Crystal Growth, Characterization and Theoretical Insights of an Organic Schiff base 2-(1-(2,4 Dichlorophenyl)Ethylidene)Hydrazine Carboxamide for NLO Applications

N. Sudha¹, R. Ananthi^1*, M. Sowmiya¹ and R. Rajesh Kanna^2
*Correspondence: R. Ananthi, Department of Physics, Sri Sarada College for Women (Autonomous), Tamilnadu, India, Email:

Keywords

Crystal growth • Quantum chemical calculations • NLO property

Introduction

The properties and performance of nonlinear optical and photonic materials depend critically on their composition, microstructure and purity. Convection and sedimentation can cause variations in processing conditions of such materials, which in turn, can affect the morphology and microstructure of the materials and thereby, also the optical properties [1,2]. The large nonlinear optical effect found in some organic crystals make the materials attractive for applications in frequency conversion [3]. Optoelectronics and NLO are expected to play a major role in photonics which is emerging as frontier areas of science and technology. The organic molecular materials have emerged as a new class of promising nonlinear optical materials because of their superior qualities over inorganic systems. Among the nonlinear phenomena frequency doubling, frequency mixing and electro-optic modulation are important in the fields of optical image storage and optical communications [4-6]. Crystal growth from solution is a powerful purification process and the purity of the obtained crystals is higher than the corresponding starting reactants. Also, the low temperature solution growth makes it easier to obtain less defective crystals than in high temperature growth methods [7]. The single crystal structure has been solved by Hoong-Kun Fun, et al. To the best of our knowledge so far, no systematic study has been carried out on the vibrational, thermal, linear and nonlinear optical properties of the title crystal. Hence our present intention is to provide a complete study of this material with respect to its physico-chemical properties. The grown crystals have been subjected to experimental characterizations and theoretical calculations to confirm its nonlinearity.

Materials and Methods

Experimental details

1 mmol of 2, 4 dichloroacetophenone is mixed with 2 mmol semicarbazide hydrochloride and 3 mmol of anhydrous sodium acetate in 10 ml of ethanol. The mixture is refluxed for 4 hours. A white precipitate is obtained and washed with water. The reaction scheme is shown in Figure 1. This precipitate is recrystallized using methanol as solvent repeatedly to obtain transparent needle shaped crystals of semicarbazone of 2,4-dichloroacetophenone (chemical name 2-(1-(2,4 dichlorophenyl)ethylidene)hydrazine carboxamide) as shown in Figure 2.

Computational details

Density functional theory is a phenomenally successful approach to find the solutions of the fundamental equation that describes the structural properties of atoms and molecules. All calculations are carried out with aid of Gaussian 09 package [8] with the help of threeparameter hybrid functional (B3) for the exchange part and Lee-Yang- Parr (LYP) correlation function, using 6-31+G(d,p) basis set [9]. An empirical uniform scaling factor of 0.956 is used to neglect the electron correlation and vibrational anharmonic [10]. The scaled frequencies were deviated <10 cm^-1 with few exceptions from the experimental frequencies. Furthermore, the dipole moment, linear polarizability, hyperpolarizability, frontier molecular orbital energies, VCD spectrum of DCAP were calculated at B3LYP/6-31+G(d,p) level. Molecular Hirshfeld surfaces calculations were performed using Crystal Explorer program.

Results and Discussion

Crystal structure analysis and molecular geometry

The DCAP crystal structure belongs to the monoclinic system with the space group C2/c. The lattice parameters are a=37.8079 (17) A, b=3.8097 (2) A,c=14.4920 A, β=98.852°. The cell volume is 2062.52 (17) A3 containing Z=8 formula units. The asymmetric unit consists the semicarbazone group (C9/C6/C7/N1/N2/C8/O1/N3) is approximately planar with an r.m.s bond length deviation of 0.011 (2) Å for atom N1, while the dihedral angle between the least-squares plane through the semicarbazone group and the benzene ring is 38.76 (9)°. The thermal displacement ellipsoid plot with 50% probability level and optimised structure are presented in Figure 3 and Figure 4 respectively. The molecules are linked via N-H…O hydrogen bonds to generate R2 2(8) ring motifs. These motifs are further connected through C-H…O hydrogen bonds to form a one-dimensional chain along the (0 1 0) direction is shown in Figure 5. The complete structure details are shown in Table 1. H atoms are positioned geometrically (C-H=0.93– 0.96 Å) and refined using a riding model with U iso(H)=1.2 Ueq(C) and 1.5 Ueq(methyl C). A rotating–group model is used for the methyl groups. The nitrogen H atoms are located from the difference Fourier map (N–H=0.85(3)–0.91(3) Å) and allowed to refine freely (Figure 5) [11].

Table 1. Crystal data and structure refinement details of DCAP crystal.

The compound is shaped by blending of 2,4-dichloroacetophenone with semicarbazide hydrochloride and due to conjugation of C=N, a Schiff base semicarbazone of 2,4-dicholoroacetophenone is formed. The compound contains a benzene ring in which the chain is substituted in ortho position. Here the formation of C=N are significant in making a non-centro symmetry structure of the molecules leading to nonlinear optical property [12]. 2,4-dichloroacetophenone is an aromatic cyclic compound which possesses number of hydrogen bonds with high degree of symmetry. The C=O at para position in phenyl ring gained higher order symmetry by the π-bond delocalization electron clouds which leads the compound to have second order of polarization and hyper polarizability. This is the main root cause to attain NLO properties when the compound is brought to crystal form [13]. The bond length C6-C7 of the chain is found to be 1.489 Å which is 0.099 Å greater than the C-C of the ring. This bond length is stretched out due to the maintaining of repulsive force between the charge clouds of the chain and ring. The bond length C8- N3 is 0.048 Å greater than C7-N1 due to the π and σ bond interactions respectively [14]. The observed value of the bond length C=O is found to be 1.23 Å however the title compound shows 1.256 Å for C=O. This is mainly due to the further substitution of NH2 group on C [15]. Usually, the bond length of C=N is 1.287 Å [16]. Where as in the grown crystal bond length is 1.369 Å which is extend by 0.081 Å due to link of azine (N-N) group. The general value of N-N bond length is 1.47 Å but in the present case the bond length has reduced to 1.37 because of steeving of the ketone group with in the chain. However, there is not much change in C-Cl bond length as it coincides with the actual value 1.78. The bond length for C8-N2 (1.369 A) is greater than the bond length of C7-N1 which is due to the attachment of NH2 and =O (electronegative groups) in the crystal structure. The observed C8–O1 bond length is higher than the theoretical value. Similarly, the N2-C8 and N3-C8 bond lengths are lesser than theoretical values. All these can be explained because of resonance involving lone pair on the nitrogen atoms. The experimental structural bond lengths and bond angles from XRD and computed optimized counterparts are tabulated in Tables 2. and 3. respectively. The small differences which originate from the theoretical calculation is due to the isolated molecule in gas phase.

Table 2. Comparison of experimental and calculated bond lengths of DCAP crystal.

Table 3. Comparison of experimental and calculated bond angles of DCAP crystal.

Here the semicarbazone segment is positioned in one plane whereas acetophenone is placed perpendicular to that plane. This is the main reason for the existence of various lobe interactions which has stipulated the optical momentum [17]. The bond angle in C6-C7- N1 is found to be 114.76º between chain and ring. This is almost the semicircle angle which is making another ring spontaneously by the existence of attractive forces between ring and N-N of chain [18]. Theoretical bond angle value of H1–N3–H2 is 120.36° which is smaller than the experimental value 131o due to electronegativity of N atom. In N1-N2-H1, the theoretical bond angle is smaller than the experimental bond angle because in experimental work molecular interactions influence the electronegativity of N. Another reason for the deviation is, in experimental work a group of molecules is taken whereas in theoretical work only one molecule is taken for study hence the bond angle is affected.

Hydrogen bonding

Hydrogen bonds are the non-covalent interactions controlling the spatial structures and their assemblies. Both intra and intermolecular hydrogen bonds (H-bonds) play a crucial role in controlling the structure of molecules as well as their functions [19]. By knowing the bond length, the strength of the hydrogen bond can be confirmed as very strong (below 2.5 Å), strong (2.5-2.7 Å), moderate (2.7-2.9 Å) and weak (above 2.9 Å). The hydrogen bonding donar-acceptor geometry is found in Table 4. It shows that both strong and weak hydrogen bonding exists.

Table 4. Hydrogen bonds of DCAP (Å and °).

Hirshfeld surface and finger print plot analysis

Hirshfeld surface analysis has recently become a popular method for the analysis of X-ray structures to investigate detailed aspects of molecular packing; for instance: polymorphism, solvatomorphism and other aspects of supramolecular arrangement. This is due to its complementary character to traditional structure viewing, where interactions are not examined in isolation but rather within the context of the whole system. Additionally, isosurface mapping is shown in Figure 6. which allows for intuitive recognition and visual analysis of interactions between the molecules. Their contributions can be expressed in terms of a percentage share. Crystal structures are imported from CIF files. Globally, the H…H contacts are the most encountered interactions in the title compound and their relative contribution reaches 26.7% followed by interactions between H…Cl about 17.3%. The next highest interactions between O…H is contributing 6.8%. The partial fingerprint plots of the title compound show that the N…H intermolecular interactions appear at the upper portion of the plot contributing 3.2% of the interactions as shown in Figure 7.

Vibrational analysis

The vibrational analysis of the grown crystal has been analyzed by FTIR and FT-Raman spectra. The present molecule DCAP consists of 24 atoms and has 66 normal modes. The title molecule belongs to C1 point group. Figure 8 and Figure 9. represent the experimental and theoretical FTIR and FT-Raman spectrum respectively. The IR and Raman scattering activities and potential energy distributions are gathered in Table 5. The realistic fundamental modes are assigned and verified along with the characteristic region of group frequencies. The entire observed modes were influenced by their nearby attractive vibrations. The molecule possesses -I effect due to the presents of chlorine in the ring. This -I effect promotes change in stretching frequencies. The carbonyl group is most important in IR spectrum because of its strong intensity absorption and high sensitivity towards minor changes in its environment. Intra and intermolecular factors affect the carbonyl absorption in common organic compounds due to inductive effect, field effect and conjugation effects.

Table 5. Observed and calculated frequencies of DCAP.

C-H vibrations

In accordance with literature, the bands observed at 3094 cm^-1 and 3068 cm^-1 in IR and 3060 cm^-1 in Raman are attributed to C-H stretching. The theoretical values at 3092 cm^-1, 3083 cm^-1 and 3053 cm^-1, are assigned as C-H stretching vibrations. The peaks in IR at 1348 cm^-1 and 1110 cm^-1 is due to C-H in-plane bending vibrations. Whereas the C-H out-of-plane bending vibrations are observed at 866 cm^-1 and 982 cm^-1 in IR. Its counterpart in Raman spectrum, the peak observed at 973 cm^-1. The theoretically computed wavenumbers are in good agreement with experimental results.

Methyl vibrations

The antisymmetric and symmetric C-H vibrations in methyl group are observed between 2920-2990 cm^-1 and 2840-2900 cm^-1. Being in excellent agreement with literature, the peaks at 2944 cm^-1, 2917 cm^-1 and 2884 cm^-1 in IR and Raman respectively, are due to antisymmetric stretching and not affected by other vibrations. The in-plane and out-ofplane vibrations are observed in the region 1340-1485 cm^-1 and 870- 630 cm^-1 respectively. The peak at 1442 cm^-1 in IR and 1440 cm^-1 in Raman is due to in-plane-bending and 982 cm^-1, 973 cm^-1 are due to out-of-plane bending. The active modes of vibrational energy levels of methyl group ensured its energetic presence in the compound.

C=O vibrations

The C=O stretching vibrations in amides occur usually around 1650-1670 cm^-1. In the present compound, the peak appeared at 1734 cm^-1 is assigned as C=O stretching vibration. The peak at 1708 cm^-1 in theoretical scaled region is in close agreement with experimental one. Due to hydrogen bonding interactions and inductive effect, the wavenumber is shifted to higher value.

C-Cl vibrations

For the substituted benzene the C-X vibrations are shifted to lower wavenumber due to inductive or mesomeric effect of the substituents. The C-Cl stretching frequency appears in the region 600-800 cm^-1 for benzene containing Cl atom. The peaks appeared at 754 cm^-1 and 686 cm^-1 in IR are assigned for C-Cl stretching vibrations. The calculated frequency is found at 688 cm^-1 and 787 cm^-1 which coincides with experimental ones. All these vibrational values are well within literature is mainly due to the consistent position of chlorine.

Ring vibrations

The C–C stretching vibrations occur in the region 1625-1430 cm^-1. In the title compound, the peaks observed at 1338 cm-1 in IR and 1586 cm^-1 and 1290 cm^-1 in Raman confirm the C-C stretching vibrations. The simulated frequencies are found to be in the expected range. The frequencies correspond to in-plane and out-of-plane bending vibrations are occurred in the expected range and coincide with literature values. Due to the extent of mixing between the ring and substituent group molecule, the wavenumbers are changed.

C=N, C-N vibrations

The heteronuclear bond named as dipole bond (C=N, C-N) which exists with azine and methyl group. Generally, the C=N stretching vibration is observed at 1600-1490 cm^-1. In the present case, it is shifted to higher energy wavenumber at 1671 cm^-1 level due to the manifestation of Schiff base from the precursor. Another evidence for the dipole bond is C-N, which is generally observed at 1382-1266 cm^-1. In DCAP, the bands at 1343 cm^-1 and 947 cm^-1 are assigned for C-N stretching in IR. The peak is shifted is lower value due to both molecular interactions. The stimulated counterparts also coincide with literature values.

N-N vibrations

The homonuclear bond N-N which is very important core bond of azine group acts as bridge point for NLO properties. Generally N-N stretching is observed around 1080 cm^-1. In the present case, it has overlapped with C-H bending group in IR and eluded in Raman and shifted to higher value due to the effect of bridge formation. The peak appeared at 1142 cm^-1 in IR and 1150 cm^-1 in Raman are assigned as N-N stretching. Molecular interaction is mainly concentrated between N and H in Schiff’s base group of compounds.

N-H vibration

Generally the N-H stretching vibrations occur in the region 3400-3500 cm^-1. In the present work, the peak at 3477 cm^-1 is observed for NH2 stretching in IR spectrum. Theoretically, the wavenumbers at 3590 cm^-1, 3448 cm^-1 and 3412 cm^-1 are due to N–H2 stretching. The N–H bending region is generally around at 1650-1580 cm-1 and 650-895 cm^-1 for in-plane and out-of-plane bending vibrations respectively. The wave numbers at 1468 cm^-1 and 1375 cm^-1 in IR attribute to N-H bending and the peak at 794 cm^-1 is NH2 deformation vibration. Theoretical counterparts are also observed in the expected region.

VCD spectrum

Vibrational Circular Dichroism (VCD) is an electro-magneto-optic effect and associated with spectroscopic technique which detects differences in attenuation of left and right circularly polarized light passing through the compound [18]. VCD is the coupling of optical activity to infrared vibrational spectroscopy. More specifically, the essence of VCD is to combine the stereo chemical sensitivity of natural optical activity with the rich structural content of vibrational spectroscopy. The result of a VCD measurement is the combination of two vibrational spectra; VCD and its parent infrared spectrum. These can be used together to deduce information regarding elucidation of optically and biologically significant molecules. The VCD spectrum of the present molecule is depicted in Figure 10.

In the present molecule the spectrum originated from zero and the intensity of the absorption is elevated in positive and negative sides due to the left and right circular polarization. The vibrational polarization bands belong to middle IR region which corresponds to the C=N, C=C, C=O, C-C, N-N and C-N stretching vibrations and N-H and C-H in-plane bending wavenumbers. All such vibrational bands stated that, the functional groups are active and eventually confirms the non-linearity [19,20].

UV-visible-NIR spectral analysis

The optical absorption spectrum is used to study the band structure and the kind of electronic transition occurring in the analysed sample. The maximum absorbance at 215 nm corresponds to π→π* transition which is due to the conjugation of semicarbazone group with the acetoketone group. The band gap energy is found to be 5.78 eV. The band gap energy also ensures that the grown crystal possess dielectric behaviour to induce the polarization when the powerful radiation is incident on the material. The calculated maximum wavelength at 287 nm is correlated with experimental λmax and the excitation energy is calculated to be 5.78 eV. The transitions are active in the UV-vis region signifying the NLO activity. The UV-visible-NIR spectrum studied experimentally and theoretically is exhibited in Figures 11a-c. Table 6 contains both experimental and theoretical values and its major contribution with oscillator strength.

Table 6. Experimental and theoretical electronic absorption spectral values of DCAP.

Photoluminescence spectral analysis

Photoluminescence is one of the effective tools to provide information about the various energy levels due to excitation energy. The photoluminescence emission spectrum of DCAP is recorded with an excitation wavelength of 215 nm. It is found that the wavelength of DCAP is red shifted to 618 nm indicating the fact that the crystal can also be used for IR applications. It is noted that during emission, an increase in intensity of DCAP spectrum shows hyperchromic shift which is shown in Figure 12.

Electrical properties

The variation of dielectric constant as a function of frequency at room temperature is shown in Figure 13a. The high value of dielectric constant at low frequencies may be due to the presence of electronic, ionic dipolar and space charge polarizations and its low value of higher frequencies may be due to loss of these polarizations gradually. According to the Miller’s rule lower value of dielectric constant is one of the suitable properties for the enhancement of SHG co-efficient. The dielectric loss is studied as a function of frequency at room temperature which is shown in Figure 13b. The minimum value of dielectric loss at high frequency indicates the enhanced optical quality of the crystal with lesser defects.

Second harmonic generation efficiency

The SHG relative efficiency for the title crystal measured by Kurtz’s Perry powder technique which generates the second harmonic output by irradiating the powder samples of randomly oriented crystallites. The powdered sample sandwiched between two glass slides is subjected to the output of a Q-Switched Nd:YAG laser emitting a fundamental wavelength of 1064 nm. The SHG efficiency is confirmed by the emission of green radiation (532 nm). The laser with input voltage 0.68mJ is given as input. The output voltage for urea and DCAP is 8.9 mJ and 3.67 mJ respectively. The SHG efficiency is compared with urea and it is found to be 0.4 times greater than that of urea crystal.

Thermal analysis

Thermogravimetric methods are largely limited to decomposition and oxidation reactions which are physical processes such as vaporization and sublimation. Thermograms provide information about decomposition patterns of materials and also weight loss. Figure 14 shows the differential thermogram of DCAP crystal. The analysis is performed between 25°C and 1400°C at a heating rate of 101°C/min in the nitrogen atmosphere. There is only one stage of weight loss noted in the thermogram. The sharp endothermic peak is observed at 249°C which shows that the compound is thermally stable up to 249°C.

Theoretical first order hyperpolarzibility

The first-order hyperpolarizability (β) is the second-order electric susceptibility per unit volume. The molecules exhibiting large hyperpolarizabilities have a strong NLO Potential and could be useful for optical devices. For exhibiting larger molecular hyperpolarizabilities, the system must be asymmetric and should contain polarizable electron donor and acceptor groups. The calculated magnitudes of second order polarizability, dipole moment and polarizibility for the DCAP crystal are shown in Table 7. The results authenticate that the grown crystal is 13.35 times greater than that of standard urea hence it can be used for non-linear applications.

Table 7. Dipole moment (μ), polarizability and first-order hyperpolarizability (β) of DCAP derived from DFT calculation at 6-31+G(d,p) basis set.

Frontier molecular orbital analysis

The frontier orbital’s (HOMO and LUMO) find out the way in which the molecule interacts with other species. The frontier orbital gap facilitates to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a small frontier orbital gap is more polarizable and is usually related with a high chemical reactivity, low kinetic stability and is also termed as soft molecule. Highest Occupied Molecular Orbital (HOMO) is directly related to the ionization potential, while energy of the Lowest Unoccupied Molecular Orbital (LUMO) is directly related to the electron affinity. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that mainly acts as the electron acceptor. It can be seen from the Figure 15. HOMO of DCAP presents a charge density localized over the benzene ring LUMO is characterized by a charge distribution on the entire molecule except of hydrogen atoms in CH3 group. The HOMO and LUMO transition implies an electron density transfer. Moreover, lower in the HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. Usually, the chemical applications would be extracted from the chemical properties of the compounds. They can also be useful for identifying new substance for the particular purpose. It is very important to know whether this compound is suitable for optical application. In this way, the ionization potential, electron affinity, electronegativity, chemical hardness and electrophilicity index are tabulated in Table 8. The chemical hardness of the present compound is 2.4152 eV. Therefore, the present compound has much chemical stability. Here, the substitutions of semicarbazone group have enhanced the chemical stability and rigidity character of the DCAP. So, in the present compound, the optical property has been tuned on the timescale of its expected NLO utility.

Table 8. Calculated molecular orbital parameters of DCAP employing 6-31+G(d,p) basis set.

The electronegativity is a chemical property which describes the tendency of a molecule or a functional group to attract electrons (or electron density) towards itself. If the compound possesses high electronegativity number, the compound will have the high degree of electron density. Usually, the accumulation of electron density is associated more with different functional groups than with individual atoms called group electronegativity. In this compound, the group electronegativity is found to be 4.148 eV which is high and the property of chemical bonds in the title molecule is virtually ionic. Accordingly, due to the combination of two groups of atoms, the bonds character of the compound reformed to be ionic.

Molecular electrostatic potential surface

The Molecular Electrostatic Surface Potential (MESP) is a technique of mapping electrostatic potential onto the iso-electron density surface. The importance of MEP is originated from it from it simultaneously displays molecular size, shape as well as positive, negative and neutral electrostatic potential regions in terms of color grading. In other words, electrostatic potential increases in the order redFigure 16.

Conclusion

In order to confirm the NLO responses of the present compound both experimental characterizations and theoretical calculations are carried out. The SXRD confirms that the structure belongs to monoclinic system. FTIR and FT-Raman spectra confirm the functional groups present in DCAP. The optical properties like UVvis, PL, SHG confirm its nonlinearity. Dielectric constant and dielecric loss confirm the polarization nature of DCAP. The crystal belongs to soft material category which is confirmed by Vickers hardness test. The theoretical investigations like vibrational analysis, HOMO-LUMO, ESP, VCD spectrum and hyperpolarizibility calculations evaluate that the compound has dynamic NLO property. The Hirshfeld analysis shows the strong interactions present in the compound. The obtained results were coherent and the entire analysis explored the presence of NLO activity present in the compound. Therefore, the DCAP crystal is very interesting material for optical applications due to its Nonlinear property.

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