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CHAPTER 3

SYNTHESIS, GROWTH, OPTICAL, THERMAL AND

MECHANICAL PROPERTIES OF 2-AMINOPYRIDINIUM

4-METHYLBENZOATE DIHYDRATE

3.1 INTRODUCTION

Organic crystals show large nonlinear optical (NLO) properties

(Williams 1983, Chemla and Zyss 1987). 2-Aminopyridinium

4-methylbenzoate Dihydrate 5 7 2 8 7 2 2( . .2 )C H N C H O H O+ − (2A4M) is an organic

NLO compound. The crystal structure of 2A4M was reported by Yun Liu and

Jie Li (2008). The title compound is composed of 4-methylbenzoate anion,

one 2-amino pyridinium cation and two water molecules. Carboxyl group of

4-methylbenzoic acid is deprotonated. In the crystal, 2-amino pyridinium

cation and 4-methylbenzoic acid anion together with water molecules are

linked into a three-dimensional supramolecular framework by multiple

N—H···O and O—H···O hydrogen bonds (Yun Liu and Jie Li 2008).

Pyridine and their organic complexes exhibit a large amount of fluorescence

in crystalline environment (Arivanandhan et al 2006). As such, pyridine

compounds are considered to be an optical material. The optical properties of

the derivatives of 2-Aminopyridine and their suitability for the fabrication of

optical devices were studied and reported (Periyasamy et al 2007).

This chapter presents the synthesis, growth and characterization of

2-Aminopyridinium 4-methylbenzoate Dihydrate (2A4M) by the slow

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evaporation solution growth technique at room temperature using water as a

solvent.

3.2 SYNTHESIS AND GROWTH OF 2-AMINOPYRIDINUM

4-METHYLBENZOATE DIHYDRATE

4-Methylbenzoic acid (1 mmol) and 2-aminopyridine (1 mmol)

were dissolved in 20 ml of double distilled water at room temperature and

stirred using a teflon coated magnetic stirrer for one hour. The solution was

refluxed at 353 K to avoid evaporation of 2-aminopyridine. The product of

2-Aminopyridinum 4-methylbenzoate Dihydrate was obtained. The reaction

mechanism of 2-Aminopyridinium 4-methylbenzoate Dihydrate is depicted in

Figure 3.1.

N-H

NH2

+

OO

_

.2H2ON-H

NH2

OO

.2H2O+

Figure 3.1 Reaction mechanism of 2-Aminopyridinium

4-methylbenzoate Dihydrate

The synthesised solution was cooled to room temperature. The

prepared solution was filtered using Whatman filter paper in a degreased

beaker and optimally covered by a thin polythin paper to avoid fast

evaporation of solvent. The solution was housed in the constant temperature

bath at room temperature for slow evaporation and closely observed every

day. After two weeks nucleation was observed in the solution and bulk

colourless crystals of size 11 × 4 × 3 mm3 were obtained in 25 days. The as

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grown 2A4M crystals are shown in Figure 3.2. The harvested crystals were

cut and polished for various characterization studies.

Figure 3.2 As grown single crystals of 2-Aminopyridinium

4-methylbenzoate Dihydrate

3.3 CHARACTERIZATION

3.3.1 Single Crystal XRD

The single crystal X-ray diffraction data of 2A4M crystal were

collected using Nonius CAD4/MACH 3 single crystal X-ray diffractometer

using MoKα radiation (λ = 0.71073 Å). It is observed that the crystal belongs

to the monoclinic system with noncentrosymmetric space group Cc. The

obtained lattice parameters are presented in Table 3.1. These values are in

good agreement with the literature (Yun Liu and Jie Li 2008). Crystal

morphologies are predicted from data stored in files in the CIF format

(crystallographic information file standard of the International Union of

Crystallography), (Bravais et al 1866, Friedel et al 1907, Donnay and Harker

et al 1935). CIF data are given as input in the Win X Morph software package

and the morphology diagram of 2A4M crystal is drawn and shown in the

Figure 3.3. As grown crystal of 2A4M from solution and the morphology of

the grown crystal resembles each other.

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Table 3.1 Crystallographic data of 2A4M

Lattice Parameter Present study Reported values

(Yun Liu and Jie Li 2008)

a 12.179(2)Å 12.2059 (14) Å

b 13.128(2) Å 13.1531 (16)Å

c 8.981(2) Å 8.9937 (11)Å

β 96.62(1)° 96.617 (2)°

V 1426.5(4) Å3 1434.3 (3) Å3

System Monoclinic Monoclinic

space group Cc Cc

Figure 3.3 Morphology diagram of 2A4M single crystal

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3.3.2 Optical Studies

3.3.2.1 Transmittance

The UV–Vis transmittance spectrum was recorded for the grown

2A4M crystal using a SHIMADZU UV-2501 PC, UV–Vis spectrophotometer

in the range 250 – 700 nm. Figure 3.4 shows the transmittance spectrum of

the 2A4M crystal, which indicates a good transmittance in the visible region

and there is no significant absorption between 349 and 700 nm. The presence

of lone electron pairs of O atoms in carbonyl groups in 4-methylbenzoic acid

is favourable to the promotion of an electron to an unoccupied orbital giving

an (n, π*) excited state. The lower cut off at 350 nm combined with very

good transparency attest the usefulness of this material for optoelectronics

applications (Bhat 1994, Sethuraman et al 2008).

Figure 3.4 Transmittance spectrum of 2-Aminopyridinium

4-methylbenzoate Dihydrate

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3.3.2.2 Optical band gap

Optical band gap of the title compound was calculated from the

transmittance spectrum. The measured transmittance (T) was used to calculate

the absorption coefficient (α) using the relation 2.1 and 2.2 given in the

second chapter. The band gap was calculated from the plot between hν and

(αhν) 1/2 as shown in the Figure 3.5 and the optical band gap is found to be

2.9 eV.

Figure 3.5 Plot between photon energy and (αhν) 1/2

of 2A4M

3.3.2.3 Refractive index (n)

The refractive index of the material can be measured from the

reflectance. The reflectance (R) in terms of the absorption coefficient can be

obtained using the equations 2.3 and 2.4 (Presented in second chapter).

Figure 3.6 shows the wavelength dependent refractive index (n) of 2A4M

crystal. The refractive index (n) is 1.40 at 1200 nm for 2A4M crystal. The

optical behaviour of crystal can be correlated with dielectric behaviour. The

complex dielectric constant is fundamental intrinsic material property. The

real part of it is associated with the term that how much it will slow down the

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speed of light in the material and imaginary part gives that how a dielectric

absorb energy from electric field due to dipole motion. The real part dielectric

constant εr and imaginary part dielectric constant εi can be calculated using

following relations

2( )

r ii n ikε ε ε= + = + (3.1)

where εr and εi are the real and imaginary parts of the dielectric constant

respectively and are given by

εr = n2-K2 and εi = 2nK. (3.2)

The value of real (εr) and imaginary (εi) dielectric constants, at

λ = 1200 nm are 1.38 and 3.82 × 10-4 respectively.

Figure 3.6 Wavelength dependent refractive index of 2A4M crystal

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3.3.2.4 Photoluminescence (PL)

Photoluminescence (PL) measurement is a prominent tool for

determining the crystalline quality of a system as well as its exciton fine

structure. The PL spectrum was recorded using a Perkin Elmer (LS 45) PL

unit at room temperature with slit width 10 nm in the wavelength range of

340–450 nm. The recorded PL spectrum of 2A4M crystal is shown in

Figure 3.7 with the excitation wavelength of 240 nm. From the PL spectrum,

a strong and broad emission peak centered at 386 nm was observed. A higher

intensity ratio specifies better purity and crystallinity of 2A4M. The

broadening of emission band is due to lattice vibrations in the 2A4M crystal.

The emission is assigned to electronic transition from π* antibonding

molecular orbital to π bonding molecular orbital of 2A4M.

Figure 3.7 The PL spectrum of 2A4M crystal

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3.3.3 FT-IR Spectral Study

The FT-IR spectrum of 2-Aminopyridinium 4-methylbenzoate

Dihydrate was recorded using Perkin Elmer spectrophotometer. Figure 3.8

shows the FT-IR spectrum recorded in the range 4000-500 cm-1. The peak due

to O-H stretch of water appears as a shoulder in the higher energy portion of

the N-H stretch vibration occurring at 3266 cm-1. The aromatic

C-H vibration gives the peak at 3021 cm-1. The aliphatic C-H vibration occurs

as a shoulder to the peak just below 3000 cm-1. The peaks at 2029 cm-1 and

1974 cm-1 are due to combination bands of 2-Aminopyridinium and

4-methylbenzoic acid. The peak at 2029 cm-1 is due to combination of the

peak at 1260 and 764 cm-1. The peak at 1974 cm-1 is the combination of the

peak at 1383 and 608 cm-1. The C═O vibration of the carboxylate group gives

its peak at 1684 cm-1. The ring skeletal vibrations of aromatic rings give peaks

at 1605, 1515 and 1436 cm-1 (Silverstein and Webster 1998).

The bending vibration of water occurs at 1639 cm-1 indicating the

presence 2 moles of water which can also be verified from the TGA analysis.

The intense sharp peak at 1383 cm-1 is due to COO– vibration. The groups of

peaks between 1660 and 850 cm-1 are due to C-N vibration. The

1, 4-disubsituted aromatic ring gives its C-H bend at 842 cm-1. All other

vibration below this is due to C - H bending modes of other aromatic rings.

From the above discussion it is verified that 2-Aminopyridinium and

4-methylbenzoic acid remains the salt of dihydrate.

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Figure 3.8 FT-IR spectrum of 2-Aminopyridinium 4-methylbenzoate

Dihydrate

3.3.4 Thermal Analysis

The thermal stability of 2A4M was identified by the

thermogravimetric (TG) and differential thermal analyses (DTA). The thermal

analyses were carried out using a NETZSCH STA 409 C/CD system between

the temperatures 30°C and 500°C at a heating rate of 10°C/ minute in nitrogen

atmosphere in the alumina crucible. The TG and DTG of the compound are

shown in the Figure 3.9. The initial weight loss up to 125°C corresponds to

13.2%, illustrating the presence of 2 moles of water in the lattice. Desorption

of water is immediately followed by decomposition of 2-Aminopyridinium

4-methylbenzoate Dihydrate. The decomposition extends up to 300°C.

The DTA trace is shown in the Figure 3.10, the initial endotherm

below 100°C is due to desorption of water. The sharp clear endothermic peak

at 125°C is due to melting point of complex. It is followed by endothermic

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decomposition of the salt. Comparison of DTA and TGA results illustrates the

NLO applicability of the material is limited up to 125°C.

Figure 3.9 TGA of 2-Aminopyridinium 4-methylbenzoate Dihydrate

Figure 3.10 DTA of 2-Aminopyridinium 4-methylbenzoate Dihydrate

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3.3.5 Microhardness Measurements

The mechanical behaviour of the grown 2A4M crystal was studied

Using Reichert MD 4000E ultra microhardness tester fitted with a Vickers

diamond pyramidal indenter in the plane (3 1 1). The indentations were made

at room temperature with a constant indentation time of 2 second. The as

grown 2A4M crystal was cut with dimension of 6 mm × 4 mm × 2 mm,

polished and subjected to Vicker’s hardness testing. The (3 1 1) plane of

prepared crystal was properly mounted on the base of the microhardness

tester attached with microscope and indented gently by applying the loads

10 to 70 g in steps of 10 g with a dwell time of 2 sec. The indented

impressions for various loads 10 g, 40 g (initial stage of crack formed) and

80 g (deformed stage) are shown in the Figure 3.11.

(a) 10 g (b) 40 g (c) 80 g

Figure 3.11 Indentation impression of 2-Aminopyridinium

4-methylbenzoate Dihydrate for (a) 10 g, (b) 40 g

and (c) 80 g

From the Figure 3.11 it can be seen that radial cracks increase in

length with increasing load and are accompanied at higher loads by severe

in-plane fracture to form lateral cracks. The lateral cracks appear as light

halos and lie parallel to the plane. The lengths of the two diagonals of the

indentations were measured by a calibrated micrometer attached to the eye

piece of the microscope after unloading. The Vickers’s hardness number was

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calculated using the relation given in equation 1.11 (in chapter 1).

Figure 3.12 (a) shows the plot between load (P) and (Hv) of 2A4M single

crystal. It is very clear that Hv increases with the increase of load. The

increase in the hardness number can be attributed to the electrostatic attraction

between the zwitterions present in the molecule. Above 30 g, cracks were

observed, owing to the release of internal stress generated locally by

indentation.

(a) (b)

Figure 3.12 (a) Variation of (Hv) with Load (P) of 2A4M single crystal,

(b) Plot of log P with log d

The Meyer's index number was calculated from the Meyer's law,

which relates the load and indentation diagonal length using the relation given

in equations 1.12 and 1.13 (presented in chapter 1). The plot between log P

and log d, (Figure 3.12 b) is a straight line and the slope of this straight line

gives the value of n. The calculated value of n is 5, from the expression Hv

should increase with the increase of P if n > 2 and decrease if n < 2. The value

of n agrees well with the experiment. According to Onitsch (1947) n should

lie between 1 and 1.6 for harder materials and above 1.6 for softer materials.

Thus 2A4M belongs to the soft material category.

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For a crystal with well-defined cracks, the resistance to fracture

indicates the toughness of a material. According to Ponton and Rawling

(1989) fracture toughness Kc is dependent on the ratio of c/a, where c is the

crack length and a is the half-diagonal length of the square indentation as

shown in Figure 1.10 in the first chapter.

For c/a ≥ 2.5, the cracks are developed with median crack system

and the fracture toughness c/a is calculated using the equation (1.14), where

(l = c–a) is the mean Palmqvist crack length. From the Figure 3.13 (a) it can

be observed that as the load increases the fracture toughness also increases for

2A4M crystal.

The brittleness index Bi is calculated using the relation (1.16) from

the hardness values, the yield strength σy can be calculated and it is defined as

the stress at which the material begins to deform plastically and depends on

Meyer's index n. For n > 2, the yield strength σy may be calculated using the

expression (1.17). For n < 2, the yield strength is calculated using the relation

(1.18). It is seen from the Figure 3.13 (b) that the yield strength also increases

as load increases. The obtained data are tabulated in Table 3.2.

Table 3.2 The data of Mechanical properties on the (311) face of 2A4M

crystal

P(g) Hv

(kg/mm2)

c (µm) K (MPa.m1/2

) Bi (m1/2

)

30 38.5 46 0.425 886.6

40 44.6 24 1.448 301.6

60 57.8 26 1.901 297.9

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The elastic stiffness constant (C11) for different loads calculated

using Wooster's empirical formula C11 = Hv7/4 (Wooster 1953).

Figure 3.13 (c) shows the plot of Hv7/4

with C11, stiffness constant increases

with hardness value which gives an idea about the tightness of bonding

between neighbouring atoms. The increase in hardness accompanied by

increase in fracture toughness, brittleness index and stiffness constant are

clearly observed.

(a) (b)

(c)

Figure 3.13 (a) Fracture toughness with load of 2A4M single crystal,

(b) Plot between load and yield strength and (c) The plot

of Load with C11

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3.3.6 Etching Studies

The etching studies were carried out on the (3 1 1) plane of the

as-grown single crystal of 2A4M using distilled water as an etchant at room

temperature for the etching time of 15s, 30s, 45s, 60s, 4 minutes and

6 minutes. In the present experimental work, transparent crystal free from

inclusions and cracks was selected. Etching of the crystal surface was carried

out by dipping the crystal in water at room temperature. The etched surfaces

were dried by gently pressing them between two filter papers and then

immediately examined and their microstructures were analyzed using an

optical microscope in the reflection mode. Some well defined and

crystallographically aligned etch pits were observed on the as grown surface.

Figure 3.14 (a) represents the photomicrograph of as grown crystal

before etching. Figures 3.14 (b) shows some rectangular growth hillocks. The

shape of the etch pits indicates the direction of the dislocation lines.

Figures 3.14 (c) (d) (e) show the bunch of rectangular growth hillocks of

2A4M crystal and there is no change in etch pits with varying etching time

(15–60 s). As etching time increases the rectangular hillock got elongated in

their size. This observation may also be associated with a decreasing

undersaturation at the dislocation source with increasing time. Figures 3.14 (f)

and (g) show elongated rectangular hillocks increases with the increase of

etching time while the pit pattern remains same. The observed etch pits, due

to layer growth, confirmed the two-dimensional nucleation (2D) mechanism

with less dislocations (Mukerji and Kar 1999).

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(a) As grown crystal surface (b) 15 sec (c) 30 sec

(d) 45 sec (e) 60 sec (f) 4 minutes

(g) 6 minutes

Figure 3.14 Etch pit patterns of 2A4M single crystal on (3 1 1) face with

water as an etchant (a) As grown crystal surface, (b) 15 s,

(c) 30 s, (d) 45 s, (e) 60s, (f) 4 min and (g) 6 min

3.3.7 Second Harmonic Generation Test

The second harmonic generation behaviour was tested by the

Kurtz-Perry powder technique using Nd: YAG laser as a source

(λ = 1064 nm). The powdered material of the crystal was packed in the

capillary tube and beam energy of 4.9 mJ/pulse was given as input. The

powdered sample of 2A4M was illuminated by the laser source. The second

harmonic signal generated in the sample was collected and detected by the

monochromator, which is coupled with the photomultiplier tube. The bright

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green emission was observed from the output of the powder of the 2A4M.

KDP sample was used as the reference material and the output power

intensity of 2A4M was compared with the output power of KDP. A second

harmonic signal of 82 mV was obtained from 2A4M and 27 mV for reference

material KDP. The output power of 2A4M is 3.03 times that of standard KDP.

3.4 CONCLUSION

A good quality bulk single crystal of 2A4M was successfully

grown by slow evaporation solution growth method. The lattice parameters

are evaluated by single crystal X-ray diffraction analysis. It revealed that the

crystal belongs to monoclinic system. The optical band gap (2.9 eV) and the

refractive index (n) were obtained from the optical transmittance data. The

functional groups were confirmed by FT-IR. The thermal behaviour of the

grown crystal was studied by TG-DTA. From the Vickers hardness studies

hardness (Hv), Fracture toughness (Kc), Brittleness index (Bi), Yield strength

(σy) and Elastic stiffness constant (C11) were calculated. The work hardening

coefficient was evaluated as 3. The etching study revealed rectangular

hillocks and one layer growth mechanism. Kurtz-Perry powder method was

used to confirm the SHG of the material. SHG relative efficiency of 2A4M is

3.03 times that of KDP.