Growth, characterization and theoretical parameter studyof benzimidazole L-tartrate single crystal: a nonlinearoptical material

HIRAL RAVAL1,2, P S RAVAL1,2, B B PAREKH1,* and M J JOSHI3

1Pandit Deendayal Petroleum University, Gandhinagar 382007, India2L D College of Engineering, Ahmedabad 380015, India3Department of Physics, Saurashtra University, Rajkot 360005, India

*Author for correspondence (dr.bharatparekh@gmail.com)

MS received 13 July 2020; accepted 30 August 2020

Abstract. Good quality, non-hygroscopic and transparent crystals of organic benzimidazole L-tartrate (BILT) were

grown successfully with a slow evaporation method. The powder X-ray diffraction patterns were analysed with

Powder-X software which confirms the monoclinic crystal structure. The charge distribution, transport mechanism

and intramolecular bonding mechanism have been investigated with the help of natural bond orbital analysis and

molecular electrostatic potential diagram. The presence of various functional groups was confirmed with the help of

FTIR–ATR response. The values were compared with the values obtained from computational output with the help of

Gaussian software. The crystalline quality was further analysed with UV–visible spectral analysis. The lower cut-off

wavelength of 288 nm and further optical parameters like band gap, change in refractive index with wavelength and

extinction coefficient values support the usage of the material for optoelectronic devices. With band gap of 4.2 eV,

the reactivity of material has been observed with the HOMO and LUMO study. The TGA and DTA analyses confirm

the thermal stability of the material up to 192�C. The lower dielectric constant and lower dielectric loss support the

usage of the material for an NLO device. The hopping motion and Joncher’s power law parameters were also

obtained. The material decomposes in single-phase which observes in a range of 180–250�C. The second harmonic

generation capacity of the material is found to be 2.69 times that of the KDP with the help of Kurtz and Perry powder

technique.

Keywords. Crystal growth; organic nonlinear optical material; second harmonic generation; DFT; UV–visible.

1. Introduction

The synthesis of multicomponent crystals like salts, moi-

eties, co-crystals and solvates attracts researchers due to

their versatile properties and applications in the area of

optoelectronic devices and NLO properties. The formation

of co-crystals has been achieved with the help of a stoi-

chiometric ratio mixture of two or more compounds. The

intramolecular assembly is mainly with the help of hydro-

gen bonding, bond interactions and charge transfer activities

[1,2]. The advanced requirements of optical image pro-

cessing, photonics, communication and storage devices

require a large spectrum of NLO materials with stability and

efficient conversion capabilities can be accomplished with

the help of such mentioned frames of molecular assemblies

[3–6]. In addition to the same, the molecular assemblies of

multi-component materials can be unfailingly formed with

the help of carboxylic acids [7]. The recent work for the

development of powerful NLO materials shows promising

results with hydrogen bonding between carboxylic groups

and other groups like hydroxyl group, heterocyclic aromatic

group [8–11]. The supramolecular assembly framework can

be established with the help of carboxylic and heterocyclic

compounds through building blocks that self-assemble via

N–H…H–O, O–H…O, N–H…O, and C–H…O hydrogen

bonds when crystallized [12,13]. Based on the same, in this

present work, the effect of a combination of the carboxyl

group with bicyclic fusion aromatic compounds in non-

linear behaviour has been reported. The various analyses

have been carried out which confirm the application of the

title compound as potential NLO material.

2. Experimental and computational details

2.1 Synthesis

The AR grade L-tartaric acid (Merk Millipore) was dis-

solved in water along with the same grade benzamida-

zolium (Sigma Aldrich), were dissolved in methanol in

Bull. Mater. Sci. (2021) 44:38 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-020-02320-2Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

equal proportion and mixed. The mixture was dried in the

oven at 60�C for 2 h. The powder was again dissolved in

water and filtered with filter paper grade 41. At every stage

of dissolution, the homogeneous solution was prepared with

the help of magnetic stirrer. The solution was kept in beaker

and covered with polythene to enable very slow evapora-

tion. After 26 days, the good quality transparent crystals

(figure 1) were harvested for analysis. The average size of

good quality and the transparent crystal was 129 89 6 mm3.

Furthermore, no hygroscopic nature of crystal was observed

and quality remains the same for a long period.

2.2 Experimental analysis

The grown crystals were subjected to various analyses

which help to identify its potential usage for non-linear

activity. The crystals were analysed by various means

like structural analysis by powder X-ray diffraction

(XRD), thermal analysis with TGA and DTA, optical

quality analysis with UV–visible photo-spectrometer,

functional group analysis with FTIR–ATR spectroscopy,

dielectric analysis with PSM analyzer and second har-

monic generation (SHG) analysis with Kurtz and Perry

powder technique.

2.3 Computational analysis

The theoretical quantum chemical computation of BILT

was carried out with the help of the Gaussian 09 software

[14]. The calculations were carried out with time-dependent

density functional theory (TD-DFT) matrix on the basis set

of B3LYP/6-311 G(d,p). The experimental correlated cal-

culations, such as vibrational response, molecular electro-

static potential (MEP) distribution, natural bond orbital

(NBO), polarizability and HOMO and LUMO calculations

were carried out. All calculations were performed on an

optimized structural output.

3. Results and discussion

3.1 Structural analysis

Structural analysis of BILT crystal was carried out with the

help of fine powder X-ray analysis. The diffraction pattern

was recorded on PANalytical X’pert PRO X-ray diffrac-

tometer from 0 to 90� range with a source of CuKa (k =

1.5418 A) radiation. The data were analysed with Powder-X

software as shown in figure 2. The analysis confirmed that

the BILT crystal belongs to monoclinic crystal structure

system with cell parameters a = 9.227 A, b = 7.278 A and

c = 10.889 A, volume = 731.241 A3 and a = c = 90� and b =

110.50�.The titled compound structure was also optimized for

further theoretical analysis with the help of the Gaussian

09W program package with the TD-SCF DFT method

with the basis point of B3LYP (6-311G). The hydrogen

sharing bond is observed between H21 and H26 with a

sharing bond length of 2.894 A. The bond parameters

observed after optimization are listed in the supplemen-

tary information. The zigzag tartrate structure with car-

bon chain C–C–C–C is observed with torsion angle of

0.0346�. The higher non-linearity and stability are pro-

jected due to the expected zigzag configuration of tartrate

molecules [15].

The benzimidazole and tartrate are aligned with head

to the tail arrangement on the axis. The formation of

chain suggests good stability of the crystalline material.

The diagram is illustrated in figure 3 with bond length.

The aromatic ring and benzene sharing bond is observed

with (C4–N5) and (C3–N2) with bond lengths of 1.407Figure 1. As grown crystal of BILT.

38 Page 2 of 16 Bull. Mater. Sci. (2021) 44:38

Figure 2. Powder X-ray pattern of single crystal BILT.

Figure 3. DFT optimized structure of title compound.

Bull. Mater. Sci. (2021) 44:38 Page 3 of 16 38

Table 1. Optimized structure parameters: bond angles.

Bond angles (�)

A1 A(2, 1, 5) 113.2446 A25 A(8, 9, 25) 122.0727

A2 A(2, 1, 20) 122.0501 A26 A(13, 10, 16) 124.6301

A3 A(5, 1, 20) 124.7053 A27 A(13, 10, 17) 111.6144

A4 A(1, 2, 3) 106.8479 A28 A(16, 10, 17) 123.7555

A5 A(1, 2, 21) 126.3753 A29 A(13, 11, 26) 108.3887

A6 A(3, 2, 21) 126.7734 A30 A(14, 12, 27) 109.6223

A7 A(2, 3, 4) 105.0491 A31 A(10, 13, 11) 106.9263

A8 A(2, 3, 6) 132.8331 A32 A(10, 13, 14) 113.1617

A9 A(4, 3, 6) 122.1177 A33 A(10, 13, 28) 107.4244

A10 A(3, 4, 5) 109.9713 A34 A(11, 13, 14) 108.5269

A11 A(3, 4, 9) 120.2394 A35 A(11, 13, 28) 110.9093

A12 A(5, 4, 9) 129.7892 A36 A(14, 13, 28) 109.8738

A13 A(1, 5, 4) 104.8871 A37 A(12, 14, 13) 107.5088

A14 A(3, 6, 7) 116.7278 A38 A(12, 14, 15) 108.4815

A15 A(3, 6, 22) 122.1184 A39 A(12, 14, 29) 108.5087

A16 A(7, 6, 22) 121.1528 A40 A(13, 14, 15) 112.8954

A17 A(6, 7, 8) 121.7074 A41 A(13, 14, 29) 108.7375

A18 A(6, 7, 23) 119.1121 A42 A(15, 14, 29) 110.5767

A19 A(8, 7, 23) 119.1805 A43 A(14, 15, 18) 123.116

A20 A(7, 8, 9) 121.1947 A44 A(14, 15, 19) 112.3854

A21 A(7, 8, 24) 119.1298 A45 A(18, 15, 19) 124.4736

A22 A(9, 8, 24) 119.6755 A46 A(10, 17, 30) 111.4637

A23 A(4, 9, 8) 118.0127 A47 A(15, 19, 31) 112.5425

A24 A(4, 9, 25) 119.9145

Dihedral angles (�)

D1 D(5, 1, 2, 3) -0.0632 D36 D(7, 8, 9, 25) 179.9462

D2 D(5, 1, 2, 21) -179.424 D37 D(24, 8, 9, 4) 179.9404

D3 D(20, 1, 2, 3) 179.9194 D38 D(24, 8, 9, 25) -0.0209

D4 D(20, 1, 2, 21) 0.5588 D39 D(16, 10, 13, 11) -112.734

D5 D(2, 1, 5, 4) 0.0566 D40 D(16, 10, 13, 14) 127.8303

D6 D(20, 1, 5, 4) -179.925 D41 D(16, 10, 13, 28) 6.3771

D7 D(1, 2, 3, 4) 0.04 D42 D(17, 10, 13, 11) 67.2116

D8 D(1, 2, 3, 6) 179.8703 D43 D(17, 10, 13, 14) -52.2242

D9 D(21, 2, 3, 4) 179.3972 D44 D(17, 10, 13, 28) -173.677

D10 D(21, 2, 3, 6) -0.7724 D45 D(13, 10, 17, 30) -177.979

D11 D(1, 2, 11, 13) -68.111 D46 D(16, 10, 17, 30) 1.9671

D12 D(3, 2, 11, 13) 133.8215 D47 D(26, 11, 13, 10) -173.965

D13 D(2, 3, 4, 5) -0.0077 D48 D(26, 11, 13, 14) -51.5809

D14 D(2, 3, 4, 9) 179.986 D49 D(26, 11, 13, 28) 69.2053

D15 D(6, 3, 4, 5) -179.861 D50 D(27, 12, 14, 13) 110.8703

D16 D(6, 3, 4, 9) 0.1329 D51 D(27, 12, 14, 15) -11.5126

D17 D(2, 3, 6, 7) -179.979 D52 D(27, 12, 14, 29) -131.69

D18 D(2, 3, 6, 22) -0.3446 D53 D(10, 13, 14, 12) 165.7882

D19 D(4, 3, 6, 7) -0.1726 D54 D(10, 13, 14, 15) -74.6044

D20 D(4, 3, 6, 22) 179.462 D55 D(10, 13, 14, 29) 48.4972

D21 D(3, 4, 5, 1) -0.0289 D56 D(11, 13, 14, 12) 47.2765

D22 D(9, 4, 5, 1) 179.9782 D57 D(11, 13, 14, 15) 166.8838

D23 D(3, 4, 9, 8) 0.0043 D58 D(11, 13, 14, 29) -70.0145

D24 D(3, 4, 9, 25) 179.9665 D59 D(28, 13, 14, 12) -74.1477

D25 D(5, 4, 9, 8) 179.9967 D60 D(28, 13, 14, 15) 45.4596

D26 D(5, 4, 9, 25) -0.0411 D61 D(28, 13, 14, 29) 168.5612

D27 D(3, 6, 7, 8) 0.0827 D62 D(12, 14, 15, 18) 7.0101

D28 D(3, 6, 7, 23) -179.964 D63 D(12, 14, 15, 19) -174.735

D29 D(22, 6, 7, 8) -179.556 D64 D(13, 14, 15, 18) -112.035

D30 D(22, 6, 7, 23) 0.3975 D65 D(13, 14, 15, 19) 66.22

D31 D(6, 7, 8, 9) 0.0492 D66 D(29, 14, 15, 18) 125.8914

D32 D(6, 7, 8, 24) -179.984 D67 D(29, 14, 15, 19) -55.8538

D33 D(23, 7, 8, 9) -179.904 D68 D(14, 15, 19, 31) -177.248

D34 D(23, 7, 8, 24) 0.0635 D69 D(18, 15, 19, 31) 0.9793

D35 D(7, 8, 9, 4) -0.0925

38 Page 4 of 16 Bull. Mater. Sci. (2021) 44:38

and 1.395 A, respectively. The detailed bond angles are

given in table 1.

3.2 Natural bond orbital analysis

The donor–acceptor analysis is very important in non-linear

organic materials. Information regarding intra- and inter-

molecular interactions for filled and virtual orbitals was

collected by NBO analysis. It is also useful in this case, to

understand hydrogen bonding interactions. The results can

be viewed as a transaction from the localized NBO of the

idealized Lewis structure into empty non-Lewis orbitals.

The high energy interaction of inter molecules and intra

molecules are given in table 2.

The basic benzene and imidazole rings have interaction

donor r(C1–N5) to acceptor r�(C3–C4) with a stabilization

energy value E(2) of 19.12 kcal mol-1. The benzene ring

holds an average stabilization intra molecular bond energy

of 20 kcal mol-1. Imidazole ring is always acknowledged

for higher stability. The same can be viewed here as

interaction between LP(1)N2 with r�(C1–N5) and r�(C3–C4) with the energies of 46.23 and 32.69 kcal mol-1,

respectively. The electron donation from r�(C1–N5) to

acceptor r�(C3–C4) with stabilization energy 57.30 kcal

mol-1 is the most important high energy interaction. The

charge transfer interaction is observed from L-tartaric

donors r(O11–H26) and r�(N2–H21) with a stabilization

value of 1.09 kcal mol-1. This confirms the hydrogen

bonding between donor and acceptor molecules. The high

energy interaction is observed in L-tartaric acid also with

oxygen, carbon and carbon–carbon pair. The same has been

mentioned in table 2.

3.3 Vibrational analysis

FTIR–ATR spectrum of BILT is shown in figure 4. The

FTIR confirms the existence of various functional groups

associated with benzene, imidazole and L-tartaric acid.

Table 2. Donor–acceptor transition through NBO analysis.

Donor NBO (i) Acceptor NBO (j)

E(2) E(j) - E(i) F(i, j)(kcal mol-1) (a.u.) (a.u.)

Within unit 1

C1–N5 C3–C4 19.12 0.34 0.079

C3–C4 C1–N5 17.38 0.25 0.060

C3–C4 C6–C7 18.29 0.28 0.065

C3–C4 C8–C9 18.51 0.29 0.067

C6–C7 C3–C4 20.48 0.28 0.071

C6–C7 C8–C9 18.28 0.29 0.065

C8–C9 C3–C4 18.28 0.27 0.066

C8–C9 C8–C9 22.61 0.27 0.070

LP N2 C1–N5 46.23 0.28 0.103

LP N2 C3–C4 32.69 0.31 0.092

C1–N5 C3–C4 57.30 0.03 0.060

From unit 1 to unit 2

N2–H21 O11–H26 0.67 1.12 0.025

From unit 2 to unit 1

O11–H26 N2–H21 1.09 1.10 0.031

LP O11 H21 0.29 1.18 0.017

LP O11 N2–H21 0.46 0.86 0.018

Within unit 2

LP O16 C10 14.49 1.61 0.137

LP O16 C10–C13 19.30 0.63 0.100

LP O16 C10–O17 30.84 0.63 0.126

LP O17 C10–O16 48.38 0.32 0.113

LP O18 C15 16.31 1.51 0.140

LP O18 C14–C15 18.08 0.61 0.095

LP O18 C15–O19 34.18 0.60 0.129

LP O19 C10–O16 22.45 0.38 0.082

LP O19 C15–O18 44.40 0.35 0.112

C15–O19 O19–H31 15.67 0.02 0.061

Bull. Mater. Sci. (2021) 44:38 Page 5 of 16 38

The theoretical (figure 4b) and experimental (figure 4a)

calculated values of each absorption peak have been

listed in table 3. The absorption peak at 3495 cm-1 is

attributed to asymmetric stretching and 3348 cm-1 is

attributed to symmetric stretching of the aromatic NH2

group of benzimidazole. The comparatively broad band is

observed at 3124 cm-1 asymmetric and symmetric

stretchings of NH2 group [16,17]. The benzene ring C–H

stretching band is observed at 2970 cm-1 [18]. The strong

absorption peak of C=O stretching corresponding to

L-tartaric bonds have also been recorded. It has contin-

uous absorption peaks up to 1500 cm-1 [18]. O–H

bending absorption is observed at 1378 cm-1 which

corresponds to L-tartaric acid. This band also involved in

intermolecular interaction. The strong C–O stretching is

observed at 1228 cm-1 [18]. The medium C–N stretching

is observed at 1130 and 1066 cm-1. The observed peak

for C–N in simulation and experimental suggest the

intramolecular charge transfer activities. The peaks

observed at 851 and 751 cm-1 corresponds to C–H

bending. The experimental and simulated peaks are in

agreement with each other. Further lower absorption at

544 cm-1 is due to deformation modes of carboxylic

group.

Figure 4. (a) Experimental FTIR–ATR response. (b) DFT calculation of FTIR analysis.

38 Page 6 of 16 Bull. Mater. Sci. (2021) 44:38

3.4 UV–visible spectral analysis

The non-linear optical process is basically the absorption

of energy and promotes the electrons to excited states.

Response of crystals in absorption of radiated energy in

UV–visible range will make available the information

about the same. The low absorption characteristic, com-

monly observed in organic crystals, supports non-linear

behaviour as well as the usage of material in optical

switching devices [19,20]. The UV–visible response of

BILT was analysed in the range of 200–1400 nm using

Perkin Elmer Lambda spectrophotometer. The transmis-

sion curve depicted in figure 5 shows lower cut-off

wavelength of 288 nm. Both are in good agreement of

each other. Further, good quality of[75% transmission

(*80% in visible range) observed throughout. This

confirms the usage of analysed crystal for optoelectronic

device [21].

Table 3. Experimental and calculated band assignment from FTIR repsonse.

Band—experimental

wavenumber (cm-1)

Band—calculated

wavenumber (cm-1) Band assignment

3495 3589 Symmetric stretching of NH2

3348

3124 3106 Asymmetric and symmetric stretching of NH2

2970 2861 C–H stretching

2766 C=O stretching

1904 1902

1700 1690

1378 1370 O–H bending

1228 1178 C–O stretching

1130 1120 C–N stretching

1066 1040

852 860 C–H bending

746 745

530 470 C–O bending

Figure 5. UV–visible transmission response of title compound.

Bull. Mater. Sci. (2021) 44:38 Page 7 of 16 38

With the help of Tauc’s plot [22] (figure 6), the optical

band gap was found to be 4.2 eV and the transition was

observed to be indirect band gap with the coefficient (m =

0.5) from the Tauc’s equation [23]. The tuning of energy

intensity during device fabrication can be subjected to

optical constants like change in refractive index (g) and

extinction coefficient (K) of the material [24]. The loss of

energy due to absorption and scattering with propagation

can be evaluated with extinction coefficient. The graph of

extinction coefficient with wavelength is given in figure 7

which suggests the value of coefficient is very low and

remains constant in the range of 280–800 nm. As shown

Figure 6. Tauc’s plot and band gap calculation.

Figure 7. Extinction coefficient observation with wavelength.

38 Page 8 of 16 Bull. Mater. Sci. (2021) 44:38

in figure 8, the refractive index remains almost constant

in the range of 280–850 nm. Furthermore, minor raise has

been observed.

3.5 Frontier molecular orbital and MEP analysis

Quantum chemical calculation was carried out with TD-

DFT using function B3LYP. The simulation (figure 9)

shows best fit absorption peak with experimental value at

305 nm which shows the transition from HOMO to LUMO.

The frontier molecular orbital plays a decisive role in

assessing the optical and electrical parameters [25,26]. The

NLO and chemical activities can also be explained with

HOMO and LUMO energy gap. The HOMO–LUMO tran-

sition is shown in figure 10. It shows delocalization of

charge density over the benzimidazole in HUMO and over

tartrate in LUMO. This confirms charge transfer interaction

[27,28]. The energy of HOMO is 6.0140 eV and LUMO is

1.9801 eV. Hence, the calculated energy gap between

HOMO and LUMO is 4.03 eV.

The net electrostatic effect and relative polarity of

molecule at point in space can be examined by overall

charge distribution of the molecule [29–31]. The MEP

describes the net charge distribution. This also helps in

visualizing reaction activity of the molecules and hydrogen

bonding as well [32–34]. The plot (figure 11) is distributed

in the range of -9.322e-2 to 9.322e-2 graded in range of

colours from red to blue, respectively. The green colour

represents central range (towards neutral or zero potential)

of the electrostatic field. The tartrate section represents the

blue (positive potential) and the benzimidazole represents

Figure 8. Change in refractive index with wavelength.

Figure 9. Quantum chemical UV–visible output and energy transition response.

Bull. Mater. Sci. (2021) 44:38 Page 9 of 16 38

yellow and red (negative potential). This is also evident that

benzene ring is mostly less reactive here. Overall, the

compound has excellent charge transfer system among two

molecules.

3.6 Mulliken charge distribution analysis

The distribution of charges in title compound helps us to analyse

the overall distribution, net charges and confirmation for

bondinganalysis.TheMullikenpopulationanalysis, as shown in

figure 12, suggests that the distribution of charges has a range of

-0.8390 to 0.5438. As given in table 4, all the hydrogen atoms

including H21 and H26 which are involved in interaction, pos-

sess positive charges in the range of 0.1402–0.4148. All the

nitrogen and oxygen atoms possess negative charge distribution

in the range of (-0.8390 to -0.3666) and (-0.6582 to

-0.3443), respectively. This analysis confirms the intramolec-

ular bond (benzene and imidazole through N2) and inter-

molecular hydrogen sharing (N2–H21—O11–H26).

Figure 10. Molecular frontier analysis: HOMO–LUMO transition.

38 Page 10 of 16 Bull. Mater. Sci. (2021) 44:38

Figure 11. Molecular electrostatic potential response.

Figure 12. Mulliken charge distribution.

Bull. Mater. Sci. (2021) 44:38 Page 11 of 16 38

3.7 Thermal analysis

Non-linear crystals are usually subjected to high energy

environment and required good stability. Thermal

decomposition of any material will give idea about its

usage range over temperature. Thermogravimetric

analysis (TGA) and differential thermal analysis (DTA)

were carried out on powder sample of grown BILT.

The analysis was carried out in nitrogen atmosphere in

the range of 30–650�C at a heating rate of 10�C per

minute. Figure 13 represents that the material starts

decomposing at 180�C and sharp decomposition is

observed at 192�C. During this single-phase decompo-

sition, the weight loss of material is observed up to

98% from 180 to 250�C. The usage range for BILT

crystal is up to 192�C which is a potential range for

any NLO devices.

3.8 Dielectric analysis

Behaviour of the material subject to external electric field

with change in frequency is observed at different tempera-

tures (30–70�C, in steps of 10�C) with ac frequency range

of 100 Hz–4 MHz. The observations were taken in PSM-

1735 impedance analyzer. The change in dielectric constant

and dielectric loss with temperature and frequency is shown

in figure 14a and b. The value of dielectric constant is found

to sustain with very minor change, temperature and same

frequency in most of the operating range. The value of the

dielectric constant is observed high at lower frequency due

to space charge polarization [35]. The space charge polar-

ization activity is high at higher temperature and lower

frequency [36,37]. The dielectric loss is very less in high

Table 4. Mulliken charge distribution.

Atom Mullikan charge value

N2 -0.839024

O12 -0.658213

O11 -0.570645

O17 -0.548349

O19 -0.537313

N5 -0.366636

O18 -0.362641

O16 -0.344361

C7 -0.193743

C8 -0.151598

C4 -0.147369

C9 -0.059228

C14 -0.024802

C6 -0.018802

C13 -0.000983

H23 0.140239

H24 0.141622

H25 0.159301

H20 0.190351

H22 0.198324

H28 0.200106

C1 0.209542

H29 0.266243

C3 0.290833

H21 0.385646

H26 0.394738

H30 0.402617

H27 0.410771

H31 0.414875

C15 0.474682

C10 0.543815

Figure 13. Thermal analysis of BILT compound.

38 Page 12 of 16 Bull. Mater. Sci. (2021) 44:38

frequency range due to less correlation with natural

frequency of bound charges and applied external field. Low

dielectric constant and dielectric loss support the second

harmonic generation activities in material [36].

The further analysis was carried out for frequency-de-

pendent conductivity (figure 15) which is also dependent on

temperature. This shows motion of weakly bound charges

with external field. As shown in figure 16, the Joncher’s

Figure 14. (a) Dielectric constant response in operating range of frequency. (b) Dielectricloss response in operating range of frequency.

Bull. Mater. Sci. (2021) 44:38 Page 13 of 16 38

power law helps us to analyse the same [38,39]. The power

law represents total conductivity as r(x) which can be

presented as:

rðxÞ ¼ rdc þ Axs;

here, rdc is the conductivity in the absence of ac field. Axs

the dispersive component of ac conduction, x the applied

angular frequency, A the strength of polarizability and s thedegree of interaction among lattice and mobile ions. At the

Figure 15. ac conductivity with frequency.

Figure 16. Joncher’s power law analysis.

38 Page 14 of 16 Bull. Mater. Sci. (2021) 44:38

lower frequency, the conductivity is observed to be

increased with temperature. So, the lower frequency con-

duction is due to thermal activity in the material. At higher

frequencies, the conduction is observed due to ionic relax-

ation phenomenon. Table 5 shows values of A and s for

various temperature ranges in high frequency region where

Joncher’s power law is obeyed. For the ionic compound,

value of s will remain between 0.6 and 1. The hopping

conduction is observed at 4 MHz frequency.

3.9 Nonlinear properties

The non-linear output and second harmonic generation

activity of BILT was examined through Kurtz and Perry

powder technique [40]. Nonlinear optical properties in

organic crystals are mainly due to electron–phonon inter-

actions [41]. After grinding, the fine crystalline powder was

packed in capillary. Fine powder was illuminated with the

laser source having wavelength of 1064 nm. The second

harmonic generation was observed as the bright emission of

green light having 532 nm wavelength, was detected by the

monochromator. The emitted output power was compared

with reference sample of same grain size crystalline powder

of potassium dihydrogen phosphate (KDP). The SHG out-

put power of BILT was 2.69 times that of KDP.

4. Conclusion

The BILT has been synthesized and successfully grown as

single crystal by slow evaporation solution growth method.

X-ray powder diffraction study confirms that the title crystal

belongs to monoclinic system. The presence of various

functional groups has been confirmed by FTIR–ATR anal-

ysis. Optical properties confirm high transparency of BILT

crystal in the entire visible region with lower cut-off

wavelength of 288 nm. The optical band gap (Eg = 4.2 eV),

refractive index (g) and extinction coefficient of the mate-

rial were calculated. The charge transition has been con-

firmed with HOMO–LUMO analysis. The supportive

information of bond and charge distribution has been con-

firmed with theoretical calculation analysis. The thermal

study reveals good thermal stability of the material thus,

proving its suitability for NLO applications. The nonlinear

output is significant as the SHG is observed higher than that

of KDP. The theoretical and experimental analyses provide

good details about usage of title compound as NLO device.

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