Organoboron-Based Photochromic Copolymers for Erasable Writing

Article pubs.acs.org/Macromolecules

Organoboron-Based Photochromic Copolymers for Erasable Writing and Patterning Junwei Wang,† Bixin Jin,‡ Nan Wang,*,† Tai Peng,† Xiaoyu Li,*,‡ Yunjun Luo,‡ and Suning Wang*,†,§ †

Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, and ‡School of Materials Science, Beijing Institute of Technology, Beijing 100081, P. R. China § Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada S Supporting Information *

ABSTRACT: We report herein the first examples of organoboron-based photochromic polymers. The synthesis of a series of blue fluorescent random copolymers bearing photochromic boron repeating units, poly[(6-B(ppy)Mes2)oxyhexyl methacrylate)m-r-(tert-butyl methacrylate)n], ppy = 2-phenylpyridyl and Mes = mesityl, via atom transfer free radical polymerization (ATRP) has been accomplished. These new polymers display thermally reversible photochromism, switching color from colorless to deep blue, and fluorescence from bright sky blue to deep blue. By controlling the monomer ratio, the photoisomerization quantum efficiencies of the polymers can be tuned effectively. In addition, the number of boron units in the polymer has been found to have a significant impact on fluorescence quenching efficiency. The new organoboron-based polymers can be used effectively as a switchable/erasable ink on glass or paper substrate or for creating switchable/erasable patterns as neat polymer films.

■

diarylethene-based photochromic polymers.24,25 Organoboronfunctionalized macromolecules and polymers have been demonstrated by Jäkle,26−31 Wagner,32−34 Chujo,35−38 Manners,39 and Liu40,41 et al. to have unique photophysical and electronic properties, which led to versatile applications in optoelectronic and sensory devices. However, to the best of our knowledge, photochromic polymers based on photochromic boron chromophores remain unknown. Combining photochromic organoboron functional groups with copolymers allows not only accessing highly processable polymer materials but also fine-tuning/modulating the photochromic and luminescent properties of the materials through controlling the ratio of different monomer units. With this in mind, we initiated the investigation on photochromic organoboroncontaining random copolymers. The system we designed is depicted in Scheme 1, in which the photochromic B(ppy)Mes2 unit is attached to the polymer backbone as a pendant group via an alkyl linker. This design is based on the consideration of minimizing π-conjugation between the boron units because it is known that extended π-conjugation along the ppy chelate unit can greatly diminish photoisomerization quantum efficiency of the boron unit.42−44 The new blue fluorescent polymers were prepared via atom transfer free radical polymerization (ATRP). We found that the

INTRODUCTION During the past decades, the design and synthesis of photoswitchable organic compounds have attracted tremendous research interest due to their potential applications in optical data storage, imaging, pharmaceuticals, and photoswitching devices.1−8 Many photochromic molecules have been extensively studied, including derivatives of azobenzene,1,9,10 fulgides,11 spiropyran,12−14 diarylethenes, etc.6,15,16 Organoboron-based photochromic systems were discovered recently and demonstrated to display versatile and highly tunable properties.17 A typical photochromic boron compound is B(ppy)Mes2 that has an N,C-chelating ligand (ppy = 2phenylpyridyl) and two bulky aryl substituents such as mesityl (Mes),17 which undergoes a highly efficient and thermally reversible photoisomerization, changing color from colorless to dark blue upon irradiation of UV light. The color of the dark isomer in the photochromic boron system was found to be highly tunable by modifying either the chelate ligand or the aryl substituents on boron.18−23 Furthermore, photochromism of boron-based compounds persists when doped into polymer matrices such as poly(methyl methacrylate) (PMMA) or polystyrene, demonstrating the feasibility of boron-based photochromism in solid state.17 Compared with small molecule counterparts, polymeric materials are much more desired for practical applications due to their flexibility, lightweight, and good processability. During the past decade, significant progress has been achieved on photochromic polymeric systems, such as spiropyran- and © 2017 American Chemical Society

Received: March 25, 2017 Revised: May 22, 2017 Published: June 5, 2017 4629

DOI: 10.1021/acs.macromol.7b00632 Macromolecules 2017, 50, 4629−4638

Article

Macromolecules

Synthesis of 2-Bromo-5-(hex-5-enyloxy)pyridine, 1. A roundbottom flask was charged with 6-bromopyridin-3-ol (3.0 g, 17.2 mmol), 6-bromohex-1-ene (5.6 g, 34.4 mmol), K2CO3 (14.2 g, 103.2 mmol), and acetone (50 mL). The reaction mixture was stirred at 65 °C for 12 h. Then the insoluble inorganic salt was removed by filtration, and the filtrate was concentrated under vacuum. The residue was purified by column chromatography on silica gel (ethyl acetate/ petroleum, 1:10) to give compound 1 as a pale yellow liquid (4.2 g, yield 96%). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.05 (d, J = 3.1 Hz, 1H), 7.36 (d, J = 8.7 Hz, 1H), 7.09 (dd, J = 8.7, 3.1 Hz, 1H), 5.82 (ddd, J = 13.3, 10.2, 5.1 Hz, 1H), 5.17−4.90 (m, 2H), 3.99 (t, J = 6.4 Hz, 2H), 2.25−2.07 (m, 2H), 1.90−1.74 (m, 2H), 1.65−1.43 (m, 2H). 13 C NMR (100 MHz, CDCl3, δ, ppm): 154.96, 138.24, 137.45, 131.90, 128.09, 124.83, 114.99, 68.61, 33.32, 28.47, 25.13. HRMS (ESI) m/z: [M + H]+ calcd for C11H15BrNO, 256.0259; found 256.0332. Synthesis of 2-(2-Bromophenyl)-5-(hex-5-enyloxy)pyridine, 2. 2-Bromo-5-(hex-5-enyloxy)pyridine (5.0 g, 19.5 mmol), 2bromophenylboronic acid (4.3 g, 21.4 mmol), K2CO3 (2 M, 50 mL), Pd(PPh3)4 (0.45 g, 0.39 mmol), and THF (100 mL) were added to a three-neck round-bottom flask equipped with reflux condenser pipe under nitrogen. The reaction mixture was stirred at 65 °C overnight, cooled to room temperature, and then concentrated in vacuo. The residue was partitioned between water and dichloromethane, and the aqueous layer was separated and extracted with dichloromethane (3 × 20 mL). The combined organic layers were dried over MgSO4, concentrated, and purified on silica gel (ethyl acetate/petroleum, 1:15) to give compound 2 as pale yellow liquid (5.0 g, yield 76%). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.40 (d, J = 2.8 Hz, 1H), 7.67 (dd, J = 8.0, 1.0 Hz, 1H), 7.61−7.49 (m, 2H), 7.39 (td, J = 7.5, 1.2 Hz, 1H), 7.32−7.17 (m, 2H), 5.86 (ddt, J = 16.9, 10.2, 6.6 Hz, 1H), 5.19−4.95 (m, 2H), 4.08 (t, J = 6.4 Hz, 2H), 2.23−2.09 (m, 2H), 1.93−1.80 (m, 2H), 1.68−1.56 (m, 2H). 13C NMR (100 MHz, CDCl3, δ, ppm): 154.36, 150.43, 140.90, 138.36, 137.23, 133.25, 131.50, 129.35, 127.51, 125.02, 122.01, 120.83, 114.95, 68.27, 33.39, 28.63, 25.25. HRMS (ESI) m/z: [M + H]+ calcd for C17H19BrNO, 332.0572; found 332.0645. Synthesis of Compound 3. n-BuLi (8.0 mL, 1.6 M) was added slowly under inert gas to a solution of compound 2 (3.88 g, 11.6 mmol) in THF (140 mL) at −78 °C, and the resulting solution was stirred for about 1 h at this temperature. Then the solution of BMes2F (3.44 g, 12.8 mmol) in THF (60 mL) was added under a stream of nitrogen by injection syringe; the solution was stirred at −78 °C for 1 h, then warmed up to room temperature, and stirred overnight. Solvent was removed under reduced pressure and then further purified by chromatography (methylene chloride/petroleum = 1/10) to give a bright yellow powder (4.4 g, yield 75%). 1H NMR (400 MHz, CD2Cl2, δ, ppm): 8.28 (d, J = 2.5 Hz, 1H), 7.94 (d, J = 8.9 Hz, 1H), 7.88−7.77 (m, 1H), 7.76−7.67 (m, 1H), 7.62 (dd, J = 8.9, 2.6 Hz, 1H), 7.35− 7.22 (m, 2H), 6.68 (s, 4H), 5.97−5.75 (m, 1H), 5.13−4.89 (m, 2H), 4.00 (t, J = 6.5 Hz, 2H), 2.20 (s, 6H), 2.14 (dd, J = 14.2, 7.1 Hz, 2H), 2.02−1.67 (m, 14H), 1.62−1.51 (m, 3H). 13C NMR (100 MHz, CD2Cl2, δ, ppm): 154.25, 152.49, 138.39, 135.02, 133.72, 132.98, 130.54, 129.74, 127.67, 125.08, 120.78, 118.16, 114.64, 69.10, 33.31, 28.29, 25.03, 20.39. 11B NMR (225 MHz, CD2Cl2, δ): 4.17 ppm. HRMS (ESI) m/z: [M + H]+ calcd for C35H41BNO, 501.3239; found 501.312. Synthesis of Compound 4. To a solution of compound 3 (2.7 g, 5.4 mmol) in dry THF (40 mL) was added slowly BH3·THF (18 mL, 1 M) by a syringe under N2 at 0 °C, and the solution was stirred for 24 h at 30 °C. After cooling to 0 °C, 2 N aqueous NaOH (8 mL, 16 mmol) and 30% H2O2 (9 mL, 79.4 mmol) were added at 0 °C. The solution was stirred at 30 °C for 24 h, then poured into water, and extracted with ethyl acetate. The organic layer was separated and dried over Na2SO4. Purification of the crude product by column chromatography (ethyl acetate/petroleum = 1/2) afforded a light yellow solid (2.4 g, yield 86%). 1H NMR (700 MHz, CD2Cl2, δ, ppm): 8.28 (d, J = 2.5 Hz, 1H), 7.94 (d, J = 8.9 Hz, 1H), 7.86−7.77 (m, 1H), 7.75−7.66 (m, 1H), 7.62 (dd, J = 8.9, 2.6 Hz, 1H), 7.36−7.16 (m, 2H), 6.68 (s, 4H), 4.05−3.92 (m, 2H), 3.64 (t, J = 6.5 Hz, 2H), 2.21 (s, 6H), 1.70 (m, 16H), 1.53−1.46 (m, 2H), 1.46−1.41 (m, 2H), 1.39

Scheme 1. New Photochromic Random Copolymers with B(ppy)Mes2 as the Pendant Unit

copolymers show a thermally reversible photoisomerization in solution and the solid state. We have established that by finely adjusting the monomer ratio (m/n), the photoisomerization quantum efficiencies of polymers can indeed be tuned effectively. Furthermore, the efficiency of fluorescence quenching accompanied by photoisomerization was also found to greatly depend on the monomer ratio. Finally, the polymer materials have been shown to be effective as invisible ink for erasable writing on glass or paper substrates. The details are presented herein.

■

EXPERIMENTAL SECTION

All solvents were freshly distilled over sodium metal with benzophenone as the indicator and stored under nitrogen prior to use. All starting materials were purchased from Energy Chemical. Compounds tert-butyl methacrylate (99%, tBMA), N,N,N′,N″,N″pentamethyldiethylenetriamine (99%, PMDETA), and the initiator methyl 2-bromo-2-methylpropanoate (98%) were distilled prior to use. Copper(I) bromide (99.9%) was washed three times with glacial acetic acid and acetone before using. 1 H NMR scale photoisomerization reactions were carried out under N2, using C6D6 as solvent, quartz J-Young NMR tubes as the reaction vessels, and a ShangHai SiLe photochemical reactor as the light source. NMR spectra were recorded on Bruker Avance 400 and 700 MHz spectrometers. UV−vis spectra were obtained on an Agilent Cary 300 UV−vis spectrophotometer. Fluorescence spectra were recorded on an Edinburgh Instruments FLS980 spectrophotometer. High-resolution mass spectra (HRMS) were obtained from an Agilent Q-TOF 6520 LC-MS spectrometer. Fluorescent quantum efficiencies were determined using a Hamamatsu Quantaurus-QY spectrometer (C11347). DSC and TGA characterizations were carried out with a Shimadzu DTG-60 and DSC-60. Glass transition temperature was determined by taking the midpoint of the glass transition from the heating curve of the second cycle with a heating rate of 10 °C/min. The decomposition temperature was defined as the onset point of the first stage weight loss in the TGA diagram. Molecular weights and polydispersity indexes (Mw/Mn) of polymers were obtained by gel permeation chromatography (GPC) using an LC-20A chromatograph at 40 °C. THF was used as the eluent, at a flow rate of 1.0 mL/min. Photoisomrization quantum yields (ΦPI) were determined with actinometry at 298 K using B(ppy)Mes2 as the reference material (ΦPI = 0.85 determined previously with a ferrioxalate actinometry45) and an Agilent Cary 300 spectrophotometer for collecting absorbance measurements. The monochromated excitation light (365 nm) from an Edinburgh Instruments FLS980 450 W spectrometer was used as the irradiation light source. The experimental details of Φ PI determination are provided in the Supporting Information. 4630

DOI: 10.1021/acs.macromol.7b00632 Macromolecules 2017, 50, 4629−4638

Article

Macromolecules Scheme 2. Synthetic Route of Monomer BHMA

(s, 1H). 13C NMR (176 MHz, CD2Cl2, δ, ppm): 154.29, 152.52, 135.06, 133.73, 133.08, 130.57, 129.74, 127.70, 125.09, 120.78, 118.14, 69.25, 62.58, 32.63, 28.84, 25.62, 25.47, 20.38. 11B NMR (225 MHz, CD2Cl2, δ): 4.20 ppm. HRMS (ESI) m/z: [M + H]+ calcd for C35H43BNO2, 519.3345; found 519.3418. Synthesis of Monomer 6-[B(ppy)Mes2]oxyhexyl Methacrylate (BHMA). To a nitrogen-purged flask were added methacrylic acid (0.33 g, 3.85 mmol), DMAP (0.47 g, 3.85 mmol), and dry CH2Cl2 (30 mL). After cooling to 0 °C, EDCI (0.73 g, 3.85 mmol) and compound 4 (1.0 g, 1.9 mmol) were added. The solution was stirred at room temperature for 24 h. Then 2 N aqueous NaOH (8 mL) was added and stirred at room temperature for 3 h. The mixture was poured into water and extracted with CH2Cl2. The organic layer were separated and dried over Na2SO4. Crude product was purified by column chromatography (methylene chloride/petroleum = 1/1). Monomer BHMA was obtained as a yellow powder (0.98 g, yield 87%). 1H NMR (400 MHz, C6D6, δ, ppm): 8.42 (d, J = 2.5 Hz, 1H), 8.04 (d, J = 7.0 Hz, 1H), 7.51 (d, J = 7.0 Hz, 1H), 7.21−7.10 (m, 2H), 7.08 (d, J = 8.9 Hz, 1H), 6.88−6.73 (m, 5H), 6.16 (s, 1H), 5.34−5.06 (m, 1H), 4.00 (t, J = 6.6 Hz, 2H), 3.24 (t, J = 6.4 Hz, 2H), 2.47−1.90 (m, 18H), 1.86 (s, 3H), 1.37−1.31 (m, 2H), 1.18 (m, 2H), 1.02−0.91 (m, 4H). 13C NMR (100 MHz, C6D6, δ, ppm): 166.65, 153.71, 152.89, 136.72, 134.96, 133.71, 131.71, 131.49, 130.32, 130.21, 125.06, 124.65, 120.51, 117.52, 68.39, 64.09, 28.43, 28.39, 25.49, 25.14, 20.61, 18.11. 11B NMR (128 MHz, C6D6, δ): 4.96 ppm. HRMS (ESI) m/z: [M + H]+ calcd for C39H47BNO3, 587.3607; found 587.3680. Synthesis of Homopolymer P1. To a Schlenk tube, monomer BHMA (0.98 g, 1.6 mmol), PMDETA (5.5 mg, 0.032 mmol), methyl 2-bromo-2-methylpropanoate (2.9 mg, 0.016 mmol), CuBr (2.3 mg, 0.016 mmol), and toluene (3.0 mL) were added under nitrogen. After three freeze−pump−thaw cycles, the mixture was stirred at 60 °C for 32 h. The reaction was quenched in liquid nitrogen. The copper salt was removed by column chromatography (Al2O3) using THF as the solvent and concentrated to ca. 10 mL. P1 was recovered by precipitation into methanol (60 mL) with vigorous stirring and dried under vacuum. The pure product was collected as a light yellow powder (0.51 g, yield 52%). 1H NMR (700 MHz, C6D6, δ, ppm): 8.44 (br, s), 8.00 (br, s), 7.57 (br, s), 7.27 (br, m), 7.15 (br, m, overlap with C6D6), 6.80 (br, s, H−Mes), 4.00 (br, s, −COO−CH2−), 3.57−3.55 (m, initiator group) 3.31 (br, s, −O−CH2−), 2.20−2.10 (m, Me− Mes), 1.51−1.11 (m, polymer backbone, side chain −(CH2)4− and Me). 11B NMR (225 MHz, C6D6, δ): 3.22 ppm. General Procedure for the Synthesis of Copolymers P2−P5. To a Schlenk tube methyl 2-bromo-2-methylpropanoate (1 mmol %), CuBr (1 mmol %), PMDETA (2 mmol %), toluene (0.5 mL/mmol),

BHMA, and tBMA were added under inert gas. The reaction mixture was heated at 60 °C for a desired period of time. Then the reaction was quenched in liquid nitrogen. The polymers were purified using the same procedures as described above for P1 and obtained as white solids for all copolymers. P2: Monomer feeding ratio (BHMA/tBMA): 1/5. Reaction time: 39 h. Yield: 55%. 1H NMR (700 MHz, C6D6, δ, ppm): 8.47 (br, s), 8.03 (br, s), 7.62 (br, s), 7.33 (br, s), 7.18 (br, m, overlap with C6D6), 7.05 (br, s, overlap with C6D6) 6.82 (br, s, H-Mes), 4.01 (br, −COO− CH2−), 3.42 (br, −O−CH2−), 2.41−0.90 (m, Me−Mes, polymer backbone, side chain −(CH2)4− and Me). 11B NMR (225 MHz, C6D6, δ): 4.23 ppm. P3: Monomer feeding ratio (BHMA/tBMA): 1/10. Reaction time: 37 h. Yield: 69%. 1H NMR (400 MHz, C6D6, δ, ppm): 8.47 (br, s), 8.03 (br, s), 7.63 (br, s), 7.35 (br, s), 7.18 (br, m, overlap with C6D6), 7.09 (br, s, overlap with C6D6) 6.82 (br, s, H−Mes), 4.01 (br, −COO−CH2−), 3.44 (br, −O−CH2−), 3.35 (s, initiator group, −OCH3), 2.43−0.90 (m, Me−Mes, polymer backbone, side chain −(CH2)4− and Me). 11B NMR (225 MHz, C6D6, δ): 4.67 ppm. P4: Monomer feeding ratio (BHMA/tBMA): 1/25. Reaction time: 39 h. Yield: 54%. 1H NMR (400 MHz, C6D6, δ, ppm): 8.47 (br, s), 8.04−8.03 (d), 7.63 (br, s), 7.35 (br, s), 7.18 (br, overlap with C6D6), 7.09 (br, s, overlap with C6D6) 6.82 (br, s, H−Mes), 4.01 (br, −COO−CH2−), 3.45 (br, −O−CH2−), 3.35 (s, initiator group, −OCH3), 2.42−0.90 (m, Me−Mes, polymer backbone, side chain −(CH2)4− and Me). 11B NMR (225 MHz, C6D6, δ): 3.89 ppm. P5: Monomer feeding ratio (BHMA/tBMA): 1/40. Reaction time: 40 h. Yield: 68%. 1H NMR (400 MHz, CD2Cl2, δ, ppm): 8.22 (br, s), 7.94−7.92 (m), 7.78 (br, s), 7.67−7.65 (m), 7.24−7.22 (m), 6.62 (br, s), 3.96 (br, s), 3.61 (initiator group, −OCH3), 2.15−0.98 (m, Me− Mes, polymer backbone, side chain −(CH2)4− and Me). 11B NMR (225 MHz, C6D6, δ): 4.32 ppm.

■

RESULTS AND DISCUSSION Synthesis and Characterization. Monomer 6-[B(ppy)Mes2]oxy hexyl methacrylate (BHMA) was synthesized according to the route shown in Scheme 2. The key precursor 2-bromo-5-(hex-5-enyloxy)pyridine (1) was prepared by the reaction of 6-bromopyridin-3-ol with hex-5-en-1-ol in acetone in the presence of K2CO3. Compound 2 (2-(2-bromophenyl)5-(hex-5-enyloxy)pyridine) was obtained via the Suzuki coupling reaction of 1 with o-Br-phenylboronic acid. Lithiation of 2 at −78 °C followed by the addition of 1 equiv of BMes2F yielded boron chelate compound 3, which was converted to 4631

DOI: 10.1021/acs.macromol.7b00632 Macromolecules 2017, 50, 4629−4638

Article

Macromolecules Scheme 3. Synthetic Route of Random Copolymers P1−P5

Table 1. Characterization Data of Polymers P1−P5 polymers

m:na

yield (%)

Mn (kDa)

Mw (kDa)

PDI

Tg (°C)

Tdc (°C)

content of BHMAb (%)

P1 P2 P3 P4 P5

36:0 (1:0) 15:75 (1:5) 19:190 (1:10) 10:240 (1:24) 5:200 (1:40)

52 55 69 54 68

21 19 38 40 31

24 23 53 47 35

1.14 1.20 1.38 1.15 1.11

146 138 137 134 130

293 210 208 207 202

100 45 29 15 9

a

Molar ratio between the two monomer units based on integrated intensities of the corresponding peaks in the 1H NMR spectra. The total number of repeating units for each monomer was obtained from Mn and the molar ratio. bCalculated from molecular weight fraction of the BHMA unit in the random copolymer. cFrom the onset temperature of the first stage weight loss in the TGA diagram (see Supporting Information).

compound 4 via hydroboration using BH3·THF as the reagent and the subsequent reaction with NaOH/H2O2. Monomer BHMA was obtained in high yield (87%) through esterification reaction of compound 4 and methacrylic acid in the presence of 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDCI) and DMAP in dry CH2Cl2. The incorporated long alkyl chain ensures the high solubility of this monomer in most common organic solvents. It also decouples electronically the vinyl group from the boron unit, preventing the vinyl group from interfering with the photoisomerization of the B(ppy)Mes2 moiety.42 Furthermore, the electron-withdrawing ester group facilitates the polymerization of this monomer. All the polymers studied in this paper were prepared via ATRP. To investigate the influence of BHMA content on the polymers’ photoisomerization efficiency, tert-butyl methacrylate (tBMA) was introduced to form random copolymers, which spatially separates the B(ppy)Mes2 moiety and reduces energy transfers from both inter- and intrachain interactions. A series of random copolymers, poly[(6-(B(ppy)Mes2)oxyhexyl methacrylate)m-r-(tert-butyl methacrylate)n] (P1−P5), of controlled molecular weight were synthesized following a typical protocol for ATRP, shown in Scheme 3. The molecular weights and the content of BHMA were controlled by tuning the ratios of BHMA and tBMA to initiator. After polymerization, the copper salt was removed by passing through a neutral alumina column, and the polymer products were collected by precipitation into methanol. After being dried under vacuum, the polymers were obtained as white powder and fully characterized by GPC and NMR analysis. Because of the incorporation of long alkyl chain, the homopolymer P1 and copolymers P2−P5 have very good solubility in most common solvents, such as toluene, benzene, dichloromethane, etc., and can be readily cast into transparent colorless films on various substrates. These features greatly

facilitate the full characterization and the study of photophysical properties of the new polymers. The key characterization data of P1−P5 including molecular weight determined from GPC analysis and the content of boron units from 1H NMR analysis are shown in Table 1. The molecular weights of these polymers range from ca. 20K to 40K Da, with good polydispersity (