCBG at 425 nm recorded by SEC-ESI-MS and simulated (red) with disproportionation and combination species shown in (c). The associated cleavage mechanism of pCBG is depicted in (b).

of dimethylgermyl diradicals (A2 and B2, Figure 5b), the complexity of the assignment situation increases further and leads to large number of isobaric structures. Scheme 5 illustrates how every sum formula of combination products consisting of benzoyldimethylgermyl radicals (CBGMe) is also described as a product of formal dimethylgermyl diradicals (CBGeMe). In addition, the specific combination product consisting of a methyl and benzoyldimethylgermyl end-group (CBGMe) is also isobaric with a combination product of trimethylgermyl and cyanobenzoyl radicals (CBGe). Thus, one has to expand the species depicted in Figure 5c by assumingin addition to all benzoyldimethylgermyl combination productsan isobaric dimethylgermyl diradical structure to obtain a scenario with diradicals. A simulation with diradicals implies two possible mechanistic pathways for the diradical formation (A2 and B2 in Figure 5b). Pathway A2 implies a subsequent cleavage of the cyanobenzoyldimethylgermyl radical, while pathway B2 describes the decomposition of the trimethylgermyl radical. The activation of subsequent cleavage pathways can be induced thermally or via photons. A light-induced cleavage of the trimethylgermyl radical (B2) seems to be unlikely because of missing chromophores, while a photoinduced cleavage of the

cyanobenzoyldimethylgermyl radical is more likely due to the π-system of the cyanobenzoyl moiety. Nevertheless, a photoinduced subsequent cleavage of a radical in a PLP experiment would imply an unreactive behavior of the primary radical toward MMA in the dark time of 10 ms between two pulses, which appears unlikely. A thermal cleavage is possible if sufficient excess energy remaining from the first cleavage to generate a vibrationally hot excited state exists, which may lead to fragmentation (vide infra, discussion of femtosecond results). In general, a polymer growth mechanism featuring diradicals is more complex in terms of diffusion-controlled termination steps. For example, if one wishes to describe the disproportionation products initiated by cyanobenzoylgermyl radicals CBGH and CBG=, which is a main product in the recorded mass spectrum, with a diradical scenario, the initiating fragment would be the dimethylgermyl diradical and the polymer would grow in two directions. After chain propagation, one end is terminated by disproportionation and the other by a cyanobenzoyl radical. This particular situation seems to be unlikely in contrast to a simple initiation by a cyanobenzoyldimethylgermyl radical with termination by disproportionation due to CBGH and CBG= as main species recorded in the H


Article

Macromolecules

polymerization initiated by pNBG leads to unexpected fragmentation patterns. Pump−Probe Femtosecond Spectroscopy. In the following the TA spectra of the five photoinitiators shown in Scheme 1 were recorded using MeOH as solvent. All samples were excited at 400 nm and probed with a CaF2 white light continuum of 350−700 nm. For the evaluation of the TA a simple model of the photophysics of each initiator is developed. Transient Absorption Spectra of the Efficient Initiators pHBG, pMBG, and pFBG. The TA spectra of the efficient initiators pHBG, pMBG, and pFBG dissolved in MeOH carried out in a pump−probe experiment with an excitation wavelength of 400 nm and probed with CaF2 white light continuum. The spectra of the three photoinitiators show similar TA behavior, which allows a joint discussion. In Figure 7, the TA spectrum for pHBG is exemplary depicted, and corresponding TA spectra for pMBG and pFBG are recorded and shown in Figures S15 and S16, respectively. At 400 nm the transient response has been omitted in all TA spectra due to scattering of the pump pulse with a spectral width of 10 nm. The dominating TA of the three efficient initiators is an ultrafast rise of absorption within a few picoseconds in the visible wavelength range around 450 nm, depicted in Figure 6a (for pMBG and pFBG in Figures S15 and S16). Subsequently, the TA slowly decreases within the assessable time window, as indicated in Figure 6b (for pMBG and pFBG in Figures S15 and S16). The TA at 350 nm decays approximately in the same time scale as the band at mainly 450 nm and is related to dynamics of the same state, which is probed into different higher lying states. Consequently, one can assume that the spectra are dominated by only one broad band from around 350 to 500 nm, which is superimposed in the range between 370 and 425 nm with the scattering of the pump pulse and bleaching responses from ground state. Neither a significant spectral shift nor stimulated emission is observed within the ΔA detection limit of 1:104. Thus, fluorescence is a negligible pathway, which is in agreement with weak fluorescence quantum yields in the visible smaller than 5 × 10−4, known from previous study.23 From TD-DFT calculations it is expected that two triplet states (T1/2) are close to or below the first excited singlet state (S1), and a higher lying triplet manifold (Tn) is available for excited state absorption.23 Therefore, the ultrafast increasing TA can be plausibly assigned to an ISC and the slow decay to triplet state relaxation. The assumption of a triplet state is supported by the analysis of TA near to 420 nm, which is the nearest to 400 nm, where ground state bleaching (GSB) is expected, but no

Table 2. Atomic Mass and Resolution of the Simulated and Measured PMMA Species Initiated by pCBG Depicted in Figure 5c for Evaluation of the Quality of Isotopic Pattern Simulation in Figure 5a species Me= MeH Ge= GeH CBG= CB= MeGe MeMe CBGH CBH GeGe CBMe GeCB/ CBGMe CBGGe CBGCBG

exact mass (simulated)

resolution at exact exact mass mass (simulated) (measured)

R at exact mass (measured)

1337.69 1339.70 1341.61 1343.62 1352.56 1352.64 1353.64 1353.72 1354.56 1354.65 1355.56 1368.67 1372.59

59922 55257 40917 48972 59591 59942 56801 59945 41176 49947 42225 59929 56834

1337.69 1339.70 1341.61 1343.62 1352.56 1352.64 1353.64 1353.72 1354.57 1354.66 1355.56 1368.67 1372.59

59424 59074 45925 52762 57920 58934 57730 57479 43921 53479 42933 58836 57858

1374.51 1389.46

56207 56190

1374.51 1389.46

54094 46110

spectrum. By these considerations, a scenario without diradicals seems to be more likely, yet both interpretations are in agreement with the current set of data. A scenario without diradicals is in line with the cleavage of dibenzoyldiethylgermane, which decomposes mainly into benzoyl and benzoyldiethylgermyl radicals without subsequent cleavage.13 The recording of the mass spectra of polymer initiated by pNBG is more complicated because under identical conditions of a total energy of 112 J only a momomer conversion of close to 0.1% is reached, and it was not possible to record a MS spectrum. With a further PLP experiment at 384 J total energy (3.2 mJ/pulse, 120 000 pulses, 429 nm) a conversion of 0.6% is determined, and a SEC-ESI-MS spectrum can be recorded, which is shown in Figure S14. The simulation of the MS spectrum of the polymer initiated by pNBG is complicated as well (simulation in Figure S14). Only a few of the occurring species can be unambiguously assigned. Species initiated by nitrobenzoyl, trimethylgermyl and nitrobenzoyltrimethyl radical fragments can be identified, yet the main species are not identifiable. Nevertheless, a very weak initiator such pNBG has a small probability to initiate polymerization, which is only observable at high total energy. In contrast to pCBG, a

Scheme 5. Example Structure for Identical Molecular Formulas of PMMA Combination Products with Cyanobenzoyldimethylgermyl/Methyl (CBGMe), Dimethylgermyl Diradical/Cyanobenzoyl/Methyl (CBGeMe), and Cyanobenzoyl/Trimethylgermyl (CBGe) End-Groups

I


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Macromolecules

Scheme 6. Jabłoński Diagram of the Efficient Initiators pMBG, pFBG, and pHBGa

a

After excitation into S1, an ultrafast ISC occurs with population of T1 or T2, respectively. Because of probing of the triplet states into a higher lying triplet manifold Tn, the two states are difficult to disentangle (refer to the main text for details).

If competing singlet relaxation pathways are neglected, one can assume that the TA reflects only triplet dynamics, and the ultrafast increase of TA can be analyzed with an exponential fitting routine to estimate ISC time constants. Therefore, TA responses are evaluated in the range of 450−515 nm as shown in Figure S25. As a result, the ISC is partly superimposed by a second process within the first few picoseconds which can be assigned to vibrational relaxation (VR) due to a slight delayed rise from 515 to 450 nm. TA responses between 450 and 460 nm are approximately free from VR and are used for exponential analysis (Figures S25−S27). Finally, the time constants for ISC of pFBG, pMBG, and pHBG are estimated between 2 and 4 ps. Transient Absorption Spectra of the Weak Initiators pCBG and pNBG. The TA spectra of the weak initiators pCBG (Figure 7a) and pNBG (Figure 7b) dissolved in MeOH were recorded after excitation at 400 nm and probed with a CaF2 white light continuum in a femtoseconds pump−probe experiment. pCBG and pNBG are dominated in the UV (360 nm) and in the visible range (pCBG 500 nm, pNBG 670 nm) by an instant TA with a subsequent decay within tens of picoseconds for pCBG and even faster for pNBG in less than 10 ps, respectively. Similar to the discussion above, only one process is observed at both wavelengths, which is superimposed with GSB and scattering of the pump pulse. The weak initiators pCBG and pNBG show a factor 2−4 higher amplitudes (ΔA = (5−8) × 10−3) at lower optical densities (OD400 nm ∼ 0.4) of the ground state as a striking difference to the efficient initiators pMBG, pFBG, and pHBG (ΔA = (1.7−2) × 10−3 at OD400 nm ∼ 1.4). In contrast to efficient initiators, where the TA does not recover to zero within the recorded time window, one can observe complete ground state recovery as shown in Figure S23 at 420 nm. Consequently, the TA of pCBG is assigned to fast

Figure 6. TA spectra of pHBG dissolved in MeOH excited at 400 nm and probed by CaF2 whitelight continuum. Arrows indicate in (a) a rise of absorption within a few piceoseconds. (b) depicts the decay on a longer time scale indicated by arrows. For clarity, the transient response at 400 nm has been omitted due to scattering of the pump pulse.

disturbances of the pump pulse scattering is observed. If singlet relaxation were involved, one should observe a decrease of the TA, but instead no significant change is observed (more details in Figures S20−S22). The processes are schematically depicted in Scheme 6. Because of the triplet manifold, one cannot differentiate whether the transient response originates from T1 or T2 dynamics because for both states there are a significant number of higher lying states accessible for resonant transition at the same probing energy. Thus, the onset of the absorption decrease on the time scale higher 100 ps can be attributed to (i) triplet−triplet relaxation from T2 to T1, (ii) radical formation, and (iii) additional relaxation from quenching processes. In contrast to our earlier benzoin-focused studies,20−22 where the decomposition starts on a tens of picoseconds scale and no remaining absorption is observed, efficient monoacyltrimethylgermyl photoinitiators seem to cleave on a considerably longer time scale which is not completed within the recorded time window. Thus, it is challenging to determine quantitative quantum yields by evaluation of the relevant amplitudes ratios. J


Article

Macromolecules Scheme 7. Jabłoński Diagram for pCBGa

a

After excitation in S1, which is probed into two higher lying singlet manifolds, an ultrafast IC back to ground state occurs.

estimated from the first time constant of a triexponential fitting function, shown in Figure S29. For explanation, it should be taken into consideration that the degrees of freedom of the nitro group can strongly influence the potential energy surface landscape, leading to ultrafast relaxation pathways as known for example from previous studies on o-nitrophenol.24 Thereby, the nitro group of the o-nitrophenol undergoes an out-of-plane rotation, eventually leading to a conical intersection as an ultrafast relaxation pathway back to the ground state. Transferring this scenario to pNBG, ultrafast IC presumably via conical intersection is highly preferred. Second, pNBG shows a small band peaking at 550 nm with a rise of approximately 10 ps and a subsequent longer-lasting decay (Figure S29)close to the detection limit. The corresponding amplitudes are at least 1 order of magnitude smaller than the one for the IC, which identifies these processes as only weak side pathways. From theory it is known that with respect to pCBG the singlet states are lowered for pNBG, and additionally, there is a manifold of six triplet states below the S2 state compared to one for pCBG.23 After excitation at 400 nm, pNBG resides in a vibrational hot S1 state close to S2 (cf. absorption spectrum of pNBG in Figure S5), and a small probability for ISC might be considered. But even under this assumption, pNBG will not act as an efficient photoinitiator due to a comparatively high energy barrier in the triplet state.23 This may lead to a reversed ISC (RISC) or collision-induced triplet−singlet (T1/S0) relaxation on a much longer time scale as possible relaxation pathways, whichunfortunatelyboth cannot be resolved with the current data set. The relevant energy diagram (without RISC) for pNBG is sketched in Scheme 8. In summary, the analysis of the transient spectra of the efficient initiators pMBG, pFBG, and pHBG resulted in an ultrafast ISC as crucial step, which is necessary for a successful radical formation. Under these considerations it is shown that

Figure 7. TA spectra of (a) pCBG and (b) pNBG dissolved in MeOH excited at 400 nm and probed by a CaF2 white light continuum. pCBG displays a fast relaxation of the TA in tens of picoseconds at 490 nm, while the TA of pNBG at 670 nm decreasing faster in less than 10 ps. For clarity, the transient response at 400 nm has been omitted due to scattering of the pump pulse.

singlet IC back to the ground state with a time constant of 13 ps as obtained from exponential fitting (Figure S28). In this scenario, ISC is not necessary to be introduced. In comparison to the even faster ISC of the efficient initiators within 2−4 ps, one would have expected that ISC to be the preferred process; however, no significant long-lasting dynamics is observed that could be attributed to triplet state contributions. Obviously, the substituents have a significant influence on the shape and location of the potential surfaces. By red-shifting the S1 state, the energy gap between S0 and S1 becomes small, which leads to a much higher probability for IC rather than ISC.23 This qualitatively explains why pCBG is a weak photoinitiator, and the resulting polymer conversion at high energy is maybe based on singlet decomposition (refer to PLP-ESI-MS section). The relevant energy diagram for pCBG is sketched in Scheme 7. At first sight, pNBG follows an analogous relaxation pathway, referred to recovery of the ground state as shown in Figure S24 at 430 nm. Differences occur in the singlet lifetime and due to small triplet contributions: First, the IC of the excited singlet state decays within 2 ps, in contrast to 13 ps for pCBG as K


Macromolecules

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Scheme 8. Jabłoński Diagram for pNBGa

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01721. Materials, methods, and synthesis route for monoacylgermanes, description of Matlab script, steady-state absorption spectra, input data for linear regression and F vs DP plots for PLP-ESI-MS-cocktail experiments, data for pulse-energy-dependent study, isotopic pattern simulations of pFBG, pHBG, and pNBG, transient spectra of all investigated systems including GSB, and exponential analysis of ISC and IC time constants (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(A.-N.U.) E-mail: [email protected]. *(C.B.-K.) E-mail: [email protected], [email protected]. ORCID

Christopher Barner-Kowollik: 0000-0002-6745-0570 Notes

a

After excitation in vibrational hot S1, which is probed in two higher lying singlet manifolds, an ultrafast IC back to ground state occurs, and in addition a very weak ISC is observed.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.B.-K. and A.-N.U. acknowledge funding from Ivoclar Vivadent for the current project as well as the excellent collaboration. C.B.-K. acknowledges additional funding by the Australian Research Council (ARC) in the context of a Laureate Fellowship as well as the Queensland University of Technology (QUT). The Karlsruhe Institute of Technology (KIT) is acknowledged for continued funding in the context of (i) the Helmholtz association program Science and Technology in Nanosystems (STN) and (ii) the GRK 2039 “Molecular Architectures for Fluorescent Cell Imaging” jointly with the state of Baden-Württemberg and the Deutsche Forschungsgemeinschaft (DFG).

the distinction between efficient initiators and weak initiators for radical polymerization is decided in the first picoseconds after excitation via the energy gaps between the relevant excited singlet and triplet states.

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CONCLUSIONS In the current study an in-depth mechanistic clarification for five monoacylgermane-based visible photoinitiators from the earliest step of light absorption to the final macromolecular growth events is carried out. Specifically, the influence of different substitution patterns on the overall initiation efficiency of the benzoyl moiety in MMA polymerization was quantified by PLP-ESI-MS cocktail experiments on the basis of the generated benzoyl fragment in the descending order of pFBG > pMBG > pHBG. Critically, pCBG and pNBG show at comparable PLP conditions no polymer conversion, yet at factor 4 higher total laser energies a small conversion to polymer was observed. Thus, a distinction between efficient and weak initiators is established to classify the five photoinitiators. Detailed isotopic pattern simulations of PLP-ESI-MS spectra unambiguously indicate that the efficient initiators follow an expected α-cleavage; for the weak initiator pCBG methyl and benzoyl dimethylgermyl radicals could additionally be identified, while pNBG shows an unexpected behavior. Femtosecond absorption spectroscopic experiments of efficient initiators reveal a fast ISC within 2−4 ps into low triplet states, which is critical for the expected α-cleavage, while weak initiators have only a negligible probability to reach the triplet states due to a fast IC in 2 ps for pNBG and 13 ps for pCBG leading to complete recovery of the ground state. Thus, we submit that the distinction between efficient (pMBG, pFBG, and pHBG) and weak initiators (pCBG and pNBG) is made in the first picosecond after excitation.

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REFERENCES

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An In-Depth Mechanistic Investigation of the Radical Initiation

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