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Crystal structure, DFT and third order non-linear optical studies of an organic bisguanidinium isophthalate monohydrate single crystal
Journal Pre-proof Crystal structure, DFT and third order non-linear optical studies of an organic bisguanidinium isophthalate monohydrate single crystal Paavai Era, R.O.M.U. Jauhar, V. Viswanathan, G. Vinitha, P. Murugakoothan PII:
S0022-2860(19)31585-6
DOI:
https://doi.org/10.1016/j.molstruc.2019.127476
Reference:
MOLSTR 127476
To appear in:
Journal of Molecular Structure
Received Date: 5 September 2019 Revised Date:
5 November 2019
Accepted Date: 22 November 2019
Please cite this article as: P. Era, R.O.M.U. Jauhar, V. Viswanathan, G. Vinitha, P. Murugakoothan, Crystal structure, DFT and third order non-linear optical studies of an organic bisguanidinium isophthalate monohydrate single crystal, Journal of Molecular Structure (2019), doi: https:// doi.org/10.1016/j.molstruc.2019.127476. 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.
Crystal structure, DFT and third order non-linear optical studies of an organic bisguanidinium isophthalate monohydrate single crystal Paavai. Eraa, RO. MU. Jauharb, V. Viswanathanc, G. Vinithab, P. Murugakoothana,d* a
MRDL, PG and Research Department of Physics, Pachaiyappa’s College, Chennai-600 030. b c
Division of Physics, School of Advanced Sciences, VIT University, Chennai - 600 127.
Department of Biophysics, All India Institute of Medical Sciences (AIIMS), New Delhi – 110029. d
Department of Physics, C. Kandaswami Naidu College for Men, Chennai – 600102.
E-mail: [email protected] Tel: +91 9444447586 Abstract A conventionally grown organic nonlinear optical crystal bisguanidinium isophthalate monohydrate was structurally investigated using single crystal X-ray diffraction study. The lattice constants, supramolecular arrangements and the hydrogen bond formation of the title compound confirm the formation of novel organic crystalline material. The various functional groups present in the crystalline material were resoluted by employing FT-IR spectral analysis. The wide-range optical transparency of the compound in the entire visible region ensures the optical quality with a lower cut-off wavelength at 298 nm was analysed experimentally using UV-vis-NIR spectrometer. The support of DFT was scrutinized to report the optimized structure, electronic state properties, hyper-conjugative interactions and frontier molecular energy gap of the bisguanidinium isophthalate compound. The charge transfer mechanism of bisguanidinium isophthalate compound from isophthalic acid to guanidine compound was also interpreted using theoretical DFT. The computational evaluation of charge stabilities as a result of charge delocalization was construed with the ratification of natural bonding orbital (NBO). The assessed thermal stability of
bisguanidinium isophthalate crystal is 120°C which was analysed by thermo gravimetric and differential thermal analyses. To determine the third-order nonlinear optical property with quantitative results of nonlinear refractive index, nonlinear absorption coefficient and optical susceptibility was ascertained using Z-scan technique. Keywords: Crystal structure; DFT; NBO; thermal decomposition; nonlinearity 1. Introduction Organic materials have been attracting much attention because of their large nonlinear optical susceptibility, high laser damage threshold, high speed electro optic response and structural flexibility compared to inorganic materials. Nonlinear optical materials are expected to play vital role in the photonics industry for several potential applications such as optical information processing, telecommunications system, frequency conversion, optical data storage and detection. In particular, high optical band gap and low dielectric constant of nonlinear optical crystals are in rich demand to be employed in optical storage devices, spectroscopy, imaging, colour display units, optical communication systems etc., [1,2]. Research work has been carried out for a long time to study the optical and spectral properties of guanidine-based crystal for their photonic applications. The structures of the guanidinium salts of aromatic and heteroaromatic polyfunctional carboxylic acids are numerous in the crystallographic literature [3-5], but they are of interest because of the capacity of the guanidinium cation to generate stable supramolecular framework structures through hydrogen-bonding associations, largely cyclic, such as those found in the structures of guanidinium carbonate [6] and guanidinium bicarbonate [7]. In particular, studies on guanidine complexes with benzenecarboxylic acids such as the complexes of guanidinium benzoate, guanidinium isophthalate (1:1) have been reported are being the subject of interest because of their capacity of Linkage-Bridge between cation-anion networks [8,9]. In this
aspect, the present investigations were intended to synthesis and grow novel bisguanidinium isophthalate (GIP) single crystals. The optimized molecular geometry of the GIP compound was computed with the support of DFT. The results of structure, absorption, optical, electronic state properties and structural activity of GIP crystal was investigated using single crystal X-ray diffraction, FTIR spectroscopic study, UV-vis-NIR spectrometer, HOMOLUMO and NBO respectively. Furthermore, the paper discusses on the thermal results of the title compound obtained from TG-DTA analyses. The third order NLO activity of GIP crystal is discussed with the results gained from Z-scan technique. 2. Experimental 2.1 Synthesis and growth The raw material of GIP crystal was synthesized using conventional solvent evaporation method by the condensation of guanidine carbonate and isophthalic acid taken in a stoichiometric 2:1 ratio. The schematic chemical reaction of the resultant bisguanidinium isophthalate compound is shown in Figure 1. The chemicals used were of analytical reagent grade. The precursors chosen were dissolved using aqueous methanol is used as a solvent in the whole experiment for preparing the saturated solutions. The calculated quantity of isophthalic acid is added into 200 ml solution of guanidine carbonate. The mixture is stirred up to the attainment of homogeneity for about 7 hours. The as prepared solution is now filtered, covered with polythene sheet and kept isolated for the process of solvent evaporation. After a span of 16 days, cluster of GIP single crystals were harvested. The purity of the collected GIP crystal is improved by repeated recrystallization process. After several times of recrystallization process, the optically good quality non-hygroscopic crystals with dimensions 16x4x2 mm3 were harvested and is evinced in Figure 2.
Figure 1. Reaction scheme of GIP
Figure 2. As grown GIP crystal
3. Results and Discussion 3.1 Single crystal X-ray diffraction study X-ray diffraction intensity data were collected at room temperature (293K) on a Bruker axs SMART APEXII single crystal X-ray diffractometer equipped with graphite monochromatic MoKα (λ=0.71073 Å) radiation and CCD detector. A crystal of dimensions 0.300 X 0.230 X 0.180 mm3 was mounted on a glass fiber using cyanoacrylate adhesive. The unit cell parameters were determined from 36 frames measured (0.5° phi-scan) from three different crystallographic zones using the method of difference vectors. The intensity data were collected with an average four-fold redundancy per reflection and optimum resolution (0.75 Å). The intensity data collection, frames integration, Lorentz and polarization corrections and decay correction were carried out using SAINT-NT(version 7.06a) software [10]. An empirical absorption correction (multi-scan) was performed using the SADABS program [10]. The crystal structure was solved by direct methods using SHELXS-14 [11] and refined by full-matrix least-squares using SHELXL-18 [12]. Molecular geometry was calculated using PARST [13]. All non-hydrogen atoms were refined using anisotropic thermal parameters. The hydrogen atoms were included in the structure factor calculation at idealized positions by using a riding model, but not refined. Images were created with the ORTEP-PLATON program [14,15]. The hydrogen atoms were placed in calculated positions with C—H = 0.93 Å, N—H = 0.83 Å to 0.93 Å and O—H = 0.87 Å to 0.90 Å refined in the riding model with fixed isotropic displacement parameters: Uiso(H) = 1.2Ueq(C) for C aromatic. The NH, OH and Hatoms were located in difference Fourier map. The N-bound H atoms and O-bound H atoms were freely refined in the final cycles of refinement as riding atoms. Table 1 shows the crystal data and structure refinement details for compound.
The compound crystallizes in the monoclinic P21/n space group with four molecules in the unit cell (a=10.6238(7) Å, b=11.4698(8) Å, c=12.1363(9) Å, α =γ=90°, β=104.491(3)°, and Z=4). The three dimensional molecular structure of this compound was determined by Xray crystallography using SHELXS-14 and later refined by SHELXL-18 to a final R-value of 4.26 %. The ORTEP of the title salt is shown in Figure 3. The asymmetric unit of the title salt comprises of two guanidinium [2(CN3H6)+], one isophthalate [C6O4H4—] and one water molecule (O1H2). In the structure of the compound, the bisguanidinium cation and isophthalate anion molecule is formed in a 2:1 ratio obtained after refinement of the X-ray diffraction data. The best mean planes of the bisguanidinium (N1,N2 & N3) and isophthalate are inclined to one another by 11.82(1) ° and the second bisguanidinium(N4,N5 & N6) and isophthalate are almost perpendicular to each other from the value of dihedral angle 73.72(1)°. The isophthalate anion is almost planar conformation with maximum deviation of atom O1 -0.1956 (2) Å. In the isophthalic acid, the carboxyl groups of (OH) hydrogen atom are deprotonated into O—. The guanidines of NH are protonated into NH2. In the structure, the bond length of C9—N1 = 1.310 Å and C10—N6 = 1.310 Å are double bond distances, but in the structure we are observing NH2 in the double bonded NH. The carboxyl substituent groups of C1 and C5 are essentially coplanar with the ring, the C6—C1—C7—O1 = -171.2 °, C2—C1—C7—O2 = -175.2 °, C6—C5—C8—O3 = 179.6 ° and C4—C5—C8—O4 = 178.0 ° which evidence from the torsion angles. These obtained parameters are comparable with that of the 1:1 guanidine isophthalate complex reported in the literature [9]. The crystallographic data of GIP crystal deposited in Cambridge Crystallographic Data Center (CCDC) (No 1941675), 12, Union Road, Cambridge CB21EZ, UK. In the crystal packing of the title salt, three dimensional network of N—H...O, O— H...N and N—H... π interactions which gives rise to the following hydrogen bonding
interactions in the Table 2. In the crystal packing, the cation of protonated nitrogen atom N1 forming an intramolecular interaction with an anion atom of O1 (N1—H1B…O1) and the cation of nitrogen atom N2 forming an intramolecular interaction with an anion of O2 atom (N2—H2A…O2) with a distance of 2.890(2) Å and 2.864(2) Å, respectively. These interaction distances are comparable with the values reported in the literature [16]. Both the N—H…O intramolecular hydrogen bond interactions generating an S(8) ring motif is shown in Figure 4. The second cation of protonated nitrogen atom of N6 forming an intramolecular interaction with an anion atom of O3 (N6—H6A…O3) and the cation of nitrogen atom N4 forming an intramolecular interaction with an anion of O3 atom (N4—H4B…O3). Both the N—H…O intramolecular hydrogen bond interaction generating an S(6) ring motif is shown in Figure 4. The water molecule is interacting with an anion of O4 atom, forming an intramolecular hydrogen bond in the structure as shown in Figure 4. The cation of protonated N1 atom is acting as a donor and forming an intermolecular hydrogen bond with an anion of O3 atom as a acceptor (N1—H1A…O3iii). The cation of N3 atom is acting as a donor and the forming an intermolecular hydrogen bond interaction with an anion of O4 atom as a acceptor (N3—H3A…O4iii). The above mentioned two intermolecular hydrogen bond interactions (Table 2) forming an R22(8) ring motif, approximately slabs lying parallel to plane (001) as shown in Figure 4. The cation molecule of N2 as a donor atom is forming an intermolecular hydrogen bond interaction with the acceptor of anion molecule of O1 atom (N2— H2B…O1iv) and its forming a C6 linear chain interaction with the intramolecule interaction of N1—H1B…O1, viewed down “b” axis as shown in Figure 5. The cation molecule of N5 as a donor atom is forming an intermolecular interaction with an anion of O2 as an acceptor atom (N5—H5A…O2ii) as shown in Figure 5. In the crystal packing, the above mentioned three intramolecular (N1—H1B…O1), (N2—H2A…O2), (N4—H4B…O3) and three intermolecular
interactions
(N1—H1A…O3iii),
(N2—H2B…O1iv),
(N5—H5A…O2ii)
forming a R66(14) ring motif in the structure as shown in Figure 4. The anion atom of O1, O2 and O3 act as bifurcated acceptors in the above ring motif. The N5—H5A…O2ii and N5— H5B…O4vi intermolecular interaction constitute pair of bifurcated donor atom of N5, bond with the oxygen acceptor of O2 and O4 atoms generating a ring of the graph-set R44(20) motif viewed down “b” axis as shown in Figure 5. The cation of protonated nitrogen N6, constitute pair of trifurcated donor atom and it’s forming interaction with the O2 (N6—H6B…O2ii), O1W (N6—H6A…O1Wii) and O3 (N6—H6A…O3) as an acceptor atoms. The N6— H6B…O2ii and N6—H6A…O3 interactions generating a ring of the graph-set motif R44(20) viewed down “b” axis as shown in Figure 6. The water molecule of O1W act as a bifurcated donor atom, bond with the acceptor of O1 (O1W—H1WB…O1i) and O4 (O1W— H1WA…O4) atoms generating a R44(20) ring motif in the structure viewed down “c” axis as shown in Figure 7. The cation and anion molecules are also held together by N3— H3B…Cg1iv interaction with the centroid of the phenyl ring (C1—C6) viewed down “b” axis as shown in Figure 8.
Figure 3. The molecular structure of title salt, showing the atom labeling and displacement ellipsoids drawn at 30% probability level.
Figure 4. The N—H...O interactions forming R22(8) and R66(14) ring motif viewed down “a” axis.
Figure 5. The N5—H5A…O2ii and N5—H5B…O4vi intermolecular interaction generating a R44(20) ring motif viewed down “b” axis.
Figure 6. The N6—H6B…O2ii and N6—H6A…O3 interactions generating a ring of the graph-set motif R44(20) viewed down “b” axis.
Figure 7. The water molecule of O1W act as a bifurcated donor atom bonded with the oxygen atoms generating a R44(20) ring motif viewed down “c” axis.
Figure 8. The N3—H3B…Cg1iv interaction with the centroid of the phenyl ring (C1— C6) viewed down “b” axis.
Table 1. Crystal data and structure refinement data. Parameters
Values
Empirical formula
C10 H18 N6 O5
Formula weight
302.3
Temperature
296(2) K
Wavelength
0.71073 Å
Crystal system, space group
Monoclinic, P21/n a = 10.6238(7) Å α = 90 °. b = 11.4698(8) Å β = 104.491(3) °.
Unit cell dimensions
c = 12.1363(9) Å γ = 90 °.
Volume
1431.80(17) Å 3
Z, Calculated density
4, 1.402 Mg/m3
Absorption coefficient
0.114 mm-1
F(000)
640
Crystal size
0.300 x 0.230 x 0.180 mm3
Theta range for data collection 2.282 to 27.508 °. Limiting indices
-13<=h<=13, -14<=k<=14, -15<=l<=15
Reflections collected / unique
21063 / 3283 [R(int) = 0.0529]
Completeness to theta
100.00 %
Refinement method
Full-matrix least-squares on F2
Data / restraints / parameters
3283 / 0 / 231
Goodness-of-fit on F2
1.007
Final R indices [I>2sigma(I)]
R1 = 0.0426, wR2 = 0.0992
R indices (all data)
R1 = 0.0940, wR2 = 0.1259
Extinction coefficient
0.012(2)
Largest diff. peak and hole
0.208 and -0.184 e. Å-3
Table 2. Hydrogen Bonds D—H…A
D—H(Å)
H…A(Å)
D…A(Å)
D—H…(Å)
N1-H1B…O1
0.99(2)
1.90(2)
2.890(2)
173(2)
N2-H2A…O2
0.92(2)
1.95(2)
2.864(2)
170(2)
N4-H4B…O3
0.84(2)
1.99(2)
2.792(2)
161(2)
N6-H6A…O3
0.91(2)
2.37(2)
3.104(2)
137.0(19)
O1W-H1WA…O4
0.90(2)
1.91(2)
2.806(2)
177(3)
O1W-H1WB…O1 (i)
0.87(2)
2.01(2)
2.878(2)
172(2)
N5-H5A…O2 (ii)
0.86
2.41
3.119(2)
140
N6-H6B…O2 (ii)
0.94(2)
1.92(2)
2.819(2)
162(2)
N6-H6A…O1W (ii)
0.91(2)
2.28(2)
2.982(3)
133.7(18)
N1-H1A…O3 (iii)
0.84(2)
2.01(2)
2.840(2)
177(2)
N3-H3A…O4 (iii)
0.86
2.00
2.835(2)
165
N2-H2B…O1 (iv)
0.92(2)
2.29(2)
3.183(2)
162.8(19)
N4-H4A…O1W (v)
0.93(2)
2.21(2)
3.006(3)
143.2(19)
N5-H5B…O4 (vi)
0.86
2.08
2.913(2)
164
N3-H3B…Cg1 (iv)
0.86
2.59
3.419(19)
164
Symmetry codes: i) 1-x,1-y,1-z; ii) 3/2-x,-1/2+y,1/2-z; iii) x,y,1+z; iv) 3/2-x,1/2+y,3/2-z; v) 1-x,1-y,-z and vi) -1/2+x,1/2-y,-1/2+z. 3.2 Molecular geometry The entire theoretical calculations of GIP crystal were executed utilizing Gaussian 03 program with the optimization of DFT/B3LYP/6-31G (d) level [17]. The optimized geometry of GIP crystal structure with the numbered atoms is exhibited in Figure 9. The selected bond lengths of GIP crystal are compared with the theoretical values in Table 3. The geometrically predicted parameters from B3LYP method shows a little difference between experimental and theoretical data. This is because of the fact that the theoretical values are obtained in the gaseous phase whereas the experimental values are obtained from the solid phase. The optimized molecular structure of GIP compound thus obtained was subjugated for the quantitative analysis to obtain HOMO-LUMO energy gap and NBO analysis.
Figure 9. Optimized molecular geometry of GIP crystal Table 3. Selected Bond lengths
Bond length
Experimental (Å)
Theoretical (Å)
O(4)-C(8)
1.260(2)
1.432
O(3)-C(8)
1.248(2)
1.287
O(2)-C(7)
1.251(2)
1.269
O(1)-C(7)
1.254(2)
1.285
N(1)-C(9)
1.310(2)
1.399
N(1)-H(1A)
0.84(2)
1.009
N(1)-H(1B)
0.99(2)
0.997
N(2)-C(9)
1.325(2)
1.392
N(2)-H(2A)
0.92(2)
1.006
N(2)-H(2B)
0.92(2)
1.001
N(4)-C(10)
1.321(2)
1.398
N(4)-H(4B)
0.84(2)
0.996
N(4)-H(4A)
0.93(2)
1.008
N(6)-C(10)
1.310(2)
1.407
N(6)-H(6A)
0.91(2)
0.998
N(6)-H(6B)
0.94(2)
0.998
N(5)-C(10)
1.319(2)
1.399
N(5)-H(5A)
0.86
0.997
N(5)-H(5B)
0.86
0.997
N(3)-C(9)
1.318(2)
1.399
N(3)-H(3A)
0.86
0.997
N(3)-H(3B)
0.86
0.997
C(5)-C(6)
1.385(2)
1.398
C(5)-C(4)
1.387(2)
1.399
C(5)-C(8)
1.503(2)
1.792
C(1)-C(2)
1.391(2)
1.398
C(1)-C(6)
1.387(2)
1.399
C(1)-C(7)
1.506(2)
1.792
C(6)-H(6)
0.93
1.099
C(2)-C(3)
1.374(3)
1.398
C(2)-H(2)
0.93
1.099
C(4)-C(3)
1.376(3)
1.399
C(4)-H(4)
0.93
1.099
C(3)-H(3)
0.93
1.099
3.3 Spectral analysis On account of obtaining the spectroscopic features of GIP crystal, Fourier transform infrared spectroscopy was carried out. The spectrum was recorded for the fine grained sample of GIP crystal in the range 4000 cm−1– 400 cm−1 using KBr pellet technique. The recorded FTIR spectrum is shown in Figure 10. The intense peaks between 2400-4000 cm-1 may be due to the symmetric and asymmetric stretching vibrations of NH2 [18]. The sharp band at 2911 cm-1 is attributed for the asymmetric C-H stretching vibration [19]. The C=O stretching vibration is observed at 1720 cm-1[20]. The spectrum appeared at 1616 cm-1 is due to the C=N stretching frequency [21]. The N-H asymmetric bending vibration of amide group is observed at 1550 cm-1. The peak observed at 1499 cm-1 is attributed to the symmetric NH2+ bending vibration. The strong band at 1094 cm-1 corresponds to C-H in-plane bending vibrations [20]. The bands in spectra at 675 cm-1 correspond to C-H out-of-plane bending vibrations [22]. The all existing functional groups present in the FTIR spectra of GIP material confirms the structure of the compound.
Figure 10. FTIR spectrum of GIP 3.4 Electronic absorption studies The electronic absorption reveals the optical properties of all optically active crystalline materials. The UV-vis-NIR transmission spectrum illustrates complete information about the optical transmission of GIP material. The structure of organic molecules could be depicted with the help of transmission spectra due to their predominantly occurring σ and π orbital electronic transitions from their corresponding lower energy (ground state) to higher energy (excited state) [23]. The UV-vis-NIR transmission spectrum of the grown GIP crystal is examined using Labindia 3032 UV-vis-NIR spectrophotometer and the resulting spectrum is exhibited in Figure 11. The maximum transmittance of the GIP crystal was observed to be around 60% with a lower cut-off wavelength at 298 nm. The observed cut-off wavelength may be due to the electronic transition of the molecule from bonding π orbital to anti-bonding π orbital (π–π*) [24]. The electronic transition assignment is confirmed with single crystal X-ray diffraction analysis results as the protonation of GIP compound is done by the proton transfer from hydroxyl group of isophthalate to the nitrogen of the guanidine molecule. This electron transition among lower and higher energy states makes a minor contribution to the hyperpolarizability of the crystal. The complete transmittance of the title crystal in the entire visible region nominates the crystal to be a befitting material for optoelectronic applications. The optical bandgap of the title material is depicted in Figure 12 and was ascertained to be 3.9 eV by extrapolating the linear part of tauc’s plot to the corresponding energy axis.
Figure 11. Transmittance spectrum of GIP crystal
Figure 12. Tauc’s plot of GIP crystal 3.5 HOMO-LUMO analysis Frontier molecular orbital (FMO) investigations were used to identify and examine the highest occupied molecular orbital and the lowest unoccupied molecular orbital abbreviated as HOMO and LUMO respectively. These FMOs play a prime role in the
evaluation of electro-optical and quantum chemical properties of the compounds [25]. The energy gap observed between HOMO and LUMO delineate the charge transfer interaction transpiring within the molecule. The spotted electronic absorption may be due to the electron transition from ground state to the corresponding excited states [26]. The linkage orientation of the title molecule facilitates the deterioration of the HOMO level and the stabilization of the LUMO level. The result of the orbital energy level scrutiny for the GIP material is exhibited in Figure 13 as a 3D plot with HOMO-LUMO energy levels. From the theoretical HOMO-LUMO calculations it is observed that the HOMO, LUMO and energy gaps are -4.81 eV, -0.05 eV and 4.75 eV respectively. Further the predominant quantum chemical molecular properties, i.e., global reactivity descriptors such as ionization energy, ionization potential, electro negativity, chemical hardness, total energy change, electrophilicity index, degree of chemical relativity and overall energy balance were also calculated using standard equations [27] and the values are provided in Table 4. The stability of the molecule expressed via global reactivity descriptors supplements the material’s resistance to polarization which may be due to an external perturbation on the molecular system. These energies indicate an enhanced intramolecular charge transfer interactions in the system that leads to make the molecule as an easily polarizable one.
Figure 13. 3D plot of HOMOs and LUMOs Table 4. Calculated molecular orbital parameters of the title compound Parameters
Calculated values
Ionization potential
4.81 eV
Electron affinity
0.05 eV
Chemical hardness
2.38 eV
Electro negativity
2.43 eV
Chemical potential
-2.38 eV
Total energy change
-0.595 eV
Electrophilicity index
1.19 eV
Degree of chemical relativity
0.2101 eV
Overall energy balance
-4.76 eV
3.6 NBO analysis The inter and intra-molecular interaction and bonding among the molecules can be studied efficiently by the natural bonding orbital (NBO) analysis. It also affords a convenient basis for the investigation of charge transfer or conjugative interactions in the molecular system. The interaction between electron donors and electron acceptors is more intense when stabilization energy of hyperconjugative interactions E(2) value is large [26], i.e., in the complete system, when the extent of conjugation interaction is higher, the donating tendency from electron donors to electron acceptors becomes greater. From the second-order perturbation theory analysis of NBO Fock matrix, the stabilization energies of GIP crystal were obtained and the stabilizing interactions between donor and acceptor NBOs obtained from Gaussian 03 program is presented in Table 5. The NBO result exhibits a quiet large number of stabilizing interactions in GIP molecule. Hence from the obtained data, the strong hyperconjugative interactions between the O(3) bond and the anti-bonding O(4)-C(20) have maximum stabilization energy of about 103.21 kJ/mol. The other significant interaction which exhibits similar strong stabilization energy at 102.68 kJ/mol., is the interaction between O(2) and O(1)-C(17). These energetic contributions to the electron delocalization interactions in the sample molecule were quantitatively estimated to identify the structure stabilizing intramolecular charge transfer interactions thereby elucidating the proper and improper bonding within the molecule [27].
Table 5 Selected second-order perturbation theory analysis of Fock matrix in NBO basis E(2)(a) (kJ/mol) E(j)-E(i)(b) (a.u)
F(i,j)(c)(a.u)
Donor NBO (i)
Acceptor NBO (j)
LP (3) O(3)
BD* (2) O(4) -C (20)
103.21
0.23
0.139
LP (3) O(2)
BD* (2) O(1) -C (17)
102.68
0.23
0.139
LP (2) O(2)
LP* (1) C (21)
83.20
0.11
0.100
BD* (2) O(4) -C (20) BD* (2) C(15) -C (24)
72.71
0.04
0.075
BD* (2) O(4) -C (20) BD* (2) C(16) -C (18)
68.44
0.04
0.072
(a)
E(2) denotes the stabilization energy of hyperconjugative interactions. Energy difference between donor (i) and acceptor (j) in NBOs. (c) F(i, j) is the Fock matrix element between donor (i) and acceptor (j) NBOs. (b)
3.7 Thermal study The thermal stability of GIP was studied by thermogravimetric (TG) and differential thermal analyses (DTA). The GIP sample weighing 5.438 mg was analysed under nitrogen atmosphere and the thermogram is depicted in Fig. 14. From the TG curve, it is evident that the GIP material is stable up to 120 °C. The TG curve showed three stage weight loss patterns when the material was heated from 32°C to 600°C. The first minor weight loss around 120°C is the evidence for the elimination of water molecule as seen in the low temperature region. The water molecule comprises a mass of 18.02 g/mol (observed 8.21%, calculated 6.02%). The second stage weight loss observed between 128°C and 240°C experiences a weight loss of about 13%. The third stage decomposition occurred between the temperatures 241°C and 322 °C incurs a weight loss of about 81%. A broad endothermic peak found 315 °C in the DTA curve is associated with the third stage weight loss of the GIP material. This broad endotherm further confirms the absorption of energy for breaking of
bonds during decomposition. The decomposition process was carried up to 600 °C with the removal of the material into gaseous products.
20
120°C
Weight %
80
TGA DTA
15
10
60
5
0
81%
40
endo
DTA (mW/mg)
100
-5
20 -10 0 0
100
200
300
400
500
600
Temperature (°C)
Figure 14. TG-DTA curve of GIP 3.8 Nonlinear optical study To investigate the nonlinear optical property of GIP crystal we opted for Z-scan technique with Nd:YAG laser radiation of wavelength 532 nm, a prominent method to examine the third order nonlinear optical chattels. The result of Z-scan technique reveals the nonlinear refractive index (n2), absorption coefficient (β) and optical susceptibility (χ(3)) of the crystal sample. The sample was allowed to move along the direction of laser beam propagation i.e., focal region say, –Z axial direction to +Z axial direction. With the same experimental setup the sample was measured at two different conditions in order to get n2 with closed aperture condition and β and χ(3) with open aperture condition. The experimentally measured normalized transmittance at closed and open aperture conditions of GIP crystal sample with thickness 0.5 mm is shown in Figure 15 and Figure 16 respectively. The self-defocussing effect of the title crystal was confirmed with the negative third-order
nonlinear refractive index (n2) calculated from closed aperture spectrum. The reverse saturation absorption with positive nonlinear absorption coefficient (β) confirms the multiphoton absorption of the grown crystal from open aperture spectrum. This would serve as a great evidence to exploit the GIP crystal for optical limiting applications [28]. The quantitatively evaluated nonlinear refractive index, nonlinear absorption coefficient and optical susceptibility values are tabulated in Table 6.
Figure 15. Closed aperture curve of GIP crystal
Figure 16. Open aperture curve of GIP crystal Table 6. Experimental results of Z-scan technique n2 x 10 -4 cm2/W -1.16
β x 10-4 cm/W 4.874
χ(3) x 10-6 esu 2.74
Table 7. Comparison of nonlinear optical parameters with some of the reported materials β cm/W
χ(3) esu
-7.7 x 10-4
3.38 x 10-6
[29]
0.311 x 10-2
3.143 x 10-4
[30]
0.08 x 10-4
2.74 x 10-6
[31]
4.874 x 10-4
2.74 x 10-6
Present work
Reference
4. Conclusion Bisguanidinium isophthalate single crystals have been grown by solvent evaporation growth method and the crystallographic parameters were estimated from single crystal XRD study. The crystal packing elucidates stabilization of π to π* stacking interactions. DFT computations were executed to manifest the optimized molecular geometry of the titular compound. The optical transmittance spectrum validates that the GIP crystal aids as a noble candidate for nonlinear optical and optoelectronic applications. The HOMO– LUMO energies and energy gap of the GIP crystal demonstrates the charge transfer interactions taking place within the crystal lattice. The strong N–H…O, C–H…O and hydrogen bonding has also been confirmed with the help of FMO investigations. Natural bonding orbital analysis was exploited to explain the occurring delocalization, intermolecular charge transfer, interacting strength and equilibrium energy among the bonds within the molecule. The thermal decomposition and the corresponding stability of the titular material were assessed from TG and DTA analyses. The evaluated nonlinear refractive index (n2), absorption coefficient (β) and third order nonlinear susceptibility (χ(3)) of GIP crystal deliberated by Zscan technique aids to promote the material for optical limiting applications.
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Highlights o
Bulk crystals of GIP has been grown obtaining slow evaporation technique
o
Crystal structure is reported for the first time
o
Third order nonlinear optical properties has been reported.
☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare no financial interests/personal relationships may be considered as potential competing interests:
S0022-2860(19)31585-6
DOI:
https://doi.org/10.1016/j.molstruc.2019.127476
Reference:
MOLSTR 127476
To appear in:
Journal of Molecular Structure
Received Date: 5 September 2019 Revised Date:
5 November 2019
Accepted Date: 22 November 2019
Please cite this article as: P. Era, R.O.M.U. Jauhar, V. Viswanathan, G. Vinitha, P. Murugakoothan, Crystal structure, DFT and third order non-linear optical studies of an organic bisguanidinium isophthalate monohydrate single crystal, Journal of Molecular Structure (2019), doi: https:// doi.org/10.1016/j.molstruc.2019.127476. 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.
Crystal structure, DFT and third order non-linear optical studies of an organic bisguanidinium isophthalate monohydrate single crystal Paavai. Eraa, RO. MU. Jauharb, V. Viswanathanc, G. Vinithab, P. Murugakoothana,d* a
MRDL, PG and Research Department of Physics, Pachaiyappa’s College, Chennai-600 030. b c
Division of Physics, School of Advanced Sciences, VIT University, Chennai - 600 127.
Department of Biophysics, All India Institute of Medical Sciences (AIIMS), New Delhi – 110029. d
Department of Physics, C. Kandaswami Naidu College for Men, Chennai – 600102.
E-mail: [email protected] Tel: +91 9444447586 Abstract A conventionally grown organic nonlinear optical crystal bisguanidinium isophthalate monohydrate was structurally investigated using single crystal X-ray diffraction study. The lattice constants, supramolecular arrangements and the hydrogen bond formation of the title compound confirm the formation of novel organic crystalline material. The various functional groups present in the crystalline material were resoluted by employing FT-IR spectral analysis. The wide-range optical transparency of the compound in the entire visible region ensures the optical quality with a lower cut-off wavelength at 298 nm was analysed experimentally using UV-vis-NIR spectrometer. The support of DFT was scrutinized to report the optimized structure, electronic state properties, hyper-conjugative interactions and frontier molecular energy gap of the bisguanidinium isophthalate compound. The charge transfer mechanism of bisguanidinium isophthalate compound from isophthalic acid to guanidine compound was also interpreted using theoretical DFT. The computational evaluation of charge stabilities as a result of charge delocalization was construed with the ratification of natural bonding orbital (NBO). The assessed thermal stability of
bisguanidinium isophthalate crystal is 120°C which was analysed by thermo gravimetric and differential thermal analyses. To determine the third-order nonlinear optical property with quantitative results of nonlinear refractive index, nonlinear absorption coefficient and optical susceptibility was ascertained using Z-scan technique. Keywords: Crystal structure; DFT; NBO; thermal decomposition; nonlinearity 1. Introduction Organic materials have been attracting much attention because of their large nonlinear optical susceptibility, high laser damage threshold, high speed electro optic response and structural flexibility compared to inorganic materials. Nonlinear optical materials are expected to play vital role in the photonics industry for several potential applications such as optical information processing, telecommunications system, frequency conversion, optical data storage and detection. In particular, high optical band gap and low dielectric constant of nonlinear optical crystals are in rich demand to be employed in optical storage devices, spectroscopy, imaging, colour display units, optical communication systems etc., [1,2]. Research work has been carried out for a long time to study the optical and spectral properties of guanidine-based crystal for their photonic applications. The structures of the guanidinium salts of aromatic and heteroaromatic polyfunctional carboxylic acids are numerous in the crystallographic literature [3-5], but they are of interest because of the capacity of the guanidinium cation to generate stable supramolecular framework structures through hydrogen-bonding associations, largely cyclic, such as those found in the structures of guanidinium carbonate [6] and guanidinium bicarbonate [7]. In particular, studies on guanidine complexes with benzenecarboxylic acids such as the complexes of guanidinium benzoate, guanidinium isophthalate (1:1) have been reported are being the subject of interest because of their capacity of Linkage-Bridge between cation-anion networks [8,9]. In this
aspect, the present investigations were intended to synthesis and grow novel bisguanidinium isophthalate (GIP) single crystals. The optimized molecular geometry of the GIP compound was computed with the support of DFT. The results of structure, absorption, optical, electronic state properties and structural activity of GIP crystal was investigated using single crystal X-ray diffraction, FTIR spectroscopic study, UV-vis-NIR spectrometer, HOMOLUMO and NBO respectively. Furthermore, the paper discusses on the thermal results of the title compound obtained from TG-DTA analyses. The third order NLO activity of GIP crystal is discussed with the results gained from Z-scan technique. 2. Experimental 2.1 Synthesis and growth The raw material of GIP crystal was synthesized using conventional solvent evaporation method by the condensation of guanidine carbonate and isophthalic acid taken in a stoichiometric 2:1 ratio. The schematic chemical reaction of the resultant bisguanidinium isophthalate compound is shown in Figure 1. The chemicals used were of analytical reagent grade. The precursors chosen were dissolved using aqueous methanol is used as a solvent in the whole experiment for preparing the saturated solutions. The calculated quantity of isophthalic acid is added into 200 ml solution of guanidine carbonate. The mixture is stirred up to the attainment of homogeneity for about 7 hours. The as prepared solution is now filtered, covered with polythene sheet and kept isolated for the process of solvent evaporation. After a span of 16 days, cluster of GIP single crystals were harvested. The purity of the collected GIP crystal is improved by repeated recrystallization process. After several times of recrystallization process, the optically good quality non-hygroscopic crystals with dimensions 16x4x2 mm3 were harvested and is evinced in Figure 2.
Figure 1. Reaction scheme of GIP
Figure 2. As grown GIP crystal
3. Results and Discussion 3.1 Single crystal X-ray diffraction study X-ray diffraction intensity data were collected at room temperature (293K) on a Bruker axs SMART APEXII single crystal X-ray diffractometer equipped with graphite monochromatic MoKα (λ=0.71073 Å) radiation and CCD detector. A crystal of dimensions 0.300 X 0.230 X 0.180 mm3 was mounted on a glass fiber using cyanoacrylate adhesive. The unit cell parameters were determined from 36 frames measured (0.5° phi-scan) from three different crystallographic zones using the method of difference vectors. The intensity data were collected with an average four-fold redundancy per reflection and optimum resolution (0.75 Å). The intensity data collection, frames integration, Lorentz and polarization corrections and decay correction were carried out using SAINT-NT(version 7.06a) software [10]. An empirical absorption correction (multi-scan) was performed using the SADABS program [10]. The crystal structure was solved by direct methods using SHELXS-14 [11] and refined by full-matrix least-squares using SHELXL-18 [12]. Molecular geometry was calculated using PARST [13]. All non-hydrogen atoms were refined using anisotropic thermal parameters. The hydrogen atoms were included in the structure factor calculation at idealized positions by using a riding model, but not refined. Images were created with the ORTEP-PLATON program [14,15]. The hydrogen atoms were placed in calculated positions with C—H = 0.93 Å, N—H = 0.83 Å to 0.93 Å and O—H = 0.87 Å to 0.90 Å refined in the riding model with fixed isotropic displacement parameters: Uiso(H) = 1.2Ueq(C) for C aromatic. The NH, OH and Hatoms were located in difference Fourier map. The N-bound H atoms and O-bound H atoms were freely refined in the final cycles of refinement as riding atoms. Table 1 shows the crystal data and structure refinement details for compound.
The compound crystallizes in the monoclinic P21/n space group with four molecules in the unit cell (a=10.6238(7) Å, b=11.4698(8) Å, c=12.1363(9) Å, α =γ=90°, β=104.491(3)°, and Z=4). The three dimensional molecular structure of this compound was determined by Xray crystallography using SHELXS-14 and later refined by SHELXL-18 to a final R-value of 4.26 %. The ORTEP of the title salt is shown in Figure 3. The asymmetric unit of the title salt comprises of two guanidinium [2(CN3H6)+], one isophthalate [C6O4H4—] and one water molecule (O1H2). In the structure of the compound, the bisguanidinium cation and isophthalate anion molecule is formed in a 2:1 ratio obtained after refinement of the X-ray diffraction data. The best mean planes of the bisguanidinium (N1,N2 & N3) and isophthalate are inclined to one another by 11.82(1) ° and the second bisguanidinium(N4,N5 & N6) and isophthalate are almost perpendicular to each other from the value of dihedral angle 73.72(1)°. The isophthalate anion is almost planar conformation with maximum deviation of atom O1 -0.1956 (2) Å. In the isophthalic acid, the carboxyl groups of (OH) hydrogen atom are deprotonated into O—. The guanidines of NH are protonated into NH2. In the structure, the bond length of C9—N1 = 1.310 Å and C10—N6 = 1.310 Å are double bond distances, but in the structure we are observing NH2 in the double bonded NH. The carboxyl substituent groups of C1 and C5 are essentially coplanar with the ring, the C6—C1—C7—O1 = -171.2 °, C2—C1—C7—O2 = -175.2 °, C6—C5—C8—O3 = 179.6 ° and C4—C5—C8—O4 = 178.0 ° which evidence from the torsion angles. These obtained parameters are comparable with that of the 1:1 guanidine isophthalate complex reported in the literature [9]. The crystallographic data of GIP crystal deposited in Cambridge Crystallographic Data Center (CCDC) (No 1941675), 12, Union Road, Cambridge CB21EZ, UK. In the crystal packing of the title salt, three dimensional network of N—H...O, O— H...N and N—H... π interactions which gives rise to the following hydrogen bonding
interactions in the Table 2. In the crystal packing, the cation of protonated nitrogen atom N1 forming an intramolecular interaction with an anion atom of O1 (N1—H1B…O1) and the cation of nitrogen atom N2 forming an intramolecular interaction with an anion of O2 atom (N2—H2A…O2) with a distance of 2.890(2) Å and 2.864(2) Å, respectively. These interaction distances are comparable with the values reported in the literature [16]. Both the N—H…O intramolecular hydrogen bond interactions generating an S(8) ring motif is shown in Figure 4. The second cation of protonated nitrogen atom of N6 forming an intramolecular interaction with an anion atom of O3 (N6—H6A…O3) and the cation of nitrogen atom N4 forming an intramolecular interaction with an anion of O3 atom (N4—H4B…O3). Both the N—H…O intramolecular hydrogen bond interaction generating an S(6) ring motif is shown in Figure 4. The water molecule is interacting with an anion of O4 atom, forming an intramolecular hydrogen bond in the structure as shown in Figure 4. The cation of protonated N1 atom is acting as a donor and forming an intermolecular hydrogen bond with an anion of O3 atom as a acceptor (N1—H1A…O3iii). The cation of N3 atom is acting as a donor and the forming an intermolecular hydrogen bond interaction with an anion of O4 atom as a acceptor (N3—H3A…O4iii). The above mentioned two intermolecular hydrogen bond interactions (Table 2) forming an R22(8) ring motif, approximately slabs lying parallel to plane (001) as shown in Figure 4. The cation molecule of N2 as a donor atom is forming an intermolecular hydrogen bond interaction with the acceptor of anion molecule of O1 atom (N2— H2B…O1iv) and its forming a C6 linear chain interaction with the intramolecule interaction of N1—H1B…O1, viewed down “b” axis as shown in Figure 5. The cation molecule of N5 as a donor atom is forming an intermolecular interaction with an anion of O2 as an acceptor atom (N5—H5A…O2ii) as shown in Figure 5. In the crystal packing, the above mentioned three intramolecular (N1—H1B…O1), (N2—H2A…O2), (N4—H4B…O3) and three intermolecular
interactions
(N1—H1A…O3iii),
(N2—H2B…O1iv),
(N5—H5A…O2ii)
forming a R66(14) ring motif in the structure as shown in Figure 4. The anion atom of O1, O2 and O3 act as bifurcated acceptors in the above ring motif. The N5—H5A…O2ii and N5— H5B…O4vi intermolecular interaction constitute pair of bifurcated donor atom of N5, bond with the oxygen acceptor of O2 and O4 atoms generating a ring of the graph-set R44(20) motif viewed down “b” axis as shown in Figure 5. The cation of protonated nitrogen N6, constitute pair of trifurcated donor atom and it’s forming interaction with the O2 (N6—H6B…O2ii), O1W (N6—H6A…O1Wii) and O3 (N6—H6A…O3) as an acceptor atoms. The N6— H6B…O2ii and N6—H6A…O3 interactions generating a ring of the graph-set motif R44(20) viewed down “b” axis as shown in Figure 6. The water molecule of O1W act as a bifurcated donor atom, bond with the acceptor of O1 (O1W—H1WB…O1i) and O4 (O1W— H1WA…O4) atoms generating a R44(20) ring motif in the structure viewed down “c” axis as shown in Figure 7. The cation and anion molecules are also held together by N3— H3B…Cg1iv interaction with the centroid of the phenyl ring (C1—C6) viewed down “b” axis as shown in Figure 8.
Figure 3. The molecular structure of title salt, showing the atom labeling and displacement ellipsoids drawn at 30% probability level.
Figure 4. The N—H...O interactions forming R22(8) and R66(14) ring motif viewed down “a” axis.
Figure 5. The N5—H5A…O2ii and N5—H5B…O4vi intermolecular interaction generating a R44(20) ring motif viewed down “b” axis.
Figure 6. The N6—H6B…O2ii and N6—H6A…O3 interactions generating a ring of the graph-set motif R44(20) viewed down “b” axis.
Figure 7. The water molecule of O1W act as a bifurcated donor atom bonded with the oxygen atoms generating a R44(20) ring motif viewed down “c” axis.
Figure 8. The N3—H3B…Cg1iv interaction with the centroid of the phenyl ring (C1— C6) viewed down “b” axis.
Table 1. Crystal data and structure refinement data. Parameters
Values
Empirical formula
C10 H18 N6 O5
Formula weight
302.3
Temperature
296(2) K
Wavelength
0.71073 Å
Crystal system, space group
Monoclinic, P21/n a = 10.6238(7) Å α = 90 °. b = 11.4698(8) Å β = 104.491(3) °.
Unit cell dimensions
c = 12.1363(9) Å γ = 90 °.
Volume
1431.80(17) Å 3
Z, Calculated density
4, 1.402 Mg/m3
Absorption coefficient
0.114 mm-1
F(000)
640
Crystal size
0.300 x 0.230 x 0.180 mm3
Theta range for data collection 2.282 to 27.508 °. Limiting indices
-13<=h<=13, -14<=k<=14, -15<=l<=15
Reflections collected / unique
21063 / 3283 [R(int) = 0.0529]
Completeness to theta
100.00 %
Refinement method
Full-matrix least-squares on F2
Data / restraints / parameters
3283 / 0 / 231
Goodness-of-fit on F2
1.007
Final R indices [I>2sigma(I)]
R1 = 0.0426, wR2 = 0.0992
R indices (all data)
R1 = 0.0940, wR2 = 0.1259
Extinction coefficient
0.012(2)
Largest diff. peak and hole
0.208 and -0.184 e. Å-3
Table 2. Hydrogen Bonds D—H…A
D—H(Å)
H…A(Å)
D…A(Å)
D—H…(Å)
N1-H1B…O1
0.99(2)
1.90(2)
2.890(2)
173(2)
N2-H2A…O2
0.92(2)
1.95(2)
2.864(2)
170(2)
N4-H4B…O3
0.84(2)
1.99(2)
2.792(2)
161(2)
N6-H6A…O3
0.91(2)
2.37(2)
3.104(2)
137.0(19)
O1W-H1WA…O4
0.90(2)
1.91(2)
2.806(2)
177(3)
O1W-H1WB…O1 (i)
0.87(2)
2.01(2)
2.878(2)
172(2)
N5-H5A…O2 (ii)
0.86
2.41
3.119(2)
140
N6-H6B…O2 (ii)
0.94(2)
1.92(2)
2.819(2)
162(2)
N6-H6A…O1W (ii)
0.91(2)
2.28(2)
2.982(3)
133.7(18)
N1-H1A…O3 (iii)
0.84(2)
2.01(2)
2.840(2)
177(2)
N3-H3A…O4 (iii)
0.86
2.00
2.835(2)
165
N2-H2B…O1 (iv)
0.92(2)
2.29(2)
3.183(2)
162.8(19)
N4-H4A…O1W (v)
0.93(2)
2.21(2)
3.006(3)
143.2(19)
N5-H5B…O4 (vi)
0.86
2.08
2.913(2)
164
N3-H3B…Cg1 (iv)
0.86
2.59
3.419(19)
164
Symmetry codes: i) 1-x,1-y,1-z; ii) 3/2-x,-1/2+y,1/2-z; iii) x,y,1+z; iv) 3/2-x,1/2+y,3/2-z; v) 1-x,1-y,-z and vi) -1/2+x,1/2-y,-1/2+z. 3.2 Molecular geometry The entire theoretical calculations of GIP crystal were executed utilizing Gaussian 03 program with the optimization of DFT/B3LYP/6-31G (d) level [17]. The optimized geometry of GIP crystal structure with the numbered atoms is exhibited in Figure 9. The selected bond lengths of GIP crystal are compared with the theoretical values in Table 3. The geometrically predicted parameters from B3LYP method shows a little difference between experimental and theoretical data. This is because of the fact that the theoretical values are obtained in the gaseous phase whereas the experimental values are obtained from the solid phase. The optimized molecular structure of GIP compound thus obtained was subjugated for the quantitative analysis to obtain HOMO-LUMO energy gap and NBO analysis.
Figure 9. Optimized molecular geometry of GIP crystal Table 3. Selected Bond lengths
Bond length
Experimental (Å)
Theoretical (Å)
O(4)-C(8)
1.260(2)
1.432
O(3)-C(8)
1.248(2)
1.287
O(2)-C(7)
1.251(2)
1.269
O(1)-C(7)
1.254(2)
1.285
N(1)-C(9)
1.310(2)
1.399
N(1)-H(1A)
0.84(2)
1.009
N(1)-H(1B)
0.99(2)
0.997
N(2)-C(9)
1.325(2)
1.392
N(2)-H(2A)
0.92(2)
1.006
N(2)-H(2B)
0.92(2)
1.001
N(4)-C(10)
1.321(2)
1.398
N(4)-H(4B)
0.84(2)
0.996
N(4)-H(4A)
0.93(2)
1.008
N(6)-C(10)
1.310(2)
1.407
N(6)-H(6A)
0.91(2)
0.998
N(6)-H(6B)
0.94(2)
0.998
N(5)-C(10)
1.319(2)
1.399
N(5)-H(5A)
0.86
0.997
N(5)-H(5B)
0.86
0.997
N(3)-C(9)
1.318(2)
1.399
N(3)-H(3A)
0.86
0.997
N(3)-H(3B)
0.86
0.997
C(5)-C(6)
1.385(2)
1.398
C(5)-C(4)
1.387(2)
1.399
C(5)-C(8)
1.503(2)
1.792
C(1)-C(2)
1.391(2)
1.398
C(1)-C(6)
1.387(2)
1.399
C(1)-C(7)
1.506(2)
1.792
C(6)-H(6)
0.93
1.099
C(2)-C(3)
1.374(3)
1.398
C(2)-H(2)
0.93
1.099
C(4)-C(3)
1.376(3)
1.399
C(4)-H(4)
0.93
1.099
C(3)-H(3)
0.93
1.099
3.3 Spectral analysis On account of obtaining the spectroscopic features of GIP crystal, Fourier transform infrared spectroscopy was carried out. The spectrum was recorded for the fine grained sample of GIP crystal in the range 4000 cm−1– 400 cm−1 using KBr pellet technique. The recorded FTIR spectrum is shown in Figure 10. The intense peaks between 2400-4000 cm-1 may be due to the symmetric and asymmetric stretching vibrations of NH2 [18]. The sharp band at 2911 cm-1 is attributed for the asymmetric C-H stretching vibration [19]. The C=O stretching vibration is observed at 1720 cm-1[20]. The spectrum appeared at 1616 cm-1 is due to the C=N stretching frequency [21]. The N-H asymmetric bending vibration of amide group is observed at 1550 cm-1. The peak observed at 1499 cm-1 is attributed to the symmetric NH2+ bending vibration. The strong band at 1094 cm-1 corresponds to C-H in-plane bending vibrations [20]. The bands in spectra at 675 cm-1 correspond to C-H out-of-plane bending vibrations [22]. The all existing functional groups present in the FTIR spectra of GIP material confirms the structure of the compound.
Figure 10. FTIR spectrum of GIP 3.4 Electronic absorption studies The electronic absorption reveals the optical properties of all optically active crystalline materials. The UV-vis-NIR transmission spectrum illustrates complete information about the optical transmission of GIP material. The structure of organic molecules could be depicted with the help of transmission spectra due to their predominantly occurring σ and π orbital electronic transitions from their corresponding lower energy (ground state) to higher energy (excited state) [23]. The UV-vis-NIR transmission spectrum of the grown GIP crystal is examined using Labindia 3032 UV-vis-NIR spectrophotometer and the resulting spectrum is exhibited in Figure 11. The maximum transmittance of the GIP crystal was observed to be around 60% with a lower cut-off wavelength at 298 nm. The observed cut-off wavelength may be due to the electronic transition of the molecule from bonding π orbital to anti-bonding π orbital (π–π*) [24]. The electronic transition assignment is confirmed with single crystal X-ray diffraction analysis results as the protonation of GIP compound is done by the proton transfer from hydroxyl group of isophthalate to the nitrogen of the guanidine molecule. This electron transition among lower and higher energy states makes a minor contribution to the hyperpolarizability of the crystal. The complete transmittance of the title crystal in the entire visible region nominates the crystal to be a befitting material for optoelectronic applications. The optical bandgap of the title material is depicted in Figure 12 and was ascertained to be 3.9 eV by extrapolating the linear part of tauc’s plot to the corresponding energy axis.
Figure 11. Transmittance spectrum of GIP crystal
Figure 12. Tauc’s plot of GIP crystal 3.5 HOMO-LUMO analysis Frontier molecular orbital (FMO) investigations were used to identify and examine the highest occupied molecular orbital and the lowest unoccupied molecular orbital abbreviated as HOMO and LUMO respectively. These FMOs play a prime role in the
evaluation of electro-optical and quantum chemical properties of the compounds [25]. The energy gap observed between HOMO and LUMO delineate the charge transfer interaction transpiring within the molecule. The spotted electronic absorption may be due to the electron transition from ground state to the corresponding excited states [26]. The linkage orientation of the title molecule facilitates the deterioration of the HOMO level and the stabilization of the LUMO level. The result of the orbital energy level scrutiny for the GIP material is exhibited in Figure 13 as a 3D plot with HOMO-LUMO energy levels. From the theoretical HOMO-LUMO calculations it is observed that the HOMO, LUMO and energy gaps are -4.81 eV, -0.05 eV and 4.75 eV respectively. Further the predominant quantum chemical molecular properties, i.e., global reactivity descriptors such as ionization energy, ionization potential, electro negativity, chemical hardness, total energy change, electrophilicity index, degree of chemical relativity and overall energy balance were also calculated using standard equations [27] and the values are provided in Table 4. The stability of the molecule expressed via global reactivity descriptors supplements the material’s resistance to polarization which may be due to an external perturbation on the molecular system. These energies indicate an enhanced intramolecular charge transfer interactions in the system that leads to make the molecule as an easily polarizable one.
Figure 13. 3D plot of HOMOs and LUMOs Table 4. Calculated molecular orbital parameters of the title compound Parameters
Calculated values
Ionization potential
4.81 eV
Electron affinity
0.05 eV
Chemical hardness
2.38 eV
Electro negativity
2.43 eV
Chemical potential
-2.38 eV
Total energy change
-0.595 eV
Electrophilicity index
1.19 eV
Degree of chemical relativity
0.2101 eV
Overall energy balance
-4.76 eV
3.6 NBO analysis The inter and intra-molecular interaction and bonding among the molecules can be studied efficiently by the natural bonding orbital (NBO) analysis. It also affords a convenient basis for the investigation of charge transfer or conjugative interactions in the molecular system. The interaction between electron donors and electron acceptors is more intense when stabilization energy of hyperconjugative interactions E(2) value is large [26], i.e., in the complete system, when the extent of conjugation interaction is higher, the donating tendency from electron donors to electron acceptors becomes greater. From the second-order perturbation theory analysis of NBO Fock matrix, the stabilization energies of GIP crystal were obtained and the stabilizing interactions between donor and acceptor NBOs obtained from Gaussian 03 program is presented in Table 5. The NBO result exhibits a quiet large number of stabilizing interactions in GIP molecule. Hence from the obtained data, the strong hyperconjugative interactions between the O(3) bond and the anti-bonding O(4)-C(20) have maximum stabilization energy of about 103.21 kJ/mol. The other significant interaction which exhibits similar strong stabilization energy at 102.68 kJ/mol., is the interaction between O(2) and O(1)-C(17). These energetic contributions to the electron delocalization interactions in the sample molecule were quantitatively estimated to identify the structure stabilizing intramolecular charge transfer interactions thereby elucidating the proper and improper bonding within the molecule [27].
Table 5 Selected second-order perturbation theory analysis of Fock matrix in NBO basis E(2)(a) (kJ/mol) E(j)-E(i)(b) (a.u)
F(i,j)(c)(a.u)
Donor NBO (i)
Acceptor NBO (j)
LP (3) O(3)
BD* (2) O(4) -C (20)
103.21
0.23
0.139
LP (3) O(2)
BD* (2) O(1) -C (17)
102.68
0.23
0.139
LP (2) O(2)
LP* (1) C (21)
83.20
0.11
0.100
BD* (2) O(4) -C (20) BD* (2) C(15) -C (24)
72.71
0.04
0.075
BD* (2) O(4) -C (20) BD* (2) C(16) -C (18)
68.44
0.04
0.072
(a)
E(2) denotes the stabilization energy of hyperconjugative interactions. Energy difference between donor (i) and acceptor (j) in NBOs. (c) F(i, j) is the Fock matrix element between donor (i) and acceptor (j) NBOs. (b)
3.7 Thermal study The thermal stability of GIP was studied by thermogravimetric (TG) and differential thermal analyses (DTA). The GIP sample weighing 5.438 mg was analysed under nitrogen atmosphere and the thermogram is depicted in Fig. 14. From the TG curve, it is evident that the GIP material is stable up to 120 °C. The TG curve showed three stage weight loss patterns when the material was heated from 32°C to 600°C. The first minor weight loss around 120°C is the evidence for the elimination of water molecule as seen in the low temperature region. The water molecule comprises a mass of 18.02 g/mol (observed 8.21%, calculated 6.02%). The second stage weight loss observed between 128°C and 240°C experiences a weight loss of about 13%. The third stage decomposition occurred between the temperatures 241°C and 322 °C incurs a weight loss of about 81%. A broad endothermic peak found 315 °C in the DTA curve is associated with the third stage weight loss of the GIP material. This broad endotherm further confirms the absorption of energy for breaking of
bonds during decomposition. The decomposition process was carried up to 600 °C with the removal of the material into gaseous products.
20
120°C
Weight %
80
TGA DTA
15
10
60
5
0
81%
40
endo
DTA (mW/mg)
100
-5
20 -10 0 0
100
200
300
400
500
600
Temperature (°C)
Figure 14. TG-DTA curve of GIP 3.8 Nonlinear optical study To investigate the nonlinear optical property of GIP crystal we opted for Z-scan technique with Nd:YAG laser radiation of wavelength 532 nm, a prominent method to examine the third order nonlinear optical chattels. The result of Z-scan technique reveals the nonlinear refractive index (n2), absorption coefficient (β) and optical susceptibility (χ(3)) of the crystal sample. The sample was allowed to move along the direction of laser beam propagation i.e., focal region say, –Z axial direction to +Z axial direction. With the same experimental setup the sample was measured at two different conditions in order to get n2 with closed aperture condition and β and χ(3) with open aperture condition. The experimentally measured normalized transmittance at closed and open aperture conditions of GIP crystal sample with thickness 0.5 mm is shown in Figure 15 and Figure 16 respectively. The self-defocussing effect of the title crystal was confirmed with the negative third-order
nonlinear refractive index (n2) calculated from closed aperture spectrum. The reverse saturation absorption with positive nonlinear absorption coefficient (β) confirms the multiphoton absorption of the grown crystal from open aperture spectrum. This would serve as a great evidence to exploit the GIP crystal for optical limiting applications [28]. The quantitatively evaluated nonlinear refractive index, nonlinear absorption coefficient and optical susceptibility values are tabulated in Table 6.
Figure 15. Closed aperture curve of GIP crystal
Figure 16. Open aperture curve of GIP crystal Table 6. Experimental results of Z-scan technique n2 x 10 -4 cm2/W -1.16
β x 10-4 cm/W 4.874
χ(3) x 10-6 esu 2.74
Table 7. Comparison of nonlinear optical parameters with some of the reported materials β cm/W
χ(3) esu
-7.7 x 10-4
3.38 x 10-6
[29]
0.311 x 10-2
3.143 x 10-4
[30]
0.08 x 10-4
2.74 x 10-6
[31]
4.874 x 10-4
2.74 x 10-6
Present work
Reference
4. Conclusion Bisguanidinium isophthalate single crystals have been grown by solvent evaporation growth method and the crystallographic parameters were estimated from single crystal XRD study. The crystal packing elucidates stabilization of π to π* stacking interactions. DFT computations were executed to manifest the optimized molecular geometry of the titular compound. The optical transmittance spectrum validates that the GIP crystal aids as a noble candidate for nonlinear optical and optoelectronic applications. The HOMO– LUMO energies and energy gap of the GIP crystal demonstrates the charge transfer interactions taking place within the crystal lattice. The strong N–H…O, C–H…O and hydrogen bonding has also been confirmed with the help of FMO investigations. Natural bonding orbital analysis was exploited to explain the occurring delocalization, intermolecular charge transfer, interacting strength and equilibrium energy among the bonds within the molecule. The thermal decomposition and the corresponding stability of the titular material were assessed from TG and DTA analyses. The evaluated nonlinear refractive index (n2), absorption coefficient (β) and third order nonlinear susceptibility (χ(3)) of GIP crystal deliberated by Zscan technique aids to promote the material for optical limiting applications.
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Highlights o
Bulk crystals of GIP has been grown obtaining slow evaporation technique
o
Crystal structure is reported for the first time
o
Third order nonlinear optical properties has been reported.
☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare no financial interests/personal relationships may be considered as potential competing interests: