Experimental and theoretical study of two new pyrazoline derivatives based on dibenzofuran

Article abstract of DOI:10.1016/j.saa.2011.08.026

Two novel pyrazoline derivatives, named 2,8-bis(1,3-diphenyl-pyrazoline-5- yl)dibenzofuran (A) and 2,8-bis(1-(4-bromophenyl)-3-phenyl-pyrazoline-5-yl) dibenzofuran (B), were synthesized and characterized by elemental analysis, NMR, MS and thermogravimetric analysis. The absorption and emission spectra of them were determined by experimental methods in different polar solvents and were computed using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) at the same time. The calculated absorption and emission wavelengths are in good agreement with the experimental data. The fluorescence quantum yields and fluorescence lifetimes of them in different polar solvents were studied by means of steady state and time resolved fluorescence. The calculated reorganization energy for hole and electron indicates that the two compounds are in favor of hole transport than electron transport. The results show the two compounds present high fluorescence quantum yields and excellent thermal stability. It makes them of great interest as novel fluorescent probes and optoelectronic materials.

Full text of DOI:10.1016/j.saa.2011.08.026

Spectrochimica Acta Part A 83 (2011) 242–249
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Experimental and theoretical study of two new pyrazoline derivatives based on
dibenzofuran
He-ping Shia,c,∗, Jian-xin Daia, Xiu-feng Zhanga, Lei Xua, Long Wanga, Li-wen Shia, Li Fanga,b,∗∗
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b Engineering Research Center of Fine Chemicals, Ministry of Education, Taiyuan 030006, PR China
c Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences, Fuzhou 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel pyrazoline derivatives, named 2,8-bis(1,3-diphenyl-pyrazoline-5-yl)dibenzofuran (A) and 2,8-
bis(1-(4-bromophenyl)-3-phenyl-pyrazoline-5-yl)dibenzofuran (B), were synthesized and characterized
by elemental analysis, NMR, MS and thermogravimetric analysis. The absorption and emission spectra of
them were determined by experimental methods in different polar solvents and were computed using the
density functional theory (DFT) and the time-dependent density functional theory (TDDFT) at the same
time. The calculated absorption and emission wavelengths are in good agreement with the experimental
data. The fluorescence quantum yields and fluorescence lifetimes of them in different polar solvents were
studied by means of steady state and time resolved fluorescence. The calculated reorganization energy for
hole and electron indicates that the two compounds are in favor of hole transport than electron transport.
The results show the two compounds present high fluorescence quantum yields and excellent thermal
stability. It makes them of great interest as novel fluorescent probes and optoelectronic materials.
© 2011 Elsevier B.V. All rights reserved.
Received 21 March 2011
Received in revised form 7 August 2011
Accepted 10 August 2011
Keywords:
Pyrazoline derivatives
Dibenzofuran
Fluorescence
TDDFT
1. Introduction
conjugated fluorescent dyes. They have high hole-transport effi-
ciency, excellent blue emission and high quantum yields [16–18],
Since the first discovery of organic light-emitting diodes (OLEDs)
by Tang and Van Slyke, light-emitting materials have attracted sig-
nificant attention because of their potential applications in flat
panel displays [1,2]. The most actively pursued areas of research
are the improvement of efficiency and stability of the device and
the tuning of color using different emitting materials. The contin-
uing efforts include ingenious device fabrication and synthesis of
materials with improved properties. Therefore, the design of novel
light-emitting molecules has been a hot issue.
At present, the development of blue-emitting molecules with
high efficiency and good color purity draws much attention,
and there have been a great number of studies for new blue
light-emitting materials that achieve higher efficiencies and have
improved characteristics [3–6].
Pyrazoline derivatives are important five-membered, nitrogen-
containing heterocyclic compounds and they have N atoms which
attain conjugation by donating electron. They are attracting
increasing interest of many researchers, not only in medici-
nal chemistry because of their bioactivity such as antimicrobial
[7,8], antiamoebic [9,10], antinociceptive [11], anticancer [12],
antidepressant [13] and antiinflammatory [14,15], but also in
which have been widely used as fluorescent brightening agents for
textiles, fabrics, plastics and papers, fluorescence chemosensors,
hole-transport materials in electrophotography, OLEDs and as a
hole-conveying medium in photoconductive materials [19–28].
Many pyrazoline derivatives have been reported as hole-
transport materials in organic electroluminescent devices (OLEDs).
Jin et al. synthesized several pyrazoline derivatives with different
electron withdrawing and pushing groups. The results indicated
that the pyrazoline derivatives with both electron withdrawing
and pushing substitutional groups were the optimistic candidate
for electroluminescent emitter due to higher transfer efficiency
from electric energy to light energy as well as larger luminance
[29]. Wang et al. synthesized a pyrazoline derivative with the
presence of anthryl substituent at position of the pyrazoline ring.
It is concluded that photo-induced intramolecular energy trans-
fer from the anthryl to pyrazoline moiety exists simultaneously
with the charge transfer from N (1) to C (3) in the pyrazoline
moiety in the excited state and both compete with each other
[30]. Bai et al. synthesized several novel fluorescence dyes in
which carbazole moiety was linked to pyrazoline molecule [31].
Recently, a series of new pyrazoline derivatives have been syn-
thesized as efficient emitting materials in organic EL devices and
their photophysical properties have been investigated in detail
[32,33].
∗
Corresponding author. Tel.: +86 351 7018094; fax: +86 351 7011688.
Corresponding author.
E-mail address: hepingshi@sxu.edu.cn (H.-p. Shi).
In our previous work, a series of compounds containing a het-
erocycle as the core have been synthesized and investigated via
∗∗
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.026

H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
243
O
O
Br
O
Br
Br2
H
n-BuLi
DMF
CH3CH2OH
KOH
H
+
O
O
O
1
2
N
N
N
N
NH-NH2
O
O
O
A
Br
Br
O
3
Br
NH-NH2HCl
N
N
N
N
O
B
Scheme 1. The synthetic route of the compound A and B.
experimental and theoretical methods [34,35]. According to our
research, we conclude the incorporation of electron-donors and
electron-acceptors into one molecule should be highly desirable
to improve optical properties.
Thermogravimetric analyses were performed with a TA TGA 2050
thermogravimetric analyzer under nitrogen atmosphere with
a heating rate of 20 ◦C/min from room temperature to 600 ◦C.
Elemental analyses were performed with an Elementar Analysen-
systeme (GmbH). Mass spectra were recorded with the LC–MS
system consisted of a Waters 1525 pump and a Micromass ZQ4000
singlequadrupole mass spectrometer detector (Waters). UV–vis
spectra were obtained on a shimadzu UV-2450 spectrophotometer.
Fluorescence spectra were obtained on a Shimadzu RF-5301PC
fluorothotometer. Fluorecence quantum yields were determined
using the a standard actinometry method. Quinine sulfate was
used in the actinometer with a known fluorescence quantum
yield of 0.55 in 0.1 mol/L sulphuric acid, the sample was excited
at 350 nm. The fluorescence decay curves were recorded with
time-correlated single photon counting (TCSPC) technique using a
commercially available Edinburgh Instruments.
In this paper, in view of the excellent optical properties of
pyrazoline derivatives and our interest in the synthesizing anal-
ogous compounds involving fascinating diverse heterocycles, we
designed and synthesized two novel pyrazoline derivatives con-
taining two pyrazoline rings as side chains (electron-donors) and
a dibenzofuran ring as the core (electron-acceptor). The synthetic
route of the investigated compounds, named 2,8-bis(1,3-diphenyl-
pyrazoline-5-yl)dibenzofuran (A) and 2,8-bis(1-(4-bromophenyl)-
3-phenyl-pyrazoline-5-yl)dibenzofuran (B), is shown in Scheme 1.
Their structures were characterized by elemental analysis, IR,
NMR and MS. Furthermore, their charge injection and transport
properties and spectral properties were studied by experimen-
tal and theoretical methods. In combination of pyrazoline rings
with dibenzofuran molecule, we anticipate the formation of novel
optical materials with excellent thermal stability and eminent pho-
tophysical properties. The results will increase the knowledge to
design and synthesize novel pyrazoline derivatives with excellent
optical properties.
2.1.2. Procedures
The syntheses of the two compounds were described in Section
2.2. All spectral experiments were carried out at room temperature.
The concentration of compounds is 1.0 × 10−5 mol/L in spectral
experiments. All emission spectra were corrected. Excitation and
emission slits width were both set at 2.5 nm.
2. Experimental
2.2. Synthesis and characterization
2.1. Materials and methods
2.2.1. Synthesis of 2,8-dibromodibenzofuran (1)
2.1.1. Reagents and apparatus
A 250 ml round bottom flask containing 8.4 g (50 mmol) of
dibenzofuran dissolved in 100 ml of glacial acetic acid was
equipped with an addition funnel. Bromine 5.13 ml (100 mmol) in
30 ml of glacial acetic acid was added dropwise via the addition
funnel to the dibenzofuran under constant stirring. This reaction
mixture was stirred at room temperature for 4 h. It was then
refluxed for 6 h, cooled. The solid was then collected by filtration
and washed with three 100 ml portions of water. Recrystallization
from 100 ml of acetic anhydride obtained 12.2 g (75%) pure 2,8-
dibromodibenzofuran (1) as a white solid; m.p. 188–190 ◦C; 1H
Dibenzofuran and n-butyl lithium were purchased from Alfa
Aesar and used without further purifications, other reagents
were purchased from Beijing Chemical plants. Hexane, ethanol,
DMF and benzene etc. were purified according to standard
methods. All the reactions were carried out under nitrogen
atmosphere.
Melting points were determined on an X-5 melting point
detector and uncorrected. All NMR spectra were measured on a
Bruker DRX-300 spectrometer with CDCl3 or DMSO-d6 as solvent.

244
H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
Br
Br
N
N
N
N
N
N
N
N
O
O
Compound A
Compound B
Fig. 1. The molecular structures as calculation models.
NMR (CDCl3, 300 MHz): ı 7.17 (d, 2H), 7.58 (d, 2H), 8.15 (d, 2H)
ppm.
Anal. Calcd for C42H32N4O: C, 82.87; H, 5.30; N, 9.20; Found: C,
82.69; H, 5.17; N, 9.06.
2.2.2. Synthesis of 2,8-diformyldibenzofuran (2)
2.2.5. Synthesis of 2,8-bis(1-(4-bromophenyl)-3-phenyl-
pyrazoline-5-yl)dibenzofuran (B)
To
a well-stirred suspension of 3 g (9.2 mmol) of 2,8-
dibromodibenzofuran (1) in 45 ml of dry ether under a nitrogen
atmosphere at −78 ◦C in a dry ice/acetone bath was added 8.1 ml
(20.24 mmol) of a 2.5 M solution of n-butyl lithium in hexane. The
resulting suspension was allowed to warm to room temperature
and was stirred for 2 h. Then it was added 2.14 ml (27.6 mmol) of
DMF under a nitrogen atmosphere at −78 ◦C in a dry ice/acetone
bath. The mixture was allowed to gradually warm to room tem-
perature for 8 h. After addition 10% aqueous hydrochloric acid
solution, the precipitated solid was collected by filtration, washed
with water, dried. The product was crystallized from dry ether
and 1.6 g of white solid was obtained. The yield was 78%; m.p.
192–194 ◦C; IR (KBr) (cm−1): 3426, ꢀC–H (dibenzofuran); 2925,
A mixture of 1 g (2.33 mmol) 3 and 4-bromophenylhydrazine
hydrochloride 1.15 g (5.13 mmol) in 30 ml of ethanol was heated
under reflux for 6 h. The solid product obtained on cooling was
filtered, washed with ethanol and crystallized from ethanol. 1.22 g
light yellow solid powder was obtained. The yield was 84%; m.p.
238–240 ◦C; IR (KBr) (cm−1): 3426, ꢀC–H (dibenzofuran-skeleton);
1598, ꢀC
(pyrazoline ring); 1490, 1351, ꢀC–H (pyrazoline ring,
N
–CH2–, –CH–). 1H NMR (300 MHz, DMSO-d6): ı 3.22 (2H, the
–CH2– of pyrazoline ring), 4.02 (2H, the –CH2– of pyrazoline ring);
5.71 (2H, the –CH of pyrazoline ring); 7.0–8.11 (25H, phenyl,
dibenzofuran-skeleton) ppm; 13C NMR (300 MHz, DMSO-d6): ı
53.5, 74.5, 115.7, 119.3, 122.0, 129.0, 131.0, 134.3, 135.5, 136.3,
143.3, 145.3, 148.8, 159.4 ppm; MS (m/z): 766.0892 (M+); Anal.
Calcd for C42H30Br2N4O: C, 65.81; H, 3.94; N, 7.31; Found: C, 65.65;
H, 3.82; N, 7.23.
2852, ꢀC–H (–CHO); 1695, ꢀC
(–CHO); 1631, 1594, 1470, ꢀC
O
C
(dibenzofuran); 900–700, ꢀC–H (dibenzofuran); 1H NMR (300 MHz,
CDCl3): ı 7.67 (s, 2H), 8.05 (s, 2H), 8.69 (s, 2H), 10.09 (s, 2H)
ppm.
The thermal stability of the two pyrazoline derivatives was mea-
sured using thermogravimetric analysis (TGA). The result reveals
that two pyrazoline derivatives exhibit excellent thermal stability
up to 250 ◦C and 280 ◦C, respectively.
2.2.3. Synthesis of chalcone based on dibenzofuran (3)
0.5 g (2.23 mmol) of 2,8-diformyldibenzofuran (2), 43 ml
(4.46 mmol) of acetophenone and 30 ml of ethanol were added
in 100 ml conical flask, and the solution was obtained by stir-
ring at room temperature. Then 10% KOH aqueous solution (5 ml)
was dropped in. The reaction was kept for 24 h at room tem-
perature. The solid was obtained after filtration, and the product
was recrystallized from anhydrous ethanol after washed by dis-
tilled water. The 0.66 g of light yellow powder was obtained.
The yield was 69%; m.p. 248–250 ◦C; IR (KBr) (cm−1): 1628
(–CO–); 1H NMR (300 MHz, CDCl3): ı 6.83 (2H), 8.37–7.47 (16H)
ppm.
2.3. Computational procedures
The ground-state geometries as well as their ionic struc-
tures of two pyrazoline derivatives were optimized at B3LYP
level with 6-31G(d, p) basis set [36,37]. The vibration fre-
quencies and the frontier molecular orbital characteristics were
analyzed on the optimized structures at the same level. The
ionization potential (IP), electron affinity (EA), reorganization
energy, and HOMO–LUMO gap of them were calculated by DFT
method based on the optimized geometry of the neutral and
ionic molecules. The excited-state geometries of two pyrazo-
line derivatives were optimized at the configuration interaction
with single excitation (CIS) level with 6-31G (d, p) basis set
[38]. The absorption spectra and the emission spectra of the two
pyrazoline derivatives were carried out using time-dependent
density functional theory (TDDFT) method based on the opti-
mized ground state structures and the lowest singlet excited-state
structures, respectively. Solvent effects were also taken into
account by using the polarized continuum model (PCM) [39,40].
All calculations were carried out with the Gaussian03 pro-
gram package [41]. All the calculations were performed using
the advanced computing facilities of supercomputing center of
computer network information center of Chinese Academy of Sci-
ences. The molecular structures as calculation models are shown
in Fig. 1.
2.2.4. Synthesis of 2,8-bis(1,3-diphenyl-pyrazoline-5-yl)
dibenzofuran (A)
A mixture of 0.506 g (1.18 mmol) 3 and phenylhydrazine 0.26 ml
(2.6 mmol) in 15 ml of glacial acetic acid and 15 ml of ethanol
was heated under reflux for 6 h. The solid product obtained on
cooling was filtered, washed with ethanol and crystallized from
ethanol. 0.438 g yellow solid powder was obtained. The yield was
80%; m.p. 176–178 ◦C; IR (KBr) (cm−1): 3416, ꢀC–H (dibenzofuran
skeleton); 1598, ꢀC
(pyrazoline ring); 1489, 1345, ꢀC–H (pyra-
N
zoline ring, –CH2–, –CH–). 1H NMR (300 MHz, DMSO-d6): ı 3.21
(2H, the –CH2– of pyrazoline ring), 4.01 (2H, the –CH2– of pyrazo-
line ring), 5.61 (2H, the –CH of pyrazoline ring), 6.71–8.31 (26H,
phenyl, dibenzofuran-skeleton) ppm; 13C NMR (300 MHz, DMSO-
d6): ı 53.4, 74.8, 114.3, 115.9, 117.9, 122.2, 128.7, 130.3, 131.4,
136.6, 143.5, 146.2, 147.6, 159.5 ppm; MS (m/z): 608.2687 (M+);

H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
245
Fig. 2. The optimized geometry of the compound A and B in the ground state.
Table 1
3. Results and discussions
HOMO and LUMO energies, and HOMO-LUMO energy gap of the compound A and B
obtained from DFT/B3LYP/6-31G(d, p) calculation (eV).
3.1. Synthesis
Methods
HOMO
LUMO
ꢂE
In this paper, we described methods for the preparation
of pyrazoline derivatives based on dibenzofuran. First, 2,8-
dibromodibenzofuran (1) was synthesized via the reaction of
dibenzofuran with bromine. Second, 2,8-diformyldibenzofuran (2)
was obtained as the key intermediate for the whole procedure
as shown in Scheme 1 via the reaction of 1 with n-butyl lithium
and DMF. Third, chalcone based on dibenzofuran (3) was prepared
by the reaction of 2 with acetophenone. Fourth, the two pyra-
zoline derivatives (A and B) were resulted by the reaction of 3
with phenylhydrazine and 4-bromophenylhydrazine hydrochlo-
ride, respectively. All of the novel compounds were characterized
by elemental analysis, IR, NMR and MS, further details are given in
Section 2.2.
Compound A
Compound B
DFT-B3LYP/6-31G(d, p)
DFT-B3LYP/6-31G(d, p)
−4.871
−5.061
−1.197
−1.388
3.674
3.674
the ground state to the excited state is mainly about an electron
flowing from the pyrazoline side to the dibenzofuran core, which
belongs to ꢁ → ꢁ* transition. The energies of the HOMO, LUMO and
energy gap are given in Table 1.
3.4. Charge injection and transport properties
The charge injection and transport ability are important param-
eters for OLEDs. Normally, the energy barrier for injection holes and
electrons are assessed by ionization potential (IP) and electronic
affinity (EA). In general, the higher EA value of the electron trans-
port layer (ETL), the easier the entrance of electrons from cathode
to ETL, and the lower IP value of the hole-transport layer (HTL), the
easier the entrance of holes from ITO to HTL. Therefore, we calcu-
lated their ionization potentials, electronic affinities. The calculated
results are listed in Table 2. As shown in Table 2, the compound A
has lower IP value, it is easier to transport hole than compound
B and the compound A has higher EA value, it is easier to trans-
port electron than compound B. The results are consistent with the
indication from the energies analysis of their HOMOs and LUMOs.
A hopping model is often used to describe the charge mobility
in organic materials, and the charge transport rate can be approxi-
mated by the Marcus electron-transfer theory with Eq. (1) [43].
3.2. Structures of pyrazoline derivatives
The optimized geometries of two pyrazoline derivatives in the
ground state are shown in Fig. 2, and the optimized geometri-
cal parameters for the two pyrazoline derivatives in the ground
state are compiled in Table S1 (in SI). According to the data listed
in Table S1, we can see the two pyrazoline derivatives possess
similar molecular structures. Each derivative contains one planar
moiety and two side chain moieties in its molecular structure,
the planar moiety is dibenzofuran ring and two side chain moi-
eties are pyrazoline rings. The dihedral angles C11–C13–C14–C15,
C11–C13–N33–N16, C6–C23–C24–C25 and C6–C23–N34–N26 of
the two moiety are 124.00◦, −124.73◦, −123.83◦ and 124.56◦ as well
as 121.78◦, −122.58◦, −121.68◦ and 122.47◦ in compound A and
B, respectively. It is indicated that the two pyrazoline derivatives
possess the distorted geometrical structures.
4ꢁ2
h
1
khole/electron
=
t2
ꢀ
4ꢁꢃhole/electronkBT
ꢁ
ꢂ
ꢃhole/electron
exp
−
(1)
4kBT
3.3. Frontier molecular orbitals
where h is Planck’s constant, ꢃhole/electron is the reorganization
energy for hole or electron transfer between both molecules,
kB is Boltzmann’s constants, T is the temperature and t is the
The frontier molecular orbitals are the most important the-
ory in determining the capability of electron or hole transport of
molecules and the spectral properties [42].
The contour plots of HOMO and LUMO of the two pyrazoline
derivatives are exhibited in Fig. 3. It can be seen from Fig. 3 that
the election densities of HOMOs of the two pyrazoline deriva-
tives are also localized on two pyrazoline side chains mainly. They
are ꢁ-bonding orbitals. The election densities of LUMOs of the
two pyrazoline derivatives are localized on the dibenzofuran core
mainly. They are ꢁ*-bonding orbitals. The electronic transition from
Table 2
Ionization potentials, electron affinities and reorganization energy of the compound
A and B obtained from DFT/B3LYP/6-31G(d, p) calculation(eV).
Compound
IPv
IPa
ꢃHol
EAv
EAa
ꢃEle
Compound A
Compound B
5.898
6.023
5.816
5.9379
0.178
0.201
0.167
0.348
0.278
0.466
0.206
0.233

246
H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
Fig. 3. The contour plots of HOMOs and LUMOs of the compound A and B in the ground state.
electronic transfer integral between the donor and acceptor
molecules. Hence, the transport process is decided by two impor-
tant Molecular factors: the intermolecular hole or electron transfer
integral (t) which describes the strength of the electronic cou-
pling between adjacent molecules and the reorganization energy
(ꢃ) which needs to be small for an efficient charge-transport [44].
The sum of two contributions, ꢃ+1 + ꢃ+2 is the reorganization
energy for hole transfer ꢃhole, that are defined as
Similarly, the sum of two contributions, ꢃ−1 + ꢃ is the reor-
−2
ganization energy for electron transfer ꢃelectron, that are defined
as
ꢃ
= E0(M−) − E0(M)
(4)
(5)
−1
ꢃ−2 = E1(M) − E1(M−)
where E0(M−) and E0(M) is the energies of the neutral molecule
at the anion geometry and the optimal ground-state geometry,
respectively. E1(M) and E1(M−) represent the energy of the charged
state at the neutral geometry and optimal anion geometry, respec-
tively.
ꢃ
= E0(M+) − E0(M)
= E1(M) − E1(M+)
(2)
(3)
+1
+2
ꢃ
where E0(M+) and E0(M) are the energies of the neutral molecule
at the cation geometry and the optimal ground-state geome-
try, respectively. E1(M) and E1(M+) represent the energy of the
charged state at the neutral geometry and optimal cation geometry,
respectively.
According to the calculated model above, the calculated reorga-
nization energy for hole and electron are listed in Table 2 where
ꢃhole exhibits lower value than corresponding ꢃelectron. It indicates
that the two compounds are in favor of hole transport than electron
transport, thus they are potential hole transport materials.
Table 3
UV–vis spectra and fluorescence spectra data of the compound A and B in different polar solvents.
Compounds
Compound A
Solvents
UV wavelengths (ꢃ, nm)
FL wavelengths (ꢃ, nm)
Stokes shift (cm−1
)
Toluene
Ethylacetate
Chloroform
DMF
360
358
359
361
360
439
446
449
452
455
4999
5511
5584
5577
5800
DMSO
Toluene
Ethylacetate
Chloroform
DMF
364
359
360
362
365
440
441
448
451
454
4745
5179
5457
5452
5371
Compound B
DMSO

H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
247
0.4
0.3
0.2
0.1
0.0
0.4
Toluene
Ethylacetate
Chloroform
DMF
Toluene
Ethylacetate
Chloroform
DMF
0.3
DMSO
DMSO
0.2
0.1
0.0
300
350
400
450
300
350
400
450
Wavelength(nm)
Wavelength(nm)
compound A
compound B
Fig. 4. Absorption spectra of the compound A and B in different polar solvents.
and 27,326–27,743 cm−1, 1.1025–1.2724, respectively. All the elec-
tronic transitions herein are strongly allowed. The calculated
S0 → S1 vertical excitation energy data of the two compounds are
in good agreement with the experimental data.
3.5. UV–vis spectra of the two pyrazoline derivatives
The UV–vis absorption spectra of the two pyrazoline derivatives
have been studied in different polar solvents and the spectral data
are collected in Table 3. The spectra are shown in Fig. 4. The two
pyrazoline derivatives have similar absorption spectra. As indicated
in the absorption spectra, each derivative exhibits two main bands
at 270–320 and 320–400 nm due to ꢁ → ꢁ* electronic transitions,
which are almost the same in different polar solvents meaning the
independence of UV absorption to the solvent polarity. The low-
energy broad band at 320–400 nm is assigned to an intramolecular
charge transfer (CT) band from the pyrazoline side rings of each
derivative to the dibenzofuran ring of each compound in different
polar solvents.
We computed singlet–singlet electronic transition in different
polar solvents based on the optimized geometries of the ground
state of the two compounds using time-dependent DFT method
at the B3LYP/6-31G (d, p) level in order to gain a detailed insight
into the nature of the UV–vis absorption of the two pyrazoline
derivatives observed experimentally. The computed data of ver-
tical electronic transitions of S0 → S1, S0 → S2, S0 → S3, S0 → S4 and
S0 → S5 are collected in Table S2 (in SI). The continuous absorp-
tion spectra were simulated with the help of SWIZARD software
with the width at half-height of 2500 cm−1 on the basis of the cal-
culated vertical excited energy and their corresponding oscillator
strengths. The simulated absorption spectra of the two compounds
are shown in Fig. S1(in SI). As seen in Fig. S1 and Table S2, the
electronic transitions are of ꢁ → ꢁ* type. The calculated S0 → S4
excitation energy, oscillator strength of the two compounds in
different polar solvents are 27,548–27,782 cm−1, 1.1763–1.2413
3.6. Fluorescence spectra of the two pyrazoline derivatives
The steady-state fluorescence spectra of two compounds were
measured in different polar solvents and shown in Fig. 5. Their
spectral data are also collected in Table 3. The emission spectra
of the two compounds consist of one broad band. This band can
be assigned to the S1 → S0 electronic transition. The spectra of the
two compounds show a small shift in different polar solvents. As
can be seen in Fig. 5 and Table 3, with the solvent changes from
toluene to DMSO, the maximum emission wavelengths of the two
compounds are redshifted from 439 to 455 nm and 440 to 454 nm,
respectively. The Stoke’s shifts for the two compounds are small
in different polar solvents showing the relative rigidity of the two
compounds.
In order to gain insight into the nature of the fluorescence
emission observed for two compounds, the geometries of the first
excited singlet state (S1) were optimized for the two compounds.
The optimized geometrical parameters for the two compounds in
the first excited state are also compiled in Table S1. According to the
data listed in Table S1, it can be see that the two compounds pos-
sess similar molecular structures in the first excited state and in the
ground state, respectively. The dihedral angles C11–C13–C14–C15,
C11–C13–N33–N16, C6–C23–C24–C25 and C6–C23–N34–N26 of
the two moiety are 137.95◦, −139.08◦, −96.93◦ and 99.21◦as well
1.0
0.8
0.6
0.4
0.2
0.0
Toluene
Ethylacetate
Chloroform
DMF
1.0
Toluene
Ethylacetate
Chloroform
0.8
DMF
DMSO
DMSO
0.6
0.4
0.2
0.0
400
450
500
550
600
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
compound A
compound B
Fig. 5. The fluorescence spectra of the compound A and B in different polar solvents.

248
H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
Fig. 6. Typical fluorescence decay profile of the compound A and B in chloroform.
as 136.84◦, −137.97◦, −97.32◦ and 99.66◦ in compound A and B,
respectively.
Table 4
The fluorescence quantum yields (Ф) and the fluorescence lifetimes (ꢄ, ns) of the
compound A and B in different polar solvents.
The optimized geometries of the two compounds in the
first excited state (S1) were used as import data to calculate
singlet–singlet electronic transition using time-dependent DFT
method at the B3LYP/6-31G(d, p) level in different polar solvents,
respectively, yielding the vertical electronic transitions energy of
S1 → S0. The computed data are collected in Table S3 (in SI). The
continuous emission spectra were simulated with the help of
SWIZARD software with the width at half-height of 2500 cm−1
on the basis of the calculated vertical excited energy and their
corresponding oscillator strengths. The simulated emission spec-
tra of two compounds are shown in Fig. S2(in SI). As can be
seen in Fig. S2 and Table S3, the electronic transitions are of
the ꢁ → ꢁ* type. The calculated S1 → S0 emission energy, oscil-
lator strength of the two compounds in different polar solvents
Solvent
Quantum
yield (Ф)
Fluorescence
lifetime (ꢄ, ns)
Toluene
Ethylacetate
chloroform
DMF
0.25
0.30
0.24
0.29
0.30
2.83
3.31
3.04
3.19
3.24
Compound A
Compound B
DMSO
Toluene
Ethylacetate
chloroform
DMF
0.25
0.25
0.10
0.25
0.22
3.55
3.74
3.56
3.38
3.22
DMSO
are 23,564–23,760 cm−1, 0.5982–0.6448 and 23,416–23,741 cm−1
,
The fluorescence decay behaviors of the compound A and B were
also studied in different polar solvents. The fluorescence decay
curves were recorded with time-correlated single photon counting
technique using a commercially available Edinburgh Instruments.
Typical decay profiles of the compound A and B are shown in Fig. 6.
The fluorescence lifetimes (ꢄ) are also collected in Table 4. For
the compound A and B, the fluorescence decay curves are mono-
exponential model.
0.7133–0.7214, respectively. All the electronic transitions are
strongly allowed. The calculated Stoke’s shifts of the two com-
pounds are 3936–4042 cm−1 and 3810–4227 cm−1 in different
polar solvents, respectively. The small Stoke’s shifts show a con-
sequence of the rigidity of the two compounds. The simulated
emission spectra are consist with the experimental fluorescence
spectra of two compounds. Simulation emission spectra show the
two compounds in different polar solvents can emit blue light and it
might be potential luminescent materials with blue light emission.
4. Conclusions
3.7. Molecular photophysical properties
In
this
paper,
two
novel
pyrazoline
derivatives
The fluorescence quantum yields of the compound A and B were
measured in different polar solvents at room temperature by a rel-
ative method using quinine sulfate in 0.1 M sulphuric acid as a
standard solution. The fluorescence quantum yields were calcu-
lated from Eq. (6) [45].
2,8-bis(1,3-diphenyl-pyrazoline-5-yl)dibenzofuran (A) and 2,8-
bis(1-(4-bromophenyl)-3-phenyl-pyrazoline-5-yl)dibenzofuran
(B) were synthesized and characterized by elemental analysis,
NMR, MS and thermogravimetric analysis. The experimental data
and conclusions about the structure, UV–vis spectra and fluores-
cence spectra are supported by quantum chemistry computations.
The calculated reorganization energy for hole and electron indi-
cates that the two compounds are in favor of hole transport than
electron transport. Our results show that the two novel pyrazoline
derivatives exhibit excellent fluorescence quantum yields and high
thermal stability, they could be used as excellent optoelectronic
materials in organic light-emitting devices. The investigations for
their applications are progressing in our laboratory, and the results
will be released soon.
ꢃ
ꢄ
2
Fs Ar nr
˚s = ˚r
(6)
Fr As ns
where Ф is the fluorescence quantum yield, F is the integration of
the emission intensities, n is the index of refraction of the solution,
and A is the absorbance at the excitation wavelength, the subscripts
“r” and “s” denote the reference and unknown samples, respec-
tively. The fluorescence quantum yields (Ф) are collected in Table 4.
As shown in Table 4, these compounds show higher fluorescence
quantum yields in different polar solvents.

H.-p. Shi et al. / Spectrochimica Acta Part A 83 (2011) 242–249
249
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This work was supported by Shanxi Province scientific and
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
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