untitled European Journal of Chemistry 3 (1) (2012) 75‐80 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2012 EURJCHEM DOI:10.5155/eurjchem.3.1.75‐80.498 European Journal of Chemistry Journal homepage: www.eurjchem.com Determination of excited singlet‐state dipole moments of hydroxy and methoxy coumarins using solvatochromic method 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: 02 August 2011 Received in revised form: 08 August 2011 Accepted: 12 August 2011 Online: 31 March 2012 KEYWORDS The effects of solvents of various polarity on the electronic absorption and fluorescence spectra of 7‐hydroxy‐4‐trifluoromethyl coumarin; 6,7‐dihydroxy‐4‐trifluoromethyl coumarin and 7‐methoxy‐4‐trifluoromethyl coumarin have been investigated. The singlet‐state excited dipole moments (μe) and ground state dipole moments (μg) are estimated from Bakshiev and Kawski‐Chamma‐Viallet equations by using the variation of Stokes’ shift with the solvent’s dielectric constant (ε) and refractive index (n). The observed singlet‐state excited dipole moments are found to be larger than the ground‐state ones. In addition, the geometry and other electronic properties are computed using ab‐initio method with correlation functions at 6‐31G basis set. Coumarins Absorption Stokes’ shift Fluorescence Dipole moment Solvatochromic 1. Introduction Coumarin and their derivatives show remarkable biological, chemical and physical properties and are useful probes in photochemical studies [1‐4]. Being highly fluorescent, coumarins act as fluorescent indicator, sunburn preventive and may be used in estimation of enzymes and has application in medicine [5‐10]. The knowledge of absorption and fluorescence characteristics of these coumarins in various solvents and at different temperatures helps in understanding their use as laser dyes [11‐14]. In the substituted coumarins; electron donating and withdrawing groups attached at various positions, alter their chemical reactivity in solvents of polar or non‐polar nature. They also alter the dipole moment in ground and excited states. This helps in understanding the behavior of these compounds in ground and excited states [15‐17]. The present work deals with the absorption and fluorescence characteristics of hydroxy and methoxy derivatives of 4‐trifluoromethyl coumarins in different solvents of polar and non‐polar nature. The absorption and fluorescence spectra of 7‐hydroxy, 6,7‐dihydroxy and 7‐methoxy derivatives of 4‐trifluoromethyl coumarin in various solvents are obtained and then the dipole moments in ground and excited states are calculated, using solvatochromic data. The excited state dipole moment of a dye is an important parameter, as it provides information about the change in electronic charge distribution due to excitation. A prior knowledge of the dipole moments in excited states is often useful in the design of non‐linear optical material [18]. The solvatochromic technique gives important information about electronic transitions. Solvatochromic technique is quite useful for assignment of n→π* or π→π* transitions. It is a popular way to determine the dipole moments in ground and excited states for short‐lived states [19‐22]. This technique is based on a linear correlation between the wave numbers of the UV‐VIS spectra of solute and solvent polarity function of ε (dielectric constant) and n (refractive index) of the solvent [23‐36]. 2. Experimental 2.1. Chemicals The laser dyes 7‐hydroxy‐4‐trifluoromethyl coumarin (7H4TFMC), 6,7‐dihydroxy‐4‐trifluoromethyl coumarin (67DH4TFMC) and 7‐methoxy‐4‐trifluoromethyl coumarin (7M4TFMC) in pure form were obtained from Sigma Aldrich Chemicals (USA) and used as received. The molecular structures of these dyes are given in Scheme 1. Where R1=H, R2= OH for 7H4TFMC; R1= OH, R2= OH for 67DH4TFMC and R1=H, R2=OCH3 for 7M4TFMC. All the solvents used were of spectroscopic grade and were chosen in such a way that they were transparent and non‐fluorescent in the range of excitation and fluorescence emission. 2.2. Spectroscopic measurements The absorption and fluorescence spectra were recorded using Shimadzu‐ UV2450 and RF‐5301PC spectrophotometer and spectrofluorometer, respectively. All the measurements were carried out at room temperature keeping dye concentration very low (~10‐6 M) in order to avoid self absorption. 76 Husain et al. / European Journal of Chemistry 3 (1) (2012) 75‐80 O O CF3 R1 R2 7H4TFMC: R1=H, R2= OH 67DH4TFMC: R1= OH, R2= OH 7M4TFMC: R1=H, R2=OCH3 Scheme 1 2.3. Computational details The molecular orbital calculations have been carried out for these coumarins in gas phase. The ab‐initio method with electron correlation function at Moller‐Plesset 2 (MP2) level by using 6‐31G standard basis set was used for this purpose. From the optimized geometry, the electronic and energetic parameters like dipole moment, in ground and excited state, frontier orbital energies, distribution of charge density and binding energies have been calculated. Throughout the computation, the r.m.s gradient and convergence limit were kept at 0.01 kcal/mol.Å and 0.01, respectively. For optimization process, HyperChem 7.52 package [37] is employed. 3. Theoretical considerations The expression normally used in fluorescence spectroscopy is a simplified form of the equation first obtained by Lippert and Mataga et al. [38‐41]. 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 a 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 difficult to estimate for non‐spherical molecule [20]. Here we report different solvent parameters, dielectric constant (ε), refractive index (n) and spectral parameters such as Stokes’ shift which is useful for determination of dipole moments suggested by Bakshiev [42] and Kawski‐Chamma‐Viallet’s [43]. They obtained a simplified quantum mechanical second order perturbation theory of absorption (νa) and fluorescence (νf) band shift in different solvents of varying permittivity (ε) and refractive index (n) relative to the band position of a solute molecule and based on which the following Equations are obtained. .),( 11 constnFmfa   (1) .),( )( 2 1 22 constnFmfa   (2) where νa and νf are the absorption and fluorescence maxima wave numbers in cm‐1, respectively, n and ε are the refractive index and dielectric constant of the solvents, respectively. The expression for F1(ε, n) [Bakshiev’s polarity function] and F2(ε, n) [Kawski‐Chamma‐Viallet polarity function] are given as               2 1 2 1 2 12 ),( 2 2 2 2 1 n n n n nF   (3) 22 4 12 )2( )1( 2 3 2 1 ),(    n n FnF  (4) with 3 2 1 )( 2 hca m ge    (5) 3 2 g 2 e 2 )( 2 hca m    (6) Where μe and μg are excited state and ground state dipole moments of solute molecule respectively, ‘a’ is Onsager cavity radius , ‘h’ is Planck’s constant and ‘c’ is the velocity of light in vacuum. The parameters m1 and m2 can be determined from absorption and fluorescence band shifts. If the ground and excited states are parallel then using above equations, the values of μg and μe can be obtained on the basis of Equations 5 and 6. 1 3 12 g 22 m hcamm   (7) 1 3 21 e 22 m hcamm   (8) or ge mm mm  12 21          ; 12 mm  (9) Hence the ratio of dipole moments in excited state and ground state is given by 12 21 mm mm g e      (10) The parameters m1 and m2 occurring for the differences (νa‐ νf) and the sum ½(νa+νf) of the wave‐numbers, are linear functions of the solvent polarity parameters F1(ε, n) and F2(ε, n), respectively and can be determined from the slopes of the linear fits. The validity of the Equations 7 and 8 is based on certain assumption like considering both the dipole moments collinear or almost so. Also the same Onsager cavity radius ‘a’ in both ground and excited states is assumed. The Onsager cavity radii ‘a’ from the molecular volume of molecules, given by Suppan’s equation [44, 45]. 3 N4π 3M  a (11) where M is the molecular weight of the dye; ρ is the density of the dye, assumed as 1.0 g/cm3 [46]; N being Avogadro’s number, have been used to calculate the radii for the three coumarin derivatives in the present work. The value of ‘a’ has also been calculated at ab‐initio level with 6‐31G basis set for comparison. Husain et al. / European Journal of Chemistry 3 (1) (2012) 75‐80 77 Table 1. Photo‐physical parameters of 7H4TFMC in different solvents used (λa and λf are wavelengths of absorption and fluorescence maxima, respectively). Solvent λa (nm) λf (nm) νa (cm‐1) νf (cm‐1) (νa‐νf) (cm‐1) ½(νa+νf) (cm‐1) Chloroform 335.3 414.0 28384 21344 7040 24864 n‐Butyl Acetate 331.9 412.5 28003 21404 6599 24703 Ethyl Benzoate 335.6 410.0 27708 21344 6364 24526 CH2Cl2 328.3 410.0 27862 21868 5994 24865 Ethyl Acetate 330.9 410.0 28121 21177 6944 24649 Methanol 336.4 433.6 27233 20064 7169 23648 DMSO 338.8 434.2 26939 19446 7163 23202 Formamide 339.2 441.0 26766 19603 7163 23184 Table 2. Photo‐physical parameters of 67DH4TFMC in different solvents used (λa and λf are wavelengths of absorption and fluorescence maxima, respectively). Solvent λa (nm) λf (nm) νa (cm‐1) νf (cm‐1) (νa‐νf) (cm‐1) ½(νa+νf) (cm‐1) Toluene 353.2 450.5 28312 22197 6115 25254 n‐Butyl Acetate 357.1 467.2 28003 21404 6599 24703 Ethyl Propionate 358.3 470.3 27909 21263 6646 24586 Ethyl Benzoate 360.9 468.5 27708 21344 6364 24526 Ethyl Acetate 355.6 472.2 28121 21177 6944 24649 Acetone 358.3 477.6 27909 20938 6971 24423 Methanol 367.2 498.4 27233 20064 7169 23648 Acetonitrile 354.5 477.1 28208 20959 7249 24583 DMF 372.4 511.9 26852 19535 7317 23193 DMSO 371.2 513.7 26939 19466 7473 23202 Formamide 373.6 510.1 26766 19603 7163 23184 Table 3. Photo‐physical parameters of 7M4TFMC in different solvents used (λa and λf are wavelengths of absorption and fluorescence maxima, respectively). Solvent λa (nm) λf (nm) νa (cm‐1) νf (cm‐1) (νa‐νf) (cm‐1) ½(νa+νf) (cm‐1) n‐Hexane 326.0 400.0 30590 25000 5590 27795 Cyclohexane 328.7 402.5 30422 24844 5578 27633 Toluene 333.7 411.1 29967 24324 5643 27145 Ethyl Benzoate 334.9 415.0 29859 24096 5763 26977 Decanol 333.7 412.9 29967 24218 5749 27092 CH2Cl2 334.9 411.6 29994 24295 5699 27144 Octanol 333.7 412.0 30048 24271 5777 27159 Heptanol 333.4 416.1 29913 24032 5881 26972 Hexanol 334.1 413.4 29931 24166 5765 27048 Butanol 333.4 414.3 29994 24137 5857 27065 Ethanol 332.7 414.3 30057 24137 5920 27097 Formamide 336.4 420.6 29726 23775 5951 26750 Table 4. Values of solvent parameters (ε, n) and solvent polarity parameters F1(ε, n) and F2(ε, n) of different solvents (ε and n are from Ref. [20,46]). Solvent ε n F1(ε, n) F2(ε, n) n‐Hexane 1.88 1.374 ‐0.0015 0.253 Cyclohexane 2.02 1.426 ‐0.003 0.287 Toluene 2.38 1.497 0.028 0.349 Chloroform 4.81 1.442 0.370 0.490 n‐Butyl Acetate 5.00 1.394 0.413 0.471 Ethyl Propionate 5.58 1.380 0.460 0.489 Ethyl Benzoate 5.99 1.503 0.430 0.550 Ethyl Acetate 6.08 1.372 0.492 0.499 Decanol 8.00 1.437 0.553 0.571 CH2Cl2 8.93 1.424 0.595 0.584 Octanol 9.80 1.429 0.614 0.604 Heptanol 11.30 1.424 0.652 0.618 Hexanol 13.30 1.418 0.686 0.627 Butanol 17.40 1.399 0.749 0.648 Acetone 21.01 1.359 0.792 0.640 Ethanol 24.30 1.361 0.810 0.650 Methanol 33.70 1.329 0.857 0.652 Acetonitrile 36.64 1.344 0.861 0.665 DMF 37.00 1.426 0.850 0.650 DMSO 47.24 1.497 0.841 0.744 Formamide 111.0 1.447 0.895 0.750 4. Results and discussion Absorption and emission spectra of the three coumarin dyes were measured in solvents of different dielectric constant (ε) and refractive index (n) values. The emission spectra were measured by exciting the sample at its longest absorption maximum. The observed absorption and emission maxima, wave numbers, Stokes’ shift (νa‐νf) and ½(νa+νf) of the three molecules are listed in Tables 1‐3. It can be seen that the charge transfer band shows a shift of about 20 nm in the absorption spectra on changing the solvent whereas emission spectra show larger shift as compared to the absorption. The smaller variation in the absorption shift with solvent 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 ground state rather than the excited state. The values of solvent parameters (ε, n) along with the polarity functions F1(ε, n) and F2(ε, n) are tabulated in Table 4. It can be seen that the Stokes’ shift is also large. The large Stokes’ shift is also indicative of the charge transfer transition. The large magnitude of the Stokes’ shift indicates that the excited state geometry could be different from that of the ground state. The general observation is that there is an increase in the Stokes’ shift values with increasing solvent polarity indicating that there is an increase in the dipole moment on excitation. In such cases, the relaxed excited singlet‐state S1 will be energetically stabilized relative to the ground state S0 and a significant red shift of fluorescence is observed. Figures 1 and 2 show graphs of the spectral shifts (νa‐νf) and ½(νa+νf) (in cm‐1) of 7H4TFMC with polarity functions F1(ε, n) and F2(ε, n). 78 Husain et al. / European Journal of Chemistry 3 (1) (2012) 75‐80 Table 5. Dipole moments (calculated experimentally), slopes (m1 and m2), Onsager radius and correlation coefficients of coumarin dyes*. Solute molecule m1 (cm‐1) m2 (cm‐1) Radius a (Å) μg (D) μe (D) Δμ = μe‐μg (D) μe/μg Correlation Coefficients 7H4TFMC 1959.90 3812.31 4.50 (Equation 1) 3.26 (Ab‐initio) 1.99 1.77 6.20 5.56 4.21 3.79 3.11 3.14 0.95 0.85 67DH4TFMC 1411.92 4707.94 4.60 (Equation 1) 3.28 (Ab‐initio) 4.31 3.52 8.01 6.56 3.70 3.04 1.85 1.86 0.92 0.88 7M4TFMC 351.35 1610.72 4.59 (Equation 1) 3.79 (Ab‐initio) 3.23 2.36 5.07 3.67 1.84 1.31 1.56 1.55 0.91 0.86 * m1 and m2 are the slopes of (νa‐νf) vs. F1(ε, n) and ½(νa+νf) vs. F2(ε, n), respectively; ‘a’ is the Onsager radius; μe and μg are the dipole moments in ground and excited state, respectively. Figure 1. Linear fit of (νa‐νf) vs. F1(ε, n) of 7H4TFMC in solvents listed in Table 1. Figure 2. Linear fit of ½(νa+νf) vs. F2(ε, n) of 7H4TFMC in solvents listed in Table 1. Figure 3a and 3b shows the electronic absorption (UV‐vis) and fluorescence emission spectra of 7H4TFMC in various solvents with different polarity. The slopes (m1 and m2) of the least square fit were taken to calculate the ground and excited state dipole moment of the three dyes. The slopes (m1 and m2) are displayed in Table 5. The values of m1 and m2 are then used in the Equations 7 and 8 to calculate the dipole moments (μg and μe) of all the three dyes. The dipole moments μg and μe depend not only on m1 and m2 but also on radius of the solute. The values of change in dipole moment (Δμ = μe‐μg) obtained are 4.21 D; 3.70 D and 1.84 D when the radius of the systems is calculated by Equations 11 and 3.79 D; 3.04 D and 1.31 D when radius is calculated by ab‐initio method for 7H4TFMC, 67DH4TFMC, and 7M4TFMC, respectively. The linear dependence of spectral shifts on polarity functions shows a good correlation (see Table 5). The first correlation coefficient corresponds to the graph between (νa‐νf) and F1(ε, n) and second corresponds to the graph between ½(νa+νf) and F2(ε, n), respectively. It is clear that on increasing solvent polarity, both absorption and emission bands undergo bathchromic shift (red shift). This means that there is charge transfer (CT) absorption of the less dipolar ground‐state molecule with dominant mesomeric structure, leading to highly dipolar excited state and with the prominent structure of coumarins. A bathchromic shift upon increasing the solvent polarity of the solvent indicates as π→π* transition. Further, substituents have a marked effect on the position of absorption (νa) and fluorescence maxima (νf). (a) (b) Figure 3. Absorption (a) and fluorescence (b) spectra of 7H4TFMC in different solvents. The molecular orbital calculations have also been carried out for these coumarin derivatives and the results are displayed in Tables 6‐7. Recently, D. Mitnik [48] carried out computational calculation of coumarin derivatives to show the usefulness of these molecules for photovoltaic application on the basis of HOMO‐LUMO orbitals. The highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, of a molecule are called the frontier orbitals. It was Fukui [49] who first noticed the prominent role played by HOMO and LUMO in governing chemical reactions. Husain et al. / European Journal of Chemistry 3 (1) (2012) 75‐80 79 Table 6. Energetic and electronic parameters of the coumarins at MP2/6‐31G level*. System Binding Energy (E) (kcal/mol) Dipole moment μ (D) Δμ = μe‐μg [D] HOMO (ev) LUMO (ev) ΔE 7H4TFMC ‐567382.7 9.181 (9.375) 0.194 ‐9.8679 (‐9.8834) 0.2224 (0.2037) 10.0904 67DH4TFMC ‐614337.8 9.165 (9.827) 0.662 ‐9.0252 (‐8.8415) 0.3803 (0.0087) 8.8867 7M4TFMC ‐591856.6 9.976 (10.68) 0.704 ‐9.7347 (‐9.5552) 0.4487 (0.3937) 11.2390 *The numbers in the parentheses are the values in singly excited states. Table 7. Net atomic charge on oxygen atom. System Ring O‐atom Atoms O (C=O) 6‐O (OH) 7‐O (OH) 7‐O (OCH3) 7H4TFMC ‐0.801 ‐0.610 ‐ ‐0.793 ‐ 67DH4TFMC ‐0.765 ‐0.615 ‐0.760 ‐0.757 ‐ 7M4TFMC ‐0.804 ‐0.610 ‐ ‐ ‐0.780 It has been revealed by recent investigation that the gap in energy between the HOMO and LUMO is an important stability index [50‐52]. A large gap implies high stability and small gap implies low stability. The high stability in turn indicates low chemical reactivity and small a gap indicates high chemical reactivity. It can be seen from the results that the energy gap (ΔE) between HOMO and LUMO state which are termed as chemical descriptor, is least in 67DH4TFMC suggestive of its high chemical reactivity and basic nature in comparison to other two dyes (Table 6). Because of this behavior, this molecule shows maxima in absorption spectra, in highly polar solvent. This observation correlates well with the solvent formamide in which it gives maximum λa (Tables 1‐3). The net atomic charge on O‐atom in these molecules is listed in Table 7. The carbonyl group in 67DH4TFMC is most reactive and capable of forming H‐bond complex with aprotic solvents. Earlier, the molecular orbital treatment was studied for the electronic absorption spectra of coumarins by Rafie and Bahget [53]. They used semi‐empirical method INDO for this purpose and concluded that π→π* transition band is more dominant than n→π* band. Later, N. Agmon [54] studied the proton transfer reaction in excited state for some α and β‐Napthols. 5. Conclusion The effect of protic, aprotic and non‐polar solvents, on the absorption spectra and fluorescence emission spectra of the coumarin derivatives show that the dipole moments in the singly excited singlet‐state are larger than the ground state. They all show π→π* transition than n→π* transition. These observations indicate the electrostatic and H‐bond interactions. 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