Growth, crystal structure, Hirshfeld surface, dielectric and mechanical properties of a new organic single crystal: ‘Bis glycine’ squarate

Abstract

A novel transparent organic crystal ‘Bis glycine’ squarate has been grown by a slow evaporation method. Single crystal XRD confirms the monoclinic lattice with space group C2/c in BGSQ crystal with a refinement value (R) of 0.06. The molecular electron charge density distribution of the BGSQ compound is described. The growth rate of the BGSQ crystal is estimated by morphological study. The Hirshfeld surface gives information about intermolecular interactions within the crystal structure and the fingerprint plot shows the nature and type of intermolecular close contact experienced by the molecules in a crystal. The FTIR spectrum confirms the various functional groups present in the grown crystal. Thermal stability and melting point (182.20 °C) were studied with TGA-DTA. The low value of the dielectric constant of BGSQ suggests that the crystal can be used in the microelectronics industry. In UV spectra, a lower cutoff value at 280 nm and a wide band gap of 4.25 eV for the BGSQ crystal are observed. Meyer's index number (1.817) in hardness studies confirmed the soft nature of the crystals. Voids have been used to describe the mechanical strength of crystalline materials.

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1. Introduction

The organic materials are well known for their wide range of applications in optoelectronics, semiconductors, telecommunication systems, frequency doubling and photonic devices.^1,2 This is because of low cost, low dielectric constant, inherent synthetic flexibility, high optical damage threshold and ultra-fast response.^3,4 In organic materials charge transfer processes, i.e. transfer of electrons between donor and acceptor molecules, play critical roles.^5,6 The structure of the organic material is constructed from electron donor and acceptor groups in a π-conjugated system, in contrast to inorganic materials, which have great attention in the field of organic optoelectronics.^7,8 These materials are much cheaper for making optical devices in comparison with their counter inorganic materials. Organic materials can be modified and tuned with respect to their chemical structure and properties of materials.⁹

Amino acids are vital components of a variety of biological, industrial, and environmental applications.^10,11 Amino acids are crystalline solids which have high melting points up to 200–300 °C.¹² Glycine is a representative amino acid which can assume cationic, anionic and zwitterionic forms.¹³ The molecule can combine with anionic, cationic and overall neutral constituents, and thus a large number of possible glycine compounds exist.^14,15 It has no asymmetric carbon atom and is optically inactive. Glycine and its methylated analogues form complexes with mineral acids exhibit astounding physical properties like ferro-elastic, ferroelectric or antiferroelectric behavior that is often associated with transitions to commensurate or incommensurate phases.¹⁶ It is ambivalent and can be inside or outside of the protein molecule. Glycine is used in stabilization applications, pharmaceutical applications and as a chemical intermediate.¹⁷

The first synthesis of squaric acid was published in 1959 by Cohen et al. Squaric acid (IUPAC name 3,4-dihydroxycyclobut-3-ene-1,2-dione) is a resourceful organic molecule that is used in a variety of fields in organic, inorganic chemistry and optoelectronic materials to bioconjugates.¹⁸ Squaric acid has strong intermolecular hydrogen bonds due to its high melting point of 275–300 °C and the low water-solubility. It is a strong dibasic acid with a four atom-ring frame work and it is can be used to generate structural assemblies in solid state via hydrogen bonding.¹⁹ The physical and chemical properties of squaric acid are used in medicinal chemistry as an equivalent functional group of a carboxylic acid. L-Glutamic acid with squaric acid formed glutamate analogs, C–Sq–Glu and N–Sq–Glu, which has exhibited a potent binding activity in AMPA and KA receptors.²⁰ Other significant organic complexes of squaric acid are dinicotin amidium squarate,²¹ di(2-amino-4-methylpyrimidinium) squarate,²² di-p-toluidinium squarate dihydrate,²³ etc.

Hence, in the present investigation, a new optical material known as ‘Bis glycine’ squarate (BGSQ) has been synthesized and grown by slow evaporation method. The morphology, structural, Hirshfeld surface, fingerprint analysis, dielectric and microhardness properties in grown single crystal were investigated.

2. Experimental section

2.1 Synthesis and crystal growth

‘Bis glycine’ squarate was synthesized by glycine and squaric acid in a 2 : 1 stoichiometric ratio. High purity materials of glycine and squaric acid (99.9%, Sigma-Aldrich) were used for growth of the single crystal.

The synthesized BGSQ material dissolved thoroughly in double distilled water at 30 °C to form a saturated solution. The reactants were stirred well using a magnetic stirrer to yield a homogenous mixture of solution. To grown the crystals of high quality, the solution was kept in temperature controlled oil bath (accuracy ± 0.1 °C) by slow evaporation at ambient temperature. During the slow evaporation, well-defined single crystals of good transparency were obtained in a period of 20 days. The purity of the solution has improved by re-crystallization processes. A photograph of as grown crystals is given in Fig. 1a.

Fig. 1 (a) A photograph of BGSQ crystals grown by the slow evaporation method. The purity of the solution has improved by re-crystallization processes. (b) The morphology of the grown crystal with plane which is solved by WinX-morph software. (c) The crystal packing with Miller plane and construct a slice of the crystal either side of this plane of grown and its relative growth of different faces which is calculated by Mercury 1.4 software.

2.2 Crystal morphology

The morphology of the grown crystal with plane was solved by WinX-morph²⁴ software and is shown in Fig. 1b. Bravais–Friedel–Donnay–Harker (BFDH) law is applicable in many cases in predicting morphology and used to solve the morphology importance of the planes.²⁵ BFDH law states that the rate of growth of the plane (hkl) is inversely proportional to the interplanar distance d_hkl. The modified BFDH law (Prywer, 2004) formula is given below²⁶
where R_hkl, R_h1k1l1, R_h2k2l2 are the normal growth rates of the hkl, h₁k₁l₁ and h₂k₂l₂ faces, α and γ are the interfacial angles for giving hkl plane. The crystal packing of BGSQ crystal with respect to different atomic arrangement along different crystal planes and its relative growth of different faces are shown in Fig. 1c. The growth rate and morphological importance (MI) of the BGSQ crystal by BFDH law have been shown in Table 1. The morphology of the crystal grown closely matches the predictions of both simple BFDH and modified BFDH law. MI of faces depend not only on the inter-planar distance of the considered face, but also on the inter-planar distances of the directly neighboring faces and on the crystal geometry represented by interfacial angles α and γ. It directly reflects the corresponding area of the crystal faces of the BGSQ crystal.^27,28 The growth rate of the crystal plane must be affected under different conditions; a slower growth crystal plane would appear with large MI.

Table 1 Growth rate and morphological importance of BGSQ crystal faces modified BFDH law

Faces (hkl)	dhkl (Å)		Morphological importance by modified BFDH law
(100)	16.536	0.2946	1.000
(112̄)	4.4588	0.2525	1.167
(1̄12)	4.8513	0.2062	1.241

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3. Characterization

In present study, BGSQ crystal has grown by slow solvent evaporation technique for the first time. The morphology of the grown crystal was solved by WinX-morph software. Single crystal diffraction data were taken using an Oxford-Diffraction X-Calibur with sapphire CCD detector and Enhance diffractometer (MoKα) radiation, graphite monochromator, (λ = 0.71073 Å) to determine the crystal structure of the BGSQ single crystals. A good quality transparent single crystal was selected and X-ray diffraction data were collected at 293 K. The crystal structure was solved by direct method (SHELXL-97) and refined by a full matrix (SHELXL-97) least square procedure employing 18 976 reflections which is satisfied I > 2r(I) criterion. Full crystallographic data (cif file) relate to the crystal structure have been deposited with the Cambridge Crystallographic Data Centre as CCDC 1052856.

A Perkin-Elmer spectrum BX spectrophotometer with the KBr pellet technique was employed to record the infrared spectrum in the range 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹ and a scanning speed of 2 mm s⁻¹. Thermal behavior of the sample was studied by thermo gravimetric (TGA) and differential thermal analysis (DTA) using a SDTQ600 TGA/DTA in the temperature range RT to 800 °C at a heating rate of 10 °C min⁻¹ in nitrogen atmosphere. The dielectric constant and dielectric loss was measured using Agilent 4284-A LCR meter with temperature ranging from room temperature to 100 °C and frequency range from 20 Hz to 2 MHz. The opposite parallel faces of the crystals were coated with high grade silver paste and placed between the two copper electrodes and thus a parallel plate capacitor was formed. The transmission spectra for the specimens were recorded using a Perkin Elmer UV spectrophotometer in the range of 190–1100 nm. The mechanical properties of the grown crystal were studied using a Vickers microhardness tester with diamond indenter. The indentations were made gently by varying the loads from 5 to 100 g for a dwell period of 20 s using Vickers diamond pyramid indenter attached to an incident ray research microscope.

4. Results and discussion

4.1 Single crystal X-ray diffraction analysis

The single crystal structural study of a BGSQ crystal was carried out by single crystal X-ray diffraction analysis. The crystal structure was solved and refined by the direct method and computerized by full matrix least square technique using the program SHELXL-97 (WINGX) program.^29,30 Single crystal X-ray diffraction analysis confirms the monoclinic crystal structure with centrosymmetric space group C2/c for BGSQ crystal. The crystallographic data and structure refinement parameters^29,31 of the compound are presented in Table 2. Refinement factor of the BGSQ structure was computed by using following relation,

R = ∑(|F_o| − |F_c|)/∑|F_o| = 0.06

Table 2 Crystal data and structure refinement of the BGSQ single crystal

Empirical formula	2(C2H5NO2)·1(C4H2O4)
Formula weight	264.20
Temperature	293(2) K
Wavelength	0.71073 Å (MoKα radiation)
Crystal system, space group	Monoclinic, C2/c
Unit cell dimensions	0.05 × 0.05 × 0.05 mm
Cell length a	16.8050 Å (16)
Cell length b	8.3008 Å (8)
Cell length c	15.7976 Å (13)
Cell angle alpha	90.00°
Cell angle beta	100.259° (9)
Cell angle gamma	90.00°
Volume	2168.4 (3) Å^3
Z, calculated density	8, 1.618 Mg m^−3
Absorption coefficient	0.15 mm^−1
F(000)	1104
Crystal size	0.05 × 0.05 × 0.05 mm^3
Theta range for data collection	3.6–29.4°
Limiting indices	−21 ≦ h ≦ 22, −10 ≦ k ≦ 11, −21 ≦ l ≦ 19
Standard reflection	2278
Independent reflection	2571 (R(int) = 0.036)
Data completeness	0.853
Absorption correction	Multi-scan
Max and min transmission	0.403 and 1.000
Refinement method	Full-matrix least-squares on F^2
Restraints/parameters	91/185
Goodness-of-fit on F^2	1.05
Final R indices [I > 2σ(I)]	R1 = 0.0375, wR2 = 0.0914
R indices (all data)	R1 = 0.0612, wR2 = 0.1813
Extinction coefficient	0.027 (2)

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The goodness of fit (S) is expressed as,

S = {∑[w(F_o² − F_c²)]/(n − p)}^1/2 = 1.05

where F_o, F_c are the experimental structure factor and structure factor calculated from the least square law, ‘n’ is the number of reflections and ‘p’ is the total number of parameters refined. Extinction corrections were also included. The experimental and crystal data are given in Table 2.

The threshold expression of F² > 2sigma (F²) is used only for calculating R-factors and is not relevant to the choice of reflections for refinement. The single crystal structure for BGSQ using Platon with 50% probability and crystal packing have been shown in Fig. 2a and b. The crystal structure model in unit cell for BGSQ and symmetry elements of BGSQ crystal with glide have been shown in Fig. 2c and d.

Fig. 2 (a) Single crystal structures for BGSQ single crystal using Platon with 50% probability. The refinement factor of grown crystal and goodness-of-fit are obtained 0.06 and 1.05 respectively. (b) The black, red, blue and white colors balls represent carbon, oxygen. Nitrogen and hydrogen atoms, respectively in crystal packing diagram. (c) Schematic illustrations of the unit cell of crystal structure model for BGSQ. (d) Figure shows symmetry elements of BGSQ crystal with glide planes. Orange color and pink color represent symmetry element and glide plane, respectively.

The final refined atomic positions and isotropic thermal parameters are listed in Table 3. The structure solution and refinement of the crystal at room temperature indicate monoclinic C2/c symmetry which was refined with anisotropic displacement parameters whose values given in Table 4. The geometric parameters, relevant bond lengths and bond angles have been shown in Table 5.

Table 3 Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.09194 (13)	0.8466 (3)	−0.03394 (13)	0.0256 (4)
C2	0.09483 (13)	0.7579 (3)	0.04585 (13)	0.0274 (5)
C3	0.17475 (12)	0.8275 (2)	0.07796 (12)	0.0226 (4)
C4	0.17068 (12)	0.9164 (3)	−0.00336 (12)	0.0238 (4)
C5	0.10038 (12)	0.6274 (3)	0.28923 (13)	0.0267 (5)
C6	0.15003 (12)	1.1974 (2)	0.15284 (12)	0.0236 (4)
C7	0.11838 (14)	0.5334 (3)	0.37159 (13)	0.0289 (5)
C8	0.12637 (13)	1.1070 (3)	0.22739 (13)	0.0265 (5)
O1	0.04847 (12)	0.6633 (3)	0.07406 (11)	0.0471 (5)
O2	0.04303 (10)	0.8593 (2)	−0.10389 (10)	0.0400 (5)
O3	0.21405 (10)	1.0121 (2)	−0.03524 (10)	0.0345 (4)
O4	0.22494 (9)	0.81570 (19)	0.14547 (9)	0.0289 (4)
O5	0.09169 (13)	0.5327 (2)	0.22184 (11)	0.0429 (5)
O6	0.09129 (10)	1.2419 (2)	0.09353 (10)	0.0358 (4)
O7	0.09515 (11)	0.7707 (2)	0.28710 (12)	0.0385 (4)
08	0.22041 (10)	1.2263 (2)	0.15006 (11)	0.0362 (4)
H1A	0.2455	1.0516	0.2677	0.032*
H1B	0.1941	0.9823	0.3263	0.032*
H2A	0.1429	0.7453	0.4364	0.043*
H2B	0.1533	0.6050	0.4961	0.043*
H3	0.0693	0.6335	0.1224	0.071*
H4	0.2077	0.7540	0.1785	0.043*
H5	0.1029	0.8062	0.3363	0.058*
H6	0.0482	1.2132	0.1063	0.054*
H7A	0.0710	0.4720	0.3791	0.035*
H7B	0.1621	0.4583	0.3691	0.035*
H8A	0.0998	1.1798	0.2616	0.032*
H8B	0.0886	1.0218	0.2059	0.032*
N1	0.19869 (11)	1.0373 (2)	0.28126 (11)	0.0266 (4)
N2	0.14138 (13)	0.6430 (3)	0.44481 (12)	0.0355 (5)

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Table 4 Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
C1	0.0256 (10)	0.0321 (11)	0.0185 (9)	−0.0026 (8)	0.0022 (7)	0.0033 (7)
C2	0.0312 (10)	0.0310 (10)	0.0182 (9)	−0.0056 (9)	0.0000 (7)	0.0030 (7)
C3	0.0258 (9)	0.0234 (9)	0.0182 (9)	−0.0011 (8)	0.0027 (7)	−0.0013 (7)
C4	0.0264 (9)	0.0273 (10)	0.0170 (8)	−0.0015 (8)	0.0019 (7)	−0.0001 (7)
C5	0.0200 (9)	0.0316 (11)	0.0270 (10)	−0.0031 (8)	0.0004 (7)	0.0047 (8)
C6	0.0270 (10)	0.0248 (9)	0.0190 (9)	−0.0035 (8)	0.0039 (7)	−0.0001 (7)
C7	0.0313 (10)	0.0284 (11)	0.0254 (10)	0.0043 (9)	0.0006 (8)	0.0018 (8)
C8	0.0256 (9)	0.0311 (10)	0.0228 (9)	0.0015 (9)	0.0039 (7)	0.0063 (8)
O1	0.0461 (10)	0.0632 (12)	0.0272 (8)	−0.0285 (9)	−0.0066 (7)	0.0181 (8)
O2	0.0302 (8)	0.0630 (12)	0.0230 (8)	−0.0074 (8)	−0.0057 (6)	0.0125 (7)
O3	0.0359 (9)	0.0436 (10)	0.0231 (7)	−0.0154 (7)	0.0027 (6)	0.0054 (6)
O4	0.0317 (8)	0.0319 (8)	0.0196 (7)	−0.0039 (7)	−0.0051 (6)	0.0029 (6)
O5	0.0625 (12)	0.0383 (9)	0.0246 (8)	−0.0048 (9)	−0.0010 (8)	0.0007 (7)
O6	0.0298 (8)	0.0510 (10)	0.0249 (8)	−0.0040 (8)	0.0001 (6)	0.0133 (7)
N1	0.0293 (9)	0.0293 (9)	0.0197 (8)	−0.0017 (7)	0.0007 (7)	0.0045 (6)
N2	0.0456 (12)	0.0364 (11)	0.0240 (9)	0.0059 (9)	0.0049 (8)	−0.0021 (7)

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Table 5 Geometric parameters (Å) relevant bond lengths [Å] and angles [deg] involving non-hydrogen atoms BGSQ

C1–C2	1.453 (3)	O1–C2	1.243 (3)
C1–C4	1.446 (3)	O2–C1	1.259 (2)
C3–C2	1.467 (3)	O3–C4	1.243 (3)
C3–C4	1.472 (3)	O4–C3	1.240 (2)
C7–C5	1.501 (3)	O5–C5	1.311 (3)
C8–N1	1.472 (3)	O6–C6	1.288 (3)
C8–C6	1.508 (3)	O7–C5	1.192 (3)
N2–C7	1.468 (3)	08–C6	1.215 (3)
C1–C2–C3	89.28 (16)	O4–C3–C2	134.97 (19)
C1–C4–C3	89.34 (16)	O4–C3–C4	135.13 (19)
C2–C3–C4	89.90 (16)	O5–C5–C7	111.59 (19)
C4–C1–C2	91.48 (16)	O6–C6–C8	115.87 (18)
O1–C2–C3	135.92 (19)	O7–C5–C7	123.1 (2)
O1–C2–C1	134.8 (2)	O7–C5–O5	125.4 (2)
O2–C1–C4	132.49 (19)	08–C6–C8	121.34 (19)
O2–C1–C2	136.0 (2)	08–C6–O6	122.79 (19)
O3–C4–C1	133.45 (18)	N1–C8–C6	109.97 (17)
O3–C4–C3	137.21 (19)	N2–C7–C5	110.16 (18)

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4.2 Residual electron density

Fourier maps are used to visualize the electron density which is an essential part of X-ray structural analysis. A Fourier map through the MAPVIEW module in WinGX is highly informative in cases of disorder or incorrect refinement.³² In single crystal XRD, the parameters of electron density are obtained by minimization of the weighted squared modulus of the difference between F(H)_obs and F(H)_cal. The F(H)_obs and F(H)_cal are represents the observed and calculated Fourier values, respectively. F(H)_obs − F(H)_cal map³³ are calculated by SHELXL software and visualized as mesh style isosurface which is shown in Fig. 3a. In residual electron density map (Fig. 3b), positive contour is shown with a solid blue line, negative contour with dotted green line and zero contours with red dot-dash line at 0.1 residual density distribution intervals. The residual electron density provides more information regarding the crystallographic analysis. The important aim of residual density analysis is to separate noise from structural information on the residual density.^34,35 The 3D surface and 3D iso-surface residual electron density³⁶ maps with zero contour level are shown in Fig. 3c and d which show radically the nature of the molecular weight function w(r).

Fig. 3 (a) Difference Fourier map F_obs − F_cal map was calculated by SHELXL software and visualized as mesh style isosurface. (b) Residual density map, the contour levels at 0.1 eÅ⁻³ intervals. Positive contours with solid blue line and negative contours with broken green line. (c and d) 3D surface and 3D isosurface residual electron density maps with zero contour level.

4.3 Hirshfeld surface analysis

Hirshfeld surfaces are a space partitioning construct that summarizes the crystal packing into a single 3D surface and the surface is reduced to a 2D fingerprint plot which summarizes the complex information on intermolecular interactions present in molecular crystals.³⁷ Hirshfeld surfaces and their associated fingerprint plots were generated using CrystalExplorer (Version 3.0) software from the crystal structure coordinates supplied in the format of Crystallographic Information File (CIF). Hirshfeld surfaces are a powerful tool for elucidating molecular crystal structures. Hirshfeld surface is an extension of Hirshfeld's stockholder concept which divides the electron density of a molecule into continuous atomic fragments. The electron density of an atomic fragment can be defined as³⁸

ρ_a(r) = w_a(r)ρ^mol(r)

where ρ^mol(r) is the molecular electron density. Atomic electron densities are sharply peaked near the nuclei and decay exponentially. A weight function of the molecule in a crystal is defined as

w(r) = ρ_promolecule(r)/ρ_procrystal(r), 0 < w(r) < 1

where the numerator is a sum over the promolecule and the denominator is an analogous sum over the crystal. It depends on the specific atomic electron density and identity of neighboring atoms. The weight function w(r) satisfies the condition 0 < w(r) < l and molecular properties are obtained by integration over the weighted electron density w(r) ρ(r). At w(r) ≧ 0.5, the promolecule contribution exceeds that from all neighboring molecules.³⁷ The electron density is counted only within this region; outside the region none is counted.

This isosurface defines the volume of space where the promolecule electron density exceeds that from all neighboring molecules. Promolecular surfaces are used to define the size and shape of a molecule in a crystal.^39,40 Different types of promolecule surfaces are shown in Fig. 4. Hirshfeld surface defines a volume around a molecule in a manner similar to a van der Waals surface or an outer surface of the electron density.³⁷ It visualizes intermolecular interactions by color-coding for short or long separation and explores the properties of all inter-contacts within the crystal structure. The d_e is the distance from a point on the surface to the nearest nucleus outside the surface and d_i is the distance from a point on the surface to the nearest nucleus inside the surface. The d_norm is very useful tool for visualizing intermolecular interactions and their contribution towards the crystal packing behavior of molecules.³⁹ It is normalized contact distance and is defined in terms of d_i, d_e and vdW radii of the atoms as follow,

d_norm = (d_i − r^vdw_i)/r^vdw_i + (d_e − r^vdw_e)/r^vdw_e

Fig. 4 Promolecule surface with the 0.002 a.u. isosurface of (a) deformation density (b) electron density (c) electrostatic potential and (d) orbital plotted, which is defined the size and shape of a molecule with visualizes the space belong to a molecule in a crystal.

Hirshfeld surfaces give the information on intermolecular contacts. The d_norm is symmetric in both d_e and d_i, close intermolecular contacts appear as two identical red spots. The red and blue colors in the Hirshfeld surface mapped with d_norm are associated with shorter and larger distances, respectively, as compared to van der Waals intermolecular separation. Hirshfeld surfaces and related graphical tools have been shown to enhance exploration of the nature of the interactions between molecules in crystal.^41,42 Other types of Hirshfeld surfaces are also described in the present study, which is shown in Fig. 5. All properties mapped on the surface have been related to the metrics of the structures: distances from surface points to nuclei or functions of the principal surface curvatures.

Fig. 5 Hirshfeld surface with the 0.002 a.u. isosurface of (a) deformation density (b) electron density (c) electrostatic potential and (d) curved type (relatively flat curvature by large regions of green color and large positive curvature separated by dark blue edges). (e) Shaped type (complementary hollows (red color) and bumps (blue color)). Hirshfeld surface defines a volume around a molecule in a manner similar to a van der Waals surface or an outer surface of the electron density.

Curvature plays an important role in condensed matter, biology, chemistry and physics.⁴³ Curvedness is defined in terms of the root-mean-square curvature of the surface and gives some information about the coordination of the molecule in the crystal structure. Curvedness show relatively flat curvature by large regions of green and large positive curvature separated by dark blue edges. Mathematically, curvedness is expressed as

C = 2/π ln{(k₁² + k₂²)/2}^1/2

where k₁ and k₂ are the maximum and minimum curvature of the surface, respectively.

Shape index (S) is defined as the local morphology of the molecular surface which is independent of the scale.³⁷ In shape index, two shapes differ by only a change of colors. Therefore, shape index on the Hirshfeld surface can be used to identify complementary hollows (red) and bumps (blue) where two molecular surfaces touch one another (Fig. 5e). It is expressed as:

S = 2/π arc tan(k₁ + k₂)/(k₁ − k₂)

Fingerprint plot is formed by d_i vs. d_e at intervals of 0.01 Å and coloring scheme of the 2D histogram as a function of the fraction of surface points, ranging from blue (few points) through green to red (many points). The molecular surface electrostatic potential is used in various packing motifs and intermolecular interactions. The electrostatic potential mapped on molecular electron density isosurfaces discuss the molecular packing of isomers of the grown crystal.³⁹

Hirshfeld surfaces clearly demonstrate the way in which complementary electropositive (blue) and electronegative (red) regions of adjacent molecules come into contact in intermolecular interactions. Fig. 6 shows the d_norm surface of BGSQ crystal, highlighted C⋯C, H⋯C, H⋯H, O⋯C and O⋯H intermolecular contacts, respectively. In these bonding, first atom is the closest atom inside the surface, and second atom the nearest atom outside the surface or vice versa, which are colored.⁴⁰ The C⋯C, H⋯C, H⋯H, O⋯C and O⋯H intermolecular contacts are highlighted separately in the fingerprint plots. The outline of the full fingerprint plot has been shown in gray color. The C⋯C, H⋯C, H⋯H, O⋯C and O⋯H interactions are comprised of 2%, 6.3%, 21.1%, 5.6%, and 53.2%, respectively in the 2D fingerprint map of the total Hirshfeld surfaces. The C⋯C contacts contribute very less (2%) and O⋯H contacts contribute large (53.2%) to the total Hirshfeld surface area of molecules. The O⋯H interaction appears as large spikes in the 2-D plot, which reveals that this interaction is around the r^vdw separation.⁴¹ The C⋯C and H⋯C contacts correspond to the π–π interaction and π-donor and π-acceptor interaction, respectively.

Fig. 6 The breakdown of the fingerprint plot for BGSQ with specific interaction. This plot shows specific interaction in C⋯C, H⋯C, H⋯H, O⋯C and O⋯H contacts. Those interactions are comprised 2%, 6.3%, 21.1%, 5.6% and 53.2% in the 2D fingerprint map of the total Hirshfeld surfaces.

4.4 FTIR analysis

Vibrational spectroscopy is an important tool for understanding the role of various functional groups present in the organic compound.^44,45 Fig. 7a shows the functional groups present in the sample by using FTIR spectroscopy. The carboxylate ion and ammonium ion are present in glycine molecule. In FTIR analysis, the pallet was prepared for taking spectrum after mixing the sample with KBr. The asymmetrical and symmetrical stretches of the COO⁻ (carboxylate ion) were found near 1596.38 cm⁻¹ and 1435.21 cm⁻¹, respectively. The C–H stretching band is superimposed upon the O–H band near 2970.13 cm⁻¹. An absorption covalent bond C=O and NH₂ bending are occurred at 1730.03 cm⁻¹ and 1517.19 cm⁻¹, respectively. In-plane vibrations give rise to a band of medium intensity at C–C stretching (832.13 cm⁻¹), C–H deformation (1080.75), the C–C deformation (505.03 cm⁻¹) and the COO⁻ vibration (648.34 cm⁻¹). The characteristic bands were slightly shifted in observation; overall the crystal has exhibited good characteristic bands of various functional groups concerned. All observed bands confirm the formation of BGSQ single crystal.

Fig. 7 (a) FTIR spectrum for BGSQ crystal which confirm the functional groups present in the sample. (b) DTA and TGA analysis of BGSQ crystal. BGSQ was stable up to 176 °C and there was no appreciable weight loss up to this temperature.

4.5 Thermal analysis

The thermal studies of the crystals were determined by differential thermal analysis and thermogravimetric analysis.⁴⁶ DTA and TGA analysis of BGSQ were carried out in the temperature range from 50 °C to 800 °C at the heating rate of 10 °C min⁻¹ in nitrogen atmosphere. Fig. 7b shows the TGA/DTA traces of BGSQ. In the TGA curve, it was inferred that BGSQ was stable up to 173 °C and there was no appreciable weight loss up to this temperature (Fig. 7b). The endothermic peak in DTA at 182.20 °C corresponds to the melting point of the BGSQ crystal.

4.6 Dielectric analysis

The variation of real dielectric constant (ε₃₃) and dielectric loss with frequency (20 Hz to 2 MHz) at different temperatures for grown crystals are shown in Fig. 8a. The real dielectric constant value decreases with increase in frequency. This effect can be attributed to the effect of charge distribution by mean carrier hopping on defects.⁴⁷ At low frequency, the charge on the defects can be rapidly redistributed so that defects closer to the positive side of the applied field become negatively charged, while defects closer to the negative side of the applied field become positively charged.⁴⁸ This leads to a screening of the field and overall reduction in the electric field as a result of increase in capacitance. At high frequency, the defects no longer have enough time to rearrange themselves in response to the applied voltage, hence the capacitance decreases.⁴⁹ It is also observed that the dielectric constant increases slowly with temperature and attains a value of 38 at 100 °C. The low value of the dielectric constant in grown crystal is useful for microwave electronic devices. The variation of dielectric loss with frequency within the temperature range RT-100 °C is shown in Fig. 8b. Due to low dielectric loss value, crystal possesses enhanced optical quality with lesser defects.

Fig. 8 (a) Variation of real parts of dielectric constant versus frequency at different temperatures. The dielectric constant increases slowly with temperature and attains a value of 38 at 100 °C. (b) The variation of dielectric loss with frequency at different temperature, which enhanced optical quality of crystal.

4.7 UV-Vis analysis

The types of band gap are not dependent on the molecule of the system. The topology of extended system is responsible for the characteristics of band gap.⁵⁰ When the intermolecular interactions are present between the atoms of the different sites, an direct band gap is appeared. Otherwise, if the intermolecular interactions belonging to the atoms of the same sites are active, indirect band gap is anticipated. Fig. 9a shows the π–π stacking pattern of the different sites of the carbon atoms in BGSQ crystal structure. This type of topological behavior results the direct band gap in the grown crystal.

Fig. 9 (a) Topology of the π–π stacking between carbon atoms of different sites in the BGSQ crystal are displayed by green and yellow color dashed lines in the elliptical region. (b) Optical transmission spectrum of BGSQ single crystal, insert figure; plots of (αhν)² function against hν with evaluation of the band gap of the material. The value of the band gap of grown crystal was found to be 4.25 eV.

The absorption in the UV-Vis region is very essential for estimation of the optical band gap and various optoelectronic applications of the material.⁵¹ The linear optical properties of the crystal are highly anisotropic in nature⁵² and have shown the lower cut-off in the UV-Vis region at 280 nm (Fig. 9b). The absorption in the UV-Vis region is due to the structural features of the organic molecules which are referred as chromophores.⁵³ Optical band gap of the grown crystal was computed by using Tauc relation as⁵⁴ αhν = A(hν − E_g)ⁿ, (n = 1/2 for allowed direct transition and n = 2 for allowed indirect transition), where α is a absorption coefficient and A is a constant. The value of absorption coefficient is determined from transmittance “T” using the relation α = ln(1/T)/t, where ‘t’ is thickness of the sample and ‘T’ is the transmittance. The inset of Fig. 9b shows the plot of photon energy (hν) vs. (αhν)² and the value of the band gap was found to be 4.25 eV. The crystallinity of the grown crystals was easily concluded from the tail of the Fig. 9b. Therefore, the wide band gap is due to low defect concentration in the grown crystal, which is more useful for the laser/optical applications.

4.8 Hardness

Hardness is a measure of resistance offers to local deformation. The hardness properties are related to the crystal structure of the material.⁵⁵ Microhardness test has been carried out to understand the plasticity of the crystals. Hardness of the crystal is also dependent on the type of chemical bonding, which may differ along different crystallographic directions. The mechanical strength of a material plays a key role in device fabrication.⁵⁴ The mechanical characterization of BGSQ crystals has been done by micro-hardness testing at room temperature. Transparent BGSQ crystal free from cracks with flat and smooth face was indented gently by loads varying from 5 g to 100 g (Fig. 10a) for a fixed dwell period of 5 s using Vickers diamond pyramid indenter. From the Fig. 10b, it is clear that crack appears at the load of 20 g. It was observed that initially up to 20 g, hardness number increases with load. Only top layer is pierced by the intender at low loads resulting in an increase in hardness with load. On increasing load after 20 g, dislocations increases in the intended region, forming closed networks of interacting dislocations resulting in a decrease in hardness with load up to 100 g. Vickers hardness number was computed using the following formula,⁵⁶

H_v = [(2P sin 136°)/2]/d²

H_v = 1.8544(P/d²)

where H_v is the Vickers hardness number for a given load, P in gram, d is the average diagonal length of the indentation in mm and 1.8544 is the constant of a geometrical factor. The load variation can be interpreted by using Meyer's law

P = Kdⁿ

where K is the material constant and the work hardening coefficient n, from the plot of P versus d uses a least square fit method has been determined (Fig. 10c). The value of n for BGSQ was found to be 1.817 suggesting a softer nature⁵⁷ of BGSQ crystal.

Fig. 10 (a) Photograph of Vickers indentation of BGSQ crystal at different load. (b) Variation of Vicker's hardness H_v with load P. (c) Plot of P vs. d. The value of n for BGSQ was found to be 1.817 by Meyer's law. (d) Voids of grown crystal at 0.002 a.u. isosurface. The volume occupied by voids is around 7.33% of the unit cell volume.

Void refers to the isolated cavity and it is an important term for describing the structure of crystalline materials.³⁵ Voids can be visualized and explored by simply constructing an isosurface of the procrystal electron density. Fig. 10d shows the void surface, with a volume of 158.89 ų per unit cell for BGSQ with 0.002 a.u. isosurface. The isovalue 0.002 a.u. covers the 99.9% of electron density region which is useful for defining the voids of the molecules.⁵⁸ It is found that the volume occupied by voids is around 7.33% of the unit cell volume. Fig. 10d displays the voids in the BGSQ crystal in regular fashion of parallel layers which is normal to the [100]. Void creates the low packing density along [100] which reduces the probability of slip along it. Moreover, it concludes that BGSQ crystal shows softer nature while it contains low value of voids percentage.

5. Conclusion

A new organic BGSQ single crystal has been grown by slow evaporation technique. The experimentally observed morphology of the crystal matches very well with the BDFH laws. The single crystal X-ray diffraction confirms that the grown crystal belongs to the monoclinic crystal system with the centro-symmetric space group. The value of refinement factor (R) and goodness of fit (S) of the BGSQ structure are found to be 0.06 and 1.05, respectively. The residual density analysis is used to show the quality of the refinement for BGSQ single crystal structure. The intermolecular interactions (C⋯C, H⋯C, H⋯H, O⋯C and O⋯H) present in BGSQ crystal were revealed from 3D Hirshfeld surfaces and 2D fingerprint plots, which enabled the decoding of the quantitative contribution of interactions present in crystal system. Because of the low value of dielectric constant (38) and dielectric loss (0.003), BGSQ crystals are an excellent candidate for microelectronic and optical devices. FTIR confirmed the functional groups present in the sample. Thermal study determines the melting (182.20 °C) and weight loss of the material. The low value of UV cut-off wavelength of 280 nm indicates that this material is a potential candidate for generating blue-violet light using a diode laser. Hardness study and voids show a softer nature of BGSQ crystal. All these properties suggest that this compound may be a promising material for optical applications.

Acknowledgements

The authors are grateful to DST for the financial support received in DRDO project (Sanction No. ARMREB/MAA/2015/163) and DU R&D Grant (Sanction No. RC/2015/9677). Dr N.S. expresses her gratitude to Dr Jaswinder Singh, Principal, SGTB Khalsa College for encouragement and support for research work. N.T. is thankful to UGC for Senior Research Fellowship. H.Y. is thankful to UGC for Meritorious Scholarship.

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Footnote

† CCDC 1052856. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18983g

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