Synthesis, characterization and structure–activity relationship of non-linear optical response of chalcone derivatives with in silico insights

Article abstract of DOI:10.1007/s11696-020-01487-6

Fifteen chalcone derivatives having D–A–D, D–A–A and A–A–D architectures have been synthesized by Claisen–Schmidt condensation reaction and characterized by UV–Vis, IR, ¹H-NMR, ¹³C-NMR and Mass spectrometry. In order to unambiguously establish the structure–activity relationship for the non-linear optical activity of these compounds, for the first time to our knowledge, we use the femtosecond degenerate four wave mixing (DFWM) technique to quantify and compare the third-order non-linear optical (NLO) activity of all the 15 compounds, under identical conditions. The second harmonics generation (SHG) efficiencies for all the compounds have also been evaluated using the Kurtz-Perry powder method. Among the compounds that we have synthesized here, the ones with A–A–D architecture show the highest NLO activity. Our results show that the NLO activity of a compound with A–A–D architecture can be further enhanced by incorporating a substituent with strong electron withdrawing ability on ring A and strong electron donating substituent on ring B. The results of the in silico studies that we have carried out correlate well with our experimental findings. The compounds (E)-3-(4-(dimethylamino)phenyl)-1-(4-nitrophenyl)prop-2-en-1-one with the compound code (4-N(CH₃)₂–4′-NO₂) and (E)-3-(4-methoxyphenyl)-1-(4-nitrophenyl)prop-2-en-1-one with the compound code (4-MeO-4′-NO₂) show the highest NLO activity among the compounds we have reported here. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11696-020-01487-6

Chemical Papers (2021) 75:2603–2615
https://doi.org/10.1007/s11696-020-01487-6
ORIGINAL PAPER
Synthesis, characterization and structure–activity relationship
of non‑linear optical response of chalcone derivatives with in silico
insights
Prashant Rai1 · Prajal Chettri2 · Swayamsiddha Kar1 · Malhar Anupam Nagar2 · Shailesh Srivastava2 ·
Nageswara Rao Golakoti1
Received: 29 September 2020 / Accepted: 19 December 2020 / Published online: 23 January 2021
© Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
Fifteen chalcone derivatives having D–A–D, D–A–A and A–A–D architectures have been synthesized by Claisen–Schmidt
condensation reaction and characterized by UV–Vis, IR, 1H-NMR, 13C-NMR and Mass spectrometry. In order to unam-
biguously establish the structure–activity relationship for the non-linear optical activity of these compounds, for the frst
time to our knowledge, we use the femtosecond degenerate four wave mixing (DFWM) technique to quantify and compare
the third-order non-linear optical (NLO) activity of all the 15 compounds, under identical conditions. The second harmon-
ics generation (SHG) efciencies for all the compounds have also been evaluated using the Kurtz-Perry powder method.
Among the compounds that we have synthesized here, the ones with A–A–D architecture show the highest NLO activity.
Our results show that the NLO activity of a compound with A–A–D architecture can be further enhanced by incorporat-
ing a substituent with strong electron withdrawing ability on ring A and strong electron donating substituent on ring B.
The results of the in silico studies that we have carried out correlate well with our experimental fndings. The compounds
(E)-3-(4-(dimethylamino)phenyl)-1-(4-nitrophenyl)prop-2-en-1-one with the compound code (4-N(CH3)2–4′-NO2) and (E)-
3-(4-methoxyphenyl)-1-(4-nitrophenyl)prop-2-en-1-one with the compound code (4-MeO-4′-NO2) show the highest NLO
activity among the compounds we have reported here.
Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s1169
6-020-01487-6.
* Nageswara Rao Golakoti
gnageswararao@sssihl.edu.in
1
Department of Chemistry, Sri Sathya Sai Institute of Higher
Learning, Prasanthi Nilayam, Andhra Pradesh 515134, India
2
Department of Physics, Sri Sathya Sai Institute of Higher
Learning, Prasanthi Nilayam, Andhra Pradesh 515134, India
Vol.:(0123456789)
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Chemical Papers (2021) 75:2603–2615
Graphical abstract
Keywords Chalcones · Synthesis and characterization · Second harmonic generation · Degenerate four wave mixing ·
Molecular modelling
donor and acceptor groups which can be modifed to tune
the non-linear response (Samoc et al. 1998; Marder et al.
1997; Norman, Luo, and Ågren 1998; Albota et al. 1998).
In the present study, we report the synthesis, characteri-
sation, Second Harmonic Generation (SHG) efciency and
Third Order Non-linear optical (NLO) properties of 15 chal-
cone derivatives having (D–A–D), (D–A–A) and (A–A–D)
architectures with the main focus on establishing the struc-
ture–activity relationship of these compounds. The third-
order non-linear optical studies were done using the Degen-
erate Four Wave Mixing (DFWM) technique, while the
SHG efciencies were characterized using the Kurtz-Perry
powder method. From the literature, we fnd that the NLO
measurements of four of the 15 chalcone derivatives that we
have reported here have been carried out using Z-scan tech-
nique (Ravindrachary et al. 2005); (Janardhana et al. 2013);
(Dharmaprakash 2017). A quick look at the existing litera-
ture tells one that the reported measured NLO properties of
compounds difer by more than an order of magnitude due to
the subtle diferences in the methods used, diferences in the
wavelengths used and diferences in the pulse widths used. It
is not possible to get the right structure–activity relationship
using such data. For efcient, uniform and consistent com-
parison of their structure-activity relationship, for the frst
time to our knowledge we report the NLO measurements of
all thee 15 chalcone derivatives by using the same technique,
at the same wavelength. For the third-order nonlinear meas-
urements carried out using dilute solutions, femtosecond
Introduction
In the quest of active non-linear optical materials for optical
devices, the organic compounds with delocalized systems
have been widely studied for their non-linear responses. The
reason behind these organic compounds taking the spotlight
in optical research lies in the fact that the organic compounds
can be easily polarised, provide greater fexibility in their
syntheses and also render the ability to tune the ultra-fast
response through structural modifcation (Kar et al. 2017;
Dhanuskodi et al. 2010). The substances with fast NLO
responses fnd their applications in optical devices which
include optical switching, optical limiting, optical storage,
etc. (Gupta et al. 2011; Perry et al. 1996). Researchers have
developed numerous design strategies such as donor–accep-
tor–donor (D–A–D), acceptor–donor–acceptor (A–D–A)
and donor–pi–donor (D–π–D) architectures for designing
the organic compounds with fast non-linear responses (Kiran
et al. 2014; He et al. 2008; Rajashekar et al. 2012).
Chalcones are known for their various biological activi-
ties (Yin et al. 2014; Liu et al. 2014; Kupcewicz et al. 2014;
Shenvi et al. 2013; Agnihotri et al. 2014) but are majorly
studied for their photonic applications(Shettigar et al. 2006;
Iliopoulos et al. 2012; Ravindra et al. 2007). These com-
pounds show high SHG efciencies and interesting non-lin-
ear properties like high absorption extinction coefcients in
the UV region (Abegão et al. 2016). Also, these compounds
possess a π conjugated bridge along with strong electron
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Scheme 1 Claisen–Schmidt condensation: R1 and R2 represent various substituents at diferent positions
pulses have been used since they rule out thermal nonlin-
earities which depend on the bulk thermal properties of the
solutions and do not truly refect the relationship between the
molecular structure and the nonlinear activities of the solute
molecules. Majority of the work done in this feld deals with
the study of non-linear optical activity of only one or two
organic/inorganic compounds at a time. Therefore, this work
on establishing structure–activity relationship of the synthe-
sized compounds with diferent architectures would lead to
a better understanding of the kind of substituents that could
be incorporated in the system for better non-linear optical
responses. In silico studies of these synthesized chalcone
derivatives have also been done to draw correlation with the
experimental fndings. Finally, thermal stability studies of
the new compounds with the highest optical nonlinearities
have also been carried out to validate their applicability in
the real world.
Experimental studies
Synthesis
The chalcone derivatives, except for those containing the
nitro groups, were synthesized by Claisen–Schmidt conden-
sation between aldehydes and ketones in a base catalysed
reaction (Scheme 1). 0.02 mol of aldehyde and 0.02 mol of
ketone were added to round-bottom fask containing 40 ml
of ethanol. 2.7 ml of 30% aqueous solution of NaOH was
then added to the reaction mixture which was then stirred at
room temperature for 12 h. The progress of the reaction was
monitored by Thin Layer Chromatography. On the comple-
tion of the reaction, the contents were poured in ice cold
water and the product was fltered. The solid obtained was
washed with water till neutral to litmus. The crude chalcone
was then dried and weighed and was recrystallized in chlo-
roform–methanol (Furniss 1989).
The chalcone derivatives with nitro groups were syn-
thesized by Claisen–Schmidt condensation in acid-cata-
lysed reaction (Scheme 1) as nitro compounds were found
to decompose in the base catalysed reaction. 0.02 mol of
aldehyde and 0.02 mol of ketones (4-nitroacetophenone or
3-nitroacetophenone) were added to round-bottom fask con-
taining 20 ml acetic acid saturated with HCl gas. The reac-
tion mixture was then stirred at room temperature for 12 hrs.
The progress of the reaction was monitored by TLC. On the
completion of reaction, the contents were poured in ice cold
water and the product was fltered. The solid obtained was
Fig. 1 General structure of the synthesized compounds
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Chemical Papers (2021) 75:2603–2615
Fig. 2 Structures of the synthesized compounds
washed with water till neutral to litmus. The crude chal-
cone was then dried and weighed and was recrystallized in
chloroform–methanol (Furniss 1989) (Figs. 1, 2). In case of
4-N(CH3)2-4′-NO2, the contents of the reaction were neutral-
ized with aq. NaOH before fltration.
Instrumentation
1H-NMR spectra were recorded on VARIAN 200 MHz
instrument. The sample preparation was done using CDCl3
as a solvent and TMS as the internal reference. 13C-NMR
spectra were recorded in Bruker Avance 100 MHz instru-
ment using CDCl3 as the solvent and TMS, the internal ref-
erence. The FT-IR spectra were recorded on Thermo-Nicolet
The IUPAC names and other details of the synthesized
compounds are given in Table 1.
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Table 1 Synthesized chalcone derivates with compound code, IUPAC name, architecture type and melting point
Structure Compound code
number
IUPAC name of chalcone derivatives
Architecture type Melting point (°C)
(1)
(2)
(3)
(4)
(5)
(6)
4-MeO-4′-NO2
4-MeO-4′-Cl
4-MeO-3′-NO2
4-MeO-4′-MeO
4-MeO-4′-Br
4-MeO-4′-Me
(E)-3-(4-methoxyphenyl)-1-(4-nitrophenyl)prop-2-en-1-one
(E)-1-(4-chlorophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one
(E)-3-(4-methoxyphenyl)-1-(3-nitrophenyl)prop-2-en-1-one
(E)-1,3-bis(4-methoxyphenyl)prop-2-en-1-one
(E)-1-(4-bromophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one
(E)-3-(4-methoxyphenyl)-1-(p-tolyl)prop-2-en-1-one
A–A–D
A–A–D
A–A–D
D–A–D
A–A–D
D–A–D
175.2–176.0
117.3–117.9
172.4–173
102.0–102.9
142.2–142.6
93.8–94.4
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
4-N(CH3)2-4′-MeO (E)-3-(4-(dimethylamino)phenyl)-1-(4methoxyphenyl)prop-2-en-1-one D–A–D
125.0–126.0
119.5–119.9
188.5–188.9
138.5–139.0
74.5–75.0
140.3–140.7
166.2–167.0
153.4–153.9
78.1–78.4
4-N(CH3)2-4′-Me
(E)-3-(4-(dimethylamino)phenyl)-1-(p-tolyl)prop-2-en-1-one
D–A–D
A–A–D
A–A–D
A–A–D
D–A–A
D–A–A
D–A–A
D–A–D
–
4-N(CH3)2-4′-NO2 (E)-3-(4-(dimethylamino)phenyl)-1-(4-nitrophenyl)prop-2-en-1-one
4-N(CH3)2-4′-Cl
4′-NO2C
4-NO2C
(E)-1-(4-chlorophenyl)-3-(4-(dimethylamino)phenyl)prop-2-en-1-one
(E)-1-(4-nitrophenyl)-3-phenylprop-2-en-1-one
(E)-3-(4-nitrophenyl)-1-phenylprop-2-en-1-one
4-NO2-4′-MeO
3-NO2-4′-MeO
4-MeOC
(E)-1-(4-methoxyphenyl)-3-(4-nitrophenyl)prop-2-en-1-one
(E)-1-(4-methoxyphenyl)-3-(3-nitrophenyl)prop-2-en-1-one
(E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one
(E)-1,3-(Bisphenyl)- prop-2-en-1-one
Chalcone
55.5–56.0
Fig. 3 Experimental confguration of degenerate four-wave mixing using folded-box CARS geometry
Avatar 370 spectrophotometer between 400 and 4000 cm−1
Non‑linear optical measurements
using KBr pellet. The UV–Vis spectra of the chalcone deriv-
atives were recorded using Shimadzu 2450 spectrophotom-
eter in the range of 200–800 nm. For mass spectra, Shi-
madzu-Q2010MS GC–MS instrument was employed. The
thermal studies were carried out in Mettler Toledo TGA/
DSC1 STARe System.
SHG efciency
The SHG efciency measurements of the synthesized chal-
cone derivatives were done using the Kurtz-Perry method
(Giuliani et al. 1971) taking urea as a reference. The
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Chemical Papers (2021) 75:2603–2615
Fig. 4 Arrangement of beams
in folded Box-CARS geom-
etry (Taylor and Saha 2007). a
Two-dimensional view of the
geometry; b Three-dimensional
view of the geometry
Nd-YAG laser having a fundamental wavelength of 1064 nm,
pulse width of 10 ns and a repetition rate of 10 Hz was used
for the Second Harmonic Generation studies for the chalcone
derivatives. The microcrystalline powder taken in a capillary
tube was exposed to the beam which was steered to be inci-
dent on the sample. The Second Harmonic (532 nm) gener-
ated was detected by a photomultiplier tube which converts
the SHG signal to an electrical signal. This electrical signal
gets displayed on the oscilloscope.
signal, well separated from the other three beams and is par-
ticularly suited for femtosecond applications. In this con-
fguration, the two pumps and the probe are aligned parallel
to each other before focusing onto the sample. The align-
ment is made in such a way that the three incident beams
form the three corners of a square on the screen. The folded
box CARS geometry gives us the advantages of easy phase
matching (Saha 1995). The output signal was detected using
a custom-built lock-in technique that used an Arduino micro-
controller interfaced to a computer. Since the nonlinear sus-
ceptibilities are sensitively dependent on the values of the
refractive indices of the compounds, we used a laser-based
lateral shift imaging technique (Singh 2002) for measuring
these values. With some averaging, this technique is accurate
to the third decimal and lends itself to automation.
Third‑order NLO studies
The third-order non-linear studies of the synthesized chal-
cone derivatives were done using the femtosecond DFWM
technique (Sahraoui et al. 1998; Sahraoui and Rivoire
1997). The experimental confguration of DFWM is shown
in Fig. 3. This process is known as degenerate since all the
four interacting waves have the same frequency. In four
wave mixing, the magnitude of the third-order non-linear
susceptibility is proportional to the strength of the fourth
beam. Thus, measurements of the observed signal yields
information about the χ(3) tensor components of the medium.
The molecular second-order hyperpolarizability (γ) can then
be computed knowing the concentrations and the refractive
indices. The laser system used for the DFWM studies was
a frequency doubled femtosecond ytterbium-doped fber
laser (MenloSystems-BlueCut) with a pulse width less than
500 fs at 515 nm. For our study, we employed the so-called
folded-Box CARS geometry. The arrangement of beams in
folded-Box CARS geometry is shown in Fig. 4. This con-
fguration gives the advantage of having a background free
The solutions of 14 chalcone derivatives in chloroform
having the concentration 0.05 mM were prepared for carry-
ing out the study. In case of 4-N(CH3)2-4′-NO2C, 0.005 mM
solution in chloroform was considered in order to avoid the
optical dispersion due to its intense colour. The third-order
susceptibility of the chalcone derivatives was calculated by
using Eq. 1:
4
ꢁ2dif
(
)
2
2ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Idif
=
ꢃ(3) IprIpu1Ipu2
,
ꢀ
(
)
(1)
ijkl
ꢀ
nc
ꢂ ꢁdif
whereꢀ(3) is the third-order nonlinear susceptibility, Ipr,
Ipu1 and Ipu2 are the intensities of the input pumps and the
probe beams, n is the refractive index, ωdif is the difracted
frequency and ꢀ is the dielectric constant of the material
(Caro and Gower 1982).
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Chemical Papers (2021) 75:2603–2615
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The second molecular hyperpolarizability (ꢀ) was calcu-
as doublet. Similarly, the proton H-7 was observed in the
range of δ 7.80–7.84 as a doublet. The coupling constants
of these protons ranged between J=14 and 16 Hz indicat-
ing that these are trans to each other. The proton at H-7
had greater chemical shift as it is in the β position with
respect to the carbonyl group. The protons H-2′ and H-6′,
H-3′ and H-5′, H-2 and H-6, H-3 and H-5 were found to
be equivalent in case of para substitutions. The chemical
shifts of these protons are dependent on the substituents
present. In case of methoxy group as a substituent, the shift
of protons at H-3′ and H-5′ and/or H-3 and H-5 were seen
in the range δ 7.1–7.3 due to the shielding from the meth-
oxy group. Similarly, for chalcones containing –NO2 group,
H-3′ and H-5′ and/or H-5 and H-3 were seen in the range
of δ 8.20–8.40 due to the electron withdrawing efect of
the –NO2 substituent. The protons attached to the methoxy
group were observed as a singlet between δ 3.10–3.20. The
ones attached to the –N(CH3)2 group were seen ~ δ 3.05.
The chemical shift of the protons attached to the carbon of
methyl group were seen between δ 2.3–2.7.
lated using Eq. 2.
ꢁe(ff3)(solution) − ꢁe(ff3)(solvent)
(2)
ꢀ =
,
Nf4
where N is the number density of the solute molecules,
n2+2
f =
is the local feld factor (this is due to the diference
3
between the applied macroscopic electric feld and the efec-
tive electric feld, or Lorentz local feld, that each atom or
molecule experiences) and n is the refractive index of the
solution. An Arduino microcontroller-based fully automated
refractive index measurement technique that utilizes the lat-
eral displacement of a light beam through the sample was
set up and used to measure the refractive index to the third
decimal. The details of this will be discussed elsewhere.
The average powers of the two pump beams were 22.3
and 59.7mW, respectively, while the probe beam had an
average power of 1.975mW.
The 13C-NMR spectra of these compounds were analysed.
The carbonyl carbon was observed at around δ 188. The aro-
matic carbon signals were observed in the range δ 120–165.
For the compounds with the methoxy substituents, the aro-
matic carbon attached to the methoxy group was observed
in the range of δ 160–165. In case of the compounds with
nitro groups, the aromatic carbon attached to the nitro group
appeared at around δ 154. Similarly, the aromatic carbon
attached to the –N(CH3)2 group was observed in the range of
δ 150–152 and the carbon at the beta position to the carbonyl
group appeared at around δ 145. The carbon at the alpha
position to the carbonyl group was seen in the range of δ
118–122. The methoxy carbon was observed at around δ 55
and N,N-dimethylamino carbons were observed at around
δ 40.
Software strategies for the computational studies
The modelling of the chalcone derivatives was done using
Gaussview5 (Dennington et al. 2003) and the energy mini-
mization was carried out using Gaussian09. The chalcone
analogues were drawn using the GaussView panel and
cleaned appropriately. The molecules were then input into
Gaussian and optimized (Opt + Freq) job was considered
using the ground state DFT method with the B3LYP func-
tional and the 6-31G (d, p) basis set to a default condition.
The charge was 0 and the spin state was set to singlet. For
the bromo-substituted chalcone derivative, DFT method was
used with the B3PW91 functional and the LanL2MB basis
set to a default condition, as the core and inner shell elec-
trons come into play and therefore there was a necessity to
factor diferent electronic condition.
The UV–Vis spectra showed absorptions at different
wavelengths corresponding to the absorption by aromatic
rings at a lower wavelength at around 205 nm, followed
by the absorption by carbonyl group (π → π* transition)
between 320 and 430 nm. In the IR spectra, some of the
prominent peaks were observed at 1650–1660 cm−1 indi-
cating the presence of a carbonyl group. The three bands
of medium intensity were observed between 1500 and
1600 cm−1 which corresponds to aromatic C = C stretch.
In case of 1, 4-disubstituted chalcones, peak was observed
at 815–830 cm−1 (aromatic C–H oop bending). The pres-
ence of methoxy groups as substituent can be concluded
by the peak at 1100–1300 cm−1. The peak was observed
at 1000–1300 cm−1 when dimethyl amine was one of the
substituents present in the chalcone derivatives. This is due
to C–N stretching. In these chalcone derivatives, peak due to
sp2 C–H stretch is observed between 3050 and 3010 cm−1.
Results and discussion
Characterization
1
The H-NMR spectra of these compounds were analysed.
The proton H-8 was observed in the range of δ 7.62–7.72
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Chemical Papers (2021) 75:2603–2615
Table 2 SHG efciency
of synthesized chalcone
Structure
number
Compound code
SHG value
with respect to
urea
derivatives with respect to urea
Methoxy group compounds
(Methoxy group on Ring B)
(5)
(1)
(2)
(4)
(6)
(3)
(7)
(8)
4-MeO-4′-Bra
4-MeO-4′-NO2
4-MeO-4′-Cld
20
5
1.4
0.9
0.5
0
4
0.5
0
c
4-MeO-4′-MeOa
4-MeO-4′-Meb
4-MeO-3′-NO2
4-N(CH3)2-4′-MeOa
4-N(CH3)2-4′-Me
4-N(CH3)2-4′-NO2
4-N(CH3)2-4′-Cle
4-MeOa
N, N-dimethyl amino group compounds
(N, N-dimethyl amino group on Ring B)
(9)
(10)
(15)
(12)
(11)
(13)
(14)
(16)
0
Other compounds in a series
8.5
2.1
0
0
0
4-NO2C
4′-NO2C
4-NO2-4′-MeOa
3-NO2-4′-MeO
Chalconea
0
SHG efciencies of some of the chalcone derivatives have been already reported in the literature
aReported by (Zhao et al. 2000)
bReported by (Ravindrachary et al. 2005
cReported by (Dharmaprakash 2017)
dReported by (Prabu et al. 2014
eReported by (Janardhana et al. 2013)
The mass spectra of the compounds showed molecular
ion peak as [M + 1]+ peaks. In case of 4-N(CH3)2-4′Cl,
[M+2] + peak with 33% intensity was observed indicating
the presence of chloro group. Similarly, the mass spectrum
of 4MeO-4′Br showed [M]+ and [M+2]+ peaks of equal
intensities indicating the presence of bromo group.
4′-Cl<4-MeO-4′-NO2. Thus, the SHG efciency was found
to increase with the increase in the electron withdrawing
ability of the substituent group in ring A, thereby show-
ing that A-A-D architecture (4-MeO-4′-NO2 and 4-MeO-
4′-Cl) gives better SHG efciency than D–A–D architecture
(4-MeO-4′-Me and 4-MeO-4′-MeO) for the methoxy group
of compounds. Also, 4MeO-3′NO2 has no SHG efciency,
whereas its isomer 4-MeO-4′-NO2 shows SHG efciency
fve times that of urea. This shows that the position of the
substituent in ring A also afects the SHG efciency of a
given compound.
SHG efciency
The values obtained from the SHG measurements of the
chalcone derivatives that we have studied are shown in
Table 2
The study of SHG efciency of the compounds with N,
N-dimethylamino group fxed at one end of the ring (B ring)
and various substituents incorporated at the other end (A
ring) has been done. The SHG efciency increases in the
order of compounds: 4-N(CH3)2-4′-​NO2 ~4-N(CH3)2–4′-​Cl
<4-N(CH3)2–4′-Me<4-N(CH3)2–4-MeO. This leads us to
the conclusion that in case of the compounds with dimethyl
amino group, the SHG efciency increases with the increase
in the electron donating ability of the substituent group at
ring A rendering superior SHG efciency of compounds wit
h dimethyl amino group having D–A–D architecture (4-N
(CH3)2-4′​-Me and 4-N(CH3)2-4′-MeO) over their A–A–D
(4-N(CH3)2-4′-NO2 and 4-N(CH3)2-4′-Cl) counterpart.
Apart from the non-centrosymmetric crystal structure,
the SHG efciency of a molecule is infuenced by various
factors that include conjugation, π–π stacking of the aro-
matic moieties and hydrogen bonding (Kiran et al. 2014).
The compounds that we have reported here exhibit cross
conjugation between the benzene–π electrons and the car-
bonyl compound.
It was previously reported in the literature that the com-
pound 4-MeO-4′-Br is 20 times more active than urea.
The compounds with bromo group as a substituent exhibit
greater SHG efects (Zhao et al. 2000). In our present study,
the SHG efciency was found to increase in the order of
compounds 4-MeO-4′-Me < 4-MeO-4′-MeO < 4-MeO-
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Table 3 Third-order non-linear susceptibility, second-order hyperpolarizability and refractive index (in CHCl3) of the synthesized chalcone
derivatives
Structure
number
Compound code
χ3 (10–14 esu)
Second order hyperpo-
larizability (10–31 esu)
Refractive index
Methoxy group compounds
(Methoxy group in Ring B)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
4-MeO-4′-NO2
4-MeO-4′-Cl
4-MeO-3′-NO2
4-MeO-4′-MeO
4-MeO-4′-Br
4-MeO-4′-Me
4-N(CH3)2-4′-MeO
4-N(CH3)2-4′-Me
3.37 0.52
3.35 0.53
3.30 0.42
3.24 0.41
2.97 0.45
2.94 0.44
3.16 0.45
3.02 0.46
2.99 0.47
2.99 0.47
3.38 0.39
3.14 0.45
3.35 0.42
3.02 0.47
3.50 0.41
3.32 0.42
2.03 0.05
1.95 0.03
1.53 0.36
1.28 0.40
0.42 0.27
0.32 0.28
1.06 0.28
0.60 0.22
4.36 1.80
0.48 0.19
1.72 0.46
0.98 0.28
1.71 0.38
0.59 0.20
2.23 0.43
1.57 0.39
1.485 0.002
1.486 0.002
1.433 0.002
1.418 0.002
1.418 0.002
1.413 0.002
1.433 0.002
1.431 0.002
1.431 0.002
1.432 0.002
1.418 0.002
1.432 0.002
1.434 0.002
1.433 0.002
1.437 0.002
1.429 0.002
N,N-dimethylamino group compounds
(N,N-dimethylamino group in Ring B)
*
(9)
4-N(CH3)2-4′-NO2
4-N(CH3)2-4′-Cl
4′-NO2C
(10)
(11)
(12)
(13)
(14)
(15)
(16)
Other compounds in a series
4-NO2C
4-NO2-4′-MeO
3-NO2-4′-MeO
4-MeOC
Chalcone
*The NLO measurement of 4-N(CH3)2-4′-NO2 was done at 0.005 mM concentration solution in CHCl3 in order to avoid the optical dispersion
due to its intense colour
For all other compounds, NLO measurement was done at 0.05 mM concentration solution in CHCl3
An interesting observation was made in the case of the
compounds 4-NO2C and 4′-NO2C. The SHG values of these
two compounds are found to be 2.16 and 0.01 times that
of urea, respectively. This indicates that the position of a
substituent in a particular ring (ring A or B) also afects the
SHG efciency of a molecule. The presence of nitro group
in A ring renders greater SHG efciency than its presence
in the B ring. Though, as mentioned earlier, SHG efciency
of a molecule is mainly infuenced by its crystal structure,
this study led us to the conclusion that the substituents and
their positions introduced in a molecule also have an efect
on its SHG efciency.
of compounds with A–A–D architecture, the second-order
hyperpolarizability increases in the order of compounds
4-MeO-4′-Br < 4-MeO-3′-NO2 < 4-MeO-4′-Cl < 4-MeO-
4′-NO2. The greater the electron accepting ability of a
substituent group at 4′ position (in A-ring), the greater the
second-order hyperpolarizability. The extended conjugation
in case of 4-MeO-4′-NO2 results in its greater second-order
hyperpolarizability value than 4-MeO-3′-NO2. Evaluating
the methoxy group of compounds with D–A–D architecture
where methoxy group is fxed in ring B, the second-order
hyperpolarizability value was found to increase with the
increase in the electron donating ability of the substituent at
ring A. Thus 4-MeO-4′-MeO showed greater second-order
hyperpolarizability value than 4-MeO-4′-Me. However, it
was observed that 4-MeOC has highest second-order hyper-
polarizability among the methoxy groups of compounds.
The simple chalcone was also found to have an appreciable
third-order optical activity.
Third order non‑linear optical susceptibility
measurements
The experimental results of the chalcone derivatives that we
have reported here are shown in the Table 3.
The chalcone derivatives we have reported here have
D–A–A, D–A–D and A–A–D architectures. Considering
the molecules with A–A–D architecture, methoxy group
is incorporated in ring B and at the other end on ring A,
the incorporated substituent group is varied in terms of its
electron withdrawing ability. It has been observed that the
increase in the electron withdrawing/accepting ability of a
substituent group in A–A–D type molecules result in better
second-order hyperpolarizability value and hence the better
third-order non-linear optical activity. In the methoxy group
Evaluating p-dimethyl amino group of chalcone deriva-
tives, the second-order hyperpolarizability of the mol-
ecules of type A–A–D, in which p-dimethyl amino group
was attached at ring B, was observed to increase with the
increasing electron withdrawing ability of the substituent
group at ring A. Thus 4-N(CH3)2-4′-NO2 showed greater
second-order hyperpolarizability than 4-N(CH3)2-4′-Cl. In
case of the p-dimethyl amino group of compounds of type
D–A–D, the second-order hyperpolarizability was observed
to increase with the increase in electron donating ability
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Chemical Papers (2021) 75:2603–2615
of the substituent group at A ring. Experimentally, it was
observed that 4-N(CH3)2-4′MeO showed greater second-
order hyperpolarizability than 4-N(CH3)2-4′Me.
4-MeO-4′-MeO, 3-NO2-4′-MeO and 4-N(CH3)2-4′-Cl have
been reported as − 1.923 × 10−13esu, − 1.619 × 10−13esu
and − 1.89×10–20 m2 V−2, respectively. For the compound
4-MeO-4′-NO2, the second-order hyperpolarizability has
been reported to be 2.9×10–31 esu. In our study, we obtained
almost the same second-order hyperpolarizability value of
2.031×10−31esu for 4-MeO-4′-NO2. The third-order suscep-
tibility values of the synthesized compounds that we have
reported here are in the order of 10−14esu.
For the molecules with D–A–A architecture, it was
observed that the non-linear optical activity of a compound
increases with the increase in the electron donating abil-
ity of a substituent on ring A as observed in case of the
compounds 4-NO2-4′-MeO and 4-NO2C, where the former
has greater second-order hyperpolarizability than the latter.
Also, it was experimentally found that 4-NO2-4′-MeO has
greater non-linear activity than 3-NO2-4′-MeO. This clearly
indicates that the position of the substituent in the ring also
infuences the non-linear activity of the compound. The
presence of extended conjugation in 4-NO2-4′-MeO results
in its greater second-order hyperpolarizability than that of
3-NO2-4′-MeO. The diference in the non-linear optical
activity was also observed when the same substituent group
was incorporated on one of the either rings (ring A or ring
B) as in case of 4′-NO2C (A–A–D architecture) and 4-NO2C
(D–A–A architecture). 4′-NO2C, i.e. nitro group when intro-
duced in the ring A, showed better non-linear activity than
4-NO2C where nitro group is placed on the ring B.
For this experiment, carbon disulphide was used as a ref-
erence to optimize the setup. The third-order susceptibility
value and the refractive index of carbon disulphide were
found to be (5.30 0.38 × 10–12) esu and (1.63 0.002),
respectively. Thus, the work we have reported here gives an
idea as to what kind of substituent group should be intro-
duced in the most suitable position so as to design and syn-
thesize the organic compounds with good non-linear optical
activity.
Thermal studies of 4‑N(CH3)2‑4′NO2
and 4‑MeO‑4′NO2
Among the synthesized compounds that we have reported
here, the compounds having A–A–D architecture showed
better non-linear optical activity. From this study, we con-
clude that the electron withdrawing group and the electron
donating group should be incorporated in ring A and ring
B, respectively, at the para positions for better non-linear
optical activity.
The thermal stability of a nonlinear compound is critically
important when it comes to real-world applications. We,
therefore, carried out the thermal analysis of 4-N(CH3)2-
4′-NO2 and 4-MeO-4′-NO2 as these showed higher NLO
activities than all the other compounds that we have
reported here. The TGA–DSC analysis of the dried crystals
of 4-N(CH3)2-4′-NO2 and 4-MeO-4′-NO2 were performed
under nitrogen fow of 100 ml/min. The equilibrate tempera-
ture was 30 °C and the instrument was heated up to 600 °C
at heating rate of 10 °C/min.
The NLO measurements of the four chalcone derivatives
4-MeO-4′-MeO, 3-NO2-4′-MeO, 4-N(CH3)2-4′-Cl, 4-MeO-
4′-NO2 have been reported in the literature where the meas-
urements have been done by using Z-scan technique (Ravin-
dra et al. 2007); (Janardhana et al. 2013); (Dharmaprakash
2017). In the mentioned studies, the Re χ3 values of
The results obtained from the thermal analysis
are depicted in Fig. 5.The endothermic peak in DSC
curves shown in Fig. 5. implies that the compounds
Fig. 5 TGA–DSC Curve for: a 4-N(CH3)2-4′-NO2 and b 4-MeO-4′-NO2
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Chemical Papers (2021) 75:2603–2615
2613
Table 4 Parameters obtained
from in silico studies of
the synthesized chalcone
derivatives
Structure
number
Compounds
Energy gap
Static dipole
Ionisation
Hardness (AU)
(AU) in CHCl3 moment (in
Debye)
potential (AU)
− 9
− 10
− 7
− 8
− 3
− 13
− 1
− 11
− 14
− 2
− 6
− 4
4-N(CH3)2-4′-NO2
4-N(CH3)2-4′-Cl
4-N(CH3)2-4′-MeO
4- N(CH3)2-4′-Me
4-MeO-3′-NO2
4-NO2-4′-MeO
4-MeO-4′-NO2
4′-NO2C
3-NO2-4′-MeO
4-MeO-4′-Cl
4-MeO-4′-Me
4-MeO-4′-MeO
4-NO2C
0.0918
0.1174
0.1224
0.1211
0.1244
0.1244
0.1247
0.1296
0.1332
0.1365
0.1399
0.141
13.1647
5.1217
6.0261
7.8197
10.5973
7.0817
7.4651
7.2269
10.0639
6.1478
4.1085
2.346
0.19514
0.19212
0.18878
0.1898
− 0.0459
− 0.0587
− 0.0612
− 0.0606
− 0.0622
− 0.0622
− 0.0624
− 0.0648
− 0.0666
− 0.0682
− 0.07
− 0.0705
− 0.0713
− 0.073
− 0.0695
− 0.0759
0.21978
0.23291
0.20953
0.23972
0.23194
0.21713
0.21457
0.21312
0.25326
0.14505
0.21549
0.23394
− 12
− 5
− 15
− 16
0.1426
0.1459
0.1389
0.1519
5.4762
7.2976
4.4986
3.8602
4-MeO-4′-Br
4-MeOC
Chalcone
Fig. 6 Plot of calculated band gap vs experimentally obtained second-order hyperpolarizability(γ) for: a N, N-dimethyl amino series of com-
pounds; b Methoxy series of compounds
4- N(CH3)2-4′-NO2 and 4-MeO-4′-NO2 undergo irrevers-
ible endothermic transition at 188 and 175 °C, respec-
tively. The peak of the endothermic transition represents
the melting point of the said compound which is 188.5 °C
for 4-N(CH3)2-4′-NO2 and 175.2 °C for compound 4-MeO-
4′-NO2. There is no phase transition of the aforementioned
compounds observed before melting. The exothermic peak
observed at 290 °C for 4-N(CH3)2-4′-NO2 and 366 °C for
4-MeO-4′-NO2 indicates that the compounds 4-N(CH3)2-
4′-NO2 and 4-MeO-4′-NO2 undergo decomposition at 290
and 366 °C, respectively.
Results obtained from computational studies
The in silico studies of the reported chalcone derivatives
have been done. The values of global reactivity parameters
like dipole moment, ionisation potential and hardness that
are related to the energy gap of a molecule have been shown
in Table 4. Lower energy gap corresponds to higher polariz-
ability which in turn results in the better non-linear activity
of a molecule. As is already known, the compounds with
larger HOMO–LUMO energy gap are considered to be hard
molecules having lower reactivity. Thus, the lower the chem-
ical hardness of a molecule, the better is its NLO response
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Chemical Papers (2021) 75:2603–2615
(Khazaal et al. 2020; Khalid et al. 2020).Our experimental
results follow the same trend.
Our results show that the NLO activity of a compound with
A–A–D architecture can be further enhanced by incorporat-
ing a substituent with strong electron withdrawing ability in
ring A and strong electron donating substituent in ring B.
The results of the in silico studies that we have carried out
correlate well with our experimental fndings. The struc-
ture activity relationship has mainly been established on the
basis of the efect of substituent incorporated on the non-
linear response of a compound which eventually enabled
us to determine the best architecture for better non-linear
response of the chalcone derivatives. 4-N(CH3)2-4′-NO2 and
4-MeO-4′-NO2 show the highest NLO activity among the
compounds we have reported here and hence these com-
pounds can be explored for their photonic applications.
From the in silico studies, for the methoxy group of com-
pounds, the energy gap was found to increase in the order
4-MeO-4′-NO2 <4-MeO-4′-Cl<4-MeO-4′-Me<4-MeO-4′-
MeO<4-MeO-4′-Br. It was observed that the compounds
having lowest energy gap showed the highest third-order
susceptibility value obtained experimentally. The correla-
tion between the calculated band gap and the experimentally
obtained second-order hyperpolarizability value has been
shown in Fig. 6. The fact that large diferences in the energy
gap results in a noticeable change in the hyperpolarizability
values is evident in these experimental observations.
For p-dimethyl amino group compounds, the energy gap
was found to increase in the order of compounds 4-N(CH3)2-
4′-NO2 <4-N(CH3)2-4′-Cl<4-N(CH3)2-4′-Me<4-N(CH3)2-
4′-MeO. Thus, the low energy gap obtained computationally
for 4- N(CH3)2–4′- NO2 correlates with our experimental
fnding of the said compound having greater non-linear
activity than the other compounds in p-dimethyl amino
series as is shown in Fig. 6. Similarly, 4′-NO2C was found
to have smaller energy gap than 4-NO2C which is in cor-
relation with our experimental fnding of 4′-NO2C having
greater non-linear activity than 4-NO2C.
Acknowledgements The authors dedicate this work to Bhagwan Sri
Sathya Sai Baba, the Founder Chancellor of SSSIHL, for being a con-
stant source of inspiration. The authors thank Central Research Instru-
ments Facility (CRIF), SSSIHL for providing all the facilities necessary
for carrying out this work.
Compliance with ethical standards
Conflict of interest The author states no confict of interest.
The compound 4-N(CH3)2-4′-NO2 which was found to
have the highest third-order susceptibility among all the
synthesized compounds reported here showed the highest
dipole moment value. Among the methoxy group of com-
pounds, 4-MeO-4′-NO2 and 4-MeO-3′-NO2 showed dipole
moments greater than other methoxy group of compounds
like 4-MeO-4′-Br, 4-MeO-4′-Cl, 4-MeOC and 4-MeO-
4′-Me. The reactivity parameters like ionisation potential
and hardness correlate with the energy gap and hence the
non-linear optical activity.
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