Hydrogen bond analysis of a third order nonlinear optical crystal, 2-amino-5-bromopyridinium benzilate, using structural and computational characterizations

Journal Pre-proof Hydrogen bond analysis of a third order nonlinear optical crystal, 2-amino-5bromopyridinium benzilate, using structural and computational characterizations S. Madadhankumar, P. Muthuraja, M. Dhandapani PII:

S0022-2860(19)31528-5

DOI:

https://doi.org/10.1016/j.molstruc.2019.127419

Reference:

MOLSTR 127419

To appear in:

Journal of Molecular Structure

Received Date: 3 August 2019 Revised Date:

11 November 2019

Accepted Date: 13 November 2019

Please cite this article as: S. Madadhankumar, P. Muthuraja, M. Dhandapani, Hydrogen bond analysis of a third order nonlinear optical crystal, 2-amino-5-bromopyridinium benzilate, using structural and computational characterizations, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/ j.molstruc.2019.127419. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical abstract

Optimized molecular geometry was obtained theoretically to understand the stability of 2-A5BPB through various inter and intramolecular hydrogen bonding interactions such as N-H…O, C-H…O and O-H…O DFT method with B3LYP functional at 6-311G(d,p) level of basis set was employed and the optimized structure. The optimized geometrical parameters for an isolated molecule are matched quit well with the observed structural parameters in SXRD and those results were discussed in the experimental section.

Hydrogen bond analysis of a third order nonlinear optical crystal, 2-amino-5-bromopyridinium benzilate, using structural and computational characterizations. S. Madadhankumar a, P. Muthuraja a and M. Dhandapani a* a*

Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science,

Coimbatore-641 020, Tamil Nadu, India Abstract The molecular

structure of a proton

transfer

organic material,

2-amino-5-

bromopyridinium benzilate crystal, grown by slow-evaporation solution growth technique has been determined by single crystal X-ray diffraction. The crystal has monoclinic structure and crystallizes in a centrosymmetric space group P21/c. The molecular network contains variety of hydrogen bonds that led to stabilization of the structure and optical properties of the crystal. The FT-IR studies confirm the functional groups present. The 1H and

13

C NMR spectra were

recorded to confirm the molecular skeleton. The crystal is transparent in entire visible region as evidenced by UV-vis spectrum. The photoluminescence spectrum of the compound shows good optical emission properties. Thermal stability of the crystal was determined using TG/DTA studies. Theoretically optimized molecular geometry shows good agreement with experimental bond length and bond angle values. Molecular electrostatic potential analysis confirmed the proton migration during the reaction. The charge transfer occurring in the compound was confirmed by both HOMO-LUMO and Mulliken atomic charge analysis, where electrons get promoted from pyridinium moiety to the benzilate through N-H…O hydrogen bonds. Natural bond orbital analysis supports the interactions and structural properties of the proton transfer compound. Z-scan technique was used to measure the effective third-order nonlinear optical susceptibility. Keywords Crystal structure, Hydrogen bond, Hyperpolarizability, Natural bond orbital, Molecular electrostatic potential *Corresponding author. E-mail address: [email protected] (Dr. M .Dhandapani) 1. Introduction In recent years, organic NLO materials have been intensively investigated due to their high nonlinearities in optical properties [1,2]. Charge-transfer based hyperpolarizable organic molecules have been shown to possess superior second and third order nonlinear optical (NLO) properties compared to more traditional inorganic materials [3,4]. Organic molecules have large

third order nonlinearity and are used to integrate the devices for signal processing applications such as harmonic generation, amplitude modulation, phase modulation and switching [5-10]. The hydrogen bonding between hydroxyl groups of carboxylic acids and heterocyclic nitrogen atoms has been proved to be a useful and powerful organizing force for the formation of supramolecular architecture in organic salt-like molecules [11]. A number of supramolecular synthons have been known to involve interactions such as hydrogen bonding, halogen bonding and π-interactions [12, 13]. Among the organic acid-base based materials, the cyclic amine based compounds exhibit excellent second and third order nonlinear optical properties [14]. Once the nitrogen is protonated, the entire positive charge present on it is readily available for ionic bonding with the negatively charged carboxylate moiety [15]. Due to the overlapping of π-orbital in these molecules, delocalization of electronic charge distribution increases the mobility of the electron density, which in turn enhances the optical nonlinearity [16]. In order to achieve good macroscopic nonlinear response in organic crystals, one requires an increase in the number of π electrons and π delocalization length, so as to lead to high molecular hyperpolarizability and also proper orientation of the molecule in the solid-state structure to facilitate high-frequency conversion efficiency [17]. Hydrogen bond motifs and contacts in a crystal structure have a greater influence on the properties of crystals. The nature of hydrogen bonding in a crystal must be well recognized in order to correlate the structure-property relationship [18]. The illustration of intermolecular hydrogen bonds by graph set method reduces the complications in understanding hydrogen bonding networks. The graph sets are also very useful in interpreting hydrogen bonding patterns in the crystal structure. Hydrogen bonding interaction is one of the tantalizing properties in the process of molecular aggregation which can be used to tune the optical nonlinearity [19]. In the present work, we report the structural, quantum chemical and thermal studies of a new organic molecular crystal, 2-amino-5-bromopyridinium benzilate (2-A5BPB) involving proton transfer mechanism. The crystalline product was characterized by both experimentally and theoretically utilizing a number of techniques.

2. Experimental details 2.1 Materials The reactants, 2-amino-5-bromopyridine (Analytical grade: Sigma-Aldrich 99% assay) and benzilic acid (Analytical grade: Sigma-Aldrich 99% assay), were utilized for the synthesis in as received condition without further purification. HPLC grade methanol was used as solvent. 2.2 Synthesis The two reactants, namely, 2-amino-5-bromopyridine and benzilic acid were taken in an equimolar stoichiometric ratio (1:1) and were dissolved in methanol. Both the methanolic solutions were mixed together and stirred for about 3 hours to get a homogeneous solution of 2-A5BPB. The chemical reaction is given in Scheme 1. The solution was filtered off and the filtrate was kept aside for crystallization without any mechanical disturbance at ambient temperature. The crude 2-A5BPB crystals obtained within a week were re-dissolved and subjected to recrystallization several times to attain ultrapure quality. Transparent colourless single crystals were obtained within a period of fifteen days. Photograph of the crystal is shown in Fig.S1.

Scheme: 1 Synthesis of 2-A5BPB 2.3 Physicochemical and computational characterization details 2.3.1 Spectroscopic and thermal analysis The Fourier transform infrared (FT-IR) spectrum was recorded in a JASCO (FP-3700) spectrophotometer in the wavenumber region 4000-400 cm-1 using KBr pellet method. The nuclear magnetic resonance (NMR) spectroscopic data were collected from a BRUKER AV III 500 MHz spectrophotometer using DMSO-d6 solvent. The electronic absorption spectrum of methanolic solution of 2-A5BPB was recorded using a JASCO (V-770) UV-Vis

spectrophotometer in the range 200-800 nm. The emission spectrum of 2-A5BPB was recorded using methanol as a solvent in the spectral region 200-800 nm by a JASCO (FP-8300) spectroflurometer. The TG/DTA thermograms were obtained in the temperature range 25-600oC under nitrogen atmosphere at a heating rate of 10oC per minute in a NETZSCH 409 C/CD thermal analyzer. 2.3.2 Crystallographic Studies The crystal structure was determined from the single crystal X-ray diffraction data obtained with a Oxford Xcalibur Gemini diffroctometer equipped with Graphite monochromator (MoKα = 0.71073 A). The structure was solved by direct methods using the program SHELXS97 [20] and refined by full matrix least square on F2 using SHELXL-97 program [21]. The thermal ellipsoid plot and crystal packing diagrams were generated using program ORTEP and MERCURY [22] software package. The presence of various hydrogen bonding interactions was determined by PLATON software [23]. 2.3.3 Computational details Quantum chemical calculations of 2-A5BPB were carried out using Gaussian'09 version E.01 software [24] program and also by the Gauss view 5’molecular visualization program.

From B3LYP (Becke’s three-parameter hybrid model employing Lee-Yang Parr

correlation functional) method at 6-311G (d,p) basis level of theory. Natural bond orbital (NBO) calculations were performed using NBO 3.1 program [25]. Molecular electrostatic potential (MEP), Frontier molecular orbitals (FMO), Mulliken atomic charge distribution and NLO parameters including dipole moment, polarizability and hyperpolarizability calculations have been carried out using B3LYP method at 6-311G(d,p) level of theory. CrystalExplorer 3.1 program was used to carry out Hirshfeld surface analysis and generate fingerprint plots [26]. The plots were used to describe various intermolecular interactions including O…H, H…H, C…H and C…C and other contacts present in crystal structures on 2-A5BPB. Crystallographic information file (.cif) was used as input for the analysis. The Hirshfeld surfaces and associated 2D fingerprints were simulated based on the electron distribution as the sum of the spherical atom electron densities [27, 28].

2.3.4 Z-Scan Technique In this experiment, the sample is translated in the Z-direction along the axis of a focussed Gaussian beam from the Nd:YAG laser of 532.8 nm wavelength and the far field intensity is measured as a function of the sample position. The sample is translated across in the +Z to -Z axial direction of the laser beam. The transmittances of open and close aperture curves for 2A5BPB crystal were recorded using a good quality 2-A5BPB crystal of 1 mm thickness. The data were collected by a photodetector through an aperture and the intensity was measured by a digital power meter attached to the detector. 3. Results and discussions 3.1 FT-IR spectroscopic analysis The FT IR spectrum of 2-A5BPB is shown in Fig.1. The frequencies at 3420 and 3340 cm

-1

are due to the N-H asymmetric and symmetric stretching vibrations of -NH2 group

respectively. The band at 3085 cm-1 is characteristic of C-H stretching vibration. The aromatic CH asymmetric and symmetric stretching vibrations of 2-A5BPB are observed at 3085 to 2850 cm-1 respectively. The appearance of +N-H group stretching vibration band at 2399 cm-1 strongly evidences the formation of 2-A5BPB through proton migration [29]. The wave numbers, 1660 and 1490 cm-1 are due to the COO- (deprotanated carboxylate anion) asymmetric and symmetric stretching vibrations respectively. The band at 1370 cm-1 is assigned to C-O stretching vibrations of carboxylate moiety. The ring skeletal vibrations of aromatic ring give a band at 1660 cm-1. The vibrational frequency at 1163 cm-1 is due to the C-N stretching vibrations in substituted pyridinium moiety. The aromatic C-H in - plane bending frequency appears at 1037 cm-1. The wave numbers at 972 and 761 cm-1 are due to out of plane bending vibrations of the O-H and N-H respectively. The band at 675 cm-1 is due to C-Br stretching vibration of pyridinium moiety. All the observed values coincide with the expected range of wavenumber reported in literature [30]. 3.2 NMR spectroscopic studies 3.2.1 Proton NMR studies The 1H NMR spectrum of 2-A5BPB is depicted in Fig.2. There are eight distinct signals in the spectrum confirming the presence of eight different types of hydrogen atoms in the

molecular skeleton of 2-A5BPB. The NH2 protons (H3A and H3B) generate a signal at δ 6.383 ppm. The hump at δ 6.406 ppm is due to +N1-H1 group proton. It is expected that aromatic hydrogens generate signals in the region 6.5 to 9.0 ppm [31]. The signals at δ 7.573 and 7.567 ppm are assigned to another pair (H2 and H3) of protons present in the amino group of 2-amino5-bromopyridinium moiety. The signal appearing at δ 7.863 ppm is due to H5 present in the second carbon from heterocyclic nitrogen. The chemically and magnetically equivalent protons H28, H32, H34 and H38 exhibit a signal at δ 7.342 ppm. The signal at δ 7.306 ppm is due to another set of equivalent protons, H29, H31, H35 and H37 of the aromatic ring of benzilate moiety. The signal at δ 7.282 ppm is due to a pair of equivalent (H30 and H36) aromatic ring protons. Due to the desheilding effect of oxygen atom, the signal for hydroxy proton is far downfield shifted, which is not seen in the spectrum [32]. 3.2.2 13C NMR studies The 13C NMR spectrum of 2-A5BPB is depicted in Fig.3. The signal due to the carboxyl carbon C26 of benzilate moiety appears in the highly deshielded region at δ 178.5 ppm is the, because of its attachment with two electronegative oxygen atoms. The strong signal at δ 155.9 ppm is assigned to C1 carbon atom bearing the -NH2 group of 2-amino-5-bromopyridinium moiety. The two signals at δ 143.2 and 142.9 ppm correspond to the C3 and C5 carbon atoms of the same cationic moiety respectively. Another set of carbons (C2 and C4) in 2-amino-5-bromo pyridinium moiety generate a signal at δ 112.4 ppm. The signal at δ 142.7 ppm is assigned to a pair of equivalent aromatic carbons, C33 and C27 of two identical benziliate moieties. The carbon signal at δ 127.9 ppm is due to a group of aromatic carbon atoms, C28, C32, C34 and C38 of benzilate moiety. The carbon signal at δ 127.5 ppm reveals the presence of another set of equivalent aromatic carbons, C29, C31, C35and C37 in benzilate moiety. The identical C36 and C30 carbon atoms of benzilate moiety generate a signal at δ 106.3 ppm, while the signal at δ 81.0 ppm corresponds to C25, which connects both the phenyl rings. 3.3 Single crystal X-ray diffraction analysis Single crystal X-ray diffraction analysis (SXRD) reveals that 2-A5BPB crystallizes in monoclinic system (P21/c space group). They are two 2-amino-5-bromopyridinium cations and two benzilate anions in the asymmetric unit. The lattice parameters obtained are a = 12.8676 (5) Å, b = 12.6839 (5) Å, c = 22.0108 (10) Å, α = 90˚, β = 90.831(1) and γ = 90˚. The unit cell volume is 3592.0 (3) (Å3) and the number of molecules per unit cell (Z) equals to 4. The

crystallographic information data are listed in Table 1. The ORTEP (50% probability) of 2-A5BPB is shown in Fig.4. The selected bond lengths and bond angles are given in Tables 2 and 3 respectively, and the complete structure were deposited at Cambridge crystallographic data centre (CCDC No: 1818870). The proton transfer occurred from the –COOH group of benzilic acid to proton acceptor of N atom in the 2-amino-5-bromopyridine molecule. The variation of bond lengths and bond angles form neutral reactants to ionic product. In the first benzilate moiety, C12-O2 and C12-O3 bond lengths are found to be 1.258 Å and 1.238 Å respectively. In the second molecule, the C26O5 and C26-O6 bond distances are found to be 1.254 and 1.242Å. These values deviate from the normal C-O distance of 1.43Å and C=O distance of 1.240 Å confirming the formation of COO- anion and prevalence of resonance in carboxylate anion developing partial double bond character. The C-N-C angles are very sensitive to protonation in amines [34]. In a pyridinium cation, the angle C-N-C is found to be always expanded in comparison with the uncharged pyridine. Protonation occurs on the aromatic nitrogen of 2-amino-5-bromopyridine as evidenced by an increase in the C10-N2-C6 angle 120.0˚ from normal angle of 116˚ in unprotonated pyridines [34]. The corresponding angle (C5-N1-C1) in the second pyridinium moiety is 122.9˚. The protonation on the N site of the cation is also confirmed from increased C-N bond distance. In general, the normal bond lengths C1-N1 and C5-N1 in an uncharged 2-amino-5-bromopyridine moiety are 1.38 Å and 1.30 Å respectively which are found to have changed in 2-A5BPB as 1.34 Å (C1-N1) and 1.35 Å (C5-N1) proving the migration of proton from benzilic acid to 2-amino-5bromopyridine during the synthesis. Similar, observations are found in the second aminopyridinium moiety with changes in bond lengths of C6-N2 (1.353Å) and C10-N2 (1.359Å). 3.4. Formation of molecular salt The complex 2-A5BPB formed during the synthesis is confirmed to be a molecular salt (a proton transfer complex) based on the outcome of the spectroscopic studies as well as single crystal X ray diffraction analysis. Further, in this type of organic acid-base reactions, we can predict the nature of the product based on pKa values of both the reactants. If ∆pKa value is less than 0, then only co-crystal is expected. If the difference is greater than 3, then the complex is no doubt, a molecular salt. If the difference is between 0 and 3, then there s a possibility of forming s either a molecular salt or a co-crystal. The value of ∆pKa can be calculated by subtracting pKa

value of the acid from pKa value of base. The pKa value of 2-amino 5-bromo pyridine is 4.65 and that of benzilic acid is 3.05.The ∆pKa value is found to be 1.60 which is less than 3. Therefore, formation of a proton transfer molecular crystal, 2-amino-5-bromopyridine benzilic acid (1:1) is confirmed [35]. 3.5 Hydrogen bonding analysis The single crystal XRD analysis of 2-A5BPB explores the role of various types of hydrogen bonding interactions stabilizing crystal network. Table.4 shows various hydrogen bonding interactions present in 2-A5BPB. In the crystal, the N-Hs of pyridinium moiety act as hydrogen donors and oxygens of carboxylate moiety act as hydrogen acceptors forming different graph sets like ܴଶଶ (8) and ܴଶଶ (11). As shown in Fig.4 (b). There are five different N-H…O hydrogen bonds that lead to the formation of supramolecular structure. The -NH2 and +N-H of pyridinium moieties get connected to benzilate oxygens through various hydrogen bonding interactions such as N1-H1N…O6, N3-H3A ...O5, N2-H2N...O2, N4-H4A…O3 and N4-H4B…O5.The atom, O5 is involved in intermolecular bifurcated hydrogen bonding with D…A distance of 2.803Å. The O5 atom of benzilate forms bifurcated hydrogen bonding with two different –NH2 groups of pyridinium cations via, N3-H3A…O5 and N4-H4B…O5 interactions. The N-H…O interactions are strong enough to lead a stable 3D network, whose D…A distances are within the limit of hydrogen bonding criteria [36]. Interestingly, atom O6 involves in a trifurcated hydrogen bonding interactions, namely, C26-O6…H37, C26-O6…H1 and C26-O6…H1N+. The structure of 2-A5BPB reveals that there are three ring graph sets further stabilizing the crystal network. There are two ܴଶଶ (8) graph sets and each graph set involves eight atoms in which N1 and N3 act as hydrogen donor in one graph set and N2 and N4 act as hydrogen donors in another ring set. In the same graph set, the sets of oxygens, O5 and O6 and O2 and O3 act as hydrogen acceptors. The hydrogen bonding interactions in the graph sets are such as N3-H3A ...O5 (2.803Å), N1-H1N…O6 (2.692Å), N2-H2N...O2 (2.741) and N4-H4A…O3 (2.782). The bond distances are given in parenthesis. The third ring graph set ܴଶଶ (11) involves 11 atoms and the interactions involved are O1-H1…O6 (2.798Å) and O4-H4…O2 (2.801Å) Apart from these interactions, a few intermolecular hydrogen bonding interactions are also observed in the structure of 2-A5BPB like C5-H5…O1 (3.088Å) and C10-H10…O4 (3.153Å) interactions. There are two intramolecular hydrogen bonding interactions too present in the

benzilic acid moieties viz., C14-H14…O1 and C28-H28…O4 with bond lengths, 2.721 Å and 2.803Å respectively. The packing diagram viewed along b axis Fig.4(c) clearly shows various hydrogen bonding interactions 2-A5BPB. 3.6 Hirshfeld surface analysis Hirshfeld surfaces are space-partitioning constructions that summarize the crystal packing into a single 3D surface and the surface is reduced to a 2D fingerprint plot which summarizes the complex information of intermolecular interactions present in molecular crystals [37] Hirshfeld surface depends on the geometry of the molecules, orientation of the molecules in the neighbourhood and the nature of atoms that make close contacts in the selected molecule. The dnorm, de, di, curvedness and shape index of 2-A5BPB are shown in Fig.5.The two distances, ‘de’ and ‘di’ in Hirshfeld surface can be defined as the distance from the point to the nearest nucleus outside the surface and the distance to the nearest nucleus inside the surface respectively. The (Fig.5 (d)) dnorm surface shows intermolecular contacts relative to the van der Waals radius by way of a simple red (higher electron density) and blue (lower electron density) colour code [37]. The large deep red spots (H3A and H1N and carboxylate oxygens) on the dnorm of Hirshfeld surfaces indicate the close-contact interactions, which are mainly responsible for the hydrogen bonding. The small red spots on the surfaces represent the C-H…π interaction. The shape index (Fig.5 (e)) indicates the π…π stacking of the molecule which is accounted from the red triangles representing concave regions and the blue triangles representing concave regions. Low values of curvedness (Fig.5 (f)) are associated with essentially flat areas of the surface while areas of sharp curvature possess a high curvedness and tend to divide the surface into patches associated with contacts between neighbouring molecules. The low curvedness reveals close contacts in the molecule, probably, due to covalent bonding interactions [38]. The large flat region shown by a blue outline on the curvedness surface indicates π…π stacking of the molecules. The H…H interactions are reflected in the middle of scattered points in the 2D finger print plot. The H-H interactions contribute to the level of 46.0% to the total Hirshfeld surfaces. This can be explained based on the fact that 2-A5BPB has various types of hydrogens such as

aromatic C-H, -NH2, -OH and N+-H groups. The C…H/H…C interactions have the second significant contribution to the total Hirshfeld surfaces (21.2%). The O…H and H…O interactions in 2-A5BPB totaling 17.6% for each molecule in the asymmetric unit controls the compactness of 2-A5BPB. The O…H/H…O interactions are represented by ‘spikes’ in the fingerprint plot from Fig.6. The interactions developed on oxygen atoms represent the closet contacts in this structure and can be viewed as dark red spot on the dnorm surface (Fig.5 (d)). Different atom - atom interactions in 2-A5BPB crystalline surface are shown in Fig.6. The 2D fingerprint map of the total Hirshfeld surfaces comprise of 21.2%, 46.0% and 17.6% of H…C, H…H and O…H interactions respectively. The C…C contacts contribute very less (2%) where as contribution of O...H contacts is large (17.6%) with respect to the total Hirshfeld surface area of molecules.. 3.7 Thermal analysis TG-DTA and DSC analyses were carried out to understand the thermal stability and to determine melting point of 2-A5BPB. The TG-DTA thermograms are shown in Fig.7 (a). From the thermograms, it is inferred that 2-A5BPB is stable up to 170˚C while DTA shows an endothermic dip at 170˚C which is assigned as a decomposition temperature of the crystal. It indicates that both decomposition and melting occur simultaneously. The decomposition of the sample starts at 170ºC which extends up to 220oC. The TG curve shows the appreciable weight change of 80% indicating rapid decomposition in this temperature range of 170-280oC. The endothermic peak in DTA at 170ºC is due to melting point of 2-A5BPB. The sharpness of the endothermic dip infers good degree of crystallinity of the grown crystal [39]. TG curve shows a weight loss which is due to complete degradation and subsequent volatilization into various gaseous fractions like CO2, NO2 and NH3. The absence of any endothermic dip before the decomposition temperature of 170oC in DTA clearly proves that the material is directly decomposed without melting. DSC analysis of both the reactants and the product (Fig, 7. (b)) reveals that the melting point of benzilic acid is 148.3˚C (Expected: 152 -153˚C), melting point of 2-amino-5brompyridine is 138.3 ˚C (Expected: 133-138˚C). The melting point of the product, 2-A5BPB is159.4 ˚C. The change in the melting point of the 2-A5BPB from both the reactants clearly

evidences the formation of a product. It is also evident that the product is a molecular salt. If the product formed is a co-crystal, then the melting point of the product would have been nearest to any one of the reactants. As the compound formed is ionic in nature, the product has higher melting point than the two reactants. 3.8 Optical activity of UV- Vis studies The electronic absorption spectrum of 2-A5BPB is shown in Fig.8. The absorption of UV and visible light promote electrons in the σ and π orbital from the ground state to higher energy states. The compound 2-A5BPB shows maximum absorption at 225 and 265 nm, in the ultraviolet region. The absorption is assigned to the n-π* and π-π* transition due to the presence of NH2 group. There is no absorption observed in the entire visible region of the spectrum which is an added advantage for the materials having NLO properties [40]. 3.9 Photoluminance spectral analysis Photoluminescence (PL) study is a prominent tool to provide relatively direct information about the physical properties of materials at the molecular level by way of shallow and deep level defects and gap-states [41]. The compound, 2-A5BPB was excited at 380 nm. The fluorescence emission spectrum was recorded in the range of 300 to 700 nm. This emission spectrum is depicted in Fig.9. Two peaks, one at 395 and another at 599 nm are seen in the emission spectrum. The results indicate that 2-A5BPB crystal has violet and yellow fluorescence emissions [42]. 4. Computational Calculations 4.1 Optimized geometry analysis The molecular structure of 2-A5BPB in the ground state was optimized and the structural parameters have been computed by using Gaussian 09 program. The optimized molecular structure of 2-A5BPB was calculated using B3LYP method at 6-311G(d,p) level of basis set [43]. The optimized structure of 2-A5BPB is shown in Fig.10 and the corresponding bond lengths and bond angles are given in Tables 2 and 3 respectively. The bond length and angle values determined from single crystal XRD analysis are also incorporated in Tables 2 and 3 respectively.

The bond lengths of all C-H bonds in 2-A5BPB are 0.92Å determined by X-ray diffraction method while the corresponding optimized calculated values are around 1.08Å. The bond length of protonated N+-H is found to be 0.86Å from X-ray structural analysis, whereas the corresponding value in optimized structure is 1.03Å. The entire C = C bond length values in the ring found from experimental determination equal 1.39Å. The calculated C62-C66 single bond (1.577Å) and C38-C33 single bond(1.552Å) bond distances are longer than other C-C (=1.54Å) bond distances in phenyl ring, because C66 and C33 carbons bear a carboxylate group which is involved in (N16-H17…O58, N3-H20…O57 and N2H3…O29 and N4-H4A…O30) hydrogen bonding interactions. The C26-N16 (1.367Å) and C18-N16 (1.367Å) bond distances are longer than the experimental values of 1.347Å and 1.347Å respectively. The same trend is seen in the second pyridinium moiety also. The C7-N2 (1.343) and C10-N2 (1.342Å) bond distances are longer than experimental values of 1.359 and 1.353 Å respectively which is also due to the involvement of hydrogen bonding interaction. It is also interesting note that the benzilate moiety forms intramolecular hydrogen bonding interaction within itself. The hydroxy group of benzilate anion acts as donor as evident from the formation of O31-H32…O30 and O59-H60…O57 interactions. 4.2 Molecular electrostatic potential analysis In order to predict electrophilic and nucleophilic reactive sites in the 2-A5BPB molecule, the molecular electrostatic potential (MEP) surface was obtained at the B3LYP/6-311G level in the optimized geometry. The MEP surface study is a tool to understand the size and shape of the molecule. It also indicates the positive, negative and neutral electrostatic potential regions denoted in different colours. Molecular structure and its physicochemical properties can be very well correlated to explore the reactivity of a system [44]. In MEP surface plots, the blue coloured positive regions are highly susceptible to nucleophilic attack while the red coloured negative regions are the preferred regions for electrophilic attack. The blue colour indicates the strongest attraction for electrons and red colour indicates the strongest repulsion for electrons. The electrostatic potential region was calculated between -7.520× e-2 and 7.520 × e-2 and the map is shown in Fig.11. The electrostatic potential surface of the molecule can be represented by different colour grading [45]. Fig.11 (a) and Fig.11 (b) represent MEP maps of 2-

amino-5-bromopyridine and benzilic acid respectively whereas the MEP map of the product (2-A5BPB) is given in Fig.11(c). It is observed that the red negative region over ring nitrogen of pyridine is changed to blue coloured positive region in the product confirming the protonation. Similarly, the blue region over free carboxylate group in the reactant benzilic acid is changed to red region in the carboxylate moiety due to deprotonation during the reaction. In 2-A5BPB crystal, it seen that regions having the negative potential with enhanced electron density are present over the electronegative of oxygen atom indicated by the large red region and the regions having the positive potential are over the hydrogen atoms which confirm the existence of the intramolecular and intermolecular interactions observed in the molecule. The MEP study proves the involvement of carboxylate oxygens and –NH, -NH2 hydrogen atoms in the N-H…O and O-H…O hydrogen bonding interactions. 4.3 Natural bond orbital analysis The NBO analysis deals with the role of electron delocalization in stability, aromaticity and reactivity in many organic compounds. The NBO analysis clearly authenticates the existence of intermolecular, intramolecular and non-covalent interactions. The second order Fock matrix is generally applied to evaluate the donor-acceptor interactions [46]. The NBO calculations were performed using NBO 3.1 program. The intermolecular interactions are formed by the orbital overlap between π(C-C) bonding orbital and π*(C-C) antibonding orbital which causes intramolecular charge transfer, promoting the stability of the system. The most important charge transfer occurs from the loan pair orbitals which give rise to the stabilization of the molecule. The hyperconjugative interactions are LP(1)N2→LP*(1)H3, LP(1)N4→LP*(1)C10 and LP(1) N19→π* N16-C18 with energy 72.74, 84.49 and 83.5 kcal/mol, respectively which is due to the N-H…O intermolecular hydrogen bonding interactions. The other hyperconjugative interactions are LP3 O29 →LP*(1) H3 and LP (3) O57→π*O58-C66 which gives the strongest stabilization by 389.83 and 79.88 kcal/mol, respectively.Table.5. The

conjugative

interactions

in

pyridinium

moiety

are

π(N2–C7)→LP*C10,

π(C9-C11)→LP(1)C13, π(C9-C11)→π*N2-C7, LP(1)C13→π*C9-C13, LP(1)C13→π*C9-C11, π*(N2-C11)→π*C9-C11, πN16-C18→π* C26-C28, πc22-C24→π*N16-C18, π* C26-C28 →π* C22-C24, and π* N16-C18→π* C22-C24

with stabilization energies, 54.84, 41.25, 35.88,

93.93, 212.72, 24.75, 29.23, 60.25 and 45.75 kcal/mol respectively.

In 2-A5BPB, all intramolecular interactions are formed by the orbitals overlap through ππ* conjugative interaction are presented in Tables 5 and S1. The high stabilization energies 255.11, 208.04, 224.91 and 237.75 kcal/mol are found in π*C36-C39→ π* C34-C49, π*C36C39→ π*C43-C47, π*C75-C83→ π*C61-C69, π*C77-C81→ π*C65-C67 conjugative interactions, which lead to intramolecular charge transfer process enhancing optical properties of 2-A5BPB molecule. 4.4 Atomic charge analysis Mulliken atomic charge was calculated using B3LYP method at 6-311G(d,p) basis set. Mulliken charge population of 2-A5BPB is shown pictorially in S2. Mulliken population analysis shows the presence of four electronegativity nitrogen atoms, namely, N2, N4, N16 and N19. The high negative charge on N2 is due to hydrogen bonding (N2-H3…O29) interactions. The atoms N16 and N19 also posses negative charge because of the presence of hydrogen bonding (N16-H17…O58). All the hydrogen atoms are positively charged. The formation of N-H…O hydrogen bonds between pyridinium and benzilate moieties is strongly supported by fact that the hydrogen atoms, H3, H5, H6, H17, H20 and H21 of pyridinium moiety show higher positive values compared to the phenolic hydrogens, H32 and H60. The highest positive value for the H3 is due to the strong intermolecular hydrogen bond interactions (N2-H3…O29). The atoms H17 and H21 also have high positive charge due to the presence of hydrogen bonding (N16-H17…O58) and (N19-H21…O57). The presence of O-H…O interaction is confirmed by the high positive charge over H32 and H60. They are also involved in intramolecular hydrogen bonding interactions (O59-H60…O57 and O31-H32 …O30). All the carbon atoms, except C10 and C18 are negatively charged. The positive charge over C10 and C18 can be explained based on their attachment with electronegative nitrogen atoms of the amino group. The carboxylic carbons C33 and C66 are also found to have positive charge due to their attachment with electronegative oxygen atoms. 4.5 HOMO-LUMO analysis and DOS-PDOS analysis The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) play a significant part in the elucidation of molecular electronic properties. [47]. The HOMO-LUMO energies have been calculated for 2-A5BPB using B3LYP method at

6-311G(d,p) basis set. The HOMO orbital represents the ability to donate electrons while LUMO orbital represents the capability to accept electrons. In general, the HOMO-LUMO orbital energy gap reveals the kinetic stability and chemical reactivity of a molecular compound. The computed HOMO-LUMO orbital energy gap value for 2-A5BPB is 3.29eV. The HOMO and LUMO are presented in Fig.12 (a). The lower energy gap value between HOMO and LUMO facilitates charge transfer in 2-A5BPB. The experimental band gap energy from UV-vis spectrum for 2-A5BPB molecule is 3.48 eV (Fig.12 (b)) which is close to the value obtained from HOMOLUMO calculations. The small HOMO-LUMO energy gap of 2-A5BPB is due to the strong electron donor-acceptor ability of the stabilized LUMO. Density of states (DOS) and partial density of states (PDOS) spectra are used to investigate the participation of each fragment molecular orbital in the overall molecular orbitals in the bonding and antibonding interactions [48]. The DOS spectrum in Fig.12(c) shows the population analysis per orbital in the energy range of 0 to -20 eV while the PDOS (Fig.12 (d)) shows the percentage contribution of NH2, Br, COO-, NH and ring fragments to each molecular orbital. It is inferred from the Fig.12 (d) that the NH group contributes more quantum to the molecular orbitals in the range of -5 to -12 eV in HOMO and 0 to -2 eV in LUMO orbitals. The fragments COO- and Br group also contribute noticeably. Therefore, the DOS and PDOS spectra clearly prove that the +NH, COO- and Br group significantly enhance the optical properties of 2-A5BPB molecule.

4.6 Hyperpolarizability calculations The quantum chemistry based calculation of nonlinear optical properties of a molecule has a significant role in the design of materials for modern communication technology and optical signal processing [49]. The NLO properties such as dipole moment (µ), polarizability (α), anisotropy polarizability (∆α) and first hyper polarizability (β) of the 2-A5BPB have been calculated using B3LYP/6-311G(d,p) level of theory and the values are provided in Table.6. These values help to understand electronic polarization responsible for the molecular nonlinear phenomenon and the structure-property relationship [50]. The calculated values of µ, α, ∆α and β for 2-A5BPB are16.4904 D, 23.9211×10-24 esu, 9.5446 × 10-24 esu and 1.286 × 10-30 esu respectively. Since urea is one of the prototypical

molecules used in the study of NLO properties of molecular systems, its value is used frequently as the threshold value for comparative purpose. The first order hyperpolarizability value is found to be 2.1 times that of urea using the same level of theory. 5. Z-scan technique The nonlinear optical materials with large intensity dependent refractive index and absorption coefficients are useful in device applications and these parameters determine whether laser beam will undergo self focussing or defocussing as it propagates in the material medium [51]. Electron donors and acceptors in organic molecules involve in intermolecular interactions that facilitate focussing properties of laser beam. Third order nonlinear optical characterization of the crystal is carried out using the single-beam, Z-scan technique. The results of the open and closed aperture Z-scan measurement of the crystal are shown in Fig.13 (a) and 13(b) respectively. The negative change in refractive index shows a self defocusing effect. The calculated value of the nonlinear refractive index n2 is 7.23 x 10-7 cm2/W, found from the open aperture Z-scan curve. The nonlinear absorbtion is a two photon absorption process as the minimum lies near the focus. The nonlinear absorption coefficient (β) is found to be 2.688 x 10-4 cm/W while the third order susceptibility (χ3) of 2-A5BPB is 6.326 x 10-7 esu. The π-electron cloud movement (delocalization) from the donor to acceptor is responsible for the absolute susceptibility which makes the molecule highly polarized. The delocalization also gradually enhances the hyperpolarizability and the nonlinear susceptibility values of 2-A5BPB. Greater the magnitude of these values, higher will be third order NLO properties [52]. Conclusion A novel organic proton transfer crystal (2-A5BPB) was synthesized and single crystals were grown by the slow - evaporation and solution growth technique from methanolic solution. The crystal structure was established by single crystal X-ray diffraction analysis, which showed that an extensive hydrogen bonding network exists between the donors and acceptors due to the proton transfer during the reaction. The +N-H group stretching vibration band at 2399 cm-1 is a strong evidence for the formation of 2-A5BPB. Both proton NMR and

13

C NMR

confirmed the formation of product. UV-vis absorption studies indicated the absence of absorption in the visible region. Photoluminance study revealed that the 2-A5BPB could be used

as a potential candidate in NLO applications. The thermal analysis showed that the crystal is stable up to 170ºC and no water molecule is present in the crystal. Complete structural analysis of the crystal 2-A5BPB, reveals the variety of hydrogen bonding interactions such as intermolecular, intramolecular, bifurcated and trifurcated and ring graph sets. Hydrogen bonding interactions including ܴଶଶ (8) and ܴଶଶ (11) facilitate optical properties. Optimized stable molecular structure of 2-A5BPB was calculated using B3LYP/6-311G(d,p) basis set. MEP map confirmed the migration of proton form benzilic acid to 2-amino-5-bromopyridine during the synthesis. The smaller HOMO-LUMO gap of 3.92eV is responsible for excellent electronic properties of the molecule. The hyperconjugative interactions found from NBO analysis are LP3 O29 →LP*(1) H3 and LP (3) O57→π*O58-C66 with 389.83 and 79.88 kcal/mol, stabilization energies respectively confirm delocalization of electrons in the product. These results correlate with the findings of SXRD analysis and theoretical calculations for the presence of excessive hydrogen bonding interactions. The calculated first-order hyperpolarizibility (β) of 2-A5BPB is 1.286 x 10-30e.s.u., which is 2.1 times greater than that of urea indicating the usefulness of 2-A5BPB in optical applications. The nonlinear refractive index and nonlinear absorption co-efficient of 2-A5BPB confirms its suitability in third order NLO applications. Acknowledgement One of the authors, M. Dhandapani, would like to thank the University Grants Commission (UGC), New Delhi, India for the financial support (Major Research Project Grant No. 43-200/2014(SR) to carry out the research work. S. Madhankumar expresses his gratitude to Dr. R. Nagarajan, School of Chemistry and UGC Networking Centre, Hyderabad, for awarding visiting research fellowship. Reference [1] D.S. Chemla, J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. [2] S. Sathiya, M. Senthilkumar, C. Ramachandra Raja, Crystal growth, Hirshfeld surface analysis, DFT study and third order NLO studies of thiourea 4 dimethyl aminobenzaldehyde,

J.

Mol.

Struct.

1180

(2019)

81-88.

Doi.10.1016/j.molstruc.2018.11.067 [3] L. Kuang, Q. Chen, E.H. Sargent, Z.Y. Wang, Fullerene-Containing Polyurethane Films with Large Ultrafast Nonresonant Third-Order Nonlinearity at Telecommunication Wavelengths, J. Am. Chem. Soc. 125 (2003) 13648-13649. Doi.10.1021/ja0376240.

[4] Q. Chen, E.H. Sargent, N. Leclerc, A.J. Attias, Ultrafast nonresonant third-order optical nonlinearity of a conjugated 3,3′-bipyridine derivative from 1150 to 1600 nm Appl. Phys. Lett. 82 (2003) 4420-4422. Doi.10.1063/1.1584517. [5] S. Manivannan, S. Dhanuskodi, Synthesis, crystal growth, structural and optical properties of an organic NLO material, J. Cryst. Growth 262 (2004) 473–478. Doi.10.1016/j.jcrysgro.2003.10.029 [6] E.D. D'Silva, G. Krishna Podagatlapalli, S. Venugopal Rao, S.M. Dharmaprakash, Study on third-order nonlinear optical properties of 4-methylsulfanyl chalcone derivatives using picosecond

pulses,

Mater.

Res.

Bull.

47

(2012)

3552–3557.

Doi.10.1016/j.materresbull.2012.06.063 [7] P.S. Patil, M.S. Bannur, D.B. Badigannavar, S.M. Dharmaprakash, Study on nonlinear optical

properties

of

2,4,5-trimethoxy-4′-bromochalcone

single

crystal,

Opt.

Laser.Technol. 55 (2014) 37–41. Doi.10.1016/j.optlastec.2013.06.037. [8] M.O. Senge, M. Fazekas, E.G.A. Notaras, W.J. Blau, M. Zawadzka, O.B. Locos, E.M.N. Mhuircheartaigh, Nonlinear Optical Properties of Porphyrins Adv. Mater 19 (2007) 2737–2774. Doi.10.1002/adma.200601850. [9] M. Thangaraj, G. Ravi, T.C. Sabari Girisun, G. Vinitha, A. Loganathan, Ethylenediaminium di(4-nitrophenolate): A third order NLO material for optical limiting applications, Spectrochim. Acta A 138 (2015) 158–163. Doi.10.1016/j.saa.2014.11.027 [10] Jerin Susan John, D. Sajan, T. Umadevi, K. Chaitanya, Pranitha Sankar, Reji Philip, Synthesis, crystal structure, vibrational spectral analysis and Z-scan studies of a new organic crystal N,N′dimethylurea ninhydrin: A scaled quantum mechanical force field study, Opt. Mat. 48 (2015) 233–242. Doi.10.1016/j.optmat.2015.08.002. [11] T. Shanmugavadivu, M. Dhandapani, S. Naveen, N.K. Lokanath, Synthesis, structural and spectral characterization of a novel NLO crystal N,N′-diphenylguanidinium picrate: diacetone

solvate,

J.

Mol.

Struct.

1144

(2017)

237–245.

Doi.10.1016/j.molstruc.2017.05.015. [12] P. Muthuraja, T. Joselin Beaula, S. Balachandar, V. Bena Jothy, M. Dhandapani, Hydrogen bonding interactions and supramolecular assemblies in 2-amino guanidinium 4-methyl benzene sulphonate crystal structure: Hirshfeld surfaces and computational calculations, J. Mol. Struct. 1146 (2017) 723–734. Doi.10.1016/j.molstruc.2017.06.055. [13] P. Muthuraja, T. Joselin Beaula, T. Shanmugavadivu, V. Bena Jothy, M. Dhandapani, Hydrogen bonded R22(8) graph set in inducing charge transfer mechanism in

guanidinium-3,5-dinitrobenzoate: A combined experimental, theoretical and Hirshfeld surface study J. Mol. Struct. 1137 (2017) 649–662. Doi.10.1016/j.molstruc.2017.02.067. [14] I.P. Bincy, R. Gopalakrishnan, Synthesis, growth and characterization of new organic crystal: 2-Aminopyridinium p-Toluenesulfonate for third order nonlinear optical applications,

J.

Cryst.

Growth

402

(2014)

22–31.

Doi.org/10.1016/j.jcrysgro.2014.03.024. [15] S. Madhankumar, P. Muthuraja, M. Dhandapani, Molecular properties, crystal structure, Hirshfeld surface analysis and computational calculations of a new third order NLO organic crystal, 2-aminopyridinium benzilate J. Mol. Struct. 1181 (2019) 118–130. Doi.10.1016/j.molstruc.2018.12.048. [16] T. Baraniraj, P. Philominathan, Growth and characterization of organic nonlinear optical material:

Benzilic

acid,

J.

Crys.

Growth

311

(2009)

3849–3854.

Doi.10.1016/j.jcrysgro.2009.05.033. [17] G. Anandha babu, R. Perumal Ramasamy, P. Ramasamy Synthesis, crystal growth and characterization of an efficient nonlinear optical D–π–A type single crystal: 2Aminopyridinium 4-nitrophenolate 4-nitrophenol, Materials Chemistry and Physics 117 (2009) 326–330. [18] P. Muthuraja, M. Sethuram, T. Shanmugavadivu, M. Dhandapani, Single crystal X-ray diffraction and Hirshfeld surface analyses of supramolecular assemblies in certain hydrogen bonded heterocyclic organic crystals, J. Mol. Struct. 1122 (2016) 146–156. Doi: 10.1016/j.molstruc.2016.05.083. [19] H. Sato, J. Dybal, R. Murakami, I. Noda, Y. Qzaki, Infrared and Raman spectroscopy and quantum chemistry calculation studies of C-H⋯O hydrogen bondings and thermal behavior of biodegradable polyhydroxyalkanoate,J. Mol. Struct. 744-747 (2005) 35-46. doi:10.1016/j.molstruc.2004.10.069. [20] G. M. Sheldrick, Crystal structure refinement with SHELX-97, Acta Crystallogr. C Cryst. Struct 71 (2015) 3-8. Doi: 10.1107/s2053229614024218. [21] G. M. Sheldrick, A short history of SHELX, Acta Crystallogr. Sect. A 64 (2008) 112-122. Doi: 10.1107/s0108767307043930. [22] C. F. Macrae, I. J. Bruno, J. A. Chisholm. P. R. Edgington, P. McCabe, E. Pidcock, L.R. Monge, R, Taylor, J.V.D. Streek, P.A. Wood, J. Appl. Crystallogr .41 (2008) 46-470. [23] A.L. Spek, A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, The Netherlands, 1999.

[24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09’, Revision E.01, Gaussian Inc., Wallingford, CT, 2013. [25] E. D. Glendening, A. E. Reed, J. ECarpenter, F. Weinhold, NBO Version 3.1, Theoretical Chemistry Institute, University of Wisconsin, Madison. [26] S. K Wolf, D. J. Grimwood, J. J. McKinnon, M. J. Turner, D. Jayatilaka, M. A. Spackman, CrystalExplorer Version 3.1, University of Western Australia, 2012. [27] M. A. Spackman, D. Jayatilaka, Hirshfeld surface analysis, Cryst.Eng.Comm 11 (2009) 19-32. Doi.10.1039/b818330a. [28] M. A. Spackman, J. J. Mckinnon, Fingerprinting intermolecular interactions in molecular crystals, Cryst.Eng.Comm 4 (2002) 378-392. Doi.10.1039/b203191b. [29] K. Sathya, P. Dhamodharan, M. Dhandapani: structural characterization and DFT study of a new optical crystal: 2-amino-3-methylpyridinium-3,5-dinitrobenzoate: Opt. Laser Technol. 101 (2018) 328-340. Doi:10.1016/j.optlastec.2017.11.027. [30] S. George, “Infrared and Raman Characteristics Group Frequencies, Tables and Charts”, Third Edition, Wiley, Chichester (2001). [31] H.O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR spectroscopy, John wiley & Sons, Chichester, 1988. Doi.10.1016/S0003-2670 (00)81981-9. [32] P. Dhamodharan, K. Sathya and M. Dhandapani: Studies on synthesis, structural, luminescent and thermal properties of a new non-linear optical crystal: 4-amino- 4H1,2,4-triazol-1-ium-3-hydroxy-2,4,6-trinitrophenolate, Physica B, 508 (2017) 33-40. Doi: 10.1016/j.physb.2016.12.009.

[33] D.L. Pavia, G.M. Lampman, G.S. Kriz, J.R. Vyvyan, “Introduction to Spectroscopy”, Fourth Edition, Wiley, Chichester (2001). [34] S. Madhankumar, P. Muthuraja, M. Dhandapani, Molecular properties, crystal structure, Hirshfeld surface analysis and computational calculations of a new third order NLO organic crystal, 2-aminopyridinium benzilate J. Mol. Struct. 1181 (2019) 118–130. Doi.10.1016/j.molstruc.2018.12.048. [35] A. Lemmerer, S. Govindraju, M. Johnston, X. Motloung, Kelsey L. Savig Co-crystals and molecular salts of carboxylic acid/ pyridine complexes: can calculated pKa's predict proton transfer? A case study of nine complexes, Cryst. Eng. Comm, 17 (2015), 3591-3595.Doi: 10.1039/c5ce00102a. [36] G.A. Jaffrey, “An Introduction to Hydrogen Bonding”, First Edition, Oxford University Press, New York (1997). [37] M. Skovsen, H.F. Christensen, J. Clausen, C. Overgaard, T. Stiewe, E. De gupta, Mueller, M.A. Spackman

and B.B. Iversen: Synthesis, Crystal Structure, Atomic

Hirshfeld Surfaces, and Physical Properties of Hexagonal CeMnNi4, Inorg. Chem., 49 (2010) 9343-9349. Doi.10.1021/ic100990a. [38] D. Jayatilaka, J.J. McKinnon and M.A. Spackman: Towards quantitative analysis of intermolecular interactions with Hirshfeld surfaces, Chem. Commun., 37 (2007) 3814. Doi.10.1039/b704980c. [39] V. Siva shankar, R. Sankar, R. Siddheswaran, R. Jayavel and P. Murugakoothanan: Growth and characterization of tetra l-lysine alanine mono hydrochloride dihydrate, a new semi-organic nonlinear optical single crystal, Mat. Chem. Phy., 109 (2008) 119-124. Doi.10.1016/j.matchemphys.2007.11.008. [40] N. Vijayan, R. Ramesh Babu, R. Gopalakrishnan, P. Ramasamy and W.T.A. Harrison: Growth and characterization of benzimidazole single crystals: a nonlinear optical material, J. Cryst. Growth 262 (2004) 490-498. Doi.10.1016/j.jcrysgro.2003.08.082. [41] M. Magesh, G. Bhagavannarayana, P. Ramasamy, Synthesis, crystal growth and characterization of an organic material: 2-Aminopyridinium succinate succinic acid single crystal, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 765 - 771. Doi.10.1016/j.saa.2015.05.077. [42] P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields, J. Phys. Chem. 98 (1994) 11623-11627. Doi: 10.1021/j100096a001.

[43] T.

Shanmugavadivu,

S. Balachandar,

K.

Senthilkumar,

M.

Dhandapani,

P.

Muthuraja,

M. SethuRaman, Theoretical and experimental evaluation of a new

organic proton transfer crystal aminoguanidinium p-nitrobenzoate monohydrate for optical

limiting

applications,

J.

Phys.

Chem.

Solids

111

(2017)

82-94.

doi:10.1016/j.jpcs.2017.07.015. [44] J. Murray and K. Sen: Molecular Electrostatic Potentials: Concepts and Applications, Relationships of Electrostatic Potentials to Intrinsic Molecular Properties, Comp. Theor. Chem., (1996) 649-660. [45] E. Scrocco and J. Tomasi: Reactivity and Intermolecular Forces: An Euristic Interpretation by Means of Electrostatic Molecular Potentials, Electronic Molecular Structure, Advances in Quantum Chemistry, 11 (1978) 115 - 193. [46] F. Weinhold, C.R. Landis, “Valency and Bonding: A Natural Bond Orbital DonorAcceptor Perspective”, Cambridge University Press, New York (2005). [47] R.S. Mulliken: Electronic Population Analysis on LCAO [Single Bond] MO Molecular Wave Functions, J. Chem. Phys., 23 (1955) 1833. [48] I. Fleming, “Frontier Orbitals and Organic Chemical Reactions”, Wiley, London (1976). [49] T. JoselinBeaula, P. Muthuraja, M. Dhandapani, I.H. Joe, V.K. Rastogi, V. BenaJothy, Biological applications and spectroscopic investigations of 4-nitrophenol-urea dimer: A DFT approach, Chem. Phys. Lett. 645 (2015) 59-70. Doi:10.1016/j.cplett.2015.12.029. [50] C. Lei, Z. Yang, B. Zhang, M.-H. Lee, Q. Jing, Z. Chen, X.-C. Huang, Y. Wang, S. Pan, M.R.S.A. Janjua, The influence of hydrogen bonding on the nonlinear optical properties of a semiorganic material NH4B[d-(+)-C4H4O5]2.H2O: a theoretical perspective, Phys. Chem. Chem. Phys., 16 (2014) 20089-20096. Doi:10.1039/c4cp02539c. [51] S. Biswas, A.K. Kole, C.S. Tiwary, P. Kumbhakar, Enhanced nonlinear optical properties of graphene oxide-silver nanocomposites measured by Z-scan technique, RSC Adv. 6 (2016) 10319-10325. Doi:10.1039/c5ra21000c.Z [52] B. Gu, X.-Q. Huang, S.-Q. Tan, M. Wang, W. Ji, Z-scan analytical theories for characterizing

multiphoton

absorbers,

Doi:10.1007/s00340-009-3426-y.

Appl.

Phys.

B.

95

(2009)

375-381.

Table 1 Crystal data and structure refinement parameters of 2-A5BPB Chemical formula

C14H11O3·C5H6BrN2

Molecular weight

802.51

Crystal system, space group

Monoclinic, P21/c

Temperature (K)

293

a, b, c (Å)

12.8676 (5), 12.6839 (5), 22.0108 (10)

b (°)

90.831 (1) 3

V (Å )

3592.0 (3)

Z

4

Radiation type

Mo Ka

m (mm-1)

2.31

Crystal size (mm)

0.20 × 0.15 × 0.10

No. of measured, independent and observed [I > 2s(I)] reflections

84761, 7925, 5808

Rint

0.054 -1

(sin q/l)max (Å )

0.642

R[F2 > 2s(F2)], wR(F2), S

0.054, 0.161, 1.10

No. of reflections

7925

No. of parameters

451

-3

Dρmax, Dρmin (e Å )

0.47, -1.50

Table 2 Bond lengths [Å]for 2-A5BPB Atoms

Observed (Å) Calculated (Å)

Br1-C9

1.879

1.902

N2-H2N

0.86

1.082

N2-C10

1.359

1.342

N2-C6

1.353

1.343

N4-H4A

0.861

1.006

N4-H4B

0.86

1.008

N4-C6

1.311

1.368

C10-H10

0.929

1.084

C10-C9

1.345

1.382

C9-C8

1.404

1.398

C6-C7

1.414

1.41

C8-H8

0.93

1.083

C8-C7

1.358

1.38

C7-H7

0.93

1.083

Br2-C4

1.880

1.904

N1-H1N

0.86

1.032

N1-C1

1.347

1.367

N1-C5

1.347

1.367

C1-N3

1.327

1.316

C1-C2

1.414

1.431

N3-H3A

0.859

1.011

N3-H3B

0.86

1.012

C2-H2

0.93

1.082

C2-C3

1.355

1.363

C3-H3

0.931

1.083

C3-C4

1.399

1.421

C5-H5

0.929

1.08

C5-C4

1.348

1.36

O3-C12

1.238

1.307

O2-C12

1.258

1.22

O1-H1

0.82

0.978

O1-C11

1.422

1.443

C12-C11

1.556

1.552

C20-H20

0.929

1.082

C20-H29

1.384

1.396

C20-C21

1.380

1.394

C19-C11

1.528

1.536

C19-C24

1.392

1.40

C13-C11

1.531

1.531

C13-C14

1.388

1.401

C13-C18

1.383

1.4

C24-H24

0.93

1.082

C24-C23

1.384

1.391

C14-H14

0.931

1.084

C14-C15

1.384

1.392

C23-H23

0.93

1.084

C23-C22

1.378

1.394

C18-H18

0.931

1.084

C18-C17

1.397

1.395

C22-H22

0.93

1.084

C22-C21

1.361

1.391

C21-H21

0.93

1.084

C17-H17

0.931

1.084

C17-C16

1.384

1.393

C15-H15

0.929

1.083

C16-C16

1.351

1.393

C16-H16

0.929

1.082

O6-C26

1.254

1.231

O5-C26

1.242

1.284

O4-H4

0.82

0.975

O4-C25

1.425

1.42

C33-C25

1.521

1.546

C33-C34

1.378

1.399

C33-C38

1.383

1.4

C25-C27

1.530

1.539

C25-C26

1.554

1.577

C32-H32

0.93

1.081

C32-C27

1.390

1.397

C32-C31

1.391

1.394

C27-C28

1.391

1.4

C28-H28

0.93

1.08

C28-C29

1.375

1.398

C34-H34

0.929

1.082

C34-C35

1.388

1.392

C38-H38

0.93

1.082

C38-C37

1.393

1.395

C31-H31

0.93

1.085

C31-C30

1.364

1.391

C35-H35

0.93

1.085

C35-C36

1.350

1.394

C29-H29

0.93

1.085

C20-C30

1.371

1.394

C37-H37

0.93

1.085

C37-C36

1.365

1.392

C30-H30

0.93

1.085

C36-H36

0.929

1.085

Table 3 Bond Angles [˚] for 2-A5BPB Bond angle

Observed (˚)

Calculated (˚)

H2N-N2-C10

118.4

116.6

H2N-N2-C6

118.6

C10-N2-C6

123.0

120.0

H4A-N4-H4B

120

116.2

H4A-N4-C6

119.9

118.0

H4B-N4-C6

120.1

118.0

N2-C10-H9

120.0

115.6

N2-C10-C9

120.3

122.1

H10-C10-C9

120.1

122.4

Br1-C9-C8

119.6

118.9

Br1-C9-C8

120.7

120.8

C10-C9-C8

119.7

118.9

N2-C6-N4

119.1

117.3

N2-C6-C7

117.2

120.9

N4-C6-C7

123.7

121.8

C9-C8-H8

120.1

120.4

C9-C8-C7

119.7

119.2

H8-C8-C7

120.2

120.4

C6-C7-C8

120.4

119.0

C6-C7-H7

119.7

120.3

C8-C7-H7

119.9

120.7

H1N-N1-C1

118.5

120.1

H1N-N1-C5

118.6

116.5

C1-N1-C5

122.9

123.5

N1-C1-N3

118.6

121.5

N1-C1-C2

117.2

116.7

N3-C1-C2

124.2

121.7

C1-N3-H3A

120.1

120.2

C1-N3-H3B

119.9

117.5

H3A-N3-H3B

120

119.6

C1-C2-H2

119.9

117.1

C1-C2-C3

120.2

120.4

H2-C2-C3

119.9

117.1

C2-C3-H3

119.9

120.1

C2-C3-C4

120.2

120.3

H3-C3-C4

119.8

119.6

N1-C5-H5

119.8

119.6

N1-C5-C4

120.6

120.2

H5-C5-C4

119.7

124.1

Br2-C4-C3

121.1

120.6

Br2-C4-C5

120.0

120.4

C3-C4-C5

118.8

119

H1-O1-C11

109.5

104.5

O3-C12-O2

126.3

125.4

O3-C12-C11

117.3

115.0

O2-C12-C11

116.4

119.6

H20-C20-C19

119.2

119.3

H20-C20-C21

119.3

120.2

C19-C20-C21

121.5

120.5

C20-C19-C11

120.7

120.3

C20-C19-C24

117.3

118.0

C11-C19-C24

121.8

120.7

C11-C13-C14

121.0

120.0

C11-C13-C18

121.3

121.0

C14-C13-C18

118.7

118.6

O1-C11-C12

109.9

104.9

O1-C11-C19

107.0

110.0

O1-C11-C13

108.6

107.4

C12-C11-C19

111.1

107.6

C12-C11-C13

107.2

112.1

C19-C11-C13

113.1

114.6

C19-C24-H24

119.5

119.8

C19-C24-C23

121.1

120.4

H24-C24-C23

119.6

119.8

C13-C14-H14

119.6

119.8

C13-C14-C15

120.8

120.7

H14-C14-C15

119.6

119.7

C24-C23-H23

119.9

119.6

C24-C23-C22

120.2

120.3

H23-C23-C22

119.9

120.1

C13-C18-H18

119.9

120.3

C13-C18-C17

120.2

120.6

H18-C18-C17

119.9

119.1

C23-C22-H22

120.2

120.2

C23-C22-C21

119.5

119.5

H22-C22-C21

120.3

120

C20-C21-C22

120.5

120.2

C20-C21-H21

119.8

119.6

C22-C21-H21

119.7

120.1

C18-C17-H17

120.3

119.6

C18-C17-C16

119.6

120.4

H17-C17-C16

120.1

120.0

C14-C15-H15

119.9

120.0

C14-C15-C16

120.3

120.5

H15-C15-C16

119.9

119.3

C17-C16-C15

120.4

119.3

C17-C16-H16

119.9

121.1

C15-C16-H16

119.7

119.5

H4-O4-C25

109.5

102.8

C25-C33-C34

121.8

118.4

C25-C33-C38

121.1

123.1

C34-C33-C38

117.0

118.4

O4-C25-C33

105.5

108.9

O4-C25-C27

111.2

109

O4-C25-C26

109.5

107.9

C33-C25-C27

114.8

110.9

C33-C25-C26

111.8

109.7

C27-C25-C26

104.7

110.4

H32-C32-C27

119.7

118.4

H32-C32-C31

119.6

120.7

C27-C32-C31

120.7

120.8

C25-C27-C32

121.1

119.6

C25-C27-C28

121.0

122

C32-C27-C28

117.7

118.5

O6-C26-O5

125.1

126.7

O6-C26-C25

116.4

120.5

O5-C26-C25

118.5

112.8

C27-C28-H28

119.5

119.7

C27-C28-C29

121.1

120.7

H28-C28-C29

119.5

119.6

C33-C34-H34

119.4

118.6

C33-C34-C35

121.2

120.9

H34-C34-C35

119.4

119.6

C33-C38-H38

119.2

119.0

C33-C38-C37

121.6

120.7

H38-C38-C37

119.2

120.1

C32-C31-H31

119.9

119.5

C32-C31-C30

120.2

120.4

H31-C31-C30

119.9

120.1

C34-C35-H35

119.6

120.6

C34-C35-C36

120.7

120.1

H35-C35-C36

119.7

120.1

C28-C29-H29

119.8

119.5

C28-C29-C30

120.4

120.5

H29-C29-C30

119.8

120.0

C38-C37-H37

120.2

119.4

C38-C37-C36

119.6

120.5

H37-C37-C36

120.1

120

C31-C30-C29

119.9

119.2

C31-C30-H30

120

120.5

C29-C30-H30

120.1

120.4

C35-C36-C37

119.8

119.2

C35-C36-H36

120

120.4

C37-C36-H36

120.2

120.4

Table 4 Hydrogen bonding geometry for 2-A5BPB D-H(Å)

H…A(Å)

D…A(Å)

D-H…A(˚)

Symmetry operation

O(1)--H(1) ..O(6)

0.82

2.31

2.798(3)

118

1-x,-y,1-z

N(1)--H(1N) ..O(6)

0.86

1.84

2.692(4)

168

1-x,-y,1-z

N(2)--H(2N) ..O(2)

0.86

1.89

2.741(4)

173

2-x,-1/2+y,1/2-z

N(3)--H(3A) ..O(5)

0.86

1.95

2.803(5)

171

1-x,-y,1-z

O(4) --H(4) ..O(2)

0.82

2.11

2.801(3)

142

1-x,-y,1-z

N(4)--H(4A) ..O(3)

0.86

1.93

2.782(4)

168

2-x,-1/2+y,1/2-z

N(4)--H(4B) ..O(5)

0.86

1.97

2.790(4)

159

1+x,y,z

C(5) --H(5) ..O(1)

0.93

2.25

3.088(5)

149

C(10)--H(10)..O(4)

0.93

2.37

3.153(4)

142

C(14)--H(14) ..O(1)

0.93

2.36

2.721(5)

103

C(28)--H(28)..O(4)

0.93

2.46

2.803(5)

102

D –H…A

1+x,-1/2-y,-1/2+z

Table.5. Selected interactions of NBO analysis Donor(i) πN 2-C 7 πN 2-C 7 π C 9 - C 11 π C 9 - C 11 LP ( 1) N 4 LP*( 1) C 10 LP ( 1) C 13 π* N 2 - C 7 LP ( 1) N 2 π C 22 - C 24 π C 22 - C 24 π C 26 - C 28 LP ( 1) N 19 LP ( 1) N 19 π* N 16 - C 18 π* N 16 - C 18 π* N 16 - C 18 LP ( 3) O 29 π C 34 - C 49 π C 34 - C 49 π C 36 - C 39 π C 36 - C 39 π C 37 - C 41 π C 37 - C 41 π C 43 - C 47 π C 43 - C 47 π C 45 - C 51 π C 45 - C 51 π C 53 - C 55 π C 53 - C 55 π* C 36 - C 39 π* C 36 - C 39 π C 61 - C 69 π C 61 - C 69 π C 63 - C 73 π C 63 - C 73 π C 65 - C 67 π C 65 - C 67 π C 71 - C 79 π C 71 - C 79 π C 75 - C 83 π C 75 - C 83 π C 77 - C 81 π C 77 - C 81 LP ( 3) O 57 π* C 75 - C 83 π* C 77 - C 81

Acceptor(j) LP*( 1) C 10 π* C 9 - C 11 LP ( 1) C 13 π* N 2 - C 7 LP*( 1) C 10 π* N 2 - C 7 π* C 9 - C 11 π* C 9 - C 11 LP*( 1) H 3 π* N 16 - C 18 π* C 26 - C 28 π* C 22 - C 24 π* N 16 - C 18 σ* N 19 - H 21 σ* N 19 - H 21 π* C 22 - C 24 π* C 26 - C 28 LP*( 1) H 3 π* C 36 - C 39 π* C 43 - C 47 π* C 34 - C 49 π* C 43 - C 47 π* C 45 - C 51 π* C 53 - C 55 π* C 34 - C 49 π* C 36 - C 39 π* C 37 - C 41 π* C 53 - C 55 π* C 37 - C 41 π* C 45 - C 51 π* C 34 - C 49 π* C 43 - C 47 π* C 71 - C 79 π* C 75 - C 83 π* C 65 - C 67 π* C 77 - C 81 π* C 63 - C 73 π* C 77 - C 81 π* C 61 - C 69 π* C 75 - C 83 π* C 61 - C 69 π* C 71 - C 79 π* C 63 - C 73 π* C 65 - C 67 π* O 58 - C 66 π* C 61 - C 69 π* C 65 - C 67

E2 54.84 9.11 41.25 35.88 84.49 61.44 93.93 212.72 72.74 29.23 14.83 17.82 83.5 13.99 0.74 45.75 39.78 389.83 21.88 20.04 18.95 18.14 19.85 19.64 19.68 23.18 20.71 20.46 20.99 19.98 255.11 208.14 20.44 21.7 20.48 20.66 21.1 22.15 20.03 20.78 19.32 19.73 20.47 19.72 79.58 224.91 237.75

I,j 0.19 0.33 0.14 0.26 0.15 0.12 0.14 0.02 0.47 0.23 0.28 0.33 0.21 0.66 0.45 0.08 0.05 0.6 0.28 0.29 0.3 0.3 0.29 0.29 0.28 0.27 0.28 0.28 0.28 0.29 0.01 0.01 0.28 0.27 0.29 0.28 0.28 0.27 0.29 0.28 0.3 0.29 0.29 0.29 0.29 0.01 0.01

F() 0.112 0.051 0.085 0.088 0.125 0.092 0.118 0.087 0.181 0.08 0.058 0.069 0.125 0.092 0.027 0.086 0.061 0.443 0.07 0.069 0.067 0.066 0.068 0.067 0.067 0.071 0.069 0.068 0.069 0.068 0.081 0.081 0.068 0.069 0.069 0.068 0.069 0.07 0.068 0.068 0.068 0.068 0.068 0.068 0.136 0.082 0.083

Table. 6. Dipole Moment, Polarizibility and hyperpolarizibility values of 2-A5BPB

Dipole moment (D) µx = 15.8953 µy = 2.7236 µz = 3.4431 µ = 16.4904

Polarizibility αxx = -335.4833 αxy = -64.2983 αyy = -290.4502 αxz = 14.4270 αyz = -11.7965 αzz = -305.3504 α0 = 23.9211x10-24 esu ∆α = 9.54466X10-24 esu

Hyperpolarizibility Xxx = 1041.9073 Xxy = -29.2064 Xyy = 306.5543 Yyy = 149.0018 Zxx = 72.2438 Xyz = -35.6992 Zyy = 9.7444 Xzz = 129.33 Yzz = 58.541 Zzz = -63.552 β tot = 1.2860X10-30 esu

Figure Captions Fig.1. FT-IR spectrum of 2-A5BPB Fig.2.1H NMR spectrum of 2-A5BPB Fig.3. 13C NMR spectrum of 2-A5BPB Fig.4. (a)ORTEP diagram, (b) various ring graphset (c) packing view of ‘b’ axis in 2-A5BPB Fig.5. (a) Optimized molecular structure and Hirshfeld surfaces of 2-A5BPB (b) de, (c) di, (d) dnorm, (e) shape–index and (f)curvedness Fig.6 Decomposed two dimensional fingerprint plos for 2-A5BPB crystal Fig.7. (a) TG – DTA thermogram of 2-A5BPB crystal (b) DSC thermograms of reactants acid, base and 2-A5BPB Fig.8. Uv-Vis Spectrum of 2-A5BPB Fig.9. Fluorescence Spectrum of 2-A5BPB Fig.10. Optimized geometry of 2-A5BPB Fig.11. Molecular electrostatic potential of 2-A5BPB Fig.12. (a) FMO analysis, (b) band energy gap spectrum, (c) DOS and (d) PDOS of 2-A5BPB Fig.13 (a) open and (b) closed aperture curve for 2-A5BPB

Fig. 1. FT-IR spectrum of 2-A5BPB

Fig.2. 1H NMR spectrum of 2-A5BPB

Fig.3. 13C NMR spectrum of 2-A5BPB

Fig.4. (a)ORTEP diagram, (b) various ring graphset (c) packing view of ‘b’ axis in 2-A5BPB

Fig.5. (a) Optimized molecular structure and Hirshfeld surfaces of 2-A5BPB (b) de (c) di (d) dnorm (e) shape–index and (f) curvedness

Fig.6 Decomposed two dimensional fingerprint plos for 2-A5BPB crystal

Fig.7. (a) TG – DTA thermogram of 2-A5BPB crystal (b) DSC thermograms of reactants acid, base and 2-A5BPB

Fig.8. Uv-Vis Spectrum of 2-A5BPB

Fig.9. Fluorescence Spectrum of 2-A5BPB

Fig.10. Optimized geometry of 2-A5BPB

Fig.11. Molecular electrostatic potential of 2-A5BPB

Fig.12. (a) FMO analysis, (b) band energy gap spectrum, (c) DOS and (d) PDOS of 2A5BPB

Fig.13 (a) open and (b) closed aperture curve for 2-A5BPB


A new THG crystal, 2-A5BPB was synthesised and characterised.
Complete structural analysis of 2-A5BPB by single crystal X-ray diffraction reveals various hydrogen bonding interactions.
Computational analysis ascertains various type of hydrogen bonding interactions.
Z scan analysis confirms third order harmonic generation in 2-A5BPB.