Thiophene substituted phenothiazine polymers: Design, synthesis and characterization

2.1 Materials

Phenothiazine, 2-ethoxyethanol, 1,4-diiodobenzene, 1,4-bis(bromomethyl)benzene, sodium hydride, tetrakis(triphenylphosphine)palladium (0), Iron (III) chloride were purchased from Sigma Aldrich, whereas N-bromosuccinimide (NBS), 1,2-bis(2-chloroethoxy)ethane and 1,5-dibromopentane were bought from Alfa Aesar. 2-thienylboronic acid was provided by Across. All solvents described herein were purchased from Merck Chemical Co. (Germany) and used without any purification.

2.2 Characterization techniques

FT-IR spectra of the synthesized monomers and polymers were recorded on a Perkin Elmer Spectrum One over the range of 4000–500 cm⁻¹. ¹H NMR and ¹³C NMR spectra were recorded on a JEOL ECX-400 II spectrometer at room temperature in DMSO‑d₆ as a solvent. Ultraviolet–visible (UV–Vis) spectra were measured by Analytikjena Specord 210 Plus to study the electronic transition of all compounds with conjugated π systems. Emission and excitation spectra of the synthesized monomers and polymers were determined by Shimadzu RF-5301PC Spectrofluorophotometer. Thermogravimetry analyses (TGA) were performed in nitrogen atmosphere (200 mL min⁻¹) at a heating rate of 10 °C min⁻¹ from 20 °C to 1000 °C with a Perkin Elmer Diamond Thermal Analysis System. Electrochemical studies of the synthesized compounds were performed using CHI 660C Electrochemical Analyzer (CH Instruments, Texas, USA). A three electrode system was used for cyclic voltammetry (CV) measurements, which glassy carbon electrode (GCE), Pt wire and Ag/AgCl as working, counter and reference electrode, were used. The HOMO-LUMO energy levels and electrochemical band gaps (E′_g) of monomers and polymers were calculated from oxidation and reduction onset values. Molecular weights were determined by Viscotek GPC max Auto sampler system. Surface morphology of the polymers were determined by using a JEOL JSM-7100F field emission scanning electron microscope.

2.3 Synthesis of the starting materials

3,7-dibromo-10H-phenothiazine (DBP) (Hsieh et al., 2015) and 3,7-Dithienyl-10H-phenothiazine (DTP) were synthesized according to literature (Hemgesberg et al., 2013). The structures of the synthesized compounds were confirmed by FT-IR, ¹H NMR and ¹³C NMR spectra.

For DBP FT-IR (cm⁻¹): 3337 v (N—H), 1592–1441 v (C⚌C), 1311 v (C—N), 737 v (C—S) 649 v (C—Br).¹H NMR (DMSO‑d₆, δ, ppm): 8.85 (s, 1H, N—H), 6.59 (d, 2H), 7.11 (s, 1H), 7.14 (dd, 2H). ¹³C NMR (DMSO‑d₆, δ, ppm):141.43, 130.12, 128.62, 118.72, 116.55, 113.19.

For DTP FT-IR (cm⁻¹): 3364 v (N—H), 1590–1465 v (C⚌C), 1298 v (C—N), 798 v (C—S).¹H NMR (DMSO‑d₆, δ,ppm): 8.96 (s, 1H, N—H), 7.49 (d, 2H), 7.42 (d, 2H), 7.33 (s, 2H), 7.13 (t, 2H), 6.75 (d, 2H). ¹³C NMR (DMSO‑d₆, δ, ppm): 143.12, 141.10, 128.86, 128.30, 125.50, 124.88, 123.36, 122.89, 117.17, 115.25.

2.4 Synthesis of the aliphatic and aromatic bridging phenothiazine monomers

3,7-dithienyl-10H-phenothiazine (0.36 g, 1 mmol) and sodium hydride (60% dispersion in mineral oil) (0.24 g, 6 mmol) were dissolved in 10 mL of DMF under the argon atmosphere at room temperature. After 30 min, dihalogene compound (0.5 mmol) in 5 mL DMF was added into the reaction mixture, which was stirred for 16 h at 90 °C. Then, reaction mixture was cooled to room temperature. This solution was poured into 100 mL of water and product was filtered, dried and recrystallized from CHCl₃/hexane (1:1). The product was dried in a vacuum oven at 70 °C (Jo et al., 2015). This step was repeated separately for each compound. The yields of BDTPP, BDTPEE, BDTPB and BDTPMB were found to be 60, 65, 53 and 45%, respectively. Melting points values of BDTPP, BDTPEE, BDTPB and BDTPMB were found to be between 181 and 183 °C, 170–171 °C, 216–217 °C and 222–224 °C, respectively.

2.5 General procedure for the polymerization reaction

To a suspension of FeCl₃ (0.810 g, 5 mmol) in chloroform (10 mL), 0.5 mmol phenothiazine monomer was added under argon atmosphere. The reaction mixture was stirred for 72 h at room temperature. This solution was then diluted with chloroform and washed with water. The organic phase was separated and stirred with ammonia (aq. 20%, 2 × 100 mL) for 30 min. Then, 0.2 M ethylene-diamino-tetra-acetic acid (EDTA) solution (100 mL) and water (100 mL) were added into the solution. The solution was poured into methanol and the product was collected by centrifugation (Aydın et al., 2013). The yields of P-BDTPP, P-BDTPEE, P-BDTPB and P-BDTPMB were found to be 48, 50, 45 and 43%, respectively. The general synthetic methods of compounds and structures of the polymers are shown in Schemes 1 and 2, respectively.

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Scheme 1 Synthetic pathway of the compounds.

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Scheme 2 The structures of synthesized polymers.

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3.1 Structural characterization of the synthesized compounds

The FT-IR spectral data of the synthesized compounds are given in Table 1. According to Table 1, the characteristic peaks like aromatic and aliphatic C—H stretching, C—Ccoupling peaks are observed as expected. The FT-IR spectra of the starting compounds DBP and DTP are given in Fig. 1 FT-IR spectrum of DBP, which possesses an N—H stretching peak and C—Br stretching peak at 3337 and 649 cm⁻¹ respectively, as shown in Fig. 1. DTP possesses N—H stretching peak and C—S stretching peak at 3364 and 798 cm⁻¹ respectively. This result showed that bromine was not bound at N position. Fig. 2 shows FT-IR spectra of the 1,2-bis(2-chloroethoxy)ethane (BDTPEE) and 1,4-bis(bromomethyl)benzene (BDTPMB) bridged monomers and polymers. As shown in Fig. 2, while —NH peak disappears, aliphatic and aromatic C—H stretching peaks appear in the structure of monomer and polymer. After the polymerization of each monomer, C—Ccoupling peaks were observed at around 1070 cm⁻¹. In addition, imine (HC⚌N) peaks were observed at around 1620 cm⁻¹ due to conjugation in phenothiazine ring. The broad bands of the polymers are related to the polyconjugation in the polymer structures. As seen Table 1, other compounds have similar results as expected. While aromatic and aliphatic C—H peaks were observed at around 3055 cm⁻¹ and 2934 cm⁻¹ respectively, the peaks at 1441–1592 cm⁻¹ were attributed to C⚌C stretching frequency of benzene ring for all the compounds.

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Fig. 1 FT-IR spectra of DBP and DTP.

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Fig. 2 FT-IR spectra BDTPEE and BDTPMB monomers and their polymers.

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The solubility behavior of the synthesized compounds is investigated in different solvents and the results are presented in Table 2. Generally, all compounds were soluble in polar organic solvents such as DMF, DMSO and DMA.

¹H NMR and ¹³C NMR spectra of BDTPEE and P-BDTPEE data are given in Figs. 3 and 4, respectively. As seen in the ¹H NMR spectra of BDTPEE, the proton values of aromatic units were observed at 7.16–7.59 ppm. In addition, the aliphatic peaks were observed at 1.35–3.85 ppm. After the polymerization, the signal of the H_g proton at 7.59 ppm as doublet in the BDTPEE compound disappeared, and the signal of H_f proton at 7.16 ppm as triplet in BDTPEE compound was observed at 7.30 ppm as doublet in the polymer compound. The ¹³C NMR spectrum of BDTPEE indicated that carbon atoms of the aliphatic chain were observed at 58.80–69.90 ppm while the aromatic carbon atoms were observed at 118–146 ppm. The ¹³C NMR spectrum of P-BDTPEE showed the resonance signals of aromatic carbon atoms at 120–148 ppm whereas carbon atoms of aliphatic chain were observed at 60–69 ppm. Additionally, in the ¹³C NMR spectrum of the P-BDTPEE a significant decline in the intensity of C₁₀ was observed compared to monomer. These results indicate that polymerization takes place predominantly at C₁₀ of thiophene units. The NMR analyses data of the synthesized compounds are listed in Table 3.

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Fig. 3 ¹H NMR spectra of BDTPEE and P-BDTPEE.

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Fig. 4 ¹³C NMR spectra of BDTPEE and P-BDTPEE.

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Table 3 NMR spectra data of the synthesized compounds.

Compounds	NMR spectra data (DMSO‑d6, δ, ppm)
	1H NMR: 7.60 (d, —Hf), 7.57 (d, —Hb), 7.44 (s, —Ha) 7.40 (d, —Hd) 7.30 (s, —Hh) 7.25 (d, —Hc) 7.20 (t,—He), 3.71 (s, —Hg). 13C NMR: 145.63 (C6), 140.82 (C7), 136.16 (C1), 132.70 (C10), 130.28 (C13), 128.89 (C12), 127.07 (C7), 126.78 (C9), 126.55 (C2), 124.06 (C3), 121.37 (C4), 120.77 (C5).
	1H NMR: 7.55 (d,—Hb), 7.47 (s —Ha) 7.44 (d,—He), 7.39 (s,—Hi) 7.32 (d, —Hc) 7.27 (d,—Hf) 3.74 (s, —Hh). 13C NMR: 147.42 (C6), 141.41 (C7), 138.27 (C1), 133.76 (C10), 131.87 (C13), 131.19 (C12), 128.93 (C7), 127.60 (C9), 127.38 (C2), 125.11 (C3), 123.31 (C4), 121.44 (C5), 54.37 (C11).
	1H NMR: 7.62 (d, —Hf), 7.54 (d, —Hd), 7.51 (t, —He), 7.37 (d, —Hc), 7.23 (d, —Hb), 7.10 (s, —Ha), 4.32 (t, —Hg), 1.75 (m, —Hh), 1.24 (m, —Hi). 13C NMR: 145.95 (C6), 143.36 (C7), 132.19 (C10), 130.35 (C2), 129.59 (C9), 128.71 (C8), 125.48 (C4), 124.78 (C3), 120.32 (C5), 117.21 (C1), 53.55 (C11), 30.73 (C12), 26.69 (C13).
Compounds	NMR spectra data (DMSO‑d6, ppm)
	1H NMR: 7.56 (d, —Hd), 7.52 (d, —He), 7.40 (d, —Hc), 7.24 (d, —Hb), 7.12 (s, —Ha), 4.34 (t, —Hg), 1.79 (m, —Hh), 1.28 (m, —Hi). 13C NMR: 148.77 (C6), 145.47 (C7), 134.15 (C10), 131.73 (C2), 130.53 (C9), 129.72 (C8), 126.58 (C4), 125.78 (C3), 122.79 (C5), 117.51 (C1), 53.70 (C11), 33.39 (C12), 27.61 (C13).
	1H NMR: 7.62 (d, —Hf), 7.54 (d, —Hd), 7.41 (t, —He), 7.13 (s, —Ha), 7.26 (d, —Hc), 7.23 (d, —Hb), 7.16 (s, —Hg). 13C NMR: 144.26 (C6), 140.91 (C7), 140.42 (C11), 136.28 (C12), 132.64 (C10), 129.10 (C9), 128.80 (C8), 126.73 (C1), 130.04 (C2), 125.55 (C3), 123.68 (C4), 121.12 (C5)
	1H NMR: 7.65 (d, —Hd), 7.56 (d, —He), 7.18 (s,—Ha), 7.28 (d, —Hc), 7.26 (d, —Hb), 7.18 (s, —Hg). 13C NMR: 145.64 (C6), 142.48 (C7), 141.86 (C11), 138.65 (C12), 133.57 (C10), 132.74 (C9), 129.42 (C8), 128.52 (C1), 127.28 (C2), 126.33 (C3), 125. 47 (C4), 123.27 (C5).

3.2 Optical and electrochemical properties

Fig. 5 shows the UV–Vis spectra that are measured for the monomers and polymers in DMSO. Band gaps of the monomers and the polymers were estimated from low energy band edges from UV–Vis absorption spectra using the following equation in the literature (Colladet et al., 2004) and were given in Table 2.

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Fig. 5 Absorption spectra of the monomers and polymers.

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According to UV–Vis spectra, a red shift occurs at the absorption edge of polymers compared to monomers due to polyconjugation (Kaya et al., 2013). Moreover, the calculated optical band gaps of polymer had lower than those of monomers. It suggested that the increase of phenothiazine-thiophene units resulted in the decrease of E_g due to extending of the conjugated length of polymers (Qui et al., 2007).

According to the obtained spectra, the band above 280 nm is mainly owing to an n → π* transition because of the presence of lone electron pairs of sulfur atom in the phenothiazine ring (Manju et al., 2012). The peak at 308–410 nm is formed because of a delocalized π → π* transition along the polymer chain, and the peak at 460–545 nm is induced from a localized transition at the state of charge transfer between phenothiazine and thiophene rings (Yun et al., 2013).

Cyclic voltammetry (CV) was carried out to investigate electrochemical properties of the synthesized monomers and polymers. As seen in Fig. 6, all compounds have two oxidation peaks except BDTPP monomer and its corresponding polymer. BDTPP monomer and its corresponding polymer have pentane chain as bridge. This means that, after the oxidation of the phenothiazine core the electron density of BDTPP compound is not sufficiently high to allow thiophene oxidation. When Fig. 6 is examined, the oxidation potential of the polymer compounds lower values compared to that of the monomer compounds. Since thiophene is a relatively electron-rich, the thienyl units should thus induce a much higher electron density within the phenothiazine, making it much more susceptible towards oxidation (Hemgesberg et al., 2013). From the onset, oxidation and reduction peak potentials HOMO-LUMO energy levels and electrochemical band gaps (E′_g) were calculated as shown in Table 4. To calculate these parameters, the following equations were used (Cervini et al., 1997):

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Fig. 6 Cyclic voltammograms of the synthesized compounds.

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According to Table 4, the electrochemical band gaps (E′_g) of polymers changes are as follows: P-BDTPB < P-BDTPMB < P-BDTPEE < P-BDTPP. P-BDTPP has a bit higher electrochemical and optical band gaps than other polymers. This can be probably because of straight aliphatic structure of P-BDTPP. The observed optical band gaps of polymers are smaller than thiophene substituted phenothiazine polymers in literature the earlier reported (Lin et al., 2015). By using solar cells fabricated from these low bandgap polymers, it is possible to capture more of the solar radiation. (Cai et.al., 2010).

3.3 Fluorescence measurement

Fluorescence properties of the synthesized compounds were determined using DMSO solutions. Fig. 7 shows fluorescence spectra of synthesized monomers and polymers. The fluorescence intensity was enhanced more significantly from P-BDTPB to P-BDTPMB and less from P-BDTPP to P-BDTPEE, probably because of conjugation. The fluorescence emission bands of the polymers were red-shifted compared to the monomers as shown in Fig. 7. Especially, P-BDTPEE emits light in a wide range from 380 to 505 nm. The shift in emission wavelength from 440 nm to 505 nm in the polymer of BDTPEE compared to the monomer shows that conjugation is higher in the polymer. Additionally, unpaired electron pairs on the oxygen atoms in the ethoxy groups used as bridge in BDTPEE have the ability to increase fluorescence intensity. The obvious reason for the observed red shift is characteristic of donor-acceptor structure, which may effect on their maximal absorption wavelength (Oleksy et al., 2011). Therefore, the fluorescence intensity of the monomer and polymer of BDTPEE is higher than this of BDTPP. Solution in DMSO of P-BDTPEE has yellow color under sunlight. The yellow solution of P-BDTPEE turned into turquoise when excited with the wavelength of 365 nm (Fig. 8).

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Fig. 7 Fluorescence spectra of the synthesized monomers and polymers in DMF.

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Fig. 8 The colours of P-BDTPEE under sunlight and UV light.

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3.4 Thermal properties of polymers

TG-DTG-DTA curves of aliphatic and aromatic bridged polymers are shown in Fig. 9. Thermal decomposition data of monomers and polymers are listed in Table 5. As seen in Fig. 9, while BDTPP, BDTPEE, BDTPMB and BDTPB demonstrated two degradation steps, P-BDTPP, P-BDTPEE, P-BDTPMB and P-BDTPB were demonstrated three degradation steps at the same thermal conditions. The initial degradation temperatures (T_on) of P-BDTPP, P-BDTPEE, P-BDTPMB and P-BDTPB were calculated as 282, 206, 196 and 189 °C, respectively. According to TGA results, the initial degradation temperatures (T_on) of BDTPEE and P-BDTPP are higher than those of the other compounds. The % char amounts of P-BDTPP, P-BDTPEE, P-BDTPMB and P-BDTPB were found to be 1.96, 3.75, 32.64 and 61.25, respectively, in the nitrogen atmosphere at 1000 °C. When %char amounts of the polymers are compared, the aromatic bridged polymers had higher %char amounts than those of the aliphatic bridged polymers at 1000 °C. Additionally, %char of P-BDTPMB was higher the others polymers. This shows that the polymer is thermally stable. The 50% weight loss temperatures of BDTPP, P-BDTPP, BDTPEE, P-BDTPEE, BDTPMB, P-BDTPMB and BDTPB were found to be 219, 422, 322, 848, 555, 428 and 522 °C, respectively. BDTPEE had a weak C—O etheric bond that could be easily broken at moderate temperatures. Also, thermal degradation process of BDTPEE was ended at 550 °C. The lowest % char amount was related to P-BDTPEE among the other polymers. The organic solvent losses of P-BDTPP, P-BDTPEE, P-BDTPB and P-BDTPMB were determined as 4.30, 2.80, 2.00 and 2.35%, respectively, until 150 °C. The glass transition temperatures (T_g) and ΔC_p of P-BDTPEE, P-BDTPP, P-BDTPMB and P-BDTPB were found to be 130, 135, 165 and 182 °C and 0.11, 0.268, 0.014 and 0.33 J g⁻¹ K⁻¹, respectively. According to DSC results of polymers, the glass transition temperatures (T_g) of the aromatic bridged polymers have higher than those of the aliphatic bridged polymers. The glass transition temperature is high due to the stiffness of the polymer chain containing the aromatic structures in the polymer main chain.

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Fig. 9 TG-DTA-DTG curves of the polymers.

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3.5 GPC analysis

The calculated number-average molecular weight (M_n), weight average molecular weight (M_w) and polydispersity index (PDI) values of the polymers were measured by using refractive index detector (RID) in DMF solutions. According to GPC results, the M_w values of P-BDTPP, P-BDTPEE, P-BDTPB and P-BDTPMB were calculated as 12,240, 14,500, 16,200 and 10,600 Da, respectively. The M_n values of P-BDTPP, P-BDTPEE, P-BDTPB and P-BDTPMB were determined as 9550, 10,850, 13,050 and 8350 Da. Also, PDI values of P-BDTPP, P-BDTPEE, P-BDTPB and P-BDTPMB were determined as 1.28, 1.33, 1.24 and 1.27, respectively. All of the polymers shows narrow PDI value. The narrow polydispersity value indicates that the polymerization reactions are selectively controlled (Bilici et al., 2010).

3.6 Surface morphological properties of polymers

The morphological properties of synthesized polymers were determined by using scanning electron microscope. SEM imaging was performed to observe how affect the morphological properties of the polymers containing aliphatic and aromatic groups used as bridge were affected. As seen in Fig. 10, P-BDTPP is composed of dense particles with different sizes, while P-BDTPEE is composed of sharp surfaces. In addition, layered structures were observed in P-BDTPB and P-BDTPMB where benzene and xylene dibromide compounds were used as bridges.

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Fig. 10 SEM images of the polymers.

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