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New derivatives of azopyrazolo dyes: Synthesis and optical characterization for application in sensitized solar cells
Optik - International Journal for Light and Electron Optics 196 (2019) 163036
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Original research article
New derivatives of azopyrazolo dyes: Synthesis and optical characterization for application in sensitized solar cells
T
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Ali Saeeda, , Muneer A. Nasherb, Ehab Abdel-Latifa, Eman M. Keshka, Abdel-Galil M. Khalila, Heba M. Metwallya a b
Department of Chemistry, Faculty of Science, Mansoura University, 35516 Mansoura, Egypt Department of Physics, Faculty of Science, Saadah University, Sadah, Yemen
A R T IC LE I N F O
ABS TRA CT
Keywords: 4-Aminoacetanilide 3-Aminocrotonitrile Pyrazoles Pyrazolotriazines Solar cells
Several novel pyrazolotriazine azo scaffolds were synthesized by the diazotization of 4-aminoacetanilide and coupling with 3-Aminocrotonitrile and then refluxed with hydrazine hydrate to furnish 4-(4-acetamidophenylazo)-3-amino-5-methyl-1H-pyrazol. The later scaffolds was diazotized and coupled with active methylene compounds namely; (malononitile, and ethyl cyanoacetate) afforded the corresponding novel pyrazolotriazines derivatives. All freshly synthesized scaffolds were elucidated by using FT-IR, 1H NMR, M S and elemental analysis, and they were evaluated for their sensitized solar cells to show promising results. Therefore, thin films of these dyes were prepared by using thermal evaporation technique. Optical properties of these films were conducted by employing the UV–vis-IR spectroscopy. Result of optical investigation indicated that the direct forbidden transition was the most probable transition. The high value and broad spectrum of absorption coefficient of these films in the UV and visible regions of spectra recommended its application in design of photovoltaic devices.
1. Introduction Organic semiconductors attracts the attention of the scientist community owing to their availability, flexibility for a wide variety of applications, such as electroluminescence, fluorescence and phosphorescence, photoconductivity and photovoltaic devices [1]. Azo dyes are the largest class of industrial synthesized organic dyes because of their wide utilized applications, such as dye sensitized solar cells [2], dyeing textile fiber [3–5] photo-sensitizers [6], sensors [7], liquid crystalline displays [8], photochromic materials [9], computational studies [10], metallochromic indicators [11], electro-optical devices [12], and biological–medical studies [13–15]. These compounds are known to have a broad range of pharmacological and medical potential applications such as antimicrobial [13,14], and antifungal activities [15]. Many of the five-membered heterocyclic scaffolds that contain two nitrogen atoms in the ring (diazoles) show significant biological activities and thus appear an important group of organic molecules [16,17]. Pyrazoles, two adjacent nitrogen atoms in fivemembered ring, are reported to possess different biological activities [18] such as antimicrobial [14,19,20], anti-inflammatory [21], antitubercular [22], antidepressant [23], anticancer [24] and dyes sensitized solar cells [25–28]. An organic molecule, particularly with macro π systems, represents an effective endeavor to develop new materials with seductive nonlinear optical properties [1]. In this context, pyrazole moieties with their five-membered heterocyclic ring in a push-pull donor–acceptor chromophore deems a π-
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Corresponding author. E-mail address: [email protected] (A. Saeed).
https://doi.org/10.1016/j.ijleo.2019.163036 Received 23 April 2019; Received in revised form 27 June 2019; Accepted 29 June 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
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electron conjugating bridge. As a result, pyrazole moieties are frequently examined in optical communication, frequency doubling and integrated optics [29–32]. Pyrazole moiety used as intermediates in the synthesis of various bioactive compounds such as pyrazolopyrimidines, pyrazolotriazines, Pyrazolopyridines, pyrazoloquinazolines and Schiff bases [33–37]. Also pyrazole possesses two N-donors on the pyrazole ring and can show flexible coordination modes when coordinates with metal ions [38–40]. The N atom of pyrazole can chelate to metal ions and form very stable complexes [41]. In view of these findings, the present work focuses on the synthesis of novel azopyrazole and pyrazolotriazine scaffolds as new organic dyes and it is designed to improve the intra-molecular charge transfer (ICT) through adopting suitable strategies such as (D-πA), and (D-π-D). We introduced the azopyrazole to elongate the π backbone, then to enhance the ICT, the electron donating groups and electron with-drawing group have been substituted, like CH3CONH (amide), at one end while COOC2H5 (ethyl acetate), CN (nitrile) at opposite side of phenyl azopyrazole moiety to boost up the electron-donor ability. The as- synthesized derivatives are addressed to clarify the possibility of inserting of these derivatives in photovoltaic devices. 2. Experimental details 2.1. General remarks All melting points (uncorrected) were measured on Gallenkamp electric melting point apparatus. Infrared spectra were determined on Mattson 5000 FTIR spectrometer (KBr discs). The 1H NMR spectra were registered on a Varian XL 400 MHz apparatus using DMSO-d6 as a solvent. The mass spectra were acquired by EI mode at 70 eV with Kratos MS equipment. Elemental analyses (C, H and N) were determined on Perkin-Elmer 2400 analyzer. The 4-aminoacetanilide used in this study was obtained from SigmaAldrich Chemical Company (St. Louis, MO, USA). 2.2. Synthesis of N-(4-acetamidophenyl)-2-oxopropanehydrazonoyl cyanide (3) To a cold suspension of 4-aminoacetanilide 1 (3.0 g, 20 mmol) in 6 mL conc. HCl, an aqueous solution of NaNO2 (1.4 g in 20 mL H2O) was added drop by drop with continuous stirring at 0–5 °C. This newly prepared diazonium chloride solution underwent drop by drop addition to a cold solution of 3-aminocrotonitrile 2 (1.64 g, 20 mmol) in ethanol (50 mL) and sodium acetate (6.0 g). After stirring of the mixture for two hours at 0–5 °C, the solid that formed was filtered and recrystallized by boiling in ethanol. Orange crystals; yield (82%); m.p. 130–132 °C. IR (ν¯ /cm−1): 3536, 3340 (2NH), 2215 (C≡N), broad at 1660 (C = O), 1583 (C = N). 1H NMR (δ/ppm): 2.04 (s, 3H, CH3), 2.39 (s, 3H, CH3), 7.48 (d, J =9 Hz, 2H, Ar-H), 7.62 (d, J =9 Hz, 2H, Ar-H), 10.02 (s, 1H, NH), 12.21 (s, 1H, NH). Anal. for C12H12N4O2 (244.25): C, 59.01; H, 4.95; N, 22.94%. Found: C, 59.12; H, 4.98; N, 22.99%. 2.3. Synthesis of 4-(4-acetamidophenylazo)-3-amino-5-methyl-1H-pyrazole (4) For a solution of compound 3 (1.22 g, 5 mmol) in ethanol (30 mL), hydrazine hydrate (0.5 mL, 10 mmol) was added. After refluxing for 4 h, the solvent was removed under vacuum and the residue recrystallized in EtOH to afford pyrazole derivative 4. Orange powder; yield (85%); m.p. 240–242 °C. IR (ν¯ /cm−1): 3409, 3294, 3262, 3194 (NH2, 2NH), 1665 (C = O), 1597 (C = N), 1530 (N = N). 1H NMR (δ/ppm): 2.06 (s, 3H, CH3), 2.35 (s, 3H, CH3), 5.81 (s, 1 H) & 6.92 (s, 1 H) (NH2), 7.65 (s, 4H, Ar-H), 10.03 (s, 1H, CONH), 11.55 (s) & 12.05 (s) (1H, NH-pyrazole). MS (m/z, %): 259 (M++1, 7.1), 258 (M+, 37.0), 236 (2.1), 191 (1.8), 119 (5.5), 77 (9.2), 56 (100.0). Anal. for C12H14N6O (258.28): C, 55.80; H, 5.46; N, 32.54%. Found: C, 55.97; H, 5.52; N, 32.46%. 2.4. General procedure for the synthesis of 5-methyl-pyrazol-3-yl-hydrazones 6 and 7 To a cold suspension of 4 (2.58 g, 10 mmol) in 3 mL conc. HCl, an aqueous solution of NaNO2 (0.7 g in 10 mL H2O) was added drop by drop with continuous stirring at 0–5 °C. This newly prepared diazonium chloride solution underwent drop by drop addition to a cold solution of active methylene compounds namely; (malononitile and/or ethyl cyanoacetate) (10 mmol) in 30 mL ethyl alcohol and 3.0 g sodium acetate. After stirring of the mixture for two hours at 0–5 °C, the solid that formed was filtered and recrystallized from EtOH-DMF mixture (3:1) to give compounds 6 and 7, respectively. 2.5. 2-(2-(4-((4-Acetamidophenyl)azo)-5-methyl-1H-pyrazol-3-yl)-hydrazono)-malononitrile (6) Red powder; yield (75%); m.p. 216–217 °C. IR (ν¯ /cm−1): 3308, 3197 (NH), 2228 (C≡N), 1667 (C = O), 1577 (C = N), 1541 (N = N). 1H NMR (δ/ppm): 2.06 (s, 3H, CH3), 2.36 (s, 3H, CH3), 7.52 (d, J =9 Hz, 2H, Ar-H), 7.70 (d, J =9 Hz, 2H, Ar-H), 10.23 (s, 1H, NH), 10.76 (s, 1H, NH), 11.76 (s, 1H, NH). Anal. for C15H13N9O (335.32): C, 53.73; H, 3.91; N, 37.59%. Found: C, 53.59; H, 3.96; N, 37.67%. 2.6. Ethyl 2-(2-(4-((4-acetamidophenyl)azo)-5-methyl-1H-pyrazol-3-yl) hydrazono)-2-cyanoacetate (7) Brown crystals; yield (75%); m.p. 201–203 °C. IR (ν¯ /cm−1): 3196, 3136 (NH), 2206 (C≡N), 1717, 1670 (2C = O), 1597 (C = N), 1556 (N = N). MS (m/z, %): 382 (M+, 8.16), 313 (15.92), 293 (18.64), 287 (19.18), 259 (13.74), 233 (8.91), 220 (14.47), 188 2
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(17.13), 172 (24.54), 157 (21.80), 148 (13.27), 121 (25.36), 106 (37.84), 94 (48.33), 83 (62.17), 77 (100.00), 57 (31.3), 43 (44.30). Anal. for C17H18N8O3 (382.38): C, 53.40; H, 4.74; N, 29.30%. Found: C, 53.22; H, 4.78; N, 29.38%. 2.7. General procedure for the Synthesis of pyrazolo[5,1-c]triazine derivatives 8 and 9 A suspension of pyrazolyl hydrazone derivatives 6 and 7 (5 mmol) in acetic acid (10 mL) was refluxed for 1 h, After cooling the mixture, it was poured into ice-cold water. The solid formed was filtered and recrystallized in EtOH/DMF mixture (4:1) to afford the pyrazolo[5,1-c]triazine derivatives 8 and 9, respectively. 2.8. 8-(4-Acetamidophenylazo)-4-amino-3-cyano-7-methylpyrazolo[5,1-c]- [1,2,4]triazine (8) Brown crystals; yield (65%); m.p. 300–302 °C. IR (ν¯ /cm−1): 3438, 3296, 3238 (NH2 and NH), 2229 (C≡N), 1665 (C = O), 1642 (C = N), 1543 (N = N). 1H NMR (δ/ppm): 2.10 (s, 3H, CH3), 2.74 (s, 3H, CH3), 7.79 (d, 2H, J =9 Hz, Ar-H), 7.84 (d, 2H, J =8.96 Hz, Ar-H), 9.54 (s, 2H, NH2), 10.26 (s, 1H, NH). MS (m/z, %): 336 (M++1, 18.23), 335 (M+, 100.0), 293 (17.39), 224 (9.73), 201 (36.21), 199 (26.33), 173 (23.39), 162 (8.65), 134 (12.79), 120 (10.91), 106 (57.25), 92 (16.00), 79 (26.75), 65 (5.74), 43 (26.73). Anal. for C15H13N9O (335.32): C, 53.73; H, 3.91; N, 37.59%. Found: C, 53.98; H, 3.96; N, 37.67%. 2.9. 8-(4-Acetamidophenylazo)-4-amino-3-ethoxycarbonyl-7-methyl-pyrazolo[5,1-c][1,2,4]triazine (9) Brown powder; yield (74%); m.p. 298–300 °C. IR (ν¯ /cm−1): 3444, 3357, 3298 (NH2 and NH), 1697, 1649 (2C = O), 1597 (C = N), 1541 (N = N). 1H NMR (δ/ppm): 1.39 (t, J =7.05 Hz, 3H, CH3), 2.09 (s, 3H, CH3), 2.74 (s, 3H, CH3), 4.44 (q, J =7 Hz, 2H, CH2), 7.78 (d, J =9 Hz, 2H, Ar-H), 7.83 (d, J =8.7 Hz, 2H, Ar-H), 8.65 & 9.41 (two s, 2H, NH2), 10.22 (s, 1H, CONH). Anal. for C17H18N8O3 (382.38): C, 53.40; H, 4.74; N, 29.30%. Found: C, 53.48; H, 4.78; N, 29.24%. 2.10. Preparation of azopyrazolo dyes thin films Thermal evaporation technique has been used to deposited films of azopyrazolo derivatives (3, 4, 6, 7, 8 and 9) at pressure around 10−5 mbar (Edwards E306 A coating system). Quartz crystal thickness monitor attached with the system were used to measure the thickness of films during deposition (Model TM-350 MAXTEK, Inc., USA). For optical measurements we used optical flat quartz. A double-beam spectrophotometer (JASCO model V-570 UV–vis–NIR) was used to measure the transmittance, T(λ), and reflectance, R (λ), and spectra of the as-deposited derivatives of azopyrazolo dyes films in the spectral range 200–2500 nm. 3. Results and discussion Diazotization of 4-aminoacetanilide (1) by using sodium nitrite in hydrochloric acid. 4-Aminoacetanilide (1) was diazotized utilizing sodium nitrite in hydrochloric acid. The diazotized 4-aminoacetanilide was then coupled with 3-aminocrotonitrile (2) in ethanol containing sodium acetate to afford the hydrazonoyl cyanide derivative 3. The formation of 3 was considered to proceed via electrophilic diazo-coupling reaction at the methylene function followed by hydrolysis of imine group (C]N) to ketone (C]O) and
Fig. 1. Synthesis of 3-amino-4-arylazopyrazole derivative 4. 3
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ammonia under the reaction conditions (Fig. 1). Heterocyclization reaction of 3 with hydrazine was hydrate proceeded by reflux in ethanol to afford the corresponding 3-amino-4-arylazopyrazole derivative 4. Spectroscopic techniques (IR, 1H NMR and MS) were used to confirm the designed structures of 3 and 4. The IR spectrum of compound 3 showed the characteristic absorption band of nitrile function (C^N) at 2215 cm−1 that disappeared in the IR spectrum of compound 4 and the characteristic absorption bands of NeH function (NH and NH2) at 3409, 3294, 3262 and 3194 cm−1 were assigned instead of this nitrile absorption. The 1H NMR spectrum of compound 3 displayed the characteristics of two singlet signals at 2.04 and 2.39 ppm corresponding to the protons of two methyl groups. The aromatic protons were resonated as two doublet signals at 7.48 and 7.62 ppm, in while the protons of two NH groups were showed as two singlet signals at 10.02 and 12.21 ppm. In addition, the 1H NMR signals of compound 4 were indicated as two singlet signals at 2.06 and 2.35 ppm for the protons of two methyl groups. The two protons of amino group (NH2) were resonated as two singlet signals at 5.81 & 6.92 ppm while the four aromatic protons were resonated as singlet at 7.65 ppm. The signal of amide NeH group was assigned as singlet at 10.03 ppm. The two singlet signals 11.55 & 12.05 indicated the NH of pyrazole formulas 4a and 4b. Diazotization of 3-amino-4-arylazopyrazole 4 with sodium nitrite in a mixture of acetic acid and hydrochloric acid led to the corresponding diazonium salt 5, which underwent coupled with malononitrile and/or ethyl cyanoacetate in ethanol containing sodium acetate furnished the corresponding hydrazonoyl cyanide derivatives 6 and 7, respectively (Fig. 1). The spectral tools (IR, 1H NMR and MS) have been utilized to identify the chemical structure of these hydrazones 6 and 7. The IR spectrum of hydrazonyl cyanide 6 exhibited absorptions at 3308 and 3197 cm−1 characteristic for the NH functions, and at 2228 and 1667 cm-1 for the nitrile (C^N) and carbonyl (C]O) functions. In the 1H NMR spectrum of 6, two singlet signals were observed at 2.06 and 2.36 ppm corresponding to the protons of two methyl groups. The aromatic protons were resonated as two doublet signals at 7.52 and 7.70 ppm while the protons of three NH functions were resonated as three singlet signals at 10.23, 10.76 and 11.76 ppm. On the other hand, the mass spectrum of compound 7 showed the molecular ion peaks at m/z 382 (relative intensity 8.16%), which was in agreement with molecular formula of the compound (C17H18N8O3). Intramolecular cyclization of scaffolds 6 and 7 was successfully performed under the influence of hot acetic acid to afford the desired pyrazolo[5,1-c]triazines 8 and 9 (Fig. 2). The designed structures of these pyrazolotriazines 8 and 9 were confirmed by spectroscopic and elemental analyses. The IR spectrum of pyrazolotriazine 9 showed absorption bands at 3444, 3357 and 3298 cm–1 assigned to the NH2 and NH groups. Meanwhile missing absorption band at 2206 cm−1 revealed the disappearance of nitrile function. The 1H NMR spectrum of pyrazolotriazine 9 exhibited ethyl protons as triplet and quartet signals at 1.39 and 4.44 ppm for the protons of ethyl group (−CH2CH3), while the other methyl protons were resonated as two singlet signals at 2.09 and 2.74 ppm. The protons on -NH2 function resonated as two singlet signals at 8.65 & 9.41 ppm while the singlet at 10.22 ppm clearly indicated the proton of amide (CONH). The mass spectrum of pyrazolotriazine scaffold 8 showed the molecular ion peaks at m/z 335 (M+, 100.0) which was in agreement with molecular formula of the compound (C15H13N9O).
Fig. 2. Synthesis of pyrazolotriazine derivatives 8 and 9. 4
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Fig. 3. Spectral behavior of transmittance, T(λ), and reflectance, R(λ), for the pristine azopyrazolo dyes thin films.
4. Optical constants The optical properties for as-deposited azopyrazolo derivatives (3, 4, 6, 7, 8 and 9) thin films of thickness 200 nm are investigated for measuring the transmittance, T(λ), and the reflectance, R(λ). The spectral of the T(λ) and R(λ) versus the light incident in the range from 200 to 2200 nm is shown in Fig. 3. The transmittance edge of pristine films divide the spectra into two regions, the first region in the range 200–700 nm; the total sum of T(λ) and R(λ) is less than unity (absorption region) and the second at long wavelengths; λ > 700 nm, the film becomes nearly transparent and no light is absorbed or scattered T(λ) + R(λ) ≈ 1 (transparent region). The spectral behavior of the T(λ) and R(λ) for the investigated thicknesses supports the film homogeneity. By using the following equations, we can calculate the values of the refractive index, n, and the extinction coefficient, k. [42–44]
R=
(n − 1)2 + k 2 (n + 1)2 + k 2
(1)
k=
αλ 4π
(2)
from the measured values of R and T, the values of the absorption coefficient, α, can be determined by the following relation [42]: 1/2
1 ⎡ (1 − R)2 ⎞ ⎛ ⎛ (1 − R ) 4 ⎞ +⎜ + R2⎟⎞ α = ⎛ ⎞ ln ⎢ ⎛ ⎝ d ⎠ ⎣ ⎝ 2T ⎠ ⎝ ⎝ 4T 2 ⎠ ⎠ ⎜
⎟
⎜
⎟
⎤ ⎥ ⎦
(3)
hence d is the film thickness. Fig. 4 shows the values of extinction coefficient as a function of incident wavelength. From this figure, it is clear the existence of two absorption bands. The first lies in the ultraviolet region of spectra and the other in the visible region of spectra. The band absorption in the UV region of spectra is attributed to n-π* orbital transition and those in the visible region of spectra are due to π -π* orbital transition [45,46]. Due to the existence of defects in structure and free charge carrier, we notice that the values of k does not approach zero at wavelength > 600 nm [47]. The calculated values of absorption coefficient (α) are plotted versus the wavelength in Fig. 5. It is clear that the values of (α) are high and greater than 2.5 × 105 cm−1. which are enough to meet the requirements of solar energy harvesting devices.
Fig. 4. Spectral behavior of absorption index, k, for the pristine azopyrazolo dyes thin films. 5
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Fig. 5. Absorption coefficient, α(hν) dependence on the incident wavelength, (λ) for pristine azopyrazolo dyes thin films.
The type of transition and the values of optical band gap energy, Eg, can be determined by the analysis of the (α) nearby the edge of absorption according to the formula [48]:
αhv = B (hv − Eg )r
(4)
(αhv )2/3
It has been found that the direct forbidden transition is the most probable transition with r = 3/2. The dependence of on the energy of photon (hv ) are illustrated in the Fig. 6 for azopyrazolo dyes thin films. The values of energy gap are summarized in Table 1. Dielectric constant of a material, ε*, is also studied, can be determined by the formula ε * = ε1 + ε2 , where ε1 and ε2 denote to the real and the imaginary parts of the dielectric constant, and they are calculated by the equations [49,50]:
ε1 = n2 − k 2, and ε2 = 2nk
(5)
Figs. 7 and 8 show the relation between the ε1 and ε2 and photon energy (hv ) , where the values of ε2 are smaller than of those ε1 at the same wavelength. The spectrum of ε1 and ε2 can be divided fundamentally into three regions, region I lies in the range energy around < 2.25 eV, hence the interaction between the electrons in azopyrazolo dyes thin films and photons is a weak, region II the energy represents the energy separation between the optical and fundamental energy gaps, region III both ε1 and ε2 increase with increasing photon energy. The surface energy loss, SEL, and the volume energy loss, VEL, can be determined by using the equations [45]:
SEL =
ε2 ε and VEL = 2 2 2 (ε1 + 1)2 + ε22 (ε1 + ε2 )
(6)
Figs. 9 and 10 shows the SEL and VEL as a function of the incident photon energy. Both functions have the same behavior and the energy lost by photons traveling through the material is greater than the energy lost by those passing by its surface. The real and imaginary optical conductivity (σ1 and σ2 ) could be calculated from dielectric constant by using this equations [44,51]: (7)
σ1 = ωε2 ε∘ and σ2 = ωε1 ε∘
For optoelectronic applications considered as study of optical conductivity is important, due to optical conductivity that gives us image of how photons can promote electrons form localized states to delocalized states and create electron-hole pairs. Figs. 11 and 12 depict the σ1 and σ2 as a functions of (hv ) . The values of optical conductivity of order are 5 × 105 (Ω.m)−1.
Fig. 6. Dependence of (αhν)2/3 on hν for pristine azopyrazolo dyes thin films. 6
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Table 1 Energy gaps for as-deposited azopyrazolo derivatives. Day film
Energy gap, Eg, (eV)
3 4 6 7 8 9
2.32 2.10 2.07 2.04 2.06 2.18
Fig. 7. Real part of dielectric constant (ε1 ), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
Fig. 8. Imaginary part of dielectric constant (ε2 ), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
Fig. 9. The surface energy loss (SEL), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
7
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Fig. 10. The volume energy loss (VEL), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
Fig. 11. Real part of optical conductivity (σ1), for azopyrazolo dyes thin films.
Fig. 12. Imaginary part of optical conductivity (σ2 ), for azopyrazolo dyes thin films.
5. Conclusion A series of novel pyrazolotriazine dyes were synthesized via a sequence involving initial diazotization of 4-aminoacetanilide, coupling with 3-Aminocrotonitrile and then refluxed with hydrazine hydrate to furnish pyrazolohydrazone that was then diazotized and coupled with active methylene compounds namely; (malononitile, and ethyl cyanoacetate) afforded the corresponding novel pyrazolotriazines dyes. Optical properties of derivatives azopyrazolo dyes films have been investigated. The films were prepared by thermal evaporation technique. The optical absorption edges are described using the band transition. The type of electron transition was found to be direct forbidden transition. Important spectral parameters, such as optical absorption coefficient (α), real and the imaginary parts of the dielectric constant (ε1 and ε2 ), the surface energy loss and the volume energy loss (SEL and VEL), and optical conductivity (σ1 and σ2 ) were calculated. The present work confirmed that the derivatives azopyrazolo dyes are recommend its application in design of photovoltaic devices. 8
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Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
New derivatives of azopyrazolo dyes: Synthesis and optical characterization for application in sensitized solar cells
T
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Ali Saeeda, , Muneer A. Nasherb, Ehab Abdel-Latifa, Eman M. Keshka, Abdel-Galil M. Khalila, Heba M. Metwallya a b
Department of Chemistry, Faculty of Science, Mansoura University, 35516 Mansoura, Egypt Department of Physics, Faculty of Science, Saadah University, Sadah, Yemen
A R T IC LE I N F O
ABS TRA CT
Keywords: 4-Aminoacetanilide 3-Aminocrotonitrile Pyrazoles Pyrazolotriazines Solar cells
Several novel pyrazolotriazine azo scaffolds were synthesized by the diazotization of 4-aminoacetanilide and coupling with 3-Aminocrotonitrile and then refluxed with hydrazine hydrate to furnish 4-(4-acetamidophenylazo)-3-amino-5-methyl-1H-pyrazol. The later scaffolds was diazotized and coupled with active methylene compounds namely; (malononitile, and ethyl cyanoacetate) afforded the corresponding novel pyrazolotriazines derivatives. All freshly synthesized scaffolds were elucidated by using FT-IR, 1H NMR, M S and elemental analysis, and they were evaluated for their sensitized solar cells to show promising results. Therefore, thin films of these dyes were prepared by using thermal evaporation technique. Optical properties of these films were conducted by employing the UV–vis-IR spectroscopy. Result of optical investigation indicated that the direct forbidden transition was the most probable transition. The high value and broad spectrum of absorption coefficient of these films in the UV and visible regions of spectra recommended its application in design of photovoltaic devices.
1. Introduction Organic semiconductors attracts the attention of the scientist community owing to their availability, flexibility for a wide variety of applications, such as electroluminescence, fluorescence and phosphorescence, photoconductivity and photovoltaic devices [1]. Azo dyes are the largest class of industrial synthesized organic dyes because of their wide utilized applications, such as dye sensitized solar cells [2], dyeing textile fiber [3–5] photo-sensitizers [6], sensors [7], liquid crystalline displays [8], photochromic materials [9], computational studies [10], metallochromic indicators [11], electro-optical devices [12], and biological–medical studies [13–15]. These compounds are known to have a broad range of pharmacological and medical potential applications such as antimicrobial [13,14], and antifungal activities [15]. Many of the five-membered heterocyclic scaffolds that contain two nitrogen atoms in the ring (diazoles) show significant biological activities and thus appear an important group of organic molecules [16,17]. Pyrazoles, two adjacent nitrogen atoms in fivemembered ring, are reported to possess different biological activities [18] such as antimicrobial [14,19,20], anti-inflammatory [21], antitubercular [22], antidepressant [23], anticancer [24] and dyes sensitized solar cells [25–28]. An organic molecule, particularly with macro π systems, represents an effective endeavor to develop new materials with seductive nonlinear optical properties [1]. In this context, pyrazole moieties with their five-membered heterocyclic ring in a push-pull donor–acceptor chromophore deems a π-
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Corresponding author. E-mail address: [email protected] (A. Saeed).
https://doi.org/10.1016/j.ijleo.2019.163036 Received 23 April 2019; Received in revised form 27 June 2019; Accepted 29 June 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
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electron conjugating bridge. As a result, pyrazole moieties are frequently examined in optical communication, frequency doubling and integrated optics [29–32]. Pyrazole moiety used as intermediates in the synthesis of various bioactive compounds such as pyrazolopyrimidines, pyrazolotriazines, Pyrazolopyridines, pyrazoloquinazolines and Schiff bases [33–37]. Also pyrazole possesses two N-donors on the pyrazole ring and can show flexible coordination modes when coordinates with metal ions [38–40]. The N atom of pyrazole can chelate to metal ions and form very stable complexes [41]. In view of these findings, the present work focuses on the synthesis of novel azopyrazole and pyrazolotriazine scaffolds as new organic dyes and it is designed to improve the intra-molecular charge transfer (ICT) through adopting suitable strategies such as (D-πA), and (D-π-D). We introduced the azopyrazole to elongate the π backbone, then to enhance the ICT, the electron donating groups and electron with-drawing group have been substituted, like CH3CONH (amide), at one end while COOC2H5 (ethyl acetate), CN (nitrile) at opposite side of phenyl azopyrazole moiety to boost up the electron-donor ability. The as- synthesized derivatives are addressed to clarify the possibility of inserting of these derivatives in photovoltaic devices. 2. Experimental details 2.1. General remarks All melting points (uncorrected) were measured on Gallenkamp electric melting point apparatus. Infrared spectra were determined on Mattson 5000 FTIR spectrometer (KBr discs). The 1H NMR spectra were registered on a Varian XL 400 MHz apparatus using DMSO-d6 as a solvent. The mass spectra were acquired by EI mode at 70 eV with Kratos MS equipment. Elemental analyses (C, H and N) were determined on Perkin-Elmer 2400 analyzer. The 4-aminoacetanilide used in this study was obtained from SigmaAldrich Chemical Company (St. Louis, MO, USA). 2.2. Synthesis of N-(4-acetamidophenyl)-2-oxopropanehydrazonoyl cyanide (3) To a cold suspension of 4-aminoacetanilide 1 (3.0 g, 20 mmol) in 6 mL conc. HCl, an aqueous solution of NaNO2 (1.4 g in 20 mL H2O) was added drop by drop with continuous stirring at 0–5 °C. This newly prepared diazonium chloride solution underwent drop by drop addition to a cold solution of 3-aminocrotonitrile 2 (1.64 g, 20 mmol) in ethanol (50 mL) and sodium acetate (6.0 g). After stirring of the mixture for two hours at 0–5 °C, the solid that formed was filtered and recrystallized by boiling in ethanol. Orange crystals; yield (82%); m.p. 130–132 °C. IR (ν¯ /cm−1): 3536, 3340 (2NH), 2215 (C≡N), broad at 1660 (C = O), 1583 (C = N). 1H NMR (δ/ppm): 2.04 (s, 3H, CH3), 2.39 (s, 3H, CH3), 7.48 (d, J =9 Hz, 2H, Ar-H), 7.62 (d, J =9 Hz, 2H, Ar-H), 10.02 (s, 1H, NH), 12.21 (s, 1H, NH). Anal. for C12H12N4O2 (244.25): C, 59.01; H, 4.95; N, 22.94%. Found: C, 59.12; H, 4.98; N, 22.99%. 2.3. Synthesis of 4-(4-acetamidophenylazo)-3-amino-5-methyl-1H-pyrazole (4) For a solution of compound 3 (1.22 g, 5 mmol) in ethanol (30 mL), hydrazine hydrate (0.5 mL, 10 mmol) was added. After refluxing for 4 h, the solvent was removed under vacuum and the residue recrystallized in EtOH to afford pyrazole derivative 4. Orange powder; yield (85%); m.p. 240–242 °C. IR (ν¯ /cm−1): 3409, 3294, 3262, 3194 (NH2, 2NH), 1665 (C = O), 1597 (C = N), 1530 (N = N). 1H NMR (δ/ppm): 2.06 (s, 3H, CH3), 2.35 (s, 3H, CH3), 5.81 (s, 1 H) & 6.92 (s, 1 H) (NH2), 7.65 (s, 4H, Ar-H), 10.03 (s, 1H, CONH), 11.55 (s) & 12.05 (s) (1H, NH-pyrazole). MS (m/z, %): 259 (M++1, 7.1), 258 (M+, 37.0), 236 (2.1), 191 (1.8), 119 (5.5), 77 (9.2), 56 (100.0). Anal. for C12H14N6O (258.28): C, 55.80; H, 5.46; N, 32.54%. Found: C, 55.97; H, 5.52; N, 32.46%. 2.4. General procedure for the synthesis of 5-methyl-pyrazol-3-yl-hydrazones 6 and 7 To a cold suspension of 4 (2.58 g, 10 mmol) in 3 mL conc. HCl, an aqueous solution of NaNO2 (0.7 g in 10 mL H2O) was added drop by drop with continuous stirring at 0–5 °C. This newly prepared diazonium chloride solution underwent drop by drop addition to a cold solution of active methylene compounds namely; (malononitile and/or ethyl cyanoacetate) (10 mmol) in 30 mL ethyl alcohol and 3.0 g sodium acetate. After stirring of the mixture for two hours at 0–5 °C, the solid that formed was filtered and recrystallized from EtOH-DMF mixture (3:1) to give compounds 6 and 7, respectively. 2.5. 2-(2-(4-((4-Acetamidophenyl)azo)-5-methyl-1H-pyrazol-3-yl)-hydrazono)-malononitrile (6) Red powder; yield (75%); m.p. 216–217 °C. IR (ν¯ /cm−1): 3308, 3197 (NH), 2228 (C≡N), 1667 (C = O), 1577 (C = N), 1541 (N = N). 1H NMR (δ/ppm): 2.06 (s, 3H, CH3), 2.36 (s, 3H, CH3), 7.52 (d, J =9 Hz, 2H, Ar-H), 7.70 (d, J =9 Hz, 2H, Ar-H), 10.23 (s, 1H, NH), 10.76 (s, 1H, NH), 11.76 (s, 1H, NH). Anal. for C15H13N9O (335.32): C, 53.73; H, 3.91; N, 37.59%. Found: C, 53.59; H, 3.96; N, 37.67%. 2.6. Ethyl 2-(2-(4-((4-acetamidophenyl)azo)-5-methyl-1H-pyrazol-3-yl) hydrazono)-2-cyanoacetate (7) Brown crystals; yield (75%); m.p. 201–203 °C. IR (ν¯ /cm−1): 3196, 3136 (NH), 2206 (C≡N), 1717, 1670 (2C = O), 1597 (C = N), 1556 (N = N). MS (m/z, %): 382 (M+, 8.16), 313 (15.92), 293 (18.64), 287 (19.18), 259 (13.74), 233 (8.91), 220 (14.47), 188 2
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(17.13), 172 (24.54), 157 (21.80), 148 (13.27), 121 (25.36), 106 (37.84), 94 (48.33), 83 (62.17), 77 (100.00), 57 (31.3), 43 (44.30). Anal. for C17H18N8O3 (382.38): C, 53.40; H, 4.74; N, 29.30%. Found: C, 53.22; H, 4.78; N, 29.38%. 2.7. General procedure for the Synthesis of pyrazolo[5,1-c]triazine derivatives 8 and 9 A suspension of pyrazolyl hydrazone derivatives 6 and 7 (5 mmol) in acetic acid (10 mL) was refluxed for 1 h, After cooling the mixture, it was poured into ice-cold water. The solid formed was filtered and recrystallized in EtOH/DMF mixture (4:1) to afford the pyrazolo[5,1-c]triazine derivatives 8 and 9, respectively. 2.8. 8-(4-Acetamidophenylazo)-4-amino-3-cyano-7-methylpyrazolo[5,1-c]- [1,2,4]triazine (8) Brown crystals; yield (65%); m.p. 300–302 °C. IR (ν¯ /cm−1): 3438, 3296, 3238 (NH2 and NH), 2229 (C≡N), 1665 (C = O), 1642 (C = N), 1543 (N = N). 1H NMR (δ/ppm): 2.10 (s, 3H, CH3), 2.74 (s, 3H, CH3), 7.79 (d, 2H, J =9 Hz, Ar-H), 7.84 (d, 2H, J =8.96 Hz, Ar-H), 9.54 (s, 2H, NH2), 10.26 (s, 1H, NH). MS (m/z, %): 336 (M++1, 18.23), 335 (M+, 100.0), 293 (17.39), 224 (9.73), 201 (36.21), 199 (26.33), 173 (23.39), 162 (8.65), 134 (12.79), 120 (10.91), 106 (57.25), 92 (16.00), 79 (26.75), 65 (5.74), 43 (26.73). Anal. for C15H13N9O (335.32): C, 53.73; H, 3.91; N, 37.59%. Found: C, 53.98; H, 3.96; N, 37.67%. 2.9. 8-(4-Acetamidophenylazo)-4-amino-3-ethoxycarbonyl-7-methyl-pyrazolo[5,1-c][1,2,4]triazine (9) Brown powder; yield (74%); m.p. 298–300 °C. IR (ν¯ /cm−1): 3444, 3357, 3298 (NH2 and NH), 1697, 1649 (2C = O), 1597 (C = N), 1541 (N = N). 1H NMR (δ/ppm): 1.39 (t, J =7.05 Hz, 3H, CH3), 2.09 (s, 3H, CH3), 2.74 (s, 3H, CH3), 4.44 (q, J =7 Hz, 2H, CH2), 7.78 (d, J =9 Hz, 2H, Ar-H), 7.83 (d, J =8.7 Hz, 2H, Ar-H), 8.65 & 9.41 (two s, 2H, NH2), 10.22 (s, 1H, CONH). Anal. for C17H18N8O3 (382.38): C, 53.40; H, 4.74; N, 29.30%. Found: C, 53.48; H, 4.78; N, 29.24%. 2.10. Preparation of azopyrazolo dyes thin films Thermal evaporation technique has been used to deposited films of azopyrazolo derivatives (3, 4, 6, 7, 8 and 9) at pressure around 10−5 mbar (Edwards E306 A coating system). Quartz crystal thickness monitor attached with the system were used to measure the thickness of films during deposition (Model TM-350 MAXTEK, Inc., USA). For optical measurements we used optical flat quartz. A double-beam spectrophotometer (JASCO model V-570 UV–vis–NIR) was used to measure the transmittance, T(λ), and reflectance, R (λ), and spectra of the as-deposited derivatives of azopyrazolo dyes films in the spectral range 200–2500 nm. 3. Results and discussion Diazotization of 4-aminoacetanilide (1) by using sodium nitrite in hydrochloric acid. 4-Aminoacetanilide (1) was diazotized utilizing sodium nitrite in hydrochloric acid. The diazotized 4-aminoacetanilide was then coupled with 3-aminocrotonitrile (2) in ethanol containing sodium acetate to afford the hydrazonoyl cyanide derivative 3. The formation of 3 was considered to proceed via electrophilic diazo-coupling reaction at the methylene function followed by hydrolysis of imine group (C]N) to ketone (C]O) and
Fig. 1. Synthesis of 3-amino-4-arylazopyrazole derivative 4. 3
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ammonia under the reaction conditions (Fig. 1). Heterocyclization reaction of 3 with hydrazine was hydrate proceeded by reflux in ethanol to afford the corresponding 3-amino-4-arylazopyrazole derivative 4. Spectroscopic techniques (IR, 1H NMR and MS) were used to confirm the designed structures of 3 and 4. The IR spectrum of compound 3 showed the characteristic absorption band of nitrile function (C^N) at 2215 cm−1 that disappeared in the IR spectrum of compound 4 and the characteristic absorption bands of NeH function (NH and NH2) at 3409, 3294, 3262 and 3194 cm−1 were assigned instead of this nitrile absorption. The 1H NMR spectrum of compound 3 displayed the characteristics of two singlet signals at 2.04 and 2.39 ppm corresponding to the protons of two methyl groups. The aromatic protons were resonated as two doublet signals at 7.48 and 7.62 ppm, in while the protons of two NH groups were showed as two singlet signals at 10.02 and 12.21 ppm. In addition, the 1H NMR signals of compound 4 were indicated as two singlet signals at 2.06 and 2.35 ppm for the protons of two methyl groups. The two protons of amino group (NH2) were resonated as two singlet signals at 5.81 & 6.92 ppm while the four aromatic protons were resonated as singlet at 7.65 ppm. The signal of amide NeH group was assigned as singlet at 10.03 ppm. The two singlet signals 11.55 & 12.05 indicated the NH of pyrazole formulas 4a and 4b. Diazotization of 3-amino-4-arylazopyrazole 4 with sodium nitrite in a mixture of acetic acid and hydrochloric acid led to the corresponding diazonium salt 5, which underwent coupled with malononitrile and/or ethyl cyanoacetate in ethanol containing sodium acetate furnished the corresponding hydrazonoyl cyanide derivatives 6 and 7, respectively (Fig. 1). The spectral tools (IR, 1H NMR and MS) have been utilized to identify the chemical structure of these hydrazones 6 and 7. The IR spectrum of hydrazonyl cyanide 6 exhibited absorptions at 3308 and 3197 cm−1 characteristic for the NH functions, and at 2228 and 1667 cm-1 for the nitrile (C^N) and carbonyl (C]O) functions. In the 1H NMR spectrum of 6, two singlet signals were observed at 2.06 and 2.36 ppm corresponding to the protons of two methyl groups. The aromatic protons were resonated as two doublet signals at 7.52 and 7.70 ppm while the protons of three NH functions were resonated as three singlet signals at 10.23, 10.76 and 11.76 ppm. On the other hand, the mass spectrum of compound 7 showed the molecular ion peaks at m/z 382 (relative intensity 8.16%), which was in agreement with molecular formula of the compound (C17H18N8O3). Intramolecular cyclization of scaffolds 6 and 7 was successfully performed under the influence of hot acetic acid to afford the desired pyrazolo[5,1-c]triazines 8 and 9 (Fig. 2). The designed structures of these pyrazolotriazines 8 and 9 were confirmed by spectroscopic and elemental analyses. The IR spectrum of pyrazolotriazine 9 showed absorption bands at 3444, 3357 and 3298 cm–1 assigned to the NH2 and NH groups. Meanwhile missing absorption band at 2206 cm−1 revealed the disappearance of nitrile function. The 1H NMR spectrum of pyrazolotriazine 9 exhibited ethyl protons as triplet and quartet signals at 1.39 and 4.44 ppm for the protons of ethyl group (−CH2CH3), while the other methyl protons were resonated as two singlet signals at 2.09 and 2.74 ppm. The protons on -NH2 function resonated as two singlet signals at 8.65 & 9.41 ppm while the singlet at 10.22 ppm clearly indicated the proton of amide (CONH). The mass spectrum of pyrazolotriazine scaffold 8 showed the molecular ion peaks at m/z 335 (M+, 100.0) which was in agreement with molecular formula of the compound (C15H13N9O).
Fig. 2. Synthesis of pyrazolotriazine derivatives 8 and 9. 4
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Fig. 3. Spectral behavior of transmittance, T(λ), and reflectance, R(λ), for the pristine azopyrazolo dyes thin films.
4. Optical constants The optical properties for as-deposited azopyrazolo derivatives (3, 4, 6, 7, 8 and 9) thin films of thickness 200 nm are investigated for measuring the transmittance, T(λ), and the reflectance, R(λ). The spectral of the T(λ) and R(λ) versus the light incident in the range from 200 to 2200 nm is shown in Fig. 3. The transmittance edge of pristine films divide the spectra into two regions, the first region in the range 200–700 nm; the total sum of T(λ) and R(λ) is less than unity (absorption region) and the second at long wavelengths; λ > 700 nm, the film becomes nearly transparent and no light is absorbed or scattered T(λ) + R(λ) ≈ 1 (transparent region). The spectral behavior of the T(λ) and R(λ) for the investigated thicknesses supports the film homogeneity. By using the following equations, we can calculate the values of the refractive index, n, and the extinction coefficient, k. [42–44]
R=
(n − 1)2 + k 2 (n + 1)2 + k 2
(1)
k=
αλ 4π
(2)
from the measured values of R and T, the values of the absorption coefficient, α, can be determined by the following relation [42]: 1/2
1 ⎡ (1 − R)2 ⎞ ⎛ ⎛ (1 − R ) 4 ⎞ +⎜ + R2⎟⎞ α = ⎛ ⎞ ln ⎢ ⎛ ⎝ d ⎠ ⎣ ⎝ 2T ⎠ ⎝ ⎝ 4T 2 ⎠ ⎠ ⎜
⎟
⎜
⎟
⎤ ⎥ ⎦
(3)
hence d is the film thickness. Fig. 4 shows the values of extinction coefficient as a function of incident wavelength. From this figure, it is clear the existence of two absorption bands. The first lies in the ultraviolet region of spectra and the other in the visible region of spectra. The band absorption in the UV region of spectra is attributed to n-π* orbital transition and those in the visible region of spectra are due to π -π* orbital transition [45,46]. Due to the existence of defects in structure and free charge carrier, we notice that the values of k does not approach zero at wavelength > 600 nm [47]. The calculated values of absorption coefficient (α) are plotted versus the wavelength in Fig. 5. It is clear that the values of (α) are high and greater than 2.5 × 105 cm−1. which are enough to meet the requirements of solar energy harvesting devices.
Fig. 4. Spectral behavior of absorption index, k, for the pristine azopyrazolo dyes thin films. 5
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Fig. 5. Absorption coefficient, α(hν) dependence on the incident wavelength, (λ) for pristine azopyrazolo dyes thin films.
The type of transition and the values of optical band gap energy, Eg, can be determined by the analysis of the (α) nearby the edge of absorption according to the formula [48]:
αhv = B (hv − Eg )r
(4)
(αhv )2/3
It has been found that the direct forbidden transition is the most probable transition with r = 3/2. The dependence of on the energy of photon (hv ) are illustrated in the Fig. 6 for azopyrazolo dyes thin films. The values of energy gap are summarized in Table 1. Dielectric constant of a material, ε*, is also studied, can be determined by the formula ε * = ε1 + ε2 , where ε1 and ε2 denote to the real and the imaginary parts of the dielectric constant, and they are calculated by the equations [49,50]:
ε1 = n2 − k 2, and ε2 = 2nk
(5)
Figs. 7 and 8 show the relation between the ε1 and ε2 and photon energy (hv ) , where the values of ε2 are smaller than of those ε1 at the same wavelength. The spectrum of ε1 and ε2 can be divided fundamentally into three regions, region I lies in the range energy around < 2.25 eV, hence the interaction between the electrons in azopyrazolo dyes thin films and photons is a weak, region II the energy represents the energy separation between the optical and fundamental energy gaps, region III both ε1 and ε2 increase with increasing photon energy. The surface energy loss, SEL, and the volume energy loss, VEL, can be determined by using the equations [45]:
SEL =
ε2 ε and VEL = 2 2 2 (ε1 + 1)2 + ε22 (ε1 + ε2 )
(6)
Figs. 9 and 10 shows the SEL and VEL as a function of the incident photon energy. Both functions have the same behavior and the energy lost by photons traveling through the material is greater than the energy lost by those passing by its surface. The real and imaginary optical conductivity (σ1 and σ2 ) could be calculated from dielectric constant by using this equations [44,51]: (7)
σ1 = ωε2 ε∘ and σ2 = ωε1 ε∘
For optoelectronic applications considered as study of optical conductivity is important, due to optical conductivity that gives us image of how photons can promote electrons form localized states to delocalized states and create electron-hole pairs. Figs. 11 and 12 depict the σ1 and σ2 as a functions of (hv ) . The values of optical conductivity of order are 5 × 105 (Ω.m)−1.
Fig. 6. Dependence of (αhν)2/3 on hν for pristine azopyrazolo dyes thin films. 6
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Table 1 Energy gaps for as-deposited azopyrazolo derivatives. Day film
Energy gap, Eg, (eV)
3 4 6 7 8 9
2.32 2.10 2.07 2.04 2.06 2.18
Fig. 7. Real part of dielectric constant (ε1 ), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
Fig. 8. Imaginary part of dielectric constant (ε2 ), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
Fig. 9. The surface energy loss (SEL), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
7
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Fig. 10. The volume energy loss (VEL), versus photon energy (hν) for the as-deposited azopyrazolo dyes thin films.
Fig. 11. Real part of optical conductivity (σ1), for azopyrazolo dyes thin films.
Fig. 12. Imaginary part of optical conductivity (σ2 ), for azopyrazolo dyes thin films.
5. Conclusion A series of novel pyrazolotriazine dyes were synthesized via a sequence involving initial diazotization of 4-aminoacetanilide, coupling with 3-Aminocrotonitrile and then refluxed with hydrazine hydrate to furnish pyrazolohydrazone that was then diazotized and coupled with active methylene compounds namely; (malononitile, and ethyl cyanoacetate) afforded the corresponding novel pyrazolotriazines dyes. Optical properties of derivatives azopyrazolo dyes films have been investigated. The films were prepared by thermal evaporation technique. The optical absorption edges are described using the band transition. The type of electron transition was found to be direct forbidden transition. Important spectral parameters, such as optical absorption coefficient (α), real and the imaginary parts of the dielectric constant (ε1 and ε2 ), the surface energy loss and the volume energy loss (SEL and VEL), and optical conductivity (σ1 and σ2 ) were calculated. The present work confirmed that the derivatives azopyrazolo dyes are recommend its application in design of photovoltaic devices. 8
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