Synthesis of Triptycene-Based Organosoluble, Thermally Stable, and

Article pubs.acs.org/Macromolecules

Synthesis of Triptycene-Based Organosoluble, Thermally Stable, and Fluorescent Polymers: Efficient Host−Guest Complexation with Fullerene Snehasish Mondal, Sourav Chakraborty, Sourav Bhowmick, and Neeladri Das* Department of Chemistry, Indian Institute of Technology Patna, Patna 800 013, Bihar, India S Supporting Information *

ABSTRACT: We report a facile synthesis of 2,6-diethynyltriptycene (DET) in high yield. Application of DET as monomer in polymer chemistry has been shown (for the first time) in syntheses of two novel polymers via Sonogashira cross-coupling reaction in high yield. The newly synthesized polymers were characterized by FT-IR, UV−vis absorption, and NMR spectroscopic techniques. The polymers prepared using DET have interesting properties such as high solubility in common organic solvents, high thermal stability [decomposition temperatures (Td) > 495 °C], and high char yield (greater than 81% at 900 °C). Additionally, polymers are fluorescent. Host−guest interaction between triptycene-based polymers and fullerene (C60) has been studied for the first time. Fluorescence quenching of our polymers by C60 has been used to study the extent of (polymer·C60) host−guest complex formation. Fluorescence quenching studies indicate binding constant for polymer· C60 complexation on the order of 105 M−1.

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INTRODUCTION Triptycene is the simplest member of the iptycene family, the members of which are molecules containing three or more arene rings connected together by a [2.2.2]bicyclic bridge. Although triptycene was first prepared by Bartlett et al. in 1942,1 research in chemistry of triptycene has gained momentum since the past decade. Research interest in triptycene-based chemistry has grown over the past few years due to reports in the literature highlighting application of triptycene derivatives in supramolecular chemistry, material science, and polymer chemistry.2 In polymer chemistry, triptycene derivatives have emerged as a new class of monomers. A polymer containing the triptycene unit was first reported by Klanderman and Faber in 1968 wherein 9,10-disubstituted triptycene moieties were used as monomers.3 For the next three decades, there were limited reports on triptycene-based polymers. However, research on iptycene-based polymers was revived by the research group of Swager.2f−j Currently, there is lot of excitement in the development of new polymers containing iptycenes.2 This is primarily because of literature reports of triptycene-based polymers having enhanced thermal stability, augmented mechanical properties (tensile strength, stiffness, ductility, higher glass transition temperatures, high modulus, and toughness), and low refractive indices.4 It was also observed that incorporation of triptycene units in the backbone of polymer enhanced the solubility of the resulting polymer in organic solvents. 4a,c,d,5 In some cases, the fluorescence quantum yield improved greatly due to the presence of triptycene units in the polymer.6−8 The aforementioned improvement in properties of polymers (synthesized from triptycene monomers) is attributed to the unique rigid three-dimensional rigid structure of triptycene core. © XXXX American Chemical Society

The steric bulk of triptycene (arising from the paddlewheel configuration of three benzene rings) disrupts π−π stacking interactions, prevents efficient packing, and thus contributes to improved solubility. Swager coined the phrase “internal free volume (IFV)” to describe void spaces produced in the cavities between the benzene rings.9 IFV thus produced in triptycenebased polymers restricts efficient packing and promotes spatial separation of polymer backbones. The favorable enhancements in mechanical properties (upon introduction of triptycene moieties in the polymer backbone) have been explained in terms of the tendency of the polymer to minimize “internal molecular free volume” (IMFV) by either molecular threading or molecular interlocking mechanisms.4,9 The IFV in triptycene polymers is also utilized to design fluorescent polymers with potential application as chemosensors, wherein the presence of bulky triptycene structural motif minimizes interchain π−π interactions.2d,e,6 This results in lesser extent of self-quenching and fluorescence quantum yield of the polymer increases.8 Electron-deficient guest molecules can penetrate in clefts generated due to IFV of triptycene motif and interact with polymer, and the resulting host−guest complexation is studied by fluorescence quenching.7,8,10 Considering these favorable properties, triptycene-based polymers have been identified for versatile applications. The enhanced solubility of triptycene-based polymers in organic solvents improves their solution processability in terms of polymer film preparation (for potential application as membrane Received: July 7, 2013 Revised: August 13, 2013

A

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Scheme 1. Synthesis of Monomer 2a

a

Reagents and conditions: (a) NaNO2, HBr, CuBr, 60 °C; (b) Pd(PPh3)4, CuI, Et3N, trimethylsilylacetylene, 60 °C, 4 h; (c) TBAF, THF, rt, 20 min.

for gas separations).5,11 Based on the aforementioned improvements in physical properties, triptycene-based polymers have been projected as promising candidates for spin-on dielectric materials,4a low dielectric constant (low-κ) materials,4a liquid crystal applications, and photoresists.4a,12 Films of iptycenebased polymers have also been incorporated into explosives detectors used in modern day warfare.2c This is due to their ability to detect trace quantities of highly explosive materials such as nitroaromatics (TNT, DNT) that are electron-deficient species.8 More recently, triptycene derivatives have been used as monomers for the design of main-chain organometallic microporous polymers (MOMPs),13 microporous organic polymers (MOPs),14 polymers of intrinsic microporosity (PIMs),15 and covalent−organic frameworks (COFs).16 In this regard, IFV generated in the polymer due to incorporation of triptycene motifs often yields a highly porous material that renders it a promising candidate to be used as specific gas adsorbent. Porous materials in general are attracting research attention due to their versatile roles in catalysis,17 gas storage16e,18 and separation,19 drug delivery,20 and chemical sensing.21 In order to design a chain polymer based on triptycene, one has to utilize a suitably disubstituted triptycene derivative as one of the monomers. The first few triptycene-based polymers reported in the literature in 1968−1969 utilized 9,10-bis(hydroxymethyl)triptycene.3,22 Most of the fluorescent polymers reported by Swager are based on incorporation of 1,4disubstituted triptycene units. 2,6-Dihalo-substituted triptycenes have been used to yield a homopolymer of triptycene5 while a series of triptycene polyimides were synthesized using 2,6diaminotriptycene as one of the monomers.4a Considering the versatile application of triptycene derivatives,2e we have developed interest in triptycene chemistry. Recently, we have reported synthesis of new triptycene-based tripods and their use as building blocks in supramolecular chemistry.23 Continuing our research interest in this area, herein we report the improved synthesis of 2,6-dibromotriptycene that has been used further to yield a new disubstituted triptycene− 2,6-diethynyltriptycene (DET) (2). In this work, we also describe synthesis of two new triptycene-based polymers (3 and 4) employing Pd(0)/Cu(I)-catalyzed Sonogashira crosscoupling of 2,6-diethynyltriptycene and an appropriate aryl dihalide. Polymers 3 and 4 were characterized by spectroscopic techniques (NMR and FTIR), gel permeation chromatography (GPC), thermal analysis (TGA and DSC), and powder X-ray diffraction (PXRD) techniques. Polymers had excellent solubility in common organic solvents. It was observed that polymers 3 and 4 are fluorescent in solution. Fullerenes have emerged as important building blocks in the design of functional nanomaterials.24 In this context, C60 containing polymer nanocomposites (PNCs) (in which C60 is noncovalently bound to polymers) have interesting applications

in modern technologies (including but not limited to photovoltaic applications and nanoparticle templating).25 In this context, it is important to first understand the interaction between a polymer and fullerene before exploring application of the polymer as a component of C60 containing PNC. Fullerenes are known to act as π-acceptors with various organic molecules (including but not limited to substituted triptycenes, tetraphenylpyranylidene, tetrathiafulvalenes, hexamethoxytriphenylene, metalla-tetraphenylporphines, and others), thereby forming donor−acceptor complexes.26a In this context, triptycene is electron donating relative to C60, and hence they can form host− guest complexes. In addition, concave cavities in triptycene (generated due to the unique orientation of three phenyl rings around the [2.2.2]bicyclic bridge) can accommodate spherical C60 molecules, and this promotes aromatic π−π interactions between them. This is also evident from solid state structures of the triptycene·C60 complex.26 In this report, for the first time host−guest interaction of a triptycene containing polymer 3 or 4 with fullerene (C60) has been studied. Monomer 2 (DET) is a new building block for design of polymers having triptycene backbone. In this report, we have efficiently used molecule 2 as monomer for facile synthesis of triptycene-based polymers that are completely soluble in a host of common organic solvents. To the best of our knowledge, polymers described in this report are unique examples of triptycene-based nonconjugated fluorescent polymers with reasonable quantum yields.

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RESULTS AND DISCUSSION Synthesis and Characterization of Monomer 2. Synthesis of 2,6-dibromotriptycene (1) has been reported previously from 2,6-diaminotriptycene via Sandmeyer reaction.5 A relatively low yield (51%) of 2,6-dibromotriptycene has been reported by authors. The reason for low yield may be partial degradation of the diazotized product upon its addition to a refluxing mixture of CuBr in HBr. We have carried out the addition of the diazotized product at a lower temperature (60 °C instead of 100 °C) (Scheme 1), and in this case we have observed that the reaction gives better yield (72%) (see Supporting Information for 1H NMR and FT-IR spectra). Thus, we have improved the reaction yield significantly during synthesis of 2,6-dibromotriptycene from 2,6-diaminotriptycene. 2,6-Diethynyltriptycene (DET) (2) has been subsequently prepared by the conventional Sonogashira coupling reaction between 1 and trimethylsilylacetylene. Removal of the trimethylsilyl protecting group yields the desired product 2 (DET) that has been isolated as a white solid after purification by column chromatography (85% isolated yield). DET has been characterized by NMR, Fourier transform infrared (FTIR) spectroscopy (see Supporting Information), single crystal X-ray diffraction (XRD), and elemental analysis. In the 1H NMR spectrum of 2 in CDCl3, signals at δ = 5.39 (s, 2H) and 2.96 ppm B

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Figure 1. Top view (left) and side view (right) of the “ORTEP” model of the single crystal X-ray diffraction structure of monomer 2. Thermal ellipsoids are drawn to 30% probability. H atoms are omitted from ORTEP (right) for clarity.

Scheme 2. Synthesis of Polymera

a

Reagents and conditions: (a) DMF or THF, Et3N, Pd(PPh3)4, CuI, 65 °C, 3 days.

under various reaction conditions using different solvents. Using tetrahydrofuran (THF) as solvent and triethylamine (TEA) as base in the presence of Pd(PPh3)4 as a catalyst and CuI as a cocatalyst, 3 was obtained with an isolated yield of 57%. The polymerization reaction was carried out at 65 °C. Gel permeation chromatography (GPC) analysis indicated that polymer obtained had number-average molecular weight (Mn) of 4.2K. The corresponding degree of polymerization (Pn) was 11, and the polydispersity index (PDI, Mw/Mn) was 1.45. However, only on changing the reaction solvent from THF to dimethylformamide (DMF), the polymerization reaction (temperature used = 65 °C) yielded a polymer with relatively higher number-average molecular weight (Mn) of 6.8K (Pn = 18, PDI = 2.07). The isolated yield of the polymer 3 also increased significantly to 93% in this case. Having obtained promising results upon using DMF as the solvent in Sonogashira polycondensation reaction, DET (2) and 4,4′-dibromobiphenyl were reacted in 1:1 stoichiometric ratio in DMF in presence of Pd(PPh3)4 (catalyst), CuI (cocatalyst), and TEA (amine base) to yield polymer 4 in 86% isolated yield. GPC analysis of the product showed that its number-average molecular weight (Mn) is 4.6K. The polydispersity index (PDI) was found to be 2.18, and this corresponds to 10 repeating units. As expected, this polymer is also soluble in common organic solvents. The results of polymerization in the two different solvents are tabulated in Table 1. In both cases, the polymer was obtained as a light brown solid. The products obtained under both reaction conditions are highly soluble in common organic solvents (such as tetrahydrofuran, dichloromethane, chloroform, toluene, and

(s, 2H) ppm were assigned to bridgehead and ethynyl protons, respectively. The 13C NMR spectrum of 2 shows two signals at δ = 76.4 and 83.7 corresponding to C1 and C2, respectively, of C2C1−H. The FTIR spectrum of DET (2) shows a strong absorption band at 3286 cm−1 corresponding to the C−H stretching frequency of terminal alkynes. The strong, broad absorption band between 600 and 661 cm−1 corresponds to the C−H bending frequency of the terminal alkynes (Supporting Information). A weak absorption band at 2102 cm−1 corresponds to the CC bond stretching frequency in 2. The structure of DET was unambiguously determined by single crystal X-ray diffraction study (Supporting Information). X-ray quality single crystals were obtained by slow evaporation of mixture of chloroform and ethanol solution of DET. The molecular structure of 2 is shown in Figure 1. Single crystal X-ray diffraction studies (T = 293 K) revealed that 2 crystallized in the orthorhombic space group P212121, a = 8.2715(9) Å, b = 10.4269(14) Å, c = 19.1790(18) Å, and Z = 4. The crystal structure analysis reveals no unusual bond lengths or angles in the disubstituted triptycene derivative (2). Synthesis of Polymers 3 and 4. The transition metal crosscoupling reaction of terminal alkynes with aryl halides is a convenient methodology for synthesis of polymers containing alkyne bridges in polymer backbone. Polymers containing an ethynyl bridge have been conveniently prepared by the Sonogashira cross-coupling reaction.7,27 Inspired by these reports, and employing DET and 1,4-dibromobenzene as monomers, polymer 3 has been prepared via Sonogashira polycondensation (Scheme 2). This reaction has been attempted C

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Table 1. Effect of Solvent on Property of Polymer (Reaction Temperature 65 °C) monomers

solvent

Mna (Da)

yield (%)

PDIa

Pnc

2 + 1,4-dibromobenzene 2 + 1,4-dibromobenzene 2 + 4,4′-dibromobiphenyl

THF DMF DMF

4.2K 6.8K 4.6K

57 93 86

1.45 2.07 2.18

11 18 10

a

Number-average molecular weight (Mn) and polydispersity index (PDI, Mw/Mn) were determined by GPC using THF as solvent and polystyrene standard as reference. cDegree of polymerization (Pn) determined from Mn.

benzene) as well as in more polar solvents like DMF and dimethyl sulfoxide (DMSO). As observed previously by Swager,4a the presence of triptycene motifs restricts efficient packing of polymer chains, and this explains for the high solubility of polymers 3 and 4 in this present case. Inherent viscosity of polymers chloroform solution was estimated to be 0.55 and 1.26 dL/g for 3 and 4, respectively . Inherent viscosities reported for triptycene polyimides (in DMF) by Swager are in the range 0.066−0.74 dL/g.4a The wide-angle X-ray diffraction (WAXD) patterns for 3 and 4 revealed broad features (Figure 2), suggesting the amorphous nature of the polymers. The presence of triptycene units in the polymer backbone apparently reduces the extent of interchain interaction resulting in amorphous nature of the polymers. Characterization of Polymers 3 and 4. The structure of each polymer was characterized by 1H NMR and FTIR spectroscopy. The proton NMR spectrum of 3 and 4 is shown in Figure 3. In the 1H NMR spectrum of 3, the multiplet in between 5.32 and 5.35 ppm corresponds to the bridgehead proton (Hf) of the triptycene unit. The signal due to protons (Hg) of the isolated phenyl ring flanked by two ethynyl groups overlap with the signals due to Hb and He protons of a triptycene unit. The peak in the range δ = 7.43−7.48 ppm corresponds to Hc protons of the triptycene ring. Similarly, in the 1H NMR spectrum of 4, the peak due to the characteristic bridgehead protons (Hf) is observed in the range δ = 5.39−5.44 ppm. The peaks due proton of biphenyl unit (Hg and Hi) and Hc, Hb, and He are observed to overlap in the range δ = 7.37−7.59 ppm. As expected, the integration ratio of all signals in the range δ = 7.03−7.59 ppm (Ha, Hb Hc, Hd, He, Hg, and Hi)

Figure 3. 1H NMR of the polymer 3 (top) and 4 (bottom) in CDCl3.

and that due to the bridgehead protons (Hf) is approximately 9:1. The FT-IR spectrum (see Supporting Information) shows a weak absorption band at 2206 and 2205 cm−1 for polymer 3 and 4, respectively, corresponding to CC bond stretching frequency. The absence of the νC−H stretching frequency (of the proton attached to the ethynyl group of 2) in polymers 3 and 4 confirms C−C bond formation between 2 and respective aryl dibromide via cross-coupling polycondensation reaction. The results of molecular weight determination of polymers 3 and 4 by GPC (THF as solvent versus polystyrene standards) are tabulated in Table 1 (see also Supporting Information for GPC trace). It was observed that when 1,4-dibromobenzene was replaced by 4,4′-dibromobiphenyl in the Sonogashira polycondensation reaction (DMF as solvent) with DET, a lower molecular weight polymer is obtained. Consequently, the degree

Figure 2. X-ray diffraction patterns for (a) polymer 3 and (b) polymer 4. D

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and 4. From DSC analysis (see Supporting Information), polymer 3 showed glass transition (Tg) at 131 °C. However, in the case of 4, no Tg was observed until 400 °C. Photophysical Properties. UV−vis absorption spectra of polymers 3 and 4 (solution in toluene) exhibited multiple bands (see Supporting Information). The highest intensity absorption band is centered at λmax = 325 and 326 nm for 3 and 4, respectively, and these are assigned to π−π* transitions of the polymer backbone. In the case of 3, additional bands are observed at 308 and 349 nm. In the case of 4, the extent of conjugation is higher in the chromophore bridging two neighboring triptycene units when compared with that in 3. Albeit, there is very negligible bathochromic shift in the absorption maxima (λmax = 326 nm) in 4 with respect to that in 3. The solution fluorescence excitation and emission spectra of polymers 3 and 4 are shown in Figure 5 (3, λex = 335 nm, λem = 364 nm, Φ = 0.12; 4, λex = 335 nm, λem = 379 nm, Φ = 0.32). The fluorescence quantum yield was measured using quinine sulfate in 1 N H2SO4 solution (Φ = 0.546) as reference. Host−Guest Interaction with Fullerene. The fluorescent property of polymers 3 and 4 has been utilized to study host− guest interaction between these polymers and fullerene (C60). It has been observed that gradual addition of C60 to the polymer (3 or 4) solution (in toluene) results in quenching of fluorescence (decrease of fluorescence intensity) as shown in Figure 6. The steady decrease in the polymer (3 or 4) fluorescence intensity with incremental addition of C60 is attributed to interaction between triptycene-based polymer (3 or 4) chains (electron donating relative to C60) and fullerene (electron deficient). The resultant decrease in concentration of free polymer is reflected in the decrease in polymer fluorescence intensity. To calculate the binding constant of the aforementioned complex formation, the modified Benesi−Hildebrand (B−H) equation for formation of 1:1 complex has been used:28

of polymerization (Pn) also decreased from 18 to 10. The molecular weight of our triptycene containing polymers 3 and 4 and yield of polymerization are higher than other polymers synthesized via the Sonogashira coupling reaction.27 However, molecular weights of alternating polymers 3 and 4 (prepared from 2,6-diethynyltriptycene as one of the monomers) are comparable with homopolymers reported by Swager (using 2,6dihalotriptycene as monomer).5 Thermal Properties. Thermal properties of the polymers 3 and 4 were studied by thermogravimetric analysis (TGA) (Figure 4) and differential scanning calorimetry (DSC). TGA has

Figure 4. TGA traces of the polymer 3 and 4 (under N2, heating rate = 10 °C/min).

been performed under a nitrogen atmosphere (heating rate: 10 °C/min). In the case of 3, 2.2% weight loss was observed at 227 °C. Similarly for polymer 4, a small weight loss of 1.3% was observed at around 100 °C. These small weight losses may be due to the evaporation of entrapped solvent molecules. Thermal decomposition temperatures (Td = 10% weight loss temperature under nitrogen) is 542 °C for polymer 3 and 500 °C for polymer 4. The char yield of polymer 3 at 900 °C is 81.8%, while that of polymer 4 is 81.5% (900 °C). These results indicate reasonably high thermal stability of these triptycene containing polymers 3

⎧⎛ 1 ⎞⎛ 1 ⎞⎫ F0 1 ⎟⎜ = + ⎨⎜ ⎟⎬ F0 − F A ⎩⎝ KA ⎠⎝ [Q] ⎠⎭

(1)

In eq 1, F0 and F are the fluorescence intensities of the polymer in absence and presence of fullerene, respectively. [Q] represents the molar concentration of the quencher (in this case C60). A is a constant associated with the difference in the emission quantum

Figure 5. Normalized excitation and emission spectra of (a) polymer 3 and (b) polymer 4 in toluene at 298 K. E

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Figure 6. Fluorescence quenching of (a) polymer 3 (10−7 mol L−1) and (b) polymer 4 (10−7 mol L−1) in the presence of fullerene C60 as quencher (concentration of quencher varied from 5.0 × 10−7 to 1.20 × 10−5 mol L−1) recorded in toluene at 298 K. Insets: fluorescent images of polymer solution in the absence of quencher (C60) (top) and in the presence of C60 (bottom) taken under UV illumination.

Figure 7. Benesi−Hildebrand fluorescence plot of 1:1 complexation between (a) C60 and polymer 3 (b) C60 and polymer 4 recorded in toluene at 298 K.

corresponding to 1:2 or 1:3 binding show curvature, thereby ruling out the probability of such host:guest complex formations (Supporting Information).] The observed magnitude of binding constant in each case indicates that C60 (acting as π-acceptor) interacts efficiently with polymers 3 and 4. The fullerene· polymer association is probably through π−π interactions between C60 (having electron-deficient convex curvature) and triptycene (that is electron donating relative to C60 and has complementary concave cavities). The order of magnitude of K is same for 3 and 4, and it can be inferred that there is no drastic difference in strength of the resultant polymer·C60 complexes. Although polymer 4 has a lower Mn and corresponding Pn (Table 1) than polymer 3, the former has a slightly higher K. Values of K obtained for these triptycene-based low molecular weight polymers are higher by an order of magnitude when compared with high molecular weight poly(p-phenylene ethynylene) (PPE) polymers reported by Bucknall and co-workers.29 It must be mentioned here that binding constants of complexes formed between C60 and small molecules (such as substituted aliphatic amines, anilines, and aromatic hydrocarbons such as naphthalenes, phenanthrene, and pyrene) evaluated using

yield of complexed and uncomplexed polymer. From a plot of F0/(F0 − F) (relative fluorescence intensity) vs 1/[Q] (Figure 7), the following correlations were obtained: polymer 3: F0 = (0.981 ± 0.104) + (3.60 × 10−6 F0 − F ⎛ 1 ⎞ 2 ± 1.47 × 10−7) × ⎜ ⎟ ; R = 0.99 ⎝ [Q] ⎠

(2)

polymer 4: F0 = (1.192 ± 0.089) + (3.91 × 10−6 F0 − F ⎛ 1 ⎞ 2 ± 1.37 × 10−7) × ⎜ ⎟ ; R = 0.99 ⎝ [Q] ⎠

(3)

The binding constant (K) for 1:1 complexation between polymer and fullerene was calculated to be 272 500 ± 40 083 M−1 and 304 859 ± 33 483 M−1 for polymer 3 and 4, respectively. [The fluorescence quenching data upon fitting with linear equation F

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mixture was then added drop by drop to a solution of CuBr (0.47 g, 3.3 mmol) in HBr (1 mL) over a period of 20 min at 60 °C. The reaction mixture was stirred at 60 °C for an additional 3.5 h. After cooling the reaction mixture to room temperature, it was extracted by DCM (30 mL × 2). The organic layer was washed with water, and the extract obtained was dried on anhydrous Na2SO4. The solution was concentrated in a rotary evaporator and purified by column chromatography on silica gel using CH2Cl2/hexane (1:20, v/v), affording the pure compound 1 as a white solid. Yield: 0.33 g, 72%; mp 252−255 °C. 1H NMR (400 MHz, CDCl3): δ 7.51 (d, J = 1.7 Hz, 2H), 7.35−7.38 (m, 2H), 7.23 (d, J = 7.8 Hz, 2H), 7.13 (dd, J = 1.8, 7.8 Hz, 2H), 7.02 (dd, J = 3.1, 5.4 Hz, 2H) 5.34 (s, 1H). Anal. Calcd for C20H12Br2: C, 58.29; H, 2.73; Br, 38.78. Found: C, 58.24; H, 2.75; Br, 38.74%. FT-IR (KBr): 3050, 3017, 2972, 2923, 2851, 1457, 1403, 1265, 1192, 1162, 1141, 1060, 917, 883, 866, 822, 774, 752, 722, 640, 615 cm−1. 2,6-Diethynyltriptycene (2). Compound 1 (0.18 g, 0.44 mmol), CuI (0.016 g, 0.08 mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol) were taken in a Schlenk flask under a N2 atmosphere. 5 mL of triethylamine was added to the reaction mixture and heated to 65 °C. Subsequently trimethylsilylacetylene (240 μL, 1.75 mmol) was added to the reaction mixture, and it was stirred for 4 h at 65 °C. The solution was evaporated to dryness in a rotary evaporator. Extracting the product obtained with n-pentane followed by filtration through Celite bed yielded the product (TMS protected DET) in reasonably pure form to be used in the next step. This product was dissolved in dry THF (5 mL) followed by addition of tetrabutylammonium fluoride (0.15 g, 0.48 mmol) under a N2 atmosphere. The reaction mixture was stirred at rt for 20 min under a N2 atmosphere. It was evaporated to dryness and extracted by DCM followed by washing with saturated brine solution. The organic layer was dried over anhydrous Na2SO4. The solution was concentrated and purified by column chromatography on silica gel with CH2Cl2/hexane (1:10, up to 1:5, v/v) affording pure compound 2 as a white solid. Yield: 0.12 g, 85%, mp 162−165 °C. 1H NMR (400 MHz, CDCl3): δ 7.50 (d, J = 1.2, 2H), 7.37 (dd, J = 3.2, 5.4 Hz, 2H), 7.32 (d, J = 7.4 Hz, 2H), 7.16 (dd, J = 1.5, 7.6 Hz, 2H), 7.02 (dd, J = 3.2, 5.4 Hz, 2H), 5.39 (s, 2H), 2.96 (s, 2H). 13C NMR (100 MHz, CDCl3): 145.5, 144.8, 144.1, 129.8, 129.6, 127.4, 127.3, 125.6, 123.8, 123.7, 119, 83.7, 53.55. Anal. Calcd for C24H14: C, 95.33; H, 4.67. Found: C, 95.21; H, 4.71%. FT-IR (KBr): 3286, 2957, 2922, 2851, 2102, 1638, 1465, 1410, 1384, 1213, 1191, 935, 894, 840, 776, 745, 661, 649, 624, 600 cm−1. General Synthesis of Polymer. Monomer 2 (60 mg, 0.2 mmol), aryl dibromide (0.2 mmol), and CuI (2 mg, 0.01 mmol) were taken in a Schlenk flask. To this solution Pd(PPh3)4 (7 mg, 0.006 mmol) was added under a N2 atmosphere followed by addition of anhydrous DMF (4 mL) and triethylamine (1 mL). The reaction mixture was then heated for 3 days at 65 °C. After cooling, the whole reaction mass was poured into methanol with continuous stirring to yield a white precipitate. The precipitate was filtered off and washed with hot methanol. The residue was dissolved in DCM and filtered through a bed of Celite to get clear reddish-yellow solution. Solution was concentrated reprecipitated by using methanol. The precipitate was filtered off and washed with hot methanol. The solid residue was dried reduced pressure overnight, affording a light brown solid product. Polymer 3. Yield: 70 mg, 93%. Mn = 6.8K Da, PDI = 2.07. 1H NMR (400 MHz, CDCl3): δ 7.43−7.48 (m, 2H), 7.24−7.39 (m, broad, 8H), 7.11 (d, J = 7.5 Hz, 2H), 6.94−6.96 (m, 2H), 5.32−5.35 (m, 2H). FT-IR (KBr): 3064, 3037, 2957, 2915, 2846, 2206, 1899, 1665, 1599, 1568, 1508, 1410, 1261, 1181, 1067, 1008, 937, 888, 834, 774, 740, 619 cm−1. Polymer 4. Yield: 92 mg, 86%. Mn = 4.6K Da, PDI = 2.18. 1H NMR (500 MHz, CDCl3): δ 7.52−7.59 (m, 8H), 7.37−7.47 (m, 6H), 7.16− 7.26 7.03−7.04 (m, 2H). FT-IR (KBr): 3064, 3031, 2920, 2850, 2205, 1899, 1671, 1599, 1463, 1410, 1262, 1186, 1073, 1002, 818, 740, 625 cm−1.

Benesi−Hildebrand theory have a significantly smaller magnitude (of the order of 10−1 M−1).29

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CONCLUSION In conclusion, we have reported an improved synthesis of 2,6dibromotriptycene, which was subsequently utilized to synthesize 2,6-diethynyltriptycene (DET) (2) in high yield. DET is a new building block in polymer chemistry. To illustrate this point, we have used DET along with commercially available aromatic dibromides to synthesize alternating copolymers 3 and 4 in high yield (>85%). 3 and 4 are soluble in common organic solvents and have high thermal stability [(Td) > 495 °C and high char yields (>81%) at 900 °C]. Interestingly, polymers 3 and 4 are fluorescent [quantum yield (Φ) = 0.12 for 3 and 0.32 for 4], although the chromophores are discrete and nonconjugating. We have studied for the first time the interaction between triptycenebased polymer and fullerene (C60). Fluorescence quenching studies suggest the polymer·C60 complex formation of the order of 105 M−1. This hints at favorable π−π interactions between C60 and triptycene-based polymer backbone. The excellent solubility of these polymers in conjugation with high thermal stability and fluorescent property makes them potential candidates for high temperature fluorescent coatings. Because of strong interaction of 3 and 4 with fullerene, these polymers may find application in synthesis of novel fullerene-based polymer nanocomposites. Our research is currently aimed at extending these new types of ethynyl-substituted triptycenes to prepare network polymers [employing 2,6,14- and 2,7,14-triethynyltriptycene (TET)]. Novel polymers thus prepared may be used as porous materials (finite/infinite) having practical applications such as selective gas-adsorption and storage materials.

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EXPERIMENTAL SECTION

Materials. Chemicals were purchased from Sigma-Aldrich or Alfa Aesar and were used without further purification. Anhydrous THF was purchased from Sigma-Aldrich. DMF and Et3N were purchased from Rankem and were dried in laboratory using a common technique. General Methods and Instrumentation. Sonogashira reactions were performed under a nitrogen atmosphere using the common Schlenk technique. Column chromatography was carried out using silica gel (60−120 mesh). NMR spectra were obtained from Bruker Avance II 400 or Jeol-500 NMR spectrometers. Melting points were determined by SRS EZ-Melt automated melting point apparatus. Polymer molecular weights were determined by Agilent PL-GPC 50 integrated GPC spectrometer using THF as the eluent (sample concentration 1 mg/mL) at a flow rate of 1.0 mL/min versus polystyrene standard at 25 °C. TGA measurements were carried out using SDT Q600 (TA Instruments) under nitrogen flow at a scan rate 10 °C/min. DSC analysis were performed by DSC8000 (PerkinElmer) under nitrogen at a scan rate 10 °C/min. FTIR spectra recorded using Shimadzu IR Affinity-1 spectrometer. UV−vis absorption and fluorescence spectra of polymer solutions (10−7 M polymer concentration in toluene) were measured in a 1 cm quartz cell. UV−vis spectra were obtained using a Shimadzu UV2550 UV−vis spectrophotometer. Fluorescence data obtained using the Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. P-XRD data were recorded using a Rigaku TTRAX III X-ray diffractometer. Single crystal XRD measurements were made on an Oxford Super-Nova X-calibur Eos CCD detector with graphite-monochromatic Cu Kα (1.541 84 Å). Fluorescence images were taken [upon irradiation of polymer solutions using a UV lamp (λ = 365 nm)] by a Canon SX-40 HS camera. Origin 6.0 software was used for plotting and fitting the data. Improved Synthesis of 2,6-Dibromotriptycene (1).5 A solution of 2,6-diaminotriptycene (0.31 g, 1.1 mmol) in HBr (1 mL) and water (5 mL) was cooled in an ice/salt bath. Then a cold solution of NaNO2 (0.21 g, 3 mmol) in water (2 mL) was added slowly during 10 min. The reaction mixture was allowed to stir for 25 min. The light yellow reaction

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ASSOCIATED CONTENT

S Supporting Information *

1

H spectrum for 1, 1H and 13C NMR{1H} spectra for 2, FTIR spectra of compounds 1, 2, 3, and 4, GPC and DSC traces of polymers 3 and 4, UV−vis absorption spectrum of polymers 3 G

dx.doi.org/10.1021/ma401421k | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

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and 4, UV−vis absorption spectra of polymers 3 and 4 in the presence of increasing concentration of C60, Benesi−Hildebrand fluorescence plot assuming 1:2 and 1:3 complexation between C60 and polymer (3 and 4), and X-ray crystallographic file (CIF) for 2; CCDC 942294 also contains the supplementary crystallographic data for 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +91 612 2552023; Fax +91 612 2277383. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

N.D. thanks the Department of Science and Technology, Govt. of India, New Delhi (Under Fast Track Scheme for Young Scientists), and the Indian Institute of Technology Patna, for financial support. S.M. thanks UGC, New Delhi, for a Junior Research Fellowship. S.C. and S.B. thank the IIT Patna for an Institute Research Fellowship. The authors also acknowledge Prof. A. K. Bhowmick (IIT Patna), Dr. D. Seth (IIT Patna) and Dr. T. K. Panda (IIT Hyderabad) for helpful discussions. The reviewers are thankfully acknowledged for their valuable comments.

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