Journal of Luminescence 130 (2010) 2271–2276

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

in (S. Ch

journal homepage: www.elsevier.com/locate/jlumin

Tuning the photophysics of a bio-active molecular probe‘3-pyrazolyl-2-pyrazoline’ derivative in different solvents:Dual effect of polarity and hydrogen bonding

Arindam Sarkar, Tapas Kumar Mandal, Dipak Kumar Rana, Sayaree Dhar, Sayantani Chall,Subhash Chandra Bhattacharya n

Department of Chemistry, Jadavpur University, Kolkata – 700032, India

a r t i c l e i n f o

Article history:

Received 31 March 2010

Received in revised form

6 July 2010

Accepted 9 July 2010Available online 14 July 2010

Keywords:

Kamlet–Taft parameter

Hydrogen bonding

Lifetime

Polarity

Fluorescence

13/$ - see front matter & 2010 Elsevier B.V. A

016/j.jlumin.2010.07.004

esponding author. Tel.: +91 33 2414 6223; fa

ail addresses: sbjuchem@yahoo.com, scbhatt

andra Bhattacharya).

a b s t r a c t

The solvatochromic behavior of 3-pyrazolyl 2-pyrazoline derivative (PYZ), a newly synthesized

molecular probe having pharmaceutical importance, has been studied in various solvents of different

polarity. The Kamlet and Taft solvatochromic comparison method was utilized to rationalize the solute–

solvent interactions from absorption and emission measurements. Spectroscopic studies reveal that the

solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the

hydrogen-bonding properties of the solvents. 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.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Pyrazoline-based fluorophores stand out due to their simplestructure and favorable photophysical properties such as largeextinction coefficient and quantum yields (0.6–0.8) [1,2]. Solva-tion studies of such probes draw attention to researchers, owingto the ability of the solvents to stabilize a probe molecule, eitherthrough dipolar interactions with the probe molecule or throughsome specific interactions like hydrogen bonding, etc. Thephotophysical properties of many fluorophores, especially thosecontaining polar substituents on the aromatic ring/s, are known tobe sensitive to the chemical and physical properties of thesolvents. Hydrogen bonding and solvent polarity are the keyfactors in controlling pathways of energy dissipation followingelectronic excitation [3,4]. Hydrogen bond formation has adifferent effect on the energies of various excited states; in anextreme case, hydrogen bonding may cause the reversal of close-lying n, pn and p, pn states [5]. Nitrogen containing heterocycles ingeneral and pyrazole and pyrazolyl-pyrazoline derivatives spe-cially possesses important biological and pharmaceutical activ-ities [6,7] and are also useful synthetic building blocks in organicchemistry including pyrazole-based bio-active materials [8,9].

ll rights reserved.

x: +91 33 24146584.

acharyya@chemistry.jdvu.ac.

2-Pyrazoline derivatives, in particular, have been effectivelyutilized as antibacterial, antiviral, antiparasitic, antitubercularand insecticidal agents [10–14]. In addition, the other mostinteresting property of 2-pyrazoline moiety is its use asnanoparticles [15]. Bipyrazoles are useful compounds and areused as cytotoxic, insecticide, herbicide and fungicide agentsalong with applications in the photographic and paint industry[16]. Synthesis of this new class of pyrazole-substituted-2-pyrazolines with inbuilt bromo substituent allow them to beused as potential bromo organics in pharmaceuticals, intermedi-ate for agrochemicals and industrially valuable new materials andprovides scope for elaboration to other functionality [17–21].Further, 2-pyrazoline derivatives are also considered to beimportant organic heterocyclic ‘transition’ materials and finduse in organic molecular crystals [22]. Photophysical studies withsome of these newly synthesized 2-pyrazoline derivatives [23–30]clearly demonstrate their potential application as fluorescentprobes in some chemosensors and as nanoparticles in optoelec-tronic device [31–33]. The compound is highly fluorescent due toextended conjugation from 2-pyrazoline ring to pyrazole. Theinterest on the photophysical study of pyrazoline derivatives insolvents originates principally from two aspects: the first stemsfrom its novel biological applications in pharmaceuticals, and thesecond one arises due to presence of electron acceptors anddonors at N – 1 and C – 3 positions, respectively. These contributeintrinsically large molecular hyperpolarizabilities of 2-pyrazolinederivatives. The ground and excited state behaviors of 2-pyrazoline

A. Sarkar et al. / Journal of Luminescence 130 (2010) 2271–22762272

derivatives are also influenced by the surrounding medium andvariation of photophysical process of 2-pyrazoline derivatives isobserved depending on the substituent [34–37].

In this study, effect of photophysical properties on thesolvation dynamics of another derivative of 2-pyrazoline (PYZ),in different solvents have been studied. The compound has phenylgroup at N – 1 position of pyrazoline with methyl and cyanosubstitution at C – 3 and C – 4 position of pyrazole, respectively.Generally phenyl substitution at N – 1 position of pyrazoline ringmakes it highly fluorescent and simultaneously extended con-jugation from 2-pyrazoline to pyrazole ring enhances thefluorescence intensity of the compound. The presence of cyanogroup at C – 4 makes it a potential hydrogen bonding site, due tothe polarity difference of the C�N bond. The bromine atom in PYZfacilitates the intersystem crossing process and generates a largeC–Br dipole that also affects the relaxation dynamics of PYZ indifferent solvents. Our target is to study the solvent dependentradiative transitions and relaxation dynamics of this derivative ofpyrazole substituted 2-pyrazoline from the excited singlet stateusing steady state and time resolved fluorescence techniques andrationalize them considering solute–solvent interactions.

2. Structure

N

N

Ph

H3C CN

N N

C6H4Br (p)

1

2

3 4

5

1'2'

3'

4'

5'

PhH

H

Ph

5-((4S, 5R)-1-(4-Bromophenyl)-4,5-dihydro-4,5-diphenyl-1H-pyrazol-3-yl)-3-methyl-1-phenyl-1H-pyrazole-4-carboni-trile

Table 1Spectroscopic parameters of PYZ in solvents of different polarity at room

temperature.

Solvent kabsmax/nm lfls

max/nm Dt/cm�1 ET(30)/kcal mol�1

Heptane 373 467 5396.37 31.1

Cyclohexane 376 468 5228.22 30.9

Dioxan 377 481 5735.18 36.0

THF 376 483 5891.81 37.4

DMF 375 492 6341.46 43.2

ACN 372 491 6515.12 45.6

Ethanol 377 493 6241.22 51.9

Methanol 377 496 6363.90 55.4

EG 379 500 6385.22 56.3

Water 391 485 4956.89 63.1

3. Experimental details

The pyrazoline derivative (PYZ) was synthesized following theprocedure mentioned in the synthesis scheme [38]. The com-pound was recrystallized using ethyl acetate-pet ether (1:1)before use. The solvents used were methanol (MeOH), ethanol(EtOH), 1,4-dioxane (Diox/Dioxan), cyclohexane (cyhx), n-heptane(hept), ethane-1,2-diol (EG), N,N-dimethylformamide (DMF),tetrahydrofuran (THF), acetonitrile (ACN). All the solvents usedwere of AR grade. The purified solvents were found to be free fromimpurities and were transparent in the spectral region of interest.The photophysics of the PYZ derivative in various solvents wasstudied applying absorbance, fluorescence and lifetime measure-ment techniques. Absorption spectra were recorded using aPharmaspec UV–vis 1700 spectrophotometer (Shimadzu), with amatched pair of silica cuvettes (path length 1 cm). The emissionspectra of the probes were recorded using spectrofluorimeter(Fluorolog FII A Spectrofluorimeter, Spex Inc, NJ, USA) with a slitwidth of 1.25 mm. Fluorescence lifetime measurements wereperformed using time correlated single photon counting methodand using a nanosecond diode laser at 370 nm (IBH, nanoLED-7)

as light source. The response time of the instrument is 1.1 ns. Thedecays were analyzed using IBH DAS-6 decay analysis software.

4. Results and discussions

4.1. Absorption study

The absorption spectra of PYZ in different solvents arecharacterized by a broad band and are sufficiently well resolvedto provide accurate excited state energies. The wavelengthmaxima (labs

max) values have been listed in Table 1. Thebathochromic shift in labs

max values of PYZ from cyclohexane(376 nm) to water (391 nm) (Fig. 1) clearly indicates that theabsorption spectrum of PYZ is sensitive to the polarity of themedium. This bathochromic band shift (Dl¼15 nm) withincreasing solvent polarity (i.e. positive solvatochromism)suggests that the molecule is polar in the ground state. All thespectra corresponds to p-pn transitions of the molecule to S1.The bathochromic shift in absorption maxima of PYZ in water,compared to the other non-aqueous solvents, reflects thedependence of the photophysical properties of the probe withthe polarity of the medium.

4.2. Steady state fluorescence study

In contrast with the ground state spectral properties, theexcited state properties of PYZ follow a different trend. Thefluorescence spectra (Fig. 2) and the emission maxima (lfls

max)values (Table 1) of PYZ demonstrate this difference. The emissionmaxima of PYZ in water is in a higher energy (lower wavelength)region than that in DMF, ACN, MeOH, EtOH and EG, although it ismore polar than the others. Earlier works by Chatterjee et al. [23]and Correa and Levinger [39] showed similar results. Thus it isvery clear that polarity alone cannot account for thesolvatochromic behavior of PYZ, in the excited state. Ifincreasing polarity of the medium is responsible for a spectralshift to lower energy, then the emission maxima of PYZ in waterwould have peaked at a lower energy region than that of theabove mentioned solvents. Hence specific interactions must alsoplay a role in the photophysics of PYZ. To account for the excitedstate behavior of PYZ in different solvents, specific probe–solventinteractions were considered in terms of hydrogen bondingaccepting (b) and hydrogen bonding donating (a) ability of thesolvents. A plot of Stokes shift vs ET (30) [40–42] of the solventswas also plotted (Fig. 3). The data points formed two separateclasses. For aprotic solvents, a line with higher slope and for proticsolvents, a line with a lower slope was obtained, indicating thatPYZ behaves differently with the polar protic and the aproticsolvents.

3000.000

0.025

0.050

0.075

0.100

Abs

orba

nce

wavelength / nm

watercyclohexane

391 nm

376 nm

350 400 450 500

Fig. 1. Absorption spectra of PYZ in water and cyclohexane. [PYZ]¼4.4�10�6 M.

425

Fluo

resc

ence

Inte

nsity

/ a.

u

wavelength / nm

ACNCyclohexane1,4-dioxanDMFEGEtOHn-heptaneMeOHTHFWater

450 475 500 525 550 575 600

Fig. 2. Emission spectra of PYZ in different solvents. [PYZ]¼4.4�10�6 M.

304800

5200

5600

6000

6400

cyclohexane

heptane

dioxan

THF

DMF

ACN

EtOHMeOH

EG

Δν /

cm-1

ET (30) / kcal mol-135 40 45 50 55 60 65

Fig. 3. Plot of Stokes shift (Dn) of PYZ vs solvent polarity parameter ET(30).

A. Sarkar et al. / Journal of Luminescence 130 (2010) 2271–2276 2273

4.3. Kamlet–Taft analysis of PYZ

The solvent effects on physical or chemical processes arefrequently studied by empirical solvent parameters to determinethe predominant interactions. One of the most useful approachesfor elucidating and quantifying different solute–solvent interac-tions is the Kamlet–Taft solvatochromic comparison method(KTSCM) [43]. According to the KTSCM, absorption [E(A)] andemission [E(F)] band energies can be correlated using the multiplelinear regression analysis approach of Abraham et al. [44] asbelow.

E¼ E0þaaþbbþsp* ð1Þ

where pn is the solvent dipolarity/polarizability parameter, a isthe H-bond donation ability of the solvent, and b is the H-bondacceptance or electron-pair donation ability to form a coordinatebond. The coefficients s, a, and b measure the relative sensitivityto the indicated solvent property.

The values obtained were:

EðFÞ ¼ 61:13-0:63a-2:67b-1:48p* ð2Þ

EðAÞ ¼ 76:37-1:19aþ1:83b-1:33p* ð3Þ

The results obtained from the PYZ absorption spectra reflectinteractions of the dye in its ground electronic state. Here, the PYZmolecule displays a bathochromic shift with a and pn and ahyspochromic shift with the b parameter. For PYZ, the s/a ratio is1.11, demonstrating that PYZ is almost equally sensitive to thedipolarity parameter of the solvent and to its H-bond donationability. This challenges the general interpretation of PYZ’ssolvatochromic behavior, that is it primarily reflects the polarityof the medium. Instead, the KTSCM parameters reveal that in itsground state PYZ is also sensitive to specific interactions besidespolarity. The KTSCM parameters from emission show that in itsexcited state PYZ interacts with its environment differently thanin its ground state. All the parameters display a bathochromicshift but the b parameter of the solvent significantly contributesto the results. The b/a ratio is 4.24 and s/a ratio is 2.35. This clearlyimplies the dominance of H-bond acceptance ability and polariz-ability of the solvents over H-bond donation ability. Thesignificance of b is clearly prominent from the results of PYZ inthe excited state than that in its ground state. In the electronicexcited state, many molecules have dipole moments larger than inthe ground state. The dynamic changes of the dipole momentsometimes induce formation of a strong hydrogen bond in theexcited state [45]. Upon excitation, polarity of PYZ increases mostlikely because of an increase in the dipole moment. As a result, thebromine atom present in the PYZ generates considerable C–Brdipole that alters the relaxation dynamics of PYZ in differentsolvents. The H-bond formation capability also becomes morefacile. These factors cause the excited-state molecule to lose itssensitivity towards the a parameter. In contrast, emission of PYZshows substantial increase in sensitivity to both pn and b for theexcited state. The fact that PYZ is more sensitive to the H-bondacceptance ability (b) of the solvent upon excitation, suggests theformation of intermolecular stable H-bond complexes withsolvents having high b values.

4.4. Fluorescence decay

The fluorescence decay behavior of the PYZ has been studied insolvents of varying polarity. The lifetime values were calculatedusing the expression

IðtÞ ¼Xn

i ¼ 1

aie�t=ti

i

4.5

5.0

5.5

EtOH

DMFACN

THFDiox

A. Sarkar et al. / Journal of Luminescence 130 (2010) 2271–22762274

where I(t) is the intensity of the fluorescence at time t, ai is thepre-exponential factor for the fraction of the fluorescenceintensity, ti is the fluorescence lifetime of the emitting speciesand n is the total number of emitting species. The averagefluorescence lifetimes were calculated using the relationtavg¼(a1t1+a2t2)/(a1+a2), where a1 and a2 are pre-exponentialfactors. The fluorescence decays of PYZ in different solvents(Fig. 4) provide additional information about the interaction of themolecule with its environment. Time constants for fluorescencedecays monitored at the emission band maxima are given inTable 2. All the decays are monoexponential when measured atthe wavelength of maximum emission except water. Previousreports suggest that the emission lifetimes of pyrazolinederivatives vary little in solvents and the emission decay tracesfit well to a single-exponential decay [23,28]. Although thelifetimes are similar in different media, they decrease in highlypolar solvents such as water and alcohols. In solvent with thelargest a and lower b values (water), biexponential decay isobserved. In other solvents the fluorescence decays aremonoexponential. The biexponential decay in water with high amost likely result from specific solvent–fluorophore interactionsthat also account for its solvatochromism. Specific interactionscan affect the ground state or the excited state or both and theseinteractions may weaken or strengthen following excitation [46].The extent of perturbation in the PYZ absorption spectrum incomparison with the excited state perturbation suggests thatspecific interactions are less important for the fluorophore in its

0

10

100

1000

log

(cou

nts)

time / ns

promptACNcyclohexaneDioxanDMFEGEtOHHeptaneMeOHTHFWater

10 20 30 40

Fig. 4. Fluorescence decay curves of PYZ in various solvents. [PYZ] ¼4.4�10�6 M.

Table 2Fluorescence decay parameters, quantum yield, radiative and non-radiative rate

constants of PYZ in various solvents.

Solvent a1 (a2) s1 (s2)/ns v2 u kr�10�7/s�1

knr x 10�7/s�1

ACN 0.1 5.05 1.15 0.41 8.12 11.67

Cyclohexane 0.09 4.73 1.13 0.56 11.85 9.28

Dioxan 0.09 5.21 1.09 0.6 11.49 7.7

DMF 0.09 4.95 1.22 0.6 12.08 8.12

EG 0.1 3.59 1.11 0.45 12.67 15.18

EtOH 0.09 4.53 1.17 0.49 10.8 11.27

Heptane 0.17 4.64 1.13 0.43 9.28 12.26

MeOH 0.43 3.74 1.10 0.34 9.21 17.52

THF 0.09 5.04 1.14 0.65 13.06 6.77

Water 0.69(0.31) 1.91(4.14) 0.98 0.18 7.11 31.34

ground state and that the observed emission shifts result fromstrong interactions between the excited solute and the solventoccurring on the time scale of the excited-state lifetime.Interactions leading to sensitivity in the absorption spectrumwith b and pn increase upon excitation where the propensity ofPYZ to form an H-bond increases. A plot of lifetime vs b (Fig. 5)gives two straight lines: one for the polar protic solvents and theother for the aprotic solvents. With increase in b values of polarprotic solvents, lifetime of PYZ increases, whereas for the aproticsolvents it is more or less the same. This indicates that theparameter b plays a major role in governing the photophysics ofPYZ in the excited state in polar solvents. In case of pn, it is just thereverse, with increase in pn values of the polar protic solvents,lifetime of PYZ decreases whereas for aprotic solvents it remainsalmost the same (figure not shown). From the plot of lifetime vsET(30) (Fig. 6), it is seen that lifetime of the probe decreases withincrease in solvent polarity of the protic solvents whereas foraprotic solvents, there is no such major difference in the lifetimevalues of PYZ. Thus one may conclude that specific hydrogen

0.1

2.5

3.0

3.5

4.0

Life

time

/ ns

β

water

EG

MeOH

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Fig. 5. Plot of lifetime (t) of PYZ vs Kamlet–Taft parameter b.

30

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Life

time

/ ns

ET (30) / kcal mol-1

water

EG

MeOH

EtOH

ACN

DMFTHFDiox

CyhxHept

35 40 45 50 55 60 65

Fig. 6. Plot of lifetime (t) of PYZ vs solvent polarity parameter ET(30).

30

7.8

7.9

8.0

8.1

8.2

8.3

8.4

8.5

logk

nr

ET (30) / kcal mol-1

heptane

cyhx

dioxan

THF

DMF

ACN

EG

EtOH

MeOH

water

35 40 45 50 55 60 65

Fig. 7. Plot of log (knr) of PYZ vs solvent polarity parameter ET(30).

A. Sarkar et al. / Journal of Luminescence 130 (2010) 2271–2276 2275

bonding interactions occurring between PYZ in the S1 state andthe protic solvent molecules lead to an increase in the non-radiative relaxation rates from S1 to S0 (discussed in the latersection). The importance of hydrogen bonding interactions ininfluencing the steady state fluorescent parameters and dynamicsof the excited state of different probes have also been observed byother workers using other probes [5,39]. Hence specificinteractions occurring in protic solvents induce an efficientvibronic coupling with the excited state of PYZ reducingfluorescence quantum yield and increasing the rate of non-radiative transitions.

4.5. Radiative and non-radiative rate constants

To investigate the effect of solvation on the dynamics of theexcited state, the radiative (kr) and non-radiative (knr) rateconstants of the systems in different solvents have beendetermined using the following relations and the values are givenin Table 2:

kr ¼ff

tfand knr ¼

ð1�ff Þ

tf

where ff and tf are the quantum yield and average lifetime valuesof the probes, respectively.

The plot of log(knr) vs ET(30) of the solvents has been shown inFig. 7. It appears that the radiative and non-radiative rateconstants for PYZ exhibit solvent sensitivity in polar andnonpolar solvents. The non-radiative rate constant in proticsolvents increases as the polarity increases. In aprotic solventsthe non-radiative rate constant decreases with increase inpolarity of the solvents. For protic solvents, specific hydrogenbonding interaction occurring between PYZ in the S1 state andsolvent molecules lead to an increase in the non-radiativerelaxation rates from S1 to S0. In ACN, dipole–dipole interactionsmay occur between the –CN groups present at C – 4 position of N-phenyl pyrazoline with ACN molecules which leads to lowerquantum yield values and higher knr values from those in otheraprotic solvents. The decrease in the fluorescence quantum yieldin protic solvents is primarily due to an increase in the non-radiative decay process. In PYZ, the cyano group present at C – 4position of the N-phenyl pyrazole is likely to be a better electronwithdrawing substituent because of its compact size and thus

more likely to retain molecular planarity and exhibit maximumeffect on conjugative interaction. The bromine atom present inPYZ assists the intersystem crossing process and simultaneouslygenerates a large C–Br dipole that alters the relaxation dynamicsin different solvents. The heavy atom effect of chlorinated andbrominated azulene derivatives has been reported by Eber et al.[47] for the enhanced intersystem crossing which plays animportant role in radiationless decay rates. With increasingpolarity of the medium, solvation dynamics of PYZ (controlledby the C–Br dipole as well as the C�N dipole) has been affectedleading to a change in the non-radiative relaxation process.

5. Conclusions

Spectroscopic studies reveal that the solvatochromic behaviorof PYZ depends not only on the polarity of the medium but also onthe hydrogen-bonding properties of the solvents. In water, EG,MeOH and EtOH, relaxation processes reflect the characteristics ofPYZ. The absorption spectra clearly depict the solvent polaritydependence of the probes, whereas the diminished emissionintensities of the probe in polar solvents indicate the formation ofa hydrogen bond resulting in radiationless decay. With ACN andDMF, the behavior is different from other aprotic solvents. Thehigh non-radiative rate constants and the short lifetime values ofthe probe in polar solvents reveal the possibility of radiationlessdecay via hydrogen bonding. A Kamlet–Taft analysis shows that inground state, PYZ appears almost equally sensitive to a and pn,demonstrating its dependence on specific interactions. In theexcited state, the sensitivity to b and pn increases, while it losesall sensitivity to a. In the excited state, PYZ forms a stable H-bondcomplex with solvents with high H-bond acceptance abilities(high b) and low H-bond donor character (low a). The bromineatom in PYZ facilitates the intersystem crossing process andgenerates a large C–Br dipole that alters the relaxation dynamicsof PYZ in different solvents. Keeping in mind of the wide spreadapplications of the pyrazoline derivatives in bio-medical fields,this photophysical study of this newly synthesized PYZ derivativewill help in assessing their potential application in differentenvironments.

Acknowledgement

The authors are indebted to Prof. K.K. Mahalanabis and Dr. A.Mukherjee of Jadavpur University for their co-operation with thesynthesis of the compound. Authors A.S. and S.D. thank UGC forproviding an SRF and JRF, respectively. Author D.K.R. thanks CSIRfor providing a JRF.

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