Letter Cite This: ACS Macro Lett. 2018, 7, 852−857
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o‑Nitrobenzyl-Based Photobase Generators: Efficient Photoinitiators for Visible-Light Induced Thiol-Michael Addition Photopolymerization Xinpeng Zhang, Weixian Xi, Guangzhe Gao, Xiance Wang, Jeffrey W. Stansbury, and Christopher N. Bowman*
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Department of Chemical and Biological Engineering, University of Colorado Boulder, UCB 596, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: In this contribution, three o-nitrobenzyl-based photobase systems were synthesized and evaluated for visible light initiated thiol-Michael addition polymerizations. With a modified structure, the (3,4-methylenedioxy-6-nitrophenyl)-propyloxycarbonyl (MNPPOC) protected base performance exceeds that of the nonsubstituted 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) protected base and an ITX sensitized photobase system, with respect to both long-wavelength light sensitivity and photolytic efficiency. In material synthesis, MNPPOC-TMG is capable of initiating photo thiol-Michael polymerization efficiently and orthogonally with only limited visible light exposure and generating a highly homogeneous cross-linked polymer network. This approach enables the thiol-Michael “click” reaction to be conducted with a low-energy, visible light irradiation and, thus, expands its applications in biocompatible and UV sensitive materials.
B
undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), or tetramethylguanidine (TMG).13,14 Applications of these new PBG systems have not only seen use in catalyzing fundamental anion-mediated reactions, such as the thiolMichael addition reaction, epoxy/cyclic esters ring-opening reactions, and thiol-urethane synthesis, but also, more practically, they have been used in the areas of photopatterning, adhesives, polymerization/depolymerization, and lithography.12,15−18 The thiol-Michael addition reaction has requirements related to its photo initiating process, which limit the range of currently available PBGs that are suitable. Compared to weak bases (hexylamine, triethylamine), stronger bases are favored to be generated due to their fast thiol deprotonation rates and efficient initiation of the thiol-Michael addition reaction.19 Second, the photo released radical side products or intermediate should be minimized during the photolysis of PBGs as radical moieties promote the radical-mediated homopolymerization, which would limit the final conversion of substrates and negatively affect the orthogonality of the thiol-Michael system. For example, Chatani and co-workers utilized TEMPO to inhibit radicals and avoid the acrylate homopolymerization in thiol-Michael photoinitiation when
earing the merits of high yields, rapid reaction rates, and the ability to be conducted at mild reaction conditions, “click” chemistry has greatly affected the development of chemistry, biology, and polymer science.1−3 The well-known thiol-Michael addition reaction stands out in the “click” reaction family due to its anion-mediated propagation mechanism and is widely used in fundamental biological and chemical research studies as well as practical applications, for example, in microparticle synthesis,4 surface modification,5 dental restoratives,6 and malleable thermosets.7 To achieve photoinitiation for the thiol-Michael reaction with high spatial and temporal control, photobase generators (PBGs) are introduced to initiate the base/nucleophile catalyzed reaction.8 However, unlike photo radical generators, the choices for PBGs are highly restricted in aspects such as photolytic efficiency and light sensitivity. As such, an ideal photoinitiated thiol-Michael system is more challenging to be achieved, as compared with other “click” coupling reactions, such as the thiol−ene reaction6,9 or the Cu(I) catalyzed alkyne−azide reaction,10 which can be readily initiated by radical photoinitiators. In the past few decades, various PBG systems have been developed and studied. Different photosensitive functional groups, such as indolines,11 amineimides,12 and tetraphenylborates,13 have been utilized to protect bases (most commonly amines). Besides weak amines, stronger basic species (pKa around 13−14) are able to be photogenerated by protecting guanidine and its derivatives, such as 1,8-diazabicyclo[5.4.0]© XXXX American Chemical Society
Received: June 8, 2018 Accepted: June 26, 2018
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DOI: 10.1021/acsmacrolett.8b00435 ACS Macro Lett. 2018, 7, 852−857
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ACS Macro Letters applying the ITX/TBDHPh4 photobase system.20 Further, in terms of visible light sensitivity, most PBGs respond only to UV light and have minimal absorption above 400 nm. Compared to UV light initiation, low-energy visible light initiation is advantageous as it minimizes UV-triggered side reactions in the photochemical process, increases curing depths in material synthesis and also avoids damaging UV sensitive cells or tissues in biomedical applications.21,22 2-(2-Nitrophenyl)propyloxycarbonyl (NPPOC) protected amines have proven to be appropriate PBGs for the thiolMichael coupling reactions upon UV irradiation.23,24 Due to the high quantum yield and photolytic efficiency, the NPPOC group has seen widespread application in the synthesis of nucleopeptide/peptide microarrays,25,26 gene assembly,27 and photoresponsive materials.28 To optimize the photo cleavage efficiency and absorption of the NPPOC group, two different strategies were introduced, namely, photosensitization and structural modification. For example, by modifying the core structure of NPPOC, the Somoza group demonstrated longwavelength responsive, highly efficient thiophenyl-2-(2-nitrophenyl) propoxycarbonyl (SPh-NPPOC) as the next-generation o-nitrobenzyl photolabile groups for light-directed chemistry.29 Similarly, (3,4-methylenedioxy-6-nitrophenyl)propyloxycarbonyl (MNPPOC), with a substituted methylene dioxy group into the benzene ring, also possessed a red-shifted absorption as well as greatly facilitated the photocleavage kinetics in microarray synthesis.30,31 Herein, we introduce MNPPOC-protected TMG (MNPPOC-TMG) as an appropriate PBG for visible lightdirected thiol-Michael addition polymerization. Three photobase systems, MNPPOC-TMG, NPPOC-TMG, and ITX sensitized NPPOC-TMG were investigated regarding their photolysis and catalytic efficiency toward the thiol-Michael addition polymerizations (Scheme 1a). Compared to the NPPOC group, the electron-donating substitution (namely,
the secondary methylene dioxy ring) in the MNPPOC structure tends to stabilize the π,π* states and induces a bathochromic shift of the absorption due to weak chargetransfer (CT) interactions. This modified structure also affects the electron transitions during the photoreaction and greatly facilitates the photolytic cleavage of MNPPOC for irradiation wavelengths above 400 nm.33,34 On the other hand, visible light active sensitizers (e.g., thioxanthone, coumarin derivatives), based on intra- or intermolecular energy transfer from the triplet state, also enable the NPPOC photoliable systems to possess a red-shifted absorption (>400 nm).32 The ITX/ NPPOC-TMG system was used to evaluate the photochemical behavior of MNPPOC-TMG, especially under visible light exposure conditions. Inducing light at appropriate wavelength cleaves these photobases and releases the desired, protected species, namely TMG, to deprotonate the thiol and initiate the thiol-Michael reaction. The synthetic route for forming MNPPOC-TMG is briefly presented in Scheme 1b, and all of the synthetic steps were conducted under mild conditions, giving a reasonable 10−20% overall yield. Previous studies have elucidated NPPOC photocleavage mechanisms (Figure S1) and proved that β-elimination and decarboxylation processes were the dominant pathways in protic solvents.33,34 Time-dependent UV−vis measurements were performed to determine the light-directed behavior of MNPPOC-TMG and further confirm the β-elimination process (Figure 1a). Upon 20 mW/cm2, 400−500 nm light irradiation, the overall absorption of MNPPOC-TMG
Scheme 1. (a) Mechanism of Photobase-Catalyzed Photo Thiol-Michael Addition Reaction and (b) Synthesis of the Photobase MNPPOC-TMG
Figure 1. UV−vis absorption evaluation of (a) MNPPOC-TMG realtime photodecomposition in methanol upon irradiation with 20 mW/ cm2, 400−500 nm light; (b) NPPOC-TMG (1.2 mM), MNPPOCTMG (0.6 mM), and ITX (9.8 × 10−2 mM) in methanol, along with the wavelength regions of the applied LED light sources (λmax at 365, 405, and 455 nm). 853
DOI: 10.1021/acsmacrolett.8b00435 ACS Macro Lett. 2018, 7, 852−857
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ACS Macro Letters increased at the beginning (0−5 min) due to the formation of the nitroso intermediate, which was generated after intramolecular benzylic H atom transformation in the triplet state. Two isosbetic points at 328 and 425 nm were observed, and the spectra decreases between and increases slightly in the range above 425 nm. The spectral change in MNPPOC-TMG is, in principle, similar and red-shifted compared with nonsubstituted NPPOC-TMG. Light-induced decomposition of MNNPOC-TMG generates TMG, which was further confirmed by a phenol red based colorimetric method (Figure S2), along with carbon dioxide and methylenedioxy onitrostyrene being observed as side products. Figure 1b shows the absorption of NPPOC-TMG, ITX, and MNPPOC-TMG along with the emission spectra of the LED light sources (λmax at 365, 405, and 455 nm). The detailed emission spectra are shown in Figure S3. Compared with NPPOC-TMG, the spectra of MNPPOC-TMG and the sensitizer ITX show long-wavelength absorption bands with λmax at 346 and 388 nm, tailing well into the visible light region (above 420 nm). Real-time FT-IR was performed to investigate the decomposition process by monitoring carbamate bond cleavage. In order to mimic the thiol-Michael reaction environment, the nonvolatile tetra-functional thiol PETMP was used as the solvent instead of methanol. In Figure 2a and Figure S4, MNNPO-TMG exhibited much faster
photodecomposition kinetics as compared to NPPOC-TMG at different UV conditions (365 and 405 nm). Due to limited absorption above 400 nm, NPPOC-TMG is almost inert upon exposure to 455 nm LED light (Figure 2b). Under the same conditions, ITX is able to sensitize NPPOC-TMG readily and induce its decomposition whereas its photolysis rate is still not comparable to MNPPOC-TMG. A linear regression curve fit for the natural logarithm of the photobase concentration versus time was observed, indicating a first-order photodecomposition (Figure S5). By using eq 1, the photolytic efficiency (εφ) was calculated based on standard values ([I], photobase concentration; I0, light intensity; λ, wavelength of induction light; NAV, Avogadro’s number; h, Planck’s constant; and c, the speed of light). −
2.303εφ[I]I0λ d[I] = dt NAVhc
(1)
Table 1 summarizes the photolysis results for these three photobase systems. The results elucidated that the photolytic Table 1. λmax of LED Light and Photolytic Efficiency for NPPOC-TMG, ITX/NPPOC-TMG, and MNPPOC-TMGa photolabile group
λmax of LED light (nm)
photolytic efficiency (εφ) (M−1 cm−1)
NPPOC MNPPOC NPPOC MNPPOC NPPOC ITX/NPPOC MNPPOC
365 365 405 405 455 455 455
55 130 8.6 38.5 ∼0 1.2 4.6
a
Photolytic efficiency (εφ) of the photobases was calculated based on eq 1.
efficiency (εφ) of MNPPOC-TMG is much greater than that of NPPOC-TMG at UV light conditions (365 and 405 nm) and ITX/NPPOC-TMG at visible light condition (455 nm). Photoinitiators with high photolytic efficiency are prone to release protected species rapidly and productively upon light irradiation, enabling reduced irradiances to be used, which limits the photochemical side reactions that occur in the medium (e.g., damage to tissues or undesired radical generation in polymer synthesis). The catalytic efficiency of MNPPOC-TMG toward the thiolMichael polymerization reactions was examined in a stoichiometric thiol−acrylate system (tetrafunctional thiol, PETMP, and difunctional acrylate, TCDDA) at different irradiation conditions. In real-time FT-IR experiments, the decreasing thiol peak around 2650 cm−1 and acrylate peak around 810 cm−1 were chosen as indicative measures to monitor the polymerization progress. Upon 30 mW/cm2, 365 nm irradiation, both MNPPOC-TMG and NPPOC-TMG were able to initiate thiol-Michael polymerization efficiently with a stoichiometric reaction observed between the thiol and acrylate functional groups (Figure S6). However, consistent with the photolytic efficiency tests, the MNPPOC-TMG catalyzed photo thiol-Michael polymerization was superior to the NPPOC-TMG catalyzed system, as it not only afforded a higher final conversion of the monomer but also exhibited a faster reaction rate. On the other hand, with the merits of redshifted absorption and good light sensitivity, the MNPPOCTMG is advantageously used in the visible light (wavelength
Figure 2. Relative amount of the photobase (percentage of the photobases left in the system) as a function of time for photobase/ PETMP mixtures (molar ratio 1:10): (a) photodecomposition of MNPPOC-TMG and NPPOC-TMG in the photobase and PETMP mixture irradiated with 5 mW/cm2, 365 nm LED light; (b) photodecomposition of MNPPOC-TMG, NPPOC-TMG, and ITX/ NPPOC-TMG in the photobases and PETMP mixture irradiated with 50 mW/cm2, 455 nm LED light. 854
DOI: 10.1021/acsmacrolett.8b00435 ACS Macro Lett. 2018, 7, 852−857
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ACS Macro Letters
promoted a distinctive and continuing polymerization reaction in the dark. Remarkably, there is no obvious change in the polymerization rate immediately after the cessation of light. Although the dark condition suspends any new base generation and slows down polymerization at later stages, this reaction still continued from 20% (when the light was turned off) to 80% conversion in 20 min. Based on this “living” character, the MNPPOC-TMG initiated thiol-Michael polymerization is able to form a high-yield cross-linked network with only limited visible light exposure time and, additionally, serve as an alternative method to solve the inhomogeneous curing problem (caused by light gradients in the incident path) in bulk materials. Finally, the polymerization kinetics study and mechanical property experiments were performed on the visible light triggered thiol-Michael polymers, and two substrates, namely, PETMP/DVS (Tg = 43 °C) and PETMP/TCDDA (Tg = 26 °C) were used as representatives for DMA tests. Standard films (0.25 mm thickness) made of a stoichiometric mixture of thiol and acrylate were cured in the presence of 2 wt % of MNPPOC-TMG and evaluated (Figure S7). Both a soft rubbery film (PETMP/TCDDA) and a stiff glassy material (PETMP/DVS) yielded relatively narrow tan δ peaks indicating the formation of homogeneous polymer network during the photopolymerization process. Moreover, combining MNPPOC-TMG with UV sensitive radical initiators enables the formation of dual cure polymer networks, whereby, amines are generated for base-catalyzed polymerization under visible light, while radicals are generated for a second stage radical polymerization upon UV exposure. MNPPOC-TMG, combined with DMPA (a radical initiator with no absorption above 410 nm), was used as the initiating system in an offstoichiometric PETMP/TCDDA (functional group molar ratio equals 1:1.5) mixture. Upon 455 nm LED light irradiation, MNPPOC-TMG cleaved and initiated thiolMichael polymerization with the consumption of all thiol moieties and a stoichiometric amount of acrylate. Subsequently, at the end of this first stage polymerization, a 365 nm LED light was used to initiate DMPA homopolymerization of the excess acrylate (Figure 4a). The Tg values observed for the two distinct polymerization stages were 4 and 39 °C (Figure 4b). The use of two wavelengths for initiating dual stage polymerizations not only provides a relatively soft intermediate polymer for optimum handling but also introduces spatial and temporal control at both polymerization stages. Potential applications of such materials include shape memory materials, holographic materials, adhesives, etc.35,36 In conclusion, an efficient photobase generator MNPPOCTMG has been successfully synthesized for visible light initiated thiol-Michael polymerization. The properties of MNPPOC-TMG, i.e., visible light sensitivity, photolytic efficiency, and catalytic activity toward photo thiol-Michael polymerization, were evaluated and proved to be superior to that of the nonsubstituted NPPOC-TMG at UV conditions and the ITX sensitized photobase system (ITX/NPPOCTMG) at visible light conditions. In material synthesis, homogeneous polymer networks and, more interestingly, a sequential two-wavelength light controlled dual cure polymerization were achieved with MNNPOC-TMG initiation of the first stage. As a whole, this approach enables the thiol-Michael “click” reaction to be conducted with low-energy, long wavelength visible light irradiation and, thus, expands its
above 400 nm) triggered thiol-Michael addition polymerizations as depicted in Figure 3a. With 70 mW/cm2, 455 nm
Figure 3. Conversion as a function of time for stoichiometric PETMP/TCDDA mixtures (tetra-functional thiol and difunctional acrylate): (a) thiol and vinyl conversions versus time for a PETMP and TCDDA mixture initiated by 2 wt % MNPPOC-TMG, NPPOCTMG, and ITX/NPPOC-TMG and initiated with 70 mW/cm2, 455 nm LED light. (b) Thiol and acrylate conversions versus reaction time in PETMP/TCDDA cross-linking systems initiated with MNPPOCTMG and irradiated with 40 mW/cm2, 400−500 nm light. The control experiment was conducted under continuous irradiation whereas the dark cure sample was irradiated for only 0.5 min and then monitored after irradiation.
LED light irradiation, NPPOC-TMG absorbs little if any light and, as expected, did not cleave to initiate thiol-Michael polymerization with almost no monomer conversion within 15 min. Under the same conditions, the sensitizer ITX worked to transfer the photon energy to decompose NPPOC-TMG and initiate the base-catalyzed thiol-Michael polymerization. However, as ITX also generates a certain amount of radicals and induces an undesired acrylate homopolymerization, the conversion of the acrylate was 8−10% higher than the corresponding thiol conversion. In contrast, the MNPPOCTMG initiated thiol-Michael polymerization was completed in 5 min and provided an orthogonally cross-linked network with stoichiometric consumption of the thiol and acrylate moieties. Due to the generation of the relatively stable anion intermediates after photoinitiation, the thiol-Michael polymerization exhibits a “living” character, which enables it to proceed and reach higher substrate conversion even without continuous irradiation. To demonstrate this concept, a dark cure experiment regarding the MNPPOC-TMG initiated thiolMichael polymerization was performed (Figure 3b). Compared to the continuous irradiation, only 0.5 min exposure still 855
DOI: 10.1021/acsmacrolett.8b00435 ACS Macro Lett. 2018, 7, 852−857
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ACS Macro Letters ORCID
Christopher N. Bowman: 0000-0001-8458-7723 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the NIH (Grant 1U01DE02377701) and the Industry/University Cooperative Research Center for Fundamentals and Applications of Photopolymerizations for providing funding for this research.
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Figure 4. (a) Thiol and acrylate functional group conversion versus time for two-wavelength off-stoichiometric PETMP/TCDDA (thiol− acrylate molar ratio = 1:1.5) dual stage polymerizations. (b) tan δ and storage modulus plots for the first stage and second stage in the PETMP/TCDDA (thiol−acrylate molar ratio = 1:1.5) dual stage polymerization. The samples were composed of 2 wt % MNPPOCTMG and 1 wt % DMPA and cured with 60 mW/cm2, 455 nm LED light during the first thiol-Michael stage and 50 mW/cm2, 365 nm light during the subsequent acrylate radical polymerization stage.
application in biocompatible and otherwise UV sensitive materials.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00435. Supporting Data: NPPOC photolabile group photo cleavage mechanism, base generation tests with pH indicator phenol red, emission spectra of LED lights (λmax = 360, 405, 455 nm), NPPOC-TMG/NPPOCTMG photolysis kinetics (405 nm), photolytic efficiency analysis and calculation, MNPPOC-TMG/NPPOCTMG initiated thiol-Michael polymerization kinetics with UV induction (365 nm), DMA for MNPPOCTMG cured PETMP/TCDDA resin, DMA for MNPPOC-TMG cured PETMP/DVS resin; Experimental Section: materials, NPPOC-TMG and MNPPOCTMG synthesis, and characterization techniques (PDF)
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REFERENCES
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DOI: 10.1021/acsmacrolett.8b00435 ACS Macro Lett. 2018, 7, 852−857
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