Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Intrinsically Porous Dual-Responsive Polydiacetylenes Based on Tetrahedral Diacetylenes Woomin Jeong,† Mohammed Iqbal Khazi,‡ Dong Geol Lee,† and Jong-Man Kim*,†,‡ †
Department of Chemical Engineering and ‡Institute of Nano Science and Technology, Hanyang University, Seoul 04763, Korea
Macromolecules Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/13/18. For personal use only.
S Supporting Information *
ABSTRACT: The combinatorial functionalization in a single molecular framework by structural integration utilizing multiple functional materials to create predefined structural morphology and multistimuli-responsive smart materials has attracted intensive attention. Herein, we constructed intrinsically porous and dualresponsive supramolecule, TeDA, by introducing a photopolymerizable diacetylene template (10,12-pentacosadiynoic acid) to the sterically rigid tetrahedral tetraphenylmethane (TPM) core. The self-assembled monomeric TeDA is transformed into the covalently cross-linked blue-phase polydiacetylene (TePDA) by UV irradiation (UV 254 nm). The BET measurement and examination of SEM images confirm the mesoporous characteristic for TeDA/PDA. Very interestingly, the blue-phase TePDA produces a naked-eye detectable colorimetric response to heat and VOCs (liquid and vapor phase). Most importantly, TePDA exhibits reversible thermochromism and excellent colorimetric response to chloroform vapors. To signify the structural influence of TPM on material properties, we also studied non-TPM derivatives. The TeDA/PDA integrated system demonstrates potential applications in developing multistimuli-responsive sensors.
1. INTRODUCTION The recent developments of multistimuli-responsive and multifunctional smart materials emphasize the design diversity through integrated materials systems that utilize the structural and functional features of multiple materials bounded to perform multitasking operations.1−5 Especially, organic molecules with a self-assembling characteristic involve cooperative noncovalent interactions as a stitching force to arrange multimolecules into functional supramolecular architectures. Polydiacetylenes (PDAs)6−13 are among the supramolecules that can be integrated via UV-promoted polymerization of selfassembled diacetylene (DA) monomers. Owing to the hierarchical conjugated architecture of the PDAs, a strong interchain interaction restricts the rotation of the side chain and therefore allows extensive π-orbital overlapping in the ene−yne backbone. As a result, PDAs absorb in the visible region and typically exhibit a blue color. Very importantly, the PDAs generate an assembly-related colorimetric response typically blue to redtoward external stimuli, and this unique property of PDAs has been extensively applied for various sensing applications.14−26 On the other hand, PDA offers intriguing morphological characteristics depending on molecular design and self-assembling behavior. In particular, these morphologies include films/sheets,27,28 vesicles,29−31 nanowires/nanofibers,32−34 tubes,35,36 crystals,37 and so forth. Over the past few decades, porous organic polymers have emerged significantly.38−41 However, porous organic polymeric materials possess similarities with those of other classes of porous materials, but organic polymers exist with very different structural features, ranging from highly cross-linked networks © XXXX American Chemical Society
to linear contorted polymers. Generally, instilling porosity into the materials enhances the material functioning performance in terms of selectivity and sensitivity. In this regard, an enormous amount of progress has been achieved with porous materials across the multidisciplinary applications such as gas capture, storage, and separation,42−44 chemical/biochemical sensors,45−48 catalysts,49,50 drug delivery systems,51,52 and energy storage.53,54 Interestingly, the ability of DAs to preorganize into well-ordered assemblies and UV-induced extended conjugated network of PDAs in combination with appropriate molecular substitution offers porous morphological features. Das and co-workers demonstrated CO2 gas sorption behavior by using a porous covalent polymer generated from dipeptideended diphenylbutadiyne precursors.55,56 By taking advantage of their colorimetric transition feature, Jelinek and co-workers developed volatile organic compound sensors by constructing a porous silica aerogel-embedded PDA matrix.57 Using PDA seems to be ideal for fabricating porous polymeric materials. However, the most widely studied linear PDAs possess limitations; origination of intrinsically porous PDA from monomer states is very rare. The self-assembly of DAs, which generate supramolecular architecture, performs functions as expected, but its conformational flexibility collapses the existing voids or circumvents empty space. Thus, the sole use of linear DA/PDA is insufficient for the generation of porous polymer and suggests adequate structural features must Received: October 25, 2018 Revised: December 3, 2018
A
DOI: 10.1021/acs.macromol.8b02294 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Illustration of porous polymer formation from self-assembled tetrahedral diacetylene. (a) Schematic representation of TeDA. (b) Structure of the tetrahedral diacetylene. (c) Self-assembly of monomeric TeDA. (d) Formation of porous structure derived from the arrangement of monomer units. (e) Topochemically polymerized TePDA and its structure.
be held by the DA monomers such as rigidity, steric demand, and contorted architecture at the molecular level to prevent tight inter-/intramolecular packing. Tetraphenylmethane (TPM) is a possible option to interfere with tight packing and create a free volume within the molecular arrangement. The tetrahedral TPM can generate highly diverse networks by imparting the functional groups at the terminal as desired. The inherent rigidity and steric demand of the tetrahedral structure effectively preclude the tight packing of terminal amphiphiles and create free volume seen as porous frameworks.58−61 In this context, to validate the combined structural features to generate intrinsically porous multistimuli-responsive supramolecular architecture, a novel tetrahedral DA derivative was designed to assess the possibility of void space formation via introducing DAs to the TPM core. The tetrahedral TPM core while maintaining a three-dimensional (3D) rigid structure was perfectly suited to generate intrinsically porous supramolecular DA architectures.62 In the investigation described herein, we synthesized the TPM-appended DA derivative (TeDA) and studied the porous characteristic, photo-cross-linking behavior, and dual-stimuliresponsive colorimetric detection. Also, we synthesized nontetraphenylmethane (triphenylmethane, diphenylmethane, and
phenylmethane) centered DAs to compare properties with tetrahedral TeDA before and after UV irradiation. The gas adsorption and morphological studies demonstrate the mesoporous characteristic exclusively for TeDA. The selfassembled TeDA monomer undergoes UV-induced topochemical polymerization, leading to blue-phase PDA, TePDA. The blue-phase TePDA generated a colorimetric sensory response to heat and organic solvents/volatile organic compounds (VOCs). Importantly, TePDA displays reversible thermochromism with the brilliant blue−red−blue color transition. Also, we confirmed the potential for VOCs sensor by performing a simple paper-based VOCs detection test. The propensity for topochemical polymerization and colorimetric responses was observed by spectral analysis.
2. RESULTS AND DISCUSSION Design and Synthesis of Diacetylene Monomers. The strategy for the creation of intrinsically porous supramolecular architecture was achieved by binding TPM with photopolymerizable DA assemblies. As seen in Figure 1, the centrally located TPM molecule linked to DA via an amide linkage. The self-assembling characteristics of DAs which arise from a π−π stacking and complementary hydrogen bonding provide a B
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phenylmethane (MoDA). For the synthesis of TrDA and DiDA, the commercially available triphenylmethane and diphenylmethane were subjected to Friedel−Crafts acylation and then converted to carboxylic acid derivatives using NaOBr. Subsequently, the carboxylic acid derivative coupled with PCDA-amine 3, using the T3P reagent. The monophenyl DA derivative, MoDA, was prepared in a one-step reaction by coupling p-toluic acid with PCDA-amine, 3 (see the Supporting Information for the scheme and experimental details). The structures of all synthesized compounds were confirmed by 1H NMR, 13C NMR, HRMS/MALDI-TOF, and IR analysis (see Figures S1−S12). Gas Adsorption Studies of Mesoporous Diacetylenes. The intrinsic porosity of TeDA was demonstrated by the N2 adsorption/desorption study. Also, the N2 uptake study of TrDA, DiDA, and MoDA was performed to compare whether the porosity selectively appeared only in TeDA because of its unique tetrahedral structure. The specific surface area calculated using the Brunauer−Emmett−Teller (BET) method for monomer assemblies of TeDA, TrDA, DiDA, and MoDA shows 6.469, 6.022, 8.989, and 9.503 m2 g−1, respectively. The BET results show the surface area relatively in a similar range for all four synthesized DAs; however, interesting results came from the N2 uptake study. For TeDA, the N2 uptake calculated at 78 K reached 30.098 cm3 g−1 at P/P0 = 0.99 and displayed type IV isotherm with H3 hysteresis loop (Figure 3, blue circle). On the other hand, non-tetraphenylmethane DAs showed negligible N2 uptake of 1.113, 1.509, and 0.879 cm3 g−1 until the pressure increased to P/P0 = 0.99 (Figure 3, red triangle, green square, and orange square, respectively). The total pore volume calculated from the amount of absorbed N2 at P/P0 = 0.99 for TeDA was found to be 0.0330 cm3 g−1, which is slightly larger or twice larger than for the other three non-tetraphenylmethane DAs (0.0161, 0.0322, and 0.0178 cm3 g−1, respectively). Isotherm type and pore size of 20.417 nm unambiguously confirmed the mesoporous characteristic for TeDA. Interestingly, upon UV irradiation (254 nm, 2 mW/ cm2, 5 h) the polymeric TePDA retains the mesoporous nature and similar porous area (5.5927 m2 g−1) originating from monomeric TeDA and possesses an additional feature of the robust covalently linked network (Figure S13). To access further details on porosity, the morphological analysis of TeDA was performed using scanning electron microscopy (SEM), as seen in Figure 4. The self-assembled monomeric structures were fabricated by employing the antisolvent precipitation technique by the addition of methanol to the dichloromethane solution of synthesized DAs. Examining the SEM image shown in Figure 4a (see also Figure S14) revealed amorphous morphology having a porous external structure for TeDA with an average pore size of about 20 nm, which is in perfect agreement with the pore size measured by BET analysis. On the other hand, the nontetraphenylmethane DAs have completely different morphology from TeDA. As can be seen in Figure 4b−d, rod- or platelike nonporous structures with smooth external surfaces appear for TrDA, DiDA, and MoDA. In addition, UVirradiated polymeric structures showed morphology similar to the monomeric state (Figure S15). Thus, the SEM analysis clearly suggests the structural impact of tetrahedral TPM on morphology. This can be attributed to the steric hindrance and rigidity induce predefine generation of void spaces in TeDA. Topochemical Cross-Linking. The self-assembled DA derivatives TeDA, TrDA, DiDA, and MoDA were integrated
polymerizable molecular geometry to generate blue-phase PDA, thus creating an opportunity to utilize colorimetric sensing applications. On the other hand, the TPM with tetrahedral symmetry and structural rigidity combined with precisely aligned terminal DAs allowed obtaining a TPM-based 3D star-shaped supramolecular network with predefined void spaces that could be used for gas adsorption. A schematic of generating a porous polymer through TeDA molecular design is shown in Figure 1. First, the tetrahedral TeDA monomers align into a star-shaped spatial arrangement. Next, the strong self-assembling feature of DAs enforces the 3D directional selforganization of TeDA to form a highly ordered porous nanoarchitecture. Eventually, monomers were covalently crosslinked with ene−yne backbone upon UV exposure, resulting in a robust and rigid porous polymer. As a proof of the structural significance of TeDA toward its viability for porous character arising from structural rigidity and a steric factor of TPM, we also synthesized three TeDA analogues by controlling the number of central phenyl rings, as shown in Figure 2.
Figure 2. Structures of diacetylene monomers with four different centers.
The synthetic strategy of TeDA is depicted in Scheme 1. Synthesis begins with the preparation of the amine-functionalized DA template from its carboxylic acid precursor, 10,12pentacosadiynoic acid (PCDA), and then coupled to terminal carboxylic acid-modified TPM. A simple two-step synthetic protocol was employed for preparing amine-functionalized PCDA (3). In the first step, the Curtius rearrangement of PCDA using DPPA followed by quenching with tert-butanol affords the Boc-protected amine derivative 2. Next, the acidhydrolyzed deprotection of 2 using TFA gave the free amine derivative 3. Finally, PCDA-amine 3 coupled with methanetetrayltetrabenzoic acid (4) using propylphosphonic anhydride (T3P) reagent to produce the desired tetrahedral TeDA (see the Supporting Information for details). Next, the TeDA analogues were synthesized by replacing the central tetrahedral TPM with the nontetrahedral core: triphenylmethane (TrDA), diphenylmethane (DiDA), and C
DOI: 10.1021/acs.macromol.8b02294 Macromolecules XXXX, XXX, XXX−XXX
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involves the topochemical polymerization mechanism, and the polymerization rate largely relies on the UV-exposed areas. The formation of PDAs was studied by using reflection− absorption spectroscopy and Raman spectroscopy. UV−vis spectra recorded for TePDA, TrPDA, DiPDA, and MoPDA showed absorption maxima in the visible region at 617, 525, 472, and 454 nm, respectively, which clearly ascertain the formation of the extended ene−yne conjugation of PDAs (Figure 5). For TeDA, TrDA, and DiDA, the absorption maximum has a distinctive intensity, and PDA formation is visually noticeable with color changes to bluish-purple, purplered, and yellow-green, respectively. Meanwhile, the absorption maxima for MoDA were observed apparently with a lower intensity and pale yellow color for PDA, which could be because of the low polymerization rate. Photographs of color transition for PDA formation are displayed as an inset in Figure 5. Further evidence for the PDA formation was confirmed by using Raman spectroscopy (Figure 6; see Figure S16 for detailed time-dependent polymerization). Figure 6a
Figure 3. N2 adsorption and desorption isotherms of synthesized diacetylene monomers at 78 K.
into corresponding PDAs by means of the cross-linking UVpolymerization process (UV 254 nm, 2 mW/cm2, 5 h). Upon exposure to UV light, the white color DA solids turn to a colored state as a result of polymerization. The PDA formation
Figure 4. SEM images of diacetylene monomers (a−d). SEM images of TeDA (a), TrDA (b), DiDA (c), and MoDA (d) obtained by precipitation with a dichloromethane/methanol mixture. D
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Figure 5. Absorption spectra of (a) TeDA/TePDA, (b) TrDA/TrPDA, (c) DiDA/DiPDA, and (d) MoDA/MoPDA before (gray line) and after (colored line) UV irradiation. The inset in each figure indicates a color change of each DA derivative upon UV irradiation.
Figure 6. Topochemical polymerization of synthesized diacetylene monomers. (a−d) Raman spectra of TeDA/TePDA (a), TrDA/TrPDA (b), DiDA/DiPDA (c), and MoDA/MoPDA (d) before (black line) and after (colored line) UV irradiation.
of PDA. A similar pattern was observed for TrDA and DiDA in Raman spectra after UV exposure. The yne−yne band at 2261 cm−1 (CC) and 2262 cm−1 (CC) corresponding to the monomer state disappeared, while two new bands appeared for conjugated ene−yne bands at 1456 cm−1 (CC), 2096 cm−1
shows the Raman spectrum of TePDA; the appearance of two bands associated with conjugated ene−yne of PDA at 2079 cm−1 (CC) and 1457 cm−1 (CC) and the disappearance of the yne−yne band at 2263 cm−1 (CC) corresponding to the monomer state serve as strong evidence for the formation E
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Figure 7. Thermochromic property of TePDA. (a) Thermochromic behavior of polymerized TePDA upon gradual heating from 30 to 220 °C. (b) Absorption spectra of polymerized TePDA recorded at phase transition temperature. (c) Raman spectra of polymerized TePDA before and after heating. (d) Absorbance spectra of polymerized TePDA showing reversible thermochromic properties upon heating and cooling cycle. (e) Plot of absorbance of a TePDA measured during 10 consecutive heating−cooling cycles.
(CC) and 1459 cm−1 (CC), 2075 cm−1 (CC) for TrPDA and DiPDA, respectively (Figure 6b,c). However, topochemical polymerization of the MoDA showed a lower conversion rate. As can be seen in Figure 6d, the significant yne−yne band remained at 2262 cm−1 (CC) for unreacted monomer (which is >50 approximately) along with new ene− yne bands appearing at 1456 cm−1 (CC), 2094 cm−1. It is important to note that TeDA appears unlikely to be polymerized because of the nonplanar and bulky tetrahedral structure, but in fact, TeDA underwent a facile polymerization. As illustrated in Figure 1, the four DA amphiphiles linked in a tetrahedral fashion self-assemble into a 3D architecture where each terminal DA was held together in separate hierarchical stacks by hydrogen-bonding interactions, thus circumventing the intramolecular structural interferences and ensuring the feasibility for topochemical polymerization. The proposed packing model was attempted to confirm by the XRD pattern (Figure S17). However, we were not able to obtain definitive information about the packing structure from the XRD pattern. The facile polymerization and hydrogen-bonding interaction between the amides which was identified by the NMR study (Figure S18) as well as the reported X-ray crystallographic
data59 with tetrapodal compounds support the packing model shown in Figure 1. GPC analysis of polymerized TePDA, TrPDA, and DiPDA was performed to determine the molecular weight of the resulting polymer. However, the molecular weight could not be obtained due to the precipitation of the polymer in the column during analysis and the difficulty in separating a large portion of unreacted monomer. Instead, the monomer conversion was determined by measuring the weight of the monomer before UV exposure and the weight of the unreacted monomer removed after UV exposure (see the Experimental Section for details). Also, our previous studies have shown that PDAs with blue color have a molecular weight in the range 50000− 200000.35,63 On the basis of this, it is believed that the generation of polymerized blue-phase PDA of TeDA effectively occurs by UV exposure, and the molecular weight lies in the range between 50000 and 200000. For TrDA and DiDA, no blue-phase transition occurred upon polymerization; however, absorption wavelength appears in the blue-color region which is indicative of the PDA formation. Colorimetric Responses. The color visualization character of PDAs and particularly their distinct color transition induced upon external stimuli have made them a possible choice for F
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for TrPDA and DiPDA occurred at 130 and 150 °C, respectively (Figure S21). However, unlike TePDA, the TrPDA and DiPDA display a low-contrast colorimetric signal response from initial purple-red to red and yellow-green to red, respectively (Figure S21). The TrPDA and DiPDA also display thermal colorimetric reversibility. The TrPDA shows thermal reversible property between 30 and 130 °C, but the color regenerated after the heating-to-cooling cycle varies from the original purple-red color. Likewise, the DiPDA displays reversible thermochromism up to 150 °C, and the original color could be completely recovered when cooled to room temperature (Figure S22). However, compared with the TrPDA and DiPDA, TePDA possesses better thermochromic property in terms of thermal reversibility and the colorimetric contrast between 30 and 100 °C for naked-eye detection. We believed that the collective noncovalent interactions within TeDA/TePDA molecule play a significant role in governing the excellent thermochromic reversibility. These interactions are arising from the internal hydrogen-bondable amide functionality and π−π stacking feature of aromatic rings of the tetrahedral core. The strong intermolecular interaction of the aromatic group in tetrahedral structure retains the rigidity and has a tendency to return to the initial structural arrangement even if distortion occurs in the diacetylene chain upon heating at a higher temperature. In addition, in a similar fashion the hydrogen bond interactions induce reversible conformation transformation in PDA chains (Figure S18).64 Significantly, the TePDA presents potential applications in the field of the thermal sensor. Considering the solubility and brilliant color visibility, TePDA and TrPDA were investigated for solvatochromic behavior. The TePDA and TrPDA solids were tested for colorimetric response by immersing in 14 organic solvents: chloroform, dichloromethane, tetrahydrofuran, benzene, toluene, xylene (a mixture of isomers), hexane, ether, methanol, ethanol, isopropyl alcohol, acetonitrile, acetone, and ethyl acetate. The TePDA display an instant bluish-purple to yellow color transition against chloroform, dichloromethane, tetrahydrofuran, benzene, toluene, and xylene (Figure S23a). The sensing behavior of TePDA was monitored by the UV−vis spectral studies, which show a dramatic blue-shift with a new band arising at 456−465 nm (Figure S23b). In contrast, TrPDA displays low colorimetric transition contrast for chloroform, dichloromethane, tetrahydrofuran, benzene, toluene, and xylene (Figure S24). It is well-known that solvatochromism is mainly influenced by the solubility of the free monomers in the polymer matrix. 35 Accordingly, solvatochromism for TePDA and TrPDA arises from solubility and availability of free monomers in PDA matrix. An immediate obvious and direct blue to yellow color transition for TePDA presumably occurred because of significant structural disintegration as a consequence of free volume created in PDA architecture upon release of the unreacted bulky monomer to the surrounding solvent medium. VOCs Detection in Vapor Phase. After confirming the excellent solvatochromic response of TePDA upon exposure to VOCs in the liquid phase, we further explored the colorimetric sensory response of TePDA against VOCs in the vapor phase. The blue-phase TePDA-coated paper-based sensor was fabricated and tested for colorimetric detection of VOCs in a closed container saturated with vapors of various solvents (Figure S25a). The colorimetric sensing response generated by TePDA toward VOCs is presented as a red chromaticity shift
colorimetric sensing applications.14−26 Because the chromogenic PDAs for TeDA, TrDA, and DiDA can be readily prepared by UV irradiation, subsequently, the colorimetric response toward heat and the organic solvent was investigated. We first began studying thermochromism of polymeric PDAs in the solid phase by heating gradually from 30 to 220 °C on a hot plate, which led to a naked-eye detectable periodic color change as a function of temperature. As can be seen in Figure 7a, the bluish-purple TePDA at 30 °C transformed into sharp red phase at 100 °C and then reaches to the yellow phase when heated continuously further to 220 °C. Real-time spectroscopic monitoring of color change undergone by the PDA at a temperature of 30 °C (bluish-purple), 100 °C (red), and 220 °C (yellow) was performed using reflection−absorption spectroscopy. As seen in Figure 7b, this multicolor thermochromic transitions of blue−red−yellow accompanied by a distinct shift in the absorption peak. The absorption maximum at 617 nm (30 °C) for the bluish-purple phase of TePDA shifts to lower wavelength at 525 nm (100 °C) in the red phase and shifts further to 470 nm (220 °C) for the yellow phase. In addition, the thermochromic blue-to-red phase transition that occurred during 30−100 °C was further elucidated by the Raman spectroscopic method. As shown in Figure 7c, the ene−yne bands in the bluish-purple phase appearing at 1456 cm−1 (CC), 2076 cm−1 (CC) shifted to 1517 cm−1 (CC), 2123 cm−1 (CC) in the red phase. These Raman shifts clearly suggest heat-induced conformational distortion in PDA backbone, leading to an interesting naked-eye detectable color transition. Very interestingly, the polymerized TePDA displayed a reversible thermochromism during the repeated thermal cycle at 30 ↔ 100 °C. As seen in Figure 7d, the absorption maxima at 617 nm for the initial blue phase (30 °C) shifted to 525 nm upon heating to the red phase (100 °C). Upon cooling, the TePDA solid completely recovered to the blue color, and the absorption spectrum changed inversely which perfectly overlapped with that of the initial blue phase spectrum. The phenomenon of reversible thermochromic transitions was also explored using Raman spectroscopy. Raman spectra of TePDA recorded during consecutive blue−red−blue phase at the corresponding temperature of 30−100−30 °C show complete reversibility on thermal cycling (Figure S19). The thermal reversibility of TePDA was further demonstrated by performing repeated heating−cooling cycles between 30 and 100 °C. Reflective UV−vis spectroscopy was used to test the reversibility and repeatability. As seen in Figure 7e, even after consecutive cycles of heating−cooling no obvious loss in absorption intensity was observed, which confirms the good reversible thermochromic ability of TePDA. The thermochromic phase transition of the TePDA was identified by performing DSC measurement for monomer precursor (TeDA). In DSC curves (Figure S20), endothermic values for TeDA are observed at 88.9 and 133.7 °C. The peak that appeared at 88.9 °C can be attributed to the solid−solid phase transition in TeDA assemblies, which promotes colorimetric transition and thermoreversibility for TePDA. Meanwhile, the TePDA appears to lose its reversible thermochromic behavior when the temperature exceeds 133.7 °C, causing significant structural disintegration. Next, TrPDA and DiPDA were tested for thermochromic behavior. In a similar manner to TePDA, the TrPDA and DiPDA display temperature-dependent chromatic change over the temperature range of 30−220 °C. The red phase transition G
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response is higher than that of PCDA and visible enough for naked-eye detection. The morphological influence on colorimetric sensitivity is evidenced by SEM analysis, which shows smooth lamellar structure for polymeric PCDA (Figure S26).
(RCS) by calculating RGB values. The TePDA-coated paper strips were exposed to vapors of chloroform, dichloromethane, tetrahydrofuran, benzene, toluene, and xylene in different concentrations ranging from 0.05% to 1.0% v/v for a duration of 1 h and subsequently dried. The RGB values were obtained by scanning image and minimizing noise by normalization. A maximum colorimetric response was observed for chloroform vapors among tested VOCs (Figure S25b). This could possibly due to the highest solubility of TePDA in chloroform and the high vapor pressure of chloroform. The colorimetric responsivity of TePDA sensor for the detection of chloroform vapors was compared with the PCDA sensor model to investigate the structural influence on sensing behavior. As seen in Figure 8a, TePDA shows a strong
3. CONCLUSION In summary, we have developed an intrinsically porous dualstimuli responsive TPM-DA/PDA conjugate by structural integration and additive functionalization originating from TPM and DA/PDA. The self-assembling characteristic of the diacetylene template created a predefined hierarchical molecular arrangement; meanwhile, the inherent sterically rigid TPM molecule instilled free volumes within supramolecular structure, leading to mesoporous morphology. Eventually, UV irradiation of the self-assembled TeDA resulted in the covalently cross-linked blue-phase polymeric network. The formation of polymeric TePDA was confirmed by UV−vis and Raman spectral analysis. The BET surface area analysis and pore size measurement confirm the mesoporous characteristic for TeDA/PDA. Examination of SEM micrographs also suggests the existence of mesoporous morphology. The bluephase TePDA displays a naked-eye detectable brilliant colorimetric response against heat and VOCs (liquid and vapor phase). The phase transition temperature was identified by performing DSC studies. TePDA shows good thermal stability and exhibits excellent reversible thermochromic properties up to 100 °C. The thermochromic reversibility and repeatability of TePDA were spectroscopically demonstrated using UV absorption and Raman spectral analysis. The existence of intermolecular hydrogen bonding responsible for reversible thermochromic behavior was confirmed by a 1H NMR study. A simple paper-based TePDA sensor was fabricated for vapor phase VOCs sensing. An interesting sensory response was observed for chloroform vapors. Moreover, TePDA shows a substantially better colorimetric response to chloroform vapors than the PCDA-based sensor. Our results confirmed that porosity of TePDA offers maximum vapor penetration, causing a distinct colorimetric response. Meanwhile, to show the structural impact of TPM, we also synthesized non-TPM derivatives by replacing the central TPM core with triphenylmethane (TrDA), diphenylmethane (DiDA), and phenylmethane (MoDA) and systematically comparing with results of TeDA/PDA. However, TeDA/ PDA shows a convincingly better property in terms of selfassembly behavior, topochemical polymerization rate, color visualization, and thermochromic as well as solvatochromic response. Significantly, the integrated TPM-DA/PDA presents a promising molecular design for constructing predefined morphology and multistimuli-responsive sensing materials. Nonetheless, the strategy induced here is not limited to the current system. It is possible to improve or optimize the morphology and functionalities using other material combinations.
Figure 8. Vapor phase colorimetric detection: TePDA versus PCDA. (a) Scanned images after exposure to a various concentration of CHCl3 vapor. (b) RGB analysis and comparison between TePDA and PCDA sensor model sensor after 1 h.
colorimetric response with an eye-sensitive color change, whereas the PCDA sensor displays a poor and indistinct color transition. The colorimetric transition response is converted to the RCS values and quantitatively expressed. The RCS values calculated for TePDA found significantly higher than the PCDA. A plot of RCS values depicting colorimetric response for vapor concentration between 0.05% and 1.0% v/v is shown in Figure 8b. The pattern of responses produced shows a maximum RCS value upon increasing the concentration. The solubility of PCDA in chloroform is about 0.35 g/mL, which is ∼4 times higher than that of TePDA (0.06 g/mL), and considering the fact that solvatochromic behavior is dominantly influenced by monomer solubility, higher chromatic transitions should normally have occurred for the PCDA sensor. However, unlike the usual case, TePDA unexpectedly shows a higher RCS value than PCDA. In other words, given the same conditions except for the difference in molecular structure, it is believed that the penetration of VOCs vapor more effectively occurred for TePDA due to the mesoporous morphology. As a result, the sensor sensitivity is increased via an increase in the contact surface area, and the colorimetric
4. EXPERIMENTAL SECTION Materials. All chemicals were purchased from Sigma-Aldrich (Korea), Tokyo Chemical Industry (Korea), Alfa Aesar (Korea), and GFS Chemical (Powell, OH), were of highest commercial quality, and were used without further purification Instruments. 1H NMR and 13C NMR spectra were recorded on a Varian Unitylnova (300 MHz) spectrometer. Proton chemical shifts were reported in ppm (δ) relative to internal tetramethylsilane (TMS, H
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Macromolecules δ 0.0 ppm) or with the solvent reference relative to TMS (CDCl3, δ 7.26 ppm). Splitting patterns were indicated as s, singlet; d, doublet; t, triplet; q, quartet; and m, multiplet. Carbon chemical shifts were reported in ppm (δ) relative to TMS with the CDCl3 (δ 77.0 ppm) as the internal standard. High-resolution mass spectra (HRMS) were recorded on a SYNAPT G2 (water, U.K.) using a time-of-flight (TOF) analyzer and MALDI-TOF using AXIMA (SHIMADZU). IR spectra were recorded on a Thermal Nicolet Nexus 670 spectrometer. Thin-layer chromatography was performed on Merck silica gel plates (60 F254; Merck) using UV light as the visualizing agent and basic aqueous potassium permanganate as the developing agent. Merck silica gel (60−120 mesh size) was used for column chromatography. The nomenclature used for all compounds was assigned with help of ChemBio Draw Ultra 12.0 software. UV−vis absorption spectra were measured on a single beam Agilent 8453 spectrophotometer. All optical measurements were performed using a quartz cell. Absorption spectra were recorded on a USB2000 miniature fiber-optic spectrometer (Ocean Optics). Raman spectra were recorded on a LabRAM HR Evolution Raman spectrometer (Horiba Scientific, 785 nm laser source). XRD spectra were recorded on a MiniFlex 600 (Rigaku, Cu Kα radiation). Evaluation of Porosity. Surface area, pore volume, and pore size before and after UV irradiation of the diacetylene derivatives were measured by a BET instrument (Micromeritics 3Flex surface characterization analyzer). Degassing of the diacetylene derivatives was performed for 12 h at 343 K to evaporate all traces of solvents and moisture followed by N2 adsorption−desorption in liquid nitrogen. Scanning electron microscopy (SEM) images of the diacetylene derivatives were obtained on a JEOL (JSM-6330 F) FE-SEM. Fabrication of Self-Assembled Structures. Self-assembled structures were fabricated using an antisolvent precipitation process. First, DA derivative (50 mg) was completely dissolved in 5 mL of dichloromethane, and then 25 mL of methanol was added slowly into the DA solution. Precipitation occurred immediately upon mixing and formed self-assembled DA aggregates with a white solid appearance. The solid was filtered and dried and analyzed by SEM. Monomer-to-Polymer Conversion Rate. TeDA was first irradiated by a hand-held UV lamp (254 nm, 2 mW/cm2, 5 h). After UV irradiation, unreacted monomer (43 mg) in the UV-exposed solid was removed by flowing hot ethyl acetate through a cotton-filled syringe filter. The residual red precipitate was triturated with hot chloroform, and the organic solvent was concentrated in vacuo to afford polymerized TePDA (20 mg, 32%). In contrast, the conversion rates for other three derivatives were not calculated either because of the lack of suitable solvents to specifically remove unreacted monomer (TrDA and DiDA) or the poor polymerization rate (MoDA). Colorimetric VOCs Detection. Fabrication of PDA-coated paper sensor began by dispersing 40 mg of TeDA in 2 mL of ethyl acetate. Eight microliters of the dispersed solution was dropped onto the filter paper of 1 × 1 cm2 size. The TeDA was then polymerized by exposure to UV light for 2 min using a hand-held UV lamp. The PDA-coated paper sensor was then affixed (using double-sided tape) inside a 62 mL closed glass container. A predetermined amount of the organic solvent was introduced into the container using a micropipet, and finally, the container was sealed and incubated at 30 °C for 1 h. All experiments were performed in triplicate. Similar experimental conditions and setup were employed for the TrDA and PCDA models. The colorimetric change was analyzed by measuring the color components before and after exposing the PDA-coated paper sensor against six different concentrations of VOCs (chloroform, dichloromethane, tetrahydrofuran, benzene, toluene, and xylene). The scanned image of the tested PDA-coated paper sensor was acquired by scanning with the HP Officejet Pro 8600 printer with a 600 dpi scanner. The scanned images (PNG format) were normalized in Adobe Photoshop (despeckle, dust and scratches, median filters) to minimize artifacts introduced by scanner hardware noise. From the scanned image to illustrate the relative red intensity represented by
each sample, the red chromaticity level (r) was calculated by extracting the RGB values.65,66 r=
R R+G+B
where R (red), G (green), and B (blue) are the primary color components. For quantitative color change comparisons, we employed the formulas red chromaticity shift (RCS) rsample − r0 RCS = × 100% rmax − r0 where rsample is the average red chromaticity level of the scanned image after exposure to VOCs vapor, r0 is the average red chromaticity level of the scanned image before exposure to VOCs vapor, and rmax is the maximum red chromaticity level when the PDA-coated paper sensor has undergone maximal color change by VOCs exposure. In principle, RCS describes the normalized color change of PDA sensor system to VOCs.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02294. Detailed information about synthesis scheme and experimental procedures; spectroscopic data (NMR, HRMS, and MALDI-TOF); BET surface area of TePDA; SEM images of DA and PDAs; time-dependent topochemical polymerization; XRD patterns of TeDA/ TePDA; hydrogen-bonding interaction of TeDA; DSC measurements of TeDA; thermochromism of TrPDA and DiPDA; solvatochromism of TePDA and TrPDA; VOCs sensing response of TePDA (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jong-Man Kim: 0000-0003-0812-2507 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This investigation was supported financially by the National Research Foundation of Korea (NRF) grant funded by a Korea government (MSIP) (NRF-2017R1A2A1A05000752).
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REFERENCES
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