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Solvatochromic fluorescence characteristics of cinnamoyl pyrone derivatives
Journal of Molecular Structure 1149 (2017) 1e7
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Solvatochromic fluorescence characteristics of cinnamoyl pyrone derivatives Nadjib Benosmane a, b, *, Baya Boutemeur b, Safouane M. Hamdi c, Maamar Hamdi b, Artur S.M. Silva d a
Department of Chemistry, Faculty of Sciences, University M'Hamed Bougara, Boumerdes (UMBB), Avenue of Independence, 35000, Algeria Laboratory of Applied Organic Chemistry (Group Heterocycles), Faculty of Chemistry, University of Sciences and Technology, BP32, El-Alia, 16111, Bab-Ezzouar, Algiers, Algeria c Department of Clinical Biochemistry, CHU Toulouse, University of Toulouse, Toulouse, France d Department of Chemistry & QOPNA, University of Aveiro, 3810-193, Aveiro, Portugal b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2016 Received in revised form 15 July 2017 Accepted 26 July 2017 Available online 27 July 2017
The solvatochromic fluorescence behavior of cinnamoyl pyrone derivatives has been studied in several polar and non-polar solvents. The fluorescence spectra of these compounds exhibit red shift from its absorption spectra and present an excellent correlation with solvent polarity. Cinnamoyl pyrones show a significant spectral shift in fluorescence emission as a function of water composition in binary aqueous solutions mixture. This change is due to the specific intermolecular hydrogen bonding of cinnamoyl pyrones with a molecules of water, due to the deactivation of the lowest excited singlet state of these compounds. The relative quantum yields are calculated. It is found that the quantum yields of the cinnamoyl pyrones vary with the change in the solvent polarity indicating the dependency of fluorescence properties on the solvent nature. It has been observed that the addition of water and pH medium can affect the fluorescence properties of cinnamoyl pyrones in ethanol. This study exhibited that due to the solvent sensitive emission, cinnamoyl pyrone derivatives are a good compound to be used as fluorescence probes. © 2017 Elsevier B.V. All rights reserved.
Keywords: Cinnamoyl pyrones Solvatochromic effect Molecular spectroscopy Fluorescence emission properties Fluorescence quantum yield Hydrogen bonding interaction
1. Introduction Fluorescence sensors have been receiving a considerable attention due to their potential applications in biochemical and medical analyses and optoelectronic devices [1e3]. It is known that the spectral behavior of an organic molecule is strongly related to its structure in both ground and excited states. The knowledge of the solvent effect on the absorption and fluorescence spectra is of particular importance. A change in the nature of solvent is accompanied by a change in polarity, dielectric constant and polarizability of the surrounding medium. Thus, the change of solvents from apolar to polar aprotic, or to polar protic affects the ground state and the excited state differently. The excited state dipole moment of a molecule control the tenability range of their emission energy as a function of solvent
* Corresponding author. Department of Chemistry, Faculty of Sciences, University M'Hamed Bougara, Boumerdes (UMBB), Avenue of Independence, 35000, Algeria. E-mail address: [email protected] (N. Benosmane). http://dx.doi.org/10.1016/j.molstruc.2017.07.089 0022-2860/© 2017 Elsevier B.V. All rights reserved.
polarity and can be useful to optimize the performance of a laser dye [4]. Solvatochromism, which describes the spectral change of solute induced by changes in solvent polarity, provides a convenient way for the monitoring of interactions between solute and solvent, including specific (such as hydrogen bonding and electron donor-acceptor interactions) and non-specific (polaritypolarizability effects) interactions [5]. Cinnamoyl pyrone derivatives were intensively investigated as photostable emitting dyes for fluorescent polymer compositions which permit to create new emitting functional materials based on polymer compositions [6]. Such molecules were proposed as sensors for the examination of medium polarity in biological and medical investigations [7]. The goal of the present work is to report the influence of chemical and physical environments, such as the effect of the substituent (nature and position), solvent, and pH medium, on the absorption and fluorescence spectra and fluorescence quantum yield (f) of some cinnamoyl pyrone derivatives. The study has been performed in various non-polar, polar aprotic and polar protic solvents to monitor general as well as specific solvent effects.
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N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
Scheme 1.
Fig. 1. UVevisible absorption spectra of some cinnamoyl pyrone derivatives B1, B10eB13 in acetonitrile (ACN) and dichloromethane (DCM) (c ¼ 6.72 106 M).
2. Experimental 2.1. Materials and methods A selection of cinnamoyl pyrone derivatives were synthesized according to literature [8] and their structure depicted in Scheme 1. All chemicals and solvents used for synthesis and spectroscopic measurements were of analytical reagent grade and obtained from Fluka and used without further purification. The absorption and fluorescence spectra were recorded at room temperature and using a 1 cm quartz cuvette on, respectively, a JENWAY-6800 UVevisible Spectrophotometer and a Jasco-FP 8200 Fluorometer. Excitation and emission slit width was fixed to 10 nm. The excitation source was a long-life xenon flash lamp and excitation
wavelengths were set on that of maximum absorption of each cinnamoyl pyrone derivatives. The method of pH measurements in non-aqueous media was based on the work of Rondinini [9]. All pH values were recorded on a multifunctional pH-meter (HANNA HI 2550). The fluorescence quantum yield (f) was measured relative to rhodamine B as standard (f ¼ 0.65) [10], according to the following Eq. (1)
f¼
fstd FAstd h2 Fstd Ah2std
(1)
where F and Fstd are the peak areas of sample and standard solutions, respectively; A and Astd are the absorption at excitation wavelength of sample and standard, respectively; h and hstd are the
Fig. 2. Fluorescence emission spectra of cinnamoyl pyrone derivatives in acetonitrile (ACN) and dichloromethane (DCM) (c ¼ 6.72 106 M).
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
3
Table 1 Spectroscopic characteristics of B1eB13 in acetonitrile (ACN) and dichloromethane (DCM) as solvents. Compounds
Solvent
labs (nm)
lex (nm)
lem (nm)
Stokes shift (na nf )(cm1)
B1
ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM
356 358 373 377 380 384 359 360 390 387 367 374 357 360 353 355 349 350 352 365 358 361 463 469 476 480
380
483 466 500 490 488 487 510 477 518 493 523 491 497 478 481 469 473 472 480 471 490 480 561 573 562 575
7387 6475 6808 6115 5824 5506 8249 6812 6336 5556 8126 6370 7889 6859 7540 6848 7510 7385 7576 6167 7526 6869 3774 3871 3222 3441
B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13
380 380 380 380 380 380 380 380 350 350 460 460
Fig. 3. a) UveVisible absorption spectra of cinnamoyl pyrone B13 in various solvents (c ¼ 6.72 106 M); b) Fluorescence emission spectra of cinnamoyl pyrone B13 in various solvents (c ¼ 6.72 106 M) (lex ¼ 460 nm).
refractive index of sample and standard solutions, respectively; f and fstd are the quantum yield of sample and standard solutions respectively.
Fmax 490 nm (Table 1). Substitutions on the aromatic ring led to wavelength shifts in the absorption and emission maxima (Figs. 1 and 2) and affected fluorescence emission efficiency.
3. Results and discussions 3.1. Absorption and emission characteristics of cinnamoyl pyrones B1eB13 The absorption and emission spectra of compounds B1-13 in dichloromethane (DCM) and acetonitrile (ACN) are presented in Figs. 1 and 2. The absorption maxima were observed at 356 nm for B7eB9 in acetonitrile. The fluorescence maximum (Fmax) was observed in the range: 470e496 nm for B7eB9 with a significant Stokes shift varying between 7511 and 7890 cm1. Emissions of the five cinnamoyl pyrone derivatives B2eB6 were very similar with
Table 2 Spectral maxima and photophysical parameters of B13 in different solvents calculated from Fig. 3a and b. Solvent
Df
labs (nm)
lem (nm)
Dn (cm1)
Toluene Acetate ethyl Ethanol Acetonitrile Dichloromethane n-propanol
0.0150 0.1995 0.2886 0.3054 0.4179 0.4619
471 472 488 476 480 468
534 545 570 562 575 562
2506 2839 2949 3222 3441 3590
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N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
Table 3 Fluorescence quantum yield (f) of B12 and B13 in different solvents. Compound
Solvent
(ε) at 20 C
(n)
m(D) of solvent
F
Rhodamine B B12
Ethanol Acetonitrile
24.5 36.8
1.3614 1.3284
1.69 3.45
n-Propanol Ethyl acetate Toluene Dichloromethane
20.65 6.19 2.39 8.90
1.3856 1.3724 1.4969 1.4244
3.09 1.88 0.43 1.58
0.65 [9] 0.018 (0.017 Lit [7]) 0.067 0.342 0.428 0.538 (0.546 Lit [7]) 0.019 0.152 0.489 0.491 0.687
B13
Acetonitrile n-Propanol Ethyl acetate Toluene Dichloromethane
The solvent used in the acquisition of the fluorescence spectra of cinnamoyl pyrone had an influence on the position and intensity of the corresponding bands. The absorption spectra of B13 in solvents of different polarity exhibited a very similar shape around 475 nm. However, its fluorescence emission spectrum exhibited a different shape according to the solvent (Fig. 3a and b). Since polar solvents induce fluorescence shift towards longer wavelengths, the dipole moment of B13 is higher in the excited state than in the ground state. This class of cinnamoyl pyrone derivatives displayed a very favorable fluorescence profile. The solvent dependent spectral shifts were also verified in terms of the simplified LippertMataga Eq. (2) [11e13]. This equation describes the Stokes shift in terms of the change in dipole moment of the fluorophore upon excitation and the dependence of the energy of the dipole on the dielectric constant and refractive index of the solvent as given by Eq. (2):
Dn ¼ Dn abs Dn em ¼
Df ¼
ε1 h2 1 2ε þ 1 2h2 þ 1
ms mg hca2
a twisted intramolecular charge transfer (ICT) state that should be a nearly full charge transfer [15]. 3.2. Effect of nature and position of substituent on the absorption and emission spectra of cinnamoyl pyrones B1eB13 In general substitution of a certain molecule with electron donating groups causes an increase of the molar absorption coefficient and a bathochromic shift of their absorption and emission spectra. It was the case of the compounds B2eB6 (Table 1). In the case of hydroxy substituents there is an ICT, confirmed by a wide and unstructured fluorescence spectra. Whereas for the nitrosubstituted compounds B10 and B11 there is a weak level of fluorescence due to an ICT that induces both efficient state crossing from the lowest excited state n-p* and S1eS0 interconversion. 3.3. Effect of addition of water on absorption and fluorescence emission spectra of cinnamoyl pyrone B13
2 $Df þ Const
(2)
(3)
where Dn is the Stokes shift in cm1 (Table 2), h ¼ 6.6260 1034 J s is Planck's constant, c ¼ 2.9979 108 m s1 is the velocity of light, (a) is Onsager cavity radius of the solute molecule being calculated from the molecule volume according to Suppan's equation [14], 1 3 a ¼ 43M , where d is the density of the solute molecule, M is the pdN
In order to study the role of the water in the absorption and fluorescence emission of cinnamoyl pyrone B13 these spectra were acquired in aqueous solutions with different percentages of water (0, 20, 40, 60 and 80%). As shown in Fig. 5a the absorption spectrum of B13 undergo only small changes in intensity and shape by increasing water content in the aqueous-ethanol solutions in a similar concentration of cinnamoyl pyrone B13, but with higher content of water 80% (V/V) the absorption intensity was drastically
=
molecular weight of solute, and N is Avogadro's number (here, a ¼ 4.704 Å), ε and h are the dielectric constant and refractive index of the solvent respectively, ms and mg is the dipole moment of molecule at excited and ground states, respectively. For this study we choose the cinnamoyl pyrone derivative B13 for which the photophysical properties are displayed on Table 2. The plot of Stokes shift (Dn) versus the term (Df ) of the polarizability orientation of the solvent are shown in Fig. 4. We found a good correlation coefficient (R2 ¼ 0.9366) in excited state. Thus, the dielectric solute-solvent interactions are responsible for the observed solvatochromic shift of the cinnamoyl pyrone derivative B13. In order to determine some quantitative characteristics of solvatochromic effects observed for a compound B13, we calculated Dm ¼ ðms mg Þ using the Lippert-Mataga equation and found Dm ¼ 1:03 D. Thus, the excited state dipole moment is only 1.03 D bigger than that of the ground state. This difference is too small for
Fig. 4. Plotting Stokes shifts versus Df of B13 in various solvents.
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
5
Fig. 5. a) Absorption spectra of B13 (c ¼ 6.72 106 M) in aqueous ethanol solutions containing different water contents (0, 20, 40, 60, 80%) (V/V); b) Fluorescence spectra of B13 (c ¼ 6.72 106 M) in aqueous ethanol solutions containing different water contents (0, 20, 40, 60, 80%) (V/V).
decreased. On the other hand, the corresponding fluorescence spectra exhibited significant changes in the spectral position and intensity of shape with water content (Fig. 5b). The spectra of solutions with water content between 20 and 60% shows a fluorescence shift to longer wavelengths, but when the water content is equal to 80% the fluorescence band was shifted to shorter wavelengths. In the term of the fluorescence intensity, the decrease was dramatic for a water proportion between 20 and 80% (Fig. 6). This phenomenon is typical of a specific water effect and is possibly due to the formation of hydrogen bonding between water molecules and the extra-cyclic amino group of B13. The fluorescence emission features of B13 are assumed to be linked to the formation of intermolecular hydrogen bonds with water molecules and the formation of mono and/or polyhydrate species. Moreover, the presence of water may induce the deactivation of the lowest excited singlet state of the probe through the hydrogen-bonded interactions [3]. The hydrogen bonding interaction is also responsible for the reduction in the absorption intensity on going from alcoholic media to water, because hydrogen bonding in pyrone ring reduces the resonance of the aromatics cycles. Solvent effect was treated within the framework of the linear solvation energy
relationships (LSER) established by Kamlet [16], in which each of specific and non specific interactions have a linear contribution to the total solvation energy of solvent. For non chloro-substituted aliphatic solvents, LSER takes the form of Eq. (4). ET ¼ A0 þ sp* þ aa þ bb
(4)
WhereET: the molar electronic transition energy in KCal mol1
ET K Cal$mol1 ¼
hcNA
lmax ðnmÞ
¼
28591:5
lmax ðnmÞ
h, c and NA are planck's constant, the velocity of light and Avogadro's number, respectively. The values of ET as a function of mole fraction of ethanol solvent are presented in Table 4. The a, b and p* are the Kamlet and Taft solvatochromic parameters (KAT) which have been developed for scaling the hydrogen-bond donor acidity, hydrogen-bond acceptor basicity and dipolarity/polarizability of solvent respectively [17e19]. The A0, a, b and s quantify the sensitivity of ET values to the acidity, basicity and dipolarity/polarizability of solvent respectively. The parameters of Eq. (4) depend physically on the nature of solute and solvent and the signification of the solute-solvent interactions. The KAT parameters for binary mixtures were extracted from the literature [20].
Table 4 The ET values of B13 and KAT parameters in different mole fraction of ethanol solvent (X2). Water-Ethanol X2
ET
p*
a
b
1 0.80 0.60 0.40 0.20
58.469 57.297 56.172 55.302 54.150
0.54 0.65 0.75 0.88 1.10
0.86 0.92 0.96 0.96 1.00
0.75 0.70 0.64 0.67 0.60
Table 5 Linear correlations obtained by plotting ET versus p*, a and b according to the Taft and Kamlet equation (r ¼ correlation coefficient). Fig. 6. Fluorescence intensity of B13 (c ¼ 6.72 106 M) in aqueous ethanol solutions containing different water contents (0, 20, 40, 60, 80%) (V/V).
Compound
A0
s
a
b
r
B13
61.639
4.652
2.740
0.389
0.995
6
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
Fig. 7. a) pH dependence of absorption spectra of B13 (c ¼ 6.72 106 M) in ethanol solutions; b) pH dependence of fluorescence spectra (lex ¼ 460 nm) of B13 (c ¼ 6.72 106 M) in ethanol solutions.
The values of Cinnamoyl pyrones for each aqueous solution were correlated with solvent properties by means of the multiple linear regressions analysis in Origin. 8 (Microcal ™ Software). Fitting results obtained from multiple linear regressions analysis are shown in Table 5. In aqueous binary mixtures, a dual-parametric KAT equation containing a and p* shows the best fit. We observed that the sign of a and p* coefficient is negative, consistent with the compensation of the negative polarity contribution to the spectral shift of cinnamoyl pyrone B13 by a strong positive contribution of hydrogen bonding interaction of cinnamoyl pyrone with solvent mixture, especially water molecules. 3.4. Effect of pH on absorption and fluorescence emission intensity of cinnamoyl pyrone B13 The absorption spectra of B13 were determined in ethanol of different pH values by addition a few gouts of acidic or basic aqueous solution (HCl, NaOH in water) (Fig. 7a). In an acid medium (pH range from 2 to 6), a strong absorption was observed at 488 nm. Increasing pH values from 8 to 12, the shapes are shifted to shorter wavelength at 402 nm and then the absorption intensity dramatically decreased. As shown in Fig. 7b, the fluorescence emission intensity was maximum in alkaline medium and decreased in acid conditions, thus one can consider B13 as a non-fluorescent compound at low pH. The spectral shifts accompanying protonation or dissociation of basic or acidic functional groups depend on whether the functional group undergoing protonation or dissociation is directly coupled to the aromatic system and whether it gains or losses of electronic charge upon going from the ground to the excited state. Since, high fluorescence intensity was observed at pH 8 [3]. 3.5. Fluorescence quantum yield of B12 and B13 The fluorescence quantum yields (f) of B12 and B13 were determined by excitation at lex 460 nm (Table 3) and they ranged between 0.018 and 0.687 (spectral maxima and photophysical parameters of B12 and B13 in different solvents are shown in Table 3). Fluorescence quantum yields of B12 and B13 showed slight variation with the change in solvent polarity indicating a negative and
positive solvatokinetic effect. The compounds are called solvatochromic when the position of their absorption spectrum and emission depends on of solvent polarity. Displacements to the longer wavelengths (bathochromic shift) or towards the shorter wavelengths (hypsochromic shift) when the polarity of the solvent increases are called respectively positive or negative solvatochromism. In other words, when a compound is surrounded by molecules of the solvent at ground state, the excited state are stabilized by solute-solvent interactions (van der Waals type and/or hydrogen bonding). The significant negative solvatochromism is explained by the high dipole moment in the ground state and almost zero in the excited state because of an intramolecular electron transfer. For negative solvatokinetic effect, quantum yield increase with a suitable enhancement of intramolecular charge transfer (ICT) which involves a n,p* electron configuration while the reduction in quantum yield by strong ICT is called positive solvatokinetic effect [7]. 4. Conclusion The aim of this work was the synthesis and the investigation of solvatochromic fluorescence characteristics of the cinnamoyl pyrone derivatives. The UVevisible absorption and fluorescence spectra, fluorescence quantum yield were determined in several solvents. It has been found that the character and position of the substituent on the aromatic ring influence the position and shape of absorption and fluorescence spectra. However, a strong effect of pH medium and addition of water on the fluorescence intensity of compound has been found. It has been observed that the quantum yield of the molecule varies with the change in the solvent polarity. References [1] A.W. Czarnik, Chemical communication in water using fluorescent chemosensors, Acc. Chem. Res. 27 (1994) 302e308. [2] A.P. de Silvan, T.P. vance, M.E.S. West, G.D. Wright, Bright molecules with sense, logic, numeracy and utility, Org. Biomol. Chem. 6 (2008) 2468e2481. [3] V.D. Suryawanshi, A.H. Gore, L.S. Walekar, P.V. Anbhule, S.R. Patil, G.B. Kolekar, Solvatochromic fluorescence behavior of 2-amino-6-hydroxy-4-(3,4dimethoxyphenyl)-pyrimidine-5-carbonitrile: a sensitive fluorescent probe for detection of pH and water composition in binary aqueous solutions, J. Mol. Liq. 184 (2013) 4e9. [4] R. Gahlaut, N. Tewari, J.P. Bridhkoti, N.K. Joshi, H.C. Joshi, S. Pant, Determination of ground and excited state dipole moments of some naphthols using
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7 solvatochromic shift method, J. Mol. Liq. 163 (2011) 141e146. [5] F. Naderi, A. Farajtabar, F. Gharib, Solvatochromic and preferential solvation of fluorescein in some water-alcoholic mixed solvents, J. Mol. Liq. 190 (2014) 126e132. [6] D.O. Tykhanov, E.V. Sanin, O.V. Grygorovych, I.I. Serikova, F.G. Yaremenko, A.D. Roshal, Funct. Mater. 18 (3) (2011) 339e347. [7] D.A. Tykhanov, I.I. Serikova, F.G. Yaremznko, A.D. Roshal, Structure and spectal properties of cinnamoyl pyrones and their vinilogs, Cent. Eur. J. Chem. 8 (2) (2010) 347e355. [8] R.H. Wiley, C.H. Jarbone, H.G. Ellert, J. Am. Chem. Soc. 77 (1955) 5102e5105. [9] S. Rondinini, P.R. Mussini, T. Mussini, Reference value standards and primary standards for pH measurements in organic solvents and water þ organic solvent mixtures of moderates to high permittivities, Pure Appl. Chem. 59 (1987) 1549e1560. [10] R.F. Kubin, A.N. Fletcher, Fluorescence quantum yields of some rhodamine dyes, J. Lumin. 27 (1982) 455e462. [11] E.Z. Lippert, Dipole moment and electronic structure of excited molecules, Electrochem 61 (1957) 962. [12] N. Mataga, T. Kubota, Molecular Interaction and Electronic Spectra, Marcel Dekker, New York, 1970. [13] F. Ito, T. Nagai, Y. Ono, K. Yamaguchi, H. Furuta, T. Nagamura, Photophysical
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[16] [17] [18]
[19]
[20]
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properties of 2-picolinoylpyrrole boron complex in solutions, Chem. Phys. Lett. 435 (2007) 283e288. P. Suppan, Excited-state dipole moments from absorption/fluorescence solvatochromic ratios, Chem. Phys. Lett. 94 (1983) 272e275. A. Kawski, P. Bojarsky, B. Kuklinski, Estimation of ground- and excited state dipole moments of nile red dye from solvatochromic effect on absorption and fluorescence spectra, Chem. Phys. Lett. 463 (2008) 410e412. R. Taft, J.-L. Abboud, M. Kamlet, M. Abraham, Linear solvation energy relations, J. Solut. Chem. 14 (1985) 153e186. M.J. Kamlet, J.-L. Abboud, R.W. Taft, The solvatochromic comparison method. 6. The p* scale of solvent polarities, J. Am. Chem. Soc. 99 (1977) 6027e6038. R.W. Taft, M.J. Kamlet, The solvatochromic comparison method. 2. The a scale of solvent hydrogen-bond donor (HBD) acidities, J. Am. Chem. Soc. 98 (1976) 2886e2894. M.J. Kamlet, R.W. Taft, The solvatochromic comparison method. I. The b-scale of solvent hydrogen-bond acceptor (HBA) basicities, J. Am. Chem. Soc. 98 (1976) 377e383. Y. Marcus, The use of chemical probes for the characterization of solvent mixtures. Part 2. Aqueous mixtures, J. Chem. Soc. Perkin Trans. 2 (1994) 1751e1758.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Solvatochromic fluorescence characteristics of cinnamoyl pyrone derivatives Nadjib Benosmane a, b, *, Baya Boutemeur b, Safouane M. Hamdi c, Maamar Hamdi b, Artur S.M. Silva d a
Department of Chemistry, Faculty of Sciences, University M'Hamed Bougara, Boumerdes (UMBB), Avenue of Independence, 35000, Algeria Laboratory of Applied Organic Chemistry (Group Heterocycles), Faculty of Chemistry, University of Sciences and Technology, BP32, El-Alia, 16111, Bab-Ezzouar, Algiers, Algeria c Department of Clinical Biochemistry, CHU Toulouse, University of Toulouse, Toulouse, France d Department of Chemistry & QOPNA, University of Aveiro, 3810-193, Aveiro, Portugal b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2016 Received in revised form 15 July 2017 Accepted 26 July 2017 Available online 27 July 2017
The solvatochromic fluorescence behavior of cinnamoyl pyrone derivatives has been studied in several polar and non-polar solvents. The fluorescence spectra of these compounds exhibit red shift from its absorption spectra and present an excellent correlation with solvent polarity. Cinnamoyl pyrones show a significant spectral shift in fluorescence emission as a function of water composition in binary aqueous solutions mixture. This change is due to the specific intermolecular hydrogen bonding of cinnamoyl pyrones with a molecules of water, due to the deactivation of the lowest excited singlet state of these compounds. The relative quantum yields are calculated. It is found that the quantum yields of the cinnamoyl pyrones vary with the change in the solvent polarity indicating the dependency of fluorescence properties on the solvent nature. It has been observed that the addition of water and pH medium can affect the fluorescence properties of cinnamoyl pyrones in ethanol. This study exhibited that due to the solvent sensitive emission, cinnamoyl pyrone derivatives are a good compound to be used as fluorescence probes. © 2017 Elsevier B.V. All rights reserved.
Keywords: Cinnamoyl pyrones Solvatochromic effect Molecular spectroscopy Fluorescence emission properties Fluorescence quantum yield Hydrogen bonding interaction
1. Introduction Fluorescence sensors have been receiving a considerable attention due to their potential applications in biochemical and medical analyses and optoelectronic devices [1e3]. It is known that the spectral behavior of an organic molecule is strongly related to its structure in both ground and excited states. The knowledge of the solvent effect on the absorption and fluorescence spectra is of particular importance. A change in the nature of solvent is accompanied by a change in polarity, dielectric constant and polarizability of the surrounding medium. Thus, the change of solvents from apolar to polar aprotic, or to polar protic affects the ground state and the excited state differently. The excited state dipole moment of a molecule control the tenability range of their emission energy as a function of solvent
* Corresponding author. Department of Chemistry, Faculty of Sciences, University M'Hamed Bougara, Boumerdes (UMBB), Avenue of Independence, 35000, Algeria. E-mail address: [email protected] (N. Benosmane). http://dx.doi.org/10.1016/j.molstruc.2017.07.089 0022-2860/© 2017 Elsevier B.V. All rights reserved.
polarity and can be useful to optimize the performance of a laser dye [4]. Solvatochromism, which describes the spectral change of solute induced by changes in solvent polarity, provides a convenient way for the monitoring of interactions between solute and solvent, including specific (such as hydrogen bonding and electron donor-acceptor interactions) and non-specific (polaritypolarizability effects) interactions [5]. Cinnamoyl pyrone derivatives were intensively investigated as photostable emitting dyes for fluorescent polymer compositions which permit to create new emitting functional materials based on polymer compositions [6]. Such molecules were proposed as sensors for the examination of medium polarity in biological and medical investigations [7]. The goal of the present work is to report the influence of chemical and physical environments, such as the effect of the substituent (nature and position), solvent, and pH medium, on the absorption and fluorescence spectra and fluorescence quantum yield (f) of some cinnamoyl pyrone derivatives. The study has been performed in various non-polar, polar aprotic and polar protic solvents to monitor general as well as specific solvent effects.
2
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
Scheme 1.
Fig. 1. UVevisible absorption spectra of some cinnamoyl pyrone derivatives B1, B10eB13 in acetonitrile (ACN) and dichloromethane (DCM) (c ¼ 6.72 106 M).
2. Experimental 2.1. Materials and methods A selection of cinnamoyl pyrone derivatives were synthesized according to literature [8] and their structure depicted in Scheme 1. All chemicals and solvents used for synthesis and spectroscopic measurements were of analytical reagent grade and obtained from Fluka and used without further purification. The absorption and fluorescence spectra were recorded at room temperature and using a 1 cm quartz cuvette on, respectively, a JENWAY-6800 UVevisible Spectrophotometer and a Jasco-FP 8200 Fluorometer. Excitation and emission slit width was fixed to 10 nm. The excitation source was a long-life xenon flash lamp and excitation
wavelengths were set on that of maximum absorption of each cinnamoyl pyrone derivatives. The method of pH measurements in non-aqueous media was based on the work of Rondinini [9]. All pH values were recorded on a multifunctional pH-meter (HANNA HI 2550). The fluorescence quantum yield (f) was measured relative to rhodamine B as standard (f ¼ 0.65) [10], according to the following Eq. (1)
f¼
fstd FAstd h2 Fstd Ah2std
(1)
where F and Fstd are the peak areas of sample and standard solutions, respectively; A and Astd are the absorption at excitation wavelength of sample and standard, respectively; h and hstd are the
Fig. 2. Fluorescence emission spectra of cinnamoyl pyrone derivatives in acetonitrile (ACN) and dichloromethane (DCM) (c ¼ 6.72 106 M).
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
3
Table 1 Spectroscopic characteristics of B1eB13 in acetonitrile (ACN) and dichloromethane (DCM) as solvents. Compounds
Solvent
labs (nm)
lex (nm)
lem (nm)
Stokes shift (na nf )(cm1)
B1
ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM ACN DCM
356 358 373 377 380 384 359 360 390 387 367 374 357 360 353 355 349 350 352 365 358 361 463 469 476 480
380
483 466 500 490 488 487 510 477 518 493 523 491 497 478 481 469 473 472 480 471 490 480 561 573 562 575
7387 6475 6808 6115 5824 5506 8249 6812 6336 5556 8126 6370 7889 6859 7540 6848 7510 7385 7576 6167 7526 6869 3774 3871 3222 3441
B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13
380 380 380 380 380 380 380 380 350 350 460 460
Fig. 3. a) UveVisible absorption spectra of cinnamoyl pyrone B13 in various solvents (c ¼ 6.72 106 M); b) Fluorescence emission spectra of cinnamoyl pyrone B13 in various solvents (c ¼ 6.72 106 M) (lex ¼ 460 nm).
refractive index of sample and standard solutions, respectively; f and fstd are the quantum yield of sample and standard solutions respectively.
Fmax 490 nm (Table 1). Substitutions on the aromatic ring led to wavelength shifts in the absorption and emission maxima (Figs. 1 and 2) and affected fluorescence emission efficiency.
3. Results and discussions 3.1. Absorption and emission characteristics of cinnamoyl pyrones B1eB13 The absorption and emission spectra of compounds B1-13 in dichloromethane (DCM) and acetonitrile (ACN) are presented in Figs. 1 and 2. The absorption maxima were observed at 356 nm for B7eB9 in acetonitrile. The fluorescence maximum (Fmax) was observed in the range: 470e496 nm for B7eB9 with a significant Stokes shift varying between 7511 and 7890 cm1. Emissions of the five cinnamoyl pyrone derivatives B2eB6 were very similar with
Table 2 Spectral maxima and photophysical parameters of B13 in different solvents calculated from Fig. 3a and b. Solvent
Df
labs (nm)
lem (nm)
Dn (cm1)
Toluene Acetate ethyl Ethanol Acetonitrile Dichloromethane n-propanol
0.0150 0.1995 0.2886 0.3054 0.4179 0.4619
471 472 488 476 480 468
534 545 570 562 575 562
2506 2839 2949 3222 3441 3590
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Table 3 Fluorescence quantum yield (f) of B12 and B13 in different solvents. Compound
Solvent
(ε) at 20 C
(n)
m(D) of solvent
F
Rhodamine B B12
Ethanol Acetonitrile
24.5 36.8
1.3614 1.3284
1.69 3.45
n-Propanol Ethyl acetate Toluene Dichloromethane
20.65 6.19 2.39 8.90
1.3856 1.3724 1.4969 1.4244
3.09 1.88 0.43 1.58
0.65 [9] 0.018 (0.017 Lit [7]) 0.067 0.342 0.428 0.538 (0.546 Lit [7]) 0.019 0.152 0.489 0.491 0.687
B13
Acetonitrile n-Propanol Ethyl acetate Toluene Dichloromethane
The solvent used in the acquisition of the fluorescence spectra of cinnamoyl pyrone had an influence on the position and intensity of the corresponding bands. The absorption spectra of B13 in solvents of different polarity exhibited a very similar shape around 475 nm. However, its fluorescence emission spectrum exhibited a different shape according to the solvent (Fig. 3a and b). Since polar solvents induce fluorescence shift towards longer wavelengths, the dipole moment of B13 is higher in the excited state than in the ground state. This class of cinnamoyl pyrone derivatives displayed a very favorable fluorescence profile. The solvent dependent spectral shifts were also verified in terms of the simplified LippertMataga Eq. (2) [11e13]. This equation describes the Stokes shift in terms of the change in dipole moment of the fluorophore upon excitation and the dependence of the energy of the dipole on the dielectric constant and refractive index of the solvent as given by Eq. (2):
Dn ¼ Dn abs Dn em ¼
Df ¼
ε1 h2 1 2ε þ 1 2h2 þ 1
ms mg hca2
a twisted intramolecular charge transfer (ICT) state that should be a nearly full charge transfer [15]. 3.2. Effect of nature and position of substituent on the absorption and emission spectra of cinnamoyl pyrones B1eB13 In general substitution of a certain molecule with electron donating groups causes an increase of the molar absorption coefficient and a bathochromic shift of their absorption and emission spectra. It was the case of the compounds B2eB6 (Table 1). In the case of hydroxy substituents there is an ICT, confirmed by a wide and unstructured fluorescence spectra. Whereas for the nitrosubstituted compounds B10 and B11 there is a weak level of fluorescence due to an ICT that induces both efficient state crossing from the lowest excited state n-p* and S1eS0 interconversion. 3.3. Effect of addition of water on absorption and fluorescence emission spectra of cinnamoyl pyrone B13
2 $Df þ Const
(2)
(3)
where Dn is the Stokes shift in cm1 (Table 2), h ¼ 6.6260 1034 J s is Planck's constant, c ¼ 2.9979 108 m s1 is the velocity of light, (a) is Onsager cavity radius of the solute molecule being calculated from the molecule volume according to Suppan's equation [14], 1 3 a ¼ 43M , where d is the density of the solute molecule, M is the pdN
In order to study the role of the water in the absorption and fluorescence emission of cinnamoyl pyrone B13 these spectra were acquired in aqueous solutions with different percentages of water (0, 20, 40, 60 and 80%). As shown in Fig. 5a the absorption spectrum of B13 undergo only small changes in intensity and shape by increasing water content in the aqueous-ethanol solutions in a similar concentration of cinnamoyl pyrone B13, but with higher content of water 80% (V/V) the absorption intensity was drastically
=
molecular weight of solute, and N is Avogadro's number (here, a ¼ 4.704 Å), ε and h are the dielectric constant and refractive index of the solvent respectively, ms and mg is the dipole moment of molecule at excited and ground states, respectively. For this study we choose the cinnamoyl pyrone derivative B13 for which the photophysical properties are displayed on Table 2. The plot of Stokes shift (Dn) versus the term (Df ) of the polarizability orientation of the solvent are shown in Fig. 4. We found a good correlation coefficient (R2 ¼ 0.9366) in excited state. Thus, the dielectric solute-solvent interactions are responsible for the observed solvatochromic shift of the cinnamoyl pyrone derivative B13. In order to determine some quantitative characteristics of solvatochromic effects observed for a compound B13, we calculated Dm ¼ ðms mg Þ using the Lippert-Mataga equation and found Dm ¼ 1:03 D. Thus, the excited state dipole moment is only 1.03 D bigger than that of the ground state. This difference is too small for
Fig. 4. Plotting Stokes shifts versus Df of B13 in various solvents.
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
5
Fig. 5. a) Absorption spectra of B13 (c ¼ 6.72 106 M) in aqueous ethanol solutions containing different water contents (0, 20, 40, 60, 80%) (V/V); b) Fluorescence spectra of B13 (c ¼ 6.72 106 M) in aqueous ethanol solutions containing different water contents (0, 20, 40, 60, 80%) (V/V).
decreased. On the other hand, the corresponding fluorescence spectra exhibited significant changes in the spectral position and intensity of shape with water content (Fig. 5b). The spectra of solutions with water content between 20 and 60% shows a fluorescence shift to longer wavelengths, but when the water content is equal to 80% the fluorescence band was shifted to shorter wavelengths. In the term of the fluorescence intensity, the decrease was dramatic for a water proportion between 20 and 80% (Fig. 6). This phenomenon is typical of a specific water effect and is possibly due to the formation of hydrogen bonding between water molecules and the extra-cyclic amino group of B13. The fluorescence emission features of B13 are assumed to be linked to the formation of intermolecular hydrogen bonds with water molecules and the formation of mono and/or polyhydrate species. Moreover, the presence of water may induce the deactivation of the lowest excited singlet state of the probe through the hydrogen-bonded interactions [3]. The hydrogen bonding interaction is also responsible for the reduction in the absorption intensity on going from alcoholic media to water, because hydrogen bonding in pyrone ring reduces the resonance of the aromatics cycles. Solvent effect was treated within the framework of the linear solvation energy
relationships (LSER) established by Kamlet [16], in which each of specific and non specific interactions have a linear contribution to the total solvation energy of solvent. For non chloro-substituted aliphatic solvents, LSER takes the form of Eq. (4). ET ¼ A0 þ sp* þ aa þ bb
(4)
WhereET: the molar electronic transition energy in KCal mol1
ET K Cal$mol1 ¼
hcNA
lmax ðnmÞ
¼
28591:5
lmax ðnmÞ
h, c and NA are planck's constant, the velocity of light and Avogadro's number, respectively. The values of ET as a function of mole fraction of ethanol solvent are presented in Table 4. The a, b and p* are the Kamlet and Taft solvatochromic parameters (KAT) which have been developed for scaling the hydrogen-bond donor acidity, hydrogen-bond acceptor basicity and dipolarity/polarizability of solvent respectively [17e19]. The A0, a, b and s quantify the sensitivity of ET values to the acidity, basicity and dipolarity/polarizability of solvent respectively. The parameters of Eq. (4) depend physically on the nature of solute and solvent and the signification of the solute-solvent interactions. The KAT parameters for binary mixtures were extracted from the literature [20].
Table 4 The ET values of B13 and KAT parameters in different mole fraction of ethanol solvent (X2). Water-Ethanol X2
ET
p*
a
b
1 0.80 0.60 0.40 0.20
58.469 57.297 56.172 55.302 54.150
0.54 0.65 0.75 0.88 1.10
0.86 0.92 0.96 0.96 1.00
0.75 0.70 0.64 0.67 0.60
Table 5 Linear correlations obtained by plotting ET versus p*, a and b according to the Taft and Kamlet equation (r ¼ correlation coefficient). Fig. 6. Fluorescence intensity of B13 (c ¼ 6.72 106 M) in aqueous ethanol solutions containing different water contents (0, 20, 40, 60, 80%) (V/V).
Compound
A0
s
a
b
r
B13
61.639
4.652
2.740
0.389
0.995
6
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7
Fig. 7. a) pH dependence of absorption spectra of B13 (c ¼ 6.72 106 M) in ethanol solutions; b) pH dependence of fluorescence spectra (lex ¼ 460 nm) of B13 (c ¼ 6.72 106 M) in ethanol solutions.
The values of Cinnamoyl pyrones for each aqueous solution were correlated with solvent properties by means of the multiple linear regressions analysis in Origin. 8 (Microcal ™ Software). Fitting results obtained from multiple linear regressions analysis are shown in Table 5. In aqueous binary mixtures, a dual-parametric KAT equation containing a and p* shows the best fit. We observed that the sign of a and p* coefficient is negative, consistent with the compensation of the negative polarity contribution to the spectral shift of cinnamoyl pyrone B13 by a strong positive contribution of hydrogen bonding interaction of cinnamoyl pyrone with solvent mixture, especially water molecules. 3.4. Effect of pH on absorption and fluorescence emission intensity of cinnamoyl pyrone B13 The absorption spectra of B13 were determined in ethanol of different pH values by addition a few gouts of acidic or basic aqueous solution (HCl, NaOH in water) (Fig. 7a). In an acid medium (pH range from 2 to 6), a strong absorption was observed at 488 nm. Increasing pH values from 8 to 12, the shapes are shifted to shorter wavelength at 402 nm and then the absorption intensity dramatically decreased. As shown in Fig. 7b, the fluorescence emission intensity was maximum in alkaline medium and decreased in acid conditions, thus one can consider B13 as a non-fluorescent compound at low pH. The spectral shifts accompanying protonation or dissociation of basic or acidic functional groups depend on whether the functional group undergoing protonation or dissociation is directly coupled to the aromatic system and whether it gains or losses of electronic charge upon going from the ground to the excited state. Since, high fluorescence intensity was observed at pH 8 [3]. 3.5. Fluorescence quantum yield of B12 and B13 The fluorescence quantum yields (f) of B12 and B13 were determined by excitation at lex 460 nm (Table 3) and they ranged between 0.018 and 0.687 (spectral maxima and photophysical parameters of B12 and B13 in different solvents are shown in Table 3). Fluorescence quantum yields of B12 and B13 showed slight variation with the change in solvent polarity indicating a negative and
positive solvatokinetic effect. The compounds are called solvatochromic when the position of their absorption spectrum and emission depends on of solvent polarity. Displacements to the longer wavelengths (bathochromic shift) or towards the shorter wavelengths (hypsochromic shift) when the polarity of the solvent increases are called respectively positive or negative solvatochromism. In other words, when a compound is surrounded by molecules of the solvent at ground state, the excited state are stabilized by solute-solvent interactions (van der Waals type and/or hydrogen bonding). The significant negative solvatochromism is explained by the high dipole moment in the ground state and almost zero in the excited state because of an intramolecular electron transfer. For negative solvatokinetic effect, quantum yield increase with a suitable enhancement of intramolecular charge transfer (ICT) which involves a n,p* electron configuration while the reduction in quantum yield by strong ICT is called positive solvatokinetic effect [7]. 4. Conclusion The aim of this work was the synthesis and the investigation of solvatochromic fluorescence characteristics of the cinnamoyl pyrone derivatives. The UVevisible absorption and fluorescence spectra, fluorescence quantum yield were determined in several solvents. It has been found that the character and position of the substituent on the aromatic ring influence the position and shape of absorption and fluorescence spectra. However, a strong effect of pH medium and addition of water on the fluorescence intensity of compound has been found. It has been observed that the quantum yield of the molecule varies with the change in the solvent polarity. References [1] A.W. Czarnik, Chemical communication in water using fluorescent chemosensors, Acc. Chem. Res. 27 (1994) 302e308. [2] A.P. de Silvan, T.P. vance, M.E.S. West, G.D. Wright, Bright molecules with sense, logic, numeracy and utility, Org. Biomol. Chem. 6 (2008) 2468e2481. [3] V.D. Suryawanshi, A.H. Gore, L.S. Walekar, P.V. Anbhule, S.R. Patil, G.B. Kolekar, Solvatochromic fluorescence behavior of 2-amino-6-hydroxy-4-(3,4dimethoxyphenyl)-pyrimidine-5-carbonitrile: a sensitive fluorescent probe for detection of pH and water composition in binary aqueous solutions, J. Mol. Liq. 184 (2013) 4e9. [4] R. Gahlaut, N. Tewari, J.P. Bridhkoti, N.K. Joshi, H.C. Joshi, S. Pant, Determination of ground and excited state dipole moments of some naphthols using
N. Benosmane et al. / Journal of Molecular Structure 1149 (2017) 1e7 solvatochromic shift method, J. Mol. Liq. 163 (2011) 141e146. [5] F. Naderi, A. Farajtabar, F. Gharib, Solvatochromic and preferential solvation of fluorescein in some water-alcoholic mixed solvents, J. Mol. Liq. 190 (2014) 126e132. [6] D.O. Tykhanov, E.V. Sanin, O.V. Grygorovych, I.I. Serikova, F.G. Yaremenko, A.D. Roshal, Funct. Mater. 18 (3) (2011) 339e347. [7] D.A. Tykhanov, I.I. Serikova, F.G. Yaremznko, A.D. Roshal, Structure and spectal properties of cinnamoyl pyrones and their vinilogs, Cent. Eur. J. Chem. 8 (2) (2010) 347e355. [8] R.H. Wiley, C.H. Jarbone, H.G. Ellert, J. Am. Chem. Soc. 77 (1955) 5102e5105. [9] S. Rondinini, P.R. Mussini, T. Mussini, Reference value standards and primary standards for pH measurements in organic solvents and water þ organic solvent mixtures of moderates to high permittivities, Pure Appl. Chem. 59 (1987) 1549e1560. [10] R.F. Kubin, A.N. Fletcher, Fluorescence quantum yields of some rhodamine dyes, J. Lumin. 27 (1982) 455e462. [11] E.Z. Lippert, Dipole moment and electronic structure of excited molecules, Electrochem 61 (1957) 962. [12] N. Mataga, T. Kubota, Molecular Interaction and Electronic Spectra, Marcel Dekker, New York, 1970. [13] F. Ito, T. Nagai, Y. Ono, K. Yamaguchi, H. Furuta, T. Nagamura, Photophysical
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