Spectroscopic investigation of 3-pyrazolyl 2-pyrazoline derivative in homogeneous solvents

Article abstract of DOI:10.1016/j.saa.2008.04.011

How solvent conditions such as solvent polarity and hydrogen-bonding affect the fluorescence of a newly synthesized 3-pyrazolyl 2-pyrazoline derivative (Pyz) having pharmaceutical activity has been explored. The solvatochromic effect of Pyz is due to a ch

Full text of DOI:10.1016/j.saa.2008.04.011

Spectrochimica Acta Part A 71 (2008) 1327–1332
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigation of 3-pyrazolyl 2-pyrazoline derivative in
homogeneous solvents
Smritimoy Pramanik, Paltu Banerjee, Arindam Sarkar, Attreyee Mukherjee,
Kumar K. Mahalanabis∗, Subhash Chandra Bhattacharya∗
Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
How solvent conditions such as solvent polarity and hydrogen-bonding affect the fluorescence of a newly
synthesized 3-pyrazolyl 2-pyrazoline derivative (Pyz) having pharmaceutical activity has been explored.
The solvatochromic effect of Pyz is due to a change in dipole moment of the compound in the excited
state. The relaxation of S1 state is perturbed in hydrogen-bonding solvents. The fluorescence properties
of the systems are strongly dependent on the polarity of the media. The non-radiative relaxation process
is facilitated by an increase in the polarity of the media. The photophysical response of Pyz in different
solvents has been explained considering solute–solvent interactions.
Received 11 January 2008
Received in revised form 25 March 2008
Accepted 14 April 2008
Keywords:
Fluorescence
Relaxation dynamics
Solvent polarity
Hydrogen bonding
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
visualize labile zinc and iron pools, or as pH sensors [12–15]. For the
design of cation specific fluorescent probes, a synthetically more
Photophysical study of pyrazoline derivatives in homogeneous
solvents originates principally from two aspects—the first one
stems from its novel biological applications in pharmaceuticals
and the second one arises due to presence of electron donors
and acceptors at N(1) and C(3) positions, respectively. 2-Pyrazoline
derivatives have been effectively utilized as antibacterial, antifun-
gal, antiviral, antiparasitic, antitubercular and insecticidal agents
[1–3] and as a hole-conveying medium in photoconductive mate-
rials and electroluminescence devices [4,5]. Pyrazoline derivatives
are well known fluorescent compounds with high quantum yields
and are used as optical brightening agents for textiles, fabrics, plas-
tics and papers [6]. These compounds have been utilized as fluo-
rescent probes in some chemosensors [7]. 2-Pyrazoline derivatives
are considered to be an important organic heterocyclic “transition”
material, i.e. a material which shares many properties with con-
ventional semiconductors and insulators like organic molecular
crystals [8]. Ranging from tens to hundreds of nanometers, recently
different groups prepared pyrazoline nanoparticles by using the
reprecipitation method and explored their size-tunable optical
properties for the application in optoelectronic device [6,9–11].
Fluorescent probes are powerful tools in cell biology for the non-
invasive measurement of intracellular ion concentrations and to
flexible fluorophore structure would be desirable. Among possible
substitutes, pyrazoline-based fluorophores stand out due to their
simple structure and favorable photophysical properties such as
large extinction coefficient and quantum yields (0.6–0.8) [16,17].
Variation of photophysical process of 2-pyrazoline derivatives
are observed depending on the substituent [18–20]. In our earlier
work on pyrazoline derivatives having fused norborane ring present
at 3a and 7a carbon of 2-pyrazoline ring fluorescence emissions
from S2 state have been observed [21].
In this study, photophysical properties of another derivative of
2-pyrazoline (Pyz) in homogeneous solvents have been studied.
The compound, ethyl 1-(4-bromophenyl)-3-(4 cyano-3-methyl-
1phenyl-1H-pyrazole-5yl)-4,5, dihydro-1H-pyrazole-5-carboxy-
late (Scheme 1) has phenyl group at N(1) position of pyrazoline
with methyl and cyano substitution at C(3) and C(4) position
of pyrazole, respectively. Generally, phenyl substitution at N(1)
position of pyrazoline ring makes it highly fluorescent and simul-
taneously extended conjugation from 2-pyrazoline to pyrazole
ring enhances the fluorescence intensity of the compound. In
this compound, the ethyl carboxylate group is substituted in C(5)
position of pyrazoline ring. Our target is to study the solvent
dependent radiative transitions and relaxation dynamics of this
derivative of pyrazole substituted 2-pyrazoline from the excited
singlet state. The results obtained from the steady state and time-
resolved fluorescence studies have been rationalized considering
solute–solvent interaction. A comparative study of the kinetics of
the probe in different solvents has been made.
∗
Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 24146584.
E-mail addresses: sbjuchem@yahoo.com, scbhattacharyya@chemistry.jdvu.ac.in
(S.C. Bhattacharya).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.04.011

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S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332
2.2. Instrumentation
UV–visible absorption spectra of Pyz in different solutions were
recorded in a Shimadzu UV-1700 spectrophotometer, Japan using
quartz cuvettes of path length 1 cm with a cell volume of 3.0 ml.
Emission spectra and steady state fluorescence anisotropy were
recorded in a Spex Fluorolog IIA spectrofluorimeter with a slit
width of 1.25 mm, and by a PerkinElmer spectrofluorimeter, respec-
tively. All measurements were done repeatedly and reproducible
results were obtained. The fluorescence quantum yields (ꢁf) of
the compound in different solvents were measured using quinine
sulphate (0.1 M in H2SO4, ꢁf = 0.57) as the reference compound.
The ꢁf values are given in Table 1. Fluorescence lifetimes were
determined from time-resolved fluorescence decay by the method
of time-correlated single-photon counting using a picosecond
diode laser at 403 nm (IBH, nanoLED-07) as light source. The typ-
ical response time of this laser system was 70 ps. The decays were
analyzed using IBH DAS-6 decay analysis software. For all the life-
time measurements, the fluorescence decay curves were analyzed
by single and biexponential iterative fitting program provided by
IBH. Mean (average) fluorescence lifetime (ꢀꢂꢁ) for biexponential
iterative fitting were calculated from the decay times (ꢂ1, ꢂ2) and
the pre-exponential factors (a1, a2) using the following relation:
Scheme 1. Structure of Pyz.
2. Experimental
2.1. Chemicals
Ethyl 1-(4-bromophenyl)-3-(4 cyano-3-methyl-1phenyl-1H-
pyrazole-5yl)-4,5, dihydro-1H-pyrazole-5-carboxylate was synthe-
sized using the method shown in Scheme 2 [22]. It was purified
by column chromatography and the purity of the compound was
checked by thin layer chromatography. The compound was further
vacuum sublimed before use. Spectroscopic grade solvents [ethanol
(EL), acetonitrile (ACN), tetrahydrofuran (THF), ethylene glycol (EG),
glycerol (GLY), dioxan (DX), heptane (HP)] were procured from E.
Merck, India. Solvents were vigorously purified according to stan-
dard procedure described elsewhere [23]. Doubly distilled water
(resistivity > 10 Mꢀ cm) was used throughout the course of exper-
iment. The purified solvents were found to be free from impurities
and were transparent in the spectral region of interest.
ꢀꢂꢁ = a1ꢂ1 + a2ꢂ2.
3. Results
3.1. Absorbance
Fig. 1a represents the absorption spectra of Pyz in different
solvents. The absorption maxima and molar extinction coefficient
values in different solvents have been presented in Table 1. ꢃmax
Scheme 2. Synthetic route of Pyz. (a) 20% H2SO4, reflux, 16 h; (b) phenyl hydrazine, heat, water bath; (c) N-bromo succinamide, CCl4, reflux, 1.5 h; (d) triethyl
amine; (e) ethyl acrylate. (1) 5-(Dichloromethyl)-3-methyl-1-phenyl-1H-pyrazole-4-carbonitrile; (2) 5-formyl-3-methyl-1-phenyl-1H-pyrazole-4-carbonitrile; (3) 3-
methyl-5-phenylhydrazino-1-phenyl-1H-pyrazole-4-carbonitrile; (4) 3-methyl-5-(4-bromophenylhydrazonoyl bromide)-1-phenyl-1H-pyrazole-4-carbonitrile; (5) ethyl
1-(4-bromophenyl)-3-(4-cyano-3-methyl-1-phenyl-1H-pyrazole-5-yl)-4,5-dihydro-1H-pyrazole-5-carboxylate.

S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332
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Table 1
Spectroscopic parameters of Pyz in different solvents
abs
Solvent
ET(30) (kcal mol−1
)
ꢃ
(ε) (nm) (dm3 mol−1 cm−1
)
ꢃflmax (nm)
Quantum yield (˚f)
Lifetime, ꢂ (ns)
max
HP
31.1
37.4
46.0
49.0
51.9
56.3
57.0
63.1
368 (10,364)
368 (9,273)
366 (10,909)
370 (12,000)
370 (11,455)
370 (7,636)
370 (8,000)
375 (8,000)
455
480
488
480
490
495
492
495
0.64
0.66
0.48
0.61
0.36
0.35
0.31
0.09
4.540
5.022
4.905
5.286
3.660
2.800
2.166
THF
ACN
DX
EL
EG
GLY
H2O
0.6, 4.204
of pyrazoline was observed to be shifted from 368 to 375 nm in
solvents of different polarity. The absorption of the compound,
characterized by a broad band with the maximum appearing
between 368 and 375 nm, corresponds to ␲ → ␲ transition of the
of dioxan–water by adjusting the ratio between dioxan and water,
where the polarity parameter ET(30) varies from 36 to 63.1.
Changes in viscosity of the solution of Pyz were produced by
adjusting the ratio between glycerol and water, where the viscosity
of glycerol at room temperature is 1490 cP, much higher than that
of water (1 cP). The absorption spectrum of Pyz is independent of
the viscosity of the media.
*
molecule to S1. The absorption is intense and the peak position is
sensitive to the polarity of the medium. The magnitude of this shift
suggests that the ground state of the molecule is polar. From the
molar absorption coefficient and the half width of the S0 and S1
transition, the oscillator strength of this transition is estimated to
be varied from 0.17 to 0.23 in different solvents. Effect of polarity
of the medium on the ground state of the compound was also esti-
mated from the absorption spectra of Pyz in the binary mixture
3.2. Steady state fluorescence
Fluorescence spectra of the compound were recorded in various
solvents of different polarity (Fig. 1b). The wavelengths correspond-
ing to the fluorescence maximum have been collected in Table 1.
An increase in polarity of the medium leads to a red shift of
the fluorescence maxima. The effect of polarity of the medium
on the fluorescence maxima is more pronounced than that on
the absorption maxima (Table 1). The effect of polarity has also
been observed from the fluorescence of Pyz in different compo-
sition of dioxan–water mixture. With decrease in polarity of the
medium blue shift of the fluorescence maxima with increase in
fluorescence intensity has been observed (Fig. 2). This observation
suggests that the emitting state of the systems is more polar than
the ground state [24–29]. The compound is highly fluorescent as
obtained from its high quantum yield (Table 1) in different sol-
vents. The fluorescence quantum yields in pure and mixed binary
solvents are sensitive towards solvent polarity. Linear variation of
quantum yield with polarity (ET(30)) implies an identical inter-
action and separate sets of points obtained for polar protic and
aprotic solvents support the presence of hydrogen-bonding inter-
Fig. 1. (a) Absorption spectra of Pyz in different solvents: (1) water, (2) THF, (3) HP,
(4) ACN, (5) EL and (6) DX. (b) Steady state fluorescence spectra of Pyz in different
solvents: (1) EL, (2) ACN, (3) THF, (4) DX and (5) HP. Inset: Steady state fluorescence
spectra of Pyz in water.
Fig. 2. Steady state fluorescence spectra of Pyz in DX–water mixture (v/v). % of DX
in the mixture: (1) 0, (2) 10, (3) 20, (4) 30, (5) 40, (6) 50, (7) 60, (8) 70, (9) 80, (10)
90 and (11) 100.

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S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332
action for polar protic solvents. ꢁf gives the information about the
solvation of the solute in these systems. With increase in polarity
of the medium, the ꢁf value decreases. The decrease is most promi-
nent in protic solvents. In dioxan–water mixture, with increasing
water the decreasing trend of ꢁf may be due to a modification of
water–Pyz hydrogen-bonding interactions by the hydrogen bonded
network present in water. More and more water molecules replace
the solvent molecules in the microenvironment, i.e. at higher frac-
tion of water owing to self aggregation of water molecules, the
microenvironment around Pyz consists mainly of associated water
clusters and these interact with water molecules in the solvation
cell decreasing H-bonding ability of bare water molecules [21].
Thus, ꢁf value tends to decrease. The modification of water–Pyz
hydrogen-bonding interaction may result the decrease in ꢁf with
increase in water content in the mixture. For other solutes such
an anomalous hydrogen-bonding behaviour in water and aprotic
solvent mixture has also been observed [30,31].
The red shift of the spectral maxima on changing the solvent
from ACN to water is clearly the result of the hydrogen bonding.
The fluorescence intensity of Pyz in aqueous solution at 495 nm
is reduced by approximately a factor of 40 from that in ACN.
Unlike absorption, the fluorescence intensity of Pyz is enhanced
in glycerol–water mixture with increasing viscosity of the mixture
by increasing glycerol content due to restricted movement of the
fluorophore in the excited state.
Fluorescence anisotropy depends upon the rotational diffusion
of the fluorophore and the rotational diffusion changes with chang-
ing viscosity of the medium as well as size and shape of the diffusing
species. Such diffusion occurs during the lifetime of the excited
state and displaces the emission dipole of the fluorophore. Gener-
ally, fluorescence occurs from the lowest singlet excited state and
fluorescence anisotropy has been measured at maximum emis-
sion wavelength. The fluorescence anisotropy of Pyz in aqueous
solution is 0.065 and enhanced to 0.309 in glycerol. So media has
strong effect on the rotational motion of Pyz in the excited state. A
higher value of anisotropy indicates a slower rotational motion of
the transition dipole in the media.
Fig. 3. (a) Fluorescence decays of Pyz in different solvents: (a) prompt, (b) water, (c)
EL, (d) HP, (e) ACN, (f) THF and (g) DX. ꢃex = 403 nm. (b) Fluorescence decays of Pyz
in DX–water mixture (v/v). ꢃex = 403 nm.
3.3. Time-resolved fluorescence study
The fluorescence decay behaviour of Pyz has been studied in the
solvents of different polarity and the data have been presented in
Table 1. The excited state decay profiles of Pyz in different solvents
have been shown in Fig. 3a. The full line is the calculated fit assum-
ing an exponential decay convoluted with the instrument response
function. Pyz exhibits single exponential decay in all solvents irre-
spective of the polarity except water. Increase in polarity of the
medium leads to a shortening of the lifetime. In water, the decay
behaviour is biexponential. The sensitivity of the luminescence
decay constant to the solvent polarity prompted us to undertake
lifetime measurement in mixed binary solvents of dioxan–water
(Fig. 3b). In dioxan–water mixture up to 15% Dx (v/v) the decay is
biexponential. The biexponential plot can be analyzed using the
function
sample within 1%. The average lifetime has been used to calculate
the rate constants in aqueous solution. Viscosity of the medium has
a profound effect on the fluorescence quantum yield and lifetime
[33]. In a viscous solution, rotational rate of a fluorophore is slow
due to rigidity of the medium. For a fluorophore dissolved in a sol-
vent, the rotational rate of the fluorophore is dependent upon the
viscous drag imposed on it by the solvent. As a result a change in
solvent viscosity will result in a change in fluorescence anisotropy.
A hypsochromic shift in the fluorescence spectra with an increase
in the glycerol concentration in the mixture has been observed. The
fluorescence intensity increases dramatically (20 times from that
in aqueous solution) when the concentration of glycerol as well
as medium viscosity increase. To study the effect of viscosity of the
medium on the dynamics of the decay of S1 state of Pyz, the ꢂ values
were determined in glycerol–water mixture of different composi-
tion. Like Dx–water mixture, with increase in water content in the
mixture (50%, v/v) the decay behaviour is biexponential. The fluo-
rescence lifetime increases with increasing viscosity in a manner
similar to that of the enhancement factor as in the fluorescence
quantum yield. The increase of the measured ꢂ and ꢁ values with
increase in viscosity of the medium and higher values of anisotropy
clearly indicate the loss of flexibility of the Pyz in the excited state
in these media.
I(t) = A1e−t/ꢂ + A2e−t/ꢂ
1
2
with two time constants ꢂ1 and ꢂ2. A1 and A2 are pre-exponential
factors. I(t) represents fluorescence intensity. The reduced ꢄ2-value
indicates an appreciable fit [32]. The biexponential fitting does
not necessarily indicate that the decay curve has only two dis-
crete time constants, it may be caused by the probe experiencing
two different localconformationsresultingfromtheintermolecular
hydrogen bonding. Despite this complexity, the average lifetimes
are extremely reproducible with repeated measurements on the

S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332
1331
Table 2
Rate constants of radiative and non-radiative transitions, oscillator strengths (f) of
Pyz in different solvents and ˛, ꢇ* values of the solvents
Solvent
kr (ns−1
)
knr (ns−1
)
f
˛
ꢇ*
HP
0.14
0.13
0.10
0.11
0.10
0.13
0.14
0.03
0.08
0.07
0.10
0.08
0.17
0.23
0.32
0.95
0.23
0.19
0.24
0.28
0.25
0.17
0.17
0.21
0a
−0.08a
0.55a
0.66a
0.49a
0.54b
0.92a
0.62a
1.09b
THF
ACN
DX
EL
EG
0a
0.19a
0a
0.83b
0.9a
1.21a
1.17b
GLY
H2O
a
Values taken from ref. [39].
Values taken from ref. [31].
b
2 and 1 and CN in 4 form a tighter hydrogen bond with protic
solvents and this influences the observed fluorescence parameters,
e.g. band maximum and the quantum yield. In Pyz, the cyano
group present at C(4) position of N phenyl pyrazole is likely to be
a better electron withdrawing substituent because of its compact
size and thus more likely to retain molecular polarity and exhibit
maximum effect on conjugative interaction. The Br atom present
in PyZ assists the intersystem crossing process and simultaneously
generates a large C–Br dipole that alters the relaxation dynamics
in different solvents. With increasing polarity of the medium,
solvation dynamics of the C–Br bond has been affected and the
non-radiative process has been perturbed in polar medium. To
investigate the effect of solvation on the dynamics of the excited
state, the radiative (kr) and non-radiative rate constants (knr) were
calculated using the expressions:
Fig. 4. Plot of ꢅ ¯ vs. ꢅf according to Lippert–Mataga equation.
3.4. Excited state dipole moment
Attempts were made to quantify the extent of charge separation
on electronic excitation of the Pyz by measuring the change in the
dipole moment (ꢆe − ꢆg) using the shift between the absorption
and fluorescence maxima (ꢅ ¯ ) as a function of solvent polar-
ity parameter. According to Lippert–Mataga equation [34–36],
ꢀ
ꢁ
2(ꢆe − ꢆg)2
n2 − 1
ꢁF
kr =
and knr = ꢂ−1 − kr
ε − 1
ꢅ ¯ =
−
+ const.
ꢂ
hca3
2n2 + 1
2ε + 1
In the above expressions, ꢁF is the fluorescence quantum yield
and ꢂ is the experimentally observed fluorescence lifetime. The plot
of kr and knr vs. ET(30) of the solvents has been shown in Fig. 5.
Table 2 shows the observed radiative and non-radiative rate con-
stants for Pyz in pure solvents. It appears that the radiative rate
constants are practically insensitive to a change in solvation except
water. Unlike the radiative rate constant, the non-radiative rate con-
stant for Pyz exhibits solvent sensitivity in polar solvents (Fig. 5).
The non-radiative rate constant in protic solvents increases as the
polarity increases. The non-radiative rate constant (knr) in water is
where ꢅ is the Stokes shift, a the Onsagar cavity radius swept
out by the fluorophore, ε and n are the dielectric constant and
refractive index of the solvent, respectively. ꢆe − ꢆg is the dif-
ference between the excited state (ꢆe) and ground state (ꢆg)
dipole moments of the probe. Various protic and aprotic sol-
vents have been used to obtain the dipole moment change
on excitation. A plot of ꢅ ¯ vs. ꢅf (Fig. 4) gives ꢅꢆ, where
ε − 1
n2 − 1
ꢅf =
−
2n2 + 1
2ε + 1
The correlation between ꢅ ¯ and ꢅf was found to be good except
water and dioxan. This may be due to the modification of water–Pyz
H-bonding interaction on excitation in water and in dioxan among
aprotic solvents there is probability of H-bond formation. Using the
AM1 calculated longest distance between the methyl proton and
˚
phenyl bromine as the Onsagar cavity radius (6.8 A), the calculated
change in dipole moment upon excitation is 10.2 Debye. The change
in dipole moment corresponds to an intramolecular displacement
of charge upon excitation of the molecule.
4. Discussion
The absorption and fluorescence band maxima in various
solvents (Table 1) show an overall red shift as the polarity of the
solvent increases. The enhanced solvent sensitivity of fluorescence
compared to absorbance may be explained in terms of increased
solute–solvent interaction in the excited state due to an increase
in the dipole moment upon excitation. For protic solvents apart
from dipolar interaction tighter hydrogen bonding is expected in
the excited state with the solvents as a result of increased charge
density on N atom and CN group. In the S1 state, the N in position
Fig. 5. Plot of radiative (kr) and non-radiative (knr) rate constants of Pyz vs. ET(30)
for different solvents.

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S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332
solvents and this is reflected in the absorption and emis-
sion properties of the probe in solvents of different polarity.
The compound is fluorescent with high fluorescence quantum
yield and considerable solvatochromism. The steady state emis-
sion spectra and relaxation dynamics have been found to be
influenced by the solute–solvent interaction. The non-radiative
decay constant for Pyz depends mainly on the hydrogen-bonding
interaction of the probe in the excited state with the solvent,
but the radiative decay constant is practically insensitive to
solvation.
Acknowledgements
Financial support from CSIR (01(2057)/06/EMR-II) is gratefully
acknowledged. One of the authors S.P. acknowledges CSIR and A.S.
acknowledges UGC for research fellowship.
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ET(30) shows a linear relationship with practically the same slope
as that observed for a plot of kr against ET(30). Such phenomenon
has also been observed by others with different probe [37]. For
protic solvents (EL, EG, GLY, H2O), specific hydrogen-bonding inter-
action occurring between the Pyz in the S1 state and protic solvent
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from S1 to S0. In ACN molecules, dipole–dipole interaction may
occur between the CN present at C(4) position of N-phenyl pyra-
zoline with ACN which leads to the lower quantum yield, kr and
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interactions on the decay constant, knr the plot of ln knr vs.
the parameters ꢇ* and ˛ representing non-specific and specific
hydrogen-bonding solute–solvent interactions, respectively, has
been considered. ln knr is linearly correlated with ˛ and no cor-
relation with ꢇ* has been observed (Fig. 6). For water, the point
is off the line which may be due to the modification of water Pyz
hydrogen-bonding interaction on excitation as explained in earlier
section. Thus, one may conclude that specific hydrogen-bonding
interactions occurring between Pyz in the S1 state and protic sol-
vent molecules leads to an increase in the non-radiative relaxation
rates from S1 to S0. The importance of hydrogen-bonding interac-
tions in influencing the steady state fluorescence parameters and
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