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Synthesis and optical properties of new alkylated pyridinium halides
Optik 139 (2017) 95–103
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Optik journal homepage: www.elsevier.de/ijleo
Original research article
Synthesis and optical properties of new alkylated pyridinium halides Ayman A. Zaki a,b,∗ , Mohamed Hagar c,d,∗∗ , Nagi R.E. Radwan c,e a b c d e
College of Sciences, Physics Department, Yanbu, Taibah University, Saudi Arabia Faculty of Sciences, Physics Department, Banha University, Banha, Egypt College of Sciences, Chemistry Department, Yanbu, Taibah University, Saudi Arabia Faculty of Science, Chemistry Department, Alexandria University, Alexandria, Egypt Faculty of Science, Chemistry Department, Suez University, Suez, Egypt
a r t i c l e
i n f o
Article history: Received 18 August 2016 Accepted 25 March 2017 Keywords: Pyridinium salts Optical properties Energy gap Refractive index
a b s t r a c t Three pyridinium salts were prepared effectively under ultrasonic irradiation. The prepared compounds were characterized by their spectral (IR, NMR) data. The optical properties were measured in water as a solvent at different concentrations. The optical characteristics of these compounds were examined by using the UV–vis spectrophotometer. The results indicate that all compounds exhibit high transmittance in the visible light spectrum. The absorption coefficient and the optical energy gap were determined for each sample. Measurement of refractive index and dispersion relation were investigated. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Recently, organic materials are intensively investigated due to their wide range of applications in electronics. The investigation of the optical constants such as refractive index, extinction coefficient and dielectric constant of the materials are important for designing of new materials. Optical constants give information for technological applications in the field of photonics technology like optical communication. Furthermore, the changes in refractive index are important for controlling optical properties of organic materials [1,2]. Pyridine and its derivatives can be used as non-linear optical. The ring of pyridine is considered as cationic bonding sites and nitrogen as proton acceptor. Optical properties of pyridine compounds are very important for different applications. They have the main role in optical communication, image and signal processing [2]. The optical transmission of 1-ethyl-2, 6-dimethyl-4-hydroxy pyridinium halide in solution was studied by UV–vis-NIR instrument and it was found in the range 275–1100 nm [3]. The addition of halides to pyridinium compounds modifies the arrangement and their chemical structures [4]. The effect of alkyl chain length, on physical properties including refractive index of polysubstituted pyridinium liquids has been studied [5]. A series of pyridinium and quinolinium salts show high hyper-polarizabilities and good transparencies in the visible region of the spectrum by using hyper-Rayleigh scattering in solution [6]. Pyridinium salts exhibit maximum absorption at 327–337 nm in different organic solvents as counter ions [7]. Moreover, the increase of alkyl chain length of substituted aryl pyridinium salts leads to decrease of thermal stability. The unsubstituted
∗ Corresponding author at: College of Sciences, Physics Department, Yanbu, Taibah University, Saudi Arabia. ∗∗ Corresponding author at: College of Sciences, Chemistry Department, Yanbu, Taibah University, Saudi Arabia. E-mail addresses: ayman [email protected] (A.A. Zaki), [email protected] (M. Hagar). http://dx.doi.org/10.1016/j.ijleo.2017.03.116 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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pyridinium cations illustrate non-luminescence compared with substituted pyridinium ring with alkyl groups in the position 2,4, and 6, which induced it for UV luminescence [8]. Amalia Pana and coworkers investigate the optical properties of pyridinium ionic liquids and the emission properties of these compounds were tested in solution [9]. The optical properties and morphology of pyridinium ionic liquids were investigated by Scanning electron microscopy, UV–vis, X-ray diffraction, and photoluminescence spectroscopy [10]. The absorption spectrum of pyridinium salts with alkyl bromides indicates that ortho-bromine group make shift to red wavelength [11]. The spectral data (UV–vis and FTIR) of 3-hydroxy pyridinium tartarate were measured [12]. Nagapandiselvi and coworkers demonstrate the optical properties of 2-carboxy pyridinium dihydrogen phosphate by UV–vis-NIR, FT-NMR, FTIR, and fluorescence techniques [13]. The spectroscopic properties of 1-methyl-4-[2-(4-hydroxyphenyl) ethenyl] pyridinium dihydrogenphosphate have been measured by spectral techniques. The measurements of refractive indices for 1-butylpyridinium tetrafluoroborate, 1-butyl3-methylpyridinium tetrafluoroborate, and 1-butyl-4-methylpyridinium tetrafluoroborate, with methyl and ethyl alcohols indicate that the pyridinium compounds in methyl alcohol have higher refractive index than the compounds in ethyl alcohol [14]. The absorption spectra and functional groups of 2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium naphthalene-2-sulfonate (DESNS) were investigated by UV–vis-NIR and FTIR spectroscopic techniques, respectively [15]. Also, the spectral data for substituted styryl-pyridinium salts by UV–vis spectral analysis showed strong absorption band higher than 400 nm [16]. Rajalakshmi and coworker investigate the optical and mechanical properties of 2-Aminopyridinium 4-methylbenzoate dihydrate [17]. It was found that the energy band and refractive index equal 2.9 eV and 1.4, respectively at 1200 nm. Optical properties of single crystals the semi-organic non-linear zinc tris (thiourea) sulphate was investigated [18]. On the other hand, UV–vis spectroscopy is benefit to find transmittance, absorption and reflectivity of solutions and thin films [19,20]. The aim of this work is to investigate the optical properties of three alkylated-2-bromo-pyridinium halides prepared under ultrasonic irradiation.
2. Experimental procedure IR spectrum were recorded with a Nicolet is 10 Thermo scientific as potassium bromide pellets and frequencies were reported in cm−1 .1 H NMR spectra were determined with a JEOL spectrometer at 500 MHz. The chemical shifts are expressed in the ı scale using tetramethylsilane as a reference. TLC was performed on Merck Kiesel gel; 60-F254 plates and the spots were detected by UV light absorption. Optical spectra in UV–vis region were recorded with a UV/visible spectrophotometer (UV-1800 SHIMADZU, Japan) by using 1 cm path length cuvettes at room temperature.
2.1. 1-Methyl-2-bromo-pyridinium iodide 2a 2-Bromo-pyridine (1.0 ml, 10.5 mmol) was dissolved in 2 ml CH2 Cl2 , followed by the addition of iodomethane (0.783 ml, 12.5 mmol). The resulting mixture was sonicated for 15 min at room temperature. water 20 ml was added The reaction mixture was extracted with CH2 Cl2 (50 ml × 3), and the organic layer was washed with water (50 ml), dried over Na2 SO4 and concentrated in vacuo to afford a yellow product (2.1 g, 88%). 1 H NMR (500 MHz, CDCl3 ): 9.31 (d, J = 6.9 Hz, 1H), 8.62–8.66 (m, 2H), 8.44 (m, 1H), 1.71 (s, 3H).
2.2. 1-Benzyl-2-bromo-pyridinium chloride 2b 2-Bromo-pyridine (1.0 ml, 10.5 mmol) was dissolved in 2 ml CH2 Cl2 , and then benzyl chloride (1.59 ml, 12.5 mmol) was added. The resulting mixture was sonicated for 15 min at room temperature. The reaction mixture was extracted with CH2 Cl2 (50 ml × 3), and the organic layer was washed with water (50 ml), dried over Na2 SO4 and concentrated in vacuo to afford colorless product (3.0 g, 90%) 1 H NMR (500 MHz, CDCl3 ): ı 9.25 (d, J = 7.0 Hz, 2H), 8.78 (d, J = 7.0 Hz, 2H), 7.58 (d, J = 3.75 Hz, 2H), 7.41 (m, 3H), 3.45 (s, 2H).
2.3. 1-Allyl-2-bromo-pyridinium bromide 2c 2-bromo-pyridine (1.0 ml, 10.5 mmol) was dissolved in 2 ml CH2 Cl2 , and then allyl bromide (1.15 ml, 10.5 mmol) was added. The resulting mixture was sonicated for 15 min at room temperature. The reaction mixture was extracted with CH2 Cl2 (50 ml × 3), and the organic layer was washed with water (50 ml), dried over Na2 SO4 and concentrated in vacuo to afford colorless oily product (2.8 g, 86%). 1 H NMR (500 MHz, CDCl3 ): 9.30 (d, J = 6.9 Hz, 1H), 8.62–8.68 (m, 2H), 8.45 (m, 1H), 6.41–6.60 (m, 1H), 5.97–6.21 (m, 2H), 5.86 (d, J = 6.0 Hz, 2H).
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RX CH2Cl2 N
Br
N X
Br
R
1
2 a. R= CH3, X= I b. R= CH2Ph, X= Cl c. R= CH2CH=CH2, X= Br
Scheme 1. Reaction of 2-bromopyridine with alkyl halides.
3. Results and discussion 3.1. Preparation of compounds 1-Methyl-2-bromo-pyridinium iodide, 1-allyl-2-bromo-pyridinium bromide and 1-benzyl-2-bromo-pyridinium chloride were prepared as shown in Scheme 1. The reaction of 2-bromopyridine (1) with alkyl halides (methyl iodide, benzyl chloride and allyl bromide) in dichloromethane was activated by ultrasound irradiation, the reaction proceeded effectively to give the corresponding 1-alkyl-2-bromopyridenium halide salts (88–95%) percentage yield in a short reaction time 15 min. 3.2. Characterization of 2a–c 3.2.1. Nuclear magnetic resonance (NMR) The products were confirmed by their spectral (NMR and IR) data. The compound 2a showed a singlet of the methyl group at ␦ = 1.71, while, 2b showed a singlet at ␦ = 3.45 which corresponding to the methylene group of the benzyl moiety. On the other hand, the allyl group of 2c showed a doublet at ␦ = 5.86 corresponding to the methylene group while CH CH2 appeared as a complex signals at ␦ = 5.86–6.60. 3.2.2. Fourier transform infrared spectroscopy (FTIR) IR data of compound 2a shows a band at 3150 cm−1 , 3050 cm−1 , 3018 cm−1 characteristic for C H of aromatic ring and alkyl group. The band located at 2918.6 cm−1 and 2850 cm−1 attributed to symmetric and asymmetric stretching vibrations of the methyl CH3 and methylene CH2 groups. The band at 1652.9 cm−1 due to C N vibration. The band at 1608 cm−1 and 1489 cm−1 attributed to C C stretch in aromatic ring. The bands at 1472.6 cm−1 and 1436.6 cm−1 characteristic for C H bend. The band at 1200 cm−1 and 1100 cm−1 refers to C C bending, and band at 775 cm−1 , 720.3 cm−1 , 694 cm−1 assigned to C Br. Compound 2b showed bands at 3050, 3032, 3020 cm−1 characteristic for C H of aromatic ring and alkyl group. The band located at 2920 cm−1 , and 2860 cm−1 attributed to symmetric and asymmetric stretching vibrations of the methyl CH3 and methylene CH2 groups. The band at 1613 cm−1 attributed to C N. The band at 1613, 1571, 1560 and 1496 cm−1 assigned to C C stretch in aromatic ring. The bands at 1449 and 1414 cm−1 characteristic for C H bend. The band at 1265, 1209, 1147, 1106, 1076, 1042, and 987, and 814 cm−1 refers to C C bending, and band at 759, 698, 676 cm−1 assigned to C Br. IR spectra of compound 2c shows a peak at 1620 cm−1 assigned to C N vibration. The band at 3081, 3050, 3012 cm−1 were characteristic for C H of aromatic ring and alkyl group. The band located at 2920 cm−1 , 2900 and 2860 cm−1 attributed to symmetric and asymmetric stretching vibrations of the methyl CH3 and methylene CH2 groups. The band at 1620 cm−1 due to C N vibration. The band at 1608, 1564 and 1489 cm−1 attributed to C C stretch in aromatic ring. The bands at 1472, 1448 and 1436.6 cm−1 characteristic for C H bend. The band at 1320, 1280, 1200, 1150, 1120, 1078, 1002, and 945 cm−1 refers to C C bending, and band at 780, 717, 696, 651 cm−1 assigned to C-Br. 3.3. UV–vis spectrophotometery UV–vis spectrum gives information about the structure of the molecules where it causes electronic excitation from lower to higher energy state. UV–vis spectral properties of new compounds (2a–c) at different concentrations in water were examined by using normal incidence light of wavelengths between 200 and 800 nm at room temperature (300 K). 3.3.1. Transmittance and absorption spectrum The transmissions of the compounds (2a–c) measured at concentrations of (0.34, 0.23 and 0.11 mM) are shown in Figs. 1–3. It was shown that at wavelengths longer than 300 nm, compounds at different concentrations are highly transparent more than 90%. Also the transmission is inversely proportional to the concentration as shown in Fig. 4. This indicates a good optical
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Fig. 1. The optical transmission spectra of compound (2a) at different concentrations.
Fig. 2. The optical transmission spectra of compound (2b) at different concentrations.
Fig. 3. The optical transmission spectra of compound (2c) at different concentrations.
Fig. 4. A plot of transmittance vs. concentration at max equals 256 nm, 240 nm and 267 nm for compounds (2a–c) respectively.
quality of these compounds. The high value of transmission in this region suggests that the samples were completely soluble in water. The absorbance spectra of the compounds (2a–c) over concentrations of (0.34, 0.23 and 0.11 mM) are shown in Figs. 5–7. As seen in these figures the maximum absorption wave length for pyridinium salts (2a–c) are 295 nm, 268 nm and 265 nm,
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Fig. 5. The optical absorption spectra of compound (2a) at different concentrations.
Fig. 6. The optical absorption spectra of compound (2b) at different concentrations.
Fig. 7. The optical absorption spectra of compound (2c) at different concentrations.
respectively and their intensity is directly proportional with concentration, this means that as the concentration of the sample increases the intensity of maximum absorbance peaks increases, which is related to the transition of electrons between the frontier molecular orbital. Moreover, the presence of the alkyl group on the nitrogen atom affects on the maximum absorbance wavelength, where, the methyl group causes red shift rather than the allyl and benzyl group. The allyl and benzyl groups show only a small difference in the maximum absorbance (only 3 nm) where they possess a similar electronic properties i.e CH2 group attached to CH = moiety. While the methyl group show higher wavelength of absorbance, this could be attributed to the higher hyper conjugation of the methyl group which causes this higher bathchromic shift. The fundamental absorption, which corresponds to the transition from valence band to the conduction band, can be used to determine the optical energy gap of the materials [21]. The plot of maximum absorbance versus concentrations is shown in Fig. 8, for pyridinium salts (2a–c) at wavelengths, 295 nm, 268 nm and 265 nm, respectively. A linear plot indicates that Lambert-Beer’s law is obeyed and the distinct absorption coefficients (␣) were (5.3 × 103 , 6.1 × 103 and 3.0 × 103 l mol−1 cm−1 ) for compounds (2a–c) respectively [22]. 3.3.2. Energy band gap The energy band gap Eg was measured by using the absorption coefficient as the following equation [10], [23–25]:
˛h = B h − Eg
m
(1)
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Fig. 8. A plot of absorbance vs. concentration at max equals 295 nm, 265 nm and 267 nm for compounds (2a–c) respectively.
Fig. 9. The plot of (␣hv)1/2 versus photon energy (h) for compounds (2a–c) at concentration 0.34 mM/L. Table 1 The values of the energy band gaps for 2a–c at certain concentrations. Samples at Concentration 0.11
Energy Gap (Eg1 ) (eV)
Energy Gap (Eg2 ) (eV)
2a 2b 2c
3.8 4.2 4.4
4.8 5.2 5.4
Where ␣ is the absorption coefficient, h is the Planck’s constant, is the frequency of the incident light and B is a constant, m is an index which can be assumed to have values of 1/2, 3/2, 2 and 3, depending on the nature of the electronic transition responsible for the absorption. If m = 1/2 the transition will be allowed while it is forbidden for m = 3/2. On the other hand, the indirect transition would be allowed or forbidden by changing m value from 2 to 3 [23,24]. Type of the electron transitions can be known either from the value of the absorption coefficient where direct transitions takes place for ␣ > 104 cm−1 , while indirect transitions occur for ␣ < 104 cm−1 [26], or from knowing the value of the exponent m. It was found that the best fit was obtained when m is equal 2 which represent the process of allowed indirect transition. From Eq. (1), the energy band gap Eg of compounds 2a–c at different concentration can be obtained as shown in Fig. 9, by plotting relation of (˛h)1/2 with photon energy (h). The energy band gap was evaluated by the extrapolation of the straightline portion of this plot to the energy axis (h), i.e., at ␣ equal zero. Table 1 shows the energy band gaps for compounds 2a–c at certain concentrations. Fig. 9 appears two linear-like regions, one region lies in the low energy range, and the other in the high energy range. The value of energy gap of compound 2a is lower than for compounds 2b and c because of methyl group in 2a has higher wavelength of absorbance than in the allyl and benzyl group in 2b and c. 3.3.3. Refractive index dispersion Refractive index plays an important role in many areas of material science with special reference for various optical technologies. Measurements of refractive indices of liquids are often required in physics and chemistry. The refractive index of liquid is measured by using various methods; a common method is the measurement of angle of minimum deviation produced by passing a beam of light through the liquid contained in a hollow prism made of transparent glass. But, this method can be limited for visible light, because glass is opaque to infrared and ultraviolet radiations. Another method is by knowing transmittance and reflectance of the materials by using UV–vis spectrophotometer. Herein, refractive index of the pyridinium salts was measured by using these two methods.
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Fig. 10. The dispersion relation between refractive indices and wavelengths for the compound (2a) at different concentrations.
If a monochromatic light is incident on the planar surface separating two isotropic media, at normal incidence, the amplitude reflection coefficients r⊥ , rII and the amplitude transmission coefficients t⊥ , tII perpendicular and parallel to the plane-of-incidence were given by the Fresnel Equations [27]: [rII ]=0 = [−r⊥ ]=0 = [tII ]i =0 = [t⊥ ]i =0 =
nt − ni nt + ni 2ni ni + nt
(2) (3)
Where, ni , nt, are denoted to refractive indices of incident and transmitted medium, respectively. These two equations could be applied to any linear, homogeneous, isotropic and dielectrics media. At normal incidence, which is of great practical interest, the transmittance, and the reflectance by using Eqs. (2) and (3) are: R = r2 = T=
n − n 2 t i
n t
ni
nt + ni t2 =
4nt ni (nt + ni )2
(4) (5)
The value of refractive index could be calculated from Eq. (4) by putting ni = 1, for air and nt = n, is the index of refraction for the sample used: √ 1+ R n= (6) √ 1− R From Eq. (5) by putting ni = 1, for air and nt = n, refractive index could be obtained also as follow: T=
4n (n + 1)2
Tn2 + (2T − 4) n + T = 0 This is a quadratic equation form, where n represents an unknown, and a = T, b = (2T-4), and c = T, represent the known values. So that the solving of this equation leads to: √ 2−T +2 1−T n= (7) T From this equation the value of refractive index was obtained by knowing the transmittance. Figs. 10–12 show the dispersion relation between refractive indices n and wavelengths for compounds 2a–c at different concentrations. The normal dispersion curve was shown in the visible region. Fig. 13, show that there is a direct proportion between refractive index and concentrations for each compound and also the indices of refraction for compound 2a were higher than for compounds 2b and c. 4. Conclusion Three pyridinium halide salts (2a–c) are prepared efficiently under ultrasonic irradiation. The optical parameters of the prepared compounds in water at different concentrations could be obtained by using UV–vis spectral data. All compounds exhibit high transmittance more than 90% in the visible light spectrum. Calculation of absorption coefficients showed allowed
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Fig. 11. The dispersion relation between refractive indices and wavelengths for the compound (2b) at different concentrations.
Fig. 12. The dispersion relation between refractive indices and wavelengths for the compound (2c) at different concentrations.
Fig. 13. The relation between refractive index and concentration at certain wavelength for the compounds (2a–c).
transition were its value ranges from 3 × 103 –6 × 103 l mol−1 cm−1 . Energy band gap of three compounds was also calculated. The refractive indices of the three compounds 2a–c were determined at different concentrations with wavelengths. A direct proportion between refractive index and concentrations for each compound were evaluated and the refractive index appeared increasing from compound 2a–c this means that by changing the alkyling moiety from methyl to benzyl then to allyl. References [1] C.C. Leznoff, A.B.P. Lever, Phthalocyaninesn. Properties and Applications, vols. 1–4, VCH, New York, 1996. [2] M. Hanack, M. Lang, Conducting stacked metallophthalocyanines and related compounds, Adv. Mater. 6 (1994) 819. [3] S. Dhanuskodi, S. Manivannan, K. Kirschbaum, Synthesis, structural, thermal and optical studies of 1-ethyl-2,6-dimethyl-4-hydroxy pyridinium halides, Spectrochim. Acta Part A 64 (2006) 504–511. [4] S. Manivannan, S.K. Tiwari, S. Dhanuskodi, Solid State Commun. 132 (2004) 123–127. [5] V. Pedro, H. Marta, J.G. Emilio, A.M. Eugénia, S. Josefa, T. Emilia, Effect of the number, position and length of alkyl chains on the physical properties of polysbstituted pyridinium ionic liquids, J. Chem. Thermodyn. 69 (2014) 19–26. [6] C.C. Eve Chauchard, H. Eric, M. Gr6goire, S. Charles, P. Andr, T. Andr, Hyperpolarization properties of pyridinium and quinolinium salts determined by hyper-Rayleigh scattering, Chem. Phys. Lett. 238 (1995) 47–53. [7] S.J. Tae, J.K. Jung, H. Haesook, K.B. Pradip, Solution thermal and optical properties of bis(pyridinium salt)s as ionic liquids, Mater. Chem. Phys. 139 (2013) 901–910.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Synthesis and optical properties of new alkylated pyridinium halides Ayman A. Zaki a,b,∗ , Mohamed Hagar c,d,∗∗ , Nagi R.E. Radwan c,e a b c d e
College of Sciences, Physics Department, Yanbu, Taibah University, Saudi Arabia Faculty of Sciences, Physics Department, Banha University, Banha, Egypt College of Sciences, Chemistry Department, Yanbu, Taibah University, Saudi Arabia Faculty of Science, Chemistry Department, Alexandria University, Alexandria, Egypt Faculty of Science, Chemistry Department, Suez University, Suez, Egypt
a r t i c l e
i n f o
Article history: Received 18 August 2016 Accepted 25 March 2017 Keywords: Pyridinium salts Optical properties Energy gap Refractive index
a b s t r a c t Three pyridinium salts were prepared effectively under ultrasonic irradiation. The prepared compounds were characterized by their spectral (IR, NMR) data. The optical properties were measured in water as a solvent at different concentrations. The optical characteristics of these compounds were examined by using the UV–vis spectrophotometer. The results indicate that all compounds exhibit high transmittance in the visible light spectrum. The absorption coefficient and the optical energy gap were determined for each sample. Measurement of refractive index and dispersion relation were investigated. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Recently, organic materials are intensively investigated due to their wide range of applications in electronics. The investigation of the optical constants such as refractive index, extinction coefficient and dielectric constant of the materials are important for designing of new materials. Optical constants give information for technological applications in the field of photonics technology like optical communication. Furthermore, the changes in refractive index are important for controlling optical properties of organic materials [1,2]. Pyridine and its derivatives can be used as non-linear optical. The ring of pyridine is considered as cationic bonding sites and nitrogen as proton acceptor. Optical properties of pyridine compounds are very important for different applications. They have the main role in optical communication, image and signal processing [2]. The optical transmission of 1-ethyl-2, 6-dimethyl-4-hydroxy pyridinium halide in solution was studied by UV–vis-NIR instrument and it was found in the range 275–1100 nm [3]. The addition of halides to pyridinium compounds modifies the arrangement and their chemical structures [4]. The effect of alkyl chain length, on physical properties including refractive index of polysubstituted pyridinium liquids has been studied [5]. A series of pyridinium and quinolinium salts show high hyper-polarizabilities and good transparencies in the visible region of the spectrum by using hyper-Rayleigh scattering in solution [6]. Pyridinium salts exhibit maximum absorption at 327–337 nm in different organic solvents as counter ions [7]. Moreover, the increase of alkyl chain length of substituted aryl pyridinium salts leads to decrease of thermal stability. The unsubstituted
∗ Corresponding author at: College of Sciences, Physics Department, Yanbu, Taibah University, Saudi Arabia. ∗∗ Corresponding author at: College of Sciences, Chemistry Department, Yanbu, Taibah University, Saudi Arabia. E-mail addresses: ayman [email protected] (A.A. Zaki), [email protected] (M. Hagar). http://dx.doi.org/10.1016/j.ijleo.2017.03.116 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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pyridinium cations illustrate non-luminescence compared with substituted pyridinium ring with alkyl groups in the position 2,4, and 6, which induced it for UV luminescence [8]. Amalia Pana and coworkers investigate the optical properties of pyridinium ionic liquids and the emission properties of these compounds were tested in solution [9]. The optical properties and morphology of pyridinium ionic liquids were investigated by Scanning electron microscopy, UV–vis, X-ray diffraction, and photoluminescence spectroscopy [10]. The absorption spectrum of pyridinium salts with alkyl bromides indicates that ortho-bromine group make shift to red wavelength [11]. The spectral data (UV–vis and FTIR) of 3-hydroxy pyridinium tartarate were measured [12]. Nagapandiselvi and coworkers demonstrate the optical properties of 2-carboxy pyridinium dihydrogen phosphate by UV–vis-NIR, FT-NMR, FTIR, and fluorescence techniques [13]. The spectroscopic properties of 1-methyl-4-[2-(4-hydroxyphenyl) ethenyl] pyridinium dihydrogenphosphate have been measured by spectral techniques. The measurements of refractive indices for 1-butylpyridinium tetrafluoroborate, 1-butyl3-methylpyridinium tetrafluoroborate, and 1-butyl-4-methylpyridinium tetrafluoroborate, with methyl and ethyl alcohols indicate that the pyridinium compounds in methyl alcohol have higher refractive index than the compounds in ethyl alcohol [14]. The absorption spectra and functional groups of 2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium naphthalene-2-sulfonate (DESNS) were investigated by UV–vis-NIR and FTIR spectroscopic techniques, respectively [15]. Also, the spectral data for substituted styryl-pyridinium salts by UV–vis spectral analysis showed strong absorption band higher than 400 nm [16]. Rajalakshmi and coworker investigate the optical and mechanical properties of 2-Aminopyridinium 4-methylbenzoate dihydrate [17]. It was found that the energy band and refractive index equal 2.9 eV and 1.4, respectively at 1200 nm. Optical properties of single crystals the semi-organic non-linear zinc tris (thiourea) sulphate was investigated [18]. On the other hand, UV–vis spectroscopy is benefit to find transmittance, absorption and reflectivity of solutions and thin films [19,20]. The aim of this work is to investigate the optical properties of three alkylated-2-bromo-pyridinium halides prepared under ultrasonic irradiation.
2. Experimental procedure IR spectrum were recorded with a Nicolet is 10 Thermo scientific as potassium bromide pellets and frequencies were reported in cm−1 .1 H NMR spectra were determined with a JEOL spectrometer at 500 MHz. The chemical shifts are expressed in the ı scale using tetramethylsilane as a reference. TLC was performed on Merck Kiesel gel; 60-F254 plates and the spots were detected by UV light absorption. Optical spectra in UV–vis region were recorded with a UV/visible spectrophotometer (UV-1800 SHIMADZU, Japan) by using 1 cm path length cuvettes at room temperature.
2.1. 1-Methyl-2-bromo-pyridinium iodide 2a 2-Bromo-pyridine (1.0 ml, 10.5 mmol) was dissolved in 2 ml CH2 Cl2 , followed by the addition of iodomethane (0.783 ml, 12.5 mmol). The resulting mixture was sonicated for 15 min at room temperature. water 20 ml was added The reaction mixture was extracted with CH2 Cl2 (50 ml × 3), and the organic layer was washed with water (50 ml), dried over Na2 SO4 and concentrated in vacuo to afford a yellow product (2.1 g, 88%). 1 H NMR (500 MHz, CDCl3 ): 9.31 (d, J = 6.9 Hz, 1H), 8.62–8.66 (m, 2H), 8.44 (m, 1H), 1.71 (s, 3H).
2.2. 1-Benzyl-2-bromo-pyridinium chloride 2b 2-Bromo-pyridine (1.0 ml, 10.5 mmol) was dissolved in 2 ml CH2 Cl2 , and then benzyl chloride (1.59 ml, 12.5 mmol) was added. The resulting mixture was sonicated for 15 min at room temperature. The reaction mixture was extracted with CH2 Cl2 (50 ml × 3), and the organic layer was washed with water (50 ml), dried over Na2 SO4 and concentrated in vacuo to afford colorless product (3.0 g, 90%) 1 H NMR (500 MHz, CDCl3 ): ı 9.25 (d, J = 7.0 Hz, 2H), 8.78 (d, J = 7.0 Hz, 2H), 7.58 (d, J = 3.75 Hz, 2H), 7.41 (m, 3H), 3.45 (s, 2H).
2.3. 1-Allyl-2-bromo-pyridinium bromide 2c 2-bromo-pyridine (1.0 ml, 10.5 mmol) was dissolved in 2 ml CH2 Cl2 , and then allyl bromide (1.15 ml, 10.5 mmol) was added. The resulting mixture was sonicated for 15 min at room temperature. The reaction mixture was extracted with CH2 Cl2 (50 ml × 3), and the organic layer was washed with water (50 ml), dried over Na2 SO4 and concentrated in vacuo to afford colorless oily product (2.8 g, 86%). 1 H NMR (500 MHz, CDCl3 ): 9.30 (d, J = 6.9 Hz, 1H), 8.62–8.68 (m, 2H), 8.45 (m, 1H), 6.41–6.60 (m, 1H), 5.97–6.21 (m, 2H), 5.86 (d, J = 6.0 Hz, 2H).
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RX CH2Cl2 N
Br
N X
Br
R
1
2 a. R= CH3, X= I b. R= CH2Ph, X= Cl c. R= CH2CH=CH2, X= Br
Scheme 1. Reaction of 2-bromopyridine with alkyl halides.
3. Results and discussion 3.1. Preparation of compounds 1-Methyl-2-bromo-pyridinium iodide, 1-allyl-2-bromo-pyridinium bromide and 1-benzyl-2-bromo-pyridinium chloride were prepared as shown in Scheme 1. The reaction of 2-bromopyridine (1) with alkyl halides (methyl iodide, benzyl chloride and allyl bromide) in dichloromethane was activated by ultrasound irradiation, the reaction proceeded effectively to give the corresponding 1-alkyl-2-bromopyridenium halide salts (88–95%) percentage yield in a short reaction time 15 min. 3.2. Characterization of 2a–c 3.2.1. Nuclear magnetic resonance (NMR) The products were confirmed by their spectral (NMR and IR) data. The compound 2a showed a singlet of the methyl group at ␦ = 1.71, while, 2b showed a singlet at ␦ = 3.45 which corresponding to the methylene group of the benzyl moiety. On the other hand, the allyl group of 2c showed a doublet at ␦ = 5.86 corresponding to the methylene group while CH CH2 appeared as a complex signals at ␦ = 5.86–6.60. 3.2.2. Fourier transform infrared spectroscopy (FTIR) IR data of compound 2a shows a band at 3150 cm−1 , 3050 cm−1 , 3018 cm−1 characteristic for C H of aromatic ring and alkyl group. The band located at 2918.6 cm−1 and 2850 cm−1 attributed to symmetric and asymmetric stretching vibrations of the methyl CH3 and methylene CH2 groups. The band at 1652.9 cm−1 due to C N vibration. The band at 1608 cm−1 and 1489 cm−1 attributed to C C stretch in aromatic ring. The bands at 1472.6 cm−1 and 1436.6 cm−1 characteristic for C H bend. The band at 1200 cm−1 and 1100 cm−1 refers to C C bending, and band at 775 cm−1 , 720.3 cm−1 , 694 cm−1 assigned to C Br. Compound 2b showed bands at 3050, 3032, 3020 cm−1 characteristic for C H of aromatic ring and alkyl group. The band located at 2920 cm−1 , and 2860 cm−1 attributed to symmetric and asymmetric stretching vibrations of the methyl CH3 and methylene CH2 groups. The band at 1613 cm−1 attributed to C N. The band at 1613, 1571, 1560 and 1496 cm−1 assigned to C C stretch in aromatic ring. The bands at 1449 and 1414 cm−1 characteristic for C H bend. The band at 1265, 1209, 1147, 1106, 1076, 1042, and 987, and 814 cm−1 refers to C C bending, and band at 759, 698, 676 cm−1 assigned to C Br. IR spectra of compound 2c shows a peak at 1620 cm−1 assigned to C N vibration. The band at 3081, 3050, 3012 cm−1 were characteristic for C H of aromatic ring and alkyl group. The band located at 2920 cm−1 , 2900 and 2860 cm−1 attributed to symmetric and asymmetric stretching vibrations of the methyl CH3 and methylene CH2 groups. The band at 1620 cm−1 due to C N vibration. The band at 1608, 1564 and 1489 cm−1 attributed to C C stretch in aromatic ring. The bands at 1472, 1448 and 1436.6 cm−1 characteristic for C H bend. The band at 1320, 1280, 1200, 1150, 1120, 1078, 1002, and 945 cm−1 refers to C C bending, and band at 780, 717, 696, 651 cm−1 assigned to C-Br. 3.3. UV–vis spectrophotometery UV–vis spectrum gives information about the structure of the molecules where it causes electronic excitation from lower to higher energy state. UV–vis spectral properties of new compounds (2a–c) at different concentrations in water were examined by using normal incidence light of wavelengths between 200 and 800 nm at room temperature (300 K). 3.3.1. Transmittance and absorption spectrum The transmissions of the compounds (2a–c) measured at concentrations of (0.34, 0.23 and 0.11 mM) are shown in Figs. 1–3. It was shown that at wavelengths longer than 300 nm, compounds at different concentrations are highly transparent more than 90%. Also the transmission is inversely proportional to the concentration as shown in Fig. 4. This indicates a good optical
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Fig. 1. The optical transmission spectra of compound (2a) at different concentrations.
Fig. 2. The optical transmission spectra of compound (2b) at different concentrations.
Fig. 3. The optical transmission spectra of compound (2c) at different concentrations.
Fig. 4. A plot of transmittance vs. concentration at max equals 256 nm, 240 nm and 267 nm for compounds (2a–c) respectively.
quality of these compounds. The high value of transmission in this region suggests that the samples were completely soluble in water. The absorbance spectra of the compounds (2a–c) over concentrations of (0.34, 0.23 and 0.11 mM) are shown in Figs. 5–7. As seen in these figures the maximum absorption wave length for pyridinium salts (2a–c) are 295 nm, 268 nm and 265 nm,
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Fig. 5. The optical absorption spectra of compound (2a) at different concentrations.
Fig. 6. The optical absorption spectra of compound (2b) at different concentrations.
Fig. 7. The optical absorption spectra of compound (2c) at different concentrations.
respectively and their intensity is directly proportional with concentration, this means that as the concentration of the sample increases the intensity of maximum absorbance peaks increases, which is related to the transition of electrons between the frontier molecular orbital. Moreover, the presence of the alkyl group on the nitrogen atom affects on the maximum absorbance wavelength, where, the methyl group causes red shift rather than the allyl and benzyl group. The allyl and benzyl groups show only a small difference in the maximum absorbance (only 3 nm) where they possess a similar electronic properties i.e CH2 group attached to CH = moiety. While the methyl group show higher wavelength of absorbance, this could be attributed to the higher hyper conjugation of the methyl group which causes this higher bathchromic shift. The fundamental absorption, which corresponds to the transition from valence band to the conduction band, can be used to determine the optical energy gap of the materials [21]. The plot of maximum absorbance versus concentrations is shown in Fig. 8, for pyridinium salts (2a–c) at wavelengths, 295 nm, 268 nm and 265 nm, respectively. A linear plot indicates that Lambert-Beer’s law is obeyed and the distinct absorption coefficients (␣) were (5.3 × 103 , 6.1 × 103 and 3.0 × 103 l mol−1 cm−1 ) for compounds (2a–c) respectively [22]. 3.3.2. Energy band gap The energy band gap Eg was measured by using the absorption coefficient as the following equation [10], [23–25]:
˛h = B h − Eg
m
(1)
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Fig. 8. A plot of absorbance vs. concentration at max equals 295 nm, 265 nm and 267 nm for compounds (2a–c) respectively.
Fig. 9. The plot of (␣hv)1/2 versus photon energy (h) for compounds (2a–c) at concentration 0.34 mM/L. Table 1 The values of the energy band gaps for 2a–c at certain concentrations. Samples at Concentration 0.11
Energy Gap (Eg1 ) (eV)
Energy Gap (Eg2 ) (eV)
2a 2b 2c
3.8 4.2 4.4
4.8 5.2 5.4
Where ␣ is the absorption coefficient, h is the Planck’s constant, is the frequency of the incident light and B is a constant, m is an index which can be assumed to have values of 1/2, 3/2, 2 and 3, depending on the nature of the electronic transition responsible for the absorption. If m = 1/2 the transition will be allowed while it is forbidden for m = 3/2. On the other hand, the indirect transition would be allowed or forbidden by changing m value from 2 to 3 [23,24]. Type of the electron transitions can be known either from the value of the absorption coefficient where direct transitions takes place for ␣ > 104 cm−1 , while indirect transitions occur for ␣ < 104 cm−1 [26], or from knowing the value of the exponent m. It was found that the best fit was obtained when m is equal 2 which represent the process of allowed indirect transition. From Eq. (1), the energy band gap Eg of compounds 2a–c at different concentration can be obtained as shown in Fig. 9, by plotting relation of (˛h)1/2 with photon energy (h). The energy band gap was evaluated by the extrapolation of the straightline portion of this plot to the energy axis (h), i.e., at ␣ equal zero. Table 1 shows the energy band gaps for compounds 2a–c at certain concentrations. Fig. 9 appears two linear-like regions, one region lies in the low energy range, and the other in the high energy range. The value of energy gap of compound 2a is lower than for compounds 2b and c because of methyl group in 2a has higher wavelength of absorbance than in the allyl and benzyl group in 2b and c. 3.3.3. Refractive index dispersion Refractive index plays an important role in many areas of material science with special reference for various optical technologies. Measurements of refractive indices of liquids are often required in physics and chemistry. The refractive index of liquid is measured by using various methods; a common method is the measurement of angle of minimum deviation produced by passing a beam of light through the liquid contained in a hollow prism made of transparent glass. But, this method can be limited for visible light, because glass is opaque to infrared and ultraviolet radiations. Another method is by knowing transmittance and reflectance of the materials by using UV–vis spectrophotometer. Herein, refractive index of the pyridinium salts was measured by using these two methods.
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Fig. 10. The dispersion relation between refractive indices and wavelengths for the compound (2a) at different concentrations.
If a monochromatic light is incident on the planar surface separating two isotropic media, at normal incidence, the amplitude reflection coefficients r⊥ , rII and the amplitude transmission coefficients t⊥ , tII perpendicular and parallel to the plane-of-incidence were given by the Fresnel Equations [27]: [rII ]=0 = [−r⊥ ]=0 = [tII ]i =0 = [t⊥ ]i =0 =
nt − ni nt + ni 2ni ni + nt
(2) (3)
Where, ni , nt, are denoted to refractive indices of incident and transmitted medium, respectively. These two equations could be applied to any linear, homogeneous, isotropic and dielectrics media. At normal incidence, which is of great practical interest, the transmittance, and the reflectance by using Eqs. (2) and (3) are: R = r2 = T=
n − n 2 t i
n t
ni
nt + ni t2 =
4nt ni (nt + ni )2
(4) (5)
The value of refractive index could be calculated from Eq. (4) by putting ni = 1, for air and nt = n, is the index of refraction for the sample used: √ 1+ R n= (6) √ 1− R From Eq. (5) by putting ni = 1, for air and nt = n, refractive index could be obtained also as follow: T=
4n (n + 1)2
Tn2 + (2T − 4) n + T = 0 This is a quadratic equation form, where n represents an unknown, and a = T, b = (2T-4), and c = T, represent the known values. So that the solving of this equation leads to: √ 2−T +2 1−T n= (7) T From this equation the value of refractive index was obtained by knowing the transmittance. Figs. 10–12 show the dispersion relation between refractive indices n and wavelengths for compounds 2a–c at different concentrations. The normal dispersion curve was shown in the visible region. Fig. 13, show that there is a direct proportion between refractive index and concentrations for each compound and also the indices of refraction for compound 2a were higher than for compounds 2b and c. 4. Conclusion Three pyridinium halide salts (2a–c) are prepared efficiently under ultrasonic irradiation. The optical parameters of the prepared compounds in water at different concentrations could be obtained by using UV–vis spectral data. All compounds exhibit high transmittance more than 90% in the visible light spectrum. Calculation of absorption coefficients showed allowed
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Fig. 11. The dispersion relation between refractive indices and wavelengths for the compound (2b) at different concentrations.
Fig. 12. The dispersion relation between refractive indices and wavelengths for the compound (2c) at different concentrations.
Fig. 13. The relation between refractive index and concentration at certain wavelength for the compounds (2a–c).
transition were its value ranges from 3 × 103 –6 × 103 l mol−1 cm−1 . Energy band gap of three compounds was also calculated. The refractive indices of the three compounds 2a–c were determined at different concentrations with wavelengths. A direct proportion between refractive index and concentrations for each compound were evaluated and the refractive index appeared increasing from compound 2a–c this means that by changing the alkyling moiety from methyl to benzyl then to allyl. References [1] C.C. Leznoff, A.B.P. Lever, Phthalocyaninesn. Properties and Applications, vols. 1–4, VCH, New York, 1996. [2] M. Hanack, M. Lang, Conducting stacked metallophthalocyanines and related compounds, Adv. Mater. 6 (1994) 819. [3] S. Dhanuskodi, S. Manivannan, K. Kirschbaum, Synthesis, structural, thermal and optical studies of 1-ethyl-2,6-dimethyl-4-hydroxy pyridinium halides, Spectrochim. Acta Part A 64 (2006) 504–511. [4] S. Manivannan, S.K. Tiwari, S. Dhanuskodi, Solid State Commun. 132 (2004) 123–127. [5] V. Pedro, H. Marta, J.G. Emilio, A.M. Eugénia, S. Josefa, T. Emilia, Effect of the number, position and length of alkyl chains on the physical properties of polysbstituted pyridinium ionic liquids, J. Chem. Thermodyn. 69 (2014) 19–26. [6] C.C. Eve Chauchard, H. Eric, M. Gr6goire, S. Charles, P. Andr, T. Andr, Hyperpolarization properties of pyridinium and quinolinium salts determined by hyper-Rayleigh scattering, Chem. Phys. Lett. 238 (1995) 47–53. [7] S.J. Tae, J.K. Jung, H. Haesook, K.B. Pradip, Solution thermal and optical properties of bis(pyridinium salt)s as ionic liquids, Mater. Chem. Phys. 139 (2013) 901–910.
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