Synthesis and characterization of new imidazole and fluorene–bisphenol based polyamides: Thermal, photophysical and antibacterial properties

Reactive & Functional Polymers 73 (2013) 555–563

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Synthesis and characterization of new imidazole and fluorene–bisphenol based polyamides: Thermal, photophysical and antibacterial properties Mousa Ghaemy a,⇑, Bahareh Aghakhani a, Mehdi Taghavi a, Seyed Mojtaba Amini Nasab a, Mojtaba Mohseni b a b

Polymer Chemistry Research Laboratory, Department of Chemistry, University of Mazandaran, Babolsar, Iran Department of Biology, University of Mazandaran, Babolsar, Iran

a r t i c l e

i n f o

Article history: Received 16 October 2012 Received in revised form 25 December 2012 Accepted 26 December 2012 Available online 2 January 2013 Keywords: Polyamides based on imidazole and fluorene–bisphenol rings Solubility Thermal stability Photophysical properties Antibacterial activity

a b s t r a c t A new para-linked diether-diamine, 9,9-bis{4-[2-(4,5-diphenylimidazol-2-yl)-4-aminophenoxy] phenyl}fluorene (III), bearing fluorene–bisphenol and two ortho-linked diaryl-substituted imidazole rings were synthesized by the catalytic reduction of the nitro groups of compound (II), 9,9-bis{4-[2-(4,5-diphenylimidazol-2-yl)-4-nitrophenoxy]phenyl}fluorene, by using hydrazine monohydrate in the presence of Pd/C. Compound (II) was synthesized by the nucleophilic chloro displacement reaction of the synthesized 2-(2-chloro-5-nitrophenyl)-4,5-diphenyl-1H-imidazole with 9,9-bis(4-hydroxyphenyl)fluorene in refluxing DMAc in the presence of potassium carbonate. This diamine was condensed directly with aliphatic and aromatic diacids via the Yamazaki–Higashi phosphorylation method in the presence of triphenylphosphite (TPP), pyridine (Py) and halide salt to give high molecular polyamides (PAs). The synthesized PAs were obtained in quantitative yields with inherent viscosities between 0.51 and 0.76 dL g 1. The structures of diamine and PAs were characterized by elemental analysis, FT-IR and NMR spectroscopy, and properties of PAs were investigated by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and UV–visible and fluorescence spectroscopy. The PAs showed good solubility in aprotic and polar organic solvents, with high thermal stability exhibiting the glass transition temperatures (Tgs) and 10% weight loss temperatures (T10%) in the range of 226–330 °C and 400–466 °C in air, respectively, and fluorescence emission with maximum wavelengths (kem) in the range of 417– 473 nm with quantum yields (Uf) of 9–35%. Two of these polymers together with compounds (II) and (III) were also screened for antibacterial activity against Gram positive and Gram negative bacteria. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Polyamides offer excellent physical and chemical properties, thermal and oxidative stability, flame resistance, and superior mechanical and dielectric properties [1,2]. However, for many applications polyamides need to have good film-forming ability and adequate organo-solubility. In order to achieve these requirements, structural modifications of polyamides often became essential [3]. Therefore, research on aromatic polyamides with heterocycles in the main chain [4–9] and in the pendant structure [10,11] is widespread in the scientific literature. Among these approaches, introducing bulky pendent substituents and heteroaromatic rings into polymers chains has been considered to be efficient, which can provide not only enhanced solubility but also good thermal stability. If these groups are carefully chosen, they are likely to increase solubility without affecting thermal and ⇑ Corresponding author. Tel.: +98 112 534 2353; fax: +98 112 534 2302. E-mail address: [email protected] (M. Ghaemy). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2012.12.005

mechanical properties to any great extent. Imidazole ring and its derivatives such as lophine, 2,4,5-triphenylimidazole, are useful n-type building blocks with high electron affinity and good thermal stability and has been successfully incorporated in small molecules and polymers as the electron-transport component [12–17]. Moreover, imidazole and its derivatives are fairly extensively studied due to new possibilities for their application in medicine as biologically active substances [18–22]. However, the use of antimicrobial agents blended in a polymer matrix can give rise to the migration towards the surface of the antibacterial agent and therefore the antimicrobial activity decreases during the lifetime of the polymer application. For this reason, in the last years, several research groups have tested the use of different antimicrobial agents covalently bonded to the polymer chain, and the vast majority of literature focuses on olefin-containing imidazole monomers [23–30]. Fluorene has a violet fluorescence and its derivatives are precursor to other useful fluorene compounds such as pharmaceuticals, dyes, and also used in preparation of many commercially important derivatives. Polyfluorene polymers are electrically conductive and

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electroluminescent, and have been much investigated for use as a luminophore in organic light-emitting diodes [31]. For all the reasons reported above the preparation of polyamides with good solubility, thermal stability and photophysical properties look very interesting. Prompted by these observations and in continuation of our research on polyamides, we hereby report the synthesis and characterization of a new diamine containing substituted imidazole and fluorene–bisphenol rings and its use in preparation of a series of polyamides. These polymers are expected to show high thermal stability and photoluminescence properties due to presence of lophine and fluorene structures in the backbone, and must also have good solubility in organic solvents as a result of presence of hetero-aromatic and ether linkages in the polymer chains. Because of the presence of substituted imidazole (lophine) in these polymers, they were also tested for antibacterial activity against Gram positive and Gram negative bacteria. 2. Experimental 2.1. Materials All chemicals were purchased from Fluka and Merck Chemical Co. (Germany), through a local agency. Ammonium acetate, 9,9bis(4-hydroxyphenyl)fluorene, hydrazine monohydrate and 10% palladium on activated carbon, and reagent grade diacids such as terephthalic acid, isophthalic acid, pyridine-2,6-dicarboxylic acid, adipic acid and sebacic acid were used as received. N-methyl-2pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and pyridine (Py) were purified by distillation under reduced pressure over calcium hydride and stored over 4 Å molecular sieves. 2.2. Diamine synthesis The synthetic pathway leading to the synthesis of target diamine is outlined in Scheme 1. 2.2.1. 2-(2-Chloro-5-nitrophenyl)-4,5-diphenyl-1H-imidazole (I) A mixture of 10 mmol (1.86 g) 2-chloro-5-nitrobenzaldehyde, 10 mmol (2.1 g) benzil, 70 mmol (5.39 g) ammonium acetate and 20 mL glacial acetic acid was refluxed for 24 h. On cooling, the precipitated white solid was collected by filtration and washed with ethanol and water. The yield of product was 3.55 g (95%) with mp = 218–220 °C. FT-IR (KBr disk) at cm 1: 3453 (NAH imidazole), 3124 (CAH aromatic), 1532, 1345 (NO2) and 1684 (C@N). 1H NMR (DMSO-d6, d in ppm): 7.34–7.38 (m, 6H), 7.54 (d, 4H, J = 8.20 Hz),

7.91 (d, 1H, J = 8.20 Hz), 8.25 (dd, 1H, J = 8.20 Hz), 8.65 (d, 1H, J = 8.20 Hz), 12.98 (s, 1H, NAH imidazole). Elemental analysis calculated for C21H14ClN3O2: C, 67.20%; H, 3.73%; N, 11.20% and found: C, 67.00%; H, 3.65%; N, 11.35%.

2.2.2. 9,9-Bis{4-[2-(4,5-diphenylimidazol-2-yl)-4nitrophenoxy]phenyl}fluorene (II) A mixture of 10 mmol (3.50 g) 9,9-bis(4-hydroxyphenyl)fluorene, 20 mmol (7.5 g) 2-(2-chloro-5-nitrophenyl)-4, 5-diphenyl1H-imidazole and 20 mmol (2.76 g) anhydrous potassium carbonate in 20 mL of dry DMAc was refluxed at 130 °C for 12 h and then cooled. The mixture was poured into water and the precipitate was collected by filtration and recrystallized from ethanol. The yield of product was 9.87 g (96%) with mp = 295–298 °C. FT-IR (KBr disk) at cm 1: 3453 (NAH imidazole), 3062 (CAH aromatic), 1622 (C@N), 1580 (C@C), 1532, 1345 (NO2) and 1243 (CAO). 1H NMR (DMSOd6, d in ppm): 6.98 (d, 2H, J = 9.2 Hz), 7.17–7.52 (m, 34H), 7.97 (d, 2H, J = 7.6 Hz), 8.19 (dd, 2H, J = 9.2 Hz), 8.92 (d, 2H, J = 2.8 Hz), 12.57 (s, 2H, NAH imidazole). Elemental analysis calculated for C67H44N6O6: C, 78.21%; H, 4.28%; N, 8.17% and found: C, 78.09%; H, 4.27%; N, 8.12%.

2.2.3. 9,9-Bis{4-[2-(4,5-diphenylimidazol-2-yl)-4-aminophenoxy] phenyl}fluorene (III) A 10 mmol (10.28 g) compound II and 0.4 g palladium on activated carbon (Pd/C, 10%) were dispersed in 120 mL ethanol. The suspension solution was heated to reflux, and 12 mL hydrazine monohydrate was slowly added to the mixture. After a further 6 h reflux, the solution was filtered hot to remove Pd/C and the filtrate was cooled to precipitate white crystals. The product was collected by filtration and dried in vacuum at 90 °C. The yield of product was 8.03 g (83%) with mp = 184–186 °C. FT-IR (KBr disk) at cm 1: 3437 (NAH imidazole), 3362, 3212 (NH2), 3032 (CAH aromatic), 1607 (C@N), 1491 (C@C) and 1221 (CAO). 1H NMR (DMSO-d6, d in ppm): 5.20 (s, 4H), 6.60 (dd, 2H, J = 7.6 Hz), 6.68 (d, 2H, J = 8.8 Hz), 6.80 (d, 2H, J = 8.8 Hz), 6.90 (d, 4H, J = 9.2 Hz), 7.14– 7.41 (m, 30H), 7.85 (d, 2H, J = 7.2 Hz), 11.94 (s, 2H, NAH imidazole). 13 C NMR (100 MHz, DMSO-d6, d in ppm): 1 (63.41), 2 (114.46), 3 (115.89), 4 (116.44), 5 (120.90), 6 (122.54), 7 (123.77), 8 (126.25), 9 (126.84), 10 (127.42), 11 (127.94), 12 (128.21), 13 (128.52), 14 (128.63), 15 (128.97), 16 (129.09), 17 (131.48), 18 (139.30), 19 (139.77), 20 (142.53), 21 (143.38), 22 (146.12), 23 (151.38), 24 (157.56). Elemental analysis calculated for C67H48N6O2: C, 83.06%; H, 4.96%; N, 8.68% and found: C, 83.14%; H, 5.05%; N, 8.64%.

Scheme 1. Synthesis of target diamine (III).

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2.3. Polymer synthesis The following general procedure, as is illustrated in Scheme 2, was used for the preparation of polyamides from diamine (III) and different aliphatic and aromatic dicarboxylic acids. Diamine (III) (1 mmol, 0.97 g), a dicarboxylic acid (1 mmol), and lithium chloride (0.30 g) were dissolved in a mixture of pyridine (1 mL), triphenylphosphite (TPP) (1.20 mmol), and NMP (5 mL). The reaction mixture was heated at 120 °C for 12 h with stirring under dry N2 atmosphere. The mixture was then cooled to room temperature and the resulting polymers were precipitated in 200 mL methanol. The precipitate was filtered and washed with hot water, and then was further purified by washing with methanol for 1 day in a Soxhlet apparatus to remove the low molecular weight fractions. The inherent viscosity (ginh) of the polymers was measured at a concentration 0.5 g/dL in NMP at 25 °C and was in the range of 0.51–0.76 dL/g. PA1: Yield 92% and ginh (dL/g) = 0.76. FT-IR (KBr disk) at cm 1: 3426 (NAH imidazole), 3273 (NAH amide), 3062 (CAH aromatic), 1664 (C@O amide), 1607 (C@N), 1495 (C@C) and 1229 (CAOAC). 1 H NMR (DMSO-d6, d in ppm): 6.89 (d, 4H, ArAH, J = 7.2 Hz), 7.02 (m, 6H, ArAH), 7.11–7.34 (m, 18H, ArAH), 7.44 (d, 4H, ArAH, J = 7.08 Hz), 7.59 (s, 1H, ArAH), 7.72 (m, 2H, ArAH), 7.92 (m, 4H, ArAH), 8.12 (d, 1H, ArAH, J = 7.2 Hz), 8.23 (d, 2H, ArAH, J = 8.2 Hz), 8.47 (m, 2H, ArAH), 8.58 (m, 1H, ArAH), 8.62 (m, 1H, ArAH), 10.60 (s, 2H, NAH amide), 12.23 (s, 2H, NAH imidazole ring). Elemental analysis calculated for (C75H50N6O4)n: C, 81.97%; H, 4.55%; N, 7.65%. Found: C, 81.84%; H, 4.63%; N, 7.62%. PA2: Yield 86% and ginh (dL/g) = 0.70. FT-IR (KBr disk) at cm 1: 3418 (NAH imidazole), 3275 (NAH amide), 3066 (CAH aromatic), 1664 (C@O amide), 1605 (C@N), 1500 (C@C) and 1226 (CAOAC). 1 H NMR (DMSO-d6, d in ppm): 6.87 (d, 4H, ArAH, J = 7.2 Hz), 7.03 (distorted d, 5H, ArAH), 7.17–7.35 (m, 19H, ArAH), 7.45 (d, 4H,

557

ArAH, J = 8.0 Hz), 7.56 (s, 1H, ArAH), 7.71 (m, 2H, ArAH), 7.88 (m, 4H, ArAH), 8.12 (d, 1H, ArAH, J = 7.2 Hz), 8.20 (d, 2H, ArAH, J = 8.0 Hz), 8.48 (m, 2H, ArAH), 8.57 (m, 1H, ArAH), 8.63 (m, 1H, ArAH), 10.63 (s, 2H, NAH amide), 12.22 (s, 2H, NAH imidazole ring). Elemental analysis calculated for (C75H50N6O4)n: C, 81.97%; H, 4.55%; N, 7.65%. Found: C, 81.88%; H, 4.69%; N, 7.70%. PA3: Yield 89% and ginh (dL/g) = 0.66. FT-IR (KBr disk) at cm 1: 3437 (NAH imidazole), 3282 (NAH amide), 3058 (CAH aromatic), 1665 (C@O amide), 1607 (C@N), 1498 (C@C) and 1230 (CAOAC). 1 H NMR (DMSO-d6, d in ppm): 6.80 (d, 4H, ArAH, J = 7.6 Hz), 7.10 (m, 6H, ArAH), 7.09–7.37 (m, 18H, ArAH), 7.48 (d, 4H, ArAH, J = 8.4 Hz), 7.51 (s, 1H, ArAH), 7.73 (m, 2H, ArAH), 7.86 (m, 4H, ArAH), 8.11–8.25 (m, 2H, ArAH, J = 7.8 Hz), 8.43 (m, 2H, ArAH), 8.59 (m, 1H, ArAH), 8.66 (m, 1H, ArAH), 10.60 (s, 2H, NAH amide), 12.28 (s, 2H, NAH imidazole ring). Elemental analysis calculated for (C74H49N7O4)n: C, 80.80%; H, 4.46%; N, 8.92%. Found: C, 80.71%; H, 4.57%; N, 8.86%. PA4: Yield 91% and ginh (dL/g) = 0.51. FT-IR (KBr disk) at cm 1: 3429 (NAH imidazole), 3270 (NAH amide), 3055 (CAH aromatic), 2934 (CAH aliphatic), 1671 (C@O amide), 1609 (C@N), 1487 (C@C) and 1220 (CAOAC). 1H NMR (DMSO-d6, d in ppm): 1.65 (m, 4H, CAH), 2.31 (t, 4H, CAH), 6.82 (d, 4H, ArAH, J = 8.0 Hz), 6.84 (m, 6H, ArAH), 7.23–7.38 (m, 22H, ArAH), 7.47 (d, 4H, ArAH, J = 7.6 Hz), 7.63 (d, 2H, ArAH, J = 8.8 Hz), 7.81 (distorted d, 2H, ArAH), 8.25 (s, 2H, ArAH), 10.14 (s, 2H, NAH amide), 12.18 (s, 2H, NAH imidazole ring). Elemental analysis calculated for (C73H54N6O4)n: C, 81.26%; H, 5.01%; N, 7.79%. Found: C, 81.17%; H, 5.13%; N, 7.73%. PA5: Yield 94% and ginh (dL/g) = 0.57. FT-IR (KBr disk) at cm 1: 3426 (NAH imidazole), 3276 (NAH amide), 3051 (CAH aromatic), 2931 (CAH aliphatic), 1671 (C@O amide), 1607 (C@N), 1487 (C@C) and 1221 (CAOAC). 1H NMR (DMSO-d6, d in ppm): 1.31 (m, 8H, CAH), 1.60 (m, 4H, CAH), 2.31 (t, 4H, CAH), 6.80 (d, 4H, ArAH,

Scheme 2. Polycondensation reaction of diamine (III) with different dicarboxylic acids.

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J = 8.4 Hz), 6.80 (m, 6H, ArAH), 7.17–7.33 (m, 22H, ArAH), 7.43 (d, 4H, ArAH, J = 7.2 Hz), 7.66 (d, 2H, ArAH, J = 9.2 Hz), 7.86 (distorted d, 2H, ArAH), 8.23 (s, 2H, ArAH), 10.04 (s, 2H, NAH amide), 12.13 (s, 2H, NAH imidazole ring). Elemental analysis calculated for (C77H62N6O4)n: C, 81.48%; H, 5.47%; N, 7.41%. Found: C, 81.44%; H, 5.52%; N, 7.40%.

100 rpm. The compounds sensitivity of the strains was assayed for positive or negative growth after 24–48 h.

2.4. Measurements

The objective of this study was two folds: synthesis and characterization of a new diamine bearing particular functional groups for optical and antibacterial activity such as imidazole and fluorene moieties and its use in preparation of corresponding polyamides. The synthetic procedure for the preparation of diamine (III) is shown in Scheme 1. Condensation of benzil with 2-chloro-5-nitrobenzaldehyde and ammonium acetate is a classical but convenient synthetic method for preparation of triaryl imidazole. The dinitro compound (II) was successfully synthesized by nucleophilic chlorodisplacement reaction between 9,9-bis(4-hydroxyphenyl)fluorene and compound (I) in DMAc in the presence of K2CO3. The diamine (III) was obtained by catalytic reduction of the dinitro compound (II) by using hydrazine hydrate and Pd/C in refluxing ethanol. The chemical structure of the synthesized compounds I– III was identified by elemental analysis, FT-IR and 1H and 13C NMR spectroscopy. The formation of imidazole ring in compound (I) was evident from the signal around 12.00 ppm of 1H NMR spectrum. The nitro groups in compounds (I) and (II) were evident from the FT-IR peaks at 1532 and 1345 cm 1 (ANO2 asymmetric and symmetric stretching). After reduction, the absorption peaks of NO2 groups disappeared and the primary amino group showed the typical absorption pair at 3362 and 3212 cm 1, as shown in Fig. 1. The 1H (A) and COSY (B) NMR spectra of diamine (III) confirms the complete transformation of nitro groups into the amine groups by the high field shift of the aromatic protons and by the appearance of characteristic signals of amine group’s protons at 5.20 ppm, as shown in Fig. 2. The 13C NMR spectrum of this compound, Fig. 3, shows 24 different carbons for the aromatic segment and heterocyclic rings. The elemental analysis results are also in good agreement with the calculated percentages of carbon, hydrogen and nitrogen contents in the intermediates and diamine (III) structures. The new PAs were synthesized by phosphorylation polycondensation between diamine (III) and various commercially available aromatic and aliphatic dicarboxylic acids in NMP using TPP and pyridine as condensing agents at 120 °C in N2 (Scheme 2). All the polycondensation reactions proceeded readily in a homogeneous solution. Since Yamazaki et al.’s [32] development, many researchers have utilized the activated polyamidation using the

1 H and 13C NMR spectra were recorded on a 400 MHz Bruker and 100 MHz, respectively, (Ettlingen, Germany) instrument using DMSO-d6 as solvent and tetramethyl silane as an internal standard. Elemental analyses performed by a CHN-600 Leco elemental analyzer. Melting point (uncorrected) was measured with a Barnstead Electrothermal engineering LTD 9200 apparatus. Inherent viscosities (at a concentration of 0.5 g/dL) were measured with an Ubbelohde suspended-level viscometer at 25 °C using NMP as solvent. Qualitative solubility was determined using 0.05 g of the polymer in 0.5 mL of solvent. Thermogravimetric analysis (TGA) was performed with the DuPont Instruments (TGA 951) analyzer well equipped with a PC at a heating rate of 10 °C/min under nitrogen atmosphere (20 cm3/min) and in the temperature range of 30– 650 °C. Differential scanning calorimetry (DSC) was recorded on a Perkin Elmer pyris 6 DSC under nitrogen atmosphere (20 cm3/ min) at a heating rate of 10 °C/min. Glass-transition temperatures (Tg) values were read at the middle of the transition in heat capacity and were taken from the second heating scan after cooling from 350 °C at a cooling rate of 20 °C min 1. Ultraviolet–visible and fluorescence emission spectra were recorded on a Cecil 5503 (Cecil Instruments, Cambridge, UK) and Perkin-Elmer LS-3B spectrophotometers (Norwalk, CT, USA) (slit width = 2 nm), respectively, using a dilute polymer solution (0.20 g/dL) in DMSO.

2.5. Antibacterial assay The in vitro biocidal screening, antibacterial activities of the synthesized compounds was assayed onto LB (Luria–Bertani) medium contained: BactoTM tryptone, 10.0 g L 1; yeast extract, 5.0 g L 1; sodium chloride, 5.0 g L 1; and glucose, 1.0 g L 1. The medium was dispensed into universal bottles and sterilised at 121 °C for 15 min. A 0.04 g of intermediate compounds (II) and (III) and polyamides PA1 and PA5 were dissolved separately in 20 mL DMSO to generate 2 mg mL 1 working solutions which were filter sterilised using a 0.22 lm Ministart (Sartorius). The sterilized work solutions were added into LB medium to give a final concentration of 1–300 lg mL 1 as required. The antibacterial activities of the compounds were compared with known antibiotic tetracycline at the same concentration. The minimum inhibitory concentration (MIC) test of the newly synthesized compounds were determined in vitro against microorganisms including the two Gram-negative bacteria Escherichia coli PTCC1533 and Pseudomonas aeruginosa PTCC 1707, and two Gram-positive bacteria Staphylococcus aureus ATCC 25923 and Bacillus subtilis PTCC 1156. MIC is typically reported as the lowest concentration of antimicrobial activity that completely inhibits growth over a set period of incubation relative to a control containing no antimicrobial. For the synthesized compounds investigated, the amount of bacterial growth at each compound concentration was reported. Late exponential phase of the bacteria were prepared by inoculating 1% (v/v) of the cultures into the fresh LB medium and incubating on an orbital shaker at 37 °C and 100 rpm overnight. Before using the cultures, they were standardized with a final cell density of approximately 108 cfu mL 1. A 1% (v/v) inoculums of each culture was inoculated into the LB medium containing different concentrations of the synthesised compounds and incubated on the orbital shaker at 37 °C and

3. Results and discussion 3.1. Synthesis and characterization of diamine and polymers

Fig. 1. FT-IR spectrum of diamine (III) and PA1.

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Fig. 2. 1H (A) and COSY (B) NMR spectra of diamine (III) in DMSO-d6.

Fig. 3.

13

C NMR spectrum of diamine (III) in DMSO-d6.

TPP activator and found that the addition of a small amount of LiCl or CaCl2 enhances the molecular weight of the polyamides. All PAs

remained soluble in the reaction medium, thus permitting an increase of their molecular weight and giving viscous solutions.

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Stringy and tough precipitates formed when the viscous polymer solutions were trickled into the stirring methanol. As shown in Table 1, the inherent viscosities of polymers were in the range of 0.51–0.76 dL/g. The elemental analysis values were in good agreement with the calculated ones. For obtaining light colored PAs, a lower capability of charge transfer is indispensable. An effective way of depressing the charge transfer of polymers is introducing non-conjugated segments into the backbone. Thus, PA4 and PA5 were synthesized from adipic acid and sebacic acid, respectively. This could effectively reduce the conjugation and capability of charge transfer and led to enlarged band gaps of the PAs. FT-IR spectrum of the representative polymer PA1 is shown in Fig. 1. These polymers exhibited characteristic absorption bands around 3300 cm 1 (NAH stretching), 1420 cm 1 (NAH bending), near 1220 cm 1 (aryl ether stretching) and at 1670 cm 1 (C@O, amide). 1 H NMR spectrum of the representative polymer PA5 is illustrated in Fig. 4, where all the peaks could be readily assigned to the protons in the repeating unit. In the 1H NMR spectra of these polymers, the NAH proton of amide groups appeared in the region of 10.04–10.63 ppm, while the NAH proton of imidazole ring appeared at the most downfield region of 12.13 ppm. 3.2. Properties of polyamides 3.2.1. Solubility The solubility behavior of these polymers was tested qualitatively in various organic solvents, and the results are summarized in Table 1. The polyamides were readily soluble in polar organic solvents such as NMP, DMAc, DMF, and DMSO at room temperature. The good solubility associated with these polymers could be attributed to the presence of propeller-shaped fluorene–bisphenol structure, bulky pendent triaryl imidazole group, and ether linkage sequences in the repeat unit. These factors increase the chain distance and decrease the interaction of the polymer chains; consequently, the solvents molecules are able to penetrate easily and interact with the polar groups and to solubilize the polymer chains. The solubility also varies depending upon the dicarboxylic acid used. The polyamides synthesized from aliphatic dicarboxylic acids PA4 and PA5 exhibited better solubility behavior in less polar solvents such as THF, m-cresol and Py. 3.2.2. Photophysical properties The UV–vis absorption and photoluminescence (PL) spectra of the diamine (2  10–5 M) and PAs (0.1 g/dL) in dilute NMP solutions are shown in Fig. 5A and B, respectively. The photophysical data for PAs in solution and in solid state were summarized in Table 2. The diamine and PAs showed strong UV–vis absorption bands with maxima (kab) in the region of 305–314 nm which was assigned to a p–p transition resulting from the conjugation systems of aromatic rings in the main chain and in the pendent group. By comparing the absorption spectra, a slightly red shift of 9 nm is observed in PAs spectra due to expansion of p system. To investigate the optical properties of thin films of these PAs, solutions of

the polymers were made by dissolving about 0.50 g of the samples in 6 mL of NMP. These solutions were poured into a 5-cm glass Petri dish, which was heated under vacuum at 50 °C for 1 h, 100 °C for 2 h, and 150 °C for 5 h to evaporate the solvent slowly. By being soaked in distilled water, the flexible and transparent thin films with almost no color was self-stripped off from the glass surface. The obtained films were then used to investigate the optical properties of the PAs. The excitation wavelength was 310 nm in all cases. The maximum emission wavelength (kem) of the diamine was observed at 385 nm, around 460–473 nm for the aromatic PAs (PA1, PA2 and PA3) and around 417–431 nm for the aliphatic PAs (PA4 and PA5). In addition, the PAs exhibited a slightly redshifted emission in the solid state when compared with that measured in NMP solutions, possibly due to an increased interchain interaction in the film form. The emission intensity of aliphatic and aromatic PAs was not the same, which confirms that the fluorescence intensity is affected by incorporation of alkyl and phenyl groups into the polymer main chain. To measure the photoluminescence (PL) quantum yields (Uf), dilute polymer solutions (0.2 g/dL) in NMP were prepared. A 0.10 N solution of quinine in H2SO4 (Uf = 0.55) was used as a reference. The Uf values were 35% and 27% for the aliphatic PA5 and PA4, respectively, and in the range of 9–12% for the aromatic PAs. The blue shift and higher fluorescence quantum yield of the aliphatic PAs compared with the aromatic PAs could be attributed to reduced conjugation and capability of charge-transfer complex formation by the aliphatic diacids with the electron-donating diamine moiety in comparison to the stronger electron-accepting aromatic diacids [33]. 3.2.3. Thermal properties The thermal behavior data of these polymers were assessed by using DSC and TGA analysis. DSC was used to determine the glasstransition temperature (Tg) values of the PAs. The DSC curves were recorded at heating rate of 10 °C/min in N2 are shown in Fig. 6. Melting endothermic peak was not observed by DSC in the temperature range of 50–350 °C, so that the PAs were considered to be essentially amorphous. The amorphous nature of these PAs can be attributed to their bulky pendent group which decreased the inter-chain interaction resulting in loose polymer chain packaging and aggregates. The Tg values were read at the midpoint of the change in slope of the baseline of DSC curves, and found to be in the range of 226–330 °C, as listed in Table 3. In general, molecular packing and chain rigidity are among the main factors influencing on Tg values. The increased rotational barrier caused by the bulky pendant in diamines and side chain-side chain and side chain-main chain interactions led to enhanced Tg values. However, the presence of flexible bond such as amide and particularly ether linkages in the main chain of these PAs reduced the rigidity of their backbones and consequently Tg of these polymers reached to a reasonable and obtainable values in the range of 226–330 °C. Therefore, Tg values of these polymers are affected by these two opposite key factors; bulky pendants and flexibility of the main

Table 1 Solubility of synthesized PA1–PA5. Code

PA1 PA2 PA3 PA4 PA5

Solvent Yield (%)

ginh (dL/g)a

DMAc

DMF

NMP

DMSO

Py

THF

CHCl3

CH3CN

m-cresol

92 86 89 91 94

0.76 0.70 0.66 0.51 0.57

++ ++ ++ ++ ++

++ ++ ++ ++ ++

++ ++ ++ ++ ++

++ ++ ++ ++ ++

± + + ++ ++

– ± ± + +

– – – – –

– – – – –

± ± + ++ ++

++, Soluble at room temperature; +, soluble on heating at 60 °C; ±, partially soluble on heating at 60 °C; –, insoluble on heating at 60 °C. a Measured at a polymer concentration of 0.5 g/dL in NMP at 25 °C.

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Fig. 4. 1H NMR spectrum of PA5 in DMSO-d6.

Fig. 5. UV–vis (A) and fluorescence (B) spectra of diamine (III) in solution and PA (1–5) films. Inside images: dilute solutions (2  10 (III), and (3) PA5 (0.1 g/dL) in DMSO.

chain by ether linkages. The Tg values also depend on the stiffness of diacid residue in the polymer main chain and the increasing order of Tg generally correlated with that of chain rigidity. Therefore, among all the prepared PAs, PA4 and PA5 based on aliphatic dicarboxylic acid showed the lowest Tg values and those based on the aromatic dicarboxylic acids such as PA(1–3) exhibited the highest Tg values. Thermal stability of these PAs was evaluated by TGA under N2 and air atmosphere at a heating rate of 10 °C min 1 and the curves are shown in Fig. 7. The data, extracted from the original TGA curves, in Table 3, show the temperature of 10% weight loss (T10%) in the range of 419–492 °C in N2 atmosphere and in the range of 396–466 °C in air, depending on the structure of diacid

5

M) of (1) dinitro (II) and (2) diamine

Table 2 Photophysical properties data of PA1–PA5. Polymer

kabs (nm)a

kem (nm)a

kabs (nm)b

kem (nm)b

Uf c (%)

PA1 PA2 PA3 PA4 PA5

311 311 311 303 300

467 460 455 429 416

314 313 313 308 306

473 463 461 431 417

12 11 9 27 35

Polymer concentration of 0.20 g/dL in NMP. a UV–visible absorption and fluorescence emission spectra of the PAs in solution. b UV–visible absorption and fluorescence emission spectra of the PAs in films. c Fluorescence quantum yield relative to 10 5 M quinine sulfate in 1 N H2SO4 (aq) (Uf = 0.55) as a standard.

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component in the polymer backbone. The residual weights for the resulting PAs were in the range of 36–63% at 750 °C in N2. Char yield (CR) can be used as criteria for evaluating limiting oxygen index (LOI) of the polymers in accordance with Van Krevelen and Hoftyzer equation [34]; LOI = 17.5 + 0.4CR. For all the PAs, LOI values calculated based on their CR at 700 °C. Due to the reasons which have been explained above, the thermal stability of these PAs are observed in order of PA(1–3) > PA(4 and 5). According to Table 3, it is clear that aromatic PAs have better thermal stability and higher LOI values as compared to the aliphatic PAs. 3.2.4. Antibacterial activity The newly synthesized compounds were screened in vitro against an assortment of two Gram-positive bacteria S. aureus and B. subtilis and two Gram-negative bacteria E. coli and P. aeruginosa. In addition, the finding towards inhibition of microorganisms was correlated with a standard antibiotic tetracycline and the screening results are summarized in Table 4. Only an aromatic and an aliphatic of the prepared polymers were tested for antibacterial activities. The antibacterial screening revealed that all the tested compounds dinitro (II), diamine (III), PA1 and PA5 showed moderate to good inhibition. The antibacterial screening indicated that among the tested bacterial strains, good inhibitory results were obtained against E. coli. As regards the relationships between the structures of the heterocyclic scaffold and the detected antibacterial properties, it showed varied biological activity. The antibacterial activity seemed to be dependent on the heterocyclic moiety as well as on the nature of substituent. Although the imidazole-containing compounds themselves are observed active. The activity was further enhanced by the presence of nitro substituent on the imidazole moiety. The nitro substituted compound has

shown good activity against all tested bacteria particularly against S. aureus. The diamine compound (III) was less active in inhibition particularly for P. aeruginosa and S. aureus in comparison with the dinitro (II) and the polymers. The results indicate that electrondonating groups such as ANH2 decrease the antibacterial effect of the imidazole-containing compounds. As can be seen from the results in Table 4, it is revealed that the dinitro (II) inhibited the growth of bacteria S. aureus at a low concentration of 50 lg mL 1 and the rest of bacteria at 100 lg mL 1 whereas the growth inhibitory effects of the diamine and PA1 on most bacteria were shown at high concentration of 150 and 200 lg mL 1 (Fig. 8). The results indicated that the presence of electron withdrawing groups is necessary for the antimicrobial activity of the synthesized compounds.

Fig. 7. TGA curves of PA1, PA3 and PA5 under a N2 and air atmosphere at a heating rate of 10 °C/min.

Table 4 Minimum inhibitory concentration (MIC) of the compounds (lg mL bacteria. Strain

E. coli P. aeruginosa S. aureus B. subtilis

Minimum inhibitory concentration (lg mL

1

) against some

1

)

Diamine (III)

Dinitro (II)

PA5

PA1

Tetracycline

150 200 200 150

100 100 50 100

100 150 150 150

150 150 100 150

25 50 10 25

Fig. 6. DSC curves of PA (1–5) under a N2 atmosphere at a heating rate of 10 °C/min.

Table 3 Thermal properties of the PAs. Code

Tg (°C)a

T10 (°C)b

T10 (°C)c

Char yieldd

LOI (%)e

PA1 PA2 PA3 PA4 PA5

330 316 305 234 226

492 476 460 428 419

466 450 433 401 396

63 57 46 36 39

43 40 36 32 33

a

Glass transition temperature was recorded at a heating rate of 10 °C/min in N2. Temperature at which 10% weight loss was recorded by TGA in N2. Temperature at which 10% weight loss was recorded by TGA in air. d Percentage weight of material left undecomposed after TGA analysis at a temperature of 800 °C in N2. e Limiting oxygen index percent evaluating at char yield 800 °C. b

c

Fig. 8. Minimum inhibitory concentration of dinitro (II), diamine, PA1 and PA5 against some bacteria.

M. Ghaemy et al. / Reactive & Functional Polymers 73 (2013) 555–563

The role of electron withdrawing group in improving antimicrobial activities is supported by the studies of Sharma et al. [35,36]. 4. Conclusions In this study, a series of novel photoactive PAs containing bulky imidazole pendant and fluorene–bisphenil rings and aryl ether linkages in the main chain have been successfully prepared by direct phosphorylation polycondensation of a new aromatic diamine with various aromatic and aliphatic dicarboxylic acids. These results support that such a modification of the polymer structure can be used as an effective strategy for imparting fluorescence emission in the visible light region and enhancing the processability of aramids, while maintaining the high thermal stability. These polymers with inherent viscosities in the range of 0.51–0.76 dL/g exhibited high enough molecular weight to give tough and flexible thin films by solution casting. Their Tg values varied from 226 °C to 330 °C and their thermal stability based on 10% weight loss temperature varied from 226 °C to 330 °C, depending on the rigidity of the polymer backbone. Some of the synthesized compounds were tested for antibacterial activity and the results indicated that the presence of electron withdrawing groups is necessary for the antimicrobial activity. In general, these compounds exhibited good antibacterial activity and can be further developed for application as effective antimicrobial agent. References [1] J.M. Garcıa, F.C. Garcıa, F. Serna, J.L. de la Peña, Prog. Polym. Sci. 35 (2010) 623– 686. [2] C.P. Yang, J.H. Lin, J. Polym. Sci. Part A: Polym. Chem. 34 (1996) 341–348. [3] J.E. Flood, J.L. White, J.F. Fellers, J. Appl. Polym. Sci. 27 (1982) 2965–2985. [4] J. Tan, C. Wang, W. Peng, G. Li, J.M. Jiang, Polym. Bull. 62 (2009) 195–207. [5] K. Faghihi, Z. Mozaffari, J. Appl. Polym. Sci. 108 (2008) 1152–1157. [6] A. Abdolmaleki, Polym. Degrad. Stab. 92 (2007) 292–298. [7] R.R. Pal, P.S. Patil, M.M. Salunkhe, N.N. Maldar, P.P. Wadgaonkar, Polym. Int. 54 (2005) 569–575. [8] S.H. Hsiao, C.P. Yang, C.W. Chen, G.S. Liou, Eur. Polym. J. 41 (2005) 511–517.

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