Journal of Molecular Liquids 158 (2011) 105–110

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Journal of Molecular Liquids

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Solvent effect on the spectroscopic properties of 6MAMC and 7MAMC

Raveendra M. Melavanki a,⁎, N.R. Patil b, S.B. Kapatkar b, N.H. Ayachit b, Siva Umapathy c,J. Thipperudrappa d, A.R. Nataraju e

a Department of Physics, M S Ramaiah Institute of Technology, Bangalore 560054, Karnataka, Indiab Department of Physics, B V B College of Engineering and Technology, Hubli 580031, Karnataka, Indiac Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, Karnataka, Indiad Department of Physics, BNM Institute of Technology, Bangalore 560070, Karnataka Indiae Department of Physics, Seshadripuram, P. U. College (Main), Bangalore 560020, Karnataka, India

⁎ Corresponding author. Tel.: +91 80 23600822, +91 980 23603124.

E-mail address: melavanki73@gmail.com (R.M. Mela

0167-7322/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.molliq.2010.11.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 August 2010Received in revised form 2 October 2010Accepted 7 November 2010Available online 11 November 2010

Keywords:Solvatochromic shift methodGround-state dipole momentsExcited-state dipole moments6MAMC and 7MAMC

The absorption and emission spectra of two dyes namely 6MAMC and 7MAMC have been recorded at roomtemperature in solvents of different polarities. The ground-state dipole moments (μg) of these two weredetermined experimentally by Guggenheim method and were compared with theoretical values obtainedusing quantum chemical method. The exited state (μe) dipole moments were estimated from Lippert's,Bakhshiev's and Chamma-Viallet's equations by using the variation of the Stokes shift with the solventdielectric constant and refractive index. The ground and excited-state dipole moments were calculated bymeans of the solvatochromic shift method and also the excited-state dipole moments are determined incombination with ground-state dipole moments. It was observed that dipole moments of excited state werehigher than those of the ground state, indicating a substantial redistribution of the π-electron densities in amore polar excited state for these two dyes.

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1. Introduction

The effect of solvent on the absorption and fluorescence character-istics of organic compounds has been a subject of interestinginvestigation [1–3]. Excitation of a molecule by photon causes aredistribution of charges leading to conformational changes in theexcited state. This can result in an increase or decrease of dipolemoment of the excited sate as compared to ground state. The dipolemoment of an electronically excited state of a molecule is an importantproperty that provides information on the electronic and geometricalstructure of the molecule in the short-lived state. Knowledge of theexcited-state dipole moment of electronically excited molecules isquite useful in designing nonlinear materials, elucidating the nature ofthe excited states and in determining the course of a photochemicaltransformation. The excited-state dipole moments of fluorescent dyemolecules such as those studied here also determine the tunabilityrange of the emission energy as a function of the polarity of themedium. All the methods available so far for the determination of

singlet excited-state dipole moment are based on the spectral shiftcaused either externally by electrochromism or internally by solvato-chromism. The electrooptic methods such as electronic polarization offluorescence, electric-dichroism, microwave conductivity and starksplitting are generally considered to be very accurate, but their use islimited because they are considered equipment sensitive and thestudies have been restricted to relatively very simple molecules. Thesolvatochromic method is based on the shift of absorption andfluorescence maxima in different solvents of varying polarity. Koutekhas shown that under suitable conditions, the solvatochromic methodyields fairly satisfactory results [4]. The solvent dependence ofabsorption and fluorescence maxima is used to estimate the excited-state dipole moments of different molecules.

Several workers have made extensive experimental and theoreticalstudies on ground-state (μg) and excited-state (μe) dipole momentsusing different techniques in variety of organic fluorescent compoundslike coumarins [5], indoles [6], purines [7], exalite dyes [8], acridine dyes[9], fluorescein [10], hemicyanine dyes [11], PRODAN, BADAN andACRYLODAN [12], acridines and phenazines [13] and in some laser dyes[14–20] etc.

Coumarins and their derivatives establish a family of dyes which areapplicable indifferentfieldsof scienceand technology. Theyexhibit strongfluorescence in the UV and VISIBLE regionwhichmakes them suitable forused as colorants, dye laser media and as nonlinear optical chromo-spheres. In medicine, coumarin derivatives are used as anticoagulants, as

106 R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110

a fluorescent indicators for the physiological pH region and as fluorescentprobes to determine the rigidity and fluidity of living cells and itssurrounding medium. They also possess distinct biological activity andhave been described as potential agents for anticancer. In the presentworkwe report the effects of solvent on absorption and emission spectra,and estimation of ground- and excited-state dipole moments ofthese two dyes namely 6-methyl 4-azidomethyl coumarin (6MAMC)and 7-methyl 4-azidomethyl coumarin (7MAMC) by solvatochromic shiftmethod and also theoretical computed results from ab initio calculationsusing DFT. However, there are no reports available in literature on thedetermination of μg and μe values of these dyes investigated. Thisprompted us to carry out the present work.

2. Materials and methods

2.1. Chemicals used

The 6MAMC and 7MAMCwere synthesized in our laboratory usingstandard methods [21–23]. The molecular structures of these dyes aregiven in Fig. 1. The solvents used in the present study namely toluene(TL), acetonitrile (AN), tetrachloroethane (TCE), dioxane (DX),diethyl ether (DEE), n-butyl alcohol (n-BA), carbon tetrachloride(CCl4), ethyl acetate (EA), tetrahydrofuran (THF) and benzene (BZ)were obtained from S-D-Fine Chemicals Ltd., India, and they were ofspectroscopic grade. The required solutions were prepared at fixedconcentration of solutes 1×10−4M/L in each solvent.

2.2. Spectroscopic measurements

The absorption spectra were recorded using Hitachi 50–20 UV–Visspectrophotometer. The fluorescence spectra were recorded usingHitachi F-2000 fluorescence spectrophotometer. The dielectric con-stants of the dilute solutionsweremeasured in a suitably fabricated cellof usually small capacitance where the accurate determination of smallchanges in the capacitancewould bepossible. The small capacitance can

6-methyl-4-azidomethyl coumarin (6MAMC)

7-methyl-4-azidomethyl coumarin (7MAMC)

O

N3

N3

H3C

H3C

O

O O

Fig. 1. The molecular structures of 6MAMC and 7MAMC.

bemeasured with the help of Forbes Tinsley (FT) 6421 LCR Data Bridgeat 10 KHz frequency. The refractive indices of various dilute solutionsof the solute for sodium D line were determined by using Abbe'srefractometer. All these measurements were carried out at roomtemperature (300 K).

2.3. Determination of the dielectric constants and refractive indices

The capacitance of air, the solvent and the solution have been usedto measure dielectric constant. By measuring the capacitance ofdifferent concentrations of the solute in toluene the dielectric constantof the solution (ε12) was calculated using the expression

ε12 =C2−CX

C1−CXð1Þ

where C2, CX and C1 represent the capacitances due to leads, solution,and air respectively. The values of dielectric constants thus deter-mined for 6MAMC and 7MAMC using toluene. The values of refractive

Fig. 2. Ground-state optimized molecular geometries of 6MAMC and 7MAMC. Thearrow indicates the direction of dipole moment.

Table 1Solvatochromic data of 6MAMC and 7MAMC along with the calculated values of polarity functions in different solvents.

Dyes Solvents νa nm νf nm ν−a (cm−1) νf− (cm−1) ν

−a −νf Þ−�

(cm−1) 1=2 ν−a + νf Þ−�

(cm−1) F1(ε,n) F2(ε,n) F3(ε,n)

6MAMC TCE 323 416.24 30,959.7 24,024.6 6935.1 27,492.15 0.035 6.357E−4 0.341BZ 326 407.07 30,674.8 24,565.8 6109.0 27,620.3 0.032 6.29E−3 0.341DX 322 409.30 31,055.9 24,432.0 6623.9 27,743.95 0.013 0.04400 0.308n-BA 274 385.84 36,496.3 25,917.5 10,578.8 10,578.8 0.192 0.5613 0.5517TL 324 407.74 30,864.2 24,525.4 6338.8 27,694.8 0.022 0.02900 0.350DEE 272 389.22 36,764.7 25,692.4 11,072.3 31,228.55 0.132 0.37100 0.425EA 270 385.33 37,037.0 25,951.8 11,085.2 31,494.4 0.174 0.49300 0.499CCL4 330 405.55 30,303.0 24,657.9 5645.1 27,480.45 0.024 0.02300 0.323

7MAMC AN 275 380.1 36,363.63 26,308.86 10,054.77 31,336.24 0.280 0.79700 0.633BZ 320 384.24 31,250.00 26,025.01 5224.99 28,637.50 0.032 6.29E−3 0.341DX 315 380.62 31,746.03 26,272.92 5473.11 29,009.47 0.013 0.04400 0.308DEE 315 389.14 31,746.03 25,697.69 6048.34 28,721.86 0.132 0.37100 0.425TCE 330 393.09 30,303.03 25,439.46 4863.57 27,871.24 0.035 6.357E−4 0.341THF 275 343 36,363.63 29,154.52 7209.11 32,759.07 0.159 0.51200 0.585TL 320 386.71 31,250.00 25,859.17 5390.83 28,554.58 0.022 0.02900 0.350

107R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110

indices (n12) of solutions were determined for 6MAMC and 7MAMCusing toluene respectively.

3. Theory

3.1. Theoretical calculations of ground-state dipole moments

The ground-state dipole moments (μ g) of these two moleculeswere calculated by quantum chemical calculations. All the computa-tions were carried out using the Gaussian 03 program [24] on aPentium-4 PC. The basis sets at the levels of theory B3LYP/6–31 g*were used for calculations and corresponding optimized moleculargeometries are shown in Fig. 2. The values of ground-state dipolemoments obtained from ab initio calculations using DFT.

3.2. Experimental calculations of ground-state dipole moments

The ground-state dipole moments (μ g) of these two dyes wereestimated experimentally using Guggenheim's method[25]. Accord-ing to this the expression for ground-state dipole moment is given by

μ2g =

27KT4πN ε1 + 2ð Þ n2

1 + 2� �X Δ

C

" #ð2Þ

where

Δ = ε12−n12ð Þ− ε1−n21

� �

where K is the Boltzmann constant, T is the absolute temperature inKelvin, N is Avogadro's number, ε12 is the dielectric constant and n12 isrefractive index of the solution,ε1and n1 are the dielectric constant andrefractive index of thepure solvent and C the concentration of the solutein given solvent. The estimated values of the ground-state dipolemoments (μg) using Eq. (2)for 6MAMC and 7MAMC respectively.

3.3. Experimental calculations of excited-state dipole moments

The three independent equations used for the estimation ofexcited-state dipole moments of two dyes are as follows

Lippert's equation [26]

ν−a −νf

−= m1F1ðε;nÞ + Constant: ð3Þ

Bakshiev's equation [27]

νa− − vf

−= −m2F2ðε;nÞ + Constant: ð4Þ

Chamma–Viallet's equation [28]

ν−a + ν−f

2= m3F3ðε;nÞ + Constant: ð5Þ

The expressions for F1(ε,n) [Lippert's polarity function], F2(ε,n)[Bakshiev's polarity equation] and F3(ε,n) [Chamma–Viallet's polarityequation] are given as

F1 ε;nð Þ = ε−12ε + 1

− n2−12n2 + 1

" #ð6Þ

F2 ε;nð Þ = ε−1ε + 2

− n2−1n2 + 2

" #2n2 + 1

� �n2 + 2� � ð7Þ

F3 ε;nð Þ = 2n2 + 12 n2 + 2� � ε−1

ε + 2− n2−1

n2 + 2

" #3 n4−1� �

2 n2 + 2� �2 ð8Þ

where ν−a and νf− are absorption and fluorescence maxima wavelength

in cm−1 respectively. The other symbols ε and n are dielectric constantand refractive index respectively. From Eqs. (6–8) it follows that

ν−a −νf Þ−�

versus F1 (ε, n), ν−a −νf Þ−�

versus F2 (ε, n) andνa−

+ νf−

2 versus

F3 (ε, n) should give linear graphs with slopes m1, m2 and m3

respectively and are given as

m1 =2 μe−μg� �2

hca3ð9Þ

m2 =2 μe−μg� �2

hca3ð10Þ

m3 =2 μ2

e−μ2g

� �2

hca3ð11Þ

where μg and μe are the ground and excited-state dipole moments ofthe solute molecules. The symbols h and c are Planck's constant andvelocity of light in vacuum respectively, ‘a’ is the Onsager radius ofthe solute molecule. If the ground state and excited states are parallel,

Fig. 3. The variation of Stokes shift with F1 (ε, n) using Lippert's equation for 6MAMC and 7MAMC.

108 R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110

the following expressions are obtained on the basis of Eqs. (11–12)[17,18]

μg =m3−m2

2hca3

2m2

" #1=2

ð12Þ

μe =m3 + m2

2hca3

2m2

" #1=2

ð13Þand

μ e =m2 + m3

m3−m2

� �μg forðm3 N m2Þ ð14Þ

4. Results and discussion

The spectral shifts ν−a −νf Þ−�

and 1=2 ν−a + νf Þ−�of two dyes and

solvent polarity function values F1(ε,n), F2(ε,n) and F3(ε,n)for varioussolvents are presented in Table 1. We have used eight and sevensolvents with dielectric constants varying from 2 to 21 for 6MAMCand 7MAMC respectively. Figs. 3–5 show the graph of ν−a −νf Þ−�

versusF1(ε,n), ν

−a −νf Þ−�

versus F2(ε,n)and 1=2 ν−a + νf Þ−�versusF3(ε,n) re-

spectively. A linear progression was done and the data was fit to astraight line, corresponding the values of the slopes are given in Table 2.In most cases ν

−a −νf Þ

−�versus F1(ε,n) and ν−a −νf Þ−�

versus F2(ε,n)correlation is established for a larger number of solvents than1=2 ν

−a + νf Þ−�

versus F3(ε,n) correlation. In most cases the correlationcoefficients are larger than 0.92 and indicate a good linearity for m1, m2

and m3 with selected number of Stokes shift data points. Generally,the deviation from linearity may be due to specific solute solventinteractions. The literature survey shows thatm2 is usually negative butfor our chemical systems, it is positive which is in agreement withfindings of several other workers [7,10,11,16].

The ground-state dipole moments of two dyes were estimated byusing the Guggenheim method [25]. The values obtained from this

method are 5.962D and 5.891D for 6MAMC and 7MAMC respectivelyand also ground-state (μ g) dipole moment values obtained fromEq. (12) are presented in Table 3. The values of the Onsager cavityradii of the 6MAMC and 7MAMC molecules were calculated bymolecular volumes and the Parachor [29] and are listed in Table 3.

The excited-state (μ e) dipolemoments of the two dyes, estimated bycomputing the values of ground-state (μg) dipole moments obtainedfrom the Guggenheim method, in Eqs. (9–11) are presented in Table 3.Also the (μg) and (μ e) values were obtained from Eqs. (12), and (13).The ratio of (μg) and (μ e) obtained from Eq. (14) are presented inTable 3. The experimental (from Eq. (2)) and theoretically calculated(ab initio calculations using DFT) values are presented in Table 3. Theexperimental and theoretical ground-state (μg) dipole moment resultsare good in agreement for our used chemical systems as shown inTable 3 [15,24]. The difference in the ground-state dipolemoment is dueto the necessity of knowing the radius of the solutemolecule in Eq. (12)as compared to experimental and theoretical values obtained fromEq. (2) and ab initio calculations using DFT. It may be noted that themeasured values of (μg) and (μ e) for 6MAMC and 7MAMC differ fromeach other. The higher values of (μ e) in the case of 6MAMC may beattributed to the structural difference between themolecules. It may benoted that the discrepancies occur between the estimated values of(μ e) for the two coumarin derivatives. These differences between thevalues of (μ e) may be in part, due to the various assumptions andsimplifications made in the use of Lippert's, Bakshiev's and Chamma–Viallet's correlations [26–28]. The large magnitude of the Stokes shiftindicates that the excited-state geometry could be different from that ofthe ground state. The general observation is that there is an increase inthe Stokes shift with an increase in solvent polarity which shows thatthere is an increase in the dipole moment on excitation.

The solvatochromic data can be used to identify the spectra, namelyπ–π*, n–π*, etc. It can be noticed from Table 1 that, with increase in thesolvent polarity, the fluorescence emission peak undergoes a bath-ochromic shift, confirming a π–π* transition. The shift of thefluorescencewavelengths towards longerwavelengths could be caused,if theexcited-state chargedistribution in the solute ismarkedlydifferent

Fig. 4. The variation of the Stokes shift with F2 (ε, n) using Bakshiev's equation for 6MAMC and 7MAMC.

109R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110

from the ground-state charge distribution, and is such as to give astronger interaction with polar solvents in the excited state.

5. Conclusion

We have studied the spectroscopic behavior of 6MAMC and7MAMC. It has been found that excited-state dipole moment (μ e) isgreater than ground-state dipole moment (μ g) for both the dyes. Theincrease in dipole moment in the excited singlet states range between

Fig. 5. The variation of arithmetic means of the Stokes shift with F3 (

about 2.5 to 3 D. This demonstrates that these two dyes aremore polarin excited states than in ground states for all the solvents studied. Theground-state dipole moments results are correlated (experimentaland theoretical) in our used chemical systems. It may be notedthat there is a difference in the ground-state and excited-state dipolemoment as estimated from Eqs. (12) and (13). It is worthwhile tostress that the discrepancies observed may be due to approximationsmade in both methods to estimate ground-state and excited singletstate dipole moments for two dyes. Also Eq. (14) can be used to

ε, n) using Chamma–Viallet's equation for 6MAMC and 7MAMC.

Table 2Statistical treatment of the correlations of solvents spectral shifts of 6MAMC and7MAMC.

Dyes Slope Correlationcoefficient

Numberof data

A B C A B C A B C

6MAMC 23,015.146 9477.105 19,451.904 0.96 0.95 0.91 8 8 87MAMC 13,685.468 5463.844 12,128.445 0.94 0.95 0.89 7 7 7

A — Lippert's equation; B — Bakshiev's equation; and C — Chamma–Viallet's equation.

Table 3Ground- and excited-state dipole moments of 6MAMC and 7MAMC.

Dyes Radius‘a’ (Ao)

μga

(D)μgb

(D)μgc

(D)μed

(D)μee

(D)μef

(D)μeg

(D)(μe/μg)h

6MAMC 3.576 5.962 6.121 3.454 16.188 12.524 11.133 10.02 2.9007MAMC 3.576 5.891 6.086 3.039 13.777 10.873 9.477 8.02 2.639

a The experimental ground-state dipole moments calculated from the Guggenheimmethod.

b Calculated by B3LYP functional with 6–31 g* basis using DFT.c The ground-state dipole moments calculated using Eq. (12).d The experimental excited-state dipole moments calculated from Lippert's equation.e The experimental excited-state dipole moments calculated from Bakshiev's equation.f The experimental excited-state dipole moments calculated from Chamma–Viallet's

equation.g The excited states dipole moments calculated using Eq. (13).h The ratio of excited-state and ground-state dipole moments values calculated using

Eq. (14).

110 R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110

estimate the value of excited-state dipole moment by pre-knowledgeof the value of ground-state dipole moment, without the necessity ofknowing the Onsager radius of the solute.

Acknowledgements

The author (RMM) is grateful to Dr. K. Rajanikanth, Principal,M.S.R.I.T,Dr. K. Venkatesh, H O D, Department of Physics, M.S.R.I.T and TheManagement ofM.S.R.I.T, Bangalore. The authors (NRP, SBK andNHA) aregrateful to Principal Dr. Ashok. S. Shettar B V B College of Engineering and

Technology,Hubli andTheManagementofK.L.E Society, Belgaumfor theirencouragement and support. Authors also thank Prof. M.V. Kulkarni,DepartmentofChemistry, KarnatakUniversity,Dharwad forproviding thenecessary coumarin compounds used in the present work.

References

[1] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, NewYork, 1991.[2] A. Elassyry, B. Benali, Z. Lazar, K. Elblidi, B. Lakhrissi, M. Massoui, D. Mondieig,

J. Mol. Liq. 128 (2006) 46.[3] B. Benali, Z. Lazar, K. Elblidi, B. Lakhrissi, M. Massoui, A. Elassyry, C. Cazeau-

Dubroca, J. Mol. Liq. 128 (2006) 42.[4] B. Koutek, Collect. Czech. Chem. Commun. 43 (1978) 2368.[5] R. Giri, M.M. Bajaj, Curr. Sci. 62 (1992) 522.[6] N. Sharma, K. Sapan, R.C. Rastogi, Spectrochim. Acta, Part A. 66 (2007) 171.[7] J.J. Aaron, M.D. Gaye, C. Parkanyi, N.S. Cho, Von SzentpalyL. , J. Mol. Struct. 156

(1987) 119.[8] S.R. Inamdar, Y.F. Nadaf, B.G. Mulimani, J. Mol. Struct. THEOCHEM 47 (2003) 624.[9] N.P. Srividya, P. Ramasamy, V.T. Radhakrishanan, Spectrochim. Acta A 53 (1997) 1743.

[10] B. Acemioglu, M. Arık, H. Efeoglu, Y. Onganer, J. Mol. Struct. THEOCHEM 548(2001) 165.

[11] J. Kabatc, B. Osmialowski, Paczkowski, J. Spectrochim. Acta A 63 (2006) 524.[12] A. Kawski, B. Kuklinski, P. Bojarski, Z. Naturforsch. 57A (2002) 716.[13] J.J. Aaron, M. Maafi, C. Parkanyi, C. Boniface, Spectrochim. Acta 51A (1995) 603.[14] J. Thipperudrappa, D.S. Biradar, S.R. Manohara, S.M. Hanagodimath, S.R. Inamadar,

J.R. Manekutla, Spectrochim. Acta A 69 (2008) 991.[15] B.R. Gayathri, J.R. Mannekutla, S.R. Inamadar, J. Mol. Struct. 889 (2008) 383.[16] D.S. Biradar, B. Siddlingeshwar, S.M. Hanagodimath, J. Mol. Struct. 875 (2008) 108.[17] A. Kawaski, in: J.F. Rabeck (Ed.), Photochem. And Photophys, vol.5, CRC Press, Boca

Raton, Ann Arbor Boston, 1992, p. 1.[18] A. Kawaski, Acta Phys. Polon 29 (1966) 507.[19] R.M. Melavanki, R.A. Kusanur, M.V. Kulakarni, J.S. Kadadevaramath, J. Lumin. 129

(2009) 1298.[20] R.M. Melavanki, R.A. Kusanur, M.V. Kulakarni, J.S. Kadadevaramath, J. Lumin. 128

(2007) 573.[21] Raviraj A. Kusanur, Manohar V. Kulakarni, Indian Council of Chemists. 22nd

Conference, Roorkee, Uttaranchal, 2003.[22] B.B. Dey, Y. Shankaranaraayan, J. Ind. Chem. Soc. 11 (1934) 687.[23] S. Rangaswamy, T.R. Sheshadri, V. Venkateshwaralu, Proc. Ind. Acad. Sci. 13A

(1941) 316.[24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian, Revision B, Gaussian, Inc.,

Pittsburg, PA, 2003.[25] E.A. Guggenheim, Trans. Faraday Soc. 47 (1951) 573.[26] LippertE. , Z. Naturforsch. 10 A (1955) 541.[27] N.G. Bakshiev, Opt. Spectrosc. 16 (1964) 821.[28] ChammaA. , VialletP. , C.R. Acad. Sci. Paris, Ser. C 270 (1970) 1901.[29] J.T. Edward, Molecular Volumes and Parachor, Chem. Ind, London, 1956, p. 774.