Independent Control of Singlet Oxygen and Radical Generation via

Article Cite This: Macromolecules 2019, 52, 4968−4978

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Independent Control of Singlet Oxygen and Radical Generation via Irradiation of a Two-Color Photosensitive Molecule Kimberly K. Childress,*,† Kangmin Kim,‡ David J. Glugla,§ Charles B. Musgrave,†,‡,∥,⊥ Christopher N. Bowman,†,∥ and Jeffrey W. Stansbury*,†,#

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Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303, United States ‡ Department of Chemistry, University of Colorado Boulder, 215 UCB, Boulder, Colorado 80309, United States § Department of Electrical, Computer, and Energy Engineering, University of Colorado Boulder, 425 UCB, Boulder, Colorado 80303, United States ∥ Materials Science and Engineering Program, University of Colorado Boulder, 596 UCB, Boulder, Colorado 80309, United States ⊥ National Renewable Energy Laboratory, Golden, Colorado 80401, United States # Department of Craniofacial Biology, School of Dental Medicine, University of Colorado Denver, 12800 East 19th Avenue, Aurora, Colorado 80045, United States S Supporting Information *

ABSTRACT: Free-radical polymerizations are used for a wide range of applications but are detrimentally impacted by the presence of oxygen. Zinc phthalocyanines have been previously used as singlet oxygen generators to excite radical-consuming ground-state triplet oxygen into its less reactive singlet state prior to photoexcitation of a photoinitiator. We report for the first time that polymerization can be achieved via irradiation of UV band of phthalocyanine and that photosensitization and photoinitiation can be independently achieved via irradiation of its two distinct absorption bands to reduce oxygen inhibition and initiate polymerization without the need for additional treatment. We propose a mechanism for this unique photoinitiation phenomenon and verify its feasibility via computational and experimental approaches. This new class of dual-photosensitive molecules shows promising utility in applications that are adversely impacted by the presence of oxygen, such as coatings and stereolithography.

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while the concentration of propagating active centers9 is on the order of 10−8 mol L−1. The extensive presence and reactivity of 3 O2 often results in a decreased rate of polymerization following an induction time,10,11 lower overall conversion,12 the formation of tacky surfaces,13,14 and less desirable final mechanical and optical properties.15,16 Inhibition by 3O2 is especially prominent in photoinduced polymerizations of thin films where open exposure to the atmosphere during polymerization of these large surface area to volume ratio structures promotes rapid oxygen diffusion into the network and thus results in poor properties throughout the film.17 Attempts to overcome oxygen inhibition have been ongoing for several decades motivated by the numerous detrimental effects it has on FRP. These efforts are generally classified as either physical or chemical methods.18 Physical techniques attempt to diminish inhibition by altering the environment in which polymerization occurs and frequently include techniques

INTRODUCTION Photoinduced polymerizations offer several advantages over other polymerization methods, including excellent spatiotemporal control and the ability to polymerize in a solvent-free environment.1 Despite these benefits, free-radical polymerizations (FRP) are particularly sensitive to the presence of oxygen. Ground-state oxygen (3O2) is a triplet biradical and efficiently reacts with primary radicals. This results in the observation that atmospheric oxygen significantly inhibits polymerizations by quenching the reactive excited states of photoinitiators (PIs) and scavenging primary initiating and propagating radicals.2 The resultant peroxy radicals do not effectively initiate vinyl monomer polymerizations3 but instead deactivate it by hydrogen abstraction or bimolecular termination, which further impedes polymerization and increases dispersity.4 The rate of reaction of 3O2 with carbon-centered radicals (koxygen ≈ 108 L mol−1 s−1)5 is at least an order of magnitude faster than the rate of the initiating radical reaction with acrylates (ki ≈ 107 L mol−1 s−1)6 and the rate of propagation (kp ≈ 103 L mol−1 s−1).7 Additionally, the equilibrium concentration of oxygen present in common acrylate systems8 exposed to air is typically ∼10−3 mol L−1 © 2019 American Chemical Society

Received: March 4, 2019 Revised: June 3, 2019 Published: June 24, 2019 4968

DOI: 10.1021/acs.macromol.9b00424 Macromolecules 2019, 52, 4968−4978

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Macromolecules such as inert gas purging,19 polymerizing in a closed or laminate system,20,21 and increasing the light intensity for greater production of initiating radicals.22,23 However, it is neither economically nor temporally feasible to implement these physical approaches industrially because of the enormous size of the annual volume of photopolymerizations. This has prompted the investigation of chemical strategies that offer greater feasibility for large-scale applications. Rather than removing dioxygen or preventing its diffusion into the polymerizing mixture, chemical mechanisms incorporate additives or suitably functionalized monomers to deplete the concentration of oxygen present in the monomer during the polymerization process. A variety of additives have been employed,24 including the use of relatively high concentrations of the PI or mixed PI,12 the incorporation of a hydrogen donor to cap the alkylperoxyl radical and upon hydrogen abstraction form a new reactive radical that reinitiates polymerization,25−28 and the addition of a singlet oxygen (1O2) generator.29,30 Singlet oxygen generators are photosensitizers (PSs) or other molecules that excite ground-state triplet oxygen to its singlet-state 1O2, which in contrast to 3O2 is relatively inert to initiating and propagating radicals. The direct radiative transition 3O2 → 1O2 is a spin-forbidden transition,31 so 1O2 is typically produced (Figure 1) by first photoexciting a PS to

Many factors must be considered while selecting a 1O2generating PS to overcome oxygen inhibition. Rapid ISC of 1 PS* relative to fluorescence is crucial to achieving high triplet quantum yields and thus greater TTET.35 Additionally, a long triplet lifetime (τT) increases the probability that 3PS* will interact with 3O2 and promote it to 1O2 by TTET. It is also preferable for the PS to absorb in the visible range (>500 nm) to avoid premature PI photolysis when used with typical PIs that absorb in the near-UV and shorter-wavelength regions (95% reflectance between 328 and 414 nm and >95% transmittance between 432 and 814 nm (DMLP425R, Thorlabs Inc.). Simultaneous UV−Vis/FT-NIR Acquisition. Samples were loaded into the center of a 0.25 mm Teflon gasket that was placed between two glass slides and held together with binder clips. The sample was mixed before loading but otherwise untreated. The glass slides were left untreated to avoid light scattering that would stem from additional interfaces between the glass and sample. The sample was then placed in a sample holder between the probing and detecting fibers and positioned normal to the curing light. The UV−vis and FT-NIR beams were coaligned using a custombifurcated optical fiber configuration (Ocean Optics, Dunedin, FL) (Figure 3). The UV−vis probing beam was transmitted through the 600 μm UV−vis leg of the bifurcated optical fiber from a deuteriumhalogen light source (DH-2000-BAL, Ocean Optics, Dunedin, FL) that was used to monitor the UV−vis spectrum over the range of 300−1100 nm. Because of the weak intensity of the UV−vis probe beam (∼100 μW/cm2), prolonged exposure did not induce polymerization. The FT-NIR probe beam (Nicolet Magna-IR Series II, Thermo Scientific, West Palm Beach, FL) was transmitted through the 600 μm vis−NIR branch of the bifurcated fiber. After passing through the sample, the combined UV−vis and NIR probe beams were separated using a second bifurcated fiber and analyzed by the respective spectrometers. The probe and detection fibers were collimated with SMA-adapted lenses (F220SMA, Thorlabs Inc.). Additionally, temperature was monitored during polymerization using a Gordon 5402 Type J Thermocouple by placing the thermocouple in the laminated sample. 4970


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Figure 4. Photopolymerization kinetics of [ZnTTP]0 = 0.2 mM (0.016 wt %) in DEGEEA. (a) Effect of ZnTTP B band irradiation on induction time. Irradiation of B band resulted in FRP for various UV intensities. (b) Irradiation of ZnTTP Q and B band for photosensitization and photoinitiation. Preirradiation of ZnTTP with red light (635 nm LED) at 34 mW/cm2 and subsequent UV irradiation (365 nm LED) at 10 mW/ cm2 resulted in a reduction or complete removal of induction time. (c) Rp is determined for various preirradiation times to determine the effect of 3 O2 on polymerization kinetics. Rp is lowest when red light preirradiation is not applied and gradually increases as 3O2 is removed prior to UV irradiation. Rp,max is greatest when no induction time is present. (d) Dose dependence of red light preirradiation on induction time. ZnTTP was irradiated with red light at varying intensities (10, 20, and 30 mW/cm2) prior to UV irradiation at 10 mW/cm2 for various times to achieve equivalent doses (2000, 4000, 6000, 6500, and 8000 mJ/cm2). A preirradiation dose of 6500 mJ/cm2 removed the induction time at all intensities. A fixed-grating FLAME-S-UV-VIS-ES spectrometer (Ocean Optics) coupled to the UV−vis leg of the detecting fiber and equipped with a Sony ILX511b detector was used to monitor the absorption spectrum of ZnTTP. The application of the Czerny− Turner configuration within the spectrometer and the incorporation of a detector with a charge-coupled device array allowed for full spectral acquisition up to 1 kHz. The integration time was set to 8−10 ms with 32 scans to average and a boxcar width of 5. The first overtone absorption band of the monomer vinyl group at 6167 cm−1 (∼1621 nm) was monitored with the Fourier transform infrared (FT-IR) spectrometer with a temporal resolution of 1 scan/4 s. By following the peak area of this functional group, monomer conversion was observed simultaneously with ZnTTP absorption and in real time. The InGaAs 2.6 μm detector and the extended range KBr beam splitter were used on the spectrometer. The averaged signal of eight scans was obtained with a wavenumber resolution of 4, an optical velocity of 3.1647 cm/s, an optical gain of 1, and an optical aperture of 73. Synchronization of UV−Vis/FT-NIR Data Acquisition and Sample Irradiation. Synchronization between the UV−vis and FTIR spectrometers and sample irradiation was achieved using an external data acquisition device (myDAQ, National Instruments, Austin, TX). Data acquisition and sample irradiation were simultaneously triggered using an arbitrary waveform generator (NI ELVIS, National Instruments). External triggering of the FT-IR spectrophotometer was accomplished using a triggering cable (Nicolet x700 Remote Trigger Cable, Thermo Scientific) on the auxiliary signal port. The analog input/output connectors on a breakout box (HR4-BREAKOUT, Ocean Optics) connected to a Flame spectrometer were accessed and used in the series with the myDAQ to achieve external triggering. Irradiation of the sample was externally triggered from the analog output of the myDAQ using two synchronized function generators (Agilent 33500B Waveform Generators, Keysight Technologies, Santa Rosa, CA) in series.

Analysis of UV−Vis and FT-NIR Spectra. OMNIC software (Thermo Scientific, West Palm Beach, FL) collected peak area data in real time between 6217.5 and 6119.1 cm−1 and was processed in MATLAB. Induction time was determined by averaging double-bond conversion prior to any vinyl conversion and visibly determining the time at which polymerization began. UV−vis data were obtained during irradiation with OceanView software (Ocean Optics) and analyzed in MATLAB using a script that was written to process the full absorbance spectrum, remove baseline shift associated with optical attenuation from refraction and light scattering, and isolate the wavelengths that were irradiated. Three replicates were performed for each experiment. Computational Method. All quantum mechanical calculations were executed within the Gaussian 16 Revision A.03 software package.68 Computations were all performed using density functional theory (DFT) at the M06 density functional69 and 6-31+G(d,p) basis set70 level of theory. The M06 functional was selected based on prior benchmarking that demonstrated its favorable performance when applied to ground-state and excited-state systems involving organic and organometallic molecules.69,71 Vibrational frequencies were calculated to verify that the stationary-state geometries were optimized to the correct structures and to compute vibrational entropies, zero-point energies, and thermal corrections at 298 K. Solvent effects were described using the universal solvation model (SMD)72 with parameters describing ethyl acetate (EtOAc) because of its structural similarity to (meth)acrylate monomers. Furthermore, the EtOAc’s dielectric constant68 of ∼6 is suitable for representing the dielectric environment produced by common bulk (meth)acrylate monomers with dielectric constants ranging from 2.5 to 11.73,74 Electronic structure calculations for excited states, including UV−vis spectra, were executed via time-dependent DFT (TD-DFT) at the same M06/6-31+G(d,p)/SMD-EtOAc level of theory. 4971


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RESULTS AND DISCUSSION Photopolymerization Kinetics. Photosensitization was previously accomplished via irradiation of the ZnTTP Q band. For TTET to occur between 3PS* and 3O2, the singlet−triplet energy gap of the PS must be greater than 0.98 eV.32 The Pc B band resides in the UV, whereas the Q band is much lower in energy and acts as an efficient visible light PS. Therefore, irradiation of the higher-energy ZnTTP B band should also result in 1O2 sensitization as the energy difference between its ground and triplet states is greater than that for 1O2. The photosensitization capabilities of the B band were investigated by incorporating the common type II photoinitiating system CQ/EDMAB, whose absorption bands minimally overlap those of ZnTTP. Initial UV irradiation of ZnTTP prior to blue light irradiation of CQ (which did not polymerize during UV irradiation) resulted in a complete removal of the induction time (Figure S1), thus confirming the B band’s utility as a PS. Although the energy of the B band is sufficient for photosensitization, it was also discovered that sustained UV exposure in the absence of a PI resulted in polymerization for a laminate sample (Figure 4a). The monofunctional acrylate DEGEEA was selected for this study to minimize baseline shift associated with optical attenuation for UV−vis analysis, although polymerization was observed for several acrylates and multifunctional (meth)acrylates. B band irradiation resulted in FRP, but irradiation of the Q band resulted in minimal polymerization and predominantly participated in 3O2 conversion (Figure S2). UV-initiated polymerization did not occur immediately as dissolved 3O2 was present in the system and resulted in an inherent induction time. As anticipated, increasing the intensity of UV irradiation resulted in greater radical generation and thus decreased the induction time, although an appreciable reduction did not occur.75 Alternatively, increasing [ZnTTP]0 would result in greater TTET and generate more 1O2 and initiating radicals. Without applying preirradiation, increasing [ZnTTP]0 drastically diminished the induction times for 0.1 mM ZnTTP from 479 s with 365 nm LED at 15 mW/cm2 to 99 and 58 s for 1 and 10 mM ZnTTP (Figure S3). Because increasing irradiation intensity and [ZnTTP]0 did not result in a reasonably reduced induction time, alternative approaches to achieve immediate polymerization with UV irradiation were pursued. ZnTTP was previously used as a PS, and a second molecule was included to initiate polymerization. It was hypothesized that ZnTTP could serve its original function as the PS while also acting as the PI by irradiating the Q and B bands separately. Unlike other photoactive molecules, the distinct peaks of ZnTTP are cleanly separated by more than 100 nm, and they are independently responsible for photosensitization and photoinitiation. To determine if photosensitization and 1 O2 generation could reduce the induction time upon UV irradiation, a solution of 0.2 mM ZnTTP in DEGEEA was first irradiated with red light at 34 mW/cm2 and subsequently irradiated with a UV LED at 10 mW/cm2 (Figure 4b). Initial preirradiation with the red LED resulted in a decreased induction time, and successful utilization of ZnTTP for both photosensitization and photoinitiation was confirmed. As expected, increasing preirradiation time linearly reduced the induction time until it was completely removed after 600 s. However, a lower [3O2]0 should greatly reduce the necessary preirradiation time. Because a substantial preirradiation time was required, the temperature was monitored during

preirradiation and polymerization to exclude a significant increase in the film (Figure S4). The presence of 3O2 not only resulted in postponed polymerization but it also continued to have a detrimental impact on polymerization kinetics after the onset of polymerization. The concentration of 3O2 needs to be significantly reduced prior to the irradiation of the PI, or a large concentration of initiating radicals will be scavenged during this process and ultimately reduce the rate of polymerization (Rp). Although a sufficiently large initiator concentration may render a negligible amount of initiator consumption prior to polymerization, solubility and potential leaching of remnant initiator remains a concern for many applications. [PI]0 can remain relatively low and undesirable PI consumption can be avoided via sufficient initial irradiation of a PS, thus eliminating the induction time and achieving a faster Rp. Rp was determined from vinyl conversion results using the same parameters as those of Figure 4b. When no red light preirradiation was initially applied prior to UV irradiation, the maximum Rp (Rp,max) was ∼7 × 10−4 mol L−1 s−1 and occurred at ∼15% conversion (Figure 4c). Longer preirradiation yielded faster initial monomer conversion and accelerated Rp. In the absence of an induction time, Rp,max increased to ∼1.3 × 10−3 mol L−1 s−1 because of reduced initial radical consumption. While the [3O2] significantly impacted Rp, the final conversions were approximately the same. Monofunctional acrylates polymerize slowly, so complete polymerization profiles were obtained once for each preirradiation time rather than averages of triplicate runs. At 34 mW/cm2, 600 s of red light irradiation was necessitated to eradicate the polymerization delay. Higher ZnTTP concentrations drastically reduced the induction time, as anticipated. However, increasing the [PI]0 is not preferable in all applications. To determine if the induction time could be lessened further with more powerful red light sources, it was necessary to explore the potential scaling between the conversion of 3O2 and its dependence on preirradiation intensity (i.e. the reciprocity law). Previously, the exposure reciprocity law was used to investigate the dependence of photopolymerization kinetics and final mechanical properties on exposure dose, where dose is the product of time irradiated and irradiation intensity.76−80 Jönsson and Bao studied the effect of exposure dose on the oxygen inhibition that occurs at the surface of an exposed film.81 Although the relationships between exposure dose and various polymerization properties have been extensively examined, the effects of irradiation intensity on photosensitization and 3O2 removal have not been studied. Three different intensities were selected (10, 20, and 30 mW/cm2), and the irradiation time was varied so that the same preirradiation doses (2000, 4000, 6000, 6500, and 8000 mJ/cm2) were achieved for each intensity (Figure 4d). As expected, increasing the 635 nm preirradiation dose reduced the induction time for all intensities. Induction time linearly decreased with increasing preirradiation dose, and a complete removal of the induction time was observed for all intensities at 6500 mJ/cm2. The additional exposure to a red light dose at 8000 mJ/cm2 resulted in a reduction of the 3O2 concentration beyond what was required for polymerization to occur. Enhanced initial Rp was observed after the additional 1O2 production (Figure S5). From these studies, induction time dependence on preirradiation dose was confirmed. A preirradiation of 6500 mJ/cm2 was required to completely remove the induction time for this system, which would 4972


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Figure 5. Vinyl conversion and PS consumption for [ZnTTP]0 = 0.2 mM in DEGEEA. (a) ZnTTP was irradiated with red light at 30 mW/cm2 for 216.67 s (shown in red box) prior to UV irradiation at 10 mW/cm2 (blue box). Minor amounts of ZnTTP were consumed during preirradiation while no polymerization occurred. Immediately upon UV irradiation, polymerization was initiated, and the rate of ZnTTP consumption increased. (b) ZnTTP was irradiated with red light at 30 mW/cm2 for 66.67 s (shown in red box) prior to UV irradiation at 10 mW/cm2 (blue box). Again, minor amounts of ZnTTP were consumed during preirradiation while no polymerization occurred. Because of insufficient preirradiation, polymerization did not begin immediately upon UV irradiation, and an induction time was present. The rate of ZnTTP consumption increased while the remaining 3O2 was consumed and increased again once polymerization began.

was consumed during UV irradiation prior to polymerization. As soon as the concentration of 3O2 was low enough to initiate polymerization, the rate of ZnTTP consumption almost doubled to 1.2 × 10−3 s−1. As before, the rate of consumption decreased as the Rp decreased. It is important to note that the overall ZnTTP consumption prior to polymerization was significantly higher when polymerization did not begin immediately upon UV irradiation. When the red light preirradiation dose was high enough to reduce [3O2] so that no induction time was present, less initiator consumption occurred. Proposed PS and PI Mechanism. Next, we attempted to elucidate the mechanism for the observed PI phenomenon. ZnTTP possesses a zinc metal core and has a peripheral tertbutyl substituent on each benzene ring. To determine if zinc metalation or the incorporation of these substituents was responsible for photoinitiation, either the zinc (TTP) or the tbutyl substituents (ZnPc) were removed (Figure 6a). Although the ability of ZnPc and TTP to initiate FRP was unknown, both are capable of acting as PSs for 1O2 generation. Because of the differences observed in the molar absorptivities (ε) for both the B and Q bands (Figure 6b), polymerization rates were taken with respect to photon absorption instead of time. UV irradiation of ZnTTP, ZnPc, and TTP at 30 mW/cm2 showed that each structure is capable of initiating polymerization (Figure 6c), albeit at significantly different rates. Nonmetallated TTP exhibited the slowest polymerization with low conversions observed over 1800 s. The 1O2 quantum yield (ΦΔ) for TTP is markedly low (0.31 ± 0.04 in benzene83) while ΦΔ is substantially higher for ZnTTP (0.62 ± 0.15 in CH2Cl2 and 0.69 ± 0.13 in toluene84) and ZnPc (0.67 ± 0.10 in DMSO85). The incorporation of Zn resulted in the heavy atom effect which enhances spin−orbit coupling.35,86 Greater spin−orbit coupling increases the rate of ISC, thus raising the population of the PS triplet state and increasing its TTET with 3 O2 and ultimately increasing the rate of polymerization. However, not all metal-functionalized Pcs resulted in polymerization. For instance, TTP with a copper(II) metal core (CuTTP) did not result in polymerization when exposed to UV irradiation at 20 mW/cm2 for 2000 s and instead seemed to cause a profound inhibitory effect (Figure S6). A low ΦΔ (∼0.16 ± 0.03 in DMSO87) was measured for copper(II) tetracarboxyphthalocyanine, and the d9 configuration of Cu

suggest that polymerization would begin after sufficient red light exposure at any intensity. Therefore, although long red light exposure times were required for the relatively low intensities used here, an industrial-grade red light source would likely have enough power to reduce the requisite preirradiation time to a few seconds. Alternatively, continuous background red light exposure during sample preparation and polymerization could be considered to circumvent induction time. Red light irradiation of 0.2 mM ZnTTP occurred for 216.67 s (precise red light and UV irradiation controlled by function generators) to achieve a dose of 6500 mJ/cm2 and convert enough 3O2 to completely remove the induction time (Figure 5a). After applying this preirradiation dose, polymerization began immediately upon UV irradiation. Red light irradiation culminated in negligible ZnTTP consumption (∼3.5%) while no polymerization occurred. This result is not surprising as the irradiation of the Q band results in the production of 1O2 from 3 O2 but not free radicals. Efficiency of 1O2 production differs based on various PS properties, including the quantum yield of ISC (ΦISC) and the lifetime of the PS in the triplet state (τT), but PSs can typically produce 103 to 105 molecules of 1Δg before degradation.82 Therefore, if 3O2 is present in the system, some initial consumption of ZnTTP should occur based on [ZnTTP]0 and the dose of the preirradiation. ZnTTP consumption during the initial preirradiation stage occurred at a rate of 1.4 × 10−4 s−1. Subsequent UV exposure resulted in instantaneous polymerization and an appreciable escalation of the ZnTTP consumption to 1.4 × 10−3 s−1. Although it is possible that some 3O2 conversion is still occurring, the concentration of 3O2 dramatically decreased during irradiation of the Q band (by several orders of magnitude to completely remove the induction time). Rather, the rapid consumption of ZnTTP and the sudden onset of polymerization observed upon UV irradiation is likely due to the same phenomenon. Insufficient preirradiation dose leads to a reduced but not completely removed induction time. During the red light preirradiation step at 30 mW/cm2 for 66.67 s (2000 mJ/cm2), the rate of ZnTTP consumption was 1.4 × 10−4 s−1 (Figure 5b). Approximately 1.2% of the ZnTTP was consumed during this stage. Subsequent UV irradiation more than quadrupled the ZnTTP consumption rate to 6.2 × 10−4 s−1, but no polymerization occurred. The increased consumption rate may be because the B band is higher in energy and may act as a more efficient PS than the Q band. Another 7% of the ZnTTP 4973


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(Scheme (b) in Figure 7).89 We hypothesize that the endoperoxide on the Pc ring can photolyze upon UV absorption, generating viable initiating radicals (Scheme (a) Figure 7). The feasibility of this proposal is demonstrated in the following Computational Method section. The Pc ring has four pyrrole groups and four benzo groups that can act as the diene for the Diels−Alder reaction with 1O2. Typically, the activation energy required for Diels−Alder reaction with an aromatic diene is higher than for a nonaromatic compound because of the stabilization provided by the resonance of the ring. However, when the dienophile is extremely reactive (e.g., 1O2) the aromatic ring can react as the diene component. ZnTTP and TTP have bulky t-butyl groups peripherally located on each benzo group, and the induced steric hindrance renders endoperoxide formation on the benzo groups unlikely. Indeed, the A-value (which provides a general representation of steric size) for t-butyl groups is notably higher (>4 kcal mol−1) than other considerably bulky groups (e.g., CF3 is 2.4−2.5 kcal mol−1 and a phenyl group is 2.8 kcal mol−1).90 The benzo groups provide separation between the tbutyl groups and the pyrrole rings, and the planarity of the molecule makes 1O2 reaction more spatially feasible. The lack of substituents for ZnPc potentially allows 1O2 to access eight reaction sites instead of the four suggested for ZnTTP and TTP, although reaction with the pyrrole rings is still more probable. It was originally anticipated that ZnPc and ZnTTP would have comparable polymerization rates because of their similar ΦΔ, but the availability of more sites may explain the faster Rp observed for ZnPc. Although this photodecomposition mechanism is feasible, Pc degradation is complex and may consist of several competitive pathways.91,92 Computational Verification for Endoperoxide Generation and Photolysis. To examine our mechanistic hypothesis of endoperoxide formation and its photolysis in phthalocyanine derivatives, we used a model chemistry of a Pc ring for our quantum chemical calculations because Zn and tbutyl functionalization were not necessary for photoinitiation. The Pc ring without a metal atom core can undergo cycloaddition with 1O2 at three distinct sites. The reaction that involves an isoindole moiety with an attached hydrogen is computed because cycloadditions of O2 at the other two sites are kinetically and thermodynamically less favorable, and the resultant products do not generate radicals. However, the addition of 1O2 to the Pc ring at these sites is described further as shown in Figures S7 and S8. The planar geometry of the Pc ring is conducive to πelectron delocalization throughout the ring, as expected for a highly prevalent dye chromophore (Figure 8a). As 1O2 approaches the diene moiety of the isoindole, it proceeds through a transition state for an asynchronous [4 + 2]cycloaddition with a moderate free-energy barrier of 9.40 kcal/ mol (Figure 8b). This cycloaddition results in an endoperoxide on the Pc ring with a reaction-free energy of −3.47 kcal/mol (Figure 8c). We note that the ring distortion shown in Figure 8c is minimal relative to those calculated for cycloaddition at the other two sites (Figures S7 and S8), suggesting that the distortion of the ring may be the cause for the high enthalpic penalty of cycloaddition at these two sites. To determine if the endoperoxide of the Pc (Pc−O2) can undergo photolysis, its UV−vis spectra was computed via TDDFT, as we did not observe the appearance of a new peak in the measured UV−vis spectra. In addition to our computationally predicted Pc ring peak maximum (λmax) at 660 nm

Figure 6. (a) Zinc, copper(II), and nonmetallated Pcs investigated in this study. (b) Molar absorptivities of TTP, ZnPc, and ZnTTP in DEGEEA. (c) UV irradiation of 0.2 mM TTP, ZnPc, and ZnTTP at 30 mW/cm2 in DEGEEA. Polymerization occurred after removal of metal core (TTP) and t-butyl groups (ZnPc). Polymerization occurred for each Pc but at significantly variable rates. In particular, the removal of the metal core significantly lowered the photoinitiating capability.

caused it to act as an electron scavenger to achieve a more stable d10 orbital. Although the polymerization profiles varied significantly, Zn and t-butyl functionalization of ZnTTP are not the primary contributing factors for photoinitiation because polymerization occurred after the removal of these substituents. For this reason, it is reasonable to assume that free-radical production is a Pc ring-mediated process. Photosensitization and subsequent [4 + 2] cycloaddition with 1O2 was previously shown to result in endoperoxide formation on ZnPc88 and ultimately the production of phthalimides as the main oxidation product 4974


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Figure 7. Proposed ZnTTP photosensitization and photoinitiation mechanism. ZnTTP is excited to its triplet state and undergoes TTET with 3O2, which is then excited to its singlet state. Highly reactive 1O2 is scavenged by ZnTTP to form endoperoxide via [4 + 2] cycloaddition. Phthalimide production via oxidative cleavage has previously been demonstrated and was the proposed product following endoperoxide formation. However, ZnTTP with the endoperoxide can generate alkoxy radicals via photolysis with UV irradiation.

Figure 8. (a) Reference geometries of Pc ring and singlet oxygen. (b) Transition-state geometry of [4 + 2] cycloaddition between the Pc ring and singlet oxygen. (c) Product geometry of endoperoxide with a slight cyclic distortion. The endoperoxide formation requires a moderate activation barrier of 9.40 kcal/mol with the overall exergonicity (−3.47 kcal/mol), indicating that this reaction is feasible and spontaneous under our experimental conditions. All geometries and their energies are computed at M06/6-31+g(d,p) with an SMD solvent model in EtOAc.

Therefore, we successfully demonstrated the novel utilization of ZnTTP as an independent PI for FRP upon continuous UV irradiation of the B band. However, an induction time was still observed because of the presence of 3O2 in the untreated monomer. To circumvent the delayed polymerization, sufficient irradiation of the Q band reduced the concentration of 3O2 enough for polymerization to begin immediately upon UV irradiation of the B band. Although previous systems required the incorporation of both a PS and a PI, independent control over photosensitization and photoinitiation was accomplished with a single molecule via the selective irradiation of ZnTTP’s two absorption bands. Unlike many PSs that undergo rapid and complete photodegradation, minor ZnTTP consumption is observed during the initial red light preirradiation stage. Therefore, most of the ZnTTP was available to initiate polymerization. Faster rates of polymerization could be achieved with multifunctional acrylates and higher concentrations of ZnTTP. Additionally, proper alteration of the Pc functionalities may increase its ability to generate 1O2 or initiate polymerization. Indeed, we saw that removing either the zinc core (TTP) or the peripheral tertbutyl substituents (ZnPc) from ZnTTP drastically affected the polymerization kinetics. Although zinc significantly enhanced polymerization, the inclusion of tert-butyl on each benzo ring negatively impacted polymerization. A mechanism was proposed to explain the photosensitizing and initiating abilities of ZnTTP based on products previously formed from the photosensitization of Pcs in the literature. Computational verification was provided as a validation of the hypothesized

congruent with the spectroscopically measured peak at 662 nm, TD-DFT computation of the excited states of Pc−O2 yielded a λmax of 426 nm that extends into the UV (Figure S9) because of the disturbed conjugation that results from the bending of the Pc ring upon O2 cycloaddition. Considering its weak absorption (oscillator strength f = 0.0165 vs 0.6522 at 660 nm of the Pc ring) and the overshadowing B band that overlaps with the Pc−O2 absorption peak, it is probable that peaks associated with Pc−O2 intermediates are not observable in the experimental UV−vis spectrum but still weakly absorb light from the UV LED and thus still undergo photolysis. Finally, our calculations predict a bond dissociation energy of the endoperoxide of approximately 50 kcal/mol, which explains the lack of polymerization under irradiation with the 635 nm LED corresponding to an energy of ∼45 kcal/mol, yet the 365 nm LED had sufficiently high energy of ∼78 kcal/mol to initiate polymerization. Our computational evidence for the facile endoperoxide generation and its photolysis verifies our proposed photoinitiation mechanism involving cycloaddition of 1O2 to the Pc ring.

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CONCLUSIONS Pcs have previously been exploited for the photosensitization and conversion of detrimental ground-state oxygen to its relatively inert singlet state via red light irradiation of its Q band. Although Pcs have two distinct absorption bands, only the Q band has been studied because of the overlap of any incorporated PI with the B band. By excluding the PI, we were able to directly study Pc photochemistry applied to FRP. 4975


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mechanism. Zn Pcs are promising for a variety of applications in which the presence of 3O2 may adversely impact polymerization kinetics and thus final mechanical properties (e.g., thin-film coatings). Their resistance to photodegradation, even under sustained red light irradiation, makes them promising candidates for continuous 1O2 generation in systems that are open to atmosphere prior to and during polymerization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00424.

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Polymerization profiles and raw computational data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.K.C.). *E-mail: jeff[email protected] (J.W.S.). ORCID

Kimberly K. Childress: 0000-0001-8525-5712 Kangmin Kim: 0000-0002-5814-9225 David J. Glugla: 0000-0002-6041-7751 Charles B. Musgrave: 0000-0002-5732-3180 Christopher N. Bowman: 0000-0001-8458-7723 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Industry/University Cooperative Research Center for Fundamentals and Applications of Photopolymerizations, ALTANA, and NIH/NIDCR (DER 01023197). The authors gratefully acknowledge the use of the Janus supercomputer, which is supported by NSF (CNS-0821794) and the University of Colorado Boulder.

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REFERENCES

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