Nanoporous Conducting Covalent Organic Polymer (COP

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Nanoporous Conducting Covalent Organic Polymer (COP) Nanostructures as Metal-Free High Performance Visible-Light Photocatalyst for Water Treatment and Enhanced CO Capture 2

Soumitra Bhowmik, Rohit G. Jadhav, and Apurba K Das J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07709 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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The Journal of Physical Chemistry

Nanoporous Conducting Covalent Organic Polymer (COP) Nanostructures as Metal-Free High Performance Visible-Light Photocatalyst for Water Treatment and Enhanced CO2 Capture Soumitra Bhowmik,† Rohit G. Jadhav† and Apurba K. Das*,† †

Department of Chemistry, Indian Institute of Technology Indore, Indore 453552, India

Email: [email protected]

ABSTRACT. The use of metal-free diacetylene based polymers to resolve environmental problems are an emerging field of research interest. In this study, two dipeptide functionalized diacetylene based compounds were synthesized. Compound 1 self-assembles to form organogels under certain conditions. Exposure of UV light irradiation on organogel results the formation of one-dimensional polydiacetylene based conjugated nanoporous covalent organic polymer (PDACOP 1) nanostructures that demonstrates significant recyclable photocatalytic dye degradation and substantial CO2 capture ability. Under visible light irradiation, 92% methyl orange degradation is achieved in presence of PDA-COP 1 after 120 min without the support of any sacrificial reagents or precious metal co-catalysts. Remarkably, surface area is tuned from 0.001 m2 g-1 (compound 1) to 260.484 m2 g-1 for light induced developed nanoporous covalent organic

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polymer (PDA-COP 1). In addition, CO2 uptake by PDA-COP 1 is increased by 2.45 times more than the CO2 uptake by respective monomeric compound 1.

INTRODUCTION. Gradual increase in fossil fuels and energy consumption triggers global warming. Industrial organic chemicals and pollutants affect environmental eco-system through various ways. Presently, several approaches have been introduced to overcome pollution and their environmental issues1 Although there is no any immediate solution for environmental and energy related problems, photocatalysis is one of the promising approach to utilize clean solar energy for various applications such as pollutant and dye degradation. Capture of greenhouse gases is one of the promising approaches for lowering effects of global warming. Over last couple of decades, metal organic frameworks (MOFs),2 covalent organic frameworks (COFs)3 and covalent organic polymers (COPs)4 have attracted significant attention in material science research.5-8 COF,9 MOF and porous COP networks displayed significant potential results in selective gas capture,10 storage and separation of heavy metal ions, photocatalytic water splitting11 and drug delivery applications.12–14 Currently, porous conjugated organic polymers (COPs) play an important role for photocatalysis as well as capturing greenhouse gases. Covalent organic polymers15 (COPs) have unique designing process in building of considerable porous material and their potential applications in gas storage and separation, optoelectronics, catalysis, sensing, energy storage and removal of hazardous dyes.16-19 Light induced synthetic route for the generation of porous COPs without having side product is attractive field of research.20-22 Out of conjugated polymers, polydiacetylene based polymers were extensively utilized as a key material for variety of applications such as molecular imprinting and sensing applications.23–25 Increasing consumption of fossil fuels enhances the rate of generation of greenhouse gas such as CO2 which triggers global warming in the environment.26 On the other hand, CO2 is used to


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manufacture several useful chemicals including methanol, methane and other hydrocarbons. Notably, Li et al designed a lithium/CO2 rechargeable battery for effective energy storage application.27 However; removal of CO2 from environment by utilizing various methods is not easy task. Comparative inert nature of CO2 gas results difficulties in CO2 capture through chemisorption.28 Capturing of CO2 by physisorption is an effective way for the removal of CO2 from environment. Designing of suitable materials for capturing CO2 by physisorption is a challenging task to fulfill this goal. In literature, there are limited reports on porous COPs which were used to capture CO2 through physisorption.29,30 Apart from gas sorption properties, COPs materials were also explored as photocatalysts for various photocatalytic applications such as dye degradation. Photocatalytic pollutant degradation provides clean and secondary pollution free approach for the environment. Semiconductor photocatalysts like TiO2, ZnO, Fe2O3, CdS and ZnS have been widely used as excellent photocatalysts.31–36 Further, these materials work under visible light range after incorporation of metal nanoparticles.37–44 Toxicity and expensiveness of metal nanoparticles forced researchers to develop eco-friendly photocatalysts. Similar to metal semiconductor based photocatalysts, MOFs and COPs were also demonstrated for photocatalytic applications under UV light as well as visible light irradiations.45,46 Metal-organic frameworks containing Fe3-µ3-oxo clusters successively disintegrated hazardous Rhodamine 6G dye in water under visible light irradiation.47 MOFs also selectively degrade organic dyes like methyl orange under visible light irradiation.41,48–50 Remembering the importance of CO2 capture and photocatalytic dye degradation, dipeptide functionalized diacetylene appended organogelator was transformed to polydiacetylene based covalent conjugated organic polymer (PDA-COP) under UV light irradiation. Light instructed


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polymerization resulted stable nanoporous networks containing covalent conjugated organic polymer. Permanent porosity of PDA-COP was measured by type I gas adsorption isotherms with N2 (g) and CO2 (g). The covalent organic polymer formed after UV irradiation adsorbed 2.94 fold N2 and 2.45 fold CO2 gases than the respective monomer. Photocatalytic activity of COPs was examined by the photocatalytic degradation of methyl orange dye under both UV and visible light irradiation. To the best of our knowledge, this is possibly one of the novel example of dipeptide functionalized diacetylene based light induced polymerized covalent organic polymer which shows significant selective greenhouse gas adsorption as well as significant metal free and recyclable photocatalytic degradation of organic pollutant under visible light.

EXPERIMENTAL SECTION

Materials. 4-Pentynoic acid, tetramethylethylenediamine (TMEDA), CuCl (copper (I) chloride), were purchased from Alfa Aesar India.

L-leucine, L-tyrosine, HOBt (1-

hydroxybenzotriazole), DCC (N, N’-dicyclohexylcarbodiimide) were obtained commercially. For chemical reactions and purification of peptide derivatives, methanol, dimethylformamide (DMF), ethyl acetate and hexane were dried according to literature. Reactions were monitored by thin-layer chromatography (TLC). All intermediates and final compounds were purified and well characterized by 1H NMR (400 MHz) and mass spectral studies.

General Experimental Procedure. Mass spectra were obtained on a Brucker MicrOTOF-Q II by positive-mode electrospray ionization. All NMR spectra were recorded on a 400 MHz Bruker AV400 NMR. FT-IR spectra of all reported compounds were performed using a Bruker (Tensor27) FT-IR spectrophotometer. Raman study was performed on Micro Raman system from Jobin Yvon Horiba LABRAM-HR visible (400-1100 nm) equipped with an Ar+ laser (488 nm, 10


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mW) excitation source and CCD detector. Transmission electron microscopic images were taken by using a PHILIPS electron microscope (model: CM 200) operated at an accelerating voltage of 200kV. Toluene gel of compound was organogel were diluted in toluene (0.5 mmol) and dried on carbon-coated copper grids (300 mesh) by slow evaporation in air, then allowed to dry separately under a vacuum at room temperature. For HPLC Chromatogram, HPLC grade acetonitrile and water were used without further purification. The purity of final compounds were characterized by reverse phase symmetry C18 column (250 x 4.6 cm, 5 µm particle size). UV-Vis absorbance was monitored at 280 nm. Separations were accomplished by running the column with acetonitrile-water as eluent at a flow rate of 1 ml min-1 at 25 °C. The sample preparation involved mixing 100 µL of gel with acetonitrile–water (900 µL, 50 : 50 mixture). A 20 µL of sample was injected into a Dionex Acclaim ® 120 C 18 column of 250 mm length with an internal diameter of 4.6 mm and 5 µm fused silica particles at a flow rate of 1 ml min-1.

Synthesis of Compounds

General Procedure for Methyl Ester Hydrolysis. Methyl esters in 10 mL MeOH was taken in a round bottom flask and 2N NaOH was added drop-wise (Scheme S2, S3). The reaction was monitored by thin layer chromatography (TLC). The reaction mixture was stirred for overnight. 15 mL of distilled water was added to the reaction mixture and MeOH was removed under vacuum. The aqueous part was washed with diethyl ether (2 × 30 mL). Then it was cooled under ice water bath for 10 minute and then pH was adjusted to 2 by drop wise addition of 1N HCl. It was extracted with ethyl acetate (3 × 50 mL) and then the ethyl acetate part was dried over anhydrous Na2SO4 and evaporated in vacuum to yield corresponding carboxylic acid, which was used for the next step without purification.


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General Procedure for Peptide Coupling. 4-pentynoic acid (1.0 equiv) was dissolved in dry-DMF (4 mL/g) and stirred on an ice water bath (Scheme S2, S3). Methyl ester protected amino acid was isolated from its corresponding methyl ester hydrochloride (2.0 equiv) by neutralization and subsequently extracted twice with ethyl acetate (2 × 30 mL). The collected ethyl acetate extracts was dried over anhydrous Na2SO4 and concentrated to 5 mL. It was then added to the pre-cooled reaction

mixture

followed

by

addition

of

(1.0

equiv)

HOBt,

(1.1

equiv)

dicyclohexylcarbodiimide (DCC). The reaction mixture was stirred for overnight. The residue was taken up in ethyl acetate (50 mL) and the DCU was filtered off. The organic layer was washed with 1 M HCl (3 × 50 mL), brine (2 × 50 mL), 1 M sodium carbonate (3 × 50 mL), brine (2 × 50 mL), dried over anhydrous Na2SO4 and evaporated in vacuum. Purification was done by silica gel column (100-200 mesh) using hexane-ethyl acetate (9:1) as eluent to yield product. General Procedure for Diacetylene Synthesis. Compound 3 or 6 (1 equiv.) was dissolved in acetone (40 mL). Copper (I) chloride (0.02 equiv.) and TMEDA (0.03 equiv.) were added and the mixture was stirred at open-air conditions for 1 day. The precipitate was filtered and washed with acetone (1 × 25 mL) and then washed with chloroform (1 × 25 mL) and solvent was removed by rotary evaporation to give crude. Purification was done by silica gel column (100200 mesh) using hexane - ethyl acetate as eluent to get compound 4 or 7 as white solid. Synthesis of compound 3. Compound 3 was obtained as a white solid (900 mg, 3.26 mmol, 32.07%). 1H NMR (400 MHz, CDCl3): 6.96 (d, 2H, J = 8 Hz), 6.74 (d, 2H, J = 8 Hz), 6.22 (d, 1H, J = 8Hz), 4.89 (t, 1H, J = 4 Hz), 3.74 (s, 3H), 3.10 (m, 2H), 2.47 (d, 2H, J = 8 Hz), 2.43 (2H, J = 4 Hz), 1.98 (s, 1H).

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C NMR (100MHz, CDCl3, δ ppm): 172.22, 170.96, 155.37, 130.36,

127.10, 115.57, 82.66, 69.6, 53.37, 52.47, 37.16, 35.13, 14.70; ESI-MS: calcd for [C15H17NO4+ Na]+ = 298.1055; found 298.1155.


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Synthesis of MeO-Tyr-(4,6-decadiyne)-Tyr-OMe 4. Compound 4 was obtained as a white solid (800 mg, 1.45 mmol, 44.47%). 1H NMR (400 MHz, CDCl3): 7.30 (s, 2H), 6.99 (d, 4H, J = 8 Hz), 6.74 (d, 4H, J = 8Hz), 5.00 (d, 2H, J = 4 Hz), 3.78 (s, 6H), 3.25 (m, 2H), 2.89 (m, 4H), 2.49 (d, 2H), 2.37 (m, 4H),;

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C NMR (100MHz, CDCl3, δ ppm): 172.23, 170.97, 155.38, 130.37,

127.11, 115.58, 82.67, 69.61, 53.38, 52.48, 37.17, 35.14, 14.71; ESI-MS: calcd for [C30H32N2O8+Na]+ = 571.2056; found 571.1942. Synthesis of OH-Tyr-(4,6-decadiyne)-Tyr-OH 5. Compound 5 was obtained as a white solid (750 mg, 1.44 mmol, 99.3%).1H NMR (400 MHz, DMSO-d6, δ ppm): 12.44 (s, 2H), 9.19 (s, 2H), 8.19 0(d, 2H, J = 4 Hz), 7.00 (d, 4H, J = 4 Hz), 6.66 (d, 4H, J = 8Hz), 4.34 (d, 2H, J = 8 Hz), 2.93 (d, 2H, J = 8 Hz), 2.77 (d, 2H, J = 8 Hz), 2.41 (m, 4H), 2.32 (m, 4H). ESI-MS: calcd for [C28H28N2O8+Na]+ = 543.1846; found 543.1519. Synthesis of MeO-Leu-Tyr-(4,6-decadiyne)-Tyr-Leu-OMe 1. Compound 1 was obtained as a white solid (625 mg, 0.806 mmol, 56%). 1H NMR (400 MHz, CDCl3, δ ppm) 7.06 (d, 4H, J = 8Hz), 6.74 (d, 4H, J = 8Hz), 6.52 (d, 2H, J =8 Hz), 6.33 (d, 2H, J =8 Hz), 4.67 (m, 2H), 4.52 (m, 2H), 3.70 (s, 6H), 2.99 (m, 4H), 2.48 (m, 4H), 2.41 (m, 4H), 1.57 (m, 4H), 1.56 (m, 2H), 0.89 (d, 12H, J = 4Hz).

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C NMR (100MHz, CDCl3, δ ppm): 172.73, 171.26, 171.16, 170.84, 155.17,

130.50, 127.81, 115.58, 82.59, 69.61, 60.44, 54.59, 52.38, 50.99, 41.38, 37.60, 35.16, 24.74, 22.67, 21.90, 14.77, 14.19. ESI-MS: calcd for [C42H54N4O10+Na]+ = 797.3738; found 797.3842. Synthesis of compound 6. Compound 6 was obtained as a white solid (900 mg, 3.99 mmol, 73%). 1H NMR (400 MHz, CDCl3, δ ppm) 6.44 (d, 1H, J = 8 Hz), 4.62 (m, 1H), 3.71 (s, 3H), 2.50 (m, 2H), 2.44 (m, 2H), 1.98 (s, 1H), 1.63 (m, 1H), 1.53 (m, 2H), 0.91 (m, 6H). ESI-MS: calcd for [C12H19NO3+Na]+ = 248.1365; found 248.1251


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Synthesis of MeO-Leu-(4,6-decadiyne)-Leu-OMe 7. Compound 7 was obtained as a white solid (820 mg, 1.82 mmol, 53%). 1H NMR (400 MHz, CDCl3, δ ppm): 6.15 (d, 2H, J = 4 Hz), 4.67 (d, 2H, J = 8 Hz), 3.75 (s, 6H), 2.61 (m, 4H), 2.44 (m, 4H), 1.68 (m, 4H), 1.56 (m, 2H), 0.96 (s, 12H).

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C NMR (100 MHz, CDCl3, δ ppm): 173.59, 170.82, 82.82, 69.31, 52.25, 50.70, 41.58,

35.07, 24.78, 22.74, 21.89, 14.74; ESI-MS: calcd for [C24H36N2O6+Na]+ = 471.2573; found 471.2314. Synthesis of HO-Leu-(4,6-decadiyne)-Leu-OH 8. Compound 8 was obtained as a white solid (800 mg, 1.9 mmol, 96%). 1H NMR (400 MHz, DMSO-d6, δ ppm): 8.19 (d, 2H, J = 4Hz), 4.24 (m, 2H, J = 8Hz), 2.51 (m, 4H), 2.48 (m, 4H), 2.37 (m, 4H), 1.65 (m, 2H), 0.90 (m, 12H).

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C

NMR (100 MHz, DMSO-d6, δ ppm): 174.61, 170.76, 84.07, 71.72, 50.61, 34.38, 24.72, 23.33, 21.77, 14.65. Synthesis of MeO-Tyr-Leu-(4,6-decadiyne)-Leu-Tyr-OMe 2. Compound 2 was obtained as a white solid (735 mg, 0.948 mmol, 50%). 1H NMR (400 MHz, DMSO-d6, δ ppm, 9.23 (s, 2H), 8.25 (d, 2H), 8.01 (d, 2H), 6.99(d, 4H, J = 8Hz), 6.65 (d, 4H, J = 8Hz), 5.59 (d, 2H), 4.36 (m, 2H), 3.56 (s, 6H), 3.81 (m, 4H), 2.33 (m, 4H), 1.71 (m, 4H), 1.62 (m, 4H), 0.88 (m, 12H).

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C

NMR (100MHz, DMSO-d6, δ ppm): 172.60, 172.39, 170.49, 156.46, 130.45, 127.51, 115.46, 84.14, 71.71, 54.31, 52.17, 51.01, 36.24, 34.47, 24.51, 23.46, 22.17, 14.70. Thermogravimetric analysis (TGA) To check the thermal stability, thermogravimetric analysis (TGA) was performed using a METTLER TOLEDO TGA instrument. The samples were heated from 25 °C to 1000 °C at a constant rate of 5 °C min-1 under nitrogen environment. Gas Sorption measurement


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Gas (N2/CO2) adsorption/desorption measurements were carried out using a Quantachrome Autosorb IQ2 Automated Gas Sorption System at 77 K (N2 sorption experiments) and 298 K (CO2 sorption experiments) in the pressure range of 0.025 bar to 1 bar (40 points system). Before sorption measurements, the dried gel of compound 1 and dried solution of PDA-COP 1 were degassed for 5 h at 373 K with increasing rate of 5 °C min-1. From the gas adsorption at low P/P0, the pore size distribution of the sample was calculated using BJH (Barrett-Joyner-Halenda analysis) method.

BET surface area was calculated from Brunauer-Emmett-Teller (BET)

equation. DFT calculation The density functional theory (DFT) calculations were performed using Gaussian 09 to evaluate the highest occupied molecular orbital energy level (HOMO) and the lowest unoccupied molecular orbital energy level (LUMO). DFT calculations of compound 1 were carried out at the B3LYP/6-31G* level in the Gaussian 09 program. DFT calculations of short oligomers (dimer and trimer) of conjugated organic polymer (PDA-COP 1) were carried out at the ωB97X-D/631G* level in the Gaussian 09 program. For efficient calculation, the molecular structure of short oligomers (dimer and trimer) of PDA-COP 1 was simplified without peptide side chain. Long dipeptide side chain creates overlapping of orbitals leading to steric strain. To avoid such problem, DFT calculations of dimer and trimer of PDA-COP 1 were carried out without considering peptide side chains.51 Photocatalytic activity measurements Photocatalytic activity of the PDA-COP 1 has been determined for photodecomposition of methyl orange (MO) in water. The decomposition reaction in quartz cell reactor containing 5.0 mL of 7x10-5 mol L-1 of MO in presence of 1.5 g L-1 of conjugated organic polymer was


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executed. The solution containing PDA-COP 1 with pollutants were irradiated with visible light and 365 nm UV lamp. Photocatalysis experiments were performed five times under identical reaction conditions to check reproducibility. After complete degradation of MO, conjugated organic polymers were recovered by filtration, and PDA-COP 1 was dried by keeping the temperature at 30 °C for overnight.

RESULTS AND DISCUSSION Two dipeptide appended diacetylene based compounds (compound 1, 2) were synthesized and placed for self-assembly (Figure 1). Compound 1 formed organogels in different solvents (Table S1). An optimized amount of organogelator compound i.e. dipeptide containing diacetylene based compound 1 and organic solvent were placed in a glass vial. Compounds were completely solubilized in organic solvents using sonication followed by heating. Clear solution of organogelator allowed gelation by standing vial at room temperature. Complete gelation was examined using inversion of the test tube method. Between two compounds, compound 1 selfassembled to form transparent thermo-reversible organogels with the solvents of toluene, chlorobenzene, o-dichlorobeneze and the mixture of hexane and ethyl acetate (1:1) (Table S1). Compound 2 did not self-assemble to form organogel in any reported organic solvents. These observations state that molecular interactions arising due to different sequence of dipeptide residues also play a major role in molecular self-assembly. Compound 1 shows better gelation property in toluene at very low concentration i.e. 1% (w/v) as the self-assembly of compound 1 is driven by strong intermolecular H-bonding of peptide motifs, hydrophobic interaction and π-π stacking interaction of aromatic amino acids.52-54 FT-IR spectra of compound 1 in solid state and toluene gel were recorded to investigate the secondary structural arrangement. Compound 1 exhibits N–H band at around 3298 cm-1 and 3275 cm-1 in solid state and gel state respectively,


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which indicates strong hydrogen bonding interactions among peptide units both in solid and gel states. For compound 1, amide I band at 1636 cm-1 in the gel state and 1638 cm-1 in solid state indicates the formation of hydrogen bonded supramolecular β-sheet structure. FT-IR spectrum of compound 2 was recorded to explore the secondary structural arrangement in solid state. Characteristic N-H band at 3438 cm-1 and amide I band at 1644 cm-1 were observed, which indicate weak hydrogen bonded random coil structure (Figure S1).54-56 Compound 2 didn't selfassemble to form organogel due to weak hydrogen bonding interactions between dipeptide moieties. Here, the peptide sequence Tyr-Leu attached with diacetylene system of compound 1 plays an important role for the organogel formation and subsequent photopolymerization. It has been previously reported by Moretto et al that a dipeptide functionalized diacetylene system undergoes topochemical polymerization reaction under self-assembled organogel conditions.57 The influence of amino acid sequences on the structure and morphology of peptide functionalized polydiacetylene backbone is also reported. The prerequisite of β-sheet structure by peptide functionalized diacetylene to undergo photopolymerization reaction is reported by Stupp et al.58 The role of dipeptide sequences in dipeptide functionalized polydiacetylenes on unusual chromatic behavior was studied in detail by Leblanc et al.59 Viscoelastic properties of compound 1 organogel were explored by rheological measurements. Rheological measurements of self-supporting toluene gel formed by compound 1 were performed with a dynamic frequency sweep at a constant strain of 0.05%.60 A typical frequency sweep experiment was performed to assess the elastic modulus (G') and viscous modulus (G"). The frequency sweep data shows that G' is 10 times higher than (G"), which is an indication of the viscoelastic nature of the organogel (Figure S2). The viscoelastic nature of


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compound 1 toluene gel can be attributed to the supramolecular ordering of dipeptide in organic solvent medium (Table S1). In order to polymerize diacetylene units to polydiacetylene, an appropriate molecular arrangement is essential to encounter with the state for topochemical reaction. An angle of 45° relative to the diyne axis and a distance of 4.9 Å between two reactive carbon atoms are compulsory to execute this reaction efficiently (Figure 1b).61 To endorse polymerization, the incorporation of the self-assembled amphiphiles, aromatic moieties with diacetylene unit is one of the best ways. In previous reports, diphenylbutadiyne, decafluorodiphenylbutadiyne and 2,3,4,5,6-pentafluorodiphenyldiacetylene were used for photopolymerization. Diphenylbutadiyne and decafluorodiphenylbutadiyne were reported as photostable compounds whereas 2,3,4,5,6pentafluorodiphenyldiacetylene and the π-stacked arrangement of diphenylbutadiyne decafluorodiphenylbutadiyne produced polydiacetylene under UV irradiation.62 Diacetylene moiety attached with self-assembled peptides through hydrogen bonding between amide groups promotes the growth of enyne during topochemical polymerization reaction.63 Covalent conjugated organic polymer was obtained by photopolymerization of compound 1 toluene gel under UV irradiation. Photopolymerization of a 1% (w/v) toluene gel of compound 1 was accomplished by irradiation at 254 nm with 72 W UV light at a distance of 10 cm for 1 hour. After irradiation, there was a loss of the gel state and yellow solution was appeared. Upon UV irradiation, transparent colorless gel was disintegrated into yellow solution (Figure 1).


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Figure 1. (a) Chemical structures of dipeptide functionalized diacetylene based compounds. (b) Schematic diagram of photopolymerization of dipeptide appended 4,6-decadiyne (Compound 1) by UV irradiation (λ = 254 nm). (c) Photographs of dried toluene gel (white powder) and selfsupporting transparent toluene gel of compound 1 (inset) prior to polymerization and dried yellow powder (PDA-COP 1) and yellow solution (inset) after light instructed polymerization.

The change in color and transparent organogel to sol transition of compound 1 affirm topochemical photopolymerization process for the formation of covalent organic polymer i.e. PDA-COP 1.64 Topochemical photopolymerization of the dipeptide functionalized diacetylene (compound 1) was confirmed by UV-Vis spectroscopy. In previous report, Wang et al. demonstrated the formation of more stable polydiacetylene based organogel after UV


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irradiation.64 However, in our case, the organogelator loses its self-assembly after UV irradiation and forms polymer. This unusual behaviour arises due to steric interaction of dipeptide residues after covalent polymerization, which induces gel to sol transition. In case of compound 2, unaltered UV-Vis spectral data after UV irradiation results the absence of photopolymerized products (Figure S3).65 Self-assembly is pre-requisite for photopolymerization of diacetylene based compounds. UV-Vis spectra were collected during the course of the topochemical reactions of compound 1 and compound 2 under 254 nm UV light. Compound 1 absorbed at 345 nm prior to UV-Vis irradiation. A new peak appeared in the visible region at 655 nm while the peak at 345 nm started to disappear during the course of the reaction. The new peak at 655 nm was appeared due to the formation of polydiacetylene conjugated polymer network (Figure 2a).66-68 Compound 2 in toluene solution absorbed at 358 nm. Polymerization does not occur for compound 2 (Figure S3).69 The distance between two reactive carbon atoms of diacetylene moieties of compound 2 does not remain 4.9 Å due to the random arrangement by compound 2, which was supported by FTIR data.70-72 The result suggests that only pre-organized monomers in the self-assembled structure can undergo photochemical polymerization, which demands an optimal and long-range organization of adjacent diacetylene groups of monomers.73 Fourier Transform Raman (FT-Raman) (Figure 2b, c) and FT-IR (Figure S4) spectroscopy were used to monitor the polymerization reaction of toluene gel of compound 1. Prior to UV irradiation, the Raman spectrum of compound 1 exhibits a peak at 2295 cm-1, which is assigned due to the regular stretching mode of 1,3-butadiyne.74 After UV irradiation, Raman spectra show that the characteristic peak at 2295 cm-1 diminishes and a new peak appears at 2182 cm-1 after 1 h of UV irradiation. The peak at 2182 cm-1 appears due to the presence of conjugated C≡C bond within the conjugated polymer after UV irradiation. It is reported that the characteristics peaks between


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1400 and 1600 cm-1 appear due to the stretching vibration of enyne.64 Prior to UV irradiation, no peak was observed between 1400 and 1600 cm-1. UV irradiated toluene gel of compound 1 shows a peak at 1496 cm-1 corresponding to (C=C) stretching vibration of the conjugated organic polymer which indicates successful polymerization of diacetylene monomer.73

(a) 1.0

0 min 60 min

(b)

1496

0 min 60 min

Raman Intensity

0.8

Absorbance

0.6 0.4 0.2 0.0 300

400

500 600 Wavelength (nm)

700

1450

1500 Raman Shift (cm-1)

(c) Raman Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000

2295

0 min

2182 60 min

2200

2400

Raman Shift (cm-1)

Figure 2. (a) UV-Vis spectra and (b) Raman spectra of compound 1 and PDA-COP 1 in the range of (1450-1560 cm-1). (c) Raman spectra of compound 1 and PDA-COP 1 in the range of (2000-2400 cm-1) at 0 minute (before) and after 60 minute of UV irradiation of compound 1


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Figure S4 shows FT-IR spectra of compound 1 prior and after photopolymerization reactions. A sharp peak at 2125 cm-1 appeared due to the presence of C≡C bonds of the compound 1. The intensity of corresponding C≡C peak decreases after UV irradiation. FT-IR results evidently support the formation of polymer as polymerization reaction occurs after UV irradiation.73 Thermal stability of compound 1 and PDA-COP 1 were examined by thermogravimetric analysis (Figure S5). To gain better insight of the nanoscale morphology of the toluene gel of compound 1, Scanning Electron Microscopy (SEM) analysis was done. Toluene gel of compound 1 reveals nanofibrous morphology (Figure 3a). After 60 min UV irradiation, the fiber morphology modifies to flower like hollow rod type of structure (Figure 3b). TEM images also reveal nanofibers of toluene gel before UV irradiation (Figure 3c) and nanorod type structures after UV irradiation (Figure 3d). Prior to UV irradiation, the average diameter of the fiber is 40 nm whereas the average diameter of the hollow rod like morphology is 60 nm.72 Both isolated and bundled hollow rod-like structures were observed for UV irradiated gel from the SEM and TEM experiments.


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Figure 3. SEM images of compound 1 (a) before UV reaction and (b) at 60 min of UV irradiation. TEM images of compound 1 (c) before UV reaction and (d) at 60 min UV irradiation.

Broad usability of PDA-COP 1 was explored by typical gas adsorption studies. Gas adsorption study evaluates the extent of porosity and hollow network structure developed after UV irradiation for dried yellow toluene solution of PDA-COP 1.74 The surface areas and porosities of dried toluene gel of compound 1 and dried solution of UV irradiated PDA-COP 1 were explored by N2 gas adsorption-desorption analysis at 77 K and CO2 gas adsorption-desorption analysis at 273 K (Figure 4). In comparison with compound 1, PDA-COP 1 exhibits larger surface area.75 Compound 1 exhibits 9.14 cc g-1 N2 uptake whereas UV irradiated polymer PDA-COP 1 polymer shows 27 cc g-1 N2 uptake (Table 1).


17


Table 1. Summary of BET isotherm model parameters for the adsorption of N2 and CO2 gases.

N2 CO2

30 25

Compound Compound 1 PDA-COP 1 Compound 1 PDA-COP 1

Surface Area (m2/gm) 0.001 260.484 5.342 116.100

0.016 0.047 0.00953 0.02264

Adsorption of compound 1 Desorption of compound 1 Adsorption of PDA-COP 1 Desorption of PDA-COP 1

16 14 12

20

10 5 0 0.0

0.2 0.4 0.6 0.8 Relative Pressure(P/P0)

Isother m Type I I I I

Max Amount Gas Ads. (cc/g) 9.14 27 4.5 11.05

Adsorption of compound 1 Desorption of compound 1 Adsorption of PDA-COP 1 Desorption of PDA-COP 1

10

2.94x

15

Pore Size (diameter in nm) 1.4875 1.5712 0.07240 1.2575

Pore Vol.

Volume of CO 2 (cc/gm)

Gas Sorption

Volume of N 2 (cc/gm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

1.0

8

2.45x

6 4 2 0 0.0

0.2 0.4 0.6 0.8 Relative Pressure(P/P0)

1.0

Figure 4. (a) N2 gas adsorption/desorption isotherms (77K) of compound 1 (dried toluene gel) and PDA-COP 1 and (b) CO2 gas adsorption/desorption isotherms (273 K) of compound 1 (dried toluene gel) and PDA-COP 1.

Surface area of compound 1 is 0.001 m2g-1. However, surface area of PDA-COP 1 becomes 260.484 m2g-1 at lower pressure range with pore size 1.57 nm (Figure S6). Compound 1 prior to UV light irradiation exhibits pore size of 0.07 nm with CO2 uptake capacity whereas PDA-COP


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1 exhibits pore size of 0.023 nm with CO2 uptake capability as high as 11.05 cc g-1 at 273 K. The light-driven polymerized nanoporous PDA-COP 1 displays almost 2.4 fold more CO2 gas uptake than the compound 1. N2 and CO2 adsorption isotherms reveal type I isotherm, which is consistent with nanoporous material. The reported polydiacetylene captures significant amount of CO2. The reported CO2-adsorbing porous organic solids such as C-COP-P-Fe (0.44 mmol g−1), C-COP-P-Mn (0.38 mmol g−1), C-COP-P-Co (0.62 mmol g−1), CMP-5 (0.63 mmol g-1), TCMP-5 (0.68 mmol g-1), CMP-3 (0.81 mmol g-1) and Py-azo-COP (8.5 wt%) adsorbed comparable amount of CO2 with the reported PDA-COP 1.75-77 The regeneration recycling feasibility of CO2 uptake by PDA-COP 1 after four cycles ascertains that PDA-COP 1 shows 2.4 times higher CO2 adsorption than the compound 1 (Figure S7). A model pollutant methyl orange dye was used for exploring the photocatalytic ability of dipeptide functionalized COP in water. Initially, PDA-COP 1 was examined for photocatalysis under visible light to utilize solar energy for clean photocatalytic applications. Decolourization of methyl orange polluted water in presence of PDA-COP 1 on irradiation with visible light irradiation states photocatalytic properties of PDA-COP 1 in visible region. Complete decolourization of water states complete degradation of MO in presence of COP photocatalyst under visible light irradiation (Figure 5a). Similar experiment was performed with monomer of compound 1. However, no methyl orange decoloration phenomenon occurred. Similarly, controlled experiment was performed without adding any compound. However, no methyl orange decoloration phenomenon occurred (Figure S8). These observations state formation of covalent conjugated network of organic polymer is responsible for visible light photocatalytic decomposition of MO.78 Figure 5a shows UV-Vis spectra for the solutions that were irradiated by visible light for various time periods.79-81 The intensity of the absorption band of methyl


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orange in the visible region decreased significantly in presence of PDA-COP 1 under visible light irradiation. PDA-COP 1 shows 92% degradation of MO after 120 min irradiation of visible light (Figure 5a). The degradation efficiency of methyl orange achieved in our polydiacetylene photocatalyst was 92% which is almost 20% higher than reported polydiacetylene photocatalyst reported by Remita et al. Not only that time required for 92% dye degradation under visible light by our polydiacetylene photocatalyst is 2h whereas 70% dye degrades after 4h visible light irradiation of reported photocatalyst.79 Photocatalytic degradation experiment was also carried out under the UV light irradiation with 365 nm wavelength (Figure 5b). The negligible changes in UV-Vis spectra of MO contaminated water without PDA-COP 1 under UV light exposure state that UV irradiation was not able to decompose MO dye. Similar results were obtained while exposure of UV irradiations on MO contaminated water in presence diacetylene based compound 1. The experiment proved that compound 1 has no photocatalytic activity under UV light (Figure 5d). Further UV-vis spectra of MO contaminated water in presence of PDA-COP 1 shows 90% degradation of MO after 9h irradiation of UV light (Figure 5b). Control experiments were also accomplished without addition of compound 1 under UV light exposure. MO was not degraded under UV light exposure (Figure S8).


20

(a) 0.25 Absorbance

0 min 15 min 30 min 45 min 60 min 75 min 90 min 105 min 120 min

Visible light exposure

0.20 0.15 0.10

(b) 0.25 0.20

0.10

0.00 300


(c)

0.25

Compound 1 PDA-COP 1

Compound 1 PDA-COP 1

0.20 C/C0 (MO)

C/C0 (MO)

0.20


(d) 0.30

0.25

0.15

0.15

0.10

0.10 0.05

0.05

0.00

0.00 0

20

40

60

80

100

2nd cycle

3rd cycle

4th cycle 5th cycle

(f)

100

Degradation Efficiency (%)

1st cycle

0

120

Time of Vis light exposure (min)

50

0

UV light exposure

0.05

0.00 300

(e) 100

0h 1h 2h 3h 4h 5h 6h 7h 8h 9h

0.15

0.05

Degradation efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance

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360 480 120 240 0 Time of Vis light exposure (min)

2 4 6 8 Time of UV light exposure (h) 1st cycle

2nd cycle 3rd cycle 4th cycle

10

5th cycle

50

0

36 27 0 9 18 Time of UV light exposure (h)

Figure 3. (a) Photocatalytic degradation of methyl orange dye in presence of PDA-COP 1 under visible light. (b) Photocatalytic degradation of methyl orange in presence of PDA-COP 1 under UV light. (c) Normalized concentration of methyl orange versus visible light irradiation time in presence of PDA-COP 1 and compound 1. (d) Normalized concentration of methyl orange versus


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UV light irradiation time in presence of PDA-COP 1 and compound 1. (e) Photocatalytic recyclability of MO degradation by PDA-COP 1 five times under visible light. (f) Photocatalytic recyclability of MO degradation by PDA-COP 1 five times under UV light.

Under UV light, degradation efficiency by our photocatalyst is 90% which is almost 10% higher than the reported photocatalyst.79 These results state potential of dipeptide containing PDACOPs as a more superior photocatalyst for photocatalytic dye degradation than reported so far metal-free photocatalyst both under visible and UV light. Generally, metal oxide semiconductor photocatalysts show high photocatalytic efficiency limited to UV region of solar spectrum. The percentage degradation of methyl orange under visible light with a particular PDA-COP was calculated using the equation given below where, A0 is the absorbance of dye at initial stage, At is the absorbance of dye at time “t”.82 % degradation= (A0-At)/A0 PDA-COP 1 showed 92% of MO photodegradation in MO contaminated water after a 120 min irradiation of visible light which was higher than of plasmonic Ag-modified TiO2 photocatalyst.83 The overall comparative study of photocatalytic degradation of MO under UV and visible light irradiations states that covalent conjugated organic polymer of compound 1 acts as highly efficient photocatalyst under visible light irradiation than under UV light.84–87 Cyclic voltammetry (CV) measurements (Figure S9, S10) and density functional theory (DFT) calculations were accomplished to detailed photocatalytic activity of conjugated polymer under visible and UV light irradiation. In case of PDA-COP 1 conjugated covalent organic polymer structures, the p-doping (oxidation) and n-doping (reduction) processes are irreversible. The values of the peak potentials are 1.2 eV (oxidation) and -0.9 eV (reduction) for conjugated


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organic polymer yielding an energy gap of 2.1 eV. From DFT calculation, the gap between valence band and conduction band is calculated as 1.8 eV for trimer of PDA-COP 1 (Table S2). Both DFT and CV show the band gap between valence band and conduction band is almost approximately 2.1 eV. Both the experimental and theoretical results demonstrate a probable photocatalytic activity under visible light and under UV light of conjugated organic polymer. Photons of energy exceeding or equal to the band gap of 2.1 eV are able to generate electronhole pair by exciting electrons (e-) from conduction band to valance band and leaving hole (h+) at conduction band (Figure S11).88-90 The highly reactive photo-excited e- and h+ migrate to polymer surface area by avoiding recombination to perform redox reactions. Photo-excited e- and h+ reacts with surrounding O2 and H2O to generate highly reactive oxygenated species like O2·-, OH· by various routes.91 These highly reactive oxygenated species disintegrate pollutant to ecofriendly molecules. The vital role of oxygen for photodegradation was examined by performing photocatalytic water decontamination under argon atmosphere. In this case, the photodegradation efficiency of conjugated polymer for methyl orange degradation was reduced to 3%. These results suggest formation of O2.− radical during degradation of dyes using conjugated organic polymers. Decomposition rate of MO at different time of visible and UV light irradiation is given by PDA-COP 1 in Figure 5c-d. To test the reusability and stability of the conjugated organic polymer photocatalyst, five consecutive reaction cycles were carried out for the degradation of methyl orange under visible light (Figure 5e) and UV light (Figure 5f). This photocatalyst is very stable and shows negligible 2-3% weight loss after five cycles of photocatalytic reaction. Progress of the reaction at different reaction time lasts in a similar way. PDA-COP 1 retained its photocatalytic activity over five reaction cycles competently. Unaltered FT-IR spectroscopy of


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PDA-COP 1 before and after photocatalytic experiment reveals stability of PDA-COP 1 (Figure S3).

CONCLUSION

In this work, we have demonstrated the scope of dipeptide containing polydiacetylene based covalent conjugated organic polymer (PDA-COP 1) for greenhouse gas capture and photocatalytic dye degradation applications. PDA-COP 1 nanostructures were obtained from the self-assembled organogel of compound 1 under UV light irradiation. The spatial arrangement of compound 1 and hydrogen bonding interactions among peptide functionalized diacetylene based compound 1 drive the formation of self-assembled organogel formation and subsequent formation of polymer PDA-COP 1 under visible light irradiation. The degradation efficiency of methyl orange was achieved as 92% in presence of photocatalyst PDA-COP 1 after 120 min under visible light without the support of any sacrificial reagents or precious metal co-catalysts. The reusable conducting metal-free PDA-COP 1 exhibited highest photocatalytic performance than other polydiacetylene based photocatalysts under visible light. The surface area was increased from 0.001 m2 g-1 (compound 1) to 260.484 m2 g-1 for PDA-COP 1. CO2 uptake was also increased by 2.45 times for PDA-COP 1 than CO2 uptake by respective compound 1. Our results may encourage the engineering of light-induced polymerized covalent conjugated organic polymer (COP) for environment friendly applications such as visible light induced metal-free photocatalytic pollutant degradation and greenhouse gas capture.


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ASSOCIATED CONTENT Supporting Information. Several experimental results and characterization of new synthesized compounds, FT-IR, DFT data, TGA data, Gas adsorption data, CV data. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Author Contributions S.B. and A.K.D. conceived and designed the projects; S.B. synthesized the compounds and performed most of the experimental work. S.B. and A.K.D. wrote the manuscript. All the authors discussed over the results and commented on the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT AKD sincerely acknowledges NanoMission, Department of Science & Technology (Project No. SR/NM/NS-1458/2014), New Delhi, India for financial support. RGJ thanks UGC, New Delhi and SB thanks MHRD for their doctoral fellowships. The sophisticated instrumentation centre (SIC), IIT Indore is acknowledged for providing access to the instrumentation. Authors thank UGC-DAE Consortium for Scientific Research, Indore for their help in recording Raman data.


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Nanoporous Conducting Covalent Organic Polymer (COP

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