Nanometer-Thick Conjugated Microporous Polymer Films for Selective

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Nanometer-Thick Conjugated Microporous Polymer Films for Selective and Sensitive Vapor-Phase TNT Detection Venkata Suresh Mothika, André Räupke, Kai Oliver Brinkmann, Thomas Riedl, Gunther Brunklaus, and Ullrich Scherf ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01779 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Nano Materials

Nanometer-Thick Conjugated Microporous Polymer Films for Selective and Sensitive Vapor-Phase TNT Detection † ‡ ‡ ‡ Venkata Suresh Mothika, * André Räupke, Kai Oliver Brinkmann, Thomas Riedl, Gunther §

Brunklaus, and Ullrich Scherf

†

†

Macromolecular Chemistry Group, Bergische Universität Wuppertal, Gaußstraße 20, 42119 Wuppertal, Germany ‡

Institute of Electronic Devices, University of Wuppertal, Rainer-Gruenter-Str. 21, 42119 Wuppertal, Germany §

I

C

lische

-

t, Corrensstr. 46 D-48149

nster, Germany KEYWORDS. Thin-films, porous polymer, electropolymerization, TNT vapors, sensing

ABSTRACT. Conjugated microporous polymer (CMP) thin films often show fast and amplified signal response, and potential for developing portable sensing devices. Here, we elucidate electrochemical generation of three CMP thin films and their fluorescence response to trinitrotoluene (TNT). A tetra(carbazolylphenyl)ethylene monomer TPETCz-derived CMP thin film (PTPETCz, SBET : 930 m2/g) displayed fluorescence (max= 525 nm) quenching to nearly 95 % in 3 min when the CMP film is exposed to 33 ppb TNT vapors. Interestingly, PTPETCz is highly sensitive (30% quenching) to TNT vapors of low concentrations (5-10 ppb), and also remarkably selective towards TNT compared to other analytes. In contrast, an only mere response was observed when a non-porous monomer TPETCz-film was exposed to 0.2 ppm TNT. So, the microporosity and extended π-conjugation of the polymer facilitating suitable hostguest interactions is found to be essential towards highly sensitive detection of TNT. Fluorenonecored CMP thin films (PFLCz) showed no response, while PTPEFLCz containing both tetraphenylethylene and fluorenone structural units showed nearly 70% of emission quenching in

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presence of 0.2 ppm TNT. Therefore, the presence of electron donating TPE core is a prerequisite for efficient photoinduced electron transfer from polymer to nitroarenes.

1. INTRODUCTION Research efforts on trace level detection of explosive chemical vapors are rapidly increasing owing to its importance in environment and defense/security applications.1,2 Several explosive sensors based on small molecules, conjugated polymers, self-assembled organic nanomaterials, metal-organic frameworks have been studied for both solution and vapor phase explosives detection up to detection limits of part per trillion (ppt).3–14 The microporous environment of metal/organic frameworks enabling fast diffusion of analytes and effective host-guest interactions was key for the enhanced sensitivity. While, extended π-conjugation in conjugated polymers was shown to be a very promising feature for amplified signal transduction.11–14 In this context, conjugated microporous polymers (CMPs)15–18 featuring both microporosity and extended π-conjugation with large surface area are unique for fluorescence-based sensing of explosive vapors. Efficient exciton migration along the polymer network and an excellent analyte diffusion opportunity can be useful for fast and amplified fluorescence response.19–21 Moreover, CMPs are thermally and chemically stable due to strong C-C linkages between connecting building blocks.27-29 Several chemically synthesized CMP powders have been studied for fluorescence based sensing of chemical analytes,30-36 however, their limited processability often restricts device fabrication. For an example, silsesquioxane-based porous polymers or porphyrin networks were solution processed onto light-weight paper or thin layer chromatography (TLC) plate substrates and used as portable sensors for chemical sensing. Nevertheless, they use expensive Pd(0) catalysts, and additional efforts were needed to remove trace catalyst impurities and to control film thickness.37-39


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Recently, the electro-oxidative polymerization technique for synthesizing CMP thin films gained significant attention.40-44This catalyst-free technique is cost effective and can provide highly pure CMPs in short times with excellent control over film thickness. These films can be grown on a large variety of electrodes including flexible substrates thus enable device fabrication.45-47 Electropolymerization approaches of carbazolyl and thienyl building blocks into CMP thin films for various applications were studied recently.48 The electrochemical sensing of nitroaromatic compounds (NACs) using various CMP thin films with detection levels of several µg/mL was also reported.49 TPE-based materials show aggregation-induced emission (AIE) and have been widely studied for sensing of various small molecules including explosives. 50-55 Recently, fluorescence quenching of CMP thin films (PTPETPOcCz, SBET = 2170 m2/g) by TNT in aqueous solution (50 ppm) was studied by us,56 while Jiang et al. reported polyTPECz-based thin films showing fluorescence quenching response to 50 ppm 2,4,6-trinitriphenol (TNP) solutions with good selectivity, low detection limits and proper fluorescence retention after washing out the analyte.57 Solution-phase sensing methods are, in principle, applicable to environmental safety devices, but with certain limitations due to the use of liquid systems, although good performance can be achieved. Having encouraging results of solution phase optical sensing based on thin films of TPE-cored networks from our previous studies, we now propose to use PTPETCz (SBET = 930 m2/g) thin films with their strongly greenish-cyan emission (max: 525 nm) also for vapor phase explosive sensing. Vapor-phase sensing can be distinctly different from solution phase processes since the nitroaromatic compounds possess quite different vapor pressures. Generally, vapor phase sensing may be more relevant to real time sensing.58 PTPETCz used in the presented study is higher fluorescent if compared to the previously studied PTPTCz 49 and, believably, it may act as superior performing optical sensor.


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There are relatively few reports on electrogenerated CMP thin films for vapor-phase sensing of explosives.59 TPBCz-CMP films were already studied for vapor-phase sensing with good performance,60 and the further development of such materials may be important for homeland security applications. CMP thin films prepared by other techniques were also shown to be promising vapor-phase sensors.58,59,62 The present work on electrogenerated PTPETCz films also documents a very encouraging sensing performance with high sensitivity and short response time. The electropolymerization approach represents a favorable approach for creating high quality CMP thin films that are suitable for device fabrication (as reported e.g. by Jiang et al. and by Ma et al.).45-47 However, reports on electropolymerized CMP thin films showing high sensitivity for trace level trinitrotoluene (TNT) vapors are scarce. We recently reported CMP thin films for vapor-phase sensing of NACs including sensing of 5 ppb TNT based on efficient fluorescence quenching.59 In our new experiments, the PTPETCz emission is strongly quenched (95%) when exposed to 33 ppb of TNT vapor in 3 min and also showed excellent selectivity if compared to the response to various other nitroaromatic analytes (Scheme 1). PTPETCz showed remarkable selective response to TNT vapors among the nitroaromatic compounds studied. To our surprise, an appreciable fluorescence response (30 % quenching) even to lower concentrations (5-10 ppb) of TNT vapors is observed. We have further studied the dependence between film thickness and sensitivity to TNT vapors for the CMP films. Intriguing AIE property of TPE cored materials was shown to be responsible for its efficient sensing properties.50-55,61 The PTPETCz-based sensing response may be similarly explained. CMP thin films grown from the fluorenone-cored carbazolyl derivative FLCz exhibited no remarkable response to TNT. In contrast, a CMP thin film prepared from a mixture of monomers TPETCz and FLCz displayed appreciable responses to TNT in the vapor phase. Although several CMP-


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based thin films have been reported for trace level explosive detection,58,59,62 we believe that the present work represents a promising new example among the already reported systems that have been studied to date for vapor-phase TNT sensing.

Scheme 1. Schematic illustration of the electropolymerization of TPETCz into fluorescent PTPETCz thin film, and the fluorescence quenching in presence of trace TNT vapors. 2. EXPERIMENTAL SECTION Materials and Methods. All reagents and chemicals required have been purchased from commercial chemical suppliers, unless stated. 1,1,2,2-Tetra[4-(carbazol-9-yl)phenyl]ethane (TPETCz) is synthesized according to a literature procedure.56 1H NMR spectra are measured using Bruker Avance III 400 MHz machine. APCI mass spectra are measured on a Bruker Daltronik microTOF system (KrF*-Laser ATLEX-SI, ATL Wermelskirchen). Bulk polymers synthesized by the chemical oxidation method are purified by washing with supercritical CO2 in a Tousimis Samdri-795 system. Bulk polymer samples and thin films are activated on a Belprepvac II at 140 °C and ~2 Pa over 10 hrs for adsorption measurements. Kr adsorption measurements are carried out using a BEL Japan Inc. Belsorp-max system at 77 K in the relative pressure range of 0-0.6, P0 = 1 atm. Ionization potential measurements are carried out with atmospheric pressure ultraviolet photoelectron spectroscopy System (Riken Keiki AC-2).


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Thermogravimetric analyses (TGA) are recorded at a Mettler Toledo TGA/DSC1 STAR machine under Ar atmosphere with a heating rate of 5 °C per min. UV/Vis absorption and FTIR measurements are carried out using JASCO V-670 and FTIR-4200 machines, respectively. Photoluminescence

(PL)

is

measured

using

a

HORIBA

Scientific

FluoroMax-4

spectrofluorometer connected to a QuantaPhi integrating sphere for determination of PL quantum yields. Photoluminescence measurements for sensing experiments are recorded using diode pumped solid state laser photoexcitation (λ= 355 nm, 11 mW/cm2) coupled into a monochromator and detected with a cooled charge-coupled device camera (Princeton Instruments). Atomic force microscopy (AFM) images of the thin films are obtained on a Bruker diInnova system operated in tapping mode, the surface roughness is extracted from the topography images. Solid-State NMR Measurements.

13

C{1H} cross-polarization magic-angle spinning

(CPMAS) spectra were recorded at 50.33 MHz using a Bruker AVANCE III 200 NMR spectrometer with a contact time of 2.5 ms, averaging 17408 transients at a relaxation delay of 2 s; the contact pulse was ramped from 70% to 100% of the optimized power level of 55 Watt. All experiments were carried out at room temperature (air-conditioned to 20°C), employing a d d B g

2.5

o 3.5 μ (

do b g

o

AS p ob

p

g

20 Hz

p

π/2-pulse

-field strength of 71.4 kHz) and SPINAL64 proton decoupling

(200.15 MHz; 10/12 pulse set to 5.8 µs) at rather modest power levels of 32 Watt (13C) and 20 Watt (1H), respectively. The

13

C spectra were referenced with 1-13C and

solid -glycine as secondary standard (176.0 ppm for

13

15

N isotope-enriched

C_carbonyl). In addition, the linewidth

of the 13C_carbonyl peak served as internal check for proper adjustment of the magic angle.


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Electropolymerization. Thin film generation follows similar procedure as reported by our group earlier.40,43,49 Monomer solutions (0.1 mM) are prepared in dichloromethane/acetonitrile (4:1) with 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. A three electrode cell connected to electrochemical workstation PAR VersaSTAT 4 is used under Ar atmosphere at 25 °C. Indium tin oxide (ITO) plates are used as working electrode (WE) combined with a platinum counter electrode (CE) and Ag°/AgNO3 (0.1 M AgNO3, 0.6 V vs NHE, non-aqueous reference) as reference electrode (RE). Potentiostatic regimes are applied for generating porous polymer films on the ITO electrodes. For sensing experiments the films are grown by applying 5 cyclic voltammograms between -0.2 to 1.1 V with a scan rate of 0.1 Vs-1. Thicker films used for gas adsorption measurements are prepared by the chronoamperometry method by applying a constant potential of 1.1 V for 20 min followed by a discharging step at 0 V for 60 s to discharge the deposited films. Free standing thin films are delaminated from the electrodes, dried after rinsing with CH3CN, CH2Cl2 and used for Kr adsorption measurements. The films are activated at 140 °C prior to the adsorption measurements. Synthesis of 2,7-di(carbazol-9-yl)-fluoren-9-one (FLCz). 2,7-dibromo-9H-fluoren-9-one (1g, 2.94 mmol), 9H-carbazole (7.34 mmol, 1.22g), 2,2'-bipyridyl (229.4 mg, 1.47 mmol), CuI (1.297 g, 7.34 mmol) and K2CO3 (2 g, 14.7 mmol) are degassed in a double neck flask and purged with Ar three times. To this mixture o-dichlorobenzene (30 mL) is added and the batch stirred at 180 °C under dark conditions for 48 hrs. Next the mixture is cooled to room temperature, filtered and washed with dichloromethane several times. The filtrate is concentrated under reduced pressure and the crude product is purified by column chromatography using chloroform/tetrahydrofuran/hexane (9:0.5:0.5) as eluent. Finally the product is recrystallized from a cyclohexane/chloroform (1:1) mixture. Red needle-like crystals are obtained after 3 days.


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Yield: 0.66 g (44 %). 1H NMR (400 MHz, CDCl3): δ = 8.20 (d 4 H) 8.0 ( 2 H) 7.89 (d 2H) 7.82 (d, 2H), 7.50 (m, 8 H), 7.37 (d, 4 H) ppm; 13C NMR (600 MHz, CDCl3) δ: 191.8 142.28 140.45, 139.01, 136.29, 133.07, 126.23, 123.75, 123.11, 121.74, 120.39, 109.72. MS (APCI) calcd for C37H23N2O: 511.1805, found m/z = 511.1808 (M+). Synthesis of PTPETCz bulk. It was synthesized using similar procedure reported by Scherf et al,56 and obtained as pale green solid. Synthesis of PFLCz bulk. FLCz (0.12 mmol, 61 mg) is dissolved in 15 mL of dry chloroform and added dropwise to a suspension of FeCl3 (0.66 mmol, 108 mg) and stirred for 24 hrs at room temperature. To this mixture 100 mL of methanol is added and stirred for 1 hr to precipitate the polymer. Precipitates are collected by filtration and washed thoroughly with methanol. Further, the powders are treated with conc. HCl (35 %) for 2 hrs and washed with water and methanol thoroughly. Compounds are purified by Soxhlet extraction with THF and methanol followed by supercritical CO2 washing (as described before) as final purification.63 PFLCz is obtained as a red solid, yield: 48 mg (78 %) Synthesis of PTPEFLCz bulk. TPETCz (0.12 mmol, 116 mg) and FLCz (0.12 mmol, 61 mg) dissolved in 30 mL of dry chloroform are added dropwise to a suspension of FeCl3 (1.32 mmol, 216 mg) and the mixture is stirred for 24 hrs at room temperature. To this mixture 200 mL of methanol is added and stirred for 1 hr to precipitate the polymer. The precipitates are collected by filtration and washed thoroughly with methanol. Further, the powder is treated with conc. HCl (35 %) for 2 hrs and washed with water and methanol thoroughly. The product is finally purified by Soxhlet extraction with THF and methanol followed by supercritical CO2 washing. PTPEFLCz is obtained as orange-red solid, yield : 87 mg (73 %) 3. RESULTS AND DISCUSSION


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Electropolymerization and Kr Adsorption. 1,1,2,2-Tetra[4-(carbazol-9-yl)phenyl]ethene (TPETCz) is prepared in a procedure previously followed by our group,56 while the 2,7di(carbazol-9-yl)-fluoren-9-one (FLCz) monomer is synthesized in a condensation reaction of 2,7-dibromo-fluoren-9-one and carbazole under reflux conditions (see Supporting Information

Figure 1. (a) Cyclic voltammograms of monomers (0.1 mM) TPETCz (black), FLCz (red) and their 1:1 mixture (TPECz/FLCz) (blue) in dichloromethane/acetonitrile (4:1) containing 0.1 M TBAP, ITO electrodes, potential range 0-1.5 V. (b-d) First 20 consecutive CV cycles showing electropolymerization and growth of CMP thin films of (b) PTPETCz, (c) PFLCz and (d) PTPEFLCz, ITO electrodes, prepared from corresponding monomer (0.1mM) solutions containing 0.1 M TBAP.


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for details). Initially, the a electrochemical behavior of the monomers is studied using 0.1 mM solution of the corresponding monomer in CH2Cl2/CH3CN (4:1) mixture with 0.1 M TBAP as supporting electrolyte. Figure 1a shows the first CV cycle of TPETCz, FLCz and their 1:1 mixture (TPETCz/FLCz). TPETCz displays first reversible oxidation peak at around 0.9 V which can be attributed to the formation of stable radial cations at the TPE cores and this signal was also observed for TPETCz/FLCz. In addition, monomers TPETCz, FLCz and TPETCz/FLCz (1:1 mixture) exhibited irreversible oxidation peak at 1.1 V which can be assigned to the oxidation of carbazole moieties of the monomers. Figure 1b-d displays the cyclic voltammograms of electrooxidation and polymerization of TPETCz, FLCz and TPETCz/FLCz with an applied oxidation potential of 1.1 V up to 20 cycles to form CMP thin films PTPETCz, PFLCz and PTPEFLCz, respectively. For PTPETCz, it can be seen that starting from the second electrochemical cycle reversible peaks of charging and discharging are developed in the potential range of 0.5 to 0.8 V with gradual rise in current with increasing cycle number. This gradual rise in the current with increasing sweep cycle number reflects continuous growth of the polymer films at the electrode (ITO). In monomer-free solution the as-grown electrogenerated CMP films showed two reversible peaks for different scan rates from 0.005 to 0.2 Vs-1 as evidence for charging/discharging processes in the polymers through reversible radical cation/dication formation (Figure S1). Further, a linear relationship between peak current and scan rate is observed suggesting deposition of uniform, electroactive polymer deposits on the electrode. Similar polymer film growth characteristics are observed for FLCz and TPECz/FLCz (Figure S1). It can be noted that polymer growth is uniform throughout the ITO electrodes with a continuous coverage of the electrode surface. Formation of CMP thin films is evident from the FTIR spectra: New bands around 744 cm-1 correspond to the C-H bending vibration mode of N-


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substituted carbazoles that are present in polymer PTPETCz films; similar bands are also observed for PFLCz and PTPEFLCz. Additional bands around 802 cm-1 appeared for all polymers and represent C-H bending vibrations of 3- or 6- substituted carbazoles, thereby d

o

g

o

o o ‘C-C’

g

b w

b zo

o

(Figure S2).

Figure 2a & S3 shows the AFM images of three different CMP films grown by applying 5 cyclovoltammetric cycles. The thicknesses are calculated from the topography images to be 15, 38 and 19 nm for PTPETCz, PFLCz and PTPEFLCz with an average roughness of 3.7, 7.5 and 4.5 nm, respectively. The low roughness values imply that the thin films are best suited for device fabrication. Free-standing polymer thin films necessary for porosity measurements are prepared using the chronoamperometry method by applying a constant oxidation potential of 1.1 V for 20 min followed by treatment at 0 V for 60 s to discharge the deposited films (Figure 2b, 2c). These free-standing CMP films were room temperature-dried and used for Kr adsorption measurements. Prior to this, the thermal stability of the thin films is studied (all films have been dried by heating under vacuum at 140 °C). As shown in Figure S4-S6, initial weight loss (

Nanometer-Thick Conjugated Microporous Polymer Films for Selective

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