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Experimental and theoretical studies of 2,5-dichloroanilinium picrate
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 53–62
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Experimental and theoretical studies of 2,5-dichloroanilinium picrate N. Sudharsana a, S. Muthunatesan b, G. Jasmine Priya a, V. Krishnakumar c, R. Nagalakshmi a,⇑ a
Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India Department of Physics, Government Arts College, Kumbakonam, India c Department of Physics, Periyar University, Salem 636 011, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The second harmonic generation
efficiency was found to be 0.8 times that of KDP. The first-order hyperpolarizability (b) was found to be 5.50321 1030 esu. NMR study confirms the molecular structure of the title compound.
a r t i c l e
i n f o
Article history: Received 17 July 2013 Received in revised form 2 October 2013 Accepted 9 October 2013 Available online 20 October 2013 Keywords: X-ray diffraction Solubility Growth from solutions Single crystal growth Organic compounds Vibrational spectroscopy
a b s t r a c t Organic 2,5-dichloroanilinium picrate crystal was grown by using slow evaporation solution technique. The lattice parameter was estimated by powder X-ray diffraction. The absence of absorption at around Nd:YAG fundamental wavelength was confirmed by ultraviolet–visible absorption study. The vibrational analyses confirm the various functional groups present in the grown crystal. The NMR study confirms the presence of chemical environment of hydrogen in the title crystal. The thermogravimetric (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) traces reveals the thermal stability of the compound. The second harmonic generation (SHG) of the crystal was confirmed by Kurtz Perry powder technique. The theoretical studies such as first-order hyperpolarizability (b), molecular orbitals, electronic excitation and electrostatic potential (ESP) were performed using Gaussian 03W software at HF/6-31G (d) level. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Organic and semiorganic materials are attracting a great deal of attention, as they have large optical susceptibilities, inherent ultrafast response time and good optical properties [1–4]. Organic materials have been found to exhibit second-harmonic generation (SHG) efficiencies that by far exceed those of inorganic materials. Picric acid (2,4,6-trinitrophenol) is an organic acid, which is used in the dyeing industry, munitions, bombs, rocket warheads and ⇑ Corresponding author. Tel.: +91 431 2503615; fax: +91 431 2500133. E-mail address: [email protected] (R. Nagalakshmi). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.047
as an explosive. The presence of three electron withdrawing nitro groups makes it as a good p-acceptor for neutral carrier donor molecules [5–7]. Moreover, it is well-known as TNP which is an organic nonlinear optical crystal by its shorter cut-off wavelength, optical quality, sufficiently large nonlinear coefficient, transparency in UV region and high damage threshold [8]. Among the organic materials picric acid draws much more attention because of its tendency to form salts or charge transfer molecular complexes with many organic compounds particularly with aromatic amines, aliphatic amines, aromatic hydrocarbons, etc. [9]. An alternative approach to obtain a large NLO response is to device molecular systems in which a charge transfer occurs between non
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covalently bound chromophores, (i.e.), by intermolecular or ‘through-space’ charge transfer [10]. Picric acid has an activating AOH group and deactivating nitro groups in its structure. It is known that picric acid acts not only as an acceptor to form various p stacking complexes with other aromatic molecules but also as an acidic ligand to form salts through specific electrostatic or hydrogen bond interactions [11]. Most of the complexes of picric acid encourage acentric packing which results in large hyperpolarizability (b) and remarkable second order NLO activity (v2) [12]. In our frame work of picrates, we have reported a new third order nonlinear optical material (hydroxyethylammonium picrate) [13]. Yoshio et al. performed ultraviolet–visible study to confirm the occurrence of charge transfer between 2,5-dichloroaniline and picric acid [14]. The structural information of 2,5-DCAP has been given by Meng et al. [15]. After that there are no reports in connection with the title compound in a full-fledged manner. In order to accomplish the complete investigations of the title compound, the authors have grown 2,5-DCAP and characterized using powder X-ray diffraction (XRD), UV–Visible spectroscopy, vibrational (Fourier Transform Infrared & Raman), Nuclear Magnetic Resonance (NMR), thermal analysis such as TG/DTA & DSC and second harmonic generation (Kurtz–Perry method). In addition, first-order hyperpolarizability, molecular orbitals (HOMO and LUMO), electrostatic potential, electronic excitation calculations and the simulation of IR and Raman spectra were also carried out on the optimized geometry using Gaussian 03W software with a view to have insights of the charge transfer mechanism. Prediction of Raman intensity The calculated Raman activities (Si) have been converted to relative Raman intensities (Ii) using the following relationship derived from the intensity theory of Raman scattering [16,17]:
Ii ¼ f ðm0 mi Þ4 Si =mi ½1 expðhcmi =K B TÞ
ð1Þ
where m0 is the exciting frequency in cm1, mi the vibrational wavenumber of the ith normal mode, h, c and KB are fundamental constants, and f is a suitably chosen common normalization factor for all peak intensities. The simulated Raman spectra have been plotted using pure Lorentizian band shape with a bandwidth of full width and half maximum (FW–HM) of 10 cm1. The HF/6-31G (d) calculated frequencies were scaled by 0.8953. Computational details The computational approach allows the determination of molecular NLO properties as an inexpensive way to design molecule by analyzing its potential before synthesis and to determine the higher order hyperpolarizability tensors of molecule [17]. The quantum chemical computation was performed at Hartree–Fock level using Gaussian 03W program package [18], invoking gradient geometry optimization. The geometry was optimized using the Hartree–Fock level of theory employing 6-31G (d) basis set. Optimized structure was confirmed to be minimum energy conformation. An optimization is complete when it has converged, i.e., when it has reached a minimum on the potential energy surface, thereby predicting the equilibrium structure of the molecule. This criterion is very important in geometry optimization [19]. The dipole moment (l), mean polarizability hai, and the total first static hyperpolarizability (btot) in terms of x, y, z components were given by following equations:
l ¼ l2x þ l2y þ l2z
1=2
hai ¼ 1=3ðaxx þ ayy þ azz Þ
ð2Þ ð3Þ
1=2 btot ¼ b2x þ b2y þ b2z 1=2
btot ¼ ½ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2
ð4Þ
The b components are converted to esu units from Gaussian output file (1 au = 8.3693 1033 esu). The electronic excitation of the title compound was calculated by CIS method keyword. This requests a calculation on excited states using single excitation. The number of excited states used in the calculation was ten (n = 10). Experimental work Synthesis and crystal growth 2,5-Dichloroanilinium picrate was synthesized by taking 2,5dichloroaniline and picric acid in 1:1 M ratio and dissolved in distilled water and heated at 40 °C to have a complete dissolution of the solute in distilled water. The resultant solution was filtered and kept undisturbed for slow evaporation at room temperature. The pH of the 2,5-DCAP solution was varied for 2.48, 3.81, 6.20, 10.93, 11.57 and 12. Above all, the pH value 2.48 yielded transparent crystal. The transparent yellow1 colored crystal was harvested after 2 months of evaporation. The pH of the solution after dissolving the grown crystal was found to be 2.43. The picture of the as-grown 2,5-DCAP crystal was shown in Fig. 1. The average size of grown crystal was found to be 9 4 2 mm3. Solubility Thermodynamically, the chemical potential of the pure solid is equal to the chemical potential of the same solute in the saturated solution. Also the growth rate of the crystal depends on its solubility and temperature. Hence solubility is one of the most important factors for bulk crystal growth [20]. The solubility of the synthesized 2,5-DCAP salt was checked in ethanol, methanol, toluene, acetone, benzene, acetonitrile and mixed solvents. The solubility was found to be moderate in water. The solubility was done for four temperatures (30 °C, 40 °C, 50 °C and 60 °C). Water was taken as a solvent and then the synthesized 2,5-DCAP salt was added to it little by little until a point where it gets supersaturated. The solution was stirred constantly using magnetic stirrer until it becomes fully soluble. The solution was poured into the Petridish. The solvent was completely evaporated by drying the solution. The amount of the salt present in the solution was measured by subtracting the empty Petridish weight. Following the same procedure, the amount of DCAP salt dissolved in water at 40 °C, 50 °C and 60 °C were determined. The solubility curve for 2,5-DCAP was shown in Fig. 2. It was observed from Fig. 2, that the solubility increases with rise in temperature. As a result, the solubility curve in the present study of 2,5-DCAP exhibits positive temperature coefficient. Results and discussion Powder X-ray diffraction and morphology studies The 2,5-DCAP crystal was subjected to powder XRD study using Panalytical 0xpert pro MPD X-ray diffractometer with Cu Ka = 1.5405 Å A radiation. The monoclinic crystal system was 1 For interpretation of color in Figs. 1, 9 and 10, the reader is referred to the web version of this article.
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Fig. 1. The picture of as-grown 2,5-DCAP crystal.
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Fig. 4a. UV–Vis spectra of 2,5-DCAP.
literature [15] and it is given Table S1. The optimized structure of the title compound is shown in Fig. 3. UV–Visible spectroscopy
Fig. 2. The solubility curve of 2,5-DCAP in water.
confirmed with the help of XRDA software. The XRD peaks were indexed using XRDA software was shown in Fig. S1 and the sharp peaks confirms the crystallinity of the sample. The experimental results are in good agreement with the previously reported
The optical absorption spectrum of the title compound was recorded using the PerkinElmer Lambda 35 spectrophotometer in the 190–1100 nm region as shown in Fig. 4a. The absorption at around 210 and 357 nm may be due to the p ? p⁄ and n ? p⁄ transitions taking place in the present complex. The chromophores (ANO2 group) present in the picric acid are responsible for the yellow colored nature of the crystal. The shift in absorption maximum to longer wavelength was due to the presence of auxochromes (ACl and ANH group) in the title compound. A good NLO material should be transparent in the optical as well as near IR region. The absence of absorption at around Nd:YAG fundamental wavelength 1064 nm indicates that the grown crystal is useful for second harmonic generation and it is completely transparent in the region between 470 nm and 1100 nm. Hence the crystal can resist laser irradiation. The predicted value at HF/6-31G (d) level was found to be 212 nm from Table 3. This excitation takes place between the orbitals composed of picrate moiety (HOMO 0) to 2,5-dichloroanilinium moiety (LUMO + 1). Hence the calculated value agree with the observed value of 210 nm. The calculated value did not support the measured excitation at 357 nm.
Fig. 3. Optimized geometry of 2,5-DCAP at HF/6-31G (d) basis set.
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Fig. 4b. (ahm)2 versus hm of 2,5-DCAP compound.
Fig. 4b shows the plot of (ahm)2 versus hm. The optical absorption coefficient (a) was calculated from the transmittance data by using the formula:
a ¼ 1=t lnð1=TÞ
ð5Þ
where t is the thickness of the crystal in mm, T is the transmittance. According to the Tauc relation [21] the absorption co-efficient a, of a crystalline solid obeys the following relation:
ðahmÞ ¼ Aðhm Eg Þ
r
ð6Þ
Eg is the optical band gap in eV, h is the Planck’s constant, A is the proportionality constant, hm is the photon energy and r is an exponent that characterizes the optical absorption process. For direct allowed transition, r = ½, for direct forbidden transition r = 3/2, for indirect allowed transition r = 2, and finally for indirect forbidden transition r = 3 [22]. The direct band gap of the crystal was found to be 5.7 eV from Fig. 4b. The band gap was determined by extrapolating the linear region near after the absorption edge to the energy axis. The band gap of the crystal indicates that the grown crystal can be used for optical applications. Fourier Transform Infrared (FTIR) and FT-Raman spectroscopic analyses The infrared spectrum of the pellet and FT-Raman spectrum have been recorded using Perkin Elmer spectrum one FTIR in the IR region 4000–400 cm1 and BRUKER RFS 27: Stand alone FT-Raman spectrometer in the region 50–4000 cm1, respectively. The recorded FTIR and FT-Raman spectra were shown in Figs. 5 and 6, respectively. For comparison, the theoretical IR and Raman spectra were simulated using Hartree Fock level of theory employing 6-31G (d) basis set. Experimentally recorded absorption wavenumbers and theoretically calculated harmonic frequencies along with their corresponding vibrational assignments were summarized in Table 1. The most prominent groups such as NHþ 3 and phenolic O confirms charge transfer in the title compound. Only those vibrations have been elaborated for the sake of understanding and the remaining vibrational assignments were shown in Table 1. In the charge transfer interactions, picric acid necessarily protonates the 2,5-dichloroaniline forming 2,5-DCAP. The NHþ 3 group gives rise to the six internal modes of vibrations. The sharp medium intense band observed at 3200 cm1 was assigned to the NHþ 3 asymmetric stretching mode (mas), whereas the symmetric 1 (ms) NHþ in 3 stretching mode of vibration appears at 3080 cm þ the FTIR spectrum. From Table 1, the mas NH3 was absent in the
Fig. 5. Comparison of the experimental and simulated IR spectrum of 2,5-DCAP: (a) FTIR spectrum in KBr; (b) IR spectrum calculated at HF/6-31G (d) level with scaling and (c) IR spectrum calculated at HF/6-31G (d) level without scaling.
Raman spectrum and the corresponding ms NHþ 3 was observed as a weak and shoulder peak at 3069 cm1 in the Raman spectrum. These vibrations were in agreement with the previously reported literature [13]. The NH2 group in 2,5-dichloroaniline has accepted þ a proton from picric acid to form salt C6 H2 O7 C6 H6 NCl2 so that þ NH3 stretching vibration was shifted to the lowest wavenumber region when compared to the uncharged various positional dichloroanilines (NH2) vibrations [23,25]. The asymmetric NHþ 3 deformation (das) vibration appears at 1611 cm1 and the corresponding symmetric (ds) NHþ 3 deformation vibration appears as a weak peak at 1541 cm1 in the FTIR spectrum. The das NHþ 3 arises as a weak peak at 1609 cm1 and the ds NHþ 3 vibration corresponds to the band at 1543 cm1 in the Raman spectrum. The medium intense and shoulder peak at 1099 cm1 in the FTIR spectrum and the weak band in the Raman spectrum at 1087 cm1 was attributed to the NHþ 3 rocking vibration. Here the agreement of the calculated wavenumber with the experimental data was fairly well for the NHþ 3 deformation vibrations and the wavenumbers showed quite good agreement with the reported literature of related compounds [23,24] as shown in Table 1. The vibrations belonging to the bond between the ring and the halogen atoms are worth to discuss here, since mixing of the vibrations were possible due to the lowering of the molecular symmetry and the presence of heavy atom on the periphery of molecule [26]. The mCACl stretching vibrations occur as a strong and a weak intense band at 710 and 460 cm1 in the FTIR spectrum, respectively and as a medium intense band at 710 cm1 in the Raman spectrum. The calculated IR wavenumber at HF level of this mode was in agreement with the experimental data (Table 1). The bending vibrations were observed in the region below 400 cm1. These
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benzene and its derivatives are observed in the region 1300– 1000 cm1 and 950–675 cm1, respectively [38–40]. The aromatic C@C stretching occurs at nearly 1650 and 1450 cm1 [41]. The mCAC vibrations of 2,5-DCAP was identified at 1611, 1541, 1508, and 1477 cm1 in the FTIR spectrum as a weak peak and the corresponding Raman spectrum exhibits peak at 1609, 1543 and 1486 cm1. In the present case, these CAH stretching, CAH inplane, CAH out-of-plane and CAC stretching vibrational frequencies were found to be well within their characteristic region. Nuclear Magnetic Resonance spectroscopy
Fig. 6. Comparison of the experimental and simulated Raman spectrum of 2,5DCAP: (a) observed FT-Raman spectrum; (b) Raman spectrum calculated at HF/631G (d) level with scaling and (c) Raman spectrum calculated at HF/6-31G (d) level without scaling.
bending vibrations were active in the predicted Raman spectrum and absent in the IR spectrum. The strong bands at 1333 cm1 and 1334(s) cm1 in FTIR and Raman spectra were assigned to the CAN stretching, respectively. The simulated Raman spectrum contains bands at 1347 and 1302 cm1 [27]. The picrate being the 2,4,6-trinitrophenolate () anion has characteristic bands of the deprotonated phenol, the bands of three NO2 groups and the bands of the six-membered aromatic ring [28]. The band at 1274 cm1 in FTIR and 1277 cm1 in FT-Raman spectrum was assigned to phenolic O vibration, m(CarAO) mode. The same wavenumber was assigned in some of the previously reported picrate compounds [10,13,24,29,30] for phenolic O vibration. The predicted Raman wavenumber was observed at 1258 cm1 and inactive in the IR spectrum for phenolic O vibration. Thus the absence of the OAH stretching vibration confirms the formation of the title compound. Usually for the free picric acid, NO2 vibration occurs at 1607 cm1 [31]. In 2,5-DCAP, weak band at 1508 cm1 and the bands at 1333 cm1 have been assigned to asymmetric (mas) and symmetric (ms) stretching modes of NO2 vibrations, respectively. The NO2 absorb at a slightly lower wavenumber at 1508 cm1 due to an increased electron density of the picric acid. The scissoring, wagging and rocking vibrations of NO2 group were identified at the characteristic frequencies and assigned [32–34]. The masCAH vibration appears as a medium peak in FTIR spectrum at 3080 cm1 and the corresponding Raman wavenumber appears as a weak band at 3069 cm1 which was in good agreement with the calculated wavenumber at 3044 cm1 in IR spectrum [34– 37]. The CAH symmetric stretching vibration appears at 2809 cm1 in FTIR spectrum and at 2840 cm1 in simulated Raman spectrum. The aromatic CAH in-plane and out-of-plane bending modes of
Nuclear Magnetic Resonance (NMR) is a very versatile nondestructive technique employed in the identification of organic compounds [42]. 2,5-DCAP molecular structure was confirmed by NMR study. The 1H NMR spectrum was recorded by dissolving the sample in DMSO solvent measured using the instrument 500 MHz FT NMR spectrometer BRUKER AV III and the obtained spectrum was shown in Fig. 7. The chemical shifts for 1H NMR is represented in d ppm. The dimethyl sulfoxide-d6 solvent peak appears at d = 2.5 ppm. The intense singlet peak at d = 8.597 ppm confirms the presence of two equivalent protons in the picric acid. The 2,5-dichloroaniline (2,5-DCA) shows three different proton peaks due to different chemical environment. The appearance of doublet signal at d = 6.834 and 6.830 ppm has been assigned to H2 aromatic proton attached to C4 carbon atom in 2,5-DCA. Another doublet signal observed at d = 7.199 and 7.182 ppm was due to the presence of H1 aromatic proton attached to C3 atom (see Fig. 3). The quartet signal present in the region d = 6.552–6.573 ppm has been assigned to H3 aromatic proton attached to C6 atom. The broad peak noticed at d = 5.633 ppm confirms the formation of title compound by the process of protonation of 2,5-dichloroaniline from picric acid as NAH O linkage. The disappearance of AOH peak at 11.94 ppm and appearance of new broad peak at d = 5.633 ppm indicates the presence of picric acid in deprotonated state [31]. Thermal study (TG/DTA and DSC) TG/DTA was carried out between 30 °C and 950 °C in the nitrogen atmosphere at a heating rate of 40 °C/min using thermal analyzer (TG/DTA6200). The obtained TG/DTA curve was shown in Fig. 8a. The DSC was carried out in the range of 30–500 °C in the argon atmosphere at a heating rate of 10 °C/min using NETZSCH DSC 204. The weight change versus temperature curve of TG so obtained gives information about the thermal stability and composition of the original sample. DTA provides information regarding endothermic exothermic effects [43]. DSC was used to study the thermal behavior of crystals. The endothermic peak observed at 125 °C in DTA was assigned to the melting point of the 2,5-DCAP which corresponds to the sharp endothermic peak in DSC curve (Fig. 8b). The enthalpy change is the area under the peak in DSC curve. The enthalpy of melting was found to be 76.86 J/g. The sharp peak indicates that there was no phase transition below melting point. There was no weight loss between room temperature to 171 °C in TG curve. The 2,5-DCAP undergoes two stages of decomposition. The first weight loss (98) in TG was observed from 172 °C to 296 °C. This major weight loss coincides exactly with exothermic peak (278 °C) of DTA curve. This may be due to the liberation of volatile substances, ammonia and nitrogen dioxide gases. The second weight loss occurs between 296 °C and 378 °C in TG curve which matches with the second exothermic peak in DTA graph. This may be due to the evolution of chlorine and hydrocarbon gases followed by ring rupture. After this temperature the crystal decomposes completely. Hence 2,5-DCAP is stable up to 171 °C
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Table 1 The observed FTIR, FT-Raman and calculated frequencies using HF/6-31G (d), along with their relative intensities and probable assignments. Observed wavenumber (cm1)
IR calculated wavenumber (cm1)
Raman calculated wavenumber (cm1)
Band assignment
FTIR
FT-Raman
Unscaled value
Scaled value
Unscaled value
Scaled value
3200 3080 2809 – – 1611 1541 1508 1477 1423 1363 1333 1274 – 1164 1099 1044 921 909 883 823 790 744 710 584 545
– 3069 – – – 1609 1543 – 1486 – 1365 1334 1277, 1215 1165 1087 1044 943 – – 824 – – 710 586 532
3684 3397 2840 1752 1731 1615 1570 – – 1364 – 1299 1228 1174 1065 1040 947 903 861 824 778 – – 589 552
3296 3044 2544 – – 1675 1570 – 1484 1443 – – – 1221 1167 1085 – 930 – – – 773 742 697 – –
3686 3460 2838 1766 1730 1615 1588 1515 – 1426 1362 1344 1299 1209 1174 1065 1039 932 903 – – 769 – – 587 –
– 3402 2840 1772 1693 1613 1586 – – – 1365 1302 1258 1213 1135 1062 1038 928 900 – – 771 – – 591 –
mas NHþ3 masCAH, mss NHþ3 msCAH mC@C mC@C das NHþ 3 , mC@C mCAN, ds NHþ3 , mC@C mCAC, ds NHþ3 , masNO2 bCAH, ds NHþ 3 , mC@C
517 460
– –
499 436
529 447
504 435
510 437
– – – – – –
– 362 331 246 170 119
408 – – – – –
392 – – – – –
407 354 328 218 148 75
407 394 358 217 150 77
sNHþ3 , dipCACAC dipCACAC, mCACl, x NHþ 3 c(CCC), mCACl s(CCCC)
–
–
–
–
–
46
bCAH msNO2 mCANH3, msNO2 m(CAO) phenolic, b(CH) b(CH) b(CH), bCAN qNHþ3 , b(CH) m(CN), b(CH), qNHþ3 d(CN), cCAH mCANO2, bCACAC, cCAH cNAH, cCAH, dNO2 cCAH cCH, xNO2 cCH, xNO2 sring, cCAC, mCACl, cCH bCAO, bCACAC q NHþ3 , dipCACAC, q(NO2)
b(CCl) b(CCCl) s(CCCCl) x NHþ3 c(CCl)
Abbreviation used ms – symmetric stretching; mas – asymmetric stretching; b – in plane bending; c – out of plane bending; q – rocking; d – scissoring; m – stretching; x – wagging, and s – torsion.
Fig. 7. 1H NMR spectrum of 2,5-DCAP.
and hence this crystal can be used for NLO application up to this temperature. First order hyperpolarizabilities (b) Molecules with p electron conjugation incorporating electron donor and acceptor groups to influence the asymmetric polarization
are the essential candidates for NLO applications such as frequency doubling [44]. Table 2 shows the linear and non-linear polarization of 2,5-dichloroanilinium picrate system calculated using Gaussian 03W employing 6-31G (d) basis set. The third rank tensor of first order hyperpolarizability (b) is described by a 3 3 3 matrix. The matrix is reduced to 10 components due to Kleinman symmetry [45]. In this study, the calculations were
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Fig. 8a. TG/DTA of 2,5-DCAP.
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performed on isolated molecules and possible intermolecular interaction was not taken into account. A large value of b is the first requirement for high NLO efficiency in a molecular material. The effect of intermolecular hydrogen bonding in the titled crystal would enhance the magnitude of b quantities. The hydrogen bond interactions play an important role in the NLO response. The NLO property is not only originated from the constituent individual molecule but also is influenced by the intermolecular interactions [46]. From Table 2, it was suggested that the title compound was polar having non-zero dipole moment, polarizability and hyperpolarizability. The static first-order hyperpolarizability (b) output was calculated using Eq. (4) from ten components to be 5.50321 1030 esu, which was nearly 9 times that of urea (0.6 1030 esu). The polarizability (atotal) and dipole moment (ltot) was calculated to be 2.39 1023 esu and 12.5815 D, respectively. It was noticed that in bxxx (which is the principal dipole moment axis and it is parallel to the charge transfer axis) direction, the biggest value of b was noticed and subsequently delocalization of electron cloud is more in that direction. The maximum value may be due to electron cloud movement from donor to acceptor which makes the molecule highly polarized and the intermolecular charge transfer possible which was also confirmed by vibrational spectral study. Molecular orbitals
Fig. 8b. DSC spectrum of 2,5-DCAP.
Table 2 Electric dipole moment (l), polarizability (a) and first-order hyperpolarizability (btotal) of 2,5-DCAP.
lx ly lz ltot Debye axx ayy axx axy axz ayz atotal 1023 esu bxxx bxxy bxyy byyy bzxx bxyz bzyy bxzz byzz bzzz btotal 1030 esu
11.9575 3.8337 0.7838 12.5815 246.9530156 5.8212833 232.7082 1.7729797 1.9670065 66.9894639 2.39 981.653 132.759 413.04 242.29 140.12 4.381 10.21 39.54 18.96 9.38 5.50321
The difference of the energies of the HOMO and LUMO, termed the band gap, can sometimes serve as a measure of the excitability of the molecule: The smaller the energy the more easily it will be excited. The HOMO is the orbital that could act as an electron donor, since it is the outermost (highest energy) orbital containing electrons. The LUMO is the orbital that could act as the electron acceptor, since it is the innermost (lowest energy) orbital that has the room to accept electrons. A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as soft molecule [47]. In Fig. 9 the red atomic orbital lobes are positive phases and the green atomic orbitals are negative phases. The HOMO–LUMO gap value of the 2,5-DCAP compound was found to be 0.378 au The HOMO ? LUMO transition and the small HOMO–LUMO energy gap explains the probable intermolecular charge transfer (ICT) taking place from 2,5-dichloroaniline electron donor group to the picric acid electron acceptor group to form 2,5-dichloroanilinium picrate charge transfer complex. The HOMO and LUMO plots were shown in Fig. 9(a) and (f), respectively. The HOMO (Fig. 9(a)) was largely delocalized over electron acceptor phenoxy (ACO) and NO2 group of picrate anion. The LUMO (Fig. 9(f)) was delocalized over NO2 and CAN group of picrate anion. Electronic excitation mechanism and electrostatic potential The static polarizability value is proportional to the optical intensity and inversely proportional to the cube of transition energy. With this concept, larger oscillator strength (fn) and Dlgn with lower transition energy (Egn) is favorable to obtain large first static polarizability values. Electronic excitation energies, oscillator strength and nature of the respective excited states were calculated by the closed-shell singlet calculation method and are summarized in Table 3. It was clearly visible from Table 3, only three excited bands have larger value of oscillator strengths. The 1st and 4th state: It mainly arises from the excitation from HOMO 0 to LUMO + 0 (Fig. 9(a) and (f)). The relevant orbitals were mainly composed of picrate group. Charge transfer happens within picrate ion, which corresponds to the transitions from the orbital
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Fig. 9. Representation of the most important orbital involved in the main electronic transition for 2,5-DCAP crystal (a) HOMO – 0, (b) HOMO – 11, (c) HOMO – 12, (d) HOMO – 13, (e) HOMO – 17, (f) LUMO + 0, (g) LUMO + 1, (h) LUMO + 3, and (i) LUMO + 8.
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N. Sudharsana et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 53–62 Table 3 Computed absorption wavelengths (kng), energy (Eng), oscillator strength (fn) and its major contribution state. S. no.
Excitation wavelength, kng (nm)
Excitation energy (eV)
Oscillator strength (f)
Assignments
1 2 3 4 5 6 7 8
252.31 248.48 241.28 241.01 234.69 228.32 226.94 212.53
4.9141 4.9897 5.1386 5.1443 5.2828 5.4303 5.4632 5.8338
0.1189 0.0316 0.0143 0.0811 0.0151 0.0003 0.0010 0.3711
HOMO – 0 ? LUMO + 0 HOMO 12 ? LUMO + 3 HOMO 11 ? LUMO + 8 HOMO 0 ? LUMO + 0 HOMO 13 ? LUMO + 3 HOMO 17 ? LUMO + 0 HOMO 17 ? LUMO + 0 HOMO 0 ? LUMO + 1
composed of electron acceptor (CarAO) group of picrate anion to that of aromatic ring (picrate anion). The 8th state: It was due to the excitation from HOMO 0 to LUMO + 1 (Fig. 9(a) and (g)). This was a major contribution state when compared to other states. The correspondent transition was between the orbital composed of amine group NHþ to that of phenoxy group (CarAO) which 3 was responsible for electron-donor acceptor-complex (2,5-dichloroanilinium picrate) formation [48,49]. The ESP surface was shown þ in Fig. 10 with different views. The NHþ 3 and CH group of C6 NH6 Cl2 (2,5-dichloroanilinium) cation, the ESP values were positive (blue color) whereas the most negative ESP values (red color) was located on NO2 and CArAO groups of C6 H2 N3 O 7 (picrate) anion. Second harmonic generation (SHG) The Kurtz–Perry powder technique remains a particularly valuable tool for testing of materials for second harmonic generation. A Q-switched Nd:YAG laser beam of wavelength 1064 nm, with 7 nm pulse length and 10 Hz repetition rate was allowed to strike the sample. 2,5-DCAP was prepared with a grain size of 40–60 lm. KDP powder with similar grain size was taken as reference. The generation of the SHG was confirmed by the emission of green light which was 0.8 times that of reference KDP. A comparison
Table 4 A comparison of relative powder SHG efficiency and first-order hyperpolarizability (b) for some of the picrate complexes. S. no.
Compound name
Powder SHG(KDP)
b(urea) in 1030 esu
1
Imidazole–imidazolium picrate monohydrate [50]
3.6
–
2
L-prolinium
3
L-tryptophanium
52
–
122
–
4 5
picrate [51] Adenosinium picrate [52] Histidinium dipicrate dihydrate [53]
0.25 2.5
– 10.54
6
L-threoninium
43
–
46
–
– 66.5
20 96
7 8 9
picrate [12]
picrate [54] 1,3-Dimethylurea dimethylammonium picrate [55] Naphthalene picrate [56]
L-asparaginium
picrate (LASP) [57,58]
10
L-leucine L-leucinium
11
L-glutamine
12
L-valinium
13 14 15 16
picrate [59]
picrate [60]
picrate [61] 2-Methylimidazolinium picrate [62] Dimethylammonium picrate [63] 2,4,6-Trinitrophenol (TNP) [8] 2,5-Dichloroanilinium picrate [present work]
1.5
–
–
27
144
36
2 2 13.4 0.8
– – – 9
b – First order hyperpolarizability, SHG – second harmonic generation, and KDP – potassium dihydrogen phosphate.
table of relative powder SHG efficiency and ‘b’ vector part for some of known picrate complexes was collected in Table 4 [50–63]. The SHG efficiency for the title compound is smaller than those obtained in case of other picrate related compounds except for adenosinium picrate as mentioned in Table 4. The SHG is 0.06 times that of parent picric acid. ‘b’ value changes with the type of method and basis set used in calculation. The proton donor (AOH) group of picric acid and proton acceptor amine (ANH) group of 2,5-dichloroaniline in the structure provide a convenient infrastructure to introduce the charge asymmetry which is required for second order nonlinearity. Conclusions The molecular charge transfer complex 2,5-DCAP was synthesized and single crystals were grown by slow evaporation solution growth technique. It was found that the crystal belongs to the monoclinic crystal system from the powder XRD study. UV–Vis study reveals that the crystal was transparent in the region 470 and 1100 nm. Molecular structure was confirmed by NMR spectral analysis and the functional groups were identified by vibrational spectral analysis. The calculated wavenumbers were compared with the observed one. Thermal behavior and stability of the crystal were studied using TG/DTA and DSC techniques. The response of SHG was found to be 0.8 times that of standard KDP. The calculated hyperpolarizability (b) and dipole moment (l) was found to be 5.50321 1030 esu and 12.5815 D, respectively. The HOMO and LUMO orbitals and electrostatic potential confirms nature of the charge transfer within the molecule. Acknowledgments
Fig. 10. The calculated electrostatic potential of 2,5-DCAP. The contour electron density isovalue is 0.0004.
The author R.N. is thankful to Council of Scientific and Industrial Research (Sanction No: 03(1158)/10/EMR-II), New Delhi for the financial assistance under major research project and N.S. is thankful to CSIR for awarding JRF in this project. The authors are grateful to thank Dr. R. Justin Joseyphus, Department of Physics, National Institute of Technology Tiruchirappalli, for providing thermal
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(DTA/TG) facilities. The authors are thankful to Mr. Nilesh Kulkarni, TIFR, Department of Condensed Matter Physics and Material Science for powder XRD measurement. The authors are also thankful to SAIF, IIT Madras for the FTIR, FT-Raman, FT-NMR and DSC measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.10.047. References [1] S. Dhanuskodi, P.A.A. Mary, J. Cryst. Growth 253 (2003) 424–428. [2] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, J. Wiley and Sons Inc., New York, 1991. [3] N. Vijayan, R. Ramesh Babu, M. Gunasekaran, R. Gopalakrishnan, R. Kumaresan, P. Ramasamy, C.W. Lan, J. Cryst. Growth 249 (2003) 309–315. [4] N. Vijayan, R. Ramesh Babu, M. Gunsekaran, R. Gopalakrishnan, P. Ramasamy, J. Cryst. Growth 256 (2003) 174–182. [5] P.G. Farrel, F. Terrier, R. Schaal, Tetrahedron. Lett. 26 (1985) 2435–2438. [6] G.C. Franchini, A. Marchetti, L. Tassi, G. Tosi, J. Chem. Soc. Faraday Trans. 184 (1988) 4427–4438. [7] M.A.F. Elmosallamy, Anal. Sci. 20 (2004) 285–290. [8] P. Srinivasan, M. Gunasekaran, T. Kanagasekaran, R. Gopalakrishnan, J. Cryst. Growth 289 (2006) 639–646. [9] R. Subramaniyan @ Raja, G. Anandha Babu, P. Ramasamy, J. Cryst. Growth 334 (2011) 159–164. [10] S.R. Dominos, P.S. Pereira Silva, et al., J. Chem. Phys. 136 (2012) 134501– 134509. [11] R.C. Maurya, P. Sharma, S. Roy, Synth. React. Inorg. Metal. Org. Chem. 33 (2003) 683–698. [12] T. Uma Devi, N. Lawrence, R. Ramesh Babu, K. Ramamurthi, J. Cryst. Growth 310 (2008) 116–123. [13] N. Sudharsana, B. Keerthana, R. Nagalakshmi, V. Krishnakumar, L. Guru Prasad, Mater. Chem. Phys. 134 (2012) 736–746. [14] Y. Matsunaga, G. Saito, N. Sakai, Bull. Chem. Soc. Jpn. 47 (1974) 2873–2874. [15] G.X. Meng, H.L. Yang, C.X. Cheng, X.T. Huang, Acta Crystallogr. C 66 (2010) 0337–0340. [16] G. Keresztury, S. Holly, G. Besenyei, J. Varga, A. Wang, J.R. Durig, Spectrochim. Acta A 49A (1993) 2007–2026. [17] G. Keresztury, J.M. Chalmers, P.R. Griffith (Eds.), Raman Spectroscopy: Theory, in Hand Book of Vibrational Spectroscopy, John Wiley & Sons Ltd., New York, 2002. [18] M.J. Frisch et al., GAUSSIAN 98, Revision A.7, Gaussian, Inc., Pittsburgh, PA, 1998. [19] A.D. Becke, J. Chem. Phys. 98 (1999) 5648–5652. [20] T. Arumanayagam, P. Murugakoothan, J. Cryst. Growth 362 (2013) 304–307. [21] J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974. pp. 159–220. [22] M.A. Gaffar, A. Abu El-Fadl, S. Bin Anooz, Physica B 327 (2003) 43–54. [23] N. Sundaraganesan, J. Karpagam, S. Sebastian, J.P. Cornard, Spectrochim. Acta A 73 (2009) 11–19. [24] V.V. Ghazaryan, M. Fleck, A.M. Petrosyan, J. Mol. Struct. 1015 (2012) 51–55. [25] R.K. Goel, S.K. Gupta, S.N. Sharma, Pramana 11 (1978) 541–546. [26] R.A. Yadav, I.S. Singh, Ind. J. Pure Appl. Phys. 23 (1985) 626–627.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Experimental and theoretical studies of 2,5-dichloroanilinium picrate N. Sudharsana a, S. Muthunatesan b, G. Jasmine Priya a, V. Krishnakumar c, R. Nagalakshmi a,⇑ a
Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India Department of Physics, Government Arts College, Kumbakonam, India c Department of Physics, Periyar University, Salem 636 011, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The second harmonic generation
efficiency was found to be 0.8 times that of KDP. The first-order hyperpolarizability (b) was found to be 5.50321 1030 esu. NMR study confirms the molecular structure of the title compound.
a r t i c l e
i n f o
Article history: Received 17 July 2013 Received in revised form 2 October 2013 Accepted 9 October 2013 Available online 20 October 2013 Keywords: X-ray diffraction Solubility Growth from solutions Single crystal growth Organic compounds Vibrational spectroscopy
a b s t r a c t Organic 2,5-dichloroanilinium picrate crystal was grown by using slow evaporation solution technique. The lattice parameter was estimated by powder X-ray diffraction. The absence of absorption at around Nd:YAG fundamental wavelength was confirmed by ultraviolet–visible absorption study. The vibrational analyses confirm the various functional groups present in the grown crystal. The NMR study confirms the presence of chemical environment of hydrogen in the title crystal. The thermogravimetric (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) traces reveals the thermal stability of the compound. The second harmonic generation (SHG) of the crystal was confirmed by Kurtz Perry powder technique. The theoretical studies such as first-order hyperpolarizability (b), molecular orbitals, electronic excitation and electrostatic potential (ESP) were performed using Gaussian 03W software at HF/6-31G (d) level. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Organic and semiorganic materials are attracting a great deal of attention, as they have large optical susceptibilities, inherent ultrafast response time and good optical properties [1–4]. Organic materials have been found to exhibit second-harmonic generation (SHG) efficiencies that by far exceed those of inorganic materials. Picric acid (2,4,6-trinitrophenol) is an organic acid, which is used in the dyeing industry, munitions, bombs, rocket warheads and ⇑ Corresponding author. Tel.: +91 431 2503615; fax: +91 431 2500133. E-mail address: [email protected] (R. Nagalakshmi). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.047
as an explosive. The presence of three electron withdrawing nitro groups makes it as a good p-acceptor for neutral carrier donor molecules [5–7]. Moreover, it is well-known as TNP which is an organic nonlinear optical crystal by its shorter cut-off wavelength, optical quality, sufficiently large nonlinear coefficient, transparency in UV region and high damage threshold [8]. Among the organic materials picric acid draws much more attention because of its tendency to form salts or charge transfer molecular complexes with many organic compounds particularly with aromatic amines, aliphatic amines, aromatic hydrocarbons, etc. [9]. An alternative approach to obtain a large NLO response is to device molecular systems in which a charge transfer occurs between non
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N. Sudharsana et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 53–62
covalently bound chromophores, (i.e.), by intermolecular or ‘through-space’ charge transfer [10]. Picric acid has an activating AOH group and deactivating nitro groups in its structure. It is known that picric acid acts not only as an acceptor to form various p stacking complexes with other aromatic molecules but also as an acidic ligand to form salts through specific electrostatic or hydrogen bond interactions [11]. Most of the complexes of picric acid encourage acentric packing which results in large hyperpolarizability (b) and remarkable second order NLO activity (v2) [12]. In our frame work of picrates, we have reported a new third order nonlinear optical material (hydroxyethylammonium picrate) [13]. Yoshio et al. performed ultraviolet–visible study to confirm the occurrence of charge transfer between 2,5-dichloroaniline and picric acid [14]. The structural information of 2,5-DCAP has been given by Meng et al. [15]. After that there are no reports in connection with the title compound in a full-fledged manner. In order to accomplish the complete investigations of the title compound, the authors have grown 2,5-DCAP and characterized using powder X-ray diffraction (XRD), UV–Visible spectroscopy, vibrational (Fourier Transform Infrared & Raman), Nuclear Magnetic Resonance (NMR), thermal analysis such as TG/DTA & DSC and second harmonic generation (Kurtz–Perry method). In addition, first-order hyperpolarizability, molecular orbitals (HOMO and LUMO), electrostatic potential, electronic excitation calculations and the simulation of IR and Raman spectra were also carried out on the optimized geometry using Gaussian 03W software with a view to have insights of the charge transfer mechanism. Prediction of Raman intensity The calculated Raman activities (Si) have been converted to relative Raman intensities (Ii) using the following relationship derived from the intensity theory of Raman scattering [16,17]:
Ii ¼ f ðm0 mi Þ4 Si =mi ½1 expðhcmi =K B TÞ
ð1Þ
where m0 is the exciting frequency in cm1, mi the vibrational wavenumber of the ith normal mode, h, c and KB are fundamental constants, and f is a suitably chosen common normalization factor for all peak intensities. The simulated Raman spectra have been plotted using pure Lorentizian band shape with a bandwidth of full width and half maximum (FW–HM) of 10 cm1. The HF/6-31G (d) calculated frequencies were scaled by 0.8953. Computational details The computational approach allows the determination of molecular NLO properties as an inexpensive way to design molecule by analyzing its potential before synthesis and to determine the higher order hyperpolarizability tensors of molecule [17]. The quantum chemical computation was performed at Hartree–Fock level using Gaussian 03W program package [18], invoking gradient geometry optimization. The geometry was optimized using the Hartree–Fock level of theory employing 6-31G (d) basis set. Optimized structure was confirmed to be minimum energy conformation. An optimization is complete when it has converged, i.e., when it has reached a minimum on the potential energy surface, thereby predicting the equilibrium structure of the molecule. This criterion is very important in geometry optimization [19]. The dipole moment (l), mean polarizability hai, and the total first static hyperpolarizability (btot) in terms of x, y, z components were given by following equations:
l ¼ l2x þ l2y þ l2z
1=2
hai ¼ 1=3ðaxx þ ayy þ azz Þ
ð2Þ ð3Þ
1=2 btot ¼ b2x þ b2y þ b2z 1=2
btot ¼ ½ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2
ð4Þ
The b components are converted to esu units from Gaussian output file (1 au = 8.3693 1033 esu). The electronic excitation of the title compound was calculated by CIS method keyword. This requests a calculation on excited states using single excitation. The number of excited states used in the calculation was ten (n = 10). Experimental work Synthesis and crystal growth 2,5-Dichloroanilinium picrate was synthesized by taking 2,5dichloroaniline and picric acid in 1:1 M ratio and dissolved in distilled water and heated at 40 °C to have a complete dissolution of the solute in distilled water. The resultant solution was filtered and kept undisturbed for slow evaporation at room temperature. The pH of the 2,5-DCAP solution was varied for 2.48, 3.81, 6.20, 10.93, 11.57 and 12. Above all, the pH value 2.48 yielded transparent crystal. The transparent yellow1 colored crystal was harvested after 2 months of evaporation. The pH of the solution after dissolving the grown crystal was found to be 2.43. The picture of the as-grown 2,5-DCAP crystal was shown in Fig. 1. The average size of grown crystal was found to be 9 4 2 mm3. Solubility Thermodynamically, the chemical potential of the pure solid is equal to the chemical potential of the same solute in the saturated solution. Also the growth rate of the crystal depends on its solubility and temperature. Hence solubility is one of the most important factors for bulk crystal growth [20]. The solubility of the synthesized 2,5-DCAP salt was checked in ethanol, methanol, toluene, acetone, benzene, acetonitrile and mixed solvents. The solubility was found to be moderate in water. The solubility was done for four temperatures (30 °C, 40 °C, 50 °C and 60 °C). Water was taken as a solvent and then the synthesized 2,5-DCAP salt was added to it little by little until a point where it gets supersaturated. The solution was stirred constantly using magnetic stirrer until it becomes fully soluble. The solution was poured into the Petridish. The solvent was completely evaporated by drying the solution. The amount of the salt present in the solution was measured by subtracting the empty Petridish weight. Following the same procedure, the amount of DCAP salt dissolved in water at 40 °C, 50 °C and 60 °C were determined. The solubility curve for 2,5-DCAP was shown in Fig. 2. It was observed from Fig. 2, that the solubility increases with rise in temperature. As a result, the solubility curve in the present study of 2,5-DCAP exhibits positive temperature coefficient. Results and discussion Powder X-ray diffraction and morphology studies The 2,5-DCAP crystal was subjected to powder XRD study using Panalytical 0xpert pro MPD X-ray diffractometer with Cu Ka = 1.5405 Å A radiation. The monoclinic crystal system was 1 For interpretation of color in Figs. 1, 9 and 10, the reader is referred to the web version of this article.
N. Sudharsana et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 53–62
Fig. 1. The picture of as-grown 2,5-DCAP crystal.
55
Fig. 4a. UV–Vis spectra of 2,5-DCAP.
literature [15] and it is given Table S1. The optimized structure of the title compound is shown in Fig. 3. UV–Visible spectroscopy
Fig. 2. The solubility curve of 2,5-DCAP in water.
confirmed with the help of XRDA software. The XRD peaks were indexed using XRDA software was shown in Fig. S1 and the sharp peaks confirms the crystallinity of the sample. The experimental results are in good agreement with the previously reported
The optical absorption spectrum of the title compound was recorded using the PerkinElmer Lambda 35 spectrophotometer in the 190–1100 nm region as shown in Fig. 4a. The absorption at around 210 and 357 nm may be due to the p ? p⁄ and n ? p⁄ transitions taking place in the present complex. The chromophores (ANO2 group) present in the picric acid are responsible for the yellow colored nature of the crystal. The shift in absorption maximum to longer wavelength was due to the presence of auxochromes (ACl and ANH group) in the title compound. A good NLO material should be transparent in the optical as well as near IR region. The absence of absorption at around Nd:YAG fundamental wavelength 1064 nm indicates that the grown crystal is useful for second harmonic generation and it is completely transparent in the region between 470 nm and 1100 nm. Hence the crystal can resist laser irradiation. The predicted value at HF/6-31G (d) level was found to be 212 nm from Table 3. This excitation takes place between the orbitals composed of picrate moiety (HOMO 0) to 2,5-dichloroanilinium moiety (LUMO + 1). Hence the calculated value agree with the observed value of 210 nm. The calculated value did not support the measured excitation at 357 nm.
Fig. 3. Optimized geometry of 2,5-DCAP at HF/6-31G (d) basis set.
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Fig. 4b. (ahm)2 versus hm of 2,5-DCAP compound.
Fig. 4b shows the plot of (ahm)2 versus hm. The optical absorption coefficient (a) was calculated from the transmittance data by using the formula:
a ¼ 1=t lnð1=TÞ
ð5Þ
where t is the thickness of the crystal in mm, T is the transmittance. According to the Tauc relation [21] the absorption co-efficient a, of a crystalline solid obeys the following relation:
ðahmÞ ¼ Aðhm Eg Þ
r
ð6Þ
Eg is the optical band gap in eV, h is the Planck’s constant, A is the proportionality constant, hm is the photon energy and r is an exponent that characterizes the optical absorption process. For direct allowed transition, r = ½, for direct forbidden transition r = 3/2, for indirect allowed transition r = 2, and finally for indirect forbidden transition r = 3 [22]. The direct band gap of the crystal was found to be 5.7 eV from Fig. 4b. The band gap was determined by extrapolating the linear region near after the absorption edge to the energy axis. The band gap of the crystal indicates that the grown crystal can be used for optical applications. Fourier Transform Infrared (FTIR) and FT-Raman spectroscopic analyses The infrared spectrum of the pellet and FT-Raman spectrum have been recorded using Perkin Elmer spectrum one FTIR in the IR region 4000–400 cm1 and BRUKER RFS 27: Stand alone FT-Raman spectrometer in the region 50–4000 cm1, respectively. The recorded FTIR and FT-Raman spectra were shown in Figs. 5 and 6, respectively. For comparison, the theoretical IR and Raman spectra were simulated using Hartree Fock level of theory employing 6-31G (d) basis set. Experimentally recorded absorption wavenumbers and theoretically calculated harmonic frequencies along with their corresponding vibrational assignments were summarized in Table 1. The most prominent groups such as NHþ 3 and phenolic O confirms charge transfer in the title compound. Only those vibrations have been elaborated for the sake of understanding and the remaining vibrational assignments were shown in Table 1. In the charge transfer interactions, picric acid necessarily protonates the 2,5-dichloroaniline forming 2,5-DCAP. The NHþ 3 group gives rise to the six internal modes of vibrations. The sharp medium intense band observed at 3200 cm1 was assigned to the NHþ 3 asymmetric stretching mode (mas), whereas the symmetric 1 (ms) NHþ in 3 stretching mode of vibration appears at 3080 cm þ the FTIR spectrum. From Table 1, the mas NH3 was absent in the
Fig. 5. Comparison of the experimental and simulated IR spectrum of 2,5-DCAP: (a) FTIR spectrum in KBr; (b) IR spectrum calculated at HF/6-31G (d) level with scaling and (c) IR spectrum calculated at HF/6-31G (d) level without scaling.
Raman spectrum and the corresponding ms NHþ 3 was observed as a weak and shoulder peak at 3069 cm1 in the Raman spectrum. These vibrations were in agreement with the previously reported literature [13]. The NH2 group in 2,5-dichloroaniline has accepted þ a proton from picric acid to form salt C6 H2 O7 C6 H6 NCl2 so that þ NH3 stretching vibration was shifted to the lowest wavenumber region when compared to the uncharged various positional dichloroanilines (NH2) vibrations [23,25]. The asymmetric NHþ 3 deformation (das) vibration appears at 1611 cm1 and the corresponding symmetric (ds) NHþ 3 deformation vibration appears as a weak peak at 1541 cm1 in the FTIR spectrum. The das NHþ 3 arises as a weak peak at 1609 cm1 and the ds NHþ 3 vibration corresponds to the band at 1543 cm1 in the Raman spectrum. The medium intense and shoulder peak at 1099 cm1 in the FTIR spectrum and the weak band in the Raman spectrum at 1087 cm1 was attributed to the NHþ 3 rocking vibration. Here the agreement of the calculated wavenumber with the experimental data was fairly well for the NHþ 3 deformation vibrations and the wavenumbers showed quite good agreement with the reported literature of related compounds [23,24] as shown in Table 1. The vibrations belonging to the bond between the ring and the halogen atoms are worth to discuss here, since mixing of the vibrations were possible due to the lowering of the molecular symmetry and the presence of heavy atom on the periphery of molecule [26]. The mCACl stretching vibrations occur as a strong and a weak intense band at 710 and 460 cm1 in the FTIR spectrum, respectively and as a medium intense band at 710 cm1 in the Raman spectrum. The calculated IR wavenumber at HF level of this mode was in agreement with the experimental data (Table 1). The bending vibrations were observed in the region below 400 cm1. These
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benzene and its derivatives are observed in the region 1300– 1000 cm1 and 950–675 cm1, respectively [38–40]. The aromatic C@C stretching occurs at nearly 1650 and 1450 cm1 [41]. The mCAC vibrations of 2,5-DCAP was identified at 1611, 1541, 1508, and 1477 cm1 in the FTIR spectrum as a weak peak and the corresponding Raman spectrum exhibits peak at 1609, 1543 and 1486 cm1. In the present case, these CAH stretching, CAH inplane, CAH out-of-plane and CAC stretching vibrational frequencies were found to be well within their characteristic region. Nuclear Magnetic Resonance spectroscopy
Fig. 6. Comparison of the experimental and simulated Raman spectrum of 2,5DCAP: (a) observed FT-Raman spectrum; (b) Raman spectrum calculated at HF/631G (d) level with scaling and (c) Raman spectrum calculated at HF/6-31G (d) level without scaling.
bending vibrations were active in the predicted Raman spectrum and absent in the IR spectrum. The strong bands at 1333 cm1 and 1334(s) cm1 in FTIR and Raman spectra were assigned to the CAN stretching, respectively. The simulated Raman spectrum contains bands at 1347 and 1302 cm1 [27]. The picrate being the 2,4,6-trinitrophenolate () anion has characteristic bands of the deprotonated phenol, the bands of three NO2 groups and the bands of the six-membered aromatic ring [28]. The band at 1274 cm1 in FTIR and 1277 cm1 in FT-Raman spectrum was assigned to phenolic O vibration, m(CarAO) mode. The same wavenumber was assigned in some of the previously reported picrate compounds [10,13,24,29,30] for phenolic O vibration. The predicted Raman wavenumber was observed at 1258 cm1 and inactive in the IR spectrum for phenolic O vibration. Thus the absence of the OAH stretching vibration confirms the formation of the title compound. Usually for the free picric acid, NO2 vibration occurs at 1607 cm1 [31]. In 2,5-DCAP, weak band at 1508 cm1 and the bands at 1333 cm1 have been assigned to asymmetric (mas) and symmetric (ms) stretching modes of NO2 vibrations, respectively. The NO2 absorb at a slightly lower wavenumber at 1508 cm1 due to an increased electron density of the picric acid. The scissoring, wagging and rocking vibrations of NO2 group were identified at the characteristic frequencies and assigned [32–34]. The masCAH vibration appears as a medium peak in FTIR spectrum at 3080 cm1 and the corresponding Raman wavenumber appears as a weak band at 3069 cm1 which was in good agreement with the calculated wavenumber at 3044 cm1 in IR spectrum [34– 37]. The CAH symmetric stretching vibration appears at 2809 cm1 in FTIR spectrum and at 2840 cm1 in simulated Raman spectrum. The aromatic CAH in-plane and out-of-plane bending modes of
Nuclear Magnetic Resonance (NMR) is a very versatile nondestructive technique employed in the identification of organic compounds [42]. 2,5-DCAP molecular structure was confirmed by NMR study. The 1H NMR spectrum was recorded by dissolving the sample in DMSO solvent measured using the instrument 500 MHz FT NMR spectrometer BRUKER AV III and the obtained spectrum was shown in Fig. 7. The chemical shifts for 1H NMR is represented in d ppm. The dimethyl sulfoxide-d6 solvent peak appears at d = 2.5 ppm. The intense singlet peak at d = 8.597 ppm confirms the presence of two equivalent protons in the picric acid. The 2,5-dichloroaniline (2,5-DCA) shows three different proton peaks due to different chemical environment. The appearance of doublet signal at d = 6.834 and 6.830 ppm has been assigned to H2 aromatic proton attached to C4 carbon atom in 2,5-DCA. Another doublet signal observed at d = 7.199 and 7.182 ppm was due to the presence of H1 aromatic proton attached to C3 atom (see Fig. 3). The quartet signal present in the region d = 6.552–6.573 ppm has been assigned to H3 aromatic proton attached to C6 atom. The broad peak noticed at d = 5.633 ppm confirms the formation of title compound by the process of protonation of 2,5-dichloroaniline from picric acid as NAH O linkage. The disappearance of AOH peak at 11.94 ppm and appearance of new broad peak at d = 5.633 ppm indicates the presence of picric acid in deprotonated state [31]. Thermal study (TG/DTA and DSC) TG/DTA was carried out between 30 °C and 950 °C in the nitrogen atmosphere at a heating rate of 40 °C/min using thermal analyzer (TG/DTA6200). The obtained TG/DTA curve was shown in Fig. 8a. The DSC was carried out in the range of 30–500 °C in the argon atmosphere at a heating rate of 10 °C/min using NETZSCH DSC 204. The weight change versus temperature curve of TG so obtained gives information about the thermal stability and composition of the original sample. DTA provides information regarding endothermic exothermic effects [43]. DSC was used to study the thermal behavior of crystals. The endothermic peak observed at 125 °C in DTA was assigned to the melting point of the 2,5-DCAP which corresponds to the sharp endothermic peak in DSC curve (Fig. 8b). The enthalpy change is the area under the peak in DSC curve. The enthalpy of melting was found to be 76.86 J/g. The sharp peak indicates that there was no phase transition below melting point. There was no weight loss between room temperature to 171 °C in TG curve. The 2,5-DCAP undergoes two stages of decomposition. The first weight loss (98) in TG was observed from 172 °C to 296 °C. This major weight loss coincides exactly with exothermic peak (278 °C) of DTA curve. This may be due to the liberation of volatile substances, ammonia and nitrogen dioxide gases. The second weight loss occurs between 296 °C and 378 °C in TG curve which matches with the second exothermic peak in DTA graph. This may be due to the evolution of chlorine and hydrocarbon gases followed by ring rupture. After this temperature the crystal decomposes completely. Hence 2,5-DCAP is stable up to 171 °C
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Table 1 The observed FTIR, FT-Raman and calculated frequencies using HF/6-31G (d), along with their relative intensities and probable assignments. Observed wavenumber (cm1)
IR calculated wavenumber (cm1)
Raman calculated wavenumber (cm1)
Band assignment
FTIR
FT-Raman
Unscaled value
Scaled value
Unscaled value
Scaled value
3200 3080 2809 – – 1611 1541 1508 1477 1423 1363 1333 1274 – 1164 1099 1044 921 909 883 823 790 744 710 584 545
– 3069 – – – 1609 1543 – 1486 – 1365 1334 1277, 1215 1165 1087 1044 943 – – 824 – – 710 586 532
3684 3397 2840 1752 1731 1615 1570 – – 1364 – 1299 1228 1174 1065 1040 947 903 861 824 778 – – 589 552
3296 3044 2544 – – 1675 1570 – 1484 1443 – – – 1221 1167 1085 – 930 – – – 773 742 697 – –
3686 3460 2838 1766 1730 1615 1588 1515 – 1426 1362 1344 1299 1209 1174 1065 1039 932 903 – – 769 – – 587 –
– 3402 2840 1772 1693 1613 1586 – – – 1365 1302 1258 1213 1135 1062 1038 928 900 – – 771 – – 591 –
mas NHþ3 masCAH, mss NHþ3 msCAH mC@C mC@C das NHþ 3 , mC@C mCAN, ds NHþ3 , mC@C mCAC, ds NHþ3 , masNO2 bCAH, ds NHþ 3 , mC@C
517 460
– –
499 436
529 447
504 435
510 437
– – – – – –
– 362 331 246 170 119
408 – – – – –
392 – – – – –
407 354 328 218 148 75
407 394 358 217 150 77
sNHþ3 , dipCACAC dipCACAC, mCACl, x NHþ 3 c(CCC), mCACl s(CCCC)
–
–
–
–
–
46
bCAH msNO2 mCANH3, msNO2 m(CAO) phenolic, b(CH) b(CH) b(CH), bCAN qNHþ3 , b(CH) m(CN), b(CH), qNHþ3 d(CN), cCAH mCANO2, bCACAC, cCAH cNAH, cCAH, dNO2 cCAH cCH, xNO2 cCH, xNO2 sring, cCAC, mCACl, cCH bCAO, bCACAC q NHþ3 , dipCACAC, q(NO2)
b(CCl) b(CCCl) s(CCCCl) x NHþ3 c(CCl)
Abbreviation used ms – symmetric stretching; mas – asymmetric stretching; b – in plane bending; c – out of plane bending; q – rocking; d – scissoring; m – stretching; x – wagging, and s – torsion.
Fig. 7. 1H NMR spectrum of 2,5-DCAP.
and hence this crystal can be used for NLO application up to this temperature. First order hyperpolarizabilities (b) Molecules with p electron conjugation incorporating electron donor and acceptor groups to influence the asymmetric polarization
are the essential candidates for NLO applications such as frequency doubling [44]. Table 2 shows the linear and non-linear polarization of 2,5-dichloroanilinium picrate system calculated using Gaussian 03W employing 6-31G (d) basis set. The third rank tensor of first order hyperpolarizability (b) is described by a 3 3 3 matrix. The matrix is reduced to 10 components due to Kleinman symmetry [45]. In this study, the calculations were
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Fig. 8a. TG/DTA of 2,5-DCAP.
59
performed on isolated molecules and possible intermolecular interaction was not taken into account. A large value of b is the first requirement for high NLO efficiency in a molecular material. The effect of intermolecular hydrogen bonding in the titled crystal would enhance the magnitude of b quantities. The hydrogen bond interactions play an important role in the NLO response. The NLO property is not only originated from the constituent individual molecule but also is influenced by the intermolecular interactions [46]. From Table 2, it was suggested that the title compound was polar having non-zero dipole moment, polarizability and hyperpolarizability. The static first-order hyperpolarizability (b) output was calculated using Eq. (4) from ten components to be 5.50321 1030 esu, which was nearly 9 times that of urea (0.6 1030 esu). The polarizability (atotal) and dipole moment (ltot) was calculated to be 2.39 1023 esu and 12.5815 D, respectively. It was noticed that in bxxx (which is the principal dipole moment axis and it is parallel to the charge transfer axis) direction, the biggest value of b was noticed and subsequently delocalization of electron cloud is more in that direction. The maximum value may be due to electron cloud movement from donor to acceptor which makes the molecule highly polarized and the intermolecular charge transfer possible which was also confirmed by vibrational spectral study. Molecular orbitals
Fig. 8b. DSC spectrum of 2,5-DCAP.
Table 2 Electric dipole moment (l), polarizability (a) and first-order hyperpolarizability (btotal) of 2,5-DCAP.
lx ly lz ltot Debye axx ayy axx axy axz ayz atotal 1023 esu bxxx bxxy bxyy byyy bzxx bxyz bzyy bxzz byzz bzzz btotal 1030 esu
11.9575 3.8337 0.7838 12.5815 246.9530156 5.8212833 232.7082 1.7729797 1.9670065 66.9894639 2.39 981.653 132.759 413.04 242.29 140.12 4.381 10.21 39.54 18.96 9.38 5.50321
The difference of the energies of the HOMO and LUMO, termed the band gap, can sometimes serve as a measure of the excitability of the molecule: The smaller the energy the more easily it will be excited. The HOMO is the orbital that could act as an electron donor, since it is the outermost (highest energy) orbital containing electrons. The LUMO is the orbital that could act as the electron acceptor, since it is the innermost (lowest energy) orbital that has the room to accept electrons. A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as soft molecule [47]. In Fig. 9 the red atomic orbital lobes are positive phases and the green atomic orbitals are negative phases. The HOMO–LUMO gap value of the 2,5-DCAP compound was found to be 0.378 au The HOMO ? LUMO transition and the small HOMO–LUMO energy gap explains the probable intermolecular charge transfer (ICT) taking place from 2,5-dichloroaniline electron donor group to the picric acid electron acceptor group to form 2,5-dichloroanilinium picrate charge transfer complex. The HOMO and LUMO plots were shown in Fig. 9(a) and (f), respectively. The HOMO (Fig. 9(a)) was largely delocalized over electron acceptor phenoxy (ACO) and NO2 group of picrate anion. The LUMO (Fig. 9(f)) was delocalized over NO2 and CAN group of picrate anion. Electronic excitation mechanism and electrostatic potential The static polarizability value is proportional to the optical intensity and inversely proportional to the cube of transition energy. With this concept, larger oscillator strength (fn) and Dlgn with lower transition energy (Egn) is favorable to obtain large first static polarizability values. Electronic excitation energies, oscillator strength and nature of the respective excited states were calculated by the closed-shell singlet calculation method and are summarized in Table 3. It was clearly visible from Table 3, only three excited bands have larger value of oscillator strengths. The 1st and 4th state: It mainly arises from the excitation from HOMO 0 to LUMO + 0 (Fig. 9(a) and (f)). The relevant orbitals were mainly composed of picrate group. Charge transfer happens within picrate ion, which corresponds to the transitions from the orbital
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Fig. 9. Representation of the most important orbital involved in the main electronic transition for 2,5-DCAP crystal (a) HOMO – 0, (b) HOMO – 11, (c) HOMO – 12, (d) HOMO – 13, (e) HOMO – 17, (f) LUMO + 0, (g) LUMO + 1, (h) LUMO + 3, and (i) LUMO + 8.
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N. Sudharsana et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 53–62 Table 3 Computed absorption wavelengths (kng), energy (Eng), oscillator strength (fn) and its major contribution state. S. no.
Excitation wavelength, kng (nm)
Excitation energy (eV)
Oscillator strength (f)
Assignments
1 2 3 4 5 6 7 8
252.31 248.48 241.28 241.01 234.69 228.32 226.94 212.53
4.9141 4.9897 5.1386 5.1443 5.2828 5.4303 5.4632 5.8338
0.1189 0.0316 0.0143 0.0811 0.0151 0.0003 0.0010 0.3711
HOMO – 0 ? LUMO + 0 HOMO 12 ? LUMO + 3 HOMO 11 ? LUMO + 8 HOMO 0 ? LUMO + 0 HOMO 13 ? LUMO + 3 HOMO 17 ? LUMO + 0 HOMO 17 ? LUMO + 0 HOMO 0 ? LUMO + 1
composed of electron acceptor (CarAO) group of picrate anion to that of aromatic ring (picrate anion). The 8th state: It was due to the excitation from HOMO 0 to LUMO + 1 (Fig. 9(a) and (g)). This was a major contribution state when compared to other states. The correspondent transition was between the orbital composed of amine group NHþ to that of phenoxy group (CarAO) which 3 was responsible for electron-donor acceptor-complex (2,5-dichloroanilinium picrate) formation [48,49]. The ESP surface was shown þ in Fig. 10 with different views. The NHþ 3 and CH group of C6 NH6 Cl2 (2,5-dichloroanilinium) cation, the ESP values were positive (blue color) whereas the most negative ESP values (red color) was located on NO2 and CArAO groups of C6 H2 N3 O 7 (picrate) anion. Second harmonic generation (SHG) The Kurtz–Perry powder technique remains a particularly valuable tool for testing of materials for second harmonic generation. A Q-switched Nd:YAG laser beam of wavelength 1064 nm, with 7 nm pulse length and 10 Hz repetition rate was allowed to strike the sample. 2,5-DCAP was prepared with a grain size of 40–60 lm. KDP powder with similar grain size was taken as reference. The generation of the SHG was confirmed by the emission of green light which was 0.8 times that of reference KDP. A comparison
Table 4 A comparison of relative powder SHG efficiency and first-order hyperpolarizability (b) for some of the picrate complexes. S. no.
Compound name
Powder SHG(KDP)
b(urea) in 1030 esu
1
Imidazole–imidazolium picrate monohydrate [50]
3.6
–
2
L-prolinium
3
L-tryptophanium
52
–
122
–
4 5
picrate [51] Adenosinium picrate [52] Histidinium dipicrate dihydrate [53]
0.25 2.5
– 10.54
6
L-threoninium
43
–
46
–
– 66.5
20 96
7 8 9
picrate [12]
picrate [54] 1,3-Dimethylurea dimethylammonium picrate [55] Naphthalene picrate [56]
L-asparaginium
picrate (LASP) [57,58]
10
L-leucine L-leucinium
11
L-glutamine
12
L-valinium
13 14 15 16
picrate [59]
picrate [60]
picrate [61] 2-Methylimidazolinium picrate [62] Dimethylammonium picrate [63] 2,4,6-Trinitrophenol (TNP) [8] 2,5-Dichloroanilinium picrate [present work]
1.5
–
–
27
144
36
2 2 13.4 0.8
– – – 9
b – First order hyperpolarizability, SHG – second harmonic generation, and KDP – potassium dihydrogen phosphate.
table of relative powder SHG efficiency and ‘b’ vector part for some of known picrate complexes was collected in Table 4 [50–63]. The SHG efficiency for the title compound is smaller than those obtained in case of other picrate related compounds except for adenosinium picrate as mentioned in Table 4. The SHG is 0.06 times that of parent picric acid. ‘b’ value changes with the type of method and basis set used in calculation. The proton donor (AOH) group of picric acid and proton acceptor amine (ANH) group of 2,5-dichloroaniline in the structure provide a convenient infrastructure to introduce the charge asymmetry which is required for second order nonlinearity. Conclusions The molecular charge transfer complex 2,5-DCAP was synthesized and single crystals were grown by slow evaporation solution growth technique. It was found that the crystal belongs to the monoclinic crystal system from the powder XRD study. UV–Vis study reveals that the crystal was transparent in the region 470 and 1100 nm. Molecular structure was confirmed by NMR spectral analysis and the functional groups were identified by vibrational spectral analysis. The calculated wavenumbers were compared with the observed one. Thermal behavior and stability of the crystal were studied using TG/DTA and DSC techniques. The response of SHG was found to be 0.8 times that of standard KDP. The calculated hyperpolarizability (b) and dipole moment (l) was found to be 5.50321 1030 esu and 12.5815 D, respectively. The HOMO and LUMO orbitals and electrostatic potential confirms nature of the charge transfer within the molecule. Acknowledgments
Fig. 10. The calculated electrostatic potential of 2,5-DCAP. The contour electron density isovalue is 0.0004.
The author R.N. is thankful to Council of Scientific and Industrial Research (Sanction No: 03(1158)/10/EMR-II), New Delhi for the financial assistance under major research project and N.S. is thankful to CSIR for awarding JRF in this project. The authors are grateful to thank Dr. R. Justin Joseyphus, Department of Physics, National Institute of Technology Tiruchirappalli, for providing thermal
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N. Sudharsana et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 53–62
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