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Conjugated Microporous Polymers Incorporating BODIPY Moieties as Light-Emitting Materials and Recyclable Visible-Light Photocatalysts Marta Liras,*,† Marta Iglesias,‡ and Félix Sánchez† †

Instituto de Química Orgánica General (IQOG-CSIC), C/Juan de la Cierva, 3 E-28006 Madrid, Spain Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), C/Sor Juana Inés de la Cruz, 3, E-28049 Madrid, Spain

‡

S Supporting Information *

ABSTRACT: Here we report the design and synthesis of a new conjugated microporous polymer based on a BODIPY dye (CMPBDP) which has shown excellent luminescence properties and an efficient heterogeneous photocatalytic activity. CMPBDP shows high thermal stability and high surface area with microporous size and efficiently catalyzes the selective oxidation of thioanisole into the corresponding sulfoxide via singlet oxygen under visible light (up to 500 nm). The photocatalytic rate of the heterogeneous materials is 4-fold faster that the corresponding soluble (used as reference) under the same reaction conditions and can be reused several times.

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INTRODUCTION The synthesis and characterization of new polymeric framework materials based on an organic skeleton are of great interest in the past decade. Thus, materials as metal−organic frameworks (MOFs)1−4 or porous organic polymer (POPs)5−7 with or without metals in their main structure have been reported. Within the POPs materials it is possible to find hyper-crosslinked polymers (HCPs),8,9 polymers with intrinsic microporosity (PIMs),10,11 covalent organic frameworks (COFs),12,13 and polymer organic frameworks (POFs).14 POFs can be classified as porous polymer aromatic frameworks (PPAFs)15 when an aryl−aryl bond was formed by Suzuki or Yamamoto coupling reaction or conjugated microporous polymers (CMPs)16 when aryl−alkyne cross-coupling takes place by Sonogashira reactions. CMPs are tridimensional polymers created by condensation polymerization with one of the two monomers bearing more than two reactive end groups (Ax + By, x > 2, y ≥ 2). So, CMPs materials are able to integrate π-electronic monomers to the covalent framework while retaining its permanent porous structure. A challenge in the synthesis of new CMPs is to find new monomers able to endow with their properties to the whole material. Thus, monomers with photophysical properties such as phenylene,17 pyrene,18 tetraphenylethene,19,20 and benzothiadiazole21,22 complement with their properties to the polymer defining its applicability range. Besides, the properties of the starting monomer can be change across the extension of the conjugation in a polymeric network, opening the door to new and exciting applications in the photonic field. In this sense, a monomer based on BODIPY dyes could be of great © XXXX American Chemical Society

interest due to their excellent photophysical properties such as high fluorescence quantum yields, low triplet−triplet absorption, high thermo- and photophysical stability, and so on.23−28 A BODIPY is a chromophore with an s-indacene core with 3aand 4a-positions occupied by N atoms; meanwhile, the 5position is occupied by a BF2 group. The important item here is that its molecular structure is almost planar and that bonds across 2- and 6-positions could lead to orthogonal structure. In fact, MOFs based on the BODIPY ligand have been recently described,29,30 and MOF postsynthetic modified with BODIPY dyes has been reported.31,32 In the present article, a stable conjugated microporous polymer incorporating BODIPYs as building blocks, CMPBDP, was synthesized and characterized. Besides, its photophysical properties have been explored and tested in selective photocatalytic oxidation of thioanisole.

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EXPERIMENTAL SECTION

Materials. Solvents were dried by standard methods or by elution through a PureSolv Innovative Technology column drying system. Unless otherwise noted, reagents were commercially available and used without further purification. Characterization Methods. Chemical Composition. Microanalyses were made with a Carlo Erba EA1108 elemental analyzer (C, H, N). Energy-dispersive X-ray spectroscopy (EDX) measurements of the CMPBDPs were made in a Philips Model XL30 ESEM. Thermogravimetric and differential thermal analyses (TGA) were Received: November 20, 2015 Revised: February 1, 2016

A

DOI: 10.1021/acs.macromol.5b02511 Macromolecules XXXX, XXX, XXX−XXX

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(CH3CH2), 13.09 (CH3C3), 11.39 (CH3C1), 9.93 (CH3C7). MS (EI), m/z (%) = 276 [nominaL mass, M+] (100). (2) A solution of iodic acid (2.0 equiv) dissolved in a minimum amount of water was added dropwise over 20 min to a solution of 2-ethyl-1,3,5,7pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (4) (350 mg, 1.3 mmol) and iodine (1 equiv) in EtOH. This mixture was stirred for 20 min at room temperature. After cooling, the mixture was evaporated under vacuum. The crude product was purified by silica gel chromatography and recrystallized from chloroform and n-hexane to afford 6-iodo-2-ethyl-1,3,5,7-pentamethyl-4,4-difluoro-4-bora-3a,4adiaza-s-indacene (2) as bright red needles; yield 440 mg, 88%. 1H NMR (300 MHz): δ = 7.01 (s, 1H, H-8), 2.56 and 2.52 (two s, 6H, CH3−C3 and CH3−C5), 2.39 (c, 2H, J = 7.6 Hz, CH2CH3), 2.22 and 2.20 (s, 6H, CH3−C1 and CH3-C7), 1.07 (t, 3H, J = 7.6 Hz, CH3CH2) ppm. 13C NMR (75 MHz): δ = 159.25, 153.77, 140.78, 139.18, 133.18, 133.79, 133.65, 131.64, 119.20 (H−C8), 17.31 (CH2CH3), 15.31 (CH3CH2), 14.33, 13.53, 12.93, 9.46 (4 × CH3− Ar). MS (EI), m/z (%) = 402 [M+] (68), 387 [M+ − 15](100). (3) The synthesis was carried out by Sonogashira−Hagihara crosscoupling reaction. In a 50 mL Schlenk tube were added 6-iodo-2ethyl-1,3,5,7-pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (2) (0.348 mmol, 0.140 g), 1,3,5-triethynylbenzene (3) (14.5 mg, 0.097 mmol), Pd(PPh3)4 (5.6 mg, 4.8 μmol), CuI (20 mg, 0.1 mmol), freshly distilled THF (5 mL), toluene (5 mL), and triethylamine (5 mL). The resulting suspension was deareated by bubbling argon at 80 °C for 20 min. The reaction mixture was stirred at 80 °C for 1 day. After removal of the solvents at reduced pressure, the residue was washed with water (100 mL) and extracted into CHCl3. The organic layer was dried on Na2SO4, and the solvent was removed under reduced pressure. Column chromatographic separation of the residue on silica gel using chloroform as eluent afforded 1,3,5-tris(1,3,5,7tetramethyl-6-ethyl-4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene-2ethynyl)benzene (BDP2); yield 66 mg, 70%. 1H NMR (300 MHz): δ = 7.5 (s, 3H−Ar), 7.01 (s, 3H, H-8), 2.68 (s, 9H, CH3−C), 2.55 (s, 9H, CH3−C), 2.42 (c, 6H, J = 7.5 Hz, 3 × CH2CH3), 2.35 (s, 3H, CH3−C), 2.16 (s, 3H, CH3−C), 1.08 ppm (t, 9H, J = 7.5 Hz, 3 × CH 3 CH 2 ). HRMS-MALDI-TOF: calculated for C 57 H 57 B 3 F 6 N 6 972.48281; found 972.4864. Synthesis of CMPBDPs. General Procedure. Di iodine BODIPY 1 (0.415 mmol, 0.21 g), 1,3,5-triethynylbenzene (0.115 mmol, 17.3 mg), [Pd-PPh3]4 (0.035 mmol, 41 mg), and CuI (0.02 mmol, 3.81 mg) were added to a 50 mL Schlenk tube containing freshly distilled DMF (10 mL) and triethylamine (5 mL). The resulting suspension was degassed with argon at 80 °C for 20 min. The reaction mixture was stirred at 80 °C for 3 days under static argon. The solid was washed with a large amount of clean DMF, water, and THF. Finally, the solid was stirred in a THF/water (1:1 v/v) solution with an excess of KCN to remove the Pd(0). Elemental analysis: %C (%Ctheo) = 64.77 (73.45); %H (%Htheo) = 5.374 (5.52); %N (%Ntheo) = 7.13 (7.67). Pd content 0.043%. General Procedure for Photocatalytic Oxidation of Thioanisole. Solvent (3 mL), thioanisole (3.17 mmol, 370 μL), and 0.5 or 0.1 mol % of CMPBDP (8.5 and 1.7 mg, respectively) or 0.1 mol % of BDP1 (1.5 mg) were irradiated at room temperature with a 150 W quartz−halogen lamp (KL 1500, Schott), using a 500 nm cutoff filter under air bubbling. The aliquots were taken directly from the solution and diluted with CDCl3 for monitoring the conversion by 1H NMR. Recycling Experiment. The reusability procedure consists in removing the solvent and sulfoxide by air current in the dark and the addition of new thioanisole (3.17 mmol, 370 μL) and 3 mL of 2ethoxyethanol over the solid residue.

conducted in an air stream with a TA Instruments Model TA-Q500 analyzer. The samples were heated under a helium stream from 40 to 800 °C with a heating rate of 10 °C min−1 in a high-resolution mode. Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer Spectrum One spectrometer and are reported in terms of the frequency of absorption (cm−1). Nuclear magnetic resonance (NMR) spectra were recorded with a Bruker Model AV300 spectrometer (Larmor frequencies of 75 and 300 MHz for 13C and 1 H, respectively) for liquids and a Bruker AV400WB spectrometer (Larmor frequencies of 400, 100, and 161 MHz for 13C and 1H, respectively) using 4 mm MAS probes spinning at a rate of 10 kHz for 13 C solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) measurements. The 13C CP-MAS spectra were obtained using a contact time of 3.5 ms and a relaxation time of 4 s. The number of scans used for the 13C CP-MAS spectra was 1024. A high-resolution mass spectrum (HRMS) was recorded in a Ultraflex III (MALDITOF/TOF) of Bruker instrument. Textural Characterization. Nitrogen adsorption and desorption isotherms were measured at 77 K, using a Micromeritics ASAP 2020 instrument. Prior to measurement, the samples were degassed for 12 h at 200 °C. The surface area was determined by BET (Brunauer− Emmett−Teller) theory, and pore size average was determined using the Barrett−Joyner−Halenda (BJH) method. Scanning electron microscopy (SEM) micrographs were obtained with a Hitachi Model SU-8000 microscope operating at 0.5 kV. The samples were prepared directly by dispersing the powder onto a double-sided adhesive surface. Fluorescence microscopy pictures were taken with Nikon Eclipse T2000-s microscope using a TexaRed filter from Nikon. Synthesis of Model 1,3,5,7-Tetramethyl-2,6-phenylethynyl4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP1). The synthesis was carried out by Sonogashira−Hagihara cross-coupling reaction (Scheme S2). In a 50 mL Schlenk tube were added 2,6diiodo-1,3,5,7,8-pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (1)33 (0.097 mmol, 50 mg), ethynylbenzene (23 mg, 0.2134 mmol), Pd(PPh3)4 (33.7 mg, 0.029 mmol), CuI (3.1 mg, 0.019 mmol), freshly distilled THF (5 mL), toluene (5 mL), and triethylamine (5 mL). The resulting suspension was deaerated by bubbling argon at 80 °C for 20 min. The reaction mixture was stirred at 80 °C for 1 day. After removal of the solvents at reduced pressure, the residue was washed with water (100 mL) and extracted into CHCl3. The organic layer was dried on Na2SO4, and the solvent was removed under reduced pressure. Column chromatographic separation of the residue on silica gel using chloroform as eluent afforded 2,6diethylbenzene-1,3,5,7-tetramethyl-4,4′-difluoro-4-bora-3a,4a-diaza-sindacene (BDP1); yield 38 mg (85%) of a red solid. 1H NMR (300 MHz): δ = 7.58−7.50 (m, 10H, H−Ar), 2.70 (s, 9H, CH3−C), 2.59 (s, 6H, CH3−C). 13C NMR (75 MHz): δ = 134.95, 134.87, 134.80, 131.02, 129.90, 128.05, 127.39, 127.32, 127.25, 96 56 (−C), 16.98. 16.13, 14.17 (CH3−Ar). MS (EI), m/z (%) = 462 [M+] (100). Synthesis of Model 1,3,5-Tris(1,3,5,7-tetramethyl-6-ethyl4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene-2-ethynyl)benzene (BDP2). The synthesis of model BDP2 was carried out following Scheme S2: (1) phosphorus oxychloride (0.66 mL, 6.6 mmol) was added to a stirred solution of 4-ethyl-3,5,dimethyl-1Hpyrrole-2-carbaldehyde (1 g, 6.6 mmol) in dichloromethane (30 mL) at room temperature under argon. After 30 min, 2,4-dimethylpyrrole (0.6 mL, 6 mmol) in dichloromethane (10 mL) was added, and the mixture was further stirred at 40 °C for 2 h. Triethylamine (4.2 mL, 30 mmol) was added and then after 15 min, boron trifluoride diethyl etherate (5.4 mL, 42 mmol). The mixture was stirred at room temperature for 2 h. The work-up yielded a brown residue that was purified by flash column chromatography (silica gel, heptane−ethyl acetate 9:1 as eluent). Pure 2-ethyl-1,3,5,7-pentamethyl-4,4-difluoro-4bora-3a,4a-diaza-s-indacene (4) was a red solid; yield 1.8 g, 80%. 1H NMR (300 MHz): δ = 7.0 (s, 1H, H-8), 6.01 (s, 1H, H-6), 2.52 (s, 6H, CH3−C3 and CH3−C5), 2.39 (c, 2H, J = 7.6 Hz, CH2CH3), 2.24 (s, 3H, CH3−C1), 2.18 (s, 3H, CH3−C7), 1.07 (t, 3H, J = 7.6 Hz, CH3CH2) ppm. 13C NMR (75 MHz): δ = 156.9 (C7), 155.48 (C1), 140.29 (C3), 138.04 (C5), 133.45 (C6), 133.20 (C8a), 132.83 (C7a), 119.66 (H−C8), 118.66 (H−C2), 17.69 (CH2CH3), 14.96 and 14.90

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RESULTS AND DISCUSSION Synthesis and Characterization. BDP1 and BDP2 were synthesized to be used as model compounds in order to establish the synthetic procedure for preparation of CMPBDP polymers and to study their photophysical properties in homogeneous media. The synthetic procedures are summarized in Schemes 1 and 2. Briefly, BDP1 and BDP2 were synthesized B


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Macromolecules Scheme 1. Synthesis of BDP1 and BDP2 Model Compounds

Scheme 2. Synthesis of Conjugated Microporous Polymers Based on BODIPY and CMPBDPa

a

Idealized geometry optimized by Materials Studio 6.0 using “universal” as force field.

C


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Macromolecules by a Sonogashira−Hagihara cross-coupling reaction between 2,6-diiodo-1,3,5,7,8-pentamethyl-4,4-difluoro-4-bora-3a,4adiaza-s-indacene (diiodoBODIPY, 1)33 or 2-ethyl-6-diiodo1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (monoiodo BODIPY, 2) and phenylacetylene or 1,3,5triethynylbenzene (3),34 respectively (Scheme 1). These syntheses established the optimal reaction conditions to prepare porous CMPBDP polymers. Thus, CMPBDP was synthesized by a palladium-catalyzed Sonogashira−Hagihara cross-coupling reaction between diiodoBODIPY (1) and compound 3 (Scheme 2). Moreover, taking into account the literature, several batches were synthesized modifying the proportion between monomers (1:1 and 1.5:1 molar ratio)35 and the solvent used36 (toluene and DMF; see Table S1 in the Supporting Information). In this case, the better conditions (in terms of porosity) were the use of an excess of alkyne groups versus iodine groups (1.5:1 monomer molar ratio) and DMF as solvent as has been reported by other authors. On other hand, the best method to remove impurities from CMPBDP as palladium nanoparticles was a treatment with aqueous cyanide for 24 h. Note that BODIPY dyes are sensitive to strong acids the typical method to eliminate palladium impurities. CMPBDP was characterized by solid-state 13C NMR (Figure 1), FTIR (Figure 2), and elemental analysis.37 Although the polymer network is amorphous (DRX pattern in Figure S1 of the Supporting Information), idealized geometry of CMPBDP optimized by Materials Studio 6.0 using “universal” as force field shows a planar configuration as illustrated in the (Scheme 2).

Figure 2. FTIR spectra of the models compounds BDP1 (a), BDP2 (b), and the polymer networks CMPBDP (c).

(Figure S8). The thermogram shows a first step corresponding to a 25% mass loss (ca. 350 °C) and a main mass loss (ca. 570 °C). We have reported previously38 that BODIPY dyes decompose thermally around 300 °C; this temperature is increased when the chromophore is covalently bound to polyhedral oligomeric silsesquixanes (POSS). If we compare CMPBDP and POSS, it is clear that the chromophore thermal stability is significantly improved when it is forming part of a polymer network. The porous nature of CMPBDP was studied by N 2 adsorption measurements at 77 K, and the surface area results ca. 299 m2 g−1 (calculated Brunauer−Emmett−Teller (BET)) (Figure 3). The total pore volume derived from the t-plot method is 0.19 cm3 g−1, and pore size distribution (calculated by Barrett−Joyner−Halenda (BJH) analyst) revealed that the micropores are below 1.3 nm whereas the mesoporous are centered at 3.7 nm (Figure 3, inset). SEM images show that CMPBDP is composed of laminar network according with the planar idealized geometry represented before (Figure 4).

Figure 1. Solid-state 13C NMR spectra of CMPBDP. Side bands have been marked with an asterisk.

The solid-state 13C NMR (Figure 1) shows broad peaks between δ 100−150 ppm due to aromatic carbons atoms and signals at δ 3−20 ppm assigned as the carbon atoms of the methyl groups from the BODIPY moiety. The FTIR spectrum of CMPBDP (Figure 2) shows the characteristic bands of BODIPY core at 1549, 1200, and 1005 cm−1. The entire network polymer shows the characteristic CC stretching band al 2194 cm−1 and aromatic C−H stretching frequencies up to 3000 cm−1. FTIR also shows the differences in the benzene substitution between the monosubstituted in model BDP1 and the network polymer (band located at 830 and 723 cm−1). The thermal stability of CMPBDP was analyzed by thermogravimetric analysis (TGA) under a N2 atmosphere

Figure 3. Nitrogen sorption isotherms (filled circles: absorption; open circles: desorption) of CMPBDP at 77 K. Inset: pore size distribution profiles using the BJH method (black square: differential pore volume; blue squares: cumulative pore volume). D


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Figure 6. Normalized absorption of dispersions of CMPBDP (black) in 2-ethoxyethanol (0.45 mg/mL) (dashed line) and DCM (solid lines) compared with BDP1 (blue), BDP2 (red), and PM567 (orange) in DCM.

broadness and hypsochromic shift of the main band observed in the CMPBDP are probably due to a high contribution of a sideon arrangement of the molecules into H-aggregates.39−41 So, the H-aggregates absorb in the region of the shoulder of the S0 → S1 transition of the BODIPY chromophore become wider the band. In all solvents the absorption pattern is independent of the concentration without changes in the proportion between the H aggregate band and the S0 → S1 transition band. In this kind of aggregate is necessary a low distance (ca. 4 Å) between the chromophore units and is indicative of the laminar self-assembly of the microporous polymer. Table 1 shows the fluorescence emission of BDP2 is strongly quenched regarding to PM567 dye used as reference and even regarding to BDP1, probably due to the free rotation of the BODIPY chromophores around ethynyl bounds and probably the subsequent increase of the internal conversion as well as the existence of H-aggregation. The fluorescence emission pattern of BDP2 exciting at 490 nm in dilute solution (ca. 5 μM) as well as in concentrated solutions (ca. 0.3 mM) shows the contribution of two peaks at 525 and 575 nm, respectively (Figure 7). Probably at 490 nm it was coexisting two species:

Figure 4. SEM micrograph of CMPBDP at different magnifications.

Photophysical Properties. The polymeric CMPBDP was an insoluble solid able to be efficiently dispersed in organic solvents such as 2-ethoxyethanol and dichloromethane (DCM) and poorly dispersed in ethyl acetate (EtOAc) and toluene and that well-dispersed samples show fluorescence under a UV lamp (Figure 5a). More significantly the CMPBDP sample is highly fluorescent even in the solid state (Figure 5b,c). Figure 6 depicts the normalized UV/vis spectra of CMPBDP dispersion in DCM and 2-ethoxyethanol solvent compared with both soluble models BDP1 and BDP2 in DCM. The spectra showed important contribution of both triethynylbenzene and BODIPY subunits. So, it can be appreciated the π−π* transition of triethynylbenzene (300 nm) and an intense absorption band (ca. 536 nm) assigned to the S0 → S1 transition of the BODIPY chromophore.

Figure 5. (a) Picture of CMPBDP dispersed in several solvents under a UV lamp. (b, c) Fluorescence microscopy images of CMPBDP. Note: 2-EE means 2-ethoxyethanol.

Noteworthy that in the case of BDP2 model the π−π* transition of triethynylbenzene is less important than the same transition of ethylbenzene in BDP1 and CMPBDP because their molar proportion is lower. In all the solvents selected the CMPBDP pattern showed an important dispersion contribution (ca. 700 nm) except in 2-ethoxyethanol solvent (Figure 6). Both CMPBDP and BDPs models show a significant bathochromic shift compared to the standard PM567 dye (ca. λabs = 520 nm)37 as well as an increase in the Stokes shift, indicative of extension of the conjugation across the alkyne bound. If the absorption band of the BODIPY chromophore in CMPBDP is compared with the absorption band of the models BDP1 and BDP2, we have found a hypsochromic shift and broadness. If we assume that the BDP1 is the correct model in terms of extension of conjugation system, we can conclude that the

Figure 7. Absorption (solid line) and fluorescence emission (dashed line) spectra of BDP1 (blue) and BDP2 (red) models in DCM.

the molecule BDP2 complete solvated and responsive of the main band at 575 nm and H-aggregates, with much less contribution, responsive of the little peak at 525 nm. In the case of BDP1 there is no evidence of the formation of H-aggregation in diluted solution, but it is obvious in concentrated solution (ca. 0.9 mM) as well as related compounds with other substituent in the s-indacene core show H aggregation.41 The fluorescence spectra were recorded exciting at 350 nm where both triethynylbenzene moiety and BODIPY core absorb E


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solvent of choice using homogeneous BODIPY photocatalyst; for the heterogeneous CMPBDP photocatalyst we have chosen 2-ethoxyethanol due to the better dispersibility compared with that observed in methanol where CMPDBD cannot be dispersed. In fact, attempts of photocatalysis reaction with CMPBDP in methanol were not successful. The photocatalytic activity of CMPBDP was evaluated under flow air in 2ethoxyethanol without additives in the presence of 0.5, 0.1, and 0.05 mol % of BODIPY catalyst (Table 2).

(Figure 8a). In all solvents the emission spectrum was composed for two regions: a broad band in the UV assigned

Table 1. Photophysical Properties of Soluble Models, BDP1 and BDP2, in DCMa

a

compd

λabs (nm)

ε (M−1 cm−1)

λem (nm)

Δν (nm)

Φflu

BDP1 BDP2

561 553

29400 41700

594 575

33 22

0.58 0.22

Using as reference Φflu PM567 (EtOH) = 0.8.37

Table 2. Photocatalyzed Oxidation Reaction of Thioanisole to Methylphenyl Sulfoxide Figure 8. Fluorescence emission spectra from dispersions of CMPBDP in 2-ethoxyethanol and DCM λexc = 350 nm (a) and 490 nm (b). Fluorescence emission spectrum of BDP2 in DCM is included for comparative purposes (red line). entry 1 2 3 4 5 6 7 8 9 10 11 12 13

to the emission from the excited estate of the triethynylbenzene and another band in the visible spectrum ascribed to the emission of the BODIPY chromophore (560 nm) with a shoulder at 515 nm probably due to H-aggregate emission. In the case of DCM and toluene solutions the UV band was weaker than in the case of 2-ethoxyethanol. In order to clarify this point, a new experiment recording the spectra exciting at 490 nm was done (Figure 8b). As was described above at 490 nm the contribution of the H aggregate seems to be high. So, the main fluorescence band is located around 512 nm, and only in the case of 2-ethoxyethanol the shoulder at 560 nm corresponding to the emission of S0 → S1 transition of the BODIPY chromophore appears. Photocatalytic Tests. As it is very well known, the BODIPY dyes have in general low probability to react with triplet oxygen (3O2) to give singlet oxygen (1O2) due to the low triplet state probability. So, the most popular strategy employed to use BODIPY as singlet oxygen sensitizer is the chromophore functionalization with heavy atoms that quench the fluorescence emission from singlet state an increase the intersystem crossing (ISC) to triplet state.25 In spite of it has been surprisingly reported halogen-free BODIPY dyes that have appropriated substituent to induce triplet sensitization.42 On other hand, Jing’s group has described that low amounts of 1O2 generated from halogen-free BODIPY moieties could be enough for selective oxidation of thioanisole to methylphenyl sulfoxide.43−45 In fact, the same authors have published a better result in terms of efficiency with iodine-functionalized BODIPY dye.46 With these ideas in mind, we ask if CMPBDP would be able to selectively oxidize thioanisole to the corresponding sulfoxide. In the photocatalytic experiment all solutions were irradiated under identical conditions at room temperature with a 150 W quartz−halogen lamp (KL 1500, Schott), using a 500 nm cutoff filter in order to ensure that the radiation was absorbed by BODIPY chromophore and that the UV radiation was filtered. According to previous literature results, methanol should be the

catalysta

solvent

CMPBDP

2-EE

BDP1

MeOH 2-EE MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH

BDP2 BODIPY244 2IPhBDP47 BODIPY348 BODIPY148 Ru(bpy)3Cl244 PHP-545 BDP-St45

cat. (mol %)

convb (%)

TONc

0.5 0.1 0.05 0.1 0.1 0.1 0.5 5 0.5 1 0.5 0.5 0.5

45 99 26 44 24 30 99 100 (3 h) 65 (6 h) 99 (6 h) 98 50 99

90 990 520 440 240 300 200

200 100 198

a

The compound from literature have been named as published. Conversion was determined by 1H NMR at 24 h. cTON: turnover number (mmol product/mmol catalyst) at 24 h. Note: 2-EE means 2ethoxyethanol.

b

Reaction with 0.1 mol % of CMPBDP photocatalyst yields 99% of product after 24 h of reaction (entry 2, Table 2) being 4-fold faster than BDP1 in the same reaction conditions (entry 5, Table 2) and being 2-fold faster and 3-fold faster than BDP1 and BDP2, respectively, when the reaction was done in methanol (entries 4 and 6, Table 2). As is expected, decreasing the CMPBDP catalyst content until 0.05 mol % had as results a decrease in the reaction rate until almost half of TON than 0.1 mol %. The kinetic curves can be seen in Figure S10 of the Supporting Information. The good photocatalytic result observed employing 2ethoxyethanol as solvent versus methanol shows that in CMPs materials although the polarity of the solvent seems to play an important role in the photocatalytic event, other effects as dispersibility and “effective surface area” need to be accounted. In fact, Vilela and co-workers related better dispersibility with a more efficient singlet oxygen generation in others CMPs photocatalyst.21 F


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ACKNOWLEDGMENTS This work was supported by MINECO through the followings projects: Consolider Ingenio 2009 (CSD-0050, MULTICAT), MAT2011-29020-C02-02 and MAT2014-52085-C2-2

To give a perspective on effectivity of CMPBDP catalyst is a comparative study with those yet published has been done (Table 2, entries 7−12). Although we have taken into account that the experimental conditions could be different (light source, time, etc.), we could conclude that both molecular catalysts, BDP1 and BDP2, show a catalyst activity similar to those halogen-free BODIPY described previously (entry 7, BODIPY2)44 and less catalytic activity that iodine-functionalized BODIPY (entries 8 and 9)47,48 or dimeric BODIPY dye, BODIPY 148 (entry 10). Also, BDP1 and BDP2 seem to have similar catalytic activity than a very well-known photosensitizer as Ru(bpy)3Cl2 (entry 11).44 On other hand, the heterogeneous catalyst CMPBDP shows better performance than the sole heterogeneous catalyst described until our knowledge, PHP-545 (entry 11) that shows lower conversion than its referable molecular catalyst (entry 13). Although PHP-5 was described as porous material, the authors no give idea about of porosity grade, and maybe this is the reason for our better result comparing both with our referable models, BDP1 and BDP2, and with PHP-5. For recycling performance of CMPBDP, we carried out three experiments of photooxidation showing that activities (TONs) and selectivity were maintained. At ca. 50 h of irradiation time the tendency change at the same time that the solution is photobleached (Figure S11). Control experiments (Table S2) have checked, and no reaction occurs in the absence of light, catalyst, and oxygen and in the presence of azide (a singlet oxygen quencher), while reaction takes place in the presence of BHT (a well-known radical quencher).

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ABBREVIATIONS BDP1 and BDP2, BODIPY model molecules; CMPBDP, conjugated microporous polymer based on BODIPY dye; 2-EE, 2-ethoxyethanol.

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CONCLUSIONS In conclusion, we report the synthesis of a conjugated microporous polymer based on BODIPY moiety which endows with emitting properties to the polymer both in liquid dispersion and in solid state. The new material shows high thermal stability and high surface area and was used as heterogeneous photocatalyst for the oxidation reaction of thioanisole to methylphenyl sulfoxide, via singlet oxygen, exhibiting 4 times higher rate than the corresponding soluble catalyst in homogeneous medium. Moreover, CMPBDP catalyst could be reused two times with similar behavior. Taking into account its high thermostability and photostability, we expect these materials will be useful like light-harvesting materials; some tests are in progress. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02511. Figures S1−S12; Tables S1 and S2 (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] and [email protected] (M.L.). Present Address

M.L.: IMDEA Energy Institute, Avda. Ramón de la Sagra, 3. Parque Tecnológico de Móstoles E-28935 Móstoles, Madrid, Spain. Notes

The authors declare no competing financial interest. G


Article

Macromolecules

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