CHAPTER - 3
Characterization of the
Crystals of Chlorine and
Bromine Substituted
Chalcone Derivatives
Chapter 3
Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Characterization of the Crystals of Chlorine and
Bromine Substituted Chalcone Derivatives
3.1 Introduction
It is of significance that novel crystals always be characterized so that they can be tagged and
categorized. The scope of “Characterization” is very broad [126, 127] that nearly every
aspects of physics and chemistry of materials can be included under this domain.
A large variety of tools and techniques are available nowadays for the detailed examination
and characterization of the crystals. In general they are classified into two groups,
(i) destructive methods and (ii) nondestructive methods. Physical examination of the crystals
using Optical Microscope, Scanning Electron Microscope (SEM), and X-ray Diffraction
(XRD) methods as well as spectroscopic methods such as Nuclear Magnetic Resonance
(NMR), Fourier Transform Infra Red (FTIR) and Laser Raman (LR) techniques for
understanding the internal structure of the crystals are non-destructive methods. Methods such
as Differential Scanning Calorimetry (DSC) Thermogravimetric Analysis (TGA), Differential
Thermal Analysis (DTA) are of destructive type. With the use of advanced computers and the
introduction of micro laser equipment the efficiency of analysis has improved considerably.
Experimental observation and results on the characterization of the solution grown chlorine
and bromine substituted crystals of chalcone derivatives by chemical purity, thermal stability,
mechanical nature, electrical properties, linear and nonlinear optical properties are discussed
in this chapter.
3.2 Compositional study of the crystals
3.2.1 Elemental analysis
The percentage compositions of the elements present in these chalcone derivatives was
determined using carbon, hydrogen and nitrogen analysis. As listed in Table 3.1, both
experimentally determined percentage compositions of elements and the calculated ones from
the molecular formulae of the respective compounds were in good agreement.
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Chapter 3
Table 3.1: CHN analysis of Chalcone derivatives
Experimental % compositions
Calculated % compositions
Compounds
Carbon Hydrogen Nitrogen Carbon Hydrogen Nitrogen
CTTMP
56.69
4.41
-
56.72
4.46
-
CTDMP
58.39
4.27
-
58.35
4.24
-
CTDCP
49.09
2.17
-
49.16
2.22
-
BTDMP
50.88
3.80
-
51.00
3.71
-
BTNP
46.04
2.28
4.11
46.17
2.38
4.14
3.2.2 Fourier transform infrared (FTIR) spectroscopy
To record the FTIR spectra of chalcone derivatives, pellet technique is used which involves
mixing a finely ground sample (0.5 – 1 mg) with potassium bromide (KBr) powder and
pressing the mixture in an evacuated die at sufficiently high pressure to produce a transparent
disk.
In the present work FTIR spectrum of the samples are recorded using SHIMADZU-8400S
FT-IR spectrometer in the wavenumber range 4000-400 cm.–1 The spectral resolution was
0.1 cm–1. The FTIR spectrum for all the chalcone derivatives is shown in Figure 3.1 a, b, c, d
and e.
The functional groups present in CTTMP, CTDMP, CTDCP, BTDMP and BTNP are
conjugated carbonyl group (–C=C–CO–, C=O), phenyl ring and thiophene ring are common
in all the samples. Each molecule differs by the presence of one or more extra substituent such
as methoxy, chlorine, bromine and nitro groups. The absorption bands corresponding to the
respective functional groups in respective chalcone derivatives are indexed and are presented
in Table 3.2.
The FTIR spectrum of CTTMP shown in Figure 3.1 a, contains three methoxy (OCH3) groups
and the absorption band corresponding to CH3 stretching of the methoxy group appeared at
2925 and 2831 cm–1 respectively. The absorption band at 1647 cm–1 confirms the presence of
C=O group in conjugation with alkene group in this molecule. The aryl C–O stretching
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Chapter 3
observed at 1215 cm–1 further confirmed the presence of methoxy group. The rest of the
functional group assignments are listed in Table 3.2.
In the FTIR spectrum of CTDMP shown in Figure 3.1 b, medium absorption band due to C=O
stretching is observed at 1643 cm–1. An absorption band at 1416 cm–1 is due to the alkyl C–H
bending vibration of CH3–O group. Two
medium intensity absorption bands owing to
asymmetric C–O–C stretching (aryl C–O stretching) and symmetric C–O–C stretching
vibrations (alkyl C–O stretching) were situated at 1271 and 1074 cm–1, respectively.
The FTIR spectrum of CTDCP is shown in Figure 3.1 c, contain chlorine as a substituent
group on either side of the molecule. The absorption band around 1645cm–1 confirms the
presence of C=O group in this molecule. The absorption band due to aromatic C–H stretching
was observed at 3086 cm–1.
Table 3.2: Fourier Transform Infrared (FTIR) spectroscopic analysis
Assignment
IR absorption bands (cm–1)
CTTMP
CTDMP
CTDCP
BTDMP
BTNP
C─H aromatic stretch
3082
3090
3086
3086
3078
Assy C─H Str. (CH3)
2925
2943
-
2939
-
Sym C─H Str. (CH3)
2831
2831
-
2824
-
C=O str.
1647
1643
1645
1643
1651
C=C stretch
1571
1575
1587
1582
1589
C─H bending
1414
1416
1417
1412
-
Aryl C─O stretching
1215
1271
1224
1273
-
Alkyl C─O stretching
-
1074
-
1072
-
802
789
785
787
-
C─Cl Str.
-
694
-
-
-
C─Br str.
-
-
-
-
756
Assy. NO2 Str.
-
-
-
-
1520
Sym. NO2 Str.
-
-
-
-
1342
Ar C─H bending
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Chapter 3
The FTIR spectrum of BTDMP, shown in Figure 3.1 d, contain bromine as a substituent
group at one side and two methoxy (OCH3) groups at the other side of the molecule. The
weak absorption bands appeared in the range of 3100-3000 cm–1 is attributed to aromatic C–H
stretching vibrations. The absorption band corresponding to C–H symmetric and asymmetric
stretching of CH3 group was observed in the wavenumber region of 2824 and 2939 cm–1
respectively, which gives the indication of presence of OCH3 group. Further the absorption
bands corresponding to aryl C–O and alkyl C–O stretching vibrations in this compound are
1273 and 1072 cm–1 respectively. The strong absorption band corresponding to C=C
stretching was observed at 1582 cm–1 and that of absorption band corresponding to C=O
stretching was observed at 1643 cm–1.
The FTIR spectrum of BTNP (Figure 3.1 e) contains a nitro group at para position of phenyl
group. The absorption band around 1651 cm–1 confirms the presence of C=O functional
group. The absorption band corresponding to asymmetric and symmetric stretching of
aromatic C–NO2 was observed at 1520 and 1342 cm–1 respectively. In addition to the
functional groups of BTNP, there is an absorption peak corresponding to C–H aromatic
stretching around 3078 cm–1. This gives the confirmation of the functional groups present in
all the chalcone derivatives and the detailed assignments are listed in Table 3.2.
Figure 3.1 a: FTIR spectrum of CTTMP
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.1 b: FTIR spectrum of CTDMP
Figure 3.1 c: FTIR spectrum of CTDCP
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.1 d: FTIR spectrum of BTDMP
Figure 3.1 e: FTIR spectrum of BTNP
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
3.2.3 Mass spectroscopy
The mass spectrum of the crystals CTTMP, CTDMP, CTDCP, BTDMP and BTNP are
recorded using the instrument Jeol AccuTOF GCV Agilent 7890 using FID detector and
results are shown in Figure 3.2 a, b, c, d and e respectively.
The mass spectrum of the compound CTTMP does not show the molecular ion peak, which
may be due to the presence of easily dissociating groups like OCH3 and Cl. However, the
compound shows a peak at m/z (272/274) due to the formation of fragment derived by the
loss of Cl and OCH3 group from the molecular ion peak. This clearly indicates that CTTMP is
indeed formed. Base peak observed at m/z 237 is may be assigned to the fragment
[M–C3H2ClS]+.
It is observed that in the case of CTDMP molecular ion peak is missing which may be due to
the presence of easily dissociating groups like OCH3 and Cl. However, the compound shows a
base peak at m/z (244/246) due to the formation of fragment derived by the loss of Cl and
OCH3 group from the molecular ion peak. This indicates the formation of the compound
CTDMP. The observed peak at m/z 202 is may be assigned to the fragment [M–C3H2ClS]+.
In the case of CTDCP molecular ion peak was observed at m/z 317 confirm the formation of
chalcone derivative. Base peak observed at 281 is assigned to the cation formed by the loss of
Cl radical from the molecular ion.
The molecular ion peak was observed at 352/354 confirm the formation of BTDMP. The
presence of bromine in the product is confirmed with equal intensity peaks observed at 352
and 354 respectively. Base peak observed at 323 is due to the loss of methoxy radical from
the molecular ion.
The molecular ion peak of BTNP also observed as a base peak at m/z 338.99 confirms the
formation of the compound. In all the cases, the remaining peaks are due to small other
fragment ions of low masses, which are products of consecutive decompositions. These may
not be the initially formed fragments, but rather arise by successive fragmentation reactions.
They are less useful than the initial fragmentation peaks for interpretation.
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.2 a: Mass spectrum of CTTMP
Figure 3.2 b: Mass spectrum of CTDMP
Figure 3.2 c: Mass spectrum of CTDCP
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Chapter 3
Figure 3.2 d: Mass spectrum of BTDMP
Figure 3.2 e: Mass spectrum of BTNP
3.2.4 Nuclear magnetic resonance (NMR) spectroscopy
The NMR spectra of the crystals – CTTMP, CTDMP, CTDCP, BTDMP and BTNP are
recorded using Bruker Ascend 400 MHz NMR Spectrometer and are shown in Figures 3.3 a,
b, c, d and e respectively. In CTTMP, the doublets observed at δ 8.022 and 7.212 each with a
coupling constant of 15.2 corresponds protons of – CH = CH – group. Other characteristic
peaks observed in the spectrum are δ: 7.543 (d, 1H, H of thiophene ring), 7.008 (d, 1H, H of
thiophene ring), 6.914 (t, 1H, H of aromatic ring), 6.444 (d, 2H, H of aromatic ring), 3.849
(3 closely packed singlets, 9H, 3 x OCH3).
In the case of CTDMP, the doublets observed at δ 8.12 and 7.42 with a coupling constant of
15.9 corresponds protons of – CH = CH – group. Other characteristic peaks observed in the
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
spectrum are δ: 7.62 (d, 1H, H of thiophene ring), 7.26 (d, 1H, H of thiophene ring), 7.10 (t,
1H, H of aromatic ring), 7.00 (d, 2H, H of aromatic ring), 3.9 (broad singlet, 6H, 2 x OCH3).
In CTDCP, the doublets observed at δ 8.1975 and 7.266 with a coupling constant of 15.9
corresponds protons of – CH = CH – group. Other characteristic peaks observed in the
spectrum are δ: 7.6115 (d, 1H, H of thiophene ring), 7.007 (d, 1H, H of thiophene ring), 7.627
(t, 1H, H of aromatic ring), 7.223 (d, 2H, H of aromatic ring), 7.265 (d, 2H, H of aromatic
ring).
In the case of BTDMP, the doublets observed at δ 8.106 and 7.4005 with a coupling constant
of 16 corresponds protons of – CH = CH – group. Other characteristic peaks observed in the
spectrum are δ: 7.576 (d, 1H, H of thiophene ring), 7.1435 (d, 1H, H of thiophene ring), 7.091
(t, 1H, H of aromatic ring), 6.97825 (d, 2H, H of aromatic ring), 7.2375 (d, 2H, H of aromatic
ring), 3.891 (2 singlet, 6H, 2 x OCH3).
In BTNP, the doublets observed at δ 7.4105 and 7.8435 with a coupling constant of 15.6
corresponds protons of – CH = CH – group. Other characteristic peaks observed are assigned
as δ: 7.18-7.19 (d, 1H, chalcone H), 7.3-7.4 (d, 1H, chalcone H), 7.63-7.65(d, 1H, H of
thiophene ring), 7.78 (d, 2H, Ar-H of nitrophenyl ring), 7.7-7.8 (d, 1H, H of thiophene ring),
8.28-8.29 (d, 2H, Ar-H of nitrophenyl ring).
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.3 a: NMR spectrum of CTTMP
Figure 3.3 b: NMR spectrum of CTDMP
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.3 c: NMR spectrum of CTDCP
Figure 3.3 d: NMR spectrum of BTDMP
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.3 e: NMR spectrum of BTNP
3.3 Thermal properties of the crystals
The Differential Scanning Calorimetry (DSC) plots for the crystals CTTMP, CTDMP,
CTDCP, BTDMP and BTNP are shown in Figures 3.4 a, b, c, d and e respectively. In all the
cases, the sample weight is of the order of 3 to 5 mg and the temperature range is 30οC to
250οC, at a rate of 10οC/min, in nitrogen atmosphere using SHIMADZU Differential
Scanning Calorimeter (DSC-60). In each case it is observed that, the first endothermic peak
occurs at the melting temperature. None of the samples show any endothermic or exothermic
peak below the melting point temperature. Also there is no solvent of crystallization in the
crystals. The melting range in each crystal is about 3 to 5 οC, which indicates a good degree of
crystallinity. Table 3.3 gives the data collected for the crystals from the DSC plots.
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.4 a: DSC plot of CTTMP
Figure 3.4 b: DSC plot of CTDMP
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Synthesis and Characterization of Nonlinear Optical Crystals of
Chlorine and Bromine Substituted Chalcone Derivatives
Figure 3.4 c: DSC plot of CTDCP
Figure 3.4 d: DSC plot of BTDMP
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Chapter 3
Figure 3.4 e: DSC plot of BTNP
Table 3.3: Thermal properties of the crystals
Crystal name
Melting point by
DSC plot (οC)
Melting range by mercury
thermometer (οC)
Latent heat of fusion
(kJ/kg)
CTTMP
112
110-113
105.61
CTDMP
111
110-112
103.18
CTDCP
179
176-180
96.00
BTDMP
118
117-120
97.35
BTNP
202
199-203
95.07
For all the crystals melting point measured by thermometer is in good agreement with the
value obtained by the DSC plot. The high melting point of the crystals is an advantage in
device fabrication. Among the chalcone derivatives studied for their thermal property, BTNP
has very high thermal stability. The substitution of strong π-electron acceptor group like nitro
results in high thermal stability in BTNP crystal. On the other hand in case of chlorine
substituted chalcone derivatives substitution of chlorine group on phenyl ring is more efficient
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Chapter 3
than substitution of methoxy group (melting point of CTTMP is 112οC, CTDMP is 111οC and
that of CTDCP is 179οC). In case of bromine substituted chalcone derivatives substitution of
nitro group on phenyl ring is more efficient than substitution of methoxy group (melting point
of BTDMP is 118οC, and that of BTNP is 202οC).
The melting point of the chalcone derivatives was compared with those of other NLO
materials such as m-nitro aniline (112οC), MMONS (110οC) and urea (130οC decomposition)
[128–130]. All the chalcone derivatives show good thermal stability than MMONS. CTDCP
and BTNP exhibits better thermal stability than urea, m-nitro aniline and MMONS.
3.4 Mechanical properties of the crystals
3.4.1 Density measurements
The density values of all the five different crystals were determined using the specific gravity
bottle at ambient temperature (27ºC). The average values of the density measurements are
listed in Table 3.4 and are compared with the values calculated from single crystal X-ray
diffraction data (Table 4.1 in chapter 4).
The bulk density observed in the laboratory-grown organic single crystals is slightly different
from that of the density values calculated from single crystal X-ray diffraction data. This
discrepancy in the values may be due to the lattice distortions induced by the presence of
point defects in the crystals.
Table 3.4: Density values of the crystals
CTTMP
Density measured
(g/cm3)
1.356
Density calculated
(single crystal XRD) (g/cm3)
1.399
CTDMP
1.446
1.452
CTDCP
1.581
1.575
BTDMP
1.654
1.648
BTNP
1.746
1.734
Crystal name
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Chapter 3
3.4.2 Microhardness studies
The mechanical hardness is one of the main factors that decide the applicability of the
nonlinear optical (NLO) materials and also in selecting the processing (cutting, grinding and
polishing) steps of bulk crystals in the fabrication of devices based on these crystals [131-133].
It is therefore, important to study the mechanical properties of NLO crystals of chalcone
derivatives. Microhardness studies of all the five crystals have been carried out to study the
surface mechanical properties using Vickers microhardness tester (CLEMEX digital micro
hardness tester “MATSUZAWA, Japan). The grown crystals with smooth and dominant faces
were selected for microhardness studies. Vickers hardness number (VHN) was calculated
using the relation
(3.1)
where P is the applied load in kgwt and d is the mean diagonal length of the indenter
impression in millimeter. Load dependence of microhardness was carried out for the loads 5g,
10g, 25g and 50g. Crack initiation and materials chipping become significant beyond 50g of
the applied load. Figures 3.5 a, b, c, d and e, indicates the variation of VHN as a function of
applied load. The VHN values calculated for different loads are listed in Table 3.5 for the
single crystals of CTTMP, CTDMP, CTDCP, BTDMP and BTNP.
Table 3.5: Microhardness data of the crystals
Crystal/Indentation face
Load (P)
(gm)
CTTMP /
(1 0 0)
VHN
(kg/mm2)
CTDMP /
CTDCP /
(1 0 0)
VHN
(kg/mm2)
BTDMP /
(1 0 0)
VHN
(kg/mm2)
BTNP /
(100)
VHN
(kg/mm2)
5
73.8
57.2
57.2
63
12.5
10
49.3
32.2
39.8
38.3
17
25
18
26
26
30.5
25
50
17
26
26
30
25
_
(2 0 1 )
VHN
(kg/mm2)
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Figure 3.5 a: Load dependence of Vicker’s hardness of CTTMP crystal
Figure 3.5 b: Load dependence of Vicker’s hardness of CTDMP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.5 c: Load dependence of Vickers hardness of CTDCP crystal
Figure 3.5 d: Load dependence of Vickers hardness of BTDMP crystal
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Chapter 3
Figure 3.5 e: Load dependence of Vickers hardness of BTNP crystal
It is observed from the study that, the hardness number of the crystals initially decreases with
the increase of the applied load and attains a constant value for higher applied loads, except
for BTNP crystal. The hardness number of BTNP increases with the increase of the applied
load and attains a constant value for higher applied loads.
3.5 Optical properties of the crystals
Optical transparency characteristics like UV-visible spectra, refractive index measurement for
the NLO crystals of chlorine and bromine substituted chalcone derivatives are reported in this
section.
3.5.1 UV-visible spectra
The UV-visible absorption spectra of the crystals have been recorded in the wavelength
region 200 nm to 800 nm using “UV-1601PC UV-visible spectrophotometer” (Figure 3.6 a, b,
c, d and e). Crystal plates with parallel surfaces and thickness of about 1 mm were used. All
the samples were found to have strong absorption band in the UV-region due to n-π* and π π* transition and is attributable to the presence of aromatic ring and C=O group. The absence
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Chapter 3
of the absorption in the visible region can be exploited for NLO applications at the room
temperature [134]. The transparency cut-off wavelength ( λo) and the corresponding band gap
energy (hc/λo) values of the crystals CTTMP, CTDMP, CTDCP, BTDMP and BTNP are
presented in Table 3.6.
Table 3.6: Transparency cut-off wavelengths and band gap energy of the crystals
Crystal
Transparency cut-off (λο)
(nm)
Band-gap energy (hc/ λο)
(eV)
CTTMP
440
2.82
CTDMP
420
2.96
CTDCP
415
2.99
BTDMP
415
2.99
BTNP
420
2.96
Figure 3.6 a: UV-visible spectra of CTTMP
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Figure 3.6 b: UV-visible spectra of CTDMP
Figure 3.6 c: UV-visible spectra of CTDCP
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Figure 3.6 d: UV-visible spectra of BTDMP
Figure 3.6 e: UV-visible spectra of BTNP
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All the varieties of the crystals are transparent in the entire visible range. The lower values for
the cut-off wavelength of the crystals are advantageous in second harmonic generation (SHG)
conversion of IR beam of diode laser into visible-light.
3.5.2 Refractive index measurement
The values of the refractive indices of the crystals CTTMP, CTDMP, CTDCP, BTDMP and
BTNP, for 632.8 nm and 543.5 nm, were measured using Brewster’s angle method and are
listed in Table 3.7. For all the crystals only the refractive index along one of the
crystallographic direction was possible due to their platy type growth. The relatively lower
refractive indices of the crystals in comparison to inorganic materials helps in achieving good
figure of merit and makes them to be better materials for nonlinear
optical (NLO)
applications.
Table 3.7: Refractive indices of the crystals
CTTMP
Refractive index for
632.8 nm
1.613
Refractive index for
543.5 nm
1.622
CTDMP
1.594
1.600
CTDCP
1.555
1.570
BTDMP
1.558
1.570
BTNP
1.582
1.594
Crystal
3.6 Nonlinear optical properties of the crystals
Some materials manifest marked changes in their optical properties as a result of the
interaction with the strong electromagnetic field of the radiation. This in turn produces
changes in the frequency, phase or amplitude of the light transmitted through the material.
Such interactions arising out of multiphoton effects are known as nonlinear optical (NLO)
processes and the materials in which such processes can be carried out efficiently are called
NLO materials.
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3.6.1 Second harmonic generation (SHG) efficiency
The SHG efficiency of all the chalcone derivatives were determined using Kurtz and Perry
[18] powder technique. Nd: YAG laser of wavelength 1064 nm with beam energy of 3.5 mJ /
pulse, pulse width of 10 ns and repetition frequency of 10 Hz was applied to the upgraded
microcrystalline powdered samples which were tightly packed in a glass capillary. The NLO
crystals of chalcone derivatives were powdered and passed through a sieve and the powders
with grain size 75-100 μm were taken as samples in glass capillary tubes and a tight packing
was ensured with the aid of mechanical vibrator. The second harmonic wave of
532 nm
generated from the sample was detected by a photomultiplier tube (Hamamatsu-R 2059) and
converted into electrical signal. The converted electrical signal was displayed on an
oscilloscope (Tektronix-TDS 3052B). The signal amplitude in volts indicates the SHG
efficiency of the sample. Urea crystals ground into identical grain size were used as the
reference material.
Among the five chalcone derivatives only BTNP is able to crystallize in noncentrosymmetric
crystal structure and rest of them crystallized in centrosymmetric crystal structure. The SHG
efficiency of the urea was measured to be 250 mV and that of BTNP is 1.0 V. Those
molecules which possess centrosymmetric structure usually do not show SHG. It is also
noteworthy to mention here that some of the molecule though they possess centrosymmetric
crystal structure reported to exhibit SHG [135]. But no such SHG activity was observed in the
CTTMP, CTDMP, CTDCP and BTDMP compounds. The SHG efficiency of the chalcone
derivatives are listed along with the frequency dependent first order hyperpolarizability (βω)
using MOPAC 2012 semi empirical computer program [136] in Table 3.8. It was not possible
to measure the SHG efficiency of the crystals in the phase matching direction, since the
accurate measurement requires high precision polishing and antireflection coating on the
surface of the crystal plate. The damage thresholds of the crystals were measured for the Nd:
YAG laser beam (1064 nm), and are listed in Table 3.8. It is observed that most of the
chalcone derivatives have reasonably high damage threshold than the other nonlinear optical
crystals such as potassium dihydrogen phosphate (KDP) (0.20 GW/cm2), urea (1.50
GW/cm2), benzimidazole (1.71 GW/cm2) etc [137]. The damage pattern of most of the
chalcone derivative crystals shows tiny circular blobs surrounding the core of the damage.
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Such circular blobs are generally seen in crystals where the damage is mainly due to thermal
effects resulting in melting and solidification or decomposition of the material [138].
The detailed studies on molecular static first and second hyperpolarizability of chalcone
derivatives using MOPAC 2012 semi empirical computer program and structure-NLO
property relationship along with the experimental results is discussed in Chapter 4
(section
4.4.6).
Table 3.8: Molecular hyperpolarizability, SHG efficiency and Laser damage threshold
of the crystals of chalcone derivatives
Compound
βω (at 1064nm)
(1x10-26 esu)
SHG efficiency
(x Urea)
Laser damage
threshold GW/cm2
CTTMP
9.51
0
2.72
CTDMP
13.3
0
3.21
CTDCP
12
0
3.50
BTDMP
106
0
2.12
BTNP
405
4
3.88
The results show that the crystal BTNP can efficiently be used for up-conversion of IR
radiation into visible green light.
3.6.2 Third order NLO properties
The crystals were dissolved in AR grade Dimethylformamide (DMF) and the solution was
prepared with the concentration of 1x10–3 mol/L. This sample was taken in a quartz cuvette
for the Z-scan measurements. Single beam Z-scan technique [139] was employed to measure
the third order optical nonlinearities of the sample. This technique enables simultaneous
measurement of nonlinear refraction (NLR) and nonlinear absorption (NLA). Basically in this
technique a gaussian laser beam is focused using a lens (25 cm focal length) on the cuvette
containing the sample. The cuvette is translated across the focal region and changes in the farfield intensity pattern are monitored. The experiments were performed using a Q-switched,
frequency doubled Nd: YAG laser (Spectra-Physics GCR170) which produces 7 ns pulses at
532 nm, with a pulse repetition rate of 10 Hz and input peak-intensity of 2.39 GW/cm2.
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The laser beam waist at the focused spot was estimated to be 18.9 m and the corresponding
Rayleigh length is 2.11 mm. The measurements were carried out using a cuvette of 1 mm
thickness, which is less than the Rayleigh length. Hence the thin sample approximation is
valid [139]. The nonlinear transmission of the sample, with and without the aperture in front
of the detector was measured in the far-field using Laser probe Rj-7620 energy meter with
pyroelectric detectors.
The nonlinear transmission of compounds without aperture (open aperture) was measured in
the far field as the sample was moved through the focal point. This allows us to determine the
nonlinear absorption coefficient β. The open aperture curve of the samples CTTMP, CTDMP,
CTDCP, BTDMP and BTNP were shown in Figure 3.7 a, 3.8 a, 3.9 a,
3.10 a and 3.11 a
respectively. It is found that the transmission is symmetric with respect to focus (z = 0), where
it has a minimum transmission, showing an intensity dependent absorption effect. The shape
of the open aperture curve suggests that the compound exhibits two-photon absorption
[140-142]. From the open aperture Z-scan data, the measured values of nonlinear absorption
coefficient β and the imaginary part of third order nonlinear optical susceptibility
(3)
values
of the samples are given in Table 3.9.
To determine the sign and magnitude of nonlinear refraction, closed-aperture Z-scan was
performed by placing an aperture in front of the detector. The closed aperture Z-scan curve of
the samples CTTMP, CTDMP, CTDCP, BTDMP and BTNP were shown in Figure 3.7 b, 3.8
b, 3.9 b, 3.10 b and 3.11 b respectively. To obtain a pure nonlinear refraction curve we adopt
the division method described in literature [139]. The samples were found to exhibit
peak-valley characteristic, indicating negative nonlinear refraction or self-defocusing effect
shown in Figure 3.7 c, 3.8 c, 3.9 c, 3.10 c and 3.11 c respectively. From the pure nonlinear
refraction Z-scan data, the real part of third order nonlinear optical susceptibility
(3)
and
nonlinear refractive index n2 values were calculated and are given in Table 3.9.
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Figure 3.7 a: Open aperture curve of CTTMP crystal
Figure 3.7 b: Closed aperture curve of CTTMP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.7 c: Pure nonlinear refraction curve of CTTMP crystal
Figure 3.8 a: Open aperture curve of CTDMP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.8 b: Closed aperture curve of CTDMP crystal
Figure 3.8 c: Pure nonlinear refraction curve of CTDMP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.9 a: Open aperture curve of CTDCP crystal
Figure 3.9 b: Closed aperture curve of CTDCP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.9 c: Pure nonlinear refraction curve of CTDCP crystal
Figure 3.10 a: Open aperture curve of BTDMP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.10 b: Closed aperture curve of BTDMP crystal
Figure 3.10 c: Pure nonlinear refraction curve of BTDMP crystal
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.11 a: Open aperture curve of BTNP crystal
Figure 3.11 b: Closed aperture curve of BTNP crystal
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Chapter 3
Figure 3.11 c: Pure nonlinear refraction curve of BTNP crystal
Table 3.9: Third order optical nonlinearities for the NLO chalcone crystals
Sample
β
(cm/GW)
n2
(x10-11 esu)
Im χ3
(x10-13esu)
Re χ3
(x10-13esu)
CTTMP
3.586
–1.555
0.552
–1.665
CTDMP
3.057
–1.532
0.470
–1.642
CTDCP
2.083
–1.042
0.321
–1.116
BTDMP
-
-
-
-
BTNP
1.494
–0.696
0.230
–0.746
3.6.3 Optical limiting
The observed nonlinear absorption which originates from two-photon absorption can be
exploited for optical limiting applications. The optical limiting behavior of the compounds
CTTMP, CTDMP, CTDCP and BTNP extracted from the open aperture Z-scan curve shown
in Figure 3.12 a, b, c and d respectively. The samples behave linear until incident fluence of
104 J/m2 and transmittance decreases for higher incident fluencies, suggesting the occurrence
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Chapter 3
of optical limiting. The limiting threshold of the sample CTTMP for the concentration of
1x10–3 mol/L is 7x104 J/m2. The optical limiting behavior was found to be in the order of
compounds CTTMP > CTDMP > CTDCP >BTNP. Optical limiting was proved to vary
according to the extent of donor strength.
Figure 3.12 a: Optical limiting in CTTMP
Figure 3.12 b: Optical limiting in CTDMP
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.12 c: Optical limiting in CTDCP
Figure 3.12 d: Optical limiting in BTNP
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3.7 Dielectric properties of the crystals
The Dielectric studies were carried out using the crystal plates with surface area (A) of about
5-10 mm2 and small thickness (d) of about 1 mm were chosen. The opposite faces were
painted with silver paste to provide electrical contact through the copper wires attached to
them. The capacitance (C) and dielectric loss factor (tan δ) of the silver painted crystal plates
were measured using Semiconductor Characterization System (Kethely 4200SCS) for
frequencies ranging from 1 kHz to 5 MHz with an applied voltage 1 mV at laboratory
temperature. The dielectric constant ( ) values for various frequencies of the applied
alternating voltage can be computed using the relation
(3.2)
Figure 3.13 a, b, c, d, e shows the graph of the dielectric constant
(solid line) and the
dielectric loss factor tan δ (dashed line) at various frequencies for the crystal plates of
CTTMP, CTDMP, CTDCP, BTDMP and BTNP respectively. The graph shows that
and
tan δ values were not varying much with higher frequencies. Also the dielectric loss factor is
very low, except at low frequencies. The slightly higher values of
and tan δ at low
frequencies may be due to the presence of grain boundaries and defects in the crystals.
Figure 3.13 a: Dielectric constant and dielectric loss factor of CTTMP
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.13 b: Dielectric constant and dielectric loss factor of CTDMP
Figure 3.13 c: Dielectric constant and dielectric loss factor of CTDCP
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Synthesis and Characterization of Nonlinear Optical Crystals of
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Figure 3.13 d: Dielectric constant and dielectric loss factor of BTDMP
Figure 3.13 e: Dielectric constant and dielectric loss factor of BTNP
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Synthesis and Characterization of Nonlinear Optical Crystals of
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3.8 Conclusion
The laboratory grown single crystals of CTTMP, CTDMP, CTDCP, BTDMP and BTNP were
characterized by various techniques. The structures of these chalcone derivatives were
confirmed by CHN analysis, FTIR spectra, mass spectra and NMR spectra. The chalcone
derivatives are found to be stable, non-hygroscopic and do not decompose at room
temperature. The DSC plots show no solvent of crystallization in the grown crystals and are
stable till their melting points. The strong endothermic peaks on the DSC plots correspond to
the melting points of the crystals. The thermal stability of BTNP was found to be more
compared to other four chalcone derivatives and is found to increase with the increase in
acceptor strength. The measured density values of the crystals differ from the calculated
density by a very small fraction, may be due to the lattice distortions induced by the presence
of point defects in the crystals. The mechanical hardness studies reveal that these chalcone
single crystals are soft and require careful handling during the cutting and polishing process.
The change in the hardness value with the applied load may be due to the plastic flow of the
material. The optical quality does not degrade in these crystals as time elapses and this
indicates that the crystals were free from the solvent inclusion.
The grown crystals have large band-gap energy. Four varieties of the crystals CTDMP,
CTDCP, BTDMP and BTNP are transparent, having the optical absorption band only in the
UV- region. All the crystals are transparent in the visible region having pale yellow colour
because of the extension of their absorption band from UV-region to the blue-violet region.
Therefore the crystals are useful for laser applications.
Among the synthesized materials BTNP shows good SHG conversion efficiency
(4 times
that of urea). Owing to the high SHG efficiency, good chemical, thermal, mechanical and
laser damage resistance of BTNP makes it to be a better candidate for frequency doubling
applications down to 420 nm.
The third order NLO properties of all the five compounds were studied using single beam Zscan technique. Except BTDMP all other chalcone crystals exhibit good nonlinear refraction
as well as nonlinear absorption and hence the third order NLO properties found to vary
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according to the design of the molecule.The study infers that the chalcone derivatives are
superior NLO materials for second and third order NLO applications.
The dielectric constant and dielectric loss factor values of the crystals do not vary much with
the applied ac frequency except at lower frequencies, where it is slightly high, may be due to
the factors like grain boundaries and defects. The dielectric loss factors were found to be very
low for all the chalcone crystals.
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