Based Metallo-Supramolecular Polymer and Imidazole System

Apr 3, 2017 - One dimensional (1D) Pt(II)-based metallo-supramolecular polymer with carboxylic acids (polyPtC) was synthesized using a new asymmetrica...
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One-Dimensional Anhydrous Proton Conducting Channel Formation at High Temperature in a Pt(II)-Based Metallo-Supramolecular Polymer and Imidazole System Chanchal Chakraborty,†,‡ Utpal Rana,† Rakesh K. Pandey,† Satoshi Moriyama,‡ and Masayoshi Higuchi*,† †

Electronic Functional Macromolecules Group, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan International Center for Materials Nanoarchitectonics (MANA), NIMS, Tsukuba 305-0044, Japan

‡

S Supporting Information *

ABSTRACT: One dimensional (1D) Pt(II)-based metallo-supramolecular polymer with carboxylic acids (polyPtC) was synthesized using a new asymmetrical ditopic ligand with a pyridine moiety bearing two carboxylic acids. The carboxylic acids in the polymer successfully served as apohosts for imidazole loaded in the polymer interlayer scaffold to generate highly ordered 1D imidazole channels through the metallo-supramolecular polymer chains. The 1D structure of imidazole loaded polymer (polyPtC-Im) was analyzed in detail by thermogravimetric analysis, powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and ultraviolet−visible and photoluminescence spectroscopic measurements. PolyPtCIm exhibited proton conductivity of 1.5 × 10−5 S cm−1 at 120 °C under completely anhydrous conditions, which is 6 orders of magnitude higher than that of the pristine metallo-supramolecular polymer. KEYWORDS: metallo-supramolecular polymer, anhydrous proton conductivity, impedance, platinum(II), imidazole

1. INTRODUCTION The tempting application of fuel cells for alternate clean energy production leads the continuous investigation of new proton conducting materials to be used as the proton exchange membrane in fuel cells.1−4 Recently, some organic polyelectrolyte (i.e., Nafion, polyacids, and polyacrylamides) and crystalline porous materials such as metal−organic framework (MOFs), porous coordination polymers (PCPs), metallopolymers, and covalent organic frameworks (COFs) show possibilities to be potential candidates for high proton conduction.5−27 However, their application is limited in a moderate temperature range (80 °C) operating proton transporting material without dependency on high humidity, nonwater media molecules, including nonvolatile acids8,30 (H2SO4 or H3PO4) and heterocyclic organic molecules such as imidazole and it is analogues have been introduced into the pores of MOFs and COFs, which creates anhydrous proton carrier pathways.31−39 These nonvolatile organic heterocyclic molecules (e.g., imidazole) with high boiling points can exist in two tautomeric forms with respect to a proton that passes between the two nitrogen atoms to provide a proton-transport pathway.40 The protonic defect may create local disorder by forming protonated and unprotonated imidazoles. In such materials, proton transport can occur through structure diffusion that involves proton transfer between the imidazole and the imidazolium ion through the hydrogen bonded Received: October 12, 2016 Accepted: April 3, 2017 Published: April 3, 2017 A

DOI: 10.1021/acsami.6b12963 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces channel.31 To date, only PCPs, COFs, and MOFs are used for this purpose to entrap these diazole or triazole molecules inside the pore.31,32,41 The effect of strong interaction of imidazoletype molecules with coordination polymer or MOF, which can produce material with high proton carrier loading, is very rarely elucidated. However, the interaction between the organic diazole or triazole guest and PCP/MOF/COF host and creation of the 1D channel with the proton carrier is important to produce such types of anhydrous proton conducting pathways. Therefore, there is always research interest to prepare an anhydrous proton conductor with the above mentioned qualities. 1D metallo-supramolecular polymer with proper functionality may be the alternate option in this perspective due to its ease of synthesis and economic benefit. Herein, we introduce a 1D Pt(II) containing luminescent nonporous metallo-supramolecular polymer (polyPtC, Scheme 1) with carboxylic acid

2.2. Instrumentation. 1H NMR and 13C NMR spectra were recorded at 300 and 75 MHz, respectively, on a JEOL AL 300/BZ instrument using CDCl3 or DMSO-d6 as solvent. Chemical shifts were given relative to TMS. Mass spectra were recorded using a Shimadzu LCMS-IT-TOF spectrometer. MALDI mass spectra was recorded using a AXIMA-CFR, Shimadzu/Kratos TOF mass spectrometer. The molecular weight of polyPtC was determined by SEC-viscometryRALLS (size exclusion chromatography-viscometry-right angle laser light scattering) on a Viscotek 270 Dual Detector instrument using poly(ethylene oxide) PEO-22K as standard in methanol solvent (flow rate 1 mL/min). Flash column chromatographic separations were performed on silica gel 60 N (neutral, 40−100 μM), Kanto Chemical Co. Inc. Wide angle X-ray diffraction (XRD) was done using Rigaku Smart Lab 3 with Cu Kα radiation (λ = 1.54 Å), a generator voltage of 40 kV, and current of 40 mA. Polymer powder was placed on a glass holder and scanned in a range of 2θ = 5−50° at a scan rate of 1 s/step with a step width of 0.01° at room temperature. Thermogravimetric analysis (TGA) measurements were done using an SII TG/DTA 6200 instrument in a N2 environment with a 10 °C/min heating rate. Scanning electron microscopy (SEM) was done using a Hitachi S4800 instrument operating at 10 kV and 10 μA current. UV−vis spectra were recorded using a Shimadzu UV-2550 UV−visible spectrophotometer. Luminescence was measured using a Shimadzu RF-5300PC spectrofluorophotometer. In both cases, the solid films were used. The Fourier transform infrared (FT-IR) spectra were measured using a Shimadzu FTIR-8400S instrument over a KBr pellet. For proton conductivity measurements, polymer powder was pressed into the form of a pellet. The thicknesses of all the polymer pellets were 1.58 mm with 3 mm diameter. The pellet was then inserted in between the two electrodes of a sample holder. The sample holder used in the conductivity experiments was composed of two electrodes with an attached micrometer on upper electrode and another fixed electrode with a guard ring which reduces the effect of stray field lines existing at the edge of the sample. The guard ring ensures that the electric field lines remain parallel all over the sample. Besides this, the AC impedance results were also normalized using the empty cell measurements by carrying out the experiment in an air-gap created with the help of the attached micrometer. This procedure eradicates the errors due to connection, equipment, etc., thus making the measurement more trustworthy. A Solartron 1260 impedance gain/ phase analyzer coupled with Solartron 1296 dielectric interface was used for the temperature dependent AC impedance measurements with high sensitivity. A frequency range of 50 Hz to 10 MHz was used to determine the resistance of the film under complete anhydrous conditions. The measurement cell was filled with nitrogen at atmospheric pressure before recording the measurements. ZView software was used to extrapolate impedance results by means of an equivalent circuit simulation to complete the Nyquist plot and obtain the resistance values. 2.3. Synthesis of Ligand and Polymer. 2.3.1. Synthesis of Dimethyl-4-bromo-2,6-pyridinedicarboxylate (1). A mixture of chelidamic acid monohydrate (3.01 g, 14.96 mmol) and PBr5 (31.01 g, 72.03 mmol) was taken in a dry Schlenk tube equipped with a reflux condenser and heated to 120 °C under N2. The formed melt was then stirred under N2 at 100 °C for 3 h. The resultant purple melt was then cooled to room temperature and transferred to a round-bottom flask by washing with CHCl3. The solution was cooled to 0 °C, and dry methanol (50 mL) was slowly added to the solution, which was then stirred overnight and concentrated in vacuo to a slurry. The compound was recrystallized from methanol, filtered, and washed with ice cold methanol. The compound was dried to yield 1 as white needle-shaped crystals (3.07 g, 75%). 1 H NMR (300 MHz, CDCl3, rt) δ ppm: 8.33 (s, 2H, Ar−H), 4.04 (s, 6H, -OCH3), 13C NMR (75 MHz, CDCl3) δ: 164.13 (carbonyl), 149.42 (aromatic), 135.35 (aromatic), 128.31 (aromatic), 53.49 (methyl). MS (MALDI-TOF) m/z: [(M + H) +] calcd for C 9 H 8 79 BrNO 4 273.97, found 274.53; [(M + H) + ]calcd for C9H881BrNO4 275.97, found 276.16. 2.3.2. Synthesis of 4-(4-([2,2′:6′,2″-Terpyridin]-4′-yl)phenyl)pyridine-2,6-dicarboxylic acid (L). To a 20 mL DMSO solution of

Scheme 1. Schematic Diagram of Synthesis of L and Its Pt(II)-Based Metallo-Supramolecular Polymer PolyPtC with Pt2+ Metal Ions

functionality to provide strong interaction with imidazole as an apohost to prepare anhydrous proton conducting material polyPtC-Im that can operate at a high temperature (120 °C).

2. EXPERIMENTAL SECTION 2.1. Materials. Unless otherwise noted, all reagents were reagent grade and used without purification. Dehydrated methanol (MeOH), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), acetonitrile (MeCN), and acetone were used as reaction solvents. The spectroscopic grade acetonitrile and methanol were used for making polymer solutions. These solvents were purchased from either Aldrich, Wako, or Kanto Chemical Co. Inc. and used as received. Deionized H2O was used in the experiment where required. Potassium pentabromide (PBr5), bis(triphenylphosphine)palladium(II) dichloride, potassium hydroxide, potassium carbonate, dichloro(1,5cyclooctadiene)platinum(II) (Pd(COD)Cl2), AgBF4 (≥99.99%), and imidazole were purchased from Sigma-Aldrich Co., Ltd. and used as received. The polyPtL1 polymer was synthesized earlier in our previous report.42 4′-(4-Pinacolatoboronphenyl)-2,2′:6′,2″-terpyridine was synthesized according to a previous report.43 B

DOI: 10.1021/acsami.6b12963 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structure of polymer polyPtL1, (b) thermogravimetric curves of polyPtL1 and polyPtL1-Im, (c) fluorogenic response of polyPtL1 and polyPtL1-Im under a 365 nm UV lamp, and (d) solid state luminescence study of polyPtL1 and polyPtL1-Im excited at 450 nm. 4′-(4-pinacolatoboronphenyl)-2,2′:6′,2″-terpyridine (0.65 g, 1.50 mmol) were added 1 (0.41 g, 1.50 mmol), potassium carbonate (0.44 g, 3.20 mmol), and PdCl2(PPh3)2 (0.11 g, 0.15 mmol) under N2 atmosphere. The solution mixture was stirred at 120 °C under N2 for 24 h. The resulting mixture was cooled to room temperature; the catalyst was removed by filtration and washed with CHCl3. The filtrate was washed with water several times, and the organic layer was dried over sodium sulfate. The solution was filtered, dried on vacuum, and purified by column chromatography on alumina using CHCl3 as eluent. The compound (0.31 g, 0.62 mmol) was then taken into a 200 mL THF-H2O mixture (1:1), and potassium hydroxide was added (0.35 g, 6.20 mmol). The mixture was refluxed for 12 h. The resulting mixture was cooled to room temperature and neutralized with 1 M aqueous HCl. THF was removed under reduced pressure. The precipitation was filtered, washed with water, THF, and diethyl ether, and dried in vacuo to give ligand L as yellowish-white solid (0.53 g, yield 74%). 1 H NMR (300 MHz, DMSO-d6, rt) δ ppm: 8.94 (m, 2H), 8.90 (s, 2H), 8.88 (m, 2H), 8.55 (s, 2H), 8.31 (t, 2H, J = 7.2 Hz), 8.22 (d, 2H, J = 8.4 Hz), 8.17 (d, 2H, J = 8.4 Hz). 7.76 (t, 2H, J = 5.7 Hz). 13C NMR (75 MHz, CDCl3) δ: 165.7 (carboxyl), 154.8, 153.7, 149.3, 149.2, 148.9, 148.5, 139.1, 138.6, 136.8, 128.3, 128.1, 125.3, 124.6, 121.9, 118.7. HRMS (ESI-TOF) m/z: [(M + H)+] calcd for C28H18N4O4 475.1408, found 475.1179. 2.3.3. Synthesis of Pt(II) Containing Polymer polyPtC. Dichloro1,5-cyclooctadieneplatinum-(II) (0.19 g, 0.50 mmol) was treated with a solution of silver tetrafluoroborate (0.21 g, 1.10 mmol) in acetone/ acetonitrile (5 mL, 4:1). The mixture was centrifuged to remove precipitated silver chloride, and the supernatant solution was added to a solution (methanol:water, 1:2) of ligand L (0.24 g, 0.50 mmol) at 60 °C and stirred for 15 min in the dark. Then, the mixture was refluxed at an elevated temperature overnight in the dark. The yellowish precipitation was filtered and washed with chloroform three times (50 mL × 3). The residue was dried to obtain polymer polyPtC as a yellow solid (0.54 g, yield 80%). 1 H NMR (300 MHz, DMSO-d6, rt) δ ppm: 9.08 (s), 8.98 (d), 8.91 (d), 8.59 (s), 8.56 (m), 8.41 (d), 8.26 (d), 8.00 (t). Molecular weight

measurement was done by the viscometry−right-angle laser light scattering (RALLS) method; Mw ≈ 2.5 × 104 Da. 2.3.4. Preparation of Imidazole Loaded Polymer polyPtC-Im. To prepare the imidazole loaded polymer polyPtC-Im, a polyPtC film over glass slide (10 mg polyPtC dispersion in 2 mL of methanol and spread over the 75 × 25 mm glass slide) was degassed by heating to 120 °C under reduced pressure for 12 h, and imidazole vapor at 120 °C was exposed over the polyPtC film for 4 h. The exposure time was optimized by thermogravimetric study of imidazole loaded material to reveal the highest imidazole loading on polyPtC. The imidazole loaded material was scratched from glass slide to yield polyPtC-Im. 1 H NMR (300 MHz, DMSO-d6, rt) δ: 9.15 (br, s), 9.03 (br, m), 8.81 (br, m), 8.61 (m), 8.57 (m), 8.44 (d), 8.31 (m), 8.19 (m), 7.85 (s, imidazole), 7.12 (s, imidazole). MS (MALDI-TOF) m/z: [(imidazole + H)+] calcd for C3H5N2 69.04, found 69.86; [L + Pt2+] calcd for C28H18N4O4Pt 334.54, found 334.11. [(L + 2Imidazole) + Pt2+] calcd for C34H26N8O4Pt 402.58, found 402.12.

3. RESULTS AND DISCUSSION To synthesize Pt(II)-based metallo-supramolecular polymer first, we synthesized 1 from chelidamic acid hydrate and then the asymmetric ligand L by Pd-catalyzed Suzuki−Miyaura cross-coupling of 4′-(4-pinacolatoboronphenyl)-2,2′:6′,2″-terpyridine with compound 1 followed by hydrolysis (Scheme 1). The detailed synthetic procedure and characterization are provided in the Experimental Section, and corresponding 1H NMR, 13C NMR, and mass spectra are given in the Supporting Information, Figures S1−S6. To prepare the Pt(II)-based metallo-supramolecular polymer, L was first treated with a solution of a Pt(II) salt prepared by sonicating and centrifuging a mixture of Pd(COD)Cl2 and AgBF4 in acetone−acetonitrile (4:1) at 60 °C, and then the mixture was refluxed for 10 h according to our previous report.42 The yellow precipitation was filtered and washed with chloroform and dried to provide polyPtC as yellow solid with 82% yield. Details of the characterizations of the polymer are provided in the Supporting C

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Figure 2. (a) Thermogravimetric curves for polyPtC and polyPtC-Im. (b) XRPD patterns of bulk imidazole, PolyPtC, and polyPtC-Im. The peaks for Pt···Pt interaction are assigned by “*” in polyPtC and PolyPtC-Im.

the imidazole did not react with the metal center of the polymer chain but interacted only with carboxylic acid groups of polyPtC to form the imidazolium cations. In the TGA study depicted in Figure 2a (also the derivative thermogravimetric (DTG) plot in Figure S13 in the Supporting Information), the polymer polyPtC exhibits one step degradation in the range of 100−370 °C. The thermogravimetric profile of polyPtC-Im shows 22% extra weight loss at the measuring temperature range due to 22% weight imidazole loading, which is calculated to be 3 imidazoles per monomer unit. The release of accommodated imidazole initiates at 100 °C and completes at ∼270 °C. The thermogravimetric curve demonstrates that the loss of imidazole molecules in polyPtC-Im befalls in two steps: in the first step, the release of imidazole starts at 100 °C and is completed around 200 °C, and in the second step, it commences at 200 °C and is completed by 270 °C. Calculation from TGA shows that the percentage of imidazole loss in the first step is 6.5% of total polyPtC-Im weight, followed by 15% of total amount in the second step. Therefore, we can assume there are two types of imidazole molecules installed in the channel in polyPtC-Im. The imidazoles with a strong interaction with the hydrophilic carboxylic acid to form imidazolium ions are released at a higher temperature (200− 270 °C), correlated to the weight loss in the second step, whereas the imidazoles that have less interaction with the polymer surface are removed in the first step (below 200 °C).31 The degradation of polyPtC-Im at above 270 °C is exclusively due to the polymer, which is clearly shown in the derivative plot in Figure S13. As three imidazole molecules are installed per monomer unit of metallo-supramolecular polymer, we can assume that two imidazole molecules interacted with the carboxylic group and another one is H-bonded with others to make the imidazole channel. These results prove that the polar −COOH functionality is eventually key factor to enhance the guest induced structural transformation with the aid of interaction between imidazole and −COOH groups in metallo-supramolecular polymer. The presence of two types of imidazole structures (imidazolium cations and pure imidazole) in polyPtC-Im is also directly proved by the MALDI-TOF mass spectroscopic study by taking the vapors of each step of degradation of loaded imidazole during the TGA study of polyPtC-Im. We collected the vapor evolved at 100− 200 and 200−270 °C ranges separately in methanol solvent and studied the MALDI-TOF results (Figure S14, Supporting Information). Interestingly, the materials in both of the degradation ranges revealed the same spectral pattern in MALDI-TOF mass spectroscopy with a prominent peak of (imidazole + H)+ at m/z ∼68.78 and 69.84, respectively. This experiment clearly confirms that the degradations are

Information, Figure S7−S8. Significant downfield shifting in the proton signals of the terpyridine and pyridine moieties in the polymer in 1H NMR in Figure S9 in the Supporting Information clearly indicates the polymerization through complexation with Pt(II). We also measured the molecular weight (Mw) of polyPtC by the RALLS method to be 2.5 × 104 Da (Figure S8, Supporting Information), which also confirms the formation of metallo-supramolecular polymer by complexing terpyridine and pyridine moieties with Pt(II) ions. The imidazole loaded polyPtC-Im was prepared by the procedure described in theExperimental Section. To evaluate how crucial the effect is of carboxylic acid functionality on the metallo-supramolecular polymer, we exposed the imidazole vapor on the same type of Pt-containing metallo-supramolecular polymer (polyPtL1, Figure 1a) without any carboxylic acid functionality, which was synthesized earlier in our previous report.42 The thermogravimetric profiles of polyPtL1 and imidazole exposed polyPtL1 (polyPtL1-Im) shown in Figure 1b indicate no imidazole loading on polyPtL1, as there is no characteristic change in thermogravimetric profile of polyPtL1-Im. However, the existence of imidazole loaded on polyPtC, without any reactions/conversion, was confirmed by 1 H NMR (Figure S10 in the Supporting Information), MALDITOF (Figure S11 in the Supporting Information) mass spectroscopy, FTIR, and TGA (Figure 2a). The comparison of 1H NMR spectra of polyPtC, polyPtC-Im, and bulk imidazole in DMSO-d6 is shown in Figure S12 in the Supporting Information. The characteristic proton signals of polyPtC are retained in polyPtC-Im along with a very little downfield shifting of peak position. This downfield shifting of polymer protons in polyPtC-Im is due to the formation of −COO− anions in the presence of basic imidazole.44 Compared to that of bulk imidazole, the imidazole proton signal also revealed a prominent downfield shifting (7.03 and 7.67 ppm in bulk imidazole to 7.12 and 7.84 ppm, respectively) in polyPtCIm. In polyPtC-Im, some of the loaded imidazole would take protons from the carboxylic acid moieties of the polymer to form imidazolium cations. The formed imidazolium cation can have a proton exchange with loaded imidazole to provide a downfield shifting of average imidazole protons in polyPtC-Im. This result confirms the formation of imidazolium cations inside the polyPtC-Im. The MALDI-TOF mass spectrum of polyPtC-Im (Figure S11 in the Supporting Information) exhibited the corresponding peaks of [(imidazole + H)+] at m/z 69.86 and [(imidazole + Na)+] at m/z 91.95 etc. The m/z peak at 402.12 clearly confirms the formation of imidazolium cation in polyPtC-Im as it is a combination of ligand in carboxylate form, imidazolium cation, and Pt2+ ion (Figure S11 in the Supporting Information). This result also confirms that D

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ACS Applied Materials & Interfaces exclusively for two forms of imidazoles in polyPtC-Im; one as pure imidazole, which does not interact with polymer (degraded at 100−200 °C range), and another is imidazolium ion, which interacts with the carboxylic group of the polyPtC polymer chain. The X-ray powder diffraction (XRPD) patterns shown in Figure 2b exhibit that the diffraction pattern of polyPtC-Im is same as that of the apohost polyPtC. However, the little peak position shifting of characteristic polymer peaks within 2θ ∼ 5− 15° range toward higher diffraction angle in polyPtC-Im is due to the structural shrinkage after installation of imidazole by the strong interaction between the imidazole and the −COOH group in polyPtC.31 Again, the broad peak at ∼2θ = 26.01 (d = 3.50 Å) in polyPtC due to π−π interaction between polymer chains is shifted to 2θ = 25.04 (d = 3.56 Å) in polyPtC-Im. The enhancement of π−π distance in polyPtC-Im might be the effect of carboxylate ion formation, which can provide repulsion between the polymer chains. Again, the peak at 2θ = 21.96° (d = 4.05 Å) in polyPtC is due to Pt···Pt interaction42,45−47 in between successive polymer chains, which are shifted to 2θ = 19.76° (d = 4.48 Å) in polyPtC-Im to enhance the Pt···Pt distance as imidazole is intercalated between polymer chains after imidazole exposure. In comparison to those of the bulk imidazole, the distinctive peaks for imidazole at 2θ = 20.55, 20.94, 26.15, 28.37, and 30.90° are fairly present in PolyPtCIm. The interaction between the carboxylic acids of metallopolymer chain and accommodated imidazole was further investigated by FTIR spectroscopy (Figure S15, Supporting Information). The ligand L exhibits the characteristic peak at 1724 cm−1 for CO stretching and a broad peak at 3428 cm−1 for −O−H stretching. The retention of these peaks in polyPtC confirms the formation of metallo-supramolecular polymer with free − COOH group. Bulk imidazole shows characteristic stretching frequencies of −N−H at 3121 cm−1 and −C−H at 3025 cm−1. After imidazole vapor exposure, the shift of CO stretching to ∼51 cm−1 lower frequency (from 1724 cm−1 of polyPtC to 1673 cm−1 of polyPtC-Im) and shift of −O−H stretching frequency to 3505 from 3428 cm−1 with decrement of relative −O−H stretching vibration intensity indicates the formation of carboxylate anions in polyPtC-Im. Also, the FTIR study of polyPtC-Im revealed two types of −N−H stretching: one at 3224 cm−1 (∼103 cm−1 enhancement over bulk imidazole) and another at 3121 cm−1 (the same as that of bulk imidazole). This result clearly confirms that two types of imidazole molecules exist in polyPtC-Im. This result is also consistent with the thermogravimetric study. To obtain further insight into the structure, we carried out SEM analysis for polyPtC and polyPtC-Im. In Figures 3a and b, the polyPtC shows a nicely assembled 1D polymer fiber with average diameter of ∼310 nm. However, after imidazole vapor exposure, though the assembled behavior of the polymer chains is not altered, the smoothness of the polymer fibers increases (Figures 3c and d) along with its average diameter, which is increased from 310 to 330 nm due to decoration of imidazole over polyPtC polymer chains. The polymer polyPtC and imidazole loaded polyPtC-Im were further investigated by UV−vis spectroscopy. The UV−vis spectra of polyPtC in thin films reveal two absorption peaks: one at 280 nm due to π−π* transition of the ligand and the other at 450 for metal to ligand charge transfer (MLCT) transition of Pt-terpyridyl complexation (Figure 4a).42,48 After imidazole exposure on polyPtC, the ligand absorption peak is red-shifted 15 nm due to imidazole incorporation. However,

Figure 3. SEM image of (a and b) polyPtC and (c and d) polyPtC-Im.

the absorption maxima of MLCT does not alter in polyPtC-Im, which denotes no change in Pt complexation after imidazole exposure. Again, the polyPtC polymer exhibits very bright yellowish orange emission when irradiated by a 365 nm UV lamp. This yellowish emission was completely quenched after imidazole vapor exposure, as shown in the inset of Figure 4b. This phenomenon is also reflected in emission spectroscopy of polyPtC and polyPtC-Im in the film state in Figure 4b. The polyPtC polymer demonstrates a yellow emission at λmax ≈ 575 nm, assigned to Pt···Pt interactions in the 3MLCT/3LLCT emissive state.42,48−50 These Pt···Pt interactions were previously identified in the wide-angle XRD studies as discussed earlier. However, in polyPtC-Im, the yellow emission is effectively quenched and blue-shifted to 550 nm. The Pt···Pt distance is effectively enhanced to quench the 3MLCT/3LLCT emissive state along with some blue-shifting of emission maxima.50,51 The enhancement of Pt···Pt distance after imidazole exposure is also evident form wide-angle XRD studies, as discussed earlier. We also checked the photoluminescence color change of polyPtL1 polymer before and after imidazole exposure. The polyPtL1 is bright red emissive, which does not change its color after imidazole exposure when irradiated by 365 nm in Figure 1c. The emission spectra of polyPtL1 and polyPtL1-Im are also identical in Figure 1d. This experiment again confirms that polyPtL1 polymer is reluctant to accommodate imidazole as it does not have any carboxylic acid functionality. Decoration of imidazole molecules along with the 1D metallo-supramolecular polymer chain can yield very interesting results regarding proton conductivity even at high temperatures. As we aimed to achieve proton conductivity at temperatures above 100 °C, we therefore planned to study the conduction behavior of polyPtC-Im and polyPtC at different temperatures in completely anhydrous conditions. For the conductivity measurements, the polyPtC and polyPtC-Im were compressed separately using a press to form a pellet structure. The pellets were then inserted between the two electrodes of a sample holder SH2Z from Toyo Inc., Japan. Conductivities of the materials were measured by AC impedance spectroscopy, which is a versatile electrochemical tool to characterize intrinsic electrical properties of materials. Figure 5a exhibit the Nyquist plots (Zimg versus Zreal) of the complex impedance measured on polyPtC-Im under anhydrous conditions at temperatures ranging from 25 to 120 °C. Usually, the diameter of the semicircle yields the conductivity. We can see that the E

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Figure 4. (a) UV−vis and (b) emission spectra of solid thin film of polyPtC and polyPtC-Im. For emission spectra, the excitation wavelength is 450 nm. The fluorogenic color change upon imidazole exposure on polyPtC is shown in the inset of panel b.

Figure 5. (a) Nyquist diagrams for polyPtC-Im at different temperatures in anhydrous conditions (inset shows zoomed-in Nyquist diagrams for polyPtC-Im). (b) Bar diagram of conductivities of polyPtC-Im at different temperatures. (c) Nyquist plot of polyPtC-Im at 120 °C and corresponding equivalent circuit, where Ri is inherent impedance of the circuit, Rb is resistance for proton conduction in bulk phase, and Cp is the constant phase element. (d) Arrhenius plot for activation energy measurement for polyPtC-Im.

conductivity arises directly from the accommodated imidazole. It also indirectly proves that the BF4− counteranions in polyPtC do not have much role in conduction behavior with increasing temperature at anhydrous conditions. Measurement of activation energy for proton conduction phenomena is important to determine the proton transport mechanism. Proton transportation typically proceeds via two generally accepted mechanisms, Grotthuss and vehicle mechanism.52 The activation energy in the range of 0.1−0.4 eV is generally considered to be associated with the Grotthuss mechanism, whereas the vehicle mechanism, the other significant model for the interpretation of fast proton conduction, is generally more energetically demanding with activation energies in the range of >0.5 eV.53,54 It is very interesting that when we fit the proton conductivity data of polyPtC-Im in the whole temperature range to the Arrhenius equation, as shown in Figure 5d, we observed two different slopes which denote two activation energies: one of 1.1 eV between 25 and 75 °C and another with lower value of 0.41 eV between 75 and 120 °C.36 Therefore, at low temperatures at

conductivity of polyPtC-Im is increased with increasing temperature (Figure 5b) as the diameter of the semicircle decreases with decreasing magnitude of Zreal. Before coming to any conclusion, we tested the reproducibility by measuring the above-mentioned proton conduction three times on different batches of polyPtC-Im samples. Each experiment revealed similar results with good consistency, highlighted by standard deviations in Figure 5b not exceeding ±10%. The conductivity of polyPtC-Im at 25 °C is calculated to be 6.0 × 10−9 S cm−1, which is increased to 1.5× 10−5 S cm−1 at 120 °C at anhydrous conditions (Figure 5c). Therefore, a massive increment (4 orders of magnitude) of conductivity is ensued as the temperature increased. For comparison purposes, we also measured the impedance of guest free polyPtC apohost shown in Figure S11. The guest free polyPtC shows the conductivity of 1.3 × 10−12 S cm−1 at 25 °C, which is not so much changed at 120 °C (1.0 × 10−11 S cm−1) at anhydrous conditions. The augmentation in the temperature-dependent conductivity of polyPtC-Im compared with the conductivity profile of apohost polyPtC indicates that a significant improvement in the proton F

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Figure 6. (a) Schematic illustration of imidazole interaction in the interpolymer chain to form 1D proton channel. (b) Possible proton hopping mechanism.

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25−75 °C, the vehicle mechanism may operate where the mobilized free imidazole (which does not interact with −COOH) molecules can transport the proton individually. However, at high temperature, the Grotthuss transport mechanism is predominantly attributable as the activation energy is very low at high temperature. In this mechanism, all of the loaded imidazole molecules (−COOH interacted or noninteracted) create a 1D proton channel along with the 1D polymer chain as shown in Figure 6 to provide proton hopping by continuous breaking and formation of alternate Hbonding. This proton hopping mechanism in the imidazole containing system is well-documented experimentally and theoretically in literature.31,55−59 As we already detected the presence of both imidazole and imidazolium cations in our system, we can assume the possible proton hopping mechanism for proton conduction as in Figure 6. When the temperature increases, the rotation of the intercalated imidazole and the vibration of their N−H bonds facilitate the hopping of protons from the one imidazole to other within the hydrogen-bonded arrays, as reflected by the change in activation energy.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12963. Characterization of the ligands and the polymers through 1 H and 13C NMR, mass spectroscopy, molecular weight measurements of the polymer, MALDI-TOF, FT-IR spectra, DTG plots polymer and imidazole loaded structures, and the Nyquist diagram of polyPtC at different temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone/Fax: +81-29860-4721. ORCID

Masayoshi Higuchi: 0000-0001-9877-1134 Notes

The authors declare no competing financial interest.

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4. CONCLUSION In conclusion, we designed and synthesized carboxylic acid containing ligand and its metallo-supramolecular polymer polyPtC by 1:1 complexation between Pt(II) salt and the ligand. The apohost polyPtC can load imidazole to produce polyPtC-Im, which was efficaciously characterized by TGA, XRD, SEM, and photophysical and emission spectroscopy. Imidazole loaded polymer exhibits proton conductivity of 1.5 × 10−5 S cm−1 at 120 °C in completely anhydrous conditions, which is 6 orders of magnitude higher than that of the pristine metallo-supramolecular polymer. The imidazole loaded polymer effectively shows the proton transport through the vehicle mechanism at low temperature range at 25−75 °C, whereas at high temperatures, the Grotthuss transport mechanism is predominantly attributable as the activation energy is very low. The carboxylic acid functionalized metallo-supramolecular polymer interlayers can serve as a scaffold for the highly ordered 1D imidazole channel that forms a Grotthuss proton transfer pathway at high temperatures, as evidenced by the very low activation energy and proton mobility at high temperatures. These results suggest that the nonporous 1D metallosupramolecular polymer with proper functionality may be the alternate option for use as an apohost for imidazole entrapping material for anhydrous proton conductivity.

ACKNOWLEDGMENTS We would like to thank JST-CREST project funding for the research. We would like to thank JST-CREST project funding for the research (No. JPMJCR1533).

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