Keywords

1 Introduction

Polymerization of bifunctional monomers usually leads to the formation of 1D-linear or branched, non-porous polymers. By chemical modification of a precursor polymer, hypercrosslinked polymers (HCPs) have been produced using different cross-likers (Tan and Tan 2017). These HCPs showed high Brunauer−Emmett−Teller (BET)-derived surface areas (SBET) up to 3000 m2 g−1 (Wang et al. 2017). A different approach for synthesizing porous polymer structures is based on the polymerization of rigid, multifunctional 3D monomers. The resulting, mostly amorphous materials are covalently bond and highly crosslinked organic structures that present inherent microporosity (porous diameter < 2 nm) (Sing et al. 1984) and high surface areas (Preis et al. 2015a). Conjugated microporous polymers (CMPs) (Lee and Cooper 2020) and porous aromatic frameworks (PAFs) (Yuan and Zhu 2019) are common names used to described these type of polymer networks. Only difference between CMPs and PAFs is the occurrence/non-occurrence of extended π-conjugation thus causing problems in the classification of (micro)porous polymers (Jiang et al. 2007). To avoid misunderstandings, we have opted for a different terminology, Microporous Polymer Networks (MPNs), which includes both CMPs and PAFs. MPNs have shown application potential in gas storage and separation (Lu et al. 2010), adsorption and encapsulation of chemicals (Wang et al. 2013), heterogeneous catalysis (Xie et al. 2013), photoredox catalysis (Zhang et al. 2013), light emittance (Bonillo et al. 2016), chemo- and biosensing (Geng et al. 2016), energy storage (Zhang et al. 2018), and in biological applications (Bhunia et al. 2018). Synthesis chemical routes for the production of MPNs are mainly dealing with C-C coupling or C-N coupling reactions such as Sonogashira-Hagihara coupling (Trunk et al. 2016), Suzuki-Miyaura coupling (Chen et al. 2010), Yamamoto coupling (Schmidt et al. 2009), Heck coupling (Sun et al. 2013), cyclotrimerizations (Buyukcakir et al. 2019), phenazine ring fusion (Marco et al. 2017), Schiff-base condensations (Xu and Hedin 2013), heterocycle linkages (Biswal et al. 2018), alkyne metathesis (Lu et al. 2015), Buchwald-Hartwig amination (Liao et al. 2018), direct arylation (Liu et al. 2013), polyolefinations (Preis et al. 2015a), and oxidative couplings (Qiao et al. 2014). MPNs produced by these chemical routes are usually insoluble and practically intractable powders or monoliths that cannot be processed into thin films for applications in the field of organic electronics. Electrochemical, oxidative generation of MPNs has proved to be an promising technique for the preparation of high quality thin MPN films (Suresh and Scherf 2018). The electropolymerization method allows for simultaneous MPN formation and deposition under mild and catalyst-free conditions, and in short times. Anodic electropolymerization is accomplished by oxidation of multifunctional monomers at the electrode followed by cascades of radical ion-radical ion coupling reactions which leads to dimers, trimers, tetramers, oligomers up to polymer networks (Ibanez et al. 2018). A controlled deposition of the growing thin MPN films on the electrode surface is driven by a progressively reduced solubility of the coupling products (Heinze et al. 2010). Thiophenes and carbazoles are the most used electroactive motifs with low oxidation potential, that are applied in combination with suitable linkers in the electrogeneration of MPN films (Palma-Cando et al. 2019a; Zhang et al. 2015).

Hereinafter, we discuss some of our efforts and contributions in the synthesis, characterization and application of microporous polymer networks during the past decade. A library of diverse monomers utilized for the generation of MPNs is presented in Scheme 1. In the first part, we review some examples of chemically synthesized MPNs based on cyclotrimerizations and Friedel-Craft-type polycondensations. Then, we will focus on the electrochemical generation of thin MPN films based on multifunctional carbazole- and thiophene-based rigid monomers and their application as luminescent or electrochemical sensors for nitroaromatic analytes. Both oxidative chemical or electrochemical polymerizations have been comparatively studied, e.g. concerning the inherent porosity of bulk powders and thin MPN films, respectively.

Scheme 1
scheme 1

Chemical structures of chemically or electrochemically polymerized monomers for generation of microporous polymer networks (MPNs)

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2 Chemically Synthesized Microporous Polymer Networks

A first selected approach in the synthesis of microporous polymer networks is based on a well-known synthesis of truxene derivatives by cyclotrimerization, here by using bifunctional s-indacene monomers (Sprick et al. 2010). Truxene itself is obtained in the cyclotrimerization of indan-1-one with protic (Dehmlow and Kelle 1997) or Lewis acids (Ansems et al. 2000), while truxenone can be synthesized in the condensation of indan-1,3-dione with protic acids (Sanguinet et al. 2006). Figure 1 shows the chemical structures of our bifunctional monomers 1–3. Polycondensation of monomer 1 was tested under different acidic conditions resulting in a MPN yield of ca. 85% by using titanium tetrachloride as Lewis acidic catalyst in 1,2-dichlorobenzene (180 °C, 72 h). Monomers 2 and 3 were polymerized under similar conditions using titanium tetrachloride (for 72 h) or methanensulfonic acid (for 3 h), respectively, with a quantitative polymer yield. Idealized structures of the resulting ladder-type MPNs are shown in Fig. 1. These network polymers showed excellent thermal stability in air up to 330 °C. Solid state 13C {1H} cross-polarization magic-angle spinning (CPMAS) NMR spectroscopy confirmed the formation of the idealized structural motifs including low-intensity carbonyl-related end group signals. Surface area (SBET) values of the bulk polymers were obtained by applying the Brunauer–Emmett–Teller equation to the corresponding nitrogen adsorption isotherms. P1 showed a SBET value of 395 m2 g−1 which was much lower than the SBET values of 1165 m2 g−1 for P2 possibly related to the presence of bulkier methyl substituents that support pore formation. P3 showed the highest SBET value of 1650 m2 g−1 for a MPN powder obtained in a metal-free, acid catalyzed cyclotrimerization.

Fig. 1
figure 1

Idealized chemical structures of MPNs a P1 and P2 (P1: R = CH3, P2: R = H), and b P3. (Sprick et al. 2010). Reproduced with permission of The Royal Society of Chemistry

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As second example of our selection, acid-catalysed Friedel-Craft-type polycondensations were applied to a series of 10 multifunctional arylamino-substituted fluorenone monomers (see monomers 4–13 in Scheme 1) and binary mixtures of them (Preis et al. 2011). A2 + B2 and AB2 polycondensations under Friedel-Craft reaction conditions between aromatic ketones and bifunctional aromatic moieties lead to the formation of linear, high molecular weight polymers (Zolotukhin et al. 2004) and hyperbranched polymers, (Smet et al. 2002) respectively. We used the similar synthetic principles for the generation of MPNs (for P4–P6 see Fig. 2) starting from A4B2 and A2B2 monomers, carried out with methane sulfonic acid as catalyst in 1,2-dichlorobenzene (at 140 °C). Formation of rigid, three-dimensional (3D) structures was obtained in a single reaction step under formation of 9,9-diphenylfluorene knot motifs in nearly quantitative yields. Thermogravimetric analysis showed good stability of the MPNs up to 250 °C. Solid-state 13C{1H} CPMAS NMR spectra for P4 showed a broad signal with three maxima at 129 ppm, 138 ppm and 148 ppm for the aromatic carbons and a weak signal at 63 ppm assigned to the aliphatic tetragonal carbon that is formed during condensation. The resulting MPN (P4) showed maximum SBET values of 1420 m2 g−1 for homopolymers due to the rigidity and high cross-linking density of the MPNs. Copolymerization of two different monomers followed by supercritical CO2 treatment (scCO2 washing) led to optimized SBET values up to 1775 m2 g−1 for a copolymer from monomers 1/3 (1:1 w/w). For comparison, the same, non-scCO2-washed copolymer showed a reduced SBET of 718 m2 g−1. Therefore, exchange of leftover organic solvents by scCO2 distinctly increases the accessible SBET surface area of the MPNs. This effect was more noticeable for MPNs of lower less cross-linking density in comparison to highly cross-linked MPNs. Optimized homopolymerization of monomer 4 (Scheme 1) with trifluoromethane sulfonic acid as catalyst in 1,2-dichlorobenzene at 140 °C led to MPNs showing high SBET values of up to 2250 m2 g−1 (Preis et al. 2013). Maximum pore volume and hydrogen storage capacity determined for this MPN (P4) are 2.16 cm3 g−1 and 1.68%, respectively.

Fig. 2
figure 2

Idealized chemical structures of MPNs a P4 and P5 (P4: R = phenyl and P5: R = methyl), and b P6. Unreacted aromatic, keto units and 9-monosubstituted fluorene cores that form end groups cores are encircled. Modified with permission (Preis et al. 2011). Reproduced with permission of The Royal Society of Chemistry

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As third example of MPN generation, the metal-free cyclotrimerization of three different aromatic diisocyanate monomers (see monomers 14–16 in Scheme 1) allows for the generation of monolithic MPNs (Preis et al. 2015b). These commercial low-priced monomers undergo cyclotrimerization under triarylcyanurate formation (see Fig. 3a), by using three different catalyst systems: sodium p-toluenesulfinate (p-TolSO2Na), (Moritsugu et al. 2011) tetrabutylammonium fluoride (TBAF) (Nambu and Endo 1993), or tetrakis(dimethylamino)ethylene (TDAE) (Giuglio-Tonolo et al. 2014). A 3 × 7 experimental matrix was set up under variation of monomers, catalysts, solvents, and reaction temperature. Five of the monolithic MPNs obtained from monomer 14 showed a combination of high yield and high SBET surface areas; P14-1 (92% yield and SBET 783 m2 g−1: with 10% TBAF in DMI at 150 °C), P14-2 (92%; 971 m2 g−1: with 10% TBAF in ODB at 150 °C), P14-3 (98%; 216 m2 g−1: with 10% TBAF in THF at 150 °C), P14-4 (100%; 115 m2 g−1: with 10% p-TolSO2Na in DMI at 150 °C), and P14-5 (86%; 711 m2 g−1: with 10% TDAE in DMI at room temperature). A second 3 × 15 experimental matrix was applied to optimize the catalyst concentrations (10%, 5% and 1%) for the five promising polymerization conditions. Herein, P14-6 was obtained under similar condition as P14-4 with a 1% TBAF resulting in a monolithic MPN with quantitative yield and a SBET value of 1320 m2 g−1 thus demonstrating the influence of the catalyst/monomer ratio on the cross-linking density of the resulting polymer networks (more than one order of magnitude of improvement in SBET). This easily produced MPN P14-6 shows reasonably high gas uptakes of 1.3 wt%, 1.1 wt%, and 14.4 wt% for hydrogen (at 77 K), methane (at 273 K), and carbon dioxide (at 273 K), respectively. Moreover, up-scaling the production of P14-6 was demonstrated by using a 100 g monomer batch resulting in compact, monolithic MPN chunks with a slightly increased SBET of 1510 m2 g−1. These results suggested potential applications of such MPN monoliths for the capture and storage of gases. It was found that increasing the monomer concentration over a certain threshold leads to the formation of MPNs of higher surface area. This behavior is probably related to an increased cross-linking density. Removal of oily contaminants from aqueous mixtures was investigated for such P14 monoliths. Figure 3b shows photographs for the dodecane (as model for an oily contamination, dyed with Sudan I) sorption from an aqueous mixture using microporous P14 chunks. Hereby, the low-density MPN material (0.18 g cm−3) is floating on the water surface and is completely trapping the oily pollutant (within short times of less than one minute, under up to fourfold mass increase).

Fig. 3
figure 3

a Idealized chemical structure of the MPNs P14-P16 made from rigid, aromatic diisocyanates (green rectangle represents the aromatic cores: (in P14) 2,2′-dimethyl-biphenyl-1,1′-diyl, (in P15) 1,5-naphthylene and (in P16) 1,4-phenylene, and b photographs showing dodecane (dyed with Sudan I) sorption into a microporous, monolithic P14 chunk from aqueous mixture (Preis et al. 2015b). Reproduced with permission of the American Chemical Society

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Fig. 4
figure 4

a Linear scan voltammograms (after background correction) for reduction of 0.5 μM aqueous TNB solution at P22-modified and bare (inset) glassy carbon electrodes (at scan rate of 0.01 V s−1) and b current ratio of the third reduction peak of TNB at MPN-modified and bare glassy carbon electrodes versus SBET of the MPN films (Palma-Cando and Scherf 2015). Adapted with permission of the American Chemical Society

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3 Electrochemically Synthesized Microporous Polymer Networks

Electrochemical oxidative polymerization of multifunctional monomers with low oxidation potential leads directly to the formation of thin MPN films, while the wet-chemistry based oxidative coupling produces MPN powders (or, in some cases, monoliths) (Palma-Cando and Scherf 2016). Hereby, non-aqueous monomer solutions are usually applied at low concentrations (µM) with supporting organic electrolytes based on tetrabutylammonium salts (Palma-Cando et al. 2014). MPN films can favorably be generated in a three-electrode cell connected to a potentiostat/galvanostat which controls potential and current applied to the system. Microporous polymer films are deposited on the working electrode (WE) by oxidative polymerization. Electronic conductors used as inert WE are usually made of gold, platinum, glassy carbon or ITO (Gurunathan et al. 1999). Our fist approach into the generation of MPN films dealt with the electrochemical polymerization of multifunctional carbazole monomers (for the six investigated monomers see 17–22 in Scheme 1) containing two, three and four carbazole units (Palma-Cando and Scherf 2015). Ambrose and Nelson described the mechanism for the electrochemical oxidative coupling of carbazoles under formation of cation radicals that experience radicalion-radicalion couplings into 3,3′-bicarbazole dimers (Ambrose and Nelson 1968). Dimers can undergo further reactions such as 6,6′-couplings, into extended oligomers and polymers. In the polymerization of monomers 17–22 we first studied the influence of the number of carbazoles attached to the core of the tectons on the resulting surface area of the deposited microporous polymers. Nitrogen and krypton adsorption isotherms were recorded for the six different MPNs made both by bulk chemical polymerization with FeCl3 (for comparison) and potentiostatic electrochemical polymerization, respectively. Electrogenerated MPN films were synthesized from diluted monomer solutions in acetonitrile/dichloromethane mixtures. Adsorption isotherms of films and powders showed similar trends with a fast N2 or Kr uptake at low relative pressure (<0.1) which is characteristic for the occurrence of permanent microporosity (Sing et al. 2008). Table 1 lists the SBET values for bulk microporous polymer powders and the respective MPN films. Similar surface area values for bulk polymers and thin films were obtained for the tetrasubstituted MPNs (P21 and P22). On the other hand, decreased SBET values were determined for films generated from bi- and trisubstituted monomers if compared to the corresponding MPN powders (P17–P20). Reduced cross-linking density for the electrogenerated films is most probably caused by an exclusive formation of carbazole dimers while FeCl3 bulk polymerization also yields 3,5-disubtituted carbazoles (Chen et al. 2012). Moreover, a direct relationship between the number of carbazole groups in the monomers and the SBET values of the resulting MPN films was observed based on increased cross-linking density and rigidity of the MPNs when increasing the functionality of the monomers. Tetrafunctionalized monomers provide an optimum amount of possible cross-linking sites (Zhang et al. 2014).

Table 1 Calculated specific BET surface areas SBET of chemically synthesized bulk polymer powders as well as of electrogenerated MPN films using N2 or Kr gas sorption, respectively
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Electrogenerated polymer films are very promising materials for the fabrication of electrochemical sensors and biosensors (Terán-Alcocer et al. 2021) for various analytes such as neurotransmitters (Moon et al. 2018), pharmaceuticals (Eslami and Alizadeh 2016), glucose (Naveen et al. 2017), hydrogen peroxide (Park et al. 2016), nitrites (Ge et al. 2020), or nitroaromatic compounds (Guo et al. 2015). We used thin, carbazole-based MPN films for the electrochemical detection of 1,3,5-trinitrobenzene (TNB) as prototypical nitroaromatic compound in aqueous solution, based on its cathodic reduction. An increased current response of up to 182 times was observed for P22 films on glassy carbon (GC) electrodes when contacted with aqueous TNB solutions, if compared to the current response of bare GC electrodes (see Fig. 4a). An interesting correlation was found between the current response increase (bare GC vs. MPN coated GC electrodes) and the SBET of the MPN films (see Fig. 4b). Hereby, the formation of charge-transfer complexes between the electron-poor nitroaromatic analytes and the electron-rich MPN films is boosted by the much higher electroactive surface area at the MPN-modified electrodes where the analyte reduction takes place (Evans et al. 2002; Zhang et al. 2006; Shamsipur et al. 2015).

Regarding civil and environmental security, the detection of explosives in the gas-phase is highly desirable (Sun et al. 2015). Here, fluorescent polymers networks have shown high application potential as luminescent sensor for explosive traces (Liu et al. 2012). We used spirobifluorene-cored MPN films (P21) for the gas-phase detection of nitroaromatic explosives due to their intense inherent fluorescence and high SBET surface area (1297 m2 g−1) (Räupke et al. 2016). Figure 5a shows 10 successive voltammetric cycles for the electrochemical polymerization of a 0.5 mM solution of monomer 21 in acetonitrile/dichloromethane (1:4) on ITO electrodes. P21 thin films with thickness of ca. 50 nm are formed by cycling in a potential range of 0 V–0.98 V with a scan rate of 0.10 Vs−1. A so-called nucleation loop with a crossing effect between the first anodic scan and the reverse sweep is observed during the first cycle. This phenomenon might be related to an autocatalytic reaction between oligomeric intermediates and monomers (Heinze et al. 2007). Tapping mode AFM image shows a rather smooth morphology with a mean roughness Rq of ~14 nm (see inset Fig. 5a). For the sensing experiment, analytes were placed in a closed chamber (analyte chamber) that is connected via a needle valve to a second chamber containing the P21 films on ITO (sample chamber). Each chamber can be separately heated. Photoluminescence (PL) spectra of a P21 film upon excitation at 355 nm show a PL peak centered at 472 nm. The so-called quenching efficiency of the P21 PL upon exposure to different analytes (e.g. acetone, ethanol, toluene, benzene, nitrobenzene—NB, and 2,4-dinitrotolune—DNT) were found to be >1 for electron-poor nitroaromatic analytes (NB and DNT), while for the other analytes only a slight effect on the PL was observed. A possible mechanism of PL quenching is an energy transfer between the excited, electron-rich host and the electron-poor nitroaromatic quencher (Toal and Trogler 2006). The higher quenching efficiency (DNT vs. NB) for the energy transfer from the LUMO level of P21 (−2.58 eV) to the LUMO level of DNT (−3.22 eV) might be ascribed to a deeper lying LUMO level of DNT if compared to the LUMO level position of NB (−2.91 eV). Finally, gas-phase detection of 2,4,6-trinitrotoluene (TNT) at low concentrations down to 5 ppb (room temperature vapor pressure) was investigated. Our P21 films showed a maximum quenching of ca. 50% after 30 min by contact with TNT vapors (see Fig. 5b). Already after 2–3 min a significant PL quenching response was recorded. For comparison, PL quenching was not detectable for non-porous films, vapor-deposited films of monomer 21 at even much higher TNT concentration of 3 ppm (3 orders of magnitude increased concentration). These results clearly demonstrate the crucial role of microporosity and high surface area of the films by facilitating the interaction between the MPN surface and the quencher molecules.

Fig. 5
figure 5

a Ten successive cyclic voltammograms at an ITO electrode for 0.5 mM solution of monomer 21 in acetonitrile/dichloromethane (1:4) and 0.1 M TBAP as supporting electrolyte, potential range: 0–0.98 V, scan rate: 0.10 Vs−1. The inset shows a tapping mode AFM image of the resulting MPN film. b Integrated PL response to different TNT vapor concentrations (455–480 nm) for a vapor-deposited non-porous film of monomer 21 (black dots) and a microporous P21-film (blue dots) (Räupke et al. 2016). Reproduced with permission

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In a second attempt, we studied the electrochemical, oxidative polymerization of tetrathienyl-substituted spirobifluorene- and tetraphenylmethane-cored monomers (see monomers 23–26 in Scheme 1) both in pure dichloromethane and dichloromethane/boron trifluoride ethyl etherate (DCM/BFFE) mixtures (Palma-Cando et al. 2015). The coupling mechanism for electron-rich heteroaromatics such as thiophenes is described in a similar way as done for carbazole-based monomers (Diaz et al. 1981). The potential required for the oxidation of the monomer is always higher than potential needed for oxidation of the oligomeric species that are formed in a cascade of condensation reactions toward formation of polymeric chains. It has been demonstrated that the addition of boron trifluoride decreases the needed potential for monomer coupling by reducing the aromaticity of the system through BF3 interaction with the lone electron pair at the thiophene sulfur (Shi et al. 1995, 1999). The resulting surface areas extracted from nitrogen or krypton sorption measurements of thin, electrogenerated MPN films and, for comparison, of bulk MPN powders obtained by wet-chemical coupling with FeCl3, respectively, are listed in Table 1. The highest SBET surface areas for thin MPN films were achieved for deposition from DCM/BFFE mixtures with SBET values like that of the corresponding bulk polymer powders. This finding indicates the importance of reducing the potential for the monomer oxidation. The connectivity of the networks made from the 3-thienylbased monomers (24 and 26) was significantly affected by the presence of boron trifluoride. The formation of hyperbranched oligothiophene structures containing 2,3,5-trisubtituted thiophene units was obtained in electrochemical polymerization in DCM (see Fig. 6a). However, MPN deposition from 24 and 26 in DCM/BFEE mixtures preferably led to the formation of 2,2′-bithiophene links similar to that what is generally observed for the corresponding 2-thienyl-substituted monomers 23 and 25 (see Fig. 6b and c). Moreover, electrochemical reduction of TNB at P25-modified GC electrodes showed a linear relationship between current response and thickness of the microporous deposits. These results confirm the role of the donor–acceptor interaction between electron-poor nitroaromatic and electron-rich moieties after modifying GC electrodes with P25-films.

Fig. 6
figure 6

Idealized chemical structure of the thienyl-thienyl links in microporous films a electrochemically prepared P24 in DCM, b electrochemically prepared P24 in DCM/BFEE mixture, and c wet-chemically prepared P23 (Palma-Cando et al. 2015). Adapted with permission of The American Chemical Society

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In a further study we utilized tetra- and octacarbazole-functionalized monomers with three dimensional, rigid tetraphenylmethane and tetraphenylsilane cores for electrochemical (and wet-chemical) MPN generation (see monomers 27–30 in Scheme 1) (Palma-Cando et al. 2016). Bulk polymer powders were isolated in high yields for the oxidative coupling of these multifunctional monomers with FeCl3. MPN powders showed good thermal stability and intense photoluminescence (PL) with maxima at 444 nm (P27), 473 nm (P28), 482 nm (P29), and 471 nm (P30). In addition, a reasonable CO2 trapping capacity was shown for P29 with a storage capacity of up to ca. 9 wt%. Quartz-crystal microbalance measurements simultaneously coupled to the electrochemical film deposition (EQCM) was used for gaining more insight into the growth of the MPN films during the electropolymerization of our multifunctional monomers. The anodic scan in the first cyclic voltammogram for polymerization of monomer 27 showed a decrease in the EQCM frequency starting at the peak potential onset at ca. 0.9 V vs. Ag/AgNO3 (see Fig. 7a). These frequency dropping is related to a mass increase that can be estimated by the Sauerbrey equation (Sauerbrey 1959) thus documenting oxidative coupling of carbazole units and subsequent irreversible deposition of a P27 layer on the Pt/quartz electrode. A continuously increasing mass was observed indicating a homogeneous film growth overlaid by a reversible uptake and release of perchlorate counteranions due to doping/de-doping events in the as-grown P27 layer. Nitrogen or krypton adsorption isotherms at 77 K for wet-chemically made bulk polymer powders or electrodeposited MPN films are depicted in Fig. 7b and c, respectively. Main gas capture at low relative pressure of <0.1 verifies the microporous nature of the materials. Slightly reduced specific surface areas were found for the electrodeposited MPN films if compared to the bulk polymer powders (see Table 1). P29 made from octacarbazolyl-substituted tetraphenylmethane tectons showed the highest SBET of 1331 m2 g−1. Both carbon-cored MPNs displayed increased SBET value in relation to the respective silicon-cored MPNs. This result can be related to electronic σ − π-interactions of aromatic substituents through the silicon centers thus decreasing the reactivity of the carbazolyl groups and leading to a lower cross-linking density (Traylor et al. 1970). Electrochemical sensing experiments for various nitroaromatic analytes were carried out on glassy carbon electrodes modified with microporous P29 films. Figure 8b depicts linear scan voltammograms for the electrochemical reduction of 0.1 μM solutions of nitrobenzene (NB), 1,3-dinitrobenzene (DNB), 2,4-dinitrotoluene (DNT), 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrophenol (TNP), and 2,4,6-trinitrotoluene (TNT) at P29-modified GC electrodes in an aqueous environment. A series of reduction peaks corresponding to the number of nitro functions in the analyte was observed for sub-µM concentrations of the analytes. Hereby, the position of the first reduction peak potential drops by increasing the number of nitro substituents in the analyte molecules, that means, less energy is required for the initial, interfacial charge transfer between the microporous polymer containing electron-rich bicarbazole-units and the electron-poor nitroaromatic compounds (Shamsipur et al. 2015). Similar results were reported by us for polymer networks deposited from multifunctional monomers with three or six carbazole units (Palma-Cando et al. 2019b).

Fig. 7
figure 7

a Ten cyclic voltammograms (black lines) and EQCM frequency changes (red lines), that reflect the mass accumulation in the electrochemical polymerization of 0.1 mM solutions of 27. Adsorption isotherms of b electropolymerized MPN films (Kr sorption) and c corresponding bulk polymer powders synthesized by oxidative coupling with FeCl3 (N2 sorption). d Linear scan voltammograms for the reduction of 0.1 μM solutions of various nitroaromatic analytes at P29-modified GC electrodes in buffered aqueous solution (pH 7.4). LSVs were obtained with a scan rate of 0.01 V s−1 (Palma-Cando et al. 2016). Adapted with permission of the American Chemical Society

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In a follow-on study, we converted wet-chemically as well as electrochemically a series of six tetra- or octa-substituted carbazole- or thiophene-based, tetraphenylethylene (TPE)-cored monomers (see monomers 31–36 in Scheme 1) into microporous polymer materials (Palma-Cando et al. 2017). TPE is a prominent propeller-shaped structural motif that shows the so-called aggregation-induced emission (AIE) phenomenon in the solid state. Main reason for occurrence of AIE effects is the restriction of intramolecular rotations and vibrations (Mei et al. 2014). Since early reports of the AIE effect, (Luo et al. 2001; Sharafy and Muszkat 1971) AIE phenomena received enormous attention from the scientific community due to potential applications in different fields (Jimenez and Rodríguez 2020). In our study, luminescent MPN powders based on TPE-cored tectons were obtained by wet-chemical oxidative coupling of monomers 31–36 with FeCl3, resulting in materials that are thermally stable up to 300 °C. Electrochemical, oxidative polymerization of the carbazole-based monomers 31–33 were carried out in dichloromethane solutions containing tetrabutylammonium tetrafluoroborate (TBABF4) as electrolyte, while the thiophene-based monomers 34–36 required the addition of 20% (V/V) BFEE to the solution for effective film formation. Nitrogen or krypton gas adsorption data at 77 K within the relative pressure window of 0–0.6 were determined for bulk polymer powders and MPN films, respectively. A fast gas uptake at low relative pressures documents the microporous nature of the materials. Slightly reduced SBET values were observed for the electrogenerated MPN films if compared to the corresponding bulk powders (see Table 1), probably indicating a reduced cross-linking density. The bulk polymer powders P31 and P34 showed quite similar SBET values of ca. 1100 m2 g−1, both containing tectons that are fourfold carbazolyl- or thienyl-substituted with the substituents directly attached to the TPE core. P32 and P35 polymers both containing 1,4-phenylene spacers between the electroactive units and the TPE core showed slightly reduced surface areas of 1039 m2 g−1 and 956 m2 g−1, respectively, for the bulk materials/powders. Electrochemically generated P34 films, as an example for electrodeposited MPN films, display a clearly lowered surface area of only 433 m2 g−1 possibly related to overoxidation effects due to the high monomer oxidation potential of 1.4 V needed for generation of the thin MPN films and/or to a reduced cross-linking density. By doubling the number of electroactive carbazolyl or thienyl substituents in one monomer, remarkably high SBET values were obtained for P33 (2203 m2 g−1) and P36 polymer powders of (1767 m2 g−1) indicating high cross-linking density and high rigidity. The presence of the 1,3,5-trisubtituted aromatic spacer units between core fragment and electroactive, peripheral substituents plays, probably, a crucial role concerning the resulting specific surface area: by decreasing the probability for chain intercalation. Within this series, the electrogenerated, carbazole-based MPN films show an intense photoluminescence. The combination of high permanent microporosity and intense PL seems promising for PL sensing applications. Thin MPN films emit green to yellow PL with maxima centered at 527 nm for P31, 542 nm for P32, and 529 nm for P33. Electrogenerated P33 films were tested as chemical sensors for TNT detection in solution, please note the importance to public health and security (Fei et al. 2014). Figure 8a shows the continuously dropped PL intensity for ppm concentrations of the TNT analyte with a clearly detectable quenching already for a concentration of ~1 ppm TNT. The sensing mechanism can be related to excited state energy transfer between the electron-rich bicarbazole units of the MPN films and the electron-poor quencher (TNT). Optimized gas-phase detection conditions for TNT in electrogenerated P31 films allowed us to detect this harmful analyte up to the 10 ppb level, in a few seconds (see Fig. 8b) (Mothika et al. 2018).

Fig. 8
figure 8

a PL spectra (upon excitation at 340 nm) of P33 films after immersing the films into TNB solutions of different concentrations. The inset shows photographic photoluminescence images of P33 films, in the pristine state and after exposure to 50 ppm TNT solutions. Reproduced with permission (Palma-Cando et al. 2017) Reproduced by permission of The Royal Society of Chemistry. b Time trace of the fluorescence quenching of a P31 film in the presence of TNT vapors at a concentration of 10 ppb (Mothika et al. 2018). Reproduced with permission of the American Chemical Society

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4 Conclusions and Perspectives

During the last decade, our research groups have tested a broad library of new monomers for oxidative generation of microporous polymer networks (MPNs) in wet-chemical and electrochemical strategies. Microporous materials both showing high specific surface areas of up to 2200 m2 g−1 and intense photoluminescence are accessible by a smart design of the tectons that form the MPN framework (in the case of P33, for example, by combining a tetraphenylethylene (TPE) core, phenylene spacers and eight electroactive carbazole units in the periphery). Potential applications in the field of optical or electrochemical sensors for detection of nitroaromatic compounds (including explosives) have been systematically explored, finally allowing for a vapor detection with sensitivities down to a few ppb (e.g. for TNT gas-phase detection). We still see big opportunities in the exploration of related MPNs nanocomposites in fields such as electrochemical sensors, supercapacitors, or heterogeneous catalysis, based e.g. on inherent microporosity, efficient photoluminescence and semiconducting nature of the films. The electrosynthesis of thin MPN films, is, hereby, an elegant and straightforward method to combine MPN synthesis directly with thin film formation.