Isomeric sp2-C-conjugated porous organic polymer-mediated photo- and sono-catalytic detoxification of sulfur mustard simulant under ambient conditions

Abstract

•Fully sp2-C-conjugated TPE-based isomeric POPs are reported for CEES detoxification•Porous photosensitizers offer ROS generation even under visible light and ultrasound•Catalytic oxidation of CEES is achieved under ambient atmosphere without toxic products•The composition of the generated ROS types is engineered by the isomeric POP structures Selective oxidation of sulfide to sulfoxide, controlling overoxidized toxic products, is a highly demanding strategy for the detoxification of sulfur mustard simulants (CEES). Singlet oxygen is a widely studied mild oxidant, but it is utilized mainly in an oxygen-abundant environment for CEES oxidation rather than atmospheric ambient conditions. Herein, we present the isomeric TPE-based porous organic polymers (POPs) to photo-oxidize CEES even under ambient conditions without toxic byproducts. Furthermore, we also discovered that superoxide and hydroxyl radicals participate in the oxidative mechanism to a greater extent in ambient conditions than in the O2 atmosphere. In addition, we demonstrate the sono-catalytic selective oxidation of CEES by POPs by ultrasound irradiation for the first time. Our findings will give inspiration for further prospective applications of POPs as photo- and sono-catalysts. The development of efficient strategies for the sustainable detoxification of mustard gas simulants has longstanding demand for human safety. Here, we present for the first time a photo- and sono-catalyzed selective detoxification of mustard gas simulants under ambient conditions using conveniently prepared porous organic polymer (POP) catalysts. We developed a microwave-assisted synthesis of three isomeric tetraphenylethylene-based POPs (TPo, TPm, and TPp) bearing fully sp2-hybridized carbon frameworks. Among the three isomers, TPm efficiently generated 1O2, whereas TPp generated both O2.– and HO. under visible light irradiation, both TPm and TPp efficiently generated ROS to selectively convert 2-chloroethyl ethyl sulfide (CEES) into nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) in the atmospheric conditions, through a known conversion mechanism. TPm and TPp can also generate 1O2 under ultrasound irradiation. This work provides insight into designing new POP photo- and sono-catalysts to generate ROS in widespread applications. The development of efficient strategies for the sustainable detoxification of mustard gas simulants has longstanding demand for human safety. Here, we present for the first time a photo- and sono-catalyzed selective detoxification of mustard gas simulants under ambient conditions using conveniently prepared porous organic polymer (POP) catalysts. We developed a microwave-assisted synthesis of three isomeric tetraphenylethylene-based POPs (TPo, TPm, and TPp) bearing fully sp2-hybridized carbon frameworks. Among the three isomers, TPm efficiently generated 1O2, whereas TPp generated both O2.– and HO. under visible light irradiation, both TPm and TPp efficiently generated ROS to selectively convert 2-chloroethyl ethyl sulfide (CEES) into nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) in the atmospheric conditions, through a known conversion mechanism. TPm and TPp can also generate 1O2 under ultrasound irradiation. This work provides insight into designing new POP photo- and sono-catalysts to generate ROS in widespread applications. IntroductionThe dangerous chemical warfare agents (CWAs), such as phosgene, lewisite, and other hazardous substances, have been a severe threat to humans since their initial use in World War I. These chemicals include a notorious sulfur mustard, 1,2-bis(2-chloroethyl)sulfide (military designation: HD), which has resulted in millions of casualties due to its low cost and convenient production.1Ganesan K. Raza S.K. Vijayaraghavan R. Chemical warfare agents.J. Pharm. Bioallied Sci. 2010; 2: 166-178https://doi.org/10.4103/0975-7406.68498Crossref PubMed Google Scholar Consequently, finding a way to degrade these CWAs safely has been urgently demanded.2Oheix E. Gravel E. Doris E. Catalytic processes for the neutralization of sulfur mustard.Chem. Eur. J. 2021; 27: 54-68https://doi.org/10.1002/chem.202003665Crossref PubMed Scopus (16) Google Scholar Commonly used detoxification methods, including dehydrohalogenation,3Wagner G.W. Koper O.B. Lucas E. Decker S. Klabunde K.J. Reactions of VX, GD, and HD with nanosize CaO: autocatalytic dehydrohalogenation of HD.J. Phys. Chem. 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Ther. 2019; 2: 1900059https://doi.org/10.1002/adtp.201900059Crossref Scopus (12) Google ScholarIn this context, for the first time, we have designed POP-based photo/sonosensitizers using tetraphenylethylene (TPE), a well-known PS modality, for CEES detoxification. We prepared three TPE-based isomeric polymers, TPo, TPm, and TPp, using ortho-, meta-, and para-xylylene dicyanide, respectively (Figure 1). We observed pronounced performance in CEES oxidation by TPm and TPp in a protic solvent, with significant conversion rates under both O2 and ambient atmospheres. We herein also propose mechanisms for CEES detoxification using POPs by reaction with 1O2 and other ROS.Results and discussionThree two-dimensional POPs (TPo, TPm, and TPp) were synthesized via the well-known Knoevenagel condensation process, the reaction of 1,1,2,2-tetrakis(4-formylphenyl)ethene (TPE-4CHO) with xylylene dicyanide isomers (ortho-, meta-, and para-) in the presence of base produced fully sp2-conjugated porous polymers.25Jin E. Asada M. Xu Q. Dalapati S. Addicoat M.A. Brady M.A. Xu H. Nakamura T. Heine T. Chen Q. et al.Two-dimensional sp2 carbon–conjugated covalent organic frameworks.Science. 2017; 357: 673-676https://doi.org/10.1126/science.aan0202Crossref PubMed Scopus (566) Google Scholar,29Yassin A. Trunk M. Czerny F. Fayon P. Trewin A. Schmidt J. Thomas A. Structure-thermodynamic-property relationships in cyanovinyl-based microporous polymer networks for the future design of advanced carbon capture materials.Adv. Funct. Mater. 2017; 27: 1700233https://doi.org/10.1002/adfm.201700233Crossref Scopus (28) Google Scholar We have utilized a much faster microwave-assisted Knoevenagel condensation reaction for the first time. The reaction time was significantly reduced, from 3 days with a conventional solvothermal method to 3 h, ascribable to homogeneous heating and rapid nucleation under microwave irradiation.30Kang D.W. Lim K.S. Lee K.J. Lee J.H. Lee W.R. Song J.H. Yeom K.H. Kim J.Y. Hong C.S. Cost-effective, high-performance porous-organic-polymer conductors functionalized with sulfonic acid groups by direct postsynthetic substitution.Angew. Chem. Int. Ed. 2016; 55: 16123-16126https://doi.org/10.1002/anie.201609049Crossref PubMed Scopus (57) Google ScholarWe anticipated that TPE and xylylene dicyanide would serve as electron donor and electron acceptor, respectively, to construct a porous donor-acceptor framework. The nitrile unit, as a strong electron-withdrawing group, lowers the lowest unoccupied molecular orbital in the sp2-conjugated system, enabling the suppression of charge recombination.31Chen W. Wang L. Mo D. He F. Wen Z. Wu X. Xu H. Chen L. Modulating benzothiadiazole-based covalent organic frameworks via halogenation for enhanced photocatalytic water splitting.Angew. Chem. Int. Ed. 2020; 59: 16902-16909https://doi.org/10.1002/anie.202006925Crossref PubMed Scopus (144) Google Scholar For this reason, the synthesized POPs can be expected to have great potential as promising PSs.Powder X-ray diffraction data indicate that the polymeric materials we prepared are amorphous (Figure S1). Following the Knoevenagel condensation, an intensity of C=O stretching peak in the infrared (IR) spectra centered at 1,693 cm−1 for the aldehyde in TPE-4CHO decreased, implying the presence of extended sp2-C conjugation forming a porous network structure (Figures S2–S4). Interestingly, the distinct C≡N stretching peaks at 2,250 cm−1 in all the xylylene dicyanide isomers were red-shifted to 2,216 cm−1 (TPm and TPp) or 2,197 cm−1 (TPo) (Figure 2A). This red-shift is attributed to the weakening of the C–N bond strength resulting from the condensation reaction. X-ray photoelectron spectroscopy analysis suggested that the elements C, N, and O were present in all the POPs, as shown in Figures S5 and S6. As the chemical environment of the nitrile group in each POP isomer is different, the binding energies of the N1s electrons were seen to be different from each other. Solid-state 13C nuclear magnetic resonance (13C ssNMR) data were recorded to obtain detailed structural information on the POPs (Figure 2B). In all the NMR spectra, peaks (region 1) were observed at approximately 170 ppm, assigned to unreacted aldehyde. The peak due to unreacted aldehyde was more intense in TPp, suggesting a lower degree of polymerization than in the other two POPs. Moreover, sp2 (region 2, phenyl ring) and sp (region 3, nitrile group) carbon peaks were observed at 150–130 and 110 ppm, respectively. Peak positioned in region 4 (below 50 ppm) indicates residual n-hexane and sp3 carbons in unreacted benzyl cyanide in the polymer backbone.Figure 2Characterization data of TPo, TPm, and TPpShow full caption(A) IR spectra of TPo, TPm, and TPp.(B) Solid-state 13C NMR spectra.(C and D) UV-vis spectra in the solid-state and dispersed in hexane, respectively.(E) Band structure diagram, the top and bottom bars indicate the conduction and valence bands of the POPs, respectively.(F) N2 isotherms at 77 K. Filled and open symbols indicate adsorption and desorption, respectively.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Furthermore, we investigated the optical and electronic properties of the POPs to verify their potential as photosensitizers. In the solid-state UV-vis absorption spectra (Figure 2C), TPo shows a broad range of absorption in the visible region, whereas both TPm and TPp display a maximum absorption peak at 418 nm with a 200–600 nm absorption range. In addition, the photoluminescence quantum yield (PLQY) was determined to be 0% (0%) for TPo, 11% (13%) for TPm, and 13% (15%) for TPp at 400 (450) nm excitation (Figure S7). However, when the POPs were dispersed in n-hexane to measure the absorption spectra, they showed a disparity among the POP isomers (Figure 2D). The maximum absorption peaks were shifted to 488 nm (TPp) and 457 nm (TPm), and no specific peak for TPo was observed. To understand the difference in optical response, we measured the particle size of POP isomers by scanning electron microscopy (Figure S8). The POPs showed diverse particle sizes (TPo: ∼900 nm, TPm: ∼50 nm, TPp: ∼500 nm), despite using the same synthetic method. Although it is difficult to obtain accurate absorbance data of TPo in solution because of its low dispersity, this corresponds to the fact that the absorption peaks of the nanoparticles could be dependent on their particle size.32Dai C. Yang D. Zhang W. Bao B. Cheng Y. Wang L. Far-red/near-infrared fluorescent conjugated polymer nanoparticles with size-dependent chirality and cell imaging applications.Polym. Chem. 2015; 6: 3962-3969https://doi.org/10.1039/C5PY00344JCrossref Google Scholar The band gaps of TPo, TPm, and TPp, calculated by Tauc plots from ssUV data, were 1.74, 2.32, and 2.31 eV, respectively (Figure S9). In addition, we used cyclic voltammetry with Ag/AgCl as a reference electrode to determine the potential levels of the POPs (Figure S10). For the oxidation process, the potential levels of the HOMOs for POPs were calculated as −5.77 eV (TPo), −5.58 eV (TPm), and −5.63 eV (TPp) versus vacuum level, respectively. By combining all the analytical data, we have profiled band diagrams of the POPs (Figure 2E) to visually identify their distinct optical and electronic properties.We next explored the thermal stability of the polymers prior to gas isotherm measurements and found that they were structurally stable up to 150°C (Figure S11). The intrinsic porosity of the POPs was investigated by measuring nitrogen isotherms at 77 K after degassing at 120°C for 12 h (Figures 2F and S12). The Brunauer-Emmett-Teller surface areas were calculated to be 8.8 m2 g−1 (TPo), 113 m2 g−1 (TPm), and 79 m2 g−1 (TPp). The main pore sizes were found to be 1.18 nm (TPm) and 1.48 nm (TPp) by pore size distribution analysis using the N2-DFT model, except for the non-porous TPo isomer. Taking all the analysis results into consideration, we propose the two-dimensional structures for TPm and TPp shown in Figure S13.The ROS generation abilities of TPE-4CHO and TPp were evaluated in a methanol solution with 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) as an indicator of 1O2 under white LED lamp irradiation (Figures 3A and S14). As the TPE monomer is a representative aggregation-induced emission luminogen (AIEgen), TPE-4CHO displays no sensitizing performance in organic solvents because it cannot aggregate.33Hu F. Xu S. Liu B. Photosensitizers with aggregation-induced emission: materials and biomedical applications.Adv. Mater. 2018; 30: 1801350https://doi.org/10.1002/adma.201801350Crossref PubMed Scopus (423) Google Scholar,34Ni J. Wang Y. Zhang H. Sun J.Z. Tang B.Z. 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Among the three isomers, TPm exhibited the most efficient generation of 1O2, presumably from faster ROS diffusion due to the larger surface area, smaller particle size, and more overlapped energy states than in the other two isomers.38Lan G. Ni K. Veroneau S.S. Luo T. You E. Lin W. Nanoscale metal–organic framework hierarchically combines high-Z components for multifarious radio-enhancement.J. Am. Chem. Soc. 2019; 141: 6859-6863https://doi.org/10.1021/jacs.9b03029Crossref PubMed Scopus (47) Google Scholar We also examined the 1O2 generation ability of the POPs using a mixture of p-nitroso dimethylaniline (RNO) and imidazole, an alternative 1O2 indicator, and found that TPm showed a marked decrease in RNO absorbance at 420 nm (Figure S16).39Herman J. Neal S.L. Efficiency comparison of the imidazole plus RNO method for singlet oxygen detection in biorelevant solvents.Anal. Bioanal. 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Role of hydroxyl, superoxide, and nitrate radicals on the fate of bromide ions in photocatalytic TiO2 suspensions.ACS Catal. 2020; 10: 7922-7931https://doi.org/10.1021/acscatal.0c02010Crossref Scopus (26) Google Scholar,42Bienert G.P. Schjoerring J.K. Jahn T.P. Membrane transport of hydrogen peroxide.Biochim. Biophys. Acta Biomembr. 2006; 1758: 994-1003https://doi.org/10.1016/j.bbamem.2006.02.015Crossref PubMed Scopus (788) Google Scholar Consequently, it is notable that TPp can efficiently generate both 1O2 and HO∙.To further validate the type of ROS generated by POPs under light irradiation, electron paramagnetic resonance (EPR) data were collected. 1O2 generation by POPs was confirmed using tetramethyl-4-piperidone (TEMP) as a spin-trapping agent (Figure 3E). The intensity of TEMP with TPm was recorded to be the highest compared with other cases, which agrees well with the observations in the absorbance spectra using ABDA and RNO + imidazole indicators (Figures S15 and S16). Moreover, we employed 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical spin trap to evaluate the existence of other radical species. Strong signals assignable to O2.– were found in the EPR spectra, with TPp exhibiting the highest intensity of DMPO (Figure 3F), consistent with the DHR123 absorbance data (Figure S17).43Gao Z. Lai Y. Tao Y. Xiao L. Li Z. Zhang L. Sun L. Luo F. Creating and tailoring ultrathin two-dimensional uranyl-organic framework nanosheets for boosting photocatalytic oxidation reactions.Appl. Catal. B: Environ. 2021; 297: 120485https://doi.org/10.1016/j.apcatb.2021.120485Crossref Scopus (7) Google Scholar We propose to explain the reason for less 1O2 catalyzing but more radical generation performance of TPp compared with TPm as follows. In general, the amount and types of generated ROS in specific PS systems can compete with each other when triplet electrons are involved in the ROS generation mechanism (Figure 3G). According to the ROS generation mechanism involving conduction band (CB) reported by Zhang and coworkers,44Wang Z.J. Ghasimi S. Landfester K. Zhang K.A.I. Molecular structural design of conjugated microporous poly (benzooxadiazole) networks for enhanced photocatalytic activity with visible light.Adv. Mater. 2015; 27: 6265-6270https://doi.org/10.1002/adma.201502735Crossref PubMed Scopus (197) Google Scholar,45Zhang K. Kopetzki D. Seeberger P.H. Antonietti M. Vilela F. Surface area control and photocatalytic activity of conjugated microporous poly (benzothiadiazole) networks.Angew. Chem. Int. Ed. 2013; 52: