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Synthesis, Structural, Spectroscopic, and Hirshfeld Surface Analysis, and DFT Investigation of Benzaldehyde Semicarbazone
By using slow evaporation, single crystals of BDSC (chemical name: 2-benzylidenehydrazine carboxamide) have been formed. A slow evaporation approach was used to generate BDSC single crystals. By using single-crystal XRD, the nonlinear characteristics of the material were studied. The presence of molecular groups was carefully examined using fundamental vibrational analysis. A Hirshfeld analysis was used to study the interactions between molecules. A series of UV-visible, photoluminescence, and VCD spectroscopic experiments were used to verify the optical characteristics of the material. In order to determine the electrical characteristics, dielectric investigations were carried out. Tests of Vickers hardness were used to measure mechanical strength. The SHG behaviour of the crystal was determined using the Kurtz–Perry method. To assess the molecule’s thermal stability, all quantitative calculations were performed using the B3LYP approach with a basis function of 6-31 + G(d,p) and TG/DTA experiments.
Molecular materials with greater hyperpolarizabilities have become a popular research area in recent years. Materials being strong optical and highly nonlinear have potential uses in data transfer, data storage, optical switching, and other fields as diverse as those mentioned above. Organic materials with significant NLO properties have already been found and synthesised. But just a handful of these were able to be crystallised and studied for second-order applications . The crystallinity and stability of semicarbazone derivatives make them particularly attractive. Using acid and a base as a catalyst, semicarbazide reacts with aldehydes or ketones to produce semicarbazone derivatives [2–6].
Semicarbazide derivatives have become the subject of a large number of investigations in recent years. Because they might be useful in both industrial and biological applications, their functional and physical features are being studied [7, 8]. Thiosemicarbazones and thiosemicarbazone analogues are two of the most significant groups of Schiff basis compounds because they share the C=N functional group. As a result of their high nonlinearities and quick reflexes, organic NLO nanocrystals are preferable to inorganic ones in electrooptic devices. Delocalized-electron systems linking donors or acceptor groups increase asymmetric polarizability in organometallic NLO materials at the microscopic level . Delocalized bonds are those where the electrons may travel across upwards of two nuclei at once. Organic materials with the best nonlinear performance are constructed using molecular units with highly delocalized pi-electron moieties upon that ring. The existence of a delocalized cloud between the ring and chain causes asymmetry in polarizability. Because of the pi-electron transit over a bridge between donors and acceptors, polarisation is quite high. Using the bridge, donors and recipients may stay in touch via technological means.
Benzaldehyde is a highly symmetrical aromatic molecule with many hydrogen bonds. Its carboxylic group CHO in the para position of either the phenyl group was delocalized in order to create higher-order symmetry, which was accomplished by bond delocalization. Hyperpolarizability and second-order polarizability are both possible for the electron cloud that leads to the molecule. Because of this, when compound is crystallised, it acquires NLO characteristics . NLO applications may benefit from Schiff base organic compounds, which have been identified to be promising materials [3, 11–14]. The characteristics of Schiff base crystal have been studied both experimentally and conceptually after a thorough review of the relevant literature.
2. Materials and Methods
2.1. Experimental Details: Synthesis
Merk supplies benzaldehyde (C7H6O) and semicarbazide hydrochloride (CH6ClN3O). Here are the steps for making the single-crystalline of benzaldehyde semicarbazone (the chemical name is 2 benzylidenehydrazine carboxamide). Alcohol and water are added until the mixture becomes somewhat cloudy, which may be eliminated with a few drops of alcohol. Then, 1.5 moles of sodium acetate and 0.1 moles of semicarbazide hydrochloride are added. In Figure 1, the reaction path is shown. About two hours of refluxing in boiling water and cooling the combination follow the well-shaken mixture. It is necessary to periodically recrystallize the white precipitate employing methanol as a solvent in order to achieve clear needle-shaped crystals, as illustrated in Figure 2, after washing with ethanol. Multiple characterisation approaches were used to verify the crystal’s suitability for use in NLO systems.
2.2. Computational Details
The Gaussian 09 software package was used for all quantum chemical computations . A DFT hybrid approach combining Becke’s nonlocal three-parameter exchange with the Lee Young as well as Parr correlation (B3LYP) and the 6-31 + G(d,p) basis set was used to optimise the molecular geometries to meet conventional convergence requirements. No imaginary frequencies were identified for the wavenumbers derived by comparing them to potential energy minima. The title molecule’s band gap energy may be determined using electromagnetic absorption spectra and frontier molecular orbitals. Reactive site charge concentrations were also measured using the B3LYP 6-31 + G(d,p) charge concentrations (d,p). Hyperpolarizability simulations have been used extensively to investigate the NLO feature of the aforementioned molecule.
3. Results and Discussion
3.1. An Investigation of the Crystal Structure and Molecular Structure
Analysis of single-crystal data from the MoK X-ray diffractometer Brucker Kappa APEXII revealed the crystalline phase of BDSC. A crystal of 0.300 × 0.250 × 0.200 mm3 was used for the X-ray data collecting. In order to establish the BDSC’s unit cell characteristics, we used least square methods and multiple reflections. Figures 3 and 4 show the ORTEP plot and indeed the optimised diagram, respectively. SHELX 97 was used to solve and improve the structure using the complete matrix least-squares approach . Table 1 summarises the results of the analysis of the unit cell parameters. The produced crystalline is monomeric with such a quasispatial class P21/c, according to X-ray diffraction studies. (a) 11.9353, (b) 5.455, (C) 12.908, (D) 8.220, and (V) 820.2 3 are the lattice characteristics available throughout the research. A total of 6814 distinct reflections were observed in the wavelength of 3.229 to 24.994° in the range of h, k, and l, respectively. The R factor reduced to 1.183 after so many refining cycles.
The B3LYPP/6-31 + G(d,p) level of theory is being used to compute the geometrical characteristics of the title compounds. The symmetry of this molecule may be attributed to its position in the C1 point group. The most significant feature of the compound was a good example of this type of binding energy and bond lengths. Tables 2 and 3 show the XRD and computationally optimised structural characteristics of the title molecule, respectively. The benzene ring in the molecule has had its chain replaced in ortho position. For C3C8 (1.408), the bond length is 0.17 nm longer than that of C8C7 (1.391 nm), C4C5 (1.395 nm), C7C8 (1.401 nm), and C5–C6 (1.395 nm). In order to maintain the force in the opposite direction between chains and rings, this increase to a maximum was extended. Because of bond interactions, the C1 N1 bond (1.393) was longer than the C2 N3 bond (1.286) . It has been shown that the experimentally measured bond length in N2H(2A) is 0.86, whereas theoretical calculations place it at 1.017. Intermolecular interactions between molecules in the experimental investigation are to blame for this huge difference.
Between the chain and the ring, the bonding angle among N2C2C3 was determined to be 122.77°. By virtue of the presence of attraction forces between this ring and NN of chain , this practically semicircular angle is creating another ring . Because of the NH2 substitution on C, the bond length in N1C1O2 is 123.7° higher. Bond angle in H(1A)N(1)H(1B) is 124°, a high value because of the electronegativity of amide group. Isolated gas-phase computations lead to minor discrepancies between experimental and theoretical results.
3.1.1. Hydrogen Bonding
3.2. Hirshfeld Surface and Fingerprint Analysis
A Hirshfeld surface study  using the Crystal Explorer 3.1 tool [18–20] revealed the several types of interactions inherent in a crystal structure. The CIF files used to store the crystal structures were opened and imported. It is possible to create Hirshfeld surfaces by dividing space inside a crystal where the ratio between the electron densities of promolecules and procrystals is equal to 0 (dnorm). Figure 6 depicts the title compound’s Hirshfeld three-dimensional dnorm surface. dnorm-coloured red hot patches on the surface are hydrogen bonds combining oxygen and nitrogen with nearby molecules that are very heated. It is possible to dissect the fingerprint scanning plots in order to emphasise a specific atom combination in close proximity. The H…H connections are by far the most common interaction inside the title compound, accounting for 43.7 percent of the total interactions. There are about 43.7 percent of the surface area of the Hirshfeld fingerprint plot accounted for by H contacts, with C…H interactions accounting for the second most prevalent kind of interaction. Other molecular interactions exist in addition to H…N and O…H. Figure 7 shows the molecule’s numerous fingerprint areas.
3.3. Vibrational Analysis
The planes of its functional groups determine the organic compound’s basic vibrations. The molecule was unravelled into 21 atoms using molecular symmetry (C1), and there are 57 major modes of vibration that are active in the IR and Raman spectroscopies. As A57 represents, the full set of basic modes may be broken down into 37 in the plane vibrations of 20 out of the plane vibrations .
Table 5 shows the computed raw frequencies scaled down to match the measured wavenumber values. Figures 8 and 9 depict the experimental and simulated FT-IR and FT-Raman spectra, respectively. Realistic fundamental modes and the distinctive area of group frequencies were assigned and validated. There was a strong impact of surrounding attractive vibrations on all of the measured modes.
3.3.1. N–H Vibrations
The N–H absorption bands typically occur between 3400 and 3500 cm−1. There are peaks at 3560 cm−1, 3459 cm−1, and 3287 cm−1 inside the NH2 stretching FT-Raman spectra in this work. The theoretical wavenumbers predicted by NH stretching have been confirmed empirically . Molecular interactions within Schiff’s base family of compounds, which generate high N–H bond forcing constants, are primarily focused on N and H. N–H deformation is responsible for IR peaks at 1512 cm−1, 1429 cm−1, and 1491 cm−1 and Raman peaks at 1429 cm−1 and 1447 cm−1. Experimental and theoretical values are quite similar.
3.3.2. C−H Vibrations
In the 3000–3100 cm−1 region, aromatic compounds often show C–H vibrational spectra. When it comes to this molecule, researchers have determined that the C–H stretching occurs at lengths ranging between 3065 and 2933 cm−1. In IR and Raman, the wavenumber of C–H bending vibrations is 1139 cm−1. In literature, these ideas are supported. It has been decided that the wavenumber range for C–H out of plane will be 850–1000 cm−1. An out-of-plane C–H vibration at 998 cm−1 Raman and 995 cm−1 IR is designated. When it comes to the wavenumbers 881.1 cm−1, 916.42 cm−1, 961.61 cm−1, or 1002.02 cm−1, there seem to be theoretical values that match those obtained experimentally .
3.3.3. C=O Vibrations
Amides have C=O stretching vibrations of 1650–1670 cm−1 as a standard. This molecule possesses a C=O asymmetric stretching at 1689–1700 cm−1, according to IR and Raman spectra. 1705 cm−1 is a good estimate for the region’s highest point .
3.3.4. Ring Vibrations
The peaks at 1312 cm−1, 1293 cm−1, or 1161 cm−1 in this investigation are attributed to the C–C stretching vibrations. The benzaldehyde semicarbazone group bridge causes the wavenumber to change to the lower wavelength region. Vibrations of bending are assigned using the Gauss View and Veda software, in and out. 525 cm−1, 625 cm−1, 628 cm−1, 692 cm−1, 758 cm−1, 856 cm−1, and 611 cm−1 are all in-plane bending vibrations in IR and Raman. As per Inverse’s Theory, the theoritical values are between 543 cm−1 and 881 cm−1. Theoretically, at 507 cm−1 and 498 cm−1, an out-of-plane bending occurs at 457 cm−1.
3.3.5. C=N, C–N, and N–N Vibrations
For C=N stretching vibrations, there is a great deal of power inside the 1600–1490 cm−1 range . Here, the infrared (IR) and Raman (Raman) vibration bands at 1600 or 1604 cm−1 are all in action, respectively. Because of its impressive exhibition, the imidazole group was able to retain a strong presence. Additionally, magnetic binding CN stretching may be seen , in addition to the 1382–1266 cm−1 values. In this condition, the IR achieves its greatest values at 1354 cm−1 as well as 1346 cm−1, or the conceptual assignments are in excellent accord with the literature on the subject. These two Raman peaks, located at 264 cm−1and 123 cm−1, are caused by in-plane and out-of-plane vibrations, respectively. A very rare nuclear link exists between the azine group and the nucleus of another azine group. In this way, the NLO is connected to the rest of the globe via this connection. An important discovery of the azine bond is the N–N stretching vibration, which occurs at a frequency of 1080 cm−1 as a rule. This stretching vibration is helped by the experimental spectrum’s 1092 and 1123 cm−1 peaks, which contribute to the stretching vibration.
3.4. VCD Spectrum
VCD is an expansion of circular dichroism spectroscopic to the infrared and near-infrared regions. For example, the VCD gives two-dimensional structural information as ligand groups are mutually aligned. Figure 10 shows the molecule’s VCD spectral profile. According to such left and right circular polarisation, the spectrum was generated from a value of zero, and the strength of the absorption was increased on the positive and negative sides. Although the absorption intensity on the two sides of the VCD spectrum is normally unequal, amalgamation was significantly more likely on the right side in this instance. The C=N, C=C, C=O, CC, NN, and CN stretching vibrations and NH and CH in bending vibration wavenumbers are all in the middle IR region, which includes both linear and circle vibrational polarisation bands. The organic molecules were active in all of these vibrational bands, which supports the induction of NLO activity.
3.5. UV-Visible NIR Spectral Analysis
In order to be used in optical applications, NLO materials must have a large transparency window in the UV-visible spectrum. Its UV-visible spectrum ranged from 200 to 1100 nanometers. It is shown in Figures 11(a) and 11(b) that perhaps the BDSC crystalline does have a reduced cut-off wavelength of 195 nanometers. To confirm that its wide transparency range inside the display area is a crucial quality for frequency doubling, a linear extrapolation of the chart’s quadratic element indicated that such glass’s band gap energy was 6.3 eV.
Absorption at 296.17 nm, 267.22 nm, and 231.76 nm is all transitions inside the moment of density functional theory (TD-DFT) of the UV-Vis spectrum, which is estimated to use the time-dependent density functional (TD-DFT). In the imine group, these changes take place. Because the quartz UV crystal zone can easily be seen in this image, all changes take place inside this region. This point of view was essential in order to produce optical pulses with second-order harmonics. A reduction in wavelength causes an increase in absorption energy . This is evidence that the frequency has doubled. Oscillator strength (f) and excitation energies, as well as spectral Figure 11(c) assignments, are reported in Table 6.
3.6. Photoluminescence Spectral Analysis
At room temperature, we measured the BDSC single-crystal photoluminescence emission spectrum using a Perkin Elmer LS45 spectrophotometer to get the graph displayed in Figure 12. Since the material was excited at 344 nm, an optical emission spectrum may be seen. Spectra show a sharp peak at 405 nm in the violet emission band. Fluorescence emission from the BDSC crystal is yellow and orange in hue, with a maximum peak at 608 nm. Because of this, the BDSC crystal is an excellent choice for optical applications.
3.7. Electrical Properties
An important feature of the dielectric analysis is its potential to provide information on the electrical characteristics of a material medium in relation to temperature and frequency. The material’s capacity to store and transport electric charges may be assessed using the results of this research.
When crystals are made of insulating materials, their dielectric characteristics are linked to their electrooptic properties [28–30]. Low dielectric constant materials are needed in the micro- and electronic sectors. A digital LCRZ metre is used to measure the BDSC crystal’s dielectric constant from 50 Hz to 200 kHz at normal temperature. Figure 13(a) depicts the relationship between log frequency and dielectric constant (r). BDSC has fewer flaws, as seen by the decreasing dielectric constant with increasing frequency. Higher frequencies show a reduction in dielectric loss, proving that BDSC may be employed in NLO applications.
Figure 13(b) depicts the relationship between dielectric loss and logarithmic frequency. Thus, the reduced dielectric loss of BDSC crystals indicates high optical quality crystals with fewer flaws, which is critical for nonlinear optical.
3.8. Microhardness Study
There must be enough mechanical strength in a material to be employed in industry for gadget manufacture. Vickers microhardness experiments were used to test the BDSC crystal’s microhardness. The material’s resistance to local deformation is measured by its hardness. An indenter load in kilogrammes (kg) and an imprint diagonal length in millimetres (mm) were used to determine the Vickers hardness number HV.
To illustrate this point, Figure 14(a) shows how the toughness of the HV changes along with the increasing pressure (p) from 25.1 to 100.0 grams. The line between log p and log d follows Meyer’s index coefficient quite closely . In Figure 14(b), the overall inclination of the graph is seen to be 2.9, which is the “n” number. As a result, BDSC crystal falls within the umbrella of soft materials.
3.9. Second Harmonic Generation Efficiency
To make optical storage systems, colour displays, and communication systems with high optical bandwidth and low dielectric constant, organic nonlinear optical crystals are in great demand. Background investigation of NLO materials may still benefit greatly from the Kurtz and Perry powder approach. The SHG production of the BDSC crystal was quantitatively measured using the method described above. A 0.68mj/pulse Nd:YAG laser operated at 1064 nm with an 8ns pulse width with a 10 HZ repetition rate. Using green irradiation from the sample, we were able to demonstrate that this molecule is a candidate for SHG applications because of its second harmonic signal. There were 9.2 mJ in production for urea and 4.5 mJ in output for the BDSC. This shows that the BDSC is 0.48 times larger than the regular urea.
3.10. Thermal Analysis
Figure 15 indicates that the produced material remains stable at room temperature up to 205°C, as can be seen from the TG/DTA graph. This proves that there is no solvent in the finished product. TGA estimates that 94% of the body’s weight is lost throughout the decomposition process. At 205 degrees Celsius, an endothermic peak is seen due to the sample’s melting point. An endothermic reaction at 205°C with a high degree of clarity suggests that the crystals created were of excellent quality.
3.11. Theoretical First-Order Hyperpolarizability
DFT has been used extensively to investigate crystals’ NLO properties. Nonzero and values are more strongly influenced by the chemical’s intermolecular hydrogen bonding. B3LYP/6-31 + G(d,p) basis set was used to determine a first-order particularly powerful and dipole moment using a finite field approach. The study of intermolecular interactions is often carried out using molecular dipole moments as a technique. The larger the dipole moment is, the more intermolecular interactions will occur. Molecular interconnection is evident from the dipole moment of BDSC, which measures 4.172 Debye (D). For this compound, the theoretically estimated NLO values mentioned in Table 7 were used to compute the linear and first-order polarizability tensors. According to the data, BDSC’s hyperpolarizability is 13.23 times more than urea’s.
3.12. Frontier Molecular Orbital Analysis
Using the B3LYP/6-31 + G(d,p) exact same position in the Gaussian 09 programme, a HOMO-LUMO energy band gap was calculated. This material’s interaction with other organisms is governed by the HOMO-LUMO boundary orbitals. A BDSC border molecular compound may be seen in the image above. C=O, C–N, N here on the NH2 group, or C–C bonds are determined to be homogeneous (HOMO) (benzene ring). C–C, C–N, or C–H bonds all have LUMOs that may be seen throughout them. Its HOMO-LUMO or energetic difference of a molecule is important for predicting its activity. Energy expenditures have been meticulously planned.
When the electrochemical reaction transition and, subsequently, the nonlinear optical responses are allowed to proceed more easily, the HOMO-LUMO overlap is evident in these orbital topologies  (Figure 16). Charge transfer interactions among molecules and a dynamic component for NLO applications are explained by the reduction in the HOMO and LUMO energy gap. Because the molecule’s theoretical value is assigned to its gas phase, the anticipated HOMO-LUMO energy gap is less than the ultraviolet light optical energy gap. Intermolecular forces, such as Vander Waals interaction in crystal field interactions, also affect the morphology of the solid-state structure. Table 8 includes a few more parameters, such as electron affinity, chemical hardness, softness, and the electrophilicity index, which are calculated and presented.
3.13. Electrostatic Potential on the Molecular Surface
With the use of electrostatic potential (MEP), molecular structure may be linked to physical properties. Molecule electrostatic potential (MEP) is a quantitative description of the molecule’s electric potential (electrons + nucleus), its shape, size, and dipole moments. As a consequence, the reactive sites of molecules may be inferred from this map of the MEP surface. MEP diagram colours indicate an electron deficit or partial positive charge. Red indicates an electron-rich zone, whereas blue indicates an electrode-deficient or partly positive ion. Because oxygen and nitrogen have stronger negative potentials than carbon, electron shortage is created by an excess of potentiality in the BDSC molecule. Additionally, Figure 17 provides an overall view of how each of these molecules’ molecular applied electric surfaces is connected, which provides an insight into how each of these molecules is wired. The MEP’s surface may be used to observe electrophilic and nucleophilic reactions.
The information presented here expands our understanding of the manufacturing process for Schiff base benzaldehyde semicarbazone. After a single XRD, the crystal was found to be monomeric with such a P21 space group. All microstructures in crystals may be explored using Hirshfeld surfaces. It was decided to investigate the functional groups of the molecule using FT-IR and FT-Raman spectroscopy. BDSC’s nonlinearity is supported by its optical properties. The dielectric studies confirm the nonlinearity. The material is soft and pliable. In addition, chemical reactivity is shown using DFT simulations such as HOMO-LUMO. The hyperpolarizability of BDSC has been determined to be 13.23 times greater than that of urea. MEP has BDSC chemical reactive sites. The VCD spectrum confirms the presence of optical activity. A new method for producing optical materials can be seen in this paper that uses a Schiff base.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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