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Investigation on structural, optical, thermal and mechanical properties of 1, 3-dinitrobenzene (1,3-DNB) single crystal
Journal Pre-proof Investigation on structural, optical, thermal and mechanical properties of 1, 3dinitrobenzene (1,3-DNB) single crystal R. Nagaraj, K. Ramachandran, K. Aravinth, S. Ranjith PII:
S0022-2860(19)31634-5
DOI:
https://doi.org/10.1016/j.molstruc.2019.127525
Reference:
MOLSTR 127525
To appear in:
Journal of Molecular Structure
Received Date: 22 April 2019 Revised Date:
30 November 2019
Accepted Date: 2 December 2019
Please cite this article as: R. Nagaraj, K. Ramachandran, K. Aravinth, S. Ranjith, Investigation on structural, optical, thermal and mechanical properties of 1, 3-dinitrobenzene (1,3-DNB) single crystal, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2019.127525. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Investigation on Structural, Optical, Thermal And Mechanical Properties of 1, 3-Dinitrobenzene (1,3-DNB) Single Crystal R. Nagaraj1,*, K. Ramachandran2, K. Aravinth2, S. Ranjith1 1)
Department of Physics, Ramapuram Campus, SRM University, Chennai – 600 089 2
SSN Research Centre, SSN College of Engineering, Chennai -603 110, *Corresponding author. E-mail: [email protected]
Abstract An organic 1,3-Dinitrobenzene (1,3-DNB) single crystal have been grown by BridgmanStockbarger method and their structural, optical, thermal and mechanical properties were analysed
by
employing
powder-XRD,
FTIR,
optical
absorption,
transmission,
photoluminescence, TG-DTA, microhardness and SHG measurements. The obtained results explore the well suitable nature of the grown crystal for the NLO and optolectronic applications. The crystalline nature and structural properties of the material has been confirmed by using powder X-ray diffraction (PXRD) analysis. FTIR spectral study reveals the presence of fundamental stretching and bending mode vibrations of grown 1,3-DNB crystal. Linear optical properties of the grown organic single crystal have been studied by using UV–Vis–NIR spectrum analysis, which exhibit the cut-off wavelength at 431 nm. From the tauc’s plot, the calculated optical band gap energy value is 2.78 eV. The photoluminescence spectrum compresses three emission bands in the region centered at 388 nm, 452 nm and 569 nm and it is characterized through the CIE 1931 chromaticity diagram to observe the dominant emission color of the studied crystal. Thermal stability and melting point of the grown single crystal was carried out through the TG-DTA analysis. The microhardness test explores the mechanical strength of the titular crystal. Nonlinear optical efficiency of grown 1,3-DNB crystal was found to be 1.75 times greater than that of standard KDP crystal. In addition, theoretical studies of polarizability and hyperpolarizability, HOMO-LUMO energy gap and electrostatic potential (ESP) were also
performed. Hirshfeld surface and two dimensional (2D) fingerprint plots were analysed from the crystal explorer 3.1 software and the results are discussed in detail. Keywords: Brindgman Technique; Crystal growth; Optical transmittance; Photoluminescence; TG-DTA; Mechanical properties; Second harmonic generation. 1. Introduction In recent years, there has been increased in the need of organic materials for nonlinear optical (NLO) materials, because of their usage in devices applications such as electro-optic modulators, optical data storage, terahertz wave generation and detection, laser fusion reactions, optical, color displays, frequency conversion, photonics and laser remote sensing [1-6]. The nonlinear optical (NLO) applications depends on various properties of the material such as optical transmittance, birefringence, refractive index, thermal and chemical stability, etc.[7, 8] Particularly, the bulk organic single crystals, possessing high optical quality and good phase matching features are in high requirement. Optical materials will be key elements for the future photonic technologies based on the fact that the photons are capable of processing information with the speed of light. In crystal growth, most of the organic, inorganic and semi-organic single crystals were grown by using different kinds of techniques such as, slow solvent evaporation technique (SEST), slow cooling and seed rotation, etc., However, the intrinsic problems can reduce the optical quality of grown single crystals in several aspects like solvent inclusion, lower growth rate, supersaturating fluctuation. In order to overcome these drawbacks and to grow bulk size, good quality organic and inorganic single crystals we employed melt growth techniques. Several technologically important bulk size single crystals are grown by Bridgman–Stockbarger technique. The advantages of these technique like the materials are melt by congruently, that
does not show possible decomposition before melting point. This is one of the best method for growing bulk size high quality crystals at particular time period and the temperature gradient across the solid–liquid interface while compared to the other growth process. Also the crystal cracking can be easily avoided during the growth process [9]. 1,3-dinitrobenzene is one of the popular representatives of benzene derivatives. Potential organic NLO material of 1,3-DNB belongs to the orthorhombic crystal system with space group Pbn21. The obtained unit cell parameters are, a = 13.25 Å, b = 14.04 Å, c = 3.80 Å, α = β = γ = 90° [10]. The small size of 1,3 DNB single crystal was grown by slow evaporation solution growth using ethanol solvent and it was previously reported by the Mohanbabu et al [11 ]. However, the bulk size organic single crystal was successfully grown by using Bridgman – Stockbarger method for device fabrication and NLO applications. In the present work, the bulk size of organic single crystal grown by low temperature vertical Bridgman-Stockbarger technique. The grown organic crystal was studied for optical, thermal, mechanical and SHG properties. 2. Experimental A 1,3-DNB single crystal has been grown by Bridgman-Stockbarger method using single zone transparent vertical Bridgman furnace and it was constructed by in our labaratery [12]. The title crystal was grown in a single wall ampoule and it is made of borosilicate glass tube. In growth ampoule, the length of the ampoule was 20 cm and inner diameter of the ampoule was 1.8 cm and the cone length is designed at 4 cm. The organic material was loaded into the growth ampoule and then evacuated to 2×10-6 milli bar after that fused. Then growth ampoule was suspended by the centre position of the single zone vertical transparent Bridgman furnace. The temperature was slowly increased from room temperature to melting point of the starting material. The melting point of the starting material is 93 ̊C and it was maintained more than 4 h,
to avoid the bubble formation during crystal growth. The growth rate was optimized at 0.75 mm/hour and the growth ampoule moved from the hot region to the cold region, which is adjusted by using nano stepper motor. As the tip of the ampoule passed through the solid-liquid interface, the crystallization started in the molten 1,3-DNB, the self-nucleation is initiated for the growth of single crystal. Once growth was completed, the temperature has been reduced to room temperature at a rate of 1° C/h in order to avoid crystal cracks. The formation of crack may occur due to the thermal expansion coefficient difference between the borosilicate glass tube and crystal. The high quality bulk size organic crystal was removed from the ampoule using a diamond wheel cutter. Figure.1 shows the as grown,cut and polished smooth surface of 1,3-DNB single crystal. 3. Material Characterization Powder X-ray diffraction measurement was carried out through the Bruker X-PERT PRO Xray diffractometer with CuKα (λ = 1.5406 Å) radiation. Perkin-Elmer FTIR spectrophotometer has been employed to record the infrared spectrum in the spectral region of 4000–400 cm–1. The optical absorption and optical transmittance spectrum of 1,3-DNB single crystal were examined using UV–visible NIR spectrum using the Perkin-Elmer Lambda-35 spectrophotometer ranging from 200–1100 nm. Horiba Jobin Yvon Fluoromax-4 spectrophotometer was used to record the excitation and emission spectrum by using 150 W Xenon arc lamp as a source with a spectral resolution of ±0.1 nm. The thermal behavior of 1,3-DNB crystal was studied by using PerkinElmer Diamond TG-DTA instrument. The heating rate of the sample was 5 °C per minutes under nitrogen gas atmosphere. The mechanical stability of the grown crystal was analyzed through Vickers microhardness test using the shimadzu HMV-G21 instrument. The optical efficiency of 1,3-DNB single crystal was studied by the Kurtz and Perry powder technique.
4. Results and discussion 4.1. Structural analysis 4.1.1. Powder X-ray diffraction (PXRD) analysis The structural properties are analyzed using powder XRD analysis. The grown organic single crystal was crushed into a fine powder and the resultant powder sample was scanned for the degree ranging from 10° to 60° at room temperature. The obtained miller indices (h, k, l) plane values are indexed by 2 theta software. The lattice parameters are calculated by the standard equations. The calculated lattice parameters are a = 13.25 Å, b = 14.04Å, c = 3.80 Å, α = β = γ= 90°. The grown single crystal belongs to orthorhombic system with non-centro symmetric space group Pbn21 [13, 14]. From figure 2, we could observe a high-intensity peak at (161) plane and the sharp peaks in the spectrum confirms that the grown crystal is highly crystalline in nature. 4.1.2. FTIR analysis Fourier transform infrared (FTIR) spectrum of the 1,3-DNB single crystal was shown in figure.3 and it is recorded in the range of 4000 cm-1 and 400 cm-1. The FTIR spectrum of 1,3DNB crystal have been carried out to identify the chemical bonding and molecular structure of the title compound. The vibrations with corresponding assignments in the spectrum are listed in the table 1. In general, the C-H stretching vibration band exists in the region 3100-2800 cm-1. The asymmetric and symmetric stretching vibrations of C-H bonds are observed at 3031 cm-1 and 2860 cm-1, respectively. The corresponding carbonyl (C=O) stretching vibration of 1,3- DNB band is observed at 1714 cm-1. The characteristic NO2 asymmetric and symmetric stretching vibrations peaks appeared at 1469 and 1303 cm-1 respectively [15]. The vibration at 1266 cm-1
corresponds to the C-O stretching vibration. The C-C bending vibration peak is observed at 1157 cm-1 and symmetric C-N bending vibration peak is 948 cm-1. Further, the deformation of nitro (NO2) group is appeared around 778 cm-1 in the infrared spectrum. 4.2. Optical properties 4.2.1. UV- Vis-NIR analysis The optical absorption, cut off wavelength and optical transmittance properties are useful tool for the optical applications. UV-Visible and NIR spectrum was analyzed in the wavelength range of 200 nm to 900 nm. At room temperature (RT), the recorded optical absorption spectrum of 1,3-DNB single crystal is shown in Figure. 4 The crystal thickness of 0.75 mm was employed for the analysis. The maximum strong absorption band is observed at 403 nm and the absence of absorption peaks in the optical window region between 434 nm and 900 nm, shows the present crystal can be used for the optical device fabrication [16]. UV-visible spectrum provides the information regarding the structure of the molecule like absorption. This generally involves shifting of the electron in the ground state to higher energy states. The energy transfer completely depends on the crystal structure, crystal symmetry and chemical composition of the grown crystal [17]. The optical transmittance of a single crystal is a main aspect in the optoelectronics and nonlinear optical (NLO) applications. The optical transmittance spectrum of 1,3-DNB single crystal with a thickness of about 1 mm was recorded and the obtained spectrum is shown in Figure.5. From the figure, it could be observed that the cut–off wavelength occurred at 431 nm, which is due to transition of an electron from a ‘nonbonding’ (lone-pair) n orbital to an ‘anti bonding’ π orbital, designated as (n→π*) electronic transition. The optical transmittance spectrum shows 67% transparency and also there is no significant absorption in the entire UV-visible to near IR region. This shows that the present
crystal can be suitable material for the optoelectronic device applications [18]. Optical absorption coefficients (α) mainly depends on the photon energy (hѵ) and crystal thickness, which in turn helps to study the electronic band structure in the crystal. The optical absorption coefficient (α) of 1,3-DNB single crystal is calculated from the following relation (1),
α=
2.303 × log(1 / T ) t
(1)
where, α is an optical absorption coefficient, T is a optical transmittance and t is a crystal thickness. The optical band gap (Eg) value has been determined from the transmittance spectrum The optical absorption coefficient (α) near the absorption edge is given by the relation [19], αhν = A (hν − Eopt)n
(2)
Where, A is an arbitrary constant, Eg is the optical band gap, h is a planck’s constant (6.626 × 10-34 joule-seconds) and ν is the frequency of incident photons. The optical band gap (Eg) value of the grown crystal was obtained by extrapolating the linear region of the Tauc’s plot [20] as shown in figure 6 and it is observed that the optical band gap energy value was found to be 2.90 eV by extrapolating the linear part of the y-axis to the energy axis. Refractive index of the grown crystal was calculated using by using relation [21],
= 1 −
(3)
where, n is the refractive index of the material, Eg is the energy band gap of the material was calculated from the optical transmittance data. The calculated refractive index of 1,3-DNB crystal is found to be 2.459 eV, which is more suitable value for the nonlinear optical material. The extinction coefficient (k) is the fraction of electromagnetic energy lost due to absorption and scattering per unit distance in a sample medium. The extinction coefficient value was evaluated from the following relation (4)
K=
αλ 4π
(4)
The reflectance (R) of 1,3-DNB single crystal mainly depends on the optical transmittance and optical absorption coefficient and it is calculated using the relation (5),
R=
exp(−αt ) ± exp(−αt )T − exp(−3αt )T + exp(−2αt )T 2 exp(−αt ) + exp(−2αt )T
(5)
Figure 8 shows the reflectance spectrum of 1,3 DNB single crystal and it is observed that the reflectance values is decreasing with increasing in the wavelength. In general, the most of the organic single crystal possess high optical transmittance, low optical absorbance and low reflectance values. From the obtained results, we could say that the grown 1,3-DNB crystal is a suitable for nonlinear optical (NLO) applications [22]. 4.2.2 Photoluminescence (PL) spectrum analysis
Figure 9 shows the photoluminescence excitation and emission spectrum of 1,3-DNB single crystal was recorded in the wavelength range for 250 − 650 nm . The excitation spectrum recorded by using the emission wavelength at 388 nm (3.20 eV) and the spectrum exhibit the excitation peak centered at 356 nm (3.47 eV), which is used as a pumping wavelength to record the emission spectrum of the 1,3-DNB. The emission spectrum gives information about the electron energy transfer from the valence band to the conduction band and it is responsible for the radiative recombination mechanism [23]. The emission spectrum has been recorded by using fixed excitation wavelength at 357 nm and the same is shown in figure 9 and the spectrum exhibits three emission bands at 388 nm (3.20 eV), 452 nm (2.74 eV) and 469 nm (2.64 eV). Among all the emission bands, the emission band centered at 388 nm is found to be higher in intensity and the same is used as a pumping wavelength to record the emission spectra of the studied crystal. This may be attributed to
radiative recombination between donors and acceptors. The obtained sharp emission peak at 388 nm shows that the violet and blue emission are possible color emission of the grown sample. This indicates that, the title crystal may be considered as a good candidate for organic light emitting diode (OLED) applications [24, 27]. 4.2.3. CIE chromaticity diagram
In order to find the dominant emission wavelength, Commission International d’Eclairage (CIE) 1931 system has been employed to analyze the emission intensities and the same is used to determine the color coordinates (x, y) of the prepared crystal. The standard equal energy point (x=0.33, y=0.33) accredited to the white light emission is always located at the center of CIE 1931 diagram. The color produced by any light source can easily be described by three dimensionless quantities called “color matching functions” such as x (l), y (l) and z (l). The real spectral color can be obtained by adding three tristimulus values (X, Y and Z). The degree of simulation required to match the color of the given spectral power density (P( λ)) can be obtained using the below given three functions, = ̅ λλλ
(6)
= λλλ
(7)
= ̅ λλλ
(8)
where X, Y and Z are the tristimulus values called artificial colors given by the simulation for each one of the three primary colors (blue, green and red) to match the color of given spectral power density (P( λ)). The x, y chromaticity coordinates of the studied glasses can be estimated from the tristimulus values using the below given expression, x=
X X +Y + Z
(9)
y=
Y X +Y + Z
(10)
From figure 10, the CIE chromaticity coordinates (x, y) of the title crystal was determined from the corresponding emission spectra using excitation wavelength at 357 nm. The emission spectrum was converted into the CIE 1931 chromaticity diagram to examine the dominant emission color and color purity of a present 1,3-DNB crystal. The CIE chromaticity coordinates of the crystal were calculated to be x= 0.1734, y= 0.0486 and marked with star symbol in the CIE 1931 chromaticity diagram. It indicates that the dominant emission color of the present crystal was located in the violet region. Hence, the present 1,3-DNB crystal may act as a violet light emitting color is desired for OLED applications [25]. 4.3. Thermal properties
Thermogravimetric (TG) and differential thermal analysis (DTA) has been studied for the thermal stability and melting point of the grown 1,3-DNB organic crystal with the heating rate at 5 ºC / minutes under the nitrogen gas atmosphere. TG-DTA measurement was recorded from the room temperature to 220 ºC. From figure 11, we could analyse the thermal stability and melting point of 1,3-DNB single crystal. TGA analysis, shows that the organic material has good thermal stability below 100ºC, after that a single stage weight loss starting from 100ºC to 179 ºC. In this stage, the organic material was completely decomposed at 179 ºC. In the DTA curve, there is a sharp endothermic peak appeared at 93ºC, which represents the melting point of grown single crystal [26,27]. 4.4. Mechanical properties
The mechanical properties of solid single crystals are significant feature for the practical device fabrication. The hardness of a crystal has measured for resistance to local deformation [28]. The mechanical properties of solid single crystals are strongly depends on the molecular
(crystal) structure and material composition. The cut and fine polished surface of organic single crystal was selected for the microhardness investigations. The 1,3-DNB crystal was properly mounted on the base of a microscope and the indentations [29] were made on the polished surface of the crystal with the applied load range of 10, 20, 30, 40, 50 and 60 g. Hardness number (Hv) is calculated from the following relation, Hv = 1.854
P Kg / mm 2 d2
(11)
where ‘P’ is the applied load and ‘d’ is the diagonal length of indentation. Figure 12A was plotted between the applied load (p) and hardness number (Hv) for the 1,3-DNB crystal and it is observed that the hardness number was increasing with increase in the applied load up to 50g. Further, increasing the applied load with slightly decrease in the hardness number at above 50g which is due to presence in the stress create by the crystal structure. The Meyer’s index (n) was calculated by the [30] following relation (12), P = Ad n
(12)
where, n is a Meyer’s index and A is constant. The plot between the log d vs log P is shown in figure.12C which represents that the linear fitting gives the straight line and the slope of the straight line gives the Meyer’s index value and it is found to be n = 3.66. From the hardness test, If n is less than 2 it represents normal ISE and if n is greater than 2 then it indicates the reverse ISE [31]. In present work, the grown crystal is representing the reverse indentation size effect character. According to Onitsch [32], if Meyer’s index value n is less than 1.6 and greater than 1.6 indicates the hard material and soft material category respectively. The present crystal belongs to soft material category. The yield strength (σy) has been calculated from the hardness values using the following relation (for n > 2),
σy =
(13)
where σy is the yield strength, Hv is the hardness of the material. From figure.12D, the load p increases with increases in the yield strength [33]. The elastic stiffness constant (C11) was calculated using Wooster's empirical formula [34]. It gives an information regarding tightness of bonding between the neighboring atoms for following relation (14), C11 = (Hv)7/4
(14)
From figure.12E we found that the the load p increasing with increases in the stiffness constant C11. The calculated stiffness constant C11, yield strength σy and Vickers hardness values for different loads from 10-50 g are accumulated in table 2. 4.5. Second Harmonic Generation efficiency
The laser frequency conversion property of the title compound was examined by KurtzPerry powder technique using Nd:YAG laser as a source [35]. It is an effective tool to evaluate the second harmonic generation of a powder material. The grown organic single crystal was finely crushed with homogeneous size and filled with the micro capillary tube. The laser source of wavelength 1064 nm with pulse width 10 ns and repetition rate of 10 Hz was illuminated to the sample. The second order nonlinear property of the 1,3-DNB crystal was confirmed through the output and the optical signal was collected by a photomultiplier tube (PMT). The optical signal incident on the PMT was converted into voltage output using CRO. The output voltage of the title compound was compared with the standard KDP crystal, which have the output voltage of 24 mV. The output of the 1,3-DNB crystal was found to be 1.75 times greater than that of KDP crystal. The obtained result reveals that the title crystal could be a suitable candidate for nonlinear applications. 4.6. Computational details (Density Functional theory)
In present work, the quantum chemical calculations of 1,3-DNB molecule were adopted by utilizing the density functional theory (DFT). The Becke-Lee-Yang-Parr (B3LYP) [36, 37] hybrid functional with standard basis set of 6-311++G(d,p) [38,39] level was used to calculate the electronic properties of title molecule. The optimized geometrical structure and electronic properties were analyzed using Gauss View 5.0.8 visualization package [40]. The natural bonding orbital (NBO) calculations were performed using NBO 3.1 program [41] at the DFT/ B3LYP/6-311++G(d,p) level included in the Gaussian 09 W software and its used to performing all the calculations [42]. 4.6.1. Optimized geometry
The structural and vibrational properties of the molecules are determined by the interaction of the substituent present in the ring.The optimized geometrical parameters such as bond length and bond angle of the 1,3-DNB have been determined by B3LYP method with 6-311++G(d,p) basis sets and is listed in Table.3. The calculated geometrical parameters are in good agreement with the XRD values. The calculated C-C bond lengths are in the range between 1.388 Å- 1.392Å and the experimental
C-C bond lengths are in the range between 1.385Å -
1.390Å. 4.6.2. First - Order Hyper polarizability
First order hyper polarizability is a third rank tensor which can be termed as 3-dimensional matrix and it is reduced to 10 components, which is due to Kleinman symmetry [43]. The Kleinman symmetry matrix such as, βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz and βzzz. The hyperpolarizability components are listed in table 4. The average first order
hyperpolarizability of 1,3-DNB molecule was estimated by (15), βtot= (β x 2+ β y2 + β z2 )1/2
(15)
The first order hyper polarizability (βtot) of 1,3-DNB molecule was calculated by the following relation (16), βtot=[(β xxx+ βxyy + βxzz )2 + (βyyy+ βyzz + βyxx )2 + (βzzz+ βzxx+ βzyy)]1/2
(16)
The electronic polarizability and hyperpolarizability of the titled molecule were calculated at the B3LYP/6-311++G(d,p) level using Gaussian 09W. The calculated total polarizability value of
present organic crystal is 1.6789 x 10-30 e.s.u. All the computed
hyperpolarizability values are converted from atomic units (1 a.u = 8.6393 x 10-33e.s.u.).to electrostatic units (e.s.u). 4.6.3. Natural bond orbital (NBO) Analysis
The nonlinear optical properties of 1,3-DNB molecule are enhanced by the delocalization of electrons, which is due to the nature of molecular stability and chemical reactivity. The important charge transfer interaction between the donor and acceptor of the orbitals are given in table.5 and it was examined using the second order Fock matrix. The stabilization energy of each donor (i) and acceptor (j) with delocalization i→j is predicted by following equation,
F (i, j ) 2 E2 = ∆Eij = qi (ε j − ε i )
(17)
Where, qi is the occupancy of donor orbital, Ei, Ej, are the diagonal elements and Fij are the off-diagonal matrix element. In this analysis, the hyper conjugative interaction and electron density transfer from electron rich part to electron deficient part. From the table.5, we could say that the π (C5-C6) → π*(N11-O13) interactions give the strongest stabilization to the title molecule by 25.72 kcal/mol. After that the highest stabilization energies are π(C1-C2)→ π* C5C6, π(C5-C6) → π*(C3-C4) and π(C3-C4) → π*(N14-O15) by 24.46 kcal/mol, 24.18 kcal/mol and 22.5 kcal/mol, respectively.
4.6.4 Molecular electrostatic potential
A 1,3-DNB molecule was exposed from the molecular electrostatic potential analysis and it’s illustrated in Figure.14. The upper negative potential (yellowish red) is located on the nitro group and oxygen atoms. The blue region is located on the hydrogen atoms of benzyl group. The molecular electrostatic potential map exhibits the negative charge regions are spread on the nitro groups and it’s favorable for the electrophilic attack, while the positive region is mainly spread over the hydrogen atoms for nucleophilic attack [44]. The donor and acceptor sites of the 1,3DNB molecule were exposed by the CHELPG (CHarges from Electrostatic Potentials using a Grid based method) analysis [45]. In order to classify the charge separation in 1,3-DNB molecule, we primarily considered the two portion: the domain which have nitro group and the other which have benzyl groups. The graphical view of CHELPG is depicted in Figure.15. The partial benzyl ring atoms exhibits the net positive charge (+0.0641e) and partial ring carbon atoms, methyl group and nitro group moiety exhibits the net negative charge (-0.0641) expose the donor and acceptor sites, respectively. 4.6.5 Hirshfeld Surface (HS) Plot
The Hirshfeld surface and two dimensional fingerprint plots were generated using Crystal Explorer 3.1 software and the molecular structure of 1,3-DNB [Number- 201615] as an input file in CIF format. The intermolecular interactions of 1,3-DNB crystal structure were analyzed from the Hirshfeld surface (HS) plot [46]. Figure. 16 shows (a) none of the molecule (b) dnorm (-0.165 to 1.080), (c) de (1.000 to 2.423), (d) di (1.000 to 2.639) (e) curvedness (-4.000 to 0.400), (f) shape index (-1.000 to 1.000) of 1,3-DNB molecule. The two dimensional finger print plots of 1,3-DNB molecule were translated for 1.0 – 2.8 Å and di and de represents the scale axis. When the dnormvalue is negative / positive, Which is the intermolecular contact is shorter /larger. The
Hirshfeld surface of the dnormvalues are found using the three different colors such as, red, blue and white. Figure.16 (b). shows the single red color denotes the closer contacts, the blue color denotes the longer contacts and the white color indicates the dnormvalue is perfectly zero. The curvedness surface of the 1,3-DNB molecule indicates the electron density surface around the molecular interactions and the shape index indicates that the shape of the electron density surface around the molecular interactions [47] The two-dimensional fingerprint plot of the1,3-DNB single molecule is shown in Figure.17. The two-dimensional fingerprint plots were translated from -0.165Å to 1.080Å, di anddeindicates the scale axis. From the Figure.17 shows the total intermolecular interaction of 1,3-DNB molecule was observed at 100%. The various bonding contribution such as, C…C (8.9 %), C…H (4.3 %), C…N (0.5 %), C…O (2.1 %), H…C ( 3.4 %),H…H ( 2.9 %), H…N (1 %), H…O ( 27.2 %), N…C (0.5 %),N…H (1.4 %),N…O (3.3 %), O…C ( 1.5 %), O…H (30.9 %), O…N (2.9 %) and O…O (9.3%). The intermolecular interaction of H…O contribution was observed with 27.2 percentage. The highest intermolecular interaction of O…H contribution was observed with 30.9 percentage compared to other molecular contributions. Finally, it can be conclude that the two-dimensional fingerprint plot of the 1,3-DNB molecule helps for better understanding of the intermolecular interaction and the crystal packing structure. 5. Conclusion
Good quality single crystals of 1,3-DNB have been successfully grown by vertical Bridgman technique. The powder XRD analysis confirmed that the unit cell dimensions of the grown crystals are in good agreement with the reported literature. The presence of various stretching and bending vibrations of C-H, N=O, NO2 and C-N network was identified using the FTIR spectra. The transmittance spectrum (67%), cut-off wavelength and optical energy band
gap are found to be 431nm and 2.90eV respectively, which are considerable parameters for NLO applications. Photoluminescence spectral study exposed the electron excitation in the grown 1,3DNB crystal and the x,y coordinates of the studied organic crystal lies in the violet region in CIE 1931 diagram, which shows that it is suitable for violet OLED applications. Thermal stability of organic crystal was studied by TG-DTA analysis and it reveals that the compound decomposes above 100oC. The various mechanical parameters are deduced from Vickers microhardness study which explores the hardness nature of the grown crystal. Second harmonic generation (SHG) efficiency of 1,3-DNB is evaluated as 1.75 times that of KDP. Based on the above facts, it could be proposed that this material can be better placed for optical applications.
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FIGURE.1. As grown, cut and polished 1,3 DNB crystal
FIGURE.2. Powder XRD spectrum of the 1,3-DNB single crystal
FIGURE.3. FTIR spectrum of the 1,3-DNB single crystal
FIGURE. 4. Optical absorption spectrum of 1,3-DNB single crystal
FIGURE.5. Optical transmittance spectrum of 1,3-DNB crystal
IGURE.6. Tauc’s plot of the 1,3-DNB single crystal
FIGURE.7. Extinction coefficient (K) of 1,3-DNB single crystal
FIGURE.8. Reflectance spectrum of 1,3-DNB single crystal
FIGURE.9. Photoluminescence spectrum of 1,3-DNB single crystal
Figure.10. CIE 1931diagram of the 1,3-DNB single crystal
Figure.11. Thermal behavior of 1,3-DNB single crystal
Figure.12. Vickers microhardness spectra. (A) variation of HV vs load p (B) variation of HV vs diagonal d (C) variation of log p vs log d (D) variation of yield strngth vs load p (E) variation of stiffness constant C11 vs load p.
Figure.13. Optimized molecular structure of 1,3-DNB material
Figure.14.Molecular electrostatic potential map of 1,3-DMB
Figure.15.The graphical view of the 1,3-DNB
(a)
(b)
(c)
(d)
(e)
(f)
Figure.16. Hirshfeld surface plot of (a) none (b) dnorm, (c) de, (d) di, (e) curvedness and (f) shape index of 1,3-DNB.
Figure.17. The two dimensional finger print plot of 1,3-DNB molecule
TABLE.1. The various functional group of 1,3-DNB single crystal
Sl.No 1 2 3 4 5 6 7 8 9 10 11
Wavenumber (cm-1) 3031 2860 1714 1469 1303 1266 1157 948 852 778 617
Assignments Asymmetric stretching vibrations of C-H group symmetric stretching vibrations of C-H group C=O stretching vibration mode N=O antisymmetric vibration N=O symmetricstretching δ (O-H) C-C bending vibration bending vibration of C-N Asymmetric ring stretching Deformation of NO2 group C-O in plane deformation
Table.2. Microhardness parameters load p (g), Hv (kg/mm2), σy (GN/m2), C11×1014 (pa)
Load P (g) 10 20 30 40 50
Hv (kg/mm2) 6.5007 11.2842 13.5186 15.4542 16.8636
σy (GN/m2) 4.7406 8.2290 9.8585 11.2701 12.2979
C11×1014 (pa) 0.4548 1.1939 1.6379 2.0701 2.4117
Table.3. Optimized structural parameter of 1,3-DNB molecule
Bond Angle (degree) B3LYP (C2-C1-C6) 120.379 (C2-C1-H7) 119.809 (C6-C1-H7) 119.811 (C1-C2-C3) 118.807 (C1-C2-H8) 121.707 (C3-C2-H8) 119.486 (C2-C3-C4) 122.466 (C2-C3-N14) 118.973 (C4-C3-N14) 118.561 (C3-C4-C5) 117.074 (C3-C4-H9) 121.462 (C5-C4-H9) 121.465 (C4-C5-C6) 122.467 (C4-C5-N11) 118.561 (C6-C5-N11) 118.972 (C1-C6-C5) 118.807 (C1-C6-H10) 121.709 (C5-C6-H10) 119.484 (C5-N11-O12) 117.267 (C5-N11-O13) 117.447 (O12-N11-O13) 125.287 (C3-N14-O15) 117.447 (C3-N14-O16) 117.267 (O15-N14-O16) 125.286
XRD 120.567 119.693 119.741 118.297 120.876 120.827 123.513 119.054 117.433 115.724 122.111 122.166 123.403 118.376 118.219 118.462 120.754 120.784 117.806 118.314 123.880 117.686 118.040 124.271
Bond length (Å) B3LYP (C1-C2) 1.392 (C1-C6) 1.392 (C1-H7) 1.083 (C2-C3) 1.392 (C2-H8) 1.081 (C3-C4) 1.388 (C3-N14) 1.485 (C4-C5) 1.388 (C4-H9) 1.080 (C5-C6) 1.392 (C5-N11) 1.485 (C6-H10) 1.081 (N11-O12) 1.223 (N11-O13) 1.222 (N14-O15) 1.222 (N14-O16) 1.223
XRD 1.388 1.389 0.931 1.390 0.930 1.385 1.473 1.387 0.930 1.387 1.471 0.930 1.226 1.221 1.225 1.221
Table.4. Hypolarizability components of 1,3-DNB crystal
β components β in a.u. βxxx 0.004 βxxy -317.65 βxyy -0.011 βyyy 75.945 βxxz 0.002 βxyz 0.006 βyyz 0.003 βxzz 0.001 βyzz 46.795 βzzz 0.01 βtot 1.6789E-30e.s.u
Table.5. Selected second-order perturbation energies E (2) (Donor → Acceptor)
ED(i) (e)
Donor (i) C 1-C 2 C 1-C 2
σ π
1.97642
C 1-C 6 C 2-C 3 C 2-H 8
σ σ σ
1.97642 1.97669 1.97560
σ
1.97247
C 3-C 4 C 3-C 4 C C C C C C C C C
3-C 4-C 4-C 4-C 4-H 4-H 5-C 5-C 5-C
4 5 5 5 9 9 6 6 6
N 11 - O 13 N 14 - O 15 N 14 - O 15 LP ( 2) O 12 LP ( 2) O 12 LP ( 2) O 13 LP ( 2) O 15 LP ( 2) O 16
π π σ σ σ σ σ π π π π π π
1.62102
1.63826
1.97248
1.97038 1.63016
1.98579 1.98579 1.89703 1.89573
1.89704
Acceptor (j)
ED(j) (e)
E(2)(kcal mol-1)
0.11064 0.34446 0.35928 0.11064 0.02202
C 3 - N14 C 3-C 4 C 5-C 6 C 5 - N11 C 3-C 4 C 3-C 4 C 2-C 3 C 5 - N11 C 1-C 2 C 5-C 6 N 14 - O15 C 3 - N 14 C 5-C 6 N 11 - O 12 C 2-C 3 C 5-C 6 C 1-C 2 C 3-C 4 N 11 - O 13 LP ( 3) O 12 C 5-C 6 N 11 - O 13 LP ( 3) O 16 N 14 - O 15 C 5 - N 11 N 11 - O 13 C 5 - N 11 N 14 - O 16 C 3 - N 14
σ* π* π* σ* σ* σ* σ* σ* π* π* π* σ* σ* σ* σ* σ* π* π* π*
0.02224 0.11064 0.28656 0.35928 0.61389 0.11064 0.02224 0.05475 0.02224 0.02224 0.28656 0.34446 0.61389 1.44086 0.35928 0.61389 1.44086 0.61389 0.11064 0.05426 0.11064 0.05475 0.11064
4.24 19.33 24.46 4.24 4.71 4.55 4.68 4.09 21.61 17.73 22.5 4.09 4.68 2.01 4.37 4.37 15.8 24.18 25.72 12.7 4.06 7.44 12.7 7.44 12.97 18.84 13.17 19.03 12.97
N 14 - O 15
σ* 0.05426
18.84
π* π*
σ* σ* σ* σ* σ*
E(j)E(i) (a.u) 0.98 0.28 0.28 0.98 1.29 1.09 1.28 1 0.3 0.29 0.15 1 1.28 1.18 1.08 1.08 0.29 0.29 0.15 0.18 0.46 0.32 0.18 0.32 0.55 0.73 0.55 0.73 0.55
0.059 0.066 0.074 0.059 0.07 0.063 0.069 0.058 0.073 0.064 0.056 0.058 0.069 0.044 0.062 0.062 0.062 0.075 0.06 0.079 0.042 0.052 0.079 0.052 0.076 0.106 0.076 0.106 0.076
0.73
0.106
F(i,j) (a.u)
Highlights Organic 1,3-Dinitrobenzene single crystal have been grown by Bridgman method. The crystal shows good transparency in the visible region. CIE diagram indicates that the dominant emission color was located in the violet region. Nonlinear optical efficiency is found to be higher than that of KDP.
Author Contribution Section
All the authors are equal contribution in the manuscript.
1. R. Nagaraj – Crystal growth, characterization and written the manuscript and also manuscript correction (technical, grammar and scientific content) 2. K. Ramachandran – contribute to the TG - DTA study 3. K. Aravinth – Some of the characterizations done (XRD and SHG) 4. S. Ranjith – contribute to the theoretical study
Declaration of interests The authors declare that they have no conflict of interest.
S0022-2860(19)31634-5
DOI:
https://doi.org/10.1016/j.molstruc.2019.127525
Reference:
MOLSTR 127525
To appear in:
Journal of Molecular Structure
Received Date: 22 April 2019 Revised Date:
30 November 2019
Accepted Date: 2 December 2019
Please cite this article as: R. Nagaraj, K. Ramachandran, K. Aravinth, S. Ranjith, Investigation on structural, optical, thermal and mechanical properties of 1, 3-dinitrobenzene (1,3-DNB) single crystal, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2019.127525. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Investigation on Structural, Optical, Thermal And Mechanical Properties of 1, 3-Dinitrobenzene (1,3-DNB) Single Crystal R. Nagaraj1,*, K. Ramachandran2, K. Aravinth2, S. Ranjith1 1)
Department of Physics, Ramapuram Campus, SRM University, Chennai – 600 089 2
SSN Research Centre, SSN College of Engineering, Chennai -603 110, *Corresponding author. E-mail: [email protected]
Abstract An organic 1,3-Dinitrobenzene (1,3-DNB) single crystal have been grown by BridgmanStockbarger method and their structural, optical, thermal and mechanical properties were analysed
by
employing
powder-XRD,
FTIR,
optical
absorption,
transmission,
photoluminescence, TG-DTA, microhardness and SHG measurements. The obtained results explore the well suitable nature of the grown crystal for the NLO and optolectronic applications. The crystalline nature and structural properties of the material has been confirmed by using powder X-ray diffraction (PXRD) analysis. FTIR spectral study reveals the presence of fundamental stretching and bending mode vibrations of grown 1,3-DNB crystal. Linear optical properties of the grown organic single crystal have been studied by using UV–Vis–NIR spectrum analysis, which exhibit the cut-off wavelength at 431 nm. From the tauc’s plot, the calculated optical band gap energy value is 2.78 eV. The photoluminescence spectrum compresses three emission bands in the region centered at 388 nm, 452 nm and 569 nm and it is characterized through the CIE 1931 chromaticity diagram to observe the dominant emission color of the studied crystal. Thermal stability and melting point of the grown single crystal was carried out through the TG-DTA analysis. The microhardness test explores the mechanical strength of the titular crystal. Nonlinear optical efficiency of grown 1,3-DNB crystal was found to be 1.75 times greater than that of standard KDP crystal. In addition, theoretical studies of polarizability and hyperpolarizability, HOMO-LUMO energy gap and electrostatic potential (ESP) were also
performed. Hirshfeld surface and two dimensional (2D) fingerprint plots were analysed from the crystal explorer 3.1 software and the results are discussed in detail. Keywords: Brindgman Technique; Crystal growth; Optical transmittance; Photoluminescence; TG-DTA; Mechanical properties; Second harmonic generation. 1. Introduction In recent years, there has been increased in the need of organic materials for nonlinear optical (NLO) materials, because of their usage in devices applications such as electro-optic modulators, optical data storage, terahertz wave generation and detection, laser fusion reactions, optical, color displays, frequency conversion, photonics and laser remote sensing [1-6]. The nonlinear optical (NLO) applications depends on various properties of the material such as optical transmittance, birefringence, refractive index, thermal and chemical stability, etc.[7, 8] Particularly, the bulk organic single crystals, possessing high optical quality and good phase matching features are in high requirement. Optical materials will be key elements for the future photonic technologies based on the fact that the photons are capable of processing information with the speed of light. In crystal growth, most of the organic, inorganic and semi-organic single crystals were grown by using different kinds of techniques such as, slow solvent evaporation technique (SEST), slow cooling and seed rotation, etc., However, the intrinsic problems can reduce the optical quality of grown single crystals in several aspects like solvent inclusion, lower growth rate, supersaturating fluctuation. In order to overcome these drawbacks and to grow bulk size, good quality organic and inorganic single crystals we employed melt growth techniques. Several technologically important bulk size single crystals are grown by Bridgman–Stockbarger technique. The advantages of these technique like the materials are melt by congruently, that
does not show possible decomposition before melting point. This is one of the best method for growing bulk size high quality crystals at particular time period and the temperature gradient across the solid–liquid interface while compared to the other growth process. Also the crystal cracking can be easily avoided during the growth process [9]. 1,3-dinitrobenzene is one of the popular representatives of benzene derivatives. Potential organic NLO material of 1,3-DNB belongs to the orthorhombic crystal system with space group Pbn21. The obtained unit cell parameters are, a = 13.25 Å, b = 14.04 Å, c = 3.80 Å, α = β = γ = 90° [10]. The small size of 1,3 DNB single crystal was grown by slow evaporation solution growth using ethanol solvent and it was previously reported by the Mohanbabu et al [11 ]. However, the bulk size organic single crystal was successfully grown by using Bridgman – Stockbarger method for device fabrication and NLO applications. In the present work, the bulk size of organic single crystal grown by low temperature vertical Bridgman-Stockbarger technique. The grown organic crystal was studied for optical, thermal, mechanical and SHG properties. 2. Experimental A 1,3-DNB single crystal has been grown by Bridgman-Stockbarger method using single zone transparent vertical Bridgman furnace and it was constructed by in our labaratery [12]. The title crystal was grown in a single wall ampoule and it is made of borosilicate glass tube. In growth ampoule, the length of the ampoule was 20 cm and inner diameter of the ampoule was 1.8 cm and the cone length is designed at 4 cm. The organic material was loaded into the growth ampoule and then evacuated to 2×10-6 milli bar after that fused. Then growth ampoule was suspended by the centre position of the single zone vertical transparent Bridgman furnace. The temperature was slowly increased from room temperature to melting point of the starting material. The melting point of the starting material is 93 ̊C and it was maintained more than 4 h,
to avoid the bubble formation during crystal growth. The growth rate was optimized at 0.75 mm/hour and the growth ampoule moved from the hot region to the cold region, which is adjusted by using nano stepper motor. As the tip of the ampoule passed through the solid-liquid interface, the crystallization started in the molten 1,3-DNB, the self-nucleation is initiated for the growth of single crystal. Once growth was completed, the temperature has been reduced to room temperature at a rate of 1° C/h in order to avoid crystal cracks. The formation of crack may occur due to the thermal expansion coefficient difference between the borosilicate glass tube and crystal. The high quality bulk size organic crystal was removed from the ampoule using a diamond wheel cutter. Figure.1 shows the as grown,cut and polished smooth surface of 1,3-DNB single crystal. 3. Material Characterization Powder X-ray diffraction measurement was carried out through the Bruker X-PERT PRO Xray diffractometer with CuKα (λ = 1.5406 Å) radiation. Perkin-Elmer FTIR spectrophotometer has been employed to record the infrared spectrum in the spectral region of 4000–400 cm–1. The optical absorption and optical transmittance spectrum of 1,3-DNB single crystal were examined using UV–visible NIR spectrum using the Perkin-Elmer Lambda-35 spectrophotometer ranging from 200–1100 nm. Horiba Jobin Yvon Fluoromax-4 spectrophotometer was used to record the excitation and emission spectrum by using 150 W Xenon arc lamp as a source with a spectral resolution of ±0.1 nm. The thermal behavior of 1,3-DNB crystal was studied by using PerkinElmer Diamond TG-DTA instrument. The heating rate of the sample was 5 °C per minutes under nitrogen gas atmosphere. The mechanical stability of the grown crystal was analyzed through Vickers microhardness test using the shimadzu HMV-G21 instrument. The optical efficiency of 1,3-DNB single crystal was studied by the Kurtz and Perry powder technique.
4. Results and discussion 4.1. Structural analysis 4.1.1. Powder X-ray diffraction (PXRD) analysis The structural properties are analyzed using powder XRD analysis. The grown organic single crystal was crushed into a fine powder and the resultant powder sample was scanned for the degree ranging from 10° to 60° at room temperature. The obtained miller indices (h, k, l) plane values are indexed by 2 theta software. The lattice parameters are calculated by the standard equations. The calculated lattice parameters are a = 13.25 Å, b = 14.04Å, c = 3.80 Å, α = β = γ= 90°. The grown single crystal belongs to orthorhombic system with non-centro symmetric space group Pbn21 [13, 14]. From figure 2, we could observe a high-intensity peak at (161) plane and the sharp peaks in the spectrum confirms that the grown crystal is highly crystalline in nature. 4.1.2. FTIR analysis Fourier transform infrared (FTIR) spectrum of the 1,3-DNB single crystal was shown in figure.3 and it is recorded in the range of 4000 cm-1 and 400 cm-1. The FTIR spectrum of 1,3DNB crystal have been carried out to identify the chemical bonding and molecular structure of the title compound. The vibrations with corresponding assignments in the spectrum are listed in the table 1. In general, the C-H stretching vibration band exists in the region 3100-2800 cm-1. The asymmetric and symmetric stretching vibrations of C-H bonds are observed at 3031 cm-1 and 2860 cm-1, respectively. The corresponding carbonyl (C=O) stretching vibration of 1,3- DNB band is observed at 1714 cm-1. The characteristic NO2 asymmetric and symmetric stretching vibrations peaks appeared at 1469 and 1303 cm-1 respectively [15]. The vibration at 1266 cm-1
corresponds to the C-O stretching vibration. The C-C bending vibration peak is observed at 1157 cm-1 and symmetric C-N bending vibration peak is 948 cm-1. Further, the deformation of nitro (NO2) group is appeared around 778 cm-1 in the infrared spectrum. 4.2. Optical properties 4.2.1. UV- Vis-NIR analysis The optical absorption, cut off wavelength and optical transmittance properties are useful tool for the optical applications. UV-Visible and NIR spectrum was analyzed in the wavelength range of 200 nm to 900 nm. At room temperature (RT), the recorded optical absorption spectrum of 1,3-DNB single crystal is shown in Figure. 4 The crystal thickness of 0.75 mm was employed for the analysis. The maximum strong absorption band is observed at 403 nm and the absence of absorption peaks in the optical window region between 434 nm and 900 nm, shows the present crystal can be used for the optical device fabrication [16]. UV-visible spectrum provides the information regarding the structure of the molecule like absorption. This generally involves shifting of the electron in the ground state to higher energy states. The energy transfer completely depends on the crystal structure, crystal symmetry and chemical composition of the grown crystal [17]. The optical transmittance of a single crystal is a main aspect in the optoelectronics and nonlinear optical (NLO) applications. The optical transmittance spectrum of 1,3-DNB single crystal with a thickness of about 1 mm was recorded and the obtained spectrum is shown in Figure.5. From the figure, it could be observed that the cut–off wavelength occurred at 431 nm, which is due to transition of an electron from a ‘nonbonding’ (lone-pair) n orbital to an ‘anti bonding’ π orbital, designated as (n→π*) electronic transition. The optical transmittance spectrum shows 67% transparency and also there is no significant absorption in the entire UV-visible to near IR region. This shows that the present
crystal can be suitable material for the optoelectronic device applications [18]. Optical absorption coefficients (α) mainly depends on the photon energy (hѵ) and crystal thickness, which in turn helps to study the electronic band structure in the crystal. The optical absorption coefficient (α) of 1,3-DNB single crystal is calculated from the following relation (1),
α=
2.303 × log(1 / T ) t
(1)
where, α is an optical absorption coefficient, T is a optical transmittance and t is a crystal thickness. The optical band gap (Eg) value has been determined from the transmittance spectrum The optical absorption coefficient (α) near the absorption edge is given by the relation [19], αhν = A (hν − Eopt)n
(2)
Where, A is an arbitrary constant, Eg is the optical band gap, h is a planck’s constant (6.626 × 10-34 joule-seconds) and ν is the frequency of incident photons. The optical band gap (Eg) value of the grown crystal was obtained by extrapolating the linear region of the Tauc’s plot [20] as shown in figure 6 and it is observed that the optical band gap energy value was found to be 2.90 eV by extrapolating the linear part of the y-axis to the energy axis. Refractive index of the grown crystal was calculated using by using relation [21],
= 1 −
(3)
where, n is the refractive index of the material, Eg is the energy band gap of the material was calculated from the optical transmittance data. The calculated refractive index of 1,3-DNB crystal is found to be 2.459 eV, which is more suitable value for the nonlinear optical material. The extinction coefficient (k) is the fraction of electromagnetic energy lost due to absorption and scattering per unit distance in a sample medium. The extinction coefficient value was evaluated from the following relation (4)
K=
αλ 4π
(4)
The reflectance (R) of 1,3-DNB single crystal mainly depends on the optical transmittance and optical absorption coefficient and it is calculated using the relation (5),
R=
exp(−αt ) ± exp(−αt )T − exp(−3αt )T + exp(−2αt )T 2 exp(−αt ) + exp(−2αt )T
(5)
Figure 8 shows the reflectance spectrum of 1,3 DNB single crystal and it is observed that the reflectance values is decreasing with increasing in the wavelength. In general, the most of the organic single crystal possess high optical transmittance, low optical absorbance and low reflectance values. From the obtained results, we could say that the grown 1,3-DNB crystal is a suitable for nonlinear optical (NLO) applications [22]. 4.2.2 Photoluminescence (PL) spectrum analysis
Figure 9 shows the photoluminescence excitation and emission spectrum of 1,3-DNB single crystal was recorded in the wavelength range for 250 − 650 nm . The excitation spectrum recorded by using the emission wavelength at 388 nm (3.20 eV) and the spectrum exhibit the excitation peak centered at 356 nm (3.47 eV), which is used as a pumping wavelength to record the emission spectrum of the 1,3-DNB. The emission spectrum gives information about the electron energy transfer from the valence band to the conduction band and it is responsible for the radiative recombination mechanism [23]. The emission spectrum has been recorded by using fixed excitation wavelength at 357 nm and the same is shown in figure 9 and the spectrum exhibits three emission bands at 388 nm (3.20 eV), 452 nm (2.74 eV) and 469 nm (2.64 eV). Among all the emission bands, the emission band centered at 388 nm is found to be higher in intensity and the same is used as a pumping wavelength to record the emission spectra of the studied crystal. This may be attributed to
radiative recombination between donors and acceptors. The obtained sharp emission peak at 388 nm shows that the violet and blue emission are possible color emission of the grown sample. This indicates that, the title crystal may be considered as a good candidate for organic light emitting diode (OLED) applications [24, 27]. 4.2.3. CIE chromaticity diagram
In order to find the dominant emission wavelength, Commission International d’Eclairage (CIE) 1931 system has been employed to analyze the emission intensities and the same is used to determine the color coordinates (x, y) of the prepared crystal. The standard equal energy point (x=0.33, y=0.33) accredited to the white light emission is always located at the center of CIE 1931 diagram. The color produced by any light source can easily be described by three dimensionless quantities called “color matching functions” such as x (l), y (l) and z (l). The real spectral color can be obtained by adding three tristimulus values (X, Y and Z). The degree of simulation required to match the color of the given spectral power density (P( λ)) can be obtained using the below given three functions, = ̅ λλλ
(6)
= λλλ
(7)
= ̅ λλλ
(8)
where X, Y and Z are the tristimulus values called artificial colors given by the simulation for each one of the three primary colors (blue, green and red) to match the color of given spectral power density (P( λ)). The x, y chromaticity coordinates of the studied glasses can be estimated from the tristimulus values using the below given expression, x=
X X +Y + Z
(9)
y=
Y X +Y + Z
(10)
From figure 10, the CIE chromaticity coordinates (x, y) of the title crystal was determined from the corresponding emission spectra using excitation wavelength at 357 nm. The emission spectrum was converted into the CIE 1931 chromaticity diagram to examine the dominant emission color and color purity of a present 1,3-DNB crystal. The CIE chromaticity coordinates of the crystal were calculated to be x= 0.1734, y= 0.0486 and marked with star symbol in the CIE 1931 chromaticity diagram. It indicates that the dominant emission color of the present crystal was located in the violet region. Hence, the present 1,3-DNB crystal may act as a violet light emitting color is desired for OLED applications [25]. 4.3. Thermal properties
Thermogravimetric (TG) and differential thermal analysis (DTA) has been studied for the thermal stability and melting point of the grown 1,3-DNB organic crystal with the heating rate at 5 ºC / minutes under the nitrogen gas atmosphere. TG-DTA measurement was recorded from the room temperature to 220 ºC. From figure 11, we could analyse the thermal stability and melting point of 1,3-DNB single crystal. TGA analysis, shows that the organic material has good thermal stability below 100ºC, after that a single stage weight loss starting from 100ºC to 179 ºC. In this stage, the organic material was completely decomposed at 179 ºC. In the DTA curve, there is a sharp endothermic peak appeared at 93ºC, which represents the melting point of grown single crystal [26,27]. 4.4. Mechanical properties
The mechanical properties of solid single crystals are significant feature for the practical device fabrication. The hardness of a crystal has measured for resistance to local deformation [28]. The mechanical properties of solid single crystals are strongly depends on the molecular
(crystal) structure and material composition. The cut and fine polished surface of organic single crystal was selected for the microhardness investigations. The 1,3-DNB crystal was properly mounted on the base of a microscope and the indentations [29] were made on the polished surface of the crystal with the applied load range of 10, 20, 30, 40, 50 and 60 g. Hardness number (Hv) is calculated from the following relation, Hv = 1.854
P Kg / mm 2 d2
(11)
where ‘P’ is the applied load and ‘d’ is the diagonal length of indentation. Figure 12A was plotted between the applied load (p) and hardness number (Hv) for the 1,3-DNB crystal and it is observed that the hardness number was increasing with increase in the applied load up to 50g. Further, increasing the applied load with slightly decrease in the hardness number at above 50g which is due to presence in the stress create by the crystal structure. The Meyer’s index (n) was calculated by the [30] following relation (12), P = Ad n
(12)
where, n is a Meyer’s index and A is constant. The plot between the log d vs log P is shown in figure.12C which represents that the linear fitting gives the straight line and the slope of the straight line gives the Meyer’s index value and it is found to be n = 3.66. From the hardness test, If n is less than 2 it represents normal ISE and if n is greater than 2 then it indicates the reverse ISE [31]. In present work, the grown crystal is representing the reverse indentation size effect character. According to Onitsch [32], if Meyer’s index value n is less than 1.6 and greater than 1.6 indicates the hard material and soft material category respectively. The present crystal belongs to soft material category. The yield strength (σy) has been calculated from the hardness values using the following relation (for n > 2),
σy =
(13)
where σy is the yield strength, Hv is the hardness of the material. From figure.12D, the load p increases with increases in the yield strength [33]. The elastic stiffness constant (C11) was calculated using Wooster's empirical formula [34]. It gives an information regarding tightness of bonding between the neighboring atoms for following relation (14), C11 = (Hv)7/4
(14)
From figure.12E we found that the the load p increasing with increases in the stiffness constant C11. The calculated stiffness constant C11, yield strength σy and Vickers hardness values for different loads from 10-50 g are accumulated in table 2. 4.5. Second Harmonic Generation efficiency
The laser frequency conversion property of the title compound was examined by KurtzPerry powder technique using Nd:YAG laser as a source [35]. It is an effective tool to evaluate the second harmonic generation of a powder material. The grown organic single crystal was finely crushed with homogeneous size and filled with the micro capillary tube. The laser source of wavelength 1064 nm with pulse width 10 ns and repetition rate of 10 Hz was illuminated to the sample. The second order nonlinear property of the 1,3-DNB crystal was confirmed through the output and the optical signal was collected by a photomultiplier tube (PMT). The optical signal incident on the PMT was converted into voltage output using CRO. The output voltage of the title compound was compared with the standard KDP crystal, which have the output voltage of 24 mV. The output of the 1,3-DNB crystal was found to be 1.75 times greater than that of KDP crystal. The obtained result reveals that the title crystal could be a suitable candidate for nonlinear applications. 4.6. Computational details (Density Functional theory)
In present work, the quantum chemical calculations of 1,3-DNB molecule were adopted by utilizing the density functional theory (DFT). The Becke-Lee-Yang-Parr (B3LYP) [36, 37] hybrid functional with standard basis set of 6-311++G(d,p) [38,39] level was used to calculate the electronic properties of title molecule. The optimized geometrical structure and electronic properties were analyzed using Gauss View 5.0.8 visualization package [40]. The natural bonding orbital (NBO) calculations were performed using NBO 3.1 program [41] at the DFT/ B3LYP/6-311++G(d,p) level included in the Gaussian 09 W software and its used to performing all the calculations [42]. 4.6.1. Optimized geometry
The structural and vibrational properties of the molecules are determined by the interaction of the substituent present in the ring.The optimized geometrical parameters such as bond length and bond angle of the 1,3-DNB have been determined by B3LYP method with 6-311++G(d,p) basis sets and is listed in Table.3. The calculated geometrical parameters are in good agreement with the XRD values. The calculated C-C bond lengths are in the range between 1.388 Å- 1.392Å and the experimental
C-C bond lengths are in the range between 1.385Å -
1.390Å. 4.6.2. First - Order Hyper polarizability
First order hyper polarizability is a third rank tensor which can be termed as 3-dimensional matrix and it is reduced to 10 components, which is due to Kleinman symmetry [43]. The Kleinman symmetry matrix such as, βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz and βzzz. The hyperpolarizability components are listed in table 4. The average first order
hyperpolarizability of 1,3-DNB molecule was estimated by (15), βtot= (β x 2+ β y2 + β z2 )1/2
(15)
The first order hyper polarizability (βtot) of 1,3-DNB molecule was calculated by the following relation (16), βtot=[(β xxx+ βxyy + βxzz )2 + (βyyy+ βyzz + βyxx )2 + (βzzz+ βzxx+ βzyy)]1/2
(16)
The electronic polarizability and hyperpolarizability of the titled molecule were calculated at the B3LYP/6-311++G(d,p) level using Gaussian 09W. The calculated total polarizability value of
present organic crystal is 1.6789 x 10-30 e.s.u. All the computed
hyperpolarizability values are converted from atomic units (1 a.u = 8.6393 x 10-33e.s.u.).to electrostatic units (e.s.u). 4.6.3. Natural bond orbital (NBO) Analysis
The nonlinear optical properties of 1,3-DNB molecule are enhanced by the delocalization of electrons, which is due to the nature of molecular stability and chemical reactivity. The important charge transfer interaction between the donor and acceptor of the orbitals are given in table.5 and it was examined using the second order Fock matrix. The stabilization energy of each donor (i) and acceptor (j) with delocalization i→j is predicted by following equation,
F (i, j ) 2 E2 = ∆Eij = qi (ε j − ε i )
(17)
Where, qi is the occupancy of donor orbital, Ei, Ej, are the diagonal elements and Fij are the off-diagonal matrix element. In this analysis, the hyper conjugative interaction and electron density transfer from electron rich part to electron deficient part. From the table.5, we could say that the π (C5-C6) → π*(N11-O13) interactions give the strongest stabilization to the title molecule by 25.72 kcal/mol. After that the highest stabilization energies are π(C1-C2)→ π* C5C6, π(C5-C6) → π*(C3-C4) and π(C3-C4) → π*(N14-O15) by 24.46 kcal/mol, 24.18 kcal/mol and 22.5 kcal/mol, respectively.
4.6.4 Molecular electrostatic potential
A 1,3-DNB molecule was exposed from the molecular electrostatic potential analysis and it’s illustrated in Figure.14. The upper negative potential (yellowish red) is located on the nitro group and oxygen atoms. The blue region is located on the hydrogen atoms of benzyl group. The molecular electrostatic potential map exhibits the negative charge regions are spread on the nitro groups and it’s favorable for the electrophilic attack, while the positive region is mainly spread over the hydrogen atoms for nucleophilic attack [44]. The donor and acceptor sites of the 1,3DNB molecule were exposed by the CHELPG (CHarges from Electrostatic Potentials using a Grid based method) analysis [45]. In order to classify the charge separation in 1,3-DNB molecule, we primarily considered the two portion: the domain which have nitro group and the other which have benzyl groups. The graphical view of CHELPG is depicted in Figure.15. The partial benzyl ring atoms exhibits the net positive charge (+0.0641e) and partial ring carbon atoms, methyl group and nitro group moiety exhibits the net negative charge (-0.0641) expose the donor and acceptor sites, respectively. 4.6.5 Hirshfeld Surface (HS) Plot
The Hirshfeld surface and two dimensional fingerprint plots were generated using Crystal Explorer 3.1 software and the molecular structure of 1,3-DNB [Number- 201615] as an input file in CIF format. The intermolecular interactions of 1,3-DNB crystal structure were analyzed from the Hirshfeld surface (HS) plot [46]. Figure. 16 shows (a) none of the molecule (b) dnorm (-0.165 to 1.080), (c) de (1.000 to 2.423), (d) di (1.000 to 2.639) (e) curvedness (-4.000 to 0.400), (f) shape index (-1.000 to 1.000) of 1,3-DNB molecule. The two dimensional finger print plots of 1,3-DNB molecule were translated for 1.0 – 2.8 Å and di and de represents the scale axis. When the dnormvalue is negative / positive, Which is the intermolecular contact is shorter /larger. The
Hirshfeld surface of the dnormvalues are found using the three different colors such as, red, blue and white. Figure.16 (b). shows the single red color denotes the closer contacts, the blue color denotes the longer contacts and the white color indicates the dnormvalue is perfectly zero. The curvedness surface of the 1,3-DNB molecule indicates the electron density surface around the molecular interactions and the shape index indicates that the shape of the electron density surface around the molecular interactions [47] The two-dimensional fingerprint plot of the1,3-DNB single molecule is shown in Figure.17. The two-dimensional fingerprint plots were translated from -0.165Å to 1.080Å, di anddeindicates the scale axis. From the Figure.17 shows the total intermolecular interaction of 1,3-DNB molecule was observed at 100%. The various bonding contribution such as, C…C (8.9 %), C…H (4.3 %), C…N (0.5 %), C…O (2.1 %), H…C ( 3.4 %),H…H ( 2.9 %), H…N (1 %), H…O ( 27.2 %), N…C (0.5 %),N…H (1.4 %),N…O (3.3 %), O…C ( 1.5 %), O…H (30.9 %), O…N (2.9 %) and O…O (9.3%). The intermolecular interaction of H…O contribution was observed with 27.2 percentage. The highest intermolecular interaction of O…H contribution was observed with 30.9 percentage compared to other molecular contributions. Finally, it can be conclude that the two-dimensional fingerprint plot of the 1,3-DNB molecule helps for better understanding of the intermolecular interaction and the crystal packing structure. 5. Conclusion
Good quality single crystals of 1,3-DNB have been successfully grown by vertical Bridgman technique. The powder XRD analysis confirmed that the unit cell dimensions of the grown crystals are in good agreement with the reported literature. The presence of various stretching and bending vibrations of C-H, N=O, NO2 and C-N network was identified using the FTIR spectra. The transmittance spectrum (67%), cut-off wavelength and optical energy band
gap are found to be 431nm and 2.90eV respectively, which are considerable parameters for NLO applications. Photoluminescence spectral study exposed the electron excitation in the grown 1,3DNB crystal and the x,y coordinates of the studied organic crystal lies in the violet region in CIE 1931 diagram, which shows that it is suitable for violet OLED applications. Thermal stability of organic crystal was studied by TG-DTA analysis and it reveals that the compound decomposes above 100oC. The various mechanical parameters are deduced from Vickers microhardness study which explores the hardness nature of the grown crystal. Second harmonic generation (SHG) efficiency of 1,3-DNB is evaluated as 1.75 times that of KDP. Based on the above facts, it could be proposed that this material can be better placed for optical applications.
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FIGURE.1. As grown, cut and polished 1,3 DNB crystal
FIGURE.2. Powder XRD spectrum of the 1,3-DNB single crystal
FIGURE.3. FTIR spectrum of the 1,3-DNB single crystal
FIGURE. 4. Optical absorption spectrum of 1,3-DNB single crystal
FIGURE.5. Optical transmittance spectrum of 1,3-DNB crystal
IGURE.6. Tauc’s plot of the 1,3-DNB single crystal
FIGURE.7. Extinction coefficient (K) of 1,3-DNB single crystal
FIGURE.8. Reflectance spectrum of 1,3-DNB single crystal
FIGURE.9. Photoluminescence spectrum of 1,3-DNB single crystal
Figure.10. CIE 1931diagram of the 1,3-DNB single crystal
Figure.11. Thermal behavior of 1,3-DNB single crystal
Figure.12. Vickers microhardness spectra. (A) variation of HV vs load p (B) variation of HV vs diagonal d (C) variation of log p vs log d (D) variation of yield strngth vs load p (E) variation of stiffness constant C11 vs load p.
Figure.13. Optimized molecular structure of 1,3-DNB material
Figure.14.Molecular electrostatic potential map of 1,3-DMB
Figure.15.The graphical view of the 1,3-DNB
(a)
(b)
(c)
(d)
(e)
(f)
Figure.16. Hirshfeld surface plot of (a) none (b) dnorm, (c) de, (d) di, (e) curvedness and (f) shape index of 1,3-DNB.
Figure.17. The two dimensional finger print plot of 1,3-DNB molecule
TABLE.1. The various functional group of 1,3-DNB single crystal
Sl.No 1 2 3 4 5 6 7 8 9 10 11
Wavenumber (cm-1) 3031 2860 1714 1469 1303 1266 1157 948 852 778 617
Assignments Asymmetric stretching vibrations of C-H group symmetric stretching vibrations of C-H group C=O stretching vibration mode N=O antisymmetric vibration N=O symmetricstretching δ (O-H) C-C bending vibration bending vibration of C-N Asymmetric ring stretching Deformation of NO2 group C-O in plane deformation
Table.2. Microhardness parameters load p (g), Hv (kg/mm2), σy (GN/m2), C11×1014 (pa)
Load P (g) 10 20 30 40 50
Hv (kg/mm2) 6.5007 11.2842 13.5186 15.4542 16.8636
σy (GN/m2) 4.7406 8.2290 9.8585 11.2701 12.2979
C11×1014 (pa) 0.4548 1.1939 1.6379 2.0701 2.4117
Table.3. Optimized structural parameter of 1,3-DNB molecule
Bond Angle (degree) B3LYP (C2-C1-C6) 120.379 (C2-C1-H7) 119.809 (C6-C1-H7) 119.811 (C1-C2-C3) 118.807 (C1-C2-H8) 121.707 (C3-C2-H8) 119.486 (C2-C3-C4) 122.466 (C2-C3-N14) 118.973 (C4-C3-N14) 118.561 (C3-C4-C5) 117.074 (C3-C4-H9) 121.462 (C5-C4-H9) 121.465 (C4-C5-C6) 122.467 (C4-C5-N11) 118.561 (C6-C5-N11) 118.972 (C1-C6-C5) 118.807 (C1-C6-H10) 121.709 (C5-C6-H10) 119.484 (C5-N11-O12) 117.267 (C5-N11-O13) 117.447 (O12-N11-O13) 125.287 (C3-N14-O15) 117.447 (C3-N14-O16) 117.267 (O15-N14-O16) 125.286
XRD 120.567 119.693 119.741 118.297 120.876 120.827 123.513 119.054 117.433 115.724 122.111 122.166 123.403 118.376 118.219 118.462 120.754 120.784 117.806 118.314 123.880 117.686 118.040 124.271
Bond length (Å) B3LYP (C1-C2) 1.392 (C1-C6) 1.392 (C1-H7) 1.083 (C2-C3) 1.392 (C2-H8) 1.081 (C3-C4) 1.388 (C3-N14) 1.485 (C4-C5) 1.388 (C4-H9) 1.080 (C5-C6) 1.392 (C5-N11) 1.485 (C6-H10) 1.081 (N11-O12) 1.223 (N11-O13) 1.222 (N14-O15) 1.222 (N14-O16) 1.223
XRD 1.388 1.389 0.931 1.390 0.930 1.385 1.473 1.387 0.930 1.387 1.471 0.930 1.226 1.221 1.225 1.221
Table.4. Hypolarizability components of 1,3-DNB crystal
β components β in a.u. βxxx 0.004 βxxy -317.65 βxyy -0.011 βyyy 75.945 βxxz 0.002 βxyz 0.006 βyyz 0.003 βxzz 0.001 βyzz 46.795 βzzz 0.01 βtot 1.6789E-30e.s.u
Table.5. Selected second-order perturbation energies E (2) (Donor → Acceptor)
ED(i) (e)
Donor (i) C 1-C 2 C 1-C 2
σ π
1.97642
C 1-C 6 C 2-C 3 C 2-H 8
σ σ σ
1.97642 1.97669 1.97560
σ
1.97247
C 3-C 4 C 3-C 4 C C C C C C C C C
3-C 4-C 4-C 4-C 4-H 4-H 5-C 5-C 5-C
4 5 5 5 9 9 6 6 6
N 11 - O 13 N 14 - O 15 N 14 - O 15 LP ( 2) O 12 LP ( 2) O 12 LP ( 2) O 13 LP ( 2) O 15 LP ( 2) O 16
π π σ σ σ σ σ π π π π π π
1.62102
1.63826
1.97248
1.97038 1.63016
1.98579 1.98579 1.89703 1.89573
1.89704
Acceptor (j)
ED(j) (e)
E(2)(kcal mol-1)
0.11064 0.34446 0.35928 0.11064 0.02202
C 3 - N14 C 3-C 4 C 5-C 6 C 5 - N11 C 3-C 4 C 3-C 4 C 2-C 3 C 5 - N11 C 1-C 2 C 5-C 6 N 14 - O15 C 3 - N 14 C 5-C 6 N 11 - O 12 C 2-C 3 C 5-C 6 C 1-C 2 C 3-C 4 N 11 - O 13 LP ( 3) O 12 C 5-C 6 N 11 - O 13 LP ( 3) O 16 N 14 - O 15 C 5 - N 11 N 11 - O 13 C 5 - N 11 N 14 - O 16 C 3 - N 14
σ* π* π* σ* σ* σ* σ* σ* π* π* π* σ* σ* σ* σ* σ* π* π* π*
0.02224 0.11064 0.28656 0.35928 0.61389 0.11064 0.02224 0.05475 0.02224 0.02224 0.28656 0.34446 0.61389 1.44086 0.35928 0.61389 1.44086 0.61389 0.11064 0.05426 0.11064 0.05475 0.11064
4.24 19.33 24.46 4.24 4.71 4.55 4.68 4.09 21.61 17.73 22.5 4.09 4.68 2.01 4.37 4.37 15.8 24.18 25.72 12.7 4.06 7.44 12.7 7.44 12.97 18.84 13.17 19.03 12.97
N 14 - O 15
σ* 0.05426
18.84
π* π*
σ* σ* σ* σ* σ*
E(j)E(i) (a.u) 0.98 0.28 0.28 0.98 1.29 1.09 1.28 1 0.3 0.29 0.15 1 1.28 1.18 1.08 1.08 0.29 0.29 0.15 0.18 0.46 0.32 0.18 0.32 0.55 0.73 0.55 0.73 0.55
0.059 0.066 0.074 0.059 0.07 0.063 0.069 0.058 0.073 0.064 0.056 0.058 0.069 0.044 0.062 0.062 0.062 0.075 0.06 0.079 0.042 0.052 0.079 0.052 0.076 0.106 0.076 0.106 0.076
0.73
0.106
F(i,j) (a.u)
Highlights Organic 1,3-Dinitrobenzene single crystal have been grown by Bridgman method. The crystal shows good transparency in the visible region. CIE diagram indicates that the dominant emission color was located in the violet region. Nonlinear optical efficiency is found to be higher than that of KDP.
Author Contribution Section
All the authors are equal contribution in the manuscript.
1. R. Nagaraj – Crystal growth, characterization and written the manuscript and also manuscript correction (technical, grammar and scientific content) 2. K. Ramachandran – contribute to the TG - DTA study 3. K. Aravinth – Some of the characterizations done (XRD and SHG) 4. S. Ranjith – contribute to the theoretical study
Declaration of interests The authors declare that they have no conflict of interest.