untitled European Journal of Chemistry 3 (1) (2012) 87‐93 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2012 EURJCHEM DOI:10.5155/eurjchem.3.1.87‐93.519 European Journal of Chemistry Journal homepage: www.eurjchem.com Photophysical properties and estimation of ground and excited state dipole moments of 7‐diethylamino and 7‐diethylamino‐4‐methyl coumarin dyes from absorption and emission spectra Mohd Mudassir Husaina,*, Rajeev Sindhua,b and Harmesh Chander Tandonc a Physics Section, Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, a Central University, New Delhi‐ 110025, India b New Green Field College of Engineering and Technology, Palwal, Haryana‐121102, India c Department of Chemistry, Sri Venketswara College, Delhi University, New Delhi‐110021, India *Corresponding author at: Physics Section, Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, a Central University, New Delhi‐110025, India. Tel.: +91.11.6831717x2512/2516; fax: +91.11.26988816. E‐mail address: mmudassirh@rediffmail.com (M.M. Husain). ARTICLE INFORMATION ABSTRACT Received: 05 September 2011 Received in revised form: 24 October 2011 Accepted: 24 October 2011 Online: 31 March 2012 KEYWORDS In the present work, the effect of solvents on absorption and fluorescence spectra and dipole moments (μg, μe) of 7‐diethylamino coumarin (7DEAC) and 7‐diethylamino‐4‐methyl coumarin (7DEA4MC) have been studied in different solvents of various polarity at room temperature. The solvents have been selected in a way to cover the full range of intermolecular interactions from non‐polar hexane to strongly polar formamide. Using the methods of solvatochromism, the difference in the first excited singlet‐state (μe) and ground state (μg) dipole moments was estimated from Lippert‐Mataga, Bakhshiev, Kawski‐Chamma‐ Viallet and McRae equations. The Onsager’s cavity radius of the probes has been calculated by AM1 and PM3 quantum chemical calculations and also by a direct relation. The change in dipole moment value (Δμ) was also calculated by using the variation of Stoke’s shift with microscopic solvent polarity parameter (ETN). The calculated dipole moments represent new results, as well as some of the solvatochromic results that were not studied earlier in such large number of solvents. It is observed that the values of excited singlet‐state dipole moments are higher than the ground state ones in both the molecules, which shows that excited states are more polar than the ground states. Coumarins Absorption Stoke’s shift Fluorescence Dipole moment Onsager’s cavity radius 1. Introduction Coumarins are well known laser dyes [1,2] and are useful probes in different chemical and photochemical studies [3‐7]. Most of the coumarins are highly fluorescent and have potential applications as fluorescent indicators [8], sunburn preventives [9], estimation of enzymes etc. [10]. In the present work we have estimated the dipole moments (μg, μe) and change in dipole moment value (Δμ) of 7‐diethyl amino coumarin (7DEAC) and 7‐diethylamino‐4‐methyl coumarin (7DEA4MC). Determination of ground and excited state dipole moments is important, because these values give information about the change in electronic distribution upon excitation. Generally the lifetime of excited states is small and dipole moments of short‐lived species are of considerable interest because just as for stable molecules, they provide important information on the electronic and geometrical structures of these transients, furthermore dipole moment represents a direct measure of electron distribution in a molecule of known geometry. A prior knowledge of the dipole moments of the electronically excited species is often useful in the design of non‐linear optical materials [11] and in the elucidation of the nature of the excited states. Experimental data on excited states are also useful in the parameterization of semi‐empirical quantum chemical procedures for such states. It is known that the electronic spectra of coumarin dyes are influenced by their immediate environment, among the major environmental factors influencing the electronic spectra, solvent effects are of particular importance. The change of solvent affects the ground and excited states differently and a systematic analysis of the solvent effect is useful in understanding the excited state behavior of the molecule. The solvent shifts can be accounted in terms of the overall effect of the interaction forces on the π‐electron system of the molecule. It is also known that, as the π‐electron system becomes more delocalized, the transition energy becomes smaller resulting in a bathchromic shift (red shift) and its opposite effect gives rise to a hypsochromic shift (blue shift). In order to assign the electronic transitions as π→π* or n→π* the solvatochromic technique is found to be very informative. It is known that π→π* bands show a red shift in the solvents of increasing polarity while n→π* bands show a blue shift [12]. The solvatochromic shifts were also used for the determination of the excited singlet‐state dipole moments of some coumarins [13‐19]. 2. Experimental Coumarin dyes 7DEAC and 7DEA4MC were obtained from Sigma Aldrich Chemicals (USA) and were used as received. The molecular structures of the systems are given in Scheme 1. All the 22 solvents (Formamide, dimethyl sulfoxide, dimethyl formamide, acetonitrile, ethanol, acetone, propanol, butanol, 1‐ pentanol, dichloromethane, ethyl acetate, ethyl benzoate, ethyl propionate, n‐butyl acetate, chloroform, toluene, p‐xylene, benzene, carbon tetrachloride, 1,4‐dioxane, cyclohexane and 88 Husain et al. / European Journal of Chemistry 3 (1) (2012) 87‐93 hexane) used were of spectroscopic grade and were found to be transparent and non‐fluorescent in the range of excitation and fluorescence emission. The absorption and fluorescence spectra were measured by Shimadzu‐UV‐Visible spectro‐ photometer (UV2450) and Shimadzu spectrofluorometer (RF‐ 5301PC), respectively. All the measurements were carried out at room temperature keeping dye concentration low (~10‐6 M) in order to avoid self absorption.     7‐diethylamino coumarin (7DEAC)     7‐diethylamino‐4‐methyl coumarin (7DEA4MC)   Scheme 1  3. Theoretical consideration In this work we report different solvent parameters e.g. dielectric constant (ε), refractive index (n) and spectral parameters such as Stoke’s shift which is useful for determination of dipole moments. The details of the methods adopted to calculate the dipole moment in ground and excited states of two molecules under consideration based on absorption and fluorescence shifts in various solvents is given below. 3.1. Determination of dipole moment The dipole moment of a molecule in the excited singlet‐ state is determined by the effect of electric field (internal and external) on the position of its spectral band. Two methods depending on the internal electric field (solvatochromism) have been employed in the present investigation. 3.1.1. Method I By employing the simplest quantum‐mechanical second order perturbation theory and taking into account Onsager’s model, Bilot and Kawski [20,21] have obtained an expression for the spectral shift given by 1 1 , , 2 2 2 - (1- ) [ (1 ) ] (2 ) (1 ) 2 1 a f a f e g m f f(1 f) f f f f f hc                          (1) where hc m ge a )( g    (2) for absorption and hc m ge f )( e    (3) for fluorescence α is the mean static isotropic polarizability of the solute. f and fˊ are the reaction field factors which depend on the shape and the Onsager’s cavity radius a of the solute, and on the relative permittivity (ε) and refractive index (n) of the solvent. νa and νf are the absorption and fluorescence maxima and h and c are Planck’s constant and velocity of light in vacuum, respectively. Based on the Equation 1, the following expressions are obtained for (νa‐νf) and (νa+νf)/2 [21,22]; constant),( 1  nfmfa  (4) constant ),( )( 2 1 2  nmfa  (5) where )(),( 2 1 ),( ngnfn   , (6) 3 2 1 )( 2 hca m ge    (7) and 3 2 g 2 e 2 )( 2 hca m    (8) The factors f and fˊ are simplified when a spherical cavity of radius a is assumed, which may be regarded as a sufficient approximation [22]. In this case, solvent polarity parameters f (ε, n) and g(n) have the form ))12/()1))(/2(1))((12/()1))((/2(1(( ))12/()1(())12/()1(( ),( 2233 22    nnaa nn nf   (9) ))12/()1))((/2(1( ))2/()1))((/(1))((12/()1(( )( 223 22322    nna nnann ng   (10) If the polarizability of the solute is neglected, i.e. α = 0, then Equation 9 leads to Lippert‐Mataga relation [23‐26]             12 1 12 1 ),( 2 2 n n nF   (11) It is based on the Onsager’s reaction field theory, which assumes that the fluorophore is a point dipole residing in the center of a spherical cavity with radius in a homogeneous and isotropic dielectric with relative permittivity ε. The Lippert‐ Mataga equation breaks down when in addition to the non‐ specific interactions, specific fluorophore‐solvent interactions e.g., hydrogen bonding, electron‐pair donor/electron‐pair acceptor interactions also contribute significantly to the solute‐ solvent interaction. Another limitation results from the cavity radius, which is not easy to estimate for non‐spherical molecule. For an isotropic polarizability of the solute, the condition 2α/a3=1 is frequently satisfied and justified [22], and functions f (ε, n) and φ(n) leads to Bakhshiev and Kawski‐Chamma‐ Viallet relations, respectively [27,28]               2 1 2 1 2 12 ),( 2 2 2 2 1 n n n n nF   (12) 22 4 12 )2( )1( 2 3 2 1 ),(    n n FnF  (13) Husain et al. / European Journal of Chemistry 3 (1) (2012) 87‐93 89 According to McRae theory [29,30] f (ε, n) can be written as             2 1 2 1 ),( 2 2 3 n n nF   (14) Using Equations 7 and 8, the values of μg and μe can be obtained as; 1 3 12 g 22 m hcamm   (15) 1 3 21 e 22 m hcamm   (16) or ge mm mm  12 21          ; 12 mm  (17) So the ratio of dipole moments in excited state and ground state is given by 12 21 mm mm g e      (18) The parameters m1 and m2 occurring for the differences (νa‐ νf) and the sum (νa+νf)/2 of the wave numbers are linear functions of the solvent polarity parameters F1(ε, n) and F2(ε, n) and can be determined from the slopes of the straight lines. 3.1.2. Method II (Molecular‐microscopic solvent polarity parameter ( N TE ) The second method is based on the empirical polarity scale proposed by Reichardt [31] and gives results with solvatochromic shift of dipolar molecules that correlates much better with molecular‐microscopic solvent polarity parameter N TE rather than the traditionally used bulk solvent polarity functions as in the later the error estimation of Onsager cavity radius ‘a’ has been minimized. The theoretical basis for the correlation as spectral shift with N TE has been developed by Ravi et al. [32] and accordingly, the excited state dipole moment is evaluated by equation constant 6.11307 32 B                         N T B fa E a a   (19) N TE [or equivalently, )30(TE ] is the solvent polarity function proposed by Reichardt, based on the absorption wave number, νa, of a standard Betaine dye in the solvent. It is expressed as )TMS()water( )TMS()solvent( TT TTN T EE EE E    (20) Here )30(TE value for a solvent is simply defined as molar transition energy of the dissolved betaine dye measured in kcal/mol for the charge transfer absorption band (νa) and is expressed as ][108591.2])[30( 131   cmmolkcalE aT  (21) ΔμB and aB are the dipole moment changes on excitation and the Onsager radius respectively, of Betaine dye. Δμ and a are the corresponding quantities of the molecule of interest. Equation 19 clearly illustrates that the Stokes shift (νa‐νf) changes linearly with the solvent polarity function N TE . Δμ can be obtained from the slope of Equation 19. Using the reported ΔμB = 9 D of Betaine dye and its Onsager radius 6.2 Å [33], the change in dipole moment is determined by 3ge 2.6 6.11307 81         a m (22) In the present work, the Onsager’s cavity radii of the dye molecules were taken as half the distance between the amino N atom and the carbonyl O atom of the systems, optimized by semiempirical models (AM1 and PM3). This is consistent with the fact that these atoms provide the strongest dipole vector component in all the molecules considered [33]. In addition to this we have also calculated the Onsager’s cavity radius by the formula [34] 3 N4π 3M = ρ a (23) where M is the molecular weight of the dye; ρ is the density of the dye; N is the Avogadro’s number. It is difficult to calculate the density of a compound and the density of the compounds are not listed in literatures so we have approximated [35] the densities of these dyes to be 1 g/cm‐3. 4. Results and discussion Absorption and fluorescence spectra of 7DEAC and 7DEA4MC were measured in solvents of different solvent parameters e.g., refractive index and dielectric constant. Estimation of ground state and excited state dipole moments are done experimentally from observed absorption and emission spectra of dye molecules in various polar and non‐ polar solvents. The photophysical parameters; absorption and emission maxima wave numbers (νa, νf), Stoke’s shift (νa‐νf) and arithmetic mean of Stoke’s shift (νa+νf)/2 values (in cm‐1) for both the coumarin dyes in different solvents are tabulated in Table 1. It has been observed that the emission peaks are more pronounced than the absorption peaks for both the molecules with increasing solvent polarity which gives a larger spectral shift in the emission spectra than in the absorption spectra of the molecules. With increase in the polarity of the solvent, the fluorescence emission peak undergoes a bathochromic shift (red shift), confirming a π → π* transition [30]. The less pronounced shift in the absorption spectra observed in all the solvents indicates that the ground‐state energy distribution is not affected to a greater extent possibly due to the less polar nature of the dyes in the ground state than in the excited state which further indicates that of μe>μg. The red shift of the fluorescence wavelengths could be due to the marked difference between the excited‐state charge distribution in the solute and the ground‐state charge distribution, resulting in a stronger interaction with polar solvents in the excited state. The values of solvent polarity function F(ε, n), F1(ε, n), F2(ε, n), F3(ε, n) and N TE for solvents used in this work are presented in Table 2. Using linear fit method, the graphs of (νa‐νf) and (νa+νf)/2 are plotted against solvent polarity functions F(ε, n), F1(ε, n), F2(ε, n) and N TE , respectively for 7DEAC (Figures 1‐4). The slopes, intercepts and correlation coefficients of these best fit lines are given in Table 3. Figure 5a shows the solvatochromic shifts of the absorption spectra of 7DEAC in different solvents at room temperature. 90 Husain et al. / European Journal of Chemistry 3 (1) (2012) 87‐93 Table 1. Spectral data of 7DEAC and 7DEA4MC in various solvents. Solvents 7DEAC 7DEA4MC νa (cm‐1) νf (cm‐1) νa‐νf (cm‐1) (νa+νf)/2 (cm‐1) νa (cm‐1) νf (cm‐1) νa‐νf (cm‐1) (νa+νf)/2 (cm‐1) Formamide 25906 21598 4308 23752 26267 21978 4289 24122 DMSO 26315 22321 3994 24318 26680 22727 3953 24703 Dimethyl formamide 26525 22371 4154 24448 27027 22988 4039 25007 Acetonitrile 26737 22301 4436 24519 27107 22946 4161 25026 Ethanol 26490 21978 4512 24234 26780 22271 4509 24525 Acetone 26881 22988 3893 24934 27322 23386 3936 25354 Propanol 26455 22172 4283 24313 26702 22371 4331 24536 Butanol 26455 22123 4332 24289 26716 22471 4245 24593 1‐Pentanol 26525 22172 4353 24348 26737 22471 4266 24604 Dichloromethane 26525 22914 3611 24719 26867 23485 3382 25176 Ethyl Acetate 27277 23752 3525 25514 27578 23877 3701 25727 Ethyl Benzoate 26737 23326 3411 25031 27173 23696 3477 25434 Ethyl Propionate 27322 23518 3804 25420 27777 23866 3911 25821 n‐Butyl Acetate 27472 23980 3492 25726 27793 23980 3813 25886 Chloroform 26385 22935 3450 24660 26723 23255 3468 24989 Toluene 27442 23866 3576 25654 27855 24213 3642 26034 p‐Xylene 27397 23529 3868 25463 27940 24319 3621 26129 Benzene 27247 23724 3523 25485 27777 24154 3623 25965 Carbon tetrachloride 27677 24131 3546 25904 28066 24691 3375 26378 1,4‐Dioxane 27397 23685 3712 25541 27700 23752 3948 25726 Cyclohexane 28089 25087 3002 26588 28506 25380 3126 26943 Hexane 28248 25316 2932 26782 28636 25641 2995 27138 Figure 1. Plot of Stoke’s shift νa‐νf (cm‐1) versus F(ε, n) of 7DEAC. Figure 2. Plot of Stoke’s shift νa‐νf (cm‐1) versus F1(ε, n) of 7DEAC. Figure 5b shows a typical example of the solvent effect on the electronic emission spectrum of 7DEAC. The geometry of these systems was optimized using semi empirical methods AM1, PM3 to obtain the Onsager’s cavity radius. The values of Onsager’s cavity radius obtained by these methods and by Equation 23 along with the calculated ground (μg) and singlet excited‐state (μe)dipole moments and the ratio (μe/μg) for both the coumarin dyes estimated by using Equations 15, 16 and 17 are given in Table 4. The difference (Δμ) in excited and ground‐ state dipole moments calculated by using method employed by Lippert‐Mataga, Bakhshiev, McRae and molecular‐microscopic based on solvent polarity relations are also given in the same table. It can be seen that the Δμ values obtained for both the molecules by Lippert‐Mataga method are large as compared to values obtained by all other methods, it is due to the fact that this method does not take into account the polarizability of the solute. A comparison of results obtained in this work with the work reported previously is shown in Table 5. Figure 3. Plot of (νa+νf)/2 (cm‐1) versus F2(ε, n) of 7DEAC. The values for the 7DEA4MC in ground and excited states are in the range of 3.48 D ‐ 6.00 D and 5.13 D ‐ 8.85 D respectively. Earlier, M. Diraisan et al. [36] reported the computed value as μg = 8.05 D and μe = 12.92 D for the said coumarin. The findings of McCarthy and Blanchard [37] shows μg = 6.35 D and μe = 9.81 D which are in good agreement with our experimental results. This inconsistency of dipole moments with these theoretical work arises because, while computing the parameters the molecule is considered as isolated system (as in gas phase), whereas the experimentally obtained values are in solution phase, where the solvent (matrix) introduces strong perturbation. Further it is evident from Table 4 that in both dye molecules, under investigation, the changes in the dipole moments on electronic excitation is rather small. Husain et al. / European Journal of Chemistry 3 (1) (2012) 87‐93 91 Table 2. Refractive index, dielectric constant and various solvent polarity functions of the solvents a. Solvents nb εb F(ε, n) F1(ε, n) F2(ε, n) F3(ε, n) )( N TE b Formamide 1.447 111.0 0.282 0.895 0.750 0.706 0.799 DMSO 1.479 47.24 0.263 0.841 0.744 0.655 0.441 Dimethyl formamide 1.426 37.0 0.274 0.850 0.650 0.664 0.404 Acetonitrile 1.344 36.64 0.304 0.861 0.665 0.710 0.472 Ethanol 1.361 24.30 0.288 0.810 0.650 0.664 0.654 Acetone 1.359 21.01 0.284 0.792 0.640 0.649 0.355 Propanol 1.385 20.6 0.274 0.781 0.652 0.632 0.617 Butanol 1.399 17.40 0.263 0.749 0.648 0.603 0.601 1‐Pentanol 1.410 14.80 0.252 0.716 0.638 0.573 0.503 Dichloromethane 1.424 8.93 0.218 0.595 0.584 0.474 0.320 Ethyl Acetate 1.372 6.08 0.199 0.492 0.499 0.398 0.228 Ethyl Benzoate 1.503 5.99 0.156 0.430 0.550 0.328 ‐ Ethyl Propionate 1.380 5.58 0.188 0.460 0.489 0.372 ‐ n‐Butyl Acetate 1.394 5.00 0.170 0.413 0.471 0.427 ‐ Chloroform 1.442 4.81 0.148 0.370 0.490 0.292 0.259 Toluene 1.497 2.38 0.012 0.029 0.349 0.020 0.099 p‐Xylene 1.496 2.27 0.003 0.009 0.342 0.005 0.077 Benzene 1.501 2.28 0.002 0.006 0.340 0.004 0.117 Carbon tetrachloride 1.459 2.24 0.010 0.299 0.446 0.016 0.055 1,4‐Dioxane 1.422 2.22 0.021 0.043 0.308 0.034 0.164 Cyclohexane 1.426 2.02 0.0015 ‐0.003 0.287 0.0024 0.015 Hexane 1.374 1.88 0.0014 ‐0.002 0.253 0.0021 0.006 a F(ε, n): Lippert, F1(ε, n): Bakhshiev, F2(ε, n): Kawski‐Chamma‐Viallet and F3(ε, n): McRae’s solvent polarity function. b The values of n, ε and N TE (Molecular‐microscopic solvent function) are from Ref. [31]. Table 3. Slopes and intercepts of linear fittings. Molecule Slope Intercept No. of data Correlation coefficient Lippert‐Mataga correlation 7DEAC 2867.56 3334.39 22 0.76 7DEA4MC 2559.68 3394.41 22 0.74 Bakhshiev correlation 7DEAC 1042.18 3311.25 22 0.64 7DEA4MC 898.17 3387.67 22 0.59 Kawski‐Chamma‐Viallet correlation 7DEAC 4654.22 27496.18 22 0.81 7DEA4MC 4679.78 27881.08 22 0.79 McRae correlation 7DEAC 1234.33 3343.44 22 0.77 7DEA4MC 1103.07 3402.01 22 0.76 N TE correlation 7DEAC 1698.68 3289.77 19 0.74 7DEA4MC 1576.36 3309.68 19 0.75 Table 4. Ground and excited‐state dipole moments (Debye) and Onsager radius (Å) of 7DEAC and 7DEA4MC. Molecule Onsager radius, a μg a μe b Δμ c Δμ d Δμ f Δμ g (μe/μg) h 7DEAC 3.50(AM1) 3.65 5.75 2.10 3.49 2.29 1.47 1.58 3.47(PM3) 3.61 5.68 2.09 3.44 2.26 1.46 1.58 4.41(Eq.19) 5.16 8.14 2.98 5.08 3.23 2.09 1.58 7DEA4MC 3.13(AM1) 3.48 5.13 1.65 2.79 1.83 1.20 1.47 3.45(PM3) 4.05 5.94 1.89 3.22 2.11 1.40 1.47 4.50(Eq.19) 6.00 8.85 2.85 4.8 3.15 2.07 1.48 a Ground state dipole moments calculated from Equation 15. b Excited state dipole moments calculated from Equation 16. c Δμ calculated from Bakhshiev model. d Δμ calculated from Lippert‐Mataga model. f Δμ calculated from McRae model. g Δμ calculated from molecular‐microscopic solvent function )( N TE . h The ratio of excited state and ground state dipole moments calculated from Equation 17. This suggests that the emission of these dyes originates from a state which although more polar than ground state is probably a locally excited intramolecular‐charge transfer (ICT) state. Charge transfer accompanying excitation to lowest excited singlet state usually results in the excited molecule having a greater dipole moment than the ground state [38].The longest absorption and fluorescence maxima shifted to the lower energy end of the spectrum on increasing the solvent polarity for coumarin molecule. It is known that the lowest excited state is n→π*, but the substituted coumarin, the energy difference between n→π* and π→π* levels becomes smaller with increasing substitution and in some cases even the state order gets inverted [39]. In the present case, there is a bathochromic shift on increasing the solvent polarity which indicates that the transition involved is a π→π* transition and lowest lying state is π→π* (see Table 1). The difference between the values of ground‐state dipole moment (μg) and excited‐state dipole moment μe of 7DEAC and 7DEA4MC is small, this could be explained as, a CH3 group at position‐4 has small influence on the energy levels of molecules [38]. The effect of amino group in case of 7DEAC and 7DEA4MC is considered where unshared pair of electrons on this group resides in molecular orbitals (largely localized to amino group). 92 Husain et al. / European Journal of Chemistry 3 (1) (2012) 87‐93 Table 5. Comparison of present results with the values reported earlier*. Solute molecule m1 [cm‐1] m2 [cm‐1] Radius, a, [Å] μg [D] μe [D] Δμ = μe‐μg [D] μe/μg 7DEAC Present work (using Method I) 1042.18 4654.22 3.50(AM1) 3.47(PM3) 4.41(Eq.19) 3.65 3.61 5.16 5.75 5.68 8.14 2.10 2.09 2.98 1.58 1.58 1.58 Previous work Ref. [40] Experimental ‐ ‐ 3.71 3.15 6.35 3.20 2.01 Previous work Ref. [41] Experimental ‐ ‐ 4.0 6.34 8.40 2.06 1.32 7DEA4MC Present work (using method I) 898.178 4679.78 3.13(AM1) 3.45 (PM3) 4.50(Eq.19) 3.48 4.05 6.00 5.13 5.94 8.85 1.65 1.89 2.85 1.47 1.47 1.48 Previous work Ref. [11] Experimental ‐ ‐ 3.48 6.35 8.60 2.25 1.35 Previous work Ref. [37] Theoretical ‐ ‐ ‐ 6.35 9.81 3.46 1.54 Previous work Ref. [41] Experimental ‐ ‐ 4.0 5.99 8.74 2.75 1.46 * m1, m2 are the slopes of νa‐νf vs. F1(ε, n) and (νa+νf)/2 vs. F2(ε, n); ‘a’ is the Onsager radius; μg, μe are the dipole moments in ground and excited state, respectively. “‐”: Values of m1 and m2 are not reported in ref. [11], ref. [40], ref. [37] and ref. [41]. Figure 4. Plot of Stoke’s shift νa‐νf (cm‐1) versus N TE of 7DEAC. Thus due to ICT the electronic charge from these functional groups gets substantially delocalized throughout the system. Hence, the energy gap between the highest occupied and lowest unoccupied orbitals of amino substituted molecule is considerably lower than the difference between the highest occupied and lowest unoccupied orbitals of unsubstituted molecules [12,38]. 5. Conclusion The present investigations of the photophysical properties of 7DEAC and 7DEA4MC, which are promising active medium for tunable lasers, show that the lowest lying excited state of the molecules is π→π*. The dipole moments of the molecules are higher in the excited state and it is about 1.6 times of the ground state value. Further, dipole moment values for the coumarin dyes differ from each other. This can be attributed to the structural difference between the molecules. The relative positions of π→π* and n→π* depend on the nature of the substituent as well as the solvents. The assumption 2α/a3=1 is justified and does not significantly influence the determined value of μe, because in most cases α/a3 is unknown [22]. Equation 17 can be used to estimate the value of the excited‐ state dipole moment by pre‐knowledge of the value of ground‐ state dipole moment, without the necessity of knowing the Onsager radius of the solute. Further the absorption and fluorescence spectra of the coumarins are studied in 22 different polar and non‐polar solvents which were limited to a few solvents in the earlier reported work. (a) (b) Figure 5. Absorption (a) and fluorescence (b) spectra of 7DEAC in different solvents. Acknowledgements This work is supported by University Grant Commission through a research grant (F. No. 36‐360/2008) provided to one of the authors (Mohammed Mudassir Husain) and Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India. Husain et al. / European Journal of Chemistry 3 (1) (2012) 87‐93 93 References [1]. Fletcher, A. N.; Bliss, D. E. Appl. Phys. 1978, 16, 289‐295. [2]. Halstead, J. A.; Reeves, R. R. Opt. 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