Synthesis and nonlinear optical properties of push-pull type stilbene and pyrazoline based chromophores

Article abstract of DOI:10.1016/j.dyepig.2017.04.009

In this work we present the synthesis, spectroscopic and nonlinear optical properties of push-pull type organic dyes based on 1-phenyl-2-pyrazoline or 4-(dimethylamino)phenyl chromophores as a donor part, and bridged by a double bound ortho- or meta-subst

Full text of DOI:10.1016/j.dyepig.2017.04.009

Accepted Manuscript
Synthesis and nonlinear optical properties of push-pull type stilbene and pyrazoline
based chromophores
A. Szukalski, K. Parafiniuk, K. Haupa, W. Goldeman, B. Sahraoui, F. Kajzar, J.
Mysliwiec
PII:
S0143-7208(17)30182-1
10.1016/j.dyepig.2017.04.009
DYPI 5904
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 January 2017
Revised Date: 4 April 2017
Accepted Date: 5 April 2017
Please cite this article as: Szukalski A, Parafiniuk K, Haupa K, Goldeman W, Sahraoui B, Kajzar F,
Mysliwiec J, Synthesis and nonlinear optical properties of push-pull type stilbene and pyrazoline based
chromophores, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.04.009.
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Synthesis and nonlinear optical properties of push-pull type
stilbene and pyrazoline based chromophores
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A. Szukalski1,@, K. Parafiniuk1, K. Haupa2, W. Goldeman3,
B. Sahraoui4, F. Kajzar5 and J. Mysliwiec1
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1 Faculty of Chemistry, Advanced Materials Engineering and Modelling Group,
Wroclaw University of Science and Technology, Wyb. Wyspianskiego 27, 50-320 Wroclaw
2 Department of Applied Chemistry and Institute of Molecular Science,
National Chiao Tung University, Hsinchu 30010, Taiwan
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3 Faculty of Chemistry, Department of Organic Chemistry,
Wroclaw University of Science and Technology, Wyb. Wyspianskiego 27, 50-320 Wroclaw
4 Laboratoire MOLTECH-Anjou, Université d'Angers, CNRS UMR 6200,
2 Boulevard Lavoisier, 49045 Angers, France
5 Faculty of Applied Chemistry and Materials Science,
University Politehnica of Bucharest, Polizu Street No 1, 011061, Bucharest, Romania
Corresponding author e-mail: adam.szukalski@pwr.edu.pl

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Abstract
In this work we present the synthesis, spectroscopic and nonlinear optical properties of push-
pull type organic dyes based on 1-phenyl-2-pyrazoline or 4-(dimethylamino)phenyl
chromophores as a donor part, and bridged by a double bound ortho- or meta- substituted
nitrophenyl group as an acceptor. For the optical measurements we have used the poly(methyl
methacrylate) based thin films doped with the dyes. Third harmonic generation measurements
were performed under the picosecond laser regime with excitation wavelength at 1064 nm.
The experimental results confirmed that investigated systems can be used as the efficient
nonlinear optical systems for harmonic of light generation (tripled but also doubled
frequency). It has also been proved the strong dependence between chemical structure
(position of electron-acceptor moiety as well as the type of electron-donor unit) and nonlinear
optical (NLO) response coming from series of isomers and derivatives.
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Keywords: organic dyes, push-pull molecules, nonlinear optics, pyrazoline, Horner–
Wadsworth–Emmons (HWE) reaction
1. Introduction
Nowadays a number of research groups put a great effort into their work to achieve
new organic materials with desired properties and better parameters, as well as develop new
solutions to be implemented in certain applications, such as sensors, photovoltaics,
electroluminescent diodes, lasers, switchers and others [1-4]. One of the very promising, and
still not enough investigated, group of organic dyes for photonic purposes is pyrazolines. The
synthesis and properties of 3-(2,2-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole
(DCNP), was patented by Gordon and Bothwell [5] and in the same year published for the
first time by group of Allen [6]. The compound became a very attractive organic photonic
material due to its great nonlinear optical (NLO) and electro-optic response. A lot of further

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studies concerning NLO [7, 8] as well as light amplification properties of DCNP were done
by Miniewicz group, emphasizing the unique features of the compound [9-11]. It has also
become reasonable to search for new analogues of the dye, that may be interesting from both,
a scientific, as well as an application point of view. Therefore, we have designed, synthesized
and characterized some derivatives of DCNP, based on 1-phenyl-2-pyrazoline electron-donor
part and various electron-withdrawing moieties conjugated by a double bond. Basing on
chosen compounds from the dyes family we have shown their high potential in NLO purposes
(second harmonic generation (SHG), third harmonic generation (THG)) [12, 16, 17], in light
amplification (amplified spontaneous emission (ASE), random lasing (RL)) [13] or in optical
switching (measurements of completely reversible photoinduced birefringence) [14, 15].
In this paper we show the synthetic route for two new derivatives of 1-phenyl-2-
pyrazoline, π-conjugated with ortho- or meta- substituted nitrophenyl group as an electron
acceptor, and characterize their 3rd order NLO response (THG), as well as basic spectroscopic
properties. In order to emphasize the role of the heterocyclic fragment in observed NLO
behavior, we compare the dyes with similar in the electron acceptor part stilbene derivatives,
where 1-phenyl-2-pyrazoline was substituted by para-(dimethylamino)phenyl donor group.
Obtained results show significant influence of the electron donor part on measured 3rd order
nonlinear susceptibilities. Moreover, taking into consideration also the earlier described third
pyrazoline isomer (para-nitrophenyl derivative [16]), we can find a correlation between
chemical structure (position of the electron-acceptor moiety as well as the type of electron-
donor unit) and NLO response coming from series of isomers and derivatives, what can be
useful in molecular design of chromophores with desired properties. The chemical structures
of the synthesized compounds are shown in Fig. 1.

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Figure 1. Synthetic route for the new push-pull type pyrazoline dyes and their stilbene
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counterparts.
2. Experimental section
General information and technical details concerning the synthetic route characterization
(Chapter S1) as well as samples preparation methodology (S2), computational details (S3) and
description of the SHG experimental set-up (S4) are presented in Supplementary Information
(SI).
2.1. Synthesis
Synthesis of the 1-phenyl-2-pyrazoline-3-carboxyaldehyde:
This compound was prepared with 37-52 % yield, according to the literature procedure
[8, 14]. 1H NMR (300 MHz, CDCl3): δ 3.12 (t, 2H, CH2, J=11.6 Hz), 4.08 (t, 2H, CH2, J=11.6
Hz), 7.05 (tt, 1H, p-ArH, J=7.3 Hz, J=1.1 Hz), 7.21 (m, 2H, o-ArH), 7.36 (m, 2H, m-ArH);

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13C NMR (75 MHz, CDCl3):
149.38, 185.80 (CHO).
δ 27.41 (CH2), 49.53 (NCH2), 114.34, 122.49, 129.44, 142.45,
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Synthesis of diethyl 2-nitrobenzylphosphonate and diethyl 3-nitrobenzylphosphonate:
A mixture of 2- or 3-nitrobenzyl bromide (5.3 g, 0.025 mol) and triethyl phosphite
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(16.6 g, 0.10 mol) was stirred and heated at about 150-160 C (oil bath temperature) for 1
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hour with continuous distillation of the ethyl bromide produced. The reaction mixture was
cooled to room temperature and evaporated to dryness (20 mmHg, boiling water bath).
Obtained residue was dissolved in ethyl acetate (100 ml) and washed with saturated solution
of NaHCO3 (6x30 ml). The organic phase was dried over anhydrate Na2SO4 and concentrated
in vacuum. Obtained thick oily residue was washed with petroleum ether (5x20 ml) and the
finally dried in vacuum (~5 mmHg) yielding diethyl 2-nitrobenzylphosphonate (5.4 g, 79 %)
and diethyl 3-nitrobenzylphosphonate (5.6 g, 82 %) as yellow oils. After confirmation of its
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structure by H and 31P NMR spectroscopy, both compounds were used in the next step
without further purification. NMR spectra of crude diethyl 2-nitrobenzylphosphonate [19, 20]
and diethyl 3-nitrobenzylphosphonate [21, 22] were in accordance with literature data.
Synthesis of AM-oNO2, AM-mNO2 stilbenes and PY-oNO2, PY-mNO2 pyrazolines:
To a cooled (water-ice bath) solution of crude diethyl 2-nitrobenzylphosphonate or
diethyl 3-nitrobenzylphosphonate (2.7 g, 10 mmol) and appropriate aldehyde (10 mmol) in a
mixture of anh. DMF (20 ml) and anh. EtOH (20 ml) a solution of EtONa (15 mmol) in
ethanol was added dropwise over 30 minutes. Obtained reaction mixture was stirred upon
cooling for 1 hour and next left standing overnight at room temperature. Afterward, a reaction
mixture was poured into a cold water (200 ml) and stirred for 30 minutes.

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In the case of stilbenes, AM-oNO2 and AM-mNO2, the aqueous phases were extracted
with dichloromethane (1x50, 3x20 ml) and dried over anh. Na2SO4. The solvent was
evaporated and obtained crude products were purified as indicated. Crude AM-oNO2 was
purified using short-column chromatography (silica gel, AcOEt) and further recrystallized
from diethyl ether, while AM-mNO2 was purified using column chromatography (silicagel,
petroleum ether/AcOEt, 2:1, V/V) and further recrystallized from dichloromethane/petroleum
ether mixture.
In the case of pyrazolines, PY-oNO2 and PY-mNO2, the precipitated solids were
filtered off, washed with water (5x20 ml) and dried on air to give the crude products which
were further purified using column chromatography (silica gel, petroleum ether/AcOEt, 2:3,
V/V) and then recrystallized from diethyl ether.
(E)-2`-nitro-4-dimethylaminostilben (AM-oNO2): yield 20 %; dark red powder; m.p. 86-87
oC (lit. m.p. 87-89oC [19]); 1H NMR (600 MHz, CDCl3):
δ 3.03 (s, 6H, 2xCH3), 6.74 (d, 2H,
J=7.8 Hz, m-ArH (C6H4NMe2)), 7.09 (d, 1H, J=16.2 Hz, =CH), 7.33 (t, 1H, J=7.8 Hz, 4-
ArH), 7.44 (d, 1H, J=16.2 Hz, =CH), 7.46 (d, 2H, J=7.8 Hz, o-ArH (C6H4NMe2)), 7.56 (t, 1H,
J=7.2 Hz, 5-ArH), 7.79 (d, 1H, J=7.8 Hz, 6-ArH), 7.92 (d, 1H, J=7.8 Hz, 3-ArH); 13C{1H}
NMR (150 MHz, CDCl3):
δ 40.35, 112.24, 118.32, 124.77, 126.82, 127.50, 128.42, 132.82,
133.77, 134.30, 147.71, 150.76 (signal of the one quaternary carbon missing due to
coalescence and overlaps); HRMS: calcd for C16H17N2O2 (M+H)+ 269.1290, found 269.1277.
(E)-3`-nitro-4-dimethylaminostilben (AM-mNO2): yield 25 %; orange powder; m.p. 146-
148 oC (lit. m.p. 148 oC [20, 21]); 1H NMR (600 MHz, CDCl3):
δ 3.04 (s, 6H, Me2N), 6.75 (d,
2H, J=7.7 Hz, m-ArH (C6H4NMe2)), 6.94 (d, 1H, J=16.2 Hz, =CH), 7.18 (d, 1H, J=16.2 Hz,
=CH), 7.46 (d, 2H, J=7.7 Hz, o-ArH (C6H4NMe2)), 7.49 (t, 1H, J=7.7 Hz, 5-ArH), 7.76 (d,
1H, J=7.7 Hz, 6-ArH), 8.04 (d, 1H, J=8.1 Hz, 4-ArH), 8.33 (s, 1H, 2-ArH) (1H NMR data are
in agreement with those reported in the literature [24]); 13C{1H} NMR (150 MHz, CDCl3):

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δ
40.36, 112.30, 120.30, 120.96, 121.48, 124.47, 128.08, 129.39, 131.70, 131.88, 140.15,
148.76, 150.64; HRMS: calcd for C16H17N2O2 (M+H)+ 269.1290, found 269.1281.
(E)-3-(2-nitrostyryl)-1-phenyl-2-pyrazoline (PY-oNO2): yield 45 %; dark red powder; m.p.
158-159 oC; 1H NMR (600 MHz, CDCl3):
δ 3.20 (t, 2H, J=10.8 Hz, CH2), 3.96 (t, 2H, J=10.8
Hz, CH2), 6.92 (tt, 1H, J=0.6 Hz, J=7.2 Hz, p-PhH), 7.12 (dd, 2H, J=0.6 Hz, J=8.4 Hz, o-
PhH), 7.15 (d, 1H, J=16.2 Hz, =CH), 7.26 (d, 1H, J=16.2 Hz, =CH), 7.33 (m, 2H, m-PhH),
7.42 (dt, 1H, J=1.2 Hz, J=7.8 Hz, 4-ArH), 7.62 (dt, 1H, J=1.2 Hz, J=7.8 Hz, 5-ArH), 7.78
(dd, 1H, J=1.2 Hz, J=8.4 Hz, 6-ArH), 7.98 (dd, 1H, J=1.2 Hz, J=8.4 Hz, 3-ArH); 13C{1H}
NMR (150 MHz, CDCl3):
δ 30.43, 48.17, 113.04, 119.81, 124.98, 125.96, 126.84, 127.70,
128.06, 129.23, 132.34, 133.15, 144.56, 147.72, 149.81; HRMS: calcd for C17H16N3O2
(M+H)+ 294.1242, found 294.1245.
(E)-3-(3-nitrostyryl)-1-phenyl-2-pyrazoline (PY-mNO2): yield 56 %; light brown powder;
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m.p. 181-183 C; H NMR (600 MHz, CDCl3):
δ 3.17 (t, 2H, J=10.2 Hz, CH2), 3.94 (t, 2H,
J=10.2 Hz, CH2), 6.62 (d, 1H, J=16.2 Hz, =CH), 6.91 (t, 1H, p-PhH), 7.10 (d, 2H, o-PhH)
7.28-7.35 (m, 3H, m-PhH+ =CH), 7.53 (t, 1H, J=7.8 Hz, 5-ArH), 7.78 (d, 1H, J=7.8 Hz, 6-
ArH), 8.10 (d, 1H, J=7.8 Hz, 4-ArH), 8.31 (s, 1H, 2-ArH); 13C{1H} NMR (150 MHz, CDCl3):
δ
30.38, 48.12, 113.00, 119.05, 119.80, 122.17, 124.76, 128.98, 129.24, 129.68, 131.76,
138.72, 144.58, 148.79, 149.27; HRMS: calcd for C17H16N3O2 (M+H)+ 294.1242, found
294.1243.
2.2. Spectroscopic and optical measurements
Basic linear optical properties of the fabricated materials, i.e. absorption and emission
spectra, were measured using the Jasco V-670 spectrophotometer and Hitachi FL-4500
spectrofluorometer. Emission spectra from the layers were gathered by the excitation of
samples at their absorption maxima.

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For the NLO characterization we have used as the fundamental wavelength 1064 nm coming
from Nd:YVO4 pulsed picosecond laser (30 ps with the 10 Hz repetition rate). The used
experimental set-up is shown schematically in Supplementary Information (Fig. S1).
For the screening and interpretation of results of both, SHG (more details in SI
materials - S4 chapter) and THG measurements, different theoretical and experimental
approaches were utilized. Third harmonic generation phenomenon serves for the
characterization of 3rd order nonlinear optical parameter - susceptibility ꢀꢁ ꢃ. In the process of
ꢂ
THG, frequency
ω of initial beam is converted to 3ω in the output. The main difference
between material restrictions for SHG and THG is that for the latter one the symmetry of
studied nonlinear optical medium does not have any demands. The Kubodera and Kobayashi
model was used for the analysis of THG in the materials (1) [26-29]:
⁄
ꢉ ꢄ
ꢄ
ꢐꢑꢒꢓ
ꢁ ꢃ
ꢂ
ꢀꢁ ꢃ = ꢅ ꢀꢆ ꢇꢆꢈ
(1)
ꢂ
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ꢔ
ꢁ
⁄ ꢃ
ꢊꢋꢌꢍꢎ ꢋꢉꢏ ꢄ
ꢑ
ꢒꢓ
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where the ꢇꢆꢈ represents coherence length of silica (reference material for THG
- linear absorption coefficient and ꢗꢂꢆꢘ, ꢗꢂꢘ are the
investigations), ꢕ is sample thickness, ꢖ
THG intensity for the reference and investigated materials, respectively. In our case the thin
ꢂ
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silica glass plate has been used as the reference material (ꢀꢆꢁ ꢃ = 2.0 × 10ꢋꢄ ꢙꢄ ∙ ꢚꢋꢄ).
3. Results and discussion
3.1. Synthesis
Synthetic route for all final dyes was shown in Fig. 1. Examined compounds were
prepared
via
the
Horner–Wadsworth–Emmons
(HWE)
reaction
of
4-
dimethylaminobenzaldehyde (in the case of AM-oNO2 and AM-mNO2) or 1-phenyl-2-
pyrazoline-3-carboxyaldehyde (in the case of PY-oNO2 and PY-mNO2) with 2- or 3-
nitrobenzylphosphonates in a mixture of anhydrous ethanol and anhydrous DMF (1:1, V/V) in
the presence of 1.5 equivalent of sodium ethoxide as a base. Starting 2- and 3-

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nitrobenzylphosphonates were synthesized by the classical Arbuzov reaction of commercial 2-
o
and 3-nitrobenzyl bromides and excess of the triethylphosphite at 150-160 C. After
1
confirmation of the structures by 31P and H NMR, the crude phosphonates were used in
HWE reaction without further purification. The 1-phenyl-2-pyrazoline-3-carboxyaldehyde
used as substrate for the synthesis of PY-oNO2 and PY-mNO2 was prepared in reaction of 1-
phenyl-2-pyrazoline [12, 18] and the Vilsmeier-Haack reagent [30] using literature procedure
[12, 31].
Using sodium ethoxide as a base and DMF-EtOH mixture as a solvent allowed us to
obtain all final products selectively as (E)-isomers. Such result is in the agreement with
literature data [32]. Structures of all final dyes were confirmed by 1H and 13C NMR, and high
resolution mass spectrometry (HRMS). Stereochemistry of the compounds double bond was
determined on the basis of the value of the coupling constant of the vinylic protons (3J) in 1H
NMR spectra.
Stilbenes AM-oNO2 and AM-mNO2 are known in the literature. In case of stilbene
AM-oNO2, its photochemical behavior was investigated [23]. Derivative AM-mNO2 was
described by Güsten and Salzwedel, who investigated NMR spectra of a series of substituted
stilbenes [24], while Syz and Zollinger analyzed its photoisomerization behavior [25].
3.2. Theoretical studies
All four compounds exist as a mixture of 3 conformers: two – trans and one – cis. The
optimized geometries of all investigated isomers are presented in Fig. 2. For pyrazoline
derivatives trans 1 isomers are the lowest in energy. The second trans isomers are 3.98 and
3.28 kcal/mol higher in energy and their contribution to population according to Boltzmann
distribution in the room temperature equals to 14 and 20 % for PY-oNO2 and PY-mNO2,
respectively. Cis isomers are less stable - 5.79 and 6.60 kcal/mol higher in energy, and their

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abundance is 4 % and 6 % for PY-oNO2 and PY-mNO2, respectively. For stilbene derivatives
both optimized trans isomers are almost equal in energy (difference of 0.01 and 0.15 kcal/mol
between trans 1 and trans 2 conformers for AM-oNO2 and AM-mNO2, respectively). Their
abundances are comparable and they are 46 % and 47 % for AM-oNO2 trans 1 and trans 2
and 41 % and 54 % for AM-mNO2 trans 1 and trans 2. The cis isomers are higher in energy
(4.46 and 5.22 kcal/mol for AM-oNO2 and AM-mNO2) and their contribution to population is
equal to 7 % and 5 %, respectively.

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Figure 2. Optimized molecular structures of PY-oNO2, PY-mNO2, AM-oNO2 and AM-mNO2
conformers. DFT/B3LYP/6-311++G(d,p). Atom colors labeling: C – dark gray, H – light
gray, S- yellow, N – blue, O - red.
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3.3. Spectroscopic and nonlinear optical properties
Normalized linear UV-Vis absorption and emission spectra from studied layers were
shown in Fig. 3. It is visible that the absorption spectra of each sample exhibits a multiband
structure, what is more pronounced for ortho isomers.
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Figure 3. Absorption and emission experimental and separated simulated spectra for all
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opitimized isomers of the investigated systems.
In order to interpret the obtained spectra vertical excitations at CAM-B3LYP/6-311++G(d,p)
level of theory were calculated. As one can see the position of transitions in experimental and
simulated spectra fit satisfactory, although the calculations underestimate the intensity of
shorter wavelength transitions in ortho substituted compounds. The multiband structure of
these spectra is connected with two transitions. The first one (363 nm in PY-oNO2 and 350
nm in AM-oNO2) can be assigned to HOMO → LUMO +1 and the second one (425 and 419

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nm, respectively) to HOMO → LUMO transitions. Selected orbitals for the most stable
isomers obtained at B3LYP/6-311+G(d,p) level of theory are presented in Fig. 4. These two
transitions for ortho isomers are predicted to be separated by ca. 90 nm. For meta isomers H
→ L+1 is significantly blue-shifted and both peaks are expected to be separated by only ca.
30-40 nm. According to the FWHM (full width at half maximum), which is above 50 nm,
they are overlapped. Moreover, the additional splitting of the lines can be connected with the
abundance of all conformers in the experimental sample.
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Figure 4. The contour plots (isodensity = 0.02 au) of HOMO (H), LUMO (L) and LUMO + 1
(L+1) of the most stable PY-oNO2, PY-mNO2, AM-oNO2 and AM-mNO2 isomers (B3LYP/6-
311++G(d,p) level of theory).
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Table 1.
λ
calc calculated at CAM-B3LYP/6-311++G(d,p) level of theory (THF solution) with
assignment of low electronic singlet excitations of PY-oNO2, PY-mNO2, AM-oNO2 and AM-
mNO2 investigated isomers
Δ
EH-L
Investigated
molecules
λcalc
f
Assignmenta
[nm]
[eV]
H→L(84%) + H-1→L(6%) + H-1→L+1(8%)
H→L+1(82%) + H→L(6%)
427.8 0.818
345.5 0.486
trans
4.76
1
H→L(80%) + H→L+1(12%)
H→L+1(79%) + H→L(12%)
H→L(86%) + H→L+1(6%)
432.7 0.486
356.5 0.593
trans
2
4.79
4.86
PY-
oNO2
409.9 0.475
323.7 0.461
311.0 0.107
H→L+1(63%) + H-7→L(12%)
H-7→L(35%) + H-4→L(9%) + H-
3→L(13%) + H→L+1(19%)
H→L(63%) + H→L+1(28%) + H-1→L(6%)
H→L+1(65%) + H→L(23%) + H-1→L(6%)
H→L+6(72%)
cis
399.2 0.721
361.4 0.714
272.0 0.111
262.0 0.173
trans
1
4.71
H-3→L(63%) + H-4→L(10%) + H-
1→L(10%)
PY-
mNO2
H→L(49%) + H→L+1(44%)
415.0 0.725
trans
2
4.69
4.86
369.6 0.223 H→L+1(49%) + H→L(39%) + H-1→L(5%)
273.0 0.101
H→L+5(69%)
H→L(75%) + H→L+1(15%)
H→L+1(77%) + H→L(15%)
369.1 0.262
350.1 0.838
cis

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H→L(86%) + H-1→L(9%)
415.6 0.727
328.8 0.483
trans
1
4.91
4.91
H→L+1(86%)
H→L(86%) + H-1→L(9%)
H→L+1(86%)
416.1 0.725
329.4 0.489
trans
2
AM-
H→L(86%) + H-1→L(8%)
H→L+1(77%)
oNO2
402.2 0.280
300.7 0269
277.2 0.149
4.98
H→L+3(20%) + H-7→L(15%) +
H→L+12(12%) + H-1→L(9%) + H-
3→L(7%)
cis
H→L+1(92%)
351.9 1.278
260.5 0.120
259.8 0.144
H→L+2(42%) + H-3→L(30%)
H→L+2(41%) + H-3→L(37%) + H-
2→L(6%)
trans
1
4.81
H→L(82%) + H-1→L(11%)
H→L+2(88%)
AM-
395.2 0.224
339.0 1.136
281.8 0.104
262.4 0.208
mNO2
trans
2
4.78
4.90
H→L+6(42%) + H→L+4(39%) + H-
1→L+1(7%)
H-3→L(63%) + H-2→L(17%)
H→L+2(86%)
321.3 0.719
cis
a Labels: H – HOMO, L – LUMO. The contribution or transitions > 5 % are included.
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Calculated parameters presented in Tab. 1 allowed us to assign experimentally
observed absorption lines. Maxima observed at 363 and 350 nm for PY-oNO2 and AM-oNO2,
respectively, can be assigned as H → L+1. Lines appeared at 425 and 419 nm for these

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compounds are due to H → L. In case of meta isomers PY-mNO2 and AM-mNO2 both H → L
and H → L+1 transitions have contribution to observed absorption maxima. In examined
group of compounds it is shown that change of -NO2 group position from meta to ortho
exhibits shift of H → L absorption into visible light.
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Simulated orbitals presented in Fig. 4 confirm also that both H → L and H → L+1
transitions have the push-pull character. The electron density transfer occurs from donor
stilbene group via -CH=CH- connector onto C6H4NO2 acceptor. The character of transition
does not depend on –NO2 position.
The performed calculations showed that in the -oNO2 compounds the lowest transition
is predominantly from HOMO to LUMO, where the electron is mainly localized in the
nitrophenyl moiety in LUMO. By contrast, in -mNO2 compounds the lowest transition has
larger involvement of LUMO+1, in which the electron is delocalized cross the entire
molecule. This is consistent with the observation that the ortho- substituted nitrophenyl group
possesses stronger electron withdrawing ability than meta- substituted ones. Therefore, the
intramolecular charge transfer character in -oNO2 compounds is more significant and
consequently it red shifts the absorption band.
Afterwards, the fluorescence spectra of the THF solutions doped by the luminescent
dyes have been acquired with the use of spectrofluorometer Hitachi F-4500. Two investigated
in this paper pyrazoline derivatives emit the light in the range of 450 - 650 nm, but absorption
maxima of PY-mNO2 and PY-oNO2 are localized at 395 and 363 nm, respectively. The latter
system possesses also the second but much weaker absorption band localized at 425 nm,
which is partially inscribed in the main range of emission for this compound. Molecule PY-
pNO2 (which is a constitutional isomer of the abovementioned ones, investigated by us
recently [15]) characterizes the highest Stokes shift among presented compounds, which is
167 nm. This system possesses also the emission band shifted strongly toward the red range.

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In the group of investigated push-pull type of low molecular weight systems doped to
the PMMA the THG phenomenon has been characterized. Table 2. contains all NLO
determined parameters of studied materials. Table S1. (Chapter S5 in SI materials) contains
also χ(2) parameters defined for both laser polarization configurations (
ꢛ and ꢜ) for the
investigated polymeric systems.
Table 2. Nonlinear optical parameters (3rd order NLO susceptibilities) for the investigated
polymeric systems.
ꢁꢂꢃ ꢝ
⁄
ꢄꢞ a, b
ꢚ
ꢄ
Compound
ꢀ
ꢙ
PY-pNO2
PY-mNO2
PY-oNO2
AM-mNO2
AM-oNO2
silica
8.1 × 10-21 c
1.1 × 10-22
4.0 × 10-22
5.6 × 10-23
3.7 × 10-23
2.00 × 10-22
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300
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a – Maker fringes set-up employing the fundamental exit (1064 nm) of a 30 ps Nd:YVO4 laser
with a repetition rate of 10 Hz at 532 nm; the energy per pulse was 40 mJ
b – calculated according to the Kubodera and Kobayashi model (taking into account the linear
absorption coefficient)
c – literature data [16]
The third harmonic generation phenomenon has been characterized by the comparable
method, where the Maker fringe patterns were measured as a function of incidence angle
using a rotational stage and picosecond pulsed laser as the incident beam. The THG results
are associated with properties of the each one molecule. As the reference material a thin silica

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glass plate was used. The characteristic incident angle dependences coming from this
measurements are shown in Fig. 5. What is clearly visible, both of the investigated stilbenes
present similar NLO answer as the used reference. However, two of the studied pyrazoline
derivatives exhibit several times larger THG signal than silica. It seems that electron donor
part in the shape of aromatic ring linked to the pyrazoline one is more effective than 3rd order
amine group used in stilbenes. Furthermore, in one case - not included experimentally in this
paper (PY-pNO2) [17], χ(3) parameter is even one order of magnitude larger than for the
reference material (Tab. 2). Anyway, among the group of pyrazoline derivatives, the ortho
isomer can induce four times higher signal than the meta one. It can be strictly involved with
the activation (ortho and para) and deactivation (meta) process (understood as the internal
electron transfer) into the aromatic ring (located in the electron acceptor part in the molecule)
by proper substituent position. Moreover, presented in this paper two isomers of pyrazoline
derivatives in the meaning of the χ(3) NLO parameter correspond with other representatives
from this family of compounds described in literature [16]. Namely, another nonlinear dyes
containing pyrazoline ring and as the electron-acceptor moiety double nitrile groups (in
various structural configuration) characterize the same order of magnitude value of the 3rd
order nonlinear optical susceptibility. It means that initial hypothesis presenting pyrazoline
derivatives group of compounds as the efficient NLO chromophores (SHG/THG converters)
was correct [16, 17].

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Figure 5. The typical Maker fringe patterns obtained in THG measurements acquired for:
silica glass plate (top) and polymeric systems doped with isomers of pyrazoline derivatives
(middle) and two isomers of the stilbene dye (bottom). The experimental results for
investigated samples and reference material were collected for the s polarization of incident
beam.
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Therefore, the concentration influence on the THG signal only for the two most effective
representatives on THG phenomenon among investigated materials - pyrazoline derivatives,
has been investigated (Fig. 6.). In these cases there is a precise value of the dye concentration
above which the increase of NLO answer is observed. In the range of 1.50 % (PY-mNO2) to
2.25 % (PY-oNO2) of the dry dye content (w/w) it is observed the threshold which indicates
the quantity of the nonlinear medium is going to be more efficient according to the linear
growth function. For each compound this dependence is different.
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Figure 6. The measured dye concentration dependence on the 3rd order nonlinear optical
susceptibility for PY-mNO2 (up) and PY-oNO2 (down) for s polarization of incident light.
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By presenting this results we would like to point out that defined values of ꢀꢁꢂꢃ were
determined for the films with 3 % dye content and it can be improved by process optimization
due to the used polymer, dye concentration and the used solvent as well. Moreover, the
crystallization or aggregation process can be responsible for this particular behavior. By
creating more condense matter in the volume of the polymeric film it is possible to gain some
optical effects, in this case - NLO response.
4. Conclusions
The represents of push-pull type of organic molecules based on the 4-
dimethylaminophenyl group and 1-phenyl-2-pyrazoline moiety have been introduced, both
the synthesis method and basic spectroscopic characterization. The THG results obtained
from picosecond pulsed laser system have been acquired and analyzed in comparison with the
theoretical models. Among the investigated compounds the pairs of derivatives and isomers
can be distinguished. They characterize different electron-donor units and spectroscopic
features (especially NLO ones) likewise. The experimental results confirmed that investigated
systems can be used as the efficient nonlinear optical systems for harmonic of light generation
(tripled but also doubled frequency). It has been also proved the strong dependence between
chemical structure (position of electron-acceptor moiety as well as the type of electron-donor
unit) and NLO response coming from series of isomers and derivatives. This correlation
directly indicates the field where the chemists and physicists can co-operate efficiently. By
simple combination of the chemists knowledge and experience in the field of synthesis with
the physicists expectations involved with the new, more useful and hybrid or composite
materials and their valuable properties can give as the result new device or design unfamiliar
solution for various processes. Also in this paper we showed that there is a particular amount

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of nonlinear dye concentration in polymeric film (threshold value), which causes sudden
increase of NLO response due to the THG, even if the active medium absorbs the light in the
range of generated harmonics of light. Accordingly, this part of the results can improve
material’s engineering as the very useful and meaningful tool in design new, more effective
materials for various applications. It also confirms that individual approach for the
optimization of already existing systems is needed and should be always done. Presented
experimental results indicate that the group of pyrazoline and stilbene derivatives have a great
potential in future photonic applications.
Acknowledgements
A. S. would like to thank for financial support for dissertation preparation under ETIUDA II
program which is financed by Polish National Science Centre (doctoral scholarship no. Dec-
2014/12/T/ST4/00233). Financial support by a statutory activity subsidy from the Polish
Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw
University of Technology (W. G.) is gratefully acknowledged. This work was also financially
supported by the National Science Centre, Poland (2016/21/B/ST8/00468).
Supplementary content
1
Supplementary data with copy of H and 13C NMR spectra of examined compounds (AM-
oNO2, AM-mNO2, PY-oNO2 and PY-mNO2) can be found, in the online version.
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Highlights:
ꢀ
ꢀ
ꢀ
ꢀ
Pairs of pyrazoline and stilbene derivatives have been analyzed
Horner–Wadsworth–Emmons (HWE) reaction was implemented
Second and third order of nonlinear optical properties were investigated
Dependence between chemical structure and NLO response was shown