Synthesis and spectral characterization of new 1,3,5-triaryl-2-pyrazolines highlighting effect of alkyloxy chain length on fluorescence

Article abstract of DOI:10.1016/j.saa.2014.02.025

A series of new 1,3,5-triaryl-2-pyrazolines (1b-12b) having one to twelve carbon alkyloxy side chains were synthesized and characterized on the basis of their spectral (IR, ¹H & ¹³C NMR and GC-MS) and microanalytical data. The UV-Vis and emission spectroscopy was used to study the effect of alkyloxy chain length on absorption and fluorescence properties of 1b-12b. All the compounds showed fluorescence in the blue region of the visible spectrum. Interestingly, the alkyloxy chain length strongly affects the emission intensity of 1,3,5-triaryl-2-pyrazoline framework without causing any major blue- or red-shift in the emission wavelength (λmaxem). The absorption and emission maxima (λmaxabs & λmaxem) for compounds (1b-12b) were observed in the range of 337-364 nm and 454-464 nm, respectively. Furthermore, the effect of fluorine substituent on aryl ring present at 3-position of pyrazoline moiety on fluorescence properties is also discussed.

Full text of DOI:10.1016/j.saa.2014.02.025

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
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 1,3,5-triaryl-2-pyrazo-
lines highlighting effect of alkyloxy chain length on fluorescence
b
c
a
a,
Asghar Abbas a, , Safdar Hussain , Noureen Hafeez , Aurangzeb Hasan , 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, 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
ꢀ
ꢀ
2-Pyrazolines are attractive organic
fluorescent compounds.
Synthesis of new 1-(3,4-
dimethylphenyl)-3-(4-fluorophenyl)-
5-(4-alkoxyphenyl)-2-pyrazolines.
Spectral characterization and
fluorescence properties.
Effect of fluoride and alkyloxy
substituents on fluorescence.
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new 1,3,5-triaryl-2-pyrazolines (1b–12b) having one to twelve carbon alkyloxy side chains
were synthesized and characterized on the basis of their spectral (IR, 1H & 13C NMR and GC–MS) and
microanalytical data. The UV–Vis and emission spectroscopy was used to study the effect of alkyloxy
chain length on absorption and fluorescence properties of 1b–12b. All the compounds showed
fluorescence in the blue region of the visible spectrum. Interestingly, the alkyloxy chain length strongly
affects the emission intensity of 1,3,5-triaryl-2-pyrazoline framework without causing any major blue- or
Received 24 November 2013
Received in revised form 28 January 2014
Accepted 9 February 2014
Available online 21 February 2014
Keywords:
1,3,5-Triaryl-2-pyrazolines
Synthesis
Spectroscopy
Alkyloxy side chain
Fluorescence
em
abs
em
red-shift in the emission wavelength (k ). The absorption and emission maxima (k
& k ) for
max
max
max
compounds (1b–12b) were observed in the range of 337–364 nm and 454–464 nm, respectively.
Furthermore, the effect of fluorine substituent on aryl ring present at 3-position of pyrazoline moiety
on fluorescence properties is also discussed.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
solar cells, optoelectronics and electronic displays [4,5]. As
compared to inorganic fluorescent materials, their advantages
Organic fluorescent compounds are attractive materials due to
their potential applications in cosmetics, surface coatings, inks
and textile industries [1,2] in addition to their use in sensors [3],
include the ease of fabrication and the tunability of emission prop-
erties through a simple chemical modification. As a consequence,
organic electroluminescence devices (OELDs) have been found
more useful due to their low cost, broad range of emission colors,
high brightness, high luminous efficiency, good life stability and
simple processing [6–8].
⇑
Corresponding authors. Tel.: +92 333 5009600; 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.2014.02.025
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
33
Despite many advantages, the major drawback associated with
General procedure for the synthesis of compounds (1b–12b)
organic fluorescent materials is the decrease in fluorescence inten-
sity due to aggregation of molecules i.e., the formation of an exci-
mer-like species and H-type molecular aggregates [9–11]. To avoid
such aggregation, structural modifications of fluorescent molecule
such as introduction of appropriate substituents have been shown
to be effective [12–14]. For example, the incorporation of an ethyl
group on the conjugated backbone of diphenylbutadiene decreases
the planarity of the molecules, leading to monomer like fluores-
cence in the bulk state [12] whereas, the introduction of quinacri-
done dye molecule into dendrimers enhances the emission
efficiency by decreasing molecular aggregation in the condensed
phase [13]. So far, a number of different classes of organic fluores-
cent compounds have been explored for their applications as fluo-
rescent materials. Luminol, the chemiluminescent compound is an
essential part of crime scene investigation as it detects blood, even
invisible blood. This fluorescent compound is used by forensic
investigators to detect trace amounts of blood left at crime scenes,
as it reacts with iron found in hemoglobin [15]. It is used by
biologists in cellular assays for the detection of copper, iron, and
cyanides, as well as of specific proteins by western blot.
For the synthesis of compounds (1b–12b), 25 mL of acetic acid
solution of the respective 4-alkoxychalcone (0.01 mol) (1a–12a)
containing a few drops of hydrochloric acid was heated at 60–
65 °C for 30 min with constant stirring in a round bottom flask.
(3,4-Dimethylphenyl)hydrazine hydrochloride (3.45 g, 0.02 mol)
was then added to the reaction flask and the reaction mixture
was heated to reflux for 5–6 h. After that, the reaction mixture
was cooled to room temperature and poured onto the crushed
ice. The precipitates thus formed, were filtered, washed with dis-
tilled water and dried. The crude products were further purified
by silica gel column chromatography using petroleum ether/ethyl
acetate (4:1) as the mobile phase to get pure compounds 1b–12b
in excellent yields for spectral characterization and fluorescence
properties.
The different protons of compounds (1b–12b) are differentiated
according to the labeling scheme shown in Fig. 1 for better under-
standing of their 1H NMR chemical shifts.
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-methoxyphenyl)-2-
pyrazoline (1b)
Pyrazolines, typical ICT (Intramolecular Charge Transfer) com-
pounds [16], symbolizes a class of organic fluorescent compounds
of great significance which are not only important due to their di-
verse biological applications [17–27] but also due to their use in
organic electroluminescent devices (OELDs) and optoelectronics
[28–32]. Among the various pyrazoline derivatives, 1,3,5-triaryl-
2-pyrazolines have gained considerable attention and now repre-
sent an important class of organic materials exhibiting blue
fluorescence with high quantum yield [33–36]. These are also
described as hole transporting media in photoconductive and
emitting materials, organic photovoltaic cells, and in OELDs
[28,37–41]. Moreover, these are also found to have many other
applications, for example, these are used as optical brightening
agents for textiles, papers and plastics [42], fluorescent switches
[43] and fluorescent probes in many chemosensors [44,45].
Our interest in 2-pyrazolines, in general [46,47], and 1,3,5-tri-
aryl-2-pyrazolines, in particular [48], led us to continue and further
investigate this class of compounds. Therefore, as continuation of
our previous study [48], herein, we report the synthesis and fluo-
rescent property evaluation of new 1,3,5-triaryl-2-pyrazoline
derivatives having fluoro-substituent at 3-aryl and one to twelve
carbon long alkyloxy chain at 5-aryl of 2-pyrazoline ring. This
study is to understand the significance of the interplay of weak
interactions and the role of alkoxy side chain length towards absor-
bance and emission properties. The present series having fluores-
cence properties in the blue region of the visible spectrum are
potential future candidates for their use as blue light emitting
materials.
Yield 85%; pale yellow solid; m.p. 141–143 °C; Rf = 0.85 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1682, 1298, 1498,
1253, 1048, 1145, 1H NMR (300 MHz, CDCl3) d 2.17 (s, 3H,
NAArA4-CH3), 2.22 (s, 3H, NAArA3-CH3), 3.08 (dd, 1H, J = 7.8,
17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.83 (s, 3H,
AOACH3), 5.20 (dd, 1H, J = 7.8, 12.3 Hz, Hx), 6.71 (d, 1H,
0
J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ), 6.94 (d, 1H,
J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m, 2H, J = 8.7 Hz,
0
0
0
ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74 (m, 2H, ArHe@e ),
13C NMR (75 MHz, CDCl3) d 19.5, 20.1, 43.6, 55.2, 64.4, 110.7, 114.9
(2C), 115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C),
134.6, 137.0, 143.1, 145.2, 158.5, 164.4 EIMS: m/z 374 (M+Å, base
peak). Anal. calcd. for C24H23FN2O: C, 76.98; H, 6.19; N, 7.48;
Found: C, 76.93; H, 6.15; N, 7.55%.
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-ethoxyphenyl)-2-
pyrazoline (2b)
Yield 84%; pale yellow solid; m.p. 138–140 °C; Rf = 0.85 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1684, 1296, 1495,
1255, 1046, 1147, 1H NMR (300 MHz, CDCl3) d 1.44 (t, 3H,
J = 7.8 Hz, AOACH2ACH3), 2.17 (s, 3H, NAArA4ACH3), 2.22 (s,
3H, NAArA3ACH3), 3.08 (dd, 1H, J = 7.5, 16.8 Hz, Ha), 3.77 (dd,
1H, J = 12.3, 17.1 Hz, Hb), 4.00 (t, 2H, J = 6.6 Hz, AOACH2A), 5.18
(dd, 1H, J = 7.8, 12.3 Hz, Hx), 6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89
0
(d, 2H, J = 8.7 Hz, ArHc@c ), 6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05
0
(s, 1H, NAArHi), 7.11 (m, 2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H,
J = 8.7 Hz, ArHd@d ), 7.69–7.74 (m, 2H, ArHe@e ), 13C NMR (75 MHz,
CDCl3) d 14.8, 19.5, 20.1, 43.6, 63.4, 64.4, 110.7, 114.9 (2C), 115.1,
115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C), 134.6,
0
0
Experimental
Materials and methods
H3C
CH3
All reagents and solvents were used as obtained from the sup-
plier or recrystallized/redistilled as required. Thin layer chroma-
tography (TLC) was performed using aluminum 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.
i
h
g
N
N
Hx
e'
f
d'
c
f '
c'
Ha Hb
e
d
R
F
O
R = CnH2n+1 n = 1-12
Fig. 1. Labelling scheme for protons of compounds (1b–12b).

34
A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
137.0, 143.1, 145.2, 158.5, 164.4 EIMS: m/z 388 (M+Å, base peak).
Anal. calcd. for C25H25FN2O: C, 77.29; H, 6.49; N, 7.21; Found: C,
77.25; H, 6.44; N, 7.29%.
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-hexyloxyphenyl)-2-
pyrazoline (6b)
Yield 82%; pale yellow solid; m.p. 90–93 °C; Rf = 0.86 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1685, 1293,
1496, 1254, 1048, 1148, 1H NMR (300 MHz, CDCl3) d 0.93 (t, 3H,
J = 7.2 Hz, AOA(CH2)5ACH3), 1.33–1.50 (m, 6H, AOACH2ACH2-
A(CH2)3A CH3), 1.79 (qn, 2H, J = 7.8 Hz, AOACH2ACH2AC4H9),
2.18 (s, 3H, NAArA4ACH3), 2.23 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.95
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.8, 12.3 Hz, Hx),
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-propyloxyphenyl)-
2-pyrazoline (3b)
Yield 86%; pale yellow solid; m.p. 135–137 °C; Rf = 0.89 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1685, 1294, 1493,
1257, 1045, 1141, 1H NMR (300 MHz, CDCl3) d 1.05 (t, 3H,
J = 7.5 Hz, AOACH2ACH2ACH3), 1.82 (sextet, 2H, J = 7.5 Hz,
AOACH2ACH2ACH3), 2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H,
NAArA3ACH3), 3.08 (dd, 1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H,
J = 12.3, 17.1 Hz, Hb), 3.91 (t, 2H, J = 6.6 Hz, AOACH2A), 5.20 (dd,
1H, J = 7.8, 12.3 Hz, Hx), 6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d,
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.2,
0
22.6, 25.7, 29.2, 31.6, 43.6, 64.4, 68.0, 110.7, 114.9 (2C), 115.1,
115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C), 134.5,
137.0, 143.1, 145.2, 158.5, 164.4, EIMS: m/z 444 (M+Å, base peak).
Anal. calcd. for C29H33FN2O: C, 78.35; H, 7.48; N, 6.30; Found: C,
78.31; H, 7.43; N, 6.37%.
0
2H, J = 8.7 Hz, ArHc@c ), 6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s,
0
1H, NAArHi), 7.11 (m, 2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H,
J = 8.7 Hz, ArHd@d ), 7.69–7.74 (m, 2H, ArHe@e ), 13C NMR (75 MHz,
CDCl3) d 10.5, 18.8, 20.2, 20.6, 43.6, 64.4, 69.5, 110.7, 114.9 (2C),
115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C),
134.6, 137.0, 143.1, 145.2, 158.5, 164.4 EIMS: m/z 402 (M+Å, base
peak). Anal. calcd. for C26H27FN2O: C, 77.58; H, 6.76; N, 6.96;
Found: C, 77.52; H, 6.72; N, 7.02%.
0
0
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-heptyloxyphenyl)-
2-pyrazoline (7b)
Yield 81%; pale yellow solid; m.p. 88–91 °C; Rf = 0.88 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1679, 1292,
1488, 1258, 1047, 1143, 1H NMR (300 MHz, CDCl3) d 0.92 (t, 3H,
J = 7.2 Hz, AOA(CH2)6ACH3), 1.33–1.47 (m, 8H, AOACH2ACH2-
A(CH2)4A CH3), 1.78 (qn, 2H, J = 7.0 Hz, AOACH2ACH2AC5H11),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 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), 5.19 (dd, 1H, J = 7.8, 12.3 Hz, Hx),
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-butyloxyphenyl)-2-
pyrazoline (4b)
Yield 83%; pale yellow solid; m.p. 115–117 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1679, 1297, 1497,
1256, 1052, 1145, 1H NMR (300 MHz, CDCl3) d 0.97 (t, 3H,
J = 7.2 Hz, AOA(CH2)3ACH3), 1.50 (sextet, 2H, J = 7.5 Hz, AOACH2-
ACH2ACH2A CH3), 1.77 (qn, 2H, J = 8.0 Hz, AOACH2ACH2AC2H5),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.5, 16.8 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.95
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.5, 12.0 Hz, Hx),
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 19.4,
0
20.2, 22.6, 26.0, 29.0, 31.8, 43.5, 64.4, 68.0, 110.7, 114.9 (2C),
115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C),
134.5, 137.0, 143.1, 145.2, 158.5, 164.4, EIMS: m/z 458 (M+Å, base
peak). Anal. calcd. for C30H35FN2O: C, 78.57; H, 7.69; N, 6.11;
Found: C, 78.53; H, 7.65; N, 6.18%.
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.04 (s, 1H, NAArHi), 7.11 (m,
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 13.9, 18.8, 19.2,
0
20.2, 31.3, 43.6, 64.4, 67.6, 110.6, 114.9 (2C), 115.1, 115.4 (2C),
115.8, 127.0 (2C), 127.3, 127.4, 129.8 (2C), 134.6, 137.0, 143.1,
145.2, 158.5, 164.4, EIMS: m/z 416 (M+Å, base peak). Anal. calcd.
for C27H29FN2O: C, 77.85; H, 7.02; N, 6.73; Found: C, 77.81; H,
6.98; N, 6.80%.
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-octyloxyphenyl)-2-
pyrazoline (8b)
Yield 86%; pale yellow solid; m.p. 87–89 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1686, 1298,
1499, 1259, 1050, 1147, 1H NMR (300 MHz, CDCl3) d 0.92 (t, 3H,
J = 7.2 Hz, AOA(CH2)7ACH3), 1.31–1.48 (m, 10H, AOACH2ACH2-
A(CH2)5A CH3), 1.78 (qn, 2H, J = 7.0 Hz, AOACH2ACH2AC6H13),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.0, 16.8 Hz, Hb), 3.93
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.5, 12.0 Hz, Hx),
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-pentyloxyphenyl)-
2-pyrazoline (5b)
Yield 85%; pale yellow solid; m.p. 113–116 °C; Rf = 0.88 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1682, 1298, 1492,
1252, 1054, 1149, 1H NMR (300 MHz, CDCl3) d 0.95 (t, 3H,
J = 7.2 Hz, AOA(CH2)4ACH3), 1.37–1.48 (m, 4H, AOACH2ACH2-
A(CH2)2A CH3), 1.79 (qn, 2H, J = 7.2 Hz, AOACH2ACH2AC3H7),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.0, 16.8 Hz, Hb), 3.94
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.8, 12.3 Hz, Hx),
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 19.5,
0
20.4, 22.7, 26.0, 29.2, 29.4, 31.8, 43.6, 64.4, 68.0, 110.7, 114.9
(2C), 115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8
(2C), 134.5, 137.0, 143.1, 145.2, 158.5, 164.4, EIMS: m/z 472 (M+Å,
base peak). Anal. calcd. for C31H37FN2O: C, 78.78; H, 7.89; N,
5.93; Found: C, 78.73; H, 7.85; N, 5.99%.
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.0, 18.8, 20.2,
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-nonyloxyphenyl)-2-
pyrazoline (9b)
Yield 83%; pale yellow solid; m.p. 93–96 °C; Rf = 0.86 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1685, 1296,
1497, 1254, 1055, 1145, 1H NMR (300 MHz, CDCl3) d 0.92 (t, 3H,
J = 7.0 Hz, AOA(CH2)8ACH3), 1.30–1.48 (m, 12H, AOACH2ACH2-
0
22.5, 28.2, 29.0, 43.6, 64.4, 67.9, 110.7, 114.9 (2C), 115.1, 115.4
(2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C), 134.5, 137.0,
143.1, 145.2, 158.5, 164.4 EIMS: m/z 430 (M+Å, base peak). Anal.
calcd. for C28H31FN2O: C, 78.11; H, 7.26; N, 6.51; Found: C, 78.08;
H, 7.21; N, 6.59%.

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
35
A(CH2)6A CH3), 1.79 (qn, 2H, J = 8.0 Hz, AOACH2ACH2AC7H15),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.94
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.5, 12.0 Hz, Hx),
m/z 514 (M+Å, base peak). Anal. calcd. for C34H43FN2O: C, 79.34;
H, 8.42; N, 5.44; Found: C, 79.31; H, 8.39; N, 5.49%.
1-(3, 4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-dodecyloxyphenyl)-
2-pyrazoline (12b)
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
Yield 83%; pale yellow solid; m.p. 75–78 °C; Rf = 0.89 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1685, 1294,
1495, 1255, 1056, 1147, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 7.0 Hz, AOA(CH2)11ACH3), 1.29–1.48 (m, 18H, AOACH2ACH2-
A(CH2)9ACH3), 1.78 (qn, 2H, J = 7.5 Hz, AOACH2ACH2AC10H21),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.94
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.5, 12.3 Hz, Hx),
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.2,
0
22.7, 26.0, 29.3, 29.4, 29.4, 29.5, 31.9, 43.6, 64.4, 68.0, 110.7,
114.9 (2C), 115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4,
129.8 (2C), 134.5, 137.0, 143.1, 145.2, 158.5, 164.4, EIMS: m/z
486 (M+Å, base peak). Anal. calcd. for C32H39FN2O: C, 78.98; H,
8.08; N, 5.76; Found: C, 78.94; H, 8.05; N, 5.82%.
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-decyloxyphenyl)-2-
pyrazoline (10b)
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ),13C NMR
(75 MHz, CDCl3) d 14.1, 18.8, 20.2, 22.7, 26.0, 29.3, 29.3, 29.4,
29.6, 29.6, 29.6, 29.6, 31.9, 43.6, 64.4, 68.0, 110.7, 114.9 (2C),
115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3, 127.4, 129.8 (2C),
134.5, 137.0, 143.1, 145.2, 158.5, 164.4, EIMS: m/z 528 (M+Å, base
peak). Anal. calcd. for C35H45FN2O: C, 79.50; H, 8.58; N, 5.30;
Found: C, 79.45; H, 8.54; N, 5.37%.
0
0
Yield 87%; pale yellow solid; m.p. 97–99 °C; Rf = 0.88 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1679, 1299,
1496, 1258, 1049, 1149, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 6.9 Hz, AOA(CH2)9ACH3), 1.30–1.46 (m, 14H, AOACH2ACH2-
A(CH2)7ACH3), 1.79 (qn, 2H, J = 7.2 Hz, AOACH2ACH2AC8H17),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.08 (dd,
1H, J = 7.5, 16.8 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 17.1 Hz, Hb), 3.94
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.8, 12.3 Hz, Hx),
Fluorescence properties of 1,3,5-triaryl-2-pyrazolines
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
The fluorescence properties of compounds (1bꢁ12b) were stud-
ied by UV–Vis and emission spectroscopy carried out at room
temperature (298 K). The UV–Vis and the emission spectra of
1,3,5-triaryl-2-pyrazolines (1bꢁ12b), were recorded in N,N-
dimethylformamideꢁwater (3:7) mixture at a concentration of
1 ꢂ 10ꢁ5 mol Lꢁ1 and 1 ꢂ 10ꢁ7 mol Lꢁ1, respectively.
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 20.2,
0
22.7, 26.0, 29.3, 29.3, 29.4, 29.6, 29.6, 31.9, 43.6, 64.4, 68.0,
110.7, 114.9 (2C), 115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3,
127.4, 129.8 (2C), 134.5, 137.0, 143.1, 145.2, 158.5, 164.4, EIMS:
m/z 500 (M+Å, base peak). Anal. calcd. for C33H41FN2O: C, 79.16; H,
8.25; N, 5.59; Found: C, 79.12; H, 8.22; N, 5.65%.
Results and discussion
Chemistry
1-(3,4-Dimethylphenyl)-3-(4-fluorophenyl)-5-(4-undecyloxyphenyl)-
2-pyrazoline (11b)
The compounds (1b–12b) were synthesized by refluxing (E)-1-
Yield 88%; pale yellow solid; m.p. 84–86 °C; Rf = 0.87 (petro-
leum ether:ethyl acetate, 4:1), FT-IR (KBr, cmꢁ1): 1685, 1292,
1497, 1256, 1057, 1142, 1H NMR (300 MHz, CDCl3) d 0.91 (t, 3H,
J = 7.0 Hz, AOA(CH2)10ACH3), 1.29–1.47 (m, 16H, AOACH2ACH2-
A(CH2)8ACH3), 1.78 (qn, 2H, J = 7.8 Hz, AOACH2ACH2AC9H19),
2.17 (s, 3H, NAArA4ACH3), 2.22 (s, 3H, NAArA3ACH3), 3.07 (dd,
1H, J = 7.8, 17.1 Hz, Ha), 3.77 (dd, 1H, J = 12.3, 16.8 Hz, Hb), 3.93
(t, 2H, J = 6.6 Hz, AOACH2A), 5.19 (dd, 1H, J = 7.5, 12.0 Hz, Hx),
(4-fluorophenyl)-3-(4-alkoxyphenyl)prop-2-en-1-one
(1aꢁ12a)
[49] with excess of (3,4-dimethylphenyl)hydrazine hydrochloride
in glacial acetic acid solvent containing catalytic amount of hydro-
chloric acid for 5–6 h (Scheme 1) and purified by silica gel column
chromatography using petroleum ether/ethyl acetate as the mobile
phase. All the products were obtained as solids in good to excellent
yields (81–88%) indicating a little influence of alkyl chain length on
reaction efficiency. The structures of all the compounds were con-
firmed on the basis of their spectral and microanalytical data.
0
6.71 (d, 1H, J = 8.1 Hz, NAArHg), 6.89 (d, 2H, J = 8.7 Hz, ArHc@c ),
6.94 (d, 1H, J = 8.1 Hz, NAArHh), 7.05 (s, 1H, NAArHi), 7.11 (m,
0
0
2H, J = 8.7 Hz, ArHf@f ), 7.25 (d, 2H, J = 8.7 Hz, ArHd@d ), 7.69–7.74
Spectral characterization of 1b–12b
(m, 2H, ArHe@e ), 13C NMR (75 MHz, CDCl3) d 14.1, 18.8, 19.5,
0
19.8, 20.2, 22.7, 26.0, 29.3, 29.4, 29.6, 29.6, 31.9, 43.6, 64.4, 68.0,
110.7, 114.9 (2C), 115.1, 115.4 (2C), 115.7, 127.0 (2C), 127.3,
127.4, 129.8 (2C), 134.5, 137.0, 143.1, 145.2, 158.5, 164.4, EIMS:
IR spectra
In the IR spectra of compounds (1b–12b), two characteristic
absorption bands in the range of 1686–1679 cmꢁ1 and
H3C
N
CH3
O
H3C
CH3
N
Glacial CH3COOH
+
R
Conc. HCl, Reflux 5-6 Hr
F
O
1a-12a
H2N NH
R
F
1b-12b
O
R = CnH2n+1 with n = 1-12
Scheme 1. Synthesis of 1,3,5-triaryl 2-pyrazolines, 1b–12b.

36
A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
Fig. 2. Mass fragmentation pattern of 3b.
1298–1292 cmꢁ1 were assigned to the stretching of carbon–nitro-
gen double bond (C@N) and carbon–nitrogen single bond (CAN),
respectively. The presence of these two frequencies indicates the
formation of cyclization product. Another band observed in the re-
gion of 1149–1147 cmꢁ1 shows the presence of ArAF bond. Two
more strong bands at stretching frequencies in the range of

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
37
1259–1252 cmꢁ1 (ArAO-stretching) and 1057–1045 cmꢁ1 (AOAR
stretching) indicate the presence of ArAOAR group. The most sig-
nificant absorption bands observed in the IR spectra of 1b–12b are
presented in the experimental section.
1H NMR spectra & 13C NMR spectra
In the 1H NMR spectra of compounds (1b–12b), all the protons
of pyrazoline nucleus and three aromatic rings were found in their
expected chemical shift regions. The presence of two methylene
protons (Ha and Hb) and one methine proton (Hx) as three doublet
of doublets confirmed the formation of five membered pyrazoline
ring. The methylenic protons Ha appeared at 3.07–3.08 ppm with
coupling constants of 7.5–7.8 Hz and 16.8–17.1 Hz due to their
coupling with neighboring Hb and Hx protons. Similarly, methylen-
ic protons Hb for compounds 1b–12b appears at 3.76–3.77 ppm
with coupling constants of 12.0–12.3 Hz and 16.8–17.1 Hz due to
their coupling with neighboring Ha and Hx protons. However, the
chemical shifts for methine protons (Hx) of 1b–12b were observed
at 5.18–5.20 ppm with coupling constant values in the range of
7.5–7.8, 12.0–12.3 Hz. The aromatic protons due to the presence
of three aryl rings appeared downfield between 6.50 and
8.00 ppm with multiplicity according to their substitution pattern.
The methylene protons (ArAOACH2A) of alkoxy side chain present
at one of the aryl ring appeared as a triplet. The chemical shifts for
these protons were found around 3.83–4.00 ppm for all pyrazo-
lines confirming the presence of ether (ArAOACA) linkage. The
chemical shifts for other aliphatic protons of the alkoxy groups
were observed between 0 and 2.0 ppm. The protons of methyl
groups at 3- and 4-positions of the aromatic ring directly attached
to the nitrogen of the pyrazoline ring in compounds 1bꢁ12b,
appeared as singlet in the range of 2.23–2.22 ppm and 2.17–
2.18 ppm respectively. The 13C NMR spectra of compounds (1b–
12b) displayed peaks at 64.4–69.5 ppm, 43.5–43.6 ppm and
55.2–64.4 ppm for C3, C4 and C5 carbons, respectively. All the aro-
matic carbons were observed in the range of 110.6–164.4 ppm. The
signals for alkoxy carbons were noticed in the range of 10.5–
31.6 ppm for all the compounds. Furthermore, the conclusions
drawn from the 1H & 13C NMR data were found to be in good agree-
ment with the results of IR spectra discussed in the previous
section.
Fig. 3. The UV–Vis absorption spectra of 1-(3,4-dimethylphenyl)-3-(4-fluoro-
phenyl)-5-(4-alkoxyphenyl)-2-pyrazolines (1bꢁ12b) in DMF-H2O (3:7) system
with the concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1
.
pattern of the representative compound 3b where molecular ion
peak and most stable fragment appeared at m/z 402 (Calcd.
402.21) of [C26H27FN2O].+ is shown in Fig. 2.
Fluorescence properties
The fluorescence properties of the synthesized 1,3,5-triaryl-2-
pyrazolines (1b–12b) bearing homologous alkyloxy groups were
studied by UV–Vis and emission spectroscopy. As shown in Table 1,
all the compounds exhibited fluorescence in the blue region of the
visible spectrum. The UV–Vis spectra of 1b–12b were recorded at a
concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1 in N,N-dimethylformamide-
water (3:7) binary solvent mixture. An intense absorption band
for 1bꢁ12b, attributed to the
p
–
p* transitions of the conjugated
abs
backbone [51] with absorption maxima (k ) wavelength in the
max
range of 337–364 nm, was observed and are presented in Table 1.
However, the spectral shapes/curves were found similar in the
absorption spectra due to the presence of the same 1,3,5-triaryl-
2-pyrazoline central nucleus for all the compounds (Fig. 3). It is
interesting to mention here that the difference of absorption
Mass spectra
The electron impact mass spectra (EIMS) of all the synthesized
pyrazoline derivatives 1b–12b are presented in the experimental
part. The mass spectral data and fragmentation pattern of all the
synthesized 2-pyrazoline derivatives [50], clearly justify the for-
mation of proposed structures discussed in IR, 1H and 13C NMR
spectroscopy. Further, a molecular ion peak (M+Å) was observed
for all the compounds at their respective molecular masses. The
most stable fragments or base peaks for 1b–12b were also found
to be the molecular ion peaks. The proposed mass fragmentation
Table 1
Absorption and fluorescence wavelengths for 1b–12b.
em
max
abs
max
Compound
Stoke’s shift
k
(nm)
k
(nm)
1b
2b
3b
4b
5b
6b
7b
8b
337
349
358
345
359
344
343
364
364
343
347
343
454
462
461
464
464
459
463
464
459
459
458
457
117
113
103
119
105
115
120
100
95
9b
10b
11b
12b
116
111
114
Fig. 4. The emission spectra of 1-(3,4-dimethylphenyl)-3-(4-fluorophenyl)-5-(4-
alkoxyphenyl)-2-pyrazolines (1bꢁ12b) in DMF-H2O (3:7) system at concentration
of 1 ꢂ 10ꢁ7 mol Lꢁ1
.

38
A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
abs
Fig. 5. Comparison of absorption maxima (k ) of 1bꢁ12b and our previously reported 1h–12h [48] in DMF-H2O (3:7) system at fixed concentration of 1 ꢂ 10ꢁ5 mol Lꢁ1
.
max
wavelengths and absorption intensities is only due to the effect of
presence of 4-alkoxyphenyl groups at the 5-position of 2-pyrazo-
line nucleus.
5-aryl on 2-pyrazoline can be expected to have some influence
on the physico-chemical properties. From the results, it is quite
clear that the change in the length of alkyloxy group strongly affect
The emission properties of compounds (1bꢁ12b) were then
the emission intensity without major blue or red shift in the
abs
max
em
studied at their corresponding excitation wavelengths (k
)
emission wavelength (k ).
max
(Table 1, Fig. 4). The same DMF-water (3:7) binary solvent system
was used for this purpose. Measured at constant concentration of
1 ꢂ 10ꢁ7 mol Lꢁ1, all the compounds (1b–12b) exhibited blue
emission in the range of 454–464 nm with variable emission inten-
sity. The compounds 4b, 5b and 8b showed maximum emission at
higher wavelengths (464 nm each) as compared to other com-
pounds of the series. This random trend may be attributed to the
different conformations of alkyloxy groups in solution [52]. Similar
to most of the previously reported triaryl-2-pyrazolines [48], these
It is also important to address here that the compounds (1b–
12b) with fluoro substitution on 3-aryl of pyrazoline revealed a
notable influence on absorption wavelength and emission intensity
of the 1,3,5-triaryl-2-pyrazolines as compared to our previously re-
ported compounds (1h–12h) [48] where there is no substitution on
3-aryl of pyrazoline ring (Figs. 5 and 6). This difference may be
attributed to the positive role of fluoro group exerting on the con-
jugated backbone for more effective intramolecular charge trans-
fer. Therefore, it is anticipated that by changing the substituent
on 3-aryl of pyrazoline ring, the emission intensity can be tuned
which is very important in controlling/optimizing the optoelec-
tronic and luminescence properties of pyrazoline based OLEDs.
To check the effect of solvent nature and polarity, emission
intensity of representative compound 8b, which exhibited maxi-
mum emission intensity was measured in pure DMF instead of
DMF:H2O mixture at a concentration of 1 ꢂ 10ꢁ7 mol Lꢁ1. Interest-
ingly, emission intensity was greatly increased. This enhancement
in emission intensity of 8b in pure DMF as compared to DMF-H2O
(3:7) mixture may possibly be due to the interaction of water
compounds (1b–12b) also showed blue fluorescence because of
em
max
the presence of their emission maxima (k ) wavelengths in the
blue region of visible spectrum.
The geometry and substitution pattern of 1,3,5-triaryl-2-pyraz-
oline ring are thought to be responsible for blue fluorescence of
such compounds [33–36,28]. The aryl groups present at 1- and 3-
position of pyrazoline form the conjugated backbone and are
mainly responsible for absorption of photons [51] 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
em
Fig. 6. Comparison of emission maxima (k ) of 1bꢁ12b and our previously reported 1h–12h [48] in DMF-H2O (3:7) system at fixed concentration of 1 ꢂ 10ꢁ7 mol Lꢁ1
.
max

A. Abbas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 32–40
39
a red shift in emission spectroscopy. It can thus be concluded that
variation in the length of alkyloxy side chain and substitution of
electron donating groups on conjugated backbone of such com-
pounds can be used to tune the emission intensity. These findings
may further play an important role in controlling/optimizing the
optoelectronic and luminescence properties of Organic Light Emit-
ting Diodes (OLEDs) based on related fluorescent compounds.
Acknowledgements
We are highly grateful to Higher Education Commission (HEC),
Government of Pakistan for financial support.
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which led to the decrease of intramolecular charge transfer. How-
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A series of 1,3,5-triaryl-2-pyrazoline derivatives (1b–12b) bear-
ing one to twelve carbon long alkyloxy side chains have been syn-
thesized starting from chalcones (1a–12a) and characterized by
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40
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