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Substituent and solvent effects on the photo-physical properties of some coumarin dyes
Spectrochimica Acta Part A 77 (2010) 337–341
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Substituent and solvent effects on the photo-physical properties of some coumarin dyes M.S. Zakerhamidi a,∗ , A. Ghanadzadeh a,b , H. Tajalli a , M. Moghadam b , M. Jassas a , R. Hosseini nia a a b
Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz, Iran Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran
a r t i c l e
i n f o
Article history: Received 4 January 2009 Accepted 18 December 2009 Keywords: Coumarin dyes Solvent–solute interactions Linear solvation energy relationships Solvent polarity scale Orientation polarizability
a b s t r a c t Absorption and fluorescence spectra of three coumarin dyes with various substituents and alkyl groups have been recorded in solvents in the range of 200–730 nm. The photo-physical behavior of dissolved dye depends on the nature of its environment, i.e. the solvatochromic behaviors of coumarin dyes and solvent/solute hydrogen bonding interactions can be analyzed by means of linear solvation energy relationships concept proposed by Kamlet and Taft. The intensity, shape, and maximum wavelength of the absorption and fluorescence band of these dyes in solution depend strongly on the solvent–solute interactions and solvent nature. Hydrogen bonding interactions (specific solute–solvent interactions) between these dye–solvent complex and dipolarity/polarizability (non-specific solute–solvent interactions) control reorientation of solvent molecules around the dye. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The coumarin dyes are important class of dyes, which are ever-present in nature, extensively studied due to their practical applications [1]. Biological and chemical sensors, fluorescent probes and laser dyes, are some of their applications [2,3]. Additionally, coumarins that contain an electron-releasing group on 7-position, and a heterocyclic electron-acceptor residue on 3position, are recognized as fluorescent dyes suitable for application to synthetic fibers [4]. Coumarin dyes are solvatochromic owing to the change in electronic distribution, resulting in an increase of dipole moments from ground state into excited state [5,6]. Strong emission of coumarins results from the polar characteristic of low-lying excited states. It is known that their immediate environment influences the electronic spectra of these dyes. Among the major environmental factors influencing the electronic spectra, solvent effects are of particular importance. Many authors have studied existing relationship between the structure of coumarin dyes and spectra (absorption and fluorescence) [6]. It is well known that the photo-physical behavior of a dissolved dye depends on the nature of its environment, i.e. the intensity, shape, and maximum absorption or emission wave-
∗ Corresponding author. Tel.: +98 411 3393003/3393027; fax: +98 411 3347050. E-mail addresses: [email protected], [email protected] (M.S. Zakerhamidi). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.12.060
length of the absorption and fluorescence band of dye in solution depends strongly on the solvent–solute interactions and solvent nature [7–10]. This effect is closely related to the nature and degree of dye–solvent interactions. The solvent-dependent spectral shifts can arise from either non-specific (dielectric enrichment) or specific solute–solvent interactions (e.g. hydrogen bonding and intermolecular charge transfer (ICT)). The solvent effect can be determined by solvent polarity scale or solvatochromic parameters [11]. Solvent polarity is a commonly used term related to the capacity of a solvent for solvating dissolved charged or neutral, apolar or dipolar, species. Attempts to express it quantitatively have mainly involved physical solvent properties such as relative permittivity, dipole moment, or refractive index, but these parameters cannot possibly account for the multitude and specific interactions of solute/solvent on the molecular-microscopic level [7]. Spectroscopic solvent polarity parameters have been derived from solvent-sensitive standard compounds absorbing or emission radiation in spectral ranges corresponding to UV/vis, fluorescence, IR, ESR, and NMR spectra [12–16]. In spite of the observation that single empirical parameters may serve as good approximations of solvent polarity in the sense defined, but there are many examples of solvent-sensitive processes known which cannot be correlated to only one empirical solvent parameter. However, multiparameter solvent polarity scale can be used for quantitative assessment of the solvent/solute interaction and the spectroscopic shifts. The effect of solvent polarity on the spectra parameter is interpreted by means of linear solvation energy
338
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341 Table 1 Coumarin dyes, structure and molecular weight. Dye
relationship (LSER) using Kamlet–Taft equation (1) [17]. = 0 + s∗ + bˇ + a˛
Molecular weight (g/mol)
Molecular structure and structural name
175/15
Coumarin440
231/30
Coumarin460
257/21
Coumarin500
3. Results and discussion (1)
where * is a measure of the solvent dipolarity/polarizability [18], ˇ is the scale of the solvent hydrogen bond acceptor (HBA) basicities [19], ˛ is the scale of the solvent hydrogen bond donor (HBD) acidities [20] and 0 is the regression value of the solute property in the reference solvent cyclohexane. The regression coefficients s, b and a in Eq. (1) measure the relative susceptibilities of the solvent-dependent solute property (absorption frequencies) to the indicated solvent parameters. In this work, three coumarin dyes, with various substituents and alkyl groups, in different solvents were studied in order to understand the specific and non-specific interactions effects of solvents and substituents on the photo-physical properties of coumarin dyes. The solvatochromic behaviors of coumarin dyes were evaluated quantitatively with linear solvation energy relationship. 2. Experimental
3.1. The solvent effect on the absorption and fluorescence spectra Solvent polarity and the local environment have profound effects on the absorption and emission spectral properties of coumarin dyes. The effects of solvent polarity on the absorption and fluorescence are one origin of Stokes’ shift. The interactions between the solvent and solute affect in the energy difference between the ground and excited states. To a primary approximation, this energy difference is a property of the refractive index (n) and dielectric constant (ε) of the solvent, and is described by the Lippert–Mataga equation, Eq. (2) [21]. The refractive index (n) is a high-frequency response and depends on the motion of electrons within the solvent molecules, which is essentially instantaneous and can occur during light absorption. In contrast, the dielectric constant (ε) is a static property, which depends on both electronic and molecular motions, the latter is connected to solvent reorganization around the excited state and may occur during light emission.
2.1. Materials The laser grade coumarin dyes (Table 1) were procured from Exciton and used without further purification. All the solvents used in the study are of highest available purity from Merck and the spectroscopic solvent polarity parameters of them are given in Table 2. 2.2. Absorption and emission spectroscopy Double beam Shimadzu UV-2450 Scan UV–Visible spectrophotometer was used to record the absorption spectra over a wavelength range 300–800 nm, which combined with a cell temperature controller. Quartz cuvettes were used for measurements in solution via 1 cm optical path length. Fluorescence of dye solutions was studied with a JASCO FP-6200 with standard quartz cuvettes. The dye concentrations were chosen as 1 × 10−6 M for each solvent.
¯ A − ¯ F =
2 hc
n2 − 1 ε−1 − 2ε + 1 2n2 + 1
(E − G )2 + Const a3
(2)
Table 2 Solvent polarity parameters. Solvent
*
ˇ
˛
f
Cyclohexane Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
0 0.62 0.49 1 0.6 0.54 0.47 1.09
0 0.48 0.37 0.76 0.66 0.75 0.84 0.47
0 0.08 0 0 0.98 0.86 0.84 1.17
0.002 0.285 0.021 0.263 0.309 0.29 0.263 0.32
f =
ε−1 2ε+1
−
n2 −1 . 2n2 +1
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341
339
Table 3 Absorption and fluorescence spectral data of coumarin dyes in various solvents. Solvent
Abs, max (nm)
Emission, max (nm)
Stokes’ shift, (cm−1 )
Coumarin 440 Cyclohexane Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
345.9 342.9 338.7 355.3 351 352.2 355 342.2
410 409 402 421 433 429 425 443
4519.846 4713.144 4649.031 4392.26 5395.34 5082.935 4639.602 6649.313
Coumarin 460 Cyclohexane Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
368.5 364.3 358.7 373.8 373 372 374 381.1
426 425 413 443 453 449 446 475
3662.864 3920.492 3665.375 4178.911 4734.596 4610.006 4316.443 5187.2
Coumarin 500 Cyclohexan Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
388.4 383.3 374.7 395.9 390 394 395 386
480 480 456 498 499 494 490 510
4913.32 5255.892 4758.193 5178.582 5600.945 5137.796 4908.292 6298.893
In this equation h is Planck’s constant, c is the speed of light, and a is the radius of the cavity in which the fluorophore resides. A and F are the wave numbers (cm−1 ) of the absorption and emission, respectively. The first parentheses in Eq. (2), is difference of two terms ((ε − 1)/(2ε + 1) and (n2 − 1)/(2n2 + 1)), the difference of these terms accounts for the spectral shifts due to reorientation of the solvent molecules (molecular motions), and hence called the orientation polarizability (f). The Lippert equation is only an approximation, but provides a useful framework for general solvent effects. The visible absorption and fluorescence spectra of the coumarin dyes (1 × 10−6 M) were obtained at room temperature in various organic solvents with different polarities (Fig. 1). The observed absorption and emission spectra of these dyes are broad which shift depending on the solvent used. Large spectral shift were observed in the emission spectra as compared to the absorption spectra on changing the solvent. The smaller shift in the absorption spectra and the large one in the emission clearly indicate that the dipole moment of the excited states is higher compared to that of ground states. The selective spectral data are also summarized in Table 3. In order to determine the solvatochromic behavior of coumarin dyes (A − F ), was plotted (Fig. 2) as a function of the polarity parameter (or orientation polarizability) f (ε, n) expressed as: f =
n2 − 1 ε−1 − 2ε + 1 2n2 + 1
Non-linear plots against the polarity parameter f (ε, n) were obtained. The non-linearity of against the polarity parameter f (ε, n) implies that this is not a valid assumption for the system under study; in other word, specific solvent effects such as hydrogen bonding exist in these systems. Coumarin dyes such as C440 and C500 have active groups for accepting and donating hydrogen bonds. Then by additional hydrogen bonding effect in protic solvents the correlations often follow two distinctive lines, one for protic and one for non-protic solvents.
Fig. 1. Absorption and fluorescence spectra of the coumarin dyes in different solvents: (a) coumarin440, (b) coumarin460 and (c) coumarin500.
In protic solvents, there is an increasing reorientation of solvents molecules around coumarin dyes molecules through hydrogen bonding. On the other hand, in protic solvents excited and ground states of these dyes stabilized (lower energy) by hydrogen bonding interactions. It is obvious that such a definition of solvent polarity cannot measure specific solvent effects by physical quantity such as the orientation polarizability (f (ε, n)). Then multiparameter solvent polarity scale can be used for the investigation of multitude of solute/solvent interactions on the molecular-microscopic level.
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M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341
Table 4 Fit regression of solvatochromic parameters for Stokes’ shift of coumarin dyes (Eq. (1)). Dye
0 (103 cm−1 )
s (103 cm−1 )
b (103 cm−1 )
a (103 cm−1 )
Ra
Fb
Coumarin440 Coumarin 460 Coumarin500
4.605 (±0.096) 3.578 (±0.117) 4.862 (±0.162)
1.335 (±0.1445) 0.674 (±0.176) 1.162 (±0.243)
−1.906 (±0.1886) −0.230 (±0.230) −1.158 (±0.318)
1.281 (±0.0905) 0.867 (±0.110) 0.658 (±0.152)
0.99 0.97 0.93
107 38 16
a b
R2 (COD). F statistic.
3.2. The multiparameter solvent polarity scale and solvatochromic behavior The effects of solvent polarity and hydrogen bonding on the coumarin dyes are interpreted by means of the linear solvation energy relationship concept using Eq. (1). The solvent parameters [7,17,22] are given in Table 2. The correlations of the spectroscopic data were carried out by means of multiple linear regression analysis. It was found that Stokes’ shifts for coumarin dyes in different solvents show satisfactory correlation with *, ˇ and ˛ parameters. The results of the multiple regressions are presented in Table 4. The negative sign of coefficient b for all coumarin dyes indicates that reorientation of solvent molecules around coumarin dyes decreases with increase in solvent hydrogen bond acceptor ability. The positive sign of s and a coefficient for all dyes points out increasing reorientation of solvent molecules with solvent hydrogen bond
Fig. 2. Plot of Stokes’ shift (cm−1 ) vs. solvent parameter f (ε, n): (a) coumarin440, (b) coumarin460 and (c) coumarin500.
Table 5 Percentage contribution of solvatochromic parameters. Dye
P* (%)
Pˇ (%)
P˛ (%)
Coumarin440 Coumarin 460 Coumarin500
30 38 39
42 13 39
28 49 22
donor acidities and dipolarity/polarizability increasing. This suggests stabilization of the excited state of dyes through hydrogen bond donor acidities and dipolarity/polarizability of solvents. The percentage contributions of solvatochromic parameters (Table 5) for coumarin dyes with electron-donating substituents in the amino group show that major solvent reorientation is due to solvent acidity and dipolarity/polarizability rather than on the solvent basicity. These results are in agreement with possible resonance structures of coumarin dyes (structures II and III, Fig. 3). In the present dyes, non-bonding electrons on the nitrogen –N(H)2 group (440), –N(CH3 )2 group (C460) and –NHC2 H5 group (C500), contribute towards the mobility of electrons on the aromatic ring. The substituent –CF3 (C500) does not produce a considerable change in the electron mobility. Upon excitation, the carbonyl group becomes strong electron donor. Also, in substrate C440 and C500, the nitrogen atoms are part of primary and secondary amino group, respectively, whereas in C460 it is part of tertiary amino group. Hence, the former can better contribute in resonance comparatively. The hydrogen bond donor solvents form strong hydrogen bonds with tertiary amino group whereas the hydrogen bond acceptor solvents form strong hydrogen bond with the primary and secondary amine. Thus the higher acidity of solvent acts more effectively on C460 compared to C440 and 500 (Table 5). In the other hand, hydrogen bonding interactions (specific solute–solvent interactions) between these dye–solvent complex and dipolarity/polarizability (non-specific solute–solvent interactions) control reorientation of solvent around the dye.
Fig. 3. Possible resonance structures of coumarin dyes.
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341
4. Conclusions These results show that the solvent effect on absorption and fluorescence spectra of investigated coumarin dyes are very complex and strongly depend on the nature of the substituent on amino group. This phenomenon is caused by the difference in the conjugational or migrating ability of electron lone pairs on nitrogen atoms and resonance structure of coumarin dyes. This also indicates that the electronic behavior of the nitrogen atom of coumarin dyes is somewhat different between derivatives with electron-donating substituents. Moreover the hydrogen bonding interactions between these dye–solvent complexes and dipolarity/polarizability control reorientation of solvent around the dye in excited state. References [1] F. Dall’Acqua, D. Vedaldi, S. Caffieri, in: H. Hönigsmann, G. Jori, A.R. Young (Eds.), The Fundamental Bases of Phototherapy, 1996, pp. 1–16. [2] R.M. Christie, H. Lui, Dyes Pigm. 42 (1999) 85. [3] N. Chandrashekharan, L. Kelly, Spectrum 15 (2002) 1. [4] J. Sokodowska, W. Czajkowski, R.aw. Podsiadly, Dyes Pigm. 49 (2001) 187.
341
[5] M. Ravi, T. Soujanya, A. Samanta, T.P. Radhakrishnan, J. Chem. Soc., Faraday Trans. 91 (1995) 2739. [6] J.R. Mannekutla, B.G. Mulimani, S.R. Inamdar, Spectrochim. Acta A 69 (2008) 419. [7] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, second edition, VCH, 1988. [8] S.E. DeBolt, P.A. Kollman, J. Am. Chem. Soc. 112 (1990) 7515. [9] T. Fox, N. Rosch, Chem. Phys. Lett. 191 (1992) 33. [10] J.E. Dubois, E. Goetz, A. Bienvenue, Spectrochim. Acta A 24 (1964) 1815. [11] M.A. Kessler, O.S. Wolfbeis, Spectrochim. Acta A 47 (1991) 187. [12] C. Reichardt, Angew. Chem. 91 (1979) 119. [13] E.M. Kosower, An Introduction to Physical Organic Chemistry, Wiley, New York, 1968, p. 293. [14] M.J. Kamlet, J.M. Abboud, M.H. Abraham, R.W. Taft, J. Org. Chem. 48 (1983) 2877. [15] J.A. Hirsch, Concepts in Theoretical Organic Chemistry, Allyn and Bacon, Boston, 1974, p. 207 (Chapter 9). [16] V. Gutmann, The Donor–Acceptor Approach to Molecular Interactions, Plenum Publ. Corp., New York, 1978. [17] M.J. Kamlet, J.M. Abboud, R.W. Taft, Prog. Phys. Org. Chem. 13 (1981) 485. [18] J.M. Abboud, M.J. Kamlet, R.W. Taft, J. Am. Chem. Soc. 99 (1977) 8325. [19] M.J. Kamlet, R.W. Taft, J. Am. Chem. Soc. 98 (1976) 377. [20] M.J. Kamlet, R.W. Taft, J. Chem. Soc., Perkin Trans. 2 (1979) 349. [21] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third edition, Springer, 2006, p. 207 (Chapter 6). [22] C. Laurence, P. Nicolet, M.T. Dalati, J.-L.M. Abboud, R. Notario, J. Phys. Chem. 98 (1994) 5807.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Substituent and solvent effects on the photo-physical properties of some coumarin dyes M.S. Zakerhamidi a,∗ , A. Ghanadzadeh a,b , H. Tajalli a , M. Moghadam b , M. Jassas a , R. Hosseini nia a a b
Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz, Iran Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran
a r t i c l e
i n f o
Article history: Received 4 January 2009 Accepted 18 December 2009 Keywords: Coumarin dyes Solvent–solute interactions Linear solvation energy relationships Solvent polarity scale Orientation polarizability
a b s t r a c t Absorption and fluorescence spectra of three coumarin dyes with various substituents and alkyl groups have been recorded in solvents in the range of 200–730 nm. The photo-physical behavior of dissolved dye depends on the nature of its environment, i.e. the solvatochromic behaviors of coumarin dyes and solvent/solute hydrogen bonding interactions can be analyzed by means of linear solvation energy relationships concept proposed by Kamlet and Taft. The intensity, shape, and maximum wavelength of the absorption and fluorescence band of these dyes in solution depend strongly on the solvent–solute interactions and solvent nature. Hydrogen bonding interactions (specific solute–solvent interactions) between these dye–solvent complex and dipolarity/polarizability (non-specific solute–solvent interactions) control reorientation of solvent molecules around the dye. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The coumarin dyes are important class of dyes, which are ever-present in nature, extensively studied due to their practical applications [1]. Biological and chemical sensors, fluorescent probes and laser dyes, are some of their applications [2,3]. Additionally, coumarins that contain an electron-releasing group on 7-position, and a heterocyclic electron-acceptor residue on 3position, are recognized as fluorescent dyes suitable for application to synthetic fibers [4]. Coumarin dyes are solvatochromic owing to the change in electronic distribution, resulting in an increase of dipole moments from ground state into excited state [5,6]. Strong emission of coumarins results from the polar characteristic of low-lying excited states. It is known that their immediate environment influences the electronic spectra of these dyes. Among the major environmental factors influencing the electronic spectra, solvent effects are of particular importance. Many authors have studied existing relationship between the structure of coumarin dyes and spectra (absorption and fluorescence) [6]. It is well known that the photo-physical behavior of a dissolved dye depends on the nature of its environment, i.e. the intensity, shape, and maximum absorption or emission wave-
∗ Corresponding author. Tel.: +98 411 3393003/3393027; fax: +98 411 3347050. E-mail addresses: [email protected], [email protected] (M.S. Zakerhamidi). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.12.060
length of the absorption and fluorescence band of dye in solution depends strongly on the solvent–solute interactions and solvent nature [7–10]. This effect is closely related to the nature and degree of dye–solvent interactions. The solvent-dependent spectral shifts can arise from either non-specific (dielectric enrichment) or specific solute–solvent interactions (e.g. hydrogen bonding and intermolecular charge transfer (ICT)). The solvent effect can be determined by solvent polarity scale or solvatochromic parameters [11]. Solvent polarity is a commonly used term related to the capacity of a solvent for solvating dissolved charged or neutral, apolar or dipolar, species. Attempts to express it quantitatively have mainly involved physical solvent properties such as relative permittivity, dipole moment, or refractive index, but these parameters cannot possibly account for the multitude and specific interactions of solute/solvent on the molecular-microscopic level [7]. Spectroscopic solvent polarity parameters have been derived from solvent-sensitive standard compounds absorbing or emission radiation in spectral ranges corresponding to UV/vis, fluorescence, IR, ESR, and NMR spectra [12–16]. In spite of the observation that single empirical parameters may serve as good approximations of solvent polarity in the sense defined, but there are many examples of solvent-sensitive processes known which cannot be correlated to only one empirical solvent parameter. However, multiparameter solvent polarity scale can be used for quantitative assessment of the solvent/solute interaction and the spectroscopic shifts. The effect of solvent polarity on the spectra parameter is interpreted by means of linear solvation energy
338
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341 Table 1 Coumarin dyes, structure and molecular weight. Dye
relationship (LSER) using Kamlet–Taft equation (1) [17]. = 0 + s∗ + bˇ + a˛
Molecular weight (g/mol)
Molecular structure and structural name
175/15
Coumarin440
231/30
Coumarin460
257/21
Coumarin500
3. Results and discussion (1)
where * is a measure of the solvent dipolarity/polarizability [18], ˇ is the scale of the solvent hydrogen bond acceptor (HBA) basicities [19], ˛ is the scale of the solvent hydrogen bond donor (HBD) acidities [20] and 0 is the regression value of the solute property in the reference solvent cyclohexane. The regression coefficients s, b and a in Eq. (1) measure the relative susceptibilities of the solvent-dependent solute property (absorption frequencies) to the indicated solvent parameters. In this work, three coumarin dyes, with various substituents and alkyl groups, in different solvents were studied in order to understand the specific and non-specific interactions effects of solvents and substituents on the photo-physical properties of coumarin dyes. The solvatochromic behaviors of coumarin dyes were evaluated quantitatively with linear solvation energy relationship. 2. Experimental
3.1. The solvent effect on the absorption and fluorescence spectra Solvent polarity and the local environment have profound effects on the absorption and emission spectral properties of coumarin dyes. The effects of solvent polarity on the absorption and fluorescence are one origin of Stokes’ shift. The interactions between the solvent and solute affect in the energy difference between the ground and excited states. To a primary approximation, this energy difference is a property of the refractive index (n) and dielectric constant (ε) of the solvent, and is described by the Lippert–Mataga equation, Eq. (2) [21]. The refractive index (n) is a high-frequency response and depends on the motion of electrons within the solvent molecules, which is essentially instantaneous and can occur during light absorption. In contrast, the dielectric constant (ε) is a static property, which depends on both electronic and molecular motions, the latter is connected to solvent reorganization around the excited state and may occur during light emission.
2.1. Materials The laser grade coumarin dyes (Table 1) were procured from Exciton and used without further purification. All the solvents used in the study are of highest available purity from Merck and the spectroscopic solvent polarity parameters of them are given in Table 2. 2.2. Absorption and emission spectroscopy Double beam Shimadzu UV-2450 Scan UV–Visible spectrophotometer was used to record the absorption spectra over a wavelength range 300–800 nm, which combined with a cell temperature controller. Quartz cuvettes were used for measurements in solution via 1 cm optical path length. Fluorescence of dye solutions was studied with a JASCO FP-6200 with standard quartz cuvettes. The dye concentrations were chosen as 1 × 10−6 M for each solvent.
¯ A − ¯ F =
2 hc
n2 − 1 ε−1 − 2ε + 1 2n2 + 1
(E − G )2 + Const a3
(2)
Table 2 Solvent polarity parameters. Solvent
*
ˇ
˛
f
Cyclohexane Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
0 0.62 0.49 1 0.6 0.54 0.47 1.09
0 0.48 0.37 0.76 0.66 0.75 0.84 0.47
0 0.08 0 0 0.98 0.86 0.84 1.17
0.002 0.285 0.021 0.263 0.309 0.29 0.263 0.32
f =
ε−1 2ε+1
−
n2 −1 . 2n2 +1
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341
339
Table 3 Absorption and fluorescence spectral data of coumarin dyes in various solvents. Solvent
Abs, max (nm)
Emission, max (nm)
Stokes’ shift, (cm−1 )
Coumarin 440 Cyclohexane Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
345.9 342.9 338.7 355.3 351 352.2 355 342.2
410 409 402 421 433 429 425 443
4519.846 4713.144 4649.031 4392.26 5395.34 5082.935 4639.602 6649.313
Coumarin 460 Cyclohexane Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
368.5 364.3 358.7 373.8 373 372 374 381.1
426 425 413 443 453 449 446 475
3662.864 3920.492 3665.375 4178.911 4734.596 4610.006 4316.443 5187.2
Coumarin 500 Cyclohexan Acetone 1,4-Dioxane DMSO Methanol Ethanol 1-Butanol Water
388.4 383.3 374.7 395.9 390 394 395 386
480 480 456 498 499 494 490 510
4913.32 5255.892 4758.193 5178.582 5600.945 5137.796 4908.292 6298.893
In this equation h is Planck’s constant, c is the speed of light, and a is the radius of the cavity in which the fluorophore resides. A and F are the wave numbers (cm−1 ) of the absorption and emission, respectively. The first parentheses in Eq. (2), is difference of two terms ((ε − 1)/(2ε + 1) and (n2 − 1)/(2n2 + 1)), the difference of these terms accounts for the spectral shifts due to reorientation of the solvent molecules (molecular motions), and hence called the orientation polarizability (f). The Lippert equation is only an approximation, but provides a useful framework for general solvent effects. The visible absorption and fluorescence spectra of the coumarin dyes (1 × 10−6 M) were obtained at room temperature in various organic solvents with different polarities (Fig. 1). The observed absorption and emission spectra of these dyes are broad which shift depending on the solvent used. Large spectral shift were observed in the emission spectra as compared to the absorption spectra on changing the solvent. The smaller shift in the absorption spectra and the large one in the emission clearly indicate that the dipole moment of the excited states is higher compared to that of ground states. The selective spectral data are also summarized in Table 3. In order to determine the solvatochromic behavior of coumarin dyes (A − F ), was plotted (Fig. 2) as a function of the polarity parameter (or orientation polarizability) f (ε, n) expressed as: f =
n2 − 1 ε−1 − 2ε + 1 2n2 + 1
Non-linear plots against the polarity parameter f (ε, n) were obtained. The non-linearity of against the polarity parameter f (ε, n) implies that this is not a valid assumption for the system under study; in other word, specific solvent effects such as hydrogen bonding exist in these systems. Coumarin dyes such as C440 and C500 have active groups for accepting and donating hydrogen bonds. Then by additional hydrogen bonding effect in protic solvents the correlations often follow two distinctive lines, one for protic and one for non-protic solvents.
Fig. 1. Absorption and fluorescence spectra of the coumarin dyes in different solvents: (a) coumarin440, (b) coumarin460 and (c) coumarin500.
In protic solvents, there is an increasing reorientation of solvents molecules around coumarin dyes molecules through hydrogen bonding. On the other hand, in protic solvents excited and ground states of these dyes stabilized (lower energy) by hydrogen bonding interactions. It is obvious that such a definition of solvent polarity cannot measure specific solvent effects by physical quantity such as the orientation polarizability (f (ε, n)). Then multiparameter solvent polarity scale can be used for the investigation of multitude of solute/solvent interactions on the molecular-microscopic level.
340
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341
Table 4 Fit regression of solvatochromic parameters for Stokes’ shift of coumarin dyes (Eq. (1)). Dye
0 (103 cm−1 )
s (103 cm−1 )
b (103 cm−1 )
a (103 cm−1 )
Ra
Fb
Coumarin440 Coumarin 460 Coumarin500
4.605 (±0.096) 3.578 (±0.117) 4.862 (±0.162)
1.335 (±0.1445) 0.674 (±0.176) 1.162 (±0.243)
−1.906 (±0.1886) −0.230 (±0.230) −1.158 (±0.318)
1.281 (±0.0905) 0.867 (±0.110) 0.658 (±0.152)
0.99 0.97 0.93
107 38 16
a b
R2 (COD). F statistic.
3.2. The multiparameter solvent polarity scale and solvatochromic behavior The effects of solvent polarity and hydrogen bonding on the coumarin dyes are interpreted by means of the linear solvation energy relationship concept using Eq. (1). The solvent parameters [7,17,22] are given in Table 2. The correlations of the spectroscopic data were carried out by means of multiple linear regression analysis. It was found that Stokes’ shifts for coumarin dyes in different solvents show satisfactory correlation with *, ˇ and ˛ parameters. The results of the multiple regressions are presented in Table 4. The negative sign of coefficient b for all coumarin dyes indicates that reorientation of solvent molecules around coumarin dyes decreases with increase in solvent hydrogen bond acceptor ability. The positive sign of s and a coefficient for all dyes points out increasing reorientation of solvent molecules with solvent hydrogen bond
Fig. 2. Plot of Stokes’ shift (cm−1 ) vs. solvent parameter f (ε, n): (a) coumarin440, (b) coumarin460 and (c) coumarin500.
Table 5 Percentage contribution of solvatochromic parameters. Dye
P* (%)
Pˇ (%)
P˛ (%)
Coumarin440 Coumarin 460 Coumarin500
30 38 39
42 13 39
28 49 22
donor acidities and dipolarity/polarizability increasing. This suggests stabilization of the excited state of dyes through hydrogen bond donor acidities and dipolarity/polarizability of solvents. The percentage contributions of solvatochromic parameters (Table 5) for coumarin dyes with electron-donating substituents in the amino group show that major solvent reorientation is due to solvent acidity and dipolarity/polarizability rather than on the solvent basicity. These results are in agreement with possible resonance structures of coumarin dyes (structures II and III, Fig. 3). In the present dyes, non-bonding electrons on the nitrogen –N(H)2 group (440), –N(CH3 )2 group (C460) and –NHC2 H5 group (C500), contribute towards the mobility of electrons on the aromatic ring. The substituent –CF3 (C500) does not produce a considerable change in the electron mobility. Upon excitation, the carbonyl group becomes strong electron donor. Also, in substrate C440 and C500, the nitrogen atoms are part of primary and secondary amino group, respectively, whereas in C460 it is part of tertiary amino group. Hence, the former can better contribute in resonance comparatively. The hydrogen bond donor solvents form strong hydrogen bonds with tertiary amino group whereas the hydrogen bond acceptor solvents form strong hydrogen bond with the primary and secondary amine. Thus the higher acidity of solvent acts more effectively on C460 compared to C440 and 500 (Table 5). In the other hand, hydrogen bonding interactions (specific solute–solvent interactions) between these dye–solvent complex and dipolarity/polarizability (non-specific solute–solvent interactions) control reorientation of solvent around the dye.
Fig. 3. Possible resonance structures of coumarin dyes.
M.S. Zakerhamidi et al. / Spectrochimica Acta Part A 77 (2010) 337–341
4. Conclusions These results show that the solvent effect on absorption and fluorescence spectra of investigated coumarin dyes are very complex and strongly depend on the nature of the substituent on amino group. This phenomenon is caused by the difference in the conjugational or migrating ability of electron lone pairs on nitrogen atoms and resonance structure of coumarin dyes. This also indicates that the electronic behavior of the nitrogen atom of coumarin dyes is somewhat different between derivatives with electron-donating substituents. Moreover the hydrogen bonding interactions between these dye–solvent complexes and dipolarity/polarizability control reorientation of solvent around the dye in excited state. References [1] F. Dall’Acqua, D. Vedaldi, S. Caffieri, in: H. Hönigsmann, G. Jori, A.R. Young (Eds.), The Fundamental Bases of Phototherapy, 1996, pp. 1–16. [2] R.M. Christie, H. Lui, Dyes Pigm. 42 (1999) 85. [3] N. Chandrashekharan, L. Kelly, Spectrum 15 (2002) 1. [4] J. Sokodowska, W. Czajkowski, R.aw. Podsiadly, Dyes Pigm. 49 (2001) 187.
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