Hydrogen-bonded structure and optical nonlinearities in the proton-transfer complex of 8-hydroxy-5-nitroquinoline with ρ-toluenesulfonic acid

Single crystals of 8-hydroxy-5-nitroquinolinium p-toluene sulfonate (HNT) were grown by the slow evaporation solution growth technique. The structure was elucidated by single-crystal X-ray diffraction analysis, and the crystal belongs to the monoclinic system with the space group C2/c. The crystallinity of HNT was studied by powder X-ray diffraction analysis. The presence of functional groups was determined by FT-IR spectral analysis. The band gap energy is estimated by the application of the Kubelka–Munk algorithm. The charge transfer characteristic of the compound was studied by frontier molecular orbital (FMO) analysis. The first-order hyperpolarizability of the HNT molecule was found to be 285.45 × 10−30 esu, which is ~ 750 times higher compared to the reference urea molecule. Investigation of the intermolecular interactions and crystal studies packing via Hirshfeld surface analysis, based on single-crystal XRD, reveals that the close contacts are associated with molecular interactions. Fingerprint plots of the Hirshfeld surfaces were used to locate and analyze the percentage of hydrogen-bonding interactions. The Mulliken charge of the present molecule was theoretically analyzed. The Kurtz-Perry powder technique has been used to estimate the second harmonic generation. Observed small SHG and large hyperpolarizability are rationalized.


Synthesis and growth
HNT was synthesized by mixing stoichiometric quantities of 8-hydroxy-5-nitroquinoline (Sigma-Aldrich) and 4methylbenzenesulfonic acid (Sigma-Aldrich) in the molar ratio of 1:1 in ethanolic medium. The mixture was stirred at room temperature for 3 h, and HNT (Scheme 1) was obtained as a yellow precipitate. The product was purified by recrystallization using ethanol as solvent. Transparent crystals were grown by the slow evaporation solution growth technique and the crystals were harvested after a period of 8-10 days. Photo image of as-grown crystal and morphology of HNT is given in Fig. 1.

Characterization techniques
The structural analysis was carried out for a selected sample of approximately 0.200 × 0.200 × 0.150 mm 3 size using BRUKER AXS (Kappa APEXII) X-ray diffractometer employing graphite monochromated MoKα radiation (λ = 0.71073 Å). The Bruker APEX3 program was used for computing data collection, APEX3/SAINT using for the cell refinement, SAINT/XPREP was used for calculating the data reduction, SHELXT-2014/5 program was used for structure solution, SHELXL-2014/7 program was used for the structure refinement process, and ORTEP3 program was used for molecular graphics. FT − IR spectra were recorded using an AVATAR 330 FT-IR by KBr pellet technique in the spectral range of 400-4000 cm −1 . Bulk samples were analyzed by PXRD with a Bruker D8 powder diffractometer (Bruker AXS, Karlsruhe, Germany). Experimental conditions: CuKα radiation (λ = 1.54056 A˚); 40 kV; 30 mA; scanning interval 5-50°, 2θ at a scan rate of 1° min −1 ; time per step 0.5 s. Diffuse reflectance spectrum of the sample is recorded using the DRA-CA-30I accessory and converted to absorption spectra using the Kubelka-Munk function. The UV-DRS spectrum was measured in the region of 200-800 nm. The thermal analysis was carried out in the nitrogen atmosphere at a heating rate of 20 °C min −1 for the temperature range of 30-500 °C for HNT. The SHG test was performed by the Kurtz powder method An Nd:YAG laser with a modulated radiation of 1064 nm was used as the optical source and directed onto the powdered sample through a filter.

Computational studies
All the theoretical calculations were performed using the GAUSSIAN 09 W [30] program package on a personal computer without any constraints on the geometry using density functional group theory (DFT) with B3LYP method 6-31G(d,p) as the basis set [31]. By the use of the GAUSSVIEW 5.0 molecular visualization program [32], the optimized structure of the molecule has been visualized. Hirshfeld surfaces and fingerprint plots were generated by CrystalExplorer (version 3.1), using DFT method with 6-31G(d,p) as basis set [33]. Crystal morphology was calculated by Winxmorph software Scheme 1 Synthetic route of complex (HNT) 1 3 [34]. Simulated powder XRD pattern and crystal packing were visualized by mercury 3.5.1 software [35].

FT-IR
The FT-IR spectrum of the compound HNT is shown in Fig. 2, and the vibrational spectrum of HNT exhibits the characteristic bands of the 8-hydroxy-5-nitroquinolinum cation and ρ-toluenesulfonate anion. The peaks at 1500 cm −1 and 1361 cm −1 are characteristic of NO 2 asymmetric and symmetric stretching vibrations, respectively. The broad peak observed in the range 2700-2250 cm −1 is due to the NH + stretching vibration. The characteristic absorption at 1332 cm −1 corresponds to CH 3 rocking vibrations. The SO 3 asymmetric and symmetric modes of vibrations of sulfonate group in the ρ-toluene-sulfonate ring are found at 1252 and 1021 cm −1 respectively. The observed characteristic IR frequencies are listed in Table 1.

Powder XRD analysis
The indexed powder XRD pattern of HNT along with simulated pattern is shown in Fig. 3. XRD profiles show that a sample is of single phase without detectable impurity. The well-defined Bragg's peaks at specific 2θ angles show good crystallinity of the material. Most of the peak positions in powder XRD and simulated XRD pattern derived from single-crystal XRD coincide. Relative intensities difference could be due to the preferred orientation of the sample used for the diffractogram measurement. Also, the mosaic spread of powder and single-crystal patterns may differ resulting in varied intensities.

Single crystal XRD
Single-crystal X-ray diffraction analysis reveals that the compound HNT crystallizes in monoclinic centrosymmetric space group C2/c with Z = 8. An ORTEP of HNT shown at the 50% probability level (Fig. 4a) clearly indicates proton transfer. The asymmetric unit of the crystal contains one molecule of the cationic form of 8-hydroxy-5-niroquinolinium and one molecule of the anionic form of p-toluene sulfonate anion. The crystal data and structure refinement details of HNT are given in Table 2. In the packing diagram (  Table 3) forming an intermolecular hydrogen bond between quinolinium N atom with sulfur atom S. The N-H…S and C-H … O interactions are given in Fig. S1. Crystal threedimensional packing showing molecular interactions along "a," "b," and "c" axes are displayed in Fig. S2.

UV-Vis-NIR diffuse reflectance spectrum
The optical absorption spectra of HNT are recorded, and band gap energies are estimated. The optical absorption spectrum shows the absorption is at minimum in the visible region and cut-off wavelength is 427 nm. The Kubelka-Munk theory provides a correlation between reflectance and concentration [36]: where F(R) is the Kubelka-Munk function, R is the reflectance of the crystal, α is the absorption coefficient, and S is the scattering coefficient; A is the absorbance, and c is the concentration of the absorbing species. The direct band gap energy of the specimens is 2.85 eV respectively from the Tauc plot, [F(R) hʋ] 2 versus hʋ (where hʋ is the photon energy in eV) (Fig. 5).

Thermal analysis
The TG-DTA response curve is shown in Fig. 6. Thermogravimetric curve shows its complete decomposition in stages. A significant weight loss of 44% is observed in the temperature range 232 to 447 °C. In the DTA curve, the endothermic peak observed at 237 °C is attributed to melting of the HNT crystal, and 49.78% of the sample remained as residue at 600 °C. The material has a good thermal stability up to 232 °C, without any decomposition before this temperature. The crystalline nature is good as evident from the sharpness of the peaks observed in DTA. No decomposition up to the melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures.

Optical nonlinear properties
The hyperpolarizability tensor β(s), the first-order molecular hyperpolarizability was computed, and the values are given in   more towards the x-axis. Generally, high β tot value is ascribed to conjugation, molecular symmetry, hyperconjugation, substitution, aromaticity, and charge transfer [37]. Intra-and intermolecular hydrogen bonding interactions and the resulting supramolecular assembly contribute mainly to observed large microscopic nonlinearity. Also, large β could be due to the dipole moment difference between the ground state and first π-electron excited state [38]. Large molecular hyperpolarizability leads to intensive dipole-dipole interactions [22] facilitating centrosymmetric crystallization of HNT (monoclinic, C2/c). Contrary to expectations, small second harmonic generation efficiency (SHG) is observed by the Kurtz and Perry technique [39] with KDP as reference material. Some of the earlier reported quinolinium derivatives exhibiting SHG efficiency are given in Table 5. Reactants and product HNT have different symmetry groups. 8-Hydroxy-5-nitroquinoline, C 9 H 6 N 2 O 3 , exhibits an acentric orthorhombic space group Fdd 2 [40] whereas p-toluenesulfonic acid belongs to centrosymmetric monoclinic space group P2 1 /c [41]. The centrosymmetric structure of proton-transfer complex HNT was confirmed by X-ray structure analysis. Small SHG observed (0.2 < times of KDP) could be ascribed to the traces of non-centrosymmetric impurity, quite likely the reactant, 8-hydroxy-5-nitroquinoline. Also, the possibility of laser-induced local non-centrosymmetry at the time of irradiation is not ruled out.

Hirshfeld surfaces analysis
The Hirshfeld surfaces of individual molecules are given in Fig. 7 for a better understanding of the molecular interactions. The Hirshfeld surface of HNT used for analyzing intermolecular interactions is displayed in Fig. 7 showing surfaces that   (19) have been mapped over a d norm , curvedness, d e , shape index and d i [42,43].

Electrostatic potential
Molecular electrostatic potential maps drawn on the Hirshfeld surfaces with potentials ranging from − 0.112 au (red) to + 0.235 au (blue) (Fig. 9). HNT crystallizes in the monoclinic space group C2/c with Z = 8. A clear separation of the electropositive and electronegative regions was observed. Positive ESP on the H2A atom (+ 0.227 au) interacts with negative ESP region over the O1 atom (− 0.099 au) resulting in the formation of the O···H contact in the crystal packing (Fig. 10a). In the N … H contact (Fig. 10b), the electropositive region (+ 0.059 au) around H2A    interacts with the highly electronegative (− 0.076 au) region around the N2 (Fig. 10b).

FMO
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of HNT are shown in Fig. 11. The red and green colors represent the positive and negative values for the wave function. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that mainly acts as an electron acceptor. HOMO is localized on SO 3 sulfonate group, and LUMO is localized on whole 5-nitro-8-hydroxyquinolinium molecule, while HOMO-1, HOMO-2, HOMO-3, and HOMO-4 are is somewhat lower than the experimental value because the alienated HNT molecule is in the gas phase whereas the obtained experimental value is in the solid phase [44]. The HOMO and LUMO energy gap explain the eventual charge transfer interactions taking place within the molecule.

MEP
The molecular electrostatic potential (MEP) plot of electrostatic potential mapped (DFT/6-311G(d, p)) on the constant electron density surface displaying electrostatic potential (electron + nuclei) distribution is shown in Fig. S4. The different values of the electrostatic potential at the surfaces are represented by different colors. Red represents regions of most negative electrostatic potential (preferred site for electrophilic attack), blue represents regions of most positive electrostatic potential (preferred site for nucleophilic attack), and green represents regions of zero potential. Red colors indicate the more electron-rich and blue the more electron-poor area. Furthermore, the polarization effect is clearly visible. The color code of this map is in the range between − 0.117 e0 to + 0.117 e0 for HNT. The negative region is mainly localized from sulfonate anion group whereas the positive region lies in the quinolinium cation system. Molecular shape, size, and dipole moments of the molecule provide a visual method to understand the relative polarity.

Mulliken population analysis
Mulliken atomic charges are calculated by determining the electron population of each atom as defined by the basis function (DFT/6-311G(d, p)). Figure 12 shows the Mulliken atomic charges and plots of HNT. From the atomic charge values, the oxygen (O1-O6), nitrogen (N2), and carbon (C1-C3, C5-C7, C9-C10, C12 and C14) in HNT had a large negative charge and behaved as electron donors. The remaining atoms (all H atoms and C4, C8, C11, C13, C15-C16, S1, and N1) are acceptors exhibiting positive charge. The negative charges on nitrogen/oxygen, which is a donor atom, and net-positive charge on hydrogen and sulfur atoms, which is an acceptor atom, suggest the presence of intra-and intermolecular hydrogen bonding interactions as visualized by Hirshfeld surface analysis. The Mulliken charge population values are given in Table 6.

Conclusions
Single crystals of hydrogen bonded assembly of the quinolinebased complex 8-hydroxy-5-nitroquinolinium p-toluenesulfonate were grown successfully from ethanol at room temperature by slow evaporation. High beta values imply that this quinoline-based proton-transfer complex is a promising NLO material and useful in optoelectronics.
Acknowledgements Author SS is grateful to the CSIR Emeritus Scientist Scheme for the award of the SRF.
Author contribution S S: data curation, investigation, visualization, validation, writing, original draft. K P: data curation, investigation; SP M: review, editing, and supervision. The final version of the manuscript submitted was approved by all the authors. Availability of data and materials Not applicable.