Multifunctional Porous Organic Polymers: Tuning of Porosity, CO2

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Multifunctional porous organic polymers: tuning of porosity, CO2 and H2 storage and visible-light-driven photocatalysis Sujoy Bandyopadhyay, Amith G Anil, Anto James, and Abhijit Patra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08331 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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ACS Applied Materials & Interfaces

Multifunctional Porous Organic Polymers: Tuning of Porosity, CO2 and H2 Storage and Visible-LightDriven Photocatalysis Sujoy Bandyopadhyay,‡ Amith G. Anil,‡ Anto James and Abhijit Patra* Indian Institute of Science Education and Research Bhopal, Indore by-pass road, Bhauri, Bhopal462066, Madhya Pradesh, India KEYWORDS porous organic polymer, BODIPY, multifunctional, photocatalyst, singlet oxygen, CO2 uptake, H2 storage

ABSTRACT

A series of porous organic polymers (POPs) were fabricated based on the boron dipyrromethene (BODIPY) core. The variation of the substituents in the BODIPY core and the fine tuning of the Sonogashira polycondenzation reaction with 1,3,5-triethynylbenzene led to the formation of POPs with a wide range of surface area and porosity. A tenfold increase in surface area from 73 m2g-1 in BDT1a polymer to 1010 m2g-1 in BDT3 was obtained. Simultaneously, the porosity was changed from mesoporous to ultramicroporous. The surface area of BDT3 turned out to be the highest reported so far for BODIPY based POPs. Molecular dynamics simulation coupled with Grand Canonical Monte Carlo simulations revealed the effect of substituents alkyl

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groups and rigidity of the core structures on the surface properties of the POPs. Detailed gas adsorption studies of the polymers revealed a high uptake of CO2 and H2. The highest uptake capacity of 16.5 wt. % for CO2 at 273 K and 2.2 wt.% for H2 at 77 K was observed for BDT3 at 1 bar pressure. Isosteric heat of adsorption (Qst) of BDT3 for CO2 was found to be as high as 30.6 kJ mol-1. Electron paramagnetic resonance studies revealed the generation of singlet oxygen upon photoexcitation of these polymers. The BODIPY based POPs turned out to be excellent catalysts for visible-light-driven photooxidation of thioanisole. The present study establishes BODIPY based POPs as a new class of multifunctional materials.

INTRODUCTION A material possessing varied properties and pertinent for different kind of applications captures the attention of scientific community for technological as well as fundamental research point of view. The exquisite control over the assembly of the building blocks and thorough understanding of the structure property relationship hold the key to the realization of a multifunctional material. Starting from the grand old activated charcoal to zeolites, porous materials have been utilized for environmental and technological benefits of mankind.1-2 Development of metal organic frameworks further enriched the myriad applications of functional materials with a well-defined pore structure.3-4 Of late, conjugated porous organic polymers (POPs) have emerged as a new class of materials capable of exhibiting diverse properties.5-6 The presence of strong C-C covalent bonds makes POPs thermally and hydrothermally extremely stable. The inherent versatility and flexibility in the design and fabrication of monomeric building blocks allow the tuning of porosity in the resultant network leading to applications ranging from selective gas adsorption,7-15 gas and liquid separation,16-19 catalysis20-24 to water purification.25 Additionally, extended π conjugation along with porosity in the network polymers open up plethora of


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optoelectronic applications like light harvesting,26-27 energy storage,28-31 chemo and biosensing.32-39 However, fabrication of a single porous material capable of exhibiting a range of different applications remain a challenge. The capture and conversion of CO2 gas are of prime importance due to its role in global warming. POPs, with narrow pore size distribution, are potential candidates for selective CO2 adsorption. The electron rich aromatic framework and extended conjugation serve as active interaction sites for CO2 molecules.14,

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Recent studies revealed that the presence of

electronegative heteroatoms like nitrogen, oxygen or fluorine in the monomer core enhance CO2philicity via polar interactions.41-43 Incorporation of CO2 active sites can be done either through functionalization of building blocks or through post-polymerization modifications.44 The reduction of pore size also enhances the hydrogen uptake capacity.45 π-conjugation shifts the absorption of the polymers towards the longer wavelengths and further tuning of the band gaps allow sensitization of singlet oxygen and visible-light-driven photocatalysis.46-48 Fluorescent POPs can also act as an energy donor and controlling the cascade energy transfer from the donor polymer to the acceptor molecules lead to efficient light harvesting.27,

49

Molecular dynamics

simulations aid to establish the structure-property relationship in POPs and have been applied to predict and understand the novel design principles.50-51 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPY) are a class of versatile fluorophores widely investigated for a variety of applications including chemosensing, photodynamic therapy, biological labelling, dye-sensitized solar cells, laser dyes and so on.52-53 Although there are multitudes of BODIPY small molecules, BODIPY based porous polymers are less known. Zhuang et al. reported dimensionality controlled heterostructures comprising BODIPY based porous polymers and nanocarbons for efficient catalysis in oxygen reduction reaction.54


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Recently, Liras and coworkers developed a conjugated microporous polymer incorporating BODIPY moiety and explored its application in photocatalysis.55 This paper focusses on design and fabrication of a novel series of BODIPY based POPs exhibiting high carbon dioxide and hydrogen uptake. The surface areas of the polymers were tuned from 73 m2g-1 to 1010 m2g-1 by the variation of the sub-structures and polymerization conditions. Computational modelling revealed the correlation between the network structure and surface properties. To the best of our knowledge, tuning of surface area and such comprehensive study of gas adsorption properties on BODIPY based POPs, have not been reported so far. Further, the POPs developed in the present study were found to be excellent materials in visible-light-driven singlet oxygen mediated photocatalysis.

RESULTS AND DISCUSSION Fabrication and characterization of polymers BODIPY cores were synthesized from 2,4-dimethyl pyrrole in a one-pot two-step synthesis involving acyl chloride and boron trifluoride diethyl etherate following a reported procedure with minor modifications.53 The iodination of meso-substituted BODIPY derivatives with Niodosuccinimide yielded monomers M1, M2 and M3 (Scheme S1). The purity of the monomers were ascertained by 1H and

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C NMR spectroscopy and mass spectrometry (Figures S1-S3).

Polymers were synthesized by Pd-catalyzed Sonogashira cross-coupling polycondenzation involving M1, M2 and M3 as monomers and 1,3,5-triethynyl benzene (TEB) as a comonomer (Scheme 1). n-Octyl substituted monomer M1 led to the polymers BDT1a and BDT1b. 4Tertiary butyl phenyl substituted BODIPY monomer M2 and phenyl substituted monomer M3 led to the polymers BDT2 and BDT3 respectively. The polymer BDT1a was obtained using bis(triphenylphosphine)palladium(II) dichloride as a catalyst in toluene at 80 oC, while rest of the


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polymers were synthesized with the tetrakis(triphenylphosphine)palladium(0) catalyst in DMF at 130 oC (Scheme 1). The polymers were precipitated in cold acidified methanol and were thoroughly washed by multiple Soxhlet extractions with methanol, acetone, chloroform and ethanol respectively. All the polymers were obtained as dark violet powder, insoluble in common organic solvents and unaffected upon exposure to strong acids and alkalis.

Scheme 1 Synthesis of BODIPY based porous organic polymers. Fabrication conditions: BDT1a: M1 monomer, [Pd(PPh3)2Cl2], toluene, 80 oC; BDT1b: M1 monomer, [Pd(PPh3)4], DMF, 130 oC; BDT2: M2 monomer, [Pd(PPh3)4], DMF, 130 oC; BDT3: M3 monomer, [Pd(PPh3)4], DMF, 130 o C. The Fourier transform infra-red spectroscopy revealed the characteristic C≡C stretching frequency near 2200 cm-1 in the polymers confirming the successful coupling of BODIPY monomers with TEB (Figure 1, Figure S4). The aliphatic and aromatic CH stretching were noticeable between 2700 cm-1 and 3000 cm-1. Furthermore, the structures of the polymers were analyzed using 13C cross polarization total suppression of spinning sidebands (CP-TOSS) NMR spectroscopy; spectral profile for BDT3 is shown in Figure 2. The peaks near 100 ppm and 90 ppm belong to the two acetylene carbons further confirming coupling between the monomers. The intense broad peak near 10-15 ppm belongs to the methyl carbons attached to the BODIPY core. The other methyl carbons of the meso-substituents in BDT1a and BDT2 give resonance


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Figure 1 FTIR spectra of (a) BDT1a, (b) BDT1b, (c) BDT2 and (d) BDT3. around 30 ppm (Figures S5, S6). Methyl carbons directly connected to the BODIPY core are shielded by the magnetic anisotropy around acetylene moiety. Hence, they resonate at a higher

Figure 2 Solid state 13C CP-TOSS spectrum of BDT3.


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field compared to other methyl carbons. Resonances generated by aromatic carbons can be found from 110-150 ppm. The thermogravimetric analysis (TGA) profiles of the polymers showed that BDT1a, BDT1b and BDT2 are thermally stable up to 300 oC; BDT3 was found to be stable up to 360 oC (Figure S7). The polymers are amorphous in nature as indicated by broad patterns in the powder X-ray diffraction (Figure S8). The morphology of the polymers, observed through field emission scanning electron microscopy (FESEM, Figure 3), was found to be well correlated with the surface properties as discussed below.

Figure 3 FESEM images of (a) BDT1a, (b) BDT1b, (c) BDT2 and (d) BDT3. Nitrogen sorption isotherms The surface areas and porosities of the polymers were determined by nitrogen adsorptiondesorption isotherms measured at 77 K (Figure 4). BDT1a and BDT1b show type IVa isotherms whereas BDT2 and BDT3 show Type IVb isotherm as per the latest IUPAC convention.56 The Brunauer-Emmett-Teller (BET) surface area of BDT1a and BDT1b were found to be 73 and 322 m2g-1 respectively. BET surface area of BDT2 and BDT3 were obtained as 573 and 1010 m2g-1 respectively (Table 1). The large hysteresis near P/P0 = 0.5 for BDT1a and BDT1b signifies mesoporous (pore size 2 - 50 nm) nature (Figure S10). High nitrogen uptakes by BDT2 and


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BDT3 at low relative pressures (P/P0 < 0.1) indicate the existence of micropores (pore dimension < 2 nm).57 Pore size distribution calculated by nonlocal density functional theory (NLDFT) method revealed that the pores are centered at 2.8, 3.2, 1.4 and 0.7 nm respectively for BDT1a, BDT1b, BDT2 and BDT3 (Figure 4a, inset). A significant contribution of mesopores ranging from 2 to 10 nm was also observed in BDT2 and BDT3. These observations establish BDT1a and BDT1b as mesoporous and BDT2 and BDT3 as microporous and ultramicroporous polymers respectively.

Figure 4 Nitrogen sorption isotherms of (a) BDT1a, (b) BDT1b, (c) BDT2 and (d) BDT3 at 77 K (filled shapes: adsorption; hollow shapes: desorption). Inset: pore size distribution profiles estimated using the NLDFT method. BDT1a shows the least surface area of 73 m2g-1 across the series. This reduced surface area is likely to be attributed to the presence of long alkyl chain in the monomeric core. Previous reports showed that longer alkyl chains impart lower surface area compared to shorter and branched alkyl chains.58 Flexible alkyl chains occupy a larger proportion of the free volume created by the porous network.59 The blocking of the pores by the long octyl chain consequently results in a lower accessible surface area for BDT1a. FESEM image of BDT1a also depicts fused sphere-


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like features accounting for a lower surface area (Figure 3a). Interestingly, a fourfold increase in surface area was obtained in BDT1b using the same monomeric core of M1 by tuning the reaction conditions. The Pd(0) catalyst was used instead of Pd(II) and the polymerization was carried out in DMF at 130 oC. The reaction solvent has important effects on surface area and porosity. There is a higher degree of condenzation in the polymers synthesized in high boiling solvent DMF compared to that of toluene.60 The FESEM images also reveal the contrasting morphologies of BDT1b and BDT1a; compared to sphere-like aggregated structures in BDT1a, BDT1b has a coarser porous morphology (Figure 3b). Table 1 Summary of the gas adsorption properties of BODIPY based POPs.

Polymer

SBET (m2g-1)

Vtot (cm3g-1)

N2 uptake H2 uptake (wt.%) (wt.%) Pore size (nm)

CO2 uptake (wt.%)

CH4 uptake (wt.%)

QSt

273 K

77 K

273 K

(kJ mol-1)

273 K

BDT1a

73

0.2

2.8

-

-

-

-

-

BDT1b

322

0.5

3.2

2.2

1

6.9

20

0.6

BDT2

571

1.3

1.4

1.1

1.5

10.5

26.8

0.8

BDT3

1010

1.5

0.7

4.6

2.2

16.5

30.6

2.9

BDT2 is composed of a comparatively rigid core. The branched tert-butyl phenyl group offers much lesser flexibility than the long octyl chain.59 As a result, pore blocking effect is reduced significantly. Additionally, non-planarity induced by the phenyl ring reduces the aggregation of polymer chains leading to lower interpenetration. Consequently, a higher surface area of 573 m2g-1 is observed. The surface area of BDT3 was found to be 1010 m2g-1. To the best of our knowledge, BDT3 exhibits the highest surface area reported so far for BODIPY based porous


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organic polymers. Replacing 4-tert-butyl group in M2 by hydrogen in M3 monomer makes the later the most rigid among others. Phenyl ring at the meso-position stays almost perpendicular to the indacene plane due to the steric bulk of β-methyl groups. Rigidity combined with the nonplanarity result in the highest surface area in BDT3. The replacement of octyl to tert-butyl phenyl to phenyl group at the meso-position of the BODIPY core led to the increase in microporosity of the resultant network polymers while decreasing the average pore diameter from 3.2 nm in BDT1b to 1.4 nm in BDT2 to 0.7 nm in BDT3. Computational modelling In-silico modelling of the polymers was carried out to gain the molecular level understanding of pore structure and to have a better insight of the gas adsorption properties. A single polymer chain was developed using dendritic approach by the successive addition of the repeat units up to 6th generation and geometry was optimized.61 Figure 5 represents the optimized structures of BODIPY polymers. A gradual decrease of interpenetration and consequent increase in the pore volume was observed from BDT1 to BDT3. As discussed earlier, the simulated structures also corroborated the fact that the flexible alkyl chains compared to shorter and branched alkyl chains reduce the free volume by blocking the pores.

Figure 5 Geometry optimized structures of polymer chains: (a) BDT1, (b) BDT2 and (c) BDT3.


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Figure 6 Snapshot of nitrogen sorption in the simulated amorphous cell at 77 K and 1 bar: (a) BDT1b, (b) BDT2 and (c) BDT3 (red dots: density field of adsorbed nitrogen molecules, cyan and grey: solvent accessible surfaces). The atomistic model of the polymers was developed using simulated polymerization and equilibration employing molecular dynamics simulation. The broad nature of the simulated PXRD patterns (Figure S12) for all three polymers showed a good resemblance to that obtained experimentally, validating the models developed for amorphous polymers. The surface area of the polymer obtained for octyl substituted BODIPY monomer (M1) was found to be similar to that of BDT1b and thus considered as a model for the same. The solvent accessible surface areas computed for BDT1b, BDT2 and BDT3, are 308, 497 and 938 m2g-1 respectively. The change in surface area values indicates the same trend as that obtained from BET analysis. The solvent accessible surface area was found to be proportional to the free volume (Table S2). The increase in the free volume having the similar core structure is likely to be an indication of lowering of the interpenetration of polymer chains (Figure S13). The slightly lower values of simulated surface area might be due to the overestimation of the density of polymer model.62 This can arise since the modelling of polymers is carried out in the absence of solvents. The snapshots of nitrogen sorption in amorphous cells obtained through Grand Canonical Monte Carlo simulations


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at 77 K and 1 bar clearly indicate that the nitrogen uptake increases from BDT1b to BDT3 (Figure 6). Carbon dioxide uptake The nitrogen rich core and the polar nature of the monomers urged us to probe the CO2 uptake capacity of the polymers. BDT1a was excluded from further gas adsorption studies due to its relatively lower surface area. CO2 sorption isotherms were measured at 273 K at 1 bar pressure. The highest CO2 uptake of 16.5 wt.% was obtained for BDT3 (Figure 7a). This value was found to be either similar or greater compared to most of the POPs, reported for selective CO2 adsorption.17, 43-44, 59, 63-64 The combined effect of three factors results in high CO2 uptake. Firstly, narrow micropores (< 1 nm) contribute largely towards CO2 adsorption rather than wider micropores and mesopores.65 The small pore size of BDT3 (0.7 nm) enhances CO2 filling via multiwall interaction.66 Secondly, the rich Nitrogen content of BODIPY units promotes favorable interaction of the porous network with CO2.63, 66 Finally, a higher BET surface area also improves CO2 adsorption. BDT2 and BDT1b showed uptake of 10.5 wt.% and 6.9 wt.% respectively (Figure 7a). Although both the polymers have nitrogen rich network as well as appreciable surface area, the low CO2 uptake is a consequence of their larger pore size. In all cases, hysteresis was observed in the CO2 adsorption-desorption isotherms, which is attributed to the interaction of CO2 molecules with porous surfaces.67 The affinity of CO2 towards the polymers was further evaluated from the measurements of isosteric heat of adsorption (Qst). The Clausius-Clayperon equation was used to calculate Qst using CO2 adsorption isotherms collected at 273 K and 298 K (Figures 7a, S14). The calculated Qst values follow the same trend as that of CO2 uptake (Figure 7). Among the polymers, BDT3


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Figure 7 (a) CO2 sorption isotherms of BDT1b, BDT2 and BDT3 at 273 K (filled shapes: adsorption; hollow shapes: desorption). (b) Isosteric heat of adsorption (Qst) profiles of the polymers for CO2. showed the highest Qst of 30.6 kJ mol-1 at the onset of adsorption. This value is quite significant compared to other reported POPs (25-33 kJ mol-1).68 The high Qst value of BDT3 supports the existence of favorable interactions of the porous network with CO2. The Qst value decreases gradually to 20 kJ mol-1 at higher CO2 pressure and then remains nearly constant (Figure 7b). The high adsorption enthalpy at low pressure is due to the initial filling of the small micropores through adsorbate-surface interactions.69 BDT2 has the initial Qst of 26.8 kJ mol-1. The Qst value of BDT2 decreases to 18.3 till 6 wt.% of CO2 loading and then remains almost constant. BDT1b has the lowest Qst of 20 kJ mol-1 which dropped rapidly at higher coverages (Figure 7b). The low value and rapid decrease in Qst are due to the larger and less energetically favorable pores of BDT1b resulting no significant interaction with CO2. Hydrogen and methane uptake The polymers were found to uptake appreciably high amount of hydrogen at 77 K. An uptake of 2.22, 1.49 and 0.99 wt.%, was observed for BDT3, BDT2 and BDT1b respectively (Figure 8a). Both BDT3 and BDT2 exhibited type Ib isotherm due to the presence of wide range of pore sizes including large micropores and narrow mesopores.56 BDT3 and BDT2 show multilayer adsorption as suggested by the steps in the isotherms (Figure 8a). BDT1b shows type III


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adsorption isotherm signifying less or no favourable interaction between the polymer network and H2 molecules. This is due to the presence of large mesopores hindering multiwall interaction. Throughout the pressure range, there is a continuous increase in the adsorption without any apparent saturation observed for all the polymers. This observation implies that the BODIPY based polymers can uptake more amount of H2 at higher pressure regime.

Figure 8 (a) Hydrogen and (b) methane adsorption isotherms of BDT1b, BDT2 and BDT3 at 77 K and 273 K respectively. Methane adsorption isotherms of the polymers were recorded at 273 K. BDT3 shows the highest uptake of 2.9 wt.% followed by 0.8 wt.% by BDT2 and 0.6 wt.% by BDT1b (Figure 8b). All the polymers have adsorption isotherm similar to type III indicating no favorable interaction of methane with the porous network. The variation in the uptake capacity also follows the trend that observed for surface area. Selectivity in gas adsorption Selective gas uptake is a desirable property for practical applications of a porous material. We determined the selectivity for CO2 over N2 in binary mixtures using experimental singlecomponent isotherms. The adsorption data of CO2 and N2 were measured at 273 K for pressures 0 - 1.0 bar. The simplified ideal adsorbed solution theory (IAST) model was employed at an equilibrium partial pressure of 0.85 bar for N2 and 0.15 bar for CO2 in the bulk phase.10, 57 It can be seen that CO2 is more selectively adsorbed compared to N2 in all polymers (Figure 9a, Figure


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S15). The higher quadrupole moment of CO2 leads to a stronger electrostatic interaction with the network polymers compared to that of N2.69 It is interesting to note that BDT2 has the highest selectivity of 11. The lower selectivity of BDT3 compared to BDT2 can be attributed to its small pores which can favorably interact with both CO2 and N2 even at 273 K. The present study reveals that pore size also has a crucial role in determining selectivity in addition to the presence of heteroatoms. The selectivity of carbon dioxide over methane was also calculated following the similar procedure as discussed above and taking 0.5 bar equilibrium partial pressure for both the gasses (Figure 9b, Figure S16).

Figure 9 Selective CO2 uptake of BDT2 over (a) nitrogen and (b) methane at 273 K. Photophysical properties Steady-state absorption and emission of the polymers were measured by preparing the dispersion of the polymers in chloroform. All the polymers showed greenish fluorescence in the dispersion upon excitation by UV light at 365 nm (Figure 10a). Spectroscopic measurements of the filtrate were carried out. All the polymer dispersions showed absorption around 500 nm and emission around 512 nm. The fluorescence quantum yields of the polymers were found to be 15 20%. It was observed that the emission of polymers is independent of the excitation wavelengths. The peak maxima of the excitation spectra were found to be similar with the absorption maxima indicating that the emission occurs from the same state where direct absorption takes place. It is noticeable that both absorption and emission spectra are blue-shifted compared to the


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corresponding monomers (Figures S17, S18, Tables S3, S4). This is likely to be due to the removal of iodine during the cross coupling of monomers.52 The fluorescence microscopy images of the polymers are shown in Figure 10b depicting green fluorescence upon excitation by blue light (460-490 nm). The optical bandgaps of the polymers estimated from the onset of absorption were found to be ~ 2 eV (Table S4).

Figure 10 (a) Digital photographs of dispersion of polymers in chloroform (i) BDT1a, (ii) BDT1b, (iii) BDT2 and (iv) BDT3 under 365 nm UV light. (b) Fluorescence microscopy images of the corresponding polymers (Scale bar = 20 µm). Singlet oxygen generation and photocatalysis Although BODIPY molecules are well known for their strong fluorescence, the synthesized polymers were found to be relatively weak fluorescent. The facile inter-system crossing from excited singlet state to the triplet state is likely to diminish the fluorescence of the polymers. The combination of a weak electron donor with an electron accepting monomer prevents fast recombination of excitons yielding more intersystem crossing.22 BODIPY being an electron acceptor whereas triethynylbenzene being a weak electron donor, facilitates the population of triplet state. Thus, the developed polymers are likely to be capable of photosensitization of singlet oxygen.48 The point was experimentally validated by electron paramagnetic resonance (EPR) spectroscopy using 2,2,6,6-tetramethylpiperidine (TEMP) as a spin trapping agent. BDT3


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polymer was added to a solution of TEMP in toluene. A weak 1:1:1 triplet signal centered at 3355 Gauss was observed in EPR after the addition of BDT3 (Figure 11). The characteristic signal is due to the formation of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radical formed by the reaction of TEMP with singlet oxygen, generated from the photoexcitation of polymer by ambient light. Upon photoexcitation of the sample using a mercury vapor lamp, the characteristic triplet signal of TEMPO radical was further intensified (Figure 11). This study clearly ascertains that the BODIPY based porous polymers are capable of singlet oxygen generation (Figure S19).

Figure 11 EPR spectra of (a) TEMP, (b) TEMP + BDT3 under ambient light and (c) TEMP + BDT3 under photoexcitation. We further explored the application of the POPs developed in the present study in singlet oxygen mediated photocatalysis. Methyl phenyl sulfoxide, an important pharmaceutical intermediate, was synthesized from thioanisole using BODIPY polymers as the photocatalyst (Table 2). Thioanisole (3 mmol) was taken in 3 mL acetonitrile and 1 wt% of polymer was added to it. The reaction mixture was irradiated at room temperature with a 100 W incandescent bulb for 24 h. 1H-NMR and gas chromatography-mass spectrometry (GCMS) data revealed 91-96 % conversion to methyl phenyl sulfoxide when POPs were employed as a photocatalyst (Table 2,


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Table 2 Photocatalytic activity of BODIPY based POPs under visible light.

Entry

Reaction Condition

Conversion#

1

BDT3, air, hν

91 %

2

Without polymer, air, hν

No reaction

3

BDT3, air, dark

No reaction

4

BDT3, argon, hν

~1%

5

BDT2, air, hν

93 %

6

BDT1b, air, hν

96 %

7

BDT3, air, hν, sodium azide

~ 1%

# Estimated through gas-chromatography (GC) analysis

Figures S19-S24, Table S5). Control experiments indicated no occurrence of reaction in the absence of light, catalyst and oxygen. The formation of the product was negligible in the presence of sodium azide, a singlet oxygen quencher (Table 2). The % of conversion was found to be similar at least up to 4 cycles indicating good stability and reusability of the POPs in photocatalytic reactions. FTIR spectra of the catalyst BDT3 were found to be similar before and after sulfoxidation reaction (Figure S25). The cyclic voltammetry measurements of the polymer and thioanisole further confirm that polymer cannot oxidize thioanisole (Figures S26, S27). Thus, the reaction is likely to be induced by singlet oxygen (Scheme S2). The photocatalysis was not only limited to thioanisole. Further investigations revealed that p-bromo/chloro/methoxysubstituted thioanisole could be converted to corresponding sulfoxide with high yield under similar reaction conditions. The sulfoxidation of diphenyl sulfide and 2-naphthyl methyl sulfide


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was also achieved with high percentage of conversion using BDT3 as a catalyst (Table S6, Figures S28-S37). Comparing with various reported porous organic polymers (Table S7), BODIPY based POPs developed in the present study turn out to be the promising multifunctional materials demonstrating high carbon dioxide and hydrogen uptake and singlet oxygen driven photocatalysis. CONCLUSIONS A novel series of multifunctional porous organic polymers capable of exhibiting high CO2 and H2 uptake, appreciable fluorescence and singlet oxygen mediated photocatalysis has been fabricated using BODIPY core. A tenfold increase in surface area from 73 m2g-1 in BDT1a to 1010 m2g-1 in BDT3 was achieved by judicious tuning of reaction conditions and monomer structures. The rigidity of the polymer backbone was found to be the chief cause of the variation of surface area. The experimental observations were further validated through molecular dynamics simulations. A comprehensive investigation of the gas adsorption behavior of the polymers revealed high uptake capacity of CO2 (up to 16.5 wt.%) at 273 K and 1 bar. The isosteric heat of adsorption (Qst) for CO2 was obtained as high as 30.6 kJ mol-1. Appreciable hydrogen uptake of 2.2 wt.% at 77 K and 1 bar was obtained for BDT3. The EPR measurements of polymer dispersions in toluene under photoexcitation in the presence of a spin trapping agent revealed the signature of singlet oxygen generation. Further application of the polymers was demonstrated in visible-light-driven photooxidation of thioanisole. BODIPY based POPs exhibited a conversion of ~ 96%. POPs developed in the present study will be further probed for supercapacitive energy storage. The facile generation of reactive singlet oxygen by POPs may find significant applications in biology. The current study paves the way for the realization of a multitude of applications of BODIPY based porous organic polymers.


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EXPERIMENTAL METHODS General methods Solution state 1H and

13

C NMR spectra were recorded on Bruker Avance III 500, 700 MHz

NMR spectrometers. Solid state

13

C NMR measurements were carried out on JEOL ECX 400

MHz (field 9.4 T) standard bore spectrometer equipped with 4 mm solid-state MAS (magic angle spinning) probe. The samples were packed into a 4 mm zirconia rotor and spun at 8 kHz at the magic angle. The magic angle was calibrated using

79

Br NMR of KBr. The spectra were

acquired with high power two-pulse phase modulation (TPPM) 1H decoupling during the time of acquisition; a ramped amplitude cross polarization with a total sideband suppression (TOSS) sequence was used. The free induction decay (FID) was processed using Bruker Topspin software.

FTIR

measurements

were

carried

on

Perkin-Elmer

Model

2000

FTIR.

Thermogravimetric analysis (TGA) was carried out using Perkin Elmer TGA-6000 instrument, heating the samples at a rate of 10 oC min−1 under the nitrogen atmosphere to a maximum of 900 o

C. The powder X-ray diffraction (PXRD) experiment was performed on PANalytical Empyrean

XRD instrument. The surface morphologies of the polymers were studied using a Carl Zeiss (Ultraplus) field emission scanning electron microscope (FESEM). Energy dispersive X-ray spectroscopy (EDS) was performed using Oxford Instruments X-MaxN spectrometer attached to FESEM. Synthesis of monomers All monomers were synthesized following minor modifications of a reported procedures52-53 (Scheme S 1). In a typical synthesis for M3, 2,4-dimethyl pyrrole (2 mL, 2 eq.) and benzoyl chloride (1.1 mL, 1 eq.) were dissolved in dry DCM (15 mL). The solution was stirred at room temperature overnight followed by the addition of 3 mL of triethylamine and 3 mL of boron


20

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trifluoride diethyl etherate. After 30 min, the reaction mixture was washed with water and dried over anhydrous magnesium sulfate. The solvent was evaporated to dryness and the residue was purified by silica gel column chromatography to afford a red solid (25% yield). Iodination of BODIPY derivatives was done with N-iodosuccinimide (NIS). BODIPY (1 mmol) and NIS (4 mmol) was dissolved in dry DCM (15 mL). The mixture was stirred at room temperature for 8 h. The solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography followed by recrystallization in ethanol-DCM mixture (85% yield) and characterized by NMR spectroscopy and mass spectrometry (Figures S1-S3). Synthesis of polymers Polymers BDT1-BDT3 were synthesized by Pd-catalyzed Sonogashira cross-coupling polycondenzation reaction of monomers M1-M3 with 1,3,5-triethynylbenzene (TEB) as a comonomer. BDT1a was synthesized using PdCl2(PPh3)2 in toluene at 80 oC while rest of the polymers were synthesized with Pd(PPh3)4 catalyst in dimethyl formamide (DMF) at 130 oC (Scheme 1). In a typical procedure for BDT3 synthesis, M3 (0.2 mmol), copper(I) iodide (1.3 mg, 0.007 mmol) and tetrakis(triphenylphosphine)palladium (10.5 mg, 0.015 mmol) were dissolved in 10 mL dry DMF and subjected to freeze–pump–thaw cycles to remove the dissolved oxygen. To this mixture, 10 mL of freshly distilled diisopropylamine was added and once again degassed by freeze–pump–thaw cycles. The reaction mixture was stirred for 72 h at 130 °C under an argon atmosphere and protection from light. The reaction mixture was allowed to cool down to room temperature and quenched using cold acidified methanol. The precipitate was collected by gravimetric filtration. Then it was washed with methanol and followed by rigorous washing by Soxhlet extraction for 24 h each in methanol, acetone, chloroform and ethanol respectively. The resulting solid was dried under vacuum at 70 oC to yield BDT3 polymer (80 mg, 75% yield).


21


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The polymers were characterized by FTIR and solid-state

Page 22 of 32

13

C-NMR spectroscopy (Figures S4-

S6). Gas adsorption and selectivity studies All the gas sorption studies were performed on Quantachrome Autosorb QUA211011 equipment. Polymer samples were degassed at 80 oC for 24 h before the measurements. Isotherms were analyzed using ASIQwin software. The surface area of the polymers was determined from nitrogen sorption isotherm at 77 K. Pore size distributions of the polymers were calculated from the nitrogen sorption isotherms using nonlocal density functional theory (NLDFT). The heat of adsorption for carbon dioxide was calculated using ASIQwin software. Selective gas adsorption in binary mixtures was estimated from single component isotherms utilizing the ideal adsorbed solution theory (IAST).57 Computational modelling All BODIPY polymers were modelled with Materials Studio 6.1 package provided by accelrys. The initial structure of the polymer model was generated using the polymatic - simulated polymerization algorithm proposed by Colina and coworkers.70 The simulation methodology was analogous to the step growth polycondenzation. The initial structures obtained after polymerization were further equilibrated using molecular dynamics simulation. The 21 steps equilibration protocol composed of annealing, cooling, compression and decompression was performed under NVT or NPT ensemble conditions.50 The geometrical surfaces were analyzed using a probe of radius 1.84 Å, which is equivalent to the kinetic radius of nitrogen molecule. Grand Canonical Monte Carlo simulation was carried out to analyze nitrogen uptake at 77 K.


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Photophysical studies UV-Visible absorption spectra were recorded on Cary 100 spectrophotometer using 10 mm path length quartz cuvette. All the steady-state fluorescence measurements were carried out on Jobin Yvon Horiba Model Fluorolog-3-21. Steady-state fluorescence of the polymers was measured by preparing their dispersion in chloroform. 0.5 mg of the polymer was stirred with 5 mL chloroform for 30 min to get a stable dispersion. Larger particles were filtered off through a 0.2 µm membrane filter and the measurements were done on the filtrate. The fluorescence microscopy was performed on Olympus Inverted System Microscope Model IX81. The samples were prepared by smearing powdered polymer on a glass slide. Images were obtained exciting the sample with blue light (460-490 nm). Singlet oxygen generation and photocatalysis Solution state EPR spectra were recorded on Bruker 9.4 GHz EMX MicroX spectrometer. The modulation frequency and modulation amplitude were 100 kHz and 0.2 G respectively. Sample was prepared by adding 100 µL TEMP to 300 µL of toluene. The dispersion of the polymer in toluene was then added to the above mixture. Photocatalysis reaction was performed in a custom-made photoreactor using 100 W incandescent bulb. In a typical procedure, thioanisole (3 mmol) and ~1 wt.% of the polymer were taken in 3 mL acetonitrile and irradiated with 100 W tungsten filament bulb for 24 h with a constant purging of air. The reaction mixture was centrifuged at 10000 rpm for 10 min, and the supernatant was used for 1H NMR and GCMS analysis.


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ASSOCIATED CONTENT Supporting Information. Details of monomer and polymer synthesis and characterizations, gas adsorption studies, computational and spectroscopic investigations, EPR and electrochemical measurements, singlet oxygen generation and photocatalysis. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions ‡ These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from DST (SB/FT/CS-081/2013), New Delhi and infrastructural support from IISERB are gratefully acknowledged. SB thanks IISERB and AA and AJ thank DST-Inspire for fellowship. We thank Dr. T. G. Ajithkumar in CSIR-National Chemical Laboratory, Pune for the help with 13C-NMR measurements.


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(60) Dawson, R.; Laybourn, A.; Khimyak, Y. Z.; Adams, D. J.; Cooper, A. I. High Surface Area Conjugated

Microporous

Polymers:

The

Importance

of

Reaction

Solvent

Choice.

Macromolecules 2010, 43 (20), 8524-8530. (61) Mansfield, M. L.; Klushin, L. I. Monte Carlo Studies of Dendrimer Macromolecules. Macromolecules 1993, 26 (16), 4262-4268. (62) Del Regno, A.; Gonciaruk, A.; Leay, L.; Carta, M.; Croad, M.; Malpass-Evans, R.; McKeown, N. B.; Siperstein, F. R. Polymers of Intrinsic Microporosity Containing Tröger Base for CO2 Capture. Ind. Eng. Chem. Res. 2013, 52 (47), 16939-16950. (63) Gu, C.; Liu, D.; Huang, W.; Liu, J.; Yang, R. Synthesis of Covalent Triazine-Based Frameworks with High CO2 Adsorption and Selectivity. Polym. Chem. 2015, 6 (42), 7410-7417. (64) Gomes, R.; Bhanja, P.; Bhaumik, A. A Triazine-Based Covalent Organic Polymer for Efficient CO2 Adsorption. Chem. Commun. 2015, 51 (49), 10050-10053. (65) Sevilla, M.; Valle-Vigón, P.; Fuertes, A. B. N-Doped Polypyrrole-Based Porous Carbons for CO2 Capture. Adv. Funct. Mater. 2011, 21 (14), 2781-2787. (66) Dawson, R.; Cooper, A. I.; Adams, D. J. Chemical Functionalization Strategies for Carbon Dioxide Capture in Microporous Organic Polymers. Polym. Int. 2013, 62 (3), 345-352. (67) Byun, Y.; Coskun, A. Bottom-up Approach for the Synthesis of a Three-Dimensional Nanoporous Graphene Nanoribbon Framework and Its Gas Sorption Properties. Chem. Mater. 2015, 27 (7), 2576-2583. (68) Dawson, R.; Stockel, E.; Holst, J. R.; Adams, D. J.; Cooper, A. I. Microporous Organic Polymers for Carbon Dioxide Capture. Energy Environ. Sci. 2011, 4 (10), 4239-4245. (69) Bhunia, A.; Boldog, I.; Moller, A.; Janiak, C. Highly Stable Nanoporous Covalent TriazineBased Frameworks with an Adamantane Core for Carbon Dioxide Sorption and Separation. J. Mater. Chem. A 2013, 1 (47), 14990-14999. (70) Abbott, L. J.; Hart, K. E.; Colina, C. M. Polymatic: A Generalized Simulated Polymerization Algorithm for Amorphous Polymers. Theor. Chem. Acc. 2013, 132 (3), 1-19.


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Multifunctional Porous Organic Polymers: Tuning of Porosity, CO2

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