Systematic evaluation of a new organic material: 1-methyl-1H-imidazol-3-ium-2,4,6-trinitrobenzene-1,3-bis(olate) for optoelectronics through spectral, structural, electrical, optical, quantum chemical and Hirshfeld surface studies

Article abstract of DOI:10.1016/j.jpcs.2017.01.019

A new organic material, 1-methyl-1H-imidazol-3-ium-2,4,6-trinitrobenzene-1,3-bis(olate) (MITB), was synthesized and crystallized by solution growth-slow evaporation technique at ambient temperature. The characteristic functional groups in MITB were identified from FT-IR spectrum. ¹H, ¹³C and DEPT-135 NMR spectroscopic techniques were used to ascertain types of carbons and protons in MITB. The compound crystallizes in the monoclinic system with a space group of P2₁/c. The electrostatic attraction between anions and cations stabilizes the crystal lattice and the N-H…O and C-H…O hydrogen bonds linking the cations and anions supplement the stable three dimensional networks. The material was thermally stable up to 178 °C. The molecular structure was optimized by Gaussian 09 program at B3LYP/6–311++G(d,p) level of basis set. Hydrogen bonding interactions are responsible for greater hyperpolarizability value of MITB and the value was found to be 34 times greater than that of reference material, urea. HOMO-LUMO, electrostatic potential surface and Mulliken atomic charges were calculated to explore covalent and non covalent interactions present in MITB. Hirshfeld surface analysis was carried out to estimate prominent covalent and non covalent interactions. Dielectric constant and dielectric loss have been determined to find MITB's suitability for optoelectronic applications.

Full text of DOI:10.1016/j.jpcs.2017.01.019

Author’s Accepted Manuscript
Systematic evaluation of a new organic material: 1-
methyl-1H-imidazol-3-ium-2,4,6-trinitrobenzene-
1,3-bis(olate) for optoelectronics through spectral,
structural, electrical, optical, quantum chemical and
Hirshfeld surface studies
P. Dhamodharan, K. Sathya, M. Dhandapani
www.elsevier.com/locate/jpcs
PII:
S0022-3697(16)30792-2
http://dx.doi.org/10.1016/j.jpcs.2017.01.019
PCS7971
DOI:
Reference:
To appear in:
Journal of Physical and Chemistry of Solids
Received date: 27 September 2016
Revised date: 21 November 2016
Accepted date: 11 January 2017
Cite this article as: P. Dhamodharan, K. Sathya and M. Dhandapani, Systematic
evaluation of a new organic material: 1-methyl-1H-imidazol-3-ium-2,4,6
trinitrobenzene-1,3-bis(olate) for optoelectronics through spectral, structural
electrical, optical, quantum chemical and Hirshfeld surface studies, Journal o
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1
Systematic evaluation of a new organic material: 1-methyl-1H-imidazol-3-ium-2,4,6-
trinitrobenzene-1,3-bis(olate) for optoelectronics through spectral, structural, electrical, optical,
quantum chemical and Hirshfeld surface studies
P. Dhamodharan, K. Sathya , M. Dhandapani*
Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science,
Coimbatore - 641 020, Tamil Nadu, India.
*Corresponding author’s. Tel.: +91 94420 01232; srmvdhandapani@gmail.com
Abstract
A
new organic material, 1-methyl-1H-imidazol-3-ium-2,4,6-trinitrobenzene-1,3-
bis(olate) (MITB), was synthesized and crystallized by solution growth-slow evaporation
technique at ambient temperature. The characteristic functional groups in MITB were identified
1
from FT-IR spectrum. H, 13C and DEPT-135 NMR spectroscopic techniques were used to
ascertain types of carbons and protons in MITB. The compound crystallizes in the monoclinic
system with a space group of P21/c. The electrostatic attraction between anions and cations
stabilizes the crystal lattice and the N-H…O and C-H…O hydrogen bonds linking the cations
and anions supplement the stable three dimensional networks extended on ‘b’ axis also. The
material was thermally stable up to 178 C. The molecular structure was optimized by Gaussian
09 program at B3LYP/6-311++G(d,p) level of basis set. Hydrogen bonding interactions are
responsible for greater hyperpolarizability value of MITB and the value was found to be 34 times
greater than that of reference material, urea. HOMO-LUMO, electrostatic potential surface and
Mulliken atomic charges were calculated to explore covalent and non covalent interactions
present in MITB. Hirshfeld surface analysis was carried out to estimate prominent covalent and
non covalent interactions. Dielectric constant and dielectric loss have been determined to find
MITB’s suitability for optoelectronic applications.

2
1. Introduction
Optoelectronics is a wide technological field that requires variety of materials for
manifold applications such as optical memories, optical information processing, colour display,
optical communication, optical computing, optical switches, optical modulation, frequency
shifting and high speed information processing [1-7]. Numerous organic crystals have been
studied in the past couple of decades for their applications in opto-electronics. Organic molecules
are always fascinating to synthetic and material chemists because of their easy tailoring and
synthesis [8-11]. The design of new crystals with specific physical and chemical properties is
influenced by intermolecular and intramolecular interactions [12]. Hydrogen bonding is an
important interaction that plays a vital role in crystal engineering, and a number of organic
molecules have the ability to form hydrogen bonding [13-16]. Heterocyclic organic skeletons
containing electronegative nitrogen can be a part of the organic material due to their electron
donating as well as proton accepting properties in facilitating excellent hydrogen bonding
network. Heterocyclic nitrogen compounds, such as 3,4,7,8-tetramethyl-1,10-phenantroline [17],
benzotriazole [18], 3,5-dimethyl pyrazole [19], 4-aminopyridine [20] were used as partners of
proton donors in proton-transfer reactions. Imidazole and their derivatives are an interesting
group of organonitrogen heterocyclics which can be exploited for the synthesis of newer organic
materials along with other proton donating organic compounds. Picric acid, 2,4,6-trinitrophenol
has been the choice of synthetic chemists in the synthesis of a huge number of proton transfer
complexes due to their proton donating ease [21-25]. Also, a good amount of reports are
available in the literature on the synthesis of organic materials by coupling imidazole derivatives
with picric acid their derivatives [26-30]. However, the use of styphnic acid (2,4,6-
trinitrobenzene-1,3-diol), an analogue of picric acid, in the synthesis of organic materials is very
limited. Styphnic acid is an attractive candidate, because of the presence of two phenolic
hydroxyl groups which favours the formation of organic salts with various organic bases in a
stable manner [31-36]. As of now, no report is available on imidazole-styphnic acid organic
crystals either on synthesis or on computational characterization for their utility in non linear
optical applications.

3
In this paper, we report for the first time the synthesis, spectral, structural, electrical,
optical and computational studies of a new organic optical material: 1-methyl-1H-imidazol-3-
ium-2,4,6-trinitrobenzene-1,3-bis(olate) (MITB) formed by a proton transfer reaction.
2. Experimental work
2.1 Synthesis and growth of crystals
2.1.1 Preparation of styphnic acid
Styphnic acid (2,4,6-trinitrobenzene-1,3-diol) was prepared by nitrating the commercial
resorcinol following Urbanski’s procedure [37]. The reaction scheme is outlined below. The
chemicals used in the synthesis were of analytical grade (Sigma-Aldrich). The final product was
recrystallized several times to get superior quality. The formation of styphnic acid was confirmed
by NMR spectroscopy and other physico - chemical techniques.
(Step:1)
2.1.2 Synthesis of 1-methyl-1H-imidazol-3-ium-2,4,6-trinitrobenzene-1,3-bis(olate) (MITB)
1-methyl-1H-imidazole (Analytical grade, Sigma-Aldrich) and laboratory-synthesized
styphnic acid were dissolved in methanol in 2:1 stoichiometric proportions separately, and the
two solutions were mixed to get a yellow coloured precipitate. Bright yellow coloured crystals
were harvested after ten days from the saturated solution of MITB in acetone solvent. The purity
of the crystal was improved by repeated recrystallization.

4
(Step:2)
2.2. Physico-chemical characterization of MITB
2.2.1 Elemental analysis and FT-IR spectral analysis
Elemental analysis (CHN) was performed in a flash EA 1112 full automatic trace element
analyzer. C, H, N analysis results are given: C14H15N7O8 (M.W: 409.03), Calculated result (%):
C, 41.08; H, 3.69; N, 23.95; Found: C, 41.15; H, 3.61; N, 24.07. There exists a good agreement
between the calculated and experimental values. Fourier-transform infrared spectrum of MITB
was recorded in the region 4000-400 cm−1 by JASCO FT/IR 5300 infrared spectrophotometer.
The FT-IR spectrum of MITB is shown in Fig. S1. The functional groups present in the MITB
were identified from its IR spectrum. The characteristic vibrational frequencies appear in the
region between 4000-400 cm-1. The formation of the charge transfer complex is strongly
evidenced by the presence of the important infrared bands in the FT-IR spectrum of MITB. The
broad band at 3433cm-1 is attributed to the N+-H stretching vibration of MITB confirming the
proton transfer process. The C-H stretching vibration is observed at 3142 cm−1. The frequency at
1295 cm−1 is characteristic of the C-O stretching vibration. The frequencies at 821 and 767 cm−1
are due to NH and CH out-of-plane bending vibrations. MITB also exhibits two sharp absorption
bands corresponding to the asymmetric and symmetric stretching vibrations of the NO2 group at
1424 and 1331 cm−1, respectively. The wagging and scissoring frequencies of NO2 group are
observed at 777 and 703 cm−1, respectively.
2.2.2 NMR Spectral analysis
1H, 13C and DEPT-135 NMR spectra were recorded in a Bruker Avance 400 MHz
FT-NMR spectrometer with tetramethylsilane as an internal standard and DMSO-d6 as the
solvent. The 1H and 13C NMR spectra of MITB are shown in Fig. 1. The singlet at  8.68 ppm is
due to the presence of a single aromatic proton in the styphnate moiety. The peak at 8.58 ppm is

5
due to presence of two chemically equivalent protons (attached to C9 and C12) in the
imidazolium moieties. The singlet peak found at δ 7.50 ppm has been assigned to two protons
(attached to C8 and C11) of the same kind in the imidazolium moieties in MITB. The singlet
signal observed at  7.39 ppm has been assigned to the two equivalent ring protons (attached to
C10 and C7) of the imidazolium moieties. The singlet at  3.81 ppm is due to six equivalent
protons of two methyl groups attached to the nitrogen atoms of two imidazolium rings. A peak
with less intensity appearing in the deshielded region at 12.9 ppm is due to the -N+- H proton of
the imidazolium moiety.
In the 13C NMR spectrum of MITB, the chemical shift of two equivalent carbons C2 and
C6 appears in the downfield at 158.2 ppm. As the carbon atoms, C1 and C5, are chemically
equivalent, a singlet signal due to these carbons occurs at  126.3 ppm. C4 carbon exhibits a
signal at  126.9 ppm. The signal observed at  144.4 ppm is due to the presence of C3 in the
styphnate moiety. The presence of two equivalent methyl carbon atoms (C10 and C14) in the
imidazolium moieties shows a signal at  34.9 ppm. The singlet signal occurring at  122.9 ppm
is due to chemically equivalent carbons C8 and C12, and the signal at  122.7 ppm is due to
another chemically equivalent pair C11 and C7 in the two imidazolium moieties. The signal for
the pair C9 and C13 is observed at  136.9 ppm in the two imidazolium moieties.
The DEPT-135 NMR spectrum of MITB is shown in Fig. S2. This technique allows us to
identify protons of various kinds such as CH, CH2 and CH3 in a molecule. MITB has three pairs
of CH carbons in its two imidazolium moieties and a single CH carbon in its styphnate moiety
apart from one pair of CH3 carbons in its imidazolium moieties. A positive signal for
C9H-C13H carbon atoms is observed at  136.9 ppm. Two positive signals occurring at  122.9
and  122.7 ppm pertain to the pair C8H-C12H and the pair C11H-C7H carbon atoms of the
identical imidazolium moieties, respectively. A single positive CH signal for C4 is found at
 126.9 ppm. The signal at  34. 9 ppm is due to the pair of CH3 carbon atoms in the two
imidazolium moieties of MITB.
3. Results and Discussion

6
3.1 Single crystal X-ray diffraction analysis
Single crystal data of MITB crystal were collected using Oxford XCalibur, Gemini
diffractometer equipped with EOS CCD detector at 25 C using CuKα radiations (λ= 1.54184 Å).
The data were reduced using CrysAlisPro Software and absorption correction using multi Ψ-scans.
The structure was solved using SHELXS-97 and refined by full-matrix least squares against F2
using SHELXL-97 software [38, 39]. Non-hydrogen atoms were refined with anisotropic thermal
parameters. The compound, MITB, crystallizes in a monoclinic system. The lattice parameters
are a = 19.9171(4) Å, b = 7.28263(17) Å, c = 12.4229(3) Å, α = γ =90°, β= 102.706(2)°. The unit
cell volume (V) is = 1757.80(7) Å3 and the number of molecules in the unit cell (Z) is 4. The
intensity data were collected for h from -21 to 24, for k from -8 to 8 and for l from -15 to 9. The
asymmetric unit of the MITB consists of two imidazolium cations and one styphnate dianion.
Crystal data and structure refinement for the MITB crystal are given in Table 1. The
crystallographic information file (cif) has been deposited in the Cambridge Crystallographic
Database Center (CCDC No.1033307).
The loss of two protons from styphnic acid during protons transfer is confirmed by the
shortening of C6-O7 and C2-O8 bond distances. The C6-O7 and C2-O8 bond lengths are
1.268(3) and 1.257(3) Å, respectively, which are a little shorter than that found in phenols (1.36
Å), which is due to delocalization of the pair of electrons of oxygen onto the adjacent nitro
groups in the styphnate moiety [40].
The bond lengths of C1-C2, C2-C3, C6-C1 and C6-C5 are found to increase from their
normal C=C bond length value of 1.34 Å. The experimental bond length values for the same C-C
bonds are 1.416, 1.452, 1.411 and 1.453 Å, respectively. The reason for this increase in the bond
lengths can be attributed to the deprotonation of neighbouring OH groups. On the other hand, the
bond lengths of C3-C4 (1.375 Å) and C4-C5 (1.382 Å) are found to decrease from the normal C-
C bond length value, 1.40 Å owing to deprotonation. The reduction in bond lengths of O7-C6
(1.268) and O8-C2 (1.251) from the normal value of 1.43 Å is due to the delocalization of
electrons in the phenyl ring. There is an increase in the bond angle values of C4-C3-C2
(121.47), C3-C4-C5 (122.5) and C4-C5-C6 (121.7) in the benzene ring compared to the
normal value of 120. A decrease in the values was also observed in the case of C1-C2-C3
(112.3) and C1-C6-C5 (112.1) bond angles from the normal value of 120 which can again be
attributed to deprotonation process [41].

7
A network of various types of hydrogen bonding was also observed in the crystal
structure. The O7 of the styphnate moiety forms a bifurcated hydrogen bond with ring carbons of
neighbouring imidazole molecules giving rise to C11-H13…O7 (2.655 Å) and C12-H8…O7
(3.459 Å) interactions whereas the other oxygen O8 forms a hydrogen bond with the protonated
nitrogen N2, generating N2-H15…O8 hydrogen bond interactions. There is a C-H…O short
contact arises from the methyl group of imidazole moiety forming C14-H10c…O2 (3.338 Å).
These N-H…O and C-H…O interactions stabilize the MITB molecule in the crystallographic
axis linking the cations and anions as a two dimensional network along the ‘b’ axis (Fig. 2).
Proton transfer from the two hydroxyl groups of styphnic acid to the nitrogen atoms of the two
methyl imidazole rings formed an ion-pair adduct. The selected bond lengths and angles along
with theoretically calculated values are presented in Table 2. The ORTEP diagram and optimized
molecular structure of MITB are shown in Fig. 3.
3.2. Quantum chemical calculations of MITB
3.2.1 Optimization of geometry
The stable molecular structure of MITB in the ground state was optimized and the
structural parameters were computed by using Gaussian 09 Program package at the Becke3-Lee–
Yang-Parr (B3LYP) level with standard 6-311++G(d,p) basis set [42-45]. It is found that most of
the optimized bond lengths are slightly higher than the experimental values. The deviations can
be attributed to the fact that the theoretical calculations were performed assuming isolated
molecules in the gaseous phase and the experiments were conducted using crystals. HOMO-
LUMO analysis, electrostatic potential surface analysis and Mulliken charge analysis were
carried out theoretically. First-order hyperpolarizability calculation and thermodynamic
calculations were also done using optimized structure [46-48].
3.2.2 Frontier Molecular Orbital analysis
Frontier molecular orbitals (FMOs) play an important role in the electric and optical
properties of the materials. The distributions and energy levels were computed at the B3LYP/6-
311++G(d,p) level of basis set to understand the important concepts such as conjugation,
aromaticity and lone pairs in the MITB molecule. The energy of the HOMO is directly related to
the ionisation potential (I) and LUMO energy is directly related to the electron affinity (A).
Energy difference between HOMO and LUMO orbital is the energy gap (E) that determines the

8
stability of a molecular structure as well as electron movement from the more aromatic part of
the π conjugated system in the electron-donor site to its electron-withdrawing part of a molecular
adduct inside the molecule. The HOMO-LUMO energy level diagram is shown in Fig. 4.
By using HOMO-LUMO energy values for a molecule, the global reactivity indices of a
molecule such as chemical hardness (), chemical potential (μ), softness (S), electronegativity
(), electrophilic index () parameters have been defined on the basis of Koopmans’s equation
[49]. The values of I, A, , μ, S,  and  are found to be 5.56 eV, 2.01 eV, 1.775, -3.785, 0.2812,
3.785 and 15.26, respectively. The electrophilic index value describes the electron accepting
nature of the material.
The HOMO-LUMO energy gap (E = 3.55 eV) reflects the chemical activity of the
molecule, which confirms the charge transfer interaction within the styphnate moiety of the
molecule. This prediction is found to be correct with the experimental results of UV-Visible
spectroscopic analysis which would be discussed elsewhere in this article. The positive and
negative phases are represented in red and green colours, respectively in the Fig 4. The π–nature
of HOMO is located mainly over the phenyl ring, oxygen and nitrogen atoms of the styphnate.
By contrast, LUMO is located only on two nitro groups of the styphnate. There is no electronic
projection of HOMO and LUMO over the 1-methyl imidazole moiety. Consequently, the
HOMO-LUMO transition implies an electron density transfer from the phenyl ring and oxygen
atoms of the phenoxy groups to the two nitro groups. The plot of HOMO and LUMO (Fig.4)
reveals the favourable n-π* and π-π* transitions in the molecule.
3.2.3 First-order hyperpolarizability calculation
Quantum chemical calculations have been shown to be useful in the description of the
relationship between the electronic structure of the systems and its non-linear optical (NLO)
response which describes the behavior of lights in the medium in which the electric polarization
varies nonlinearly to the electric field of the light. The computational approach allows the
determination of molecular NLO properties as an inexpensive way to design molecules by
analyzing their potential before synthesis and to determine the higher order hyperpolarizability
tensors of molecules. Organic molecules able to influence photonic signals efficiently are of
importance in optical communication, optical computing and dynamic image processing [50].

9
The calculated first-order hyperpolarizability (β) and the ground state dipole moment (μg)
of the MITB molecule are (βtot) 4.405*10-30esu and 10.0294 Debye respectively. It is observed
from Table 3 that the calculated βtot value of MITB is larger than the βtot values of the reactants,
namely, styphnic acid and 1-methyl imidazole. The first-order hyperpolarizability value (βtot) of
MITB is approximately 34 times greater than that of the value of urea (0.13*10-30 esu). The
higher βtot value and low HOMO-LUMO energy gap value indicate that the charge transfer
prevails between the imidazolium cation and styphnate anion in the MITB molecule [51].
3.2.4 Mulliken population analysis
The Mulliken population analysis on atomic charge was calculated based on the density
functional theory (DFT) are calculated at B3LYP/6-311++G(d,p) level of basis set by
determining the electron population of each atom as defined. The bonding capability of a
molecule depends on the electronic charge on the atoms [52]. The Mulliken charge diagram is
shown in the Fig. S3. All the nitrogen atoms in styphnate moiety have negative values due to
presence of electronegative O and N atoms in the nitro groups. Similarly, phenolate oxygen
atoms have the highest negative charge because of their anionic nature. All the hydrogen atoms
and carbon atoms acquire positive charge except those carbons attached to nitro groups. The
delocalization of π electrons from the NO2 group reduces the positive charge over these carbon
atoms.
3.2.5 Electrostatic potential surface analysis
Electrostatic potential (ESP) mapping is a very useful method in the investigation of the
molecular structure with its physico-chemical property relationships. The ESP surfaces of MITB
molecule were constructed by using B3LYP/6-311++G(d,p) level of basis set. The electrostatic
potential surfaces of the MITB are shown in Fig. S4. This has been used for predicting reactive
sites towards electrophilic attack and in turn understanding the biological activity and hydrogen
bonding interactions [53].
The different values of the electrostatic potential are represented by different colours; red,
blue and green represent the regions of most negative, most positive and zero electrostatic
potential, respectively. The colour code of this map is in the range from -6.810 to 6.810 eV. It is
seen from the Fig S4, the negative region (red) is mainly spread over the oxygen atoms of the

10
phenolate and nitro groups of styphnate moiety. The positive region (blue) lies in the five
membered imidazolium moieties. The red and blue regions indicate the preferred sites for
electrophilic and nucleophilic attacks respectively.
3.2.6 Thermodynamic properties
Thermodynamic parameters such as thermal energy (Etotal), specific heat capacity (Cv),
rotational constants, entropy (S) and dipole moment (μ) of MITB were calculated by B3LYP
level with 6-311++G(d,p) basis set and the results are presented in the form of plots in Fig. 5. It
is observed that the values of heat capacity, entropy and enthalpy increase with temperature
starting from 100 to 1000 K which can be attributed to the greater molecular vibrations at higher
temperature [54]. The correlation equations between different parameters such as heat capacity,
entropy, enthalpy (H) and temperature were fitted by quadratic formula and the respective R2
factors for are 0.949, 0.987 and 0.982. The fitting equations are as follows.
Som =1.864x + 614.6, R² = 0.949
Com =1.026x +350.9, R² = 0.987
Ho m =0.950x-147.8 R² = 0.982
3.3 Electrical properties
Defect free and good transparent crystals of MITB with polished surface were selected
for the dielectric measurements in a HIOKI 3532-5D LCR HITESTER instrument in the
frequency range 50 Hz to 5 MHz. Dielectric constant and dielectric loss were calculated for
varying frequencies at room temperature. The dielectric constant (εr) is calculated using the
relation,
----------------------------
(1)
where C is the capacitance, d is the thickness, A is the area and ε0 is the absolute
permittivity of the free space. Fig. 6 shows the variations of dielectric constant and dielectric loss
with applied frequency at 300 K. At low frequency, the properties such as space charge,
orientation, electronic and ionic polarizations are found to be active. With increase in frequency,
dielectric loss (tan δ) and ε0values are found to decrease which are dependent on the excitation of

11
bound electrons, lattice vibrations, dipole orientations and charge polarization. The low value of
dielectric constant (εr) at high frequencies is due to the loss of these polarizations at low
temperature [55]. Conferring to the Miller rule, the lower values of dielectric constant (εr) at
higher frequencies is a vital factor for the enhancement of SHG coefficient. In the present study
the dielectric constant of the MITB was found to be lower (εr 95) than the other well-known
photonic crystals such as imidazolium picrate (εr 140), 2-methylimidazolium hydrogen
succinate (εr 600), 4-aminopyridinium p-aminobenzoate dihydrate (εr 420), Dimethyl amino
pyridinium 4-nitrophenolate (εr 1180) and an inorganic electro-optic crystal KNbO3 (εr 154).
In addition, MITB has low dielectric loss at higher frequencies which endows the MITB as a
good optical material with minimum defects [56-60].
3.4 Hirshfeld surface analysis
CrystalExplorer, a computer program was used for calculation of Hirshfeld surfaces and
construction of 2D fingerprint plots [61]. Hirshfeld parameters such as curvedness, shape index
and dnorm, and 2D fingerprint plots of MITB crystal are shown in Fig. 7. Hirshfeld surface
depends on the geometry of the molecules, orientation of the molecules in their neighborhood,
and the nature of atoms that make close contacts in a crystal. The dnorm surface shows
intermolecular contacts relative to the van der Waals radii by way of a simple red, white and blue
colour scheme: red indicating higher electron density and hydrogen bonding capability and blue
representing lower electron density sites [62]. The low curvedness suggests close contacts and
this is possibly indicative of covalent bonding interactions [63]. The large flat region indicated
by a blue outline on the curvedness surfaces indicates π…π stacking of the molecules. The red
triangles on the shape index represent concave regions indicating atoms of the π…π stacked
molecule above them and the blue triangles represent convex regions, indicating the ring atoms
of the molecule inside these surfaces. The larger circular depressions (deep red) visible on the
side of the Hirshfeld surfaces correspond to the significant hydrogen bonding contacts while the
red colour points indicate the short contacts of H...H, N…H and H...O interactions.
Hirshfeld two-dimensional (2D) fingerprint plots render a quick and easy identification of
the significant intermolecular interactions map on the molecular surface. The proportion of
O…H/H…O interactions covers 49.6 % of the total Hirshfeld surfaces with two distinct spikes in
the 2D fingerprint plots, indicating hydrogen bonding interactions are the most significant

12
interactions in the crystal. The H…H interactions, which are reflected in the middle of scattered
points in the 2D fingerprint plot, comprise 18.8 % of the total Hirshfeld surfaces. Apart from the
above interactions, the presence of π…π (C…C), lone pair….π (N…C, O…C) and lone
pair…lone pair (O…O, N…N and O…N) interactions are also observed.
3.5 UV-Visible spectral analysis
3.5.1 UV-Visible Absorption measurement
The electronic absorption spectrum of MITB dissolved in methanol solvent was
measured in a Varian Model Carry 100 UV-Vis spectrometer in a range of 200-800 nm at room
temperature. The UV-visible absorption spectrum of MITB is shown in Fig. S5. The compound
shows absorptions at 251, 351, and 420 nm which are due to the π–π* and n-π* transitions in the
molecule. The presence of the π–π* transition is due to the charge transfer in the styphnate
moiety. This result is found to match with the result of HOMO- LUMO study discussed earlier in
this article. The presence of the n–π* transition is due to the presence of lone pairs of electrons
over more electronegative atoms like nitrogen and oxygen. The higher intensity of the absorption
band observed in the UV region is due to the conjugated systems in the molecule.
3.5.2 Solid state UV-Vis-NIR transmittance spectral analysis
The optical transmittance of the solid MITB was measured in a JASCO (V-570)
UV/VIS/NIR spectrophotometer. The UV-Vis-NIR spectrum of MITB is shown in Fig. 8. The
transmittance spectrum shows that the crystal is highly transparent from 450 to 1000 nm. The
lower cut-off wavelength of the crystal was found to be 370 nm and is short enough for the SHG
of Nd: YAG laser radiation at 1064 nm or other applications in the blue region. Therefore, MITB
crystal can be used in optoelectronics and diode fabrications [64].
3.5.3 Photoluminescent studies
The photoluminescence spectrum of MITB crystal was recorded in the wavelength range
of 440-700 nm by using a Horiba Jobin-Yvon-FL3-22 Flurolog spectroflurometer. Fig. 9. shows
the emission spectrum of the MITB crystal When the powdered sample of the MITB was excited
at 450 nm, it showed an efficient luminescence in the longer wavelength at 525 nm with high
intensity. The results indicate that the MITB crystal has a green emission, suggesting that it can
be used as a new green-light emitting material for optical applications.

13
3.6. Thermal analysis
Thermal analyses (TG/DTA) were performed by a Mettler Toledo Star SW 8.10 TG/DTA
thermal analyzer between 30 and 800 C under nitrogen atmosphere at a heating rate of 20C per
minute. TG/DTA curves of MITB crystal are shown in Fig. 10. In the TG/DTA thermogram,
there is no thermal event till 178 C before the start of the decomposition, illustrating the absence
of physically adsorbed or lattice water in the MITB crystal. There is an endothermic peak at
178 C, in the DTA curve of the MITB which can be attributed to the melting point of the
sample. The sharpness of endothermic peak shows the good degree of crystalline nature and
purity of the sample [65]. The TG thermogram shows that the MITB is stable up to 178 C and
there is a single stage weight loss occurring between 178 and 800 C. The weight loss is
accounted for the liberation of a number of volatile substances such as ammonia and CH4, NO2
and CO molecule.
4. Conclusion
Single crystals of MITB were grown by a slow evaporation method at room temperature.
The compound MITB was characterized through elemental analysis, thermal analysis, NMR and
FT-IR for its formation and chemical composition. Various kinds of protons and carbons present
1
13
in the molecule were confirmed by H, C and DEPT 135 NMR spectroscopic techniques. The
crystal was found to be thermally stable up to 178 C. The photoluminescence study indicates
that the MITB crystal has a green emission suggesting that it can be used as a new green-light
emitting material for optical applications. The single crystal XRD study reveals that the presence
of nitro groups of styphnate moiety and N+-H of imidazolium moieties in the molecular
framework favour prominent hydrogen bonding. The electrostatic potential colour map derived
from optimized geometry of the MITB describes clearly the possible reactive sites for
electrophilic and nucleophilic attacks. The calculated values of HOMO-LUMO energies,
hardness, chemical potential, softness, electronegativity, electrophilicity index attest to the
superiority of the material. The correlations between the thermodynamical parameters and
temperature show an increase in the heat capacities, entropies and enthalpies with increasing
temperature due to the molecular vibrations. Quantum chemical calculations illustrate that the
first-order hyperpolarizability (β) of the MITB crystal is 4.405*10-30 esu, which is 34 times
greater than that of urea, a reference material for NLO applications. A large transparent window

14
from 450 to 1000 nm, lower cut off wavelength of 370 nm, and low dielectric loss and dielectric
constant values approve the MITB crystals as a superior material for optoelectronic applications.
Hirshfeld surface analysis of MITB crystal demonstrates that among the different kinds of
interactions, O…H/H…O is the most prominent interaction.
Acknowledgements
One of the authors, M. Dhandapani, would like to thank the University Grants
Commission (UGC), New Delhi, India for the financial support (Major Research Project Grant
No. 43-200/2014(SR) to carry out the research work. P. Dhamodharan, thanks the Dr. R.
Nagarajan, School of Chemistry and UGC Networking Centre, Hyderabad, for awarding visiting
research fellowship.
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Fig. 1 1H and 13C NMR spectra of MITB
Fig. 2 Packing view of MITB
Fig. 3 ORTEP view and optimized geometry of MITB

17
Fig. 4 Frontier molecular orbitals of MITB
Fig. 5 Thermodynamic parameters of MITB
Fig. 6 Dielectric loss and dielectric constant of MITB
Fig. 7 Hirshfeld dnorm, Shape index, Curvedness and 2D finger-print plots of MITB
Fig. 8 UV-Vis-NIR spectrum of MITB
Fig. 9 Photoluminescence spectrum of MITB
Fig. 10 TG/DTA curves of MITB

18
Fig. 1

19
Fig. 2

20
Fig. 3

21
Fig. 4

22
Fig. 5

23
Fig. 6

24
Fig. 7

25
Fig. 8

26
Fig. 9

27
Fig. 10

28
Table 1. Crystal data and structure refinement for MITB crystal
Empirical formula
Formula weight
Temperature
C14H15N7O8
409.03
298(2) K
Wavelength
1.54184 Å
Monoclinic
P21/c
Crystal system
Space group
Unit cell dimensions
a = 19.9171 (4) Å
b = 7.28263 (17) Å
α= 90°.
β= 102.706
(2)°
c = 12.4229 (3) Å
 = 90°.
1757.80 (7) Å3
Volume
Z
4
1.547 Mg/m3
Density (calculated)
Absorption coefficient
-1
0.161 mm
726
F(000)
0.18 x 0.15 x 0.12 mm3
Crystal size
Theta range for data collection
Index ranges
6.49 to 71.48°.
-21<=h<=24, -
8<=k<=8, -15<=l<=9
6674
Reflections collected
Independent reflections
Completeness to theta = 26.24°
Refinement method
3334 [R(int) = 0.0183]
97.5 %
Full-matrix least-
squares on F2
3334 / 0 / 265
1.049
Data / restraints / parameters
2
Goodness-of-fit on F
Final R indices [I>2sigma (I)]
R1 = 0.0645, wR2 =
0.1915

29
R indices (all data)
R1 = 0.0680, wR2 =
0.1951
Extinction coefficient
0.068(4)
0.700 and -0.677 e.Å-3
Largest diff. peak and hole
Table 2. Comparison of experimental and calculated bond lengths (Å) and bond angles () of MITB
Atoms
Observed
Calculated
1.230
1.246
1.437
1.421
1.459
1.243
1.234
1.433
1.387
1.340
1.462
1.388
1.436
1.461
1.380
1.080
1.362
1.373
1.075
1.241
1.223
1.455
1.076
1.328
1.083
1.088
1.090
1.090
1.041
1.346
1.385
1.460
1.322
1.372
1.140
1.077
Atoms
Observed
Calculated
O(2)-N(5)
N(5)-O(1)
N(5)-C(5)
C(2)-C(1)
1.217(3)
1.226(3)
1.427(3)
1.416(3)
1.452(3)
1.225(3)
1.233(3)
1.431(3)
1.319(3)
1.380(3)
1.467(3)
1.375(3)
1.411(3)
1.453(3)
1.382(3)
0.93
O(2)-N(5)-O(1)
O(2)-N(5)-C(5)
O(1)-N(5)-C(5)
O(8)-C(2)-C(1)
O(8)-C(2)-C(3)
C(1)-C(2)-C(3)
O(5)-N(7)-O(6)
O(5)-N(7)-C(3)
O(6)-N(7)-C(3)
C(13)-N(3)-C(11)
C(13)-N(3)-C(14)
C(11)-N(3)-C(14)
C(4)-C(3)-N(7)
C(4)-C(3)-C(2)
N(7)-C(3)-C(2)
O(7)-C(6)-C(1)
O(7)-C(6)-C(5)
C(1)-C(6)-C(5)
C(3)-C(4)-C(5)
C(3)-C(4)-H(16)
C(5)-C(4)-H(16)
C(13)-C(12)-N(4)
C(13)-C(12)-H(8)
N(4)-C(12)-H(8)
O(4)-N(6)-O(3)
O(4)-N(6)-C(1)
O(3)-N(6)-C(1)
C(4)-C(5)-N(5)
C(4)-C(5)-C(6)
N(5)-C(5)-C(6)
C(6)-C(1)-C(2)
C(6)-C(1)-N(6)
C(2)-C(1)-N(6)
N(3)-C(13)-C(12)
N(3)-C(13)-H(9)
C(12)-C(13)-H(9)
120.7(2)
119.8(2)
119.5(2)
121.5(2)
126.2(2)
112.3(19)
121.0(2)
119.7(2)
119.3(2)
108.2(2)
125.7(2)
126.1(2)
117.1(2)
121.47(19)
121.4(2)
121.8(2)
126.1(2)
112.13(19)
122.5(2)
118.8
121.7
119.0
119.1
120.8
124.6
114.3
121.9
118.7
119.2
108.5
126.3
125.0
116.6
121.3
121.9
122.7
123.4
113.7
121.9
118.7
119.2
106.8
130.9
122.1
122.6
118.0
119.2
117.4
121.6
120.9
126.6
116.2
117.1
106.7
122.1
131.1
C(2)-C(3)
N(7)-O(5)
N(7)-O(6)
N(7)-C(3)
N(3)-C(13)
N(3)-C(11)
N(3)-C(14)
C(3)-C(4)
C(6)-C(1)
C(6)-C(5)
C(4)-C(5)
C(4)-H(16)
C(12)-C(13)
C(12)-N(4)
C(12)-H(8)
N(6)-O(4)
N(6)-O(3)
N(6)-C(1)
C(13)-H(9)
C(11)-N(4)
C(11)-H(13)
C(14)-H(11A)
C(14)-H(12B)
C(14)-H(10C)
N(4)-H(14)
N(1)-C(7)
N(1)-C(9)
N(1)-C(10)
N(2)-C(7)
N(2)-C(8)
N(2)-H(15)
C(7)-H(2)
1.323(3)
1.359(4)
0.93
1.191(3)
1.202(4)
1.459(3)
0.93
118.8
108.2(2)
125.9
1.339(4)
0.93
125.9
122.8(3)
119.6(2)
117.6(2)
116.5(2)
121.7(2)
121.81(19)
129.8(2)
115.63(18)
114.57(18)
109.1(2)
125.5
0.96
0.96
0.96
0.86
1.324(3)
1.372(4)
1.459(3)
1.320(3)
1.348(4)
0.86
0.93
125.5