Semiconductor Quantum Dots as Photocatalysts for Controlled Light-Mediated Radical Polymerization

Transcription

1 Supporting Information Semiconductor Quantum Dots as Photocatalysts for Controlled Light-Mediated Radical Polymerization Yiming Huang, Yifan Zhu, Eilaf Egap* Department of Materials Science and NanoEngineering, and Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, USA * Contents General Procedures....2 Preparation of the CdSe quantum dots (QD)...2 General procedure for photocatalyzed polymerization Synthesis of poly(methyl methacrylate) (PMMA)...3 Example of quantum yield calculation...7 Chain extension of macroinitiators....7 Quenching of CdSe QD fluorescence....9 Proposed mechanisms of QD catalyzed polymerization....9 Discussions on proposed mechanisms H-NMR spectra of block copolymers obtained from chain extension of a macroinitiator References...13 Page S1

2 General Procedures. All commercially available chemicals were purchased from Sigma-Aldrich or Alfa Aesar and were used as received unless otherwise specified. All reagents used in CdSe QD preparation were degassed and stored in a glovebox prior to use. Trioctylphosphine selenide (TOPSe) and cadmium oleate precursor stock solutions were prepared according to reported procedures. 1 Commercial monomers were purified by filtering through silica and neutral alumina plugs immediately prior to polymerization. 1 H-NMR spectra were obtained on a Varian VNMR 400 or Inova 400 spectrometers operated at 400 MHz at room temperature. Gel permeation chromatography (GPC) was carried out on an Agilent 1260 Infinity LC system and calibrated with polystyrene standards in THF. Absorption and photoluminescence spectra were measured with Agilent Cary-60 UV-Vis spectrometer and Cary Eclipse fluorometer, respectively. Preparation of the CdSe quantum dots (QD). In a glovebox, selenium pellets (0.790 g, 10.0 mmol) were combined with 10 ml of tri-n-octylphosphine (20 mmol) and was stirred and heated to 100 C until the pellets were fully dissolved. In a separate procedure, precursor solution (0.2 M) was prepared according to previous reports. 1 A mixture of cadmium oxide (0.318 g, 25.0 mmol), oleic acid (3.45 ml, 100 mmol), and 9 ml 1-octadecene was heated to 220 C under a nitrogen atmosphere until the solution turned clear and colorless, in approximately one hour. The product was a white solid at room temperature and needs to be heated to 60 C before use. CdSe QD was synthesized using an adapted hot injection method as previously reported. 1,2 In a 100 ml 3- neck flask, 10 ml of ODE was degassed for 30 minutes by evacuating the flask to below 20 mtorr. Then, 15 ml 0.2 M cadmium oleate was added under nitrogen and the mixture was heated to 270 C. Once the solution reached the set temperature, 1.5 ml 1 M TOPSe solution was injected and the temperature was set to 220 C. The reaction was allowed to grow for six minutes and quenched by cooling the flask, first with cool air followed by a water bath. The QD was washed with methanol and hexanes, and the resulting QD product in hexanes was stored in a glove box. Page S2

3 Figure S1. Normalized absorption (black) and fluorescence emission (red) spectra of CdSe QD photocatalyst in THF. The emission spectrum was recorded with 400 nm excitation. General procedure for photocatalyzed polymerization Synthesis of poly(methyl methacrylate) (PMMA). To a mixture of MMA (0.50 ml, 4.7 mmol, 100 eq), DIPEA (40 µl, 0.24 mmol), EBP (8.2 µl, mmol, 1 eq), and 0.50 ml anhydrous solvent, 0.10 ml QD stock solution (0.6 mol% with respect to MMA) was added. The mixture was deaerated by three freeze-pump-thaw cycles, backfilled with argon, and stirred at ambient temperature under irradiation of a 10 W household 480 nm blue LED lamp for 8 h. The reaction vial was placed approx. 15 cm from the lamp, where the light intensity was 10 mw/cm 2, unless otherwise specified. The whole setup was covered with aluminum foil to block exposure any other light sources. (In control experiments when the reaction was conducted in the dark, the reaction vial was completely wrapped with aluminum foil to block exposure to any light source from entering the vial.) To determine the MMA conversion by 1 H NMR, 2-3 drops of the polymerization mixture was dissolved in 0.7 ml CDCl 3 and used as such for 1 H NMR analysis. The rest of the polymerization mixture was diluted with 0.5 ml DCM, precipitated in MeOH and filtered to give the crude PMMA as a pale red solid (260 mg, 77.0% conversion on 1 H NMR, 55.6% isolated yield): M n = 41.5 kda, Đ = Purification of crude PMMA consisted of re-dissolved the polymers in minimal acetone and the mixture was passed through a 0.2-micron syringe filter to remove the insoluble red QD residue. The PMMA solution was then added dropwise into cold MeOH to recover the purified PMMA as a white solid. A separate polymerization under natural sunlight was carried out on the top level of Peavine parking deck at Emory (29 Eagle Row, Atlanta, GA 30322) on May 27, 2017, from 10 am to 6 pm (partly cloudy, C). The reaction mixture was prepared as described above (without stirring), sealed with an airtight septum and brought to the top of the building. Synthesis of poly(butyl methacrylate) (PBMA) According to the general procedure, a mixture of BMA (0.74 ml, 4.7 mmol), DIPEA (0.80 ml, 4.7 mmol), EBP (8.2 µl, mmol), and 0.10 ml QD stock solution generated PBMA as a colorless sticky oil (363 mg, 82.6% conversion, 54.8% isolated yield): M n = 47.7 kda, Đ = Synthesis of poly(2-ethoxyethyl methacrylate) (PEEMA) According to the general procedure, a mixture of EEMA (0.77 ml, 4.7 mmol), DIPEA (0.80 ml, 4.7 mmol), EBP (8.2 µl, mmol), and 0.10 ml QD stock solution generated PEEMA as a colorless sticky oil (652 mg, 99.0% conversion, 74.2% isolated yield): M n = 36.1 kda, Đ = Page S3

4 Synthesis of poly(butyl acrylate) (PBA) According to the general procedure, a mixture of BA (0.74 ml, 4.7 mmol), DIPEA (0.80 ml, 4.7 mmol), EBP (16.4 µl, mmol), and 0.10 ml QD stock solution generated PBA as a colorless oil (80.1% conversion): M n = 4.30 kda, Đ = Synthesis of poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) According to the general procedure, a mixture of TFEMA (0.67 ml, 4.7 mmol), DIPEA (0.16 ml, 0.94 mmol), EBP (8.2 µl, mmol), and 0.10 ml QD stock solution generated PTFEMA as a pale red solid (76.0% conversion): M n = 30.3 kda, Đ = Synthesis of poly(2,2,3,3,3-pentafluoropropyl acrylate) (PPFPA) According to the general procedure, a mixture of PFPA (0.36 ml, 2.3 mmol), DIPEA (0.08 ml, 0.47 mmol), EBP (4.1 µl, mmol), and 0.10 ml QD stock solution generated PPFPA as a pale red solid (30.5% conversion): M n = 30.3 kda, Đ = Synthesis of polystyrene (PS) According to the general procedure, a mixture of styrene (0.54 ml, 4.7 mmol), DIPEA (1.6 ml, 9.4 mmol), EBP (8.2 µl, mmol), and 0.10 ml QD stock solution generated PS as a pale red solid (10.0% isolated): M n = 6.28 kda, Đ = In a separate experiment, a mixture of styrene (0.54 ml, 4.7 mmol), DIPEA (0.16 ml, 9.4 mmol), EBP (8.2 µl, mmol), and 0.10 ml QD stock solution generated PS as a pale red solid (35.7% conversion, 20.6% isolated yield): M n = 9.69 kda, Đ = Page S4

5 Table S1. Photo-mediated polymerizations of MMA. entry [M]:[I]:[DIPEA] # Solvent Time (h) conv (%) Theoretical M n (kda) Experi- mental M n (kda) Đ ϕ TON Notes 1 [1]:[0.01]:[2] THF * [1]:[0.005]:[2] THF * [1]:[0.002]:[2] THF * [1]:[0.01]:[0.05] THF * [1]:[0.01]:[0.05] DMF [1]:[0.01]:[0] DMF QD replaced by perylene 3 7 [1]:[0.01]:[0.05] DMF QD replaced by fluorescein 4 8 [1]:[0.01]:[2] neat * [1]:[0.01]:[1] neat * [1]:[0.01]:[1] C 6H [1]:[0.01]:[0.1] C 6H [1]:[0.01]:[0.05] C 6H [1]:[0.01]:[0.05] C 6H [1]:[0.01]:[0] C 6H QD replaced by perylene 3 15 [1]:[0.01]:[0.05] C 6H QD replaced by fluorescein 4 16 [1]:[0.01]:[0.05] C 6H EBP replaced by DBMM 17 [1]:[0.01]:[0.05] C 6H EBP replaced by EBiB 18 [1]:[0.01]:[0.05] C 6H Sunlight 19 [1]:[0.01]:[0.05] C 6H Blue LED, 10 mw/cm 2 20 [1]:[0.01]:[0.05] C 6H Blue LED, 5.6 mw/cm 2 21 [1]:[0.01]:[0.05] C 6H Blue LED, 2.5 mw/cm 2 22 [1]:[0.01]:[0.05] C 6H Not isolated Green 530 nm LED, 10 mw/cm 2 23 [1]:[0.01]:[2] THF 16 No QD or light 24 [1]:[0.01]:[0.05] C 6H 6 16 No QD or light 25 [1]:[0.01]:[0] THF 16 No DIPEA 26 [1]:[0.01]:[0] neat 16 No DIPEA 27 [1]:[0]:[0.05] C 6H No initiator 28 [1]:[0.01]:[2] THF * Not deaerated Unless otherwise noted, reactions were carried out in the deaerated environment at an ambient temperature and were irradiated with a 10 W household blue (480 nm) LED lamp (10 mw/cm 2 ). # Molar ratio of [monomer]:[initiator]:dipea. Monomer conversion determined by 1 H- NMR spectrum of the reaction liquor. *Yield of the isolated polymer product. Number-average (M n ) and weight-average (M w ) molecular weights were determined by gel permeation chromatography (GPC). Quantum yield (ϕ) and turnover number (TON) were calculated as described in literature. 2 Polydispersity index defined as Đ = M w / M n. DBMM: diethyl 2-bromo-2-methylmalonate; EBiB: ethyl α- bromoisobutyrate. Page S5

6 Table S2. Photo-mediated polymerizations of other monomers. entry Monomer [M]:[I]:[DIPEA] # Solvent Time (h) conv (%) Theoretical M n (kda) Experimental M n (kda) Đ ϕ TON Notes 1 BMA [1]:[0.01]:[0.05] C 6H BMA [1]:[0.01]:[2] THF * BMA [1]:[0.01]:[1] neat BA [1]:[0.02]:[0.05] C 6H BA [1]:[0.02]:[1] neat BA [1]:[0.02]:[0.2] neat EEMA [1]:[0.02]:[0.05] C 6H EEMA [1]:[0.01]:[2] THF * EEMA [1]:[0.01]:[1] neat TFEMA [1]:[0.02]:[0.05] C 6H TFEMA [1]:[0.02]:[0.05] C 6H TFEMA [1]:[0.01]:[2] THF * TFEMA [1]:[0.01]:[0.2] neat PFPA [1]:[0.01]:[0.05] C 6H PFPA [1]:[0.01]:[0.2] neat Styrene [1]:[0.01]:[2] neat * Styrene [1]:[0.01]:[0.2] neat EEMA [1]:[0.01]:[1] neat 16 No QD 19 EEMA [1]:[0]:[1] neat 16 No initiator Reactions were carried out in the deaerated environment at an ambient temperature and were irradiated with a 10 W household blue (480 nm) LED lamp (10 mw/cm 2 ). # Molar ratio of [monomer]:[initiator]:dipea. Monomer conversion determined by 1 H-NMR spectrum of the reaction liquor. *Yield of the isolated polymer product. Number-average (M n ) and weight-average (M w ) molecular weights were determined by gel permeation chromatography (GPC). Quantum yield (ϕ) and turnover number (TON) were calculated as described in literature. 2 Polydispersity index defined as Đ = M w / M n. Table S3. Change of temperature during the polymerization of MMA. Time (h) Ambient ( C) Reaction ( C) Page S6

7 Example of quantum yield calculation The quantum yield can be determined by the molar ratio of monomer consumed to photon absorbed. 2 Moles of monomer consumed can be obtained with initial moles of monomer multiplied by conversion (Table S1, entry 13): MMA 4.7 mmol mmol Photons absorbed can be calculated with light intensity (I), the surface area of the reaction vessel (S), duration of irradiation (t), photon energy at the wavelength of 450 nm (E 450 ) and Avogadro constant (N A ): 10 mw cm 6.3 cm s J mol 6.9 mmol Therefore, the quantum yield (ϕ) of this reaction is: 100% 3.6 mmol 6.9 mmol 100% 52% Chain extension of macroinitiators. Figure S2. (a) Molecular structure of PMMA and PS macroinitiators, PMMA-b-PTFEMA, PS-b-PMMA and PSb-PTFEMA block copolymers, and their molecular weight (M n ) and polydispersity, (b) the corresponding GPC traces of macroinitiators (black) and the block copolymers (red). General procedure Synthesis of PS-b-PMMA copolymer. A mixture of aforementioned polystyrene (20 mg, M n = 6.28 kda), MMA (0.10 ml, 0.94 mmol), DIPEA (0.32 ml, 1.9 mmol), 20 µl QD stock solution and 0.10 ml THF was deaerated by three freeze-pump-thaw cycles, backfilled with argon, and stirred at ambient temperature under irradiation of a 10 W household blue LED Page S7

8 floodlight for 24 h. The reaction mixture was dissolved in minimal DCM, precipitated in MeOH and filtered to give the PS-b-PMMA copolymer as a pale red solid: M n = 11.0 kda, Đ = Synthesis of PS-b-PBMA from isolated PS macroinitiator According to the general procedure, a mixture of polystyrene (10 mg, M n = 6.28 kda), BMA (0.15 ml, 0.94 mmol), DIPEA (0.32 ml, 1.88 mmol), 20 µl QD stock solution and 0.10 ml THF was allowed to react for 24 h and generated PS-b-PBMA copolymer as a pale red solid: M n = 16.1 kda, Đ = Synthesis of PS-b-PTFEMA from isolated PS macroinitiator According to the general procedure, a mixture of polystyrene (10 mg, M n = 6.28 kda), TFEMA (0.13 ml, 0.94 mmol), DIPEA (0.32 ml, 1.88 mmol), 20 µl QD stock solution and 0.10 ml THF was allowed to react for 24 h and generated PS-b-PTFEMA copolymer as a pale red solid: M n = 9.40 kda, Đ = Synthesis of PMMA-b-PTFEMA from isolated PMMA macroinitiator According to the general procedure, a mixture of PMMA (10 mg, M n = 17.1 kda), 0.14 ml TFEMA, 40 µl DIPEA, 40 µl QD stock solution and 0.10 ml THF was allowed to react for 24 h and generated PMMA-b- PTFEMA copolymer as a pale red solid: M n = 34.0 kda, Đ = Synthesis of PTFEMA-b-PMMA from isolated PTFEMA macroinitiator According to the general procedure, a mixture of PTFEMA (10 mg, M n = 8.70 kda), 0.14 ml MMA, 40 µl DIPEA, 40 µl QD stock solution and 0.10 ml THF was allowed to react for 24 h and generated PTFEMA-b- PMMA copolymer as a pale red solid: M n = 9.40 kda, Đ = Table S4. Results of the block copolymers obtained from macroinitiator. entry M n (Đ) Molar ratio Copolymer M n (Đ) M n calcd. Copolymer [block 2]:[block 1] [block 1]-b-[block 2] Macroinitiator from NMR from GPC from NMR 1 PS-b-PMMA (1.64) (2.41) PS-b-PBMA (1.64) (1.70) PS-b-PTFEMA (1.64) (1.31) PTFEMA-b-PMMA (1.49) (1.42) PMMA-b-PTFEMA (1.65) (2.28) Page S8

9 Quenching of CdSe QD fluorescence. Figure S3. (a) Enlarged view of Stern-Volmer plot to show the weak quenching of CdSe QD fluorescence by DIPEA (blue triangles). (b) Digital image of QD fluorescence (under a 365 nm UV lamp) when QD is separately mixed with only EBP (left) and only DIPEA (right), in identical ratio as in general reaction procedures. Proposed mechanisms of QD catalyzed polymerization. Figure S4. Two possible mechanisms for the photoredox catalysis involving oxidative quenching (left, identical to Figure 4b) and reductive quenching (right) pathways of QD*. The oxidative pathway was supported by fluorescence quenching experiments. Figure S5. Redox potentials (vs SCE) of CdSe QD 2, MMA 5, EBP 6, and DIPEA (NR 3 ) 7. Page S9

10 Discussions on proposed mechanisms To further probe the mechanism and redox processes in this living radical polymerization, we considered two possible and similar mechanisms and examined the role of the DIPEA (Figure S4). Both mechanisms start with the photoexcitation of the QD to generate the excited state QD* which subsequently reduce the dormant alkyl bromide chain to a radical, and a reversible termination of the propagating radical chain. A major difference between the two proposed mechanisms is the quenching pathway of QD*. The QD* can play a role as a strong oxidant or a reductant. In the oxidative pathway (Figure 4b), a single electron transfer (SET) takes place as QD* donates an electron to the alkyl bromide and forms a radical cation (QD + ). On the other hand, QD* can also accept an electron from the trialkylamine (NR 3 ) and become a QD radical anion (QD ) in the reductive pathway (Figure S4). 8 Through SET process with QD*, the alkyl bromide (P n Br) or the trialkylamine (NR 3 ) is a potential QD* fluorescence quencher in each pathway. While most ATRP photoredox catalysts proceed through an oxidative pathway without an electron donor, 3,6,9 amines often act as an important sacrificial electron donor in catalytic photoredox reactions. 10 However, in this polymerization method, DIPEA as little as 0.05 eq of MMA proved to be sufficient to produce conversion of > 90%. To identify the dominant SET pathway in this polymerization, we used the Stern-Volmer relationship to evaluate the change in the QD* photoluminescence (PL) at various concentrations of EBP or DIPEA (Figure 4a). The Stern-Volmer plot shows a linear relationship in the ratio of I 0 /I PL intensity (I 0 = PL intensity in the absence of quencher, and I = PL intensity with a given concentration of a quencher), as an increase in EBP concentration, indicative of CdSe fluorescence quenching by EBP. In contrast, DIPEA was far less effective in quenching CdSe fluorescence (Figure S3), contrary to previous reports that showed strong quenching effect by trialkylamine. 2 These results and the redox potentials (Figure S5) suggest that the excited state of CdSe is more likely undergoing a redox process with EBP and not with the DIPEA, and therefore an oxidative quenching pathway could be the major contribution in this radical polymerization. We note that the polymerization proceeds in the absence of the EBP as previously reported. 5,11 In that case, there are other possible mechanisms, either a ligand exchange with amines facilitates radical generation on the QD surface, followed by direct radical transfer to MMA monomer as suggested previously, 5 or DIPEA radical cation, formed as result of electron transfer to QD, initiates the radical polymerization. We think the later mechanism is unlikely based on fluorescence studies (Figures 4a and S3) and redox potentials (Figure S5). In both cases, the polymerization is better classified as photo-initiated rather than photo-controlled polymerization. Page S10

11 1 H-NMR spectra of block copolymers obtained from chain extension of a macroinitiator. Figure S6. H-NMR spectrum of PS-b-PMMA block copolymer obtained from a PS macroinitiator. Figure S7. H-NMR spectrum of PS-b-PBMA block copolymer obtained from a PS macroinitiator. Page S11

12 Figure S8. H-NMR spectrum of PS-b-PTFEMA block copolymer obtained from a PS macroinitiator. Figure S9. H-NMR spectrum of PTFEMA-b-PMMA block copolymer obtained from a PTFEMA macroinitiator. Page S12

13 Figure S10. H-NMR spectrum of PMMA-b-PTFEMA block copolymer obtained from a PMMA macroinitiator. References (1) Yu, W. W.; Peng, X. Angew. Chemie Int. Ed. 2002, 41 (13), (2) Caputo, J. A.; Frenette, L. C.; Zhao, N.; Sowers, K. L.; Krauss, T. D.; Weix, D. J. J. Am. Chem. Soc. 2017, 139 (12), (3) Miyake, G. M.; Theriot, J. C. Macromolecules 2014, 47 (23), (4) Liu, X.; Zhang, L.; Cheng, Z.; Zhu, X. Polym. Chem. 2016, 7 (3), 689. (5) Strandwitz, N. C.; Khan, A.; Boettcher, S. W.; Mikhailovsky, A. A.; Hawker, C. J.; Nguyen, T.-Q.; Stucky, G. D. J. Am. Chem. Soc. 2008, 130 (26), (6) Theriot, J. C.; Lim, C.-H.; Yang, H.; Ryan, M. D.; Musgrave, C. B.; Miyake, G. M. Science (80-. ). 2016, 352 (6289), (7) Srivastava, V.; Singh, P. P. RSC Adv. 2017, 7 (50), (8) Lim, C.-H.; Ryan, M. D.; McCarthy, B. G.; Theriot, J. C.; Sartor, S. M.; Damrauer, N. H.; Musgrave, C. B.; Miyake, G. M. J. Am. Chem. Soc. 2017, 139 (1), 348. (9) Treat, N. J.; Sprafke, H.; Kramer, J. W.; Clark, P. G.; Barton, B. E.; Read de Alaniz, J.; Fors, B. P.; Hawker, C. J. J. Am. Chem. Soc. 2014, 136 (45), (10) Zhang, G.; Song, I. Y.; Ahn, K. H.; Park, T.; Choi, W. Macromolecules 2011, 44 (19), (11) Barichard, A.; Galstian, T.; Israëli, Y. Phys. Chem. Chem. Phys. 2012, 14 (22), Page S13