Facile Synthesis of β-Diketone Alcohols for Combined Functionality: Initiation, Catalysis, and Luminescence Xuepeng Zhang, Minxin Cui, Rui Zhou, Changle Chen,* Guoqing Zhang*
Primary alcohol-functionalized β-diketones (bdks) are successfully synthesized via facile one-step Claisen condensation between aromatic monoketones and ε-caprolactone (ε-CL). To demonstrate application potentials, these bdk alcohols are used to chelate with various Lewis acids, including Tb (III), Eu (III), and B (III). It is discovered that the resulting Tb (III) and Eu (III) diketonate complexes can serve as both catalysts and initiators for ring-opening polymerization (ROP) under solvent-free conditions, using lactide monomer as an example. The polylactides (PLAs) thus obtained exhibit luminescence properties characteristic of Tb (III) and Eu (III), respectively. On the other hand, boron-chelated diketone can initiate ROP of lactide in the presence of Sn(oct)2, and affords a PLA material with dual-emission, i.e., ﬂuorescence and room temperature phosphorescence. The synthesis described here represents a shortcut for the preparation of bdk-based macroligands and subsequent functional materials.
1. Introduction Polymer-conjugated ligands or macroligands have been extensively used in producing materials with desirable functions such as optoelectronic and stimuli-responsive properties.[1–4] Examples of common chelating moieties include derivatives of bipyridine,[5–10] terpyridine,[11–14] hydroxyquinoline,[15–21] and diketone.[22–31] Among these investigations, Fraser et al. pioneered in the use of β-diketone (bdk) macroligands for site-isolated Eu-centered, luminescent heterocomplexes,[22,23] Fe-centered, self-catalyzing polymerization,[24,25] as well as B-centered, ratiometric in vivo nanosensors for molecular oxygen in tumor tissues. More recently, we reported highly ﬂuorescent X. Zhang, M. Cui, R. Zhou, Prof. C. Chen, Prof. G. Zhang, CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026, China E-mail: [email protected]; [email protected]
dye-aggregate-enhanced energy-transfer nanoparticles for neuronal cell imaging based on bdk macroligands. In all cases, PLA macroligands were initiated by hydroxylfunctionalized diketone ligands developed by Fraser et al., in that the advantage of ligand initiation (vs coupling reaction) towards macroligand synthesis is the high homogeneity of the resulted systems.[1,33] The preparation typically includes substitution, protection, condensation, and deprotection.[22,23] Consequently, the multiple steps involved may limit the scalability and commercialization potentials. Here, we report a strategy, which signiﬁcantly simpliﬁes the procedure for obtaining primary alcoholfunctionalized bdk ligands, using Claisen condensation reactions between a ketone and a cyclic ester. In this way, the diketone and the alcohol moieties can be generated in the same step simultaneously, eliminating the need for addition, protection, and deprotection of a primary alcohol group as previously reported.[22,23] Claisen condensation was ﬁrst developed in 1887 and has been widely used in generating β-keto esters or bdks through carbon–carbon
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Facile Synthesis of β-Diketone Alcohols for Combined Functionality: . . .
bond formation.[34–37] To the best of our knowledge, however, this is the ﬁrst example of Claisen condensation involving a cyclic ester to produce hydroxy bdks. To demonstrate the practical application of such bdk derivatives, we take advantage of the combined functionality in initiation, catalysis, and luminescence of the bdk-lanthanide [Eu (III) and Tb (III)] complexes and demonstrate that single-component luminescent polymeric materials could be prepared by simply mixing bdk complexes with lactide under heating. We also show that, consistent with previous reports,[26,27] when BF3 was used as the chelating Lewis acid, not only intense ﬂuorescence was observed, but also strong room-temperature phosphorescence was induced in the boron-PLA material.
2. Experimental Section 2.1. Materials Tetrahydrofuran (THF) and 1,4-dioxane were distilled by reﬂuxing with sodium for 3 h before use. Dichloromethane (DCM) was puriﬁed by drying over calcium hydride for 2 h prior to use. Sodium hydride (95%) and boron triﬂuoride diethyl etherate (99.8%, puriﬁed by redistillation) were purchased from Sigma–Aldrich. Lactide of high purity was obtained from PURAC Biomaterials. All other reagents and solvents were obtained from Aladdin Reagent (Shanghai) and were used as received.
2.2. Methods 1H
NMR (300 MHz) spectra were recorded on a Bruker AV300 NMR spectrometer operated in the Fourier transform mode. 1H NMR spectra were referenced to the signal for residual protiochloroform at 7.26 ppm and coupling constants are given in hertz. Mn (NMR) of polymers were calculated on the basis of the integration ratio of PLA CH and residual initiator proton. Relative molecular weight and molecular weight distributions of polymer samples were measured by conventional gel permeation chromatography (GPC) system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR0.5, HR2, and HR4, 7.8 × 300 mm). GPC measurements were run at 25 °C using THF as eluent with a ﬂow rate of 0.3 mL min−1, and the system was calibrated with linear polystyrene standards. Electrospray ionization (ESI) mass spectra were recorded on a LTQ ORBITRAP XL mass spectrometer (Thermo Scientiﬁc). Matrix-assisted laser desorption/ionization time of ﬂight mass spectrometry (MALDI-TOF MS) were conducted on an Autoﬂex Speed MALDI-TOF mass spectrometer (Bruker Daltonics) using DCTB as matrix. Elemental analysis for C and H was performed by a vario EL cube elemental analyzer. The temperatures of the combustion tube and the reduction tube were 950 °C and 550 °C, respectively. UV–vis absorption spectra were recorded on a Beijing Persee TU-1901 UV–vis spectrometer. Photographs were taken by a Cannon 500D digital camera. Steady-state emission spectra were recorded on a Horiba FluoroMax-4 spectroﬂuorometer (Japan). Fluorescence lifetime
data were acquired with a 1 MHz LED laser with the excitation peak at 372 nm (NanoLED-370). Phosphorescence lifetime data were acquired with a 1 MHz LED laser with the excitation peak at 374 nm (SpectraLED-370). Lifetime data were analyzed with DataStation v6.6 (Horiba Scientiﬁc).
2.3. Syntheses 2.3.1. 8-Hydroxy-1-(4-methoxyphenyl)octane-1,3-dione (bdk1) Route A (Ketone:Lactone = 1:1): To a dry round-bottom ﬂask ε-CL (2.28 g, 20 mmol), 4-methoxyacetophenone (3.00 g, 20 mmol), THF (26 mL), and sodium hydride (960 mg, 40 mmol) were introduced successively and the mixture was stirred at 60 °C under N2 for ≈48 h. Then, the reaction mixture was neutralized with HCl (1 M, aqueous solution) and extracted with ethyl acetate. The brown oil obtained after solvent removal was puriﬁed by ﬂash chromatography on silica gel to yield bdk1 as colorless crystalline powder (2.06 g, 40%). 1H NMR (300 MHz, CDCl3,δ): 16.33 (s, 0.8 H, COCHCOH), 7.92 (m, 0.3H, 2, 6 -ArH in the diketone form), 7.87 (m, 1.7H, 2, 6 -ArH in the enol form), 6.94 (m, 2H, 3, 5 -ArH), 6.11 (s, 0.8H, COCHC(OH)), 4.03 (s, 0.3H, COCH2CO), 3.87 (s, 0.45H, OCH3 in the diketone form), 3.86 (s, 2.55H, OCH3 in the enol form), 3.64 (m, 2H, CH2CH2CH2CH2CH2OH), 2.59 (t, 0.3H, CH2CH2CH2CH2CH2OH in the diketone form), 2.41 (t, 1.7H, CH2CH2CH2CH2CH2OH in the enol form), 1.72 (dt, 2H, CH2CH2CH2CH2CH2OH), 1.59 (m, 2H, CH2CH2CH2CH2CH2OH), 1.45 (ddd, 2H, CH2CH2CH2CH2CH2OH). HRMS (ESI) m/z: [M+H]+ calcd for C15H21O4, 265.14398; found, 265.14334; [M+Na]+ calcd for C15H20NaO4, 287.12593; found, 287.12512. Anal. Calcd for C15H20O4: C, 68.16, H, 7.63; found: C, 68.26, H, 7.64. UV/ vis (DCM): λmax (ε) = 323 nm (24 500 M−1 cm−1). Route B (Ketone:Lactone = 1:4): To a dry round-bottom ﬂask ε-CL (9.12 g, 80 mmol), 4-methoxyacetophenone (3.00 g, 20 mmol), THF (26 mL) and sodium hydride (960 mg, 40 mmol) were introduced successively and the mixture was stirred at 60 °C under N2 for ≈36 h. Then the reaction mixture was neutralized with HCl (1 M, aqueous solution) and extracted with ethyl acetate. The brown oil obtained after the removal of the solvent was puriﬁed by ﬂash chromatography on silica gel and components with Rf value ranging from 0.4 to 0.2 (TLC, 20% ethyl acetate in dichloromethane) were collected as pale yellow soft solid (about 3.8 g, 73%) (see the HRMS in the Supporting Information). This crude product (490 mg), sodium hydroxide (500 mg, 12.5 mmol), and methanol (8 mL) were introduced to a dry round-bottom ﬂask and the mixture was stirred at 60 °C for ≈2 h. Then the reaction mixture was neutralized with HCl (1 M, aqueous solution) and extracted with ethyl acetate. The solvent was removed and the residue was puriﬁed by column chromatography on silica gel to obtain bdk1 as colorless crystalline powder (190 mg, 39%)
2.3.2. 8-Hydroxy-1-(naphthalen-2-yl)octane-1,3-dione (bdk2) To a dry round-bottom ﬂask ε-CL (2.01 g, 17 mmol), 1-(naphthalen-2-yl)ethanone (3 g, 17 mmol), 1,4-dioxane (15 mL) and sodium hydride (830 mg, 35 mmol) were introduced successively and the mixture was stirred at 80 °C under N2 for
≈2 h. Then the reaction mixture was neutralized with HCl (1 M, aqueous solution) and extracted with ethyl acetate. The viscous liquid obtained after solvent removal was puriﬁed by silica gel chromatography to get bdk2 as pale yellow oil (2.1 g, 42%). 1H NMR (300 MHz, CDCl3,δ): 16.16 (s, 0.8 H, COCHCOH), 8.8 ≈ 7.4 (m, 7H, ArH), 6.03 (s, 0.88H, COCHCOH), 4.21 (s, 0.12H, COCH2CO), 3.68 (m, 2H, CH2CH2CH2CH2CH2OH), 2.65 (t, J = 7.3 Hz, 0.15H, CH2CH2CH2CH2CH2OH in the diketone form), 2.38 (t, J = 7.5 Hz, 1.85H, CH2CH2CH2CH2CH2OH in the enol form), 1.75 (dt, J = 15 Hz, 2H, CH2CH2CH2CH2CH2OH), 1.63 ((dt, J = 13.2 Hz, 2H, CH2CH2CH2CH2CH2OH), 1.48 (m, 2H, CH2CH2CH2CH2CH2OH). HRMS (ESI) m/z: [M+H]+ calcd for C18H21O3, 285.14907; found, 285.14868; [M+Na]+ calcd for C18H20NaO3, 307.13101; found, 307.13064. Anal. Calcd for C18H20O3: C, 76.03, H, 7.09; found: C, 75.82, H, 6.98. UV–vis (DCM): λmax (ε) = 322 nm (16 000 M−1 cm−1).
2.3.3. 8-Hydroxy-1-(thiophen-2-yl)octane-1,3-dione (bdk3) To a dry round-bottom ﬂask ε-CL (2.71 g, 24 mmol), 1-(thiophen2-yl)ethanone (3g, 24 mmol), THF (30 mL) and sodium hydride (711 mg, 30 mmol) were introduced successively and the mixture was stirred at 60 °C under N2 for ≈33 h. Then the reaction mixture was neutralized with HCl (1 M, aqueous solution) and extracted with ethyl acetate. The viscous liquid obtained after the removal of the solvent was puriﬁed by silica gel chromatography to get bdk3 as brownish oil (2.86 g, 50%). 1H NMR (300 MHz, CDCl3,δ): 15.68 (s, 0.8 H, COCHCOH), 7.73 (d, J = 3.8 Hz, 0.2H, 5-ArH in the diketone form), 7.69 (dd, J = 3.7 Hz, 0.8H, 5-ArH in the enol form), 7.59 (dd, J = 4.9 Hz, 1H, 3-ArH), 7.13 (dd, J = 4.9 Hz, 1H, 4-ArH), 6.01 (s, 0.8H, COCHCOH), 4.01 (s, 0.4H, COCH2CO), 3.65 (m, 2H, CH2CH2CH2CH2CH2OH), 2.62 (t, J = 7.2 Hz, 0.4H, CH2CH2CH2CH2CH2OH in the diketone form), 2.38 (t, J = 7.5 Hz, 1.6H, CH2CH2CH2CH2CH2OH in the enol form), 1.71 (dt, J = 15 Hz, 2H, CH2CH2CH2CH2CH2OH), 1.59 (m, 2H, CH2CH2CH2CH2CH2OH), 1.46 (m, 2H, CH2CH2CH+ calcd for C12H17O3S, 2CH2CH2OH). HRMS (ESI) m/z: [M+H] 241.08984; found, 241.08943; [M+Na]+ calcd for C12H16 NaO3S, 263.07178; found, 263.07123. Anal. Calcd for C12H16O3S: C, 59.97, H, 6.71; found: C, 59.74, H, 6.68. UV–vis (DCM): λmax (ε) = 324 nm (18 000 M−1 cm−1).
2.3.4. Comp1 Bdk1 (200 mg, 0.76 mmol), sodium hydroxide (30 mg, 0.76 mmol), and ethanol (20 mL) were introduced successively to a roundbottom ﬂask and the mixture was stirred at room temperature for ≈30 min. Then TbCl3 · 6H2O (94 mg, 0.25 mmol) was introduced and the mixture was stirred for another 20 h. The insoluble was removed by ﬁltration and the ﬁltrate was dried under reduced pressure to get comp1 as pale yellow viscous oil (167 mg, 70%). MALDI-TOF-MS m/z: [M-H]− calcd for C45H56O12Tb, 947.3025; found, 947.3037. Anal. Calcd for C45H57O12Tb: C, 56.96, H, 6.05; found: C, 56.87, H, 6.02. UV/vis (DCM): λmax (ε) = 323 nm (35 700 M−1 cm−1).
2.3.5. Comp2 Bdk2 (100 mg, 0.35 mmol), sodium hydroxide (14 mg, 0.35 mmol), and ethanol (20 mL) were introduced successively to a round-bottom ﬂask and the mixture was stirred at
room temperature for ≈30 min. Then Eu(NO3)3 · 6H2O (52 mg, 0.117 mmol) was introduced and the mixture was stirred for another 2 h. The insoluble was removed by ﬁltration and the ﬁltrate was dried under reduced pressure to get comp2 as pale yellow powder (90 mg, 75%). MALDI-TOF-MS m/z: [M+Na]+ calcd for C54H57EuNaO9, 1025.3113; found, 1025.3122. Anal. Calcd for C54H57O9Eu: C, 64.73, H, 5.73; found: C, 64.56, H, 5.69. UV/vis (DCM): λmax (ε) = 322 nm (66 000 M−1 cm−1).
2.3.6. Comp3 Bdk3 (633 mg, 2.64 mmol), DCM (20 mL), and boron triﬂuoride diethyl etherate (1.4 mL, 9.91 mmol) were introduced successively to a round-bottom ﬂask and the mixture was stirred at room temperature under N2 for ≈3 h. Then ﬂash chromatography on silica gel (eluted with DCM) was used to get comp3 as brown oil (418 mg, 55%). 1H NMR (300 MHz, CDCl3,δ): 8.01 (d, J = 3.3 Hz, 1H, 5-ArH), 7.87 (d, J = 4.8 Hz, 1H, 3-ArH) 7.25 (m, 1H, 4-ArH), 6.36 (s, 1H, COCHCO), 3.68 (t, J = 6.0 Hz, 2H, CH2CH2CH2CH2CH2OH), 2.60 (t, J = 7.4 Hz, 2H, CH2CH2CH2CH2CH2OH), 1.80–1.49 (m, 6H, CH2CH2CH2CH2CH2OH). HRMS (ESI) m/z: [M+Na]+ calcd for C12H15BF2NaO3S, 311.07007; found, 311.06930. Anal. Calcd for C12H15BF2O3S: C, 50.02, H, 5.25; found: C, 49.83, H, 5.12. UV/vis (DCM): λmax (ε) = 355 nm (33 500 M−1 cm−1).
2.3.7. P1 Comp1 (16.4 mg, 0.017 mmol) and D,L-lactide (1.00 g, 6.92 mmol) (loading: 1:400) were combined in a sealed Kontes ﬂask under N2. The entire bulb of the ﬂask was submerged in a 130 °C oil bath for 4 h. Crude polymer was puriﬁed by precipitation from DCM/cold MeOH (3×). The polymer was collected by centrifugation, the ﬁltrate was decanted, and the gummy solid was washed with additional cold MeOH (2×), then dried in vacuo to give P1 as colorless foam (420 mg, 42%). Mn (GPC/RI) = 12 800, PDI = 1.30; Mn (NMR) = 32 000. 1H NMR (300MHz, CDCl3,δ): 7.87 (s, 6H, 2, 6 –ArH), 6.92 (s, 6H, 3, 5 –ArH), 6.1 (s, 3H, COCHCO), 5.16 (m, broad, 446H, PLA CH), 4.36 (m, 3H, –CH3CHOH), 4.13 (s, 6H, –CH2CH2CH2CH2CH2O–), 3.87 (s, 9H, OCH3), 3.75 (s, 3H, –OH), 2.40 (m, 6H, –CH2CH2CH2CH2CH2O–), 1.56 (m, 1350H, CH2CH2CH2CH2CH2OH and PLA CH3), 1.25 (m, 9H, -CH3CHOH). UV/vis (DCM): λmax (ε) = 320 nm (11 200 M−1cm−1).
2.3.8. P2 Comp2 (11.6 mg, 0.0115 mmol) and D,L-lactide (1.00 g, 6.92 mmol) (loading: 1:600) were combined in a sealed Kontes ﬂask under N2. The entire bulb of the ﬂask was submerged in a 130 °C oil bath for 6 h. Crude polymer was puriﬁed by precipitation from DCM/cold MeOH (3×). The polymer was collected by centrifugation, the ﬁltrate was decanted, and the gummy solid was washed with additional cold MeOH (2×), then dried in vacuo to give P2 as colorless foam (516 mg, 51.6%). Mn (GPC/RI) = 18 000, PDI = 1.21; Mn (NMR) = 62 000. 1H NMR (300MHz, CDCl3,δ): 8.44–7.61 (m, 21H, ArH), 6.02 (s, 3H, COCHCO), 5.16 (m, broad, 858 H, PLA CH), 4.35 (m, 3H, –CH3CHOH), 4.15 (m, –CH2CH2CH2CH2CH2O–), 2.44 (m, 6H, –CH2CH2CH2CH2CH2O–), 1.72 (m, 18H, –CH2CH2CH2 CH2CH2O–), 1.55 (m, 2500 H, PLA CH3), 1.39 (m, 9H, –CH3CHOH). UV/vis (DCM): λmax (ε) = 322 nm (24 500 M−1 cm−1).
Facile Synthesis of β-Diketone Alcohols for Combined Functionality: . . .
2.3.9. P3. Comp3 (10 mg, 0.035 mmol), D,L-lactide (1.00 g, 6.92 mmol), and Sn(oct)2 (0.28 mg, 0.69 μmol) (loading: 1:200:1/50) in n-hexane were combined in a sealed Kontes ﬂask under N2. The entire bulb of the ﬂask was submerged in a 130 °C oil bath for 10 h. Crude polymer was puriﬁed by precipitation from DCM/cold MeOH (3×). The polymer was collected by centrifugation, the ﬁltrate was decanted, and the gummy solid was washed with additional cold MeOH (2×). The resulting solid was reprecipitated from DCM/nhexanes, collected by centrifugation, washed with hexanes (2×), and dried in vacuo to give P3 as colorless foam (520 mg, 52%). Mn (GPC/RI) = 12 300, PDI = 1.10; Mn (NMR) = 13 000. 1H NMR (300MHz, CDCl3,δ): 8.01 (s, 1H, 5-ArH), 7.89 (s, 1H, 3-ArH), 7.16 (m, 1H, 4-ArH), 6.36 (s, 1H, COCHCO), 5.18 (m, broad, 180 H, PLA CH), 4.35 (m, 1H, –CH3CHOH), 4.15 (m, 2H, –CH2CH2CH2CH2CH2O–), 2.93 (s, 1H, –OH), 2.59 (m, 2H, –CH2CH2CH2CH2CH2O–), 1.76 (m, 6H, –CH2CH2CH2CH2CH2O–), 1.57 (t, J = 7.4, 541 H, PLA CH3), 1.35 (t, 3H, –CH3CHOH). UV/vis (DCM): λmax (ε) = 355 nm (13 700 M−1 cm−1).
3. Results and Discussion 3.1. Synthesis The current strategy employs commercially available monoketones and ε-caprolactone (ε-CL) as the starting materials to demonstrate the great potential for largescale applications. The synthetic route is illustrated in Figure 1. When 4-methoxyacetophenone and ε-CL were used at 1:1 ratio, 8-hydroxy-1-(4-methoxyphenyl)octane-1,3-dione (bdk1, Figure 2A) was generated at ≈40% yield from the Claisen condensation in dry THF at 60 °C. The yield is signiﬁcantly higher than the overall yield of ≈10%–15% in previously reported methods for same type of bdk alcohols.[22,23] When the ratio between the starting ketone (e.g., 4-methoxyacetophenone) and ε-CL was changed to 1:4 or 1:10, hydroxyl end-capped diketone ester oligomers [bdkPCL-OH], instead of pure bdk1, were obtained at ≈73%
yield. The oligomers were characterized with 1H NMR and electrospray ionization-mass spectrometry (ESI-MS). ESI-MS spectra reveal that the main product remains to be the desired initiator bdk1; however, signiﬁcant oligo-ester products with polymerization degrees up to 9–10 are also present (Figure 2C,D). This is very likely caused by anionic polymerization of ε-CL in the Claisen condensation conditions, where the RO− can attack the lactone monomer for chain propagation. However, these oligomers can undergo alcoholysis in the presence of sodium hydroxide in methanol to give pure bdk1 at ≈39% yield (Figure 2B). The two-step reaction gave an overall yield of 28%. As such, the optimized 1:1 reaction ratio was used for the rest of the study, which affords higher yield with less starting materials. 8-Hydroxy-1-(naphthalen-2-yl)octane-1,3-dione (bdk2) was obtained at ≈42% yield from the reaction of 1-(naphthalen-2-yl)ethanone with ε-CL in 1,4-dioxane at 80 °C after 2 h, while 8-hydroxy-1-(thiophen-2-yl)octane1,3-dione (bdk3) was generated at ≈50% yield from the reaction of 1-(thiophen-2-yl)ethanone with ε-CL in THF at 60 °C after 33 h. It should be noted that the choice of ether solvents was largely dependent on the temperature needed to conduct reactions required by different monoketones. The three diketones were characterized by elemental analysis, 1H NMR, and HRMS (Figure 2 and Supporting Information). As has been previously reviewed,[1,33] polymeric metal complexes have many distinct advantages such as diversity and tunability in structures as well as functions. In this case, in order to prepare PLA materials with luminescence properties, chelation products of bdk1, bdk2, and bdk3 with Tb (III), Eu (III), and B (III), respectively, were prepared to obtain luminophore-containing initiators for ROP of lactide (Figure 1). At room temperature in ethanol using NaOH as the deprotonating agent, bdk1 and bdk2 reacted with TbCl3·6H2O and Eu(NO3)3·6H2O to afford lanthanide-bdk initiators comp1 and comp2 in 70% and 75% yields, respectively. Boron initiator comp3 was obtained
Figure 1. Chemical structures and syntheses of β-diketone ligands, complexes, and polymers.
Figure 2. A) 1H NMR spectrum (CDCl3) of bdk1 prepared from the Claisen condensation between 4-methoxyacetophenone and ε-CL at 1:1 molar ratio. B) 1H NMR spectrum (CDCl3) of bdk1 prepared from the alcoholysis of the bdk-PCL-OH oligomers. C) ESI-MS spectrum of bdk-PCLOH oligomers prepared from the Claisen condensation between 4-methoxyacetophenone and ε-CL at 1:4 molar ratio. D) ESI-MS spectrum of bdk-PCL-OH oligomers prepared from the Claisen condensation between 4-methoxyacetophenone and ε-CL at 1:10 molar ratio. E) 1H NMR spectrum of bdk2 in CDCl3. F) 1H NMR spectrum of bdk3 in CDCl3. (EA: ethyl acetate; Ace: acetone)
in 55% yield from the reaction of bdk3 with BF3·OEt2 in DCM at room temperature. Comp1 and comp2 were characterized with elemental analysis and HRMS (Figures S8 and S9, Supporting Information), since NMR spectra could not be employed in this case due to the paramagnetic effect of the central metal ion, while comp3 could be characterized with elemental analysis, 1H NMR and HRMS (Figure S1 and S10, Supporting Information). The chelation number for both Eu (III) and Tb (III) is three according to the characterization data (Figure 1, S8 and S9, Supporting Information). Since many organometallic lanthanide complexes have been found to exhibit interesting catalytical properties in oleﬁn and lactone polymerizations,[40–45] it is quite conceivable that these Ln (III) (bdk)3 complexes may also serve as ROP catalyst for lactide. Furthermore, the Ln (III) complexes already contain primary alcohols, which can be used as initiating sites for lactide ROP as well. In other words, comp1
and comp2 may trigger polymerization without the presence of additional ROP catalysts such as Sn(oct)2, Al(O-iPr)3, or DMAP. To test the hypothesis, comp1 and lactide ([comp1]:[lactide] = 1:400) were heated under N2 to generate a transparent melt in a sealed reaction vessel at 130 °C. The melt started to impede the magnetic bar after ≈2 h due to viscosity increase and completely stopped the stirring after 4 h, presumably due to high monomer conversion during ROP. The percentage conversion is calculated to be 89% and the polymer molecular weight is 32 000 Da (vs ca. 51 000 Da theoretically) based on 1H NMR integration. The polymer was puriﬁed by standard precipitation from DCM into cold methanol (3×) and the ﬁnal product was obtained as colorless foam after vacuum drying. Measurement with gel permeation chromatography (GPC) in THF gives an Mn of 12 800 Da with a polydispersity index (PDI) of 1.30 (42% yield, Figure S11, Supporting Information). The rather large discrepancy between NMR and GPC molecular weight data
Facile Synthesis of β-Diketone Alcohols for Combined Functionality: . . .
Table 1. Characterization data of polymers P1–P3 obtained from ROP of D,L-lactide initiated by comp1–comp3
[M]/[I]a) Time Conv. [h] [%]b)
32 000 12 800d) 1.30
62 000 18 000d) 1.21
13 000 12 300
feed ratio; on NMR integration in CDCl3; c)Based on GPC data (THF eluent) using polystyrene standards; d)Discrepancy could be due to macroligand dissociation in GPC columns. Table 2. Optical characterization of complexes comp1–comp3 and polymers P1 –P3.
a)Steady-state emission data acquired from DCM solutions excited
at 365 nm; b)In DCM; excitation source: 370 nm light-emitting diode; c)Steady-state emission data acquired from the bulk samples excited at 365 nm; d)In bulk state; excitation source: 370 nm light-emitting diode; e)Signal too weak for data acquisition.
could be due to macroligand dissociation in GPC column, which has been previously documented.[23–25] Similar results were obtained for comp2: under the same reaction condition at an initiator to monomer ratio of 1:600, the reaction melt started to thicken drastically after 3 h and the stir bar stopped completely after 6 h. For comp3, additional Sn(oct)2 catalyst was needed to conduct ROP since no active catalytical properties were observed for the boron complex. The characterization data for all three polymers are shown in Table 1. The kinetics details of lactone ROPs in the presence of Ln(III) are under careful investigation but it is speculated the mechanism is similar to that of Fe (III)-catalyzed ROP. Nonetheless, it is clear that the two Ln (III) luminophores are capable of not only initiating but also catalyzing lactide ROP at a moderately fast speed and a controlled manner, revealing unprecedented combination of functionality of initiating, catalyzing, and luminescing (shown in the next section) in polymer chemistry and physics. 3.2. Optical Characterization The optical properties of the β-diketones, complexes, and polymers were investigated in both solutions and the bulk state (Table 2). The DCM solutions of bdk1 (λmax = 323 nm,
ε = 24 500 M−1 cm−1), bdk2 (λmax = 322 nm, ε = 16 000 M−1 cm−1) and bdk3 (λmax = 324 nm, ε = 18 000 M−1 cm−1) all exhibit strong absorptions in the band of 260–370 nm, which are assigned to be from π–π* transitions of the these bdk ligands. Compared to the absorption spectrum of bdk1 (Figure 3A), a new peak centered at 285 nm appears in those of both comp1 (λmax = 323 nm, ε = 35 700 M−1 cm−1) and P1 (λmax = 320 nm, ε = 11 200 M−1 cm−1), which is characteristic of Tb (III) transition. In contrast, comp2 (λmax = 322 nm, ε = 66 000 M−1 cm−1) and P2 (λmax = 322 nm, ε = 24 500 M−1 cm−1) share almost the same absorption spectra with that of bdk2. On the other hand, comp3 (λmax = 355 nm, ε = 33 500 M−1 cm−1) and P3 (λmax = 355 nm, ε = 13 700 M−1 cm−1) exhibit a signiﬁcant bathochromic shift in the absorption spectra compared with that of bdk3. The absorption bands of both comp3 and P3 may involve a mixture of π–π* and intramolecular charge-transfer state, which can be induced by the strong electron-withdrawing boron-diketone moiety in the presence of electron-rich sulfur atom on the thiophene ring. Next the luminescence properties of these complexes as well as polymers were investigated. Comp1, a viscous pale yellow oil at room temperature, does not display notable luminescence in bulk and is weakly luminescent in DCM solution (λmax = 544 nm), which may be due to intermolecular collision or fast intrinsic thermal decay of the complex, given the close triplet-state energy levels between bdk1 and Tb(III). By contrast, comp2, a pale yellow powder at room temperature, exhibits bright red luminescence (Figure 3B) when illuminated by 365 nm UV light. The rigidity of the comp2 molecule in bulk state may inhibits the thermal decay pathway and thus enhances the luminescence (λmax = 610 nm, τ = 0.14 ms). The luminescence in DCM solution is comparable to that of the solid state (λmax = 610 nm, τ = 0.16 ms). The emission spectra both in DCM solution and bulk state are characteristic of 5D-7F transitions of Eu (III), among which, consistent with literature report, the strongest emission around 610 nm is ascribed to 5D0– 7 F2 transition. After polymerization, the luminophores are presumably site-isolated and embedded in rigid PLA matrix. Therefore, bright green emission was observed for P1 foam under 365 nm light excitation, while red emission with moderate intensity was observed for P2 under the same condition (Figure 3C). The steady-state emission spectrum of P1 denotes the 5D–7F transition nature of Tb (III), in which the strongest emission centered at 545 nm is assigned to 5D4–7F5 transition. Compared to comp2, a weak and broad peak centered at ≈420 nm characteristic of ligand emission appeared in the emission spectrum of P2, indicating partial dissociation of the Eu (III)-diketone moiety may have occurred during the precipitation process in methanol. As for comp3, a viscous dark brown oil at room temperature, it shows very weak yellow emission in bulk state (λmax = 525 nm, τ = 600 ps) and notable blue
Figure 3. A) Normalized UV–vis absorption spectra of bdk ligands, complexes, and polymers in DCM. B) Normalized steady-state emission spectra of comp2 and comp3 in DCM solution and in bulk (left, λex = 365 nm); photos showing the corresponding samples in glass vials under 365 nm UV light (right). C) Normalized steady-state emission spectra of P1-P3 in air and P3 in DCM and in vacuo (left, λex = 365 nm); photos of P1-P3 solids and ﬂuorescence (FL)/room-temperature phosphorescence (RTP) of a thin ﬁlm of P3 in a glass vial ﬁlled with N2 under 365 nm UV light (right).
ﬂuorescence (λmax = 397 nm, τ = 15 ps) in DCM solution. P3 in DCM (λmax = 397 nm, τ = 107 ps) shares almost identical emission spectrum with that of comp3 DCM solution, indicating the stability of the chromophore during ROP and post-processing. Interestingly, in the solid-state P3 exhibits bright blue emission when illuminated by 365 nm UV light. Upon deoxygenation, long-lived green “after-glow” lasting for ≈1 s was observed at room temperature after the UV light has ceased (Figure 3C). Corresponding steadystate emission spectra were recorded for P3 foam under air as well as in vacuo (Figure 3C). Compared to the unimodal nature (λmax = 413 nm, τ = 0.69 ns, ﬂuorescence) under air, the emission in vacuo is bimodal with an emergence of a conspicuous new peak centered at 530 nm (τ = 104 ms). The 530 nm band is assigned to the triplet-state emission or room-temperature phosphorescence. This serves as another example of dual-emissive polymer that can be
facilely synthesized and may be of great application potential in oxygen sensing.
4. Conclusions In this communication, we have demonstrated that previously reported time-consuming synthesis of primary alcohol-functionalized β-diketone initiators for ringopening polymerization can be simpliﬁed through onestep Claisen condensation between monoketones and ε-caprolactone. The resulting β-diketone initiators can be modiﬁed by reacting with various Lewis acids such as Tb (III), Eu(III), and B(III) to obtain complexes with desired optical properties. The two lanthanide-diketone chelates can act as an ROP initiator, catalyst, and emitting center simultaneously, enabling quite facile preparation
Facile Synthesis of β-Diketone Alcohols for Combined Functionality: . . .
of single-component luminescent PLAs. Meanwhile the boron-diketone chelate can initiate lactide ROP in the presence of Sn(oct)2 catalyst, generating another example of dual-emissive polymer with application potential for oxygen detection. We are currently investigating the full details of above-mentioned systems, including the mechanism of catalysis, polymerization kinetics, alternative monomers, and application in optical sensing and imaging, as well as exploring more organometallic systems with similarly combined functionalities.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: We thank the Natural Science Foundation of China (21222405 and 21374108) as well as the Specialized Research Fund for the Doctoral Program of Higher Education of China (20123402120020 and 20133402120019) for support. We also thank Mr. Joshua D. Vaughn for taking the photos used in this paper. We are very grateful to PURAC Biomaterials for providing high purity lactide. Received: November 10, 2013; Revised: December 1, 2013; Published online: December 19, 2013; DOI: 10.1002/marc.201300834
Keywords: catalysts; functionalization of polymers; initiators; luminescence; metal–polymer complexes
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