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Studies of solvation in homogeneous and heterogeneous media by electronic spectroscopic method
Spectrochimica Acta Part A 59 (2003) 2921 /2933 www.elsevier.com/locate/saa
Studies of solvation in homogeneous and heterogeneous media by electronic spectroscopic method Mrinmoy Shannigrahi, Ramkrishna Pramanik, Sanjib Bagchi * Department of Chemistry, Burdwan University, Burdwan 713104, India Received 24 September 2002; received in revised form 14 January 2003; accepted 5 February 2003
Abstract Solvation characteristics in homogenous (pure and mixed binary solvents) and heterogeneous media (aqueous micelles, b- and g-cyclodextrine solutions) have been studied by monitoring the emission characteristics of a newly synthesised dye. The longest wavelength absorption and emission band of the dye arise due to transition between S0 and S1 state. The maximum energy of electronic transition involving intramolecular charge transfer is found to be dependent on both the hydrogen-bond donating ability and the polarity /polarisability in pure solvent. The dipole moment in the S1 state, as determined by solvatochromic procedure, agrees well with the value obtained by theoretical calculation at the AM1 level. Preferential solvation of the dye by alcohols has been found to occur in ethanol/water, propan-1-ol/ water, propan-2-ol/water binary mixtures. In aqueous micellar media the dye molecule is located at the water /micelle interface. The binding constant for the dye /micelle interaction has also been determined. The results have been compared with those for a structurally related symmetrical ketocyanine dye. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Solvation; Fluorescence probes; Mixed binary solvents; Preferential solvation; Micellar media
1. Introduction It is well known that the electronic transition in molecules, involving intramolecular charge transfer (ICT), is very sensitive to the nature of the microenvironment around the solute and the spectral parameters can be used for studying solute/solvent interaction at the microscopic level [1]. In general several modes of solute /solvent
* Corresponding author. Tel.: /91-342-558545; fax: /91342-564452. E-mail address: [email protected] (S. Bagchi).
interaction have been identified, viz. non-specific (e.g. dipolarity /polarisability) and specific (acidity, basicity) modes and it is customary to write an observed parameter in a solute(s) /solvent(i) system in terms of linear solvation energy relationship [2 /5] X aa (s)Pa (i) (1) P(s; i) P0 (s) where the suffix ‘a’ denotes a particular mode of solute /solvent interaction. P0 and aa terms depend only on the solute and Pas are solvent properties pertinent to a-mode of interaction. As the solventsensitivity of optical response of a solute depends on the solute through aa(s) in Eq. (1), several
1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(03)00120-3
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solutes have been used for studying solvation characteristics [6]. We have, in our laboratory, used ketocyanine dyes [7] as indicator solutes for studying solvation interaction in homogeneous and heterogeneous media [8 /12]. These compounds are characterised by solvent-sensitive absorption and fluorescence bands, which depend to a large extent on the hydrogen bond donation ability of a solvent. But the insolubility of the compounds in water imposes a limitation, for these compounds, to study solvation in aqueous media. In the present work, we have investigated the absorption and fluorescence characteristics of the dye, 2-[3-(N -methyl-N -phenyl amino)-2-propenylidene] indanone (henceforth called dye A) (Fig. 1), in various pure solvents including alcohols, ketones, nitriles, hydrocarbons, chlorohydrocarbon, ester and cyclic ether. The contribution of different modes of solute/solvent interaction towards solvation in pure solvents have been addressed. Solvation characteristics and the nature of transition at 77 K for the dye have been investigated. The non-ideal solvation in mixed aqueous alcohols (water/ethanol, water/propan-1-ol and water/ propan-2-ol) and interaction of the dyes in heterogeneous media (aqueous micelles, b- and g-cyclodextrine solutions) have also been studied. Semiempirical molecular orbital (MO) calculations at the AM1 level have been carried out to confirm some of the measured properties of the dye. The results have been compared with those for a
Fig. 1. Solutes used in the work.
structurally similar ketocyanine dye, 2,5 di-[3-(N methyl-N -phenyl amino)-2-propenylidene] cyclopentanone (dye B, Fig. 1).
2. Materials and methods The dye B was prepared from cyclopentanone, 1,1,3,3-tetramethoxy propane and N -methylaniline by the method described in the literature [7]. The dye A was prepared as follows. Perchloric acid was added to a solution of 1,1,3,3-tetramethoxy propane (25 mM) and N -methylaniline (50 mM) in ethanol. The mixture was kept at 50 8C for 30 min and stored overnight in a refrigerator to give yellow needle-like intermediate compound. The product (14.25 mM) was added to a solution of sodium methoxide and indanone (14 mM). The mixture was refluxed for 2 h and the golden yellow precipitate was filtered off, washed with water, methanol and diethyl ether. Purity of the prepared compound was checked by C, H, N analysis (calculated: C /82.88%, H /6.22%, N /5.09%; observed: C /82.54%, H /5.96%, N /5.08%) and IR data (peaks at 1613, 1565, 1506, 1466 cm 1 in KBr disc). All the solvents used were of spectroscopic grade and were distilled from calcium hydride immediately before use to ensure the absence of peroxides and oxidising agents. Solvent parameters like the refractive index, dielectric constant, and ET (30)-values were taken from the literature [6,13]. Sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide (CTAB) and Triton X100 (TX-100), b- and g-cyclodextrine were received from Sigma and used without purification. For preparing a heterogeneous medium, the required amount of surfactant/cyclodextrine were added to triply distilled water, and the contents kept in an ultrasound sonicator (Jencons, UK Model T80) for a considerable period. Solutions of dyes were prepared by adding the dye to the heterogeneous media with proper sonication. The UV /Vis absorption spectra were recorded on a Shimadzu UV 2101 PC spectrophotometer fitted with a temperature-controlled unit (Model TB-85 Thermobath, Shimadzu). The emission and excitation spectra were taken on a Hitachi F4500
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spectrofluorimeter equipped with polarisation accessory and a temperature-controlled cell holder. For measurement at different temperatures, water was made to circulate through the cell holder from a constant temperature bath (Heto Holten, temperature range: 243 /373 K). The low temperature attachment of the spectrofluorimeter was used for measurements at 77 K. The concentration of dye in solution was in the range 10 5 /10 6 M.
3. Results and discussions 3.1. Theoretical calculations Semi-empirical MO calculations at the AM1 [14,15] level using the MOPAC PC program were carried out for the two dyes. The initial optimisation of the ground state geometries of the dye molecules were achieved using a molecular mechanics program followed by unrestricted geometry optimisation. Electronic energy calculations for the S1 state were obtained from the optimised ground state using configuration interaction using the MICROS option. The choice of a large number of microstates was to ensure true representation of the correlation effect. The method, although not very rigorous, can however be used for confirming some of the measured properties. Charge density on nitrogen and oxygen centre for the dye molecules in the S0 state are shown in Fig. 1. The molecules are planar, except the aliphatic hydrogen atoms. Table 1 lists some of the molecular properties. Note that the energy of S0 0/S1 transition as obtained from the theoretical calculations is in the order: dye A /dye B. This is also the
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order observed experimentally. The absolute magnitude of the calculated energy of the S0 0/S1 transition, for a particular dye, however, shows a deviation from the experimentally observed value. Thus the calculated values are 86.4 and 84.7 kcal mol 1 for dye A and B, respectively, while the corresponding observed values are 70.8 and 64.3 kcal mol 1 in n-hexane. This modest deviation is expected because the calculated values refer to the gas phase. The experimental data in solution would expectedly differ from the gas phase values due to the existence of dielectric effects. Moreover the calculated value refers to S0 (v /0) 0/S1 (v /0) transition while the experimental data refer to S0 (v /0) 0/S1 (v /n ) transition where n is largely unidentified. 3.2. Spectroscopic measurements 3.2.1. Pure solvents at 298 K The longest wavelength absorption and fluorescence spectra of the dye A in various pure solvents at 298 K are almost structureless. The transition energies corresponding to the maximum absorption and fluorescence, E(A) and E(F), respectively, are listed in Table 2. Fig. 2 shows some representative absorption and fluorescence spectra in pure solvents. A mirror image relationship between modified absorption and fluorescence spectra is apparent from the figure. For pure solvents, both E(A) and E(F) shift to lower energies as the solvent becomes more polar. A linear correlation of E(A) with the empirical polarity parameter, ET(30), indicates that longest wavelength band of the dye originates due to ICT presumably from the nitrogen atom to oxygen of the carbonyl group. A
Table 1 Semi-empirical MO calculated parameters at the AM1 level for the two dyes Parameters
Dye A S0 state
Dipole moment (D) Ionisation potential (eV) Heat of formation /(kcal mol 1) Total energy (eV) Transition energy (eV)
4.17 8.21 61.69 /3191.76
Dye B S1 state 6.16 2.83 157.53 /3187.6 3.74
S0 state 3.70 9.91 91.28 /4290.04
S1 state 5.79 7.92 158.54 /4286.78 3.67
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Table 2 Spectroscopic parameters for the dye A in various media at 298 K Media
E(A)a
E(F)a
Water Acetic acid 2,2,2 Trifluoroethanol Ethanol Acetone Acetonitrile Tetrahydrofuran 2-Me Tetrahydrofuran Benzene Ethyl acetate Dichloromethane n -Hexane SDS CTAB TX-100 b-Cyclodextrine g-Cyclodextrine Ethanol glass MTHF glass Toluene glass
61.4 64.0 61.7 65.1 67.3 67.3 68.1 66.0 65.0 68.4 66.8 70.8 63.5 63.5 63.5 63.5 62.2 / / /
54.7 56.3 56.2 56.1 58.6 57.8 60.6 60.8 60.3 60.7 59.1 64.1 55.7 56.6 56.1 55.7 55.1 58.3 60.2 60.2
a
kcal mol 1.
red shift of 2/4 kcal mol 1 is obtained for the dye as one goes from aprotic to protic solvents.
Moreover, the band shifts dramatically to the red as a few drops of alcohol is added to the solution of the dye in aprotic solvents. These facts point to the existence of a solvato-complex of the dye with alcohols, presumably through hydrogen bond interaction of the solvent with the carbonyl oxygen of the dye molecule. A similar conclusion has been obtained for the symmetrical dye B [8]. The red shift of the absorption band with increasing polarity indicates that there is an increase in the dipole moment (m) on excitation. The dipole moment of the solute in the excited state has been determined from the solvatochromism of the absorption and fluorescence band by a method described in earlier communications [8,12]. In this procedure E(A)/E(F) and E(A) /E(F) are plotted against appropriate dielectric functions, 2(o /1)/ (2o/1) and [2(o /1)/(2o/1) /2(n2 /1)/(2n2/1)], respectively, where n is the refractive index and o is the dielectric constant of the solvent. The ratio of the dipole moment in the S1 state (m1) to that in the S0 state (m0) is obtained from the ratio of the slopes of the linear plots. This method does not require the knowledge of the cavity radius. The ratio m1/m0 is found to be 1.66 for dye A, which is
Fig. 2. Representative modified absorption spectrum [o (/n) ¯ ///n¯/vs. n] ¯ (solid line) and modified emission spectrum [I (/n) ¯ ///n¯/3 vs. n] ¯ (dotted line) of dye A in ethanol (1) and dichloromethane (2).
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slightly greater than its symmetric counterpart [9]. The value of the dipole moment in the S1 state has been calculated by using the value of m0 obtained from semi-empirical AM1 calculation. This procedure gives a value of m1 /6.95 for the dye A, which agrees well with the theoretically calculated value of 6.16 (Table 1). The emission band for the dye A may be characterised as S1 0/S0 fluorescence. No phosphorescence could be detected even at 77 K. Plot of E(F) versus ET(30) shows a double linear correlation as shown in Fig. 3. Data points for aprotic solvents fall on a line with a higher slope. Similar behaviour has been observed for the dye B [9]. A double linear correlation indicates that the nature of the emitting state is significantly different in the two classes of solvents. To obtain an insight into the contribution of specific and nonspecific modes of interaction towards solvation, a multiple linear regression analysis has been done. Correlations of E(A) or E(F) were sought with Kamlet and Traft’s parameters, p*, a and b representing, respectively, the solvent polarity / polarisability, hydrogen bond donating (HBD) and accepting (HBA) ability of the solvent [16]. The following regression equations were obtained
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E(A) (kcal mol 1 ) 70:0 3:41a0:84b 4:30p ; n 12;
(2)
r 0:88
E(F) (kcal mol 1 ) 63:8 2:62a 2:08b 4:30p ; n 12;
(3)
r 0:96
Eqs. (2) and (3) indicate that E(A) is determined mainly by a and p*, but E(F) has a significant correlation with b, the HBA ability of the solvent. From the correlation Eq. (3), it appears that the coefficient of p* term is greater than that of the a or b term indicating that dipolarity /polarisability interaction is more important than the hydrogen bonding interaction for this newly synthesised unsymmetrical dye A. This result is also in conformity with our previous work with other unsymmetrical dyes [12]. But our previous work using the symmetric dye B indicates that the specific HBD interaction of the dye with the solvent is relatively more important than the non-specific modes of solvation interaction [9]. For the symmetric dye, the increased electron density on the carbonyl oxygen in the S1 state,
Fig. 3. Plot of E(F) vs. ET(30) for the dye A in pure solvents.
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which favours the HBD interaction, also leads to an extension of conjugation, giving extra stability. An interesting observation is obtained when dry HCl gas is passed into the ethanolic solution of the two dyes. For the symmetric dye B, the absorption band around 500 nm in pure ethanol gets reduced in intensity while a new band with maximum at 660 nm appears. Similar result was obtained when ethanol was replaced by another aprotic solvent. For the unsymmetrical dye A, however, only a small shift of the absorption band is obtained (Fig. 4). These observations suggest that HCl reacts with the dye B as shown in Scheme 1. The product formed has an extensive conjugation between the two N atoms which was absent in the dye molecule due to the presence of an electron withdrawing carbonyl group at the middle. The enhancement of delocalisation leads to an extra stability of the product. Such extensive conjugation, however, is not possible in the case of unsymmetrical dye and no chemical reaction thus takes place. 3.2.2. Pure solvents at 77 K The emission bands of the dyes in ethanol, 2methyl tetrahydrofuran (2-MTHF) and toluene at
different temperatures are shown in Fig. 5. In all the cases the observed spectrum is independent of the concentration of the solute and the excitation wavelength. The shape of the band at 77 K also differs from that at 298 K. The excitation spectrum at 298 K in all the cases is identical with the absorption spectrum. However, the excitation spectra at 77 K show prominent vibrational structure, which can be assigned as carbonyl vibration. The fluorescence band for both the dyes in ethanol suddenly changes shape as the glass melts. The peak in the glass (/490 nm for dye A and /570 nm for dye B) gets reduced in intensity and simultaneously the intensity at higher wavelength increases. On further increase in temperature the spectrum approaches that obtained at 298 K. For 2-MTHF and toluene the fluorescence band at 77 K shows permanent structures for the dye A; the position of maximum absorption does not changes. However, the relative intensity of the lower wavelength band increases over that of the higher wavelength band at 77 K. For the dye B, however, the relative intensity of the higher wavelength band increases at 77 K. Since the excitation spectrum does not change on going from 298 to 77 K, any change in the fluorescence
Fig. 4. Absorption spectrum for dye A (dotted line) and dye B (solid line) in pure ethanol (1) and acidic ethanol solution (2).
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Scheme 1.
spectrum can be rationalized in terms of modification of the S1 state of the solute by interaction with the solvent molecules. The S0 state of the dyes in ethanol are stabilised by hydrogen bonding. This hydrogen bonding becomes tighter in the S1 state due to an increased charge density on the carbonyl oxygen atom. This H-bonded state is the emitting state in ethanol at 77 K leading to emission band at 490 nm for the dye A and 570 nm for the dye B. Appearance of this band in ethanol/methanol glass supports this view. The abrupt change in the shape of the fluorescence band in ethanol on melting points to a possible molecular motion and related geometry change in the S1 state in the fluid phase. As stated earlier, the double linear correlation of E(F) with ET(30) at 298 K indicates that the emitting state in an alcohol solvent is different from that in aprotic solvents. Our results indicate that the emitting state for ethanol at 298 K is not the initially prepared ICT state. One possibility is the formation of a TICT state [17] involving rotation of the amino nitrogen in the S1 state. Formation of TICT state in this case will be facilitated due to greater solvent stabilisation and enhanced affinity of the hydrogen bonded carbonyl group [18]. For aprotic solvents like 2-MTHF and toluene, appearance of emission band maximum at 77 K with almost unchanged position (relative to that at 298 K) but with a change of relative intensity suggests that different conformations are possible in the S1 state and the population differs as temperature is changed. There are two (four) ethylenic double bonds in the chromophoric part of the molecule of dye A (dye B) and several conformers formed by rotation about
quasi-single C /C in the S1 state is possible. Existence of such conformers has been documented by earlier studies on retinyl acetate [19,20] and other ketocyanine dyes [18,21]. 3.2.3. Binary aqueous solvents In the present work three binary aqueous mixtures, namely, water/ethanol, water/propan-1-ol and water/propan-2-ol have been studied using the dye A as the probe molecule. Table 3 lists the various absorption and fluorescence spectral parameters. Both the absorption and fluorescence maxima of the solute shifts towards shorter energy as the percentage of water in the binary solvent mixture increases. For all the three binary aqueous solvent mixtures, non-linear relationship between the spectral property [E(A) or E(F)] and mole fraction of water has been observed indicating a non-ideal solvation behaviour of the solute [13]. The nature of the deviation from linearity also suggests that the alcohols are preferred by the solute over water. The preferential solvation of the dye by alcohol over water can be explained in terms of strong self-association of water molecule through hydrogen bonding. The molecule of the solute having a hydrophobic wing is unable to break the water structure. Thus, the percentage of free water molecule that takes part in the solvation process decreases. Plots of Stoke’s shift versus the mole fraction of water in the binary mixtures are given in Fig. 6. It is known that the Stoke’s shift (Dn ) is related to the solvent reorganisation energy (l ). The solvent reorganisation about the CT excited state of the solute is expected to increase with the polarity of the solvent. Thus,
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Fig. 5. Emission spectra (a) for dye A and (b) for dye B in pure solvents (1) ethanol, (2) 2-methyltetrahydrofuran and (3) toluene at room temperature (solid line) and at 77 K (dotted line).
of the four solvents viz, water, ethanol, propan-1ol and propan-2-ol, the highest value of Stoke’s shift is expected for water. But our experimental results indicate the order of solvent reorganisation energy as ethanol /water /propanols. Moreover,
the value of l increases as the mole fraction of ethanol in ethanol/water system increases. The lower value of l for water than that for ethanol probably arises due to lack of solubility in water. Although water has a relatively high polarity it
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Table 3 Spectroscopic parameters of the dye A in various mixed aqueous solvents as a function of solvent composition xw
1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50 0.40 0.30 0.25 0.20 0.15 0.10 0.00 a
Water /ethanol E(A)a
E(F)
61.4 62.0 62.5 62.9 63.2 63.8 64.1 64.4 64.5 64.6 64.7 64.7 64.8 64.9 65.1
54.7 54.9 55.0 55.3 55.5 55.8 56.1 56.2 56.3 56.3 56.4 56.4 56.4 56.5 56.6
Water /propan-1-ol a
E(A) /E(F) 6.7 7.1 7.5 7.6 7.7 8.0 8.0 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
a
E(A) 61.4 61.3 61.2 61.2 61.2 61.2 61.3 61.4 61.5 61.7 61.8 61.9 62.0 62.1 62.3
a
E(F) 54.7 55.1 55.3 55.5 55.7 56.0 56.1 56.2 56.2 56.3 56.3 56.4 56.4 56.5 56.8
a
Water /propan-2-ol E(A) /E(F) 6.7 6.2 5.9 5.7 5.5 5.2 5.2 5.2 5.3 5.4 5.5 5.5 5.6 5.6 5.5
a
E(A) 61.4 61.3 61.3 61.4 61.4 61.4 61.5 61.6 61.7 61.9 62.0 62.1 62.2 62.3 62.6
a
E(F) 54.7 55.1 55.5 55.7 55.9 56.2 56.4 56.5 56.5 56.5 56.6 56.6 56.7 56.9 57.3
a
E(A) /E(F)
a
6.7 6.2 5.8 5.7 5.5 5.2 5.1 5.1 5.2 5.4 5.4 5.5 5.5 5.4 5.3
kcal mol 1.
does not contribute to the reorganisation stabilisation of the CT excited state presumably due to hydrophobicity of the dye molecule. Our results indicate that solvent reorganisation energy increases as the percentage of ethanol in the water/ethanol mixture increases. The dependence
of the reorganisation energy with xwater, the mole fraction of water, is also non-linear. In water/ propanol systems, the solvent reorganisation energy, however, decreases as the percentage of propanol in the mixture increases. The nature of variation of l in these cases are also non-linear.
Fig. 6. Plot of Stoke’s shift as a function of solvent composition (xwater) in water/ethanol (j), water/propan-1-ol (m) and water/ propan-2-ol (') binary mixtures for dye A.
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The nature of deviation from linearity also suggests that the solute is preferentially solvated by alcohols. From Fig. 6, it appears that for small percentage of water (0 B/xwater B/0.1) a linear variation of the solvent reorganisation is followed. The deviation from ideality increases as the mole fraction of water increases. Note that, the maximum deviation from linearity appears at xwater :/ 0.7 for all the aqueous alcohols. Thus in this region the solvent /solvent interaction leading to nonideal solvation behaviour is maximum. Similar types of variation of E(A), E(F) and the Stoke’s shift has been observed by Shirota et al. for water/propan-1-ol using the coumarine 153 probe [22]. 3.2.4. Heterogeneous media The maximum energy of fluorescence of the dye A in the aqueous micelles, b- and g-cyclodextrine (CD) media at 298 K are listed in Table 2. Fig. 7 shows some representative spectra. It appears that there is a general blue shift as one goes from water to micellar media. The maximum energy of transition is in the order of CTAB /TX-100/ SDS :/b-CD /g-CD /water. Similar order of maximum transition energy has been observed for the dye B [11]. Moreover, the band is structured in water and also in an aqueous surfactant solution when the concentration is below the critical micelle concentration (CMC). In a micellar medium the intensity of fluorescence increases and the band becomes structureless. This points to a substantial dye/micelle interaction. For a particular concentration of the micelle, the intensity of fluorescence increases as the temperature is decreased. The presence of a dye/micelle interaction is also substantiated by the greater solubility of the dyes in the micellar media. The binding of the dyes in the micellar media also finds support from the high value of fluorescence anisotropy which indicates slower rate of rotational motion of the transition dipole. As discussed earlier, the fluorescence characteristics of the dyes originate from ICT transition. In a protic solvent maximum energy of transition is largely determined by the HBD interactions between the solvent and the carbonyl oxygen atom of the dye in the excited state. The greater the hydrogen bonding ability,
the lower is the energy of transition. The present study indicates that for the dye A the HBD interaction increases in the order CTAB /TX100/SDS :/b-CD/g-CD /water. The position of the fluorescence band maximum for the dye A indicates that the immediate environment around the chromophoric part of the molecule is in between that of water and ethanol. Thus we infer that the dye molecule is located at the micelle / water interface. The negative charge density of the carbonyl group of the dye will be involved in the electrostatic interaction with the polar head group of the ionic micelles. Hydrophobic interaction of the hydrocarbon moiety of the dye with micelle also plays a role. In the CTAB micellar medium, the positively charge head groups will interact favourably with the negative charge density of the carbonyl oxygen of the dye, particularly in the S1 state. Thus the dye will be transferred to a more hydrophobic environment and the extent of hydrogen-bonded interaction with water will be diminished, leading to an increase in the transition energy of fluorescence. In the SDS micelle, on the contrary, the negatively charged head groups will repel the negative charge density on the carbonyl oxygen; thus the chromophoric part of the dye will be in a more hydrophilic environment and will be more effectively hydrogen bonded by water. TX100 consists of bulky phenyl head group and a long polyoxy-ethylene chain that terminates in an /OH group. Here also the interaction of the dye with the terminal /OH group of the micelle is important. The appearance of the fluorescence maximum of the dye A in TX-100 in a position intermediate between that of CTAB and SDS micelle indicates that the micro-polarity of the local region of the fluorophore is in between those for the cationic and anionic micelles. The micropolarity, as measured by the maximum energy of fluorescence of dye A, in cyclodextrine solution is very similar to that in the case of SDS micelle. Thus the dye is located away from the CD-bucket and the H-bonding interactions of the dye with the terminal /OH groups of CD are important. The higher value of Stoke’s shift for dye A in the cationic micelles over that in the anionic micelle suggest that the electrostatic contribution to the reorganisation stabilisation of the CT excited state
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Fig. 7. (a) Representative emission spectra of dye A in CTAB (1), TX-100 (2), SDS (3) and water (4) (concentration of micelle 10 CMC) (b) Representative plot of Eq. (5) for dye A in TX-100.
of the dye is relatively large compared to the contribution due to HBD interaction. In the case of dye B molecule the reverse order of Stoke’s shift has been reported [11]. This probably originates due to the difference in the orientation of the dye with respect to the micelle. For the unsymmetrical dye A, one end (positive/negative) of the dipolar
molecule remains attached to the micelle surface keeping the other end away from the micelle, thus minimising electrostatic interaction. But this is, however, not possible for the symmetric structure. A quantitative estimate of the binding constant (K ) for the equilibrium, representing dye /micelle interaction, can be obtained by using the equation
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proposed by Almgren et al. [12,23,24]. Thus, for the equilibrium DMX D M
(4)
where D and M represents dye molecule and micelles respectively, we have (Ia I0 )=( I I0 ) 1(K [M]) 1
(5)
where Ia, I0, and I are the fluorescence intensities under complete micellisation of the dye, in absence of micellisation, and at any intermediate micelle concentration, respectively. [M] represents the concentration of the micelle, which is given by [M] ([Surf] CMC)=N
(6)
In the above equation [Surf] represents the surfactant concentration and N is the aggregation number. For the calculation of [M] the values of N /62, 60, and 143 for SDS, CTAB, and TX-100, respectively, were assumed. Fig. 7 shows a representative plot of Eq. (5). The measured K values (9/10%) are given in Table 4. Note that the value of the binding constant of both the dyes with cationic micelle, CTAB, is about ten times higher than that with the anionic micelle, SDS. This can be explained in terms of the electrostatic interaction between the dye and the micelle. The negative part of the molecules of the dye (the carbonyl oxygen) can come closer to the cationic micelles while the positive part of the dye molecule, being sterically hindered, remains away from the micelle. In the case of the neutral micelle, hydrogen bonding, rather than the electrostatic interaction is important and the unhindered oxygen atom in the dye molecule can approach the micellar surface closely. Table 4 Values of binding constant for the two dyes at 298 K Micellar media
Triton-X (100) SDS CTAB
Binding constant Dye A
Dye B
1.8 /105 2.0 /104 1.9 /105
2.3 /104 1.0 /103 1.2 /104
4. Conclusion 1) For the dye A, dipolarity /polarisability mode of solute/solvent interaction is more important than the H-bonding interaction, particularly at the S1 state. The fluorescence characteristics of the symmetrical dye B, on the other hand, are largely influenced by hydrogen bonding of the protic solvent with the carbonyl oxygen. 2) Measurements at 77 K indicate that the emitting state in an alcohol solvent in the fluid state is not the initially prepared state on excitation. 3) Although the fluorescence maximum is largely determined by H-bonding interaction of a protic solvent with the chromophoric part of the dye molecules, the solvent reorganisation leading to the stability of the CT excited state is largely determined by the hydrophobic interaction of the dye molecule as a whole with the solvent. 4) The enhanced solubility of the newly synthesised dye A in water makes it a better fluorescence probe for studying solvation interaction in aqueous media than its symmetrical counterpart. 5) The dye /micelle binding for ionic micelles is mainly determined by electrostatic interaction.
Acknowledgements M.S. thanks CSIR (India) for a fellowship. The authors also thank UGC (India) [DSA Programme] for financial assistance.
References [1] E.M. Itskovitch, J. Ulstrup, M.A. Vorotyntsev, in: R.R. Dogonadze, E. Kalman, A.A. Kornyshev, J. Ulstrup (Eds.), The Chemical Physics of Solvation Part B, Elsevier, Amsterdam, 1986. [2] I.A. Koppel, V.A. Palm, in: N.B. Chapmen, J. Shorter (Eds.), Advances in Linear Free Energy Relationships, Plenum Press, New York, 1972. [3] (a) M.J. Kamlet, J.L.M. Abboud, R.W. Taft, Prog. Phys. Org. Chem. 13 (1980) 485;
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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
(b) M.H. Abraham, P.L. Grellier, J.L.M. Abboud, R.M. Doherty, R.W. Taft, Can. J. Chem. 66 (1988) 2673. R.M.C. Goncalves, A.M.N. Simoes, L.M.P.C. Albuquerque, J. Chem. Soc. Perkin Trans. 2 (1990) 1379. K. Medda, P. Chatterjee, A.K. Chandra, S. Bagchi, J. Chem. Soc. Perkin Trans. 2 (1992) 343. C. Reichardt, Chem. Rev. 94 (1994) 2319. M.A. Kessler, O.S. Wolfbeis, Spectrochim. Acta A 47 (1991) 187. D. Banerjee, A.K. Laha, S. Bagchi, Indian J. Chem. 34A (1995) 94. D. Banerjee, A.K. Laha, S. Bagchi, J. Photochem. Photobiol. A: Chem. 85 (1995) 153. D. Banerjee, S. Mondal, S. Ghosh, S. Bagchi, J. Photochem. Photobiol. A: Chem. 90 (1995) 171. D. Banerjee, P.K. Das, S. Mondal, S. Ghosh, S. Bagchi, J. Photochem. Photobiol. A: Chem. 98 (1996) 183. P.K. Das, R. Pramanik, D. Banerjee, S. Bagchi, Spectrochim. Acta A 56 (2000) 2763. Y. Marcus, Ion Solvation, Wiley, Chichester, 1985. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902.
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[15] M.J.S. Dewar, K.M. Dieter, J. Am. Chem. Soc. 108 (1986) 8075. [16] J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, J. Org. Chem. 48 (1983) 2877. [17] W. Rettig, R. Lapouyade, in: J.R. Lakowicz (Ed.), Topics in Fluorescence Spectroscopy, vol. 4, Plenum Press, New York, 1994. [18] R. Pramanik, P.K. Das, D. Banerjee, S. Bagchi, Chem. Phys. Lett. 341 (2001) 507. [19] S.L. Bondarev, M.V. Belkov, S.M. Dmitriev, Khim. Fizika 3 (1984) 715. [20] J. Everaeri, K.M. Reintzepis, Photochem. Photobiol. 36 (1982) 543. [21] I.A. Vasileva, M.D. Galanin, Z.A. Krasnaya, A.N. Nikitina, Z.A. Chizhikova, Opt. Spectrosk 73 (1992) 301. [22] H. Shirota, E.W. Castner, Jr., J. Chem. Phys. 112 (5) (2000) 2367. [23] M. Almgren, F. Griesser, J.K. Thomas, J. Am. Chem. Soc. 101 (1979) 279. [24] G. Saroja, B. Ramachandran, S. Saha, A. Samanta, J. Phys. Chem. B 103 (1999) 2906.
Studies of solvation in homogeneous and heterogeneous media by electronic spectroscopic method Mrinmoy Shannigrahi, Ramkrishna Pramanik, Sanjib Bagchi * Department of Chemistry, Burdwan University, Burdwan 713104, India Received 24 September 2002; received in revised form 14 January 2003; accepted 5 February 2003
Abstract Solvation characteristics in homogenous (pure and mixed binary solvents) and heterogeneous media (aqueous micelles, b- and g-cyclodextrine solutions) have been studied by monitoring the emission characteristics of a newly synthesised dye. The longest wavelength absorption and emission band of the dye arise due to transition between S0 and S1 state. The maximum energy of electronic transition involving intramolecular charge transfer is found to be dependent on both the hydrogen-bond donating ability and the polarity /polarisability in pure solvent. The dipole moment in the S1 state, as determined by solvatochromic procedure, agrees well with the value obtained by theoretical calculation at the AM1 level. Preferential solvation of the dye by alcohols has been found to occur in ethanol/water, propan-1-ol/ water, propan-2-ol/water binary mixtures. In aqueous micellar media the dye molecule is located at the water /micelle interface. The binding constant for the dye /micelle interaction has also been determined. The results have been compared with those for a structurally related symmetrical ketocyanine dye. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Solvation; Fluorescence probes; Mixed binary solvents; Preferential solvation; Micellar media
1. Introduction It is well known that the electronic transition in molecules, involving intramolecular charge transfer (ICT), is very sensitive to the nature of the microenvironment around the solute and the spectral parameters can be used for studying solute/solvent interaction at the microscopic level [1]. In general several modes of solute /solvent
* Corresponding author. Tel.: /91-342-558545; fax: /91342-564452. E-mail address: [email protected] (S. Bagchi).
interaction have been identified, viz. non-specific (e.g. dipolarity /polarisability) and specific (acidity, basicity) modes and it is customary to write an observed parameter in a solute(s) /solvent(i) system in terms of linear solvation energy relationship [2 /5] X aa (s)Pa (i) (1) P(s; i) P0 (s) where the suffix ‘a’ denotes a particular mode of solute /solvent interaction. P0 and aa terms depend only on the solute and Pas are solvent properties pertinent to a-mode of interaction. As the solventsensitivity of optical response of a solute depends on the solute through aa(s) in Eq. (1), several
1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(03)00120-3
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solutes have been used for studying solvation characteristics [6]. We have, in our laboratory, used ketocyanine dyes [7] as indicator solutes for studying solvation interaction in homogeneous and heterogeneous media [8 /12]. These compounds are characterised by solvent-sensitive absorption and fluorescence bands, which depend to a large extent on the hydrogen bond donation ability of a solvent. But the insolubility of the compounds in water imposes a limitation, for these compounds, to study solvation in aqueous media. In the present work, we have investigated the absorption and fluorescence characteristics of the dye, 2-[3-(N -methyl-N -phenyl amino)-2-propenylidene] indanone (henceforth called dye A) (Fig. 1), in various pure solvents including alcohols, ketones, nitriles, hydrocarbons, chlorohydrocarbon, ester and cyclic ether. The contribution of different modes of solute/solvent interaction towards solvation in pure solvents have been addressed. Solvation characteristics and the nature of transition at 77 K for the dye have been investigated. The non-ideal solvation in mixed aqueous alcohols (water/ethanol, water/propan-1-ol and water/ propan-2-ol) and interaction of the dyes in heterogeneous media (aqueous micelles, b- and g-cyclodextrine solutions) have also been studied. Semiempirical molecular orbital (MO) calculations at the AM1 level have been carried out to confirm some of the measured properties of the dye. The results have been compared with those for a
Fig. 1. Solutes used in the work.
structurally similar ketocyanine dye, 2,5 di-[3-(N methyl-N -phenyl amino)-2-propenylidene] cyclopentanone (dye B, Fig. 1).
2. Materials and methods The dye B was prepared from cyclopentanone, 1,1,3,3-tetramethoxy propane and N -methylaniline by the method described in the literature [7]. The dye A was prepared as follows. Perchloric acid was added to a solution of 1,1,3,3-tetramethoxy propane (25 mM) and N -methylaniline (50 mM) in ethanol. The mixture was kept at 50 8C for 30 min and stored overnight in a refrigerator to give yellow needle-like intermediate compound. The product (14.25 mM) was added to a solution of sodium methoxide and indanone (14 mM). The mixture was refluxed for 2 h and the golden yellow precipitate was filtered off, washed with water, methanol and diethyl ether. Purity of the prepared compound was checked by C, H, N analysis (calculated: C /82.88%, H /6.22%, N /5.09%; observed: C /82.54%, H /5.96%, N /5.08%) and IR data (peaks at 1613, 1565, 1506, 1466 cm 1 in KBr disc). All the solvents used were of spectroscopic grade and were distilled from calcium hydride immediately before use to ensure the absence of peroxides and oxidising agents. Solvent parameters like the refractive index, dielectric constant, and ET (30)-values were taken from the literature [6,13]. Sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide (CTAB) and Triton X100 (TX-100), b- and g-cyclodextrine were received from Sigma and used without purification. For preparing a heterogeneous medium, the required amount of surfactant/cyclodextrine were added to triply distilled water, and the contents kept in an ultrasound sonicator (Jencons, UK Model T80) for a considerable period. Solutions of dyes were prepared by adding the dye to the heterogeneous media with proper sonication. The UV /Vis absorption spectra were recorded on a Shimadzu UV 2101 PC spectrophotometer fitted with a temperature-controlled unit (Model TB-85 Thermobath, Shimadzu). The emission and excitation spectra were taken on a Hitachi F4500
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spectrofluorimeter equipped with polarisation accessory and a temperature-controlled cell holder. For measurement at different temperatures, water was made to circulate through the cell holder from a constant temperature bath (Heto Holten, temperature range: 243 /373 K). The low temperature attachment of the spectrofluorimeter was used for measurements at 77 K. The concentration of dye in solution was in the range 10 5 /10 6 M.
3. Results and discussions 3.1. Theoretical calculations Semi-empirical MO calculations at the AM1 [14,15] level using the MOPAC PC program were carried out for the two dyes. The initial optimisation of the ground state geometries of the dye molecules were achieved using a molecular mechanics program followed by unrestricted geometry optimisation. Electronic energy calculations for the S1 state were obtained from the optimised ground state using configuration interaction using the MICROS option. The choice of a large number of microstates was to ensure true representation of the correlation effect. The method, although not very rigorous, can however be used for confirming some of the measured properties. Charge density on nitrogen and oxygen centre for the dye molecules in the S0 state are shown in Fig. 1. The molecules are planar, except the aliphatic hydrogen atoms. Table 1 lists some of the molecular properties. Note that the energy of S0 0/S1 transition as obtained from the theoretical calculations is in the order: dye A /dye B. This is also the
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order observed experimentally. The absolute magnitude of the calculated energy of the S0 0/S1 transition, for a particular dye, however, shows a deviation from the experimentally observed value. Thus the calculated values are 86.4 and 84.7 kcal mol 1 for dye A and B, respectively, while the corresponding observed values are 70.8 and 64.3 kcal mol 1 in n-hexane. This modest deviation is expected because the calculated values refer to the gas phase. The experimental data in solution would expectedly differ from the gas phase values due to the existence of dielectric effects. Moreover the calculated value refers to S0 (v /0) 0/S1 (v /0) transition while the experimental data refer to S0 (v /0) 0/S1 (v /n ) transition where n is largely unidentified. 3.2. Spectroscopic measurements 3.2.1. Pure solvents at 298 K The longest wavelength absorption and fluorescence spectra of the dye A in various pure solvents at 298 K are almost structureless. The transition energies corresponding to the maximum absorption and fluorescence, E(A) and E(F), respectively, are listed in Table 2. Fig. 2 shows some representative absorption and fluorescence spectra in pure solvents. A mirror image relationship between modified absorption and fluorescence spectra is apparent from the figure. For pure solvents, both E(A) and E(F) shift to lower energies as the solvent becomes more polar. A linear correlation of E(A) with the empirical polarity parameter, ET(30), indicates that longest wavelength band of the dye originates due to ICT presumably from the nitrogen atom to oxygen of the carbonyl group. A
Table 1 Semi-empirical MO calculated parameters at the AM1 level for the two dyes Parameters
Dye A S0 state
Dipole moment (D) Ionisation potential (eV) Heat of formation /(kcal mol 1) Total energy (eV) Transition energy (eV)
4.17 8.21 61.69 /3191.76
Dye B S1 state 6.16 2.83 157.53 /3187.6 3.74
S0 state 3.70 9.91 91.28 /4290.04
S1 state 5.79 7.92 158.54 /4286.78 3.67
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Table 2 Spectroscopic parameters for the dye A in various media at 298 K Media
E(A)a
E(F)a
Water Acetic acid 2,2,2 Trifluoroethanol Ethanol Acetone Acetonitrile Tetrahydrofuran 2-Me Tetrahydrofuran Benzene Ethyl acetate Dichloromethane n -Hexane SDS CTAB TX-100 b-Cyclodextrine g-Cyclodextrine Ethanol glass MTHF glass Toluene glass
61.4 64.0 61.7 65.1 67.3 67.3 68.1 66.0 65.0 68.4 66.8 70.8 63.5 63.5 63.5 63.5 62.2 / / /
54.7 56.3 56.2 56.1 58.6 57.8 60.6 60.8 60.3 60.7 59.1 64.1 55.7 56.6 56.1 55.7 55.1 58.3 60.2 60.2
a
kcal mol 1.
red shift of 2/4 kcal mol 1 is obtained for the dye as one goes from aprotic to protic solvents.
Moreover, the band shifts dramatically to the red as a few drops of alcohol is added to the solution of the dye in aprotic solvents. These facts point to the existence of a solvato-complex of the dye with alcohols, presumably through hydrogen bond interaction of the solvent with the carbonyl oxygen of the dye molecule. A similar conclusion has been obtained for the symmetrical dye B [8]. The red shift of the absorption band with increasing polarity indicates that there is an increase in the dipole moment (m) on excitation. The dipole moment of the solute in the excited state has been determined from the solvatochromism of the absorption and fluorescence band by a method described in earlier communications [8,12]. In this procedure E(A)/E(F) and E(A) /E(F) are plotted against appropriate dielectric functions, 2(o /1)/ (2o/1) and [2(o /1)/(2o/1) /2(n2 /1)/(2n2/1)], respectively, where n is the refractive index and o is the dielectric constant of the solvent. The ratio of the dipole moment in the S1 state (m1) to that in the S0 state (m0) is obtained from the ratio of the slopes of the linear plots. This method does not require the knowledge of the cavity radius. The ratio m1/m0 is found to be 1.66 for dye A, which is
Fig. 2. Representative modified absorption spectrum [o (/n) ¯ ///n¯/vs. n] ¯ (solid line) and modified emission spectrum [I (/n) ¯ ///n¯/3 vs. n] ¯ (dotted line) of dye A in ethanol (1) and dichloromethane (2).
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slightly greater than its symmetric counterpart [9]. The value of the dipole moment in the S1 state has been calculated by using the value of m0 obtained from semi-empirical AM1 calculation. This procedure gives a value of m1 /6.95 for the dye A, which agrees well with the theoretically calculated value of 6.16 (Table 1). The emission band for the dye A may be characterised as S1 0/S0 fluorescence. No phosphorescence could be detected even at 77 K. Plot of E(F) versus ET(30) shows a double linear correlation as shown in Fig. 3. Data points for aprotic solvents fall on a line with a higher slope. Similar behaviour has been observed for the dye B [9]. A double linear correlation indicates that the nature of the emitting state is significantly different in the two classes of solvents. To obtain an insight into the contribution of specific and nonspecific modes of interaction towards solvation, a multiple linear regression analysis has been done. Correlations of E(A) or E(F) were sought with Kamlet and Traft’s parameters, p*, a and b representing, respectively, the solvent polarity / polarisability, hydrogen bond donating (HBD) and accepting (HBA) ability of the solvent [16]. The following regression equations were obtained
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E(A) (kcal mol 1 ) 70:0 3:41a0:84b 4:30p ; n 12;
(2)
r 0:88
E(F) (kcal mol 1 ) 63:8 2:62a 2:08b 4:30p ; n 12;
(3)
r 0:96
Eqs. (2) and (3) indicate that E(A) is determined mainly by a and p*, but E(F) has a significant correlation with b, the HBA ability of the solvent. From the correlation Eq. (3), it appears that the coefficient of p* term is greater than that of the a or b term indicating that dipolarity /polarisability interaction is more important than the hydrogen bonding interaction for this newly synthesised unsymmetrical dye A. This result is also in conformity with our previous work with other unsymmetrical dyes [12]. But our previous work using the symmetric dye B indicates that the specific HBD interaction of the dye with the solvent is relatively more important than the non-specific modes of solvation interaction [9]. For the symmetric dye, the increased electron density on the carbonyl oxygen in the S1 state,
Fig. 3. Plot of E(F) vs. ET(30) for the dye A in pure solvents.
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which favours the HBD interaction, also leads to an extension of conjugation, giving extra stability. An interesting observation is obtained when dry HCl gas is passed into the ethanolic solution of the two dyes. For the symmetric dye B, the absorption band around 500 nm in pure ethanol gets reduced in intensity while a new band with maximum at 660 nm appears. Similar result was obtained when ethanol was replaced by another aprotic solvent. For the unsymmetrical dye A, however, only a small shift of the absorption band is obtained (Fig. 4). These observations suggest that HCl reacts with the dye B as shown in Scheme 1. The product formed has an extensive conjugation between the two N atoms which was absent in the dye molecule due to the presence of an electron withdrawing carbonyl group at the middle. The enhancement of delocalisation leads to an extra stability of the product. Such extensive conjugation, however, is not possible in the case of unsymmetrical dye and no chemical reaction thus takes place. 3.2.2. Pure solvents at 77 K The emission bands of the dyes in ethanol, 2methyl tetrahydrofuran (2-MTHF) and toluene at
different temperatures are shown in Fig. 5. In all the cases the observed spectrum is independent of the concentration of the solute and the excitation wavelength. The shape of the band at 77 K also differs from that at 298 K. The excitation spectrum at 298 K in all the cases is identical with the absorption spectrum. However, the excitation spectra at 77 K show prominent vibrational structure, which can be assigned as carbonyl vibration. The fluorescence band for both the dyes in ethanol suddenly changes shape as the glass melts. The peak in the glass (/490 nm for dye A and /570 nm for dye B) gets reduced in intensity and simultaneously the intensity at higher wavelength increases. On further increase in temperature the spectrum approaches that obtained at 298 K. For 2-MTHF and toluene the fluorescence band at 77 K shows permanent structures for the dye A; the position of maximum absorption does not changes. However, the relative intensity of the lower wavelength band increases over that of the higher wavelength band at 77 K. For the dye B, however, the relative intensity of the higher wavelength band increases at 77 K. Since the excitation spectrum does not change on going from 298 to 77 K, any change in the fluorescence
Fig. 4. Absorption spectrum for dye A (dotted line) and dye B (solid line) in pure ethanol (1) and acidic ethanol solution (2).
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Scheme 1.
spectrum can be rationalized in terms of modification of the S1 state of the solute by interaction with the solvent molecules. The S0 state of the dyes in ethanol are stabilised by hydrogen bonding. This hydrogen bonding becomes tighter in the S1 state due to an increased charge density on the carbonyl oxygen atom. This H-bonded state is the emitting state in ethanol at 77 K leading to emission band at 490 nm for the dye A and 570 nm for the dye B. Appearance of this band in ethanol/methanol glass supports this view. The abrupt change in the shape of the fluorescence band in ethanol on melting points to a possible molecular motion and related geometry change in the S1 state in the fluid phase. As stated earlier, the double linear correlation of E(F) with ET(30) at 298 K indicates that the emitting state in an alcohol solvent is different from that in aprotic solvents. Our results indicate that the emitting state for ethanol at 298 K is not the initially prepared ICT state. One possibility is the formation of a TICT state [17] involving rotation of the amino nitrogen in the S1 state. Formation of TICT state in this case will be facilitated due to greater solvent stabilisation and enhanced affinity of the hydrogen bonded carbonyl group [18]. For aprotic solvents like 2-MTHF and toluene, appearance of emission band maximum at 77 K with almost unchanged position (relative to that at 298 K) but with a change of relative intensity suggests that different conformations are possible in the S1 state and the population differs as temperature is changed. There are two (four) ethylenic double bonds in the chromophoric part of the molecule of dye A (dye B) and several conformers formed by rotation about
quasi-single C /C in the S1 state is possible. Existence of such conformers has been documented by earlier studies on retinyl acetate [19,20] and other ketocyanine dyes [18,21]. 3.2.3. Binary aqueous solvents In the present work three binary aqueous mixtures, namely, water/ethanol, water/propan-1-ol and water/propan-2-ol have been studied using the dye A as the probe molecule. Table 3 lists the various absorption and fluorescence spectral parameters. Both the absorption and fluorescence maxima of the solute shifts towards shorter energy as the percentage of water in the binary solvent mixture increases. For all the three binary aqueous solvent mixtures, non-linear relationship between the spectral property [E(A) or E(F)] and mole fraction of water has been observed indicating a non-ideal solvation behaviour of the solute [13]. The nature of the deviation from linearity also suggests that the alcohols are preferred by the solute over water. The preferential solvation of the dye by alcohol over water can be explained in terms of strong self-association of water molecule through hydrogen bonding. The molecule of the solute having a hydrophobic wing is unable to break the water structure. Thus, the percentage of free water molecule that takes part in the solvation process decreases. Plots of Stoke’s shift versus the mole fraction of water in the binary mixtures are given in Fig. 6. It is known that the Stoke’s shift (Dn ) is related to the solvent reorganisation energy (l ). The solvent reorganisation about the CT excited state of the solute is expected to increase with the polarity of the solvent. Thus,
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Fig. 5. Emission spectra (a) for dye A and (b) for dye B in pure solvents (1) ethanol, (2) 2-methyltetrahydrofuran and (3) toluene at room temperature (solid line) and at 77 K (dotted line).
of the four solvents viz, water, ethanol, propan-1ol and propan-2-ol, the highest value of Stoke’s shift is expected for water. But our experimental results indicate the order of solvent reorganisation energy as ethanol /water /propanols. Moreover,
the value of l increases as the mole fraction of ethanol in ethanol/water system increases. The lower value of l for water than that for ethanol probably arises due to lack of solubility in water. Although water has a relatively high polarity it
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Table 3 Spectroscopic parameters of the dye A in various mixed aqueous solvents as a function of solvent composition xw
1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50 0.40 0.30 0.25 0.20 0.15 0.10 0.00 a
Water /ethanol E(A)a
E(F)
61.4 62.0 62.5 62.9 63.2 63.8 64.1 64.4 64.5 64.6 64.7 64.7 64.8 64.9 65.1
54.7 54.9 55.0 55.3 55.5 55.8 56.1 56.2 56.3 56.3 56.4 56.4 56.4 56.5 56.6
Water /propan-1-ol a
E(A) /E(F) 6.7 7.1 7.5 7.6 7.7 8.0 8.0 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
a
E(A) 61.4 61.3 61.2 61.2 61.2 61.2 61.3 61.4 61.5 61.7 61.8 61.9 62.0 62.1 62.3
a
E(F) 54.7 55.1 55.3 55.5 55.7 56.0 56.1 56.2 56.2 56.3 56.3 56.4 56.4 56.5 56.8
a
Water /propan-2-ol E(A) /E(F) 6.7 6.2 5.9 5.7 5.5 5.2 5.2 5.2 5.3 5.4 5.5 5.5 5.6 5.6 5.5
a
E(A) 61.4 61.3 61.3 61.4 61.4 61.4 61.5 61.6 61.7 61.9 62.0 62.1 62.2 62.3 62.6
a
E(F) 54.7 55.1 55.5 55.7 55.9 56.2 56.4 56.5 56.5 56.5 56.6 56.6 56.7 56.9 57.3
a
E(A) /E(F)
a
6.7 6.2 5.8 5.7 5.5 5.2 5.1 5.1 5.2 5.4 5.4 5.5 5.5 5.4 5.3
kcal mol 1.
does not contribute to the reorganisation stabilisation of the CT excited state presumably due to hydrophobicity of the dye molecule. Our results indicate that solvent reorganisation energy increases as the percentage of ethanol in the water/ethanol mixture increases. The dependence
of the reorganisation energy with xwater, the mole fraction of water, is also non-linear. In water/ propanol systems, the solvent reorganisation energy, however, decreases as the percentage of propanol in the mixture increases. The nature of variation of l in these cases are also non-linear.
Fig. 6. Plot of Stoke’s shift as a function of solvent composition (xwater) in water/ethanol (j), water/propan-1-ol (m) and water/ propan-2-ol (') binary mixtures for dye A.
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The nature of deviation from linearity also suggests that the solute is preferentially solvated by alcohols. From Fig. 6, it appears that for small percentage of water (0 B/xwater B/0.1) a linear variation of the solvent reorganisation is followed. The deviation from ideality increases as the mole fraction of water increases. Note that, the maximum deviation from linearity appears at xwater :/ 0.7 for all the aqueous alcohols. Thus in this region the solvent /solvent interaction leading to nonideal solvation behaviour is maximum. Similar types of variation of E(A), E(F) and the Stoke’s shift has been observed by Shirota et al. for water/propan-1-ol using the coumarine 153 probe [22]. 3.2.4. Heterogeneous media The maximum energy of fluorescence of the dye A in the aqueous micelles, b- and g-cyclodextrine (CD) media at 298 K are listed in Table 2. Fig. 7 shows some representative spectra. It appears that there is a general blue shift as one goes from water to micellar media. The maximum energy of transition is in the order of CTAB /TX-100/ SDS :/b-CD /g-CD /water. Similar order of maximum transition energy has been observed for the dye B [11]. Moreover, the band is structured in water and also in an aqueous surfactant solution when the concentration is below the critical micelle concentration (CMC). In a micellar medium the intensity of fluorescence increases and the band becomes structureless. This points to a substantial dye/micelle interaction. For a particular concentration of the micelle, the intensity of fluorescence increases as the temperature is decreased. The presence of a dye/micelle interaction is also substantiated by the greater solubility of the dyes in the micellar media. The binding of the dyes in the micellar media also finds support from the high value of fluorescence anisotropy which indicates slower rate of rotational motion of the transition dipole. As discussed earlier, the fluorescence characteristics of the dyes originate from ICT transition. In a protic solvent maximum energy of transition is largely determined by the HBD interactions between the solvent and the carbonyl oxygen atom of the dye in the excited state. The greater the hydrogen bonding ability,
the lower is the energy of transition. The present study indicates that for the dye A the HBD interaction increases in the order CTAB /TX100/SDS :/b-CD/g-CD /water. The position of the fluorescence band maximum for the dye A indicates that the immediate environment around the chromophoric part of the molecule is in between that of water and ethanol. Thus we infer that the dye molecule is located at the micelle / water interface. The negative charge density of the carbonyl group of the dye will be involved in the electrostatic interaction with the polar head group of the ionic micelles. Hydrophobic interaction of the hydrocarbon moiety of the dye with micelle also plays a role. In the CTAB micellar medium, the positively charge head groups will interact favourably with the negative charge density of the carbonyl oxygen of the dye, particularly in the S1 state. Thus the dye will be transferred to a more hydrophobic environment and the extent of hydrogen-bonded interaction with water will be diminished, leading to an increase in the transition energy of fluorescence. In the SDS micelle, on the contrary, the negatively charged head groups will repel the negative charge density on the carbonyl oxygen; thus the chromophoric part of the dye will be in a more hydrophilic environment and will be more effectively hydrogen bonded by water. TX100 consists of bulky phenyl head group and a long polyoxy-ethylene chain that terminates in an /OH group. Here also the interaction of the dye with the terminal /OH group of the micelle is important. The appearance of the fluorescence maximum of the dye A in TX-100 in a position intermediate between that of CTAB and SDS micelle indicates that the micro-polarity of the local region of the fluorophore is in between those for the cationic and anionic micelles. The micropolarity, as measured by the maximum energy of fluorescence of dye A, in cyclodextrine solution is very similar to that in the case of SDS micelle. Thus the dye is located away from the CD-bucket and the H-bonding interactions of the dye with the terminal /OH groups of CD are important. The higher value of Stoke’s shift for dye A in the cationic micelles over that in the anionic micelle suggest that the electrostatic contribution to the reorganisation stabilisation of the CT excited state
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Fig. 7. (a) Representative emission spectra of dye A in CTAB (1), TX-100 (2), SDS (3) and water (4) (concentration of micelle 10 CMC) (b) Representative plot of Eq. (5) for dye A in TX-100.
of the dye is relatively large compared to the contribution due to HBD interaction. In the case of dye B molecule the reverse order of Stoke’s shift has been reported [11]. This probably originates due to the difference in the orientation of the dye with respect to the micelle. For the unsymmetrical dye A, one end (positive/negative) of the dipolar
molecule remains attached to the micelle surface keeping the other end away from the micelle, thus minimising electrostatic interaction. But this is, however, not possible for the symmetric structure. A quantitative estimate of the binding constant (K ) for the equilibrium, representing dye /micelle interaction, can be obtained by using the equation
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proposed by Almgren et al. [12,23,24]. Thus, for the equilibrium DMX D M
(4)
where D and M represents dye molecule and micelles respectively, we have (Ia I0 )=( I I0 ) 1(K [M]) 1
(5)
where Ia, I0, and I are the fluorescence intensities under complete micellisation of the dye, in absence of micellisation, and at any intermediate micelle concentration, respectively. [M] represents the concentration of the micelle, which is given by [M] ([Surf] CMC)=N
(6)
In the above equation [Surf] represents the surfactant concentration and N is the aggregation number. For the calculation of [M] the values of N /62, 60, and 143 for SDS, CTAB, and TX-100, respectively, were assumed. Fig. 7 shows a representative plot of Eq. (5). The measured K values (9/10%) are given in Table 4. Note that the value of the binding constant of both the dyes with cationic micelle, CTAB, is about ten times higher than that with the anionic micelle, SDS. This can be explained in terms of the electrostatic interaction between the dye and the micelle. The negative part of the molecules of the dye (the carbonyl oxygen) can come closer to the cationic micelles while the positive part of the dye molecule, being sterically hindered, remains away from the micelle. In the case of the neutral micelle, hydrogen bonding, rather than the electrostatic interaction is important and the unhindered oxygen atom in the dye molecule can approach the micellar surface closely. Table 4 Values of binding constant for the two dyes at 298 K Micellar media
Triton-X (100) SDS CTAB
Binding constant Dye A
Dye B
1.8 /105 2.0 /104 1.9 /105
2.3 /104 1.0 /103 1.2 /104
4. Conclusion 1) For the dye A, dipolarity /polarisability mode of solute/solvent interaction is more important than the H-bonding interaction, particularly at the S1 state. The fluorescence characteristics of the symmetrical dye B, on the other hand, are largely influenced by hydrogen bonding of the protic solvent with the carbonyl oxygen. 2) Measurements at 77 K indicate that the emitting state in an alcohol solvent in the fluid state is not the initially prepared state on excitation. 3) Although the fluorescence maximum is largely determined by H-bonding interaction of a protic solvent with the chromophoric part of the dye molecules, the solvent reorganisation leading to the stability of the CT excited state is largely determined by the hydrophobic interaction of the dye molecule as a whole with the solvent. 4) The enhanced solubility of the newly synthesised dye A in water makes it a better fluorescence probe for studying solvation interaction in aqueous media than its symmetrical counterpart. 5) The dye /micelle binding for ionic micelles is mainly determined by electrostatic interaction.
Acknowledgements M.S. thanks CSIR (India) for a fellowship. The authors also thank UGC (India) [DSA Programme] for financial assistance.
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