Synthesis of a Novel m-Substituted Poly(phenoxy-imine)

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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry (2013) 50, 709–719 C Taylor & Francis Group, LLC Copyright  ISSN: 1060-1325 print / 1520-5738 online DOI: 10.1080/10601325.2013.792210

Synthesis of a Novel m-Substituted Poly(phenoxy-imine) and Investigation of its Fluorescence and Some Properties 1 3 ˘ ¨ ¨ UREN ¨ ¨ HACI OKKES ¸ DEMI˙R1∗, TAHI˙R AGIRG OT , KADEM MERAL2, I˙LKAY OZAYTEKIN , 4 5 5 ˘ ˙ ˙ ¨ ¨ ¨ ¨ ASHABIL AYGAN , C ¸ IGDEM KUC ¸ UKTURKMEN , and MERT OZHALLAC ¸

Kahramanmaras¸ S¨utc¸u¨ I˙mam University, Faculty of Science and Arts, Department of Chemistry, Kahramanmaras¸, Turkey Atat¨urk University, Faculty of Science, Department of Chemistry, Erzurum, Turkey 3 Selc¸uk University, Engineering and Architecture Faculty, Department of Chemical Engineering, Konya, Turkey 4 Kahramanmaras¸ S¨utc¸u¨ I˙mam University, Department of Biology, Kahramanmaras¸, Turkey 5 Kilis 7 Aralık University, Faculty of Science and Arts, Department of Chemistry, Kilis, Turkey 1 2

Received December 2012, Accepted January 2013

Oxidative polycondensation of 3-((2-phenylhydrazono)methyl)phenol (3-PHMP), a new m-substituted poly(phenoxy-imine), was studied using oxidants such as sodium hypochlorite, air (O2 ) and hydrogen peroxide in an aqueous alkaline medium under various polymerization conditions. The macromolecular structure and optical properties of the polymer were characterized with elemental analysis, Size Exclusion Chromatography (SEC), Fourier Transform Infrared (FTIR), Nuclear Magnetic Resonance (NMR), absorption and fluorescence spectroscopy techniques. As a result of fluorescence measurement, the fluorescence lifetime of poly(3-PHMP) in DMF was calculated as 2.88 ns (χ 2 = 1.12). An electrochemical property the monomer and polymer were also studied using Cyclic Voltammetry (CV) technology. According to the CV measurements, the electrochemical band gaps (Eg ) of 3-PHMP and poly(3-PHMP) were found to be 2.64 and 1.94 eV, respectively. Electrical conductivity of the polymer was measured by the four-point probe technique. The electrical conductivity of poly(3-PHMP) was found to be ∼3.2 × 10−2 S/cm. Thermo Gravimetric Analysis (TGA) revealed poly(3-PHMP) to be stable against thermo-oxidative decomposition. In addition, the in vitro antimicrobial activities of the synthesized compounds were tested on various microorganisms. Keywords: Oxidative polycondensation, conjugated polymers, fluorescence property, semi-conductive polymer, band gap

1 Introduction Recently, considerable attentions has been paid to polyphenols since they include the active hydroxyl (-OH) group and conjugated bond system. They are promising candidates for a wide variety of applications such as optoelectronics (1, 2), semiconductor (3), electrochromic (4), antistatic (5) and antimicrobial (6) materials. Polyphenols are synthesized by enzymatic polymerization (7, 8), oxidative polycondensation methods (9) and via reduction of gold ions into gold metal (10). Oxidative polycondensation is advantageous when compared with other methods about simple, inexpensive and mild reaction conditions. This method has been used extensively by several research groups for the polymerization of a wide variety of substituted phenols and anilines (3, 5, 11–15). Poly(phenoxy-imine)s or ∗

¨ Address correspondence to: Haci Okkes Demir, Kahramanmaras Sutcu Imam University, Faculty of Science and Arts, Department of Chemistry, Avsar Campus, 46100 Kahramanmaras, Turkey. Tel: +903442801455; Fax: +903442801352; Email: [email protected]

poly(phenoxy-ketimine)s, varieties of substituted phenols, are one of the most important synthetic polyphenols due to their interesting electrochemical and optical properties and environmental stability. To date, the detailed polymerization behaviors of oxidative polycondensation of o-substituted (12, 16–21) and psubstituted (22–26) phenoxy-imines have been investigated, whereas there have been a few studies on the oxidative polycondensation of m-substituted phenoxy-imines (27). Structural studies of the oxidatively prepared poly(phenoxyimine)s by spectroscopic techniques indicated that the linkages are the C-C (phenylene) and C-O-C (oxyphenylene) coupling (28, 29). The C-O-C coupling linkage induces to interrupt conjugated bond system on the main chain of polymer. Mostly, m-substituted polyphenols include phenylene units (27, 30). The ratio of phenylene and oxyphenylene units (regioselectivity) can be related to changes at the position of substituent. In addition, a part of the imine group on poly(phenoxy-imine)s can also be oxidized to carboxylic group during the polymerization reaction. Kobayashi et al. reported that m-substituents prevent the oxidation of the phenolic group at the terminal (30). So, efforts were

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Several chemicals, including m-hydroxybenzaldehyde, phenylhydrazine, sodium hypochlorite (NaOCl) (11%), hydrogen peroxide (H2 O2 ) (30%), hydrochloric acid (HCl) (37%), sulfuric acid (H2 SO4 ) (98%), potassium hydroxide (KOH), silver nitrate (AgNO3 ), diethyl ether, dichloromethane, ethanol, ethyl acetate, acetone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), methanol, chloroform, n-hexane, and acetonitrile, were purchased from Merck Schuchardt OHG (Hohenbrunn, Germany). All of these chemicals were used without further purification.

Differential Thermal Analysis (DTA) measurements were made between 20◦ C and 1000◦ C (under N2 ; flow rate, 10◦ C/minute). The number average molecular weight (Mn ), weight average molecular weight (Mw ) and polydispersity index (PDI) were determined with Size Exclusion Chromatography (SEC) (Shimadzu Co., Kyoto, Japan). The following conditions were used for chromatography: SGX (100 A◦ and 7 nm diameter loading material) 3.3 mm i.d. x 300 mm column; eluent: DMF (0.4 mL/minute), polystyrene standards. A refractive index detector was used to analyze the product at 25◦ C. The optical band gaps (Eg ) of the synthesized compounds were calculated from their absorption edges obtained by UV-Vis measurements. Excitation and fluorescence spectra were taken with a Shimadzu RF-5301 PC Spectrofluorophotometer. The polymer samples were dissolved in DMF and measurements were carried out by using a quartz cuvette with a 1.0 cm × 1.0 cm dimension. Fluorescence decays for the fluorescence lifetime were carried out with a Laser Strobe Model TM-3 lifetime fluorometer from Photon Technology International. The details of this method have been given elsewhere (31). Cyclic Voltammetry (CV) measurements at a potential scan rate of 20 mV/s. The electrochemical cell consists of an Ag wire as the reference electrode, a Pt wire as the counter electrode, and glassy carbon electrode as the working electrode immersed in tetrabutylammonium hexafluorophosphate (TBAPF6 )(0.1 M) as the supporting electrolyte. The voltammetric measurements were carried out in acetonitrile for the monomer and acetonitrile/DMSO mixture (v/v, 2:1) for the polymer. An ultrasonic bath was used to dissolve the samples. The Highest Occupied Molecular Orbital (HOMO)-Lowest Unoccupied Molecular Orbital (LUMO) energy levels and electrochemical band gaps (Eg ) were calculated from the oxidation and reduction onset values. Electrical properties of the polymer were determined with a four-point probe technique at room temperature and atmospheric pressure using Four Point Probe Measuring System FPP 470. The pellets were pressed on hydraulic press developing up to 100 bar/cm2.

2.2 Characterization

2.3 Microorganisms and Antimicrobial Activity Assay

Elemental analysis was carried out with a LECO CHNS932 (LECO Corporation, St Joseph, MI, USA). The infrared and UV–Vis absorption spectra were measured using a Perkin-Elmer FTIR Spectrum one (Perkin-Elmer, Llantrisant, UK) and Perkin-Elmer Lambda 25 (PerkinElmer, Shelton, CT, USA), respectively. The FTIR spectra were recorded using universal attenuated total reflectance sampling accessories (4000–650 1/cm) whose brand is Pike GladiATR (Madison, WI). 1H-NMR spectra (Varian XL-400 NMR) was also recorded at 25◦ C in DMSO with tetramethylsilane as an internal standard. Thermal data were obtained with a Perkin-Elmer Diamond Thermal Analyser. The Thermo Gravimetric Analysis (TGA), Derivative Thermo Gravimetric (DTG) and

The antimicrobial activities of 3-PHMP and poly(3PHMP) were assayed against Sarcina lutea ATCC 9341NA, Enterobacter aerogenes ATCC 13048, Bacillus subtilis ATCC 6633, Escherichia coli ATCC 39628, Enterococcus feacalis ATCC 29212, Serratia marcescens, MRSA (Methicillin Resistant Staphylococcus aureus), Pseudomonas aeruginosa, Klebsiella pneumonia, Listeria monocytogenes, Saccharomyces cerevisiae, and Candida albicans obtained from Celal Bayar University, Biology Department and Kahramanmaras Sutcu Imam University, Medical Faculty, Microbiology Laboratory. The antimicrobial assay was accomplished using Mueller-Hinton agar and Sabouraud dextrose agar by disc diffusion method. The culture suspensions of bacteria

Sch. 1. The structures poly(phenoxy-ketimine)s.

of

poly(phenoxy-imine)s

and

focused on synthesis of new m-substituted polyphenols derivates. In our previous paper, we synthesized a new msubstituted poly(phenoxy-ketimine) for the first time. On the basis of these studies, we decided to synthesize of a new m-substituted polyphenol containing the phenylhydrazono pendent group. Then, we evaluated the fluorescence, optical-electrochemical, conductivity, thermal and antimicrobial properties of the newly synthesized compounds and compared the results with those from previous studies. In addition, the calculation of fluorescence lifetime for poly(phenoxy-imine)s or poly(phenoxy-ketimine)s has been put forward with this study for the first time (Sch. 1).

2 Experimental 2.1 Materials

Synthesis of a Novel Poly(m-phenoxy-imine)

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and yeast were prepared with standardized inoculums, 108 cfu/mL and 106 cfu/mL, respectively (32). All of the compounds were dissolved in acetone and loaded to a sterile disc (Whatman no 1; 6 mm in diameter), at a concentration of 100 μg/disc and 200 μg/disc (33). Nystatine (100 U) and Chloramphenicol (30 μg/disc) were used as the standard and acetone as the solvent control. The plates were incubated at 36◦ C and 30◦ C for 24 h. After incubation, the diameter of the zone of inhibition was measured and evaluated. 2.4 Synthesis 2.4.1 Preparation of 3-((2-phenylhydrazono)methyl)phenol (3-PHMP) The monomer was prepared by a standard condensation method (34). Solutions of phenylhydrazine (1.08 g, 10 mmol) in ethanol (2 mL) and m-hydroxybenzaldehyde (1.22 g, 10 mmol) in ethanol (3 mL) were mixed and stirred for ∼ 30 min at room temperature. The precipitated product was filtered and washed with cold ethanol. The monomer (Sch. 2) was purified by recrystallization in ethanol to provide 2.06 g of the compound as a pale yellow-white solid. The purity was confirmed by silica plates and melting point determination. 3-PHMP: Yield: 97%. Rf : 0.38 (SiO2 , n-hexane/ethyl acetate, v/v, 3:1). Melting point: 142◦ C. 1H-NMR (400 MHz, DMSO-d6 , δ, ppm): 7.59 (1H, s, CH = N), 7.35–7.20 (4H, m, ArH), 7.19 (1H, bs, OH), 7.16 (1H, d, J = 7.5 Hz, ArH), 7.10 (2H, d, J = 7.6 Hz, ArH), 6.88 (1H, t, J = 7.2 Hz, ArH), 6.78 (1H, dd, J = 7.7, 2.5 Hz, ArH), 4.91 (1H, bs, NH). FTIR (υ max /1/cm): 3472 (O–H), 3307 (N–H), 3053 (C–H aryl), 2983–2900 (C–H aliphatic), 1593 (HC N), 1515–1446 (C C), 1253 (C–N), 1132 (C–O), 1071 (N–N). UV-Vis (λmax /nm): 308 and 353. Elemental analysis for C13 H12 N2 O: Theoretical (%): C, 73.56; H, 5.70; N, 13.20, found (%): C, 72.74; H, 5.65; N, 13.24. 2.4.2 The Oxidative Polycondensation of 3-PHMP using NaOCl The compound 3-PHMP (0.53 g, 2.5 mmol) was dissolved in an aqueous solution of KOH (0.14 g, 2.5 mmol, 10%) and placed into a 25-mL three-necked round bottomed

Sch. 2. The synthesis route of 3-PHMP and poly(3-PHMP).

Fig. 1. Effect of reaction temperature on the yield of polymer {time: 5 h, [3-PHMP]0 = [NaOCl]0 = [KOH]0 = 0.25 mol/L, and flow rate of air oxygen = 0.5 L/h}.

flask that was fitted with a condenser, a thermometer and a stirrer, in addition to a funnel containing NaOCl. After heating to the appropriate temperature, NaOCl was added as dropwise for ∼ 30 min. The reaction mixture was stirred under different conditions (Figs. 1–4). At the end of the reaction, the mixture was cooled to room temperature and neutralized with HCl (1.0 M, 0.5 mL). The mixture was filtered and washed with hot water (25 mL, three times), and the removal of mineral salts was confirmed using an AgNO3 solution. The unreacted 3-PHMP was then separated from the reaction products by washing with a diethyl ether/nhexane mixture (7 mL, v/v, 1:1). The polymeric product, dark brown powder, was dried in an oven at 105◦ C. The purity was confirmed with silica plates and melting point determination (Sch. 2). Poly(3-PHMP): Yield: 56%. Rf : 0 (SiO2 , n-hexane/ethyl acetate, v/v, 3:1) and Rf : 0.71 (SiO2 , methanol). 1HNMR (400 MHz, DMSO-d6 , δ, ppm): 7.94–6.32 (10H, m, CH = N, OH, NH and ArH). FTIR (υ max /1/cm): 3646 (O–H), 3325 (N–H), 3059 (C–H aryl), 2988–2904 (C–H aliphatic), 1595 (HC N), 1493–1451 (C C), 1250 (C-N),

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Fig. 2. Effect of KOH concentration on the yield of polymer {temperature: 70◦ C, time: 5 h, [3-PHMP]0 = [NaOCl]0 = 0.25 mol/L, and flow rate of air oxygen = 0.5 L/h}.

1157 (C–O), 1073 (N–N). UV-Vis (λmax /nm): 291(shoulder) and 345(shoulder). Elemental analysis for C13 H10 N2 O: Theoretical (%): C, 74.26; H, 4.80; N, 13.33, found (%): C, 74.66; H, 4.74; N, 13.26.

2.4.3 The Oxidative Polycondensation of 3-PHMP using Air Oxygen The compound 3-PHMP (0.53 g, 2.5 mmol) was dissolved in an aqueous solution of KOH (0.14 g, 2.5 mmol, 10%) and placed into a 25-mL three-necked round bottomed flask that was fitted with a condenser, a thermometer and a stirrer, in addition to a glass tubing condenser for sending air. In order to prevent water loss in the reaction mixture and to deneutralize the CO2 of the air to KOH, air oxygen was passed into 200 mL of an aqueous solution of KOH (20%) before passing through the reaction mixture. After heating to the appropriate temperature, air oxygen was bubbled through the reaction mixture at a rate ranging from 0.25 to 1 L/h (Figs. 1–4). At the end of the reaction, the mixture was cooled to room temperature and neutralized with HCl (1.0 M, 0.5 mL), and the solid product was filtered and washed with hot water (25 mL, three times), and the removal of mineral salts was confirmed using an AgNO3 solution. The unreacted 3-PHMP was then separated from the reaction products by washing with a diethyl ether/nhexane mixture (7 mL, v/v, 1:1). The polymeric product, dark brown powder, was dried in an oven at 105◦ C. The purity was confirmed with silica plates and melting point determination (Sch. 2).

Fig. 3. Effect of NaOCl concentration (a) and flow rate of air oxygen (b) on the yield of polymer {temperature: 70◦ C, time: 5 h and [3-PHMP]0 = [KOH]0 = 0.25 mol/L}.

3 Results and Discussion 3.1 The Investigation of Synthesis Conditions of the Polymer The oxidative polycondensation of 3-PHMP was investigated in various media such as acidic, alkaline and organic. No polymerization reaction was observed in an acidic and/or organic using an oxidant such as sodium hypochlorite or air (O2 ). Although 3-PHMP was oxidized in the aqueous alkaline medium by air oxygen and sodium hypochlorite, it was not oxidized by hydrogen peroxide in the same conditions. Additionally, the effects of various

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Fig. 5. UV spectra of 3-PHMP (a) and poly(3-PHMP) (b). (Color figure available online.)

3.2 Characterization of Poly(3-PHMP) Fig. 4. Effect of reaction time on the yield of polymer {temperature: 70◦ C, [3-PHMP]0 = [NaOCl]0 = [KOH]0 = 0.25 mol/L and flow rate of air oxygen = 0.5 L/h}.

parameters, such as the initial concentrations of KOH and NaOCl, the flow rates of air oxygen, reaction time and temperature, on the yield of oxidative polycondensation products were examined; the results were examined as given in Figures 1–4. The yield of oxidative polycondensation products from 3-PHMP was 56% under the optimum reaction conditions: [3-PHMP]0 = [KOH]0 = [NaOCl]0 = 0.25 mol/L at 70◦ C for 25 h. When air oxygen was used as an oxidant, the yield of oxidative polycondensation product of 3-PHMP was determined to be 39% under optimum reaction conditions: [3-PHMP]0 = 0.25 mol/L, [KOH]0 = 0.25 mol/L, flow rate of air oxygen = 0.5 L/hour at 70◦ C for 10 h. In comparison of the data obtained, it was observed that NaOCl is more active than air (O2 ) for the synthesis of the polymer (also as seen in Table 1). Additionally, the yield of the polymer was dependent upon temperature, time, the initial concentrations and the type of oxidant.

3.2.1 Solubility The solubility tests were performed with 1 mg sample and 1 mL solvent at 25◦ C. The compounds 3-PHMP and poly(3-PHMP) were stable at room temperature. Both compounds were soluble in highly polar solvents such as ethanol, DMSO, DMF, and solutions of KOH and concentrated H2 SO4 , although they were insoluble in nhexane. Nevertheless, the monomer was completely soluble in CH2 Cl2 , CHCl3 , ethyl acetate, acetonitrile, and THF while poly(3-PHMP) was partly soluble in these solvents. Furthermore, the monomer was soluble in diethyl ether, whereas poly(3-PHMP) was insoluble. 3.2.2 Structural Analysis In the previous papers describing the oxidative polycondensation of phenol and substituted phenols, NMR and FTIR analyses showed that the resulting polyphenols had a structure consisting of a mixture of phenylene and oxyphenylene units (7, 35). In the present study, the structures of synthesized compounds were analysed with UV-Vis, FTIR and 1 H-NMR spectroscopies. The electronic absorption spectra (Fig. 5) for 3-PHMP and poly(3-PHMP) were obtained in DMSO at room temperature and the spectral data are presented in the

Table 1. Molecular weight distribution parameters of poly(3-PHMP) Total Compounds Poly(3-PHMP)a Poly(3-PHMP)b a

Fraction I

Mn (g/mol)

Mw (g/mol)

PDI

Mn (g/mol)

Mw (g/mol)

PDI

%

Mn (g/mol)

Mw (g/mol)

PDI

%

15,888 17,912

16,843 19,893

1.06 1.11

24,230 22,670

25,560 26,920

1.05 1.19

52 39

6,850 14,870

7,400 15,400

1.08 1.04

48 61

NaOCl oxidant. Air oxygen oxidant (despite of similar reaction conditions).

b

Fraction II

714 experimental section. The UV-Vis spectrum of monomer revealed two bands centered at 308, 353 nm which are attributed to π→π ∗ transition of benzene -C C- and azomethine -C N- and, n→π ∗ transition of phenolic -OH and azomethine -C N- groups, respectively (14). On the other hand, poly(3-PHMP) displayed similar transitions which are centered at 291 (shoulder), 345 (shoulder) nm. The resemblance of the UV–Vis spectra of 3-PHMP and poly(3PHMP) points out the similarly of resonance-inductive effects on both structures and the unperturbed nature of monomer during polymerization. In addition, band shifting until 690 nm was observed in the poly(3-PHMP) spectrum, which was attributed to an increase in conjugation due to coupling between aromatic rings (16, 36). In comparison of the FTIR spectra of 3-PHMP and poly(3-PHMP), the observed differences were in the reduction of the band strength and peak numbers, and in the formation of new bands strengths between 650–900 1/cm (Fig. 6). The peak at 1595 1/cm was assigned to the characteristic absorption of C N vibration. In the IR spectrum of poly(3-PHMP), a broad peak centred at 3646 1/cm was due to the vibration of the O-H linkage of the phenolic group, and a peak at 3059 1/cm was attributed to the strength vibrations of the N-H linkage. The bands at 650–900 1/cm correspond to different substitution patterns in the aromatic ring (37, 38). In the IR spectrum of 3-PHMP, peaks at 688 and 742 1/cm show the presence of five adjacent aromatic hydrogen atoms located in 9–13

Demir et al.

Sch. 3. The numbers for C-H groups of 3-PHMP.

positions of the benzyl ring; the peak at 778 1/cm shows the presence of three adjacent aromatic hydrogen atoms located in 4-, 5-, and 6-positions, and the peak at 884 1/cm shows the presence of isolated hydrogen atom located in 2-position the phenyl ring. In the case of poly(3-PHMP), the peak intensity at ∼780 1/cm decreases considerably, while the peak intensity at ∼880 1/cm increases. Furthermore, the FTIR spectrum of poly(3-PHMP) showed a new small broad absorption band at 842 1/cm, which was most intense and characteristic absorption of two adjacent aromatic hydrogen atoms, was located in 4- and 5- or 5- and 6-positions of the phenyl ring appears at terminal groups (as seen in Scheme 2 b, c and 3). Since no additional peaks at 1100–1250 1/cm for C-O-C linkages were observed in the FTIR spectrum of polymer, it seems obvious that poly(3PHMP) consists of phenylene units in accordance with the reported value in the literatures (27, 39). Additionally, the polyphenols obtained by the oxidative polycondensation of phenol and its derivatives show

Fig. 6. FTIR spectra of 3-PHMP (a) and poly(3-PHMP) (b). (Color figure available online.)

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Fig. 7. 1H-NMR spectra of poly(3-PHMP) (1) and 3-PHMP (2).

a small peak at ∼1660 1/cm which is attributed to the carbonyl (C O) stretching vibration and may be formed by the oxidation of the imines groups at the polymer pendent groups (6, 40). Interestingly, in the spectrum of the synthesized polymer cannot be detected a peak originated from the carbonyl group because the m-substituent prevents the oxidation of the imine groups. The 1H-NMR spectra of the 3-PHMP and its polymer are presented in Figure 7. In the NMR spectrum of poly(3PHMP), the signals were shown broadly and it was one of the proofs of polymerization. A broad multiplet peak in the 6.32–7.94 ppm range can be attributed to the aromatic protons (Ar-H), imine proton (CH = N), hydroxy proton (OH) and secondary amine proton (NH) groups. The FTIR and 1H-NMR results showed the formation of polymeric macromolecules from the 3-PHMP residue via polymerization on C2 , C4 and C6 positions (Schs. 2, 3). This suggests that the polymerization proceeds primarily through ortho–para coupling, and poly(3-PHMP) may have three possible repeating units (Sch. 2). The calculated number average molecular weight (Mn ), weight average molecular weight (Mw ) and polydispersity index (PDI) values of the polymer are presented in Table 1. According to SEC analysis, the molecular mass distribution values of the polymer prepared shows the bimodal character. In other words, poly(3-PHMP) consists of two section. The first section (39–52%) have Mn = 22670–24230 and Mw = 25560–26920 and the second (48–61%) have Mn = 6850–14870 and Mw = 7400–15400. The obtained

bimodality of molecular mass distribution values of poly(3PHMP) may be attributed to the polymer coupling occurring at active centers of various (radical) natures (41). According to the total values, the synthesized polyphenol have quite high molecular weights. The obtained results confirm the polymer structures. These results show that poly(3-PHMP) has the higher Mn and Mw values compared to the other m-substituted poly(phenoxy-imine) and poly(phenoxy-ketimine) such as O-B-3’-HA (27) and poly(3-PHEP) (42). Analytical data of the compounds are presented in the experimental section. The experimental elemental analyses results of the two compounds are in good compliance with the theoretical calculations. The analytical and spectroscopic data enable us to predict the possible structure of the compounds as shown Figures 5–7. Based on the elemental analyses and spectroscopic characterization, these compounds are presumed to have the structures shown in Scheme 2.

3.3 The Investigation of Properties of the Synthesized Compounds 3.3.1 Fluorescence Properties of poly(3-PHMP) The fluorescence properties of poly(3-PHMP) in DMF were investigated at room temperature. The fluorescence spectra of the polymer were taken for various concentrations in order to determine the effect of polymer

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Fig. 9. Fluorescence decay of poly(3-PHMP) in DMF (λex = 337 nm). (Color figure available online.)

Fig. 8. The excitation (a) and fluorescence spectra (b) of poly(3PHMP) in DMF solution at different concentrations. λem = 343 nm for excitation spectra and λex = 300 nm for emission spectra were used. (Color figure available online.)

concentration on the fluorescence properties. In this regard, the concentration of poly(3-PHMP) from 100.0 mg/L to 3.125 mg/L in DMF was altered. Figure 8 shows the excitation and fluorescence spectra of poly(3-PHMP) in a DMF solution. Taking into account Figure 8, the intensity of excitation and fluorescence are dependent on the concentration of poly(3-PHMP), which the low concentration of poly(3PHMP) in DMF possess strong fluorescence. In the excitation spectra (Fig. 8a), one excitation band at ∼295 nm was observed and its relative intensity was decreased by increasing the concentration of the polymer in DMF. In contrast, the fluorescence properties of poly(3-PHMP) depending on the concentration of the polymer is interesting (Fig. 8b). In the diluted polymer concentration, it was observed one intense fluorescence band at ∼343 nm. When the concentration of poly(3-PHMP) in DMF is increased, the fluorescence band at 343 nm was decreased and a new fluorescence band at ∼ 430 nm was aroused. The change in the

fluorescence spectrum of the polymer could be attributed to the arrangement of poly(3-PHMP) molecules in DMF at the higher concentration. The fluorescence properties of poly(3-PHMP) in DMF resemble that of 3-PHMP in DMF. The fluorescence spectra of 3-PHMP in DMF was given in the inset of Figure 8a. The main fluorescence band of 3-PHMP in DMF was located at ∼393 nm and the fluorescence intensity of 3-PHMP was strongly quenched increasing the amount of 3-PHMP. It was observed that the fluorescence band maximum of poly(3-PHMP) was redshifted due to the conjugation effect of the polymer when the obtained results are compared (Fig. 8b). In addition, we evaluated the fluorescence lifetime of poly(3-PHMP) in DMF at a moderate polymer concentration (25.0 mg/L). In order to determine the fluorescence lifetime of poly(3PHMP) in DMF, the fluorescence decay spectrum of the polymer sample was recorded at an 337 nm excitation wavelength (Fig. 9).

Fig. 10. Cyclic voltammograms of 3-PHMP (a) and poly(3PHMP) (b). (Color figure available online.)

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Synthesis of a Novel Poly(m-phenoxy-imine) Table 2. Electrochemical data for the monomer and the polymer Compounds 3-PHMP Poly(3-PHMP)

Eox (V) 1.072 1.015

Ered (V) −1.061 −0.927

HOMO (eV) −5.972 −5.405

LUMO (eV) −3.329 −3.463

Eg (eV)

Table 3. Thermal decomposition values of the monomer and the polymer Compounds

2.64 1.94

3-PHMP Poly(3-PHMP)

Tinitial (◦ C)

50% mass loss (◦ C)

Carb. Res.(%) at 1000◦ C

123 151

270 500

9.3 32

Abbreviations: Carb. Res., Carbine Residue

Exponential analyses of the fluorescence decay of poly(3PHMP) in DMF was fitted to the single-exponential decays with the acceptable statistical χ 2 values. As a result, the fluorescence lifetime of poly(3-PHMP) in DMF was calculated as 2.88 ns (χ 2 = 1.12). Under the same conditions, the lifetime of 3-PHMP in DMF was found to be 0.92 ns (χ 2 = 1.10). The higher lifetime of poly(3-PHMP) in DMF is attributed to increasing conjugation of the polymer compared to that in 3-PHMP. 3.3.2 Optical Properties The poly(3-PHMP) and 3-PHMP have absorption spectrum in UV-Vis region (Fig. 5). The poly(3-PHMP) shows a broad absorption band extending to ∼ 690 nm as can be seen in Figure 5. The absorption edge of the poly(3PHMP) is shifted to higher wavelengths than that of 3PHMP monomer due to increasing conjugation. Optical band gap (Eg ) values calculated from absorption edges (43) for 3-PHMP and poly(3-PHMP) were 3.15 eV and 2.98 eV, respectively (Fig. 5). As expected, the polymer has a lower optical band gap than the monomer. 3.3.3 Electrochemical Properties Figure 10 shows cyclic voltammograms of 3-PHMP and poly(3-PHMP). The HOMO–LUMO energy levels and the electrochemical band gaps (Eg ) were estimated by using the oxidation onset (Eox ) and reduction onset (Ered ) values (44),

as shown below. EHOMO = −(4.39 + Eox ) ELUMO = −(4.39 + Ered ) Eg = ELUMO − EHOMO 3-PHMP and poly(3-PHMP) were electrochemically active in both oxidation and reduction regions. The electrochemical data of 3-PHMP and poly(3-PHMP) are listed in Table 2. As can be seen, Figure 10 and Table 2, poly(3-PHMP) has a lower band gap compared to that of the monomer because of the polyconjugated structures of the polymers, which increase HOMO and decrease LUMO energy levels resulting in lower band gaps. The value of Eg measured for poly(3-PHMP) is low compared to that of other NO (29), NNO (42, 45, 46) and N2O2 (47) types Schiff base polyphenols. The lowest band gap; i.e., 1.52 eV, was found among these polyphenols (46). The results obtained in the present study are compatible with the literature values. It is well known that lower band gaps facilitate electronic transitions between HOMO and LUMO energy levels and make the polymers more electro-conductive than monomers.

Fig. 11. TGA-DTG-DTA curves of the monomer (1) and the polymer (2). (Color figure available online.)

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Table 4. Antimicrobial activities of 3-PHMP and polymer(3-PHMP) 3-PHMP Microorganisms Sarcina lutea ATCC 9341NA Enterobacter aerogenes ATCC 13048 Bacillus subtilis ATCC 6633 Escherichia coli ATCC 39628 Serratia marcescens∗ Enterococcus feacalis ATCC 29212 MRSA∗ Pseudomonas aeruginosa∗ Klebsiella pneumonia ∗ Saccharomyces cerevisiae Candida albicans∗

Poly(3-PHMP)

Control

100 (μg/disc)

200 (μg/disc)

100 (μg/disc)

200 (μg/disc)

chl (30 μg/disc)

Nys (100 U)

22 0 17 18 0 25 14 0 8 15 16

25 0 18 28 9 26 16 0 8 20 17

0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 15 0 0 0 0 0

35 35 14 21 34 23 44 0 22 NT NT

NT NT NT NT NT NT NT NT NT 18 18

Symbol: ∗ , Clinical isolate. Abbreviations: NT, Not Tested; Chl, Chloramphenicol (30 μg/disc); Nys, Nystatine.

3.3.4 Conductivity of poly(3-PHMP) Conductivity measurements of the poly(3-PHMP) were carried out with an electrometer using a four-point probe technique. The conductivity value for poly(3-PHMP) was found to be approximately 3.2 × 10−2 S/cm. According to the results, poly(3-PHMP) has higher conductivity values then other poly(phenoxy-imine)s. Among these poly(phenoxy-imine)s, the highest electrical conductivity; i.e., 5.86 × 10−5 S/cm, was found (25). When the values of electrical conductivity at the range of 10−2 S/cm, which was obtained for poly(3-PHMP), is compared with the values obtained for o-substituted, m-substituted and p-substituted poly(phenoxy-imine)s in the literature, it can be seen that the highest values belong to poly(3-PHMP). This is because of the polyconjugated structures of the polymers with phenylhydrazono pendent groups, which increase electrical conductivity values. Having this feature, these polyphenols step forwards in comparison with other imine-substituted polyphenols. The results obtained in this study are compatible with the values in the literature. 3.3.5 Thermal Properties The TGA-DTG-DTA curves of monomer and polymer are presented in Figure 11 and the obtained data are summarized in Table 3. The polyphenols contain absorbed water molecules in their structures in that they have a hygroscopic nature (36, 48). Due to loss of absorbed water molecules (2.5%), the initial decomposition temperature of 3-PHMP was higher than that of poly(3-PHMP). The presence of water can be seen in the TGA curves of poly(3-PHMP) (Figure 11). 50% Mass loss of 3-PHMP and poly(3-PHMP) were observed 270◦ C and 500◦ C, respectively. Similarly, the level of carbine residue quantity of poly(3-PHMP) was higher than that of the monomer. Although the initial degradation temperature of poly(3-PHMP) was low compared with

that of 3-PHMP, the polymer has a higher carbine residue and 50% degradation temperature. Also, in the DTA curve of the monomer, endothermic peak was observed at 142◦ C (Fig. 11). 3.3.6 Antimicrobial Activities of the Synthesized Compounds The synthesized monomer and the polymer were tested for antimicrobial activity with respect to certain standard strains and clinical isolates with the conventional disc diffusion method. The antimicrobial activities of substances are presented in Table 4. The results revealed that 3-PHMP had an antimicrobial activity against all the tested microorganisms, with the except of E.aerogenes and P.aeruginosa. In contrast, the polymer had no antimicrobial effect on the microorganisms tested. The only inhibitory effect was observed with poly(3-PHMP) against the E.feacalis. As can be seen in Table 4, the polymer does not have an inhibitory effect, while the monomer does. This circumstance may be explained by the long chain structure of the polymer, which prevents entry into the cell.

4 Conclusions m-Substituted polyphenol with phenylhydrazono pendent groups, poly(3-PHMP), has been successfully synthesized by oxidative polycondensation. The reaction temperature, time and initial concentrations of alkaline and NaOCl have dramatic effects on the yield of polymer. Poly(3-PHMP) with reactive imine and hydroxyl group is readily soluble in common polar organic solvents such as DMSO, DMF and ethanol, and therefore will have useful applications, such as synthesis of resins, and copolymer. The observed band gaps are low enough to make the polymer promising

Synthesis of a Novel Poly(m-phenoxy-imine) for photovoltaic applications. The polymer also exhibited reasonable electrical conductivity and good fluorescence property. The fluorescence lifetime value of poly(phenoxyimine)s or poly(phenoxy-ketimine)s has been put forward with this study for the first time. The electrical conductivity value is encouraging and indicates that the oxidative polycondensation might be used to obtain these electro active polymers. Those interesting properties of poly(3-PHMP) will have potentially beneficial applications in various fields such as semi-conductive materials and photovoltaic cells.

Acknowledgments We would like to thank Professor Dr. A. U. Tamer for his kind help providing some of the microorganisms. We also thank KSU, Biology Department, Biotechnology Laboratory for allowing us to carry out the antimicrobial tests.

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