Synthesis and spectral characterization of new homologous 1,3,5-triaryl-2-pyrazolines: Influence of alkyloxy chain length on fluorescence

Article abstract of DOI:10.1016/j.saa.2014.05.065

Twelve new homologous 1,3,5-triaryl-2-pyrazolines (1c-12c) have been synthesized and characterized on the basis of their spectral (IR, ¹H and ¹³C NMR and MS) data and microanalysis. The influence of alkyloxy chain length on absorption and fluorescence properties of 1c-12c was studied by UV-Vis and emission spectroscopy. For all the compounds, fluorescence was observed in the blue region of the visible spectrum. Furthermore, a strong influence of alkyloxy chain length was found on the emission intensity of 1,3,5-triaryl-2-pyrazolines, without causing any major blue- or red-shift in the emission wavelength (λ_max^em). The absorption and emission maxima (λ_max^abs and λ _max^em) for all the compounds were observed in the range of 408-416 nm and 471-476 nm, respectively. The effect of chloro-substituent present on the conjugated backbone of 1,3,5-triaryl-2-pyrazoline moiety is also discussed.

Full text of DOI:10.1016/j.saa.2014.05.065

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and spectral characterization of new homologous
1,3,5-triaryl-2-pyrazolines: Influence of alkyloxy chain length
on fluorescence
b
c
a,
Asghar Abbas a, , Safdar Hussain , Noureen Hafeez , Muhammad Moazzam Naseer
⇑
⇑
a Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
b Department of Forensic Medicine & Toxicology, National University of Sciences and Technology, Islamabad 44000, Pakistan
c Department of Forensic Medicine & Toxicology, Rawalpindi Institute of Health Sciences (RIHS), Bahria University, Islamabad, Pakistan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Organic fluorescent compounds are
important and attracted considerable
attention.
H3C
CH3
ꢀ
New 1-(3,4-dimethylphenyl)-3-(4-
chlorophenyl)-5-(4-alkoxyphenyl)-2-
pyrazolines.
ꢀ
ꢀ
Synthesis and fluorescence property
evaluation.
Effect of chloro and alkyloxy
substituents on fluorescence.
N
N
R
Cl
O
R = CnH2n+1 with n = 1-12
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2014
Received in revised form 7 May 2014
Accepted 14 May 2014
Available online 2 June 2014
Twelve new homologous 1,3,5-triaryl-2-pyrazolines (1c–12c) have been synthesized and characterized
on the basis of their spectral (IR, 1H and 13C NMR and MS) data and microanalysis. The influence of
alkyloxy chain length on absorption and fluorescence properties of 1c–12c was studied by UV–Vis and
emission spectroscopy. For all the compounds, fluorescence was observed in the blue region of the visible
spectrum. Furthermore, a strong influence of alkyloxy chain length was found on the emission intensity
of 1,3,5-triaryl-2-pyrazolines, without causing any major blue- or red-shift in the emission wavelength
Keywords:
em
abs
em
(k ). The absorption and emission maxima (k
max
and k ) for all the compounds were observed in
max
max
1,3,5-triaryl-2-pyrazolines
Spectral characterization
Fluorescence
the range of 408–416 nm and 471–476 nm, respectively. The effect of chloro-substituent present on
the conjugated backbone of 1,3,5-triaryl-2-pyrazoline moiety is also discussed.
Ó 2014 Elsevier B.V. All rights reserved.
Substituents effect
Introduction
optoelectronics and electronic displays [5,6]. They are generally
considered superior to inorganic fluorescent materials, mainly
Organic fluorescent compounds have attracted considerable
attention in cosmetics, surface coatings, inks and textile industries
[1–3]. They have also been used in sensors [4], solar cells,
due to the ease of fabrication and tunability of emission properties
by a simple chemical modification, as evidenced by organic elec-
troluminescence devices (OELDs) having low cost, broad range of
emission colors, high brightness, high luminous efficiency, good
life stability and simple processing [7–9]. However, the major
drawback of organic fluorescent materials is aggregation of mole-
cules i.e. the formation of an excimer-like species and H-type
⇑
Corresponding author. Tel.: +92 51 90642129; fax: +92 51 90642241.
E-mail addresses: profazmi@hotmail.com (A. Abbas), moazzam@qau.edu.pk
(M.M. Naseer).
http://dx.doi.org/10.1016/j.saa.2014.05.065
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
183
molecular aggregates, leading to the decrease in fluorescence
intensity [10–12]. To circumvent this problem, slight structural
changes in fluorescent molecules have been found useful and effec-
tive [13–15]. The examples include the introduction of an ethyl
group on the conjugated backbone of diphenylbutadiene [13] and
incorporation of quinacridone dye molecule into dendrimers [14]
leading to enhanced emission efficiency due to decreased molecu-
lar aggregation.
Pyrazolines in general [16] and 1,3,5-triaryl-2-pyrazolines in
particular [17–20] are typical intramolecular charge transfer
compounds of immense importance, not only due to their use in
organic electroluminescent devices (OELDs) and optoelectronics
[21–25] but also due to their diverse biological applications
[26–36]. 1,3,5-Triaryl-2-pyrazolines with blue fluorescence and
high quantum yield are described as hole transporting media in
photoconductive and emitting materials, organic photovoltaic
cells, and in OELDs [37–41]. In addition, they are also used as
optical brightening agents for textiles, papers and plastics [42],
fluorescent probes in many chemosensors [43,44] and fluorescent
switches [45].
For better understanding of 1H NMR chemical shift values, the
different protons of compounds 1c–12c are differentiated accord-
ing to the labeling shown in Fig. 1.
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-methoxyphenyl)-2-
pyrazoline (1c)
Yield 87%; pale yellow solid; m.p. 144–147 °C; Rf = 0.85 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1688, 1295, 1492,
1255, 1047, 1080, 1H NMR (300 MHz, CDCl3) d 2.17 (s, 3H, N-Ar-4-
CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H, J = 7.5, 16.6 Hz, Ha),
3.80 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.84 (s, 3H, AOACH3), 5.22
(dd, 1H, J = 7.5, 12.0 Hz, Hx), 6.71 (d, 1H, J = 8.1 Hz, N-ArHg), 6.87
0
(d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d, 1H, J = 8.1 Hz, N-ArHh), 7.04 (s,
0
1H, N-ArHi), 7.24 (d, 2H, J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H,
J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H, J = 8.7 Hz, ArHe=e ), 13C NMR
(75 MHz, CDCl3) d 18.8, 20.1, 43.4, 55.2, 64.4, 110.8, 114.4 (2C),
115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5,
133.9, 134.5, 137.0, 142.9, 144.9, 158.3, EIMS: m/z 390 (M+Å, base
peak). Anal. calcd. for C24H23ClN2O: C, 73.74; H, 5.93; N, 7.17;
Found: C, 73.71; H, 5.90; N, 7.22%.
0
0
In continuation of our previous studies on synthesis and
properties of pyrazolines [46–49], herein, we report the synthesis
and spectral characterization of new homologous 1,3,5-triaryl-2-
pyrazolines having chloro-substituent at 3-aryl and one to twelve
carbon alkyloxy side chain at 5-aryl of 2-pyrazoline ring highlight-
ing the effect of alkyloxy side chain on fluorescence property.
The study is helpful in understanding the interplay of weak
interactions with the change in alkyloxy chain length and their
significance in absorption and emission properties. The com-
pounds of the present series with fluorescence properties in the
blue region of the visible spectrum are potential future candidates
for their use as blue light emitting materials.
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-ethoxyphenyl)-2-
pyrazoline (2c)
Yield 84%; pale yellow solid; m.p. 128–131 °C; Rf = 0.86 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1686, 1297, 1495,
1253, 1042, 1087, 1H NMR (300 MHz, CDCl3) d 1.42 (t, 3H,
J = 7.0 Hz, AOACH2ACH3), 2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H,
N-Ar-3-CH3), 3.07 (dd, 1H, J = 7.5, 16.8 Hz, Ha), 3.75 (dd,
1H, J = 12.3, 16.8 Hz, Hb), 4.02 (q, 2H, J = 7.0 Hz, AOACH2A), 5.21
(dd, 1H, J = 7.5, 12.3 Hz, Hx), 6.71 (d, 1H, J = 8.1 Hz, N-ArHg), 6.87
0
(d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d, 1H, J = 8.1 Hz, N-ArHh), 7.04 (s,
0
0
1H, N-ArHi), 7.24 (d, 2H, J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, ArHf=f ),
7.66 (d, 2H, J = 8.7 Hz, ArHe=e ), 13C NMR (75 MHz, CDCl3) d 14.8,
0
18.8, 20.1, 43.2, 63.4, 64.4, 110.8, 115.0 (2C), 115.2, 126.7(2C),
127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5, 133.9, 134.5, 137.0,
142.9, 144.9, 158.3, EIMS: m/z 404 (M+Å, base peak). Anal. calcd.
for C25H25ClN2O: C, 74.15; H, 6.22; N, 6.92; Found: C, 74.11; H,
6.18; N, 6.99%.
Experimental
Materials and methods
All reagents and solvents were used as obtained from the
supplier or recrystallized/redistilled as required. Thin layer chro-
matography (TLC) was performed using aluminium sheets coated
with silica gel 60 F254 (Merck). Elemental analyses were carried
out with a LECO-183 CHNS model. 1H and 13C NMR spectra of com-
pounds were recorded on a Bruker 300 MHz spectrometer using
deuterated solvents and TMS as internal standard. IR spectra of
compounds were recorded on a Bio-Rad FTS 3000 MX spectropho-
tometer (400–4000 cmꢁ1). The melting points of compounds were
determined using capillary tubes and an electrothermal melting
point apparatus, model MP-D Mitamura Riken Kogyo, Japan.
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-propyloxyphenyl)-
2-pyrazoline (3c)
Yield 81%; pale yellow solid; m.p. 98–101 °C; Rf = 0.89 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1684, 1292,
1493, 1250, 1044, 1081, 1H NMR (300 MHz, CDCl3) d 1.05 (t, 3H,
J = 7.25 Hz, AOACH2ACH2ACH3), 1.81 (sextet, 2H, J = 7.2 Hz,
AOACH2ACH2ACH3), 2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H,
N-Ar-3-CH3), 3.07 (dd, 1H, J = 7.5, 16.8 Hz, Ha), 3.76 (dd, 1H,
J = 12.3, 17.1 Hz, Hb), 3.91 (t, 2H, J = 6.6 Hz, AOACH2A), 5.21 (dd,
1H, J = 7.5, 12.3 Hz, Hx), 6.71 (d, 1H, J = 8.1 Hz, N-ArHg), 6.87 (d,
0
2H, J = 8.7 Hz, ArHc=c ), 6.93 (d, 1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H,
General procedure for the synthesis of compounds (1c–12c)
0
N-ArHi), 7.24 (d, 2H, J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz,
ArHf=f ), 7.66 (d, 2H, J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3)
0
0
The respective 4-alkoxychalcone (0.01 mol) (1a–12a) [50] in
25 mL acetic acid containing a few drops of hydrochloric acid
was heated at 60–65 °C for half an hour with constant stirring in
a
round bottom flask before the addition of (3,4-dimethyl-
phenyl)hydrazine hydrochloride (3.45 g, 0.02 mol) (1b). After the
addition of 1b to the reaction flask, the reaction mixture was
heated to reflux for 5–6 h. The reaction mixture was then cooled
to room temperature and poured onto the crushed ice. The precip-
itates thus appeared, were filtered, washed thoroughly with
distilled water and dried. To get highly pure compounds
(1c–12c) for spectral characterization and fluorescence properties,
the obtained crude products were subjected to silica gel column
chromatography using petroleum ether/ethyl acetate (4:1) as the
mobile phase.
Fig. 1. Labelling scheme for protons of compounds (1c–12c).

184
A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
d 10.5, 18.8, 20.1, 22.6, 43.2, 64.4, 69.5, 110.8, 115.0 (2C), 115.2,
126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5, 133.9,
134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z 418 (M+Å, base peak).
Anal. calcd. for C26H27ClN2O: C, 74.54; H, 6.50; N, 6.69; Found: C,
74.51; H, 6.47; N, 6.75%.
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-heptyloxyphenyl)-
2-pyrazoline (7c)
Yield 84%; pale yellow solid; m.p. 105–108 °C; Rf = 0.86 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1684, 1291, 1491,
1255, 1046, 1083, 1H NMR (300 MHz, CDCl3) d 0.92 (t, 3H,
J = 7.2 Hz, AOA(CH2)6ACH3), 1.29–1.48 (m, 8H, AOACH2ACH2-
A(CH2)4ACH3), 1.78 (qn, 2H, J = 8.0 Hz, AOACH2ACH2AC5H11),
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.5, 16.8 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.94 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.5, 12.3 Hz, Hx), 6.71
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-butyloxyphenyl)-2-
pyrazoline (4c)
Yield 83%; pale yellow solid; m.p. 110–113 °C; Rf = 0.85 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1679, 1293, 1497,
1254, 1050, 1085, 1H NMR (300 MHz, CDCl3) d 0.99 (t, 3H,
J = 7.2 Hz, AOA(CH2)3ACH3), 1.50 (sextet, 2H, J = 7.5 Hz, AOACH2-
ACH2ACH2ACH3), 1.77 (qn, 2H, J = 8.0 Hz, AOACH2ACH2AC2H5),
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.8, 17.1 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.95 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.5, 12.3 Hz, Hx), 6.71
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.1,
0
22.6, 26.0, 29.0, 29.2, 31.7, 43.2, 64.4, 68.0, 110.8, 115.0 (2C),
115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5,
133.9, 134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z 474 (M+Å, base
peak). Anal. calcd. for C30H35ClN2O: C, 75.85; H, 7.43; N, 5.90;
Found: C, 75.81; H, 7.40; N, 5.97%.
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 13.8, 18.8, 19.2,
0
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-octyloxyphenyl)-2-
pyrazoline (8c)
20.1, 31.3, 43.4, 64.4, 67.6, 110.8, 115.0 (2C), 115.2, 126.7(2C),
127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5, 133.9, 134.5, 137.0,
142.9, 144.9, 158.5, EIMS: m/z 432 (M+Å, base peak). Anal. calcd.
for C27H29ClN2O: C, 74.90; H, 6.75; N, 6.47; Found: C, 74.90; H,
6.75; N, 6.47%.
Yield 86%; pale yellow solid; m.p. 102–105 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1683, 1295, 1497,
1252, 1052, 1087, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 7.2 Hz, AOA(CH2)7ACH3), 1.31–1.49 (m, 10H, AOACH2ACH2-
A(CH2)5ACH3), 1.78 (qn, 2H, J = 7.2 Hz, AOACH2ACH2AC6H13),
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.5, 16.8 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.94 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.5, 12.3 Hz, Hx), 6.71
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-pentyloxyphenyl)-
2-pyrazoline (5c)
Yield 85%; pale yellow solid; m.p. 117–120 °C; Rf = 0.86 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1682, 1298, 1496,
1257, 1051, 1089, 1H NMR (300 MHz, CDCl3) d 0.96 (t, 3H,
J = 7.0 Hz, AOA(CH2)4ACH3), 1.28–1.48 (m, AOACH2ACH2A(CH2)2-
ACH3), 1.78 (qn, 2H, J = 7.2 Hz, AOACH2ACH2AC3H7), 2.17 (s, 3H,
N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.06 (dd, 1H, J = 7.8,
17.1 Hz, Ha), 3.75 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.95 (t, 2H,
J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.5, 12.3 Hz, Hx), 6.71
0
(d, 1H, J = 8.1 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.1,
0
22.6, 26.0, 29.0, 29.2, 29.3, 31.8, 43.2, 64.4, 68.0, 110.8, 115.0
(2C), 115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5,
133.9, 134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z 488 (M+Å, base
peak). Anal. calcd. for C31H37ClN2O: C, 76.13; H, 7.63; N, 5.73;
Found: C, 76.09; H, 7.60; N, 5.79%.
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.0, 18.8, 19.2,
0
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-nonyloxyphenyl)-2-
pyrazoline (9c)
20.1,22.4, 31.3, 43.4, 53.4, 67.6, 110.8, 115.0 (2C), 115.2,
126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5, 133.9,
134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z 446 (M+Å, base peak).
Anal. calcd. for C28H31ClN2O: C, 75.23; H, 6.99; N, 6.27; Found: C,
75.19; H, 6.97; N, 6.34%.
Yield 88%; pale yellow solid; m.p. 94–97 °C; Rf = 0.88 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1678, 1293,
1495, 1259, 1054, 1085, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 6.9 Hz, AOA(CH2)8ACH3), 1.30–1.48 (m, 12H, AOACH2ACH2-
A(CH2)6ACH3), 1.78 (qn, 2H, J = 7.8 Hz, AOACH2ACH2AC7H15),
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.8, 17.1 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.93 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.8, 12.3 Hz, Hx), 6.71
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-hexyloxyphenyl)-2-
pyrazoline (6c)
Yield 87%; pale yellow solid; m.p. 115–118 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1678, 1295, 1492,
1258, 1048, 1088, 1H NMR (300 MHz, CDCl3) d 0.92 (t, 3H,
J = 7.2 Hz, AOA(CH2)5ACH3), 1.32–1.48 (m, 6H, AOACH2ACH2-
A(CH2)3ACH3), 1.77 (qn, 2H, J = 8.0 Hz, AOACH2ACH2AC4H9),
2.17 (s, 3H, N-Ar-4-CH3), 2.21 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.5, 16.8 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.93 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.5, 12.3 Hz, Hx), 6.71
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.1,
0
22.6, 26.0, 29.0, 29.2, 29.3, 29.4, 31.8, 43.2, 64.4, 68.0, 110.8,
115.0 (2C), 115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9,
131.5, 133.9, 134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z 502
(M+Å, base peak). Anal. calcd. for C32H39ClN2O C, 76.39; H, 7.81; N,
5.57; Found: C, 76.35; H, 7.78; N, 5.63%.
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93
(d, 1H, J = 8.1 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.0, 18.8, 19.2,
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-decyloxyphenyl)-2-
pyrazoline (10c)
Yield 83%; pale yellow solid; m.p. 92–95 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1682, 1292,
1493, 1250, 1049, 1089, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 7.2 Hz, AOA(CH2)9ACH3), 1.30–1.48 (m, 14H, AOACH2ACH2-
0
20.1, 22.6, 29.2, 31.5, 43.4, 53.4, 68.7, 110.8, 115.0 (2C), 115.2,
126.7(2C), 127.0 (2C), 127.4, 128.7 (2C), 129.9, 131.5, 133.9,
134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z 460 (M+Å, base peak).
Anal. calcd. for C29H33ClN2O: C, 75.55; H, 7.21; N, 6.08; Found: C,
75.55; H, 7.21; N, 6.08%.

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
185
A(CH2)7ACH3), 1.78 (qn, 2H, J = 7.8 Hz, AOACH2ACH2AC8H17),
Fluorescence properties of homologous 1,3,5-triaryl-2-pyrazolines
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.5, 17.1 Hz, Ha), 3.76 (dd, 1H, J = 12.1, 17.1 Hz, Hb), 3.93 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.5, 12.0 Hz, Hx), 6.71
The UV–Vis and emission spectroscopy was used to study
fluorescence properties of compounds 1c–12c. The UV–Vis and
emission spectra of homologous 1,3,5-triaryl-2-pyrazolines (1c–12c),
were recorded in N,N-dimethylformamide–water (3:7) mixture at
a concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1 and 1 ꢂ 10ꢁ7 mol Lꢁ1, respec-
tively at room temperature (298 K).
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.4 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.1,
0
22.7, 26.0, 29.3, 29.3, 29.4, 29.5, 29.5, 31.8, 43.2, 64.4, 68.0,
110.8, 115.0 (2C), 115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7 (2C),
129.9, 131.5, 133.9, 134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z
516 (M+Å, base peak). Anal. calcd. for C33H41ClN2O: C, 76.64; H,
7.99; N, 5.42; Found: C, 76.60; H, 7.93; N, 5.49%.
Results and discussion
Chemistry
The homologous 1,3,5-triaryl-pyrazolines (1c–12c) were syn-
thesized by reacting (E)-1-(4-chlorophenyl)-3-(4-alkoxyphe-
nyl)prop-2-en-1-one (1a–12a) [50] with two equivalents of
(3,4-dimethylphenyl)hydrazine hydrochloride in glacial acetic acid
solvent having catalytic amount of hydrochloric acid for 5–6 h
(Scheme 1). After pouring the cooled reaction mixture on crushed
ice, the crude solids of desired products were isolated by filteration
which were further purified by silica gel column chromatography
using petroleum ether/ethyl acetate (4:1) as the mobile phase. A
little influence of alkyloxy chain length was observed on the
reaction efficiency as all the products were obtained in good to
excellent yields ranging from 81% to 88%. The structures of all
the compounds were deduced from their spectral (IR, 1H and 13C
NMR, MS) and microanalytical data.
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-undecyloxyphenyl)-
2-pyrazoline (11c)
Yield 81%; pale yellow solid; m.p. 95–97 °C; Rf = 0.89 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1681, 1295,
1497, 1253, 1053, 1082, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 7.2 Hz, AOA(CH2)10ACH3), 1.29–1.47 (m, 16H, AOACH2ACH2-
A(CH2)8ACH3), 1.77 (qn, 2H, J = 7.8 Hz, AOACH2ACH2AC9H19),
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.8, 17.1 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.93 (t,
2H, J = 6.6 Hz, AOACH2A), 5.21 (dd, 1H, J = 7.8, 12.3 Hz, Hx), 6.71
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.87 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.4 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.1,
0
Spectral characterization of 1c–12c
22.7, 26.0, 29.2, 29.2, 29.3, 29.4, 29.5, 29.6, 31.8, 43.2, 64.4, 68.0,
110.8, 115.0 (2C), 115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7 (2C),
129.9, 131.5, 133.9, 134.5, 137.0, 142.9, 144.9, 158.5, EIMS: m/z
530 (M+Å, base peak). Anal. calcd. for C34H43ClN2O: C, 76.88; H,
8.16; N, 5.27; Found: C, 76.84; H, 8.11; N, 5.33%.
IR spectra
Two characteristic absorption bands, one in the range of 1688–
1678 cmꢁ1 and the second in the range of 1298–1291 cmꢁ1 in the
IR spectra of compounds (1c–12c) were assigned to the stretching
of carbon–nitrogen double bond (C@N) and carbon–nitrogen single
bond (CAN) [46–49,51], respectively that indicates the formation
of pyrazoline skeleton. The ArACl functionality in all the products
was identified by the presence of strong band in the region of
1089–1180 cmꢁ1. Two more strong bands due ArAO-stretching
and AOAR stretching were observed in the range of 1259–
1-(3,4-Dimethylphenyl)-3-(4-chlorophenyl)-5-(4-dodecyloxyphenyl)-
2-pyrazoline (12c)
Yield 87%; pale yellow solid; m.p. 89–91 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1683, 1294,
1495, 1252, 1052, 1087, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 7.2 Hz, AOA(CH2)11ACH3), 1.29–1.47 (m, 18H, AOACH2ACH2-
A(CH2)9ACH3), 1.78 (qn, 2H, J = 8.0 Hz, AOACH2ACH2AC10H21),
2.17 (s, 3H, N-Ar-4-CH3), 2.22 (s, 3H, N-Ar-3-CH3), 3.07 (dd, 1H,
J = 7.5, 16.8 Hz, Ha), 3.76 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.93 (t,
2H, J = 6.6 Hz, –O–CH2–), 5.21 (dd, 1H, J = 7.8, 12.3 Hz, Hx), 6.71
1250 cmꢁ1 and 1054–1042 cmꢁ1
, respectively, confirming the
presence of alkyloxy chain on one of the aryl ring of the products.
The most significant absorption bands selected from the IR spectra
of compound 1c–12c are presented in the experimental section.
0
(d, 1H, J = 8.4 Hz, N-ArHg), 6.86 (d, 2H, J = 8.7 Hz, ArHc=c ), 6.93 (d,
1H, J = 8.4 Hz, N-ArHh), 7.04 (s, 1H, N-ArHi), 7.24 (d, 2H,
1H NMR spectra and 13C NMR spectra
0
0
J = 8.7 Hz, ArHd=d ), 7.36 (d, 2H, J = 8.7 Hz, ArHf=f ), 7.66 (d, 2H,
The formation of pyrazoline skeleton of compounds 1c–12c was
further confirmed from NMR spectroscopy [46–49]. In 1H NMR,
three doublet of doublets, two due to methylenic protons (Ha
and Hb) and one due to methine proton (Hx) were observed, con-
firming the formation of five membered pyrazoline ring. The
methylenic protons Ha appeared at 3.06–3.07 ppm with coupling
constants of 7.5–7.8 Hz and 16.6–17.1 Hz due to their coupling
J = 8.4 Hz, ArHe=e ); 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.1,
0
22.7, 26.0, 29.2, 29.3, 29.4, 29.5, 29.6, 29.6, 29.6, 31.8, 43.2, 64.4,
68.0, 110.8, 115.0 (2C), 115.2, 126.7(2C), 127.0 (2C), 127.4, 128.7
(2C), 129.9, 131.5, 133.9, 134.5, 137.0, 142.9, 144.9, 158.5, EIMS:
m/z 544 (M+Å, base peak). Anal. calcd. for C35H45ClN2O: C, 77.10;
H, 8.32; N, 5.14; Found: C, 77.04; H, 8.28; N, 5.21%.
Scheme 1. Synthesis of homologous 1,3,5-triaryl 2-pyrazolines, 1c–12c.

186
A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
with neighbouring Hb and Hx protons. Similarly, methylenic pro-
tons Hb of compounds 1c–12c were observed at 3.75–3.80 ppm
with coupling constants of 12.1–12.3 Hz and 16.8–17.1 Hz due to
their coupling with neighbouring Ha and Hx protons. The methine
protons (Hx) of 1c–12c were appeared slightly downfield as
compared to Ha and Hb at 5.21–5.22 ppm with coupling constant
values in the range of 7.5–7.8 and 12.0–12.3 Hz due to its coupling
with two neighbouring non-equivalent protons. The aromatic pro-
tons of three aryl rings appeared in the aromatic region at their
respective places between 6.71 and 7.66 ppm with multiplicity
according to their substitution pattern. The methylenic protons
(ArAOACH2A) of alkoxy side chain directly attached to oxygen
were observed as a triplet in the range of 3.84–4.02 ppm for all
pyrazolines confirming the presence of ether (ArAOACA) linkage.
All other aliphatic protons of the alkoxy side chains were found
between 0 and 2.0 ppm. Two methyl groups at 3- and 4-positions
of one of the aromatic ring installed on pyrazoline ring of
compounds 1c–12c were present as singlet in the range of
2.21–2.22 ppm and 2.17 ppm, respectively. In 13C NMR spectra of
compounds (1c–12c), all the carbons were found at their
respective places. The peaks for C3, C4 and C5 carbons of pyrazoline
skeleton were found at 64.4–69.5 ppm, 43.2–43.4 ppm and
Fig. 2. Mass fragmentation pattern of representative compound 3c.

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
187
53.4–64.4 ppm, respectively and the carbons of three aryl rings
were observed in the range of 110.8–158.5 ppm. Furthermore, all
the carbons of alkyloxy side chains were also present in the range
of 10.5–31.8 ppm.
(3:7) binary solvent mixture, showed an intense absorption band
attributed to the
[53,54] with absorption maxima (k ) wavelength in the range
p
–
p⁄ transitions of the conjugated backbone
abs
max
of 408–416 nm (Table 1). However, due to the same 1,3,5-triaryl-
2-pyrazoline central nucleus, similar spectral shapes/curves in
the absorption spectra were achieved (Fig. 3). Therefore, it can be
concluded that the difference in absorption wavelengths and
absorption intensities is only due to the presence of homologous
alkoxy substituents at one of the phenyl ring of 2-pyrazoline
nucleus.
Mass spectra
The formation of proposed structures of compounds 1c–12c
discussed in IR, 1H and 13C NMR spectroscopy were also confirmed
through their electron impact mass spectral (EIMS) data. Not only,
the molecular ion peak (M+Å) for all the compounds was observed
at their respective molecular masses, fragmentation pattern was
also in good agreement with already reported 2-pyrazoline deriv-
atives [46–49,52]. Furthermore, the molecular ion peak was found
to be the most stable fragment or base peak in all the synthesized
compounds. The molecular mass data of all the synthesized
pyrazoline derivatives 1c–12c are provided in the experimental
part. However, a representative example of compound 3c with
proposed mass fragmentation pattern where molecular ion peak
and most stable fragment appeared at m/z 402 (calcd. 402.21) of
[C26H27FN2O]+ is shown in Fig. 2.
The emission properties of compounds (1c–12c) were then
abs
studied at their corresponding excitation wavelengths (k
)
max
(Table 1, Fig. 4). The same DMF–water (3:7) binary solvent system
was again used. All the compounds (1c–12c) displayed blue emis-
sion in the range of 471–476 nm with variable emission intensity,
when measured at constant concentration of 1 ꢂ 10ꢁ7 mol Lꢁ1. The
compounds 1c, 8c and 11c showed emission at higher wavelengths
(476 nm each) in comparison to other compounds of the series.
This random trend may be credited to the different conformations
of alkyloxy groups in solution [53]. Similar to most of the hitherto
reported triaryl-2-pyrazolines [46–48], the present series also
em
showed blue fluorescence, as their emission maxima (k ) wave-
Fluorescence properties
max
lengths fall in the blue region of visible spectrum.
The geometry and substitution pattern of 1,3,5-triaryl-2-pyraz-
oline ring are considered to be important/responsible for blue
fluorescence of such compounds [17–21]. The aryl groups present
at 1- and 3-position of pyrazoline ring form the conjugated back-
bone and are mainly responsible for absorption of photons [55]
whereas the aryl group present at 5-position of the pyrazoline is
not a part of that conjugated system. Therefore, the substitution
of alkoxy group at aryl group (not part of the conjugated backbone)
can be expected to have some influence on the physico-chemical
properties. From the results of the present series, it is quite clear
that the change in the length of alkyloxy group strongly affect
The UV–Vis and emission spectroscopy was used to evaluate
fluorescence properties of the synthesized homologous 1,3,5-tri-
aryl-2-pyrazolines (1c–12c). In general, all the compounds
displayed fluorescence in the blue region of the visible spectrum
(Table 1).
The UV–Vis spectra of compounds 1c–12c, recorded at a con-
centration of 1 ꢂ 10ꢁ5 mol Lꢁ1 in N,N-dimethylformamide-water
Table 1
Absorption and fluorescence wavelengths for 1c–12c.
abs
em
Compound
k
(nm)
k
(nm)
Stoke’s shift
max
max
the emission intensity without causing major blue or red shift in
em
the emission wavelength (k ).
1c
2c
3c
4c
5c
6c
7c
8c
413
414
408
409
412
413
412
414
408
416
413
416
476
474
472
472
472
474
475
476
471
472
476
474
63
60
64
63
60
61
63
62
63
56
63
58
max
It is also worth mentioning here that the compounds 1c–12c
with chloro substitution on aryl ring present at 3-position of
pyrazoline moiety, revealed a notable influence on absorption
wavelength and emission intensity of the 1,3,5-triaryl-2-pyrazo-
lines as compared to our previously reported compounds 1h–12h
[48] and 1b–12b [46] where there is no substitution and fluoro
9c
10c
11c
12c
Fig. 3. The UV–Vis absorption spectra of 1-(3,4-dimethylphenyl)-3-(4-chloro-
phenyl)-5-(4-alkoxyphenyl)-2-pyrazolines 1c–12c in DMF–H2O (3:7) system with
Fig. 4. The emission spectra of 1-(3,4-dimethylphenyl)-3-(4-chlorophenyl)-5-(4-
alkoxyphenyl)-2-pyrazolines 1c–12c in DMF–H2O (3:7) system with the
the concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1
.
concentration of 1 ꢂ 10ꢁ7 mol Lꢁ1
.

188
A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
substitution, respectively on 3-aryl of pyrazoline ring (Figs. 5 and
6). In case of chloro group, the influence is more prominent as com-
pared to fluoro group (Fig. 6). This difference may be attributed to
the increased electron donating ability of chloro group towards the
conjugated backbone for more effective intramolecular charge
transfer. Consequently, it is anticipated that by changing the sub-
stituent on 3-aryl of pyrazoline ring, the emission intensity can
be tuned, an important and useful approach in controlling/optimiz-
ing the optoelectronic and luminescence properties of pyrazoline
based OLEDs.
To judge the effect of solvent nature and polarity, compound 4c
which exhibited maximum emission intensity was remeasured in
pure DMF instead of DMF:H2O mixture at a concentration of
1 ꢂ 10ꢁ7 mol Lꢁ1. Fascinatingly, emission intensity was greatly
increased. This increase of emission intensity of 4c in pure DMF
as compared to DMF–H2O (3:7) mixture may probably be due to
the interaction of water molecules with chloro substituent through
intermolecular hydrogen bonding leading to its decreased electron
donating ability to the conjugated backbone and hence decreasing
intramolecular charge transfer. However, no such interactions
are present in pure DMF solvent due to its polar aprotic nature.
The emission intensity measurements of compounds 1c–12c in
pure water could not be made because of their insolubility in
Fig. 7. Photoluminescence of 1-(3,4-dimethylphenyl)-3-(4-chlorophenyl)-5-(4-
butoxyphenyl)-2-pyrazoline 4c in ethyl ethanoate (1 ꢂ 10ꢁ3 mol Lꢁ1).
pure water. Fig. 7 demonstrates photoluminescence of 1-(3,4-
dimethylphenyl)-3-(4-chlorophenyl)-5-(4-butoxyphenyl)-2-pyr-
azoline (4c) in ethyl ethanoate, however, it was not quantitatively
measured. This variable fluorescence emission behaviour of
compounds (1c–12c) can be described as the combined influence
of chloro and alkoxy substituents present in these molecules.
Conclusions
In summary, a series of new homologous 1,3,5-triaryl-2-pyraz-
olines (1c–12c) were synthesized and characterized by their
physical, spectral and microanalytical data. The effect of alkyloxy
chain length on fluorescence properties of these compounds were
studied by UV–Vis and emission spectroscopy. All the compounds
were found to have fluorescence in the blue region of the visible
abs
Fig. 5. Comparison of absorption maxima (k ) of 1c–12c and our previously
max
reported 1h–12h [48] and 1b–12b [46] in DMF–H2O (3:7) system at fixed
abs
em
concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1
.
spectrum. The absorption and emission maxima (k
and k
)
max
max
in the range of 408–416 nm and 471–476 nm for 1c–12c clearly
demonstrates the effect of alkyloxy chain length on otherwise
same fluorophore for all the compounds. It is also quite clear that
the chain length of alkyloxy group governs the aggregation/self
assembly in ground/excited states in DMF:water binary solution.
Furthermore, in comparison to our previously reported com-
pounds, 1h–12h, 1b–12b with no and fluoro-substituent, respec-
tively, the compounds of the present series 1c–12c having chloro
substitution on conjugated backbone of 1,3,5-triaryl-2-pyrazoline
system showed a red shift in both absorption and emission spec-
troscopy. Therefore, it can be concluded that by varying the chain
length of the substituents and substituting electron donors on
conjugated backbone of such compounds is a promising approach
to tune the emission intensity. The results of this study may
provide a route towards controlling/optimizing the optoelectronic
and luminescence properties of Organic Light Emitting Diodes
(OLEDs).
Acknowledgement
em
Fig. 6. Comparison of absorption maxima (k ) of 1c–12c and our previously
max
Financial support from Higher Education Commission (HEC),
Government of Pakistan is gratefully acknowledged.
reported 1h–12h [48] and 1b–12b [46] in DMF–H2O (3:7) system at fixed
concentration of 1 ꢂ 10ꢁ7 mol Lꢁ1
.

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 182–189
189
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