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Synthesis and characterization of poly(pyridinium salt)s derived from various aromatic diamines
Polymer 53 (2012) 1063e1071
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Synthesis and characterization of poly(pyridinium salt)s derived from various aromatic diaminesq Tae Soo Jo, Alexi K. Nedeltchev, Bidyut Biswas, Haesook Han, Pradip K. Bhowmik* Department of Chemistry, University of Nevada at Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, NV 89154, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2011 Received in revised form 28 December 2011 Accepted 11 January 2012 Available online 17 January 2012
Several poly(pyridinium salt)s containing various aromatic diamine moieties and tosylate counterions were prepared by the ring-transmutation polymerization reaction of bis(pyrylium tosylate) with aromatic diamines in dimethyl sulfoxide at 130135 C for 48 h and their tosylate counterions were exchanged to triflimide polymers by a metatheses reaction in an organic solvent. Their chemical structures were established by using various spectroscopic techniques. Their number-average molecular weights (Mn) were in the range of 38e46 kg/mol and polydispersities in the range of 1.13e1.43 as determined by gel permeation chromatography. They showed excellent thermal stabilities in nitrogen in the range of 326 e477 C. They exhibited lyotropic liquid-crystalline phase in polar aprotic and protic organic solvents above their critical concentrations depending on their microstructures and counterions. Their optical properties were examined by using UVeVis and photoluminescent spectroscopy, which revealed that some polymers emitted UV light, some emitted blue light, and some emitted green light (both in solutions and solid states) depending on their microstructures, counterions, and on solvent polarity of organic solvents. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Liquid-crystalline polymers Gel permeation chromatography (GPC) UVeVis spectroscopy
1. Introduction The pconjugated polymers have emerged as potential candidates for many optical devices. Due to their straightforward preparation methods, unique properties, and stability in air, these conducting polymers have been applied to energy storage, memory devices, chemical sensors, electrocatalyst, organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaic cells (OPVs) [1]. Many classes of conjugated polymers have been developed such as poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(terthiophene)s, poly(aniline)s, poly(fluorine)s, poly(3-alkylthiophene)s, polytetrathiafulvalenes, polynaphthalenes, poly(p-phenylene sulfide), and poly(p-phenylene vinylene)s over the past decade [2,3]. The pconjugated polymers with electrondonating and electron-withdrawing moieties are currently of interest because the built-in intramolecular charge transfer can facilitate easy manipulation of their electronic structures (HOMO/ LUMO levels) [46], leading to small band gap semiconducting polymers or materials with enhanced third-order non-linear optical properties [7]. Through the design and synthesis of new conjugated polymers can extend to systems with efficient photo-induced charge q This article is dedicated to the memory of Professor David W. Emerson who passed away on January 12, 2012. * Corresponding author. Tel.: þ1 702 895 0885; fax: þ1 702 895 4072. E-mail address: [email protected] (P.K. Bhowmik). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.017
transfer and separation for photovoltaic devices [8] and to bipolar charge transport materials for light-emitting diodes [9], lasers [10], and other applications [11]. Among many polymers, nitrogencontaining conjugated materials have been considered as a suitable candidate for electron transporting layers (ETLs) in OLEDs. Because of their electron-deficient heterocycles, transporting electrons from the cathode through the ETL materials can be more efficient [12]. Furthermore, their light-emitting properties could be easily tuned by protonating nitrogen heterocycles. One of the well known examples is polyaniline (PANI). PANI has been drawn much attention for its excellent electronic, optical, redox properties, and environmental stability [13]. It has been applied to biosensors, electrochemical displays, rechargeable batteries, liquid crystal devices, and separation membranes [14]. Even though it demonstrated good thermal stabilities and high thermal transitions, it had very poor solubility in common organic solvents that lead to difficulties in the fabrication or processing. Therefore, to increase the solubility of the rigid pconjugated polymers, flexible long alkyl chains are generally introduced into polymer backbones. The modification can occur via copolymerization with a functionalized comonomer or postfunctionalization of the parent polymer. However, functionalization of the comonomer during the polymerization step generally resulted in low-molecular-weight polymers [15]. In the latter approach, several chemical modifications have been developed [16], but they usually resulted in the alternation of the original molecular weight. Another method of increased solubility of a polymer is to introduce an
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ionic charge along the backbone of the polymer chain. Introduction of organic counterions such as tosylate or triflimide into polymer backbone can also increase the solubility in common organic solvents because of their reduced electrostatic interaction between polymer and counterions. Recently, viologen polymers and poly(pyridinium salt)s have emerged as a new class of main-chain ionic polymers due to their high solubility in common organic solvents. Although earlier poly(pyridinium salt)s had very low molecular weights, limited solubility, and low thermal stability [17], their properties were significantly improved by preparing them via ring-transmutation polymerization reaction in dimethyl sulfoxide (DMSO) and by introducing of different size of organic counterions [18,19]. These recent development of ionic polymers is of great significance for the preparation of novel electrondeficient pconjugated polymers. In this article, we report the synthesis of both wholly aromatic and semiflexible poly(pyridinium salt)s containing organic counterions from various aromatic diamines via the ring-transmutation polymerization and metathesis reactions as shown in Scheme 1. Our objective was to directly study the influence of the aromatic diamine structural moieties on the solution, thermal, and optical properties of this class of ionic polymers. Also unlike the previous studies of poly(pyridinium salt)s, the current approach enables us to synthesize both rigid and semiflexible ionic polymers with high molecular weights and good solubility in common organic solvents. The effect of aromatic diamine structures and counterions on their solution, thermal and optical properties was examined by using various experimental techniques. 2. Experimental section 2.1. Materials The 2,20 -Bis(trifluoromethyl)benzidine was received as a gift from Polaroid Corporation and it was then purified by recrystallization
from ethanol/water mixture before use. The 2,20 -Bis(4-aminophenyl) hexafluoropropane and 4-aminophenyl sulfone were purchased from SigmaeAldrich and TCI America and used without further purification. Lithium triflimide and common organic solvents were purchased from commercial vendors (SigmaeAldrich, Alfa-Aesar, Acros Organics, and TCI America) and used without any further purification. 2.2. Monomer synthesis 2.2.1. Synthesis of monomer, M The 4,40 -(1,4-phenylene)bis(2,6-diphenylpyrylium p-toluene sulfonates), M, was synthesized according to the reported procedure [19,20]. 2.2.2. Synthesis of 9,10-bis(4-aminophenyl)anthracene The 9,10-bis(4-aminophenyl)anthracene was synthesized according to the reported procedure [21]. 2.3. Polymer synthesis 2.3.1. Synthesis of Polymer I-1I-4 The bis(pyrylium) salt, M, was reacted with an appropriate diamine by a ring-transmutation reaction to yield each of the target polymers. Equal mole ratio of monomers was placed in threenecked round-bottomed flask equipped with a magnetic stirrer in DMSO. A small amount of toluene was added to remove the water generated during the reaction by azeotrope, which was facilitated by using DeaneStark trap and water condenser. The temperature was gradually increased up to 130135 C and the solution was stirred for 48 h under nitrogen atmosphere. After the end of polymerization reaction, the solution was cooled to room temperature. Excess DMSO was removed by a rotary evaporator until the solution became viscous. It was then poured into water to
Scheme 1. Synthesis of poly(pyridinium salt)s.
T.S. Jo et al. / Polymer 53 (2012) 1063e1071
precipitate the polymer. The precipitated solid was washed with water several times more to remove any residual impurities. The process of dissolution in MeOH or DMSO and precipitation with H2O was repeated one more time. The collected polymer was additionally washed a few times with boiling water and it was dried in vacuo at 110 C for 72 h. Data for polymer I-1: Anal. Cacld for C68H48N2O6F6S2 (1167.24): C, 69.97; H, 4.14; N, 2.40; S, 5.49. Found: C, 68.23; H, 4.15; N, 2.44; S, 5.26; for polymer I-2: Anal. Cacld for C69H50N2O6F6S2 (1181.27): C, 70.16; H, 4.27; N, 2.37; S, 5.43. Found: C, 68.89; H, 4.27; N, 2.43; S, 4.94; for polymer I-3: Anal. Cacld for C66H50N2O8S3 (1095.31): C, 72.37; H, 4.60; N, 2.56 S, 8.78. Found: C, 71.44; H, 4.15; N, 2.79; S, 8.34; for polymer I-4: Anal. Cacld for C80H58N2O6S2 (1207.46): C, 79.58; H, 4.84; N, 2.32; S, 5.31. Found: C, 78.05; H, 4.83; N, 2.29; S, 5.07. 2.3.2. Synthesis of polymers II-1II-4 Polymers II-1II-4 were prepared by a metathesis reaction from the respective tosylate polymer with excess lithium triflimide salt in DMSO at 50 C for 48 h. At the end of the metathesis reaction, excess DMSO solvent was reduced by a rotary evaporator to form a viscous solution, and it was poured into water to precipitate the polymer. The reaction step was repeated once or twice more until all the tosylate counterions were completely exchanged to triflimide counterions, which was confirmed by 1H NMR spectrum. Collected polymer was washed with boiling water a few times more to remove any entrapped DMSO or organic salts. The washed polymers were dried in vacuo at 110 C for 72 h. Data for polymer II-1: Anal. Cacld for C58H34N4O8F18S4 (1385.14): C, 50.29; H, 2.47; N, 4.04; S, 9.26. Found: C, 50.61; H, 2.86; N, 4.17; S, 8.88; for polymer II-2: Anal. Cacld for C59H36N4O8F18S4 (1399.18): C, 50.65; H, 2.59; N, 4.00; S, 9.15. Found: C, 51.00; H, 2.68; N, 4.08; S, 9.50; for polymer II-3: C56H36N4O10F12S5 (1313.21): Anal. Cacld for C, 51.22; H, 2.76; N, 4.27; S, 12.21. Found: C, 51.60; H, 2.80; N, 4.43; S, 12.61; for polymer II-4: Anal. Cacld for C70H44N4O8F12S4 (1425.37): C, 58.99; H, 3.11; N, 3.93; S, 9.00. Found: C, 58.82; H, 3.50; N, 3.98; S, 8.66. 2.4. Polymer characterization The FTIR spectra were recorded with a Shimadzu infrared spectrometer. Polymer samples were prepared by coating NaCl plates with various polymers and subsequently vacuum dried at 70 C overnight. The 1H, 19F and 13C NMR spectra were obtained using a Varian NMR spectrometer (400 MHz for 1H, 376 MHz for 19F and 100 MHz for 13C) with three RF channels at room temperature, and chemical shifts were referenced to tetramethylsilane (TMS). The NMR samples were prepared by applying gentle heating to dissolve the polymer in d6-DMSO. The 1H and 13C NMR samples were usually prepared at a concentration of 10 and 30 mg/mL, respectively. Differential scanning calorimetry (DSC) measurements of polymers were conducted on TA module DSC Q200 series in nitrogen at heating and cooling rates of 10 C/min. The temperature axis of the DSC thermograms was calibrated before using the reference standard of high purity indium and tin. Thermogravimetric analyses (TGA) of polymers were performed using a TGA Q50 instrument in nitrogen. The TGA data were collected at temperatures between 30 and 600 C at a heating rate of 10 C/min. Dilute solution for UVeVis and photoluminescence spectroscopy study was prepared by dissolving 0.5 mg of the dry polymer in 5 mL of the organic solvent at room temperature. 50 mL of the resulting solution was transferred into a volumetric flask and further diluted with 5 mL of the solvents to adjust the optical density. The UVeVis absorption spectra of polymer solutions in organic solvents were recorded at room temperature using Varian Cary 50 Bio UVeVisible spectrophotometer in quartz cuvettes. Photoluminescence spectra in solutions and thin films were recorded with a PerkineElmer LS 55 luminescence spectrometer
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with a xenon lamp light source. Lyotropic liquid crystal (LC) properties of the polymers were obtained using a polarized optical microscopy (POM, Nikon, Model Labophot 2) equipped with crossed polarizers. Samples of polymers for lyotropic LC properties were made by dissolving known amounts of polymer into known amounts of organic solvents (DMSO, MeOH or CH3CN). To assess the molecular weight of polymer, gel permeation chromatography (GPC) was run at 50 C with a flow rate of 1 mL/min. The GPC instrument had Water 515 pump simultaneously with a Viscotek Model 301 Triple Detector Array. The array contained a laser refractometer, a differential viscometer, and a light scattering detector both right angle laser light scattering (RALS) and low angle laser light scattering (LALS) in a single instrument with a fixed interdetector system and temperature control that can be regulated up to 80 C. The instrument was calibrated with a pullulan standard of P-50 obtained from Polymer Standard Services USA, Inc. Separations were accomplished using ViscoGel I-MBHMW-3078 columns purchased from Viscotek. An aliquot of 100e200 mL of 2 mg/mL polymer solution in DMSO containing 0.01 M LiBr was injected. The dn/dc values were corrected by injecting different volumes to assess the trend. All data analyses were performed by using Viscotek TriSEC software. 3. Results and discussion 3.1. Chemical structures Polymers I-1I-4 were synthesized by a ring-transmutation polymerization reaction; whereas polymers II-1II-4 were made by a metathesis reaction in DMSO by changing the counter ion from tosylate to triflimide as outlined in Scheme 1. Their chemical structures were confirmed by FTIR, 1H, 19F and 13C NMR spectroscopy and elemental analyses. For example, the FTIR spectra displayed characteristic peaks, among other peaks, for polymer I-1: 3063 (Csp2 H aromatic stretching), 1319 (CF stretching of CF3benzidine moiety), 1195 (CNþ), 1126 (S]O asymmetric stretching), and 1034 (S]O symmetric stretching) as displayed in Fig. S1 and polymer II-1: 3071 (C2spH aromatic stretching), 1350 (CeF of triflimide moiety), 1319 (CF stretching of CF3benzidine moiety), 1188 (CNþ), 1134 (S]O asymmetric stretching), and 1057 (S]O symmetric stretching) as displayed in Fig. S2. The 1H NMR spectra of polymers I-2 and II-2 (Fig. 1) showed unique resonances at d ¼ 8.93 and 8.70 ppm for
Fig. 1. 1H NMR spectra of poly(pyridinium salt)s (delay time ¼ 1 s, number of scans ¼ 16 scans): (a) I-2 (10 mg/mL in d6-DMSO) and (b) II-2 (10 mg/mL in d6-DMSO).
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the protons of the aromatic moieties of poly(pyridinium salt) and a set of resonances at d ¼ 7.44, 7.05 and 2.25 ppm for the protons of the aromatic moiety and methyl group in the tosylate counter ion. Vinylogous signals in both 1H and 13C NMR spectra of polymers I-2 were not observed suggesting that the ring-transmutation polymerization reaction underwent to completion. After exchange of counter ion from tosylate to triflimide, the disappearance of tosylate peaks and appearance of a new signal at 78.7 ppm in 19F NMR spectrum indicated that the metathesis reaction also proceeded to completion. The other polymers in this series, which had different functional groups in their repeating units, showed the expected proton signals in their NMR spectra (Figs. S3, S4, S7, S9, S11 and S13); and the signals of carbon nuclei of polymers I-2, II-2, I-3, and II-3 were well resolved and are consistent with their chemical structures (Figs. S5, S6, S8, and S10). Those signals of polymers I-1 and II-1 could not be obtained even at elevated temperature because of viscous solutions of these polymers in DMSO; but those of polymers I-4 and II-4 were obtained as somewhat less well-resolved when compared with those of other spectra in the series (Figs. S12 and S14). The relatively high viscosity of each of these polymers and the broadness of its proton spectrum were suggestive of the high molecular weight of this polymer. 3.2. Molecular weights characterization Since poly(pyridinium salt)s prepared in this study showed much better solubility in DMSO than other peconjugated polymers, the number-average molecular weight (Mn) and polydispersity index (PDI ¼ Mw/Mn) of the representative polymers were measured using gel permeation chromatography (GPC) technique. As shown in Table 1, some representative polymers showed high number-average molecular weight (Mn) in the range of 38e46 kg/mol. Their PDI values were between 1.13 and 1.43. After the methathesis reaction completed, their Mn and PDI values increased slightly, as expected. These changes implied that the weight and size of counter ion affected their molecular weights. These results enabled us to study their solution, thermal, and optical properties without concerning the effect of molecular weight on these properties. 3.3. Solution properties To process materials into thin films or fibers, solubility of polymers becomes an important issue. Furthermore, highly peconjugated polymers usually have limited solubility in common organic solvents due to their rigid structures or pep stacking interactions [1c]. Even though several of the poly(pyridinium salt)s (I-1, II-1, I-4, and II-4) used in this study are conjugated, they showed good solubility in organic solvents such as DMSO and CH3CN. In contrast, other poly(pyridinium salts) having non-linear moieties like sulfone and hexafluoroisopropylidene moieties in their backbones were not only soluble in aprotic polar solvent but
Table 1 GPC data of poly(pyridinium salt)s. Polymer
IV (dL/g)
Mna
Mwb
Mw/Mnc
dn/dc (ml/g)
ad
Kd
I-1 II-1 I-4 II-4
1.53 1.27 0.55 0.53
44.5 46.3 38.4 44.8
54.5 62.1 43.6 64.2
1.22 1.27 1.13 1.43
0.1100 0.0950 0.1000 0.0900
1.31 1.40 1.14 1.02
1.03 4.68 2.66 7.26
a b c d
Number-average molecular weight in kg/mol. Weight-average molecular weight in kg/mol. Polydispersity index (Mw/Mn). a and K were calculated by using Mark-Houwink equation: [s] ¼ KMa.
106 107 106 106
also in protic polar solvent (Table 2). It has been reported that many poly(pyridinium salt)s exhibited lyotropic LC properties in both aprotic polar solvents and protic polar solvents at various critical concentrations depending on the rigidity of backbones of polymer structures [19]. These results motivated us to study the lyotropic LC properties of new series of poly(pyridinium salt)s containing both rigid and semiflexible moieties in their backbones in several organic solvents. Polymers I-1 and II-1 showed good solubility in DMSO and CH3CN. Polymer I-1 containing tosylate as counter ion formed an isotropic solution in the range of 0e15 wt % and a lyotropic phase at 20 wt % in DMSO. In this solvent, there was no development of biphase (anisotropic and isotropic) for this polymer. However, such lyotropic properties of this polymer were observed in neither acetonitrile nor methanol. Similarly, polymer II-1 containing triflimide as counter ion displayed a lyotropic phase at identical concentration to that of polymer I-1 in DMSO and also did not form a biphasic solution. In contrast, it formed an isotropic solution between 0 and 20 wt %, biphasic solution at 25 wt % and lyotropic solution at 30 wt % in acetonitrile, respectively. The solution properties of polymers I-2 and II-2 are quite different because of the presence of hexafluoroisopropylidene moieties in their backbones. Polymer I-2 exhibited an isotropic solution from 0 to 10 wt %, biphasic solution at 20 wt %, and lyotropic property at 30 wt % in DMSO, respectively. In methanol, it showed only isotropic and biphasic solutions. Polymer II-2 formed an isotropic solution up to 20 wt %, even though it had good solubility in DMSO. On further increasing its concentration in this solvent, a biphasic solution appeared at 40 wt % and retained its biphasic solution up to 60 wt %, but no lyotropic phase was detected in this solvent with polarizing optical microscopy (POM) studies. Similar solution properties were also observed for this polymer in acetonitrile. Unlike I-2, polymer II-2 did not show isotropic and biphasic solutions. In contrast to polymer I-2, polymer I-3 was highly soluble in DMSO and acetonitrile up to 60 wt % in these solvents because of the presence of non-linear sulfone moieties, but exhibited neither biphasic nor lyotropic properties in these solvents. However, in methanol, it exhibited biphasic solutions over a broad range of concentrations (5e40 wt %). Polymer II-3 exhibited an isotropic solution in DMSO and biphasic properties over a broad range of concentrations. Similarly, it formed biphasic solutions in acetonitrile over a broad range of concentrations, but no lyotropic phase. Polymer I-4 displayed good solubility in DMSO, but not in acetonitrile, and its lyotropic property was observed at a relatively low concentration at 20 wt % in this solvent because of rigid aromatic 9,10-diphenylantharcene moieties in the backbone. Contrastingly, polymer II-4 containing triflimide as counterions had improved solubility in acetonitrile, but decreased solubility in DMSO. Consequently, it exhibited a fully-grown lyotropic LC phase at 20 and 10 wt % in these solvents, respectively (Table 2). In general, the formation of the lyotropic phase in a polymer is determined by several key factors such as rod-like structures with an extended chain character to facilitate the alignment of the polymer chain along a particular direction and sufficient solubility to exceed the critical concentration for the formation of LC phase [22]. The solubility and chain stiffness of a polymer are dependent on the microstructure, molecular weight, polymerepolymer and polymeresolvent interactions, and temperature [23]. Therefore, the presence of organic counterions in the poly(pyridinium salt)s might increase these interactions resulting in the increased solubility and the prospect of formation of a lyotropic phase in common organic solvents. However, the flexible moieties such as sulfone and hexafluoroisopropylidene moieties in the repeating units can disrupt the crystallinity of the poly(pyridinium salt)s, and hence an increased solubility in various organic solvents. Simultaneously, these moieties also decreased the chain rigidity of the polymer
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Table 2 Solution properties of poly(pyridinium salt)s in various organic solvents. Polymer
I-1
II-1
I-2
II-2
I-3
II-3
I-4
II-4
DMSO (ε ¼ 48.9)
0e15% I 20% L
0e15% I 20% L
0e30% I 40e60% B
0e60% I
0e20% I 25e60% B
20% L
10% L
CH3CN (ε ¼ 37.5)
e
0e20% I 35e60% B
0e60% I
10e50% B
e
20% L
CH3OH (ε ¼ 32.6)
e
0e20% I 25% B 30% L e
010% Ia 20% Ba 30% La e
e
5e40% B
e
e
e
a
0e10% I 20e50% B
I ¼ Isotropic; B ¼ Biphasic (anisotropic þ isotropic); and L ¼ Lyotropic.
chains thus reducing the propensity for the formation of lyotropic LC phase. Consequently, the formation of a lyotropic phase of the polymers I-1, II-1, I-4 and II-4 in DMSO usually occurred at a relatively low concentration (10e20 wt %); whereas II-2, I-3 and II-3 showed isotropic or biphasic LC phases at high concentration (60 wt %). Similar observations were found in acetonitrile and methanol as well. Some of the representative photomicrographs of lyotropic LC phases of these polymers in DMSO recorded at room temperature are shown in Fig. 2. 3.4. Thermal properties High thermal stability of a polymer is one of the essential properties for its high-operating temperature applications. To enhance the thermal stabilities, highly peconjugated aromatic moieties are introduced into the polymer backbones. However, the
introduction of rigid aromatic structures in the repeating units generally results in the decreased solubility in organic solvents and in the increase of the melting transition of a polymer [2426]. In this study, polymers I-1eI-4 displayed high thermal stability in the range of 326e371 C and polymers II-1eII-4 exhibited excellent thermal stability in the range of 420e477 C (Fig. S16) while showing their good solubility in various organic solvents. Generally, polymers with triflimide had higher thermal stability than that of polymers with tosylate counterions which is consistent with the previously reported results of other poly(pyridinium salt) s [19]. These results can be related to the fact that the thermal stability of sodium tosylate (155 C) is lower than that of lithium triflimide (363 C). Therefore, the stability of counter ion may affect the decomposition behavior of an ionic polymer or an ionic compound significantly. Furthermore, triflimide has much weaker nucleophilicity than the tosylate counter ion and, therefore, it acts
Fig. 2. Photomicrographs of (a) polymer I-1 at 20 wt % in DMSO, (b) polymer II-1 at 30 wt % in DMSO, (c) polymer I-2 at 20 wt % in DMSO, (d) polymer II-2 at 50 wt % in DMSO, (e) polymer I-4 at 20 wt % in DMSO, and (f) polymer II-4 at 20 wt % in DMSO under crossed polarizers recorded at room temperature exhibiting lyotropic LC phase, respectively (magnification 400).
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3.5. Optical properties
Table 3 Optical properties of poly(pyridinium salt)s. Polymer
I-1
II-1
I-2
II-2
I-3
II-3
I-4
II-4
UV labs (nm) Band gap (eV) PL lem DMSO (nm) PL lem CH3CN (nm) PL lem CH3OH (nm) PL lem CHCl3 (nm) PL lem THF (nm) PL lem film (nm)
345 3.06 385 e 413 e e 390b
345 3.08 439 443 e e e 388c
340 3.12 523 448 487 447 438 455b
339 3.12 527 496 e 437 420 452c
339 3.10 403 444 414 450 430 455b
339 3.10 381 444 389 e 378 e
338a 2.84 536 485 494 412 445 435b
338a 2.92 531 462 478 416 450 562c
a Shoulder peaks from 9,10-diphenylanthracene moiety in various solvents are provided in Table S1. b Thin film cast from methanol. c Thin film cast from acetonitrile.
as a nucleophile causing decomposition of the main chain of the polymer at high temperatures [27]. The DSC thermograms, obtained from all of these polymers, could not provide any meaningful information with regard to their glass transition temperatures and melting transitions. However, polymers containing non-linear moieties and tosylate as counterions formed viscous birefringence melts at ca. 350 C and those with triflimide formed viscous birefringence melts at slightly lower temperatures of 320 C as verified by using Fisher Jones melting point apparatus and POM study.
Because of the presence of organic counterions in combination with phenylated pyridinium moieties and rigid/semiflexible aromatic diamine moieties, these ionic polymers showed good solubility in common organic solvents. These properties allowed us to measure their optical properties in solutions and in thin-film states cast from various organic solvents by UVeVis and photoluminescent spectroscopy (PL). Each of the polymers I-1eI-3 and II-1eII-3 showed essentially an identical lmax in the narrow range of 338e345 nm (Table 3) as detected in their absorption spectra recorded in DMSO, acetonitrile, methanol, chloroform, and tetrahydrofuran (THF) (not shown). These results could be an indication of closely spaced pep* transitions common to various aromatic moieties suggesting that their absorption maxima were less sensitive to the polarity of solvents examined. In other words, the interactions of various organic solvents with the backbones of polymers did not cause any changes in the energies of their ground states. However, each of the polymers I-4 and II-4 in various organic solvents showed not only a major labs at 338 nm, like other polymers in the series, but also contained shoulder peaks. The shoulder peaks are related to the presence of 9,10diphenylanthracene moieties. All the shoulder peaks in various organic solvents are collected from their absorption spectra and provided in Table S1. The optical band gaps (Eg) of these polymers as determined from the onset of wavelength (low energy region) in
Fig. 3. Emission spectra of (a) polymer I-1 in methanol, (b) polymer II-1 in acetonitrile, (c) polymer I-2 in methanol, and (d) polymer II-2 in acetonitrile at various excitation wavelengths.
T.S. Jo et al. / Polymer 53 (2012) 1063e1071
each of the UVeVis absorption spectra were summarized in Table 3. These band gaps were higher than those of peconjugated lightemitting polymers [1b,c], but similar to those of previously reported poly(pyridinium salt)s [20]. The lem peaks for all of these ionic polymers in various organic solvents of varying polarities are compiled in Table 3. The diverse structural parameters including the presence and absence of the trifluoromethyl groups in the main chain and in the side chain, and interactions of solvent molecules with the polymer chains (both excited and ground states) make them quite difficult to establish a clear trend in their emission spectra. However, Fig. 3 showed the PL spectra of polymers I-1 and I-2 in methanol and II-1 and II-2 acetonitrile. Despite the presence of 2,20 -trifluoromethyl-benzidine moiety present in polymer I-1 it showed a hypsochromic shift at lem peak at 413 nm when compared with that (487 nm) of polymer I-2 in methanol (Fig. 3). Similarly, polymer II-1 also showed a hypsochromic lem peak when compared with that of polymer II-2 in acetonitrile (Fig. 3). These results suggested that the presence of trifluoromethyl groups in 2- and 20 -positions of biphenylene moieties caused the twisting of the two phenylene rings of diamine moiety resulting in the disruption of conjugation lengths. The large bathochromic shifts of polymers I-4 and II-4 when compared with those of polymers I-3 and II-3 in acetonitrile (Fig. 4) were related to the presence of 9,10-diphenylanthracence which caused an increased conjugation length of the repeating units. Additionally, the sulfone moieties in polymers I-3 and II-3 a non-linear (kinked) group caused both the break down of the conjugation of the repeating units significantly and prevented the extensive pep stacking phenomena of aromatic moieties resulting in
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a hypsochromic effect. These results are in excellent agreement with those of other poly(pyridinium salt)s having the bent structures in the main chain [27]. In contrast, polymers I-4 and II-4 clearly exhibited a bathochromic shift in both methanol and acetonitrile when compared with those of polymers I-3 and II-3. These results suggested that the extensive pep interactions in polymers I-4 and II-4 due to the peconjugated aromatic structures from 9,10-diphenylanthracene moieties caused a decreased in energy of the anti-bonding p* orbital that was conducive for the lowering HOMOLUMO energy gap. These interactions account for the red shift in the light emission of these polymers than those of polymers I-3 and II-3 that lack of these pep interactions. Furthermore, polymers I-4 and II-4 showed the highest bathochromic shifts in their lem peaks in DMSO when compared with those in other solvents, which is probably related to its highest ε ¼ 48.9 among the solvents examined (Figures S17 and S18). These two ionic polymers exhibited blue and green light when excited over a broad range of excitation wavelengths used depending on the polarity of organic solvents (Table 3). Note here that poly(pyridinium salts) of identical counterions tosylate and triflimide containing 2,6-anthracene moieties emitted UV light both in organic solvents and in solid states [20b]. To our knowledge, these are the first examples of ionic polymers containing 9,10diphenylanthracene moieties in the main chain exhibited blue and green light. In the literature, two other peconjugated polymers containing diphenylanthracene vinylne biphenylene and diphenylanthracene vinylne terphenylene in the main chain and fluorene pendant groups exhibited blue light both in solution and in solid states [28]. The lem peaks of I-2eI-4 were in the range of
Fig. 4. Emission spectra of (a) polymer I-3, (b) polymer II-3, (c) polymer I-4, and (d) polymer II-4 in acetonitrile at various excitation wavelengths.
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444e485 nm (blue) in acetonitrile and those of polymers II-1II-4 also in this solvent were in the range of 443e496 nm (blue) (Table 3). To understand the influence of solvent polarity in their lem peaks, the data from PL spectra in various organic solvents were compiled in Table 3. These data suggested that a positive solvatochromism phenomenon was observed in several cases with some exceptions, that is, the lem peaks were shifted to the longer wavelengths with the increasing in solvent polarity. Full-width at half-maximum (fwhm) values of PL spectra were greater than 100 nm suggesting that their light-emission stemmed from a number of chromophoric species. They emitted UV, blue and green light in various organic solvents depending on the polymer microstructures, on the counterions associated with them and polarity of solvents. To evaluate the potential application for optoelectronic devices, we also examined their light-emitting properties in the solid state. Fig. 5a shows the PL spectra of thin films of polymers I-2eI-4 cast from methanol; and Fig. 5b shows the PL spectra of thin films of polymers II-2 and II-4 cast from acetonitrile. Tosylate polymers exhibited lem peaks at 390 (UV), 455, 455 and 435 nm (blue) at various excitation wavelengths; and triflimide polymers exhibited
lem peaks at 388 (UV), 452 (blue), and 562 nm (green) at various excitation wavelengths. The overall shapes of the fluorescence spectra in their solid states were quite different and were not comparable to those in solution spectra except polymer II-4. Some polymers exhibited hypsochromic shifts in their light emission when compared with those of their solution spectra (Table 3) because of the less-ordered structures in the solid state. These results are consistent with other poly(pyridinium salt) reported earlier [28]. The emission spectrum of polymer II-4 displayed a large red-shift emission when compared with its solution spectrum in acetonitrile as shown in Fig. 4d. This bathochromic shift may be attributed to the more ordered structures in the solid state of this polymer via strong pep interactions when the film was cast from acetonitrile. However, they emitted UV, blue, and green light in the solid states depending on the microstructures of polymer backbones, on the counterions and on the nature of solvents used for the preparation of films. 4. Conclusions In summary, novel poly(pyridinium salt)s with organic counterions were prepared by using ring-transmutation and metatheses reactions. Their chemical structures were characterized by spectroscopic techniques. Their weight-average molecular weights (Mw) were in the range of 44e64 kg/mol and polydispersities in the range of 1.13e1.43 as determined by gel permeation chromatography. They exhibited excellent thermal stabilities in the range of 327e477 C as determined by thermogravimetric analysis. Several of these polymers exhibited lyotropic liquid-crystalline phases in organic solvents above their critical concentrations depending on their microstructures of the polymer backbones and on the counterions. The analyses of their photoluminescent spectra revealed that some of the polymers emitted UV light (<400 nm), some emitted blue light (430e480 nm) and some emitted green light (480e560 nm) both in solutions and solid states. Currently, we are exploring them as a dispersing aid for the dispersion of singlewalled carbon nanotubes via non-covalent interactions [29]. Acknowledgments P.K.B. acknowledges the University of Nevada Las Vegas (UNLV) for New Investigation Award (NIA), Planning Initiative Award (PIA), and Applied Research Initiative (ARI) grants, and the donors of the Petroleum Research Fund (PRF# 35903-B7), administered by the American Chemical Society, and an award (CCSA# CC5589) from Research Corporation for the support of this research. The work is in part supported by the NSF under Grant No. 0447416 (NSF EPSCoR RING-TRUE III), NSF-Small Business Innovation Research (SBIR) Award (Grant OII-0610753), NSF-STTR Phase I Grant No. IIP0740289, and NASA GRC Contract No. NNX10CD25P. T.S.J. acknowledges the Graduate College (UNLV) for providing him a financial support in the form of SPGRA for the academic year 2010e2011. We thank the anonymous reviewers for their constructive comments on the contents of this article. Appendix. Supplementary information Supplementary data related to this article can be found online at doi:10.1016/j.polymer.2012.01.017. References
Fig. 5. Emission spectra of (a) polymers I-2eI-4 in thin films cast from methanol, and (b) polymer II-2 and II-4 in thin films cast from acetonitrile at various excitation wavelengths.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of poly(pyridinium salt)s derived from various aromatic diaminesq Tae Soo Jo, Alexi K. Nedeltchev, Bidyut Biswas, Haesook Han, Pradip K. Bhowmik* Department of Chemistry, University of Nevada at Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, NV 89154, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2011 Received in revised form 28 December 2011 Accepted 11 January 2012 Available online 17 January 2012
Several poly(pyridinium salt)s containing various aromatic diamine moieties and tosylate counterions were prepared by the ring-transmutation polymerization reaction of bis(pyrylium tosylate) with aromatic diamines in dimethyl sulfoxide at 130135 C for 48 h and their tosylate counterions were exchanged to triflimide polymers by a metatheses reaction in an organic solvent. Their chemical structures were established by using various spectroscopic techniques. Their number-average molecular weights (Mn) were in the range of 38e46 kg/mol and polydispersities in the range of 1.13e1.43 as determined by gel permeation chromatography. They showed excellent thermal stabilities in nitrogen in the range of 326 e477 C. They exhibited lyotropic liquid-crystalline phase in polar aprotic and protic organic solvents above their critical concentrations depending on their microstructures and counterions. Their optical properties were examined by using UVeVis and photoluminescent spectroscopy, which revealed that some polymers emitted UV light, some emitted blue light, and some emitted green light (both in solutions and solid states) depending on their microstructures, counterions, and on solvent polarity of organic solvents. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Liquid-crystalline polymers Gel permeation chromatography (GPC) UVeVis spectroscopy
1. Introduction The pconjugated polymers have emerged as potential candidates for many optical devices. Due to their straightforward preparation methods, unique properties, and stability in air, these conducting polymers have been applied to energy storage, memory devices, chemical sensors, electrocatalyst, organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaic cells (OPVs) [1]. Many classes of conjugated polymers have been developed such as poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(terthiophene)s, poly(aniline)s, poly(fluorine)s, poly(3-alkylthiophene)s, polytetrathiafulvalenes, polynaphthalenes, poly(p-phenylene sulfide), and poly(p-phenylene vinylene)s over the past decade [2,3]. The pconjugated polymers with electrondonating and electron-withdrawing moieties are currently of interest because the built-in intramolecular charge transfer can facilitate easy manipulation of their electronic structures (HOMO/ LUMO levels) [46], leading to small band gap semiconducting polymers or materials with enhanced third-order non-linear optical properties [7]. Through the design and synthesis of new conjugated polymers can extend to systems with efficient photo-induced charge q This article is dedicated to the memory of Professor David W. Emerson who passed away on January 12, 2012. * Corresponding author. Tel.: þ1 702 895 0885; fax: þ1 702 895 4072. E-mail address: [email protected] (P.K. Bhowmik). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.017
transfer and separation for photovoltaic devices [8] and to bipolar charge transport materials for light-emitting diodes [9], lasers [10], and other applications [11]. Among many polymers, nitrogencontaining conjugated materials have been considered as a suitable candidate for electron transporting layers (ETLs) in OLEDs. Because of their electron-deficient heterocycles, transporting electrons from the cathode through the ETL materials can be more efficient [12]. Furthermore, their light-emitting properties could be easily tuned by protonating nitrogen heterocycles. One of the well known examples is polyaniline (PANI). PANI has been drawn much attention for its excellent electronic, optical, redox properties, and environmental stability [13]. It has been applied to biosensors, electrochemical displays, rechargeable batteries, liquid crystal devices, and separation membranes [14]. Even though it demonstrated good thermal stabilities and high thermal transitions, it had very poor solubility in common organic solvents that lead to difficulties in the fabrication or processing. Therefore, to increase the solubility of the rigid pconjugated polymers, flexible long alkyl chains are generally introduced into polymer backbones. The modification can occur via copolymerization with a functionalized comonomer or postfunctionalization of the parent polymer. However, functionalization of the comonomer during the polymerization step generally resulted in low-molecular-weight polymers [15]. In the latter approach, several chemical modifications have been developed [16], but they usually resulted in the alternation of the original molecular weight. Another method of increased solubility of a polymer is to introduce an
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ionic charge along the backbone of the polymer chain. Introduction of organic counterions such as tosylate or triflimide into polymer backbone can also increase the solubility in common organic solvents because of their reduced electrostatic interaction between polymer and counterions. Recently, viologen polymers and poly(pyridinium salt)s have emerged as a new class of main-chain ionic polymers due to their high solubility in common organic solvents. Although earlier poly(pyridinium salt)s had very low molecular weights, limited solubility, and low thermal stability [17], their properties were significantly improved by preparing them via ring-transmutation polymerization reaction in dimethyl sulfoxide (DMSO) and by introducing of different size of organic counterions [18,19]. These recent development of ionic polymers is of great significance for the preparation of novel electrondeficient pconjugated polymers. In this article, we report the synthesis of both wholly aromatic and semiflexible poly(pyridinium salt)s containing organic counterions from various aromatic diamines via the ring-transmutation polymerization and metathesis reactions as shown in Scheme 1. Our objective was to directly study the influence of the aromatic diamine structural moieties on the solution, thermal, and optical properties of this class of ionic polymers. Also unlike the previous studies of poly(pyridinium salt)s, the current approach enables us to synthesize both rigid and semiflexible ionic polymers with high molecular weights and good solubility in common organic solvents. The effect of aromatic diamine structures and counterions on their solution, thermal and optical properties was examined by using various experimental techniques. 2. Experimental section 2.1. Materials The 2,20 -Bis(trifluoromethyl)benzidine was received as a gift from Polaroid Corporation and it was then purified by recrystallization
from ethanol/water mixture before use. The 2,20 -Bis(4-aminophenyl) hexafluoropropane and 4-aminophenyl sulfone were purchased from SigmaeAldrich and TCI America and used without further purification. Lithium triflimide and common organic solvents were purchased from commercial vendors (SigmaeAldrich, Alfa-Aesar, Acros Organics, and TCI America) and used without any further purification. 2.2. Monomer synthesis 2.2.1. Synthesis of monomer, M The 4,40 -(1,4-phenylene)bis(2,6-diphenylpyrylium p-toluene sulfonates), M, was synthesized according to the reported procedure [19,20]. 2.2.2. Synthesis of 9,10-bis(4-aminophenyl)anthracene The 9,10-bis(4-aminophenyl)anthracene was synthesized according to the reported procedure [21]. 2.3. Polymer synthesis 2.3.1. Synthesis of Polymer I-1I-4 The bis(pyrylium) salt, M, was reacted with an appropriate diamine by a ring-transmutation reaction to yield each of the target polymers. Equal mole ratio of monomers was placed in threenecked round-bottomed flask equipped with a magnetic stirrer in DMSO. A small amount of toluene was added to remove the water generated during the reaction by azeotrope, which was facilitated by using DeaneStark trap and water condenser. The temperature was gradually increased up to 130135 C and the solution was stirred for 48 h under nitrogen atmosphere. After the end of polymerization reaction, the solution was cooled to room temperature. Excess DMSO was removed by a rotary evaporator until the solution became viscous. It was then poured into water to
Scheme 1. Synthesis of poly(pyridinium salt)s.
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precipitate the polymer. The precipitated solid was washed with water several times more to remove any residual impurities. The process of dissolution in MeOH or DMSO and precipitation with H2O was repeated one more time. The collected polymer was additionally washed a few times with boiling water and it was dried in vacuo at 110 C for 72 h. Data for polymer I-1: Anal. Cacld for C68H48N2O6F6S2 (1167.24): C, 69.97; H, 4.14; N, 2.40; S, 5.49. Found: C, 68.23; H, 4.15; N, 2.44; S, 5.26; for polymer I-2: Anal. Cacld for C69H50N2O6F6S2 (1181.27): C, 70.16; H, 4.27; N, 2.37; S, 5.43. Found: C, 68.89; H, 4.27; N, 2.43; S, 4.94; for polymer I-3: Anal. Cacld for C66H50N2O8S3 (1095.31): C, 72.37; H, 4.60; N, 2.56 S, 8.78. Found: C, 71.44; H, 4.15; N, 2.79; S, 8.34; for polymer I-4: Anal. Cacld for C80H58N2O6S2 (1207.46): C, 79.58; H, 4.84; N, 2.32; S, 5.31. Found: C, 78.05; H, 4.83; N, 2.29; S, 5.07. 2.3.2. Synthesis of polymers II-1II-4 Polymers II-1II-4 were prepared by a metathesis reaction from the respective tosylate polymer with excess lithium triflimide salt in DMSO at 50 C for 48 h. At the end of the metathesis reaction, excess DMSO solvent was reduced by a rotary evaporator to form a viscous solution, and it was poured into water to precipitate the polymer. The reaction step was repeated once or twice more until all the tosylate counterions were completely exchanged to triflimide counterions, which was confirmed by 1H NMR spectrum. Collected polymer was washed with boiling water a few times more to remove any entrapped DMSO or organic salts. The washed polymers were dried in vacuo at 110 C for 72 h. Data for polymer II-1: Anal. Cacld for C58H34N4O8F18S4 (1385.14): C, 50.29; H, 2.47; N, 4.04; S, 9.26. Found: C, 50.61; H, 2.86; N, 4.17; S, 8.88; for polymer II-2: Anal. Cacld for C59H36N4O8F18S4 (1399.18): C, 50.65; H, 2.59; N, 4.00; S, 9.15. Found: C, 51.00; H, 2.68; N, 4.08; S, 9.50; for polymer II-3: C56H36N4O10F12S5 (1313.21): Anal. Cacld for C, 51.22; H, 2.76; N, 4.27; S, 12.21. Found: C, 51.60; H, 2.80; N, 4.43; S, 12.61; for polymer II-4: Anal. Cacld for C70H44N4O8F12S4 (1425.37): C, 58.99; H, 3.11; N, 3.93; S, 9.00. Found: C, 58.82; H, 3.50; N, 3.98; S, 8.66. 2.4. Polymer characterization The FTIR spectra were recorded with a Shimadzu infrared spectrometer. Polymer samples were prepared by coating NaCl plates with various polymers and subsequently vacuum dried at 70 C overnight. The 1H, 19F and 13C NMR spectra were obtained using a Varian NMR spectrometer (400 MHz for 1H, 376 MHz for 19F and 100 MHz for 13C) with three RF channels at room temperature, and chemical shifts were referenced to tetramethylsilane (TMS). The NMR samples were prepared by applying gentle heating to dissolve the polymer in d6-DMSO. The 1H and 13C NMR samples were usually prepared at a concentration of 10 and 30 mg/mL, respectively. Differential scanning calorimetry (DSC) measurements of polymers were conducted on TA module DSC Q200 series in nitrogen at heating and cooling rates of 10 C/min. The temperature axis of the DSC thermograms was calibrated before using the reference standard of high purity indium and tin. Thermogravimetric analyses (TGA) of polymers were performed using a TGA Q50 instrument in nitrogen. The TGA data were collected at temperatures between 30 and 600 C at a heating rate of 10 C/min. Dilute solution for UVeVis and photoluminescence spectroscopy study was prepared by dissolving 0.5 mg of the dry polymer in 5 mL of the organic solvent at room temperature. 50 mL of the resulting solution was transferred into a volumetric flask and further diluted with 5 mL of the solvents to adjust the optical density. The UVeVis absorption spectra of polymer solutions in organic solvents were recorded at room temperature using Varian Cary 50 Bio UVeVisible spectrophotometer in quartz cuvettes. Photoluminescence spectra in solutions and thin films were recorded with a PerkineElmer LS 55 luminescence spectrometer
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with a xenon lamp light source. Lyotropic liquid crystal (LC) properties of the polymers were obtained using a polarized optical microscopy (POM, Nikon, Model Labophot 2) equipped with crossed polarizers. Samples of polymers for lyotropic LC properties were made by dissolving known amounts of polymer into known amounts of organic solvents (DMSO, MeOH or CH3CN). To assess the molecular weight of polymer, gel permeation chromatography (GPC) was run at 50 C with a flow rate of 1 mL/min. The GPC instrument had Water 515 pump simultaneously with a Viscotek Model 301 Triple Detector Array. The array contained a laser refractometer, a differential viscometer, and a light scattering detector both right angle laser light scattering (RALS) and low angle laser light scattering (LALS) in a single instrument with a fixed interdetector system and temperature control that can be regulated up to 80 C. The instrument was calibrated with a pullulan standard of P-50 obtained from Polymer Standard Services USA, Inc. Separations were accomplished using ViscoGel I-MBHMW-3078 columns purchased from Viscotek. An aliquot of 100e200 mL of 2 mg/mL polymer solution in DMSO containing 0.01 M LiBr was injected. The dn/dc values were corrected by injecting different volumes to assess the trend. All data analyses were performed by using Viscotek TriSEC software. 3. Results and discussion 3.1. Chemical structures Polymers I-1I-4 were synthesized by a ring-transmutation polymerization reaction; whereas polymers II-1II-4 were made by a metathesis reaction in DMSO by changing the counter ion from tosylate to triflimide as outlined in Scheme 1. Their chemical structures were confirmed by FTIR, 1H, 19F and 13C NMR spectroscopy and elemental analyses. For example, the FTIR spectra displayed characteristic peaks, among other peaks, for polymer I-1: 3063 (Csp2 H aromatic stretching), 1319 (CF stretching of CF3benzidine moiety), 1195 (CNþ), 1126 (S]O asymmetric stretching), and 1034 (S]O symmetric stretching) as displayed in Fig. S1 and polymer II-1: 3071 (C2spH aromatic stretching), 1350 (CeF of triflimide moiety), 1319 (CF stretching of CF3benzidine moiety), 1188 (CNþ), 1134 (S]O asymmetric stretching), and 1057 (S]O symmetric stretching) as displayed in Fig. S2. The 1H NMR spectra of polymers I-2 and II-2 (Fig. 1) showed unique resonances at d ¼ 8.93 and 8.70 ppm for
Fig. 1. 1H NMR spectra of poly(pyridinium salt)s (delay time ¼ 1 s, number of scans ¼ 16 scans): (a) I-2 (10 mg/mL in d6-DMSO) and (b) II-2 (10 mg/mL in d6-DMSO).
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the protons of the aromatic moieties of poly(pyridinium salt) and a set of resonances at d ¼ 7.44, 7.05 and 2.25 ppm for the protons of the aromatic moiety and methyl group in the tosylate counter ion. Vinylogous signals in both 1H and 13C NMR spectra of polymers I-2 were not observed suggesting that the ring-transmutation polymerization reaction underwent to completion. After exchange of counter ion from tosylate to triflimide, the disappearance of tosylate peaks and appearance of a new signal at 78.7 ppm in 19F NMR spectrum indicated that the metathesis reaction also proceeded to completion. The other polymers in this series, which had different functional groups in their repeating units, showed the expected proton signals in their NMR spectra (Figs. S3, S4, S7, S9, S11 and S13); and the signals of carbon nuclei of polymers I-2, II-2, I-3, and II-3 were well resolved and are consistent with their chemical structures (Figs. S5, S6, S8, and S10). Those signals of polymers I-1 and II-1 could not be obtained even at elevated temperature because of viscous solutions of these polymers in DMSO; but those of polymers I-4 and II-4 were obtained as somewhat less well-resolved when compared with those of other spectra in the series (Figs. S12 and S14). The relatively high viscosity of each of these polymers and the broadness of its proton spectrum were suggestive of the high molecular weight of this polymer. 3.2. Molecular weights characterization Since poly(pyridinium salt)s prepared in this study showed much better solubility in DMSO than other peconjugated polymers, the number-average molecular weight (Mn) and polydispersity index (PDI ¼ Mw/Mn) of the representative polymers were measured using gel permeation chromatography (GPC) technique. As shown in Table 1, some representative polymers showed high number-average molecular weight (Mn) in the range of 38e46 kg/mol. Their PDI values were between 1.13 and 1.43. After the methathesis reaction completed, their Mn and PDI values increased slightly, as expected. These changes implied that the weight and size of counter ion affected their molecular weights. These results enabled us to study their solution, thermal, and optical properties without concerning the effect of molecular weight on these properties. 3.3. Solution properties To process materials into thin films or fibers, solubility of polymers becomes an important issue. Furthermore, highly peconjugated polymers usually have limited solubility in common organic solvents due to their rigid structures or pep stacking interactions [1c]. Even though several of the poly(pyridinium salt)s (I-1, II-1, I-4, and II-4) used in this study are conjugated, they showed good solubility in organic solvents such as DMSO and CH3CN. In contrast, other poly(pyridinium salts) having non-linear moieties like sulfone and hexafluoroisopropylidene moieties in their backbones were not only soluble in aprotic polar solvent but
Table 1 GPC data of poly(pyridinium salt)s. Polymer
IV (dL/g)
Mna
Mwb
Mw/Mnc
dn/dc (ml/g)
ad
Kd
I-1 II-1 I-4 II-4
1.53 1.27 0.55 0.53
44.5 46.3 38.4 44.8
54.5 62.1 43.6 64.2
1.22 1.27 1.13 1.43
0.1100 0.0950 0.1000 0.0900
1.31 1.40 1.14 1.02
1.03 4.68 2.66 7.26
a b c d
Number-average molecular weight in kg/mol. Weight-average molecular weight in kg/mol. Polydispersity index (Mw/Mn). a and K were calculated by using Mark-Houwink equation: [s] ¼ KMa.
106 107 106 106
also in protic polar solvent (Table 2). It has been reported that many poly(pyridinium salt)s exhibited lyotropic LC properties in both aprotic polar solvents and protic polar solvents at various critical concentrations depending on the rigidity of backbones of polymer structures [19]. These results motivated us to study the lyotropic LC properties of new series of poly(pyridinium salt)s containing both rigid and semiflexible moieties in their backbones in several organic solvents. Polymers I-1 and II-1 showed good solubility in DMSO and CH3CN. Polymer I-1 containing tosylate as counter ion formed an isotropic solution in the range of 0e15 wt % and a lyotropic phase at 20 wt % in DMSO. In this solvent, there was no development of biphase (anisotropic and isotropic) for this polymer. However, such lyotropic properties of this polymer were observed in neither acetonitrile nor methanol. Similarly, polymer II-1 containing triflimide as counter ion displayed a lyotropic phase at identical concentration to that of polymer I-1 in DMSO and also did not form a biphasic solution. In contrast, it formed an isotropic solution between 0 and 20 wt %, biphasic solution at 25 wt % and lyotropic solution at 30 wt % in acetonitrile, respectively. The solution properties of polymers I-2 and II-2 are quite different because of the presence of hexafluoroisopropylidene moieties in their backbones. Polymer I-2 exhibited an isotropic solution from 0 to 10 wt %, biphasic solution at 20 wt %, and lyotropic property at 30 wt % in DMSO, respectively. In methanol, it showed only isotropic and biphasic solutions. Polymer II-2 formed an isotropic solution up to 20 wt %, even though it had good solubility in DMSO. On further increasing its concentration in this solvent, a biphasic solution appeared at 40 wt % and retained its biphasic solution up to 60 wt %, but no lyotropic phase was detected in this solvent with polarizing optical microscopy (POM) studies. Similar solution properties were also observed for this polymer in acetonitrile. Unlike I-2, polymer II-2 did not show isotropic and biphasic solutions. In contrast to polymer I-2, polymer I-3 was highly soluble in DMSO and acetonitrile up to 60 wt % in these solvents because of the presence of non-linear sulfone moieties, but exhibited neither biphasic nor lyotropic properties in these solvents. However, in methanol, it exhibited biphasic solutions over a broad range of concentrations (5e40 wt %). Polymer II-3 exhibited an isotropic solution in DMSO and biphasic properties over a broad range of concentrations. Similarly, it formed biphasic solutions in acetonitrile over a broad range of concentrations, but no lyotropic phase. Polymer I-4 displayed good solubility in DMSO, but not in acetonitrile, and its lyotropic property was observed at a relatively low concentration at 20 wt % in this solvent because of rigid aromatic 9,10-diphenylantharcene moieties in the backbone. Contrastingly, polymer II-4 containing triflimide as counterions had improved solubility in acetonitrile, but decreased solubility in DMSO. Consequently, it exhibited a fully-grown lyotropic LC phase at 20 and 10 wt % in these solvents, respectively (Table 2). In general, the formation of the lyotropic phase in a polymer is determined by several key factors such as rod-like structures with an extended chain character to facilitate the alignment of the polymer chain along a particular direction and sufficient solubility to exceed the critical concentration for the formation of LC phase [22]. The solubility and chain stiffness of a polymer are dependent on the microstructure, molecular weight, polymerepolymer and polymeresolvent interactions, and temperature [23]. Therefore, the presence of organic counterions in the poly(pyridinium salt)s might increase these interactions resulting in the increased solubility and the prospect of formation of a lyotropic phase in common organic solvents. However, the flexible moieties such as sulfone and hexafluoroisopropylidene moieties in the repeating units can disrupt the crystallinity of the poly(pyridinium salt)s, and hence an increased solubility in various organic solvents. Simultaneously, these moieties also decreased the chain rigidity of the polymer
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Table 2 Solution properties of poly(pyridinium salt)s in various organic solvents. Polymer
I-1
II-1
I-2
II-2
I-3
II-3
I-4
II-4
DMSO (ε ¼ 48.9)
0e15% I 20% L
0e15% I 20% L
0e30% I 40e60% B
0e60% I
0e20% I 25e60% B
20% L
10% L
CH3CN (ε ¼ 37.5)
e
0e20% I 35e60% B
0e60% I
10e50% B
e
20% L
CH3OH (ε ¼ 32.6)
e
0e20% I 25% B 30% L e
010% Ia 20% Ba 30% La e
e
5e40% B
e
e
e
a
0e10% I 20e50% B
I ¼ Isotropic; B ¼ Biphasic (anisotropic þ isotropic); and L ¼ Lyotropic.
chains thus reducing the propensity for the formation of lyotropic LC phase. Consequently, the formation of a lyotropic phase of the polymers I-1, II-1, I-4 and II-4 in DMSO usually occurred at a relatively low concentration (10e20 wt %); whereas II-2, I-3 and II-3 showed isotropic or biphasic LC phases at high concentration (60 wt %). Similar observations were found in acetonitrile and methanol as well. Some of the representative photomicrographs of lyotropic LC phases of these polymers in DMSO recorded at room temperature are shown in Fig. 2. 3.4. Thermal properties High thermal stability of a polymer is one of the essential properties for its high-operating temperature applications. To enhance the thermal stabilities, highly peconjugated aromatic moieties are introduced into the polymer backbones. However, the
introduction of rigid aromatic structures in the repeating units generally results in the decreased solubility in organic solvents and in the increase of the melting transition of a polymer [2426]. In this study, polymers I-1eI-4 displayed high thermal stability in the range of 326e371 C and polymers II-1eII-4 exhibited excellent thermal stability in the range of 420e477 C (Fig. S16) while showing their good solubility in various organic solvents. Generally, polymers with triflimide had higher thermal stability than that of polymers with tosylate counterions which is consistent with the previously reported results of other poly(pyridinium salt) s [19]. These results can be related to the fact that the thermal stability of sodium tosylate (155 C) is lower than that of lithium triflimide (363 C). Therefore, the stability of counter ion may affect the decomposition behavior of an ionic polymer or an ionic compound significantly. Furthermore, triflimide has much weaker nucleophilicity than the tosylate counter ion and, therefore, it acts
Fig. 2. Photomicrographs of (a) polymer I-1 at 20 wt % in DMSO, (b) polymer II-1 at 30 wt % in DMSO, (c) polymer I-2 at 20 wt % in DMSO, (d) polymer II-2 at 50 wt % in DMSO, (e) polymer I-4 at 20 wt % in DMSO, and (f) polymer II-4 at 20 wt % in DMSO under crossed polarizers recorded at room temperature exhibiting lyotropic LC phase, respectively (magnification 400).
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3.5. Optical properties
Table 3 Optical properties of poly(pyridinium salt)s. Polymer
I-1
II-1
I-2
II-2
I-3
II-3
I-4
II-4
UV labs (nm) Band gap (eV) PL lem DMSO (nm) PL lem CH3CN (nm) PL lem CH3OH (nm) PL lem CHCl3 (nm) PL lem THF (nm) PL lem film (nm)
345 3.06 385 e 413 e e 390b
345 3.08 439 443 e e e 388c
340 3.12 523 448 487 447 438 455b
339 3.12 527 496 e 437 420 452c
339 3.10 403 444 414 450 430 455b
339 3.10 381 444 389 e 378 e
338a 2.84 536 485 494 412 445 435b
338a 2.92 531 462 478 416 450 562c
a Shoulder peaks from 9,10-diphenylanthracene moiety in various solvents are provided in Table S1. b Thin film cast from methanol. c Thin film cast from acetonitrile.
as a nucleophile causing decomposition of the main chain of the polymer at high temperatures [27]. The DSC thermograms, obtained from all of these polymers, could not provide any meaningful information with regard to their glass transition temperatures and melting transitions. However, polymers containing non-linear moieties and tosylate as counterions formed viscous birefringence melts at ca. 350 C and those with triflimide formed viscous birefringence melts at slightly lower temperatures of 320 C as verified by using Fisher Jones melting point apparatus and POM study.
Because of the presence of organic counterions in combination with phenylated pyridinium moieties and rigid/semiflexible aromatic diamine moieties, these ionic polymers showed good solubility in common organic solvents. These properties allowed us to measure their optical properties in solutions and in thin-film states cast from various organic solvents by UVeVis and photoluminescent spectroscopy (PL). Each of the polymers I-1eI-3 and II-1eII-3 showed essentially an identical lmax in the narrow range of 338e345 nm (Table 3) as detected in their absorption spectra recorded in DMSO, acetonitrile, methanol, chloroform, and tetrahydrofuran (THF) (not shown). These results could be an indication of closely spaced pep* transitions common to various aromatic moieties suggesting that their absorption maxima were less sensitive to the polarity of solvents examined. In other words, the interactions of various organic solvents with the backbones of polymers did not cause any changes in the energies of their ground states. However, each of the polymers I-4 and II-4 in various organic solvents showed not only a major labs at 338 nm, like other polymers in the series, but also contained shoulder peaks. The shoulder peaks are related to the presence of 9,10diphenylanthracene moieties. All the shoulder peaks in various organic solvents are collected from their absorption spectra and provided in Table S1. The optical band gaps (Eg) of these polymers as determined from the onset of wavelength (low energy region) in
Fig. 3. Emission spectra of (a) polymer I-1 in methanol, (b) polymer II-1 in acetonitrile, (c) polymer I-2 in methanol, and (d) polymer II-2 in acetonitrile at various excitation wavelengths.
T.S. Jo et al. / Polymer 53 (2012) 1063e1071
each of the UVeVis absorption spectra were summarized in Table 3. These band gaps were higher than those of peconjugated lightemitting polymers [1b,c], but similar to those of previously reported poly(pyridinium salt)s [20]. The lem peaks for all of these ionic polymers in various organic solvents of varying polarities are compiled in Table 3. The diverse structural parameters including the presence and absence of the trifluoromethyl groups in the main chain and in the side chain, and interactions of solvent molecules with the polymer chains (both excited and ground states) make them quite difficult to establish a clear trend in their emission spectra. However, Fig. 3 showed the PL spectra of polymers I-1 and I-2 in methanol and II-1 and II-2 acetonitrile. Despite the presence of 2,20 -trifluoromethyl-benzidine moiety present in polymer I-1 it showed a hypsochromic shift at lem peak at 413 nm when compared with that (487 nm) of polymer I-2 in methanol (Fig. 3). Similarly, polymer II-1 also showed a hypsochromic lem peak when compared with that of polymer II-2 in acetonitrile (Fig. 3). These results suggested that the presence of trifluoromethyl groups in 2- and 20 -positions of biphenylene moieties caused the twisting of the two phenylene rings of diamine moiety resulting in the disruption of conjugation lengths. The large bathochromic shifts of polymers I-4 and II-4 when compared with those of polymers I-3 and II-3 in acetonitrile (Fig. 4) were related to the presence of 9,10-diphenylanthracence which caused an increased conjugation length of the repeating units. Additionally, the sulfone moieties in polymers I-3 and II-3 a non-linear (kinked) group caused both the break down of the conjugation of the repeating units significantly and prevented the extensive pep stacking phenomena of aromatic moieties resulting in
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a hypsochromic effect. These results are in excellent agreement with those of other poly(pyridinium salt)s having the bent structures in the main chain [27]. In contrast, polymers I-4 and II-4 clearly exhibited a bathochromic shift in both methanol and acetonitrile when compared with those of polymers I-3 and II-3. These results suggested that the extensive pep interactions in polymers I-4 and II-4 due to the peconjugated aromatic structures from 9,10-diphenylanthracene moieties caused a decreased in energy of the anti-bonding p* orbital that was conducive for the lowering HOMOLUMO energy gap. These interactions account for the red shift in the light emission of these polymers than those of polymers I-3 and II-3 that lack of these pep interactions. Furthermore, polymers I-4 and II-4 showed the highest bathochromic shifts in their lem peaks in DMSO when compared with those in other solvents, which is probably related to its highest ε ¼ 48.9 among the solvents examined (Figures S17 and S18). These two ionic polymers exhibited blue and green light when excited over a broad range of excitation wavelengths used depending on the polarity of organic solvents (Table 3). Note here that poly(pyridinium salts) of identical counterions tosylate and triflimide containing 2,6-anthracene moieties emitted UV light both in organic solvents and in solid states [20b]. To our knowledge, these are the first examples of ionic polymers containing 9,10diphenylanthracene moieties in the main chain exhibited blue and green light. In the literature, two other peconjugated polymers containing diphenylanthracene vinylne biphenylene and diphenylanthracene vinylne terphenylene in the main chain and fluorene pendant groups exhibited blue light both in solution and in solid states [28]. The lem peaks of I-2eI-4 were in the range of
Fig. 4. Emission spectra of (a) polymer I-3, (b) polymer II-3, (c) polymer I-4, and (d) polymer II-4 in acetonitrile at various excitation wavelengths.
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444e485 nm (blue) in acetonitrile and those of polymers II-1II-4 also in this solvent were in the range of 443e496 nm (blue) (Table 3). To understand the influence of solvent polarity in their lem peaks, the data from PL spectra in various organic solvents were compiled in Table 3. These data suggested that a positive solvatochromism phenomenon was observed in several cases with some exceptions, that is, the lem peaks were shifted to the longer wavelengths with the increasing in solvent polarity. Full-width at half-maximum (fwhm) values of PL spectra were greater than 100 nm suggesting that their light-emission stemmed from a number of chromophoric species. They emitted UV, blue and green light in various organic solvents depending on the polymer microstructures, on the counterions associated with them and polarity of solvents. To evaluate the potential application for optoelectronic devices, we also examined their light-emitting properties in the solid state. Fig. 5a shows the PL spectra of thin films of polymers I-2eI-4 cast from methanol; and Fig. 5b shows the PL spectra of thin films of polymers II-2 and II-4 cast from acetonitrile. Tosylate polymers exhibited lem peaks at 390 (UV), 455, 455 and 435 nm (blue) at various excitation wavelengths; and triflimide polymers exhibited
lem peaks at 388 (UV), 452 (blue), and 562 nm (green) at various excitation wavelengths. The overall shapes of the fluorescence spectra in their solid states were quite different and were not comparable to those in solution spectra except polymer II-4. Some polymers exhibited hypsochromic shifts in their light emission when compared with those of their solution spectra (Table 3) because of the less-ordered structures in the solid state. These results are consistent with other poly(pyridinium salt) reported earlier [28]. The emission spectrum of polymer II-4 displayed a large red-shift emission when compared with its solution spectrum in acetonitrile as shown in Fig. 4d. This bathochromic shift may be attributed to the more ordered structures in the solid state of this polymer via strong pep interactions when the film was cast from acetonitrile. However, they emitted UV, blue, and green light in the solid states depending on the microstructures of polymer backbones, on the counterions and on the nature of solvents used for the preparation of films. 4. Conclusions In summary, novel poly(pyridinium salt)s with organic counterions were prepared by using ring-transmutation and metatheses reactions. Their chemical structures were characterized by spectroscopic techniques. Their weight-average molecular weights (Mw) were in the range of 44e64 kg/mol and polydispersities in the range of 1.13e1.43 as determined by gel permeation chromatography. They exhibited excellent thermal stabilities in the range of 327e477 C as determined by thermogravimetric analysis. Several of these polymers exhibited lyotropic liquid-crystalline phases in organic solvents above their critical concentrations depending on their microstructures of the polymer backbones and on the counterions. The analyses of their photoluminescent spectra revealed that some of the polymers emitted UV light (<400 nm), some emitted blue light (430e480 nm) and some emitted green light (480e560 nm) both in solutions and solid states. Currently, we are exploring them as a dispersing aid for the dispersion of singlewalled carbon nanotubes via non-covalent interactions [29]. Acknowledgments P.K.B. acknowledges the University of Nevada Las Vegas (UNLV) for New Investigation Award (NIA), Planning Initiative Award (PIA), and Applied Research Initiative (ARI) grants, and the donors of the Petroleum Research Fund (PRF# 35903-B7), administered by the American Chemical Society, and an award (CCSA# CC5589) from Research Corporation for the support of this research. The work is in part supported by the NSF under Grant No. 0447416 (NSF EPSCoR RING-TRUE III), NSF-Small Business Innovation Research (SBIR) Award (Grant OII-0610753), NSF-STTR Phase I Grant No. IIP0740289, and NASA GRC Contract No. NNX10CD25P. T.S.J. acknowledges the Graduate College (UNLV) for providing him a financial support in the form of SPGRA for the academic year 2010e2011. We thank the anonymous reviewers for their constructive comments on the contents of this article. Appendix. Supplementary information Supplementary data related to this article can be found online at doi:10.1016/j.polymer.2012.01.017. References
Fig. 5. Emission spectra of (a) polymers I-2eI-4 in thin films cast from methanol, and (b) polymer II-2 and II-4 in thin films cast from acetonitrile at various excitation wavelengths.
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