New anthracene-based semi-conducting polymer analogue of poly(phenylene sulfide): Synthesis and photophysical properties

Optical Materials 46 (2015) 401–408

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

New anthracene-based semi-conducting polymer analogue of poly(phenylene sulfide): Synthesis and photophysical properties Khaled Hriz a,⇑, Nejmeddine Jaballah a, Jean-Louis Fave b, Mustapha Majdoub a a b

Laboratory of Interfaces and Advanced Materials, University of Monastir, Faculty of Science, Boulevard of the Environment, 5019 Monastir, Tunisia Institut des Nanosciences de Paris (INSP), UMR 7588-CNRS, Université Pierre et Marie Curie (UPMC), 4 Place Jussieu, Case 840, 75252 Paris Cedex 05, France

a r t i c l e

i n f o

Article history: Received 20 April 2015 Received in revised form 27 April 2015 Accepted 28 April 2015 Available online 14 May 2015 Keywords: Organic semiconductors Anthracene Poly(phenylene sulfide) Optical properties Thin films

a b s t r a c t A new anthracene-based polymer analogue of poly(phenylene sulfide) has been synthesized via Wittig polycondensation. The polymer is soluble and shows a good film quality. This organic material showed an amorphous behavior with a Tg of 70 °C. The absorption and fluorescence properties of the polymer were investigated. The HOMO/LUMO energy levels were estimated by cyclic voltammetry measurements. The PAnS thin film exhibits an optical gap of 2.56 eV and emits in orange region. The fluorescence quantum efficiency in dilute solution of PAnS was of 66%. A PAnS-based single-layer diode has been fabricated and shows relatively low turn-on voltage of 4.8 V. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Organic electronics has been the focus of a growing body of investigation in applied chemistry field for more than 50 years. Conjugated polymers are promising organic analogues of inorganic semi-conducting materials, and their exploitation in other electronic devices, such as thin-film transistors [1,2], photovoltaic cells [3,4], polymer solar cells (PSCs) [5,6], chemical sensors [7] and organic lasers [8] are currently expanding. On the applied research side, while not destined to replace silicon-based technologies, organic semi-conductors promise the advent of fully flexible electronic devices for large-area displays. In fact, these synthetic materials are compatible with solution processing techniques, thereby eliminating the need for the expensive lithography and vacuum deposition steps required for inorganic semi-conducting film elaboration. Solution processing also expands the repertoire of tolerant substrates and processing options, allowing flexible plastics to be used in conjunction with relatively simple methods such as stamping, inkjet printing and spin coating. However, the major characteristic of organic semi-conducting materials is their tunable opto-electronic properties, benefiting from the richness of the organic synthesis and therefore from an adjustable molecular structure [9,10]. ⇑ Corresponding author. Tel.: +216 73 500 280; fax: +216 73 500 278. E-mail address: [email protected] (K. Hriz). http://dx.doi.org/10.1016/j.optmat.2015.04.055 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

Anthracene was one of the first aromatic materials employed in organic light emitting diodes (OLEDs). The first experiments were carried out by Pope in the early 1960s [11]. Soon after, several reports on single-crystal anthracene-based OLEDs were published and good quantum yields were obtained (up to 5%) [12]. Nevertheless, such devices are thick and hence require very high operating voltage (over 100 V). The improvement in the operating voltage was achieved by vacuum evaporating thin layers of anthracene; in this case the operating voltage was lowered up to 30 V [13]. However, the anthracene tends to recrystallize with diode operating time, which led to a degradation of device performance. Recently, the anthracene derivatives represent the most heavily studied and exploited organics semiconductors [14–16]; and frequently used in OLEDs [17,18], as well as in other organic thin-layer-based electronic devices such as transistors [19,20] and photovoltaic cells [21,22]. Besides, anthracene is a promising fluorescent building block for highly photoluminescent semi-conducting polymers, which are valuable for the fabrication of stable blue-emitting electroluminescent devices [23]. Poly(p-phenylene sulfide) (PPS) is a high-performance thermoplastic with many desirable characteristics, including outstanding thermal, oxidative and chemical resistance [24]. In particular, PPS has a Tg of 85 °C and a Td above 420 °C [25] and shows electrical conductivity greater than 10 Scm1 when doped with AsF5 [26]. In this contribution, we developed a synthetic strategy for conception of new PPS analogue, in which the phenylene group has

402

K. Hriz et al. / Optical Materials 46 (2015) 401–408

been replaced by a more extended distyrylanthracene conjugated system. The optical and electrical behaviors of this organic semi-conducting material were investigated.

polymers. The measurements were performed at 25 °C, and the cell was deoxygenated with argon before each reductive scan. 2.3. Synthesis of the monomers M1 and M2

2. Experimental 2.1. Materials 4,40 -Thiodiphenol (TDP) (Aldrich, 99%), bromoethane (Acros, 98%), potassium carbonate (Acros, 99%), paraformaldehyde (Acros, 96%), triphenylphosphine (Acros, 99%) and potassium tert-butoxide (Acros, 98%), pyridinium chlorochromate (Sigma– Aldrich, 98%), sodium acetate anhydrous (Sigma–Aldrich, >99%) were used as received. Tetrahydrofuran (THF) was dried over Na/benzophenone and freshly distilled before use. Acetone was stirred over and distilled from potassium carbonate under argon. The solvents were commercially available and were used without purification. 2.2. Measurements 1 H NMR and 13C NMR spectral data were obtained on a Bruker AV300 spectrometer. FT-IR spectra were acquired on a Perkin-Elmer BX FT-IR system spectrometer (0.5 cm1 resolution) by dispersing samples in KBr disks. Steric exclusion chromatography (SEC) was performed on an Agilent Technologies 1200 HPLC. The experiment was done at room temperature using THF as eluent with standard polystyrene calibration. DSC was performed on a Mettler Toledo DSC 1 with a heating rate of 10 °C min1. The atomic force microscopy (AFM) analysis of the polymer coated glass surface was carried out using a Nanoscope III (Digital Instruments, Santa Barbara, CA) operating in the tapping mode. UV–vis absorption spectra were recorded on a Cary 2300 spectrophotometer. Photoluminescence (PL) spectra were obtained on a Jobin-Yvon spectrometer HR460 coupled to a nitrogen cooled Si Charged-Coupled Device (CCD) detector (2000 pixels). Samples were excited with a 450 W Xenon lamp at 370 nm. For solid state measurements, the films were spin-coated onto quartz substrate from 50 L of a chloroform solution (2  102 M) and using 2000 tr min1 speed. The film thicknesses were measured by a Dektak profilometer and were about 60 nm. For solid state measurements, the films were deposited onto a quartz substrate from a chloroform solution. All measurements were performed at room temperature. The solution PL quantum yields were measured in dilute chloroform solution according to a relative method using quinine sulfate (105 M solution of 0.5 M H2SO4) as reference [27]. Absorbance of the sample solutions was kept below 0.05 to avoid inner filter effect. Measurements were performed at room temperature using freshly prepared solutions. Both sample and reference solutions were excited at the same wavelength (365 nm) and the PL quantum efficiency of the quinine sulfate solution (/r) was assumed to be 0.54 [28]. Hence, the PL quantum efficiency of the sample (/s) can be calculated using the following relation: /s//r = (Ar/As)(Fs/Fr)(ns2/nr2), where, Ar and Fr are absorbance at excitation wavelength and emission integration area for the reference, while As and Fs are absorbance and emission integration for the sample. Cyclic voltammetry (CV) was performed on a CHI 660B electrochemical station in a three-electrode cell and using material films that were drop-cast onto an indium tin oxide (ITO/1 cm2) working electrode. The measurements were carried out at a scanning rate of 50 mV s1 against an Ag/AgCl reference electrode, a counter electrode made with a Platinum wire (1 cm of length) using 0.1 M tetrabutylammoniumfluoroborate ((n-Bu)4NBF4) in acetonitrile as supporting electrolyte. The electrochemical cell was externally calibrated by ferrocene in the same conditions as

2.3.1. 1,10 -Thiodi(4-ethoxybenzene) (a) To a stirred mixture of TDP (2.18 g, 10 mmol) and potassium carbonate (3.48 g, 25 mmol) in 20 mL of dimethylformamide, was added dropwise bromoethane (2 mL, 26 mmol). After stirring for 8 h at room temperature (25 °C), the reaction mixture was poured into distilled water and extracted with diethyl ether. The extract was washed with distilled water, dried over anhydrous magnesium sulfate, and then evaporated. The resultant crude product was purified by recrystallization from ethanol to afford (a) as white crystals. Yield: 92%; Tf: 66 °C; 1H NMR (300 MHz, CDCl3, d): 7.29 (d, J = 8.7 Hz, 4H, ArAH); 6.84 (d, J = 8.7 Hz, 4H, ArAH); 4.02 (q, J = 6.9 Hz, 4H OCH2); 1.40 (t, J = 6.90 Hz, 6H, CH3). 13C NMR (75.5 MHz, CDCl3, d): 158.4, 133.8, 127.3, 115.3, 63.7, 14.9; FTIR (cm1): 3064, 3035 (w, aromatic CAH stretching), 2983, 2878 (w, aliphatic CAH stretching), 1608, 1394 (s, C@C stretching), 1254 (s, CAOAC asymmetric stretching), 1043 (s, CAOAC asymmetric stretching), 822 (s, aromatic CAH out-of-plane bending). 2.3.2. 1,10 -Thiodi[3-(chlorométhyl)-4-éthoxyoxybenzène] (b) A mixture of (a) (2.74 g, 10 mmol), paraformaldehyde (2.50 g, 80 mmol) and 37% aqueous HCl (8.5 mL, 102 mmol) in acetic acid (30 mL) was stirred at room temperature (25 °C) for 10 h. The resulting mixture was then poured into distilled water and extracted with diethyl ether. The organic layer was washed several times with distilled water and dried over anhydrous magnesium sulfate. After solvent removal and recrystallization from ethanol, we obtained (b) as white crystals. Yield: 75%; Tf: 52 °C; 1H NMR (300 MHz, CDCl3, d): 7.43 (d, J = 2.4 Hz; 2H, ArAH); 7.32 (d, J = 2.1 Hz; 2H, ArAH); 6.80 (d, J = 8,6 Hz, 2H, ArAH); 4,63 (s, 4H, CH2Cl); 4.92 (q, J = 6.9 Hz, 4H, OCH2); 0.88 (t, J = 6.9 Hz, 6H, CH3). 13 C NMR (75.5 MHz, CDCl3, d): 154.8, 142.8, 133.8, 125.1, 125.2, 116.1, 64.6, 42.6, 15.1. FTIR (cm1): 3064–3035 (w, aromatic CAH stretching), 2983–2878 (w, aliphatic CAH stretching), 1608, 1394 (s, C@C stretching), 1254 (s, CAOAC asymmetric stretching + CH2Cl out-of-plane bending), 1045 (s, CAOAC asymmetric stretching), 822 (s, aromatic CAH out-of-plane bending), 610 (s, CACl stretching). 2.3.3. 1,10 -Thiodi[3-(acétylméthyl)-4-éthoxybenzène] (c) To stirred a mixture of (b) (3.71 g, 10 mmol) and sodium acetate (3.28 g, 40 mmol) in 20 mL of dimethylformamide. After stirring for 2 h at 90 °C, the reaction mixture was then precipitated into distilled water. The obtained white powder (c) was filtered and dried under vacuum for 24 h. Yield: 92%; 1H NMR (300 MHz, CDCl3, d): 7.31 (d; J = 7,8 Hz; 2H; ArAH); 7.26 (dd, 3J = 6.9 Hz, 4 J = 3.0 Hz, 2H; ArAH); 6.81 (d; J = 7.8 Hz; 2H; ArAH); 5.10 (s; 4H; O@COCH2); 4.08 (q; J = 6.3 Hz; 4H; OCH2); 2.09 (s; 6H; O@CCH3); 1.42 (t; J = 6.9 Hz; 6H; CH3). 13C NMR (75.5 MHz, CDCl3, d): 170.1, 156.1, 132.7, 132.2, 126.9, 125.3, 113.5, 63.8, 61.8, 21.1, 15.1. FTIR (cm1): 1742 (s, asymmetric stretching, C@O), 1201 (s, CAO stretching). 2.3.4. 1,10 -Thiodi[3-(hydroxyméthyl)-4-éthoxybenzène] (d) A mixture of sodium hydroxide (1.6 g, 40 mmol) and diacetate (c) (4.18 g, 10 mmol) was heated at reflux for 4 h. The reaction mixture is then concentrated and was then precipitated into distilled water. The obtained white powder (d) was filtered and dried under vacuum for 24 h. Yield: 92%; Tf = 153 °C; 1H NMR (300 MHz, CDCl3, d): 7.29 (d; J = 7.8 Hz; 2H; ArAH); 7.27 (dd, 3J = 6.9 Hz, 4 J = 3.0 Hz, 2H; ArAH); 6.81 (d; J = 7.8 Hz; 2H; ArAH); 4.64 (s;

K. Hriz et al. / Optical Materials 46 (2015) 401–408

4H; CH2O); 4.08 (q; J = 6.3 Hz; 4H; OCH2); 2.44 (s; 2H; OH); 1.42 (t; J = 6,9 Hz; 6H; CH3). 13C NMR (75.5 MHz, CDCl3, d): 156.1, 132.7, 132.2, 130.9, 127.3, 112.5, 64.8, 61.8, 15.1. FTIR (cm1): 3340 (s, asymmetric stretching, OH), 1021 (s, CAO stretching). 2.3.5. 1,10 -Thiodi[3-(diformyl)-4-ethoxybenzene] (M1) To a solution of (d) (3.34 g, 10 mmol) in 20 mL of anhydrous methylene chloride was added dropwise pyridinium chlorochromate (PCC) (8.62 g, 40 mmol) in a bath of ice. After stirring for 4 h at room temperature (25 °C), the reaction mixture was filtered and poured into distilled water and extracted with methylene chloride. The extract was washed with distilled water, dried over anhydrous magnesium sulfate, and then evaporated. The resultant crude product was purified by recrystallization from ethyl acetate to afford M2 as yellow-greenish crystals. Yield: 80%; Tf: 88 °C; 1H NMR (300 MHz, CDCl3, d): 10.51 (s; 2H; aldehyde proton);7.80 (s; 2H; ArAH); 7.50 (dd; J = 8.4 Hz; J = 3.0 Hz; 2H; ArAH); 7.10 (d; 3J = 8.1 Hz; 2H; ArAH); 4.07 (q; J = 6.3 Hz; 4H; OCH2); 1.44 (t; J = 6,9 Hz; 6H; CH3). 13C NMR (75.5 MHz, CDCl3, d): 189.4, 163.5, 139.2, 132.4, 127.9, 125.9, 114.1, 65.3, 14.9. FTIR (cm1): 1688 (s, C@O stretching), 1254 (s, CAOAC asymmetric stretching), 1043 (s, CAOAC asymmetric stretching). 2.3.6. 9,10-Dichlormethylanthracene (AnCl) A mixture of anthracene (1.83 g, 10 mmol), paraformaldehyde (1.56 g, 50 mmol of CH2O) and 37% aqueous HCl (5 mL, 60 mmol) in acetic acid (30 mL) was heated at 50 °C for 24 h. The resulting mixture was then cooled to room temperature, poured into distilled water and extracted with chloroform. The organic layer was washed several times with distilled water and dried over anhydrous magnesium sulfate. The resulting solution was then concentrated and precipitated in diethyl ether. To obtained AnCl as a yellow powder. Yield: 86%. 1H NMR (300 MHz, CDCl3, d): 8.38 (dd, 3J = 6.9 Hz, 4J = 3.0 Hz, 4H, ArAH), 7.66 (dd, 3J = 6.9 Hz, 4 J = 3.0 Hz, 4H, ArAH), 5.61 (s, 4H, CH2Cl); 13C NMR (75.5 MHz, CDCl3, d) = 129.8, 129.8, 126.7, 124.4, 38.8; FT-IR (cm-1) 3086 (w, aromatic CAH stretching), 1517 (s, C@C stretching), 796 (s, aromatic CAH out-of-plane bending), 625 (s, CACl stretching). 2.3.7. 9,10-Bis(triphenylphosphoniomethyl)anthracène dichloride (M2) A solution of AnCl (2.75 g, 10 mmol) and triphenylphosphine (5.82 g, 22 mmol) in anhydrous acetone (50 mL) was stirred and heated at reflux for 24 h under argon atmosphere. After cooling the reaction mixture, the resulting yellow precipitate was filtered off, washed several times with diethyl ether and dried under vacuum. Yield: 90%; 1H NMR (300 MHz, CDCl3, d): 8.02–8.00 (dd, 3 J = 6.3 Hz, 4J = 2.7 Hz, 4H, anthracene), 6.98–6.94 (dd, 3J = 6.3 Hz, 4 J = 2.7 Hz, 4H, anthracene), 7.81–7.49 (m, 30H, P(Ph)3), 6.22 (d, 2 JH-P = 13.8 Hz, 4H, CH2-P. 13C NMR (75.5 MHz, CDCl3, d): 135.4, 134.8, 134.7, 134.6, 130.7, 130.2, 130.1, 130.0, 125.7, 125.5, 122.2, 118.2, 117.1, 31.6, 30.6. 2.4. Synthesis of the polymer PAnS To a stirred mixture of an equimolar amount of a M1 (0.33 g, 1 mmol) and M2 (0.799 g, 1 mmol) in 10 mL of anhydrous THF, 10 mL of a 0.5 M t-BuOK solution in THF (0.572 g, 5 mmol) was added dropwise under a nitrogen atmosphere. The reaction mixture was stirred at room temperature for 24 h and then heated at reflux during 4 h. The resulting mixture was then cooled to room temperature, acidified with 3 M aqueous hydrochloric acid, poured into water and extracted with chloroform. The organic phase was washed with water, concentrated and then precipitated into methanol. The obtained orange powder (PAnS) was filtered and dried under vacuum for 24 h. Yield: 57%, yellow powder. 1H NMR

403

(300 MHz, CDCl3, d): 8.52–6.30 (m, aromatic and vinylic protons), 4.12–3.89 (m, OACH2), 3.22 (m, ArACH3 end-group), 1.33– 0.82(m, aliphatic protons). 13C NMR (75.5 MHz, CDCl3, d): 148.9– 122.9 (aromatic and vinylic C), 64.4 (PhOC), 29.8–14.1 (aliphatic C). FTIR (cm-1): 3078 (w, aromatic and vinylic CAH stretching), 2967, 2870 (w, aliphatic CAH stretching), 1690, 1592 (m, C@C stretching), 1244 (s, CAOAC asymmetric stretching), 1040 (m, CAOAC symmetric stretching), 926 (m, trans-HC@CH out-of-plane bending), 808 (w, cis-HC@CH out-of-plane bending), 750 (s, aromatic CAH out-of-plane bending). 2.5. Fabrication and characterization of diodes Single-layer device was elaborated as sandwich structures between an aluminium (Al) cathode and an indium tin oxide (ITO) (ITO-thickness of 100 nm, sheet resistance of 20 X/square) anode. Polymer solution (2  102 M in chloroform) was spin-cast (2500 rpm) onto ITO glass to obtain a film about 60 nm thick after annealing at 40 °C for 1 h. A thin aluminium layer (150 nm) was deposited by thermal evaporation at 3  106 Torr. The current– voltage (I–V) characteristics of the devices were recorded with a Keithley 236 source meter. 3. Results and discussion 3.1. Synthesis and structural characterization The polymer was synthesized by the synthetic route shown in Scheme 2. A new aromatic dialdehyde (M1), used as monomer, was prepared from 4,40 -thiodiphenol (TDP) according to a four steps synthetic route as showed in Scheme 1. The first step consists on a direct chloromethylation of the benzene ring of the corresponding ethylated ether (a) in an HCl/paraformaldehyde/acetic acid system following a conventional described procedure [29]. Then, the diacetate derivative (c) was obtained by nucleophilic substitution of chloride by acetate group. The dialcohol (d) was prepared by saponification reaction of (c). The last step is an oxidation of the primary alcohols in (d) by using the pyridinium chlorochromate (PCC). The 9,10-bis(chloromethyl)anthracene (AnCl) was synthesized by direct chloromethylation of the anthracene using the HCl/paraformaldehyde/acetic acid system, following our previously reported procedure [29]. The correspondent triphenylphosphonium salts (M2) was then obtained by acetone reflux of the AnCl in presence of triphenylphosphine. The polymer (PAnS) was synthesized via the Wittig reaction by condensing the M2 with the dialdehyde M1, according to a previously reported procedure using the t-BuOK/THF system (Scheme 2) [30]. The obtained polymer was found to have good solubility in common organic solvents such as tetrahydrofuran, chloroform and methylene chloride. The polymer structure was confirmed by NMR and FTIR spectroscopic analysis. The IR spectrum showed the presence of both Z-(808 cm1) and E-(926 cm1) vinylic absorptions [31]. Indeed, according to their chemical structures, the yilde used in this work can be classified as semi-stabilized yilde, and its normal Wittig reaction with aldehydes produces mixtures of Z- and E-configurations nonstereospecifically [32]. The 1H NMR spectrum shows a broad peak between 8.5 and 6.3 ppm assigned to aromatic and vinylic protons (Fig. 1). The OCH2 groups appear between 4.1 and 3.8 ppm. The aliphatic protons showed a broad peak between 1.3 and 0.8 ppm. The absence of the aldehyde terminal groups was supported by the absence of the corresponding peak in the NMR spectrum. In return, the appearance of a weak signal at 3.2 ppm in 1H NMR data and a pick about 14.1 ppm in 13C NMR data suggests the obtaining of aromatic methyl terminal groups (ArACH3). A number-average weight of 4950 g mol1 was

404

K. Hriz et al. / Optical Materials 46 (2015) 401–408

Scheme 1. Synthetic routes to the monomers M1 and M2.

Scheme 2. Synthesis of the polymer PAnS.

405

K. Hriz et al. / Optical Materials 46 (2015) 401–408

estimated for PAnS, by comparing the 1H NMR signal integration for ArACH3 groups and AOCH2A units. The SEC analysis showed a polydispersity index (Ip) around 1.2; nevertheless, the polymer weights were underestimated comparing to the NMR calculated values. In fact, contrary to NMR method which gives absolute exact weights, the SEC analysis is related to the polymer hydrodynamic volume and whose results depend on the nature of the polymer

used as reference. Indeed, the conjugated polymers present rigid molecular structures, which make using the flexible polystyrene as reference unreliable. The thermal stability of PAnS was investigated by differential scanning calorimetry (DSC). The DSC curve reveals a glass transition temperature (Tg) of 70 °C (Fig. 2). No melting or other thermal events were seen before thermal decomposition, indicating that the polymer was fully amorphous. A PAnS organic film on a glass substrate was prepared and characterized by atomic force microscopy (AFM) (Fig. 3). The result shows a smooth surface with a root-mean-square (RMS) of 0.36 nm. 3.2. UV–vis absorption

Fig. 1. 1H NMR spectrum of PAnS.

The UV–visible absorption spectrum of PAnS was recorded at room temperature, in chloroform dilute solution and in thin solid film. The Table 1 summarizes the obtained spectral data. The absorption spectra in dilute solution and in thin film of polymer were depicted in Fig. 4. The solution spectrum shows three maxima at 364, 386 and 409 nm; these characteristic bands are attributed to the p–p⁄ electronic transitions of the anthracene group [33]. The spectrum was qualitatively similar to that of the anthracene [34], but significantly shifted to the highest wavelengths (20 nm), indicating longer effective conjugation length in the distyrylanthracene system. The PAnS exhibits almost the same absorption feature of our previously reported distyrylanthracene-based polymer (P1, in Scheme 1) containing an aliphatic chain as non-conjugated spacer group [35]. Nevertheless, the absorption-onset of PAnS was significantly shifted to the highest wavelengths (15 nm) in comparison with Table 1 UV–visible absorption data for PAnS. Dilute solution in chloroform kmax (nm) 364a; 386; 409 a

Shoulder.

Fig. 2. DSC thermogram of PAnS.

Fig. 3. AFM image of [glass/PAnS] layer.

emax

Thin film kmax (nm)

(104 M1 cm1)

Eg-op (eV)

konset (nm)

Eg-op (eV)

0.10; 0.16; 0.18

2.77

368; 389; 411

483

2.56

406

K. Hriz et al. / Optical Materials 46 (2015) 401–408 Table 2 Photoluminescence data for PAnS. Dilute solution in chloroform b

kmax (nm) a

440; 495 ; 560 a b c

Fig. 4. UV–vis absorption spectra of the PAnS, in chloroform (5  105 M of conjugated unit) and in thin solid film (60 nm).

P1. This behavior was attributed to the mesmeric effect of the sulfur spacer in PAnS [36]. In solid film state, the PAnS absorption profile was comparable to that of the solution (Fig. 4); however, a broader and unstructured spectrum was obtained in thin film. In fact, similar behavior was generally observed in semi-conducting polymers and was attributed to the p–p interaction of the conjugated segments and aggregates formation in the solid state [37]. The optical gap (Eg-op) was estimated from the absorption-onset of the PAnS film and was of 2.56 eV. In comparison with our previously reported PPS-analogue based on the distyrylbenzene chromophore units (PPVS) [30], the PAnS showed a lower optical gap indicating a higher effective conjugation length in the polymer film. 3.3. Photoluminescence properties The PL spectra of the polymer were acquired at room temperature, in dilute solutions and as thin solid film (Fig. 5); the results were summarized in Table 2. The PAnS solution presents a green emission with a relatively broad PL spectrum which reveals one maximum at 440 nm and two shoulders at 495 and 560 nm. In comparison with PPVS, a broader and red-shifted spectrum PanS was obtained, which can be attributed to the more extended fluorophore units. The fluorescence maximum of the PAnS thin film was significantly shifted (150 nm) to the orange region, in comparison with the dilute

Fig. 5. Fluorescence spectra of the PAnS, in chloroform (2  107 M of conjugated unit) and in thin solid film (60 nm).

a

Thin film c

FWHM (nm)

UPl

152

0.66

kmax (nm)

FWHMb (nm)

446; 605

137

Shoulder. Spectrum full width at half maximum. PL quantum yield.

solution (Fig. 5). This behavior was ascribed to the p–p interaction of the excited distyrylanthracene segments and excimer formation in the solid state [38]. The fluorescence quantum efficiency of the polymer was determined in dilute chloroform solution by a relative method using quinine sulfate as standard [39]; the obtained value was of 0.66. Hence, the PAnS exhibits a higher PL yield compared to P1 (0.37) [40]. This difference can be explained by the remarkable flexibility of P1 macromolecular structure due to the introduction of aliphatic chain as spacer group. Consequently, the available vibrational and rotational degrees of freedom were increased and the loss of PL by such processes was considerably amplified. 3.4. Electrochemical and electrical characterization Cyclic voltammetry (CV) was employed to investigate the redox behavior of the polymers and to estimate their HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels. Knowledge of these energy levels is in fact of crucial importance to the selection of cathode and anode materials for OLED devices [41]. The use of CV analysis is reliable as the electrochemical processes are similar to those involved in charge injection and transport processes in OLEDs [42]. The polymer film was drop-coated onto an ITO glass substrate and scanned both positively and negatively in (n-Bu)4NBF4 /acetonitrile. The cyclic obtained voltammograms are shown in Fig. 6. According to an empirical method [43] and by assuming that the energy level of the ferrocene/ferrocenium couple is 4.8 V below the vacuum level, the HOMO energy level (EHOMO), LUMO energy level (ELUMO) and the electrochemical gap (Eg-el) can be calculated as follows:

EHOMO ðIP; ionization potentialÞ ¼ ðVonsetox  VFOC þ 4:8Þ eV ELUMO ðEA; electron affinityÞ ¼ ðVonsetred  VFOC þ 4:8Þ eV Egel ¼ ðELUMO  EHOMO Þ eV

Fig. 6. Cyclic voltammograms for the PAnS film coated onto ITO electrodes; in 0.1 M (n-Bu)4NBF4/acetonitrile, at scan rate of 50 mV s1.

K. Hriz et al. / Optical Materials 46 (2015) 401–408 Table 3 Electrochemical data for PAnS.

407

Acknowledgments

Vonset-ox (V)

Vonset-red (V)

EHOMO (eV)

ELUMO (eV)

Eg-el (eV)

1.05

1.29

4.90

2.56

2.34

This study was supported by the Ministry of Higher Education and Scientific Research–Tunisia.

References

Fig. 7. Current–voltage characteristics of the [ITO/PAnS/Al] device.

where VFOC is the ferrocene half-wave potential (0.95 V), Vonset-ox the polymer oxidation onset and Vonset-red the polymer reduction onset, all measured versus Ag/AgCl. The calculated EHOMO, ELUMO and Eg-el values are summarized in Table 3. The difference between the bandgaps obtained from the optical method (2.56 eV) and from electrochemical analysis (2.34 eV) has been previously reported for some other conjugated polymers, and was attributed to the interface barrier between the polymer film and the electrode surface [44]. In fact, the optical value corresponds to the pure bandgap between the valence band and the conduction band, while the electrochemical value may be the results of the optical bandgap coupled with the interface barrier for charge injection, which makes it larger. In comparison with PPVS, an improvement of 0.23 eV for electron affinity was observed. However, a lower ionization potential (0.21 eV) was detected; indicating lower air stability [45]. These results can be explained by a higher effective conjugation length in the PAnS compared to PPVS. A single-layer device with the ITO/PAnS/Al configuration was fabricated to investigate the current–voltage (I–V) characteristic of the anthracene-based semi-conducting polymer analogue of poly(p-phenylene sulfide). As shown in Fig. 7, the I–V curve of PAnS indicates typical diode behavior with relatively low turn-on voltage of 4.8 V.

4. Conclusion We have synthesized a new anthracene-based semi-conducting polymer analogue of poly(phenylene sulfide) via the Wittig polycondensation. The synthesized polymer has good solubility in common organic solvents. The polymer is fully amorphous, exhibits good thermal stability and shows a glass transition temperature of 70 °C. The surface property of the PAnS film was analyzed by atomic force microscopy (AFM) and presents a root mean square (RMS) of 0.36 nm. The optical gap (Eg-op) determined from the absorption-onset of the PAnS film was of 2.56 eV. The distyrylanthracene-based polymer containing sulfur spacer group atom exhibits a green emission in dilute solution and an orange fluorescence in solid thin film. The I–V characteristic of the [ITO/PAnS/Al] device demonstrates a typical diode behavior and a relatively low turn-on voltage.

[1] S. Cataldo, B. Pignataro, Polymeric thin films for organic electronics: properties and adaptive structures, Materials 6 (2013) 1159–1190. [2] J. Lee, D.H. Kim, J.Y. Kim, B. Yoo, J.W. Chung, J. Park II, B.L. Lee, J.Y. Jung, J.S. Park, B. Koo, S. Im, J.W. Kim, B. Song, M.H. Jung, J.E. Jang, Y.W. Jin, S. Y Lee, Reliable and uniform thin-film transistor arrays based on inkjet-printed polymer semiconductors for full color reflective displays, Adv. Mater. 25 (2013) 5886– 5892. [3] Y. Kim, E. Lim, Development of polymer acceptors for organic photovoltaic cells, Polymer 6 (2014) 382–407. [4] R. Berger, A.L. Domanski, S.A.L. Weber, Electrical characterization of organic solar cell materials based on scanning force microscopy, Eur. Polym. J. 49 (2013) 1907–1915. [5] B.C. Thompson, J.M.J. Fréchet, Polymer-fullerene composite solar cells, Angew. Chem. Int. Ed. 47 (2008) 58–77. [6] C.L. Chochos, S.A. Choulis, How the structural deviations on the backbone of conjugated polymers influence their optoelectronic properties and photovoltaic performance, Prog. Polym. Sci. 36 (2011) 1326–1414. [7] V.S. Palaparthy, M.B. Baghini, D.N. Singh, Review of polymer-based sensors for agriculture-related applications, Emerg. Mater. Res. 2 (2013) 166–180. [8] S. Zulkarnaen Bisri, T. Takenobu, Y. Iwasa, The pursuit of electrically-driven organic semiconductor lasers, J. Mater. Chem. C 2 (2014) 2827–2836. [9] J.L. Segura, The chemistry of electroluminescent organic materials, Acta Polym. 49 (1998) 319–344. [10] L. Akcelrud, Electroluminescent polymers, Prog. Polym. Sci. 28 (2003) 875– 962. [11] M. Pope, H.S. Kallmann, P. Magnante, Electroluminescence in organic crystals, J. Chem. Phys. 38 (1963) 2042–2043. [12] D.F. Williams, M. Schadt, Dc and pulsed electroluminescence in anthracene and doped anthracene crystals, J. Chem. Phys. 53 (1970) 3480–3487. [13] P.S. Vincett, W.A. Barlow, R.A. Hann, G.G. Roberts, Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films, Thin Solid Films 94 (1982) 171–183. [14] R. Ben, N. Chaâbane, M. Jaballah, A. Benzarti-Ghédira, M. Chaieb, H. Majdoub, H. Ben Ouad, Synthesis and thin films characterization of new anthracene-core molecules for opto-electronic applications, Physica B 404 (2009) 1912–1916. [15] E. Gondek, I.V. Kityk, A. Danel, Some anthracene derivatives with N,Ndimethylamine moieties as materials for photovoltaic devices, Mater. Chem. Phys. 112 (2008) 301–304. [16] H. Meng, F. Sun, M.B. Goldfinger, F. Gao, D.J. Londono, W.J. Marshal, G.S. Blackman, K.D. Dobbs, E.K. Dalen, 2,6-Bis[2-(4-pentylphenyl)vinyl]anthracene: a stable and high charge mobility organic semiconductor with densely packed crystal structure, J. Am. Chem. Soc. 128 (2006) 9304–9305. [17] P. Raghunath, M. Ananth Reddy, C. Gouri, K. Bhanuprakash, V. Jayathirtha, Rao, Electronic properties of anthracene derivatives for blue light emitting electroluminescent layers in organic light emitting diodes: a density functional theory study, J. Phys. Chem. A 110 (2006) 1152–1159. [18] T. Serevicius, R. Komskis, P. Adomenas, O. Adomeniene, V. Jankauskas, A. Gruodis, K. Kazlauskasa, S. Jursenas, Non-symmetric 9,10diphenylanthracene-based deep-blue emitters with enhanced charge transport properties, Phys. Chem. Chem. Phys. 16 (2014) 7089–7101. [19] Y. Li, T.H. Kim, Q. Zhao, E.K. Kim, S.H. Han, Y.H. Kim, J. Jang, S.K. Kwon, Synthesis and characterization of a novel polymer based on anthracene moiety for organic thin film transistor, J. Polym. Sci., Part A: Polym. Chem. 46 (2008) 5115–5122. [20] D.S. Chung, J.W. Park, J.H. Park, D. Moon, G.H. Kim, H.S. Lee, D.H. Lee, H.K. Shim, S.K. Kwon, C.E. Park, High mobility organic single crystal transistors based on soluble triisopropylsilylethynyl anthracene derivatives, J. Mater. Chem. 20 (2010) 524–530. [21] L. Valentini, D. Bagnis, A. Marrocchi, M. Seri, A. Taticchi, J.M. Kenny, Novel anthracene-core molecule for the development of efficient PCBM-based solar cells, Chem. Mater. 20 (2008) 32–34. [22] D.S. Chung, J.W. Park, W.M. Yun, H. Cha, Y.H. Kim, S.K. Kwon, C.E. Park, Solution-processed organic photovoltaic cells with anthracene derivatives, ChemSusChem 3 (2010) 742–748. [23] H. Hrichi, K. Hriz, M. Benzarti-Ghédira, N. Jaballah, R. Ben Chaabane, M. Majdoub, H. Ben Ouada, Optical and electrical characterization of thin films based on anthracene polyether polymers, Mater. Sci. Semicond. Process. 16 (2013) 851–858. [24] L.C. Lopez, G.L. Wilkes, Poly(p-phenylene sulfide)-an overview of an important engineering thermoplastic, J. Macromol. Sci., Rev. Macromol. Chem. Phys. C29 (10) (1989) 83–151. [25] D.G. Brady, The crystallinity of poly(phenylene sulfide) and its effect on polymer properties, J. Appl. Polym. Sci. 20 (1976) 2541–2551. [26] R.H. Baughmann, J.L. Bredas, R.R. Chance, R.L. Elsenbaumer, L.W. Shacklette, Structural basis fos semiconducting and metallic polymer dopant systems, Chem. Rev. 82 (1982) 209–222.

408

K. Hriz et al. / Optical Materials 46 (2015) 401–408

[27] L. Feng, Z. Chen, Synthesis and photoluminescent properties of polymer containing perylene and fluorene units, Polymer 46 (2005) 3952–3956. [28] W.H. Melhuish, Quantum efficiencies of fluorescence of organic substances: effect of solvent and concentration of the fluorescent solute, J. Phys. Chem. 65 (1961) 229–235. [29] K. Hriz, N. Jaballah, M. Chemli, J.L. Fave, M. Majdoub, Synthesis and characterization of new anthracene-based semiconducting polyethers, J. Appl. Polym. Sci. 119 (2011) 1443–1449. [30] N. Jaballah, M. Majdoub, J.L. Fave, C. Barthou, M. Jouini, J. Tanguy, New confined p-phenylenevinylene (PPV)-type polymer analogue of poly(phenylene sulfide), Eur. Polym. J. 44 (2008) 2886–2892. [31] F. Wang, F. He, Z. Xie, M. Li, M. Hanif, X. Gu, B. Yang, H. Zhang, P. Lu, Y. Ma, A solution-processible poly(p-phenylene vinylene) without alkyl substitution: Introducing the cis-vinylene segments in polymer chain for improved solubility, blue emission, and high efficiency, J. Polym. Sci., Part A: Polym. Chem. 46 (2008) 5242–5250. [32] H. Ndayikengurukiye, S. Jacobs, W. Tachelet, J. Van Der Looy, A. Pollaris, H.J. Geise, M. Ciaeys, J.M. Kauffmann, S. Janietz, Alkoxylated p-phenylenevinylene oligomers: synthesis and spectroscopic and electrochemical properties, Tetrahedron 53 (1997) 13811–13828. [33] E. Gondeka, I.V. Kityk, A. Danel, Ome anthracene derivatives with N,Ndimethylamine moieties as materials for photovoltaic devices, Mater. Chem. Phys. 112 (2008) 301–304. [34] K. Rameshbabu, Y. Kim, T. Kwon, J. Yoo, E. Kim, Facile one-pot synthesis of a photo patternable anthracene polymer, Tetrahedron Lett. 48 (2007) 4755– 4760. [35] K. Hriz, M. Chemli, N. Jaballah, J.L. Fave, M. Majdoub, Synthesis, characterization and optical properties of distyrylanthracene-based polymers, High Perform. Polym. 23 (2011) 290–299.

[36] J. Cornil, D.A. Dos Santos, D. Beljonne, J.L. Bredas, Electronic structure of phenylene vinylene oligomers: influence of donor/acceptor substitutions, J. Phys. Chem. 99 (1995) 5604–5611. [37] K.Y. Peng, S.A. Chen, W.S. Fann, S.H. Chen, A.C. Su, Well-packed chains and aggregates in the emission mechanism of conjugated polymers, J. Phys. Chem. B 109 (2005) 9368–9373. [38] Y.F. Huang, Y.J. Shiu, J.H. Hsu, S.H. Lin, A.C. Su, K.Y. Peng, S.A. Chen, W.S. Fann, Aggregate versus excimer emissions from poly(2,5-di-n-octyloxy-1,4phenylenevinylene), J. Phys. Chem. C 111 (2007) 5533–5540. [39] L. Feng, Z. Chen, Synthesis and photoluminescent properties of polymer containing perylene and fluorene units, Polymer 46 (2005) 3952–3960. [40] H. Hrichi, K. Hriz, N. Jaballah, R. Ben Chaâbane, M. Majdoub, New anthracenebased soluble polymers: optical and charge carrier transport properties, J. Polym. Res. 20 (2013) 241–300. [41] B. Fan, Q. Sun, N. Song, H. Wang, H. Fan, Y. Li, Electroluminescent properties of a partially-conjugated hyperbranched poly(p-phenylene vinylene), Polym. Adv. Technol. 17 (2006) 145–149. [42] M. Cheng, Y. Xiao, W.L. Yu, Z.K. Chen, Y.H. Lai, W. Huang, Synthesis and characterization of a cyano-substituted electroluminescent polymer with well-defined conjugation length, Thin Solid Films 363 (2000) 110–113. [43] J.L. Bredas, R. Silbey, D.S. Bordeaux, R.R. Chance, Chain-length dependence of electronic and electrochemical properties of conjugated systems: polyacetylene, polyphenylene, polythiophene, and polypyrrole, J. Am. Chem. Soc. 105 (1983) 6555–6559. [44] Y.F. Huang, Y.J. Shiu, J.H. Hsu, S.H. Lin, A.C. Su, K.Y. Peng, S.A. Chen, W.S. Fann, Aggregate versus excimer emissions from poly(2,5-di-n-octyloxy-1,4phenylenevinylene), J. Phys. Chem. C 111 (2007) 5533–5540. [45] N. ThejoKalyani, S.J. Dhoble, Novel materials for fabrication and encapsulation of OLEDs, Renew. Sust. Energy Rev. 44 (2015) 319–347.