Pyrazole-3,4-dicarboxylic acid 2 was synthesized via the hydrolysis of pyrazole-3-carboxylic acid 1 and subsequently heated with thionyl chloride to give the novel pyrazole-3,4-dicarbonyl dichloride 3, which was easily converted into oligo-pyrazole 4 1 13 upon its reaction with p-phenylene-diamine. These newly synthesized compounds were characterized by H-NMR, C-NMR, and FT-IR spectroscopy, and gel permission chromatography (GPC). Three novel oligo-pyrazole thin films were prepared using oligo-pyrazole 4 with these respective values of thickness: 20, 21, and 24 μm. The optical properties of the films, including the absorbance, transmittance, and optical band gap, were determined using UV-vis spectroscopy. The E values of the films were found to be 1.426, 1.537, and 1.648 eV for the 20, 21, and 24 μm thick organic films, respectively. Atomic force microscopy (AFM) was used to examine the surface morphology and properties of the organic films. In the AFM images, a few black regions were observed and several yellow regions appeared over a large area, and the surface of the oligo-pyrazole films had an extremely low roughness value. The as-synthesized oligo-pyrazole has great potential in optoelectronic applications according to the optical properties of the as-prepared films. . . . . Keywords Absorbance Conjugated polymer Organic semiconductor Pyrazole Thin film Introduction photovoltaics, organic light emitting diodes, and thin film transistors, as well as their industrial applications [1–3]. Conjugated polymers are important target molecules for The search for new materials depends on the rapid devel- synthetic chemists due to their use in solar cells, organic opment of science and technology, which has created an intense area of research on conducting polymers and in- creased the importance of the projects in this area. Recently, the synthesis of new polyamide derivatives has Highlights • A new different asymmetric oligo-pyrazole is synthesized and its thin become an increasingly popular topic and widely studied films were prepared. research area [4–6]. In particular, their optical properties � The optical properties of thin films were investigated. and physicochemical properties have guided researchers � Thepreparedthin films canbea candidateas a semiconductor due to towards the synthesis of polyamide derivatives using having the optical band gap (E ) values (1.426, 1.537, and 1.648 eV). existing methods and applications [7, 8]. Electronic supplementary material The online version of this article Poly-pyrazoles are an important class of heterocyclic poly- (https://doi.org/10.1007/s00396-018-4342-7) contains supplementary material, which is available to authorized users. mers, which have become more popular than before due to their wide range of properties such as film formation and high * Adnan Cetin thermal stability [9, 10]. Poly-pyrazoles and pyrazoles have firstname.lastname@example.org also been successfully applied in many fields including biol- ogy, industry, and pharmaceutical chemistry [11–13]. Past Faculty of Education, Department of Science, Muş Alparslan studies used conjugated polyamides (e.g., amide groups) for University, Muş, Turkey drug delivery in medicine [14, 15]. Furthermore, poly- School of Health, Muş Alparslan University, Muş, Turkey pyrazoles and pyrazoles have many advantages in several ap- plication fields such as optics and optoelectronic technology Faculty of Pharmacy Department of Pharmaceutical Chemistry, Yüzüncü YılUniversity,Van, Turkey [9, 16]. 1250 Colloid Polym Sci (2018) 296:1249–1257 One of the most common applications of polymer materials The solution was cooled down to room temperature. It was is in thin film devices. Thin films that are made of these poly- added with concentrated hydrochloric acid (1.5 mL) and mers have several advantages when used as optical coatings, water (1.5 mL). The white solid product was occurred. It coatings in electronics, and decorative protective coatings, was filtered and it was washed with water. Yield 90%. Color −1 thanks to the fundamental characteristics they have. For in- white. mp 126–128 °C. FT-IR (ν,cm ) 3350 (br, -OH), stance, optical, mechanical, and electrical properties are sig- 3064 (aromatic C-H), 2936 (aliph. C-H), 1721–1710 nificantly increased when polymer materials are used as a thin (C=O, acide), H NMR (400 MHz, CDCl ) δ (ppm) 11.6 film [17, 18]. In particular, thin films have been used in the (br.s,2H,-OH), 8.1(m,1H),7.9(m, 2H),7.6(m,2H),7.1 construction of semiconductors and superconducting devices, (m, 3H), 2.1 (s, 3H, Ar-CH ), 1.8 (s, 3H, Ar-CH ). C 3 3 insulation and transmission coatings, and circuits due to their NMR (100 MHz, CDCl ) δ (ppm) 171.3, 167.9 (C=O, ac- electrical properties. They are also used in reflective and non- id), 145.2 (C ), 142.1 (C ), 135.0, 134.0, 132.9, 132.8, 3 5 reflective coatings, interference filters, and optical discs due to 132.0, 128.0, 127.2, 125.0, 115.8, 115.7 (C ), 29.4 (Ar- their optical properties. In addition, thin films are used in CH ), 20.7 (Ar-CH ). (+)ESI-HRMS m/z calculated for 3 3 memory disks due to their magnetic properties [19, 20]. [C H N O +H ] 337.3562; observed 337.3565. 19 16 2 4 This study aimed to synthesize different oligo-pyrazole- based thin films that are newly developed and to investigate 1-(3,4-Dimethylphenyl) their optical properties such as the optical band gap and trans- -5-phenyl-1H-pyrazole-3,4-dicarbonyl dichloride (3) mittance. Furthermore, the study determined the surface mor- phology of the new thin film (21 μm) that had different thick- Compound 2 (0.397 g, 1 mmol) was added to the reaction ness levels using atomic force microscopy (AFM). flask that included excessive thionyl chloride and refluxed at 80 °C. After 6 h, the reaction mixture, which dissolved over time, was cooled down to room temperature. Experimental Excessive thionyl chloride was evaporated. The remaining oily product was purified in dry ether and then was crystal- Materials and equipment lized from toluene, to yield 75%. Color milk white. FT-IR −1 (ν,cm ) 3062 (aromatic C-H), 2928 (aliph. C-H), 1720, All reagents and solvents were purchased from Merck, Sigma 1700 (C=O, acyl), H NMR (400 MHz, CDCl ) δ (ppm) 8.0 and Aldrich companies. These materials were used without (s,1H),7.8 (s,1H), 7.6(m, 2H),7.4(m,2H), 7.2 (m, 2H), purification. Infrared spectra were recorded on a Shimadzu 2.6 (s, 3H, Ar-CH ), 2.1 (s, 3H, Ar-CH ). CNMR 3 3 1 13 IR-470 spectrophotometer. H (400 MHz) and C (100 MHz, CDCl ) δ (ppm) 171.0, 167.5 (C=O, acyl), (100 MHz) NMR spectra were recorded on a Bruker DRX- 142.3 (C ), 141.8 (C ), 135.6, 133.6, 133.0, 132.4, 130.5, 3 5 400 high-performance digital FT-NMR spectrometer. NMR 130.1, 128.4, 128.1, 127.7, 126.3, 114.1 (C ), 30.8(Ar- spectra were obtained in solutions of deuterated chloroform. CH ), 22.8 (Ar-CH ). (+)ESI-HRMS m/z calculated for 3 3 Molecular weight and PDI of the synthesized oligo-pyrazole [C H Cl N O +H ] 374.2438, observed 374.2439. 19 14 2 2 2 were determined by gel permeation chromatography (GPC) using Agilent 1100 Series, equipped with refractive index de- Synthesis tector. Optical measurements of thin films at different thick- of poly(p-phenylene-1-(3,4-dimethylphenyl) nesses were carried out with a Shimadzu model UV-1800 -5-phenyl-1H-pyrazole-3,4-dicarboxyamide (4) spectrophotometer in the wavelength range of 1100–190 nm at room temperature. An Ambios Q-Scope AFM device was Pyrazole-3,4-dicarbonyl dichloride 3 (0.08 g 0.25 mmol) was used to study the surface structures of the films. dissolved in anhydrous tetrahydrofuran (10 ml). The p- phenylene-diamine (0.028 g 0.25 mmol) was added to the 1-(3,4-Dimethylphenyl)-4-(ethoxycarbonyl) reaction pot and the mixture was refluxed for 24 h under an -5-phenyl-1H-pyrazole-3-carboxylic acid (1) atmosphere of nitrogen. Then, it was cooled to room temper- ature. The solvent was evaporated. The precipitated product To synthesize compound 1, procedure existed in literature was was washed with diethyl ether. Then, it was filtered and dried. −1 followed . The yield of 1 was 75%; mp 163.02 °C. Yield 80%. Color brown. FT-IR (ν,cm ) 3385 (N-H, amide), 2967 (aromatic -CH), 1731 (C=O, acyl), 1652 (C=O, amide), 1-(3,4-Dimethylphenyl) 1595–1579 (C=N), 1536–1449 (aromatic, C=C), 1313 (C-N). -5-phenyl-1H-pyrazole-3,4-dicarboxylic acid (2) H NMR (400 MHz, CDCl ) δ (ppm) 8.7, 8.4 (-NH, amide), 7.9–6.3 (m, Ar-H), 4.4 (-NH, aromatic), 2.5, 2.2, 2.1, 2.0, 1.7 Compound 1 (0.364g, 1mmol) washeatedinsolutionof (s, Ar-CH ). C NMR (400 MHz, CDCl ) δ (ppm) 172.5, 3 3 sodium hydroxide (0.1 g 2.5 mmol) in 20 mL water for 1 h. 165.8, 162.3, 159.9 (-C=O), 149.8 (C ), 142.1 (C ), 140.6, 3 5 Colloid Polym Sci (2018) 296:1249–1257 1251 Fig. 1 Images of the thin films of oligo-pyrazole with thickness values of 20, 21, and 24 μm 139.6, 139.3, 139.2, 134.5, 130.4, 130.2, 130.1, 129.4, 127.2, structure of 2 was confirmed using NMR spectroscopy by 123.1, 121.7, 117.5, 114.1, 111.3, 110.3, 108.3 (C ), 30.9, the two carboxylic acid proton signals at δ = 11.68 ppm and 4’ 30.3, 22.4, 22.7, 21.5 (Ar-CH ). Mn = 1333 g/mol, Mw = the two carbonyl carbon signals at δ = 171.3 and δ 167.9 ppm, 2347 g/mol, (polydispersity index, PDI, 1.872). and using FT-IR spectroscopy by the characteristic IR absorp- −1 −1 tion bands at 3350 cm (COOH), 3064 cm (Ar-C-H), 1721 −1 Preparation of the thin films that synthesized and 1710 cm (acid, C=O). oligo-pyrazole (4) The monomeric starting material, pyrazole-3,4- dicarbonyl dichloride 3, was prepared upon heating 2 with Oligo-pyrazole 4 (0.05 g) was added to 1 mL of DMF and the excess thionyl chloride. All the new compounds (1–3)were resulting solution was stirred at room temperature for 1 h. Any confirmed using spectroscopic methods, which were con- insoluble oligo-pyrazole was removed by filtration. The sur- sistent with the previous studies . The preparation of face of a glass substrate was cleaned using piranha solution oligo-pyrazole was performed using the one-step procedure (sulfuric acid and hydrogen peroxide) and then rinsed with showninScheme 1. Poly(p-phenylene-1-(3,4- water. The solution of oligo-pyrazole was added dropwise dimethylphenyl)-5-phenyl-1H-pyrazole-3,4- on the glass substrate and left to dry to form a thin film of dicarboxyamide 4 was synthesized upon the reaction of 3 oligo-pyrazole 4. To obtain films with different thickness and p-phenylene-diamine in refluxing THF for 1 day under levels, the process was repeated several times for each film. an atmosphere of argon gas. The average molecular weight Using this method, three films with thickness values of 20, 21, (Mn), average molecular weight (Mw), and polydispersity and 24 μm were obtained. The thickness values of the films index (PDI) of the as-synthesized oligo-pyrazole 4 were were measured using a micrometer (sensitivity = 0.001 mm), determined by GPC using poly(methyl methacrylate). The as shown in Fig. 1. Mn, Mw, and PDI of the as-synthesized oligo-pyrazole −1 were observed to be 1333, 2347, and 1.872 g mol ,respec- tively. As shown Scheme 1, the as-synthesized oligo- Results and discussion pyrazole 4 was determined be a trimeric structure using gel permeation chromatography. The as-synthesized oligo- Synthesis and characterization pyrazole 4 has three pyrazole structural units and ten phenyl groups. In addition, the as-obtained oligo-pyrazole was Poly-pyrazoles are the most extensively studied subject in asymmetric due to the structure of the monomer. The better polymer chemistry and organic chemistry because of their conjugation was used as a warranty since there were many reliability, accessibility, and chemo-selectivity. Pyrazole-3- aromatic structures. This also explains the low optical band gap value. carboxylic acid 1 was synthesized according to a literature procedure  and pyrazole-3,4-dicarboxylic acid 2 was ob- The structure of the as-obtained oligo-pyrazole was also 1 13 confirmed by the FT-IR, H NMR, and C NMR spectra. tained from the basic hydrolysis of 1 (Scheme 1). The Scheme 1 The synthesis of oligo-pyrazole 4 1252 Colloid Polym Sci (2018) 296:1249–1257 Fig. 2 The graph of absorbance vs wavelength obtained for the oligo-pyrazole-coated and uncoated glass samples −1 The FT-IR bands at 3385 cm correspond to the -NH amide The optical properties of the thin films groups. The bands corresponding to the amide (C=O) groups −1 appear in the region of 1652–1731 cm . The band observed The absorbance curves recorded for the oligo-pyrazole coated −1 at 3066 cm was attributed to the aromatic C-H stretching glass samples and uncoated blank glass were recorded and −1 vibrations. The strong bands at 1595 and 1579 cm corre- shown in Fig. 2. The absorbance value of the oligo-pyrazole spond to the C-C stretching of the phenyl rings. In the case of coated glass samples was increased when compared to the the as-synthesized oligo-pyrazole 4, the correct structure was blank glass sample . This effect was easily observed at 1 13 established by H NMR and C NMR spectroscopy in which all the wavelengths studied. In particular, the absorption be- the characteristic peak for the C=NH proton in was observed tween 550 and 900 nm increased sharply. An increase in the at δ = 9.94 ppm and the NH protons were observed as singlet absorbance value was also observed for the thin films with peaks at δ = 6.0 and 5.1 ppm, and as multiplets in the region of different thickness levels. δ =6.9–8.0 due to the aromatic protons in the as-synthesized The increase in absorbance could be greater if the oligo- oligo-pyrazole 4. pyrazole film was thick. The highest absorbance value was Fig. 3 The graph of transmittance vs wavelength obtained for the oligo-pyrazole-coated and uncoated glass samples Colloid Polym Sci (2018) 296:1249–1257 1253 Fig. 4 The graph of (αhν) vs photon energy obtained for oligo-pyrazole observed for the 24 μm film, while the lowest absorbance value transmittance values of the same films were calculated 12, 10, was observed for the 20 μm film. The transmittance graph and 5% in the visible range (between 380 and 780 nm), also obtained for the 20, 21, and 24 μmfilmsisshowninFig. 3. respectively. As a result, the transmittance value decreased after After coating the glass substrate with oligo-pyrazole, the increasing the film thickness . Also, we can say that the transmittance was significantly reduced. The transmittance val- transparent quality of the oligo-pyrazole films is very low in ue of the blank glass sample was in the range of 90–100% (Fig. the visible region due to the remarkably low light transmission 3). Upon coating the glass substrate with oligo-pyrazole, the . One of the most important parameters for the optical prop- transmittance value was at most 55%. The transmittance value erties of a film coating is the Eg value. The photon energy vs for the oligo-pyrazole-coated films remained constant in the (α.hν) plot was used to obtain the direct band gap of oligo- range of 300–550 nm, whereas it increased in the range of pyrazole using the Tauc equation (Fig. 4). 550–1100 nm. The average transmittance values of the 20, As shown in Fig. 4,the E values observed for the films 21, and 24 μm oligo-pyrazole films were 26, 19, and 13% with thickness values of 20, 21, and 24 μmwere 1.646, 1.537, between 294 and 1100 nm, respectively. The average and 1.428 eV, respectively. The E value decreased upon 1/2 Fig. 5 The graph of (αhν) vs photon energy obtained for the thin films of oligo-pyrazole 1254 Colloid Polym Sci (2018) 296:1249–1257 Table 1 The E values obtained for the oligo-pyrazole films with thick- The optical band gap is an important parameter for semi- ness values of 20, 21, and 24 μm conductors. It particularly has a shifting effect in the design of semiconductor materials . For this reason, it is very im- No Thickness Direct E Indirect E Forbidden indirect Eg g g (μm) values (eV) values (eV) values (eV) portant that these materials may be modeled and have the desired optical energy gap value. Also, organic photovoltaic 1 20 1648 1324 1466 devices are among the most popular research areas, because 2 21 1537 1182 1339 the organic molecules used in these devices have certain prop- 3 24 1426 1117 1145 erties that are superior to photovoltaic applications. These properties include being low cost, flexible, light weight, and suitable for many new designs [29, 30]. The oligo-pyrazole increasing the thickness of the oligo-pyrazole film and it was compound has a very low band gap value, although the re- possible to further reduce the E value by increasing the film peating unit numbers were very small. This low value (1.42– thickness further. In addition, the E value of the newly syn- 1.64 eV) is desirable for photovoltaic applications . The thesized oligo-pyrazole was very low. When the coated oligo- minimum band gap value of organic materials that were stud- pyrazole film is set to the desired thickness, it acts as the ied in photovoltaic applications was 1.6–2.2 eV [32, 33]. It semiconducting material . Obviously, the desired E value appears that the oligo-pyrazole compound has a low Eg value can be obtained for a particular optical application by 2 as the polymers used in photovoltaic applications. Thus, the adjusting the film thickness. The photon energy vs (α.hν) synthesized oligo-pyrazole molecule has a potential to be used plot was used to determine the indirect E values for the thin in photovoltaic applications. The oligo-pyrazole molecule in- films of oligo-pyrazole (Fig. 5). creases the thermal stability by lowering the internal energy The indirect band gap values observed for the films with because it shows high conjugation . thickness values of 20, 21, and 24 μm were found to be 1.324, 1.182, and 1.117 eV, respectively. The E value decreased after increasing the film thickness (Fig. 5). The direct transi- The surface morphology of thin film tion was sharper than the indirect transition (Figs. 4 and 5)and the direct transition was more dominant than the indirect tran- In this study, AFM was used to investigate the surface proper- sition. The direct E and indirect E values observed for the ties of the oligo-pyrazole thin films. Two- and three- g g oligo-pyrazole films with different thickness levels are shown dimensional images (9 μm×9 μm) of the surface morphol- in Table 1. ogies of the thin films deposited on the glass substrate were 1/3 The graph of E (eV) vs (αhν) was plotted for the oligo- recorded at a scanning speed of 0.7 Hz using the AFM device pyrazole films with thickness values of 20, 21, and 24 μm in non-contact mode . The surface features, such as the (Fig. 6). The forbidden indirect band gap values observed surface roughness, skewness, kurtosis, and height, were deter- for the 20, 21, and 24 μm films were 1.466, 1.339, and mined using the software on the AFM instrument. Figure 7 1.145 eV, respectively. The forbidden indirect band gap value shows the two-dimensional and three-dimensional AFM image decreased after increasing the film thickness. of the oligo-pyrazole thin film 21 μmcoatedonaglass 1/3 Fig. 6 The graph of (αhν) vs photon energy obtained for the thin films of oligo-pyrazole Colloid Polym Sci (2018) 296:1249–1257 1255 Fig. 7 The 2D and 3D AFM images of the as-synthesized oligo-pyrazole thin film coated on a glass substrate substrate [36, 37]. In the AFM images, a few black regions The height distribution graph was obtained using the AFM and several yellow regions appeared over a large area. The tool (Fig. 7). The heightdistributionplotisassociatedwiththe surface properties of the films were analyzed using high- homogeneous grain distribution observed on the samples’ sur- resolution AFM imaging and image processing evaluation face. The thin film coated on the glass substrate exhibits a software. The AFM surface analysis results obtained for the perfect Gauss curve . To summarize, three thin films were thin film coated on the glass substrate showed the average obtained with different thickness levels and their optical prop- surface roughness, which gives the deviation in height, was erties were examined. Desired optical properties can be 3.34 nm; the average square root roughness, which repre- achieved by adjusting the oligo-pyrazole film thickness. sents the standard deviation of the surface height, was 5.65 nm and the skewness, which represents the symmetry, was − 0.26 nm. The Kurtosis value, which is a measure of Conclusions pressure, was 8.84 nm. The total roughness, which is the sum of the maximum height and the maximum depth for the This study synthesized a novel conjugated oligomer that entire measurement length, was 43.80 nm for the red line in was comprised of pyrazole skeleton. Thin films made from the 2D-AFM image, average surface roughness (3.90 nm), this oligomer with different thickness (20, 21, and 24 μm) average square root roughness (5.74 nm), and skewness were prepared and their optical properties were investigat- (1.18). The Kurtosis value was 5.43. The total roughness ed. The band gap values of the oligo-pyrazole films were was 33.79 nm for the green line in the 2D-AFM graph, as very low (1.426, 1.537, and 1.648 eV, respectively) and shown in Fig. 6. The skewness value in the produced film were suitable for optical applications. Furthermore, the was negative for the red line, which showed that the pits on band gap value was observed to decrease after increasing the film surface were more dominant than the peaks. the thickness of the thin film, which showed that their 1256 Colloid Polym Sci (2018) 296:1249–1257 9. 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Colloid Polymer Science – Springer Journals
Published: Jun 5, 2018
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