Study on the fluorescence properties of a new intramolecular charge transfer compound 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-2-pyrazoline

Article abstract of DOI:10.1016/j.saa.2006.12.010

The fluorescence properties of a newly synthesized compound, 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-2-pyrazoline (DEP) have been studied. On excitation at 352 nm, the fluorescence spectrum exhibits a large red shift with an increase in the polarity of solvents. The intensity of the band is different in different solvents as well. The change in the dipole moment in various solvents at room temperature has been characterized by the absorption and steady state fluorescence techniques and calculated based on the Lippert-Mataga equation. DEP has an increase of dipole moment of 2.83 D units on excitation to the lowest singlet state. It is concluded that photo-induced charge transfer from N (1) to C (3) actually exists in the excited state of the pyrazoline moiety. Its fluorescence property is relative to viscosity and temperature of solvents. The φ{symbol}_f of DEP in neutral medium or basic medium is higher than acidic medium. In addition, when the concentration of DEP is higher than 10^-3 M, its fluorescence is quenched by the collision of each molecule. The red shift of the maximum emission of DEP attributes to the formation of aggregates and the conjugate system is strengthened.

Full text of DOI:10.1016/j.saa.2006.12.010

Spectrochimica Acta Part A 68 (2007) 404–408
Study on the fluorescence properties of a new intramolecular charge transfer
compound 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-2-pyrazoline
J.F. Li, B. Guan, D.X. Li, C. Dong∗
Institute of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, People’s Republic of China
Received 3 October 2006; accepted 8 December 2006
Abstract
The fluorescence properties of a newly synthesized compound, 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-2-pyrazoline (DEP) have been studied.
On excitation at 352 nm, the fluorescence spectrum exhibits a large red shift with an increase in the polarity of solvents. The intensity of the band
is different in different solvents as well. The change in the dipole moment in various solvents at room temperature has been characterized by the
absorption and steady state fluorescence techniques and calculated based on the Lippert–Mataga equation. DEP has an increase of dipole moment
of 2.83 D units on excitation to the lowest singlet state. It is concluded that photo-induced charge transfer from N (1) to C (3) actually exists in
the excited state of the pyrazoline moiety. Its fluorescence property is relative to viscosity and temperature of solvents. The φf of DEP in neutral
medium or basic medium is higher than acidic medium. In addition, when the concentration of DEP is higher than 10−3 M, its fluorescence is
quenched by the collision of each molecule. The red shift of the maximum emission of DEP attributes to the formation of aggregates and the
conjugate system is strengthened.
© 2007 Elsevier B.V. All rights reserved.
Keywords: 2-Pyrazoline; Fluorescence property; Intramolecular charge transfer
1. Introduction
pyrazolines containing electron donors and acceptors at 1- and
3-positions attract many attentions from Chemists.
Pyrazoline derivatives are an important class of conjugated
fluorescent dyes emitting blue fluorescence. It has been reported
that substituted pyrazoline derivatives possess a wide spectrum
of biological activities and are crucial intermediates in the syn-
thesis of substituted cyclopropane derivatives [1–3]. Pyrazolines
have been extensively applied in the industry due to the high flu-
orescence quantum yield as well [4,5]. For example, pyrazoline
derivatives have been widely used as optical brightening agents
for textiles, paper and fabrics [6,7]. In recent years, this class
of compounds with high glass-transition temperature has been
known as hole transport materials in organic or emitting materi-
als in electroluminescence (EL) devices [8,9]. PYB nanocrystals
with different sizes have been prepared using the reprecipitation
method and their size-dependent properties have also been stud-
ied [10]. Attempts have been made to synthesize and elucidate
the effects of substituents on the absorption and fluorescence
properties of this class of compounds [11–14]. Specifically, 2-
Intramolecular charge transfer (ICT) has always been attract-
ing continued interest as a topic of central importance both in
photochemistry and in biochemistry and deeply studied because
of their special functions and behaviors [15–17]. It is a basic phe-
nomenon underlying many chemical reactions that play a major
role in a variety of biological processes and laser dye dynam-
ics [18–20]. It is well known that pyrazoline compounds are
typical ICT compounds. An intramolecular conjugated charge
transfer process has been reported to exist in pyrazoline moiety
in the excited state [10]. In the conjugated part (–N1–N2 C3–)
of the pyrazoline ring, the nitrogen atom at the 1-position and the
carbon atom at the 3-position are, respectively, electron donat-
ing and withdrawing moieties [21]. The carbon atoms at 4- and
5-positions do not conjugate with the above conjugated part.
In this report, a new derivative of 2-pyrazoline, 1,5-diphenyl-
3-(N-ethylcarbazole-3-yl)-2-pyrazoline (DEP), was synthe-
sized. Polarity of different solvents has a great influence on the
fluorescence emission wavelength and intensity of DEP com-
pound. It is found that ICT process exists between the nitrogen
atom at the 1-position and the carbon atom at the 3-position in
pyrazoline moiety. DEP has an increase of dipole moment of
∗
Corresponding author. Tel.: +86 3517011322; fax: +86 3517011322.
E-mail address: dc@sxu.edu.cn (C. Dong).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.12.010

J.F. Li et al. / Spectrochimica Acta Part A 68 (2007) 404–408
405
Fig. 1. Synthetic route of DEP.
Compound 3: 1H NMR (CDCl3) δ: 1.4 (3H, –CH3); 4.4 (2H,
–CH2–); 7.15–8.18 (12H, Ph+ carbozole); 7.3 (1H, –COH );
7.7 (1H, CH–Ph). IR (KBr) cm−1: 1647 (–CO–).
2.83 D units on excitation to the lowest singlet state. For a thor-
ough understanding of its fluorescence properties, its behaviors
in solvents with different viscosity, temperature and pH have
been studied. Concentration on the fluorescence spectra has been
studied as well. It shows that this compound is very sensitive to
the microenvironment. It can be a new and good fluorescent
dye and used as the probe of environment or hole transporting
material in organic EL devices.
Compound 4: 1H NMR (CDCl3) δ: 1.4 (3H, –CH3); 4.4 (2H,
–CH2–); 3.5 (2H, –CH2, pyrazole); 3.7 (1H, –CH , pyrazole);
7.2–8.2 (17H, Ph+ pyrazole). IR (KBr) cm−1: 1540 (C N).
3. Results and discussion
2. Experimental
3.1. Fluorescence spectral characteristics of DEP in
various solutions
2.1. Chemicals and apparatus
The fluorescence spectra show one maximum in the solvents
and are independent of the exciting wavelength. Fig. 2 presents
the normalized fluorescence spectra of DEP in solvents
of different polarity and the corresponding absorption and
fluorescence spectral characteristics are listed in Table 1.
As can be seen, DEP exhibits a broad absorption at about
325–420 nm which is a result of the combination of two chro-
mophores (the pyrazoline and carbazole moieties) and due to
the different electronic excitations. The absorption maximum λa
shows minimal dependence on the polarity of the solvents with
red shift of about 20 nm. This indicates that there is no charge
transfer in the ground state. While the fluorescence emission
spectrum suffers a strongly bathochromic shift as the solvent
polarity increase. When the solvent is changed from cyclohax-
DEP stock solution was prepared to 1 × 10−3 M using
CHCl3 as solvent. The solvents, cyclohexane, tetrahydrofuran,
trichloromethane, acetone, N,N-dimethylformamide, acetoni-
trile, ethanol and methanol were all purchased from Beijing
Chemical Plant or Tianjing Second Chemical Plant without
further purification prior to use. No interference fluorescence
was observed under the experimental conditions for all the
reagents used.
The absorption spectra were recorded on a TU-1901 UV–vis
spectrophotometer (Puxin Company, Shanghai, China) and cor-
rected emission spectra were obtained on a Carry Eclipse
fluorescence spectrophotometer (VARIAN, USA) connected to
an ultrathermostate of temperature precision 0.1 ◦C with sin-
gle cell peltier accessory. Standard 1 cm quartz cells were used.
Excitation and emission bandwidths were set at 5.0 and 10 nm,
respectively. Fluorescence quantum yields were measured using
quinine sulfate in 0.05 M H2SO4 (φf = 0.55) as standard. All
experiments were performed at 20 1 ◦C.
2.2. Synthesis of DEP
DEP was synthesized in our lab and the procedure is shown
as Fig. 1. The detailed synthesis method is reported otherwise.
Here just shows the main structural NMR and IR data of the
relative compounds.
Compound 1: 1H NMR (CDCl3) δ: 1.4 (3H, –CH3); 4.4 (2H,
–CH2–); 7.2–8.2 (8H, carbozole).
Compound 2: 1H NMR (CDCl3) δ: 1.4 (3H, –CH3); 2.8 (3H,
–COCH3); 4.4 (2H, –CH2–); 7.3–8.2 (7H, carbozole). IR
(KBr) cm−1: 1660(–COCH3).
Fig. 2. Normalized fluorescence spectra of DEP in various solvents. Solvents
from1to7are:cyclohexane, tetrahydrofuran, acetone, N,N-dimethylformamide,
acetonitrile, ethanol and methanol, respectively.

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J.F. Li et al. / Spectrochimica Acta Part A 68 (2007) 404–408
Table 1
Maximum wavelength of absorption and fluorescence and Stokes shift for DEP in the solvents studied
Solvent
ET(30)a (kcal/mol)
λa (nm)
νa (cm−1
)
λf (nm)
νf (cm−1
)
ꢀfb
ꢀν (cm−1
)
Cyclohexane
Tetrahydrofuran
Trichloromethane
Acetone
N,N-dimethylformamide
Acetibutrile
31.2
37.4
39.1
42.2
43.8
46.0
51.9
55.4
320
323
328
328
329
328
336
338
31250
30959
30488
30488
30395
30488
29762
29585
396
410
415
419
424
421
440
448
25253
24396
21096
23981
23585
23585
22727
22321
0.0021
0.2107
0.148
0.1452
0.2751
0.3055
0.2878
0.309
5997
6563
6392
6507
6810
6903
7035
7264
Ethanol
Methanol
The subscripts “a” and “f” refer to the absorption and fluorescence, respectively.
a
Values taken from Ref. [26].
b
Polarity function for each solvent is based on the value of dielectric constant and refractive index of the solvents given in Ref. [27].
ane to methanol, the emission maximum shift observably from
396 to 448 nm. Bathochromic shift proves the ␲–␲* transition
in the excited state. DEP has two excitation wavelengths of
283 and 335 nm, and they did not change with the increasing
polarity. In addition, it is noted that the fluorescence intensity
or fluorescence quantum yield increases as well.
˚
by 0.8 [23] and gives 13.0 A. ꢀf is the orientation polarizability
parameter of the solvent where n is the refractive index of the
medium and ε is the static dielectric constant of the solvent (both
at room temperature).
A plot of ꢀν versus ꢀf (ε, n) is shown in Fig. 3. Least square
linear regression analysis gave a good correlation according
to:
The great increase of the Stokes shift about 1267 cm−1 of
the fluorescence maximum with the increasing solvent polarity
suggests that the potential energy surface of the emitting state
is different from that of the ground state and a photo-induced
ICT takes place in the fluorescence states. That is to say that
the molecule is solvated significantly in the S1 excited state,
resultinginadifferenceindipolemomentbetweentheS1 excited
state and the ground state.
ꢀν = 3520ꢀf (ε, n) + 5939.8 (r = 0.95)
Deviation from linearity in the Lippert–Mataga treatment, spe-
cially in case of alcohol, is the characteristic of the specific
solvent effect, such as hydrogen bonding, etc. [24]. Fig. 4 shows
the absorption maximum νa and emission maximum νf of DEP
to the different solvent polarity parameter ET(30) values. The
stokes shift and solvent parameter ET(30) are of linear relation.
This is commonly considered as the result of stabilizing the state
of charge transfer by solvent molecule [25].
3.2. Measurement of the magnitude of the change in the
dipole moment
The increase in the dipole moment ꢀμ is obtained from the
slope of the plot of ꢀν versus ꢀf (ε, n) and gives approximately
2.83 D. ꢀμvalue reveals the CT character of the excited state.
This is caused by the redistribution of atomic charges in excited
state, which is only possible due to photo-induced charge trans-
fer from election rich donor moiety N (1) to acceptor moiety C
(3) within the molecule. It is evident that the introduction of a
Determination of the extent of increase in the dipole moment
of DEP on electronic excitation
2 ꢀu2
4πε0hcρ3
νa − νf =
ꢀf (ε, n) + constant
can be made from the Lippert–Mataga relation and the solvent
parameter ꢀf (ε, n) are expressed as follows [22]:
ꢀ
ꢁ
ꢀ
ꢁ
ε − 1
n2 − 1
ꢀf (ε, n) =
−
2ε + 1
2n2 + 1
In these relations, νa − νf represents the Stokes shift (in
cm−1), where νa and νf are the spectral positions of the absorp-
tion maximum and solvent-equilibrated fluorescence maximum,
respectively. ꢀμ is given by μe − μg, which is the magnitude
of the change in the dipole moment from the ground state to the
excited state. The other terms are: h corresponds to the Plank’s
constant (6.6 × 10−34 J S), c corresponds to the velocity of light
in vacuum (3.0 × 108 m s−1), ε0 corresponds to the permittivity
of vacuum (8.85 × 10−12 V C−1 m−1) and ρ corresponds to the
Onsager cavity radius (in meter), respectively. Onsager cavity
radius was estimated from the optimized distance between the
two farthest atoms in the direction of charge separation within
the molecules, and the Onsagar cavity radius ρ can be derived
Fig. 3. Plot of νf − νa vs. the solvent polarity–polarizability parameter (ꢀf) for
compound DEP (ꢀ νa, ꢁ νf).
˚
from the value of the length of the molecule (16.3 A) multiplied

J.F. Li et al. / Spectrochimica Acta Part A 68 (2007) 404–408
407
Fig. 4. Dependence of νa and νf of DEP on solvent polarity ET(30) value.
Fig. 6. The effect of solvent viscosity on fluorescence spectra of DEP. The ratios
of glycerol:methanol (volume) from up to below are 7:3, 5:5, 6:4, 8:2, 9:1.
3.4. The effect of viscosity to fluorescence spectra
The effect of viscosity to fluorescence spectra was studied in
glycerol containing different ratio of methanol. Because of their
similar polarity but very different viscosity, different volume
ratio of glycerol and methanol could make different viscosity.
Fig. 6 presents the spectra of DEP in solvents with different
viscosity. When glycerol:methanol = 9:1 (volume), fluorescence
intensity is lowest, and reaches to the biggest when the ratio
is 7:3. Then if the viscosity increases, fluorescence intensity
decreases again and the spectrum blue shifts 35 nm. This proves
that DEP molecules are fixed in the system due to the viscos-
ity. It is difficult for them to be relaxed to the best luminescent
conformation after being excited and even blue shift when the
viscosity is high enough.
Fig. 5. The effect of pH on fluorescence spectra of DEP. Curves from up to
below are pH 6.40, 8.05, 7.65, 8.60, 4.60, 8.28, 2.10, 1.40.
3.5. The effect of temperature to fluorescence spectra of
DEP
carbazone group at position 3 is benefit to the DEP molecular
polarization in the excited state.
Temperature effect on the fluorescence of DEP in three
selected solvents of various polarity (acetonitrile, cyclohexane
and THF) was studied. The emission spectra characteristics of
DEP solution show that the fluorescence intensities just drop
down and there is no obvious blue shift observed with the ele-
vation of temperature (20, 25, 30, 35, 45, 50 and 60 ◦C). The
fluorescence intensity is most extensive to the temperature in
acetonitrile solvent than in cyclohexane and THF. In general,
when the temperature decreases, the viscosity of the medium
increases, and the collision quenching for fluorescent molecules
and solvents is easier. As a result, the fluorescence intensity
increases with the decrease of temperature.
3.3. The effect of pH to fluorescence quantum yield
The fluorescence spectra of DEP at different pH are shown
in Fig. 5. It is obvious that in strong acidic medium pH 1.40,
the fluorescence is almost quenched and blue shifts about 5 nm,
while in neutral or basic medium, the intensities are much higher.
The fluorescence quantum yields of DEP in solutions of differ-
ent pH were obtained as literature [28] reported. The results are
listed in Table 2. It can be seen that DEP is highly fluorescent
and the fluorescence quantum yields depend strongly on the pH.
When pH reaches to 4.60, fluorescence quantum yield increase
sharply from 0.079 to 0.21. It proves that in medium or basic
medium, the φf of DEP is higher than acidic medium and con-
stant which may be due to the damaged conjugation structure of
DEP molecule.
3.6. The effect of concentration on fluorescence emission
The effect of concentrations on fluorescence intensities
of DEP in CHCl3 was studied. The corresponding emission
maximums are listed in Table 3. It is noted that the flu-
orescence intensities increased initially and then decreased
with the increasing concentration. With the increase of the
concentration, the number of luminescent molecules of DEP
increase and fluorescence intensity increased. But when the
Table 2
The fluorescence quantum yields of DEP in solutions of different pH
pH 1.41
2.10
3.50
4.62 6.40 7.65
58.05 8.28 8.60
0.22 0.20 0.23
φf 0.018 0.060 0.079 0.21 0.23 0.071

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J.F. Li et al. / Spectrochimica Acta Part A 68 (2007) 404–408
Table 3
the Youth Science Foundation of Shanxi Province (20051005).
All the authors express their deep thanks.
The maximum emission wavelengths of DEP of different concentrations in
CHCl3
C (mol/L) 1.0 × 10−3 1.0 × 10−4 1.0 × 10−5 1.0 × 10−6 1.0 × 10−7
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A new compound 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-
2-pyrazoline has been synthesized and characterized. Its
fluorescence spectrum exhibits a large red shift with an increase
in the polarity of solvents. An increase of dipole moment of
2.83 D units has been characterized by the absorption and fluo-
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