Spectrochimica Acta Part A 71 (2008) 1327–1332

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Spectroscopic investigation of 3-pyrazolyl 2-pyrazoline derivative inhomogeneous 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

Article history:Received 11 January 2008Received in revised form 25 March 2008Accepted 14 April 2008

Keywords:

a b s t r a c t

How solvent conditions such as solvent polarity and hydrogen-bonding affect the fluorescence of a newlysynthesized 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 excitedstate. The relaxation of S1 state is perturbed in hydrogen-bonding solvents. The fluorescence propertiesof the systems are strongly dependent on the polarity of the media. The non-radiative relaxation process

FluorescenceRSH

is facilitated by an increase in the polarity of the media. The photophysical response of Pyz in differentsolvents has been explained considering solute–solvent interactions.

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elaxation dynamicsolvent polarityydrogen bonding

. Introduction

Photophysical study of pyrazoline derivatives in homogeneousolvents originates principally from two aspects—the first onetems from its novel biological applications in pharmaceuticalsnd the second one arises due to presence of electron donorsnd acceptors at N(1) and C(3) positions, respectively. 2-Pyrazolineerivatives have been effectively utilized as antibacterial, antifun-al, antiviral, antiparasitic, antitubercular and insecticidal agents1–3] and as a hole-conveying medium in photoconductive mate-ials and electroluminescence devices [4,5]. Pyrazoline derivativesre well known fluorescent compounds with high quantum yieldsnd are used as optical brightening agents for textiles, fabrics, plas-ics and papers [6]. These compounds have been utilized as fluo-escent probes in some chemosensors [7]. 2-Pyrazoline derivativesre considered to be an important organic heterocyclic “transition”aterial, i.e. a material which shares many properties with con-

entional semiconductors and insulators like organic molecularrystals [8]. Ranging from tens to hundreds of nanometers, recentlyifferent groups prepared pyrazoline nanoparticles by using the

eprecipitation method and explored their size-tunable opticalroperties for the application in optoelectronic device [6,9–11].

Fluorescent probes are powerful tools in cell biology for the non-nvasive measurement of intracellular ion concentrations and to

∗ 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).

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386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2008.04.011

© 2008 Elsevier B.V. All rights reserved.

isualize labile zinc and iron pools, or as pH sensors [12–15]. For theesign of cation specific fluorescent probes, a synthetically moreexible fluorophore structure would be desirable. Among possibleubstitutes, pyrazoline-based fluorophores stand out due to theirimple structure and favorable photophysical properties such asarge extinction coefficient and quantum yields (0.6–0.8) [16,17].

Variation of photophysical process of 2-pyrazoline derivativesre observed depending on the substituent [18–20]. In our earlierork on pyrazoline derivatives having fused norborane ring present

t 3a and 7a carbon of 2-pyrazoline ring fluorescence emissionsrom S2 state have been observed [21].

In this study, photophysical properties of another derivative of-pyrazoline (Pyz) in homogeneous solvents have been studied.he compound, ethyl 1-(4-bromophenyl)-3-(4 cyano-3-methyl-phenyl-1H-pyrazole-5yl)-4,5, dihydro-1H-pyrazole-5-carboxy-

ate (Scheme 1) has phenyl group at N(1) position of pyrazolineith methyl and cyano substitution at C(3) and C(4) position

f pyrazole, respectively. Generally, phenyl substitution at N(1)osition of pyrazoline ring makes it highly fluorescent and simul-aneously extended conjugation from 2-pyrazoline to pyrazoleing enhances the fluorescence intensity of the compound. Inhis compound, the ethyl carboxylate group is substituted in C(5)osition of pyrazoline ring. Our target is to study the solventependent radiative transitions and relaxation dynamics of this

erivative of pyrazole substituted 2-pyrazoline from the excitedinglet state. The results obtained from the steady state and time-esolved fluorescence studies have been rationalized consideringolute–solvent interaction. A comparative study of the kinetics ofhe probe in different solvents has been made.

1328 S. Pramanik et al. / Spectrochimica Ac

2

2

psbcv(gMd(ia

2

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〈

3

3.1. Absorbance

Fig. 1a represents the absorption spectra of Pyz in different

Sam1

Scheme 1. Structure of Pyz.

. Experimental

.1. Chemicals

Ethyl 1-(4-bromophenyl)-3-(4 cyano-3-methyl-1phenyl-1H-yrazole-5yl)-4,5, dihydro-1H-pyrazole-5-carboxylate was synthe-ized using the method shown in Scheme 2 [22]. It was purifiedy column chromatography and the purity of the compound washecked by thin layer chromatography. The compound was furtheracuum sublimed before use. Spectroscopic grade solvents [ethanolEL), acetonitrile (ACN), tetrahydrofuran (THF), ethylene glycol (EG),lycerol (GLY), dioxan (DX), heptane (HP)] were procured from E.erck, India. Solvents were vigorously purified according to stan-

ard procedure described elsewhere [23]. Doubly distilled waterresistivity > 10 M� cm) was used throughout the course of exper-

ment. The purified solvents were found to be free from impuritiesnd were transparent in the spectral region of interest.

sv

cheme 2. Synthetic route of Pyz. (a) 20% H2SO4, reflux, 16 h; (b) phenyl hydrazinmine; (e) ethyl acrylate. (1) 5-(Dichloromethyl)-3-methyl-1-phenyl-1H-pyrazole-4-cethyl-5-phenylhydrazino-1-phenyl-1H-pyrazole-4-carbonitrile; (4) 3-methyl-5-(4-bro

-(4-bromophenyl)-3-(4-cyano-3-methyl-1-phenyl-1H-pyrazole-5-yl)-4,5-dihydro-1H-p

ta Part A 71 (2008) 1327–1332

.2. Instrumentation

UV–visible absorption spectra of Pyz in different solutions wereecorded in a Shimadzu UV-1700 spectrophotometer, Japan usinguartz cuvettes of path length 1 cm with a cell volume of 3.0 ml.mission spectra and steady state fluorescence anisotropy wereecorded in a Spex Fluorolog IIA spectrofluorimeter with a slitidth of 1.25 mm, and by a PerkinElmer spectrofluorimeter, respec-

ively. All measurements were done repeatedly and reproducibleesults were obtained. The fluorescence quantum yields (�f) ofhe compound in different solvents were measured using quinineulphate (0.1 M in H2SO4, �f = 0.57) as the reference compound.he �f values are given in Table 1. Fluorescence lifetimes wereetermined from time-resolved fluorescence decay by the methodf time-correlated single-photon counting using a picosecondiode laser at 403 nm (IBH, nanoLED-07) as light source. The typ-

cal response time of this laser system was 70 ps. The decays werenalyzed using IBH DAS-6 decay analysis software. For all the life-ime measurements, the fluorescence decay curves were analyzedy single and biexponential iterative fitting program provided byBH. Mean (average) fluorescence lifetime (〈�〉) for biexponentialterative fitting were calculated from the decay times (�1, �2) andhe pre-exponential factors (a1, a2) using the following relation:

�〉 = a1�1 + a2�2.

. Results

olvents. The absorption maxima and molar extinction coefficientalues in different solvents have been presented in Table 1. �max

e, heat, water bath; (c) N-bromo succinamide, CCl4, reflux, 1.5 h; (d) triethylarbonitrile; (2) 5-formyl-3-methyl-1-phenyl-1H-pyrazole-4-carbonitrile; (3) 3-mophenylhydrazonoyl bromide)-1-phenyl-1H-pyrazole-4-carbonitrile; (5) ethylyrazole-5-carboxylate.

S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332 1329

Table 1Spectroscopic parameters of Pyz in different solvents

Solvent ET(30) (kcal mol−1) �absmax (ε) (nm) (dm3 mol−1 cm−1) �fl

max (nm) Quantum yield (˚f) Lifetime, � (ns)

HP 31.1 368 (10,364) 455 0.64 4.540THF 37.4 368 (9,273) 480 0.66 5.022ACN 46.0 366 (10,909) 488 0.48 4.905DX 49.0 370 (12,000) 480 0.61 5.286EL 51.9 370 (11,455) 490 0.36 3.660EGH

oscbmssmtbom

F(ss

ow

aoot

G 56.3 370 (7,636)LY 57.0 370 (8,000)2O 63.1 375 (8,000)

f pyrazoline was observed to be shifted from 368 to 375 nm inolvents of different polarity. The absorption of the compound,haracterized by a broad band with the maximum appearingetween 368 and 375 nm, corresponds to � → �* transition of theolecule to S1. The absorption is intense and the peak position is

ensitive to the polarity of the medium. The magnitude of this shiftuggests that the ground state of the molecule is polar. From the

olar absorption coefficient and the half width of the S0 and S1

ransition, the oscillator strength of this transition is estimated toe varied from 0.17 to 0.23 in different solvents. Effect of polarityf the medium on the ground state of the compound was also esti-ated from the absorption spectra of Pyz in the binary mixture

ig. 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 differentolvents: (1) EL, (2) ACN, (3) THF, (4) DX and (5) HP. Inset: Steady state fluorescencepectra of Pyz in water.

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495 0.35 2.800492 0.31 2.166495 0.09 0.6, 4.204

f dioxan–water by adjusting the ratio between dioxan and water,here the polarity parameter ET(30) varies from 36 to 63.1.

Changes in viscosity of the solution of Pyz were produced bydjusting the ratio between glycerol and water, where the viscosityf glycerol at room temperature is 1490 cP, much higher than thatf water (1 cP). The absorption spectrum of Pyz is independent ofhe viscosity of the media.

.2. Steady state fluorescence

Fluorescence spectra of the compound were recorded in variousolvents of different polarity (Fig. 1b). The wavelengths correspond-ng to the fluorescence maximum have been collected in Table 1.n increase in polarity of the medium leads to a red shift of

he fluorescence maxima. The effect of polarity of the mediumn the fluorescence maxima is more pronounced than that onhe absorption maxima (Table 1). The effect of polarity has alsoeen observed from the fluorescence of Pyz in different compo-ition of dioxan–water mixture. With decrease in polarity of theedium blue shift of the fluorescence maxima with increase in

uorescence intensity has been observed (Fig. 2). This observationuggests that the emitting state of the systems is more polar thanhe ground state [24–29]. The compound is highly fluorescent asbtained from its high quantum yield (Table 1) in different sol-ents. The fluorescence quantum yields in pure and mixed binary

olvents are sensitive towards solvent polarity. Linear variation ofuantum yield with polarity (ET(30)) implies an identical inter-ction and separate sets of points obtained for polar protic andprotic solvents support the presence of hydrogen-bonding inter-

ig. 2. Steady state fluorescence spectra of Pyz in DX–water mixture (v/v). % of DXn the mixture: (1) 0, (2) 10, (3) 20, (4) 30, (5) 40, (6) 50, (7) 60, (8) 70, (9) 80, (10)0 and (11) 100.

1 ica Acta Part A 71 (2008) 1327–1332

asonwwnttmccThias

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oissaflsssht

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Fig. 3. (a) Fluorescence decays of Pyz in different solvents: (a) prompt, (b) water, (c)Ei

sta[dvvsAifliamwtmr

330 S. Pramanik et al. / Spectrochim

ction for polar protic solvents. �f gives the information about theolvation of the solute in these systems. With increase in polarityf the medium, the �f value decreases. The decrease is most promi-ent in protic solvents. In dioxan–water mixture, with increasingater the decreasing trend of �f may be due to a modification ofater–Pyz hydrogen-bonding interactions by the hydrogen bondedetwork present in water. More and more water molecules replacehe solvent molecules in the microenvironment, i.e. at higher frac-ion of water owing to self aggregation of water molecules, the

icroenvironment around Pyz consists mainly of associated waterlusters and these interact with water molecules in the solvationell decreasing H-bonding ability of bare water molecules [21].hus, �f value tends to decrease. The modification of water–Pyzydrogen-bonding interaction may result the decrease in �f with

ncrease in water content in the mixture. For other solutes suchn anomalous hydrogen-bonding behaviour in water and aproticolvent mixture has also been observed [30,31].

The red shift of the spectral maxima on changing the solventrom ACN to water is clearly the result of the hydrogen bonding.he fluorescence intensity of Pyz in aqueous solution at 495 nms reduced by approximately a factor of 40 from that in ACN.nlike absorption, the fluorescence intensity of Pyz is enhanced

n glycerol–water mixture with increasing viscosity of the mixturey increasing glycerol content due to restricted movement of theuorophore in the excited state.

Fluorescence anisotropy depends upon the rotational diffusionf the fluorophore and the rotational diffusion changes with chang-ng viscosity of the medium as well as size and shape of the diffusingpecies. Such diffusion occurs during the lifetime of the excitedtate and displaces the emission dipole of the fluorophore. Gener-lly, fluorescence occurs from the lowest singlet excited state anduorescence anisotropy has been measured at maximum emis-ion wavelength. The fluorescence anisotropy of Pyz in aqueousolution is 0.065 and enhanced to 0.309 in glycerol. So media hastrong effect on the rotational motion of Pyz in the excited state. Aigher value of anisotropy indicates a slower rotational motion ofhe transition dipole in the media.

.3. Time-resolved fluorescence study

The fluorescence decay behaviour of Pyz has been studied in theolvents of different polarity and the data have been presented inable 1. The excited state decay profiles of Pyz in different solventsave been shown in Fig. 3a. The full line is the calculated fit assum-

ng an exponential decay convoluted with the instrument responseunction. Pyz exhibits single exponential decay in all solvents irre-pective of the polarity except water. Increase in polarity of theedium leads to a shortening of the lifetime. In water, the decay

ehaviour is biexponential. The sensitivity of the luminescenceecay constant to the solvent polarity prompted us to undertake

ifetime measurement in mixed binary solvents of dioxan–waterFig. 3b). In dioxan–water mixture up to 15% Dx (v/v) the decay isiexponential. The biexponential plot can be analyzed using theunction

(t) = A1e−t/�1 + A2e−t/�2

ith two time constants �1 and �2. A1 and A2 are pre-exponentialactors. I(t) represents fluorescence intensity. The reduced �2-valuendicates an appreciable fit [32]. The biexponential fitting does

ot necessarily indicate that the decay curve has only two dis-rete time constants, it may be caused by the probe experiencingwo different local conformations resulting from the intermolecularydrogen bonding. Despite this complexity, the average lifetimesre extremely reproducible with repeated measurements on the

sqici

L, (d) HP, (e) ACN, (f) THF and (g) DX. �ex = 403 nm. (b) Fluorescence decays of Pyzn DX–water mixture (v/v). �ex = 403 nm.

ample within 1%. The average lifetime has been used to calculatehe rate constants in aqueous solution. Viscosity of the medium hasprofound effect on the fluorescence quantum yield and lifetime

33]. In a viscous solution, rotational rate of a fluorophore is slowue to rigidity of the medium. For a fluorophore dissolved in a sol-ent, the rotational rate of the fluorophore is dependent upon theiscous drag imposed on it by the solvent. As a result a change inolvent viscosity will result in a change in fluorescence anisotropy.hypsochromic shift in the fluorescence spectra with an increase

n the glycerol concentration in the mixture has been observed. Theuorescence intensity increases dramatically (20 times from that

n aqueous solution) when the concentration of glycerol as wells medium viscosity increase. To study the effect of viscosity of theedium on the dynamics of the decay of S1 state of Pyz, the � valuesere determined in glycerol–water mixture of different composi-

ion. Like Dx–water mixture, with increase in water content in theixture (50%, v/v) the decay behaviour is biexponential. The fluo-

escence lifetime increases with increasing viscosity in a mannerimilar to that of the enhancement factor as in the fluorescenceuantum yield. The increase of the measured � and � values with

ncrease in viscosity of the medium and higher values of anisotropylearly indicate the loss of flexibility of the Pyz in the excited staten these media.

S. Pramanik et al. / Spectrochimica Acta Part A 71 (2008) 1327–1332 1331

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Table 2Rate constants of radiative and non-radiative transitions, oscillator strengths (f) ofPyz in different solvents and ˛, �* values of the solvents

Solvent kr (ns−1) knr (ns−1) f ˛ �*

HP 0.14 0.08 0.23 0a −0.08a

THF 0.13 0.07 0.19 0a 0.55a

ACN 0.10 0.10 0.24 0.19a 0.66a

DX 0.11 0.08 0.28 0a 0.49a

EL 0.10 0.17 0.25 0.83b 0.54b

EG 0.13 0.23 0.17 0.9a 0.92a

GLY 0.14 0.32 0.17 1.21a 0.62a

H

2segasmigisnisc

k

aoTsconstants are practically insensitive to a change in solvation exceptwater. 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 thepolarity increases. The non-radiative rate constant (knr) in water is

Fig. 4. Plot of � vs. �f according to Lippert–Mataga equation.

.4. Excited state dipole moment

Attempts were made to quantify the extent of charge separationn electronic excitation of the Pyz by measuring the change in theipole moment (e − g) using the shift between the absorptionnd fluorescence maxima (�) as a function of solvent polar-ty parameter. According to Lippert–Mataga equation [34–36],

= 2(e − g)2

hca3

[ε − 1

2ε + 1− n2 − 1

2n2 + 1

]+ const.

here � is the Stokes shift, a the Onsagar cavity radius sweptut by the fluorophore, ε and n are the dielectric constant andefractive index of the solvent, respectively. e − g is the dif-erence between the excited state (e) and ground state (g)ipole moments of the probe. Various protic and aprotic sol-ents have been used to obtain the dipole moment changen excitation. A plot of � vs. �f (Fig. 4) gives �, where

f = ε − 12ε + 1

− n2 − 12n2 + 1

The correlation between � and �f was found to be good exceptater and dioxan. This may be due to the modification of water–Pyz-bonding interaction on excitation in water and in dioxan amongprotic solvents there is probability of H-bond formation. Using theM1 calculated longest distance between the methyl proton andhenyl bromine as the Onsagar cavity radius (6.8 A), the calculatedhange in dipole moment upon excitation is 10.2 Debye. The changen dipole moment corresponds to an intramolecular displacementf charge upon excitation of the molecule.

. Discussion

The absorption and fluorescence band maxima in variousolvents (Table 1) show an overall red shift as the polarity of theolvent increases. The enhanced solvent sensitivity of fluorescenceompared to absorbance may be explained in terms of increased

olute–solvent interaction in the excited state due to an increasen the dipole moment upon excitation. For protic solvents apartrom dipolar interaction tighter hydrogen bonding is expected inhe excited state with the solvents as a result of increased chargeensity on N atom and CN group. In the S1 state, the N in position

Ff

2O 0.03 0.95 0.21 1.17b 1.09b

a Values taken from ref. [39].b Values taken from ref. [31].

and 1 and CN in 4 form a tighter hydrogen bond with proticolvents and this influences the observed fluorescence parameters,.g. band maximum and the quantum yield. In Pyz, the cyanoroup present at C(4) position of N phenyl pyrazole is likely to bebetter electron withdrawing substituent because of its compact

ize and thus more likely to retain molecular polarity and exhibitaximum effect on conjugative interaction. The Br atom present

n PyZ assists the intersystem crossing process and simultaneouslyenerates a large C–Br dipole that alters the relaxation dynamicsn different solvents. With increasing polarity of the medium,olvation dynamics of the C–Br bond has been affected and theon-radiative process has been perturbed in polar medium. To

nvestigate the effect of solvation on the dynamics of the excitedtate, the radiative (kr) and non-radiative rate constants (knr) werealculated using the expressions:

r = �F

�and knr = �−1 − kr

In the above expressions, �F is the fluorescence quantum yieldnd � is the experimentally observed fluorescence lifetime. The plotf kr and knr vs. ET(30) of the solvents has been shown in Fig. 5.able 2 shows the observed radiative and non-radiative rate con-tants for Pyz in pure solvents. It appears that the radiative rate

ig. 5. Plot of radiative (kr) and non-radiative (knr) rate constants of Pyz vs. ET(30)or different solvents.

1332 S. Pramanik et al. / Spectrochimica Ac

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[[[35] E.Z. Lippert, Z. Elektrochem. 61 (1957) 962.[36] N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 29 (1956) 465.

Fig. 6. Plot of ln knr vs. ˛ (�) and �* (�).

igher than in other solvents. For aprotic solvents, a plot of knr vs.T(30) shows a linear relationship with practically the same slopes that observed for a plot of kr against ET(30). Such phenomenonas also been observed by others with different probe [37]. Forrotic solvents (EL, EG, GLY, H2O), specific hydrogen-bonding inter-ction occurring between the Pyz in the S1 state and protic solventolecules lead to an increase in the non-radiative relaxation rates

rom S1 to S0. In ACN molecules, dipole–dipole interaction mayccur between the CN present at C(4) position of N-phenyl pyra-oline with ACN which leads to the lower quantum yield, kr andigher knr values from those in other aprotic solvents. These resultsemonstrate that having almost the same oscillator strength valuesf Pyz in different solvents, the decrease in the fluorescence quan-um yield in protic solvents is primarily due to an increase in theon-radiative decay process.

To find the role of non-specific and specific solute–solventnteractions on the decay constant, knr the plot of ln knr vs.he parameters �* and ˛ representing non-specific and specificydrogen-bonding solute–solvent interactions, respectively, haseen considered. ln knr is linearly correlated with ˛ and no cor-elation with �* has been observed (Fig. 6). For water, the points off the line which may be due to the modification of water Pyzydrogen-bonding interaction on excitation as explained in earlierection. Thus, one may conclude that specific hydrogen-bondingnteractions occurring between Pyz in the S1 state and protic sol-ent molecules leads to an increase in the non-radiative relaxationates from S1 to S0. The importance of hydrogen-bonding interac-ions in influencing the steady state fluorescence parameters andynamics of the excited state of different probes have also beenbserved by other workers [31,38] using other probes.

Hence, specific interactions induced by protic solvents seemso induce an efficient vibronic coupling with the excited state ofyz reducing fluorescence quantum yield and increasing the ratef non-radiative transition. Thus, a study of different propertiesrovides the same information regarding the local environmentround the probe in different solvents.

. Conclusions

The newly synthesized pyrazoline compound shows a strongendency towards a hydrogen-bonding interaction with protic

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ta Part A 71 (2008) 1327–1332

olvents and this is reflected in the absorption and emis-ion properties of the probe in solvents of different polarity.he compound is fluorescent with high fluorescence quantumield and considerable solvatochromism. The steady state emis-ion spectra and relaxation dynamics have been found to benfluenced by the solute–solvent interaction. The non-radiativeecay constant for Pyz depends mainly on the hydrogen-bonding

nteraction of the probe in the excited state with the solvent,ut the radiative decay constant is practically insensitive toolvation.

cknowledgements

Financial support from CSIR (01(2057)/06/EMR-II) is gratefullycknowledged. One of the authors S.P. acknowledges CSIR and A.S.cknowledges UGC for research fellowship.

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