Letter Cite This: ACS Macro Lett. 2019, 8, 616−622
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Far-Red Light-Induced Reversible Addition−Fragmentation Chain Transfer Polymerization Using a Man-Made Bacteriochlorin Hongliang Cao, Guicheng Wang, Yudong Xue, Guoliang Yang, Jia Tian, Feng Liu, and Weian Zhang* Shanghai Key Laboratory of Functional Materials Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
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ABSTRACT: To overcome the challenge of photoregulated living radical polymerization in long-wavelength radiation, a photoinduced electron transfer reversible addition−fragmentation chain transfer (PET-RAFT) polymerization in far-red wavelength (λmax = 740 nm) is reported by using a man-made bacteriochlorin as a photocatalyst. A reduced tetraphenylporphyrin (RTPP) having a natural bacteriochlorin macrocycle ring with two reduced pyrrole rings was synthesized with strong absorption in the far-red light region (700−765 nm) and applied for the PET-RAFT polymerization as a photoredox catalyst, which offered excellent control over molecular weight and polydispersities and oxygen tolerance for the polymerization of (methyl) acrylates monomers, and exhibited attractive features of “living” radical polymerization. Benefiting from high penetration of far-red light, the polymerization was also well-controlled when the reaction vessel was covered by translucent animal tissue barriers, for example, skin.
P
and inorganic semiconductor nanoparticles33 (e.g., TiO234,35 and ZnO36), can activate a controlled radical polymerization under a specific wavelength, for example, the blue and UV regions. The UV−vis spectra of photocatalyst located on the peak or near the peak are not essential,37 however, a high energy wavelength under 450 nm can be absorbed by organism, which causes safety concerns and considerably reduces the potential applications in biomedical applications. To overcome the limitations, chlorophyll a (Chl a),38 extracted from natural spinach leaves, was used as a photoredox catalyst to initiate RAFT polymerization by electron transfer between the catalyst and the RAFT agent at the red light region. Furthermore, photocatalysis based on a single molecule metal porphyrin (MTPP),39 such as zinc porphyrin (ZnTPP), can efficiently modulate the polymerization rates. Dependent on the absorption profile of the catalyst, tunable working wavelengths can be achieved over a wide range of visible light from blue to red radiation Further expanding the range of photoactivation wavelengths can reduce the energy consumption. More importantly, it can also improve photon penetration through soft tissues and gels when longer wavelengths are employed. Recently, Boyer et al. developed a PET-RAFT system that was well regulated by farred and near-infrared (NIR) radiation utilizing bacteriochlorophyll a as a biocatalyst.40 Bacteriochlorophyll a has strong
hotosynthesis is one of the most fundamental processes to life on Earth. In the last few decades, many efforts have been made to transfer the solar energy into chemical energy by mimicking the natural photosynthesis process.1 By mimicking the chloroplast pigments, a number of photocatalytic systems have been developed to perform the artificial photosynthesis.2−5 Among these strategies, photoactivation for the reversible addition−fragmentation chain transfer (RAFT) polymerization has been attracted wide attention because light is more environment-friendly and the polymerization is easily operated at ambient temperature.6−13 In addition, the unique ability switched between “ON” and “OFF” is triggered by light offer convenience in spatiotemporal control of polymerizations. There are mainly three ways to start the photoactivated RAFT polymerization,14 including direct,15,16 photoinitiatior-assistant,17 and photoredox catalytic photoinduced RAFT polymerizations. Inspired by enantioselective intermolecular α-alkylation of photoredox aldehyde compounds catalyzed by Ru(bpy)3Cl2 in organic chemistry,18−21 Boyer and coworkers demonstrated iridium and ruthenium complexes, such as fac-[Ir(ppy)3] and Ru(bpy)3Cl2, can be used as photoredox catalysts to reduce thiocarbonylthio compounds via photoinduced electron transfer (PET) and generate radicals (Pn•) under the radiation of a blue LED source.22−25 This approach extended the photoactivation of RAFT polymerization from UV to the visible light region, and PET-RAFT polymerization can be carried out by a wide range of monomers. Nowadays, most of the photoredox catalysts, including organic compounds (e.g., 1,10-phenylphenothiazine (PTH),26,27 eosin Y,28−30 fluorescein,31 and organic amines,32) © 2019 American Chemical Society
Received: April 30, 2019 Accepted: May 6, 2019 Published: May 8, 2019 616
DOI: 10.1021/acsmacrolett.9b00320 ACS Macro Lett. 2019, 8, 616−622
Letter
ACS Macro Letters absorption in the NIR region due to the reduction of two βdouble bonds located at para-pyrroles in the porphyrin macrocycle. However, rare sources of the photocatalyst from phototrophic bacteria in deep sea are limited for its widespread applications. Expanding light wavelengths to long wavelength regions of the solar spectrum maintains a big challenge. To date, except bacteriochlorophyll a, there is very few synthetic organic photocatalyst concerned in far-red (700−780 nm) and NIR-induced PET-RAFT polymerization. Inspired by the reductive structure of bacteriochlorophyll a, in this work, we reduce β-double bonds of two unsaturated para-pyrroles in porphyrin macrocycle of tetraphenylporphyrin (TPP) into a 4H-porphine (two reduced pyrrole rings, RTPP) with a bacteriochlorin macrocycle ring, whose Q-absorption wavelength range is adjusted to be 700−765 nm. The photocatalyst was developed by a chemical synthetic route and applied for the PET-RAFT polymerization regulated by far-red light (Scheme 1).
shows the calculated molecular weight (618.7717 g/mol [M + H]+) is consistent with its found one (618.2786 g/mol). In previous reports, Bruhn et al. used computed average local ionization energies (ALIEs) to show opposite low ionization energy surfaces in tautomer of chlorin structure and explained bacteriochlorin structure was the only product of an electrophilic attack.44 The absorption spectrum of RTPP (Figure 1, red line) exhibits strong absorption in the range of 400−550 nm and in
Scheme 1. Proposed Mechanism for PET-RAFT Polymerization (Top-Left) Mediated by a Photoredox Catalyst of RTPP (Top-Right) under 740 nm Far-Red Light Radiation; CDTPA and CDB Were Used as RAFT Agents and (Methyl)Acrylates Were Used as Model Monomers (Bottom)
Figure 1. Absorption spectra of TPP, CDTPA, and RTPP and a mixture of CDTPA/RTPP, showing strong absorption of RTPP in the far-red region (700−765 nm).
the far-red light region of 700−765 nm. Compared to the TPP (Figure 1, purple line), the Q-band of RTPP has a big red-shift. The reduction of TPP to RTPP reduced the energy space between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which resulted in a red-shift of the Q-band for RTPP in UV−vis absorption spectrum.45 The absorbance profile of RTPP shows an extremely enhanced molar extinction coefficient (ε = 7404 M−1 cm−1 at λmax = 740 nm, see Figure S10 and Table S1) and low fluorescence quantum yield (Q = 0.135, see Figure S9 and Table S1), which indicates the significantly enhanced absorption of the far-red light. In addition, in the work, we selected 4-cyano-4-(((dodecylthio) carbonothioyl)thio) pentanoic acid (CDTPA) as a RAFT agent owing to its high transfer constant (ktr) for widespread monomers.46,47 The molar absorption coefficient of CDTPA (Figure 1, black line) is negligible owing to deficient absorption by the trithiocarbonate groups in far-red regions. The mixture of RTPP and CDTPA showed no competitive absorption in the 700−765 nm (Figure 1, green line), indicating the RTPP photoredox catalyst was not quenched by CDTPA, which would benefit the photopolymerization by the high absorption efficiency in these wavelengths. PET-RAFT polymerization under 740 nm radiation: To study the photoactivation of RTPP for RAFT agent under farred radiation, methyl methacrylate (MMA) was selected as a model monomer, together with CDTPA as a RAFT agent and DMSO as the solvent. A far-red LED bulb (λmax = 740 nm wavelength, 66 mW cm−2) was employed as an irradiation source. The mole ratio of MMA/CDTPA/RTPP was set as 200:1:0.01 (50 ppm catalyst relative to monomer concentration) and the volume ratio of monomer to DMSO was 1:1. The PET-RAFT polymerization was carried out at room temperature. After 12 h of irradiation under 740 nm, 89% monomer conversion was detected by NMR with Mn,GPC =
Synthesis and characterization of photoredox RTPP: In previous studies, TPP could hardly regulate photopolymerization of methyl acrylates in green and red LED light, even at high catalyst concentrations, compared with its metal complexes, for example, ZnTPP.41 The outer sphere electron transfer between electron donor and acceptor under the light radiation restricts the reaction depending on collision frequency or the distance of the donor and acceptor.42 Instead, for the metal complex porphyrins, electron resonance from the donor and acceptor plays the major role,41 Interestingly, pheophorbide A (PheoA), as a nonmetal complex porphyrin could regulate the photopolymerization very well due to its strong absorption of the red-light radiation enhanced the electron transfer efficiency.43 PheoA has a strong absorption in red light region, and the electron transfer efficiency may be largely enhanced between the donor and acceptor under red light. Based on the consideration, we first synthesized the TPP (Scheme S1) and reduced it into a 4H-porphine (RTPP; Scheme S2). RTPP was characterized by NMR and mass spectrum, respectively. A new chemical shift (δ ≈ 3.8−4.0 ppm) in 1H NMR spectrum of RTPP (Figure S5) compared with that of TPP (Figure S4) can be attributed to 8H of two reduced pyrrole rings. Mass spectrum of RTPP (Figure S6) 617
DOI: 10.1021/acsmacrolett.9b00320 ACS Macro Lett. 2019, 8, 616−622
Letter
ACS Macro Letters
Table 1. Polymerization of Different Monomers and Concentration with CDTPA by PET-RAFT Using RTPP as Photoredox Catalyst under Far-Red LED Light (λmax = 740 nm) entry
[M]/[RAFT]/ [RTPP]
monomer
RAFT agent
[catalyst]/[M] (ppm)
time (h)
1 2 3e 4 5 6 7 8 9 10 11 12 13 14 15 16 17
200:1:0 200:0:0.01 200:1:0.01 200:1:0.01 200:1:0.005 200:1:0.002 200:1:0.01 200:1:0.005 200:1:0.002 200:1:0.01 200:1:0.002 200:1:0.01 200:1:0.002 200:1:0.01 200:1:0.01 200:1:0.002 200:1:0.01
MMA MMA MMA MMA MMA MMA MMA MMA MMA MMA MMA GMA GMA GMA BA BA BA
CDTPA
0 50 50 50 25 10 50 25 10 50 10 50 10 50 50 10 50
6 6 12 12 12 12 6 6 6 6 6 6 6 6 6 6 6
CDTPA CDTPA CDTPA CDTPA CDTPA CDTPA CDTPA CDB CDB CDTPA CDTPA CDB CDTPA CDTPA CDB
conv.a (%)
Mn,thb (g/mol)
Mn,GPC.c (g/mol)
Mw/Mnd (Đ)
185740
1.42
17800 16800 15000 12900 12000 10400 10280
17500 17300 13400 14600 12400 10300 9210
1.10 1.08 1.13 1.09 1.15 1.17 1.18
15170 10330 17330 8090 5520
12250 10050 16050 8020 10030
1.31 1.27 1.27 1.08 1.09
14 89 84 75 63 58 50 50 4 52 35 60 30 20 1
a Monomer conversion is calculated from 1H NMR. bMn,th is calculated by [M]0/[RAFT] × Mmonomer × conv. %. cMn,GPC is determined by gel permeation chromatography (GPC). dPolydispersities Đ = Mw/Mn is determined by GPC. eThe reaction is in dark.
17500 g/mol and a polydispersity (Đ = Mw/Mn) of 1.10, measured by gel permeation chromatography (GPC; entry 4, Table 1). The less deviation between experimental and theoretical molecular weight (Mn,th = 17800 g/mol) and low molecular weight polydispersity indicated the polymerization was well controlled. In addition, it did not affect the high monomer conversion and well-controlled polymerization when reducing the amount of RTPP to 25 ppm (84%) or 10 ppm (75%; entries 5 and 6, Table 1). The experimental molecular weights (Mn,GPC = 17300 and 13400 g/mol) are close to their theoretical values (M n,th = 16800 and 15000 g/mol, respectively) and polydispersities are also low (entries 5 and 6, Table 1). Our control experiments, for example, omission of any single component (RTPP, CDTPA, and light source), showed either uncontrolled polymerization or no polymer products (entries 1−3, Table 1). Without photocatalyst or light, the conversion of monomer is zero when the reaction time is over 6 h (entries 1 and 3, Table 1). In the case of absence of CDTPA, uncontrolled polymers (Mn = 185700, Đ = 1.42) were obtained when the reaction mixture was exposed to far-red light for 6 h to afford around 14% conversion (entry 2, Table 1). These results demonstrated both RTPP and CDTPA were essential to activate the controlled radical photopolymerization. To further verify the living characteristic of the polymerization process, the kinetic rate plots for the PET-RAFT polymerization of MMA were studied. The polymerization of MMA was carried out without induction period to proceed with linear first-order kinetics (Ln([M]0/[M]t) vs time) for the varied testing RTPP contents. The propagation apparent rate constants kpapp are 0.1934 h−1, 0.1607 h−1 and 0.1221 h−1 at different RTPP concentrations of 50, 25, and 10 ppm, respectively, indicating that propagation rate of polymerization is increased with the increase of RTPP concentrations due to stronger adsorption ability of RTPP under higher concentration (Figure 2A). The linear relationship for apparent propagation rate (kpapp) with the square root of [RTPP] is not very good like reported one, where Ir(ppy)3 was utilized as a photocatalyst,48 maybe because we used high power light
Figure 2. PET-RAFT polymerization for MMA and chain extending. (A) Polymerization kinetics corresponding to different photoredox catalyst concentrations (50, 25, and 10 ppm relative to monomer concentration) and 25 ppm TPP as a comparative photoredox catalyst showing no polymerization happening. (B) Dependence of molecular weight and polydispersity (Đ = Mw/Mn) on conversion. (C) GPC traces of PMMAs corresponding to different polymerization time (50 ppm). (D) “ON/OFF” monomer conversion corresponding to different time (50 ppm). (E) PMMA-b-PMMA and (F) PMMA-bPBA of diblock copolymers for GPC traces using PMMA as macroinitiator, [macroinitiator]/[monomer]/[RTPP] = 1:870:0.01 for 3 h (total polymerizations condition: 740 nm wavelength LED light, 66 mW cm−2, 20 °C; MMA (50% w/v) in DMSO utilizing RTPP as the photoredox catalyst). 618
DOI: 10.1021/acsmacrolett.9b00320 ACS Macro Lett. 2019, 8, 616−622
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ACS Macro Letters
narily by GPC and NMR spectrum, respectively. The BCPs were obtained by chain extension under 740 nm light using RTPP in DMSO with [monomer]:[macro-CTA]:[catalyst] = 870:1:0.01 (Table S2). Successful chain extensions were confirmed by the shift of the GPC trace to lower retention time. For PMMA-b-PMMA, Mn,GPC (41900 g/mol) is consistent with its Mn, th (43400 g/mol), and the low polymeric Đ (1.33) reveals the evidence of a living/controlled manner (Figure 2E). From the 1H NMR spectrum of PMMAb-PBA block copolymers, it can be seen that the characteristic methylene peaks (adjacent to the ester) for PBA block at δ 3.8−4.2 ppm (Figure S14), confirming the incorporation of BA into the BCP. Mn,GPC is increased from 14600 to 43500 g/mol with low polydispersity (Đ = 1.12) after 3 h radiation (Figure 2F). For both BCPs from GPC, a few low molecular polymers were observed that were attributed to the presence of macroCTA dead chains and were assumed to have some degradation of the RAFT end groups during the purifying process. Tissue penetration and oxygen tolerance of the PET-RAFT polymerization under 740 nm: Considering high penetration performances of biomass in far-red light, we carried out the PET-RAFT polymerization in a vessel shielded by translucent biotissue barriers, for example, skin of animal. When 1.2 and 1.7 mm thickness of chicken and pork skin were chosen at the RTPP concentration of 50 ppm, we still observed the linear increases of Ln([M]0/[M]t) versus time for both tissue (Figure 3A,B). The propagation apparent rate constants kpapp are
source, which would lead to less differences between 50 and 10 ppm catalyst concentration. In contrast, the control of TPP did not active the polymerization under 740 nm light illumination (Figure 2A, black line). It is well understood because there is no absorption at the wavelength of 740 nm for TPP to produce PET-RAFT polymerization process (Figure 1, purple line). Figure 2B showed the relationship of number-average molecular weights (Mn,GPC) and polydispersities (Đ) versus monomer conversion during photopolymerization with 50 ppm of RTPP. The Mn,GPC of the products were linearly increased with the monomer conversion and were in agreement with their theoretical values (Mn,th). Polydispersities (Đ) of all the samples were less than 1.3 and decreased with the increase of monomer conversion. Similar trends were observed in the reaction system with the RTPP concentration of 25 and 10 ppm (Figures S11 and S12). The change of Mn,GPC and Đ, as well as symmetrical GPC spectra (Figures 2C, S11, and S12), further verified the features of active polymerization. To explore photoswitchable behaviors, we used NMR spectrum to calculate the monomer conversion at different time points, and observed whether the polymerization could be restarted or terminated by “ON” or “OFF” of the light (Figure 2D). After 3 h of irradiation, the reaction mixture was removed from the light for 3 h. Repeating this process, the polymerization of MMA could be stopped easily by turning off the light and restarted steadily by turning on the light, without obvious induction period. The temporal control of PET-RAFT polymerization can be realized by regulating the intermittent light irradiation sequence. We then applied the PET-RAFT process to the polymerization of other monomers such as glycidyl methacrylate (GMA) and butyl acrylate (BA). As shown in Table 1 (entries 12, 13, 15, and 16), polymers with low molecular weight distributions were obtained for GMA and BA. The molar masses of Mn,GPC are consistent with the corresponding results calculated based on conversions, suggesting that the polymerizations are well controlled for both monomers. Then we used another typical dithiobenzoate RAFT agent of CDB to test the PET-RAFT polymerization for the tolerance of RTPP. The high monomer conversion and low Đ can be obtained for methacrylate monomers of MMA and GMA at 50 ppm of CDB concentration in 6 h (entries 10 and 14, Table 1), while low monomer conversion for 10 ppm of CDB concentration (entry 11, Table 1) or using acrylate monomer of BA (entry 17, Table 1). The results indicated the activation of trithiocarbonate was more efficient than dithiobenzoate at low catalyst concentration in our study. In addition, methacrylate monomers showed more active than acrylate monomers under the far-red radiation. PMMA, obtained via the PET−RAFT polymerization, was purified and analyzed by 1H NMR spectroscopy in CDCl3. Calculations of the molecular weights using NMR spectra were performed by comparing the characteristic signals of CDTPA (−CH2−) at δ ≈ 3.3 ppm and methyl methacrylate (−OCH3) at δ ≈ 3.5−3.7 ppm (Figure S13), which showed good agreement of the molecular weight between Mn,NMR (13300 g/ mol) and its theoretical value Mn,th (12500 g/mol). The purified PMMA homopolymer was utilized as a macro-RAFT agent in the chain-extension experiment with MMA or BA as the monomer. PMMA-b-PMMA and PMMA-b-PBA block copolymers (BCPs) were synthesized by similar PET-RAFT process and their structures were also characterized prelimi-
Figure 3. PET-RAFT for tissue penetration and oxygen tolerance. Kinetics plots of photopolymerization when the polymerization vessel was shielded by chicken skin with a thickness of 1.2 mm (A) and a pork skin with a thickness of 1.7 mm (B), respectively. (C) Conversion vs tissue thickness of pork skin, 740 nm LED light, 1 cm distance from vessel for 15 h. (D) Monomer conversion vs volume of reaction solution, no removal of air in a closed 2 mL vessel for 15 h. Reaction condition: [MMA]:[RAFT agent]:[RTPP] = 200:1:0.01, MMA (50% w/v) in DMSO, 740 nm LED light (66 mW cm−2).
0.1263 and 0.1168 h−1 for the skin of chicken and pork, respectively. Compared with nonscreened reaction, kpapp have a slight decrease. Both plots of Mn and the Đ against monomer conversion and symmetric shifts from low to high molecular weights determined by GPC revealed characteristics of a living radical polymerization during the screened polymerization (Figure S15). Furthermore, we increased the thickness of skin tissue to explore the boundaries of the far-red radiation. We observed the monomer conversion tended to decrease as 619
DOI: 10.1021/acsmacrolett.9b00320 ACS Macro Lett. 2019, 8, 616−622
Letter
ACS Macro Letters Author Contributions
increasing pork skin thickness (Figure 3C). Surprisingly, even the pork skin was up to around 7 mm, ∼50% conversion could also be retained, showing excellent tissue penetration ability of far-red light. Mn,GPC are also well agreement to their Mn,th with low Đ (Figure S16). Further increasing the thickness to 10 mm, the conversion was very low. The investigation of intensity and distribution of wavelength light source was performed by covering pork skin on the surface of the light source. Compare with Figure S8, the wavelength distribution was not changed (Figure S17B). However, the intensity was found to be decreased with the increase of the thickness and there was a plateau in the range of 1−7 mm (Figure S17A), which was very similar to the polymerization experiments (Figure 3C). The results indicated the conversion of monomers was closely related to the intensity of light radiation. To investigate the oxygen tolerance of the PET-RAFT polymerization, the reaction was carried out in DMSO with the presence of oxygen. The polymerization without prior deoxygenation can offer convenient for the complex polymeric manufacture,49,50 especially for small volume and high throughput synthesis in a 96-well plate.51−53 In this study, we found the monomer conversion was largely increased by adding more volume reaction solution in no removal of oxygen and closed 2 mL glass vial (Figure 3D). When the solution volume was less than 0.7 mL (35% occupation of vial), the conversion was negligible. If the occupation volume of reaction solution was increased to 0.9 mL (45% occupation of vial), the conversion was rapidly increased to 50%, and 98% conversion with well-controlled polymerization was obtained when the volume was more than 1 mL (Figure S18). In conclusion, we have demonstrated a successful PETRAFT polymerization in the far-red light region. A reduced tetraphenylporphyrin was synthesized and showed strong absorption in the range of 700−765 nm, which was employed for the activation of PET-RAFT polymerization as an efficient photocatalyst under far-red LED light (740 nm). Monomers, such as various (meth) acrylates, were successfully polymerized with controlled molecular weight and low polydispersities. In addition, far-red light has been utilized to penetrate thick skin tissue to perform the PET-RAFT polymerization for the first time. Such a photopolymerization approach via long-wavelength radiation with low energy provides great potential in biomaterial applications.
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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The support from the Shanghai Natural Science Foundation (18ZR1408300) is acknowledged. This work was also financially supported by the National Natural Science Foundation of China (Nos. 21574039 and 51173044).
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00320. Detailed experimental descriptions and characterizations and 18 figures, 3 schemes, and 2 tables (PDF)
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REFERENCES
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Jia Tian: 0000-0001-7391-4522 Weian Zhang: 0000-0002-1717-597X 620
DOI: 10.1021/acsmacrolett.9b00320 ACS Macro Lett. 2019, 8, 616−622
Letter
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DOI: 10.1021/acsmacrolett.9b00320 ACS Macro Lett. 2019, 8, 616−622
