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Photophysics of 4-N,N-dimethylamino cinnamaldehyde in AOT reverse micelles and exploration of its position and orientation
Chemical Physics Letters 367 (2003) 330–338 www.elsevier.com/locate/cplett
Photophysics of 4-N,N-dimethylamino cinnamaldehyde in AOT reverse micelles and exploration of its position and orientation Subhasis Panja, Sankar Chakravorti
*
Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Received 21 August 2002; in final form 8 October 2002
Abstract An attempt has been made in this Letter to locate the position and orientation of 4-N,N-dimethylamino cinnamaldehyde (DMACA) inside sodium bis(2-ethylhexyl) sulfosuccinate (AOT)–n-heptane reverse micelle based on change in photophysical properties of DMACA compared to that in n-heptane. It has been proposed that the possibility of finding the donor moiety inside the small water pool of reverse micelle is maximum while the acceptor group straddles in the remaining part of the reverse micelle. The micropolarity in the vicinity of the donor moiety has been computed in terms of dielectric constant with varying water pool size. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Reverse micelles are thermodynamically stabilized organized systems formed by surfactant molecules in hydrocarbon solvents with polar head groups pointing inwards. When water is added to surfactant in hydrocarbon solution a spherical discrete droplet of nanometer size is built by aggregation of inward pointing polar head groups. In sodium bis (2-ethylhexyl) sulfosuccinate (AOT)– n-heptane reverse micellar system the radii of these water pools are defined by twice the molar ratio
*
Corresponding author. Fax: +91-3347-32805. E-mail address: [email protected] (S. Chakravorti).
(2W0 ) between the polar component and the surfactant {AOT} where W0 ¼ ½H2 O=½AOT. The value of W0 for reverse micelles lies between 10 and ) and those with radii 15 (radius 2W0 ¼ 30 A are called microemulmore than hundreds of A sions [1–3]. As reverse micelles provide interesting microenvironments there has been an engaging interest in the studies for chemical, enzymatic catalysis [4,5] and biological systems [1,6–11]. Some processes like isomerization [12], energy transfer [13], proton transfer [14,15] get modified greatly inside the restricted pool of water probably due to the special properties of reverse micelles. Reverse micelles are charged inside due to which a huge change of solvent relaxation time between inner side of the droplet compared to outside [16] could be observed. Several groups have studied relaxa-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 6 9 4 - 9
S. Panja, S. Chakravorti / Chemical Physics Letters 367 (2003) 330–338
tion dynamics of water in various self-organized assemblies with different techniques [17–22] and found that the relaxation time is much slower in water filled in nanospace than that in bulk. Some molecules upon photoexcitation undergo internal rotation that leads to twisted intramolecular charge transfer (TICT) state [23]. This process leads to an additional fluorescence whose quantum yield strongly depends on solvent polarity and viscosity [24]. Kim and Lee [25] observed that fluorescence lifetime of a hemicyanine dye in the water pool of reverse micelle increases with increasing pool size, implying the significant influence of microstructure of reverse micelle on excited state twisting motion. We have identified one molecule 4-N,N-dimethylamino cinnamaldehyde (DMACA) [26] which forms TICT state in aprotic solvents but in protic solvent hydrogen bonding of acceptor part of the molecule and the solvent trammels the formation of TICT state. This property of DMACA has prompted us to use this molecule as a probe in exploring different regions of reverse micelle, water pool and the micellar interface, through the twisting dynamics of DMACA and related photophysical change.
2. Experimental details The compound DMACA was purchased from Aldrich Chemical, USA and was purified by vacuum sublimation. The solvents acetonitrile (ACN), tetrahydrofuran (THF), chloroform (CHCl3 ), N,N-dimethyl formamide (DMF), dichloro methane (DCM), n-heptane, methanol, ethanol and butanol (E. Merck, spectroscopic grade) were used as supplied, but only after checking the purity fluorimetrically in the wavelength range of interest. Sodium bis(2-ethylhexyl) sulfosuccinate received from Fluka was dried in vacuum before use. For aqueous solution, we used deionized MilliPore water. Sufficient time 12 h was given for the equilibration of micellar solution. The absorption spectra at 300 K were recorded with a Shimadzu absorption Spectrophotometer model UV-2101PC, and the fluorescence spectra were obtained with a Hitachi F-4500 fluorescence spectrophotometer. For emission measurement,
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the sample concentration was maintained at 105 M, in each case in order to avoid aggregation. The fluorescence lifetime >500 ps was measured by time-correlated single photon counting coupled to a micro channel plate photomultiplier (Model 28090, Hamamatsu, Edinburgh Instrument). The degree of polarization of fluorescence at room temperature was obtained by using a modified polarizer accessory with the Hitachi F-4500 spectrophotometer.
3. Results and discussion 3.1. Absorption spectra DMACA in n-heptane shows a strong absorption band 350 nm. With addition of AOT the absorption band shows a gradual decrease of its absorbance along with a redshift. Fig. 1a depicts the AOT concentration dependent absorption spectra of DMACA in n-heptane. A new absorption shoulder in the lower energy side (380 nm) appears with the addition of AOT. Absorption spectra of DMACA have been recorded by varying amount of water in a fixed concentration of AOT in n-heptane, i.e., varying the water pool size (W0 ) of the reverse micelle. Fig. 1b depicts the water pool size (W0 ) dependent absorption spectra of DMACA in 0.1 M of AOT in n-heptane solution. The absorbance shows a sharp increase along with a little redshift with the increase of water pool size. 3.2. Emission spectra DMACA in all polar solvents shows distinct dual fluorescence, the higher energy emission (350 nm) assigned to be arising from the locally excited (LE) state, called the LE emission [23]. The lower energy emission above 440 nm assigned to be arising from the twisted intramolecular charge transfer (TICT) state [23] and is called as TICT emission. The TICT emission could only be observed in polar solvents but in non-polar solvents it is absent. In all polar protic solvents hydrogen bonding between the C@O group of solute and the hydroxylated part of the solvent plays a crucial role to stabilize the TICT state. Accordingly the
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Fig. 1. (a) AOT concentration dependent absorption spectra of DMACA in n-heptane. (b) W0 dependent absorption spectra of DMACA in AOT–n-heptane reverse micelle.
Fig. 2. (a) AOT concentration dependent emission spectra of DMACA in n-heptane. (b) W0 dependent emission spectra of DMACA in AOT–n-heptane reverse micelle.
TICT emission shows more redshift in polar protic solvents compared to the solvent polarity with respect to the other polar aprotic solvents. Hydrogen bond formation act as one of the nonradiative channel of the TICT emission and the TICT emission intensity gradually decreases with the increase of hydrogen bond donating tendency of the solvent [26]. In n-heptane DMACA hardly shows any TICT emission due to the lower polarity of the solvent but with the addition of AOT in n-heptane an emission band arises at 440 nm (being excited at 345 nm) as the TICT emission. Fig. 2a shows the AOT concentration dependent
TICT emission of DMACA in n-heptane. The TICT emission of DMACA in n-heptane–AOT reverse micellar media shows sharp change with the increase of water concentration. Fig. 2b shows the variation of the fluorescence spectra of DMACA in n-heptane–AOT reverse micelle as function of water pool sizes (W0 ). For initial addition of water (up to W0 3) the TICT emission shows an strong enhancement along with a large redshift (up to 470 nm) but with further increase of water pool size the band position shows a little change though the emission intensity increases with increase of water pool size. The emission
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shows a maximum intensity for water pool size ðW0 Þ 20 and addition of further water does not effect the emission significantly. The TICT fluorescence lifetime measurement showed that the lifetime does not change appreciably in the nanosecond range with varying water pool size. The excitation wavelength dependent emission spectra have been recorded for a fixed value of W0 ð 15Þ which shows that the emission band position remains unaltered for different excitation wavelengths (350–390 nm). The excitation spectra of the same solution have been recorded for different emission wavelengths, which reflect the absorption spectrum. This fact indicates the presence of single type emitting species in the reverse micellar system. The gradual decrease of the absorbance as well as the redshift with the addition of AOT in nheptane indicates the change of polarity of the surrounding environment of the fluorophore. The sharp change of the absorption spectra with water pool sizes (W0 ) in n-heptane–AOT reverse micellar system suggests that water pool is accessible to DMACA molecules. So the DMACA molecule resides either in the micellar–water pool interface region or in the water pool of the reverse micelle or spreading in both regions to get the polarity effect in ground state of DMACA. In pure n-heptane the TICT band is completely absent due to the lower polarity of the solvent, as we know the TICT state formation barrier depends upon the solvent polarity. Eisenthal and coworkers [27,28] proposed a semiempirical formula that the TICT state formation barrier (EB ) decreases linearly with the increase of solvent polarity following the relation: EB ¼ EB0 A½ET ð30Þ 30; EB0
ð1Þ
where is the barrier in hydrocarbon having ET ð30Þ values 30 kcal mol1 . So with the initial addition of AOT in n-heptane the DMACA goes to polar state and consequently shows the TICT emission according to the semi-empirical formula of Eisenthal et al. The W0 dependent emission spectra show an initial strong enhancement and a large redshift with the increase of water amount. So the water pool size dependent emission spectra also confirm that water pool is accessible to
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DMACA to get the polarity effect in the excited state in n-heptane–AOT reverse micelle. Up till now we have gathered from absorption and emission spectra that a part or the whole DMACA molecule is inside the water pool. In our next part of this work we will try to find the solubulization position and the orientation of the DMACA molecule within the n-heptane–AOT reverse micellar system. In our earlier work [26] we found that the TICT emission of DMACA shows an unusually large redshift in polar protic solvents than those expected on the basis of solvent polarity in polar aprotic solvent alone. The fluorescence band position bears a linear correlation with hydrogen donating parameter (a) of the solvent (Fig. 4a). The fluorescence yield decreases with the increase of hydrogen bonding tendency of the solvents. These facts suggest that hydrogen bond formation between the carbonyl group (C@O) of the fluorophore (hydrogen bond acceptor) and the hydroxylated solvents (hydrogen bond donor) acts as a non-radiative channel of the TICT state as proposed by Testa [29]. In AOT reverse micellar system the TICT emission of DMACA shows an initial enhancement and red shift with the increase of water pool size (W0 ) whereas for water pool size greater than 15 the emission spectra does not show any sharp response. In this regard we have studied the effect of addition of water in its absorption and emission spectra of DMACA in acetonitrile (ACN) solvent. ACN is a polar aprotic solvent so hydrogen bond formation between the solute and solvent is not possible. With addition of water in acetonitrile, hydrogen bond formation between the solute DMACA and the added water molecule is possible. Initially the absorption spectra of DMACA in ACN show a little increase along with a small red shift with the increase of water concentration (Fig. 3a). The emission spectra of DMACA in ACN (Fig. 3b) show a sharp decrease in its intensity as well as a red shift with the increase of water concentration which clearly indicate the formation of hydrogen bond between the carbonyl (C@O) group of DMACA and hydroxylated part of the water molecule. Fig. 3b (inset) depicts the variation of normalized emission of DMACA with the increase of water concentration
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Fig. 3. Variation of absorption spectra (a) and emission spectra (b) of DMACA in ACN with the increase of water concentration. Inset shows the variation of band position of normalized emission spectra.
in ACN. Considering the above facts and the spectral observations relating to DMACA in nheptane–reverse micelle we can infer that the hydrogen bond formation between the C@O group of DMACA and the water molecule of the water pool of the n-heptane–AOT reverse micelle does not occur as no indication of decrease of intensity or extra redshift of emission could be observed even for the larger water pool size or greater water concentration. So it is sure that the carbonyl (C@O) group of the DMACA does not reside inside the water pool of the n-heptane–AOT reverse micellar system.
Fig. 4. (a) Variation of band position of DMACA with hydrogen bonding parameter (a) in polar protic solvent (b) Lippert–Mataga parameter (Df) corresponding to the Stokes shift in different water pool sizes.
Having established the above we now try to locate the orientation of the molecule in reverse micelle. The TICT state is the most polar state of the molecule DMACA and in its excited state the donor (amino) group releases an electron to the acceptor carbonyl (C@O) group as a result of complete decoupling between dimethylamino and the carbonyl moieties by twisting the amino group in a perpendicular position. The donor [NðCH3 Þ2 ] group becomes cationic and the acceptor (C@O) group becomes anionic in nature in the TICT
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Scheme 1. Approximate presentation of orientation of DMACA within AOT–n-heptane reverse micelle.
state. Now we consider the structure of the reverse micelle formed by the anionic surfactant AOT. The polar heads ()ve ion) of the AOT surround the water pool. So the acceptor part of the molecule would like to go away from the water due to the electrostatic repulsion by the polar head of AOT and also due to the relative hydrophobicity of C@O group of DMACA [26]. Earlier we have observed that DMACA is able to reach water in n-heptane–AOT reverse micelle. So considering the electrostatic attraction between the polar head of the AOT (anionic) and the fluorophore (cationic, donor group) DMACA and hydrophobic force on acceptor C@O we can predict that, the donor [NðCH3 Þ2 ] group of DMACA would try to reside near the polar head of the AOT reverse
micelle in the water pool. At this point we may consider two types of possible orientations of DMACA within the AOT–n-heptane reverse micelle. In one case the donor group is inside the water pool and near the polar head of AOT due to electrostatic attraction and the acceptor is situated outside the water pool (Scheme 1a). The other case being both the donor and acceptor situated outside the water pool with the acceptor part near the water pool–micelle interface (Scheme 1b). To have a better idea about the exact location and the orientations of the molecule DMACA in n-heptane–AOT reverse micelle we have measured the variation of TICT fluorescence polarization of DMACA as a function of water pool size of the
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reverse micelle. The degree of polarization (P) of fluorescence is defined as P¼
Ik GI? ; Ik þ GI?
ð2Þ
where Ik and I? are the intensity of the emitted light polarized with electric wavevector parallel and perpendicular, respectively, to the excited light which is polarized with electric vector perpendicular to the plane containing the excited and emitted beam. G is the correction factor of the polarization of the fluorescence spectrophotometer defined as G ¼ Ik =I? . Fig. 5 shows the variation the fluorescence polarization of DMACA with water pool sizes (W0 ) of the AOT reverse micelle. DMACA shows a very negligible fluorescence polarization in pure n-heptane or in pure aqueous media but just after the water droplet is formed (W0 < 4) the fluorescence polarization shoots up and with increasing water pool size (W0 5 or more) the fluorescence polarization decreases exponentially. It seems that the molecule faces initial restriction in small pool size which decreases with bigger pool size.
Fig. 5. Variation of polarization with water pool size in AOT– n-heptane reverse micelle.
For the sake of understanding we take help of Scheme 1, a possible position in dynamic equilibrium. In Scheme 1b the fluorophore DMACA is situated in the interfacial region with possible random orientations in such a way that the acceptor group does not enter inside the water pool. In such a case with the increase of water pool size the intermolecular distance between the surfactant (AOT) molecule will increase and the donor group would get much more space for its free rotation. So the restriction imposed upon the donor group by the AOT molecule will gradually decrease with the increase of the water pool size which will cause a decrease in the fluorescence polarization [31]. No such decrease of polarization could be observed in our case that indicates that the donor group is certainly not in the interfacial region of the reverse micelle. So Scheme 1b is untenable on the basis of the observed polarization data. In n-heptane–AOT reverse micelle and the the radius of the water pool is 2W0 A [26]. Let length of the DMACA molecule is 4 A us now consider Scheme 1a where the donor group resides inside the water pool of the reverse micelle. Before adding water the AOT and DMACA molecules roam about randomly. As soon as the water droplet is formed with smallest radius the donor group moves into the pool and faces restriction of free rotation of NðCH3 Þ2 group but the restriction is decreased with bigger pool size (W0 > 5). Beyond this pool size (W0 > 5) the polarization remains almost unaltered (Fig. 5). Thereby we may infer that beyond this size the restriction for free rotation of donor molecule) is constant. So group (a part of 4 A Scheme 1a efficiently explains polarization and other data. We have tried to estimate the micropolarity of the solubilization position of the fluorophore DMACA inside the reverse micellar media by monitoring the TICT band position. The TICT state possesses a large dipole moment and according to Lippert–Mataga [30] the energy of such polar band depends in a linear way with the solvent polarity following the relation: ðDmÞ ¼ ma mf ¼
2l2e f ðD; nÞ; 4pe0 hcq3
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Table 1 Variation of calculated dielectric constant of the surrounding environment of DMACA with the increase of water pool sizes in nheptane–AOT reverse micelle [water]/[AOT]ðW0 Þ
ðDmÞ in cm1 ð 103 Þ
Calculated f ðD; nÞ from Lippert–Mataga plot
Calculated dielectric constant
0 3 7 15 20
4.27 4.59 4.72 4.81 4.91
0.181 0.241 0.263 0.281 0.30
4.5 7.8 10.8 14.5 22.9
where the Lippert–Mataga parameter ðD 1Þ ðn2 1Þ f ðD; nÞ ¼ ; ð2D þ 1Þ ð2n2 þ 1Þ ma ; mf are the absorption and emission wavelength, D and n are the dielectric constant and refractive index of the medium h; e0 ; c; q are PlanckÕs constant, permittivity of the vacuum, velocity of light and the radius of the solvent cavity, le represents the excited state dipole moment. In all polar protic solvent the TICT emission shows a linear relationship with the hydrogen bond formation tendency of the solvent but in all polar aprotic solvents the Stokes shift ðDmÞ shows a linear dependence with the Lippert– Mataga f ðD; nÞ parameter of the solvent. Fig. 4b shows the variation of Dm of DMACA with f ðD; nÞ in different solvents. The different f ðD; nÞ of varying water pool sizes corresponding to the solubilization position of the DMACA can be estimated from the corresponding Dm. Assuming the refractive index of water pool of the reverse micelle remains same (1.33) we have calculated the dielectric constant D of the surrounding environment of the DMACA and have been recorded in Table 1. Acknowledgements The authors express their deep sense of gratitude to Professor S. Basak, SINP, Kolkata for kindly allowing them to use the fluorescence lifetime measuring instrument. The authors express their sincere thanks to the reviewer for his critical comments and suggestions towards the improvement of the Letter.
References [1] M. Zulauf, H.-F. Eicke, J. Phys. Chem. 83 (1979) 480. [2] P.L. Luisi, M. Giomini, M.P. Pileni, B.H. Robinson, Biochim. Biophys. Acta 947 (1988) 209. [3] N.M. Correa, M.A. Biasutti, J.J. Sibler, J. Coll. Interface Sci. 172 (1995) 71. [4] J.H. Fendlar, E.J. Fendlar, Catalysis in Micellar and Macromolecular systems, Academic Press, New York, 1975. [5] P.L. Luisi, B.E. Straub, Reverse Micelles, Amphiphilic Structures in Polar media, Plenum Press, New York, 1984. [6] P.L. Luisi, Angew. Chem. Int. Ed. Engl. 24 (1985) 439. [7] J.H. Fendlar, Annu. Rev. Phys. Chem. 35 (1984) 137. [8] G. Cassin, J.P. Badiali, M.P. Pileni, J. Phys. Chem. 99 (1995) 12941. [9] J. Eastoe, W.K. Young, B.H. Robinson, J. Chem. Soc., Faraday Trans. 86 (1990) 2883. [10] Y.C. Jean, J.H. Ache, J. Am. Chem. Soc. 100 (1978) 984. [11] M. Kotlarchyk, J.S. Huang, S.H. Chen, J. Phys. Chem. 89 (1985) 4382. [12] D.-M. Shin, K.S. Schanze, D.G. Whitten, J. Am. Chem. Soc. 111 (1989) 8494. [13] G. Rothenberger, P.P. Infelta, M. Gr€atzel, J. Phys. Chem. 83 (1979) 1871. [14] M.J. Politi, H. Chaimovich, J. Phys. Chem. 90 (1986) 282. [15] P.D.I. Fletcher, A.M. Howe, B.H. Robinson, J. Chem. Soc., Faraday Trans. I 83 (1987) 985. [16] S. Vajda, R. Jimenez, S. Rosenthal, V. Fiddler, G.R. Fleming, E. Castner Jr., J. Chem. Soc., Faraday Trans. 91 (1995) 867. [17] N. Nandi, K. Bhattacharya, B. Bagchi, Chem. Rev. 100 (2000) 2013. [18] R.B. Gregory (Ed.), Protein Solvent Interactions, Marcel Dekker, New York, 1995. [19] R. Pethig, Annu. Rev. Phys. Chem. 43 (1992) 177. [20] K. Bhattacharya, B. Bagchi, J. Phys. Chem. 104 (2000) 10603. [21] D.M. Willard, R.E. Riter, N.E. Levinger, J. Am. Chem. Soc. 120 (1998) 4151. [22] S.K. Pal, D. Mandal, K. Bhattacharya, J. Phys. Chem. B 102 (1998) 11017. [23] K. Rothkiewicz, K.H. Grellmann, Z.R. Grabowski, Chem. Phys. Lett. 19 (1973) 315.
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[24] S. Panja, P.R. Bangal, S. Chakravorti, Chem. Phys. Lett. 329 (2000) 377 (and references cited therein). [25] J. Kim, M. Lee, J. Phys. Chem. 103 (1999) 3378. [26] P.R. Bangal, S. Panja, S. Chakravorti, J. Photochem. Photobiol. A 139 (2001) 5. [27] J.M. Hicks, M.T. Vandersall, Z. Babarogic, K.B. Eisenthal, Chem. Phys. Lett. 116 (1985) 1.
[28] J.M. Hicks, M.T. Vandersall, E.V. Sitzmann, K.B. Eisenthal, Chem. Phys. Lett. 135 (1987) 413. [29] A.C. Testa, J. Lumin. 50 (1991) 243. [30] N. Mataga, Y. Kaifu, M. Koiumi, Bull. Chem. Soc. Jpn. 29 (1956) 465. [31] Y.-B. Jiang, M.-G. Jin, Spect. Chem. Acta A 56 (2000) 623.
Photophysics of 4-N,N-dimethylamino cinnamaldehyde in AOT reverse micelles and exploration of its position and orientation Subhasis Panja, Sankar Chakravorti
*
Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Received 21 August 2002; in final form 8 October 2002
Abstract An attempt has been made in this Letter to locate the position and orientation of 4-N,N-dimethylamino cinnamaldehyde (DMACA) inside sodium bis(2-ethylhexyl) sulfosuccinate (AOT)–n-heptane reverse micelle based on change in photophysical properties of DMACA compared to that in n-heptane. It has been proposed that the possibility of finding the donor moiety inside the small water pool of reverse micelle is maximum while the acceptor group straddles in the remaining part of the reverse micelle. The micropolarity in the vicinity of the donor moiety has been computed in terms of dielectric constant with varying water pool size. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Reverse micelles are thermodynamically stabilized organized systems formed by surfactant molecules in hydrocarbon solvents with polar head groups pointing inwards. When water is added to surfactant in hydrocarbon solution a spherical discrete droplet of nanometer size is built by aggregation of inward pointing polar head groups. In sodium bis (2-ethylhexyl) sulfosuccinate (AOT)– n-heptane reverse micellar system the radii of these water pools are defined by twice the molar ratio
*
Corresponding author. Fax: +91-3347-32805. E-mail address: [email protected] (S. Chakravorti).
(2W0 ) between the polar component and the surfactant {AOT} where W0 ¼ ½H2 O=½AOT. The value of W0 for reverse micelles lies between 10 and ) and those with radii 15 (radius 2W0 ¼ 30 A are called microemulmore than hundreds of A sions [1–3]. As reverse micelles provide interesting microenvironments there has been an engaging interest in the studies for chemical, enzymatic catalysis [4,5] and biological systems [1,6–11]. Some processes like isomerization [12], energy transfer [13], proton transfer [14,15] get modified greatly inside the restricted pool of water probably due to the special properties of reverse micelles. Reverse micelles are charged inside due to which a huge change of solvent relaxation time between inner side of the droplet compared to outside [16] could be observed. Several groups have studied relaxa-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 6 9 4 - 9
S. Panja, S. Chakravorti / Chemical Physics Letters 367 (2003) 330–338
tion dynamics of water in various self-organized assemblies with different techniques [17–22] and found that the relaxation time is much slower in water filled in nanospace than that in bulk. Some molecules upon photoexcitation undergo internal rotation that leads to twisted intramolecular charge transfer (TICT) state [23]. This process leads to an additional fluorescence whose quantum yield strongly depends on solvent polarity and viscosity [24]. Kim and Lee [25] observed that fluorescence lifetime of a hemicyanine dye in the water pool of reverse micelle increases with increasing pool size, implying the significant influence of microstructure of reverse micelle on excited state twisting motion. We have identified one molecule 4-N,N-dimethylamino cinnamaldehyde (DMACA) [26] which forms TICT state in aprotic solvents but in protic solvent hydrogen bonding of acceptor part of the molecule and the solvent trammels the formation of TICT state. This property of DMACA has prompted us to use this molecule as a probe in exploring different regions of reverse micelle, water pool and the micellar interface, through the twisting dynamics of DMACA and related photophysical change.
2. Experimental details The compound DMACA was purchased from Aldrich Chemical, USA and was purified by vacuum sublimation. The solvents acetonitrile (ACN), tetrahydrofuran (THF), chloroform (CHCl3 ), N,N-dimethyl formamide (DMF), dichloro methane (DCM), n-heptane, methanol, ethanol and butanol (E. Merck, spectroscopic grade) were used as supplied, but only after checking the purity fluorimetrically in the wavelength range of interest. Sodium bis(2-ethylhexyl) sulfosuccinate received from Fluka was dried in vacuum before use. For aqueous solution, we used deionized MilliPore water. Sufficient time 12 h was given for the equilibration of micellar solution. The absorption spectra at 300 K were recorded with a Shimadzu absorption Spectrophotometer model UV-2101PC, and the fluorescence spectra were obtained with a Hitachi F-4500 fluorescence spectrophotometer. For emission measurement,
331
the sample concentration was maintained at 105 M, in each case in order to avoid aggregation. The fluorescence lifetime >500 ps was measured by time-correlated single photon counting coupled to a micro channel plate photomultiplier (Model 28090, Hamamatsu, Edinburgh Instrument). The degree of polarization of fluorescence at room temperature was obtained by using a modified polarizer accessory with the Hitachi F-4500 spectrophotometer.
3. Results and discussion 3.1. Absorption spectra DMACA in n-heptane shows a strong absorption band 350 nm. With addition of AOT the absorption band shows a gradual decrease of its absorbance along with a redshift. Fig. 1a depicts the AOT concentration dependent absorption spectra of DMACA in n-heptane. A new absorption shoulder in the lower energy side (380 nm) appears with the addition of AOT. Absorption spectra of DMACA have been recorded by varying amount of water in a fixed concentration of AOT in n-heptane, i.e., varying the water pool size (W0 ) of the reverse micelle. Fig. 1b depicts the water pool size (W0 ) dependent absorption spectra of DMACA in 0.1 M of AOT in n-heptane solution. The absorbance shows a sharp increase along with a little redshift with the increase of water pool size. 3.2. Emission spectra DMACA in all polar solvents shows distinct dual fluorescence, the higher energy emission (350 nm) assigned to be arising from the locally excited (LE) state, called the LE emission [23]. The lower energy emission above 440 nm assigned to be arising from the twisted intramolecular charge transfer (TICT) state [23] and is called as TICT emission. The TICT emission could only be observed in polar solvents but in non-polar solvents it is absent. In all polar protic solvents hydrogen bonding between the C@O group of solute and the hydroxylated part of the solvent plays a crucial role to stabilize the TICT state. Accordingly the
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Fig. 1. (a) AOT concentration dependent absorption spectra of DMACA in n-heptane. (b) W0 dependent absorption spectra of DMACA in AOT–n-heptane reverse micelle.
Fig. 2. (a) AOT concentration dependent emission spectra of DMACA in n-heptane. (b) W0 dependent emission spectra of DMACA in AOT–n-heptane reverse micelle.
TICT emission shows more redshift in polar protic solvents compared to the solvent polarity with respect to the other polar aprotic solvents. Hydrogen bond formation act as one of the nonradiative channel of the TICT emission and the TICT emission intensity gradually decreases with the increase of hydrogen bond donating tendency of the solvent [26]. In n-heptane DMACA hardly shows any TICT emission due to the lower polarity of the solvent but with the addition of AOT in n-heptane an emission band arises at 440 nm (being excited at 345 nm) as the TICT emission. Fig. 2a shows the AOT concentration dependent
TICT emission of DMACA in n-heptane. The TICT emission of DMACA in n-heptane–AOT reverse micellar media shows sharp change with the increase of water concentration. Fig. 2b shows the variation of the fluorescence spectra of DMACA in n-heptane–AOT reverse micelle as function of water pool sizes (W0 ). For initial addition of water (up to W0 3) the TICT emission shows an strong enhancement along with a large redshift (up to 470 nm) but with further increase of water pool size the band position shows a little change though the emission intensity increases with increase of water pool size. The emission
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shows a maximum intensity for water pool size ðW0 Þ 20 and addition of further water does not effect the emission significantly. The TICT fluorescence lifetime measurement showed that the lifetime does not change appreciably in the nanosecond range with varying water pool size. The excitation wavelength dependent emission spectra have been recorded for a fixed value of W0 ð 15Þ which shows that the emission band position remains unaltered for different excitation wavelengths (350–390 nm). The excitation spectra of the same solution have been recorded for different emission wavelengths, which reflect the absorption spectrum. This fact indicates the presence of single type emitting species in the reverse micellar system. The gradual decrease of the absorbance as well as the redshift with the addition of AOT in nheptane indicates the change of polarity of the surrounding environment of the fluorophore. The sharp change of the absorption spectra with water pool sizes (W0 ) in n-heptane–AOT reverse micellar system suggests that water pool is accessible to DMACA molecules. So the DMACA molecule resides either in the micellar–water pool interface region or in the water pool of the reverse micelle or spreading in both regions to get the polarity effect in ground state of DMACA. In pure n-heptane the TICT band is completely absent due to the lower polarity of the solvent, as we know the TICT state formation barrier depends upon the solvent polarity. Eisenthal and coworkers [27,28] proposed a semiempirical formula that the TICT state formation barrier (EB ) decreases linearly with the increase of solvent polarity following the relation: EB ¼ EB0 A½ET ð30Þ 30; EB0
ð1Þ
where is the barrier in hydrocarbon having ET ð30Þ values 30 kcal mol1 . So with the initial addition of AOT in n-heptane the DMACA goes to polar state and consequently shows the TICT emission according to the semi-empirical formula of Eisenthal et al. The W0 dependent emission spectra show an initial strong enhancement and a large redshift with the increase of water amount. So the water pool size dependent emission spectra also confirm that water pool is accessible to
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DMACA to get the polarity effect in the excited state in n-heptane–AOT reverse micelle. Up till now we have gathered from absorption and emission spectra that a part or the whole DMACA molecule is inside the water pool. In our next part of this work we will try to find the solubulization position and the orientation of the DMACA molecule within the n-heptane–AOT reverse micellar system. In our earlier work [26] we found that the TICT emission of DMACA shows an unusually large redshift in polar protic solvents than those expected on the basis of solvent polarity in polar aprotic solvent alone. The fluorescence band position bears a linear correlation with hydrogen donating parameter (a) of the solvent (Fig. 4a). The fluorescence yield decreases with the increase of hydrogen bonding tendency of the solvents. These facts suggest that hydrogen bond formation between the carbonyl group (C@O) of the fluorophore (hydrogen bond acceptor) and the hydroxylated solvents (hydrogen bond donor) acts as a non-radiative channel of the TICT state as proposed by Testa [29]. In AOT reverse micellar system the TICT emission of DMACA shows an initial enhancement and red shift with the increase of water pool size (W0 ) whereas for water pool size greater than 15 the emission spectra does not show any sharp response. In this regard we have studied the effect of addition of water in its absorption and emission spectra of DMACA in acetonitrile (ACN) solvent. ACN is a polar aprotic solvent so hydrogen bond formation between the solute and solvent is not possible. With addition of water in acetonitrile, hydrogen bond formation between the solute DMACA and the added water molecule is possible. Initially the absorption spectra of DMACA in ACN show a little increase along with a small red shift with the increase of water concentration (Fig. 3a). The emission spectra of DMACA in ACN (Fig. 3b) show a sharp decrease in its intensity as well as a red shift with the increase of water concentration which clearly indicate the formation of hydrogen bond between the carbonyl (C@O) group of DMACA and hydroxylated part of the water molecule. Fig. 3b (inset) depicts the variation of normalized emission of DMACA with the increase of water concentration
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Fig. 3. Variation of absorption spectra (a) and emission spectra (b) of DMACA in ACN with the increase of water concentration. Inset shows the variation of band position of normalized emission spectra.
in ACN. Considering the above facts and the spectral observations relating to DMACA in nheptane–reverse micelle we can infer that the hydrogen bond formation between the C@O group of DMACA and the water molecule of the water pool of the n-heptane–AOT reverse micelle does not occur as no indication of decrease of intensity or extra redshift of emission could be observed even for the larger water pool size or greater water concentration. So it is sure that the carbonyl (C@O) group of the DMACA does not reside inside the water pool of the n-heptane–AOT reverse micellar system.
Fig. 4. (a) Variation of band position of DMACA with hydrogen bonding parameter (a) in polar protic solvent (b) Lippert–Mataga parameter (Df) corresponding to the Stokes shift in different water pool sizes.
Having established the above we now try to locate the orientation of the molecule in reverse micelle. The TICT state is the most polar state of the molecule DMACA and in its excited state the donor (amino) group releases an electron to the acceptor carbonyl (C@O) group as a result of complete decoupling between dimethylamino and the carbonyl moieties by twisting the amino group in a perpendicular position. The donor [NðCH3 Þ2 ] group becomes cationic and the acceptor (C@O) group becomes anionic in nature in the TICT
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Scheme 1. Approximate presentation of orientation of DMACA within AOT–n-heptane reverse micelle.
state. Now we consider the structure of the reverse micelle formed by the anionic surfactant AOT. The polar heads ()ve ion) of the AOT surround the water pool. So the acceptor part of the molecule would like to go away from the water due to the electrostatic repulsion by the polar head of AOT and also due to the relative hydrophobicity of C@O group of DMACA [26]. Earlier we have observed that DMACA is able to reach water in n-heptane–AOT reverse micelle. So considering the electrostatic attraction between the polar head of the AOT (anionic) and the fluorophore (cationic, donor group) DMACA and hydrophobic force on acceptor C@O we can predict that, the donor [NðCH3 Þ2 ] group of DMACA would try to reside near the polar head of the AOT reverse
micelle in the water pool. At this point we may consider two types of possible orientations of DMACA within the AOT–n-heptane reverse micelle. In one case the donor group is inside the water pool and near the polar head of AOT due to electrostatic attraction and the acceptor is situated outside the water pool (Scheme 1a). The other case being both the donor and acceptor situated outside the water pool with the acceptor part near the water pool–micelle interface (Scheme 1b). To have a better idea about the exact location and the orientations of the molecule DMACA in n-heptane–AOT reverse micelle we have measured the variation of TICT fluorescence polarization of DMACA as a function of water pool size of the
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reverse micelle. The degree of polarization (P) of fluorescence is defined as P¼
Ik GI? ; Ik þ GI?
ð2Þ
where Ik and I? are the intensity of the emitted light polarized with electric wavevector parallel and perpendicular, respectively, to the excited light which is polarized with electric vector perpendicular to the plane containing the excited and emitted beam. G is the correction factor of the polarization of the fluorescence spectrophotometer defined as G ¼ Ik =I? . Fig. 5 shows the variation the fluorescence polarization of DMACA with water pool sizes (W0 ) of the AOT reverse micelle. DMACA shows a very negligible fluorescence polarization in pure n-heptane or in pure aqueous media but just after the water droplet is formed (W0 < 4) the fluorescence polarization shoots up and with increasing water pool size (W0 5 or more) the fluorescence polarization decreases exponentially. It seems that the molecule faces initial restriction in small pool size which decreases with bigger pool size.
Fig. 5. Variation of polarization with water pool size in AOT– n-heptane reverse micelle.
For the sake of understanding we take help of Scheme 1, a possible position in dynamic equilibrium. In Scheme 1b the fluorophore DMACA is situated in the interfacial region with possible random orientations in such a way that the acceptor group does not enter inside the water pool. In such a case with the increase of water pool size the intermolecular distance between the surfactant (AOT) molecule will increase and the donor group would get much more space for its free rotation. So the restriction imposed upon the donor group by the AOT molecule will gradually decrease with the increase of the water pool size which will cause a decrease in the fluorescence polarization [31]. No such decrease of polarization could be observed in our case that indicates that the donor group is certainly not in the interfacial region of the reverse micelle. So Scheme 1b is untenable on the basis of the observed polarization data. In n-heptane–AOT reverse micelle and the the radius of the water pool is 2W0 A [26]. Let length of the DMACA molecule is 4 A us now consider Scheme 1a where the donor group resides inside the water pool of the reverse micelle. Before adding water the AOT and DMACA molecules roam about randomly. As soon as the water droplet is formed with smallest radius the donor group moves into the pool and faces restriction of free rotation of NðCH3 Þ2 group but the restriction is decreased with bigger pool size (W0 > 5). Beyond this pool size (W0 > 5) the polarization remains almost unaltered (Fig. 5). Thereby we may infer that beyond this size the restriction for free rotation of donor molecule) is constant. So group (a part of 4 A Scheme 1a efficiently explains polarization and other data. We have tried to estimate the micropolarity of the solubilization position of the fluorophore DMACA inside the reverse micellar media by monitoring the TICT band position. The TICT state possesses a large dipole moment and according to Lippert–Mataga [30] the energy of such polar band depends in a linear way with the solvent polarity following the relation: ðDmÞ ¼ ma mf ¼
2l2e f ðD; nÞ; 4pe0 hcq3
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Table 1 Variation of calculated dielectric constant of the surrounding environment of DMACA with the increase of water pool sizes in nheptane–AOT reverse micelle [water]/[AOT]ðW0 Þ
ðDmÞ in cm1 ð 103 Þ
Calculated f ðD; nÞ from Lippert–Mataga plot
Calculated dielectric constant
0 3 7 15 20
4.27 4.59 4.72 4.81 4.91
0.181 0.241 0.263 0.281 0.30
4.5 7.8 10.8 14.5 22.9
where the Lippert–Mataga parameter ðD 1Þ ðn2 1Þ f ðD; nÞ ¼ ; ð2D þ 1Þ ð2n2 þ 1Þ ma ; mf are the absorption and emission wavelength, D and n are the dielectric constant and refractive index of the medium h; e0 ; c; q are PlanckÕs constant, permittivity of the vacuum, velocity of light and the radius of the solvent cavity, le represents the excited state dipole moment. In all polar protic solvent the TICT emission shows a linear relationship with the hydrogen bond formation tendency of the solvent but in all polar aprotic solvents the Stokes shift ðDmÞ shows a linear dependence with the Lippert– Mataga f ðD; nÞ parameter of the solvent. Fig. 4b shows the variation of Dm of DMACA with f ðD; nÞ in different solvents. The different f ðD; nÞ of varying water pool sizes corresponding to the solubilization position of the DMACA can be estimated from the corresponding Dm. Assuming the refractive index of water pool of the reverse micelle remains same (1.33) we have calculated the dielectric constant D of the surrounding environment of the DMACA and have been recorded in Table 1. Acknowledgements The authors express their deep sense of gratitude to Professor S. Basak, SINP, Kolkata for kindly allowing them to use the fluorescence lifetime measuring instrument. The authors express their sincere thanks to the reviewer for his critical comments and suggestions towards the improvement of the Letter.
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