spectrochimica acta part a: molecular and biomolecular...

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 286–296

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Solvent effect on the absorption and fluorescence spectraof 7-acetoxy-6-(2,3-dibromopropyl)-4,8-dimethylcoumarin: Determinationof ground and excited state dipole moments

Yadigar Gülseven Sıdır ⇑, _Isa SıdırBitlis Eren University, Faculty of Arts & Science, Department of Physics, 13000 Bitlis, Turkey

h i g h l i g h t s

" The excited state dipole moment isestimated from solvatochromic shiftmethods.

" The results reveal that compound ismore polar in the excited state thanin the ground state.

" It is observed that shifts inabsorption band are controlled bydispersion–polarization forces.

" Fluorescence band shifts iscontrolled by induction–orientationinteractions.

1386-1425/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.saa.2012.10.018

⇑ Corresponding author. Tel.: +90 434 228 5170; faE-mail address: [email protected] (Y. Gülse

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 August 2012Received in revised form 27 September 2012Accepted 13 October 2012Available online 24 October 2012

Keywords:CoumarinExcited state dipole momentSolvatochromic shiftEN

T parameterLSERMEP

a b s t r a c t

The ground state (lg) and excited state (le) dipole moments of 7-acetoxy-6-(2,3-dibromopropyl)-4,8-dimethylcoumarin (abbreviated as 7ADDC) are estimated from solvatochromic shifts of absorption andfluorescence spectra as a function of the dielectric constant (e) and refractive index (n). While the groundstate dipole moment is determined by using Bilot–Kawski method, the excited state dipole moment iscalculated by using Bilot–Kawski, Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet and Reichardtcorrelation methods. Excited state dipole moment is observed as larger than the ground state dipolemoment due to substantial p-electron density redistribution. The ground state and excited state dipolemoments are observed as parallel to each other with angle of 0�. Solute–solvent interactions are analyzedby means of linear solvation free energy relationships (LSER) using dielectric constant function (f(e)),refractive index function (f(n)) and Kamlet–Taft parameters (a and b). Atomic charges, electron densitiesand molecular orbitals are calculated in vacuum and with solvent effect by using both DFT and TDDFTmethods. Solvent accessible surface, molecular electrostatic potential (MEP) and electrostatic potential(ESP) are visualized as a result of DFT calculations.

� 2012 Elsevier B.V. All rights reserved.

Introduction When a compound is dissolved in different solvents, the effect

Coumarins, which are organic laser dyes, have many applica-tions. The widespread of these compounds have been used as fluo-rescence derivatization reagents for high-performance liquidchromatography [1], fluorescence probes for protein studies [2],fluorescent ionophores [3] and fluorescent indicators [4].

ll rights reserved.

x: +90 434 228 5171.ven Sıdır).

of solvent on the absorption and fluorescence spectra has been asubject of interesting investigation [5]. Therefore, the systematicanalysis on solvent effect provides beneficial information in study-ing the excited states behavior of the molecule. A change in solventis accompanied by a change in polarity, dielectric constant orchange in polarizability of the surrounding medium. Therefore, achange in solvent affects the ground and excited states differently.

The knowledge of dipole moments of electronically excited spe-cies is often useful in the design of non-linear optical materials [6]

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Y. Gülseven Sıdır, _I. Sıdır / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 286–296 287

and elucidation of the nature of excited states. Describing of ex-cited state properties helps not only in the design of new moleculesbut also for the best performance in analysis of specific application.Excitation of a molecule gives rise to redistribution of charges andelectron densities leading to conformational changes in the excitedstate. Thus, dipole moment of the excited state can increase and/ordecrease as compared to ground state.

The ground and excited state dipole moments of some couma-rin derivatives were previously determined experimentally byusing different methods [7–11].

In this research paper, we have attempted to determine theground state (lg) and excited state (le) dipole moments of 7ADDCby using Bilot–Kawski [12,13], Lippert–Mataga [14,15], Bakhshiev[16], Kawski–Chamma–Viallet [17,18] and Reichardt [19] methods.We also reported the angle between the ground state and excitedstates dipole moments. The influence of solvents on absorptionand fluorescence spectra along with solvatochromic behavior isinvestigated using multiple linear regression analysis (MLRA). Inorder to investigate intramolecular charge transfer and interactionsites of 7ADDC with solvent, DFT and TDDFT calculations were per-formed in vacuum and with solvent effect [20].

Materials and methods

Theoretical background

By employing the simplest quantum–mechanical second orderperturbation theory and taking into account Onsager’s model, Bilotand Kawski [12,13] have obtained expressions for the solvent spec-tral shift given by

�ma � �mf ¼ mð1Þf ðe;nÞ þ constant ð1Þ

�ma þ �mf ¼ �mð2Þuðe;nÞ þ constant ð2Þ

where u(e,n) = f(e,n) + 2g(n)

mð1Þ ¼2ðle � lgÞ

2

hca3 ð3Þ

mð2Þ ¼2ðl2

e � l2gÞ

hca3 ð4Þ

where lg and le are the dipole moments in the ground and excitedstates, respectively, h Planck’s constant (6.63 � 10�34 J s), c thevelocity of light in vacuum (3.0 � 108 m s�1), a Onsager’sinteraction radius of solute, f(e,n) and g(n) are the solvent polarityfunctions given by following equations:

f ðe;nÞ ¼ 2n2 þ 1n2 þ 2

e� 1eþ 2

� n2 � 1n2 þ 2

� �ð5Þ

gðnÞ ¼ 32

n4 � 1

ðn2 þ 2Þ2

!ð6Þ

Assuming that the symmetry of the investigated solute mole-cule remains unchanged upon electronic transition and the groundand excited state dipole moments are parallel, based on Eqs. (3)and (4) one obtains [13],

lg ¼mð2Þ �mð1Þ

2hca3

2m1

!1=2

ð7Þ

le ¼mð2Þ þmð1Þ

2hca3

2m1

!1=2

ð8Þ

le ¼mð2Þ þmð1Þ

mð2Þ �mð1Þlg for ðmð2Þimð1ÞÞ ð9Þ

The parameters m(1) and m(2) can be determined from the slopesof the straight lines occurs in Eqs. (1) and (2). Generally, the groundstate and excited state dipole moments are not parallel to eachother but make an angle u. The use of Eqs. (3) and (4) leads thanto equation given by Eq. (10) [21].

cosu ¼ 12lgle

l2g þ l2

e

� ��mð1Þ

mð2Þl2

e � l2g

� �� ð10Þ

The electric dipole moment of polar solute polarizes the solventso that the solute itself experiences an electric field, the reactionfield, which is proportional to the solute dipole moment in theground and excited states. Such proportionalities for the differenceand sum of absorption, �ma, and fluorescence, �mf , maxima (in cm�1)have been defined by following independent equations [14–18]used for the estimation of ground and excited state dipole mo-ments of dyes:

�ma � �mf ¼ m1FLippert—Matagaðe;nÞ þ constant ð11Þ

�ma � �mf ¼ m2FBakhshievðe;nÞ þ constant ð12Þ

�ma þ �mf

2¼ �m3FKawski—Chamma�Vialletðe;nÞ þ constant ð13Þ

where m1, m2 and m3 are the slopes of the linear relationships cor-responding to Eqs. (11)–(13), respectively.

m1 ¼2ðle � lgÞ

2

hca3 ð14Þ

m2 ¼2ðle � lgÞ

2

hca3 ð15Þ

m3 ¼2ðl2

e � l2gÞ

hca3 ð16Þ

FLippert–Mataga [14,15], FBakhshiev [16] and FKawski–Chamma–Viallet

[17,18] are solvent polarity functions and are given as:

FLippert—Matagaðe;nÞ ¼e� 1

2eþ 1� n2 � 1

2n2 þ 1ð17Þ

FBakhshievðe;nÞ ¼2n2 þ 1n2 þ 2

e� 1eþ 2

� n2 � 1n2 þ 2

� �ð18Þ

FKawski—Chamma�Vialletðe;nÞ¼2n2þ1

2ðn2þ2Þe�1eþ2

�n2�1n2þ2

� �þ 3ðn4�1Þ

2ðn2þ2Þ2

" #

ð19Þ

Moreover, Reichardt method based on the empirical polarityscale EN

T [19] was used for estimating dipole variation (dl) fromsolvatochromic shift. The spectral data correlation with micro-scopic solvent polarity generally provides superior results as com-pared to traditionally used bulk solvent polarity functions.

�ma � �mf ¼ m4ENT þ constant ð20Þ

ENT ; the normalized solvent polarity of Reichardt [19] is a solvato-

chromic parameter based on the absorption wavenumber of a stan-dard betaine dye in the corresponding solvent.

ENT ¼

ETð30Þsolvent � ETð30ÞTMSETð30Þwater � ETð30ÞTMS

¼ ETð30Þsolvent � 30:732:4

ð21Þ

Fig. 2. Gaussian curves fitted for the fluorescence spectra in ethanol.

Table 1Electronic absorption and fluorescence spectral data of the studied compound.

Solventsa �ma �mf �ma � �mf �ma þ �mf �ma þ �mf Þ=2

1 1,4-Dioxane 31,447 25,189 6258 56,635 28,3182 CCl4 31,348 25,253 6095 56,600 28,3003 Toluene 31,496 25,284 6212 56,781 28,3904 Chloroform 31,348 24,938 6410 56,286 28,1435 Ethyl acetate 31,348 24,814 6534 56,162 28,0816 Butyl acetate 31,546 24,876 6670 56,421 28,2117 THF 31,397 24,814 6583 56,211 28,1068 1-Butanol 31,447 24,752 6694 56,199 28,1009 2M1P 31,348 24,631 6717 55,979 27,989

10 2-Propanol 31,447 24,570 6877 56,017 28,00811 Ethanol 31,546 24,510 7036 56,056 28,02812 Methanol 31,447 24,390 7056 55,837 27,91813 DMF 31,348 24,510 6838 55,858 27,92914 DMSO 31,496 24,450 7046 55,946 27,973

a Solvents are listed in the order of increasing dielectric constants.

288 Y. Gülseven Sıdır, _I. Sıdır / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 286–296

where TMS represents tetramethyl-silane, known as nonpolarsolvent.

ETð30Þi ¼ hcN�mai ¼ 2:8591� 10�3�maiðin kcal mol�1Þ ð22Þ

where �mai (cm�1) is the absorption maxima of the standard betainedye in solvent (i).

m4 ¼ 11307:6dldlB

� �2 aB

a

� �3" #

ð23Þ

where dl = le � lg for the solute molecule and dlB = 9D for thebetaine dye; a and aB are the Onsager radius cavities of the soluteand betaine, respectively, with aB = 6.2 Å.

From the relation (23) we obtain:

le ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

m481

11307:6ð6:2=aÞ3

sþ lg ð24Þ

Wavenumbers, f ðe;nÞ;uðe;nÞ; FLippert—Matagaðe;nÞ; FBakhshievðe; nÞ;FKawski—Chamma�Vialletðe;nÞ and EN

T along with solvent parameters eand n are given in Tables 1 and 2, respectively.

Experimental details

7-Acetoxy-6-(2,3-dibromopropyl)-4,8-dimethylcoumarin (Fig. 1)is purchased from Aldrich Company by authors and used withoutfurther purification. All of the solvents are of spectroscopic gradecommercially available from Sigma–Aldrich. Solvents are checkedin steady-state fluorescence apparatuses for the lack of fluores-cence impurities in the wavelength ranges of interest. Theconcentrations of the solutions are prepared as 10�4–10�5 M.Ultraviolet–visible (UV–vis) absorption spectra are recorded on

O O

BrBr

O

O

Fig. 1. The molecular structure of 7-acetoxy-6-(

Perkin Elmer Lambda-35 UV–vis. spectrophotometer. Steady-statefluorescence spectra are recorded on Perkin Elmer LS-55 Fluores-cence Spectrometer. The positions of the absorption and fluores-cence bands are determined by Gaussian curve fit analyses usingOriginPro 7.5 (see Fig. 2). Linear correlation and data fit are alsoperformed by using OriginPro 7.5.

All of the measurements are performed by using 1 cm quartzcell at room temperature. Dielectric constant, e, refractive index,n and Kamlet–Taft parameters (a and b) were taken from the liter-ature [5,22].

Onsager cavity radius

The value of Onsager cavity radius (a) of 7ADDC is calculatedfrom the following equation [23],

a ¼ 3M4pdNA

� �1=3

ð25Þ

where d is the density of solute molecule, M the molecular weight ofthe solute molecule and NA is the Avagadro’s number. For the stud-ied compound, d = 1.63 g/cm3 and M = 432.10 g/mol and thus, a isfound to be 4.816 Å.

Multiple linear regression analysis details

The wavelengths of absorption and fluorescence spectral bandsshow changes with the effect of solvent parameters. In this section,the solvent-induced spectral shifts have been analyzed in relation to

2,3-dibromopropyl)-4,8-dimethylcoumarin.

350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

flu

ores

cenc

e in

tens

ity

λ /nm

Toluene Chlorofom Butyl acetate 2M1P 2-Propanol DMSO

300 320 340 3600.00

0.25

0.50

0.75 A

bsor

banc

e

λ /nm

Toluene Chloroform Butyl acetate 2M1P 2-Propanol DMSO

Fig. 3. UV–vis. absorption spectra (top) and normalized fluorescence spectra(bottom) of the titled compound in toluene, chloroform, butyl acetate, 2M1P (2-methy-1-propanol), 2-propanol, DMSO solvents.


the different solute–solvent interaction mechanism. The solvatochro-mic properties of a molecule are analyzed by linear solvation energyrelationships (LSER) depending on solvent parameters, such asrefractive index (n), relative permittivity (e) and Kamlet–Taft param-eters (b and a). LSER is derived by multiple linear regression method.This method is carried out using solvent parameters as independentvariables and wavelengths of absorption and emission spectra asdependent variables. Multiple linear regression method has beenused to correlate electronic transition by applying following equation:

mmax ¼ C0 þ C1f ðnÞ þ C2f ðeÞ þ C3ðbÞ þ C4ðaÞ ð26Þ

Table 2Solvent parameters and solvent polarity functions.

No. Solvent e n u(e, n)a

1 1,4-Dioxane 2.21 1.4224 0.61482 CCl4 2.24 1.4601 0.64653 Toluene 2.38 1.4969 0.69984 Chloroform 4.81 1.4459 0.97535 Ethyl acetate 6.02 1.3724 0.99586 Butyl acetate 6.17 1.3719 1.00387 THF 7.58 1.4072 1.10238 1-Butanol 17.51 1.3993 1.29319 2M1P 17.93 1.3959 1.2938

10 2-Propanol 19.92 1.3772 1.291911 Ethanol 24.55 1.3614 1.304912 Methanol 32.66 1.3284 1.302113 DMF 36.71 1.4305 1.419614 DMSO 46.45 1.4793 1.4880

f(e,n) value has the same value with FB(e, n).a Solvent function from Eqs. (5) and (6).b Lippert–Mataga solvent function (Eq. (17)).c Bakhshiev solvent function (Eq. (18)).d Kawski–Chamma–Viallet solvent function (Eq. (19)).e Reichardt solvent parameter values taken from Ref. [5].

where f(n) = (n2 � 1)/(n2 + 1) is electronic polarizability function,f(e) = (e � 1)/(e + 2) is polarity functions and a, b (a, hydrogen bonddonor and b, hydrogen bond acceptor) are Kamlet–Taft parameters.C coefficient gives information about the types of interactions be-tween the solute and solvents. In Eq. (26), C0 is the wavenumber(cm�1) of electronic spectra for gaseous phase of investigated mol-ecule. The C1, C2, C3 and C4 coefficients reveal the relative contribu-tions of the considered solvatochromic parameters to the totalspectral shifts [24]. The multiple linear regressions for LSER havebeen performed with SPSS15.0 statistical package program.

Results and discussion

Solvent effects on the absorption and fluorescence spectra

Absorption and fluorescence spectra were investigated in non-polar, polar aprotic and polar protic solvents with different polar-ities. The absorption and fluorescence wavenumbers are listed inTable 1. It is clear from Fig. 3 and Table 1 that Stokes shift increaseswith increasing solvent polarity. The magnitude of Stokes shiftindicates that the excited state geometry could be significantly dif-ferent from that of the ground state and thus large values of ex-cited state dipole moments are expected. On changing solventsfrom nonpolar to polar, the magnitude of Stokes shift varies be-tween 6095 cm�1 and 7056 cm�1 (see Table 1).

The absorption spectra show the bands in the regions of 317–319 nm. These bands correspond to low-lying p–p� states of themain chromophore. Absorption bands do not show solvent sensi-tive shift. The absorption maxima for different solvents remain al-most constant with polarity function. 7ADDC shows twofluorescence bands in all of the solvents on excitation. The firstemission band is observed in the region of 395.5–410 nm whilethe second one observed in the region of 477–505 nm. As can beseen from Table 1, increasing in solvent polarity gives rise to thefirst emission band shift to red. This trend confirms the existenceof p–p� electronic transitions. The second fluorescence band doesnot sensitive to polarity function. Unchanged absorption spectrumwith solvent polarity implies that the ground state energy distribu-tion is not affected possibly due to less polar nature of the mole-cule in the ground state. On the other hand, red shift of theemission maximum with the solvent polarity indicates greater sta-bilization of excited state in polar solvents [25].

FL–M(e, n)b FB(e, n)c FK–C–W(e, n)dEN

Te

0.0205 0.0415 0.3692 0.1640.0112 0.0236 0.3818 0.0520.0132 0.0291 0.4055 0.0990.1483 0.3709 0.5474 0.2590.1996 0.4891 0.5642 0.2280.2024 0.4977 0.5683 0.2410.2096 0.5491 0.6143 0.2070.2635 0.7504 0.7104 0.6020.2656 0.7556 0.7110 0.5520.2762 0.7787 0.7118 0.5400.2887 0.8129 0.7198 0.6540.3086 0.8546 0.7216 0.7620.2744 0.8356 0.7709 0.4040.2630 0.8400 0.8010 0.444

Fig. 5. Plots of ma � mf (cm�1) versus F1(e, n) and ma � mf (cm�1) versus F2(e, n) for the studied compound in different solvents.

Fig. 4. Plots of ma � mf (cm�1) versus f(e,n) and ma + mf (cm�1) versus u(e, n) for the studied compound in different solvents.


Fig. 6. Plots of (ma + mf)/2 (cm�1) versus F3(e, n) and ma � mf (cm�1) versus ENT for the studied compound in different solvents.

Table 4Onsager cavity radius, ground state and excited state dipole moments (in Debye, D).

Onsager radiusa (Å) lgb le

c le(I)d le(II)

e le(III)f le(IV)

g (le/lg)h uj

4.816 0.385 3.427 5.828 3.635 3.347 2.458 8.901 0�

a 1D = 3.33564 � 10�30 C. m. = 10�18 esu cm.b Ground state dipole moment calculated according to Bilot–Kawski, Eq. (7).c Excited state dipole moment calculated according to Bilot–Kawski, Eq. (8).d Calculated according to Lippert–Mataga correlation, Eq. (14).e Calculated according to Bakhshiev correlation, Eq. (15).f Calculated according to Kawski–Chamma–Viallet correlation, Eq. (16).g Calculated according to Reichardt correlation, Eq. (24).h Calculated according to Eq. (9).j The angle between the ground state and excited state dipole moments calculated from Eq. (10).

Table 3Spectral treatment of the Bilot–Kawski, Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet and Reichardt correlations of the studied compound.

Equation Slope (m) Intercept Correlation (R2) Solvents used in correlation na

Bilot–Kawski (Eq. (1)) m(1) = 828.27 6157.4 0.9433 Except for 8,11,12,14 10Bilot–Kawski (Eq. (2)) m(2) = 1038 57337.0 0.9112 Except for 8,11,12,14 10Lippert–Mataga m1 = 2652.10 6107.5 0.9033 Except for 14 13Bakhshiev m2 = 945.73 6129.4 0.9044 All of the solvents 14Kawski–Chamma–Viallet m3 = 989.36 28707.0 0.9011 Except for 8 13Reichardt m4 = 1279.90 6088.2 0.9207 Except for 6,7,13,14 10

a Number of data.


Estimation of ground state and excited state dipole moments

In order to determine the ground state and excited state dipolemoments of 7ADDC, we first apply Eqs. (1), (2), (11)–(13) and (20)based on the various solvatochromic shift methods. We plotted�ma � �mf versus to f(e, n), �ma þ �mf versus to u(e, n), �ma � �mf versus to

F1(e, n), �ma � �mf versus to F2(e, n), ð�ma þ �mf Þ=2 versus to F3(e, n) and�ma � �mf versus to solvent polarity parameter EN

T by using Bilot–Kawski, Lippert–Mataga, Bakhshiev, Kawski–Chamma–Vialletand Reichardt equations, respectively (Figs. 4–6). �ma � �mf ; �maþ�mf andð�ma þ �mf Þ=2 values and solvent functions and parameters aregiven in Tables 1 and 2, respectively.

Table 6Mulliken atomic charges (in a.u.) and electron density (in a.u.) of 7ADDC in vacuumand ethanol calculated by using DFT/B3LYP and TDDFT/B3LYP methods along with 6-311++G(d, p) basis set.

Atoms DFT/B3LYP (in vacuum) TDDFT/B3LYP (in ethanol)

Atomiccharge

Electrondensity

Atomiccharge

Electrondensity

C(1) 0.917 13.582 0.908 12.955C(2) �1.260 13.649 �1.253 12.910O(3) �0.108 8.392 �0.132 8.400C(4) 0.357 6.521 0.443 6.269C(5) �1.154 19.083 �1.203 18.111C(6) 1.231 13.489 1.244 12.823C(7) �0.482 7.419 �0.456 7.363C(8) 1.607 30.685 1.583 30.549C(9) �0.764 10.637 �0.779 10.632C(10) �0.040 22.561 �0.072 22.139C(11) �0.407 8.323 �0.428 8.206O(12) �0.275 8.027 �0.354 8.130C(13) �0.324 9.368 �0.329 9.156O(14) 0.039 8.577 �0.032 8.516C(15) �0.062 6.882 0.005 6.706C(16) �0.492 6.272 �0.508 6.143O(17) �0.142 7.758 �0.206 7.824C(18) �0.986 12.240 �0.972 11.818C(19) 0.234 10.216 0.268 9.788Br(20) �0.082 35.077 �0.107 35.128C(21) �0.774 6.841 �0.810 6.767Br(22) �0.071 35.021 �0.102 35.077H(23) 0.184 0.532 0.205 0.499H(24) 0.087 0.631 0.114 0.591H(25) 0.168 0.520 0.181 0.502H(26) 0.169 0.565 0.180 0.547H(27) 0.180 0.560 0.198 0.529H(28) 0.208 0.513 0.198 0.526H(29) 0.172 0.577 0.189 0.545H(30) 0.210 0.522 0.214 0.514H(31) 0.175 0.532 0.195 0.505H(32) 0.176 0.531 0.190 0.522H(33) 0.187 0.495 0.189 0.493H(34) 0.210 0.625 0.211 0.616H(35) 0.248 0.557 0.276 0.514H(36) 0.228 0.584 0.223 0.576H(37) 0.187 0.529 0.211 0.491H(38) 0.248 0.492 0.263 0.470

Table 5Experimental and calculated band maxima as a result of multiple linear regression analysis of electronic transition for investigated molecule and solvent parameters.

Solvent e n b a vf-exp.a vf-calc.

a va-exp.a va-calc.

a

1.4-Dioxane 2.21 1.4224 0.37 0 25,189 25260 31,447 31,434CCl4 2.24 1.4601 0 0 25,253 25,252 31,348 31,361Toluene 2.38 1.4969 0.11 0 25,284 25,275 31,496 –Chloroform 4.81 1.4459 0 0.44 24,938 24,869 31,348 31,354Ethyl acetate 6.02 1.3724 0.45 0 24,814 24,773 31,348 31,376Butyl acetate 6.17 1.3719 0.45 1.12 24,876 24,709 31,546 31,516THF 7.58 1.4072 0.55 0 24,814 24,747 31,397 31,3701-Butanol 17.51 1.3993 0.84 0.84 24,752 24,526 31,447 31,4892M1P 17.93 1.3959 0.84 0.79 24,631 24,520 31,348 –2-Propanol 19.92 1.3772 0.84 0.76 24,570 24,482 31,447 31,480Ethanol 24.55 1.3614 0.75 0.86 24,510 24,416 31,546 31,475Methanol 32.66 1.3284 0.66 0.98 24,390 24,324 31,447 31,476DMF 36.71 1.4305 0.69 0 24,510 24,486 31,348 31,331DMSO 46.45 1.4793 0.76 0 24,450 24,528 31,496 –

a v is given in cm�1.


The results of the statistical treatment of the Bilot–Kawski, Lip-pert–Mataga, Bakhshiev, Kawski–Chamma–Viallet and Reichardtcorrelations, namely the slopes and number of data are shown inTable 3. As can be seen from Table 3, correlation coefficients arelarger than 0.9011 in most cases, which indicate a good linearityfor these correlations (Figs. 4–6).

Dipole moments of ground state (lg) and excited state (le) aredetermined from the slopes m(1) and m(2) of the Bilot–Kawski cor-relations by applying Eqs. (7) and (8). The excited state dipole mo-ments (le) are also determined from the slopes (m1, m2, m3 and m4)of Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet and Reic-hardt correlations by applying Eqs. (14)–(16) and (24). All of the re-sults are presented in Table 4.

As can be noted in Table 4, excited state dipole moment(3.427D) is found to be higher than ground state dipole moment(0.385D). The differences are ranged from about 2.073–5.443D,which indicate that the studied molecule is significantly more po-lar in its excited state than in its ground state. Therefore, the sol-vent–solute interactions should be stronger in the excited statethan in the ground state, demonstrating an important redistribu-tion of charge densities between both electronic states. We haveobserved a relatively good agreement between the excited state di-pole moments observed from the Bilot–Kawski, Bakhshiev, Kaw-ski–Chamma–Viallet and Reichardt correlations. It is noticed thatDl (5.443D) obtained from Lippert–Mataga method is large com-pared to value obtained by the other methods, since it does notconsider polarizability effect of the solute. The change in dipolemoment on excitation can be considered as a result of the natureof emitting state and charge transfer.

The estimation of present like problem by many authors [26–30] assumed that excited state dipole moment is almost parallelwith the ground state. Accordingly we have calculated the anglebetween ground and excited state dipole moments and the ob-tained 0� indicates that dipole moments are parallel. Dipole mo-ment is a measure of the molecular charge distribution given asa vector given in three dimensions. Therefore, it can be used as adescriptor to depict the charge movement across the molecule.Direction of the dipole moment vector in a molecule depends onthe centers of positive and negative charges. The parallelism be-tween the ground and excited state dipole moments indicatesthe larger charge movement across the molecule in the excitedstate than those of ground state in the same way.

Ground state dipole moment calculated by using differenttheoretical methods can be seen in the literature [31]. Excited statedipole moments of 7ADDC calculated at the TDDFT/B3LYP/6-311++G(d,p) level of theory are found as 5.118 Debye and 7.150Debye in vacuum and in ethanol, respectively. Discrepanciesbetween experimental and theoretically obtained results can beattributed to the fact that theoretical calculations belong to

gaseous phase of isolated molecule and the experimental resultsbelong to solid-state.

Solvatochromic behavior of 7-acetoxy-6-(2,3-dibromopropyl)-4,8-dimethylcoumarin

The wavenumbers of the electronic absorption and fluorescencebands and values of parameters used in multiple linear regression


analysis are collected in Table 5. The spectral shifts do not dependon a regular variation from nonpolar to polar solvents. The correla-tion plot of absorption and fluorescence spectra versus solventparameters (functions of e and n and Kamlet–Taft parameters)are shown in Supplementary material (Figs. S1 and S2). The wave-number in the maxima of electronic absorption bands of this mol-ecule does not depend linearly on the dielectric function, refractiveindex function and Kamlet–Taft parameters. Thus, LSER should beused to explain the mechanism of electronic transition; because itcould provide information on different interactions between soluteand solvent. LSER equation constituted for absorption spectra ofthis molecule is shown in Eq. (27).

Absorbance wavelength:

mabs ¼ 31556:762þ ð�340:683Þf ðnÞ þ ð�245:820Þf ðeÞþ ð172:373Þbþ ð127:102Þa ð27Þ

n = 11 (except for toluene, 2M1P and DMSO solvents); R = 0.888;R2 = 0.788; P = 0.032.

In order to derive the LSER equation for absorption spectra withthe satisfactory regression coefficient as R = 0.888 and R2 = 0.788,11 solvents (except for toluene, 2M1P and DMSO) have been used.As can be seen from Eq. (27), absolute value of C1 coefficient is big-ger than absolute value of C2 coefficients. Consequently, effect ofdispersion–polarization forces is bigger than the effect of orienta-tion induction interactions. It is observed that C3 coefficient is sig-nificantly higher than the C4 coefficient, also. Thereby, H-bonddonating ability is powerful than the ability to accept the H-bond.The high absolute value of C3 coefficient compared to C4 coefficientreveals to tendency of bathochromic effect [32].

As can be seen from Fig. S2 (see Supplementary material), wehave observed very good correlation between fluorescence

Fig. 7. Calculated atomic charges and electron densities of the titled molecule by using Dvacuum and ethanol.

maximum band and dielectric function, f(e) that plays an impor-tant role in determining orientation–induction interactions. So,the mechanism of the shift in fluorescence bands is almost com-pletely controlled by dielectric function which indicative of induc-tion–orientation interactions since correlation coefficient betweenvf and f(e) is R2 = 0.9513. But, vf values of this molecule do not cor-relate Kamlet–Taft parameters and refractive index function. How-ever, LSER for fluorescence have been performed to completelyelucidate the electronic transition mechanism. Therefore, Eq. (28)is derived with MLR method by using 14 solvents. According toEq. (28), statistical parameters of LSER provides very good correla-tion with R = 0.984 and R2 = 0.969. The absolute value of C1 is big-ger than the absolute value of C2. This means that fluorescenceband shifts are controlled by induction–orientation interactionsof solute–solvent. The fluorescence spectra show both red shiftand stronger H-bond donor capability than H-bond acceptor capa-bility due to the bigger C3 coefficient compared to C4 coefficient.

Fluorescence wavelength:

mf ¼ 24965:310þ ð1840:955Þf ðnÞ þ ð�1293:900Þf ðeÞþ ð119:499Þbþ ð�48:298Þa ð28Þ

n = 14; R = 0.984; R2 = 0.969; P = 0.000.C1, C2 and C3 coefficients have negative sign and we can say that

electronic absorption and fluorescence spectra of 7ADDC are even-tuating in red shift (bathochromic effect). This means that mole-cule is more polar in the excited state than in the ground state.The negative sign of C2 designate that ground state dipole momentvalue is smaller than the excited dipole moment value. The resultsof experimentally determined ground state and excited state di-pole moments confirm these findings (see Table 4).

FT/B3LYP and TDDFT/B3LYP methods supplemented with 6-311++G(d,p) basis set in


Quantum chemical calculations

DFT and TDDFT [20] calculations were used to investigate theelectronic structure in order to support and explain the experimen-tal observations. Solvent effect in ethanol was calculated by meansof the IEFPCM, which is SCRF method [33,34]. Atomic charges andelectron densities of each atom of 7ADDC were investigated in vac-uum and ethanol to provide information about dipole momentresulting from non-uniform distribution of charges. Mullikenatomic charges and electron density in vacuum and ethanol areillustrated in Table 6. The better represented graphical form ofthe results has been done in Fig. 7. Some of the atomic charges cal-culated in vacuum are presented in the earlier study [31]. How-ever, in the solvent phase, O(3), O(12), O(14), O(17), Br(20),Br(22) and most of the carbon atoms exhibit a substantial negativecharge, which are donor atoms, whereas C(1), C(4), C(6), C(8) andC(19) atoms exhibit positive charge, which are acceptor atoms.The charge distribution of both in vacuum and in solvent showsthat there exists a strong polar character, directing from C(8)(QD = �+1.60e) to C(2) (QA = ��1.26e) atom. Such a charge transfercharacter behavior results in a difference in dipole moment be-tween the ground and first excited state. It is clear that TDDFT/B3LYP method gives rise to the atomic charges increase due tothe inductive effect of solvent. The magnitude of the hydrogenatomic charges in both vacuum and ethanol were reported to beonly positive values indicating the charge transfer from hydrogento the other atoms. Solvent effect gives rise to hydrogen atomshave bigger atomic charges.

Fig. 8. 3D plots of HOMO and LUMO calculated by DFT and

Atomic charges describe a polarized character of the bonds,while electron density characterizes the magnitude of electroncloud located on atom in three-dimensions. There is not directlyrelationship between atomic charge and electron density for anatom. However, neighbor atom type and its radius give rise to vari-ety of these parameters. As can be seen from Table 6 and Fig. 7,electron densities along CAC, CAO, C@O and CABr bonds are ob-served not in the center of the bonds. Electron densities of Br atomsboth in vacuum and in solvent are almost equal (�35 a.u.). Magni-tudes of electron densities (�8 a.u.) on oxygen atoms are in the or-der of O(14) > O(3) > O(12) > O(17). Solvent effect gives rise tolowering in electron density of O(14) atom due to the presenceof p-bond of C(15)@O(17) whereas increasing in electron densityof the other oxygen atoms. Among the carbon atoms, C(8) hasthe biggest electron density with the value of �30 a.u. Electrondensities of C5, C8 and C10 atoms are relatively bigger becausethey are affected by the electron densities of C@O, BrCH2CHBrCH2

and CH3COO moieties, respectively.The second way in order to exploring the direction of intramo-

lecular charge transfer comes from orbital topologies. HOMO andLUMO calculated in vacuum and solvent (ethanol) phase have beenused to elucidate information regarding charge transfer within themolecule. The HOMO (highest occupied molecular orbital) energycharacterizes the ability of electron giving while LUMO (lowestunoccupied molecular orbital) energy characterizes the ability ofelectron accepting. Generally, energy gap of HOMO and LUMO re-flect the chemical activity of the molecule. The smaller energy gap,the easier it is for the HOMO electrons to be excited to the LUMO

TDDFT in vacuum and ethanol for the titled compound.


levels. The energies of HOMO and LUMO were computed as�6.799 eV and �2.027 eV in vacuum and �6.787 eV and�2.163 eV in ethanol, respectively. Obviously, LUMO–HOMO en-ergy gap in vacuum is slightly higher than those in ethanol withthe value of 0.148 eV. This is stabilization energy of ethanol. The3D HOMO and LUMO graphs of the 7ADDC are shown in Fig. 8.The positive phase is red and the negative phase is green. As canbe seen from Fig. 8, the HOMO is localized on ABrCH2CHBrA moi-ety both in vacuum and in solvent phase. The LUMO is localizedover the entire molecule except for ABrCH2CHBrCH2A and methylmoieties. In LUMO plot, inductive effect of solvent gives rise to theentire electron in ABrCH2CHBrA and methyl moieties migratewhereas some of them are kept in on Br and H atoms in vacuum.

In addition, after the frontier molecular orbitals (FMOs), molec-ular electrostatic potential (MEP), electrostatic potential (ESP) andsolvent accessible surfaces of 7ADDC are visualized in Fig. 9 andFig. 10, respectively.

The solvent accessible surfaces according to atomic colors andelectron densities are displayed in Figs. 9a and b, respectively. Ascan be seen from Fig. 9a, the light blue regions show the interactionzone of the oxygen atoms with the solvent, while the mat greenishblue regions indicate the interaction zone of bromide atoms withthe solvent. Purple zone shows the interaction sites of some carbonatoms with the surrounding of the compound. The largest region,

Fig. 9. Solvent accessible surfaces: (a) according to atomic charge colors and (b)according to electron density colors. Specified dots indicate the interaction radius ofatom with its surroundings. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 10. 3D plots of the (a) molecular electrostatic potential (MEP) and (b)electrostatic potential (ESP) map. (Red color shows the negative regions whileblue and yellow colors show the positive region of MEP and ESP, respectively.) (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

mat greenish yellow zone, is indicative of interaction betweenhydrogen atoms and solvent. Magnitude of the mat greenish yel-low region is larger than those of the other regions. It is clear thatinteraction between title compound and solvent is generally con-trolled by oxygen, bromide and hydrogen atoms. Electron densitysurface shown in Fig. 9b also supports this result.

The MEP is a plot of electrostatic potential drawn on the basis ofthe constant electron density surface and is a very useful descriptorin determining sites for electrophilic attack and nucleophilic reac-tions as well as hydrogen bonding interactions in solvent [35,36].Different values of electrostatic potential at the surface are repre-sented by different colors. The negative (red) regions of MEP wererelated to electrophilic reactivity and the positive (blue) regions tonucleophilic reactivity (Fig. 10a). As can be seen from the MEP sur-face, the negative regions are mainly localized around oxygen andbromide atoms. The regions having positive potential are over thehydrogen atoms.

As can be seen from ESP figure (Fig. 10b), negative ESP is local-ized over the molecule and is reflected as a yellowish blob. ESP is ameasure of electronegativity and partial charges of a molecule.

Conclusions

We have calculated the ground state dipole moment by usingBilot–Kawski’s method and excited state dipole moments by using


Bilot–Kawski’s, Lippert–Mataga’s, Bakhshiev’s, Kawski–Chamma–Viallet’s and Reichardt’s solvatochromic shift methods. The excitedstate dipole moment of investigated compound is found biggerthan ground state dipole moment in all of the methods. This re-veals that molecule is more polar in the excited state than in theground state, and, therefore, is more sensitive to solvent effects.Solute–solvent interaction mechanism in absorption and fluores-cence spectra has been investigated by using multiple linearregression methods using solvent parameters such as shifts inabsorption band wavelength is controlled by dispersion–polariza-tion forces, while the shifts observed in fluorescence band wave-length is controlled by induction–orientation interactions ofsolute–solvent. DFT and TDDFT calculations indicate that 7ADDCis reactive in interacting with the solvent.

Acknowledgements

The authors grateful the Bitlis Eren University, Scientific andTechnological Application and Research Center for providing thePerkin Elmer Lambda-35 UV–vis. spectrophotometer and PerkinElmer LS-55 Fluorescence Spectrometer. Author would also liketo thank Dear Assist. Prof. Dr. Halil Berber from Anadolu University,Faculty of Science, and Department of Chemistry for providingGaussian 03W software.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2012.10.018.

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